Flexible and Transparent SWCNT Electrodes for Alternating Current Electroluminescence (ACEL) Devices
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Flexible and transparent SWCNT electrodes for alternating current electroluminescence (ACEL) devices Christian Schrage1 and Stefan Kaskel1 1 Dresden University of Technology, Mommsenstrasse 6, Dresden, 01062, Germany
ABSTRACT The application of transparent single-walled carbon nanotube (SWCNT) electrodes in rigid and flexible alternating current electroluminescence (ACEL) devices is demonstrated. SWCNT thin film electrodes (50 – 160 nm) were made using a spray-coating process suitable for adjusting transparency and sheet resistance. The emission intensity was as high as for indium tin oxide (ITO) based ACEL devices with transparencies comparable to ITO coated polymer slides. INTRODUCTION Because of their high resolution and brightness, uniform light emission and low power consumption together with the possibility of a thin architecture (60 – 100µm), alternating current electroluminescence devices (ACEL) have a high potential for commercial application.[1] An efficient EL-phosphor (usually doped zinc sulfide) sandwiched between two electrodes is excited in a strong electric field. Dopant materials (Cu, Al, Cl, Mn etc.) and phosphors allow color tuning over the whole visible range, and even the creation of white light emitting devices has been reported.[2] Up to now, transparent conducting oxides such as indium tin oxide (ITO) electrodes prepared by cost intensive sputtering techniques constitute the electrode through which the light is extracted. However, low temperature deposition techniques compatible with flexible polymer substrates lead to higher sheet resistances and surface roughness of the ITO electrode.[3] Moreover, flexibility of ITO layers is limited leading to cracking and delamination after repeated bending.[4] Referring to these facts and the extremely increasing demand on indium leading to a strong price advance, transparent single-walled carbon nanotube (SWCNT) electrodes seem to be a valuable alternative for ITO based electrodes for applications with moderate requirements on conductivity. The recent attention on single-wall carbon nanotubes driven from their exceptional electrical and mechanical properties has led to a large number of applications in the area of optoelectronics and macroelectronics. A full exploitation of the materials potential has been accomplished through an effective debundling and dispersion of SWCNT aggregates usually obtained from the production process. Surfactant-assisted ultrasonication is a method to obtain aqueous dispersions with sufficiently high concentrations for solution casting of SWCNT films. So far, a variety of coating processes such as spin-coating [5], drying from solvent [6], vacuum filtration [7] or spray-coating [4] were proposed in order to produce SWCNT thin films. The resulting two-dimensional random SWCNT networks are highly conductive and mechanically robust, while transparency is retained. In our study we adopt the spray-coating process to investigate the use of transparent SWCNT electrodes in ACEL device structures.
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