Recent Progress in Organic TADF Emitters Containing Heavy Atoms

Abstract All organic compounds exhibiting thermally activated delayed fluorescence (TADF) are useful materials for applications in organic light‐emitting diodes (OLED), bioimaging, and photocatalysis. In addition to their low cost, organic TADF materials can have high brightness, excellent colour purity, and good stability. Despite this, many emitters still suffer from slow TADF which can reduce the operational lifetime of TADF‐based OLEDs due to biexcitonic processes that lead to decomposition of the emitters. In recent years, the incorporation of heavy elements into organic TADF emitters has emerged as a useful strategy for accelerating both forward and reverse intersystem crossing through the heavy atom effect. This approach can increase device efficiencies and lifetimes, but relies on a detailed, and often subtle, understanding of molecular design. This review summarizes recent advances in the use of the heavy atom effect in organic TADF systems, providing a detailed overview of the photophysics and, where applicable, OLED performance of the studied materials. Design principles, challenges, and opportunities for heavy atom incorporation in TADF systems are also discussed.