Layered post transition metal chalcogenides such as SnSe, SnSe₂, In₂Se₃, and In₄Se₃ have attracted attention as promising thermoelectric materials due to their intrinsically low lattice thermal conductivities. Recently, <i>n</i>-type indium selenide (InSe) based materials have also been suggested as good candidates for thermoelectric materials by optimizing their electrical properties, i.e., increasing carrier concentration. Here, we report enhancement of the thermoelectric properties of <i>n</i>-type InSe by Sn substitutional doping at the In site. A series of In_1-xSn_xSe for x = 0, 0.03, 0.05, 0.15 and 0.2 was examined. The carrier concentration and electrical conductivity increased due to the Sn substitution, since Sn behaves as a shallow electron donor in InSe, while the Seebeck coefficient decreased moderately. In addition, it was found that effective mass was increased by more than 10 times by Sn doping. As a result, the power factor was enhanced from 0.07 mW/mK² to 0.13 mW/mK² at 800 K. The total thermal conductivity was unchanged despite Sn doping because the electrical contribution to the total thermal conductivity was very small. Consequently, Sn doping in InSe enhanced the dimensionless thermoelectric figure of merit <i>zT</i> from 0.04 to 0.14 at 800 K, mainly due to enhanced electrical properties.

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Significant bipolar conduction of the carriers in Bi2Te3-based alloys occurs at high temperatures due to their narrow bandgaps. Therefore, at high temperatures, their Seebeck coefficients decrease, the bipolar thermal conductivities rapidly increase, and the thermoelectric figure of merit, zT, rapidly decreases. In this study, band modification of n-type Cu0.008Bi2(Te,Se)3 alloys by sulfur (S) doping, which could widen the bandgap, is investigated regarding carrier transport properties and bipolar thermal conductivity. The increase in bandgap by S doping is demonstrated by the Goldsmid–Sharp estimation. The bipolar conduction reduction is shown in the carrier transport characteristics and thermal conductivity. In addition, S doping induces an additional point-defect scattering of phonons, which decreases the lattice thermal conductivity. Thus, the total thermal conductivity of the S-doped sample is reduced. Despite the reduced power factor due to the unfavorable change in the conduction band, zT at high temperatures is increased by S doping with simultaneous reductions in bipolar and lattice thermal conductivity.

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