We have extracted J90 from published data on a wide range of OLEDs together with their EQE at a practical luminance of 1000 cd m -2 . These are plotted for each class of OLEDs and each colour in Fig. . The ideal behaviour would be high J90 (for low efficiency roll-off) and high EQE, i.e., the top right quadrant of the graph. Most fluorescent OLEDs fall in the green rectangle marked A, which is a region of high J90 but low EQE. Most phosphorescent OLEDs (and a few others) fall in the blue rectangle marked B. The upper half of this rectangle represents OLEDs with high efficiency and fairly high J90. TADF OLEDs fall mainly in region C. It is noticeable that there is a much wider spread of both EQE and J90 than for the other classes of OLEDs -possibly because TADF OLEDs is a much younger field. The upper right part of region C shows that there are some reports of TADF OLEDs with high EQE and moderately high J90, although lower than for good phosphorescent devices. However, region C also extends to extremely low values of J90 i.e., there are many TADF devices experiencing significant efficiency roll-off at current densities below 0.1 mA cm -2 . Even for a green device, this would correspond to a brightness of at most 100 cd m -2 , whereas typical displays run at 400 cd m -2 and their individual pixels often run at much higher brightness to achieve an average 400 cd m -2 on the display 13 . Hence, many reported TADF OLEDs have very severe efficiency roll-off and even the best has significant efficiency roll-off (J90 of a few mA cm -2 ). This brings us to the central question of this analysis, which is what can be done in terms of emitter design to reduce efficiency roll-off (i.e., increase J90)? In order to answer this, we first need to consider what causes efficiency roll-off. Studies in phosphorescent OLEDs have shown that triplet-triplet (TTA) and triplet-polaron (TPA) annihilation are the main loss mechanisms as the current density is increased . A similar understanding is developing in TADF OLEDs in which TTA, TPA and singlettriplet (STA) may all contribute . These are all bimolecular processes and thus much more severe at higher excitation densities. Furthermore, as all these processes involve triplets, they can be mitigated by reducing the triplet lifetime and hence reducing the triplet population. This has been achieved very successfully in phosphorescent OLEDs by engineering the light-emitting material (for example by using an iridium complex) to achieve a relatively short triplet lifetime of around 1 µs. For comparison, delayed fluorescence lifetimes in organic TADF materials range from 1 to beyond 500 µs (Extended Data Fig. ). We briefly note that as well as its deleterious effect on efficiency, bimolecular processes involving triplets are also a major mechanism of device degradation, providing a further reason to reduce the triplet population in operating devices .