Progress in Science and Technology of Polyaniline

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Progress in Science and Technology of Polyaniline

Progress in Science and Technology of Polyaniline and Poly-ethylenedioxythiophene B. Wessling,* Ormecon Chemie GmbH & Co. KG, Ferdinand-Harten-Str. 7, D-22949 Ammersbek, Received June 30, 2002; accepted Abstract New results in morphology, conductivity, dispersion and technological applications for PAni are presented. A dispersible PAni powder with > 200 S/cm has been created. The first dispersible PEDT powder is available. Keywords: Dispersion, self-organisation, neutron scattering, UV-VIS-NIR, metallic films, Polyaniline, Polythiophene

1. Background Conductive polymers (CP) have been impossible to process, until dispersion techniques had been developed by us [1]. and later other groups, too. Alternative approaches to solving the processing problem, like producing true solutions, polymerizing moldable CP or sterically stabilized colloids [2], have basically failed or did not meet the requirements for industrial applications, like reproducibility or longterm stability. Actually, there are three commercially available CP, all of them insoluble and infusible, provided by Bayer AG (poly-ethylenedioxy-thiophene, PEDT) [3], available in form of a water-based colloidal dispersion [4], by Panipol Inc. (a polyaniline, protonated with dodecylbenezene sulfonic acid, and complexed with Zndodecylbenzene sulfonate) [5], available in powder form to be dispersed in polyolefines [6], and by Ormecon Chemie (a polyaniline protonated with p-toluene sulfonic acid) available as a dispersible powder and in predispersed form for applications in all kinds of media from unpolar to polar ones, e.g. xylene, alcohols, water, paints and various polymers [7]. Insofar, the "dispersion hypothesis" as created end of the 70s and later theoretically well supported [8], has been challenged by many different practical tasks, and has met industrial requirements which can be considered a significant progress, as it is also in line with scientific conclusions. More recently, Ormecon's polyaniline has crossed the "insulator-to-metal transition" during a special dispersion step [9] and can be considered a true organic metal, though still resembling an amorphous and a nanometal, due to its poorly crystalline structure and nanoscopic morphology (10 nanometer small primary particles). Applications have been developed and are in use in industrial scale in printed circuit board manufacturing (solderable surface finish), hole injection in OLEDs, corrosion protection, conductive and antistatic (transparent) coatings and others. New applications are being introduced based on recent scientific and technological progress. 2. Morphology The morphology of our dispersible polyaniline powder ORMECONÔ is now fully understood. Small angle x-ray scattering [10] revealed that the primary particle with a particle size of around 10 - 15 nm are built from 3.5 nm diameter units, which are not identical with individual molecules, as can be seen from MALDITOFF experiments [11]: polyaniline exhibits a broad molecular weight distribution, starting with > 16 monomer units, up to > 100 units (Mw > 10,000). This broad Mw distribution is www.bernhard-wessling.com/pani/www2/Research/ICSM2002/vortrag_wessling.htm

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understandable from the fact, that the polymerisation is carried out in a heterogeneous way, i.e., the chains are growing on insoluble particles generated in the first stage of the polymerisation, whereby monomer and oxidant are soluble in the acid water reaction medium. The 3.5 nm subunits, confirmed by recent preliminary neutron scattering experiments [12], had also been detected earlier in emeraldine base dispersions in NMP and measured by Laser-Doppler technique [13]. As we had shown with the results of microwave absorption experiments, that the metallic unit of the primary particle in the emeraldine salt is in the range of 8 nm in diameter [14], we conclude now that the morphological subunit, composed from many molecules of different length, is a 3.5 nm unit. In the protonated, metallic form, they aggregate further to particles with 10 - 15 nm diameter (as proven by SAXS [ cf 10] and AFM [ 15]) which are the "primary particle" of the organic metal, the morphological entity of the conductive form of polyaniline, which is the smallest conductive unit (with a coherent electron "gas" within the 8 nm metallic core) and the smallest particle size accessible by dispersion. Above the morphological subunit and the primary particle, there is a well developed "hierarchy" of morphological units, i.e. agglomerates of the primary particles. The size of the agglomerates is not random, but seems to follow certain rules. By SAXS, we can detect all these defined sizes, namely 30, 40, 50, 80, and 100 nm. These occur (and can be detected by independant methods) as dominant units in water (30 nm), paints (50 and 80 nm) and thermoplastic polymer blends (100 nm). 3. Hole injection in OLEDs Polyaniline has first been proposed for use as hole injection layer (HIL) in OLEDs many years ago [16]. However, it did not get any broader attention, nor was it practically used mainly due to very poor reproducibility of the products. The reason for the failure can be assigned to the strategy applied: to produce true solutions. These systems (which in fact also were colloidal dispersions) tend to gel so that neither the particle size of the PAni to begin with, nor during storage time, even not the processability due to gelation was stable. Therefore, research groups as well as industrial development teams did not focus on polyaniline, but on PEDT (Baytron P water dispersion) which established as a standard HIL. Only when we began to introduce water based dispersions of PAni with a defined particle size, it was possible to work with an alternative and to evaluate the scope of potential improvements. We achieved an order of magnitude smaller particle size than the commercial PEDT dispersion (Fig. 1). This allows to apply much smaller HIL layer thicknesses, i.e. reduce the layer thickness from 150 - 200 nm to about 50 nm, and increase performance. Fig 1. Particle size distribution of PAni D 1005 W and PEDT (here: Baytron P) water dispersions

We have attempted to evaluate the basic reason for differences in performance between various PAni products and between these and PE DT as HIL [17]. For this purpose, we prepared different grades of PAni and measured their conductivity (by impedance spectroscopy), work function (by Kelvin probe) and luminescence [18]. As a result we concluded preliminarily, that (for Covion's "SuperYellow") an optimal range of work function seems to be around 4.9 eV (Fig 2). Fig. 2. Work function of Covion's "SuperYellow (SY), two PEDT grades, various PAni grades grouped by performance, and ITO. www.bernhard-wessling.com/pani/www2/Research/ICSM2002/vortrag_wessling.htm

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The work function of ITO was determined to be - 4.6 eV, for PEDT around - 4.85 eV, for PAni grades showing a performance of < 100% (compared to standard PEDTHIL) around (or greater than) 4.4 eV, PAni grades with a performance of about 100% at - 4.8 or - 5.05 eV, whereby grades exhibiting a performance of > 100% showed a tight distribution of work function values around 4.9 eV. As HIL for Polyspiro Blue and Green, a performance increase of 13% and 26%, resp., was found compared to standard PEDT-HIL. This is in line with the consideration of a HIL offering a work function less negative than ITO. For passive matrix displays, it is important to avoid any "cross talk" between neighboured pixels. This requires a conductivity as low as possible, without affecting the HI efficiency. Whereas previous commercial (PEDT) HIL offer a conductivity of slightly higher than 10-5 S/cm, we have now achieved values well below 10-6 S/cm, without deterioration of the luminescence efficiency. 4. Towards higher conductivity It is a common assumption that only with certain counterions and secondary dopants a conductivity of > 102 S/cm can be achieved [19]. Based on earlier published work [9], we had suspected that a conductivity higher than in the 101 S/cm range should be possible also if one would not use the "secondary doping" process involving camphor sulfonic acid and m-cresole, or comparable approaches. These processes did not only prove to be quite irreproducible, but also fully unacceptable for industrial applications (due to toxicity of the secondary dopants and lack of thermal stability). Recently we succeeded in finding synthesis conditions for a new PAni powder grade and suitable dispersion parameters for it leading to specific conductivity values well above > 102 S/cm, without any secondary dopant involved, and using p-toluene sulfonic acid for protonation [20]. We achieved to disperse this new grade in colloidal form in xylene, suitable to be spin coated on various substrates. It was used in first experiments for replacement of vacuum sputtered metals as source, drain and gate in organic field effect transistors. The results have been very promising as there was no difference in the FET performance when using the higher conducting polyaniline instead of vacuum sputtered conventional metals [21]. Another challenging demand has been the formulation of highly conducting screen printing inks (e.g. xylene based) in two specifications, one as transparent as possible, the other opaque but light coloured. Such paints are being printed on various substrates (like PET or PC film or glass) and serve as front and back electrode, resp., in electroluminescent (EL) displays, where the EL active species is not a semiconductor, but a properly doped inorganic pigment (ZnS), and instead of a DC current (as in OLEDs), an AC bias is applied, e.g. 400 Hz@12 V. Such displays consume almost no power. Such displays are not as bright as OLEDs, but can serve as backlights. With the new screen printing inks involving a dispersed 200 S/cm PAni powder, it was possible to replace ITO as front electrode and carbon black containing inks as back electrode, thereby increasing light emission by orders of magnitude, and even more important: www.bernhard-wessling.com/pani/www2/Research/ICSM2002/vortrag_wessling.htm

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providing a composite which can be vacuum-formed and back-injection molded. Hence, all kinds of illuminated products are now possible to be manufactured, from license plates for cars, velocity and motor rpm display backlights and switches, up to mobile phone covers and large area displays. 5. The first dispersible PEDT powder Apart from polyaniline which is available in dispersible form, there is a continuous need for a generally dispersible PEDT powder. and a scientific debate whether such a powder is principally feasable. It was stated earlier by other research groups, that PEDT, when polymerized in form of a dry powder, is absolutely insoluble and undispersible [cf 4]. However, when applying the synthetic and dispersion principles described in previous papers [cf 8, 9] to the polymerisation of EDT and the dispersion of a dry PEDT powder recovered from it, it is possible to make dispersions in all kinds of media, e.g. in thermoplastic polymers or (without any binder) in xylene or screen printing inks. The conductivity of the resulting pure PEDT layers is in the range of 5 - 10 S/cm, the surface resistance of a transparent screen printed layer around 10-3 S/cm. In light of earlier publications [cf 4], according to which the transparency of PEDT is higher than a comparable PAni layer, we have measured the VIS absorption up into the NIR. The optical appearance of comparably thin (1.5 µm) layers does not resemble a "pale blue", but in contrast exhibits a very strong blue, which (except for the range between 400 and 500 nm) shows a stronger absorbance at all wavelengths. It is evident, that the new higher conducting PAni (dispersion in screen print ink) allows for a transmission between 45 and close to 80%, around 65% in average. PEDT in a comparable ink formulation, however, offers a transmission between 70 and 35%, less than 50% in average (fig. 3).

Fig 3. VIS absorption of PAni and PEDT screen print inks (1.5 µm thickness)

Obviously, the absorption ("pale blue" transmission) of PEDT-PSS (deposited from water dispersion) can not be generalized to any PEDT dispersion. Actually, PAni dispersions offer a higher conductivity and a more favorable conductivity / transmission relation. On the other side, there is plenty of room for improvement in PEDT conductivity, dispersion and conductivity. 6. Experimental Synthesis and dispersion have been performed according to principles described earlier [cf 9]. Neutron scattering was made on blends of PAni in PMMA (as described in [9]) equilibrated in D2O atmosphere to provide contrast. Particle size distribution was measured by Laser Doppler technique with a Microtrac UPA 150 Particle Analyzer. Conductivity investigations were performed by Fourier Transformation Electrochemical Impedance Spectroscopy (FT-EIS) with an EIS model 6416B. Impedance spectra were obtained at open circuit potential (E » 0 V) by evaluation of perturbation and response signal by FT. Data analysis was carried out using an Rs and C in parallel equivalent circuit (Rs = sheet resistance; C = capacitance of the cell). Work function was determined using a Scanning Kelvin Probe (SKP, UBM Messtechnik GmbH) in a chamber equipped with silica gel giving a relative humidity of 0%. As vibrating reference electrode a Cr/Ni wire with a tip diameter of 80 µm was www.bernhard-wessling.com/pani/www2/Research/ICSM2002/vortrag_wessling.htm

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used. The tip was positioned about 20 µm above the specimen, the vibration amplitude was ± 10 µm and the vibration frequency of the needle was 1.75 kHz. Acknowledgements The results presented here have been achieved by the continuous collaboration within our Ormecon team. Thanks go especially to Dr. B. Werner, Dr. J. Posdorfer, S. Lehmann, S. Nissen and S. Gleeson. We are grateful for the productive collaboration with Covion Organic Semiconductors and Avecia. Thanks go to Prof. M. Winokur for arranging the neutron scattering experiments at the Chicago Fermilab. References 1] B. Wessling, in T. Skotheim, R. Elsenbaumer, J. Reynolds, (eds.), Handbook of Conducting Polymers, Marcel Dekker, Inc., 1998, p. 467. [ 2] cf. various authors in H. S. Nalwa, (ed.) Handbook of Organic Conductive Molecules and Polymers, J. Wiley & Sons, 1997, and T. Skotheim, R. Elsenbaumer, J. Reynolds, (eds.), Handbook of Conducting Polymers, Marcel Dekker, Inc., 1998. [ 3] cf. Baytron P technical information, Bayer AG, Leverkusen, Germany. [ 4] L. B. Groenendaal, F. J. Jonas, D. Freitag, H. Pielartzik, J. R Reynolds, Adv. Mater. 12, No. 7 (2000) 481. [ 5] cf. Panipol technical information.. [ 6] Bayer AG and Panipol are working under license provided by Zipperling Kessler / Ormecon Chemie. [ 7] cf. technical information by Ormecon Chemie. [ 8] a) B. Wessling, Synth. Met. 45 (1991) 119. b) B. Wessling, J. Phys. Chem. 191 (1995) 119. c) B. Wessling, in B. Wessling in: Handbook of Nanostructured Materials and Nanotechnology, H. S. Nalwa, (ed.), Academic Press, 1999, Vol. 5, Chapter 10, Section 2., p. 525 . [ 9] B. Wessling, D. Srinivasan, G. Rangarajan, T. Mietzner, W. Lennartz, Eur. Phys. J. E 2 (2000) 207. [10] W. Lennartz, T. Mietzner, G. Nimtz, B. Wessling, Synth. Met. 119 (2001) 425. [11] Bev Brown (Avecia), personal communication. [12] M. Winokur, B. Wessling, unpublished results. [13] Wessling, B., in: H.S. Nalwa (ed), Handbook of Nanostructured Materials, Vol. 5, Academic Press, 1999, p. 525. [14] R. Pelster, G. Nimtz, B. Wessling, Phys. Rev. B 49, No. 18 (1994) 12718. [15] B. Wessling, R. Hiesgen, D. Meissner, Acta Polymer. 44 (1993) 132. [16] Y. Yang, A. J. Heeger, Appl. Phys. Lett. 64 , No. 10 (1994) 1245. [17] B. Werner, J. Posdorfer, B. Wessling, S. Heun, H. Becker, www.bernhard-wessling.com/pani/www2/Research/ICSM2002/vortrag_wessling.htm

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H. Vestweber, T. Hassenkam, presentation at SPIE 2002, Seattle, July 2002, to be published. [18] conductivity and work function measured at Ormecon, luminescence performance at Covion Organic Semiconductors. [19] A.G. MacDiarmid, A.J. Epstein, Synth. Met. 65, No. 2-3 (1994) 103. [20] experimental details to be published later, general process information cf [9]. [21] J. Veres (Avecia), personal communication.

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