Indium substitution effect on thermoelectric and optical properties of Sn_1-xIn_xSe compounds

Recent investigations of the high thermoelectric figure-of-merit ZT of 2.6 for SnSe crystalline compound demonstrated the state-of-the-art highest thermoelectric performance in spite of the high ZT value in a narrow temperature range and along a specified crystallographic direction. We investigated the thermoelectric properties of polycrystalline Sn_1-x In_xSe (x = 0.0, 0.01, 0.05, and 0.1) compounds and the optical properties of single crystalline compounds. Sn_1-x In_xSe exhibits increased electrical resistivity, an increase in the Seebeck coefficient and band gap energies, and a significant decrease in the Hall mobility with an increase in the In-doping concentration, whereas the Hall carrier density does not changed significantly. These abnormal results and the decrease in the lattice volume with an increase in the In-doping concentration strongly imply the existence of band renormalization by In 4d- and Se 4p-orbital hybridization. We argue that the significant flat bands by band renormalization induce significantly decreased Hall mobility and increases in the electrical resistivity and Seebeck coefficient, resulting in the deterioration of the thermoelectric properties.