Organic Dyes Containing Pyrenylamine-Based Cascade Donor Systems with Different Aromatic π Linkers for Dye-Sensitized Solar Cells: Optical, Electrochemical, and Device Characteristics

New organic dyes containing pyrenylamine donors in a cascade arrangement and cyanoacrylic acid acceptors have been synthesized and characterized by optical, electrochemical, and theoretical studies. The dyes inherit a D-p1-D-p2-A (D=donor, A=acceptor) molecular architecture where the p linkers p1 are changed from phenyl to biphenyl and fluorene, whereas the p linker p2 that connects the donor fragment with the acceptor is a phenyl unit. The conjugation pathway linking the two donor segments has been found to play a major role in the optical and electrochemical properties. Shorter p linkers such as phenyl groups facilitate the donoracceptor interaction while the nonplanar biphenyl spacer decreases the electronic communication between the donors and enhances the oxidation propensity of the corresponding dye. All the dyes display an intense longer wavelength electronic transition,which is attributable to the amine-to-cyanoacrylic acid charge transfer. The extinction coefficient of this peak grows dramatically on increasing the conjugation pathway length between the two donor segments. The dyes were used as sensitizers in nanocrystalline TiO2-based dye-sensitized solar cells (DSSCs) and the cascade donor system contributed to the enhancement in the device efficiency due to favorable absorption and redox properties.