Electrospun Nanofibers of Blends of Conjugated Polymers

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Macromolecules 2005, 38, 4705-4711

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Electrospun Nanofibers of Blends of Conjugated Polymers: Morphology, Optical Properties, and Field-Effect Transistors Amit Babel,† Dan Li,‡ Younan Xia,‡ and Samson A. Jenekhe*,†,‡ Department of Chemical Engineering and Department of Chemistry, University of Washington, Seattle, Washington 98195-1750 Received November 30, 2004; Revised Manuscript Received March 27, 2005

ABSTRACT: Electrospun nanofibers of two series of binary blends of poly[2-methoxy-5-(2-ethylhexoxy)1,4-phenylenevinylene] (MEH-PPV) with regioregular poly(3-hexylthiophene) (PHT) and with poly(9,9dioctylfluorene) (PFO) were prepared, and their morphology and optical and electrical properties were characterized. Morphological and photophysical studies showed that the phase-separated domains in MEH-PPV/PHT nanofibers (30-50 nm) are much smaller as compared to blend thin films (100-150 nm), and efficient energy transfer was observed in these blend nanofibers. The MEH-PPV/PFO blend nanofibers had cocontinuous or core-shell structures, and significant energy transfer was absent in these blend nanofibers as compared to bulk thin films. Field-effect transistors based on MEH-PPV/PHT blend nanofibers showed exponential dependence of hole mobility on blend composition. The hole mobility decreased from 1 × 10-4 cm2/(V s) in 20 wt % MEH-PPV blend nanofibers to 5 × 10-6 cm2/(V s) at 70 wt %. If corrected for the reduced channel area of the transistors, the effective hole mobility varies from 5 × 10-5 to 1 × 10-3 cm2/(V s) and is similar to that of spin-coated blend thin films. Our results demonstrate that electrospun nanofibers of binary blends of conjugated polymers have tunable, composition-dependent, optical, and electronic properties that can be exploited in field-effect transistors.

Introduction Semiconductor nanowires and nanotubes are of growing broad interest in fundamental studies of confinement effects on electronic, optical, and magnetic properties and for applications in nanoscale electronics, photonics, and sensors.1 The vast majority of the focus has been on inorganic semiconductor nanostructures1a-c and carbon nanotubes.1d,e Conjugated polymer semiconductors, which combine a range of electronic and optical properties with good mechanical properties and easy processing,2-6 are being exploited in organic electronics including light-emitting diodes,2-4 thin film transistors,2,5 and photovoltaic cells.2,6 It is expected that nanowires and nanotubes of polymer semiconductors will similarly offer opportunities for fundamental studies of nanoscale confinement effects on electronic and optical properties while serving as building blocks for nanoelectronic and nanophotonic devices and systems. Nanowires and nanofibers of conducting polymers have been prepared by a variety of methods, including polymerization in nanoporous templates,7 dip-pen nanolithography,8 self-assembly,9 and electrospinning.10-13 Polypyrrole, poly(3-methylthiophene), and polyaniline nanofibrils and nanotubules, made by polymerization within the pores of track-etched polycarbonate or polyester membranes, showed more than 1 order of magnitude enhancement in dc conductivity as compared to those of the bulk samples.7 Nanowires of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene]) (MEHPPV)8a and poly(3,4-ethylenedioxythiophene)8b were fabricated by dip-pen nanolithography; however, their physical properties were not reported. Poly(3-hexylthiophene) (PHT) was shown to self-assemble into nanowires (or whiskers) from poor solvents such as p-xylene †

Department of Chemical Engineering . Department of Chemistry. * Corresponding author: E-mail [email protected].

‡

and cyclohexanone.9a,b Recently, field-effect conductance measurements on such self-assembled nanofibers of regioregular PHT showed that the field-effect mobility of holes in single PHT nanofiber was among the best reported for PHT field-effect transistors.9c Microphase separation of rod-coil block copolymers has also been used to prepare nanowires of conjugated polymers.9d Diblock and triblock copolymers of PHT with polystyrene or poly(methyl acrylate) self-assembled into a welldefined nanowire morphology with high electrical conductivity.9d Electrospinning is a simple technique that uses electrostatic forces to produce polymeric, ceramic, and composite nanofibers with diameters ranging from micrometers to tens of nanometers.10-13 Because of the limitations on molecular weight and solvents suitable for electrospinning, only a few conjugated polymers such as polyaniline, poly(dodecylthiophene), and MEH-PPV have been electrospun by either blending or forming core/sheath structure with easily spinnable polymers such as poly(ethylene oxide) and poly(vinylpyrrolidone) (PVP).11,12,13a-c Acid-doped polyaniline (PANI), PANI/ poly(ethylene oxide), and PANI/polystyrene blends were electrospun into nanofibers with diameters in the 96275 nm range and dc conductivities of 0.1-33 S/cm.11c Uniform electrospun composite fibers of MEH-PPV/ silica with a mean diameter of 700-800 nm were recently demonstrated.12 The photoluminescence of the MEH-PPV/silica fibers was blue-shifted from that of MEH-PPV thin films.12 We also recently demonstrated that conjugated polymers, such as MEH-PPV, and their blends can be electrospun into high-quality nanofibers.13c In this paper, we exploit the recently developed method of electrospinning through a coaxial two-capillary spinneret13 to prepare and investigate two series of binary blends of π-conjugated polymers: MEH-PPV with regioregular PHT and MEH-PPV with poly(9,9dioctylfluorene) (PFO) (Figure 1). One goal is to achieve

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Figure 1. Molecular structures of polymers used for electrospinning and schematic of field-effect transistor.

electrospun polymer fibers with tunable, compositiondependent, optical, and charge transport properties that could be exploited in nanoscale devices. Another goal is to understand the effects of confinement on the phase separation and properties of polymer blend nanofibers. We also aim to explore the nanofibers in organic electronics, especially field-effect transistors. We point out that based on prior thin film studies blends of conjugated polymer semiconductors can exhibit novel properties and phenomena not found in the components as a result of intermolecular interactions, self-organization/phase separation, and confinement effects.14-17 Examples include enhanced electroluminescence,14 efficient energy transfer,15 efficient photoinduced charge transfer,16 and ambipolar charge transport.17 What is not clear a priori is how the change from two-dimensional (2-D) confinement in thin films to 1-D confinement in nanowires and nanofibers will modify these and other electronic and optical properties of multicomponent polymer semiconductors. Experimental Section Materials. The PHT sample with regioregular head-to-tail (HT) coupling exceeding 98.5% (Mw ∼19 400) was purchased from Aldrich. MEH-PPV (Mw ∼ 1000 000) and PFO (Mw ∼ 65 000) were obtained from American Dye Source, Inc. Highpurity chloroform (HPLC grade) was used to make polymer solutions and the corresponding blends. Two series of binary blends of MEH-PPV with PHT (20, 30, 40, 50, 60, and 70 wt % MEH-PPV) and PFO (5, 14, 28, 44, and 55 wt % MEH-PPV) were prepared by mixing the appropriate amounts of the corresponding homopolymers. Blend composition in this paper refers to weight percentage (or weight fraction x) of MEH-PPV, which is the common component in both blend systems. All thin films of the homopolymers and blends were spin-coated from chloroform solutions at a spin rate of 1200 rpm for 30 s. The films were dried overnight (10-12 h) at 60 °C in a vacuum oven to remove any residual solvent. Electrospinning Setup. The spinneret consisting of two coaxial capillaries was fabricated according to our previously reported procedure.13c,d A polyimide-coated silica capillary was guided to penetrate the wall of a plastic syringe and then

Macromolecules, Vol. 38, No. 11, 2005 inserted into a stainless steel needle. In a typical procedure for electrospinning, the solution containing 0.6 g of PVP (Aldrich, Mw ≈ 1 300 000), 1.5 mL of water, and 8.5 mL of ethanol was added to the syringe connected to the metallic needle, and a conjugated polymer or its blend solution in chloroform was added to another syringe connected to a silica capillary. The solutions were fed by two syringe pumps (KDS200, Stoelting, Wood Dale, IL). The feed rate for the PVP solution was set at 0.6 mL/h. The feed rate for the conjugated polymer blend solution was varied in the range of 0.05-0.3 mL/h. The metallic needle was connected to a high-voltage power supply (ES30P-5W, Gamma High Voltage Research, FL), and a piece of aluminum foil or silicon wafer (Silicon Sense, Nashua, NH) was placed 9 cm below the tip of the needle to collect the nanofibers. The spinning voltage was set at 7.5 kV. PVP was extracted by immersing the as-spun fibers in ethanol for 2 h. The samples were then dried overnight at 60 °C in a vacuum oven. Morphology of Nanofibers. Scanning electron microscope (SEM) images were taken using a field-emission microscope (Sirion, FEI, Hillsboro, OR) operated at an accelerating voltage of 5 kV. Before imaging, a thin layer of Au/Pd (∼5 nm thick) was sputtered on the samples. TEM images were taken using a Philips EM-420 microscope operated at 80 keV. Optical Absorption and Photoluminescence Spectroscopy. Optical absorption spectra were obtained by using a Perkin-Elmer Lambda 900 UV/vis/near-IR spectrophotometer. Steady-state photoluminescence (PL) spectra were obtained on a PTI QM-2001-4 spectrofluorimeter. The detailed procedures for PL spectroscopy are as described previously.4b,c The films were positioned such that the emitted light was detected at 22.5° from the incident beam. Fabrication and Characterization of Thin Film Transistors. The thin film field-effect transistors (FETs) were fabricated by using a bottom contact geometry, as shown in Figure 1. Heavily doped Si with a conductivity of 103 S/cm was used as the gate electrode with a 300 nm thick SiO2 layer as the gate dielectric. By means of photolithography and vacuum sputtering (2 × 10-6 Torr), two 90 nm thick gold electrodes (source and drain) with 10 nm thick Ti-W alloy adhesive layer were fabricated onto the SiO2/Si substrates. A channel length (L) of 25 µm and a channel width of 500 µm were used. A gold contact pad was also deposited on the gate electrode to make ohmic contact. Finally, on top of this device a layer of electrospun blend fibers of MEH-PPV/PHT was deposited. Electrical characteristics of the devices were measured using a Keithley 4200 semiconductor parameter analyzer (Keithley Instruments, Inc., Cleveland, OH). All fabrication and measurements were done under ambient laboratory conditions.

Results and Discussion Morphology of MEH-PPV/PHT Blend Nanofibers. Figure 2A,B shows the typical SEM images of some MEH-PPV/PHT blend fibers with different concentrations of MEH-PPV after PVP had already been removed by ethanol extraction. The presence of PHT in these fiber products was confirmed by energy-dispersive X-ray (EDX) analysis (not shown here). As was reported earlier, MEH-PPV/PVP fibers with diameters in the range of 150-500 nm were featureless but contained some beads of ∼1 µm in size whereas the pure MEHPPV fibers, after the removal of PVP, form a ribbonlike structure with wrinkled surfaces, and the thickness of these ribbons was on the scale of ∼30 nm.13c The morphology and diameter of these MEH-PPV fibers could be controlled by adjusting the electrospinning parameters such as the concentration of MEH-PPV solution and the feed rate. Figure 2A shows an SEM image of 50 wt % MEH-PPV blend fibers with diameter in the range of 150-500 nm. Compared to the pure MEH-PPV fibers, which were smooth and uniform, the surfaces of MEH-PPV/PHT blend fibers are much

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Figure 3. Morphology of electrospun MEH-PPV/PFO blend nanofibers SEM images (A-D) and TEM images (E, F) with different concentrations of MEH-PPV. The scale bars in the inset are 500 nm. Figure 2. SEM images of electrospun MEH-PPV/PHT blend nanofibers with 50 wt % MEH-PPV (A) and 30 wt % MEHPPV (B). The scale bars in the inset are 250 nm.

rougher. The high-magnification SEM image in the inset of Figure 2A reveals that the surfaces of these fibers are composed of small particles with dimensions in the range of 30-50 nm. These nanoparticles could be assigned to PHT since the PHT phase exhibited a darker contrast under TEM (not shown here) due to the presence of sulfur element. On increasing the content of PHT in these nanofibers the surface of the fibers become rougher, and the number densities of the nanoparticles corresponding to PHT phase have also increased. The diameter of nanofibers containing 30 wt % MEH-PPV was in the range of 150-300 nm (Figure 2B). It should be noted that when the pure PHT was electrospun along with PVP, the fibers were destroyed after extraction of PVP, and only small aggregates of PHT were left. This nonspinnability of PHT was attributed to its low molecular weight. The SEM images clearly show the phase-separated morphology in these blend nanofibers. However, because of confinement of the liquid jets during electrospinning, the length scales of the phase separation in these blend fibers are much smaller than those of the MEH-PPV/PHT blend thin films prepared by spin-casting where the length scales of the phase-separated domains were on the order of 100-150 nm.18 Morphology of MEH-PPV/PFO Blend Nanofibers. Figure 3 shows SEM images of MEH-PPV/PFO blend nanofibers with different concentrations of MEHPPV. The homopolymer PFO can be co-electrospun with PVP, but after extraction of PVP in ethanol the PFO fibers shrink in length and formed short fibers with a

very low density of fibers remaining on the substrate. For 5 wt % MEH-PPV blend, electrospinning results in fibers with diameters in the range of 100-200 nm along with some particles (Figure 3A). The high-resolution image shown in the inset of Figure 3A clearly shows the small particles with dimensions in the range of 4050 nm dispersed in the fibers, indicating phase separation in the fibers. Increase of the MEH-PPV concentration (28-50 wt %) gave long continuous fibers with diameter in the range of 100-500 nm. The highmagnification images did not show any particle formation, indicating uniform and smooth surfaces in these fibers (Figure 3B-D). As can be seen from the highresolution image of the 50 wt % blend fiber that, it is clear that these fibers also form ribbonlike structures similar to electrospun MEH-PPV fibers.13c TEM images of the MEH-PPV/PFO blend nanofibers (Figure 3E,F) also confirm the absence of particulate morphology that was observed in the MEH-PPV/PHT blends. TEM image of the 44 wt % the blend nanofibers (Figure 3E) shows the presence of core-shell structures. In the 50 wt % blend nanofibers (Figure 3F), contrast variation was not observed in single nanofibers. These single contrast fibers (dark or light) probably suggest the presence of single-component nanofibers in the mat of blend nanofibers. Since the SEM images showed smooth surface morphology, it is possible that PFO/MEH-PPV blends phase-separate on a larger length scale and form either core-shell structures or continuous bundles of single-component nanofibers. Optical Properties of MEH-PPV/PHT Blend Nanofibers. Figure 4 shows the absorption and the photoluminescence (PL) emission spectra of nonwoven mats of the electrospun blend nanofibers and the corresponding spin-cast thin films. Figure 4A shows the

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Figure 4. Optical absorption (A, B) and photoluminescence emission (C, D) spectra of MEH-PPV/PHT blend nanofibers (B, D) and spin-cast thin films (A, C). The number on each curve is the wt % of MEH-PPV.

absorption spectra of spin-cast thin films of MEH-PPV/ PHT blends. The absorption band of PHT clearly shows up as a lower energy shoulder (610 nm) with a band at 520 nm corresponding to MEH-PPV in the absorption spectra of the blend thin films. The absorption spectra of the blends thin films are simple superposition of the absorption spectra of the homopolymers. Compared to the absorption of spin-cast thin films, the absorption band at 520 nm corresponding to MEH-PPV is redshifted by 30 nm to 550 nm in the blend nanofibers (Figure 4B). However, the lower energy shoulder band at around 600 nm is unchanged. In addition, the absorption band of the nanofibers is broadened; this broadening is typical of an inhomogeneous environment and is similar to those seen in MEH-PPV prepared in aligned mesoporous silica.19 The red shift in the absorption peak suggests that the polymer chains in the fibers are more extended, which may lead to the increase of π-conjugation length.19 Moreover, the extended polymer chains should be oriented along the fiber axis due to the strong stretching of the liquid jet during electrospinning. The extension and the orientation of polymer chains have also been observed in mechanically stretched films of MEH-PPV/polyethylene20 and also in other electrospun fibers. As suggested by the morphology of MEH-PPV/PHT blend fibers, the PHT is dispersed in the fibrous matrix of MEH-PPV as particles; the stretching of PHT chains is less significant as compared to that of MEH-PPV. Hence, no red shift was observed in the absorption band corresponding to PHT. Parts C and D of Figure 4 show the PL emission spectra of thin films and electrospun nanofibers, respectively. The thin film PL emission spectra of MEHPPV/PHT blends normalized with respect to the PHT emission peak at 640 nm are shown in Figure 4C. It can be seen that the emission spectra of the blends are composed of contributions from both MEH-PPV (peak at 580 nm) and PHT (peak at 640 nm). As the PHT concentration in the blends increased, the intensity of the 580 nm band in the blend PL emission spectra

decreased relative to the PHT emission band at 640 nm. The observed decrease in the intensity of the MEHPPV emission band (Figure 4C) in the blend thin films suggests some energy transfer from MEH-PPV to PHT. The emission spectra of the MEH-PPV/PHT blend nanofibers (Figure 4D) were also composed of contributions from both MEH-PPV (580 nm) and PHT (640 nm). However, it can be seen that the emission band near 580 nm (corresponding to the MEH-PPV in the electrospun fibers) is much weaker than that of the spincast films. For instance, in the 20 wt % MEH-PPV blend thin film the emission peak at 580 can be clearly resolved, whereas it is almost completely quenched in the emission spectrum of the 20 wt % MEH-PPV blend nanofibers. The enhanced quenching efficiency of the MEH-PPV emission in the nanofibers clearly indicates a more efficient energy transfer process from MEHPPV to PHT due to the stronger interaction between MEH-PPV and PHT in these confined nanostructures as compared to the bulk thin films. The morphology of these MEH-PPV/PHT blend nanofibers, as discussed earlier, showed that the PHT phase is dispersed as 3050 nm nanoparticles in the fibrous matrix of MEHPPV. The small domain size of the PHT phase, as compared to 100-150 nm in the MEH-PPV/PHT blend thin films prepared by spin-casting,18 increases the effective interfacial area between MEH-PPV and PHT and hence leading to more efficient energy transfer in the blend nanofibers. Optical Properties of MEH-PPV/PFO Blend Nanofibers. Figure 5 shows the absorption and the PL emission spectra of the electrospun blend nanofibers and the spin-casted thin films. Figure 5A shows the optical absorption spectra of thin films of MEH-PPV/PFO blends, spin-coated from their solution in chloroform, with peaks at 380 nm due to PFO and 500 nm due to MEH-PPV. Compared to the thin film absorption spectra, the absorption spectra of the blend nanofibers (Figure 5B) are broadened, similar to the MEH-PPV/ PHT blend system. The absorption peak corresponding

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Figure 5. Optical absorption (A, B) and photoluminescence emission (C, D) spectra of MEH-PPV/PFO blend nanofibers (B, D) and spin-cast thin films (A, C). The number on each curve is the wt % of MEH-PPV.

to MEH-PPV (500 nm) is significantly red-shifted by 50 nm to 550 nm in blend nanofibers, implying more extended chain conformation and better π-electron delocalization. An interesting feature in the absorption spectra of MEH-PPV/PFO blend nanofibers is a 2030 nm red shift of the PFO absorption band to 400410 nm, suggesting that the PFO chains are also extended and oriented along the fiber axis. This interpretation is supported by the fact that the PFO homopolymer could also be electrospun into nanofibers with the assistance of PVP. The thin film PL spectra of MEH-PPV/PFO blends (380 nm excitation) are shown in Figure 4C. PFO homopolymer has structured PL emission with peaks at 436, 450, and 490 nm. The PL emission spectra in Figure 4C are normalized with respect to the PFO emission peak at 436 nm. Large enhancements in the MEH-PPV PL emission band are observed in the blend thin films. Efficient Fo¨rster energy transfer from the blue-emitting PFO to the orange-emitting MEH-PPV fully account for this observation. The PL emission spectra of MEH-PPV/PFO blend nanofibers, normalized with respect to the 436 nm PFO emission peak, clearly show intense PL emission bands due to PFO (436, 450, 490 nm) and those due to MEH-PPV (Figure 4D). In the 28% and 50% blend nanofibers, broad white light emission in the 400-650 nm region is observed (Figure 4D). There is thus no evidence of significant energy transfer from PFO to MEH-PPV in the blend nanofibers. This observation contrasts sharply with MEH-PPV/PHT blend nanofibers where very efficient energy transfer from MEH-PPV to PHT was seen. This reduced efficiency of energy transfer from PFO to MEH-PPV in blend nanofibers indicates reduced interaction between two polymers. As suggested by the observed morphology (Figure 3), the MEH-PPV/PFO blend nanofibers form either core-shell structures or continuous bundles of individual nanofibers. This kind of morphology implies less interfacial surface area between two polymers (PFO and MEH-PPV) in the blend nanofibers as compared to bulk thin films, leading

to reduced interaction between the blend components. Hence, energy transfer is inefficient in these blend nanofibers. Polymer Blend Nanofiber Thin Film Transistors. Thin-film transistors based on a mesh of nanofibers of MEH-PPV/PHT blends showed typical p-channel output characteristics (plot of drain current Id vs drain voltage Vd at different gate voltages Vg), with saturation of drain current at higher drain voltages, when operated in the accumulation mode.21 Figure 6A shows the output characteristics of a 20 wt % MEHPPV blend nanofiber FET. The field-effect mobility calculated from the saturation region21 of the 20 wt % MEH-PPV blend FET was 1 × 10-4 cm2/(V s). Similarly, the I-V curves show typical p-channel operation for a 60 wt % MEH-PPV blend nanofiber FET with field-effect mobility of holes of 2.5 × 10-5 cm2/(V s) (Figure 6B). The hole mobility measured in the 40 wt % MEH-PPV nanofiber FET was 3 × 10-5 cm2/(V s), and that for the 70 wt % device was 5 × 10-6 cm2/(V s). The blend nanofiber devices have off-state (Vg ) 0) drain currents of 1-20 nA, which indicates that these nanofibers may be partially doped (unintentionally). The compositional dependence of the saturation region field-effect mobility in these blend nanofibers is shown in Figure 7. Compared to the hole mobility in the 20 wt % MEH-PPV (1 × 10-4 cm2/(V s)) nanofibers, there is more than an order of magnitude decrease in the hole mobility when the concentration of MEH-PPV in the nanofibers is increased to 70 wt % (5 × 10-6 cm2/ (V s)). The hole mobility shows an exponential dependence on blend composition (µh ) 3 × 10-4 exp(-5x)). This exponential dependence of charge carrier mobility is similar to our recently observed charge transport in PHT/polystyrene and MEH-PPV/PHT blend thin films. The hole mobilities in these blend systems decreased exponentially with the PHT concentration in blend thin films. However, compared to the hole mobilities in MEH-PPV/PHT blend thin film transistors,18 the fieldeffect hole mobilities in the present nanofiber blend TFT are about an order of magnitude lower. This is not an

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estimated the effective area covered by the blend nanofibers is 10% of the actual channel area (W × L). Considering the channel length (L) to be constant, the effective hole mobilities in MEH-PPV/PHT blend nanofibers would be (0.05-1) × 10-3 cm2/(V s).22 These estimated mobilities are similar to the field-effect mobilities of hole in MEH-PPV/PHT blend thin films.18 Conclusions

Figure 6. Output characteristics of a 20 wt % (A) and 60 wt % (B) MEH-PPV/PHT blend nanofiber FETs. Id1/2 vs Vg plot of 20 wt % MEH-PPV/PHT blend device is shown in the inset of (A).

Electrospun fibers of MEH-PPV/PHT blends and MEH-PPV/PFO blends were found to have diameters of 100-500 nm and tunable optical and charge transport properties. SEM and TEM studies revealed that both series of blend nanofibers were phase-separated. The MEH-PPV/PHT blend nanofibers had 30-50 nm phaseseparated domains whereas the MEH-PPV/PFO blend nanofibers had cocontinuous or core-shell structures. Efficient energy transfer and enhanced red emission from PHT were observed in the MEH-PPV/PHT blend nanofibers. In contrast, significant energy transfer was absent in the MEH-PPV/PFO blend nanofibers, which showed broad white light emission contributed from both components. The MEH-PPV/PHT blend nanofibers exhibited p-channel transistor characteristics with hole mobility in the range of (0.05-1) × 10-4 cm2/ (V s) as nonwoven mats. The effective field-effect mobility of holes in these blend nanofibers are 1 order of magnitude higher, (0.05-1) × 10-3 cm2/(V s), if the fact that the web of nanofibers occupy only 10% of the FET channel area is taken into account. Acknowledgment. This research was supported by the Air Force Office of Scientific Research (Grant F49620-03-1-0162) and in part by the NSF (Grant CTS0437912) and the AFOSR Smart Skins MURI program. A.B. thanks the Center for Nanotechnology at University of Washington for a UIF research fellowship. We also thank Jesse T. McCann for helping with some of the electrospinning experiments. References and Notes

Figure 7. Compositional dependence of the field-effect mobility in mats of MEH-PPV/PHT blend nanofibers.

unexpected result since the present measurements were done on a mat of nanofibers and not on an individual fiber. As can be seen from the SEM micrograph of the electrospun fibers (Figure 2), the nonwoven mat is highly porous, and therefore the area covered in the active channel region (W × L) is much less than that of a cast film. A similar effect of the porous nature of nonwoven mat was observed in electrospun polyaniline/ poly(ethylene oxide) (PEO) blend fibers. The dc conductivity of the nonwoven mat of polyaniline/PEO blend fibers was significantly lower than that for a cast film for the same concentration of polyaniline.11a The above hole mobility values in the MEH-PPV/ PHT blend nanofibers were calculated using the physical channel width (W) of the device, which clearly yields a lower bound on the carrier mobility since the conduction area between source and drain electrodes is not fully covered by the nanofiber mesh. On the basis of the SEM images of MEH-PPV/PHT blend nanofibers, we

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