The Research on Molecular Orientation and Characterization of the OLED Emitters

Abstract

We analyzed the scattered X-ray intensities of the host and host:dopant emitters, which have the spectra of in-plane and out-of-plane scattering to the plane and azimuthal intensity profiles for a plot of orientation distribution using grazing incidence wide-angle X-ray diffraction (GI-WAXD) measurement for high-efficiency organic light-emitting diodes (OLEDs). In blue fluorescent emitters, we found that a non-doped anthracene derivatives have the highly rigid structure and thermal stability because pnaphthylxylene groups in 9,10-positions were highly twisted to anthracene core with steric hindrance of xylene groups. The OLEDs with non-doped emitter with 2,3-diphenyl groups showed high-efficiency of 5.21 cd/A due to enhanced carrier mobility with well-aligned π-stacking structure toward outof- plane by the face-on orientation by GI-WAXD analysis. In phosphorescent emitters, we have used the tetrahedral silicon-based host materials with electron transporting (ET) property and investigated on the preferential host orientation. The ET hosts had good morphological stability as well as balanced charge-carrier transport by pyridyl-substituted linker in the devices. The Ir(ppy)3-doped or Firpic-doped devices showed high-efficiency of 53.54 cd/A or 11.08 cd/A, respectively. In thermally activated delayed fluorescence emitters, we found that the molecular orientation of emitting materials with diphenylamine derivative containing malononitrile had different horizontal and/or random orientations as a function of para or meta linker position. It showed 12.5 times difference of the device efficiency. These improved device efficiency was confirmed by the enhanced out-coupling efficiency with wellaligned stacking structure in the emitters with horizontal molecular orientation to in-plane and out-of-plane analysis using GI-WAXD analysis. A study on the relationship between device characteristics and molecular orientation using GI-WAXD analysis for the emitting materials will contribute to basic research on molecular design for high efficiency OLEDs.