Minimizing Voltage Loss in Efficient All-Inorganic CsPbI₂Br Perovskite Solar Cells through Energy Level Alignment

All-inorganic CsPbI2Br, prized for its strong stability against thermal aging and light soaking, has attracted intensive attention. However, a large energy loss results from the serious energy level offset of 1.05 eV between CsPbI2Br and Spiro-MeOTAD, hindering the further efficiency improvement of perovskite solar cells. To address this issue, a moderate energy level (CsPbI2Br)1–x(CsPbI3)x layer has been introduced at the interface between CsPbI2Br and Spiro-MeOTAD to form a graded energy level alignment, the interpolation of which has offered the energy level gradient for reducing the resistance of hole transport. Correspondingly, the energy level tailoring has minimized the energy loss, and a remarkable VOC improved from 1.12 to 1.32 V, which is one of the highest values for CsPbI2Br-based solar cells. A relatively good thermal stability has also been validated. These good performances indicate that setting an intermediate energy level alignment will be a potential strategy for idealized device architecture to minimize energy loss.