Flexible solar cells made of organic semiconductors are promising for the application to the photovoltaic energy sources for flexible devices and those can be attached to any curved surfaces. Mixing and phase separation of different molecules are important for the formation of bulk-hetero junctions. We found that DNTT (dinaphtotienothiophene) and C₆₀ spontaneously mixes when they are sequentially deposited. The driving force does not seem to be the charge transfer between p-type and n-type molecules but the maximization of van der Waals attraction coming from the molecular shapes.