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Effect of Solution Shearing Method on Packing and Disorder of Organic Semiconductor Polymers Gaurav Giri, Dean M. DeLongchamp, Julia A. Reinspach, Daniel A Fischer, Lee J. Richter, Jie Xu, Stephanie J Benight, Alexander L. Ayzner, Mingqian He, Lei Fang, Gi Xue, Michael F. Toney, and Zhenan Bao Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm503780u • Publication Date (Web): 17 Mar 2015 Downloaded from http://pubs.acs.org on March 23, 2015
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Chemistry of Materials
Effect of Solution Shearing Method on Packing and Disorder of Organic Semiconductor Polymers Gaurav Giri1, Dean M. DeLongchamp2, Julia Reinspach1,3, Daniel A. Fischer2, Lee J. Richter2, Jie Xu4, Stephanie Benight1, Alex Ayzner3, Mingqian He5, Lei Fang1, Gi Xue4, Michael F. Toney3, Zhenan Bao1 * 1 2
Department of Chemical Engineering, Stanford University, Stanford, CA 94305 National Institute of Standards and Technology, 100 Bureau Drive, Gaithersburg, MD 20899
3
Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo Park, CA 94025
4
Department of Polymer Science and Engineering, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing, 210093, P.R. China. 5
Corning Incorporated, Corning, NY 14831
ABSTRACT: The solution shearing method has previously been used to tune the molecular packing and crystal thin film morphology of small molecular organic semiconductors (OSCs). Here, we study how the solution shearing method impacts the thin film morphology and causes structural rearrangements of two polymeric OSCs with interdigitated sidechain packing, namely P2TDC17FT4 and PBTTT-C16. The conjugated backbone tilt angle and the thin film morphology of the P2TDC17FT4 polymer were changed by the solution shearing conditions, and an accompanying change in the charge carrier mobility was observed. For PBTTT-C16, the out-of-plane lamellar spacing was increased by solution shearing, due to increased disorder of side chains. The ability to induce structural rearrangement of polymers through solution shearing allows for an easy and alternative method to modify OSC charge transport properties.
Introduction: Organic electronic devices are actively being pursued for flexible, transparent, large area and low cost applications.1-6 The basic device element of organic electronics is the thin film transistor (TFT), and current research effort is focused on improving the charge transport properties of the organic semiconductors (OSCs) in TFTs. TFTs can be fabricated over a large area by using solution deposition methods such as rollto-roll coating, spin coating, inkjet printing etc.7-17 Solution processable conjugated polymers are promising candidates for organic electronics due to their excellent mechanical properties and low temperature deposition from solution.18-20 However, the charge transport properties of polymeric OSCs, specifically the charge carrier mobility, must be improved to create high performance TFTs.21-23 Recently, several high mobility polymers have been reported, showing great promise of these materials.23-33
Charge transport in polymers is generally limited by a low structural order and poor interchain and intergrain connectivity in the thin films.34, 35 A small molecule OSC, rubrene, has shown mobility as high as 40 cm2/Vs as a vapor grown single crystal, and 2,7-dioctyl[1]benzothieno[3,2b][1]benzothiophene (C8-BTBT) have shown mobilities as high as 43 cm2/Vs, both from inkjet printed single crystals and solution coated thin films.17, 36, 37 Recently, new polymers have been reported with comparably high charge carrier mobilities.23, 33, 38, 39 For example, polymers based on diketopyrrolo-pyrrole (DPP) units have exhibited hole mobility higher than 10 cm2/Vs.22, 30 More recently, with an alignment layer, a polymer semiconductor was reported to give TFT mobility as high as 36.3 cm2/Vs.40 For polymeric OSCs, a promising design has been to incorporate monomers with fused ring aromatic structures to give large intermolecular π-π stacking overlapping areas. Additionally, to increase the electronic coupling, it is reported that the donor-acceptor copolymer design enhances the intermolecular interaction and shortens the distance between the polymer backbones with proper sidechains.23, 25 In general, the TFT´s charge carrier transport can be tuned by changing the molecular packing of the OSC. Both the π-π stacking distance (as measured by distance between the polymer backbone) and polymer backbone orientation are critical parameters that impact charge transport mobility.23 Polymers with interdigitated lamellar packing have been shown to give highly crystalline thin films.38, 39, 41, 42 In addition to molecular design, processing conditions can also have a great impact on polymer morphology, packing and disorder. Thus, it is important to gain understanding on ways to modify polymer packing structure and control disorder using solution processing methods. Using solution processing for polymer structural rearrangements would allow for expeditious roll-to-roll coating of high performance organic electronics.
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Chemistry of Materials
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We have previously demonstrated that the solution shearing method can be used to tune the molecular packing and crystalline texture of OSC small molecules.11 Figure 1a shows a conceptual image of the solution shearing process. During solution shearing, the top plate drags the OSC solution across a heated substrate, with only the evaporation front exposed.10, 11 The molecular packing of 6,13 bis[triisopropyl-silylethynyl] pentacene (TIPS-pentacene) thin film was found to change depending on the solution shearing conditions. Under certain conditions, the electronic overlap between molecules increased as a result of the shorter molecular distance and the charge carrier mobility increased by an order of magnitude.11 Currently, the use of solution processing methods to structurally rearrange polymer packing, and thus possibly impacting charge transport properties, has not been demonstrated. This is due to: (1) the disorder present in polymeric systems, and (2) the small crystalline domains.43, 44
Figure 1.(a) Schematic illustration of solution shearing. (b) Chemical structure of P2TDC17FT4. (c) Chemical structure of
PBTTT-C16. Here, we use the solution shearing method to explore the change in disorder and molecular packing of two OSC polymers, poly(2,5-bis(thiophene-2-yl)-(3,7-diheptadecanyltetrathienoacene) (P2TDC17FT4) and poly(2,5-bis(3hexadecylthiophene-2-yl)thieno[3,2-b]thiophene)(PBTTTC16).13, 39, 43, 45-49 These thienophene-based polymers are reported to have relatively high crystallinity with the alkylated side chains interdigitated. Here, we employ grazing incidence X-ray diffraction (GIXD) to show that the lamellar spacing of thin films of both P2TDC17FT4 and PBTTT-C16 changed as a function of shearing speed, indicating that solution shearing can introduce disorder into the polymer thin film. While we observed that the side chain disorder of PBTTT-C16 increased with increasing shearing speed, the π-π stacking distance and backbone tilt remained unaffected. Additionally, through a combination of near edge X-ray fine structure (NEXAFS) spectroscopy and infra-red (IR) spectroscopy, it was revealed that the polymer backbone tilt also changed as a function of shearing speed for the P2TDC17FT4 polymer. We measured the electrical behavior of the above structures in a TFT configuration and investigated the critical issue of how polymer packing structure rearrangement impacted charge transport. Experimental Section: Materials: Poly(2,5-bis(thiophene-2-yl)-(3,7-dihepta-decanyl tetrathienoacene) (P2TDC17FT4) (MW = 14000 g/mol), poly(3-hexylthiophen-2,5-diyl) (P3HT) (MW = 12000 g/mol) (purchased from BASF) and poly(2,5-bis(3-
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hexadecylthiophene-2-yl)thieno[3,2-b]thiophene)(PBTTTC16) (MW = 40000 – 80000 g/mol, purchased from Ossila Co.) were used without further purification. Phenyltrichlorosilane (PTS) and octadecyltrichlorosilane (OTS) was purchased from Sigma-Aldrich and were used as received (storage under an argon atmosphere to prevent hydrolysis). Highly doped n-type silicon wafers (resistivity
