Method for measuring exciton-exciton annihilation rate constants in thermally activated delayed fluorescent (TADF) emitters to understand efficiency roll-off in OLEDs

Efficiency roll-off at high luminance is a significant issue in organic light-emitting diodes (OLEDs) made using thermally activated delayed fluorescence (TADF) emitters. It is often attributed to singlet–triplet annihilation (STA) or triplet–triplet annihilation (TTA) in the light-emitting layer, but their contributions are not properly quantified because of uncertainty about the magnitude of the rate constants. Here we show that a combination of time-resolved photoluminescence (PL) measurements with variable pulse repetition rate and with background illumination provides a reliable method to study STA and TTA. We find that the STA rate constant in a multi-resonant TADF emitter DABNA-2 dispersed in mCBP increases from about 3 × 10⁻¹¹ to 1 × 10⁻¹⁰ cm³ s⁻¹ with increasing emitter concentration from 3 to 10 wt%. The rate constant for TTA in the film with 10 wt% of DABNA-2 is four orders of magnitude smaller than that for STA and below the sensitivity limit at lower concentration of DABNA-2. Modelling of the OLED efficiency roll-off with experimentally determined parameters shows that STA is responsible for more than half of the efficiency loss at high luminance in the device with this emitter, whilst TTA can slightly reduce the effect of STA. The combination of these measurements and modelling is a powerful tool that can be employed for selecting the optimal host material for a particular emitter and the emitter concentration to minimize efficiency roll-off.