Solvent Additive-Assisted Anisotropic Assembly and Enhanced

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Organic Electronic Devices

Solvent Additive-Assisted Anisotropic Assembly and Enhanced Charge Transport of #-Conjugated Polymer Thin Films Jae Won Jeong, Gyounglyul Jo, Solip Choi, Yoong Ahm Kim, Hyeonseok Yoon, Sang-Wan Ryu, Jaehan Jung, and Mincheol Chang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03221 • Publication Date (Web): 04 May 2018 Downloaded from http://pubs.acs.org on May 5, 2018

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ACS Applied Materials & Interfaces

Solvent Additive-Assisted Anisotropic Assembly and Enhanced Charge Transport of π−Conjugated Polymer Thin Films Jae Won Jeong,1 Gyounglyul Jo,1 Solip Choi,1 Yoong Ahm Kim,1,2 Hyeonseok Yoon,1,2 Sang-Wan Ryu,3 Jaehan Jung,4,* and Mincheol Chang,1,2,* 1

School of Polymer Science and Engineering, 2Alan G. MacDiarmid Energy Research Institute,

and 3Department of Physics, Chonnam National University, Gwangju 61186, South Korea 4

Department of Materials Science and Engineering, Hongik University, Sejongsi 30016, South

Korea

KEYWORDS: solvent additive, poly(3-hexylthiophene), molecular ordering, anisotropic assembly, charge carrier mobility ABSTRACT: Charge transport in π−conjugated polymer films involves π−π interactions within or between polymer chains. Here, we demonstrate a facile solution processing strategy that provides enhanced intra- and interchain π−π interactions of the resultant polymer films using a good solvent additive with low volatility. These increased interactions result in enhanced charge transport properties. The effect of the good solvent additive on the intra- and intermolecular interactions, morphologies, and charge transport properties of poly(3-hexylthiophene) (P3HT)

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films is systematically investigated. We found that the good solvent additive facilitates the selfassembly of P3HT chains into crystalline fibrillar nanostructures by extending the solvent drying time during thin-film formation. As compared to the prior approach using a nonsolvent additive with low volatility, the solvent blend system containing a good solvent additive results in enhanced charge transport in P3HT organic field-effect transistor (OFET) devices (from ca. 1.7 × 10-2 to ca. 8.2 × 10-2 cm2 V-1 s-1 for dichlorobenzene (DCB) versus 4.4 × 10-2 cm2 V-1 s-1 for acetonitrile (AN)). The mobility appears to be maximized over a broad spectrum of additive concentrations (1–7 vol %), indicative of a wide processing window. Detailed analysis results regarding the charge injection and transport characteristics of the OFET devices reveal that a high-boiling-point solvent additive decreases both the contact resistance (Rc) and channel resistance (Rch), contributing to the mobility enhancement of the devices. Finally, the platform presented here is proven to be applicable to alternative good solvent additives with low volatility, such as chlorobenzene (CB) and trichlorobenzene (TCB). Specifically, the mobility enhancement of the resultant P3HT films increases in the order CB (bp 131°C) < DCB (bp 180 °C) < TCB (bp 214 °C), suggesting that solvent additives with higher boiling points provide resultant films with preferable molecular ordering and morphologies for efficient charge transport.

1. INTRODUCTION Conjugated polymers have been of great interest as a new class of electronic materials promising a diversity of practical applications including light-emitting diodes (LEDs)1,2, field-effect transistors (FETs)3,4, and photovoltaic cells (PVs),5,6 as well as electronic skins7,8 and chemical sensors and biosensors9-11 due to the substantial benefits of the solution-based and lowtemperature processability and good mechanical flexibility.12–16 However, compared to organic


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small-molecule semiconductors, conjugated polymers generally have lower device performance arising from the relatively lower degree of crystallinity (i.e., intra- and intermolecular interactions); the amorphous regions, which consist of largely disordered polymer chains, hamper the charge transport occurring via a hopping of carriers between transporting sites within the polymer films.17–19 Many efforts have been made to develop new processing strategies (e.g., modification of semiconductor-dielectric interface,20 thermal treatment,21 solvent vapor treatment,22 and solution treatments,15,23–25), combined with molecular engineering approaches in order to improve the crystallinity and, concomitantly, enhance charge transport characteristics of the solidified polymer films. In particular, solution processing using a good solvent with a high-boiling-point (e.g., chlorobenzene (CB) (bp 131 °C), dichlorobenzene (DCB) (bp 180 °C), or trichlorobenzene (TCB) (bp 214 °C) has been intensively studied because it can readily lead to crystalline structured films with high charge carrier mobilities through simple steps such as polymer dissolution and film deposition.26–28 It is believed that the low volatility of the solvent provides sufficient time for supramolecular assembly via a favorable intra- and interchain interactions during film formation. However, utilization of a high-boiling-point solvent means that an additional step is required for the removal of the residual solvent after film coating, which is generally performed at a high temperature and therefore could give rise to film deformation; thus, the use of flexible, low glass transition temperature substrates is limited.26–29 In addition, highboiling-point solvents generally present poor wettability on substrates because of their high surface tension (e.g., CB: ca. 33.6 mNm-1, DCB: ca. 37.0 mNm-1, and TCB: ca. 39.9 mNm-1).30– 32

Therefore, the use of such solvents requires additional surface modification, which is


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performed by depositing a self-assembled monolayer of organic molecules on to the surface of the substrates before film deposition.25,33–35 In contrast, good solvents with low boiling points have relatively lower surface tensions (e.g., chloroform (bp 61 °C): ca. 27.5 mNm-1, tetrahydrofuran (bp 66 °C): ca. 26.4 mNm-1, and toluene (bp 110 °C): ca. 28.4 mNm-1), providing a more uniform film morphology on substrates in the deposition process.15,24,36 Furthermore, approaches using low-boiling-point solvents require no additional steps to remove the residual solvent after film deposition because of their high volatility.15,24,37 However, the conjugated polymer films deposited from a low-boiling-point solvent tend to be more amorphous, which is attributed to the rapid solvent evaporation that hinders the supramolecular assembly of polymer chains.15,24,37 To overcome these problems, several independent groups have recently reported alternative solution processing strategies using a mixed solvent system consisting mostly of a good solvent with higher volatility and an additive non-solvent with lower volatility.15,38-40 Such co-solvent systems have been demonstrated to provide improved molecular ordering (i.e., crystallinity) and charge transport, as well as yielding uniform morphologies for solidified polymer films without any additional treatments, including modification of semiconductordielectric interface, thermal treatment, and solvent vapor treatment.15,38 It is theorized that a crystalline morphology in the resultant films is led by the less volatile non-solvent; which remains longer time during film formation and, thus, promotes the supramolecular assembly of polymer chains.15,38 Although significant advances have been made by using the co-solvent systems, some challenges remain. In particular, the processing parameters such as the nonsolvent content and the aging time of the polymer solution should be carefully controlled and optimized because subtle variations in these parameters give rise to significant alterations in the


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morphological properties and, thus, the charge transport characteristics of the resultant films.15,38 In addition, these co-solvent systems provide no enough time during evaporation for the favorable self-assembly of π−conjugated polymer chains, limiting further maximization of the charge carrier mobility of the solidified films. The temperature or fraction of non-solvent additive in the solution varies rapidly by the solvent evaporation, which can alter the solvent mixture quickly from a good to a poor solvent and vice versa.15,38 In this paper, we demonstrate that solvent blend systems using a good solvent additive with low volatility lead to anisotropic supramolecular assemblies of π−conjugated polymers during the coating process, thus enhancing the crystallinity of the resultant films and resulting in improved charge transport characteristics without additional pre- and/or post-deposition steps. Further, compared to existing approaches using a non-solvent additive with lower volatility, the approach presented here is facile, effective, and scalable for the preparation of uniform films of π−conjugated polymers with excellent charge carrier transport characteristics. In addition, we have studied how the solvent additive effects charge injection and transport properties of organic FET (OFET) devices by measuring the contact resistance (Rc) and channel resistance (Rch). Lastly, we investigate the generality of the approach using a good solvent additive with low volatility by adopting alternative good solvent additives with low volatility.

2. RESULTS AND DISCUSSION To investigate the influence of good solvent additives with low volatility on the morphological and electronic properties, two poly(3-hexylthiophene) (P3HT) were used as model polymers (Table S1); one has 96 % regioregularity (RR) and 43.7 kDa molecular weight (MW) and the other has 92 % RR and 69.0 kDa MW. Chloroform (CF, bp 61 °C) was selected as


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Figure 1. (a) Photographs and (b) AFM phase images of 96% RR P3HT films obtained by spin-coating CF solutions containing different DCB ratios. (c) Magnified AFM image of the 96% RR P3HT film obtained from a CF/DCB-5 vol % solution. the primary good solvent because of the ease of evaporation at room temperature and its good film-forming ability, and chlorobenzene (CB, bp 131 °C), dichlorobenzene (DCB, bp 180 °C), and trichlorobenzene (TCB, bp 214 °C) were chosen as the good solvent additives with higher boiling points. To obtain the 96% RR P3HT films, the polymer solutions were prepared by dissolving P3HT in CF mixed with a requisite amount of a good solvent additive, and then, the as-prepared solutions were deposited onto the substrates by spin-coating. As shown in Figure 1, the morphologies of the corresponding films were observed by the naked eye and atomic force microscopy (AFM). The pristine film prepared from a pure CF solution appears uniformly wetted on the glass substrate, whereas the film obtained from a pure DCB solution exhibits a


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dewetted morphology owing to the high surface tension of the solvent (Figure 1a). The film thickness changed with increasing DCB content, but the film morphology remained robustly homogeneous. The measurement of the film thickness was performed by using a Veeco Dektak 150 profiler; the film thickness somewhat increased from 44.5 nm to 56.1 nm with increasing DCB content up to 2 vol %, and then decreased to 32.5 nm with the further addition of DCB up to 40 vol % (Figure S1). However, the dewetted morphologies began to appear as the content of DCB increased over 40 vol % (Figure S2). Interestingly, the color of resultant films transformed from pinkish red to bluish purple upon the addition of DCB, indicative of the enhanced molecular ordering of polymer chains.24 The morphological analysis of the resultant films was accomplished by using AFM (Figure 1b). The morphologies of the P3HT films prepared from CF/DCB solutions were compared with those of the films obtained from the CF/acetonitrile (AN, bp 81 °C) solutions. In a prior report, the introduction of a small quantity of AN to a P3HT/CF solution led to significantly enhanced molecular ordering and a concomitant improvement in mobility of the resultant P3HT films.38 For a systematic comparison, the concentration of the solvent additives was varied up to 5 vol % because the polymer solutions with a concentration of AN higher than 5 vol % gave rise to severe precipitation of P3HT aggregates (Figure S3). Upon the addition of DCB or AN, the nano- and microstructure of the P3HT film clearly changed from the initial featureless and amorphous-like structure typical of pristine P3HT films obtained by spin-coating a pure CF solution. Consistent with prior reports, the P3HT films prepared by the addition of AN contain randomly shaped, nano-sized P3HT aggregates, whereas the P3HT films prepared by spin-coating the solutions containing DCB show distinct nanofibrillar structures (Figures 1b and 1c), suggesting that the presence of DCB facilitates the anisotropic self-assembly of P3HT chains. As the DCB content increased to 2 vol %, the fibrillar aggregates begin to


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Figure 2. Schematic illustration for the influence of the solvent additives, DCB and AN on the morphology of P3HT films. appear, and then mature in shape and size. The further addition of DCB over 5 vol % did not result in a discernable change in the microscale morphology (Figure S4). The surface roughness of the resultant films was varied upon the addition of DCB. The pristine films exhibited a rootmean-square (RMS) of ca. 0.44 nm, and the RMS of the films prepared through the addition of DCB gradually increased up to ca. 4.63 nm with increasing DCB content up to 2 vol % (Figure S5), then remaining at a value of ca. 3.75 nm with further increase of the additive content. The increased roughness is attributed to the generation of fibrillar aggregates in the films. A mechanism to account for the influence of the solvent additives, DCB and AN, on the morphological properties of solidified polymer films during the deposition process is presented in Figure 2. DCB remains for a longer period owing to its lower volatility relative to CF during


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film formation, which provides sufficient time for self-assembly via favorable π−π interactions between P3HT chains and thereby results in the formation of crystalline fibrillar aggregates in the films. On the contrary, a non-solvent additive, AN molecules stay a longer time within the evolving film compared to CF molecules when a solvent mixture consisting mostly of CF and AN evaporates during spin-coating, which means that the solvent mixtures alter quickly from a good to a poor solvent to P3HT, and thereby the rapid aggregations (i.e., prominent nucleation followed by growth) of P3HT chains occur via unfavorable interactions between AN molecules and P3HT chains. As a result, nanosized P3HT aggregates with random shapes are formed within the resultant films. UV-vis absorption spectra of the resultant films were collected to assess the influence of DCB on the intra- and intermolecular interactions of P3HT chains. Figure 3a shows the spectra of 96% RR P3HT films spin-coated from CF solutions containing small amounts of DCB (0, 1, 2, 3, and 5 vol %). The resultant films exhibit two features in the UV-vis absorption spectra: a higher energy band (π−π* interband transition) at ca. 520 nm that correlates with disordered single polymer chains, and lower energy features (i.e., (0−1) transition at ca. 558 nm and (0−0) transition at ca. 617 nm) arising from the well-ordered structures. P3HT films spin-coated from CF/DCB solutions show spectral features apparently different from those of the pristine films (Figure 3a). Upon the addition of DCB (< 5.0 vol %), the lower energy absorption bands significantly increased in intensity in the resultant P3HT films, indicative of an increase in the amount of ordered structures formed via strong π−π interactions between the P3HT polymer chains. Interestingly, no significant intensity variation of the lower energy bands appeared over the range of DCB concentrations. The intensity slightly increased as the DCB concentration increased to 5 vol %.


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Figure 3. (a) UV-vis absorption spectra of 96% RR P3HT films prepared by spin-coating CF solutions with a small amount of DCB: 0, 1, 2, 3, and 5 vol %. (b) The absorption spectrum of the film obtained from a 96% RR P3HT solution containing 3 vol % DCB subjected to deconvolution analysis using eq 1. The red line indicates the spectrum of amorphous P3HT chains, and the blue line represents the spectrum of P3HT aggregates in the solidified film. The black line represents the experimental spectrum. (c) The intensity ratios of the (0−0) and (0−1) transitions obtained from 96% RR P3HT films prepared via spin-coating CF solutions with various DCB contents (i.e., 0, 1, 2, 3, and 5 vol %). (d) The left axis indicates calculated exciton bandwidth, W and the right axis indicates percentage of aggregates in the corresponding films.

Spano’s model fitting of UV-vis absorption spectra was employed to perform quantitative analysis on the intra- and interchain interactions of P3HT chains (Figure 3b and Figure S6). According to the assumptions in Spano’s model, the crystalline region that exhibits the lowerenergy vibronic bands consisting of weakly interacting H-aggregates.37,41 The theoretical absorption spectra of the crystalline P3HT aggregates could be described by eq 1.37,41 The


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theoretical spectra of the ordered aggregates were fit to the experimental spectra of resultant P3HT films to calculate the free exciton bandwidth (w), which is related to intrachain molecular ordering (i.e., polymer conjugation length),37,41 and to obtain the relative proportions (interchain molecular ordering) of the ordered aggregates, as shown in Figure 3b. 2

  ( E − E0−0 − mEP − 1/ 2WS m e− S ) 2   e − S S m   We− S A ∝ ∑ G × 1 − × exp −    m m!   2 EP 2σ 2 m=0    

(1)

Where, A is the absorbance of ordered conjugated polymer aggregates, E is the photon energy, Ep is the vibrational energy of the C=C symmetric stretch,41 S is the Huang–Rhys factor (ca. 1.0), W is the exciton bandwidth, σ is the Gaussian linewidth, Gm is a constant as defined by Gm = ∑n(≠m)Sn/n!(n - m), where m and n are differing vibrational levels, and E0-0 is the 0-0 transition energy. W inversely correlates with the intramolecular interaction of an individual chain in P3HT aggregates, which is related to the ratio between the intensities of the (0−0) and (0−1) transitions, as given by eq 2. I 0−0  1 − 0.24W / E p  ≈ I 0−1  1 + 0.073W / E p 

2

(2)

I0−0 and I0−1 represent the intensities of the (0−0) and (0−1) transitions, respectively. The ratios calculated for the P3HT films obtained by spin-coating CF solutions containing different amounts of DCB are compared in Figure 3c. A dramatic increase in the ratio was observed with the addition of 1 vol % DCB (i.e., from 0.81 to 0.88). Upon a further increase in the DCB content up to 5 vol %, the value slightly increased to 0.89 and then became saturated. This result mirrors the reduction in the W value from ca. 114.2 meV, followed by its saturation at ca. 64.0 meV with increasing DCB content. This behavior indicates improved intramolecular ordering of individual polymer chains (Figure 3d). The fraction of P3HT aggregates (i.e., intermolecular


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Figure 4. (a) Grazing incidence X-ray diffraction spectra for 96% RR P3HT films prepared from CF solutions with varied proportions of DCB: 0, 1, 2, 3, and 5 vol %. (b) The corresponding (100) d-spacing values and grain size as a function of DCB vol %.

ordering of P3HT chains) clearly depends on the intramolecular interaction of the polymer chains, as shown in Figure 3d. The relative proportions of aggregated versus amorphous polymer chains for each P3HT film were obtained by using a previously reported calculation method.34 The percentage of P3HT aggregates appeared to be inversely proportional to the W value, which increased from ca. 48.3 % and then saturated at ca. 51.6 % as the DCB content increased to 5 vol %. This result suggests that the enhanced intramolecular ordering led to favorable P3HT π−π interchain interactions; a more rigid backbone is preferable for extended supramolecular assemblies of the polymer chains.24,37 Grazing incidence X-ray diffraction (GIXRD) measurements provide further insights relating to the molecular packing of polymer thin films. The effect of the solvent additive, DCB, on the P3HT microstructure was systematically studied (Figure 4). Upon the addition of DCB, the intensity of the (100) peak corresponding to the lamellar packing of the polymer chains increased, which indicates enhanced intermolecular ordering (Figure 4a).18,38,42 This increase could result from an increase in the size and/or number of the polymer crystallites.18,38,42 Also,


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the (100) peak position slightly shifted from 5.34° to 5.39° upon the addition of DCB, which reveals that the (100) d-spacing had decreased from 1.65 to 1.64 nm as shown in Figure 4b. Detailed analysis of the XRD patterns provided additional insight into the average grain size relating to P3HT lamellar stacking. The average grain size was calculated using the Scherrer equation; 18,43 the crystal grain size correlates with the full width at half-maximum (FWHM) of the (100) peak.18,43 On the basis of the calculations with the Scherrer analysis, the grain size increased upon the addition of DCB; the grain size increased from ca. 6.6 to ca. 11.5 nm as the DCB content increased to 2 vol %, then plateauing with a further increase of DCB content to 5 vol %, as shown in Figure 4b. The increased grain size may be attributed to the preferred crystal growth; the slow evaporation of the solvent facilitates the crystal growth relative to the nucleation process.18,38 The charge transport characteristics of the resultant 96% RR P3HT thin films were investigated by measuring the field-effect mobilities of the corresponding films deposited on FETs with a bottom-gate, bottom-contact configuration. The macroscopic charge transport was significantly improved by the addition of DCB because of the enhanced molecular ordering (i.e., intra- and intermolecular interactions), which agrees well with the AFM, UV-vis, and XRD analysis of the resultant P3HT films. Figure 5a shows the comparison of the mobilities of P3HT films obtained by the addition of DCB and AN, respectively, as a function of the additive concentration. As the DCB concentration increased to 2 vol %, the field-effect mobility dramatically increased from 1.7 × 10-2 to 8.2 × 10-2 cm2 V-1 s-1 and became saturated with a further increase of the DCB content to 5 vol %. The trend in mobility saturation continued as the DCB content increased to 7 vol %, and, then, the charge carrier mobility began to decrease with increasing DCB content, as shown in Figure S7, presumably owing to the decreased wettability


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Figure 5. (a) Average field-effect mobilities of 96% RR P3HT films obtained from P3HT/CF solutions containing various amount of DCB and AN, respectively. (b) Transfer characteristics of OFETs based on 96% RR P3HT films deposited from a pure CF solution, CF/AN (97/3) solution, and CF/DCB (97/3) solution. The channel width (W) and length (L) of the OFETs tested here are 2000 µm and 50 µm, respectively.

of the P3HT films (Figures S1 and S2b). These results suggest that a solvent system containing a good solvent additive with low volatility could provide a scalable processibility in that the charge carrier mobility is maximized over a wide range of additive concentrations. In contrast to the CF/DCB solvent systems, the CF/AN blends yielded no enhanced mobility saturation regime; a remarkable reduction to 7.8 × 10-3 cm2 V-1 s-1 occurred on the further addition of AN up to 5 vol %, whereas an increase in the mobility from 1.7 × 10-2 to 4.2 × 10-2 cm2 V-1 s-1 appeared as the AN concentration increased up to 2 vol %. The abrupt reduction in the mobility at concentrations over 2 vol % of AN is ascribed to the increased density of grain boundaries between crystal domains, which prevent efficient charge transport, as shown in Figure 1b.38 In general, as the state of conjugated polymer solution changes from metastable to highly supersaturated, the nucleation of the solute is preferred to crystal growth.18,24 As described above, a subtle variation in nonsolvent content would cause significant alterations in the morphological properties and, concomitantly, the charge transport characteristics of the resultant


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films. In this respect, the solvent blends using a nonsolvent additive with low volatility are unsuitable for the reproducible fabrication of conjugated polymer films with maximized mobility compared to the solvent systems containing a good solvent additive with low volatility. Figure 5b displays the drain current (ID) versus gate voltage (VG) plots of the OFETs fabricated from three different P3HT films: (i) the pristine P3HT film prepared by spin-coating; (ii) the P3HT film deposited by spin-coating a CF/DCB (97/3) solution; and (iii) the P3HT film deposited by spin-coating a CF/AN (97/3) solution. The CF/AN solvent system led to a lower on-current compared to the CF/DCB blend, although the former afforded higher molecular ordering (intraand intermolecular interactions), as shown in Figures S8a and S8b. This result may be ascribed to the reduction in the grain size (Figure S8c) and inhomogeneity of the resultant films (Figure 1b).15,37 The field-effect mobility of OFETs is directly governed by the contact and channel resistance. Typically, the contact resistance Rc is assumed to affect the charge transport characteristics in an ideal OFET negligibly because the contact resistance Rc is generally much lower than the channel resistance Rch of the active semiconducting polymer layers.44 However, the contact resistance begins to dominate the charge transport characteristics of OFETs as the intrinsic mobility of polymer semiconductors improves.44 To investigate the effect of the solvent additives on the contact and channel resistance in OFET devices, we fabricated a series of OFETs with different channel lengths: 30, 50, 100, 200, and 300 µm. The contact resistance Rc was obtained by extrapolating the measured total device resistance (Rtot = ∂VD/∂ID) as a function of the channel length (L) (Figure 6a). The contact resistance Rc of the P3HT films decreases in the following order: pristine (ca. 8.8 × 106 Ω) > 2% AN (ca. 1.0 × 106 Ω) > 2% DCB (ca. 0.5 × 106 Ω), as shown in Figure 6b.


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Figure 6. (a) Plot of total resistance (Rtot) versus the channel length of OFETs based on P3HT films prepared by spin-coating a pure CF, CF/AN-2%, and CF/DCB-2% solution, respectively. The contact resistance (Rc) is obtained by extrapolating the data to L = 0. (b) The contact resistance (Rc) of the corresponding films, (c) channel resistance (Rch), and (d) ratio of contact resistance (Rc) over the total resistance (Rtot) of the corresponding films as a function of the channel length.

The channel resistance, Rch, which was obtained by subtracting Rc from Rtot, is plotted as a function of the channel length (L) in Figure 6c. The Rch value of the P3HT films is lower for all the different channel lengths in the same order as shown in Figure 6b (i.e., pristine > 2% AN > 2% DCB). It should be noticed that the plot of Rtot versus L (Figure 6a) is similar to that of Rch versus L (Figure 6c) of the OFET devices because the contribution of Rc to Rtot is negligible (Figure 6b) relative to that of Rch to Rtot. As shown in Figures 6b and 6c, the values of Rc and Rch of the OFET devices are much lower for DCB than those of AN, which is thought to result from


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the relatively uniform surface morphology near the metal electrodes and reduced grain boundaries of the resultant P3HT films, respectively.44,45 Figure 6d depicts the ratio of contact resistance to the total resistance measured between the source (S) and drain (D) electrode as a function of the channel length (L). The ratio significantly decreases as L increases for all the P3HT films, indicating that Rc is less important than Rch as the channel becomes longer. Regioregularity (RR) is one of the key parameters that affect the microscopic structure and, thus, the charge transport characteristics of polymer thin film because it directly influences properties of single chain and, thus, assemblies of multichain.37,42,46 Therefore, the influence of RR on the morphological properties and the charge transport characteristics of resultant P3HT thin films through a solvent blend system with a good solvent additive were investigated. As seen in the 96% RR P3HT films (Figures 1b and 1c), nanofiber-like structures appear in the 92% RR films on the addition of DCB, whereas a featureless and amorphous-like morphology is observed in the pristine film (Figures 7a and 7b). However, the nanofibers were smaller in width and length compared to those of 96% RR P3HT, which is consistent with previous reports.37,46 The higher RR P3HT exhibits wider and longer fibrillar structures because of the more planar polymer chain conformation and favorable π−π interactions between polymer chains.37,46 As shown in Figure 7c, the mobility of the 92% RR P3HT films was significantly improved by the addition of DCB. As the DCB concentration increased to 2 vol %, the charge carrier mobility dramatically increases (approximately 6-fold from 3.2 × 10-3 to 1.8 × 10-2 cm2 V-1 s-1), followed by saturation upon the further addition of DCB. Meanwhile, the addition of AN results in a considerable decrease in the mobility as the concentration increases beyond 1 vol %. These trends are similar to those observed from the 96% RR P3HT films (Figure 5a).


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Figure 7. AFM phase images of 92% RR P3HT films prepared by spin-coating CF solutions containing (a) 0 and (b) 2 vol % DCB. (c) Comparison of mobilities of 92% RR P3HT films obtained from CF solutions containing various amount of DCB and AN.

Of the good solvents for P3HT, in addition to DCB, CB, and TCB are well-known highboiling-point solvents (relative to CF). To verify the generality of our method using high-boilingpoint good solvent additives that promote the anisotropic assembly of the polymer chains and, thus, improve the mobility of the resultant films, CB and TCB were also tested as solvent additives for the P3HT/CF solution. As in the case of DCB, anisotropic assemblies of P3HT


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appeared on the respective addition of a small amount (3 vol %) of CB and TCB (Figures 1b and 8a). The lower energy features at ca. 558 and 617 nm increased in the order CB < DCB < TCB, as shown in Figure 8b. In other words, the molecular ordering of the P3HT films is enhanced in the sequence of CB < DCB < TCB, which is consistent with the order of boiling point (i.e., inverse order of volatility) of the solvents. As expected from the AFM and UV-vis data (Figures 8a and 8b), the OFETs based on the P3HT films prepared by the respective addition of CB and TCB also exhibit enhanced charge carrier mobilities as shown in Figure 8c, supporting the generality of the approach using a good solvent additive with low volatility. Specifically, the mobility is enhanced in the order of CB < DCB < TCB, consistent with UV-vis data in Figure 8b, and the highest average mobility (ca. 9.5 × 10-2 cm2 V-1 s-1) was recorded on the addition of TCB (< 5 vol %). It is believed that good solvent additives with low volatility yield films with preferable molecular ordering and morphologies for efficient charge transport; as evidenced in previous reports,47,48 the use of good solvent with low volatility allows the formation of large crystalline grains of P3HT. In contrast, a non-solvent additive, dimethylformamide (DMF) (bp 153 °C) with low volatility led to a lower enhancement of the charge mobility compared to AN (3.4 × 10-2 vs. 4.2 × 10-2 cm2 V-1 s-1) (Figure 5a and Figure S9). We have shown that the addition of a solvent additive (DCB and AN) can profoundly impact the crystal size of P3HT, resulting in improved charge transport properties of the resultant films. However, some research groups presented a counter report that the addition of the solvent additive, 1,8 diiodooctane (DIO) had a negligible effect on the crystal size of the low bandgap polymer,

poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-

[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7), while resulting in very good efficiencies of the solar cells based on the polymer.40,49 The difference in the molecular


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Figure 8. (a) AFM phase images of 96% RR P3HT films obtained by spin-coating a CF/CB3% and CF/TCB-3% solution, respectively. (b) UV-vis absorption spectra of 96% RR P3HT films obtained by spin-coating a respective CF, CF/CB-3 %, CF/DCB-3%, and CF/TCB-3% solution. (c) Comparison of mobilities of films deposited from 96% RR P3HT/CF solutions containing various amount of CB, DCB, and TCB, respectively. UV-vis spectrum of 96% RR P3HT obtained from a CF/DCB-3% solution and average mobility data of 96% RR P3HT films as a function of DCB content were reused for comparison.

structures could be a substantial factor that results in such a difference in the effect of solvent


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additives on the crystalline nature of polymer films.40,49 In general, PTB7 exhibits relatively low crystallinity owing to the incorporation of a less symmetric monomer unit compared to P3HT.50

3. CONCLUSIONS In summary, we have demonstrated the anisotropic self-assembly of P3HT chains during film formation by simply adding a good solvent additive with low volatility into solutions of P3HT in a good solvent with high volatility. Specifically, the addition of a small amount of DCB to P3HT/CF solutions facilitated the formation of crystalline nanofibrillar structures of P3HT during spin-coating. The appearance of nanofibrils is indicative of improved molecular ordering and a reduced grain boundary density of resultant films, and no costly and tedious pre- and/or post-deposition processes were required. The formation of nanofibrillar structures is attributed to the extended drying time of P3HT solution films, which is preferable for the self-assembly of P3HT chains via π−π interactions. Consequently, P3HT films obtained by the addition of a good solvent additive exhibited enhanced charge transport characteristics compared to those prepared by the addition of a non-solvent additive. As compared to the use of a solvent blend containing a non-solvent additive (AN), the use of a solvent system with a good solvent additive, DCB, was revealed to be a facile, effective, and scalable approach. The charge carrier mobility was maximized over a wide range of the additive concentrations, indicative of scalable processibility. The contact and channel resistances were found to be lower in the P3HT films prepared by the addition of DCB rather than AN, attributable to the relatively uniform surface morphology and reduced grain boundaries of the polymer films, which are indicative of efficient charge injection and transport in the OFETs. The platform presented here has been shown to be applicable to other good solvent additives with low volatility, such as CB and TCB. The good solvent additive


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with lower volatility has been revealed to be more effective to enhance the molecular interactions and, thus, the charge transport properties of resultant films; the charge carrier mobility was improved up to ca. 9.5 × 10-2 cm2 V-1 s-1 on the addition of less than 5 vol % TCB. Combined with film deposition techniques including shear-coating, ink-jet printing, stamping, and bladecoating, the approach presented here is expected to be attractive for fabricating high-performance flexible electronic and photonic devices for a broad spectrum of applications.

4. EXPERIMENTAL SECTION

Materials. The 96% regioregular (RR) P3HT (Mw of 43.7 kDa and Mn of 19.7 kDa) and 92% RR P3HT (Mw of 69.0 kDa and Mn of 34.2 kDa) were purchased from Rieke Metals Inc. and Sigma Aldrich Chemical Co., respectively. Chloroform (anhydrous grade), chlorobenzene (anhydrous grade), 1,2-dichlorobenzene (anhydrous grade), 1,2,4-trichlorobenzene (anhydrous grade), dimethylformamide (anhydrous grade), and acetonitrile (anhydrous grade) were purchased from Sigma-Aldrich. All chemicals were used without further purification.

Sample Preparation. All polymer solutions were prepared at 55 °C in a sealed glass vial in air. The solvent additives, chlorobenzene, 1,2-dichlorobenzene, 1,2,4-trichlorobenzene, and acetonitrile were added in the requisite quantity to chloroform to prepare 2 mL of solvent mixtures with various concentrations (0, 1, 2, 3, and 5 vol %) in a 20 mL vial. Ten milligrams (10 mg) of P3HT was added into each of the as-prepared solvent mixtures. Subsequently, the solutions were stirred at 55 °C for 60 min in a sealed vial to ensure the complete dissolution of the polymer.

Device Fabrication and Characterization. The OFETs with a bottom-gate, bottomcontact configuration were constructed following the procedures reported in a previous paper.34


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Prior to spin-coating, all substrates were cleaned in a UV-ozone cleaner (Novascan PSD-UV) to remove any organic residues. Subsequently, polymer thin films were spin-coated onto precleaned substrates at 1500 rpm for 60 s in the air using a spinner (WS-650MZ-23NPP, Laurell). The asprepared OFETs were placed in a vacuum oven (1 Torr) overnight at 55 °C to remove residual solvent. Finally, the OFETs were characterized in a N2 atmosphere glovebox using a Keithley 4200 Semiconductor Characterization System. The field-effect mobility was estimated in the saturation regime (VDS = − 80 V) by using the eq 3.

IDS =

WCOX µ (VGS − VT) 2 2L

(3)

Where, W and L are the transistor channel width and length, respectively, IDS is the drain current, VGS is the gate-source voltage, COX is the capacitance of the SiO2 gate dielectric per unit area, and VT is the threshold voltage.

Characterizations. UV-vis absorption spectra were recorded using An Agilent 8510 UVvis spectrometer. GIXRD-ray diffraction data were collected using a Panalytical X’Pert Pro system. The films deposited on the substrate was aligned with an incidence angle of 0.2°, while the XRD measurement was performed in a scanning interval of 2θ between 3° to 20°. An ICON dimension scanning probe microscope (Bruker) operated in tapping mode with a silicon tip (RTESP, Bruker) was employed to characterize the film morphologies.

ASSOCIATED CONTENT

Supporting Information Photographs of the P3HT solutions and the corresponding films; thickness and surface roughness; average field-effect mobilities; AFM images; UV-vis absorption spectra; GIXRD


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profiles; d-spacings and grain sizes. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected] *E-mail: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was financially supported by Chonnam National University (Grant number: 20162427) and the National Research Foundation of Korea (NRF) grant by the Korea government (MSIP) (No. 2017R1C1B1004605). REFERENCES (1)

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