The effect of additional electron donating group on the photophysics and photovoltaic performance of two new metal free D-π-A sensitizers

• Synthesis of D-π-A organic sensitizers with benzimidazole group as π-bridge.
• Solar cell conversion efficiency of 3.4% with the D-π-A sensitizers.
• Electron injection dynamics on TiO2 and Al2O3 films sensitized with the two dyes.

The synthesis, photophysical, electrochemical and photovoltaic properties of two D-π-A organic sensitizers, having triphenylamine as electron donor (with or without additional electron donating hexyloxy group), carboxylic acid as electron acceptor and benzimidazole group as π-bridge, are described in detail. The photophysical measurements were carried out for the dyes in solutions and on TiO2 and Al2O3 nanostructured films using time resolved spectroscopy in the fs-ps and ns time scale. The electron injection dynamics on a series of TiO2 nanostructured films sensitized with the two dyes, have shown a faster electron injection for the dye with additional electron donating group. The photovoltaic characterization has shown a maximum overall conversion efficiency of 3.4% for dye sensitized solar cells with a quasi-solid state electrolyte. The performance of the solar cells is explained in terms of the photophysical properties, electron injection dynamics and local resistance to charge transfer across the TiO2-electrolyte interface measured with electrochemical impedance spectroscopy.