Two-step surface modification for bottom-contact structured pentacene thin-film transistors Sihan Wang a , Jin-Ho Kim a , Eung-Kyu Park a , Jongsu Oh a , KeeChan Park b , Yong-Sang Kim a, * a School of Electronic and Electrical Engineering, Sungkyunkwan University, Gyeonggi 16419, South Korea b Department of Electronic Engineering, Konkuk University, Seoul 143-701, South Korea article info Article history: Received 1 August 2016 Received in revised form 6 November 2016 Accepted 16 December 2016 Available online 23 December 2016 Keywords: Two-step surface treatment Bottom-contact Organic thin-film transistor abstract We investigated surface treatment effects of hexamethyldisilazane (HMDS), poly(3,4- ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) and L-cysteine on gold source/drain elec- trodes in bottom-contact structured pentacene thin-film transistors (TFTs). The treatment methods include spin coating and immersing. We have also researched on two-step treatment based on the combination of each treatment methods. The highest device performance was achieved by treating gold S/D electrodes with L-cysteine first and PEDOT:PSS afterwards, showing field effect mobility up to 0.35 cm 2 /V$s. L-cysteine can reduce the contact resistance between metal and semiconductor layer, and PEDOT:PSS acted as a hole transporting layer while HMDS decreased the surface energy, which enlarged the grain size of pentacene on it. © 2016 Elsevier B.V. All rights reserved. 1. Introduction There are two common device configurations used in organic thin-film transistors (OTFTs) for possible source and drain (S/D) contacts: top contact (TC) and bottom-contact (BC) structures. In TC structured devices, S/D electrodes are deposited onto the organic semiconducting layer while in BC structured devices, this deposi- tion sequence is reversed. TC structure has revealed superior electrical properties than its BC counterpart [1] due to smaller contact resistance [2,3]. However BC structure has more commer- cial potential than TC configuration as photolithography has to be used in contacts patterning but pentacene cannot be exposed to solvents involved in this process. ‘Recently, orthogonal processing has been developed for patterning pentacene [4] and has great potential to be applied in industry. At elevate temperatures, pentacene is in polycrystalline phase through thermal evaporation and its film quality influences the device performance [5,6]. Larger grain size and less grain bound- aries can help increase the mobility of the device. In BC structure, pentacene consists small grain size on metal S/D electrodes which results in large contact resistance especially in the linear region [7]. If pentacene molecules are perpendicular to the substrate, large grain size will be formed [8]. However, metal has the ability of rearranging its surface energy to accommodate nearby molecules. As a result, pentacene molecules tend to lie on metal film surface [7]. Self-assemble monolayers (SAMs) containing thiol group have been widely studied to enlarge pentacene grain size on metal [7e10]. SAMs of alkanethiol derivatives can form closely packed monolayers on the gold surface, with the alkyl chains extending towards the surface. Repulsion between pentacene molecules and alkanethiol molecules help form larger grains on SAM treated gold surface than on bare surface [8,10]. However, SAM modification methods were reported with only inorganic insulators because devices should be immersed in SAM material dissolved ethanol for several hours and ethanol will damage the organic insulator surface. Aparting from modifying the S/D electrodes, adding a hole in- jection layer is also a common way to improve the performance of OTFTs. For electrode and organic film interface, low energy barrier is preferred for easier charge injection. Molybdenum trioxide (MoO 3 ) [11] and PEDOT:PSS [12] have been studied as hole injection layer in OTFTs. In our work, we focused on the gold surface treatment method in devices with polymer insulator. We introduced hexamethyldi- silazane (HMDS), poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) and L-cysteine as modification material on * Corresponding author. E-mail address: yongsang@skku.edu (Y.-S. Kim). Contents lists available at ScienceDirect Organic Electronics journal homepage: www.elsevier.com/locate/orgel http://dx.doi.org/10.1016/j.orgel.2016.12.040 1566-1199/© 2016 Elsevier B.V. All rights reserved. Organic Electronics 43 (2017) 21e26