Morphological, Optical and Electrical Characterization of the Interfaces in Fullerene-Porphyrin Thin Films

The role of the interfaces on the optoelectronical properties of porphyrin-fullerene composites has been studied by means of ultraviolet-visible (UV-Vis) and Raman spectroscopy, atomic force microscopy (AFM) and electric conductivity measurements. A simple method of synthesis of donor- acceptor complexes has been performed by subsequent deposition of C₆₀ fullerene and tetraphenylporphyrin (H₂TPP) thin films, using physical vapor deposition (PVD) on a (100) silicon substrate. UV-Vis spectra showed that the interaction of π-orbitals leads to a more ordering for the dipole moments arrangement and the π-orbitals overlapping between C₆₀ and H₂TPP molecules. Besides, Raman spectra presented intensity changes at 960 and 1000 cm^-1, both related to the vibration of the pyrrole ring and the rocking of the H on the C atoms within the macrocycle. Therefore, it can be expected that the interface C₆₀-H₂TPP should have a main role in the electric response of the multilayer films. The measurements of surface conductivity indicated that interface has specific contribution, and the value of surface conductivity is enhanced by charge delocalization mechanisms occur by π-π stacking interactions. It was found that the transversal conductivity of 3-layer films was enhanced by a factor of 4 in comparison to 2-layer film, due to charge transfer mechanisms occur in the junctions that could extend the diffusion length of the charge carriers. Finally, the interface generated between C₆₀ and H₂TPP films, without any linking molecule, enhance charge transport mechanism through the films.