Alignment and Charge Transport of One-Dimensional Conjugated

Dec 6, 2016 - We demonstrate that solution shear coating of P3HT-NWs/PS nanocomposites is an effective strategy in aligning P3HT NWs in the presence o...
0 downloads 0 Views 2MB Size
Download PDF
Article pubs.acs.org/Macromolecules

Alignment and Charge Transport of One-Dimensional Conjugated Polymer Nanowires in Insulating Polymer Blends Mincheol Chang,† Zhe Su,† and Eilaf Egap*,†,‡ †

Department of Chemistry, Emory University, Atlanta, Georgia 30322, United States The Wallace H. Coulter Department of Biomedical Engineering, Georgia Institute of Technology and Emory University, Atlanta, Georgia 30332, United States

‡

S Supporting Information *

ABSTRACT: Self-assembled and well-aligned nanowires (NWs) of poly(3-hexylthiophenes) (P3HT) embedded within insulating polystyrene (PS) matrix were found to have a high field-effect carrier mobility. We demonstrate that solution shear coating of P3HT-NWs/PS nanocomposites is an effective strategy in aligning P3HT NWs in the presence of PS and has a significant impact on the molecular order, morphology, and consequently charge transport. Shear-coated P3HT-NWs/PS nanocomposites consistently exhibited higher carrier mobilities compared to P3HT NWs or pristine P3HT/PS films by up to 10.2-fold. P3HT-NWs/PS nanocomposites containing only 3 wt % P3HT exhibit a mobility of ∼0.053 cm2 V−1 s−1, which is comparable to that of the 30 wt % P3HT (∼0.064 cm2 V−1 s−1) and even higher than that of 100 wt % P3HT (∼0.024 cm2 V−1 s−1).

■

INTRODUCTION Conjugated polymer semiconductors have attracted significant attention in organic electronics and optoelectronics including light-emitting diodes,1 thin-film transistors,2 solar cells,3 and sensors4,5 due to their low-cost and large-area solution-based processability.6−8 However, facile degradation and photooxidation of polymer semiconductors in ambient conditions combined with the lack of controlling the morphology and molecular packing limits their widespread in low-cost and largearea applications.9−11 In addition, the trade-off between the electrical properties and mechanical properties of polymer semiconductor films has been raised.12,13 In other words, while enhanced π−π interchain interaction gives rise to an increase in the charge carrier mobility, it leads to densely packed molecules and increased rigidity that is detrimental to the mechanical properties of polymer semiconductors and for flexible electronics.12,13 Several strategies have been empolyed to overcome these drawbacks including the use of conjugated polymer semiconductor/insulating polymer blends which may result in reduced-cost, enhanced mechanical properties, and improved environmental stability while maintaining the electronic performance properties.7,10,11,14−16 It should be noted that phase separation between conjugated polymer semiconductor and insulating polymer has been revealed to play a crucial role that affects the electronic performance of the blends.11,15,16 In general, incorporation of insulating polymers to polymer semiconductors active layer leads to a decrease in charge © XXXX American Chemical Society

transport due to phase segregations and lack of interconnectivity of conjugated polymer domains in the blend films.10,11,15,16 One-dimensional nanostructures, such as nanowires (NWs) or nanofibers of conjugated polymers, have proved to be an effective strategy to overcome the severe phase segregation and lend for sufficient connectivity within an insulating polymer matrix. Charge transport of polymer semiconductor NWs/insulating nanocomposites with composition of the polymer semiconductors as a low as 2−20 wt % can result in carrier mobilities that are comparable to pure conjugated polymer films.10,11,15−18 Recently, we and others have demonstrated that solution shear coating can be an effective and a simple strategy for the alignment of conjugated polymer NWs,19 thin films,20−22 and small molecules23 and lead to significant enhancement in the carrier mobilities. The solution shear coating method consists of the controlled slow uniaxial shear of a solution of the polymer semiconductors between two flat substrates coupled with slow evaporation of the solvent.19−23 The shear coating and shearing parameters were found to have a profound impact on molecular order, morphologies, and charge transport properties.19−23 Despite great efforts in aligning polymer chains and crystalline domains, solution shear coating has seldomly been Received: August 6, 2016 Revised: November 1, 2016

A

DOI: 10.1021/acs.macromol.6b01721 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Figure 1. Schematic illustrations of (a) the PTFE cover plate and (b) procedure for aligning P3HT NWs within PS matrix via solution shear coating. allowing P3HT nanowire self-assembly as described in a previous report.25 Organic Field-Effect Transistor (OFET) Fabrication and Characterization. The FET structure was constructed on a 300 nm thick SiO2 substrate on which source and drain metal electrodes were photolithographically defined with a channel length of 50 μm and width 2000 μm and deposited using e-beam evaporation, following the procedures in previous reports.15 OFET devices were prepared by spin-coating (WS-650MZ-23NPP, Laurell) the solutions at a spin speed of 1500 rpm for 60 s or by shear-coating the solutions onto precleaned substrates at various shearing speeds (i.e., 0.5, 1.0, 1.5, 2.0, 3.0, and 4.0 mm/s) in ambient conditions, followed by a thermal annealing at 120 °C for 10 min in a nitrogen-filled glovebox. The solution shear coating process consisted of the following steps: First, FET devices were placed onto a homemade vacuum chuck equipped on the microstage (Zaber A-LSQ150A) for solution shear coating. Second, the PTFE cover plate was placed on the electrode substrate, and approximately 70 μL of the polymer solution was dropped inside the front groove of the PTFE cover (Figure 1a) on the substrate with a pipet. Third, a stepper motor controlled the translation of the microstage at various speeds (0.5 and 4.0 mm/s), while the electrode substrate was fixed. Consequently, the polymer solution was sheared by the rear groove of the cover plate (Figure 1a) on the electrode substrate. Finally, the shear-coated devices were tested in the glovebox using an Agilent 4155C semiconductor parameter analyzer. The fieldeffect mobility was calculated in the linear regime of transistor operation (VDS = −3 V) by plotting the drain current (IDS) versus gate voltage (VGS) and fitting the data to the equation

employed in polymer semiconductor nanocomposites embedded in insulating polymer blends, particularly to align polymer semiconductor NWs within an insulating polymer matrix. During our ongoing work, a recent study reported only on the self-assembly and shear coating of polymer blends but not on the impact on charge transport or the electrical properties.24 Given that insulating polymers have different physical properties such as viscosity, polarity, and solubility compared to conjugated polymers, the presence of an insulating polymer could significantly influence the alignment of the conjugated polymer chains and crystalline domains during the solution shearing process. In this paper, we investigate solution shear coating of poly(3hexylthiophene)-NWs/polystyrene (P3HT-NWs/PS) nanocomposites and demonstrate that solution shearing can result in the alignment of P3HT NWs within the matrix of PS. The effect of solution shearing on the molecular ordering, morphologies, and charge transports of P3HT NWs embedded in PS blend films was systematically studied using static absorption spectroscopy, atomic force microscopy (AFM), polarized optical microscopy (POM), and macroscale charge carrier mobility measurements. The shear speed was revealed to profoundly influence the molecular order, morphology, and charge transport of the blend. We investigated the correlation between the content of P3HT in P3HT-NWs/PS nanocomposites, orientation of P3HT NWs, and charge transport properties and discovered that 3 wt % of P3HT content in the nanocomposites can result in superior carrier mobility compared to 100 wt % of P3HT thin films or NWs. The correlation of molecular ordering, morphologies, and charge transport characteristics was also interrogated.

■

IDS =

WCOX μ(VGS − VT)VD L

(1)

where W (2000 μm) and L (50 μm) are the transistor channel width and length, respectively, VT is the threshold voltage, and COX is the capacitance per unit area of the silicon dioxide gate dielectric (1.15 × 10−8 F/cm2). UV−Vis Spectroscopy. Spin-coated or shear-coated thin films UV−vis spectra were recorded with an Agilent 8510 UV−vis spectrometer. Thin films were prepared by spin-coating or shearcoating the requisite solution onto precleaned glass slides following the same procedures used to prepare OFET devices. Atomic Force Microscopy (AFM). The AFM measurements were performed on films spin-coated or shear-coated onto bottom contact OFET devices using an ICON dimension scanning probe microscope (Bruker) operating in tapping mode with a silicon tip (NCS-14, Mikromasch).

MATERIALS AND METHODS

Materials. All chemicals were used without additional purification. Poly(3-hexylthiophene) with regioregularity of 96%, molecular average weight (Mw) of 90 kDa, and polydispersity of 2.3 was purchased from Rieke Metals Inc. Chloroform (anhydrous grade), dioxane (anhydrous grade), and polystyrene (Mw of 350 kDa and Mn of 170 kDa) were purchased from Sigma-Aldrich. Preparation of P3HT 1-D Aggregates in Solution. The P3HT/ PS blend solutions were heated at 60 °C until the polymers completely dissolved, cooled to ambient temperature, and then aged for 3 days, B

DOI: 10.1021/acs.macromol.6b01721 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

■

RESULTS AND DISCUSSION Figure 1 represents a schematic illustration of P3HT-NWs/PS nanocomposites alignment via the solution shear coating method. The polytetrafluoroethylene (PTFE) substrate was used as a cover plate due to its outstanding chemical resistance and good abrasion resistance. The design and the dimensions of the cover plate are shown in Figure 1a. During the solution shearing process, part “I” of the cover plate is filled with a mother blend solution and serves as a reservoir, while part “II” drags the solution across the substrate at a constant speed. Subsequently, the blend solution is continuously and mechanically sheared and deposited onto the substrate generating a macroscopically aligned P3HT NWs embedded in PS matrix along the shearing direction (Figure 1b). P3HT-NWs/PS blend solutions were prepared by dissolving P3HT and amorphous polystyrene (PS) in chloroform (CHCl3)/dioxane solvent mixture, followed by aging at room temperature for 3 days.25 Chloroform was used as a common good solvent for both P3HT and PS while dioxane was used as a poor solvent to promote the formation of P3HT NWs. To minimize the content of rigid polymer semiconductors in devices and concomitantly improve the mechanical properties, relatively low composition weight (%) ratios of P3HT (i.e., 3, 5, 10, 20, and 30 wt %) were blended with PS and studied. We initially focused our studies on the 20/80 P3HT/PS blend system to probe the self-assembly, molecular order, and the alignment of P3HT NWs within the blend. The role of the poor solvent and the aging process on the self-assembly of P3HT were examined. We discovered that aging P3HT/PS blend solution for 3 days results in a dark-purple solution regardless of the content of dioxane volume contents, which is indicative of aggregates formation (Figure 2a).15,25 As the content of dioxane in the solvent increased from 0 to 7 vol %, the solution appeared gradually darker. This is attributed to an increase in P3HT aggregates as the poor solvent vol % increased. However, as the content of dioxane increased to 10 vol %, the blend solutions began to form a gel that does not form uniform thin films (Figure S1). Figure 2b shows normalized absorption spectra of P3HT/PS (20/80) blend solution as a function of the dioxane content and confirms the formation of NWs as observed in Figure 2a. All solutions absorption spectra show additional low-energy vibronic peaks at ∼515, 565, and 615 nm corresponding to interchain exciton delocalization and a significant coupling to vibrations arising from ordered aggregates as well as the typical high energy π−π* intraband transition peak at ∼450 nm, which is mainly related to intrachain states of individual P3HT chains in a random-coil conformation.15,25−27 Interestingly, solution absorption spectra of P3HT/PS with 3 vol % dioxane and without dioxane are essentially the same. Only upon further increase of dioxane to 7 vol %, the intensity of low-energy vibronic bands increases relative to that of high-energy bands (π−π* intraband transition), indicative of an increase in aggregates formation.15,25−27 Atomic force microscopy (AFM) in tapping mode was performed to investigate the blend morphology depending on various processing techniques, i.e., solution-shearing or spincoating (Figure 3). Figures 3a and 3b show a pristine P3HT film spin-coated from a nonaged CHCl3 solution and P3HTNWs/PS (20/80) blend films spin-coated from aged CHCl3/ dioxane (93/7) solution, respectively. Spin-coated pristine P3HT film appears featureless (Figure 3a) while P3HT-NWs/

Figure 2. (a) Photo image of 20 mL vials containing P3HT/PS (20/ 80) blend solutions in CHCl3/dioxane solvent mixtures with 0, 3, 5, and 7% dioxane. (b) Normalized UV−vis absorption spectra of corresponding P3HT/PS blend solutions. The corresponding solutions were prepared via an aging at room temperature for 3 days.

PS blend films show randomly orientated but well-interconnected P3HT NWs within the PS matrix (Figure 3b). These results further confirm that the solvent-exchange method combined with the aging process facilitates the formation of P3HT NWs.10,25 Solution shear coating of P3HT-NWs/PS blend results in well-aligned NWs along the shearing direction (Figures 3c−h). This suggests that solution shear coating is effective in the macroscopic alignment of conjugated polymer NWs in spite of the presence of PS. The orientation and the morphology of the blend films were influenced by the variation of the shearing speed. The orientation of P3HT-NWs/PS blend improved preferentially along the shearing direction as the shearing speed increased from 0.5 to 1.0 mm/s, whereas further increase in the speed up to 4.0 mm/s led to a slight decrease in the alignment of P3HT-NWs. Interestingly, in the case of P3HT-NWs/PS nanocomposites blend, the increase in shearing speed has a significantly less impact in leading to isotropic morphology compared to pure P3HT NWs as we observed in a previous work.19 This is likely due to the higher viscosity of PS compared to P3HT, which would result in effective transfer of shearing and thus promote disentanglement and alignment of P3HT NWs during shearing.28 As the shearing speed increased from 1.0 to 4.0 mm/s, we observed a decrease in the length of P3HT NWs presumably due to an increase in the shear force. Solution-sheared P3HT-NWs/PS blend films were studied using polarized optical microscopy (POM) (Figure 4). Bright textures under the crossed polarizers are observed when both spin-coated and shear-coated P3HT-NWs/PS (20/80) blend films were rotated with respect to the crossed polarization axes, suggesting crystalline and anisotropic morphology (Figure 4a,b).19 However, the birefringent textures of solution-shearC

DOI: 10.1021/acs.macromol.6b01721 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Figure 3. AFM images of (a) pristine P3HT thin-film spin-coated from a nonaged CHCl3 solution, (b) P3HT-NWs/PS (20/80) blend film spincoated from an aged CHCl3/dioxane (93/7) solution, and P3HT-NWs/PS (20/80) blend films shear-coated at (c) 0.5, (d) 1.0, (e) 1.5, (f) 2.0, (g) 3.0, and (h) 4.0 mm/s from the corresponding solution. Arrows in the figures indicate shearing direction.

Figure 5a shows thin-film UV−vis absorption spectra of pristine P3HT, P3HT/PS (20/80) blend spin-coated from a nonaged CHCl3, P3HT-NWs/PS blend (20/80) aged from solvent mixtures of CHCl3/dioxane (93/7), and shear-coated films of P3HT-NWs/PS (20/80) blend using various shearing speeds (i.e., 0.5, 1.0, 1.5, 2.0, 3.0, and 4.0 mm/s). Two features are observed in pristine P3HT absorption spectrum: low-energy bands in the range of ∼558 to 617 nm that correspond to a crystalline phase and a high-energy band at ∼525 nm that correlates with an amorphous phase.26,31 Based on the Spano’s model, the low-energy vibronic bands arise from the crystalline region composed of weakly interacting H-aggregates. The theoretical absorption contributions of the aggregates to the vibronic bands are described according to eq 2. Gaussian peak fittings to the experimental spectra are performed to obtain the free exciton bandwidth (W) and calculate the fraction of ordered aggregates within the films (Figure 5b and Figure S2).19,26,31 2 ⎛ e − S S m ⎞⎛ W e −S ⎞ Gm⎟ A∝ ∑⎜ ⎟ ⎜1 − 2E P ⎠ m = 0 ⎝ m ! ⎠⎝ m −S 2 ⎞ ⎛ (E − E 0 − 0 − mE P − 1/2WS e ) ⎟ × exp⎜ − 2σ 2 ⎝ ⎠

Figure 4. Polarized optical microscopy images of P3HT-NWs/PS (20/80) blend films (a) shear-coated and (b) spin-coated from an aged CHCl3/dioxane (93/7) solution. Shear coating was conducted at a speed of 1.0 mm/s. P and A indicate the axes of the microscope polarizer and of the light vibration plane, respectively.

(2)

A is the absorbance as a function of the photon energy (E), W is exciton bandwidth, S is the Huang−Rhys factor (∼1.0),31 EP is the vibrational energy of the symmetric vinyl stretch (∼0.18 eV), Gm is a constant that depends on the vibrational level, m (for e.g. m = 0 for the (0−0) transition) as given by the equation Gm = ∑n(≠m)Sn/n!(n − m), where n is the vibrational quantum number, E0−0 is the 0−0 transition energy, and σ is the Gaussian line width. The W value correlates with intrachain ordering of an individual polymer chain within the aggregates, which is calculated using the intensities of the (0−0) and (0−1) transitions (i.e, the intensities of the peaks at ∼617 and 558 nm,

coated P3HT-NWs/PS blend films displayed higher brightness compared to spin-coated films, which is primarily attributed to enhanced anisotropy and alignment. This trend is consistent with the results observed in the AFM images (Figure 3b−d). Quantitative analysis of intra- and intermolecular ordering of P3HT polymer chains can be performed by means of UV−vis absorption spectroscopy and Spano’s model (Figure 5).26,31 D

DOI: 10.1021/acs.macromol.6b01721 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Figure 5. (a) UV−vis absorption spectra of a pristine P3HT film and P3HT/PS (20/80) blend film spin-coated from a nonaged CHCl3 and an aged CHCl3/dioxane (93/7) solution, respectively, and P3HT/PS (20/80) blend films shear-coated from the corresponding blend solution under different shear speeds (i.e., 0.5, 1.0, 1.5, 2.0, 3.0, and 4.0 mm/s). (b) Absorption spectrum of the blend film shear-coated at 1.0 mm/s subjected to the Spano analysis using eq 1. The blue lines indicate the spectra of aggregates, and the red lines indicate the absorption spectra associated with amorphous P3HT chains in the respective films. The black lines depict the experimental absorption spectra. Exciton bandwidth W (left axis) and percentage of ordered aggregates (right axis) of (c) the pristine P3HT film and P3HT/PS (20/80) blend film spin-coated from a fresh CHCl3 and an aged CHCl3/dioxane (93/7) solution, respectively, and (d) P3HT/PS (20/80) blend films shear-coated from the corresponding blend solution under different shear speeds. PSp = pristine P3HT spin-coated; NSp = P3HT/PS blend spin-coated; NSh (0.5 mm/s) = P3HT/PS blend sheared at 0.5 mm/s; NSh (1.0 mm/s) = P3HT/PS blend sheared at 1.0 mm/s; NSh (1.5 mm/s) = P3HT/PS blend sheared at 1.5 mm/s; NSh (2.0 mm/s) = P3HT/PS blend sheared at 2.0 mm/s; NSh (3.0 mm/s) = P3HT/PS blend sheared at 3.0 mm/s; NSh (4.0 mm/s) = P3HT/PS blend sheared at 4.0 mm/s.

Figure 6. (a) Transfer characteristics of OFET devices based on pristine P3HT film spin-coated from a nonaged CHCl3 solution and P3HT-NWs/ PS (20/80) blend films spin-coated and shear-coated under a speed of 1.0 mm/s from an aged CHCl3/dioxane (93/7) solution, respectively. Average field-effect mobilities of (b) pristine P3HT film spin-coated from a nonaged CHCl3 solution and P3HT-NWs/PS (20/80) blend film spincoated from an aged CHCl3/dioxane (93/7) solution and (c) P3HT-NWs/PS (20/80) blend films shear-coated from an aged CHCl3/dioxane (93/ 7) solution under different speeds (0.5, 1.0, 1.5, 2.0, 3.0, and 4.0 mm/s). Mobilities were calculated in the linear regime of operation with VDS = −3 V.

speeds, respectively. Spin-coated P3HT-NWs/PS blend film exhibits lower W value (∼38.7 meV) and higher amount of aggregates (∼53.5%) compared to pristine P3HT film (W ≈ 116.7 meV, aggregate fraction ≈46.8%). In general, shearaligned P3HT-NWs/PS blend films show relatively lower W values (∼22.5 to 33.1 meV) and percentage of P3HT aggregates (∼47.1 to 49.5%) compared to the spin-coated blend film (Figure 5d). Specifically, the W value decreased from ∼26.1 to 22.5 meV with an increase in the shear coating speed from 0.5 to 1.0 mm/s, respectively. This suggests that solution

respectively).26,31 W is inversely related to the conjugation length (intrachain ordering) of an individual chain in P3HT aggregates.26,31 Thus, a decrease in W indicates an increase in average conjugation length, namely intrachain ordering. The intermolecular ordering of polymer chains can be reflected by the fraction of aggregates in the films given that the interchain coupling leads to aggregation.26,31 Figures 5c and 5d show the W values and fractions of P3HT aggregates within the spincoated pristine P3HT and P3HT-NWs/PS blend films and shear-coated P3HT-NWs/PS blend films at different shear E

DOI: 10.1021/acs.macromol.6b01721 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Figure 7. AFM phase images of P3HT/PS blend films shear-coated from aged CHCl3/dioxane (93/7) solutions with different weight ratios: (a) 3/ 97, (b) 5/95, (c) 10/90, (d) 20/80, and (e) 30/70. (f) AFM phase image of P3HT-NWs/PS (20/80) blend films shear-coated from a nonaged CHCl3 solution. Arrows in the figures indicate shearing direction.

blend films (μ = 0.028 cm2 V−1 s−1), respectively (Figure 6b). Shear-coated P3HT-NWs/PS films displayed higher saturation drain current (IDS) compared to spin-coated pristine P3HT and blend thin films, presumably due to the well-aligned P3HT nanowires that facilitate efficient charge transport.19 A high turn-on voltage (VON) is observed in devices fabricated from P3HT-NWs/PS compared to devices of pristine P3HT. This may be attributed to the effects of additional doping and/or charge trapping at grain boundaries/interfaces.15,32,33 The charge carrier mobility depended on the shear speed (Figure 6C). We attribute the enhancement in the charge carrier mobility as a function of the shear speed to improvement in the intramolecular ordering and/or alignment of P3HT NWs in the blend films. Shear coating speed of 1.0 mm/s seems to result in the optimum carrier mobility in this system. This result is consistent with the UV−vis and AFM images that indicate films shear-coated at 1.0 mm/s yield the best alignment. An increase in the shearing speed of 4.0 mm/s resulted in an overall decrease in the average carrier mobility (μ = 0.036 cm2 V−1 s−1). We further investigated the morphology and the charge transport of shear-coated P3HT/PS blend as we varied the weight ratios of P3HT to PS. Figure 7 shows AFM images of P3HT/PS blend films shear-coated from aged CHCl3/dioxane (93/7) solution with different blend weight ratios of P3HT/PS (i.e., 3/97, 5/95, 10/90, 20/80, and 30/70) and P3HT/PS (20/80) deposited from a nonaged CHCl3 solution at a shear speed of 1.0 mm/s. The blend films show two distinct phases: a light and a dark phase. The light features increase as the P3HT content increases, suggesting that these regions correspond to the P3HT phase (Figures 7a−e). Figures 7a−e clearly show that the number density of P3HT NWs within the P3HTNWs/PS matrix various accordingly with the amount (wt %) of P3HT. As the number density of P3HT NWs increases, there seems to be an improvement in the alignment of P3HT NWs along the shearing direction. Conceivably, an increase in PS content results in a slower evaporation rate of the solvents due

shear coating would extend or coplanarize the individual polymer backbone and thus resulted in enhanced intramolecular ordering. Further increase in the shear speed to 4.0 mm/s led to an increase in the W value of up to ∼33.1 meV, indicative of a decrease in the planarization of the polymer backbone.19,25 In regard to P3HT-NWs/PS blend, we did not observe a direct corrleation between the percentage of P3HT aggregates and the evolution of W value as a function of shear speed. As the shear-coating speed increased from 0.5 to 1.0 mm/s, the fraction of P3HT aggregates decreased from ∼49.5 to 47.1% but essentially remained unchanged as the shear speed increased to 4.0 mm/s. Together, these results combined with AFM images suggest that while solution shear coating at low speed would effectively extend or coplanarize individual polymer backbone, high shear speed (≥1.0 mm/s) would result in small degradation of P3HT aggregates into an amorphous film due to shear force. This result is contrary to previous report, where solution shear coating of P3HT-NWs led to enhancement in both intra- and intermolecular interactions.19 Presumably, in the P3HT-NWs/PS blend, single P3HT chains are diluted and isolated in the PS-rich phase, which would prevent the self-assembly of the chains during film formation. Grazing incidence X-ray measurements of P3HTNWs/PS films were not successful owing to the significantly dilute P3HT-NWs phase in PS matrix and weak intensity peaks (100) to calculate either d-spacing or crystal grain size in the blend films.15 The charge-transport properties of the shear-coated blend films were measured by field-effect transistors (FETs) using a bottom-contact and a bottom-gate geometry. Figure 6a shows typical transfer curves of p-channel OFETs operation in the accumulation mode of spin-coated pristine P3HT, spin-coated blend films, and P3HT-NWs/PS thin films sheared at 1.0 mm/ s. A maximum carrier mobility of 0.061 cm2 V−1 s−1 was measured in shear-coated P3HT-NWs/PS thin films, which is about 10.2- and 2.2-fold improvement compared to that of pristine P3HT films (μ = 0.006 cm2 V−1 s−1) and spin-coated F

DOI: 10.1021/acs.macromol.6b01721 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

platform for efficient charge transport pathways and high charge carrier mobilities. Our results show that solution shear coating of polymer semiconductor NWs such as poly(3hexylthiophene) within an insulting matrix is a simple and an efficient approach to controlling the macroscopic alignment of nanocomposites films. Solution shear coating was found to impact the molecular order, morphologies, and charge transport properties of P3HT-NWs/PS. The alignment of P3HT-NWs/ PS blend was found to depend on the shearing parameters and the composition of the P3HT NWs within the blend. In particular, P3HT-NWs/PS blends of only 3 wt % content of P3HT sheared-coated at a speed of 1.0 mm/s exhibit carrier mobility as a high as ∼0.053 cm2 V−1 s−1 and 10.2-fold enhancement in the carrier mobility compared to pristine P3HT films obtained via a conventional spin-coating method. The content of PS had a significant influence on the orientation and alignment of P3HT NWs. Compared to pure P3HT NWs films, P3HT-NWs/PS blend films with ∼80 wt % of PS exhibit better degree of alignment of P3HT NWs even at a higher shear speed. This is attributed to the higher viscosity of PS compared to P3HT. However, as the PS content relatively increased up to ∼97 wt %, the degree of alignment of P3HT NWs gradually decreased, presumably due to a decrease in evaporation rate of solvents which facilitates disorientation of the aligned P3HT NWs. Consequently, such decrease in the orientation led to a slight decrease in mobility of the resultant films (∼0.053 cm2 V−1 s−1 for 97 wt % PS vs ∼0.064 cm2 V−1 s−1 for 70 wt % PS). The studies presented here offer new insights into the complex interplay between molecular order, processing, morphology, and charge transport properties and may facilitate the development of active layer formulations suitable for the fabrication of large-area, low-cost flexible organic electronics with high environmental stability and good mechanical properties.

to stronger noncovalent interactions between the solvents and PS compared to P3HT. In other words, there is sufficient time for aligned P3HT NWs to move in the sheared solution films as the PS content increases, resulting in a decrease in orientation of the NWs in the nanocomposite films. Thin films of P3HT/ PS (20/80) blend deposited from nonaged solution do not show formation of NWs and no preferential orientation of P3HT regions within the PS matrix as the result of solution shear coating (Figure 7f and Figure S3). The shear-coated P3HT/PS blend films deposited from nonaged solution exhibit roughly two different island-like of P3HT features, large and small islands (Figure 7f), while spin-coated nonaged blend films show dominantly large islands (Figure S4). The phase segegration and formation of islands is due to phase separation between P3HT and PS.15,16 The correlation between the charge transport properties and the content of P3HT in P3HT/PS nanocomposites blend was examined by field-effect transistors (Figure 8). Shear-coated

Figure 8. Average field-effect mobilities of pristine P3HT/PS (30/70 and 100/0) blends shear-coated from a fresh CHCl3 solution and P3HT/PS (3/97, 5/95, 10/90, 20/80, 30/70, and 100/0) blend films shear-coated from an aged CHCl3/dioxane (93/7) solution at a speed of 1.0 mm/s. Mobilities were calculated in the linear regime of operation with VDS = −3 V.

■

ASSOCIATED CONTENT

S Supporting Information *

P3HT-NWs/PS blends consistently exhibited higher carrier mobilities compared to pristine P3HT/PS blend, regardless of the composition content (wt %) of P3HT. The observed higher carrier mobility is most likely due to the presence of excellent interconnectivity and alignment of P3HT nanowires. The carrier mobility increased from 0.024 to ∼0.064 cm2 V−1 s−1 in P3HT-NWs/PS nanocomposites as the content of P3HT decreased from 100 to 30 wt %, respectively. As P3HT concentration decreased to as low as 3 wt %, the P3HT-NWs/ PS films continued to exhibit comparable carrier mobility (∼0.053 cm2 V−1 s−1) to that of the 30 wt % P3HT blend. In contrast, the carrier mobilities of pure P3HT and P3HT/PS blend films deposited by shear-coating from nonaged CHCl3 solution decreased dramatically from 0.020 to 0.003 cm2 V−1 s−1 as the P3HT content decreased from 100 to 30 wt %. Charge transport was not observed within the P3HT/PS blend as the content of P3HT decreased to 20 wt %. The significant decrease in carrier mobilities in pure P3HT and P3HT/PS blend system is attributed to the severe phase segregation as evidence from AFM and POM images (Figure S5).

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b01721. A photo image of 20 mL vials containing P3HT/PS (20/ 80) blend solutions in aged CHCl3/dioxane solvent mixtures with 10 and 20 vol % dioxane, respectively; absorption spectra of pristine P3HT and P3HT/PS blend films subjected to the Spano analysis; POM images of P3HT/PS (20/80) blend film shear-coated from a nonaged CHCl3 solution; an AFM image of P3HT/PS (20/80) blend film spin-coated from a nonaged CHCl3 solution; an optical micrograph image of pure P3HT film shear-coated from an aged CHCl3/dioxane (93/7) at a speed of 1.0 mm/s and AFM image of P3HT/PS (30/ 70) blend film shear-coated from a nonaged CHCl3 solution (PDF)

■

■

AUTHOR INFORMATION

Corresponding Author

CONCLUSIONS In conclusion, we demonstrate that well-aligned, self-assembled NWs of polymer semiconductors with low polymer content embedded in an insulating polymer matrix is an effective

*E-mail: [email protected] (E.E.). Notes

The authors declare no competing financial interest. G

DOI: 10.1021/acs.macromol.6b01721 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

■

(19) Chang, M.; Choi, D.; Egap, E. Macroscopic Alignment of OneDimensional Conjugated Polymer Nanocrystallites for High Mobility Organic Field Effect Transistors. ACS Appl. Mater. Interfaces 2016, 8, 13484−13491. (20) Giri, G.; DeLongchamp, D. M.; Reinspach, J.; Fischer, D. A.; Richter, L. J.; Xu, J.; Benight, S.; Ayzner, A.; He, M.; Fang, L.; Xue, G.; Toney, M. F.; Bao, Z. Effect of Solution Shearing Method on Packing and Disorder of Organic Semiconductor Polymers. Chem. Mater. 2015, 27, 2350−2359. (21) Dorling, B.; Vohra, V.; Dao, T. T.; Garriga, M.; Murata, H.; Campoy-Quiles, M. Uniaxial Macroscopic Alignment of Conjugated Polymer Systems by Directional Crystallization during Blade Coating. J. Mater. Chem. C 2014, 2, 3303−3310. (22) Reinspach, J. A.; Diao, Y.; Giri, G.; Sachse, T.; England, K.; Zhou, Y.; Tassone, C.; Worfolk, B. J.; Presselt, M.; Toney, M. F.; Mannsfeld, S.; Bao, Z. Tuning the Morphology of Solution-Sheared P3HT:PCBM Films. ACS Appl. Mater. Interfaces 2016, 8, 1742−1751. (23) Diao, Y.; Tee, B. C. K.; Giri, G.; Xu, J.; Kim, D. H.; Becerril, H. A.; Stoltenberg, R. M.; Lee, T. H.; Xue, G.; Mannsfeld, S. C. B.; Bao, Z. Solution Coating of Larger-Area Organic Semiconductor Thin Films with Aligned Single-Crystalline Domains. Nat. Mater. 2013, 12, 665− 671. (24) Chen, L.; Wang, H.; Liu, J.; Xing, R.; Yu, X.; Han, Y. Tuning the π−π Stacking Distance and J-Aggregation of DPP-Based Conjugated Polymer via Introducing Insulating Polymer. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 838−847. (25) Qiu, L.; Wang, X.; Lee, W. H.; Lim, J. A.; Kim, J. S.; Kwak, D.; Cho, K. Organic Thin-Film Transistors Based on Blends of Poly(3hexylthiophene) and Polystyrene with a Solubility-Induced Low Percolation Threshold. Chem. Mater. 2009, 21, 4380−4386. (26) Zhao, K.; Khan, H. U.; Li, R. P.; Su, Y. S.; Amassian, A. Entanglement of Conjugated Polymer Chains Influences Molecular Self-Assembly and Carrier Transport. Adv. Funct. Mater. 2013, 23, 6024−6035. (27) Wang, W.; Guo, S.; Herzig, E. M.; Sarkar, K.; Schindler, M.; Magerl, D.; Philipp, M.; Perlich, J.; Müller-Buschbaum, P. Investigation of Morphological Degradation of P3HT:PCBM Bulk Heterojunction Films Exposed to Long-Term Host Solvent Vapor. J. Mater. Chem. A 2016, 4, 3743−3753. (28) Wang, H.; Chen, L.; Xing, R.; Liu, J.; Han, Y. Simultaneous Control over both Molecular Order and Long-Range Alignment in Films of the Donor-Acceptor Copolymer. Langmuir 2015, 31, 469− 479. (29) Lovinger, A. J.; Wang, T. T. Investigation of the Properties of Directionally Solidfied Poly(vinylidene fluoride). Polymer 1979, 20, 725−732. (30) Crossland, E. J. W.; Rahimi, K.; Reiter, G.; Steiner, U.; Ludwigs, S. Systematic Control of Nucleation Density in Poly(3-hexylthiophene) Thin Films. Adv. Funct. Mater. 2011, 21, 518−524. (31) Clark, J.; Chang, J. F.; Spano, F. C.; Friend, R. H.; Silva, C. Determining Exciton Bandwidth and Film Microstructure in Polythiophene Films Using linear Absorption Spectroscopy. Appl. Phys. Lett. 2009, 94, 163306. (32) Chua, L. L.; Zaumseil, J.; Chang, J. F.; Ou, E. C. W.; Ho, P. K. H.; Sirringhaus, H.; Friend, R. H. General Observation of n-type FieldEffect Behaviour in Organic Semiconductors. Nature 2005, 434, 194− 199. (33) Wu, P. T.; Xin, H.; Kim, F. S.; Ren, G. Q.; Jenekhe, S. A. Regioregular Poly(3-hexylthiophene): Synthesis, Self-Assembly of Nanowires, High-Mobility Field-Effect Transistors, and Efficient Photovoltaic Cells. Macromolecules 2009, 42, 8817−8826.

ACKNOWLEDGMENTS The authors thank Rui Chang at the Georgia Institute of Technology for help with the POM analyses. This study was supported by Emory University.

■

REFERENCES

(1) Di Benedetto, F.; Camposeo, A.; Pagliara, S.; Mele, E.; Persano, L.; Stabile, R.; Cingolani, R.; Pisignano, D. Patterning of LightEmitting Conjugated Polymer Nanofibres. Nat. Nanotechnol. 2008, 3, 614−619. (2) Kline, R. J.; McGehee, M. D.; Toney, M. F. Highly Oriented Crystals at the Buried Interface in Polythiophene Thin-Film Transistors. Nat. Mater. 2006, 5, 222−228. (3) Li, G.; Zhu, R.; Yang, Y. Polymer Solar Cells. Nat. Photonics 2012, 6, 153−161. (4) Lin, P.; Yan, F. Organic Thin-Film Transistors for Chemical and Biological Sensing. Adv. Mater. 2012, 24, 34−51. (5) Yoon, H. Current Trends in Sensors Based on Conducting Polymer Nanomaterials. Nanomaterials 2013, 3, 524−549. (6) Kim, B. G.; Jeong, E. J.; Chung, J. W.; Seo, S.; Koo, B.; Kim, J. S. A Molecular Design Principle of Lyotropic Liquid-Crystaline Conjugated Polymers with Direct Alignment Capability for Plastic Electronics. Nat. Mater. 2013, 12, 659−664. (7) Ahmed, E.; Kim, F. S.; Xin, H.; Jenekhe, S. A. Benzobisthiazolethiophene copolymer semiconductors: synthesis, enhanced stability, field-effect transistors, and efficient solar cells. Macromolecules 2009, 42 (22), 8615−8618. (8) Li, J.; Sun, Z.; Yan, F. Solution Processable Low-Voltage Organic Thin Film Transistors with High-K Relaxor Ferro-Electric Polymer as Gate Insulator. Adv. Mater. 2012, 24, 88−93. (9) Burke, D. J.; Lipomi, D. J. Green Chemistry for Organic Solar Cells. Energy Environ. Sci. 2013, 6, 2053−2066. (10) Lim, J. A.; Kim, J. − H.; Qiu, L.; Lee, W. H.; Lee, H. S.; Kwak, D.; Cho, K. Inkjet-Printed Single-Droplet Organic Transistors Based on Semiconductor Nanowires Embedded in Insulating Polymers. Adv. Funct. Mater. 2010, 20, 3292−3297. (11) Arias, A. C.; Endicott, F.; Street, R. A. Surface-Induced SelfEncapsulation of Polymer Thin-Film Transistors. Adv. Mater. 2006, 18, 2900−2904. (12) O’Connor, B.; Chan, E. P.; Chan, C.; Conrad, B. R.; Richter, L. J.; Kline, R. J.; Heeney, M.; McCulloch, I.; Soles, C. L.; DeLongchamp, D. M. Correlations between Mechanical and Electrical Properties of Polythiophenes. ACS Nano 2010, 4, 7538−7544. (13) Savagatrup, S.; Makaram, A. S.; Burke, D. J.; Lipomi, D. J. Mechanical Properties of Conjugated Polymers and Polymer-Fullerene Composites as a Function of Molecular Structure. Adv. Funct. Mater. 2014, 24, 1169−1181. (14) Xu, Y. Z.; Liu, J. G.; Wang, H. Y.; Han, Y. C. Hierarchical Network-Like Structure of Poly(3-hexylthiophene) (P3HT) by Accelerating the Disentanglement of P3HT in a P3HT/PS (Polystyrene) Blend. RSC Adv. 2013, 3, 17195−17202. (15) Chang, M.; Choi, D.; Wang, G.; Kleinhenz, N.; Persson, N.; Park, B.; Reichmanis, E. Photoinduced Anisotropic Assembly of Conjugated Polymers in Insulating Polymer Blends. ACS Appl. Mater. Interfaces 2015, 7, 14095−14103. (16) Qiu, L.; Lee, W. H.; Wang, X.; Kim, J. S.; Lim, J. A.; Kwak, D.; Lee, S.; Cho, K. Organic Thin-Film Transistors Based on Polythiophene Nanowires Embedded in Insulating Polymer. Adv. Mater. 2009, 21, 1349−1353. (17) Lu, G. H.; Blakesley, J.; Himmelberger, S.; Pingel, P.; Frisch, J.; Lieberwirth, I.; Salzmann, I.; Oehzelt, M.; Di Pietro, R.; Salleo, A.; Koch, N.; Neher, D. Moderate Dopping Leads to High Performance of Semiconductor/Insulator Polymer Blend Transistores. Nat. Commun. 2013, 4, 1588. (18) Kim, F. S.; Jenekhe, S. A. Charge transport in poly (3butylthiophene) nanowires and their nanocomposites with an insulating polymer. Macromolecules 2012, 45, 7514−7519. H

DOI: 10.1021/acs.macromol.6b01721 Macromolecules XXXX, XXX, XXX−XXX