大共轭高能隙电致磷光主体材料的研究进展
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摘要
目前,有机电致磷光材料亟待解决的一个核心科学问题是如何获得高能隙与双极载流子注入/传输能力兼备的主体材料。现已报道的主体材料可分为小分子主体材料、树枝状主体材料和聚合物主体材料。树枝状化合物可实现主客体一体化修饰,具有热稳定性高、载流子传输能力优异和可通过旋涂等溶液加工方式制备器件等优点,但其合成难度相对较大。同时,受限于现有的溶液加工技术,其器件效率仍有进一步提升的空间。聚合物主体材料结构易于修饰,因而易于进行功能性扩展。但其化合物成分的单分散性较弱,因受聚合物分子量分布的影响,器件性能的重现性和稳定性有待进一步提升。就小分子主体材料而言,虽然其化学结构明确、易提纯。但是,小的共轭体系往往功能相对单一,难以体现"多功能合一"的分子设计思想。相对而言,大共轭型小分子主体材料一方面可以通过合理地组合多功能基团对分子的光电性质进行有目的的细致调节,另一方面可通过恰当的连接方式来抑制共轭扩展和分子内电子相互作用所导致的激发态能量损失,从而获得光电性质明确可调的高性能主体材料体系。本文对近年来大共轭型电致磷光主体材料的研究进展进行了简单的梳理,对分子的设计思想和材料的构效关系进行了讨论,为今后本领域的研究提供借鉴和参考。
Currently,to obtain high-energy-gap host materials with bipolar carrier injection / transportation abilities,is the central scientific problem to be solved in the organic electrophosphorescent materials.Host materials,reported so far,can be divided into three categories: small molecules,dendrimers and polymers. Dendrimers,the combination of the host and guest,possess many advantages,such as high thermal stabilities,good carrier transporting properties,and solution processible. However,the synthesis is still very difficult,while its device efficiency can be improved due to the intrinsic drawbacks of solution processing approaches. Polymers are simple to functionally extend owing to the easily modified structures.Unfortunately,the monodispersity of polymers is poor,and the distribution of molecular weight affects the reproducibility and stability of the device. Small molecular host materials,although they possess certain chemical structures and simplicity in purification,are difficult to reach multiple functions in one limited conjugated system. By contrast,large-conjugated small molecules,their photoelectric properties can be carefully adjusted on purpose by reasonable combination of multifunctional groups,can inhibit the loss of excitation energy caused by the conjugation extension and intramolecular electron interaction through appropriate linking mode. Therefore,highly efficient host material system with adjustable photoelectric properties is achieved. This brief review gives the research progress of large-conjugated electrophosphorescent host materials in recent years. We discuss prominently the molecular design ideas and the structure-activity relationship of materials,and provide guidance for future research in this field.
Currently,to obtain high-energy-gap host materials with bipolar carrier injection / transportation abilities,is the central scientific problem to be solved in the organic electrophosphorescent materials.Host materials,reported so far,can be divided into three categories: small molecules,dendrimers and polymers. Dendrimers,the combination of the host and guest,possess many advantages,such as high thermal stabilities,good carrier transporting properties,and solution processible. However,the synthesis is still very difficult,while its device efficiency can be improved due to the intrinsic drawbacks of solution processing approaches. Polymers are simple to functionally extend owing to the easily modified structures.Unfortunately,the monodispersity of polymers is poor,and the distribution of molecular weight affects the reproducibility and stability of the device. Small molecular host materials,although they possess certain chemical structures and simplicity in purification,are difficult to reach multiple functions in one limited conjugated system. By contrast,large-conjugated small molecules,their photoelectric properties can be carefully adjusted on purpose by reasonable combination of multifunctional groups,can inhibit the loss of excitation energy caused by the conjugation extension and intramolecular electron interaction through appropriate linking mode. Therefore,highly efficient host material system with adjustable photoelectric properties is achieved. This brief review gives the research progress of large-conjugated electrophosphorescent host materials in recent years. We discuss prominently the molecular design ideas and the structure-activity relationship of materials,and provide guidance for future research in this field.
引文
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[6]BALDO M A,OBRIEN D F,YOU Y,et al.Highly efficient phosphorescent emission from organic electroluminescent devices[J].Nature,1998,39:151-154.
[7]KALINOWSKI J,STAMPOR W,MEZ·YK J,et al.Quenching effects in organic electrophosphorescence[J].Phys Rev B,2002,66:235321.
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[10]KAWAMURA Y,YANAGIDA S,FORREST S R.Energy transfer in polymer electrophosphorescent light emitting devices with single and multiple doped luminescent layers[J].J Appl Phys,2002,92:87-93.
[11]BRUNNER K,DIJKEN A V,BASTIAANSEN J A M,et al.Carbazole compounds as host materials for triplet emitters in organic light-emitting diodes:tuning the HOMO level without influencing the triplet energy in small molecules[J].J Am Chem Soc,2004,126:6035-6042.
[12]DING J Q,ZHANG B H,LU J H,et al.Solution-processable carbazole-based conjugated dendritic hosts for power-efficient blue-electrophosphorescent devices[J].Adv Mater,2009,21:4983-4986.
[13]ZHANG B,TAN G,LAM C S,et al.High-efficiency single emissive layer white organic light-emitting diodes based on solution-processed dendritic host and new orange-emitting iridium complex[J].Adv Mater,2012,24:1873-1877.
[14]WU H,ZHOU G,ZOU J,et al.Efficient polymer white-light-emitting devices for solid-state lighting[J].Adv Mater,2009,21:4181-4184.
[15]KAMTEKAR K T,MONKMAN A P,BRYCE M R.Recent advances in white organic light-emitting materials and devices(WOLEDs)[J].Adv Mater,2010,22:572-582.
[16]ZOU J,WU H,LAM C S,et al.Simultaneous optimization of charge-carrier balance and luminous efficacy in highly efficient white polymer lightemitting devices[J].Adv Mater,2011,23:2976-2980.
[17]LI J,ZHANG T,LIANG Y,et al.Solution-processible carbazole dendrimers as host materials for highly efficient phosphorescent organic light-emitting diodes[J].Adv Funct Mater,2013,23(5):619-628.
[18]YANG W,CHEN Y,JIANG W,et al.A carbazole-based dendritic host material for efficient solution-processed blue phosphorescent OLEDs[J].Dyes Pigments,2013,97(2):286-290.
[19]HUANG H,FU Q,PAN B,et al.Butterfly-shaped tetrasubstituted carbazole derivatives as a new class of hosts for highly efficient solution-processable green phosphorescent organic light-emitting diodes[J].Org Lett,2012,14:4786-4789.
[20]SASABE H,PU Y J,NAKAYAMA K,et al.m-Terphenyl-modified carbazole host material for highly efficient blue and green PHOLEDS[J].Chem Commun,2009,43:6655-6657.
[21]BURKHART R D,CHAKRABORTY D K.Binding energies of triplet excimers in poly(N-vinylcarbazole)solid films from laser-based kinetic spectroscopy between 15 and 55 K[J].J Phys Chem,1990,94(10):4143-4147.
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