Large-Size Single-Crystal Oligothiophene-Based Monolayers for Field

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Organic Electronic Devices

Large-Size Single-Crystal OligothiopheneBased Monolayers for Field-Effect Transistors Vladimir Bruevich, Anastasia V. Glushkova, Olena Yu. Poimanova, Roman S. Fedorenko, Yuriy N Luponosov, Artem V Bakirov, Maxim Anatolievich Shcherbina, Sergei N. Chvalun, Andrey Yurievich Sosorev, Linda Grodd, Souren Grigorian, Sergei A. Ponomarenko, and Dmitry Yu. Paraschuk ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b20700 • Publication Date (Web): 21 Jan 2019 Downloaded from http://pubs.acs.org on January 23, 2019

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ACS Applied Materials & Interfaces

Large-Size Single-Crystal Oligothiophene-Based Monolayers for Field-Effect Transistors Vladimir V. Bruevich1,3,9,+, Anastasia V. Glushkova1,+, Olena Yu. Poimanova2, Roman S. Fedorenko1,3, Yuriy N. Luponosov3, Artem V. Bakirov3,7, Maxim A. Shcherbina6,7, Sergei N. Chvalun3,7, Andrey Yu. Sosorev1,3,9, Linda Grodd4, Souren Grigorian4,5, Sergei A. Ponomarenko3,8, Dmitry Yu. Paraschuk*,1,3 Faculty of Physics & International Laser Centre of Lomonosov Moscow State University, Leninskiye gory 1/62, 119991 Moscow, Russia Department of Chemistry of Donetsk National University, Universitetskaya Str. 24, 83001 Donetsk, Ukraine 3Enikolopov Institute of Synthetic Polymeric Materials of Russian Academy of Sciences, Profsoyuznaya Str. 70, 117393 Moscow, Russia 4Department of Physics, University of Siegen, Walter-Flex-Strasse 3, 57072 Siegen, Germany 5Aix-Marseille Université, Université Toulon, CNRS, IM2NP, Avenue Escadrille Normandie Niemen – Case 142, F-13397 Marseille, France 6 Moscow Institute of Physics and Technology, 4 Institutsky line, 141700 Dolgoprudny, Moscow region, Russian Federation 7 National Research Center "Kurchatov Institute", 1 pl. Akademika Kurchatova, 123182 Moscow, Russia 8 Chemistry Department, Lomonosov Moscow State University, Leninskiye gory 1/3, 119991 Moscow, Russia 9 Institute of Spectroscopy of Russian Academy of Sciences, Fizicheskaya Str., 5, 108840 Troitsk, Moscow, Russia 1 2

+ These authors contributed equally to this work. *E-mail: [email protected]

Abstract High structural quality of crystalline organic semiconductors is the basis of their superior electrical performance. Recent progress in quasi two-dimensional (2D) ultrathin organic semiconductor films challenges bulk single crystals as both demonstrate competing charge carrier mobilities. As the thinnest molecular semiconductors, monolayers offer numerous advantages such as unmatched flexibility and light transparency as well is an excellent platform for sensing. Oligothiophene-based materials are among the most promising for light-emitting applications due to combination of efficient luminescence and decent charge carrier mobility. Here we demonstrate single-crystal monolayers of unprecedented structural order grown from four alkyl-substituted thiophene and thiophene-phenylene oligomers. The monolayer crystals with lateral dimensions up to 3 mm were grown from solution on substrates with various surface energies and roughness by drop or spin casting with subsequent slow solvent evaporation. Our data indicate that 2D crystallization resulting in single-crystal monolayers occur at the receding gas-solution-substrate contact line. The structural properties of the monolayers were studied by grazing-incidence X-ray diffraction/reflectivity, atomic force and differential interference contrast microscopies and imaging spectroscopic ellipsometry. These highly ordered monolayers demonstrated excellent performance in organic field-effect transistors approaching the best values reported for the thiophene or thiophene-phenylene oligomers. Our findings pave the way to efficient monolayer organic electronics highlighting the high potential of simple solution-processing techniques for growth of large-size single-crystal monolayers with excellent structural order and electrical performance competing against bulk single crystals.

Keywords organic field effect transistor, organic monolayer, oligo(thiophene-phenylene), grazing-incidence X-ray diffraction, organic single crystal, charge carrier mobility

Introduction Organic electronics gradually steps into its technological implementation setting high demands to its main building block – organic field field-effect transistor (OFET). Basic requirements to OFETs include energy- and cost-effective processing as well as high and reproducible electrical performance. Single crystals are 1 ACS Paragon Plus Environment

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recognized among the most industry relevant active layers for OFETs as they benefit from highly-ordered structure and low concentration of defects, which are crucial for high OFET performance.1 The best OFETs were realized on single crystals grown as free-standing samples.2 However, for printing technologies, single crystals grown directly on substrate would be much more beneficial, but the lack of morphological homogeneity and thickness uniformity due to step-and-terrace growth, dislocations and other defects in largearea crystalline films cause significant variability of the electrical characteristics 3 and, therefore, diminishes their applicability. A variety of solution-based methods to fabricate thin films with large single-crystal domains were developed recently, e.g., meniscus-guided coating,4 edge-casting,5 printing,6 and surface-mediated crystallization.7 These works exhibit a clear benefit of enlargement in the area of single crystals grown directly on substrate. Taking in account that charge transport in OFETs occurs in a few semiconductor monolayers adjacent to the semiconductor-dielectric interface,8-9 the perfect OFET active layer would be a large-area single crystal that is not thicker than а few molecular layers — two-dimensional (2D) single crystal. Even though a significant effort has been made in the area of 2D single-crystal OFETs over the recent years,10 a range of devices exhibit less than optimal OFET characteristics, which has led to erroneous evaluation of charge carrier mobility in many cases.11 Moreover, the reported x-ray data for single-crystal monolayers are still too scarce to determine their atomic and molecular structure. Accordingly, more focused and detailed effort is necessary to carefully address the structure, charge transport properties, and device performance of 2D OFET. Semiconductor monolayers — the thinnest 2D materials — can be fabricated of wafer-size area by selforganization of functionalized semiconductor molecules on the liquid-solid interface, i.e., by self-assembled monolayer technique,12 or on the gas-liquid interface with subsequent transfer of the film onto a solid substrate, i.e., by Langmuir technique.13-16 Both techniques are robust and result in monolayer coverage of the whole substrate. However, these techniques usually result in polycrystalline films with sizes of crystalline domains not larger than 10 µm 17 with the best OFET charge mobilities for monolayers reaching 0.04 cm2V-1s-1 12 obtained on quinquethiophene-based oligomers. Though the highest mobilities in small-molecule OFETs are achieved on fused carbocyclic compounds,2, 18 oligothiophene-based materials containing benzene or furan rings in the conjugated core are of high interest due to their ability to combine bright luminescence and efficient ambipolar charge transport.19-20 Such cooligomers enable fabrication of state-of-the-art organic light-emitting transistors (OLETs),21-22 which are now based on bulk single crystals and could benefit from the use of ultrathin semiconductor layers. In fact, optical confinement in bulk luminescent single crystals leads to light emission mainly from their edges.19, 23-24 Using ultrathin single crystals would strongly decrease luminescence reabsorption and exclude light waveguiding so that light emission directed perpendicular to the crystal plane would be more intense strongly improving the OLET performance. Combination of luminescent and semiconductor properties was recently demonstrated in Langmuir monolayers based on a thiophene-phenylene co-oligomer.25 The advantages of semiconductor monolayers expand further to the field of sensing devices. OFET-based gas sensors reach the highest sensitivity with monolayers used as a semiconductor material.26 Moreover, semiconductor single-crystal monolayers are intrinsically optically transparent and highly flexible, which is very beneficial for numerous applications. With all this background, the need for ultrathin large-size semiconductor single crystals for various organic field-effect devices, such as OFETs, OLETs and sensors, is highly pronounced.

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In this work, we report on oligothiophene and oligo(thiophene-phenylene) large-size single-crystal monolayers with lateral dimensions up to 3 mm. Using the 5-ring long oligomers with hexyl and decyl terminal substituents (Figure S1), we grew ultrathin films from solution by common drop or spin casting combined with subsequent slow solvent evaporation on different substrates and show that the films consist of high-quality monolayer single crystals by using grazing incidence X-ray diffraction and reflectivity as well as differential interference contrast and atomic force microscopies. Monolayer OFETs with printed electrodes demonstrated high performance with the measurement reliability factor11 approaching 100% and charge carrier mobilities of 0.04–0.25 cm2V-1s-1, which are the highest among oligothiophene-based monolayers12 and exceed or approach the values for the bulk samples. The mechanism of 2D crystallization was suggested based on in situ growth monitoring.

Results and Discussion Large-size monolayers: optical and AFM characterization Figure 1a-b, d-e demonstrates C-DIC optical microscopy data for 1,4-bis(5′-hexyl-2,2′-bithiene-5-yl)benzene (DH-TTPTT) and 5,5''''-didecyl-2,2':5',2'':5'',2''':5''',2''''-quinquethiophene (DD-5T) films grown on SiO2. The films exhibit domain structure, each domain is visually uniform with sizes up to 1.5 mm for DH-TTPTT and 0.6 mm for DD-5T. The maximum domain size exceeded 3 mm for DH-TTPTT (Figure S2). The images recorded at different angular positions of the Nomarski prism: Figure 1a-b, d-e shows different brightness and colors of the domains; hence, they are optically anisotropic. The domain color and brightness are constant within one domain; therefore, each domain looks as a single crystal of even thickness. The domains of the DD-5T film demonstrate clear faceted shapes, which also indicates that the domains are single crystals. The habit of DHTTPTT domains is elongated that corresponds to that of the bulk single crystals.27 The C-DIC microscopy data for ultrathin films of the other oligomers — 5,5''''-dihexyl-2,2':5',2'':5'',2''':5''',2''''-quinquethiophene (DH-5T) and 1,4-bis(5′-decyl-2,2′-bithiene-5-yl)benzene (DD-TTPTT) — are similar: they also show optically anisotropic domains of ~1 mm size, the domains for both films are faceted (Figure S3). Thus, the optical microscopy data indicate that all the four oligomers studied crystallize in thin large-size smooth single crystals. Figure 1c, f depicts atomic-force microscopy (AFM) topography for DH-TTPTT and DD-5T films, the corresponding images for DH-5T and DD-TTPTT films are presented in Figure S4. All the films studied show molecular smoothness with the mean-square roughness close to that of the Si/SiO2 substrates (0.3 nm). Thicknesses of the films were measured by AFM at 2–3 different locations on several samples of each oligomer, the typical AFM profiles DH-TTPTT and DD-5T for are presented in Figure 1c,f by white lines. The AFM profiles for DH-5T and DD-TTPTT are shown in Figure S4b,d. Thicknesses of all the films studied are in the range of 30– 40 Å (Figure S4) and correspond to the calculated lengths of the oligomers (Table S1). Assuming that the oligomers are oriented more or less vertically against the substrate, from the optical and AFM data we suggest that the observed large-size ultrathin domains consist of one molecular layer, i.e., they are monolayers. Furthermore, junctions between adjacent single-crystal monolayer domains can be molecularly smooth without any steps as demonstrated for the DH-TTPTT film in Figure 1 by white arrows; the junctions were observed by C-DIC microscopy (not shown). The C-DIC and AFM data for smooth junctions formed by DH-5T single-crystal monolayers are presented in Section 6, Supporting Information (SI). Further evidence for largesize crystalline monolayers follows from the imaging spectroscopic ellipsometry data presented for DH-TTPTT films in Section 7, SI. The ellipsometry data also revealed differently oriented adjoining single-crystal domains in the smooth monolayer film (Figure S12, SI).



φ

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φ + 45°

Figure 1. Microscopy data: C-DIC images of DH-TTPTT (a-b) and DD-5T (d-e) monolayer crystals on SiO2 captured at different angular positions of the Nomarski prism (the angle between (a)/(b) and (d)/(e) is 45°), the crystals with different orientations are outlined by dashed lines and marked by numbers; (c, f) AFM topography images of DH-TTPTT and DD-5T respectively; white lines show typical AFM profiles. We found that large-size smooth ultrathin crystalline films of all the oligomers studied also grow on silicon substrates treated by hexamethyldisilazane (HMDS), i.e., on surfaces with the lower surface energy (43.6 mJ m-2) than that of SiO2 (61.4 mJ m-2).28 The C-DIC microscopy images of these films are presented in Figure S13a-d. Moreover, the growth of the smooth ultrathin films is not interrupted by scratches on the substrate (Figure S13e-f). As the growth of the ultrathin films is weakly sensitive to the substrate, we suggest that crystallization occurs on the liquid-gas interface, the details of the growth mechanism are discussed below.

X-ray studies Figure 2a plots X-ray reflectivity (XRR) scan for a DH-TTPTT sample, it shows three definite minima corresponding to the layer thickness of 33.5 Å, which is in a perfect agreement with the calculated length of the isolated molecule (inset in Figure 2b and Table S1), the molecule length in single crystal,27 and the film thickness from the AFM data (Figure 1c). Assuming almost the vertical orientation of DH-TTPTT molecules in the film as usually observed for thin films of conjugated oligomers, we reconstructed the electron density distribution along the film normal, the result is shown in Figure 2b. The reconstructed electron density shows a pronounced dip near the substrate, this dip corresponds to the aliphatic tails. Note that the edges of regions with strongly different electron densities are smeared as the reconstruction procedure implies the determination of an inherently continuous function.29-30 Farther from the substrate, there is a region with the electron density of 0.62 Å-3, which is typical for densely packed oligothiophenes 31-32 and close to that observed for Langmuir oligothiophene and oligo(thiophene-phenylene) monolayers.25,33 Therefore, the XRR data demonstrate that the major part of the DH-TTPTT sample is a monolayer film. 4 ACS Paragon Plus Environment

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Insets in Figure 2a display 2D grazing incidence X-ray diffraction (GIXD) patterns for the DH-TTPTT sample recorded for its different positions. The GIXD patterns shows intense Bragg rods, which are signatures of a 2D crystal structure.34 Therefore, we conclude that the crystals generating the Bragg rods are monolayers. The scattering vectors indexed to reflections 110 (1.23 Å-1), 020 (1.30 Å-1), and 210 (2.22 Å-1) correspond to a 2D rectangular unit cell with parameters 5.96 and 9.68 Å. Such a set of reflections is typically observed for oligothiophene-based monolayers 12, 17 and polycrystalline films 35 and is assigned to packing of the conjugated fragments in a herringbone structure with two molecules in the unit cell. Remarkably, a single 2D GIXD scan recorded at a particular in-plane orientation does not show all the reflections found, we assign this to large monolayer single-crystal domains, whose sizes are comparable to that of the X-ray beam footprint on the sample. As a result, the X-ray beam touches only one or a few 2D single crystals so that, depending on their orientations, the Bragg diffraction conditions for them are either fulfilled or not. Therefore, a GIXD scan samples a limited number of directions in the reciprocal lattice and hence does not reveal all the possible reflections. This result demonstrates that a single-crystal analysis is indeed possible, but it is a challenging task as one needs to rotate the sample recording multiple GIXD patterns from a single domain. The observed GIXD features are in strong contrast with the earlier reported GIXD data on polycrystalline organic monolayers (e.g.,12, 17), where randomly oriented domains allow the observation of all reflections in a single GIXD scan. Moreover, the Bragg rods in Figure 2a are very narrow (a few pixels on the detector) and correspond to δqxy~0.005 Å-1. Therefore, the x-ray coherence length evaluated as 2π/δqxy for monolayer DH-TTPTT crystals is longer than 1000 Å. To the best of our knowledge, this is the first observation of such a high crystalline order in an organic monolayer. While the intensity of the reflection 210 peaks at qz=0, the maximum of the most intense reflection 020 is shifted along qz direction (Figure 2a, top inset), which indicates a tilt of the molecules in the 2D crystals.34, 36-37 The azimuthal position of reflection 020 testifies for the tilt of 10° of the conjugated backbone from the normal to the substrate. Note that tilted oligothiophene conjugated fragments were observed earlier in crystalline self-assembled monolayers 17, 38 and thin films.39 The tilt of the conjugated backbones explains the optical anisotropy in the monolayer plane revealed by imaging spectroscopic ellipsometry (Figure S9). The structure of the peaks along qz signifies that multilayered species (in agreement with the microscopy data presented in Figure 2a, inset) with an average thickness of up to 300 Å also present on the substrate. This observation is in full agreement with the ellipsometry data (Section 7, SI).



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Figure 2. X-ray data for DH-TTPTT. (a) Experimental XRR data (black dots) and fit (red line); insets show GIXD patterns recorded at different sample positions, the in-plane rods are assigned to 110, 020, and 210 reflections corresponding to d-spacings of 5.09, 4.84, and 2.83 Å, respectively; and C-DIC microscopy image of the sample; (b) reconstructed absolute electron density profile; the inset depicts the calculated molecule geometry (Table S1), the molecule is shifted from the substrate by a typical interatomic (intermolecular) distance, the colored regions are guides to the eye. DH-5T and DD-TTPTT films included a substantial part of multilayered structures. As a result, their XRR data did not allow us to evaluate the film thickness and other parameters, while their 2D GIXD patterns were indicative of highly crystalline structures as follows from Figure S14 and Figure S15, correspondingly. Locallyresolved 2D GIXD studies on DH-5T films showed Bragg rods indicating the crystalline monolayers with thickness corresponding to the length of DH-5T molecule (Section 9.1, SI). The DD-TTPTT film showed a very narrow in-plane reflection similar to the DH-TTPTT films. The only recorded reflection at 1.48 Å-1 does not correspond to any of those of DH-TTPTT films; this can be explained by that the packing of the TTPTT conjugated fragments is sensitive to the length of terminal alkyl substituents (decyl vs hexyl). The x-ray data for DD-5T films are similar to those of DH-TTPTT films. Figure 3a plots XRR scan for a DD-5T film demonstrating minima corresponding to the layer thickness of 46.6 Å, which is in good agreement with the calculated length of the molecule (Figure 3b and Table S1) and the AFM data (Figure 1f). Figure 3b represents the reconstructed absolute electron density, whose profile is similar to that of DH-TTPTT molecule (Figure 2b). The region nearest to the substrate with the lower electron density region corresponding to the aliphatic tails is longer than that in DH-TTPTT by ~7 Å, which is in good agreement with the difference in length of decyl and hexyl terminal substituents. The maximal electron density is 0.56 Å-3, which is in perfect agreement with that of Langmuir monolayers formed by oligothiophenes with 5T conjugated fragment.33 The top inset in Figure 3a demonstrates 2D GIXD data for the DD-5T sample. The GIXD patterns show Bragg rods recorded at two orientations of the substrate along its normal. As for DH-TTPTT films, the Bragg rod widths (δqxy ≤ 0.005 Å-1) correspond to the coherence length longer than 1000 Å and indicate high-quality low-defect crystals. The observed scattering vectors correspond to a 2D rectangular unit cell with parameters 5.45 and 7.58 Å. The scattering vectors are very close to 110 and 020 reflections reported for DH-5T films 35 (see also the GIXD data for DH-5T in Section 9.1, SI). Therefore, our data indicate very similar molecular packing of 5T fragments in DD-5T and DH-5T ultrathin films. All in all, the XRR and GIXD data for the DH-TTPTT and DD-5T films demonstrate that the substantial part of the substrate is covered by highly ordered monolayer single crystals. The DH-5T and DD-TTPTT ultrathin films 6 ACS Paragon Plus Environment

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are also highly crystalline, but apart from monolayer domains the substrate is covered by multilayered structures resulting in nearly structureless XRR curves, which did not allow us to model the electron density along the normal to the substrate. The x-ray data indicate that the conjugated fragments of the oligomers studied are slightly tilted against the normal to the substrate. This provides in-plane optical anisotropy of the crystalline domains resulting in birefringence for polarized light incident normally to the substrate. This explains why variously oriented domains of single-crystal monolayers are easily visualized with the help of CDIC microscopy.

Figure 3. X-ray data for DD-5T. (a) Experimental XRR data (black dots) and fit (red line); insets show GIXD scans recorded at different sample positions, the in-plane rods are assigned to 110 and 020 corresponding to d-spacings of 4.42 and 3.79 Å, respectively; and C-DIC microscopy image of the sample; (b) the reconstructed absolute electron density profile (for other details see the caption to Figure 2).

Electrical characteristics Single-crystal monolayers of all the oligomers studied were investigated as active layers in OFETs. For each oligomer, 4 working devices were fabricated and tested. The devices demonstrated clear p-type OFET behavior. Figure 4 shows typical output and transfer characteristics for DH-TTPTT and DD-5T in the saturation regime, the transfer characteristics in the linear regime are presented in Figure S16. The output characteristics at low drain voltages show Ohmic-like behavior, which indicates negligible contact effects. The OFET characteristics for DH-5T and DD-TTPTT look very similar and are presented in Section 10, SI. All the monolayer devices showed the charge carrier mobility in the range of 0.04–0.25 cm2V-1s-1, the average mobility in the saturation and linear regimes were very close for all devices (Figure 4c, Table S3) indicating that the OFET operation nicely fits to the Shockley model used for OFET. The linear mobility measurement reliability factor 11 for the most samples is close to 100% (Table S3). Figure 4c summarizes the mobility data measured in the linear and saturation regimes. Note, that linear regime usually provides more accurate evaluation of the charge mobility as compared to the saturated regime.2 The full statistics on the OFET data is collected in Table S3. DH-TTPTT monolayers exhibited the average charge carrier mobility of 0.13 cm2V-1s-1 (linear regime), which is significantly higher than the mobility values reported for vacuum-evaporated DH-TTPTT thin films.40-41 Moreover, the average charge mobility in DH-TTPTT monolayers is about twice higher than that in bulk DHTTPTT solution-grown single crystals,27 which corroborates the high structural quality of the monolayers. The average charge mobility for DH-5T monolayers amounted to only 0.046 cm2V-1s-1 with the maximum of 0.053 7 ACS Paragon Plus Environment


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cm2V-1s-1. The significantly lower mobility for DH-5T can be explained by the lower monolayer quality. AFM and optical microscopy revealed the presence of cracks over the DH-5T 2D crystalline domains (see Figure S6). As discussed in the next section, we believe that these cracks emerge during the film transfer from the solution drop surface to the substrate. The presence of these cracks effectively decreases the width to length ratio of the OFET channel and thus the measured charge mobility. Nevertheless, it is still in a reasonable agreement with the charge mobility values reported for DH-5T vacuum-evaporated bulk films.39 Unlike the monolayers of DH-substituted oligomers, both of DD-substituted ones demonstrated high OFET performance. The averaged mobility in the DD-TTPTT monolayers amounted 0.09 cm2V-1s-1 in the linear regime, while that in the DD-5T ones was 0.18 cm2V-1s-1. The maximum charge mobility (0.22 cm2V-1s-1) for the DD-5T monolayers approaches that reported for DD-5T vacuum evaporated films of 30-nm thickness.42-43 Note that all OFETs demonstrated a positive threshold voltage meaning that the p-channel transistors behave in the depletion mode. We assign the positive threshold voltages to doping of the active layers (probably by oxygen) as commonly observed for oligo- and polythiophenes.39, 44-45 The shelf-life stability of the device characteristics was studied for the DD-5T monolayers stored in ambient air under normal laboratory conditions. Figure 4f shows that DD-5T mobility is reduced by about 5% after two months of storage. The threshold voltage increased with the storing time (Section 10.4, SI; Table S5), which is consistent with the effect of doping by atmospheric oxygen. To check the effect of SiO2 surface on performance of the monolayer OFETs, we studied OFETs on monolayers grown on SiO2 surface passivated by HMDS (Section 10.3, SI). The OFETs on HMDS-treated SiO2 showed considerably lower charge mobilities, which we tentatively assign to the lower structural quality of the monolayers grown on HMDS-treated SiO2. This lower quality can be assigned to the lower surface energy of HMDS-treated SiO2,28 which leads to larger solution contact angle resulting in less optimal film transfer to the substrate. Taking also in account that the DH-TTPTT and DD-TTPTT monolayer OFETs show very close OFET performances, which are insensitive to the alkyl length and hence the distance from the substrate, we suggest that the effect of unpassivated SiO2 on performance of the monolayer OFETs is not essential. The monolayer OFETs reported herein demonstrated performances close or even superior to those of the corresponding bulk OFETs. The OFET charge carrier mobilities in monolayers exceed the highest reported for oligothiophene-based monolayers.12, 17, 46-48 The demonstrated combination of good charge transport and contact properties of single-crystal monolayers in air-stable OFETs opens a variety of novel OFET-based applications including chemical and biological sensors. Such devices would benefit from the ultimately thin device architecture that literally exposes the transistor conducting channel to any molecular species on top of it. Moreover, the highly luminescent nature of thiophene-phenylene oligomers could open a way for creating efficient ultrathin light-emitting transistors, which are fundamentally free from light-outcoupling problems inherent to modern organic light-emitting devices such as OLEDs. All these devices could be ultraflexible due to the intrinsic flexibility of the monolayer.


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Figure 4. Electrical characterization of oligothiophene-based monolayer single crystal devices: output characteristics of DH-TTPTT (a) and DD-5T (d); transfer characteristics in the saturation regime of DH-TTPTT (b) and DD-5T (e), blue squares depict calculated OFET mobilities (right axes), insets show OFET images; average and highest measured mobilities (c), error bar depicts the standard deviation; long-term device stability after storage in air (f).

Discussion of 2D crystallization To explore the mechanism of 2D crystallization, specifically the growth of large-size single-crystal monolayers, multiple in situ experiments were conducted using C-DIC microscopy (see Supporting Movies). From these observations, the most important persistent features typical for 2D crystallization of all the oligomers studied were drawn out. These features are summarized in the following three paragraphs. Crystallization near the freely receding contact line. Crystallization occurs mainly close to the contact line of the solution drop. Moreover, large size ultrathin crystals grow only if the contact line recedes freely as illustrated in Figure 5 showing the crystallization dynamics for DD-TTPTT. If the contact line is pinned, the nearcontact-line crystallization typically results in small bulk crystals (Figure S20). Pinning of the contact line stimulates outward hydrodynamic flows in the liquid phase and leads to the coffee-ring effect —49 the formation of thicker crystals near the pinned contact line (Figure S21). The contact line is typically pinned if the initial solution concentration is higher than the optimal one (Table S1) and also at the later stages of crystallization due to the increased concentration resulting from solvent evaporation. As a result, 3D crystallization starts to dominate over 2D one (Supporting Movies S1, after 6 s; S2 after 40 s). We suggest that 2D nucleation seeds appear near the drop contact line. This is substantiated by mainly radial distribution of the elongated 2D DH-TTPTT and DH-5T crystals, which are differently oriented and grow towards the drop center (see Supporting Movies S1 and S2). At later crystallization stages, the variously oriented monolayer crystals meet and form smooth inter-domain junctions (Figure S6a, Figure S12b), which are discussed in Section 6, SI.



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Figure 5. Growth dynamics of 2D DD-TTPTT crystals on SiO2 surface from a solution drop with a freely receding contact line, one of the crystals is outlined by a red dash line. The arrow shows the growth direction. Slowed drop evaporation. During the crystallization, as the drop size decreases, the growing 2D film can encircle the shrinking droplet. At this stage, decreasing the drop size is considerably slowed down (Supporting Movie S1 after 3 s and S2 after 8 s), which we assign to the slower solvent evaporation rate resulting from high coverage of the drop surface by the crystallizing solid film. Therefore, we conclude that the solid film surrounding the drop also covers a major part of its surface. Gas-solution interface 2D crystallization at the receding contact line. As the 2D crystals float on the drop surface, and their growth is weakly sensitive to the type of substrate, we suggest that the 2D crystallization occurs mainly at the gas-solution interface as we have reported earlier for bulk platy crystals of terminalsubstituted thiophene-phenylene co-oligomers.50 Such growth mechanism agrees with the surface-mediated crystallization from solution drop on substrate reported for a condensed aromatic molecule with methylpentyl terminal substituents.7 Moreover, the importance of high solvent surface energy for the gas-solution interface 2D crystallization is supported by inability to grow large-size 2D crystals from toluene, whose surface energy (29 mJm-2) is lower than that of ortho-dichlorobenzene (37 mJm-2) mainly used in this study. This is also in line with Ref.7 claiming that the solvent surface energy is a key factor (neither its viscosity nor boiling temperature) for the surface-mediated crystallization. Another important factor for crystallization and film transfer from the drop surface to the substrate is the substrate surface energy, which affects the contact angle of the solution drop. Most of our experiments were performed on clean SiO2 surface with a very small contact angle (

Large-Size Single-Crystal Oligothiophene-Based Monolayers for Field

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