Organic Thin-Film Transistors Processed from Relatively Nontoxic

Sep 29, 2010 - ... Biomedical Engineering, Nanyang Technological University, Singapore 637457 ... Thin-film transistors utilizing polymer semiconducto...
0 downloads 0 Views 5MB Size
Download PDF
Chem. Mater. 2010, 22, 5747–5753 5747 DOI:10.1021/cm102071m

Organic Thin-Film Transistors Processed from Relatively Nontoxic, Environmentally Friendlier Solvents Jun Li,†,‡ Qiaoliang Bao,† Chang M. Li,*,† Wei Zhang,§ Cheng Gong, Mary B. Chan-Park,† Jingui Qin,§ and Beng S. Ong*,†,‡ †

School of Chemical & Biomedical Engineering, Nanyang Technological University, Singapore 637457, Institute of Materials Research & Engineering (IMRE), Agency for Science, Technology and Research Singapore 117602, and §Chemistry Department, Hubei Key Lab on Organic and Polymeric Optoelectronic Materials, Wuhan University, Wuhan 430072, P. R. China ‡

Received July 26, 2010. Revised Manuscript Received September 8, 2010

Poly(2,6-bis(3-alkylthiophen-2-yl)dithieno- [3,2-b;20 ,30 -d]-thiophene)s represent a new class of solution-processable polymer semiconductors for organic thin-film transistors. This class of polymer semiconductors offers not only high field-effect transistor mobility, but also processability from relatively nontoxic, environmentally friendlier solvents. Thin-film transistors utilizing these polymer semiconductors fabricated even from environmentally friendlier organic solvents of generally low solvency such as hexane and toluene have exhibited high field-effect mobility and current on/off ratio as well as excellent thermal stability and high resistance to photoinduced oxidative doping. This is in sharp contrast to most high-mobility polymer semiconductors, which require toxic chlorinated aromatic solvents for fabrication to achieve high mobility. The use of relatively nontoxic processing solvent would eliminate the safety concerns associated with use of chlorinated aromatic solvents, potentially enabling low-cost mass manufacturing processes for thin-film transistor devices. Introduction In recent years, there has been a phenomenal increase in interest in organic thin-film transistors (OTFTs) as a lowcost alternative to traditional devices based on inorganic semiconducting materials such as silicon and gallium arsenide.1-6 Both solution processability and air stability of organic semiconductors are central to achieving lowcost OTFTs for commercial applications. Solution processability permits utilization of printing processes for circuit fabrication, which are traditionally high-throughput mass-manufacturing processes considered to be significantly much lower cost than traditional photolithography, whereas air stability ensures that the semiconductor preserves its functionality after fabrication to provide a long serviceable life. In general, polymer semiconductors are more amenable than small-molecule semiconductors to high-speed solution processing techniques.7 However, until recently, the fieldeffect transistor (FET) performance of most solutionprocessed polymer semiconductors, particularly those *Corresponding author. E-mail: [email protected] (C.M.L.); ongb@ imre.a-star.edu.sg (B.S.O.); [email protected] (B.S.O.).

(1) Sirringhaus, H.; Brown, P. J.; Friend, R. H.; Nielsen, M. M.; Bechgaard, K.; Langeveld-Voss, B. M. W.; Spiering, A. J. H.; Janssen, R. A. J.; Meijer, E. W.; Herwig, P.; de Leeuw, D. M. Nature 1999, 401, 685. (2) Forrest, S. R. Nature 2004, 428, 911. (3) Facchetti, A. Mater. Today 2007, 10, 28. (4) Murphy, A. R.; Frechet, J. M. J. Chem. Rev. 2007, 107, 1066. (5) Allard, S.; Forster, M.; Souharce, B.; Thiem, H.; Scherf, U. Angew. Chem., Int. Ed. 2008, 47, 4070. (6) Street, R. A. Adv. Mater. 2009, 21, 1. (7) Ong, B. S.; Wu, Y.; Li, Y.; Liu, P.; Pan, H. Chem.;Eur. J. 2008, 14, 4766. r 2010 American Chemical Society

fabricated in air under ambient conditions, was generally poor. Regioregular head-to-tail poly(3-hexylthiophene) was the first reported high-mobility polymer semiconductor for OTFTs when fabricated in an inert atmosphere. However, it is very susceptible to photoinduced oxidative doping in air, and thus its FET performance degrades rapidly in ambient conditions.8,9 Since then, several research groups reported development of high-mobility polymer semiconductors with lesser air sensitivities.10-21 For example, Xerox group first reported a relatively air-stable, high-mobility polymer (8) Bao, Z.; Dodabalapur, A.; Lovinger, A. Appl. Phys. Lett. 1996, 69, 4108. (9) Sirringhaus, H.; Tessler, N.; Friend, R. H. Science 1998, 280, 1741. (10) Ong, B. S.; Wu, Y.; Liu, P.; Gardner, S. J. Am. Chem. Soc. 2004, 126, 3378. (11) Pan, H.; Li, Y.; Wu, Y.; Liu, P.; Ong, B. S.; Zhu, S.; Xu, G. Chem. Mater. 2006, 18, 3237. (12) Li, Y.; Wu, Y.; Liu, P.; Birau, M.; Pan, H.; Ong, B. S. Adv. Mater. 2006, 18, 3029. (13) McCulloch, I.; Heeney, M.; Bailey, C.; Genevicius, K.; Macdonald, I.; Shkunov, M.; Sparrowe, D.; Tierney, S.; Wagner, R.; Zhang, W. M.; Chabinyc, M. L.; Kline, R. J.; Mcgehee, M. D.; Toney, M. F. Nat. Mater. 2006, 5, 328. (14) Pan, H.; Li, Y.; Wu, Y.; Liu, P.; Ong, B. S.; Zhu, S.; Xu, G. J. Am. Chem. Soc. 2007, 129, 4112. (15) Zhang, M.; Tsao, H. N.; Pisula, W.; Yang, C.; Mishra, A. K.; Mullen, K. J. Am. Chem. Soc. 2007, 129, 3472. (16) Li, J.; Qin, F.; Li, C. M.; Bao, Q.; Chan-Park, M. B.; Zhang, W.; Qin, J.; Ong, B. S. Chem. Mater. 2008, 20, 2057. (17) Fong, H. H.; Pozdin, V. A.; Amassian, A.; Malliaras, G. G.; Smilgies, D.-M.; He, M.; Gasper, S.; Zhang, F.; Sorensen, M. J. Am. Chem. Soc. 2008, 130, 13202. (18) Yan, H.; Chen, Z.; Zheng, Y.; Newman, C.; Quinn, J. R.; Dotz, F.; Kastler, M.; Facchetti, A. Nature 2009, 457, 679. (19) Tsao, H. N.; Cho, D.; Andreasen, J. W.; Rouhanipour, A.; Breiby, D. W.; Pisula, W.; Mullen, K. Adv. Mater. 2009, 21, 209–212. (20) Osaka, I.; Zhang, R.; Sauve, G.; Smilgies, D.-M.; Kowalewski, T.; McCullough, R. D. J. Am. Chem. Soc. 2009, 131, 2521.

Published on Web 09/29/2010

pubs.acs.org/cm

5748

Chem. Mater., Vol. 22, No. 20, 2010

semiconductor, poly(3,3000 -didodecylquaterthiophene) or PQT-12.10 Subsequently, variants of PQT with two fused thienylene rings (i.e., thieno[3,2-b]thiophene) as well as other polythiophene systems with higher mobilities were reported.12-14 Although these latest advances in semiconductor development are very encouraging from the perspective of achieving high mobility and air stability, there is, however, a critical limitation in that these materials require processing in environmentally undesirable, toxic solvents such as chlorinated aromatic solvents (e.g., chlorobenzene) to achieve high mobility. It is extremely difficult if not impossible to fabricate OTFTs from these polymer semiconductors in environmentally friendlier solvents such as hydrocarbons, toluene, THF, etc., because of severely limited solubility. The necessity of requiring environmentally undesirable solvents for processing would preclude these materials from being adopted in low-cost OTFT manufacturing for commercial applications. To resolve the environmental and health and safety concerns of using toxic chlorinated solvents for manufacturing, we conducted a structural study of polymer semiconductors aimed at enhancing solution processability through crystallization structural defects. This is because highly crystalline forms of materials generally have a lower solubility or solution processability than those with significant crystalline defects under similar conditions. Dithieno[3,2-b:20 ,30 -d]thiophene (DTT) was selected for our studies as it had exhibited high FET mobility as a vacuum-deposited thin-film semiconductor in OTFTs;22,23 more importantly, the DTT structure would enable construction of a rational polymer semiconductor system with varying degrees of conformational defects to enhance its solution processability (see Discussion). The synthesis and characterization of this class of poly(2,6-bis(3-alkylthiophen-2-yl)dithieno- [3,2-b;20 ,30 -d]thiophene)s (PBTDT) have been described in our previous communication.16 In this paper, we will further show that these polymer semiconductors containing extended fusedring aromatic repeat units with apparently limited solubility, could be made to exhibit solution processability in even organic solvents of very low solvency (e.g., hydrocarbon solvents), together with excellent FET characteristics in solution-processed OTFTs. Experimental Section The synthesis and characterization of poly(2,6-bis(3-alkylthiophen-2-yl)dithieno- [3,2-b;20 ,30 -d]- thiophene)s (PBTDT) was reported previously.16 The UV-vis absorption spectra were obtained with a Shimadzu UV-2450 UV-visible spectrophotometer. The AFM experiments were performed with tapping mode at ambient temperature by an atomic force microscope (AFM) (SPM 3000, Veeco Instruments Inc., USA). The images were (21) Osaka, I.; Abe, T.; Shinamura, S.; Miyazaki, E.; Takimiya, K. J. Am. Chem. Soc. 2010, 132, 5000. (22) Li, X.-C.; Sirringhaus, H.; Garnier, F.; Holmes, A. B.; Moratti, S. C.; Feeder, N.; Clegg, W.; Teat, S. J.; Friend, R. H. J. Am. Chem. Soc. 1998, 120, 2206. (23) Sun, Y; Ma, Y; Liu, Y.; Wang, J.; Pei, J.; Yu, G.; Zhu, D. Adv. Funct . Mater. 2006, 16, 426.

Li et al. acquired from the top surface of the thin film on the OTFT devices. A HRTEM image and selected area electron diffraction (SAED) pattern were obtained on a high-resolution transmission electron microscope (HRTEM, JEOL JEM-2100F, Japan) with an acceleration voltage of 200 kV. The point resolution of HRTEM image is 0.19 nm. To minimize the electron irradiation effect, we employed optimized parameters for imaging, reducing the exposure time and electron dose to as low as possible. The sample was prepared by depositing the PBTDT-12 suspension in hexane onto a carbon-coated copper grid and then drying in a vacuum oven. STM measurements were performed at the n-tetradecane/substrate interface at room temperature in constant current mode using a Pico LE SPM system (Molecular Imaging). STM tips were prepared from Pt/Ir (90:10) wire by mechanical cutting. HOPG substrates were freshly cleaved prior to deposition. A drop of very dilute solution of PBTDT-12 in n-tetradecane (Aldrich, 99þ%) was deposited onto the HOPG substrate and used for measurement. The experimental transistor devices were built on a heavily doped n-type silicon wafer, which served as a common gate electrode, whereas a 100 nm thermal SiO2 surface layer functioned as the gate dielectric. The SiO2 surface was first cleaned with Piranha solution (mixture of 70% sulfuric acid and 30% hydrogen peroxide) and then modified with organosilane by immersing the clean wafer in 10 mM hexane solution of octyltrichlorosilane (OTS-8) for 45 min at room temperature, thoroughly rinsing the substrate with hexane and 2-isopropol and then blowing dry with nitrogen gas. The gold source and drain electrodes were patterned on the OTS-8-modified SiO2 via conventional photolithography method, with electrode features of channel lengths (L) of 10 to 100 μm and widths (W) from 3 mm to 16 mm so as to create a series of transistors of various dimensions. A 10 nm layer of titanium was used as an adhesion layer for the gold electrodes on SiO2. A ring-type geometry of source and drain electrodes was employed to eliminate the parasitic leakage path from source to drain. In addition, the gold source/drain electrode pairs were modified with octanethiol self-assembled monolayer to reduce the contact resistance with the semiconductor layer by immersing into a 5 mM solution of octanethiol in ethanol for about 24 h, and then rinsed with ethanol before the semiconductor layer was deposited. OTFT devices were prepared from a particle suspension of PBTDT-12 in hexane. The particle suspension in hexane was prepared by agitating a mixture of PBTDT-12 and hexane (4 mg/mL) in a vial in an ultrasonic bath for 1 h. The suspension of PBTDT-12 in hexane was subsequently deposited on the aboveprepared featured wafer substrate by drop-casting. The devices were annealed in a vacuum oven at 160 °C for 45 min and then slowly cooled to room temperature before evaluation. Electrical properties of the transistors were characterized using a Cascade Microtech probe station fitted with an in situ thermal system controller and an Agilent E5270B 8-Slot Precision Measurement Mainframe at ambient conditions without taking any precautions to isolate the material and device from exposure to ambient oxygen, moisture, or light. In the mobility versus temperature experiments, the OTFT device was subjected to heating for a temperature range of 25-200 °C, and the device was held at a selected temperature for 5 min before the mobility was measured. The OFET mobility was extracted from the I-V curves according to the following equations Linear regime ðV D , V G Þ : I D ¼ V SD C i μðV G - V T ÞW =L Saturated regime ðV D > V G Þ : I D ¼ C i μðW =2LÞðV G - V T Þ2

Article

Chem. Mater., Vol. 22, No. 20, 2010

where VSD is the drain voltage with the source electrode being grounded. W and L are, respectively, the transistor channel width and length, and Ci is the capacitance per unit area of the dielectric layer.

Results and Discussion Our structural study was aimed at improving the solution processability of high-mobility polymer semiconductors, and more importantly, circumventing the inherent difficulties of processing these materials in nontoxic, environmentally friendlier solvents. In this regard, we explored the possibility of creating a high-mobility polymer semiconductor structural design which would display conformational defects in its crystalline forms to increase solubility characteristics. The DTT building block was thought to provide the flexibility for achieving this objective. First, DTT had earlier been shown to exhibit high FET mobility as a vacuum-deposited thin-film semiconductor in OTFTs.22,23 Second, the DTT structure would enable design of polymer structures with varying degrees of conformational crystalline defects to enhance solution processability. Other considerations included (i) the relatively large and planar structure of DTT, which when incorporated on the polymer backbone would promote strong π-π stacking to facilitate charge carrier transport via hopping mechanism; and (ii) the large resonance stabilization energy of DTT structure would suppress extended π-delocalization along the backbone to cut down on effective π-conjugation length for greater stability against oxidative doping.7,10 From these perspectives, the polymer system that would be of interest would be one comprising the monomer unit, 2,6-bis(3-alkylthiophen2-yl)dithieno[3,2-b;20 ,30 -d]thiophene (BTDT), 2.

Thermodynamically, adjacent thienylene moieties in an oligothiophene and polythiophene would preferentially adopt an anti conformation (I) to avoid the steric interference of 3,30 -hydrogen substituents as experienced in the syn conformation (II). Accordingly, the BTDT monomer repeat units, 2, in poly(2,6-bis(3-alkylthiophen-2-yl)dithieno[3,2-b;20 ,30 -d]thiophene), (PBTDT), 3, would be expected to assume a preferential “anti-anti” conformation described by (III) in which the alkyl side chains are pointed in the same direction as opposed to opposite

5749

directions as in the less preferred “anti-syn” conformation (IV). However, in molecular self-organization of PBTDT with long alkyl side-chains such as poly(2,6-bis(3-dodecylthiophen-2-yl)dithieno[3,2-b;20 ,30 -d]- thiophene) (PBTDT-12; 3, R = n-C12H25), the driving force to achieve higher molecular orders via intermolecular alkyl side-chain interdigitation would overcome the energy barrier imposed by the “anti-syn” conformation (IV) and force the monomer repeat units of 3 into predominantly anti-syn conformation (IV).24 Conformation (III) of the repeat units would then show up as conformational defects of crystallization, leading to greatly enhanced solution processability of 3. As expected on the basis of conformational crystalline defect considerations, PBTDT-12 was found to be processable in a wide range of organic solvents including chlorinated solvents and nonchlorinated solvents such as hexane, THF and toluene under appropriate conditions. A closer look at the “solution” of PBTDT-12 in chlorobenzene revealed that the polymer existed predominantly as suspended nanoparticles having a mean particle size of around 90 nm.16 The polymer nanoparticles were very stable and gelation was not observed at room temperature, thus permitting low-temperature solution processing of PBTDT-12 for printed electronics applications. We capitalized on this unique nanoparticle-forming capability of PBTDT-12 and developed a viable processing technique for easy processing in environmentally friendlier solvents (e.g., hexane, THF, etc.), thus potentially enabling mass-manufacturing without the usual safety concerns of most high-mobility polymer semiconductors associated with using toxic solvents such as chlorobenzene. Specifically, ultrasonication of PBTDT12 in a solvent such as hexane over a period of 1 h led to facile formation of a purplish submicrometer particle suspension in the solvent. Light-scattering measurements of a hexane particle dispersion showed that these particles were uniformly dispersed in the solvent, and had an average particle size of about 200-500 nm. UV-vis spectrum of a dilute particle suspension in hexane (1  10-5 M) exhibited distinct vibronic splitting with strong absorption peaks at 519, 558, and 605 nm (Figure 1), characteristic of an orderly assembled polymer system. The absorption spectrum of the suspension solution and a thin film cast from hexane suspension were almost the same, which were also quite similar to that of a thin film cast from a chlorobenzene solution. The spectral similarity implied that these submicrometer particles possessed the same molecular ordering as that in the thin film. The absorption spectrum of the filtrate from filtration of particle suspension in hexane through a 0.2 μm PTFE membrane displayed only an absorption peak at 443 nm with an 80-fold decrease in absorbance intensity from that of the unfiltered hexane suspension. This was attributable to the residual oligomers in the solution, suggesting that (24) Frey, J.; Bond, A. D.; Holmes, A. B. Chem. Commun. 2002, 20, 2424.

5750

Chem. Mater., Vol. 22, No. 20, 2010

Figure 1. UV-vis absorption spectra of PBTDT-12: (a) thin film of PBTDT-12 cast from particle suspension in hexane (black line); (b) dilute solution of PBTDT-12 in hexane (red line); (c) thin film of PBTDT-12 cast from chlorobenzene solution (green line); and (d) filtrate of dilute hexane solution of PBTDT-12 after filtration through 0.2 μm membrane (blue line).

the hexane suspension of PBTDT-12 contained nearly 99% in submicrometer particle form. The particle suspension was relatively stable as the particles settled to the bottom of the container only after standing for a long period of time, ranging from 2 days to more than 1 week depending on the samples. Nonetheless, the settlement could be redispersed easily by vigorous hand shaking to give a uniform particle suspension in the solvent again. Figure 2a shows the 3D AFM image of the surface of a thin film of PBTDT-12 that was prepared by drop casting without thermal annealing. It displayed a distinctive surface structure comprising of loosely packed, coalesced particles, which became a featureless smooth surface upon annealing to 160 °C (Figure 2b). The internal microstructure of the hexane-suspended particles appeared to be a highly ordered lamellar structure, consisting of parallel stacks of lamellar sheets of polymer chains ordered via intermolecular side-chain interdigitation with the stacking separation being characterized by π-π stacking as schematically depicted in Figure 3. A typical high-resolution transmission electron microscope (HRTEM) image of a PBTDT-12 thin film on carbon grid fabricated from its particle dispersion in hexane, and the corresponding selective area electron diffraction (SAED) pattern are shown in Figure 4. The HRTEM image clearly revealed the crystal lattice domains of well-ordered lamellar stacks of tens of nanometers in size. The d-spacing of 3.8 A˚ corresponded to the cofacial π-π stacking distance, which was supported by the XRD measurement of a powder sample.16 The SAED pattern further demonstrates the high degree of crystallinity with a π-π stacking structure. The long-range π-π stacking lamellar order in the cast thin film was particularly striking since the film was not annealed, further confirming the well-ordered inner structure of PBTDT-12 submicrometer particles. However, the crystalline domains were much smaller than the particle size (tens of nanometer versus 200-500 nm), implying that these particles were polycrystalline and that postdeposition

Li et al.

annealing would potentially lead to improved long-range ordering and thus device performance. The efficacy of self-organization of PBTDT-12 was further supported by scanning tunneling microscopy (STM) examination. For our studies, molecular layers of PBTDT-12 were absorbed on the basal plane of highly oriented pyrolytic graphite (HOPG) at the solid/liquid interface from a solution of PBTDT-12 in n-tetradecane. Figure 5 is a representative STM image of a PBTDT-12 layer, showing long-range molecular orders of wellresolved PBTDT-12 molecules, exhibiting linear strands (Figure 5, A) and chain-folded strands. Complete chain folding (Figure 5, B) and chain foldings at angles of 120° (Figure 5, C) and to a lesser extent, 60° (Figure 5, D) of polymer strands corresponding to folding along the three main crystallographic axes of HOPG substrate.25 Also noted was the strand-to-strand separation of about 19-20 A˚ in the lamellae, which was in excellent agreement with earlier XRD measurements.16 These results reflected an efficient intermolecular interdigitation of fully extended alkyl side-chains on adjacent polymer chains of PBTDT-12. The parallel strand-to-strand and strand-folding arrangements of PBTDT-12 as observed by STM strongly supported the presence of respectively anti-syn (i.e., IV in Chart 1) and anti-anti (i.e., III in Chart 1) conformations of the monomer units of PBTDT12 along its polymer backbone. A bottom-gate, bottom-contact TFT device configuration was utilized in the characterization of FET properties of PBTDT polymer semiconductors. The TFTs were built on an n-doped silicon wafer which also served as a common gate electrode. The thermal silicon oxide layer (SiO2 ∼100 nm thick) on the wafer surface was modified with octyltrimethoxysilane (OTS-8) and functioned as the gate dielectric. A series of gold source-drain electrode pairs was vacuum evaporated onto the modified SiO2 dielectric through a shadow mask carrying source-drain electrode features of various sizes, thus enabling creation of TFTs of various dimensions. The gold source-drain electrodes thus formed on the wafer were modified with octanethiol to reduce contact resistance as generally observed with bottom-contact devices by dipping in a dilute solution of octanethiol in hexane, and then washing with hexane and drying in air. Subsequently, the channel semiconductor was deposited by drop-casting from a particle suspension of PBTDT-12 in hexane. The devices thus formed were dried in an oven and annealed at 160 °C before evaluation. The TFTs were characterized in air without taking precautionary measures to isolate the devices from exposure to atmospheric oxygen, moisture, and ambient light. Our evaluation showed that the TFTs exhibited the characteristic p-type FET behaviors. The output curve displayed very good saturation behavior with no obvious contact resistance and clear saturation currents that are (25) Mena-Osteritz, E.; Meyer, A.; Langeveld-Voss, B. M. W.; Janssen, R. A. J.; Meijer, E. W.; Bauerle, P. Angew. Chem., Int. Ed. 2000, 39, 2680.

Article

Chem. Mater., Vol. 22, No. 20, 2010

5751

Figure 2. 3-D AFM topographical images of the top surface of the thin film on OTFT device at different condition: (a) at room temperature without annealing; (b) after annealing at 160 °C.

Figure 4. HRTEM image of crystalline domains in a thin film drop-cast on carbon grid from a PBTDT-12 particle suspension in hexane (inset shows the corresponding SAED pattern).

Figure 3. Schematic representation of highly ordered lamellar structure of PBTDT-12.

quadratic to the gate bias; the transfer curve showed a nearzero turn-on voltage, a threshold voltage of -3.57 V, and a subthreshold swing of ∼1.7 V dec-1. The extracted mobility in the saturated regime was about 0.2 cm2 V-1 s-1 and a current on/off ratio of 106 (Figure 6). These results of OTFTs fabricated from an environmentally benign solvent, hexane, were very competitive with the performance of some of the best polymer semiconductors fabricated from toxic chlorobenzene solutions.10-15,17 Although the air stability of organic and polymer semiconductors for TFTs has received much attention because of its importance in enabling low-cost fabrication in ambient conditions, the thermal stability of TFTs haas rarely received any attention despite its importance to the

Figure 5. STM image of long-rang orderings of PBTDT-12 at the solutionHOPG interface (100  100 nm2): (A) linear strands; (B) complete chain folding; (C) chain folding at 120°; and (D) chain folding at 60°.

5752

Chem. Mater., Vol. 22, No. 20, 2010

Li et al.

Figure 6. I-V characteristics of a representative TFT device fabricated from a PBTDT-12 particle suspension in hexane after annealing at 160 °C: (a) output curves at different gate voltages; and (b) transfer curve in saturated regime at constant source-drain voltage of -40 V and square root of the absolute value of the drain current as a function of gate voltage.

Chart 1

operational stability of TFTs. In practical operation, TFTs may constantly be exposed to high temperatures arising from heat generation in the device during operation, thus thermal stability of channel semiconductors would be critical to TFT device’s stability and thus lifetime. Thermal stability of semiconductors is a complex issue for TFTs as it relates not only to the intrinsic thermal stability of the semiconductor but also to the stability of molecular ordering and oxidative doping resistance of the semiconductor at higher temperatures. Generally, it is difficult to sustain molecular ordering at elevated temperatures as thermal energy promote and accelerate phase transition from ordered to disordered phase, leading to drastic degradation in charge-carrier transport properties, thus FET performance. In addition, thermal energy also accelerates photooxidative doping by oxygen, leading again to loss in FET mobility and current on/off ratio. The thermal properties of PBTDT-12 and the effects of temperature on the FFT performance of PBTDT-12 TFTs were thus conducted. Figure 7 shows the DSC of PBTDT-12 during two heating/cooling scans at 5 °C/min; no obvious phase

Figure 7. DSC of PBTDT-12 during the two heating/cooling scans at 5 °C/min.

transitions were noted between the temperature ranging from 25 to 300 °C, suggesting that PBTDT would be capable maintaining the pre-established molecular ordering

Article

Chem. Mater., Vol. 22, No. 20, 2010

5753

range under ambient conditions in the presence of atmospheric oxygen, moisture and light, thus affirming the excellent thermal stability and oxidative doping stability of PBTDT-12 even at high temperatures. These results were reproducible not only from a freshly prepared nanoparticle suspension in hexane, but also from the hand-shaken redispersed suspension of a settled particle sample of PBTDT-12 in hexane, thus representing the first report of highperformance polymer TFTs fabricated from a relatively nontoxic, environmentally friendlier solvent (hexane) under ambient conditions. Conclusion

Figure 8. FET mobility versus temperature plot of an OTFT fabricated from a PBTDT-12 particle suspension in hexane.

even when subjected to a high temperature of 300 °C. The dependence of FET mobility of PBTDT-12 on temperature was monitored at temperatures from 25 to 200 °C using a freshly prepared TFT device. Figure 8 shows the increase of mobility with temperature, from a low of 4.2  10-3 cm2 V-1 s-1 at 25 °C to a maximum of about 0.32 cm2 V-1 s-1 at 180 °C, and then a slight drop-off to 0.25 cm2 V-1 s-1 at 200 °C. The output characteristics showed very good saturation behavior over the tested temperature range and a current on/off ratio of 1  106 was achieved even at 200 °C. This was in sharp contrast to most high-mobility polymer TFTs (such as PQTs10 and PBTTTs13) whose performance degraded rapidly when exposed to temperatures beyond 180 °C in our experiments. The PBTDT-12 TFT also provided a stable mobility of 0.17 cm2 V-1 s-1 when cooled down to room temperature. These were very desirable OTFT properties as they were demonstrated over a wide temperature

In conclusion, we have developed a class of efficient polymer semiconductors, PBTDTs, which are processable in relatively nontoxic, environmentally friendlier solvents such as hexane, toluene and THF. The excellent solution processability of PBTDT is attributable to the presence of crystalline defects in the polymer material, which greatly enhance its solubility characteristics and thus processability in even organic solvents of low solvency. In “solution”, the polymer semiconductor is present in the form of structurally ordered submicrometer particles, thus enabling fabrication of highly ordered semiconductor channel layer for high-mobility OTFTs from environmentally friendlier solvents. The processability in relatively nontoxic solvents to provide high-mobility OTFTs would potentially resolve the current difficult issue of using high-mobility polymer semiconductors for massmanufacturing OTFT processes. Acknowledgment. This work is financially supported by Singapore A*STAR Grant 052 117 0031 and National Science Foundation of China (20774072, 20202007).