Changes in the photoelectrical properties and generation of photoinduced defects under light/air exposure of C60 thin films

Abstract

We report on the time development of surface photovoltage (SPV) and electron paramagnetic resonance (EPR) spectra for C60 films irradiated by room light in air. Such exposure is shown to result in the progressive reduction of the SPV signal at 0.9–1.1 eV and in an increase in the signal at approximately 1.3 eV, as well as in the reduction of the signals at about 1.6 and 2.3 eV. A decrease in the dark contact potential difference signal as a result of surface band-bending reduction is also observed after exposure. These results are explained assuming that gap states at Ec−(0.9–1.1) eV in our samples are attributed to nonbonded intercalated O2 in the C60 lattice while other states at Ev+1.3 eV are related to oxygen chemically bonded to the C60 molecules. EPR measurements reveal that light/air exposure causes an increase in the number of C60+ paramagnetic defects. A hypothesis has been suggested that the recombination centers at Ev+1.3 eV and the C60+ paramagnetic centers have the same origin and are attributable to carbon dangling bonds.