For OLEDs with 3CzBPz, blue emission is achieved with λEL of 464 nm and CIE coordinates of (0.16, 0.22). The EQEmax for the device with 3CzBPz reached 9.6% at 1 cd m -2 and this reduced to 3.1% and 2.0% at 100 cd m -2 and 1000 cd m -2 , respectively. The severe efficiency roll-off of the devices with 4CzPyz and 3CzBPz are ascribed to the relatively larger ΔEST values and thus longer triplet exciton lifetimes, a correlation that has also been noted in previously reported pyrazine-or pyrimidine-based blue emitter (Table ). 12, The EQE is a product of four parameters and can be expressed as: