Templating Highly Crystalline Organic Semiconductors Using Atomic Membranes of Graphene at the Anode/Organic Interface

Abstract

Charge and energy transport in organic semiconductors is highly anisotropic and dependent on crystalline ordering. Here, we demonstrate a novel approach for ordering crystalline organic semiconductors, with orientations optimized for optoelectronics applications, by using a single monolayer of graphene as a molecular template. We show, in particular, that large-area graphene can be integrated on metals and oxides to modify their surface energies and used to template copper phthalocyanine (CuPc), a prototypical organic semiconductor. On unmodified substrates, thermally evaporated films of CuPc are small-grained, and the molecules are in the "standing-up" (100) orientation. On graphene modified substrates, CuPc is templated in favorable "lying-down" (112̅) and (012̅) orientations with drastically larger crystal sizes. This results in an 86% increase in the absorption coefficient at 700 nm and should furthermore result in enhanced energy and charge transport. The use of highly conductive and transparent (>95%) graphene membranes as templates is expected to be a foundation for developing future planar and nanostructured organic light-emitting diodes and organic photovoltaics with improved performance.

Controllable growth of copper-phthalocyanine thin film on rough graphene substrate

“…[27][28][29][30] From these studies, one seemly can draw the same conclusion that the planar molecules prefer to be stacked with face-on geometry on graphene substrates, namely, the planar aromatic ring is parallel to the graphene sheets, due to the favorable p-p type MS interaction. They also observed that the molecules prefer to nucleate at grain boundaries or wrinkle positions of the graphene substrates and then gradually grow into larger islands as the film thickness increases.…”

“…38 This observation is also in good consistent with studies on CuPc film growing on other graphene substrates such as transferred graphene substrates. 29,30 On the RG substrate, however, the morphology of CuPc film differs considerably from that on the SG substrate. Shown in Fig.…”

“…While much attention has been paid to the growth of OTFs on metal and conductive oxide substrates, 20-26 only a few studies were targeted to the growth behavior of OTFs on graphene substrates. [27][28][29] Compared with molecule-substrate (MS) interaction between organic molecules and metal substrates, the MS interaction between the organic molecules, especially planar aromatic molecules such as pentacene and metalphthalocyanines (MPcs) and graphene, is weaker, which usually leads to a weakly epitaxial growth (WEG) for these organic films on graphene substrates. A thorough understanding about such WEG behavior is need in order to achieve controllable growth of organic single crystals on graphene substrates.…”

Towards functionalization of graphene:in situstudy of the nucleation of copper-phtalocyanine on graphene

“…A wide range of applications for graphene has been proposed following the isolation of individual monolayer sheets by Geim in 2005 [1]. These applications include the use of graphene as a transparent electrode material for organic light emitting diodes or organic photovoltaics [2][3][4][5], as a growth modifier for organic crystals [6][7][8], in single molecule sensors [9][10][11], or as a replacement of silicon in future high performance electronics [12]. All of these applications require some means of functionalization of the pristine graphene sheets, either simply for electronic contacting or to alter its properties as needed, e.g., to induce a band gap for the use in a field effect transistor.…”

Magnetic-field-induced enhancement of crystallinity and field-effect mobilities in phthalocyanine thin films

“…5 Several methods for improving the crystallinity of organic thin films have been proposed, such as thermal heating of the substrate during deposition, thermal-or solvent-annealing after thin-film fabrication, and surface treatment of the substrate. [1][2][3][4][6][7][8][9] However, applying external fields during the thin-film fabrication process might make it possible to control both the orientation and crystallinity of thin films with ease over large areas without a time-consuming and complicated fabrication process. [10][11][12][13] In particular, the use of a magnetic field is a powerful technique for controlling the orientation of various kinds of materials such as carbon nanotubes 14,15 and liquid crystalline (LC) mesogens.…”