Side chain effect on electronic structure of spin-coated films of [6,6]-phenyl-C61-butyric acid methyl ester and its bis-adduct

► Electronic structure of spin-coated films of PCBM and bis-PCBM was investigated. ► Ionization energy and electron affinity of bis-PCBM are smaller than those of PCBM. ► Electron donation from the side chain to C60-backbone raises the HOMO and LUMO. ► Open circuit voltages of PCBM-based solar cells relates to electron affinities.