Photovoltaic performance of dye-sensitized solar cells based on donor-acceptor pi-conjugated benzofuro[2,3-c]oxazolo[4,5-a]carbazole-type fluorescent dyes with a carboxyl group at different positions of the chromophore skeleton

Donor-acceptor pi-conjugated benzofuro2,3-c]oxazolo4,5-a]carbazole-type fluorescent dyes 3a, 3b, 8a, and 8b with a carboxyl group at different positions of the chromophore skeleton have been designed and synthesized. The absorption and fluorescence spectra and cyclic voltammograms of the fluorescent dyes agree very well, showing that the position of the carboxyl group has a negligible influence on the photophysical and electrochemical properties of these dyes. When these dyes are used in dye-sensitized solar cells, however, their photovolatic performances are considerably different. The short-circuit photocurrents and energy conversion efficiencies under a simulated solar light increase in the order: 3a (2.12 mA cm(-2), 1.00%) approximately 3b (2.10 mA cm(-2), 1.06%) > 8b (1.50 mA cm(-2), 0.67%) > 8a (0.84 mA cm(-2), 0.34%). Based on semi-empirical molecular orbital calculations (AM1 and INDO/S) together with spectral analyses and their photovolatic performance, the relationships between the observed photovolatic properties and the chemical structures of the benzofuro2,3-c]oxazolo4,5-a]carbazole-type fluorescent dyes are discussed. It is found that strong interaction between a TiO(2) surface and the electron accepting moiety of the dye leads to a high photovoltaic performance.