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 •− ) 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 •− is a product of iodide oxidation. Rapid disproportionation of I 2 •− to yield I 3 − and I − products that are not easily reduced by electrons injected into TiO 2 is proposed to be key to the success of the I − /I 3 − redox mediator in dye-sensitized solar cells.