Incorporating CsF into the PbI2 Film for Stable Mixed Cation‐Halide Perovskite Solar Cells

Adding a small amount of CsI into mixed cation‐halide perovskite film via a one‐step method has been demonstrated as an excellent strategy for high‐performance perovskite solar cells (PSCs). However, the one‐step method generally relies on an antisolvent washing process, which is hard to control and not suitable for fabricating large‐area devices. Here, CsF is employed and Cs is incorporated into perovskite film via a two‐step method. It is revealed that CsF can effectively diffuse into the PbI₂ seed film, and drastically enhances perovskite crystallization, leading to high‐quality Cs‐doped perovskite film with a very long photoluminescence carrier lifetime (1413 ns), remarkable light stability, thermal stability, and humidity stability. The fabricated PSCs show power conversion efficiency (PCE) of over 21%, and they are highly thermally stable: in the aging test at 60 °C for 300 h, 96% of the original PCE remains. The CsF incorporation process provides a new avenue for stable high‐performance PSCs.     

Halogen Engineering for Operationally Stable Perovskite Solar Cells via Sequential Deposition

The performance of perovskite solar cells (PSCs) relies on the synthesis method and chemical composition of the perovskite materials. So far, PSCs that have adopted two‐step sequential deposited perovskite with the state‐of‐art composition (FAPbI₃)_1?x(MAPbBr₃)_x (x < 0.05) have achieved record power conversion efficiency (PCE), while their one‐step antisolvent dripping counterparts with typical composition Cs_0.05FA_0.81MA_0.14Pb(I_0.85Br_0.15)₃ with more bromine have exhibited much better long‐term operational stability. Thus, halogen engineering that aims to elevate bromine content in sequential deposited perovskite film would push operational stability of PSCs toward that of antisolvent dripping deposited perovskite materials. Here, a Br‐rich seeding growth method is devised and perovskite seed solution with high bromine content is introduced into a PbI₂ precursor, leading to bromine incorporation in the resulting perovskite film. Photovoltaic devices fabricated by Br‐rich seeding growth method exhibit a PCE of 21.5%, similar to 21.6% for PSCs having lower bromine content. Whereas, the operational stability of PSCs with higher bromine content is significantly enhanced, with over 80% of initial PCE retained after 500 h tracking at maximum power point under 1‐sun illumination. This work highlights the vital importance of halogen composition for the operational stability of PSCs, and introduces an effective way to incorporate bromine into mixed‐cation‐halide perovskite film via sequential deposition method.     

Cold Antisolvent Bathing Derived Highly Efficient Large‐Area Perovskite Solar Cells

Scaling large‐area solar cells is in high demand for the commercialization of perovskite solar cells (PSCs) with a high power‐conversion efficiency (PCE). However, few roll‐to‐roll‐compatible deposition methods for the formation of highly oriented uniform perovskite films are reported. Herein, a facile cold antisolvent bathing approach compatible with large‐area fabrication is introduced. The wet precursor films are submerged in a cold antisolvent bath at 0 °C, and the retarded nucleation and growth kinetics allow highly oriented perovskite to be grown along the [110] and [220] directions, perpendicular to the substrate. The high degree of the preferred crystal orientation benefits the effective charge extraction and reduces the amount of inter‐ and intra‐grain defects inside the perovskite films, improving the PCE from 16.48% (ambient‐bathed solar cell) to 18.50% (cold‐bathed counterpart). The cold antisolvent bathing method is employed for the fabrication of large‐area (8 × 10 cm²) PSCs with uniform photovoltaic device parameters, thereby verifying the scale‐up capability of the method.     

Precursor Engineering for Ambient‐Compatible Antisolvent‐Free Fabrication of High‐Efficiency CsPbI2Br Perovskite Solar Cells

High temperature stable inorganic CsPbX₃ (X: I, Br, or mixed halides) perovskites with their bandgap tailored by tuning the halide composition offer promising opportunities in the design of ideal top cells for high‐efficiency tandem solar cells. Unfortunately, the current high‐efficiency CsPbX₃ perovskite solar cells (PSCs) are prepared in vacuum, a moisture‐free glovebox or other low‐humidity conditions due to their poor moisture stability. Herein, a new precursor system (HCOOCs, HPbI₃, and HPbBr₃) is developed to replace the traditional precursors (CsI, PbI₂, and PbBr₂) commonly used for solar cells of this type. Both the experiments and calculations reveal that a new complex (HCOOH?Cs⁺) is generated in this precursor system. The new complex is not only stable against aging in humid air ambient at 91% relative humidity, but also effectively slows the perovskite crystallization, making it possible to eliminate the popular antisolvent used in the perovskite CsPbI₂Br film deposition. The CsPbI₂Br PSCs based on the new precursor system achieve a champion efficiency of 16.14%, the highest for inorganic PSCs prepared in ambient air conditions. Meanwhile, high air stability is demonstrated for an unencapsulated CsPbI₂Br PSC with 92% of the original efficiency remaining after more than 800 h aging in ambient air.     

Processing‐Performance Evolution of Perovskite Solar Cells: From Large Grain Polycrystalline Films to Single Crystals

Solution‐processable halide perovskites have emerged as strong contenders for next‐generation solar cells owing to their favorable optoelectronic properties. To maintain the efficiency momentum of perovskite solar cells (PSCs), development of advanced processing techniques, particularly for the perovskite layer, is imperative. There is a close correlation between the quality of the perovskite layer and its photophysical properties: Highly crystalline large grains with uniform morphology of the perovskite layer and their interface with charge transporters are crucial for achieving high performance. Significant efforts have been dedicated to achieve perovskite films with large grains reaching the millimeter‐scale for high‐efficiency PSCs. Recent work showcases a transition from large grain polycrystalline to single‐crystalline (SC) PSCs made possible by the facile growth of perovskite single crystals. In this review, the recent progress of the large grain polycrystalline PSCs and grain boundary‐free SC‐PSCs is reported, particularly focusing on the recent approach of depositing large‐grained perovskite layers and single crystal growth technique, that have been adopted for fabrication of efficient PSCs. In addition, prospects of SC‐PSCs and their further development in terms of efficiency, device design, scalability, and stability are discussed.     

Morphology Controlling of All‐Inorganic Perovskite at Low Temperature for Efficient Rigid and Flexible Solar Cells

All‐inorganic cesium lead halide (CsPbX₃) perovskites have emerged as promising photovoltaic materials owing to their superior thermal stability compared to traditional organic–inorganic hybrid counterparts. However, the CsPbX₃ perovskites generally need to be prepared at high‐temperature, which restricts their application in multilayer or flexible solar cells. Herein, the formation of CsPbX₃ perovskites at room‐temperature (RT) induced by dimethylsulphoxide (DMSO) coordination is reported. It is further found that a RT solvent (DMSO) annealing (RTSA) treatment is valid to control the perovskite crystallization dynamics, leading to uniform and void‐free films, and consequently a maximum power conversion efficiency (PCE) of 6.4% in the device indium tin oxide (ITO)/NiO _x /RT‐CsPbI₂Br/C₆₀/Bathocuproine (BCP)/Ag, which is, as far as it is known, the first report of RT solution‐processed CsPbX₃‐based perovskite solar cells (PSCs). Moreover, the efficiency can be boosted up to 10.4% by postannealing the RTSA‐treated perovskite film at an optimal temperature of 120 °C. Profiting from the moderate temperature, flexible PSCs are also demonstrated with a maximum PCE of 7.3% for the first time. These results may stimulate further development of all‐inorganic CsPbX₃ perovskites and their application in flexible electronics.     

Fusing Nanowires into Thin Films: Fabrication of Graded‐Heterojunction Perovskite Solar Cells with Enhanced Performance

Perovskite solar cells (PSCs) have recently experienced a rapid rise in power conversion efficiency (PCE), but the prevailing PSCs with conventional mesoscopic or planar device architectures still contain nonideal perovskite/hole‐transporting‐layer (HTL) interfaces, limiting further enhancement in PCE and device stability. In this work, CsPbBr₃ perovskite nanowires are employed for modifying the surface electronic states of bulk perovskite thin films, forming compositionally‐graded heterojunction at the perovskite/HTL interface of PSCs. The nanowire morphology is found to be key to achieving lateral homogeneity in the perovskite film surface states resulting in a near‐ideal graded heterojunction. The hidden role of such lateral homogeneity on the performance of graded‐heterojunction PSCs is revealed for the first time. The resulting PSCs show high PCE up to 21.4%, as well as high operational stability, which is superior to control PSCs fabricated without CsPbBr₃‐nanocrystals modification and with CsPbBr₃‐nanocubes modification. This study demonstrates the promise of controlled hybridization of perovskite nanowires and bulk thin films for more efficient and stable PSCs.     

Carrier Interfacial Engineering by Bismuth Modification for Efficient and Thermoresistant Perovskite Solar Cells

Organic–inorganic halide perovskite solar cells (PSCs) have emerged as attractive alternatives to conventional solar cells. It is still a challenge to obtain PSCs with good thermal stability and high permanence, especially at extreme outdoor temperatures. This work systematically studies the effects of Bi³⁺ modification on structural, electrical, and optical properties of perovskite films (FA_0.83MA_0.17Pb(I_0.83Br_0.17)₃) and the performance of corresponding PSCs. The results indicate that Bi³⁺ modified PSCs can achieve better thermal stability, photovoltaic response, and reproducibility compared with control cells due to the decreased grain boundaries, enhanced crystallization, and improved electron extraction from perovskite film. As a result, the modified PSC exhibits an optimized power conversion efficiency (PCE) of 19.4% compared with 18.3% for the optimized control device, accompanied by better thermoresistant ability under 100–180 °C and enhanced long‐term stability. The degradation rate of the modified device is reduced by an order of magnitude due to effective structural defect modification in perovskite photoactive layer. It could maintain more than two months at 60 °C. These results shed light on the origin of crystallization and thermal stability of perovskite films, and provide an approach to solve thermal stability issue of PSCs.     

CsI‐Antisolvent Adduct Formation in All‐Inorganic Metal Halide Perovskites

The excellent optoelectronic properties demonstrated by hybrid organic/inorganic metal halide perovskites are all predicated on precisely controlling the exact nucleation and crystallization dynamics that occur during film formation. In general, high‐performance thin films are obtained by a method commonly called solvent engineering (or antisolvent quench) processing. The solvent engineering method removes excess solvent, but importantly leaves behind solvent that forms chemical adducts with the lead‐halide precursor salts. These adduct‐based precursor phases control nucleation and the growth of the polycrystalline domains. There has not yet been a comprehensive study comparing the various antisolvents used in different perovskite compositions containing cesium. In addition, there have been no reports of solvent engineering for high efficiency in all‐inorganic perovskites such as CsPbI₃. In this work, inorganic perovskite composition CsPbI₃ is specifically targeted and unique adducts formed between CsI and precursor solvents and antisolvents are found that have not been observed for other A‐site cation salts. These CsI adducts control nucleation more so than the PbI₂–dimethyl sulfoxide (DMSO) adduct and demonstrate how the A‐site plays a significant role in crystallization. The use of methyl acetate (MeOAc) in this solvent engineering approach dictates crystallization through the formation of a CsI–MeOAc adduct and results in solar cells with a power conversion efficiency of 14.4%.     

The Synergism of DMSO and Diethyl Ether for Highly Reproducible and Efficient MA0.5FA0.5PbI3 Perovskite Solar Cells

Composition and film quality of perovskite are crucial for the further improvement of perovskite solar cells (PSCs), including efficiency, reproducibility, and stability. Here, it is demonstrated that by simply mixing 50% of formamidinium (FA⁺) into methylammonium lead iodide (MAPbI₃), a highly crystalline, stable phase, and compact, polycrystalline grain morphology perovskite is formed by using a solvent‐mediated phase transformation process via the synergism of dimethyl sulfoxide and diethyl ether, which shows long carrier lifetime, low trap state density, and a record certified 21.8% power conversion efficiency (PCE) in pure‐iodide, alkaline‐metal‐free MA_0.5FA_0.5PbI₃ perovskite‐based PSCs. These PSCs show very high operational stability, with 85% PCE retention upon 1000 h 1 Sun intensity illumination. A 17.33% PCE module (6.5 × 7 cm²) is also demonstrated, attesting to the scalability of such devices.     

Adamantanes Enhance the Photovoltaic Performance and Operational Stability of Perovskite Solar Cells by Effective Mitigation of Interfacial Defect States

Passivation of electronic defects is an effective strategy to boost the performance and operational stability of perovskite solar cells (PSCs). Identifying molecular materials that achieve this purpose is a key target of current research efforts. Here, adamantane (AD) and 1‐adamantylamine (ADA) are introduced as molecular modulators to abate electronic defects present within the bulk and at the perovskite–hole conductor interface. To this effect, the modulator is added either into the antisolvent (AS) to precipitate it together with the perovskite (AS method) or they are spin coated (SC) onto its surface (SC method). Time‐resolved photoluminescence measurements show substantially longer lifetimes for perovskite films treated with AD and ADA compared to the reference sample. In line with this observation, it is found that the presence of AD and ADA molecules at the interface between the perovskite film and the hole conductor increases all photovoltaic metrics, in particular the open circuit photovoltage (V _oc) as well as the operational stability of the PSC.     

Stable Formamidinium‐Based Perovskite Solar Cells via In Situ Grain Encapsulation

Formamidinium (FA)‐based lead iodide perovskites have emerged as the most promising light‐absorber materials in the prevailing perovskite solar cells (PSCs). However, they suffer from the phase‐instability issue in the ambient atmosphere, which is holding back the realization of the full potential of FA‐based PSCs in the context of high efficiency and stability. Herein, the tetraethylorthosilicate hydrolysis process is integrated with the solution crystallization of FA‐based perovskites, forming a new film structure with individual perovskite grains encapsulated by amorphous silica layers that are in situ formed at the nanoscale. The silica not only protects perovskite grains from the degradation but also enhances the charge‐carrier dynamics of perovskite films. The underlying mechanism is discussed using a joint experiment‐theory approach. Through this in situ grain encapsulation method, PSCs show an efficiency close to 20% with an impressive 97% retention after 1000‐h storage under ambient conditions.     

Room‐Temperature Vapor Deposition of Cobalt Nitride Nanofilms for Mesoscopic and Perovskite Solar Cells

Organic/inorganic hybrid solar cells, typically mesoscopic and perovskite solar cells, are regarded as promising candidates to replace conventional silicon or thin film photovoltaics. There have been intensive investigations on the development of advanced materials for improved power conversion efficiencies, however, economical feasibilities and reliabilities of the organic/inorganic photovoltaics are yet to reach at a sufficient level for practical utilizations. In this study, cobalt nitride (CoN) nanofilms prepared by room‐temperature vapor deposition in an inert N₂ atmosphere, which is a facile and highly reproducible procedure, are proposed as a low‐cost counter electrode in mesoscopic dye‐sensitized solar cells (DSCs) and a hole transport material in inverted planar perovskite solar cells (PSCs) for the first time. The CoN film successfully replaces conventional Pt in DSCs, resulting in a power conversion efficiency comparable to the ones based on Pt. In addition, PSCs employing the CoN manifest high efficiency even up to 15.0%, which is comparable to state‐of‐the‐art performance in the cases of PSCs employing inorganic hole transporters. Furthermore, flexible solar cell applications of the CoN are performed in both mesoscopic and perovskite solar cells, verifying the advantages of the room‐temperature deposition process and feasibilities of the CoN nanofilms in various fields.     

Controllable Perovskite Crystallization via Antisolvent Technique Using Chloride Additives for Highly Efficient Planar Perovskite Solar Cells

The presence of surface and grain boundary defects in organic–inorganic halide perovskite films can be detrimental to both the performance and operational stability of perovskite solar cells (PSCs). Here, the effect of chloride additives is studied on the bulk and surface defects of the mixed cation and halide PSCs. It is found that using an antisolvent technique, the perovskite film is divided into two layers, i.e., a bottom layer with large grains and a thin capping layer with small grains. The addition of formamidinium chloride (FACl) into the precursor solution removes the small‐grained perovskite capping layer and suppresses the formation of bulk and surface defects, providing a perovskite film with enhanced crystallinity and large grain size of over 1 µm. Time‐resolved photoluminescence measurements show longer lifetimes for perovskite films modified by FACl and subsequently passivated by 1‐adamantylamine hydrochloride as compared to the reference sample. Impedance spectroscopy measurements show that these treatments reduce the recombination in the PSCs, leading to a champion device with power conversion efficiency (PCE) of 21.2%, an open circuit voltage of 1152 mV and negligible hysteresis. The Cl treated PSC also shows improved operational stability with only 12% PCE loss after 700 h under continuous illumination.     

Greener,Nonhalogenated Solvent Systems for Highly Efficient Perovskite Solar Cells

All current highest efficiency perovskite solar cells (PSCs) use highly toxic, halogenated solvents, such as chlorobenzene (CB) or toluene (TLN), in an antisolvent step or as solvent for the hole transporter material (HTM). A more environmentally friendly antisolvent is highly desirable for decreasing chronic health risk. Here, the efficacy of anisole (ANS), as a greener antisolvent for highest efficiency PSCs, is investigated. The fabrication inside and outside of the glovebox showing high power conversion efficiencies of 19.9% and 15.5%, respectively. Importantly, a fully nonhalogenated solvent system is demonstrated where ANS is used as both the antisolvent and the solvent for the HTM. With this, state‐of‐the‐art efficiencies close to 20.5%, the highest to date without using toxic CB or TLN, are reached. Through scanning electron microscopy, UV–vis, photoluminescence, and X‐ray diffraction, it is shown that ANS results in similar mixed‐ion perovskite films under glovebox atmosphere as CB and TLN. This underlines that ANS is indeed a viable green solvent system for PSCs and should urgently be adopted by labs and companies to avoid systematic health risks for researchers and employees.     

Copper Salts Doped Spiro‐OMeTAD for High‐Performance Perovskite Solar Cells

The development of effective and stable hole transporting materials (HTMs) is very important for achieving high‐performance planar perovskite solar cells (PSCs). Herein, copper salts (cuprous thiocyanate (CuSCN) or cuprous iodide (CuI)) doped 2,2,7,7‐tetrakis(N,N‐di‐p‐methoxyphenylamine)‐9,9‐spirobifluorene (spiro‐OMeTAD) based on a solution processing as the HTM in PSCs is demonstrated. The incorporation of CuSCN (or CuI) realizes a p‐type doping with efficient charge transfer complex, which results in improved film conductivity and hole mobility in spiro‐OMeTAD:CuSCN (or CuI) composite films. As a result, the PCE is largely improved from 14.82% to 18.02% due to obvious enhancements in the cell parameters of short‐circuit current density and fill factor. Besides the HTM role, the composite film can suppress the film aggregation and crystallization of spiro‐OMeTAD films with reduced pinholes and voids, which slows down the perovskite decomposition by avoiding the moisture infiltration to some extent. The finding in this work provides a simple method to improve the efficiency and stability of planar perovskite solar cells.     

Pb‐Reduced CsPb0.9Zn0.1I2Br Thin Films for Efficient Perovskite Solar Cells

Fabrication of efficient Pb reduced inorganic CsPbI₂Br perovskite solar cells (PSC) are an important part of environment‐friendly perovskite technology. In this work, 10% Pb reduction in CsPb_0.9Zn_0.1I₂Br promotes the efficiency of PSCs to 13.6% (AM1.5, 1sun), much higher than the 11.8% of the pure CsPbI₂Br solar cell. Zn²⁺ has stronger interaction with the anions to manipulate crystal growth, resulting in size‐enlarged crystallite with enhanced growth orientation. Moreover, the grain boundaries (GBs) are passivated by the Cs‐Zn‐I/Br compound. The high quality CsPb_0.9Zn_0.1I₂Br greatly diminishes the GB trap states and facilitates the charge transport. Furthermore, the Zn4s‐I5p states slightly reduce the energy bandgap, accounting for the wider solar spectrum absorption. Both the crystalline morphology and energy state change benefit the device performance. This work highlights a nontoxic and stable Pb reduction method to achieve efficient inorganic PSCs.     

Elucidating the Role of a Tetrafluoroborate‐Based Ionic Liquid at the n‐Type Oxide/Perovskite Interface

Halide perovskites are currently one of the most heavily researched emerging photovoltaic materials. Despite achieving remarkable power conversion efficiencies, perovskite solar cells have not yet achieved their full potential, with the interfaces between the perovskite and the charge‐selective layers being where most recombination losses occur. In this study, a fluorinated ionic liquid (IL) is employed to modify the perovskite/SnO₂ interface. Using Kelvin probe and photoelectron spectroscopy measurements, it is shown that depositing the perovskite onto an IL‐treated substrate results in the crystallization of a perovskite film which has a more n‐type character, evidenced by a decrease of the work function and a shift of the Fermi level toward the conduction band. Photoluminescence spectroscopy and time‐resolved microwave conductivity are used to investigate the optoelectronic properties of the perovskite grown on neat and IL‐modified surfaces and it is found that the modified substrate yields a perovskite film which exhibits an order of magnitude lower trap density than the control. When incorporated into solar cells, this interface modification results in a reduction in the current–voltage hysteresis and an improvement in device performance, with the best performing devices achieving steady‐state PCEs exceeding 20%.     

Inkjet‐Printed Micrometer‐Thick Perovskite Solar Cells with Large Columnar Grains

Transferring the high power conversion efficiencies (PCEs) of spin‐coated perovskite solar cells (PSCs) on the laboratory scale to large‐area photovoltaic modules requires a significant advance in scalable fabrication methods. Digital inkjet printing promises scalable, material, and cost‐efficient deposition of perovskite thin films on a wide range of substrates and in arbitrary shapes. In this work, high‐quality inkjet‐printed triple‐cation (methylammonium, formamidinium, and cesium) perovskite layers with exceptional thicknesses of >1 µm are demonstrated, enabling unprecedentedly high PCEs > 21% and stabilized power output efficiencies > 18% for inkjet‐printed PSCs. In‐depth characterization shows that the thick inkjet‐printed perovskite thin films deposited using the process developed herein exhibit a columnar crystal structure, free of horizontal grain boundaries, which extend over the entire thickness. A thin film thickness of around 1.5 µm is determined as optimal for PSC for this process. Up to this layer thickness X‐ray photoemission spectroscopy analysis confirms the expected stoichiometric perovskite composition at the surface and shows strong deviations and inhomogeneities for thicker thin films. The micrometer‐thick perovskite thin films exhibit remarkably long charge carrier lifetimes, highlighting their excellent optoelectronic characteristics. They are particularly promising for next‐generation inkjet‐printed perovskite solar cells, photodetectors, and X‐ray detectors.     

Spontaneously Self‐Assembly of a 2D/3D Heterostructure Enhances the Efficiency and Stability in Printed Perovskite Solar Cells

As perovskite solar cells (PSCs) are highly efficient, demonstration of high‐performance printed devices becomes important. 2D/3D heterostructures have recently emerged as an attractive way to relieving the film inhomogeneity and instability in perovskite devices. In this work, a 2D/3D ensemble with 2D perovskites self‐assembled atop 3D methylammonium lead triiodide (MAPbI₃) via a one‐step printing process is shown. A clean and flat interface is observed in the 2D/3D bilayer heterostructure for the first time. The 2D perovskite capping layer significantly suppresses nonradiative charge recombination, resulting in a marked increase in open‐circuit voltage (V_OC) of the devices by up to 100 mV. An ultrahigh V_OC of 1.20 V is achieved for MAPbI₃ PSCs, corresponding to 91% of the Shockley–Queisser limit. Moreover, notable enhancement in light, thermal, and moisture stability is obtained as a result of the protective barrier of the 2D perovskites. These results suggest a viable approach for scalable fabrication of highly efficient perovskite solar cells with enhanced environmental stability.