Diffuse reflectance of CuBiSeCl2 powder was measured using an Agilent Cary 5000 between 200 and 2500 nm with a step size of 1 nm. Calibration to 100% and 0% reflectance was performed prior to measurement using a PTFE standard and a light trap, respectively. The band gap was determined from a Tauc plot using a method described by Makuła et al.