The EQEmax (PEmax) were 8.5% (2.06 lm W -1 ), 11.2% (3.51 lm W -1 ) and 10.2% (5.19 lm W -1 ) for devices with cD of 1.5 wt%, 3.0 wt% to 6.0 wt%, respectively. The high EQE (11.2%) of the device at this deep blue chromaticity (CIEy < 0.08) is among the highest recorded to date (Figure ). The device data are summarized in Table . A linear dependence of luminescence to current density (Figure ) and the presence of delayed component in the transient PL analysis provide strong indications that triplet harvesting in the device occurs through TADF. Despite the high EQEmax, the efficiency roll-off in each system is severe, which is likely due in part to the large DEST of the emitter, the long delayed lifetimes that result in increased TTA and STA processes, and the instability of the phosphine oxide-based host .