Organic light-emitting diodes (OLEDs): Materials, photophysics, and device physics

Currently, organic light-emitting diodes (OLEDs) have reached the stage of commercialization, and there has been an intense drive to use them in various applications from small-and medium-sized mobile devices to illumination equipment and large television screens. In particular, room-temperature phosphorescent materials have become core OLED components as alternatives to conventionally used fluorescent materials because devices made with phosphorescent materials exhibit excellent light-emitting performance with internal electroluminescence efficiencies (η_int) of nearly 100 %. However, phosphorescent materials have several intrinsic problems, such as being limited to metal-organic compounds containing rare metals, for example, Ir, Pt, Au, and Os, and difficulty in realizing stable blue light emission. As a result, researchers have attempted to develop new materials for use as emissive dopants in OLEDs that overcome these limitations. In this chapter, first we briefly review the progress of OLED materials and device architectures mainly based on fluorescent (first-generation) and phosphorescent (secondgeneration) emitters. Then, we discuss third-generation OLEDs that use 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, allowing access to the unlimited freedom of molecular design using carbon-based materials. This has led to the production of ultimate OLEDs that are made of common organic compounds without precious metals and can convert electricity to light at η_int of nearly 100 %.