Synthesis and characterization of thieno[3,4-d]imidazole-based organic sensitizers for photoelectrochemical cells

Six organic sensitizers containing thieno[3,4-d]imidazole chromophore were synthesized and characterized by photophysical, electrochemical and photovoltaic studies. N-alkylated derivatives containing thiophene substituents showed blue-shifted absorption when compared to the dyes containing 4-tert-butyl-phenyl group attributable to the disturbance in the conjugation due to tilting in the structure. However, the former dyes exhibited red-shifted absorption when anchored on TiO₂ due to enhanced aggregation. All dyes displayed negative solvatochromism in the absorption spectra indicating the charge transfer character and polar ground state. They exhibited low oxidation potentials than the dyes, known in the literature, which possess imidazole-based donor. Among the dyes, a dye containing bithiophene in the conjugation and thiophenes on thieno[3,4-d]imidazole nucleus showed best photovoltaic performance with η = 2.78%, J_sc = 7.91 mA cm^-2 and ff = 0.63. Electrochemical impedance spectroscopy studies on the devices established the relationship between the functional chromophores on the electron injection and electron recombination kinetics.