Nanotube Sheet Employed as OLED Electrode

Daniel S. Burgess

Organic LEDs (OLEDs) incorporating an anode made from a thin sheet of single-walled carbon nanotubes display performance similar to that of devices that employ indium tin oxide, report investigators in Montreal. The finding offers device designers an alternative to the transparent conductive ITO, which is brittle, heavy and relatively expensive.

Richard Martel of Université de Montréal said that the most significant hurdle to the use of the nanotube sheets in OLEDs involves the availability of the material. He noted that companies such as Raymor Industries Inc. of Montreal are addressing this issue, and he predicted that the manufacture of the nanotube sheets will be as scalable as the production of paper in the pulp and paper industry.

Thin sheets of single-walled carbon nanotubes used as the anode in organic LEDs yield devices that perform about the same as those employing an ITO anode. Courtesy of Carla M. Aguirre-Carmona, École Polytechnique de Montréal.

In a demonstration of the application of the electrode material, the scientists from the university and from École Polytechnique de Montréal, OLA Display Corp. and Université du Québec à Montréal fabricated their own 130-nm-thick nanotube sheets. P-type single-walled nanotubes obtained via pulsed laser vaporization were suspended in a solution of sodium cholate and filtered using a 0.2-μm cellulose filter. They then dissolved the filter with acetone and transferred the liberated nanotube sheet onto glass slides.

The researchers created an electrical contact by evaporating a 50-nm-thick layer of titanium on one end of the sheet and deposited an organic stack atop the anode. A 50-nm-thick layer of aluminum tris(8-hydroxyquinoline) served as the electron-transport and light-emitting layer, and lithium fluoride and aluminum were evaporated atop the structure to serve as the cathode. To assess the effect of using the nanotube sheet, they produced a device with an ITO anode but otherwise identical.

Under testing, the OLED with the nanotube sheet and the one with ITO displayed similar turn-on voltages of 6.6 and 6.2 V, respectively. At 2800 cd/m2, the maximum brightness of the nanotube-based OLED was less than half that of the other, but its external quantum efficiency was 1.4 cd/A, compared with 1.9 cd/A for the emitter using ITO. The investigators thus suggest that the difference in performance is largely the result of the nanotube film’s lower transmittance at the 520-nm peak emission wavelength and of current loss, both of which may be improved through optimization of design and materials.

Martel said that they will focus on means of avoiding bad contact resistance to enable the further development of organic diodes and photovoltaic cells.

Applied Physics Letters, May 1, 2006, 183104.