Defect tolerance in chalcogenide perovskite photovoltaic material BaZrS₃

Chalcogenide perovskites (CPs) exhibiting lower band gaps than oxide perovskites and higher stability than halide perovskites are promising materials for photovoltaic and optoelectronic applications. For such applications, the absence of deep defect levels serving as recombination centers (dubbed defect tolerance) is a highly desirable property. Here, using density functional theory (DFT) calculations, we study the intrinsic defects in BaZrS₃, a representative CP material. We compare Hubbard-U and hybrid functional methods, both of which have been widely used in addressing the band gap problem of semi-local functionals in DFT. We find that tuning the U value to obtain experimental bulk band gap and then using the obtained U value for defect calculations may result in over-localization of defect states. In the hybrid functional calculation, the band gap of BaZrS₃ can be accurately obtained. We observe the formation of small S-atom clusters in both methods, which tend to self-passivate the defects from forming mid-gap levels. Even though in the hybrid functional calculations several relatively deep defects are observed, all of them exhibit too high formation energy to play a significant role if the materials are prepared under thermal equilibrium. BaZrS₃ is thus expected to exhibit sufficient defect tolerance promising for photovoltaic and optoelectronic applications.[Figure not available: see fulltext.]