DOI: 10.1002/adma.200602575 Microscopic Understanding of the Anisotropic Conductivity of PEDOT:PSS Thin Films** By Alexandre Mantovani Nardes , Martijn Kemerink,* René A. J. Janssen, Jolanda A. M. Bastiaansen, Nicole M. M. Kiggen, Bea M. W. Langeveld, Albert J. J. M. van Breemen, and Margreet M. de Kok Conjugated polymers combine the advantageous character- istics of conventional polymers, such as low weight, process- ability, and flexibility, with the functional physical properties of conventional semiconductors, such as absorption and emis- sion of light and a tunable conductivity, to provide innovative materials for (opto)electronic devices. In polymer light-emit- ting diodes, solar cells, memory storage devices, and field-ef- fect transistors, as well as in other high-volume, low-cost applications of plastic electronics, there is a need for polymers with high conductivity to act as charge transport layer or elec- trical interconnect. In the large majority of devices, this role is played by poly(3,4-ethylenedioxythiophene):poly(styrenesul- fonate) (PEDOT:PSS), a blend of an oxidatively doped, cat- ionic, conducting polythiophene derivative (PEDOT) that is electrostatically bound to a PSS polyanion. [1] PEDOT:PSS combines high conductivity and good transparency in the visi- ble region with excellent stability under ambient conditions and can be easily processed from aqueous dispersions by spin coating. A common feature of conducting and semiconducting p-conjugated polymers is the strong influence of morphology and, hence, processing conditions on the material properties. This subject has been extensively investigated for semicon- ducting polymers. [2] Despite the ubiquitous use of PEDOT:PSS, relatively little attention has been given to the basics of the charge transport in this material. [3,4] The lateral conductivity of PEDOT:PSS blends has been addressed for various compositions with variation of pH, [3] solvents, [5] and water content. [6] From the observed temperature dependence, it was concluded that lateral electric conductivity occurs via hopping of charge carriers. However, a detailed understand- ing of the role of the morphology in this process is currently lacking. Moreover, possible anisotropies in the conductivity have not been systematically addressed, despite the observed anisotropy in the optical response. [7] A recent conductive atomic force microscopy (AFM) experiment on 1 lm thick drop-cast PEDOT:PSS films suggests that the charge trans- port perpendicular to the substrate is dominated by space– charge effects, in striking contrast to the transport parallel to the substrate. [4] For the use in pixilated displays, the issue of in- versus out-of-plane anisotropy is of utmost importance since in-plane conductivity causes crosstalk between neigh- boring pixels, whereas the out-of-plane conductivity has a di- rect effect on the contact resistance. The currently accepted morphology of spin-cast PEDOT:PSS films is that of a phase segregated material con- sisting of PEDOT:PSS grains surrounded by a shell formed by excess PSS. [8] The thickness of the grain boundary has been found to be about 30–40 Å. [9,10] The PEDOT-rich core of the grains has a much higher intrinsic conductivity than the PEDOT-depleted grain boundary, which is essentially insulat- ing because PSS is only a weak ionic conductor. Consequently, the main obstacle is to transport electric current between the PEDOT-rich grains, [3,11] while the electronic current is easily transported within the grains. Although this description ratio- nalizes some of the properties of PEDOT:PSS thin films, a de- tailed and consistent correlation between morphology and electrical properties has not yet been established. In the present study we combine scanning probe microsco- py with macroscopic conductivity measurements to come to a full 3D morphological model that explains the observed an- isotropic conductivity of spin coated PEDOT:PSS thin films. In particular, we find that the vertical conductivity, that is, per- pendicular to the substrate, can be up to three orders of mag- nitude lower than the lateral conductivity in the plane of the film. Observation of pancake-shaped PEDOT-rich islands separated by lamellas of PSS in cross-sectional AFM and to- pographic scanning tunneling microscopy (STM) images ex- plains not only the large difference in conductivity, but also the difference in the conduction mechanism as observed in temperature-dependent conductivity measurements. Despite COMMUNICATION 1196 © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2007, 19, 1196–1200 – [*] Dr. M. Kemerink, A. M. Nardes, Prof. R. A. J. Janssen Molecular Materials and Nanosystems Department of Applied Physics Eindhoven University of Technology PO Box 513, 5600 MB Eindhoven (The Netherlands) E-mail: m.kemerink@tue.nl J. A. M. Bastiaansen, N. M. M. Kiggen, Dr. B. M. W. Langeveld, Dr. A. J. J. M. van Breemen TNO Science and Industry Multilayer Devices and Coatings De Rondom 1 PO Box 6235, 5600 HE Eindhoven (The Netherlands) Dr. M. M. de Kok Philips Research Laboratories Prof. Holstlaan 4 WAG 1.514 5656 AA Eindhoven (The Netherlands) [**] A.M.N. would like to thank the Alßan Program (the European Union Programme of High Level Scholarships for Latin America, ID#E03D19439BR) and the Eindhoven University of Technology (TU/e) for their financial support.