Charge Recombination to Oxidized Iodide in Dye-Sensitized Solar Cells

Abstract

The goal of this study was to determine whether electrons injected into TiO2 in dye-sensitized solar cells (DSSCs) react with di-iodide, I2•–, a known intermediate in sensitized iodide oxidation. The approach was to utilize time-resolved absorption spectroscopy to quantify the yield of I2•– disproportionation under conditions where I2•– reduction by electrons photoinjected into TiO2, TiO2(e–)s, could be competitive. The DSSC was based on [Ru(dtb)2(dcb)]2+, where dtb is 4,4′-(C(CH3)3)2-2,2′-bipyridine and dcb is 4,4′-(COOH)2-2,2′-bipyridine, sensitized mesoporous nanocrystalline TiO2 thin films sintered onto an optically transparent fluorine-doped tin oxide (FTO) conductive substrate. A transparent Pt counter-electrode and a 0.5 M LiI/0.05 M I2/acetonitrile electrolyte completed the DSSC. After pulsed 532 nm laser excitation, the first iodide oxidation product observed spectroscopically was I2•–. Under all conditions studied, there was no direct evidence for the reaction between TiO2(e–) and I2•–, and the kinetics for I2•– loss indicated quantitative disproportionation of I2•– to yield I3– and I– with a rate constant that was, within experimental error, the same as that measured in fluid acetonitrile solution, 2.2 + 1 × 109 M–1 s–1. This was true even when steady state illumination was utilized to increase the TiO2(e–) concentration. Data consistent with charge recombination to I3–, from TiO2(e–) or electrons at the Pt counter electrode, were quantified spectroscopically, with the Kohlrausch–Williams–Watts (KWW) function, at specific points on the current–potential curve. This reaction was found to be sensitive to steady state illumination incident on the DSSC. Transient absorption changes assigned to a Stark effect that were intimately coupled to the presence of transiently generated TiO2(e–) complicated charge recombination analysis.