Controlled n‐Doping in Air‐Stable CsPbI2Br Perovskite Solar Cells with a Record Efficiency of 16.79%

Cesium‐based inorganic perovskites, such as CsPbI₂Br, are promising candidates for photovoltaic applications owing to their exceptional optoelectronic properties and outstanding thermal stability. However, the power conversion efficiency of CsPbI₂Br perovskite solar cells (PSCs) is still lower than those of hybrid PSCs and inorganic CsPbI₃ PSCs. In this work, passivation and n‐type doping by adding CaCl₂ to CsPbI₂Br is demonstrated. The crystallinity of the CsPbI₂Br perovskite film is enhanced, and the trap density is suppressed after adding CaCl₂. In addition, the Fermi level of the CsPbI₂Br is changed by the added CaCl₂ to show heavy n‐type doping. As a result, the optimized CsPbI₂Br PSC shows a highest open circuit voltage of 1.32 V and a record efficiency of 16.79%. Meanwhile, high air stability is demonstrated for a CsPbI₂Br PSC with 90% of the initial efficiency remaining after more than 1000 h aging in air.     

Chromium-Based Metal–Organic Framework as A-Site Cation in CsPbI2Br Perovskite Solar Cells

Inorganic CsPbI_xBr_3−x perovskite solar cells (PSCs) have gained enormous interest due to their excellent thermal stabilities. However, their intrinsically poor moisture stability hampers their further development. Herein, a chromium-based metal–organic framework group is intercalated inside the inorganic Pb I framework, resulting in a new multiple-dimensional electronically coupled CsPbI₂Br perovskite. In this structurally and electronically coupled perovskite, the π-conjugated terpyridyl can delocalize the excited valence electrons of metal Cr³⁺ ion, enabling multi-interactive charge-carrier transport channels within CsPbI₂Br perovskites. The stability and efficiency of the produced devices are substantially enhanced in comparison to their counterparts with only a pristine CsPbI₂Br active layer. The optimized all-inorganic PSC yields a power conversion efficiency (PCE) as high as 17.02%. Remarkably, the stabilized device retains 80% of its PCE after 1000 h in the ambient atmosphere. This study provides a new paradigm toward addressing the stability challenge of the inorganic perovskite while enhancing its carrier transport ability.     

Constructing All-Inorganic Perovskite/Fluoride Nanocomposites for Efficient and Ultra-Stable Perovskite Solar Cells

Organic-inorganic hybrid perovskite solar cells (PSCs) have rapidly developed over the past decade and have achieved the latest certified power conversion efficiency (PCE) up to 25.5%. However, unsatisfactory long-term operational stability for these hybrid PSCs remains a huge obstacle to further development and commercialization. Herein, a unique hetero-structured CsPbI₃/CaF₂ perovskite/fluoride nanocomposites (PFNCs) is fabricated via a newly developed facile two-step hetero-epitaxial growth strategy to deliver efficient and ultra-stable PSCs. After being incorporated into the crystal lattice of α-phase CsPbI₃ perovskite, the cubic-phase CaF₂ in the resultant CsPbI₃/CaF₂ PFNCs can not only passivate the intrinsic defects of CsPbI₃ perovskite itself but also effectively suppress the notorious ion migration in hybrid perovskite Cs_0.05FA_0.81MA_0.14PbI_2.55Br_0.45 (CsFAMA) thin-films of PSCs. As such, the CsFAMA PSC devices based on CsPbI₃/CaF₂-deposited perovskite thin-film achieve a mean PCE of 20.45%, in sharp contrast to 19.33% of the control devices without deposition. Specifically, the CsPbI₃/CaF₂-deposited PSC retains 85% of its original PCE after 1000 h continuous operation at the maximum power point under AM 1.5G solar light, far better than those of the control and CsPbI₃-deposited PSCs with a device T₈₅ lifetime of 315 and 125 h, respectively.     

Uniform Stepped Interfacial Energy Level Structure Boosts Efficiency and Stability of CsPbI2Br Solar Cells

All-inorganic CsPbI₂Br perovskite has attracted great attention as an absorber for perovskite solar cells (PSCs) due to its excellent thermal and light resistance. However, its device performance is restricted by the large energy level offset between CsPbI₂Br and the most commonly used hole-transporting layer (HTL). Herein, multicarbazolyl-substituted benzonitrile (4t-5CzBn) is inserted into the interface between CsPbI₂Br and HTL to form a uniform stepped (0.24 eV) interfacial energy level structure, which reduces the energy loss and boosts the hole extraction of CsPbI₂Br PSCs. The incorporation of 4t-5CzBn induces the increase in open-circuit voltage and fill factor from 1.256 V and 74.5% to 1.335 V and 82.3%, respectively. The optimized device achieves a power conversion efficiency of 17.34%, which is among the highest reported values of CsPbI₂Br PSCs. Besides the energy level tuning effect, the tert-butyl groups in 4t-5CzBn improve the moisture-resistance of CsPbI₂Br PSCs. The unencapsulated device maintains over 75% of its initial efficiency after 700 h storage in air. These results demonstrate that the rational tuned energy level step benefits the performance improvement of CsPbI₂Br PSCs.     

Modification of Energy Level Alignment for Boosting Carbon-Based CsPbI2Br Solar Cells with 14% Certified Efficiency

Hole transfer material (HTM)-free, carbon-based all-inorganic perovskite solar cells (C-PSCs) are promising alternatives to conventional organic–inorganic hybrid PSCs in addressing thermal and moisture instability issues. However, the energy level mismatch between the inorganic perovskite and carbon electrode coupled, together with the incapability of the carbon electrode to reflect incident light for reabsorption, limits the power conversion efficiency (PCE) of C-PSCs. To address these issues, herein, a new strategy of a hexyltrimethylammonium bromide (HTAB)-modified CsPbI₂Br perovskite surface is devised to reduce this energy offset from 0.70 to 0.32 eV and increase the built-in potential by 70 mV for the final devices. Additionally, a CsPbI₂Br perovskite film with a thickness of up to 800 nm is realized via a hot-flow-assisted spin coating approach in an ambient atmosphere with humidity of less than 80%. Reduced energy offset coupled with suppressed charge recombination and thick perovskite layer boosts the champion PCE of CsPbI₂Br C-PSCs to 14.3% (J_sc = 14.1 mA cm⁻², V_oc = 1.26 V, and fill factor = 0.806), and the average PCE to 13.9% under one sun illumination. A new certified efficiency record of 14.0% is obtained for HTM-free inorganic C-PSCs. Meanwhile, the moisture-resistant barrier from the alkyl chain in HTAB improves the stability of the final devices.     

Composition Engineering of All‐Inorganic Perovskite Film for Efficient and Operationally Stable Solar Cells

Cesium‐based inorganic perovskites have recently attracted great research focus due to their excellent optoelectronic properties and thermal stability. However, the operational instability of all‐inorganic perovskites is still a main hindrance for the commercialization. Herein, a facile approach is reported to simultaneously enhance both the efficiency and long‐term stability for all‐inorganic CsPbI_2.5Br_0.5 perovskite solar cells via inducing excess lead iodide (PbI₂) into the precursors. Comprehensive film and device characterizations are conducted to study the influences of excess PbI₂ on the crystal quality, passivation effect, charge dynamics, and photovoltaic performance. It is found that excess PbI₂ improves the crystallization process, producing high‐quality CsPbI_2.5Br_0.5 films with enlarged grain sizes, enhanced crystal orientation, and unchanged phase composition. The residual PbI₂ at the grain boundaries also provides a passivation effect, which improves the optoelectronic properties and charge collection property in optimized devices, leading to a power conversion efficiency up to 17.1% with a high open‐circuit voltage of 1.25 V. More importantly, a remarkable long‐term operational stability is also achieved for the optimized CsPbI_2.5Br_0.5 solar cells, with less than 24% degradation drop at the maximum power point under continuous illumination for 420 h.     

Enhanced Performance of Planar Perovskite Solar Cells Induced by Van Der Waals Epitaxial Growth of Mixed Perovskite Films on WS2 Flakes

Organic–inorganic metal halide perovskite solar cells (PSCs) have attracted much research interest owing to their high power conversion efficiency (PCE), solution processability, and the great potential for commercialization. However, the device performance is closely related to the quality of the perovskite film and the interface properties, which cannot be easily controlled by solution processes. Here, 2D WS₂ flakes with defect‐free surfaces are introduced as a template for van der Waals epitaxial growth of mixed perovskite films by solution process for the first time. The mixed perovskite films demonstrate a preferable growth along (001) direction on WS₂ surfaces. In addition, the WS₂/perovskite heterojunction forms a cascade energy alignment for efficient charge extraction and reduced interfacial recombination. The inverted PSCs with WS₂ interlayers show high PCEs up to 21.1%, which is among the highest efficiency of inverted planar PSCs. This work demonstrates that high‐mobility 2D materials can find important applications in PSCs as well as other perovskite‐based optoelectronic devices.     

One-Source Strategy Boosting Dopant-Free Hole Transporting Layers for Highly Efficient and Stable CsPbI2Br Perovskite Solar Cells

All-inorganic perovskites have emerged as promising photovoltaic materials due to their superior thermal stability compared to their organic–inorganic hybrid counterparts. However, the inferior film quality and doped hole transport layer (HTL) have a strong tendency to degrade the perovskite under high temperatures or harsh operating conditions. To solve these problems, a one-source strategy using the same polymer donor material (PDM) to simultaneously dope CsPbI₂Br perovskite films via antisolvent engineering and fabricating the HTL is proposed. The doping assists perovskite film growth and forms a top–down gradient distribution, generating CsPbI₂Br with enlarged grain size and reduced defect density. The PDM as the HTL suppresses the energy barrier and forms favorable electrical contacts for hole extraction, and assemble into a fingerprint-like morphology that improves the conductivity, facilitating the creation of a dopant-free HTL. Based on this one-source strategy using PBDB-T as PDM, the CsPbI₂Br perovskite solar cell with a dopant-free HTL achieves a power conversion efficiency (PCE) of 16.40%, which is one of the highest PCEs reported among all-inorganic CsPbI₂Br pero-SCs with a dopant-free HTL. Importantly, the devices exhibit the highest thermal stability at 85 °C and operational stability under continuous illumination even with Ag as the top electrode and present good universality.     

Illumination Enhanced Crystallization and Defect Passivation for High Performance CsPbI3 Perovskite Solar Cells by Sacrificing Dye

All-inorganic perovskite solar cells (PSCs) have been the research focus due to their high thermal stability and proper band gap for tandem solar cells. However, their power conversion efficiency (PCE) is still lower than that of organic-inorganic hybrid PSCs. Herein, a sacrificing dye (Rhodamine B isothiocyanate, RBITC) is developed to regulate the growth of perovskite film by in situ release of ethylammonium cations, isothiocyanate anions and benzoic acid molecules upon annealing and illumination. The ethylammonium cations can efficiently passivate surface defects. The isothiocyanate anions incorporate with uncoordinated Pb to regulate the crystallization process. The benzoic acid molecules facilitate the nucleation of the perovskite crystals. Especially, the illumination can accelerate the release of these beneficial ions/molecules to improve the quality of perovskite films further. After optimization with RBITC, a high open circuit voltage (V_OC) of 1.24 V and a champion PCE of 20.95% are obtained, which are among the highest Voc and PCE values of CsPbI₃ PSCs. Accordingly, the operational stability of the PSC devices is significantly improved. The results provide an efficient chemical strategy to regulate the formation of perovskite films in whole crystallization process for high performance all-inorganic PSCs.     

Enhancing Stability of Efficient Perovskite Solar Cells (PCE ≈ 24.5%) by Suppressing PbI2 Inclusion Formation

Excess lead(II) iodide (PbI₂) has controversial roles in affecting the efficiency of perovskite solar cells (PSCs). Since the photoinstability of PbI₂ is now known to largely accelerate perovskite degradation, suppressing and/or eliminating excess PbI₂ is key to improving the stability of PSCs. Herein, process-dependent PbI₂ formation on the surfaces of formamidinium lead triiodide (FAPbI₃) films is examined. Due to the faster evaporation rate of organic substances, crystalline PbI₂ as an inclusion is found within the triple junction grain boundaries. With this hypothesis, two strategies are suggested: control of the 1) vapor pressure and 2) stoichiometry of precursor solutions to induce sufficient reaction of FAPbI₃. Although both strategies successfully eliminate the PbI₂ as inclusions, due to the slower evaporation rate, vapor pressure control films also exhibit a larger grain size (≈1.18 µm) with a good film quality to attain the highest power conversion efficiency (PCE) of 24.5%. Furthermore, the phase stability of α-FAPbI₃ is improved due to the elimination of the degradation sites induced by the photoinstability of PbI₂. The findings explore the formation process of unwanted PbI₂ (≈2.8%) and provide a simple method to effectively suppress its formation. This may further boost the PCE and stability, especially for FA-based perovskites.     

Efficient and Ambient‐Air‐Stable Solar Cell with Highly Oriented 2D@3D Perovskites

3D organic–inorganic lead halide perovskites have shown great potential in efficient photovoltaic devices. However, the low stability of the 3D perovskite layer and random arrangement of the perovskite crystals hinder its commercialization road. Herein, a highly oriented 2D@3D ((AVA)₂PbI₄@MAPbI₃) perovskite structure combining the advantages of both 2D and 3D perovskite is fabricated through an in situ route. The highest power conversion efficiency (PCE) of 18.0% is observed from a 2D@3D perovskite solar cell (PSC), and it also shows significantly enhanced device stability under both inert (90% of initial PCE for 32 d) and ambient conditions (72% of initial PCE for 20 d) without encapsulation. The high efficiency of 18.0% and nearly twofold improvement of device stability in ambient compared with pure 3D PSCs confirm that such 2D@3D perovskite structure is an effective strategy for high performance and increasing stability and thus will enable the timely commercialization of PSCs.     

FAPbI3 Flexible Solar Cells with a Record Efficiency of 19.38% Fabricated in Air via Ligand and Additive Synergetic Process

Compared with silicon‐based solar cells, organic–inorganic hybrid perovskite solar cells (PSCs) possess a distinct advantage, i.e., its application in the flexible field. However, the efficiency of the flexible device is still lower than that of the rigid one. First, it is found that the dense formamidinium (FA)‐based perovskite film can be obtained with the help of N‐methyl‐2‐pyrrolidone (NMP) via low pressure‐assisted method. In addition, CH₃NH₃Cl (MACl) as the additive can preferentially form MAPbCl_3?xI_x perovskite seeds to induce perovskite phase transition and crystal growth. Finally, by using FAI·PbI₂·NMP+x%MACl as the precursor, i.e., ligand and additive synergetic process, a FA‐based perovskite film with a large grain size, high crystallinity, and low trap density is obtained on a flexible substrate under ambient conditions due to the synergetic effect, e.g., MACl can enhance the crystallization of the intermediate phase of FAI·PbI₂·NMP. As a result, a record efficiency of 19.38% in flexible planar PSCs is achieved, and it can retain about 89% of its initial power conversion efficiency (PCE) after 230 days without encapsulation under ambient conditions. The PCE retains 92% of the initial value after 500 bending cycles with a bending radii of 10 mm. The results show a robust way to fabricate highly efficient flexible PSCs.     

Synergetic Excess PbI2 and Reduced Pb Leakage Management Strategy for 24.28% Efficient,Stable and Eco-Friendly Perovskite Solar Cells

Introducing excess PbI₂ has proven to be an effective in situ passivation strategy for enhancing efficiency of perovskite solar cells (PSCs). Nevertheless, the photoinstability and hysteresis are still tough issues owing to the photolysis nature of PbI₂. Moreover, the humidity-related degradation of perovskite films is also a difficult territory to cover in such an in situ passivation strategy. Herein, a synergistic strategy is reported via initiatively inducing vertical graded PbI₂ distribution (GPD) in the whole perovskite film and capping a cis-Ru(H₂dcbpy)(dnbpy)(NCS)₂ (Z907) internal encapsulation (IE) layer on the surface to ameliorate the above issues. The GPD design can enhance luminescence, prolong carrier lifetimes, ascertaining the improvement of efficiency and elimination of photoinstability in the PSCs. Besides, the introduced IE layer not only can promote the moisture and thermal resistance, but also inhibit Pb leakage and ion migration in the PSCs. Through the synergetic regulations, the resultant PSCs exhibit an impressive open circuit voltage (V_OC) of 1.253 V, fill factor of 81.25%, and power conversion efficiency (PCE) of 24.28%. Moreover, the PSCs maintain 91% of its initial PCE at relative humidity of 85% after 500 h aging and 94% under continuous heating at 85 °C after 750 h aging.     

Function Follows Form: Correlation between the Growth and Local Emission of Perovskite Structures and the Performance of Solar Cells

Understanding the relationship between the growth and local emission of hybrid perovskite structures and the performance of the devices based on them demands attention. This study investigates the local structural and emission features of CH₃NH₃PbI₃, CH₃NH₃PbBr₃, and CH(NH₂)₂PbBr₃ perovskite films deposited under different yet optimized conditions using X‐ray scattering and cathodoluminescence spectroscopy, respectively. X‐ray scattering shows that a CH₃NH₃PbI₃ film involving spin coating of CH₃NH₃I instead of dipping is composed of perovskite structures exhibiting a preferred orientation with [202] direction perpendicular to the surface plane. The device based on the CH₃NH₃PbI₃ film composed of oriented crystals yields a relatively higher photovoltage. In the case of CH₃NH₃PbBr₃, while the crystallinity decreases when the HBr solution is used in a single‐step method, the photovoltage enhancement from 1.1 to 1.46 V seems largely stemming from the morphological improvements, i.e., a better connection between the crystallites due to a higher nucleation density. Furthermore, a high photovoltage of 1.47 V obtained from CH(NH₂)₂PbBr₃ devices could be attributed to the formation of perovskite films displaying uniform cathodoluminescence emission. The comparative analysis of the local structural, morphological, and emission characteristics of the different perovskite films supports the higher photovoltage yielded by the relatively better performing devices.     

Stable α‐CsPbI3 Perovskite Nanowire Arrays with Preferential Crystallographic Orientation for Highly Sensitive Photodetectors

All‐inorganic metal‐halide perovskites CsPbX₃ (X = Cl, Br, I) exhibit higher stability than their organic–inorganic hybrid counterparts, but the thermodynamically instable perovskite α phase at room temperature of CsPbI₃ restricts the practical optoelectronic applications. Although the stabilization of α‐CsPbI₃ polycrystalline thin films is extensively studied, the creation of highly crystalline micro/nanostructures of α‐CsPbI₃ with large grain size and suppressed grain boundary remains challenging, which impedes the implementations of α‐CsPbI₃ for lateral devices, such as photoconductor‐type photodetectors. In this work, stable α‐CsPbI₃ perovskite nanowire arrays are demonstrated with large grain size, high crystallinity, regulated alignment, and position by controlling the dewetting dynamics of precursor solution on an asymmetric‐wettability topographical template. The correlation between the higher photoluminescence (PL) intensity and longer PL lifetime indicates the nanowires exhibit stable α phase and suppressed trap density. The preferential (100) orientation is characterized by discrete diffraction spots in grazing incidence wide‐angle scattering patterns, suggesting the long‐range crystallographic order of these nanowires. Based on these high‐quality nanowire arrays, highly sensitive photodetectors are realized with a responsivity of 1294 A W^?1 and long‐term stability with 90% performance retention after 30‐day ambient storage.     

Simultaneous Power Conversion Efficiency and Stability Enhancement of Cs2AgBiBr6 Lead‐Free Inorganic Perovskite Solar Cell through Adopting a Multifunctional Dye Interlayer

Perovskite solar cells (PSCs) are highly promising next‐generation photovoltaic devices because of the cheap raw materials, ideal band gap of ≈1.5 eV, broad absorption range, and high absorption coefficient. Although lead‐based inorganic‐organic PSC has achieved the highest power conversion efficiency (PCE) of 25.2%, the toxic nature of lead and poor stability strongly limits the commercialization. Lead‐free inorganic PSCs are potential alternatives to toxic and unstable organic‐inorganic PSCs. Particularly, double‐perovskite Cs₂AgBiBr₆‐based PSC has received interests for its all inorganic and lead‐free features. However, the PCE is limited by the inherent and extrinsic defects of Cs₂AgBiBr₆ films. Herein, an effective and facile strategy is reported for improving the PCE and stability by introducing an N719 dye interlayer, which plays multifunctional roles such as broadening the absorption spectrum, suppressing the charge carrier recombination, accelerating the hole extraction, and constructing an appropriate energy level alignment. Consequently, the optimizing cell delivers an outstanding PCE of 2.84%, much improved as compared with other Cs₂AgBiBr₆‐based PSCs reported so far in the literature. Moreover, the N719 interlayer greatly enhances the stability of PSCs under ambient conditions. This work highlights a useful strategy to boost the PCE and stability of lead‐free Cs₂AgBiBr₆‐based PSCs simultaneously, accelerating the commercialization of PSC technology.     

Spray‐Coated Colloidal Perovskite Quantum Dot Films for Highly Efficient Solar Cells

A fully automated spray‐coated technology with ultrathin‐film purification is exploited for the commercial large‐scale solution‐based processing of colloidal inorganic perovskite CsPbI₃ quantum dot (QD) films toward solar cells. This process is in the air outside the glove box. To further improve the performance of QD solar cells, the short‐chain ligand of phenyltrimethylammonium bromide (PTABr) with a benzene group is introduced to partially substitute for the original long‐chain ligands of the colloidal QD surface (namely PTABr‐CsPbI₃). This process not only enhances the carrier charge mobility within the QD film due to shortening length between adjacent QDs, but also passivates the halide vacancy defects of QD by Br^? from PTABr. The colloidal QD solar cells show a power conversion efficiency (PCE) of 11.2% with an open voltage of 1.11 V, a short current density of 14.4 mA cm^?2, and a fill factor of 0.70. Due to the hydrophobic surface chemistry of the PTABr–CsPbI₃ film, the solar cell can maintain 80% of the initial PCE in ambient conditions for one month without any encapsulation. Such a low‐cost and efficient spray‐coating technology also offers an avenue to the film fabrication of colloidal nanocrystals for electronic devices.     

Impact of the Solvation State of Lead Iodide on Its Two‐Step Conversion to MAPbI3: An In Situ Investigation

Producing high efficiency solar cells without high‐temperature processing or use of additives still remains a challenge with the two‐step process. Here, the solution processing of MAPbI₃ from PbI₂ films in N,N‐dimethylformamide (DMF) is investigated. In‐situ grazing incidence wide‐angle X‐ray scattering (GIWAXS) measurements reveal a sol–gel process involving three PbI₂‐DMF solvate complexes—disordered (P₀) and ordered (P₁, P₂)—prior to PbI₂ formation. When the appropriate solvated state of PbI₂ is exposed to MAI (methylammonium Iodide), it can lead to rapid and complete room temperature conversion into MAPbI₃ with higher quality films and improved solar cell performance. Complementary in‐situ optical reflectance, absorbance, and quartz crystal microbalance with dissipation (QCM‐D) measurements show that dry PbI₂ can take up only one third of the MAI taken up by the solvated‐crystalline P₂ phase of PbI₂, requiring additional annealing and yet still underperforming. The perovskite solar cells fabricated from the ordered P₂ precursor show higher power conversion efficiency (PCE) and reproducibility than devices fabricated from other cases. The average PCE of the solar cells is greatly improved from 13.2(±0.53)% (from annealed PbI₂) to 15.7(±0.35)% (from P₂) reaching up to 16.2%. This work demonstrates the importance of controlling the solvation of PbI₂ as an effective strategy for the growth of high‐quality perovskite films and their application in high efficiency and reproducible solar cells.     

Improvement and Regeneration of Perovskite Solar Cells via Methylamine Gas Post‐Treatment

The control of film morphology is crucial in achieving high‐performance perovskite solar cells (PSCs). Herein, the crystals of the perovskite films are reconstructed by post‐treating the MAPbI₃ devices with methylamine gas, yielding a homogeneous nucleation and crystallization of the perovskite in the triple mesoscopic inorganic layers structured PSCs. As a result, a uniform, compact, and crystalline perovskite layer is obtained after the methylamine gas post‐treatment, yielding high power conversion efficiency (PCE) of 15.26%, 128.8% higher than that of the device before processing. More importantly, this post‐treatment process allows the regeneration of the photodegraded PSCs via the crystal reconstruction and the PCE can recover to 91% of the initial value after two cycles of the photodegradation‐recovery process. This simple method allows for the regeneration of perovskite solar cells on site without reconstruction or replacing any components, thus prolonging the service life of the perovskite solar cells and distinguishing from any other photovoltaic devices in practice.     

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).