Molecular orientation in organic light emitting devices

Abstract

The research on molecular orientation in organic light emitting device(OLEDs) has been working because it is known as one of key factor to OLED efficiency by considering the outcoupling enhancement as well as charge mobility, which are mainly analyzed using angular dependent photoluminescence spectra or X-ray diffraction. However, the two dimensional grazing incidence X-ray diffraction(2D-GIXD) anlysis using scttered X-rays has not been widely used to clarify the molecular orientation in OLEDs. The 2D-GIXD analysis with synchrotron light offers the unique opportunity to study the diffraction from weakly scattering of thin films. In this study, we found that instead of about 1, 6, and 1.8 cd/A for an isotropic emitter(mixed orientation), thus, the OLED efficiency with horizontal emitter orientation can be improved over 5, 11, and 12 cd/A due to increased light outcoupling, as shown in Fig. 1(a), (b), and (c). To clarify the molecular orientations with various parameters in synthesized materials, we analyzed the scattered X-ray intensity in the 2D-GIXD patterns of emitting materials with spectra of scttering in-plane and out-of-plane to the plane and azimuthal angle scan. In summay, we have discussed the relationship between molecular orientation and device efficiency in the synthesized FL, PH, and TADF-based emitting, which can be determined by the substituent group or linker position.