Cobalt Redox Mediators for Ruthenium-Based Dye-Sensitized Solar Cells: A Combined Impedance Spectroscopy and Near-IR Transmittance Study

Abstract

Dye-sensitized solar cells with power conversion efficiencies of up to 6.5% have been fabricated using a cobalt tris-bipyridyl redox mediator with the cis-diisothiocyanato-(2,2′-bipyridyl-4,4′-dicarboxylic acid)-(2,2′-bipyridyl-4,4′-dinonyl) ruthenium(II) (Z907) sensitizer. This represents a significant improvement in efficiency compared with previous reports using ruthenium sensitizers. In situ near-IR transmittance measurements in conjunction with electrochemical impedance spectroscopy have been used to explain the difference in performance between DSCs using Z907 and another benchmark sensitizer cis-diisothiocyanato-bis(2,2′-bipyridyl-4,4′-dicarboxylic acid) ruthenium(II) bis(tetrabutylammonium) (N719). It is found that the small-perturbation electron diffusion length (Ln) is significantly longer in Z907 cells compared with that in N719 cells, which can explain most of the difference in performance. It is also shown that the longer Ln in Z907 cells is caused by inhibited recombination, as opposed to faster transport, and possible reasons for this are discussed. Our methodological approach is especially useful for accurately determining Ln when it is shorter than the TiO2 layer thickness, where standard dynamic techniques start to become unreliable.