Efficient polymer-fullerene photovoltaic devices require close proximity of the two components to ensure photoexcited electron transfer from the semiconducting polymer to the fullerene acceptor. We describe studies in which a bilayer system consisting of spin-cast 2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylenevinylene copolymer (MEH-PPV) and sublimed C60 is heated above the MEH-PPV glass transition temperature in an inert environment, inducing an interdiffusion of the polymer and the fullerene layers. With this process, a controlled, bulk gradient heterojunction is created bringing the fullerene molecules within the exciton diffusion radius of the MEH-PPV throughout the film to achieve highly efficient charge separation [1]. The interdiffused devices show a dramatic decrease in photoluminescence and concomitant order-of-magnitude increase in short circuit currents, demonstrating the improved interface. Figure 1 shows the spectral photoresponsivity (from bottom to top) of devices consisting of an MEH-PPV single layer, MEH-PPV/C60 bilayer, MEH-PPV/C60 bilayer interdiffused at 150 °C, and MEH-PPV/C60 bilayer interdiffused at 250 °C. The figure also shows that a valley in the spectral photoresponsivity of the MEH-PPV/C60 bilayer system, ascribed to the short optical penetration depth at the peak of the MEH-PPV absorbance, is eliminated following the interdiffusion process. Detailed studies of the effects of the initial MEH-PPV and C60 thicknesses and the time-temperature profile of the heat treatment on device efficiency will be presented. Furthermore, transmission electron microscopy and Auger spectroscopy studies of the nanoscale film composition have demonstrated that C60 intrdiffuses into the MEH-PPV polymer as rather large aggregates, most likely as a result of the low miscibility of the two materials. Further enhancements in the photovoltaic efficiency have been achieved with a photoexciited electron donor-acceptor pair that is more miscible, providing a smoother gradient cocentration profile.