The p - n junction p-CuI/nano-porous n- made by coating CuI on a nano-porous film of n- does not show a photocurrent action spectrum corresponding to the bandgap excitation of . However, if the surface is lightly coated with the dye cyanidin the peak corresponding to bandgap excitation of is clearly manifested in addition to the sensitization peak. The effect is explained as originating from the relay action of the dye molecules adsorbed at the surface of mediating hole transfer from the valence band of to the valence band of p-CuI.