The current density -voltage -luminescence (JVL) characteristics of the devices (Error! Reference source not found.c) all showed a low turn-on voltage of 2.9 V for all concentrations of the emitter. An EQEmax of 24.4% was obtained using 2 wt% of TPPO-tBu-DiKTa (Error! Reference source not found.d) with a drop in EQE to 10.0% at 100 cd m -2 (EQE100). The devices showed blue electroluminescence (EL) at lEL of 480 nm (Figure ), which is close to the PL emission in mCP at 476 nm for this emitter (Figure ). However, the FWHM of the EL spectrum was slightly wider (46 nm) than that of the PL spectrum (34 nm), which may be due to the differences in polarity of the co-host system compared to mCP. The Commission International de l'Éclairage, CIE, coordinates (Error! Reference source not found.f) for the 2 wt% TPPO-tBu-DiKTa devices were (0.133, 0.323). With increase in concentration of the emitter to 5 wt% the EQEmax dropped to 18.8% and continued to decrease for 10 and 15 wt% doping of the emitter to 14.6 and 13.7%, respectively. This decrease in efficiency is consistent with the progressively lower FPL observed for TPPO-tBu-DiKTa, which decreased from 85% (for 2 wt% doped films) to 63% (for 15 wt% doped show faster kRISC and shorter τd. The EL of the devices with greater than 10 wt% emitter doping concentration was significantly red-shifted to 541 nm and slightly broadened from a FWHM of 77 nm (2 wt%) to 84 nm (10 wt%). This is in accordance with the shifts in the PL spectra observed for the higher concentrations of TPA-tBu-DiKTa (Figure ) and might be attributed to the emission from intermolecular CT states.