Effect of External Electric Field on the Ordered Structure of Molecular

Oct 30, 2018 - The optimal magnitude of the external electric field was 5000 V/cm. With the optimized electric field applied to a series of P3HT films...
1 downloads 0 Views 6MB Size
Download PDF
Article Cite This: Langmuir 2018, 34, 13871−13881

pubs.acs.org/Langmuir

Effect of External Electric Field on the Ordered Structure of Molecular Chains and Hole Mobility in Regioregular Poly(3hexylthiophene) with Different Molecular Weights Jiaxuan Ren, Yanchun Tao, Xiaona Li, Tengning Ma, Bin Liu, and Dan Lu* State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, No. 2699 Qianjin Avenue, Changchun, 130012, China

Downloaded via UNIV OF WINNIPEG on November 27, 2018 at 14:26:20 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

S Supporting Information *

ABSTRACT: This research investigated the effect of a highvoltage external electric field on the ordered structure of molecular chains and hole mobility in regioregular poly(3hexylthiophene) (P3HT) with different molecular weights through X-ray diffraction, atomic force microscopy, transmission electron microscopy, Fourier transform infrared spectroscopy, micro-Raman spectroscopy, UV−vis spectroscopy, photoluminescence spectroscopy, and organic fieldeffect transistors. The optimal magnitude of the external electric field was 5000 V/cm. With the optimized electric field applied to a series of P3HT films, the carrier mobility of all P3HT films increased, and the increase rate changed from 105% to 56%, closely depending on the increase in molecular weight from 33 kg/mol to 100 kg/mol. The results indicated that the increase in carrier mobility was attributed to the P3HT conformation order, which was controlled by the external electric field. Molecular weight was a critical factor in determining the P3HT conformation response to the external electric field. The external electric field orientated lower-molecular-weight (33 kg/mol) P3HT into ordered structures more obviously than higher-molecular-weight (100 kg/mol) P3HT. This research contributes to the understanding of the effect of an external electric field on the ordered structure of the chains and carrier mobility in P3HT with different molecular weights. This research also reveals the regularity and mechanism of the formation of ordered structures and essentially enhances the carrier mobility of P3HT films with different molecular weights, to fabricate photovoltaic devices with high efficiency, based on polymer physics. crucial than those of the processing conditions.10 Nevertheless, few studies have investigated this aspect. The regioregularity of P3HT indicates the percentage of stereoregular head-to-tail (HT) attachments of hexyl side chains to the 3-position of thiophene rings (Figure 1).11 P3HT is a microcrystalline and self-organizing polymer.12 Samples with high regioregularity (>91%) have high carrier mobility, because the high-RR P3HT extends conjugated length of main chains and imparts a nearly planar conformation with intraand intermolecular overlap; thus, hole mobility can be as high as μ = 0.1−0.3 cm2/(V s).13,14 Unlike small molecules, the Mw

1. INTRODUCTION As a conjugated polymer used for organic electronics, poly(3hexylthiophene) (P3HT) has been widely considered for many applications, such as organic light-emitting diodes (OLED) and organic field effect transistors (OFET).1,2 P3HT has also been used as an electron donor with the fullerene derivative ([6,6]-phenyl-C61-butyric acid methyl ester, PCBM) in polymer solar cells with a high energy conversion efficiency.3,4 However, at present, its efficiency remains the main issue limiting its photovoltaic device applications, and one of the main factors that restricts the enhancement of efficiency is its low carrier mobility. Previous studies have focused on the synthesis of high-efficiency materials,5,6 photovoltaic device architectures, and operating principles,7,8 but the essential relationship between structure and performance is fundamental to enhance the performance of photovoltaic devices in terms of polymer physics. For conjugated polymers, chain conformation is the basis for macromolecular structure, and a slight change in conformation affects the photophysical properties and electron structures of polymer systems.9 The intrinsic properties, such as the regioregularity (RR) and molecular weight (Mw) of polymers, usually influence chain conformation. The effects of the intrinsic properties on photovoltaic performance are more © 2018 American Chemical Society

Figure 1. Structural formula of P3HT. Received: August 21, 2018 Revised: October 26, 2018 Published: October 30, 2018 13871

DOI: 10.1021/acs.langmuir.8b02838 Langmuir 2018, 34, 13871−13881

Article

Langmuir

the conformations of P3HT chains, and the mechanism of the change in P3HT conformation. More details will be discussed below.

of polymer is polydisperse, which can directly affect chain conformation, arrangement, and packing.15,16 Several reviews have demonstrated that the chain length of a conjugated polymer is a principal factor affecting active layers with high carrier mobility.17,18 Kline et al.15 found that carrier mobility increased from 10−5 cm2/(V s) for short chains (Mn corrected = 1 kDa) to 0.04 cm2/(V s) for long ones (Mn corrected = 27 kDa). In our research, we focused on P3HT with Mws between 33 kg/mol and 100 kg/mol, because P3HT has chemical defects and impurities at low Mws (270 kDa).19 The polydispersity (PDI) of P3HT in our study was relatively narrow (PDI ≈ 2), and its regioregularity was relatively high (RR> 91%) in each P3HT sample. These parameters (Table 1) were necessary to explore the changes in the physical properties of P3HT and their effects on the photovoltaic performance of devices.

2. EXPERIMENTAL SECTION 2.1. Device Fabrication and Sample Preparation. 2.1.1. Device Fabrication, Cleaning, and Preparation. An organic field effect transistor (OFET) was fabricated with Si/SiO2/P3HT/Au layers. Highly n+-doped Si was used as a gate electrode, purchased from SiMat Silicon Materials, Germany. An SiO2 oxide layer with a thickness of 300 nm was thermally evaporated onto the Si substrate as an insulator. The Si/SiO2 substrate was immersed in a piranha solution (H2SO4/H2O2 = 3:1) for 4 h and then cleaned with deionized water, ethanol, 2-propanol, and acetone. The cleaned substrates were dried in a vacuum oven at 150 °C. Then a P3HT film was spin-coated onto the oxide layer. After the effect of the external electric field during annealing, the source/drain gold electrodes were deposited onto the P3HT film by using a KVET-C500200 electron beam evaporator (Korea Vacuum Inc., Korea). The electrodes were designed with a channel width of 500 μm and a channel length of 20 μm. The electrical properties of the samples were obtained at various gate voltages with a high-speed source monitor unit (E5264A, Agilent Technologies, Santa Clara, CA). 2.1.2. P3HT Solution Processing and Deposition. RR-P3HT samples with different Mws (33, 55, 75, and 100 kg/mol) were purchased from Rieke Metals Inc. (Lincoln, NE) and used as received without further purification. Their specific parameters are shown in Table 1. The RR-P3HT samples were first dissolved in chlorobenzene, diluted to a concentration of 20 mg/mL, and magnetically stirred at 40 °C for 24 h. Afterward, the P3HT films with different Mws were spin-coated onto the oxide layers at 1000 rpm for 20 s in a nitrogenfilled glovebox, to a thickness of ca. 80 nm. 2.1.3. Annealing and Electric Field. For the first case, the spincoated film was dried by annealing above the glass transition temperature (Tg) of P3HT for 20 min and labeled “annealed” for comparison. The Tg, Mw, and regioregularity of P3HT reported by Salim et al.31 are 110 °C, 48.3 kg/mol, and >90%, respectively, which are similar to those of our samples. According to Ballantyne’s and Hiorns’s reports,32,33 the Tg of P3HT increases with its Mw. Hence, we chose the annealing temperature range of 110 °C to 160 °C to achieve the optimum annealing temperature of P3HT for each Mw. To ensure the accuracy of the optimal annealing temperature, the samples were repeatedly put through ten groups of parallel experiments and measured by UV−vis absorption spectra (Figure S1). According to the results, we chose 120 °C as the optimal annealing temperature for 33 kg/mol and 55 kg/mol P3HT and chose 150 °C as the optimal annealing temperature for 75 kg/mol and 100 kg/mol P3HT. For the second case, an “electric field device” was designed to apply a high-voltage DC between the highly n+-doped Si and the upper copper electrode. The distance between the two electrodes was 2 mm. The upper electrode did not directly contact the P3HT film surface, because the distance was larger than the sample’s thickness. The designed electric field device was similar to a capacitor. In the working process of the device, the steady-state current was below the detection limit (i.e., less than 1 fA). The external electric field was used to induce the change in the P3HT conformation before the samples were made into OFETs. After optimizing the annealing temperature for each Mw P3HT, we tried to optimize the intensity of the electric field. For each Mw sample, the external electric field could realize the most beneficial regulating effect at its optimal annealing temperature.28 The optimum intensity of external electric field, 5 kV/ cm, was achieved by XRD and UV−vis absorption spectra analysis through repeated experiments and measurements (Figures S2−S4). Spano’s model, R = A0−0/A0−1, was used to quantify the change in absorption spectra. The ratio R, shown in Tables S1 and S2, was the average value of ten parallel experimental results, which illustrated that changing the processing conditions could result in statistical differences in the UV−vis curves. The P3HT films for the second case

Table 1. P3HT Samples with Different Molecular Weights (Mws), Polydispersity Index (PDI), and Regioregularity (RR) samples

Mw (kg/mol)

PDI

RR (%)

1 2 3 4

33 55 75 100

1.7 1.9 2.0 2.1

91 93 91 94

As a soft matter, P3HT elicits a strong response when it is affected by a weak external field.20 The effect of solvent and temperature field on conjugated polymers has been reported; that is, these external fields can induce a polymer chain conformation to form an ordered structure and sequentially enhance the carrier mobility of photovoltaic devices.21,22 An electric field is a special external field with directionality, which is different from other external fields. It can align dipoles along the electric field direction in a phenomenon known as dipole polarization.23 Previous studies have reported that an external electric field can orient biological macromolecules24 and liquid crystal molecules25 along its direction. External electric fields can also control the nanomorphology of active layers to improve photoabsorption and charge carrier mobility.26 P3HT is a polar molecule with a dipole moment from 1.0 to 1.6 D, and its polarity is attributed to the electronegative sulfur atom in the heterocyclic molecule.27 Electric fields can induce variations in P3HT conformations to form ordered structures.28,29 Sirringhaus et al.30 showed that a high hole mobility was achieved in P3HT films with an edge-on orientation, indicating that the heterocyclic molecules were perpendicular to the substrate, and the main chains of conjugated molecules were arranged in parallel to the substrate. Applying an in-plane high-voltage electric field on films can cause polar heterocyclic molecules to adopt a more “edge-on” conformation. This orientation is beneficial to the movement of charge carriers through the active layer to an electrode.30 Although some studies have performed on external electric fields, no studies have focused on the effects of external electric field on the molecular chain conformations of P3HT with different Mws. In this work, we present the effects of high-voltage external electric fields on the ordered structures of molecular chains and hole mobility in regioregular P3HT with Mws ranging from 33 kg/mol to 100 kg/mol. We also explore the morphological and crystalline characteristics of P3HT films, 13872

DOI: 10.1021/acs.langmuir.8b02838 Langmuir 2018, 34, 13871−13881

Article

Langmuir

Figure 2. Schematic of the OFET device with applied electric field on P3HT film during annealing (without source/drain electrodes).

Figure 3. (a) Transfer characteristic curves of 33 kg/mol P3HT transistor with and without electric field. (b) Schematic of mobility (cm2/(V s)) of P3HT samples with molecular weights varying from 33 kg/mol to 100 kg/mol (e.f. denotes electric field). were named “annealed and electric field”. The P3HT films with different Mws (33−100 kg/mol) were cured under two conditions, in accordance with “annealed” and “annealed and electric field”. A schematic of the fabricated device and the electric-field annealing process is shown in Figure 2. 2.2. X-ray Diffraction (XRD) Measurement. The P3HT films with different Mws were spin-coated on Si wafers for XRD to measure their crystallinity under two different conditions. XRD analysis was performed using a high-resolution X-ray diffractometer (SmartLab, Rigaku Corporation, Japan) with monochromatized Cu Kα radiation (λ = 0.15406 nm). 2.3. Atomic Force Microscopy (AFM) Measurement. AFM was carried out with FastScan Bruker equipment in a tapping mode to examine the morphological characteristics of low-Mw and high-Mw P3HT films under different conditions. The measurement region was 2 μm × 2 μm. The values of surface roughness were calculated using the NanoScope Analysis software. 2.4. Transmission Electron Microscopy (TEM) Measurement. The images of TEM and high-resolution transmission electron microscopy (HR-TEM) were carried out via JEM-2100F, Japan. The magnified lattice fringes in HR-TEM can illustrate the degree of polymer orderliness. 2.5. Micro-Raman Spectroscopy Measurement. Raman spectra were recorded at an excitation wavelength of 488 nm (Ar ion laser), a power of 0.01 mW, and an acquisition time of 10 s with a JY-T64000 Raman microscope (HORIBA Jobin Yvon, Japan). The testing range was from 200 to 2000 cm−1. The peak position of the CC mode, the full width at half-maximum (fwhm) of the CC mode, and the relative peak intensity IC−C/ICC calculated by integrating the peak areas could illustrate the P3HT conformational changes under the external electric field.

2.6. Fourier Transform Infrared Spectroscopy (FTIR) Measurement. The P3HT films were spin-coated onto Si wafers for FTIR measurement (VERTEX 80 V, Bruker, Billerica, MA). FTIR spectra were obtained to characterize the changes in the conformations of P3HT with different Mws in the range of 3500− 400 cm−1 under the external electric field. 2.7. UV−vis Absorption Spectra. A UV−vis absorption spectrum was obtained to characterize the characteristic peak of P3HT with a Shimadzu UV-3000 spectrophotometer. The testing range was from 800 to 400 nm. The peak position shift could indicate the variations in the conjugated length of the P3HT chains under different conditions. The intensity of the shoulder peak could illustrate the status of the P3HT chain packing. Spectrosil glass substrates were used instead of Si wafers for the measurement as the former could transmit light. A copper electrode was utilized instead of the highly n+-doped Si as the lower electrode to apply the external electric field during annealing. 2.8. Photoluminescence (PL) Spectra. PL spectra were obtained with a fluorescence spectrophotometer (RF-5301PC, Shimadzu, Japan) equipped with a xenon lamp as an excitation source. The differences of ordered structures in P3HT with Mws of 33−100 kg/mol were more evident in the PL measurement than in the UV−vis spectra. The measurements were recorded from 620 to 750 nm with an excitation wavelength of 550 nm at room temperature.

3. RESULTS AND DISCUSSION 3.1. OFET Performance. The field-effect mobility (μ) was extracted in the saturation regime (VDS = −80 V) by the following standard equation: 13873

DOI: 10.1021/acs.langmuir.8b02838 Langmuir 2018, 34, 13871−13881

Article

Langmuir

2L ijj ∂ IDS jj WCi jk ∂VGS

yz zz zz { where Ci is the gate capacitance, W is the channel width, and L is the channel length. Figure 3a shows the transfer characteristic curves of the 33 kg/mol P3HT spin-coated film with and without external electric field upon annealing. This result indicated that the source-drain current (IDS) increased markedly after the electric field was applied, and the slope of (−IDS)1/2 versus VGS (right axis) increased from 3.14 × 10−5 to 4.45 × 10−5 after the electric field was applied, thereby increasing the hole mobility of P3HT from 3.87 × 10−2 cm2/(V s) to 7.92 × 10−2 cm2/(V s). Figure 3b illustrates the mobility of all P3HT samples with different Mws under two conditions (i.e., with and without electric field), and the reported data are the average values obtained from five different transistors fabricated under the same conditions (Table 2). In Table 2, the mobility of each μ=

2

decrease in mobility might be attributed to the change in the P3HT condensed state structure. The P3HT molecular chains lengthened as the Mw increased from 33 kg/mol to 100 kg/ mol. In this case, the carriers experienced difficulty in transporting along these long chains because of chain disorder and entanglement. Moreover, the effect of the electric field on P3HT films with different Mws varied. As the Mw increased, the growth rate of P3HT mobility decreased from 105% to 56%, suggesting that the effect of the external electric field on increasing mobility weakened as the chain length of P3HT increased. It is worth noting that the change in mobility might also be influenced by the morphological characteristics and roughness of the films, thereby affecting the contact resistance between the P3HT films and source/drain electrodes (section 3.2.2). However, the above standard equation neglected the existence of contact resistance. The true mobility should exclude the influence of the contact resistance,34,35 and thus our results underestimated the true mobility. However, the mobility we calculated did not affect the trend of the effect of the external electric field on P3HT films with different Mws, which could be demonstrated by subsequent characterizations. The mobility of P3HT was found to be lower than that described in previous studies that focused on photovoltaic device efficiency, possibly because we considered polymer physics rather than device structure modification. In this research, annealing above the Tg of P3HT was crucial to the effect of the electric field. Annealing is an effective method to adjust the chain conformation of polymers. Heating thin films above their Tg could provide not only the energy to force molecular chain segments to move but also the free volume to make such motions.36,37 External electric fields could easily alter the conformation during annealing. Therefore, the “annealed” P3HT samples were used for comparison in this research. Chloroform and chlorobenzene are good solvents of P3HT, and the mobility of P3HT in chloroform is higher than

Table 2. OFET Mobility with Different Mw P3HT Films under Two Conditions mobility [cm2/(V s)] samples 33 kg/mol 55 kg/mol 75 kg/mol 100 kg/mol

annealed 3.87 8.52 1.91 1.04

× × × ×

10−2 10−3 10−3 10−3

annealed and e.f. 7.92 1.57 3.25 1.62

× × × ×

10−2 10−2 10−3 10−3

mobility growth rate (%) 105 84 70 56

P3HT sample increased after the electric field was applied, suggesting that the high-voltage electric field could induce the polar P3HT molecular chains to align themselves into a more ordered structure, which was beneficial to charge transfer. However, the mobility decreased as the Mw of P3HT increased, regardless of the electric field (Figure 3b). This

Figure 4. X-ray diffraction of P3HT films with different molecular weights deposited on Si/SiO2 under “annealed” and “annealed and electric field” conditions (e.f. denotes electric field). 13874

DOI: 10.1021/acs.langmuir.8b02838 Langmuir 2018, 34, 13871−13881

Article

Langmuir that in chlorobenzene.13 However, the boiling point of chlorobenzene is 132.2 °C, whereas the boiling point of chloroform is 61.3 °C, obviously lower than the Tg of P3HT (110 °C),31 suggesting that chloroform has been evaporated at the annealing temperature of P3HT, when the P3HT chain segments begin to move. Using a solvent with a higher boiling point in spin-coated films can lengthen the evaporation time of the solvents, causing the P3HT chains to self-organize and form ordered structures, in a process similar to solution casting.38 Here, we observed the effect of the external electric field during annealing rather than attempting to achieve a high device performance. Therefore, chlorobenzene was used as a P3HT solvent in this research. 3.2. Thin-Film Properties. 3.2.1. Crystalline Characteristic of P3HT Films. XRD analysis (Figure 4) shows the crystalline characteristics of the P3HT films with four different Mws. The “annealed” and “annealed and electric field” samples showed an obvious single peak of (100) at 2θ = 5.4 ± 0.1°. This peak originated from polymer crystallites with an alkyl spacing direction normal to the substrate.39,40 The height of the peak at 2θ = 5.4° was proportional to the crystallinity of the P3HT thin films for the same thickness. From Figure 4a to Figure 4d, the crystallinity of the P3HT films at each Mw could be increased by applying the external electric field during annealing. The increase in thin film crystallinity was a direct consequence of the increase in the number of microcrystallites distributed across the films.41 Although the crystallinity of the P3HT films with four different Mws increased, the XRD analysis revealed that the increase rate of crystallinity differed. In Figure 4a, the diffraction peak intensity of the 33 kg/mol P3HT film increased obviously after the electric field was applied. To deeply understand the crystalline characteristics of P3HT film, interplanar distance (d) and crystallite size (D) were determined by XRD analysis in terms of the Bragg equation and Scherrer formula. In Table 3, the interplanar

3.2.2. Morphological Analysis of the P3HT Films. To investigate the effect of the external electric field on the morphologies of P3HT films with different Mws, we observed two typical samples of low-Mw P3HT (33 kg/mol) and highMw P3HT (100 kg/mol) with and without the application of electric field upon annealing through AFM in the tapping mode along the lateral direction. The AFM images of the P3HT films with low (33 kg/mol) and high (100 kg/mol) Mw under different treatment conditions are presented in Figure 5, and the corresponding roughness of the films is shown in Table 4. In Figure 5a, the low-Mw P3HT film (33 kg/mol) showed macroscopic nanofibers, indicating that P3HT had an ordered conformation in the low-Mw sample. In Figure 5b, the morphological characteristics of P3HT showed a large number of obvious nanofibers under the applied external electric field. This finding indicated that the application of the electric field during annealing caused the P3HT chains to form a more ordered conformation and led to an increased number of nanofibers. These results further indicated that the low-Mw P3HT films (33 kg/mol) had an obvious crystal morphology, and their crystallinity was enhanced with the application of the electric field. The application of electric field during annealing led to the formation of π−π stacked aggregates, which acted as nucleation centers for nanofiber formation.41 By comparison, high-Mw P3HT (100 kg/mol) showed nodulated morphology (Figure 5c), and no obvious change occurred after the electric field was applied (Figure 5d). This observation suggested that more amorphous states were present in the high-Mw P3HT films and that the effect of the electric field on the morphological characteristics of the high-Mw P3HT films was weaker than that on the low-Mw P3HT films. In short, the P3HT film morphology was altered as the Mw increased. In addition to bulk film microstructures, other factors, such as the interface between the P3HT film and the upper electrodes, influenced P3HT mobility. Thus, the mobility was also related to P3HT film roughness. The large rough morphology was detrimental to mobility, because it could increase the contact resistance between the P3HT films and electrodes, thereby causing poor charge injection.32,43 In Table 4, the average roughness increased obviously from 2.04 nm for the 33 kg/mol P3HT film to 6.50 nm for the 100 kg/mol P3HT film. Therefore, the amorphous state and large roughness of the 100 kg/mol P3HT film collectively resulted in poor mobility. Although the film roughness of the 33 kg/mol P3HT increased slightly from 2.04 to 2.20 nm after the electric field was applied,44 the effects of its crystallinity and ordered morphology on mobility were stronger than that of film roughness. Therefore, the mobility of the 33 kg/mol P3HT increased noticeably after the electric field was applied (Table 2). To further analyze the change in microscopic morphology and ordered structures of P3HT films under the external electric field, we studied the TEM and HR-TEM images of low-Mw and high-Mw P3HT (Figure S5). HR-TEM results indicated that the regulatory effect of the electric field on lowMw P3HT chains was more obvious than that on the high-Mw P3HT. This result was well consistent with XRD and AFM analyses. 3.3. Ordered Structure of the P3HT Chains. 3.3.1. Micro-Raman Spectroscopy Measurement. To investigate the effects of the external electric field on the chain ordered structures of P3HT, we characterized low-Mw P3HT (33 kg/mol) and high-Mw P3HT (100 kg/mol) using Raman

Table 3. Interplanar Distance (d) and Crystallite Size (D) Determined by XRD Analysis from Figure 4 plane distance (Å)

crystallite size (nm)

samples

annealed

annealed and e.f.

annealed

annealed and e.f.

33 kg/mol 55 kg/mol 75 kg/mol 100 kg/mol

16.23 16.27 16.60 16.61

15.99 16.16 16.54 16.57

22.81 19.70 18.59 17.70

25.59 21.00 19.66 17.96

distance (d) of the 33 kg/mol P3HT film decreased from 16.23 to15.99 Å, indicating that the density of the chain arrangements increased in the (100) direction under the electric field. The crystallite size (D) of the 33 kg/mol P3HT film increased from 22.81 to 25.59 nm with the application of the electric field, thereby contributing to an increase in mobility. Because the carriers transported faster in the crystalline region than in the grain boundary, the large grain could decrease the grain boundary density, thereby reducing the scattering action of the grain boundary to carriers and increasing the mobility.29,42 However, Figure 4d shows that the crystallinity of the 100 kg/ mol P3HT film increased weakly, and interplanar distance (d) and crystallite size (D) changed slightly after the electric field was applied. Therefore, the increase rate of the crystallinity of the P3HT films gradually decreased as the Mw increased from 33 kg/mol to 100 kg/mol. 13875

DOI: 10.1021/acs.langmuir.8b02838 Langmuir 2018, 34, 13871−13881

Article

Langmuir

Figure 5. Tapping mode AFM images and corresponding RMS images of P3HT films with low-Mw (33 kg/mol) (a, b) and high-Mw (100 kg/mol) (c, d), under the conditions of “annealed” and “annealed and electric field”.

respectively.45,46 The mode at 1208 cm−1 corresponds to C−C inter-ring stretching. The modes at 1380 and 1450 cm−1 are related to intraring C−C skeletal stretching and CC symmetric ring stretching, respectively.45,46 To observe the effect of the external electric field on the conformation of the P3HT chains with different Mws, we analyzed the regions from 1350 to 1500 cm−1 (Figures 6b,c). This region was deemed to be sensitive to the conjugation length and π-electron delocalization of P3HT molecules.47 For low-Mw P3HT (33 kg/mol), the peak position of the CC mode shifted from 1450 cm−1 to a lower wavenumber of 1448 cm−1 after the external electric field was applied, indicating that P3HT with a highly ordered structure was formed under the applied electric field, because this CC peak position was sensitive to the degree of the molecular order of P3HT.45 The fwhm of the CC mode for the 33 kg/mol “annealed and

Table 4. Average RMS (Root Mean Square) Surface Roughness of Different P3HT Films under “Annealed” and “Annealed and Electric Field” Conditions film roughness (nm) samples 33 kg/mol 55 kg/mol 75 kg/mol 100 kg/mol

annealed 2.04 3.43 5.84 6.50

± ± ± ±

0.10 0.10 0.05 0.05

annealed and e.f. 2.20 3.51 5.88 6.53

± ± ± ±

0.10 0.10 0.05 0.05

spectra. Figure 6a represents the micro-Raman spectra of lowand high-Mw P3HT thin films with and without the external electric field, showing various Raman modes from 600 to 1600 cm−1. The Raman modes observed at 725 and 1186 cm−1 are attributed to C−S−C ring deformation and C−H bending, 13876

DOI: 10.1021/acs.langmuir.8b02838 Langmuir 2018, 34, 13871−13881

Article

Langmuir

electric field” film (32.48 cm−1) was smaller than that of corresponding film without electric field (33.84 cm−1), suggesting that the electric field induced the low-Mw P3HT chains to form a highly ordered structure.46,48 The relative peak intensity, IC−C/ICC, was calculated by integrating the peak areas of the C−C mode (1380 cm−1) and the CC mode (1450 cm−1). The IC−C/ICC ratio increased from 0.09 for the 33 kg/mol “annealed” P3HT film to 0.10 for the 33 kg/ mol “annealed and electric field” sample, implying that the electric field could effectively enhance the conjugated length of P3HT chains, causing P3HT to form a more ordered film.45,46 However, the Raman spectrum of the high-Mw P3HT (100 kg/mol, Figure 6c) changed little. This observation indicated that external electric field could not induce the high-Mw P3HT conformation to orient into more ordered structures. The fwhm of the CC mode (1450 cm−1) of the high-Mw P3HT was 34.56 cm−1, which was obviously larger than that of the low-Mw P3HT (33.84 and 32.48 cm−1), and the IC−C/ICC of high-Mw P3HT was 0.08, also lower than that of low-Mw P3HT (0.09 and 0.10). All of these characterized results illustrated that P3HT with a high Mw presented disordered structures. To complement the Raman analysis, we subjected the P3HT thin films with low (33 kg/mol) and high (100 kg/mol) Mw to FTIR studies (Figures S6 and S7). In Figure S6a, the ratio of two peak intensities I1510/I1460 increased after the external electric field was applied, suggesting that the action of the electric field could increase the conjugated lengths of the main chains of the low-Mw P3HT and enhance the planarity of the P3HT skeleton.49 Conversely, the ratio of the high-Mw P3HT had no obvious change (Figure S7b). In Figure S7c, the peak intensity of the 3054 cm−1 position increased after the external electric field was applied to low-Mw P3HT, whose intensity increasd as the ordered structure enhanced.50 Conversely, the peak intensity of the high-Mw P3HT did not increase apparently (Figure S7d). These results indicated that the regulatory effect of the external electric field on the low-Mw P3HT conformation was better than that on the high-Mw

Figure 6. (a) Raman spectra of RR-P3HT thin films from 400 to 2000 cm−1 under 488 nm excitation. (b, c) Normalized Raman spectra (C− C and CC modes) of low-Mw and high-Mw P3HT, respectively, under different conditions from 1350 to 1600 cm−1.

Figure 7. UV−vis absorption spectra of four P3HT films with different molecular weights, treated as “annealed” and “annealed and electric field”. 13877

DOI: 10.1021/acs.langmuir.8b02838 Langmuir 2018, 34, 13871−13881

Article

Langmuir

coupled to the electronic transition. Assuming the CC symmetric stretch (Ep = 0.18 eV) dominates the coupling to the electronic transition, W could be estimated.52 3.3.3. Photoluminescence Spectroscopy Measurement. The normalized PL spectra of low- and high-Mw P3HT thin films are shown in Figure 8 to support the UV−vis absorption

P3HT. The results of the FTIR analysis were consistent with those of the Raman spectra. 3.3.2. UV−vis Spectroscopy Measurement. The effects of the external electric field on the chain conformation of P3HT with different Mws were also investigated through UV−vis absorption and PL spectroscopy. The UV−vis absorption spectra of the P3HT films with different Mws (33, 55, 75, and 100 kg/mol) treated under two conditions are shown in Figure 7a−d. In Figure 7, two characteristic absorption peaks of P3HT were seen at 555 and 607 nm. According to Spano’s model,51 the low energy 0−0 (at 607 nm) is dominant in the crystalline region of the film, which forms weakly interacting H-aggregate states, and the high energy 0−1 (at 555 nm) creates intrachain states.52 In our study, we referred to the first two peak intensities in the absorption spectra as A0−0 and A0−1. A0−0 was the intensity of the low-energy absorption peak, and A0−1 was the intensity of the high-energy absorption peak. The ratio of A0−0/A0−1 quantitatively increased with the increase in conjugated length and ordered degree of the P3HT chains (Table 4).53,54 For the 33 kg/mol P3HT films (Figure 7a), an obvious redshift of the absorption peak occurred at ca. 555 nm, and the relative intensity of the absorption peak increased distinctly at ca. 607 nm with the applied external electric field during annealing, indicating that the external electric field caused an increase in not only the intrachain conjugated length but also the interchain π−π stacking.55,56 In Table 5, the ratio

Figure 8. Normalized PL spectra of different Mw P3HT thin films under different conditions.

spectra. In comparison with the low-Mw (33 kg/mol, black lines) P3HT, the high-Mw (100 kg/mol, red lines) P3HT showed a blueshift of the emission peak, and this finding was the primary evidence supporting the decrease in the conjugation length of the main chains of high-Mw P3HT and the formation of a twisted, disordered structure.18 Considering that P3HT presented H-aggregates in this research, we termed the first two peak intensities in the emission spectrum as PL0−0 and PL0−1, and found that the ratio RPL = PL0−0/PL0−1 increased with the disorder of the P3HT molecular chains.57,58 This PL Mw-dependent behavior indicated that the P3HT conformation transformed from an ordered aggregated phase (RPL < 1) for the low-Mw P3HT (33 kg/mol) to a coiled amorphous phase (RPL > 1) for the highMw P3HT (100 kg/mol).36 These results highlighted the difference in P3HT conformation between low and high Mw (Table 6).

Table 5. Ratio R = A0‑0/A0‑1 for Four Kinds of P3HT Films under Different Conditions in UV-vis Absorption Spectra R = A0−0/A0−1 samples

annealed

annealed and e.f.

33 kg/mol 55 kg/mol 75 kg/mol 100 kg/mol

0.60 0.53 0.50 0.48

0.68 0.56 0.51 0.49

R = A0−0/A0−1 increased from 0.60 to 0.68, suggesting that the electric field increased the conjugated length and ordered degree of the 33 kg/mol P3HT chains. These results demonstrated that the low-Mw P3HT segments can arrange and pack into a more ordered structure with electric field application. However, the redshift and intensity of the UV−vis absorption peaks did not obviously increase as the P3HT Mws increased (Figures 7b−d), indicating that the gap between the “annealed” and “electric field annealed” samples narrowed with the increase in Mws. In other words, the effect of the external electric field weakened as the Mw of P3HT increased, and this observation could be ascribed to the appearance of long and twisted P3HT chains as the Mws increased, so that the external electric field induced P3HT to undergo a difficult transformation of ordered chain conformation. Note: For the data summarized from the UV−vis absorption spectra in Figure 7, the ratio of A0−0 and A0−1 was related to the free exciton bandwidth of the aggregates (W) and the energy of the main intramolecular vibration (Ep) by the following equation:

Table 6. Ratio R = PL0‑0/PL0‑1 for Four Kinds of P3HT Films under Different Conditions in PL Spectra RPL = PL0−0/PL0−1 samples

annealed

annealed and e.f.

33 kg/mol 100 kg/mol

0.91 1.31

0.84 1.29

For the low-Mw P3HT (33 kg/mol), the ratio RPL = PL0−0/ PL0−1 decreased from 0.91 to 0.84 under the external electric field, indicating that the electric field enhanced the ordered structure of the low-Mw P3HT. Conversely, in the high-Mw P3HT (100 kg/mol) PL spectrum, there was no marked change after the electric field was applied, which meant the external electric field could not induce the high-Mw P3HT to orient into more ordered structures. These results verified the UV−vis, FTIR, and Raman spectra. 3.4. Mechanism of the Change in the Ordered Structure of P3HT Chains. The process and mechanism of the effect of the external electric field on P3HT chain ordered structure are summarized in Figure 9. In the absence of an electric field, the molecular chains of P3HT likely exhibited a relatively random orientation. The structures of the high-Mw

A 0 − 0 ijj 1 − 0.24W /Ep yzz zz ≈ jj A 0 − 1 jj 1 + 0.073W /Ep zz k { Here W is the free exciton bandwidth of the aggregates and Ep is the energy of the main intramolecular vibration, which is 2

13878

DOI: 10.1021/acs.langmuir.8b02838 Langmuir 2018, 34, 13871−13881

Article

Langmuir

Figure 9. A schematic illustration of P3HT chain conformation orderliness with (a) low Mw (33 kg/mol) and (b) high Mw (100 kg/mol) under the action of an external electric field.

result, the electric field could not easily overcome the energy barrier caused by the rotational resistance of the structural units, steric hindrance of the side chains, and the twisting and entanglement of main chains to orient the long P3HT chains into ordered structures. These conclusions contributed to the understanding of the effect of the external electric field on chain conformation with different Mws in terms of polymer physics and provided a basis for fabricating photovoltaic devices with high charge carrier mobility and efficiency.

P3HT with longer molecular chains were more disordered than those of the films with shorter molecular chains (Figure 9b), because long chains were more flexible than short ones due to the lack of rigidity of the former, leading to the more curly and entangled conformations of these chains.59,60 After electric field annealing was performed, the polymer chains were expected to align along the electric field direction (Figure 9a). However, orienting the high-Mw P3HT chains (100 kg/mol) into ordered structures was more difficult than orienting the low-Mw ones (33 kg/mol) under the applied electric field. In this instance, the energy provided by the electric field was insufficient to overcome the energy consumption from the rotational resistance of the structural units, steric hindrance of the side chains, and main chain twisting and entanglement, so that the energy provided by the electric field could not cross the height barrier of the ordered rearrangement of the chains. Therefore, the effect of the electric field on regulating and controlling the molecular chain conformation weakened with the increasing of P3HT Mw.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.8b02838.

4. CONCLUSIONS The effect of an external electric field on the molecular chain ordered structures and hole mobility in regioregular P3HT with different molecular weights ranging from 33 kg/mol to 100 kg/mol was revealed. The hole mobility of all samples was enhanced with the application of the external electric field. The growth rate of mobility gradually decreased from 105% to 56% as the molecular weight increased from 33 kg/mol to 100 kg/ mol under the optimized external electric field magnitude (5000 V/cm) during annealing. This result indicated that the response of the P3HT conformation to the effect of external electric field was closely correlated with molecular weight, an intrinsic property. The optimal molecular weight of P3HT in this research was 33 kg/mol, and the external electric field could achieve its optimized effect on the P3HT films to enhance the ordered structures of the chain and consequently increase hole mobility. However, high-Mw (100 kg/mol) P3HT chains had more complicated conformations than lowMw ones, as the former tended to twist and entangle with one another, thereby limiting the conjugation length, creating a disordered chain packing, and yielding low crystallinity. As a

■

UV−vis spectra of the P3HT films at different annealing temperatures; XRD and UV−vis spectra of the P3HT films at different intensities of the external electric field; TEM and HR-TEM results of the P3HT films; FTIR measurement results of the P3HT films (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +86-136-2079-2963. ORCID

Dan Lu: 0000-0002-7537-3173 Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS This work has been supported by the Natural Science Foundation of China (91333103 and 21574053). REFERENCES

(1) Sirringhaus, H.; Tessler, N.; Friend, R. H. Integrated optoelectronic devices based on conjugated polymers. Science 1998, 280, 1741−1744. (2) Fix, W.; Ullmann, A.; Ficker, J.; Clemens, W. Fast polymer integrated circuits. Appl. Phys. Lett. 2002, 81, 1735−1737.

13879

DOI: 10.1021/acs.langmuir.8b02838 Langmuir 2018, 34, 13871−13881

Article

Langmuir

and exciton bandwidth in high-mobility conjugated polymers. Phys. Rev. B: Condens. Matter Mater. Phys. 2006, 74, 115318. (20) Qian, B. In The Transition and Relaxation of Polymers, Xu, G. P., Yu, F. S., Eds.; Science Publishing Company: Beijing, 1986; p 101. (21) Azuma, H.; Asada, K.; Kobayashi, T.; Naito, H. Fabrication of α- and β-phase poly(9,9-dioctylfluorene) thin films. Thin Solid Films 2006, 509, 182−184. (22) Dias, F. B.; Morgado, J.; Macanita, A. L.; da Costa, F. P.; Burrows, H. D.; Monkman, A. P. Kinetics and thermodynamics of poly(9,9-dioctylfluorene) β-phase formation in dilute solution. Macromolecules 2006, 39, 5854−5864. (23) Hao, X.- T.; Chan, N. Y.; Dunstan, D. E.; Smith, T. A. Conformational changes and photophysical behavior in poly[2methoxy-5-(2′-ethyl-hexyloxy-1,4-phenylenevinylene] thin films cast under an electric field. J. Phys. Chem. C 2009, 113, 11657−11661. (24) Yamaoka, K.; Fukudome, K. Electric field orientation of nucleic acids in aqueous solutions. 2. Dependence of the intrinsic electric dichroism and electric dipole moments of rodlike DNA on molecular weight and ionic strength. J. Phys. Chem. 1990, 94, 6896−6903. (25) Zhou, W.; Shi, J.; Lv, L.; Chen, L.; Chen, Y. A mechanistic investigation of morphology evolution in P3HT-PCBM films induced by liquid crystalline molecules under external electric field. Phys. Chem. Chem. Phys. 2015, 17, 387−397. (26) Lin, C.- C.; Lin, Y.- Y.; Li, S.- S.; Yu, C.- C.; Huang, C. L.; Lee, S.- H.; Du, C.- H.; Lee, J.- J.; Chen, H.- L.; Chen, C.- W. Electric fieldassisted self-organization of polymer: fullerene hybrids on the photovoltaic performance. Energy Environ. Sci. 2011, 4, 2134−2139. (27) Marchiori, C. F. N.; Koehler, M. Dipole assisted exciton dissociation at conjugated polymer/fullerene photovoltaic interfaces: a molecular study using density functional theory calculations. Synth. Met. 2010, 160, 643−650. (28) Bagui, A.; Iyer, S. S. K. Effect of solvent annealing in the presence of electric field on P3HT:PCBM films used in organic solar cells. IEEE Trans. Electron Devices 2011, 58, 4061−4066. (29) Lee, S.- W.; Kim, C.- H.; Lee, S.- G.; Jeong, J.- H.; Choi, J.- H.; Lee, E.- S. Mobility improvement of P3HT thin film by high-voltage electrostatic field-assisted crystallization. Electron. Mater. Lett. 2013, 9, 471−476. (30) Sirringhaus, H.; Brown, P. J.; Friend, R. H.; Nielsen, M. M.; Bechgaard, K.; Langeveld-Voss, B. M. W.; Spiering, A. J. H.; Janssen, R. A. J.; Meijer, E. W.; Herwig, P.; deLeeuw, D. M. Two-dimensional charge transport in self-organized, high-mobility conjugated polymers. Nature 1999, 401, 685−688. (31) Salim, T.; Sun, S.; Wong, L. H.; Xi, L.; Foo, Y. L.; Lam, Y. M. The role of poly(3-hexylthiophene) nanofibers in an all-polymer blend with a polyfluorene copolymer for solar cell applications. J. Phys. Chem. C 2010, 114, 9459−9468. (32) Ballantyne, A. M.; Chen, L.; Dane, J.; Hammant, T.; Braun, F. M.; Heeney, M.; Duffy, W.; McCulloch, I.; Bradley, D. D. C.; Nelson, J. The effect of poly(3-hexylthiophene) molecular weight on charge transport and the performance of polymer:fullerene solar cells. Adv. Funct. Mater. 2008, 18, 2373−2380. (33) Hiorns, R. C.; de Bettignies, R.; Leroy, J.; Bailly, S.; Firon, M.; Sentein, C.; Khoukh, A.; Preud’homme, H.; Dagron-Lartigau, C. High molecular weights, polydispersities, and annealing temperatures in the optimization of bulk-heterojunction photovoltaic cells based on poly(3-hexylthiophene) or poly(3-butylthiophene). Adv. Funct. Mater. 2006, 16, 2263−2273. (34) Hamadani, B. H.; Natelson, D. Temperature-dependent contact resistances in high-quality polymer field-effect transistors. Appl. Phys. Lett. 2004, 84, 443−445. (35) Bürgi, L.; Richards, T. J.; Friend, R. H.; Sirringhaus, H. Close look at charge carrier injection in polymer field-effect transistors. J. Appl. Phys. 2003, 94, 6129−6137. (36) Brown, P. J.; Thomas, D. S.; Köhler, A.; Wilson, J. S.; Kim, J.S.; Ramsdale, C. M.; Sirringhaus, H.; Friend, R. H. Effect of interchain interactions on the absorption and emission of poly(3-hexylthiophene). Phys. Rev. B: Condens. Matter Mater. Phys. 2003, 67, 064203.

(3) Kim, Y.; Cook, S.; Tuladhar, S. M.; Choulis, S. A.; Nelson, J.; Durrant, J. R.; Bradley, D. D. C.; Giles, M.; McCulloch, I.; Ha, C.- S.; Ree, M. A strong regioregularity effect in self-organizing conjugated polymer films and high-efficiency polythiophene: fullerene solar cells. Nat. Mater. 2006, 5, 197−203. (4) Ma, W.; Yang, C.; Gong, X.; Lee, K.; Heeger, A. J. Thermally stable, efficient polymer solar cells with nanoscale control of the interpenetrating network morphology. Adv. Funct. Mater. 2005, 15, 1617−1622. (5) Kim, H. J.; Kim, J. W.; Kim, J.- J. Invited Paper: nanostructures of a mixed donor and acceptor layer in organic photovoltaics. Electron. Mater. Lett. 2011, 7, 93−104. (6) Kim, D.- I.; Hwang, B.- U.; Tien, N. T.; Kim, I.- J.; Lee, N.- E. Effects of piezoresistivity of pentacene channel in organic thin film transistors under mechanical bending. Electron. Mater. Lett. 2012, 8, 11−16. (7) You, J.; Dou, L.; Yoshimura, K.; Kato, T.; Ohya, K.; Moriarty, T.; Emery, K.; Chen, C.- C.; Gao, J.; Li, G.; Yang, Y. A polymer tandem solar cell with 10.6% power conversion efficiency. Nat. Commun. 2013, 4, 1446−1457. (8) Liao, S.- H.; Jhuo, H.- J.; Yeh, P.- N.; Cheng, Y.- S.; Li, Y.- L.; Lee, Y.- H.; Sharma, S.; Chen, S.- A. Single junction inverted polymer solar cell reaching power conversion efficiency 10.31% by employing dual-doped zinc oxide nano-film as cathode interlayer. Sci. Rep. 2015, 4, 6813. (9) Yin, J. H. In Modern Polymer Physics, 1st ed.; Mo, Z. S.; Science Press: Beijing, 2001; p 21. (10) Verilhac, J. M.; LeBlevennec, G.; Djurado, D.; Rieutord, F.; Chouiki, M.; Travers, J. P.; Pron, A. Effect of macromolecular parameters and processing conditions on supramolecular organisation, morphology and electrical transport properties in thin layers of regioregular poly(3-hexylthiophene). Synth. Met. 2006, 156, 815−823. (11) McCullough, R. D. The chemistry of conducting polythiophenes. Adv. Mater. 1998, 10, 93−116. (12) Jeong, J.- W.; Park, Y.- W.; Park, T.- H.; Choi, J.- H.; Choi, H.J.; Song, E.- H.; Lee, J.- I.; Chu, H.- Y.; Ju, B. K. The emission properties of integrated organic light emitting diodes with organic photo sensor for emotional lighting applications. IEEE Electron Device Lett. 2011, 32, 348−350. (13) Dang, M. T.; Hirsch, L.; Wantz, G.; Wuest, J. D. Controlling the morphology and performance of bulk heterojunctions in solar cells. Lessons learned from the benchmark poly(3-hexylthiophene): [6,6]-phenyl-C61-butyric acid methyl ester system. Chem. Rev. 2013, 113, 3734−3765. (14) Cho, S.; Lee, K.; Yuen, J.; Wang, G.; Moses, D.; Heeger, A. J.; Surin, M.; Lazzaroni, R. Thermal annealing-induced enhancement of the field-effect mobility of regioregular poly(3-hexylthiophene) films. J. Appl. Phys. 2006, 100, 114503. (15) Kline, R. J.; McGehee, M. D.; Kadnikova, E. N.; Liu, J.; Fréchet, J. M. J. Controlling the field-effect mobility of regioregular polythiophene by changing the molecular weight. Adv. Mater. 2003, 15, 1519−1522. (16) Zen, A.; Pflaum, J.; Hirschmann, S.; Zhuang, W.; Jaiser, F.; Asawapirom, U.; Rabe, J. P.; Scherf, U.; Neher, D. Effect of molecular weight and annealing of poly(3-hexythiophene)s on the performance of organic field-effect transistors. Adv. Funct. Mater. 2004, 14, 757− 764. (17) Kline, R. J.; McGehee, M. D.; Kadnikova, E. N.; Liu, J.; Fréchet, J. M. J.; Toney, M. F. Dependence of regioregular poly(3hexylthiophene) film morphology and field-effect mobility on molecular weight. Macromolecules 2005, 38, 3312−3319. (18) Zen, A.; Saphiannikova, M.; Neher, D.; Grenzer, J.; Grigorian, S.; Pietsch, U.; Asawapirom, U.; Janietz, S.; Scherf, U.; Lieberwirth, I.; Wegner, G. Effect of molecular weight on the structure and crystallinity of poly(3-hexylthiophene). Macromolecules 2006, 39, 2162−2171. (19) Chang, J.- F.; Clark, J.; Zhao, N.; Sirringhaus, H.; Breiby, D. W.; Andreasen, J. W.; Nielsen, M. M.; Giles, M.; Heeney, M.; McCulloch, I. Molecular-weight dependence of interchain polaron delocalization 13880

DOI: 10.1021/acs.langmuir.8b02838 Langmuir 2018, 34, 13871−13881

Article

Langmuir (37) Gurau, M. C.; Delongchamp, D. M.; Vogel, B. M.; Lin, E. K.; Fischer, D. A.; Sambasivan, S.; Richter, L. J. Measuring molecular order in poly(3-alkylthiophene) thin films with polarizing spectroscopies. Langmuir 2007, 23, 834−842. (38) Chang, J.- F.; Sun, B.; Breiby, D. W.; Nielsen, M. M.; Sölling, T. I.; Giles, M.; McCulloch, I.; Sirringhaus, H. Enhanced mobility of poly(3-hexylthiophene) transistors by spin-coating from high-boilingpoint solvents. Chem. Mater. 2004, 16, 4772−4776. (39) Yang, X.; Loos, J.; Veenstra, S. C.; Verhees, W. J. H.; Wienk, M. M.; Kroon, J. M.; Michels, M. A. J.; Janssen, R. A. J. Nanoscale morphology of high-performance polymer solar cells. Nano Lett. 2005, 5, 579−583. (40) Erb, T.; Zhokhavets, U.; Gobsch, G.; Raleva, S.; Stühn, B.; Schilinsky, P.; Waldauf, C.; Brabec, C. J. Correlation of structural and optical properties of composite polymer/fullerene films for organic solar cells. Adv. Funct. Mater. 2005, 15, 1193−1196. (41) Aiyar, A. R.; Hong, J.; Nambiar, R.; Collard, D. M.; Reichmanis, E. Tunable crystallinity in regioregular poly(3-Hexylthiophene) thin films and its impact on field effect mobility. Adv. Funct. Mater. 2011, 21, 2652−2659. (42) Braun, D.; Heeger, A. J. Visible light emission from semiconducting polymer diodes. Appl. Phys. Lett. 1991, 58, 1982− 1984. (43) Alam, M. W.; Wang, Z. K.; Naka, S.; Okada, H. Mobility enhancement of top contact pentacene based organic thin film transistor with bi-layer GeO/Au electrodes. Appl. Phys. Lett. 2013, 102, 061105. (44) Shen, Y.- M.; Chen, C.- S.; Yang, P.- C.; Ma, S.- Y.; Lin, C.- F. Improvement of surface morphology of thin films and performance by applying electric field on P3HT:PCBM based solar cells. Sol. Energy Mater. Sol. Cells 2012, 99, 263−267. (45) Tsoi, W. C.; James, D. T.; Kim, J. S.; Nicholson, P. G.; Murphy, C. E.; Bradley, D. D. C.; Nelson, J.; Kim, J.- S. The nature of in-plane skeleton Raman modes of P3HT and their correlation to the degree of molecular order in P3HT:PCBM blend thin films. J. Am. Chem. Soc. 2011, 133, 9834−9843. (46) Baibarac, M.; Lapkowski, M.; Pron, A.; Lefrant, S.; Baltog, I. SERS spectra of poly(3-hexylthiophene) in oxidized and unoxidized states. J. Raman Spectrosc. 1998, 29, 825−832. (47) Gao, Y.; Grey, J. K. Resonance chemical imaging of polythiophene/fullerene photovoltaic thin films: mapping morphology-dependent aggregated and unaggregated CC species. J. Am. Chem. Soc. 2009, 131, 9654−9662. (48) Sharma, T.; Singhal, R.; Vishnoi, R.; Sharma, G. D.; Biswas, S. K. Effect of high energy ions on the electrical and morphological properties of poly(3-hexylthiophene) (P3HT) thin film. Phys. B 2018, 537, 306−313. (49) Trznadel, M.; Pron, A.; Zagorska, M.; Chrzaszcz, R.; Pielichowski, J. Effect of molecular weight on spectroscopic and spectroelectrochemical properties of regioregular poly(3-hexylthiophene). Macromolecules 1998, 31, 5051−5058. (50) Motaung, D. E.; Malgas, G. F.; Arendse, C. J. Comparative study: The effects of solvent on the morphology, optical and structural features of regioregular poly(3-hexylthiophene): fullerene thin films. Synth. Met. 2010, 160, 876−882. (51) Spano, F. C. Modeling disorder in polymer aggregates: The optical spectroscopy of regioregular poly(3-hexylthiophene) thin films. J. Chem. Phys. 2005, 122, 234701. (52) Clark, J.; Chang, J.- F.; Spano, F. C.; Friend, R. H.; Silva, C. Determining exciton bandwidth and film microstructure in polythiophene films using linear absorption spectroscopy. Appl. Phys. Lett. 2009, 94, 163306. (53) Beljonne, D.; Cornil, J.; Silbey, R.; Millié, P.; Brédas, J. L. Interchain interactions in conjugated materials: the exciton model versus the supermolecular approach. J. Chem. Phys. 2000, 112, 4749− 4758. (54) Barford, W. Exciton transfer integrals between polymer chains. J. Chem. Phys. 2007, 126, 134905.

(55) Ruderer, M. A.; Guo, S.; Meier, R.; Chiang, H.- Y.; Körstgens, V.; Wiedersich, J.; Perlich, J.; Roth, S. V.; Müller-Buschbaum, P. Solvent-induced morphology in polymer-based systems for organic photovoltaics. Adv. Funct. Mater. 2011, 21, 3382−3391. (56) Chang, H.; Wang, P.; Li, H.; Zhang, J.; Yan, D. Solvent vapor assisted spin-coating: A simple method to directly achieve high mobility from P3HT based thin film transistors. Synth. Met. 2013, 184, 1−4. (57) Yamagata, H.; Spano, F. C. Interplay between intrachain and interchain interactions in semiconducting polymer assemblies: the H J-aggregate model. J. Chem. Phys. 2012, 136, 184901. (58) Spano, F. C. The spectral signatures of Frenkel polarons in Hand J-aggregates. Acc. Chem. Res. 2010, 43, 429−439. (59) Linse, P.; Källrot, N. Polymer adsorption from bulk solution onto planar surfaces: Effect of polymer flexibility and surface attraction in good solvent. Macromolecules 2010, 43, 2054−2086. (60) Gillissen, J. J. J. Polymer flexibility and turbulent drag reduction. Phys. Rev. E 2008, 78, 046311.

13881

DOI: 10.1021/acs.langmuir.8b02838 Langmuir 2018, 34, 13871−13881