High open-circuit voltage photovoltaic devices from carbon-nanotube-polymer composites

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Organic photovoltaic devices based on the bulk heterojunction concept, containing a blend of single-wall carbon nanotubes (SWNTs) and soluble polythiophenes (P3OT) were studied. The open circuit voltage Voc of the devices was found to be 0.75 V, which is larger than the theoretical limit calculated by the metal–insulator–metal (MIM) model. In order to investigate the origin of this unusually high Voc, we have prepared P3OT–SWNT based devices with different metal negative electrodes. The Voc measured is only very weakly dependent on the work function of the metal, suggesting that the MIM model does not apply in this case. From the analysis of the current–voltage characteristics and electron microscopy imaging of the composite structure, it is proposed that the photovoltaic response of these devices is based on the introduction of internal polymer/nanotube junctions within the polymer matrix, which due to a photoinduced electron transfer from the polymer to the nanotube contribute to enhanced charge separation and collection. The data suggest that the negligible influence of the negative electrode work function on Voc can be explained by the metal negative electrode forming ohmic contacts to the nanotube percolation paths.