Enhanced Thermoelectric Properties of Oxyselenide Bi₂O₂Se via Cl Doping and Microstructure Modulation

Bi₂O₂Se is an emerging oxyselenide semiconductor, noted for its promising thermoelectric properties and excellent chemical stability, and it is often regarded as an n-type counterpart to p-type BiCuSeO. However, its intrinsic thermoelectric figure of merit (zT) is severely limited by low electron concentration and high thermal conductivity. In this work, we employed Cl element as donator dopant to substantially enhance the carrier concentration. The room-temperature carrier concentration of Bi₂O₂Se_0.98Cl_0.02 thereby reached 2.70 × 10²⁰ cm^-3, representing an improvement of two orders of magnitude compared to that of pristine Bi₂O₂Se. Subsequently, multiple hot-pressing cycles were applied to the optimized composition Bi₂O₂Se_0.98Cl_0.02. The process induced significant grain refinement, and the resulting high density of grain boundaries effectively suppressed the lattice thermal conductivity, reducing it to 0.95 W·m^-1·K^-1 at 823 K. Eventually, a maximum zT of 0.24 was achieved at 823 K for the three-time hot-pressed Bi₂O₂Se_0.98Cl_0.02 sample, representing a 71% improvement compared with the pristine Bi₂O₂Se sample prepared by single hot-pressing (zT = 0.14 at 823 K). This work provides a new strategy for enhancing the thermoelectric performance of oxyselenide through the synergistic regulation of doping and microstructure.