Inductive Effects of Diphenylphosphoryl Moieties on Carbazole Host Materials: Design Rules for Blue Electrophosphorescent Organic Light-Emitting Devices

Abstract

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 ≈ 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 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/cm2) 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.

Theoretical Study of Dibenzothiophene Based Electron Transport Materials

“…From this work we established that the B3LYP/6-31G* level of theory gave values that closely matched the experimental values for bond lengths, bond angles and orbital energies [18]. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energies were determined from the minimized singlet geometry ( Table 3).…”

A Highly Efficient Universal Bipolar Host for Blue, Green, and Red Phosphorescent OLEDs

“…[2] However, a drawback is that the introduction of an electron-donating and -withdrawing moiety to the host molecule would lead to an intramolecular charge transfer resulting in the reduction of energy gap of the molecule. Several bipolar hosts for sky-blue phosphorescent devices were synthesized including a pyridine/ carbazole hybrid, 2,6-bis(3-(carbazol-9-yl)phenyl)pyridine (26DCzPPy) by Kido et al, [3] a phosphine oxide/carbazole hybrid, N-(4-diphenylphosphoryl phenyl) carbazole (MPO12) by Sapochak and his coworkers and phosphine oxide/phenylamine bipolar materials, [4,5] 2,7-bis(diphenylphosphineoxide)-9-(9-phenylcarbazol-3-yl)-9-phenylfluorene (PCF) and 2,7-bis(diphenylphosphoryl)-9-[4-(N,N-diphenylamino) phenyl]-9-phenylfluorene (POAPF) by Shu and Chou et al [6,7] . Bipolar hosts for green phosphorescent devices were also known.…”

A Novel, Bipolar Polymeric Host for Highly Efficient Blue Electrophosphorescence: a Non‐Conjugated Poly(aryl ether) Containing Triphenylphosphine Oxide Units in the Electron‐Transporting Main Chain and Carbazole Units in Hole‐Transporting Side Chains

“…We propose that FIrpic can transport the carriers and, herein, PVK needs more FIrpic molecules than PCzPO to help balance the hole and electron fl ow in the emissive layer. [ 20 ] In addition, the slightly higher shoulder at 499 nm in the EL spectrum of PCzPO, compared with that of PVK, may give further evidence for the bipolarity of PCzPO. In the hole-transport-only host PVK, the emission zone was close to the cathode, while in PCzPO it was far from the cathode because of the balanced charge-injection/ transport capability.…”