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Triacetate cellulose gate dielectric organic thin-film transistors
Organic Electronics xxx (2016) 1e4
Contents lists available at ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Letter
Triacetate cellulose gate dielectric organic thin-film transistors Jin Woo Bae a, *, Hyung-Seok Jang a, Won-Hyeong Park b, Sang-Youn Kim b, ** a
Department of Mechanical Engineering, Berkeley Sensor and Actuator Center, University of California, Berkeley, CA, 94720, United States Interaction Laboratory of Advanced Technology Research Center, Korea University of Technology and Education, 1800 Chungjeollo, Byeongcheon-Myeon, Cheonan-City 330-708, Chungnam Province, South Korea
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 July 2016 Received in revised form 25 October 2016 Accepted 2 November 2016 Available online xxx
Here, we report on the performance and the characterization of all solution-processable top-contact organic thin-film transistors (OTFTs) consisting of a natural-resourced triacetate cellulose gate dielectric and a representative hole-transport poly[2,5-bis(3-dodecylthiophen-2-yl)thieno[3,2-b]thiophene] (pBTTT) semiconductor layer on rigid or flexible substrates. The bio-based triacetate cellulose layer has an important role in the OTFT fabrication because it provides the pBTTT semiconducting polymer with highly suitable gate dielectric properties including a low surface roughness, hydrophobic surface, appropriate dielectric constant, and low leakage current. The triacetate cellulose gate dielectric-based pBTTT OTFTs exhibit an average filed-effect mobility of 0.031 cm2/Vs similar to that obtained from a SiO2 gate dielectric-based OTFT device in ambient conditions. Even after a bending stimulation of 100 times and in an outward bending state, the flexible triacetate cellulose gate pBTTT OTFT device still showed excellent electrical device performance without any hysteresis. © 2016 Elsevier B.V. All rights reserved.
Keywords: Triacetate cellulose Eco-friendly gate dielectric polymer pBTTT semiconductor Flexible organic thin film transistors
1. Introduction Organic thin-film transistors (OTFTs) have been used in various flexible, bendable, and electrical consumer devices such as active matrix displays, radiofrequency identification tags (RFID) and sensing applications [1e3]. Recently, several bio-based dielectric polymers in OTFT components have received increasing interest because of their highly ordered structure and unique properties [4e9]. Because the biomaterials are a renewable, non-toxic, abundant, and inherently biodegradable or biocompatible resource, in particular, it is expected to widely open up applications in bioelectronics technology as environmentally-friendly materials [5e9]. Even though several natural-based polymers as ideal organic dielectric materials have been introduced, there are still critical limitations in achieving ‘green electronics’ with stable operation. First, natural resources generally possess hydrophilic properties which are detrimental to device operations because of their water sensitivity [6]. Additionally, their low surface energy makes it quite difficult to introduce a solution-processed organic semiconductor
* Corresponding author. ** Corresponding author. E-mail address: [email protected] (S.-Y. Kim).
onto their surface [10]. Even natural resource-based gate dielectrics are likely to suffer from high hysteresis which is easily observed in unstable device operations [11]. Therefore, natural-resourced dielectric materials need to overcome their intrinsic problems for practical applications in OTFT devices. ‘Cellulose derivatives’ are one of the typically renewable natural-resources which are derived from cellulose by acetylating cellulose with acetic acid and/or acetic anhydride [12e14]. Previously, a few cellulose-based polymers have been successfully synthesized, commercialized and utilized as excellent environmental protective coating materials or next generation water-treatment membranes [15e19]. In particular, chemical functionalization of triacetate cellulose (TAC) completely converts all hydroxyl groups in cellulose to acetyl groups rendering the cellulose polymer much more soluble in organic solvents as well as more hydrophobic and heat-resistant [12e19]. In this study, we introduced solutionprocessable TAC as the gate dielectric in bio-based OTFT (BiOTFT) devices. The hydrophobic TAC showed good compatibility with the solution-processed poly[2,5-bis(3-dodecylthiophen-2-yl)thieno [3,2-b]thiophene] (pBTTT) semiconductor. As a result, BiOTFT devices prepared by cellulose derivatives as the gate dielectric onto both a rigid ITO-glass and a flexible ITO-PET substrate exhibited excellent and stable device performance, especially similar to that obtained from a SiO2 gate dielectric-based OTFT device in ambient conditions. In particular, in comparison to the rigid BiOTFT device,
http://dx.doi.org/10.1016/j.orgel.2016.11.002 1566-1199/© 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: J.W. Bae, et al., Triacetate cellulose gate dielectric organic thin-film transistors, Organic Electronics (2016), http://dx.doi.org/10.1016/j.orgel.2016.11.002
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Fig. 1. Schematic diagram of the top-contact pBTTT-OTFT device fabricated with an eco-friendly TAC dielectric layer on an ITO PET or ITO glass substrate. Molecular structures of the TAC gate insulator and pBTTT semiconducting polymer (insert).
the flexible TAC gate dielectric BiOTFT arrays prepared onto an ITOPET substrate still maintained a high field-effect mobility and good current on-off ratios without any hysteresis after several bending stresses and even in a bending state. 2. Experimental Top-contact pBTTT-BiOTFT devices were fabricated with a TAC layer on both rigid ITO-glass or flexible ITO-PET substrates (20 mm 20 mm) shown in Fig. 1. The insert in Fig. 1 shows the molecular structures of TAC and pBTTT used as a gate insulating layer and semiconducting polymer, respectively. First, all substrates were cleaned in acetone, ethanol and deionized water using ultra sonication for 10 min. A 2 wt% TAC (Sigma-Aldrich) solution in CHCl3 (Sigma-Aldrich, 99.9%) was spin-coated on the substrates at 3000 rpm for 60 s followed by thermal treatment at 200 C for 30 min. Then, as a semiconductor layer, the pBTTT (Mw ¼ 40,000e80,000) solution (10 mg/mL) in 1,2dichlorobenzene was directly coated onto the thermally treated TAC layer by spin-coating at 2000 rpm for 60 s followed by annealing at 200 C for 30 min. The TAC gate dielectric pBTTTBiOTFTs were then completed by applying thermal evaporation to add 50 nm thick source and drain gold electrodes on top of the pBTTT active layer creating transistors with a channel length (L) of 20e100 mm and a width (W) of 0.5e1 mm, respectively. The evaporation rate of the gold was 1 Å/s. The surface morphology of the PHCPM film was studied with an atomic force microscope (AFM, Park System XE-70). The static contact angles of deionized water and diiodomethane were measured at room temperature and ambient relative humidity using a Kruss DSA 10 contact angle analyzer interfaced with drop shape analysis software. The Owens-Wendt-Rabel-Kaelble (OWRK) method [20] was used to calculate the surface energy of the polymer films. The thicknesses of the TAC films determined by a surface profilometer (Kosaka ET-3000) were about 325 nm. To determine the capacitance and leakage current of the PHCPM film, metaleinsulator-metal (MIM) capacitor structures were prepared by deposition of an Al electrode on the TAC spin-coated Si wafer using a thermal evaporator (DAEKI HI-TECH Co.). The thickness and active area of the MIM device were 100 nm and 0.65 mm2, respectively. The frequency dependence of the capacitance and leakage current measurements was evaluated with an Agilent 4284A LCR meter and 4156C semiconductor parameter analyzer, respectively. The electrical properties of the TAC gate dielectric BiOTFTs were measured with an Agilent 4156C semiconductor parameter analyzer under ambient conditions. 3. Results and discussion TAC film as a natural-based biomaterial was developed using cellulose which is one of the world's most abundant representative
renewable materials [12e19]. TAC polymers originating from cellulose can be synthesized by acetylation through substitution of the hydroxyl groups in the cellulose with acetic acid and/or acetic anhydride functional groups. Accordingly, the TAC polymers can be dissolved in some kinds of organic solvents, although the cellulose, a polysaccharide consisting of b-1,4 glycosidic linkages of anhydroglucose units, is commonly insoluble in water and most organic solvents due to the strong intermolecular hydrogen bonding within and between the cellulose molecules. The solution-processable TAC film with uniform thickness can easily be handled by various processes including the spin-coating and bar coating methods. In this study, a 2 wt% TAC solution in chloroform was spin-coated onto ITO-glass and ITO-PET, respectively. After thermal annealing of the TAC film at 200 C for 30 min, the various properties of the TAC film as a good gate dielectric were confirmed, so it could be used in suitable and stable BiOTFT devices. The surface properties of the TAC film were investigated by measuring the water contact angle and surface roughness. The TAC film has a low surface energy (41.3 mN/m) which is much smaller or similar to that of SiO2 (61.4 mN/m) or HMDS-treated SiO2 (43.6 mN/ m) [21]. Accordingly, the TAC film was found to be hydrophobic which could be advantageous for a water-sensitive device. Moreover, the root-mean-square roughness (RMS) of the spin-coated TAC film was determined to be 1.016 nm by AFM which is similar to that of natural resource-based dielectrics such as silk fibroin, cellulose, and chicken albumin [7e9]. Therefore, the smooth surface and low surface energy of the TAC film are appropriate enough to effectively utilize further enhancements of the structural order of solution-processable thin-film pBTTT semiconducting polymers in BiOTFT devices [9,21]. The capacitance, gate leakage current density, and breakdown voltage values of the prepared thin TAC films were measured with the MIM capacitor structure. The measured capacitance and dielectric constant values of the TAC films were 8.1 nF/cm2 and 4.57, respectively, at 1 kHz (Fig. 2(a)). Considering that natural-resourcebased gate dielectrics often suffer from a high dependence of capacitance based on the frequency, the capacitance of the TAC film did not significantly change in the range of 100 Hz to 1 MHz shown in Fig. 2(a) which indicates that there are few mobile impurities in the TAC dielectric films [22]. Additionally, the TAC film showed a relatively stable gate leakage current density exceeding 107 A/cm2 at a breakdown field of over 1.54 MV/cm (Fig. 2(b)). The low leakage current density and high break-down voltage (50 V) of the TAC film may be due to the acetylation effect of the hydroxyl groups in the cellulose [23e25]. Because the hydroxyl groups in the dielectric layer deteriorate the gate leakage current densities [25], chemical functionalization of the TAC films converting the hydroxyl groups in the cellulose to acetyl groups are expected to form a stable gate dielectric layer. Taking the surface and the dielectric properties into account, it is therefore clear that the TAC films can be used as a good gate dielectric in BiOTFT devices.
Please cite this article in press as: J.W. Bae, et al., Triacetate cellulose gate dielectric organic thin-film transistors, Organic Electronics (2016), http://dx.doi.org/10.1016/j.orgel.2016.11.002
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Fig. 2. (a) Frequency dependent capacitance density and (b) electric field dependent leakage current density plots for the TAC dielectric layer with the MIM capacitor structure.
As shown in the BiOTFT device diagram of Fig. 1, ITO-glass or ITO-PET as a rigid (or flexible) substrate was chosen, and the TAC film was used as a gate dielectric. Moreover, pBTTT solution (10 mg/ ml) in 1,2-dichlorobenzene was spin-coated onto the TAC film as a solution-processable hole-transport semiconducting polymer. Fig. 3(a) shows the output characteristics of the pBTTT BiOTFT with the TAC dielectric on the rigid ITO-glass. The output characteristic curves of the pBTTT BiOTFT have good pinch-off and current saturation features. The saturated mobility value can be obtained from the transfer characteristics (Fig. 3(b)). All electrical performances of 60 triacetate cellulose gate pBTTT OTFT devices fabricated onto each ITO-glass (or ITO-PET) were evaluated in an ambient atmosphere and summarized in Table 1. The average fieldeffect mobility of the pBTTT BiOTFTs with the TAC gate insulators on the rigid ITO-glass was 0.031 ± 0.007 cm2/Vs, which is comparable to that of an untreated silicon oxide gate dielectric-based OTFT device [26e28]. This mobility value might be attributed to the good dielectric characteristics of the TAC film and the good compatibility between the TAC dielectric film and the pBTTT semiconducting layer. To apply the flexible BiOTFT device system, the TAC dielectric layer was coated onto the flexible substrate of an ITO-coated PET film followed by the introduction of the pBTTT semiconducting layer. The device geometry and working condition were identical with that of the previous ITO-glass type device. However, bending stimulation of the flexible BiOTFT device arrays was done with a custom-built bending apparatus with a dimension of 15 mm. Inward and outward bending was repeated 100 times shown in Fig. 4(a) and (b); after which, all electrical characteristics were measured in an outward bending state. Generally, flexible BiOTFT devices easily suffer from an increase in the leakage current after bending leading to low device performance [9]. Surprisingly, the flexible TAC pBTTT BiOTFT devices still exhibited excellent device
Table 1 Electrical characteristics of triacetate cellulose gate pBTTT OTFT devices fabricated onto different substrates. Substrates type
mavga (cm2/Vs)
Ion/Ioff
Vth (V)
ITO-glass (uncurved state) ITO-PET (curved state)
0.031 ± 0.007 0.027 ± 0.004
1.2Eþ03 1.2Eþ03
0.15 0.12
a Average field-effect mobilities and standard deviations are calculated from 60 individual BiOTFT devices.
performances without a significant difference from the rigid TAC pBTTT BiOTFT devices. As shown in Table 1, the average mobility value and on/off ratio were found to be 0.027 ± 0.004 cm2/Vs and 1.2 103, respectively, which are almost identical with those obtained from the rigid ITO glass substrate. Even though the flexible device performances were severely measured in an outward bending state after 100 times of inward and outward bending, as seen in Fig. 4(c), hysteresis of the transfer characteristics was not observed during 10 cycle-operations suggesting that the mechanical and dielectric properties of the TAC dielectric layer is sufficient enough for application in flexible TAC pBTTT BiOTFT devices. Therefore, the results show that a natural-resourced, thin, hydrophobic and solution-processable TAC film as a gate dielectric layer, even in a bending state, has an important role in the high mobility value and reliably stable performances of flexible BiOTFT devices without significant hysteresis. 4. Conclusions Natural-resource-based, hydrophobic, thin, and solutionprocessable TAC films as a gate dielectric were used in rigid and flexible BiOTFT devices. The eco-friendly TAC films had a smooth surface with low RMS roughness, an appropriate dielectric constant
Fig. 3. a) Output and b) transfer curves obtained from the pBTTT-based OTFT devices with the triacetate cellulose gate dielectric.
Please cite this article in press as: J.W. Bae, et al., Triacetate cellulose gate dielectric organic thin-film transistors, Organic Electronics (2016), http://dx.doi.org/10.1016/j.orgel.2016.11.002
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Fig. 4. a) A schematic diagram of the procedure for the bending stimulation of the flexible BiOTFT device arrays using a custom-built bending apparatus with a dimension of 15 mm. After inward and outward bending was repeated 100 times, then b) a photo and c) the transfer characteristics of the flexible pBTTT-based OTFT device using the TAC gate dielectric were done in an outward bending state. The transfer curves were repeated 10 times.
and a stable low leakage current density. Due to these good surface and dielectric properties of the TAC-gate dielectric layer, fully solution-processed BiOTFT arrays onto rigid ITO-glass exhibited high mobility values and reliably stable device performance which were similar to that of the representative silicon dioxide gate dielectric-based BiOTFT device. In addition, like the rigid BiOTFT arrays, the flexible BiOTFT devices with the TAC gate dielectric layer maintained the high BiOTFT mobility and current on/off ratio without any hysteresis after several bending stimulations. This result might be attributed to the superior surface and dielectric properties of the TAC dielectric layer. Therefore, the inherently environmentally-friendly TAC film as a renewable resource-based polymeric gate dielectric is expected to be applied to a variety of new generation medical, flexible, and portable electronic devices. Acknowledgements We gratefully acknowledge helpful discussions with Ms. JeongHyun Ryu within Research & Analytical Centre at KOREATECH. This research was supported by the Pioneer Research Centre Program through the National Research Foundation of Korea funded by the Ministry of Science, ICT & Future Planning (NRF2013M3C1A3059588). References [1] C.D. Dimitrakopoulos, P.R.L. Malenfant, Adv. Mater. 14 (2002) 99. [2] M. Fadlallah, G. Billiot, W. Eccleston, D. Barclay, Solid State Electron. 51 (2007) 1047. [3] K. Fukuda, Y. Takeda, M. Mizukami, D. Kumaki, S. Tokito, Sci. Rep. 4 (2014) 3947. [4] M. Irimia-Vladu, Chem. Soc. Rev. 43 (2014) 588.
[5] M. Irimia-Vladu, P.A. Troshin, M. Reisinger, L. Shmygleva, Y. Kanbur, €diauer, A. Mumyatov, J.W. Fergus, G. Schwabegger, M. Bodea, R. Schwo V.F. Razumov, N.S. Sariciftci, S. Bauer, Adv. Func. Mater. 20 (2010) 4069. [6] C. Yumusak, T.B. Singh, N.S. Sariciftci, J.G. Grote, Appl. Phys. Lett. 95 (2009) 263304. [7] C.H. Wang, C.Y. Hsieh, J.C. Hwang, Adv. Mater. 23 (2011) 1630. [8] A. Petritz, A. Wolfberger, A. Fian, T. Griesser, M. Irimia-Vladu, B. Stadlober, Adv. Mater. 27 (2015) 7645. [9] J.W. Chang, C.G. Wang, C.Y. Huang, T.D. Tsai, T.F. Guo, T.C. Wen, Adv. Mater. 23 (2011) 4077. [10] R. Hamilton, J. Smith, S. Ogier, M. Heeney, J.E. Anthony, I. McCulloch, J. Veres, D.D.C. Bradley, T.D. Anthopoulos, Adv. Mater. 21 (2009) 1166. [11] B. Singh, N.S. Sariciftci, J.G. Grote, F.K. Hopkins, J. Appl. Phys. 100 (2006) 24514. [12] K. Kamide, Cellulose and Cellulose Derivatives, Molecular Characterization and its Applications, first ed., Elsevier Science, 2005. [13] H. Sata, M. Murayama, S. Shimamoto, Macromol. Symp. 208 (2004) 323. [14] Balser, K., Hoppe, L., Eicher, T., Wandel, M., Astheimer, H.-J., Steinmeier, H., Allen, J. M., 7 (2012) 333. [15] M.T.M. Pendergast, E.M.V. Hoek, Energy Environ. Sci 4 (2011) 1946. [16] Garcia, M. A., Ferrero, C., Bertola, N., Martino, M., Zaritzky, N. 3 (2002) 391. [17] T.P.N. Nguyen, E.-T. Yun, I.-C. Kim, Y.-N. Kwon, J. Membr. Sci. 433 (2013) 49. [18] H. Sata, M. Murayama, S. Shimamoto, Macromol. Symp. 208 (2004) 323. [19] S. Nobukawa, H. Shimada, Y. Aoki, A. Miyagawa, V.A. Doan, H. Yoshimura, Y. Tachikawa, M. Yamaguchi, Polymer 55 (2014) 3247. [20] D.K. Owens, R.C. Wendt, J. Appl. Polym. Sci. 13 (1969) 1741. [21] S.C. Lim, S.H. Kim, J.H. Lee, M.K. Kim, T. Zyung, Synth. Met. 148 (2005) 75. [22] J. Veres, S. Ogier, G. Lloyd, D. De Leeuw, Chem. Mater. 16 (2004) 4543. [23] S.H. Kim, S.Y. Yang, K. Shin, H. Jeon, J.W. Lee, K.P. Hong, C.E. Park, Appl. Phys. Lett. 89 (2006) 183516. [24] L. Kergoat, N. Battaglini, L. Miozzo, B. Piro, M.C. Pham, A. Yassar, G. Horowitz, Org. Electron. 12 (2011) 1253. [25] S. Lee, B. Koo, J. Shin, E. Lee, H. Park, Appl. Phys. Lett. 88 (2006) 162109. [26] J.W. Bae, K. Song, Org. Electron. 30 (2016) 143. [27] R.J. Kline, D.M. DeLongchamp, D.A. Fischer, E.K. Lin, M. Heeney, I. McCulloch, M.F. Toney, Appl. Phys. Lett. 90 (2007) 062117. [28] C. Wang, L.H. Jimison, L. Goris, I. McCulloch, M. Heeney, A. Ziegler, A. Salleo, Adv. Mater. 22 (2010) 697.
Please cite this article in press as: J.W. Bae, et al., Triacetate cellulose gate dielectric organic thin-film transistors, Organic Electronics (2016), http://dx.doi.org/10.1016/j.orgel.2016.11.002
Contents lists available at ScienceDirect
Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Letter
Triacetate cellulose gate dielectric organic thin-film transistors Jin Woo Bae a, *, Hyung-Seok Jang a, Won-Hyeong Park b, Sang-Youn Kim b, ** a
Department of Mechanical Engineering, Berkeley Sensor and Actuator Center, University of California, Berkeley, CA, 94720, United States Interaction Laboratory of Advanced Technology Research Center, Korea University of Technology and Education, 1800 Chungjeollo, Byeongcheon-Myeon, Cheonan-City 330-708, Chungnam Province, South Korea
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 July 2016 Received in revised form 25 October 2016 Accepted 2 November 2016 Available online xxx
Here, we report on the performance and the characterization of all solution-processable top-contact organic thin-film transistors (OTFTs) consisting of a natural-resourced triacetate cellulose gate dielectric and a representative hole-transport poly[2,5-bis(3-dodecylthiophen-2-yl)thieno[3,2-b]thiophene] (pBTTT) semiconductor layer on rigid or flexible substrates. The bio-based triacetate cellulose layer has an important role in the OTFT fabrication because it provides the pBTTT semiconducting polymer with highly suitable gate dielectric properties including a low surface roughness, hydrophobic surface, appropriate dielectric constant, and low leakage current. The triacetate cellulose gate dielectric-based pBTTT OTFTs exhibit an average filed-effect mobility of 0.031 cm2/Vs similar to that obtained from a SiO2 gate dielectric-based OTFT device in ambient conditions. Even after a bending stimulation of 100 times and in an outward bending state, the flexible triacetate cellulose gate pBTTT OTFT device still showed excellent electrical device performance without any hysteresis. © 2016 Elsevier B.V. All rights reserved.
Keywords: Triacetate cellulose Eco-friendly gate dielectric polymer pBTTT semiconductor Flexible organic thin film transistors
1. Introduction Organic thin-film transistors (OTFTs) have been used in various flexible, bendable, and electrical consumer devices such as active matrix displays, radiofrequency identification tags (RFID) and sensing applications [1e3]. Recently, several bio-based dielectric polymers in OTFT components have received increasing interest because of their highly ordered structure and unique properties [4e9]. Because the biomaterials are a renewable, non-toxic, abundant, and inherently biodegradable or biocompatible resource, in particular, it is expected to widely open up applications in bioelectronics technology as environmentally-friendly materials [5e9]. Even though several natural-based polymers as ideal organic dielectric materials have been introduced, there are still critical limitations in achieving ‘green electronics’ with stable operation. First, natural resources generally possess hydrophilic properties which are detrimental to device operations because of their water sensitivity [6]. Additionally, their low surface energy makes it quite difficult to introduce a solution-processed organic semiconductor
* Corresponding author. ** Corresponding author. E-mail address: [email protected] (S.-Y. Kim).
onto their surface [10]. Even natural resource-based gate dielectrics are likely to suffer from high hysteresis which is easily observed in unstable device operations [11]. Therefore, natural-resourced dielectric materials need to overcome their intrinsic problems for practical applications in OTFT devices. ‘Cellulose derivatives’ are one of the typically renewable natural-resources which are derived from cellulose by acetylating cellulose with acetic acid and/or acetic anhydride [12e14]. Previously, a few cellulose-based polymers have been successfully synthesized, commercialized and utilized as excellent environmental protective coating materials or next generation water-treatment membranes [15e19]. In particular, chemical functionalization of triacetate cellulose (TAC) completely converts all hydroxyl groups in cellulose to acetyl groups rendering the cellulose polymer much more soluble in organic solvents as well as more hydrophobic and heat-resistant [12e19]. In this study, we introduced solutionprocessable TAC as the gate dielectric in bio-based OTFT (BiOTFT) devices. The hydrophobic TAC showed good compatibility with the solution-processed poly[2,5-bis(3-dodecylthiophen-2-yl)thieno [3,2-b]thiophene] (pBTTT) semiconductor. As a result, BiOTFT devices prepared by cellulose derivatives as the gate dielectric onto both a rigid ITO-glass and a flexible ITO-PET substrate exhibited excellent and stable device performance, especially similar to that obtained from a SiO2 gate dielectric-based OTFT device in ambient conditions. In particular, in comparison to the rigid BiOTFT device,
http://dx.doi.org/10.1016/j.orgel.2016.11.002 1566-1199/© 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: J.W. Bae, et al., Triacetate cellulose gate dielectric organic thin-film transistors, Organic Electronics (2016), http://dx.doi.org/10.1016/j.orgel.2016.11.002
2
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Fig. 1. Schematic diagram of the top-contact pBTTT-OTFT device fabricated with an eco-friendly TAC dielectric layer on an ITO PET or ITO glass substrate. Molecular structures of the TAC gate insulator and pBTTT semiconducting polymer (insert).
the flexible TAC gate dielectric BiOTFT arrays prepared onto an ITOPET substrate still maintained a high field-effect mobility and good current on-off ratios without any hysteresis after several bending stresses and even in a bending state. 2. Experimental Top-contact pBTTT-BiOTFT devices were fabricated with a TAC layer on both rigid ITO-glass or flexible ITO-PET substrates (20 mm 20 mm) shown in Fig. 1. The insert in Fig. 1 shows the molecular structures of TAC and pBTTT used as a gate insulating layer and semiconducting polymer, respectively. First, all substrates were cleaned in acetone, ethanol and deionized water using ultra sonication for 10 min. A 2 wt% TAC (Sigma-Aldrich) solution in CHCl3 (Sigma-Aldrich, 99.9%) was spin-coated on the substrates at 3000 rpm for 60 s followed by thermal treatment at 200 C for 30 min. Then, as a semiconductor layer, the pBTTT (Mw ¼ 40,000e80,000) solution (10 mg/mL) in 1,2dichlorobenzene was directly coated onto the thermally treated TAC layer by spin-coating at 2000 rpm for 60 s followed by annealing at 200 C for 30 min. The TAC gate dielectric pBTTTBiOTFTs were then completed by applying thermal evaporation to add 50 nm thick source and drain gold electrodes on top of the pBTTT active layer creating transistors with a channel length (L) of 20e100 mm and a width (W) of 0.5e1 mm, respectively. The evaporation rate of the gold was 1 Å/s. The surface morphology of the PHCPM film was studied with an atomic force microscope (AFM, Park System XE-70). The static contact angles of deionized water and diiodomethane were measured at room temperature and ambient relative humidity using a Kruss DSA 10 contact angle analyzer interfaced with drop shape analysis software. The Owens-Wendt-Rabel-Kaelble (OWRK) method [20] was used to calculate the surface energy of the polymer films. The thicknesses of the TAC films determined by a surface profilometer (Kosaka ET-3000) were about 325 nm. To determine the capacitance and leakage current of the PHCPM film, metaleinsulator-metal (MIM) capacitor structures were prepared by deposition of an Al electrode on the TAC spin-coated Si wafer using a thermal evaporator (DAEKI HI-TECH Co.). The thickness and active area of the MIM device were 100 nm and 0.65 mm2, respectively. The frequency dependence of the capacitance and leakage current measurements was evaluated with an Agilent 4284A LCR meter and 4156C semiconductor parameter analyzer, respectively. The electrical properties of the TAC gate dielectric BiOTFTs were measured with an Agilent 4156C semiconductor parameter analyzer under ambient conditions. 3. Results and discussion TAC film as a natural-based biomaterial was developed using cellulose which is one of the world's most abundant representative
renewable materials [12e19]. TAC polymers originating from cellulose can be synthesized by acetylation through substitution of the hydroxyl groups in the cellulose with acetic acid and/or acetic anhydride functional groups. Accordingly, the TAC polymers can be dissolved in some kinds of organic solvents, although the cellulose, a polysaccharide consisting of b-1,4 glycosidic linkages of anhydroglucose units, is commonly insoluble in water and most organic solvents due to the strong intermolecular hydrogen bonding within and between the cellulose molecules. The solution-processable TAC film with uniform thickness can easily be handled by various processes including the spin-coating and bar coating methods. In this study, a 2 wt% TAC solution in chloroform was spin-coated onto ITO-glass and ITO-PET, respectively. After thermal annealing of the TAC film at 200 C for 30 min, the various properties of the TAC film as a good gate dielectric were confirmed, so it could be used in suitable and stable BiOTFT devices. The surface properties of the TAC film were investigated by measuring the water contact angle and surface roughness. The TAC film has a low surface energy (41.3 mN/m) which is much smaller or similar to that of SiO2 (61.4 mN/m) or HMDS-treated SiO2 (43.6 mN/ m) [21]. Accordingly, the TAC film was found to be hydrophobic which could be advantageous for a water-sensitive device. Moreover, the root-mean-square roughness (RMS) of the spin-coated TAC film was determined to be 1.016 nm by AFM which is similar to that of natural resource-based dielectrics such as silk fibroin, cellulose, and chicken albumin [7e9]. Therefore, the smooth surface and low surface energy of the TAC film are appropriate enough to effectively utilize further enhancements of the structural order of solution-processable thin-film pBTTT semiconducting polymers in BiOTFT devices [9,21]. The capacitance, gate leakage current density, and breakdown voltage values of the prepared thin TAC films were measured with the MIM capacitor structure. The measured capacitance and dielectric constant values of the TAC films were 8.1 nF/cm2 and 4.57, respectively, at 1 kHz (Fig. 2(a)). Considering that natural-resourcebased gate dielectrics often suffer from a high dependence of capacitance based on the frequency, the capacitance of the TAC film did not significantly change in the range of 100 Hz to 1 MHz shown in Fig. 2(a) which indicates that there are few mobile impurities in the TAC dielectric films [22]. Additionally, the TAC film showed a relatively stable gate leakage current density exceeding 107 A/cm2 at a breakdown field of over 1.54 MV/cm (Fig. 2(b)). The low leakage current density and high break-down voltage (50 V) of the TAC film may be due to the acetylation effect of the hydroxyl groups in the cellulose [23e25]. Because the hydroxyl groups in the dielectric layer deteriorate the gate leakage current densities [25], chemical functionalization of the TAC films converting the hydroxyl groups in the cellulose to acetyl groups are expected to form a stable gate dielectric layer. Taking the surface and the dielectric properties into account, it is therefore clear that the TAC films can be used as a good gate dielectric in BiOTFT devices.
Please cite this article in press as: J.W. Bae, et al., Triacetate cellulose gate dielectric organic thin-film transistors, Organic Electronics (2016), http://dx.doi.org/10.1016/j.orgel.2016.11.002
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3
Fig. 2. (a) Frequency dependent capacitance density and (b) electric field dependent leakage current density plots for the TAC dielectric layer with the MIM capacitor structure.
As shown in the BiOTFT device diagram of Fig. 1, ITO-glass or ITO-PET as a rigid (or flexible) substrate was chosen, and the TAC film was used as a gate dielectric. Moreover, pBTTT solution (10 mg/ ml) in 1,2-dichlorobenzene was spin-coated onto the TAC film as a solution-processable hole-transport semiconducting polymer. Fig. 3(a) shows the output characteristics of the pBTTT BiOTFT with the TAC dielectric on the rigid ITO-glass. The output characteristic curves of the pBTTT BiOTFT have good pinch-off and current saturation features. The saturated mobility value can be obtained from the transfer characteristics (Fig. 3(b)). All electrical performances of 60 triacetate cellulose gate pBTTT OTFT devices fabricated onto each ITO-glass (or ITO-PET) were evaluated in an ambient atmosphere and summarized in Table 1. The average fieldeffect mobility of the pBTTT BiOTFTs with the TAC gate insulators on the rigid ITO-glass was 0.031 ± 0.007 cm2/Vs, which is comparable to that of an untreated silicon oxide gate dielectric-based OTFT device [26e28]. This mobility value might be attributed to the good dielectric characteristics of the TAC film and the good compatibility between the TAC dielectric film and the pBTTT semiconducting layer. To apply the flexible BiOTFT device system, the TAC dielectric layer was coated onto the flexible substrate of an ITO-coated PET film followed by the introduction of the pBTTT semiconducting layer. The device geometry and working condition were identical with that of the previous ITO-glass type device. However, bending stimulation of the flexible BiOTFT device arrays was done with a custom-built bending apparatus with a dimension of 15 mm. Inward and outward bending was repeated 100 times shown in Fig. 4(a) and (b); after which, all electrical characteristics were measured in an outward bending state. Generally, flexible BiOTFT devices easily suffer from an increase in the leakage current after bending leading to low device performance [9]. Surprisingly, the flexible TAC pBTTT BiOTFT devices still exhibited excellent device
Table 1 Electrical characteristics of triacetate cellulose gate pBTTT OTFT devices fabricated onto different substrates. Substrates type
mavga (cm2/Vs)
Ion/Ioff
Vth (V)
ITO-glass (uncurved state) ITO-PET (curved state)
0.031 ± 0.007 0.027 ± 0.004
1.2Eþ03 1.2Eþ03
0.15 0.12
a Average field-effect mobilities and standard deviations are calculated from 60 individual BiOTFT devices.
performances without a significant difference from the rigid TAC pBTTT BiOTFT devices. As shown in Table 1, the average mobility value and on/off ratio were found to be 0.027 ± 0.004 cm2/Vs and 1.2 103, respectively, which are almost identical with those obtained from the rigid ITO glass substrate. Even though the flexible device performances were severely measured in an outward bending state after 100 times of inward and outward bending, as seen in Fig. 4(c), hysteresis of the transfer characteristics was not observed during 10 cycle-operations suggesting that the mechanical and dielectric properties of the TAC dielectric layer is sufficient enough for application in flexible TAC pBTTT BiOTFT devices. Therefore, the results show that a natural-resourced, thin, hydrophobic and solution-processable TAC film as a gate dielectric layer, even in a bending state, has an important role in the high mobility value and reliably stable performances of flexible BiOTFT devices without significant hysteresis. 4. Conclusions Natural-resource-based, hydrophobic, thin, and solutionprocessable TAC films as a gate dielectric were used in rigid and flexible BiOTFT devices. The eco-friendly TAC films had a smooth surface with low RMS roughness, an appropriate dielectric constant
Fig. 3. a) Output and b) transfer curves obtained from the pBTTT-based OTFT devices with the triacetate cellulose gate dielectric.
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Fig. 4. a) A schematic diagram of the procedure for the bending stimulation of the flexible BiOTFT device arrays using a custom-built bending apparatus with a dimension of 15 mm. After inward and outward bending was repeated 100 times, then b) a photo and c) the transfer characteristics of the flexible pBTTT-based OTFT device using the TAC gate dielectric were done in an outward bending state. The transfer curves were repeated 10 times.
and a stable low leakage current density. Due to these good surface and dielectric properties of the TAC-gate dielectric layer, fully solution-processed BiOTFT arrays onto rigid ITO-glass exhibited high mobility values and reliably stable device performance which were similar to that of the representative silicon dioxide gate dielectric-based BiOTFT device. In addition, like the rigid BiOTFT arrays, the flexible BiOTFT devices with the TAC gate dielectric layer maintained the high BiOTFT mobility and current on/off ratio without any hysteresis after several bending stimulations. This result might be attributed to the superior surface and dielectric properties of the TAC dielectric layer. Therefore, the inherently environmentally-friendly TAC film as a renewable resource-based polymeric gate dielectric is expected to be applied to a variety of new generation medical, flexible, and portable electronic devices. Acknowledgements We gratefully acknowledge helpful discussions with Ms. JeongHyun Ryu within Research & Analytical Centre at KOREATECH. This research was supported by the Pioneer Research Centre Program through the National Research Foundation of Korea funded by the Ministry of Science, ICT & Future Planning (NRF2013M3C1A3059588). References [1] C.D. Dimitrakopoulos, P.R.L. Malenfant, Adv. Mater. 14 (2002) 99. [2] M. Fadlallah, G. Billiot, W. Eccleston, D. Barclay, Solid State Electron. 51 (2007) 1047. [3] K. Fukuda, Y. Takeda, M. Mizukami, D. Kumaki, S. Tokito, Sci. Rep. 4 (2014) 3947. [4] M. Irimia-Vladu, Chem. Soc. Rev. 43 (2014) 588.
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Please cite this article in press as: J.W. Bae, et al., Triacetate cellulose gate dielectric organic thin-film transistors, Organic Electronics (2016), http://dx.doi.org/10.1016/j.orgel.2016.11.002