Large Scale Alignment and Charge Transport ... - ACS Publications

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Large Scale Alignment and Charge Transport Anisotropy of pBTTT Films Oriented by High Temperature Rubbing Laure Biniek,† Nicolas Leclerc,‡ Thomas Heiser,§ Rony Bechara,§ and Martin Brinkmann†,* †

Institut Charles Sadron, CNRSUniversité de Strasbourg, 23 rue du Loess, Strasbourg 67034, France Institut de Chimie et Procédés pour l’Energie, l’Environnement et la Santé, UMR 7515, ECPM, 25 rue Becquerel, 67087 Strasbourg Cedex2, France § Laboratoire ICube, Département ESSP, CNRSUniversité de Strasbourg, 23 rue du Loess, Strasbourg, 67034, France ‡

S Supporting Information *

ABSTRACT: A large-scale alignment method is used to orient the conjugated polymer poly(2,5-bis(3-dodecyl-2-yl)thieno[3,2-b]thiophene) (C12-pBTTT). This fast one-step alignment method does not use any alignment layer and does not necessarily require postalignment annealing of the films. It exploits the increased plasticity of the conjugated polymer films for 50 °C ≤ T ≤ 125 °C to obtain high in-plane orientations by mechanical rubbing of the films. As visualized by HR-TEM, the in-plane alignment of C12-pBTTT chains and size of the oriented domains increase with the temperature of the film during rubbing (Tr). The domains have a preferential face-on orientation; i.e., the π-stacking direction is along the film normal and the chain direction parallel to the rubbing direction. Postrubbing annealing at T < 200 °C can further improve in-plane alignment whereas for T ≥ 200 °C, edge-on oriented C12-pBTTT crystals are formed. The anisotropy of hole transport for highly in-plane oriented face-on as well as edge-on oriented films was measured in OFET devices. Depending on the annealing conditions, this anisotropy of hole mobility varies in the range 7−70 with the highest mobilities along the rubbing direction and the highest anisotropies for the oriented face-on films.

I. INTRODUCTION In polymeric semiconducting materials, both molecular and crystalline orientations determine optical, electronic, and optoelectronic properties in thin films since these properties are by essence highly anisotropic. In regioregular poly(3-hexylthiophene), charge mobilities are highest along the chain and the πstacking directions and much lower along the side chain direction.1−4 Therefore, it is of interest to control the contact plane of crystalline domains in the active layers as it determines, for instance, the direction of facile charge transport in the cases of organic solar cells and organic field effect transistors.5 However, the absence of in-plane preferential orientation further limits charge transport as adjacent domains may be separated by high-tilt grain boundaries that are detrimental to long-range charge transport.6 Uni- or biaxial orientation in thin films are also of interest for structural investigations that e.g. help determine macromolecular packing schemes in semiconducting polymers.7,8 For organic electronic device applications, the design of dense arrays of OFETs implies to find procedures that reduce parasitic current paths between adjacent OFETs. One such method implies preferential alignment of the active layer to achieve anisotropic charge transport.9 In most previous reports, alignment of semiconducting polymers was obtained by resorting to some alignment layer such as rubbed polyimide,10−12 friction-transferred poly(tetrafluoroethylene),13 or hot-drawn polyethylene.14 The presence of the alignment © 2013 American Chemical Society

layers often requires the design of specific device architectures, e.g., inverted OFETs, to exploit the resulting anisotropic properties (charge transport, electroluminescence, fluorescence). Therefore, developing simple orientation methods that avoid the use of alignment layers and are applicable to different types of substrates (SiO2, flexible polymers) is highly desirable. These processes should be readily upscaled especially in the perspective of roll-to-roll fabrication schemes. This is not the case of other methods such as directional epitaxial crystallization15,16 or zone casting.17,18 The latter methods can align polymers such as pBTTT or P3HT without the use of an orienting substrate. Although very effective in generating highly oriented and crystalline films, these methods imply a slow oriented growth with characteristic growth rates of ∼20 μm/s. Such slow growth processes are however difficult to transpose to large scale device manufacturing. Herein, we focus on a simple alignment method applied to poly(2,5-bis(3dodecylthiophene-2-yl)thieno[3,2-b]thiophene) (C12-pBTTT) based on mechanical rubbing. This polymer is particularly relevant as it belongs to a class of conjugated polymers that exhibit very high field effect mobilities in the range 0.1−1 cm2/ V·s as reported for thin films of the C14-analogue.19,20 The Received: March 11, 2013 Revised: April 24, 2013 Published: May 10, 2013 4014

dx.doi.org/10.1021/ma400516d | Macromolecules 2013, 46, 4014−4023

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contact plane of the crystalline domains can be changed. The face-on → edge-on transformation induced by annealing has been followed in situ by transmission electron microscopy (TEM) using electron diffraction. Finally, charge transport measurements in bottom gate, bottom contact OFET configuration were performed to probe the anisotropy of charge transport.

molecular structure of C12-pBTTT is depicted in Figure 1. Orientation control of pBTTT has been achieved using flow

II. EXPERIMENTAL SECTION Materials. All reagents and chemicals were purchased from commercial sources (Aldrich, Across, Fluka) and used without further purification. All reactions were carried out in an argon atmosphere. All solvents were distilled over an appropriate drying agent prior to use and were purged with nitrogen. The synthesis of 2,7-bis(trimethylstannyl)thieno[3,2-b]thiophene has been described in the literature.19 1H and 13C NMR spectra were recorded on a Bruker Avance 300 and a Bruker 400 Ultrashield NMR spectrometers, with an internal lock on the 2H-signal of the solvent (CDCl3). Size Exclusion Chromatography (SEC) measurements were performed with a Waters Alliance GPCV 2000 instrument (Milford/MA) that incorporates a differential refractive index and a viscosimeter. 1,2,4-Trichlorobenzene was used as the mobile phase at a flow rate of 1 mL/min at 150°C. It was stabilized with 2,6-di(tert-butyl)-4-methylphenol. The polymer was injected at a concentration of 1 mg·mL‑1. The separation was carried out on three Agilent columns (PL gel Olexis 7 × 300 mm) protected by a guard column (PL gel 5 μm). Columns and detectors were maintained at 150 °C. The Empower software was used for data acquisition and analysis. The molecular weight distributions were calculated with a calibration curve based on narrow polystyrene standards (Polymer Standard Service, Mainz), using only the refractometer detector. The polymer was synthesized by a Stille cross-coupling reaction in toluene using tris(dibenzylideneacetone)dipalladium and tri(o-tolyl)phosphine as catalyst. At the end of the polymerization time, 2bromothiophene and 2-trimethylstannylthiophene were added to endcap and stabilize the polymers. After purification by Soxhlet extractions, the chlorobenzene polymer fraction was isolated and analyzed by means of high temperature SEC (HT-SEC) at 150°C in 1,2,4-trichlorobenzene (TCB) solutions (these copolymers tend to aggregate in solution at room temperature). A reasonable numberaverage molecular weight (Mn) of 27 kDa and a satisfactory PDI of 1.68 have been obtained. Thin Film Preparation and Orientation. Orientation of the films by mechanical rubbing follows the procedure described in an earlier work for the preparation of polycarbonate alignment layers.25−27 It involves mainly two steps: (i) the preparation of a pBTTT film by the doctor blade method from a 4 wt % solution in ortho-dichlorobenzene (o-DCB) on a clean glass substrate maintained at 180 °C and (ii) rubbing of the as-deposited films with a microfiber tissue. As shown in Figure 1, the rubbing machine is composed of a rotating cylinder (4 cm diameter) covered by a microfiber cloth. The rubbing is performed by applying the rotating cylinder with a 2 bar pressure on the translating sample holder (1 cm/s) and it takes accordingly 5 s to align a 5 cm long pBTTT film. The sample holder can be heated to the desired temperature during the rubbing process. The sample temperature is allowed to equilibrate for 1−2 min before rubbing. A rubbing cycle is characterized by the so-called rubbing length i.e. the length of the rubbing tissue applied on a given point of the sample. In the present case, it is 50 cm. Structural Analysis. TEM oriented areas were identified for TEM analysis by optical microscopy (Leica DMR-X microscope). The C12pBTTT films were coated with a thin amorphous carbon film and removed from the glass substrate by floating on a diluted aqueous HF solution (10 wt %) and subsequent recovery on TEM copper grids. TEM was performed in bright field, high resolution and diffraction modes using a CM12 Philips microscope equipped with a MVIII (Soft Imaging System) charge coupled device camera. In situ annealing TEM measurements were performed by using a PW6592 sample holder and a PW6363 heating temperature controller (Phillips). To estimate

Figure 1. (a) Schematic illustration of the experimental setup used for the rubbing of polymer films at high temperature. (b) Structure of C12-pBTTT polymer. (c) Schematic representation of edge-on and face-on oriented C12-pBTTT nanocrystals. The orientation of the unit cell axes is indicated.

coating or stretch-orientation on a polymeric substrate.17,18 While flow coating leads indeed to high in-plane alignments, it requires a subsequent annealing step at T ≥ 240 °C to obtain highly oriented films. In the present communication, we introduce a different orientation process that leads to very high in-plane alignment in a very simple and fast one-step process without the need for subsequent annealing. However, such an annealing can further improve orientation and/or change the preferential contact plane of the crystalline domains on the substrate while preserving in-plane orientation. The method is based on a mechanical rubbing, as illustrated recently in the case of P3HT.16,21 Rubbing (or buffing) is a well-known orientation method, widely used in the liquid crystal display industry to prepare polyimide alignment layers.22,23 During rubbing, shear forces align polymeric chains over a thickness of up to 60 nm in polyimide layers.23 Recently, rubbing was applied to the orientation of P3HT. In addition to the expected alignment of P3HT chains along the rubbing direction R, P3HT domains were observed to change from initially edge-on to face-on orientation (π-stacking perpendicular to the film plane), which is of interest for photovoltaic applications. In the present study, the temperature of the polymeric film (Tr) is controlled during mechanical rubbing. This is at variance with most studies reported so far for which rubbing is performed at room temperature.23,24 As shown hereafter, Tr controls the level of in-plane alignment, hence the charge transport anisotropy. In particular, rubbing at 75 °C ≤ Tr ≤ 125 °C induces a single biaxial alignment, i.e., a well-defined inplane orientation of the pBTTT chains and a unique contact plane of the domains corresponding to pure face-on orientation. Depending on the temperature used in a subsequent annealing, the in-plane alignment can be further increased and/or the 4015


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Figure 2. (a and b) POM images under crossed polarizers showing the highly oriented C12-pBTTT film obtained by rubbing at T = 125 °C. The rubbing direction R is indicated by a blue arrow. The orientation of the crossed polarizers is indicated in both images by two arrows. (c) ED pattern of as-deposited C12-pBTTT films and (d) ED pattern after rubbing at T = 125 °C. (e) UV−vis polarized optical absorption of the oriented C12pBTTT film with parallel and perpendicular incident light polarization. beam damage of the C12-pBTTT sample during the in situ TEM measurement, a spot size of 11 was selected and the time-dependence of the 1 0 0 peak intensity evolution was recorded. It varied less than 10% over 3 min continuous exposure to the electron beam, which indicates little structural beam damage during the entire in situ experiment. The temperature was increased by steps of 12−13 °C and allowed to stabilize for 1 min at each step. In those periods, the electron beam was blanked by a shutter. After temperature stabilization, the beam shutter was opened for 2 s to acquire the ED pattern. Calibration of the reticular distances in the ED patterns was made with an oriented PTFE film. Differential scanning calorimetry (DSC) experiments were performed on a DSC Q2000 from TA Instrument under argon. The DSC temperature ramps were 10 °C/ min and 5 °C/min for heating and cooling cycles, respectively. Anisotropy of Charge Transport and Optical Absorption. The orientation of the C12-pBTTT films was probed by UV−visible absorption (300−800 nm) using a Shimadzu UV-2101PC spectrometer with polarized incident light and spectral resolution of 1 nm. Bottom contact field-effect transistors (FETs) were elaborated on prepatterned test structures whose source and drain contacts were composed of a 30 nm thick gold layer on top of a 10 nm thick indium tin oxide (ITO) layer. A 230 nm thick silicon oxide was used as gate dielectric and n-doped (3 × 1017 /cm3) silicon crystal as gate electrode. The channel length (L) and channel width (W) were 20 μm and 10 mm, respectively. These electrode patterns were oriented along two directions on the substrates at 90° relative orientation so that charge transport could be measured both parallel and perpendicular to the rubbing direction. The transistor substrates were cleaned by sonication in acetone and isopropanol at 45 °C for 15 min in each solvent. After drying under nitrogen, the substrates were subsequently exposed to an ultraviolet ozone atmosphere for 30 min. Polymer films of ∼50 nm thickness were spin-coated from 4 wt % solutions in o-DCB at 1250 rpm (for 2 min). The polymer solutions were prepared by dissolution at 100 °C for 2h under continuous stirring. All polymer solutions and films were prepared in a nitrogen atmosphere. To ensure identical rubbing conditions for both structural and electronic investigations, two C12-pBTTT films were rubbed during the same run under ambient conditions. After rubbing of the films, the electronic

characterization of the OFETs was carried out in a nitrogen atmosphere using a Keithley semiconductor parametric test system. Field effect mobilities (μsat) were determined from the current− voltage transfer characteristics in the saturation regime using the following equation: μsat (Vg) =

∂Ids,sat L 1 ∂Vg WC i (Vg − VTh)

(1)

Here Ids,sat is the source-drain current, Vg is the gate voltage, Ci is the capacitance per unit area of the gate dielectric, and VTh is the threshold voltage. Postalignment thermal annealing was done in a Linkam hot stage under inert atmosphere at either 100, 200, or 240 °C for 1 min. Heating and cooling rates were 20 and 0.5 °C/min, respectively.

III. RESULTS AND DISCUSSION a. Orientation of the C12-pBTTT Films. Figure 2 shows the POM image of a C12-pBTTT film rubbed at T = 125 °C with the corresponding UV−vis absorption spectra recorded for parallel and perpendicular orientations of the incident light. When the film temperature during rubbing is set to 125 °C, a very high orientation of the films is obtained as evidenced by a very strong birefringence of the films (Figure 2, parts a and b). The UV−vis absorption is highly polarized with a maximum absorbance when the light polarization is parallel to R. The C12-pBTTT films rubbed at 125 °C have typically a dichroic ratio in the 8 to 10 range at 550 nm. When moving from parallel to perpendicular orientation relative to incident light polarization one observes a clear color change of the films from violet to red-brown. It corresponds to a change of the overall UV−vis absorption spectrum whose maximum is shifted from 550 to 520 nm respectively (Figure 2.e). Orientation was further investigated by TEM using electron diffraction. As seen in Figure 2.c, the initial C12-pBTTT films exhibit a typical edge-on orientation of crystalline domains: the 4016


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two characteristic Scherrer rings correspond to the 0 1 0 and the 0 0 3 reflections at d010 = 3.65 Å and d003 = 4.42 Å and the h 0 0 reflections are absent. The C12-pBTTT films rubbed at T = 125 °C have a very different ED pattern. It consists of a sequence of 0 0 l meridional reflections along the rubbing direction with l = 3, 4, and 5 and well-defined and rather sharp h 0 0 equatorial reflections with h = 1−4. This ED pattern indicates that C12-pBTTT chains are aligned along the rubbing direction R and that the crystalline domains have changed the preferential contact plane from (b, c) (edge-on) to (a, c) (faceon) after rubbing (see Figure 1c).28 This behavior is similar to that of P3HT when rubbed at room temperature16 and suggests that the reorientation induced by rubbing from edge-on to faceon is a general behavior for alkyl-substituted thiophene-based semiconducting polymers. b. Effect of Increasing Film Temperature during Rubbing. The influence of the film temperature during the rubbing was further investigated in a systematic way by comparing the level of orientation achieved in C12-pBTTT films rubbed at Tr ranging from 23 to 125 °C. Figure 3.a shows

Table 1. Summary of Hole Mobilities Measured in Rubbed C12-pBTTT Thin Films as a Function of Rubbing Temperature Tr and as a Function of Annealing Temperature for a Film Rubbed at Tr = 100 °Ca rubbing temperature (°C) 23 50 100 annealing temperature (°C) [Tr = 100 °C] 100 200 240

μ∥ (cm2/V·s) −7

3 × 10 2 × 10−4 4 × 10−3 μ∥ (cm2/V·s) −2

1.8 × 10 2 × 10−2 1 × 10−2

μ⊥ (cm2/V·s)

dichroic ratio

− 3 × 10−6 6 × 10−4

1.4 1.9 6

μ⊥ (cm2/V·s)

dichroic ratio

−4

2.5 × 10 1.5 × 10−3 1 × 10−3

6 13 7.5

a

As a comparison of the charge transport anisotropy, the dichroic ratio of the absorption spectrum, measured at 550 nm, has been reported here.

to the liquid crystalline phase formed at around T = 130 °C as indicated by DSC on as-prepared C12-pBTTT powders (see Figure S1, Supporting Information). Accordingly, alignment of C12-pBTTT (Mn = 27 kDa) by rubbing must be performed in the crystalline phase with a partial disordering of the alkyl side chains but cannot be made in the mesophase. The structural modifications in the rubbed C12-pBTTT films with increasing Tr have been investigated by electron diffraction (Figure 3b−f). Films rubbed at RT show only a very weak 1 0 0 reflection in the direction perpendicular to the rubbing R and the initial Scherrer rings characteristic of the edge-on oriented domains are still visible and dominant. Rubbing at 23 °C thus results in marginal alignment of C12-pBTTT chains. For T = 50 °C, additional 2 0 0 and 3 0 0 reflections appear on the meridian together with an arced 0 0 3 reflection along the rubbing direction, both indicative of an onset of chain alignment. As for rubbed P3HT films, the emergence of h 0 0 reflections (h = 1−3) demonstrates the change of contact plane of the crystalline domains from initially edge-on to face-on after rubbing. However, the 0 1 0 reflection characteristic of the edge-on oriented domains is still present, showing that only partial alignment/reorientation of C12-pBTTT domains occurred. For Tr ≥ 75 °C, the 0 1 0 reflection is no longer visible and all ED patterns show essentially the h 0 0 reflections with h = 1−3 and a sequence of 0 0 l reflections with l = 3, 4, 5, and 9. The crystalline domain orientation has changed to faceon over the entire film thickness (50 nm). The presence of high-order 0 0 l reflections indicates a high level of order along the chain direction. This is quite different from rubbed P3HT films that show only smectic-like order upon rubbing i.e. regularly spaced layers of π-stacked chains with a layer spacing of 1.6 nm but no translational order along the chain direction.16 The different ED patterns collected for 75 °C ≤ Tr ≤ 125 °C relates mainly to the quality of in-plane chain alignment. To quantify this effect, the azimutal in-plane distribution of the chain orientation was extracted from the intensity profile of the 1 0 0 reflection distributed around the rubbing direction R. Figure 4 depicts such azimuthal intensity profiles and the inset shows the corresponding fwhm as a function of Tr. The trend is very clear: the higher the Tr, the lower the fwhm of the 1 0 0 peak, i.e., the higher the in-plane alignment of the C12-pBTTT chains. For Tr = 125 °C, the fwhm is 16°, which is quite low and indicates a very high level of chain alignment. Accordingly, the observed increase in the DR of the films at 550 nm is

Figure 3. (a) Dependence of the dichroic ratio at 550 nm with the temperature during rubbing Tr of C12-pBTTT thin films. (b−f) ED patterns of the C12-pBTTT films rubbed at different temperatures in the range 23−125 °C. The rubbing direction R is indicated by a vertical arrow.

the evolution of the dichroic ratio (DR) at 550 nm versus Tr (see also Table 1). The DR increases gradually with Tr from less than 2 for Tr = 23 °C to 8−10 for Tr = 125 °C, indicating that the in-plane alignment of C12-pBTTT chains improves substantially at higher Tr. Rubbing temperatures over 125 °C were not used as most of the polymer layer tended to detach from the substrate for Tr > 125 °C. This behavior must be due 4017


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Figure 4. In-plane azimuthal intensity profiles of the 1 0 0 reflection of rubbed C12-pBTTT thin films as extracted from the nonsaturated intensity of the 1 0 0 reflection in the ED patterns of Figure 3. Inset: Extracted fwhm of the 1 0 0 peak as a function of film temperature during rubbing.

certainly related to the improved in-plane alignment of C12pBTTT chains at higher Tr. c. Evolution of Thin Film Nanomorphology As Probed by Low Dose HR-TEM. Conventional bright field TEM was not suited to investigate the morphology of the rubbed films. It did not show any crystalline lamellae or nanoribbons. Low dose high resolution TEM turned out to be the technique of choice for this analysis and allowed us to observe the nanomorphology of the oriented films as a function of Tr at the length scale of individual layers of π-stacked chains. As for P3HT, the presence of sulfur in the backbone of C12-pBTTT generates a Z-contrast relatively to the layers of interdigitated alkyl side chains.29,30 If successive backbones are regularly π-stacked along the film normal, one can observe a corresponding periodic “fringed“ pattern with a regular layer period corresponding to d100 observed by ED. Figure 5 collects a few representative HR-TEM images of oriented films prepared at different Tr. Although not all areas exhibit fringed patterns, which correspond to stacks of conjugated C12-pBTTT backbones alternating with layers of alkyl side chains in face-on orientation, such patterns were observed for all rubbed films. The typical periodicity of the patterns is in the range 1.88−1.94 nm (see the FFT in Figure 5). As seen in Figure 5, the temperature at which the C12pBTTT films are rubbed has a direct impact on the size of the oriented domains both along the chain direction (lc) and along the C12 side chains (la). The higher Tr, the longer the average stems length and the higher the number of stems along the side chain direction inside an ordered domain. However, the boundaries between ordered and less ordered areas are not as clear as for semicrystalline P3HT films epitaxied on TCB, which show a clear periodic semicrystalline structure.16 The overall nanoscale morphology of the rubbed C12-pBTTT films is more alike that of rubbed P3HT films although translational order along the chain direction is present for C12-pBTTT. Interestingly, for Tr = 125 °C, the C12-pBTTT domain size lc can exceed 50 nm, a value substantially larger than for the best prepared P3HT thin films for which lc hardly exceeds 20 nm.31 Compared to P3HT, the C12-pBTTT marked ordering along

Figure 5. Comparison of low dose HR-TEM images of some highly oriented areas in rubbed C12-pBTTT thin films as a function of Tr. Inset: calculated FFT showing the periodicity of the observed fringed patterns.

the chain direction must result from its higher chain rigidity and the interdigitation of side chains. For Tr = 50 °C, the contrast modulation between backbones and side chains is less regular, indicating disorder at the level of π-stacked C12-pBTTT backbones. The Z-contrast may be reduced or lost in the HRTEM images because: (i) the π-stacking direction differs from the direction of the incident electron beam, (ii) the π-stacking of the chains along the film normal is discontinuous, and (iii) some edge-on oriented domains are present. The present ED and HR-TEM results do not give enough information on the orientation distribution inside the films. For films rubbed at Tr ≤ 50 °C, ED results indicate that only a part of the film is oriented by rubbing. Taken together, ED and HR-TEM results indicate that the alignment of C12-pBTTT chains is limited by the plasticity of the domains. Increasing Tr generates disorder in the layers of alkyl side chains, which results in increased plasticity necessary for efficient chain alignment and crystal reorientation. d. Impact of Thermal Annealing on Structure As Observed by in Situ TEM. Thermal annealing is known to improve in some cases the transport properties in thin films, e.g., by allowing for a partial recrystallization or coarsening of domains. Accordingly, in situ annealing in the TEM was used to follow the structural modifications in thin films of C12-pBTTT and evaluate the optimal thermal annealing for the rubbed films. Since beam damage occurs in a TEM, the exposure conditions necessary to avoid structural alteration of the C12pBTTT films under the electron beam were determined first (see Experimental Section). The evolution of the ED pattern 4018


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upon thermal annealing of the C12-pBTTT film is reported in Figure 6 and 7 for a film rubbed at 100 °C. Figure 6 shows (a)

Figure 6. Results of an in situ TEM experiment on a rubbed C12pBTTT thin film (Tr = 100 °C). The ED patterns are recorded in situ using a heating sample holder: (a) ED recorded in situ at 35 °C, (b) ED for T = 208 °C (note the very intense 1 0 0 reflection), and (c) ED pattern observed after cooling from 240 to 35 °C. (d) Section profiles of the ED patterns along the equator as a function of the temperature.

the ED pattern of as-rubbed films, (b) the pattern of the same area recorded at T = 208 °C, and (c) the ED pattern after subsequent cooling to 35 °C. Figure 6d shows the corresponding section profiles of the ED patterns along the equator. Figure 7 illustrates the temperature dependence of the 1 0 0 peak intensity (amplitude) I100 and the reticular distances d100 and d003 upon heating to 240 °C and subsequent cooling to room temperature. The temperature dependence of I100 shows three ranges: For T < 140 °C, I100 increases monotonously by a factor of 2. The intensity variation becomes steeper around T = 140 °C (which corresponds to the crystal → liquid crystal transition seen in DSC around 130 °C) and decreases rapidly for T > 200 °C. The T-dependence is reversible upon cooling, indicating that the increase in intensity is not due to some improvement of the overall crystallinity but is rather due to a change in the scattering factor of the layered structure when going from the crystalline to the liquid crystalline phase. The intensity of the 0 0 3 reflection follows a different trend compared to the 1 0 0 reflection: it decreases continuously in the entire T-range up to 240 °C. For T > 210 °C, the 0 0 3 reflection has almost disappeared whereas the 1 0 0 reflection reaches its maximum intensity. This result is a clear evidence for the progressive disordering of the interdigitated side chains as T increases while the “lamellar“ structure made of layers of π-stacked C12pBTTT backbones separated by more or less disordered side chains is preserved up to 200°C. The variation of intensity of the 1 0 0 reflection is also accompanied by an expansion of the lattice as d100 increases between 19.4 and 20.2 Å as the temperature increases to 240 °C. This expansion is also reversible upon cooling and overall

Figure 7. Results of the in situ TEM experiment upon annealing and cooling of a rubbed C12-pBTTT film. T-dependence of the 1 0 0 peak amplitude (a), d100 spacing (b), and d003 spacing (c). Measurements performed upon heating and cooling are shown in red and blue, respectively.

similar to that reported for C14-pBTTT films and measured by conventional X-ray diffraction.20 The difference in the temperature effect along the chain axis and along the side chain direction is also manifested in the T-dependence of the d003 distance. Contrary to the expansion of the lattice along the side chain direction, only a very small but sizable and reversible contraction along the chain direction (see Figure 7.c) is observed. More importantly, Figure 6 shows that the annealing at T = 240 °C of the rubbed samples results in a partial reorientation of the domains from a face-on orientation to some edge-on orientation as inferred from the appearance of the 0 1 0 reflection on the equator of the ED pattern in Figure 6.c and the corresponding section profile in Figure 6.d. As a matter of fact, when annealing is performed at a temperature below 200 °C, no 0 1 0 reflection is observed: ED patterns are very similar intensity-wise both before and after annealing with only a slight narrowing of the 1 0 0 peak width along the equator. Therefore, thermal annealing at T ≤ 200 °C does not seem to increase substantially in-plane order. In sharp contrast, for T ≥ 200 °C a face-on → edge-on reorientation of 4019


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for 5 min on spin-coated films prepared initially in a glovebox. The charge mobilities measured in both cases are 0.085−0.10 cm2/V·s and 0.12−0.14 cm2/V·s respectively, indicating no degradation of the field effect mobility upon annealing of the films in ambient. Accordingly, annealing in ambient is not responsible for the lower hole mobilites observed in rubbed films. A second origin lies in the damaging of the crystalline structure of the films during rubbing. To verify this, the evolution of the vibronic structure of the UV−vis absorption spectrum was followed as a function of rubbing and postrubbing annealing (see Figure S4, Supporting Information). Indeed, as reported by Clark et al., the presence of a welldefined vibronic structure in absorption is a sign of high crystallinity in the films.32 The vibronic structure is best defined in the initial doctor-bladed films from o-DCB at 180 °C with two clear components centered at 555 and 600 nm. After rubbing, the vibronic structure is less visible and the spectrum shows a small blue-shift, indicating that some disordering process has occurred. Annealing at T ≥ 200 °C helps to recover some of the 600 nm feature in the spectra, indicating that local crystalline order has been partly restored. This behavior is similar to that reported for rubbed P3HT films.16 To improve the intrinsic hole mobilities in the devices, annealing of the rubbed films was performed in a hot stage under inert atmosphere. Table 1 collects the average parallel and perpendicular hole mobilities, μ∥ and μ⊥ respectively. For the films rubbed at Tr = 100 °C, annealing at 100 °C results in an increase of the mobilities by about 1 order of magnitude along both directions. The same holds true for annealing temperatures of 200 and 240 °C. However, annealing has a more marked effect on μ⊥: it increases from 2.5· 10−4 cm2/V·s for T = 100 °C to 1 × 10−3 cm2/V·s for T = 240 °C. The substantial increase of charge mobilities μ∥ and μ⊥ with annealing must be analyzed in the light of the structural modifications evidenced by TEM. Thus, TEM samples have been prepared directly from the C12-pBTTT thin films rubbed on the OFET devices and submitted to various annealings at 100 °C, 200 and 240 °C for 1 min and slowly cooled to RT. Figure 8 shows the ED patterns corresponding to these annealed samples. As observed in the in situ experiment on doctor-bladed films, the initial face-on configuration after rubbing is still observed after annealing at 100 °C on spin-coated films. Only a slight sharpening of the h 0 0 reflections is observed, indicating a possible increase of the domain size along the alkyl side chain direction. By contrast, for T = 200 °C the face-on → edge-on reorientation of C12-pBTTT domains takes place as indicated by the appearance of a strong equatorial 0 1 0 reflection whereas the h 0 0 reflections nearly disappeared. For T = 240 °C, a total “switch” to the edge-on orientation has occurred. Morphology-wise, these annealed layers do not show the socalled ribbon-phase but a rather featureless and continuous morphology. The trends observed on the annealed samples are similar to those reported for the in situ TEM investigations. Neither the film process condition (spin-coated vs doctor bladed) nor the substrate nature (SiO2 vs glass) modify significantly the morphology and orientation in the rubbed films. However, a marginal discrepancy may be noticed for T = 200 °C as a result of different cooling rates. Indeed, a very slow and controlled cooling rate of 0.5 °C/min was applied in the case of OFET devices while a faster cooling rate of 7−8 °C/min was applied

C12-pBTTT domains takes place. Most interestingly, these in situ experiments also show that the in-plane alignment of pBTTT in rubbed films is not lost upon annealing even to quite high temperatures. This is at strong variance with rubbed P3HT films that loose in-plane orientation readily upon annealing at 100 °C. e. Effect of Tr and Annealing on Charge Transport. The impact of alignment on charge transport was investigated by using the oriented fims of C12-pBTTT as semiconducting layers in field effect transistors. The C12-pBTTT films were spin-coated on the substrates in a glovebox. They were rubbed outside the glovebox to align the polymer films in an ambient atmosphere and then transferred back to the glovebox. Prior to measurements, all samples were placed in a vacuum chamber (10−7 mbar) and pumped overnight in order to remove H2O and O2 to the extent possible. The as-spin coated C12-pBTTT (27 kDa) films show a preferred edge-on orientation of domains and exhibit a hole mobility in the range 0.06−0.1 cm2/V·s. This value lies slightly below that reported by McCulloch et al., i.e., 0.3 cm2/V·s19 for a similar device configuration and a similar polymer molecular weight. However, our measurements are essentially performed on bare SiO2 substrates without OTS treatment which explains the lower values observed in our case. As Tr has been shown to impact the level of in-plane alignment, we have prepared OFETs with an oriented C12pBTTT layer for different Tr. Figures S2 and S3 in the Supporting Information show typical transfer and output characteristics of OFET devices using an oriented C12pBTTT film as semiconducting layer before and after annealing. C12-pBTTT films rubbed at room temperature have a poor in-plane orientation with very low drain currents (below 10−8 A), indicating severe damaging of the film morphology upon rubbing via scratching and removal of material. In contrast, when rubbing is performed at only 50 °C, a very high anisotropy of charge transport is observed: hole mobilities measured parallel (μ∥) and perpendicular (μ⊥) to the rubbing are: μ∥ = 2 × 10−4cm2/V·s and μ⊥ = 3 × 10−6 cm2/V·s, respectively. For Tr = 100 °C, the high in-plane alignment further translates into a higher hole mobility along both parallel and perpendicular directions with μ∥ = 4 × 10−3cm2/V·s and μ⊥ = 6 × 10−4 cm2/V·s. The mobility anisotropies observed in both cases are very high and exceed the previously reported values for the so-called ribbon phase of C14-pBTTT.18 These results are in line with the high orientation observed in TEM for C12pBTTT films rubbed at Tr ≥ 75 °C. Interestingly, the charge transport anisotropy is already quite high for the films rubbed at Tr = 50 °C even though only partial orientation is evidenced by TEM. Since transport in the D/S channel occurs essentially within the few molecular layers at the interface with the dielectric, this result suggests that mechanical rubbing at Tr = 50 °C has aligned the polymer chains at the interface with the SiO2 layer. Although the rubbed films are highly oriented, the absolute values of the charge mobilities lie below those of spin-coated films of C12-pBTTT. We can consider two main explanations for this observation: (i) thermal degradation of the C12pBTTT films occurs during annealing in ambient during rubbing or (ii) the high initial crystalline order is strongly disrupted by rubbing. The possible impact of thermal degradation of a C12-pBTTT film upon annealing at T = 100 °C in ambient has been verified by performing transport measurements prior and after annealing at 100 °C in ambient 4020


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The alignment mechanism of C12-pBTTT is distinct from that observed for P3HT in terms of temperature necessary to achieve orientation. When rubbed at room temperature, P3HT films of low-Mw (7 kDa) align very well leading to anisotropy of mobility in excess of 20.16 Under similar conditions, high-Mw P3HT does not orient well possibly because chain entanglements prevent efficient reorientation of crystalline domains. By contrast, C12-pBTTT films orient only when rubbing is performed at Tr ≥ 50 °C. These differences must be related to the structural differences of C12-pBTTT and P3HT i.e. to the backbone flexibility. First, the C12-pBTTT backbone structure is more rigid than that of P3HT and there is so far no clear evidence for folding in C12-pBTTT. The structure of C12-pBTTT differs from that of form I P3HT in that side chains are strongly interdigitated.28 These features confer strong 3D order and crystallinity to the domains of C12pBTTT in as deposited films (see Figure 9).28 Adjacent layers of π-stacked C12-pBTTT chains are strongly interconnected due to the interdigitation of the side chains, which is not the case for form I P3HT. As a result, at room temperature, mechanical shearing is not able to disrupt ordered crystalline domains, i.e., no plastic deformation of the films is possible. Under these conditions, rubbing at 23 °C causes essentially scratching and rupture of the initially continuous C12-pBTTT film, thus the charge transport is severely decreased. To align efficiently the polymer, rubbing must to some extent disrupt the initial 3D order inside crystalline domains. Increasing the temperature of the C12-pBTTT film results in a progressive disordering of the alkyl side chains, which confers a higher plasticity to the films. This softening of the films allows for the plastic deformation of the layers under shear stress e.g. the conditions for the in-plane orientation of C12-pBTTT backbones along the rubbing direction and reorientation of layers of π-stacked C12-pBTTT chains. However, the temperature for the rubbing does not need to reach the temperature at which C12-pBTTT becomes liquid crystalline (150 °C). b. Charge Transport Anisotropy. Rubbing of C12pBTTT films makes it possible to prepare essentially face-on oriented films with a high in-plane orientation of the chain axis,

Figure 8. Evolution of the ED pattern of the rubbed spin-coated C12pBTTT thin films (Tr = 100 °C) recovered from the OFET devices after annealing at different temperatures. (a) ED of an as-rubbed film at 100 °C. (b−d) ED of the films annealed at 100, 200, and 240 °C, respectively.

during the in situ TEM experiments. Slower cooling rates result in higher face-on → edge-on conversion rates.

IV. DISCUSSION a. Orientation Mechanism, Comparison with P3HT. The gradual increase of DR with Tr seems to relate to the progressive increase in plasticity/softening of the C12-pBTTT films. The plasticity of the C12-pBTTT crystal is due to the progressive disordering of the C12 alkyl chains with an increasing amount of gauche defects as Tr increases between 23 and 100 °C.20 This is supported by the TEM in situ experiments that evidence the loss of translational order along the chain axis whereas the “lamellar” structure along the a axis is preserved.

Figure 9. Schematic illustration of the reorientation of crystalline domains of C12-pBTTT upon mechanical rubbing. The initial films show edge-on oriented domains with a random in-plane orientation of the b and c axes. At T ≥ 50 °C, partial disordering of the dodecyl side chains results in an increased plasticity of the films which is necessary to align and reorient C12-pBTTT nanocrystals toward face-on orientation. 4021


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spectra of C12-pBTTT films prior to and after rubbing and after annealing at different temperatures (Figure S4). This material is available free of charge via the Internet at http:// pubs.acs.org/.

especially for Tr = 100 °C. Subsequent annealing of the rubbed layers at T ≥ 200 °C maintains the film orientation and induces edge-on oriented domains. It becomes thus possible to compare the charge transport anisotropy in essentially similar films but with π-stacking directions at right angles. For face-on orientation, one probes transport along the backbone versus the alkyl side chain direction whereas for edge-on orientation, transport is measured along the backbone versus the π-stacking direction. Table 1 shows that face-on orientation yields the highest hole transport anisotropies. For films annealed at 100 °C, μ∥/μ⊥ > 70 whereas for the best edge-on films (annealed at 240 °C) μ∥/μ⊥∼10. Thus, transport measurements show that μ⊥ is substantially higher for the films with edge-on oriented domains which in turn indicates that the charge mobility along the π-stacking direction is significantly larger than that along the side chain direction. Accordingly, the observed increase of μ⊥ with annealing temperature reflects a more efficient transport perpendicular to the rubbing direction in films composed of a majority of edge-on domains along the π-stacking direction over transport along the alkyl side chain direction in face-on oriented films. This result is in line with those reported for the ribbon-phase of C14-pBTTT oriented by flow-coating. Schuettfort et al. measured an anisotropy of hole mobility μ∥/ μ⊥∼ 431 where μ⊥ was measured along the π-stacking direction and μ∥ along the backbone direction.

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*E-mail: (M.B.) [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the European project Interreg IV Rhin Solar (n° C25) is gratefully acknowledged. The authors acknowledge B. Lotz for fruitful discussions and careful reading of the manuscript, O. Boyron for performing the GPC analysis and C. Saettel for the DSC measurements.

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REFERENCES

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V. CONCLUSION Control of the film temperature during alignment by mechanical rubbing allows to prepare highly in-plane aligned C12-pBTTT thin films especially when the film temperature during rubbing approaches Tr = 100 °C. Besides in-plane orientation of the chains, mechanical rubbing induces the edgeon→face-on reorientation of crystalline domains. For Tr ≥ 75 °C, the films consist of purely face-on oriented domains. The increase of in-plane orientation with Tr is due to the higher plasticity of the films upon disordering of the dodecyl alkyl side chains that give sufficient molecular mobility for the reorientation of crystalline domains. The high in-plane orientation of C12-pBTTT films results in high hole mobility anisotropies as measured in bottom gate, bottom contact OFET devices. Postrubbing thermal annealing at various temperatures modifies both the transport properties and the structure of the films. Highly in-plane oriented films with a pure edge-on orientation of C12-pBTTT nanocrystals are obtained by annealing at 240 °C. The highest hole transport anisotropies are obtained for films with pure face-on orientation. Preliminar results demonstrate that a large palette of both hole and electron-conducting polymers can be efficiently oriented by fine-tuning the film temperature during rubbing. This opens the possibility to probe charge transport anisotropy in a more systematic way in semiconducting polymer films. Finally, the present alignment method is not substrate-dependent and is compatible with alternative device architectures, e.g., top gate top contact OFETs.

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AUTHOR INFORMATION

Corresponding Author

ASSOCIATED CONTENT

S Supporting Information *

DSC data of the C12-pBTTT (Figure S1), output and transfer characteristics of the film rubbed at 100 °C along the direction parallel and perpendicular to the rubbing (Figure S2), output and transfer characteristics of the film rubbed at 100 °C and subsequently annealed at T = 100 °C along the directions parallel and perpendicular to the rubbing (Figure S3), UV−vis 4022


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Large Scale Alignment and Charge Transport ... - ACS Publications

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