showed lower EQEmax values of 22.0% and 22.6%, and significantly more severe efficiency roll-off, with EQE1000 of 9.6% and 9.0%, respectively. The high EQEmax of tCzBT1B and tCzBT2B is in part ascribed to their high ΦPL (over 85%) and improved light out-coupling efficiency originated from their strongly horizontally oriented transition dipole moment (TDM) in PhCzBCz film. As shown in Figure , the 4 wt% films of tCzBT1B and tCzBT2B in PhCzBCz exhibit anisotropy factors, α, of 0.12 and 0.13, corresponding to 88% and 87% horizontally oriented TDM, respectively. Thus, the theoretical EQEmax of the devices are 35.2% and 34.8%, comparable to the experimental EQEmax of 34.9% and 34.3%, respectively, assuming that in the devices the charge balance (γ) is unity and the efficiency of producing radiative excitons (ηr) is equal to their ΦPL. However, the α values for tDPABT1B and tDPABT2B are 0.28 and 0.26, and the corresponding theoretical EQEmax are 22.3% and 23.5%, respectively, which also are close the experimental EQEmax of 22.0% and 22.6%.