Influence of Diffusion Coefficient of Cobalt Redox Mediator Using Triphenylamine Dyes with Various Number of Anchoring Groups: Photovoltaic Performance of DSSCs

The development of electrolytes containing a novel redox shuttle is essential for achieving highly efficient and stable dye-sensitized solar cells (DSSCs). Here, the effect of the diffusion coefficient of redox shuttles in the electrolyte on the photovoltaic performance of DSSCs using triphenylamine dyes by a different number of anchoring groups was investigated. Two different types of cobalt (II)/(III) polypyridine complexes, namely, [Co(bpy)(3)](2+/3+) and [Co(dtb)(3)](2+/3+) (where bpy = (2,2'-bipyridine) and dtb = (4,4'-di-tert-butyl-2,2'-bipyridine)), are applied to the hole transporting redox shuttles in DSSCs. Their diffusion coefficients are estimated by linear sweep voltammetry (LSV) at various scan rates. The diffusion coefficient of [Co(bpy)(3)](2+) and [Co(dtb)(3)](2+) in the prepared electrolyte are 1.91 x 10(-7) and 2.27 x 10(-8) cm(2)/s, respectively. The high diffusion coefficient of [Co(bpy)(3)](2+) leads to a high ionic conductivity of the electrolyte that is nearly two times higher than the [Co(dtb)(3)](2+/3+)-based electrolyte. This [Co(bpy)(3)](2+/3+)-based electrolyte leads to an increase in the DSSCs photovoltaic performance with an increase in electronic coupling with the organic dye and TiO2. This is due to the photovoltaic performance is not limited by the mass-transportation process.