Conformational effect of symmetrical squaraine dyes on the performance of dye-sensitized solar cells

Abstract

A symmetrical squaraine dye altered to possess dicyanovinylene functionality on a carbonyl group of the cyclobutene core (SQM1a) was synthesized to elucidate the effects of the conformationally locked-in cis-like form ensured by the steric repulsion of dicyanovinylenes on the performance of dye-sensitized solar cells (DSSCs) through a comparison with an unsymmetrical analogue bearing one carboxy group (SQM1b) and a standard squaraine dye (MSQ). The FT-IR spectrum of the TiO₂ thin film immersed in the dye solution suggested that the linkages between the dye and TiO₂ were formed by both carboxy groups symmetrically incorporated on the π-conjugation of dyes. The symmetrical and conformationally locked structure of SQM1a brought about a decrease in the non-radiative decay rate, which may have been induced by suppression of the possible photoisomerization of squaraines. In accordance with the finding of non-radiative decay, a DSSC fabricated with SQM1a exhibited a significantly enhanced short-circuit current density, which revealed an efficient electron injection derived from the double-anchored and conformationally locked structure of SQM1a. Consequently, the performance of DSSCs fabricated with SQM1a was higher than the performance of those fabricated with SQM1b and MSQ. This indicates that substantial advantages can be derived by restricting the conformation of symmetrical squaraine dyes.