Thermally assisted delayed fluorescence (TADF): fluorescence delayed is fluorescence denied†
High luminescence efficiency is a prerequisite for achieving an electroluminescence (EL) efficiency approaching 100% (photons/electrons). Since the EL process involves the formation of both singlet (S1) and triplet (T1) excitons, this requirement can be further refined to be high luminescence efficiency from both singlet and triplet states, or rapid interconversion between them coupled with high luminescence efficiency from either the singlet or triplet states. A common approach to harvesting S1 and T1 excitons in EL is to use a heavy metal-based emitter, relying on the high spin orbit coupling of a metal ion, such as Ir3+, to promote rapid intersystem crossing and mixing of the S1 and T1 states. An alternative to the use of rare elements like Ir to achieve high EL efficiency involves the use of emitters that harvest S1 and T1 excitons via thermally assisted delayed fluorescence (TADF). In this report, we introduce the routes for efficient radiative decay from S1 and T1 states and describe the TADF process in some depth. The kinetics and efficiency of TADF are analyzed as a function of the energy difference between the S1 and T1 and the rates of emission from each state.