Novel Bis[5-(fluoren-2-yl)thiophen-2-yl]benzothiadiazole End-Capped with Carbazole Dendrons as Highly Efficient Solution-Processed Nondoped Red Emitters for Organic Light-Emitting Diodes

A series of novel red-emitting bis­[5-(fluoren-2-yl)­thiophen-2-yl]­benzothiadiazole-cored dendrimers containing carbazole dendrons up to the third generation are synthesized. Their photophysical, thermal, electrochemical, and electroluminescent properties as nondoped solution-processed red light-emitters for OLEDs are investigated. By using carbazole dendrons as the end caps, we are able to reduce the crystallization and retain the high emissive ability of a planar fluorescent core in the solid state as well as improve the thermal stability of the material. These dendrimers show a bright-red fluorescence and can form morphologically stable amorphous thin films with glass-transition temperatures as high as 283 °C. Simple structured solution-processed OLEDs using these materials as hole-transporting nondoped emitters and BCP as the hole-blocking layer emit a stable red color around 622–645 nm, with high luminance efficiencies (up to 4.80 cd A^–1 at 1.2 mA cm^–2) and CIE coordinates of (0.65, 0.33), which are close to the pure red color

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 π-spacer, and anchoring acceptor groups (D–D−π–A type) were synthesized and characterized by experimental and theoretical methods for dye-sensitized solar cells. (<i>E</i>)-2-Cyano-3-(5″-(4-((4-(3,6-di-<i>tert</i>-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%, <i>J</i>_sc value of 10.89 mA cm^–2, <i>V</i>_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 π-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−π–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−π–A structure with direct electron injection would simplify the strategy for designing organic sensitizers