Article pubs.acs.org/cm
Thiol−ene Cross-Linked Polymer Gate Dielectrics for Low-Voltage Organic Thin-Film Transistors Chao Wang,† Wen-Ya Lee,† Reina Nakajima,† Jianguo Mei, Do Hwan Kim,‡ and Zhenan Bao* Department of Chemical Engineering, Stanford University, Stanford, California 94305, United States S Supporting Information *
ABSTRACT: We report a low-temperature processed, hydroxyl-free poly(vinyl phenyl) (PVP) dielectric layer cross-linked using thiol−ene chemistry. This new dielectric material exhibited a high dielectric constant as compared to conventional hydroxyl-free polymer dielectrics, e.g. polystyrene, and allowed for cross-linking at 80 °C, which is lower than the glass transition temperature of most commonly used plastic substrates, e.g. poly(ethylene terathalate) (PET). Due to the absence of hydroxyl groups, the dielectric layer displayed more stable performance than other previously reported cross-linked PVP dielectrics. The lowtemperature processing, high air stability, and low current−voltage hysteresis while retaining high device performances are important advantages of this new dielectric material. KEYWORDS: organic thin-film transistors, polymer gate dielectrics, thiol−ene chemistry
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where W and L are channel width and length, respectively, Vth the threshold voltage, and Ci the capacitance per unit area of the dielectric layer. For identical device’s geometry and semiconducting material, the increase in Ci allows obtaining the equivalent output current in low-voltage operation. Ci can be expressed by
rganic thin-film transistors (OTFTs) have attracted increasing attention over the past two decades due to their numerous advantages such as lightweight, inexpensive large area processing, and compatibility with flexible plastic substrates.1,2 While extensive research has been focused on semiconductive materials to understand and improve their charge transport properties,3,4 limited attention has been devoted to the gate dielectric material. We believed that developing dielectric components possessing desirable properties, e.g. stable performances, high throughput, and lowtemperature solution-processing, will greatly benefit the realization of commercial electronics driven by organic transistors.5 For OTFTs, low-voltage transistor operation is of particular interest for the development of portable electronics due to its low power consumption, e.g. chemical detection in aqueous media.6 To achieve low-power applications, it is critical to maintain a high output drain current (IDS), even in low voltage operation. The magnitude of IDS is related to the applied gate voltage (Vg) and the capacitance of the dielectric layers used in most OTFTs. In general, the saturation IDS is defined by the follow equation:2 IDS = W /2LμCi(Vg − Vth) © 2013 American Chemical Society
Ci = kε0 /d
(2)
where ε0 is the vacuum permittivity, k is the dielectric constant, and d is the thickness of the insulator layer. Reducing the thickness is an efficient approach to obtain high current for lowvoltage operation. However, thin dielectric films usually lead to high leakage current due to poor film uniformity and increased number of pin holes, causing low on−off current ratios. The use of ultrathin self-assembled nanodielectrics, such as selfassembled monolayer7,8 and multiple layers,9 is an alternative way to enhance output current. However, obtaining highquality film and full coverage of the self-assembly organic layers requires precise processing controls and complex multiple processes. Increased capacitance of gate dielectricsis another Received: September 30, 2013 Revised: November 19, 2013 Published: November 19, 2013
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successfully fabricated a low-temperature processing, hydroxylfree PVP dielectric layer. This new dielectric layer exhibited a high dielectric constant as compared to conventional hydroxylfree polymer dielectrics, e.g. polystyrene, and allowed for crosslinking at 80 °C, which is similar to, or even lower than, the glass transition temperature of most conventional plastic substrates, e.g. poly(ethylene terathalate) (PET) and polyimide. Due to the absence of hydroxyl groups, the dielectric layer displayed more stable performance than any other previously reported PVP−HDA dielectrics.
path to efficiently reduce driving voltage of OTFTs. The Frisbie group has demonstrated ion-gel dielectrics for low-voltagedriven OTFTs.10,11 Ion-gels consisting of a mobile ions within a polymer electrolyte provide ultra high capacitance exceeding 10 μF cm−2. However, slow polarization and high leakage current are still major challenges for the ion-gel gate dielectrics. Insulating polymers have been considered as a promising material for gate dielectrics due to their solution processability. However, the operation of OTFTs made from polymer dielectrics generally require high voltages (>60 V), mainly attributed to the resulting thick film (>300 nm) needed to reduce pin-holes and in obtaining low dielectric constant. Recently, a reduction in OTFT operating voltages has been accomplished by incorporating known insulator materials or developing new systems based on cross-linkable polymers.12 For the latter, various cross-linked polymer systems have been used for organic thin-film transistors, including polyimides,13 glass resins,14 poly(methyl methacrylate) (PMMA),15 poly(vinyl alcohol), 1 6 divinyltetramethyldisiloxane-bis(benzocyclobutene) (BCB),17,18 polystyrene (PS),19 and photoalignment layers.20 On the other hand, among the various polymer dielectrics, poly(4-vinylphenol) (PVP) based cross-linking systems are especially attractive due to their high dielectric constant (∼4). Klauk et al. showed the first example of PVP-based dielectrics by mixing PVP with a cross-linking agent, poly(melamine-coformaldehyde) (PMF), spin-coating the mixture from solution, and then curing at 180 °C.21 Yoon et al. have also demonstrated low-voltage OTFTs based on PVP films crosslinked with trichlorosilanes.22 Furthermore, we have also reported low-voltage OTFT operation by incorporating ultrathin films of poly(4-vinylphenol) cross-linked with 4,4′(hexafluoroisopropylidene)diphthalic anhydride (HDA).23−26 These PVP-based dielectrics were observed to have good dielectric properties, and the hydroxyl groups of PVP can be modified by self-assembly monolayers (SAMs) to improve molecular packing of organic semiconductors. A high hole mobility of 1.2 cm2 V−1 s−1 was obtained on a SAM-modified PVP dielectrics when using pentacene as the semiconductor.22 Despite the observed high performances, PVP-based polymer dielectrics have some key limitations. One major limitation is its high curing temperature. The cross-linking of PVP is mostly achieved via the formation of ester groups between the crosslinker and the hydroxyl groups on the polymer backbones. This reaction requires a high temperature process (usually >100 °C), which may distort the plastic substrate. The other major limitation is the high density of moisture-sensitive hydroxyl groups, since the unreacted free hydroxyl groups are remained in the cross-linked PVP. The hydroxyl groups in the polymer are hydrophilic and can thus form hydrogen bonds with water molecules. Therefore, PVP dielectrics are highly sensitive to moisture in the air. The swelling effect of the PVP dielectrics is even more severe when the devices are used in humid conditions or even under water. Besides, devices with PVP dielectrics usually show both large hysteresis and large leakage current, since the hydroxyl groups may act as electron traps. Therefore, developing PVP-based polymer dielectrics that are (i) devoid of hydroxyl groups and (ii) cross-linked at low temperature is important for flexible polymer electronics. Recently, thiol−ene chemistry has emerged as a versatile and efficient tool for cross-linking reaction, polymer functionalization, and surface modification.27−29 Herein, by introducing thiol−ene chemistry as a cross-linking approach, we have
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EXPERIMENTAL SECTION
Materials. All materials were purchased from Sigma Aldrich and used without further purification, unless otherwise noted. Propylene glycol monomethyl ether acetate (PGMEA) was purchased from Alfa Aesar. Poly(tetrathienoacene-diketopyrrolopyrrole) (PTDPPTFT4) was provided by Corning Incorporated,30 and the TIPS-pentacene was provided by 3M. Synthesis of PVP−Alkylene. Poly-4-vinylphenol (7.5 g, 50 mmol, MW = 25 000) was dissolved in a mixture solvent of acetone and toluene (60 mL, V:V = 2:1), to which was added potassium carbonate (41.5 g, 300 mmol) and a phase transfer catalyst tetrabutylammonium bromide (750 mg), followed by addition of allyl bromide (26 mL, 300 mmol). The resulting mixture was refluxed for 10 h. Upon cooling to room temperature, the solvent was evaporated. The precipitates were then suspended into chloroform and passed through a pad of Celite and silica gel and washed with chloroform (3 × 30 mL). The collected chloroform solution was concentrated and precipitated into methanol. The white precipitates were collected and dried under vacuum at 60 °C to give 9.1 g PVP−alkylene (96%, MW ∼ 33 000, dielectric constant ∼ 4). H NMR (δ, CHCl3): 7.81−6.24 (m, 4H), 5.38−5.50 (m, 1H), 5.36−5.20 (m, 1H), 4.62−4.38 (bs, 2H), 2.20−1.62 (m, 1H), 1.60−1.18 (m, 2H). Dielectric Film Preparation. PVP−alkylene, pentaerythritol tetra(3-mercapto propionate) (4T), and azobisisobutyronitrile (AIBN) were dissolved in PGMEA with concentrations of 40, 7.5, 40 mg mL−1, respectively. The solutions were left for stirring overnight to obtain homogeneous solutions. The PVP-alkylene solution (2 mL) and AIBN solution (160 μL) were mixed together with 4T at various ratios (molar ratios between alkene groups and thiol groups are 1:1 (R = 1), 2:1 (R = 2), 4:1 (R = 4), and 8:1 (R = 8)) and vigorously stirred for 3 min before being applied for spin-coating. Highly doped silicon wafers were cut into small pieces (2.5 cm × 2.5 cm). The wafers were first cleaned with compressed air and washed with toluene, acetone, and isopropanol, respectively. The cleaned Si wafers were then treated with UV-ozone for 20 min before spincoating with dielectric polymers. Spin-coating: 0.5 mL of the mixed solution was filtered through a 0.2 μm syringe filter and dropped onto a piece of Si wafer. After 20 s, it was spin-coated at a spin rate of 7000 rpm for 1 min. The as-prepared wafers were put on a hot-plate and cured at 80 °C in air for at least 3 h. The second layer of dielectrics was prepared with a similar procedure. The thickness of dielectrics, as measured by a profilometer, was determined to be around 40−50 nm. Dielectric Film of PVP−HDA Preparation. PVP−HDA dielectrics were prepared according to previous reports23 by mixing 1 mL of poly-4-vinylphenol solution (40 mg/mL), 1 mL of HDA solution (4 mg/mL), and 1 μL of triethylamine as a cross-linking catalyst. Two layers were deposited and cured at 110 °C for crosslinking at least 3 h. The thickness of obtained PVP−HDA double layers is about 45 nm. Device Fabrication. A top-contact bottom-gate structure was used for our OTFTs. The polymer dielectrics were deposited on highly doped silicon as described above. Four different organic semiconductors were tested as a charge transport layer for OTFTs, respectively. Pentacene, 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene), and PTDPPTFT4 were used as p-channel materials, while hexadecafluorocopper phthalocyanine (F16CuPC) 4807
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Figure 1. Chemical structures of the crosslinked PVP dielectric polymer based on thiol-ene chemistry. was employed as an n-channel material. Pentacene and F-CuPc films were deposited by thermal evaporation (Angstrom Engineering) at a rate of around 0.3 and 0.05−0.13 Å s−1 to thickness of 40 and 30 nm, respectively, under a pressure of 5.0 × 10−7 Torr. The substrate temperatures (Tsub) for pentacene and F-CuPc were controlled to be 60 and 105 °C respectively by heating a copper block during deposition. PTDPPTFT4 was spin-coated at 1000 rpm for 60 s from a 1,2,4-trichlorobenzene solution and then thermally annealed at 190 °C for an hour. TIPS-pentacene in toluene was deposited by our previously reported solution-shearing method with a shearing speed of 2.8 mm s−1 and substrate temperature at 90 °C. The gold electrodes for source and drain electrodes were thermally evaporated at 0.5 Å s−1 to be 40 nm on a rotating substrate. Electrode dimensions were defined by a shadow mask with a channel width (W) of 1 mm and channel length (L) of 50 μm. The electrical measurements of OTFTs were carried out under ambient conditions or in an inert atmosphere by using a Keithley 4200-SCS semiconductor parameter analyzer (Keithley Instruments, Cleveland, OH). Characterization. Tapping-mode AFM images of the films was recorded using a Multimode Nanoscope III with Extender electronics (Digital Instruments/Veeco Metrology Group, Santa Barbara, CA). The capacitance of the polymer dielectrics was measured with Agilent E4980A Precision LCR Meter for frequencies ranging from 20 Hz to 10 kHz in ambient condition. Water stability was evaluated by capacitance changes. A sandwich electrode structure with gold electrodes (thickness 40 nm, gold area 0.0225 cm2) is soaked in deionized water in corresponding time, and then water residue is removed from the surface using an air gun prior to measuring capacitance. Contact angles were measured by using a goniometer (First Ten Angstroms FTA200) equipped with a CCD camera.
film was subsequently cured at 80 °C to obtain our desired PVP−4T dielectrics. To investigate the optimized ratio between PVP−alkene polymer and 4T cross-linkers, we prepared a series of PVP−4T films of similar film thicknesses (at 40 nm) but with different molar ratios. We define R as the molar ratios between alkene groups and thiol groups. The morphologies of the films with different R values are shown in Figure 2. When R = 1 or 2, the
RESULTS AND DISCUSSION Polymer Dielectrics PVP−4T. The design of our new dielectric layer was shown in Figure 1. The precursors of the dielectric layer were consisted of three components: PVP− alkene polymer, multithiol cross-linker, and radical initiator. PVP−alkene polymer was synthesized by converting the hydroxyl groups of PVP into alkene groups. NMR results showed that nearly all the hydroxyl groups on PVP were reacted. We used pentaerythritol tetra(3-mercaptopropionate) (4T), a commercially available nonvolatile tetra-thiol as the cross-linker as it provides four reactive sites and can potentially result in a more densely cross-linked network, which is desirable for low leakage. AIBN, which can generate radicals when heated to over 70 °C, was chosen as radical initiator. These three components were dissolved and mixed in PGMEA and spin-coated onto a Si wafer into a thin film. The yielded
Figure 2. AFM images of the morphologies of the PVP−4T dielectrics with molar ratios of (a) 1:1, (b) 2:1, (c) 4:1, and (d) 8:1.
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films were rough and exhibited many pin-holes. However, when the R value was >4, smooth PVP−4T films (i.e., without any pin-holes) were observed. The roughness of PVP−4T (R = 4) and PVP−4T (R = 8) were 0.31 and 0.35 nm, respectively. Dielectric Characterizations. The dielectric characteristics of the gate dielectric layer were evaluated by measuring capacitance and leakage current. Capacitance is the amount of charges stored at a specific voltage. The capacitance is mainly determined by dielectric constant and thin-film thickness. The capacitance and leakage current were estimated in a metal− insulator−metal sandwich structure. A gold electrode with an area of 0.0225 cm2 was thermally deposited on polymer thin films on a heavily doped Si substrate. To evaluate PVP−4T dielectric characteristics, capacitance of PVP−4T with different 4808
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R value (thickness: 40 nm) was measured. Interestingly, Supporting Information Figure S3 showed the decrease of capacitance when R value was decreased. This was attributed to the decreased percentage of PVP polymer, which contributes most to the capacitance. For further device fabrication, R = 4 was used due to less pinholes and lowest roughness. In addition, previously reported PVP−HDA was used as a reference for comparison. The capacitances of PVP−4T and PVP−HDA films were 58 and 89 nF cm−2 (at 20 Hz) in 50 and 45 nm thick films, respectively. The relative dielectric constant k of PVP−4T and PVP−HDA, as extracted from the slope of the capacitance versus inverse thickness, showed values of 3.1 and 4.5, respectively. The lower dielectric constant of PVP−4T is attributed to the lack of polar hydroxyl groups in polymer chains. Figure 3a shows the
angles also supported this idea (Supporting Information Figure S9). PVP−4T showed a water contact angle of 77−81°, while PVP−HDA showed a contact angle of ∼58−60°. This indicates that PVP−4T is less hydrophilic and possesses higher water stability. Performance of OTFT by Using Different Semiconductors. To demonstrate the compatibility of PVP−4T dielectrics with various p- and n-channel semiconducting materials, we used several previously reported organic semiconductors, including p-channel pentacene, poly(tetrathienoacene- dithiophenyl-diketopyrrolopyrrole) (PTDPPTFT4), and 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene), and n-channel perfluorinated copper phthalocyanine (F16CuPc), for OTFT fabrications. All the TIPS-pentacene and PTDPPTFT4 devices were fabricated using solution processing approaches. They were selected to investigate the compatibility of PVP−4T film via solution processing. After cross-linking, PVP−4T was found to be insoluble in any commonly used solvents, such as chloroform, chlorobenzene, and toluene. This indicated that the crosslinked polymer film exhibited a high solvent resistance. The performance of top-contact bottom-gate OTFTs based on cross-linked polymer dielectrics was measured in an inert atmosphere. Table 1 summarizes our obtained results. The
Figure 3. (a) Capacitance versus frequency for PVP−4T and PVP− HDA dielectrics. (b) Water swelling impact of PVP−4T and PVP− HDA measured by capacitance changes.
Table 1. OTFT Performance of TIPS-Pentacene, F16CuPc, Pentacene, and PTDPPTFT4 Based on PVP−4T Dielectricsa mobility avg (cm2 V−1 s−1)
capacitance of cross-linked PVP films as a function of frequency (20−104 Hz). A slight frequency dependency for PVP−4T was observed. In contrast, the capacitance of PVP−HDA showed a significant frequency dependence at higher frequency. This may be attributed to the hydroxyl group absorbing moisture in the air, causing the dipolar relaxation. The leakage current as a function of applied electrical field of PVP−4T and PVP−HDA is shown in Supporting Information Figure S4. PVP−4T and PVP−HDA displayed current densities below 10−7 and 10−6 A cm−2 for 50 and 45 nm, respectively, for operation voltages
