Inductive effects of diphenylphosphoryl moieties on carbazole host materials: Design rules for blue electrophosphorescent organic light-emitting devices

We show that the inductive electron-withdrawing effect of diphenylphosphoryl (Ph2P=O) groups lowers both the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO) of a carbazole chromophore. This improves electron injection from a cathode without affecting the high triplet exciton energy (E-T approximate to 3.0 eV) of the host material. Three new carbazole derivatives, 3,6-bis(diphenylphosphoryl)-9-ethylcarbazole (PO 10), 3,6-bis(diphenylphosphoryl)-9-phenylcarbazole (PO9), and N-(4-diphenylphosphoryl phenyl) carbazole (MPO12), were investigated as host materials in blue phosphor-doped organic light-emitting devices (OLEDs). Photophysical characterization showed all three carbazole derivatives exhibit monomer UV fluorescence (367-385 nm) in solution and contributions from molecular aggregates or excimers in solid-state films (378-395 nm). The polar MPO12 derivative exhibited solvatochromism and had the highest propensity for aggregate formation in the solid state. Testing of OLEDs using PO9, PO10, and MPO12 as host materials for the sky blue organometallic phosphor iridium(III) bis(4,6-(difluorophenyl)-pyridinato-N,C-2') picolinate (FIrpic) gave external quantum efficiencies (EQE) and operating voltages at a similar current density (J = 13 mA/cm(2)) of 6-8% at <7 V. The best device performance was exhibited using MPO12 as the host when an appropriate hole-blocking layer was implemented. The higher performance of MPO12 was attributed to the ambipolar charge-transporting character of the polar carbazole derivative. However, exciton relaxation on nonradiative aggregate states of all host materials studied may limit further improvements in device efficiencies.