Semiconductive Polymer Blends: Correlating Structure with Transport Properties at the Nanoscale** By Cristian Ionescu-Zanetti, Adam Mechler, Sue A. Carter, and Ratnesh Lal* Conjugated polymer blends are the focus of scientific and commercial interest due to their wide range of applications, from organic light-emitting diodes (LEDs) [1] and organic polymer electronics [2] to chemical sensors, [3] biological sen- sors, [4] and microactuators. [5] Poly(3,4-ethylenedioxythio- phene) (PEDOT) is a semiconducting polymer that becomes conductive when doped with either poly(styrene sulfonate) (PSS) or other charged groups, such as PF 6 ± . PEDOT±PSS is a member of a class of polymers used as charge-injecting elec- trodes. In particular, it holds promise for applications such as polymer LEDs [1] and solar cells. [6] For electro-optical devices such as LEDs and solar cells, improving efficiency requires a detailed understanding of the molecular superstructure of the blend and control of transport properties. [7] The role of inter- faces (e.g., indium tin oxide (ITO)±polymer) in injection effi- ciency was recently highlighted, [8] and previous scanning tun- neling microscopy (STM) studies suggest that the polymer morphological inhomogenities strongly influence the charge carrier injection and transport properties. [9] For structural studies, macroscopic methods such as X-ray diffraction (XRD) are often employed. [10] These methods average over a large area, typically over 1 mm 2 , giving a view of the overall structural properties. The paracrystalline struc- ture of tosylate doped PEDOT blends was shown by XRD. [10] Anisotropies in sample conductivity have been determined by bulk methods, [11] and comparison with X-ray data demon- strates that the orientation of the lamellae shows a clear correlation with the conductivity±anisotropy. In particular, conductivity is higher parallel to the plane of lamellar orienta- tion. There is a lack of information, however, about the local morphology of the blend, as well as about the effect of struc- tural features on charge injection at the polymer surface. Atomic force microscopy (AFM) is a widely used surface characterization tool. In addition to morphological character- ization, AFM also has spectroscopic capabilities. Phase imag- ing AFM (PI-AFM) is sensitive to the nature of the tip±sur- face interaction, and therefore to the material composition. [12] In addition, when combined with the new high-sensitivity con- ductive working mode (C-AFM), [13] local structural, composi- tional, and transfer properties can be studied on a nanometer scale. Unlike transmission electron microscopy (TEM) mea- surements, [14] C-AFM allows imaging morphology in parallel with local currents. Recording such a multi-dimensional map assures that matching sets of the different sample properties are collected. In the present study we apply multimodal AFM to map the local charge transfer properties in correlation to the molecu- lar superstructure of the polymer blend. We identify the struc- tural basis of the correlation between charge injection effi- ciency and local blend composition. At the polymer surface, our results show that efficient charge injection occurs in re- gions where the lamellar edges are exposed to the probe. This correlation suggests that the efficiency of charge injection at the polymer±electrode interface can be enhanced by control- ling lamellar orientation. Large-scale scanning electron microscopy (SEM) imaging of the PEDOT±PSS surface (Fig. 1) reveals an anisotropic sur- face, in terms of metallic properties. Electron-rich regions vary in size from 5±30 lm (bright regions, Fig. 1). The PEDOT±PSS blend has an average thickness of 1 lm (marked A on the SEM COMMUNICATIONS Adv. Mater. 2004, 16, No. 5, March 5 DOI: 10.1002/adma.200305747  2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 385 ± [*] Prof. R. Lal, Dr. C. Ionescu-Zanetti, Dr. A. Mechler [+] Neuroscience Research Institute, University of California Santa Barbara, CA 93106 (USA) E-mail: lal@lifesci.ucsb.edu Dr. S. A. Carter Physics Department, University of California Santa Cruz, CA 95064 (USA) [+] Dr. A. MechlerisonleavefromtheResearchGrouponLaserPhysics of the Hungarian Academy of Sciences, Szeged, Hungary 6701. [**] The authors thank M. Lefevre, R. Puestow, and P. Harris of Digital Instruments for access to conductive AFM hardware. We also thank Luisa Bozano and Campbell Scott (IBM Almaden) for helpful dis- cussions. This work was supported by Phillip Morris External Grant Program and NIH (CIZ, RL), NSF-NATO (AM, RL), BIOSTAR-UC- ABT Inc. (CIZ, RL) NSF grant ECS0101794 (SAC). - PEDOT-PSS A B - ITO Drive Signal Tip Displacement ϕ ϕ(x,y) Pt/Ir probe i v i(x,Y) Figure 1. A schematic representation of the AFM imaging modes em- ployed. A) In C-AFM, a conductive tip is biased with respect to the sam- ple at a voltage V . Current measurements are taken while the tip raster scans the surface and assembled into a map of current intensity. B) Phase images (PI-AFM) are obtained in tapping mode AFM. The phase shift between the piezo drive signal and the cantilever oscillation is measured at each point and assembled into a phase image of the sur- face.