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Solvent-type-dependent polymorphism and charge transport in a long fused-ring organic semiconductor† Jihua Chen,*a Ming Shao,a Kai Xiao,a Adam J. Rondinone,a Yueh-Lin Loo,b Paul R. C. Kent,af Bobby G. Sumpter,af Dawen Li,c Jong K. Keum,a Peter J. Diemer,d John E. Anthony,e Oana D. Jurchescu*d and Jingsong Huangaf Crystalline polymorphism of organic semiconductors is among the critical factors in determining the structure and properties of the resultant organic electronic devices. Herein we report for the first time a solvent-type-dependent polymorphism of a long fused-ring organic semiconductor and its crucial effects on charge transport. A new polymorph of 5,11-bis(triethylsilylethynyl)anthradithiophene (TES ADT) is obtained using solvent-assisted crystallization, and the crystalline polymorphism of TES ADT thin films is correlated with their measured hole mobilities. The best-performing organic thin film transistors of the two TES ADT polymorphs show subthreshold slopes close to 1 V dec
1, and threshold voltages close to zero, indicating that the density of traps at the semiconductor–dielectric interface is negligible in these devices and the observed up to 10-fold differences in hole mobilities of devices fabricated with different

Received 15th August 2013 Accepted 18th October 2013

solvents are largely resultant from the presence of two TES ADT polymorphs. Moreover, our results suggest that the best-performing TES ADT devices reported in the literature correspond to the new

DOI: 10.1039/c3nr04341j

polymorph identified in this study, which involves crystallization from a weakly polar solvent (such as

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toluene and chloroform).

Introduction Crystalline polymorphism is relevant to the physical properties of organic solids including pharmaceutical crystals, energetic materials, dyes, and organic semiconductors.1 This phenomenon is widely encountered in organic solids, mostly because their intermolecular interactions are weak, of van der Waals type. For organic semiconductors, polymorphism was shown to strongly affect long-range order,2 electron–phonon coupling,3,4 as well as band structures and charge transport.5 Previously reported polymorphic organic semiconductors include oligoacenes,6 oligothiophenes,7–9 tetracyanoquinodimethane (TCNQ),10 functionalized pentacene,11,12 dibenzo- or dithiophene-tetrathiafulvalene (DB-, or DT-TTF),4,13 and a

Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA. E-mail: [email protected]; [email protected]

b

Department of Chemical and Biological Engineering, Princeton University, Princeton, NJ 08544, USA

c Department of Electrical and Computer Engineering, Center for Materials for Information Technology, University of Alabama, Tuscaloosa, AL 35487, USA d

Department of Physics, Wake Forest University, Winston-Salem, NC 27109, USA

e

Department of Chemistry, University of Kentucky, Lexington, KY 40506, USA

f

Computer Science & Mathematics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA † Electronic supplementary 10.1039/c3nr04341j

information

(ESI)

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available.

See

DOI:

uorinated anthradithiophene.14 So far the typical approaches used to manipulate polymorphs of organic semiconductors include heating,11,14,15 substrate connement (thin lm vs. bulk),16,17 substrate modication,18 deposition temperature control,7,19 pressure,20 and solution shear.12 Solvent-dependent polymorphism has been previously reported in single crystals of DB-TTF and DT-TTF with short (2) fused rings due to their backbone (p–p) and S/S interactions.4,13 In addition, single crystals of non-fused ring didodecyl-quarterthiophenes exhibit polymorphs induced by the conformational changes of their pedant side groups during crystallization from good or poor solvents.9 To the best of our knowledge, solvent-dependent polymorphism of a linear, long fused ring organic semiconductor (n > 4) has not been reported, although conjugated molecules with 4 or more fused rings represent an extremely important class of high performance organic semiconductors. 5,11-Bis(triethylsilylethynyl)anthradithiophene (TES ADT) is a solution-processable, high-performance small molecule organic semiconductor with ve fused rings, showing hole mobilities as high as 1 cm2 V
1 s
1.21 However, the reproducibility of devices fabricated with this material spans three to four orders of magnitude in transistors fabricated by various methods, such as spin-coating coupled with solvent vapor annealing22–24 or aging,25 drop casting,26–28 and blending with polymers to form vertically phase separated semiconductor– dielectric structures.29 Table 1 lists some representative mobility

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Table 1 Previously reported hole mobility values of TES ADT-based OTFTs along with the corresponding solution process conditions, solvent choice, and dipole moments31 of the chosen solvents

Mobility (cm2 V
1 s
1)

Solvent (dipole moment)

Process conditions

Reference

1.0 0.11 0.05 0.02 0.01 0.002 0.43 0.42 0.38–0.40

Toluene (0.375 D) 1,2-Dichloroethane (1.48 D) Toluene (0.375 D) THF (1.75 D) Acetone (2.88 D) Hexane (0 D) Chloroform (1.04 D) Chloroform (1.04 D) Toluene (0.375 D)

Solution cast with a blade (1–2 wt%) Solvent vapor annealing Solvent vapor annealing Solvent vapor annealing Solvent vapor annealing Solvent vapor annealing Solvent vapor annealing Solution cast at 5  C (4 wt%) Drop cast on Mylar (8 wt%)

Payne et al., 2005 (ref. 21) Dickey et al., 2006 (ref. 24) Dickey et al., 2006 (ref. 24) Dickey et al., 2006 (ref. 24) Dickey et al., 2006 (ref. 24) Dickey et al., 2006 (ref. 24) Lee et al., 2007 (ref. 23) Yu et al., 2011 (ref. 27) Yi et al., 2012 (ref. 28)

values reported in the literature for TES ADT based organic thin lm transistors (OTFTs), along with their solvent choices and process conditions. As we demonstrate in this work, a solventtype-dependent polymorphism can contribute signicantly to this large spread in TES ADT device performance. The reported bulk TES ADT crystal has a triclinic unit cell with parameters of ˚ b ¼ 7.4163 A, ˚ c ¼ 16.7167 A, ˚ a ¼ 96.4022 , b ¼ a ¼ 6.9107 A,   ˚ 3).21,30 92.0203 , and g ¼ 106.0026 (cell volume V ¼ 816.47 A Herein we show that this is not the only crystalline packing that TES ADT can display when processed at room temperature, and that the observed polymorphism critically affects device performance, changing the maximum mobility by up to 10 times under the same testing conditions. Moreover, our results suggest that the highest-performing TES ADT devices reported in the literature21,23,27,28 correspond to the new polymorph identied in this study (instead of the well-known triclinic-type bulk unit cell), which involves slow solution crystallization processes using a weakly polar solvent (such as toluene and chloroform). In this study, we demonstrate solvent-dependent polymorphism of TES ADT (and long fused-ring organic semiconductor), by using solvent-assisted crystallization (SAC), which was proven to be a simple yet powerful method to fabricate high-quality crystalline thin lms of functionalized heteroacenes including uorinated TES ADT.32

Experimental section Materials TES ADT is synthesized based on a previously published procedure.21 Anhydrous toluene, chloroform, and THF are purchased from Sigma-Aldrich and EMD Millipore. Film formation TES ADT powder is dissolved in anhydrous solvent at a total solid concentration of 0.2 wt%. Drop casting of TES ADT solution is performed in a solvent–vapor-rich glass Petri dish with cover under ambient conditions. TEM, electron diffraction and simulation Electron diffraction and bright-eld TEM are conducted with a Zeiss Libra 120 at 120 kV with an in-column energy lter. Electron diffraction experiments are conducted with an

450 | Nanoscale, 2014, 6, 449–456

emission current as small as 5  10
6 A and a 1-microndiameter selected area aperture. The Al (111) (0.234 nm) ring is used to calibrate all electron diffraction patterns. WebEMAPS (http://emaps.mrl.uiuc.edu) and Mercury2.3 (www.ccdc.cam.ac.uk/mercury/) are used to simulate electron diffraction patterns and unit cell views. Organic thin lm transistors The substrates used for OTFT fabrication consist of highly doped Si gate electrodes with thermally grown SiO2 gate dielectrics and Ti/Au source and drain contacts dened by photolithography and deposited by e-beam evaporation. These substrates are sequentially cleaned in heated baths of acetone and then isopropyl alcohol, followed by exposure to UV/ozone, each for 10 minutes. The substrates were treated with phenyltrichlorosilane (PTS) and/or a pentauorobenzenethiol (PFBT). The details of the phenyltrichlorosilane (PTS) treatment on wafer substrates are available elsewhere.12 For PFBT treatment, they are placed in individual solutions of 30 mM PFBT in ethanol and le undisturbed for 30 minutes.33 They are removed from solution, rinsed in ethanol, and further sonicated in ethanol for 5 minutes and dried with nitrogen. The substrate is placed in a Petri dish and a TES ADT solution (0.2% in room-temperature toluene or THF) is deposited by drop casting. Extra solvent is syringed around the substrate and a cover is placed over the Petri dish to keep the solution and substrate in a solvent-rich environment while the lm crystallizes.32 The devices are measured using an Agilent 4155C Semiconductor Parameter Analyzer connected to a probe station. The transistor active layers are also used in GIXRD experiments (Philips X’Pert). UV-Vis absorption and simulation Experimental UV-Vis spectra are obtained on TES ADT thin lms crystallized on a quartz substrate under conditions consistent with the ones fabricated as OTFT active layers. The theoretical UV-Vis spectrum is calculated for a one-molecularlayer lm constructed based on the bulk unit cell structure at the level of time-dependent density funcitonal theory (TDDFT) with the adiabatic local density approximation (ALDA) approximation using the Yambo code. (Details and references are available in the ESI.†) Sixty valence bands and sixty conduction

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(a) GIXRD of TES ADT thin films slowly crystallized from chloroform, THF and toluene solutions by the SAC approach in a solventrich environment. The films from chloroform and toluene share the same (001) d-spacing of 1.630 nm, which is slightly smaller than that of the TES ADT films from THF solution (1.637 nm). (b) A unit cell view of TES ADT films on the substrate based on its bulk unit cell. The discussion on TES ADT films from chloroform solution is given later in the text after the detailed comparison between the films from THF and toluene solutions.

Fig. 2

Fig. 1 Molecular structure (a) and a unit cell view of TES ADT down the “b” axis in its bulk phase (b). Optical micrographs of TES ADT films prepared from SAC are shown in (c) and (d), with toluene and THF solvents, respectively. (e) Experimental and calculated UV-Vis spectra of TES ADT thin films. The theoretical UV-Vis spectrum is calculated for a one-molecular-layer film constructed based on the bulk unit cell structure at the level of time-dependent DFT (TDDFT) with the adiabatic local density approximation (ALDA) (details in the Experimental section and ESI†).

bands around the Fermi level were correlated in the calculation. The local eld effects were considered by setting the dimension of the response function to 300 to account for the charge oscillations induced by the external potential. The dimension of the exchange-correlation kernel was set to be equal to that of the response function size.

Results and discussion Fig. 1a and b show the molecular structure of TES ADT and a unit cell view of bulk TES ADT. Fig. 1c and d show representative optical micrographs of the resultant TES ADT lms with toluene and tetrahydrofuran (THF) as the solvents, respectively. In Fig. 1e, a calculated UV-Vis spectrum for a TES ADT thin lm constructed based on the bulk unit cell structure using TDDFT

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is compared with the experimental results. A close match between the simulation and experimental spectra is obtained. Experimental UV-Vis spectra of SAC fabricated TES ADT lms yield an optical absorption onset of 614 nm (2.02 eV) for lms from THF solution, and 643 nm (1.93 eV) for lms from toluene solution (Fig. 1e). Fig. 2a presents the grazing-incidence X-ray diffraction (GIXRD) results of TES ADT lms fabricated from THF, chloroform, and toluene solutions. TES ADT lms fabricated from both THF and toluene solutions are highly crystalline as evidenced by their strong diffraction peak intensities. Both lms have dominating (00l) type reections, which indicate that the TES ADT molecules are present on the substrate with the bulky side group touching down (Fig. 2b). Fig. 2b provides a simulated side view of TES ADT lms on the substrate down the “a” axis using their bulk unit cell. However, the (001) reection in the lm produced from the toluene solution (1.630 nm) is slightly smaller than that from THF (1.637 nm), indicating only a very small difference (