Organic solar cells. Supramolecular composites of porphyrins and fullerenes organized by polypeptide structures as light harvesters

We have constructed supramolecular solar cells composed of a series of porphyrin-peptide oligomers [porphyrin functionalized alpha-polypeptides, P(H2P)(n) or P(ZnP)(n) (n = 1, 2, 4, 8, 16)], and fullerenes assembled on a nanostructured SnO2 electrode using an electrophoretic deposition method. Remarkable enhancement in the photoelectrochemical performance as well as the broader photoresponse in the visible and near-infrared regions is seen with increasing the number of porphyrin units in alpha-polypeptide structures. Formation of supramolecular clusters of porphyrins and fullerenes prepared in acetonitrile-toluene = 3 : 1 has been confirmed by transmission electron micrographs (TEM) and the absorption spectra. The highly colored composite clusters of porphyrin-peptide oligomers and fullerenes have been assembled as three-dimensional arrays onto nanostructured SnO2 films using an electrophoretic deposition method. A high power conversion efficiency (g) of similar to 1.6% and the maximum incident photon-tophotocurrent efficiency (IPCE = 56%) were attained using composite clusters of free base and zinc porphyrin-peptide hexadecamers [P(H2P)(16) and P(ZnP)(16)] with fullerenes, respectively. Femtosecond transient absorption and fluorescence measurements of porphyrin-fullerene composite films confirm improved electron-transfer properties with increasing number of porphyrins in a polypeptide unit. The formation of molecular assemblies between porphyrins and fullerenes with a polypeptide structure controls the electron-transfer efficiency in the supramolecular complexes, meeting the criteria required for efficient light energy conversion.