Journal article

Characteristics of the Iodide/Triiodide Redox Mediator in Dye-Sensitized Solar Cells

Dye-sensitized solar cells (DSCs) are of interest because of their potential for low-cost solar energy conversion. Currently, the certified record efficiency of these solar cells is 11.1%, and measurements of their durability and stability suggest lifetimes exceeding 10 years under operational conditions. The DSC is a photoelectrochem. system: a monolayer of sensitizing dye is adsorbed onto a mesoporous TiO2 electrode, and the electrode is sandwiched together with a counter electrode. An electrolyte contg. a redox couple fills the gap between the electrodes. The redox couple is a key component of the DSC. The reduced part of the couple regenerates the photooxidized dye. The formed oxidized species diffuses to the counter electrode, where it is reduced. The photovoltage of the device depends on the redox couple because it sets the electrochem. potential at the counter electrode. The redox couple also affects the electrochem. potential of the TiO2 electrode through the recombination kinetics between electrons in TiO2 and oxidized redox species. The special properties of the I-/I3- redox couple in dye-sensitized solar cells were studied. It was the preferred redox couple since the beginning of DSC development and still yields the most stable and efficient DSCs. Overall, the iodide/triiodide couple has good soly., does not absorb too much light, has a suitable redox potential, and provides rapid dye regeneration. But what distinguishes I-/I3- from most redox mediators is the slow recombination kinetics between electrons in TiO2 and the oxidized part of the redox couple, triiodide. Certain dyes adsorbed at TiO2 catalyze this recombination reaction, presumably by binding I or triiodide. The std. potential of the iodide/triiodide redox couple is 0.35 V (vs. the normal H electrode, normal H electrode), and the oxidn. potential of the std. DSC-sensitizer (Ru(dcbpy)2(NCS)2) is 1.1 V. The driving force for redn. of oxidized dye is therefore as large as 0.75 V. This process leads to the largest internal potential loss in DSC devices. Overall efficiencies >15% might be achieved if half of this internal potential loss could be gained. The regeneration of oxidized dye with iodide gives the diiodide radical, I2-·. The redox potential of the I2-·/I- couple must therefore be considered when detg. the actual driving force for dye regeneration. The formed I2-· disproportionates to I3- and I-, which leads to a large loss in potential energy.


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