Surface Halide Inversion Mitigates Voltage Losses in Wide-Bandgap Perovskite for Efficient Tandem

Substantial energy loss at the wide-bandgap (WBG) perovskite/fullerene interface poses a fundamental bottleneck for efficient all-perovskite tandem solar cells (APTSCs). Surface defects of WBG perovskite are a known source of non-radiative recombination and have been well-passivated, but this interface still remains with low quasi-Fermi level splitting (QFLS) with considerable voltage loss. Here, we reveal another critical source of the interfacial loss, that WBG perovskite surface features an iodine-rich nature with a locally narrower bandgap (Eg)_. The simulations reveal that the narrow local Eg induces serious non-radiative recombination at both surface and bulk regions, and the bromine-rich is preferable with aligned energy levels. From this mechanistic insight, we systematically compared distinct strategies of surface polishing, bromine compensation and halide inversions via functional agents. Comparably, inverting surface into a bromine-rich via cesium fluoride (CsF) demonstrates as the most effective pathway for less undesirable recombination and higher QFLS. Additionally, Cs/F substitutions can strengthen surface lattice, significantly enhancing the phase stability. Consequently, we achieve a champion efficiency of 20.89% with an open-circuit voltage (V_OC) of 1.368 V for WBG perovskite photovoltaics. After stacking to tin-lead subcell, an impressive efficiency of 29.15% in APTSCs is realized, coupled with good T₉₀ operational stability of 1000 h.