Host Material for Multi‐Color Display with High and Stable Electro‐Phosphorescent Efficiencies: Meta‐Linkage for Balanced Hole and Electron Injection

Abstract Novel host materials and their molecular design methods for phosphorescent materials are crucial for the application of phosphorescent organic light emitting diodes (PhOLEDs), which require balanced carrier injection and sufficient triplet energy levels ( E T ). Herein, two host materials, namely PPI22PPPBO and PPI33PPPBO, are designed by varying the linkage of benzoxazole (PBO) and phenanthroimidazole (PPI) groups with appropriate E T for green, yellow, and red phosphors. The meta‐link PPI33PPPBO is not only of smaller π‐conjugation, but also of more ordered face‐to‐face stacking for enhanced and more balanced carrier mobility. As a result, the green, yellow, and red PhOLEDs utilizing PPI33PPPBO as host materials show low turn‐on voltages of 2.8 V. The maximum external quantum efficiency (EQE max ) of the corresponding devices reaches 22.8%, 26.7%, and 17.6%, which is superior to that of the traditional host materials CBP and mCP, showing great application potential. More importantly, when the luminance is 1000 cd m −2 , their EQE can still be as high as 21.9%, 25.5%, and 16.4%, corresponding to negligible efficiency roll‐offs of only 3.9%, 4.5%, and 6.8%. To the best of authors knowledge, it is the first time that PBO is applied to PhOLED host materials using a twisted connection method.