Rhodanine dyes for dye-sensitized solar cells: spectroscopy, energy levels and photovoltaic performance
Three new sensitizers for photoelectrochem. solar cells were synthesized consisting of a triphenylamine donor, a rhodanine-3-acetic acid acceptor and a polyene connection. The conjugation length was systematically increased, which resulted in two effects. First, it led to a red shift of the optical absorption of the dyes, resulting in an improved spectral overlap with the solar spectrum. Secondly, the oxidn. potential decreased systematically. The excited state levels were, however, calcd. to be nearly stationary. The exptl. trends were in excellent agreement with d. functional theory computations. The photovoltaic performance of this set of dyes as sensitizers in mesoporous TiO2 solar cells was investigated using electrolytes contg. the iodide/triiodide redox couple. The dye with the best absorption characteristics showed the poorest solar cell efficiency, due to losses by recombination of electrons in TiO2 with triiodide. Addn. of 4-tert-butylpyridine to the electrolyte led to a strongly reduced photocurrent for all dyes due to a reduced electron injection efficiency, caused by a 0.15 V neg. shift of the TiO2 conduction band potential.