AN INVESTIGATION OF PHOTOCURRENT GENERATION BY SQUARAINE AGGREGATES IN MONOLAYER-MODIFIED SNO2 ELECTRODES

The photocurrent generations from modified electrodes, consisting of monolayers of a surfactant squaraine DSSQ (4-(distearylamino)phenyl-4'-(dimethylamino)phenylsquarine) deposited on SnO2 substrates by the Langmuir-Blodgett (LB) technique, have been studied, in a conventional photoelectrochemical cell. Absorption studies show that the lambda(max) of the DSSQ-SnO2 electrode is at 530 nm and is significantly blue-shifted from the monomeric absorption of DSSQ (lambda(max) 633 nm), indicating that the squarine chromophores form aggregates on the electrode. Under ambient conditions, a cathodic photocurrent is observed when the DSSQ-SnO2 electrode is illuminated by visible light. The action spectrum of the photocurrent generation is coincident with the absorption of the electrode, indicating that the squarine aggregate in the LB film is responsible for the photocurrent. The observation suggests that electrons flow from the electrode through the LB film to the electrolyte solution, which is an aqueous solution of 1 M NaNO3. We have demonstrated that O-2 is vital in the photocurrent generation process. While a 2-fold increase in photocurrent is obtained when the 1 M NaNO3 solution is saturated with O-2 to form the superoxide anion radical with a subsequent electron transfer from the conduction band of the electrode to the holes residing in the squarine aggregate. This model is supported by studies of the effects of insulating layer, applied bias voltage, and added electron donors and acceptors on the photocurrent generation. The implication of this work to the photogeneration mechanism of squarine photoconductors in xerographic devices is discussed.