Improved Efficiency and Stability in Inverted-Structure Solar Cells with Lead-Free All-Inorganic Halide Perovskite CsSn _{1– <i>x</i>} Zn <i> _x </i> Br ₃

We report the fabrication of an inverted structure solar cell with all-inorganic lead-free perovskite CsSn_1-<i>x</i>Zn<i>_x</i>Br₃ alloy thin films grown by physical vapor codeposition of CsBr, SnBr₂, and ZnBr₂. It was found that the deposited CsSn_1-<i>x</i>Zn<i>_x</i>Br₃ perovskite alloy thin films exhibited improved morphological characteristics (larger grain sizes, lower pinhole density, and improved flatness) compared to the CsSnBr₃ thin film. The incorporation of 4% Zn (CsSn_0.96Zn_0.04Br₃, abbreviated as 4Zn) resulted in a bandgap narrowing of ∼20 meV compared to CsSnBr₃ with the upshift of the valence band maximum and conduction band minimum of ∼0.3 eV. The inverted-structure perovskite solar cells (PSCs) (ITO/PEDOT:PSS/4Zn/C₆₀/BCP/Ag) exhibit improved energy level alignment with the transport layers of C₆₀ and PEDOT:PSS. The 4Zn solar cell showed an open-circuit voltage (<i>V</i>_OC: 0.35 V), short-circuit current density (<i>J</i>_SC: 13.99 mA/cm²), and fill factor (FF: 54%), yielding a power conversion efficiency (PCE) of 2.59%. The Zn-alloyed PSCs were more efficient and stable than the pure CsSnBr₃ PSCs under ambient air conditions. The 4% Zn device preserved 96% of <i>V</i>_OC, 86% of <i>J</i>_SC, 91% of FF, and 83% of the initial PCE after preservation for 6 days under 60% humidity at 25 °C. This result offers a potential strategy for the fabrication of air-stable all-inorganic lead-free PSCs.