Efficiency Potential and Voltage Loss of Inorganic CsPbI₂Br Perovskite Solar Cells

Inorganic perovskite solar cells show excellent thermal stability, but the reported power conversion efficiencies are still lower than for organic–inorganic perovskites. This is mainly caused by lower open‐circuit voltages (V_OCs). Herein, the reasons for the low V_OC in inorganic CsPbI₂Br perovskite solar cells are investigated. Intensity‐dependent photoluminescence measurements for different layer stacks reveal that n–i–p and p–i–n CsPbI₂Br solar cells exhibit a strong mismatch between quasi‐Fermi level splitting (QFLS) and V_OC. Specifically, the CsPbI₂Br p–i–n perovskite solar cell has a QFLS–e ·V_OC mismatch of 179 meV, compared with 11 meV for a reference cell with an organic–inorganic perovskite of similar bandgap. On the other hand, this study shows that the CsPbI₂Br films with a bandgap of 1.9 eV have a very low defect density, resulting in an efficiency potential of 20.3% with a MeO–2PACz hole‐transporting layer and 20.8% on compact TiO₂. Using ultraviolet photoelectron spectroscopy measurements, energy level misalignment is identified as a possible reason for the QFLS–e ·V_OC mismatch and strategies for overcoming this V_OC limitation are discussed. This work highlights the need to control the interfacial energetics in inorganic perovskite solar cells, but also gives promise for high efficiencies once this issue is resolved.