Recent progress and prospects of inverted singlet-triplet energy gap (INVEST) materials in OLEDs

Abstract

The complete harvesting of singlet and triplet excitons offers a pathway to surpass the spin-statistics limit of conventional fluorescent materials. This opens the door to highly efficient molecular light-emission mechanisms such as thermally activated delayed fluorescence (TADF). This TADF mechanism is aided by low singlet-triplet energy gaps (ΔE_ST), which are usually positive (following Hund's rule). Recent research has explored the violation of Hund's rule by inverting the excited states. Consequently, thermodynamically advantageous down conversion by the negative ΔE_ST eliminates the need for thermal activation and permits more effective reverse intersystem crossing (RISC), which enables solving problems of short lifetime and fast efficiency roll off of organic light emitting diode (OLED) devices. The potential uses of molecules that violate Hund's law have garnered considerable interest in OLEDs, bioimaging, and photocatalysis. Therefore, the success of INVEST materials holds the potential to enable the next generation of OLEDs. This mini-review focuses on significant progress in INVEST materials (molecular design and photophysical properties). Furthermore, the outlook and prospects for future developments in INVEST materials are described.