Halide Perovskite solar cells have attracted much recent interest because of their high power conversion efficiency and low fabrication cost. However, perovskite materials suffer from some significant challenges including long term stability issue before the solar cells can be practically used at large scale. Herein we report our recent progress in addressing the stability of perovskite solar cells, including introduction of capping layers to improve the stability against moisture and heat, down-conversion elemental doping to protect from UV damage, and perovskite size engineering to suppress phase segregation. In particular, quantum dots (QDs) have the advantages of quantum confinement effect, defect-tolerant nature, and the capability of developing stable lightweight and flexible films, we will discuss our recent progress on a novel surface ligand engineering strategy in designing new hybrid perovskite QDs, which leads to not only fundamental understanding on the optoelectronic working mechanism of the QDs, but also remarkably improve the optoelectronic quality and stability of the perovskite QDs. The new classes of perovskite quantum dots have been used as building blocks in Quantum Dot Solar Cells with a certified world record efficiency of 16.6% (https://en.wikipedia.org/wiki/Solar_cell_efficiency) with excellent long-term operation stability. By using QDs as light absorbing materials, the QD based photocatalysts also exhibited good stable performance in photocatalytic gaseous hydrogen production. The integration of perovskite solar cells and rechargeable batteries have led to a single module type rechargeable solar batteries with an overall storable solar energy conversion efficiency of >12%.

The grain size, grain boundary defects and ion migration of lattice as well as compactness of interface buffer layer significantly affect the stability of perovskite solar cells. While, the process conversion from spin-coating to printing with an increasing scale is the only way for large-area perovskite solar cells fabrication. The problems including preparation of flexible transparent electrodes, stability and matching of interface layer, and obstructing function, the ion migration suppression and defects repair of grain boundaries, fabrication of flexible perovskite solar cells and its self-healing need to be solved to finally realize the large scale perovskite solar cells with efficiency and stability via printing process. Aiming at the stability problems caused by ion migration and water and oxygen erosion of perovskite solar cells, a "pin-through" strategy is proposed to comprehensively improve the stability and bending resistance of perovskite solar cells. The carbon nanotubes, fluorine-containing semiconductors and fluorescent up-conversion molecules are used to fill the grain boundary defects of perovskite solar cells and inhibit ion migration and realize grain boundary linkage, which greatly improve device stability and broaden the absorption response. The spontaneous gradient 2D/3D structure can achieve the effective regulation of crystallization kinetics and carrier dynamics to the realization of high-quality perovskite with vertical crystal orientation and the optimal regulation of carrier transport. Moreover, the self-grown oriented scaffold and elastomer are proposed to repair the perovskite grain boundary defects, solve the problem of PbI2 conversion and endow the active layer with bending resistance. Systematically adopting integrated technology and vertebral structure to release stress and solve the self-repairing problem of perovskite fracture, the manufacture of flexible perovskite solar cells is fully realized. By optimizing printing equipment, adjusting ink stability and colloidal chemistry as well as perovskite grain growth kinetics during the printing process, the printing preparation of high-efficiency rigid and flexible large-area modules can be fulfilled.

To date, perovskite solar cells (PSCs) have achieved a record certified power conversion efficiency (PCE) of 25.5%. There is still much room for PCE improvement as compared to theoretical Shockley–Queisser limit efficiency over 30%. In addition, the poor long-term operational stability still restricts its large-scale commercial application. The bulk and interfacial carrier non-radiative recombination losses can significantly reduce the efficiency and stability of devices. In order to minimize bulk and interfacial non-radiative recombination losses, Prof. Chen’s research group has carried out systematic and in-depth research work on composition and solvent engineering, additive engineering and interface engineering. The obtained research achievements are as follows: (1) The BF₄^- anionic doping strategy is developed, which effectively solves the serious carrier recombination problem. The inhibition mechanism of halogen ion migration by large-size organic cations is revealed. (2) The multifunctional additive ligand molecule (11MA) is developed to regulate perovskite crystallization, passivate grain boundary defects, inhibit halogen ion migration and improve humidity stability, achieving an efficiency of 23.3%. The potassium salt additive Molecules (SAMS) containing large size strongly coordinated organic anions is developed. A large size strongly coordinated organic anion grain boundary anchoring strategy was proposed. The mechanism of grain boundary defect passivation and halogen ion migration inhibition is revealed. The device based on CsFA-based perovskite and SAMS delivers a PCE of 22.7%. (3) The interface between electron transport layer and perovskite layer is modified by self-assembled ionic liquid (ImAcHCl) or sulfonium salt (CDSC), achieving interfacial defect passivation and energy band modulation. It is revealed that non-halogen anions (PF₆^- and SO₄^2-) can effectively chemically bridge the metal oxide electron transport layer and the perovskite layer, thereby improving the interface contact. Two-dimensional perovskite, perovskite grain boundary passivation layer, multi-active-site ligand molecule (MTDAA) and novel phosphonium salt are developed to modulate the interface between perovskite and hole transport layer. The multiple-active-site defect passivation mechanism is revealed.

Understanding and regulating the crystallization behavior of quasi-2D perovskite is the key to obtain high quality perovskite thin films. This report will firstly discuss the heterogeneous nucleation, orientated growth, tuned crystal growth rate of quasi-2D perovskites. And then we will introduce our recent advances in healing the buried cavities and defects in quasi-2D Perovskite films by a self-generated methylamine gas method. When quasi-two-dimensional RP perovskite films, which adopt a downward growth mode, possess large grain size, it is found that defective contacts tend to be formed at their bottom interfaces together with many nanocavities. This is attributed to the angular growing fronts of RP perovskite grains which adopt [111] (or/and [101]) growth direction. Herein, a self-generated methylamine gas, by a replacement reaction in precursor solution, is developed to in-situly heal the bottom interfacial nanocavities during the crystallization process. The amount of self-generated methylamine gas can be adequately controlled to avoid homogenous nucleation of perovskite phase from the liquid perovskite-amine intermediate phase, which is a key to healing the buried irregular cavities deep in the perovskite layer without ruining the large grains and composition of the RP perovskites. As a consequence, this interfacial-defect healing strategy enables efficient hole extraction as well as improved interfacial adhesion, providing significantly enhanced efficiency and stability in RP perovskite solar cells.

Over ten-year development of organic-inorganic perovskite solar cells (PSCs), the highest power conversion efficiency (PCE) has exceeded 25%, competitive with the silicon based solar cells. However, due to the intrinsic ionic lead-containing feature of organic-inorganic perovskite crystals, the stability against moisture/heat and the potential risk of lead leakage still limits its commercialization and long-term utilization in civilian circumstances. Generally, scholars use polymers as external encapsulating layer (EEL) or interface to solve these problems. However, the small quantity of dopant or the thin thickness of interface is not enough to guarantee their long-term protection against moisture/heat.

Inspired by the multi-level encapsulating method for electronic components in order to achieve a long-term stability of circuits, we propose to utilize the internal encapsulating layer (IEL) as a complement of EEL to improve the stability of perovskite films as well as the corresponding devices.

Flexible perovskite solar cells (f-PSCs) have great potential prospects in flexible energy and mobile energy systems. Rigid PSCs have made important progress, and the power conversion efficiency (PCE) has exceeded 25%, but the PCE of f-PSCs is still much lower than rigid ones. Based on the device structure of PEN/ITO/SnO₂/FAMAPbBr_xI_3-x/Spiro- OMeTAD/Ag, the PCEs up to 19.51% were achieved through the precise control on electron transport layer SnO₂[1]. Furthermore, CF₃PEAI molecule was introduced to construct 2D perovskite modified layer between 3D perovskite and hole transport layer[2]. It could passivate 3D perovskite film defects, adjust energy level structure between perovskite and hole transport layer and improve 3D perovskite charge transport performance, leading to the PCEs up to 21.1% (verified PCE was 20.5%). The f-PSCs exhibited excellent stability and bending resistance. Meanwhile, based on ink formula regulation and interface optimization, efficient f-PSCs were fabricated by roll-to-roll printing process, showing potential application prospects [3].

Gas-quenching is a robust and highly-reproducible approach for upscalable manufacturing of high-quality perovskite films. However, the requirement of high gas pressure on MA-free perovskite materials has limited the accessibility for upscalable manufacturing of stable perovskite films and devices. In this work, by employing tetramethylene sulfoxide (TMSO) as a ligand in the precursor solution, high-quality FA0.9Cs0.1PbI3 perovskite films have been successfully obtained by gas-quenching. Study on the precursor solution and film-forming process revealed the effect of TMSO on the formation of large-grain films, as well as the critical role of gas-quenching in regulating the intermediate films. The absolute MA-free perovskite device exhibits superior stability over the PSCs obtained with MACl additive. The fabricated MA- and Br-free PSCs shows a power conversion efficiency of 21.3% without any passivation treatment. Moreover, gas-quenching with TMSO enables wide gas pressure processing window and superior accessibility to low-pressure processing, demonstrating its promising potential in up-scaling manufacturing of high-efficiency MA-free PSCs.

Perovskite solar cell (PSC) has seen rapid progress in stability in recent years. We report here our perovskite solar modules (PSM) passing key ageing tests described in IEC61215 under multi-folds stricter conditions in Dec 2020 carried out by an independent third party certification center. The tests include 3000hours damp-heat (DH) test, 100kWh UV preconditioning test and 1000hours one sun 70°C light soaking test . The modules passed these tests with less than 5% P_mpp degradation. We will discuss the challenges we faced while preparing for these tests and the methods taken to overcome them involving multi-level engineering on materials, device architectures, module structures and encapsulation. With the commissioning of our first 100MW production line underway, we will also introduce its progress and future implementations of perovskite products.

III−V type indium phosphide (InP) quantum dots (QDs) are now firmly positioned as heavy metal-free, environment-benign visible emitters of particularly green and red colors in next-generation display devices. On the basis of synthetic and core/shell heterostructural advances of InP QDs toward bright and sharp emissivity, they have been successfully exploited in the platform of light-emitting diode (LED) based on color conversion or self-emissive electroluminescence (EL). In contrast to substantial progress of green and red InP QDs, synthesis of blue InP ones with a bright, deep-blue emissivity is not likely viable, which is primarily associated with their intrinsic size limitation Meanwhile, although green InP QDs can exhibit comparable photoluminescence (PL) features in quantum yield (QY) and full-width-at-half-maximum (fwhm) to red InP ones, performances of green QD-based devices are inferior in both color conversion and EL to those of red QD-based ones. This is again likely correlated to the size issue, that is, smaller size of green InP core relative to red one, thus necessitating the development of alternative non-InP green emitters. In this talk, we introduce II−VI type ternary ZnSeTe QDs as blue-to-red emitters, which may be potential alternatives to InP counterparts. Taking full advantage of state-of-the-art multiple shelling scheme, high PL and EL performances of ZnSeTe QDs are demonstrated.

Surface passivation is an effective way to boost the efficiency and stability of perovskite devices. However, a key challenge faced by most of the passivation strategies is reducing the interface charge recombination without imposing energy barriers to charge extraction. In this talk, I will present a novel multi-functional semiconducting organic ammonium cationic interface modifier inserted between the light-harvesting perovskite film and the hole transporting layer. We show that the conjugated cations can directly extract holes from perovskite efficiently, and simultaneously reduce interface non-radiative recombination. Together with improved energy level alignment and stabilized interface in device, we demonstrate a triple-cation mixed-halide medium-bandgap perovskite solar cell with an excellent power conversion efficiency of 22.06% (improved from 19.94%), and the suppressed ion migration and halide phase segregation, which lead to a long-term operational stability over 1000 hours. Using a similar concept, we achieved high performance red LEDs with external quantum efficiency over 15%. Our strategy provides a new practical method of interface engineering in perovskite solar cells and LEDs towards improved efficiency and stability.

Fibrous controllable liquid transfer:

towards high-performance thin-film devices

Huan Liu*

School of Chemistry, Beihang University, Beijing, 100191, China.

E-mail: liuh@buaa.edu.cn

In nature, various biological fibrous systems exhibit unique dynamic wetting properties, which has shown many advantages in inhabiting local environments. However, controllable liquid transfer by an open fibrous system is still poorly understood and remains a challenge, because capillary coalescence is frequently encountered when fiber array interacted with a liquid. Here, we revealed the fundamental of the Chinese brush for its capability in controllable liquid transfer: the unique anisotropic multi-scale structure of the freshly emergent hairs. Drawing inspirations, we developed model devices with flexible conical fibers that allows for direct writing functional micro-lines with 10 µm resolution and nano-thin films, with well-defined profile and uniform distribution on diverse substrates. To be noticed, the fibers-guided directional liquid transfer enables fine controlling the liquid/solid/gas tri-phase contact line under multiple directional stresses. Taking advantages, highly oriented polymer thin film and aligned AgNWs film were fabricated in large scale, based on which high performance of polymer TFTs devices and the anisotropic flexible conductive electrode were developed, respectively. We also demonstrated that the conical fiber array enables direct preparing ultra-smooth QD micro-patterns, and thus a high-performance QLED devices was allowed. We envision that the controllable liquid transfer guided by the conical fibers will shed light on the novel template-free printing of functional photoelectric devices.

Reference

1. M. Zhang, H. Deng, L. Meng, H. Wang, Y. Wang, H. Liu* Angew. Chem. Int. Ed., 2021, 60, 680-684.

2. L. Meng, M. Zhang, H. Deng, B. Xu, H. Wang, Y. Wang, L. Jiang, H. Liu* CCS Chemistry, 2020, 2, 2194-2202.

3. Q. Wang,+ B. Xu,+ Q. Hao, D. Wang, H. Liu*, L. Jiang Nat. Commun., 2019, DOI: 10.1038/s41467-019-09201-1.

4. R. Bian, L. Meng, C. Guo, Z. Tang, H. Liu*, Adv. Mater., 2019, 31, 1900534.

5. M. Zhang, B. Hu, L. Meng, R. Bian, S. Wang, Y. Wang, H. Liu*, L. Jiang, J. Am. Chem. Soc., 2018, 140, 8690-8695.

6. L. Meng, R. Bian, C. Guo, B. Xu, H. Liu*, L. Jiang, Adv. Mater., 2018, 30, 1706938.

7. P. Wang, R. Bian, Q. Meng, H. Liu*, L. Jiang, Adv. Mater., 2017, 29, 1703042.

Quantum dot light emitting diodes (QLEDs) that electrically excite quantum dots (QDs) are the future of QD displays, owning to the excellent optical properties of QDs, such as narrow emission spectra, size-controlled emission wavelength, high quantum yield and inherent stability. During the past twenty years, the efficiency of QLEDs has dramatically increased from less than 0.1% to more than 20%, but the operation lifetime of these devices and other key factors were still far below the requirement of display applications. In this report, we will discuss our concerns and progress in IJP QLEDs Display, and talk about what we should do to commercialized this technique in the future.

Recent breakthrough in synthesizing arbitrary vertical heterostructures of Ruddlesden–Popper (RP) perovskites opens doors to myriad quantum optoelectronic applications. However, it is not clear whether moiré excitons and flat bands can be formed in such heterostructures. Here, we predict from first principles that twisted homobilayers of RP perovskite, MA2PbI4, can host moiré excitons and yield flat energy bands. The moiré excitons exhibit unique and hybridized characteristics with electrons confined in a single layer of a striped distribution while holes localized in both layers. Nearly flat valence bands can be formed in the bilayers with relatively large twist angles, thanks to the presence of hydrogen bonds that strengthen the interlayer coupling. External pressures can further increase the interlayer coupling, yielding more localized moiré excitons and flatter valence bands. Finally, electrostatic gating is predicted to tune the degree of hybridization, energy, position and localization of moiré excitons in twisted MA2PbI4 bilayers.

Photovoltaics and electroluminescence has fundamental reciprocity relationship in the PIN diode but it is difficult to be demonstrated in traditional photovoltaics. In the work, we present our recent study on perovskite solar cell  (PSC) and light emitting diode (LED) and illustrate the reciprocity relation in the device. The interface engineering and  passivation approaches are performed to optimize the performance thus it is demonstrated that a great solar cell needs to be a great LED, leading to the perovskite bifunctional device (PBD). We also fabricate PBD from LED with high comprehensive performance. The PBD can ultilize the intermittent solar energy more efficiently and can be employed for building integrated photovoltaics in the future.

Metal halide perovskites serving as one of the most promising photovoltaic materials are gaining considerable attention worldwide. For achieving high performance as well as long-term stability of the perovskite solar cells (PSCs), a good quality of perovskite film featuring in smooth, pinhole-free morphology, full coverage over substrates, good heterojunction contacts, and stable photoactive phase, is of great importance. During solution fabrication for perovskite films, intermediate phase, which refers to the state of precursor composition before final annealing, plays essential roles in determining the film quality, especially in the state-of-the-art PSCs. We review the research involving mechanism and applications of intermediate phase engineering (IPE) processes in various solution-processing technologies. Challenges and perspectives of IPE in high-quality perovskite films are further discussed.

Compared with the single junction solar cell technology, perovskite/silicon tandem solar cells (PSTSCs) provide a foreseeable strategic opportunity to further increase the power conversion efficiency (PCE). Nowadays, a certified power conversion efficiency (PCE) of 29.8% for the monolithic PSTSCs has been obtained, which exceeds the single junction Shockley–Queisser limit (29.3%), indicating its tremendous potential in the photovoltaic energy field. Here, a n-i-p type PSTSCs was fabricated by solution two-step sequential deposition method. Lithium chloride (LiCl) was added into the tin oxide (SnO₂) precursor solution, which simultaneously passivated the defects and increased the electron injection driving force at the electron transfer layer (ETL)/perovskite interface. Eventually, we achieved monolithic PSTSCs with an efficiency of 25.42% and V_OC of 1.92 V, which is the highest PCE and V_OC in N-I-P type perovskite/Si tandem devices . Moreover, we also deposited p-i-n type perovskite top cells on fully-textured c-Si solar cells to construct conformal-grown monolithic PSTSCs by evaporation-solution combination two-step method. A thermal-evaporated CsBr thin layer between the perovskite layer and the hole transport layer is introduced to construct a gradient perovskite absorber for optimized energy level alignment, so as to improve the V_OC and fill factor (FF) of the device. Finally, the PSTSCs achieved an efficiency of 27.48% and was stable in nitrogen over 10,000 h.

Ion migration is a well-known problem in perovskite materials. It causes baseline drift, lowers imaging resolution, accelerated decomposition and device performance degradation. In particular in X-ray detectors, the effect of ion migration is more obvious under working bias. The first principals study reveals that the 0D structure perovskite would show effectively reduced ion migration between neighbouring unit cells compared with the popular 2D and 3D perovskites. A nucleation-controlled strategy is developed to grow superior inch-sized high-quality 0D-structured lead-free (CH₃NH₃)₃Bi₂I₉ perovskite single crystals (MA₃Bi₂I₉ PSCs) with significantly lower ion migration, much reduced dark current and better environmental stability compared to other perovskite materials, enabling us to design and fabricate a new type of 0D-structured lead-free perovskite X-ray detector. It is found that the X-ray detectors show surprisingly high sensitivity, 15 times more than that of the state-of-the-art commercial α-Se detectors, with very low detection limit that is desired for medical diagnostics, material inspection, etc. Furthermore, their response time is as short as 0.98 ms, the shortest among all X-ray detectors reported in literature, which may allow us to develop an X-ray screening system with reduced X-ray dose and improved resolution.

In this talk, I will introduce some works of our group on materials design and compositional engineering for high efficiency and highly stable perovskite solar cells (PSCs). First, through molecular design for hole transporting materials (HTMs), we developed a new, dopant-free conjugated polymer HTM called DTB with very simple molecule structure but excellent abilities both on defect passivation and on hole extraction. The DTB based PSC has the PCE of 19.68% and the J_SC achieved 25.75 mA/cm², which was the world's highest level based on the dopant-free organic HTLs. Then we further developed an "inorganic/organic" double-layer HTL, achieving 22.0% efficiency with excellent light, humidity and temperature stabilities. Secondly, through material design of the perovskite absorb layer, we successfully incorporated GABr into the perovskite material which can suppress the oxidation of Sn2+ ions. Consequently, the ideal-bandgap (1.35eV) Sn/Pb perovskite solar cells with GABr doping can have the PCE of 20.6%, which is the highest efficiency among all values reported to date for ideal-bandgap Sn/Pb PSCs. In addition, we developed an in-situ green approach utilizing nontoxic cetyltrimethylammonium chloride (CTAC) and isopropanol (IPA) as anti-solvent to effectively passivate both surface and grain boundary defects in hybrid perovskites. Anion vacancies can be readily passivated by the chloride group due to its high electronegativity, and cation defects can be synchronously passivated by the more stable cetyltrimethylammonium group, which leads to the cell device with 23.4% PCE and excellent light, humidity and temperature stabilities. Thirdly, we developed a high-throughput inkjet printing based technique for high speed perovskite composition screening, which can fabricate 30-50 high-quality mixed perovskite films in several minutes, and the corresponding film properties database allowed to accelerate the screening and optimization of perovskite compositions. As a demonstration, we screened 30 tribromide perovskite materials, and these accelerated optimized novel compositions yielding a high open-circuit voltage exceeding 1.6 V.

Organic-inorganic hybrid lead halogenide perovskites have attracted vast attention ever since it was first applied to solar cells as light absorber in 2009. Efficiency as high as 25.5% was obtained for small area lab cells after a decade’s efforts made by researchers all over the world. However, prior to the mass production of this type of photovoltaic devices, some key issues, such as large area deposition, monolithically interconnected module fabrication and stability improvement, have to be solved. In this talk, we will present our recent progress in the efficiency enhancement in all-inorganic and hybrid perovskite solar cells and modules. Aperture area efficiencies of 21.37% and 19.21%, certified an independent third party, were achieved for inverted perovskite mini-modules on glass and flexible polymer substrates, respectively. In addition, mini-module with efficiency over 30% measured under LED indoor light illumination was obtained for indoor light harvesting application, either.

Opto-electronic measurements on perovskite solar cells show several characteristic features, such as IV curve hysteresis [1-2] and slow transient current rise [3]. It is believed, that these effects are caused by mobile ionic charges in the perovskite layer and simulation models including ionic charges could qualitatively reproduce these individual features [1-3]. To increase the trustability of the model, we systematically studied the influence of individual model parameters to find a single set of parameters to explain the combined results from various measurement techniques [4].

Here we present measurements of methylammonium lead iodide (MAPI) perovskite solar cells in the DC, AC and transient regime. The applied techniques include IV curves with different scan rates, light‑intensity dependent open‑circuit voltage, impedance spectra, intensity-modulated photocurrent spectra (IMPS), transient photocurrents and transient voltage step responses. These experimental data sets are successfully reproduced by a drift‑diffusion simulation using a single set of parameters.

This allows for a better understanding of the governing physical effects and provides a powerful tool to study the influence of device parameters on the solar cell performance and efficiency. Our in‑depth parameter study suggests possible paths towards an optimized device.

Organic-inorganic lead halide perovskites have revolutionized the field of thin-film solar cells. In recent years, the record efficiency of lead halide perovskite-based thin-film solar cells has increased rapidly from 3.8% in 2009 to 25.5% in 2020. Such rapid progress has never been seen before in the history of solar cell development. The unique optoelectronic properties and low-temperature synthesis also provided unprecedented opportunities for the next generation thin-film solar cells including low-cost tandem solar cells and bifacial solar cells. However, despite the exciting progress and opportunities, metal halide perovskite solar cells also face significant challenges, due to the inclusion of toxic Pb and instability against moisture, heat, and irradiation. In this talk, I will provide an overview of the unique opportunities and challenges of metal halide perovskite solar cells.

With the unprecedent efficiency progress, perovskite solar cells have become one of most promising next-generation photovoltaics technologies to realize the goal of low cost and high efficiency. However, their commercialization faces the challenge of stability. Various technologies have been developed to enhance the stability of perovskite solar cell without decreasing efficiency. The Cs based perovskites are one of the most promising ones. The Cs based FA-Cs with tunable tolerant factor and high stability are promising choice to realize high efficiency and stability. However, the crystallization of FA-Cs perovskite face the serious challenge of phase segregation. We develop the MA and DMA cation induced crystallization of phase pure FA-Cs mixed cation perovskite solar cell, which exhibiting both the high efficiency and high stability. Furthermore. the all-inorganic lead halide perovskite without volatile component would be a promising alternative candidate for high efficiency photovoltaics. However, the all inorganic CsPbI3 with most suitable band gap face the challenges of low room temperature phase stability and relative low efficiency. To enhance the performance and stability of all-inorganic CsPbI3 perovskite, the 2D/3D configuration especially the (110) oriented 2D perovskite component was introduced to stabilize the black phase CsPbI3. Later, facile organic cation surface termination approach was developed to significantly enhance the stability and performance of black phase CsPbI3 solar cell with >15% efficiency. The bifunctional stabilization of CsPbI3 with gradient Br doping and organic cation termination finally improve the efficiency of CsPbI3 perovskite solar cells to a value of 17% with enhanced stabilities. With crystallization dynamics control, we developed a highly stable and efficient beta-phase CsPbI3 with impressing efficiency up to 18%. Recently, with more passivation approaches, we push the inorganic perovskties’ efficiencies approaching 20%.

Recently, solution processed hybrid perovskites show intriguing electronic and optoelectronic properties applicable in various device application, and the development of high-quality materials with improved homogeneity and less defects is crucial. In this talk, I mainly focused on the perovskite films growth and defects manipulation, and the construction of efficient and stable photovoltaic devices. Two related aspects are discussed: 1) Develop liquid medium annealing process that leads to the films with high crystallinity, less defects, desired stoichiometry, and overall film homogeneity, resulting in the solar cells with certified efficiency of 22.3% at 1 cm²; 2) Explore defects self-elimination approach, as well as phase evolution approach to fabricate high-efficiency solar cells, with certified efficiency of 24.5% at 0.1 cm²; 3) Propose methods such as "redox ion pair" and "multiple non-covalent bond synergistic effect" to suppress defect pairs in a sustainable way, which greatly improved the long-term stability of perovskite solar cells for over 2000 h under the stressors, e.g. light, heat, and electricity. Corresponding degradation mechanism of hybrid perovskite films and devices under operational conditions are revealed on the molecular and atomic scales.

When droplets containing non-volatile solutes dry on a substrate, the solution in the interior prefers migrating to the periphery caused by the capillary force and ultimately the solutes are carried to the edges of droplets forming “coffee rings” after drying, namely coffee ring effect (CRE). CRE is a general phenomenon in the printing process, however, seriously affects the uniformity and quality of films, especially in the dispersed deposition technology based on droplets such as spray coating.

In recent years, spray coating, a mature industrial technology, has been widely used in the fields of perovskite solar cells (PSCs) owing to its advantages of high-throughput, outstanding compatibility with diverse substrates and scalability. However, the inhomogeneity of sprayed films caused by CRE leads to poor reproducibility of devices and limits the improvement of devices performance. The power conversion efficiencies (PCEs) of sprayed perovskite devices are still far behind that of their spin-coated counterparts. Therefore, the elimination of the CRE is of great practical interest for the development of spraying technology in the field of perovskite.

In recent, Prof. Yong Peng and colleagues in Wuhan University of Technology reported a reaction-dependent regulating strategy. Unlike the inert conditions under which the solution cannot react with the substrate, here, the liquid droplets (FAI/Br) are reactive to the solid films (CsI/PbI₂) which will affect the distribution of the solutes. Considering this characteristic, authors regulated the reaction process via solvent selection and found that the solvent with proper drying rate, such as n-butyl alcohol (NBA), can effectively suppress the formation of “coffee rings”. On the one hand, an appropriate prolonged-drying-time enhances droplet spreading and permits the formation of an integrated wet film reducing the “edge” counts, on the other hand, the increased reaction between FAI/Br and CsI/PbI₂ film reduces the accumulation of solutes in the periphery, resulting in uniform deposition. Finally, dense and homogeneous Cs_0.19FA_0.81PbI_0.5Br_2.5 perovskite films were achieved and benefiting from the high-quality perovskite films, the PCEs of corresponding devices reached 19.17%, which is among the highest PCEs for spray-coated PSCs. In addition, the enlarged perovskite film exhibits excellent uniformity. This work provides not only an effective approach for CRE controlment in spraying technology to achieve high-performance devices, but a new idea for the development of other printing technologies.