Conjugated Polymer Alignment: Synergisms Derived from Microfluidic

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Conjugated Polymer Alignment: Synergisms Derived from Microfluidic Shear Design and UV Irradiation Gang Wang, Ping Hsun Chu, Boyi Fu, Zhongyuan He, Nabil Kleinhenz, Zhibo Yuan, Yimin Mao, Hongzhi Wang, and Elsa Reichmanis ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b07548 • Publication Date (Web): 26 Aug 2016 Downloaded from http://pubs.acs.org on August 30, 2016

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ACS Applied Materials & Interfaces

Conjugated Polymer Alignment: Synergisms Derived from Microfluidic Shear Design and UV Irradiation

Gang Wang,† ‡ Ping-Hsun Chu,† Boyi Fu,† Zhongyuan He, ‡ Nabil Kleinhenz, ⊥ Zhibo Yuan,⊥ Yimin Mao,∥, δ Hongzhi Wang, *,‡ Elsa Reichmanis*,†,⊥,§ †

School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta,

Georgia 30332, United States ‡

State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of

Material Science and Engineering, Donghua University, Shanghai 201620, People’s Republic of China ⊥

School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia

30332, United States § School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States ∥

Department of Materials Science and Engineering University of Maryland, College Park,

Maryland 20742, United States δ

NIST Center for Neutron Research, National Institute of Standards and Technology, 100

Bureau Dr., Gaithersburg, Maryland 20899, United States

ACS Paragon Plus Environment


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KEYWORDS: microfluidic shear; conjugated polymers; charge transport; alignment

ABSTRACT: Solution shearing has attracted great interest for the fabrication of robust and reliable, high performance organic electronic devices, owing to applicability of the method to large area and continuous fabrication, as well as its propensity to enhance semiconductor charge transport characteristics. To date, effects of the design of the blade shear features (especially the microfluidic shear design) and the prospect of synergistically combining the shear approach with an alternate process strategy have not been investigated. Here, a generic thin film fabrication concept that enhanced conjugated polymer intermolecular alignment and aggregation, improved orientation (both nanoscale and long range), and narrowed the π-π stacking distance is demonstrated for the first time. The impact of the design of shearing blade microfluidic channels and synergistic effects of fluid shearing design with concomitant irradiation strategies were demonstrated, enabling fabrication of polymer-based devices with requisite morphologies for a range of applications.


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Introduction The solution processability of conjugated polymer semiconductors has enabled the development of organic electronic devices, including organic photovoltaics (OPV), organic lightemitting diodes (OLED), and organic field-effect transistors (OFET).1,2,3,4,5 Many applications, however, require enhancement in organic semiconductor charge transport properties.6,7,8,9,10 While various synthetic approaches have led to the identification of high-mobility conjugated systems, an essential element related to achieving desired morphology with concomitant regulation of crystallization and orientation of the conjugated polymers in solution processed thin films remains a key challenge for the fabrication of robust and reliable, high performance OFET devices.11,12,13,14,15 While techniques to increase polymer charge carrier mobility by manipulation of the microstructure have been developed, they rely on mechanical stretching, or as in very recent reports of high-mobility semiconducting polymers, on long post-processing techniques such as high-temperature rubbing, solution shearing or the use of confined fluid flows on preengraved substrates.16,17,18,19,20,21,22 Among the available processing methods, solution shearing has attracted great interest owing to applicability of the method to large area and continuous fabrication, as well as its propensity to enhance semiconductor charge transport characteristics. Solution shearing, especially microfluidic shear, also allows for tunable film formation conditions enabling control of the nucleation, crystallization and phase separation of conjugated small molecules and polymers.4,23,24,25,26,27 For example, Bao et al designed a printing blade with micropillars embedded in the blade surface that mix the ink, inducing microfluidic shear, so as to form a uniform film of large, well-aligned small molecule crystals, providing for a 10-fold improvement in macroscale mobility.7 Most recently, based on the hypothesis of flow-induced polymer crystallization, a microstructured printing blade to induce microfluidic shear was introduced for the fabrication of semiconducting films composed of tiny nanometer-sized crystals of uniform size that exhibited enhanced electrical properties.25 To date, effects of the design of the blade features, especially features that induce microfluidic shear, have not been investigated. In particular, the precise geometry of shear blade patterns is expected to significantly impact molecular stacking, crystal structure and long range



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orientation of small molecule and polymer semiconductors, in turn affecting charge transport performance.4,21 In addition, to date research related to blade coating largely focused on the effects of shear, with no consideration given to synergistic integration of shear with alternative approaches for manipulating thin film microstructure. For instance, UV irradiation and sonication can be applied to conjugated polymer solutions to promote molecular π-π stacking and aggregation, thereby enhancing molecular ordering and as a consequence, charge transport performance of conjugated polymer films for OFET applications.20,28,29,30 It is anticipated that the integration of shear effects with external irradiation during direct film formation will facilitate microstructural development for high charge transport performance. Here, a generic, synergistic thin film fabrication concept to enhance intermolecular alignment and aggregation (including narrowing π-π stacking distance and tuning molecular ordering), and improve orientation (both nanoscale and long range) is demonstrated for the first time. The approach couples solution shear using a blade having an appropriately patterned microfluidic channel design with UV irradiation (BPD-UV), as shown in Figure 1A. Three different microfluidic channels (Figure 1B, C and D) were designed and systematic investigations (UV-vis, AFM, GIWAXS and charge transport performance) of the conjugated polymer films were conducted. Using a thin film transistor as a test vehicle, the most significant increase in charge transport characteristics was observed when using BPD-UV with wave-like channel features (wBPD-UV). The importance of the microfluidic shear design for conjugated polymer film fabrication was confirmed: the design principle is proposed here. Moreover, BPD-UV thin film fabrication exhibited obvious advantages in facilitating desirable conjugated polymer microstructure formation with enhanced charge transport vs. BPD (BPD-UV fabrication without UV irradiation) or spin coating, which suggests benefits associated with the synergistic approach. Additionally, BPD-UV can be applied to different solvent/conjugated-polymer systems. Both halogenated (chloroform, CHCl3) and nonhalogenated (toluene and mesitylene) solvents were evaluated; and hole-transport and electron transport polymers, poly(3-hexylthiophene) (P3HT) and the naphthaplene diimide based conjugated polymer, poly(NDI2OD-T2), respectively, were tested. Enhanced mobility was achieved for both: P3HT exhibited an 11-fold enhancement, while poly(NDI2OD-T2) showed an enhancement in mobility by a factor of 2.4. The BPD-UV concept provides a strategy for the


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direct fabrication of conjugated organic/polymer thin-films with morphologies that are controlled at scales ranging from the molecular level to long-range macroscale orientation, enabling a wide range of polymer-based devices.



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Experimental Section Materials P3HT was purchased from Reike Metals Inc. and used without further purification. The P3HT used in this study had a M w of 43.9 kDa, a PDI of 1.8 and a regioregularity of >96% (batch number is BS2186).

Electron-transport poly(NDI2OD-T2) was purchased from Polyera

Corporation (Activink N2200) and used without further purification. The poly(NDI2OD-T2) used in this study had a Mw of 48.0 kDa and PDI of 1.9. Chloroform (anhydrous), mesitylene (anhydrous), toluene (anhydrous) and dichlorobenzene (anhydrous) were purchased from SigmaAldrich and used without further purification. The molecular weight and PDI were obtained through gel permeation chromatography (GPC) using trichlorobenzene as the eluent and polystyrene as the standard. Fabrication of shearing blade with various patterns For blade fabrication, a silicon mold was prepared first by patterning a photoresist mask with a Cr mask on top of a silicon wafer using standard photolithography followed by Surface Technology Systems (STS) - Inductively Coupled Plasma (ICP) Etching. The silicon was etched by 50–100 µm. The patterned blade was then rinsed with acetone to remove the photoresist layer. The above processing steps were conducted in the Marcus clean room at the Georgia Institute of Technology. The blade was then plasma-activated for 15 min with UV ozone and immersed in a 0.1 vol.% OTS/trichloroethylene (anhydrous) solution for 40 min at room temperature. After rinsing with toluene and isopropanol, the wafer was blown dry by N2 flow and the annealed at 150 oC for 30 min in the vacuum. The patterned blade was stored in N2 glovebox. It is important to note that the pattern structures play a crucial role in the BPD-UV fabrication process, even the post-clean procedures. BPD-UV fabrication of P3HT solutions A BPD-UV fabrication concept was used to rapidly form a uniform P3HT film. First, a tolueneP3HT solution was prepared: 6 mg of P3HT was introduced into 2 mL of toluene in a 20-mL borosilicate glass vial. Subsequently, the vial was capped and placed on top of a hot plate for at least 30 min at ≈85 °C to ensure complete dissolution of P3HT. A syringe pump (Remote PHD ULTRA™ CP Syringe Pump) with a glass syringe (10 mL, Model 1010 LTN SYR) was used to introduce the P3HT solution onto the substrates for BPD-UV fabrication.


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As shown in the experimental setup in Figure 1A and Figure S1, the processing setup can be divided into three parts: (1) The BPD-UV fabrication blade: The BPD-UV fabrication blade was kept stable at a distance of 10 um above the substrate (contact angle is 15o) and the shearing speed was set at 100 ~ 300 µm/s. (2) The UV irradiation light: The UV lamp (Entela UV, 254 nm) was held stable at a distance of 4~5 cm from the substrate, irradiation took place during the entire BPD-UV fabrication process. (3) The pre-heated substrates: The substrates were pre-heated at 35 ~ 40 oC and transfered to the vacuum stage immediately prior to BPD-UV fabrication. The experiments were performed in an ambient atmosphere, in an exhaust hood on to remove evaporated solvent. The processing of poly(NDIOD-T2) solutions 10 mg of poly(NDIOD-T2) was introduced into 2 mL of mixed solvents (chloroform: mesitylene=1:1 in volume) in a 20-mL borosilicate glass vial. The processing conditions and procedures were similar to those described above for P3HT. Organic field-Effect transistor (OFET) fabrication and characterization The bottom-gate, bottom-contact OFET devices were fabricated to perform electrical characterization of P3HT films fabricated from solutions prepared as above. Silicon wafers (ntype) with a 300-nm-thick SiO2 dielectric surface were used as the substrate. The source and drain contacts (Au/Cr) were fabricated using a standard photolithography based lift-off process inside a cleanroom, followed by E-beam evaporation (Denton Explorer) of 50 nm Au contacts with 3 nm of Cr as the adhesion layer. Before spin-coating or BPD-UV fabrications, all devices were cleaned for 15 min in a UV-ozone cleaner (Novascan PSD-UV) to completely clean the surface. An Agilent 4155C semiconductor parameter analyzer was used for the test of OFET devices under a nitrogen atmosphere. The devices were stored in a vacuum oven overnight at 50 oC and 1 torr to remove residual solvent. The field-effect hole mobility was calculated in the saturation regime of transistor operation (VDS = −80 V) by plotting the drain current ( ID ) versus gate voltage ( VG ) and fitting the data to the following equation 1 ௐ

‫ܫ‬஽ = μC୭ ଶ௅ ሺܸீ − ்ܸ ሻଶ


(1)


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where W (2000 µm) and L (50 µm) are the transistor channel width and length, respectively, VT is the threshold voltage, and CO is the capacitance per unit area of the silicon dioxide gate dielectric (1.15 × 10−8 F cm−2). For the top-gate, top-contact N-type transistors, W (1000 µm) and L (50 µm) are the transistor channel width and length, respectively. UV-Vis spectroscopy An Agilent 8510 UV-vis spectrometer was used for the record of the UV-Vis spectra of the conjugated polymer films. Films for solid state studies were prepared by spin-coating or BPDUV fabrication the requisite solution onto pre-cleaned 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 BPD-UV fabrications onto OFET devices, using an ICON Dimension scanning probe microscope (Bruker) operating in standard tapping mode with a silicon tip (RTESP, Bruker) in air. Polarized optical microscopy (POM) A Leica DMRX optical microscope equipped with two polarizers and a Nikon D300 digital SLR camera was used to obtain the POM images. Polarized Raman Raman spectra were obtained using a 50× objective, 20 mW laser power and a 785 nm laser light source (Kaiser Optic System) that has 4 cm−1 resolution in the backscattering geometry. The spectra were acquired using exposure times of 6 s with six accumulations using parallel polarizers. The sample stage was rotated from 0 to 90°. For each spectrum, a Holograms software was used to obtain the C=C stretching peak heights and positions. Synchrotron radiation characterization GIWAXS measurements were carried out at beamline 11-3 in the Stanford Synchrotron Radiation Light Source (SSRL). The X-ray beam was kept at an energy of 13 KeV and the incident angle of measurement was 0.12°. A LaB6 standard sample was used to calibrate the beam position and sample-to-detector distance. 2D diffraction images were corrected and presented in q-spacing using the software WxDiff (Figure S8), from which 1D scattering profiles can be reduced for relevant analysis. Peak fitting was done using the software MagicPlot Pro.


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Results BPD-UV thin film fabrication -impact of blade microfluidic channel design

Figure 1. (A) The scheme of the BPD-UV fabrication concept; The SEM images of the BPD-UV fabrication blade: (B) parallel; (D) prismatic; (F) wave-like features; and corresponding conjugated polymer films (C), (E) and (G) in the same printing conditions (Tapping mode AFM phase images (5 × 5 µm and 2 × 2 µm).). All the scale bars in the SEM images are 40 µm.

A photograph of the BPD-UV fabrication apparatus can be found in Figure S1 in the supporting information. The shearing blade, patterned with the desired microfluidic channels was



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fabricated by photolithography followed by etching the design into a silicon substrate (details can be found in the experimental section). As shown in the schematic of the experimental apparatus in Figure 1A, the conjugated polymer solution was injected onto the surface of the substrate at a constant pumping speed, after which the patterned blade was positioned about 10 µm above the substrate at a contact angle of 15o. The blade was then induced to travel across the device substrate at constant speed to create a shear environment. During the shearing process, the blade/substrate interface was irradiated with 254 nm UV light from a distance of about 5 cm. To evaluate the effect of shearing blade microfluidic channel design, three distinct geometries were fabricated. The surface morphology of the blade microfluidic channels themselves can be observed in the Scanning Electron Microscope (SEM) images shown in Figure 1 B, D and F. More details concerning the surface geometry and dimensions of the microfluidic channels can be found in Figure S2. Atomic Force Microscopy (AFM) phase images of P3HT films obtained via BPD-UV fabrication with the respective blades are presented in Figure 1 C, E and G, demonstrating the microstructural evolution from a pristine featureless film to a heterogeneous, anisotropic film with clearly defined P3HT nanofibers. Nanofibers exhibiting large variations in density and scale corresponding to the different microfluidic channel design features are readily obtained in thin conjugated polymer films fabricated using the blade shearing process. From the AFM phase images, the blade patterned with simple, parallel-like microfluidic channels (Figure 1b) affords only limited quantities of randomly distributed nanofibers ca. 200~400 nm in length (Figure 1c). For the prismatic-like featured blade (Figure 1d), a high density of nanorod structures (ca. 100 nm in length) with random orientations were obtained (Figure 1e). In the case of the blade patterned with wave-like microfluidic channels (Figure 1f), a high-density of longer (ca. 1~2 µm in length) nanofibers with uniform orientation along the shearing direction was achieved (Figure 1g). The AFM images presented here convey the importance of microfluidic channel geometry in the development of thin-film morphology: uniformly distributed parallel fibers can be realized via the wave-like patterned blade. In addition, AFM height images (Figure S3) show obvious variation in surface microstructure. The Root Mean Square (RMS) surface roughness is 3.34 nm, 1.23 nm and 0.805 nm, corresponding to parallel, prismatic and wave-like blade features, respectively. The results suggest that more uniform surface structure with less roughness can be


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attained via shearing with a blade patterned with microfluidic channels, especially when the channels comprise continuous, wave-like features.

Figure 2. (A) Polarized optical microscopy (POM) images of the P3HT films via wBPD-UV fabrication. (B) Change in absorption as a function of shearing direction with respect to polarizer. Image shown is P3HT films via wBPD-UV fabrication, displaying linear dichroism. (C) Representative AFM images of P3HT nanofibers uniformly oriented along the shearing direction with expected π-π stacking direction along the long axis of the nanofiber. (D) Scheme illustrating the molecular chain stacking of the sheared conjugated polymer film.

Polarized optical microscopy (POM) characterization was used to further interrogate the long-range order associated with P3HT films fabricated with the blade having wave-like microfluidic channels. Figure 2A presents POM images of the microfluidic channel as viewed through crossed polarizers as the substrate is rotated from 0° to 45° with respect to the analyzer. While an isotropic material would appear dark at all angles, the observed transmitted light intensity indicates a birefringent fluid with requisite anisotropy. The near complete extinction and reemergence of brightness upon rotation of the channel long axis from 0° to 45° suggests long range ordering along the wBPD-UV fabrication direction.31 Figure 2B depicts the linear dichroism displayed by the P3HT films. The observed dichroism is believed to emanate from elongated π-π stacked P3HT assemblies preferentially oriented parallel to the shearing direction, as depicted in Figure 2D, with individual P3HT chains perpendicular to the shearing direction. From the AFM (phase, 2 µm ×2 µm) image (Figure 2C), the surface geometry was comprised of distributed, oriented fibers. The polarized Raman spectra in Figure S4 and Table S1 further confirm the molecular orientation of assemblies within the wBPD-UV fabricated films. The



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results presented above strongly suggest that long range nanofiber alignment can be facilitated using an appropriately patterned blade. AFM images associated with films prepared with the wave-like geometry at different shearing speeds (Figure S5) show that the oriented nanofiber distribution can be manipulated in terms of length and density. POM images (Figure S6) also suggest that shearing speed impacts the formation of films having long range order.

Photo-physical properties

Figure 3. (A) Normalized UV-visible absorption spectra of P3HT films obtained by spin-coating and BPD-UV fabrication. (B) Fraction of aggregated P3HT in films via spin coating and BPD-UV fabrication and corresponding exciton bandwidth W was calculated using the ratios of the A0-0 and A0-1 peak intensities. Lines were added to guide the eye.

Figure 3A depicts the electronic absorption spectra obtained from P3HT films fabricated via spin coating and the three alternative microfluidic channel patterned blade coating protocols. As discerned from Figure 3A, the microfluidic channels appear to effect an increase in intensity of the low energy (0-1) and (0-0) absorption bands in resultant polymer films.29,32 Thus, consistent with previous reports, nanoaggregates formed in the solution appear to survive the coating process, and in turn provide for increased molecular order in the solidified thin-films. In addition, processing with a shearing blade effects a red-shift in P3HT absorption peaks compared to those obtained via spin coating, which suggests enhanced co-facial π-π stacking and/or planarization of the conjugated backbone.33 Figure 3B shows the fraction of aggregated P3HT and exciton energy within the films as extracted by the Spano’s model.34,35 The peak fit results can be found in Figure S7 in the supporting information. As qualitatively observed from the UV-vis spectra, the geometry of the shearing blade microfluidic channel patterns results in differences in the fraction of aggregates.


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According to Spano’s model, interchain coupling leads to vibronic bands in the absorption spectrum, which can be related to the free exciton bandwidth (W). In turn, W correlates with intrachain ordering of an individual polymer chain within the aggregates.36 An increase in intramolecular order is associated with a decrease in W, and is used as a general method to compare the extent of conjugation in polymer based semiconductors. As shown in Figure 3B, the value of W for P3HT films obtained from spin coating appears at 130 meV, but decreases to 98, 73, and 64 meV when the films are prepared using blades with the respective straight, prismatic and wave-like microfluidic shearing channels; suggesting enhanced intramolecular ordering. The lowest solidified thin-film free exciton bandwidth, and thus the sample with the most well-ordered polymer aggregates was observed for films fabricated using the blade patterned with wave-like microfluidic channels.

GIWAXS analysis

Figure 4. (A) Geometry of grazing incidence wide angle GIWAXS with a 2D planar detector; 2D-GIWAXS patterns of pristine P3HT film via (B) spin coating, BPD-UV fabrication using (C) parallel patterns, (D) prismatic patterns, and (E) wave patterns. (F) The 1D integration profiles of 2D GIWAXS patterns; (G) The line cut profile of 2D GIWAXS patterns at 85o.



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Table 1. GIWAXS peak analysis of P3HT films fabricated via different processing methods. d100

d010

Crystallite Size

fH

(Å)

(Å)

(Å)

Spin coating

16.91

---

78.09

0.10

BPD-UV (Parallel)

16.75

---

97.45

0.29

BPD-UV (Prismatic)

16.68

3.84

109.50

0.42

BPD-UV (Wave)

16.56

3.72

114.12

0.64

The crystallinity and microstructure of the as prepared P3HT films were characterized by 2D synchrotron grazing incidence wide angle X-ray scattering (2D GIWAXS) (BL 11-3) at the SLAC National Accelerator Laboratory. The diffraction patterns shown in Figure 4 B–E were recorded on a planar area detector (Figure 4A). The 2D GIWAXS profiles of the films present ( 1 0 0 ) reflection peaks along the out-of-plane (qz) direction, corresponding to P3HT lamellar stacking oriented normal to the substrate. However, halo-like patterns are apparent along the azimuthal angle, which indicate a randomly oriented thin film microstructure. 37 In contrast to the relatively isotropic patterns obtained from the P3HT samples fabricated via spin coating, BPD-UV fabricated samples exhibit a series of intense characteristic ( h 0 0 ) ( h = 1, 2, 3) spots along the qz direction. Additionally, the apparently improved π–π interchain stacking of the polymer backbones leads to stronger ( 0 1 0 ) arcs along the in-plane (qxy) axis, suggesting that the crystallites are well organized and preferentially in an edge-on orientation. 38 Figure 4F depicts the 1D integration profiles of structures (with background strip). Strong ( 1 0 0 ) peaks at q~0.371, 0.375, 0.376 and 0.379 (Å−1) can be observed for spin coated and BPD-UV fabricated (from left to right, via parallel, prismatic and wave-like channels) P3HT films, respectively. The ( 1 0 0 ) peak intensity of P3HT films obtained via BPD-UV fabrication was significantly increased in comparison to spin coated analogs. The appearance of these high order reflections is a strong indicator of enhanced thin film crystallinity. The position and full width at half-maximum of the diffraction peaks can be correlated to the lamellar spacing and crystalline correlation lengths (crystalline domain sizes) via Scherrer's equation in reciprocal space.39 The calculated average d100–spacing of BPD-UV fabricated films was slightly improved compared to spin coated alternatives, ranging from 16.91 to 16.75, 16.68, and 16.56 Å, respectively, as show in Table 1. An obvious decrease in d100–spacing can be observed. From results presented in


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Table 1, tighter molecular stacking and edge-on orientation can be obtained via wBPD-UV fabrication. For the wBPD-UV film fabrication process, the alkyl lamellar stacking became more close-packed (d100-spacing ~16.56 Å), and the crystalline domain size increased to 114 Å. Note that no ( 0 1 0 ) peak cannot be observed from spin coated P3HT films or those prepared by BPD-UV fabrication using parallel -like microfluidic features. For the BPD-UV fabrication using prismatic or wave-like microfluidic channels, the π-π stacking interactions were improved and significant ( 0 1 0 ) peak can be observed. From calculation of the average π-π stacking distance (calculated from the ( 0 1 0 ) peak in the line cut profiles of structures at 85o in Figure 4G), BPD-UV fabrication can facilitate stacking of the conjugated backbone. From the π-π stacking values (no values can be calculated from the parallel-like patterns, owing to the weak π-π stacking.) obtained using the three different microchannel patterns, blade pattern geometry played a dominant role, whereby the wave-like channels provided for the tightest packing, with a reduction in π-π stacking distance from 3.84 Å to 3.72 Å. These results support the UV–vis data that suggested a strong intermolecular overlap integral for BPD-UV thin film fabrication, especially using the wave-like geometry. The electrical properties of semiconducting polymers are intimately related to crystal orientation, which can be quantified by Hermans’ orientation factor, fH.40 Calculated from the first order alkyl stacking ( 1 0 0 ) peaks, these factors can range from -0.5 to 1, where 1 corresponds to perfectly oriented crystal planes perpendicular to the substrate (edge-on), -0.5 denotes a lattice plane strictly oriented parallel (face-on), and randomly oriented structures afford a value of 0. As shown in Table 1, compared with the other two patterned blade channel geometries, e.g. parallel and prismatic, the films obtained from the blade with wave-like channels provides for the highest fH at 0.64, suggesting a highly oriented microstructure with primarily edge-on orientation. The highly oriented edge-on stacking direction (parallel to the charge transport direction in the OFETs) with narrower molecular chain stacking distance can be attributed to the synergistic effect of the rigid molecular chain induced by UV irradiation and shear stress induced by extensional flow during the direct BPD-UV fabrication process. It is known that thiophene ring orientation and π-π stacking distance significantly impact charge carrier mobility. Analysis of GIWAXS and AFM data demonstrated that BPD-UV fabrication, especially the wBPD-UV processing, can promote fiber growth, shorten P3HT π-π



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stacking distance, and induce enhanced edge-on orientation. All of these microfluidic channel features are expected to confer the films with more efficient charge carrier transport pathways, providing the potential for improved macroscopic semiconductor device performance.

Charge transport performance

Figure 5. Electrical characterization of BPD-UV fabrication P3HT FETs. (A) The scheme of the film structure of the OFET device; (B) Overlaid transfer curves of 60 OFETs at −80 VDS. (C) Histogram of mobility of 60 OFETs via wBPD-UV fabrication. (D) Average field-effect mobilities of P3HT films via spin coating and BPD-UV fabrication via various print blade patterns. Mobilities were calculated in the saturation regime of operation with VDS= -80 V. (E) The average mobility under different shearing speeds (each data point was collected from 10 devices). (F) The average mobility with spin coating, BPD and BPD-UV fabrication (each data point was collected from 10 devices). (G) The plot of the charge transport mobility of P3HT films from literature values compared with the current work highlighted with a red circle.).41, 29, 42, 28, 43, 44, 45, 46, 47

Based upon AFM analysis, Figure 5A depicts the proposed schematic representation of the film structure with oriented conjugated polymer nanofiber distribution on an OFET device. The printing (shearing) direction was parallel to the source-drain direction. The channel length and width for the OFET devices used in this study were 50 µm and 2,000 µm, respectively. A total of 6 substrates, each with an area of ∼6 × 2 cm2, were coated with P3HT using the wBPDUV process. Sixty randomly selected devices were characterized, and a yield of >90% can be achieved. Overlaid in Figure 5B are the transfer curves measured at VDS = −80 V. As shown in Figure 5C, the average macroscale mobility was 0.13±0.03 cm2·V−1·s−1 (highest mobility at 0.16 cm2 V−1·s−1) with a relative variance of 23% over the 60 tested devices (5 devices per substrate).


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The variance of the mobility for devices from one individual substrate was within 10%, which suggests that the conjugated polymer films were relatively uniform. In comparison, other patterning methods used to create patterned organic semiconductors for isolated OFETs have shown relative variances ranging from 20% to 80%.48 The relatively low variance in the electrical characteristics of the current devices along with their enhanced charge transport performance can be attributed to both the presence of highly packed nanofibers that connect the electrodes and uniform surface microstructure (ascertained from AFM images). Figure 5D presents a histogram of charge (hole) transport performance using spin coating and BPD-UV fabrication via the three distinct microchannel patterns. The BPD-UV fabrication concept, as well as the microfluidic channel patterns, significantly enhance charge transport performance. With the optimal design (wave-like microfluidic channels), the average mobility reached as high as 0.13 cm2 V-1 s-1, with a maximum of 0.16 cm2 V-1 s-1. While BPDUV fabrication using parallel and prismatic geometries afforded approximately a 3–fold and 4.5fold increase in mobility, respectively (from 1.30±0.15×10-2 to 4.58±1.05×10-2 cm2 V-1 s-1 and 5.85±1.35×10-2 cm2 V-1 s-1), using the blade patterned with wave-like microfluidic channels provided for more than an 11–fold performance enhancement (from 1.30±0.15×10-2 to 13±3.0×10-2 cm2 V-1 s-1). As shown in Figure 5E, shearing speed impacts mobility. With an increase in shearing speed from 50 µm s-1 to 150 µm s-1, the average mobility increased from 0.08 to 0.13 cm2 V-1 s-1. A continued increase in shearing speed to 300 µm s-1 led to a decrease in mobility (from 0.13 to 0.09 cm2 V-1 s-1). In light of these results, the optimized speed was set at 150 µm s-1. Conjugated polymer charge transport performance depends on the overall film morphology, which includes polymer orientation, polymer−polymer orientation, polymer domain orientation, and aggregated domain connectivity.49 From the AFM images, nanofiber length, density and orientation varied significantly with shearing speed. At the shearing speed of 150 µm s-1, an optimized microstructure favorable to charge transport was obtained, thereby leading to the highest observed mobility. From the above results, shear enhancement techniques, especially those derived from shear effects associated with the introduction of microfluidic channels, may increase the uniformity and charge transport performance of semiconducting polymer films, which in turn may provide for yield enhancement. As seen from Figure 5F, compared with the mobility of only 0.01 cm2 V-1 s-1 via spin coating methods, the 4-fold enhancement of the



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mobility via blade coating using a blade patterned with wave-like microfluidic channels in the absence of UV irradiation (wBPD fabrication), confirmed the superiority of blade vs. spin coating. Moreover, wBPD-UV strategy achieved a mobility as high as 0.13 cm2 V-1 s-1, about 3.25- fold enhancement compared with BPD processing, confirming the importance of synergistic effects associated with integration of shear with UV irradiation. In addition, a plot of reported P3HT charge carrier mobility with respect to the current investigation is provided in Figure 5G. Spin coating, which was used in most reported results, is incompatible with large area and continuous fabrication of organic electronic devices. Obvious enhancement in charge transport characteristics can be achieved via the wBPD-UV printing strategy. While solution preaggregation afforded higher mobility,47 that process approach is expected to be more challenging to control and be more time and energy intensive.

Process versatility – an N-channel conjugated polymer example


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Figure 6. (A) Chemical structure of naphthaplene diimide based conjugated polymer poly(NDI2OD-T2); (B) POM images; (C) AFM images with various scale bars of the P(NDI2OD-T2) films on the silicon wafer via wBPD-UV fabrication. (D) Transfer characteristics of poly(NDI2OD-T2) OFETs fabricated from polymer solutions with different processing methods. (E) Average field-effect mobilities of poly(NDI2OD-T2) films spin coated from poly(NDI2OD-T2) solutions via different processing methods (each data point was collected from 10 devices). Mobilities were calculated in the saturation regime of operation with VDS= 80 V.

The chemical structure of the electron transport conjugated polymer, poly(NDI2OD-T2), can be found in Figure 6A. As shown in Figure 6B, the sheared sample was viewed through crossed polarizers as the substrate was rotated from 0° to 45° with respect to the analyzer: the observed transmitted light intensity indicates a birefringent film with requisite anisotropy. From the AFM images (Figure 6C) (2 µm to 10 µm viewing area), directionally grown fibrillar microstructures with high density are sharply aligned. Together with birefringence observed under POM, the results show long range molecular alignment over a few hundred micrometers. As shown in Figure 6D and 6E, the wBPD-UV fabrication strategy, effected a 2.4 fold increase in mobility (wBPD-UV fabrication vs. spin coating, 1.2±0.3 vs. 0.5±0.2 cm2 V-1 s-1,



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respectively). Further enhancements are expected with process and device fabrication optimization. As observed above for P3HT hole transport performance, the enhancement in electron transport performance for the NDI system can be attributed to long range ordering of the obtained nanofiber like structures.


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Discussion

Figure 7. (A) The proposed film formation mechanism via wBPD-UV fabrication; (B) Schematic of possible packing arrangement of crystalline regions and tie-chain interactions (marked with pink line) and the corresponding surface morphology of the conjugated polymer film.

Thin film formation using wBPD-UV fabrication leads to fibril growth, long range ordering and enhanced charge transport performance, which can be explained by the combined application of shear stress, extensional flow and UV irradiation. In addition, the AFM images suggest the presence of tie-chain interactions. More obvious tie-chain interactions can be found at the shearing speed of 50 µm s-1 in Figure S5. The lamellar and π-π stacking distance both decreased, which implies that the molecular backbones and side chains become increasingly close-packed with the introduction of shear and extensional flow. The synergistic effects of shear stress, extensional flow and UV irradiation were expected to promote formation of a microstructure favorable for charge transport. Previously, it was demonstrated that UV irradiation promotes polymer chain crystallization and aggregation.20 Here, microfluidic shear combined with UV irradiation was shown to tailor conjugated polymer film



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structure at the molecular scale. The microfluidic channels patterned into the shear blade play a crucial role during fluid flow, thereby affecting fluid shear stress, chain alignment and extensional flow. From the fluid dynamics perspective, flow viscosity varied significantly with a change in the dimensions of the confined structure.25,50 A proposed wBPD-UV film formation mechanism is presented in Figure 7A. Molecular chain extension, tie-chain interactions, 90 degree molecular inversion, and intense π-π intermolecular stacking can be expected due to the effects of shear stress, extensional flow and UV irradiation. The results of Thiele et al, who showed that a macromolecular (protein) chain can be inverted by 90 degrees while flowing through a narrowed region of a microfluidic channel, provide precedent for the proposed behavior of conjugated polymer chains flowing through microfluidic channels where the diameter of the channel varies.50 The hypothesis presented here is supported by AFM, POM and GIWAXS analysis, as well as photo-physical characterization. To further understand the synergistic effects of shear stress and UV irradiation, in-situ characterization strategies, e.g. Xray diffraction and neutron scattering, will be utilized in the future studies. The phenomenon observed with wBPD-UV fabrication is closely related to flow-induced changes in polymer conformation, as shown in Figure 7A. In particular, flow-induced chain extension and alignment is known to promote polymer crystallization, owing to a lowered entropic barrier to the formation of ordered structures.25,50,51,52,53,54 Extensional flow has been shown to be effective in stretching polymer chains, thereby inducing nucleation and crystallization. Also, shear stress may increase chain alignment, thus promote crystallization kinetics.55,56,57 The microfluidic channel patterned with the wave-like features was designed to induce extensional flow as well as to increase the shear rate across various shearing conditions. The channels serve to combine and direct the ink during the shearing process thereby guiding chain alignment, possibly inducing tie-chain interactions, and also promoting long range order. We hypothesize that the high extensional strain rate facilitates stretching of the polymer chains, which are subsequently aligned under the high shear rate. Combined with UV irradiation, above three effects cooperate to promote polymer crystallization, aggregation and orientation. The mechanism of polymer crystallization and aggregation under extensional flow and shear stress is the subject of intense research and is not yet fully understood.24,51,52 However, flow-induced crystallization has been extensively studied in the context of isotatic polyethylene


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and polypropylene crystallization from the melt.51,53,55,58 Flow-induced crystallization is due not only to extensional flow, but also to shear flow-induced orientational ordering. These studies further support the design concept and hypothesis presented for solution based processing of conjugated polymers.54 The shear speed dependent experiment (surface structure analysis in Figure S5 and charge transport performance in Figure 4E) supports the importance of shear stress. In addition, polyethylene solutions under flow have been observed to form bundle-like fibers termed "shish-kebab nuclei."59,60 The shish-kebab structure offers a potential explanation for the increased fiber alignment and charge transport observed in BPD-UV fabricated devices.20 Combined with the GIWAXS and AFM images, inter-fiber connections may have been formed during the BPD-UV coating process. Furthermore, the inter-fiber connections that would be afforded by a shish-kebab morphology provide additional support to the growing body of evidence that inter-grain “tie chains” play a major role in determining P3HT macroscopic hole mobility (as shown in Figure 7B), given the improved charge transport performance demonstrated by BPD-UV fabricated devices.20 A similar mechanism is expected to be at play for the electron transport materials, obvious buddle-like structures can be observed in Figure 6C. Thus, wBPD-UV fabrication, which imparts shear stress, extensional flow and UV irradiation to the system, will lead to desirable microstructures on the molecular scale through to the mesoscale, and up to the macro scale with a range of a few hundred micrometers. In addition, owing to the effects of the microchannel geometric design on molecular chain stacking and crystal structure, the wBPD-UV fabrication strategy may also apply in bulk-heterojunction organic photovoltaic systems, especially in the regulation of crystal domain size and donoracceptor interfacial design.



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Conclusion The generic BPD-UV thin film fabrication concept presented here opens new avenues for continuous, large scale fabrication of conjugated polymer films with molecular level control of thin film microstructure. Through experiments on hole (P3HT) and electron (poly(NDI2OD-T2)) transporting conjugated polymers, halogenated solvent (CHCl3) and non-halogenated solvent (toluene, and mesitylene), obvious charge transport enhancements were attained. The impact of the design of microfluidic shear was demonstrated for the first time: a design principle whereby microfluidic channels can promote polymer chain alignment and extensional flow was proposed, and is expected to deliver a blueprint for the further design of fully optimized shearing microfluidic channels. Further, the synergistic effects of fluid shearing design with alternative irradiation strategies (e. g. infrared radiation annealing, sonication and microwave) are expected to be promising for the design of high performance organic electronics. Given the synergistic effects of shear flow with external irradiation on molecular conformation and the modulation of molecular scale interactions, BPD-UV fabrication may extend beyond organic electronics, enabling a wide range of printed materials systems where morphology design and regulation of charge transport pathways is crucial.


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected]

ACKNOWLEDGMENT The financial support of the Georgia Institute of Technology and the National Science Foundation (CBET 1264555) is gratefully acknowledged. G. Wang thanks the China Scholarship Council for Fellowship support. N. Kleihnenz thanks the NSF NESAC IGERT Traineeship program (DGE-1069138). G. Wang, Z. He and H. Wang are grateful for support from NSF of China (51572046, 51503035), and Eastern Scholar. E. Reichmanis is also grateful for support from the Brook Byers Institute for Sustainable Systems at Georgia Tech, and the Georgia Tech Polymer Network. We also appreciate time spent with Prof. Martha Grover and Nils Persson in helpful discussions.

AUTHOR CONTRIBUTIONS G. Wang, P. Chu, and Z. Yuan performed the experiments. G. Wang, E. Reichmanis and H. Wang conceived and designed the experiments. P. Chu, B. Fu, Z. He, N. Kleihnenz, Y. Mao and G. Wang analyzed the data. G. Wang and E. Reichmanis wrote the manuscript.

COMPETING FINANCIAL INTERESTS The authors declare no competing financial interests.



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SUPPORTING INFORMATION Details on the photograph of the BPD-UV instrument setup, SEM images with various scale bars of the shear blade features, tapping mode AFM height images of conjugated polymer films using various microfluidic channels; tapping mode AFM phase images and POM images of conjugated polymer films using wave-like microfluidic channels at various shearing speed, Polarized Raman spectra of the conjugated polymer films, and detailed Raman data including peak heights, area, Peak fitting to obtain exciton energy and aggregation fraction from the UV-vis of various films calculation results and Correction images of 2D GIWAXS data to intensity vs q.

PRESENT ADDRESS G. Wang: Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208, United States. B. Fu: Emerging Technology Product, Applied Materials, Inc., 3050 Bowers Ave, Santa Clara, CA, 95054-3299, United States.


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The table of contents entry

Conjugated Polymer Alignment: Synergisms Derived Microfluidic Shear Design and UV Irradiation

A generic, synergistic thin film fabrication concept — couples solution shearing using a blade having an appropriately patterned microfluidic channel design with UV irradiation, was demonstrated for the first time. Enhanced intermolecular alignment and aggregation (including narrowing π-π stacking distance and tuning molecular ordering), improved orientation (both nanoscale and long range), and high charge transport can be achieved.


Conjugated Polymer Alignment: Synergisms Derived from Microfluidic

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