Photosensitized Hole Injection from Langmuir-Blodgett Films into Molecular Crystals; The Influence of Image Potential and Through-Bond Effects

Abstract

The quantum yield of photoinduced charge generation processes depends upon the yield of the primary charge transfer step ana that of the consecutive separation steps which are in competition with charge recombination steps. When the charge generation occurs at the interface between an electrolyte and a low dielectric molecular crystal, the injected charge remains trapped near the interface, where the competition of the recombination process is efficient, due to the effect of the image potential. The interposition of a thin (8 Å) alkane layer between the electrolyte and the surface of the molecular crystal leads to a decrease of this image potential and of the rate of the recombination process. Both lead to an increase of the yield of the separation process. Upon increasing the length of the alkane spacer the rate of the primary charge transfer step is decreased. The decrease is however considerably slower than what would be expected for an electron tunneling through vacuum between the HOMO of the sensitizer and that of an aromatic moiety. Analogous results have been observed by Kuhn et al. for tunneling of electrons from a ground state dye molecule to an excited dye molecule. They probably must be explained by interaction of the tunneling electron with the HOMO or the LUMO of the fatty acid chain (superexchange).