Fundamentals in Organic Light Emitting Diodes

有机发光二极管(OLED)是由夹在透明导电阳极和反射金属阴极之间的一堆有机半导体层组成的。在阳极和阴极之间的外部偏压下，来自阳极的空穴或正电荷和来自阴极的电子被强迫进入有机半导体层以形成激子(紧密结合的空穴-电子对)。激子的湮灭可以辐射地释放它的能量来发光，或者非辐射地释放它的能量来产生热。由于光波导和其他光学效应，一些发射的光可能被捕获在器件内部。从OLED发出的光的百分比(即光耦合系数)与有机分子在这些薄膜中的取向有关。器件的外量子效率由以下三个过程决定：电子-空穴平衡因子、激子和光学耦合。对于电子-空穴平衡因子，研究重点是利用X射线光电子能谱(XPS)和紫外光电子能谱(UPS)来了解电极-有机界面和有机异质结的能垒。对于激子，时间分辨的发光和电致发光将被用来量化几个相互竞争的激子过程：迁移和辐射复合，非辐射复合，以及激子能量从一个发射体分子到另一个发射体分子的转移。对于光学耦合，研究目标将是了解控制客体-宿主系统中发射体分子取向的材料科学。

An organic light-emitting diode (OLED) is made of a stack of organic semiconductor layers sandwiched between a transparent conducting anode and a reflective metal cathode. Under an external electric bias between the anode and the cathode, holes, or positive charges, from the anode and electrons from the cathode are forced into the organic semiconductor layers to form excitons (tightly bound hole-electron pairs). The annihilation of an exciton may release its energy radiatively to emit light or non-radiatively to generate heat. Some of the emitted light may be trapped inside the device due to optical waveguide and other optical effects. The percentage of the emitted light coming out of an OLED (i.e. optical coupling factor) is related to the orientation of organic molecules in these thin-films. The device’s external quantum efficiency is determined by these three processes: electron-hole balance factor, excitonics, and optical coupling. For electron-hole balance factor, the research will focus on understanding energy barriers at electrode-organic interfaces and organic hetero-junctions by using X-ray photoelectron spectroscopy (XPS) and ultra-violet photoelectron spectroscopy (UPS). For excitonics, time-resolved PL and EL will be used to quantify several competing excitonic processes: migration and radiative recombination, non-radiative recombination, and exciton energy transfer from one emitter molecule to another. For optical coupling, the research objective will be understanding of materials science governing emitter molecular orientations in guest-host systems.