In order to increase the efficiency of photo-electric conversion by A-S-D molecular photodiodes, the second donor layer, D', the ionization potential of which was smaller than that of the donor moiety of the A-S-D triad, was deposited on top of the donor of the A-S-D monolayer. The photo-induced charge separation in the monolayer assemblies was observed by a scanning surface potential microscope. The addition of the second donor layer promoted photo-induced charge separation due to the longer charge separation distance by the presence of the D' monolayer and lateral diffusion of charges through the D' monolayer. The effect of the D' concentration of the second monolayer was studied.