D-D-π-A-Type Organic Dyes for Dye-Sensitized Solar Cells with a Potential for Direct Electron Injection and a High Extinction Coefficient: Synthesis, Characterization, and Theoretical Investigation

A series of organic sensitizers with the direct electron injection mechanism and a high molar extinction coefficient comprising double donors, a pi-spacer, and anchoring acceptor groups (D-D-pi-A type) were synthesized and characterized by experimental and theoretical methods for dye-sensitized solar cells. (E)-2-Cyano-3-(5 ''-(4-((4-(3,6-di-tert-butylcarbazol-9-yl)phenyl)dodecylamino)- phenyl)-[2,2':5',2 ''-terthiophene]-5-yl)acrylic acid showed performance with a maximal incident photon to electron conversion efficiency of 83%, J(sc) value of 10.89 mA cm(-2), V-oc value of 0.70 V, and fill factor of 0.67, which correspond to an overall conversion efficiency of 5.12% under AM 1.5G illumination. The molecular geometry, electronic structure, and excited states were investigated with density functional theory, time-dependent density functional theory, and the symmetry-adapted cluster-configuration interaction method. The double donor moieties not only contribute to enhancement of the electron-donating ability, but also inhibit aggregation between dye molecules and prevent iodide/triiodide in the electrolyte from recombining with injected electrons in TiO2. Detailed assignments of the UV-vis spectra below the ionization threshold are given. The low-lying light-harvesting state has intramolecular charge transfer character with a high molar extinction coefficient because of the long pi-spacer. Our experimental and theoretical findings support the potential of direct electron injection from the dye to TiO2 in one step with electronic excitation for the present D-D-pi-A sensitizers. The direct electron injection, inhibited aggregation, and high molar extinction coefficient may be the origin of the observed high efficiency. This type of D-D-pi-A structure with direct electron injection would simplify the strategy for designing organic sensitizers.