Near-edge X-ray absorption fine structure study of the C60-derivative PCBM

The fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester plays a key role for electron transport in polymer solar cells. We have studied the unoccupied molecular orbitals of PCBM by near edge X-ray absorption fine structure spectroscopy and were able to assign the main resonances to molecular moieties by comparison with calculated sum spectra of individual carbons. We analyzed specifically the origin of the high-energy shoulder to the first π∗-resonance and identified contributions from the lowest-energy transition of a specific carbon in the phenyl and from transitions to higher unoccupied orbitals of the unmodified carbons in the C60-cage.