Synergistic Effect of Solvent and Epitaxy on the Formation of

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Synergistic Effect of Solvent and Epitaxy on the Formation of Anisotropic Structures of P3HT and P3HT/PCBM Films Waqar Ali Memon, Jiali Li, Qunqun Fang, Zhongjie Ren, Shouke Yan, and Xiaoli Sun J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.9b03522 • Publication Date (Web): 25 Jul 2019 Downloaded from pubs.acs.org on July 26, 2019

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The Journal of Physical Chemistry

Synergistic Effect of Solvent and Epitaxy on the Formation of Anisotropic Structures of P3HT and P3HT/PCBM Films Waqar Ali Memon a‡, Jiali Li a‡, Qunqun Fang a, Zhongjie Ren a, Shouke Yana, b* and Xiaoli Suna* a State Key Laboratory of Chemical Resource Engineering, College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China. b Key Laboratory of Rubber-Plastics Ministry of Education, Qingdao University of Science & Technology, Qingdao 266042, China.

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ABSTRACT Oriented organic semiconductor blends can confer desirable properties, such as enhanced charge transport properties and polarized light emission or absorption. A technique that is not only adapted to solution processing but also producing anisotropic conducting blend films is realized by epitaxial crystallization of blends on oriented polymer substrate. The epitaxial structure of P3HT and PCBM on oriented polyethylene (PE) substrate is affected by the boiling point of the used solvent. The P3HT spin-coated from o-dichlorobenzene with high boiling point on PE forms the “side-on” and “face-on” molecular chain orientation with c-axis parallel to c-axis of PE. While the orientation of “side-on” and “face-on” orientation is poor when P3HT is spincoated from chloroform with low boiling point. The addition of PCBM does not affect the epitaxial crystallization behavior of P3HT. Moreover, anisotropic structure of PCBM is also obtained on PE substrate. PE substrate efficiently increases the amount of “face-on” structure and the ratio of “face-on” to “side-on” is 7 times as much as that on PSS:PEDOT substrate. Anisotropic structures lead to anisotropic absorption and photoluminescence (PL) properties. The anisotropic optical properties are better for sample spin-coated from o-dichlorobenzene with the dichroic ratio of 2. The technique of employing oriented PE film to regulate the formation of oriented conducting polymer combing with the analytical method provides guidance to fabrication and characterization of anisotropic functional film.


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INTRODUCTION Conjugated polymer with uniaxial orientation attracts great interest for their excellent properties, such as their increasing charge transfer characteristics.1-7 Very recently, a new emerging concept is to fabricate aligned organic semiconductor blends, which have a variety of applications. For the use in polymer solar cell, studies are mainly focused on enhancing the power conversion efficiency.8-12 Vohra et al.

8

improved the device performance through

mechanical rubbing bilayer of regioregular poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM). Two factors affect the performance of device: reorientation of the P3HT molecules from “side-on” to “face-on” configuration within thin active layer and better donor-acceptor vertical gradient within active layers. Going beyond simple lightharvesting applications, the polarized organic semiconductor blends can facilitate new device concepts for polarization-dependent optoelectrical applications.13,14 For example, the polymer solar cells that contain aligned P3HT:PCBM photoactive films exhibit greater degree of anisotropy of the photovoltaic effects under polarized illumination along the two principle axes. Zhu et al. 13 use polarizing polymer solar cell as a polarizing filter in liquid-crystal displays that harvests some of the back light for energy harvesting and recycling. Besides application, oriented donor-acceptor blend films can assist in fundamental photo-physical studies, such as to probe the polarization anisotropy of charge-transfer absorption and emission.8, 15,16 Several alignment techniques are mainly employed to structure oriented organic semiconductor blends. Mechanical rubbing is one of the main ones. Oriented thin films can be prepared by two ways: rubbing donor and acceptor simultaneously or rubbing a donor layer (P3HT) followed by spin-coating a top acceptor layer (PCBM).8,9 Other methods adapt for both the design of polarized solar cell and bipolar field-effect transistors during solution processing 3 ACS Paragon Plus Environment


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are developed by using a crystallizable solvent to induce orientation of conjugated polymer, offering significant advantages.3-5,17-19 The solvent initially dissolves the polymer or polymer blend and then crystallizes first to induce epitaxial solidification of the polymer with crystallization the evaporation of the solvent. The occurrence of oriented crystallization of solvent is the premise of obtaining aligned polymers. Brinkmann et al.4,18,19 adopt such method to directionally solidify many kinds of conjugated polymer. To simplify the procedure, Muller et al.10 develop a new method to align not only single-conjugated polymer but also polymerfullerene bulk-heterojunction blends. They achieve the highly anisotropic thin films through codeposition of the semiconductor blend and crystallizable solvent (1,3,5-trichlorobenzene) from a second critical solvent (o-dichlorobenzene). Depending on the solvent evaporation method, the spherulite-like structure or uniaxial alignment of the thin film can be obtained. Employing a crystallizable solvent to prepare oriented film may lead to the large roughness, which is caused by the evaporation of crystallizable solvent. A technique, processed by solution, can easily produce anisotropic conducting blend films with quite small roughness through epitaxial crystallization of conjugated polymer on oriented polymer substrate. Theoretically, (002) or (020) crystal plane distance of P3HT is similar to the (110) or (200) crystal plane distance of polyethylene (PE) and epitaxial crystallization of P3HT is expected to occur on highly oriented PE, which was evidenced in previous work.20 Considering epitaxial crystallization of P3HT may be affected by the solvent evaporation rate, it is essential to compare the epitaxial crystallization of conjugated polymer spin-coated from different kind of solvents to improve the anisotropic structure. Moreover, it is known that the charge transfer of P3HT films closely connected with the used solvent during solution processing.21 Here, we further extend our work to fabricate highly oriented polymer-fullerene 4 ACS Paragon Plus Environment

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bulk-heterojunction blends through epitaxial crystallization on oriented PE substrate. Two solvents with extremely different boiling point are chosen: o-dichlorobenzene and chloroform. We find that the P3HT and P3HT/PCBM films display anisotropic optical properties. The orientation degree is highly dependent on solvent type. By using higher boiling point solvent instead of chloroform, for spin-coating, “face-on” and “side-on” molecular chain orientation of P3HT in both neat and blend films orient excellent in the film plane with c-axis parallel to drawing direction of PE. Moreover, even PCBM shows anisotropic structures on PE substrate. EXPERIMENTAL SECTION Film Preparation. Regioregular P3HT was supplied by Rieke Metals Inc. The weight-average molecular weights, polydispersity index and regioregularity are 54.2 kg/mol, 2.3 and 94.2%, respectively. PCBM was purchased from API Service, Inc. All the materials were used as received. P3HT solution was prepared by dissolving 20 mg of P3HT in 1 mL of odichlorobenzene or chloroform. The P3HT:PCBM solution was prepared according to mass ratio of 1:1 by using o-dichlorobenzene or chloroform. Films about 100 nm thick (as measured by DektakXT) were prepared by spin-coating at 1500 rpm for 40s on different substrates: poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), which were first spin-coated on quarz and highly oriented PE film with thickness around 30 ‒ 50 nm supported by quartz. The method to prepare oriented PE film can be seen in Figure S1 (Supporting Information). In order to quantify the uniaxial alignment of PE film, polarized FTIR was used to obtain the orientation function (f = 0.8) which was calculated by:

f 

D 1 2  D  2 3cos 2  

(1)



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Where D is the dichroic ratio, and  is the angle between the transition moment vector and the chain axis. The dichroic ratio D can be easily calculated by A∥/A⊥ (A∥ and A⊥ are the intensities of a characteristic band measured with electron vectors parallel (0˚) and perpendicular (90˚) to the reference direction, respectively) of the absorption peak at 2848 cm-1 as shown in Figure S2. It is noted here that the names of substrates are simplified as PSS and PE, respectively. After complete evaporation of the solvent, the samples were annealed at 120 C for 30 min inside a nitrogen glove box. Morphology and structure characterization. The polarized optical microscopy (POM) images were obtained by using an Axioskop 40A Pol optical microscope (Carl Zeiss). Two-dimensional grazing incidence X-ray diffraction (GIXRD) data were obtained at beam line BL14B1 of the Shanghai Synchrotron Radiation Facility (SSRF) using X-ray with a wavelength of 0.154 nm and the incident angle of 0.2. The 2D-GIXRD experiments were performed with incident beam parallel “‖” and perpendicular “” to the drawing direction of PE film, respectively. For transmission electron microscopy (TEM) observations, a JEOL JEM‒2100 TEM operated at 200 kV was used in this study. Optical Absorption and PL Measurements. Absorption spectra were recorded using a UVvis spectrophotometer (U‒2900, Hitachi Limited). For the polarized absorption characterization, a prism polarizer accessory was placed between the light source and the samples to provide the polarized incident light. The photoluminescence (PL) spectra were obtained using a spectrofluorimeter (F-7000, Hitachi Limited) with excitation at 540 nm in air.


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RESULTS AND DISCUSSION To understand the molecular arrangement in P3HT and P3HT/PCBM films, 2D-GIXRD are performed at an incidence angle of 0.2°. The samples of P3HT and P3HT/PCBM on PSS substrate shown in Figure 1a and b are firstly studied. It is noted that for the P3HT films obtained by spin coating, most of the crystals are “side-on” or “face-on” orientations and both of them are belong to edge-on lamellae (molecular main chain is parallel to the substrate). There is a third kind of orientation, e. g. flat-on lamellae (molecular main chain is perpendicular to the substrate). The flat-on lamellae can only be obtained through some special preparation methods.22,23 In Figure 1a, the appearance of (h00) peaks and (020) peak in the out-of-plane and in-plane directions respectively demonstrates a preferential “side-on” orientation of P3HT on the PSS substrate. The addition of PCBM alters the orientation of P3HT crystals and leads to the formation of a small amount of P3HT crystals with “face-on” orientation, as evidenced by the appearance of (100) peak and (020) peak in the in-plane and out-of-plane directions, respectively. Note that these two samples were also examined with 2D-GIXRD by converting the direction in the plane, rotating the samples in the plane, but no difference was observed. This suggests that the polycrystalline “side-on” and “face-on” P3HT crystals exhibit an orientation with molecular chains randomly arranged in film plane as sketched in Figure 1c and d.



qz (Å-1)

2

1

Out of Plane

a

(020)

(200)

qz (Å-1)

b

(020)

Face-on

Face-on Side-on Face-on Face-on

(200)

0

d

Side-on

PCBM

(300)

(100) (100) (100)

0

a c

Side-on

0

1

Side-on

b

(300)

(100)

2

c

In Plane

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1

qxy (Å-1)

2

Figure 1. 2D-GIXRD maps obtained for (a) P3HT and (b) P3HT/PCBM films spin-coated from o-dichlorobenzene on PSS:PEDOT substrate. The sketch for the molecular orientation is shown respectively in (c) and (d). For the P3HT crystallized on oriented PE substrate, the 2D-GIXRD experiments were performed with incident beam parallel “‖” and perpendicular “” to the drawing direction of PE, respectively. Figure 2a shows the 2D-GIXRD patterns of P3HT crystallized from odichlorobenzene on oriented PE substrate. The corresponding out-of-plane and in-plane 1D profiles, extracted from the 2D-GIXRD results, are presented in Figure S3. Two strong diffraction peaks locating at qz = 15.07 nm-1 and 16.85 nm-1 belong respectively to the (110) and (200) peaks of PE are observed in the “”X-ray measurements see bottom frame of Figure 2a and Figure S3b. They disappear in the “‖” X-ray measurements. This originates from the high uniaxial c-axis orientation of the used PE substrate film with molecular chains aligned in the drawing direction but the a- and b-axes rotated randomly around the c-axis. In this configuration, 8 ACS Paragon Plus Environment

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there are always (hk0) planes, e.g., the strongest (110) and (200) diffraction planes, that fulfil the Bragg diffraction condition and contribute the diffraction spots in the out-of-plane direction under “” mode. When the incident X-ray beam is along to the molecular chain of PE, the situation is totally different. According to the Bragg equation, an incident angle of approximately 10.5 is needed for bring the (110) diffraction. However, as judged from the sharp (110) and (200) reflection spots in the electron diffraction patterns, the (110) and (200) crystal plane of the PE crystals are oriented almost perfectly in the direction perpendicular to the c-axis direction. An inclination angle of ca.  3 is estimated from the width of the (110) diffraction spots. The incidence angle of 0.2° was used in performing the X-ray measurement, meaning that a maximum incident angle of ca. 3.2 can be achieved when measurement is performed along the molecular chain direction of PE. It is definitely far away from the diffraction condition. As a result, all of the (hk0) diffraction spots of PE substrate film disappeared when measured along the c-axis direction.

2

//

(020)

a b

(020)P3HT

d

//

2 (020)

qz (Å-1)

(200)PE

1

(300) (200)

Side-on

(100)

Side-on

Face-on

(200)

Face-on

0 ⊥

(200)PE

(200)

c

⊥

(020)P3HT or (002)P3HT

(200)PE (110)PE

2 1/nm

Side-on

(100) (020)

0

0

1

2

qxy (Å-1)

2

(020)

(200)

Side-on Face-on

qz (Å-1)

qz (Å-1)

(110)PE

1

1

(100)

0 2

qz (Å-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

(100)

0 2

1

qxy (Å-1)

0

2 1/nm



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Figure 2. 2D-GIXRD maps obtained for P3HT films spin-coated from (a) o-dichlorobenzene and (d) chloroform on highly oriented PE substrate. Whereas symbol (‖) indicates an incident Xray beam is parallel to the stretching direction of oriented PE substrate and the symbol () indicates an incident X-ray beam is perpendicular stretching direction of oriented PE. Electron diffraction pattern of P3HT thin films spin-coated from (b) o-dichlorobenzene and (c) chloroform. As for P3HT, under “”X-ray measurement condition, sharp (100) diffraction spot is observed in the out-of-plane direction as shown in the bottom frame of Figure 2a and the 1D profile shown in Figure S3b. This demonstrates a preferred orientation of P3HT crystals with crystallographic a-axis aligned in normal direction of the thin film, i.e., a “side-on” molecular arrangement. This has been further confirmed by electron diffraction. As shown in Figure 2b, the PE reflection spots has been well accounted for by the orthorhombic unit cells with axes a = 0.74, b = 0.494, and c = 0.2534 nm.24 Diffraction spots related closely to the (020) or (002) of P3HT aligned along the 110 and 200 directions of PE substrate are also observed. As identified by polarized infrared spectroscopy in our previous work,20 it reflects actually a preferred orientation of P3HT on oriented PE substrate a parallel chain alignment. This demonstrates the occurrence of heteroepitaxy of P3HT on oriented PE substrate. The X-ray diffraction measured under “‖” mode is quite different from that obtained under “” mode. As presented in the top frame of Figure 2a, now (100) diffraction ring with the maximum intensity distributed in the out-of-plane direction is observed. Also, the 1D profile shown in Figure S3a shows a weak (100) diffraction peak in the in-plane direction. This implies that, in the plane perpendicular to the c-axis of both P3HT and PE, the a-axis of P3HT crystals 10 ACS Paragon Plus Environment

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rotated to some extent about the c-axis. It is acceptable when a fiber orientation of the used PE substrate is considered. Nevertheless, a “side-on” molecular arrangement with crystallographic a-axis aligned in the normal direction of the thin film is predominant. The existing small amount crystal with “face-on” molecular arrangement contributes a weak in-plane (100) reflection. A sketch showing the orientation of P3HT crystals in films spin-coated from o-dichlorobenzene on PE is shown in Figure 3a. The 2D-GIXRD patterns of P3HT spin-coated from chloroform on PE are shown in Figure 2d. It is clearly different from that spin-coated from o-dichlorobenzene. Now, (100) diffraction rings appear under both the “‖” and “” X-ray measurement conditions. This implies essentially a random orientation of P3HT on the PE oriented substrate from chloroform solution. The existence of diffraction maximum intensity in the out-of-plane direction indicates a more or less preferred “side-on” molecular alignment. The corresponding electron diffraction pattern, Figure 2c, shows also (002) or/and (020) diffraction ring of P3HT. The intensity of this reflection ring is somewhat stronger in the c-axis direction of PE. This demonstrates the existence of thin epitaxial layer of P3HT on the PE substrate.



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a

CPE//CP3HT

Oriented PE lamellae

b

Figure 3. Schematic representation for P3HT structural ordering spin-coated from (a) odichlorobenzene (b) chloroform on PE substrate. In other words, crystallization of P3HT on PE substrate from chloroform leads to the formation of layered structure. A sketch illustrating the structure of P3HT on PE substrate from chloroform solution is shown in Figure 3b, where “side-on” and “face-on” crystals does not show clear orientation on oriented PE substrate and they are rotating. The different orientation structure is caused by different evaporation rates of the used solvents. Actually, the influence of solvent evaporation rate on the crystallinity has been reported.21,25 In the present case, a quick evaporation of chloroform results in a faster crystallization of P3HT. While epitaxial


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crystallization takes place at the interface of P3HT and PE, randomly oriented crystals are produced in the top P3HT layer. The 2D-GIXRD patterns of P3HT/PCBM blends spin-coated from o-dichlorobenzene and chloroform solutions on PE are shown in Figure 4. Under “” X-ray measurement condition, the diffraction pattern of the sample spin-coated from o-dichlorobenzene has a close remembrance as that of neat P3HT film except for an even sharper (100) diffraction spot (compare Figure 4a with Figure 2a). In the 1D profile of Figure S4a, even a weak (200) of P3HT is observed. Under “‖” X-ray measurement condition, the (100) diffraction ring is clearly seen as shown in the top frame of Figure 4a. This indicates a continuous rotation of a-axis about the c-axis. However, the existence of diffraction maximum in the out-of-plane direction tells a more preferred side-on molecular orientation of P3HT. With close inspection, one may find a diffused diffraction ring associated with PCBM aggregates locating at q = 14 nm-1. Its intensity is slightly stronger in perpendicular direction than that in parallel direction (as shown in Figure 4a). The corresponding 1D diffraction profiles further identify such difference as shown in Figure S4. This result suggests that the PCBM aggregates adopt also a more or less anisotropic structure in the film plane. Considering that the plane distance of PCBM aggregates (q = 14 nm-1) is similar to that of (110) plane distance of PE crystals, it can be inferred that anisotropic structures of PCBM could be induced by epitaxial effect of oriented PE substrate. In order to reflect the surface quality of the resulting films, the morphology of P3HT and P3HT/PCBM blend thin films is observed by Atomic Force Microscope (AFM), as shown in Figure S5. For films spincoated from chloroform or o-dichlorobenzene, the obtained roughness Rq is about 1.8 nm. While for the P3HT/PCBM blend thin films, the roughness Rq is about 3 nm.



2

//

qz (Å-1)

(020)

1

Face-on

qz (Å-1)

c (020)

P3HT or (002)P3HT

0 ⊥

PCBM PCBM PCBM

(110)PE

(200)

Side-on Face-on

(100)

PCBM

2

1

(100)

0

2

1

(200)PE

PCBM

2 1/nm PCBM

Side-on

0

(100)

qz (Å-1)

(200)

(200)

PCBM

⊥

(200)PE

1

Side-on Face-on

Side-on

(110)PE

0

(020)

(300) (200)

0

1

2

PCBM

PCBM

(100)

2

d //

a b

qz (Å-1)

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2

qxy (Å-1)

1

qxy (Å-1)

0

2 1/nm

Figure 4. 2D-GIXRD maps obtained for P3HT/PCBM films on highly oriented PE substrate. The sample spin-coated from (a) o-dichlorobenzene (d) chloroform is measured. Whereas symbol (‖) indicates an incident X-ray beam is parallel to the stretching direction of oriented PE substrate and the symbol () indicates an incident X-ray beam is perpendicular stretching direction of oriented PE. Electron diffraction pattern of P3HT/PCBM thin films spin-coated from (b) o-dichlorobenzene and (c) chloroform. To further explore the effect of solvent type and the addition of PCBM on the orientation of P3HT crystals, the pole angle () dependence of the (100) diffraction intensities for different sample is integrated. The P3HT film spin-coated from o-dichlorobenzene on PE is taken as an example and shown in Figure 5a. The intensities for  = 0 to 45 and  = 45 to 90 denote “faceon” and “side-on” orientations, respectively. Thus, on PE substrate, the “side-on” crystals 14 ACS Paragon Plus Environment

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decrease and the component of “face-on” crystals increases relatively.26-31 The amount of “faceon” crystals relative to “side-on” crystals calculated as shown in Figure 5b. The ratio on PE substrate is much larger than that on PSS substrate. It can be clearly seen that oriented PE substrate can efficiently increase the amount of P3HT crystals with “face-on” orientation. On both kinds of substrates, the addition of PCBM favors the formation “face-on” crystals. To check the effect of orientation of PE substrate on the crystallization of P3HT, P3HT and blends of P3HT and PCBM films were also fabricated on unoriented PE film. The intensity of P3HT (100) diffraction peak measured by 2D-GIXRD is nearly zero in the in-plane profiles suggesting “faceon” orientation cannot form on the unoriented PE film, shown in Figure S6. Thus the orientation of PE substrate plays the most important role on formation of “face-on” orientation.

600

a Face-on Side-on

400

Side-on

)

300

200

Face-on

100

0

0

15

30

45



( )

60

75

Chloroform o-Dichlorobenzene

0.8

Iface-on/Iside-on

500

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


90

b

0.6

0.4

0.2

0.0

P3HT PE

P3HT/PCBM PE

P3HT P3HT/PCBM PSS PSS

Figure 5. (a) Taking the sample of P3HT film spin-coated from o-dichlorobenzene on oriented PE as an example to show the amount of “face-on” and “side-on” molecular chain orientation. The pole angle (χ) dependence of the (100) diffraction intensities is integrated. The intensities for  = 0 to 45 and  = 45 to 90 denote “face-on” and “side-on” orientations, respectively. (b) The ratio of “face-on” crystal to “side-on” crystal for P3HT on different substrates. 15 ACS Paragon Plus Environment


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It is well established that anisotropic structures lead to anisotropic optical properties. The optical properties of P3HT and its blends with PCBM on different substrates were therefore investigated. Figure 6 shows the polarized UV‒vis absorption spectra of P3HT and its PCBM blends on PE substrates with respect to the molecular chain direction of PE. For a comparison, the polarized UV‒vis spectra of the films on PSS substrate are also shown in Figure S7 in Supporting Information. The films on PSS substrates show no intensity difference between two polarization directions. By contrast, both P3HT and its P3HT/PCBM blend films on oriented PE substrate spin-coated either from o-dichlorobenzene or from chloroform display anisotropic absorption properties. In all cases, the absorption intensities are stronger when measured with polarization vector parallel (I‖) than perpendicular (I) to the PE chain direction. Moreover, the dichroic ratios (I‖/I) for P3HT and P3HT/PCBM spin-coated from o-dichlorobenzene (2) are much higher than that spin-coated from chloroform (1.2).


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Figure 6. The polarized UV‒vis absorption spectra of (a) P3HT and (b) P3HT/PCBM films on PE spin-coated from o-dichlorobenzene. The spectra of (c) P3HT and (d) P3HT/PCBM films on PE spin-coated from chloroform; the normalized absorption spectra in two different directions are inserted. Whereas the symbol (‖) indicates the parallel absorption and the symbol () indicates the perpendicular absorption to the c-axis of oriented PE substrate. Samples spin-coated from chloroform show a much wider range absorption as shown in the inset of Figure 6c and 6d. UV‒vis spectral features strongly correlate with the status of molecular ordering (intra- and intermolecular ordering) in π-conjugated polymer. The low energy peaks appeared at 550 and 596 nm correlate to the vibronic bands associated with the (01) and (00) transitions, respectively.32,33 The high intensity of lower energy peaks is associated with higher amount of ordered aggregates featuring interchain ππ stacking interactions. According to the inset of Figure 6, the relative intensity of the (00) and (01) transition is much stronger for the sample spin-coated from o-dichlorobenzene than that spin-coated from chloroform suggesting the o-dichlorobenzene favors improving the molecular order.



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Figure 7. (a) The sketch representation of polarized photoluminescence setup, (b) P3HT film, (c) P3HT/PCBM blends from o-DCB on oriented PE. (d) P3HT film, (e) P3HT/PCBM blends from chloroform on oriented PE. The samples are annealed at 120 oC for 30 min. Where the ratio of IVH/IVV of P3HT and P3HT/PCBM blend on PE from o-DCB is 3.84 and 2.57, on the other hand the ratio of IVH/IVV of P3HT and P3HT/PCBM blend on PE from chloroform is 1.681 and 2.381, respectively.


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The ordered structure also leads to luminescence anisotropy. The polarized PL spectra of thin films of P3HT and its PCBM blends are shown in Figure 7 and the setup is presented in Figure 7a.34 As expected, both P3HT films and blend films display an anisotropic PL property as illustrated in Figure 7b-e. The perpendicular emission intensities (IHH and IVH) are much higher compared with those of the parallel emission intensities (IVV). It can be seen that the IHH are almost similar to IVV, which is caused by the fact that the excitation of electrons from ground state to excitation state is only sensitive to the polarization of emission light rather than excitation light.

a

b

100μm

100μm

Figure 8. Optical micrographs of P3HT/PCBM blend film on the oriented PE at 45˚ (a) and 0˚ (b) with respect to the analyzer. Orientation of the polarizer pair indicated by solid arrows; Oriented PE direction indicated with dashed arrows. The above experimental results indicate that the preferred alignment of P3HT molecular chains in the chain direction of PE enhance the optical properties of P3HT. This is helpful for preparing high performance thin films of P3HT. It is worth to be noted that large-area highly-oriented P3HT and P3HT/PCBM films can be obtained by epitaxial crystallization on the oriented PE substrate. Figure 8a and 8b present the polarized optical micrographs of annealed P3HT/PCBM 19 ACS Paragon Plus Environment


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blend film spin-coated from o-dichlorobenzene on oriented PE substrate. The related images for pure P3HT on PE can be seen in Figure S8. It is clearly seen that uniform large area thin film has been obtained. The occurrence of light extinction when the samples were rotated ± 45 about the beam axis illustrates the high orientation of P3HT, and most probably the PCBM, crystals on the PE substrate. CONCLUSION Highly oriented P3HT and P3HT/PCBM blend thin films were successfully prepared via epitaxial crystallization on a highly oriented PE substrate. The anisotropic structures of P3HT and blend films were identified and effect of solvent is clarified. GIXRD and TEM experiments demonstrate that the epitaxial crystallization of P3HT and PCBM on PE substrate is determined by solvent type. When o-dichlorobenzene is used, the anisotropic structures with higher orientation degree are produced. Both “side-on” and “face-on” crystals adopt the orientation with c-axis of P3HT parallel to c-axis of PE. Moreover, anisotropic structure of PCBM is also obtained on PE substrate. By contrast, the sample spin-coated from chloroform does not show obvious anisotropic structures in the film plane. PE substrate efficiently increases the amount of “face-on” molecular chain orientation, which is several times higher than that on PSS:PEDOT substrate. This technique provides a new way to prepare large area uniform ordered P3HT/PCBM blend films with anisotropic absorption and luminescence property, which can be used as polarized luminophors or polarizing filter.


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ASSOCIATED CONTENT Supporting Information: Schematic representation of melt drawn PE procedure, polarized FTIR spectra of oriented PE thin film, 1D correlation profiles extracted from 2D-GIXRD maps for the P3HT and P3HT/PCBM spin-coated from o-dichlorobenzene and chloroform, AFM images, 2D-GIXRD maps obtained on unoriented PE substrate, polarized UV‒vis absorption spectra and polarized microscope photos for P3HT and P3HT/PCBM films. These materials are available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Sun, X.); [email protected] (Yan, S.). Author Contributions ‡These authors contributed equally: Waqar Ali Memon, Jiali Li. CONFLICTS OF INTEREST There are no conflicts to declare. ACKNOWLEDGMENT This study was financially supported by the National Natural Science Foundations of China (No.21274010, 21774011& 51221002). REFERENCES



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2

-1

1

PCBM (020)

PCBM

(300) (100)

TOC Graphic

Side-on Face-on

Side-on Face-on

(200) (100)

0

-1

o-Dichlorobenzene (200)PE ⊥ PCBM (110) PCBM

0

Chloroform

⊥ PCBM

(200)PE (110)PE

PCBM PCBM

PE

1

1

Side-on Face-on

Side-on

(100)

0 0 -1 1 qxy (Å )

2

2

1 -1 0 qxy (Å )

Iface-on/Iside-on

0

Chloroform o-Dichlorobenzene

0.8

CPE//CP3HT

2

-1

(100)

1

qz (Å )

qz (Å )

2

2 -1

qz (Å )

For Table of Contents use only (020)

qz (Å )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.6

0.4

0.2

0.0

P3HT PE

P3HT/PCBM PE

P3HT P3HT/PCBM PSS PSS


Synergistic Effect of Solvent and Epitaxy on the Formation of

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