Photooxidation of indene-C-60 bisadduct (ICBA) and indene-C-60 monoadduct (ICMA) was studied using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS), IR spectroscopy, and density functional theory (DFT) calculations. MALDI-TOF MS suggested that the oxidation occurred on the fullerene cage of ICBA or ICMA and that the oxidation of ICBA proceeded faster than that of ICMA. The IR study and DFT calculation indicated that carbonyl substituents were generated upon photoirradiation. The effects of the oxidized ICMA were examined by measuring the device properties of organic photovoltaic (OPV) cells. The increase in energy levels of fullerene derivatives affects the stability against photooxidation negatively, while it increases the open circuit voltage of OPV devices. These results indicate that it is important to balance the OPV performance with stability against photooxidation. (c) 2015 Elsevier B.V. All rights reserved.