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Easy processible highly ordered Langmuir-Blodgett films of quaterthiophene disiloxane dimer for monolayer organic field-effect transistors Alexey S Sizov, Daniil S Anisimov, Elena V. Agina, Oleg V Borshchev, Artem V Bakirov, Maxim A Shcherbina, Souren Grigorian, Vladimir Vasilievich Bruevich, Sergei N Chvalun, Dmitry Yurievich Paraschuk, and Sergei A. Ponomarenko Langmuir, Just Accepted Manuscript • DOI: 10.1021/la504037b • Publication Date (Web): 25 Nov 2014 Downloaded from http://pubs.acs.org on November 25, 2014

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Easy processible highly ordered Langmuir-Blodgett films of quaterthiophene disiloxane dimer for monolayer organic field-effect transistors Alexey S. Sizova, Daniil S. Anisimovb, Elena V. Aginaa, Oleg V. Borshcheva, Artem V. Bakirova, Maxim A. Shcherbinaa, Souren Grigorianc, Vladimir V. Bruevichb, Sergei N. Chvaluna, Dmitry Yu. Paraschukb, and Sergei A. Ponomarenkoa* a

Institute of Synthetic Polymeric Materials of Russian Academy of Sciences, Profsoyuznaya st. 70, 117393 Moscow, Russia; bFaculty of Physics & International Laser Center, Lomonosov

Moscow State University, 119991 Moscow, Russia; cInstitute of Physics, University of Siegen, Emmy-Noether-Campus, Walter-Flex-Str. 3, D-57068 Siegen, Germany; e-mail: [email protected] Keywords: oligothiophenes, OFETs, Langmuir-Blodgett films, self-assembly, charge carrier mobility ABSTRACT Self-assembly of highly soluble water-stable tetramethyldisiloxane-based dimer of α,α’dialkylquaterthiophene on the water-air interface was investigated by Langmuir, grazing incidence X-ray diffraction and X-ray reflectivity techniques. The conditions for formation of very homogeneous crystalline monolayer Langmuir-Blodgett (LB) films of the oligomer were 1 ACS Paragon Plus Environment

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found. Monolayer organic field-effect transistors (OFETs) based on these LB films as a semiconducting layer showed hole mobilities up to 3×10-3 cm2/Vs, on-off ratio of 105, small hysteresis, and high long-term stability. The electrical performance of the LB films studied is close to that for the same material in the bulk or in the monolayer OFETs prepared from water vapor sensitive chlorosilyl derivatives of quaterthiophene by self-assembling from solution. These findings show high potential of disiloxane-based LB films in monolayer OFETs for largearea organic electronics.

TEXT Organic field-effect transistors (OFETs) attract significant attention due to possibility of enabling low cost, flexible and transparent large area electronics.1 It was established that the charge transport in OFETs takes place only in a thin layer having a thickness of several nanometers, close to the semiconductor-dielectric interface, which corresponds to 1-3 molecular layers.2,3 Therefore usage of monolayer OFETs can lead to a significant reduction of the organic semiconducting material consumption as compared to the traditional thin-film OFETs and could result in a new generation of ultrathin organic electronic devices. However, due to high inhomogeneity of the monomolecular film and high concentration of defects such devices usually demonstrate significantly lower performance than the conventional “thick film” devices.4,5 Preparation of homogeneous low defect organic monolayer films with high electrical performance is still a challenging task. Self-assembled monolayers (SAMs) of functional semiconducting molecules have been successfully applied to monolayer OFETs.6-8 Typically, molecules for SAM-based OFETs (SAMFETs) consist of a semiconducting core, an aliphatic spacer and an anchor group, which is responsible for chemical binding with the substrate. Nowadays several different anchor groups 2 ACS Paragon Plus Environment

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have been used in SAMFETs, including hydroxy-,9 carboxy-,3 chlorosilyl-10,11 and phosphonic acid groups.12,13 The last two are the most promising because of their ability to form dense SAMs with high electrical properties on silicon oxide or aluminium oxide dielectrics, respectively. Nowadays the highest field-effect mobility of 0.04 cm2/Vs was reported for SAMFETs prepared by self-assembly of chlorosilyl derivative of quinquethiophene from solution.10 However, this process takes quite a lot of time (from a few hours up to 1-2 days), thus limiting its practical application. The other drawback of this approach is sensitivity of chlorosilanes to the water traces, which result in their storage and treatment complexity. Oligothiophenes are promising class of semiconducting materials in organic electronics due to their high charge mobility combined with air- and thermal stability.14-16 It was shown that oligothiophene-based OFETs can show mobilities up to 1.1 cm2/Vs for the vacuum evaporated active layer.17,18 The device performance significantly depends on the number of conjugated thiophene rings in the molecule.17,19 Oligomers containing at least four conjugated thiophene units demonstrate high field-effect mobilities up to 0.5 cm2/Vs.18,20,21 Increasing the conjugating length of the oligothiophene fragment leads to higher mobility in OFETs. However, at the same time it dramatically reduces the molecule's solubility, limiting its potential for application in solution processible large area electronics. Only a few examples of oligomers with six and more conjugated thiophene rings with sufficient solubility are described in the literature.22,23 Thus, the search for a reasonable compromise between the electronic properties and the ease of processing is very important.24-26 Langmuir-Blodgett (LB) technique is a process of self-assembling of molecules at the air-water interface under a barrier pressure followed by transfer of the thin film formed onto a substrate under a constant surface pressure. LB technique can be successfully applied to OFET fabrication as a fast alternative to the solution self-assembling.27-33 In the case of diphilic molecules LB can 3 ACS Paragon Plus Environment

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lead to formation of a monolayer film,34 which can be used as an active layer in OFETs. The functionalized oligothiophenes deposited with Langmuir-Blodgett technique show mobilities up to 0.01 cm2/Vs.35 Similar results were recently reported for multilayer LB films of diperylene bisimide.36 Therefore, LB OFETs can show the electrical performance close to that of the solution processed SAMFETs. Moreover, in LB OFETs, the interface chemistry between the dielectric and the active layer is not essential35 as opposite to SAMFETs allowing fabrication of monolayer LB OFETs from appropriate chemically inert semiconducting molecules. Herein we report the self-assembly of solution-processible non-functionalized (chemically inert) tetramethyldisiloxane dimer of α,α’-dialkylquaterthiophene D2-Und-4T-Hex (Figure 1) on the water-air interface, which leads to formation of a highly-ordered self-assembled monolayer film. These films were transferred to a Si/SiO2 dielectric substrate by LB technique and studied by grazing-incidence X-ray diffraction, X-Ray reflectivity and AFM methods, which proved their monolayer thickness and 2D crystalline structure. OFETs with the dimer LB films as a semiconducting layer show hole mobilities up to 0.003 cm2/Vs, which are as high as those measured for the same material in the bulk37 or measured for SAMs prepared from water vapor sensitive chlorosilyl derivatives of quaterthiophene by solution processing.38

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Figure 1. Chemical structure of the quaterthiophene dimer D2-Und-4T-Hex studied in its extended (a) and closed (b) conformation and schematic of its self-assembly on the water-air interface without (c) and under (d) barrier pressure as well as the final structure.


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Quaterthiophene dimer The

quaterthiophene

dimer,

1,3-bis[11-(5‴-hexyl-2,2′:5′,2″:5″,2‴-quaterthiophen-5-yl)-

undecyl]-1,1,3,3-tetramethyldisiloxane,

D2-Und-4T-Hex

(Figure

1)

was

designed

and

synthesized as described earlier.37 The inclusion of long alkyl chains in α and α’-positions of oligothiophenes provides their good solubility in widely used organic solvents, such as toluene, THF or chloroform, as well as improves their air-stability and crystallinity.16,39 The presence of tetramethyldisiloxane (-Si(CH3)2-O-Si(CH3)2-) group capable to hydrogen bond formation with water molecules enables processing of this dimer by LB self-assembling technique. The dimer D2-Und-4T-Hex is stable under normal conditions and does not hydrolyze in the presence of water

as

opposite

to

the

chlorosilane

derivatives

of

oligothiophenes

investigated

before10,11,15,16,37,40. Due to unique flexibility of the disiloxane unit, the dimer can easily transform from its extended (Figure 1,a) to the closed conformation (Figure 1,b), which can be additionally stabilized by the intramolecular π-π staking interactions of the quaterthiophene units, leading to a 2D crystalline monolayer on the air-water interface (Figure 1,d), as will be shown below. It should be noted that self-assembly of the dimer D2-Und-4T-Hex into a monolayer film directly on the substrate (Figure 1,e) is not possible by the standard procedure from solution since the dimer does not have any reactive anchor functions like chlorosilyl- or phosphonic acid groups.

Self-assembly on the air-water interface First of all, self-assembly of D2-Und-4T-Hex on the air-water interface was investigated by Langmuir technique. Figure 2 presents a Langmuir isotherm with characteristic Brewster angle microscopy (BAM) images of the water surface. The Langmuir isotherm of the first compression has four clearly distinguished regions that could be interpreted by the following way. The first region (from the surface pressure of ca. 900 to ca. 400 Å2) corresponds to the presence of 6 ACS Paragon Plus Environment

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separate islands of the dimer molecules interacting with each other; thus the surface pressure does not increase with the area decreasing. As for the internal island structure, Brewster micrographs allow to suppose that some of the islands are crystalline and oligothiophene fragments are oriented vertically inside of these islands, while the other part of them are amorphous with the oligothiophene fragments lying on the water surface as shown in Figure 1,c. They look like areas with different brightness on the Brewster micrographs that correspond to different layer thicknesses. The 2nd region (from the surface pressure of ca. 400 to ca. 230 Å2) corresponds to the monolayer compression when the same islands are in contact. That is why increasing the surface pressure with decreasing the film area is observed on the Langmuir isotherm. The 3rd region (around the surface pressure of ca. 200 Å2) corresponds to some kind of a “phase transition”, when the lying oligothiophene fragments (Figure 1,c) change their horizontal orientation to the vertical one (Figure 1,d), like it was described before for chlorosilyl quinquethiophene SAMs34 and similar rigid mesogenic groups of disiloxane-containing dendrimers41 on the water-air interface. For this region the surface pressure again ceases to increase with the layer compression. The 3rd region is relatively short as compared to the similar one observed in Ref.34, which could be explained by a tendency of D2-Und-4T-Hex to crystallize and partially form crystalline islands with vertical orientation of the oligothiophene fragments directly after spreading (i.e., even before compression). The 4th region (from the surface pressure of ca. 180 to ca. 100 Å2) corresponds to compression of the crystalline monolayer and it is characterized by a sharp increase of the surface pressure. Additionally, the Langmuir isotherm under the 1st cycle of the film compression-decompression shows significant hysteresis that could be explained by a weak tendency of the crystallized oligothiophene units to spread again under a reduced surface pressure and slow return of the compressed monolayer to the initial state during the film decompression.34 This suggestion could be illustrated by a Brewster micrograph in Figure 2 obtained at a surface 7 ACS Paragon Plus Environment

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pressure of 1.5 mN/m during the film decompression: the image looks like “crushed ice” with clearly defined edges corresponding to the crushed crystalline monolayer.

Figure 2. Langmuir isotherm for D2-Und-4T-Hex (1st compression-decompression cycle) and BAM micrographs of Langmuir films at different surface pressures.

Preliminary investigations of the structure and morphology of LB films obtained were performed by polarization optical microscopy (POM) and AFM techniques (Figure 3). The POM data clearly indicate that the LB film is macroscopically homogeneous and has very nice coverage with just small cracks looking like light stripes crossing the electrodes. The AFM data confirm that the film consists of homogeneous roundish domains having the overall dimensions of approximately 5 µm with small regions where the domains overlap along their boundaries that look like brighter stripes in the AFM image. Each domain corresponds to a uniform monolayer with a thickness of around 4 nm. Note that the experimentally determined monolayer thickness is in a good agreement with the half of the theoretically calculated length of D2-Und-4T-Hex molecule in its extended conformation (86 Å), that corresponds to a vertical orientation of the 4T units in the monolayer. More detailed structural investigation of the LB films obtained was made


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by grazing incidence X-ray diffraction and X-ray reflectivity techniques, which deserves a special consideration.

Figure 3. POM (a) and AFM (b,c) data of a LB film of D2-Und-4T-Hex on a silicon substrate with gold electrodes. The film was transferred at the surface pressure of 25 mN/m. The surface profile (c) was extracted along the vertical white line in the AFM image (b).


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X-Ray investigation of the LB films structure X-Ray reflectivity scans (Figure 4) of the LB films obtained by the transfer of Langmuir layers onto the silicon substrate have shown the presence of monolayers with the thickness δ of about 45.5 Å for both π = 8 and 25 mN/m. As the length of the dimer molecule in its’ extended state (Figure 1,a) is 86 Å, while in the closed conformation (Figure 1,b) it is 43.2 Å, one can suppose that the layer is formed by self-assembling of the dimer molecules to the silicon substrate through disiloxane groups in their closed conformations. However, the layers investigated are characterized by substantial roughness, probably, formed by overlapping of the monolayer platelets edges clearly seen on the AFM images (Figure 3,b), which manifests itself in the shape of the reflectivity curves possessing only two pronounced minima. Unfortunately, such poor patterns do not allow a reliable reconstruction of the electron density profile along the normal to the substrate.

Figure 4. X-Ray reflectivity curves for LB films formed at the surface pressure of 8 mN/m and 25 mN/m.

A driving force for the layer self-assembly is formation of extended crystallites by oligothiophene fragments through their π-π stacking, resulting in the two-dimensional centered orthogonal herring-bone lattice with parameters close to those found in polythiophene.42 Grazing 10 ACS Paragon Plus Environment

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incidence X-Ray diffraction patterns of the films prepared at the surface pressure π = 25 mN/m (Figure 5b) revealed the development of highly ordered crystal lattice with parameters a = 7.96 Å and b = 5.65 Å. Table 1 lists the reflections observed and parameters of the crystal lattice for thin films of the dimer studied, bulk samples of the same material,37 and for the polythiophene.

Figure 5. Grazing incidence X-Ray diffraction patterns for LB formed at surface pressures of 8 mN/m (a) and 25 mN/m (b). Plot (c) is the intensity scan of the pattern (b) along qxy.

d110, Å d200, Å d210, Å a, Å b, Å

LB Film

Bulk Material

Polythiophene

4.63 3.99 3.27 7.98 5.69

4.53 3.99 3.23 7.98 5.52

4.52 3.92 3.19 7.83 5.52

Table 1. The reflections observed and parameters calculated for crystal lattice of LB films of the quaterthiophene dimer studied, bulk samples of the same dimer and for the polythiophene.


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Their comparative analysis allows drawing several important conclusions. First, parameter a is the same for LB films and the bulk quaterthiophene dimer, while it is higher than those of the polythiophene due to the effect of attached aliphatic tails. Second, parameter b is very close for the bulk quaterthiophene dimer and pure polythiophene, while in the LB films it is 3.1 % higher. One can find an explanation of this effect by investigating the shape of 110 and 200 Bragg rods on Figure 5c (210 reflection is too weak for such analysis). While the latter reflection is virtually perpendicular to qx axis, the former one is substantially tilted. Thus, looking along vector a direction, quaterthiophene fragments are perpendicular to the substrate, while the tilt director close to vector b, with the tilt angle of 14°. This value is consistent with the structure of the thin monolayer films prepared from the chlorosilyl derivative of quinquethiophene reported earlier.34 Furthermore, changing of the corresponding d-spacing calculated from the observed tilt angle ϕ is ∆ܾ 1 = − 1 = 3.1% ܾ cos ߮ Thus, very good coincidence of ∆b calculated using two different methods – from the positions of Bragg rods and from the tilt angle, proves that the director of tilt could not substantially deviate from vector b of the crystal lattice. The thickness of crystalline layer estimated from the azimuthal half-width of 110 reflection is 19 Å, which is in a good agreement with the length of quaterthiophene segments (17 Å). This value is different from the total thickness of the LB monolayer, since it consists of not only crystalline 4T fragments, discussed above, but also undecyl and hexyl aliphatic tails as well as tetramethyldisiloxane groups, which are not crystallized in the LB monolayer due to flexibility of the former and bulkiness of the latter.


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The results of molecular modelling (Figure 6) proved the conclusions drawn from the X-Ray structural data. The model suggests that oligothiophene segments of the same molecule are arranged along the direction of crystallographic vector b, their long axis being tilted respectively to the substrate normal by 14°. Note that the charge carrier mobility in oligothiophenes-based systems has a direct correlation with the packing density of the oligothiophene blocks manifesting itself in the cross-section area of the crystalline sublattice formed: the less the crosssection area – the higher the charge carrier mobility.37 Monolayer LB films described in this paper possess the crystalline sublattice of quaterthiophene, which is only 5% less dense than that of polythiophene. This fact explains the observed good semiconducting properties of the dimer D2-Und-4T-Hex, discussed below.

Figure 6. Structure of the monolayer of D2-Und-4T-Hex as devised from structural data and molecular modelling. Side views in ac (a) and bc (b) projections, for the top view (c, ab projection) only the quaterthiophene lattice is shown. The same colors correspond to the segments belonging to one dimer molecule.


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Another important parameter having close correlation with charge carrier mobility in thiophene-based materials is a stacking distance between two neighboring thiophene rings. As one can see on Figure 6c, face-to-face π−π stacking takes place along [010] direction, while edge-to-face π−π stacking is along [110] vector of the crystal lattice. Corresponding values of parameter b and d110 are given in Table 1. It can be seen from the data presented that these values very close for the LB film, bulk material and the literature data for polythiophene.42

Electrical studies To estimate the semiconducting properties of the LB films obtained, they were transferred onto pre-structured Si/SiO2 substrates with gold electrodes. Figure 7 demonstrates transfer (a) and output (b) characteristics of a monolayer LB OFET with 30 um channel length. The latter shows that the drain current at low voltages is proportional to the drain voltage, while at higher voltages the current saturates. Such behavior is typical for p-type OFETs. Note that the contact effects are small that follows from the linear output characteristics near zero drain voltage, which are focused into the zero point.43,44 A threshold voltage VT was found to be around 15V, while it varied in the range of ±10V for several transistors on the same substrate. The linear and saturated hole mobilities are virtually the same in the range of 10-3 cm2/Vs with the highest measured value of 3×10-3 cm2/Vs. The OFETs show current modulation of five orders of magnitude between open and closed states for all measured devices. Note that all 24 devices produced have demonstrated electrical performance with small mobility distribution (see Supporting Information, Figure S1). The transistors were found to be very stable under normal conditions and the hole mobility decreased only by 4% after one month storage of the devices in air (see Supporting Information, Figure S2). Moreover, the devices showed very small hysteresis that was 14 ACS Paragon Plus Environment

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visible only near and below the threshold voltages (V>Vg, Figure 7b) indicating very good electrical performance of the devices.

Figure 7. Transfer (a) and output (b) characteristics of a monolayer LB OFET with 30 µm channel length. The ×10-3 cm2/Vs correspondingly. estimated saturation and linear mobilities were found to be 3.12× ×10-3 cm2/Vs and 2.64×

Since LB technique allows fabrication of not only mono-, but also multilayered devices, this possibility was used to prepare three-layers LB OFETs as well. Remarkably, these devices showed the electrical performance similar to that of the monolayer OFETs (see Supporting Information, Figure S3). This is in strong contrast with the data reported in Ref. 36 for LB OFETs on the base of diperylene bisimide, where the monolayer device showed negligible mobility, and only two or more molecular layers of the organic semiconductor lead to reasonable electrical performance. According to Ref. 45, in multilayer organic thin film transistors, the charge transport is governed by percolation across the first layer and mediated by the second layer. The similar mobility observed for one-, three-layered LB OFETs in this work can be explained by a very high quality of the first layer, which is enough to provide a path for the efficient charge transport. Therefore, no second or further semiconducting layers are required to mediate the charge transport. Moreover, the hole mobility in bulk films of D2-Und-4T-Hex deposited by spincoating was 0.003 cm2/Vs,37 which is exactly the same as the value measured in LB films in this 15 ACS Paragon Plus Environment

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work. This fact confirms that in the bulk film charge transport in OFETs occurs mainly in onetwo molecular layers close to the gate dielectric. It is worth to mention that LB film of D2-Und-4T-Hex interacts with the substrate only via weak hydrogen bonds without any strong covalent bonding (as described in Section 2). The similar situation takes place in Ref. 22,23, where oligothiophene monolayer is formed by physical adsorption on the substrate. However, the presence of disiloxane (Si-O-Si) group in D2-Und-4THex capable to hydrogen bond formation with water molecules enables processing of this dimer by self-assembling on the water-air interface by LB technique. It is not possible to make such monolayer by self-assembly from solution as in the case of chlorosilanes, which involves chemical reactions. Moreover, the electrical properties of SAMs based on quaterthiophene chlorosilane described in Ref.

38

and D2-Und-4T-Hex LB films are virtually the same, allowing

us to conclude that the presence of strong covalent binding between the semiconducting layer and the substrate is not necessary for effective monolayer OFET fabrication.

Conclusion To sum up, we have developed a monolayer OFET based on highly ordered Langmuir-Blodgett film of quaterthiophene disiloxane dimer D2-Und-4T-Hex. This substance is nicely soluble in widely used organic solvents, such as toluene, THF or chloroform, due to relatively short thiophene core, while the film deposition does not require special environment conditions due to high chemical stability of the disiloxane group as compared to the chlorosilane group reported before.38 The field-effect hole mobility was measured as high as 3×10-3 cm2/Vs, which is among the best results obtained for monolayer devices. The mobility has very close value to those reported for the same material in the bulk or for SAMs prepared from chlorosilyl derivatives of quaterthiophene by solution processing. The electrical performance of the LB OFETs did not 16 ACS Paragon Plus Environment

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significantly decrease after one month storage in air. These findings show high potential of the disiloxane-based LB films in monolayer organic field-effect transistors for large-area electronics.

Methods Materials. Synthesis of 1,3-bis[11-(5'''-hexyl-2,2':5',2'':5'',2'''-quaterthiophen-5-yl)undecyl]1,1,3,3-tetramethyldisiloxane (quaterthiophene-based disiloxane dimer D2-Und-4T-Hex) was described elsewhere.37 Toluene (Acros) was distilled over sodium before use. Isopropanol (Acros) was used as received. Langmuir and LB films. The spreading solution was prepared by dissolving D2-Und-4T-Hex in toluene at the concentration of 0.33 g/L. The solution was spread on the water surface with a microsyringe, and the film was then left for 5 min to equilibrate before the compression started. Data were collected with a Nima 712BAM system equipped with Brewster angle microscope MicroBAM2 using a Teflon trough and barriers at room temperature. Ultrapure water obtained from an Akvilon deionizer D-301 system was used for the subphase. The monolayers were compressed with the speed equal to 100 mm/min. LB films were obtained by transfer on silicon substrates with or without gold contacts that were freshly activated by oxygen plasma treatment during 15 min followed by a cleaning in pure isopropanol. The vertical dipping method with a dipping speed of 8 mm/min was used to obtain monolayer films. Film transfers were performed at different surface pressures close to the collapse point: 1) at 25 mN/m, corresponding to the most condensed phase in the monolayer and 2) at 8 mN/m in order to estimate structure and morphology of the layer at lower compression. Substrates. OFETs were fabricated in the bottom-contact bottom-gate geometry. The heavily p-doped silicon wafer and thermally grown 280-nm-thick oxide SiO2 layer was used as a gate electrode and gate insulator, respectively. Gold electrodes thermally evaporated through the 17 ACS Paragon Plus Environment

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shadow mask were used as a source and drain electrodes with channel lengths 30 µm and channel width 1000 µm. On each substrate five OFETs were tested. The gate dielectric capacitance was 13 nF/cm2. Thin film characterization. AFM studies were performed with NT-MDT Solver NEXT instrument in a semicontact mode under ambient environment. Commercially available silicon probes Brücker FESPA with resonance frequency 70 kHz were used. Polarized optical microscopy images were obtained with Carl Zeiss Axioscop A40Pol. X-ray measurements. Grazing incidence X-Ray diffraction and X-Ray reflectivity experiments were performed on at P08 beamline, PETRA III synchrotron radiation source (Hamburg, Germany). Incident X-Ray beam of the energy of 15 keV probed monolayers with the beam sizes of 200 µm and 50 µm in vertical and horizontal directions, respectively. Scattered radiation was monitored by a 2D CCD detector. During the experiment, samples were kept under helium atmosphere to reduce radiation damage. X-Ray analysis. X-Ray reflectivity data were analyzed using StochFit program which utilizes stochastic fitting methods to model specular reflectivity curves. The obtained distributions of electron density were afterwards interpolated by three slab models with subsequent solution of scattering problem and following reconstruction of reflectivity curves. The electron density ρ = 2πδ/λ2γe where γe is the classical electron radius equal to 2.814·10-5Å, and δ is the dispersion coefficient,46 as well as thickness d and roughness R of monolayers were calculated. Electrical Measurements were performed with Keithley 2636A source-meter in air at room temperature in darkness. Linear and saturated field-effect mobilities were extracted from the transfer characteristics using Shockley’s gradual-channel model with linear fit in the corresponding voltage range. The total measurement time for one curve in both direct and reverse directions was about 20 seconds. 18 ACS Paragon Plus Environment

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Molecular modeling. Accelrys Materials Studio® program set was employed for molecular modeling of the studied compounds. We used two sets of potentials, which allow taking into account non-covalent interactions of oligothiophene groups: COMPASS (Condensed-phase Optimized Molecular Potentials for Atomistic Simulation Studies) and UFF (Universal Force Field). The COMPASS set is suitable for modeling of isolated molecules and condensed phases of mainly organic, polymeric and of some inorganic compounds,47-49 it also allows parameterizing partial charges and valence ab initio with the subsequent system optimization. To prove the results of the modelling, we applied UFF potentials, used for the calculation of geometry of different organic molecules as it does not have any limitation on the chemistry of compounds involved.50-52 ASSOCIATED CONTENT Supporting Information. Supplementary data consists of electrical properties of D2-Und-4THex three-layered transistor and monolayer OFET after one month air exposure. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding author Sergey A. Ponomarenko e-mail: [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. 19 ACS Paragon Plus Environment

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ACKNOWLEDGMENT This work was supported by RFBR (grants 14-03-00873 and 13-03-12472), Council on grants of the President of Russian Federation for State Support of Young Russian Scientists (grant MK6878.2013.3) and Russian Academy of Sciences (program OKh-3), SG is grateful to BMBF (project No. 05K13PS4) for financial support. The authors are thankful to the P08 beamline staffs at PETRA III, Hamburg for experimental support.

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