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 ͑SPV͒ and electron paramagnetic resonance ͑EPR͒ spectra for C 60 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 E c Ϫ(0.9-1.1) eV in our samples are attributed to nonbonded intercalated O 2 in the C 60 lattice while other states at E v ϩ1.3 eV are related to oxygen chemically bonded to the C 60 molecules. EPR measurements reveal that light/air exposure causes an increase in the number of C 60 ϩ paramagnetic defects. A hypothesis has been suggested that the recombination centers at E v ϩ1.3 eV and the C 60 ϩ paramagnetic centers have the same origin and are attributable to carbon dangling bonds.

Discrete Fulleride Anions and Fullerenium Cations

“…367 This area is in need of further study, particularly since neutral C 60 has very recently been found to react with O 2 to form C 120 O. 375 Such studies are also relevant to the effects of oxygen on the photoconductivity of fullerene films 428 and the reactivity of O 2 with fullerenes in general. 429,430 …”

Physical Properties of Organic Fullerene Cocrystals

“…In compounds of C60 with H2, NH4, and CO which is obtained via gas-phase intercalation, for example, the room-temperature symmetry is fcc, with variations of the lattice parameter of less than 0.2% compared to pristine fullerite. Only a minor lattice distortion is observed in N2-intercalated C60, which is obtained at high pressure (Galtsov et al, 2007), while O2 can diffuse even at atmospheric pressure through the interstices of C60 (Assink et al, 1992), which rationalizes its observed degradability by photooxidation (Hamed et al, 1993;Katz et al, 1998). These intermolecular interstices, especially the larger octahedral ones, are large enough to accommodate also the heavier alkali ions without distortion nor expansion of the fcc lattice, and cocrystallization of alkali fullerides with ammonia leads to only minor tuning of the lattice spacing.…”

Solid State Physicochemical Properties and Applications of Organic and Metallo- Organic Fullerene Derivatives

“…At very high stoichiometric ratios of the solvent, the abundance of the solvent leads to the formation of solvent-rich islands in which the C 60 -C 60 van der Waals interactions are negligible, as demonstrated e.g. using CCl 4 and CBrCl 3 with a fullerene-solvent ratio of 1:12 [93,94,95]. Ternary molecular complexes can also be formed (e.g.…”

“…Subsequent illumination by light triggers photochemical reactions leading to C-O binding and disruption of the fullerene cages. [3,4] For this reason, C 60 is normally stored under noillumination conditions, and C 60 films and devices are only achieved and characterized under vacuum or inert atmosphere, often requiring in situ measurements on freshly deposited samples. Fullerene functionalization can prevent photo-oxidation, either by changing the chemical reactivity of charged fullerene moieties by modifying the energy position of the lowest unoccupied molecular orbital (LUMO) with respect to the trap energy of adsorbed atmospheric oxidants (oxygen and water), or by introducing sideadducts that modify the solid-state structure and act as a barrier against oxygen or water diffusion [11,12].…”

“…However, the utilization of these remarkable all-carbon molecules for applications in biochemistry and medicine or low-cost device fabrication is hindered by their insolubility in a large number of solvents (besides organic ones [1]) including especially water, where clustering of the molecules is reported [2], and by their low stability under ambient conditions in powder and thin-film form [3,4]. Fullerene derivatization aims on one hand at overcoming these shortcomings of the parent moieties, and on the other hand to confer the all-carbon cages enhanced physicochemical functionalities.…”