Determination of the Kinetic Rates and Intrinsic Nonradiative Losses in Organic Thermally Activated Delayed Fluorescence Emitters

Knowledge of kinetic rates of intersystem crossing (ISC) k_ISC, reverse ISC k_rISC, singlet k_S, and triplet k_T relaxation processes, respectively, is indispensable for material design as well as understanding of the operation of organic light-emitting diodes (OLEDs) based on organic emitters with thermally activated delayed fluorescence (TADF). Typically, the various rate constants are obtained by numerically fitting of the photoluminescence (PL) decay, but it is overlooked whether or not the solution of such a fit is unique. Using an analytical model, it is demonstrated that for typical TADF emitters only the sum of k_S and k_ISC, and the sum of k_T and k_rISC can be uniquely obtained from the transient PL decay and PL quantum yield (PLQY) experiments. Depending whether nonradiative losses stem from singlets or triplets, various combinations of k_T and k_rISC, k_S and k_ISC can be used to obtain the same transient PL decay, while keeping the PLQY constant. The only exception is the case of unity PLQY that allows for a direct determination of all relevant kinetic rates from just a single transient PL measurement. It is further shown that additional PL measurements in oxygen atmosphere with complete triplet quenching are redundant, since the results can be explicitly predicted from PL measurements in inert atmosphere. Using the TADF emitter CzDBA with a PLQY as high as 90% as a model system, it is demonstrated that by a combination of PL decay, PLQY, and OLED device modeling it is possible to quantify all the individual kinetic rates. For CzDBA neat film with PLQY as high as 90%, the majority of nonradiative losses stems from triplets, showing it is vital to conduct comprehensive studies on the molecular origin of nonradiative losses in the future.