Triple-halide wide-bandgap perovskites tailored via facile organic halide treatment for high-performance perovskite/Cu(In,Ga)Se2 tandem solar cells

Wide-bandgap (Eg) perovskites (PVSK) are important materials for realizing high-efficiency tandem devices that surpass the efficiency limit of single-junction solar cells. However, they suffer from phase separation and opencircuit voltage (Voc) loss. This study demonstrated a facile fabrication strategy of highly efficient and stable wideEg PVSKs through an organic halide surface treatment with formamidinium bromide to CH3NH3PbI3-xClx. The concurrent diffusion of organic cations and halide ions from the surface into bulk PVSK results in the complete conversion of the PVSK crystallinity to a cubic phase, eliminating segregated secondary phases and reducing unreacted PbI2. Br incorporation induces halide redistribution, resulting in the formation of triple-halide wide-Eg PVSK. The complete reconstruction of the bulk and surface of PVSK by surface post-treatment suppresses trapinduced nonradiative recombination and decreases the Urbach tail energy. Inverted PVSK solar cells based on the reconstructed PVSK exhibit enhanced photovoltaic performance with a low Voc deficit and high stability. In addition, we demonstrated a four-terminal (4-T) tandem device based on the triple-halide wide-Eg PVSK as a top cell combining with a Cu(In,Ga)Se2 bottom cell, achieving a power conversion efficiency of 24.5 %.