Solvent Processible Composite Carbon Nanotube Cathode for Polymer LED Applications
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Solvent Processible Composite Carbon Nanotube Cathode for Polymer LED Applications Gao Liu, Steve Johnson and John B. Kerr Environmental Energy Technology Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Rd., Berkeley, CA 94720, U.S.A. ABSTRACT Composites of single wall carbon nanotubes (SWNT) and conductive polymers were studied as potential cathode materials for application in polymer light emitting devices. A new conductive poly(2,7-9,9 (di(oxy-2,5,8-trioxadecane))fluorene) (PFO) possessing surfactant properties was used to stabilize SWNTs in solutions. The rigid PFO backbones act as a template while the ethyleneoxide side chains appear to wrap around the SWNTs. Up to 0.02% (by weight) of SWNTs are stabilized in the solution phase. The current vs. voltage behavior of the SWNT/PFO composite film (2% SWNT in PFO by weight) shows that most of the current is carried by SWNTs. INTRODUCTION Organic light emitting materials have attracted increasing interest in the past two decades.1 Soluble light emitting polymers are even more appealing for manufacture of large area, low cost light emitting devices.2 In recent years, screen and inkjet printing have been successfully applied to this area, promising lower cost for area lighting, patterning and display applications.3 In order to achieve low cost and high efficiency, it is crucial to have an air stable cathode with efficient electron injection properties. Presently, efficient electron injection for polymer device is achieved by using a low work function metal such as Calcium or Magnesium, which are very reactive towards oxygen and moisture. Therefore, development of air stable cathode composite materials with good electron injection properties is a key step for lowering the overall cost of OLED devices. Single wall carbon nanotubes (SWNT) have been demonstrated to be viable as electron injection material for application in OLEDs.4 However, the applications are limited by the available materials to render the SWNT soluble as well as its compatibility to the light emitting materials. Poly(m-phenylene-vinylene-co-2,5-dioctyloxy-p-phenylene-vinylene) (PmPV) is so far the only conducting polymer which has been found to be able to wrap around the SWNT to stabilize the tubes in solution phase.5 Low solubility of SWNT in other conductive polymer systems is one of the major obstacles for wide range applications. The established methods of dissolving carbon nanotubes are either using surfactants or polymer wrapping agents.6 Those molecules are able to associate on the nanotube surface. The carbon nanotube concentrations are typically 0.1-0.01% (by weight) in these surfactant stabilized systems. In situ polymerization of the surfactant in the solution system can provide a permanent coating on the nanotube. A new poly(2,7-9,9-(di(oxy-2,5,8-trioxadecane))fluorene) (PFO) (Figure 1) conductive polymer has been synthesized that possesses surfactant properties. The surfactant conductive polymer is able to stabilize the SWNT dispersions in organic solutions at 0.02% (by
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