A model for recombination in Type II dye-sensitized solar cells: Catechol–thiophene dyes

Recombination in dye-sensitized solar cells with direct injection is cast as internal conversion in the dye–Ti(OH)2 complex. For catechol–thiophene dyes with 1, 2, or 3 thiophene units, the complex reproduces the previously observed dye-to-semiconductor bands. We compare the decomposition of the internal conversion rate by vibrational mode and predict a trend in recombination with the extension of conjugation, which offers an explanation for the trend in DSSC efficiency. We employ a simple model for the vibrational factors and show that they are only important in the presence of vibrational modes with ℏ ω ⩽ kT and strong electronic factors, as is the case here.