Synergistic optimization of thermoelectric performance in earth-abundant Cu2ZnSnS4 by inclusion of graphene nanosheets.

Earth-abundant quaternary chalcogenide semiconductors with complex structures, such as copper zinc tin sulphide (Cu2ZnSnS4; CZTS), have the potential to become economic and non-toxic thermoelectric materials. However, the inferior power factor of CZTS, due to its insignificant electrical conductivity, negates the advantage of inherent small thermal conductivity. In the present report, the thermoelectric properties of CZTS composites integrated with graphene nanosheets (GNs) CZTS/x (x = 0.25, 0.5, 0.75 or 1 wt% GNs) were synergistically optimized. The inclusion of GNs (⩽0.75 wt%) simultaneously enhanced the carrier transport (electrical conductivity σ) by providing conductive pathways as well as suppressed lattice thermal conductivity (κ L) due to the enhanced grain barriers in addition to interface scattering. This synergistic optimization enhanced the figure of merit, ZT, of CZTS/GN nanocomposites to its highest value (∼0.5) at 623 K for the addition of 0.75 wt% GNs, which is nearly a seven-fold enhancement over the pristine sample. The novel strategy of fabricating CZTS/GN nanocomposites by utilizing GNs is an alternative way to obtain the highest thermoelectric performance (ZT ∼ 0.5) in CZTS, and can be extended to other environmentally friendly quaternary chalcogenides.

Tailoring of Seebeck coefficient with surface roughness effects in silicon sub-50-nm films.

The effect of surface roughness on the Seebeck coefficient in the sub-50-nm scale silicon ultra thin films is investigated theoretically using nonequilibrium Green's function formalism. For systematic studies, the surface roughness is modelled by varying thickness periodically with square wave profile characterized by two parameters: amplitude (A 0) and wavelength (λ). Since high Seebeck coefficient is obtained if the temperature difference between the ends of device produces higher currents and higher induced voltages, we investigate how the generated current and induced voltage is affected with increasing A 0 and λ. The theoretical investigations show that pseudoperiodicity of the device structure gives rise to two effects: firstly the threshold energy at which the transmission of current starts is shifted towards higher energy sides and secondly transmission spectra of current possess pseudobands and pseudogaps. The width of the pseudobands and their occupancies determine the total generated current. It is found that current decreases with increasing A 0 but shows a complicated trend with λ. The trends of threshold energy determine the trends of Seebeck voltage with roughness parameters. The increase in threshold energy makes the current flow in higher energy levels. Thus, the Seebeck voltage, i.e. voltage required to nullify this current, increases. Increase in Seebeck voltage results in increase in Seebeck coefficient. We find that threshold energy increases with increasing A 0 and frequency (1/λ). Hence, Seebeck voltage and Seebeck coefficient increase vice versa. It is observed that Seebeck coefficient is tuneable with surface roughness parameters.