The Mixed Phases of α and γ-CsPbI₃ Enable Efficient and Stable Semitransparent Solar Cells

Cesium lead iodide (CsPbI₃) is a promising material for semitransparent perovskite solar cells (ST-PSCs) in building-integrated photovoltaics (BIPV) due to its favorable bandgap and thermal stability. However, the phase instability of CsPbI₃ limits its device performance. Developing a simple strategy to regulate the phase transition is essential for enhancing both device efficiency and stability. In this study, synchrotron-based grazing-incidence wide-angle X-ray scattering and in situ X-ray diffraction results revealed that adding HBr promotes the formation of γ-CsPbI₃ at low temperatures. Upon annealing at 100 C, this γ-phase partially converts to α-CsPbI₃, resulting in a mixed-phase film. This mixed-phase film effectively mitigates substrate strain caused by mismatched thermal expansion coefficients and strain generated during phase transitions. Moreover, the mixed-phase CsPbI₃ films exhibit a pinhole-free morphology with reduced defect density, leading to enhanced moisture stability under ≈85% relative humidity. Consequently, the mixed-phase CsPbI₃-based ST-PSC achieved a remarkable power conversion efficiency (PCE) of 19.89% and retained 90.89% of its initial PCE after 500 h of continuous illumination.