Enhanced efficiency of polymeric solar cells via alignment of carbon nanotubes

Carbon nanotubes (CNTs) are well-known electron acceptor materials for use in optoelectronic devices due to their appropriate electronic properties In this work, we report on their use as a part of the multi-component active layer in organic solar cells to provide better transport of photogenerated charges To optimize transport, orientation of CNTs perpendicular to the electrodes of solar cells with an active layer of poly(3-hexylthiophene)-CNT- (6,6-phenyl C-61 butyric acid methyl ester) (P3HT-CNT-PCBM) composite is preferable In order to achieve this preferred orientation by applying an electric field on the device electrodes, polarization of CNT solutions was studied using dielectric spectroscopy We find that an improved orientation can be achieved with a high frequency AC electric field due to the alternating force exerted rapidly on field-induced dipoles of the nanotubes Following these experiments, the orientation method was applied to organic solar cells with a sandwich structure in which the active layer consists of P3HT-CNT-PCBM The electric field was applied continuously on the electrodes during annealing at a temperature near the glass transition of the polymer matrix We demonstrate that a significant increase in cell performance (i e open-circuit voltage = 0 61 V. efficiency = 4 53%, fill factor = 0 46) can be achieved, which may signify a better path created by the oriented nanotubes for carrier transport to the electrodes (C) 2010 Society of Chemical Industry