Solid-state nuclear magnetic resonance (NMR) has been applied to "amorphous" active layers consisting of donor-acceptor self-assembled composites in organic solar cells. Several stoichiometric supramolecular complexation states as well as the charge-transfer states are revealed by the solid-state NMR, which have been difficult to access by conventional spectroscopy. The spectra show clear correlation between local self-assembled supramolecular structures and the organic solar cell performances.