Creation of a gradient polymer-fullerene interface in photovoltaic devices by thermally controlled interdiffusion

Efficient polymer-fullerene photovoltaic devices require close proximity of the two materials 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 $\text{2-}\mathrm{methoxy}{\text{-5-(2}}^{\prime }-\mathrm{ethylhexyloxy}\text{)-1,4-}\mathrm{phenylenevinylene}$ copolymer (MEH-PPV) and sublimed ${\mathrm{C}}_{60}$ 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. The interdiffused devices show a dramatic decrease in photoluminescence and concomitant increase in short circuit currents, demonstrating the improved interface.