Retardation of interfacial charge recombination by addition of quaternary ammonium cation and its application to low temperature processed dye-sensitized solar cells

Abstract

Electron lifetime (τ) in dye-sensitized solar cells (DSC), using electrolytes containing I⁻/I₃⁻ redox couple, was investigated with quaternary ammonium cations having various alkyl chain lengths, and it was found that the lifetime increased in the order τ_TPA ≈ τ_TBA < τ_THA ≈ τ_THpA, where τ_TPA, τ_TBA, τ_THA and τ_THpA were the lifetimes with tetra-propyl, butyl, hexyl and heptyl ammonium cations respectively. On the other hand, little influence of the cations on electron diffusion coefficients was observed, indicating that the DSCs using these cations have longer electron diffusion length (L). New electrolyte compositions for DSCs were sought with THA⁺ to increase the lifetime and L without compromising charge injection yield from the dye to TiO₂. The electrolyte was applied for DSCs using TiO₂ electrode prepared by 150 °C sintering. Under one-sun conditions (100 mW cm⁻²), newly prepared electrolytes showed at most 40% increase of energy conversion efficiency in comparison to conventional electrolytes and the highest value was 4.7%, which was the highest among the reported values of low temperature processed DSCs. The increase of the efficiency was achieved by higher short-circuit current induced by longer electron diffusion length and higher open-circuit voltage due to higher electron density.