Effect of amount of dye in the TiO2 photoanode on electron transport, recombination, Jsc and Voc of dye-sensitized solar cells

The present study shows that although the dye amount in TiO2 films increased continuously with dye soaking time (up to 24 h) photocurrent density (J(sc)) in the dye sensitized solar cell (DSSC) increased until an initial 50 min of soaking and remained unchanged beyond that. The electron transport rate, as measured by intensity modulated photocurrent spectroscopy, in these films also increased up to the initial 50 min of dye soaking and remained almost the same for films soaked for a longer time. Despite faster diffusion of electrons, current per mole of dye continuously decreased with increase in amount of dye. This decrease in current per mole of dye was because of the continuous decrease in percent light reaching the dye molecules in the TiO2 layers away from the substrate, which resulted in a fraction of dye that did not undergo photoexcitation. However, this excess amount of dye reduced recombination at the TiO2/electrolyte interface and resulted in continuous increase in open circuit voltage (V-oc) of the cells with increasing amount of dye. Photoluminescence measurements confirmed that these dye molecules, which did not contribute to the current but reduced recombination, are also chemisorbed on TiO2 in a monolayer.