ABSTRACT Thermally activated delayed fluorescent (TADF) OLEDs have attracted widespread interest for display applications in recent years due to their ability to harvest close to 100% of excitons for light emission using materials free from heavy metals. However, TADF OLEDs often suffer from a severe drop in efficiency at high luminance—an effect known as efficiency roll‐off. Up to now, identifying the processes responsible for efficiency roll‐off in TADF OLEDs has been challenging due to the difficulty of accurately measuring the rate constants governing these processes with conventional methods. To overcome this, we develop an in‐operando frequency‐domain technique to understand the efficiency roll‐off of TADF OLEDs based on 1,2,3,5‐tetrakis(carbazol‐9‐yl)‐4,6‐dicyanobenzene (4CzIPN). We find an interplay of singlet‐triplet annihilation (STA), singlet‐polaron annihilation (SPA) and triplet‐polaron annihilation (TPA) are responsible for the efficiency roll‐off in these OLEDs and quantify the contributions of each process. This technique could be applied to understand roll‐off in a wide range of materials and devices – aiding the development of TADF materials and OLEDs.
King et al. (Fri,) studied this question.