Harnessing High Power Factors with Enhanced Stability in Heavy Metal-Free Solution-Processed Thermoelectric Copper Sulfoselenide Thin Films

Abstract

Thermoelectric devices have the potential to recover waste heat from inefficient energy conversion processes. State-of-the-art thermoelectrics demonstrate low efficiency and incorporate materials containing rare and toxic elements. In this regard, p-type copper selenide (Cu 2 Se) has been identified as a promising and environmentally benign alternative. Unfortunately, the high diffusivity of liquid-like copper ions results in structural instability and performance degradation during operation, especially at moderate to high temperatures above 200 °C. Sulfur substitution has been utilized in melt-annealed samples to improve the stability of Cu 2 Se during operation, however this fabrication process is energy intensive and does not allow for use of flexible substrates. In this work, we report a solution-based direct thin film route to tune carrier concentration in copper sulfoselenide (Cu 2-y S x Se 1-x ) thin films by controlling sulfur content and degree of copper saturation. We observe that improved thermoelectric performance through copper saturation in nominally copper-deficient Cu 2-y Se films comes at a huge cost, with significantly reduced material stability due to enhanced copper migration resulting in severe degradation of the thermopower. Circumventing copper saturation, we show that controlled sulfur addition and tuning of annealing temperature has a synergistic effect, resulting in improved stability of the thermoelectric properties during continuous operation for mildly copper-deficient films while sustaining a high power factor of 800 μW/mK 2 at room temperature. Our results demonstrate a pathway for generating high performance solution processed thermoelectric devices with flexible form factors, and reinforce the case for Cu 2-y S x Se 1-x thin films as a heavy metal free alternative for scavenging low grade waste heat.