Novel Composites of Carbon Nanotubes with a Poly(Phenylene Ethynylene) by Non-Covalent Wrapping for Photovoltaic Devices
J.C. Ramos, P. Cortes, I. Moggio, E. Arias, C. Martínez, K.J. Moreno
Universidad Autonoma de Ciudad Juarez, MX
Keywords: conjugated polymers, carbon nanotubes, photovoltaics
Abstract:Conjugated polymers exhibit optoelectronic properties comparable with those of inorganic semiconductors and with some advantage over them. By their side, carbon nanotubes (CNTs) offer a great opportunity for exciton dissociation. For this reason, the combination of conjugated polymer and carbon nanotubes in composites has garnered interest in recent advances for solar cells technology. In this work we present an extensive study on nanocomposites based on a poly(phenylene ethynylene) sequenced with thioester moieties (pPET3OC12-sqS) with single (SWCNTs) and multiwall (MWCNTs) carbon nanotubes in order to develop novel hybrid materials for photovoltaic cells. 1HNMR analysis suggests a strong interaction type π stacking between the rigid conjugated backbone of the polymer and the backbone of the nanotubes in agreement with previous works. This interaction is also reflected in the resonant peaks of both, the tioester-ethyldisulfide and dodecanoxy side chains of pPET3OC12-sqS, and likely due to the fact that these spins need more relaxation time to show signals. The absorption and emission maximal wavelengths of the polymer are maintained when it is wrapped on the nanotubes even if the baseline of the composites UV-Vis spectra is very marked due to the scattering contribution from the nanotubes. The optical band gap decreases in the nanocomposite suggesting an enhancement of the electrical conductivity as confirmed by cyclic voltammetry and by electrical measurement in the films, where the electrical conductivity of the composites resulted one order of magnitude higher than that of pPET3OC12-sqS. A quenching of the polymer fluorescence is found, which is attributed to the efficient energy transfer between pPET3Oc12-sqS and CNTs, more than the disruption of conjugation by a conformational change as reported and in agreement with the fact that the absorption and emission maxims are unchanged. Based on these results, a molecular model was proposed. Studies of SEM and TEM microscopy indicate a good dispersion of the nanotubes in the polymer matrix as well as the absence of bundles of nanotubes in the composite which is consistent with a homogeneous adhesion of the PPS around the carbon nanostructures.