Impact of Molecular Orientation and Spontaneous Interfacial Mixing on the Performance of Organic Solar Cells

A critically important question that must be answered to understand how organic solar cells operate and should be improved is how the orientation of the donor and acceptor molecules at the interface influences exciton diffusion, exciton dissociation by electron transfer, and recombination. It is exceedingly difficult to probe the orientation in bulk heterojunctions because there are many interfaces and they are arranged with varying angles with respect to the substrate. One of the best ways to study the interface is to make bilayer solar cells with just one donor–acceptor interface. Zinc phthalocyanine is particularly interesting to study because its orientation can be adjusted by using a 2 nm-thick copper iodide seed layer before it is deposited. Previous studies have claimed that solar cells in which fullerene acceptor molecules touch the face of zinc phthalocyanine have more current than ones in which the fullerenes touch the edge of zinc phthalocyanine because of suppressed recombination. We have more...