Bismuth and tellurium co-doping: a route to improve thermoelectric efficiency in InSe polycrystals

Indium selenide (InSe), a layered chalcogenide material, has gained substantial scientific interest as a thermoelectric material due to its intrinsic low thermal conductivity. However, its intrinsic carrier concentration is notably minimal (∼10¹⁴ cm⁻³) due to a significant bandgap of 1.3 eV limiting its thermoelectric efficiency. Therefore, to optimize InSe-based materials for thermoelectric applications, it is essential to increase the carrier concentration through precise doping methodologies. In this study, co-doping at both the anion and cation sites of InSe was achieved by introducing Bi to the In site and Te to the Se site. The impact of this co-doping on the thermoelectric performance of InSe-based materials was thoroughly investigated. The increase in carrier concentration due to the electron-donating nature of Bi significantly enhanced the electrical transport properties and the Seebeck coefficient (S) experienced a minor reduction, and the incorporation of Bi atoms resulted in a substantial improvement in the power factor (PF) across the temperature range. Among all the samples studied, In_0.96Bi_0.04Se_0.97Te_0.03 exhibited the highest PF throughout the temperature range. The dopants Bi/Te acted as an effective phonon scattering center, reducing lattice thermal conductivity. The synergistic effect of cation-anion co-doping resulted in a maximum ZT of ∼0.13 at 630 K in the In_0.96Bi_0.04Se_0.97Te_0.03 sample, which is nearly 11 times higher compared to the pristine sample. Considering these findings, Bi-Te co-doped InSe emerged as a highly promising material for thermoelectric applications.