Recent progress and challenges in molecular design for hyperfluorescent based organic light emitting diodes (OLEDs)

Hyperfluorescence also known as thermally activated delayed fluorescence (TADF) sensitized fluorescence is an innovative approach to achieve high performance organic light emitting diodes (OLEDs). Lighting applications rely on parameters such as external quantum efficiency (EQE) and colour purity. In this context, hyperfluorescence OLEDs (HF-OLEDs) are the leading technology due to its capability to combine the benefits of fluorescent dopants and TADF emitters to obtain saturated colour and long-term operational stability. Hyperfluorescence enabled through Förster resonance energy transfer (FRET) process which takes place between sensitizer and fluorescent emitter. Obtaining highly efficient fluorescent emitter, it is important to reduce the loss mechanisms such as dexter energy transfer (DET), direct charge trapping on terminal emitter by precise optimization and selection of materials. This review gives a broad overview of the development of HF-OLEDs including the device design approaches, and evolution of molecular design of the emitters for the entire range of colors. This review also provides a detailed discussion on various issues and possible solutions related to multi-resonance (MR)-TADF emitters, their molecular designs and inert peripheral substitution. These detailed insights in this review are expected to provide further impetus to the field of HF-OLEDs to enable commercial realization and application of the technology.