Highly soluble fullerene derivatives (HSFD) and a low soluble polymer (LSP) were investigated as modifiers of the active layer morphology in conventional P3HT/PCBM bulk heterojunction solar cells. The observed changes in photovoltaic and electrical characteristics of the devices after addition of one or two modifiers suggest that they induced favourable vertical phase separation in the blends simply due to different solubilities of the components. In particular, HSFD is supposed to accumulate at the top of the film serving as a hole-blocking interlayer at the cathode/active layer interface. On the contrary, LSP seems to form electron-blocking buffer layer at the bottom of the device at the active layer/anode interface. Thus, the differential material solubility was suggested as a tool for adjustment of vertical morphology of organic bulk heterojunction solar cells. (C) 2010 Elsevier B.V. All rights reserved.