PNNL

Abstract

We show that the inductive electron-withdrawing effect of diphenylphosphoryl (Ph2P=O) groups stabilizes 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 (ET ~ 3.0 eV) of the host material. Three new carbazole derivatives 3,6-bis(diphenylphosphoryl)-9-ethylcarbazole (PO10), 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 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-(di-fluorophenyl)-pyridinato-N,C2') picolinate (FIrpic) gave external quantum efficiencies (EQE) and operating voltages at a similar current density (J = 13 mA/cm2) of 6 - 8 % at