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Hanlin Wang, Cheng Cheng, Lei Zhang, Hongtao Liu, Yan Zhao, Yunlong Guo, Wenping Hu,* Gui Yu, and Yunqi Liu* Inkjet printing, in the past decade, has proved to be a powerful tool in the cost-effective ambient deposition of electrodes,[1–3] organic semiconductors,[4–6] and plastic dielectric layers.[7] With this patterning methodology and proper inks, high performance organic field-effect transistors,[8] organic inverters,[9] organic photodetectors, and polymer solar-cells have been realized.[10–12] However, it is highly desirable that the resolution of inkjet printing need further improvement, and in fact continuous efforts have been made to reduce the channel length and linewidth of inkjet printed transistors.[13–18] For example, we have developed a method named “coffee ring lithography” to successfuly fabricate a channel length with high resolution of 1−2 µm.[18] It has been acknowledged that the performance of transistors would greatly be enhanced if a shorter-channel geometry is adopted since a large source−drain current and a high transition frequency are extremely crucial for faster and more sensitive organic logic circuits.[19,20] For example, three-stage polymer ring oscillators exhibiting a working frequency of 182 KHz was achieved by reducing the channel length to 5 µm,[21] and the photoresponsity of perylene bisimide transistors could be enhanced.[22] Hence, cost-effective fabrication of short-channel polymer transisors is very important although it remains a big challenge. In this work we demonstrate a simple but effective way to mass-fabricate short-channel electrodes, from several micrometers down to 700 nm via a 50 µm orifice nozzle. Organic solvents are inkjetted to pattern/etch ultrathin polymethylmethacrylate (PMMA) resist. By optimizing the spacing between two adjacent droplets, a rectangular PMMA border/ridge with a submicron width, a length of 12 to 20 µm can be obtained between two etched patterns (source and drain) on the resist, which allows the subsequent fabrication of two isolated electrodes with a spacing of a submicrometer scale after metal deposition and lift-off. Compared with direct inkjet printing
H. L. Wang, C. Cheng, Dr. L. Zhang, Dr. H. T. Liu, Y. Zhao, Dr. Y. L. Guo, Prof. W. P. Hu, Prof. G. Yu, Prof. Y. Q. Liu Beijing National Laboratory for Molecular Sciences Institute of Chemistry Chinese Academy of Sciences Beijing 100190, P. R. China E-mail: [email protected]; [email protected] H. L. Wang, C. Cheng, Y. Zhao University of Chinese Academy of Sciences Beijing 100049, P. R. China
DOI: 10.1002/adma.201400697
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Inkjet Printing Short-Channel Polymer Transistors with High-Performance and Ultrahigh Photoresponsivity
of metallic inks, it is independent of nozzle-clogging, photolithographic prepatterning and surface treatments with organosilanes. Besides, short-channel polymer transistor array with good uniformity and reproducibility based on a p-type polymeric semiconductor, PDPPTzBT[23] (a diketopyrrolopyrrolethiazolothiazole copolymer, its molecular structure is given in Figure S1), exhibited high performances with a maximum hole mobility of 1.80 cm2 V−1 s−1 and an on/off ratio of 108. Moreover, benefiting from the short-channel device geometry, ultrasensitive photodetectors were obtained with a photoresponsivity up to 106 A W−1 together with gigantic shift of onset voltage under a weak illumination. The flow chart for fabricating short-channel electrodes are illustrated in Figure 1: First, ultrathin (3 nm) PMMA was deposited on a silica substrate as resist via a high-speed spincoating from its dilute solution in toluene (8000 rpm). Then, droplets of n-butyl acetate were inkjet printed onto PMMA to define source and drain electrode-pattern for polymer transistors, which could corrode PMMA thoroughly and leaving the silica surface exposed. With an optimized reduction of interdroplet intervals (between 118−120 µm), a sub-micron spacing made of a PMMA-border between two adjacent electrode-pattern (source and drain) was obtained. Finally, 3 nm of titanium as an adhesion layer and 30 nm of gold were consecutively thermally-evaporated. In this step, lift-off was performed in n-butyl acetate through a gentle ultrasonic-cleaning process. Since the etched PMMA layer was complementary to the metallic electrodes, as the layer of metal film on the outer surface of resist was lifted off, the metal film deposited on the substrate showed a pattern as shown in Figures 1e and 1f. Compared to photolithography, our inkjet printing exhibits several distinctive characteristics. First, 3 nm of PMMA resist used here is far thinner than conventional photoresists (1−2 µm). In the conventional photolithography, the thickness of metal film deposited should be kept to be less than one-third of the resist; metal with a thickness of tens of nanometers seems infeasible here. However, since the height of a PMMA-border between two source and drain patterns is 150 nm, metal atoms will not penetrate through the PMMA-border and pollute the channel region during the thermal evaporation, thus effectively isolating the source and drain (Figure S2). Besides, exposure and developing procedures necessary for photolithography are omitted here, which simplifies fabrication procedures. Second, it is found that solvents with lower boiling point (b. p.) yield higher resolution. However, droplets of solvents with higher b. p. are more controllable and ensure a much steadier uniformity. The resolutions of line width versus solvents with deviation are
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limit of inter-electrode spacing.[24–27] In contrast, in our case the coffee-ring effect was utilized to pattern PMMA with high resolution. When a solvent droplet is inkjet printed onto the substrate, the resist is dissolved in it. The solution containing resist is then transported by the flow from the center to the edge and solidifies there.[28] As the volatilizing rate of etchants can not match that of the solute migrates on the substrate, the dissolved resist continuously flows towards the outline of the droplet and accumulates into a craterlike-ridge (see the schema in Equation S1). This results in the formation of a three-phase contact line with a remarkably high aspect-ratio and leaving bare substrates exposed in the center.[29] Here, to obtain an estimation of the channel lengths, it is suggested that the mass of resist formerly deposited in the center equals to that of the newlyborn craterlike-ridge. The transversal surface of the ridge takes the form of a half ellipse according to atomic force microscopy (AFM). Hence, the channel length could be given by (see supporting information, Equation S1 for detailed derivation):
L ≈ Wridge = 4Rt (π h ridge − 2t )
−1
(1)
In this equation, Wridge is the width of a ridge. R, the radius of a droplet the instance it landed on resist, while t and hridge refer to Figure 1. (a) 3 nm of PMMA resist is deposited on a silica substrate via spin-coating. (b) Patterning of PMMA is carried out by inkjet printing pure solvents. (c) Subsequent deposition of the resist thickness and the peak height of a adhesion layer (titanium) and gold. (d) Lift-off is performed in n-butyl acetate through an ultra- ridge, respectively. It is clear that the smaller sonic-cleaning process. Then, metal electrodes show a pattern. (e) A PMMA-border formed droplets, thinner resist and an appropriate between two adjacent patterns (source and drain) prior to metal-deposition. The mazarine polymer resist simultaneously contribute to background is the silica surface while the wathet blue ridge is PMMA. (f) a short-channel gap the shorter channel lengths. corresponding to PMMA-ridge in (e) after titanium-gold deposition and lift-off. Note that the Compared to vacuum-deposited small PMMA-ridge is complementary to the gap. molecular semiconductors, polymer semiconductors possess a few merits such as its solution-processability with a variety of printing techniques and summarized in Figure S3. Interestingly, it is also notable that higher-performance, which paves the way for the pratical applithe molecular weights (MW) of PMMA played a crucial influcation of organic electronics. Bottom-gate, gold bottom-contact ence on the resolution. To elucidate this, in Figure 2a, PMMA transistors were fabricated on heavily n-doped silicon subwith various MW were used as resists for comparison and we strates. The gate dielectric used was a silica layer with a thickresorted to AFM to determine the width (channel length) of the ness of 300 nm passivated with octadecyltrichlorosilane (OTS). ridges. It was confirmed that the minimum width, Wridge, was The active layer of PDPPTzBT was inkjet printed and patterned achieved by PMMA with the smallest MW after lift-off, which across the source and drain electrodes. It is worth mentioning reached 700 nm, as determined by SEM as shown in Figure 2b. that common solvents such as dichlorobenzene leads to highly In order to demonstrate the reproducibility and reliability of uneven films on OTS-treated substrates due to large surface tenthis technique, in Figure 2c, an uniform and well-defined array sion, to address this problem, a mixed solvent of dichlorobenconsisting of 30 gold electrode-pairs on SiO2/Si is displayed, zene and toluene (v/v = 95/5) was chosen to yield a uniform indicating its potential use in integrated circuits. Apart from polymer layer. As reported previously that PDPPTzBT tended the patterning of high-resolution electrodes, this method is also to form fibrillar intercalating networks upon being spin-coated. applicable for substrates with a large area up to several tens of In this case, the inkjet printed film also possesses similar morsquare centimeters. In Figures 2d and 2e, resists bearing short pholgy due to the strong intermolecular interact brought by the phrases were patterned prior to deposition of metals on a oneshort π−π stacking distance.[23] In Figure 3a, an AFM image of inch wafer. After lift-off, gold features showed a pattern. inkjet printed PDPPTzBT thin film across the channel region is In inkjet printing, coffee-ring effect severely hampers resoshown. The bundles of PDPPTzBT fibers packed uniformly in lution and uniformity of features, which usually restricts the
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the channel as well as on the electrode as demonstrated in its phase image (Figure 3b), which was different from thermallyevaporated small molecule semiconductors.[18] Electrical characteristics of sub-micron PDPPTzBT transistors are depicted in Figure 3. The semiconductor layer carefully patterned via inkjet printing resulted in a very low gate leakage current (Figure 3c). Besides, VT (threshold voltage) of short-channel transistors was not constant when different Vds applied, which might be explained by the less-effective gate-modulation when the channel length (700 nm) is comparable with 300 nm gate-dielectric (Figure S4). A constant gate voltage of −40 V and a constant source-drain voltage of −40 V were applied for a period of 500 s and only a minor shift of VT was observed in Figure S5, eliminating concerns about the bias stress effect. Benefiting from increasing Vds, a clear saturation in the output characteristics was observed in the shortchannel transistors (Figure S6). Here, it is crucial to note that as the applied Vds reached −60 V, a maximum saturation mobility of 1.80 cm2 V−1 s−1 was obtained with a high on/off ratio of 108, which is a considerbly high performance in inkjet printed polymer transistors. We attribute this high performance to these: (i) A very low off-state current with an magnitude of 10−12 A is superior to those of reference devices with lithography-patterned short-channel gold source–drain electrodes, which indicates the no electrically-conductive pollutant is detectable in the channel region.[20] (ii) The calculated contact resistance of inkjet-printed devices was 90 kΩ cm at Vg = −10 V in Figure S7, therefore, holes could be injected efficiently from the gold electrodes to the channel region to give a high carriermobility. Finally, to assess the reproducibility of inkjet-printing, the statistical distribution of 100 transistors mobility is summarized in Figure 3d, which exhibited an average mobility of 1.20 cm2 V−1 s−1. To evaluate the photoresponsivity of the device, the transfer curves of the devices under light illumination with different
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intensities were characterized. With the increase of illumination intensity, the carrier mobility remained unaffected while transfer curves shifted to a higher gate voltage with a gradual reduced on/off ratio, as shown in the Figure 3f, which has been observed in the past report.[30] Upon illumination, photoinduced excitons were generated in the PDPPTzBT thin film, which were instantaneously dissociated into electrons and holes under the positive gate voltage; the holes were transferred into the active layer while the electrons were brought to the interface of the dielectric layer. Then, photo-induced holes transport dominated the channel. Specifically, gigantic shift of onset voltage (VOn) was observed in a sub-micron channel transistor in Figure 3f, which mounted to 90 V under the illumination of 40 µW cm−2. Photoresponsivity, a crucial parameter assessing the performance of phototransistors, is defined as R = ΔIds P−1 A−1, where ΔIds = Iphoto − Idark and P is illumination intensity while A refers to the area of channel region exposed to illumination. As for a sub-micron transistor, thanks to gigantic shift of VT, under the weak illumintion intensity of 700 nW cm−2, ΔIds reached 0.5 µA when Vg was kept at 20 V, with Vds of −20 V, which corresponded to an ultrahigh photosensitivity of 1.0 × 106 A W−1, as calculated from Figure 3e, which can rival that of the best result of organic multifiber photodetectors.[22] To further study photoresponse reliance’s on channel lengths, the dependence of onset gate voltage’s shift on channel lengths (Vg was applied from 140 V to −50 V) were shown under parallel illmination in the transfer mode. Interestingly, it was found that the reduction of channel lengths is accompanied by an increasing trend of ΔVOn , as summarized in Figure 4a. Previous report attributed the improvement in photoresponse mainly to inverse proportionally enlarged current with reduction of channel lengths.[22] However, in our case, photoresponse of short-channel transistors is primarily due to modulation of the gate voltage, which is greatly
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Figure 3. (a) The morphology of PDPPTzBT film inkjet-printed across the sub-micron channel region (L = 0.7 µm). The color scale is 35 nm. (b) is the coresponding phase image of the polymer film. (c) Typical transfer characteristics of a polymer transistor (L = 0.7 µm). (d) Statistical distribution of the transistor mobility with PDPPTzBT as the active layer. (e) Comparison of drain currents under illumination with increasing intensity (wavelength 650 nm) at fixed Vg and calculated photoresponsivity under different illumination intensity (inset). (f) Transfer characteristics of the transistor under illumination compared with darkness.
benefited from enlarged ΔVOn. Here, the shift of VOn exhibited a power function relationship with incident light power, which is given by:
ΔVOn = α P β
(2)
However, the coefficent β is not a constant with changing channel length; in Figure 4b, it is shown that the shorter channel lengths lead to higher β. Additionally, in the linear region, it's evident that the ΔIds is in linear proportion to ΔVOn when Vds was kept fixed, which is given by: 4686
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ΔIds = CμΔVOn Vds WL−1
(3)
In this equation, large ΔIds contributes to high photoresponse. Moreover, ΔIds will further be enlarged due to shortened channel lengths. Judging from these equations, we attribute the ultrahigh photoresponsivity of short-channel devices to: (i) gigantic shift of gate voltage leads to a much larger ΔIds induced by short-channel device geometry. (ii) The ΔIds is greatly enhanced by short channel length. In Figure 4c, we show the transient behaviour of the device under illumination. The Vds was kept constant at −20 V while the gate voltage was set constant at 20 V. Negligible dark
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current of picoampere was detected under darkness. With shortened transit time of photogenerated carriers given as τ = L2 µ−1 Vds−1, once irradiation was on, the immediate current response took place, which jumped from picoampere to microampere-current within one second. However, switching the illumination off does not recover the device to the off-state. Alternatively, the drain current decays and turns steady at a lower value in Figure S8, which indicates persistent photoconductivity (PPC).[31,32] This phenomenon is caused by the photogenerated electrons trapped in the interface of dielectric or the shift of VOn under the combined influence of illumination and positive gate-bias. A retention test was performed over a period of 10000 seconds and there was not a significant attenuation of drain current observed, which indicated the long lifetime of trapped electrons (Figure S8). However, the performance as a photodetector was seriously affected by its long recovery time, which restricts its practical application. In Figure 4d, it was found that a pulse of negative gate bias at −80 V for 1 s can eliminate PPC and regain its original VOn by releasing the stored electrons in the interface and recombining them with
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holes in the active layer, indicating that it can be used a gatetuned photodetector.[33,34] A further examination showed that the sub-micron transistor can function as a reproducible light-triggered memory device. Our controlled experiments with PDPPTzBT transistors with various channel lengths all exhibited PPC, implying that this property is not a byproduct of short channel, but an instrinc property of PDPPTzBT. The persistent Ids after switching off illumination is attributed to the effect of gate field that hinders the recombination of photogenerated electrons and holes in the interface of dielectric layer and channel. However, PPC is beneficial for a memory device. In Figure 4d, as a PDPPTzBT transistor is exposed to illumination with an intensity of 7.5 µW cm−2, which acts an light-triggered writing process with a gate voltage of 20 V and on-stage current of 1.5 µA was obtained after tuning light off. As for the second stage, the gate voltage was reduced to 0 V with the illumination off, which increased Ids by two times to 3 µA. Consecutively, the gate voltage was reduced to −10 V, the drain current was enhanced by 3 times, compared with the first stage. As the gate voltage
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was turned to −20 V, the drain current of the device mounted up to a gate-dependent saturation of 10−5 A. In the final stage, an erasing process was performed by applying a pulse of −80 V for 1 s and PPC was eliminated within one second with the drastic decay of Ids. To test its reproducibility, ten cycles of light-writing process and gate modulation were conducted and the currents exhibited well controllability. Due to PPC in asfabricated devices under illumination, light-triggered memory characteristics with multi-stage can be acquired simply through light-writing once and tuning the gate voltage several times, which shows its prospective application in multi-functional memory device. In this research, a lithography-free method is developed aiming at mass-production of short-channel electrode-pair at low cost. This method is characterized by high resolution, low cost and high yields over a large area by inkjet printing pure solvents, thus making the process more economical and less laborious. Moreover, with this method, high performance short-channel PDPPTzBT transistors were fabricated, with an average hole mobility reaching 1.20 cm2 V−1 s−1 in ambient atmosphere. Remarkably, the sub-micron transistors exhibit ultrahigh photoresponsivity of 1.0 × 106 A W−1 towards weak light. We attribute this record-high performance to remarkable change of source-drain current due to the gigantic shift of VOn under illumination. Due to a strong persistent photoconductivity, we demonstate the device can also function as a lighttriggered memory device.
Experimental Section Materials: PMMA with increasing MW (120 KDa, 350 KDa, 550 KDa and 996 KDa) was purchased from Aldrich Chemical. PDPPTzBT was synthsized according to literature.[23] PMMA granules were thoroughly dissolved in toluene over night and filtered before spin-coating. After deposition of PMMA layer, inkjet etching was conducted with Jetlab II inkjet printing equipment from MicroFab. The orifice of the nozzle in this research was fixed to 50 µm. Device Fabrication and Characterization: (1) Silicon wafers with 300 nm silica (dielectric) were successively rinsed with detergent, deionized water, isopropyl alcohol, and acetone and were finally dried with nitrogen. (2) PMMA solutions with conc. of 2 mg ml−1 were spin-coated on wafers. (3) Inkjet printing of organic solvents on ultrathin PMMA were carried out in a point-to-point mode, while the moving-speed and the temperature of the sample stage were set at 250 µm s−1 and 80 °C, respectively. (4) 3 nm titanium and 30 nm gold were consecutively thermally-evaporated on the silicon wafer as source and drain electrodes. Note that the edges of wafers should be sheltered by a tape to facilitate lift-off. (5) PMMA was lift-off in n-butyl acetate by ultrasonic-cleaning. (6) After the deposition of electrodes, the wafers were cleaned by oxygen plasma. Then, OTS treatment was performed on silica in a vacuum oven at 120 °C. Subsequently, the treated substrates were rinsed with heptane, ethanol, and chloroform. (7) PDPPTzBT was dissolved in a mixed solvents of dichlorobenzene/ toluene (v/v = 95/5) and filtered with a 0.45 µm PTFE syringe filter before use. At least 50 droplets of semiconductor ink were necessary to complete an individual transistor. After polymer deposition, the wafer was annealed in a glovebox filled with nitrogen at 120 °C for 5 min. All devices were characterized with a Keithley 4200 semiconductor characterization system in ambient atmosphere. The light source was a LED with the wavelength of 650 nm and illumination intensity was measured by an irradiatometer. The mobility in the saturation regime was extracted from the following equation: Ids = Ciµ(W/2L)(Vg – Vt)2,
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where Ids is the drain current, Ci is the capacitance per unit area of the gate dielectric layer, and Vg and Vt are the gate voltage and threshold voltage, respectively.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was supported by the National Natural Science Foundation of China (60911130231 and 51233006), and the Major State Basic Research Development Program (2011CB932303, 2013CB733700, 2013CBA01602), and Chinese Academy of Sciences. Received: February 12, 2014 Revised: March 15, 2014 Published online: April 6, 2014
[1] P. Calvert, Chem. Mater. 2001, 13, 3299. [2] M. Hiraoka, T. Hasegawa, T. Yamada, Y. Takahashi, S. Horiuchi, Y. Tokura, Adv. Mater. 2007, 19, 3248. [3] S. F. Jahn, T. Blaudeck, R. R. Baumann, A. Jakob, P. Ecorchard, T. Rüffer, H. Lang, P. Schmidt, Chem. Mater. 2010, 22, 3067. [4] B. K. Kjellander, W. T. Smaal, J. E. Anthony, G. H. Gelinck, Adv. Mater. 2010, 22, 4612. [5] J. Lee, D. H. Kim, J. Y. Kim, B. Yoo, J. W. Chung, J. I. Park, B. L. Lee, J. Y. Jung, J. S. Park, B. Koo, S. Im, J. W. Kim, B. Song, M. H. Jung, J. E. Jang, Y. W. Jin, S. Y. Lee, Adv. Mater. 2013, 25, 5886. [6] H. Minemawari, T. Yamada, H. Matsui, J. Tsutsumi, S. Haas, R. Chiba, R. Kumai, T. Hasegawa, Nature 2011, 475, 364. [7] S. Jeong, D. Kim, J. Moon, J.Phys.Chem.C. 2008, 112, 5245. [8] Y. Zhao, C. A. Di, X. Gao, Y. Hu, Y. Guo, L. Zhang, Y. Liu, J. Wang, W. Hu, D. Zhu, Adv. Mater. 2011, 23, 2448. [9] S. Chung, M. Jang, S. B. Ji, H. Im, N. Seong, J. Ha, S. K. Kwon, Y. H. Kim, H. Yang, Y. Hong, Adv. Mater. 2013, 25, 4773. [10] G. Azzellino, A. Grimoldi, M. Binda, M. Caironi, D. Natali, M. Sampietro, Adv. Mater. 2013, 25, 6829. [11] C. N. Hoth, S. A. Choulis, P. Schilinsky, C. J. Brabec, Adv. Mater. 2007, 19, 3973. [12] K. K. Manga, S. Wang, M. Jaiswal, Q. Bao, K. P. Loh, Adv. Mater. 2010, 22, 5265. [13] J. Z. Wang, Z. H. Zheng, H. W. Li, W. T. Huck, H. Sirringhaus, Nat. Mater. 2004, 3, 171. [14] C. W. Sele, T. von. Werne, R. H. Friend, H. Sirringhaus, Adv. Mater. 2005, 17, 997. [15] Y. Y. Noh, N. Zhao, M. Caironi, H. Sirringhaus, Nat. Nanotechnol. 2007, 2, 784. [16] T. Sekitani, Y. Noguchi, U. Zschieschang, H. Klauk, T. Someya, Proc. Natl. Acad. Sci. USA 2008, 105, 4976. [17] Z. Zhang, X. Zhang, Z. Xin, M. Deng, Y. Wen, Y. Song, Adv. Mater. 2013, 25, 6714. [18] L. Zhang, H. Liu, Y. Zhao, X. Sun, Y. Wen, Y. Guo, X. Gao, C. A. Di, G. Yu, Y. Liu, Adv. Mater. 2012, 24, 436. [19] H. Kang, R. Kitsomboonloha, J. Jang, V. Subramanian, Adv. Mater. 2012, 24, 3065. [20] M. Uno, K. Nakayama, J. Soeda, Y. Hirose, K. Miwa, T. Uemura, A. Nakao, K. Takimiya, J. Takeya, Adv. Mater. 2011, 23, 3047. [21] A. J. Kronemeijer, E. Gili, M. Shahid, J. Rivnay, A. Salleo, M. Heeney, H. Sirringhaus, Adv. Mater. 2012, 24, 1558.
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[30] K. Wasapinyokul, W. I. Milne, D. P. Chu, J. Appl. Phys. 2011, 109, 084510. [31] S. E. Ahn, S. Jeon, Y. W. Jeon, C. Kim, M. J. Lee, C. W. Lee, J. Park, I. Song, A. Nathan, S. Lee, U. I. Chung, Adv. Mater. 2013, 25, 5549. [32] K. Roy, M. Padmanabhan, S. Goswami, T. Phanindra Sai,G. Ramalingam, S. Raghavan, A. Ghosh, Nat. Nanotechnol. 2013, 8, 826. [33] M. H. Hoang, Y. Kim, M. Kim, K. H. Kim, T. W. Lee, D. N. Nguyen, S. J. Kim, K. Lee, S. J. Lee, D. H. Choi, Adv. Mater. 2012, 24, 5363. [34] K. H. Kim, S. Y. Bae, Y. S. Kim, J. A. Hur, M. H. Hoang, T. W. Lee, M. J. Cho, Y. Kim, M. Kim, J. I. Jin, S. J. Kim, K. Lee, S. J. Lee, D. H. Choi, Adv. Mater. 2011, 23, 3095.
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[22] M. El Gemayel, M. Treier, C. Musumeci, C. Li, K. Mullen, P. Samori, J. Am. Chem. Soc. 2012, 134, 2429. [23] C. Cheng, C. Yu, Y. Guo, H. Chen, Y. Fang, G. Yu, Y. Liu, Chem. Commun. 2013, 49, 1998. [24] D. Soltman, V. Subramanian, Langmuir 2008, 24, 2224. [25] H. Hu, R. G. Larson, J. Phys. Chem. B. 2006, 110, 7090. [26] H. Kang, D. Soltman, V. Subramanian, Langmuir 2010, 26, 11568. [27] S. Choi, S. Stassi, A. P. Pisano, T. I. Zohdi, Langmuir 2010, 26, 11690. [28] T. Kawase, H. Sirringhaus, R. H. Friend, T. Shimoda, Adv. Mater. 2001, 13, 1601. [29] B. J. de Gans, S. Hoeppener, U. S. Schubert, Adv. Mater. 2006, 18, 910.
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Hanlin Wang, Cheng Cheng, Lei Zhang, Hongtao Liu, Yan Zhao, Yunlong Guo, Wenping Hu,* Gui Yu, and Yunqi Liu* Inkjet printing, in the past decade, has proved to be a powerful tool in the cost-effective ambient deposition of electrodes,[1–3] organic semiconductors,[4–6] and plastic dielectric layers.[7] With this patterning methodology and proper inks, high performance organic field-effect transistors,[8] organic inverters,[9] organic photodetectors, and polymer solar-cells have been realized.[10–12] However, it is highly desirable that the resolution of inkjet printing need further improvement, and in fact continuous efforts have been made to reduce the channel length and linewidth of inkjet printed transistors.[13–18] For example, we have developed a method named “coffee ring lithography” to successfuly fabricate a channel length with high resolution of 1−2 µm.[18] It has been acknowledged that the performance of transistors would greatly be enhanced if a shorter-channel geometry is adopted since a large source−drain current and a high transition frequency are extremely crucial for faster and more sensitive organic logic circuits.[19,20] For example, three-stage polymer ring oscillators exhibiting a working frequency of 182 KHz was achieved by reducing the channel length to 5 µm,[21] and the photoresponsity of perylene bisimide transistors could be enhanced.[22] Hence, cost-effective fabrication of short-channel polymer transisors is very important although it remains a big challenge. In this work we demonstrate a simple but effective way to mass-fabricate short-channel electrodes, from several micrometers down to 700 nm via a 50 µm orifice nozzle. Organic solvents are inkjetted to pattern/etch ultrathin polymethylmethacrylate (PMMA) resist. By optimizing the spacing between two adjacent droplets, a rectangular PMMA border/ridge with a submicron width, a length of 12 to 20 µm can be obtained between two etched patterns (source and drain) on the resist, which allows the subsequent fabrication of two isolated electrodes with a spacing of a submicrometer scale after metal deposition and lift-off. Compared with direct inkjet printing
H. L. Wang, C. Cheng, Dr. L. Zhang, Dr. H. T. Liu, Y. Zhao, Dr. Y. L. Guo, Prof. W. P. Hu, Prof. G. Yu, Prof. Y. Q. Liu Beijing National Laboratory for Molecular Sciences Institute of Chemistry Chinese Academy of Sciences Beijing 100190, P. R. China E-mail: [email protected]; [email protected] H. L. Wang, C. Cheng, Y. Zhao University of Chinese Academy of Sciences Beijing 100049, P. R. China
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Inkjet Printing Short-Channel Polymer Transistors with High-Performance and Ultrahigh Photoresponsivity
of metallic inks, it is independent of nozzle-clogging, photolithographic prepatterning and surface treatments with organosilanes. Besides, short-channel polymer transistor array with good uniformity and reproducibility based on a p-type polymeric semiconductor, PDPPTzBT[23] (a diketopyrrolopyrrolethiazolothiazole copolymer, its molecular structure is given in Figure S1), exhibited high performances with a maximum hole mobility of 1.80 cm2 V−1 s−1 and an on/off ratio of 108. Moreover, benefiting from the short-channel device geometry, ultrasensitive photodetectors were obtained with a photoresponsivity up to 106 A W−1 together with gigantic shift of onset voltage under a weak illumination. The flow chart for fabricating short-channel electrodes are illustrated in Figure 1: First, ultrathin (3 nm) PMMA was deposited on a silica substrate as resist via a high-speed spincoating from its dilute solution in toluene (8000 rpm). Then, droplets of n-butyl acetate were inkjet printed onto PMMA to define source and drain electrode-pattern for polymer transistors, which could corrode PMMA thoroughly and leaving the silica surface exposed. With an optimized reduction of interdroplet intervals (between 118−120 µm), a sub-micron spacing made of a PMMA-border between two adjacent electrode-pattern (source and drain) was obtained. Finally, 3 nm of titanium as an adhesion layer and 30 nm of gold were consecutively thermally-evaporated. In this step, lift-off was performed in n-butyl acetate through a gentle ultrasonic-cleaning process. Since the etched PMMA layer was complementary to the metallic electrodes, as the layer of metal film on the outer surface of resist was lifted off, the metal film deposited on the substrate showed a pattern as shown in Figures 1e and 1f. Compared to photolithography, our inkjet printing exhibits several distinctive characteristics. First, 3 nm of PMMA resist used here is far thinner than conventional photoresists (1−2 µm). In the conventional photolithography, the thickness of metal film deposited should be kept to be less than one-third of the resist; metal with a thickness of tens of nanometers seems infeasible here. However, since the height of a PMMA-border between two source and drain patterns is 150 nm, metal atoms will not penetrate through the PMMA-border and pollute the channel region during the thermal evaporation, thus effectively isolating the source and drain (Figure S2). Besides, exposure and developing procedures necessary for photolithography are omitted here, which simplifies fabrication procedures. Second, it is found that solvents with lower boiling point (b. p.) yield higher resolution. However, droplets of solvents with higher b. p. are more controllable and ensure a much steadier uniformity. The resolutions of line width versus solvents with deviation are
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limit of inter-electrode spacing.[24–27] In contrast, in our case the coffee-ring effect was utilized to pattern PMMA with high resolution. When a solvent droplet is inkjet printed onto the substrate, the resist is dissolved in it. The solution containing resist is then transported by the flow from the center to the edge and solidifies there.[28] As the volatilizing rate of etchants can not match that of the solute migrates on the substrate, the dissolved resist continuously flows towards the outline of the droplet and accumulates into a craterlike-ridge (see the schema in Equation S1). This results in the formation of a three-phase contact line with a remarkably high aspect-ratio and leaving bare substrates exposed in the center.[29] Here, to obtain an estimation of the channel lengths, it is suggested that the mass of resist formerly deposited in the center equals to that of the newlyborn craterlike-ridge. The transversal surface of the ridge takes the form of a half ellipse according to atomic force microscopy (AFM). Hence, the channel length could be given by (see supporting information, Equation S1 for detailed derivation):
L ≈ Wridge = 4Rt (π h ridge − 2t )
−1
(1)
In this equation, Wridge is the width of a ridge. R, the radius of a droplet the instance it landed on resist, while t and hridge refer to Figure 1. (a) 3 nm of PMMA resist is deposited on a silica substrate via spin-coating. (b) Patterning of PMMA is carried out by inkjet printing pure solvents. (c) Subsequent deposition of the resist thickness and the peak height of a adhesion layer (titanium) and gold. (d) Lift-off is performed in n-butyl acetate through an ultra- ridge, respectively. It is clear that the smaller sonic-cleaning process. Then, metal electrodes show a pattern. (e) A PMMA-border formed droplets, thinner resist and an appropriate between two adjacent patterns (source and drain) prior to metal-deposition. The mazarine polymer resist simultaneously contribute to background is the silica surface while the wathet blue ridge is PMMA. (f) a short-channel gap the shorter channel lengths. corresponding to PMMA-ridge in (e) after titanium-gold deposition and lift-off. Note that the Compared to vacuum-deposited small PMMA-ridge is complementary to the gap. molecular semiconductors, polymer semiconductors possess a few merits such as its solution-processability with a variety of printing techniques and summarized in Figure S3. Interestingly, it is also notable that higher-performance, which paves the way for the pratical applithe molecular weights (MW) of PMMA played a crucial influcation of organic electronics. Bottom-gate, gold bottom-contact ence on the resolution. To elucidate this, in Figure 2a, PMMA transistors were fabricated on heavily n-doped silicon subwith various MW were used as resists for comparison and we strates. The gate dielectric used was a silica layer with a thickresorted to AFM to determine the width (channel length) of the ness of 300 nm passivated with octadecyltrichlorosilane (OTS). ridges. It was confirmed that the minimum width, Wridge, was The active layer of PDPPTzBT was inkjet printed and patterned achieved by PMMA with the smallest MW after lift-off, which across the source and drain electrodes. It is worth mentioning reached 700 nm, as determined by SEM as shown in Figure 2b. that common solvents such as dichlorobenzene leads to highly In order to demonstrate the reproducibility and reliability of uneven films on OTS-treated substrates due to large surface tenthis technique, in Figure 2c, an uniform and well-defined array sion, to address this problem, a mixed solvent of dichlorobenconsisting of 30 gold electrode-pairs on SiO2/Si is displayed, zene and toluene (v/v = 95/5) was chosen to yield a uniform indicating its potential use in integrated circuits. Apart from polymer layer. As reported previously that PDPPTzBT tended the patterning of high-resolution electrodes, this method is also to form fibrillar intercalating networks upon being spin-coated. applicable for substrates with a large area up to several tens of In this case, the inkjet printed film also possesses similar morsquare centimeters. In Figures 2d and 2e, resists bearing short pholgy due to the strong intermolecular interact brought by the phrases were patterned prior to deposition of metals on a oneshort π−π stacking distance.[23] In Figure 3a, an AFM image of inch wafer. After lift-off, gold features showed a pattern. inkjet printed PDPPTzBT thin film across the channel region is In inkjet printing, coffee-ring effect severely hampers resoshown. The bundles of PDPPTzBT fibers packed uniformly in lution and uniformity of features, which usually restricts the
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the channel as well as on the electrode as demonstrated in its phase image (Figure 3b), which was different from thermallyevaporated small molecule semiconductors.[18] Electrical characteristics of sub-micron PDPPTzBT transistors are depicted in Figure 3. The semiconductor layer carefully patterned via inkjet printing resulted in a very low gate leakage current (Figure 3c). Besides, VT (threshold voltage) of short-channel transistors was not constant when different Vds applied, which might be explained by the less-effective gate-modulation when the channel length (700 nm) is comparable with 300 nm gate-dielectric (Figure S4). A constant gate voltage of −40 V and a constant source-drain voltage of −40 V were applied for a period of 500 s and only a minor shift of VT was observed in Figure S5, eliminating concerns about the bias stress effect. Benefiting from increasing Vds, a clear saturation in the output characteristics was observed in the shortchannel transistors (Figure S6). Here, it is crucial to note that as the applied Vds reached −60 V, a maximum saturation mobility of 1.80 cm2 V−1 s−1 was obtained with a high on/off ratio of 108, which is a considerbly high performance in inkjet printed polymer transistors. We attribute this high performance to these: (i) A very low off-state current with an magnitude of 10−12 A is superior to those of reference devices with lithography-patterned short-channel gold source–drain electrodes, which indicates the no electrically-conductive pollutant is detectable in the channel region.[20] (ii) The calculated contact resistance of inkjet-printed devices was 90 kΩ cm at Vg = −10 V in Figure S7, therefore, holes could be injected efficiently from the gold electrodes to the channel region to give a high carriermobility. Finally, to assess the reproducibility of inkjet-printing, the statistical distribution of 100 transistors mobility is summarized in Figure 3d, which exhibited an average mobility of 1.20 cm2 V−1 s−1. To evaluate the photoresponsivity of the device, the transfer curves of the devices under light illumination with different
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intensities were characterized. With the increase of illumination intensity, the carrier mobility remained unaffected while transfer curves shifted to a higher gate voltage with a gradual reduced on/off ratio, as shown in the Figure 3f, which has been observed in the past report.[30] Upon illumination, photoinduced excitons were generated in the PDPPTzBT thin film, which were instantaneously dissociated into electrons and holes under the positive gate voltage; the holes were transferred into the active layer while the electrons were brought to the interface of the dielectric layer. Then, photo-induced holes transport dominated the channel. Specifically, gigantic shift of onset voltage (VOn) was observed in a sub-micron channel transistor in Figure 3f, which mounted to 90 V under the illumination of 40 µW cm−2. Photoresponsivity, a crucial parameter assessing the performance of phototransistors, is defined as R = ΔIds P−1 A−1, where ΔIds = Iphoto − Idark and P is illumination intensity while A refers to the area of channel region exposed to illumination. As for a sub-micron transistor, thanks to gigantic shift of VT, under the weak illumintion intensity of 700 nW cm−2, ΔIds reached 0.5 µA when Vg was kept at 20 V, with Vds of −20 V, which corresponded to an ultrahigh photosensitivity of 1.0 × 106 A W−1, as calculated from Figure 3e, which can rival that of the best result of organic multifiber photodetectors.[22] To further study photoresponse reliance’s on channel lengths, the dependence of onset gate voltage’s shift on channel lengths (Vg was applied from 140 V to −50 V) were shown under parallel illmination in the transfer mode. Interestingly, it was found that the reduction of channel lengths is accompanied by an increasing trend of ΔVOn , as summarized in Figure 4a. Previous report attributed the improvement in photoresponse mainly to inverse proportionally enlarged current with reduction of channel lengths.[22] However, in our case, photoresponse of short-channel transistors is primarily due to modulation of the gate voltage, which is greatly
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Figure 3. (a) The morphology of PDPPTzBT film inkjet-printed across the sub-micron channel region (L = 0.7 µm). The color scale is 35 nm. (b) is the coresponding phase image of the polymer film. (c) Typical transfer characteristics of a polymer transistor (L = 0.7 µm). (d) Statistical distribution of the transistor mobility with PDPPTzBT as the active layer. (e) Comparison of drain currents under illumination with increasing intensity (wavelength 650 nm) at fixed Vg and calculated photoresponsivity under different illumination intensity (inset). (f) Transfer characteristics of the transistor under illumination compared with darkness.
benefited from enlarged ΔVOn. Here, the shift of VOn exhibited a power function relationship with incident light power, which is given by:
ΔVOn = α P β
(2)
However, the coefficent β is not a constant with changing channel length; in Figure 4b, it is shown that the shorter channel lengths lead to higher β. Additionally, in the linear region, it's evident that the ΔIds is in linear proportion to ΔVOn when Vds was kept fixed, which is given by: 4686
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ΔIds = CμΔVOn Vds WL−1
(3)
In this equation, large ΔIds contributes to high photoresponse. Moreover, ΔIds will further be enlarged due to shortened channel lengths. Judging from these equations, we attribute the ultrahigh photoresponsivity of short-channel devices to: (i) gigantic shift of gate voltage leads to a much larger ΔIds induced by short-channel device geometry. (ii) The ΔIds is greatly enhanced by short channel length. In Figure 4c, we show the transient behaviour of the device under illumination. The Vds was kept constant at −20 V while the gate voltage was set constant at 20 V. Negligible dark
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current of picoampere was detected under darkness. With shortened transit time of photogenerated carriers given as τ = L2 µ−1 Vds−1, once irradiation was on, the immediate current response took place, which jumped from picoampere to microampere-current within one second. However, switching the illumination off does not recover the device to the off-state. Alternatively, the drain current decays and turns steady at a lower value in Figure S8, which indicates persistent photoconductivity (PPC).[31,32] This phenomenon is caused by the photogenerated electrons trapped in the interface of dielectric or the shift of VOn under the combined influence of illumination and positive gate-bias. A retention test was performed over a period of 10000 seconds and there was not a significant attenuation of drain current observed, which indicated the long lifetime of trapped electrons (Figure S8). However, the performance as a photodetector was seriously affected by its long recovery time, which restricts its practical application. In Figure 4d, it was found that a pulse of negative gate bias at −80 V for 1 s can eliminate PPC and regain its original VOn by releasing the stored electrons in the interface and recombining them with
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holes in the active layer, indicating that it can be used a gatetuned photodetector.[33,34] A further examination showed that the sub-micron transistor can function as a reproducible light-triggered memory device. Our controlled experiments with PDPPTzBT transistors with various channel lengths all exhibited PPC, implying that this property is not a byproduct of short channel, but an instrinc property of PDPPTzBT. The persistent Ids after switching off illumination is attributed to the effect of gate field that hinders the recombination of photogenerated electrons and holes in the interface of dielectric layer and channel. However, PPC is beneficial for a memory device. In Figure 4d, as a PDPPTzBT transistor is exposed to illumination with an intensity of 7.5 µW cm−2, which acts an light-triggered writing process with a gate voltage of 20 V and on-stage current of 1.5 µA was obtained after tuning light off. As for the second stage, the gate voltage was reduced to 0 V with the illumination off, which increased Ids by two times to 3 µA. Consecutively, the gate voltage was reduced to −10 V, the drain current was enhanced by 3 times, compared with the first stage. As the gate voltage
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was turned to −20 V, the drain current of the device mounted up to a gate-dependent saturation of 10−5 A. In the final stage, an erasing process was performed by applying a pulse of −80 V for 1 s and PPC was eliminated within one second with the drastic decay of Ids. To test its reproducibility, ten cycles of light-writing process and gate modulation were conducted and the currents exhibited well controllability. Due to PPC in asfabricated devices under illumination, light-triggered memory characteristics with multi-stage can be acquired simply through light-writing once and tuning the gate voltage several times, which shows its prospective application in multi-functional memory device. In this research, a lithography-free method is developed aiming at mass-production of short-channel electrode-pair at low cost. This method is characterized by high resolution, low cost and high yields over a large area by inkjet printing pure solvents, thus making the process more economical and less laborious. Moreover, with this method, high performance short-channel PDPPTzBT transistors were fabricated, with an average hole mobility reaching 1.20 cm2 V−1 s−1 in ambient atmosphere. Remarkably, the sub-micron transistors exhibit ultrahigh photoresponsivity of 1.0 × 106 A W−1 towards weak light. We attribute this record-high performance to remarkable change of source-drain current due to the gigantic shift of VOn under illumination. Due to a strong persistent photoconductivity, we demonstate the device can also function as a lighttriggered memory device.
Experimental Section Materials: PMMA with increasing MW (120 KDa, 350 KDa, 550 KDa and 996 KDa) was purchased from Aldrich Chemical. PDPPTzBT was synthsized according to literature.[23] PMMA granules were thoroughly dissolved in toluene over night and filtered before spin-coating. After deposition of PMMA layer, inkjet etching was conducted with Jetlab II inkjet printing equipment from MicroFab. The orifice of the nozzle in this research was fixed to 50 µm. Device Fabrication and Characterization: (1) Silicon wafers with 300 nm silica (dielectric) were successively rinsed with detergent, deionized water, isopropyl alcohol, and acetone and were finally dried with nitrogen. (2) PMMA solutions with conc. of 2 mg ml−1 were spin-coated on wafers. (3) Inkjet printing of organic solvents on ultrathin PMMA were carried out in a point-to-point mode, while the moving-speed and the temperature of the sample stage were set at 250 µm s−1 and 80 °C, respectively. (4) 3 nm titanium and 30 nm gold were consecutively thermally-evaporated on the silicon wafer as source and drain electrodes. Note that the edges of wafers should be sheltered by a tape to facilitate lift-off. (5) PMMA was lift-off in n-butyl acetate by ultrasonic-cleaning. (6) After the deposition of electrodes, the wafers were cleaned by oxygen plasma. Then, OTS treatment was performed on silica in a vacuum oven at 120 °C. Subsequently, the treated substrates were rinsed with heptane, ethanol, and chloroform. (7) PDPPTzBT was dissolved in a mixed solvents of dichlorobenzene/ toluene (v/v = 95/5) and filtered with a 0.45 µm PTFE syringe filter before use. At least 50 droplets of semiconductor ink were necessary to complete an individual transistor. After polymer deposition, the wafer was annealed in a glovebox filled with nitrogen at 120 °C for 5 min. All devices were characterized with a Keithley 4200 semiconductor characterization system in ambient atmosphere. The light source was a LED with the wavelength of 650 nm and illumination intensity was measured by an irradiatometer. The mobility in the saturation regime was extracted from the following equation: Ids = Ciµ(W/2L)(Vg – Vt)2,
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where Ids is the drain current, Ci is the capacitance per unit area of the gate dielectric layer, and Vg and Vt are the gate voltage and threshold voltage, respectively.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was supported by the National Natural Science Foundation of China (60911130231 and 51233006), and the Major State Basic Research Development Program (2011CB932303, 2013CB733700, 2013CBA01602), and Chinese Academy of Sciences. Received: February 12, 2014 Revised: March 15, 2014 Published online: April 6, 2014
[1] P. Calvert, Chem. Mater. 2001, 13, 3299. [2] M. Hiraoka, T. Hasegawa, T. Yamada, Y. Takahashi, S. Horiuchi, Y. Tokura, Adv. Mater. 2007, 19, 3248. [3] S. F. Jahn, T. Blaudeck, R. R. Baumann, A. Jakob, P. Ecorchard, T. Rüffer, H. Lang, P. Schmidt, Chem. Mater. 2010, 22, 3067. [4] B. K. Kjellander, W. T. Smaal, J. E. Anthony, G. H. Gelinck, Adv. Mater. 2010, 22, 4612. [5] J. Lee, D. H. Kim, J. Y. Kim, B. Yoo, J. W. Chung, J. I. Park, B. L. Lee, J. Y. Jung, J. S. Park, B. Koo, S. Im, J. W. Kim, B. Song, M. H. Jung, J. E. Jang, Y. W. Jin, S. Y. Lee, Adv. Mater. 2013, 25, 5886. [6] H. Minemawari, T. Yamada, H. Matsui, J. Tsutsumi, S. Haas, R. Chiba, R. Kumai, T. Hasegawa, Nature 2011, 475, 364. [7] S. Jeong, D. Kim, J. Moon, J.Phys.Chem.C. 2008, 112, 5245. [8] Y. Zhao, C. A. Di, X. Gao, Y. Hu, Y. Guo, L. Zhang, Y. Liu, J. Wang, W. Hu, D. Zhu, Adv. Mater. 2011, 23, 2448. [9] S. Chung, M. Jang, S. B. Ji, H. Im, N. Seong, J. Ha, S. K. Kwon, Y. H. Kim, H. Yang, Y. Hong, Adv. Mater. 2013, 25, 4773. [10] G. Azzellino, A. Grimoldi, M. Binda, M. Caironi, D. Natali, M. Sampietro, Adv. Mater. 2013, 25, 6829. [11] C. N. Hoth, S. A. Choulis, P. Schilinsky, C. J. Brabec, Adv. Mater. 2007, 19, 3973. [12] K. K. Manga, S. Wang, M. Jaiswal, Q. Bao, K. P. Loh, Adv. Mater. 2010, 22, 5265. [13] J. Z. Wang, Z. H. Zheng, H. W. Li, W. T. Huck, H. Sirringhaus, Nat. Mater. 2004, 3, 171. [14] C. W. Sele, T. von. Werne, R. H. Friend, H. Sirringhaus, Adv. Mater. 2005, 17, 997. [15] Y. Y. Noh, N. Zhao, M. Caironi, H. Sirringhaus, Nat. Nanotechnol. 2007, 2, 784. [16] T. Sekitani, Y. Noguchi, U. Zschieschang, H. Klauk, T. Someya, Proc. Natl. Acad. Sci. USA 2008, 105, 4976. [17] Z. Zhang, X. Zhang, Z. Xin, M. Deng, Y. Wen, Y. Song, Adv. Mater. 2013, 25, 6714. [18] L. Zhang, H. Liu, Y. Zhao, X. Sun, Y. Wen, Y. Guo, X. Gao, C. A. Di, G. Yu, Y. Liu, Adv. Mater. 2012, 24, 436. [19] H. Kang, R. Kitsomboonloha, J. Jang, V. Subramanian, Adv. Mater. 2012, 24, 3065. [20] M. Uno, K. Nakayama, J. Soeda, Y. Hirose, K. Miwa, T. Uemura, A. Nakao, K. Takimiya, J. Takeya, Adv. Mater. 2011, 23, 3047. [21] A. J. Kronemeijer, E. Gili, M. Shahid, J. Rivnay, A. Salleo, M. Heeney, H. Sirringhaus, Adv. Mater. 2012, 24, 1558.
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[30] K. Wasapinyokul, W. I. Milne, D. P. Chu, J. Appl. Phys. 2011, 109, 084510. [31] S. E. Ahn, S. Jeon, Y. W. Jeon, C. Kim, M. J. Lee, C. W. Lee, J. Park, I. Song, A. Nathan, S. Lee, U. I. Chung, Adv. Mater. 2013, 25, 5549. [32] K. Roy, M. Padmanabhan, S. Goswami, T. Phanindra Sai,G. Ramalingam, S. Raghavan, A. Ghosh, Nat. Nanotechnol. 2013, 8, 826. [33] M. H. Hoang, Y. Kim, M. Kim, K. H. Kim, T. W. Lee, D. N. Nguyen, S. J. Kim, K. Lee, S. J. Lee, D. H. Choi, Adv. Mater. 2012, 24, 5363. [34] K. H. Kim, S. Y. Bae, Y. S. Kim, J. A. Hur, M. H. Hoang, T. W. Lee, M. J. Cho, Y. Kim, M. Kim, J. I. Jin, S. J. Kim, K. Lee, S. J. Lee, D. H. Choi, Adv. Mater. 2011, 23, 3095.
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[22] M. El Gemayel, M. Treier, C. Musumeci, C. Li, K. Mullen, P. Samori, J. Am. Chem. Soc. 2012, 134, 2429. [23] C. Cheng, C. Yu, Y. Guo, H. Chen, Y. Fang, G. Yu, Y. Liu, Chem. Commun. 2013, 49, 1998. [24] D. Soltman, V. Subramanian, Langmuir 2008, 24, 2224. [25] H. Hu, R. G. Larson, J. Phys. Chem. B. 2006, 110, 7090. [26] H. Kang, D. Soltman, V. Subramanian, Langmuir 2010, 26, 11568. [27] S. Choi, S. Stassi, A. P. Pisano, T. I. Zohdi, Langmuir 2010, 26, 11690. [28] T. Kawase, H. Sirringhaus, R. H. Friend, T. Shimoda, Adv. Mater. 2001, 13, 1601. [29] B. J. de Gans, S. Hoeppener, U. S. Schubert, Adv. Mater. 2006, 18, 910.
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