Defect Engineering in Bi₂SeO₂ by Tuning Synthesis Conditions - A Combined Experimental and Theoretical Study

Recently, Bi₂SeO₂ has emerged as a promising n-type counterpart to p-type BiCuSeO in thermoelectric (TE) devices. However, there are significant variations in the reported charge carrier concentration of Bi₂SeO₂ spanning several orders of magnitude, resulting in diverging reports of TE properties. These predominantly arise from the disparate synthesis routes used, which in turn affect the defect thermodynamics in the system. In this study, we employ a combined experimental and theoretical approach to demonstrate how defect engineering controlled by synthesis conditions can be used to tailor the transport properties of n-type Bi₂SeO₂. Through first-principles calculations, we identify the dominant native defect in Bi₂SeO₂ as the donor-like selenium vacancy (V_Se). We predict that increasing the synthesis temperature (T_SSR) of nominally undoped Bi₂SeO₂ will lead to an increase in V_Se and, consequently, electron concentration. We observe a rise in carrier concentration by nearly two orders of magnitude between samples synthesized at T_SSR = 773 K and T_SSR = 1173 K. This results in the enhancement in both the quality factor and the TE performance with a remarkable zT value of 0.25 at 773 K achieved for self-doped Bi₂SeO₂. Our findings underscore a noteworthy relationship between processing conditions and the transport properties of Bi₂SeO₂ rooted in defect engineering.