The next factor leading to a spread of data is that the effect of a given triplet population depends on the material. In particular, reported TTA rate constants gTT are widely spread over 8 orders of magnitude (10 -18 -10 -10 cm 3 s -1 ) . So, increasing the figure of merit will reduce triplet population, and is beneficial (increases J90) but the improvement arising from the reduced triplet population depends on the value of gTT. Similar considerations apply to singlet-triplet annihilation, where again there is a range of gST, and the relative importance of STA and TTA depends on the relative values of gTT and gST. As these rate constants are not yet widely measured, we have not at this stage attempted to incorporate them into a figure of merit. However, we show their potential effect in Extended Data Fig. , which shows calculations of how J90 would depend on figure of merit for systems with 𝑘 r S between 10 5 s -1 and 10 10 s -1 , 𝑘 ISC /𝑘 r S between 10 -1 , and 10 3 and 𝑘 r S 𝐾 eq between 10 2 s -1 and 10 8 s -1 for a range of values of gST, gTT and 𝑘 r S .