In situ epitaxial growth of blocking structure in mixed-halide wide-band-gap perovskites for efficient photovoltaics

Mixed-halide perovskite solar cells (PSCs) with wide band gap (WBG) have attracted increasingly intensive attention because they are ideally suited for tandem photovoltaics. However, the device performance and stability are still undesirable due to the existence of photoinduced phase segregation. Here, we precisely assembled a highly crystalline blocking structure onto the WBG perovskite plane via in situ epitaxial growth to address this issue. It was found that the blocking structure contributes to the release of lattice strain, the enhancement of the halide-ion migration barrier, and effective defect passivation of WBG perovskites. Consequently, an impressive open-circuit voltage of 1.25 V (maximum 1.26 V) with a champion efficiency of 21.80% (certified 21.52%) was obtained in a 1.65 eV mixed-halide PSC with outstanding photostability. The semitransparent device with a certified efficiency of 19.15% was overlaid on a silicon subcell to make a four-terminal tandem configuration, demonstrating a high efficiency of 28.83%.