Charge Recombination to Oxidized Iodide in Dye-Sensitized Solar Cells

The goal of this study was to determine whether electrons injected into TiO 2 in dye-sensitized solar cells (DSSCs) react with di-iodide, I 2 •–, a known intermediate in sensitized iodide oxidation. The approach was to utilize time-resolved absorption spectroscopy to quantify the yield of I 2 •– disproportionation under conditions where I 2 •– reduction by electrons photoinjected into TiO 2, TiO 2 (e – )s, could be competitive. The DSSC was based on [Ru(dtb) 2 (dcb)] 2+, where dtb is 4,4′-(C(CH 3 ) 3 ) 2 -2,2′-bipyridine and dcb is 4,4′-(COOH) 2 -2,2′-bipyridine, sensitized mesoporous nanocrystalline TiO 2 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 I 2 /acetonitrile electrolyte completed the DSSC. After pulsed 532 nm laser excitation, the first iodide oxidation product observed spectroscopically was I 2 •– . Under all conditions studied, there was no direct evidence for the reaction between TiO 2 (e – ) and I 2 •–, and the kinetics for I 2 •– loss indicated quantitative disproportionation of I 2 •– to yield I 3 – and I – with a rate constant that was, within experimental error, the same as that measured in fluid acetonitrile solution, 2.2 + 1 × 10 9 M –1 s –1 . This was true even when steady state illumination was utilized to increase the TiO 2 (e – ) concentration. Data consistent with charge recombination to I 3 –, from TiO 2 (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 TiO 2 (e – ) complicated charge recombination analysis.