Intermolecular Exchange Boosts Efficiency of Air‐Stable, Carbon‐Based All‐Inorganic Planar CsPbIBr2 Perovskite Solar Cells to Over 9%

Abstract Among all inorganic halide perovskite photovoltaic materials, CsPbIBr 2 exhibits the most balanced features in terms of bandgap and stability. However, the poor quality of solution‐processed CsPbIBr 2 films impedes further optimization of cells performance. Herein, a facile intermolecular exchange strategy for CsPbIBr 2 film is demonstrated, wherein an optimized methanol solution of CsI is spin‐coated on CsPbIBr 2 precursor film in conventional one‐step solution route. It surprisingly produces full‐coverage and pure‐phase CsPbIBr 2 films featured with average grain size of ≈0.65 µm, few grain boundaries, high crystallinity, preferable (100) orientation, stoichiometric composition along with favorable electronic structures for effective dissociation and transfer of carriers. Hence, the cost‐effective, carbon‐based all‐inorganic planar perovskite solar cells based on them, yield an optimized efficiency of 9.16% with a stabilized value of 8.46% in ambient air conditions that highlight a particularly superb open‐circuit voltage of 1.245 V, all of which represent the highest values reported in pure CsPbIBr 2 based cells so far. Moreover, the optimized cell without encapsulation shows excellent long‐term stability because it can retain 90% over 60 days and 97% over 7 days of its initial efficiency, when is stored controllably in ≈45% relative humidity at 25 or 85 °C at zero humidity, respectively.