Thermal annealing-induced vertical phase separation of copper phthalocyanine: Fullerene bulk heterojunction in organic photovoltaic cells

The effect of thermal annealing treatment on the morphology change in bulk heterojunction (BHJ) in organic photovoltaic cells was studied by photoemission spectroscopy. The results reveal that vertical phase separation upon annealing occurs in typical BHJ layer formed between planar molecule CuPc and spherical C₆₀ with inhomogeneous concentration distribution in profile, varying from CuPc-rich near the air surface to C₆₀ -rich adjacent to the substrate interface. The morphology variation is associated with the difference in surface energy of CuPc and C₆₀, leading to the accumulation of CuPc to the interface with air to lower the overall energy of the free surface.