Polythiophene coated poly(methyl methacrylate) sheet as a new candidate for flexible organic electrode applications
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Poly(methylmethacrylate) (PMMA) is an amorphous, flexible and transparent polymer. We benefited these properties of PMMA to replace glass in solar cells. To replace rigid indium-tin oxide coated glass, PMMA sheet was coated by polythiophene. To achieve this goal, PMMA was in situ coated by FeCl3-catalyzed polymerization of thiophene in acetronitrile at room temperature. Different catalyst concentrations and reaction times were checked. The optimum catalyst concentration was found to be 1.8 g. Surface and cross-section morphologies of the coated layer were investigated using optical and scanning electron microscopes. At high catalysts concentration, a prolonged reaction time increased the polythiophene particles’ diameters, whereas at low catalyst concentrations longer reaction timeswere found to be almost of no effect on particles’ sizes. However, it was noted that the reaction time affects the thickness of the coated conductive layer on the substrate. The highest electrical conductivity was observed for a layer coated under optimized reaction conditions (that is, 1.8 g catalyst along with 20 min reaction time) with an electrical conductivity of 0.74 S/m and a conductive layer thickness of 37 micrometers. The prepared fully polymer electrode is supposed to replace ITO or FTO coated glasses in some applications such as solar windows.
KeywordsPolythiophene PMMA Electrically-conductive polymer Surface morphology Organic electrode Solar window Transparent electrode
The flexibility and light-weight are two main characteristics of polymers. Some polymers are semi-crystalline and opaque but some other polymers are amorphous and optically transparent. These properties of polymers altogether lead to very specific features which can be profited in different applications. PMMA is known as a substituent for glass  due to its transparency, good light transmittance and chemical resistance . It is believed that if an amorphous transparent polymer can replace glass in solar cells, the demand for this polymer can be doubled . In spite of favorable mechanical, optical and surface properties of PMMA, it is difficult to cover its films or sheets with a thin layer made from asemi-flexible conductive polymer such as polythiophene. Whenever difficulties of electrical conductive layercoating are overcome, PMMAwould be afavorite material to replace conventional materials such as glass sheets coated by conductive metallic oxides (e.g. indium thin oxide, ITO) which are usually used in electronics and energy harvesting applications. Notwithstanding excellent electrical conductivity and transparency of ITO layer, some inherent drawbacks of ITO are high price and temperature of manufacturing, rareness of indium, brittleness of ITO layer and increase in electrical resistance in large areas. Hence, the future of flexible electronics seeks for any commodity substitution for ITO in many different routes [4, 5, 6]. Moreover, in organic light emitting diodes (OLEDs) the large mismatch (~ 1.2 eV) between work function of ITO (anode) and the typical highest occupied molecular orbital (HOMO) of active polymer/organic molecule prohibits surmounting the barrier of hole transfer from ITO . Meanwhile, some tried to prepare highly flexible and transparent layers at room temperature through some modifications [8, 9]. This thin transparent conductive ITO layers on a given substrate are usually prepared using layer-deposition technique . Transparency and electrical conductivity of ITO films are acceptable, but ITO layer are still brittle on the flexible substrates and show some bending limitations [11, 12].
Carbon nanotubes along with different conductive polymers are used to improve electrical conductivity of polymeric layers [13, 14]. Graphene sheets are also coated on polymer substrates to prepare conductive transparent layers [15, 16]. Nowadays, conductive polymers in different applications have replaced conventional conductive materials such as ITO. Flexible electronic devices are the main target of conductive polymers . Some polymer composite fibers  and textiles  based on inherently non-conductive polymers are also developed which show electrical conductivity. The inherent conductivity of conjugated conductive polymers such as derivatives of polythiophene grants the polymer some special properties such as electroluminescence as well . These polymers emit light in full range of visible spectrum and become very attractive in the field of printed optoelectronics. Conductive polymers were admixed with metallic powders to yield conductive inks for divers printing techniques applicable in flexible electronics. PEDOT:PSS [poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate)] is used in inkjet conductive printing in pure form  or in form of nanocomposite with single-walled carbon nanotubes . The pure film of this conductive polymer is used to inkjet print patterns of flexible solar cells . A review of the printing techniques such as inkjet which could be employed in flexible electronics is available . The state of the art of the technologies of printing polymer solutions  and flexible electronics is developed in roll-to-roll process . This technology leads to a cost effective process to produce light-weight, low-price and highly flexible circuit boards for reliable and heavy duty electronic devices. In addition to the electronic boards, the conductive polymers are also used in rechargeable lithium batteries . Some research teams have worked on conductive polymer coatings on polymeric substrates such as transparent flexible films of poly ethylene terephthalate (PET) [28, 29, 30, 31] and piezoelectric, semi-transparent and flexible PVDF nano-composite films .
As said, poly(methylmethacrylate) (PMMA), a solution-cast low-temperature processed material, is a transparent amorphous polymer with a relatively high glass-transition temperature (Tg = 105 °C) . Due to its low cost, PMMA can be integrated in building components such as windows and floating tiles while it bears a conductive polymer coating such as polythiophene, polyaniline or polypyrrole . A review of scientific literature reveals that PET films and sheets are overwhelmingly used as substrate for conductive polymeric coatings. PET, a semi-crystalline polymer, its transparency is less than that of PMMA. PET thin films’ transparency is good, but its thicker sheets cannot be used in building integrated photovoltaics (BIPVs) . PMMA is an effective polymer in protection of conductive polymers such as poly(3-hexylthiophene) (P3HT)and poly[2-methoxy-5-(39,79-dimethyloctyloxy)-1,4-phenylenevinylene](MDMO-PPV) against oxidation. Therefore, using the couple of PMMA and a conductive polymer layer could be resulted in a replacement for glass-ITO brittle couple .
This paper report facile and low-price technique for preparation of a transparent flexible electrode using fully amorphous, transparent and flexible poly(methylmethacrylate) sheet under proper conductive polymer layer deposition conditions using CH3CN as polymerization medium. The morphology of the surface and cross-section of the coated layers were also investigated using scanning electron and optical microscopes. The electrical properties of the coated layers are also reported.
2.2.1 Layer deposition
Reaction conditions, morphological and optical parameters of polythiophene layers coated on PMMA substrate
Reaction time (min)
Mean particle diameter (nm)
Conductive layer thickness (μm)
113 ± 35
281 ± 81
101 ± 36
94 ± 30
76 ± 35
106 ± 31
The electrical conductivity of coated samples was measured using four-point microprobe device from Sanat Nama Javan Company (FPP-SN-554 model, Iran). Surface morphology and EDX pattern of layers was studied using scanning electron microscopy (VEGA/TESCAN, Czech Republic). The SEM samples were sputtered with gold before the test. The Fourier transformed infrared spectroscopy (FT-IR) spectra of the layers was obtained using Bruker Equinox 55 model in attenuated total reflectance (ATR) mode. The optical microscope images of the coated surfaces were obtained using an Olympus CX21 microscope at 10× magnification.
3 Results and discussion
3.1 FT-IR spectroscopy
3.2.1 Scanning electron microscopy
The diameter of the distinguishable particles forming the conductive layers Figs. 3b, 4a, b are measured using Image J software and reported in Table 1 (other high-magnification SEM micrographs are not reported for the sake of brevity). Finding a relation between reaction conditions and particle size is complicated. According to the figures and Table 1, both reduced catalyst concentration and shortened reaction time result in particle size reduction. A reverse effect on polymer particle size in FeCl3/H2O2 (catalyst/oxidant) found with H2O2 concentration . It resulted from difference in initiator system for non-coated polymer particles. It seems that at different radical concentrations, reaction time differently affects particles morphology. For example, at the same reaction time (say 12 min) reducing catalyst concentration leads to a reduction in number of active centers and a lower total polymer formation rate. This reduces size of particles in the conductive coating layer. At the same catalyst concentration, a shorter reaction time leads to coagulation of polymer chains and favors formation of larger particles.
The SEM micrograph of the cross-section of other samples is not reported here for the sake of brevity but the measured thicknesses are reported in Table 1. As seen in Table 1 the thickness of the coated conductive layers ranges between 30 and 70 microns. As a general conclusion both catalyst (FeCl3) concentration and reaction time affect the thickness of the coated conductive layer.
3.2.2 Optical microscopy
3.2.3 Electrical properties
In case of the second two samples the catalyst concentration is lower than that of the first two samples. Here a prolonged reaction time leads to 30% reduction in conductive layer thickness. This could be a result of preferential polymer precipitation during longer reaction time in the presence of lower catalyst concentration. A lower catalyst concentration does not favor polymer deposition. Consequently, a thinner polymer layer is deposited on the PMMA substrate. However, based on the argument made for the first couple of the samples due to shorter path formed between the electrodes of the measuring device, Th1.8-20 sample represents the highest electrical conductivity. A stepwise reduction of catalyst concentration in case of the last two samples at constant reaction time (12 min) leads to a constant conductivity of the coated layer followed by conductivity reduction. After polymerization the catalyst also acts as doping agent for the conductive polymer. Therefore, it is normal to expect that at low catalyst concentration the conductivity of the deposited layer decreases. As it is the case for the two last samples.
A transparent conductive polymer layer on a transparent polymer substrate was prepared through polymerization deposition technique using acetonitrile as reaction system solvent. Formation of polythiophene conductive polymer was approved using FT-IR technique. The SEM images proved the formation of layer and its morphology. Depending on catalyst concentration and reaction time variations result in the particle size of the deposited polymer changes. Particle size increases at high catalyst concentration whereas the particle size almost remains constant at lower concentrations. Thinner layers show lower electrical resistance (higher electrical conductivity) due to shorter electrical path between measuring electrodes. The 4-probe device is recommended for thin-polymer layers. The thickness of the coated conductive layer of different samples varies between 30 and 70 micrometers. The optimum catalyst concentration was found to be around 1.8 g for a reaction time of 20 min.
This study was funded by Iranian National Science Foundation (Grant Number 93008296).
Compliance with ethical standards
Conflict of interest
Author declares that there is no conflict of interests.
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