AbstractPoly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is among the most widely used polymers that are used as printed transparent electrodes for flexible Organic Electronic (OE) devices, such as Organic Photovoltaics (OPVs). The understanding of their optical properties and the correlation of the optical properties with their electronic properties and metallic-like behavior can lead to the optimization of their functionality as transparent electrodes in multilayer OE device architectures. In this work, we study the optical properties of different PEDOT:PSS formulations by non-destructive Spectroscopic Ellipsometry (SE), from the infrared to the far ultraviolet spectral regions. The optical response of PEDOT:PSS includes an intense optical absorption originated from the conductive part (PEDOT) at lower photon energies, whereas the electronic transition energies of the non-conductive PSS part have been measured at higher photon energies. Based on the different PEDOT:PSS formulations, the optical investigation revealed significant information on the relative contribution of conductive PEDOT and insulating PSS parts of the PEDOT:PSS formulation in the overall optical response, which can strongly impact the final device functionality and its optical transparency. View Full-Text
Keywords: Organic Electronics; Organic Photovoltaics; transparent electrodes; PEDOT:PSS; Spectroscopic Ellipsometry; optical propertiesOrganic Electronics; Organic Photovoltaics; transparent electrodes; PEDOT:PSS; Spectroscopic Ellipsometry; optical properties►▼ Figures
This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. (CC BY 4.0).
Share & Cite This Article
MDPI and ACS Style
Laskarakis, A.; Karagkiozaki, V.; Georgiou, D.; Gravalidis, C.; Logothetidis, S. Insights on the Optical Properties of Poly(3,4-Ethylenedioxythiophene):Poly(styrenesulfonate) Formulations by Optical Metrology. Materials2017, 10, 959.
Electrically conductive polymers are of great interest in the field of bioelectronics as materials that can improve the interface between electronics and biology. Among different conducting polymers, poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is the most promising due to its high conductivity, easy processing and commercial availability [1,2]. PEDOT:PSS is commercialized in the form of an aqueous dispersion which can be processed in the form of thin films by spin-coating and solvent-casting methods. PEDOT:PSS is widely used in the field of organic electronics as transparent conductive oxides (TCO), and as a hole-conducting layer or electrochromic layer in a variety of devices from organic light-emitting diodes (OLEDs) and organic photovoltaic devices (OPVs) [3,4] to electrochromics [5,6]. In recent years, PEDOT:PSS has also been widely applied in bioelectronic devices for applications as electrodes for electrophysiology, a variety of biosensors, organic electrochemical transistors (OECTs) and small bioelectrode coatings [6,7,8,9,10,11,12,13,14,15]. However, in order to connect well the two fields of biology and electronics, PEDOT:PSS presents some limitations, mostly due to its low biofunctionality and biocompatibility.
In most bioelectronic applications, the fine tuning of the interface of conducting polymers and biological molecules or tissues/organisms is a crucial parameter. As one illustrative example, the biotic/abiotic interface for interfacing with live cells can be improved by the incorporation of biological molecules such as nucleotides or proteins for functionality, e.g., for sensing. In this way, the biofunctionalized conductive polymer can enhance their ultimate properties such as biocompatibility and adhesion, and could help to reduce the inflammatory response of a device in living tissue. The two components of PEDOT:PSS are limited, due on the one hand to the lack of functionality of PEDOT and on the other hand to the low biocompatibility of PSS. To overcome these limitations, much effort has been devoted to design innovative poly(dioxythiophene) polymers containing different functional groups for improved biocompatibility [16,17].
In this review, we summarize recent advances in the design of innovative conducting polydioxythiophene (PEDOT poly(3,4-ethylenedioxythiophene) or ProDOT poly(3,4-Propylenedioxythiophene))-type materials for bioelectronics (Figure 1). The broadest used material today is the commercially available poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) aqueous dispersion which has two main components: PEDOT and PSS. First, we focus our efforts on discussing the different synthetic routes to modify the first component PEDOT. Thus, we will review the different functional EDOT and ProDOT monomers and their derivatives that have been reported in the scientific literature. Next, we review the different trials to substitute PSS, the second component of the material. Here, we review the synthesis of PEDOT:biopolymer aqueous dispersions. Finally, we discuss how these innovative functional PEDOT materials are widening the scope of the applications of conducting polymers in bioelectronics.
2. Synthesis of Functional Ethylenedioxythiophene (EDOT-ProDOT) Monomers
There are two main different methods to synthesize functional 3,4-ethylenedioxothiophene monomers. One synthetic route involves the creation of the thiophene and dioxolane derivatives (Figure 2 pathway #1). In the second synthetic route, the starting material is a 3,4-substituted thiophene ring, for example 3,4-dimethoxythiophene or 3,4-dibromothiophene (Figure 2 pathway #2). The macroscopic difference between the two synthetic routes is the number of steps to reach the final product: in the case of pathway #1, more steps are needed, but the price of the starting material for pathway #2 is about 10 to 100 times more expensive than for pathway #1. It is worth mentioning that pathway #1 is the oldest synthetic route and has only been employed for the synthesis of 2-hydroxymethyl-EDOT (EDOT-CH2OH) and carboxy-EDOT (EDOT-COOH) [13,16]. In the case of EDOT-CH2OH following pathway #2, the reaction with glycerol tends to yield a mixture of EDOT and ProDOT that is difficult to separate. In the case of EDOT-COOH, PCC (pyridinium chlorochromate) is used in an oxidative step and the groups in position 2 and 5 on the thiophene ring protect the molecule from further polymerization when pathway #1 is used . For the ProDOT derivatives, pathway #2 is the only route used in the literature, whereby 1,3-diol derivatives are used to create the dioxepane ring.
The most representative functional EDOT monomers are shown in Figure 3. These molecules, which can be synthesized using both pathways previously described, include hydroxymethyl-EDOT, chloromethyl-EDOT, azidomethyl-EDOT and carboxy-EDOT. These functional monomers have been used in order to obtain functional PEDOT polymers including hydroxyl, halide or carboxylic acid functionalities. Furthermore, as will be shown in the following paragraphs, they have been widely used as basic reagents to synthesize a large variety of EDOT monomers with other functionalities.
Hydroxymethyl-EDOT is the most widely studied and used EDOT derivative which makes the molecule commercially available [16,17,31,41,42,43]. As a monomer, it offers the advantage of improving the solubility of EDOT in water, and it can be co-polymerized giving PEDOTs of high electrical conductivity. As a synthetic brick, it opens the way to several functionalities by chemically modifying the hydroxyl group. Two main strategies have been exploited to obtain functional EDOT monomers. One involves the etherification of the hydroxyl group by nucleophilic substitution using an alkylbromide group, and the other uses an esterification with a carboxylic acid derivative, as shown in Figure 4. In the first case, new EDOT molecules including aromatic and aliphatic carboxylic acid groups, alkene, alkyne, alkyl halide and sulfonate groups have been synthesized (Figure 4, left side). In the second case, EDOT monomers including activated alkenes and halides, pyridinium, alkylammonium, protected amines, naphthalene diimide and nitroxide functionalities have been reported, as shown in Figure 4 on the right side.
Another important reagent is chloromethyl-EDOT, first synthesized by Segura et al. [56,57] and further optimized by many other research groups [18,49,50,58,59,60,61,62]. Using this molecule, functionalities can be incorporated by using the chlorine atom as a leaving group to form ether, amine or ester linkages. Using this monomer, new EDOT derivatives including imidazolium, thiol, azide, primary amine, uracil and methacrylate functionalities have been synthesized (Figure 5). Similar chemistry can be applied to the bromomethyl EDOT derivative. Among the EDOT molecules produced by this route, azidomethyl-EDOT is a powerful scaffold used in the Cu-catalysed cycloadditions of click chemistry in combination with terminal alkynes (Figure 6). Using azidomethyl-EDOT, further molecules having a triazole spacer have been synthesized containing different functionalities such as alkyl, aromatic, fluorinated, hydroxyl, sulfonate, ferrocenium, viologen, tetrathiafulvalene, naphthalene diimide, fullerene or saccharide moieties.
The closest brother of the EDOT molecule is ProDOT which has a propylenedioxy ring instead of a ethylenedioxy ring attached to the thiophene in the 3,4-position. ProDOT was originally considered a side product in the synthesis. In fact, ProDOT is both easier to obtain and is more stable than EDOT, and even if ProDOT’s electrical conductivity is less than EDOT, it can still be used in applications such as OPV, OECT, tissue engineering and more, after doping or coupling with EDOT. The synthesis of the ProDOT derivatives in literature has been performed mostly only with pathway #2 as previously described. As shown in Figure 7, a 1,3-diol was reacted with a 3,4-dimethoxythiophene, leading to the substituted dioxepine ring. Many different diols have been used, leading to a wide library of functional ProDOT monomers. Summarizing them, monomers—including aliphatic moieties, perfluorinated aliphatic chains, hydroxyl groups, aromatic groups such as substituted and unsubstituted benzenes or naphthalene—have been inserted, principally to enhance optical properties [67,68,69,70]. ProDOTs functionalized with alkyl bromide [71,72,73,74,75,76,77], cyano , allyl , azide [30,71,72,73,74,75,76,77,80], or di- and mono-carboxylic acid  groups have been obtained as starting materials for further monomer and polymer functionalizations, as detailed in Figure 7.
3. Innovative PEDOT Biopolymer Aqueous Dispersions
Today, it is well known that EDOT/ProDOT monomers can be polymerized using different methods such as electrochemical polymerization, vapor phase polymerization (VPP) or chemical oxidative polymerization. Electrochemical and VPP polymerization methods usually give polymer films with very good properties such as surface quality, high conductivity and very stable redox chemistry. However, for large-scale applications, chemical polymerization is the preferred route due to its easy scale-up. For instance, this is the method used to produce industrially the PEDOT:PSS dispersions that are commercially available. PEDOT:PSS is currently being used in the area of bioelectronics due to its low toxicity with several cell types including endothelial, epithelial, fibroblast, macrophage, and most importantly human neuronal cell lines in vitro [101,102,103,104]. However, regardless of how promising this material has proven to be, the long-term effects such as PEDOT chains degradation and possible release of acidic PSS degradation products remains a potential issue .
A demonstrated strategy to enhance the biocompatibility and reduce the cytotoxicity of conducting polymers like PEDOT is the use of biomolecules as dopants. Thus, the incorporation of biopolymers could be the way to overcome the limitations of PEDOT:PSS dispersions for specific applications. Although PEDOT:PSS has proven to be an appropriate material for cell culture [102,106], the aim is to provide an environment that stimulates and persuades cell growth . The first attempts to synthetize PEDOT doped by biopolymers was pioneered by Inganäs et al. by the electropolymerization of EDOT in the presence of biomolecules (heparin, hyaluronic acid, fibrinogen, gellan gum, carboxymethyl cellulose, xanthan gum, pectin and gellan gum) [107,108,109] showing their potential due to their non-toxicity  and finding applications as electrode interfaces for cell recordings [105,108]. Inspired by these polymer composites formed by PEDOT, different water-based PEDOT:biopolymer dispersions have been more recently synthetized by different groups using chemical polymerization.
PEDOT:biopolymer aqueous dispersions synthesized by chemical polymerization have been reported using DNA , sulfated cellulose , dextran sulfonate , hyaluronic acid, heparin, chondroitin sulfate , pectin  and guar gum . The synthesis of all of them is very similar and it can be exemplified in Figure 8 for the case of PEDOT:hyalunoric acid dispersions. A typical PEDOT:biopolymer dispersion is synthesized by chemical oxidative polymerization of the EDOT monomer using an oxidant in the presence of a biopolymer as a stabilizer and dopant. However, some parameters vary from one synthesis to another, and these include the PEDOT:biopolymer ratio, reaction temperature, concentration, time and oxidant used. In a typical experimental set-up, these biomolecules and EDOT are firstly dissolved in water; once dissolved, the oxidant is added to the solution. This oxidant can be ammoniumpersulfate ((NH4)2S2O8), potassium persulfate (K2S2O8), iron (III) chloride (FeCl3) or iron (III) p-toluenesulfonate ((CH3C6H4SO3)3Fe). A catalyst is often employed such as iron (II) sulfate (Fe2(SO4)3) to accelerate the reaction kinetics. Once the reaction is complete, the dispersions are purified by ion exchange, filtered and/or dialyzed.
The PEDOT:biopolymer dispersions have a macroscopic aspect similar to PEDOT:PSS dispersions. As can be seen in the picture of Figure 8, dark blue dispersions are obtained. The dispersions are formed by PEDOT particles of sizes between 100–500 nm stabilized by the biopolymer. Particle size and morphology can be studied by UV-spectroscopy, light-scattering, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Similarly to PEDOT:PSS, the PEDOT:biopolymer dispersions can be processed in the form of thin films or the solution formulated to be inkjet printed, extrusion printed and spray coated. The electrochemical properties of the PEDOT:biopolymer films, for instance PEDOT:dextran sulfate or PEDOT:DNA present similar features to PEDOT:PSS. Furthermore, the electrical conductivity of drop-casted or spin-coated films presents similar values to pristine PEDOT:PSS without further treatments of between 10−1–10 S·cm−1.
Using this method, different PEDOT:biopolymer dispersions have been prepared as shown in Figure 9 and summarized in Table 1. The advantages of each PEDOT:biopolymer material in comparison to PEDOT:PSS are discussed here. PEDOT:dextran sulfate presents two advantages. Firstly, it does not interfere with cell growth of L-929 cells in media in contrast with decreased cell numbers in culture when PEDOT:PSS is tested . Secondly, PEDOT:dextran sulfate was absorbed into the PC12 cells while PEDOT:PSS was not. In the case of the PEDOT:DNA complex, the main advantages are its higher conductivity with respect to PEDOT:PSS and its non-acidic nature . In the case of PEDOT:sulfate cellulose, it shows a higher conductivity than PEDOT:PSS which has been attributed to a higher proportion of PEDOT chains of quinoid structure than in PEDOT:PSS. In the case of PEDOT:glycosaminoglycans, they provide functional support in neuroregenerative processes and in the case of chondroitin sulfate, additional protection in oxidative milieu .
In particular, PEDOT:biopolymers have been biologically tested by cell proliferation of a fibroblast cell line (L-929 cells) in the cases of dextran, heparin, hyaluronic acid and chondroitin sulfate. The biological study was more extended in the case of the glycosaminoglycans , including cytotoxicity assays, SH-SY5Y differentiation studies and immunocytochemistry, and intracellular calcium measurements. In these studies, there are many biological findings. Hyaluronic acid, heparin and chondroitin sulfate do not interfere with physiological functions. They are more supportive for neuroregenerative processes compared to PEDOT:PSS and provide functional support. Moreover, chondroitin sulfate was found to have a neuroprotective effect against H2O2-induced cell death on SH-SY5Y cells. In the case of PEDOT:dextran sulfate, the studies showed higher L-929 cell line proliferation than PEDOT:PSS.
Although there are thorough biocompatibility studies of these dispersions as mentioned above, little is known about their long-term effects when implanted. The longest cytotoxic test performed so far had a duration of 96 h in the case of PEDOT:dextran sulfate. Although promising because of their satisfactory findings, these dispersions should undergo deeper study regarding their stability and possible long-term effects when in contact with living tissue.
PEDOT:biopolymer dispersions have great potential due to their improved biocompatibility, combined features and bio-based nature. The methods to increase the conductivity of these materials remains an unaddressed issue with respect to the well-known methods used to increase the conductivity of PEDOT:PSS. This relatively low conductivity of PEDOT:biomolecules limits their applicability in some applications such as bioelectrodes . The study of the effect of solvents or secondary dopants should be addressed. For instance, this effect was studied in the case of PEDOT:dextran sulfate, where ethylene glycol was added to the dispersion achieving higher conductivities (286% increase in conductivity reaching 20 S·cm−1