One very important factor limiting the power conversion efficiency of the current state-of-the-art organic solar cells is the low energy conversion efficiency during the conversion process of an absorbed photon to an electron-hole pair collected at the electrodes. The absorption of a 2 to 3 eV photon typically leads to an open-circuit voltage of 0.5-0.6 V, representing approximately 80% energy loss. In this paper, we show that the open-circuit voltage of an organic donor-acceptor heterojunction cell is related to both the photocurrent and the dark current. Many factors, such as illumination intensity, organic heterojunction structure, electrode properties, operating temperature, can have significant impact on the open-circuit voltage. We also show that the conventional wisdom of using the "effective" gap of an organic donoracceptor heterojunction to determine the maximum open-circuit voltage needs to be carefully re-examined. While the study shows that the open-circuit voltage in the copper phthalocyanine-C60 heterojunction cell still has some room for improvement, ultimately new materials will have to be used to boost the power conversion efficiency of organic solar cells to the 20% regime.