Zinc oxide (ZnO) thin films were grown on p-Si substrates under various oxygen partial pressure (p(O-2)) from 5.3 to 9.3 Pa by using pulsed laser deposition. In x-ray diffraction analysis, n-ZnO thin film grown under an p(O-2) of 8 Pa showed the highest intensity of (002) diffraction peak and highly c-axis oriented. At room temperature, all the n-ZnO thin films grown at various p(O-2) showed near band edge emissions about 385 nm, and the performance of n-ZnO/p-Si heterojunction grown at p(O-2) of 8 Pa shows better than that of the heterojunction with n-ZnO layer grown at p(O-2) of 5.3, 6.7, and 9.3 Pa. The performance of the heterojunction with and without Al-doped ZnO (AZO) layer was more improved by post-annealing at 200 degrees C, so that the heterojunction with and without AZO layer showed power conversion efficiency (PCE) of 0.61% and 1.5%, respectively. By measurement of external quantum efficiency (EQE), it was found that the improved PCE of the heterojunction with AZO layer was attributed to the overall enhanced EQE values from ultraviolet to near infrared.