基于腈类荧光染料的有机电致发光器件及其机理研究
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摘要
有机电致发光器件（organic light emitting devices, OLEDs）拥有自发光、能耗小、响应快、对比度高、厚度薄、耐高低温、制备工艺简单、材料来源广泛、可实现柔性和大面积显示等突出优点，是目前最具发展潜力的平板显示和固体照明技术。尽管OLEDs的发展已进入商业化阶段，但其居高不下的制造成本，有待于提高的发光效率和稳定性，仍然需要继续发展高性能新材料和进行器件结构优化，并对器件内部的发光过程和能量衰减机理进行深入的分析和研究。针对上述问题，本论文主要围绕两种新型腈类荧光染料，系统研究了基于腈类荧光染料的互补色白光OLEDs、三原色白光OLEDs和高性能的红色磷光敏化荧光OLEDs。并在增强载流子传输平衡、控制色坐标稳定、提高显色指数、降低能量损失、探讨功能材料的本征参数对器件性的影响等方面做了一系列探索性和创新性的工作，主要内容分为以下四方面：
1.采用一种黄色腈类荧光染料(2Z,2’Z)-3,3’-bis(2-phenylacrylonitrile)(1,4-phenylene)（BPhAN），通过旋涂工艺与聚合物poly(N-vinylcarbazole)（PVK）混合作为发光层制备了基于蓝黄互补色的白光OLEDs。三刺激值的理论计算表明PVK和BPhAN的混合发光可以形成标准白光，在实验上进行材料掺杂比例优化，得到了蓝黄发光成分平衡、光谱稳定、Commission Internationale d’Eclairage(CIE)色坐标为（0.32，0.33）互补色白光器件。讨论了不同浓度比例和不同驱动电压对器件光谱和色坐标的影响，发现PVK对BPhAN的不完全能量转移和BPhAN形成的载流子陷阱效应是促进激子复合发光的两个主要因素。另外，针对器件发光效率不理想的问题，设计了基于NPB:PVK复合空穴传输层的载流子传输平衡实验，发现NPB的加入有助于提高空穴迁移率，增大薄膜的表面粗糙度，从而改善载流子注入和传输。与单一空穴传输层的器件相比，复合空穴传输层的器件效率提高了164%。
2.采用一种红色腈类荧光染料3-(dicyanomethylene)-5,5-dimethyl-1-(4-dimethylamino-styryl) cyclohexene（DCDDC），通过真空蒸镀工艺分别制备了掺杂型和超薄层型三原色白光OLEDs。其中，掺杂型白光OLEDs采用双发光层掺杂结构，启亮电压为5V，最大功率效率为1.4lm/W，最大正向观察亮度为12580cd/m2。当外加电压大于9V后，白光显色指数达到85，CIE色坐标接近(0.33,0.33)的标准白光区域。器件光谱随电压变化而移动的原因是由于掺杂发光层无法有效限制载流子复合中心的移动。超薄层型白光OLEDs采用三发光层超薄膜结构，启亮电压为4V，最大功率效率为2.4lm/W，最大正向观察亮度为16690cd/m2。当外加电压为5V时，白光显色指数为80。CIE色坐标为（0.330，0.300），在5-13V电压变化范围内仅有(-0.020,+0.002)的偏移。器件发光性能的改善归结于超薄发光层对载流子复合中心的限制和直接载流子俘获对能量利用率的提高。
3.研究了红色腈类荧光染料DCDDC与磷光敏化剂的能级匹配对磷光敏化荧光OLEDs的影响。将两种铱配合物磷光染料fac tris(2-phenylpyridine)iridium（Ir(ppy)3）和bis[2-(4-tertbutylphenyl)benzothiazolato-N,C2']iridium(acetylacetonate)（(t-bt)2Ir(acac)）作为磷光敏化剂，分别与DCDDC共同掺杂到主体材料4,4’-N,N’-dicarbazole-biphenyl（CBP）中做发光层，在浓度优化的基础上，制备了高性能的红色磷光敏化荧光OLEDs。Ir(ppy)3和(t-bt)2Ir(acac)对应器件的启亮电压均为3.6V，最大正向观察亮度分别为11307cd/m2和15871cd/m2，最大功率效率分别为4.7lm/W和8.5lm/W，比纯荧光器件的效率分别高2倍和3倍。Ir(ppy)3敏化器件的EL光谱的主发光峰位于625nm，有明显的绿光和蓝光成分肩峰，CIE色坐标为(0.54±0.02,0.41±0.02)。(t-bt)2Ir(acac)敏化器件的EL光谱的主发光峰位于635nm，有微弱的黄光成分肩峰，CIE色坐标为（0.62±0.01,0.36±0.01）。相比Ir(ppy)3敏化器件，(t-bt)2Ir(acac)敏化器件的发光效率更高，红光色纯度更好。这是由于(t-bt)2Ir(acac)与DCDDC的能级更匹配，可以有效地限制荧光染料的载流子陷阱效应，减少三重态能量损失，并提高DCDDC在瀑布式能量转移中获得激子能量效率。
4.研究了不同电子传输材料、空穴传输材料和主体材料的三重态能级和其他本征参数对以(t-bt)2Ir(acac):DCDDC共掺系统为发光层的红色磷光敏化荧光器件的影响。完成了基于不同功能材料组合的器件制备和性能表征，发现电子传输材料的三重态能级与器件效率关系不大，但具有高电子迁移率的电子传输材料更有利于平衡载流子传输和提高器件效率；空穴传输材料的三重态能级与器件性能有很强的关联。对磷光染料的三重态激子有限制作用的空穴传输材料可以降低能量转移中的损失，改善器件效率；此外，具有双载流子传输特性的主体材料能在很大程度上提高载流子复合几率。因此，在选择合适功能材料的基础上，磷光敏化荧光器件的性能得到了很大提高。最大电流效率和最大功率效率分别为20.5cd/A和16.5lm/W。最大正向观察亮度为25530cd/m2。EL光谱主发光峰位于635nm，CIE色坐标为（0.61，0.37），为饱和的红光发射。
Organic light emitting devices (OLEDs) have been considered as one of the mostpotential flat-panel display and solid-state lighting due to their significant advantages,e.g. self-emission, low-energy cost, fast response, high contrast, super-thin thickness,temperature enduring, simple production process, widely sources of materials,flexibility, as well as large-scale fabrication. Although OLEDs have stepped to thecommercialization stage, there are some shortcomings to prevent their rapiddevelopment such as high processing cost, low external quantum efficiency and shortstability. Therefore, the basic research on the synthesis of novel materials, optimizationof device architecture and understanding of device mechanism are still playing animportant role to promote the device performance. Aiming at these problems, this paperpresent a systematic works on the complementary-colors white OLEDs, three-primarycolors white OLEDs (WOLEDs) and red phosphor-sensitized-fluorescent OLEDs byusing two novel nitrile fluorescent dyes. The balance of charge carriers trasporting,stability of chromaticity coordinate, enhancement of color rendering index and lowerenergy loss are especially focused on.
1. A yellow nitrile fluorescent dye (2Z,2’Z)-3,3’-bis(2-phenylacrylonitrile)(1,4-phenylene)(BPhAN) as dopant and poly(N-vinylcarbazole)(PVK) as host wereblending together to fabricate complementary-colors white OLEDs via spin-castingprocess. The theoretical calculation of tristimulus values for BPhAN and PVK indicatesthat it is possible to obtain white light by using the two materials. In experimentally,balanced white emission with Commission Internationale d’Eclairage (CIE) coordinatesat (0.32,0.33) was observed. The effects of different doping ratio and driving voltage onthe device performance were discussed. The incomplete energy transfer from PVK toBPhAN and the charge trapping effect of BPhAN are the main factors to promoterecombination. Moreover, to solve the problem of low efficiency, a charge carriertransporting balanced experiment was designed based on a composite hole transportinglayer (CHTL) comprised of NPB and PVK. The results showed that dopingconcentration of NPB not only enhanced the competence of hole transporting ability, but also modified the recombination region of charge as well as the surface morphologyof doped film. The efficiency of the CHTL devices exhibited a power efficiency of164%higher than that of the single HTL devices.
2. A red nitrile fluorescent dye3-(dicyanomethylene)-5,5-dimethyl-1-(4-dimethylamino-styryl) cyclohexene (DCDDC) was employed to fabricate three-primarycolors WOLEDs by using doped structure and ultrathin structure via thermalevaporation process, respectively. The doped WOLEDs of dual emitting layers (EMLs)showed a turn on voltage of5V. The maximum power efficiency and brightness in thefront viewing direction is1.4lm/W and12580cd/m2, respectively. The CIE coordinatesnear (0.33,0.33) with CRI of85were obtained as the bias voltage over9V. This wasattributed to the shift of recombination zone between the EMLs. The ultrathin WOLEDsof three EMLs showed a turn on voltage of4V. The maximum power efficiency andbrightness in the front viewing direction is2.4lm/W and16690cd/m2. Pure whiteemission with a good color rendering index (CRI) of80was achieved as low as5V.The CIE coordinates near (0.33.0.30) shows slight variation of (-0.020,+0.002) from5-13V. The achievement of white emission at low-operation voltages and high-colorstability were attributed to the confined emission zone by the thin EML and enhancedenergy utilization by direct carrier trapping within ultrathin layer.
3. The effect of energy level matched system incorporated of DCDDC and phosphorsensitizers on the performance of red phosphor-sensitized-fluorescent OLEDs werestudied. Two phosphorescent materials fac tris(2-phenylpyridine)iridium (Ir(ppy)3) andbis[2-(4-tertbutylphenyl)benzothiazolato-N,C2']iridium(acetylacetonate)[(t-bt)2Ir(acac)]were used as phosphor sensitizers to dope with DCDDC and CBP as the emitting layer,respectively. By optimizing the doping ratio and layer thickness, two redphosphor-sensitized-fluorescent OLEDs were constructed. The Ir(ppy)3based deviceand (t-bt)2Ir(acac) based device illustrated a turn on voltage of3.6V, a maximumbrightness of11307cd/m2and15871cd/m2, and a power efficiency of4.7lm/W and8.5lm/W, respectively, which were two (or three) times higher than the correspondingfluorescent devices. The electroluminescence (EL) spectra of Ir(ppy)3based deviceshowed a main red emission peaked at625nm, with obvious green and blue emissiveshoulders, resulting in the CIE coordinates located at (0.54±0.02,0.41±0.02). In contrast,the EL spectra of (t-bt)2Ir(acac) based device exhibited a main red emission peaked at 635nm, with a weak yellow emissive shoulder, resulting in the CIE coordinates locatedat (0.62±0.01,0.36±0.01). The improved efficiency and red emission purity wereascribed to a lower energy barrier between (t-bt)2Ir(acac) and DCDDC, whichinherently suppresses the strong charge trapping on DCDDC molecules as well asreduce the energy loss.
4. The effect of triplet energy level and other intrinsic parameters of electrontransporting materials (ETMs), hole transporting materials (HTMs) and host materialson the performance of red fluorescent-phosphor-sensitized OLEDs were investigated.By combining the codoped system of (t-bt)2Ir(acac):DCDDC:CBP as EML and differentfunctional materials as the host or charge transporting layers, the devices werefabricated and analyzed. The results showed that there is no apparent correlationbetween the triplet energy level of ETMs and luminous efficiency. Instead, a highelectron mobility of ETM can effectively balance the charge carriers transporting andenhance the efficiency. Interesting, the device performance was sensitive to the tripletenergy level of HTMs. A HTM with a higher triplet energy level could confine thetriplet excitons and reduce the energy loss. Moreover, a host possessed a bipolar chargetransporting characteristic can greatly enhance the carrier recombination efficiency.Therefore, by selecting the proper functional materials, the performance of redfluorescent-phosphor-sensitized OLEDs is significantly improved. A maximum currentefficiency and power efficiency were20.5cd/A and16.5lm/W, respectively. and amaximum brightness of25530cd/m2at the front viewing direction was achieved. TheEL spectrum showed a saturated red emission peaked at635nm, with CIE coordinatesat (0.61,0.37).
1.采用一种黄色腈类荧光染料(2Z,2’Z)-3,3’-bis(2-phenylacrylonitrile)(1,4-phenylene)（BPhAN），通过旋涂工艺与聚合物poly(N-vinylcarbazole)（PVK）混合作为发光层制备了基于蓝黄互补色的白光OLEDs。三刺激值的理论计算表明PVK和BPhAN的混合发光可以形成标准白光，在实验上进行材料掺杂比例优化，得到了蓝黄发光成分平衡、光谱稳定、Commission Internationale d’Eclairage(CIE)色坐标为（0.32，0.33）互补色白光器件。讨论了不同浓度比例和不同驱动电压对器件光谱和色坐标的影响，发现PVK对BPhAN的不完全能量转移和BPhAN形成的载流子陷阱效应是促进激子复合发光的两个主要因素。另外，针对器件发光效率不理想的问题，设计了基于NPB:PVK复合空穴传输层的载流子传输平衡实验，发现NPB的加入有助于提高空穴迁移率，增大薄膜的表面粗糙度，从而改善载流子注入和传输。与单一空穴传输层的器件相比，复合空穴传输层的器件效率提高了164%。
2.采用一种红色腈类荧光染料3-(dicyanomethylene)-5,5-dimethyl-1-(4-dimethylamino-styryl) cyclohexene（DCDDC），通过真空蒸镀工艺分别制备了掺杂型和超薄层型三原色白光OLEDs。其中，掺杂型白光OLEDs采用双发光层掺杂结构，启亮电压为5V，最大功率效率为1.4lm/W，最大正向观察亮度为12580cd/m2。当外加电压大于9V后，白光显色指数达到85，CIE色坐标接近(0.33,0.33)的标准白光区域。器件光谱随电压变化而移动的原因是由于掺杂发光层无法有效限制载流子复合中心的移动。超薄层型白光OLEDs采用三发光层超薄膜结构，启亮电压为4V，最大功率效率为2.4lm/W，最大正向观察亮度为16690cd/m2。当外加电压为5V时，白光显色指数为80。CIE色坐标为（0.330，0.300），在5-13V电压变化范围内仅有(-0.020,+0.002)的偏移。器件发光性能的改善归结于超薄发光层对载流子复合中心的限制和直接载流子俘获对能量利用率的提高。
3.研究了红色腈类荧光染料DCDDC与磷光敏化剂的能级匹配对磷光敏化荧光OLEDs的影响。将两种铱配合物磷光染料fac tris(2-phenylpyridine)iridium（Ir(ppy)3）和bis[2-(4-tertbutylphenyl)benzothiazolato-N,C2']iridium(acetylacetonate)（(t-bt)2Ir(acac)）作为磷光敏化剂，分别与DCDDC共同掺杂到主体材料4,4’-N,N’-dicarbazole-biphenyl（CBP）中做发光层，在浓度优化的基础上，制备了高性能的红色磷光敏化荧光OLEDs。Ir(ppy)3和(t-bt)2Ir(acac)对应器件的启亮电压均为3.6V，最大正向观察亮度分别为11307cd/m2和15871cd/m2，最大功率效率分别为4.7lm/W和8.5lm/W，比纯荧光器件的效率分别高2倍和3倍。Ir(ppy)3敏化器件的EL光谱的主发光峰位于625nm，有明显的绿光和蓝光成分肩峰，CIE色坐标为(0.54±0.02,0.41±0.02)。(t-bt)2Ir(acac)敏化器件的EL光谱的主发光峰位于635nm，有微弱的黄光成分肩峰，CIE色坐标为（0.62±0.01,0.36±0.01）。相比Ir(ppy)3敏化器件，(t-bt)2Ir(acac)敏化器件的发光效率更高，红光色纯度更好。这是由于(t-bt)2Ir(acac)与DCDDC的能级更匹配，可以有效地限制荧光染料的载流子陷阱效应，减少三重态能量损失，并提高DCDDC在瀑布式能量转移中获得激子能量效率。
4.研究了不同电子传输材料、空穴传输材料和主体材料的三重态能级和其他本征参数对以(t-bt)2Ir(acac):DCDDC共掺系统为发光层的红色磷光敏化荧光器件的影响。完成了基于不同功能材料组合的器件制备和性能表征，发现电子传输材料的三重态能级与器件效率关系不大，但具有高电子迁移率的电子传输材料更有利于平衡载流子传输和提高器件效率；空穴传输材料的三重态能级与器件性能有很强的关联。对磷光染料的三重态激子有限制作用的空穴传输材料可以降低能量转移中的损失，改善器件效率；此外，具有双载流子传输特性的主体材料能在很大程度上提高载流子复合几率。因此，在选择合适功能材料的基础上，磷光敏化荧光器件的性能得到了很大提高。最大电流效率和最大功率效率分别为20.5cd/A和16.5lm/W。最大正向观察亮度为25530cd/m2。EL光谱主发光峰位于635nm，CIE色坐标为（0.61，0.37），为饱和的红光发射。
Organic light emitting devices (OLEDs) have been considered as one of the mostpotential flat-panel display and solid-state lighting due to their significant advantages,e.g. self-emission, low-energy cost, fast response, high contrast, super-thin thickness,temperature enduring, simple production process, widely sources of materials,flexibility, as well as large-scale fabrication. Although OLEDs have stepped to thecommercialization stage, there are some shortcomings to prevent their rapiddevelopment such as high processing cost, low external quantum efficiency and shortstability. Therefore, the basic research on the synthesis of novel materials, optimizationof device architecture and understanding of device mechanism are still playing animportant role to promote the device performance. Aiming at these problems, this paperpresent a systematic works on the complementary-colors white OLEDs, three-primarycolors white OLEDs (WOLEDs) and red phosphor-sensitized-fluorescent OLEDs byusing two novel nitrile fluorescent dyes. The balance of charge carriers trasporting,stability of chromaticity coordinate, enhancement of color rendering index and lowerenergy loss are especially focused on.
1. A yellow nitrile fluorescent dye (2Z,2’Z)-3,3’-bis(2-phenylacrylonitrile)(1,4-phenylene)(BPhAN) as dopant and poly(N-vinylcarbazole)(PVK) as host wereblending together to fabricate complementary-colors white OLEDs via spin-castingprocess. The theoretical calculation of tristimulus values for BPhAN and PVK indicatesthat it is possible to obtain white light by using the two materials. In experimentally,balanced white emission with Commission Internationale d’Eclairage (CIE) coordinatesat (0.32,0.33) was observed. The effects of different doping ratio and driving voltage onthe device performance were discussed. The incomplete energy transfer from PVK toBPhAN and the charge trapping effect of BPhAN are the main factors to promoterecombination. Moreover, to solve the problem of low efficiency, a charge carriertransporting balanced experiment was designed based on a composite hole transportinglayer (CHTL) comprised of NPB and PVK. The results showed that dopingconcentration of NPB not only enhanced the competence of hole transporting ability, but also modified the recombination region of charge as well as the surface morphologyof doped film. The efficiency of the CHTL devices exhibited a power efficiency of164%higher than that of the single HTL devices.
2. A red nitrile fluorescent dye3-(dicyanomethylene)-5,5-dimethyl-1-(4-dimethylamino-styryl) cyclohexene (DCDDC) was employed to fabricate three-primarycolors WOLEDs by using doped structure and ultrathin structure via thermalevaporation process, respectively. The doped WOLEDs of dual emitting layers (EMLs)showed a turn on voltage of5V. The maximum power efficiency and brightness in thefront viewing direction is1.4lm/W and12580cd/m2, respectively. The CIE coordinatesnear (0.33,0.33) with CRI of85were obtained as the bias voltage over9V. This wasattributed to the shift of recombination zone between the EMLs. The ultrathin WOLEDsof three EMLs showed a turn on voltage of4V. The maximum power efficiency andbrightness in the front viewing direction is2.4lm/W and16690cd/m2. Pure whiteemission with a good color rendering index (CRI) of80was achieved as low as5V.The CIE coordinates near (0.33.0.30) shows slight variation of (-0.020,+0.002) from5-13V. The achievement of white emission at low-operation voltages and high-colorstability were attributed to the confined emission zone by the thin EML and enhancedenergy utilization by direct carrier trapping within ultrathin layer.
3. The effect of energy level matched system incorporated of DCDDC and phosphorsensitizers on the performance of red phosphor-sensitized-fluorescent OLEDs werestudied. Two phosphorescent materials fac tris(2-phenylpyridine)iridium (Ir(ppy)3) andbis[2-(4-tertbutylphenyl)benzothiazolato-N,C2']iridium(acetylacetonate)[(t-bt)2Ir(acac)]were used as phosphor sensitizers to dope with DCDDC and CBP as the emitting layer,respectively. By optimizing the doping ratio and layer thickness, two redphosphor-sensitized-fluorescent OLEDs were constructed. The Ir(ppy)3based deviceand (t-bt)2Ir(acac) based device illustrated a turn on voltage of3.6V, a maximumbrightness of11307cd/m2and15871cd/m2, and a power efficiency of4.7lm/W and8.5lm/W, respectively, which were two (or three) times higher than the correspondingfluorescent devices. The electroluminescence (EL) spectra of Ir(ppy)3based deviceshowed a main red emission peaked at625nm, with obvious green and blue emissiveshoulders, resulting in the CIE coordinates located at (0.54±0.02,0.41±0.02). In contrast,the EL spectra of (t-bt)2Ir(acac) based device exhibited a main red emission peaked at 635nm, with a weak yellow emissive shoulder, resulting in the CIE coordinates locatedat (0.62±0.01,0.36±0.01). The improved efficiency and red emission purity wereascribed to a lower energy barrier between (t-bt)2Ir(acac) and DCDDC, whichinherently suppresses the strong charge trapping on DCDDC molecules as well asreduce the energy loss.
4. The effect of triplet energy level and other intrinsic parameters of electrontransporting materials (ETMs), hole transporting materials (HTMs) and host materialson the performance of red fluorescent-phosphor-sensitized OLEDs were investigated.By combining the codoped system of (t-bt)2Ir(acac):DCDDC:CBP as EML and differentfunctional materials as the host or charge transporting layers, the devices werefabricated and analyzed. The results showed that there is no apparent correlationbetween the triplet energy level of ETMs and luminous efficiency. Instead, a highelectron mobility of ETM can effectively balance the charge carriers transporting andenhance the efficiency. Interesting, the device performance was sensitive to the tripletenergy level of HTMs. A HTM with a higher triplet energy level could confine thetriplet excitons and reduce the energy loss. Moreover, a host possessed a bipolar chargetransporting characteristic can greatly enhance the carrier recombination efficiency.Therefore, by selecting the proper functional materials, the performance of redfluorescent-phosphor-sensitized OLEDs is significantly improved. A maximum currentefficiency and power efficiency were20.5cd/A and16.5lm/W, respectively. and amaximum brightness of25530cd/m2at the front viewing direction was achieved. TheEL spectrum showed a saturated red emission peaked at635nm, with CIE coordinatesat (0.61,0.37).
引文
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