Organic dyes containing fluoren-9-ylidene chromophores for efficient dye-sensitized solar cells

Novel organic sensitizers comprising one or two fluorenylidene moieties in the donor part of triarylamine, cyanoacrylic acid as the acceptor/anchoring group and a fluorene and oligothiophene spacer in a D-π-A architecture have been synthesized and characterized as sensitizers for nanocrystalline TiO ₂-based dye-sensitized solar cells. Their optical, electrochemical and photovoltaic properties are compared with the electron accepting dicyanovinyl unit containing the organic sensitizer. Incorporation of the fluorenylidene moiety dominates the optical properties of the dyes in terms of relatively broad and high molar extinction coefficient absorption when compared to the dicyanovinyl derivatives. Theoretical investigations using TDDFT simulations indicate that the trends in the excitation energies are consistent with the solution spectral data for higher wavelength absorption and the lower wavelength absorptions attributed to the amine to auxiliary acceptor charge transfer. The electrochemical properties are influenced by the number of fluorenylidene chromophores and the electron richness of the linking segment. The dye-sensitized solar cells fabricated using fluorenylidene-based sensitizers showed higher power conversion efficiency than the dicyanovinyl derivatives attributed to their higher photocurrent density. A fluorenylidene-based dye exhibited a high power conversion efficiency of 6.13% under full sunlight (AM 1.5G, 100 mW cm^-2).