DOI: 10.1002/adma.200601792 High-Performance Bottom-Contact Organic Thin-Film Transistors with Controlled Molecule-Crystal/Electrode Interface** By Mingsheng Xu,* Masakazu Nakamura,* Masatoshi Sakai, and Kazuhiro Kudo Organic thin-film transistors (OTFTs) are emerging as an inexpensive alternative to amorphous silicon devices because of their many attractive features, such as a simple fabrication process, low cost, and mechanical flexibility. Recently, the per- formance of OTFTs has been significantly improved by modi- fying the dielectric surface and/or source and drain (S/D) elec- trodes, [1] or by using novel dielectric materials. [2–5] However, high-performance OTFTs were mostly achieved in top-con- tact (TC) configurations [6] and not in bottom-contact (BC) configurations. As organic active materials are sensitive to chemical wet and ion-beam processes, shadow-mask evapora- tion is usually used to form S/D electrodes in TC configura- tions. Such a process, however, is incompatible with large- scale integration and does not allow one to produce a channel length shorter than a few tens of micrometers. It is therefore desirable to use a BC configuration, which is compatible with fine lithography processing, for promising applications such as high-resolution flexible displays. The performance of OTFTs is mainly dependent on i) mo- lecular ordering in the organic active layer, ii) charge-injec- tion ability of the source electrode, and iii) charge transport at the interface between the organic active layer and the di- electric layer; the inferior performance of BC-OTFTs com- pared with TC counterparts is believed to mostly derive from factors (i) and (ii). The step formed by S/D electrodes, in par- ticular thick S/D electrodes on a dielectric layer, disturbs the continuous growth of a single-phase domain in the organic ac- tive layer. [7] Additionally, electrode edges with a relatively ir- regular shape may cause disordered growth of organic crystals adjacent to the electrodes. Despite this, not much effort is re- quired to overcome the inherent drawbacks. A simple ap- proach to completely eliminate the electrode-induced effects in BC-OTFTs is to embed the S/D electrodes in the dielectric layer and planarize the S/D electrodes, making the S/D elec- trodes and dielectric layer coplanar. In this Communication, we report, for the first time, planar- ized BC-OTFTs (pBC-OTFTs) with S/D electrodes em- bedded in the dielectric layer (Fig. 1a); this led to a preferen- tial orientation of organic crystals adjacent to the S/D electrodes and a continuous growth of organic crystals across the S/D edges. Our pentacene pBC transistors on SiO 2 /Si showed a superior performance in controlling conventional BC-OTFTs (cBC-OTFTs) (Fig. 1d) and TC-OTFTs, and the mobility was comparable with the best value ever reported for TC-OTFTs made with similar materials. We investigated bottom-gate pBC-OTFTs and cBC-OTFTs built on a SiO 2 /Si platform. In the present study, Pt (ca. 31 nm)/Cr (ca. 2 nm) were used as the S/D electrodes, and pentacene thin films, fabricated by using the molecular- beam-deposition technique, [8,9] were used as the active organic semiconductor. The width and length of our transistors was 5000 and 22.9 lm, respectively. We tested eight devices for each type of transistor and show the properties of the de- vice with the best mobility obtained. As shown in Figure 1b, the current–voltage characteristics of a representative penta- cene pBC transistor showed a strong field-effect modulation of the channel conductance. By fitting the data to the linear- and saturation-regime standard field-effect transistor equa- tions, [7] the linear and saturation mobilities were estimated to be 0.36 cm 2 V –1 s –1 (the bias applied between the drain and the source electrodes V ds = –20 V) and 0.48 cm 2 V –1 s –1 (V ds = –80 V), respectively. In contrast, the linear and sat- uration mobilities of the control cBC transistor (Fig. 1e) are 0.17 cm 2 V –1 s –1 (V ds = –20 V) and 0.30 cm 2 V –1 s –1 (V ds =–80 V), respectively. Thus, the linear mobility of the pBC-OTFT is more than twice that of the cBC-OTFT. Furthermore, the mobility of our pentacene pBC-OTFT is higher than that of our TC counterpart with Au as the S/D electrodes (see Table 1) and comparable with the best values for that of TC-OTFTs fabricated with similar materials, re- ported to date (which is typically around 0.5 cm 2 V –1 s –1 ). [5,10] All our transistors showed a gate leakage current less than a nanoampere. In addition, the magnitude of the on-state drain current of the pBC-OTFT was about twice that of the control transistors (see Table 1). This sort of high-saturation mobility and large current is necessary to drive organic light-emitting diodes. The device with planar S/D electrodes had high mobil- ity, improving the state of the art, at least for BC-OTFTs on SiO 2 /Si without surface modification. By modifying the di- electric surface using a self-assembled monolayer, treating the COMMUNICATION Adv. Mater. 2007, 19, 371–375 © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 371 – [*] Dr. M. S. Xu Venture Business Laboratory, Chiba University 1-33 Yayoi-cho, Inage-ku, Chiba 263-8355 (Japan) E-mail: msxu@restaff.chiba-u.jp Prof. M. Nakamura, Dr. M. Sakai, Prof. K. Kudo Department of Electronics and Mechanical Engineering Chiba University 1-33 Yayoi-cho, Inage-ku, Chiba 263-8355 (Japan) E-mail: nakamura@faculty.chiba-u.jp [**] The work is partially supported by “21 st Century Center of Excel- lence Program: Frontiers of Super-Functionality Organic Devices” at Chiba University. The authorsthank Mr. H. Yamauchi, Mr. H. Tomii, and Mr. N. Ohashi for help with the techniques used.