Organic Light-Emitting Diodes (OLEDs): Materials, Photophysics, and Device Physics

Currently, organic light-emitting diodes (OLEDs) have reached the stage of commercialization, and there have been intense efforts to use them in various applications from small- and medium-sized mobile devices to illumination equipment and large TV screens. In particular, room-temperature phosphorescent materials have become core OLED materials as alternatives to conventionally used fluorescent materials because devices made with phosphorescent materials exhibit excellent light-emitting performance with internal electroluminescence efficiency (ηint) of nearly 100%. However, phosphorescent materials have several intrinsic problems, such as their structure being limited to organic metal compounds containing rare metals, for example, Ir, Pt, Au, and Os, and difficulty in realizing stable blue light emission. Therefore, the development of new materials has been anticipated. In this chapter, first, we shortly review the progress of OLED materials and device architectures mainly based on fluorescence (the first generation) and phosphorescence (the second generation) emitters. Then, we mention the third-generation OLED using a new light-emitting mechanism called thermally activated delayed fluorescence (TADF). Recently, highly efficient TADF, which had been difficult to realize with conventional molecular design, has been achieved by very sophisticated molecular structures, indicating unlimited freedom of molecular design in carbon-based materials. This has led to the realization of ultimate OLEDs that are made of common organic compounds without precious metals and can convert electricity to light at nearly ηint = 100%. Further, we mention the recent progress of NIR-OLEDs.