SPECTRAL SENSITIZATION OF SnO2 THIN FILM ELECTRODE BY THE J-AGGREGATE OF CYANINE DYES IN MONOLAYERS

Arachidic acid monolayers containing 5,6,5',6'-dibenzo-I,I-diethyl-2,2'-cyanine chloride were prepared by the spreading method and deposited on the SnO₂ surface using the Langmuir-Blodgett technique. The SnO₂ : dye assembly prepared in this manner was used as a photoelectrode of the conventional electrochemical cell. A distinct J-band was observed in the action spectra of sensitized anodic and cathodic photocurrents. Effects of calcium arachidate barriers inserted between dye layer and either SnO₂ electrode (inside barrier) or electrolyte solution (outside barrier) on the photocurrent were examined. Although the inside barrier was effective in decreasing both anodic and cathodic photocurrents, the outside barrier did not suppress cathodic photocurrent. The following mechanism involving a molecular exciton of the J-aggregate is proposed for the sensitized photocurrent. The anodic photocurrent is caused by hole trapping by some reducing agent and concomitant injection of the electron from molecular exciton to the conduction band of SnO₂. Extraction of conduction-band electron of SnO₂ by molecular exciton and supplying to some oxidizing agent such as dissolved oxygen are responsible for the cathodic photocurrent.