Organic light emitting diodes (OLEDs) are already commercially applied for smartphone and TV displays. And OLED lighting comes in the focus of interests. Currently in commercial applications, the process of OLED light emission is mainly based on iridium metal complexes that are, however, expensive since the iridium metal is very rare. The Ir(III) center has the advantage of inducing high spin–orbit coupling that is important for harvesting all generated singlet (25%) and triplet (75%) excitons in the lowest triplet state and for transforming the excitation energy with almost 100% internal efficiency into light. This triplet harvesting mechanism was already proposed about 20 years ago. However, a disadvantage of Ir‐based light emitters is not only given by the high costs of Ir, in particular, if mass production for OLED illumination is aspired, but its use is also problematic for blue light emission due to stability problems. As a consequence, an alternative OLED light emitting mechanism has been proposed at the University of Regensburg already in 2006. It allows one to exploit the effect of thermally activated delayed fluorescence (TADF) in OLEDs for harvesting all generated triplet and singlet excitons through emission from the lowest excited singlet state. This leads to the singlet harvesting mechanism. The great advantage of this mechanism is given by the fact that high TADF and device efficiency can be achieved with low‐cost Cu or Ag complexes or purely organic molecules, even for the blue spectral range. Meanwhile, the development of this new technology is characterized by an exceptional interdisciplinary research in the fields of chemistry, physics, and material sciences.

In this volume, leading scientists present comprehensive reviews, which provide insight into TADF properties of organometallic and purely organic emitters, the mechanisms of electroluminescence, the development of new emitter and host materials, and device structures. The different contributions are written in a style that enables researchers from related fields and industrial laboratories as well as graduate students to follow the highly informative presentations. I am convinced that this book demonstrates the attractiveness and the great potential of TADF compounds and triggers further studies towards a better understanding of optoelectronic properties and mechanisms. These studies will not only open larger‐scale application of OLED displays and lighting systems, but will also stimulate future progress in organic electronics. For instance, it is attractive to advance a new OLED generation based on direct singlet harvesting, using emitter molecules with almost zero-engery gap between the lowest excited triplet and singlet state, or to develop electrically pumped OLED lasers.