Annealing-induced phase separation in small-molecular bulk heterojunctions.

Morphology optimization of donor–acceptor bulk heterojunctions at microscopic scale is critical for improving performance of organic photovoltaic devices. Here, effects of thermal annealing on phase separation processes in small-molecular bulk heterojunctions with different geometrical structures (i.e., PTCDA, TiOPc, CuPc and C 60 ) are investigated with ultraviolet and X-ray photoemission spectroscopies. It was identified that post-annealing treatment caused the different degrees of vertical diffusions at the bulk heterojunctions, leading to non-uniform composition distributions. Variations in phase separations are mainly due to the differences in surface energy of the involved materials, which play a crucial role in the intermolecular interactions and the molecular diffusion. Low-surface-energy materials were found to segregate preferentially on the surface for minimizing total energy of the systems. [ABSTRACT FROM AUTHOR]