ITO Alternative: solution deposited Clevios PEDOT:PSS for transparent conductive applications

Transcription

1 Trade article Leverkusen, Germany June 6 th 2012 ITO Alternative: solution deposited Clevios PEDOT:PSS for transparent conductive applications Andreas Elschner, Wilfried Loevenich, Aloys Eiling, John Bayley, Heraeus Precious Metals GmbH & Co KG, Conductive Polymers Division, Building B202, Chempark Leverkusen, Leverkusen, Germany History/Abstract Seldom in the history of materials development has there been a product technology that creates more new opportunities or challenges as conductive polymers. As the team that invented PEDOT:PSS (in the laboratories of our former owner Bayer AG) Heraeus Precious Metals, Conductive Polymers Division stands in a unique position of depth of knowledge in the development, production and delivery of commercially available materials for a broad range of applications. This paper offers an introduction to Clevios PEDOT:PSS materials and discusses the material as an alternative to inorganic transparent conductive oxides. It summarizes current applications and the future potential of highly conductive poly(3,4-ethylenedioxythiophene) also known as PEDOT. The main focus lies on the water dispersed complex with poly (styrene sulfonic acid), also known as PSS, as the counter-ion. The basic chemical and physical properties of the Clevios PEDOT:PSS complex offered by Heraeus are discussed in order to show the fundamentals that lead to the use of PEDOT:PSS in transparent conductive applications. Due to our ability to increase conductivity and transparency in recent years this versatile material has now reached the requirements for current driven devices such as displays, touch panels, and solar cells. Besides conductivity, advantages of this polymer are flexibility, the use in low-cost production processes such as printing, its safe handling and availability on a ton scale. Introduction In the 35 years since intrinsically conductive polymers (ICPs) were discovered, such materials have found many applications not only due to their conductivity but also because they are easily processed and have low drying temperatures. 1 2 Although the conductivity of ICPs is below that of metals, due to their specific properties they have found a wide range of industrial applications.

2 Page 2 Figure 1 shows some important ICPs in their neutral form Free charge carriers in ICPs can be introduced by oxidation or reduction. As a result the electrical behavior of these materials changes from a semiconductor to a conductor. For example when polyacetylene is oxidized by AsF 5 the conductivity increases from 10-5 S/cm to 200 S/cm 3. This process is often referred to as doping and the resulting polymer is called a doped polymer. Apart from polyacetylene, a number of ICPs are very stable. For example, polypyrrol is used in polyelectrolyte capacitors and polyaniline is used as antistatic coating on plastic films. Clevios PEDOT:PSS outperforms the other ICPs. In comparison, PEDOT coated films are exceptionally transparent and conductive. PEDOT:PSS was first discovered in , and its first application was its use as an antistatic agent in photographic films. Since then Clevios PEDOT:PSS has penetrated into other markets including polymer capacitors, printed wiring boards, and packaging films, it also functions as a hole transport layer in organic light emitting diodes (OLEDs) and organic photovoltaics (OPV). 567 However, in this article we focus on the use of Clevios PEDOT:PSS as transparent electrode. Therefore, the properties of the PEDOT dispersion, the physics of PEDOT films and applications of PEDOT as transparent electrode e.g. in electroluminescent lamps, touch screens and OLEDs will be described.

3 Page 3 Processable PEDOT PEDOT was first discovered by oxidation of ethylenedioxythiophene (EDOT) by iron salts.4. The resulting PEDOT forms a dark blue intractable precipitate, which cannot be re-dissolved, re-dispersed or used in any other way for transparent conductive coatings. However, if the reaction mixture itself is deposited onto a substrate and the polymerization takes place on the substrate a transparent conductive film can be obtained. The disadvantages for this so called in situ PEDOT is the need for rinsing steps to remove the oxidation agent from the film and the resulting surface roughness. In order to obtain a re-dispersible PEDOT powder, the option is to polymerize in the presence of surfactants. 8 This process results in PEDOT particles surrounded by surfactant shells which can be washed and redispersed. However, such PEDOT particles suffer from poor film forming properties and the conductivity is limited. The most versatile form of PEDOT for processing is the synthesis of PEDOT as a polyelectrolyte complex. 7 The complex consists of polymeric cationic PEDOT and a polymeric counter anion. The most effective counterion for PEDOT is polystyrenesulfonic acid (PSS). PSS is easily soluble in water, film forming and transparent in visible light. Due to the ionic nature of the polyelectrolyte complex water is the preferred solvent. Clevios PEDOT:PSS can be obtained in the form of discreet swollen so called gel particles ranging between nm. The long loops and tails of PSS stabilize the particles against coagulation and precipitation. 9 PEDOT:PSS dispersions are stable at room temperature under ambient conditions. They can be processed in a wide range of processing methods and yield the most transparent, conductive films achievable by use of today s ICPs. Figure 2 shows the chemical reaction that is used for the formation of the PEDOT:PSS polyelectrolyte complex. The oxidation of EDOT is performed in the presence of PSS. The resulting PEDOT forms a complex with the dissolved PSS. The chain length of PEDOT is relatively short. 10 Typically every third or fourth thiophene unit carries a positive charge. The Coulomb interactions between the positively charged PEDOT chains and the negatively charged PSS chains results in a high viscosity of the dispersion of up to 1 Pa s even at low solids content of 1%. Since the complex is present in the form of gel particles, the most suitable description of its morphology is that of a dispersion rather than a solution. Shearing of the dispersion leads to a decrease of particle size and viscosity. Concentration is feasible up to a solids content of 5% and increases the viscosity. Binders, surfactants, cross-linkers and solvents can be used as additives in

4 Page 4 order to improve the film deposition, drying speed, adhesion and film hardness. Figure 2. Chemical polymerization of ethylenedioxythiophene in the presence of polystyrenesulfonic acid. Application of Materials The Clevios PEDOT:PSS dispersion can be deposited using a wide range of methods. Spin-coating allows the deposition of very smooth layers in the range of nm layer thickness of the dry film. Thicker layers of up to several µm can be deposited via silk screen printing. Fine lines can be printed by ink-jet printing. Slot-die coating allows the deposition of thin films in rectangular areas. Furthermore, the dispersion can be deposited by roll-to-roll printing techniques such as gravure printing or flexo printing. The specific requirements for each printing method such as viscosity, solids content, adhesion, drying speed etc can be obtained by a number of methods. A very specific group of additives are so called conductivity enhancers. For example, dimethylsulfoxide and ethylene glycol are two widely used conductivity enhancing agents. The effect of these materials is due to morphological changes in the film forming process. During the drying process the additive becomes the predominant medium and its effect is

5 Page 5 similar to a plasticizer; rearranging PEDOT and PSS chains within the film.1 In the dispersion the gel particles are encased by PSS chains on its outside, whereas the PEDOT is mainly in the center of the gel particle. In the absence of a conductivity enhancement agent the composition remains unchanged in the film forming process and many barriers are formed due to insulating properties of the PSS on the outside of each gel particle. The presence of a conductivity enhancement agent allows a rearrangement in which larger domains of PEDOT rich areas are formed and the charge carriers can move more freely. The following table shows some commercial Clevios PEDOT:PSS dispersions and their properties: Trade name %Solids content in water (w/w) PEDOT:PSS ratio (w/w) Viscosity at 20 C (mpas) Average nm. particle Clevios PH : Clevios PH : size Conductivity in S/cm (a) Clevios PH : Clevios P 1.3 1: Clevios PH 1.3 1: Table 1: Typical values for solids content, PEDOT:PSS ratio, viscosity, particle size and conductivity of commercial PEDOT:PSS dispersions. (a) Values for conductivities were measured in the presence of 5% (w/w) dimethylsulfoxide. Physical properties of Clevios PEDOT:PSS based films The physical properties of solid Clevios PEDOT:PSS based films are determined by a number of factors: 1. The ratio of PEDOT to PSS within the dispersion determines the film s conductivity and its optical absorption. Both properties will increase proportionally to the volume fraction of PEDOT. The PEDOT concentration in dispersion is limited on the other hand to approx. 0.5wt% otherwise the dispersion will become instable. A PEDOT to PSS ratio of approx. 1:2.5 is found to be the best compromise between high conductivity of the finished film and sufficient stability of the dispersion. 2. The size distribution of the particles in the dispersion affects conductivity and surface roughness. The mean value of size distribution can be shifted from 90 nm as in Clevios P down to 30 nm as in Clevios PH by applying

6 Page 6 shear stress to the dispersion. This size reduction decreases the film s conductivity from 1 S/cm to 0.3 S/cm. 3. The addition of high boiling solvents, also known as conductivity enhancers, will alter the film s morphology when drying. This will lead to lower energy barriers for charge carrier transport between the individual PEDOT:PSS clusters and generate higher conductivity. Other waterborne additives that can be added to the PEDOT:PSS dispersion such as crosslinking polymers, film forming polymers, plasticizers and stabilizers modify the film s mechanical properties and thermal stability. 4. The drying conditions will have an impact on the remaining water in the film and consequently mechanical properties and electrical properties, e.g. the work function will be affected. All these factors define the materials performance and allow tailoring the material with respect to its intended application. The optical properties of PEDOT:PSS make this material unique within the class of intrinsic conducting polymers. PEDOT in its neutral form has a broad absorption band in the visible spectral range similar to other well known semiconducting thiophene polymers, e.g. poly(3-hexylthiophene). In its doped form the absorption shifts to the infrared and the material becomes transparent. By changing the redox state of PEDOT electrically the film s transmission can be altered by applying a bias as it has been demonstrated e.g. in electrochromic devices. 11 For transparent conductive electrodes the uniformity of transmission within the whole visible spectral range and the ratio of conductivity to absorption are of upmost relevance. Figure 3 depicts the optical properties of highly conductive Clevios PEDOT: PSS.

7 Page 7 Figure 3: (a) Optical spectra of a 190 nm thick film of Clevios PH1000. Transmission, absorption and reflection spectra are normalized to 100%. (b) Transmission spectra of Clevios PH1000 films on glass substrates deposited at different layer thickness. The luminous transmittance Y of each sample is denoted accordingly. ICPs are often considered to suffer with respect to stability since organic molecules bear the inherent risk of degradation due to oxidation. Moreover, for some ICPs the doping is reversible and the loss of doping agent results in conductivity losses over time. In contrast, the neutral, low-band gap polymer PEDOT is easily oxidized and in the presence of acid a stable complex is formed. This reaction is not reversible. Stability tests performed under accelerated outdoor conditions show only a minor increase of film resistance even when exposed for 1000 hours at 85 C/85rH. No changes in sheet resistance are observed when the films are protected hermetically against air. Important to note: The integral current carried through the conducting films does not degrade the material s conductivity. Application of Clevios PEDOT:PSS as a transparent conductive film The development of PEDOT:PSS dispersions were triggered by the demand for a waterborne, transparent, conductive material to furnish photographic films with an antistatic coating. This has been the first application of PEDOT:PSS and was accomplished in the early 90s. For antistatic coatings a sheet resistance covering the wide range of Ohm/sq is required. Early PEDOT formulations fulfilled these requirements easily. The advantage to deposit the material by slot dye coating in a continuous process onto polymer foils and its high level of transparency made it the standard for ICPs in antistatic coatings. Such films are employed to protect items against high voltage electrical charges residing on packages used for storage of integrated circuits stored in transparent plastic trays or to avoid dust attraction, e.g. coated protection foils employed in the LCD production. An additional advantage comes here into play that PEDOT: PSS-coatings are clean room compatible in contrast to polymers filled that contain conductive particles such as carbon black. First applications of Clevios PEDOT:PSS as transparent conductive electrode replacing transparent conductive oxides (TCO), especially Indium Tin Oxide (ITO) were in thick film electroluminescent (EL) devices. The device set up consists of a several µm thick layer of ZnS-particles embedded in a polymer matrix sandwiched between a transparent and a metal electrode. By applying an AC-bias of typically V at a frequency of Hz the semiconducting particles emit and visible

8 Page 8 light. Owing to the fact that these EL lamps are not direct-current driven the sheet resistance of the transparent electrode requires conductivity in the order of Ohm/sq which is easily accessible by adding high boiling solvents to standard Clevios PEDOT:PSS. The possibility to silk screen print structured transparent electrodes offers an additional advantage over TCOs. In the past years the conductivity of PEDOT:PSS has been significantly improved. Clevios TM PH1000, a material dispersion, has been made available exhibiting a conductivity of 1000 S/cm. Applications that require electrodes with a sheet resistance in the order of Ohm/sq. at high transparency levels have become accessible. Figure 4 shows the luminous transmission Y depicted as a function of sheet resistance R/sq. The stars denote the correspondent layer thickness, e.g. a 100 nm thick layer has a sheet resistance of 100 Ohm/sq. and its Y is 91. For comparison the same plot has been calculated for ITO exhibiting a conductivity of 2000 S/cm on plastic substrates. Figure 4: Luminous transmittance of Clevios PEDOT:PSS and ITO layers normalized to the substrate as a function of sheet resistance. The calculation of Y is based on the materials spectral absorption and the layer

9 Page 9 thickness. Stars denote specific layer thickness of PEDOT: PSS-films in nm. Although the ratio of conductivity to absorption is larger for ITO the properties of Clevios PEDOT:PSS are already sufficient to meet the requirements of many applications. They include electrophoretic displays, touch panel screens and transparent electrodes for organic light-emitting diodes (OLEDs) and organic solar cells (OSCs). All these applications face a strong demand to lower the total costs especially for the transparent electrode. The deposition of the polymer from solution in a continuous rollto-roll process meets this requirement better than vacuum deposited TCO layers. Besides cost, the polymer layers offer an additional feature that cannot be met by TCOs. Polymers are flexible and will not crack under bending as brittle TCO-layers do. In touch panel displays this is an important property particularly in case of glass-free devices that are expected to withstand high mechanical stress. In need of high current densities transparent electrodes, ICPs can be combined with metallic bus-bar lines to avoid a voltage drop across the electrode. A wide mesh of thin metallic lines is deposited onto the substrate first to distribute the current uniformly. The transparent areas in between the lines are made conductive by a layer of Clevios PEDOT:PSS coated on top. This concept has been proven to be reliable even on large aspect ratios of transparent to non-transparent areas e.g. for OLED-lamps or Organic Solar Cells. 12,13 Another function of PEDOT:PSS films is exploited in OLEDs and OSCs. Here the pristine material and special designed Clevios TM formulations functions as a buffer layers being placed between the electrode and the organic semiconductor. The work function of these layers can be tuned between ev facilitating the injection or extraction of holes in OLEDs or OSCs respectively. Apart of this electrical functionality, this layer planarizes or smoothes the surface of the substrate. Frequently, ITO-electrodes suffer spikes due to their production process and crystalline nature. These spikes may stick out of the surface and cause electrical shorts in the finished device. By planarizing the ITO with a thin PEDOT-layer the number of electrical shorts is reduced and consequently the yield of well-functioning devices will improve.

10 Page 10 Summary Dispersions of Clevios PEDOT:PSS have developed from antistatic layers into new applications of transparent conducting electrodes. The conductivity and transparency of this versatile material have been increased in recent years to a level that makes applications in devices such as displays, touch panel screens and solar cells possible. Clevios PEDOT:PSS enables easy deposition on mechanically flexible substrates and the films will not crack when bent. Due to its aqueous dispersion nature various deposition techniques can be applied including roll-to-roll coating that reduces production costs. Lastly, the material is environmentally friendly as it is water based and does not contain any hazardous elements or compounds. Foreseeable, the demand for touch panels increases rapidly applications not only on mobile phones but computer systems, touch information systems and even as a replacement of membrane switches. As this innovation gathers pace in the display industries reliability and cost will play an increasingly important role in the process. Clevios PEDOT:PSS materials solve this challenge. With the support of our new parent company Heraeus Precious Metals we commit ourselves be at the center of the new generation of electronics. References 1. C. K. Chiang, C. R. Fincher, Jr., Y. W. Park, A. J. Heeger, H. Shirakawa, E. J. Louis, S. C. Gau, A. G. MacDiarmid, Phys Rev Lett (1977). 2. H. Shirakawa, E. J. Louis, A. G. MacDiarmid, C. K. Chiang, A. J. Heeger, J Chem Soc, Chem Commun. 578 (1977). 3. A. J. Heeger, J Phys Chem B (2001). 4. F. Jonas and G. Heywang. DE (Bayer AG), Prior: April 22, A. Elschner, S. Kirchmeyer, W. Loevenich, U. Merker, K. Reuter, PEDOT: Principles and Applications of an Intrinsically Conductive Polymer page 126 (CRC Press, Boca Raton, Fl, USA ISBN ) 6. L.Groenendaal, F.Jonas, D. Freitag, H. Pielartzik, J.R. Reynolds, Adv. Mater (2000)

11 Page S. Kirchmeyer, K. Reuter, J. Mater Chem (2005) 8. Y.Kudoh, K. Akami, Y. Matsuya, Synth. Met (1998) 9. N.Karibyants, H. Dautzenberg, H. Cölfen, Marcomolecules (1997) 10. S. Timpanaro, M. Kemerink, F.J.Touwslager, M.M.De Kok, S. Schrader, Chem. Phys. Lett (2004) 11. H.W. Heuer, R. Wehrmann, S. Kirchmeyer, Adv.Funct. Mater (2002). 12. K. Fehse, K. Walzer, K. Leo, W. Lövenich, A. Elschner, Adv. Mater. 19, 441 (2007). 13. Y. Galagan, J.E.J.M. Rubingh, R. Andriessen, C.C. Fan, P.W.M. Blom, S.C. Veenstra, J.M. Kroon, Sol. Energy Mater. Sol. Cells (2011) Heraeus, the precious metals and technology group headquartered in Hanau, Germany, is a global, private company with more than 160 years of tradition. Our fields of competence include precious metals, materials, and technologies, sensors, biomaterials, and medical products, as well as dental products, quartz glass, and specialty light sources. With product revenues of 4.8 billion and precious metal trading revenues of 21.3 billion, as well as more than 13,300 employees in over 120 subsidiaries worldwide, Heraeus holds a leading position in its global markets. For additional information, please contact: John Bayley Sales and Marketing Manager Europe Conductive Polymers Division Heraeus Precious Metals CHEMPARK Leverkusen Leverkusen, Germany Phone +49 (0) Fax: +49 (0) john.bayley@heraeus.com