| Using electrophosphorescence to improve the quantum efficiency of organic light-emitting dioldes (OLED) is one of the forefront and hot areas of the photoelectronic information materials in recent years, and the research of phosphorescent host materials is indispensible for the commercial fabrication of phosphorescent OLEDs. As for effective host materials, how to improve thermal and chemical stability, charge transport balance and compatibility with the guest materials under the premise of ensuring high triplet energy has become the research focus. On the other hand, in terms of fabrication technology of OLEDs, solution process has many advantages, such as simple equipment, low cost and so on, and the solution-processed film-forming ability is also an important factor for phosphorescent host materials design.In this study, we aim at designing and synthesizing solution processable phosphorescent host materials with high triplet energy, balanced charge transport and good compatibility with guest materials. A series of steric monodisperse triphenylamine-fluorene, carbazole-fluorene and diazafluorene-triphenylamine oligomers were designed and synthesized as phosphorescent host materials and deep-blue fluorescent dyes using Friedel-Crafts electrophilic substitution reaction. The designs make full use of extended fluorene arms to effectively avoid aggregation of carbazole (triphenylamine) and fluorene, endowing host materials with stable morphology; the steric structure helps to maintain good compatibility with the guest materials; the nonconjugated linkages between fluorene arms and core have a synergy of the groups’optoelectronic properties and guarantee the stability of the film morphology and charge transport balance in light-emitting layer. As a result, the stable and efficient phosphorescent (or fluorescent) OLEDs are finally achieved.The main achievements are summarized as the followings:(1) Synthesis and optoelectronic properties of triphenylamine-fluorene based cyclic oligomer phosphorescent host materialBased on Friedel-Crafts electrophilic substitution reaction between triphenylamine and fluorenol, a novel cyclic triphenylamine-fluorene based oligomer (TPAF)3 was synthesized as a phosphorescent host. Here, the triphenylamine group is connected by C-9 of fluorene group, which limits the effective conjugation length and ensures the host of high triplet energy (2.79 eV); the higher molecular weight and large cycle space structure endow it with good thermal stability (no obvious Tg below 300℃, Td5%>500 ℃) and compatibility with guest materials (a root-mean-square roughness of 0.42 nm).It is found that the mixture of (TPAF)3, and TPBI (1,3,5-Tris(1-phenyl-1H-benzimidazol-2-yl)benzene) or CBP (4,4’-bis(9-carbazolyl)-1,1’-biphenyl) generates exciplex. Based on this exciplex, solution-processed white PhOLED was finally obtained with a simple structure of ITO/PEDOT:PSS (25 nm)/(TPAF)3:OXD-7:FIrpic (1:0.3:0.15,40 nm)/CBP (0-35 nm)/TPBI (35 nm)/Ca (10 nm)/Ag. The maximum luminance and current efficiency of the device are 14213 cd/m2 and 22.6 cd/A respectively, with a CIE coordinate of (0.27,0.32). The device structure differs from that of the common wihte OLEDs:the latter usually utilizes the fabrication technology of multi guest material doping or the stack of light-emitting layers with different colors, while the former obtains white light-emitting only with a blue phosphorescent guest Bis[2-(4,6-difluorophenyl)pyridinato-C2,N] (picolinato)iridium(Ⅲ) (FIrpic) as mono-chromatic dopant. The result provides a new idea for the future implementation of white display.(2) Synthesis and optoelectronic properties of carbazole-fluorene based cyclic oligomer phosphorescent host materialBased on Friedel-Crafts electrophilic substitution reaction between carbazole and fluorenol, a novel carbazole-fluorene based oligomer (PhCzF)4 was synthesized as a phosphorescent host material. In the produced compound, carbazole is connected to the sp3-hybridized carbon atom and the rodlike fluorenes intercept the ring plane to form a rigid three-dimensional structure, affording (PhCzF)4 with ET of 2.82 eV and good film-forming and doping ability. Using (PhCzF)4 and TAPC (Di-[4-(N,N-ditolyl-amino)-phenyl]cyclohexan) as mixed host materials, solution-processed blue PhOLED with the structure of ITO/PEDOT:PSS (25 nm)/[(PhCzF)4:TAPC=1:1]:Firpic 23 wt% (30 nm)/TPBI (35 nm)/Ca (10 nm)/Ag was fabricated. The optimized device has a low turn-on voltage of 3.3 V and the maximum luminance, current efficiency and quantum efficiency are 11243 cd/m2,18.8 cd/A and 8.7%, respectively. Besides, the device based on (PhCzF)4:OXD-7:TAPC shows a lower turn-on voltage of 2.9 V. At the luminances of 100 and 1000 cd/m2, the operating voltages are only 3.8 and 4.6 V, and current efficiencies are also as high as 12.0 and 14.1 cd/A, respectively. In this study we obtain cyclic monodisperse oligomer phosphorescent host material by a simple two-step reaction and the solution-processed OLEDs exibit good performances.(3) Synthesis and optoelectronic properties of diazafluorene-triphenylamine based bipolar phosphorescent host materialsBased on Friedel-Crafts electrophilic substitution reaction between diazafluorene and fluorenol, a series of bipolar phosphorescent hosts TPA-DAFn (n=1～3) with different ratios of electron-transporting unit were synthesized. The results of single carrier devices demonstrate the TPA-DAF has the most balanced charge fluxes. Solution-processed red, green and blue PhOLEDs were fabricated utilizing TPA-DAFn as hosts, respectively. TPA-DAF-based devices show the best performance with maximum current efficiencies of 20.6,24.5, and 12.5 cd/A, and maximum EQE of 8.5, 7.6, and 4.5% for blue, green, and red PhOLEDs, respectively. What’s more, all TPA-DAF-based devices exhibit slight efficiency roll-off. For examples, blue PhOLEDs based on TPA-DAF have current efficiencies of 20.0,19.5 and 17.6 cd/A at luminance of 973,5586 and 93109310 cd/m2, respectively. That is to say, from 1000 to 10000 cd/m2, the efficiency roll-off is only 12%. This paper introduces some new host molecules with different ratios of electron-transporting units using simple Friedel-Crafts reaction, and optimized PhOLEDs with good performance are obtained.(4) Synthesis and optoelectronic properties of pyrene-fluorene based deep-blue fluorescent dyesBased on Friedel-Crafts electrophilic substitution reaction between triphenylamine and pyrene-functionalized fluorenol, two monodisperse pyrene-fluorene based oligomers, TPA-PyF3 and CBP-PyF3, were prepared as deep-blue fluorescent dyes. In the oligomer, nonconjugated linkage of fluorene, triphenylamine, CBP and pyrene coordinates the optoelectronic properties of each group and affords the molecules with high fluorescence quantum efficiencies, good charge transporting and hole-injection ability; the core and surface structure effectively suppresses the aggregation of pyrene and improves the thermal stability and film-forming ability. The fluorescent emitting peaks are 394 and 418 nm with solution fluorescence quantum yields of 0.84 and 0.89 for TPA-PyF3 and CBP-PyF3, respectively. Solution-processed deep-blue OLEDs with a simple structure of ITO/PEDOT:PSS (25 nm)/TPA-PyF3 or CBP-PyF3 (40 nm)/TPBI (35 nm)/Ca (10 nm)/Ag were fabricated. Device based on the CBP-PyF3 shows higher performance and achieves the highest efficiency at 200 cd/m2 with the maximum current efficiency of 1.68 cd/A, maximum external quantum efficiency of 2.07%. The CIE coordinates are all around (0.16,0.08). Besides, the device shows the low efficiency roll-off with current efficiencies of 1.44 and 1.20 cd/A at 100 and 1000 cd/m2, respectively. |
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