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Blending effect of 6,13-bis(triisopropylsilylethynyl) pentacene–graphene composite layers for flexible thin film transistors with a polymer gate dielectric
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 085201 (http://iopscience.iop.org/0957-4484/25/8/085201) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 155.69.4.4 This content was downloaded on 21/05/2015 at 05:52
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Nanotechnology Nanotechnology 25 (2014) 085201 (11pp)
doi:10.1088/0957-4484/25/8/085201
Blending effect of 6,13-bis (triisopropylsilylethynyl) pentacene–graphene composite layers for flexible thin film transistors with a polymer gate dielectric Sarbani Basu, Feri Adriyanto and Yeong-Her Wang Institute of Microelectronics, Department of Electrical Engineering, Advanced Optoelectronic Technology Center, National Cheng-Kung University, Tainan, 701, Taiwan E-mail: [email protected] Received 12 April 2013, revised 11 June 2013 Published 4 February 2014
Abstract
Solution processible poly(4-vinylphenol) is employed as a transistor dielectric material for low cost processing on flexible substrates at low temperatures. A 6,13-bis (triisopropylsilylethynyl) (TIPS) pentacene–graphene hybrid semiconductor is drop cast to fabricate bottom-gate and bottom-contact field-effect transistor devices on flexible and glass substrates under an ambient air environment. A few layers of graphene flakes increase the area in the conduction channel, and form bridge connections between the crystalline regions of the semiconductor layer which can change the surface morphology of TIPS pentacene films. The TIPS pentacene–graphene hybrid semiconductor-based organic thin film transistors (OTFTs) cross-linked with a poly(4-vinylphenol) gate dielectric exhibit an effective field-effect mobility of 0.076 cm2 V−1 s−1 and a threshold voltage of −0.7 V at Vgs = −40 V. By contrast, typical TIPS pentacene shows four times lower mobility of 0.019 cm2 V−1 s−1 and a threshold voltage of 5 V. The graphene/TIPS pentacene hybrids presented in this paper can enhance the electrical characteristics of OTFTs due to their high crystallinity, uniform large-grain distribution, and effective reduction of crystal misorientation of the organic semiconductor layer, as confirmed by x-ray diffraction spectroscopy, atomic force microscopy, and optical microscopy studies. (Some figures may appear in colour only in the online journal)
1. Introduction
large-area flexible devices due to their lower solubility in common organic solvents. In general, solution-processed OTFTs have higher on–off ratios but suffer from poor field-effect mobility, while a carbon-based material such as graphene or carbon nanotubes demonstrates very high mobility but suffers from low on/off ratio [3, 4]. The density of CNTs has a higher threshold for the formation of a percolation network. Therefore, such materials are limited by their high cost and process complexity. Thus, our aim is to make a semiconductor composite layer with an organic semiconductor and graphene flakes to improve the field-effect mobility as well as the
In recent years, solution-processed organic semiconductors have offered potential applications for display drivers, radio frequency identification tags, e-paper, and chemical sensors due to their low cost, low temperature, large-area manufacture, and flexibility [1–4]. Pentacene is one of the most promising and widely investigated organic semiconductors for organic thin film transistors (OTFTs) as an active layer. However, the fabrication of pentacene thin films requires a vacuum deposition process, which is not suitable for the fabrication of 0957-4484/14/085201+11$33.00
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pentacene semiconductor with inorganic and polymeric gate dielectrics was thoroughly discussed in recently published research articles [15, 16]. In this study, we investigated the effect of graphene, instead of polymer additives, on OTFT performance. The addition of graphene flakes does not disrupt the 5–5 stacking of the TIPS pentacene molecule. Two factors are essential when graphene is dispersed in organic solvents: polydispersity and flake size. The first is related to the graphene flakes in the solvent, i.e. monolayer, bilayers, multilayers. Following the centrifugation method, removal of the thicker graphene flakes could enhance the transmittance and resistance properties. It improves the uniformity of the molecular morphology and increases the semiconductor layer coverage within the devices and reduces the variation in molecular orientation. Graphene is a two-dimensional (2D) graphite film with monoatomic layer thickness, unique transport properties, and high conductivity. It is not easy to enhance the mobility without degrading the on–off ratio in solution-processed OTFTs. Therefore, it is expected that the incorporation of a few layers of graphene flakes may modify TIPS pentacene crystallization and increase the area in the conduction channel of OTFTs without lowering the on–off ratio. We report Raman spectroscopy to characterize the local molecular order, orientation and coupling energy (ω1 ) between two molecules of TIPS pentacene–graphene composite films. The grain size distribution, crystallinity and surface morphology also have been studied through XRD, optical microscopy, and AFM measurements. The dielectric properties of the metal/insulating layer/metal structure (MIM) were investigated with an HP 4284 C–V plotter. The electrical characteristics of the devices were analyzed with a semiconductor parameter analyzer (Agilent B1500A) in ambient air environment. This is the first example of the fabrication and characterization of graphene/TIPS pentacene OTFTs. Overall, this attempt is a general route to enhance the performance of semiconductor graphene-based composite OTFTs.
on–off ratio. Improvement of TIPS pentacene–graphene-based OTFTs was achieved after controlling and optimizing the grain size, crystallinity and surface morphology. The small molecule of 6,13-bis(triisopropylsilylethynyl) (TIPS) pentacene is an excellent candidate for the p-type OTFT active layer due to its high mobility, air stability, and good solubility; moreover, its face-to-face stacking in crystalline form is highly beneficial for 5-orbital overlap. Polymeric dielectrics are ideal because of their diverse properties, favorable film-forming characteristics, and tunable surface chemistry for the control of device-critical interfacial trap state densities [5–7]. However, the OTFTs with TIPS pentacene reported thus far have been fabricated using inorganic gate dielectrics on highly conductive silicon substrates, thereby ruling out their use in solution-processing techniques because of the high cost entailed and their inapplicability in largearea flexible devices. On the other hand, Shin et al have demonstrated that drop-cast TIPS pentacene thin film transistors (TFTs) (bottom-gate indium tin oxide electrode) with poly(4-vinylphenol) (PVP) gate dielectric exhibit a field-effect mobility of 0.002 cm2 V−1 s−1 on a flexible substrate [8]. To date, many efforts have been made to increase the mobility of TIPS pentacene devices. However, the anisotropic nature of the films does not support film uniformity and crystal misorientations have occurred. Polystyrene (PS) and PVP with small-molecule TIPS pentacene blends were recently reported by Kim et al [9]. The mobilities of self-organized PSand PVP-blended OTFTs (10 µm gate length) are 0.85 and 0.14 cm2 V−1 s−1 , respectively. Compared with PS-blended OTFTs, the performance degradation of threshold voltage, mobility, and hysteresis of PVP-blended OTFTs is due to the presence of hydroxyl groups in PVP, which introduce hole trapping at the oxide–semiconductor interfaces. Sim et al conducted research on solution-processable OTFTs (L = 50 µm) with the polymide gate dielectrics 1,2,3,4-butanetetracarboxylic dianhydride and 4,4-diaminodiphyeyl methane (BTDA–DADM) and PVP. The active layer was prepared by blending a polymeric binder, poly(α-methyl styrene) (0.1 wt%), and TIPS pentacene (0.1 wt%) in a high solvent (odichlorobenzene) [10]. Better performance of the pαMS/TIPS pentacene with polymide gate dielectric BTDA–DADM (µ ∼ 0.15 cm2 V−1 s−1 , Vth ∼ −2.7 V, on–off ratio ∼ 106 ) was observed over the cross-linked PVP gate dielectric (µ ∼ 0.001 cm2 V−1 s−1 , Vth ∼ −2.4 V, on–off ratio ∼ 104 ). Thus, the polymer gate dielectric chain dynamics affect the organic semiconductor growth and the polymer gate dielectric surface viscoelasticity modulates the OTFT’s performance, which can be substantially enhanced by manipulating the semiconductor/dielectric interfacial properties by optimizing the gate dielectric [11–13]. He et al demonstrated that the enhanced performance consistency of 10 wt% SiO2 nanoparticle–TIPS pentacene composite semiconductor OTFTs (50 µm gate length) with a mobility of 0.15 cm2 V−1 s−1 , which is three times higher than that of pure TIPS pentacene OTFTs (µ ∼ 0.05 cm2 V−1 s−1 ) [14]. The incorporation of SiO2 nanoparticles enables the modification of TIPS pentacene crystallization by nanoparticle aggregation at TIPS pentacene grain boundaries. A more detailed solution-processed TIPS
2. Fabrication and device structure
Liquid exfoliation of graphene is a cost-effective method to prepare graphene dispersions. In this research, graphene oxide (GO) was prepared from natural graphite powder (300 mesh, Alfa Aesar) by a modified Hummers method [17]. Graphene nanopowder (3 nm flake size, black powder) purchased from Graphene Supermarket (USA) was dispersed in tetralin solvent. The dispersion process followed Tour and Barron’s method [18]. Tetralin solvent with high boiling temperature was found to be more effective for achieving distinct phase segregation/crystallization of TIPS pentacene. Up to 5 mg of graphene flakes in 1 ml of tetralin solvent was vigorously shaken and subjected to ultrasonication for 1 h. The thick graphene flakes were settled in the solution by leaving them to stand for over 30 min. The top half portion of the graphene solution (less concentration of graphene dispersion) that was collected and dispersed was further centrifuged at 4500 rpm for 30 min to remove the precipitates. The final solution contained single-layered and multilayered graphene dispersion. 2
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Figure 1. Schematic illustration of the device fabrication steps and the cross-sectional schematics of the TIPS pentacene–graphene-based TFTs fabricated on plastic and glass substrates. The chemical structures of TIPS pentacene and the PVP gate dielectric are also shown. The thickness of the cross-linked PVP layer was 650 nm, as measured by the alpha step.
The concentration of the graphene dispersion was approximately 100 times lower than that of the starting concentration. Bottom-gate and bottom-contact configurations were used for the OTFTs. Al and ITO served as the gate metals for plastic and glass substrates, respectively. A 650 nm thick cross-linked PVP (Mw ∼ 25 000) layer was spin coated (5000 rpm) on patterned gate metal used as the polymeric gate dielectric (specific capacitance = 4.9 nF cm−2 ). The concentration of the cross-linking agent poly melamine-co-formaldehyde (PMF, Mw = 511 g mol−1 ) was fixed at 4 wt%. The PVP was fixed at 10 wt%. Propylene glycol monomethyl ether acetate (PGMEA ∼ 1 ml) was used as the solvent. The curing temperature was maintained at 175 ◦ C for 1 h on a hot plate at room temperature. Au source/drain electrodes were deposited by thermal evaporation without any adhesion layer. Selfassembled monolayers (SAMs) of pentafluorobenzenethiol (PFBT) and octyltrichlorosilane (OTS-8) formed on the Au S/D electrodes and gate dielectric, respectively. To reduce the injection barrier between the organic semiconductor and the source/drain electrodes, the Au electrodes were treated with 10 mM PFBT, rinsed with anhydrous ethanol, and then dried with N2 . Furthermore, the OTS-8 self-assembled monolayer was deposited on the channel region by immersion into a 1 mM solution of OTS-8 in toluene at 60 ◦ C for 15 min. Subsequent rinsing with toluene and ethanol and then blow-drying with nitrogen were performed before use. The pure TIPS pentacene and graphene/TIPS pentacene hybrid semiconductors were then drop cast in the active region. Device fabrication, measurements, solution preparation, and drying of the semiconductor layer were performed in a clean room either in ambient air or in solvent-rich ambient atmosphere. Figure 1 shows the complete OTFT fabrication steps, a cross-sectional diagram of the bottom-gate and bottom-contact pentacene-based TFT fabricated on plastic as well as glass
substrates and the molecular structure of TIPS pentacene and PVP films. Figures 2(a) and (b) indicate the optical microscopy images of the TIPS pentacene–graphene flake hybrid semiconductor layer drop cast from tetralin solution on the channel and source–drain region for both types of OTFTs fabricated on plastic and glass substrates. The surface area of the graphene nanoflakes used is 510 m2 g−1 , containing carbon ∼97%, hydrogen ∼1%, and oxygen ∼2%. The average flake thickness and particle lateral sizes are 1 nm and 10 µm, respectively, as shown from the SEM study in figure 2(c). The surface morphology of the graphene sheet was confirmed as hexagonal shape. Figure 2(d) shows the SEM images of the graphene flakes with a scale bar of 100 nm. 3. Results and discussion
Figure 3 shows the grazing incidence x-ray diffraction results and the corresponding optical images from the TIPS pentacene–graphene composite and typical TIPS pentacene thin films drop cast from a 1 wt% tetralin solution on plastic and ITO-coated glass substrates, respectively. The crystallinity of the film is strongly dependent on the solvent. The XRD curve exhibits strong (001)-type reflections and other preferential orientations in the (002) and (003) directions, which match well with the same diffraction peaks of bulk crystal TIPS pentacene. The strong and sharp peaks are observed at 5.4◦ , 10.7◦ , and 16.1◦ for n = 1, 2, and 3, respectively. The average interlayer spacing between the 001 planes of the drop-cast TIPS pentacene is 16.8 Å, calculated using the well-known Bragg diffraction formula as follows: 2d sin θ = nλ, where d, θ, n, and λ represent the interlayer spacing, angle of the diffracted wave, wavelength of the x-ray used, and order of the diffracted beam, respectively. This value is consistent 3
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Figure 2. The graphene dry nanopowder directly purchased from Graphene Supermarket. Optical microscope images of the TIPS pentacene–graphene flake hybrid semiconductor layer drop cast by using tetralin solution on the channel and source–drain region. (a) OTFTs configured with bottom-gate (aluminum) and top-contact (gold) on flexible substrates and (b) OTFTs configured with bottom-gate indium tin oxide and top-contact gold on glass substrates. (c) SEM images of graphene flakes with size ∼3 nm, which were dispersed in tetralin solution with TIPS pentacene and used as a hybrid semiconductor layer for OTFT application with a scale bar of 5 µm. (d) Higher magnification of image (c) with a scale bar of 100 nm.
which are hydrophobic in nature and can easily adhere to dielectric PVP (low surface energy of 34 mJ m−2 ) surfaces. The smooth surface of the PVP layer (roughness ∼ 1.2 nm) is suitable for maintaining good interface quality between the TIPS pentacene and the dielectric layer. The drop-cast films have stronger molecular ordering due to longer film formation time. The solvent tetralin which has a higher boiling point enables slower solvent evaporation and gives the semiconductor material adequate time to self-assemble, which is beneficial to the highly ordered polycrystalline structure. The crystalline sizes (t) of the TIPS pentacene–graphene and the sole TIPS pentacene layers were calculated using the Scherrer formula (t = 0.9λ/β cos θ ), where λ denotes x-ray wavelength, β is the FWHM for the diffraction peaks, and θ , which is half of the diffraction angle, has values of 27.7 and 18 nm, respectively. The crystallite size of the TIPS pentacene–graphene composite film increases to 27.7 nm, which may be attributed to the improved crystallinity. The XRD curves and corresponding optical microscopy image show the stem-like and needleshaped morphologies of the TIPS pentacene, proving the good crystalline property obtained from the drop-casting method. However, the crystal formation of typical TIPS pentacene shows large gaps between the TIPS pentacene stems compared with the composite one. An increase in the concentration of the graphene flakes compared with TIPS pentacene facilitates the aggregation of flakes, thus reducing device mobility. Figures 4(a)–(c) show the Raman spectra of a TIPS pentacene (single droplet) thin film on plastic substrate (sample T1), TIPS pentacene with graphene composite films on plastic substrate (sample T2), and TIPS pentacene with graphene
Figure 3. Grazing incidence x-ray diffraction of TIPS pentacene–graphene hybrid and typical TIPS pentacene films on PVP/Al/plastic substrate and PVP/ITO/glass substrate. The films exhibited prominent (001)-type reflections. The pictures on the left indicate the optical microscopic images of the corresponding films. Only the TIPS pentacene/PVP/ITO glass substrate OTS treated for 20 min showed some black spots.
with previously reported data for TIPS pentacene having a c-axis of well-ordered triclinic structure [5]. The (001) plane of TIPS pentacene is composed of tri-isopropylsilyl groups 4
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Using the polarizations of the incident and scattered light either parallel or perpendicular to each other, we calculated the intermolecular coupling energy (ω1 ) that can be estimated by 1ω = ω12 /2ωo , where ωo is the frequency of the intermolecular band and 1ω is the splitting frequency based on the coupled oscillator mode. The calculated coupling energy (ω1 ) of 5.36 meV between two molecules of the TIPS pentacene–graphene composite (sample T2) semiconductors is illustrated in figure 4(b); it is higher than the 4.46 eV coupling energy of two pentacene molecules on PVP dielectric previously reported [23]. Raman spectroscopy is a powerful and nondestructive technique that can determine molecular packing conjugated backbone orientation and thin film uniformity (homogeneity) in small-molecule thin films, and can provide information on the reorganization energy (λ), which indicates the charge–phonon interaction inside the molecules. The hopping rate (W) of carriers in the organic semiconductors was estimated by the Marcus–Hush equation which is expressed as follows: W = V 2 / h[5 /(λkB T )1/2 ] exp (−λ/4kB T )αV 2 , where kB is the Boltzmann constant, V represents the intermolecular coupling (also called the transfer integral), and T is the absolute temperature. The calculated value of the hopping rate is 1010 s−1 . V and λ were calculated from the polarized Raman spectra and UV–vis spectra experiments, as shown in figures 4(b) and (c), respectively. The estimated V and λ are 5.36 meV and 0.271 eV for the TIPS pentacene–graphene hybrid semiconductor, whereas for the as-vacuum-deposited pentacene molecules on the PVP dielectric the values are 4.46 meV and 0.2 eV [23]. To further investigate the influence of the inserted graphene in the TIPS pentacene composite film, the surface morphologies of typical TIPS pentacene and TIPS pentacene–graphene hybrid films on PVP-coated ITO/glass and Al/plastic were examined. The root-mean-square (RMS) values for the 1 wt% TIPS pentacene without and with graphene are 0.926 and 0.566 nm on PVP/ITO/glass, as observed as in figures 5(a) and (c). Similarly, the TIPS pentacene and TIPS pentacene– graphene hybrid on PVP/Al/plastic substrate show RMS roughness values of 1.8 and 1.012 nm, respectively, as shown in figures 5(d) and (e). The TIPS pentacene film drop cast from tetralin has relatively low surface roughness compared with the vacuumdeposited films. We further measured the average grain size from the 2D AFM films. The grain sizes of the TIPS pentacene–graphene hybrid films are larger at 730 and 530 nm compared with those of typical TIPS pentacene with 330 and 320 nm for plastic and glass substrates, respectively. According to Gundlach et al the higher mobility of TIPS pentacene–graphene hybrid films can be partially attributed to the enlargement of grain size [24]. Figure 6 shows high resolution transmission microscopy images of graphene flakes with various concentrations in tetralin solvent and the deposits suspended over the lacey carbon film of the TEM grid. Structural characterization of the deposited flakes was performed by diffraction techniques. The flakes’ lateral sizes reduced from several µm to the nm level after the ultrasonication and centrifugation process (see the details of the dispersion process in section 2). With
Figure 4. (a) Raman spectra (λexc = 532 nm) of (a) TIPS
pentacene-based OTFTs on plastic substrate (sample T1), TIPS pentacene–graphene hybrid-based OTFTs on plastic substrate (sample T2) and TIPS pentacene–graphene hybrid-based OTFTs on glass substrate (sample T3). (b) Raman spectra of TIPS pentacene–graphene hybrid on PVP gate dielectric using (x, x) and (x, y) polarizers indicating the splitting frequency (1ω) in the range of 1150–1165 cm−1 . (c) UV–vis spectra of TIPS pentacene–graphene hybrid semiconductor layer. TIPS pentacene and TIPS pentacene–graphene composite thin films were drop cast on a PFBT-treated gold source–drain metal. All data are plotted after device fabrication.
on ITO substrate (sample T3), respectively. TIPS pentacene was drop cast on PFBT-treated gold metal. A Jobin Yvon LabRam HR spectrometer with a 532 nm solid state laser was used as the excitation light source. The spatial resolution of the beam spot was approximately 1 µm. To avoid thermal damage on the TIPS pentacene film, the laser light beam was defocused from the surface of the TIPS pentacene film. The bands at 1157.71 and 1191.53 cm−1 are related to the C–H intra-molecular vibration from the ends and sides of the pentacene backbone [19]. The bands at 1373.21 and 1577.85 cm−1 are assigned to the C–C ring stretch modes when the carbon atoms mainly vibrate along the short and long molecular axes, respectively. James et al [20] observed the maximum Raman scattering intensity for the 1373.21 cm−1 (short axis) mode and for the 1577.85 cm−1 (long axis) mode when the laser polarization was parallel to the short and long axes, respectively. Samples T1 and T2 on the plastic substrate further indicate no change in the peak intensity of the different vibration modes after the blending of graphene with TIPS pentacene, which indicates that the addition of graphene does not disrupt the 5–5 stacking of the TIPS molecules. All the bending modes described above are in good agreement with previously assigned modes in pentacene thin films [21, 22]. 5
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the concentration varied from 25 mg/5 ml to 2 mg/5 ml, which is comparable with the transmittance of ∼97% of CVD grown monolayer graphene film. We have observed that, independent of the concentration, our films have a flat optical transmission profile across the visible region. Due to the measurement limitations, we are presently unable to measure the transmittance in the near infrared region, however in the future we will verify it. Ideal transparent electrodes should have high transparency (>80%), therefore our results are potentially attractive for transparent conductors in display applications. Bottom-gate and bottom-contact OTFT configurations were considered throughout the experiment. The channel width and length were 1500 µm and 150 µm, respectively. All the fabricated devices were associated with PFBT-modified S–D and OTS-treated dielectric layers. The OTFTs fabricated on plastic substrate used aluminum gate metal and those on glass substrate used ITO gate metal. The leakage current and capacitance of the PVP insulator measured through the Al/PVP/Au metal–insulator–semiconductor structure were 10−9 A cm−2 at ±0.6 MV cm−1 and 4.9 nF cm−2 , respectively. Figures 8(a) and (b) show the current–voltage (Ids –Vds ) characteristics of the TIPS pentacene–graphene and typical TIPS pentacene OTFTs, respectively, at different gate–source voltages (Vgs ) from 10 to −60 V with a 650 nm thick PVP gate insulator fabricated on plastic substrate. A clear transition was observed from linear to saturation behavior. At a given Vgs , Ids initially increases linearly and then saturates due to a pinch-off in the accumulation layer located in the interface between the TIPS pentacene and the PVP. However, we can observe from figure 8(b) that ‘current crowding’ occurs at low Vds , which corresponds to the non-Ohmic behavior of the drain current (Ids ) in the linear region. The non-linearity of the output curves in the low-Vds region may result from the combined effect of both the contact barrier and the field-dependent mobility [25, 26]. Figures 8(c) and (d) show the transfer characteristics of the corresponding OTFTs. The devices demonstrate the characteristics of typical pchannel thin film transistors with three times larger saturation drain current of TIPS pentacene–graphene OTFTs compared with TIPS pentacene OTFTs at Vgs = −60 V. The field-effect mobility µsat of the devices was estimated from the saturation region with the following equation:
Figure 5. (a)–(c) AFM images of the semiconductor layer of untreated TIPS pentacene, OTS-treated PVP gate dielectric of TIPS pentacene, and OTS-treated TIPS pentacene–graphene hybrids on PVP/ITO-coated glass substrates. (d)–(e) AFM images of OTS-treated PVP gate dielectric of TIPS pentacene and TIPS pentacene–graphene hybrids on plastic substrate.
increasing concentration, the graphene flakes are stacked together, wrinkled and folded several times over themselves, as shown in figure 6(f). TEM study reveals a monolayer to a few layered structure. Figures 6(d), (g) and (h) show the presence of graphene lattice fringes and carbon nanotubes, respectively. The electron diffraction pattern acquired over the flake, shows a ring-like structure compatible with randomly stacked graphene. One of the most promising optoelectronic applications of graphene is probably its use as a transparent conducting film, e.g. for flexible electronics and solar cells. High conductivity, transparency and stability may allow graphene to challenge the existing ITO-based films. Moreover, the fragile nature of ITO limits its application in some conditions. ITO is also limited by the scarcity and thus increasing cost of indium. Figure 7 illustrates the transmittance curves of various concentrations of graphene dispersion in solvent of tetralin. The transmittance increased from 77 to 95% as
Ids = (Ci µsat W )/2L(Vgs − VT )2
for Vds > Vgs − VT (1)
where Ids is the drain current in log scale, Vgs is the gate voltage, VT is the threshold voltage, W and L are the channel width and length, and Ci is the capacitance per unit area of the gate insulator. The field-effect mobility (at Vgs = −40 V) and threshold voltage (Vth ) were calculated by fitting a straight line to the plot of the square root of Ids versus Vgs curve (figures 8(c) and (d)). The field-effect mobilities are 0.076 and 0.019 cm2 V−1 s−1 and the threshold voltages are −0.7 and 5 V for TIPS pentacene–graphene and TIPS pentacene OTFTs, respectively. The presence of graphene strongly influences the threshold voltage shift, which moves towards −0.7 V for TIPS pentacene–graphene OTFTs. A threshold voltage shift towards negative voltage is preferable in logic circuits. In the 6
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Figure 6. HRTEM images of graphene flakes dispersed in tetralin solvent with different concentrations: (a)–(c) 5 mg/5 ml, (d)–(e) 10 mg/5 ml, (f)–(j) 15 mg/5 ml, and (k)–(m) 20 mg/5 ml. High resolution imaging of the clear folded edge indicating single layer to few layer graphene film.
observed that the subthreshold slope was enhanced from 10 to 6 V/dec for TIPS pentacene–graphene-based OTFTs. These improvements may be due to a reduction in the interface trap densities suggesting that the scattering of carriers by trapped charges at the PVP/TIPS pentacene–graphene interface is limited. Recently, James et al presented an improvement in saturation mobility of 3.27 times over pristine devices [20]. From the plot in the logarithmic scale, the on–off ratios weres estimated to be 104 and 103 for the two types of transistors. The electrical properties of the TIPS pentacene–graphene TFTs and typical TIPS pentacene on ITO/glass substrate were characterized by measuring the output and transfer characteristics of the devices, as illustrated in figures 9(a)–(d), respectively. Clear pinch-off and ideal saturation behavior with clear gate-voltage dependence can be observed. The field-effect mobility was calculated from measurements taken from an average of ten devices. The measured TIPS pentacene– graphene devices have significantly higher field-effect mobility ( ∼ 0.04 cm2 V−1 s−1 ). The field-effect mobility of TFTs using TIPS pentacene is 0.0096 cm2 V−1 s−1 (figure 9(d)), which is much lower than the mobilities of TFTs fabricated with a TIPS pentacene–graphene semiconductor layer. The two devices exhibit on–off ratios of 104 and 103 , and threshold voltage of approximately ∼−0.2 and 6 V, respectively. The measured average mobilities of TIPS pentacene–graphene OTFTs fabricated on plastic and ITO substrates are plotted in figure 10, as a function of graphene flake concentration. The mobility increases as the concentration of graphene flakes increases from 0 mg to 2, 5, 10, 15 and reaches a
Figure 7. Transmission characteristic curves of graphene flakes dispersed in tetralin solvent. The concentration was (1) 2 mg/5 ml, (2) 5 mg/5 ml, (3) 10 mg/5 ml, (4) 15 mg/5 ml, (5) 20 mg/5 ml, (6) 22 mg/5 ml, and (7) 25 mg/5 ml.
case of TIPS pentacene, the threshold voltage shifts towards the positive due to the generation of negative ions in the interface between the pentacene and the cross-linked PVP gate insulator. This negative ion generation is caused by attraction of electrons by the hydroxyl radical at the positive polar end. Therefore, the insertion of graphene flakes inside TIPS pentacene reduces the generation of negative ions in the interface between the pentacene and the cross-linked PVP insulator. We 7
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Figure 8. Output characteristic curves of (a) TIPS pentacene–graphene hybrid OTFTs and (b) TIPS pentacene OTFTs. The gate voltage was varied from 10 to −60 V in steps of −5 V. The transfer curves of (c) TIPS pentacene–graphene hybrid OTFTs and (d) TIPS pentacene OTFTs. This device was fabricated on PVP/Al/plastic substrate. The gate leakage current (Ig , A) versus gate–source voltage (Vgs ) is plotted in figure 8(d). The threshold voltage (VT ) of TIPS pentacene–graphene hybrid OTFTs is −0.7 V lower than that of the pure TIPS pentacene OTFTs (5 V) indicated by the black arrow.
Figure 9. Output characteristic curves of (a) TIPS pentacene–graphene hybrid OTFTs and (b) TIPS pentacene OTFTs. The gate voltage was varied from 10 to −60 V in steps of −10 V. Transfer curves of (c) TIPS pentacene–graphene hybrid OTFTs and (d) TIPS pentacene OTFTs. This device was fabricated on PVP/ITO/glass substrate. The threshold voltage of TIPS pentacene–graphene hybrid OTFTs is −0.2 V lower than that of pure TIPS pentacene OTFTs (6 V).
maximum at 20 mg in 5 ml of tetralin solvent. However, the average mobility reduces to six times lower than the highest mobility value when the concentration of graphene flakes is increased further to 20 mg/5 ml, as clearly shown in figure 10. The high density graphene flakes could form a disordered
film, which is supposed to exceed the percolation threshold of graphene flakes. These results indicate that the polymeric dielectric has better interface properties with plastic substrate than with ITO/glass substrate. The saturation field-effect mobility and on–off ratio of the TIPS pentacene–graphene 8
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Table 1. Performance of recently published PVP/SiO2 -related TIPS pentacene-based transistors.
Semiconductor layer
Insulator thickness (nm)
Device dimension W/L (µm)
On/off
µ (cm2 V−1 s−1 ) 0.12 0.3 0.05 0.1
0
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Blending effect of 6,13-bis(triisopropylsilylethynyl) pentacene–graphene composite layers for flexible thin film transistors with a polymer gate dielectric
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 085201 (http://iopscience.iop.org/0957-4484/25/8/085201) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 155.69.4.4 This content was downloaded on 21/05/2015 at 05:52
Please note that terms and conditions apply.
Nanotechnology Nanotechnology 25 (2014) 085201 (11pp)
doi:10.1088/0957-4484/25/8/085201
Blending effect of 6,13-bis (triisopropylsilylethynyl) pentacene–graphene composite layers for flexible thin film transistors with a polymer gate dielectric Sarbani Basu, Feri Adriyanto and Yeong-Her Wang Institute of Microelectronics, Department of Electrical Engineering, Advanced Optoelectronic Technology Center, National Cheng-Kung University, Tainan, 701, Taiwan E-mail: [email protected] Received 12 April 2013, revised 11 June 2013 Published 4 February 2014
Abstract
Solution processible poly(4-vinylphenol) is employed as a transistor dielectric material for low cost processing on flexible substrates at low temperatures. A 6,13-bis (triisopropylsilylethynyl) (TIPS) pentacene–graphene hybrid semiconductor is drop cast to fabricate bottom-gate and bottom-contact field-effect transistor devices on flexible and glass substrates under an ambient air environment. A few layers of graphene flakes increase the area in the conduction channel, and form bridge connections between the crystalline regions of the semiconductor layer which can change the surface morphology of TIPS pentacene films. The TIPS pentacene–graphene hybrid semiconductor-based organic thin film transistors (OTFTs) cross-linked with a poly(4-vinylphenol) gate dielectric exhibit an effective field-effect mobility of 0.076 cm2 V−1 s−1 and a threshold voltage of −0.7 V at Vgs = −40 V. By contrast, typical TIPS pentacene shows four times lower mobility of 0.019 cm2 V−1 s−1 and a threshold voltage of 5 V. The graphene/TIPS pentacene hybrids presented in this paper can enhance the electrical characteristics of OTFTs due to their high crystallinity, uniform large-grain distribution, and effective reduction of crystal misorientation of the organic semiconductor layer, as confirmed by x-ray diffraction spectroscopy, atomic force microscopy, and optical microscopy studies. (Some figures may appear in colour only in the online journal)
1. Introduction
large-area flexible devices due to their lower solubility in common organic solvents. In general, solution-processed OTFTs have higher on–off ratios but suffer from poor field-effect mobility, while a carbon-based material such as graphene or carbon nanotubes demonstrates very high mobility but suffers from low on/off ratio [3, 4]. The density of CNTs has a higher threshold for the formation of a percolation network. Therefore, such materials are limited by their high cost and process complexity. Thus, our aim is to make a semiconductor composite layer with an organic semiconductor and graphene flakes to improve the field-effect mobility as well as the
In recent years, solution-processed organic semiconductors have offered potential applications for display drivers, radio frequency identification tags, e-paper, and chemical sensors due to their low cost, low temperature, large-area manufacture, and flexibility [1–4]. Pentacene is one of the most promising and widely investigated organic semiconductors for organic thin film transistors (OTFTs) as an active layer. However, the fabrication of pentacene thin films requires a vacuum deposition process, which is not suitable for the fabrication of 0957-4484/14/085201+11$33.00
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pentacene semiconductor with inorganic and polymeric gate dielectrics was thoroughly discussed in recently published research articles [15, 16]. In this study, we investigated the effect of graphene, instead of polymer additives, on OTFT performance. The addition of graphene flakes does not disrupt the 5–5 stacking of the TIPS pentacene molecule. Two factors are essential when graphene is dispersed in organic solvents: polydispersity and flake size. The first is related to the graphene flakes in the solvent, i.e. monolayer, bilayers, multilayers. Following the centrifugation method, removal of the thicker graphene flakes could enhance the transmittance and resistance properties. It improves the uniformity of the molecular morphology and increases the semiconductor layer coverage within the devices and reduces the variation in molecular orientation. Graphene is a two-dimensional (2D) graphite film with monoatomic layer thickness, unique transport properties, and high conductivity. It is not easy to enhance the mobility without degrading the on–off ratio in solution-processed OTFTs. Therefore, it is expected that the incorporation of a few layers of graphene flakes may modify TIPS pentacene crystallization and increase the area in the conduction channel of OTFTs without lowering the on–off ratio. We report Raman spectroscopy to characterize the local molecular order, orientation and coupling energy (ω1 ) between two molecules of TIPS pentacene–graphene composite films. The grain size distribution, crystallinity and surface morphology also have been studied through XRD, optical microscopy, and AFM measurements. The dielectric properties of the metal/insulating layer/metal structure (MIM) were investigated with an HP 4284 C–V plotter. The electrical characteristics of the devices were analyzed with a semiconductor parameter analyzer (Agilent B1500A) in ambient air environment. This is the first example of the fabrication and characterization of graphene/TIPS pentacene OTFTs. Overall, this attempt is a general route to enhance the performance of semiconductor graphene-based composite OTFTs.
on–off ratio. Improvement of TIPS pentacene–graphene-based OTFTs was achieved after controlling and optimizing the grain size, crystallinity and surface morphology. The small molecule of 6,13-bis(triisopropylsilylethynyl) (TIPS) pentacene is an excellent candidate for the p-type OTFT active layer due to its high mobility, air stability, and good solubility; moreover, its face-to-face stacking in crystalline form is highly beneficial for 5-orbital overlap. Polymeric dielectrics are ideal because of their diverse properties, favorable film-forming characteristics, and tunable surface chemistry for the control of device-critical interfacial trap state densities [5–7]. However, the OTFTs with TIPS pentacene reported thus far have been fabricated using inorganic gate dielectrics on highly conductive silicon substrates, thereby ruling out their use in solution-processing techniques because of the high cost entailed and their inapplicability in largearea flexible devices. On the other hand, Shin et al have demonstrated that drop-cast TIPS pentacene thin film transistors (TFTs) (bottom-gate indium tin oxide electrode) with poly(4-vinylphenol) (PVP) gate dielectric exhibit a field-effect mobility of 0.002 cm2 V−1 s−1 on a flexible substrate [8]. To date, many efforts have been made to increase the mobility of TIPS pentacene devices. However, the anisotropic nature of the films does not support film uniformity and crystal misorientations have occurred. Polystyrene (PS) and PVP with small-molecule TIPS pentacene blends were recently reported by Kim et al [9]. The mobilities of self-organized PSand PVP-blended OTFTs (10 µm gate length) are 0.85 and 0.14 cm2 V−1 s−1 , respectively. Compared with PS-blended OTFTs, the performance degradation of threshold voltage, mobility, and hysteresis of PVP-blended OTFTs is due to the presence of hydroxyl groups in PVP, which introduce hole trapping at the oxide–semiconductor interfaces. Sim et al conducted research on solution-processable OTFTs (L = 50 µm) with the polymide gate dielectrics 1,2,3,4-butanetetracarboxylic dianhydride and 4,4-diaminodiphyeyl methane (BTDA–DADM) and PVP. The active layer was prepared by blending a polymeric binder, poly(α-methyl styrene) (0.1 wt%), and TIPS pentacene (0.1 wt%) in a high solvent (odichlorobenzene) [10]. Better performance of the pαMS/TIPS pentacene with polymide gate dielectric BTDA–DADM (µ ∼ 0.15 cm2 V−1 s−1 , Vth ∼ −2.7 V, on–off ratio ∼ 106 ) was observed over the cross-linked PVP gate dielectric (µ ∼ 0.001 cm2 V−1 s−1 , Vth ∼ −2.4 V, on–off ratio ∼ 104 ). Thus, the polymer gate dielectric chain dynamics affect the organic semiconductor growth and the polymer gate dielectric surface viscoelasticity modulates the OTFT’s performance, which can be substantially enhanced by manipulating the semiconductor/dielectric interfacial properties by optimizing the gate dielectric [11–13]. He et al demonstrated that the enhanced performance consistency of 10 wt% SiO2 nanoparticle–TIPS pentacene composite semiconductor OTFTs (50 µm gate length) with a mobility of 0.15 cm2 V−1 s−1 , which is three times higher than that of pure TIPS pentacene OTFTs (µ ∼ 0.05 cm2 V−1 s−1 ) [14]. The incorporation of SiO2 nanoparticles enables the modification of TIPS pentacene crystallization by nanoparticle aggregation at TIPS pentacene grain boundaries. A more detailed solution-processed TIPS
2. Fabrication and device structure
Liquid exfoliation of graphene is a cost-effective method to prepare graphene dispersions. In this research, graphene oxide (GO) was prepared from natural graphite powder (300 mesh, Alfa Aesar) by a modified Hummers method [17]. Graphene nanopowder (3 nm flake size, black powder) purchased from Graphene Supermarket (USA) was dispersed in tetralin solvent. The dispersion process followed Tour and Barron’s method [18]. Tetralin solvent with high boiling temperature was found to be more effective for achieving distinct phase segregation/crystallization of TIPS pentacene. Up to 5 mg of graphene flakes in 1 ml of tetralin solvent was vigorously shaken and subjected to ultrasonication for 1 h. The thick graphene flakes were settled in the solution by leaving them to stand for over 30 min. The top half portion of the graphene solution (less concentration of graphene dispersion) that was collected and dispersed was further centrifuged at 4500 rpm for 30 min to remove the precipitates. The final solution contained single-layered and multilayered graphene dispersion. 2
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Figure 1. Schematic illustration of the device fabrication steps and the cross-sectional schematics of the TIPS pentacene–graphene-based TFTs fabricated on plastic and glass substrates. The chemical structures of TIPS pentacene and the PVP gate dielectric are also shown. The thickness of the cross-linked PVP layer was 650 nm, as measured by the alpha step.
The concentration of the graphene dispersion was approximately 100 times lower than that of the starting concentration. Bottom-gate and bottom-contact configurations were used for the OTFTs. Al and ITO served as the gate metals for plastic and glass substrates, respectively. A 650 nm thick cross-linked PVP (Mw ∼ 25 000) layer was spin coated (5000 rpm) on patterned gate metal used as the polymeric gate dielectric (specific capacitance = 4.9 nF cm−2 ). The concentration of the cross-linking agent poly melamine-co-formaldehyde (PMF, Mw = 511 g mol−1 ) was fixed at 4 wt%. The PVP was fixed at 10 wt%. Propylene glycol monomethyl ether acetate (PGMEA ∼ 1 ml) was used as the solvent. The curing temperature was maintained at 175 ◦ C for 1 h on a hot plate at room temperature. Au source/drain electrodes were deposited by thermal evaporation without any adhesion layer. Selfassembled monolayers (SAMs) of pentafluorobenzenethiol (PFBT) and octyltrichlorosilane (OTS-8) formed on the Au S/D electrodes and gate dielectric, respectively. To reduce the injection barrier between the organic semiconductor and the source/drain electrodes, the Au electrodes were treated with 10 mM PFBT, rinsed with anhydrous ethanol, and then dried with N2 . Furthermore, the OTS-8 self-assembled monolayer was deposited on the channel region by immersion into a 1 mM solution of OTS-8 in toluene at 60 ◦ C for 15 min. Subsequent rinsing with toluene and ethanol and then blow-drying with nitrogen were performed before use. The pure TIPS pentacene and graphene/TIPS pentacene hybrid semiconductors were then drop cast in the active region. Device fabrication, measurements, solution preparation, and drying of the semiconductor layer were performed in a clean room either in ambient air or in solvent-rich ambient atmosphere. Figure 1 shows the complete OTFT fabrication steps, a cross-sectional diagram of the bottom-gate and bottom-contact pentacene-based TFT fabricated on plastic as well as glass
substrates and the molecular structure of TIPS pentacene and PVP films. Figures 2(a) and (b) indicate the optical microscopy images of the TIPS pentacene–graphene flake hybrid semiconductor layer drop cast from tetralin solution on the channel and source–drain region for both types of OTFTs fabricated on plastic and glass substrates. The surface area of the graphene nanoflakes used is 510 m2 g−1 , containing carbon ∼97%, hydrogen ∼1%, and oxygen ∼2%. The average flake thickness and particle lateral sizes are 1 nm and 10 µm, respectively, as shown from the SEM study in figure 2(c). The surface morphology of the graphene sheet was confirmed as hexagonal shape. Figure 2(d) shows the SEM images of the graphene flakes with a scale bar of 100 nm. 3. Results and discussion
Figure 3 shows the grazing incidence x-ray diffraction results and the corresponding optical images from the TIPS pentacene–graphene composite and typical TIPS pentacene thin films drop cast from a 1 wt% tetralin solution on plastic and ITO-coated glass substrates, respectively. The crystallinity of the film is strongly dependent on the solvent. The XRD curve exhibits strong (001)-type reflections and other preferential orientations in the (002) and (003) directions, which match well with the same diffraction peaks of bulk crystal TIPS pentacene. The strong and sharp peaks are observed at 5.4◦ , 10.7◦ , and 16.1◦ for n = 1, 2, and 3, respectively. The average interlayer spacing between the 001 planes of the drop-cast TIPS pentacene is 16.8 Å, calculated using the well-known Bragg diffraction formula as follows: 2d sin θ = nλ, where d, θ, n, and λ represent the interlayer spacing, angle of the diffracted wave, wavelength of the x-ray used, and order of the diffracted beam, respectively. This value is consistent 3
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Figure 2. The graphene dry nanopowder directly purchased from Graphene Supermarket. Optical microscope images of the TIPS pentacene–graphene flake hybrid semiconductor layer drop cast by using tetralin solution on the channel and source–drain region. (a) OTFTs configured with bottom-gate (aluminum) and top-contact (gold) on flexible substrates and (b) OTFTs configured with bottom-gate indium tin oxide and top-contact gold on glass substrates. (c) SEM images of graphene flakes with size ∼3 nm, which were dispersed in tetralin solution with TIPS pentacene and used as a hybrid semiconductor layer for OTFT application with a scale bar of 5 µm. (d) Higher magnification of image (c) with a scale bar of 100 nm.
which are hydrophobic in nature and can easily adhere to dielectric PVP (low surface energy of 34 mJ m−2 ) surfaces. The smooth surface of the PVP layer (roughness ∼ 1.2 nm) is suitable for maintaining good interface quality between the TIPS pentacene and the dielectric layer. The drop-cast films have stronger molecular ordering due to longer film formation time. The solvent tetralin which has a higher boiling point enables slower solvent evaporation and gives the semiconductor material adequate time to self-assemble, which is beneficial to the highly ordered polycrystalline structure. The crystalline sizes (t) of the TIPS pentacene–graphene and the sole TIPS pentacene layers were calculated using the Scherrer formula (t = 0.9λ/β cos θ ), where λ denotes x-ray wavelength, β is the FWHM for the diffraction peaks, and θ , which is half of the diffraction angle, has values of 27.7 and 18 nm, respectively. The crystallite size of the TIPS pentacene–graphene composite film increases to 27.7 nm, which may be attributed to the improved crystallinity. The XRD curves and corresponding optical microscopy image show the stem-like and needleshaped morphologies of the TIPS pentacene, proving the good crystalline property obtained from the drop-casting method. However, the crystal formation of typical TIPS pentacene shows large gaps between the TIPS pentacene stems compared with the composite one. An increase in the concentration of the graphene flakes compared with TIPS pentacene facilitates the aggregation of flakes, thus reducing device mobility. Figures 4(a)–(c) show the Raman spectra of a TIPS pentacene (single droplet) thin film on plastic substrate (sample T1), TIPS pentacene with graphene composite films on plastic substrate (sample T2), and TIPS pentacene with graphene
Figure 3. Grazing incidence x-ray diffraction of TIPS pentacene–graphene hybrid and typical TIPS pentacene films on PVP/Al/plastic substrate and PVP/ITO/glass substrate. The films exhibited prominent (001)-type reflections. The pictures on the left indicate the optical microscopic images of the corresponding films. Only the TIPS pentacene/PVP/ITO glass substrate OTS treated for 20 min showed some black spots.
with previously reported data for TIPS pentacene having a c-axis of well-ordered triclinic structure [5]. The (001) plane of TIPS pentacene is composed of tri-isopropylsilyl groups 4
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Using the polarizations of the incident and scattered light either parallel or perpendicular to each other, we calculated the intermolecular coupling energy (ω1 ) that can be estimated by 1ω = ω12 /2ωo , where ωo is the frequency of the intermolecular band and 1ω is the splitting frequency based on the coupled oscillator mode. The calculated coupling energy (ω1 ) of 5.36 meV between two molecules of the TIPS pentacene–graphene composite (sample T2) semiconductors is illustrated in figure 4(b); it is higher than the 4.46 eV coupling energy of two pentacene molecules on PVP dielectric previously reported [23]. Raman spectroscopy is a powerful and nondestructive technique that can determine molecular packing conjugated backbone orientation and thin film uniformity (homogeneity) in small-molecule thin films, and can provide information on the reorganization energy (λ), which indicates the charge–phonon interaction inside the molecules. The hopping rate (W) of carriers in the organic semiconductors was estimated by the Marcus–Hush equation which is expressed as follows: W = V 2 / h[5 /(λkB T )1/2 ] exp (−λ/4kB T )αV 2 , where kB is the Boltzmann constant, V represents the intermolecular coupling (also called the transfer integral), and T is the absolute temperature. The calculated value of the hopping rate is 1010 s−1 . V and λ were calculated from the polarized Raman spectra and UV–vis spectra experiments, as shown in figures 4(b) and (c), respectively. The estimated V and λ are 5.36 meV and 0.271 eV for the TIPS pentacene–graphene hybrid semiconductor, whereas for the as-vacuum-deposited pentacene molecules on the PVP dielectric the values are 4.46 meV and 0.2 eV [23]. To further investigate the influence of the inserted graphene in the TIPS pentacene composite film, the surface morphologies of typical TIPS pentacene and TIPS pentacene–graphene hybrid films on PVP-coated ITO/glass and Al/plastic were examined. The root-mean-square (RMS) values for the 1 wt% TIPS pentacene without and with graphene are 0.926 and 0.566 nm on PVP/ITO/glass, as observed as in figures 5(a) and (c). Similarly, the TIPS pentacene and TIPS pentacene– graphene hybrid on PVP/Al/plastic substrate show RMS roughness values of 1.8 and 1.012 nm, respectively, as shown in figures 5(d) and (e). The TIPS pentacene film drop cast from tetralin has relatively low surface roughness compared with the vacuumdeposited films. We further measured the average grain size from the 2D AFM films. The grain sizes of the TIPS pentacene–graphene hybrid films are larger at 730 and 530 nm compared with those of typical TIPS pentacene with 330 and 320 nm for plastic and glass substrates, respectively. According to Gundlach et al the higher mobility of TIPS pentacene–graphene hybrid films can be partially attributed to the enlargement of grain size [24]. Figure 6 shows high resolution transmission microscopy images of graphene flakes with various concentrations in tetralin solvent and the deposits suspended over the lacey carbon film of the TEM grid. Structural characterization of the deposited flakes was performed by diffraction techniques. The flakes’ lateral sizes reduced from several µm to the nm level after the ultrasonication and centrifugation process (see the details of the dispersion process in section 2). With
Figure 4. (a) Raman spectra (λexc = 532 nm) of (a) TIPS
pentacene-based OTFTs on plastic substrate (sample T1), TIPS pentacene–graphene hybrid-based OTFTs on plastic substrate (sample T2) and TIPS pentacene–graphene hybrid-based OTFTs on glass substrate (sample T3). (b) Raman spectra of TIPS pentacene–graphene hybrid on PVP gate dielectric using (x, x) and (x, y) polarizers indicating the splitting frequency (1ω) in the range of 1150–1165 cm−1 . (c) UV–vis spectra of TIPS pentacene–graphene hybrid semiconductor layer. TIPS pentacene and TIPS pentacene–graphene composite thin films were drop cast on a PFBT-treated gold source–drain metal. All data are plotted after device fabrication.
on ITO substrate (sample T3), respectively. TIPS pentacene was drop cast on PFBT-treated gold metal. A Jobin Yvon LabRam HR spectrometer with a 532 nm solid state laser was used as the excitation light source. The spatial resolution of the beam spot was approximately 1 µm. To avoid thermal damage on the TIPS pentacene film, the laser light beam was defocused from the surface of the TIPS pentacene film. The bands at 1157.71 and 1191.53 cm−1 are related to the C–H intra-molecular vibration from the ends and sides of the pentacene backbone [19]. The bands at 1373.21 and 1577.85 cm−1 are assigned to the C–C ring stretch modes when the carbon atoms mainly vibrate along the short and long molecular axes, respectively. James et al [20] observed the maximum Raman scattering intensity for the 1373.21 cm−1 (short axis) mode and for the 1577.85 cm−1 (long axis) mode when the laser polarization was parallel to the short and long axes, respectively. Samples T1 and T2 on the plastic substrate further indicate no change in the peak intensity of the different vibration modes after the blending of graphene with TIPS pentacene, which indicates that the addition of graphene does not disrupt the 5–5 stacking of the TIPS molecules. All the bending modes described above are in good agreement with previously assigned modes in pentacene thin films [21, 22]. 5
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the concentration varied from 25 mg/5 ml to 2 mg/5 ml, which is comparable with the transmittance of ∼97% of CVD grown monolayer graphene film. We have observed that, independent of the concentration, our films have a flat optical transmission profile across the visible region. Due to the measurement limitations, we are presently unable to measure the transmittance in the near infrared region, however in the future we will verify it. Ideal transparent electrodes should have high transparency (>80%), therefore our results are potentially attractive for transparent conductors in display applications. Bottom-gate and bottom-contact OTFT configurations were considered throughout the experiment. The channel width and length were 1500 µm and 150 µm, respectively. All the fabricated devices were associated with PFBT-modified S–D and OTS-treated dielectric layers. The OTFTs fabricated on plastic substrate used aluminum gate metal and those on glass substrate used ITO gate metal. The leakage current and capacitance of the PVP insulator measured through the Al/PVP/Au metal–insulator–semiconductor structure were 10−9 A cm−2 at ±0.6 MV cm−1 and 4.9 nF cm−2 , respectively. Figures 8(a) and (b) show the current–voltage (Ids –Vds ) characteristics of the TIPS pentacene–graphene and typical TIPS pentacene OTFTs, respectively, at different gate–source voltages (Vgs ) from 10 to −60 V with a 650 nm thick PVP gate insulator fabricated on plastic substrate. A clear transition was observed from linear to saturation behavior. At a given Vgs , Ids initially increases linearly and then saturates due to a pinch-off in the accumulation layer located in the interface between the TIPS pentacene and the PVP. However, we can observe from figure 8(b) that ‘current crowding’ occurs at low Vds , which corresponds to the non-Ohmic behavior of the drain current (Ids ) in the linear region. The non-linearity of the output curves in the low-Vds region may result from the combined effect of both the contact barrier and the field-dependent mobility [25, 26]. Figures 8(c) and (d) show the transfer characteristics of the corresponding OTFTs. The devices demonstrate the characteristics of typical pchannel thin film transistors with three times larger saturation drain current of TIPS pentacene–graphene OTFTs compared with TIPS pentacene OTFTs at Vgs = −60 V. The field-effect mobility µsat of the devices was estimated from the saturation region with the following equation:
Figure 5. (a)–(c) AFM images of the semiconductor layer of untreated TIPS pentacene, OTS-treated PVP gate dielectric of TIPS pentacene, and OTS-treated TIPS pentacene–graphene hybrids on PVP/ITO-coated glass substrates. (d)–(e) AFM images of OTS-treated PVP gate dielectric of TIPS pentacene and TIPS pentacene–graphene hybrids on plastic substrate.
increasing concentration, the graphene flakes are stacked together, wrinkled and folded several times over themselves, as shown in figure 6(f). TEM study reveals a monolayer to a few layered structure. Figures 6(d), (g) and (h) show the presence of graphene lattice fringes and carbon nanotubes, respectively. The electron diffraction pattern acquired over the flake, shows a ring-like structure compatible with randomly stacked graphene. One of the most promising optoelectronic applications of graphene is probably its use as a transparent conducting film, e.g. for flexible electronics and solar cells. High conductivity, transparency and stability may allow graphene to challenge the existing ITO-based films. Moreover, the fragile nature of ITO limits its application in some conditions. ITO is also limited by the scarcity and thus increasing cost of indium. Figure 7 illustrates the transmittance curves of various concentrations of graphene dispersion in solvent of tetralin. The transmittance increased from 77 to 95% as
Ids = (Ci µsat W )/2L(Vgs − VT )2
for Vds > Vgs − VT (1)
where Ids is the drain current in log scale, Vgs is the gate voltage, VT is the threshold voltage, W and L are the channel width and length, and Ci is the capacitance per unit area of the gate insulator. The field-effect mobility (at Vgs = −40 V) and threshold voltage (Vth ) were calculated by fitting a straight line to the plot of the square root of Ids versus Vgs curve (figures 8(c) and (d)). The field-effect mobilities are 0.076 and 0.019 cm2 V−1 s−1 and the threshold voltages are −0.7 and 5 V for TIPS pentacene–graphene and TIPS pentacene OTFTs, respectively. The presence of graphene strongly influences the threshold voltage shift, which moves towards −0.7 V for TIPS pentacene–graphene OTFTs. A threshold voltage shift towards negative voltage is preferable in logic circuits. In the 6
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Figure 6. HRTEM images of graphene flakes dispersed in tetralin solvent with different concentrations: (a)–(c) 5 mg/5 ml, (d)–(e) 10 mg/5 ml, (f)–(j) 15 mg/5 ml, and (k)–(m) 20 mg/5 ml. High resolution imaging of the clear folded edge indicating single layer to few layer graphene film.
observed that the subthreshold slope was enhanced from 10 to 6 V/dec for TIPS pentacene–graphene-based OTFTs. These improvements may be due to a reduction in the interface trap densities suggesting that the scattering of carriers by trapped charges at the PVP/TIPS pentacene–graphene interface is limited. Recently, James et al presented an improvement in saturation mobility of 3.27 times over pristine devices [20]. From the plot in the logarithmic scale, the on–off ratios weres estimated to be 104 and 103 for the two types of transistors. The electrical properties of the TIPS pentacene–graphene TFTs and typical TIPS pentacene on ITO/glass substrate were characterized by measuring the output and transfer characteristics of the devices, as illustrated in figures 9(a)–(d), respectively. Clear pinch-off and ideal saturation behavior with clear gate-voltage dependence can be observed. The field-effect mobility was calculated from measurements taken from an average of ten devices. The measured TIPS pentacene– graphene devices have significantly higher field-effect mobility ( ∼ 0.04 cm2 V−1 s−1 ). The field-effect mobility of TFTs using TIPS pentacene is 0.0096 cm2 V−1 s−1 (figure 9(d)), which is much lower than the mobilities of TFTs fabricated with a TIPS pentacene–graphene semiconductor layer. The two devices exhibit on–off ratios of 104 and 103 , and threshold voltage of approximately ∼−0.2 and 6 V, respectively. The measured average mobilities of TIPS pentacene–graphene OTFTs fabricated on plastic and ITO substrates are plotted in figure 10, as a function of graphene flake concentration. The mobility increases as the concentration of graphene flakes increases from 0 mg to 2, 5, 10, 15 and reaches a
Figure 7. Transmission characteristic curves of graphene flakes dispersed in tetralin solvent. The concentration was (1) 2 mg/5 ml, (2) 5 mg/5 ml, (3) 10 mg/5 ml, (4) 15 mg/5 ml, (5) 20 mg/5 ml, (6) 22 mg/5 ml, and (7) 25 mg/5 ml.
case of TIPS pentacene, the threshold voltage shifts towards the positive due to the generation of negative ions in the interface between the pentacene and the cross-linked PVP gate insulator. This negative ion generation is caused by attraction of electrons by the hydroxyl radical at the positive polar end. Therefore, the insertion of graphene flakes inside TIPS pentacene reduces the generation of negative ions in the interface between the pentacene and the cross-linked PVP insulator. We 7
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Figure 8. Output characteristic curves of (a) TIPS pentacene–graphene hybrid OTFTs and (b) TIPS pentacene OTFTs. The gate voltage was varied from 10 to −60 V in steps of −5 V. The transfer curves of (c) TIPS pentacene–graphene hybrid OTFTs and (d) TIPS pentacene OTFTs. This device was fabricated on PVP/Al/plastic substrate. The gate leakage current (Ig , A) versus gate–source voltage (Vgs ) is plotted in figure 8(d). The threshold voltage (VT ) of TIPS pentacene–graphene hybrid OTFTs is −0.7 V lower than that of the pure TIPS pentacene OTFTs (5 V) indicated by the black arrow.
Figure 9. Output characteristic curves of (a) TIPS pentacene–graphene hybrid OTFTs and (b) TIPS pentacene OTFTs. The gate voltage was varied from 10 to −60 V in steps of −10 V. Transfer curves of (c) TIPS pentacene–graphene hybrid OTFTs and (d) TIPS pentacene OTFTs. This device was fabricated on PVP/ITO/glass substrate. The threshold voltage of TIPS pentacene–graphene hybrid OTFTs is −0.2 V lower than that of pure TIPS pentacene OTFTs (6 V).
maximum at 20 mg in 5 ml of tetralin solvent. However, the average mobility reduces to six times lower than the highest mobility value when the concentration of graphene flakes is increased further to 20 mg/5 ml, as clearly shown in figure 10. The high density graphene flakes could form a disordered
film, which is supposed to exceed the percolation threshold of graphene flakes. These results indicate that the polymeric dielectric has better interface properties with plastic substrate than with ITO/glass substrate. The saturation field-effect mobility and on–off ratio of the TIPS pentacene–graphene 8
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Table 1. Performance of recently published PVP/SiO2 -related TIPS pentacene-based transistors.
Semiconductor layer
Insulator thickness (nm)
Device dimension W/L (µm)
On/off
µ (cm2 V−1 s−1 ) 0.12 0.3 0.05 0.1
0
