Vapor phase oxidative synthesis of conjugated polymers and applications

Authors

  • Dhiman Bhattacharyya,

    1. Department of Chemical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139
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  • Rachel M. Howden,

    1. Department of Chemical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139
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  • David C. Borrelli,

    1. Department of Chemical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139
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  • Karen K. Gleason

    Corresponding author
    1. Department of Chemical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139
    • Department of Chemical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139
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Abstract

Since their discovery, electrically conductive polymers have gained immense interest both in the fields of basic and applied research. Despite their vast potential in the fabrication of efficient, flexible, and low-cost electronic and optoelectronic devices, they are often difficult to process by wet-chemical methods due to their very low to poor solubility in organic solvents. The use of vapor-based synthetic routes, in which conductive polymers can be synthesized and deposited as a thin film directly on a substrate from the vapor phase, provides many unique advantages. This article discusses oxidative vapor deposition processes, primarily vapor phase polymerization and oxidative chemical vapor deposition, of conjugated polymers and their applications. The mild operating conditions (near room temperature processing) allow conformal and functional coatings of conjugated polymers on delicate substrates. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012

INTRODUCTION

The pioneering discovery of electrical conductivity in doped polyacetylene in the 1970s and in polyaromatic systems such as polypyrrole (PPy), polythiophene, and polyaniline in the early 1980s has introduced a new dimension to the field of electronics.1, 2 The introduction of conductive polymers, a new class of materials, lead to basic research on the chemical synthesis of these polymers and applied research on the utilization of these polymers in various devices, such as organic photovoltaics (OPVs),3 light-emitting diodes,4 organic field-effect transistors (OFETs),5 and sensors.6 Various successful solvent-based technologies, such as electrochemical polymerization and spin coating, have been developed to fabricate thin conductive polymer films for device applications. However, the toxicity, corrosive nature, and/or costs of the required solvents and chemicals are often concerns. For example, unsubstituted polythiophenes are intractable and soluble only in solutions like mixtures of arsenic trifluoride and arsenic pentafluoride,7 whereas substituted poly(3-hexylthiophene) dissolves in tetrahydrofuran.8 There is also the potential for solvent to swell or degrade the substrate and for residual solvent to remain in the films after drying. Major research efforts in this area are aimed at structurally modifying monomer molecules or synthesizing new monomer precursors to make the resulting polymers soluble in commonly used solvents. The fabrication processes that involve solvents are often restricted to limited substrates because of solvent-substrate incompatibility. However, these difficulties may be overcome by depositing polymer thin films directly from the vapor phase, as solubility of the conjugated monomer is no longer required.

Chemical vapor deposition (CVD) of doped conjugated polymers was first reported in 1986 when pyrrole monomer vapor was exposed to iron chloride vapor under vacuum conditions and obtained similar chemical structures and physical properties of the polymer as solution processed PPy.9 One of the major advantages of this process is elimination of solvents associated with solution processing. Various techniques are currently available for the deposition of conjugated polymers from vapor phase, including thermal evaporation, pulsed laser deposition, vapor phase polymerization (VPP), vapor deposition polymerization (VDP), pyrolysis, molecular layer deposition, oxidative chemical vapor deposition (oCVD), plasma-enhanced chemical vapor deposition, and various other CVD techniques.10, 13 Herein, this review will focus only on the step growth oxidative synthesis processes, namely VPP and oCVD.

For both VPP and oCVD, one or more monomer precursors are delivered through the vapor phase. VPP is a two-step process. The first step is the preapplication of the oxidants to the substrate, which is typically carried out using wet chemistry. The second step is the exposure of the pretreated substrate to monomer vapors inside of a vacuum chamber. The oCVD method is a single step process, in which the oxidant and monomer are both delivered through the vapor phase. Thus, oCVD film growth can proceed from the interface up and is compatible with volatile oxidants, such as bromine. As will be discussed in more detail later in this article, the method and timing of monomer and oxidant delivery has multiple practical impacts, including the compatibility of a process with a given substrate, the degree to which the conjugated polymer film formed conforms to micro- and nano-scale features on the substrate, and whether or not covalent bonds are formed between the film and the substrate (e.g., grafting).

Table 1 lists the molecular structure, molecular weight, and the vapor pressure at 25 °C for the most commonly used monomers for vapor-based synthesis of conjugated polymers by VPP and oCVD. As the monomers are delivered in the vapor phase, high volatility and relatively high vapor pressure of the monomers are one of the important properties for successful vapor phase synthesis of the corresponding conjugated polymers. It should be noted that VPP has been extended to solid, nonvolatile monomers using a flow of solvent vapors as a carrier gas.14 Additionally, in CVD methods for other types of films, such as metal organic CVD, low volatility monomers that are soluble can be used. The resulting solutions are delivered to the vacuum chamber using bubblers or atomizers.15, 16

Table 1. Molecular Structures and Vapor Pressures of Conjugated Monomers
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Vapor-based polymerization closely follows the mechanism of oxidative polymerization in solution, in which a desired monomer reacts with an oxidizing species resulting in step growth polymerization. Figure 1 shows this mechanism for the polymerization of poly(3,4-ethylenedioxythiohene (PEDOT) starting with EDOT monomer and using iron(III) chloride as the oxidizing agent. Here, the cation radical formation step is the rate determining step, which is followed by combination of the cation radicals and formation of conjugated bonds by deprotonation. The doping by the counter anion of the oxidant occurs as a simultaneous process in situ.17

Figure 1.

Proposed mechanism for oxidative polymerization of PEDOT. (1) Oxidation of EDOT monomer to form cation radical, (2) combination of cation radicals, (3) formation of conjugation by deprotonation, (4) polymerization continues from n-mer to (n + 1)-mer, and (5) undoped and doped states of PEDOT. (Reproduced from Ref.17, with permission from American Chemical Society.)

In this review, we first discuss the VPP and oCVD processes and the benefits they provide, as summarized in Table 2. We will then discuss relevant processing parameters, film properties, and various applications for vapor deposited conducting polymer films. A majority of the existing body of research investigates these conductive films, especially for use as transparent conductors in optoelectronic devices. Therefore, the review will have a special focus on film properties related to optoelectronic applications and will frequently refer to PEDOT, a widely studied conductive polymer. The use of VPP and oCVD for the deposition of semiconducting polymers will also be discussed.

Table 2. Comparison Between the Available Techniques to Deposit Conjugated Polymer Thin Films
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OVERVIEW OF OXIDATIVE VAPOR PHASE SYNTHESIS PROCESSES

Vapor Phase Polymerization

Vapor deposition processes for the synthesis of conjugated polymer thin films have evolved through many different approaches, particularly the oxidant delivery method for oxidative step-growth polymerization. In this review, the term VPP, also sometimes referred to as VDP, refers to a two-step process in which (i) an oxidizing agent is first deposited onto a substrate and (ii) the oxidant-coated substrate is exposed to monomer vapor inside an enclosed chamber, either at ambient pressure or under controlled low pressure vacuum that results in polymer film synthesis and deposition. The deposited film is then washed with alcohol to remove any unreacted oxidant and adsorbed monomer, and other byproducts. Common steps in the VPP process are shown in Figure 2. Further, post-treatment of the films, such as annealing, may also occur.

Figure 2.

A typical procedure for deposition of conjugated polymer films by VPP. Alternative methods, such as sublimation, have been used to achieve oxidant preapplication (Step I).

To the best of our knowledge, the term VPP was first used to refer to deposition of conductive polymer films in 1986 when Ojio et al. deposited conductive PPy by exposing pyrrole monomer vapor to a poly(vinyl alcohol) (PVA) film containing iron(III) chloride.22 They also reported that the conductivity and transparency of the PVA-PP composites depended significantly on the pyrrole polymerization time, temperature, and FeCl3 concentration. A different approach was taken to deliver the oxidant layer when thermally evaporated or sputter-coated copper thin films were chlorinated to form copper(II) chloride. The freshly prepared copper(II) chloride layer was used as the oxidant for polymerization of pyrrole vapor.23 Submicron patterned films of a conductive polymer on nonconductive substrates by VPP was first achieved by exposing pyrrole vapor to CuCl2 patterns.24 Chlorination of other metallic films, such as gold, palladium, and iron, were also successfully used as oxidants, whereas chlorination of aluminum, tin, lead, nickel, and indium resulted in polymer formation. The authors attributed this difference to the differences in the salts' hygroscopicity.25

Despite successful synthesis of PPy by VPP, the electrical conductivity of the polymers was much lower than commercially available soluble polymers, such as PEDOT:PSS. The earliest use of VPP to produce PEDOT films gave conductivities around 70 S cm−1, using FeCl3 as an oxidant.26 A thin layer of iron(III) chloride was initially prepared by dip or micro-gravure roll coating a solution of iron (III) chloride in methanol onto polymer substrates and exposing EDOT vapor to the dried oxidant-coated substrate at ambient conditions. VPP of highly conductive PEDOT, >1000 S cm−1, was first reported using a similar procedure with a different oxidant solution, iron(III) p-toluenesulfonate mixed with a volatile base pyridine.27 Since Levermore et al. first introduced a low pressure vacuum chamber in the VPP process for exposure of EDOT vapor to oxidant layer, it has become a standard experimental procedure for most of the current VPP experiments.28

Mechanistic studies of the polymer film growth mechanism during the VPP process have been done by tracking the mass and conductivity changes of the growing film.29, 30 Figure 3 shows the quartz crystal microbalance (QCM) and 4-point probe conductivity measurements recorded during polymerization of PPy on iron(III) tosylate. The quartz crystal and 4-point probe were coated with iron(III) tosylate from a 5% solution in butanol and placed inside the pyrrole vapor filled container after drying the probes. Instantly, a large increase in mass was observed in the quartz crystal due to immediate onset of polymerization along with a simultaneous rapid drop in the resistance, monitored by the 4-point probe. After 1250 s, when the probes were removed from the pyrrole filled chamber, the QCM showed a drop in mass due to evaporation of pyrrole; however, the measured resistance remained at a constant level.

Figure 3.

Vapor phase polymerization experiment monitored using QCM (connected circles) and 4-point probe conductivity measurements. (Reproduced from Ref.29, with permission from American Chemical Society.)

To the best of our knowledge, Table 3 lists the combinations of oxidants and monomers that have been used to date in VPP, citing representative references for each.

Table 3. A List of Combinations of Oxidants, Solvents, and Monomers Used in VPP
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Extensive research has been reported in the last decade, especially in recent years, on the effects of various parameters and external additives on the growth mechanism and properties of conductive polymers grown via VPP. These parameters and the various applications of films grown using VPP will be discussed later.

Oxidative Chemical Vapor Deposition

oCVD differs from VPP particularly in terms of the oxidant delivery method. In oCVD, both the monomer and oxidant are delivered to the substrate surface in the vapor phase in a single step. A standard oCVD process involves placement of the substrate inside a vacuum chamber onto an inverted, temperature-controlled stage (Fig. 4). Iron(III) chloride, a solid state oxidant at room temperature, is placed in a crucible directly inside the vacuum chamber oriented upward at the inverted stage and heated to a high temperature to sublime it.17 The monomer, frequently in liquid form, is placed in a heated vacuum jar and fed into the system through a heated line at a controlled flow rate. The operating pressure during deposition is often controlled using a butterfly valve.

Figure 4.

Schematic of an oCVD process chamber. (Reproduced from Ref.17, with permission from American Chemical Society.)

Lock et al. first reported deposition of a PEDOT thin film by oCVD, in which they resistively heated iron(III) chloride in a crucible within a vacuum reactor and reacted the sublimed iron(III) chloride vapor with EDOT monomer vapor.55 This solventless deposition technique yielded PEDOT films with conductivities as high as 105 S cm−1 and 84% transparency. Later, surface roughness of the deposited PEDOT was significantly improved by modifying the oCVD reactor configuration. The substrate was placed inverted above the oxidant crucible, which helped to avoid the accumulation of iron(III) chloride particulates that might fall on the substrate.17

For techniques that require the use of solvents during polymer deposition, the substrate properties are often an important consideration. However, because both the monomer and oxidant are delivered in the vapor phase during oCVD, this technique is substrate independent. Figure 5 demonstrates this property. The hydrophobic graphene surface makes it difficult to obtain an uniform film of PEDOT:PSS, which is dissolved in water. However, PEDOT is easily deposited uniformly on graphene using oCVD in a single step. This work will be discussed further in the section “Organic Photovoltaics.”

Figure 5.

Optical micrographs of (a) PEDOT:PSS and (b) oCVD PEDOT on graphene (c) diagram and photograph of PEDOT:PSS spun coat on graphene (d) diagram and photograph of oCVD PEDOT on graphene.56 Copyright 2012 American Chemical Society.

Different types of oxidants, both in solid and liquid forms, have been used for oCVD synthesis of conjugated polymers. More recently, bromine, a liquid at room temperature, has been successfully used as an oxidant for synthesis of homopolymers and copolymers of EDOT and 3-thiopheneacetic acid (TAA).57, 58 Because bromine undergoes the oxidative reaction in vapor phase and leaves no unreacted oxidant or byproducts in the as-deposited polymer, the washing and drying step that is often required for polymers deposited with solid oxidants, is avoided. Bromine being a very highly volatile liquid oxidant at room temperature can have continuous and more controlled flow rate compared with solid iron(III) chloride.58 A comprehensive list of oxidants and monomers used in oCVD is provided in Table 4.

Table 4. A Comprehensive List of Oxidants and Monomers Used in oCVD
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Shared Attributes of VPP and oCVD

Copolymerization

In addition to the synthesis of homopolymers of conjugated monomers, attempts have been made to copolymerize two monomers to tune optoelectronic properties and also to incorporate chemically active functional groups. The functional groups can then be used to bind nanoparticles66, 70 or biomolecules.57, 67 Synthesis of copolymers by a vapor mediated process in the presence of oxidant is advantageous in terms of easier control of the monomer feed ratio. Because solubility characteristics of the monomers and polymers can be easily ignored in these solventless processes, almost any sets of monomers with sufficient vapor pressures can be copolymerized.

Pyrrole and N-methylpyrrole were first copolymerized in situ within preformed polyurethane with iron(III) chloride by VPP in an attempt to make conductive elastomeric foams.74 Conductivity of the copolymer loaded polyurethane foams ranged from 10−7 to 10−1 S cm−1. The conductivity mainly depended on the density of the foam, copolymer composition, and copolymer loading. Recently, a successful vapor phase copolymerization of EDOT with 3-hexylthiophene (3HT) on iron(III) chloride coated substrates has been demonstrated by VPP.31 Because the vapor pressures and polymerization rates for the two monomers are different, the PEDOT to P3HT ratios were kinetically controlled by adjusting the feed concentrations of the monomers to the VPP reaction chamber. The absorption spectra and the electrical conductivities of thin P(EDOT-co-3HT) polymer films were successfully tuned, with conductivities in the range of 3.0 × 10−2 to 2.3 × 101 S cm−1.

The oCVD process has been used to synthesize copolymers of pyrrole and TAA,70 EDOT and 3-TE,67 and EDOT and TAA57, 66 using iron(III) chloride and bromine as oxidants. In these cases, copolymerization of TAA or 3-TE with pyrrole or EDOT produced films with much higher conductivity compared with the homopolymers of TAA and 3-TE. Additionally, the mechanical properties of the films improved upon copolymerization; for example, P(Py-co-TAA) is flexible compared with brittle PTAA films. The ratio of the two monomers, pyrrole and TAA, and hence the concentration of [BOND]COOH functional groups in the films was varied by controlling the ratio of feed concentrations of monomers by vapor phase. The conductivity of the as obtained copolymers, P(Py-co-TAA), increased from 4 × 10−4 S cm−1 to 0.306 S cm−1 as the mole fraction of pyrrole in the feed increased from 0.32 to 0.69.70 EDOT and TAA were copolymerized by using both iron(III) chloride66 and bromine57 as oxidants. It was shown that the bromine deposited P(EDOT-co-TAA) had two orders of magnitude higher conductivity than the iron(III) chloride deposited copolymer.57

Nonplanar Substrates

Effective deposition of polymers on nonplanar substrates is critical for many applications, such as when high surface areas are needed. The ability to achieve conformal coatings on microstructured surfaces by solution-based coating techniques, such as spin coating and dip coating, are limited by wetting effects, as illustrated in Figure 6(a). The nonconformal coating from solution-based processes often results from the solvent flowing due to the action of gravitational force as well as solvent surface tension. In contrast, oCVD and VPP processed films show more uniformity in thickness. Figure 6(b–d) shows SEM images of PEDOT films deposited by oCVD using different oxidants, CuCl2, FeCl3, and Br2, respectively. Conformality of PEDOT films on silicon trench wafers was significantly improved when Br2 was used in the polymerization process.

Figure 6.

Cross-sectional SEM images of micro-trench silicon wafer with (a) spin-coated EDOT and FeCl3 mixture solution; (b), (c), and (d) conformal oCVD PEDOT deposited using CuCl2, FeCl3, and Br2, respectively. The high volatility of the Br2 oxidant leads to excellent conformality. (a)–(c) Reproduced from Ref.63, with permission from American Chemical Society. (d) Reproduced from Ref.58, with permission from American Chemical Society.

Conformal coverage was also demonstrated on various substrates with complex features like copy paper and electrospun nanofiber mats. Because oCVD is a vapor-mediated process, oCVD can grow conformal films on various soft, fragile substrates without damaging the substrate structure. Figure 7(a) demonstrates a 10-nm conformal oCVD PEDOT coating on 8 nm diameter and 1-mm-long carbon nanotubes. Figure 7(b) shows a SEM image of high aspect ratio (50:1), 10-μm-long multifunctional and electrically conductive nanobundles that were templated via oCVD of a copolymer of EDOT and 3TE. The presence of [BOND]OH groups provides a platform to fabricate high surface area hybrid nanomatrials.76 Delicate substrates like paper substrates and electrospun mats have been successfully conformally coated with oCVD polymers while maintaining the original substrate features [Fig. 7(c–e)]. Fluorescently labeled protein molecules were covalently attached to the [BOND]OH functional groups of a copolymer of EDOT and 3TE deposited on electrospun fibers as shown in Figure 7(e) for sensor application. Similarly, VPP has also been used for coating delicate substrates, like cellulose fibers as shown in Figure 7(f–h).32

Figure 7.

SEM images of conformal oCVD polymer coatings on nanostructures: (a) oCVD PEDOT coated carbon nanotubes, and (b) templated high aspect ratio (50:1) features functional conducting oCVD copolymer nanoforest; vapor-based conductive polymer coating on fibrous substrates: (c) and (d) before and after oCVD PEDOT coating on paper mat; (e) fluorescently labeled avidin attached to [BOND]OH functional groups of conformal oCVD copolymer coated on electrospun fiber mat; (f) and (g) uncoated and VPP polypyrrole coated viscose fibers, respectively; (h) VPP PEDOT coated electrospun polycarbonate fibers. (a) Reproduced from Ref.64, with permission from American Chemical Society. (c) and (d) Reproduced from Ref.63, with permission from American Chemical Society. (e) Reproduced from Ref.67, with permission from John Wiley and Sons. (f) and (g) Reproduced from Ref.32, with permission from Elsevier. (h) Reproduced from Ref.75, with permission from Copyright NRC Canada [2010].

Patterning and Nanostructures

Electronic device applications of polymer thin films often require patterning the films in certain geometries. Additionally, nanostructured polymers provide significantly higher surface area which in turn may improve device performance and efficiencies. Inkjet printing has been successfully used to pattern an oxidant layer on unmodified flexible PET substrates, and a conductive polyaniline pattern at 80 μm resolution was formed when aniline and HCl vapors were exposed to the patterned oxidant layer as shown in Figure 8(f).52 A variety of methods including conventional photolithography, capillary force lithography, and e-beam lithography were used combining oCVD technique to fabricate patterns, which were then successfully used to fabricate PEDOT films with features down to 60 nm on flexible PET substrates [Fig. 8(c,d)].61

Figure 8.

(a, b) Vapor-patterned 200-nm-thick oCVD PEDOT printed in 15 pt. bold Verdana font on Saran™ wrap and Tissue Paper, respectively; (c) 60 nm line pattern of oCVD PEDOT film grafted on flexible PET substrate; (d) pattern on Si wafers with grafted oCVD PEDOT; (e) oCVD PVs vapor-printed on tissue paper; (f) VPP patterning of polyaniline at 80 μm resolution using inkjet printing of oxidants; (g) a well-defined colloidal pattern of in situ grafted oCVD PEDOT nanostructure; (h) templated VPP PEDOT nanofibers; (i) AFM image of VPP polypyrrole nanostructure on patterned CuCl2. (a), (b), and (e) Reproduced from Ref.68, with permission from John Wiley and Sons. (c) and (d) Reproduced from Ref.61, with permission from John Wiley and Sons. (f) Reproduced from Ref.52, with permission from Elsevier. (g) Reproduced from Ref.65, with permission from Royal Society of Chemistry. (h) Reproduced from Ref.75, with permission pending copyright permission. (i) Reproduced from Ref.25, with permission from American Institute of Physics.

Inkjet printing of oxidant and lithography techniques for pattering may require special pretreatment of substrates to reduce the solvent-substrate wettability issues.77 However, vapor printing by oCVD is a solvent-free and dry process. Many successful vapor printing processes have recently been demonstrated on various substrates [Fig. 8(a,b,e)]. Very recently, vapor printed PEDOT films by oCVD process have been successfully used as electrodes to fabricate inexpensive OPVs on paper substrates.68

In addition to patterning conjugated polymers for device fabrication, device performance can be significantly improved by introducing a high surface area at the interface of its various layers. High surface area interfaces can be constructed by creating micro- or nano-structures of the same polymers. Trujillo et al. created patterned and amine functionalized biocompatible PEDOT nanostructures, using grafting reactions between oCVD PEDOT and amine functionalized polystyrene (PS) colloidal particles as shown in Figure 8(g). The robust interface was prepared by covalently attaching PEDOT to the aromatic surface groups of the PS particles. The functionality of the colloidal particles was directly transferred to the surface of the patterned PEDOT nanobowls.65 VPP has also been used to make high surface area conjugated polymer structures. For example, highly conductive PEDOT nanofiber mats were created by a combination of electrospinning and VPP techniques [Fig. 8(h)]. The fibers displayed diameters around 350 ± 60 nm and conductivity around 60 ± 10 S cm−1.75 Micron and submicron featured structures were fabricated by exposing pyrrole vapor to patterned CuCl2 on nonconductive substrates as shown in Figure 8(i).25

Scalability

The ability of a process to be scaled up for large throughput production is an important consideration for the adoption of any technique for many applications. CVD is a standard industrial process for the manufacture of inorganic thin films used in the fabrication of microelectronic and micro-electromechanical systems. The desirable features of CVD for inorganic systems include growth of materials having low impurity levels, uniformity over even larger wafer sizes, and the ability to systematically tune film properties using CVD process parameters.78 Although the scale-up of VPP and CVD processes for conjugated polymers has not yet been published, the ability to scale to larger CVD reactor sizes for dielectric vinyl polymers has been documented. Batch reactors for coating substrates >1 m in width and semicontinuous roll-to-roll systems have been commercialized for dielectric polymers.79 Dimensional analysis has been used to scale up a CVD process for dielectric materials from a small batch reactor to a 300-mm wide roll-to-roll system in which the substrate moved at speeds between 20 and 60 mm min−1.80 Using similar strategies, the scale-up and transition to roll-to-roll processing of CVD processes for conjugated polymers is anticipated because the flow rates and pressures used are in a similar range. A primary challenge is the uniform spatial delivery of monomers and oxidants as these often have relatively low volatility.

CONDUCTIVE POLYMERS

Optoelectronic Properties

Transparency Versus Conductivity Tradeoff

One of the most important properties for a transparent conductor material (whether it be an electrode or buffer layer within a device) is the tradeoff between transparency and conductivity. The films must thin enough to limit losses from optical absorption yet be thick enough to provide low enough sheet resistance to effectively transport or extract charge. The desired optical and electrical requirements for transparent electrode materials are transmittance (T) >90% and sheet resistance Rsh < 100 Ω sq−1. T and Rsh can be related by the following equation:81

equation image

where Z0 = 377 Ω is the impedance of free space, and σop and σdc are the optical and dc conductivities, respectively. The standard industry value for transparent oxide conductors, such as indium tin oxide (ITO), is σdcop > 35.81

As seen in Figure 9, conducting polymer films such as PEDOT have a less desirable trend between transmittance and film conductivity or sheet resistance than standard metal oxide transparent conductor layers, reaching values of ∼9 for σdcop.68

Figure 9.

Tradeoff between transmittance (λ = 550 nm) and sheet resistance of PEDOT films deposited via oCVD, σdcop, ∼9 compared with industry standard (dashed black line), σdcop = 35. The inset is sheet resistance Rsh versus film thickness. (Reproduced from Ref.68, with permission from John Wiley and Sons.)

Researchers are working to improve this relationship by optimizing film conductivities using a number of different processing techniques, several of which are described in the following sections.

Conductivity

Processing Temperature and Pressure

Two important process parameters for both VPP and oCVD are pressure and temperature. Substrate temperature during the oCVD has been found to have a significant effect on the final conductivity of the deposited polymer. The electrical conductivity of oCVD PEDOT films substantially increases as the substrate temperature increases from 15 °C to 110 °C keeping the other processing parameters identical. The range of conductivity achieved was from a low of 9.1 × 10−4 S cm−1 to a high of 348 S cm−1 as shown in Figure 10(a). Thus, the conductivity of oCVD PEDOT can be systematically controlled over a range of more than five orders of magnitude by simply varying substrate temperature.

Figure 10.

(a) Electrical conductivity of oCVD PEDOT as a function of substrate temperature (Arrhenius plot) and (b) effect of operating reaction pressure for PEDOT film growth rate in VPP. (a) Reproduced from Ref.17, with permission from American Chemical Society. (b) Reproduced from Ref.42, with permission from Elsevier.

The Arrhenius plot of conductivity with the substrate temperature revealed an apparent single activation energy of 28.1 ± 1.1 kcal mol−1. The trend observed in conductivity was in accordance with the optical and chemical properties of the corresponding oCVD PEDOT films. It showed that increased substrate temperature resulted polymers with longer conjugation length and higher doping level which helped obtain higher electrical conductivity.17, 59, 60

A systematic study of the effects of the reaction pressure and monomer temperature for VPP of EDOT was conducted by Fabretto et al.42 The change in mass upon EDOT polymerization, that is, growth rate of the PEDOT film, was recorded by a QCM for a range of operating pressures (1.5–10 kPa). The observed rate of polymerization was found to increase by lowering operating pressure as shown in Figure 10(b). In a parallel experiment, when the temperature of the EDOT containing crucible was decreased from 80 °C to 35 °C, the maximum conductivity was achieved between 45 °C and 55 °C. Higher temperatures resulted in an increased rate of polymerization, whereas too low a temperature produced a slow growth rate resulting in a patchy film.

Additives to Oxidant Layer Forming Solution in VPP

Another important factor in controlling film characteristics is the properties of the pretreatment solution, which may be altered using chemical additives or by changing solution concentration or pH.

In the oxidative synthetic processes, the acidity of the oxidant is an important parameter. For example, the oxidation strength of Fe(III) increases with increasing acidity of the salt. A related study demonstrated that pyrrole polymerization time varied from a few minutes to hours when the acidity of the iron(III) sulfonate salt decreased.29 Too strong or too acidic nature of an oxidizing agent can sometimes over oxidize the growing polymer, which results in disruption in conjugation and reduced electrical conductivity.55, 82

Organic bases, such as pyridine and imidazole, are known to reduce the acidity of the oxidants, lower the rate of polymerization, and increase the electrical conductivity of the deposited polymers.55, 82

In addition to controlling the humidity in the VPP chamber, incorporation of a surfactant such as poly(ethylene glycol)-ran-poly(propylene glycol) (PEG-ran-PPG) to the oxidant solution also inhibits the adverse effects of water absorption and prevents the oxidant from forming crystallite domains during the VPP process resulting in a significant increase in the conductivity from 84 S cm−1 to 528 S cm−1.83, 84 It is believed that PEG-ran-PPG co-ordinates to the Fe3+ ions in the oxidant iron(III) tosylate and reduces the effective reactivity of the oxidant which reduces the rate of polymerization leading to increased conductivity as shown in Figure 11.84 Further studies revealed that the surfactant can potentially replace the weak bases such as pyridine and imidazole in reducing the apparent reactivity of the oxidant, thus eliminating the problems associated with volatility of low molecular weight organic bases.85

Figure 11.

(a) Electrical conductivity and film thickness of VPP PEDOT as a function of PEG-ran-PPG surfactant loading and (b) effect of the presence of PEG-ran-PPG surfactant on rate of EDOT vapor phase polymerization. (Reproduced from Ref.84, with permission from John Wiley and Sons.)

Humidity

One of the many external conditions that can have an impact on the vapor deposited polymer film properties is humidity. Fabretto et al. have reported on the role of water during PEDOT polymerization via VPP.40 Water was introduced into the polymerization chamber to vary relative humidity from 10 to 70% and films were compared according to conductivity. Substrates pretreated with Fe(III) tosylate as an oxidizing agent were unable to grow polymer films without the presence of any water vapor. Above a level of ∼46% relative humidity, crystallization of the oxidant layer was observed which hindered further polymerization. Another study investigating both the effects of humidity and PEG-ran-PPG as an additive found an optimal relative humidity of 35%, yielding the highest conductivity film, as shown in Figure 12.83 A similar dependence on the presence of water vapor during polymerization was seen in depositing PPy.86 Fabretto et al. suggest that the water is acting as an effective proton scavenger allowing polymerization to proceed.36 For oCVD polymerization of PEDOT using FeCl3 as the oxidizing agent, the proposed mechanism of proton scavenging is in the formation of HCl, which is easily evaporated away from the growing polymer chain.17 The different proton scavenging pathways may be a result of the dissimilar oxidant delivery methods in the VPP and oCVD processes. In oCVD, there is a continuous supply of vapor phase oxidant to the reaction site, which could account for the anion's ability to act as a proton scavenger.

Figure 12.

PEDOT film conductivity versus % relative humidity in vacuum chamber during VPP using Fe(III) tosylate as the oxidant. (Reproduced from Ref.83, with permission from John Wiley and Sons.)

Post-Deposition Treatment

Post-deposition treatments are often done for solution processed polymer layers, for example, annealing to remove excess solvent and improve charge mobility. Similarly, a number of pretreatments may be considered for vapor deposited films including annealing, chemical rinsing or dopant exchange, plasma treatment, UV exposure, and possibly many others. Films deposited via VPP or oCVD are often rinsed with methanol or ethanol to remove unreacted monomer or oxidant as well as short oligomers and byproducts remaining after the polymerization.27, 60 Films may be dried before or after this rinsing step either in ambient conditions or in the presence of an inert gas. Annealing at temperatures in excess of 80 °C has shown significant degradation of VPP PEDOT films, characterized by a drop in conductivity and work function. Films annealed at 200 °C saw a decrease in conductivity from 870 S cm−1 to 290 S cm−1 after just 10 min, whereas this effect was less pronounced for films annealed at 80 °C, falling from 870 S cm−1 to 770 S cm−1 over the same time period.28

A study comparing vapor deposited PEDOT to PEDOT bearing an alkyl methacrylate side group (PEDOT-MA) found that UV exposure was significantly more harmful to the PEDOT-MA films than the pure PEDOT films. The conductivity of PEDOT-MA films exposed to 3.9 and 19.5 J cm−2 UV doses suffered conductivity losses of 55.6 and 96.6%, respectively.38

Work Function

Another benefit of working with vapor deposited conductive polymers is the tunability of the films' electrical properties, such as work function. Im et al. demonstrated that the work function of PEDOT films deposited via oCVD could be varied from 5.1 to 5.4 eV by changing the polymerization process temperature [Fig. 13(a)].59

Figure 13.

(a) Work function dependence on deposition temperature for oCVD PEDOT films and (b) conductivity and work function of VPP PEDOT films after treatment with 30 s of oxygen plasma at different power levels. (a) Reproduced from Ref.59, with permission from American Institute of Physics. (b) Reproduced from Ref.28, with permission from John Wiley and Sons.

In addition to controlling film properties by varying process parameters, post-treatment of the film can be used to alter film characteristics. As shown in Figure 13(b), the application of 30 s of oxygen plasma to VPP PEDOT films after polymerization has been shown to increase work function with only a slight decrease in conductivity with increasing plasma power.28 The oxygen plasma is thought to increase the concentration of surface negatively charged oxygen species, which increase work function by forming strong dipoles.

Further tunability of the polymer film work function has been demonstrated by using tetrakis-(dimethylamino)ethylene (TDAE), a strong reduction agent, to modify the surface of VPP and oCVD PEDOT films. Results from UPS show that the work function of VPP PEDOT reaches 3.8 eV, which is even lower than aluminum-(oxide) electrodes (4.1 eV), known as a standard cathode material.87

VPP PEDOT films have been found to generally have a lower work function (≈4.3 eV) than solution processed PEDOT:PSS films (≈5.1 eV).88, 89

Mechanical Properties

A significant benefit of using organic polymeric materials is their good mechanical properties. Conductive polymer layers may be vapor deposited onto a number of flexible, fragile, and otherwise difficult to work with substrates. Unlike traditional electrode materials, such as ITO, vapor deposited PEDOT has been shown to retain conductivity upon flexing. It was demonstrated that PEDOT deposited on a PET substrate can withstand over 1000 compressive flexing cycles with negligible losses in conductivity, whereas ITO-coated PET suffers more than a 400-fold decrease in conductivity after flexing due to cracking.68 As shown in Figure 14(a,b), the ITO forms microcracks after folding, whereas the PEDOT has no visible damage. A comparison between PEDOT, PEDOT bearing an alkyl methacrylate side group (PEDOT-MA), and ITO films on flexible PET substrates showed that the PEDOT-MA films had the lowest increase in resistivity with increasing bending angle, a ∼10% increase at a bending angle of 80° (bending radius r = 0.65 cm bending radius) and only a 30% increase at a severe bending angle of 40° (r = 0.4 cm) [Fig. 14(c)]. There was no significant change in morphology of the films observed at the bending point before and after bending.38

Figure 14.

Optical microscope images of oCVD PEDOT (a) and ITO (b) on PET after 100 compressive flexing cycles to a <5 mm radius. (c) Increased resistance of ∼100 nm VPP PEDOT and PEDOT-MA films compared with ITO films on PET at decreasing bending angles. (a, b) Reproduced from Ref.68, with permission from John Wiley and Sons. (c) Reproduced from Ref.38, with permission from American Chemical Society.

A method for in situ grafting of oCVD PEDOT films was demonstrated, which minimized delamination problems associated with patterning and handling the films. The grafting process takes advantage of the adhesion of the conducting polymer films to aromatic groups by using silane coupling agents containing an aromatic ring to graft films to a number of different substrates (PET, Si wafers, and glass).61 The superior adhesion by covalent bonds between the grafted oCVD polymer reduces delamination and hence enables high resolution patterning [Fig. 7(d,e,g)].

Stability

Thermal stability and shelf-life of the polymer films are necessary attributes for practical use in applications. In comparison to other conductive polymers, PEDOT is recognized for having a relatively high stability in the oxidized state.90 During one study, VPP PEDOT films stored in air for a year and a half gradually decreased in conductivity, whereas samples stored in water for the same amount of time quickly dropped in conductivity but then remained relatively constant, as shown in Figure 15.91 After exposure to 1M p-toluene sulfonic acid (pH ∼ 1) for 3 h, the samples stored in air and in water recovered 75% and 94% of their original conductivities, respectively.

Figure 15.

Stability of VPP PEDOT films, in air (dashed line) and water (two separate experiments, solid lines). Conductivity values after treatment in 1M p-toluene sulfonic acid are shown by filled symbols connected by arrows to the pre-rinsed film. (Reproduced from Ref.91, with permission from Elsevier.)

In another study, VPP PEDOT lost about 45% of its starting conductivity of 575 S cm−1 when exposed to ambient conditions for 60 days.92 However, over this same period of time, PEDOT:PSS samples only lost 25% of their original conductivity, although the initial conductivity was only 200 S cm−1.

Changes in pH of an aqueous rinsing solution affected a reversible change in conductivity, with the highest conductivity at the lowest pH values (Fig. 16).91 This reversibility suggests that the PEDOT backbone is undamaged and that the conductivity change is due to changes in the doping level. It is suggested that the increase in doping levels of PEDOT are due to oxygen, which is dissolved in solution, being a stronger oxidizing agent in more acidic solutions.

Figure 16.

Resistivity of VPP PEDOT films when subjected to alternating solutions of pH 1 and pH 13. (Reproduced from Ref.91, with permission from Elsevier.)

In the same study, it was also found that high conductivity composite films of PEDOT and Loctite 3311, a UV and heat curable glue, could withstand very high current density before material breakdown.

The counter anion dopant has also been shown to influence the stability of conducting polymers. oCVD PEDOT films using Br2 as the oxidizing agent, and thus doped with Br anions, were demonstrated to have a ∼5-fold increase in stability during accelerated lifetime tests over films deposited with FeCl3.58

Four-probe resistors were fabricated using films of PPy and poly(3,3′-dipentoxy-2,2′-bithiophene) (P-DPOBT) deposited by VPP for stability studies.93 In addition to stability tests at constant room temperature, tests were also performed in which the films' conductivities were measured as they were heated and then cooled back down to room temperature. The PPy films were not permanently damaged upon heating up to 100 °C in inert atmosphere and cooled back to room temperature, but their resistances quickly increased over the course of 40 days at room temperature. In contrast, films of conductive P-DPOBT prepared in the same way exhibited irreversible, continuously decreasing conductivity upon heating above 50 °C in vacuum. Yet, these films showed a less than 10% increase in film resistance after 18 days, stored both in air and in N2. A separate study on VPP P-DPOBT showed a 51% decrease in film conductivity in 60 days at room temperature, with a 22% drop in conductivity in the first 3 days.14 Higher temperatures appeared to accelerate degradation; samples held at −10 °C only decreased by 13% in conductivity over 2 months, whereas samples held at 40 °C decreased by 75% conductivity in 20 days.

Applications

Organic Photovoltaics

One of the main applications that has been extensively explored for vapor deposited conjugated polymers is in OPVs.

There has been a large research effort in the organic optoelectronic community to replace traditional metal oxide electrodes, such as ITO, with another transparent conductive material. Conductive polymer electrodes, among the various alternatives being investigated along with graphene,94 carbon nanotubes,95 thin metal layers, and metal grids,96 offer a promising alternative to ITO or other electrode and buffer layer materials. Conductive polymer electrodes have the benefit of cheap, scalable large area processing, and the ability to be used with lightweight and flexible substrates.

PEDOT:PSS commonly functions as a buffer layer material [hole transporting layer (HTL)] that is spin-coated directly onto an ITO anode. oCVD PEDOT films have been demonstrated to work as a buffer layer material replacing solution processed PEDOT:PSS with equivalent performance, reporting values of 4.5 and 4.5 mA cm−2 for short circuit current, 0.48 and 0.47 V for open circuit voltage, and 0.60 and 0.56 for fill factor for oCVD PEDOT and PEDOT:PSS HTLs on ITO, respectively [Fig. 17(a)].68 oCVD PEDOT has also been demonstrated as an HTL on graphene electrodes. Whereas PEDOT:PSS has difficulty being spin coated onto the graphene surface due to the graphene surface hydrophobicity, the oCVD PEDOT was patterned directly onto the graphene surface in a single step, providing conformal coverage. The graphene electrode devices with oCVD PEDOT HTLs had similar performance to the ITO control devices, 3.0% and 3.1%, respectively.56 An example of devices using a VPP PEDOT HTLs resulted in significantly lower current and voltage values than the control devices using PEDOT:PSS, which is thought to be a manifestation of the lower work function of the VPP PEDOT.88 Vapor deposited PEDOT has also been demonstrated to replace ITO as the anode in a variety of device architectures including those with small molecule planar bilayers68 as well as polymer bulk heterojunction active layers.34

Figure 17.

(a) Comparison of IV curves for devices made using pure ITO and oCVD PEDOT electrodes and ITO devices using oCVD PEDOT and PEDOT:PSS hole transporting layers; (b) architecture of devices made using a laminated VPP PEDOT cathode; (c) solar cell array connected in series fabricated on tracing paper using oCVD PEDOT electrodes; (d) top-illuminated solar cells fabricated on various paper and plastic substrates. (a, c) Reproduced from Ref.68, with permission from John Wiley and Sons. (b) Reproduced from Ref.88, with permission from Elsevier. (d) Reproduced from Ref.97, with permission from John Wiley and Sons.

Vapor deposition techniques also enable the fabrication of OPV devices on nontraditional substrates. Fully dry-processed devices have been fabricated directly onto paper substrates using oCVD PEDOT electrodes [Fig. 17(c,d)], and Figure 17(d) shows an array of OPVs connected in series.68 The incorporation of OPVs into textiles still suffers from poor reproducibility and low performance due to handling, making electrical connections, low stability, and short circuiting due to rough surfaces. The technique described earlier in section “Applications” for lowering PEDOT work function using a post-treatment reduction agent (TDAE) has allowed vapor deposited PEDOT to serve as both a cathode and as an electron transporting layer within photovoltaic devices. Inverted devices, with a transparent low-work function vapor deposited PEDOT cathode, were achieved using evaporated small molecule active layer materials (a DBP/C60 planar bilayer). Vapor-deposited PEDOT cathode devices were also fabricated using a Cs2CO3 buffer layer to improve charge transport from the acceptor to the PEDOT cathode.69

Taking advantage of the lower work function of VPP PEDOT (as opposed to PEDOT:PSS), laminated devices were demonstrated using a VPP PEDOT top cathode with an ITO/PEDOT:PSS anode. The devices had low current values (<0.1 mA cm−2) due largely to the high series resistance at the laminated polymer/PEDOT interface; however, the devices displayed rectification behavior confirming that the VPP PEDOT acted as a cathode.88

Vapor-deposited PEDOT films have also been used as a top transparent electrode by using a stamp print transfer process to put the film on top of the other photovoltaic layers grown on the substrate material, as shown in Figure 17(b). The PEDOT electrode is transferred after first depositing the PEDOT onto a spatially patterned PDMS stamp. Devices with up to 2.7% power conversion efficiency have been fabricated using this method for a P3HT:PCBM ([6,6]-phenyl-C61-butyric acid methyl ester) bulk heterojunction active layer.98 This stamping process, which has also been applied to solution processed P3HT and PEDOT:PSS, can be read about in more detail in ref.99.

Top-illuminated inverted devices were fabricated using oCVD to pattern the PEDOT electrode directly onto the rest of the fabricated cell. The use of a thin molybdenum trioxide (MoO3) buffer layer allowed the PEDOT layer to be deposited without damage to the underlying active layer materials. This technique allowed for the fabrication of OPVs on opaque substrates including paper [Fig. 17(d)], yielding efficiencies > 2%, a more than 10-fold increase over previous paper devices.97

Large-area (4.2 cm2) devices were fabricated using thick semitransparent VPP PEDOT electrodes (20 Ω sq−1) on glass with (poly[2-methoxy-5-(2′-ethylhexyloxy)-p-phenylene vinylene]) MEH-PPV/PCBM bulk heterojunction active layers. The PCE of these devices reached 0.22%. The low efficiency is mostly due to the low current as a result of poor transmission (<50% across the spectrum) of the thick PEDOT film.100

Vapor-deposited polymers have potential application for dye sensitized solar cells as well as OPVs. An insoluble conductive polymer electrode may be a good choice taking into account the regular solvent use in DSSCs. Vapor-deposited PEDOT:PTS has been used as a counter electrode in dye sensitized solar cells. Winther-Jensen and coworkers reported a PCE of 5.25% with a glass-based wet type DSSC and observed a lower resistance during operation for the poly[3,4-ethylenedioxythiophene:para-toluenesulfonate](PEDOT:PTS) coated cathodes than for the standard platinum (Pt) coated fluorine doped tin oxide.101 PPy has also been applied as counter electrode in DSSCs showing good catalytic behavior and a moderate fill factor (FF = 0.42–0.54) in comparison to regularly used Pt counter electrodes (FF = 0.65).47

Organic Light Emitting Diodes

Organic light emitting diodes (OLEDs) are light emitting diodes in which the emissive layer consists of organic materials. OLEDs have received increasing demand in the display market (such as in screens for televisions, computers, and mobile devices) because of their many advantages, such as bright light output and low energy consumption.102, 105

Similar to the work to replace inorganic electrode materials in OPVs by vapor-deposited PEDOT, OLED devices using VPP PEDOT anodes have been successfully demonstrated by Levermore et al.28, 37 They optimized the film thickness (40 nm), conductivity (≤1180 S cm−1), and work function (4.6 eV) of VPP PEDOT layers for OLED application. ITO-free OLEDs were fabricated on glass and plastic substrates with the following structure: substrate/VPP PEDOT/PEDOT:PSS/emissive layer/cathode. The thickness of the VPP PEDOT anode was kept at 40 nm with either a 50 nm or 80 nm PEDOT:PSS hole injection layer coated on top. Bright green electroluminescent OLEDs were obtained with efficiencies of 16.8 and 9.8 lm W−1 at ∼100 cd m−2 on glass and plastic substrates, respectively. These efficiencies were comparable with equivalent standard ITO anode OLEDs.28 A similar study revealed that the anodes not pretreated with PEDOT:PSS exhibited low luminosities and efficiencies due to poor hole injection at the anode compared with the devices with PEDOT:PSS-coated anodes.37

Electrocatalysis

Many important energy conversion technologies, such as fuel cells, batteries, photo-electrochemical solar cells, and water-splitting devices for hydrogen generation, require catalysts that are often limited by performance or economics. The Pt catalysts commonly used for many of the electrodes are expensive and degrade in performance over time due to CO poisoning and a drift phenomenon in which Pt particles diffuse and agglomerate, which reduces the active catalytic area.106, 107 Because conducting polymers are inexpensive and not subject to these degradation modes, they have received increasing attention to replace the metal electrodes in these types of applications.108

PEDOT has been shown to be capable of serving as the reduction electrode for O2, which is an important reaction in fuel cell and battery applications. VPP PEDOT deposited onto a gold-coated Goretex membrane provided a substantial oxygen reduction current, comparable with that of a Pt electrode of the same geometrical area at several different pH values [Fig. 18(a–c)].109

Figure 18.

Comparison of conversion current versus potential for oxygen reduction from air at VPP PEDOT electrode (black line) and Pt electrode (gray line). Measurements at steady-state conditions at: (a) pH 1, (b) pH 7, and (c) pH 13. (d) Current versus time in air contaminated by 10% CO. (Reproduced from Ref.109, with permission from AAAS.)

Performance using air contaminated with 10% CO demonstrated that the Pt electrode was very quickly poisoned by the formation of carbonyl complexes, whereas the PEDOT electrode remained unaffected [Fig. 18(d)]. It is suggested that the mechanism of the air reduction electrocatalysis involves a redox cycling process in which the PEDOT is reduced by the electrochemical cell, followed by the absorption and oxidation of the PEDOT by an O2 molecule which in turn reduces that molecule.

Zn/air and Mg/O2 batteries with VPP PEDOT cathodes demonstrated superior performance to those that used Pt and Au cathodes, respectively.109, 110 Furthermore, a flexible Zn/air battery was printed on paper by depositing a VPP PEDOT cathode on one side, absorbing the liquid electrolyte into the paper matrix, and air-brushing a Zn/carbon/polycarbonate anode on the other side of the paper.111

Electrodes made using composite films of nonconducting polyethylene glycol (PEG) and VPP PEDOT films were shown to catalyze the reduction of protons to form hydrogen with comparable performance with that of Pt.112 The presence of PEG was necessary to coordinate the protons to bring them close to the PEDOT, and the addition of PEG did not significantly change the electrical conductivity of the composite from that of pure PEDOT. Continuous operation of the PEDOT-PEG electrodes at constant potential demonstrated stability over the 80 days tested.

Organic Field-Effect Transistors

OFETs have received significant attention for their potential to serve as the main component of flexible and inexpensive electronic circuits.5 Despite this, limited work has been performed applying VPP or oCVD polymers to the fabrication of these devices.

One group fabricated pentacene organic thin-film transistors on flexible plastic substrates using VPP PPy as the source and drain electrodes that showed comparable performance to devices fabricated with Au electrodes.53 The oxidizing agent (ammonium persulfate) was patterned by inkjet printing onto the pentacene before exposure to pyrrole vapors [Fig. 19(a)]. The contact resistance between the organic semiconductor and the VPP PPy was lower than for identical devices made with Au top electrodes, as shown in Figure 19.

Figure 19.

(a) Schematic of organic thin-film transistor (OTFT) fabrication with inkjet printing assisted-VPP PPy top contacts. (b). Total contact resistance (RS/D) as a function of applied gate voltage of OTFTs with VPP PPy and Au top electrodes. (Reproduced from Ref.53, with permission from Elsevier.)

This difference was attributed to a smaller hole-injection barrier between the polymer and organic semiconductor than between the Au/pentacene contact.

Electrochromic Devices

Electrochromic materials have received much attention for their use in “smart windows” and other light management applications.113 Many conjugated polymers exhibit electrochromic properties including PEDOT, which is transparent lighter blue in the oxidized state and darker blue in the reduced form.114 Thus, PEDOT has been explored for electrochromic applications by using an applied voltage to switch between the two electronic states.

Electrochemical testing was used to study the electrochromic properties of PEDOT deposited by VPP89 and by oCVD.62 Film transmission contrasts up to 45% at 566 nm were achieved for the oCVD films, and the contrast remained 85% of the original value after 150 cycles. A patterned electrochromic device [Fig. 20(a,b)], made by depositing oCVD PEDOT onto ITO-coated PET, exhibited dark-to-light switching times of less than 10 s and a light-to-dark transition of about 1 min.62

Figure 20.

An electrochromic device using patterned oCVD PEDOT in the (a) light state and (b) dark state. Prototype large-scale electrochromic device using VPP PEDOT in the (c) light state and (d) dark state. (a, b) Reproduced from Ref.62, with permission from Elsevier; (c, d) Reproduced from Ref.42, with permission from Elsevier.

Large-scale electrochromic devices using VPP PEDOT were configured as car rear view mirrors [Fig. 20(c,d)].42 Scale-up of the VPP process to accommodate large substrates (up to 30 × 10 cm) necessitated the use of a larger chamber that required process optimization, resulting in PEDOT films with conductivities up to 1485 S cm−1. The large-scale prototype electrochromic devices achieved a maximum transmission contrast of 52.2% at 555 nm.

Very stable electrochromic devices were made using VPP PEDOT cathodes with VPP PPy anodes. Ionic liquids were developed that were based on the same anion (tosylate) as the dopant in the conducting polymer films. Electrochromic devices made with a gel-form of the ionic liquid electrolytes, and the VPP films demonstrated transmission contrasts of 42% at 560 nm and were cycled 10,000 times without a decrease in performance.115

Sensing

Conducting polymers have been successfully used in small devices for their applications in sensing chemical and biological species. Conducting polymer-based sensors can be classified by the nature of their detection methods such as amperometric, conductometric, chemiresistive, and others.6, 116

VPP conductive polymer films were first used for detecting 3 mM concentrations of alcohols (C2–C4) by using patterned poly(3,3′-dipentoxy-2,2′-bithiophene) on nonconductive substrates like glass and surface oxidized silicon wafers. An increase in conductivity was detected in less than 5 s for all alcohols tested, and responses were observed to decrease as the chain length of the alcohol increased.14 In another experiment done by the same group, responses of VPP PPy films of thicknesses from 500 nm to 3 μm were tested against a 20,000 ppm concentration of toluene. Here, toluene reversibly interacted with active sites within PPy by forming charge transfer complexes where electrons were transferred to the polymer. Because PPy is a p-doped polymer, this resulted in a decrease in conductivity being observed in the interaction with toluene vapor. As shown in Figure 21(a), an 8-fold increase in response was observed for a 2-fold increase in polymer film thickness.117

Figure 21.

(a) Effect of film thickness on chemiresistive responses of VPP PPy films to toluene as a function of time [(a) 500 nm; (b) 1000 nm; (c) 2000 nm; (d) 3000 nm]; (b) Amerometric response of VPP PEDOT based sensors as a function of glucose concentration; (c) chemiresistive response of avidin bound oCVD copolymer film on electrospun mat against biotin solutions of 5 nM (blue), 50 nM (purple), 500 nM (green), and 5 μM (red) concentrations; and (d) effect of surface area on the maximum response [Rp (%) (max.)] and response time for devices made on flat substrates and high surface area electrospun mats to a 5 μM biotin solution. (a) Reproduced from Ref.117, with permission from Elsevier. (b) Reproduced from Ref.116, with permission from Japan Society of Applied Physics. (c, d) Reproduced from Ref.67, with permission from John Wiley and Sons.

VPP PEDOT has been successfully used to fabricate amperometric biosensors to detect hydrogen peroxide (H2O2) by absorbing horseradish peroxidase from an ethanol solution. Amperometric detection of 5 mM H2O2 in phosphate buffered saline solution was reported with a sensitivity of 190 μA cm−2.118 An amperometric glucose sensor based on VPP PEDOT was demonstrated, which showed a linear relationship between the response against micro- to mili-molar glucose concentration. The optimized condition showed high sensitivity and a detection limit below 10 μM. The response of the PEDOT-based sensor to various glucose concentrations at a gate bias, Vg = 0:4 V with a source-drain bias, Vd = −0:4 V (open circles) and Vd = −1 V with Vg = 0:8 V (filled squares) is shown in Figure 21(b).116

Chemiresistive sensing properties of conductive polymers can be improved by covalent immobilization of an analyte specific molecule to the conducting polymer matrices. For example, direct analysis of the changes in the work functions of metal nanoparticles when exposed to a vapor is very difficult. However, it can be overcome by incorporating metals in conducting polymer platforms and converting these changes in work function of the metals into changes in resistances of the hybrids.66, 70 Despite having very high electrical conductivity in PEDOT, a lack of any active chemical functional group makes it difficult to attach any analyte-specific molecules to it. In contrast, polymers made from functional monomers, for example, TAA or 3TE, have very poor to no conductivity, and sometimes their mechanical properties are also poor.57, 66, 70 The oCVD technique has been successfully used to synthesize functional conductive copolymers by polymerizing 3-substituted thiophenes containing [BOND]COOH or [BOND]OH functional groups with either pyrrole or EDOT.57, 66, 70 oCVD poly(EDOT-co-TAA) films containing [BOND]COOH groups were used to covalently attach palladium and nickel nanoparticles using 4-aminothiophenol linker molecules. The chemiresistive response of hybrid Pd/poly(EDOT-co-TAA) and Ni/poly(EDOT-co-TAA) films to acetone and toluene was found to be specific and proportional to their concentrations. The detections limits of toluene and acetone were 10–20 and 40–50 ppm, respectively. The minimum detectable concentration of the analytes decreased (or sensitivity increased) with decreasing copolymer film thickness. The response times of these hybrid sensors were found to be in the range of 60–70 and 200–300 s for acetone and toluene, respectively, for palladium and nickel nanoparticles assembled on top of 50-nm-thick conducting copolymer films.

oCVD copolymers containing [BOND]COOH and [BOND]OH groups were also used to fabricate chemiresistive biosensors. Poly(EDOT-co-TAA) copolymer was deposited on a nonconductive glass substrate using bromine as the oxidant. A 2-fold increase in the sheet resistance of the conductive copolymer was observed when a biomolecule, bovine serum albumin, was covalently attached to the [BOND]COOH functional groups of the copolymer.57 In a separate study, an [BOND]OH functional copolymer poly(EDOT-co-3TE) was deposited on 2 cm × 2 cm electrospun nylon fiber mats by oCVD.67 Avidin, a biomolecule, was covalently immobilized to the [BOND]OH functional groups as the analyte specific molecule, and the change in resistance of the sensor was monitored against 5 nM to 5 μM concentrations of another protein, biotin. Because avidin and biotin are specific and selective to each other, no change in resistance was observed when another protein, albumin, which is nonspecific to avidin, was used as an analyte. As the biotin concentration was increased, an increased response and faster steady state were achieved [Fig. 21(c)]. In this study, electrospun fiber mats provided a very high surface area compared with flat glass substrates, which resulted in a 6-fold increase in response and 30% reduction in response time as shown in Figure 21(d). The high selectivity of this sensor can be used for detection of other biomolecules of interest, for example, food toxins, and shows promise for manufacturing ultra-light weight, flexible, and field deployable biosensors for fast detection of various analytes.

Resistive Heaters

The application of current through a conductor results in heat generation caused by resistance to the current flow, which is known as the Joule effect. This can be used to create simple designs for various applications.

VPP PEDOT nanofiber nonwoven mats were prepared on textiles by electrospinning the oxidant solution into nanofibers, and their temperatures were monitored with an infrared camera under various applied currents [Fig. 22(b)].119

Figure 22.

(a) Temperature measurements of a VPP PEDOT mat with as a function of applied current. (b) Infrared camera images of the PEDOT mat under various applied currents, with an outline of the electrodes drawn on the first image. (c) Application of 0 and 120 mA to VPP PEDOT mat coated with thermochromic microcapsules. (Reproduced from Ref.119, with pending copyright permission.)

The high heating power of 5470 W m−2 of the samples allowed for a high temperature of ∼100 °C to be reached at low voltages (10 V) within seconds. The samples also demonstrated good stability upon cycling and holding a constant temperature for 2 h. Color-changing textiles were then fabricated by depositing thermochromic microcapsules onto the PEDOT and adjusting the current to obtain the desired temperature for a color transition (37 °C), as shown in Figure 22(c).

Conductive films deposited by oCVD were used to resistively heat thermally activated shape memory polymer (SMP) foams.120 Penetration of the oCVD film into the foam's internal structure compensates for the foam's low thermal conductivity and enables uniform heating throughout the sample upon application of an electrical current. The polymer-coated SMP foams, which were stiff at room temperature, softened and deformed under a small mechanical load when a voltage potential was applied. Heating of the SMP foam to temperatures above 100 °C could be obtained in this way.

oCVD PEDOT films have also been used to resistively heat temperature-responsive films of poly(N-isopropylacryalmide-co-di(ethylene glycol) divinylether) (p(NIPAAm-co-DEGDVE)) deposited by initiative chemical vapor deposition (iCVD).121 Membranes coated with the oCVD and iCVD films showed a temperature change of 10 °C in water upon applying a voltage, which was sufficient to heat the iCVD film above its lower critical solution temperature (LCST) of 28 °C. By cycling the applied voltage and thus cycling the film temperature around its LCST, water flux through the coated membranes could be controlled.

SEMICONDUCTING POLYMERS

Although VPP and oCVD have traditionally been used to obtain conductive polymers, semiconducting polymers are also of great importance as active layer materials for OPVs,3, 122 OLEDs,4 OFETs,5 and other applications.

Recently, oCVD has been used to deposit semiconducting unsubstituted polythiophene films.71, 73 Unsubstituted polythiophene's insolubility has made it difficult to process using most traditional techniques but it is easily deposited using vapor phase techniques. Using oCVD with vanadium oxytrichloride (VOCl3) as the oxidizing agent, as-deposited films exhibited conductivities up to 20 mS cm−1.73 Rinsing the films in hydrochloric acid followed by sodium hydroxide and water removed vanadium compounds and resulted in the neutral, semiconducting polythiophene. Figure 23(a) distinctively shows the differences between absorption characteristics of oCVD made polythiophene films both in the doped and dedoped states.73 Whereas the doped, conductive polymer shows broad polaron absorption, dedoped, semiconducting polymers exhibit a characteristic absorption peak due to the π-π* transition. Film properties, including conductivity and absorption maximum of the washed films, were tuned by controlling the surface concentrations of the monomer and oxidizing agent by adjusting their flow rates. Unsubstituted polythiophene was also prepared by oCVD using iron (III) chloride as the oxidizing agent.71 As-deposited films had conductivities up to 20 S cm−1, and methanol was used to dedope the films to achieve neutral, semiconducting polythiophene. The oCVD-polythiophene was used as an electron donor in bilayer heterojunction organic solar cells with a thermally evaporated C60 electron acceptor layer. Power conversion efficiencies up to 0.8% were achieved, which is the highest efficiency to date using a vapor-phase deposited donor polymer for a polymer solar cell [Fig. 23(b)].

Figure 23.

(a) UV–vis absorbance spectra of oCVD polythiophene as-deposited (doped) and washed (dedoped); (b) JV curves of photovoltaic devices fabricated using oCVD polythiophene as the electron donor material with device structure ITO/oCVD PT/C60 (30 nm)/BCP(8 nm)/Ag. A maximum efficiency of 0.8% was obtained using 25 nm of oCVD. (a) Reproduced from Ref.73, with permission from American Chemical Society. (b) Reproduced from Ref.71, with permission from Elsevier.

Polyselenophene, an insoluble polymer with a lower bandgap than polythiophene, was prepared using oCVD.72 Deposited films exhibited significant increases in roughness from 11 nm to 80 nm with an increase of film thickness from 100 nm to 750 nm.

Semiconducting polymers have recently been identified as a possible way to enhance electro-catalytic performance of electrode materials by serving the function of photo-excitation as well as electro-catalysis.108 In this way, the rate of the redox reaction could be controlled by adjusting the incident light on the electrode. This idea was demonstrated with light-enhanced proton reduction by VPP polybithiophene composite films and photo-stimulated electrolysis of water by a VPP polyterthiophene film.46, 108

CONCLUSIONS

Since their development, vapor deposition techniques have been used in creating a wide variety of polymeric materials for use in various applications. Specific processes have been developed that enable vapor deposition of conjugated polymers. Two of these processes, VPP and oCVD, mimic oxidative step growth polymerization in solution and allow for polymerization of conductive and semiconductive thin films onto a variety of substrates. These vapor deposition techniques demonstrate a number of benefits including conformality, grafting, low temperature processing, and in the case of oCVD, substrate independence. Performing polymerization in the vapor phase allows for the fabrication of homo- or co-polymer layers that are insoluble or otherwise difficult to make by solution-based methods. A number of process parameters may be controlled to tune the optoelectronic and physical properties of the thin films. Of high importance for electronic applications is the ability to control film conductivity and work function by varying such process parameters as relative humidity, substrate temperature, or controlling oxidant reactivity by use of additives. Varying pre- or post-processing steps such as annealing or chemical treatments to the substrate also impact the final film characteristics, such as roughness and adhesion. Tunability and optimization of vapor deposited conductive and semiconductive thin films have allowed these materials to be integrated into a number of applications, including OPVs, OLEDs, OFETs, sensors, electrochromic devices, and in electrocatalysis applications. The dry nature of the vapor-based processes has allowed devices to be fabricated on unique paper and textile substrates and can integrate well with the fabrication of flexible devices by such techniques as roll-to-roll processing or large area batch vacuum depositions. New materials are continuing to be explored, especially in the areas of copolymers and semiconducting layers, and vapor deposited polymer systems are becoming more controllable, expanding the ranges of tunable properties, and enabling the creation of novel applications.

Acknowledgements

This work was supported in part by the U.S. Army through the Institute for Soldier Nanotechnologies under Contract DAAD-19-02-D-0002 with the U.S. Army Research Office. R. M. Howden gratefully acknowledges support from the Department of Energy (DOE) Office of Science Graduate Fellowship Program (DOE SCGF). The DOE SCGF Program was made possible in part by the American Recovery and Reinvestment Act of 2009. The DOE SCGF program is administered by the Oak Ridge Institute for Science and Education for the DOE. ORISE is managed by Oak Ridge Associated Universities (ORAU) under DOE contract number DE-AC05-06OR23100. D. C. Borrelli gratefully acknowledges support from the National Science Foundation Graduate Research Fellowship Program. The content does not necessarily reflect the position of the Government, and no official endorsement should be inferred.

Biographical Information

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Dhiman Bhattacharyya is currently working on his postdoctoral research with Prof. Gleason at MIT. He received a Ph.D. in Chemistry from the University of Texas at Arlington in 2008. He also has B.Sc. in Chemistry, B.Tech. and M.Tech. in Polymer Science from the University of Calcutta in India. He has authored and co-authored seven international and U.S. patent applications and 16 journal articles. His research interests include renewable energies, composites, sensors, drug deliveries, and biomaterials.

Biographical Information

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Rachel M. Howden is currently working on her Ph.D. in Chemical Engineering with Dr. Karen Gleason at MIT. She received her B.S. in Engineering from Harvey Mudd College in 2008. Her current research focus is on chemical vapor deposition techniques for organic optoelectronics. Other research interests include paper-based substrates, polymer electrodes, hybrid photovoltaics, and device fabrication techniques.

Biographical Information

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David C. Borrelli is currently pursuing his Ph.D. in Chemical Engineering under the supervision of Dr. Karen Gleason at MIT. He received his B.S. in Chemical Engineering from the University of Rochester in 2009 and a M.S. in Chemical Engineering Practice from MIT in 2011. His research focuses on the use of oxidative chemical vapor deposition for polymer thin films for organic photovoltaics.

Biographical Information

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Karen K. Gleason is the Alexander and I. Michael Kasser Professor of Chemical Engineering at the Massachusetts Institute of Technology (MIT). In 1982, she simultaneously received her B.S. in Chemistry and M.S. in Chemical Engineering from MIT. In 1987, she was awarded a Ph.D. in Chemical Engineering from the University of California at Berkeley. Prof. Gleason has authored more than 200 publications and registered more than a dozen U.S. patents.