Molecular Design of Unsymmetrical Squaraine Dyes for High Efficiency Conversion of Low Energy Photons into Electrons Using TiO 2 Nanocrystalline Films

An optimized unsymmetrical squaraine dye 5‐carboxy‐2‐[[3‐[(2,3‐dihydro‐1, 1‐dimethyl‐3‐ethyl‐1 H ‐benzo[ e ]indol‐2‐ylidene)methyl]‐2‐hydroxy‐4‐oxo‐2‐cyclobuten‐1‐ylidene]methyl]‐3,3‐dimethyl‐1‐octyl‐3 H ‐indolium ( SQ02 ) with carboxylic acid as anchoring group is synthesized for dye‐sensitized solar cells (DSCs). Although the π ‐framework of SQ02 is insignificantly extended compared to its antecessor squaraine dye SQ01 , photophysical measurements show that the new sensitizer has a much higher overall conversion efficiency η of 5.40% which is improved by 20% when compared to SQ01 . UV‐vis spectroscopy, cyclic voltammetry and time dependent density functional theory calculations are accomplished to rationalize the higher conversion efficiency of SQ02 . A smaller optical band gap including a higher molar absorption coefficient leads to improved light harvesting of the solar cell and a broadened photocurrent spectrum. Furthermore, all excited state orbitals relevant for the π – π * transition in SQ02 are delocalized over the carboxylic acid anchoring group, ensuring a strong electronic coupling to the conduction band of TiO 2 and hence a fast electron transfer.