The ionisation potential (IP) of CuBiSeCl2 was determined by collecting the secondary electron cut-off (SEC) region using a monochromatic Al Kα SPECS X-ray source with a PSP Vacuum Technology hemispherical electron energy analyser with mean radius of 120 mm. Pass energies of 2 eV for the SEC, 10 eV for core levels and 50 eV for survey scans were used to measure the emitted photoelectrons. Measurements were performed in an ultrahigh vacuum chamber with a base pressure of 2 × 10 -10 mbar. Secondary electron cut-off data were recorded at 16 W to prevent overloading the analyser. A 10 V bias was applied to the sample to remove any effects from the material work function.