Antimony sulfide photovoltaics with high open-circuit voltage not limited by self-trapped excitons

Sb₂S₃ is a promising material for low-toxicity, high-stability next-generation photovoltaics, but its device performance is constrained by large open-circuit voltage (V_OC) deficits. From recent spectroscopic investigations, it was hypothesized that this arises from self-trapping, limiting V_OCs to approximately 800 mV, which is indeed the level nearly all Sb₂S₃ solar cells have asymptotically approached. Herein, it is revealed through temperature-dependent mobility measurements that band-like transport, rather than self-trapping, occurs in Sb₂S₃. By lowering the defect density in Sb₂S₃ thin films, the 800 mV threshold is surpassed to achieve a V_OC of 824 mV. This is accomplished by adding citrate ligands to the precursor solution used for chemical bath deposition, lowering the grain boundary density in Sb₂S₃ films from 1114 ± 52 nm μm⁻² to 586 ± 11 nm μm⁻². The likely performance-limiting defects in Sb₂S₃ are identified to be S vacancies or Sb on S anti-sites by comparing deep level transient spectroscopy measurements with defect calculations. This work addresses the debate in the field around whether Sb₂S₃ is limited by defects or self-trapping, showing that it is possible to improve the performance towards the radiative limit through careful defect engineering.