Controlled Molecular Orientation of Inkjet Printed Semiconducting

Article Cite This: Chem. Mater. 2017, 29, 10150−10158

pubs.acs.org/cm

Controlled Molecular Orientation of Inkjet Printed Semiconducting Polymer Fibers by Crystallization Templating Tobias Rödlmeier,†,‡ Tomasz Marszalek,§ Martin Held,∥ Sebastian Beck,‡,⊥ Christian Müller,‡,# Ralph Eckstein,†,‡ Anthony J. Morfa,†,‡ Robert Lovrincic,‡,# Annemarie Pucci,‡,⊥,∇ Uli Lemmer,†,¶ Jana Zaumseil,∥,∇ Wojciech Pisula,○,◆ and Gerardo Hernandez-Sosa*,†,‡ †

Light Technology Institute, Karlsruhe Institute of Technology, Engesser Str. 13, 76131 Karlsruhe, Germany InnovationLab, Speyerer Str. 4, 69115 Heidelberg, Germany § Institute of Organic Chemistry, Heidelberg University, Im Neuenheimer Feld 253, 69120 Heidelberg, Germany ∥ Institute for Physical Chemistry, Heidelberg University, Im Neuenheimer Feld 253, 69120 Heidelberg, Germany ⊥ Kirchhoff-Institute for Physics, Heidelberg University, Im Neuenheimer Feld 227, 69120 Heidelberg, Germany # Institute for High-Frequency Technology, Technische Universität Braunschweig, Schleinitzstr. 22, 38106 Braunschweig, Germany ∇ Centre for Advanced Materials, Heidelberg University, Im Neuenheimer Feld 225, 69120 Heidelberg, Germany ¶ Institute of Microstructure Technology, Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany ○ Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany ◆ Department of Molecular Physics, Faculty of Chemistry, Lodz University of Technology, Zeromskiego 116, 90-924 Lodz, Poland ‡

S Supporting Information *

ABSTRACT: Here we present the controlled deposition of highly aligned poly(3-hexylthiophene-2,5-diyl) (P3HT) fibers by inkjet printing. The functional ink consists of the crystallization agent 1,3,5-trichlorobenzene (TCB), the carrier solvent chlorobenzene, and the semiconducting polymer P3HT. The inkjet printing process was designed in such a way that the drying zone migrates in the printing direction, effectively growing the TCB out of solution and forcing the P3HT chains to align in the printing direction. The films are deposited in arbitrary shapes on a variety of substrates, thus demonstrating the full freedom of design necessary for the digital fabrication of future integrated circuits. We demonstrate by optical and structural investigations that P3HT arranges in a nontrivial empty-core−shell structure with the long molecular axis in the fiber direction while the short axis extends in a radial fashion. Such arrangement induces a fourfold increase in field-effect mobility along the fiber direction as compared to the isotropic printed reference.

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INTRODUCTION The macroscopic optoelectronic properties of polymeric organic semiconductor thin films rely on the inherent anisotropy of their molecular structure.1−4 This has been observed in the performance of optoelectronic devices such as organic field-effect transistors (OFETs), organic light-emitting diodes (OLEDs), and photodetectors.5,6 In the first case, the current density and carrier mobility can differ by orders of magnitude depending on the direction of charge-carrier transport.7−9 In OLEDs, the emission efficiency is strongly dependent on the orientation of the emitter materials,10,11 while in photodetectors the detection of polarized light was enabled by the preferential orientation of carbon nanotubes.12,13 Therefore, the development of techniques that enable control of thin film deposition on the macroscopic scale, with molecular-scale control, is of the utmost importance for © 2017 American Chemical Society

the optimization of device performance and the search for novel applications. Solution processability of organic electronic materials provides access to industrial coating and printing techniques, which can permit the low-cost, high-throughput manufacturing of electronics on flexible substrates. However, it typically introduces a high degree of disorder in the molecular arrangement during solvent drying, resulting in undesired or uncontrolled morphology (e.g., grain boundaries, glass-like structures, phase segregation, etc.).14−17 Recently, a growing effort to develop solution-based processes that yield highly oriented thin films has been under way. Processes like offReceived: September 18, 2017 Revised: November 15, 2017 Published: November 15, 2017 10150

DOI: 10.1021/acs.chemmater.7b03948 Chem. Mater. 2017, 29, 10150−10158

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Chemistry of Materials center spin coating,18 chemical design,19 bar coating,20 wetting and dewetting structures,21 mechanical rubbing and shearing,22−25 nanopatterning the processed layer by imprinting,26,27 microfluidic shear design,28 microfluidic crystal engineering,29 dip coating,30 or processing with crystalline templates31−35 have shown promising results for controlling the orientation of macroscopic films on the molecular level. Nevertheless, few of these techniques are compatible or applicable to industrial processes, or they lack the lateral resolution required for the integration of devices into more complex systems (e.g., electronic circuits). The usage of a crystalline template is a versatile method to form highly ordered structures. In this method, a solid organic material, with a melting point higher than the deposition temperature, dissolves the functional material when in its liquid state orientating the film upon recrystallization. With respect to organic semiconductors, Brinkmann et al. induced the uniaxial growth of regioregular poly(3-hexylthiophene-2,5-diyl) (P3HT) by depositing solid 1,3,5-trichlorobenzene (TCB) onto drop cast films prior to melting and recrystallizing the films.31,36,37 Müller et al. obtained continuous thin films of spherulite-like structures for a variety of different material systems commonly used in organic electronics by spin and dip coating,32 while most recently, Dörling et al. demonstrated larger areas of uniaxial P3HT fibers by blade coating.38 In this work, we demonstrate a single step process for the formation of highly oriented P3HT fibers by inkjet printing. The functional ink consists of the crystallization agent TCB, the carrier solvent chlorobenzene (CB), and the functional polymer P3HT. TCB is a low vapor pressure solid that can be dissolved in CB and that serves as a template for P3HT during film drying independently of the used substrate. TCB can subsequently be removed from the printed film by a vacuum sublimation step. In contrast to previous work, we utilize a contact-free printing technique with the full possibilities and advantages of freedom of design conveyed by digital printing, enabling us to deposit structures with a high degree of order in arbitrary shapes and placements on the substrate.39−42 The process is designed in such a way that the drying front of the subsequent printed lines determines the direction of the TCB crystallization and by this the epitaxial alignment of P3HT chains. This enables a one-step deposition method for aligned and structured semiconducting polymer films suitable for a variety of applications. Furthermore, we resolve by spectroscopic techniques the arrangement of the P3HT chains within the fibers in a nontrivial empty-core−shell arrangement and quantify its positive influence on the film electronic properties by organic field-effect transistor (OFET) measurements.

Figure 1. (a) Schematic illustration of the printing process with the sequential deposition of band like structures in the print head direction and crystallization of TCB along the printing direction forming P3HT fibers with the migration of the drying zone. (b) Polarized light microscopy images of the aligned P3HT fibers. (c) Scanning electron microscopy image of a bundle of P3HT fibers. The magnification shows a cross section of the bundle, cut with a focused ion beam.

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remove the residual TCB from the film. It is readily observed that the fibers extend over millimeters in length with gaps in between fiber bundles. A scanning electron microscope (SEM) image of a fiber bundle is presented in Figure 1c alongside with its corresponding cross section. The bundles are clearly made of single fibers, which appear to be hollow, suggesting that P3HT has assembled around the TCB crystallites. Atomic force microscopy (AFM) measurements taken in between the P3HT fibers (Figure S1) reveal the characteristic Shish Kebab structure previously reported by Brinkmann et al. by a directional epitaxial crystallization method.37 The optimization of the ink formulation was carried out by varying the CB:TCB mass ratio. Figure S2 of the Supporting Information (SI) presents optical microscopy images of films printed using different mass ratios at a fixed P3HT concentration of 20 g L−1

RESULTS AND DISCUSSION The printing process used herein to form aligned P3HT fibers is schematically illustrated in Figure 1a. The inkjet process was designed in such a way that the sequentially deposited bands in the print head direction coincide with the drying zone of the ink in the printing direction. By this method, the drying zone migrates in the printing direction, effectively growing the TCB from solution and forcing the P3HT in the residual CB to grow epitaxially on the previously formed crystals, analogous to the process described by Müller et al. for spin coated P3HT samples.32 The optical micrograph images in Figure 1b show the printed P3HT fibers extending along the printing direction (from left to right) on a glass substrate after a vacuum step was used to 10151


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Figure 2. (a) Absorbance spectra of aligned P3HT fibers with the light source being unpolarized or polarized parallel or perpendicular to the fibers. (b) Crossed-polarized and polarized images of printed P3HT fibers; solid arrows indicate the orientation of the polarizers, and dashed arrows point in the printing direction. (c, d) Images of star-shaped printed P3HT fibers taken with polarizer; polarizer orientation is indicated by the solid arrow, and the printing direction is indicated by dashed arrows. (e) Photograph of fibers on flexible PET.

In order to gain a further insight in the molecular arrangement of P3HT within the printed fibers, we performed FTIR spectroscopy measurements as a function of polarization of the excitation source. The spectra of a printed fiber film on a silicon substrate measured with light polarized parallel and perpendicular to the fiber axis as well as their relative difference are shown in Figure 3a. They reveal a strong polarization dependence of the characteristic vibrational features of P3HT. The vibrational bands at 1460 and 1510 cm−1, attributed to inplane CC stretching vibrations in the thiophene rings, are much stronger when the light is polarized parallel to the P3HT fiber axis. In contrast, the absorption bands of the C−H stretching vibrations between 2830 and 3000 cm−1 that are mainly located on the alkyl chains show the opposite behavior.45,46 For comparison, the corresponding transmission FTIR spectra of a single fiber are presented in Figure 3b. The single fiber was selected from a drop cast dispersion of fibers in acetonitrile, and an appropriate spot size of 40 μm was chosen so that only the fiber was illuminated. The measured spectra present very similar peak positions and the same polarization dependence compared to the fiber thin film spectra demonstrating that optical properties of the single fiber dictates the characteristics of the fiber assembly. A clear correlation exists between the conjugation length of polythiophenes and the position of the antisymmetric CC stretching vibration: the lower the peak position, the higher the conjugation length.47 GIWAXS measurements were performed in order to investigate the organization of the P3HT polymer chains in the printed fiber structures. As a reference sample, the GIWAXS pattern of the P3HT film printed only from CB is shown in Figure 4a, which displayed neither a long-range orientation nor a fiber microstructure. This reference film reveals a typical lamellar packing for edge-on ordered P3HT, which is well described in the literature (Figure 4b).48 The outof-plane (h00) reflections with the main (100) peak at qz = 0.285 Å−1 are assigned to an interlamellar distance of 1.60 nm. The edge-on organization is additionally confirmed by the inplane (010) reflection corresponding to a π−π stacking of 0.38 nm. The GIWAXS measurements of the aligned fiber film deposited from CB:TCB were performed perpendicular and parallel to the printing direction, as schematically presented in

and the same printing conditions. By analyzing the change in the fast Fourier transform (FFT) pattern of each image we qualitatively determined that the highest fiber alignment is achieved for a CB:TCB mass ratio of 1:0.22. The high degree of order of the printed fibers is evident in in the polarization dependent absorption measurements presented in Figure 2a. The spectra show a higher absorption when the light is polarized parallel to the long fiber axis (i.e., printing direction). This anisotropic optical property can be attributed to P3HT uniaxially aligned in the fibers. The absorption spectra show two main peaks at 555 and 610 nm. Their ratio is known to be a qualitative measure of increased π−π stacking in P3HT.43,44 Figure S3 compares the polarization absorption spectra for the different CB:TCB ratios and the pristine P3HT samples. We observed that the dichroic ratio as well as the characteristic peak ratio increase with higher TCB contents, suggesting an increased degree of alignment (Figure S4). The fibers used in the remainder of this work were printed with a CB:TCB mass ratio of 1:0.22. Figure 2b presents micrographs of 2.5 cm glass substrates covered with printed fibers, where a polarized light source illuminated the samples from the back and a corresponding crossed polarizer placed in front of the camera. In relation to its anisotropic structure, the P3HT fiber film shows a significant change in intensity when rotating the polarization filter by 45° whereas the printed films without fibers (shown in Figure S5) exhibited no color changes. The printed fiber patterns shown in Figure 2c,d demonstrate the freedom of design and placement of our method. The patterns were fabricated with different sizes and printing directions. A larger image of the 8 × 15 cm substrate and other printed examples are shown in Figure S6. The two panels in Figures 2c,d and S6 correspond to different directions of the polarization filter and show the dependence of the transmitted light intensity to the fiber direction. Video S1 of the SI presents the printed stars “blinking” while rotating the polarizer in front of the camera as a demonstration of the anisotropic optical properties of the films over square-centimeter areas. Furthermore, the formation of the fibers is independent of the substrate as we demonstrate throughout this work by printing on glass, polyethylene-terephthalate (Figure 2e), silicon oxide wafers, indium tin oxide covered glass, and parylene C. 10152


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circumference, while the interlayer is radially arranged toward the central void leading to an isotropic intensity distribution of the corresponding reflections in the parallel measurement. When the sample is rotated by 90°, scattering occurs only for the out-of-plane arranged (h00) crystal planes with the interlayer yielding the corresponding reflections solely on the z-plane of the pattern (Figure 4f). When the π−π stacking coincides exactly with the in- and out-of-plane of the film as indicated in the illustration by green arrows in Figure 4f, the corresponding reflections appear in the same planes of the pattern (Figure 4e). The presented GIWAX data is consistent with the detailed analysis of polarization dependent angle-resolved IR spectra presented in the SI, which also revealed a preferred orientation of the polymer backbone along the fiber long axis as well as a radial symmetric distribution of the polymer chains (Figures S8 and S9). Thus, the fiber structure is confirmed by two independent spectroscopic techniques. The difference to the molecular packing reported by Brinkmann et al.31 suggests that the chosen inkjet printing parameters and ink formulation promote different drying dynamics that than that of drop/spincast films. A careful parameter variation study with different polymers of defined molecular structure could shed light on these questions in the future. As a tool to investigate the charge transport in the P3HT fibers, bottom-contact electrolyte-bottom-gated OFETs (bEGOFETs, Figure 5a) were fabricated.50,51 To charge the entire P3HT bundles by the gate voltage field, the fibers were printed on top of a parylene dielectric layer and covered with an [EMIM][TFSI]−PVDF-HFP ion gel by drop-casting. The ion gel penetrates the layer of fibers and bundles, and a part of the ions even drifts and diffuses into the parylene layer; hence, they are capable of charging the P3HT from top and bottom. Details on conventional bottom-gating and electrolyte-side-gating are described in the SI. The P3HT fibers were printed perpendicular and in parallel to the current direction (Figure 5b,c). Ohmic contacts are indicated by the output characteristics (Figure S12d,e). The averaged transfer (Figure 5d) and the output characteristics reveal higher currents for the transistors with the fibers compared to the printed and spin coated isotropic reference OFETs. This is confirmed by the linear mobility, where the fiber transistors exceed the references (Table S1) by fourfold on average and across a large gate voltage range (Figure 5e,f). Transfer characteristics and mobilities in the saturation regime corroborate this interpretation (Figure S12a−c, Table S1), as well as bottom-gated OFETs (Figures S13 and S14). In fact, the mobility is underestimated for the fibers, as the geometric channel width was used while the true W is much smaller due to gaps between the fibers and the bundles. Both reference films have a randomchain edge-on morphology, which is expected from spin coating a P3HT polymer with a high regioregularity (>98% head-totail),48 as the GIWAXS data shows for the printed isotropic film. The differences in mobility between the two references can be assigned to oxygen- and water-induced traps, introduced when printing in ambient atmosphere but not when spin coating P3HT in inert atmosphere. At the same time, oxygen and water lead to high off-currents in the printed isotropic film by unintentional doping (Figure S12a). This has to be counteracted by the cations at positive Vg, which is much more effective for the fibers than the film as a result of their large surface area. Along with the enhanced on-currents, the fiber bEG-OFETs display a significantly higher on−off ratio

Figure 3. Experimental relative transmission spectra of a printed (a) P3HT fiber film deposited on a silicon substrate and (b) a single P3HT fiber on silicon measured near normal incidence (7°) using ppolarized light. IR spectra for parallel (black) and perpendicular (orange) orientation of the polymer fibers with respect to the polarization of the light were measured in both cases. For comparison, the relative differences (parallel divided by perpendicular) are shown in green. Compared to literature values, the relatively low peak position of ∼1510 cm−1 reveals a high conjugation length along the polymer backbone which is beneficial for charge transport.

Figure S10, to gain information about the macroscopic structural anisotropy of the polymer morphology (Figure 4c,e). The GIWAXS pattern recorded parallel to the fiber long axis exhibits an isotropic intensity distribution of the interlayer and π-stacking reflections (Figure 4c), typically characteristic for a random orientation of domains, with respect to the substrate. It should be noted that the appearance of up to a third order reflection indicates, still, a high degree of crystallinity. The crystal lattice parameters, in comparison to the pristine CB deposited film, remain unchanged. Interestingly, the reflection distribution significantly changes for the measurement perpendicular to the fiber structure. In this case, the interlayer (h00) reflections are solely located in the out-ofplane region of the pattern, while the π−π stacking peak is found on both in-plane and out-of-plane locations of the pattern. See in- and out-of-plane line-cuts presented in Figure S11. Taking the parallel and perpendicular patterns into account, a model for the polymer organization can be derived where the polymer chains are oriented along the fiber direction, which is in agreement with TCB grown fibers reported in the literature.49 In our case, the oriented chains crystallize radially around the TCB crystals, which are later evaporated resulting in hollow rod-like voids and the formation of a tube structure as illustrated in Figure 4d,f. Such an arrangement is congruent to the film morphology observed in the SEM images (Figures 1c and S7). In this organization, the π−π stacking follows the 10153


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Figure 4. GIWAXS patterns of P3HT films (a) printed from CB and (b) illustration of the edge-on polymer organization; P3HT fibers printed from the CB:TCB 1:0.22 mixture, measured (c) parallel and (e) perpendicular to the printing direction; (d) and (f) illustration of the polymer tube structure obtained from the CB:TCB 1:0.22 mixture. The reflections in the patterns are indexed by the Miller indices which are also used in the illustrations. The gray rod in the drawings indicates the hollow void after TCB removal. Stars in part f added to hkl:010 reflections indicate the possible observed π−π interactions (*) which are parallel or (**) perpendicular to the X-ray beam direction.

printed fibers to be used in device applications. In fact, the method of inkjet-printed semiconducting polymer fibers can be extended for a variety of different polymers employed in OFETs, solar cells, and OLEDs (Figure S16).

(parallel: Ion/Ioff = 4 × 102) than the printed reference (Ion/Ioff = 1 × 101, Table S1). The microscopic images (Figure 5 b,c) reveal that the arrangement of the fibers being aligned perpendicularly to the current direction is branched rather than side-by-side, due to the wetting properties of the parylene layer underneath. Consequently, some fibers bridge the transistor channel, while others bridge the tips of the interdigitated electrode fingers, leading to comparable averaged currents and averaged linear mobility in OFETs with perpendicular and parallel aligned fibers. When printed on glass, however, the fibers arrange side-by-side and were side-gated with ion gel (Figure S15a). In this case, the drain current of the parallel fibers exceeds the perpendicular case by a factor 5 (Figure S15d,e). Summing up, the presented P3HT OFET devices in various device architectures demonstrate the adaptability of the inkjet-

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CONCLUSION In this article, we presented the controlled deposition of aligned P3HT fibers by digital printing. The structures can be deposited on a variety of substrates, with arbitrary designs and placements over centimeter large areas. We have elucidated, by GIWAX and FTIR spectroscopy, the nontrivial molecular arrangement of P3HT on a radial shell structure with the long axis extending in the direction of the fibers. The anisotropic structure of the fibers does not only trigger anisotropic optical properties but also improves OFET charge carrier mobility which exceeds that of randomly ordered films. 10154


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Figure 5. (a) Schematic layout of the electrolyte-bottom-gated OFETs with printed P3HT fibers. (b, c) Microscopic images of active layer being printed from CB:TCB 1:0.22 either (b) perpedicular or (c) parallel to the current direction. Printed and spin coated P3HT from CB serve as isotropic references without fibers. (d) Averaged transfer characteristics of the bEG-OFETs (Vd = −0.05 V) normalized to the ratio of channel width W = 470 μm to channel length L = 50 or 20 μm. (e) Averaged linear mobilities of the corresponding transistors (from 10 devices with L = 20 μm or L = 50 μm at Vd = −0.05 V using C = 7 μF/cm2), as dependent on the gate voltage Vg. (f) Linear mobility, averaged from Vg = −2 V to −1 V. perpendicular to the current direction (i.e., channel length) by aligning the substrate with respect to the printing direction. The thickness of the printed layer without fibers (CB:TCB 1:0) ranged around 272 nm. The spin coated reference film without fibers was produced from the same solution (CB:TCB 1:0; 20 g·L−1) by spinning at 6000 rpm for 120 s, giving a 57 nm thickness. All P3HT layers were annealed for 12 h at 100 °C in dry nitrogen atmosphere. An iongel solution of poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP, Mw = 400 kg mol−1, Sigma-Aldrich), 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI], Merck), and acetone (anhydrous, Sigma-Aldrich) in the mass ratio of 1:4:14 was drop casted on top of the P3HT covered channel area and annealed at 100 °C for 12 h in a nitrogen filled glovebox. The bottom-contact bottomgate transistors (BG-OFETs) were fabricated in the same way, without drop-casting the ion gel on top. Bottom-contact electrolyte-side-gated transistors (sEG-OFETs) were prepared by patterning source/drain electrodes (2 nm Cr, 30 nm Au) on glass substrates by photolithography and lift-off, channel length L = 20 μm and W = 4000 μm. The P3HT ink was printed on top as described above. An ion gel solution with 1-ethyl-3methylimidazolium tris(pentafluoroethyl)trifluorophosphate ([EMIM][FAP], Merck) was prepared with PVDF-HFP and acetone as described above, spin coated at 2000 rpm for 30 s, and annealed at 90 °C for 12 h in a nitrogen filled glovebox. Current−voltage characteristics were recorded with an Agilent 4156C Semiconductor Parameter Analyzer. The parylene gate dielectric capacitance was measured with a Solartron ModuLab XM MTS S impedance analyzer at 500 Hz from −8 to 8 V on five capacitors per sample. The mobilities were calculated only from transistors (10 to 19 samples) that were macroscopically fully covered by bundles, using the geometric channel width W. The free volume between the fibers will reduce the true channel width; hence, we underestimated the mobilities of the fiber OFETs with respect to the isotropic reference. Infrared Spectroscopy. IR spectra of single P3HT fibers were measured utilizing an IR microscope (Bruker Hyperion 1000) coupled to a Fourier-transform IR spectrometer (Bruker Tensor 27). Transmission measurements were carried out using a liquid nitrogen cooled mercury cadmium telluride (MCT) detector and a spectral resolution of 4 cm−1. The linearly polarized IR beam was focused onto the top of the silicon substrate with a spot size of 40 μm. For each spectrum 2000 scans were averaged and afterward referenced to a spectrum of the bare substrate. The polarizer was rotated by 90° to achieve parallel and perpendicular orientation of the P3HT fibers relative to the polarization of the incoming IR beam. To avoid atmospheric influences, such as water and CO2 absorptions, the whole

The presented method is based on a validated industrial printing technique and is therefore promising for the digital deposition of highly ordered domains of functional polymers. The approach can be advantageous for the fabrication of optoelectronic systems ranging from OFETs and OLEDs to sensing devices where molecular orientation or functionalization of their hollow core plays a decisive role in their functionality.

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METHODS AND EXPERIMENTAL DETAILS

Inkjet Printing. Glass slides (Schott BOROFLOAT 33 borosilicate glass), indium tin oxide covered glass slides (180 nm, 10 Ω/□, Kintec Company), and silicon wafer chips (Dummy CZ-Si, polished, Microchemicals and for the IR measurements 1 mm, FZ, intrinsic >5000 Ω cm, both sides polished, native oxide, Siltronix) were used as substrates for all processes and measurements. The substrates were cleaned with acetone and isopropanol subsequently in an ultrasonication bath before an oxygen plasma treatment of 5 min (Tetra 30, Diener electronics GmbH + Co. KG). Chlorobenzene and 1,3,5trichlorobenzene were mixed in the CB:TCB mass ratios of 1:0.22, 1:0.11, 1:0.07, and 1:0 to prepare the corresponding P3HT solutions (average Mn = 54 000−75 000, >98% head-to-tail regioregular, SigmaAldrich) with a concentration of 20 g L−1 in a N2 glovebox and annealed overnight at 80 °C. All layers were printed with a 20 μm drop spacing and two nozzles in ambient conditions with a Dimatix DMP2800 and Fujifilm Dimatix 10 pL cartridge at maximum jetting frequency of 5 kHz with a print head temperature of 47 °C and a custom-designed waveform. The printing plate temperature was kept at 25 °C. The experiments were executed in cleanroom conditions (20 °C; 50% RH). The printed layers were transferred to a vacuum oven and dried at 15 mbar at room temperature for 10 min. Optical Characterization. Microscope images were captured with a Nikon Eclipse 80i. Photography images were captured with a Canon 600D and a linear polarization filter (heliopan ES 67). UV−vis absorption spectra were recorded using an AvaLight-DHS-Bal light source and an AvaSpec-ULS3648 spectrometer. For polarized absorption measurements a linear polarization filter (heliopan ES 67) with varying orientations was used. OFET Fabrication. The bottom-contact electrolyte-bottom-gate (bEG-OFET) transistors were fabricated on glass substrates by thermally evaporating a 70 nm silver gate electrode through a shadow-mask, depositing 500 nm of parylene C dielectric, and evaporating shadow-mask patterned gold source−drain electrodes (45 nm) with channel lengths L = 20 or 50 μm and W = 470 μm. The P3HT inks were inkjet printed as described above, parallel and 10155


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Chemistry of Materials beam path was purged with dry air for at least 60 min before each measurement. IR spectra of macroscopic films of P3HT fibers were measured utilizing a Fourier-transform IR spectrometer Bruker Vertex80v. Transmission measurements were carried out using a MCT detector, a spectral resolution of 4 cm−1, and a spot size of 2 mm. For each spectrum 200 scans were averaged and afterward referenced to a spectrum of the bare substrate. The sample was rotated by 90° with respect to the fiber axes to achieve parallel and perpendicular orientation of the P3HT fibers relative to the polarization of the incoming IR beam. To avoid atmospheric influences, such as water and CO2 absorptions, the whole beam path was evacuated to 2 mbar for at least 30 min before each measurement. For the single fiber measurements, a printed layer of P3HT fibers was ultrasonically treated in acetonitrile for 5 min to obtain a fiber dispersion which was subsequently drop cast on a silicon substrate. Grazing-Incidence Wide-Angle X-ray Scattering (GIWAXS). To investigate the molecular ordering, GIWAXS measurements were performed at the DELTA Synchrotron using beamline BL09 with a photon energy of 10 keV (λ = 1.239 Å). The beam size was 1.0 mm × 0.2 mm (width × height), and samples were irradiated just below the critical angle for total reflection with respect to the incoming X-ray beam (∼0.1°). The scattering intensity was detected on a 2-D image plate (MAR-345) with a pixel size of 150 μm (2300 × 2300 pixels), and the detector was placed 381 mm from the sample center. The raw detector image needs to be converted into reciprocal-space. This was done by using a calibration standard (silver behenate), which has rings at known 2Θ positions. Scattering data are expressed as a function of the scattering vector: q = 4π/λ sin(Θ), where Θ is a half the scattering angle and λ = 1.239 Å is the wavelength of the incident radiation. Here qxy (qz) is a component of the scattering vector in-plane (out-of-plane) to the sample surface. All X-ray scattering measurements were performed under vacuum (∼1 mbar) to reduce air scattering and beam damage to the sample. All GIWAXS data processing and analysis was performed by using the software package Datasqueeze (http://www. datasqueezesoftware.com). Scanning Electron Microscopy. Images with cross sections prepared by FIB milling with Ga+ ions were recorded in a Crossbeam Workstation AURIGA by Carl Zeiss Microscopy Oberkochen. Atomic Force Microscopy. Images were recorded with a Bruker Dimension Icon Atomic Force Microscope in tapping mode.

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Gerardo Hernandez-Sosa: 0000-0002-2871-6401 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are thankful to Paul Heimel, Lars Müller, and Guillaume Gomard for useful discussion and help with the experiments. This work was financially supported by the German Federal Ministry of Education and Research (BMBF) through Grant FKZ:13N13691 and Grant FKZ: 13N13657. Wojciech Pisula acknowledges National Science Centre, Poland, through Grant UMO-2015/18/E/ST3/00322 and the beamline 9 of the DELTA electron storage ring in Dortmund for providing synchrotron radiation and technical support for the GIWAXS measurements.

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(1) Brinkmann, M.; Hartmann, L.; Biniek, L.; Tremel, K.; Kayunkid, N. Orienting Semi-Conducting Pi-Conjugated Polymers. Macromol. Rapid Commun. 2014, 35, 9−26. (2) Lim, J. A.; Liu, F.; Ferdous, S.; Muthukumar, M.; Briseno, A. L. Polymer Semiconductor Crystals. Mater. Today 2010, 13, 14−24. (3) Peng, B.; Huang, S.; Zhou, Z.; Chan, P. K. L. Solution-Processed Monolayer Organic Crystals for High-Performance Field-Effect Transistors and Ultrasensitive Gas Sensors. Adv. Funct. Mater. 2017, 27, 1700999. (4) Ren, X.; Schmidt, E.; Walter, J.; Ganguly, K.; Leighton, C.; Frisbie, C. D. Rubrene Single-Crystal Transistors with Perfluoropolyether Liquid Dielectric: Exploiting Free Dipoles to Induce Charge Carriers at Organic Surfaces. J. Phys. Chem. C 2017, 121, 6540−6545. (5) Salleo, A. Charge Transport in Polymeric Transistors. Mater. Today 2007, 10, 38−45. (6) Liu, S.; Wang, W. M.; Briseno, A. L.; Mannsfeld, S. C. B.; Bao, Z. Controlled Deposition of Crystalline Organic Semiconductors for Field-Effect-Transistor Applications. Adv. Mater. 2009, 21, 1217− 1232. (7) Persson, N. E.; Chu, P. H.; McBride, M.; Grover, M.; Reichmanis, E. Nucleation, Growth, and Alignment of Poly(3-Hexylthiophene) Nanofibers for High-Performance OFETs. Acc. Chem. Res. 2017, 50, 932−942. (8) Zaumseil, J.; Groves, C.; Winfield, J. M.; Greenham, N. C.; Sirringhaus, H. Electron-Hole Recombination in Uniaxially Aligned Semiconducting Polymers. Adv. Funct. Mater. 2008, 18, 3630−3637. (9) Kim, D. H.; Han, J. T.; Park, Y. D.; Jang, Y.; Cho, J. H.; Hwang, M.; Cho, K. Single-Crystal Polythiophene Microwires Grown by SelfAssembly. Adv. Mater. 2006, 18, 719−723. (10) Flämmich, M.; Frischeisen, J.; Setz, D. S.; Michaelis, D.; Krummacher, B. C.; Schmidt, T. D.; Brütting, W.; Danz, N. Oriented Phosphorescent Emitters Boost OLED Efficiency. Org. Electron. 2011, 12, 1663−1668. (11) Jurow, M. J.; Mayr, C.; Schmidt, T. D.; Lampe, T.; Djurovich, P. I.; Brütting, W.; Thompson, M. E. Understanding and Predicting the Orientation of Heteroleptic Phosphors in Organic Light-Emitting Materials. Nat. Mater. 2015, 15, 85−91. (12) Nanot, S.; Cummings, A. W.; Pint, C. L.; Ikeuchi, A.; Akiho, T.; Sueoka, K.; Hauge, R. H.; Léonard, F.; Kono, J. Broadband, Polarization-Sensitive Photodetector Based on Optically-Thick Films of Macroscopically Long, Dense, and Aligned Carbon Nanotubes. Sci. Rep. 2013, 3, 1335. (13) Freitag, M.; Martin, Y.; Misewich, J.; Martel, R.; Avouris, P. Photoconductivity of Single Carbon Nanotubes. Nano Lett. 2003, 3, 1067−1071.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b03948. Additional micrograph images and UV/vis absorption measurements of printed P3HT films with different ratios of CB:TCB, SEM images of P3HT fibers on ITO coated glass, AFM images, IR measurements, GIWAXS line-cuts, detailed analysis of bEG OFETS, bottom-gate OFETs and sEG-OFETs, and micrograph images of printed fibers from different semiconducting polymers (PDF) Video with inkjet printed P3HT fibers and a rotating polarizer in front of the camera (AVI)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*(G.H.-S.) E-mail: [email protected]. ORCID

Robert Lovrincic: 0000-0001-5429-5586 Annemarie Pucci: 0000-0002-9038-4110 Jana Zaumseil: 0000-0002-2048-217X Wojciech Pisula: 0000-0002-5853-1889 10156


Article

Chemistry of Materials (14) Smith, J.; Zhang, W.; Sougrat, R.; Zhao, K.; Li, R.; Cha, D.; Amassian, A.; Heeney, M.; McCulloch, I.; Anthopoulos, T. D. Solution-Processed Small Molecule-Polymer Blend Organic ThinFilm Transistors with Hole Mobility Greater than 5 Cm 2/Vs. Adv. Mater. 2012, 24, 2441−2446. (15) Schmidt-Hansberg, B.; Sanyal, M.; Grossiord, N.; Galagan, Y.; Baunach, M.; Klein, M. F. G.; Colsmann, A.; Scharfer, P.; Lemmer, U.; Dosch, H.; Michels, J.; Barrena, E.; Schabel, W. Investigation of NonHalogenated Solvent Mixtures for High Throughput Fabrication of Polymerfullerene Solar Cells. Sol. Energy Mater. Sol. Cells 2012, 96, 195−201. (16) Jimison, L. H.; Toney, M. F.; McCulloch, I.; Heeney, M.; Salleo, A. Charge-Transport Anisotropy Due to Grain Boundaries in Directionally Crystallized Thin Films of Regioregular Poly(3Hexylthiophene). Adv. Mater. 2009, 21, 1568−1572. (17) Kline, R. J.; McGehee, M. D.; Kadnikova, E. N.; Liu, J.; Fréchet, J. M. J.; Toney, M. F. Dependence of Regioregular poly(3Hexylthiophene) Film Morphology and Field-Effect Mobility on Molecular Weight. Macromolecules 2005, 38, 3312−3319. (18) Yuan, Y.; Giri, G.; Ayzner, A. L.; Zoombelt, A. P.; Mannsfeld, S. C. B.; Chen, J.; Nordlund, D.; Toney, M. F.; Huang, J.; Bao, Z. UltraHigh Mobility Transparent Organic Thin Film Transistors Grown by an off-Centre Spin-Coating Method. Nat. Commun. 2014, 5, 3005. (19) Li, X.; Wolanin, P. J.; MacFarlane, L. R.; Harniman, R. L.; Qian, J.; Gould, O. E. C.; Dane, T. G.; Rudin, J.; Cryan, M. J.; Schmaltz, T.; Frauenrath, H.; Winnik, M. A.; Faul, C. F. J.; Manners, I. Uniform Electroactive Fibre-like Micelle Nanowires for Organic Electronics. Nat. Commun. 2017, 8, 15909. (20) Bucella, S. G.; Luzio, A.; Gann, E.; Thomsen, L.; McNeill, C. R.; Pace, G.; Perinot, A.; Chen, Z.; Facchetti, A.; Caironi, M. Macroscopic and High-Throughput Printing of Aligned Nanostructured Polymer Semiconductors for MHz Large-Area Electronics. Nat. Commun. 2015, 6, 8394. (21) 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 Large-Area Organic Semiconductor Thin Films with Aligned Single-Crystalline Domains. Nat. Mater. 2013, 12, 665− 671. (22) Nagamatsu, S.; Takashima, W.; Kaneto, K.; Yoshida, Y.; Tanigaki, N.; Yase, K.; Omote, K. Backbone Arrangement In “frictionTransferred” regioregular poly(3-Alkylthiophene)s. Macromolecules 2003, 36, 5252−5257. (23) Hartmann, L.; Tremel, K.; Uttiya, S.; Crossland, E.; Ludwigs, S.; Kayunkid, N.; Vergnat, C.; Brinkmann, M. 2D versus 3D Crystalline Order in Thin Films of Regioregular poly(3-Hexylthiophene) Oriented by Mechanical Rubbing and Epitaxy. Adv. Funct. Mater. 2011, 21, 4047−4057. (24) Lin, F.; Guo, C.; Chuang, W.; Wang, C.; Wang, Q.; Liu, H.; Hsu, C.-S.; Jiang, L. Directional Solution Coating by the Chinese Brush: A Facile Approach to Improving Molecular Alignment for High-Performance Polymer TFTs. Adv. Mater. 2017, 29, 1606987. (25) Wittmann, J. C.; Smith, P. Highly Oriented Thin Films of Poly(tetrafluoroethylene) as a Substrate for Oriented Growth of Materials. Nature 1991, 352, 414−417. (26) Yang, Y.; Lee, K.; Mielczarek, K.; Hu, W.; Zakhidov, A. Nanoimprint of Dehydrated PEDOT:PSS for Organic Photovoltaics. Nanotechnology 2011, 22, 485301. (27) Chen, D.; Zhao, W.; Russell, T. P. P3HT Nanopillars for Organic Photovoltaic Devices Nanoimprinted by AAO Templates. ACS Nano 2012, 6, 1479−1485. (28) Wang, G.; Chu, P. H.; Fu, B.; He, Z.; Kleinhenz, N.; Yuan, Z.; Mao, Y.; Wang, H.; Reichmanis, E. Conjugated Polymer Alignment: Synergisms Derived from Microfluidic Shear Design and UV Irradiation. ACS Appl. Mater. Interfaces 2016, 8, 24761−24772. (29) Wang, G.; Persson, N.; Chu, P. H.; Kleinhenz, N.; Fu, B.; Chang, M.; Deb, N.; Mao, Y.; Wang, H.; Grover, M. A.; Reichmanis, E. Microfluidic Crystal Engineering of π-Conjugated Polymers. ACS Nano 2015, 9, 8220−8230.

(30) Wu, K.; Li, H.; Li, L.; Zhang, S.; Chen, X.; Xu, Z.; Zhang, X.; Hu, W.; Chi, L.; Gao, X.; Meng, Y. Controlled Growth of Ultrathin Film of Organic Semiconductors by Balancing the Competitive Processes in Dip-Coating for Organic Transistors. Langmuir 2016, 32, 6246−6254. (31) Brinkmann, M.; Wittmann, J. C. Orientation of Regioregular poly(3-Hexylthiophene) by Directional Solidification: A Simple Method to Reveal the Semicrystalline Structure of a Conjugated Polymer. Adv. Mater. 2006, 18, 860−863. (32) Müller, C.; Aghamohammadi, M.; Himmelberger, S.; Sonar, P.; Garriga, M.; Salleo, A.; Campoy-quiles, M. One-Step Macroscopic Alignment of Conjugated Polymer Systems by Epitaxial Crystallization during Spin-Coating. Adv. Funct. Mater. 2013, 23, 2368−2377. (33) Wittmann, J. C.; Lotz, B. Epitaxial Crystallization of Polyethylene on Organic Substrates: A Reappraisal of the Mode of Action of Selected Nucleating Agents. J. Polym. Sci., Polym. Phys. Ed. 1981, 19, 1837−1851. (34) Wittmann, J. C.; Hodge, A. M.; Lotz, B. Epitaxial Crystallization of Polymers Onto Benzoic Acid: Polyethylene and Paraffins, Aliphatic Polyesters, and Polyamides. J. Polym. Sci., Polym. Phys. Ed. 1983, 21, 2495−2509. (35) Wittmann, J. C.; Lotz, B. Epitaxial Crystallization of Aliphatic Polyesters on Trioxane and Various Aromatic Hydrocarbons. J. Polym. Sci., Polym. Phys. Ed. 1981, 19, 1853−1864. (36) Brinkmann, M. Directional Epitaxial Crystallization and Tentative Crystal Structure of poly(9,9′-Di-N-Octyl-2,7-Fluorene). Macromolecules 2007, 40, 7532−7541. (37) Brinkmann, M.; Chandezon, F.; Pansu, R. B.; Julien-Rabant, C. Epitaxial Growth of Highly Oriented Fibers of Semiconducting Polymers with a Shish-Kebab-like Superstructure. Adv. Funct. Mater. 2009, 19, 2759−2766. (38) Dörling, 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. (39) Minemawari, H.; Yamada, T.; Matsui, H.; Tsutsumi, J.; Haas, S.; Chiba, R.; Kumai, R.; Hasegawa, T. Inkjet Printing of Single-Crystal Films. Nature 2011, 475, 364−367. (40) Kang, B.; Lee, W. H.; Cho, K. Recent Advances in Organic Transistor Printing Processes. ACS Appl. Mater. Interfaces 2013, 5, 2302−2315. (41) Conti, S.; Lai, S.; Cosseddu, P.; Bonfiglio, A. An Inkjet-Printed, Ultralow Voltage, Flexible Organic Field Effect Transistor. Adv. Mater. Technol. 2017, 2, 1600212. (42) Yan, H.; Chen, Z.; Zheng, Y.; Newman, C.; Quinn, J. R.; Dötz, F.; Kastler, M.; Facchetti, A. A High-Mobility Electron-Transporting Polymer for Printed Transistors. Nature 2009, 457, 679−686. (43) Morfa, A. J.; Barnes, T. M.; Ferguson, A. J.; Levi, D. H.; Rumbles, G.; Rowlen, K. L.; Van De Lagemaat, J. Optical Characterization of Pristine poly(3-Hexyl Thiophene) Films. J. Polym. Sci., Part B: Polym. Phys. 2011, 49, 186−194. (44) Spano, F. C. Modeling Disorder in Polymer Aggregates: The Optical Spectroscopy of Regioregular poly(3-Hexylthiophene) Thin Films. J. Chem. Phys. 2005, 122, 234701. (45) Hotta, S.; Rughooputh, S. D. D. V.; Heeger, A. J.; Wudl, F. Spectroscopic Studies of Soluble poly(3-Alkylthienylenes). Macromolecules 1987, 20, 212−215. (46) Delongchamp, D. M.; Kline, R. J.; Fischer, D. A.; Richter, L. J.; Toney, M. F. Molecular Characterization of Organic Electronic Films. Adv. Mater. 2011, 23, 319−337. (47) Hernandez, V.; Casado, J.; Ramirez, F. J.; Zotti, G.; Hotta, S.; Navarrete, J. T. L. Efficient Pi Electrons Delocalization in Alpha,alpha’dimethyl End-Capped Oligothiophenes: A Vibrational Spectroscopic Study. J. Chem. Phys. 1996, 104, 9271−9282. (48) Sirringhaus, H.; Brown, P. J.; Friend, R. H.; Nielsen, M. M.; Bechgaard, K.; Langeveld-Voss, B. M. W.; Spiering, A. J. H.; Janssen, R. A. J.; Meijer, E. W.; Herwig, P.; de Leeuw, D. M. Two-Dimensional Charge Transport in Self-Organized, High-Mobility Conjugated Polymers. Nature 1999, 401, 685−688. 10157


Article

Chemistry of Materials (49) Qu, S.; Yao, Q.; Wang, L.; Chen, Z.; Xu, K.; Zeng, H.; Shi, W.; Zhang, T.; Uher, C.; Chen, L. Highly Anisotropic P3HT Films with Enhanced Thermoelectric Performance via Organic Small Molecule Epitaxy. NPG Asia Mater. 2016, 8, e292. (50) Kettner, M.; Vladimirov, I.; Strudwick, A. J.; Schwab, M. G.; Weitz, R. T. Ionic Gel as Gate Dielectric for the Easy Characterization of Graphene and Polymer Field-Effect Transistors and Electrochemical Resistance Modification of Graphene. J. Appl. Phys. 2015, 118, 025501. (51) Kim, S. H.; Hong, K.; Xie, W.; Lee, K. H.; Zhang, S.; Lodge, T. P.; Frisbie, C. D. Electrolyte-Gated Transistors for Organic and Printed Electronics. Adv. Mater. 2013, 25, 1822−1846.

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Controlled Molecular Orientation of Inkjet Printed Semiconducting

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