Molecular design of unsymmetrical squaraine dyes for high efficiency conversion of low energy photons into electrons using TiO2 nanocrystalline films

Abstract

An optimized unsymmetrical squaraine dye 5-carboxy-2-[[3-[(2,3-dihydro-1, 1-dimethyl-3-ethyl-1H-benzo[e]indol-2-ylidene)methyl]-2-hydroxy-4-oxo- 2cyclobuten-1 -ylidene]methyl]-3,3-dimethyl-1 -octy 1-3H-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 orbitais 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 TiO2 and hence a fast electron transfer. © 2009 WILEY-VCH Verlag GmbH & Co. KGaA.