Dye-sensitized solar cells based on novel donor-acceptor π-conjugated benzofuro[2,3-c]oxazolo[4,5-a]carbazole-type fluorescent dyes exhibiting solid-state fluorescence

Novel donor-acceptor pi-conjugated benzofuro[2,3-c]oxazolo[4,5-a]carbazole-type fluorescent dyes ( 2a-2d) with substituents ( R = H, butyl, benzyl and 5-nonyl) on the nitrogen of carbazole ring have been designed and synthesized as sensitizers in dye-sensitized solar cells ( DSSCs). Absorption and fluorescence properties of 2a-2d are similar in solution. In the solid state, however, the dyes 2c ( R = benzyl) and 2d ( R = 5-nonyl) with bulky substituents exhibit strong solid-state fluorescence properties, which are attributed to the reduction of the pi-pi interaction between the dyes. Photovoltaic parameters of DSSCs based on 2a-2d are measured with the amount of dyes adsorbed on TiO2 film as a parameter. It is found that the 2d exhibits a 100% efficiency for absorbed photon-to-current conversion, demonstrating that bulky substituents in 2d can efficiently prevent intermolecular energy transfer between the dyes in molecular aggregation states.