Enhanced thermoelectric properties in Cu₁.₈Se thin films: Achieving superior power factor through phase control and optimal deposition temperature

Thermoelectric thin films are vital for efficient energy conversion and thermal management. XRD analysis reveals that Cu₁.₈Se films exhibit a mixed α-phase when deposited at room temperature, 100 °C, and 300 °C, but transition to a pure β-phase at 200 °C. The formation of the pure β-Cu₁.₈Se phase at 200 °C significantly improves the crystallinity of the films. Increased annealing temperatures lead to greater surface roughness and grain size as observed by AFM and FESEM. Electrical conductivity decreases with higher measurement temperatures, reflecting degenerate semiconductor behavior due to Cu vacancies. The sample deposited at 200 °C, exhibiting the pure β-phase, achieves the highest power factor of 5456 μWm⁻¹K⁻² and improved Seebeck coefficient, underscoring the importance of phase purity and controlled surface roughness for optimal thermoelectric performance.