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 Cs2PdBr6–xIx 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 Cs2PdBr6–xIx 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.