Reduction in Urbach energy and density of states for pyrite (FeS2) thin films: Healing of sulfur vacancies during hematite to pyrite transformation

Pyrite, an inexpensive, earth-abundant photovoltaic material has potential to compete with silicon. Unfortunately, pyrite based devices suffer from low power conversion efficiencies and open-circuit voltages, which can be overcome by improved understanding of electronic and optical properties of pyrite. In the electronic and optical properties of pyrite role of sulfur vacancies is very crucial but less explored and still ambiguous. In the present study, effect of sulfur vacancies on the optical and charge transport properties of phase pure pyrite films has been investigated where the sulfur vacancies have been tamed by increasing the sulfurization pressure from subatmospheric to overatomspheric value. Lower Urbach energy in the pyrite films indicates a decrease in density of localized states, which is believed to be due to reduced sulfur vacancies at higher sulfurization pressure. Temperature dependent transport properties reveal the presence of Mott-variable range hoping conduction through the high density of localized states arising from sulfur vacancies. Decreased density of localized states reflects in increased hoping energy associated with longer hoping distance in the pyrite films.