An optical intensity distribution under light irradiation in the organic photovoltaic cell affects the absorbance of the active layer, which determines the photovoltaic performance. In this research, we evaluated the optimum thickness of the organic active layer with poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b']dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)] and [6,6]-phenyl C71-butyric acid methyl ester. The spectral response of external quantum efficiency was good agreement with the simulated optical intensity distribution within a device stack as a function of the position and the wavelength. As a result, the highest photoconversion efficiency of 10.1% was achieved for the inverted device structure.