Enhancement of thermoelectric properties through alloying in two-dimensional halide materials

Two-dimensional (2D) copper halides represent an emerging class of semiconductors with tunable electronic and optical properties, yet their structural stability and alloying behavior remain largely unexplored. Alloying is an effective strategy for enhancing the thermoelectric response of two-dimensional materials. Using first-principles density functional theory, we investigate the structural, electronic, and thermoelectric properties of monolayer CuI–CuBr alloys. The calculations predict thermodynamically stable CuIxBr1−x solid solutions across the full compositional range, as evidenced by uniformly negative mixing enthalpies for all considered crystal symmetries. Halide substitution induces systematic lattice contraction and symmetry-dependent distortions of the Cu-centered coordination environment, leading to a monotonic reduction of the electronic bandgap with increasing Br content. Within the constant relaxation time approximation, alloying significantly enhances the electronic contribution to the thermoelectric figure of merit, leading to high calculated ZT values within the constant relaxation time approximation framework, which should be interpreted as electronic upper-limit estimates at elevated temperatures for all investigated polymorphs. These results demonstrate that alloying provides a robust and controllable route to improve thermoelectric performance in two-dimensional materials.