Defect Engineering of Bi2SeO2 Thermoelectrics

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 same nominal compositions. Such unsystematic variations in the electron concentration has a thermodynamic origin related to the variations in native defect concentrations. In this study, we use first-principles calculations 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. We predict 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. We validate our computational predictions through solid-state synthesis of Bi2SeO2. We observe more than two orders of magnitude increase in the electron concentration simply by adjusting the synthesis conditions. Additionally, we reveal the significant effect of grain boundary scattering on electron mobility in Bi2SeO2, which can also be controlled by adjusting T_SSR. By simultaneously optimizing the electron concentration and mobility, we achieve a zT of ~0.2 at 773 K for self-doped n-type Bi2SeO2. Our study highlights the need for careful control of thermodynamic growth conditions and demonstrates TE performance improvement by varying synthesis parameters according to thermodynamic guidelines.