Optical Properties of Sputtered SnS Thin Films for Photovoltaic Absorbers

Tin monusulfide (SnS) is an absorber with promising optoelectronic properties and low environmental constraints of interest for high-efficiency solar cells. The optical properties of SnS thin films are investigated to assess their compatibility with the solar spectrum. SnS thin films were RF magnetron sputter-deposited at target powers of 105-155 W and total pressures of 5 to 60 mtorr in argon at room temperature. X-ray diffraction patterns confirmed a dominant tin monosulfide herzenbergite phase. The absorption coefficient was determined by spectroscopic ellipsometry and unpolarized spectrophotometry measurements. Both methods show that the films have absorption coefficients above the band gap in the range of 10⁵ -10⁶ cm^-1. The direct gap, indirect gap, and forbidden direct gap for the films were found to be in the range of 1.2-1.6 eV, indicating a strong match with the solar irradiance spectrum.