LAUSR

We have studied the reaction of PC61BM ([6,6]-phenyl-C61-butyric acid methyl ester) film with K atoms using photoemission spectroscopy measurements and density functional theory calculations. It is found that the molecular structure of PC61BM keeps intact until the intercalation stage of K3PC61BM. This is because that the C60 cage of the molecule attracts the three first intercalated K atoms (per molecule) through the electron transfer from the K 4s states to the LUMO and LUMO+1 orbitals. The fourth intercalated K atom then detaches the methyl group and bonds with the two O atoms of the molecule. Additional K atoms can still bond with the molecule until the LUMO+2 orbital is filled, and the highest stoichiometry of K-intercalated PC61BM is K7–8PC61BM. The results indicate that the electrode interface of PC61BM-based devices is far from understood.