Investigation of substrate effect on molecular orientation of organic thin films by pMAIRS

Abstract

Organic semiconductors generally have asymmetric molecular structures with π-conjugated planes. Since the overlap of the π-orbitals favors carrier transport, it is important to control the orientation of the molecular π-stacking direction in films in order to optimize the performance of organic electronics [1]. It is known that the molecular orientation is affected by the molecular interaction with the substrate and kinetic energy during thin film growth [2]. However, there was no clear quantitative indicator as to whether the molecules formed a vertical or horizontal orientation. Therefore, we proposed a method to estimate an energy barrier for molecules to form vertical orientation in the organic thin film formation process, collective orientation barrier (COB), using p-polarized multiple-angle incidence resolution spectroscopy (pMAIRS) [3]. pMAIRS is a spectroscopic method that can quantitatively evaluate the molecular orientation of organic thin films for each functional group [4]. The COB is estimated by pMAIRS results of temperature-dependence of molecular orientation in organic thin film systems. Pentacene, a typical organic semiconductor, is generally known to be prone to form a standing molecular orientation on the SiO2 surface and a lying molecular orientation on the graphene surface. The pentacene on SiO2 and that on graphene systems were deposited with different growth temperature in a constant deposition rate. The difference in film morphology and temperature-dependent molecular orientation between two systems was experimentally investigated by pMAIRS, atomic force microscopy, and grazing incidence x-ray diffraction. The COB was estimated by the temperature dependence of molecular orientation of the two systems. The results show that the COB of the pentacene/graphene system was about 10 times larger than that of pentacene/SiO2 system, which clearly indicates the degree of formation in the standing orientation is affected by the substrate surface [5]. Our methodology using pMAIRS is promising for understanding the molecular orientation formation of organic thin films.