Appl. Phys. Lett. 102, 101905 (2013).
Masahito Oh-e, Hidenori Ogata, Yoshimasa Fujita, Mitsuhiro Koden.
Materials & Energy Technology Laboratories, Sharp Co., 273-1 Kashiwa, Kashiwa-shi, Chiba-ken 277-0005, Japan
Variable angle spectroscopic ellipsometry and sum-frequency vibrational spectroscopy have been used to study molecular orientations in thin films used in an organic light-emitting-diode. The films consist of sterically bulky and cross-shaped molecules that have small anisotropy in shape, 2-methyl-9,10-di(2-naphthyl)anthracene (MADN). As a result, anisotropic molecular orientation in the amorphous films has been observed with respect to the surface normal. The short axis of anthracene in MADN molecules, more or less, slightly tilts from the surface plane but preferentially close to the surface with a certain orientational distribution, while the long axis of anthracene is, on average, oriented close to the magic angle from the surface normal. This anisotropic molecular orientation gives rise to better carrier transportation properties than the isotropic orientation.
Although the recent findings of anisotropy in the amorphous films, which create a sensation in the community of OLEDs, are significantly important, it is not so surprising to often observe such anisotropy with respect to the surface normal in organic thin films from a different viewpoint of surface science. The thin films can be divided into three regions: air/film interface, bulk, and film/substrate interface. At interfaces, inversion symmetry is always broken, which makes molecules to be oriented in a certain way. On the other hand, the area surrounding a single molecule in the bulk is environmentally isotropic; thereby, molecular orientation in the bulk is isotropic. From the interface to the bulk, there must be a transition layer in which molecular orientation gradually changes. If films are thin, which correspond to transition layers, one would expect a certain molecular orientation in the film. A difficulty, however, in detecting subtle molecular orientations may be due to low sensitivity in linear optics. Molecular shape would amplify molecular orientations; it has recently been reported that linear- or planarshaped molecules are rather horizontally oriented in the amorphous films.
In this article, we study molecular orientations of a uniquely shaped molecule, keeping it in mind whether or not anisotropy can be observed with relatively bulky molecules. We make a difference in a more quantitative analysis over how the molecules sit in a thin film on average, introducing the advanced technique beyond linear optics; sum-frequency vibrational spectroscopy (SFVS) has been used to deduce more detailed molecular orientations. We also make clear how anisotropy accompanies an effect on carrier mobility at the end.
Fig. A schematic of the averaged molecular orientation in a MADN thin layer for OLED devices.