More Se Vacancies in Sb₂ Se₃ under Se-Rich Conditions: An Abnormal Behavior Induced by Defect-Correlation in...

It was believed that the Se-rich synthesis condition can suppress the formation of deep-level donor defect V_Se (selenium vacancy) in Sb₂ Se₃ and is thus critical for fabricating high-efficiency Sb₂ Se₃ solar cells. However, here it is shown that by first-principles calculations the density of V_Se increases unexpectedly to 10¹⁶ cm^-3 when the Se chemical potential increases, so Se-rich condition promotes rather than suppresses the formation of V_Se . Therefore, high density of V_Se is thermodynamically inevitable, no matter under Se-poor or Se-rich conditions. This abnormal behavior can be explained by a physical concept "defect-correlation", i.e., when donor and acceptor defects compensate each other, all defects become correlated with each other due to the formation energy dependence on Fermi level which is determined by densities of all ionized defects. In quasi-1D Sb₂ Se₃ , there are many defects and the complicated defect-correlation can give rise to abnormal behaviors, e.g., lowering Fermi level and thus decreasing the formation energy of ionized donor V_Se ²⁺ in Se-rich Sb₂ Se₃ . Such behavior exists also in Sb₂ S₃ . It explains the recent experiments that the extremely Se-rich condition causes the efficiency drop of Sb₂ Se₃ solar cells, and demonstrates that the common chemical intuition and defect engineering strategies may be invalid in compensated semiconductors.