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Doping of polythiophene by microwave plasma deposition
Surface & Coatings Technology 204 (2010) 3053–3058
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Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Doping of polythiophene by microwave plasma deposition Boonchoat Paosawatyanyong a,b, Phensupa Kamphiranon c,d, Wanna Bannarakkul c, Yongsak Srithana-anant e, Worawan Bhanthumnavin e,⁎ a
Department of Physics, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand THEP Center, Commission on Higher Education, Ministry of Education, Bangkok 10400, Thailand Program of Petrochemistry and Polymer Science, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand d Center for Petroleum, Petrochemicals, and Advanced Materials, Chulalongkorn University, Bangkok 10330, Thailand e Organic Synthesis Research Unit, Department of Chemistry, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand b c
a r t i c l e
i n f o
Available online 3 March 2010 Keywords: Microwave Plasma polymerization Polythiophene In situ doping Electrical conductivity
a b s t r a c t The work represents an efficient method in deposition of electrical conducting polythiophene thin films utilizing a well-assembled microwave plasma system in combination with an in situ doping with iodine. Microwave (2.54 GHz at 150–250 W) plasma polymerization method has been used to fabricate dense and pinhole-free polythiophene films in high uniformity as evidenced by Scanning Electron Microscopic analysis. Moreover, a much improved electrical conductivity of the otherwise insulating polythiophene has been achieved by in situ doping with iodine (I2). Although initial conductivities (1.4× 10− 5 to 1.0 × 10− 4 S/cm) are lower than those of plasma-polymerized films obtained from regular ex situ doping (1.5× 10− 4 to 1.9× 10− 3 S/cm), the latter decreases rapidly and reaches an undoped value in 24 h. The conductivity of the in situ-doped material, on the other hand, decreases at a rate approximately seven-fold more slowly and does not reach the undoped value. Characterization of plasma-polymerized polythiophene films with various spectroscopic methods has been carried out. Incorporation of iodine is evident in Infrared spectroscopy. Ultraviolet–visible spectra of the doped polythiophene are at longer wavelengths (440–535 nm) than the undoped (374–535 nm) suggesting a longer conjugating framework. Energy-dispersive X-ray spectroscopic results are also supportive of iodine incorporation of up to 10%. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Since the structurally simple conjugated polymer, polyacetylene, was first synthesized by Heeger, MacDiarmid, and Shirakawa [1,2] which earned them the Nobel prize in 2000 [3–5], an increased level of interest has been drawn to this class of organic semiconductors. However, because of the instability of polyacetylene in the environment, other types of conjugated molecules were studied to be used as substitutes. A number of conjugated polymer chains consisting of only unsaturated carbon atoms in the backbone or carbon atoms with electron-rich heteroatoms or even totally non-carbon atom backbones have since been synthesized. Many polyheterocyclic aromatic compounds having good thermal stabilities and high electrical conductivities could also be added to the list of organic conducting polymers. Polythiophene (PTh) [6] is one of the polyheterocyclics widely studied and plasma polymerization methods have also been utilized for their synthesis to alleviate problems associated with insolubility of PTh in most organic solvents [7–17] which usually result in difficulties in the processing stage. PTh exhibits high
⁎ Corresponding author. Tel.: + 66 2218 7626; fax: + 66 2218 7598. E-mail address: [email protected] (W. Bhanthumnavin). 0257-8972/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2010.02.066
electrical conductivity upon doping, therefore, potential applications exist in many areas including sensors, organic light emitting diodes, solar cells, applications requiring photonic and conductive properties and polymeric batteries. Doping of plasma-polymerized films by various methods have been reported, however, with the ex situ protocol, conductivity may decrease due to the desorption of the dopant over time [8–10,14,18–20]. The purpose of this study is to synthesize PTh and increase electrical conductivity by simultaneously incorporating dopant species into the films during fabrication process. It was envisaged that this means of doping would result in a higher and more sustainable electrical conductivity of the films. Results on the fabrication of plasmapolymerized polythiophene (PPPTh) as well as in situ iodine-doped plasma-polymerized polythiophene (PPPTh/I2) using a low cost microwave (MW) plasma assembly are reported. 2. Experimental 2.1. Materials and methods Thiophene (Sigma-Aldrich) was used without further purification. A plasma reactor was assembled based on a waveguide-based axial-type microwave plasma source configuration which has been previously
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described [17]. Microwave can be fed into the middle reactor chamber via the air-filled aluminum rectangular waveguide which is designed based on the WR340 standard; with internal cross section dimensions of 86.4 mm by 43.2 mm. Quartz plates were placed at both ends of the reactor chamber to isolate the vacuum. A magnetron head, which was utilized to generate MW at a frequency of 2.45 GHz at different output powers, was coupled to a WR340 waveguide at a distance of 22 mm from the waveguide end cap identical to the distance used in a standard launcher for microwave coupling. In addition, the system also consisted of an in-line 3-stub tuner, and movable sliding short at the waveguide far end. The total length of the waveguide can be varied by moving the sliding short in or out along the reactor axis. These features make it possible to match the impedance of the system to the microwave source at the desirable electromagnetic resonance. During the polymerization process, glass substrates were placed inside the middle reactor chamber at approximately 5 cm from the near end quartz plate. The reactor chamber was evacuated to the base pressure of 0.03 torr. Film fabrication was carried out by using argon as a carrier gas to transport thiophene vapor from an in-line monomer vessel into the reactor chamber through a port located inside just next to the near end quartz plate. A cold trap in conjunction with a rotary pump is used to collect condensation produced from residue monomer through a port at the far end of the reactor chamber. The input power was supplied to the magnetron to generate a microwave which was then transmitted through the waveguide to produce plasma discharge in a violetish pale blue color in the reactor chamber. Plasma phase was analyzed by OES. An HR4000 spectrophotometer including an OOIBase 32 software program was employed. Spectra were recorded from 300 to 1100 nm. The measuring probe was placed outside of the reactor chamber. The plasma discharge could be observed through a mess shield quartz window on the side wall of the reactor chamber. The syntheses of PPPTh were done in 1 and 2 min at a constant pressure of 2.5 torr and an argon flow rate of 10 sccm (mL/min). MW power of 150, 200, 250, and 300 W were utilized. In the in situ iodine doping method, iodine crystals were placed inside the reactor chamber. The experiments were carried out at 250, and 300 W with the reaction time of 1 and 2 min.
3. Results and discussion 3.1. Characterization of plasma-polymerized polythiophene (PPPTh) films Functional groups and chemical characteristics of the PPPTh films were analyzed by ATR FT-IR spectroscopic analysis. Representative spectra obtained from films grown at 150, 200, 250, and 300 W at 1 and 2 min reaction times are presented in Fig. 1. A spectrum of chemically-synthesized PTh by conventional solution polymerization was also included for comparison. The transmittance spectra exhibit the following characteristic peaks: aromatic or olefinic C–H stretching at around 3084 cm− 1; –C–H stretching around 2924 cm− 1; symmetric C = C stretching mode around 1450 cm− 1; C–C stretching of the thiophene rings around 1370 cm− 1; C–H bending around 1046 cm− 1 and C–S stretching around 795 cm− 1. In general, observed peaks of the films are broader than those of chemically-synthesized PTh. This is consistent with amorphous structure of the plasma-polymerized material. Since numerous different fragments, radicals, ions, etc. are generated in the plasma process due to its high energy, a high degree of crosslinking would take place. It is not surprising to observe that in the spectra of PPPTh a C–H stretching of the thiophene ring around 3080 cm− 1 are weakened or almost disappeared. In general, signals for sp2–C–H bond are already rather weak in nature. Amorphous structure of the PPPTh resulting from numerous structural differences would no doubt result in even weaker signals. The previously nonexisting wave number at 1660 cm− 1 representing the C = C stretching of alkene in the spectrum of the PPPTh is somewhat suggestive of fragmentation of thiophene ring. The most distinct feature in PPPTh spectra is the weakened band at 817 cm− 1 assigned to the C–S stretching. This result also suggests a certain degree of thiophene ring fragmentation which is in good agreement with literature report [19]. In spite of some evidence for ring fragmentation, aromatic C = C stretching around 1427 and 1370 cm− 1 confirm existence of cyclic structure. In addition, spectra in the fingerprint region still show relatively well maintained characteristics of the thiophene rings. The morphology of the PPPTh films was observed by SEM analysis to be a globular structure under most conditions used. At 1 min reaction
2.2. Film characterization The films were analyzed by various spectroscopic methods. Scanning electron microscopy (SEM) was performed using the JEOL, JSM-6480LV electron microscope. Data from Energy-dispersive X-ray spectroscopy (EDS) were obtained on an OXFORD, INCAX-sight 7573. Attenuated total reflectance Fourier transform infrared (ATR FT-IR) spectra were obtained on a Continμum infrared microscope attached to the Nicolet 6700 FT-IR spectrometer. UV–visible spectrophotometer model UV2550 SHIMADZU was used to investigate surface absorption of the films with BaSO4 as reference. The electrical conductivity was determined through the resistance measured using a two-point probe system. Authentic samples of chemically-synthesized PTh were also prepared and characterized for comparison.
2.3. Chemical synthesis of poly(thiophene) Thiophene monomer (3 mmol) was dissolved in 5 mL of dichloromethane. The solution was slowly added into a stirred suspension of 4 mmol of anhydrous ferric chloride in 5 mL of dichloromethane. When the addition was complete, the mixture was stirred for an additional 6 h at 0 °C. After that the reaction mixture was allowed to warm to room temperature and stirred overnight for 18h, the precipitate was filtered off and rinsed with methanol until the washed solution was colorless. The remaining ferric chloride in the precipitate was extracted out by Soxhlet extraction with methanol for 24h.
Fig. 1. ATR FT-IR spectra of PPPTh at a different MW power and time compared with chemically-synthesized PTh.
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time, as the microwave power increased, the extended spherical agglomerate afforded smoother surface as shown in Fig. 2(a). Likewise, Fig. 2(b) reveals similar trend for materials obtained at 2 min. Fig. 2(c) shows surface morphology and cross section of PPPTh/I2 the detail of which will be discussed later. SEM analysis also revealed a relatively uniformly distributed deposition of the films. The thickness ranges from 1.04–1.34 μm for plasma polymerization time 1 min and from 1.39–1.90 μm at 2 min. In addition, SEM cross section realizes that PPPTh films are dense and pinhole-free. The thickness values of the films obtained from different experiments using different lengths of time were presented to give the readers an idea of the range of thickness which can be obtained at different conditions. A direct relationship of the polymerization time to the film thickness has not been emphasized. The film growth rate is not linear and this may be the consequence of the following. It is quite well known that the net growth in most CVD processes is likely to be the result from a competition between deposition and etching reactions. Although, the control of these two competitive channels is an interesting subject, however, it is not the main purpose of this study. It is envisaged that longer deposition time would lead to thicker films. However, long deposition time also allows longer exposure to the Ar plasma etching. Therefore, longer processing time could also retard the net growth rate of the PPPTh due to the hydrocarbon ion bombardment. The combination of hydrocarbon ion bombardment and etching effects could result in the etching process being gradually more dominant compared to the film deposition process in longer processing time. Elemental composition of the PPPTh films were investigated by EDS analysis. Results are shown in Table 1. The data for the iodine-doped materials which will be discussed later are also included for comparison. Theoretical carbon to sulfur ratio (C/S) of 4:1 was used as a reference. Most PPPTh films have a C/S ratio higher than 4:1 ranging from
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Table 1 The elemental composition of plasma-polymerized polythiophene. MW (W)
150 200 250 300
C/S in PPPTh films
C/S in PPPTh/I2 films
1 min
2 min
1 min
8.7 7.8 8.6 7.5
7.0 8.7 9.3 10.4
N/A N/A 15.1 15.0
Iodine (%)
C/S in PPPTh/I2 films
Iodine (%)
2 min N/A N/A 8.9 9.2
N/A N/A 13.2 14.7
N/A N/A 9.8 10.1
approximately 7:1 to 10:1. This is suggestive of a decrease in the content of the sulfur element from the main structure of thiophene rings. It is noteworthy that as the microwave power increased, the C/S ratio is higher. This is possible since a more energetic plasma environment may be experienced by the existing film. Moreover, C–C and C = C bonds are stronger (290 and 720 kJ mol− 1, respectively) than the C–S bond (272 kJ mol− 1), therefore, the C–S bond is relatively more easily broken. The effect of the microwave power on the C/S ratio may be explained by an increase in electron density with increasing power input. At higher power input more electrons with adequate energy to break the C–S bond attack the already deposited polythiophene layer which resulted in a higher C/S ratio. Furthermore, the increase in electron density may result in an increase in the number of collisions between electrons and other plasma species [14]. The electrical conductivity of the PPPTh films deposited on glass slides were measured using a two-point probe method. All of the measurements were performed on a Signatone S-1160 general purpose analytical probe station. A Hewlett Packard model 4140B DC voltage source-pA meter unit was used to obtain the DC current-voltage characteristics of the samples between −100 and 100 V. A typical plot for current versus voltage is depicted in Fig. 3. In this study I–V
Fig. 2. Surface morphology of materials grown at various microwave powers (a) PPPTh grown at 1 min; (b) PPPTh grown at 2 min; and (c) PPPTh/I2.
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B. Paosawatyanyong et al. / Surface & Coatings Technology 204 (2010) 3053–3058 Table 3 UV–Vis maximum absorption (λmax) of PPPTh and PPPTh/I2. MW power (W)
λmax (nm) of PPPTh
λmax (nm) of PPPTh/I2
1 min
2 min
1 min
2 min
150 200 250 300
376 374 398 415
389 397 406 418
N/A N/A 478 439
N/A N/A 535 511
3.2. Characterization of iodine-doped plasma-polymerized polythiophene (PPPTh/I2) films
Fig. 3. The plot of current versus voltage at room temperature.
characteristics showed a linear response as can be expected from ohmic devices. Table 2 summarized conductivity of the fabricated films at various powers and reaction times. The conductivity of the microwave PPPTh films is in the range of 5.4 × 10− 7 to 1.9× 10− 6 S/cm. It has been observed that film conductivity appeared to decrease with increasing processing time employed in the polymerization. Similar observation was also noticed in the case where higher MW power was utilized. This may be attributed to a higher chance of thiophene monomer ring fragmentation or decomposition when the already deposited polythiophene film is exposed to the high energy environment caused by the plasma at longer period of time. This may result in partial destruction or disruption of conjugating framework, hence decrease in conductivity. UV–Vis is a preliminary tool to determine conjugating nature of the materials obtained especially for organic-related materials and has been used to investigate optical characteristics of the films. The UV–Vis maximum absorption of the PPPTh films as well as the iodinedoped materials (vide infra) at different MW power and polymerization time is summarized in Table 3. The UV–Vis spectra illustrated that the absorption maxima of PPPTh at 1 min and 2 min reaction time which were assigned to the π–π⁎ transition commonly the observed in conjugated π-system [21] were noticed at around 375–415 nm and 389–418 nm, respectively. It was clearly observed that PPPTh films exhibited the wavelengths in a longer region than that of monomeric thiophene [22]. Moreover, a shift to a longer wavelength with an increasing MW power was also observed. These are both suggestive of a longer π-electron conjugation length in the PPPTh films and high degree of crosslinking which resulted in a red shift with MW power increasing. Furthermore, the broadening of absorption band was observed in a higher degree with an increase in the microwave power used. It is expected that electrical conductivity of films grown under this condition would be higher than those at other conditions since a longer π-framework would allow better electron transports.
Table 2 Electrical conductivity of microwave plasma-polymerized polythiophene. MW power (W) 150 200 250 300
Conductivity of PPPTh (S/cm)
Conductivity of PPPTh/I2 (S/cm)
1 min
1 min
2 min
N/A N/A 1.0 × 10− 4 1.8 × 10− 5
N/A N/A 4.6 × 10− 5 1.4 × 10− 5
2 min −6
1.4 × 10 1.9 × 10− 6 5.8 × 10− 7 1.5 × 10− 6
−7
9.3 × 10 6.9 × 10− 7 6.3 × 10− 7 5.4 × 10− 7
In this study iodine, as one of the most studied doping agents in plasma polymers, was used as a dopant. The high vapor pressure facilitates an introduction of its vapor into vacuum plasma reactors [23]. Identification of reactive species and information on the energetic properties of plasma and on plasma processes was investigated by optical emission spectroscopy (OES). The main advantage of OES comparing to other plasma diagnostic techniques is its non-intrusive character [24]. Electron temperature has been measured to be 1.62 eV. The doping was performed by placing iodine crystals on a glass slide inside the chamber. When polymerization was taking place, OES analysis was performed to ensure the presence of iodine as well as to identify active iodine species involved in the experiments. OES spectra taken at various microwave powers are as shown in Fig. 4 and the position of iodine peaks (804.34, 885.64, 890.09, 902.18, 905.88, 911.30, 933.67 and 973.33) are indicated [25]. In the doping studies, only the microwave power at 250 W and 300 W as well as polymerization times of 1 and 2 min were used. Attempts to utilize microwave power at 150 W and 200 W failed to generate plasma discharge. This was accounted for by the fact that while iodine was introduced in the chamber, the pressure of the system increased. As a result, higher microwave power must be applied in order to generate plasma. Functional groups and chemical characteristics of the PPPTh/I2 films were analyzed by ATR FT-IR, spectra of which are shown in Fig. 5. As mentioned earlier, complex chemical structures of PPPTh are expected. Moreover, many reactions with iodine may occur during the in situ doping process. This may also result in an even more amorphous structure of the films. Consequently, the ATR FT-IR spectra are broad. However, attempts are made here to identify some transmittance which might be suggestive of certain structural features related to incorporation of iodine in the PPPTh/I2 films. For instance, a peak around 960 cm− 1 might be assigned to the C–I vibration. Furthermore, the new band around 1540–1590 cm− 1 may be ascribed to–CH2I or –CI = CH2 groups. Iodine could probably also react with residual radicals in the PPPTh films. In addition, iodine radicals may be able to remove hydrogen atoms from thiophene because the aromatic structure can stabilize the resulting radical. Subsequently, the so-formed HI can react with C≡C as evident from a report by Groenewoud [26]. The radical on the thiophene structure can in its turn react with other iodine radicals to form monosubstituted thiophene, etc. Film morphology of the PPPTh/I2 by MW plasma polymerization was investigated by SEM and the micrographs are shown in Fig. 2(c). These appeared as thinner films compared to those of the PPPTh films. PPPTh/I2 films were found to be in the range of 100–440 nm thickness. It was evident that at higher MW power and longer polymerization time, a more globular particle and smoother surface was observed. EDS verified the presence of iodine in the in situ-doped films. The atomic percentage of iodine in the PPPTh/I2 ranged from 8.9 to 10.1 (Table 1). This is in good agreement with the FT-IR data confirming the incorporation of the iodine dopant in the films. As for the optical characteristics of the PPPTh/I2 films, they exhibit λmax around 439 to 535 nm. The data were also included in
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Fig. 4. OES spectra of plasma illumination during PPPTh/I2 fabrication at various MW powers.
Table 3 together with those of PPPTh for comparison. A red shift to a longer wavelength than those of undoped polythiophene films was evident. This confirms the incorporation of iodine in the films. The shift of UV absorption is strongly suggestive of the formation of the (PPT)+I− charge-transfer complex. Consequently, an enhanced electrical conductivity was also observed. The conductivity of an in situ-doped I2 plasma polythiophene is summarized in Table 2. The measured values (1.4×10−5 to 1.0×10−4 S/ cm) are up to three orders of magnitude higher than the usual range of conductivity of undoped PPPTh (5.4×10−7 to 1.9×10−6 S/cm). This result indicated that the in situ doping can indeed enhance the electrical conductivity of the films. For comparison purposes, reported electrical conductivity values of I2-doped conventional chemically-synthesized,
electrochemically-synthesized polythiophene and other plasma-polymerized polythiophene are tabulated in Table 4. From literature reports, most of the doping processes were carried out after the synthesis of the films by exposing the films to an iodine vapor in a sealed container at various lengths of time [14,19,20]. Only in the case of plasma polymerization of 2-iodothiophene monomer has the in situ doping been carried out. By this means, the dopant iodine was introduced directly with the monomer into the plasma process [9,10]. In general it can been seen that in the case of conventional electrochemical as well as chemically-polymerized films, after being doped, a relatively high initial electrical conductivity was observed. However, over a period of time, conductivities were reported to decrease to more or less a value of an undoped material. In order to study and compare the difference in the lifetime of the doped state of polythiophene obtained from different methods, the following experiments were carried out. Polythiophene films prepared by microwave plasma polymerization at 250 W for 1 min were doped by placing them in a sealed container containing iodine crystals for 24 h (ex situ). Subsequently, the conductivity of these materials was determined by a two-point probe method. The measurements revealed that these films exhibited conductivity in the range of 1.5 × 10− 4 to 1.9 × 10− 3 S/cm which are higher than PPPTh/I2 obtained in this study (1.4 × 10− 5 to 1.0 × 10− 4 S/cm). After the doping, the films were taken out and were let stand in ambient atmosphere. In situ-doped films prepared at 250 W for 1 min were also studied in comparison. Conductivity of both the in situ- and Table 4 Electrical conductivity of I2-doped polythiophene by conventional synthetic and different plasma polymerization methods.
Fig. 5. ATR FT-IR spectra of PPPTh/I2 films at a different MW power and time compared with PPPTh and chemically-synthesized PTh.
Entry Polymerization method
Doping method (doping period)
Conductivity (S/cm)
Ref.
1 2 3 4 5 6 7
I2 chamber,a (unspecified) I2 chamber,a (unspecified) I2 chamber,a 5 h I2 chamber,a 5 h In situ dopingb I2 chamber,a 5 min I2 chamber,a 5 min
1.5 × 101 1.4 × 101 2.2 × 10− 3 4.3 × 10− 4 10− 6 to 2.6 × 10− 1 5.6 × 10− 5 3 × 10− 8 to 5 × 10− 6
[18,19] [8] [19] [19] [9,10] [20] [14]
Electrochemical Chemical AF-plasma RF-plasma MW plasma Pulsed plasma Pulsed plasma
Remark: AF = audio frequency, RF = radio frequency, MW = microwave. a Preformed films were exposed to a saturated vapor of I2 in a sealed container. b In situ doping by using an iodine containing monomer (2-iodothiophene) was performed.
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solvents, of PTh obtained from traditional oxidative or electrochemical polymerization. Doping can be carried out simultaneously resulting in a higher electrical conductivity compared to those obtained from other methods. Films doped under the in situ doping conditions have been shown to retain their electrical conductivity for at least seven-fold longer period of time compared to the ex situ counterparts.
References
Fig. 6. A relationship between standing time and conductivity of the in situ- and the ex situdoped films measured at intervals of time.
ex situ-doped films was measured at intervals of time and a relationship between standing time and conductivity was plotted as depicted in Fig. 6 where triangle and square data points represent ex situ-doped and in situ-doped materials, respectively. It was found that during the post-doping period, conductivity of the ex situ-doped materials rapidly decreased to more or less that of the undoped material in approximately 48h (2880 min). This fast decay is not surprising since iodine could only be adsorbed mostly on the surface of the polythiophene films. This physical interaction is rather weak. As a result, diffusion of the adsorbed iodine can occur over time. Apparently, the in situ-doped films maintained their conductivity in a longer period of time compared to the results obtained from traditional doping methods. Therefore, the in situ doping had presumably evenly distributed iodine dopant all over the entire bulk of the films [27]. It can be proven that in situ doping provides the increased conductivity and high stability for a long time. 4. Conclusions It has been demonstrated that the MW plasma polymerization process can be an efficient method for fabricating conducting polymer films. This overcomes major fabrication limitation, due to the insolubility in most
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Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Doping of polythiophene by microwave plasma deposition Boonchoat Paosawatyanyong a,b, Phensupa Kamphiranon c,d, Wanna Bannarakkul c, Yongsak Srithana-anant e, Worawan Bhanthumnavin e,⁎ a
Department of Physics, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand THEP Center, Commission on Higher Education, Ministry of Education, Bangkok 10400, Thailand Program of Petrochemistry and Polymer Science, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand d Center for Petroleum, Petrochemicals, and Advanced Materials, Chulalongkorn University, Bangkok 10330, Thailand e Organic Synthesis Research Unit, Department of Chemistry, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand b c
a r t i c l e
i n f o
Available online 3 March 2010 Keywords: Microwave Plasma polymerization Polythiophene In situ doping Electrical conductivity
a b s t r a c t The work represents an efficient method in deposition of electrical conducting polythiophene thin films utilizing a well-assembled microwave plasma system in combination with an in situ doping with iodine. Microwave (2.54 GHz at 150–250 W) plasma polymerization method has been used to fabricate dense and pinhole-free polythiophene films in high uniformity as evidenced by Scanning Electron Microscopic analysis. Moreover, a much improved electrical conductivity of the otherwise insulating polythiophene has been achieved by in situ doping with iodine (I2). Although initial conductivities (1.4× 10− 5 to 1.0 × 10− 4 S/cm) are lower than those of plasma-polymerized films obtained from regular ex situ doping (1.5× 10− 4 to 1.9× 10− 3 S/cm), the latter decreases rapidly and reaches an undoped value in 24 h. The conductivity of the in situ-doped material, on the other hand, decreases at a rate approximately seven-fold more slowly and does not reach the undoped value. Characterization of plasma-polymerized polythiophene films with various spectroscopic methods has been carried out. Incorporation of iodine is evident in Infrared spectroscopy. Ultraviolet–visible spectra of the doped polythiophene are at longer wavelengths (440–535 nm) than the undoped (374–535 nm) suggesting a longer conjugating framework. Energy-dispersive X-ray spectroscopic results are also supportive of iodine incorporation of up to 10%. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Since the structurally simple conjugated polymer, polyacetylene, was first synthesized by Heeger, MacDiarmid, and Shirakawa [1,2] which earned them the Nobel prize in 2000 [3–5], an increased level of interest has been drawn to this class of organic semiconductors. However, because of the instability of polyacetylene in the environment, other types of conjugated molecules were studied to be used as substitutes. A number of conjugated polymer chains consisting of only unsaturated carbon atoms in the backbone or carbon atoms with electron-rich heteroatoms or even totally non-carbon atom backbones have since been synthesized. Many polyheterocyclic aromatic compounds having good thermal stabilities and high electrical conductivities could also be added to the list of organic conducting polymers. Polythiophene (PTh) [6] is one of the polyheterocyclics widely studied and plasma polymerization methods have also been utilized for their synthesis to alleviate problems associated with insolubility of PTh in most organic solvents [7–17] which usually result in difficulties in the processing stage. PTh exhibits high
⁎ Corresponding author. Tel.: + 66 2218 7626; fax: + 66 2218 7598. E-mail address: [email protected] (W. Bhanthumnavin). 0257-8972/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2010.02.066
electrical conductivity upon doping, therefore, potential applications exist in many areas including sensors, organic light emitting diodes, solar cells, applications requiring photonic and conductive properties and polymeric batteries. Doping of plasma-polymerized films by various methods have been reported, however, with the ex situ protocol, conductivity may decrease due to the desorption of the dopant over time [8–10,14,18–20]. The purpose of this study is to synthesize PTh and increase electrical conductivity by simultaneously incorporating dopant species into the films during fabrication process. It was envisaged that this means of doping would result in a higher and more sustainable electrical conductivity of the films. Results on the fabrication of plasmapolymerized polythiophene (PPPTh) as well as in situ iodine-doped plasma-polymerized polythiophene (PPPTh/I2) using a low cost microwave (MW) plasma assembly are reported. 2. Experimental 2.1. Materials and methods Thiophene (Sigma-Aldrich) was used without further purification. A plasma reactor was assembled based on a waveguide-based axial-type microwave plasma source configuration which has been previously
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described [17]. Microwave can be fed into the middle reactor chamber via the air-filled aluminum rectangular waveguide which is designed based on the WR340 standard; with internal cross section dimensions of 86.4 mm by 43.2 mm. Quartz plates were placed at both ends of the reactor chamber to isolate the vacuum. A magnetron head, which was utilized to generate MW at a frequency of 2.45 GHz at different output powers, was coupled to a WR340 waveguide at a distance of 22 mm from the waveguide end cap identical to the distance used in a standard launcher for microwave coupling. In addition, the system also consisted of an in-line 3-stub tuner, and movable sliding short at the waveguide far end. The total length of the waveguide can be varied by moving the sliding short in or out along the reactor axis. These features make it possible to match the impedance of the system to the microwave source at the desirable electromagnetic resonance. During the polymerization process, glass substrates were placed inside the middle reactor chamber at approximately 5 cm from the near end quartz plate. The reactor chamber was evacuated to the base pressure of 0.03 torr. Film fabrication was carried out by using argon as a carrier gas to transport thiophene vapor from an in-line monomer vessel into the reactor chamber through a port located inside just next to the near end quartz plate. A cold trap in conjunction with a rotary pump is used to collect condensation produced from residue monomer through a port at the far end of the reactor chamber. The input power was supplied to the magnetron to generate a microwave which was then transmitted through the waveguide to produce plasma discharge in a violetish pale blue color in the reactor chamber. Plasma phase was analyzed by OES. An HR4000 spectrophotometer including an OOIBase 32 software program was employed. Spectra were recorded from 300 to 1100 nm. The measuring probe was placed outside of the reactor chamber. The plasma discharge could be observed through a mess shield quartz window on the side wall of the reactor chamber. The syntheses of PPPTh were done in 1 and 2 min at a constant pressure of 2.5 torr and an argon flow rate of 10 sccm (mL/min). MW power of 150, 200, 250, and 300 W were utilized. In the in situ iodine doping method, iodine crystals were placed inside the reactor chamber. The experiments were carried out at 250, and 300 W with the reaction time of 1 and 2 min.
3. Results and discussion 3.1. Characterization of plasma-polymerized polythiophene (PPPTh) films Functional groups and chemical characteristics of the PPPTh films were analyzed by ATR FT-IR spectroscopic analysis. Representative spectra obtained from films grown at 150, 200, 250, and 300 W at 1 and 2 min reaction times are presented in Fig. 1. A spectrum of chemically-synthesized PTh by conventional solution polymerization was also included for comparison. The transmittance spectra exhibit the following characteristic peaks: aromatic or olefinic C–H stretching at around 3084 cm− 1; –C–H stretching around 2924 cm− 1; symmetric C = C stretching mode around 1450 cm− 1; C–C stretching of the thiophene rings around 1370 cm− 1; C–H bending around 1046 cm− 1 and C–S stretching around 795 cm− 1. In general, observed peaks of the films are broader than those of chemically-synthesized PTh. This is consistent with amorphous structure of the plasma-polymerized material. Since numerous different fragments, radicals, ions, etc. are generated in the plasma process due to its high energy, a high degree of crosslinking would take place. It is not surprising to observe that in the spectra of PPPTh a C–H stretching of the thiophene ring around 3080 cm− 1 are weakened or almost disappeared. In general, signals for sp2–C–H bond are already rather weak in nature. Amorphous structure of the PPPTh resulting from numerous structural differences would no doubt result in even weaker signals. The previously nonexisting wave number at 1660 cm− 1 representing the C = C stretching of alkene in the spectrum of the PPPTh is somewhat suggestive of fragmentation of thiophene ring. The most distinct feature in PPPTh spectra is the weakened band at 817 cm− 1 assigned to the C–S stretching. This result also suggests a certain degree of thiophene ring fragmentation which is in good agreement with literature report [19]. In spite of some evidence for ring fragmentation, aromatic C = C stretching around 1427 and 1370 cm− 1 confirm existence of cyclic structure. In addition, spectra in the fingerprint region still show relatively well maintained characteristics of the thiophene rings. The morphology of the PPPTh films was observed by SEM analysis to be a globular structure under most conditions used. At 1 min reaction
2.2. Film characterization The films were analyzed by various spectroscopic methods. Scanning electron microscopy (SEM) was performed using the JEOL, JSM-6480LV electron microscope. Data from Energy-dispersive X-ray spectroscopy (EDS) were obtained on an OXFORD, INCAX-sight 7573. Attenuated total reflectance Fourier transform infrared (ATR FT-IR) spectra were obtained on a Continμum infrared microscope attached to the Nicolet 6700 FT-IR spectrometer. UV–visible spectrophotometer model UV2550 SHIMADZU was used to investigate surface absorption of the films with BaSO4 as reference. The electrical conductivity was determined through the resistance measured using a two-point probe system. Authentic samples of chemically-synthesized PTh were also prepared and characterized for comparison.
2.3. Chemical synthesis of poly(thiophene) Thiophene monomer (3 mmol) was dissolved in 5 mL of dichloromethane. The solution was slowly added into a stirred suspension of 4 mmol of anhydrous ferric chloride in 5 mL of dichloromethane. When the addition was complete, the mixture was stirred for an additional 6 h at 0 °C. After that the reaction mixture was allowed to warm to room temperature and stirred overnight for 18h, the precipitate was filtered off and rinsed with methanol until the washed solution was colorless. The remaining ferric chloride in the precipitate was extracted out by Soxhlet extraction with methanol for 24h.
Fig. 1. ATR FT-IR spectra of PPPTh at a different MW power and time compared with chemically-synthesized PTh.
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time, as the microwave power increased, the extended spherical agglomerate afforded smoother surface as shown in Fig. 2(a). Likewise, Fig. 2(b) reveals similar trend for materials obtained at 2 min. Fig. 2(c) shows surface morphology and cross section of PPPTh/I2 the detail of which will be discussed later. SEM analysis also revealed a relatively uniformly distributed deposition of the films. The thickness ranges from 1.04–1.34 μm for plasma polymerization time 1 min and from 1.39–1.90 μm at 2 min. In addition, SEM cross section realizes that PPPTh films are dense and pinhole-free. The thickness values of the films obtained from different experiments using different lengths of time were presented to give the readers an idea of the range of thickness which can be obtained at different conditions. A direct relationship of the polymerization time to the film thickness has not been emphasized. The film growth rate is not linear and this may be the consequence of the following. It is quite well known that the net growth in most CVD processes is likely to be the result from a competition between deposition and etching reactions. Although, the control of these two competitive channels is an interesting subject, however, it is not the main purpose of this study. It is envisaged that longer deposition time would lead to thicker films. However, long deposition time also allows longer exposure to the Ar plasma etching. Therefore, longer processing time could also retard the net growth rate of the PPPTh due to the hydrocarbon ion bombardment. The combination of hydrocarbon ion bombardment and etching effects could result in the etching process being gradually more dominant compared to the film deposition process in longer processing time. Elemental composition of the PPPTh films were investigated by EDS analysis. Results are shown in Table 1. The data for the iodine-doped materials which will be discussed later are also included for comparison. Theoretical carbon to sulfur ratio (C/S) of 4:1 was used as a reference. Most PPPTh films have a C/S ratio higher than 4:1 ranging from
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Table 1 The elemental composition of plasma-polymerized polythiophene. MW (W)
150 200 250 300
C/S in PPPTh films
C/S in PPPTh/I2 films
1 min
2 min
1 min
8.7 7.8 8.6 7.5
7.0 8.7 9.3 10.4
N/A N/A 15.1 15.0
Iodine (%)
C/S in PPPTh/I2 films
Iodine (%)
2 min N/A N/A 8.9 9.2
N/A N/A 13.2 14.7
N/A N/A 9.8 10.1
approximately 7:1 to 10:1. This is suggestive of a decrease in the content of the sulfur element from the main structure of thiophene rings. It is noteworthy that as the microwave power increased, the C/S ratio is higher. This is possible since a more energetic plasma environment may be experienced by the existing film. Moreover, C–C and C = C bonds are stronger (290 and 720 kJ mol− 1, respectively) than the C–S bond (272 kJ mol− 1), therefore, the C–S bond is relatively more easily broken. The effect of the microwave power on the C/S ratio may be explained by an increase in electron density with increasing power input. At higher power input more electrons with adequate energy to break the C–S bond attack the already deposited polythiophene layer which resulted in a higher C/S ratio. Furthermore, the increase in electron density may result in an increase in the number of collisions between electrons and other plasma species [14]. The electrical conductivity of the PPPTh films deposited on glass slides were measured using a two-point probe method. All of the measurements were performed on a Signatone S-1160 general purpose analytical probe station. A Hewlett Packard model 4140B DC voltage source-pA meter unit was used to obtain the DC current-voltage characteristics of the samples between −100 and 100 V. A typical plot for current versus voltage is depicted in Fig. 3. In this study I–V
Fig. 2. Surface morphology of materials grown at various microwave powers (a) PPPTh grown at 1 min; (b) PPPTh grown at 2 min; and (c) PPPTh/I2.
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λmax (nm) of PPPTh
λmax (nm) of PPPTh/I2
1 min
2 min
1 min
2 min
150 200 250 300
376 374 398 415
389 397 406 418
N/A N/A 478 439
N/A N/A 535 511
3.2. Characterization of iodine-doped plasma-polymerized polythiophene (PPPTh/I2) films
Fig. 3. The plot of current versus voltage at room temperature.
characteristics showed a linear response as can be expected from ohmic devices. Table 2 summarized conductivity of the fabricated films at various powers and reaction times. The conductivity of the microwave PPPTh films is in the range of 5.4 × 10− 7 to 1.9× 10− 6 S/cm. It has been observed that film conductivity appeared to decrease with increasing processing time employed in the polymerization. Similar observation was also noticed in the case where higher MW power was utilized. This may be attributed to a higher chance of thiophene monomer ring fragmentation or decomposition when the already deposited polythiophene film is exposed to the high energy environment caused by the plasma at longer period of time. This may result in partial destruction or disruption of conjugating framework, hence decrease in conductivity. UV–Vis is a preliminary tool to determine conjugating nature of the materials obtained especially for organic-related materials and has been used to investigate optical characteristics of the films. The UV–Vis maximum absorption of the PPPTh films as well as the iodinedoped materials (vide infra) at different MW power and polymerization time is summarized in Table 3. The UV–Vis spectra illustrated that the absorption maxima of PPPTh at 1 min and 2 min reaction time which were assigned to the π–π⁎ transition commonly the observed in conjugated π-system [21] were noticed at around 375–415 nm and 389–418 nm, respectively. It was clearly observed that PPPTh films exhibited the wavelengths in a longer region than that of monomeric thiophene [22]. Moreover, a shift to a longer wavelength with an increasing MW power was also observed. These are both suggestive of a longer π-electron conjugation length in the PPPTh films and high degree of crosslinking which resulted in a red shift with MW power increasing. Furthermore, the broadening of absorption band was observed in a higher degree with an increase in the microwave power used. It is expected that electrical conductivity of films grown under this condition would be higher than those at other conditions since a longer π-framework would allow better electron transports.
Table 2 Electrical conductivity of microwave plasma-polymerized polythiophene. MW power (W) 150 200 250 300
Conductivity of PPPTh (S/cm)
Conductivity of PPPTh/I2 (S/cm)
1 min
1 min
2 min
N/A N/A 1.0 × 10− 4 1.8 × 10− 5
N/A N/A 4.6 × 10− 5 1.4 × 10− 5
2 min −6
1.4 × 10 1.9 × 10− 6 5.8 × 10− 7 1.5 × 10− 6
−7
9.3 × 10 6.9 × 10− 7 6.3 × 10− 7 5.4 × 10− 7
In this study iodine, as one of the most studied doping agents in plasma polymers, was used as a dopant. The high vapor pressure facilitates an introduction of its vapor into vacuum plasma reactors [23]. Identification of reactive species and information on the energetic properties of plasma and on plasma processes was investigated by optical emission spectroscopy (OES). The main advantage of OES comparing to other plasma diagnostic techniques is its non-intrusive character [24]. Electron temperature has been measured to be 1.62 eV. The doping was performed by placing iodine crystals on a glass slide inside the chamber. When polymerization was taking place, OES analysis was performed to ensure the presence of iodine as well as to identify active iodine species involved in the experiments. OES spectra taken at various microwave powers are as shown in Fig. 4 and the position of iodine peaks (804.34, 885.64, 890.09, 902.18, 905.88, 911.30, 933.67 and 973.33) are indicated [25]. In the doping studies, only the microwave power at 250 W and 300 W as well as polymerization times of 1 and 2 min were used. Attempts to utilize microwave power at 150 W and 200 W failed to generate plasma discharge. This was accounted for by the fact that while iodine was introduced in the chamber, the pressure of the system increased. As a result, higher microwave power must be applied in order to generate plasma. Functional groups and chemical characteristics of the PPPTh/I2 films were analyzed by ATR FT-IR, spectra of which are shown in Fig. 5. As mentioned earlier, complex chemical structures of PPPTh are expected. Moreover, many reactions with iodine may occur during the in situ doping process. This may also result in an even more amorphous structure of the films. Consequently, the ATR FT-IR spectra are broad. However, attempts are made here to identify some transmittance which might be suggestive of certain structural features related to incorporation of iodine in the PPPTh/I2 films. For instance, a peak around 960 cm− 1 might be assigned to the C–I vibration. Furthermore, the new band around 1540–1590 cm− 1 may be ascribed to–CH2I or –CI = CH2 groups. Iodine could probably also react with residual radicals in the PPPTh films. In addition, iodine radicals may be able to remove hydrogen atoms from thiophene because the aromatic structure can stabilize the resulting radical. Subsequently, the so-formed HI can react with C≡C as evident from a report by Groenewoud [26]. The radical on the thiophene structure can in its turn react with other iodine radicals to form monosubstituted thiophene, etc. Film morphology of the PPPTh/I2 by MW plasma polymerization was investigated by SEM and the micrographs are shown in Fig. 2(c). These appeared as thinner films compared to those of the PPPTh films. PPPTh/I2 films were found to be in the range of 100–440 nm thickness. It was evident that at higher MW power and longer polymerization time, a more globular particle and smoother surface was observed. EDS verified the presence of iodine in the in situ-doped films. The atomic percentage of iodine in the PPPTh/I2 ranged from 8.9 to 10.1 (Table 1). This is in good agreement with the FT-IR data confirming the incorporation of the iodine dopant in the films. As for the optical characteristics of the PPPTh/I2 films, they exhibit λmax around 439 to 535 nm. The data were also included in
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Fig. 4. OES spectra of plasma illumination during PPPTh/I2 fabrication at various MW powers.
Table 3 together with those of PPPTh for comparison. A red shift to a longer wavelength than those of undoped polythiophene films was evident. This confirms the incorporation of iodine in the films. The shift of UV absorption is strongly suggestive of the formation of the (PPT)+I− charge-transfer complex. Consequently, an enhanced electrical conductivity was also observed. The conductivity of an in situ-doped I2 plasma polythiophene is summarized in Table 2. The measured values (1.4×10−5 to 1.0×10−4 S/ cm) are up to three orders of magnitude higher than the usual range of conductivity of undoped PPPTh (5.4×10−7 to 1.9×10−6 S/cm). This result indicated that the in situ doping can indeed enhance the electrical conductivity of the films. For comparison purposes, reported electrical conductivity values of I2-doped conventional chemically-synthesized,
electrochemically-synthesized polythiophene and other plasma-polymerized polythiophene are tabulated in Table 4. From literature reports, most of the doping processes were carried out after the synthesis of the films by exposing the films to an iodine vapor in a sealed container at various lengths of time [14,19,20]. Only in the case of plasma polymerization of 2-iodothiophene monomer has the in situ doping been carried out. By this means, the dopant iodine was introduced directly with the monomer into the plasma process [9,10]. In general it can been seen that in the case of conventional electrochemical as well as chemically-polymerized films, after being doped, a relatively high initial electrical conductivity was observed. However, over a period of time, conductivities were reported to decrease to more or less a value of an undoped material. In order to study and compare the difference in the lifetime of the doped state of polythiophene obtained from different methods, the following experiments were carried out. Polythiophene films prepared by microwave plasma polymerization at 250 W for 1 min were doped by placing them in a sealed container containing iodine crystals for 24 h (ex situ). Subsequently, the conductivity of these materials was determined by a two-point probe method. The measurements revealed that these films exhibited conductivity in the range of 1.5 × 10− 4 to 1.9 × 10− 3 S/cm which are higher than PPPTh/I2 obtained in this study (1.4 × 10− 5 to 1.0 × 10− 4 S/cm). After the doping, the films were taken out and were let stand in ambient atmosphere. In situ-doped films prepared at 250 W for 1 min were also studied in comparison. Conductivity of both the in situ- and Table 4 Electrical conductivity of I2-doped polythiophene by conventional synthetic and different plasma polymerization methods.
Fig. 5. ATR FT-IR spectra of PPPTh/I2 films at a different MW power and time compared with PPPTh and chemically-synthesized PTh.
Entry Polymerization method
Doping method (doping period)
Conductivity (S/cm)
Ref.
1 2 3 4 5 6 7
I2 chamber,a (unspecified) I2 chamber,a (unspecified) I2 chamber,a 5 h I2 chamber,a 5 h In situ dopingb I2 chamber,a 5 min I2 chamber,a 5 min
1.5 × 101 1.4 × 101 2.2 × 10− 3 4.3 × 10− 4 10− 6 to 2.6 × 10− 1 5.6 × 10− 5 3 × 10− 8 to 5 × 10− 6
[18,19] [8] [19] [19] [9,10] [20] [14]
Electrochemical Chemical AF-plasma RF-plasma MW plasma Pulsed plasma Pulsed plasma
Remark: AF = audio frequency, RF = radio frequency, MW = microwave. a Preformed films were exposed to a saturated vapor of I2 in a sealed container. b In situ doping by using an iodine containing monomer (2-iodothiophene) was performed.
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solvents, of PTh obtained from traditional oxidative or electrochemical polymerization. Doping can be carried out simultaneously resulting in a higher electrical conductivity compared to those obtained from other methods. Films doped under the in situ doping conditions have been shown to retain their electrical conductivity for at least seven-fold longer period of time compared to the ex situ counterparts.
References
Fig. 6. A relationship between standing time and conductivity of the in situ- and the ex situdoped films measured at intervals of time.
ex situ-doped films was measured at intervals of time and a relationship between standing time and conductivity was plotted as depicted in Fig. 6 where triangle and square data points represent ex situ-doped and in situ-doped materials, respectively. It was found that during the post-doping period, conductivity of the ex situ-doped materials rapidly decreased to more or less that of the undoped material in approximately 48h (2880 min). This fast decay is not surprising since iodine could only be adsorbed mostly on the surface of the polythiophene films. This physical interaction is rather weak. As a result, diffusion of the adsorbed iodine can occur over time. Apparently, the in situ-doped films maintained their conductivity in a longer period of time compared to the results obtained from traditional doping methods. Therefore, the in situ doping had presumably evenly distributed iodine dopant all over the entire bulk of the films [27]. It can be proven that in situ doping provides the increased conductivity and high stability for a long time. 4. Conclusions It has been demonstrated that the MW plasma polymerization process can be an efficient method for fabricating conducting polymer films. This overcomes major fabrication limitation, due to the insolubility in most
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