However, the performance of the devices using AZB-Ph-TRZ varies more significantly depending on the host employed, both in terms of the EL spectrum, as well as the EQEmax and efficiency roll-off. Based on the shapes and positions of the EL spectra, we consider the mCP: AZB-Ph-TRZ device to be emitting from the 1 LE state (as in toluene and zeonex), which frustrates efficient triplet harvesting by RISC, and leads to lower efficiency. Indeed, comparing the maximum efficiency (2.1%) with the film FPL (27%) reveals that triplet harvesting is hardly active at all in these devices, consistent with the very large ΔEST of 0.39 eV in mCP host. Conversely, with DPEPO employed as the host the EL spectrum is red-shifted and broader, indicative of 1 CT emission; this switchover is attributed to the larger polarity/polarizability of the DPEPO host. This difference in exciton character together with reduced ΔEST (0.15 eV) results in more efficient RISC and a higher EQEmax of 5.6% despite a lower film FPL of 20%. The efficiency roll-off is still considerably worse in DPEPO though, a consequence of its well-documented electrical instability.