Novel organic D-π-2A sensitizer for dye sensitized solar cells and its electron transfer kinetics on TiO₂ surface

Although numerous donor-π-acceptor (D-π-A) type organic dyes were investigated in order to replace the ruthenium polypyridyl complexes, there have been few reports of the D-π-2A system and the related electron transfer processes. In this work, a novel D-π-2A dye (coded as B2) was designed and synthesized for applications in dye-sensitized solar cells (DSSC). Obvious intramolecular charge transfer (ICT) between the donor and acceptor takes place under photoexcitation. Three frontier LUMOs (LUMO, LUMO+1, LUMO+2) of B2 are all located on the acceptor part, which is highly favorable for intramolecular electron transfer from the donor to acceptors and enhances the electron injection into the semiconductors. DSSC based on B2 showed a maximum monochromatic incident photon-to-current efficiency (IPCE) of 68% at 425 nm and an overall power conversion efficiency of 3.62% under simulated solar light (AM 1.5G, 100 mW cm^-2) irradiation. Femtosecond and nanosecond TA, and TCSPC techniques were used to monitor the photophysical properties of B2 and the electron transfer processes taking place between B2 and the semiconducting nanoparticles. It is found that electrons in the delocalized π→π* transition could be further injected into the semiconductor, while such injection process hardly happens for electrons in the localized π→π* transition.