Iodide Chemistry in Dye-Sensitized Solar Cells: Making and Breaking I-I Bonds for Solar Energy Conversion

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 (I-2(center dot-)) and triiodide (I-3(-)). Studies at the solution-semiconductor interface present in dye-sensitized solar cells have also revealed that I-I bonds are formed, and I-2(center dot-) is a product of iodide oxidation. Rapid disproportionation of I-2(center dot-) to yield I-3(-) and I- products that are not easily reduced by electrons injected into TiO2 is proposed to be key to the success of the I-/I-3(-) redox mediator in dye-sensitized solar cells.