Profound influence of surface trap states on the utilization of charge carriers in CdS photoanodes

Abstract

CdS is one of the most promising photoanode candidates for photoelectrochemical (PEC) water splitting because of its narrow bandgap, suitable band-edge alignment, and facile fabrication. However, it suffers from photocorrosion and instability due to surface charge recombination. Extensive PEC characterization studies showed that hole utilization at the photoanode/electrolyte interface is the bottleneck for CdS photoanodes. In this study, the fundamental charge carrier dynamics processes on CdS surfaces were investigated and a deactivation mechanism was proposed where surface sulfur vacancies (S_v) served as electron trap centers that caused surface charge recombination. Detailed photocurrent transients showed that the extent of trapping was affected by S_v content and oxygen evolution via the water oxidation reaction. To circumvent this issue, a surface decoration method was demonstrated by using hexadecyltrimethylammonium bromide (CTAB) which passivated electron trapping via occupation of the surface S_vs by Br⁻. This work may contribute to further studies on CdS and other semiconductor photoelectrodes to enhance the understanding of charge utilization processes at photoanode–electrolyte interfaces.