Structure and Thermoelectric Transport Analysis of Defect-Containing CuGaTe2 Prepared by Room-Temperature High-Pressure Treatment

The effect of room-temperature high-pressure (RTHP) treatment on the transport properties, crystal structure, and electronic structure of CuGaTe2 with chalcopyrite structure was investigated. Dense bulk samples were obtained without the application of heat. In addition, the thermal conductivity of the RTHP-treated CuGaTe2 was greatly reduced compared with that of its hot-pressed counterpart. Theoretical analysis based on the Debye–Callaway model indicated that this reduction in the thermal conductivity originated from point defects, stacking faults, and increased grain boundaries. These defects also decreased the carrier mobility and increased the carrier concentration, resulting in reductions of the electrical conductivity and Seebeck coefficient. Crystal structure analysis revealed that the RTHP-treated CuGaTe2 contained microstrains, which partially arose from a series of disorder at the Cu/Ga cation site with various degrees and geometries. From the calculated formation energies, we predicted that such disorder can be induced as a metastable structure by the RTHP treatment applied in this study. Electronic structure calculations clarified that this disorder at the Cu/Ga cation site affected the orbital hybridization. Specifically, the shape of the band structure near the Fermi level was modified, and the bandgap was considered to decrease compared with that of pristine CuGaTe2; these findings partially explain the experimentally observed transport properties of the RTHP-treated CuGaTe2.