Kwon Sun-Kap, Baek Ji-Ho, Choi Hyun-Chul, Kim Seong Keun, Lampande Raju, Pode Ramchandra, Kwon Jang Hyuk
Department of Information Display, Kyung Hee University Dongdaemun-gu Seoul 02447 South Korea
LG Display Wollong-myeon/Paju-Si Gyeonggi-do 413-779 South Korea.
RSC Adv. 2019 Dec 23;9(72):42561-42568. doi: 10.1039/c9ra09730a. eCollection 2019 Dec 18.
Organic light-emitting diode (OLED) displays are highly susceptible to the harsh environmental conditions found outdoors, like exposure to direct sunlight as well as UV radiation and storage temperature, resulting in a loss of luminance and lifespan, pixel shrinkage, and permanent damage and/or malfunction of the panel. Here, we fabricated top emission OLEDs (TEOLEDs) using Yb : LiF (1 : 1, 2 nm)/Ag : Mg (10 : 1, 16 nm) and Mg : LiF (1 : 1, 2 nm)/Ag : Mg (10 : 1, 16 nm) cathode units and the performances of the devices were investigated by subjecting them to UV radiation. A fabricated red TEOLED (control device), employing a standard Mg : LiF (1 : 1, 2 nm) electron injection layer (EIL) and an Ag : Mg (16 nm) cathode, showed a rapid decrease in luminance and a fast increase in driving voltage at 10 mA cm over time after UV irradiation for 300 h. However, a cathode unit comprising a Yb : LiF (1 : 1, 2 nm) EIL and an Ag : Mg (10 : 1, 16 nm) cathode showed no loss of luminance or increase in driving voltage at 10 mA cm over time after UV irradiation for 300 h. Therefore, we investigated the changes occurring in both cathode units due to UV irradiation using the lift-out FIB-TEM technique and EDS mapping. With UV irradiation for 300 h, Ag atoms migrated toward the center of the cathode, Mg atoms migrated toward the CPL, and no Mg atoms were observed in the EIL area. In contrast, we observed (i) no substantial migration of Ag atoms and they were located at the center of the cathode, (ii) no migration of Mg atoms toward the CPL layer, and (iii) no movement of Yb atoms after UV irradiation. Furthermore, the UV irradiated red TEOLED with an Mg : LiF (1 : 1, 2 nm) EIL showed (i) deterioration in electron injection into the emissive layer (EML) and an increase in the EIL/metal interface resistance, and (ii) a remarkable shift of the - curve to the higher voltage side, while almost no such changes were observed in the TEOLD with a Yb : LiF (1 : 1, 2 nm) EIL. Also, an almost identical RGB pixel emitting area was noticed in the Yb : LiF (1 : 1, 2 nm) based devices after UV irradiation for 300 h. These results suggest that Yb could become a good candidate for the cathode unit, providing better device stability against harsh environmental conditions as well as excellent electron injection properties.
有机发光二极管（OLED）显示屏极易受到户外恶劣环境条件的影响，如暴露在直射阳光下、紫外线辐射以及储存温度下，这会导致亮度和寿命损失、像素收缩以及面板的永久性损坏和/或故障。在此，我们使用Yb : LiF（1 : 1，2 nm）/Ag : Mg（10 : 1，16 nm）和Mg : LiF（1 : 1，2 nm）/Ag : Mg（10 : 1，16 nm）阴极单元制备了顶部发射OLED（TEOLED），并通过对其进行紫外线辐射来研究器件的性能。制备的红色TEOLED（对照器件）采用标准的Mg : LiF（1 : 1，2 nm）电子注入层（EIL）和Ag : Mg（16 nm）阴极，在紫外线照射300小时后，在10 mA cm下，其亮度迅速下降，驱动电压随时间快速增加。然而，由Yb : LiF（1 : 1，2 nm）EIL和Ag : Mg（10 : 1，16 nm）阴极组成的阴极单元在紫外线照射300小时后，在10 mA cm下，亮度没有损失，驱动电压也没有增加。因此，我们使用剥离式聚焦离子束-透射电子显微镜（FIB-TEM）技术和能谱分析（EDS）映射研究了两个阴极单元在紫外线照射下发生的变化。在紫外线照射300小时后，Ag原子向阴极中心迁移，Mg原子向阴极保护层（CPL）迁移，并且在EIL区域未观察到Mg原子。相比之下，我们观察到：（i）Ag原子没有大量迁移，它们位于阴极中心；（ii）Mg原子没有向CPL层迁移；（iii）紫外线照射后Yb原子没有移动。此外，具有Mg : LiF（1 : 1，2 nm）EIL的紫外线照射红色TEOLED显示出：（i）向发射层（EML）的电子注入恶化以及EIL/金属界面电阻增加；（ii） - 曲线明显向更高电压侧偏移，而在具有Yb : LiF（1 : 1，2 nm）EIL的TEOLD中几乎没有观察到这种变化。同样，在基于Yb : LiF（1 : 1，2 nm）的器件中，在紫外线照射300小时后，注意到几乎相同的RGB像素发光面积。这些结果表明，Yb可能成为阴极单元的良好候选材料，可提供更好的器件稳定性以抵抗恶劣环境条件，并具有优异的电子注入特性。
