Article pubs.acs.org/Langmuir
Volatilize-Controlled Oriented Growth of the Single-Crystal Layer for Organic Field-Effect Transistors Haoyan Zhao, Dong Li, Guifang Dong,* Lian Duan, Xiaohui Liu, and Liduo Wang Key Laboratory of Organic Optoelectronics and Molecular Engineering of Ministry of Education, Department of Chemistry, Tsinghua University, Beijing 100084, People’s Republic of China S Supporting Information *
ABSTRACT: We demonstrate a solution method of volatilize-controlled oriented growth (VOG) to fabricate aligned single crystals of 6,13bis(triisopropylsilylethynyl) pentacene (TIPS pentacene) on a Si/SiO2 substrate. Through controlling the evaporation rate of the solvent, largearea-aligned single-crystal layers can be achieved on several substrates at the same time, covering over 90% on 2 × 1 cm substrates. The method provides a low-cost, maneuverable technology, which has potential to be used in batch production. We find that the atmosphere of the solvent with high dissolving capacity is in favor of aligned single-crystal growth. Besides, the growth mechanism of the VOG method is investigated in this paper. Topcontact organic field-effect transistors based on the single crystals of TIPS pentacene are achieved on a Si/SiO2 substrate. The optimal device exhibits a field-effect mobility of 0.42 cm2 V−1 s−1 and an on/off current ratio of 105. Our research indicates that the VOG method is promising in single-crystal growth on a Si/SiO2 substrate for commercial production.
1. INTRODUCTION Organic field-effect transistors (OFETs) offer several advantages, such as low cost, solution processability, mechanical flexibility, and chemical versatility, over their inorganic counterparts. As a result, OFETs have been the subject of intensive academic and commercial attention as a potential alternative in applications, such as flexible circuits, display backplanes, and sensors. Single-crystalline OFETs have been attracting considerable attention because of their high mobility and environmental stability compared to the amorphous and polycrystalline OFETs. Various solution methods have been reported for aligned films, including external-induced methods, such as photoalignment,1 magnetic alignment,2 and rubbing alignment.3 Besides, droplet-pinned crystallization,4,5 solution shearing,6,7 slot-die coating,8 electrostatic spray deposition,9 and post-annealing method10 are also used to align organic crystals on a restricted substrate. In the previous reports, 6,13bis(triisopropylsilylethynyl) pentacene (TIPS pentacene) is widely adopted because of its solution processability and large π-orbital overlap. As for the solution process, crystal quality and preferred orientation are influenced by many choices, such as the type of solvent, solution concentration, surface wettability of substrate, and deposition method. Taking the solvent type into account, using higher boiling point solvents generally affords OFETs with high field-effect mobilities. In contrast, solvents with low boiling point could result in a crystal film with amorphous-like morphology and a lower degree of crystallinity, leading to the mobility of less than 0.002 cm2 V−1 s−1.11,12 © 2014 American Chemical Society
However, to remove a high boiling point solvent, a high annealing temperature is often required, which is proven to cause cracks in TIPS pentacene crystals.13−15 To address this issue, some groups have attempted to fabricate high-quality TIPS pentacene crystals from a low-boiling-point solvent by confined solution deposition (CSD). However, even the crystals on the same piece of substrate showed different morphology and orientation.14 Therefore, it is necessary to fabricate large-area-aligned single crystals using low-boilingpoint solvents. In the present study, we employed a simple and large-areacompatible deposition technique, volatilize-controlled oriented growth (VOG) and succeeded in preparing TIPS pentacene single crystals on substrates with a low-boiling-point solvent (lower than 90 °C) at room temperature. It should be noted that the TIPS pentacene crystals are highly oriented and the coverage on substrates is over 90%. This method is based on the fact that the evaporation behavior during the drying process plays a vital role in mediating the film quality and, hence, dominating the device performance. By exploitation of a pumping-rate-controllable vacuum system and appropriate atmosphere, the evaporation rate of the solvent can be tuned to realize steady growth of the single crystals on the substrate. The VOG system with common vacuum equipment was set up as illustrated in Figure 1. The Received: April 7, 2014 Revised: September 15, 2014 Published: September 16, 2014 12082
dx.doi.org/10.1021/la503481r | Langmuir 2014, 30, 12082−12088
Langmuir
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Figure 1. Schematic of the VOG system. The left is the zoomed side view of one vial and the substrate. θ is the included angle between the substrate and horizontal direction, which was fixed as 60° in this paper. As the solvent volatilizes, both solvents A and B volatilize and single crystals grow on the substrate. The speed of the solvent volatilization can be adjusted through the valve. Prior to film deposition process, the solutions were heated for 6 h at 50 °C. Then, the silicon wafers were put inside the glass vials, which contained the TIPS pentacene solutions. The included angle of the substrate with the horizontal direction was set at 60°. The vacuum was always maintained at 15 000 Pa during the fabrication of the single crystals. Several hours later, all of the solution in the glass vials had evaporated, leaving aligned crystals on the substrates. 2.4. Characterization. The morphologies of the TIPS pentacene crystals were characterized using optical microscopy (Olympus BX51M). Grazing-incidence X-ray diffraction (GIXRD) measurements in the θ/2θ mode were performed using the synchrotron source at the 1W1A beamline of the Beijing Synchrotron Radiation Facility (BSRF) in China. The electrical properties of the transistors were characterized by measuring their output and transfer curves (Keithley 4200) under ambient conditions. The average field-effect mobility (μ) of the fieldeffect transistor (FET) devices was calculated in the saturation region by plotting the square root of the drain current (Isd) versus the gate voltage (Vg) using the following equation: IDS = (WCi/2L)μ(Vg − Vth)2, where Ci (1.02 × 10−8 F cm−2) is the unit area capacitance of the dielectric layer and W and L are the channel width and length, respectively. 2.5. Fabrication of the OFETs. FETs with top-contact structures were fabricated using heavily doped n-type Si wafers as gate electrodes, with a 300 nm thick thermally oxidized SiO2 layer as the gate dielectric. After the modification of the SiO2 surface, TIPS pentacene single crystals were formed by the VOG method and used as the semiconductors. The source and drain electrodes were Au thin films (50 nm) deposited by a conventional evaporation method with a metal mask. The channel width was 1000 μm, and the channel length was 60 μm.
chamber was connected with a vacuum pump. Between the chamber and the vacuum pump, we added a solvent recovery system to recover the solvent and protect the environment. Several glass vials containing solutions of TIPS pentacene (marked as A) were put in the chamber, and the substrates were set in the glass vials at a certain included angle marked as θ. Several glass vials containing other solvents (marked as B) were also put in the chamber to provide an assistant atmosphere. The speed of the solvent volatilization could be adjusted through a gas valve. With this VOG method, we succeeded in depositing TIPS pentacene single crystals on the Si/SiO2 substrate and fabricated OFETs based on it. The highest field-effect mobility of our optimized device is 0.42 cm2 V−1 s−1 and the on/off current ratio is 105. Our VOG method featured by easy control of single-crystal growth from a low-boiling-point solvent is promising for a variety of semiconductor materials. We believe this method would push forward the development of low-cost, high-performance, organic single-crystal semiconductor devices.
2. EXPERIMENTAL SECTION 2.1. Materials. TIPS pentacene (received from Sigma-Aldrich) was used without further purification. Phenyltrichlorosilane (PTS) and octadecyltrichlorosilane (OTS) were purchased from Sigma-Aldrich and used as received (storage under an argon atmosphere to prevent hydrolysis). Highly doped n-type silicon wafers (
