(Invited) Who Shifts Where, When, and Why? Experimental Insights into Cu(In,Ga)(S,Se)2 Solar Absorbers for Photoelectrochemical Water Splitting

Abstract

The electronic surface level positions of different chalcopyrite [Cu(In,Ga)(S,Se)2] thin-film solar absorbers are presented and their suitability for photoelectrochemical (PEC) water splitting is discussed. For efficient PEC water splitting, electrode surfaces must exhibit suitable band edge energies (i.e., the conduction band minimum, CBM, and the valence band maximum, VBM) to enable hydrogen and oxygen evolution. The VBM and CBM at the sample surfaces were experimentally derived under vacuum conditions using direct and inverse photoemission (XPS, UPS, and IPES). By measuring the work function at the surface, the band edge energies can be correlated to the normal hydrogen electrode and compared with the reduction and oxidation potentials necessary to drive PEC water splitting. By studying several chalcopyrite variants differing in growth process, composition, stoichiometry, and surface treatment, strategies are derived to optimize chalcopyrite PEC devices with respect to the redox potentials for solar water splitting.