www.advmat.de www.MaterialsViews.com COMMUNICATION 100 © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2011, 23, 100–105 wileyonlinelibrary.com Sangchul Lee, Gunho Jo, Seok-Ju Kang, Gunuk Wang, Minhyeok Choe, Woojin Park, Dong-Yu Kim, Yung Ho Kahng,* and Takhee Lee* Enhanced Charge Injection in Pentacene Field-Effect Transistors with Graphene Electrodes S. Lee, G. Jo, S.-J. Kang, G. Wang, M. Choe, W. Park, Prof. D.-Y. Kim, Dr. Y. H. Kahng, Prof. T. Lee Department of Nanobio Materials and Electronics Department of Materials Science and Engineering Gwangju Institute of Science and Technology Gwangju 500–712, Korea E-mail: yhkahng@gist.ac.kr; tlee@gist.ac.kr Dr. Y. H. Kahng Research Institute for Solar and Sustainable Energies Gwangju Institute of Science and Technology Gwangju 500–712, Korea DOI: 10.1002/adma.201003165 Organic electronic devices have generated a great deal of research interest because of their low-cost fabrication, limitless material variety, and the myriad of potential applications. [1–9] Sig- nificant research efforts have been directed towards improving the performance of devices such as organic field-effect transis- tors (OFETs). [4–9] One important approach for improving this performance is to select contact-electrode materials that yield efficient charge injection into the active channels. [7–9] Con- tact electrodes affect the device performance by determining the carrier type from the energy-level alignment and by gov- erning charge injection, which is related to the quality of the contacting interface. [10,11] Noble metals and doped metal-oxide films have conventionally been used as electrode materials. However, problems with metal-oxide films include their inflex- ibility and high cost. [12] In addition, Au, which is predominantly used as an electrode in OFETs, has shown a significant charge- injection barrier because of unfavorable interface dipole layer formation. [13,14] Therefore, alternative electrode materials must be found to avoid such shortfalls. [7–9,15–17] Recently, graphene-based thin films have attracted great attention as an alternative electrode material for organic elec- tronic devices. [18] Graphene, a two-dimensional conducting sheet, is transparent and conducting and has good mechan- ical stability and flexibility. [19,20] Large samples of multilayer graphene (MLG) films have been synthesized by chemical vapor deposition (CVD) or solution-based methods, rendering them realistic prospects as electrodes. [21–23] To date, MLG films have exhibited a promising performance as electrodes for many organic devices such as organic transistors, [15–17] light-emitting diodes, [24,25] and photovoltaics. [26–29] However, to establish MLG films as viable alternative electrodes for organic devices and to provide fundamental engineering routes for better perform- ances, detailed studies on the interface between the MLG film and organic materials are necessary. Because the interfacial characteristics such as contact resistance, dipole effect, and charge injection barrier height strongly dominate the charge- injection properties, these characteristics need to be analyzed. In this work, we report on the efficient charge injection in pentacene OFETs with graphene electrodes. Pentacene OFETs are an excellent choice to investigate the properties of the interface because they are known to be reliable, exhibit a high mobility, and have been studied for many years as organic sem- iconductor devices. [30,31] We show that the OFET performance was significantly improved by using MLG electrodes; the output and transfer currents and mobility increased in comparison to conventional OFETs with Au electrodes. Detailed investigations on the contact resistance and charge-injection barrier height revealed that such improvements were a result of the superior interfacial contact between the MLG electrode and the penta- cene organic channels. Our findings will aid in establishing graphene-based thin films as an efficient electrode material for improving OFETs and other types of organic electronic devices. The fabrication process of pentacene OFETs with patterned MLG electrodes is illustrated in Figure 1. The graphene syn- thesis is explained in the Experimental section. The grown MLG film was detached from the growth substrate by using a catalyst-etching and scooping-up technique with aqueous iron chloride (FeCl 3 ) solution (around 1 M) as the etchant (Figure 1a), [21,32] and transferred onto a 300-nm-thick SiO 2 layer on a heavily doped Si wafer (Figure 1b). Then, a 50-nm-thick Ni layer was evaporated through a shadow mask onto the MLG film (Figure 1c), and the substrate was exposed to oxygen plasma at 200 mTorr and 50 W to remove the MLG film from the unprotected regions (Figure 1d). The Ni mask patterns were then etched away in FeCl 3 solution (Figure 1e). The patterned graphene electrodes were coated with poly(methylmethacrylate) (PMMA). Subsequently, the PMMA-coated graphene electrodes were detached from the SiO 2 substrate by etching the SiO 2 layer with buffered oxide etchant. Then, the detached graphene elec- trodes were transferred onto an octadecyltrichlorosilane (OTS)- treated substrate. After this, the PMMA was removed by ace- tone. Finally, a pentacene layer (ca. 60 nm thick) was deposited using a thermal evaporator under vacuum through a shadow mask onto the patterned MLG electrodes (Figure 1f). The fab- ricated OFETs had a width of 1000 μm and channel lengths of 50, 100, 200, and 300 μm. Figure 2 shows the typical characteristics of the graphene-elec- trode pentacene OFETs (denoted as GR-pentacene OFETs). Also shown are the characteristics of Au-electrode pentacene OFETs (denoted as Au-pentacene OFETs) for comparison. Figure 2a presents the output characteristics, that is, the drain current versus drain voltage ( I D – V D ) curves at a fixed gate voltage ( V G ) of -50 V for GR-pentacene and Au-pentacene OFETs with