A heterogeneous bilayer structure of a Langmuir-Blodgett film was formed at an Au/water interface as a molecular photodiode in a photoelectrochemical cell. In this structure, an A-S-D triad monolayer, as a photo-responsive monolayer, was deposited first on an Au electrode and then the resulting surface was covered with a protective monolayer of arachidic acid. The protective layer was deposited in order to stabilize orientation of the triad molecules in the first monolayer, where a hydrophilic head group A was expected to orient towards the Au electrode, a hydrophobic D moiety towards the protective layer, and an S moiety to be located in between them. In the absence of the protective layer, the hydrophobic tail group D was supposed to face to the electrolyte solution with unfavorable surface energetics. The effect of the presence and the absence of the protective monolayer on measured photocurrents was examined.