Augmentation of thermoelectric power factor in the magnetron sputtered Cu2Se thin films by air annealing

Abstract

Copper selenide (Cu2Se) emerged as a potential thermoelectric (TE) material owing to its composition from earth-abundant elements, extraordinary electrical conductivity, and inherently lower thermal conductivity, making it suitable for mid-temperature waste heat recovery. In thin film form, Cu2Se offers additional advantages such as ease of integration into compact thermoelectric devices, sensors, and energy harvesters, due to its scalability, potential mechanical flexibility, and compatibility with microfabrication technologies. In this work, deposition of Cu2Se thin films was carried out employing radio frequency magnetron sputtering onto a glass substrate and subjected to air annealing across a temperature gradient spanning 200 degrees C-350 degrees C with an interim of 50 degrees C each. An extensive analysis of the structural, optical, morphological, chemical constitutions, and electrical transport properties of the films was done. The X-ray Diffraction (XRD) patterns and energy-dispersive X-ray spectroscopy (EDS) analysis revealed a progressive formation of oxides with increasing annealing temperatures, with a pronounced augmentation observed at 350 degrees C, indicating notable Cu2Se dissociation. The UV-vis analysis revealed that the film annealed at 250 degrees C exhibited 1.3 eV optical band gap, suggesting a composition closer to the ideal stoichiometry that favors the Cu2Se phase. Mechanical studies showed that films exhibited maximum hardness of 2.27 f 0.27 GPa at 250 degrees C. The films annealed at 250 degrees C showed a power factor of 18.2 mu W/cmK2 at 450 degrees C, indicating that moderate temperature annealing significantly improves film quality and performance. These findings establish 250 degrees C as the optimal annealing temperature for enhancing the electrical characteristics of sputtered Cu2Se thin films.