Understanding the intrinsic degradation processes of organic light-emitting diodes is necessary to improve their lifetimes. This intrinsic degradation is typically caused by carrier injection at the interface between the hole transport layer (HTL) and the emissive layer (EML). However, revealing the charge behavior in this local region with a high spatial resolution remains challenging. Thus, this study employed electron holography, a transmission electron microscopy (TEM) technique, to measure the nanometer scale potential distribution inside an OLED composed of N,N′-di-[(1-naphthyl)-N,N′-diphenyl]-(1,1′-biphenyl)-4,4′-diamine (α-NPD) and tris-(8-hydroxyquinoline)aluminum (Alq3) that was degraded via continuous voltage application. The α-NPD and Alq3 functioned as the HTL and EML, respectively. The degraded OLED was found to exhibit several potential distributions, depending on the local positions from which the TEM samples were lifted out of the same bulk sample. The distributions included (i) formation of a potential valley at the α-NPD/Alq3 interface, (ii) disappearance of electric fields within the organic layers, and (iii) similar distribution to original before degradation. We suggest that the degradation was caused by charge accumulation, cationization of Alq3, and local failures. Thus, this study revealed the influence of electric degradation at the nanometer scale because of charge injection to the α-NPD/Alq3 interface. Electron holographic degradation analysis near the HTL/EML interface is expected to aid in the development of design guidelines for preventing device degradation and thus extend device lifetime.

中文翻译：

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通过电子全息术评估有机发光二极管内部电位分布的劣化现象


了解有机发光二极管的本征降解过程对于延长其使用寿命是必要的。这种本征退化通常是由空穴传输层 （HTL） 和发射层 （EML） 之间界面的载流子注入引起的。然而，以高空间分辨率揭示该局部区域中的电荷行为仍然具有挑战性。因此，本研究采用电子全息术，一种透射电子显微镜 （TEM） 技术，测量由 N，N′-di-[（1-萘基）-N，N′-二苯]-（1,1′-联苯）-4,4′-二胺 （α-NPD） 和三-（8-羟基喹啉）铝 （Alq3） 组成的 OLED 内部的纳米级电位分布通过连续电压施加降解。α-NPD 和 Alq3 分别作为 HTL 和 EML 发挥作用。发现降解的 OLED 表现出多种潜在分布，具体取决于从同一块状样品中取出 TEM 样品的局部位置。分布包括 （i） 在 α-NPD/Alq3 界面处形成电位谷，（ii） 有机层内电场消失，以及 （iii） 与降解前的原始分布相似。我们认为降解是由电荷积累、Alq3 的阳离子化和局部故障引起的。因此，本研究揭示了由于电荷注入 α-NPD/Alq3 界面而在纳米尺度上电降解的影响。HTL/EML 接口附近的电子全息退化分析有望有助于制定防止器件退化的设计指南，从而延长器件使用寿命。