An organic redox mediator for dye-sensitized solar cells with near unity quantum efficiency

The organic redox mediator tetramethylformaminium disulfide/tetramethylthiourea was evaluated in dye-sensitized nanocrystalline TiO2 solar cells, as an alternative to the conventional I3-/I- redox couple. Devices were optimized by judicious variation of electrolyte composition and selection of sensitizing dye. The best performance of the dye-sensitized solar cells incorporating this redox mediator was achieved by using a metal-free indoline-based sensitizer (D131). It was found that conventional ruthenium based sensitizers (e.g. Z907) exhibited inferior performance, possibly as a result of an insufficient driving force for sensitizer regeneration. A near unity incident photon-to-collected-electron conversion efficiency was achieved at low light intensity for optimized devices. The overall light-to-electric power conversion efficiency under AM 1.5 1 Sun illumination reached 3.88%. This represents an increase of ca. 25% compared with previously reported DSCs using this redox mediator. Factors limiting cell performance were further investigated using transient absorption spectroscopy and electrochemical impedance spectroscopy.