Photovoltage generation in bilayer lipid membrane-cadmium sulfide junctions

Flat lipid films of bimolecular thickness separating two aqueous solutions, known as black lipid membranes (BLM), provide matrices for formation of semiconductor particles and films. The formation process of cadmium sulfide particulate films on BLM of glycerol monooleate is quantitatively described by measuring potential differences across the membrane, membrane capacitance, and pH variations of the solutions. The values of trans-membrane potential differences induced by hydrogen sulfide gradient suggest that H2S, used to form CdS, makes the BLM selectively permeable to protons. This selectivity is responsible for the dark voltage established across the membrane after the CdS formation. Biphasic photovoltages are induced by step illumination. Control experiments have been performed to check the influence of dark potentials, temperature gradients, and proton permeability on the photopotential. On the basis of the results of these experiments and those reported by other authors, a quantitative model which describes the kinetics of the photopotential is proposed. According to this model the light creates nonlocalized electron/hole pairs in the CdS which become localized when electrons interact with O2 in the solution and its corresponding hole interacts with H2S in the membrane. The charged species move through the membrane under electric fields and concentration gradients. Four differential equations are used to describe the charge separation and motion and one other equation links charges and voltage. The equations have been numerically solved and the kinetics parameters determined by fitting the experimental results.