Minimizing Open-Circuit voltage deficit via interface engineering for highly efficient CsPbI₂Br perovskite solar cells

All-inorganic perovskite CsPbI₂Br is a promising wide bandgap light absorber for tandem solar cells owing to its appropriate bandgap and great thermal stability. However, high open-circuit voltage (V_OC) deficit has been a major obstacle for obtaining high efficiency in CsPbI₂Br-based solar cells and thus limiting their application. Herein, we employ a strategy of interface engineering that deploys a bilayer electron transporting layer (ETL) and dopant-free hole transporting layer (HTL) to significantly mitigate the energy loss. A bilayer ETL ZnO/Mg_xZn_1-xO and Poly[(2,6-(4,8-bis(5-(2-ethylhexyl-3-fluoro)thiophen-2-yl)-benzo[1,2-b:4,5-b′]dithiophene))-alt-(5,5-(1′,3′-di-2-thienyl-5′,7′-bis(2-ethylhexyl)benzo[1′,2′-c:4′,5′-c′]dithiophene-4,8-dione)] (PM6) are energetically more compatible with the energy band of CsPbI₂Br than ZnO and 2,2′,7,7′-tetrakis (N, N-di-p-methoxy-phenylamine)-9,9′-spirobifluorene (spiro-OMeTAD), which facilitate the photogenerated electron and hole transfer at perovskite/charge transporting layer interfaces. Moreover, the quality of the CsPbI₂Br perovskite film deposited atop bilayer ETL ZnO/Mg_xZn_1-xO is substantially enhanced compared to that on top of ZnO. Both the more favorable energy level alignment and reduced defect density alleviate energy loss in the resultant solar cells. Perovskite solar cells with the structure of ITO/ZnO/Mg_xZn_1-xO/CsPbI₂Br/PM6/MoO₃/Ag give rise to an open-circuit voltage (V_OC) of 1.34 V, which is one of the highest V_OC among all-inorganic CsPbI₂Br solar cells. The resultant V_OC deficit drops to be lower than 0.6 V. As a result, the optimized all-inorganic CsPbI₂Br-based solar cells yield a champion power conversion efficiency of 16.04%.