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

Abstract

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 BiS1–xSexI films into both electrochemical and solid state solar cells. Tuning the band gap of BiS1–xSexI 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 BiS1–xSexI 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.