Morphological Studies of Bulk Heterojunction Organic Solar Cells

A new approach offers the potential of introducing structural control during device fabrication without the need of thermal annealing. It was shown that incorporating of small concentrations of solvent additives in the solutions of conjugated polymer and fullerene derivatives from which the bulk hetero junction (BHJ) films are cast leads to a modification of the blend morphology. Despite the excellent power conversion efficiency of these films, important questions regarding the BHJ domain morphology remain unanswered. To probe the internal structures we performed complementary grazing incidence small and wide angle scattering experiments (GISAXS and GIWAXS). In addition, we conducted NEXAFS experiments to determine the blend composition at the interfaces. The results indicate a pronounced effect of polymer chain architecture on the degree of chain ordering within the films. The ordering can be further improved by incorporation of solvent additives during the casting process. The morphological changes in the BHJ systems are well correlated with the device characteristics.