Exciplexes in OLEDs: Principles and promises

Science has always been full of surprises, and organic light-emitting diodes (OLEDs) are no exception. What exasperates scientists today may entice society tomorrow. The world has witnessed a revolution in display technology in the past two decades. In this period, the displays with bulky and heavy cathode ray tubes were transformed into ultra-slim OLED display panels. Commercialization of any new technology requires a delicate balance between device efficiency and manufacturing cost. OLEDs are still costlier than the popular liquid crystal-based displays (LCDs) because of their higher production cost. However, OLEDs are gradually replacing LCDs. The current OLED-based devices mainly utilize phosphorescent emitters that exhibit some cost and environmental concerns. Therefore, developing OLEDs with only organic components has remained the primary goal in OLED research. The discovery of thermally activated delayed fluorescence (TADF) OLEDs presents a major impetus in this area. Organic TADF molecules can be feasibly realized by manipulating the intramolecular or intermolecular charge transfer between electron-donor and electron-acceptor. The current article discusses the exciplexes formed by intermolecular charge transfer and their versatile applications in OLEDs. In the past OLED research, scientists were often bothered by the red-shifted electroluminescence band along with the desired excitonic emission, which was later identified as the exciplex emission. Exciplexes are virtual emitters that exist only in the electronically excited states. They are formed when electron-rich and -deficient compounds with appropriate frontier molecular orbitals come close enough for orbital participation, one of them being in the excited state. Given the lack of sophisticated characterization technology, exciplex emission was not fully understood back then and was considered a defect in device design. However, some were also inquisitive about the possibilities. The importance of exciplexes in OLEDs was finally recognized in the early 2010s when scientists were elated about the enormous opportunities they could offer. These emitters can be easily generated in a blend of donor and acceptor molecules without the need for complicated syntheses. They also exhibit excellent TADF emission. The researchers were exuberant about the discovery, and a slew of publications have followed the original article published by Adachi and coworkers in 2012. Exciplexes can be applied as emitters and hosts in OLEDs. The efficiency of the relevant devices gradually approaches the phosphorescent OLEDs. In the present article, we have systematically reviewed this success story, reminding the OLED community of the transformation of annoyance into ecstasy, to assist this community in developing high-performing devices based on the data accumulated herewith.