Complicated and Unconventional Defect Properties of the Quasi-One-Dimensional Photovoltaic Semiconductor Sb₂Se₃

It was expected that the properties of intrinsic point defects would be simple in the binary semiconductor Sb₂Se₃. However, we show using first-principles calculations that the intrinsic defects in this quasi-one-dimensional (Q1D) semiconductor are unexpectedly complicated and different from those in conventional photovoltaic semiconductors such as CdTe or GaAs. First, the same type of defects located on non-equivalent atomic sites can have very different properties due to the low symmetry of the Q1D structure, which makes the properties of point defects complicated, even though there are only a few point defects. Second, uncommon defects such as the cation-replace-anion antisite SbSe, anion-replace-cation antisite SeSb, and even two-anion-replace-one-cation antisite 2SeSb, which are difficult to form in CdTe and GaAs, can have high concentrations and even be dominant in Sb₂Se₃ due to the weak van der Waals interactions and the large void space between different [Sb₄Se₆] n atomic chains of the Q1D structure. These defects produce a series of acceptor and donor levels in the band gap and make Sb₂Se₃ p-type under the Se-rich condition but n-type under the Se-poor condition. Five deep-level recombination-center defects are identified, and their formation is difficult to suppress, imposing a serious limit to the development of high-efficiency Sb₂Se₃ solar cells. Our study demonstrates that the defects can be complicated and unconventional in the binary compound semiconductors with low symmetry and Q1D structures, which can be classified as chemically binary while structurally multinary.