Electrochemical reversibility and fast bimolecular exchange reaction found for VO(salen) gave rise to a highly efficient redox mediation to enhance the photocurrent of a dye-sensitized solar cell, leading to an excellent photovoltaic performance with a conversion efficiency of 5.4%. A heterogeneous electron-transfer rate constant at an electrode (k0) and a second-order rate constant for an electron self-exchange reaction (k(ex)) were proposed as key parameters that dominate the charge transport property, which afforded a novel design concept for the mediators based on their kinetic aspects.

Organic-based solar cells potentially offer a photovoltaic module with low production costs and low hazard risk of the components. We report organic dye-sensitized solar cells, fabricated with molecular designed indoline dyes in conjunction with highly reactive but robust nitroxide radical molecules as redox mediator in a quasi-solid gel form of the electrolyte. The cells achieve conversion efficiencies of 10.1% at 1 sun, and maintain the output performance even under interior lighting. The indoline dyes, customized by introducing long alkyl chains, specifically interact with the radical mediator to suppress a charge-recombination process at the dye interface. The radical mediator also facilitates the charge-transport with remarkably high electron self-exchange rate even in the quasi-solid state electrolyte to lead to a high fill factor.

A ferrocenylimidazolium salt was found to be an efficient iodine/iodide-free redox mediator for a dye-sensitized solar cell (DSSC), giving rise to a photoconversion efficiency of 4% under optimized electrolyte conditions. The excellent redox mediation process was ascribed not only to the electrochemically reversible nature of the molecule with a large heterogeneous electron-transfer rate constant in the order of 10−2 cm s−1 at electrodes, but also to an exceptionally high solubility up to 2.7 M to produce a large redox gradient in the mediation layer, thus allowing the increase of diffusion-limited current. The high solubility originating from the imidazolium group in the mediator molecule was reminiscent of ionic liquids, which are typically characterized by small absorption coefficients in a visible region. These properties provided a mass-transfer-based concept of molecular design for organic redox mediators in DSSCs.