Functional tuning of phenothiazine-based dyes by a benzimidazole auxiliary chromophore: An account of optical and photovoltaic studies

We have designed and synthesized a series of organic dyes featuring phenothiazine donors containing an N-phenylbenzimidazole substituent and a cyanoacrylic acid acceptor. Benzimidazole incorporation red-shifted the charge transfer transition and increased the molar extinction coefficient of the peak. It also helped to fine-tune the HOMO and LUMO energies of the dyes due to the electron-withdrawing nature of the benzimidazole. The dye-sensitized solar cells fabricated using the dyes possessing N-phenylbenzimidazole showed efficiency better than the analogous dye without benzimidazole. The hike in the efficiency resulted from the increase in short-circuit current and open circuit voltage. The electrochemical impedance spectroscopy of the devices revealed that the integration of benzimidazole in the dye enhances the life time of the injected electrons in the conduction band of TiO₂ and resists electron recombination at the TiO₂/dye/electrolyte interface.