The effects of film thickness, codeposition and introduction of exciton-blocking layers are investigated for heterojunction photovoltaic devices based on copper phthalocyanine (CuPc) and fullerene (C-60). Device performance peaks at low film thicknesses and is improved by the introduction of a mixed layer between continuous pure layers at the electrodes. The control afforded by organic molecular beam deposition enables a detailed analysis of the effects of mixed-layer composition on the device performance. The highest efficiency is reached for devices containing 75% CuPc and is further improved in structures containing multiple mixed layers that lead to the creation of a gradient of donor-acceptor compositions. The mixed-layer properties were assessed using atomic force microscopy and electronic absorption spectroscopy. In most cases, amorphous films with intimate molecular mixing are formed, but several exceptions rationalise the efficiencies observed in corresponding devices. Finally, the degradation of the devices under solar illumination highlights the destructive effects of oxygen, which are found to be particularly severe in mixed-layer devices. (C) 2004 Elsevier B.V. All rights reserved.