Interface-Induced Face-on Orientation of Organic Semiconductors with a Template Layer and Its Application to Vertical-Type Organic Transistors
Authors: Yamada Keitaro , Shinoda Wataru , Nakayama Kenichi 
Published: February 2020

Molecular orientation to the substrate surface is a fundamental problem in organic semiconductor devices due to the anisotropic nature of optoelectronic properties of organic semiconductors. A face-on molecular orientation with the molecular planes parallel to the substrate, is ideal for out-of-plane charge carrier transport for vertical-type organic transistors where charge carriers flow perpendicularly to the substrate. It is usually difficult to induce the face-on molecular orientation on the substrate where intermolecular interactions are stronger than the molecule–substrate surface interactions. Most of the organic semiconductor materials show an edge-on orientation with the molecular planes vertical to the substrate, which is unsuitable for out-of-plane charge carrier transport. Template layer techniques have been proven to enhance the molecule–substrate surface interactions by inserting template materials between the substrate and organic layer. One of the template materials, perylene 3,4,8,9-tetracarboxylic dianhydride (PTCDA) exceptionally show the face-on orientation on the substrate and act as the template layer due to the π-π interactions between PTCDA and the upper side molecules. It has been known that PTCDA is effective for orientation control of phthalocyanine and its derivatives. However, it is still not clear about templating effect for other materials and vertical carrier transport property of organic semiconductors oriented with PTCDA. In this study, we investigated molecular orientation of various π-conjugated crystalline molecules on PTCDA, and the mechanism of orientation control from the crystal structure view point. We also measured the vertical carrier mobility of the oriented molecules and the performance of vertical-type organic transistors, and revealed the effect of orientation control for vertical carrier transport.
Dimethyl-3,4,8,9-perylenedicarboximide (MePTC), copper-phthalocyanine (CuPc), dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT), pentacene, and diindenoperylene (DIP) were deposited on glass substrates by thermal vacuum evaporation after deposition of PTCDA. X-ray diffraction (XRD) measurements were carried out in the out-of-plane configuration to evaluate molecular orientation. In these organic semiconductor molecules, MePTC and CuPc on PTCDA gave a diffraction peak at 27.7° with d-spacing of 3.33 Å corresponding to the distance of π-π stacking, which indicates the face-on orientation of the molecules. DNTT, pentacene, and DIP had no diffraction peaks showing the face-on orientation. The difference of molecular orientation results from molecular packing in the crystal structure. Vertical carrier mobility of MePTC was measured by the space charge limited current (SCLC) technique. The electron mobility improved from 1.3×10-4 cm2/Vs to 4.3×10-4 cm2/Vs due to the face-on orientation control. The MePTC film oriented with PTCDA was applied to the metal base organic transisitor (MBOT) which is one of the vertical-type organic transistors, resulting in higher output current compared to the device without PTCDA.