Effects of Aqueous Electrolyte, Active Layer Thickness and Bias Irradiation on Charge Transfer Rates in Solar Cells Sensitized with Top Efficient Carbazole Dyes

The photovoltaic performance and electron-transfer dynamics from femtoseconds to milliseconds are explored for very efficient carbazole sensitizers applied in solar cells, including the champion dye ADEKA-1. The photocurrent in the novel, environmentally friendly aqueous electrolyte is about 40% of that in the standard acetonitrile one, both based on cobalt complexes as a redox pair. The drop in the photocurrent is found to be correlated with increased electron recombination between sensitized titania particles and dyes, taking place with a time constant of several hundreds of picoseconds. Electron injection and recombination between titania and dye are slowed down under the presence of water (about 10 times) and with additional 1 sun bias irradiation (about 2 times). These effects are interpreted as due to reduced electronic coupling between the electron donors (dyes) and acceptor framework (titania). Moreover, a decrease in the relative photocurrent, photovoltage, and fill factor of the cells with increasing thickness of titania layers are observed. The obtained results should be relevant for the optimization of solar cells with a large class of organic sensitizers, especially when looking for nontoxic configurations and determining true charge transfer rates under operating solar cell conditions