Halide Double Perovskites Cs₂PdBr_6-I with Tunable Bandgaps for Solar Cells

Inorganic lead-free vacancy-ordered double perovskites with the chemical formula A₂BX₆ are promising candidates to overcome Pb-based organic-inorganic perovskite's toxicity and instability issues. We designed the mixed-halide double perovskites Cs₂PdBr_6-I by halogen anions substitution. The structure, stability, and electronic and photoelectric properties were explored using density functional theory (DFT). The negative value of the formation energy indicated that the Cs₂PdBr_6-I perovskites are thermodynamically stable. These perovskites exhibit tunable bandgap values in the range of 0.77-1.73 eV, which are direct or quasi-direct bandgaps except for Cs₂PdBr₃I₃. Their absorption spectrum shows that the absorption range of visible light expands significantly. The theoretical spectral limit maximum efficiency (SLME) of Cs₂PdBr₅I with 1.3 eV and Cs₂PdBr₄I₂ with 1.04 eV reached 32 and 30.4%, respectively, which are becoming comparable to or slightly surpassing CH₃NH₃PbI₃, indicating they could be candidates for single-junction solar cells. In addition, the Cs₂PdBr₃I₃ and the Cs₂PdBr₄I₂, with the bandgap of 1.12 and 1.04 eV, respectively, could be the bottom cell to form the homogeneous tandem solar cells with the Cs₂PdBr₆, which could be the top cell with the bandgap of 1.73 eV.