Fundamentals in Organic Light Emitting Diodes

有机发光二极管（OLED）是由一堆有机半导体层夹在透明导电阳极和反射金属阴极之间制成的。在阳极和阴极之间的外部电偏压下，来自阳极的空穴或正电荷和来自阴极的电子被迫进入有机半导体层形成激子（紧密结合的空穴-电子对）。激子的湮灭可以以辐射方式释放能量以发光，也可以以非辐射方式释放能量以产生热量。由于光波导和其他光学效应，一些发射的光可能被困在器件内部。从OLED中发出的光的百分比（即光学耦合因子）与这些薄膜中有机分子的取向有关。器件的外量子效率由电子-空穴平衡因子、激子学和光耦合这三个过程决定。对于电子-空穴平衡因子，将重点研究利用x射线光电子能谱（XPS）和紫外光电子能谱（UPS）了解电极-有机界面和有机异质结的能量势垒。对于激子学，时间分辨PL和EL将用于量化几个相互竞争的激子过程：迁移和辐射重组，非辐射重组，以及激子能量从一个发射器分子转移到另一个发射器分子。对于光学耦合，研究目标将是理解主客系统中控制发射器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. In additional to application as display modules in smart phones and televisions, OLED technology has been identified as the greenest possible technology for energy-efficient lightings: no toxic mercury used in the product, no toxic gases used in device fabrication. According to a recent report released in September, 2016, by the US Department of Energy, a 75% energy saving will be reached by year 2035 if there is a broad societal adoption of energy-efficient lightings. This proposed research program on OLED fundamentals bridging device functions and materials science (energetics and molecular orientation) will lead to innovations in future OLED technology. A successful research outcome is expected to accelerate commercial adoption of OLEDs as a new generation of green and energy-efficient “light bulbs”. Moreover, there are numerous solid-state lighting companies and OLED technology players in Canada. This proposed NSERC Discovery program will provide an exceptional opportunity to train a steady stream of HQPs (eight Ph.D. and ten Master/MEng students) for Canadian companies having business interests in energy, environment, and information and communication technologies.