Diphenylamine/triazine hybrids as bipolar hosts for phosphorescent organic light-emitting diodes†
Bipolar host materials employing diphenylamine (DPA) as a donor unit and triazine (TRZ) as an acceptor unit could achieve suitable energy levels and balanced charge transporting properties, which are necessary for high performance organic light-emitting diodes (OLEDs). In this work, three DPA/TRZ based host materials, named 2-(4-(N,N-diphenylamino)phenyl)-4,6-diphenyl-1,3,5-triazine (DPA–TRZ), 2,4-bis(4-(N,N-diphenylamino)phenyl)-6-phenyl-1,3,5-triazine (DDPA–TRZ), and 2,4,6-tris(4-(N,N-diphenylamino)phenyl)-1,3,5-triazine (TDPA–TRZ), were designed and synthesized. Their thermal, photophysical, electrochemical, and carrier transporting properties were investigated to reveal the relationship between their molecular structure and properties. The number of DPA units could regularly tune the optical and electrical properties of the hybrids. The triplet excited state energy (ET) values of DPA–TRZ, DDPA–TRZ, and TDPA–TRZ are 2.38, 2.51, and 2.73 eV, respectively. To evaluate the electroluminescence properties of the three host materials, blue, green, red, and white phosphorescent OLEDs (PhOLEDs) were fabricated and characterized. With the highest ET value, TDPA–TRZ can afford from blue to red phosphorescent emitters. All these devices exhibited EQEs of over 20% with relatively low efficiency roll-offs. The single-host white PhOLEDs based on TDPA–TRZ showed impressive color stability and superior color rendition. These results demonstrate that the combination of DPA and TRZ is an effective way to construct highly efficient bipolar host materials.