基于芴和苯并咪唑的蓝光发射共聚物的合成、表征及电致发光器件研究
摘要
有机电致发光是近年来国际上的一个研究热点。有机发光二极管具有全固态、主动发光、亮度高、对比度高、视角宽、响应速度快、厚度薄、低电压直流驱动、功耗低、成本低、工作范围宽、可实现软屏显示等特点,被认为是平板显示的“明日之星”。经过近20多年来的飞速发展,基本实现了红、绿、蓝三基色发光。其中绿色发光材料发展最快,基本达到了商业化的要求,而红色和蓝色发光材料的问题较多,特别是稳定、高效率的蓝光更具有挑战性。因而制备高效的蓝光器件一直是该领域的研究重点。
聚芴及芴的衍生物由于具有良好的热稳定性和化学稳定性、较好的空穴传输能力以及很高的荧光量子效率,成为一种倍受关注的蓝色发光的共轭聚合物。为了提高聚芴的电子传输能力,平衡空穴和电子的注入和传输,通常采用的方法将p-型的芴(电子给体)与n-型单体(电子受体),形成电子受体-给体结构的共聚物。
咪唑基类化合物的咪唑环是缺电子芳香环,属于非中心对称结构,能发生分子内电荷转移,具有良好的电子传输性能。
因此,我们设计了合理的合成路线制备出2,2'-(1,4-苯撑)-二(5-溴-苯并咪唑)(单体5),并与2,7-二硼酸-9,9-二辛基芴(单体9)通过Suzuki反应制得共聚物(PFBI0)。但是由于苯并咪唑强烈的分子间氢键作用,导致PEBI0不溶于常用的有机溶剂。所以,我们又将单体5进行N-烷基取代,得到烷基化的单体7,然后将单体7和单体9共聚得到共聚物PFBI8。由于咪唑环上的NH全部被烷基取代,所以共聚物的溶解性能得到极大的改善,溶于四氢呋喃、二氯甲烷、三氯甲烷、甲苯等常见的有机溶剂。且PFBI8具有良好的热性能(Tg = 103οC,Td = 428 oC),以及很高的荧光量子效率,在氯仿溶液中量子效率高达99%。PFBI8薄膜的最大发射波长为450 nm。
我们将PFBI8作为发光层,制备了三种不同结构的聚合物发光二极管,研究PFBI8的电致发光性能。其中,器件(ITO/PEDOT:PSS/PFBI8/LiF/Al)的最大发射波长为448 nm,最大亮度达到1534 cd/m~2 (电流效率0.67 cd/A,功率效率0.20 lm/W)。
总之,PFBI8具有作为蓝色发光的聚合物电致发光材料的潜力。
Organic/Polymeric light-emitting diodes have received increasing attention due to their potential applications in the next generation flat-panel displays. The benefits of PLEDs include thin, low-cost displays with a low driving voltage, wide viewing angle, high contrast, color gamut and flexible. Among the three prime colors, many good red and green emitters have been developed to satisfy the practical requirements for OLEDs. In contrast, the development of blue emitters with high electroluminescence (EL) efficiencies remains a challenge that has attracted significant research effort.
Polyfluorenes and fluorine-containing copolymers have emerged as very promising materials for blue emission due to their unique combination of high thermal stability, chemical stability, high photoluminescence (PL) quantum yields, good film-forming and hole-transporting properties. To improve the electron transport properties and achieve balanced charge carrier injection/transport in polyfluorenes, a common strategy is to incorporate n-type (electron acceptor) comonomers into the predominantly p-type polyfluorenes backbone.
Imidazole ring is a non-center symmetrical structure with electron deficient characteristic. Benzimidazole based molecules, such as 1,3,5-tris(1-phenyl-1H-benzimidazole-2-yl)benzene (TPBI) are reported to show large bandgap and effective electron-transport property.
In the present study, we designed and synthesized 2,2'-(1,4-phenylene)-bis(benzimidazole) (monomer 5), and copolymerized with 9,9-dioctylfluorene-2,7-diboronic acid (monomer 9) though Suzuki coupling reaction to obtain copolymer PFBI0. But PFBI0 shows poor solubility in common solvents because of intense intra- and intermolecular interactions. For this reason, we synthesized N-octyl substituted 5 (monomer 7) to avoid the strong hydrogen bond of benzimidazole. And as expected, copolymer of 7 and 9 (PFBI8) exhibits excellent solubility due to the diminished interactions. PFBI8 also enjoys good thermal stability, whose decomposition temperature and glass transition temperature are as high as 428 and 103 oC, respectively. It emits blue light in solvents with a quantum yield up to 99% in CHCl3.
The electroluminescence (EL) peak of PFBI8 in the device with the configuration of ITO/PEDOT:PSS/ PFBI8/TPBI/LiF/Al is 448 nm, which is comparable to the film photoluminescence maximum of 450 nm. Such polymer light-emitting diode (PLED) exhibits a maximum luminance of 1534 cd/m~2 with the current efficiency and power efficiency of 0.67 cd/A and 0.20 lm/W, respectively.
The pure blue emission and good EL performance make PFBI8 promising for blue light-emitting PLED applications.
聚芴及芴的衍生物由于具有良好的热稳定性和化学稳定性、较好的空穴传输能力以及很高的荧光量子效率,成为一种倍受关注的蓝色发光的共轭聚合物。为了提高聚芴的电子传输能力,平衡空穴和电子的注入和传输,通常采用的方法将p-型的芴(电子给体)与n-型单体(电子受体),形成电子受体-给体结构的共聚物。
咪唑基类化合物的咪唑环是缺电子芳香环,属于非中心对称结构,能发生分子内电荷转移,具有良好的电子传输性能。
因此,我们设计了合理的合成路线制备出2,2'-(1,4-苯撑)-二(5-溴-苯并咪唑)(单体5),并与2,7-二硼酸-9,9-二辛基芴(单体9)通过Suzuki反应制得共聚物(PFBI0)。但是由于苯并咪唑强烈的分子间氢键作用,导致PEBI0不溶于常用的有机溶剂。所以,我们又将单体5进行N-烷基取代,得到烷基化的单体7,然后将单体7和单体9共聚得到共聚物PFBI8。由于咪唑环上的NH全部被烷基取代,所以共聚物的溶解性能得到极大的改善,溶于四氢呋喃、二氯甲烷、三氯甲烷、甲苯等常见的有机溶剂。且PFBI8具有良好的热性能(Tg = 103οC,Td = 428 oC),以及很高的荧光量子效率,在氯仿溶液中量子效率高达99%。PFBI8薄膜的最大发射波长为450 nm。
我们将PFBI8作为发光层,制备了三种不同结构的聚合物发光二极管,研究PFBI8的电致发光性能。其中,器件(ITO/PEDOT:PSS/PFBI8/LiF/Al)的最大发射波长为448 nm,最大亮度达到1534 cd/m~2 (电流效率0.67 cd/A,功率效率0.20 lm/W)。
总之,PFBI8具有作为蓝色发光的聚合物电致发光材料的潜力。
Organic/Polymeric light-emitting diodes have received increasing attention due to their potential applications in the next generation flat-panel displays. The benefits of PLEDs include thin, low-cost displays with a low driving voltage, wide viewing angle, high contrast, color gamut and flexible. Among the three prime colors, many good red and green emitters have been developed to satisfy the practical requirements for OLEDs. In contrast, the development of blue emitters with high electroluminescence (EL) efficiencies remains a challenge that has attracted significant research effort.
Polyfluorenes and fluorine-containing copolymers have emerged as very promising materials for blue emission due to their unique combination of high thermal stability, chemical stability, high photoluminescence (PL) quantum yields, good film-forming and hole-transporting properties. To improve the electron transport properties and achieve balanced charge carrier injection/transport in polyfluorenes, a common strategy is to incorporate n-type (electron acceptor) comonomers into the predominantly p-type polyfluorenes backbone.
Imidazole ring is a non-center symmetrical structure with electron deficient characteristic. Benzimidazole based molecules, such as 1,3,5-tris(1-phenyl-1H-benzimidazole-2-yl)benzene (TPBI) are reported to show large bandgap and effective electron-transport property.
In the present study, we designed and synthesized 2,2'-(1,4-phenylene)-bis(benzimidazole) (monomer 5), and copolymerized with 9,9-dioctylfluorene-2,7-diboronic acid (monomer 9) though Suzuki coupling reaction to obtain copolymer PFBI0. But PFBI0 shows poor solubility in common solvents because of intense intra- and intermolecular interactions. For this reason, we synthesized N-octyl substituted 5 (monomer 7) to avoid the strong hydrogen bond of benzimidazole. And as expected, copolymer of 7 and 9 (PFBI8) exhibits excellent solubility due to the diminished interactions. PFBI8 also enjoys good thermal stability, whose decomposition temperature and glass transition temperature are as high as 428 and 103 oC, respectively. It emits blue light in solvents with a quantum yield up to 99% in CHCl3.
The electroluminescence (EL) peak of PFBI8 in the device with the configuration of ITO/PEDOT:PSS/ PFBI8/TPBI/LiF/Al is 448 nm, which is comparable to the film photoluminescence maximum of 450 nm. Such polymer light-emitting diode (PLED) exhibits a maximum luminance of 1534 cd/m~2 with the current efficiency and power efficiency of 0.67 cd/A and 0.20 lm/W, respectively.
The pure blue emission and good EL performance make PFBI8 promising for blue light-emitting PLED applications.
引文
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[3] Lohmann F., Mehl W., Dark injection and radiative recombination of electrons and holes in naphthalene crystals, J. Chem. Phys. 1969, 50, 500-506.
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[10] Zimmerman R. G., Wessling R. A., Polyelectrolytes from bis sulfonium salts, USP: 3401152, 1968, 09-10.
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[12] Braun D., Heeger A. J., Visible light emission from semiconducting polymer diodes, Appl. Phys. Lett. 1991, 58, 1982-1984.
[13] Greenham N. C., Moratti S. C., Bradley D. D. C., et al, Efficient light-emittingdiodes based on polymers with high electron affinities, Nature 1993, 365, 628-630.
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