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Polythiophene-based field-effect transistors with enhanced air stability
Synthetic Metals 142 (2004) 49–52
Polythiophene-based field-effect transistors with enhanced air stability Beng Ong∗ , Yiliang Wu, Lu Jiang, Ping Liu, Krish Murti Xerox Research Centre of Canada, 2660 Speakman Drive, Mississauga, Ont., Canada L5K 2L1 Received 20 August 2002; received in revised form 17 June 2003; accepted 23 July 2003
Abstract We have demonstrated that relatively air stable field-effect transistors (TFTs) with mobility up to 0.01 cm2 /V s, near-zero turn-on voltage, and current on/off ratio over 105 could be fabricated in entirety under ambient conditions with poly(3 ,4 -dialkyl-2,2 ;5 ,2 -terthiophene) as active channel layer. This class of polythiophenes, which comprise of regioregularly arranged 2,5-thienylene and disubstituted-2,5-thientylene moieties, have shown enhanced stability against p-doping by atmospheric oxygen. When exposed to atmospheric oxygen, the unprotected TFTs fabricated with these materials had exhibited significantly higher stability than those of regioregular poly(3-hexylthiophene)s under similar conditions. © 2003 Elsevier B.V. All rights reserved. Keywords: Organic thin-film transistor; Polythiophene; Field-effect mobility; Current on/off ratio
Organic semiconductors have in recent years attracted growing interest as active channel materials for thin-film field-effect transistors (FETs). One of the primary appeals of organic semiconductors is their potential in fabricating low-cost integrated circuit elements for large-area (e.g. displays, image sensors) and low-end microelectronic (e.g. smart cards, radio frequency identification tags) devices via simple solution processes such as spin- and dip-coating, screen printing, stamping, jet printing, etc. [1,2]. Other attributes of organic semiconductors, particularly polymeric materials, include compatibility with flexible substrates, mechanical durability, lightweight characteristics, etc. For low-cost manufacturing, the processing needs to be carried out in ambient conditions, which requires the materials to be stable in air. Any precautionary measures to insulate manufacturing processes from ambient environmental effects (e.g. exposure to oxygen) would lead to increased cost, thus nullifying the fundamental attribute of organic electronics as a low-cost alternative to their amorphous silicon counterpart. Accordingly, it is of paramount importance that the active materials have sufficient air stability to enable fabrication in ambient conditions to achieve and maintain useful functional performance characteristics before the devices are protected or shielded (e.g. via encapsulation) from environment. Recently, a great deal of attention has been paid to the ambient stability of organic
∗ Corresponding author. E-mail address: [email protected] (B. Ong).
0379-6779/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2003.07.004
small molecular and oligomeric semiconducting materials [3–5]. However, these materials are either insoluble, difficult to process in solution, or low performing in nature. On the other hand, most solution processable semiconductor polymers (e.g. polythiophenes) only provide functionally useful properties when processed in an inert atmosphere. For example, regioregular head-to-tail polythiophenes such as poly(3-hexylthiophene), P3HT (1), gave a mobility of about 0.1 cm2 /V s and current on/off ratio of about 106 when processed in an inert atmosphere [6]. When processed in ambient conditions, it exhibited lower mobility and significantly lower current on/off ratio [7]. In addition, its performance also rapidly degraded upon exposure to air. A more stable polythiophene derivative, F8T2, was recently reported [8]. Mobility of up to 0.01–0.02 cm2 /V s was obtained through proper molecular alignment induced with a rubbed polyimide alignment layer and thermal annealing at 270 ◦ C. This high annealing temperature may not however be compatible with most common flexible substrate materials. We report here a class of solution processable polythiophenes, poly(3 ,4 -dialkyl-2,2 ;5 ,2 -terthiophene)s, PTTs (2), which exhibit both enhanced air stability and functionally useful FET properties without high-temperature annealing. A mobility of up to 0.01 cm2 /Vs and current on/off ratio of more than 105 had been obtained with the devices fabricated entirely in ambient atmosphere. These devices were also relatively stable in the dark on exposure to air for extended period of time without protective encapsulation. PTT is derived from the monomer comprised of 3,4dialkylthiophene flanked by two non-substituted thienylene
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B. Ong et al. / Synthetic Metals 142 (2004) 49–52
units. The dialkylthiophene moiety imparts solubility characteristics to the polymer and may help induce and facilitate molecular organization through intermolecular side-chain ordering. The non-substituted thienylene moieties, which are not held in place by intermolecular side-chain interactions, can assume certain torsional deviation from being coplanar with the 3,4-dialkylthiophene moieties. Torsional angles of 6◦ –9◦ of this nature have been reported [9,10]. This slight deviation from the otherwise planar conformation typified of regioregular poly(3-alkylthiophene) effectively shortens PTT’s extended -conjugation to an extent that is sufficient to suppress its oxidative doping sensitivity, thus enabling achievement of near-intrinsic FET properties. PTT was synthesized by either FeCl3 -mediated oxidative polymerization of 3 ,4 -dialkyl-2,2 ;5 ,2 -terthiophene monomer [11], or dehalogenative coupling reaction of 5,5 -dibromo-3 ,4 -dialkyl-2,2 ;5 ,2 -terthiophene monomer [12]. For the present studies, PTT was obtained from FeCl3 -mediated oxidative polymerization. The polymer was first dedoped by stirring with an aqueous ammonia solution, followed by purification by precipitation from methanol, and then by successive Soxhlet extractions with methanol, hexane and chlorobenzene. The synthesis and effects of polymerization methods on device performance will be discussed separately [13]. The polymers had an average molecular weight (Mw) of 20,000–35,000 Da with a polydispersity of about 2 relative to polystyrene standards. Commercially available regioregular P3HT (Aldrich), which was purified by repeated reprecipitations of its chlorobenzene solution from methanol, followed by extraction with acetone for 24 h [7], was used as our reference. Thin-film FETs were fabricated under ambient conditions without precautionary measurements being taken in excluding ambient light, oxygen and moisture. Fig. 1 shows a schematic cross section of the FET device configuration. An n-doped Si wafer with a 110 nm thick thermally grown silicon dioxide layer (capacitance of 32 nF/cm2 ) was used as the substrate. The surface of silicon dioxide layer was modified with octyltrichlorosilane to promote self-organization of the polythiophene on the substrate. Semiconductor films were deposited by spin coating 1 wt.% of chlorobenzene solutions of polythiophenes at 1000 rpm for 30 s, followed by drying in vacuo at 80 ◦ C for several hours to drive off the solvent. The gold source and drain electrodes were then deposited on top of the semiconductor layer by vacuum evaporation through a shadow mask with various channel lengths and widths, thus creating a series of transistors of various dimensions (L = 60–370 m; W = 1000 and 5000 m). The devices were characterized using Keithley 4200 SCS semiconductor characterization system in a black metal box. Fig. 2 shows the typical I–V curves for the FET devices with PTT-10 (2, R: n-C10 H21 ) as the p-channel semiconductor. As can be noted, the output characteristics displayed very good saturation behavior, clear saturation currents which behaved quadratically to the gate bias. The saturation current was more than −2 A at the gate and source–drain
Fig. 1. Schematic diagram of a thin-film field-effect transistor and chemical structures of the polymeric semiconductors.
voltages of −40 V. The device switched on at around 0 V, with a good sub-threshold slope of around 1.5 V per decade (Fig. 3A). The current on/off ratio between Vg = −80 and 0 V was around 4.0 × 105 , which is quite similar to that of P3HT devices fabricated in a nitrogen atmosphere [6]. For comparison, similar devices were fabricated with carefully purified P3HT as the active layer under ambient conditions. The devices showed a current on/off ratio which varied from 2.1×103 to 2.0 ×105 between Vg = −80 and 20 V, together with a larger sub-threshold slope and a positive turn-on voltage. (The best I–V curve was shown in Fig. 3B.) The observed depletion regime of the P3HT device indicates the presence of a high free carrier density in the semiconductor
Fig. 2. Output characteristics of a typical thin-film field-effect transistor with a PTT-10 active layer, channel length (L) = 165 m, channel width (W) = 5000 m.
B. Ong et al. / Synthetic Metals 142 (2004) 49–52
51
stacking axis parallel to the substrate, which are conducive to charge carrier transport [15]. However, the extended -conjugation elevates its highest occupied molecular orbital (HOMO) energy levels, resulting in its propensity towards oxidation in air. For PTT, the presence of unsubstituted thiophene moieties, which have certain degree of rotation freedom and assume some torsional deviations from the otherwise planar conformation, leads to shorter effective -conjugation lengths. This perturbation to the extended -delocalization lowers the HOMO levels of PTT, thus the greater resistance to p-doping by atmospheric oxygen. Further support came from the cyclic voltammetric behavior of PTT-10 thin film,2 which exhibited an onset potential of oxidative p-doping of 0.84 eV against Ag/AgCl electrode as opposed to 0.60 eV for P3HT. These values correspond to HOMO energies of 5.22 and 4.98 eV, respectively, for PTT-10 and P3HT, indicating that PTT-10 is far more stable than P3HT to p-doping by 0.24 eV. The field-effect mobility was calculated in the saturated regime using the following equation: W ISD = Ci µ (1) (Vg − Vt )2 2L
Fig. 3. Transfer characteristics in the saturation regime of typical thin-film field-effect transistors as a function of time: (A) PTT-10 device and (B) P3HT device. The source–drain voltage is –80 V, L = 165 m, and W = 5000 m. Devices were kept in the dark and exposed to air with humidity of less than 30%.
layer due to p-doping by atmospheric oxygen [7]. P3HT was known to form charge transfer complexes with oxygen when exposed to air and illuminated with light [14]. The ambient stability of the devices was monitored through the time dependence of the electrical characteristics of the devices. The PTT-10 device provided a high initial on/off ratio of 4.4 × 105 , which degraded slightly to 1.1 × 105 after standing 30 days in an environment of dry air (<30% RH) in the dark.1 Conversely, the P3HT device exhibited a lower initial on/off ratio of 1.9 × 105 , and it essentially lost most of its FET activity with an on/off ratio of 2.6 × 102 after 10 days under similar conditions. These results conclusively attest to the enhanced stability of PTT-10 against p-doping by atmosphere oxygen. Regioregular P3HT self-organized into an edge-on lamellar structure when spin-coated or solution cast from dilute solution, forming extended -conjugation states with the 1
We have to note that humidity has an effect on device performance. To study the effect of oxygen only, sample were stored in dry air in the dark.
where ISD is the drain current at the saturated regime; W and L are, respectively, the semiconductor channel width and length; Ci the capacitance per unit area of the gate dielectric layer; and Vg and Vt are, respectively, the gate voltage and threshold voltage. Vt of the device was determined from the relationship between the square root of ISD at the saturated regime and Vg of the device by extrapolating the measured data to ISD = 0. Our P3HT devices showed a field-effect mobility of around 0.01–0.03 cm2 /V s when processed under ambient conditions. This value was about same as that reported in [7], which was a factor of 2–3 lower than that obtained under an inert atmosphere [6]. The mobility of PTT-10 devices was calculated to be 0.007–0.01 cm2 /V s, which is in the same order as that of P3HT when processed under ambient conditions. These levels of mobility and on/off ratio have been shown to be potentially useful for active matrix addressing circuits in polymer dispersed liquid crystal display [16]. We believe that the mobility could be further improved through optimizing the fabrication process and tailoring the length of alkyl chain. In summary, we have demonstrated for the first time that for regioregular poly(alkylthiophene) semiconductor materials, a fine balance between functionally useful FET mobility and air stability can be achieved through minor perturbation to the extended -conjugation. This perturbation can be accomplished via torsional deviation from the planar conformation of the regioregular poly(alklylthiophene) system. Through PTTs, we have shown that relatively air stable FETs with mobility up to 0.01 cm2 /V s, near-zero turn-on 2 Experiments were performed on a BAS 100 voltammetric system with a three-electrode cell in a solution of Bu4 NClO4 (0.10 M) in acetronitrile at a scanning rate of 40 mV/s.
52
B. Ong et al. / Synthetic Metals 142 (2004) 49–52
voltage, and current on/off ratio over 105 could be fabricated under ambient conditions. These devices were also relatively stable for an extended period of time on exposure to ambient oxygen in the dark. These results affirm the potential of achieving low-cost microelectronic devices through organic materials that enable simple solution fabrication processes such as spin coating or jet printing under ambient conditions.
Acknowledgements Partial financial support of this work from US National Institute of Standards and Technology through ATP grant 70NANB80H3033 is gratefully acknowledged.
References [1] F. Garnier, R. Hajlaoui, A. Yassar, P. Srivastava, Science 265 (1994) 1684. [2] H. Sirringhaus, T. Kawase, R.H. Friend, T. Shimoda, M. Inbasekaran, W. Wu, E.P. Woo, Science 290 (2000) 2123.
[3] W. Li, H.E. Katz, A.J. Lovinger, J.G. Laquindanum, Chem. Mater. 11 (1999) 458. [4] X.M. Hong, H.E. Katz, A.J. Lovinger, B.C. Wang, K. Raghavachari, Chem. Mater. 13 (2001) 4686. [5] H. Meng, Z. Bao, J. Lovinger, B.C. Wang, A.M. Mujsc, J. Am. Chem. Soc. 123 (2001) 9214. [6] H. Sirringhaus, N. Tessler, R.H. Friend, Science 280 (1998) 1741. [7] Z. Bao, A. Dodabalapur, A.J. Lovinger, Appl. Phys. Lett. 69 (1996) 4108. [8] H. Sirringhaus, R.J. Wilson, R.H. Friend, M. Inbasekaran, W. Wu, E.P. Woo, M. Grell, D.D.C. Bradley, Appl. Phys. Lett. 77 (2000) 406. [9] G. Barbarella, M. Zambianchi, A. Bongini, L. Antolini, Adv. Mater. 6 (1994) 561. [10] F. Van Bolhuis, H. Wynberg, E.E. Hvinga, E.W. Meijer, E.G.J. Staring, Synth. Met. 30 (1989) 381. [11] C. Wang, M.E. Benz, E. leGoff, J.L. Schindler, J. Allbritton-Thomas, C.R. Kannewurf, M.G. Kanatzidis, Chem. Mater. 6 (1994) 401. [12] T. Chen, X. Wu, R.D. Rieke, J. Am. Chem. Soc. 117 (1995) 233. [13] L. Jiang, Y. Wu, P. Liu, B. Ong, manuscript in preparation. [14] M.S.A. Abdou, F.P. Orfino, Y. Son, S. Holdcroft, J. Am. Chem. Soc. 119 (1997) 4518. [15] H. Sirringhaus, P.J. Brown, R.H. Friend, M.M. Nielsen, K. Bechgaard, B.M.W. Langeveld-Voss, A.J.H. Spiering, R.A.J. Janssen, E.W. Meijer, P. Herwig, D.M. de Leeuw, Nature 401 (1999) 685. [16] H.E.A. Huitena, G.H. Gelinck, J.B.P.H. van der Putter, K.E. Kuijk, C.M. Hart, E. Cantatore, P.T. Herwig, A.J.J.M. van Breemen, D.M. de Leeuw, Nature 414 (2001) 599.
Polythiophene-based field-effect transistors with enhanced air stability Beng Ong∗ , Yiliang Wu, Lu Jiang, Ping Liu, Krish Murti Xerox Research Centre of Canada, 2660 Speakman Drive, Mississauga, Ont., Canada L5K 2L1 Received 20 August 2002; received in revised form 17 June 2003; accepted 23 July 2003
Abstract We have demonstrated that relatively air stable field-effect transistors (TFTs) with mobility up to 0.01 cm2 /V s, near-zero turn-on voltage, and current on/off ratio over 105 could be fabricated in entirety under ambient conditions with poly(3 ,4 -dialkyl-2,2 ;5 ,2 -terthiophene) as active channel layer. This class of polythiophenes, which comprise of regioregularly arranged 2,5-thienylene and disubstituted-2,5-thientylene moieties, have shown enhanced stability against p-doping by atmospheric oxygen. When exposed to atmospheric oxygen, the unprotected TFTs fabricated with these materials had exhibited significantly higher stability than those of regioregular poly(3-hexylthiophene)s under similar conditions. © 2003 Elsevier B.V. All rights reserved. Keywords: Organic thin-film transistor; Polythiophene; Field-effect mobility; Current on/off ratio
Organic semiconductors have in recent years attracted growing interest as active channel materials for thin-film field-effect transistors (FETs). One of the primary appeals of organic semiconductors is their potential in fabricating low-cost integrated circuit elements for large-area (e.g. displays, image sensors) and low-end microelectronic (e.g. smart cards, radio frequency identification tags) devices via simple solution processes such as spin- and dip-coating, screen printing, stamping, jet printing, etc. [1,2]. Other attributes of organic semiconductors, particularly polymeric materials, include compatibility with flexible substrates, mechanical durability, lightweight characteristics, etc. For low-cost manufacturing, the processing needs to be carried out in ambient conditions, which requires the materials to be stable in air. Any precautionary measures to insulate manufacturing processes from ambient environmental effects (e.g. exposure to oxygen) would lead to increased cost, thus nullifying the fundamental attribute of organic electronics as a low-cost alternative to their amorphous silicon counterpart. Accordingly, it is of paramount importance that the active materials have sufficient air stability to enable fabrication in ambient conditions to achieve and maintain useful functional performance characteristics before the devices are protected or shielded (e.g. via encapsulation) from environment. Recently, a great deal of attention has been paid to the ambient stability of organic
∗ Corresponding author. E-mail address: [email protected] (B. Ong).
0379-6779/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2003.07.004
small molecular and oligomeric semiconducting materials [3–5]. However, these materials are either insoluble, difficult to process in solution, or low performing in nature. On the other hand, most solution processable semiconductor polymers (e.g. polythiophenes) only provide functionally useful properties when processed in an inert atmosphere. For example, regioregular head-to-tail polythiophenes such as poly(3-hexylthiophene), P3HT (1), gave a mobility of about 0.1 cm2 /V s and current on/off ratio of about 106 when processed in an inert atmosphere [6]. When processed in ambient conditions, it exhibited lower mobility and significantly lower current on/off ratio [7]. In addition, its performance also rapidly degraded upon exposure to air. A more stable polythiophene derivative, F8T2, was recently reported [8]. Mobility of up to 0.01–0.02 cm2 /V s was obtained through proper molecular alignment induced with a rubbed polyimide alignment layer and thermal annealing at 270 ◦ C. This high annealing temperature may not however be compatible with most common flexible substrate materials. We report here a class of solution processable polythiophenes, poly(3 ,4 -dialkyl-2,2 ;5 ,2 -terthiophene)s, PTTs (2), which exhibit both enhanced air stability and functionally useful FET properties without high-temperature annealing. A mobility of up to 0.01 cm2 /Vs and current on/off ratio of more than 105 had been obtained with the devices fabricated entirely in ambient atmosphere. These devices were also relatively stable in the dark on exposure to air for extended period of time without protective encapsulation. PTT is derived from the monomer comprised of 3,4dialkylthiophene flanked by two non-substituted thienylene
50
B. Ong et al. / Synthetic Metals 142 (2004) 49–52
units. The dialkylthiophene moiety imparts solubility characteristics to the polymer and may help induce and facilitate molecular organization through intermolecular side-chain ordering. The non-substituted thienylene moieties, which are not held in place by intermolecular side-chain interactions, can assume certain torsional deviation from being coplanar with the 3,4-dialkylthiophene moieties. Torsional angles of 6◦ –9◦ of this nature have been reported [9,10]. This slight deviation from the otherwise planar conformation typified of regioregular poly(3-alkylthiophene) effectively shortens PTT’s extended -conjugation to an extent that is sufficient to suppress its oxidative doping sensitivity, thus enabling achievement of near-intrinsic FET properties. PTT was synthesized by either FeCl3 -mediated oxidative polymerization of 3 ,4 -dialkyl-2,2 ;5 ,2 -terthiophene monomer [11], or dehalogenative coupling reaction of 5,5 -dibromo-3 ,4 -dialkyl-2,2 ;5 ,2 -terthiophene monomer [12]. For the present studies, PTT was obtained from FeCl3 -mediated oxidative polymerization. The polymer was first dedoped by stirring with an aqueous ammonia solution, followed by purification by precipitation from methanol, and then by successive Soxhlet extractions with methanol, hexane and chlorobenzene. The synthesis and effects of polymerization methods on device performance will be discussed separately [13]. The polymers had an average molecular weight (Mw) of 20,000–35,000 Da with a polydispersity of about 2 relative to polystyrene standards. Commercially available regioregular P3HT (Aldrich), which was purified by repeated reprecipitations of its chlorobenzene solution from methanol, followed by extraction with acetone for 24 h [7], was used as our reference. Thin-film FETs were fabricated under ambient conditions without precautionary measurements being taken in excluding ambient light, oxygen and moisture. Fig. 1 shows a schematic cross section of the FET device configuration. An n-doped Si wafer with a 110 nm thick thermally grown silicon dioxide layer (capacitance of 32 nF/cm2 ) was used as the substrate. The surface of silicon dioxide layer was modified with octyltrichlorosilane to promote self-organization of the polythiophene on the substrate. Semiconductor films were deposited by spin coating 1 wt.% of chlorobenzene solutions of polythiophenes at 1000 rpm for 30 s, followed by drying in vacuo at 80 ◦ C for several hours to drive off the solvent. The gold source and drain electrodes were then deposited on top of the semiconductor layer by vacuum evaporation through a shadow mask with various channel lengths and widths, thus creating a series of transistors of various dimensions (L = 60–370 m; W = 1000 and 5000 m). The devices were characterized using Keithley 4200 SCS semiconductor characterization system in a black metal box. Fig. 2 shows the typical I–V curves for the FET devices with PTT-10 (2, R: n-C10 H21 ) as the p-channel semiconductor. As can be noted, the output characteristics displayed very good saturation behavior, clear saturation currents which behaved quadratically to the gate bias. The saturation current was more than −2 A at the gate and source–drain
Fig. 1. Schematic diagram of a thin-film field-effect transistor and chemical structures of the polymeric semiconductors.
voltages of −40 V. The device switched on at around 0 V, with a good sub-threshold slope of around 1.5 V per decade (Fig. 3A). The current on/off ratio between Vg = −80 and 0 V was around 4.0 × 105 , which is quite similar to that of P3HT devices fabricated in a nitrogen atmosphere [6]. For comparison, similar devices were fabricated with carefully purified P3HT as the active layer under ambient conditions. The devices showed a current on/off ratio which varied from 2.1×103 to 2.0 ×105 between Vg = −80 and 20 V, together with a larger sub-threshold slope and a positive turn-on voltage. (The best I–V curve was shown in Fig. 3B.) The observed depletion regime of the P3HT device indicates the presence of a high free carrier density in the semiconductor
Fig. 2. Output characteristics of a typical thin-film field-effect transistor with a PTT-10 active layer, channel length (L) = 165 m, channel width (W) = 5000 m.
B. Ong et al. / Synthetic Metals 142 (2004) 49–52
51
stacking axis parallel to the substrate, which are conducive to charge carrier transport [15]. However, the extended -conjugation elevates its highest occupied molecular orbital (HOMO) energy levels, resulting in its propensity towards oxidation in air. For PTT, the presence of unsubstituted thiophene moieties, which have certain degree of rotation freedom and assume some torsional deviations from the otherwise planar conformation, leads to shorter effective -conjugation lengths. This perturbation to the extended -delocalization lowers the HOMO levels of PTT, thus the greater resistance to p-doping by atmospheric oxygen. Further support came from the cyclic voltammetric behavior of PTT-10 thin film,2 which exhibited an onset potential of oxidative p-doping of 0.84 eV against Ag/AgCl electrode as opposed to 0.60 eV for P3HT. These values correspond to HOMO energies of 5.22 and 4.98 eV, respectively, for PTT-10 and P3HT, indicating that PTT-10 is far more stable than P3HT to p-doping by 0.24 eV. The field-effect mobility was calculated in the saturated regime using the following equation: W ISD = Ci µ (1) (Vg − Vt )2 2L
Fig. 3. Transfer characteristics in the saturation regime of typical thin-film field-effect transistors as a function of time: (A) PTT-10 device and (B) P3HT device. The source–drain voltage is –80 V, L = 165 m, and W = 5000 m. Devices were kept in the dark and exposed to air with humidity of less than 30%.
layer due to p-doping by atmospheric oxygen [7]. P3HT was known to form charge transfer complexes with oxygen when exposed to air and illuminated with light [14]. The ambient stability of the devices was monitored through the time dependence of the electrical characteristics of the devices. The PTT-10 device provided a high initial on/off ratio of 4.4 × 105 , which degraded slightly to 1.1 × 105 after standing 30 days in an environment of dry air (<30% RH) in the dark.1 Conversely, the P3HT device exhibited a lower initial on/off ratio of 1.9 × 105 , and it essentially lost most of its FET activity with an on/off ratio of 2.6 × 102 after 10 days under similar conditions. These results conclusively attest to the enhanced stability of PTT-10 against p-doping by atmosphere oxygen. Regioregular P3HT self-organized into an edge-on lamellar structure when spin-coated or solution cast from dilute solution, forming extended -conjugation states with the 1
We have to note that humidity has an effect on device performance. To study the effect of oxygen only, sample were stored in dry air in the dark.
where ISD is the drain current at the saturated regime; W and L are, respectively, the semiconductor channel width and length; Ci the capacitance per unit area of the gate dielectric layer; and Vg and Vt are, respectively, the gate voltage and threshold voltage. Vt of the device was determined from the relationship between the square root of ISD at the saturated regime and Vg of the device by extrapolating the measured data to ISD = 0. Our P3HT devices showed a field-effect mobility of around 0.01–0.03 cm2 /V s when processed under ambient conditions. This value was about same as that reported in [7], which was a factor of 2–3 lower than that obtained under an inert atmosphere [6]. The mobility of PTT-10 devices was calculated to be 0.007–0.01 cm2 /V s, which is in the same order as that of P3HT when processed under ambient conditions. These levels of mobility and on/off ratio have been shown to be potentially useful for active matrix addressing circuits in polymer dispersed liquid crystal display [16]. We believe that the mobility could be further improved through optimizing the fabrication process and tailoring the length of alkyl chain. In summary, we have demonstrated for the first time that for regioregular poly(alkylthiophene) semiconductor materials, a fine balance between functionally useful FET mobility and air stability can be achieved through minor perturbation to the extended -conjugation. This perturbation can be accomplished via torsional deviation from the planar conformation of the regioregular poly(alklylthiophene) system. Through PTTs, we have shown that relatively air stable FETs with mobility up to 0.01 cm2 /V s, near-zero turn-on 2 Experiments were performed on a BAS 100 voltammetric system with a three-electrode cell in a solution of Bu4 NClO4 (0.10 M) in acetronitrile at a scanning rate of 40 mV/s.
52
B. Ong et al. / Synthetic Metals 142 (2004) 49–52
voltage, and current on/off ratio over 105 could be fabricated under ambient conditions. These devices were also relatively stable for an extended period of time on exposure to ambient oxygen in the dark. These results affirm the potential of achieving low-cost microelectronic devices through organic materials that enable simple solution fabrication processes such as spin coating or jet printing under ambient conditions.
Acknowledgements Partial financial support of this work from US National Institute of Standards and Technology through ATP grant 70NANB80H3033 is gratefully acknowledged.
References [1] F. Garnier, R. Hajlaoui, A. Yassar, P. Srivastava, Science 265 (1994) 1684. [2] H. Sirringhaus, T. Kawase, R.H. Friend, T. Shimoda, M. Inbasekaran, W. Wu, E.P. Woo, Science 290 (2000) 2123.
[3] W. Li, H.E. Katz, A.J. Lovinger, J.G. Laquindanum, Chem. Mater. 11 (1999) 458. [4] X.M. Hong, H.E. Katz, A.J. Lovinger, B.C. Wang, K. Raghavachari, Chem. Mater. 13 (2001) 4686. [5] H. Meng, Z. Bao, J. Lovinger, B.C. Wang, A.M. Mujsc, J. Am. Chem. Soc. 123 (2001) 9214. [6] H. Sirringhaus, N. Tessler, R.H. Friend, Science 280 (1998) 1741. [7] Z. Bao, A. Dodabalapur, A.J. Lovinger, Appl. Phys. Lett. 69 (1996) 4108. [8] H. Sirringhaus, R.J. Wilson, R.H. Friend, M. Inbasekaran, W. Wu, E.P. Woo, M. Grell, D.D.C. Bradley, Appl. Phys. Lett. 77 (2000) 406. [9] G. Barbarella, M. Zambianchi, A. Bongini, L. Antolini, Adv. Mater. 6 (1994) 561. [10] F. Van Bolhuis, H. Wynberg, E.E. Hvinga, E.W. Meijer, E.G.J. Staring, Synth. Met. 30 (1989) 381. [11] C. Wang, M.E. Benz, E. leGoff, J.L. Schindler, J. Allbritton-Thomas, C.R. Kannewurf, M.G. Kanatzidis, Chem. Mater. 6 (1994) 401. [12] T. Chen, X. Wu, R.D. Rieke, J. Am. Chem. Soc. 117 (1995) 233. [13] L. Jiang, Y. Wu, P. Liu, B. Ong, manuscript in preparation. [14] M.S.A. Abdou, F.P. Orfino, Y. Son, S. Holdcroft, J. Am. Chem. Soc. 119 (1997) 4518. [15] H. Sirringhaus, P.J. Brown, R.H. Friend, M.M. Nielsen, K. Bechgaard, B.M.W. Langeveld-Voss, A.J.H. Spiering, R.A.J. Janssen, E.W. Meijer, P. Herwig, D.M. de Leeuw, Nature 401 (1999) 685. [16] H.E.A. Huitena, G.H. Gelinck, J.B.P.H. van der Putter, K.E. Kuijk, C.M. Hart, E. Cantatore, P.T. Herwig, A.J.J.M. van Breemen, D.M. de Leeuw, Nature 414 (2001) 599.