Enhancing Thermoelectric Performance through Semiconductor–Semimetal Heterostructure via Entropy- and Enthalpy-Driven Phase Regulation in Al/S-Doped BiSbSe3

This study presents a phase-regulation strategy for enhancing the thermoelectric performance of BiSbSe3-based materials through the controlled formation of a semiconductor–semimetal heterostructure. By incorporating sulfur and aluminum dopants, a hexagonal phase is induced within an orthorhombic BiSbSe3 matrix, thereby establishing a bidirectionally adjustable phase composition through compositional regulation. The hexagonal phase exhibits semimetallic behavior, with a Fermi level positioned higher than that of the semiconductor matrix. This electronic structure difference is consistent with low-barrier interfacial charge transfer and a modulation-doping-like redistribution of electrons across the phase boundaries, contributing to enhanced carrier concentration without severely sacrificing carrier mobility. Moreover, the inherently low thermal conductivity of both phases, combined with enhanced phonon scattering at the interfaces and simultaneous control over grain refinement during phase regulation, effectively suppresses the lattice thermal conductivity. The resulting biphasic structure enables coupled regulation of electronic and thermal transport, synergistically improving thermoelectric performance, achieving a peak thermoelectric figure of merit (zT) of 1.50 at 773 K in Bi0.98Sb0.98Al0.04Se2.8S0.2 under the fixed 2 wt % CuI donor-additive condition. This improvement relative to BiSbSe3 materials demonstrates the effectiveness of phase engineering in decoupling electrical and thermal transport properties. These results highlight interface-engineered semiconductor–semimetal heterostructures as an effective design route for high-performance thermoelectric materials.