Simple triarylamine-based dyes containing fluorene and biphenyl linkers for efficient dye-sensitized solar cells

New dipolar dyes featuring arylamine donors, fluorene or biphenyl conjugation, and cyanoacetic acid acceptor units have been synthesized and characterized by spectral, optical, and electrochemical measurements. Absorption measurements indicate that the fluorene conjugation is beneficial for red shifting the charge transfer transition and increasing the optical density. On the contrary, the biphenyl linker leads to a blue-shifted hypochromic charge-transfer transition due to the inefficiency in the π-conjugation. For a particular linker, the diphenylamine unit acts as an efficient donor when compared to the 1-naphthylphenylamine moiety. However, the red shift observed in the absorption profile for the diphenylamine-containing dyes is compensated for by a reduction in the molar extinction coefficient, that is, the naphthylamine derivatives are beneficial for increasing the molar extinction coefficient. Electrochemical investigations show that the fluorene-based dyes are more redox stable than the corresponding biphenyl derivatives. Effects of deprotonation on the absorption maxima and redox potential have been investigated by absorption and electrochemical measurements. These studies reveal a blue shift in the absorption profile and a cathodic shift for the oxidation potential originating due to deprotonation of the dyes. The dye-sensitized solar cells fabricated using these dyes exhibit moderate to good efficiencies, and the fluorene-based dyes are better than the biphenyl analogues. It is attributed to the diminished charge separation in the biphenyl derivatives due to the nonplanar structural feature which hinders donor-acceptor interactions and electron delocalization across the molecule.