Here we add DMACI to the FA_0.83Cs_0.17Pb(I_xBr_1-x) perovskite precursor with x = 0.1 to 0.4 and study its impact on the perovskite formation. The DMACl changes the intermediate phase during the formation of the perovskite. A series of 2D XRD shows that the crystal orientation changes from a preferred corner-up to a face-up orientation for the FACs perovskite. It also impacts the surface morphology of the perovskite significantly leading to an almost needle-like growth of the perovskite for 100 mol% excess DMA. The DMA: FACs perovskite shows also a significant increase in stability under light, heat and humidity exposure compared to the commonly DMF/DMSO FACs perovskite. Our champion device reaches stabilised 18.2% and 16.1% in an n-i-p and a p-i-n stack respectively. The DMF/DMSO FACs control achieves stabilised 18.3% (n-i-p) and 14.8% (p-i-n).  

Achieving long-term stability of perovskite solar cells is arguably the most important challenge required to enable widespread commercialization. Understanding the impact of the perovskite material quality on device stability is critical to overcoming this hurdle. Surprisingly, we found that the fabrication methods had significant impact on the overall perovskite stability. The commonly employed “DMF/DMSO” solvent system preparation method resulted in poor crystal quality and microstructure, which ultimately lead to an inferior material stability compared the “DMF/acid” or “DMF/DMA” processing method, proposed in this work. These fabrication methods exhibited a high degree of texturing and crystallinity. Furthermore, we observe residual DMSO in the perovskite film, which could adversely affect the stability of the material. In this work, we introduce a high temperature “DMSO-free” processing method that utilizes DMACl to accurately control the perovskite precursor phases. By precisely controlling the 2H to 3C crystallization sequence rate, we can tune the size, texturing, orientation (corner-up vs face-up) and crystallinity of the crystal grain to extend the long‑term operational lifetime of the (FA,Cs)Pb(Br,I)3 perovskite system. We investigated the impact of these various crystal quality film on the stability of the perovskite by decoupling the main degradation mechanisms (humidity, heat and light). A population of encapsulated devices showed a t80 lifetime, for the stabilized PCE, of 1190 h as median value and a champion device showing a t80 of 1410 h, under simulated sun light at 65 °C in air, under open‑circuit conditions, in contrast to a median value of t80 = 1040 h and a champion t80 = 780 h for conventional DMF/DMSO devices. Our work introduces an innovative processing method that allows higher overall perovskite device stability, by controlling the intermediate phases domains during the perovskite formation. This work highlights the importance of material quality in order to achieve long-term operational stability of perovskite solar cells.

Mixed lead–tin (Pb-Sn) perovskites have emerged as promising low band gap absorber materials with recent efficiencies exceeding 20% for single junctions[1], [2] and 24.8% for all-perovskite tandem solar cells[3]. While these results reflect the great potential of this class of absorber materials, mixed Pb-Sn perovskites are inherently less stable than their neat Pb counterparts. Therefore, the development of practical perovskite multi-junctions requires first understanding the degradation mechanisms inherent to these materials so that routes can be designed to address them. Previous studies have shown that their main instability is chemical in nature and stems from the tendency of Sn²⁺ to oxidise, which leads to the formation of Sn vacancies in the lattice, subsequent p-doping, and reduced optoelectronic quality.[4]–[7] Moreover, it has been reported that exposure to oxygen results in the facile oxidation of Sn and formation of SnO₂[8], [9] and that the presence of Pb in neighbouring lattice sites can influence how degradation proceeds.[10] A variety of different perovskite compositions that blend different ionic species at various lattice sites can achieve high power conversion efficiencies with nominally the same band gap value. While the detrimental effects of Sn oxidation are already known and some general strategies have been proposed to mitigate it, less effort has been applied to understanding how composition impacts material degradation.

Herein, a side-by-side degradation assessment of several different low band gap compositions is presented. The evolution of structural and optoelectronic properties under either thermal or humidity stressing over time is monitored in order to identify compositional trends. Our results highlight how ion choice can impact the speed of degradation and alter the degradation footprint by triggering different degradation modes, which we expect will inform future material selection for reliable multi-junction development.

It has been proposed that halide defect states determine the photoinduced ionic dynamics of metal-halide perovskites, including molecular iodine formation and removal, and consequent perovskite degradation.[1–3] Conversely, it has been suggested that the volatility of methylammonium (MA) is the primary component compromising MA-based perovskite devices.[4] In this study, we investigate the effect of photoinduced ion migration from metal halide perovskites upon commonly used electron and hole transport layer (ETL/HTL) materials and the perovskite itself. By placing the separate perovskite and TL films together under illumination, we allow species to photomigrate from the perovskite to the TL films, whilst still able to independently probe the films. Through optical, vibrational, chemical and electrical measurements, we propose a degradation pathway and identify the most critical ionic species upon perovskite device stability.

Halide perovskite semiconductors have recently gathered significant attention due to their intriguing optoelectronic properties combined with low-cost and simple fabrication method. In addition, the easy bandgap tunability of this material by changing the ratio of halides in the chemical composition, makes them promising candidate for LEDs and tandem solar cells in combination with silicon. However, illuminating mixed-halide perovskites results in the formation of segregated phases enriched in a single halide. Phase segregation affects the homogeneity of the bandgap compromising the purity and the quality of the absorption/emission and therefore its applications. This segregation occurs through ion migration, which is also observed in pure-halide compositions, and whose control is essential to enhance lifetime and stability.

In this work, we investigate how pressure-induced compression of the unit cell volume affects the kinetics aspects of phase segregation in mixed halide perovskite MAPb(Br_xI_1-x)₃ (where x is the bromide concentration equal to 25%, 50% and 70%). Using pressure-dependent transient absorption spectroscopy, we find that the formation rates of both iodide- and bromide-rich phases in MAPb(Br_xI_1-x)₃ reduce by ~2 orders of magnitude upon increasing pressure to 0.3 GPa. We interpret this change as a compression-induced difference in the activation energy for ion migration, which is supported by first-principle calculations. A similar mechanism occurs when the unit cell volume is reduced by incorporating a smaller cation. These findings reveal that stability with respect to halide segregation can be achieved either physically through compressive stress or chemically through compositional engineering and that in principle any iodide-bromide ratio can be thermodynamically stabilized by tuning the unit cell volume.

Unlike typical inorganic semiconductors, lead-halide perovskites (LHPs) exhibit significant ionic conductivity, which is believed to affect their performance and stability. Motivated by a recent experimental study that suggested pressure as a means to control ionic conductivity in CsPbBr₃ [1], we present a detailed theoretical analysis of the atomic scale effects of pressure on anion migration in the low-temperature orthorhombic Pnma phase of CsPbBr₃ [2]. Using density functional theory, we compute the transition state structures and activation enthalpies and entropies associated with anion vacancy hopping between nearby lattice sites. We use those data to parametrise a kinetic model for anion vacancy migration, which takes into account the non-trivial topology of the Pnma anion sublattice, and solve it to determine the mobility tensor as a function of applied pressure.

As the pressure is increased, we find that the mobility tends to become increasingly anisotropic, such that at 2.0 GPa the mobility in the [010] direction is three orders of magnitude lower than the mobility in the (010) plane to which it is normal. This can be explained by the fact that a network of only a small subset of possible hops dominates the mobility at elevated pressures, leading to a strongly anisotropic response of the mobility tensor to increasing pressure. Our results demonstrate the potential importance of pressure in controlling both the rate and direction of anion migration in LHPs.

Co-evaporation of perovskites becomes more and more popular these days, as its advantages over other processing methods, are low-temperature processing of multi-layered structures as well as precise control over film thickness and composition.¹ These benefits make the sublimation technique also an interesting and suitable method for large-scale fabrication of high-purity systems. To achieve homogeneous, defect-free devices on a large scale, many engineering challenges need to be solved as a typical co-evaporated perovskite film, has multiple grain-sizes and surface defects. Understanding therefore all key-parameter that control the perovskite crystallization, are necessary to fabricate defect-free large-area devices with a high efficiency. In our work we have identified different factors, that give insight into these processes which will lead in the future to higher reproducibility of device performance for co-evaporated solar cells.

[1] Sessolo, M., Momblona, C., Gil-Escrig, L., and Bolink, H.J. (2015). Photovoltaic devices employing vacuum-deposited perovskite layers. MRS Bull. 40, 660–666.

Understanding the atomic-scale crystallographic properties of photovoltaic semiconductor materials such as silicon, GaAs, and CdTe has been essential in their development from interesting materials to large-scale energy conversion industries. However, studying photoactive hybrid perovskites by transmission electron microscopy (TEM) has proved particularly challenging due to the large electron energies typically employed in these studies.[1] In particular, the very close structural relationship between a number of crystallographic orientations of the pristine perovskite and lead iodide has resulted in severe ambiguity in the interpretation of EM-derived information, severely impeding the advance of atomic resolution understanding of the materials.

Here, we successfully image the archetypal CH(NH₂)₂PbI₃ (FAPbI₃) and CH₃NH₃PbI₃ (MAPbI₃­) hybrid perovskites in their thin-film form with atomic resolution using a carefully developed protocol of low-dose STEM.[2] Our images enable a wide range of previously undescribed phenomena to be observed, including a remarkably highly ordered atomic arrangement of sharp grain boundaries and coherent perovskite/PbI₂ interfaces, with a striking absence of long-range disorder in the crystal. These findings explain why inter-grain interfaces are not necessarily detrimental to perovskite solar cell performance, in contrast to what is commonly observed for other polycrystalline semiconductors. Additionally, we observe aligned point defects and dislocations that we identify to be climb-dissociated, and confirm the room-temperature phase of CH(NH₂)₂PbI₃ to be cubic. We further demonstrate that degradation of the perovskite under electron irradiation leads to an initial loss of CH(NH₂)²⁺ ions, leaving behind a partially unoccupied, but structurally intact, perovskite lattice, explaining the unusual regenerative properties of partly degraded perovskite films. Our findings thus provide a significant shift in our atomic-level understanding of this technologically important class of lead-halide perovskites.

Impedance (IS), intensity-modulated photocurrent (IMPS) and photovoltage (IMVS) spectroscopies have been used extensively to characterize the operating mechanisms of photosensitive devices.[1, 2] These three techniques are normally used independently, their connection and interpretation are still under discussion.[3, 4] The responses obtained with IS are generally analyzed through the modeling of the internal electronic mechanism using an equivalent circuit (EC), the selection of which is often difficult since many EC may provide the same electrical response.[5] On the other hand, the analysis of the spectra obtained with IMPS and IMVS are commonly limited to the electronic characteristic times of the devices.[6-8] In this talk, we present a procedure that allows the simultaneous analysis of these three spectroscopic techniques (IS, IMPS and IMVS), as well as the identification of the more appropriate EC for a photosensitive device. In the particular case of a silicon photodiode, by using this procedure, we expose that the experimental spectra obtained with the three techniques can be correlated with the same EC. Furthermore, parameters inaccessible from IS are obtained, such as the separation efficiency, external and internal quantum efficiency. The application of these results in different photovoltaic and photoelectrochemical devices will allow more precise identification and quantification of their operating mechanisms.

Authors: Tiarnan A.S. Doherty1§, Andrew J. Winchester2§, Stuart Macpherson1, Duncan N. Johnstone3, Vivek Pareek2, Elizabeth M. Tennyson1, Sofiia Kosar2, Felix U. Kosasih3, Miguel Anaya1, Mojtaba Abdi-Jalebi1, Zahra Andaji-Garmaroudi1, E Laine Wong2, Julien Madéo2, Yu-Hsien Chiang1, Ji-Sang Park4, Young-Kwang Jung5, Christopher E. Petoukhoff2, Giorgio Divitini3, Michael K. L. Man2, Caterina Ducati3, Aron Walsh4,5, Paul A. Midgley3, Keshav Dani2*, Samuel D. Stranks1*

Metal halide perovskite (MHP) materials exhibit exceptional performance characteristics for low-cost optoelectronic applications. Though widely considered defect tolerant materials, perovskites still exhibit a sizeable density of deep sub-gap non-radiative trap states, which create local variations in photoluminescence [doi: 10.1126/science.aaa5333] that fundamentally limit device performance. These trap states have also been associated with light-induced halide segregation in mixed halide perovskite compositions [doi: 10.1021/acsenergylett.8b02002] and local strain [doi:10.1039/C8EE02751J], both of which can detrimentally impact device stability. The origin and distribution of these trap states remains unknown as multiple, complimentary multi-modal techniques are required to probe their location and surrounding structure and composition and MHPs damage rapidly under the electron beam. Understanding the nature of these traps will be critical to ultimately eliminate losses and yield devices operating at their theoretical performance limits with optimal stability. In this talk, we outline a low dose, multiple – metrological framework to reveal the structural origins of non-radiative recombination sites in (Cs0.05FA0.78MA0.17)Pb(I0.83Br0.17)3 thin films. By combining scanning electron diffraction and energy dispersive X-ray spectroscopy, with photoemission electron microscopy (PEEM) measurements we reveal that nanoscale trap clusters are distributed non-homogenously across the surface of high performing perovskite films and that there are distinct structural and compositional fingerprints associated with the generation of these detrimental sites.

The commercialization of perovskite solar cells struggles despite the booming power conversion efficiencies, facile processability, and good compatibility with large‐area deposition techniques. The primary reason being the several instabilities of the perovskite devices. Here we show that enabling an indium tin oxide (ITO) buffer layer in the inverted architecture can significantly improve the stability of PSCs. Our holistic approach either dramatically slows down or completely prevents most of the degradation processes. Further, we demonstrate superior light soaking stabilities retaining 80% of initial efficiency under continuous illumination for over 2000h in ambient air and excellent thermal stabilities for more than 1500 hours with only less than 5% degradation under 85 °C thermal aging. Our barrier layer design enables excellent moisture, thermal and light-soaking stabilities to the perovskite solar cells, which is a crucial step to commercialization. The application of this stabilization strategy to large area cells and modules will be shown.

Perovskite Solar Modules (PSMs) are attracting the photovoltaic market showing low manufacturing costs and process versatility. The upscaling of perovskite solar cells is one of the challenges that must be addressed to pave the way toward the commercial development of this technology. As for other thin-film photovoltaic technologies, upscaling requires the fabrication of modules composed of series-connected cells. The use of flexible substrates gives the possibility to explore new applications and could further increase the production throughput. However, the current state of art of Flexible Perovskite Solar Modules (FPSMs) does not show any data on light soaking stability, revealing that the scientific community is still far from the potential marketing of the product. In this work, we demonstrate, the use of double-cation perovskite (forsaking the unstable methylammonium (MA) cation) by employing potassium-doped graphene oxide (GO-K) as an interlayer, between the mesoporous TiO₂ and the perovskite layer and using infrared annealing (IRA). We upscaled the device active area from 0.09 to 16 cm² by blade coating the perovskite layer, exhibiting power conversion efficiencies (PCEs) of 18.3 and 16.10% for 0.1 and 16 cm² active area devices, respectively. We demonstrated how the efficiency and stability of MA-free-based perovskite deposition by blade coating have been improved by employing GO-K and IRA. We further demonstrate on inverted structure using flexible substrate, a light stability of FPSMs over 1000 hours considering the recovering time (T80=730 h), exhibiting a PCE of 10.51% over 15.7cm² active area obtained with scalable processes by exploiting blade deposition of PTAA in air and stable Double Cation Perovskite (CsFA) absorber. We finally conclude our work by demonstrating the interconnection of inverted modules with NiO_x using a UV ns laser, obtaining a 10.2 cm² minimodule with a 15.9% efficiency on the active area, the highest for a NiO_x based perovskite module. The results are implemented in a complete electrical simulation of the cell-to-module losses to evaluate the experimental results and to provide an outlook on further development of single junction and multijunction perovskite modules.

Halide perovskites have been widely explored in photovoltaics research due to their outstanding properties as light absorbing materials. The most efficient devices use Pb-based halide perovskite. PbI2 and PbBr2, together possibly with oxide/hydroxides, are the most likely decomposition products if cells are broken and the perovskite is exposed to the ambient. These decomposition products show some solubility in water, so rain can lead to Pb reaching the groundwater. Such a scenario carries environmental and public health risk. We show here how to mitigate and even eliminate this scenario by efficient sequestering of lead. We show a scheme of ligands for sequestration as part of an encapsulation, using commercially viable materials and methods, reaching till now 85% lead retention within insoluble products (that can be removed from a disaster zone, i.e., reduce the problem to that of commercial CdTe-based cells), using commercially viable materials and methods.

Some years back we introduced alkoxy functionalized donor groups as a building block in organic dyes as light harvesters for DSSC. This donor group provides a desirable 3-dimensional structure that aids in surface protection of electrons injected into the semiconductor from oxidants in the electrolyte, allowing for record-setting cobalt- and copper-based redox shuttles to be utilized more frequently. With these systems we recently set a certified world record efficiency for DSSC of 13%. DSSCs are ideally suited for ambient light and indoor applications where efficiencies up to 35% have been reached calculated with respect to the fluorescent light source.

For perovskite solar cells we have developed methods to accomplish a stable FAPbI₃ phase. We found, for example, that a film of the photoinactive yellow δ- phase was converted to a highly crystalline black α-phase by vapor exposure to methylammonium (or formamidinium) thiocyanate at 100°C, and it retained this structure after 500 hours at 85°C. We have also introduced an anion engineering concept that uses the pseudo-halide anion formate (HCOO−) to suppress anion-vacancy defects that are present at grain boundaries and at the surface of the FAPbI₃ films. The resulting solar cell devices attain a power conversion efficiency of 25.6 per cent (certified 25.2 per cent). In our most recent work we have obtained record level efficiencies of of 23.3%, 21.7% and 20.6% with active areas of 1, 20 and 64 cm2, respectively.

Metal-halide perovskite semiconductors have received tremendous attention in research due to their excellent optoelectronic properties, making them interesting materials for solar cells and light emitting diodes (LEDs). It is fascinating that these perovskites are highly tolerant against electronic defects and at the same time show pronounced ionic conductivity mediated through mobile lattice defects.

In this talk the various effects of ion migration on device performance are described. They range from hysteresis in the current-voltage curve of solar cells, to reversible degradation during long-term operation, to phase-segregation in mixed-halide systems, to electroluminescence in LEDs that depends on the mode of operation. Key in understanding these phenomena is the interplay between ionic and electronic conductivity, where the ionic response belatedly modifies the electronic response, which is the one commonly observed in devices. This interplay has consequences on how to interpret results of common characterization techniques such as impedance spectroscopy and can also be seen in lead-free double perovskite solar cells, which conclude the talk.

I will discuss here our studies on the nature of defects and their photo-chemistry in tin-halide perovskites thin films. First I show that, in inert conditions, tin, p-doped, and lead (intrinsic) based perovskite thin film show comparable photoluminescence quantum yield, at comparable morphology, while photovoltaic devices, showing comparable device architecture show a dramatic reduction in power conversion efficiency for tin based devices. Tin perovskite thin film are also extremely stable under light soaking. However, the use of additives, meant to control the level of doping in the semiconductor, or the mixing of lead-tin metal cation for modulating the semiconductor bandgap, induces strong instabilities instabilities.  I will show how these behaviours are all related to the to the energetics of defects and their photochemistry activity

Perovskite and organic solar cells must be manufactured via low-cost, high-throughput methods in order to achieve meaningful penetration into the global energy infrastructure in the coming years. Slot-die coating is an established thin film production method with several decades of history in commercial manufacturing of thin film products such as adhesives, capacitors, and Li-ion batteries. It has been shown to satisfy the manufacturing requirements of thin film PV devices, but has typically seen limited lab-scale adoption. This is due to its origin as an industrial manufacturing method, which has made slot-die processes historically challenging and cost-prohibitive to develop at the lab scale.

Over the past decade, FOM Technologies has miniaturized and optimized slot-die technology towards repeatable, scalable R&D of novel thin film coatings and devices. This short presentation provides an overview FOM Technologies slot-die hardware, as well as the benefits of slot-die technology compared to other lab-scale thin film production techniques in thin film PV development.

Device optimization by modulating carrier concentration with doping to reduce recombination losses is well known in many technologies such as c−Si solar cells [1]. In emerging photovoltaic technologies such as the halide perovskites some recent publications have discussed the effect of doping on solar cell efficiency [2–5]. Also, Feldmann et. al [6] proposed that photo−doping by bandgap variation leads to increased photoluminescence quantum efficiency Qilum which then might lead to the increase in photovoltaic efficiency.  In the current work, presented here, we aim to provide a theoretical basis for the findings of Feldmann et al. and develop models to perform a critical assessment of the role of doping on PL quantum yield and photovoltaic device performance. To tackle the different questions, we use two approaches. First, we develop a simple analytical model to study the effect of lateral band gap variations and come to the conclusion that it cannot possibly be beneficial for device performance. Then we explore doping by homogeneous concentrations of charged defects that are either always ionized (doping) or only ionized under illumination (photodoping) and describe the consequences of these two forms of doping on luminescence, charge transport and efficiency using numerical device modelling.Device optimization by modulating carrier concentration with doping to reduce recombination losses is well known in many technologies such as c−Si solar cells [1]. In emerging photovoltaic technologies such as the halide perovskites some recent publications have discussed the effect of doping on solar cell efficiency [2–5]. Also, Feldmann et. al [6] proposed that photo−doping by bandgap variation leads to increased photoluminescence quantum efficiency Qilum which then might lead to the increase in photovoltaic efficiency.  In the current work, presented here, we aim to provide a theoretical basis for the findings of Feldmann et al. and develop models to perform a critical assessment of the role of doping on PL quantum yield and photovoltaic device performance. To tackle the different questions, we use two approaches. First, we develop a simple analytical model to study the effect of lateral band gap variations and come to the conclusion that it cannot possibly be beneficial for device performance. Then we explore doping by homogeneous concentrations of charged defects that are either always ionized (doping) or only ionized under illumination (photodoping) and describe the consequences of these two forms of doping on luminescence, charge transport and efficiency using numerical device modelling.

The ability to accurately model, understand and predict the behaviour of crystalline defects would constitute a significant step towards improving photovoltaic device efficiencies and semiconductor doping control, accelerating materials discovery and design.¹ In this work, we apply state-of-the-art ab initio techniques - hybrid Density Functional Theory (DFT) including spin-orbit coupling - to accurately model the atomistic behaviour of the cadmium vacancy (V_Cd) in cadmium telluride (CdTe).² In doing so, we resolve several longstanding discrepancies in the extensive literature on this subject.

CdTe is a champion thin-film absorber for which defects, through facilitation of non-radiative recombination, significantly impact photovoltaic (PV) performance, contributing to a reduction in efficiency from an ideal (Shockley-Queisser) value of 32% to a current record of 22.1%. Despite over 70 years of experimental and theoretical research, many of the relevant defects in CdTe are still not well understood, with the definitive identification of the atomistic origins of experimentally-observed defect levels remaining elusive.

In this work, through identification of a tellurium dimer ground-state structure for the neutral Cd vacancy, we obtain a single negative-U defect level for V_Cd at 0.35 eV above the VBM, finally reconciling theoretical predictions with experimental observations. Moreover, we reproduce the polaronic, optical and magnetic behaviour of V_Cd^-1 in excellent agreement with previous Electron Paramagnetic Resonance (EPR) characterisation.

The origins of previous discrepancies between theory and experiment, namely incomplete mapping of the defect potential energy surface (PES) and inherent qualitative errors in lower levels of electronic structure theory, are analysed in detail. Accordingly, this work helps to establish robust procedures for accurate and reliable modelling of defect processes in emerging materials, informing future investigations and enabling the acceleration of materials discovery and design procedures.

Metal halide perovskites are a highly promising class of semiconductors for the fabrication of cheap and efficient optoelectronic devices such as solar cells, LED’s, and photodetectors. Intense research effort has resulted in efficiencies for lab scale perovskite solar cells of over 25%. [1] More recently perovskite solar cells are also attracting a lot of commercial interest which is increasing the importance of factors such as long-term stability, processability at a large scale, and cost. Co-evaporation is a promising deposition technique for the upscaling of perovskites because it is additive, results in very uniform films, and does not require toxic solvents [2].

Recently two-dimensional organic metal halide materials which are similar to the well-known 3D perovskites have attracted a lot of attention. They are frequently called 2D perovskites, even so they do not possess a perovskite structure. Long organic molecules are used on the A site which are too big to be incorporated into the perovskite structure and a layered structure forms instead. These 2D materials are promising as stabilisation and passivation layers used together with 3D perovskites or for use on their own as active materials in LEDs and solar cells.

It has previously been shown by La-Placa et al. [3] that it is also possible to co-evaporate 2D metal halide semiconductors and integrate them into solar cells. Unfortunately, La-Placa et al. also found that the 2D layers can hinder charge transport in the device and while they may be helpful for the overall stability, they did not improve the solar cell efficiency. The orientation of the 2D sheets is crucial for the charge transport through the 2D layer. Being able to determine and ideally influence that orientation is therefore decisive for usability of these films in efficient devices.

In this study we co-evaporate PEA₂PbI₄ (phenethyl ammonium lead iodide) 2D metal halide thin films. We probe their properties in detail using electron microscopy, GIWAXS, and optical measurements. We find that using evaporation, it is possible to fabricate very uniform, planar, pin-hole free thin-films. Using GIWAXS measurements we probe the details of the texture and orientation in these films and how they are influenced by fabrication conditions. Understanding the direction of and degree of orientation in these films is an important step towards being able to influence the orientation which will be crucial for the integration of 2D metal halide perovskites into devices such as LEDs and solar cells.

Organometal halide perovskites have emerged as one of the most promising photoabsorbent materials for efficient and cheap solar cells.¹ The research in Perovskite Solar Cells (PSC) underwent an inflexion point with the replacement of the liquid electrolyte by solid‐state hole conductors (SSHCs), which increased significantly the Power Conversion Efficiency (PCE) and stability of the devices. The first and still most used SSHC is the organic molecule Spiro‐OMeTAD.²

Whereas the use of Spiro‐OMeTAD is widely spread, its undoped form is reported to have very poor intrinsic hole mobility and conductivity that can be increased by means of dopants.³ Unfortunately, the main handicap still hindering the eventual exploitation of PSCs is their poor stability under prolonged illumination, ambient conditions, and increased temperatures, which is partially hindered by the use of dopants that contribute to the degradation. For instance, Li⁺ cations, a common dopant, are highly hygroscopic and induce moisture absorption.³

However, negligible attention has been paid to the effect of the crystalline structure of Spiro-OMeTAD layers mainly due to the highly disordered nature of solution‐processed films (the most widely spread).⁴ Nevertheless, improved hole‐transporting properties of pristine Spiro‐OMeTAD single crystals have been reported.⁴ Even though these results show the potential of crystalline Spiro‐OMeTAD layers to enhance the photovoltaic properties of PSCs, their implementation is not straightforward due to the antisolvent experimental strategy used to grow this organic molecule in its crystalline form.⁴

In this work,⁵ we report the unprecedented sublimation under vacuum conditions of the Spiro‐OMeTAD, in the form of dopant‐free crystalline layers. In addition, we demonstrated the enhanced stability of these layers acting as SSHC in PSCs in comparison with the solution‐processed counterpart. Our results reveal that the substrate temperature is a critical parameter controlling the microstructure and crystallinity of the layers. On the other hand, the implementation of these vacuum sublimated Spiro‐OMeTAD layers on PSCs have demonstrated two key aspects: i) a considerably increased PCE in comparison to the dopant‐free Spiro‐OMeTAD layers fabricated by a standard wet approach and ii) a significant enhancement of the stability of the cells, which have been tested under continuous illumination during 40 h and after annealing in air up to 200 °C.⁵

The free carrier interaction with defects and critical defect properties remain unclear in methylammonium lead halide perovskites despite tremendous efforts in the study of their electric properties. Here we use a multi-method approach to quantify defects in single crystal MAPbI3. Time of Flight current waveform spectroscopy reveals the interaction of carriers with five shallow and deep defects. Photo Hall [1] and Thermoelectric effect spectroscopy assess the defect density, cross-section, and relative (to the valence band) energy. The detailed reconstruction of free carrier relaxation through Monte Carlo simulation[2] allowed quantifying the lifetime, mobility, and diffusion length of hole and electron separately. We demonstrate that the dominant part of defects releases free chargers after trapping, without non-radiative recombination with expected positive effects on the photoconversion and charge transport properties. On the other hand, shallow traps decrease sensibly drift mobility. Our results provide a trustworthy picture for future consideration on the defect properties in MAPbI3 since their properties have been crosschecked with multiple methods, which showed full consistency. Our results will allow optimizing charge transport properties and defects in MHP and will help improve their stability. Also, we pave the way for doping and defect control, enhancing the scalability of perovskite devices with large diffusion lengths and lifetimes.

[1] A. Musiienko, P. Moravec, R. Grill, P. Praus, I. Vasylchenko, J. Pekarek, J. Tisdale, K. Ridzonova, E. Belas, L. Landová, B. Hu, E. Lukosi, M. Ahmadi, Deep levels, charge transport and mixed conductivity in organometallic halide perovskites. Energy Environ. Sci. 12, 1413–1425 (2019).
[2]  A. Musiienko, J. Pipek, P. Praus, M. Brynza, E. Belas, B. Dryzhakov, M.-H. Du, M. Ahmadi, R. Grill, Deciphering the effect of traps on electronic charge transport properties of methylammonium lead tribromide perovskite. Sci. Adv. 6, 6393–6404 (2020).

Rapid advances in perovskite photovoltaics have produced efficient solar cells, with stability and duration improving thanks to variations in materials composition, including the use of layered 2D perovskites. A major reason for the success of perovskite photovoltaics is the presence of free carriers as majority optical excitations in 3D materials at room temperature. On the other hand, the current understanding is that in 2D perovskites or at cryogenic temperatures insulating bound excitons form, which need to be split in solar cells and are not beneficial to photoconversion. Here we apply a tandem spectroscopy technique that combines ultrafast photoluminescence and differential transmission to demonstrate a plasma of unbound charge carriers in chemical equilibrium with a minority phase of light-emitting excitons, even in 2D perovskites and at cryogenic temperatures. We validate the technique with 3D perovskites and investigate 2D compounds basded on both Pb and Sn as metal cation. The underlying photophysics is interpreted as formation of large polarons, charge carriers coupled to lattice deformations, in place of excitons. A conductive polaron plasma foresees novel mechanisms for LEDs and lasers, as well as a prominent role for 2D perovskites in photovoltaics.

In this work, the synthesis of the mixed tin-lead MAPb0.75Sn0.25(I0.4Br0.6)3 perovskite is investigated. The presence of tin renders the understanding and control of the perovskite crystallization crucial. Spin-coating is the most common deposition technique employed to synthesize perovskite. This work aims to discriminate the process parameters, which are playing a key role in the properties of the perovskite layer.A statistical design of experiment (DoE) was used to avoid varying one factor at a time. This DoE allows an assessment of the process parameters influence and their possible interaction, and can predict further outputs and optimization.

Spin-coating process is broken down into two parts; first the perovskite ink deposition, then the perovskite nucleation and growth. Here, the speed and duration of the second step have been varied from 2000 to 8000 rpm and from 8 to 22s respectively. Simultaneously, anti-solvent washing has also been investigated; the delay of anti-solvent pouring has been varied from -3s before apparition of the first perovskite crystals to +3s after.

Crystallinity, morphology, composition and absorption of the perovskite layers were compared in the DoE. It was observed that, thickness aside, rotation speed of the substrate does not impact the final properties of the layer. No major change can either be imputed on a lengthening of the second step. DoE showed that anti-solvent washing determines all perovskite layer properties. Anti-solvent pouring after first perovskite crystals formation leads to incomplete conversion of intermediate into cubic perovskite, larger FWHM, and preferred grain growth along the {001} planes. Even though the elemental composition is identical, a slight band gap variation is observed for the substrates made from a late anti-solvent washing. This variation may come from perovskite intermediates still present in the perovskite layer. Thus, to synthesize a homogenous cubic perovskite with larger crystallite sizes, anti-solvent can be poured just before the vaporization of the last uncoordinated solvent. However, the crystallite will be less oriented along the same planes and the absorption will decrease slightly.

To investigate the incidence of film crystallinity on its opto-electrical properties, time resolved luminescence of perovskites exhibiting different crystallinities have been analyzed. Measurements show slower decay and more intense photoluminescence for fully converted films. Orientation of growth seems to take precedence over crystallites sizes and crystallinity. As a conclusion, to manufacture the optimal perovskite film, full perovskite conversion must be assured and attention should be paid to crystal orientation.

Investigations of photovoltaic devices and semiconductors are essential to enhance the efficiency of preparation methods as well as their electronic and optical properties. Different parameters define these properties, e.g., number of defects and trap states, interface interactions, energy and electron transfer behavior and, of course, absorption properties and response to photon stimulation.

Surface characterization of such materials is usually done with scanning electron, scanning tunnel and atomic-force microscopes, which can gather information about homogeneity, conductivity, carrier mobility and defect center of films, semiconductors or within devices. But there still is a lack of information about important parameters that are necessary for understanding and optimizing the preparation and efficiencies of these materials.

We present here how combining time-resolved laser scanning microscopy – offering a broad range of techniques – with a spectrometer results in a valuable and powerful toolbox. This combination of microscopic (e.g., FLIM, PLIM, fast switch between widefield and confocal resolution, or carrier diffusion measurements) and spectroscopic methods (such as time-resolved photoluminescence or wavelength dependent emission scanning) allows investigating the photophysical properties of semiconductors, nanoparticles, QDs, polymers, solid-states as well as nanostructures on a whole new level. This additional information enables gaining a deeper understanding of both photophysical processes as well as structure-property relationships, which will help for the optimization of properties and efficiencies in practical applications.

Organic solar cells (OSCs) typically employ electron donor and acceptor materials to faciliate efficient charge generation from excitons. Minimizing the energy offset between the lowest exciton and charge-transfer (CT) states is a widely employed strategy to suppress the energy loss (E_g/q – V_OC) in polymer:non-fullerene acceptor (NFA) OSCs. While a lot of studies have investigated the CT state energetics and its correlation with device photovoltage as well as voltage losses, the CT state dynamics still remains relatively unexplored. In my talk, I will discuss how transient absorption spectroscopy (TAS) is employed to determine CT state lifetimes in a series of low energy loss polymer:NFA blends. TAS measurements show that the CT state lifetime follows an inverse energy gap law dependence and decreases as the energy loss is reduced. This behavior is assigned to increased mixing between these CT states and shorter-lived singlet excitons of the lower gap component as the energy offset is reduced. These results highlight how achieving longer exciton and CT state lifetimes has the potential for further enhancement of OSC efficiencies.

Doping of  hybrid and all-inorganic metal halide perovskites with alkali metal ions is a prominent means of enhancing their optoelectronic performance and stability. [1-3] In particular, sodium doping has been used to enhance the optoelectronic and solar cell metrics of CsPbBr₃ and MAPbI₃, [4-6] and the effect has been ascribed to the passivation of trap states associated with iodide vacancies. [7] However, the atomic-level mechanism of action of sodium in these compositions has been uncertain. Here, we use high-resolution solid-state sodium NMR to elucidate the speciation of sodium in MAPbI₃, the triple cation composition, and CsPbBr₃. We unambiguously show that sodium has no capacity to incorporate into the structure of these lead halide perovskites and remains in the materials as the unreacted sodium halide. However, owing to the exceptionally high affinity of sodium halides to ambient humidity, the unreacted halides facilitate the reaction of water vapour with the surface of the perovskites under ambient conditions, leading to complex sodium-doped hydrated surface layers. The results are corroborated by first principles calculations of ²³Na chemical shifts and are the first example of NMR crystallography applied to the highly disorder surfaces of metal halide perovskites.

The great progress in OPV over the past few years was achieved by the development of non-fullerene acceptors (NFAs), increasing the power conversion efficiency of organic solar cells up to 18.2%.[1] To further enhance device performance, loss mechanisms have to be identified and minimized. Especially triplet excitons (TE) are detrimental for efficient free charge generation, since they can form energetic trap states, responsible for non-radiative losses or even material degradation. Using spin sensitive measurement techniques, such as optically detected magnetic resonance (ODMR) and transient electron paramagnetic resonance (trEPR), we analyse exciton pathways in NFA-based OSC blends, employing PBDB-T, PM6 and PM7, as donors and Y6 and ITIC, as NFAs. We identify and assign long-living triplet excitons on NFAs, which are generated either via geminate intersystem crossing (ISC) or non-geminate hole back transfer (HBT). In comparison to fullerene-based blends with acceptor PC₇₀BM, trap states on fullerene were partially absent due to higher lying triplet energies, instead TE formed rather on the donor. However, NFA-based solar cells show higher efficiencies due to improved absorption. The good performance suggests that this advantage has higher impact on device efficiency than trapped triplet formation on NFAs and resulting degradation mechanisms. This result is further confirmed by the ternary blend PM6:O-IDTBR:Y6, which possess higher stability than the binary blend PM6:Y6.[2] Formation of long-lived triplets is determined on both NFAs in the ternary blend, supporting the assumption, that triplet excitons are less responsible for degradation in OPV.

Two-dimensional (2D) lead halide Ruddlesden-Popper perovskites (RPP) recently emerged as a prospective material system for optoelectronic applications. Their self-assembled multi quantum-well structure gives rise to the novel inter-well energy funnelling phenomenon, which is of broad interests for photovoltaics, light-emission applications and in emerging technologies (e.g., spintronics). Herein, we developed a realistic finite quantum-well superlattice model that corroborates the hypothesis of exciton delocalization across different quantum-wells in RPP. Such delocalization leads to a sub-50 fs coherent energy transfer between adjacent wells, with the efficiency depending on the RPP phase matching and the organic large cation barrier lengths. Our approach provides a coherent and comprehensive account for both steady-state and transient dynamical experimental results in RPPs. Importantly, these findings pave the way for a deeper understanding of the physics underpinning these systems crucial for establishing materials design-rules to realize efficient RPP-based devices.Two-dimensional (2D) lead halide Ruddlesden-Popper perovskites (RPP) recently emerged as a prospective material system for optoelectronic applications. Their self-assembled multi quantum-well structure gives rise to the novel inter-well energy funnelling phenomenon, which is of broad interests for photovoltaics, light-emission applications and in emerging technologies (e.g., spintronics). Herein, we developed a realistic finite quantum-well superlattice model that corroborates the hypothesis of exciton delocalization across different quantum-wells in RPP. Such delocalization leads to a sub-50 fs coherent energy transfer between adjacent wells, with the efficiency depending on the RPP phase matching and the organic large cation barrier lengths. Our approach provides a coherent and comprehensive account for both steady-state and transient dynamical experimental results in RPPs. Importantly, these findings pave the way for a deeper understanding of the physics underpinning these systems crucial for establishing materials design-rules to realize efficient RPP-based devices.

Recent literature provides a constant stream of new recipes and processing techniques, leading to improved power conversion efficiencies for photovoltaics (PVs), although often with limited detailed understanding of the mechanisms for performance improvement. Additive engineering and stoichiometric variations are particularly interesting in terms of optimizing performance levels within solar cells and related devices. However, the underlying impact of additives and stoichiometry on film grain structure, transport, and recombination properties is often not well understood. This talk will address our group’s recent efforts to examine shoichiometry control and additive engineering in the model system CH₃NH₃PbI₃, using a variety of tools, including a new carrier-resolved photo-Hall (CRPH) technique [1,2] and in-situ X-ray and photoluminescence characterization during film deposition [3,4]. In particular, the CRPH approach [1] provides unique insights into the majority and minority carrier properties, as a function of light intensity and using the same sample and measurement. A second direction for the talk will involve another important aspect of additive engineering, namely doping (Fermi level control). A recent study [5] has demonstrated the careful selection of molecular dopant and perovskite band positions to enable five orders of magnitude control over conductivity and carrier density in a mixed-metal CH₃NH₃Sn_0.5Pb_0.5I₃ perovskite. Such fundamental studies related to grain structure, defect properties and doping are expected to provide an enhanced degree of control over semiconducting properties for PV and related optoelectronic devices.

Impedance spectroscopy has been widely applied to study electrochemical and solid state energy conversion and storage devices. Performing impedance spectroscopy, however, on emerging photovoltaic materials, such as metal-halide perovskites, presents new challenges related to the unusual material properties and complex device architectures. In this talk I will give a brief introduction to impedance spectroscopy, and discuss how impedance can be applied to extrapolate information about resistive and capacitive signatures, as well as relevant timescales of dynamic processes. Practical tips for performing experiments are discussed, as well as guidelines for extracting meaningful data from common analyses, such as equivalent circuit modelling and capacitance-frequency spectroscopy. The underlying assumptions of each analysis approach, as well as the advantages, limitations, and potential pitfalls are discussed. Ultimately impedance spectroscopy can yield useful information over material and interface stability, but should be combined with complementary,  corroborative measurements in order to extract quantitative information about physical processes [1,2].

Ion motion remains an important topic in halide perovskite semiconductors. We discuss measurements of light- and bias-driven ion motion in halide perovskites, from early chemical evidence confirming photoinduced halide motion using imaging mass-spectroscopy, to recent scanning probe studies of ion motion under bias. Importantly, we show that the ability of bias stress (poling) to induce non-radiative defects due to ion migration hinges on a combination of both ion motion and redox processes associated with injected charge carriers. We study low-dimensional Ruddlesden-Popper phases that are of interest as materials for potentially improved stability and reduced ion motion, and we find that ion motion exists in 2D perovskite phases, and that it depends on the layer number and dimensionality of the perovskite.

Metal halide perovskites have shown remarkable success in photovoltaic applications by hitting efficiencies over 25%.[1,2] Despite the enormous contributions from material and device engineering, understandings of device physics, which is critical to optimize the design of perovskite solar cells towards their theoretical limit, are still lacking and debating. One is the kinetics and coherence of charges transport within the perovskite layer and transfer from the perovskite to its interlayers.  

Herein, we demonstrate a simple and time-resolved photoluminescence (TRPL) method to characterize charge transport across bulk perovskite and charge transfer from perovskite to the interlayers. An asymmetric charge carrier distribution between the charge transport layer (CTL) and methylammonium lead iodide (MAPbI₃) surface and its opposite side was created by using a much higher energy excitation light with a very short penetration depth of ~30 nm. As such, most charge carriers will be generated very locally at the selected surface, where charge transfer kinetics of TRPL will be more pronounced if the charges are generated at the CTL/MAPbI₃ interface, while charge diffusion processes will be more distinguished when generated at the opposite side. This is because, under the later condition, charges need to diffuse across the MAPbI₃ film before reaching the interface and to be transferred. We then conducted this TRPL measurement on three typical MAPbI₃ perovskites with two varying thicknesses of 250 nm and 750 nm, and another 750 nm perovskite with an additional post-treatment: aerosol assisted solvent (AAS) annealing for removing grain boundaries (GBs) in the vertical direction. Finally, a numerical calculation was conducted based on these TRPL kinetics to interpret charge transfer and diffusion parameters.

Our results elucidate the dependence of these kinetics on film thickness, GBs, and interlayers. Affected by film thickness and GBs, MAPI₃ shows an asymmetry between electron and hole in terms of charge transport across bulk MAPI₃ as well as charge transfer from MAPI₃ to the interlayers.

Efficient energy transport is highly desirable for organic semiconductor (OSC) devices such as photovoltaics, photodetectors, and photocatalytic systems. However, photo-generated excitons in OSC films mostly occupy highly localized states over their lifetime. Energy transport is hence thought to be mainly mediated by the site-to-site hopping of localized excitons, limiting exciton diffusion coefficients to below ~10^-2 cm²/s with corresponding diffusion lengths below ~50 nm. Here, using ultrafast optical microscopy combined with non-adiabatic molecular dynamics simulations, we present evidence for a new highly-efficient energy transport regime: transient exciton delocalization. In this regime, long-range electrostatic interactions enable the presence of low-lying spatially-extended states which excitons can temporarily re-access via energy exchange with vibrational modes under equilibrium conditions. In films of highly-ordered poly(3-hexylthiophene) nanofibers, prepared using living crystallization-driven self-assembly, we show that this enables exciton diffusion constants up to 1.1+-0.1 cm²/s and diffusion lengths of 300+-50 nm. Our results reveal the dynamic interplay between localized and delocalized exciton configurations at equilibrium conditions, calling for a re-evaluation of the basic picture of exciton dynamics. This establishes new design rules based on the power of long-range electrostatics to engineer efficient energy transport in OSC films, which will enable new devices architectures not based on restrictive bulk heterojunctions.

Some fundamental questions regarding to the solar cell efficiencies of are related to morphology and structures of the semiconductor thin films and transport layers. Research on solution-processable semiconductors has achieved significant fundamental and technological advancements over the last decade, in large part due to improvements in characterization techniques to understand these materials at different length scales. In a device stack, photoactive layer consisting of planar heterojunction (e.g., metal halide perovskites) or bulk heterojunction (e.g., donor-acceptor D-A conjugated polymers, and their blends) morphology is sandwiched between with hole- (HTL) and electron transport layers (ETL). The charge transport in such devices strongly depends on morphology and solid-state organization of molecular entities. Specifically, interfaces between the device layers, or with in the photoactive layer itself, manifest compositional and structural heterogeneities which are difficult to measure and understand at molecular-level details. Here, we show that the high field solid-state (ss)NMR spectroscopy is a valuable tool to address a number of pertinent questions related to interfacial interactions in metal halide perovskites (MHPs) and D-A contacts in BHJ thin films, and HTLs. SsNMR spectroscopy does not require long-range order and is best suited to study short-range (sub-nanometer to nanometer) structures in ordered and disordered regions of these materials.^[1-5] Interfacial structures elucidated by 2D ssNMR techniques in 3D perovskites, 2D layered perovskites and non-fullerene acceptor (NFA) BHJ solar cells ^[2,3] and chemically doped polymers will be discussed.^[6] In addition, we demonstrate that ssNMR results and analyses are best used when supported by complementary methods such as X-ray scattering and modelling techniques for the study of organic and hybrid semiconductors and their blends.

References:

[1] C. Dahlman et al, Chemistry of Materials, 2021, 33, 642-656

[2] A. Kazemi et al, Small Methods, 2021, 5, 2000834

[3] A. Karki et al, Energy & Environmental Science, 2020, 13, 3679-3692

[4] A. Karki et al, Advanced Materials, 2019, 31, 1903868

[5] M. Seifrid et al, Nature Reviwes Materials, 2020, 5, 910-930

[6] B. Yurash et al, Chemistry of Materials, 2019, 31, 17, 6715-6725

Mixed halide perovskite semiconductors can provide optimal bandgaps for multi-junction solar cells which are key to improve cost-efficiencies. However, these materials can suffer from detrimental illumination-induced phase segregation. In this work we employ Optical-Pump Terahertz-Probe spectroscopy to investigate the impact of halide segregation on the charge-carrier dynamics and transport properties of mixed halide perovskite films. We reveal that, surprisingly, halide segregation results in negligible impact to the THz charge-carrier mobilities, and that charge carriers within the I-rich phase are not strongly localised. We further demonstrate enhanced lattice anharmonicity in the segregated I-rich domains, which is likely to support ionic migration. These phonon anharmonicity effects also serve as evidence of a remarkably fast, picosecond charge funnelling into the narrow-bandgap I-rich domains. By modelling the charge-carrier dynamics and incorporating the charge funnelling, we show how although the high mobilities are preserved, the long-range transport can be hampered by the enhanced recombination rates resulting from the charge concentration. Our analysis demonstrates how minimal structural transformations during phase segregation have a dramatic effect on the charge-carrier dynamics as a result of the film heterogeneities. Given that the such an enhanced recombination is mainly radiative, we suggest that performance losses may be mitigated by deployment of careful light management strategies in solar cells.

Hybrid perovskites are being commercialized using roll-to-roll processing and are attractive for flexible optoelectronics. This raises questions about what happens to the film structure and stability after bending. Here, we examine the consequences of bending on the sub-grain structure of MAPbI₃, the prototypical perovskite. MAPbI₃ is a ferroelastic, which means that it forms sub-grain domains with identical crystal structure and different crystallographic orientation. Repeated bending causes changes to the proportions of these sub-grain domains, and the applied strains required for these changes are mapped on the stress-strain curve. The effects of thermal stress from the substrate are also decoupled. Bending the films outwards caused faster degradation; this is correlated with nucleation of new sub-grain domains causing a greater amount of defects. The impacts for ion migration, carrier trapping, and degradation are also discussed, as well as how these behaviors might be differently impacted in single crystals and thin films. [1]

Unraveling the structure-property relationships in hybrid lead halide perovskites is instrumental for the fundamental understanding not only of their exceptional optoelectronic performance but also their in-operando stability. Surprisingly, hybrid halide perovskites exhibit a great defect tolerance in their performance, despite their mechanical softness and the consequent low energetic barriers for point defect formation and ion migration. Recently, we have constructed the complete phase diagram of organic-cation solid-solutions of lead iodide perovskites (FA_xMA_1-xPbI₃, where MA stands for methylammonium and FA for formamidinium) with compositions x ranging from 0 to 1 in steps of 0.1 and in the temperature range from 10 to 365 K by combining Raman scattering and photoluminescence (PL) measurements [1]. We have extended this work by studying the evolution of shallow-defect signatures observed in the PL spectra at low temperatures [2]. The strong free-exciton PL of the perovskites, apart from the discontinuous changes in the energy of its maximum at the different structural transitions, also exhibits a strong decrease in linewidth at low temperatures. This allows for the observation of the emission related to radiative recombination of bound exciton complexes (BECs) associated with shallow defects (donor and/or acceptor). We report a tentative assignment of all PL features to the different shallow-defects typically present in hybrid perovskites, attained with the aid of state of the art ab-initio calculations [3]. The defect-related signatures exhibit a clear trend regarding the composition of the mixed crystals, indicating that the material becomes less prone to defect formation with increasing FA content.

Formamidinium lead iodide (FAPbI₃) can be used in its black (cubic perovskite) phase as a light absorber in single-junction solar cells. This material has a relatively narrow bandgap (~1.55 eV) and high decomposition temperature (320°C). However, the black phase is unstable at room temperature and transforms in less than 1 day into a yellow, non-photoactive form. Black phase can be recovered by the following thermal annealing at 180°C for 2 min. These two states, as-synthesized and recovered, were considered as the same phase in literature. Even more, this phase was usually defined as a perfectly cubic perovskite. Indeed, XRD analysis is not capable to discern between the photo-active FAPbI₃ states, including the high temperature one (T > 180°C). In contrast, our Raman spectroscopy and photoluminescence measurements have shown clear difference between as-synthesized, degraded and recovered FAPbI₃ samples. A temperature dependent study revealed phase transition in black FAPbI₃ at around 110°C. Detailed consideration of this transition disclosed that the room temperature polymorphs have a distorted cubic structure, such as Im-3. The structural stability deduced from the Raman spectra was discussed for each polymorph.

In solar cells, 33% of the energy of sunlight is lost as heat during the thermalization and cooling processes. Slow HCC is desired for thermoelectric devices and hot-carrier solar cells where extracting carriers before they have cooled could enable breaking the thermodynamic limit for single-junction solar cells. Emissive applications such as lasers, single-photon sources, and optical modulators require short HCC times for efficient radiative recombination and to prevent carrier trapping. Hot-carrier cooling (HCC) in metal halide perovskites above the Mott transition is significantly slower than in conventional semiconductors. This effect is commonly attributed to a hot-phonon bottleneck, but the influence of the lattice properties on the HCC behavior is poorly understood. Using pressure-dependent transient absorption spectroscopy, we find that at an excitation density below the Mott transition, pressure does not affect the HCC. On the contrary, above the Mott transition, HCC in methylammonium lead iodide is around 2–3 times faster at 0.3 GPa than at ambient pressure. Our electron–phonon coupling calculations reveal ∼2-fold stronger electron–phonon coupling for the inorganic cage mode at 0.3 GPa. However, our experiments reveal that pressure promotes faster HCC only above the Mott transition. Altogether, these findings suggest a change in the nature of excited carriers above the Mott transition threshold, providing insights into the electronic behavior of devices operating at such high charge-carrier densities.

Reference:

Accelerated Hot-Carrier Cooling in MAPbI3 Perovskite by Pressure-Induced Lattice Compression

Loreta A. Muscarella, Eline M. Hutter, Jarvist M. Frost, Gianluca G. Grimaldi, Jan Versluis, Huib J. Bakker, and Bruno Ehrler

The Journal of Physical Chemistry Letters 0, 12

DOI: 10.1021/acs.jpclett.1c00676

Low-dimensional (2D or 1D) hybrid perovskites are receiving increased attention due to their structural flexibility and a generally enhanced stability compared to their 3D counterparts. Understanding the phase formation and degradation behavior of these materials is crucial towards their use in optoelectronic devices since different crystal phases possess different optical and electronic properties.[1-4]

In the first part of this talk, I will discuss the phase formation and degradation behavior of a series of lead iodide hybrids containing bithiophene, terthiophene and quaterthiophene derivatives. We show that two crystal phases can be formed for each of these systems, depending on the processing conditions. One of these phases corresponds to a 2D layered perovskite and the other phase has optical properties corresponding to a dimensionality intermediate between typical 2D and 1D hybrids.

In the second part of this talk, I will discuss the influence of halide substitution on the phase formation of 2D layered perovskites (Bit-C3)2PbX4 (with X= Cl, Br, and I), containing a bithiophene derivative (Bit-C3). The crystal structure and phase behavior of the 2D layered HOIPs were studied in detail. It is suggested that via halide substitution from iodide to bromide and chloride, the molecular degrees of freedom of the Bit-C3 ammonium cations are reduced by spatial confinement due to a smaller inorganic framework. Therefore, limiting the formation of lower-dimensional hybrids besides the targeted 2D layered HOIP.

Layered Ruddlesden-Popper (RP) perovskites are promising candidates for optoelectronic applications due to their excellent stability and excitonic properties. These natural quantum well structures consisting of inorganic layers separated by large organic spacers have been known since several decades. However, only lately attention has turned towards investigating the distinctive nature of the exciton-phonon coupling as the driving mechanism for  many of their unique properties [1].

In this work we show that despite the assumed confinement of the excitons withing the inorganic layers, for certain ligands significant coupling of the excitons to the vibrations of the organic part can occur. We investigated the RP perovskite (PEA)₂(MA)_n-1Pb_nI_3n+1 (PEPI) [2] with a varying number of inorganic layers n=1,2,3 using magneto-transmission in magnetic fields up to 68 T [3]. In the transmission spectra we could identify periodically spaced features separated by around 40 meV which we assign as phonon replicas of the main excitonic transition. Our interpretation is strongly supported by the identical shifts of the features in magnetic field suggesting their common origin. The mode around 40 meV has been previously identified as the vibration of the PEA cation [1]. The observed coupling is an evidence of the leakage of the excitonic wavefunction into the organic barrier layer due to weaker dielectric confinement  induced by the PEA cations as compared to alkyl chains.

The quantification of quasi-Fermi level splitting (QFLS) in organic photovoltaic devices would provide researchers with much needed information about the origins of non-radiative losses to the open circuit voltage. Difficulties in measuring the QFLS in the bulk of organic semiconductors (OSC's) arise due to the excitonic nature of photoexcitation in OSC's and non-radiative electrode-induced photovoltage losses occurring at the interface between an organic active-layer and the bordering electrode layers. Here a novel experimental technique called electro-modulated photoluminescence quantum yield is introduced, which quantifies the QFLS within the bulk of OSC devices at operational conditions regardless of electrode-induced losses. Drift-diffusion simulations are used to validate the technique. The QFLS is quantified for PM6:Y6 active layer devices and is shown to be independent of both device architecture and varying total non-radiative losses.

Dye-sensitized photoelectrochemical cell is one of the prospective systems for photocatalytic water splitting. Recently, a remarkable achievement has been made based on ruthenium molecular complexes as sensitizer and catalyst coadsorbed on titania [1]. The maximum incident photon to current efficiency (IPCE) has reached 25% and stable photocurrent of 1.7 mA/cm² has been obtained [1]. We have recently studied the systems with popular sensitizer RuP and two types of ruthenium complexes with different pyridine rings and anchoring group [2,3]. In both cases we have observed a very fast quenching of the oxidized RuP by the catalyst, taking place with the time constant below 200 ps and the quantum yield close to 100%. The dynamics of this process was independent on the water-based electrolyte composition [2], excitation fluence and dimer formation of the catalyst [3].

The initial photocurrent recorded in water-based electrolyte is significantly (about two times) higher for the photoanodes sensitized with a mixture of RuP and the catalyst than the sum of the separate individual contributions of photoanodes with RuP and the catalyst. Due to the observed fast electron transfer process from the catalyst, RuP is quickly regenerated, allowing more frequent photon absorption and injection into titania, resulting in higher initial photocurrents. However, the photocurrent quickly decays into low stationary values (comparable to the sum of separated RuP and catalyst contributions), most likely due to the disappearance of the catalyst in the lowest oxidation state. So far, the interpretations of the initial photocurrent spike were based on fast interfacial charge recombination and/or slow catalytic turnover.

Our results reveal that ruthenium complexes-based chromophore-catalyst-assemblies on titania should serve as efficient systems for simple photocatalytic processes involving single electron transfer. For more complex mechanisms such as water splitting, its optimization requires finding solutions to accelerate the electron relay from higher valent catalyst species.

As technologies based on metal halide perovskites have rapidly developed, so too has our understanding of the underpinning chemistry and physics. There has been close collaboration and interplay between measurement and modelling throughout this process. The impact of in silico methods has ranged from understanding the microscopic nature of chemical bonding to the macroscopic responses of the crystal. These include studies involving lattice vibrations and molecular rotations, charged point defects and dielectric response, to describing complex emergent behavior including current-voltage hysteresis, ferroelectricity, and photoinstabilities. In this talk, beyond covering some of my own research [1-5], I will give a perspective on current directions and future challenges for modelling soft crystalline semiconductors.

1. “Atomistic origins of high-performance in hybrid halide perovskite solar cells” Nano Letters, 14, 2584 (2014); https://pubs.acs.org/doi/10.1021/nl500390f

2. "Direct observation of dynamic symmetry breaking above room temperature in methylammonium lead iodide perovskite" ACS Energy Lett. 1, 880 (2016); https://doi.org/10.1021/acsenergylett.6b00381

3. "Slow cooling of hot polarons in halide perovskite solar cells" ACS Energy Letters 2, 2647 (2017); https://doi.org/10.1021/acsenergylett.7b00862

4. "Dynamic symmetry breaking and spin splitting in metal halide perovskites" Physical Review B 98, 085108 (2018); https://doi.org/10.1103/PhysRevB.98.085108

5. "Performance-limiting nanoscale trap clusters at grain junctions in halide perovskites" Nature 580, 360 (2020); https://doi.org/10.1038/s41586-020-2184-1

Common (PV) technologies are absorbing the visible solar spectrum as it includes the highest photon flux domain and it is where the theoretical Shockley-Queisser limit is maximum. The general concept of PV technology relies on black absorbers.  As a corollary, PV devices are aesthetically not transparent, fulfilling plenty performance aspects. For higher-tech or architectural applications, see-through technologies are bringing important innovation paving the way to new integration opportunities. In this case, the level of average visible transparency (AVT) can be modulated either through partially segmented p-n junction or by decreasing non selective absorber thickness. Another approach is to develop selective absorbers. For this, the NIR part is a target since it composes ca. 51% of the total solar spectrum. Selective NIR absorption can only be achieved by molecular-based technologies. This is the case for luminescence solar concentrators, organicPV and dye-sensitized solar cells. The practical PCE limit of a single junction NIR-selective transparent PV (TPV) with an AVT of 100% is 10.8%.[1] LSC reaches the highest level of AVT, up to 88%, however with PCEs lying well-below 1%.[2] OPV reached a PCE of 4% with an AVT of 64% by adopting a bulk heterojunction. The American company Ubiquitous Energy achieved 5.1% PCE with an AVT of 51.5%.[3-4]

The optical rendering of DSSC is particularly advantageous since both coloration and transparency can be adjusted. Only semi-transparent DSSC has been proposed so far. Han et al. reported a green see-through DSSC with a PCE of 3.7% and a transmittance maximum of 60% at 560nm[5], Mallick et al. reported a semi-transparent DSSC based on N719 achieving a PCE of 2.4% and an AVT of 44%[6]. Demadrille et al. also reported a photochromic semi-transparent DSSC achieving AVT of 59% in dark and 27% under illumination affording in the latter case a PCE of 3.7%.[7] 

In this communication, we will describe our recent achievements in the development of NIR-selective dyes which design principle relies in reaching intense S0-S1 transition beyond 800nm while rejecting the upper S0-Sn transitions far in the blue where the human retina is poorly sensitive.[8]  Thanks to a holistic reduction of all internal optical losses and coloration tracking, we demonstrate the relevance of NIR-DSSC for TPV applications, achieving PCEs over 4%, AVT above 80% and color rendering index above 96%. Such characteristics confer to the selective NIR-DSSC never reached aesthetic features in terms of transparency and coloration.

Lead halide perovskites have drastically changed the solar cell research field due to their ease of synthesis and high power conversion efficiencies, which now reach over 25%. Improving stability and understanding degradation pathways in these devices is of high importance for their further development and potential commercialisation. X-ray based techniques such as photoelectron spectroscopy (PES) are powerful tools for obtaining chemical and electronic structure information of material surfaces and therefore interfaces of perovskites with contact materials can be studied. By combining measurements with visible illumination and/or dosing of atmospheric gasses, photo-induced reactions and therefore the stability of materials can be studied in-situ. However, the X-rays themselves used for measurement can also cause changes in the perovskite materials.

In this presentation, I will show how we have used photoelectron spectroscopy to investigate the X-ray stability of different perovskite active materials. We were able to follow the kinetics chemical and electronic structure changes in our materials induced by the X-ray illumination. Different degradation pathways and kinetics are found depending on the perovskite composition, some of which might be similar to degradation pathways under visible illumination [1,2]. Furthermore, I will show results of studies of the interface formation and interface degradation of a perovskite active layer with metals such as silver [3] and copper. Reactions with these metals can lead to a degradation of the perovskite materials.

Greatcell Solar, part of the Greatcell Energy group has been a major force in the DSSC and PSC domain for many years. The latest developments for the group will be introduced along with the latest record modules optimised for low light environments. The latest innovations in materials and equipment will be presented along with the HYPERION IV the latest and most advanced LED A+ solar simulator.

The complete interpretation of small perturbation frequency‐domain measurements on perovskite solar cells has proven to be challenging. This is particularly true in the case of intensity‐modulated photocurrent/photovoltage spectroscopy (IMPS/IMVS) measurements in which the high frequency response is obscured by instrument limitations. A new experimental methodology capable of accurately resolving the high frequency response—often observable in the second and third quadrants of the complex plane—of a range of perovskite devices is demonstrated. By combining single‐frequency IMPS/IMVS measurements, it is able to construct the time dependence of the IMPS/IMVS response of these devices during their initial response to illumination. This reveals significant negative photocurrent/photovoltage signals at high frequency while devices reach steady state, which is in keeping with observations made from comparable time‐domain transient measurements. These techniques allow the underlying interfacial recombination and ion migration processes to be assessed, which are not always evident using steady‐state measurements. The ability to study and mitigate these processes is vital in optimizing the real‐world operation of devices.

The characterization of the operating mechanisms of photosensitive devices is essential for their improvement. Impedance spectroscopy (IS) has been thoroughly used for that purpose. The analysis of the IS response is generally done through the modeling of the internal processes into an equivalent circuit (EC), whose selection is usually difficult given that many circuits can give the same response. The analysis of intensity-modulated photocurrent (IMPS) and photovoltage (IMVS) spectroscopies have also proven to be useful in optoelectronic characterization, generally through their time constants. Here a procedure is established for the direct analysis of IMPS and IMVS spectra with an EC[A1] [A2] . It is exhibited how this procedure allows accurate identification of the appropriate EC. Using that analysis, it is shown that the three spectra, obtained by measuring the techniques for a silicon photodiode, are reproduced with the same EC. As a result, parameters inaccessible from IS are obtained, such as the separation efficiency, external and internal quantum efficiency. The application of these results on other photosensitive devices will allow for more accurate identification and quantification of their operational mechanisms.

Measuring the diffusion-recombination parameters in perovskite solar cells by small perturbation frequency methods has not been successful so far. We present a new approach that opens interesting possibilities.  We have associated a negative spiraling loop to Intensity Modulated Photocurrent Spectroscopy (IMPS transfer function at high frequencies to the generation of electronic carriers in a thin region and the subsequent diffusion transport across the sample thickness, up to the collecting contact. This spectrum is obtained from classical diffusion-recombination theory and the negative current at high frequency is quite real. We showed that the application of the method enables a quantitative determination of the carrier diffusion coefficient and lifetime in standard sandwhich cells and also in back contact interdigitated planar perovskite solar cells. This is the first consistent determination of electron diffusion by small amplitude spectral method, since these features have not been obtained in Impedance Spectroscopy and emerge in IMPS.

Halide perovskite stability has solidified as the major commercial barrier for these materials as photovoltaics, and may eventually be the primary barrier to their adoption into a broader suite of optoelectronic applications. In light of this situation, effort must be directed at understanding the broad array of degradation processes and pathways present in these materials in order to predict material stabiltiy and lifetime in varied real-world conditions. Toward this broader goal, we track the kinetics of the perovskite to non-perovskite phase transition in the model compound, cesium lead triiodide (CsPbI3). We explore the effects of temperature on the phase transition kinetics, and investigate the role of varied initiating gas-phase species (e.g., water vapor, ethanol, and tetrahydro furan) and their partial pressure. Further work is ongoing to more thoroughly characterize the nature of these effects. Characterization of these effects should lead to better predictions of material and device stability and more accurate predictions of real-world lifetime from accelerated testing data.

The high performance of hybrid perovskite based devices is attributed to its excellent bulk-transport properties. However, carrier dynamics at the metal-perovskite interface and its influence on device operation is not widely understood. Here we explore dominant transport mechanisms in methyl ammonium lead iodide (MAPbI₃) perovskite based asymmetric metal-electrode lateral devices, with inter-electrode length varying from 4 μm to 120 μm. The device operation characteristics indicate distinct transport signatures in the ohmic and space-charge limited current (SCLC) regimes, which were observed to be dependent on inter-electrode length and applied bias. Further, the influence of charge carrier dynamics at the metal-perovskite interface were understood using spatially resolved Kelvin probe microscopy. The potential mapping across the device indicates minimal ion-screening effects and the presence of a transport barrier at the metal-MAPbI₃ interface. Additionally, the effects of photo-excitation, studied using near-field scanning photocurrent microscopy show dominant recombination and charge-separation zones in the lateral devices. This spatially resolved photocurrent tends to a uniformly distributed profile in the case of short channel devices. This trend correlates with bulk light response in a short channel length (4 um) device, which exhibits a responsivity of ~ 6 A/W at 5 V bias. Owing to the low device capacitance, the transient photocurrent indicates a fast response component of ~11 ns, which allows for high speed operational applications.

Halide perovskites have attracted tremendous attention due to their excellent opto-electronic properties. This has enabled perovskite PV single junction to reach record efficiencies of 25.5%[1]. The progress of perovskite in PV applications is the result of the improvement of the opto-electrical properties of the perovskite absorber, as well as of the quality of the interface between transport layers and the perovskite absorber. Specifically, the recent introduction of self-assembled monolayers (SAMs) as charge transport layers on ITO has led to highly efficient devices[2-3], as well as a recent tandem perovskite/Si record of 29.5%[4]. Thus, SAM-based transport layers are expected to be part of the technological roadmap of perovskite PV.

So far, limited investigation has been carried out on the surface coverage of SAMs on ITO. Surface coverage is important because non-covered areas can result in low open circuit voltage and electrical shunts, thus reducing the device efficiency. To date, it has been shown that the SAM quality depends on the ITO crystallinity and morphology, as well as its surface treatment prior to SAMs processing[5-6]. In this study, we investigate the surface coverage of SAM on ITO using MeO-2PACz ([2-(3,6-dimethoxy-9H-carbazol-9-yl)ethyl]phosphonic acid) employed in a p-i-n structure of ITO/SAM/CsFAMA/C₆₀/BCP/Cu by transmission electron microscopy (TEM) in combination with conductive atomic force microscopy (c-AFM) mapping. We observe that processing SAM directly on ITO can result in an inhomogeneous layer, with areas having low molecular density in TEM images. Supporting the TEM images analysis, c-AFM map also reveals exposed ITO areas due to insufficient SAM coverage in ITO+SAM sample. We attribute this inhomogeneity to large device performance spread seen in ITO+SAM devices. On the other hand, when adopting an atomic layer deposited NiO[7-8] between ITO and MeO-2PACz, the homogeneity, and thus, the surface coverage of the SAM improve. The cross-sectional TEM images of NiO+SAM device point out to a homogeneous SAM layer on NiO in contrast to direct growth on ITO. The homogeneous distribution of SAM molecules on NiO and the low lateral conductivity of NiO lead to narrower device efficiency distribution with high shunt resistance on average reaching more than 20% efficiency with NiO+SAM device. Notably, NiO layers do not require pre-treatment prior to the SAM solution process, which offers an advantage over ITO - eliminating the discrepancy in ITO pre-treatment and properties. We trust that this finding can further promote the benefit of using phosphonic acid-based molecules as contact layers in PSCs.

Most models of charge transport in disordered organic semiconductors assume the carriers are localised to individual molecules and move by hopping from one to another. I this talk, I will report the development of delocalised kinetic Monte Carlo (dKMC), the first three-dimensional treatment of partially delocalised carrier motion in disordered materials, showing the critical role delocalisation plays in both conduction and charge separation in organic photovoltaics.

dKMC shows that the fundamental physics of transport in moderately disordered materials is that of charges hopping between partially delocalised electronic states [1]. Our approach is the first to treat, in three dimensions, all the processes crucial in organic semiconductors: disorder, delocalisation, noise, and polaron formation. As a result, it is able to treat the intermediate transport regime, between band conduction and hopping conduction. Applying dKMC to carrier transport reveals that even a small amount of delocalisation can increase carrier mobilities by an order of magnitude, explaining the underestimation by conventional hopping models [1].

I will also report recent work on using dKMC to resolve the question of how charges in organic photovoltaics are able to overcome their significant Coulombic attraction and separate efficiently. The low dielectric constants in organic semiconductors produce Coulombic attractions an order of magnitude greater than the available thermal energy. Delocalisation has been suggested as an explanation, because it could increase the initial separation of charges in the CT state. By applying dKMC to the problem, we overcame the considerable computational hurdle of tracking the correlated quantum-mechanical motion of two delocalised particles. We find that even small amounts of delocalisation, across less than two molecules, can produce large enhancements in the efficiency at which charges separate, even if they start out in a thermalised CT state. Importantly, these delocalisation enhancements are a kinetic effect, rather than the common hypothesis that delocalisation increases efficiency by reducing the Coulomb attraction.

Halide perovskites have garnered tremendous attention as potential materials for next-generation photovoltaic technologies as they possess slow hot-carrier cooling properties [1]. The hot-carrier cooling rates can  extend into the ps-time range in halide perovskite nanocrystals due to an enhanced phonon bottleneck, which underlies the possibility of achieving elevated carrier temperatures and low threshold multiple exciton generation in these materials [2, 3]. These properties posit halide perovskites as a promising absorber material to overcome the Shockley-Queisser limit, for instance, by their implementation in hot carrier solar cells (HCSCs).

Elevated carrier temperatures are a prerequisite for high open circuit voltages in a HCSC and ultimately determines their maximum efficiency [4]. Hence, the correct determination of the carrier temperature is of utmost importance. In this presentation, we will critically examine the commonly adopted procedures for determining the carrier temperatures in halide perovskites and the degree of comparability of the results between studies. A proposed fitting approach for determining the carrier temperatures from the transient absorption spectra more consistently and reliably will also be presented. Ultimately, the focus of this talk is to discuss how these discrepancies may influence the interpretations of hot carrier cooling processes in halide perovskites, how to overcome these pitfalls, and how it may affect the development of actual hot carrier devices in the real-world.

We report our effort to understand the stability of nonfullerene OSCs, made with the binary blend poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-b’]dithiophene))-alt-(5,5-(1’,3’-di-2-thienyl-5’,7’-bis(2-ethylhexyl)benzo[1’,2’-c:4’,5’-c’]dithiophene-4,8-dione)] (PBDB-T) : 3,9- bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)- dithieno[2,3-d:2’,3’-d’]-s-indaceno[1,2-b:5,6-b’] dithiophene (ITIC) system. It shows that a continuous vertical phase separation process occurred, forming a PBDB-T-rich top surface and an ITIC-rich bottom surface in PBDB-T:ITIC BHJ during the aging period. It is found that a gradual decrease in the built-in potential (V₀) in the regular configuration PBDB-T:ITIC OSCs, due to the interfacial reaction between the poly(3,4-ethylenedioxythiophene)- poly(styrenesulfonate) (PEDOT:PSS) hole transporting layer (HTL) and ITIC acceptor, is one of the reasons responsible for the performance deterioration. Retaining a stable and high V₀ across the BHJ via interfacial modification and device engineering, e.g., as seen in the inverted PBDB-T:ITIC OSCs, is a prerequisite for efficient and stable operation of the nonfullerene OSCs.^[1]

The stable built-in potential in the OSCs is realized through suppression of the interfacial reaction between the BHJ and PEDOT:PSS HTL. The impact of interfacial modification, molybdenum oxide (MoO₃) induced oxidation doping of the PEDOT:PSS HTL, on the operational stability of poly[(2,6-(4,8-bis(5-(2-ethylhexyl-3-fluoro)thiophen-2-yl)-benzo[1,2-b:4,5-b’]dithiophene))-alt-(5,5-(1’,3’-di-2-thienyl-5’,7’-bis(2-ethylhexyl)benzo[1’,2’-c:4’,5’-c’] dithiophene-4,8-dione)] (PM6) : 3,9-bis(2-methylene-((3-(1,1-dicyanomethylene)-6,7-difluoro)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2’,3’-d’]-s-indaceno [1,2-b:5,6-b’]dithiophene (IT-4F) nonfullerene OSCs has been analyzed. We found that the MoO₃-induced oxidation doping in PEDOT:PSS can effectively suppress the interfacial chemical reactions between IT-4F and PEDOT:PSS, a recently identified major degradation mechanism in NFA with 2-(3-oxo-2,3-dihydroinden-1-ylidene)malononitrile moieties-based OSCs. Our findings highlight the importance of retaining high built-in potential to mitigate any associated degradation mechanisms, to accompany the rapid advance in the molecular synthesis of NFAs, toward enhanced operational stability of NFA-based OSCs.^[2]

A  compromise  between  high  power  conversion  efficiency  and  long-term  stability  of  hybrid organic-inorganic metal halide perovskite solar cells is necessary for their outdoor photovoltaic application and commercialization. Herein, We have developed a method to improve the stability of perovskite solar cells under water and moisture exposure consisting in the encapsulation of the cell with an ultrathin  plasma  polymer. The  deposition  of  the  polymer  is  carried  out  at  room temperature by the remote plasma vacuum deposition of adamantane powder. This encapsulation method  does  not  affect  the  photovoltaic  performance  of  the  tested  devices  and  is  virtually compatible with any device configuration independently of the chemical composition. After 30 days under ambient conditions with a relative humidity in the 35% – 60% range, the absorbance of  encapsulated  perovskite  films  remains  practically  unaltered.  The  deterioration  in  the photovoltaic performance of the corresponding encapsulated devices becomes also significantly delayed  with  respect  to  devices  without  encapsulation  when  vented  continuously  with  very humid air (RH > 85%). More impressively, when encapsulated solar devices were immersed in liquid water, the photovoltaic performance was not affected at least within the first 60 seconds. In fact, it has been possible to measure the power conversion efficiency of encapsulated devices under operation in water. The proposed method opens up a new promising strategy to develop stable photovoltaic and photocatalytic perovskite devices.

The open circuit voltage decay (OCVD) is a characteristic transient response from operating solar cells observed after the illumination is turned off. By analyzing such transients correctly, it is possible to obtain valuable information about charge carrier dynamics in fully functional devices. For perovskite solar cells, additional charged species like mobile ions are often present, leading to extraordinary features in the OCVD. Here, we use a self programmed transient drift diffusion simulator accounting for mobile ions with sub-nanoseconds time resolution, to show how quantitative information about the ionic species, such as concentration and diffusion coefficient can be obtained directly from OCVD. The simulated transients, showing characteristic ionic displacement, are in excellent agreement with a multitude of experimental data from methylammonium lead iodide (CH₃NH₃PbI₃) devices, without requiring any change in material input parameters. Further, the ionic contributions to the OCVD are clearly visualized by specially resolved carrier profiles. Our new approach opens up new ways to interpret transient electrical measurements on perovskite based devices.

Stability issue is the core restriction for the application of organic solar cells (OSCs). However, state-of-the-art OSCs based on bulk-heterojunction concept suffer from stability problems caused by the severe morphological changes upon thermal or illumination stress [1]. Single-component concept, enabled by the covalently-bonded structure with donor and acceptor in one molecule, presents attractive advantages such as a simplification of device fabrication and a stabilization of microstructure [2]. Recently, with the rapid improvement of efficiency from 2-3% to 8.4% for single-component organic solar cells (SCOSCs), the reports on their stability however are scanty. In this work, we for the first time systematically investigated the stability under thermal and illumination stress of a series of SCOSCs based on polymeric (SCP3, PBDBPBI-Cl) and molecular (Dyad1, 2, 3, 4) materials. Under thermal pressure, double-cable polymer-based SCOSCs exhibited excellent thermal stability with no degradation at 90 ^oC for 3000 hours, partially attributed to the high temperatures (around 200 ^oC) for post-treating active layer during device fabrication. Such rather high annealing temperatures are plausible to create microstructure arrangements which are in thermodynamic equilibrium after cooling down. Furthermore, without precedent, we studied the thermal stability comparison among a series of SCOSCs based on D-A small molecules (Dyad1, 2 and 3) with the same donor and acceptor units but differently long alkyl space linkers. Since macroscopic diffusion of molecules is excluded in these dyads, the length of the spacer can only provide the necessary flexibility for sub-nm rearrangements caused by thermal stress. Interestingly, dyads showed a distinctly different behavior, Dyad1 with the shortest linker exhibiting the highest thermal stability, while Dyad3 with the longest linker showing the worst thermal stability [3]. This highlights the need for further in-depth studies on the dual importance of spacer length for performance and stability. Moreover, Dyad1-based SCOSCs exhibited extraordinary illumination stability, retaining 98% of the initial PCE under concentrated light equal to 7.5-suns for over 1000 hours, which could be among the highest illumination stability for solution-processed OSCs. Based on the outstanding stability, SCOSCs could be an ideal candidate to study the ultimate stability under extremely rugged accelerating conditions such as high temperature and concentrated light. Since the morphological evolution is excluded, SCOSCs could serve as a model system to selectively study interface degradation. In the near future, SCOSCs will see a prospective renaissance with 10% efficiency and 20 years lifetime for industrial applications.

While c-Si appears untouchable as leading PV main stream technology for a longer time, many of the new applications which rely on flexibility, transparency, colour management, integrability or simply elegant appearance require novel photovoltaic materials and technologies.

Organic photovoltaics (OPV), like other emerging thin film PV technologies, are not yet part of this global TW scenario. The first printed PV products were launched in 2008/2009 for portable chargers at an efficiency of about 2 %. Despite the rather low performance at that time, these first products already showed the characteristic “OPV features” like flexibility, transparency and colour variability. Since then, the printed PV community has concentrated on developing novel material systems for higher efficiency – a development which was outstandingly successful. In the last 10 years. Organic solar modules with close to 13 % efficiency were certified in 2020, while single junction cells already reach 18 % power conversion efficiency. Despite the great process, OPV is still facing multiple challenges, from lifetime to cost and to environmental aspects. The most urgent question therefore is – how can we accelerate the development of organic solar cells towards a market ready product.

In this talk I will discuss concepts and strategies to speed-up the development of OPV technologies towards an earlier deployment on the market. Automated, robot-based research lines with shared interfaces to multi-objective machine learning based optimization routines are introduced as a powerful concept to accelerate the development of new materials towards markets. Despite the commonly accepted understanding that machine learning algorithms can accelerate optimization, we demonstrate that physical model-based AI is superior in discovering hidden relations. We will demonstrate that in specific cases it is becoming possible to predict efficiency or even stability of organic solar cells based on a simple absorption measurement.

In previous works, we have used ultrafast charge extraction and optical probes to show that photocreated charge carriers in typical OPV devices do not completely thermalize before extraction. After a short recap of these findings, we will show how the non-equilibrium nature of the charge carrier populations in OPV affects the open circuit voltage (Voc) and how the underlying slow thermalization can be rectified to boost Voc and the fill factor (FF).

Specifically, we experimentally calibrate a kinetic Monte Carlo model to reproduce temperature- and thickness-dependent jV-characteristics of both fullerene and non-fullerene OPV devices. This then allows us to assess the Voc gain that is associated with the incomplete thermalization, and we find gains of 0.1-0.2 V as compared to –hypothetical- OPV devices where thermalization does fully complete. In the second part of the talk, I will show how composition gradients can be used to rectify a significant fraction of the thermalization loss to boost Voc and FF and thereby the overall power conversion efficiency.

Photovoltaic (PV) devices based on metal halide perovskite (MHP) absorbers have reached outstanding performance over the past few years, surpassing power conversion efficiency of over 25% for lab cells and with large area devices in excess of 18%. For the solar application stability, the most demanding requirement to assess for PV and remains the outstanding issue for MHP based devices. The problem of stability motivates basic science driven work on MHP based PV at NREL and work by industrial partners. Material and device insight can enable MHP PV stability along with the associated opportunities to further improve efficiency with multijunction while maintaining scalability and manufacturability is critical. This talk will highlight the latest work at NREL to develop understanding of critical roadblocks, aspects of solar cell performance, device architectures (e.g., single junction and tandems), stability and operational dynamic to enable the next generation of photovoltaics. In addition, work on related hybrid semiconductor systems and devices will also be highlighted.

Dye-sensitized solar cells (DSSCs) have been regarded as one of the most prospective solar cells, due to low-cost, flexibility, simple device fabrication and high conversion efficiency, in comparison to the conventional photovoltaic devices. Recently, G2E in Swiss and Exeger in Sweden including Asian companies have demonstrated prototyped components based on DSSC technology employing liquid electrolytes. A state of the art DSSC based on organic sensitizer-, porphyrin-based SCs as well as Ru-complex-based SCs has exceeded the power conversion efficiency (PCE) of over 14.3 %, 13% and 11.9%, respectively. However, the unit costs, long-term device stability and power conversion efficiency must be further improved for real-life applications. In this regard, we demonstrated that D–π–A structured Zn(II)–porphyrin and organic sensitizers for efficient retardation of charge recombination and fast dye regeneration were newly designed and synthesized [1-4]. The device with new porphyrin sensitizer exhibited the higher PCE than those of the devices with SM315 as a world champion dye. To further improve the maximum efficiency of the DSSCs, very recently, the cocktailed co-sensitization of new organic sensitizer with a porphyrin dye showed state-of-the-art PCEs of 14.20% [5]. Also, to improve the long-term device stability, triblock copolymer-based quasi-solid state (QSS) DSSCs with significantly improved long-term device stability exhibited an overall photovoltaic PCE of 10.49%, which is higher than a liquid electrolyte-based DSSC [5]. Also, the best PGE was applied to QSS-DSSCs based on the co-sensitization of organic dye and porphyrin dye. As a result, the PCEs of both polymer gel electrolytes and polymer/TiO₂ composite gel electrolytes based QSS-DSSCs were comparable with liquid-state DSSCs. The highest PCE measured for polymer and polymer/TiO2 composite gel electrolytes was 10.97% and 11.05%, respectively [6-7]. These are the highest values reported for QSS-DSSCs. The long-term stability of QSS-DSSCs was better than liquid state DSSCs, retaining > 80% of its initial PCE after 2000 hours of testing at 50°C under 1-sun condition. Furthermore, we have searched low-cost, scalable metal-free counter electrodes (CEs) based on carbon-based nanomaterials with improved fill factor and low-cost for alternative to expensive and noble Pt metal CEs[8-9], those factors of which limit large scale production and thus prohibit the practical application of DSSCs. In this presentation, new strategy on materials paradigm for low-cost, long-term stable, highly efficient dye-sensitized solar cells will be described to give right answers in overcoming the limitation of the existing technology for the practical use.

Organic solar cell (OSC) is a very promising photovoltaic technology. Over the past decades, the power conversion efficiencies (PCEs) of OSCs have been promoted from about 1% to currently over 18%. The development of new organic photovoltaic materials, including donor, acceptor and interlayer material, contributed greatly to the rapid increase of PCEs. In the past a few years, we developed many polymer donors based on benzothiophene (BDT) units and applied them to fabricate the PSC devices. For instance, we reported a polymer donor named PBDB-T in 2013 and then modified its chemical structure to obtain a polymer named PBDB-TF in 2015. These two polymers exhibit outstanding photovoltaic properties when blending with fullerene and non-fullerene acceptors. Very recently, the researchers in the field and our team employed PBDB-TF to blend with the newly developed acceptor and realized over 18% efficiencies. In this presentation, some general and efficient molecular design strategies related to this two polymer donors will be summarized and introduced. What is more, some of our recent progress including the OSCs with over 19% efficiencies and new considerations for interpreting the low driving force in the devices will also be briefly introduced.

Light is ubiquitous in the urban environment – from the sun that shines down upon us to the artificial sources that light-up our devices and homes. While some of this light is used very effectively, for example by plants in the process of photosynthesis, much is wasted, either due to inefficient harvesting or poor recycling of the broad spectrum of wavelengths available. Spectral conversion materials provide a potential solution to this problem, using a photoluminescence process to convert available photons into energies that can be used more effectively.[1] If such materials are integrated within a suitable host, they may provide additional features such as concentration of diffuse light, improved mechanical properties and the potential to retrofit to existing photovoltaic device installations.

In this talk, recent highlights from our research into the bottom-up design of spectral converters based on organic-inorganic hybrids will be presented. It will be shown that materials chemistry design strategies can be used to control the packing, orientation and placement of lumophores, which provides a means of modulating the optical properties – from enhanced photoluminescence quantum yields[2], to tunable emission colour via Förster resonance energy transfer.[3,4] These characteristics can be exploited to improve light-harvesting and trapping within the integrated material, which can be used to develop highly efficient spectral converters for luminescent solar concentrators[5] or as optical amplifiers for visible light communications.[6] New results on the use of these hosts for triplet-triplet annihilation upconversion will also be presented.

TCI supports HOPV21 and your PV research by providing high performance materials as same as ever. Some recent topics in this research area are focusing on stability, scalability, and cost-effectiveness. In order to solve above issues, we will present some high performance HTMs such as TOP-HTM and 2PACz series. The latter 2PACz series recently received good attention as it forms self-assembling monolayers (SAM) as a hole contact layer on metal oxide surface. Our latest SAM material, Me-4PACz, was quite recently commercialized following a great collaboration with top researchers. We will introduce you to possibilities for your PV research by using TCI materials.

Long-term stability of Perovskite Solar Cells (PSCs) is the main issue to be solved for a forthcoming commercialization of this technology. The stability of PSCs mainly suffers for water and oxygen infiltration as well as prolonged exposition to UV radiation. Hence, encapsulation of devices is mandatory to achieve good long-term stability [1,2]. Polyurethanes (PU) are cheap and environmental-friendly materials whose mechanical, chemical and physical properties could be easily tuned by thoughtful choice of their precursor.

In this work we proposed to use this class of materials as encapsulant for (flexible) perovskite solar cells [3]. We focused our attention on the barrier properties of PU, especially in the prevention of degradation caused by both moisture and oxygen. Furthermore, some UV-filtering molecules were added into the PU matrix to prevent their degradation during the illumination of the device [4]. 

To test their UV and thermal stability, polyurethanes were stressed by means of various approached: they were soaked with UV radiation for different times (i.e. 2, 4 and 8 hours under continuous irradiation) and thermally stressed (i.e. 100 °C up to one week and -50 °C for 30 minutes). Remarkably, both UV-vis and IR analyses of polymers did not show any significant modification as a consequence of the activity.

The polymers were then deposited on perovskite solar cells, the photoelectrochemical properties of encapsulated device remain constant up to three months. Notably, the incapsulated cells showed higher stability stored under ambient light with 50% RH and temperature ranging from 18 to 32 °C by retaining 94% of the initial power conversion efficiency after more than 2500 hours whereas unprotected device lost more than 90% of their performance during the same period.

This result is quite remarkable as it allowed to demonstrate for the first time the use of modified polyurethanes as promising encapsulant materials for PSCs.

Photoelectrochemical (PEC) cells utilizing dye-sensitized electrodes are promising candidates for generating solar fuels using only water as a chemical feedstock.[1] Dye-sensitized photoelectrochemical (DS-PEC) systems build on knowledge gained from dye-sensitized solar cells (DSSCs). Using such an approach for proton reduction to H₂ or CO₂ reduction at the photocathode has a number of advantages over homogeneous systems in solution while still taking advantage of the best performing molecular photocatalysts. Charge-transfer kinetics and energetics at the catalyst-semiconductor interface are readily optimised through design of the molecular catalyst/sensitizer. PEC systems dispense with the need for a sacrificial electron donor in the water oxidation process[2] by introducing an external bias from a solar cell or by addition of a sensitized counter electrode, in a tandem device, for overall water-splitting.[3]

In the present study we interrogate recent results for DS-PEC systems utilizing novel Ruthenium-Rhenium molecular photocatalysts deposited on mesoporous NiO films for CO₂ reduction, building on previous efforts from water-splitting studies. Particular interest is taken in optimizing experimental conditions, dye adsorptivity, and stability during extended periods of irradiation. Transient absorption spectroscopy is used to comment on the longevity of the charge separated state. Electrochemical measurements and gas chromatography are used to measure PEC device performance and X-ray photoelectron spectroscopy is used to monitor chemical changes on the semiconductor surface. It is apparent that more work is needed to develop efficient PEC systems by engineering longer-lived charge separated excited states on the semiconductor surface.

The performance of perovskite solar cells/modules (PSC/Ms) has improved through optimisation of the perovskite and charge transport layers (CTLs). However, little attention has been paid to the substrate. Almost all thin film photovoltaics rely on soda lime glass (SLG) which contains alkali ions. Na⁺ is the most likely to diffuse into the active layers due to its small radius and low charge. This is especially true in PSMs which use the P1-P2-P3 monolithic interconnection because in the P1 lines, the bottom CTL is in contact with SLG. Previous studies on Na doping of PSCs have found beneficial effects but also detrimental ones when the Na concentration is too high. Therefore, studying inadvertent Na diffusion from SLG in PSMs is of interest.

Here, we used spectroscopy and microscopy techniques to study Na diffusion in PSMs with a Cs_0.05(CH₃NH₃)_0.14(CH(NH₂)₂)_0.81PbI_2.7Br_0.3 perovskite. We used XRD to compare the crystallography of perovskite deposited on SLG and quartz and found that the perovskite peaks of SLG samples are shifted to lower angles, indicating wider lattice plane spacings. We examined the perovskite grain morphology with SEM imaging and observed Brownian tree-shaped areas (trees) growing perpendicularly from the edges of P1 lines up to ~250 μm into the active area. While the bulk of the film contains plate-shaped grains on top of the perovskite grains, very few of these plates are found inside the trees. AFM, KPFM, and cathodoluminescence (CL) mapping showed that these plates are excess PbI₂ from the precursor solution. CL also shows that near the P1 lines, the perovskite’s luminescence is redshifted by 19 meV and stronger by 6-7x, indicating less non-radiative recombination. The Brownian tree shape of these PbI₂-less areas suggests that their formation was controlled by diffusion from the P1 lines, with Na being the most likely diffusant. We performed cross-sectional elemental mapping with STEM-EDX and SIMS to confirm that Na diffused from SLG to CTL/perovskite inside P1 lines and then from P1 lines into active areas. We found strong Na-Br correlation, indicating NaBr formation inside the perovskite. A mechanism can thus be proposed: annealing of the CTL/perovskite provided enough energy for Na to diffuse from SLG. In the perovskite layer close to P1 lines, Na bonds with Br, leaving the perovskite precursor Br-poor. To compensate, more PbI₂ reacted to form I-rich, Br-poor perovskite which explains the XRD and CL peak shifts. NaBr then boosts the perovskite’s local emission by passivating defect sites, as previously observed with KBr.

Novel molecules are key drivers in the development of efficient organic solar cells (OSCs). Two fabrication routes have proven successful to make devices from molecules [1]: casting from solution - mostly involving polymers in the blend - and thermal evaporation of small molecules in vacuum, similar to the industrial fabrication of OLEDs. Until about 2015, the best results for both routes were achieved by tailoring the donor molecule, yielding power conversion efficiencies of around 10% [2,3]. For the acceptor, both fabrication technologies relied on fullerene or its derivatives. The advent of non-fullerene acceptors (NFAs) in solution processing pushed OSC efficiency by 50% to around 18% [4] as of now, outpacing the development of vacuum-deposited OSCs. So far, highly efficient evaporated NFAs have not been reported.

Here, we take an important first step towards efficient NFA-based evaporated OSCs by demonstrating that also vacuum deposited donors would benefit from NFAs. We do so by depositing evaporated donors onto solution-processed NFAs to form a planar heterojunction. We find that voltage losses of donor/NFA systems are reduced by up to 400mV compared to corresponding donor/C60 systems, without compromising photocurrent. In-depth analysis of voltage losses is carried out via sensitive external quantum efficiency and electroluminescence measurements.

Our findings show that evaporable donor molecules are well-suited for high-performance OSCs and stress the need for evaporable non-fullerene acceptors. Once such molecules are available, a significant increase in efficiency can be expected. Together with the existing technological advantages of evaporated OSCs - industrial scalability as proven by OLEDs and the relative ease of fabricating multijunction solar cells – our findings highlight the further potential of evaporated organic solar cells.

In comparison to their wet-chemical counterparts, the film formation of the dry, vacuum-based synthesis of perovskite absorbers is still rather obscure. Even for the standard absorber, methyl ammonium lead iodide (MAPbI₃), the growth requirements and synthesis routes are not sufficiently developed to date. In this contribution, we analyze the film formation of MAPbI₃ perovskite absorbers in quasi real-time during their growth by co-evaporation with the aid of in situ X-ray diffraction (XRD). The detailed analysis of phase evolutions allows us to pinpoint structure-property relationships between the involved crystalline phases as a function of perovskite composition and processing conditions. For example, the X-ray diagnostics allows us to identify the presence and monitor the evolution of secondary phases such as PbI₂ or the film orientation during the growth process. In addition, the time resolved phase analysis gives us direct access to the kinetics of the film formation. More specifically, we analyze the effect of different precursors flux rate ratios and substrate temperatures on the crystal growth. We find the perovskite formation to strongly depend on the processing temperature and determine suitable process windows for optimal absorber compositions, where small amounts of PbI₂ secondary phases seem to benefit its electronic properties. Ultimately, we report on our progress in fabricating efficient devices with co-evaporated perovskite absorbers (>14%) in a regular n-i-p structure using a ITO/SnO₂/Perovskite/PTAA/Au device configuration.

Metal halide perovskite solar cells (PSCs) are one of the most promising photovoltaic technologies to date, with their reported power conversion efficiencies (PCEs) increasing from 3.8 % in 2009 to over 25 % today. In this study, we aim to improve the photovoltaic parameters of PSCs through interface engineering. Specifically, we focus on enhancing device open circuit voltage (V_oc), which is one of the most limiting parameters associated with losses in PCE.

Numerous studies have shown that V_oc losses can be attributed to non-radiative recombination within bulk perovskite films and/or at the interface between charge transport layers. Here, strategies to improve V_oc by suppressing non-radiative losses through interface engineering will be discussed. Specifically, we investigate two triarylamine polymer derivatives- PTAA and PF8TAA -as hole transport layers (HTLs) in FACsPbI₃ PSCs. Using a suite of optical, structural and electrical characterisation techniques we study modifications in microstructure, absorption and stability of PSCs prepared using these HTLs. Our results focus on the impact of the energy of the highest occupied molecular orbital (HOMO) level of the HTL and the impact this has on device performance and lifetime.

Introducing molecular additives into perovskite precursors has become one of the most effective and prevailing strategies to improve the performance of metal halide perovskite optoelectronic devices, which recently has boosted the external quantum efficiency (EQE) of perovskite light-emitting diodes (PeLEDs) to above ~20%^1,2. The performance enhancement results from suppressed non-radiative recombination which is generally believed to be associated with defect passivation – surface dangling bonds in perovskites are said to be healed by additional coordination or ionic bonding with the additives, leading to the annihilation of trap states. Accordingly, a wide range of passivating molecular additives have been investigated for PeLEDs, especially those containing Lewis base moieties such as amino- and carboxyl-functionalized molecules However, a general and puzzling observation that can hardly be rationalized by passivation alone is that most of the molecular additives enabling high-efficiency perovskite light-PeLEDs are chelating (multidentate) molecules, while their respective monodentate counterparts receive limited attention. Here, we reveal the largely ignored yet critical role of the chelate effect on governing crystallization dynamics of perovskite emitters and mitigating trap-mediated non-radiative losses. Specifically, we discover that the chelate effect enhances lead-additive coordination affinity, enabling the formation of thermodynamically stable intermediate phases and inhibition of halide coordination-driven perovskite nucleation. The retarded perovskite nucleation and crystal growth are key to high crystal quality and thus efficient electroluminescence. Our work elucidates the full effects of molecular additives on PeLEDs by uncovering the chelate effect as an important feature within perovskite crystallization, as well as opening new prospects for the rationalized screening of highly effective molecular additives.

Bright and efficient blue emission is key to further development of metal halide perovskite light-emitting diodes. Although modifying the bromide/chloride composition is the most straightforward process to achieve blue emission, practical implementation of this strategy has been challenging due to poor colour stability and severe photoluminescence quenching. Both detrimental effects become increasingly prominent in perovskites with the high chloride content needed to produce blue emission.

Here, we demonstrate a correlation between spectral instability and compositional heterogeneity in the perovskite films. We provide a vapor-assisted crystallization technique which largely mitigates ion migration by compositional homogenization and show spectrally stable blue perovskite light-emitting diodes over a wide range of emission wavelengths from 490 to 451 nanometres.Particularly, our blue and deep-blue light-emitting diodes based on three-dimensional perovskites show high EQE values of 11.0% and 5.5% with emission peaks at 477 and 467 nm, respectively.

Beyond that, stabilized mixed halide perovskites are of great interest for a wide range of perovskite applications where the bandgap needs to be finely controlled, for instance, in tandem solar cells, photo detectors and lasing.

Several techniques have already been developed to elaborate high-quality perovskite films for photovoltaic (PV) application, and to further improve the crystallization of the active layer [1]. Up to now, spin coating is the most used technique for perovskite solar cells deposition, either by a one-step or a two-step method [2]. It gives good results but presents limitations in terms of deposition area’s size and compatible type of substrates. Alternative methods should be developed. The biggest challenge is to find a better deposition technique that gives high-quality perovskite layers on large size substrates, with a minimum manufacturing cost. Herein, the electrodeposition method was explored as an efficient alternative for perovskite fabrication. It possesses the ability to ideally satisfy the all above-mentioned advantages [3],[4]. In this work, two routes have been studied to elaborate the perovskite layer. The first one consist in an immediate conversion of PbO₂ into PK1 by immersion in MAI (CH₃NH₃I) solution and the second route is a two-step conversion: first conversion into PbI₂ by immersion of PbO₂ in HI, and then immersion of PbI₂ in MAI to convert into PK2. For further evaluation of the impact of the conversion route and the substrate nature, a perovskite solar cell has been developed using the electrodeposited active layers. Its structure is the following: an ITO (indium tin oxide) substrate as transparent cathode, a spin coated SnO₂ or mesoporous TiO₂ ETL (Electron Transport Layer) sub-layer, the electrodeposited CH₃NH₃PbI₃ perovskite layer (PK), a spin coated P3HT HTL layer (Hole Transport Layer) and a carbon paste top-layer as conducting anode. PK1 shows negligible photovoltaic activity whereas PK2 presents favorable results. When changing the ETL from SnO₂ to TiO₂, a percentage increase from 3% to 8% in terms of efficiency was detected, with the Voc increasing from 0.65 V to 0.87 V and the Jsc from 12 mA/cm² to 21 mA/cm². This opens the way to promising performances using electrodeposition.

Self-assembled monolayers of para-substituted phenylphosphonic acids were used to modify the surface of nickel oxide layer, which served as the hole-transport layer in the fabrication of inverted perovskite solar cells. The monolayer was installed to modulate the work function of nickel oxide-based on their electron-withdrawing or electron-donating substituent, while the perovskite film morphology and quality were not significantly altered due to the analogous hydrophilic nature of the modified surfaces. The modification impacted the device performance based on the work function modification and the dipole direction of the molecules. The best-performing device was observed with electron-withdrawing cyano-substituted phosphonic acid modification, with a maximum power conversion efficiency of 18.45%.

Inverted perovskite solar cells (PSCs) have attracted great attention due to their low fabrication cost, high performance, and negligible hysteresis. To achieve their commercialization, many researchers on inverted PSCs have been tried to improve the power conversion efficiency over the past few years. It is extremely important to develop high-quality hole transport layer (HTL) for fabricating inverted PSCs with highly performance. This work describes a facile, one-step, solution process method for the preparation of zinc-doped nickel oxide (Zn-doped NiO) hole transport layer for efficient PSCs. Inverted PSCs with the configuration of FTO/Zn:NiO/Cs_0.05FA_0.81MA_0.14Pb(I_0.90Br_0.10)₃/PCBM/BCP/Ag were fabricated and evaluated. We detected that a Zn-doped NiO hole transport layer can directly effect growth dynamics of perovskite on HTL layer (from SEM images). Also, photoluminescence results, we found that Zn-doped NiO hole transport layer improved the hole extraction efficiency due to reduced defect states. As a result of all these improvements, we report Zn-doping strategy for NiO improves photovoltaic performance of inverted PSCs.

Numerous studies have shown that perovskite solar cells with carbon-based contacts (C-PSCs) provide strong potential for delivering stable and up-scalable perovskite photovoltaic devices. However, their power conversion efficiencies (PCEs) are still lagging behind the conventional solar cells with metallic back-contacts. This necessitates a deeper understanding of the power losses present in C-PSCs in order to find effective strategies to reduce them. In principle, one can distinguish between two types of C-PSCs: (i) where the back-contact is cured at high temperatures (typically > 400°C), thereby allowing perovskite to be integrated into cell stack only after its deposition and (ii) where the back-electrode is deposited at low temperatures (typically < 120°C), which enables layer-by-layer deposition. Both cell structures have significant differences not only in the processing conditions, but also in the dominant losses present in the corresponding PV devices. For the first time, we conducted an objective experimental study to identify the main losses in both types of C-PSCs. We found that the major limitation of the cells with cathodes treated at high-temperatures is the non-radiative recombination happening at the numerous grain-boundaries, which are present in the mesoscopic cell stack of such cell. In contrast, the cells with low-temperature treated contacts can have large perovskite crystals due to more favorable crystallization techniques, allowed via layer-by-layer deposition. By combining experimental results with our numerical simulation we quantitatively demonstrate that the low-number of grain-boundaries reduces non-radiative recombination, thus increasing the quasi-Fermi level splitting of perovskite, prolonging charge carrier lifetime, which results in an impressive Voc > 1.1V in the HTL-free C-PSCs with low-temperature treated contacts. However, we note that in such cells the transport is hindered by the perovskite/carbon contact which significantly reduces the fill factor of such PV devices. Finally, we outline the promising methods of reducing non-radiative recombination and improving charge carrier transport in both types of C-PSCs to fulfill their potential. We further highlight the advantages of the C-PSCs with low-temperature treated electrodes due to higher flexibility of processing conditions, which allows to integrate wide range of charge-transport layer with favorable properties, enhanced crystallization, compatibility with roll-to-roll manufacturing and faster fabrication. [1]

I will describe electrochemical reactions that occur rapidly when perovskite solar cells are operated in reverse bias, which happens when one solar cell in a panel is shaded and must be current matched to illuminated cells.  Potential solutions for preventing long-term degradation will be presented.

I will also present dynamic windows based on reversible metal electrodeposition.  These windows have a wider dynamic range and more neutral color than the industry leading tungsten oxide electrochromic windows.  They have potential to be sufficiently inexpensive to make dynamic windows ubiquitous.  There are many similarities between this device and dye sensitized solar cells.

Among emerging photovoltaic technologies, Dye-Sensitized Solar Cells (DSSC) represent a promising and appealing class of solar cells. In the last years, they have attained quite high efficiencies and stability, and they have demonstrated their potential to convert energy under non-ideal operating environments, including indoor. [1-2] They can be fabricated using low-cost manufacturing process and materials. More interestingly, these solar cells can be colourful and semi-transparent, which makes them appealing for use in Building-Integrated Photovoltaics (BIPV). [3-4]

However, when developing semi-transparent DSSCs, a trade-off has to be found between transparency and efficiency. Current state-of-the-art only allow for the fabrication of solar cells showing an optical transmission that is fixed during the fabrication process. For the development of smart photovoltaic windows and their massive integration in buildings, variable and self-adaptable optical properties would be very valuable.

To tackle this challenge, we recently proposed a new approach based on the development of photochromic sensitizers. In this communication, we will present our strategy to prepare photochromic dyes for use in DSSCs. We will discuss the structure-properties relationships of these photochromic molecules and we will show how they can be used to develop a new generation of functional solar cells capable to change their colour, self-adjust their transparency and their photovoltaic energy conversion depending on the daylight intensity. [5]

We will also discuss the factors influencing the photochromic behaviour and the photovoltaic properties of this new class of dyes in device configuration.

Hybrid halide perovskites combine top-notch optoelectronic properties with solution-deposition. This unprecedented combination has led to the development of solar cells approaching power conversion efficiencies achieved by industry staples such as poly-Si. The road towards these achievements has been marked by a constant improvement of perovskite deposition techniques fuelled by our increasing understanding of the crystallization processes, yet stability is still a challenge. Perovskites are particularly susceptible to moisture, which catalyses degradation. In this talk I will show how the mechanism works, how a popular strategy of incorporating layered perovskites as a 'hydrophobic' layer, in fact does not keep water out; and alternative approaches to minimise the issue.

Among photovoltaics technologies, dye-sensitized solar cells (DSSCs) offer high conversion efficiencies (15% record efficiency) and low-cost manufacturing. Unfortunately, one of the major drawbacks in these record cells is the presence of toxic volatile organic solvents (VOCs) in the electrolyte.

To overcome this problem, we have successfully tested eco-friendly reaction media such as Deep Eutectic Solvents (DESs), made of two or three safe and cheap components which are able to express hydrogen-bond interactions with each other to form an eutectic mixture with a melting point much lower than either of the individual components. DESs are simple and low-cost to synthesize, do not need purification, and they are usually biodegradable. One of the most common DES components, choline chloride (ChCl), is largely used as an additive for chicken feed. We tested both hydrophilic and hydrophobic DESs in DSSCs with promising results [1,2]. As a prototypical hydrophilic DES, we used ChCl/glycerol (1:2 mol mol^–1) with 40% water jointly with an hydrophilic dye, and performed an extensive optimization of the device, including different co-adsorbents and TiO₂ film thicknesses. Conversely, when using a hydrophobic DES made of menthol and acetic acid we chose a phenothiazine-based dye already studied in our group. DSSCs filled with DESs displayed a lower recombination resistance and a higher V_oc when compared to cells filled with an electrolyte based on standard VOCs.

We then focused on DSSCs containing innovative sugar-based natural DES electrolytes, that is ChCl with different monosaccharides, sensitized with multi-branched phenothiazine dyes developed in our group, and characterized by the presence of an alkyl or a sugar substituent [3,4]. In particular, we systematically varied the dye (alkyl functionality vs. sugar moiety), the co-adsorbent (chenodeoxycholic acid vs. glucuronic acid), and the monosaccharide present in the DES. Overall, results are consistent with a cooperative interaction among all the components containing a sugar functionality leading to a performance boost.

In the past decade, the emergence of non-fullerene small molecules for organic photovoltaics (OPV) have led to an enhancement of the power conversion efficiency (PCE) up to over 18% for bulk heterojunction (BHJ) devices [1]. Not only the favorable electronic and optical properties of these chemically tunable materials are fundamental to achieve such high performance, but also obtaining optimal nanoscale phase separation and appropriate crystallization is crucial, while unique for each combination of donor and acceptor. In this work, we present the influence of different post-annealing processes on the morphology and performance of additive-free BHJ solar cells based on donor Poly[[ 2,2'-[[4,8-Bis[4-fluoro-5-(2-hexyldecyl)-2-thienyl]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl]-2,5-thiophenediyl(5,6-dihydro-5-octyl-4,6-dioxo-4Hthieno[3,4-c]pyrrole-1,3-diyl)-2,5-thiophenediyl] (TPD-3F) and small molecule acceptor 9-Bis(2-methylene-((3-(1,1-dicyanomethylene)-6,7-difluoro)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2’,3’-d’]-s-indaceno[1,2-b:5,6-b’]dithiophene (IT-4F) [2]. An increase of 25% in PCE compared to the device with as-cast blend film was observed when using an optimized solvent vapor annealing post-treatment process [3], leading to over 14% of average PCE and a maximum 76% of geometric fill factor, which is among the highest reported values for additive-free BHJ solar cells. Thermal annealing and vacuum drying processes were performed in parallel for further comparison. Electrical, morphological and photophysical characterizations were performed to gain insight into these results.

The addition of a small electron donor (tris(p-anisyl)amine, TPAA) to cobalt bipyridine redox electrolyte can lead to significant improvement of the performance dye-sensitized solar cells (DSC) [1]. Here, we investigate the addition of TPAA to traditional iodide/triiodide electrolytes for DSCs. When using 3-methoxy proprionitrile (MPN) as non-volatile solvent for the electrolyte and the organic dye LEG4 as sensitizer, a power conversion efficiency (PCE) of 4.3% is obtained for the I^-/I₃^- electrolyte, which improved to 5.7% upon addition of 0.1 M TPAA, due to small increases in voltage, photocurrent and fill factor. In contrast, using the ruthenium dye N719 as sensitizer, a slight decrease in PCE was found upon TPAA addition, from 5.0 to 4.7 %, due to a decrease in open-circuit potential.

Ultrafast dye regeneration takes place when TPAA is present in the electrolyte [1]. Using nanosecond transient absorption spectroscopy (ns-TAS) it is shown that this leads to the formation of TPAA⁺, which is subsequently reduced by iodide under the formation of the diodide radical, I₂^-. The latter is a rather slow process, 28 µs, which is due to the very low driving force in the reaction TPAA⁺ + 2 I^- = TPAA + I₂^-.

The reason for the decreased performance of N719 in the presence of TPAA is the recombination between electrons in TiO₂ and TPAA⁺, which is not sufficiently blocked by the dye, in contrast to LEG4.

Despite the rapid progress in perovskite solar cells (PSCs), their commercialization is still prohibited by various issues, with the most critical one being their instability. Part of the cause, and therefore the solution, lies at the charge selective contacts and their interfaces with halide perovskites. With MoO3 being one of the most successful hole transport layers in organic photovoltaics, the disastrous results of its combination with MAPbI3 came as a surprise but was later attributed to severe chemical instability at the MoO3/MAPbI3 interface. To discover the atomistic origin of this instability, we used density functional theory (DFT) calculations to investigate the interaction of MoO3 with perovskite precursors, which is the starting point of the formation of the interfaces. Besides MAPbI3 we also extended the study to FA and Br compositions. Two possible degradation routes leading to halogen oxidation and Mo reduction were identified, which are triggered by oxygen vacancies on the MoO3 surface. Iodine was found to be especially reactive, in contrast to Br, which does not significantly affect the oxide. These results were confirmed by XPS measurements revealing severe reduction of MoO3 accompanied by loss of iodine, when the oxide is interfaced with I-containing precursors. Based on these results, we can conclude that by avoiding I or applying interface passivation MoO3 could be employed as an effective HTL in PSCs and other perovskite optoelectronic devices, such as LEDs.

Environmentally friendly lead-free halide double perovskites with improved stability are regarded as one of the most promising alternatives to lead halide perovskites. The benchmark double perovskite Cs₂AgBiBr₆ is the most famous and most studied compound. It shows attractive optical and electronic features, making it promising for high-efficiency optoelectronic devices. However, the poor absorption profiles caused by the large bandgap limits its further applications, especially for photovoltaics. Here, we applied two strategies, namely metal doping and crystal synthesis engineering, to improve the absorption properties of the benchmark Cs₂AgBiBr₆. For metal doping strategy, we choose Cu as dopant and achieve new modified double perovskites Cs₂(Ag:Cu)BiBr₆. The absorption band edge of Cs₂AgBiBr₆ to the near‐infrared range is significantly broadened after Cu-doping. This due to Cu ions introduces defect state (subbandgap state) in the bandgap. More interestingly, the subbandgap state can generate band carriers upon excitation, which provides a great potential for using such doped material for near‐infrared light detection. Another straightforward method to broaden the absorption band edge of Cs₂AgBiBr₆ is to decrease its bandgap. We achieve the smallest reported band gap Cs₂AgBiBr₆ through simply controlling the growth temperature of single crystals. The Cs₂AgBiBr₆ crystal prepared from high evaporation temperature (150 °C) shows a significant band gap narrowing (ca. 0.26 eV) compared with that prepared from the low evaporation temperature (60 °C). We hypothesize that this band gap narrowing is caused by an increased level of Ag–Bi disorder. These two works shed new light on the absorption modulation of halide double perovskites for future efficient optoelectronic devices.

Metal-halide perovskites (MHP) based solar cells stand on the brink of revolutionizing the field of photovoltaics through their highly versatile nature, their bandgap tenability making them ideal candidates for tandem applications. Vapor deposition of these materials, where the precursors are heated in ultra-high vacuum such that they sublime onto the substrate [1], is the most straightforward way of depositing multiple layers in succession and hence fabricating tandem solar cells. However, this method still suffers from poor understanding of the sublimation and crystallization dynamics, particularly for the organic precursor [2]. The highest performance co-evaporated devices use the prototypical MHP CH₃NH₃PbI₃ (MAPbI₃) as the absorber, but show grains much smaller than those typically seen in solution processed devices. Understanding the film formation process and what leads to grain nucleation versus additive growth is likely to lead to improved performance and reproducibility of these devices.

In this study, we show that it is possible to control the grain size of vapor deposited MAPbI₃ by tuning the temperature of the substrate on which the precursor vapor sublimes. We find that, surprisingly, it is cold temperatures (-2 °C) that lead to the growth of large, micrometer-sized grains, while films grown at room temperature (23 °C) exhibit much smaller grains of size 100 nm. While the large grain samples show improved crystallographic properties, their performance is significantly worse than the ones with small grains. We find that the smaller grains and enhanced performance are linked to the presence of excess PbI₂, but that the enhanced performance remains even when all excess PbI₂ is converted to MAPbI₃. To more accurately control the sublimation rate of MAI we developed a novel technique based on the sublimation rate measured near the substrate, which is crucial to disentangle the effects of temperature and stoichiometry. As such, substrate temperature significantly affects the rate of adsorption of the organic MAI vapor and hence the rate of conversion of PbI₂ into MAPbI₃, and that both a balanced stoichiometry and high adsorption rate are necessary to form larger grains. However, a small excess of PbI₂ plays a beneficial, passivating role both during the film growth process and in the finished device, and the stoichiometry of the interfaces can be tuned to give optimized performance [3].

Recently, perovskite solar cells have gained significant interest as an alternative photovoltaic technology due to an impressive power conversion efficiency raise of up to 25% in the last decades. However, the commercialization of perovskite solar cells is hampered by environmental concerns due to the toxicity of the used lead. Hence, there is a high interest to substitute the lead by less toxic elements. Tin is a promising candidate, since it has similar electronic properties as lead and so can be easily replaced in the ABX₃ perovskite structure. But tin perovskite solar cells are known for their fast degradation in ambient atmosphere, due to the oxidation of tin from Sn²⁺ to Sn⁴⁺. A way to reduce toxicity and retard oxidation is to replace lead partly by tin to form tin-lead mixed perovskite solar cells. [1]–[4] Beside reduced toxicity, those perovskite solar cells are also beneficial for the bottom cell in tandem devices due to their low bandgap of around 1.2 eV. [4]

However, many publications about these mixed perovskites report on methylammonium (MA) containing perovskites and/or on PEDOT:PSS as hole transport layer (HTL) [5], although both should be detrimental for long-term stability of the cell stack.

Hence, we present a strategy to replace PEDOT:PSS by PTAA in MA-free devices. First, PEDOT:PSS can be easily replaced by PTAA with similar solar cell performance for a reference with a MA-containing perovskite of the composition FA_0.75MA_0.25Sn_0.5Pb_0.5I₃ (12.8 % vs. 10.5 %), respectively. Second, comparable solar cell efficiencies of 12.0 % could also be achieved for MA-free FA_0.915Cs_0.085Sn_0.5Pb_0.5I₃ on PEDOT:PSS. However, combining PTAA with the MA-free perovskite revealed a strong S-shape solar cell performance (2.7%). This was thought to be due to an energetic barrier at the PTAA/perovskite interface. Increasing the Cs to FA ratio in the perovskite composition reduces the s-shape which is expected to be due to a decreased valence band, but constant band-gaps of around 1.26 eV, and thus a better band alignment. By this a solar cell efficiency of 9.8 % for the perovskite composition FA_0.65Cs_0.35Sn_0.5Pb_0.5I₃ could be achieved.

We use a multi-technique approach to determine the phase diagram and molecular cation dynamics of mixed methylammonium-formamidinium MA1-xFAxPbBr3 (0 ≤ x ≤ 1) hybrid perovskites. The calorimetric, ultrasonic and X-ray diffraction experiments show a substantial suppression of the structural phase transitions and stabilization of the cubic phase upon mixing. We use the broadband dielectric spectroscopy to study the MA and FA cations dynamics in these compounds. Our results indicate absence of the MA cation ordering and a gradual increase of the rotation barrier upon mixing. The room-temperature dielectric permittivity substantially decreases as the fraction of the FA cations is increased. No significant changes of the permittivity are detected at temperatures where the dielectric relaxations are absent. We discuss the possibility of the glass phase formation in these compounds.

One of the most important photocurrent loss mechanisms limiting the power-conversion efficiency in all solar cells is trap-assisted recombination caused by localized sub-gap states. The presence and relevance of this first-order recombination in organic photovoltaic devices is still a subject of current debate, hindering the field as it seeks to push the boundaries of efficiency towards inorganic and perovskite semiconductor counterparts. In this work we conduct wide dynamic range, sensitive intensity-dependent photocurrent measurements combined with one-dimensional drift-diffusion simulations. Our key finding is that first-order, trap-assisted recombination appears to be universally present in a large variety of fullerene and non-fullerene acceptor systems - including state-of-the-art PM6:BTP-eC9 achieving above 15 % power conversion efficiency. The trap states are found to be situated ~ 0.35-0.6 eV below the transport level edges of acceptor: donor blends with trap densities lying between 10¹⁶-10¹⁷ cm^-3. We show that the trap-assisted recombination via these deep sub-gap states induces not only losses in the photocurrent but also limits the open-circuit voltage leading to ideality factors between 1 and 2. Hence, our findings deliver new insight into the role and nature of trap states in organic light-harvesting devices, and shed new light on the complexity and variability of ideality factors in solar cells.

Perovskite solar cells with high power-per-weight have great potential to be used for aerospace applications such as satellites or high-altitude pseudo-satellites. The latter are unmanned aircraft exclusively powered by solar energy, typically flying in the stratosphere where the conditions of pressure, temperature and illumination are critically different from that on the earth's surface. In this work, we evaluate the performance and stability of high efficiency perovskite solar cells under a mimic stratospheric environment. In situ measurements under controlled conditions of pressure, temperature and illumination were developed. We show that the cells can operate efficiently in a large range of temperature from −50 °C to +20 °C, with a maximum power conversion efficiency at −20 °C, which is ideal for use in the stratosphere. Besides, performances are maintained after a number of temperature cycles down to −85 °C, representative of temperature variations due to diurnal cycles. An efficient encapsulation is developed, which could be critical to avoid the accelerated degradation of the cells under vacuum. Finally, a promising stability for 25 days of day–night cycles was demonstrated, which suggests that perovskite solar cells could be used to power high altitude pseudo-satellites.

Organic solar cells (OSCs) have experienced an explosive development in the last 5 years with the emergence of the so-called fused-ring-based non-fullerene acceptors (NFAs), leading to power conversion efficiencies (PCEs) surpassing 18% recently and with 20% firmly on the radar.  Such performance metrics are now challenging the traditional inorganic semiconductor photovoltaics and have aroused increased interest in manufacturing. In this regard, realizing high-efficiency OSCs with thick-junction active layer while understanding the charge generation/recombination mechanisms is an acknowledged prerequisite for high-throughput deposition processing such as roll-to-roll. In this contribution, we advance a new approach for accurate measurement of charge generation yield (CGY) in a series of polymer: non-fullerene OSCs. We show that the state-of-art PM6:Y6 system possess a near unity yield of 98.4%, and this value can be further improved to 99.3% in a Y6 derivative based PM6:BTP-eC9 system. Importantly, we find this apparently modest improvement of 0.9% in CGY can translate to a dramatic impact on photo-generated carrier recombination, leading to a reduced factor of 2.5 relative to the Langevin limit and delivering an unprecedented PCE of >16% (fill factor exceeding 70%) in a 300nm PM6:BTP-eC9 device, which we also probe and understand using drift diffusion simulations. Our work not only reveals the interplay between charge generation, recombination, and device efficiency, but also provides a correlation between basic device physics and practical solar cell engineering. 

Tin halide perovskites are currently the most promising alternative to address the toxicity of lead halide perovskite photovoltaics, although their poor ambient stability remains as the main impediment for this lead-free technology to attain competitive efficiencies. Hence, a detailed understanding of their degradation pathways is required to tackle their stability issues. In this talk, we present the degradation mechanism of tin perovskite thin films based on (PEA)_0.2(FA)_0.8SnI₃ (where PEA is phenylethylammonium and FA is formamidinium). We show that SnI₄, a product of the oxygen-mediated tin perovskite decomposition, promptly evolves to give iodine via the combined effect of atmospheric water and oxygen. Iodine is then found to be a highly aggressive species that leads to further perovskite degradation to produce more SnI₄, giving rise to a cyclic degradation mechanism. We find the ambient stability of perovskite films to be strongly influenced by the hole transport layer chosen as the substrate, which is used to mitigate the oxidative decomposition of the material. We anticipate the key findings described here to inspire effective design rules towards stable lead-free, tin perovskite solar cells.   

Intel corporation predict that by the end of 2020, up to 200 billion connected Internet of Things (IoT) devices came online and this is predicted to consume over 1000 TWh yr^-1 by 2025. To put this in perspective, this is over 1300 x the energy output of EU’s largest solar plant, Núñez de Balboa. The increasing digital interconnectivity of our everyday lives means that this energy burden will only increase in the coming decades. In addition, many portable IoT devices incorporate primary batteries, which consume valuable materials. One solution to this growing demand is the coupling of IoT nodes with energy harvesting devices, such as photovoltaics cells which could provide power to IoT nodes and avoid the need to replace primary batteries.

Hybrid solar cells such as perovskites, OPVs and DSSCs, have all shown promising performance in ambient light, meaning that any of these technologies could be candidates to power future IoT devices. Despite the promise of these PV technologies, work needs to be done especially around standards for testing. There is currently no consensus on what constitutes “ambient light” both in terms of spectral and power output and it is difficult to compare results from one laboratory to the next. We can predict the path of the Sun for any given point on Earth, but predicting how much light power is available in any given ambient scenario is difficult and reliant on human factors such as available light sources (e.g. CFT, LED, natural, or combination) and position of the solar cell relative to the light source.

This presentation will explore these issues, showing the results of maximum power measurement in real and simulated scenarios and how in some cases, an IV curve measured at single lux value is not an accurate predicator of real-world performance. Module design will also be discussed in relation to series resistance at low light intensity and how this may be able to reduce module costs. Finally, the presentation will explore some ideas from the field of Human-Computer-Interaction (HCI) showing the result of collaborative work whereby hybrid solar cells not only provide power to IoT nodes, but also provide means of user interaction and control of the technologies themselves.

Efficient dye-sensitized photocathodes offer new opportunities for converting sunlight into storable energy cheaply and sustainably.[1] We are developing dye-sensitized NiO cathodes for light-driven reduction of carbon dioxide or water to high energy products (solar fuels) using the lessons we have learnt from solar cells.[2] This strategy combines the selectivity of a molecular catalyst with the charge transport properties of inorganic semiconductors in a robust device. When photoelectrodes are assembled in a tandem configuration (see figure), the electrons released in water oxidation at the photoanode can be consumed by the photocathode in the reduction of e.g. H⁺ to H₂ or CO₂  to CO. Generating hydrogen on one electrode and oxygen on another enables the two gasses to be collected separately. The device performance can be optimised by tuning the properties of the individual components responsible for light absorption, charge transport and catalysis, rather than relying on one material to have all the necessary credentials. The electron-transfer dynamics are key to the performance and a major challenge is slowing down charge recombination between the photoreduced dye and the oxidised NiO so that chemistry can take place.[4] Highlights from recent work examining charge-transfer at the interface between NiO and new supramolecular photocatalysts using transient absorption spectroscopy and time-resolved infrared spectroscopy will be presented [4-7]. The effect of the environment on the kinetics will be discussed [5,6]. The relationship between the structure dynamics and performance of the photoelectrocatalytic devices will be summarized [2].

Climate change has made transforming our energy economy into one that is clean, renewable, and sustainable a global imperative. Our perovskite/silicon tandem solar cell solution delivers high efficiency at a low cost, which is essential for solar to replace fossil fuels and meet growing energy demand. Our technology uses existing production routes taken from the Si industry and drastically enhances the power output with the addition of a perovskite top-cell in a tandem configuration. Supported by excellent advancements in the field of perovskite photovoltaics in recent years, we have demonstrated a certified world-record efficiency of 29.5%, which far exceeds the record Si single junction performance (26.7%) and even exceeds the physical limit of Si PV (29.3%). With this presentation, we will give a brief look at the activities at both our R&D site in England and our industrial site in Germany, where we will commence production in 2022.

In this we talk we present an investigation of charge extraction in solar cell stacks using steady-state and time-resolved photoluminescence spectroscopy and microscopy.

Owing to efficiency of perovskite solar cells is rapidly reaching the one of conventional PV, stability and scalability are becoming important topics toward commercialization of this ultra-low-cost photovoltaic system[1–3]. In this work, we develop a new molecular and structural engineering approaches for 3D/2D perovskite absorber as well as polymeric hole transport layer that allow us to successfully fabricated high efficiency, stable perovskite solar cells and modules reaching above 21% photoconversion efficiency (PCE) on small area cells as well as 19.9% PCE on 9.0 cm² active area and above 18% on 48 cm² active area of perovskite solar modules. Stable perovskite modules have been fabricated by an improved up-scaling program showing that our strategy is quite scalable and reproducible and compatible with low-cost solution-based manufacture strategies. Light stability (under atmospheric air and 55% RH) of PSC has been carefully investigated via ex-situ GIWAX analysis for ≈1000 hours and the results shown that applying novel 2D materials and using polaron arrangement and doping strategy of PTAA it is possible to improve the efficiency as well as the stability of the PSCs and Modules[4,5].  The obtained performances of the fabricated modules can be classified among the best-reported values for this area. The interpretation of optical, electrochemical and photovoltaic characteristics of the devices has been assessed by thorough analyses including UV-Vis absorbance, Photoluminscence (PL), Electrochemical Impedance Spectroscopy (EIS), Cyclic Voltammetry (CV), Transient Photovoltage and Photocurrent Fall/Rise analyses in addition to surficial analysis methods e.g., FE-SEM and XRD measurements.

‡NYN and MZ contributed equally to this work.

Perovskite-based tandem solar cells have rapidly developed in the last few years, to a current record power conversion efficiency (PCE) of 29.5% and several research groups overcoming 27% PCE. The predicted realistic target PCE of around 33% is nevertheless still far away. The set of measurement techniques suitable for tandem solar cells is smaller compared to single junction solar cells due to the series connection of subcells in monolithic architecture, especially when characterizing individual subcells. Here, we present a setup for advanced characterization of tandem solar cells, based on a bichromatic LED light source. Two spectrally independent LED arrays are used to independently bias individual subcells. Blue LEDs with a central wavelength of λ = 470 nm are used to bias the top cell, while IR LEDs (λ = 940 nm) bias the bottom cell. This enables accessing properties of each subcell through e.g. light intensity dependent I-V measurements and extracting one-diode model parameters, shunt resistance, series resistance, ideality factor and saturation current density. With this method and using one-diode model for each subcell we are able to reconstruct tandem J-V and disentangle it into subcell J-Vs. We validate the procedure using simulations that return excellent agreement, while for the fabricated tandem device, we observe a slight discrepancy around maximum power point, attributed to voltage-dependent ideality factor. The developed BCLED tool can therefore be used for subcell sensitive analysis and also long-term stability testing due to longevity of LEDs.

A two-terminal hybrid tandem solar cell consists of two sub-cells (a high-bandgap hybrid perovskite sub-cell and a low-bandgap Si sub-cell) connected in series with each other using ITO as the recombination layer. The efficiency of a tandem solar cell depends on the performance of the individual sub-cells as well as their current matching capability. We investigate here the charge carrier dynamics in these individual sub-cells in response to a supercontinuum picosecond light pulse, where the spectral range provides the possibility to photoexcite single sub-cell as well as collectively both the sub-cells. The transient photocurrent responses were also characterized at different intensity levels of the dc-light bias. We observe a spectral dependence of transient photocurrent lifetime which can be classified into two distinct timescales. The first timescale in the range of ~ 500 ns represents the top-perovskite sub-cell (absorption range from 300 nm – 750nm) and the other timescale regime of ~ 10 - 30 µs corresponds to the bottom Si sub-cell (> 750 nm). These lifetimes observed under single sub-cell excitation are comparable to the lifetimes measured for individual single-junction cells for the respective materials. The measurements also indicate that the carrier-lifetimes in each of the sub-cells are independent when the other sub-cell is specifically excited with different intensities of steady-state illumination. The results obtained from the transient photocurrent measurements were modelled using the scattering matrix formulation for generating the carrier generation profiles and drift-diffusion equations for understanding the carrier transport process. These studies quantify the important effects of imbalance on the charge carrier dynamics.

Dye-sensitized solar cells (DSCs) based on copper(II/I) redox mediators have readily attained high open-circuit voltages (Voc) of over 1 V, but relatively low short-circuit photocurrent densities. To achieve high power conversion efficiency (PCE), the development of extended spectral-response photosensitizers compatible with the copper-based electrolytes while attaining a high Voc is crucial. Herein, a blue photosensitizer coded R7 was reported, which was purposely designed for highly efficient DSCs with copper-based electrolytes. R7 features a strong electron-donating segment of 9,19-dihydrobenzo[1',10']phenanthro[3',4':4,5]thieno[3,2-b]benzo[1,10]phenanthro[3,4-d]thiophene conjugated with an bulky auxiliary donor N-(2',4'-bis(hexyloxy)-[1,1'-biphenyl]-4-yl)-2',4'-bis(hexyloxy)-N-methyl-[1,1'-biphenyl]-4-amine and the electron acceptor 4-(7-ethynylbenzo[c][1,2,5]thiadiazol-4-yl)benzoic acid. The blue dye R7 with bulkier auxiliary donor moiety in the DSC largely outperforms the reference dye R6 with a smaller bis(4-(hexyloxy)phenyl)amine donor unit.[1] Transient absorption spectroscopy and electrochemical impedance spectroscopy measurements show that the R7 based DSC has a higher charge separation yield, a higher charge collection efficiency, and a slower interfacial charge recombination than the R6 based counterpart. A co-sensitized system of R7 and Y123 for the DSC presents an outstanding PCE of 12.7% and maintains 90% of its initial value after 1,000 h of continuous full sunlight soaking at 45 oC. The work sheds light on future designs of organic photosensitizers for highly efficient copper-based DSCs.

Transient photoluminescence (TRPL) experiments became an indispensable tool for characterization of radiative and non-radiative loss channels in perovskite-based device architectures [1,2] through the use of detailed balance relations or fitting of rate equations. However, due to the sharp absorption edge of perovskites resulting in a strong overlap of absorption and emission spectra [3], as well as the low non-radiative losses in high-quality perovskites [4], significant photon recycling occurs which will alter the shape and strength of the resulting spectral TRPL signal [5].

It is therefore crucial to take all these effects into account carefully in order to predict and correctly interpret such experiments. However, up to this day, most theoretical or simulation-based investigations into this topic were either limited to simple device architectures with photon recycling taken into account using only ray-optical considerations [5] or more complex architectures but lacking any impact of photon recycling [6].

In our talk, we will present fully-coupled opto-electronic simulations of TRPL where photon recycling, based on a transverse Green’s function model, is taken into account in a full-wave picture and self-consistently coupled to the electronic transport solved by a drift-diffusion approach, resulting in a more accurate prediction of the emitted TRPL spectrum. Due to consistent consideration of the detailed balance principle in the local radiative rates, as well as the inclusion of the correct local photonic density of states, the resulting PL spectrum is conforming with global detailed balance relations such as generalized Kirchhoff [7].

Furthermore, our approach is not limited to single layer structures, instead, more complex device structures can also be analyzed, where the full optoelectronic approach allows to consider non-radiative loss channels, especially trap-assisted recombination at interfaces.

The upscaling of perovskite solar cells is one of the challenges that must be addressed to pave to way for the commercial exploitation of this technology. While a large part of the upscaling efforts has been focusing on n-i-p cells, here we will show the development of highly efficient inverted solar modules based on NiO_x. The first step consists of the development of an efficient P1 P2 P3 laser ablation process for the series connection of the sub-cells of a module.^[1] By combining SEM-EDX analysis with a transfer length measurement, we optimized the use of a low-cost UV ns laser to provide a robust process compatible with narrow interconnections. This led to 10 cm² minimodules with PCE of up to 15.9% on active area. To further improve in these results, we developed a novel n-side passivation of the perovskite layer that enabled the fabrication of inverted cells with NiOx with an efficiency of up to 20%, with remarkable fill factors of up to 83%. By combining the improved cell stack with the optimized interconnection, we were able to demonstrate 10 cm² minimodules with an efficiency of up to 18.1%, the highest value reported for modules based on NiO_x. Finally, we will show how electroluminescence mapping can quickly help in the characterization of the defects that can limit the performance of a perovskite module.

Efficient and stable perovskite solar cells rely on the use of water-soluble Pb²⁺ species potentially challenging the technologies’ commercialisation. In this study, the environmental fate of perovskite-based Pb²⁺ species is evaluated in soil-water microcosm experiments, simulating worst-case leaching from the solar cells under various biogeochemical conditions. The rapid and efficient removal of Pb²⁺ from the aqueous phase is demonstrated by inductively coupled plasma mass spectrometry. Sequential soil extraction results reveal that a substantial amount of Pb²⁺ is naturally immobilised, while a minor proportion of Pb²⁺ may become available again in the long term, when oxygen is depleted. X-ray absorption spectroscopy results reveal that the sorption of Pb²⁺ on mineral phases (such as birnessite-type MnO₂) represents the most likely sequestration mechanism. The obtained results suggest that the availability of leached Pb²⁺ from perovskite solar cells is naturally limited in soils and that its adverse effects on soil biota are probably negligible in oxic soils.

The efficiency of flexible perovskite solar cells (f-PSCs) has recently reached power conversion efficiency (PCE) as high as 19.5% [1]. Although still lagging behind their rigid counterparts, which in very short time have rocketed 25.2% efficiency [2], f-PSCs present several appealing features, such as bendability, conformability and high power-to-weight ratio [3], that make them good candidates for several applications, from consumer electronics to avionics and spacecrafts [4].

We present the synthesis of poly-3-hexylthiophene (P3HT)-derivated HTMs, embodying benzothiadiazole (BDT) moieties as electron-poor host. BDT was inserted along P3HT polymeric backbone, creating a donor-acceptor system able to promote the charge mobility throughout the HTM. Benzothiadiazole-modified P3HT (BTD-P3HT)[5] was used as hole transport material (HTM) in f-PSCs and led to PCE comparable to commercially available P3HT and showed improved stability under continuous illumination. It was also employed in 6×6 cm² modules, delivering 6.9% efficiency on 16 cm² of active area and demonstrating its feasibility for large area manufacture.

[1]        K. Huang, Y. Peng, Y.Y. Gao, J. Shi, H. Li, X. Mo, H. Huang, Y.Y. Gao, L. Ding, J. Yang, High-Performance Flexible Perovskite Solar Cells via Precise Control of Electron Transport Layer, Adv. Energy Mater. 9 (2019) 1901419. https://doi.org/10.1002/aenm.201901419.

[2]        NREL, Best Research-Cell Efficiency Chart | Photovoltaic Research | NREL, Best Res. Effic. Chart | Photovolt. Res. | NREL. (2021) https://www.nrel.gov/pv/cell-efficiency.html. https://www.nrel.gov/pv/cell-efficiency.html (accessed April 28, 2021).

[3]        M. Kaltenbrunner, G. Adam, E.D. Głowacki, M. Drack, R. Schwödiauer, L. Leonat, D.H. Apaydin, H. Groiss, M.C. Scharber, M.S. White, N.S. Sariciftci, S. Bauer, Flexible high power-per-weight perovskite solar cells with chromium oxide-metal contacts for improved stability in air, Nat. Mater. 14 (2015) 1032–1039. https://doi.org/10.1038/nmat4388.

[4]        J. Zhang, W. Zhang, H.M. Cheng, S.R.P. Silva, Critical review of recent progress of flexible perovskite solar cells, Mater. Today. (2020). https://doi.org/10.1016/j.mattod.2020.05.002.

[5]        F. De Rossi, G. Renno, B. Taheri, N. Yaghoobi Nia, V. Ilieva, A. Fin, A. Di Carlo, M. Bonomo, C. Barolo, F. Brunetti, Modified P3HT materials as hole transport layers for flexible perovskite solar cells, J. Power Sources. 494 (2021) 229735. https://doi.org/10.1016/j.jpowsour.2021.229735.

Monolithic perovskite/silicon tandem solar cells are of interest in the photovoltaic community thanks to their potential to combine high power conversion efficiency (PCE) with affordable cost. In the last decade, significant advancements have been reported towards this goal. However, to make perovskite/silicon tandems fully industry-relevant, exclusively scalable fabrication methods and materials need to be employed. Vacuum-based processing techniques can provide a conformal coverage on the pyramidal texture, typical for single-junction silicon solar cells. For such tandems, we reported 25% certified PCE with record current densities of 19.8 mA cm-2. Specifically, we used the vacuum/solution hybrid technique for the perovskite layer, combined with nanocrystalline recombination junctions to keep possible electrical micro shunts localized.[1] Solution-based techniques, specifically one-step perovskite spin-casting, have shown rapid advancements for single-junction perovskite solar cells. However, fully covering perovskite films on micron-scale textured interfaces with this technique requires process sophistication. To achieve end-to-end coverage, we reduced the pyramid size to 1-2 mm and adjusted the perovskite precursor solution concentration. Combining this with 1-butanethiol surface passivation enabled a certified PCE of 25.7% with negligible losses after 400 hours of operation.[2] Next, to translate the solution-based method to large-scale deposition, we adopted slot-die-coated perovskite top cells on textured surfaces since it offers significant advantages in throughput and material utilization. With this approach, we reported 23.7% PCE for the first proof-of-concept device.[3]

Beyond the requirement towards the use of industry-compatible silicon bottom cells (avoiding mirror-polished surfaces), which dictates appropriate perovskite processing techniques, the best choice for the device polarity is still to be settled as well. The initial perovskite/silicon tandems were in the n-i-p configuration but were limited by a high parasitic absorption in the hole-collecting contact stacks at the front (as well as the non-ideal optical design of the bottom cells, using double-side polished wafers). Global tandem research refocused, therefore, onto the p-i-n configuration. However, as a result, perovskite/silicon tandem research no longer stood to benefit from impressive progress made for efficient n-i-p perovskite single-junction solar cells. Nevertheless, adopting these advancements to tandem solar cells may be key towards perovskite/silicon tandems with PCEs well over 30%. Therefore, in this contribution, we will also discuss the existing challenges and our recent advancement on the n-i-p configuration tandems. Overall, this talk will give insight into the future directions to be taken to push the PCE of the perovskite/silicon tandem solar cells beyond 30%.

In the last decade, perovskite solar cell (PSC) technology showed an efficiency improvement approaching the Si record ^[1,2]. The low-cost perspective of PSCs is achievable only if scalable and reliable processes in manufacturing conditions, such as pilot line or plant factory, are designed and optimized for the full device stack ^[3–5]. In literature, no reports have been presented for scaling up to module size the efficient and stable CsMAFA perovskite ^[6] with scalable coating technique in ambient condition. Here, a full semi-automatic scalable process based on the blade coating technique is demonstrated to fabricate the full perovskite solar modules (PSMs) n-i-p stack in real fabrication conditions ^[7]. The triple cation CsMAFA perovskite is deposited with a double step process assisted by air quenching and green anti-solvent. The developed material formulation and coating procedure allow the fabrication of several highly reproducible small area cells on a module size substrate with an efficiency exceeding 17%. Corresponding reproducible modules with a 90% geometrical fill factor, achieved a champion efficiency of 16.1% and a T₈₀=750 h in light soaking condition at MPP and RT/ambient thanks to the low defect density of the coated layers. The properties and the homogeneity of the film depositions are assessed by different characterization techniques such as Scanning Electron Microscopy, profilometry, UV-vis and Photo-luminescence spectroscopy, Photo- and Electro-luminescence imaging. The last two techniques confirmed fewer defects and local coating variations of the ambient air/bladed devices with respect to the reference procedure based on the spin-coating technique in glovebox and ambient air. Finally, this work provides a scalable and industry-compatible route to realize efficient and stable triple cation perovskite solar modules in real ambient conditions.

The field of photovoltaics gives the opportunity to make our buildings ‘‘smart’’ and our portable devices
“independent”, provided effective energy sources can be developed for use in ambient indoor
conditions. To address this important issue, ambient light photovoltaic cells were developed to power
autonomous Internet of Things (IoT) devices, capable of machine learning, allowing the on-device
implementation of artificial intelligence. Through a novel co-sensitization strategy, we tailored dyesensitized
photovoltaic cells based on a copper(II/I) electrolyte for the generation of power under
ambient lighting with an unprecedented conversion efficiency (34%, 103 mW cm
^-2 at 1000 lux; 32.7%,
50 mW cm^-2 at 500 lux and 31.4%, 19 mW cm^-2 at 200 lux from a fluorescent lamp). A small array of
DSCs with a joint active area of 16 cm2 was then used to power machine learning on wireless nodes.
The collection of 0.947 mJ or 2.72  10¹⁵ photons is needed to compute one inference of a pre-trained
artificial neural network for MNIST image classification in the employed set up. The inference accuracy
of the network exceeded 90% for standard test images and 80% using camera-acquired printed MNISTdigits.
Quantization of the neural network significantly reduced memory requirements with a less than
0.1% loss in accuracy compared to a full-precision network, making machine learning inferences on
low-power microcontrollers possible. 152 J or 4.41  10²⁰ photons required for training and verification
of an artificial neural network were harvested with 64 cm² photovoltaic area in less than 24 hours under
1000 lux illumination. Ambient light harvesters provide a new generation of self-powered and “smart” IoT
devices powered through an energy source that is largely untapped.

Perovskite silicon tandem solar cells promise lower levelized costs of electricity than single-junction silicon and perovskite solar cells. Achieving this goal requires high power conversion efficiencies >27% and stability for long device lifetime. [1] Furthermore, deposition on textured silicon would enable higher energy yields, [2] and lead-free perovskite absorbers would avoid acceptance issues due to environmental concerns. Applicability to different bottom solar cell concepts, such as PERC, TOPCon or SHJ, would further facilitate an evolutionary transition of the PV industry. [3]

Our work addresses the above described challenges towards an industrialization of perovskite silicon tandem solar cells. We present tandem devices in the n‑i‑p and the p‑i‑n device architecture reaching 25.1% certified stabilized efficiency using the photo-stable bandgap adapted perovskite composition FA_0.75Cs_0.25Pb(I_0.8Br_0.2)₃. We further show the deposition of the absorber on industry-relevant random pyramid silicon texture for improved light management and discuss possible options towards upscaling. Regarding lead-free perovskite absorbers, we present process-engineering of Cs₂AgBiBr₆ films and their application in solar cell devices.

Over the last years molecular photovoltaics, such as dye sensitized cells (DSCs) and perovskite solar cells (PSCs) have emerged as credible contenders to conventional p-n junction photovoltaics. The certified power conversion efficiency of PSCs currently attains 25.5 %, exceeding that of the market leader polycrystalline silicon. This lecture covers the genesis and recent evolution of DSCs and PSCs, describing their operational principles, current performance. DSCs have now found commercial applications for ambient light harvesting and electric power producing glazing. The scale up and pilote production of PSCs are progressing rapidly but there remain challenges that still need to be met to implement PSCs on a large commercial scale. PSCs can produce high photovoltages rendering them attractive for applications in tandem cells, e.g. with silicon and for the generation of fuels from sunlight. Examples are the solar generation of hydrogen from water and the conversion of CO₂ to chemical feedstocks such as ethylene, mimicking natural photosynthesis.

High open-circuit voltage (Voc) is the essential benefit of perovskite photovoltaic cells that has enabled high efficiency in power conversion. Voc is improved by successful passivation of defects at grain boundaries and hetero-junction interfaces. Various approaches of interfacial engineering have been attempted in the past decade by using modulator molecules and mixing 2D and 3D structures to perovskites, as overviewed by our group as a comprehensive review.¹ These efforts also lead to enhance the stability of the organic—inorganic hybrid perovskite devices. However, organic cations in hybrid perovskites and use of diffusible ionic dopants in hole transport materials (HTMs) are responsible for low stability of perovskites at high temperatures (>120^oC). In this respect, combination of all-inorganic perovskite materials and dopant-free HTMs will be a main direction of perovskite photovoltaics.² Although CsPbX₃(X=Br, I) has been studied as a popular candidate of the all-inorganic perovskite, the stability of its photo-active black phase depends on the halide composition and CsPbI₃ lacks in high stability. We have focused our work on CsPbI₂Br as the stable visible light absorber. The CsPbI₂Br film was made into junction with a solution-processed dopant-free HTMs which are copolymer materials and thermally stable up to 300^oC. The quality of interfacial structure at the CsPbI₂Br and HTM junction was improved by inserting an ultra-thin layer (<3 nm) of amorphous SnOx. The all-inorganic and dopant-free perovskite device exhibits high Voc value beyond 1.4V with efficiencies >15%.³ The high level of Voc was maintained even under low light intensity as demonstrated by ideality factors as small as <1.4, indicating that defect-assisted recombination is suppressed well at the junctions of perovskite with HTM and SnOx. When the device performance was assessed under indoor illumination (200 lx luminance of LED), efficiency reached 35% or more, holding the value of Voc at 1.1V or more. It is very rare to find a photovoltaic device capable of Voc >1.1V under indoor illumination. Therefore, high Voc perovskite devices definitely work as the best power supply for IoT applications. For consumer electronics, however, a big challenge should also be directed to development of lead-free perovskite materials for environmental safety in practical applications.²

Integrating metal halide perovskite top cells with crystalline silicon or CIGS bottom cells into monolithic tandem devices has recently attracted increased attention due to the high efficiency potential of these cell architectures. To further increase the performance of these fascinating tandem solar cells to a level of predicted efficiency limits well above 30%, optical and electrical optimizations as well as a detailed device understanding of this advanced tandem architecture need to be developed. Here we present our recent results on monolithic tandem combinations of perovskite with crystalline silicon and CIGS, as well as tandem relevant aspects of perovskite single junction solar cells. 

Recently we have shown that self-assembled monolayers (SAM) could be implemented as appropriate hole selective contacts.^[1] The implementation of new generation SAM molecules enabled further reduction of non-radiative recombination losses with Voc’s up to 1.19 V and efficiency of 21.2% for p-i-n perovskite single junctions with band gaps of 1.63 eV and 1.55 eV, respectively. By fine-tuning the SAM molecular structure even further, the photostability of perovskite composition with tandem-ideal band gaps of 1.68 eV could be enhanced by reduction of defect density and fast hole extraction. That enabled a certified world record perovskite/silicon tandem solar cell efficiency at 29.15%.^[2]

In addition to the experimental material and device development, also main scientific and technological challenges and empirical efficiency limits as well as advanced analysis methods will be discussed for perovskite based tandem solar cells. ^[3] In addition, first results for upscaling of these tandem solar cells by thermal evaporation and slot-die coating will be shown.

References:

[1] DOI: 10.1039/C9EE02268F

[2] DOI: 10.1126/science.abd4016

[3] DOI: 10.1002/aenm.201904102

With the development of a broad range of strongly absorbing non-fullerene absorbers in the last five years, the power conversion efficiencies of organic solar cells have increased rapidly and are now approaching 20% [1],[2]. The market of the internet of things is emerging remarkably and demands to drive high amounts of off-grid low power consumption devices [3],[4]. The possibility to produce solution-based, low-cost and flexible solar foils makes OPV (organic photovoltaics) a good candidate to fulfil this demand.

Although efficiencies for iOPV (indoor OPV) have improved substantially [5]–[9], it is hard to determine champion solar cells due to a lack of standardized comparison methods [4],[10],[11]. Different authors use different conditions to evaluate the performance of devices. The set-ups differ in the source of light with varying color temperatures and intensities. The overlap of the materials’ absorption spectra and the spectral emission power of the light-source defines the short circuit current density [12]. Consequently, the performance depends on the intensity and color temperature of the light-source, which makes the comparability between publications difficult. Furthermore, the intensity of the emission power determines the input power and therefore the efficiency as well.

To address the problem of unavoidable arbitrariness of light-sources one needs to find figures of merit, which are easy to use by the OPV community. In this meta-analysis, we present an evaluation method based on the measurement of the external quantum efficiency combined with relative measurements of the spectral irradiance and current-voltage characteristics at different light intensities with one light-source. This set of experimental data, enables us to calculate the efficiency of organic solar cells under the illumination of several different light-sources and intensities which now allows cross-publication between published or in house data.

We investigate the dependence of the efficiency of the solar cells on the spectra of the light-sources and found the efficiency to deviate ~20 % as a function of the color temperature of the LED. As our approach ensures equal evaluation conditions, we present a meta-analysis of organic solar cells for indoor applications and compare the state of the art with thermodynamic efficiency limits under indoor illumination. We find that the optimal bandgap of the absorber material depends on the used light-source and ranges between 1.75 eV and 2 eV. The presented calculation is unique in literature and a powerful tool to detect possible candidates for high indoor performance solar cells.

Hybrid halide perovskites are excellent materials for next generation photovoltaics, demonstrating outstanding power conversion efficiencies over 25% measured in lab-scale devices.[1,2] Despite the extraordinary progresses, such record efficiencies are obtained by perovskite processing in a controlled glove-box environment, by means of non-scalable techniques (i.e. spin-coating often associated with solvent dripping) and with the use of highly toxic solvents.[3] The inherent limitations interfering with large-scale production of perovskite solar cells are related to the critical material deposition/reproducibility, which relies on film formation occurring throughout a complex self-assembly process driven by weak interactions.[4-6]  That being said, there has been significant and exciting developments in recent years to bring viability to the large scale manufacturing of perovskite photovoltaics in roll-to-roll facilities.[7] To this purpose herein we propose a simple, yet, effective, material preparation allowing a facile and scalable process at mild-temperature and ambient air conditions, with the use of low toxicity dimethyl sulfoxide (DMSO). .

To elegantly simplify the solvent based deposition, we exploit polysaccharides as rheological modifiers to tune the viscosity of perovskite-polymer formulation, which positively influences the formation of perovskite films via single step coating. The hydroxyl groups on the biopolymer chains establish hydrogen interactions with organic cations and at the same time, with the DMSO solvent leading to solution gelation that allows for a convenient and finely tuned viscosity modulation.[8] Moreover, due to the organic polymeric nature and to the non-covalent interactions between adjacent chains, they confer superior flexibility, moisture/stability to the perovskite-polymer films, enabling the nanocomposite material to accommodate a strain, whilst maintaining transport properties suitable for devices, thus very attractive for flexible photovoltaics.

Herein, we demonstrate that, thanks to the easily tuneable viscosity, such perovskite-polymer inks can be adapted to the requirements of scalable slot-die and gravure printing techniques. The superior film forming properties of polymeric materials guarantees the deposition of perovskites on large area flexible substrates without the use of the antisolvent-bath, thus significantly simplifying the large-scale processing that is a mandatory prerequisite in view of the large-scale manufacturing of perovskite solar cells at low-cost.

Recently, we have demonstrated using VTT’s pilot manufacturing lines the roll-to-roll gravure printing of flexible solar cell devices by depositing the aforementioned perovskite-polymer inks in ambient conditions via a single step printing method. The flexible and fully printed, except the electrodes, solar cells, were fabricated on 50-meter-long rolls featuring promising power conversion efficiencies near 10%.

Energy consumption of building sector has been increasing rapidly due to population growth, improved levels of wealth and lifestyle changes. Moreover, the buildings and buildings construction sectors combined are responsible for 40 % of total carbon dioxide emissions. In order to reduce CO₂ emission, energy generation from fossil fuel has to be replaced with renewable energy. Therefore, demands of the Building Integration Photovoltaics (BIPV), which makes possible the power generation from roofs, windows or facades have increased. In order to integrate PV into windows or facades of buildings, semitransparency or transparency of the solar cells is essential. The organometallic halide perovskite (PK) solar cells can be semitransparent by tuning their thickness and with transparent contact. Moreover, PK solar cells have achieved a very high efficiency (>25 % certified efficiency) and could be processed with low costs. Hence, PK solar cells satisfy almost all the requirement for BIPV applications.

In this talk we present our work on the semitransparent PK modules fully laser patterned. We optimized thickness and transmittance of sputtered ITO/Ag/ITO multilayers for transparent contact. The ITO/Ag/ITO multilayers have high transmittance of 75 % in visible region from 400nm to 800nm and very low sheet resistance of 4-5Ω/□. Combination of thin NiO layer and semiconducting polymer significantly reduced shunts on large area substrates and patterning by femtosecond laser increased geometric fill factor (GFF) over 95% compared to nanosecond laser. Finally, semitransparent perovskite modules achieved a power conversion efficiency of 15.6 % with an aperture area of 13.68 cm². Moreover, there is almost no efficiency difference between semitransparent modules and 1 cm² cells.

Lab-scale perovskite solar cells (PSCs) have recently reached power conversion efficiencies (PCEs) of up to 25.5%[1] almost exclusively fabricated from precursor solutions containing harmful and polluting solvents such as dimethylformamide (DMF).

A fully sustainable and green solvent[2] such as pure dimethyl sulfoxide (DMSO) would be more desirable[3], [4] as recently presented by Vidal et al.[5] since DMSO offers the lowest human health and environmental impact compared to all other typical polar aprotic solvents for producing PSCs.[5], [6]

However, utilizing pure DMSO for solution processing via scalable printing techniques implies two main challenges in contrast to DMF or DMF:DMSO mixtures: (i) increased dewetting on the subjacent layer[7], [8] due to the high DMSO surface tension of 42.8 mN m^-1[9] and a high viscosity of 2.0 cP[10] resulting in inhomogeneous perovskite films on large-area;[11] (ii) complex quenching and longer solvent evaporation periods of the wet film before crystallization[12] due to its low vapor pressure and high boiling point of 189 °C.[5]

Here, we report on one-step blade coating methylammonium (MA)-free double-cation perovskite for inverted (p-i-n) PSCs using solely the sustainable precursor solvent DMSO at low processing temperatures. To prevent dewetting of the precursor solution and realize sufficient quenching, we apply a blade coated silicon oxide (SiO₂) nanoparticle (NP) wetting agent at the hole transport layer (HTL)/perovskite interface[13], [14] and gas stream-assisted drying[14], respectively. Trends in perovskite grain size, morphology, crystallinity and elemental composition of samples from both, toxic and green, solvent concepts are compared revealing analogical results. Thus, PSCs with 0.24 cm² active area blade coated from purely DMSO achieve comparable PCE of up to 16.7% versus the ones from a DMF:DMSO mixture (16.9%) and thereby showing that the use of toxic DMF is avoidable. This represents an important step for bringing solution processing of PSCs with environmentally friendly precursor solvents closer to industrial realization and application.

The direct conversion of sunlight into chemical fuels like Hydrogen by Photoelectrochemical (PEC) water splitting involves efficient oxidation and reduction half reactions at the electrodes. Since PEC electrodes combine light harvestation and catalysis processes into one unit, engineering each layer of these electrodes is essential to harness their complete capability. Inorganic Semiconductors (ISCs), while high performing and stable in most regards, fall short of Organic Semiconductors (OSCs) in many opto-electronic properties. In our lab, we have pioneered in PEC water splitting by combining OSCs which absorb light and split excitons and inorganic metal oxides nanoparticles, which catalyze the water oxidation or reduction reactions. While earlier attempts to split water were focused on using robust, stable, OSCs with ideal energy levels for water oxidation, the photoanode was restricted due to intrinsic limitations in charge separation and poor catalytic conversion to oxygen [1]. Subsequent research involved use of high performing Bulk Heterojunction structures (BHJs) to reduce water with RuO_x inorganic metal oxide catalysts, and led to stable photocathodes without any protecting layers [2]. The current focus of research is to use low energy level donors and acceptors with metal oxide oxygen evolution catalysts to get stable, high performing photoanodes. We were successful in developing a robust OSC photoanode capable of tandem hydroiodic acid splitting with the previously developed OSC photocathode [3]. Additionally, initial attempts resulted in a BHJ photoanode oxidizing water, albeit temporarily. This presentation aims to summarize our work, the challenges overcome, and the major roadblocks to realize an all OSC based tandem PEC water splitting cell.

Due to their high power-to-weight ratio (specific power) and potential to be fabricated as flexible devices, Perovskite solar cells (PSCs) have gained increasing interest from the aerospace sector to supersede the current PV technology. However, before they can be deployed into space, their resistance to ionizing radiations, such as high‐energy protons, must be demonstrated. Here, we investigate the effect of 150 keV protons on the performance of PSCs based on aluminium‐doped zinc oxide (AZO) transparent conducting oxide (TCO). AZO was used due to its low-cost, nontoxicity and abundance. A record power conversion efficiency of 15% and 13.6% is obtained for cells based on AZO under AM1.5G and AM0 illumination, respectively. It is demonstrated that the PSCs can withstand proton irradiation up to 1x10¹³ protons/cm² without significant loss in efficiency. At this proton irradiation dose, Si or GaAs solar cells would be completely or severely degraded. From 1x10¹⁴ protons/cm², a decrease in short‐circuit current of PSCs is observed. Through non-destructive characterisation techniques such as Raman spectroscopy, Photoluminescence (PL) and Transient Photovoltage (TPV), the results highlight interfacial degradation due to the deterioration of the Spiro‐OMeTAD holes transport layer during proton irradiation. The structural and optical properties of perovskite remain intact up to high fluence levels and although shallow trap states are induced by proton irradiation in perovskite bulk at low fluence levels, charges are released efficiently and are not detrimental to the cell's performance. This work highlights the potential of PSCs based on AZO TCO to be used for space applications and gives a deeper understanding of interfacial degradation due to proton irradiation.

Limitation of the moisture ingress thanks to encapsulation is a key challenge to improve the lifetime of Perovskite solar cells. The side permeation through sealing materials and interfaces, in a glass/glass encapsulation for example, cannot be neglected[1],[2] because of the rapid degradation rate of Perovskite absorber with few amount of water[3].

The side permeation can be studied using an optical test monitoring the degradation of a Perovskite layer in damp heat conditions[2]. Such a method already described the role of interfaces, particularly those between the sealing material and the glass covers.

In this study, we investigated the degradation rate of a glass/glass encapsulated Perovskite layer depending on the composition of the Perovskite stack, for a given sealing material (Acrylic glue). We chose a transparent architecture which mimics a Perovskite sub-cell in a tandem architecture, with silicon for example. We investigated the role of HTL and ETL regarding the degradation rate of the perovskite due to water side permeation, in damp heat conditions. We highlighted the role of these layers regarding the side permeation and established the important increase of the degradation rate with Perovskite stack including the transport layers ETL and HTL we selected, respectively SnO2 nanoparticles and PTAA polymer. This result highlights the importance of the transport layers, not only regarding the electrical performances, but also to manage the side permeation in an encapsulated device. This method we developed allows the study of alternative transport layers with better characteristics regarding gas permeation as dense layers deposited with vacuum technics.

We report here the design and synthesis of a series of conjugated molecules based on thieno[3,4-c]pyrrole-4,6-dione central core in a D–π-A–π-D molecular configuration for perovskite solar cells application. The thermal, morphological, optical and electrochemical properties of all prepared compounds have been investigated in detail and a comparative discussion has been presented.

Their characteristics have suggested that these molecules could be suitable for use as hole transporting materials in perovskite photovoltaic devices. The preliminary photovoltaic application have given devices with power conversion efficiency (PCE) around 17 %. One of these molecules has been selected for further device optimization. Interface engineering with 2-(2-aminoethyl)thiophene hydroiodide (2-TEAI) between perovskite and hole transport layers improves PCE from 19.60% (untreated) to 21.98% (treated) and this champion PCE is even higher than that of the spiro-MeOTAD-based device (21.15%). Thermal stability test at 85 ^oC for over 1000 h showed that the PSC employed novel HTM retains 85.9% of initial PCE (from 21.9% (0 h) to 18.8% (1032 h)), while the spiro-MeOTAD-based PSC degrades unrecoverably from 21.1% to 5.8%. Time-of-flight secondary ion mass spectrometry studies combined with Fourier transformed infrared spectroscopy reveal that novel HTM shows much lower lithium ion diffusivity than spiro-MeOTAD due to a strong complexation of the lithium ion with HTM, which is responsible for the higher degree of thermal stability. Under optimized condition, the perovskite solar cells employed additive-free HTM gave a PCE of 15.91%. This work delivers an important message that capturing mobile Li⁺ in hole transporting layer is critical in designing novel HTM for improving thermal stability of PSCs. In addition, it also highlights the importance of interfacial engineering on the non-conventional HTM.

Long-term operational stability remains the primary concern for perovskite solar cells. Consequently, there is a quest for searching for new compositions that enable stable and efficient perovskites. We report a new molecular-level interface engineering strategy using a multifunctional ligand that augments long-term operational and thermal stability by chemically modifying the formamidinium lead iodide rich photoactive layer. The surface derivatized solar cells exhibited high operational stability (maximum powering point tracking at 1 sun) with a stabilized T₈₀ (the time over which the device efficiency reduces to 80% of its initial value of post-burn-in) of ≈5950 h at 40 ºC and stabilized efficiency over 23%. The origin of high device stability and performance is correlated to the nano/sub-nanoscale molecular level interactions between ligand and perovskite layer, which is corroborated by comprehensive multiscale characterization. Our results provide key insights into the modulation of the grain boundaries, local density of states, surface bandgap, and interfacial recombination. Chemical analysis of the aged devices showed that interface passivation inhibited ion migration and prevented photoinduced I₂ release that irreversibly degrades the perovskite. This study shows that passivating ligands have the potential to overcome stability issues associated with the high performing hybrid perovskite compositions, thus allowing a step closer to achieving long-standing stability of perovskite-based solar cells.