The Role of Hole Transport between Dyes in Solid-State Dye-Sensitized Solar Cells

In dye-sensitized solar cells (DSSCs) photogenerated positive charges are normally considered to be carried away from the dyes by a separate phase of hole-transporting material (HTM). We show that there can also be significant transport within the dye monolayer itself before the hole reaches the HTM. We quantify the fraction of dye regeneration in solid-state DSSCs that can be attributed to this process. By using cyclic voltammetry and transient anisotropy spectroscopy, we demonstrate that the rate of interdye hole transport is prevented both on micrometer and nanometer length scales by reducing the dye loading on the TiO₂ surface. The dye regeneration yield is quantified for films with high and low dye loadings (with and without hole percolation in the dye monolayer) infiltrated with varying levels of HTM. Interdye hole transport can account for >50% of the overall dye regeneration with low HTM pore filling. This is reduced to about 5% when the infiltration of the HTM in the pores is optimized in 2 μm thick films. Finally, we use hole transport in the dye monolayer to characterize the spatial distribution of the HTM phase in the pores of the dyed mesoporous TiO<sub>2