In organic photovoltaic (OPV) devices fabricated with a double small-molecular layer, the power conversion efficiency strongly depends on the thickness of the organic donor layer (here, copper phthalocyanine). In other words, the power conversion efficiency increases with the donor layer thickness up to a specific thickness (∼12.7nm) and then decreases beyond that thickness. This trend is associated with the light absorption and carrier transport resistance of the small-molecular donor layer, both of which strongly depend on the layer thickness. Experimental and calculated results showed that the short-circuit current due to light absorption increased with the donor layer thickness, while that due to current through the donor layer decreased with 1/R. Since the total short-circuit current is the product of the light absorption current and current through the donor layer, there is a trade-off, and the maximum power conversion efficiency occurs at a specific organic donor layer thickness (e.g. ∼12.7nm in this experiment).