Tuning the thermoelectric performance of flexible copper selenide thin films through sputtering pressure and hybrid microwave annealing

Abstract

Flexible copper selenide (Cu₂₋ₓSe) thin films were deposited on polyimide substrates by direct current magnetron sputtering under varying pressures (0.8–4.0 × 10⁻² mbar) and subsequently annealed using hybrid microwave irradiation at 250 °C for 10–30 min. Increasing sputtering pressure raised the copper content (62.2–63.8 at%) and suppressed the formation of Cu₃Se₂ impurities. Hybrid microwave annealing promoted the transformation to stoichiometric β-Cu₂Se, removed oxide phases such as selenium dioxide and copper oxide, and improved crystallinity, as confirmed by x-ray diffraction and x-ray photoelectron spectroscopy. Field-emission scanning electron microscopy revealed microstructural densification at 10–20 min, whereas 30 min induced cracks and porosity that degraded transport properties. The optimized 20-minute annealed film achieved a peak power factor of 81.5 × 10⁻⁵ W/m·K² at 300 °C—over 130 times higher than that of the as-deposited film and comparable to other flexible Cu₂Se systems. Stability tests confirmed excellent retention after three months of ambient storage. These results establish sputtering pressure control and hybrid microwave annealing as scalable strategies for high-performance, stable Cu₂Se thermoelectric films.