VISIBLE ABSORPTION AND PHOTOCURRENT SPECTRA OF EPITAXIALLY DEPOSITED PHTHALOCYANINE THIN-FILMS - INTERPRETATION OF EXCITON COUPLING EFFECTS

The Q-band absorbance spectra or photocurrent spectral maxima from epitaxially grown AlPc-Cl, GaPc-Cl, and InPc-Cl thin films, grown on the metal dichalcogenide SnS2, were compared with the spectral peak positions predicted from an extension of the molecular exciton model, adapted for thin film aggregates of these same Pc's. These comparisons are made possible by the narrow line shapes in the Q-band spectra of these molecules when ordered Pc films are produced. Such narrow line shapes result from the highly homogeneous environments for these dyes. Correlations between the spectral shift in the Q-band and the number of monolayers in the ultrathin film were made and suggest that reasonable approximations can be made to the perturbation in exciton energies in closely spaced 4-fold symmetric chromophores such as the phthalocyanines. The calculations also show that the Q-band position and full width are sensitive to the packing geometries of adjacent chromophores, which may be useful in characterizing a variety of highly ordered thin film dyes and organic superlattices.