Abstract Submitted to the    NANOTUBE'04 Conference:

Organic photovoltaic cells based in composites of conjugated polymers and soluble derivatives of fullerenes as the 1-(3-methoxycarbonyl)-propyl-1-1phenyl-(6,6)C₆₁ (PCBM) have given efficiencies around 2.5%. Here we present a similar approach in which the electron acceptor molecule is a single walled carbon nanotube. Our cells have been prepared by spin coating solutions of poly-(3-octylthiophene), hereafter referred as P3OT, and single walled carbon nanotubes onto a glass substrate covered with indium tin oxide (ITO). In order to improve the solubility of the carbon nanotubes, we have functionalized them by attaching alkyl chains to the carboxylic acid groups on the open ends of commercially available single walled carbon nanotubes (NanoCyl S. A., Belgium). Aluminium back electrodes were prepared by magnetron sputtering.

We have tested with a surface force microscope the topography and adhesion of the composites and found that 1,2-dichlorobencene is the best solvent in order to avoid phase segregation of polymers and carbon nanotubes, thus providing two interpenetrating subnetworks to ensure carrier transport to the corresponding electrodes (holes to ITO and electrons to Al).

We have measured the photocurrent by illuminating the cells from the ITO side with a Xe lamp at 100mW/cm^-2, the cells have been driven by a Keithley Mod.230 voltage source, and the current measured with a Keithley Mod.6514 electrometer. The obtained efficiencies are about 0.1%, with an open circuit voltage around 0.6V. This is a low value, but it is worth mentioning that it represents a two orders of magnitude increase compared with the efficiency obtained with a photovoltaic cell without carbon nanotubes (using only a P3OT film between the electrodes).

In conclusion, single walled carbon nanotubes are a promising candidate to act as electron acceptor in bulk heterojunction organic photovoltaic cells. Their processability have been increased by a functionalization which greatly improves their solubility. Further investigation to control the nanostructuration of such composites is needed in order to increase their efficiency.