Near-Infrared Absorbing Squaraine Dyes for Solar Cells: Relationship between Architecture and Performance

To further investigate the influence of dye architecture on dye-sensitized solar cell (DSSC) performance, some near-infrared absorbing quinoline-based squaraine dyes (4a, 4b, and 4c) with different conjugated degrees carrying carboxylic or sulfonic groups as anchoring groups were designed, prepared, and applied as sensitizers in solar cells. The photophysical and photochemical studies showed that photoelectric conversion efficiencies (eta) are significantly affected by molecular structures; i.e., eta of 4c with the strongest polarity and anchoring ability and the longest pi electron conjugated degree was two times more than that of 4b and six times more than that of 4a. That is, the molecules with smaller optical band gap and higher molar absorption coefficient will possess better light-harvesting properties and enhanced conversion efficiency. Furthermore, all excited state orbitals relevant for the pi-pi electron transition both in 4b and 4c are delocalized over the anchoring groups, ensuring a strong electronic coupling to the conduction band of TiO2 and hence a fast electron transfer.