Composite from Conjugated Polymers and Carbon Nanotubes: a novel material for molecular optoelectronics
S.Currana, P.Ajayanb, W.J. Blau*, a, D. Carrollc, J. Colemana, A. Daltona, A.P. Daveya, A. Drurya, B. McCarthya, S.Maiera, and A. Strevensa
aPhysics Department, Trinity College Dublin,
Dublin 2, Ireland bMaterials Science and Engineering Department, Rensselaer Polytechnic Institute Troy, New York, USA cDepartment of Physics and Astronomy, Clemson, South Carolina, USA
As research progresses towards smaller and more efficient devices, the need to develop alternative molecular scale electronic materials becomes apparent. Integrated electronic component fabrication from organics has been recognised theoretically as the ultimate goal. In order to gain a comprehensive insight into these materials, extensive research has been carried out on conjugated carbon systems to optimise their optical and electrical properties. For example, doping polyacetylene with I2 has been shown to result in a large increase in conductivity from the pristine material. However, doping polymers tends to retard their optical properties as regards luminescence by reducing their bandgaps and introduce trapping sites such as solitons, polarons or bipolarons. The simple lesson over the years is that if materials are to be considered for luminescence, doping should not be carried out despite the desire to improve charge transport properties. We report the first physical 'doping', to use the traditional term, using small concentrations of multiwalled nanotubes in a conjugated luminescent polymer, poly(m-phenylenevinylene-co-2,5-dioctoxy-p-phenylenevinylene) in a polymer/nanotube composite. This can increase electrical conductivity of the polymer by up to eight orders of magnitude. In addition, this composite material is far more robust than the pristine polymer in terms of mechanical strength. The nanotubes appear to act as nanometric heat-sinks, preventing the build up of large thermal effects, caused either optically (photobleaching) or electrically which degrade these conjugated systems. We also report that electroluminescence was achieved from an organic light emitting diode (LED) using the composite as the emissive layer in the device. Furthermore, we have observed a substantial third-order nonlinear optical response in the near infrared in thin film samples.