Unravelling charge-heat interconversion and transport in Ag2Se thermoelectrics

Abstract

Thermoelectric materials must have a balance of high electronic conductivity and low thermal conductivity. Doping is one approach to achieve this goal, both to enhance electronic conductivity and suppress thermal transport. Because it has these multiple effects, however, it can both increase or reduce thermoelectric efficiency. We use stroboSCAT spatiotemporal microscopy to investigate doping in thermoelectrics, providing direct access to electronic and thermal diffusivities inaccessible to bulk methods. With measurements spanning a large fs-μs dynamic range, we observe that heat transport is strongly influenced by material morphology. While long-time thermal diffusivity measured by stroboSCAT agrees with bulk measurements, in the first 100ps heat diffuses within crystalline domains with three orders of magnitude greater diffusivity. The transition to bulk-like diffusion occurs when heat encounters grain boundaries. Intercrystalline diffusion is strongly modulated by doping, while intracrystalline diffusion is unaffected. Our work offers microscopic mechanistic insights into how doping affects thermoelectricity.