Fabrication of Efficient and Stable CsPbI3 Perovskite Solar Cells through Cation Exchange Process

Inorganic lead halide perovskites have attracted attention due to their tolerance to higher processing temperature and higher bandgap suitable for tandem solar cell application. Not only do they improve cell stability and efficiency, they also reveal many interesting and un‐anticipated material qualities. This work reports a simple cation exchange growth (CEG) method for fabricating inorganic high‐quality cesium lead iodide (CsPbI₃) by adding methylammonium iodide (MAI) additive in the precursor. X‐ray diffraction results reveal a multi‐stage film formation process whereby i) MAPbI₃ perovskite first formed that acts as a perovskite template for ii) subsequent ion exchange whereby the MA⁺ ions in the MAPbI₃ are replaced by Cs⁺ (as temperature ramps up) and iii) form g‐phase perovskite CsPbI₃. Optical microscopy, photoluminescence, and electrical characterizations reveal that the CEG process produces high‐quality film with better absorption, uniform and dense film with better interface, lower defects, and better stability. Using the CEG approach, the power conversion efficiency of the best CsPbI₃ solar cell is significantly increased up to 14.1% for the device fabricated using 1.0 m MAI additive. The outcome is beneficial for further improvement of inorganic perovskite solar cells and their application in perovskite‐silicon tandem devices.     

Compositional and Solvent Engineering in Dion–Jacobson 2D Perovskites Boosts Solar Cell Efficiency and Stability

Hybrid halide 2D perovskites deserve special attention because they exhibit superior environmental stability compared with their 3D analogs. The closer interlayer distance discovered in 2D Dion–Jacobson (DJ) type of halide perovskites relative to 2D Ruddlesden–Popper (RP) perovskites implies better carrier charge transport and superior performance in solar cells. Here, the structure and properties of 2D DJ perovskites employing 3‐(aminomethyl)piperidinium (3AMP²⁺) as the spacing cation and a mixture of methylammonium (MA⁺) and formamidinium (FA⁺) cations in the perovskite cages are presented. Using single‐crystal X‐ray crystallography, it is found that the mixed‐cation (3AMP)(MA_0.75FA_0.25)₃Pb₄I₁₃ perovskite has a narrower bandgap, less distorted inorganic framework, and larger Pb? I? Pb angles than the single‐cation (3AMP)(MA)₃Pb₄I₁₃. Furthermore, the (3AMP)(MA_0.75FA_0.25)₃Pb₄I₁₃ films made by a solvent‐engineering method with a small amount of hydriodic acid have a much better film morphology and crystalline quality and more preferred perpendicular orientation. As a result, the (3AMP)(MA_0.75FA_0.25)₃Pb₄I₁₃‐based solar cells exhibit a champion power conversion efficiency of 12.04% with a high fill factor of 81.04% and a 50% average efficiency improvement compared to the pristine (3AMP)(MA)₃Pb₄I₁₃ cells. Most importantly, the 2D DJ 3AMP‐based perovskite films and devices show better air and light stability than the 2D RP butylammonium‐based perovskites and their 3D analogs.     

Trap States,Electric Fields,and Phase Segregation in Mixed‐Halide Perovskite Photovoltaic Devices

Mixed‐halide perovskites are essential for use in all‐perovskite or perovskite–silicon tandem solar cells due to their tunable bandgap. However, trap states and halide segregation currently present the two main challenges for efficient mixed‐halide perovskite technologies. Here photoluminescence techniques are used to study trap states and halide segregation in full mixed‐halide perovskite photovoltaic devices. This work identifies three distinct defect species in the perovskite material: a charged, mobile defect that traps charge‐carriers in the perovskite, a charge‐neutral defect that induces halide segregation, and a charged, mobile defect that screens the perovskite from external electric fields. These three defects are proposed to be MA⁺ interstitials, crystal distortions, and halide vacancies and/or interstitials, respectively. Finally, external quantum efficiency measurements show that photoexcited charge‐carriers can be extracted from the iodide‐rich low‐bandgap regions of the phase‐segregated perovskite formed under illumination, suggesting the existence of charge‐carrier percolation pathways through grain boundaries where phase‐segregation may occur.     

Understanding the Role of Cesium and Rubidium Additives in Perovskite Solar Cells: Trap States,Charge Transport,and Recombination

Adding cesium (Cs) and rubidium (Rb) cations to FA_0.83MA_0.17Pb(I_0.83Br_0.17)₃ hybrid lead halide perovskites results in a remarkable improvement in solar cell performance, but the origin of the enhancement has not been fully understood yet. In this work, time‐of‐flight, time‐resolved microwave conductivity, and thermally stimulated current measurements are performed to elucidate the impact of the inorganic cation additives on the trap landscape and charge transport properties within perovskite solar cells. These complementary techniques allow for the assessment of both local features within the perovskite crystals and macroscopic properties of films and full devices. Strikingly, Cs‐incorporation is shown to reduce the trap density and charge recombination rates in the perovskite layer. This is consistent with the significant improvements in the open‐circuit voltage and fill factor of Cs‐containing devices. By comparison, Rb‐addition results in an increased charge carrier mobility, which is accompanied by a minor increase in device efficiency and reduced current–voltage hysteresis. By mixing Cs and Rb in quadruple cation (Cs‐Rb‐FA‐MA) perovskites, the advantages of both inorganic cations can be combined. This study provides valuable insights into the role of these additives in multiple‐cation perovskite solar cells, which are essential for the design of high‐performance devices.     

Highly Efficient and Stable Solar Cells with 2D MA3Bi2I9/3D MAPbI3 Heterostructured Perovskites

Organic–inorganic hybrid lead halide perovskites are emerging as highly promising candidates for highly efficient thin film photovoltaics due to their excellent optoelectronic properties and low‐temperature process capability. However, the long‐term stability in ambient air still is a key issue limiting their further practical applications. Herein, the enhancement of both performance and stability of perovskite solar cells is reported by employing 2D and 3D heterostructured perovskite films with unique nanoplate/nanocrystalline morphology. The 2D/3D heterostructured perovskites combine advantages of the high‐performance lead‐based perovskite 3D CH₃NH₃PbI₃ (MAPbI₃) and the air‐stable bismuth‐based quasi‐perovskite 2D MA₃Bi₂I₉. In the 2D/3D heterostructure, the hydrophobic MA₃Bi₂I₉ platelets vertically situate between the MAPbI₃ grains, forming a lattice‐like structure to tightly enclose the 3D MAPbI₃ perovskite grains. The solar cell based on the optimal 2D/3D (9.2%) heterostructured film achieves a high efficiency of 18.97%, with remarkably reduced hysteresis and significantly improved stability. The work demonstrates that construction of 2D/3D heterostructured films by hybridizing different species of perovskite materials is a feasible way to simultaneously enhance both efficiency and stability of perovskite solar cells.     

MACl‐Assisted Ge Doping of Pb‐Hybrid Perovskite: A Universal Route to Stabilize High Performance Perovskite Solar Cells

Interfacial engineering, grain boundary, and surface passivation in organic–inorganic hybrid perovskite solar cells (HyPSCs) are effective in achieving high performance and enhanced durability. Organic additives and inorganic doping are generally used to chemically modify the surface contacting charge transport layers, and/or grain boundaries so as to reduce the defect density. Here, a simple but tricky one‐step method to dope organic–inorganic hybrid perovskite with Ge for the first time is reported. Unlike Ge doping to all‐inorganic perovskites, application of GeI₂ in organic–inorganic perovskite precursors is challenging due to the extremely poor solubility of GeI₂ in hybrid perovskite ink, leading to failure in the formation of uniform films. However, it is found that addition of methylammonium chloride (MACl) into the precursor remarkably increases the solubility of GeI₂. This MACl‐assisted Ge doping of hybrid perovskites produces high‐quality crystalline film with its surface passivated with nonvolatile GeI₂ (GeO₂) and the volatile MACl additive also improves the uniformity of GeO₂ distribution in the perovskite films. The resulting Ge‐doped mixed cation and mixed halide perovskite films with composition FA_0.83MA_0.17Ge_0.03Pb_0.97(I_0.9Br_0.1)₃ show superior photoluminescence lifetime, power conversion efficiency above 22%, and greater stability toward illumination and humidity, outperforming photovoltaic properties of HyPSCs prepared without the Ge doping.     

Benzylamine‐Treated Wide‐Bandgap Perovskite with High Thermal‐Photostability and Photovoltaic Performance

Mixed iodide‐bromide organolead perovskites with a bandgap of 1.70–1.80 eV have great potential to boost the efficiency of current silicon solar cells by forming a perovskite‐silicon tandem structure. Yet, the stability of the perovskites under various application conditions, and in particular combined light and heat stress, is not well studied. Here, FA_0.15Cs_0.85Pb(I_0.73Br_0.27)₃, with an optical bandgap of ≈1.72 eV, is used as a model system to investigate the thermal‐photostability of wide‐bandgap mixed halide perovskites. It is found that the concerted effect of heat and light can induce both phase segregation and decomposition in a pristine perovskite film. On the other hand, through a postdeposition film treatment with benzylamine (BA) molecules, the highly defective regions (e.g., film surface and grain boundaries) of the film can be well passivated, thus preventing the progression of decomposition or phase segregation in the film. Besides the stability improvement, the BA‐modified perovskite solar cells also exhibit excellent photovoltaic performance, with the champion device reaching a power conversion efficiency of 18.1%, a stabilized power output efficiency of 17.1% and an open‐circuit voltage (V _oc) of 1.24 V.     

Metal Halide Perovskites for Solar‐to‐Chemical Fuel Conversion

This review article presents and discusses the recent progress made in the stabilization, protection, improvement, and design of halide perovskite‐based photocatalysts, photoelectrodes, and devices for solar‐to‐chemical fuel conversion. With the target of water splitting, hydrogen iodide splitting, and CO₂ reduction reactions, the strategies established for halide perovskites used in photocatalytic particle‐suspension systems, photoelectrode thin‐film systems, and photovoltaic‐(photo)electrocatalysis tandem systems are organized and introduced. Moreover, recent achievements in discovering new and stable halide perovskite materials, developing protective and functional shells and layers, designing proper reaction solution systems, and tandem device configurations are emphasized and discussed. Perspectives on the future design of halide perovskite materials and devices for solar‐to‐chemical fuel conversion are provided. This review may serve as a guide for researchers interested in utilizing halide perovskite materials for solar‐to‐chemical fuel conversion.     

Rapid Aqueous Spray Fabrication of Robust NiOx: A Simple and Scalable Platform for Efficient Perovskite Solar Cells

Organometal halide perovskites have powerful intrinsic potential to drive next‐generation solar technology, but their insufficient thermomechanical reliability and unproven large‐area manufacturability limit competition with incumbent silicon photovoltaics. This work addresses these limitations by leveraging large‐area processing and robust inorganic hole transport layers (HTLs). Inverted perovskite solar cells utilizing NiO_x HTLs deposited by rapid aqueous spray‐coating that outperform spin‐coated NiO_x and lead to a 5× improvement in the fracture energy (G_c), a primary metric of thermomechanical stability, are presented. The morphology, chemical composition, and optoelectronic properties of the NiO_x films are characterized to understand and optimize compatibility with an archetypal double cation perovskite, Cs_.17FA_.83Pb(Br_.17I_.83)₃. Perovskite solar cells with sprayed NiO_x show higher photovoltaic performance, exhibiting up to 82% fill factor and 17.7% power conversion efficiency (PCE)—the highest PCE reported for inverted cell with scalable charge transport layers—as well as excellent stability under full illumination and after 4000 h aging in inert conditions at room temperature. By utilizing open‐air techniques and aqueous precursors, this combination of robust materials and low‐cost processing provides a platform for scaling perovskite modules with long‐term reliability.     

The Spacer Cations Interplay for Efficient and Stable Layered 2D Perovskite Solar Cells

Organic spacer cations in layered 2D (A₁)₂(A₂)_n?1B_nX_3n+1 (where A₁ is an organic cation acting as a spacer between the perovskite layers, A₂ is a monovalent cation, e.g., Cs⁺,CH₃NH₃⁺, CH(NH₂)₂⁺) perovskite materials improve the long‐term stability of the resulting solar cells, but hamper their power conversion efficiency due to poor carrier generation/transportation. Rational guidelines are thus required to enable the design of organic spacer cations. Herein, mixed A₁ cations are employed in layered 2D perovskites to investigate the interplay between alkylamine cations and unsaturated alkylamine cations. It is revealed that alkylamine spacer cations are able to facilitate precursor assembly, which results in the orientated growth of perovskite crystals. Unsaturated alkylamine cations further lead to reduced exciton binding energy, which improves carrier pathway in the 2D perovskites. By mixing both cations, substantially improved open circuit voltage is observed in the resultant photovoltaic cells with the efficiency of 15.46%, one of the highest one based on (A₁)₂(A₂)₃Pb₄I₁₃ layered 2D perovskites. The generality of the design principle is further extended to other cation combinations.     

Correlating Phase Behavior with Photophysical Properties in Mixed‐Cation Mixed‐Halide Perovskite Thin Films

Mixed cation perovskites currently achieve very promising efficiency and operational stability when used as the active semiconductor in thin‐film photovoltaic devices. However, an in‐depth understanding of the structural and photophysical properties that drive this enhanced performance is still lacking. Here the prototypical mixed‐cation mixed‐halide perovskite (FAPbI₃)_0.85(MAPbBr₃)_0.15 is explored, and temperature‐dependent X‐ray diffraction measurements that are correlated with steady state and time‐resolved photoluminescence data are presented. The measurements indicate that this material adopts a pseudocubic perovskite α phase at room temperature, with a transition to a pseudotetragonal β phase occurring at ≈260 K. It is found that the temperature dependence of the radiative recombination rates correlates with temperature‐dependent changes in the structural configuration, and observed phase transitions also mark changes in the gradient of the optical bandgap. The work illustrates that temperature‐dependent changes in the perovskite crystal structure alter the charge carrier recombination processes and photoluminescence properties within such hybrid organic–inorganic materials. The findings have significant implications for photovoltaic performance at different operating temperatures, as well as providing new insight on the effect of alloying cations and halides on the phase behavior of hybrid perovskite materials.     

Triggering the Passivation Effect of Potassium Doping in Mixed‐Cation Mixed‐Halide Perovskite by Light Illumination

Potassium (K⁺) doping has been recently discovered as an effective route to suppress hysteresis and improve the performance stability of perovskite solar cells. However, the mechanism of these K⁺ doping effects is still under debate, and rationalization of the improved performance in these perovskites is needed. Herein, the photoluminescence (PL) properties and device performance of mixed‐cation mixed‐halide perovskite are dynamically monitored with and without K⁺ doping under bias light illumination via a confocal fluorescence microscope, together with ultrafast transient absorption as well as time‐dependent and time‐resolved PL measurements. It is demonstrated that illumination is essential to trigger the passivation effect of K⁺ by forming KBr‐like compounds, leading to the elimination of interface trapping defects and suppression of mobile ion migration, thus resulting in improved power conversion efficiency and negligible current–voltage hysteresis of solar cells. This work provides novel insight into the hysteresis suppression upon K⁺ doping and highlights the significance of light illumination when using this protocol.     

Progress in Theoretical Study of Metal Halide Perovskite Solar Cell Materials

Lead halide perovskites have recently emerged as promising absorbers for fabricating low‐cost and high‐efficiency thin‐film solar cells. The record power conversion efficiency of lead halide perovskite‐based solar cells has rapidly increased from 3.8% in 2009 to 22.1% in early 2016. Such rapid improvement is attributed to the superior and unique photovoltaic properties of lead halide perovskites, such as the extremely high optical absorption coefficients and super‐long photogenerated carrier lifetimes and diffusion lengths that are not seen in any other polycrystalline thin‐film solar cell materials. In the past a few years, theoretical approaches have been extensively applied to understand the fundamental mechanisms responsible for the superior photovoltaic properties of lead halide perovskites and have gained significant insights. This review article highlights the important theoretical results reported in literature for the understanding of the unique structural, electronic, optical, and defect properties of lead halide perovskite materials. For comparison, we also review the theoretical results reported in literature for some lead‐free perovskites, double perovskites, and nonperovskites.     

Thermodynamically Self‐Healing 1D–3D Hybrid Perovskite Solar Cells

Thermal degradation in perovskite solar cells is still an unsettled issue that limits its further development. In this study, 2‐(1H‐pyrazol‐1‐yl)pyridine is introduced into lead halide 3D perovskites, which allows 1D–3D hybrid perovskite materials to be obtained. The heterostructural 1D–3D perovskites are proved to be capable of remarkably prolonging the photoluminescence decay lifetime and suppressing charge carrier recombination in comparison to conventional 3D perovskites. The intrinsic properties of thermodynamically stable yet kinetically labile 1D materials allow the system to alleviate the lattice mismatch and passivate the interface traps of heterojunction region of 1D–3D hybrid perovskites that may occur during the crystal growth process. Importantly, the as‐fabricated 1D–3D perovskite solar cells display a thermodynamic self‐healing ability, which is induced through blocking the ion‐migration channels of A‐site ions by the flexible 1D perovskite with less densely close‐packed structure. Particularly, the power conversion efficiency of as‐fabricated unencapsulated 1D–3D perovskite solar cells is demonstrated to be reversible under temperature cycling (25–85 °C) at 55% relative humidity, which largely outperforms the pure 3D perovskite solar cell. The present study provides a facile approach to fabricate 1D–3D perovskite solar cells with high efficiency and long‐term stability.     

Ionic Reactivity at Contacts and Aging of Methylammonium Lead Triiodide Perovskite Solar Cells

Hybrid lead halide perovskites have reached very large solar to electricity power conversion efficiencies, in some cases exceeding 20%. The most extensively used perovskite‐based solar cell configuration comprises CH₃NH₃PbI₃ (MAPbI₃) in combination with electron (TiO₂) and hole 2,2′,7,7′‐tetrakis(N,N‐di‐p‐methoxyphenylamine)‐9,9‐spiro‐bifluorene (spiro‐OMeTAD) selective contacts. The recognition that the solar cell performance is heavily affected by time scale of the measurement and preconditioning procedures has raised many concerns about the stability of the device and reliability for long‐time operation. Mechanisms at contacts originate observable current–voltage distortions. Two types of reactivity sources have been identified here: (i) weak Ti–I–Pb bonds that facilitate interfacial accommodation of moving iodine ions. This interaction produces a highly reversible capacitive current originated at the TiO₂/MAPbI₃ interface, and it does not alter steady‐state photovoltaic features. (ii) An irreversible redox peak only observable after positive poling at slow scan rates. It corresponds to the chemical reaction between spiro‐OMeTAD⁺ and migrating I^? which progressively reduces the hole transporting material conductivity and deteriorates solar cell performance.     

A Review and Perspective on Cathodoluminescence Analysis of Halide Perovskites

Halide perovskite solar cells have achieved a certified efficiency of 25.2%, surpassing CdTe and CuInGaSe₂, which have long been regarded as the most‐efficient thin‐film photovoltaic materials. As this exciting class of materials continues to mature, researchers will require characterization techniques capable of exposing the interplay among structure, chemistry, and optoelectronic properties to inform processing strategies and increase both device efficiencies and long‐term stability. Cathodoluminescence microscopy is an ideal technique to provide such information due to the high spatial resolution and robust optical information acquired. Here, the current body of work related to cathodoluminescence analysis of halide perovskite materials for optoelectronic applications is surveyed. This review demonstrates how cathodoluminescence can monitor degradation due to environmental stressors, phase segregation resulting from material processing, and other halide perovskite‐centric material issues. A persistent concern associated with e‐beam‐based analysis of halide perovskites is what effect the electron beam has on the material properties being probed. Addressing this, a detailed discussion is provided on the origin of the cathodoluminescence signal and a review of studies focused on revealing changes in the properties of halide perovskites resulting from e‐beam excitation. Finally, a perspective on future opportunities to expand the role of cathodoluminescence analysis for halide perovskites is provided.     

Interfacing Pristine C60 onto TiO2 for Viable Flexibility in Perovskite Solar Cells by a Low‐Temperature All‐Solution Process

A low‐temperature solution‐processed strategy is critical for cost‐effective manufacture of flexible perovskite solar cells (PSCs). Based on an aqueous‐processed TiO₂ layer, and conventional fullerene derivatives replaced by a pristine fullerene interlayer of C₆₀, herein a facile interface engineering for making all‐solution‐processed TiO₂/C₆₀ layers in flexible n‐i‐p PSCs is reported. Due to the improvement of the perovskite grain quality, promotion of interfacial charge transfer and suppression of interfacial charge recombination, the stabilized power conversion efficiency for the flexible PSCs reaches as high as 16% with high bending resistance retention (≈80% after 1500 cycles) and high light‐soaking retention (≈100% after 100 min). In addition, the stabilized efficiency is over 19% for the rigid TiO₂/C₆₀‐based PSCs. The present work with the facile low‐temperature solution process renders the practicability for high‐performance flexible PSCs applied to wearable devices, portable equipment, and electric vehicles.     

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.     

Exploring the Stability of Novel Wide Bandgap Perovskites by a Robot Based High Throughput Approach

Currently, lead‐based perovskites with mixed multiple cations and hybrid halides are attracting intense research interests due to their promising stability and high efficiency. A tremendous amount of 3D and 2D perovskite compositions and configurations are causing a strong demand for high throughput synthesis and characterization. Furthermore, wide bandgap (≈1.75 eV) perovskites as promising top‐cell materials for perovskite–silicon tandem configurations require the screening of different compositions to overcome photoinduced halide segregation and still yielding a high open‐circuit voltage (V_oc). Herein, a home‐made high throughput robot setup is introduced performing automatic perovskite synthesis and characterization. Subsequently, four kinds of compositions (i.e., cation mixtures of Cs–methylammonium (MA), Cs– formamidinium (FA), MA–FA, and FA–MA) with an optical bandgap of ≈1.75 eV are identified as promising device candidates. For Cs–MA and Cs–FA films it is found that the Br–I phase segregation indeed can be overcome. Moreover, Cs–MA, MA–FA, and Cs–FA based devices exhibit an average V_oc of 1.17, 1.17, 1.12 V, and their maximum values approached 1.18, 1.19, 1.14 V, respectively, which are among the highest V_oc (≈1.2 V) values for ≈40% Br perovskite. These findings highlight that the high throughput approach can effectively and efficiently accelerate the invention of novel perovskites for advanced applications.     

Enhanced Efficiency of Hot‐Cast Large‐Area Planar Perovskite Solar Cells/Modules Having Controlled Chloride Incorporation

Organic–inorganic perovskite photovoltaics are an emerging solar technology. Developing materials and processing techniques that can be implemented in large‐scale manufacturing is extremely important for realizing the potential of commercialization. Here we report a hot‐casting process with controlled Cl^? incorporation which enables high stability and high power‐conversion‐efficiencies (PCEs) of 18.2% for small area (0.09 cm²) and 15.4% for large‐area (≈1 cm²) single solar cells. The enhanced performance versus tri‐iodide perovskites can be ascribed to longer carrier diffusion lengths, improved uniformity of the perovskite film morphology, favorable perovskite crystallite orientation, a halide concentration gradient in the perovskite film, and reduced recombination by introducing Cl^?. Additionally, Cl^? improves the device stability by passivating the reaction between I^? and the silver electrode. High‐quality thin films deployed over a large‐area 5 cm × 5 cm eight‐cell module have been fabricated and exhibit an active‐area PCE of 12.0%. The feasibility of material and processing strategies in industrial large‐scale coating techniques is then shown by demonstrating a “dip‐coating” process which shows promise for large throughput production of perovskite solar modules.