Mesoporous TiO2 Beads Offer Improved Mass Transport for Cobalt-Based Redox Couples Leading to High Efficiency Dye-Sensitized Solar Cells
Overcoming ionic diffusion limitations is essential for the development of high-efficiency dye-sensitized solar cells based on cobalt redox mediators. Here, improved mass transport is reported for photoanodes composed of mesoporous TiO2 beads of varying pore sizes and porosities in combination with the high extinction YD2-o-C8 porphyrin dye. Compared to a photoanode made of 20 nm-sized TiO2 particles, electrolyte diffusion through these films is greatly improved due to the large interstitial pores between the TiO2 beads, resulting in up to 70% increase in diffusion-limited current. Simultaneously, transient photocurrent measurements reveal no mass transport limitations for films of up to 10 mu m thickness. In contrast, standard photoanodes made of 20 nm-sized TiO2 particles show non-linear behavior in photocurrent under 1 sun illumination for a film thickness as low as 7 mu m. By including a transparent thin mesoporous TiO2 underlayer in order to reduce optical losses at the fluorine-doped tin oxide (FTO)-TiO2 interface, an efficiency of 11.4% under AM1.5G 1 sun illumination is achieved. The combination of high surface area, strong scattering behavior, and high porosity makes these mesoporous TiO2 beads particularly suitable for dye-sensitized solar cells using bulky redox couples and/or viscous electrolytes.