Halide Double Perovskites Cs2PdBr6–xIx 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-xI_x 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-xI_x 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.