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

The photocurrent generations from modified electrodes, consisting of m onolayers of a surfactant squaraine DSSQ arylamino)phenyl-4'-(dimethyl amino)phenylsquarine) deposited on SnO2 substrates by the Langmuir-Blo dgett (LB) technique, have been studied, in a conventional photoelectr ochemical cell. Absorption studies show that the lambda(max) of the DS SQ-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 amb ient conditions, a cathodic photocurrent is observed when the DSSQ-SnO 2 electrode is illuminated by visible light. The action spectrum of th e photocurrent generation is coincident with the absorption of the ele ctrode, indicating that the squarine aggregate in the LB film is respo nsible for the photocurrent. The observation suggests that electrons f low 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 i ncrease in photocurrent is obtained when the 1 M NaNO3 solution is sat urated with O-2 to form the superoxide anion radical with a subsequent electron transfer from the conduction band of the electrode to the ho les residing in the squarine aggregate. This model is supported by stu dies of the effects of insulating layer, applied bias voltage, and add ed electron donors and acceptors on the photocurrent generation. The i mplication of this work to the photogeneration mechanism of squarine p hotoconductors in xerographic devices is discussed.