J. Phys. Chem. C 2008, 112, 1705-1710
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Enhancement of Field-Effect Mobility and Stability of Poly(3-hexylthiophene) Field-Effect Transistors by Conformational Change Yeong Don Park, Do Hwan Kim,† Jung Ah Lim, Jeong Ho Cho, Yunseok Jang, Wi Hyoung Lee, Jong Hwan Park, and Kilwon Cho* Department of Chemical Engineering, Pohang UniVersity of Science and Technology, Pohang, 790-784, Korea ReceiVed: September 5, 2007; In Final Form: October 27, 2007
With the aim of enhancing the field-effect mobility and air stability of self-aligned regioregular poly(3hexylthiophene), P3HT, by promoting two-dimensional molecular ordering, we transformed the chemical structure of the P3HT chain from a benzoid to a quinoid structure by doping the P3HT solution with HAuCl4 prior to film formation. We found that, for the appropriate HAuCl4 concentration, the P3HT nanocrystals adopt a highly ordered molecular structure with a field-effect mobility of 0.03 cm2 V-1 s-1, which is an improvement by a factor of more than 100. This increase in field-effect mobility is due to a significant enhancement of molecular ordering and the perpendicular orientation of the nanocrystals with respect to the insulator substrate. This resulted from the change in the P3HT chain conformation from a benzoid to a quinoid structure due to oxidation by HAuCl4. The quinoid structure favors a linear or expanded-coil conformation, so the thiophene inter-ring bonds have increased double-bond character, which improves the molecular ordering. Furthermore, the electrical properties of a doped P3HT device had highly improved stability to air without encapsulation. These results suggest that the effect of unintentional doping by oxygen on HAuCl4-doped P3HT film is insignificant because the p-type doping effect of HAuCl4 is the major contributor.
1. Introduction The high field-effect mobility and solution processability of regioregular poly(3-hexylthiophene)s (P3HTs)1,2 have stimulated considerable interest in the utilization of these fascinating polymers as the active materials in organic field-effect transistors (OFETs).3-10 Further, self-organized regioregular P3HT has a supramolecular two-dimensional structure that is of special interest because the one-dimensional electronic properties of its π-conjugated polymer chains are modified by the increased interchain stacking that results from its π-π interactions.11-15 The π-stacking of the polymer chains results in increased interchain interactions; optical spectroscopy has shown that polarons in regioregular P3HT are not one-dimensional entities isolated on single chains but are instead two-dimensional species influenced by the presence of neighboring polymer chains. Thus it is possible to achieve high field-effect mobilities in P3HT thin films as a result of two-dimensional transport in their selforganized conjugated lamellae. P3HT FETs with preferential supramolecular two-dimensional ordering of the P3HT chains have a high field-effect mobility of up to 0.1 cm2 V-1 s-1, approaching that of single-crystal oligothiophenes.16-20 The efficiency of charge transport in such systems is expected to improve with better control of the structural order and with the case of polymers with stronger π-π interactions between building blocks. However, the chargecarrier mobilities of macroscopic samples of solution-processed conjugated polymer films are generally limited by the hopping process between polymer chains in disordered regions of the material because of their low crystallinity. In particular, the spin* To whom correspondence should be addressed. E-mail: kwcho@postech. ac.kr. † Current address: Display Device & Materials Laboratory, Samsung Advanced Institute of Technology, Yongin-si, Gyeonggi-do, 446-712, Korea.
coating method cannot produce nanowire morphologies with molecularly ordered structures, unlike the solvent-dropping method, because of fast solvent evaporation rates. Furthermore, when polymer FETs are exposed to air, they tend to degrade easily, limiting the lifetime of polymer devices.21-25 Therefore, many groups have tried to enhance the molecular ordering of conducting polymer films14,17,20,26-28 and their electrical properties through doping19,29-37 or to improve the stability through encapsulation.38,39 McCullough and co-workers recently reported the one-pot synthesis of regioregular polythiophene-stabilized gold nanoparticles with narrow size distributions, which can form fibrillike structures, and succeeded in producing films with enhanced conductivity.40 Janssen and co-workers demonstrated that long poly(3-alkylthiophene)s with an irregular substitution pattern, which lack any intrinsic tendency to form well-ordered supramolecular aggregates, can attain a surprisingly high degree of organization in films, simply by oxidation in solution; however, the mobilities of the resulting films were not significantly improved.27 No experimental evidence showing that π-stacked polymers with a highly ordered structure can exhibit low resistance and desirable electrical properties in polymer field-effect transistors has previously been reported. In this paper, we describe how high molecular weight regioregular P3HTs, which lack any intrinsic tendency to form supramolecular, well-ordered structures when spin-coated due to the fast solvent evaporation rate, can attain a favorable alignment and stability to air in films simply by doping in solution. We focus on controlling the structural ordering that results from the intermolecular interactions with the aim of enhancing the two-dimensional molecular ordering of regioregular P3HT in thin films and increasing the lifetime of polymer devices. A clear correlation is shown between the field-effect mobility of regioregular P3HT and the molecular ordering of
10.1021/jp077125b CCC: $40.75 © 2008 American Chemical Society Published on Web 01/15/2008
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Figure 1. (a) Photographs of the diluted P3HT solution containing various HAuCl4 concentrations (pristine P3HT, 0.014 mM, 0.07 mM, 0.14 mM, 0.7 mM, 1 mM, 5 mM, 10 mM from left). (b) Absorption spectra of the diluted P3HT solutions as a function of HAuCl4 concentration. The concentration increases in the direction of the arrow. The inset shows magnified intensities of the absorption maximum.
Park et al. Highly doped Si was used as the transistor substrate as well as the gate electrode. A thermally grown silicon dioxide (SiO2) layer of thickness 300 nm was employed as the gate dielectric. A simple spin-coating method was used to fabricate regioregular P3HT thin films of 35-55 nm thickness. Oxidation of the P3HT chains in 0.5 wt % chloroform (CHCl3) solution was accomplished by adding HAuCl4 at various concentrations: sample 1, pristine P3HT; 2, 0.014 mM; 3, 0.07 mM; 4, 0.14 mM; 5, 0.7 mM; 6, 1 mM; 7, 5 mM; 8, 10 mM HAuCl4. Solutions were filtered through a 1 µm pore poly(tetrafluoro ethylene) membrane syringe filter before use. The source-drain electrodes (channel length 100 µm, channel width 1000 µm) were used from a water-based ink of the conducting polymer poly(3,4ethylenedioxythiophene) doped with polystyrene sulfonic acid (PEDOT/PSS) (from Bayer AG, Bytron P). A line of PEDOT droplets was deposited on top of the P3HT film by inkjet printing. 2.2. Characterization of the P3HT Thin Films. Solutionstate UV-vis absorption spectra were recorded using a UVvis spectrophotometer (Varian, CARY-5000). Grazing-incidence X-ray diffraction (GIXD) measurements were performed at the 3C2, 4C2, 8C1, and 10C1 beamlines (wavelength ca. 1.54 Å) at the Pohang Accelerator Laboratory (PAL), Korea. An atomic force microscope (Digital Instruments Multimode) operating in tapping mode using a silicon cantilever was employed to characterize the surface morphologies of the samples. Film thickness was determined using an ellipsometer (M-2000V, H.A. Woollam Co., Inc.). In our study of the current-voltage characteristics of the prepared devices, the OFETs were operated in accumulation mode by applying a negative gate bias, where the source electrode was grounded and the drain electrode was negatively biased. All the measurement results were obtained at room temperature in ambient conditions using Keithley 2400 and 236 source/measure units. 3. Results and Discussion
Figure 2. (a) Out-of-plane grazing incidence angle X-ray diffraction intensities as a function of the scattering angle 2θ for P3HT thin films on SiO2/Si substrates with various HAuCl4 concentrations (1, pristine P3HT; 2, 0.014 mM; 3, 0.07 mM; 4, 0.14 mM; 5, 0.7 mM; 6, 1 mM; 7, 5 mM; 8, 10 mM). (b) Magnified intensities of the out-of-plane (010) reflection in the wide angle 2θ region of the P3HT thin films. The HAuCl4 concentration increases in the direction of the arrows. The inset shows a schematic representation of the edge-on structure for P3HT.
the P3HT chains. This goal is achieved by adding various amounts of HAuCl4 to P3HT solutions. In this report, eight P3HT thin films were fabricated with varying amounts of HAuCl4. The pristine P3HT film was sample 1, and samples 2-8 contained increasing amounts of HAuCl4. Details regarding the amounts of HAuCl4 and the fabrication method are provided in the Experimental section. 2. Experimental Section 2.1. Preparation of Polymer Thin Film and Electronic Devices. The regioregular P3HT used in this study was obtained from Rieke Metals Inc (Mn ) 45-50 kg/mol). The coupling ratio of head-tail to head-head and tail-tail was estimated to be about 93% by NMR integration. Hydrogen tetrachloro aurate(III) trihydrate (HAuCl4‚3H2O) was used as received from Aldrich.
Various concentrations HAuCl4 were added to a 0.5 wt % P3HT chloroform (CHCl3) solution and the color change of the P3HT solution was observed. As shown in Figure 1a, the solution changes from the transparent pristine P3HT solution to a dark-colored P3HT solution when 0.7 or 1 mM of HAuCl4 was added and eventually became turbid when above 5 mM of HAuCl4 was added. Figure 1b, the UV-vis absorption spectra of the P3HT solutions according to HAuCl4 concentration, shows the changes of the molecular structure of P3HT in solution. The pristine P3HT solution shows no sign of evolution of molecular ordering, which indicates that the P3HT chains remain well-dissolved as in a good solvent. As the HAuCl4 concentration increases, a redshift of the absorption maximum (λmax) and additional absorption bands (∼610 nm) at lower energy begin to appear and additional bands intensity increases gradually. Both features are associated with the increased molecular order.13,41 Broadly speaking, these are usually attributed to a increase in the effective conjugation lengths of the chain segments in the P3HT solution, therefore decreasing its energy.42,43 The structures obtained through this process seem to form highly ordered P3HT chains. When P3HT concentrations are higher than 0.5 wt %, the solution forms a gel whose viscosity increases strongly, which makes it difficult to fabricate a uniform film by spin coating. To investigate the variation of the molecular structure (chain orientation and crystallinity) of the P3HT thin films as a function
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Figure 3. Tapping-mode AFM phase images of P3HT films with various HAuCl4 concentrations: (a) 0 mM, (b) 0.014 mM, (c) 0.07 mM, (d) 0.14 mM, (e) 0.7 mM, (f) 1 mM, (g) 5 mM, (h) 10 mM.
of the HAuCl4 concentration, GIXD measurements were carried out on the spin-coated films.14,17,20,27 Figure 2 shows the outof-plane X-ray diffraction patterns of the P3HT films for various HAuCl4 concentration. In the case of the pristine P3HT thin film, the (100) reflection due to the lamellar layer structure (16.4 Å) is weak (Figure 2a), but the out-of-plane (010) reflection due to π-π interchain stacking (3.8 Å) is relatively strong (Figure 2b). However, as the amount of the HAuCl4 added to the solution increases, the first Bragg peak (100) is more easily detected, and the (010) reflection disappears. The intensities of the Bragg peak (100) dramatically increase as the amount of HAuCl4 increases but decrease at the two highest concentrations, even though the total film thickness is high. Films 1-6 were found to have approximately the same thicknesses, in the range of 35-38 nm, whereas the highly doped samples 7 and 8 were found to have thicknesses of about 55 nm, as measured by ellipsometry. We conclude that the crystallinity of the HAuCl4doped P3HT films is much higher than that of the film produced from the pristine P3HT solution. Furthermore, whereas the pristine P3HT film has a face-on structure with its (010)-axis normal to the P3HT film,13,17 as the amount of added HAuCl4 increases, the molecular orientation of the HAuCl4-doped P3HT films is converted to an edge-on structure with a preferential orientation of its (100) axis normal to the P3HT film. From GIXD, transmission electron microscopy (TEM), and X-ray photoemission spectroscopy (XPS) results, Au nanoparticles were not observed, which indicates that Au nanoparticles were not made by just adding HAuCl4. Our GIXD measurements reveal the molecular structure that is present in the P3HT films, and the atomic force microscopy (AFM) observations were used to provide images of the top surface. Figure 3 shows AFM phase images of the P3HT thin films. The pristine P3HT film is featureless (Figure 3a). However, as the concentration of HAuCl4 increases, peculiar morphologies indicative of nanoribbon structures with widths of 25-35 nm become more and more common in the phase images (parts d-f of Figure 3) and are less common at high HAuCl4 concentrations (parts g and h of Figure 3). To determine the relationship between molecular ordering and electrical characteristics, the field-effect mobilities of the P3HT films were measured using a top-contact thin-film FET
Figure 4. Drain current (ID) vs drain-source voltage (VDS) at various gate voltages (VG) for the FETs (100 µm long and 1000 µm wide): (a) pristine P3HT and (b) 0.14 mM HAuCl4 doped P3HT. (c) Plot of -IDS vs VG at a fixed VDS of -80 V on both linear (left axis) and log(right axis) scales for devices: 1, pristine P3HT; 4, 0.14 mM. (d) Field-effect mobility (left axis) and on-off ratio (right axis) obtained in the saturation regime of HAuCl4-doped P3HT FETs as a function of HAuCl4 concentration.
geometry. Typical source-drain current (IDS) vs source-drain voltage (VDS) plots at various gate voltages are shown for the pristine P3HT and 0.14 mM HAuCl4 doped P3HT FETs operating in accumulation mode in parts a and b of Figure 4, respectively. The 0.14 mM HAuCl4 doping increases the drain current of the FET by a factor of about 100, from 0.14 to 14 µA. The average field-effect mobility of each transistor was calculated in the saturation regime (VDS ) -80 V) by plotting the square root of the drain current versus the gate voltage (Figure 4c) and fitting the data to the following equation44
IDS )
WCi µ(VGS - VT)2 2L
where Ci ) 10.8 × 10-9 F cm-2, W ) 1000 µm, and L ) 100 µm.
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Park et al.
SCHEME 1: Reaction Mechanism for the Oxidation of P3HT by HAuCl4
Figure 4d shows the field-effect mobilities of the films. The field-effect mobility of the P3HT FET with 0.14 mM HAuCl4 (3.0 × 10-2 cm2 V-1 s-1) is about 100-fold greater than that of the pristine P3HT FET (2.8 × 10-4 cm2 V-1 s-1), which has low field-effect mobilities mainly due to the very low film thickness (∼35 nm),45 the poor crystallinity and the unfavorable orientation of conjugated plane caused by the fast solvent evaporation rates (CHCl3) of the spin coating method. This increase in the field-effect mobility is probably due to the increase in the molecular ordering of the P3HT chains and their perpendicular orientation with respect to the insulator surface. The perpendicularly oriented layers enable the transport of the charge carrier in the two-dimensional conjugation direction, resulting in a higher field-effect mobility.17,20 However, the fieldeffect mobilities of the P3HT FETs doped with high HAuCl4 concentrations are lower due to decreased molecular ordering. This effect can be understood, in part, in terms of the structural changes that accompany the oxidation of the polymer. The P3HT chains are oxidized in the HAuCl4 solution,29,30 as shown in Scheme 1, which results in a new chain conformation. The chemical structure of the oxidized P3HT chains changes from a benzoid to a quinoid structure. The benzoid structure favors the coil conformation, whereas the quinoid structure favors a linear or expanded-coil conformation.46 Therefore, the partial quinoid geometry of the P3HT chains results in an increase in the double bond character of the thiophene interring bonds, which improves the molecular ordering. It makes sense that the interaction between P3HT chains in a linear conformation will be stronger than that between P3HT chains in a coil conformation.27 Figures 5 and 6 illustrate the electrical characteristics obtained for pristine (sample 1) and doped (sample 3) P3HT FETs, respectively. Both devices were fabricated and measured in air. The output characteristics for pristine P3HT devices show poor saturation behavior and increased conduction at VG ) 0 V after 1 week (parts a and b of Figure 5). The on/off ratio for pristine P3HT devices is on the order of 103 initially. However, after these devices were exposed to ambient atmosphere in the dark, the on/off ratio drops below 10 after 6 days (Figure 5c). Figure 6 demonstrates successful stability of the doped (sample 3) P3HT FETs without encapsulation. Devices were measured every day. The device has very stable characteristics when operating in air for a period of 1 week. The output characteristics for doped P3HT devices maintained similar current levels (parts a and b of Figure 6). The transfer characteristics shown in Figure 6c indicate that the on/off ratio decreased only by a factor of 10 during that period, indicating an improved environmental stability for polymer-based devices. P3HT FETs made with the other HAuCl4 concentrations show similar trends. Generally, polymer FETs can only operate successfully for a long time when there is the encapsulation coating on the entire top surface of the channel.38,39 However,
Figure 5. Drain current (ID) vs drain-source voltage (VDS) at various gate voltages (VG) for the pristine P3HT FETs measured (a) immediately and (b) after 1 week. Plot of -IDS vs VG at a fixed VDS of -80 V on both (c) log and (d) linear scales as a function of time. The number of days increases in the direction of the arrows (from 0 to 6 days).
Figure 6. Drain current (ID) vs drain-source voltage (VDS) at various gate voltages (VG) for the doped P3HT FETs measured (a) immediately and (b) after 1 week. Plot of -IDS vs VG at a fixed VDS of -80 V on both (c) log and (d) linear scales as a function of time. The number of days increases in the direction of the arrows (from 0 to 6 days).
HAuCl4-doped P3HT FETs provide clear evidence of stability to air without encapsulation as well as increased field-effect mobility.47 The effects of environmental exposure are clearly summarized in Figure 7. The degradation of pristine P3HT devices is very pronounced. The threshold voltage for pristine P3HT devices increases to 120 V after exposure to air for 6 days, giving a high onset voltage and high off current. The on/off ratio also decreases below 10. However, device performance for doped P3HT was maintained near the initial current level and electrical properties. These results demonstrate that the effect of unintentional doping of oxygen on doped P3HT film is insignificant because the p-type doping effect of HAuCl4 rather than that of oxygen is the major contributor. Such device stability without
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J. Phys. Chem. C, Vol. 112, No. 5, 2008 1709 References and Notes
Figure 7. The change of electrical properties as a function of time are summarized: (a) field-effect mobility, (b) on-off ratio, (c) threshold voltage, and (d) turn-on voltage. (9, pristine P3HT; b, doped P3HT).
encapsulation is obviously useful in the commercialization of polymer-based electronics. 4. Conclusion In conclusion, the field-effect mobility and air stability of regioregular P3HT were enhanced by making use of twodimensional molecular ordering; the chemical structure of the P3HT chains was changed from a benzoid to a quinoid structure by doping with various amount of HAuCl4. Comparing the results obtained for these systems with those for the pristine P3HT thin film showed that, depending on the HAuCl4 concentration, the P3HT chains adopt an edge-on orientation perpendicular to the insulator substrate, resulting in more ordered P3HT films with a higher field-effect mobility, on average 0.03 cm2 V-1 s-1. This remarkable increase in the field-effect mobility over that of the pristine P3HT is due to the enhancement of the supramolecular two-dimensional ordering of the P3HT chains, which is probably due to the conformational change. The chemical structure of the P3HT chains is changed from a benzoid to a quinoid structure, which favors a linear or expanded-coil conformation, via oxidation with HAuCl4. The partial quinoid geometry of the polymers results in an increase in the double bond character of the thiophene inter-ring bonds, which improves the molecular ordering. Furthermore, the doped P3HT FETs were stable in air without encapsulation for a long time because the p-type doping of HAuCl4 rather than that of oxygen is the major contributor. This approach to enhancing the molecular ordering and air stability of semiconducting polymer materials should prove useful in the development of robust and practical polymer devices for a wide range of commercial applications. Acknowledgment. This work was supported by a grant (F0004022-2007-23) from the Information Display R&D Center under the 21st Century Frontier R&D Program, ERC Program (R11-2003-006-05004-0) of the MOST/KOSEF, and the Regional Technology Innovation Program of the MOCIE (RT10401-04), POSTECH Core Research Program, and the Pohang Acceleratory Laboratory for providing the synchrotron radiation source at the 3C2, 4C2, 8C1, and 10C1 beamlines used in this study.
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