Defect Engineering of Bi2SeO2 Thermoelectrics

Abstract

Bi₂SeO₂ is a promising n-type semiconductor to pair with p-type BiCuSeO in a thermoelectric (TE) device. The TE figure of merit zT and, therefore, the device efficiency must be optimized by tuning the carrier concentration. However, electron concentrations in self-doped n-type Bi₂SeO₂ span several orders of magnitude, even in samples with the same nominal compositions. Such unsystematic variations in the electron concentration have a thermodynamic origin related to the variations in native defect concentrations. In this study, first-principles calculations are used to show that the selenium vacancy, which is the source of n-type conductivity in Bi₂SeO₂, varies by 1–2 orders of magnitude depending on the thermodynamic conditions. It is predicted that the electron concentration can be enhanced by synthesizing under more Se-poor conditions and/or at higher solid-state reaction temperatures (T_SSR), which promote the formation of selenium vacancies without introducing extrinsic dopants. The computational predictions are validated through solid-state synthesis of Bi₂SeO₂. More than two orders of magnitude increase are observed in the electron concentration simply by adjusting the synthesis conditions. Additionally, a significant effect of grain boundary scattering on the electron mobility in Bi₂SeO₂ is revealed, which can also be controlled by adjusting T_SSR. By simultaneously optimizing the electron concentration and mobility, a zT of ≈0.2 is achieved at 773 K for self-doped n-type Bi₂SeO₂. The study highlights the need for careful control of thermodynamic growth conditions and demonstrates TE performance improvement by varying synthesis parameters according to thermodynamic guidelines.