We numerically investigate the effects of the exciton generation rate G, temperature T, the active layer thickness d and the position of LUMO level E_L related to the cathode work function W_c at a given energy gap on the open-circuit voltage V_oc of single layer organic solar cells with Schottky contact. It is demonstrated that open-circuit voltage increases concomitantly with the decreasing cathode work function W_c for given anode work functions and exciton generation rates. In the case of given cathode and anode work functions, the open-circuit voltage first increases with the exciton generation rate and then reaches a saturation value, which equals to the built-in voltage. Additionally, it is worth noting that a significant improvement to V_oc could be made by selecting an organic material which has a relative high LUMO level (low |E_L| value). However, V_oc decreases as the temperature increases, and the decreasing rate reduces with the enhancement of exciton generation rate. Our study also shows that it is of no benefit to improve the open-circuit voltage by increasing the device thickness because of an enhanced charge recombination in thicker devices.