Thin film thermoelectric elements of p–n tin chalcogenides from chemically deposited SnS–SnSe stacks of cubic crystalline structure

Abstract

SnS and SnSe thin films of cubic (CUB) crystalline structure prepared by chemical deposition are recent additions to their orthorhombic polymorphs. These structures are simple cubes with lattice constants 11.587 and 11.963 Å, respectively, holding 32 formula units. Their chemically deposited thin films have direct optical bandgaps, 1.76 and 1.4 eV, respectively. Electrical conductivity of these films are 10–8 to 10–6 Ω–1 cm–1, which are low for photovoltaic or thermoelectric applications. We report that SnS-CUB/SnSe-CUB thin film stacks convert to p-type SnSxSe1−x or n-type SnSxSe2−x when heated at 350 °C for 30 min in a nitrogen ambient in presence of selenium vapor, depending on its abundance. They have an onset of optical absorption at 1.3–1.5 eV. The electrical conductivity of the stack increases from 1.3 × 10–5 Ω–1 cm–1 toward 0.3 Ω–1 cm–1 for p-type SnSxSe1−x or 0.8 Ω–1 cm–1 for n-type SnSxSe2−x. Seebeck coefficients (S) are, + 0.6 mV K−1 for the p-type and −0.4 mV K−1 for the n-type material in the 25 °C–45 °C interval. Three-element n-p-n thermoelectric devices have S, 1.1–1.6 mV K−1. We analyze the feasibility of solar radiation as the heat source for such devices.