The photo-oxidation of iodide (I-) results in the formation of I-I bonds relevant to solar energy conversion. The making (and breaking) of I-I bonds is specifically important to the operation of high-efficiency dye-sensitized solar cells. In this Perspective, the redox chemistry of iodide in aqueous solution is briefly reviewed, followed by recent photoinduced studies in nonaqueous solution. Analogous to thermal electron-transfer studies, two mechanisms have been identified for photodriven I-I bond formation in solution. With regard to breaking I-I bonds, the photodriven cleavage of I-I bonds has been quantified by the reduction of diiodide (I2•-) and triiodide (I3-). Studies at the solution-semiconductor interface present in dye-sensitized solar cells have also revealed that I-I bonds are formed, and I2•- is a product of iodide oxidation. Rapid disproportionation of I2•- to yield I3- and I- products that are not easily reduced by electrons injected into TiO2 is proposed to be key to the success of the I-/I3- redox mediator in dye-sensitized solar cells. © 2010 American Chemical Society.

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 I 2← 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. © 2011 American Chemical Society.