pubs.acs.org/NanoLett
Periodic Array of Polyelectrolyte-Gated Organic Transistors from Electrospun Poly(3-hexylthiophene) Nanofibers Sung W. Lee,† Hyun J. Lee,‡ Ji H. Choi,† Won G. Koh,‡ Jae M. Myoung,† Jae H. Hur,§ Jong J. Park,§ Jeong H. Cho,*,| and Unyong Jeong*,† †
Department of Materials Science and Engineering, ‡ Department of Chemical and Biomoleucular Engineering, Yonsei University, 134 Shinchon-dong, Seoul, Korea, § Samsung Advanced Institute of Technology, Mt.14-1, Nongseo-Dong, Giheung-Gu, Yongin-Si, Gyeonggi-Do 446-712, Korea, and | Department of Organic Materials and Fiber Engineering, Soongsil University, 511 Sangdo-dong, Seoul, Korea ABSTRACT High-performance organic field-effect transistors (OFETs) based on polyelectrolyte gate dielectric and electrospun poly(3hexylthiophene) (P3HT) nanofibers were fabricated on a flexible polymer substrate. The use of UV-crosslinked hydrogel including ionic liquids for the insulating layer enabled fast and large-area fabrication of transistor arrays. The P3HT nanofibers were directly deposited on the methacrylated polymer substrate. During UV irradiation through a patterned mask, the methacrylate groups formed covalent bonds with the patterned polyelectrolyte dielectric layer, which provides mechanical stability to the devices. The OFETs operate at voltages of less than 2 V. The average field-effect mobility and on/off ratio were ∼2 cm2/(Vs) and 105, respectively. KEYWORDS Organic field effect transistors, polyelectrolyte, P3HT nanofiber, electrospinning, ion gel
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response up to kilohertz.11,12 However, since their ion gels were made via physical gelation of triblock copolymers in ionic liquids, the structures of the ion gels were not stable upon temperature change. In addition, the poor interfacial adhesion between the ion gel and polymer semiconductor may induce delamination upon temperature change or substrate bending. Therefore, patterning of the ion gel dielectric layer to fabricate transistor arrays has not been realized so far.
rganic field-effect transistors (OFETs) have been intensively studied due to their ability to form integrated circuits on plastic substrates, enabling the fabrication of flexible electronic devices.1 The transistors will find a variety of applications in radio frequency identification (RFID) tags, next-generation displays, and chemical sensors.1,2 The possibility of solution processing in device fabrication has been another unique feature of organic-based electronics. In spite of the enormous efforts,3 OFETs fabricated in the solution process have been far behind the performance requested for real electronic devices. To fully realize flexible OFETs, the organic transistors must operate at low voltages and possess high field-effect mobility and large on/off ratios. Both the high on/off ratio and low voltage operation have been demonstrated by utilizing a gate insulator with a large capacitance.4 Since electrolytes are insulators for electrons and holes, they form electric double layers upon contact with a charged electrode, resulting in specific capacitances in excess of 10 µF/cm2, allowing for low switching voltages and high on-currents.5,6 Recently, several groups have demonstrated the fabrication of solid polymer electrolyte-gated transistors.7-10 Unfortunately, the slow polarization response of the solid polymer electrolyte restricted their practical applications in fast switching devices. Very recently, Frisbie and co-workers have first demonstrated the high capacitance “ion gel” gate dielectric with fast polarization
In this study, we demonstrated an easy method for fabricating arrays of ion gel-gated OFETs on flexible polymer substrates. We employed a UV-curable hydrogel having ionic liquids inside. Photo patterning through a shadow mask enabled the fabrication of OFETs on flexible substrates over large areas. Key factors to form mechanically stable ion-gel patterns include the employment of electrospun semiconductor nanofibers instead of films and the use of a methacrylated polymer substrate. The poly(3-hexylthiophene) (P3HT) nanofibers directly deposited on the substrate allowed the reactive surface of the substrate exposed to air. UV irradiation through a patterned mask formed hydrogel patterns that were chemically bonded to the substrate via covalent bonds. Such permanent chemical binding of the dielectric layer with the substrate cannot be achieved by using film-type semiconductor layers. Since the films do not allow any direct contact between the polyelectrolyte layer and the substrate, the polyelectrolyte layer will have simple physical contact with the semiconductor layer instead of forming chemical bonding. The poor interfacial adhesion with the semiconductor will lead to delamination or peeling-
* Towhomcorrespondenceshouldbeaddressed.E-mail:(U.J.)
[email protected]; (J.H.C.)
[email protected]. Received for review: 11/6/2009 Published on Web: 12/08/2009 © 2010 American Chemical Society
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DOI: 10.1021/nl903722z | Nano Lett. 2010, 10, 347-351
solution of P3HT:PCL (7:3, w/w) in chloroform was used instead of pure P3HT solution. The total polymer content in the solution was 10 wt %. Electrospinning of P3HT was carried out using a single nozzle (inner diameter ) 110 µm) at 18 kV. The nozzle-to-collector distance was 7 cm. The diameter of the fibers was ∼400 nm. There are wellestablished ways to align electrospun nanofibers.15 The P3HT nanofibers were first collected between parallel metal wires (separated by ∼3 cm) to uniaxially align the fibers, and then transferred onto the substrate. The fibers were annealed at 120 °C for 30 min to reduce contact resistance with Au electrodes. For the UV crosslinkable polyelectrolyte dielectric layer, [EMIM][TFSI] ionic liquid, PEG-DA monomer, and HOMPP (UV cross-linking initiator) were mixed at a ratio of 88:8:4 (w/w) and the mixed solution was dropped on the entire substrate. After doctor blading the electrolyte liquid, a square-patterned film mask was placed on the substrate. UV exposure (365 nm, 100 mW/cm2) generated free radicals from HOMPP that initiated polymerization of acrylate end groups on PEG derivatives. Methacrylate moieties on the substrate surface also participated in the free radical polymerization and created covalent bonding between acrylate groups present in the bulk gel, thus permanently fixing the hydrogel micropatterns to the substrate. Such a process is already well-established in the scaffold applications.16 Complete cross-linking of PEG-DA required less than 2 s, producing square gel patterns between electrodes. Since the hydrogel pattern prevents the penetration of organic solvents, the underlying P3HT fibers were intact from most organic solvents. The unexposed part of the ionic liquid solution and the underlying P3HT fibers were removed by dipping in pure chloroform for 5 s. The solvent barrier of the dielectric patterns prevented the generation of charge trap sites in the P3HT fibers, which are frequently encountered during solvent drying processes. Finally the substrate was flipped over and placed on a PEDOT:PSS thin film. Annealing at 50 °C for 5 min allowed clear transfer of PEDOT:PSS layer to the top surface of the ion gel patterns. The densely arrayed device was highly flexible and transparent. The same approach can be applied to any substrates if their surfaces are functionalized to involve in the polymerization of the polyelectrolyte. The ion gel can have perfect contact with the P3HT fibers, which is considered one of the reasons of the high performance for the devices. Capacitance-voltage (C-V) measurements were performed on the polyelectrolyte gate insulator by varying the frequency (10-106 Hz) in an impedance analyzer (HP 4284A, Agilent technologies). A metal-insulator-semiconductor (MIS) structure was employed for the measurement. A 1 mm insulator layer was formed on the p-type silicon substrates and a round shape gold electrode (200 µm diameter) was contacted on the insulator layer. The chemical structures of the components constituting the gate insulator are shown in Figure 2A. Figure 2B shows the C-V curves at four frequencies. The specific capacitance increased upon
FIGURE 1. Schematic illustration of the process for fabricating an array of transistors. The procedure involves (i) P3HT fiber deposition, (ii) formation of polyelectrolyte ion gel pattern, (iii) removing unexposed polyelectrolyte and underlying fibers, and (iv) PEDOT: PSS transfer.
off of the dielectric layer from the substrates upon repeated bending or stretching. The use of electrospun nanofibers provides other advantages including high surface-to-volume ratios, compatibility with many substrates, extreme flexibility, easy alignment, and scalability. The continuous production of semiconducting nanofibers also enables highly efficient and convenient assembly of the device components at low costs for flexible electronics.13 Our group previously demonstrated the preparation of P3HT nanofibers.14 Electrospinning highly regioregular P3HT has been a challenge because their fast crystallization during electrospinning can readily block the nozzles. Electrospinning of P3HT was achieved with the aid of extra solvent flowing through the outer nozzle in the coaxial setup. We also prepared composite fibers of P3HT and poly(ε-caprolactone) (PCL) to improve the uniformity of the fiber diameter and to guarantee the continuous production without nozzle blocking. The bottom-gated transistors from the P3HT fibers on the SiO2 insulating layer exhibited excellent charge mobility (10-2 cm2/(Vs)) in air. The composite fibers with 30 wt % PCL also showed good mobility (10-3 cm2/(Vs)), which is comparable with those obtained from P3HT thin film transistors. Figure 1 illustrates the fabrication of the patterned OFETs on a flexible substrate. For the experiments, regioregular P3HT (Mw ) 87 000, 95% regioregularity), PCL(Mw ) 80 000), poly(ethyleneglycol) diacrylate(PEG-DA) (Mw ) 575), 2-hydroxy-2-methylpropiophenone (HOMPP), and 3-(trichlorosilyl)propyl methacrylate (TPM) were purchased from Aldrich. The ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide or [EMIM][TFSI], was purchased from Merck. The methacrylated polyethylene terephthalate (PET) films were kindly donated by Toraysaehan Co. For electrospinning the semiconductor polymers, a mixture © 2010 American Chemical Society
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DOI: 10.1021/nl903722z | Nano Lett. 2010, 10, 347-351
FIGURE 3. (A) Device structure of the arrayed OTFTs. The scheme and the optical microscopy image show a transistor. (B) Output characteristics of the arrayed OFET (ID-VD). (C) Corresponding transfer characteristics (ID vs VG).
FIGURE 2. (A) Chemical structures of the ionic liquid and UV crosslinkable monomer, (B) capacitance-voltage characteristics at four different frequencies, and (C) frequency dependence of the maximum capacitance on a function of frequency (10-106 Hz). The inset shows the device structure of p type-Si/polyelectrolyte insulator/Au.
drain electrodes. The average field-effect mobility was calculated to be ∼2 cm2/(Vs), which is much higher than those reported in other P3HT-based thin film transistors. The on/ off current ratios were ∼105. The maximum on-current was stable with increasing the humidity and kept the same value even after storage in an ambient condition for a few days. However, the off current slightly increased and the turn-on voltage also shifted from -0.5 to -1.0 V. This behavior is considered to originate from the hydration of the hydrophilic PEG (8 wt %) in the polyelectrolyte gate insulator. The water molecules may act as charge trapping sites and provide leakage current paths. The electrospun organic semiconductor nanofibers are advantageous in large contact area with dielectric layer, compatibility with any substrate, extreme flexibility, and easy scale-up. Although the number and orientation of the fibers bridging the source/drain electrodes are not precisely controllable so far, reasonable control was possible by adjusting collection time, flow rate of the polymer solution, and the distance between the nozzle and the collector. Figure 4 shows the dependence of the maximum on-current values on the number of fibers connecting the electrodes. The transfer curve was measured by sweeping VG from +1 to -4 V at VD) -1 V. Average values from five devices (L ) 10 µm) were presented in the figure. Transfer characteristics of the devices based on different number of P3HT nanofibers are presented in Supporting Information. We found that nonparallel orientation and crossover between the fibers do not affect the electrical properties of the transistors. Devices based on a single P3HT fiber and 10 fibers showed oncurrent values of 0.7 and 3.8 mA, respectively. The devices exhibited a very good linear relationship (the slope of 3.6 × 10-4) between the number of fibers and maximum oncurrent, which indicates that the electrical properties are controllable by adjusting the number of fibers. The deviation in the number of fibers could be controlled within 2 in this
sweeping the bias from positive to negative voltages due to the voltage-dependent hole accumulation at the polyelectrolyte/p-Si interface for negative biases. The capacitance reached a maximum value (∼30 µF/cm2) at 10 Hz, which is far higher than those achievable with typical 100 nm thick SiO2 dielectric layers (∼25 nF/cm2). Figure 2C demonstrates the frequency dependence of capacitance. The capacitance value decreased from ∼30 to ∼1 µF/cm2 with increasing frequency from 10 Hz to 1 MHz, which may be due to the limited polarization response at high frequency. However, the capacitance of the hydrogel polyelectrolyte insulator is still around 1 µF/cm2 at 1 MHz, which is two orders larger than what was reported for physically cross-linked gel (∼0.01 µF/cm2 at 1 MHz).11 The large capacitance opens a possible use for high frequency operation of the OFETs over 1 MHz. Figure 3A shows a schematic of the OFET device with a channel length of 10 µm and the corresponding optical microscopy image (Olympus BX-51). The current-voltage (I-V) measurements were carried out in air at room temperature with an I-V tester (E5270D, Agilent technologies). Typical output curves, drain current (ID)-drain voltage (VD), at different gate voltages (VG) were shown in Figure 3B. The device exhibited reasonable gate modulation of the drain current. Figure 3C gives the transfer (ID-VG) characteristics at VD ) -1 V. From the slope of the VG vs |ID|1/2 curves obtained from more than five devices, average field-effect mobility was calculated in the saturation regime (VD ) -1 V) by the following equation,18 IDS ) WCµ/2L(VG s Vth),2 where C is the capacitance of the gate dielectric layer, W is the width of the active layer, L is the channel length, Vth is the threshold voltage, and µ is the field-effect mobility. We calculated the width of the devices as the sum of the circumferences of all P3HT fibers bridging the source and © 2010 American Chemical Society
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DOI: 10.1021/nl903722z | Nano Lett. 2010, 10, 347-351
terning is a unique feature of the fiber-based transistors. The capacitance of the UV-crosslinked hydrogel gate insulator including [EMIM][TFSI] ions was around 30 µF/ cm2 at 10 Hz and 1 µF/cm2 at 1 MHz, which facilitated high on-current and low-voltage operation in air. The field effect mobility of the transistors was higher than ∼2 cm2/ (Vs) at the saturation regime and the on/off ratio was ∼105. The on-current showed a linear relationship with the number of P3HT fibers bridging the electrodes within the polyelectrolyte gate insulator. Control over the deposition rate of the nanofibers on the substrate minimized the possible deviation of device performance. The simple solution process in a continuous way and the high performance of the devices may find immediate applications in practical electronic devices. Study on the crystallization behavior of P3HT during electrospinning and optimization of the humidity-insensitive polyelectrolyte insulator patterns are remained as further studies.
FIGURE 4. Plot of the maximum on-current versus number of fibers bridging the source and drain electrodes within each polyelecrolyte gate dielectric pattern. The insets are typical optical microscopy images of the OFETs containing one fiber and six P3HT fibers, respectively.
study. For devices based on 6 P3HT fibers in average, the number of fibers in each device was within a range of 4-8, giving maximum on-current deviation of ∼0.65 mA from the whole devices. Since typical off-currents of the devices were from 10 to 100 nA, such on-current deviation is tolerable for stable device operation. Therefore, this fiber-based device fabrication is reliable for the preparation of highly dense OFETs. The high on-currents and low operation voltages of the transistors result from the high capacitance of polyelectrolyte gate dielectrics, which induces large carrier density in the source/drain channel. It is well-known that the carrier mobility in polymer semiconductors dramatically increases with increasing carrier concentration.17 However, this explanation is not enough in our devices because we used blend fiber containing 30 wt % PCL in the P3HT matrix. Our previous results14 demonstrated that 30 wt % PCL in the as-spun P3HT fibers exists as small grains in the P3HT matrix. Annealing at 120 °C did not lead to a noticeable difference in the morphology. As the concentration of PCL increased, macroscopic phase separation during electrospinning formed long-range interfaces between P3HT and PCL phases. Despite the possible defect sites, the hole mobility still exceeded the literature values of P3HT-based TFTs. Since the electrical properties of polymer semiconductors are strongly dependent on the chain orientation and crystal structure,18 we believe the performance using this process can be further improved. The correlation between the electrical properties and crystal structure of the electrospun polymer semiconductors are currently being investigated. In summary, we have demonstrated a periodic array of high performance ion gel-based OFETs with the assistance of electrospun P3HT nanofibers. Continuous production of the nanofibers enabled the fabrication of transistors over large areas. The polyelectrolyte gate insulator patterns could be covalently bonded to the PET substrate. The mechanically stable polyelectrolyte pat© 2010 American Chemical Society
Acknowledgment. This work was supported by the National Research Foundation (NRF) grant funded by the Korea Government (MEST) through the Active Polymer Center Pattern Integration (No. R11-2007-050-01004-0) and World Class University (R32-20031). J. H.C. thanks the KOSEF Grant (2009-0073278). Supporting Information Available. Transfer characteristics (ID vs VG) depending on the number of nanofibers. This material is available free of charge via the Internet at http:// pubs.acs.org. REFERENCES AND NOTES (1) (2)
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