Molecular Design of Organic Dye toward Retardation of Charge Recombination at Semiconductor/Dye/Electrolyte Interface: Introduction of Twisted π-Linker

A triphenylamine (TPA) dye with two hexyl groups at the opposable 3,3′-positions of a bithiophene linker ( 1a ) was prepared and compared to a dye with a regio regular 4,3′-dihexylbithiophene unit ( 1b ). The absorption spectrum of 1a showed a blue shift in comparison to 1b, suggesting the structure of 1a was twisted in comparison to 1b . The twisted structure agreed with the structure optimized by DFT calculations. By replacing one thiophene unit of 1a and 1b with a pyridine ring ( 2a and 2b, respectively), a further blue shift was observed. Dye-sensitized solar cells (DSSCs) were prepared from these dyes and a conventional Ru dye (N719). Under one sun conditions, DSSCs/ 2a showed comparable or higher open-circuit voltage ( V oc ) than did DSSCs/N719. The high V oc was attributed solely to long electron lifetime in the DSSCs/ 2a . A previous study has suggested that TPA dyes with long π-conjugation unit suffer from larger dispersion forces between the dyes and acceptors, I 3 − and/or I 2, causing short electron lifetime and thus low V oc . The present study shows that this problem can be overcome by increasing steric hindrance by attaching obstacle units to the π-linker without a significant increase of polarizability. The obstacle unit is to increase the intermolecular distance between the π-linker and acceptor species in electrolytes. Twisted structure is suggested to be one strategy to add such an effect.