Control and Monitoring of Dye Distribution in Mesoporous TiO₂ Film for Improving Photovoltaic Performance

Dye distribution in a mesoporous TiO 2 film is a key factor in the performance of dye-sensitized solar cells, but there has been little research on it. Here we report even dye distribution within the porous TiO 2 film achieved by a physical driving force of gas flow. Gas-assisted dye arrangement, gas bubbling soaking (GBS), significantly accelerates the dye infiltration compared to conventional overnight soaking (OS). As a demonstration, we investigated the time-dependent dye infiltration using plasmon sensors. GBS produces an even vertical dispersion throughout the film, as illustrated by time-of-flight secondary ion mass spectrometry depth profiles. For devices using a 7-μm-thick active layer and a ruthenium-based dye (N719), only 15 min of GBS treatment produced better power conversion efficiency (PCE) than the optimal result from OS treatment (15 h), despite a lower dye capacity. Dual-GBS treatment (20 min for N719 and 10 min for YD2, a porphyrin dye) produced the best PCE (9.0%) in the device, which was ∼17% higher than that treated with dual-OS (10 h for N719 and 5 h for YD2). Such improvements are associated with reduced dye-free sites inside the porous TiO 2 film after GBS treatment, leading to faster charge transport and slower charge loss.