Organic solar cells have been intensively investigated as next-generation solar cells, because of their fascinating advantages of lightweight, low fabrication cost, resource-free, and flexibility. However, the fundamental aspects for improving the energy conversion efficiency (η) have been unclear satisfactorily. In this review, I report on the effects of molecular crystallinity/orientation of a donor film on the external quantum efficiency, which is one of the most important factors to determine the η, of Zinc-porphyrin/C₆₀ hetero-junction solar cells.