The last decade has seen the most spectacular rise of the halide perovskites with the general formula of ABX₃, where X = I, Br or Cl. Their photovoltaic and light emissive properties, almost invariably with B = Pb, have reached superlative levels of performance within this exceptionally short span of time. The A site is known to accommodate methyl ammonium (MA)⁺, (CH₃NH₃)⁺, formamidinium (FA), (CH(NH₂)₂)⁺ and Cs⁺ ions while retaining the perovskite structure. MAPbI₃ is possibly the most investigated among the pure systems. Despite the unprecedented rise in the photovoltaic efficiency with MAPbI₃ as the active material in solar cells, rapid degradation of the performance arising from multiple contributing factors has been a vexing issue in this field. Through numerous experiments, it has now been established that A-site cationic substitutions, such as FA for MA to form MA_1-xFA_xPbX₃, can increase the stability and the performance of these materials under real-life operating conditions. However, the possible mechanisms behind such improvements or even the microscopic changes brought about by such substitutions are not very well understood at present. We address these issues by first making a comparative study of the pure end-members, MAPbX₃ and FAPbX₃ to understand their distinctive features at some fundamental aspects. Our results establish that the dielectric properties of the two are very different arising from a strong contribution from nearly freely rotating dipoles present for the MA containing compounds, while being absent for the FA series.¹ Quasi-elastic neutron scattering experiments over a wide temperature range also underline the differences in the rotational dynamics of these two organic moieties.² We then investigated³ the substitutional series, MA_1–xFA_xPbI₃, for a range of compositions (x) and over a wide temperature range to cover all crystallographic forms present to map out the structural phase diagram in the temperature–composition phase space; four crystallographic phases are found to exist for this solid solution series, namely, cubic (Pm-3m), tetragonal (I4/mcm), orthorhombic (Pnma), and large-cell cubic (Im-3). Temperature and frequency dependent dielectric measurements show remarkable dependency of dielectric properties on the specific crystal structures; significantly, it is seen that for the most relevant compositions, the presence of FA ions hinder the nearly-free rotations of the MA units, giving rise to a glassy dipolar state. It is tempting to associate this glassy, locked state of significantly FA doped MA_1–xFA_xPbI₃ with the enhanced stability of similar compositions. We also find evidence of this systematically enhanced stability on doping of FA from our analysis of the temperature dependent photoluminescence from this solid solution with different composition, x.⁴ We shall present these results to emphasize the structure-property correlation in MA_1–xFA_xPbI₃, touching on the possible origin of the enhanced stability with FA doping of MAPbI3. If time permits, I may briefly discuss the consequences of small amount of Cs doping, forming Cs_δ(MA_1–xFA_x)_1-δ PbI₃ on these properties reported here.⁴

Lead halide perovskite nanocrystals are recently emerged as one of the most efficient energy materials for photovoltaic and optoelectronic applications. These nanocrystals typically retained the cube or platelet shapes having six facets. These could tune colors with tuning halide compositions and also enhance the emission intensity. Extensive research has been carried out to understand the chemistry and physics of these nanocrystals to optimize their phase stability and optical properties. However, entire developments on these nanocrystals till date are related to their six facets, ligands on these surfaces and ions interactions mostly halide exchange for changing their optical bands; but stories behind other facets were still largely unknown. This talk would present the chemistry and physics of some of the new facets in perovskite nanocrystals and would discuss their success stories, challenges in designing and variations in optical properties.

Photovoltaics are a modern and promising solution for the direct conversion of solar energy into electricity. The efficiency of Perovskite Solar Cells (PSCs) has increased rapidly in recent years, surpassing other 3rd generation PV technologies in terms of efficiency (power conversion efficiency (PCE) exceeding 25 %). However, several issues including further efficiency increase and performance stabilization have not been effectively addressed yet. PSCs show different yields and different degrees of maturity depending on the materials involved (inorganic, organic) and find very promising applications. Their application field is further expanded following deposition of the perovskite absorber in the form of thin films on top of glass and flexible plastic substrates. The device presents a layered architecture based on the deposition of successive thin films (compact layer-CL and / or intermediate layer electron transfer-ETL, perovskite, HTM, metal contacts). An important role in the behavior of PSCs is played by the nature, structure and morphology of each layer but also the functionality of the respective interfaces (ETL / perovskite and perovskite / HTM). [1]

In this work, we mainly focused on the modification of the ETL / perovskite interface by graphitic carbon nitride incorporation (g-C3N4). This material has attracted research interest due to its significant physical and chemical properties and its potential integration into energy storage and conversion devices. Most importantly, modern graphitic carbon nitride 2D nanostructures can provide interesting ion and electron diffusion properties, high electrochemical activity and create functional interfaces with high electron conductivity and improved chemical stability. [2]

In this contribution we present the results of a comprehensive study on the synthesis of innovative graphitic carbon nitride materials and their application as electron transport mediators in planar perovskite solar cells. Specifically, we have prepared nanostructured derivatives of g-C3N4 which were used to modify the ETL of planar PSCs, resulting in devices with high power conversion efficiency (PCE) and improved stability. The investigation of the photoelectrochemical properties confirmed that g-C3N4–based devices present enhanced short-circuit photocurrent density and greater stability. The obtained results are attributed to the particular structure and the morphology of the graphitic carbon nitride materials and open new perspectives in the field of PSCs.

Perovskite solar cells are low cost, easy to fabricate, and most importantly efficient. However, there are challenges. Stability is an issue. So is repeatability and scalability. In this talk I will highlight some of the advances made by the group in IISc to solve these problems using compositions engineering. We shall demonstrate how a novel cation, acetamidinium, can lead to more stable solar cells that are also more efficient. The stronger hydrogen bonding and restricted rotation make for a more stable lattice. We shall show novel transport layers that are potentially cheaper, and more stable than spiro-OMeTAD. While it is well-known that band-alignment and passivation are important, the interfaces in perovskite are still mysterious. We shall also discuss methodologies to improve morphology, which impact both stability and scalability. Some of these techniques are very important for upscaling, because anti-solvent treatment does not lend itself to printing.

Perovskite solar cell fabrication using solution-based approaches is prone to appearance of pinholes and other defects, which reflect adversely on the device performance and stability.

The interfacial carrier recombination in perovskite solar cells (PSC) is one of the dominant efficiency loss mechanisms, which also results in the simultaneous loss of potential efficiency [1]. In order to achieve a stable long-term conversion of energy and a good performance of organometal halide perovskite solar cells, an interface passivation between perovskite and charge transporting materials is required. The insertion of polymeric interlayer permits to treat defects, suppress the current leakage and align energy level, maximizing the V_OC of devices [2].

In this work we developed p-i-n PSC in planar configuration with polymer interlayer (PEO and PMMA) between perovskite (CH₃HN₃PbI₃) and hole transporting layer (NiO_x). We provided comparison of device performance fabricated on different concentrations of polymers interlayers to define the impact of thickness on output characteristics. Both kind of devices have hysteresis-free behavior which is related to the reduction interfacial defects. Maximum power point tracking (MPPT) shows satisfactory light stability of PSC with polymer interlayers in comparison with pristine perovskite solar cells for 180 hours of measurements under continuous light soaking (LED source, 1 Sun, 50°C) (figure 1).

PSC with PEO and PMMA interlayer exhibits a promising power conversion efficiency of 18.32 % and 18.02%, respectively which is higher than pristine PSC value of 17.89 %. At the same time, the light soaking analysis under the Sun simulator demonstrates the improved stability of PSCs fabricated with polymers (PEO and PMMA) which lost 20% of initial power conversion  after 140 hours whereas in the sample without polymer treatment this lost is reached after  10 hours. Photovoltaic measurements (JV, TPV, charge extraction, dark JV, IPCE) confirmed that polymer passivation leads to lower concentration of defects at the interface

The long-standing interpretations for the exceptional photovoltaic and optoelectronic properties showcased by the perovskite family largely pertain to the underlying complicated interplay of polaron formation and hot carrier cooling.  In the present talk we are going to discuss the existing status of the polaron studies conducted on CsPbBr₃ based systems within the frame work ultrafast Femtosecond spectroscopic studies. We are going to discuss the key aspect which is accountable to polaron formation i.e. the carrier- Longitudinal Optical (LO) phonon coupling in different conditions which affect this FrÖhlich interaction mediated coupling. We have elaborately discussed the changes in the lattice polarity, surrounding dielectric medium, lattice temperature and the system dimensionality which can influence the charge screening extents and thereby the polaron formation process. Our investigation demonstrated the experimental conditions for polaron formation in CsPbBr₃ based systems which are highly relevant for technological advancements.

Absorption of an energetic photon in a semiconductor leads to generation of charge carriers with excess energy equal to the photon energy and the semiconductor band gap. Loss of this excess energy as heat is a major contribution to the efficiency limit of solar cells (~30% for Si solar cells). Carrier multiplication is a process of utilizing the excess energy to produce additional charge carriers. In this way a single photon can excite two or more electrons across the semiconductor band gap. This can boost the solar cell efficiency up to 40%. In this presentation, recent results on efficient carrier multiplication in mixed Sn/Pb halide perovskites (band gap 1.28 eV) will be discussed. The onset of carrier multiplication was close to twice the band gap with the number of charge carriers produced reaching 2 at 2.8 times the band gap. The results will be helpful to design efficient multi-excitonic perovskite solar cells.

Recently the metal halide perovskite nanoparticles have attracted a lot of attention for various opto-electronic applications like micro/nano lasers, light emitting diodes, photodetectors along with solar cells due to their low fabrication costs as well as excellent electronic and optical properties.  However, dominant surface trapping continues to plague the field, despite their high defect tolerance, as evidenced by the several fold improvements in the external quantum efficiency of perovskite nanocrystals (NCs) upon appropriate surface passivation or physical confinement between high band gap materials.  In a bid towards minimizing energy loss, in this talk I introduce unique pathways to harvest additional photons via enhanced absorption and additional excitonic recombination pathways that are so far not extensively explored through doping and heterostructure formation.

Lead halide based perovskite has seen a phenomenal research interest in the last decade documenting record solar cell efficiency. Typically, 3D perovskites have been reported for optoelectronics and dilute magnetic semiconductor doping. In this talk, I will talk about dilute magnetic doping in lower dimensional 2D perovskites highlighting the superiority of the here utilized 2D perovskites. I will demonstrate the optoelectronic properties of such bulk phase 2D materials and discuss the exciton dynamics in such materials. This will be followed by highlighting the utility of main group metal halide based hybrid low dimensional perovskites with ns² electronic configuration (Sb³⁺) for applications in white light sources. The mechanism of such broad band white light emission in these low D perovskites, involving self-trapped excitons and the stereo-chemical activity of the ns² electron lone pair, will be discussed. I will conclude the talk with research outlook of our current research work highlighting the existing issues in white light emission using main group metal halide based hybrid low D perovskites

Metal halide perovskites have a key advantage of having tuneable optical and electronic properties and possess the ability to deliver very high quality semiconductor materials with a range of band gaps. This opens the possibility of creating multi-junction PV cells which have the capacity to delivering efficiencies far beyond single junction PV technologies. However, when the band gap is tuned, new problems arise with respect to minimising performance limiting defects and selection of  idea semiconductors with which to interface the perovskite, for low loss charge extraction and high long term stability. Here, I will present work on perovskite-on-silicon and perovskite-on-perovskite multi-junction cells, efforts upon understanding and improving the long term stability and work upon understanding fundamental processes occurring in these materials and devices.

Atomic Layer Deposition of ultrathin Al₂O₃ on hybrid perovskite solar cells drew significant attention due to the considerable improvement in the overall device stability. In our laboratory, with intermittent current-voltage measurements, the coated devices show the value of T80>7500 hours under ambient conditions. Subsequently, these coated devices are found highly stable when measured in a cyclic manner for 7 days, replicating the real-life day-night sequences. Such encapsulation is found very effective as an oxygen barrier-layer and water-impermeable membrane; hence contribute to the overall stability of these devices.

In this presentation, I would like to emphasize on our experimental findings, subsequently supported by device simulation, which undoubtedly reveals that the perovskite-spiroOMeTAD interfacial band-structure play a detrimental role in initiating the degradation processes in the pristine devices (device structure). We try to provide a comprehensive insight depicting an apparently non-trivial active phenomenon resulted due to the ALD grown Al₂O₃ layer that supposedly be a passive component of the entire device stack. Favored electronic modification of the spiro-OMeTAD/perovskite interface resulted due to the Al₂O₃ ALD provides better charge extraction and lesser ionic accumulation, unlike the unencapsulated devices, and hence offers better performance stability. Our study indicates that essentially the ionic accumulation triggers the device degradation that is eventually followed by materials degradation.

Both lead and lead-free halide perovskites have emerged as revolutionary optoelectronic materials for a wide variety of applications^1,2. While lead-based systems have made tremendous progress in photovoltaics, the lead-free counterparts are promising game-changers in photocatalytic energy generation reactions. With a focus on nanostructures, the first part of the lecture will discuss the nanocrystal-assisted passivation of pin-holes in CH₃NH₃PbI₃ photoactive layer of solar cells to achieve photoconversion efficiency up to 20% along with long term ambient stability^3-5. The second part will focus on CsPbX₃ (X = Br/I) nanosheets, their increased charge separation and facile carrier mobility that enable excellent performances in photodetectors and field-effect transistors. The lecture will conclude with a discussion on the application of Cs₃Bi₂I₉ nanocrystals in photocatalytic reactions.

References

(1) Chaudhary, D. K., Ghosh, A., Ali, Y. M., Bhattacharyya, S. Charge Transport between Coaxial Polymer Nanorods and Grafted All Inorganic Perovskite Nanocrystals for Hybrid Organic Solar Cells with Enhanced Photoconversion Efficiency. J. Phys. Chem. C 2020, 124, 246-255.

(2) Roy, S.; Mandal, A.; Raj R., A.; Bhattacharyya, S.; Pal, B. Thermal Nonlinear Refraction in Cesium Lead Halide Perovskite Nanostructure Colloids. J. Phys. Chem. C 2020, 124, 28, 15558–15564.

(3) Ghosh, D.; Chaudhary, D.; Ali, M.; Chauhan, K.; Prodhan, S.; Bhattacharya, S.; Ghosh, B.; Datta, P.; Ray, S.; Bhattacharyya, S. All-Inorganic Quantum Dot Assisted Enhanced Charge Extraction across the Interfaces of Bulk Organo-Halide Perovskites for Efficient and Stable Pin-Hole Free Perovskite Solar Cells. Chem. Sci. 2019, 10, 9530-9541.

(4) Ghosh, A.; Chaudhary, D. K.; Mandal, A.; Prodhan, S.; Chauhan, K. K.; Vihari, S.; Gupta, G.; Datta, P. K.; Bhattacharyya, S. Core/shell Nanocrystal Tailored Carrier Dynamics in Hysteresis-less Perovskite Solar Cell with ~20% Efficiency and Long Operational Stability. J. Phys. Chem. Lett. 2020, 11, 591-600.

(5) Ghosh, D.; Ali, M.; Chaudhary, D.; Bhattacharyya, S. Dependence of Halide Composition on The Stability of Highly Efficient All-Inorganic Cesium Lead Halide Perovskite Quantum Dot Solar Cells. Sol. Energy Mater. Sol. Cells 2018, 185, 28-35.

Perovskite light-emitting diodes (PeLEDs) have recently attracted great research luminescence at room temperature in interest for their narrow emissions and solution processability. Remarkable progress has been achieved PeLEDs in recent years [1]. Here we present the new configuration of ambipolar transparent perovskite light emitting device. The combination of voltage induced p-i-n formation and ionically doped carbon electrodes and allows electroluminescence in forward and reverse bias. For this experiment high quality films of randomly oriented SWCNTs were produced by the aerosol (floating catalyst) chemical vapor deposition (CVD) method [1]. PeLEDs were assembled using a glass substrate with ITO stripes as bottom electrode; spin-coated CsPbBr3/I3:PEO composite as emissive layer; SWCNT deposited by a simple press transfer process at room temperature as top electrode.

SWCNT has a sensitive response of electrical properties to doping, due to having only one graphene layer. Any changes on the external graphene layer leads to sensitive response of CNT properties. In direct mode on LED device (+ITO/pero/SWCNT–) positively charged ions coat SWCNT. P-type doping leads to downshifting and change in the Fermi level, that helps to holes injection from SWCNT instead of ITO. Changing the direction of the electrical field coats the surface with oppositely charged ions.

We demonstrate a concept of stacked multicolor tandem pixel. Stack of transparent light emitting units might allow fine color tuning in parallel tandem connection without segregation compared to mixed halide perovskites. This configuration conforms pixel downsizing and make to fabrication of emissive multijunction pixels in a stack. Stacked pixel designs have potential application in head-up displays and augmented reality technologies, due to smaller pixel area compared to conventional display with active matrix.

Figure 1: IV curve of single layer ambipolar PeLED (ITO/pero/SWCNT). Orange curves corresponds to forward sweep, SWCNT doped by Cs+ injects electrons; grey curves correspond to switched mode: SWCNT doped by Br- injects holes. Switching realize by the voltage induced ions migration.

The field of perovskite based solar cells have witnessed unprecedented activity in the last decade. While impressive gains have achieved in the efficiency, long time stability is an aspect that is under active research for a variety of reasons. Among the many factors that affect long term stability, ion migration and phase segregation have attracted significant recent interest.

Here, we explore the influence of these phenomena on the efficiency of solar cells through detailed numerical simulations. The influence of abovementioned phenomena is studied through self-consistent simulation of relevant semiconductor equations – Poisson’s equation and continuity equations for electrons, holes and mobile ions in the presence of photogeneration of carriers. Both bulk as well as interface recombination phenomena are considered in simulations. The equations are solved self-consistently, with Scharfetter-Gummel discretization for continuity equations and backward Euler scheme for time integration with adaptive time discretization to account for the slow varying ions as compared to the charge carriers. To explore phase segregation, the active material is treated as several domains of identical size. Depending on the extent of phase segregation, each domain is identified as either mixed halide, iodide or bromide with appropriate parameters.

In this contribution, we discuss the results from extensive statistical simulations to ascertain the influence of critical parameters like geometry or spatial pattern of phase segregation, energy level alignments, and transport parameters. Further, we highlight the influence of interface recombination and ion dynamics on the hysteresis in light JV characteristics.

Inorganic organic hybrid perovskite materials have shown great promise in optoelectronic device application due to their superior optoelectronic properties. Starting with photovoltaic cells perovskites are also used in light emitting diodes, photoelectrochemical cells, sensors, detectors and memristors. Despite tremendous progress in device efficiencies there have been various issues in perovskite materials preventing them for immediate commercialization. Hybrid perovskites are highly unstable under heat and sunlight due to photoinduced ion migration which can be characterized by electrochemical impedance spectroscopy.[1] However, there have been many controversial reports on ion migration and their corresponding activation energy barriers estimated from EIS data fitting. In this talk I will discuss various methods to analyse impedance spectroscopy data and the fit parameters which are useful for device performance analysis. I will also discuss about the advantages and drawback of EIS data analysis in perovskite-based optoelectronic devices. It can be demonstrated that the strong electronic-ionic coupling in perovskite materials gives rise anomalous charge transport properties in solid state devices.[2,3] Interplay between electronic and ion charge transport can give rise to negative capacitance in perovskite based solar cells and LEDs.[4] AC ionic conductivity measurement shows anomalously high and non-linear conductivity at high frequency regime due to week interaction between A-site cations and X-site halide ions in ABX₃ perovskites while blocking nature of electrode materials as well as strong hydrogen bonding between A-site cations and X-site halide ions reduces the ac conductivity at low frequency regime. Therefore, a tuneable ac ionic conductivity can be achieved in mixed-cation mixed halide perovskite materials. Interfaces play a crucial role in deciding charge extraction at the electron and hole transporting layers. At the end I will discuss some interesting observations in spectroelectrochemistry where these perovskite materials form semiconductor/electrolyte junctions. These analyses will be helpful to understand electrolyte gated perovskite field effect transistors as well.

How to design a defect tolerant solar cell?

The term defect tolerance is widely used in literature to describe materials which exhibit long non-radiative lifetimes of carriers despite possessing a large concentration of point defects. The defect tolerance of a material is credited to the presence of an antibonding valence band that causes intrinsic point defects to lie either in the bands or at least not deep in the band gap. The family of lead-halide perovskites is one such material class with an antibonding valence band and solar cells made from these materials have shown a remarkable rise in photovoltaic performance in the past decade. However, most studies on defect tolerance are concerned with host material properties that affect the recombination coefficients of defects and not how the electrostatics and the design of the layer stack of a device affects the recombination activity.

Here we discuss defect tolerance from a device perspective and try to understand how certain device geometries will enhance or slow down non-radiative recombination via defects in the absorber or interface layers. The dependence of the recombination activity on device structure is a direct consequence of the fact that recombination inside a device is a function of both the properties of the defect and that of the device. The defect is characterized by the capture coefficients of the defect which encompass all the microscopic properties of the host material and the energetic position of the defect within the bandgap of the host material. The device structure constituting the absorber layer sandwiched between transport layers determines the carrier concentration inside the device as a function of the mobilities, workfunctions and doping concentrations of the different layers. The recombination efficiency of a defect, which is the number of recombination events occurring at a defect per unit time, is a function of both the capture coefficients and the carrier concentration inside the device. By studying different device geometries of a perovskite solar cell with an iodine interstitial defect level as the dominant recombination level, we explain how and why asymmetric thicknesses of hole and electron transport layers improve the solar cell performance. We also offer generic design principles which when implemented after identifying the dominant recombination levels will help reduce the recombination through the device.

Trapping of holes at iodide sites in mixed halide perovskite, MHP (MAPbBr_1.5I_1.5 ) films cause iodide ions to migrate toward grain boundaries, thus inducing the formation of iodide rich phases and bromide rich phases. When mixed halide perovskite films are in contact with solution, the migration of iodine extends beyond phase segregation as it gets expelled into solution. The selective removal of iodine from MHP upon continued irradiation transforms the perovskite film into a bromide-rich perovskite film. Substituting A-site cation of MHP with cesium (Cs) slows down iodide expulsion due to increased thermodynamic stabilization of MHP lattices. Similar lattice stabilization has also been observed in Cl-alloyed perovskites. Furthermore, photoinduced iodide expulsion process in MHPs can be modulated through externally applied electrochemical bias. At anodic potentials, electron extraction at TiO₂/MHP interfaces becomes more efficient, leading to hole build-up within MHP films. This improved charge separation, in turn, favors iodine migration as evident from the increased apparent rate constant of iodine expulsion (k_expulsion = 0.0030 s^-1). Conversely, at cathodic bias (-0.3 V vs. Ag/AgCl potential) electron-hole recombination is facilitated within MHP films, slowing down iodine expulsion by an order of magnitude (k_expulsion = 0.00018 s^-1). The tuning of the E_Fermi level through external bias modulates electron extraction at the TiO₂/MHP interface indirectly controls the build-up of holes, which ultimately induces iodine migration/expulsion. Suppressing iodine migration in perovskite solar cells is important for attaining greater stability since they operate under internal electrical bias.

In this talk, I would start with the brief introduction of first principles electronic structure calculations within the framework of Density Functional Theory (DFT) in hybrid perovskite materials, and how it could be connected to the Computational High-throughput Screening for achieving highly efficient and stable solar cell materials [1, 2]. Next, I will be talking about the fundamentals and possible implications of Rashba phenomena hybrid perovskite materials [3]. The rest of the talk would be devoted to the theoretical understanding of piezochromism in lead free metal chalcogenide perovskites. Hydrostatic pressure is an effective tool, which can give rise to novel crystal structures and physical properties, while it has proven to be an alternative to chemical pressure [4]. Therefore new functional materials with intriguing properties can be designed by exerting external pressure. We have recently [5] envisaged the structural, electronic and optical properties under the influence of hydrostatic pressure along with the effective mass evolution of the charge carriers. This work elucidates the effects of pressure on the sensitive tuning piezochromism in zirconium based chalcogenide perovskites.

References:

1 - Sudip Chakraborty* et al. ACS Energy Letters, 2, 837 (2017).

2 - H. Arfin, J. Kaur, T. Sheikh, Sudip Chakraborty*, Angshuman Nag*, Angewandte Chemie, 59, 11307 (2020).

3 - Sudip Chakraborty*, M. K. Nazeeruddin*, Submitted (2020)

4 - H. Banerjee, Sudip Chakraborty*, M. K. Nazeeruddin*, Submitted (2020)

5 - A. Majumdar, A. Adeleke, Sudip Chakraborty* et al. Journal of Materials Chemistry C, in press (2020)

Lead Halide Perovskites both in their inorganic and hybrid forms have become an important candidate for photovoltaic devices. The optoelectronic properties of such materials can be tuned by controlling the bandgap, with the structure playing the determining role. An unusual aspect of the hybrid perovskites of the form ABO₃ are that one must consider the hydrogen bonding of the molecule at the A-site with the inorganic cage, while discussing changes induced on the structure [1]. This in the case of a molecule like MA (Methylammonium) leads to the molecule being displaced from the center of the inorganic cage towards one end [2] leading to shorter bonds between the hydrogens attached to the ammonium end of the molecule and the anions. This could be as large as 0.30 Å for MA in MAPbBr₃, and leads to an increase in the dipole moment. The strong hydrogen bonding also leads to deviations in the Pb-anion-Pb angles from 180^o which one has for an ideal perovskite. We quantify these parameters as a function of pressure as well as strain, for hybrid and inorganic perovskites in a comparative manner to show how these evolve and the implications on the structural properties.

Recently, lead free all-inorganic double perovskites have revolutionized photovoltaic research, showing promising light emitting efficiency and tunability via modification of inherent structural/chemical properties. Carrier-lattice interactions via Frohlich mechanism is known to be the dominant scattering mechanism, dictating carrier mobility near room temperature for these compounds. In this talk, I will present a combined theoretical and experimental study on the variation of carrier-lattice interaction and optoelectronic properties of Cs2AgIn1-xBixCl6 double perovskite with varying alloying concentration. Using a careful analysis of Raman spectra assisted with first-principles simulations, we assign the possible three types of active modes to intrinsic atomic vibrations; 2 T2g modes, 1 Eg and 1 A1g for various stretching of Ag-Cl octahedra. Ab-initio simulation reveals dominant carrier-phonon scattering via Fröhlich mechanism near room temperature, with longitudinal optical phonons being effectively activated around 230 K. We observe a noticeable increase in hole mobility with small Bi alloying (~4 times). This is attributed to the valence band maxima acquiring Bis orbital characteristics, thus increasing its dispersive nature. We believe that our results should help to gain a better understanding of the intrinsic electronic and lattice dynamical properties of similar class compounds. Reference(s):

[3] Debjit Manna, J. Kangsabanik, T K Das, D Das, Aftab Alam, A. Yella, A. Alam, J. Phys. Chem. Lett. 11, 2113-2120 (2020).

Electron-phonon coupling (EP-coupling) in a material drives multi-phonon mediated transitions, such as broadening of optical transitions, intra-band cooling, and nonradiative recombination of carriers, imposing fundamental limits to the efficiency of devices. We have developed an efficient method to extract EP-coupling strengths for nanomaterals from ab-initio Molecular Dynamics (AIMD) simulations, and have applied these techniques to study phonon-mediated transitions in lead-halide perovskite nanocrystals. We find an enhancement of electron-phonon coupling stengths with decreasing nanocrystal size, leading to broad radiative transitions and efficient charge carrier thermalization. Our findings provide guidlines for optimizing the surface structure of the nanocrystals to reduce these coupling strengths, providing narrower emmision and hampering charge carrier cooling.

Halide perovskites show good photovoltaic performance surpassing conventional solar cell technologies. A good photovoltaic material with good stability is expected to be implemented in solar fuel systems. However the poor stability of these materials hampers them from being employed as such in photo/Photoelectrocatalysis and surface protection deemed necessary to implement them in these systems. We have recently discovered that Cs₂PtI₆ that comes under a class of vacancy ordered perovskites to be stable even in strong acidic and basic environment. This material also shows panchromatic absorption upto ~950 nm. This talk will focus on the electrochemistry of Cs₂PtI₆ and their application in solar water oxidation.

Abstract: Additive Engineering is a key approach to achieve high performance hybrid perovskite solar cells (PSCs) by passivating the bulk defects. However, a clear understanding about the nature of defects and which charge carrier (either hole or electron or both) gets trapped in the defect states, is still required. Herein, we present a comparative study on the estimation of charge transport length scale (L) in pristine versus passivated MAPbI₃-based PSC via scanning photocurrent microscopy (SPM). The SPM study suggested an improved L and degree of ambipolarity of photo-generated charge carriers (electron and hole) in passivated as compared to pristine MAPbI₃-based PSCs. These results were found to be correlated with frequency dependent photocurrent measurement, which shows that the relaxation time of the charge carrier is relatively lower in passivated MAPbI₃-based PSCs. This mechanism could be explained by trap-assisted recombination, where trap states are induced by ion migration in halide perovskite films. Furthermore, passivation of traps showed an increased degree of ambipolarity in the perovskite semiconductor thin film and provides an insight about charge carrier trapping in the pristine MAPbI₃-based PSC.

Keywords: MAPbI₃, defects, scanning photocurrent microscopy, dielectric relaxation time constant, degree of ambipolarity.

Hybrid perovskites have attracted much attention as a promising photovoltaic material in the past few years. Typically these hybrid perovskites like methyl ammonium lead halides (MAPbX₃) undergo dimensionality reduction from 3-D to 0-D and finally to PbX₂ upon continuous moisture exposure. In the talk I will be showing that 0-D perovskite related structures exhibit reversible transformation from transparent state to colored 3-D state upon exposure to humidity. Fluorescence imaging of individual microcrystals revealed that the structural phase transition could be visualized in solid state, where in the shape of the crystals transform to cubic crystals. The thermal and the moisture stability were found to be greatly enhanced in the transformed 3-D perovskite. Excellent device stability was also demonstrated when the devices were kept under moist (~70 %RH) conditions.

In current days, development and search of new materials for solid state thin film gas sensor to detect toxic gases, particularly using nanomaterial for their enhanced functionality, is growing rapidly as view point of both environmental and non invasive clinical application. Perovskite halides are popular as photovoltaic and optoelectronic materials. We have recently found highly sensitive and highly selective room temperature ammonia gas sensors can also be made from nano  structured lead halide perovskite. In this abstract a heap, paper electronics based solid state gas sensor to detect NH₃ gas selectively with sub ppm detection capability. The sensor is fabricated by perovskite halide CH₃NH₃PbI₃ (MAPI) as the active sensor material grown on a paper. This paper based sensor works at room temperature. The paper based sensor has a visual detection limit of 10 ppm by simple color change method  [1]. In this electrical sensor, the current increases by one order through the channel on exposure to only 10 ppm NH₃ gas . The calibrated sensitivity is ~55% for 1ppm of NH₃ gas in Nitrogen or Air. The current noise limited resolution estimated to be ~ 10 ppb [2]. This work establishes perovskite halide as a new solid state gas sensing material that can reach sub ppm sensitivity using simple paper electronics. Use of paper and also solution method used to grow the active material makes the sensor cost effective and easy to manufacture.  This type of disposable high sensitive paper sensor can be used for detection of NH₃  gas as a marker in exhaled breathes for non-invasive diagnosis. Being grown on the paper and since it supports unheated operation, needs less than few nanowatt powers for its operation. This makes the sensor very low power sensor as compared with the common metal oxide gas sensors. Thus, lead halide perovskites may act as next generation solid state gas sensor in a cost effective manner for rapid and selective detection of ammonia for room temperature operation.

Lateral two-terminal MSM (metal-semiconductor-metal) devices provide a model structure to probe and understand the interface and bulk semiconductor processes. These devices come under a category that is explored widely for back-contact solar cells and light-emitting transistors. We systematically probe pristine hybrid perovskite in lateral M₁SM₂ configuration where M₁ and M₂ are two different electrode-metals. Specifically, results for methylammonium lead iodide with Au and Al electrodes will be discussed. I(V) measurements, kelvin-probe microscopy, and spatially varying photocurrent were performed for different combinations of M₁ and M₂ and different inter-electrode distances spanning from (10 mm to 100 mm). A model to explain these observations and results are proposed. Future directions and implications of these studies will be highlighted.

The understanding of the nature and properties of the hetero-structures between hybrid/halide perovskite systems and other functional materials is of significant interest to emergent device configurations. In this short talk I will briefly discuss three interesting examples of the interfaces between CsPbBr₃ quantum dots (QDs) and a) few layer black phosphorous (FLBP), b) Ti₃C₂T_x MXene QDs, and c) Au-Ag bimetallic film configuration without and with an ultrathin MgO separating layer. The results reveal interesting consequences of these interfaces for charge transfer between these functional materials, which could be exploited in interesting applications.

Funding Support: UK-India SUNRISE (GCRF and GRTA), DST Nanomission (Govt. of India)

References:

ACS Applied Nano Materials 3 (4), 3305-3314 (2020), ACS Omega 5, 21, 11915–11922 (2020) and Angew Chem 57, 26, 7682-7686 (2018)

Lead halide perovskites have been developed primarily for use in solar cells and now show remarkable levels of efficiency.  One of the reasons for this excellent performance is that these materials can be strongly luminescent, showing unexpectedly low levels of non-radiative recombination, so that charge extraction can proceed at high cell voltages without significant recombination losses.  This high luminescence yield can be exploited in LEDs. Light-emitting diodes based on halide perovskites have recently reached external quantum efficiencies of over 20 percent. I will review the factors that control the performance of perovskite LEDs, including the management of non-radiative recombination losses and selection of charge-transport layers that are compatible with perovskite deposition.  One of the recent developments that has advanced the level of performance is the use of mixed two-dimensional and three-dimensional pervoskites, which allow very high luminescence yields from the three-dimensional phases.


(1)        Zhao, B.; Lian, Y.; Cui, L.; Divitini, G.; Kusch, G.; Ruggeri, E.; Auras, F.; Li, W.; Yang, D.; Zhu, B.; Oliver, R. A.; MacManus-Driscoll, J. L.; Stranks, S. D.; Di, D.; Friend, R. H. Efficient Light-Emitting Diodes from Mixed-Dimensional Perovskites on a Fluoride Interface. Nat. Electron. 2020. https://doi.org/10.1038/s41928-020-00487-4.

Transition metal dichalcogenide (TMD) nanosheets with defect-rich and vertically aligned edges are highly advantageous for various catalytic applications. Synthesis of TMDs using the colloidal techniques opens various possibilities to tune the electronic and optical properties of these 2D materials. As an example, we choose MoSe₂ nanosheets that have plenty of defects. The defect sites are responsible for adsorption on the surface thereby yielding excellent electrocatalytic hydrogen evolution and other catalytic activities on the surface.

Further, these defects can be employed as seeding points to grow other materials on them. Cu₂S in these defect sites leads to a Type-II semiconductor heterojunction that allows for charge separation and therefore the MoSe₂-Cu₂S forms a superior material for generation of photocurrent.

Now even heterojunctions of MoSe₂, a hexagonal crystal with CsPbBr₃ – a perovskite have been enabled by use of a linker molecule 4 – aminothiophenol. Enhanced photocurrents are obtained with such a nanoheterostructure. This methodology further opens up avenues for forming heterostructures with large lattice mismatches and can therefore be of great potential use.

References

Colloidally Synthesized Defect-Rich MoSe2 Nanosheets for Superior Catalytic Activity, Md. S. Hassan, A. Jana, S. Gahlawat, N. Bhandary, S. Bera, P. P. Ingole, S. Sapra, Bull. Mater. Sci. (2018)..

Functionalized 2D-MoS₂ Incorporated Polymer Ternary Solar Cells: Role of Nanosheets Induced Long range Ordering of Polymer Chains on Charge Transport, R. Ahmad, R. Srivastava, S. Yadav, S. Chand, S. Sapra, ACS Appl. Mater. Interfaces 9, 34111 (2017).

Functionalized MoS₂ Nanosheets for 0D-2D Hybrid Nanostructure: Photoinduced Charge Transfer and Enhanced Photoresponse, R. Ahmad, R. Srivastava, S. Yadav, D. Singh, G. Gupta, S. Chand, S. Sapra, J. Phys. Chem. Lett. 8,1729 (2017).

Design of MoSe₂-Cu₂S Vertical p-n Nanoheterostructures through Passivation of Defects to Tune the Optoelectronic Properties for Photonic Applications, Md. S. Hassan, S. Bera, D. Gupta, S. K. Ray, S. Sapra, ACS Appl. Mater. Interfaces 11, 4074 (2019).

Enhanced Photocurrent owing to Shuttling of Charge Carriers across 4-Aminothiophenol Functionalized MoSe2-CsPbBr3 Nanohybrids, Md. S. Hassan, P. Basera, S. Bera, M. Mittal, S. K. Ray, S. Bhattacharya, S. Sapra, ACS Appl. Mater. Interfaces 12, 7317 (2020).

Cesium Lead Halide perovskite nanocrystals (NCs) CsPbX₃ (X=Cl,Br,I) have been prominent materials from last few years due to their high photoluminescence quantum yield (PLQY) owing for light emitting diodes (LEDs) and other significant applications in photovoltaic and optoelectronics. Meanwhile, there is still a challenge of maintaining its photo-stability and PLQY which degrades because of ligand loss via proton transfer between oleic acid and oleylamine (commonly used surface capping ligands). In this presentation we will discuss, a facile and efficient completely amine- free synthesis of cesium lead bromide perovskite nanocrystals using hydrobromic acid as halide source and n-trioctylphosphine (TOP) as ligand in open atmospheric conditions. The use of hydrobromic acid helped to eliminate oleylamine, thus we can completely exclude the formation of labile oleylammonium ion halide. These completely amine free CsPbBr3 perovskite NCs synthesized using bromine-rich condition ( excess HBr) exhibit good stability and durability for more than three months in the form of colloidal solutions and films respectively. This amine free CsPbBr3 NCs films exhibiting high photoluminescence (PL) at open atmospheric conditions which can be further used for opto- electronic device applications.

Materials exhibiting ferroelectric and piezoelectric properties are of immense attention due to their promising technology applications and recently as mechanical energy harvesters. Our group is interested in the generation of phosphonium and ammonium cations for the halo-, oxo- and metallate-derived anions with ferroelectric properties and employ them to fabricate nanogenerator devices (Figure). We have initially shown that the PF₆^‒ salt of diphenyl diisopropyl phosphonium cation is ferroelectric in nature. Its various weight percentages composites with poly (dimethyl) siloxane yielded the first examples of all-organic mechanical energy harvesting devices.¹ Use of tetrahedral anions such as BF₄^‒, ClO_4^‒ and IO₄^‒ ions gave rise to a family of ferro- and piezoelectric phosphonium salts with excellent ferroelectric and energy harvesting properties.²  Inspired by these findings, ferroelectric salts with A₂MX₄, AMX₃ and A₄(ML₆) composition were prepared for the halogenometallate and pseudo-halogenometallate anions. Composites of these frameworks with commercial polymers such as PDMS and TPU were prepared and shown to yield high performances for their mechanical energy harvesting devices.^3,4 Recently, using an ammonium cation containing metal-coordinating hydroxyl functionality, we have synthesized a zwitterionic ABX₃ type perovskite that shows not only a high ferroelectric polarization but also very high nanogenerator device characteristics.⁵