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Effect of gate electrode conductivity on operation frequency of inkjet-printed complementary polymer ring oscillators
Thin Solid Films 546 (2013) 141–146
Contents lists available at ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Effect of gate electrode conductivity on operation frequency of inkjet-printed complementary polymer ring oscillators Hyun Han a, Paul S.K. Amegadze a, Jongwoon Park c, Kang-Jun Baeg b,⁎, Yong-Young Noh a,⁎ a
Department of Energy and Materials Engineering, Dongguk University, 26 Pil-dong, 3-ga, Jung-gu, Seoul 100-715, Republic of Korea Creative and Fundamental Research Division, Korea Electrotechnology Research Institute (KERI), 12 Bulmosan-ro 10beon-gil, Seongsan-gu, Changwon-si, Gyeongsangnam-do 642-120, Republic of Korea c School of Electrical, Electronics & Communication Engineering, Korea University of Technology and Education, Cheonan 330-708, Republic of Korea b
a r t i c l e
i n f o
Available online 1 May 2013 Keywords: Organic field effect transistors Inkjet printing Conjugated polymers Organic complementary circuits
a b s t r a c t We report the effect of the conductivity of the gate electrode on operation speeds in printed organic ring oscillators (RO). The highly conducting gate electrode leads to a superior oscillation frequency (as high as ~30 kHz) for the printed ROs. Above the optimum thickness of the gate electrodes (~30 nm), inkjet-printed p-type poly(3hexylthiophene) (P3HT) and n-type poly([N,N-9-bis(2-octyldodecyl)naphthalene-1,4,5,8-bis(dicarboximide)-2,6diyl]-alt-5,59-(2,29-bithiophene)) (P(NDI2OD-T2)) organic field-effect transistors showed reasonably high hole and electron mobilities of ~0.05 cm2 V−1 s−1 and ~0.25 cm2 V−1 s−1, respectively. Complementary inverters and ring oscillators based on these p- and n-type semiconductor transistors were constructed, where the inverters showed the inverting voltage, (Vinv) near the ideal switching points at 1/2 the drain voltage (VDD), high gain (~10), low static power consumptions, as well as high noise margin (~60% of 1/2VDD). Finally, printed P3HT complementary ring oscillators with a gate thickness over 30 nm exhibited the highest oscillation frequency (~30 kHz). © 2013 Elsevier B.V. All rights reserved.
1. Introduction In the past two decades, remarkable progress in organic electronics and optoelectronics has been achieved using π-conjugated organic semiconductors. The unique advantages of these organic materials, including mechanical flexibility, the fine tuning of their properties by versatile molecular design and chemical synthesis, as well as their straightforward solution processability, enable a variety of unconventional flexible and stretchable optoelectronic applications via simple and low-cost graphic-art printing processes [1]. In particular, solutionprocessed organic field-effect transistors (OFETs) are crucial components in a majority of promising printed electronic device applications, such as digital and analog circuits in radio frequency identification tags, drivers of flexible active-matrix displays, sensors, and non-volatile memory [2–5]. Importantly, the versatile applications of solutionprocessed OFETs are predominantly determined by the operation speed of the individual devices and integrated circuits. Although promising field-effect mobilities (μFET) of over 3 cm2 V−1 s−1 (for polymers) and 10 cm2 V−1 s −1 (for single crystals) have been achieved in stateof-the-art OFETs [6,7], most low-mobility organic semiconductors still remain to be employed in suitable applications. The electrical performance of OFETs is mainly determined by the intrinsic properties of the organic semiconductors as well as their thin film morphologies, and the interface properties between semiconductors ⁎ Corresponding authors. Tel.: +82 2 2260 4974; fax: +82 2 2268 8550. E-mail addresses: [email protected] (Y.-Y. Noh), [email protected] (K.-J. Baeg). 0040-6090/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2013.04.060
and charge injection electrodes or semiconductors and gate dielectrics [8]. In addition, the circuit configuration (unipolar or complementary), device architecture (coplanar or staggered), the dimensions of the active channel (such as the channel's width/length ratio (W/L)), and the overlap between the gate and source/drain (S/D) electrodes (Loverlap), might also critically affect the overall performance of the electronic circuits [9]. The transition frequency (fT) of the individual transistor is given by Eqs. (1)–(3): f T≈
gm μ ðV GS −V Th Þ ¼ 2 2πC G 2πL 2L overlap þ 3 L
g m ≈μC ox
W ðV GS −V Th Þ L
2 2 C G ≅2C overlap þ WLC ox ¼ 2WLoverlap C ox þ WLC ox 3 3
ð1Þ
ð2Þ ð3Þ
where Cox and VTh are the gate dielectric capacitance and threshold voltage, respectively. Furthermore, for more complex electronic circuitry, including complementary inverters and ring oscillators, electrical conductivities of the gate, S/D electrodes, and the interconnects that bridge the transistors become important parameters. Obviously, the resistive–capacitive delay hinders any further increase in the speed of state-of-the-art microelectronic integrated circuits, and it will also present a serious problem when the complexity and down-scaling of the feature sizes of the printed organic electronic devices and integrated
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circuits are increased [2]. Most conductive inks typically used in flexible and printed electronics applications, such as conducting polymers, metal pastes, and transparent conducting oxides, have a higher resistance than conventional bulk metal electrodes do. The low conductivities found in the printed conductive materials constituting the electrodes and interconnects may cause certain problems for the development of high-performance organic electronic devices and circuits based on graphic-art printing processes [10]. Therefore, the effects of electrode conductivity on the physical characteristics of the printed electronic devices and circuits should be studied. In this paper, we investigate the effects of the conductivity of the gate electrode and the interconnects on the operation speeds of printed organic ring oscillators. The sheet resistance of the gate electrodes was controlled by simply varying the thicknesses of thermally evaporated Au films. We found a strong correlation between the conductivity of the gate electrode and the operation speed of printed organic ring oscillators. Above the optimum thickness of the gate electrodes (~30 nm), the inkjet-printed p-type poly(3hexylthiophene) (P3HT) and n-type poly([N,N-9-bis(2-octyldodecyl) naphthalene- 1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,59-(2,29-bithiophene)) (P(NDI2OD-T2)) OFETs showed reasonably high hole and electron mobilities of ~0.05 cm2 V−1 s−1 and ~0.25 cm2 V−1 s−1, respectively. Complementary inverters and ring oscillators were composed based on these p- and n-type semiconductor transistors, where the inverters showed the inverting voltage (Vinv) near the ideal switching point at 1/2 the drain voltage (VDD), high gain (~10), low static power consumption, and high noise margin (~60% of 1/2VDD). Finally, printed P3HT:N2200 complementary ring oscillators with a gate thickness of over 30 nm exhibited the highest oscillation frequency (~30 kHz).
2.2. Complementary inverter and ring oscillator fabrication The described Au/Ni S/D electrodes on glass substrates were used for building complementary inverters and ring oscillators with varying W/L ratios. Each p-type P3HT (5 mg/ml in CB) and n-type P(NDI2OD-T2) (5 mg/ml in CB) was sequentially inkjet-printed onto Au/Ni patterned substrates in air. S/D electrode patterning, inkjet-printing, and thermal annealing were performed using the same procedure as used for the fabrication of OFETs. After spin-coating the PMMA gate dielectric, pure solvent (CB) was inkjet-printed onto the PMMA-coated devices to make via holes. Top-gated and inkjet-printed complementary polymer ring oscillators were completed by the formation of gate electrodes via thermal evaporation of Au with various film thicknesses (~ 1–50 nm) using a metal shadow mask. 2.3. Characterization The electrical and static characteristics of OFETs and complementary inverters were measured using a semiconductor parameter analyzer (HP-4156A) in a N2-filled glove box. The μFET and the VTh were calculated at the saturation region (Vd = ± 60 V) using gradual channel approximations [11]. The dynamic characteristics of the ring oscillators were measured using a DC voltage power supplier with a built-in oscilloscope system. Scanning electron microscope (SEM) images of the thermally deposited Au electrodes with different film thicknesses were obtained using an S-4800 SEM (Hitachi Co. Ltd.) while the sheet resistance of the Au gate electrodes was obtained using a four-point probe measurement (AiT Co. Ltd., CMTSR2000N).
2. Experimental details 3. Results and discussion 2.1. Field-effect transistor fabrication Corning Eagle 2000 glass substrates were cleaned sequentially in an ultrasonic bath with deionized water, acetone, and isopropanol (10 min for each cycle). The gold/nickel (Au/Ni) (12 nm/3 nm thick) patterns for the S/D electrodes (Ni being the adhesion layer) were fabricated using a conventional lift-off photolithography technique. The p-type and n-type polymer semiconductors prepared from regioregular poly(3-hexylthiophene) (rr-P3HT, Rieke Metals, Inc.) and poly([N, N-9-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,59-(2,29-bithiophene)) (P(NDI2OD-T2), Polyera Corporation), respectively, were used as received. P3HT and P(NDI2OD-T2) were dissolved in anhydrous chlorobenzene (CB) to obtain solutions of ~5 mg/ml. After heating overnight at 80 °C on a hotplate, the solutions were filtered through a 0.2 μm polytetrafluoroethylene (PTFE) syringe filter before use. The semiconductor solutions were spin-coated or inkjet-printed onto Au/Ni patterned substrates in air, while maintaining the substrates at room temperature using a custom-built research inkjet printer (UJ100MF, Unijet). A piezoelectric-type drop-on-demand dispensing head (Microfab Technologies) with a 50 μm orifice diameter was used at an operating frequency of 500 Hz. Inkjet-printed P3HT and P(NDI2OD-T2) films were annealed at 150 °C for 30 min in a N2-filled glove box to remove residual solvent and to induce an optimized microcrystalline morphology of the films. For the polymer gate dielectric layers, poly(methyl methacrylate) (PMMA, Aldrich, MW = 120 kD) was used without further purification. PMMA (80 mg ml−1) was dissolved in n-butyl acetate and the solution was filtered through a 0.2 μm PTFE syringe filter before being spin-coated at 2000 rpm for 1 min. After dielectric layer coating, the samples were finally annealed at 80 °C for 1–2 h in the same glove box to remove the residual solvent. Top-gated OFET devices were completed by forming gate electrodes (Al, 50 nm) on the PMMA-coated active regions via thermal evaporation with a metal shadow mask.
Fig. 1a shows the molecular structure of the P3HT and P(NDI2OD-T2) semiconductors as well as the configuration of the top-gate/bottomcontact (TG/BC) OFET employed in this study. The TG/BC staggered geometry typically enables high charge carrier mobilities for both holes and electrons by proper selection of the gate dielectrics and the fact that the orthogonal solvents are free from specific mobile charge trapping moieties such as hydroxyl groups on SiO2 [12]. Moreover, the relatively low contact resistance provided by the large charge injection area, the ambient stability improved by the overlaid gate dielectrics and gate electrodes, and most importantly, the easy formation of the gate electrodes on top of the gate dielectric layer, make this geometry perfectly suited for studying the effect of the conductivity of the gate electrode on the characteristics of printed organic electronic circuits. OFET devices were fabricated either by inkjet-printing or spin-coating methods, where the latter served for the construction of the reference device. Fig. 1b and c show CCD camera images of the P(NDI2OD-T2) ink droplet formation with delay times varying from 0 to 100 μs and the corresponding inkjet-printed active features of the Au S/D electrode patterned on a glass substrate, respectively. Here, we used optimized conditions to generate droplets with diameters of 30 to 33 μm and volumes of 15–19 pl at a drop velocity of 3.0–3.6 m s−1. The printed conjugated polymer films typically showed the strong coffee ring effect which leads to uneven surface morphology. We already reported the effect of the rough surface, which is mainly induced by ink-jetting processes, on the characteristics of P3HT OFETs [13]. The height difference from boundary to center parts in the ink-jet printed conjugated polymer feature was around 10–30 nm [13]. However, the over-coated PMMA dielectric layer in top gated FET geometry is relatively thick enough (~400 nm) to make flat surface (rms roughness b 0.1 nm) for thin gate electrode film. Therefore, we can exclude the rough surface effect of ink-jet printed active layers even for the very thin gate electrode ~10 nm.
H. Han et al. / Thin Solid Films 546 (2013) 141–146
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Fig. 1. (a) TG/BC OFET device configuration and molecular structures of the p-type (P3HT) and n-type (P(NDI2OD-T2)) organic semiconductors used as an active layer. CCD camera images showing, (b) the evolution of inkjet droplet formation, and (c) inkjet-printed active features of the P(NDI2OD-T2).
Fig. 2. (a, b) Output and, (c, d), transfer characteristics of the TG/BC OFET devices based on inkjet-printed, (a, c), p-type P3HT and, (b, d), n-type P(NDI2OD-T2) polymer semiconductors.
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Fig. 2 shows the transfer and output characteristics of the inkjetprinted p-type (P3HT) and n-type (P(NDI2OD-T2)) OFETs. The output plots for both p- and n-channel transistors (Fig. 2a and b) exhibited distinct linear and saturation regions with a negligible contact resistance (i.e., a linear shape in the lower Vd range). Fig. 2c and d show the transfer plots obtained at the saturation region (Vd = ±60 V). μFET and VTh were measured by the gradual channel approximation at Vd = ±60 V and are listed in Table 1. The average μFET and VTh values were 0.053 cm2 V−1 s−1 and −12.2 V, respectively, for P3HT OFETs and 0.248 cm2 V−1 s−1 and 8.3 V, respectively, for P(NDI2OD-T2) OFETs. Note that the inkjet-printed OFETs exhibited device performances that were quite similar to those of the same device configurations obtained by spin-coating (W/L and PMMA gate dielectrics), and they showed μFET,h and μFET,e of 0.08 cm 2 V − 1 s − 1 and 0.228 cm 2 V − 1 s − 1, respectively, at the same Vd value. The slight difference between inkjet-printed and spin-coated devices can be attributed mainly to the relatively rough top-surface of the inkjetprinted active films and the reduced leakage current pathways through effective patterning of the active area [10]. The relatively high off-current of the P(NDI2OD-T2) OFETs at Vd = 60 V (see Fig. 2d) mostly resulted from counter charge injection (i.e., holes) and transport, which in turn resulted from the enhanced capability of P(NDI2OD-T2) molecules to transport both electrons and holes [14]. The inkjet-printed P3HT and P(NDI2OD-T2) OFETs were placed on an organic complementary inverter and ring oscillators (ROs). Fig. 3a and c show the voltage transfer characteristics (VTCs) and corresponding gains of the inverters at supplied voltages (VDD) of 20 V to 100 V, respectively. For ideal complementary inverters, the VTC curves have to be narrow transition zones with inverting voltage (Vinv) at 1/2VDD for high gain during switching transients. These characteristic features are necessary to obtain significant outputs upon small changes in input voltage. The Vinv of the complementary inverter is reached when both the p- and n-channel OFETs are operating in the saturation region as follows:
V inv
V DD þ V pTh þ V nTh qffiffiffiffi ¼ 1 þ ββn
qffiffiffiffi βn βp
ð4Þ
p
where β equals (W/L) μFET Ci and is a design factor to adjust the ON currents of the p- and n-channel transistors, with the subscripts p and n denoting the semiconductor types. In this study, the inkjet-printed n-type transistors showed higher μFET and lower VTh than p-type transistors, and thus, β of each p- and n-type transistor should be modified to obtain balanced ON currents. Here, we simply changed the W/L ratios to Wp/Lp = 5 mm/20 μm and Wn/Ln = 1 mm/20 μm for the p-type and n-type OFETs, respectively. Therefore, the complementary inverter devices showed Vinv near 1/2 drain voltage (VDD) (see Fig. 3a and b) as well as gains as high as ~10. There are two characteristic features for the complementary circuits: (i) a negligible static power consumption and (ii) a high noise margin. The noise margins both at high logical levels (NMH) and low logical levels (NML) are represented in Fig. 3b, where the fabricated inverters exhibited relatively good noise margins (~60%) at the theoretical maximum values (1/2VDD). Moreover, as shown in Fig. 3d, in the case of both p- and n-type OFETs, at the Table 1 Fundamental device parameters of the OFETs fabricated by spin-coating and inkjet-printing methods. The field-effect mobilities (μFET) and threshold voltage (VTh) were calculated at the saturation region (Vd = ± 60 V) using the gradual channel approximation (W/L = 1 mm/20 μm and PMMA gate dielectric thickness = ~ 500 nm, Ci ~ 6.2 nF cm−2). Printing method
Organic semiconductor
μFET [cm2 V−1 s−1]
VTh [V]
Ion/Ioff
Spin coating
P3HT P(NDI2OD-T2) P3HT P(NDI2OD-T2)
0.080 0.228 0.053 0.248
−14.5 17.4 −12.2 8.3
104 104 105 105
Inkjet printing
Fig. 3. Complementary inverter characteristics of the inkjet-printed p- and n-type polymer OFET: (a) voltage transfer characteristics, (b) high-level (NMH) and low-level (NML) noise margins, (c) voltage gain, and (d) power consumption under various VDD conditions from 20 V to 100 V (Wp/Lp = 5 mm/20 μm, Wn/Ln = 1 mm/20 μm).
saturation region, power was only significantly dissipated in the transient states. To investigate the dependence of the operating speeds of the printed ROs on gate electrode conductivity, five-stage complementary polymer ring oscillators were fabricated by the inkjet-printing process (see Fig. 4a). Fig. 4b shows the dependency of the oscillation frequencies (fosc) of the ROs on VDD and W/L. fosc typically increases in proportion to VDD and L −2. In this study, we obtained oscillation frequencies as high as ~ 30 kHz using a short channel length of 2 μm (Wp/Wn = 10.5:5 mm) at relatively high VDD values of over 150 V. The high VDD required to effect high fosc was mostly attributed to the small gate dielectric capacitance (Ci = ~ 6.2 nF cm −2 for a PMMA dielectric of ~ 500 nm in thickness), and the operation bias could therefore be reduced sufficiently by using high-k and/or thin-gate dielectrics [12–15]. However, VDD = 150 V is almost the maximum voltage to obtain the highest fosc. Normally, the ring oscillators operated quite well over VDD = 50 V and this operating voltage value is consistent with previous reported values with same thickness PMMA films [10]. It was notably observed that the high frequencies of the printed ROs were achieved only at gate electrodes with sufficiently high thickness (>30-nm-thick Au electrodes), which enables low resistivity of the gate electrodes. To linearly control the gate conductivities using the same electrode materials, we used thermally deposited Au electrodes of varying thicknesses. As can be observed in Table 2, the electrical conductivity of the Au electrode significantly decreased from ~ 235.7 Ω/□ to ~ 0.008 Ω/□ at a deposition thickness of ~ 1.7 nm and ~ 30 nm, respectively. As can be seen in Fig. 4c, it was verified that the stage delay time (td) of the complementary inverters (which corresponds to td = 1/(2nfosc), where n is the number of RO
H. Han et al. / Thin Solid Films 546 (2013) 141–146
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Fig. 4. (a) Circuit and CCD camera image with circuit configuration of the fabricated five-stage ring oscillator. (b) Dependency of the oscillation frequencies of the ring oscillators on VDD and W/L. (c) Stage delay time vs. VDD of the ring oscillator at different thicknesses of the evaporated Au gate electrode.
stages) is strongly affected by the conductivities of the gate electrode. For an Au thickness between 1.7 and 5.1 nm, the ROs did not operate at all, but the fosc significantly increased from ~40 Hz to ~3.8 kHz at sufficiently high Au thicknesses of 9.6 nm and 30 nm under identical conditions (i.e., VDD = 120 V, PMMA thickness = ~ 500 nm, Wp/Lp = 5 mm/20 μm and Wn/Ln = 1 mm/20 μm). The drastic increase in fosc of the ROs, which depended on the thickness of the deposited Au electrode (and thereby on an increase in electrical conductivity), mainly resulted from a transition from non-continuous to continuous gate electrode films and this transition resulted in a change in the bulk conductivity of the gate metal. The scanning electron microscope images in Fig. 5 show that the high sheet resistance recorded at low deposition thickness of the gate electrode was a consequence of the non-continuous coverage of the film on the metal electrode. Such non-continuous films serve as grain boundaries that impede the flow of current, thereby resulting in a
Table 2 Electrical conductivities of the thermally deposited Au gate electrodes with varying thicknesses and operation frequencies of the complementary organic ring oscillators. Electrode
Thickness [nm]
Sheet resistance [Ω/□]
Operation frequency at VDD = 120 V
Au
1.7 3.4 5.1 9.6 12 30
NA 235.7 21.3 6.7 4.2 0.008
Failed Failed Failed 40.6 Hz 220 Hz 3.8 kHz
high sheet resistance. Apparently, the progressive reduction in sheet resistance with increasing metal electrode thickness can be attributed to an increase in electrode coverage and diminished grain boundaries. Note that further increases in electrode thickness (beyond 30 nm thick) are not expected to affect the sheet resistance because the bulk conductivity of the metal electrode was achieved above this optimal thickness. 4. Conclusion In this study, we investigated the effect of the electrical conductivity of the gate electrode on the operation frequency of printed complementary integrated circuits. The sheet resistance of the gate electrode was readily controlled by using different deposition thicknesses of the Au electrode. As expected, a highly conducting gate electrode leads to the highest oscillation frequency (as high as ~ 30 kHz) for the printed ROs. Above the optimum thickness of the gate electrodes, the inkjet-printed p-type (P3HT) and n-type (P(NDI2OD-T2)) OFETs showed high hole and electron mobilities of ~ 0.05 cm 2 V − 1 s − 1 and ~ 0.25 cm 2 V − 1 s − 1, respectively. Complementary inverters and ROs based on the p- and n-type semiconductor transistors were composed, where the inverter devices showed Vinv near the ideal switching points at 1/2VDD, a high gain (~ 10), low static state power consumptions, and high noise margin (~ 60% of 1/2VDD). The circuit speeds that depend on gate electrode conductivity are important, especially in printed electronics, because most conductive inks still have a relatively high sheet resistance. This indicated the necessity of developing highly conducting inks for application in printed electronics.
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Fig. 5. SEM images of the thermally evaporated Au electrode surfaces at various thicknesses: (a) 1.7 nm, (b) 3.3 nm, (c) 5 nm, (d) 9.6 nm, (e) 12 nm, and (f) 30 nm.
Acknowledgments This work was supported by the Industrial Strategic Technology Development Program (10041957, the Design and Development of Fiber-Based Flexible Display) funded by the Ministry of Knowledge Economy (MKE, Korea) and was supported by the Dongguk University Research Fund of 2013. References [1] [2] [3] [4]
A. Facchetti, Chem. Mater. 23 (2011) 733. W. Clemens, I. Fix, J. Ficker, A. Knobloch, A. Ullmann, J. Mater. Res. 19 (2004) 1963. G. Gelinck, P. Heremans, K. Nomoto, T.D. Anthopoulos, Adv. Mater. 22 (2010) 3778. M. Zirkl, A. Sawatdee, U. Helbig, M. Krause, G. Scheipl, E. Kraker, P.A. Ersman, D. Nilsson, D. Platt, P. Bodo, S. Bauer, G. Domann, B. Stadlober, Adv. Mater. 23 (2011) 2069.
[5] K.-J. Baeg, D. Khim, J. Kim, B.-D. Yang, M. Kang, S.-W. Jung, I.-K. You, D.-Y. Kim, Y.-Y. Noh, Adv. Funct. Mater. 22 (2012) 2915. [6] H.N. Tsao, D.M. Cho, I. Park, M.R. Hansen, A. Mavrinskiy, D.Y. Yoon, R. Graf, W. Pisula, H.W. Spiess, K. Müllen, J. Am. Chem. Soc. 133 (2011) 2605. [7] H. Minemawari, T. Yamada, H. Matsui, J. Tsutsumi, S. Haas, R. Chiba, R. Kumai, T. Hasegawa, Nature 475 (2011) 364. [8] H. Sirringhuas, Adv. Mater. 17 (2005) 2411. [9] Y.-Y. Noh, N. Zhao, M. Caironi, H. Sirringhaus, Nat. Nanotechnol. 2 (2007) 784. [10] K.-J. Baeg, D. Khim, D.-Y. Kim, S.-W. Jung, J.B. Koo, I.-K. You, H. Yan, A. Facchetti, Y.-Y. Noh, J. Polym. Sci. Part B: Polym. Phys. 49 (2011) 62. [11] S.M. Sze, Physics of Semiconductor Devices, Wiley, New-York NY, 1981. [12] Y.-Y. Noh, H. Sirringhaus, Org. Electron. 10 (2009) 174. [13] K.-J. Baeg, D. Khim, S.-W. Jung, J.B. Koo, I.-K. You, Y.-C. Nah, D.-Y. Kim, Y.-Y. Noh, ETRI J. 33 (2011) 887. [14] M.-H. Yoon, H. Yan, A. Facchetti, T.J. Marks, J. Am. Chem. Soc. 127 (2005) 10338. [15] H. Klauk, U. Zschieschang, J. Pflaum, M. Halik, Nature 445 (2007) 745.
Contents lists available at ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Effect of gate electrode conductivity on operation frequency of inkjet-printed complementary polymer ring oscillators Hyun Han a, Paul S.K. Amegadze a, Jongwoon Park c, Kang-Jun Baeg b,⁎, Yong-Young Noh a,⁎ a
Department of Energy and Materials Engineering, Dongguk University, 26 Pil-dong, 3-ga, Jung-gu, Seoul 100-715, Republic of Korea Creative and Fundamental Research Division, Korea Electrotechnology Research Institute (KERI), 12 Bulmosan-ro 10beon-gil, Seongsan-gu, Changwon-si, Gyeongsangnam-do 642-120, Republic of Korea c School of Electrical, Electronics & Communication Engineering, Korea University of Technology and Education, Cheonan 330-708, Republic of Korea b
a r t i c l e
i n f o
Available online 1 May 2013 Keywords: Organic field effect transistors Inkjet printing Conjugated polymers Organic complementary circuits
a b s t r a c t We report the effect of the conductivity of the gate electrode on operation speeds in printed organic ring oscillators (RO). The highly conducting gate electrode leads to a superior oscillation frequency (as high as ~30 kHz) for the printed ROs. Above the optimum thickness of the gate electrodes (~30 nm), inkjet-printed p-type poly(3hexylthiophene) (P3HT) and n-type poly([N,N-9-bis(2-octyldodecyl)naphthalene-1,4,5,8-bis(dicarboximide)-2,6diyl]-alt-5,59-(2,29-bithiophene)) (P(NDI2OD-T2)) organic field-effect transistors showed reasonably high hole and electron mobilities of ~0.05 cm2 V−1 s−1 and ~0.25 cm2 V−1 s−1, respectively. Complementary inverters and ring oscillators based on these p- and n-type semiconductor transistors were constructed, where the inverters showed the inverting voltage, (Vinv) near the ideal switching points at 1/2 the drain voltage (VDD), high gain (~10), low static power consumptions, as well as high noise margin (~60% of 1/2VDD). Finally, printed P3HT complementary ring oscillators with a gate thickness over 30 nm exhibited the highest oscillation frequency (~30 kHz). © 2013 Elsevier B.V. All rights reserved.
1. Introduction In the past two decades, remarkable progress in organic electronics and optoelectronics has been achieved using π-conjugated organic semiconductors. The unique advantages of these organic materials, including mechanical flexibility, the fine tuning of their properties by versatile molecular design and chemical synthesis, as well as their straightforward solution processability, enable a variety of unconventional flexible and stretchable optoelectronic applications via simple and low-cost graphic-art printing processes [1]. In particular, solutionprocessed organic field-effect transistors (OFETs) are crucial components in a majority of promising printed electronic device applications, such as digital and analog circuits in radio frequency identification tags, drivers of flexible active-matrix displays, sensors, and non-volatile memory [2–5]. Importantly, the versatile applications of solutionprocessed OFETs are predominantly determined by the operation speed of the individual devices and integrated circuits. Although promising field-effect mobilities (μFET) of over 3 cm2 V−1 s−1 (for polymers) and 10 cm2 V−1 s −1 (for single crystals) have been achieved in stateof-the-art OFETs [6,7], most low-mobility organic semiconductors still remain to be employed in suitable applications. The electrical performance of OFETs is mainly determined by the intrinsic properties of the organic semiconductors as well as their thin film morphologies, and the interface properties between semiconductors ⁎ Corresponding authors. Tel.: +82 2 2260 4974; fax: +82 2 2268 8550. E-mail addresses: [email protected] (Y.-Y. Noh), [email protected] (K.-J. Baeg). 0040-6090/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2013.04.060
and charge injection electrodes or semiconductors and gate dielectrics [8]. In addition, the circuit configuration (unipolar or complementary), device architecture (coplanar or staggered), the dimensions of the active channel (such as the channel's width/length ratio (W/L)), and the overlap between the gate and source/drain (S/D) electrodes (Loverlap), might also critically affect the overall performance of the electronic circuits [9]. The transition frequency (fT) of the individual transistor is given by Eqs. (1)–(3): f T≈
gm μ ðV GS −V Th Þ ¼ 2 2πC G 2πL 2L overlap þ 3 L
g m ≈μC ox
W ðV GS −V Th Þ L
2 2 C G ≅2C overlap þ WLC ox ¼ 2WLoverlap C ox þ WLC ox 3 3
ð1Þ
ð2Þ ð3Þ
where Cox and VTh are the gate dielectric capacitance and threshold voltage, respectively. Furthermore, for more complex electronic circuitry, including complementary inverters and ring oscillators, electrical conductivities of the gate, S/D electrodes, and the interconnects that bridge the transistors become important parameters. Obviously, the resistive–capacitive delay hinders any further increase in the speed of state-of-the-art microelectronic integrated circuits, and it will also present a serious problem when the complexity and down-scaling of the feature sizes of the printed organic electronic devices and integrated
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circuits are increased [2]. Most conductive inks typically used in flexible and printed electronics applications, such as conducting polymers, metal pastes, and transparent conducting oxides, have a higher resistance than conventional bulk metal electrodes do. The low conductivities found in the printed conductive materials constituting the electrodes and interconnects may cause certain problems for the development of high-performance organic electronic devices and circuits based on graphic-art printing processes [10]. Therefore, the effects of electrode conductivity on the physical characteristics of the printed electronic devices and circuits should be studied. In this paper, we investigate the effects of the conductivity of the gate electrode and the interconnects on the operation speeds of printed organic ring oscillators. The sheet resistance of the gate electrodes was controlled by simply varying the thicknesses of thermally evaporated Au films. We found a strong correlation between the conductivity of the gate electrode and the operation speed of printed organic ring oscillators. Above the optimum thickness of the gate electrodes (~30 nm), the inkjet-printed p-type poly(3hexylthiophene) (P3HT) and n-type poly([N,N-9-bis(2-octyldodecyl) naphthalene- 1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,59-(2,29-bithiophene)) (P(NDI2OD-T2)) OFETs showed reasonably high hole and electron mobilities of ~0.05 cm2 V−1 s−1 and ~0.25 cm2 V−1 s−1, respectively. Complementary inverters and ring oscillators were composed based on these p- and n-type semiconductor transistors, where the inverters showed the inverting voltage (Vinv) near the ideal switching point at 1/2 the drain voltage (VDD), high gain (~10), low static power consumption, and high noise margin (~60% of 1/2VDD). Finally, printed P3HT:N2200 complementary ring oscillators with a gate thickness of over 30 nm exhibited the highest oscillation frequency (~30 kHz).
2.2. Complementary inverter and ring oscillator fabrication The described Au/Ni S/D electrodes on glass substrates were used for building complementary inverters and ring oscillators with varying W/L ratios. Each p-type P3HT (5 mg/ml in CB) and n-type P(NDI2OD-T2) (5 mg/ml in CB) was sequentially inkjet-printed onto Au/Ni patterned substrates in air. S/D electrode patterning, inkjet-printing, and thermal annealing were performed using the same procedure as used for the fabrication of OFETs. After spin-coating the PMMA gate dielectric, pure solvent (CB) was inkjet-printed onto the PMMA-coated devices to make via holes. Top-gated and inkjet-printed complementary polymer ring oscillators were completed by the formation of gate electrodes via thermal evaporation of Au with various film thicknesses (~ 1–50 nm) using a metal shadow mask. 2.3. Characterization The electrical and static characteristics of OFETs and complementary inverters were measured using a semiconductor parameter analyzer (HP-4156A) in a N2-filled glove box. The μFET and the VTh were calculated at the saturation region (Vd = ± 60 V) using gradual channel approximations [11]. The dynamic characteristics of the ring oscillators were measured using a DC voltage power supplier with a built-in oscilloscope system. Scanning electron microscope (SEM) images of the thermally deposited Au electrodes with different film thicknesses were obtained using an S-4800 SEM (Hitachi Co. Ltd.) while the sheet resistance of the Au gate electrodes was obtained using a four-point probe measurement (AiT Co. Ltd., CMTSR2000N).
2. Experimental details 3. Results and discussion 2.1. Field-effect transistor fabrication Corning Eagle 2000 glass substrates were cleaned sequentially in an ultrasonic bath with deionized water, acetone, and isopropanol (10 min for each cycle). The gold/nickel (Au/Ni) (12 nm/3 nm thick) patterns for the S/D electrodes (Ni being the adhesion layer) were fabricated using a conventional lift-off photolithography technique. The p-type and n-type polymer semiconductors prepared from regioregular poly(3-hexylthiophene) (rr-P3HT, Rieke Metals, Inc.) and poly([N, N-9-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,59-(2,29-bithiophene)) (P(NDI2OD-T2), Polyera Corporation), respectively, were used as received. P3HT and P(NDI2OD-T2) were dissolved in anhydrous chlorobenzene (CB) to obtain solutions of ~5 mg/ml. After heating overnight at 80 °C on a hotplate, the solutions were filtered through a 0.2 μm polytetrafluoroethylene (PTFE) syringe filter before use. The semiconductor solutions were spin-coated or inkjet-printed onto Au/Ni patterned substrates in air, while maintaining the substrates at room temperature using a custom-built research inkjet printer (UJ100MF, Unijet). A piezoelectric-type drop-on-demand dispensing head (Microfab Technologies) with a 50 μm orifice diameter was used at an operating frequency of 500 Hz. Inkjet-printed P3HT and P(NDI2OD-T2) films were annealed at 150 °C for 30 min in a N2-filled glove box to remove residual solvent and to induce an optimized microcrystalline morphology of the films. For the polymer gate dielectric layers, poly(methyl methacrylate) (PMMA, Aldrich, MW = 120 kD) was used without further purification. PMMA (80 mg ml−1) was dissolved in n-butyl acetate and the solution was filtered through a 0.2 μm PTFE syringe filter before being spin-coated at 2000 rpm for 1 min. After dielectric layer coating, the samples were finally annealed at 80 °C for 1–2 h in the same glove box to remove the residual solvent. Top-gated OFET devices were completed by forming gate electrodes (Al, 50 nm) on the PMMA-coated active regions via thermal evaporation with a metal shadow mask.
Fig. 1a shows the molecular structure of the P3HT and P(NDI2OD-T2) semiconductors as well as the configuration of the top-gate/bottomcontact (TG/BC) OFET employed in this study. The TG/BC staggered geometry typically enables high charge carrier mobilities for both holes and electrons by proper selection of the gate dielectrics and the fact that the orthogonal solvents are free from specific mobile charge trapping moieties such as hydroxyl groups on SiO2 [12]. Moreover, the relatively low contact resistance provided by the large charge injection area, the ambient stability improved by the overlaid gate dielectrics and gate electrodes, and most importantly, the easy formation of the gate electrodes on top of the gate dielectric layer, make this geometry perfectly suited for studying the effect of the conductivity of the gate electrode on the characteristics of printed organic electronic circuits. OFET devices were fabricated either by inkjet-printing or spin-coating methods, where the latter served for the construction of the reference device. Fig. 1b and c show CCD camera images of the P(NDI2OD-T2) ink droplet formation with delay times varying from 0 to 100 μs and the corresponding inkjet-printed active features of the Au S/D electrode patterned on a glass substrate, respectively. Here, we used optimized conditions to generate droplets with diameters of 30 to 33 μm and volumes of 15–19 pl at a drop velocity of 3.0–3.6 m s−1. The printed conjugated polymer films typically showed the strong coffee ring effect which leads to uneven surface morphology. We already reported the effect of the rough surface, which is mainly induced by ink-jetting processes, on the characteristics of P3HT OFETs [13]. The height difference from boundary to center parts in the ink-jet printed conjugated polymer feature was around 10–30 nm [13]. However, the over-coated PMMA dielectric layer in top gated FET geometry is relatively thick enough (~400 nm) to make flat surface (rms roughness b 0.1 nm) for thin gate electrode film. Therefore, we can exclude the rough surface effect of ink-jet printed active layers even for the very thin gate electrode ~10 nm.
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Fig. 1. (a) TG/BC OFET device configuration and molecular structures of the p-type (P3HT) and n-type (P(NDI2OD-T2)) organic semiconductors used as an active layer. CCD camera images showing, (b) the evolution of inkjet droplet formation, and (c) inkjet-printed active features of the P(NDI2OD-T2).
Fig. 2. (a, b) Output and, (c, d), transfer characteristics of the TG/BC OFET devices based on inkjet-printed, (a, c), p-type P3HT and, (b, d), n-type P(NDI2OD-T2) polymer semiconductors.
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Fig. 2 shows the transfer and output characteristics of the inkjetprinted p-type (P3HT) and n-type (P(NDI2OD-T2)) OFETs. The output plots for both p- and n-channel transistors (Fig. 2a and b) exhibited distinct linear and saturation regions with a negligible contact resistance (i.e., a linear shape in the lower Vd range). Fig. 2c and d show the transfer plots obtained at the saturation region (Vd = ±60 V). μFET and VTh were measured by the gradual channel approximation at Vd = ±60 V and are listed in Table 1. The average μFET and VTh values were 0.053 cm2 V−1 s−1 and −12.2 V, respectively, for P3HT OFETs and 0.248 cm2 V−1 s−1 and 8.3 V, respectively, for P(NDI2OD-T2) OFETs. Note that the inkjet-printed OFETs exhibited device performances that were quite similar to those of the same device configurations obtained by spin-coating (W/L and PMMA gate dielectrics), and they showed μFET,h and μFET,e of 0.08 cm 2 V − 1 s − 1 and 0.228 cm 2 V − 1 s − 1, respectively, at the same Vd value. The slight difference between inkjet-printed and spin-coated devices can be attributed mainly to the relatively rough top-surface of the inkjetprinted active films and the reduced leakage current pathways through effective patterning of the active area [10]. The relatively high off-current of the P(NDI2OD-T2) OFETs at Vd = 60 V (see Fig. 2d) mostly resulted from counter charge injection (i.e., holes) and transport, which in turn resulted from the enhanced capability of P(NDI2OD-T2) molecules to transport both electrons and holes [14]. The inkjet-printed P3HT and P(NDI2OD-T2) OFETs were placed on an organic complementary inverter and ring oscillators (ROs). Fig. 3a and c show the voltage transfer characteristics (VTCs) and corresponding gains of the inverters at supplied voltages (VDD) of 20 V to 100 V, respectively. For ideal complementary inverters, the VTC curves have to be narrow transition zones with inverting voltage (Vinv) at 1/2VDD for high gain during switching transients. These characteristic features are necessary to obtain significant outputs upon small changes in input voltage. The Vinv of the complementary inverter is reached when both the p- and n-channel OFETs are operating in the saturation region as follows:
V inv
V DD þ V pTh þ V nTh qffiffiffiffi ¼ 1 þ ββn
qffiffiffiffi βn βp
ð4Þ
p
where β equals (W/L) μFET Ci and is a design factor to adjust the ON currents of the p- and n-channel transistors, with the subscripts p and n denoting the semiconductor types. In this study, the inkjet-printed n-type transistors showed higher μFET and lower VTh than p-type transistors, and thus, β of each p- and n-type transistor should be modified to obtain balanced ON currents. Here, we simply changed the W/L ratios to Wp/Lp = 5 mm/20 μm and Wn/Ln = 1 mm/20 μm for the p-type and n-type OFETs, respectively. Therefore, the complementary inverter devices showed Vinv near 1/2 drain voltage (VDD) (see Fig. 3a and b) as well as gains as high as ~10. There are two characteristic features for the complementary circuits: (i) a negligible static power consumption and (ii) a high noise margin. The noise margins both at high logical levels (NMH) and low logical levels (NML) are represented in Fig. 3b, where the fabricated inverters exhibited relatively good noise margins (~60%) at the theoretical maximum values (1/2VDD). Moreover, as shown in Fig. 3d, in the case of both p- and n-type OFETs, at the Table 1 Fundamental device parameters of the OFETs fabricated by spin-coating and inkjet-printing methods. The field-effect mobilities (μFET) and threshold voltage (VTh) were calculated at the saturation region (Vd = ± 60 V) using the gradual channel approximation (W/L = 1 mm/20 μm and PMMA gate dielectric thickness = ~ 500 nm, Ci ~ 6.2 nF cm−2). Printing method
Organic semiconductor
μFET [cm2 V−1 s−1]
VTh [V]
Ion/Ioff
Spin coating
P3HT P(NDI2OD-T2) P3HT P(NDI2OD-T2)
0.080 0.228 0.053 0.248
−14.5 17.4 −12.2 8.3
104 104 105 105
Inkjet printing
Fig. 3. Complementary inverter characteristics of the inkjet-printed p- and n-type polymer OFET: (a) voltage transfer characteristics, (b) high-level (NMH) and low-level (NML) noise margins, (c) voltage gain, and (d) power consumption under various VDD conditions from 20 V to 100 V (Wp/Lp = 5 mm/20 μm, Wn/Ln = 1 mm/20 μm).
saturation region, power was only significantly dissipated in the transient states. To investigate the dependence of the operating speeds of the printed ROs on gate electrode conductivity, five-stage complementary polymer ring oscillators were fabricated by the inkjet-printing process (see Fig. 4a). Fig. 4b shows the dependency of the oscillation frequencies (fosc) of the ROs on VDD and W/L. fosc typically increases in proportion to VDD and L −2. In this study, we obtained oscillation frequencies as high as ~ 30 kHz using a short channel length of 2 μm (Wp/Wn = 10.5:5 mm) at relatively high VDD values of over 150 V. The high VDD required to effect high fosc was mostly attributed to the small gate dielectric capacitance (Ci = ~ 6.2 nF cm −2 for a PMMA dielectric of ~ 500 nm in thickness), and the operation bias could therefore be reduced sufficiently by using high-k and/or thin-gate dielectrics [12–15]. However, VDD = 150 V is almost the maximum voltage to obtain the highest fosc. Normally, the ring oscillators operated quite well over VDD = 50 V and this operating voltage value is consistent with previous reported values with same thickness PMMA films [10]. It was notably observed that the high frequencies of the printed ROs were achieved only at gate electrodes with sufficiently high thickness (>30-nm-thick Au electrodes), which enables low resistivity of the gate electrodes. To linearly control the gate conductivities using the same electrode materials, we used thermally deposited Au electrodes of varying thicknesses. As can be observed in Table 2, the electrical conductivity of the Au electrode significantly decreased from ~ 235.7 Ω/□ to ~ 0.008 Ω/□ at a deposition thickness of ~ 1.7 nm and ~ 30 nm, respectively. As can be seen in Fig. 4c, it was verified that the stage delay time (td) of the complementary inverters (which corresponds to td = 1/(2nfosc), where n is the number of RO
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Fig. 4. (a) Circuit and CCD camera image with circuit configuration of the fabricated five-stage ring oscillator. (b) Dependency of the oscillation frequencies of the ring oscillators on VDD and W/L. (c) Stage delay time vs. VDD of the ring oscillator at different thicknesses of the evaporated Au gate electrode.
stages) is strongly affected by the conductivities of the gate electrode. For an Au thickness between 1.7 and 5.1 nm, the ROs did not operate at all, but the fosc significantly increased from ~40 Hz to ~3.8 kHz at sufficiently high Au thicknesses of 9.6 nm and 30 nm under identical conditions (i.e., VDD = 120 V, PMMA thickness = ~ 500 nm, Wp/Lp = 5 mm/20 μm and Wn/Ln = 1 mm/20 μm). The drastic increase in fosc of the ROs, which depended on the thickness of the deposited Au electrode (and thereby on an increase in electrical conductivity), mainly resulted from a transition from non-continuous to continuous gate electrode films and this transition resulted in a change in the bulk conductivity of the gate metal. The scanning electron microscope images in Fig. 5 show that the high sheet resistance recorded at low deposition thickness of the gate electrode was a consequence of the non-continuous coverage of the film on the metal electrode. Such non-continuous films serve as grain boundaries that impede the flow of current, thereby resulting in a
Table 2 Electrical conductivities of the thermally deposited Au gate electrodes with varying thicknesses and operation frequencies of the complementary organic ring oscillators. Electrode
Thickness [nm]
Sheet resistance [Ω/□]
Operation frequency at VDD = 120 V
Au
1.7 3.4 5.1 9.6 12 30
NA 235.7 21.3 6.7 4.2 0.008
Failed Failed Failed 40.6 Hz 220 Hz 3.8 kHz
high sheet resistance. Apparently, the progressive reduction in sheet resistance with increasing metal electrode thickness can be attributed to an increase in electrode coverage and diminished grain boundaries. Note that further increases in electrode thickness (beyond 30 nm thick) are not expected to affect the sheet resistance because the bulk conductivity of the metal electrode was achieved above this optimal thickness. 4. Conclusion In this study, we investigated the effect of the electrical conductivity of the gate electrode on the operation frequency of printed complementary integrated circuits. The sheet resistance of the gate electrode was readily controlled by using different deposition thicknesses of the Au electrode. As expected, a highly conducting gate electrode leads to the highest oscillation frequency (as high as ~ 30 kHz) for the printed ROs. Above the optimum thickness of the gate electrodes, the inkjet-printed p-type (P3HT) and n-type (P(NDI2OD-T2)) OFETs showed high hole and electron mobilities of ~ 0.05 cm 2 V − 1 s − 1 and ~ 0.25 cm 2 V − 1 s − 1, respectively. Complementary inverters and ROs based on the p- and n-type semiconductor transistors were composed, where the inverter devices showed Vinv near the ideal switching points at 1/2VDD, a high gain (~ 10), low static state power consumptions, and high noise margin (~ 60% of 1/2VDD). The circuit speeds that depend on gate electrode conductivity are important, especially in printed electronics, because most conductive inks still have a relatively high sheet resistance. This indicated the necessity of developing highly conducting inks for application in printed electronics.
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Fig. 5. SEM images of the thermally evaporated Au electrode surfaces at various thicknesses: (a) 1.7 nm, (b) 3.3 nm, (c) 5 nm, (d) 9.6 nm, (e) 12 nm, and (f) 30 nm.
Acknowledgments This work was supported by the Industrial Strategic Technology Development Program (10041957, the Design and Development of Fiber-Based Flexible Display) funded by the Ministry of Knowledge Economy (MKE, Korea) and was supported by the Dongguk University Research Fund of 2013. References [1] [2] [3] [4]
A. Facchetti, Chem. Mater. 23 (2011) 733. W. Clemens, I. Fix, J. Ficker, A. Knobloch, A. Ullmann, J. Mater. Res. 19 (2004) 1963. G. Gelinck, P. Heremans, K. Nomoto, T.D. Anthopoulos, Adv. Mater. 22 (2010) 3778. M. Zirkl, A. Sawatdee, U. Helbig, M. Krause, G. Scheipl, E. Kraker, P.A. Ersman, D. Nilsson, D. Platt, P. Bodo, S. Bauer, G. Domann, B. Stadlober, Adv. Mater. 23 (2011) 2069.
[5] K.-J. Baeg, D. Khim, J. Kim, B.-D. Yang, M. Kang, S.-W. Jung, I.-K. You, D.-Y. Kim, Y.-Y. Noh, Adv. Funct. Mater. 22 (2012) 2915. [6] H.N. Tsao, D.M. Cho, I. Park, M.R. Hansen, A. Mavrinskiy, D.Y. Yoon, R. Graf, W. Pisula, H.W. Spiess, K. Müllen, J. Am. Chem. Soc. 133 (2011) 2605. [7] H. Minemawari, T. Yamada, H. Matsui, J. Tsutsumi, S. Haas, R. Chiba, R. Kumai, T. Hasegawa, Nature 475 (2011) 364. [8] H. Sirringhuas, Adv. Mater. 17 (2005) 2411. [9] Y.-Y. Noh, N. Zhao, M. Caironi, H. Sirringhaus, Nat. Nanotechnol. 2 (2007) 784. [10] K.-J. Baeg, D. Khim, D.-Y. Kim, S.-W. Jung, J.B. Koo, I.-K. You, H. Yan, A. Facchetti, Y.-Y. Noh, J. Polym. Sci. Part B: Polym. Phys. 49 (2011) 62. [11] S.M. Sze, Physics of Semiconductor Devices, Wiley, New-York NY, 1981. [12] Y.-Y. Noh, H. Sirringhaus, Org. Electron. 10 (2009) 174. [13] K.-J. Baeg, D. Khim, S.-W. Jung, J.B. Koo, I.-K. You, Y.-C. Nah, D.-Y. Kim, Y.-Y. Noh, ETRI J. 33 (2011) 887. [14] M.-H. Yoon, H. Yan, A. Facchetti, T.J. Marks, J. Am. Chem. Soc. 127 (2005) 10338. [15] H. Klauk, U. Zschieschang, J. Pflaum, M. Halik, Nature 445 (2007) 745.