Relationship between diffusion of Co3+/Co2+ redox species in nanopores of porous titania stained with dye molecules, dye molecular structures, and photovoltaic performances

Abstract

Diffusion of Co²⁺ and Co³⁺ in nanopores of porous titania stained with various dyes were evaluated and were compared with those of I⁻, and I₃⁻. Diffusion of Co³⁺ in the bare nanopore was slower than that of I₃⁻. Dyes adsorbed on the wall of nanopores affected these diffusion coefficients remarkably. The Co³⁺ diffusion in nanopores became faster when the nanopore wall was covered with dyes with long alkyl groups, probably because the long alkyl groups suppress the interaction between TiO₂ surface and Co³⁺. Higher open circuit voltage (V_oc) was obtained for the DSSC with fast Co³⁺ diffusion in nanopores because the fast diffusion suppresses the charge recombination between electrons in titania and Co³⁺ remaining in the nanopores. The fast diffusion in nanopores was applied to thin nanoparticle spacer (th: 5 μm) of DSSCs. After nanoparticle spacer was stained with dyes with long alkyl groups, the efficiency drastically increased from 2.8% to 5.3%.