基于有机光伏型连接层的叠层有机电致发光器件

The tandem organic light emitting device (TOLED) can irradiate doubled luminance under the same driving current comparing to conventional devices. So there are no such problems caused by high driving current in TOLEDs. But the introduction of connector between two emissive unites also causes some disadvantages, such as driving high voltage, imbalance charge accumulation in recombination zone, and caused low luminous efficiency. We think outside the box and propose an idea about a new TOLED based on organic photovoltaic-type (OPV) charge generation connector, which can absorbs a portion of photons irradiated from emission zone and form excitons. Such axcitons can disassociate into free charges without any energy barrier and effectively under external electrical field, just like an organic photodetector, which can balance the opposite charges in both recombination zones well, leading to high luminous efficiency and providing enough photons for connector. So such processes are included inside a positive cycle. Normally in OLED only 20% irradiation can be extracted, and remained 80% irradiation is wasted through variety ways. Because the whole positive cycle processes we proposed work inside of the TOLED, the OPV type connector actually utilize the 80% irradiation much more, which should have been wasted, and the negative influences caused by connector absorption can be neglected. So the overall influences of the OPV type connector for the TOLED are helpful.

作为一种能够实现小电流驱动下高亮度发光的有机电致发光器件，叠层器件能够解决高驱动电流带来的诸多负面效应。但是叠层器件同样也面临着一些问题，如连接层电压负载造成的器件功耗增加以及电荷产生效率不足引起的的复合区内电荷不平衡。我们突破当前的设计思路，提出一种基于有机光伏效应的叠层有机电致发光器件设计理念。由于有机电致发光器件结构的特殊性，80%的光辐射会损失掉，因此我们在叠层器件两发光单元之间插入一个基于光伏效应连接层，使其可以吸收部分由器件复合区发出的光子并形成激子，并在外电场的促进下，实现无能量势垒、高效率的电荷产生效果。产生的电荷可以很好的实现其两侧发光单元复合区内的电荷平衡以及高的发光效率，而高效率的发光又为连接层电荷产生提供了光源，因此这是一个良性循环过程。因为整个循环过程都发生在器件内部，利用更多的是80%本该损失掉的光子，因此该光伏型连接层的吸收对器件的效率造成的损失实际很低。

在常规三明治结构的organic light emitting diode (OLED)中，由于表面等离子模式、波导模式等的作用，是OLED内部产生的光子只有20%能够耦合输出到器件外部，导致OLED低的器件效率。鉴于此，本项目设计了一种光伏型电荷生成层(OPV-type CGL：organic photovoltaic–type charge generation layer)，通过OPV-type CGL对限制在器件内部的光子进行吸收，然后将其进一步转换成激子，用于辐射发光，来提高OLED器件的效率。为了验证本项目的可行性，将OPV-type CGL应用到叠层单色光及白光OLEDs和传统单发光单元OLED中，发现三种OLEDs的亮度和效率等性能都有提高。具体地，以Ir(piq)2acac为红光发光材料，制备基于LiF/C60/CuPc/MoO3为光伏型CGL的叠层OLED。调节C60和CuPc的厚度，优化出最佳的CGL结构为LiF(1 nm)/C60(5 nm)/CuPc(5 nm)/MoO3(3 nm)，实现了叠层OLED电流效率相对于单层OLED成倍增长，而电压相对于单层OLED则不足两倍。进一步，制备了基于OPV-type CGL(C60/CuPc)的蓝/红全磷光叠层白光OLED (WOLED)和蓝/黄荧光-磷光叠层WOLED，通过对比基于非光伏型CGL的叠层WOLED和基于C60/CuPc的叠层WOLED的性能，论证了C60/CuPc在叠层WOLED中也可以通过光伏特性提升器件性能。最后，将OPV-type CGL(C60/CuPc)作为缓冲层引入传统单发光单元OLED中，制备了基于OPV-type CGL的红、绿、红单发光单元OLEDs。优化OPV-type CGL的结构和引入到OLED中的位置，发现OPV-type CGL为C60(5 nm)/CuPc(15 nm)时，且被引入到OLED中的ETL和阴极之间，器件的性能得到了有效提高。最后比较OPV-type CGL对红光OLED，绿光OLED和蓝光OLED性能的提升幅度，验证了OPV-type CGL在单发光单元OLED中可以通过吸收光子产生电子，实现器件性能的提升。