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

•1. Bilayer ETL ZnO/MgxZn1-xO and HTL dopant-free PM6 are introduced.•2. ZnO/MgxZn1-xO provides a cascaded energy level alignment with CsPbI2Br.•3. ZnO/MgxZn1-xO improves CsPbI2Br film quality.•4. PM6 energetically aligns better with CsPbI2Br than spiro-OMeTAD.•5. CsPbI2Br-based PSCs delivers a champion PCE over 16% with VOC deficit of 0.58 V. 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 (VOC) 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/MgxZn1-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/MgxZn1-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/MgxZn1-xO/CsPbI2Br/PM6/MoO3/Ag give rise to an open-circuit voltage (VOC) of 1.34 V, which is one of the highest VOC among all-inorganic CsPbI2Br solar cells. The resultant VOC 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%.