ON THE PHOTO-SENSITIZED INJECTION OF ELECTRONS INTO SNO2 FROM CYANINE DYES INCORPORATED IN LANGMUIR-BLODGETT-FILMS

Upon excitation of cyanine dyes incorporated in Langmuir-Blodgett films, deposited on SnO2, an anodic photocurrent (SEC) is observed at electrode potentials more anodic than -200 +/- 51 mV versus the SCE. The quantum yield of the photo-sensitized electron current obtained with mixed monolayers of the dye and arachidic acid is always smaller than 13% and in most experiments even smaller than 5% and does not depend on the nature of the dye. When the mixing ratio between the dye and arachidic acid is less than 1/5 the quantum yield of the photo-sensitized electron current does not depend in a significant way on the amount of dye in the Langmuir-Blodgett film. These data suggest a high efficiency for the primary electron transfer step. The electrolyte pH and the concentration or chemical structure of the regenerator influence the quantum yield of the observed photocurrents. The influence of the intensity of the excitation, the regenerator concentration and the electrode potential on the photocurrent suggest competition between the reduction of the oxidized dye by electrons in the conduction band and regeneration of the oxidized dye. The saturation of the photocurrent at high regenerator concentrations and anodic electrode potentials at values below 0.1 suggests that the electrons are in the first step injected in a surface state where competition between escape to the conduction band and various recombination processes occurs.