where 𝛾 represents the charge carrier balance of the device, ΦPL is the photoluminescence quantum efficiency of the emitter, 𝜂𝑆/𝑇 accounts for the ratio of excitons that are able to decay radiatively, and 𝜂𝑜𝑢𝑡 is the outcoupling efficiency. Considering the ΦPL of 56% in 3CzBPz doped PPT film and presuming 20% of typical light outcoupling for OLEDs and 100% for hole-electron combination efficiency, the 𝜂𝑆/𝑇 of the 3CzBPz based OLEDs reaches 86%, and the slightly sub-linear relationship between luminance and current density of the device with 3CzBPz (Figure ) rules out triplet-triplet annihilation (TTA) as a dominant exciton harvesting process in the OLED. The device results are consistent with a higher energy triplet state being involved in the exciton harvesting mechanism. The high EQEmax value also makes 3CzBPz amongst the most efficient HLCT emitters (Table ). The device based on 2CzBPz exhibited deep blue emission with λEL of 446 nm and CIE coordinates of (0.15,0.12). The EQEmax for the OLED with 2CzBPz