More Se Vacancies in Sb₂Se₃ Under Se-Rich Conditions—An Abnormal Behavior Induced by Defect-Correlation in Compensated Compound Semiconductors

Anion vacancies are the most important defects in compound semiconductors. They usually act as deep donors, limiting the p-type conductivity and causing the non-radiative recombination, so suppressing their formation and reducing their density are critical for developing high-performance electronic and optoelectronic devices. One widely used method is through adopting the synthesis condition in which the anion element is rich with a high elemental chemical potential, <i>e.g.</i>, Te-rich condition suppresses V_Te in CdTe, and N-rich condition suppresses V_N in GaN. The method is based on the simple chemical intuition that it is difficult for the atoms to leave the lattice and form vacancy defects when the element is rich in the environment. This intuition is direct and simple, so the method has become the fundamental defect-engineering strategy in many compound semiconductors.<br/>In this work, however, we will demonstrate that this simple chemical intuition and the method are invalid in donor-acceptor compensated semiconductors, <i>e.g.</i>, the binary compound semiconductors Sb₂Se₃ and Sb₂S₃. Our first-principles calculations show that the density of Se vacancies (V_Se) can NOT be reduced through very Se-rich condition, and on the contrary, its density can increase to a very high level as Se becomes very rich. Therefore, more Se anions leave the Sb₂Se₃ lattice in the Se-rich environment, in sharp contrast with the simple chemical intuition.<br/>To explain such an abnormal behavior, we introduce a physical concept of the so-called defect-correlation, <i>i.e.</i>, the formation energy and density of an ionized defect depend not only on the elemental chemical potential but also on the Fermi level, which is determined by densities of all ionized donor and acceptor defects, so all ionized defects are correlated with each other. Therefore, the density of ionized anion vacancy depends not only on itself but also on other ionized defects. In the low-symmetry semiconductors such as Sb₂Se₃, there are many types of donor and acceptor defects which can compensate each other, so the defect-correlation can become complicated and thus make the abnormal behavior possible, <i>e.g.</i>, the formation of other ionized defects can lower the Fermi level and lower the formation energy of ionized Se vacancies, thus increasing their density under Se-rich condition. The defect-correlation can be derived from the well-established defect formation energy theory, however, the abnormal behavior has seldom been reported in previous studies because most of the previous studies were focused on the traditional compound semiconductors (zincblende or wurtzite II-VI and III-V compounds) in which the number of possible defects is limited and the defect-correlation is simple. However, in many new low-symmetry or multinary semiconductors, the more serious donor-acceptor compensation and more complicated defect-correlation make the abnormal behavior highly possible.<br/>The abnormal behavior highlights the importance of revisiting the defect physics and fundamental defect-engineering strategies in new low-symmetry or multinary semiconductors, because the widely used strategy may be not valid. Among the intensive studies of solar cells based on Sb₂Se₃ and Sb₂S₃ during the past decade, it was widely believed that the Se-rich condition is optimal for reducing Se vacancy density, so the post selenization process is required for improving photovoltaic performance. However, our finding challenges this common belief. Interestingly, the recent experiment indeed showed that the extremely Se-rich condition causes the efficiency drop of Sb₂Se₃ solar cells, consistent with our finding.<br/>References:<br/>[1] M. Huang, <i>et al.</i>, Small 2021, 17, 2102429.<br/>[2] M. Huang, <i>et al.</i>, ACS Applied Materials & Interfaces, 2019, 11, 15564.