Benzodithiophene based organic dyes for DSSC: Effect of alkyl chain substitution on dye efficiency

A series of pushepull triarylamine organic dyes containing the benzo[1,2-b:4,5-b0]dithiophene unit as a spacer and bearing alkyl chains in different positions of the molecule were synthesized. The new dyes 1 e4 were characterized by optical and electrochemical measurements and density functional theory calculations and used as sensitizers in liquid dye-sensitized solar cells. The effect of alkyl chain position on dye properties and performances was investigated and dye 1, bearing two 3,7-dimethyloctyl groups on the benzodithiophene core, exhibited the best behaviour in term of light absorption and cell performance (PCE of 6.6%, to be compared with a PCE of 8.1% for N719-based device). The efficiency of the DSSC was highly dependent on the solvent used for the dye-sensitizing bath, which affects the position of the COOH/COO equilibrium. © 2015 Elsevier Ltd. All rights reserved. 1. Introduction In recent years metal-free organic dyes for DSSC [1e4] have been the object of a growing interest by organic chemists, because a great number of diverse structures can be easily prepared using well established synthetic methodologies and, through appropriate molecular design, their optical, electronic and electrochemical properties can be finely modulated [5e7]. The most efficient metalfree dyes for DSSC are characterized by a pushepull structure in which an