The optimization of CsPbIBr₂ top sub-cells for the application in monolithic all-perovskite tandem solar cells

More and more attention has been paid to CsPbIBr₂ material for perovskite solar cells (PSCs), owing to the trade-off between bandgap and stability. However, the reported maximum power conversion efficiency (PCE) of CsPbIBr₂ PSCs is lag behind that of the congeners due to the serious interface recombination of charge carriers. In this work, the effects of interface defect density and interface band offset on the properties of CsPbIBr₂ PSCs are investigated. It is found that the characteristic of the front electron transfer layer (ETL)/perovskite interface has a significant influence on the performance of PSCs than that of the back perovskite/hole transfer layer (HTL) interface. Additionally, a PCE of 15.05% with a high open-circuit voltage (V_OC) of 1.54 V is obtained for the PSCs with FTO/ZnOS/CsPbIBr₂/CuAlO₂/Au structure. Finally, two-terminal monolithic all-perovskite double-junction tandem solar cells (TSCs) with the architecture of FTO/ZnOS/CsPbIBr₂/CuAlO₂/ITO/TiO₂/MAPbI₃/Spiro-MeOTAD/Au are constructed, and a maximum PCE of 27.4% (V_OC of 2.60 V, J_SC of 12.21 mA/cm², and FF of 86.42%) is obtained for the TSCs with top and bottom absorber thicknesses of 600 nm and 500 nm respectively. Furthermore, four-terminal triple-junction TSCs are designed with silicon solar cells as bottom sub-cell, and a maximum PCE of 35.35% is achieved. These results will provide theoretical guidance for the fabrication of high performance TSCs.