Photocurrents from the Direct Irradiation of a Donor–Acceptor Complex Contained in a Thin Film on Indium Tin Oxide

Abstract

We describe the performance of a photoelectrochemical cell based on an electron donor–acceptor (EDA) complex formed by the association of tetraphenylborate anion with a viologen dication siloxane film immobilized on an indium tin oxide (ITO) electrode. Visible irradiation of a weak absorbance band of the EDA complex leads to simultaneous photoreduction of the viologen and photooxidation of the tetraphenylborate, forming a viologen monocation radical and tetraphenylborate radical. Spontaneous decomposition of the latter to products incapable of further reaction inhibits reverse electron transfer, permitting harvesting of the redox equivalents stored as the viologen monocation radical at the electrode. Results of statistically designed two-level factorial experiments indicate that the photocurrent increases with light intensity (L) and viologen coverage (P), but decreases with viologen siloxane film age (A). Although the solution BPh4– concentration (F) does not significantly affect the photocurrent response under our experimental conditions, the nature of the electrolyte anion (C) does. Although significant interactions among the L, P, A, and C variables exist, our analyses provide a model that (1) describes the viologen film aging; (2) adequately predicts cell photocurrent responses as functions of levels of the L, P, and, to a somewhat lesser extent, A variables; and (3) identifies areas for improvement and optimization of cell performance.