Although there is a good correlation between J90 and the proposed figures of merit there is a significant spread of data points in Fig. . We consider that two main factors contribute to this spread. The first is that the figure of merit describes the best that could be achieved with a particular light-emitting material in a device limited by the triplet population. In real devices many factors, especially imperfect charge balance could lead to worse performance than this ideal, and hence can explain the spread of the data in figure 4. In addition, at low current density some devices show efficiency increasing with current density, as can be seen for the devices in Fig . As J90 is taken as a reduction from peak efficiency, this will lead to higher values of J90 than in devices with peak efficiency at very low current. This effect is exemplified by comparing two 2CzPN devices in Extended Data Fig. 6,39 .