Design of Organic Dyes and Cobalt Polypyridine Redox Mediators for High-Efficiency Dye-Sensitized Solar Cells

Dye-sensitized solar cells (DSCs) with Co-based mediators with efficiencies surpassing the record for DSCs with iodide-free electrolytes were developed by selecting a suitable combination of a Co polypyridine complex and an org. sensitizer. The effect of the steric properties of 2 triphenylamine-based org. sensitizers and Co polypyridine redox mediators on the overall device performance in DSCs as well as on transport and recombination processes in these devices was compared. The recombination and mass-transport limitations that, previously, limit the performance of these mediators were avoided by matching the properties of the dye and the Co redox mediator. Org. dyes with higher extinction coeffs. than the std. Ru sensitizers were employed in DSCs in combination with outer-sphere redox mediators, enabling thinner TiO2 films to be used. Recombination was reduced further by introducing insulating butoxyl chains on the dye rather than on the Co redox mediator, enabling redox couples with higher diffusion coeffs. and more suitable redox potential to be used, simultaneously improving the photocurrent and photovoltage of the device. Optimization of DSCs sensitized with a triphenylamine-based org. dye in combination with tris(2,2'-bipyridyl)cobalt(II/III) yielded solar cells with overall conversion efficiencies of 6.7% and open-circuit potentials of >0.9 V under 1000 W m-2 AM1.5 G illumination. Good performance was found under low light intensity indoor conditions.