Highly Anisotropic P3HT Film Fabricated via Epitaxy on an Oriented

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Highly Anisotropic P3HT Film Fabricated via Epitaxy on Oriented Polyethylene Film and Solvent Vapor Treatment Jiali Li, Meiling Xue, Ning Xue, Huihui Li, Lei Zhang, Zhongjie Ren, Shouke Yan, and Xiaoli Sun Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.9b00402 • Publication Date (Web): 13 May 2019 Downloaded from http://pubs.acs.org on May 16, 2019

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Highly Anisotropic P3HT Film Fabricated via Epitaxy on Oriented Polyethylene Film and Solvent Vapor Treatment Jiali Li,a Meiling Xue,b Ning Xue,c Huihui Li,a Lei Zhang,c 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. c Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China.

*E-mail: [email protected] (Sun, X.); [email protected] (Yan, S.).

ABSTRACT

To improve the epitaxial crystallization ability of poly(3-hexylthiophene) (P3HT) on highly oriented polyethylene (PE) substrate, controlled solvent vapor treatment (CSVT) is employed.

ACS Paragon Plus Environment

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The anisotropic structures and related optical properties depend not only on the solvent used for preparing film but also on subsequent solvent vapor treatment pressure and time. Highly oriented PE film facilitates “side-on” chain orientation of P3HT with c-axis parallel to the drawing direction of PE film. The dichroic ratio (DR) of P3HT film reflected by UV vis spectra can reach as high as 7.1, which is much larger than the value treated by thermal annealing. Moreover, the excitation bandwidth W, indicating the effective conjugation length and molecular order, shows significant anisotropic features. Solvent used for solution processing with high boiling point is more favorable for inducing anisotropic multi-scale structures. In particular, the oriented structures lead to obvious anisotropic carrier mobility. The carrier mobility of P3HT after CSVT along the PE molecular chain direction is 7.5 times higher than that measured perpendicular to the PE chain direction. This is of great importance in fabricating anisotropic thin films of conjugated polymeric semiconductors with enhanced performance.


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INTRODUCTION

Organic semiconductors are considered as promising materials in the field of organic electronics. Solution-processed organic semiconductors have attracted considerable attention due to their flexibility, low cost and solution process ability, such as spin-coating,1-5 dipcoating6-9 and jet-printing.10-12 The performance of organic semiconductor devices is closely related to the structure and morphology of semiconductor thin layers. Therefore, controlling the multi-scale structure of solution-processed organic semiconductors is crucial for fabricating high quality devices. Regioregular P3HT with high solubility in common organic solvents is one of the typical I/

polymers for applications in organic field effect

transistors (OFETs),13-16 organic thin-film transistors (OTFTs) 17,18 and organic photovoltaic cells (OPVs).19-23 Actually, it has frequently been employed as model system to study the relationship between structure and performance of organic semiconductors.

Highly oriented conducting film enables anisotropic charge transport properties, which can be applied to the fabrication of polymer light-emitting diodes (PLEDs) 24 and enhancement of the mobility of charges in polymer field effect transistors (PFETs).25,26 Various procedures have been reported to align the main chains of the organic conducting polymers to fabricate oriented films. Mechanical rubbing is one of the most frequently utilized technique.27,

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Brinkmann et al.27 prepared oriented conducting polymer thin films through mechanical rubbing. In the rubbed P3HT films, the molecular chains were arranged parallel to the rubbing direction, and the crystalline domains orientation changes from preferential edge-on to flat-


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on orientation. The measured field effect mobility shows significant anisotropy. In addition, Imanishi et al.29 used a soft friction transfer method to obtain a uniaxial oriented P3HT film. Besides, dip-coating is another technique that permits uniaxial orientation of the molecules in thin film. For example, Müller et al.7 obtained oriented P3HT films via dip-coating through immersing glass sheet in the 1,3,5-trichlorobenzene/chlorobenzene (TCB/CB) solution of P3HT and pulling out of the solution at a certain speed. To overcome the roughness induced by rubbing and evaporation of TCB, our research group has fabricated oriented conducting polymer films through epitaxial crystallization on oriented PE or polypropylene films.30-32 Originally, we spin-coated P3HT solution from chloroform (CHCl3) on highly oriented PE films. Although oriented P3HT structures were evidenced by polarized infrared spectroscopy, the orientation degree of the prepared film was low.30 This is associated to two factors: (i) the rigid molecular chains of P3HT, leading to a slow organization process of it; (ii) the lower boiling point of CHCl3, resulting in a rapid evaporation of it. Considering this, slow down the evaporation of the solvent will provide longer time for arrangement of P3HT and therefore enhance the orientation.

Recently, CSVT is reported to effectively control nucleation and growth process of conducting polymer at room temperature.33-35 As presented in Figure 1, a small amount of carbon disulfide (CS2) or tetrahydrofuran (THF) was put at the bottom of a long pipe with the length of L0. After the vapor reached a balanced state in the tube, conducting polymer film was firstly put at the position (L1) which was near the bottom of the pipe with high solvent


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In this paper, we add CSVT to regulate the orientation structure of P3HT crystal on highly oriented PE films after solution processing with two solvents: o-dichlorobenzene (o-DCB) and CHCl3. CSVT is much efficient to facilitate the epitaxial crystallization of P3HT on highly oriented PE film. The “side-on” structures of P3HT with c-axis parallel to the drawing direction of PE film were obtained. The DR of P3HT obtained from UV vis spectra increases with P1 and P2 and can reach as high as 7.1. Solvent used for solution processing with high boiling point is more favorable for inducing anisotropic structures. In particular, the oriented structure of P3HT also supports anisotropic charge transport characteristics with Ion/Ioff upon 103 and the carrier mobility of 6.64×10-2 cm2V-1s-1 parallel to PE chain direction, which is 7.5 times higher than that in direction perpendicular to PE chain.

EXPERIMENTAL SECTION

Materials.

Regioregular P3HT was supplied by Rieke Metals Inc. The weight-average

molecular weights, polydispersity index and regioregularity are 34 kg/mol, 2.3 and 94.2%, respectively. High density PE (Lupolen 6021DX), was obtained from BASF AG Ludwigshafen, Germany. Film Preparation. Highly oriented PE thin films were prepared according to a melt-draw technique by a motor driven cylinder.36 P3HT solution was dissolved in o-DCB or CHCl3 with 5 mg/ml at 60 C. The P3HT films ranging from 25 to 35 nm were prepared by spin-coating on the oriented PE films at 1500 rpm for 60 s at room temperature. After spin-coating, the film was dried in vacuum at room temperature for 24 h to remove residual solvent in the film.


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Then, the CSVT was achieved by a long glass tube (6 cm in diameter and 120 cm in length) as sketched in Figure 1. There was also a water bath soaking the tube to keep the temperature constant (26 C). CS2 was injected into the bottom of the tube. As the solvent vapor pressure in the tube reaches equilibrium, a gradient of solvent vapor pressure along the tube forms with the saturation pressure at the bottom and near zero at the top. The sample was firstly placed at L1 with the solvent vapor pressure of P1 in the tube for dissolution, then placed at L2 with P2 for crystallization.34

Characterization. For transmission electron microscopy (TEM) observation, the P3HT/PE thin films were detached from the glass slide with the help of poly(acrylic acid) and mounted onto 400 mesh TEM copper grids. TEM observations were performed using a JEOL JEM-2100 with an accelerating voltage of 200 kV. Two-dimensional grazing incidence X-ray diffraction (2DGIXRD) results were obtained at a diffuse scattering station of Beijing Synchrotron Radiation Facility (BSRF, 1W1A), the wavelength and the incident angle of the X-ray beam are 0.1548 nm and 0.2 , respectively. Absorption spectra were recorded by using a UV vis spectrophotometer (UV 2550, Shimadzu). For the polarized absorption characterization, a polarizer accessory was placed between the light source and the samples to provide the polarized incident light.

The gate electrode and the dielectric layers are Si wafer and 300 nm heavy N-doped SiO2 layers. Prior to deposition of the organic semiconductor, the gate dielectric was treated with octadecyltrichlorosilane (OTS) by the vapor-deposition method. The wafer was dried under


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vacuum at 90 °C for 2 hours to remove moisture. PE films were transferred onto clean substrates and P3HT films were spin coated onto the PE film from solution. Figure S1 is the sketch of transistors based on the P3HT/PE-SiO2/Si with P3HT deposited on the oriented PE film. The devices were fabricated in a “top contact” model. The mobilities of the devices based on the oriented P3HT films were calculated in the saturation regime with the following equation: (1) where ISD is the drain-source current, W is the channel width, L is the channel length, µ is the fieldeffect mobility, Ci is the capacitance per unit area of the gate dielectric layer, while the VG and VTH are the gate and threshold voltages, respectively. A copper grid was used as mask, which makes the channel length L = 40 8

and the channel width W = 240 8 ! The capacitance of the OTS

modified SiO2 dielectric layer is 7.5 nF/cm2. However, in this work, the SiO2/Si with 500 nmthickness SiO2 together with the PE film act as the dielectric layer and the capacitance is 6.9 nF/cm2 obtained through experimental tests. All electrical characteristics of the devices were measured at room temperature using a semiconductor parameter analyzer (Keithley 4200 SCS) in air atmosphere.

RESULTS AND DISCUSSION

2.4

(a)

0.4

As-prepared P2= 68%, P1(83-91%)

4.0

DR

0.1 500

600

700

3.6 3.2 2.8

1.2

Wavelength(nm)

As prepared P2 = 68%, P1(83-91%) P1 = 89%, P2(50-82%)

1.6

0.2

(c)

P1= 89%, P2(50-82%)

2.0

//

0.3

0.0 400

(b)

DR

0.5

Absorbance

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

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0

20

40

60

80

Vapor Pressure (%)

100


0

20

40

60

80

Vapor Pressure(%)

100

8

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Figure 2. (a) UV vis absorption spectra of P3HT films cast from o-DCB onto oriented PE film before (dotted line) and after (solid line) CSVT. The “//” and “ ” indicate that the polarization directions of incident light are parallel and perpendicular to the molecular chain direction of PE, respectively. The dependence of DR on the vapor pressure P1 (t1 = 10 s) and P2 (t2 = 30 min) for P3HT films cast from CHCl3 (b) and o-DCB (c). Polarized UV vis spectra were used to characterize the anisotropic optical properties of the prepared thin films. Figure 2a shows the polarized UV vis spectra of the P3HT thin films spuncast from o-DCB on PE substrate before and after CSVT, where the “//” and “ ” indicate that the polarization directions of the incident light are parallel and perpendicular to the molecular chain direction of PE, respectively. Vapor pressure exerts great influence on the orientation of the P3HT film which was firstly swollen to a yellow liquid state at P1, implying the erasing of the existing P3HT crystals. When the film was moved to P2, its color turned to purple, suggesting the occurrence of recrystallization under the reduced solvent vapor. A strong bathochromic shift as well as an absorption peak with a maximum at 552 nm and two small overlapping shoulders at 520 nm and 606 nm for both before and after CSVT are shown in Figure 2a. Besides, the absorption intensity is stronger under the “//” direction than under the “ ” direction, implying anisotropic structure of P3HT formed on the PE film. Compared with the as-prepared sample (before CSVT), the anisotropy increases obviously after CSVT. Herein, DR is adopted to quantitatively characterize the anisotropic ratio of the absorbance of the sample, which can be calculated by the formula (DR = Ap/A0), where Ap is the polarized light parallel to the maximum absorbance of the polymer molecular chain, A0 represents the polarized light perpendicular to the maximum absorbance of the polymer molecular chain. As shown in Figure 2b, for fixed P2 at 68%, the DR


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increases with P1 as long as the film does not dewet. Once the vapor pressure of P1 exceeded 90%, the DR decreases notably with respect to the value of as-prepared sample caused by the dewetting of the film. When Pl is 89%, P3HT film shows the highest anisotropic optical properties. Therefore, Pl is fixed at 89% to further study the influence of P2 on the orientation of the P3HT. Similarly, DR also increases with P2. Yet, once the P2 exceeds 82%, the color of film gradually get yellow, implying that the vapor pressure is too high for recrystallization. As can be seen in Figure 2b, the DR value of the P3HT film treated under P1 = 89% for 10 s and P2 = 82% for 30 min is nearly twice the value of the as-prepared P3HT film from CHCl3. Similar DR variation trend of P3HT spun-cast from o-DCB with P1 and P2 (see Figure 2c) as that from CHCl3 is observed, and the DR value for P3HT cast from o-DCB is much larger than that from CHCl3.

150

8 7

(a)

(b)

120

W(meV)

6

DR

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5 4 3

//-CHCl3

-o-DCB

-CHCl3 -o-DCB

90 60

2 1 0

20

40

t1 (s)

60

80

100

As-prepared Film CSVT Film

Figure 3. (a) The DR of P3HT film spun-cast from DCB treated at P1 = 89% for different t1 time, under fixed P2 = 82% (t2 =30min). (b)The excitation bandwidth W of P3HT films spuncast from CHCl3 or o-DCB before or after CSVT at the condition (P1 = 89%, t1 = 60s; P2 = 82%, t2 = 30min).


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It should be pointed out that the treatment time at each pressure exhibits absolutely impact on the orientation of P3HT. Thus, the effect of solvent vapor treatment time on the orientation status of P3HT has also been explored. Figure 3a shows the changes of DR with time for the P3HT spuncast from o-DCB on PE substrate under P1 (t1) and P2 = 82% (t2 =30min). When extending the processing time under P1 (t1), the DR rises as long as the film does not dewet. The DR value increases from 2.8 to 7.1 when t1 extends to 60 s. The DR also increases with time t2 under P2, but further extending the time longer than 30 minutes has little effect on DR value. Thus, at conditions P1 = 89% for 60 s and P2 = 82% for 30 min, the P3HT film gets largest anisotropy with DR=7.1. The P3HT molecules are better uniaxially aligned after CSVT with increased DR, which proves that the solvent vapor treatment favors the epitaxial crystallization of P3HT on oriented PE substrate. To further investigate the uniaxial alignment of P3HT molecular chains, the H-aggregation model reported by Spano et al. is employed to calculate the free exciton bandwidth W from the > F

spectrum according to: 37 A0 - 0 1 - 0.24W/Ep 2 =( ) A0 - 1 1 + 0.073W/Ep

(2)

Where Ep stands for the energy of the main intramolecular transition (using as 0.18 eV), A0-0 and A0-1 denote the absorption at 606 nm and 552 nm (where subscripts denote respective vibronic transitions). A decrease of W suggests an increase of effective conjugation length and higher order of the molecular chains.38, 39 The calculated W value is shown in Figure 3b. Three main results related to W can be observed in Figure 3b. First, W decreases sharply after CSVT, especially for the film cast from CHCl3. Second, it is much smaller for the sample cast from o-DCB than that from CHCl3, demonstrating that solvent with high boiling point favors enhancing the molecular


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Figure 4. 2D-GIXRD maps obtained for P3HT films cast from o-DCB on highly oriented PE substrate before (a & b) and after (c & d) CSVT. In the pictures, the “//” and “ ” indicate that the incidence X-ray beams are parallel and perpendicular to the PE molecular chain direction, respectively. 2D-GIXRD was employed to gain a better understanding of how does the solvent vapor affect the crystal structure of oriented P3HT films. Figure 4a-d shows the 2D-GIXRD patterns of the P3HT films spun-cast from o-DCB before and after CSVT. GIXRD experiments were performed in two ways with PE molecular chain direction parallel and perpendicular to the incident X-ray beam, respectively. The 1D diffraction profiles with respect to the out-of-plane (along the qz) and inplane (along the qx and qy) directions are displayed in Figure S3, as integrated from the 2DGIXRD pattern using the intensities of 0~45 and 45~90 to denote “face-on” and “side-on” orientations, respectively. Two obvious diffraction peaks at qz = 15.17 nm-1 and 16.77 nm-1 associated to the (110) and (200) reflections of PE integrated from the 2D-GIXRD in the out-ofplane mode under “ ” direction. The reason why these two peaks disappear under “//” direction is due to the high uniaxial c-axis orientation along the drawing direction. For P3HT, the (h00) reflection refers to the repeat distance in the direction of alkyl stacking, while (0k0) reflection refers to the repeat distance in the direction of L–L stacking. For the P3HT films before CSVT, the (100) peak was observed in the in-plane mode under “//” direction, but disappears under “ ” direction. It suggests that the c-axis of “face-on” )L–L stacking direction is perpendicular to the substrate) molecular orientation is parallel to the c-axis of PE due to the epitaxial crystallization of P3HT on PE during spin-coating process. The epitaxial mechanism can be crystal lattice matching, considering the L–L stacking distance is similar to the lattice distance of (200) of PE. But it may also stem from specific, directional adsorption of the P3HT chains on PE surface which


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been well indexed as the orthorhombic unit cells with parameters a = 1.663, b = 0.775, and c = 0.777 nm41,42 and a = 0.74, b = 0.494, and c = 0.2534 nm, respectively. 43 As presented in Figure 5a and b, the appearance of diffraction arcs of P3HT on PE substrate indicates the occurrence of epitaxial crystallization of P3HT on oriented PE substrate. It has already been confirmed that the c-axis of P3HT is parallel to the c-axis of PE by polarized infrared spectroscopy, so that the diffraction arc of (002)P3HT is indexed in the same direction along the (002)PE.30 It is noted that the (020)P3HT plane has similar interplanar spacing with the (002)P3HT planes. Thus, the diffraction arc in the direction of the (hk0)PE direction has been assigned to the (020)P3HT. The (020)P3HT reflection just locates between (110)PE and (200)PE reflections suggesting lattice matching promotes the epitaxial crystallization of P3HT on PE with the (100)P3HT as the contacting plane. The sharper diffraction arc and higher intensity of (020)P3HT and (002)P3HT in ED pattern indicate that the solvent vapor treatment induces higher crystallinity and more amounts of “side-on” molecular chain orientation with c-axis parallel to the PE molecular chain direction. Moreover, the “side-on” orientation in P3HT film cast from o-DCB is much better than that cast from CHCl3, as indicated by the reflection ring in ED (see in Figure S5a and b). The bright-field (BF) electron micrograph shown in Figure 5c also suggests the highly oriented lamellar structures of P3HT forming on oriented PE film. The obvious lamellar structures of PE had already been demonstrated, while the blacker parts which stand for thick contrast are attributed to the contribution of P3HT crystals.


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//

0.006 ± 0.002

4.1×102

~12

0.06 ± 0.004

1.1×103

~1

0.008 ± 0.001

1.96×102

~19

Vapor-treated

Figure 6 shows the transfer characteristics obtained from OFETs devices based on P3HT cast from o-DCB on PE substrate after CSVT measured along and vertical to PE molecular chain direction, respectively. The oriented structures of P3HT apparently lead to anisotropic charge transport characteristics. The electrical performance along the direction of PE molecular chains is superior to that in the vertical direction. Compared to the mobility of P3HT films cast from o-DCB in the direction vertical to the PE chains, which exhibits the carrier mobility of 8.9×10-3 cm2V-1 s-1 and the Ion/Ioff of upon 102, a higher charge transport mobility of 6.64×10-2 cm2V-1 s-1 and Ion/Ioff upon 103 have been achieved along the PE chain direction after CSVT. Corresponding output curve is shown in Figure S6. Meanwhile, the calculated average charge transport properties of different samples are shown in Table 1. The mobility of the P3HT after CSVT is much higher than no CSVT devices. Moreover, Ion/Ioff of P3HT film is also increasing accordingly after CSVT and shows anisotropy. However, as for P3HT cast from CHCl3, the overall electric properties are much worse than that cast from o-DCB due to the inadequate self-assembly of semiconductor molecules under fast volatilization rates.44 Although they also show anisotropic features on carrier mobility and Ion/Ioff, the anisotropic ratio is much smaller than the sample cast from o-DCB which shows apparent electrical anisotropy not only due to the large amount of “side-on” structures along PE molecular chain orientation, but also the higher order of molecules and longer effective conjugation length indicated by much lower value of W.


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CONCLUSION

Highly oriented conducting film enables anisotropic charge transport in electronic devices which can find many applications. Compared with rubbing technique and epitaxial crystallization method by employing small molecular of TCB, fabricating oriented conducting polymer through epitaxial crystallization on oriented PE has the advantages of solution processing and producing film with low roughness. However, fast solvent evaporation leads to rather thin epitaxial crystallization layer of P3HT film with worse anisotropic structure. In this study, by combining CSVT method, the epitaxial crystallization ability of conducting polymer on highly oriented PE substrate is improved and large-scale P3HT films with highly anisotropic molecular order and properties are fabricated. The largest DR of P3HT film reflected by UV vis spectra can reach as high as 7.1. The oriented structures lead to obvious anisotropic charge transport properties. The carrier mobility of P3HT after CSVT in the molecular chain direction is 7.5 times higher than in the direction perpendicular to the molecular chains. These results rendered here not only clearly demonstrate the synergism between epitaxy and solvent vapor treatment, but also revealed the intrinsic influence of molecular arrangement on the charge transport characteristics for conductive polymers. In particular, we provide a new method to enhance anisotropic performance for P3HT, which may also suitable for other new-style polymer semiconductors.

ASSOCIATED CONTENT


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Supporting Information: OFET based on the P3HT/PE-SiO2/Si, UV vis absorption spectra before and after CSVT by using THF, 1D diffraction profiles for P3HT films cast from o-DCB on highly oriented PE substrate, 2D-GIXRD maps obtained for P3HT films cast from CHCl3, electron diffraction patterns of P3HT films cast from CHCl3 and output curves obtained from OFETs devices of P3HT thin 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

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT This study was financially supported by the National Natural Science Foundations of China (No.21574010, 21774011 & 51221002).

REFERENCES


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Highly Anisotropic P3HT Film Fabricated via Epitaxy on an Oriented

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