Room‐Temperature Partial Conversion of α‐FAPbI3 Perovskite Phase via PbI2 Solvation Enables High‐Performance Solar Cells

The two‐step conversion process consisting of metal halide deposition followed by conversion to hybrid perovskite has been successfully applied toward producing high‐quality solar cells of the archetypal MAPbI₃ hybrid perovskite, but the conversion of other halide perovskites, such as the lower bandgap FAPbI₃, is more challenging and tends to be hampered by the formation of hexagonal nonperovskite polymorph of FAPbI₃, requiring Cs addition and/or extensive thermal annealing. Here, an efficient room‐temperature conversion route of PbI₂ into the α‐FAPbI₃ perovskite phase without the use of cesium is demonstrated. Using in situ grazing incidence wide‐angle X‐ray scattering (GIWAXS) and quartz crystal microbalance with dissipation (QCM‐D), the conversion behaviors of the PbI₂ precursor from its different states are compared. α‐FAPbI₃ forms spontaneously and efficiently at room temperature from P₂ (ordered solvated polymorphs with DMF) without hexagonal phase formation and leads to complete conversion after thermal annealing. The average power conversion efficiency (PCE) of the fabricated solar cells is greatly improved from 16.0(±0.32)% (conversion from annealed PbI₂) to 17.23(±0.28)% (from solvated PbI₂) with a champion device PCE > 18% due to reduction of carrier recombination rate. This work provides new design rules toward the room‐temperature phase transformation and processing of hybrid perovskite films based on FA⁺ cation without the need for Cs⁺ or mixed halide formulation.     

Oriented Crystallization of Mixed‐Cation Tin Halides for Highly Efficient and Stable Lead‐Free Perovskite Solar Cells

As the most promising lead‐free branch, tin halide perovskites suffer from the severe oxidation from Sn²⁺ to Sn⁴⁺, which results in the unsatisfactory conversion efficiency far from what they deserve. In this work, by facile incorporation of methylammonium bromide in composition engineering, formamidinium and methylammonium mixed cations tin halide perovskite films with ultrahighly oriented crystallization are synthesized with the preferential facet of (001), and that oxidation is suppressed with obviously declined trap density. MA⁺ ions are responsible for that impressive orientation while Br^‐ ions account for their bandgap modulation. Depending on high quality of the optimal MA_0.25FA_0.75SnI_2.75Br_0.25 perovskite films, their device conversion efficiency surges to 9.31% in contrast to 5.02% of the control formamidinium tin triiodide perovskite (FASnI₃) device, along with almost eliminated hysteresis. That also results in the outstanding device stability, maintaining above 80% of the initial efficiency after 300 h of light soaking while the control FASnI₃ device fails within 120 h. This paper definitely paves a facile and effective way to develop high‐efficiency tin halide perovskites solar cells, optoelectronic devices, and beyond.     

HPbI3: A New Precursor Compound for Highly Efficient Solution‐Processed Perovskite Solar Cells

Recently, there have been extensive research efforts on developing high performance organolead halide based perovskite solar cells. While most studies focused on optimizing the deposition processes of the perovskite films, the selection of the precursors has been rather limited to the lead halide/methylammonium (or formamidium) halide combination. In this work, we developed a new precursor, HPbI₃, to replace lead halide. The new precursor enables formation of highly uniform formamidium lead iodide (FAPbI₃) films through a one‐step spin‐coating process. Furthermore, the FAPbI₃ perovskite films exhibit a highly crystalline phase with strong (110) preferred orientation and excellent thermal stability. The planar heterojunction solar cells based on these perovskite films exhibit an average efficiency of 15.4% and champion efficiency of 17.5% under AM 1.5 G illumination. By comparing the morphology and formation process of the perovskite films fabricated from the formamidium iodide (FAI)/HPbI₃, FAI/PbI₂, and FAI/PbI₂ with HI additive precursor combinations, it is shown that the superior property of the HPbI₃ based perovskite films may originate from 1) a slow crystallization process involving exchange of H⁺ and FA⁺ ions in the PbI₆ octahedral framework and 2) elimination of water in the precursor solution state.     

All‐Inorganic Bismuth‐Based Perovskite Quantum Dots with Bright Blue Photoluminescence and Excellent Stability

Lead halide perovskite quantum dots (QDs) possess color‐tunable and narrow‐band emissions and are very promising for lighting and display applications, but they suffer from lead toxicity and instability. Although lead‐free Bi‐based and Sn‐based perovskite QDs (CsSnX₃, Cs₂SnX₆, and (CH₃NH₃)₃Bi₂X₉) are reported, they all show low photoluminescence quantum yield (PLQY) and poor stability. Here, the synthesis of Cs₃Bi₂Br₉ perovskite QDs with high PLQY and excellent stability is reported. Via a green and facile process using ethanol as the antisolvent, as‐synthesized Cs₃Bi₂Br₉ QDs show a blue emission at 410 nm with a PLQY up to 19.4%. The whole series of Cs₃Bi₂X₉ (X = Cl, Br, and I) QDs by mixing precursors can cover the photoluminescence emission range from 393 to 545 nm. Furthermore, Cs₃Bi₂Br₉ QDs show excellent photostability and moisture stability due to the all‐inorganic nature and the surface passivation by BiOBr, which enables the one‐pot synthesis of Cs₃Bi₂Br₉ QD/silica composite. A lead‐free perovskite white light‐emitting diode is fabricated by simply combining the composite of Cs₃Bi₂Br₉ QD/silica with Y₃Al₅O₁₂ phosphor. As a new member of lead‐free perovskite QDs, Cs₃Bi₂Br₉ QDs open up a new route for the fabrication of optoelectronic devices due to their excellent stability and photophysical characteristics.     

Combination of Hybrid CVD and Cation Exchange for Upscaling Cs‐Substituted Mixed Cation Perovskite Solar Cells with High Efficiency and Stability

Mixed cation hybrid perovskites such as Cs_xFA_1?xPbI₃ are promising materials for solar cell applications, due to their excellent photoelectronic properties and improved stability. Although power conversion efficiencies (PCEs) as high as 18.16% have been reported, devices are mostly processed by the anti‐solvent method, which is difficult for further scaling‐up. Here, a method to fabricate Cs_xFA_1?xPbI₃ by performing Cs cation exchange on hybrid chemical vapor deposition grown FAPbI₃ with the Cs⁺ ratio adjustable from 0 to 24% is reported. The champion perovskite module based on Cs_0.07FA_0.93PbI₃ with an active area of 12.0 cm² shows a module PCE of 14.6% and PCE loss/area of 0.17% cm^?2, demonstrating the significant advantage of this method toward scaling‐up. This in‐depth study shows that when the perovskite films prepared by this method contain 6.6% Cs⁺ in bulk and 15.0% at the surface, that is, Cs_0.07FA_0.93PbI₃, solar cell devices show not only significantly increased PCEs but also substantially improved stability, due to favorable energy level alignment with TiO₂ electron transport layer and spiro‐MeOTAD hole transport layer, increased grain size, and improved perovskite phase stability.     

A Dual‐Retarded Reaction Processed Mixed‐Cation Perovskite Layer for High‐Efficiency Solar Cells

Mixed‐cation perovskite solar cells (PSCs) have become of enormous interest because of their excellent efficiency, which is now crossing 23.7%. Their broader absorption, relatively high stability with low fabrication cost compared to conventional single phase ABX₃ perovskites (where A: organic cation; B: divalent metal ion; and X: halide anion) are key properties of mixed‐halide mixed‐cation perovskites. However, the controlling reaction rate and formation of extremely dense, textured, smooth, and large grains of perovskite layer is a crucial task in order to achieve highly efficient PSCs. Herein, a new simple dual‐retarded reaction processing (DRP) method is developed to synthesize a high‐quality mixed‐cation (FAPbI₃)_0.85(MAPbBr₃)_0.15 (where MAPbBr₃ stands for methylammonium lead bromide and FAPbI₃ stands for formamidinium lead iodide) perovskite thin film via intermediate phase and incorporation of nitrogen‐doped reduced graphene oxide (N‐rGO). The reaction rate is retarded via two steps: first the formation of intermediate phase and second the interaction of the nitrogen groups on N‐rGO with hydrogen atoms from formamidinium cations. This DRP process allows for the fabrication of PSCs with maximum conversion efficiency higher than 20.3%.     

Air‐Stable Highly Crystalline Formamidinium Perovskite 1D Structures for Ultrasensitive Photodetectors

State‐of‐the‐art optoelectronic devices based on metal‐halide perovskites demand solution‐processed structures with high crystallinity, controlled crystallographic orientation, and enhanced environmental stability. Formamidinium lead iodide (α‐FAPbI₃) possesses a suitable bandgap of 1.48 eV and enhanced thermal stability, whereas perovskite‐type polymorph (α‐phase) is thermodynamically instable at ambient temperatures. Stable α‐FAPbI₃ perovskite 1D structure arrays with high crystallinity and ordered crystallographic orientation are developed by controlled nucleation and growth in capillary bridges. By surface functionalization with phenylethylammonium ions (PEA⁺), FAPbI₃ wires sustain a stable α‐phase after 28 day storage in the ambient environment with a relative humidity of 50%. Enhanced photoluminescence (PL) intensity and elongated PL lifetime demonstrate suppressed trap density and high crystallinity in these 1D structures, which is also reflected by the enhanced diffraction density and pure (001) crystallographic orientation in the grazing‐incidence wide‐angle X‐ray scattering (GIWAXS) pattern. Based on these high‐quality 1D structures, sensitive photodetectors are achieved with average responsivities of 5282 A W^?1, average specific detectivities of more than 1.45 × 10¹⁴ Jones, and a fast response speed with a 3 dB bandwidth of 15 kHz.     

Introduction of Hydrophobic Ammonium Salts with Halogen Functional Groups for High‐Efficiency and Stable 2D/3D Perovskite Solar Cells

2D perovskites have attracted extensive attention due to their excellent stability compared with 3D perovskites. However, the intrinsic hydrophilicity of introduced alkylammonium salts effects the humidity stability of 2D/3D perovskites. Devices based on longer chain alkylammonium salts show improvement in hydrophobicity but lower efficiency due to the poorer charge transport among various layers. To solve this issue, two hydrophobic short‐chain alkylammonium salts with halogen functional groups (2‐chloroethylamine, CEA⁺ and 2‐bromoethylamine, BEA⁺) are introduced into (Cs_0.1FA_0.9)Pb(I_0.9Br_0.1)₃ 3D perovskites to form 2D/3D perovskite structure, which achieve high‐quality perovskite films with better crystallization and morphology. The optimal 2D/3D perovskite solar cells (PSCs) with 5% CEA⁺ display a power conversion efficiency (PCE) as high as 20.08% under 1 sun irradiation. Because of the notable hydrophobicity of alkylammonium cations with halogen functional groups and the formed 2D/3D perovskite structure, the optimal PSCs exhibit superior moisture resistance and retain 92% initial PCE after aging at 50 ± 5% relative humidity for 2400 h. This work opens up a new direction for the design of new‐type 2D/3D PSCs with improved performance by employing proper alkylammonium salts with different functional groups.     

An Interdiffusion Method for Highly Performing Cesium/Formamidinium Double Cation Perovskites

The fabrication of high‐quality cesium (Cs)/formamidinium (FA) double‐cation perovskite films through a two‐step interdiffusion method is reported. Cs_x FA_1‐x PbI_{3‐y(1‐x )}Br_{y(1‐x )} films with different compositions are achieved by controlling the amount of CsI and formamidinium bromide (FABr) in the respective precursor solutions. The effects of incorporating Cs⁺ and Br^? on the properties of the resulting perovskite films and on the performance of the corresponding perovskite solar cells are systematically studied. Small area perovskite solar cells with a power conversion efficiency (PCE) of 19.3% and a perovskite module (4 cm²) with an aperture PCE of 16.4%, using the Cs/FA double cation perovskite made with 10 mol% CsI and 15 mol% FABr (Cs_0.1FA_0.9PbI_2.865Br_0.135) are achieved. The Cs/FA double cation perovskites show negligible degradation after annealing at 85 °C for 336 h, outperforming the perovskite materials containing methylammonium (MA).     

Triamine‐Based Aromatic Cation as a Novel Stabilizer for Efficient Perovskite Solar Cells

Operational stability of perovskite solar cells has been a challenge from the beginning of perovskite research. In general, humidity and heat are the most well‐known degradation sources for perovskites, requiring ideal design of perovskite chemistry to withstand them. Although triple‐cation perovskite (Cs_0.05(FA_0.85MA_0.15)_0.95Pb(I_0.85Br_0.15)₃) has been already introduced as the stable perovskite material, the high reactivity of methylammonium and formamidinium in the cation sites demands further modification. Herein, 1,2,4‐triazole is suggested as an effective cation solute to improve the performance and stability of perovskite solar cells. 1,2,4‐Triazole is an aromatic cation with low dipole moment that is stable under humidity and heat. It also possesses three nitrogen atoms, forming additional hydrogen bonds in the lattice, stabilizing the material. In this study, the solar cell utilizing 1,2,4‐triazole alloying achieves a power conversion efficiency of 20.9% with superior stability under extreme condition (85 °C/85% of relative humidity (RH), encapsulated) for 700 h. The 1,2,4‐triazole‐alloyed perovskite exhibits reduced trap density and film roughness and enhanced carrier lifetime with electrical conductivity, suggesting an ideal perovskite structure for efficient and stable optoelectronic applications.     

Mixed Halide Formation in Lead-Free Antimony-Based Halide Perovskite for Boosted CO2 Photoreduction: Beyond Band Gap Tuning

Photocatalytic conversion of carbon dioxide (CO₂) into value-added fuels is a vastly promising anthropogenic chemical carbon cycle to combat the greenhouse effect while meeting the ever-increasing energy demand. Recently, lead-based halide perovskites have demonstrated great potential in various applications including photochemical reduction of CO₂. However, in view of lead toxicity, the exploration of a lead-free alternative is crucial for long term application. Herein, a series of lead-free mixed halide perovskites Cs₃Sb₂Cl_xBr_9−x (0 ≤ x ≤ 9) is prepared via a facile antisolvent recrystallization technique, where the incorporation of a secondary halide enhances the charge transfer and separation while allowing precise tuning of bandgap between 2.59 and 2.90 eV. Theoretical calculations further reveal that the formation of mixed Cl/Br halides engenders favorable charge redistribution due to lower octahedral distortion, which in turn strengthens CO₂ adsorption and activation. Under visible light illumination, the optimal dual halide perovskite, Cs₃Sb₂Cl₄Br₅ manifests substantial twofold and fourfold enhancements of CH₄ yield over the single halide perovskite, Cs₃Sb₂Br₉ and Cs₃Sb₂Cl₉, respectively. In brief, this study provides a compelling demonstration of lead-free mixed halide perovskites for photocatalytic CO₂ reduction, and it is anticipated to drive further application of perovskite-based photocatalysts toward a diverse range of artificial photoredox reactions.     

Impact of Processing on Structural and Compositional Evolution in Mixed Metal Halide Perovskites during Film Formation

Understanding crystallization processes and their pathways in metal‐halide perovskites is of crucial importance as this strongly affects the film microstructure, its stability, and device performance. While many approaches are developed to control perovskite formation, the mechanisms of film formation are still poorly known. Using time‐resolved in situ grazing incidence wide‐angle X‐ray scattering, the film formation of perovskites is investigated with average stoichiometry Cs_0.15FA_0.85PbI₃, where FA is formamidinium, using the popular antisolvent dropping and gas jet treatments and this is contrasted with untreated films. i) The crystallization pathways during spin coating, ii) the subsequent postdeposition thermal annealing, and iii) crystallization during blade coating are studied. The findings reveal that the formation of a nonperovskite FAPbI₃ phase during spin coating is initially dominant regardless of the processing and that the processing treatment (e.g., antisolvent dropping, gas jet) has a significant impact on the as‐cast film structure and affects the phase evolution during subsequent thermal treatment. It is shown that blade coating can be used to overcome the nonperovskite phase formation via solvothermal direct crystallization of perovskite phase. This work shows how real‐time investigation of perovskite formation can help to establish processing–microstructure–functionality relationships.     

Thermochromic Lead‐Free Halide Double Perovskites

Lead‐free halide double perovskites with diverse electronic structures and optical responses, as well as superior material stability show great promise for a range of optoelectronic applications. However, their large bandgaps limit their applications in the visible light range such as solar cells. In this work, an efficient temperature‐derived bandgap modulation, that is, an exotic fully reversible thermochromism in both single crystals and thin films of Cs₂AgBiBr₆ double perovskites is demonstrated. Along with the thermochromism, temperature‐dependent changes in the bond lengths of Ag? Br (R_{Ag? Br}) and Bi? Br (R_{Bi? Br}) are observed. The first‐principle molecular dynamics simulations reveal substantial anharmonic fluctuations of the R_{Ag? Br} and R_{Bi? Br} at high temperatures. The synergy of anharmonic fluctuations and associated electron–phonon coupling, and the peculiar spin–orbit coupling effect, is responsible for the thermochromism. In addition, the intrinsic bandgap of Cs₂AgBiBr₆ shows negligible changes after repeated heating/cooling cycles under ambient conditions, indicating excellent thermal and environmental stability. This work demonstrates a stable thermochromic lead‐free double perovskite that has great potential in the applications of smart windows and temperature sensors. Moreover, the findings on the structure modulation‐induced bandgap narrowing of Cs₂AgBiBr₆ provide new insights for the further development of optoelectronic devices based on the lead‐free halide double perovskites.     

FAPbI3‐Based Perovskite Solar Cells Employing Hexyl‐Based Ionic Liquid with an Efficiency Over 20% and Excellent Long‐Term Stability

Formamidinium lead triiodide (FAPbI₃)‐based perovskite materials are of interest for photovoltaics in view of their close‐to‐ideal bandgap, allowing absorption of photons over a broad solar spectrum. However, FAPbI₃‐based materials suffer from a notorious phase transition from the photoactive black phase (α‐FAPbI₃) to nonperovskite yellow phase (δ‐FAPbI₃) under ambient conditions. This transition dramatically reduces light absorbtion, thus, degrading the photovoltaic performance and stability of ensuring solar cells. In this study, 1‐hexyl‐3‐methylimidazolium iodide (HMII) ionic liquid (IL) is employed as an additive for the first time in FAPbI₃ perovskite to overcome the above‐mentioned issues. HMII incorporation facilitates the grain coarsening of FAPbI₃ crystal owing to its high‐polarity and high‐boiling point, which yields liquid domains between neighboring grains to reduce the activation energy of the grain‐boundary migration. As a result, the FAPbI₃ active layer exhibits micron‐sized grains with substantially suppressed parasitic traps with an Urbach energy reduced by 2 meV. Hence, the resulting perovskite solar cell achieves an efficiency of 20.6% with notable increase in open circuit voltage (V_OC) of 80 mV compared with HMII‐free cells (17.1%). More importantly, the HMII‐doped FAPbI₃‐based cells show a striking enhancement in shelf‐stability under high humidity and thermal stress, retaining >80% of their initial efficiencies at 60 ± 10% relative humidity and ≈95% at 65 °C.     

Lead‐Free Cs2BiAgBr6 Double Perovskite‐Based Humidity Sensor with Superfast Recovery Time

Lead halide perovskites have demonstrated outstanding achievements in photoelectric applications owing to their unique properties. However, the moisture sensitivity of lead halide perovskite has rarely been developed into an applicable humidity sensor due to the intrinsic instability and toxicity issue. Herein, as a highly stable lead‐free perovskite, a Cs₂BiAgBr₆ thin film is chosen to be the active material for humidity sensor due to its extraordinary humidity‐dependent electrical properties and good stability. This Cs₂BiAgBr₆ thin film humidity sensor demonstrates a superfast response time (1.78 s) and recovery time (0.45 s). The superfast response and recovery properties can be attributed to the reversible physisorption of water molecules, which can be easily adsorbed onto or desorbed from the thin film surface. Moreover, the sensor also shows an excellent reliability and stability properties as well as logarithmic linearity in a relative humidity's range of 15% to 78%. The lead‐free Cs₂BiAgBr₆ perovskite possesses great potential for application in real‐time humidity sensing.     

Octadecylamine‐Functionalized Single‐Walled Carbon Nanotubes for Facilitating the Formation of a Monolithic Perovskite Layer and Stable Solar Cells

Organic–inorganic lead halide perovskites have shown great future for application in solar cells owing to their exceptional optical and electronic properties. To achieve high‐performance perovskite solar cells, a perovskite light absorbing layer with large grains is desirable in order to minimize grain boundaries and recombination during the operation of the device. Herein, a simple yet efficient approach is developed to synthesize perovskite films consisting of monolithic‐like grains with micrometer size through in situ deposition of octadecylamine functionalized single‐walled carbon nanotubes (ODA‐SWCNTs) onto the surface of the perovskite layer. The ODA‐SWCNTs form a capping layer that controls the evaporation rate of organic solvents in the perovskite film during the postthermal treatment. This favorable morphology in turn dramatically enhances the short‐circuit current density of the perovskite solar cells and almost completely eliminates the hysteresis. A maximum power conversion efficiency of 16.1% is achieved with an ODA‐SWCNT incorporated planar solar cell using (FA_0.83MA_0.17)_0.95Cs_0.05Pb(I_0.83Br_0.17)₃ as light absorber. Furthermore, the perovskite solar cells with ODA‐SWCNT demonstrate extraordinary stability with performance retention of 80% after 45 d stability testing under high humidity (60–90%) environment. This work opens up a new avenue for morphology manipulation of perovskite films and enhances the device stability using carbon material.     

Ultrastable and Reversible Fluorescent Perovskite Films Used for Flexible Instantaneous Display

All‐inorganic halide perovskite materials are regarded as promising materials in information display applications owing to their tunable color, narrow emission peak, and easy processability. However, the photoluminescence (PL) stability of halide perovskite films is still inferior due to their poor thermal stability and hygroscopic properties. Herein, all‐inorganic perovskite films are prepared through vacuum thermal deposition method to enhance thermal and hygroscopic stability. By intentionally adding extra bromide source, a structure of CsPbBr₃ nanocrystals embedded in a CsPb₂Br₅ matrix (CsPbBr₃/CsPb₂Br₅) is formed via an air exposure process, leading to impressive PL stability in ambient atmosphere. In addition, the as‐fabricated CsPbBr₃/CsPb₂Br₅ structure shows enhanced PL intensity due to the dielectric confinement. The CsPbBr₃/CsPb₂Br₅ structure film can almost reserve its initial PL intensity after four months, even stored in ambient atmosphere. The PL intensity for CsPbBr₃/CsPb₂Br₅ films vanishes at elevated temperature and recovers by cooling down in a short time. The reversible PL conversion process can be repeated over hundreds of times. Based on the reversible PL property, prototype thermal‐driven information display devices are demonstrated by employing heating circuits on flexible transparent substrates. These robust perovskite films with reversible PL characteristics promise an alternative solid‐state emitting display.     

Inorganic Rubidium Cation as an Enhancer for Photovoltaic Performance and Moisture Stability of HC(NH2)2PbI3 Perovskite Solar Cells

Perovskite solar cells (PSCs) based on organic monovalent cation (methylammonium or formamidinium) have shown excellent optoelectronic properties with high efficiencies above 22%, threatening the status of silicon solar cells. However, critical issues of long‐term stability have to be solved for commercialization. The severe weakness of the state‐of‐the‐art PSCs against moisture originates mainly from the hygroscopic organic cations. Here, rubidium (Rb) is suggested as a promising candidate for an inorganic–organic mixed cation system to enhance moisture‐tolerance and photovoltaic performances of formamidinium lead iodide (FAPbI₃). Partial incorporation of Rb in FAPbI₃ tunes the tolerance factor and stabilizes the photoactive perovskite structure. Phase conversion from hexagonal yellow FAPbI₃ to trigonal black FAPbI₃ becomes favored when Rb is introduced. The authors find that the absorbance and fluorescence lifetime of 5% Rb‐incorporated FAPbI₃ (Rb_0.05FA_0.95PbI₃) are enhanced than bare FAPbI₃. Rb_0.05FA_0.95PbI₃‐based PSCs exhibit a best power conversion efficiency of 17.16%, which is much higher than that of the FAPbI₃ device (13.56%). Moreover, it is demonstrated that the Rb_0.05FA_0.95PbI₃ film shows superior stability against high humidity (85%) and the full device made with the mixed perovskite exhibits remarkable long‐term stability under ambient condition without encapsulation, retaining the high performance for 1000 h.     

Boosting Photovoltaic Performance for Lead Halide Perovskites Solar Cells with BF4− Anion Substitutions

Composition engineering is a particularly simple and effective approach especially using mixed cations and halide anions to optimize the morphology, crystallinity, and light absorption of perovskite films. However, there are very few reports on the use of anion substitutions to develop uniform and highly crystalline perovskite films with large grain size and reduced defects. Here, the first report of employing tetrafluoroborate (BF₄^?) anion substitutions to improve the properties of (FA = formamidinium, MA = methylammonium (FAPbI₃)_0.83(MAPbBr₃)_0.17) perovskite films is demonstrated. The BF₄^? can be successfully incorporated into a mixed‐ion perovskite crystal frame, leading to lattice relaxation and a longer photoluminescence lifetime, higher recombination resistance, and 1–2 orders magnitude lower trap density in prepared perovskite films and derived solar cells. These advantages benefit the performance of perovskite solar cells (PVSCs), resulting in an improved power conversion efficiency (PCE) of 20.16% from 17.55% due to enhanced open‐circuit voltage (V_OC) and fill factor. This is the highest PCE for BF₄^? anion substituted lead halide PVSCs reported to date. This work provides insight for further exploration of anion substitutions in perovskites to enhance the performance of PVSCs and other optoelectronic devices.     

Metal Halide Perovskite Surfaces with Mixed A-Site Cations: Atomic Structure and Device Stability

Mixing cations in the perovskite structure has been shown to improve optoelectronic device performance and stability. In particular, Cs_xMA_1-xPbBr₃ (MA = CH₃NH₃) has been used to build high-efficiency light-emitting diodes. Despite those advantages, little is known about the exact location of the cations in the mixed perovskite film, and how cation distribution affects device properties and stability. By using scanning tunneling microscopy , the exact atomic structure of the mixed cation Cs_xMA_1-xPbBr₃ perovskite interface is revealed. In addition, X-ray photoelectron spectroscopy, ultraviolet photoemission spectroscopy and inverse photoemission spectroscopy are used to study the stability and electronic properties of the Cs_xMA_1-xPbBr₃ perovskite film. Partial substitution of MA⁺ by Cs⁺ induces a modification of the perovskite surface structure, leading to improved device stability is shown. These results provide a better understanding of the key parameters involved in the stability of mixed cation perovskite solar cells.