An organic redox couple tetramethylthiourea/tetramethylformaminium disulfide (TMTU/TMFDS2+) is evaluated in dye-sensitized solar cells in conjunction with a series of indoline and ruthenium-based dyes. Of these, devices with indoline dye D205 show the best performance, with an optimized power conversion efficiency of 7.6 % under AM 1.5G 1 sun illumination. Charge collection and injection are highly efficient in all TMTU-based DSCs studied. Regeneration of indoline dyes is highly efficient, whereas regeneration of ruthenium dyes by TMTU is less efficient, accounting for their inferior performance. Impedance spectroscopy results reveal that using an optimized TMTU/TMFDS2+ electrolyte solution, the TiO2 conduction band edge is 300–400 meV lower than when an optimized I3−/I− electrolyte is used. The would-be loss in open-circuit voltage caused by the downward conduction band shift is mostly compensated by approximately the 200 meV lower redox level of the TMTU/TMFDS2+ electrolyte and up to 1000 times slower recombination rates. This makes TMTU/TMFDS2+ a promising redox couple in the development of highly efficient solar energy conversion devices.
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