Burning TADF solids reveals their excitons’ mobility

Abstract

Nowadays, Thermally Activated Delayed Fluorescence (TADF) compounds have proven to be attractive for highly efficient optoelectronic devices as they can exploit both singlet and triplet excited states generated by charge recombinations that occur in such devices. However, quenching of excited states limits such benefits at high current densities. We have studied the quenching in the case of cationic copper(I) complexes coordinated by both N-heterolytic carbene (NHC) and 2,2′-dipyridylamine (dpa) ligands. Defects are created in their crystals by a strong laser irradiation. Then we have investigated, by analysing their fluorescence decay as a function of temperature, the quenching mechanism and the exciton mobility. Using a time-resolved version of the Perrin’s quenching model, we show that the quenching volume depends on temperature according to the singlet/triplet equilibrium. At room temperature, the singlet states are responsible for 75% of the quenching although they contribute for 2% to the excited states population. From the analysis of the decay curves, we show that the excitons are not mobile in the crystals and are quenched through Förster Resonant Energy Transfer (FRET).