Organic Light Emitting Devices Fabricated from Semiconducting Nanospheres
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Organic Light Emitting Devices Fabricated from Semiconducting Nanospheres Thomas Piok1,2, Franz P. Wenzl1, Stefan Gamerith1,2, Christoph Gadermaier1,2, Satish Patil3, Rivelino Montenegro4, Thomas Kietzke5, Dieter Neher5, Ullrich Scherf 3, Katharina Landfester4 and Emil J.W. List1,2 1 Christian Doppler Laboratory Advanced Functional Materials, Institute of Solid State Physics, Graz University of Technology, Petersgasse 16, A-8010 Graz, Austria. 2 Christian Doppler Laboratory Advanced Functional Materials, Institute of Nanostructured Materials and Photonics, Franz-Pichler-Strasse 30, A-8160 Weiz, Austria. 3 Department of Chemistry, BUGH Wuppertal, Gauss-Str. 20 D-42097 Wuppertal, Germany. 4 Max Planck Institute of Colloids and Interfaces, Research Campus Golm, D-14424Potsdam Germany. 5 Institute of Physics, University of Potsdam, Am Neuen Palais 10, D-14469 Potsdam, Germany
ABSTRACT Organic light emitting diodes (OLEDs) have been fabricated from organic semiconducting polymer nanospheres (SPNs) which have been deposited from aqueous dispersions. The active layer of the devices consists of a single, homogeneous layer of light emitting SPNs, as verified by optical, interferometric and surface probe measurements. Different batches of SPNs with different SPN diameters have been tested (69nm, 95nm, 126nm and 150 nm). All SPN-based OLEDs exhibit a light emission onset corresponding to the SPN energy gap (ca. 2.7 eV for mLPPP, a semiconducting para-phenylene ladder polymer). The low onset is attributed to field enhanced injection of charge carriers at the aluminum cathode due to the formation of stalactitetype nanostructures. A detailed comparison of the SPN-based and bulk semiconducting polymer films reveals no differences in the basic optoelectronic properties. INTRODUCTION The trend towards nano-scale organic electronics raises a quest for novel materials and concepts which allow for the controlled deposition of the active material on the nano- to mesoscopic scale. Gaining control over the material at such a scale will enable the fabrication of novel as well as improved devices from organic semiconductors such as conjugated polymers in a cost-effective way. As demonstrated over the last decade, conjugated polymers can be utilized as the active medium in organic light emitting diodes (OLEDs) [1], light-emitting electrochemical cells (LECs) [2], solar cells [3], photo detectors [4], lasers [5], field-effect transistors [6], and allpolymer integrated electronic circuits [7]. By adopting the so-called miniemulsion process one can combine the properties of semiconducting polymers with those of nanostructured matter as shown recently [8]. Miniemulsions are understood as stable emulsions consisting of droplets with a distinct size of 50 - 500 nm, made by shearing a system containing oil (solvent), water, a small amount of a surfactant, and a highly water insoluble compound (e.g. a polymer), the so-called hydrophobe [9]. Polymer particles can be obtained from such droplets after evaporation of the solvent. Using such an
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