Physics of large thermoelectric power factors in SnSe nanoflakes in mid-temperature range

We have theoretically investigated the underlying physics of observed high electrical conductivity (& sigma;), simultaneous increase of & sigma; and Seebeck coefficient (S) with temperature, and large power factors (PFs) in nominally undoped SnSe nanoflakes sintered at different temperatures, reported recently in Mandava et al (2022 Nanotechnology 33 155710). Given the fact that S and & sigma; show unusual temperature trends and that the undoped SnSe samples are highly porous and disordered, the conventional Boltzmann theory does not appear to be an appropriate model to describe their transport properties. We have, instead, used a strong disorder model based on percolation theory where charge and energy transport take place through hopping between localized states to understand these observations. Our model is able to explain the observed temperature dependence of & sigma; and S with temperature. Large & sigma; can be explained by a high density of localized states and a large hopping rate. The sample sintered at a higher temperature has lower disorder (& sigma; DOS) and higher hopping rate (1/& tau; 0). We find & sigma; DOS = 0.151 eV and 1/& tau; 0 = 0.143 x 1015 s-1 for sample sintered at 673 K and & sigma; DOS = 0.044 eV and 1/& tau; 0 = 2.023 x 1015 s-1 for sample sintered at 703 K. These values are comparable to the reported values of transition frequencies, confirming that the dominant charge transport mechanism in these SnSe nanoflakes is hopping transport. Finally, we suggest that hopping transport via localized states can result in enhanced thermoelectric properties in disordered polycrystalline materials.