Improved Efficiency in Blue Phosphorescent Organic Light-Emitting Devices Using Host Materials of Lower Triplet Energy than the Phosphorescent Blue Emitter

Data from a series of phosphorescent blue organic light‐emitting devices with emissive layers consisting of either 4,4′‐bis(N‐carbazolyl)‐2,2′‐biphenyl (CBP):6% bis[(4,6‐difluorophenyl)pyridinato‐N,C2](picolinato)iridium(III) (FIrpic) or bis(9‐carbazolyl)benzene (mCP):6% FIrpic show that the triplet energy of the hole and electron transport layers can have a larger influence on the external quantum efficiency of an operating device than the triplet energy of the host material. A maximum external quantum efficiency of 14% was obtained for CBP:6% FIrpic devices which is nearly double all other published CBP:6% FIrpic results. A new host material, 4‐(diphenylphosphoryl)‐N,N‐di‐p‐tolylaniline (DHM‐A2), which has a triplet energy lower than that of FIrpic is also reported. Devices fabricated using DHM‐A2 show improved performance (lower drive voltage and higher external quantum efficiency) over devices using 4‐(diphenylphosphoryl)‐N,N‐diphenylaniline (HM‐A1), a high performance ambipolar DHM‐A2 analogue with a triplet energy greater than FIrpic. Nearly 18% external quantum efficiency was obtained for the DHM‐A2:5% FIrpic devices. The results suggest modified design rules for the development of high performance host materials: more focus can be placed on molecular structures that provide good charge transport (ambipolarity for charge balance) and good molecular stability (for long lifetimes) rather than first focusing on the triplet energy of the host material. High external quantum efficiency (EQE) is obtained for low‐triplet‐energy hosts when bis[(4,6‐difluorophenyl)pyridinato‐N,C2](picolinato)iridium(III) (FIrpic) is used as the emitter in spite of the endothermic energy transfer between Flrpic and the host. 14% EQE was obtained for 4,4′‐bis(N‐carbazolyl)‐2,2′‐biphenyl (CBP) 6% FIrpic. A new low‐triplet‐energy host, 4‐ (diphenylphosphoryl)‐N,N‐di‐p‐tolylaniline (DHM‐A2), is introduced. Organic light‐ emitting devices (OLEDs) using a DHM‐ A2:5% FIrpic emissive layer had a maximum of 18% EQE.