ABSTRACT
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 review a series of studies in which thermally controlled interdiffusion is used to establish a gradient bulk heterojunction in photovoltaic devices incorporating conjugated polymers and fullerenes. First, a bilayer system consisting of spin-cast 2-methoxy-5-(2′-ethylhex-yloxy)-1,4-phenylenevinylene (MEH-PPV) copolymer and sublimed C60 is heated above the MEH-PPV glass transition temperature (T g) 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 fuller-ene 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. The dependence of the photovoltaic performance on the thickness of the MEH-PPV layer is examined, and it is found that, within the range examined, thinner layers yield enhanced efficiency as a result of improved charge transport. Cross-sectional transmission electron microscopy (TEM) images show that the interdiffused films contain large aggregates of C60 as a result of the relatively low miscibility of MEH-PPV and C60. Further improved device performance is achieved using poly(3-octylthio-phene-2,5-diyl), which is more miscible with C60. Auger spectroscopy combined with ion beam milling reveals a smooth concentration gradient from one surface of the film to the other in this latter case.
