Electronic Structure of Chalcopyrite Surfaces for Photoelectrochemical Hydrogen Production

Abstract

The electronic surface level positions of different chalcopyrite [Cu­(In,Ga)­(S,Se)2] thin-film 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. 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.

Conduction Band Cliff at the CdS/CuIn0.1Ga0.9Se2 Thin-Film Solar Cell Interface

“…Despite the high efficiencies achieved for GGI ≈ 0.3, wide-band-gap chalcopyrite absorbers have recently drawn attention due to higher theoretical open-circuit voltages, their potential uses as a top cell in a tandem solar-cell device, − and in photoelectrochemical water splitting. − However, cells with high GGI absorbers lead to photovoltaic devices with much lower efficiencies than predicted, mainly caused by an increase of the open-circuit voltage ( V OC ) deficit (i.e., E g / e – V OC ) for increasing GGI. ,,− For example, the highest efficiency reported for (In-free) CdS/CuGaSe 2 -based thin-film solar cells is only 11% …”