Enhanced brightness in organic light-emitting diodes using a carbon nanotube composite as an electron-transport layer

We have studied the effects of using a composite fabricated from carbon nanotubes and a host polymer, poly(m-phenylene-vinylene-co-2,5-dioctyloxy-p-phenylene-vinylene) (PmPV), as an electron-transport layer in organic light-emitting diodes. Double layer devices using this composite as an electron-transport layer, triple layer devices with a composite electron-transport layer, and poly(9-vinylcarbazole) as a hole-transport layer, as well as poly(2,5-dimethoxy-1,4phenylene-vinylene-2-methoxy- 5(2'-ethylhexyloxy)-1,4-phenylene-vinylene (M3EH-PPV) single layer devices were prepared. Current-voltage-luminance and electroluminescent spectral measurements were performed using six different nanotube powder to polymer mass ratios (0%, 2%, 4%, 8%, 16%, and 32%) for all device structures studied. dc transport and photoluminescence behavior of the polymer-nanotube composite were also investigated. Although a potential barrier is introduced at the M3EH-PPV/composite interface, a significant increase in efficiency was observed using the composite. The best efficiency was obtained for those devices with an electron-transport layer of mass ratio 8%. In addition, on doping with nanotubes, electron conductivity in the composite increased by over 4 orders of magnitude with little quenching of photoluminescence. (C) 2000 American Institute of Physics.