First-Principles Insights into Cs2BiCuBr6 and Cs2BiCuI6 Double Perovskites for Energy Applications

The structural, electrical, optical and thermoelectric characteristics of the Pb-free Cs2BiCuBr6 and Cs2BiCuI6 double perovskites are examined in this work using first-principles density functional theory (DFT). Both compounds are established to be structurally stable and exhibit semiconducting character with narrow bandgaps significantly influenced by spin–orbit coupling (SOC) effects due to presence of the heavy Bi atoms. Optical analysis reveals that Cs2BiCuBr6 and Cs2BiCuI6 possess superior visible-light absorption, higher dielectric response, and stronger optical conductivity, making it a promising candidate for optoelectronic devices. Thermoelectric analysis shows that Cs2BiCuI6 offers enhanced performance at high temperatures, while Cs2BiCuBr6 is more effective at room temperature due to its lower thermal conductivity. Cs2BiCuBr6 and Cs2BiCuI6 support flexible doping strategies for indirect bandgap semiconducting (p-type and n-type) applications.