The horizontal transition dipole orientation of these materials in 20 wt% doped films are investigated by using the angle-dependent p-polarized PL spectra (Supplementary Fig. ). It is found that CBP-1 has a good Θ// of 81.2%, owing to the presence of spiro SXAC moiety, while CCO-1, CCO-2 and CCO-3 show much superior Θ//s of 91.7%, 93.6% and 90.1%, respectively, validating that the enhancement in molecular planarity contributes significantly to promote horizontal transition dipole orientation of these materials. With the data of Θ//, reflex index and active layer thickness, the ηouts of CCO-1, CCO-2 and CCO-3 are calculated as high as 43.0%, 43.9% and 42.3%, respectively, approaching the theoretical limit of ηout (~45%) , and higher than that of CBP-1 (38.0%). Assuming that exciton utilization and exciton recombination efficiency are 100%, the corresponding theoretical maximum ηexts of CCO-1, CCO-2 and CCO-3 are 42.6%, 43.5% and 41.9%, respectively, in good agreement with device data. Clearly, these excellent ηouts have significantly contributed to boost the EL efficiencies of the devices.