High Thermoelectric Power Factor in Cuprous Sulfide Thin Films for Low-Temperature Thermoelectric Applications

Cuprous sulfide (Cu2S) is a promising sustainable thermoelectric material for waste heat recovery due to its earth abundance and favorable electronic properties. However, there is limited information on its performance at low temperatures and in reduced-dimensional systems. This paper reports the thermoelectric performance of Cu-deficient Cu2−xS (x > 0) thin films fabricated by ion beam sputtering and thermal treatment (Tanneal = 100 – 400 °C). X-ray diffraction patterns showed monoclinic djurleite Cu1.94S for the as-deposited and 100 °C annealed film and the cubic digenite Cu1.80S phase for 200 and 300 °C annealed Cu2−xS films. Tanneal = 400 °C showed the formation of cubic Cu2O, indicating oxidation of the film. X-ray photoelectron results confirmed Cu1+ and S2− valence states, indicating the Cu2S phase. Results of XPS and UV−visible spectroscopies showed band-gap widening due to an increase in defect concentration in the annealed films. High electrical conductivity (2291.5 Ω−1 cm−1) and Seebeck coefficient (35.4 μV K−1) yield the highest power factor (α2σ ≈ 286.9 μW m−1 K−2) in the as-deposited Cu1.94S film compared to α2σ ≈ 132.9 μW m−1 K−2 in the digenite Cu1.80S in the 300 °C annealed Cu2−xS film. This research highlights the potential of djurleite Cu1.94S for low-temperature thermoelectric applications.