Band Alignments, Band Gap, Core Levels, and Valence Band States in Cu 3 BiS 3 for Photovoltaics.

The earth-abundant semiconductor CuBiS (CBS) offers promising photovoltaic properties, and is often considered analogous to solar absorbers copper indium gallium diselenide (CIGS) and copper zinc tin sulphide (CZTS) despite few device reports. The extent to which this is justifiable is explored via a thorough x-ray photoemission spectroscopy (XPS) analysis: spanning core levels, ionisation potential, work function, surface contamination, cleaning, band alignment, and valence band density of states. The XPS analysis overcomes addresses the shortcomings of prior XPS studies of this material. Temperature-dependent absorption spectra determine a 1.2 eV direct band gap at room temperature; the widely reported 1.4-1.5 eV band gap is attributed to weak transitions from the low density of states of the topmost valence band previously being undetected. Density functional theory HSE06+SoC calculations determine band structure, optical transitions, and well-fitted absorption and Raman spectra. Valence band XPS spectra and model calculations find the CBS bonding to be superficially similar to CIGS and CZTS, but the Bi cation (and formally occupied Bi-6s orbital) have fundamental impacts: giving a low ionisation potential (4.98 eV), suggesting that the CdS window layer favoured for CIGS and CZTS gives detrimental band alignment, and should be rejected in favour of a better aligned material in order for CBS devices to progress.