Transformation of n-type FeSxOy semiconductor to p-type by sulfur annealing in vacuum and air for homostructure solar cell applications

The effects of sulfur annealing temperature (300–600 °C) in vacuum (V) and air (A) conditions for 1 h on the properties of n-type iron-sulfide-oxide (FeSxOy: as-deposited) film were studied. Increasing the annealing temperature for both conditions resulted in larger agglomerations of the grain/particle consisting of various sizes, increased film resistance, and lowered the bandgap due to oxygen reduction. The transformation of n-type to p-type occurred at an annealing temperature > 400 °C, and pure marcasite (FeS2) appeared in Raman spectra for the films annealed in air at 500 °C. The average efficiency of the FeSxOy homostructure solar cell containing 50 mM tartaric acid (0.57%) is slightly higher than that of 100 mM lactic acid (0.39%). The low efficiency of our fabricated solar cells was primarily due to the high resistance of the p-type film, which partly hindered the free motion of carrier charges.