Understanding Comparable Charge Transport Between Edge-on and

Article Cite This: ACS Appl. Polym. Mater. 2019, 1, 1257−1262

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Understanding Comparable Charge Transport Between Edge-on and Face-on Polymers in a Thiazolothiazole Polymer System Masahiko Saito,† Tomoyuki Koganezawa,‡ and Itaru Osaka*,† †

Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi-Hiroshima, Hiroshima, 739-8527, Japan ‡ Japan Synchrotron Radiation Research Institute, Sayo-gun, Hyogo 679-5198, Japan

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ABSTRACT: The orientation of semiconducting polymers is one of the most important factors for the charge transport and thus the performance of polymer-based devices such as organic field-effect transistors and organic photovoltaics. Here, we study thiophene−thiazolothiazole polymers with different alkyl side chains, which apparently form the “edge-on” or “face-on” orientation depending on the side chain composition. Interestingly, hole mobilities observed in transistors were seemingly independent of the backbone orientation. However, in-depth X-ray analysis revealed that while the edge-on polymers evenly had the edge-on to face-on fraction throughout the film thickness, the face-on polymers had the unevenly distributed edge-on to faceon fraction: the edge-on orientation was abundant at the film−substrate interface and the face-on orientation was abundant in the bulk. Thus, in fact the edge-on was the dominant orientation at the film−substrate interface in all the polymers, which agrees well with the high hole mobility in the face-on polymers that was comparable to the edge-on polymers. Our findings provide good guidelines for in-depth understanding of structure− property relationship in semiconducting polymers and their optoelectronic devices. KEYWORDS: organic field-effect transistors, semiconducting polymers, orientation, interfacial orientation, thiazolothiazole

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polymers with the edge-on orientation.16−19 One such example is a naphthalenedicarboximide (NDI)-based semiconducting polymer, P(NDI2OD-T2). P(NDI2OD-T2) was reported to show a change in the backbone orientation from the face-on to edge-on when annealing the film to the polymer melting point followed by slow cooling to ambient temperature. Although the clear mechanism has not yet been investigated, the polymer provided comparable OFET mobilities in both face-on and edge-on orientations.19 Recently, we have reported that in thiophene−thiazolothiazole copolymers (PTzBTs), the backbone orientation can be controlled by carefully tuning the side chain composition, which largely enhanced the photovoltaic performance of the polymer cells.20 More recently, we found that in thin films of PTzBTs blended with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) the orientation was not evenly distributed through film thickness, which correlated well with the photovoltaic performance of the polymer cells.21 Furthermore, through these studies we also found that PTzBTs showed similar charge carrier mobilities in OFETs regardless of the backbone orientation. A series of the results inspired us to further investigate the correlation between the OFET perform-

INTRODUCTION Semiconducting polymers have been attracting much attention because of their potential applications in low-cost, lightweight, large-area, and flexible devices that can be made by solution processes.1−3 Numerous semiconducting polymers have been designed and synthesized in the past decade, which have realized charge carrier mobilities surpassing that of amorphous silicon in p-type, n-type, and ambipolar organic field-effect transistors (OFETs).4,5 Tuning of the frontier molecular orbital energy levels, that is, highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO) energy levels, improves the ambient oxidation of the polymers and charge trapping by oxygen and moisture, while it also facilitates the charge injection at the polymer/electrode interface, all of which influences the OFET performance.6,7 Control of polymer crystallinity and orientation is equally important as they determine the charge carrier transport of semiconducting polymers in the solid state, which greatly dictates the OFET performance.8−14 In principle, the edge-on orientation where the polymer backbones π-stack along the substrate plane is thought to be advantageous for high in-plane charge transport and thus for OFETs.9,10,15 However, it has been reported that several semiconducting polymers with the face-on orientation where the backbones π-stack along the substrate normal can give high OFET performances comparable or even better than the © 2019 American Chemical Society

Received: November 9, 2018 Accepted: April 29, 2019 Published: April 29, 2019 1257

DOI: 10.1021/acsapm.8b00111 ACS Appl. Polym. Mater. 2019, 1, 1257−1262

Article

ACS Applied Polymer Materials

Figure 1. (a) Chemical structure of PTzBTs with different side chain composition (14HD, EHOD, 12OD, and BOHD). (b) The 2D GIXD patterns of the PTzBT films with the thickness of around 200 nm.

ance and the detailed backbone orientation to understand the mechanism of the charge carrier transport in this polymer system. Herein, we performed the grazing incident X-ray diffraction (GIXD) measurements using the thin films of PTzBTs with various thickness and conducted the pole figure analysis. As a result, we found that while the backbone orientation is evenly distributed through the film thickness in PTzBTs that apparently form the edge-on orientation, the backbone orientation is unevenly distributed through the film thickness in PTzBT that apparently form the face-on orientation, in which the edge-on fraction is dominant at the film−substrate interface and gradually decreases (face-on fraction increases) through the bulk. The results wellrationalize the fact that the charge carrier mobility in OFETs is comparable between PTzBT with the edge-on and face-on backbone orientation.

Table 1. Transistor Properties and GIXD Results of PTzBTs polymer

μFET (cm−2 V−1 s−1)a

dπ (Å)b

LC (Å)c

14HD EHOD 12OD BOHD

0.19−0.28 0.082−0.13 0.15−0.28 0.010−0.015

3.52 3.58 3.52 3.65

48 37 42 22

a Field-effect mobility. bπ−π stacking distance determined from the edge-on crystallite. cCrystallite coherence length calculated by a simplified Scherrer’s equation (LC = 2π/fwhm) using the full width at half-maximum (fwhm) of the π−π stacking peak for the edge-on crystalline in the thinnest film.

the polymer chains have strong intermolecular interaction, they would aggregate strongly with each other and avoid interacting with the substrate surface, resulting in the edge-on orientation. On the other hand, when the polymer chains have relatively weak intermolecular interaction, the aggregation property becomes weaker and they may interact with the substrate surface, leading to the face-on orientation. OFET characteristics of the polymers were measured using devices with a bottom-gate top-contact architecture. OFETs were fabricated by spin-coating the polymer solution in chlorobenzene onto the 1H,1H,2H,2H-perfuluorodecyltriethoxysilane (FDTS)-modified Si/SiO2 substrates and then were annealed at 150 °C. Figure 2a,b depicts typical transfer and output curves of the OFETs, respectively, in which all the polymers showed p-channel behavior. Table 1 summarizes the mobilities (μFET) calculated from the saturation regime (see Table S1 for more information). The μFET for 14HD and EHOD, which formed edge-on orientation, were as high as 0.28 and 0.13 cm2 V−1 s−1, respectively. Interestingly, 12OD showed μFET of 0.28 cm2 V−1 s−1 that was comparable to 14HD, even though it formed the face-on orientation that is thought to be unfavorable for OFETs. BOHD with the face-on orientation, however, gave 1 order of magnitude lower μFET (0.015 cm2 V−1 s−1). Seemingly, the μFETs observed for these polymers were independent of the backbone orientation but were rather correlated with the π−π stacking distance. It should be noted that, however, in the bottom-gate OFETs, the charge transport occurs near the interface between the film and the substrate (gate dielectric). Therefore, the orientation at the interface

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RESULTS AND DISCUSSION Figure 1a shows the chemical structure of PTzBTs used in this study.20 Each PTzBT has two different alkyl groups as the side chains. PTzBTs with n-tetradecyl and 2-hexyldecyl groups (14HD) and with 2-ethylhexyl and 2-octyldodecyl groups (EHOD) form the edge-on orientation: the lamellar and π−π stacking diffractions appeared along the qz and qxy axes, respectively, in the two-dimensional (2D) GIXD patterns (Figure 1b).22,23 In the meantime, PTzBTs with n-dodecyl and 2-octyldodecyl groups (12OD) and with 2-butyloctyl and 2hexyldecyl groups (BOHD) mainly form the face-on orientation: although the lamellar diffraction appeared along both the qz and qxy axes, the π−π stacking diffraction mostly appeared along the qz axis in the 2D GIXD patterns (Figure 1b). The π−π stacking distances, determined using the diffraction peak in the qxy axis (edge-on fraction), were found to be 3.52, 3.58, 3.52, and 3.65 Å for 14HD, EHOD, 12OD, and BOHD, respectively (Table 1). Thus, PTzBTs that possess linear and branched alkyl groups had shorter π−π stacking distances than PTzBTs that possess only branched alkyl groups. The change in the orientation likely originates in the subtle change in the intermolecular interaction as we reported previously: the polymers with bulkier side chains would have weaker intermolecular interaction and tend to form face-on orientation.20 This can be explained as follows. When 1258


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Figure 2. Typical OFET characteristics of the OFETs based on PTzBTs. (a) Transfer curves. (b) Output curves.

Figure 3. Two-dimensional GIXD images of the polymers with different film thickness at the small angle region (left) and pole figure plots extracted from the lamellar diffraction in the 2D GIXD images (right). (a) 14HD, (b) EHOD, (c) 12OD, (d) BOHD.

must be carefully investigated. One good methodology to investigate the orientation at the buried interface, as well as the distribution of the orientation through the film thickness, is to perform the GIXD measurements and the pole-figure analysis for the films with various thickness and to investigate the variation of the face-on/edge-on fraction by the thickness.24

Although this may be an indirect methodology, several reports have shown reasonable correlations between the difference in the orientation through the film thickness and device performances.21,24,25 In order to discuss the backbone orientation at the interfacial layer, we measured 2D GIXD patterns of the 1259


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orientation in the film of 12OD and BOHD varied with the thickness: whereas the face-on is more abundant in the thicker films, the edge-on orientation is more abundant in the thinner films. Furthermore, it can be assumed that the interfacial orientation is insensitive to the film thickness. Therefore, the decrease in the Axy/Az values with the decrease in the film thickness means that the face-on population decreased mainly in the bulk. This implies that the edge-on is the dominant orientation motif at the film−substrate interface whereas the face-on is the dominant motif in the bulk even in the thicker film (Figure 5b).21,24,25 Thus, in all PTzBTs the orientation motif at the film−substrate interface is most likely edge-on regardless of the composition of alkyl side chains, though there is also the possibility that the edge-on fraction is localized at the film−air interface (film surface) as well. Thus, in 12OD the charge transport would occur through the edge-on fractions localized at the gate dielectric (SiO2) surface that is the main charge carrier path for the bottom-gate OFET. It is also possible to explain that even though there are small fractions of face-on orientation at the film−substrate interface in the 12OD film, those face-on fractions would not interfere the charge transport. This agrees well with the fact that 12OD showed μFET comparable to 14HD. The lower μFET observed for EHOD and BOHD, despite the fact that the predominant interfacial orientation is also edgeon, can be explained by the wider π−π stacking distance (dπ) and lower crystallinity than 14HD and 12OD (Table 1). In fact, 14HD and 12OD had the same dπ of 3.52 Å and the similar crystallite coherence length (LC) of 48 and 42 Å, when these parameters were calculated in the thinnest films, which is consistent with the transistor properties. On the other hand, EHOD and BOHD had dπ of 3.58 and 3.65 Å, and LC of 37 and 22 Å, respectively. In addition, we investigated the morphology of the polymer films on the OFETs. As displayed in Figure 6, there is only a marginal difference among the four polymers, and the surface roughness is mostly the same for all the films. We also estimated the ratio of crystalline and amorphous phases in the thin film from UV−vis absorption spectra of PTzBT thin films. We assumed that the low-energy region (approximately 550− 700 nm) and the high-energy region (approximately 400−550 nm) correspond to the absorption bands contributed from the crystalline and amorphous phases, respectively (Figure S4).26,27 In all the polymers, the ratio of crystalline phase to amorphous phase was found to be roughly 70:30. This indicates that the fraction of crystalline phase in these polymers was mostly the same, and that the effect of the crystal/ amorphous ratio on the difference in the mobility among these polymers are ignored. Thus, we concluded that the transistor performance of this polymer system is determined by the polymer π−π stacking distance and crystallinity at the interfacial layer as the orientation at the interfacial layer is mostly the same.

polymer neat films with the thickness of 20−200 nm (Figures S1−S3). Figure 3 shows the 2D GIXD patterns at the small angle region (qxy = −0.4−0.4 Å−1, qz = 0−0.5 Å−1) corresponding to the first order lamellar diffraction (100) (left) and the pole figure plots extracted along the azimuth angle (χ) (right). Note that we used the first order lamellar diffraction because the inaccessibility of the diffraction along the qz axis in the small-angle region is very limited compared to that in the wide-angle region. The areas of χ = 0−45° and 135−180° (Axy) and χ = 55−125° (Az) were defined as the face-on and edge-on fractions, respectively. In 14HD and EHOD, the texture of the 2D patterns and the shape of the profile did not change by the film thickness, indicating that the orientation motif (edge-on) was independent of the film thickness (Figure 3a,b). On the other hand, in 12OD and BOHD the diffraction for the face-on fraction that appeared along the qxy axis in the 2D patterns was weakened with the decrease of the film thickness relative to the edge-on fraction that appeared along the qz axis (Figure 3c,d). This is clearer in the profiles as the intensity at χ = 0−45° and 135−180° corresponding to the face-on fraction decreased with decreasing the film thickness relative to the intensity at χ = 55−125° corresponding to the edge-on fraction. This indicates that the population of the face-on fraction became smaller (edge-on fraction became larger) in the thinner films. To further quantify this, we determined the face-on to edge-on ratio using the profiles for all the films. The face-on/edge-on ratio was evaluated by using the peak area for the face-on fraction (Axy) and the edge-on fraction (Az). We then plotted Axy/Az as a function of the film thickness (Figure 4). Note that

Figure 4. Change in Axy/Az as a function of film thickness.

calculated Axy/Az does not necessarily indicate the real faceon/edge-on ratio because the scattering from the direct beam overlaps with the first order lamellar diffraction along the qz axis, which overestimates the fraction of edge-on orientation. Nevertheless, the change in Axy/Az can be compared among these polymers as all the experiments were carried out in the same manner. As expected, 14HD and EHOD showed very small Axy/Az values of around 0.025 for all film thicknesses, confirming that these polymers almost completely form edgeon orientation (Figure 5a). However, 12OD and BOHD showed the Axy/Az values of around 0.25 at the relatively thick film (∼200 nm), which was ten times larger than 14HD and EHOD, indicating that there is large population of face-on orientation. Interestingly, the Axy/Az values then gradually decreased to less than 0.1 as the film thickness decreased to below 50 nm. This again indicates that the average backbone

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CONCLUSIONS We fabricated OFET devices using PTzBTs with different orientation motifs, that is, edge-on and face-on orientations. Interestingly, the field-effect mobilites of PTzBTs were apparently independent of primarily orientation: 12OD with the predominant face-on orientation showed hole mobilities comparable to 14HD with the complete edge-on orientation. Interestingly, however, in-depth analysis of the GIXD patterns of the PTzBT films revealed that, in 12OD as well as another 1260


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Figure 5. Schematic illustration of the orientation distribution in the polymer thin films. (a) Edge-on polymers (14HD and EHOD). (b) Face-on polymers (12OD and BOHD). Notes: (1) The face-on/edge-on ratio in this figure is exaggerated in order to visualize the distribution of the orientation. (2) There is the possibility that the edge-on fraction may also localize at the film−air interface (film surface).

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Parameters of the OFETs, 2D GIXD patterns of various thickness, cross-sectional profiles of 2D GIXD patterns (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Itaru Osaka: 0000-0002-9879-2098 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Advanced Low Carbon Technology Research and Development Program (ALCA) from JST (Grant JPMJAL1404) and JSPS KAKENHI (Grant 16H04196). The 2D GIXD experiments were performed at the BL46XU of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal No. 2015A1952, 2015B1904).

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Figure 6. AFM images of the polymer films on the Si/SiO2 substrate. (a) 14HD, (b) EHOD, (c) 12OD, (d) BOHD.

“face-on” polymer BOHD, the face-on to edge-on fraction was unevenly distributed through the film thickness: the edge-on orientation was the dominant motif at the film−substrate interface where the charge carriers flow. This explains well the high mobility observed in 12OD. These results indicate that the distribution of the backbone orientation is an important factor for understanding the performance of OFET devices.

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsapm.8b00111. 1261


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Understanding Comparable Charge Transport Between Edge-on and

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