Charge Transport and Defects in Sulfur-Deficient Chalcogenide Perovskite BaZrS₃

Exploring the conduction mechanism in the chalcogenide perovskite BaZrS₃ is of significant interest due to its potential suitability as a top absorber layer in silicon-based tandem solar cells and other optoelectronic applications. Theoretical and experimental studies anticipate native ambipolar doping in BaZrS₃, although experimental validation remains limited. This study reveals a transition from highly insulating behavior to n-type conductivity in BaZrS₃ through annealing in an S-poor environment. BaZrS₃ thin films are synthesized via a two step process: co-sputtering of Ba-Zr followed by sulfurization at 600 ∘C, and subsequent annealing in high vacuum. UV-Vis measurement reveal a red-shift in the absorption edge concurrent with sample color darkening after annealing. The increase in defect density with vacuum annealing, coupled with low activation energy and n-type character of defects, strongly suggests that sulfur vacancies (VS) are responsible, in agreement with theoretical predictions. The shift of the Fermi level towards conduction band minimum, quantified by Hard X-ray Photoelectron Spectroscopy (Ga Kα, 9.25 keV), further corroborates the induced n-type of conductivity in annealed samples. Our findings indicate that vacuum annealing induces VS defects that dominate the charge transport, thereby making BaZrS₃ an n-type semiconductor under S-poor conditions. This study offers crucial insights into understanding the defect properties of BaZrS₃, facilitating further improvements for its use in solar cell applications.