Ultrafast photoinduced charge transfer from conjugated polymers to fullerenes has led to extensive studies of these systems as photovoltaic devices. The charge transfer process prevents radiative electron-hole recombination, resulting in free, mobile charges. A limiting factor is the exciton diffusion distance, which is of the order of 10 nm. If the fullerene is not within this distance of the optical excitation site, no charge separation will take place. The simplest system for such devices is a bilayer system in which a film of C60 is sublimed onto a spin-cast film of MEH-PPV, for example. We describe studies in which the polymer is heated above the 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. The photoluminescence and the short circuit currents of the devices show a dramatic decrease in photoluminescence and concomitant increase in short circuit currents, demonstrating the improved interface.