The role of π bridges in high-efficiency DSCs based on unsymmetrical squaraines

A series of squaraine-based sensitizers with various π bridges and anchors were prepared and examined in dye-sensitized solar cells. The carboxylic anchor group was attached onto a squaraine dye through π bridges with and without an ethynyl spacer. DFT studies indicate that the LUMO is delocalized throughout the dyes, whilst the HOMO resides on the squaraine core. The dye that incorporates a 4,4-di-n-hexyl-cyclopentadithiophene group that is directly attached onto the π bridge, JD10, exhibits the highest power conversion efficiency in a DSC; this result is attributed, in part, to the deaggregative properties that are associated with the gem-di-n-hexyl substituents, which extend above and below the π-conjugated dye plane. Dye JD10 demonstrates a power-conversion efficiency of 7.3% for liquid-electrolyte dye-sensitized solar cells and 7.9% for cells that are co-sensitized by another metal-free dye, D35, which substantially exceed the performance of any previously tested squaraine sensitizer. A panchromatic incident-photon-to-current-conversion efficiency curve is realized for this dye with an excellent short-circuit current of 18.0 mA cm(-2). This current is higher than that seen for other squaraine dyes, partially owing to a high molar absorptivity of >5,000 M(-1) cm(-1) from 400 nm to the long-wavelength onset of 724 nm for dye JD10.