Exciton Diffusion Drives Efficiency Roll-off in TADF OLEDs

2 Abstract Despite their potential to deliver high-efficiency organic light-emitting diodes (OLEDs), those using thermally activated delayed fluorescent (TADF) emitters often suffer from a large drop in efficiency at high luminance. This effect is known as efficiency roll-off and is a longstanding issue for these devices, which has limited their commercial deployment. We have discovered that the degree of overlap between the photoluminescence (PL) and absorption spectra drives singlet exciton diffusion and is a strong predictor of the magnitude of efficiency roll-off in TADF OLEDs. This effect is especially important in OLEDs made with multiresonant (MR)-TADF materials given their intense low-energy short-range charge transfer absorption bands. MR-TADF OLEDs are very appealing due to their narrow emission spectra; however, their small Stokes shift leads to large spectral overlap and contributes to these devices showing severe efficiency roll-off. Developing TADF materials that have a larger Stokes shift whilst maintaining their bright, narrow emission would help minimize efficiency roll-off while securing high internal quantum efficiencies, features particularly relevant for displays.