We have investigated the current-voltage characteristics of the multi-layered photovoltaic devices consisting of ITO/oxide /p-type (donor)/fullerene/ bathocuproine (BCP)/ Al structures. We chose various p-type (donors) small molecules and polymers in order to tune the values of ionization potential (IP) of donor molecules. The open-circuit voltage (Voc) increases with the increment of IP of donor materials. However, VOC was limited at ~0.6-0.7V for the devices without oxide layer. On the other hand, the VOC increases up to 0.9V for the devices with NiO and to ~ 1.1V for the devices with MoOX as a hole extraction buffer layer, respectively. We also estimated the work-function differences between Al and the oxide as 0.7, 0.9-1.0, and 1.2-1.3 eV for the device without oxide, with NiO, and with MoOX, respectively. We therefore concluded the value of VOC is limited by the lower part of VOC and energy difference between the LUMO of fullerene and the HOMO of donor ΔE.