To combine the benefits from each host, i.e., high EQE in PPT, and large luminance and good color purity in mCP, a co-host strategy was employed with a 1:1 ratio of mCP:PPT, and the same doping level of emitter (device 3). The photophysical properties are shown in Figure , and this device shows almost the same photoluminescence with lPL at 461 nm and FWHM of 38 nm as well as similar decay kinetics with tp of 2.6 ns and td of 60 µs. The FPL of DOBDiKTa in the 1:1 co-host thin film increased to 86%. The corresponding OLED showed an improved EQEmax of 17.4%. Due to the bipolar character of the co-host, the Von was significantly reduced from 4.3 V to 2.9 eV, a value that is similar to the emitted photon energy.