Study of the defect chemistry in Ag2Q (Q = S, Se, Te) by first-principles calculations

Ag2Q-based (Q = S, Se, Te) silver chalcogenides show great potential in thermoelectrics due to their suitable band gaps, high electron mobilities, and even remarkable ductility. Particularly, Ag2S and Ag2S/Se/Te solid solutions have been reported with both good ductility and thermoelectric performance, which are extremely suitable for the application in flexible wearables. However, the underlying mechanism of the native n-type conduction and p-type undopability for Ag2Q remains elusive. Herein, we use first-principles calculations based on density functional theory combined with GW correction to investigate the defect chemistry in Ag2Q. It is found that the site potential and Voronoi volume deviations resulting from Ag interstitials are noticeably smaller than those caused by Ag vacancies, which makes Ag interstitials with low formation energy more desirable during preparation, contributing to the native n-type conduction. The small and even negative dopability win-dows, on the other hand, account for the p-type undopability of Ag2Q. The calculated carrier concentrations of pristine Ag2Q are well consistent with the experimental observations, validating the reliability of our defect calculations. This work provides valuable guidance for first-principles calculation of defect chemistry in other narrow-gap semiconductors.