Polymer Thin Film: Effect of Annealing Temperature - American

Sep 18, 2014 - School of Materials Science and Technology, Indian Institute of ... Motilal Nehru National Institute of Technology, Allahabad-211004, I...
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Directed Self-Assembly of Poly(3,3‴-dialkylquarterthiophene) Polymer Thin Film: Effect of Annealing Temperature Rajiv K. Pandey,† Arun Kumar Singh,‡ and Rajiv Prakash*,† †

School of Materials Science and Technology, Indian Institute of Technology (Banaras Hindu University), Varanasi-221005, India Department of Physics, Motilal Nehru National Institute of Technology, Allahabad-211004, India

‡

S Supporting Information *

ABSTRACT: Self-assembly of π-conjugated polymers in desired manner plays a vital role in structure, orientations, crystalline packing, and also in electrical charge transport properties. Despite this, there is lack of thorough study about the direct formation of smooth, oriented, crystalline, and aligned films using selfassembly property of π-conjugated polymers. In this study, we have discussed the crystallization behavior and an easy method to study face-on orientation, crystallization, and alignment in organic films, giving as an example poly(3,3‴dialkylquarterthiophene) (PQT-12). The effect of annealing temperature (80 and 120 °C) is also studied for this polymer film as the ordering of the polymer backbone and side chains highly depends on temperature. We have directed the self-assembly of PQT-12 using facile “floating film transfer method (FTM)” for obtaining crystalline, oriented, smooth, and aligned polymer films directly without further processing. Unpolarized, polarized UV−vis spectra and selected area electron diffraction (SAED) pattern are used to investigate the ordering/crystallinity, orientation, and alignment (optical anisotropy) of PQT-12 polymer films. Further, an easy electrochemical method is explored to study the crystalline and amorphous phases in the polymer films. Atomic force microscopy (AFM) topography is carried out to study the surface morphology, which shows formation of very smooth films with roughness below 1 nm. Raman spectra show the increase in intensity of signal-to-noise ratio (SNR) (1457 cm−1) and decrease in ratio of SNR intensity (1457 cm−1/1393 cm−1) as a function of annealing temperature. Finally, this study helps in improving the charge transport properties of films and is characterized into two modes, perpendicular and along the films surface with the effect of annealing temperature on PQT-12 films.

1. INTRODUCTION Self-assembly is the process of autonomous organization into a regular pattern or structure without any external intervention.1 The intervention in self-assembly by designing external factors such as templates, directing field, and interfaces, etc., is called directed/guided self-assembly.2−5 These external factors play a vital role in polymer thin film technology to obtain directly crystalline, smooth, ordered, and aligned polymers thin film.6−8 In addition to this, there are some other factors that determine the crystallinity and alignment of polymer in thin films such as molecular weight, temperature, processing conditions, and film forming techniques.9−11The low molecular weight of polymer chains forms well-packed crystalline structure with localization of charge over crystallites, while high molecular weight forms a relatively extended pathway for charge transport, which causes better charge transport properties.12−16 By tuning the preparation conditions, molecular parameters with selfassembly property, the size of crystalline domains can be spatially extended to several micrometers in length and a few hundred nanometers in polymer thin films width, which is desirable to achieve the electrical and optical properties up to a benchmark. In the past few years, there has been great development in the polymer thin film processing methodologies, and their role has been studied for electronic application.17−19 The thin film © 2014 American Chemical Society

processing methodologies are of great important because they provide the information about the crystallization behavior, and decide the formation of smooth, aligned, and large crystalline films. The charge transport properties of organic materials strongly depend on the microstructure of the active semiconducting layer, which is directly governed by the film forming techniques.20−22 The amorphous phase or highly dense grain boundaries generated during the film formation limit the electrical conductivity. Thus, there is requirement of post higher temperature annealing of films to improve the crystallinity and uniformity, which limit the use of common flexible substrate such as poly(ethyleneterephthalate) for device fabrication. Therefore, annealing is another major challenge for cost-effective large-scale production of organic electronic devices. Because of these difficulties, there is need for a deposition technique that is facile and provides crystalline domains with precise control over the molecular ordering, alignment, and orientation. To resolve all of the concerns associated with the reported techniques, our group has developed a facile and cheap technique to achieve the goals in a single thin film processing methodology called “floating Received: July 22, 2014 Revised: September 18, 2014 Published: September 18, 2014 22943

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film transfer methods (FTM)”.23 This technique is almost similar to the Langmuir−Schaefer (LS) technique except for absence of pressure during transferring the film over solid substrate. The advantage of FTM over the LS technique is the formation of compact solid film over air−liquid interface after evaporation of a volatile solvent of polymer solution. This causes the coalescence of the multi domains and the formation of large crystalline film with well connectivity.23 Recently, there has been great progress in the development of organic polymer-based semiconducting materials like polyaniline, polythiophene, etc.17,22,24 Among these, a solution processable derivative of polythiophene conducting polymer, PQT-12, has attracted extensive scientific interest due to its superior charge transporting characteristics and ability to selforganize into higher structure.25,26 Self-organizing property of PQT-12 provides a special interest because one-dimensional electronic property of π-conjugated polymer chains can be modified by interchain stacking that results from π−π interaction. The alkyl side chains of PQT-12 provide additional ordering due to intermolecular interdigitation and also provide the solubility in common organic solvents. The crystallization induced by π−π stacking, alkyl side-chains interaction, and changes in solubility allows an increase of crystallization length, and efficient charge transfer in PQT-12 film.27−32 Since the report of the first polythiophene transistor in 1986 by Tsumura and co-workers, much work has been focused on obtaining the uniform and ordered film of polythiophene to improve the electrical and optoelectronic properties with complex fabrication methodologies like vapor deposition in vacuum or inert environment, post annealing of spin-coated film, solvent of different boiling points, and annealing between liquid crystalline phases of polymer. Yet, these methodologies neither provide better understanding of the crystallization behavior nor control the assembly, alignment of polymer chains. Further, the film obtained by using such methodologies is isotropic in nature and also may deteriorate the electrical and optical properties. Keeping bottlenecks associated with the methods previously explored, we have introduced FTM as a new technique for the formation of thin film of polymers. It turns out that the FTM technique provides better alignment and crystallinity of polymer chains in comparison to other techniques, which results in the formation of oriented, crystalline, and anisotropic films with better charge transportation. In the present work, we report the FTM technique that is designed to assemble the PQT-12 polymer into aligned, oriented, as well as large crystalline domains in thin film on crystallization from a solution phase, and to possess an extended, planar π electron system that allows close intermolecular π−π distances. We also report the effect of higher temperature annealing (80 and 120 °C) on polymer film property such as alignment, orientation, as well as size of crystalline domains using polarized, unpolarized UV−vis spectroscopy and SAED pattern. The dependency of Raman spectral peak intensity on annealing temperature was also investigated. Further, a sandwiched structure of Al/PQT-12/ ITO and source−drain configuration (Au/PQT-12/Au) were used to investigate the charge transport properties of PQT-12 film as a function of annealing temperature.

polymer has been published in our previous report.23 In this report, we have used the isolated and extracted PQT-12 polymer from CH2Cl2 by using the Soxhlet extraction method for obtaining higher molecular weight because high molecular weight forms a crystalline film with a well connected pathway between the crystalline domains through longer chains.33 To obtain interconnected crystalline domains film, a 10 mg/mL concentrated solution of PQT-12 was made in chloroform (Merck, India) and directly casted over liquid surface with air− liquid interface for formation of thin films in ambient. The films formed over liquid surface were transferred over unmodified hydrophilic solid substrates by just stamping of substrates. The whole procedure of film formation over solid substrate is called FTM, which has been described in detail in our previous paper.23 An ∼15 μL PQT-12 solution in chloroform was dropped over liquid surface (ethylene glycol and glycerol mixture in 1:1 (Merck India)) for formation of a thin film of PQT-12 on air−liquid interface. The solution spreads over the liquid surface with rapid evaporation of chloroform, and a floating compact solid film of PQT-12 polymer remains over the liquid surface with starfish structure. The as-deposited FTM films were well dried below 50 °C for 1 h in ambient and named as “as-deposited”. The as-deposited films were also annealed at 80 and 120 °C for 1 h and named as “80 °C” and “120 °C”, respectively. The thicknesses of the films were measured by AFM tip and found to be 22 ± 1 nm in all cases. Indium tin oxide (ITO), aluminum (Al), and gold (Au) were used as electrodes to study the charge transport property. The charge transport property was studied perpendicular to the film surface using a sandwiched structure of device Al/PQT12/ITO, and along the film surface with polymer backbone in source−drain configuration Au/PQT-12/Au. 2.2. Characterizations. A thermal and liquid crystal (LC) property of PQT-12 was studied by using differential scanning calorimetry (DSC) (Mettler-Toledo model 823) in N 2 environment. Polarized and unpolarized UV−vis spectra were recorded using PerkinElmer Lamda-25 spectrophotometer attached to Glan Thomson polarizer. Three electrode cell assemblies with Ag/AgCl as reference electrode were used for cyclic voltammetry (CV) measurement over ITO as working electrode. The CV measurements were performed in 0.1 M tetrabutyl ammonium perchlorate (TBAP) and acetonitrile solution with 50 mV scanning rate in range from 0.0 to 1.5 V vs Ag/AgCl using an electrochemical workstation (CHI 7041C instruments Inc., U.S.). Transmission electron microscopy (TEM) and SAED pattern were obtained by using Tecnai G2 (U.S.) over Cu grid. Raman spectroscopy was conducted using a Micro Raman spectrometer at 180° scattering geometry (Renishaw, Germany) with 514.5 nm line of Ar+ laser of 50 mW. AFM topographies of PQT-12 films prepared by the FTM technique over SiO2 substrate were investigated under tapping mode of AFM (JEOL SPM5200, Japan). All solid substrates were ultrasonically cleaned using acetone, chloroform, and toluene and dried at 50 °C for 1 h before samples preparation. Charge transport properties of fabricated devices were investigated using a Keithley source meter (model 2612A, U.S.). The Al and Au metal electrodes (sandwich structure Al/ PQT-12/ITO and source−drain structure Au/PQT-12/Au on SiO2 with channel length 20 μm and width 3 mm) were deposited by vacuum evaporation method using the “Hind HIVAC” model No-12A4D (India) vacuum coating system with a metal mask.

2. EXPERIMENTAL SECTION 2.1. Material Synthesis and Sample Preparation. The synthesis procedure of monomer and its corresponding 22944

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Figure 1. DSC thermogram of PQT-12: (a) heating at 10 °C/min for second, third, and fourth runs, respectively; and (b) cooling at 10 °C/min for first, 30 °C/min for second, and 80 °C/min for third run, respectively. All measurements were performed in inert N2 environment.

Figure 2. (a) Normalized UV−vis spectra of (i) as-deposited, (ii) annealed at 80 °C, (iii) at 120 °C FTM film of PQT-12, and (iv) in liquid of concentration 0.25 mg/mL (insets show the presence of 688 nm peak in as-deposited and 80 °C annealed films). (b) CV of as-deposited and 80 °C annealed PQT-12 FTM film (inset shows UV−vis spectra of as-deposited and 80 °C annealed PQT-12 FTM films). (c) Scheme for charge transfer transition from amorphous phase to crystalline/ordered phase of PQT-12 films. (d) Polarized UV−vis spectra of as-deposited, annealed at 80 °C, and 120 °C films.

first run is being discarded because it contains the thermal history of the polymer. Therefore, we have recorded DSC endotherms from the second run of heating to avoid the information due to thermal history and to investigate only the melt crystallization behavior. Figure 1a shows the two peaks of endotherms at 100 and 130 °C (heating rate 10 °C/min),

3. RESULTS AND DISCUSSION DSC study is carried out for thermal properties, LC property, and stability of polymer at high temperature before studying the annealing effect on optical, morphology, and electrical properties of polymer films. Figure 1 shows the DSC endotherms and exotherms of PQT-12 polymer in N2 environment. In DSC, the 22945

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chain) also causes the decrease in intensity of the peak at 575 nm. However, further annealing at 120 °C (complete disordering of alkyl side chains) causes a blue shift of strong absorptions peak with disappearance of shoulder peak in comparison to asdeposited and annealed at 80 °C films. Kohn et al. have already estimated the crystallinity using the ratio of A0−0 (575 nm) to A0−1.35 The ratio A0−0/A0−1 obtained for as-deposited and 80 °C annealed films is 0.826 and 0.708 (Supporting Information Figure S1), respectively, which also reveals the decrease in the crystallinity. Further annealing of the sample at 120 °C (in LC phase) shows a disappearance of A0−0 vibronic peak with a blue shift of the strong absorption peak (A0−1). The blue shift and disappearance of peaks are due to disordering of side chains at 120 °C, which causes the formation of one-dimensional π−π stacked lamellar structure in the films. The decrease in crystallinity due to annealing is similar to our previous report.23 The weak absorption peak at 688 nm in as-deposited and 80 °C samples (inset of Figure 2a) was also investigated by using a cyclic voltammetry measurement, which reveals the two oxidation peaks at 0.77 and 0.97 V vs Ag/AgCl (Figure 2b). The two oxidation peaks show the two electrons ejection (oxidation). The two oxidation peaks in the CV may be due to either two-step oxidation process of this polymer or two difference phases of the polymers, that is, crystalline and amorphous. In this case, probably it is due to two phases of the polymers, crystalline and amorphous, as the two oxidation peaks’ current ratio is different in the two cases, that is, film formed at room temperature and at higher temperature. The peak current ratio is clearly showing the change in the ratio of the crystalline and amorphous phases as it is indicated by UV− vis studies also. In addition, the irreversible oxidation is also an indication that the two oxidation peaks are due to two difference phases of the polymers. Ostrowski et al.10 have also proved the different energy level of electrons, which was directly connected to morphology and electronic coupling between the polymer chains. Further, he has shown the work function difference for different phases of polymer chains. The self-organized polymer chains show higher work function and higher wavelength emission of light, while disordered polymer chains show the low work function and low wavelength light emission. CV is also connected to the work function of electron in materials because it is ejection of the electron from energy level of molecule (HOMO) to out of the surface. Ordered phases are highly stable in comparison to disordered phases and require higher voltage to eject the electron from the energy level. Therefore, 0.97 V corresponds to oxidation of ordered phase and 0.77 V as oxidation of disordered phase. Using the CV results, the estimation of HOMO level of PQT-12 films by half wave oxidation potential (E1/2) shows the 5.41 eV for disordered phase and 5.84 eV for ordered phase, respectively. The lowest unoccupied molecular orbital (LUMO) was also estimated from HOMO and the peak value of UV−vis spectra (Figure 2b). It is observed as 2.94 and 3.57 eV, respectively. The energy diagram scheme for two phases of PQT-12 is shown in Figure 2c, which shows that the difference of energy between the HOMO of the disordered phase and the LUMO ordered phase is almost equal to 688 nm. The 688 nm transition is thus attributed to the charge transfer (688 nm) from the ordered phase to disordered phase as shown in Figure 2c. Figure 2d shows the optical investigation of PQT-12 films under polarized light. In polythiophenes, the absorption and

which correspond to melting/disordering of alkyl side chains, and melting/disordering of facial π−π stacking, respectively. It has been already reported in the literature that backbone stacking remains in an ordered state in the LC phase (retains π−π stacking); only the melting and disordering of side chains occurs.23 The first endotherms show the transition from anisotropic solid to anisotropic liquid (LC) phase, and the second endotherm shows the anisotropic liquid to isotropic liquid (absence of interdigitation of side chains and π−π stacking). We have also examined the stability of PQT-12 polymer by heating the same sample four times at the same heating rate (10 °C/min). It has already been reported that the endotherm peak shifts toward higher temperature with increase in molecular weight of PQT-12 polymer.34 We observed that the nature and endotherm peak position remain unchanged as shown in Figure 1a, which is a clear indication of high stability because degradation of polymer chains should shift the endotherm peak toward lower temperature. Thus, the results show the high stability of PQT-12 polymer chains and are also consistent with a previous report.32 The dependency of crystallization temperature of polymer on cooling rate has been investigated using the exotherm peak of DSC as shown in Figure 1b. The different cooling rates (10, 30, and 80 °C per minute) show the change in position of DSC exotherm peak position, which reveals the dependency of crystallization temperature of polymer on cooling rates. The increasing cooling rates cause the shift of exotherm peak position toward lower temperature due to shift of crystallization of polymers at low temperature. Figure 2a shows the normalized UV−vis spectra of PQT-12 films prepared by FTM technique and in chloroform solution. The as-deposited, 80, and 120 °C UV−vis spectra of films are marked as (i), (ii), and (iii), respectively, and in chloroform solution (0.25 mg/mL) is marked as (iv) in Figure 2a. A strong absorption peak at 473 nm and a weak absorption shoulder peak at 597 nm appear for PQT-12 in solution where the strong peak is attributed to π−π* transition and absorption at 597 nm is attributed to charge transfer transition from PQT-12 lamella to chloroform solvent.27 It is important to note that at higher concentrations (10 mg/mL) and at low temperature (room temperature), PQT-12 solution forms a gel due to the formation of lamellar structures from coplanar PQT-12 molecules through intermolecular interactions, which results in a three-dimensional structure in films. The use of liquid surface in FTM provides a hydrophilic mobile substrate on which the three-dimensional structure can align, orient, and further fuse together during evaporation of solvent and form extended crystalline domains, and results in a red shift of UV− vis spectra as shown by the black line in Figure 2a(i). In general, there is a consistent red shift of the strong absorption peak in the thin films as compared to in solution due to more planar conformations and stronger interchain interactions. The “as-deposited” films display two shoulder peaks (502 and 575 nm result due to side-chain ordering) and one weak absorption peak at 688 nm (transition between crystalline and noncrystalline phases) with strong absorption at 540 nm (π−π* transition). However, the absorption peak at 540 nm slightly shifts toward the lower wavelength (blue) after annealing at 80 °C as depicted by the red line in Figure 2a(ii). The peak positions at 575, 540, and 502 nm are assigned as A0−0, A0−1, and A0−2 transitions, respectively. The annealing of as-deposited films at 80 °C (at start of disordering of side 22946

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peak at shoulder, which also supports the explanation of our UV−vis spectra obtained at different annealing temperatures. Nagamatsu et al. have reported the facial orientation of the backbone of polythiophene (face on) on unmodified hydrophilic surface during film formation.36 The optical anisotropy and SAED pattern confirm the alignment and parallel orientation of polymer backbone to the substrate. The hydrophilic nature of substrate in FTM orients the polythiophene backbone as face on (Figure 4a). The appearance of

emission dipoles components are essentially oriented along the polymer backbone, with a very small perpendicular component that is experimentally insignificant. When illuminated with polarized light, the predominant orientation of the molecular axis with respect to the polarized light axes causes a relative macroscopic dichroism “D” defined according to I − Imin D = max × 100 Imax + Imin (1) Here, Imin and Imax denote the extreme values of absorption intensity when polarized light passes through a films. For the data displayed in Figure 2d, the degree of dichroism is 33% for as-deposited films and 52% for 80 °C annealed films. This suggests that the annealing of PQT-12 films starts as melting/ disordering of alky side chains get further aligned and results in increased optical anisotropy at macroscopic level. The optical anisotropy on the macroscopic scale also confirms an orientation of the PQT-12 molecules parallel to the substrate.29 Figure 3a−d shows the SAED pattern of PQT-12 FTM films ((a) as-deposited, (b) annealed at 80 °C, and (c,d) annealed at

Figure 4. (a) Scheme for interaction of polythiophene chains with hydrophilic surface. (b) Scheme for self-assembled π−π stacked and alkyl side-chain interdigitated PQT-12 crystallites. (c) Scheme for only π−π stacked lamella in high temperature annealed films.

(100) rings in electron diffraction also reveals the same. However, the presence of (100) and (010) planes shows the formation of three-dimensional crystalline structure in FTM (Figure 4b). The consistent red shift of the strong absorption spectra with appearance of vibronic peak at shoulder also reveals the formation of three-dimensional structure in films through interchain interactions, π−π stacking and alkyl sidechains interdigitation. Thus, we have successfully directed/guided the assembly of polythiophene chains over liquid surface through FTM technique and obtained the extended crystalline/ordered domains and face on oriented thin film with alignment of polymer chains in the desired manner. The annealing effect on self-assembled chains has been depicted in a schematic diagram in Figure 4c, which shows disordering/melting of alkyl side chains in lamella of PQT-12 with decrease in dimensionality due to disordering of side chains. Thus, this technique clearly shows the direct formation of three-dimensional crystalline structure of aligned and ordered polymer chains from solution phase and did not require any annealing for ordering of polymer chains or enhancing the ordering of chains. Figure 5 shows the AFM topography (scan area 2 μm × 2 μm) of PQT-12 thin film with average area roughness of 0.8 ± 0.02 nm for as-deposited films, 2.7 ± 0.05 nm for 80 °C, and 0.9 ± 0.03 nm for 120 °C annealed films, which confirms the formations of very smooth film with nano level roughness. The different annealing temperatures cause the change in average area roughness with height profile. The high average area roughness and height profile of film annealed at 80 °C show the agglomerations of polymer chains (Figure 5b). This agglomeration might be the cause of alignment of polymer chains and increase in optical anisotropy as shown in Figure 2d. Further, annealing at 120 °C shows the formation of very smooth films (average area roughness 0.9 ± 0.03 nm), and very slight changes in topography in comparison to as-deposited films. However, UV−vis and SAED pattern have already shown the decrease in crystallinity due to disordering of side chains. Thus, FTM has the potential to direct/guide the assembly of PQT-12 polymer to obtain a face on oriented, aligned, and extended domain of crystallites with very smooth, surface roughness of films below 1 nm. These qualities in films are

Figure 3. Selected area electron diffraction (SAED) pattern of FTM film of PQT-12 transferred over Cu grid: (a) as-deposited, (b) annealed at 80 °C, and (c,d) annealed at 120 °C.

120 °C onto Cu grid). The PQT-12 FTM films were directly transferred onto Cu grids by just stamping. SAED pattern of “as-deposited” PQT-12 films showed very distinct pattern of chains ordering, indicative of lamellar stacking. The estimation of structure of PQT-12 crystallites from electron diffraction pattern reveals that the bright spot that appears in Figure 3a corresponds to the (100) ring, which is equivalent to “d” spacing of 1.8 nm. The obtained values are in excellent agreement with the reported value that reveals that this ordering arises due to the alkyl side chain.27 The SAED pattern also shows a bright spot (Supporting Information Figure S2) for the (010) plane equivalent to a d spacing of 0.37 nm. This ordering arises due to the presence of cofacial π−π stacked polymer chains in crystallites. The appearance of these diffraction spots affirmed the two types of ordering (alkyl side chains and cofacial π−π stacked) with three-dimensional lamellar structure in “as-deposited” films. The appearance of spots at (100) and (010) ring again exhibits the parallel orientation of polymer backbone to the substrates as shown in Figure 2d. The three-dimensional structure of polymer crystallites in films causes the red shift of strong absorption peak with the appearance of vibronic peak at shoulder as shown in Figure 2a. The annealing of films (at 80 and 120 °C) causes successive disappearance of spots or ring equivalents to (100) plane and a d spacing 1.8 nm (Figure 3b,c), while the (010) plane with d spacing of 0.37 nm remains in the diffraction pattern (Figure 3d). This is indicative of loss in the ordering of side chain and three-dimensional structures due to higher temperature annealing. The decrease in dimensionality of crystallites in PQT-12 films causes the blue shift of the strong absorption peak in UV−vis with disappearance of the vibronic 22947

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Figure 5. AFM topography of PQT-12 FTM film: (a) as-deposited, and annealed at (b) 80 °C and (c) 120 °C.

Figure 6. (a) Normalized Raman spectra, and (b) Raman spectra of as-deposited, annealed at 80 °C, and at 120 °C PQT-12 FTM films. Inset shows the change in SNR intensity of peak (1457 cm−1) for different films with thicknesses remaining the same. Error bar shows the 5× standard deviation (5σ) estimation.

Figure 7. (a) Current density (J)−voltage (V) characteristics of PQT-12 perpendicular to film in diode configurations and (b) current (I) versus electric field (E) along the film in source-drain configuration formed by FTM technique as-deposited, and annealed at 80 and 120 °C, respectively.

respectively, appears due to higher temperature annealing (at 120 °C, between LC phase and complete melting/disordering of side chains).37 It is important to note that the polarizability of bonds in organic polymer set the SNR intensity of the peak in Raman spectra, which directly governs the degree of crysatllinity/ ordering and orientations as reported in literature.18,38 UV−vis spectra and SAED study have already revealed the formations of face on oriented and crystalline domains in as-deposited films, which cause reduction in polarizability of bonds in polymer. Therefore, the as-deposited films show the low peak

bottleneck for organic materials and highly desirable to improve the performance of organic electronic devices up to a bench mark. Figure 6a shows the normalized Raman spectra of asdeposited, 80, and 120 °C annealed films. The thickness of films remains the same in all cases. The as-deposited films show a weak SNR signal and display only a peak at 1457 cm−1 (C C symmetrical stretching). The annealing of films at 80 °C causes the appearance of a new spectral peak at 1393 cm−1 (ring C−C stretching). An additional peak such as 1057 cm−1 (C−H bending), 1565 cm−1 (CC asymmetric stretch), 22948

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high current density in as-deposited films is due to the formations of very crysatlline structure. The annealing of films at 80 and 120 °C causes a decrease in crystallinity. The decrease in crystallinity causes localization of charge, which results in reduction in current density as shown in Figure 7a.42 The device parameters listed in Table 1 also show the changes with annealing temperature. The transport property in source−drain configurations (inset of Figure 7b) shows the significant change in current with annealing temperature. The as-deposited films having aligned and ordered chains cause the delocalization of charge carrier over the crystalline domains. The 80 °C annealing (moderate temperature) shows the increase in current in comparison to as-deposited films, while unpolarized UV−vis spectra and SAED patterns show the decrease in crystallinity. The annealing of samples again at 120 °C causes a sharp decrease in current in comparison to as-deposited and 80 °C annealed films. The increase in current of 80 °C annealed films after decrease in crystallinity can be explained by using polarized UV−vis spectra. Figure 2d shows the increase in alignment of polymer chains after annealing at 80 °C. It is important to note that conductivity is maximum along the polymer backbone. The alignment of polymer chains between the electrodes causes the increase in current. The annealing of films at 120 °C shows complete vanishing in polymer alignment (Figure 2d) with decrease in ordering and crystallinity (Figures 2a and 3c). The decrease in ordering and crystallinity causes the localization of charge carrier as a result of decrease in current.27,43

SNR intensity. However, annealing of films at higher temperature (80 and 120 °C) shows the decrease in crystallinity/ordering due to melting/disordering of side chains, which cause the increase in polarizability of bonds in polymer chains. Therefore, the annealed films show the high peak intensity of SNR. The annealing of films also changes the ratio of SNR (1457 cm−1/1393 cm−1) from 14 and 4 for 80 and 120 °C, which may be due to the change in orientation as well as decrease in crystallinity/ordering of polymer chains (Figure 6b) (we cannot extract the correct peak ratio of SNR intensity in asdeposited samples due to a weak signal). The charge transport properties of PQT-12 films were characterized into two modes, first perpendicular to the film surface or along facial π−π stacking direction by sandwiched configuration Al/PQT-12/ITO (structure of device is shown in the inset of Figure 7a), and second along the film surface or along the backbone of polymer chains by Au/PQT-12/Au, source−drain configuration (structure of device shown in inset of Figure 7b). The sandwich structure Al/PQT-12/ITO Schottky diode configurations was characterized by current density (J)−voltage (V) measurement (Figure 7a), while Au/ PQT-12/Au source−drain was examined by current (I)− electric field (E) measurement (Figure 7b). The conducting polymer in Schottky diode (Al/PQT-12/ ITO) follows the thermionic emission, and the diode equation can be given as39,40 ⎡ ⎛ qV ⎞ ⎤ J = J0 ⎢exp⎜ ⎟ − 1⎥ ⎣ ⎝ ηkT ⎠ ⎦

(2)

⎛ qΦ ⎞ J0 = A * T 2 exp⎜ − B ⎟ ⎝ kT ⎠

(3)

4. CONCLUSIONS We have used the facile FTM technique for formation of smooth, crystalline, oriented, and aligned film of PQT-12 polymer. Thin films of PQT-12 are annealed at different temperatures (80 and 120 °C), and the effects of annealing temperatures on film morphology, ordering, electronic, and optical properties are examined and discussed. UV−vis spectra revealed the crystalline, anisotropic, and parallel oriented polymer backbone films of PQT-12. However, annealing caused reduction in crystallinity, except for an increase in optical anisotropy for annealing at 80 °C. SAED pattern showed the formation of crystalline and face on oriented films, while similar annealing effects were observed on films. The effect of annealing on films also appeared in the form of an increase in Raman spectral peak intensity and change in ratio of peaks. Charge transport property in diode configuration (perpendicular to the surface of the film) revealed the decrease in the current with rise in annealing temperature. However, the source−drain configuration (along the surface of the film with polymer backbone) revealed the increase in current for annealing at 80 °C due to enhancement in alignment of polymer backbone. Furthermore, higher temperature caused a decrease in current drastically due to misalignment and disordering. This study showed facile methods to obtain and characterize the face on orientation, alignments, and crystallization of PQT-12 films and effect of temperatures. These qualities in films are the bottleneck of organic materials and highly desirable to improve the performance of organic electronic devices. Further, our study showed the potential to fabricate organic devices with enhanced performance.

where A* is the effective Richardson constant having a value of 120 A/cm2 for free electron, J0 is saturation current density,41 T is temperature in Kelvin, ΦB is barrier height, k is Boltzmann constant, and η is ideality factor. The value of J0 is less at room temperature, so equations become

⎡ ⎛ qV ⎞⎤ J = J0 ⎢exp⎜ ⎟⎥ ⎣ ⎝ ηkT ⎠⎦

(4)

where η is the ideality factor of Schottky diode:

η=

q ⎡ ∂V ⎤ ⎥ ⎢ kT ⎣ ∂ ln(J ) ⎦

(5)

On the basis of the above equation, we have calculated the η, J0, and ΦB, which have been listed in Table 1. Table 1. Electronic Parameter, η, J0, ΦB of Devices devices

η

J0 (A/cm2)

ΦB (eV)

as-deposited 80 °C 120 °C

3.90 ± 0.2 4.28 ± 0.14 4.0 ± 0.18

(6.57 ± 0.09) × 10−7 (1.34 ± 0.06) × 10−8 (1.25 ± 0.06) × 10−7

1.024 ± 0.013 1.036 ± 0.011 0.978 ± 0.010

Table 1 shows the change in electronic parametes like barrier height (ΦB), saturation current density (J0), ideality factor (η), and forward current with annealing of film. Our aim is to invastigate the effect of annealing temperature on electronic properties of PQT-12 films. The annealing of films at the starting of the disordering of side chains and between the LC phase causes again reduction in forward current density. The 22949

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(16) Cha, H.; Park, J. W.; Chung, D. S.; An, T. K.; Kim, Y. H.; Kwon, S. K.; Park, C. E. A Side Chain-Modified Quarterthiophene Derivative for Enhancing the Performance of Organic Solar Cell Devices. J. Mater. Chem. 2012, 22, 15141−15145. (17) Bazylewski, P.; Kim, K. H.; Choi, D. H.; Chang, G. S. SelfOrdering Properties of Functionalized Acenes for Annealing- Free Organic Thin Film Transistors. J. Phys. Chem. B 2013, 117, 10658− 10664. (18) Pandey, R. K.; Upadhyay, C.; Prakash, R. Pressure Dependent Surface Morphology and Raman Studies of Semicrystalline Poly(indole-5-carboxylic acid) by Langmuir−Blodgett Technique. RSC Adv. 2013, 3, 15712−15718. (19) Nakanishi, N.; Tada, K.; Onoda, M.; Nakayama, H. Electronic States at Conducting Polymer/Conducting Oxide Interfaces Observed Using a Low-Energy Photoelectron Spectroscopic Method. Appl. Phys. Lett. 1999, 75, 226−228. (20) Lee, W. H.; Cho, J. H.; Cho, K. Control of Mesoscale and Nanoscale Ordering of Organic Semiconductors at the Gate Dielectric/Semiconductor Interface for Organic Transistors. J. Mater. Chem. 2010, 20, 2549−2561. (21) Cho, C. H.; Kim, H. J.; Kang, H.; Shin, T. J.; Kim, B. J. The Effect of Side-Chain Length on Regioregular Poly [3-(4-n-alkyl) Phenylthiophene]/PCBM and ICBA Polymer Solar Cells. J. Mater. Chem. 2012, 22, 14236−14245. (22) Suryanarayanan, C.; Ravindran, E.; Ananthakrishnan, S. J.; Somanathan, N.; Mandal, A. B. A Molecular Insight on the Supramolecular Assembly of Thiophene Polymers. J. Mater. Chem. 2012, 22, 18975−18982. (23) Pandey, R. K.; Takashima, W.; Nagamatsu, S.; Deaundorffer, A.; Kaneto, K.; Prakash, R. Macroscopic Self Ordering of Solution Processible Poly(3,3″′-dialkylquarterthiophene) by Floating Film Transfer Method. J. Appl. Phys. 2013, 114, 054309−8. (24) Kim, N.-J.; Kwon, J.-H.; Kim, M. Highly Oriented Self-Assembly of Conducting Polymer Chains: Extended-Chain Crystallization during Long-Range Polymerization. J. Phys. Chem. C 2013, 117, 15402−15408. (25) McCulloch, I.; Heeney, M.; Bailey, C.; Genevvcius, K.; Macdonald, I.; Shkunov, M.; Sparrowe, D.; Tierney, S.; Wagner, R.; Zhang, W.; Chabinyc, M. L.; Kline, R. J.; McGehee, M. D.; Toney, M. F. Liquid-Crystalline Semiconducting Polymers with High Charge Carrier Mobility. Nat. Mater. 2006, 5, 328−333. (26) Jimison, L. H.; Salleo, A.; Chabinyc, M. L.; Bernstein, D. P.; Toney, M. F. Correlating the Microstructure of Thin Films of Poly [5, 5-bis (3- dodecyl-2-thienyl)-2, 2-bithiophene] with Charge Transport: Effect of Dielectric Surface Energy and Thermal Anealing. Phys. Rev. B 2008, 78, 125319−18. (27) Ong, B. S.; Wu, Y.; Liu, P.; Gardner, S. Structurally Ordered Polythiophene Nanoparticles for High-Performance Organic ThinFilm Transistors. Adv. Mater. 2005, 17, 1141−1144. (28) Xu, Y.; Liu, J.; Wang, H.; Zheng, L.; Han, Y. Formation of Parallel Aligned Nano-Fibrils of poly(3,3‴-didodecylquaterthiophene) Induced by the Unimer Coils in Solution. RSC Adv. 2013, 3, 12069− 1274. (29) Kuehn, S.; Pingel, P.; Breusing, M.; Fisher, T.; Stumpe, J.; Neher, D.; Elsesser, T. High-Resolution Near-Field Optical Investigation of Crystalline Domains in Oligomeric PQT-12 Thin Films. Adv. Funct. Mater. 2011, 21, 860−868. (30) Ong, B. S.; Wu, Y.; Liu, P.; Gardner, S. High-Performance Semiconducting Polythiophenes for Organic Thin-Film Transistors. J. Am. Chem. Soc. 2004, 126, 3378−3379. (31) Keg, P.; Lohani, A.; Fichou, D.; Lam, Y. M.; Wu, Y.; Ong, B. S.; Mhaisalkar, S. G. Direct Observation of Alkyl Chain Interdigitation in Conjugated Polyquarterthiophene Self-Organized on Graphite Surfaces. Macromol. Rapid Commun. 2008, 29, 1197−1202. (32) Zhao, N.; Botton, G. A.; Zhu, S.; Duft, A.; Ong, B. S.; Wu, Y.; Liu, P. Microscopic Studies on Liquid Crystal Poly(3,3‴-dialkylquarterthiophene) Semiconductor. Macromolecules 2004, 37, 8307−8312. (33) Pandey, R. K.; Singh, A. K.; Upadhyay, C.; Prakash, R. Molecular Self ordering and Charge Transport in Layer by Layer

ASSOCIATED CONTENT

S Supporting Information *

Deconvolution of UV−vis spectra of PQT-12 films, TEM image, SAED patterns, and three-dimensional AFM image. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone/fax: +91-542-2368707. E-mail: rprakash.mst@iitbhu. ac.in. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are thankful to Prof. R. Chandra, IIT Roorkee, Prof. R. Singh, Department of Physics BHU, K. Kaneto, and Prof. W. Takashima, KIT, Japan, for providing the HRTEM, Raman, and AFM facility. We also acknowledge UGC, MHRD, DST, and DST-JSPS for various support.

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REFERENCES

(1) Whitesides, G. M.; Grzybowski, B. Self-Assembly at All Scales. Science 2002, 295, 2418−2421. (2) Kumpfer, J. R.; Rowan, S. J. Directed Self-Assembly of Metallosupramolecular Polymers at the Polymer−Polymer Interface. ACS Macro Lett. 2012, 1, 882−887. (3) Grzelczak, M.; Vermant, J.; Furst, E. M.; Liz-Marzan, L. M. Directed Self − Assembly of Nanoparticles. ACS Nano 2010, 4, 3591− 3605. (4) Ariga, K.; Hill, J. P.; Ji, Q. Layer-by-Layer Assembly as a Versatile Bottom-Up Nanofabrication Technique for Exploratory Research and Realistic Application. Phys. Chem. Chem. Phys. 2007, 9, 2319−2340. (5) Krishnaswamy, R.; Sood, A. K. Growth, Self-Assembly and Dynamics of Nano-Scale Films at Fluid Interfaces. J. Mater. Chem. 2010, 20, 3539−3552. (6) Mattu, J.; Johansson, T.; Holdcroft, S.; Leach, G. W. Highly Ordered Polymer Films of Amphiphilic, Regioregular Polythiophene Derivatives. J. Phys. Chem. B 2006, 110, 15328−15337. (7) Bantz, K. C.; Nelson, H. D.; Haynes, C. L. Plasmon-Enabled Study of Self-Assembled Alkanethiol Ordering on Roughened Ag Substrates. J. Phys. Chem. C 2012, 116, 3585−3593. (8) Cheng, L.; Cao, D. Self-Assembly of Star-Polymer-Attached Nanospheres for Polymer Nanocomposites. J. Phys. Chem. C 2010, 114, 5732−5740. (9) Schuettfort, T.; Watts, B.; Thomsen, L.; Lee, M.; Sirringhaus, H.; McNeill, C. R. Microstructure of Polycrystalline PBTTT Films: Domain Mapping and Structure Formation. ACS Nano 2012, 6, 1849−1864. (10) Ostrowski, D. P.; Lytwak, L. A.; Mejia, M. L.; Stevenson, K. J.; Holliday, B. J.; Bout, D. A. V. The Effects of Aggregation on Electronic and Optical Properties of Oligothiophene Particles. ACS Nano 2012, 6, 5507−5513. (11) Joshi, S.; Pingel, P.; Grigorian, S.; Panzner, T.; Pietsch, U.; Neher, D.; Forster, M.; Scherf, U. Bimodal Temperature Behavior of Structure and Mobility in High Molecular Weight P3HT Thin Films. Macromolecules 2009, 42, 4651−4660. (12) Katz, H. E.; Bao, Z. The Physical Chemistry of Organic FieldEffect Transistors. J. Phys. Chem. B 2000, 104, 671−678. (13) Coropceanu, V.; Cornil, J.; Filho, D. A. D. S.; Olivier, Y.; Silbey, R.; Brédas, J.-L. Charge Transport in Organic Semiconductors. Chem. Rev. 2007, 107, 926−952. (14) Lim, J. A.; Liu, F.; Ferdous, S.; Muthukumar, M.; Briseno, A. L. Polymer Semiconductor Crystals. Mater. Today 2010, 13, 14−24. (15) Chen, H.; Josowicz, M.; Janata, J. Chemical Effect in Organic Electronics. Chem. Mater. 2004, 16, 4728−4735. 22950

dx.doi.org/10.1021/jp507321z | J. Phys. Chem. C 2014, 118, 22943−22951

The Journal of Physical Chemistry C

Article

Deposited Poly (3,3‴-dialkylquarterthiophene) Films Formed by Langmuir-Schaefer Technique. J. Appl. Phys. 2014, 116, 094311−8. (34) Zhang, L.; Colella, N. S.; Liu, F.; Trahan, S.; Baral, J. K.; Winter, H. H.; Mannsfeld, S. C. B.; Alejandro, L.; Briseno, A. L. Synthesis, Electronic Structure, Molecular Packing/Morphology Evolution, and Carrier Mobilities of Pure Oligo-/ Poly(alkylthiophenes). J. Am. Chem. Soc. 2013, 135, 844−854. (35) Kohn, P.; Huettner, S.; Komber, H.; Senkovskyy, V.; Tkachov, R.; Kiriy, A.; Friend, R. H.; Steiner, U.; Huck, W. T. S.; Sommer, J.-U.; Sommer, M. On the Role of Single Regiodefects and Polydispersity in Regioregular Poly(3-hexylthiophene): Defect Distribution, Synthesis of Defect-Free Chains, and a Simple Model for the Determination of Crystallinity. J. Am. Chem. Soc. 2012, 134, 4790−4805. (36) Nagamatsu, S.; Takashima, W.; Kaneto, K.; Yoshida, Y.; Tanigaki, N.; Yase, K.; Omote, K. Backbone Arrangement in “FrictionTransferred” Regioregular Poly(3-alkylthiophene)s. Macromolecules 2003, 36, 5252−5257. (37) Brown, P. J.; Thomas, D. S.; Köler, 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 2003, 67, 064203−16. (38) Cabalo, J.; Guicheteau, J. A.; Christesen, S. Toward Understanding the Influence of Intermolecular Interactions and Molecular Orientation on the Chemical Enhancement of SERS. J. Phys. Chem. A 2013, 117, 9028−9038. (39) Pandey, R. K.; Singh, A. K.; Prakash, R. Enhancement in Performance of Polycarbazole-Graphene Nanocomposite Schottky Diode. AIP Adv. 2013, 3, 122120−9. (40) Singh, A. K.; Dwivedi, A. D. D.; Chakrabarti, P.; Prakash, R. Electronic and Optical Properties of Electrochemically Polymerized Polycarbazole/Aluminum Schottky Diodes. J. Appl. Phys. 2009, 105, 114506−6. (41) Singh, A. K.; Prakash, R. Organic Schottky Diode Based on Conducting Polymer -Nanoclay Composite. RSC Adv. 2012, 2, 5277− 5283. (42) Salleo, A.; Chen, T. W.; Völkel, A. R.; Wu, Y.; Liu, P.; Ong, B. S.; Street, R. A. Intrinsic Hole Mobility and Trapping in a Regioregular Poly(thiophene). Phys. Rev. B 2004, 70, 115311−10. (43) Street, R. A.; Northrup, J. E.; Salleo, A. Transport in Polycrystalline Polymer Thin-Film Transistors. Phys. Rev. B 2005, 71, 1652−13.

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