Photoinduced Charge Transport in Films of Near-Infrared Dye Assisted by Nanoscale Inhomogeneity of Dye Aggregates

To clarify the mechanism of a photoinduced charge transport in films of relatively small organic molecules, i.e., whether it is stimulated by energetic and/or structural disorder, three types of films composed of a specific tricarbocyanine near-infrared dye were prepared, i.e., a polycrystalline film, an amorphous film composed of a mixture of monomers and J- and H-aggregates, and a composite dye film in a polymer matrix composed predominantly of dye J-aggregates. It was found that in most cases the charge transport is governed by trap-limited conduction; however, the suggested driving force for the photoinduced transport of charge carriers can be interpreted to be due to the structural inhomogeneity (i.e., different aggregate types) and energetic disorder in the dye film, where Gaussian and discrete, rather than exponential, trap distributions play a decisive role. It is concluded that intermolecular interactions in J- and H-aggregates of pi-conjugated molecules provide fascinating incentives for charge transfer pathways.