Cation-Dependent Photostability of Co(II/III)-Mediated Dye-Sensitized Solar Cells

The electrolyte composition has a significant effect on the performance and stability of cobalt-based, dye-sensitized solar cells (DSSCs). The stability of DSSCs incorporating Co(II/III) tris(bipyridine) redox mediator has been investigated over 1000 h under full solar irradiation (with UV cutoff) at a temperature of 60 degrees C, the main focus being on monitoring the photovoltaic performance of the device and analyzing the internal charge-transfer dynamics in the presence of different cation coadditives (preferably added as tetracyanoborate salts). A clear cation-dependence is shown, not only of the early light-induced performance but also of the long-term photostability of the photovoltage of the device. These light-induced changes, which are attributed to the promotion of electron injection and less electron recombination loss, by transient spectral and electrochemical studies at the TiO2/dye/electrolyte interface, indicate that the main cation effects involve the TiO2 surface electric field and energy-state distribution. By examining the stability of adsorbed and solvated dye during aging, it has been found that the dye photodegradation is probably responsible for the decline in the photovoltage and that this is extremely dependent on the nature of the cation coadditives in the electrolyte. It is therefore suggested that optimization of the electrolyte cation composition is essential for improving the stability of cobalt-based DSSCs.