n‑BiSI Thin Films: Selenium Doping and Solar Cell Behavior

BiSI (indirect band gap = 1.57 eV) is a recently discovered photoelectrode material possessing promising optical properties for use in alternative thin film solar cells. In this work, we study the effects of selenium doping on BiSI film properties and also demonstrate the incorporation of BiS_1–xSe_xI films into both electrochemical and solid state solar cells. Tuning the band gap of BiS_1–xSe_xI by substituting selenium for sulfur was accomplished by substituting various amounts of SeO₂ for thiourea in the BiSI spray pyrolysis precursor solutions. This strategy was employed to reduce the direct band gap of BiS_1–xSe_xI films from 1.63 eV to as low as 1.48 eV, as measured by UV–vis–NIR diffuse reflectance spectroscopy for x = 0.4. Both electrochemical and solid state solar cell devices utilizing n-BiSI as the light absorbing material demonstrated open circuit voltages of nearly 0.4 V. The electrochemical devices showed much higher short circuit currents and power conversion efficiencies than the solid state devices. Power conversion efficiencies of up to 0.25 and 0.012% were measured for electrochemical and solid state devices, respectively, under AM1.5G illumination.