Fundamental insights into the iodide/triiodide redox mediator utilized in dye-sensitized solar cells

Dye-sensitized solar cells (DSSCs) that employ the iodide/triiodide redox mediator are able to achieve solar energy power conversion efficiencies >10 %. The redox properties of iodide/triiodide are intriguing in that iodide (I-) is oxidized by 1e- at the sensitized TiO2 photoanode, yet triiodide (I3 -) is reduced by 2e- at the Pt counter electrode. This offset in electrochemical reactivity allows for efficient charge separation within the DSSC. Specifically, sensitizer regeneration at TiO2 occurs by rapid I- oxidation to the diiodide radical anion (I2·-). Near diffusion limited kinetics for the disproportionation of I2·- results in I3- formation. The disproportionation reaction inherently results in an oxidized product (I3- ) that is more easily reduced by 2e- than by 1e-. The recombination of electrons injected in TiO2 (TiO2(e-)) is predicted to be limited to 1e- reactivity. Therefore, reduction of I3- by TiO2( e-) is inhibited within DSSCs, thus aiding to solar energy power conversion.;Throughout this thesis, fundamental details related to the processes of sensitizer regeneration by iodide and charge recombination to triiodide are discussed. Chapter 2 reports the formation of iodine atoms upon the oxidation of iodide by Ru-tris(diimine) excited-states in acetonitrile. The presented data provide compelling evidence for possible iodine atom formation in the sensitizer regeneration step within DSSCs. Chapter 3 focuses on the advantages toward iodide oxidation within ion-paired species in low polarity solvents. Here, the bond formation chemistry that accompanies iodide oxidation was greatly enhanced in dichloromethane compared to acetonitrile. The use of modified electrodes to study sensitizer regeneration at interfaces is detailed in Chapter 4. This novel technique provided new insights into iodide oxidation at heterogeneous surfaces. Finally, Chapter 5 provides evidence for I2·- product formation in the 1e- reduction of I3- in fluid solution. This was the first such report of triiodide reduction accompanied by identification of products and was representative of the reduction of I3- by TiO 2(e-) within DSSCs.