Rapid Stoichiometry Control in Cu2Se Thin Films for Room-Temperature Power Factor Improvement

Cu2Se thin films provide a promising route toward relatively safe, sustainable and solution processed thermoelectric (TE) modules in contrast to more expensive and toxic materials currently on the market such as Bi2Te3. Cu2Se is known in the TE community for its high performance at high temperature and has recently attracted attention from its large theoretically predicted figure of merit at room temperature. Unfortunately, one of the main limitations encountered so far in Cu2Se thin films is that the carrier concentrations are not optimized for TE operation after solution processing. In this work, we conduct a comprehensive study of the structural, optical, and TE properties of Cu2Se thin films and demonstrate that nonoptimized carrier concentrations in these films lead to observations of poor performance at room temperature. Through a simple soaking procedure in a Cu+ ion solution for only a few minutes, we demonstrate a 200-300% increase in power factor. This soaking process pushes the carrier concentration of the Cu2Se thin film toward its optimal value for TE operation and marks the highest TE performance for any solution processed Cu2Se thin film at room temperature thus far. If the performance can be further optimized at room temperature, Cu2Se thin films will be the material of choice to utilize in TE modules for powering miniature electronics and sensors, which has been an increasingly popular and rapidly expanding market.