The impact of non-uniform photogeneration on mass transport in dye-sensitised solar cells

Abstract

Following the introduction of cobalt(ii/iii)tris(2,2′-bipyridyl)-based redox mediators, dye-sensitised solar cells (DSSCs) have greatly advanced in power conversion efficiency (PCE). However, significant limiting factors include the fast electron recombination and slow mass transport of the oxidised redox mediator ([Co(bipy)3]3+). In this work, the effect of non-uniform photogeneration on the mass transport of [Co(bipy)3]3+ through an electrolyte-infiltrated mesoporous TiO2 film was investigated. Different illumination conditions were used to control the photogeneration profile and the subsequent spatial distribution of [Co(bipy)3]3+ throughout the TiO2 film. They included parameters such as the light intensity, substrate-electrode/electrolyte-electrode (SE/EE) illumination direction, wavelength, and TiO2 photoanode thickness. Using large and small optical perturbation photocurrent transients, electron recombination kinetics with [Co(bipy)3]3+ were analysed in the time domain. Importantly, strong SE-absorption was shown to significantly contribute to the gradual depletion of [Co(bipy)3]3+ at the counter electrode, along with an increased film thickness and light intensity, resulting in excess recombination with [Co(bipy)3]3+ on the 10-2-1 s timescale. Furthermore, charge extraction current decay transients showed that a substantial amount of [Co(bipy)3]3+ can accumulate inside the TiO2 film, resulting in significant recombination at the collecting fluorine-doped tin oxide (FTO) contact on the 10-3-10-2 s timescale. The sub-linear scaling of recombination with light intensity leads to deviating trends in charge extraction and electron transport measurements. Mass transport limitations and recombination losses at the FTO can be significantly reduced by maximising light absorption from the EE-side, which can increase PCE and reduce J-V hysteresis.