Stable Inverted Antimony-Bismuth Alloyed Inorganic Perovskite Solar Cells via Crystallization Control and Defect Passivation.

Triple-junction tandem solar cells require top-cell absorbers with suitable wide-bandgap (2.0-2.3 eV), low toxicity, and high stability. Bismuth-based perovskites with the A3B2X9 structure have emerged as promising candidates due to their nontoxic nature and favorable optoelectronic properties. However, their efficiency is constrained by intrinsic deep-level defects, low-dimensional morphology, poor film quality, and mismatched energy alignment with charge transport layers. Here, we address these challenges using an antimony-bismuth alloyed Cs3SbBiI6Br3 perovskite as an absorbing layer. Incorporating methylammonium chloride (MACl) into the precursor solution yields mixed-halide films of Cs3SbBi(I,Br)9-xClx, retarding crystallization and enhancing both film quality and two-dimensional phase purity. Furthermore, we introduce potassium hexafluorophosphate (KPF6) on the films. KPF6 forms a dipole layer on the surface, which upshifts the perovskite Fermi level and suppresses defect-mediated recombination. The resulting inverted (p-i-n) devices achieve a champion power conversion efficiency of 1.8% and an open-circuit voltage of 0.88 V, which is the highest reported for A3B2X9 perovskites in inverted architecture. Moreover, the KPF6-treated devices retain 90% of their initial efficiency after 3200 h of storage in nitrogen, demonstrating exceptional stability. Our work highlights the potential of Sb-Bi alloyed perovskites as top-cell absorbers for triple-junction tandems.