Organic dyes containing indolo[2,3-b]quinoxaline as a donor: synthesis,optical and photovoltaic properties

New organic dyes containing indolo[2,3-b]quinoxaline donor have been synthesized for application in dye sensitized solar cells as sensitizers. The dyes exhibited interesting optical properties tuneable by the nature of the conjugation bridge. Origin of the optical transitions in the dyes was corroborated by the TDDFT computations. The LUMO and HOMO of the dyes were found to be favourable for electron injection into the conduction band of TiO₂ and the regeneration of the dye by the redox shuttle in the electrolyte, respectively. Since the dyes exhibited solvent dependent absorption properties, DSSC were fabricated from two different dye bath solutions, DCM and acetonitrile/tert-butanol/dimethylsulphoxide mixture. The dyes adsorbed from the later bath solution led to better device performance owing to the comparatively less charge transfer resistance arising from the well-spaced dye packing. Among all dyes, a dye containing thiophene in the π-spacer exhibited the highest device efficiency.