Electronic structure and transport properties of TlInSe2 and Tl0.5Li0.5InSe2

We report calculations of the electronic structure of thermoelectric ternary chalcogenide TlInSe2 in the pressure range 0-30 GPa and the Li-substituted compound Tl0.5Li0.5InSe2 using density functional theory. Moreover, with Boltzmann transport theory the electronic transport properties of these compounds are investigated at the optimal p-doping level for a maximized power factor. We follow two possible band engineering routes by applying pressure and elemental substitution with Li to investigate a possible enhancement of the electronic properties for thermoelectric applications. Our study employs several exchange-correlation functionals including the spin-orbit interaction as well as the B3LYP hybrid functional. The band gap in TlInSe2 obtained by using the Tran-Blaha modified Becke-Johnson functional is in good agreement with experimental data. We find a direct band gap for TlInSe2 at the M-point and a slightly larger energy gap at the Z-point. The spin-orbit (SO) splitting is extracted from the calculated electronic band structure. When applying pressure to TlInSe2 the Seebeck coefficient strongly decreases and band crossing results in metallic properties above 20 GPa. In contrast to TlInSe2, an indirect band gap is found for Tl0.5Li0.5InSe2 with the valence band maximum located at an off-symmetry point along the M-X direction and the conduction band minimum located at an off-symmetry point along the X-P direction. In contrast to TlInSe2 at ambient pressure, taking the SO coupling into account for Tl0.5Li0.5InSe2 and TlInSe2 at 20 GPa is necessary as it markedly changes the transport properties. Optimally doped p-type TlInSe2 at ambient pressure has the most favorable electronic band structure for thermoelectric applications superior to both, optimally doped p-type TlInSe2 under pressure and optimally doped p-type Tl0.5Li0.5InSe2. (C) 2018 Elsevier Ltd. All rights reserved.