Surface Self-Assembly, Film Morphology, and Charge Transport

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Surface Self-Assembly, Film Morphology, and Charge Transport Properties of Semiconducting Triazoloarenes David Wisman, Seyong Kim, Tobias W. Morris, Jihwan Choi, Christopher D. Tempas, Colleen Q. Trainor, Dongwhan Lee, and Steven L. Tait Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.9b00512 • Publication Date (Web): 12 Apr 2019 Downloaded from http://pubs.acs.org on April 13, 2019

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Langmuir

Surface Self-Assembly, Film Morphology, and Charge Transport Properties of Semiconducting Triazoloarenes David L. Wisman,†,§ Seyong Kim,‡ Tobias W. Morris,† Jihwan Choi,‡ Christopher D. Tempas,† Colleen Q. Trainor,†,¦ Dongwhan Lee,‡,* and Steven L. Tait†,* †Department

of Chemistry, Indiana University, Bloomington, Indiana 47405, United States Crane, Crane, Indiana 47522, United States ‡Department of Chemistry, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Korea ¦Department of Chemistry, Hillsdale College, Hillsdale, Michigan 49242, United States KEYWORDS: supramolecular packing, scanning tunneling microscopy, nitrogen-rich heterocycles, organic semiconductor thin film, four-point probe van der Pauw measurement, thin film morphology. §NAVSEA

ABSTRACT: Surface-assisted molecular self-assembly is a powerful strategy for forming molecular-scale architectures on surfaces. These molecular self-assemblies have potential applications in organic electronics, catalysis, photovoltaics, and many other technologies. Understanding the intermolecular interactions on a surface can help predict packing, stacking, and charge transport properties of films, and allow for new molecular designs to be tailored for a required function. We have previously studied a molecular platform, tris(N-phenyltriazole) (TPT), that exhibits planar stacking through > 20 molecular layers through donor–acceptor (D–A) type intermolecular π–π contacts between the electrondeficient tris(triazole) core and electron-rich peripheral phenyl units. Here, we investigate an expanded family of TPTbased molecules with variations made on the peripheral aryl groups to modulate the molecular electron distribution and examine the impact on molecular packing and charge-transport properties. Molecular-resolution scanning tunneling microscopy was used to compare the molecular packing in the monolayer and to investigate the effects that the structural and electronic modifications have on the stacking in subsequent layers. Conductivity measurements were made using the four-point probe van der Pauw technique to demonstrate charge transport properties comparable to pentacene. Although molecular packing is clearly impacted by the chemical structure, we find that the charge transport efficiency is quite tolerant to small variations in the molecular structure.

Introduction Organic semiconductor (OSC) devices have the potential to lower manufacturing costs and require less harsh conditions for the production of larger area, more flexible displays1-4 and more efficient photovoltaic devices.5-7 One of the major challenges to OSCs is the inability to precisely predict and control the packing structure of the molecules. Additionally, due to poor molecular arrangement, they often have inefficient charge transport through film layers, making them less appealing than materials currently in use. A better understanding of the relationship between charge transport and molecular orientation is needed to facilitate the development of novel organic semiconducting materials that are competitive with conventional inorganic-based materials. Well-aligned liquid crystals (LCs) are one class of materials that have been studied for OSC devices.8-9 For example, discotic LCs having good columnar π–π contacts have shown promise for increasing charge transport.10 Tunable optoelectronic properties of LCs are also appealing for applications in sensing and lasers.11-12 Such LC-based materials complement other organic materials, such as acenes, in OSC-based devices.

Pentacene is one of the more widely studied molecules for use in OSCs13-16 due to the high mobility of its charge carriers.1718 Pentacene has been studied on several single-crystal metal surfaces to gain insights into the molecular orientation of the molecules as the thickness is increased.13, 19 These studies have found that the molecules lie flat in the monolayer but transition to a herringbone-type orientation after the first few layers.20-21 Due to the large amount of work that has previously been reported for pentacene, and the relative ease of synthetic modifications on the acene backbone, substantial work has shifted to investigating analogues or functionalized derivatives of pentacene.22-26 Another major problem with pentacene is its susceptibility to oxidation in air, in particular through C–H activation at the 6- and 13-position,27 which degrades the function of the material.28 Thus, there is a clear need to investigate other types of molecular OSC platforms, in particular, tunable molecular libraries. Dispersion forces between π-faces are not strongly directional and thus allow for multiple crystalline packing modes of flat polyaromatics.29 Such polymorphism poses significant challenges in correlating intermolecular packing to the observed electronic properties of the bulk material. A

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logical approach to improve the structural ordering of π-stacked molecules is introducing donor–acceptor (D–A) type electrostatic complementarity.30-32 The use of electron-deficient aromatic systems,33-36 particularly those having nitrogen-rich cores,37 has gained attention due to their ability to take advantage of such D–A arrangement. One such molecule is tris(N-phenyl)triazole (TPT) shown in Figure 1, which is comprised of an electron-deficient tris(triazole) core and electron-rich phenyl groups on the periphery.38

Figure 1. Chemical structures of TPT, TPT-2M, and TPT-3M.

We have recently shown that the TPT molecules can selfassemble onto metallic surfaces, and grow into highly-ordered and conductive multi-layer structures.38 Extensive scanning tunneling microscopy (STM) imaging studies, in conjunction with in situ X-ray photoelectron spectroscopy (XPS) measurements, have established that TPT maintains precise offset π–π stacking that maximizes interlayer electrostatic complementary through phenyl···tris(triazole) contacts (Figure 2). In general, repetitive intermolecular D–A contacts inevitably serve as Coulombic traps that suppress efficient charge transport across the bulk material. Unlike simple D–A diads, however, the three-fold symmetric TPT has three D units surrounding a single A unit at the center (Figure 1). As such, intermolecular D–A contact (to direct ordering) of π-faces still allow for the co-facial stacking of remaining D units (to serve as energy-aligned conduits for electron mobility), as illustrated schematically in Figure 2. The result is a room-temperature stable, single-component film with uniform structure and good charge transport over at least 20 molecular layers.38

Figure 2. Schematic representations of interlayer π–π stacking of TPT molecules in the solid state.

Building on this initial discovery, we wished to investigate how the structural variations of the N-aryl groups could affect the charge transport properties of TPT and its derivatives in device settings, and compare their performance with pentacene as a benchmark organic semiconductor. Toward this objective, systematic variations were made on the N-aryl groups from the simple phenyl in the parent TPT to more electron-donating groups, such as 3,4-dimethoxyphenyl in TPT-2M, and 3,4,5trimethoxyphenyl in TPT-3M (Figure 1). According to an intuitive structure–property prediction, an increasing electrondonating ability of the N-aryl groups along the series TPT  TPT-2M  TPT-3M could enhance the stability of the D–A type π–π stacking (Figure S10). At the same time, changes in molecular footprint and overall symmetry would result in different packing patterns and electrical properties. Using STM, we examined how the addition of these electrondonating aryl units modifies the 2D monolayer packing and 3D stacking of the molecules on a single-crystal metal surface. We also investigated the properties of 100 nm thin films grown on SiO2, including conductivity and film morphology analysis. Our results provide insights into a new OSC platform, in which the solid-state packing structure can be tuned through functional group substitution around the molecular core while maintaining good charge transport performance.

Experimental Details STM measurements were made in an ultra-high vacuum (UHV) chamber (< 5  10–10 Torr) using a variable temperature STM microscope (SPM UHV 750, RHK Technologies). XPS measurements were made using a Mg/Al dual anode X-ray source and hemispherical electron energy analyzer (XR-50 and PHOIBOS 150, SPECS GmbH). The Ag(111) single crystal (Princeton Scientific Corp.) surface was prepared by repeated cycles of argon ion sputtering at 200 °C and thermal annealing to 480–500 °C. TPT, TPT-2M, and TPT-3M (see the Supporting Information for synthetic procedures, thermal analysis, and spectral characterizations) were vapor deposited from a quartz crucible Knudsen-type evaporator at 230 °C, 270 °C, and 280 °C, respectively, onto the Ag(111) surface, which was maintained at room temperature during deposition. Although solution processing would offer higher sample throughput,39-40 thermal evaporation was chosen for these experiments to maintain the extreme purity of the samples required for UHV analysis and to allow precise film thickness control. Thin film samples were made by thermally evaporating the molecules onto Si wafers with a SiO2 insulating layer. Metal contacts (Au) were evaporated onto the substrates to allow for the measurement of the film resistivity (Figure S19). These thin film samples were further characterized using scanning electron microscopy (SEM) in a high-vacuum environment. Atomic force microscopy (AFM) measurements were made in ambient conditions using a Nanosurf easyScan 2 microscope. Additional experimental details and procedures can be found in the Supporting Information.

Results and Discussion

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Langmuir Design and Synthesis of Three-Fold Symmetric Triazoloarenes. The parent TPT molecule consists of an electron-deficient tris(triazole) core and three electron-rich Nphenyl substituents on the periphery (Figure 1).38 To investigate the effects of electrostatic D–A type intermolecular π–π stacking on the molecular self-assembly and layer growth, we made systematic changes in the peripheral N-aryl groups. Two structural derivatives of TPT have been prepared, in which either two (TPT-2M) or three (TPT-3M) methoxy groups were installed onto each of the three N-phenyl groups (Figure 1). As shown in Scheme 1, these TPT derivatives were readily prepared by adapting the synthetic protocol used for the synthesis of TPT.38 The tris(azo)-precursors 1 and 2 were prepared in good yields (88–89%) by triple azo-coupling reactions of 1,3,5-triaminobenzene with the aryldiazonium salts derived from 3,4-dimethoxyaniline (for TPT-2M) and 3,4,5trimethoxyaniline (for TPT-3M), respectively. Oxidative cyclization reactions using copper(II) in basic conditions cleanly produced the desired products in 70–76% yields, which were purified and fully characterized. Scheme 1. Synthetic Routes to TPT Derivatives

TPT-2M forms a less dense, porous, monolayer structure (Figure 3b) with unit cell dimensions of 2.28 ± 0.03 nm and 2.16 ± 0.03 nm, and an internal angle of 65 ± 1° (Figure 3e). Apparently, the bulky peripheral units of TPT-2M disrupt the close-packed 2D organization exhibited by TPT. The unit cell of TPT-2M, which has a reduced symmetry, has a twomolecule basis with the two molecules separated by a 180degree rotation. STM images of TPT-2M show a regular and uniform pattern that would be consistent with a uniform conformation of the molecules, but the STM resolution does not allow unambiguous determination of the exact positions of the methoxy units with respect to the rotatable aryl–triazole bonds. TPT-3M forms a regular 2D packing on the surface, which results in unit cell dimensions of 1.76 ± 0.03 nm and 3.72 ± 0.04 nm, and an internal angle of 93 ± 1° (Figure 3f). The packing densities of TPT-2M and TPT-3M are 39% and 57% lower than that of TPT, respectively (Table S2). These are larger decreases than would be expected simply from differences in the molecular weight or van der Waals volume (Table S2). In fact, there is a significant increase in the porosity of the monolayer packing for the larger molecules; TPT-2M and TPT-3M are not close-packed (Figure 3). These results show a clear difference in packing as a result of structural modifications. Initial Growth of 3D Stacking. The monolayer packing shown in Figure 3 is important for seeding ordered multilayer stacking. A disordered monolayer packing negatively impacts long-range order in the multilayer, which often leads to a loss in charge transport efficiency.31, 41 The offset stacking of TPT (Figure 4a) is a result of interlayer D–A interactions: the triazole core and a phenyl group of a molecule in one layer interacts with a phenyl group and triazole core, respectively, of two different molecules in each adjacent layer (Figure 2). The remaining two phenyl groups are aligned with phenyl groups of other molecules in adjacent layers, thereby assisting charge transport between layers.38

2D Molecular Packing on Metallic Surfaces. As was previously reported,38 TPT forms an ordered, close-packed structure in the monolayer when deposited on the Ag(111) surface (Figure 3a). The unit cell dimensions for the TPT molecule are 1.42 ± 0.02 nm with an internal angle of 60 ± 6° (Figure 3d). Figure 4. Space-filling model overlays for the offset stacking of (a) TPT and (b) TPT-2M. Blue- and green-shaded molecules indicate the first and the second layer, respectively.

Figure 3. Monolayer packing of TPT, TPT-2M, and TPT-3M. STM images of (a) TPT, (b) TPT-2M, and (c) TPT-3M on Ag(111) are shown with (d–f) corresponding packing models.

As shown in Figure 4b, TPT-2M also exhibits an offset stacking in the second layer, but the porosity in the monolayer makes it impossible for the molecules to have a significant triazole···phenyl overlap with multiple molecules in the adjacent layers. Although the first layer is complete before the second layer growth, higher layer growth initiates before completion of the second layer (Figure 5), as a quasi-StranskiKrastanov mode.42 For a sample with just over one monolayer of TPT-2M, we find that the molecules aggregate to form small islands of several layers in localized areas (Figure 5). XPS measurements indicate that the overall molecular coverage for this sample is just 1.1 ± 0.1 monolayers (ML); there are other regions of the sample with just a flat, single layer of TPT-2M, but the region in Figure 5 shows the interlayer stacking. The line profile in

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Figure 5d shows layer heights of 2.9 ± 0.2 Å, which are consistent with D–A type π–π stacking distances in typical organic crystals,43 but larger than what is expected for Ag(111) step edges (2.2–2.3 Å, Figure S11).38, 44 Thus, the islands observed here are due to molecular stacking and are not simply underlying steps in the Ag surface. This growth pattern is different from the layer-by-layer (Frank-van der Merwe) growth45 observed for TPT films,38 suggesting a strain-induced transition in growth for TPT-2M after satisfying the first adsorbed monolayer. In Figure 5c, a magnified region is shown where three layers of molecules are present. The space-filling model overlaid on the STM image highlights the offset stacking between layers. Electrostatic repulsion between electron-rich π-faces is expected to lower the stability of the co-facial π–π stack.46-47 Previous studies on acene multilayers have demonstrated a transition from a flat molecular orientation in the monolayer (where the surface provides a stabilizing interaction) to a herringbone packing in higher layers.20-21 In contrast, TPT-2M seems to maintain sufficient interlayer triazole···phenyl contacts to maintain a flat-lying geometry and regular porous packing features in the first stages of multilayer formation, as were observed in the monolayer. When the overall surface coverage of TPT-2M is increased above 2 ML, however, a disorganized and less stable structure is observed via STM (Figure S12).

Figure 5. STM images of TPT-2M on Ag(111) showing stacking of several molecular layers. In this local area, the third molecular layer has started to grow before completion of the second layer. The image in (a) was blended with its own derivative to enhance the contrast in (b) for better visualization of the molecular ordering. (c) Closer zoom area image (also derivative merged) with molecular model overlay showing the slightly offset stacking mode between the TPT-2M layers (highlighted by blue and green lines through the centers of molecules in different layers). (d) STM line profile across several layers of molecular stacking along the black line in (a). Inset in (a) shows molecular electrostatic potential (MEP) map of TPT-2M.

Figure 6. (a) STM image of 2.0 ML TPT-3M sample showing molecular islands. (b–d) STM height profiles of the three lines annotated in (a) showing the height of the layers in the islands. The average height of the molecules in the islands is 3.7 ± 0.5 Å; larger than was observed for TPT or TPT-2M.

Coverages above 1 ML for TPT-3M have proven to be difficult to scan with STM since the molecule is mobile in the multilayer, as indicated by the changing island shapes shown in Figure S13. At a coverage of 2.0 ML (Figure 6a), a region with three molecular islands of TPT-3M was observed but molecular resolution could not be obtained. Image height profiles indicate that these islands are taller (3.7 ± 0.5 Å, Figure 6b–6d) than those observed for TPT and TPT-2M, suggesting that the molecules are not in a flat-lying geometry. This is likely a result of the significantly increased spacing in the monolayer (Figure 3 and Table S2). A reduction in the intimate stacking of the molecules in the multilayer would cause them to adopt more energetically favorable arrangements as the surface coverage increases, leading to a loss of order in the layer. Bulk Resistivity Measurements. Resistivity values were obtained for 100 nm thick films of each molecule using the van der Pauw method.48-50 Thin-film samples were evaporated onto p-type Si wafers with a 300 nm SiO2 coating. XPS was performed on a selection of the samples to verify intact molecule deposition (Table S3 and Figures S15–S18). Two different electrode designs were subsequently used to contact the films: bottom contacts (Figure S19a), and top contacts (Figure S19b). Current–voltage (I–V) graphs for one probe configuration of the four molecules are presented in Figure 7; all others are shown in Figures S21–S24. Each of the molecules was tested in forward and reverse voltage sweep directions over –20 V to +20 V in order to check for anisotropy in the flow of charge in the material. The curves in Figure 7 are representative of the I–V scans, and show a transition from a linear to a polynomial I–V relationship, similar to what has been previously reported for the transition from the linear Ohmic region to a space-charge limited current (SCLC) with a V2 dependence on I.23, 51-52 Resistance values were calculated from the slope of a linear fit to the Ohmic region of the graph for each van der Pauw configuration (Figure S20). Average film resistivity was calculated using these resistance values and equation S1 for all

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Langmuir samples; results are summarized in Table 1 with supporting data and discussion in the Supporting Information (Tables S4–S7).

Figure 7. Resistivity measurements for one configuration of the van der Pauw setup in a forward current direction. The linear region of the graph corresponds to Ohmic charge transport, while the curved portion is more closely related to SCLC charge transport. Table 1. Average film resistivity values for all four materials studied. Compound

ρ (Ω∙cm)

Std. Dev. (Ω∙cm)

TPT

1.48 

105

1.03  105

TPT-2M

1.18  105

4.77  104

TPT-3M

1.25  105

3.50  104

Pentacenea

1.13 

2.86  104

105

a The

resistivity of pentacene is in good agreement with previously reported values.53

These resistivity values indicate that TPT, TPT-2M, and TPT-3M each have film conductivities similar to pentacene.53 The charge transport efficiency in these films does not seem to vary strongly with the changes made in the chemical structure of the peripheral N-aryl groups, suggesting that some desirable aspects of pentacene as an organic semiconductor may be accessible to a broader range of polyaromatics, even heteroatom-rich platforms such as N-aryl tris(triazole)s. The charge transport must be affected by intermolecular contacts, but our results show that there exists certain tolerance in transport efficiency for slight variations in the local packing structure. Film Morphology Characterization. Low-voltage SEM54-56 was used to characterize the morphology of the films and inspect for uniformity of the depositions. Strikingly different film morphologies were observed for the molecules. As shown in Figure 8a, samples of TPT exhibit denselypacked, but disordered filaments at the surface of the film. These features are not observed on the substrate in areas without the organic molecule present (Figure S26). At the extreme edge of the film deposition, where the film thickness is small (Figure 8b), we observe isolated filament structures. We used a focused ion beam (FIB) to examine a cross-section of the film-substrate interface (Figures 8c and 8d) to investigate this morphology.

Contrast between the different layers (gold, organic, oxide, and SiO2 substrate) can be observed most clearly in Figure 8d. This cross-section shows that the organic layer is still connected beneath the filaments, which allows for consistent charge conduction paths across the film. These structures are similar to those previously observed for other organic molecules.57

Figure 8. (a) Low magnification SEM image at 3 kV of 100 nm TPT film on SiO2 with 20 Å Au sputter coating; (b) is a higher magnification image with the same beam conditions, but at the extreme edge of the film deposition area to more clearly observe the microstructure of TPT. (c) SE image of a FIB crater milled into the sample to observe a cross-section of the film-substrate interface, which is magnified in (d) to show the various layers (bright outline on the filaments is the gold layer, which is on top of the darker organic layer). FIB images were obtained at 5 kV, and utilized an additional carbon deposition near the FIB crater to aid in the crater face smoothing.

SEM examination of the TPT-2M film (Figure 9a) shows fibrous structures with long noodle-like backbones and small dendrites branching off, which are similar to other conducting organic polymers.58 Examining the edge of the film deposition, we see a less-densely packed region with particle-like features, similar to what was observed at the edge of the TPT film. The amount of fiber entanglement observed likely allows for the charge transport over the film.

Figure 9. SEM image of TPT-2M sample sputter-coated with 20 Å Au. (a) High magnification image at the edge of the filmsubstrate boundary showing the film structure with a lower molecule density. (b) Image near the center of the film showing a network of connecting fibers.

We were not able to observe structured features for TPT-3M even with a 20 Å gold sputter coat (Figure S27). Electron dispersive spectroscopy (EDS) confirmed the existence of the TPT-3M organic film on the surface (Figure S28), and SEM images of the film to substrate boundary show a change in contrast, confirming the deposition of the film. The lack of any observable structure, even at the film edge, suggests that the

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molecules are lying flat in a uniform orientation. Images of pentacene were confirmed to be similar to other literature reported images of films with comparable thicknesses and deposition conditions (Figure S29).54, 59 To check that the variations in film morphology were not due to perturbation by the electron beam, tapping mode AFM measurements were also performed on the samples (Figure S25); these confirm the morphologies measured by SEM.

Conclusions In this work, we have compared the parent TPT molecule with its N-aryl modified derivatives TPT-2M and TPT-3M to examine the effect of structural variations on the 2D molecular packing and 3D stacking. STM images on Ag(111) show that all three molecules have distinctly different 2D packing arrangements in the monolayer due to the structural and symmetry differences. At coverages above one monolayer, the differences become much more significant; the close-packed TPT monolayer seems most effective for maintaining flatlying, ordered multilayer stacking. This observation also points to the importance of balancing electrostatic complementarity and shape/size match. While TPT-2M and TPT-3M having progressively more electron-donating N-aryl groups were anticipated to engage in tighter D–A type π–π stacking, their bulkier size and lower molecular symmetry resulted in loose 2D packing and apparent lack of long-range 3D ordering. Physical characterization of thin films showed vastly different morphologies for each of the molecules. In spite of these structural differences, however, bulk resistivity measurements of thin-films showed that the TPT-based polyheteroaromatic molecules have comparable film conductivities to each other and to pentacene.

ASSOCIATED CONTENT Supporting Information Experimental procedures, TGA results, NMR spectra, packing density analysis, supplemental STM and XPS results, van der Pauw measurement data, AFM, and SEM data. The Supporting Information is available free of charge on the ACS Publications website.

AUTHOR INFORMATION Corresponding Author * [email protected]; [email protected]

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

ACKNOWLEDGMENT This work at Indiana University was supported by the NSF DMREF program, award number DMR 1533988. Support for the work at Seoul National University was provided by the Basic Research Grant (2017R1A2B2006605) and Creative Materials Discovery Program (2017M3D1A1039558) through the National Research Foundation of Korea (NRF). The authors would like to thank Mr. Daniel Wilcox of the Purdue University POWER

laboratory for assistance in making thin film samples, Mr. Jacob Huston and Mr. Hanjin Oh for assistance with TGA analysis, Mr. Tommy Derflinger and Dr. Aaron Pedigo for assistance with FIB imaging, and Mr. Derek Lengacher for useful discussions. DLW acknowledges funding and support from the United States Department of Defense (DoD) Naval Surface Warfare Center, Crane Division (NSWC CD) under the Naval Innovative Science and Engineering (NISE) and PhD fellowship programs. Research with NSWC CD and Indiana University was facilitated under a Cooperative Research and Development agreement (CRADA) NCRADA-NSWCCD-18-280.

REFERENCES (1) Brütting, W. Introduction to the Physics of Organic Semiconductors. 1 ed.; Wiley-VCH Verlag GmbH: Weinheim, Germany, 2005; p 40. (2) Kelley, T. W.; Baude, P. F.; Gerlach, C.; Ender, D. E.; Muyres, D.; Haase, M. A.; Vogel, D. E.; Theiss, S. D. Recent Progress in Organic Electronics: Materials, Devices, and Processes. Chem. Mater. 2004, 16, 4413–4422. (3) Kraft, A. Organic Field‐Effect Transistors—the Breakthrough at Last. ChemPhysChem 2001, 2, 163–165. (4) Meng, Q.; Dong, H.; Hu, W.; Zhu, D. Recent Progress of High Performance Organic Thin Film Field-Effect Transistors. J. Mater. Chem. 2011, 21, 11708–11721. (5) Mishra, A.; Bäuerle, P. Small Molecule Organic Semiconductors on the Move: Promises for Future Solar Energy Technology. Angew. Chem. Int. Ed. 2012, 51, 2020–2067. (6) Günes, S.; Neugebauer, H.; Sariciftci, N. S. Conjugated PolymerBased Organic Solar Cells. Chem. Rev. 2007, 107, 1324–1338. (7) Spanggaard, H.; Krebs, F. C. A Brief History of the Development of Organic and Polymeric Photovoltaics. Sol. Energy Mater. Sol. Cells 2004, 83, 125–146. (8) O'Neill, M.; Kelly, S. M. Liquid Crystals for Charge Transport, Luminescence, and Photonics. Adv. Mater. 2003, 15, 1135-1146. (9) Iino, H.; Usui, T.; Hanna, J. Liquid Crystals for Organic ThinFilm Transistors. Nat. Commun. 2015, 6, 6828. (10) An, Z.; Yu, J.; Jones, S. C.; Barlow, S.; Yoo, S.; Domercq, B.; Prins, P.; Siebbeles, L. D.; Kippelen, B.; Marder, S. High Electron Mobility in Room‐Temperature Discotic Liquid‐Crystalline Perylene Diimides. Adv. Mater. 2005, 17, 2580-2583. (11) Klinkhammer, S.; Heussner, N.; Huska, K.; Bocksrocker, T.; Geislhöringer, F.; Vannahme, C.; Mappes, T.; Lemmer, U. VoltageControlled Tuning of an Organic Semiconductor Distributed Feedback Laser Using Liquid Crystals. Appl. Phys. Lett. 2011, 99, 023307. (12) Yap, B. K.; Xia, R.; Campoy-Quiles, M.; Stavrinou, P. N.; Bradley, D. D. Simultaneous Optimization of Charge-Carrier Mobility and Optical Gain in Semiconducting Polymer Films. Nat. Mater. 2008, 7, 376-380. (13) Lu, M. C.; Wang, R. B.; Yang, A.; Duhm, S. Pentacene on Au (111), Ag (111) and Cu (111): From Physisorption to Chemisorption. J. Phys.: Condens. Matter 2016, 28, 094005. (14) Park, B.; Seo, S.; Evans, P. G. Channel Formation in SingleMonolayer Pentacene Thin Film Transistors. J. Phys. D: Appl. Phys. 2007, 40, 3506. (15) Lin, Y. J.; Tsao, H. Y.; Liu, D. S. Hall-Effect Mobility of Pentacene Films Prepared by the Thermal Evaporating Method with Different Substrate Temperature. Appl. Phys. Lett. 2012, 101, 013302. (16) Kitamura, M.; Arakawa, Y. Pentacene-Based Organic FieldEffect Transistors. J. Phys.: Condens. Matter 2008, 20, 184011. (17) Kelley, T. W.; Boardman, L. D.; Dunbar, T. D.; Muyres, D. V.; Pellerite, M. J.; Smith, T. P. High-Performance OTFTs Using Surface-Modified Alumina Dielectrics. J. Phys. Chem. B 2003, 107, 5877–5881. (18) Jurchescu, O. D.; Popinciuc, M.; van Wees, B. J.; Palstra, T. T. M. Interface-Controlled, High-Mobility Organic Transistors. Adv. Mater. 2007, 19, 688–692.

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