Simultaneous Control over both Molecular Order and Long-Range

Dec 1, 2014 - ABSTRACT: Control over both molecular order and long-range alignment order in films of the donor−acceptor copolymer of 3,6-...
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Simultaneous Control over both Molecular Order and Long-Range Alignment in Films of the Donor−Acceptor Copolymer Haiyang Wang,† Liang Chen,† Rubo Xing,† Jiangang Liu,† and Yanchun Han*,†,‡ †

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin Street, Changchun 130022, People’s Republic of China ‡ University of the Chinese Academy of Sciences, No. 19A Yuquan Road, Beijing 100049, People’s Republic of China S Supporting Information *

ABSTRACT: Control over both molecular order and long-range alignment order in films of the donor−acceptor copolymer of 3,6bis(thiophen-2-yl)-N,N′-bis(2-octyl-1-dodecyl)-1,4-dioxo-pyrrolo[3,4-c]pyrrole and thieno[3,2-b]thiophene (PDBT-TT) was demonstrated via off-center spin-coating (OCSC) from its blend solution with polystyrene (PS). It was found that the dichroic ratio (DR) of OCSC blend films was dependent on both the physical process of spin-coating and the effect of PS chains. The highest DR of 2.75 was obtained via OCSC from the blend solution in oDCB at 1500 rpm. Meanwhile, both the intrachain and interchain molecular order were improved in blend films compared with neat ones, which were indicated by the red-shift of the max absorption, enhanced J-aggregation absorption, and smaller π−π stacking distance (from 3.77 to 3.70 Å). According to the results of the investigation into the macro anisotropy, micro morphology, solution rheology properties, and photophysics features of films, an overall mechanism of simultaneous control over molecular and long-range order of D−A copolymer films was proposed. On the one hand, a larger viscosity and the pseudoplastic nature of the solution tuned by choosing good solvents with high boiling points and adding PS resulted in a better chain disentanglement, better shear transfer, and a slower contact line receding velocity to induce an enhanced alignment of chains and thus fibrillar aggregates. Also, the critical contact line receding velocity for alignment dominated by the solvent evaporation rate accounted for the variation of DR with OCSC rates. A vertical phase separation accompanying the formation of aligned fibrils during OCSC was also confirmed due to the friction shear between air and solution surface. On the other hand, the negligible dependence of the blend OCSC film’s photophysical and morphological features on the solvent suggested the critical role of PS in determining the better intrachain conjugation in blend films, which was attributed to multiple reasons, like limited phase separation room, a coil-toward conformation promotion, and a high surface energy. Furthermore, the enhanced π-attraction and smaller steric hindrance induced by improved intrachain conjugation accounted for the smaller π−π stacking distance in the blend films than that in the neat ones.

1. INTRODUCTION Conjugated polymers (CPs) have attracted a lot of attention and been extensively researched due to their good electronic properties and solution processable advantages in field-effect transistor (FET) production.1,2 In quantum mechanics theory, a good charge carrier transfer in (semi)crystalline CP films relies on the continuous π-conjugation pathways built by assembly of chains. Thus, the carrier transfer in a crystalline region and through different regions both contribute to the final apparent device performance.3−5 First, the carrier mobility in crystalline region, which determines the electronic properties of CPs in nature,3 is dependent on two quantum mechanics parameters, reorganization energy and transfer integral.5−7 A smaller reorganization energy and a larger transfer integral lead to a higher carrier mobility, which can be realized through increasing conjugation length, improving backbone planarity, and decreasing interchain π−π stacking period. Accordingly, a © XXXX American Chemical Society

lot of effort has been made by chemists to synthesize novel CPs with the structural features mentioned above.8−10 Donor− acceptor (D−A) copolymers are a promising group of these materials due to their improved planarity,10 strong intrachain electron transfer, and interchain interactions assisted by D−A interaction,11 which are all driving forces to form thermodynamically stable structures. A carrier mobility record of 12.04 cm2V−1s−1 has been reported in the example of DPPDTSE, with a short π−π stacking period of 3.58 Å.12 Second, the distribution order of crystalline regions in films also plays an important role in tuning carrier transport. A continuous transfer pathway with low barrier and grain boundary angle, i.e., one made up of smooth, interconnected crystalline regions with Received: September 21, 2014 Revised: November 25, 2014

A

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prevalent intrachain transfer, is preferred by carrier transport.4,5 Furthermore, the alignment of such pathways can decrease the amount of grains and improve the carrier mobility a lot.13 So far, the alignment of microstructures in films has been demonstrated by physical methods like dip-coating,14−16 zone-casting,17 doctor-blading,18,19 off-center spin-coating,13 etc. During these processes, kinetic factors such as solvent evaporation, contact line recession, and shearing have an important effect on the alignment of crystalline regions, which implies an important origin of alignment.14,20 Therefore, tuning carrier transport in CP films can be regarded as a combination of both thermodynamic and kinetic control over chain assembly. Apart from a great deal of work on the neat films of CPs, blending with flexible polymers (FPs) has been found to be an effective strategy to control the molecular and morphological order in CP films.21−26 First, Goffri et al. and Lu et al. did systematic research into the morphology and electronic properties of blend films of P3AT and FPs via meltcrystallization and solution process, respectively.21,22 Although the crystallization induced phase separation between P3HT and PE or i-PS crystals corresponded to the enhanced carrier mobility when cooling the molten blend down to the crystallization temperature of PE or i-PS, in blend films directly prepared from solution, it led to a greatly decreased interface area between the flexible polymer matrix and the P3BT fibrillar aggregates and therefore lower electrical conductivity. Thus, it was thought that an amorphous FP matrix could promote a maximal dispersion of CP fibrils, larger interface area between CPs and matrix, and more transport pathways. Second, the introduction of FP regardless of crystallinity has been proven to improve the intrachain molecular order and morphological order in CP films. Hellmann et al. reported the universal feature of max absorption red-shift after being blended with PEO for CPs like P3HT, pBTTT, PPV, PFO, and F8BT.23 Wang et al. reported the formation of well aligned bundle-like morphology containing 10 nm wide nanofibrils with a spectral feature indicating improved intrachain conjugation and J-aggregation in PDBT-TT films.24 Xu et al. and Benetti et al. prepared a P3HT hierarchical fibrillar network in blend with PS via melt crystallization and P3HT nanorods with a much elongated conjugation length in 2% cross-linked PMMA gels compared to nanowhisker network in film spin-coated from anisole solution.25,26 In their works, the ordered assembly of chains was attributed to the dispersion of the matrix and template effect of gel cages, respectively. Thus, blending with FPs can be regarded as a promising method to improve the order in CP films, considering the effects mentioned above. In this work, we chose a D−A copolymer with excellent electronic properties, PDBT-TT, and its blend with amorphous polystyrene to prepare films via off-center spin-coating (OCSC) for clarifying the overall mechanism of tuning shortrange and long-range order in films, respectively.13 The physical parameters related to the OCSC process and their correlation with film anisotropy was systematically investigated. The effects of the PS matrix on the intrachain conjugation and the interchain packing were also discussed. The results here were not only helpful to understand the physics of CP chain assembly, but also practical to efficient solution processing of CPs.

2. EXPERIMENTAL SECTION 2.1. Materials. The copolymer of 3,6-bis(thiophen-2-yl)-N,N′ -bis(2-octyl-1-dodecyl)-1,4-dioxo-pyrrolo[3,4-c]pyrrole and thieno[3,2-b]thiophene (PDBT-TT) (Mn = 29k, PDI = 3.9) was purchased from Solarmer Inc (Figure 1). The polystyrene (PS, Mw = 13.2k) was

Figure 1. Chemical structure of PDBT-TT. purchased from Sigma-Aldrich Co. The solvent chloroform (CF) was from Beijing Chemical, China; chlorobenzene (CB) and orthodichlorobenzene (oDCB) were purchased from Sigma-Aldrich Co. The glass slides were cleaned in piranha solution (70/30 v/v of concentrated H2SO4 and 30% H2O2) at 90 °C for 20 min and then rinsed with deionized water and finally blown dry by nitrogen. 2.2. Sample Preparation. All solutions of PDBT-TT in the neat form or blend form were prepared at a concentration of 5 mg/mL and total 55 mg/mL (PDBT-TT/PS = 1/10, w/w), respectively. Solutions were heated for 10 min (at 50 °C for CF and 90 °C for CB and oDCB) and then placed at room temperature overnight before use. The film samples were prepared according to the method called offcenter spin-coating (OCSC) reported in ref 13 and also via central spin-coating (SC). The off-center distance was 2 cm, and spin-coating rates of 500, 750, 1000, 1500, 2000, and 2500 rpm were chosen. For solutions in CF, CB, and oDCB, the spin-coating time was 1, 3, and 5 min for total evaporation of solvents, respectively. 2.3. Characterization. The macroscopic morphology of films were characterized in the polarized light mode on a Karl-Zeiss optic microscopy (Germany) at 50× magnification with crossed polarizer and analyzer. Before TEM characterization of microscopic morphology, PS in blend films was removed by being rinsed in ethyl acetate. Thus, only PDBT-TT layers remained on the substrates. Neat films and rinsed blend films were floated from the substrate with deionized water after being placed in an atmosphere of hydrogen fluoride vapor for several seconds, and then picked up by copper grids. TEM images and selected area electron diffraction patterns were obtained with a JEOL JEM-1011 transmission electron microscope (Japan) operated at an accelerating voltage of 100 kV. According to the diameters of diffraction rings or arcs (L) and the camera constant of the microscopy (C), the π−π stacking period (d) was calculated as follows:

C = L1d1 = L 2d 2 To quantitatively measure the anisotropy of OCSC films, the UV− Vis−NIR absorption spectra with radial directions parallel and perpendicular to polarization direction were recorded on an AvaSpect-3648 optical fiber spectrometer (Netherlands) equipped with a polarizer. The anisotropy was defined as the dichroic ratio between max absorption parallel and perpendicular to polarization direction. Besides, absorption spectra of all SC and OCSC films under natural light were also recorded on this spectrometer. The viscosity of solutions was measured on a Brookfield DV-III Ultra programmable rheometer to investigate the role of shearing during OCSC. For each solution, a volume of 500 μL was injected to the liquid cell. During measurement, the rotation rate of the cone was increased until it reached 90% of the maximal torque and the viscosity at each specific rate was recorded.

3. RESULTS AND DISCUSSIONS 3.1. Anisotropy of PDBT-TT Films Controlled by Physical Process of OCSC. 3.1.1. Formation of Macro B

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Figure 2. Optic microscope images of films prepared from solutions of (1) PDBT neat by central spin-coating, (2) PDBT-TT/PS blend by central spin-coating, (3) PDBT-TT neat by OCSC and (4) PDBT-TT/PS blend by OCSC in (a) CF, (b) CB, and (c) oDCB under crossed polarizer and analyzer. The arrow indicates the radial direction during OCSC. The scale bar represents 1 mm. The directions of crossed polarizer and analyzer are also shown. The images of OCSC films were obtained at 45° with respect to the directions of crossed polarizer and analyzer. All films were prepared at a spin-coating rate of 1500 rpm.

Figure 3. TEM images of OCSC films coated at 1500 rpm: (a) CF neat, (b) CB neat, (c) oDCB neat, (d) CF blend, (e) CB blend, and (f) oDCB blend. The scale bar represents a length of 1 μm. The wide arrow indicates the radial direction during OCSC, and the red and white arrows in electron diffraction patterns represent π−π stacking and backbone directions, respectively.

and Micro Anisotropy in PDBT-TT Films with the Assistance of PS and Centrifugal Force. The optic microscope images of OCSC films obtained at 45°with respect to crossed polarizer and analyzer (Figure 2, Columns 3 and 4) displayed large areas of bright zones in contrast to the dark images obtained at 0° (Supporting Information (SI) Figure S1), which indicated that OCSC films had more significant apparent optic anisotropy than optic isotropic SC ones in macro scales. Although bright domains were observed in SC films coated from solutions in

CF and oDCB, they were not as continuous as those in the OCSC films. It indicated that the alignment of crystalline domains in SC films was random. Instead, the bright aligned zones in OCSC films showed a very good continuity, especially for blend films. In CB and oDCB blend OCSC films, the stripelike zones parallel to radial direction implied a good alignment of PDBT-TT chains in micro scale (Figure 2b4,c4). In fact, the stripe-like microscopic morphology with the long axis of stripes parallel to radial direction in OCSC blend films also implied C

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Figure 4. Optic microscope images of PDBT-TT/PS films prepared from solutions in (a, a′) CF, (b, b′) CB, and (c, c′) oDCB at different spincoating rates at (a, b, c) 45° and (a′, b′, c′) 0° with respect to the crossed polarizer and analyzer. The single arrow indicates the radial direction during OCSC, and the crossed arrows indicate the directions of polarizer and analyzer. The scale bar represents 1 mm.

(Figure 4a). For films coated at above 1000 rpm, large areas of continuously aligned domains were obtained, except for that at 2500 rpm, which showed a slightly decreased continuity (Figure 4a). The enhanced continuity of aligned domains corresponded to the increased dichroic ratio (DR) of CF blend films in Figure 5 and Table 1. For the CB blend solution (Figure 4b,b′), the good anisotropy of films coated from 500 to 1500 rpm seemed unchanged. However, the continuity was destroyed when the rate was above 1500 rpm. For the oDCB blend solution (Figure 4c,c′), the alignment was improved when the rate was also increased from 500 to 1500 rpm and decreased above 1500 rpm, as in the CB solution. The apparent variation of macroscopic morphology for CB and oDCB blend films with coating rate (Figure 4b,c) corresponded to the quantitative variation of DR (Figure 5 and Table 1). Accordingly, it was confirmed that the spin-coating rate did play an important role in alignment of PDBT-TT chains and crystalline domains. In the following, the detailed origin for this shear-induced alignment would be investigated and discussed. 3.1.2. Rheology Effect of Solutions Brought by PS and Solvents as an Intrinsic Origin of Anisotropy. In Figure 5 and Table 1, it was found that the alignment degree of OCSC blend films was significantly dependent on the solvents. With the increase of solvent viscosity and boiling point, CF(0.57 cP, 60

that the packing of chains were along the off-center radial direction (Figure 3d−f), which was different from the isotropic feature of OCSC neat films (Figure 3a−c). According to interchain π−π stacking direction indicated by the arcs representing (0k0) diffractions perpendicular to both radial shear direction indicated by the arrows in TEM images and backbone direction, it was proven that the direction of PDBTTT backbones was mainly parallel to the radial direction (Figure 3d−f). It suggested that the radial shearing in OCSC process played an important role in alignment, and the better microscopic alignment of PDBT-TT stripe domains led to the better optic anisotropy of OCSC blend films than OCSC neat ones. Considering that OCSC blend films displayed the best optic anisotropy, they were chosen to investigate the formation mechanism of macro and micro anisotropic morphology. In Figure 4, it was found that all blend OCSC films displayed a good anisotropy covering a large area, as indicated by the intense brightness contrast between the optic microscopy images at 45° and 0° with respect to the directions of crossed polarizer and analyzer. Meanwhile, the macro-scale anisotropic features of blend films coated from solutions in different solvents were dependent on OCSC rate. In films from CF solution, an increase of spin-coating rate from 500 to 1000 rpm led to a stepwise improvement of aligned domain continuity D

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where ρ, ω, η, r, and H0 were solution density, angular velocity, viscosity, distance from center, and constant, respectively. Viscosity of blend solutions (SI Figure S2) was an important parameter to determine Vr. For a unit volume of solution with the same distance from the center under the same angular velocity, Vr was mainly related to the value of ρ/η. In Table 2, the ρ/η for CB blend (0.174) and oDCB blend (0.223) solutions was much smaller than that for the CF blend solution (0.512). According to the theory proposed by Rogowski et al., a slow receding velocity below the critical alignment velocity (Uth) could lead to a large concentration gradient of solute, making the crystallization rate match the receding vecolity of the solution contact line to realize crystalline domain alignment.14 In this view, the slow receding of solution promoted the increase of DR in CB and oDCB blend films than CF blend ones. Along with influencing the contact line receding, the rheological properties reflected by viscosity had an important effect on shear thinning. The friction of solution against air and substate could be regarded as an origin of shear, and the rheological properties of solution might determine the practical effect of shear on chain conformation and motions during the high-speed process of spin-coating.27 The scale index of viscosity against shearing rate (n; logη ∼ nlogω) could indicate the Newtonian property of the fluid. When n is zero, the fluid is Newtonian, and when n is minus, the fluid is pseudoplastic. For CF blend solutions, its viscosity was the smallest (3.00 cP) and n was nearly zero. Instead, the viscosity of CB and oDCB blend solutions were 6.64 cP and 6.08 cP, respectively, indicating more chain entanglements and domain cross-linking points (Scheme 1a) than in CF solutions. Fortunately, the values of n for CB and oDCB blend solutions were −0.294 and −0.223, respectively, indicating their pseudoplastic nature. Shear thinning was a common feature of pseudoplastic fluid, which promoted chain disentangling for a further ordered restacking along the shear direction (Figure 3d−f, Scheme 1b,c). Meanwhile, the remaining cross-linking points could transfer shear to the following chains nearby (Scheme 1b). Thus, the disentanglement was dramatically promoted by stronger shear arising from larger coating rate (500−1500 rpm) to increase the DR for these pseudoplastic fluids. In a word, the more significant pseudoplastic nature was thought to be critical to promote the alignment of the crystalline domains. As to the maximal DR at 1500 rpm for CB and oDCB blend films, the origin was their smaller Uth. The critical receding rate could be roughly defined as follows according to results by Rogowski et al.:14

Figure 5. (a) An example of absorption with the the radial direction parallel and perpendicular to the polarized direction, with the arrow indicating the peak for calculating DR and (b) the variation of dichroic ratio of OCSC films with spin-coating rates for different solutions.

°C) < CB(0.80 cP, 131 °C) < oDCB (1.32 cP, 180 °C), the maximal DR of OCSC blend films had an opposite order of oDCB(2.75) > CB(2.36) > CF(1.97). Besides, the maximal DR of CB and oDCB blend films were obtained at the coating rate of 1500 rpm, unlike at 2500 rpm for CF blend films. To find the origin of these results, insights had to be put into the kinetic process of OCSC. In the beginning stage of OCSC, the large centrifugal force caused the contact line of the solution (threephase-line) to recede from the center along radial direction. Two important factors dominated in the alignment of crystalline domains associated with contact line recession, that is, the receding velocity and its relation to the critical alignment velocity. At first, the receding of solution threephase-line (Vr) could be defined as follows:20 Vr ≈ (ρω 2rH0)/η

Uth ≈ Q ev 0.6σ 0.1ρ−0.3 η−0.4

(2)

where Qev and σ were solvent evaporation flux and solution surface energy, respectively. In Table 2, it was found that the

(1)

Table 1. Variation of Dichroic Ratio of OCSC Films with Spin-Coating Rate for Different Solutionsa

a

DR

500 rpm

750 rpm

1000 rpm

1500 rpm

2000 rpm

2500 rpm

CF neat CB neat oDCB neat CF blend CB blend oDCB blend

1.114

1.111 1.136 1.298 1.642 1.960 1.972

1.172 1.402 1.739 2.061 2.104

1.461 1.837 2.364 2.746

1.445 1.970 1.475 2.387

1.468 1.970 1.605 2.065

1.393 1.713 2.033 1.592

The blank in table represents a DR of almost 1. E

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Table 2. Parameters Associated with Dynamic Process during Off-Center Spin-Coating for Different Solutions ηa/cP σ/mN m−1 ρ/g cm−3 n Qev/mmol s−1 Qev0.6σ0.1ρ−0.3η−0.4/10−4 mol0.6m1.3kg−0.6s−0.4 ρη−1/106s m−2 a

CF neat

CF blend

CB neat

CB blend

oDCB neat

oDCB blend

1.91 27.14 1.485 −0.046 5.47 × 10−3 6.62 0.777

3.00 27.14 1.535 −0.025 5.47 × 10−3 5.47 0.512

3.35 33.14 1.105 −0.124 4.74 × 10−4 1.36 0.330

6.64 31.81 1.155 −0.294 4.74 × 10−4 1.01 0.174

5.00 36.58 1.305 −0.171 5.67 × 10−5 0.32 0.281

6.08 35.80 1.355 −0.223 5.67 × 10−5 0.28 0.223

measured at a shearing rate of 40 rpm on the rheometer.

Scheme 1. Formation Process of PDBT-TT Aligned Fibrillar Bundles

value of Qev played an important role on the Uth value. Compared to Uth of CF blend solution (5.47), that for the CB (1.01) and oDCB (0.28) blend were much smaller. It resulted in the fact that coating rates above 1500 rpm exceeded Uth, and the crystallization of PDBT-TT became evaporation controlled. Therefore, the alignment continuity was destroyed and large cracks on the macro scale appeared, causing DR to decrease (2000 rpm and 2500 rpm in Figures 4b and Figure 5). As to the reason why oDCB blend films had larger dichroic ratios than CB blend ones, it might be (i) the longer growth time due to the slower evaporation of oDCB with a higher boiling point, and (ii) a better match between rates of contact line recession, crystallization, and evaporation, in other words, a smaller difference between the spin-coating rate and Uth. 3.1.3. Vertical Phase Separation due to Alignment. Besides the in-plane alignment of PDBT-TT blend films, a vertical distribution of phases related to nucleation of PDBT-TT was also observed. When rinsing CF blend films into ethyl acetate, a layer of green PDBT-TT layer was peeled off the substrate after the PS was dissolved. This layer was made of aligned PDBT-TT fibrillar bundles (Figure 2d) and displayed optic anisotropy (SI Figure S3). After removing the PS, another PDBT-TT layer remained on the substrates and had no anisotropic features both in POM and TEM images (SI Figure S3). For CB and oDCB blend films, the continuous PDBT-TT layer on substrates might break somewhere when being rinsed in ethyl acetate for a long time though it was not peeled off substrates. Thus, it indicated that the aligned PDBT-TT structures formed near the liquid−air interface and might precipitate to substrate

owing to gravitational force (Scheme 1d−f) before the total solidification of the liquid layers.28 As to CF blend films, the aligned PDBT-TT structures did not precipitate onto substrate and stayed in the upper layer because of the short evaporation time of solvent. Fortunately, the longer evaporation time of CB and oDCB blend solutions allowed the precipitation of aligned structures across PS matrix and finally onto substrates. Combining the shear-induced alignment and vertical phase separation, the formation process of PDBT-TT layers with aligned domains could be understood better in a threedimensional mode. 3.2. Molecular Order in Films Controlled by PS Matrix. 3.2.1. PS As the Dominating Factor of the Improved Intrachain Conjugation. As mentioned above, the crystalline domains of PDBT-TT were aligned with backbones parallel to radial shearing direction and fibrillar long axis, which was potentially good for carrier transport because of the efficient intrachain pathways.5,17,29 Besides, the alignment of domains was also favored to decrease the grain boundary angle and hopping barriers adverse to carrier transport.4 However, it was noted that the carrier transport in crystalline domains played an intrinsic and important role in determining the electronic properties of the conjugated polymers.3 The carrier mobility in crystalline domains was dependent on the molecular order of chain packing comprising an intrachain part and an interchain part.30 In the following, the morphology and molecular order in OCSC films will be investigated and discussed. In Figure 3, a large contrast between neat and blend OCSC films was observed. In neat films, random small domains F

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displayed more isotropic features than that of the blend films, corresponding to their smaller DR (maximal 1.468). Among neat films, oDCB neat films displayed clearer fibrillar and anisotropic features (Figure 3c). The enhanced morphological order indicated the underlying higher molecular order of chains reflected in the absorption spectra (Figure 6). The larger AJ/AH

AJ/AH (a decreasing H-absorbance here) indicated a more prevailing and intense J-aggregation and intrachain conjugation when it was larger than 1 according to HJ-model. In the morphological view, these photophysical results corresponded to clear fibrillar bundles with more anisotropic features in blend films (Figure 3d−f). The microscale morphology of films spincoated at different rates from blend solution in the same solvent was nearly the same (SI Figure S4). The differences between blend films coated from solutions in varying solvent resided in the bundle width. The width in CB and oDCB blend films was smaller than that of the CF ones. However, this morphological variation did not correspond to a large difference in intrachain molecular order. In absorption spectra, the max absorption position and AJ/AH ratio for blend films were similar. Therefore, it indicated that the factors dominating the molecular order of PDBT-TT was the PS matrix and not the solvent. In other words, the molecular order in crystalline domains depended on thermodynamic factors. The flexible PS matrix as an immiscible environment played a quite different role from that of the conjugated polymer matrix on improving intrachain conjugation, perhaps in terms of disentanglement, promoting diffusion, the template effect, etc.24 First, the molecular weight of PS had a critical effect on the morphological order. In our previous work, it was shown that PS with a molecular of 1k led to narrow PDBT-TT stripes with random orientations in blend films and a more blue-shifted Jabsorption compared with blend films with PS13.7k. It was attributed to the better miscibility between PS1k and PDBTTT causing too narrow room for PDBT-TT to orderly aggregate.24 Here, we also investigated the morphology of PDBT-TT blend films with PS97.4k and found the separated leaf-like fibrillar domain with poor connectivity to each other and random orientations (SI Figure S5a). Meanwhile, the absorption spectrum of PDBT-TT/PS97.4k film was nearly the same as neat one, and displayed a smaller AJ/AH ratio than PDBT-TT/PS13.7k film (SI Figure S5b). The less ordered morphology corresponded to the less prevailing J-aggregation. It could be attributed to the fact that longer PS chains hindered the motion of PDBT-TT chains heavily and caused the poor miscibility between PS97.4k and PDBT-TT. Thus, the domains of PDBT-TT were separated from each other and lost their alignment order. Therefore, a proper molecular weight of PS controlled an optimal miscibility with PDBT-TT, which ensured both a sufficient diffusion ability of PDBT-TT and a proper phase separation degree for alignment of fibrillar domains. Second, the surface energy of PS13.7k matrix might also play an important role in promoting the extension of intramolecular conjugation. According to the contact angle of water and glycol on surface of neat films, the surface energies of PS and PDBT-TT were calculated to be 40.4 mJ/m2 and 20.8 mJ/m2, respectively (SI Table S1). The higher surface energy of the PS matrix than that of the PDBT-TT and solvents led to a more planar conformation of PDBT-TT with a better thermodynamic stability, which was possibly due to a better wetting of PDBT-TT domains on its interface against PS with a higher surface energy. However, in neat solutions, PDBT-TT domains had a poorer wettability on the “interface” against itself with a lower energy surface, which was not favored for PDBT-TT chains to adopt a more stable conformation. 3.2.2. Stronger Interchain Interaction Induced by an Elongated Intrachain Conjugation. Besides intrachain order, the interchain packing indicated by π−π stacking distance also had a profound effect on the carrier transport through tuning

Figure 6. UV−Vis−NIR absorption spectra for neat and blend films coated at 1500 rpm from solutions in different solvents (a) and its magnification from 675 to 900 nm (b). The direction of the arrows indicates an enhanced AJ/AH ratio.

ratio of oDCB neat films indicated more prevailing Jaggregation and extended intrachain conjugation, which corresponded to more ordered molecular packing in turn.24 It might be partly attributed to the slow evaporation of oDCB providing sufficient time for chains to crystallize into a more stable structure. Besides, a larger surface tension of solvent could lead to a thermodynamically stable crystalline structure, according to the work of Giri et al.31 In this work, the surface tension of oDCB was largest (37.40 mJ/m) in all solvents (27.14 mJ/m for CF and 33.28 mJ/m for CB), which corresponded to the best molecular order in oDCB films. Compared with neat films, the J-aggregation and intrachain conjugation in blend films with PS13.7k were largely improved, indicated by the larger AJ/AH ratio and red-shifted max absorption regardless of solvent. In this work, and ref 24, we took both H- and J-aggregates into account, and assigned 0−1 absorption at ca. 750 nm to a contribution of H-aggregates and 0−0 absorption at ca. 830 nm to a contribution of J-aggregates according to work by Kirkus et al. and the HJ-model of conjugated polymers proposed by Yamagata et al.,32,33 which were named as AH and AJ here, respectively. Therefore, a higher G

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transfer integral according to quantum mechanics.5,7 In this work, it was found that the π−π stacking distance (dπ−π) in PDBT-TT crystals was decreased significantly after being blended with PS, which was often demonstrated via chemical synthesis or thermal annealing for conjugated polymers. Coating rates did not alter dπ−π largely, in spite of a slight increase of dπ−π (Figure 7 and Table 3) and tiny blue-shift of

the weight content of backbones in blend with PS was no more than 10%, but the PS matrix also resulted in an obvious decrease of dπ−π as a blend component, unlike the results of Fang et al.34 It suggested that the origin of the smaller dπ−π in this work was different from the effect of PS chains linked with backbones covalently. However, there was a significant red-shift of max absorption of PII2T containing 33% monomer with PS side chains compared with PII2T containing 0−10%, corresponding to the smaller dπ−π of the former in the work of Fang et al. This relationship between absorption and dπ−π was similar to our results. However, although blending with PS seemed different from making the branching point further from the backbones, the importance of relieving the steric hindrance for a closer conjugated plane packing proposed in the sidechain design had to be noted. Accordingly, a mechanism of decreasing dπ−π could be proposed as follows. First, an extended intrachain conjugation and better planarity in blend films were indicated by the red-shift of max absorption and higher AJ/AH ratio than those of the neat ones (Figure 6). On the one hand, it increased the attraction between conjugated segment planes due to enhanced donor−acceptor interactions and van-derWaals force.11 On the other hand, good planarity was favored to decrease the face-to-face dihedral angle between adjacent conjugated planes, which could also result in a larger transfer integral to induce a smaller dπ−π.37 Second, the improved Jaggregation of chains also reflected a more ordered distribution of side-chains with less chance to move outside the backbone plane, which helped to relieve steric hindrance of bulky sidechains against a closer interchain stacking.35,38 Therefore, the more planar conformation of PDBT-TT chains reflected in absorption spectra (Figure 6) was thought to be a driving force for smaller dπ−π in blend films. As to the reason why intrachain conjugation was improved by blending with PS, the pretuning of PDBT-TT chain conformation, promoting molecular motion, template effect, and high surface energy of PS matrix were possible factors as mentioned in section 3.2.1.24,39 It was unclear and needed a further research in the future. 3.3. Overall Formation Mechanism of PDBT-TT Morphology in Blend Films with Simultaneously Improved Molecular Order and Long-range Alignment Order. According to the discussions above, we could propose an overall formation mechanism of PDBT-TT anisotropic morphology in blend OCSC films in a three-dimensional space, as shown in Scheme 1. At the beginning of OCSC, the entangling of PDBT-TT chains in blend solutions were severe (Scheme 1a). As the OCSC process went on, the radial friction between air and the top layer of liquid layers as the shear force started to exert its effect on in-plane alignment of PDBT-TT domains (Scheme 1b,d). The more significant pseudoplastic nature of the blend solution in CB and oDCB resulted in more effective shear-thinning than CF solutions, which promoted disentangling of chains for their reorganization into fibrillar

Figure 7. Variation of π−π stacking distances with spin-coating rates for OCSC neat and blend films coated from solution in different solvents. The direction of arrows represents the decrease of dπ−π in blend films compared with neat ones.

spectra with faster coating rate (SI Figure S6). Therefore, it suggested again that interchain molecular packing was dominated by PS. Fang et al. introduced polystyrene as sidechains into the isoindigo-containing donor−acceptor backbones to decrease the weight content of backbones.34 When the weight content of backbones decreased from 36.3% to 26.7%, dπ−π in films decreased from 3.70 to 3.61 Å. However, a content of PS-containing unit lower than 10% was proven not to alter the crystalline structure of copolymers in their work. Besides, Zhang et al. did systematic research into the effect of branching point in side-chains on the interchain packing of naphthalene diimides-based molecules.35 They proposed that molecular packing arose from a competition between π−π interactions, hydrophobic interactions between long branched alkyl chains, and steric effect of N-alkyl substitutes. A branching point farther from backbones facilitated a decrease of the steric hindrance from branched alkyl chains and thus closer π−π stacking. Many groups have obtained smaller dπ−π according to this synthesis strategy.12,35,36 Kang et al. introduced longer branched side-chains with a branching point farther from backbones into DPPDTSE backbones and its dπ−π decreased to 3.58 Å compared with its derivative with shorter side-chains and closer branching point (dπ−π > 3.7 Å), resulting in a much enhanced carrier mobility to 12.04 cm2V−1s−1.12 In this work,

Table 3. Variation of π−π Stacking Distances of Off-Center Spin-Coating Films with Spin-Coating Rate for Different Solutions dπ−π/Å

500 rpm

750 rpm

1000 rpm

1500 rpm

2000 rpm

2500 rpm

CF neat CB neat oDCB neat CF blend CB blend oDCB blend

3.74 3.74 3.68 3.69 3.77

3.81 3.78 3.73 3.68 3.78 3.73

3.77 3.80 3.72 3.70 3.78 3.64

3.78 3.78 3.77 3.70 3.75 3.72

3.78 3.77 3.77 3.70 3.76 3.71

3.81 3.82 3.76 3.72 3.80 3.71

H

average 3.78 3.78 3.74 3.70 3.77 3.70

± ± ± ± ± ±

0.03 0.03 0.03 0.01 0.02 0.03

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intrachain and interchain molecular order were improved in the blend with PS13.7k, in spite of their insensitivity to coating rates, which was indicated by the red-shifted max absorption, more prevailing J-aggregation absorption, and smaller π−π stacking period. We attributed it to (i) a proper interaction during the phase separation between PDBT-TT and PS13.7k chains that could promote disentanglement and diffusion of PDBT-TT chains, and (ii) the much higher surface energy of PS than solvents and PDBT-TT. Besides, the smaller interchain stacking period was thought to be driven by stronger attraction and smaller steric hindrance between adjacent segments with elongated intrachain conjugation. In summary, a simultaneous control over molecular order and long-range alignment order was demonstrated in a simple process, which was helpful to understand the self-assembly of conjugated polymers and to better guide device preparation.

domains with an enhanced alignment. Besides, the remaining cross-linking points with the following chains could transfer the alignment effect of shear to induce a further growth of anisotropic PDBT-TT domains (Scheme 1b). Meanwhile, in the vertical direction, aligned fibrillar domains formed due to frictional shear on top surface (Scheme 1d). Thus, the role of shear played in alignment was illustrated. In the following, the contact line of solution on substrates went on receding due to centrifugal force, and the receding velocity played an important role in determining variation of in-plane anisotropy with coating rates. A slower contact line receding velocity than the critical velocity (Uth), which was determined mainly by evaporation and solution viscosity, could match the rate of aligned crystal growth with solvent evaporation to maintain alignment and increase anisotropy. Therefore, the dichroic ratio of CB and oDCB films were higher due to their high solution viscosity and slower receding than CF films, but reached a maximum at 1500 rpm due to slower solvent evaporation and smaller Uth than that of CF solutions. At last, a macro anisotropic film was constructed by aligned microscale domains (Scheme 1c). Meanwhile, in the vertical direction, the original aligned domains formed near the top surface tended to sink due to gravity (Scheme 1d). Given sufficient evaporation time, like that in CB or oDCB, the aligned domains could fall to the bottom layer and pull the ones formed later down through the cross-linking points (Scheme 1e,f). Otherwise, they stayed near the top surface, as in CF films. Thus, we proposed a threedimensional formation picture of anisotropic blend films. Fortunately, the simultaneously enhanced molecular order including elongated intrachain conjugation and smaller π−π stacking distance was demonstrated with improved long-range order. Along with the pseudoplastic nature induced by PS mentioned above (Scheme 1b), the PS matrix played another important role in improving molecular order due to disentangling, coil-toward conformation promotion, flexible phase separation room (Scheme 1b,c), high surface energy, etc.24,31 Thus, combining PS matrix and shear force, a simultaneous enhancement in both molecular and long-range order of D-A copolymer films was realized experimentally and explained by the corresponding mechanism.

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ASSOCIATED CONTENT

S Supporting Information *

The calculation of surface energy of PDBT-TT and PS. The microscopy images of OCSC films at 0° and 45° with respect to the crossed polarizer and analyzer. The variation of viscosity with shear rate on the rheometer for neat and blend solutions in different solvents. The polarized optic microscope image indicating the top layer and bottom layer in CF blend films rinsed by ethyl acetate and TEM image of the bottom layer. TEM images of PDBT-TT/PS films prepared from solutions in different solvents at different spin-coating rates. The TEM image and absorption spectrum of PDBT-TT/PS97.4k blend OCSC film. The variation of absorption spectra with OCSC rates for blend solutions in different solvents. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: 86-431-85262175; fax: 86-431-85262126; e-mail: ychan@ ciac.ac.cn. Notes

The authors declare no competing financial interest.

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4. CONCLUSIONS In this work, PDBT-TT films with a good optic anisotropy and aligned fibrillar morphology were easily obtained by off-center spin-coating the PDBT-TT/PS13.7k blend solution in their good solvents. A dichroic ratio as high as 2.75 was observed in the oDCB blend film. The anisotropy in films resulting from the long-range alignment of PDBT-TT fibrillar bundles was proven to be dependent on factors associated with the OCSC process, such as shear, contact line recession, solvent evaporation, etc. First, the addition of PS enhanced the viscosity and pseudoplastic property of the PDBT-TT solutions, resulting in a better transfer of shear and promotion of shear-induced disentanglement and alignment for chains. Second, the higher viscosity of the blend solution in the high boiling-point solvent led to a slower receding contact line, which was preferred for a better alignment under the threshold. Third, the threshold for best alignment was mainly controlled by the solvent evaporation rate. The slow evaporation in CB and oDCB led to a smaller threshold and therefore the max DR at 1500 rpm. Besides, the aligned structures were found to form at the air/solution interface and fell toward the substrate given sufficient time. Besides the enhanced anisotropy, both the

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21334006, 21474113), the National Basic Research Program of China (973 Program2014CB643505), and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB12020300).

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