A critical overview of rate models for the determination of the rate constants associated with thermally activated delayed fluorescence

Thermally activated delayed fluorescence (TADF) is a photophysical phenomenon that involves electronically coupled singlet and triplet excited states. Materials exhibiting TADF have most prominently been employed in organic light-emitting diodes (OLEDs). Electroluminescent devices with TADF emitters are capable of converting up to 100% of the excitons generated to light. The microsecond long delayed lifetimes and the sensitivity of the emission to the environment have been exploited in sensing, imaging, and photocatalysis applications. TADF relies on there being energetically similar singlet and triplet excited states, which enables not only intersystem crossing (ISC) but also the endothermic conversion of triplet excitons to singlet excitons via reverse intersystem crossing (RISC). The coupling of the singlet and triplet excited states leads to a biexponential decay of the emission that is observed in the transient photoluminescence (PL) of the material. It means that although emission is from the singlet, at long time its dynamics are controlled by the triplet population via the RISC process. This review provides an overview of the methods used in the literature to analyze the PL decay of TADF compounds and to infer the rate constants that govern all facets of the TADF process. While the photophysics of TADF is often analyzed using transient PL, most applications of TADF emitters occur in a steady-state regime facilitated by constant exciton generation and recombination. Thus, this review also discusses the link between parameters of the kinetics and the performance of TADF OLEDs.