Understanding the phase changes and optical properties in the solvothermal synthesis of iron pyrite.

Iron pyrite (FeS₂) has emerged as a promising photovoltaic material due to its high absorption coefficient, earth abundance, and non-toxicity. However, its low power conversion efficiency, largely attributed to structural defects and phase impurities, has limited its application in solar cells. This study explores the solvothermal synthesis of iron pyrite under varying reaction conditions to optimize its phase purity and optical properties. X-ray diffraction and scanning electron microscopy confirm that phase-pure pyrite is obtained at 180 °C with a stoichiometric sulfur ratio, while higher temperatures and non-stoichiometric sulfur concentrations lead to the formation of secondary phases such as pyrrhotite and marcasite. Spectroscopic ellipsometry is used to determine the optical properties, revealing a direct band gap of 2.8 eV and an indirect band gap of 0.95 eV for phase-pure pyrite. The presence of secondary phases significantly alters the band structure and optical properties, leading to defect-related recombination highlighting the importance of precise synthesis control to achieve phase-pure pyrite with desirable optical characteristics, providing valuable insights into its potential for photovoltaic applications.