Solar cells with structure of ZnPc (20 nm)/C60 (10 nm) are fabricated by thermal evaporation. The photocurrent density (Jph) expressed by effective applied voltage (V0-Va) and light intensity (I) is derived from an analytical method, and it is shown that the results from the model are in good agreement with experimental results. The short-circuit current density (JSC) exhibits a nearly linear dependence on light intensity, which is partially determined by effective applied voltage. Thermal annealing treatment under an optimal condition of 100 ℃ and 20 min, improves JSC by 43.8%, lengthening its attenuation constant from 5.6h to 22.2 h, which is attributed to the improvement on interfacial morphology and the decrease of series resistance due to better contact of the layers.