| Star-shaped molecules usually comprise a central core and multiple conjugated arms as the functionalized units. They can be considered to be intermediate of conjugated small molecules and polymers, and can feature the best of both sets of attributes, e.g., precise HOMO/LUMO energy levels, high thermal stability, good solubility and excellent film-forming properties. The star-shaped molecules have attracted much attention as functional optoelectronic materials in solar cells, organic light-emitting diodes(OLEDs), field effect transistors, sensors and laser gain materials. In this thesis, hexakis(fluoren-2-yl)benzene was designed and synthesized, and it was further extended with functional units to construct a series of star-shaped compounds. The application of these compounds as functional materials in OLEDs and solar cells is investigated.The main contents and results are described as follows:In Chapter1, we give an introduction on star-shaped molecules, including its background, structure characteristic, synthetic strategies and some typical star-shaped molecules, such as oligofluorenes, phenylamines and oligothiophenes. The structures-property relationship is discussed. Then we also give a brief introduction on organic light emitting diodes and polymer solar cells. Finally, the design strategies and main contents of this thesis are presented.In Chapter2, hexakis(fluoren-2-yl)benzenes with different alkyl chains were designed an synthesized. Their crystal structures, thermal, photophysical and electrochemical properties were fully characterized. X-ray crystallography indicates that hexakis(fluoren-2-yl)benzenes is a single molecule without rotamers. TGA and DSC measurement indicated these compounds with highly strained hexakis(fluoren-2-yl)benzenes architecture possess high thermal stabilities and high glass transition temperatures. OLEDs based on hexyl-HFB as hole-transporting materials showed a maximum current density of5.50cd/A, which is higher than the control device based on NPB.In Chapter3, two new hole-transporting materials, namely HFB-Cz and HFB-Dpa, were designed and synthesized by attaching carbazole and diphenylamine units to the hexakis(9,9-dihexyl-9H-fluoren-2-yl)benzene (HFB) core via Buchwald-Hartwig coupling reaction. The long alkyl chain and core rigidity endow these compounds with good solution processability and high thermal stability. HFB-Cz and HFB-Dpa exhibit high glass transition temperatures (225and154℃). Solution-processed green OLED devices using HFB-Cz and HFB-Dpa as hole-transporting materials exhibited the maximum current efficiency up to6.20cd/A. These efficiencies are sustantially higher than the NPB-based control device, and are among the highest for the hole transporting materials in similar device configuration. These results demonstrate that using hexakis(9,9-dihexyl-9H-fluoren-2-yl)benzene as the core is an effective approach to design high Tg solution-processable hole-transporting materials.In Chapter4, hexakis(9,9-dihexyl-9Hfluoren-2-yl)benzene was further extended with oligofluorenes. A series of propeller-like star-shaped oligofluorenes were synthesized. DSC measurement indicated that the benzene ring is not the core of these star-shaped molecules as we initially thought, instead, the propeller-like hexakis(fluoren-2-yl)benzene is the new core. These star-shaped oligofluorenes show amorphous feature and deep-blue emisson with high quantum efficiency. Simple double-layer electroluminescence (EL) devices using T1-T3as non-doped solution-processed emitters displayed deep-blue emissions with Commission Internationale de l’Eclairage (CIE) coordinates of (0.17,0.08) for T1(0.16,0.08) for T2, and (0.16,0.07) for3T. These devices exhibited excellent performance, with maximum current efficiency of up to5.40cd/A, and maximum external quantum efficiency of up to6.80%, which is the highest efficiency for non-doped solution-processed deep-blue organic light-emitting diodes (OLEDs) based on starburst oligofluorenes, and is even comparable with other solution-processed deep-blue fluorescent OLEDs. Furthermore, T2-and T3-based devices show striking blue EL color stability independent of driving voltage.In Chapter5, star-shaped oligothiophens with HFB as the core were designed and synthesized. Grafting thiophene units to HFB results a bond flattening of the core, making molecule a flipping-twist shape. Their thermal, photophysical and electrochemical properties were fully characterized. S1-S3show aggregate emission in condensed state. A highly cross-linked conjugated polymer can be obtained by electropolymerization of S1-S3. In Chapter6, alcohol-soluble units were introduced to star-shaped oligofluorenes to combine the advantages of star-shaped molecules and alcohol-soluble conjugated polymers. Polymer solar cells based on TO-OH and T1-OH as interlayer and PCDTBT as active layer were fabricated. The devices with TO-OH and T1-OH as interlayer showed simultaneous enhancement of open-circuit voltage(Voc), short-circuit current density(Jsc) and fill factor(FF) compared to control device without interlayer. A maximum power conversion efficiency(PCE) of6.20%was achieved, which is among the highest for polymer solar cells with PCDTBT as active layer. Inverted polymer solar cell free of PEDOT:PSS and with T1-OH as interlayer was also fabricated. The device showed a Voc of0.88V, a Jsc of9.47mA/cm2, a FF of60%and a maximum PCE of6.04%, which is33%higher than the control device based on PFN. These results demonstrated that alcohol-soluble star-shaped oligofluorenes were very promising interlayer materials.In Chapter7, Scholl oxidation of HFB was investigated. The resulting product ox-HFB were characterized by1H NMR,13C NMR, EA, MALDI-TOF MS and X-ray crystallography. The mechanism of the oxidation process was discussed. Photophysical and electrochemical properties of ox-HFB was studied. ox-HFB not only shows aggregation induced quenching in solution, but also shows crystal induced enhanced emission(CIEE) in solid state. |
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