有机电致发光二极管的界面行为及其对器件性能的影响
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摘要
作为一种新型的电致发光器件,有机电致发光二极管(OLED:Organic Light Emitting Device)具有驱动电压低、节能环保、响应速度快以及面光源等特点,能够应用在固态照明、平板显示器以及LCD的背光源。目前,OLED产业化应用还存在一些问题,尤其是OLED的性能和稳定性。利用荧光效率较高的有机发光分子,设计局限激子的多层器件结构及采用金属阴极和ITO阳极的界面修饰等手段,已经被证明是提高器件效率非常有效的方法。本论文首先对发光层内分子-分子间界而分析,提出了改善平衡载流子、最大限度地提高激子产生和复合几率的方法；其次,从有机-有机层界面角度,开发出带有纳米柱界面模式的发光层结构器件,通过扩大激子复合区范围、提高外部模式的光取出提高器件的发光效率；论文最后,讨论了因OLED界面劣化导致OLED失效的机理,提出了有效避免长期驱动情况下的OLED老化途径。
该论文的具体研究内容如下：
首先,调查了两种磷光发光分子掺杂的有机薄膜的空穴-电子的传导特性以及发光特性。本文研究不同掺杂浓度的CBP:Ir(ppy)3薄膜的单载流子特性,发现当Ir(PPY)3的掺杂浓度为10%时,有助于提高CBP:Ir(ppy)3薄膜的空穴迁移率,抑制电子的传输能力,这将有助于构建载流子平衡的高效率发光器件。通过器件结构优化,本文制备低效率滚降(roll-off)的绿光磷光OLED,在100cd/m2的低亮度下器件的最大电流效率为52.5cd/A,而在10000cd/m2的高亮度下器件的电流效率也高于30.0cd/A。另外,本文分别选用两种材料作为磷光发光层和三线态激子的空间阻隔层,分析了空间阻隔层对基于Pt-4磷光OLED发光效率的激子限制作用,结果发现：选择三重态能级较高的空间阻隔层材料和电子传输层材料,能有效抑制发光层中发光分子的三重态能量传递到相邻功能层中,减少了能量传递过程中的损失,最终得到了低效率roll-off的磷光OLED。
其次,通过优化绿色磷光OLED的异质界面结构,对发光层/空穴传输层界面或者阴极/有机层界面进行研究,通过纳米柱修饰来优化界面结构,实现提高器件发光效率的目的。本文以典型的绿色磷光器件结构ITO/NPB/CBP:Ir(ppy)3/TPBi/Alq3/LiF/Al为研究对象,在CBP:Ir(ppy)3发光层两侧制备了具有两种不同纳米柱阵列模式的器件,实验结果发现：纳米柱的导入在有效增加发光层激子复合区的范围的前提下,还可以在一定程度上提高光的取出效率,而纳米柱的高度和个数直接决定着器件的发光效率。
最后,本文研究探讨了OLED在过程中劣化机理,采用斜切方法分析施加电压后OLED的各斜切点的光致退化效果。通过测试每个点的光致发光强度,与未进行加速老化驱动OLED进行对比研究,我们发现:OLED工作过程中,在能级势垒较大的异质界面区域内发生轻微的衰减,而器件内各有机功能层均没有出现明显的衰减。此外,本文也分析热引起的OLED退化以及热退化的特征和发光面积的影响。
The Organic Light-Emitting Diodes (OLEDs) can be widely applied to the solid-state lighting, flat panel displays, as well as the LCD backlight source because its low power-consumption, fast response and surface-emitting characteristics. There are still some problems to be solved for industry applications of OLED, especially for those related to OLED efficiency and lifetime. The uses of light emitting material with high fluorescent efficiency, the multilayer structure controlling exciton recombination and interfacial modification through metal cathode or ITO anode, have been proven to be a very effective method to improve the efficiency of the device. Firstly, molecule-molecule interface in light-emitting layers has been studied. Through optimized device structure and improved carrier balance device, the study aims at maximizing the probability of exciton generation and recombination. Secondly, from the viewpoint of organic layer-organic layer interface, we have developed a device with a light emitting layer structure of the nanopillar, and improved light out-coupling to some degree. Finally, the characteristics and mechanism of device degradation due to interfacial degradation has been proposed to avoid the degradation of device driven under a long operation time.
The main research contents include the following three aspects. By investigating on the carrier transporting ability and the light emitting characteristics of the phosphorescent material doped organic thin film, we have found that10%of the Ir(ppy)3doped hole-only device can improve the conductivity of holes, whereas it inhibits the transport of electrons. For the film of Ir(ppy)3doped CBP with different concentrations, its fluorescence lifetime and PL emission are varied, which is mainly caused by the exciton quenching. According to optimized device structure, we have prepared a low efficiency roll-off green phosphorescent OLED. At low luminance, the maximum current efficiency reached52.5cd/A, and at the high brightness of10000cd/m2the device efficiency is also higher than30.0cd/A. Further, the triplet exciton space barrier layer has been designed for achieving a Pt-4based phosphorescent OLED. We found that the species and thickness of the carrier transporting materials adjacent to light-emitting layer can effectively affect the transfer of the triplet energy to the adjacent function layers; thereby reducing the loss of energy and efficiency roll-off at high luminance. The triplet-triplet annihilation of exciton is different for different phosphorescent emttiers, in which that of Ir complex is heavier than that of Pt complex.
Next, by patterning the light-emitting layer/carrier transporting layer interface with nanopillar for the device,ITO/NPB/CBP:Ir(ppy)3/TPBi/Alq3/LiF/Al, the enhancement in light extraction efficiency of the device has been demonstrated. Nanopillars were prepared on both sides of the light emitting layer of the device having two different nano-pillar array modes with varied number and height. We have found that the use of nanopillar could increase the exciton recombination zone, therefore improve the light extraction efficiency to some extent to. In addition, the improvement in light efficiency is the proportion of the height and spacing of the nanopillars.
Finally, in order to study the impact factor of device deterioration, oblique cutting technology has been used by observing each oblique-cutting point of light-induced degradation. By testing the PL intensity of each point for operated device compared to device without operated, we found that except a slight degradation at the interface device between the light emitting layer and a hole transport layer, the device has no significant degradation. The author also studied the PL decay at70℃, the results showed that in addition to the carrier injection layer/transport layer interface, temperature had no effect on the device degradation. We concluded that the degradation of the device at high temperatures is mainly because the carrier accumulation at the interface with energy barrier, as well as a low glass transition temperature. The effects of heat-induced degradation characteristics and active light-emitting area have been studied too.
该论文的具体研究内容如下：
首先,调查了两种磷光发光分子掺杂的有机薄膜的空穴-电子的传导特性以及发光特性。本文研究不同掺杂浓度的CBP:Ir(ppy)3薄膜的单载流子特性,发现当Ir(PPY)3的掺杂浓度为10%时,有助于提高CBP:Ir(ppy)3薄膜的空穴迁移率,抑制电子的传输能力,这将有助于构建载流子平衡的高效率发光器件。通过器件结构优化,本文制备低效率滚降(roll-off)的绿光磷光OLED,在100cd/m2的低亮度下器件的最大电流效率为52.5cd/A,而在10000cd/m2的高亮度下器件的电流效率也高于30.0cd/A。另外,本文分别选用两种材料作为磷光发光层和三线态激子的空间阻隔层,分析了空间阻隔层对基于Pt-4磷光OLED发光效率的激子限制作用,结果发现：选择三重态能级较高的空间阻隔层材料和电子传输层材料,能有效抑制发光层中发光分子的三重态能量传递到相邻功能层中,减少了能量传递过程中的损失,最终得到了低效率roll-off的磷光OLED。
其次,通过优化绿色磷光OLED的异质界面结构,对发光层/空穴传输层界面或者阴极/有机层界面进行研究,通过纳米柱修饰来优化界面结构,实现提高器件发光效率的目的。本文以典型的绿色磷光器件结构ITO/NPB/CBP:Ir(ppy)3/TPBi/Alq3/LiF/Al为研究对象,在CBP:Ir(ppy)3发光层两侧制备了具有两种不同纳米柱阵列模式的器件,实验结果发现：纳米柱的导入在有效增加发光层激子复合区的范围的前提下,还可以在一定程度上提高光的取出效率,而纳米柱的高度和个数直接决定着器件的发光效率。
最后,本文研究探讨了OLED在过程中劣化机理,采用斜切方法分析施加电压后OLED的各斜切点的光致退化效果。通过测试每个点的光致发光强度,与未进行加速老化驱动OLED进行对比研究,我们发现:OLED工作过程中,在能级势垒较大的异质界面区域内发生轻微的衰减,而器件内各有机功能层均没有出现明显的衰减。此外,本文也分析热引起的OLED退化以及热退化的特征和发光面积的影响。
The Organic Light-Emitting Diodes (OLEDs) can be widely applied to the solid-state lighting, flat panel displays, as well as the LCD backlight source because its low power-consumption, fast response and surface-emitting characteristics. There are still some problems to be solved for industry applications of OLED, especially for those related to OLED efficiency and lifetime. The uses of light emitting material with high fluorescent efficiency, the multilayer structure controlling exciton recombination and interfacial modification through metal cathode or ITO anode, have been proven to be a very effective method to improve the efficiency of the device. Firstly, molecule-molecule interface in light-emitting layers has been studied. Through optimized device structure and improved carrier balance device, the study aims at maximizing the probability of exciton generation and recombination. Secondly, from the viewpoint of organic layer-organic layer interface, we have developed a device with a light emitting layer structure of the nanopillar, and improved light out-coupling to some degree. Finally, the characteristics and mechanism of device degradation due to interfacial degradation has been proposed to avoid the degradation of device driven under a long operation time.
The main research contents include the following three aspects. By investigating on the carrier transporting ability and the light emitting characteristics of the phosphorescent material doped organic thin film, we have found that10%of the Ir(ppy)3doped hole-only device can improve the conductivity of holes, whereas it inhibits the transport of electrons. For the film of Ir(ppy)3doped CBP with different concentrations, its fluorescence lifetime and PL emission are varied, which is mainly caused by the exciton quenching. According to optimized device structure, we have prepared a low efficiency roll-off green phosphorescent OLED. At low luminance, the maximum current efficiency reached52.5cd/A, and at the high brightness of10000cd/m2the device efficiency is also higher than30.0cd/A. Further, the triplet exciton space barrier layer has been designed for achieving a Pt-4based phosphorescent OLED. We found that the species and thickness of the carrier transporting materials adjacent to light-emitting layer can effectively affect the transfer of the triplet energy to the adjacent function layers; thereby reducing the loss of energy and efficiency roll-off at high luminance. The triplet-triplet annihilation of exciton is different for different phosphorescent emttiers, in which that of Ir complex is heavier than that of Pt complex.
Next, by patterning the light-emitting layer/carrier transporting layer interface with nanopillar for the device,ITO/NPB/CBP:Ir(ppy)3/TPBi/Alq3/LiF/Al, the enhancement in light extraction efficiency of the device has been demonstrated. Nanopillars were prepared on both sides of the light emitting layer of the device having two different nano-pillar array modes with varied number and height. We have found that the use of nanopillar could increase the exciton recombination zone, therefore improve the light extraction efficiency to some extent to. In addition, the improvement in light efficiency is the proportion of the height and spacing of the nanopillars.
Finally, in order to study the impact factor of device deterioration, oblique cutting technology has been used by observing each oblique-cutting point of light-induced degradation. By testing the PL intensity of each point for operated device compared to device without operated, we found that except a slight degradation at the interface device between the light emitting layer and a hole transport layer, the device has no significant degradation. The author also studied the PL decay at70℃, the results showed that in addition to the carrier injection layer/transport layer interface, temperature had no effect on the device degradation. We concluded that the degradation of the device at high temperatures is mainly because the carrier accumulation at the interface with energy barrier, as well as a low glass transition temperature. The effects of heat-induced degradation characteristics and active light-emitting area have been studied too.
引文
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