摘要
作为新一代平板显示技术,有机发光二极管(OLED)又称为有机电致发光(OEL)因为具有主动发光,响应快,可视角度大及制作工艺简单等优点,受到学术和产业界的高度重视。目前阻碍OLED实用化和市场化的关键问题是有些颜色的发光效率较低、工作寿命短、量产难度大。开发高效率与量产性稳定的有机电致发光材料、探索新的器件制备工艺、优化器件结构、提高器件效率和寿命及探索彩色化的最佳方案等,仍然是研究工作的主要目标。本文针对上述问题,从材料合成、纯化、新型器件结构设计到器件优化做了一系列研究,主要内容如下:
合成了典型有机电致发光材料,如电子传输材料Alq与空穴传输材料NPB,通过对反应条件的控制及各项工艺参数的优化,我们得到了高产率的Alq与NPB的粗品材料,其纯度分别达到了98.2%与95.5%。通过三温区同时升温的升华方法,我们对Alq与NPB进行了进一步的升华纯化,得到了有机电子级纯的Alq与NPB产品(纯度高于99.9%)。三温区同时升温升华的方法不但提高了材料的纯度,而且还大大提高了提纯效率,降低了提纯过程中产物的损失。X射线衍射的结果表明,经多次提纯后Alq与NPB材料的晶体粒度增大,材料的性能有所改善。以各种纯度的Alq与NPB分别作为电子传输层与空穴传输层材料制作了发光器件。实验结果表明随着材料纯度的提高,器件的性能也依次提高,这是因为随着材料纯度的提高,材料中杂质或缺陷浓度都会降低,这将减少杂质或缺陷对发光的淬灭与载流子传输的陷阱作用。杂质或缺陷浓度的减少将有利于激子的运动,提高器件中电子和空穴复合的几率,从而获得更高的发光效率。
通过共掺杂方法,我们成功制备了以宽禁带材料ADN为基质的红、绿、蓝三色发光器件,实现了以一种材料为主体的三基色发光。其中蓝光器件的结构为ITO (80nm)/NPB (30nm)/ ADN: DPAVB: TBPE (30nm)/ Alq (30nm)/LiF (1 nm)/Al(100nm),经优化后器件的最大发光亮度达到5626cd/m2,最高发光效率6.2 cd/A ,CIE色坐标为x,y=0.15,0.19;绿色发光器件的结构为ITO (80 nm)/NPB (40 nm)/ ADN: C545T: DMQA (30nm)/ Alq (30nm)/LiF (1nm)/Al(100nm),经优化后器件的最大发光亮度达到15153cd/m2,最高发光效率为10.8 cd/A,CIE色坐标为x,y=0.30,0.62。红色发光器件的结构为ITO (80nm)/NPB (40nm)/ ADN: C6: DCJTB (30nm)/ Alq (30nm)/LiF (1 nm)/Al(100nm),经优化后器件的最大发光亮度达到12847cd/m2,最高发光效率为4.9 cd/A,CIE色坐标为x,y=0.61,0.38;ADN材料具有的双极性载流子传输特征,能够捕获多余的空穴,从而使器件中载流子的注入更加平衡,器件的性能得到提高。同时在一种主体材料中掺入双掺杂客体材料,能使得主体材料与客体材料之间的能量传递更加充分有效,并且不同的客体分子同时掺入到主体材料中也可以减少同种分子间的自淬灭几率,从而在很大程度上抑制掺杂发光分子的浓度淬灭现象,使器件性能得到大幅提高。
从分子设计的角度出发,我们合成了一种新的蓝色非掺杂发光材料TOBP及一种新的红色掺杂材料DADIN。其中蓝光材料TOBP是一种含噁二唑基团的邻菲啰啉衍生物，该材料具有良好的热稳定性，睞Щ湮露任?Tg=142℃,热分解温度Td=325℃,以该材料为发光层制备的蓝色发光器件ITO (80 nm)/NPB (30nm)/ TOBP (30nm)/ Alq (30nm)/LiF (1nm)/Al(100nm)的最大亮度达到4078cd/m2,器件的最高发光效率为2.7cd/A,CIE色坐标为x,y =0.15, 0.10。而红色发光材料DADIN是一种含吡喃腈、具有对称结构的掺杂型电致发光材料,与柯达公司经典掺杂型红色发光材料DCJTB相比,DADIN的合成、提纯工艺简单,产率高,更容易实现规模化制备。以DADIN为掺杂材料制备的红色发光器件ITO (80nm)/NPB (40nm)/ Alq:DADIN (30nm)/ Alq (30nm)/LiF (1nm)/Al(100nm),其发光峰值波长约在650nm处,CIE色坐标为x,y=0.64, 0.34,非常接近于NTSC标准红色,其最高电致发光效率达到2.3 cd/A。相比于以DCJTB掺杂材料制备的器件,以DADIN掺杂制备的OLED器件具有更高的色纯度及发光电流效率。
Organic light-emitting diodes (OLED) have attracted much attention because of their potential applications for multicolor emission with many advantages such as active emission, fast response, wide view-angle and simple fabrication process. As the present time, low power-conversion efficiency, short useful life and bad long-term stability are the critical problems to block the utility and marketization of OLED. However, exploiting organic light-emitting materials with high efficiency and stable physic characteristics, choosing appropriate electrode materials, searching for new film fabrication process, and optimizing device configuration, improving the efficiency and useful life of device, and questing for the best scheme to realize full color are still the primary aims of study work. The investigation of essential mechanism and characterization in organic electroluminescence (EL) field is imperative for fulfilling commercialization requirements. Consequently, this work is dedicated to the systematic study of following issues.
By controlling the reaction conditions and optimizing the process parameters, we get the high purity crude products of electron-transporting material Alq and hole-transporting material NPB, whose purity reached 98.2% and 95.5% respectively. Through the method of elevating the temperature of the three zones, we carry out further purification sublimation and get the higher purity material of Alq and NPB, the purity of which is higher than 99.9%.This method not only increased the purity of the materials, but also greatly enhanced the efficiency of the purification and reduced the product losses during the purification process. The data of XRD showed that with the increase of the purification times, the crystal size of the Alq and NPB material increased and the performance of the material was improved. The OLED devices were made with the various purity of Alq and NPB as electron-transporting and hole-transporting layer respectively. The experiment results showed that with the increase of the material purity, the performance of the devices was improved. With the improvement of the material purity, the concentration of the impurities or defects in the materials was reduced, which would reduce the luminescence quenching effect caused by impurities or defects. And the concentration decrease of impurities or defects will help to the movement of the exciton and increasing the possibility of the recombination of holes and electrons in the devices, resulting in the performance improvement of the devices.
Through the co-doping method, we obtained blue, green and red light-emitting devices based on a wide band gap host material ADN. The structure of the blue device was ITO (80nm)/NPB (30nm)/ ADN: DPAVB: TBPE (30nm)/ Alq (30nm)/ LiF (1 nm)/Al(100nm), the maximum luminance brightness of which reached 5626 cd/m2 and the maximum current luminance efficiency of which got to 6.2 cd/A with CIEx,y=0.15,0.19.The structure of the green device was ITO (80 nm)/NPB (40nm)/ ADN: C545T: DMQA (30nm)/ Alq (30nm)/LiF (1nm)/Al(100nm), the maximum luminance brightness of which reached 15153 cd/m2 and the maximum current luminance efficiency of which got to 10.8 cd/A with CIEx,y=0.30,0.62.The structure of the blue devise was ITO (80 nm)/NPB (40 nm)/ ADN: C6: DCJTB (30 nm)/ Alq (30nm)/LiF (1nm)/Al(100 nm), the maximum luminance brightness of which reached 12847cd/m2 and the maximum current luminance efficiency of which got to 4.9 cd/A with CIEx,y=0.61,0.38.The bipolar character of ADN serves to trap the excess holes generated at high current drive condition thus preventing the imbalance of the carriers injected in the device and improving the performance of the device. In addition, co-dopants of the heterogeneous light-emitting molecules may decrease the possibility of self-quenching from the interaction of the homogenous molecules at the same doping concentration, which will also result in the improvement of the color purity and emission efficiency.
Based on the design of the molecule, we have synthesized a new non-doped blue light-emitting material TOBP and a new red-doped material DADIN. TOBP is one kind of phenanthroline derivatives with oxadiazole group and it has good thermal stability with glass transition temperature Tg = 142℃and the thermal decomposition temperature Td = 325℃. The structure of the blue device with TOBP as the emitting layer is ITO (80nm)/NPB (30nm)/ TOBP (30nm)/ Alq (30nm)/LiF (1 nm)/Al(100nm), the maximum luminance brightness of which reached 4078 cd/m2 and the maximum current luminance efficiency of which got to 2.7 cd/A with CIEx,y=0.15,0.10. DADIN is an electroluminescent material with symmetrical structure. Compared with Kodak classic red dopant material DCJTB, the synthesis and purification process of DADIN is simpler and it is easier to achieve large-scale preparation. The structure of the red device with DADIN as the dopant is ITO (80 nm)/NPB (40nm)/ Alq:DADIN (30 nm)/ Alq (30nm)/LiF (1nm)/Al(100nm), the EL wavelength of which was 650 nm and the maximum current luminance efficiency of which got to 2.3 cd/A with CIEx,y=0.64,0.36 that was very close to the standard red color. Compared to the device with DCJTB as the dopant material, the device with DADIN as the dopant material had showed higher color purity of and higher current luminance efficiency.
引文
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