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 In showed a blue shift in comparison to 1b, suggesting the structure or 1 a was Misled in comparison to 11 I I 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 pi-conjugation unit suffer from larger dispersion forces between the dyes and acceptors. 1(3)(-) and/or 1(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 pi-linker without a significant increase of polarizability. The obstacle unit is to increase the intermolecular distance between the pi-linker and acceptor species in electrolytes. Twisted structure is suggested to he one strategy to add such an effect.