New Type II Catechol-Thiophene Sensitizers for Dye-Sensitized Solar Cells

We report a new class of thiophene-catechol light harvesting molecules for dye-sensitized solar cells. The catechol-titania combination has been described as a Type II injection system where it is thought charge injection occurs directly from the highest occupied molecular orbital of the dye to states near the Mania conduction band edge. We show that the optical absorption of the catechol-titania complex moves to longer wavelengths with extension of the configuration (1, 2, or 3 thiophene units). Binding, of the dyes is via a bidentate catechol-titania bridge with the strength of the binding following the acidity of the catechol: the more acidic the stronger the binding. The binding strength was found to increase with increasing chromophore length. We have optimized device performance based upon dye loading and find open circuit voltage and short circuit current density are consistent with the observed improvements in light harvesting efficiency with extended conjugation. However, the maximum open circuit voltage is observed at low dye loadings, and dark current-voltage and open circuit voltage decay measurements indicate that recombination plays a significant role in suppressing, device performance as dye loading increases. The interplay between open circuit voltage and short circuit current as a function of dye uptake means that efficiencies in these catechol-thiophene sensitizers are limited. It is clear that these factors need to be independently controlled if Type II systems are to challenge Type I injection systems for high performance dye-sensitized solar cells.