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 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 < 7V. The best device performance was exhibited using MPO12 as the host when an appropriate hole blocking layer was implemented. At a brightness of 800 cd/m2 an EQE of 9.09 ± 0.12% at 4.8 V was achieved. The higher performance of MPO12 was attributed to the ambipolar charge transporting character of the polar carbazole derivative. However, exciton relaxation on nonradiative aggregate or excimer states of all host materials studied may limit further improvements in device efficiencies.
- Research Organization:
- Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC05-76RL01830
- OSTI ID:
- 937035
- Report Number(s):
- PNNL-SA-58590; 830403000; TRN: US0806060
- Journal Information:
- Journal of Physical Chemistry C, 112(21):7989-7996, Vol. 112, Issue 21
- Country of Publication:
- United States
- Language:
- English
Similar Records
Designing Organic Phosphine Oxide Host Materials Using Heteroaromatic Building Blocks: Inductive Effects on Electroluminescence
Design strategies for achieving high triplet energy electron transporting host materials for blue electrophosphorescence