Theoretical Study of Electron Transfer Properties of Squaraine Dyes for Dye Sensitized Solar Cell

The environmental issues and high cost of Ru create many scientists to explore cheaper and safer sensitizer as alternative for dye sensitized solar cells (DSCs). Dyes play an important role in solar energy conversion efficiency. The squaraine (SQ) dyes has good spectral match with the solar spectra, therefore, SQ dyes have great potential for the applications in DSCs. SQ01-CA is an unsymmetrical SQ dye, reported by Grätzel and colleagues in 2007, featuring a D-π-spacer-A framework and has a carboxylic acid anchoring group. The electron donating ability of indolium in SQ01-CA and SQ01-CAA dyes is relatively weak, better performance may be achieved by introducing an additional donor moiety into indolium [1]. In this study, we investigate six unsymmetrical SQ dyes adsorbed on a (TiO₂)₃₈ cluster [2] using density functional theory (DFT) and time-dependent DFT to study electron transfer properties of squaraine dyes on their photophysical. SQ01-CA, WH-SQ01-CA, and WH-SQ02-CA use a carboxylic acid group as its electron acceptor. Furthermore, SQ01-CAA, WH-SQ01-CAA, and WH-SQ02-CAA use a cyanoacrylic acid group as its electron acceptor. WH-SQ01-CA and WH-SQ01-CAA have an alkyl, while WH-SQ02-CA and WH-SQ02-CAA have alkoxyl substituted diarylamines to the indolium donor of sensitizer SQ01-CA. Our calculations show with additional diarylamines in donor tail of WH-SQ02-CAA, the SQ dyes have red-shifted absorption and have slightly larger probability of electron density transferred to TiO₂ moiety. Furthermore, an additional -CN group as electron a withdrawing group in the acceptor exhibits red-shifted absorption and enhances the electron density transferred to TiO₂ and anchoring moiety after photo-excitation. The tendency of calculated probabilities of electron density being delocalized into TiO₂ and driving force for excited-state electron injection of these studied SQ dyes is compatible with their experimentally observed.