Analytical description of the charge-to-photon conversion efficiency for electroluminescent devices based on thermally activated delayed fluorescence

The efficiency of electroluminescent devices based on thermally activated delayed fluorescence (TADF) is a complex interplay of spin-allowed radiative and nonradiative singlet and triplet recombination as well as spin-flip (reverse) intersystem crossing processes. An analytical description of exciton dynamics based on the various singlet and triplet transition rates would strongly facilitate the evaluation of the effects of different processes on device performance. We present unified and analytical expressions for the exciton densities, photoluminescence quantum yield (PLQY), and charge-to-photon (CTP) conversion efficiency in TADF-based electroluminescent devices. The derivation is based on a three-level model that can also be applied to conventional fluorescence- or phosphorescence-based devices. The model allows us to analytically calculate the fundamental kinetic rates in TADF systems as well as the CTP efficiency in devices with only PLQY and transient photoluminescence decay as experimental inputs. Evaluation of the individual kinetic processes reveals that TADF emitters with PLQY as high as around 90% that exhibit pronounced delayed fluorescence, intuitively treated as potential candidates for high-performance electroluminescent devices, can still result in a CTP efficiency of only 50%–60% due to the direct competition between triplet recombination and reverse intersystem crossing.