Joint electrical, photophysical and computational studies on D-π-A dye sensitized solar cells: the impacts of dithiophene rigidification

The rigidification of pi-conjugated segments represents a feasible tactic towards energy-level engineering of organic D-pi-A dyes in mesoscopic titania solar cells. In this work, comparions of four dyes with the di(3-hexylthiophene), dihexyldithienosilole, dihexylcyclopentadithiophene and N-hexyldithienopyrrole linkers have revealed some general influences of pi-linker rigidification on the optoelectronic features of titania solar cells employing a cobalt(II/III) redox electrolyte, in terms of energetic and kinetic viewpoints. Compared to a dye with the di(3-hexylthiophene) linker, its three counterparts with rigidified dithiophene blocks present bathochromic and hyperchromic absorptions of solar photons. Transient absorption measurements have shown that the incorporation of Si-, C-and N-bridged dithiophene segments decelerates the dye regeneration kinetics. The rigidification of pi-conjugated dithiophene linkers brings forth a general open-circuit photovoltage diminishment, in the range from 60 to 190 mV. Further insightful impedance analyses have disclosed that the open-circuit photovoltage reduction, due to the pi-linker alternation from di(3-hexylthiophene) to N-hexyldithienopyrrole, is predominantly caused by an adverse downward displacement of the titania conduction band edge, despite a positive contribution from attenuated charge recombination at the titania/electrolyte interface.