Uncovering the origin of efficiency roll-off in TADF OLEDs

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.