Enhancement of Out-of-Plane Mobilities of Three Poly(3

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Enhancement of Out-of-Plane Mobilities of Three Poly(3-alkylthiophene)s and Associated Mechanism Daisuke Kajiya, Tomoyuki Koganezawa, and Ken-ichi Saitow J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b06833 • Publication Date (Web): 26 Sep 2016 Downloaded from http://pubs.acs.org on October 3, 2016

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The Journal of Physical Chemistry

Enhancement of Out-of-Plane Mobilities of Three Poly(3-alkylthiophene)s and Associated Mechanism Daisuke Kajiya,† Tomoyuki Koganezawa,‡ and Ken-ichi Saitow†,§,* †

Natural Science Center for Basic Research and Development (N-BARD), Hiroshima University, 1-3-1 Kagamiyama, Higashi-hiroshima, Hiroshima 739-8526, Japan

‡

Japan Synchrotron Radiation Research Institute (JASRI), SPring-8, 1-1-1 Kouto, Sayo, Hyogo 679-5198, Japan

§

Department of Chemistry, Graduate School of Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-hiroshima, Hiroshima 739-8526, Japan

*Corresponding author Natural Science Center for Basic Research and Development (N-BARD), Hiroshima University, 1-3-1 Kagamiyama, Higashi-hiroshima, Hiroshima 739-8526, Japan Telephone & fax: +81-82-424-7487, e-mail address: [email protected]

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ABSTRACT: Poly(3-alkylthiophene) (P3AT) is a popular family of π-conjugated polymers used to study polymer-based flexible electronic devices. To enhance the performance of a smart material for wearable or stretchable-skin devices, it is crucial to optimize both the carrier mobility and the film structure. Here, we present the enhancements of hole mobilities of three P3ATs films along the out-of-plane direction using photoconductivity measurements by rubbing. The three P3ATs have the same conjugated backbone but different alky-side chain (CnH2n+1) lengths of n = 4, 6, and 12. The hole mobility increased with the alkyl-chain length up to 5-fold after rubbing. Polarized absorption and grazing-incidence X-ray diffraction measurements indicate the P3AT (n = 12) film has a highly oriented structure in the in-plane direction and increased π−π stacking in the out-of-plane direction after rubbing due to a low tensile modulus from the long alkyl-side chain.


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INTRODUCTION Conjugated-polymer thin films have been widely investigated to understand the optoelectronic properties of organic films and to develop films for flexible, lightweight, printed electronics.1–3 Poly(3-alkylthiophene) (P3AT) is a popular polymer associated with flexible organic electronic devices such as solar cells,4–8 field-effect transistors,9–11 tactile sensors,12 thermoelectric sensors,13 and nonvolatile memory.14 The P3AT molecule, which is shown in Figure 1a, consists of a polythiophene backbone for carrier transport and an alkyl-side chain (CnH2n+1) to increase solubility. The relationships between the alkyl chain length and the electronic and physical properties has also been investigated using P3AT compounds with n ranging from 4 to 12.2,3,15–33 The results of these studies show that an increase of the alkyl chain length increases solubility but decreases carrier mobility.22,25,29,30 In solar cell applications, P3AT with longer alkyl chains exhibit lower current density (Jsc)28 and higher open-circuit voltage (Voc).20,27 Based on the achievable balance between properties such as solubility, carrier mobility, and solar-cell performance, poly(3-hexylthiophene) (P3HT, n = 6) has been the most popular material to obtain the highest photoconversion efficiency (PCE) in the series of P3AT solar cells.23,25,27,28 From the P3AT family, poly(3-dodecylthiophene) (P3DDT, n = 12) has other distinct properties.15–17,19,20 First, the photostability is higher than those of P3HT and poly(3butylthiophene) (P3BT, n = 4) films.15 Second is the high durability; the PCE of a solar cell composed of P3DDT does not decrease under application of 10% strain, although that of P3HT significantly decreases from 0.594% to 0.0008% under the same strain.20 Third, the film can be elongated up to 1.47 times without cracking, because the tensile modulus of P3DDT is 7- and 12fold lower than those of P3HT and P3BT, respectively.20 These excellent properties of P3DDT



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are attributed to its high flexibility due to a low glass transition temperature (Tg), which is attributed to the long side chain.26,31 Thus, P3DDT can be expected to be used as a nextgeneration smart material for wearable and stretchable-skin devices.1,17,20 However, there is a crucial task to be overcome with P3DDT in that its carrier mobility is not as high as the other P3AT polymers.30,33 The carrier mobility in polymer films has been improved by control of the molecular orientation,34–38 and rubbing is a simple method to achieve this.11,39–46 In addition, according to a commercial rubbing instrument for preparing large-sized oriented film,39,40 the wearable and stretchable-skin devices (≅ 5–30 cm)12,17,47 are expected to be prepared by rubbing. Oriented P3HT films by rubbing have been obtained by several groups.11,39–46 Brinkmann et al. has reported pioneering works,11,43,45,46 and the rubbing produced the orientation of P3HT backbone along the rubbing direction,11 which depends on molecular weight and heating temperature during the rubbing.43,45 Vohra et al. observed the increase of PCE for solar cell using the rubbed P3HT layer.42 In our previous study, the mobility in P3HT film was enhanced along the out-ofplane direction, where carrier migrates in a solar cell.44 However, all the studies have used P3HT, and there have not been reports for the orientation and the mobility of rubbed films of P3AT with the other chain lengths yet so far. In the present study, the out-of-plane hole mobilities of three P3AT-based compounds, i.e., P3DDT (n = 12), P3HT (n = 6), and P3BT (n = 4), were evaluated from photoconductivity measurements before and after rubbing. The out-of-plane mobilities of the three P3AT films were enhanced by rubbing. Note that the maximum enhancement was observed for the P3DDT film. The in-plane and out-of-plane structures of the three films were investigated using polarized electronic absorption spectroscopy and two-dimensional grazing-incidence X-ray


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diffraction (2D-GIXD), respectively, before and after rubbing. As a result, the enhancement of mobility was attributed to both the orientation of the molecular backbone in the in-plane direction and to an increase of π−π stacking along the out-of-plane direction (Fig. 1b). The structural change by rubbing was the most significant for the P3DDT film, which has the lowest tensile modulus among the three P3ATs.

EXPERIMENTAL SECTION P3AT materials used were P3DDT (regioregularity 97%, Mw = 27 kDa, PDI = 1.5, Sigma Aldrich), P3HT (regioregularity 93%, Mw = 62 kDa, PDI = 2.3, Rieke Metals), and P3BT (regioregularity 80-90%, Mw = 54 kDa, PDI = 2.3, Sigma Aldrich). The ITO-coated glass (FLAT-ITO, Geomatec) was used after washing with a detergent solution (PK-LCG201, Parker Corp.) in an ultrasonic bath (3510J, Branson) at 40 °C for 20 min, and subsequent rinsing with distilled water. Chlorobenzene (Nacalai Tesque) was used as-received. P3AT films were prepared by the drop-casting of P3AT dissolved in chlorobenzene at the concentration of 20 mg/mL onto indium tin oxide (ITO)-coated glass substrates in air at room temperature. After the P3AT film was dried for 12 h in air, it was rubbed 30 times with a velvet cloth at a constant pressure of 1 kgf/cm2 and at a rate of 1 cm/s. The size of substrates was 15×15 mm2. A 4 mm diameter and 50 nm thick Al electrode was deposited on the rubbed P3AT films using a vacuum evaporation system (SVC-700TM, Sanyu Electron). The photoconductivity of the P3AT films were measured using an in-house-built instrument described elsewhere.48 The light source was the second harmonic of a Nd:YAG laser (λ = 532 nm, 7 ns, and 6 Hz). To conduct accurate measurements, the pulsed energy was adjusted to be as small as possible (0.5 µJ/pulse) using an attenuator and a laser energy sensor (J-



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10MB-LE, Coherent). The signal obtained from the time-of-flight (TOF) method was collected using a sample enclosed in an aluminum alloy Faraday cage to minimize electrical noise. TOF time profiles due to the carrier dynamics were stored in a digital oscilloscope (TDS3054, Tektronix) for 512 accumulations. The film thicknesses were 4–10 µm, which were measured using a confocal laser microscope (OLS4000, Shimadzu) equipped with a 100× objective. Polarized absorption spectra of the films were measured as described in elsewhere.44 P3AT thin films for absorption spectroscopy measurements were prepared by spin-coating films with thicknesses measured as 200 nm, which gave accurate absorbance data due to the high molar extinction coefficients of the P3AT compounds. 2D-GIXD measurements of the P3AT films were conducted at the BL19B2 beamline of SPring-8 with an X-ray energy of 12.39 keV (λ = 1 Å).44 Briefly, X-rays were irradiated at an incident angle of 0.12° and the scattered X-rays were recorded using a 2-D image detector (Pilatus 300K, Dectris). The direction of the incident X-rays was parallel to the rubbing direction. All the films for 2D-GIXD measurements were 9 µm by careful selection from the various prepared films.

RESULTS AND DISCUSSION Figure 1c shows typical time-of-flight (TOF) signals for photoconductivity measurements of P3DDT films before and after rubbing, which indicate hole migration along the out-of-plane direction. The out-of-plane hole mobility (µ) is obtained from the equation µ = d/Ettr, where d is the film thickness, E is the electrical field applied to the film, and ttr is the transit time. The values of ttr were determined from the intersection between the plateau and subsequent decay in the TOF signal, as described elsewhere.48,49 Figure 1d shows out-of-plane mobilities for the P3AT films before and after rubbing, and these values are listed in Table 1. The out-of-plane


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mobilities for all of the P3AT films are enhanced after rubbing, and this enhancement is most significant with the P3DDT film. An enhancement factor (EF) was estimated to quantify the enhancement from the equation µafter/µbefore, where µbefore and µafter are the mobilities before and after rubbing, respectively. Table 1 lists the EFs of the three films, from which the mobility of P3DDT is shown to be significantly improved by rubbing. Polarized electronic absorption spectra of the films were measured, which indicated a change in the in-plane film-structure after rubbing, as shown in Fig. 2. The solid and dotted lines represent the spectra measured with incident light polarized parallel (//) and perpendicular (⊥) to the rubbing direction, respectively. The absorbance of the // configuration is higher than that of the ⊥ configuration. The direction of the transition dipole moment, the HOMO-LUMO transition of P3AT, is // to the backbone structure;51,52 therefore, the orientation of the P3AT backbone is increased by rubbing. The orientation factor, S = (A///A⊥ – 1)/(A///A⊥ + 1), was estimated to be 0.8 (P3DDT), 0.7 (P3HT), and 0.1 (P3BT), where A is the absorbance. Here, S values of 1 and 0 would represent a complete uniaxial alignment and a random isotropic distribution of the backbone, respectively. Based on the high S value observed, the in-plane structure in the P3DDT film is significantly changed by rubbing, up to 8 times more than that of the P3BT film. As another feature in Fig. 2, the peak maxima at // configuration locate at longer wavelength than those at ⊥, and the red shifts are observed in // configuration. The spectral components in longer and shorter wavelength of an absorption spectrum for P3AT have been attributed to the ordered (aggregate, crystalline) and disordered (amorphous) structures, respectively.13,17–19,28,53,54 As a result, the red shifts at // configuration can be due to larger amount of ordered structure in // direction than that in ⊥ direction.



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The out-of-plane structures before and after rubbing were investigated using 2D-GIXD. Figure 3a shows 2D-GIXD images for the P3DDT, P3HT, and P3BT films. The intensity in the qz direction is increased after rubbing for all the films (P3DDT, P3HT, and P3BT). This indicates an increase of π−π stacking, which corresponds to a face-on orientation,53,55,56 as illustrated in Fig. 3b. To quantify the amount of face-on orientation, the pole angle (χ) dependence of the intensity of the (100) diffraction was analyzed, and the results are shown in Fig. 3c. The intensity changed as a function of χ and the data for P3DDT reveals a significant difference after rubbing at around χ ≈ 0°. The (100) diffractions at lower (χ ≈ 0°) and higher (χ ≈ 90°) angles correspond to the face-on and edge-on orientations, respectively.54,57,58 The diffraction intensity for P3DDT at around χ ≈ 0° increases 3-fold after rubbing; therefore, the face-on component of P3DDT is significantly increased by rubbing. The increased amount of face-on is responsible for the enhancement of out-of-plane mobility, because a preferable pathway for carrier migration is given by the increase of face-on component. Namely, the pathway is shortened by the face-on components that partially overlap π-orbitals among P3AT molecules, and the carrier rapidly migrates in the film. Such a situation after rubbing is the most significant for P3DDT, i.e. before and after rubbing, amount of face-on in Fig. 3c and the out-of-plane mobility listed in Table 1. Next, we discuss the π−π stacking distance (dπ−π) and alkyl-side chain distance (dalkyl, lamellar periodicity) of the face-on components in P3AT, which are analyzed from GIXD data of the (010) diffraction along the qz and the (100) diffraction along the qxy, respectively, as shown in Fig. 3a. The values of dπ−π for the P3AT films before and after rubbing was obtained, as shown in Fig. S1 in Supporting Information, using the equation dπ−π = 2π/qz. For the clarity, the distance difference before and after rubbing, ∆dπ−π = dπ−π (after) − dπ−π (before), are shown in Fig. 4a. A negative value of ∆dπ−π indicates a decrease of the π−π stacking distance by rubbing. The


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magnitude of ∆dπ−π for P3DDT is larger than those for P3HT and P3BT. Note that the decrease of the π−π stacking distance also enhances out-of-plane mobility, because carrier rapidly migrates along the shortened pathway. As for dalkyl, the distance difference of dalkyl before and after rubbing, ∆dalkyl = dalkyl (after) −dalkyl (before), was similarly estimated from dalkyl = 2π/qxy and (100) diffraction along the qxy, as shown in Fig. 3b. Since the ∆dalky is positive value, the lamellar periodicity is increased by rubbing. Thus, the two large deformations in P3DDT film are caused by rubbing at the same time, as illustrated in Fig. 4c, i.e. the decrease of π−π stacking distance along the qz (out-of-plane direction) and the increase of lamellar periodicity in qxy (inplane direction). In summary, as the alkyl-side chain length of P3AT increased, the mobility increased after rubbing. There were highly oriented structure in the in-plane direction and increased π−π stacking in the out-of-plane direction. In addition, these changes were significant when the polymer structure was significantly deformed by rubbing. In the previous report, the similar feature between the orientation and the polymer deformation was observed in the similar system.43 Namely, heating P3HT film during rubbing disorders the alkyl-side chain and gives higher orientation, i.e. a polymer film softened by heating at around Tg results in a deformation by mechanical force such as rubbing. The situation in the previous study is consistent with that observed in the present study. Namely, P3DDT with long flexible alkyl-side chains has a low tensile modulus20 and exhibits the most pronounced deformation, giving higher orientation and greater mobility. Finally, let us mention the orientation and the molecular weight (Mw) of polymer for rubbing study, briefly. In the previous reports,11,43 higher orientation was observed in the film of P3HT with lower Mw, according to measurements of absorption spectra as a function of Mw. If



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this result were applied to the current system, the P3DDT (Mw = 27 kDa) could give higher orientation than those of P3HT (Mw = 62 kDa) and P3BT (Mw = 54 kDa). However, these polymers have many different physicochemical properties, e.g. not only Mw but also alkyl-side chain length, regioregularity, Tg, tensile modulus, and so on. Based on these situations, it can be considered that the systematic synthesis of P3DDT, P3HT and P3BT with different Mw gives the relation between molecular orientation of P3AT and Mw.

CONCLUSION The P3DDT, P3HT, and P3BT films were investigated before and after rubbing using photoconductivity, polarized electronic absorption spectroscopy, and 2D-GIXD measurements. The results indicated that rubbing provided the following: i) Enhancement of the out-of-plane hole mobility in all of the P3ATs examined and the EF increased with the length of the alkyl side chain. The maximum enhancement was observed for P3DDT, of which the EF was 5-fold. ii) Orientation of the π-conjugated backbone in all the P3AT films and significant molecular alignment in the in-plane direction of the P3DDT film. iii) An increased amount of π−π stacking in the out-of-plane direction. The π−π stacking distance decreased with an increase in the distance between alkyl-chains. Based on i)-iii), the significant enhancement of the out-of-plane mobility for the P3DDT film by rubbing was attributed to the changes of the in-plane and out-ofplane structures, which improved the molecular alignment and π-orbital overlapping, respectively. Accordingly, P3DDT with long flexible alkyl chains and a low tensile modulus exhibited the most pronounced improvement of mobility. The increase of mobility achieved for a


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stretchable polymer is expected to be a useful contribution to the development of smart materials for next-generation wearable and stretchable-skin devices.

ACCOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. dπ−π for P3AT films.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS KS acknowledges financial support from the Funding Program for the Next Generation WorldLeading Researchers (GR073) of the Japan Society for the Promotion of Science (JSPS) and the PRESTO Structure Control and Function program of the Japan Science and Technology agency (JST). DK acknowledges a Grant-in-Aid for Young Scientists (B) (No. 26790015) from the Ministry of Education, Culture, Sports, Science, and Technology of Japan. GIXD experiments



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were performed at the BL19B2 beamline of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal Nos. 2014B1629 and 2015B1630).

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Table 1. Hole mobilities for P3AT films before and after rubbing. EF denotes the enhancement factor, estimated as the ratio of mobilities before and after rubbing. materials P3DDT P3HT P3BT

mobility (cm2/Vs) before

after

6.6×10－5 2.2×10－4 7.5×10－3

3.2×10－4 9.0×10－4 1.0×10－2


EF 5 4 1.3

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Figure 1. (a) Chemical structures of the three P3ATs investigated in this study. (b) Schematic diagram of out-of-plane and in-plane directions. (c) Typical TOF signals for P3DDT films before and after rubbing, where ttr represents the transit time. (d) Out-of-plane hole mobilities for P3AT films before (blue) and after (red) rubbing. The standard deviation (±σ) was obtained from data for five measurements.



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Figure 2. Polarized electronic absorption spectra for P3AT films after rubbing. Solid and dotted lines represent the spectra obtained using incident light polarized parallel (//) and perpendicular (⊥) to the rubbing direction, respectively. The dichroic ratio (DR) is given by DR = A///A⊥. The film thicknesses are 200 nm.50


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Figure 3. (a) 2D-GIXD images for P3DDT (top), P3HT (middle), and P3BT (bottom) films before (left) and after (right) rubbing. The direction of the incident X-rays is parallel to the rubbing direction. The film thicknesses are 9 µm.50 (b) Schematic diagram of the face-on orientation. (c) Left panels: GIXD intensities of the (100) diffraction as a function of χ for the P3DDT (top), P3HT (middle), and P3BT (bottom) films. Black and red lines denote intensities from the films before and after rubbing, respectively. Right panels: expanded 2D-GIXD images around the (100) diffraction after rubbing, from Fig. 3a. The intensities for the left panel figures were obtained as integrated intensities from between two yellow dashed lines in the right panel 2D-GIXD images.



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Figure 4. (a) ∆dπ−π for the face-on orientation of P3AT films, analyzed from GIXD data of the (010) diffraction along the qz. (b) ∆dalkyl for the face-on orientation of P3AT films, analyzed from GIXD data of the (100) diffraction along the qxy. The direction of the incident X-rays is parallel to the rubbing direction. (c) Schematic diagram of the structural change caused by rubbing.


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Table of Contents Graphic


Enhancement of Out-of-Plane Mobilities of Three Poly(3

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