A problem in both organic and inorganic LEDs is that as the drive current is increased for more light output, the efficiency decreases 10 . This is known as efficiency roll-off and is illustrated in Fig. , which shows the efficiency as a function of current density for prototypical examples of fluorescent, phosphorescent and TADF OLEDs . The figure shows that the phosphorescent and TADF OLEDs have more than four times the efficiency of the fluorescent OLEDs, but that their efficiency decreases, and particularly severely for the TADF OLEDs as the current density is increased. To compare the behaviour of a wide range of OLEDs of each type, we define J90 as the current density at which the external quantum efficiency (EQE) falls to 90% of its peak value, as illustrated in Fig. .