| Since the earliest reports of organic/polymeric light-emitting diodes (OLEDs/PLEDs), fabricating ultrathin, full-color, free-angle and large-area displays have stimulated intensive research interest around the world. To realize full-color displays, high performance red, green, and blue light-emitting materials are required. In contrast to red and green emitters, only a few blue emitters showed application potential but much inferior performance in efficiency, stability and color purity. The highly efficient blue light-emitting materials can be used as not only emissive layer in OLEDs/PLEDs, but also host materials for efficient blue and white light source. Among the promising blue emitters, fluorene-based oligomers and polymers have displayed excellent thermal stabilities, fluorescent quantum yields, and electroluminescent behaviors. However, the fluorene-based electroluminescent blue materials usually exhibited inferior electroluminescence efficiency, poor color purity and spectral stability caused by weak charge injection and transport, aggregation/excimer formation, and/or the fluorenone of photooxidized fluorene. To address these problems, covalently incorporating bulky electron-donating and-withdrawing groups onto the polymer backbone is an effective way to enhance the charge injection/transport and suppress aggregation/excimer formation. In addition, if the bulky groups were attached on the C-9 position of fluorene unit, the photooxidization could be inhibited.This thesis was divided into nine parts, as follows:The progress of the fluorene-based blue light-emitting compounds and polymers was reviewed in Chapter 1. The key point of the review was paid to the design, synthesis, and electroluminescent performance of oligofluorenes and polyfluorenes with electron-donors and/or acceptors. From the literatures reviewed, it was concluded that the materials containing both electron-donating and electron-withdrawing segments could have better performance than those corresponding "hole-only" or "electron-only" materials.In Chapter 2, aπ-conjugated copolymer (CNF-TPA)n was synthesized by Knoevenagel polycondensation. Fast and efficient photoinduced electron transfer from triphenylamine (TPA) to cyanofluorene (CNF) produced the long-lived charge-separated state (90 s) in benzonitrile. The charge-recombination process of (CNF·-TPA·+)n was much slower than the charge-separation in polar benzonitrile. To further confirm the electron-donating property of TPA units, a new blue-emitting polymer poly[5-(diphenylamino)-1,3-phenylenevinylene] (Yul) was prepared via McMurry condensation reaction of YuO. In Ar-saturated toluene, the fluorescence decay profile of this blue-emitting polymer exhibited single exponential decay with lifetime of 7.12 ns.Chapter 3 described a series of blue-light-emitting copolymers PSF, PCC-1, PCC-2, PCC-3, and PCF composed of different ratios of cyanophenyl-spirobifluorenes and carbazole-triphenylamines. Incorporation of the rigid spirobifluorene units substituted with cyanophenyl groups into the polymer backbone improved not only the thermal stabilities but also the photoluminescence efficiencies. With the device configuration of ITO/PEDOT:PSS/polymers:PBD/CsF/Ca/Al, PCC-2 showed the best performance with the lowest turn-on voltage of 3.1 V, the highest luminance of 6369 cd/m2, the highest current efficiency of 1.97 cd/A, and the best power efficiency of 1.40 lm/W.Since cyanophenyl-spirobifluorene is an excellent monomer with electron-transporting capability, in Chapter 4, we use it to polymerize with the electron-donating tricarbazole-triphenylamine and 9,9-dihexylfluorene-2,7-bis(trimethyleneborate). By tuning the feed ratio, a series of blue-light-emitting copolymers PTC-1, PTC-2, PTC-3, and PTCF were prepared. It was found that increasing the content of the donors raised both the HOMO and LUMO energy levels. With the device configuration of ITO/PEDOT:PSS/polymers:PBD/CsF/Ca/Al, PTC-2 showed the best performance with the turn-on voltage of 3.0 V, maximum brightness of 7257 cd/m2, maximum current efficiency of 1.76 cd/A, and EL emission peak at 460 nm.In Chapter 5, a novel bipolar copolymer PTHCF with triphenyamine and cyanophenylfluorene side chains was synthesized for studying the polymer backbone emission. In contrast to the electronic absorption spectrum in dilute solution, the absorbance of PTHCF in thin film was slightly, blue-shifted. An electroluminescence (EL) device with configuration of ITO/PEDOT:PSS/PTHCF70%+PBD30%/CsF/Ca/Al exhibited a deep-blue emission as result of excitons formed by the charges migrating along the full-fluorene mainchain. The incorporation of the bipolar side chains into the polymer structure prevented the inter-molecular interaction of the fluorene moieties, improved charge injection/transport, increased the yield of exciton formation in main chain, and thereby enhanced the polymer backbone emission.Chapter 6 innovatively reported a series of oligofluorenes with ambipolar cyanophenyl and carbazole end groups. The existence of the bipolar end groups could effectively tune the energy levels of the oligofluorenes. By using the device configuration of ITO/PEDOT:PSS/oligofluorenes/TPBi/LiF/Al, F4 with four fluorene spacers displayed the best performance:the lowest turn-on voltage (4.1 V),the highest maximum luminance (2180 cd/m2) and maximal current efficiency (1.17 cd/A). The optimized device of ITO/MoO3/NPB/CBP:F4(1:4)/TPBi/LiF/Al by vapor deposition showed highest brightness of 5135 cd/m2, current efficiency of 1.76 cd/A, and CIE coordinates of (0.16,0.09).Quinoline was also one of the outstanding electron acceptors. In Chapter 7 we synthesized a blue light-emitting copolymer PTHD containing electron-rich triphenylamine and electron-poor diphenylquinoline side chains in the C-9 positions of fluorene units. In contrast to the reference polymer poly{[9,9-dihexylfluorene]-alt-[9,9-di(2,4-diphenylquinoline)fluorene]} (PHD), PTHD exhibited higher HOMO energy level and maximum brightness.Besides cyano group and quinoline, another excellent electron-transporting candidate was oxadiazole. Thus, Chapter 8 introduced two copolymers POFPA and POFCPA, in which diphenyloxadiazole and carbazole/tricarbazole-triphenylamine were chosen as charge transport segments. Either in dilute toluene solution or in the thin film, the polymer with higher content of carbazole possessed shorter absorption and photoluminescence, as well as much higher fluorescent quantum yield, in comparison with the other polymer. With the device configuration of ITO/PEDOT:PSS/polymer/TPBi/LiF/Al, efficient graded LUMO route for electron injection and transport was obtained in the POFCPA device, leading to higher maximum current efficiency (1.79 cd/A) and power efficiency (0.87 lm/W) than the POFPA device (1.60 cd/A,0.83 lm/W). The doped device based on POFCPA showed maximum luminance of 13613 cd/m2, highest current efficiency of 3.38 cd/A with the CIE coordinates of (0.15,0.24).In Chapter 9, the research results from Chapter 2 to Chapter 8 were summarized. Keywords:electroluminescence; blue light-emitting materials; fluorene; electron-withdrawing groups; electron donating groups... |
