LONG-TERM EVOLUTION OF PHOTOINDUCED LIGHT-ABSORPTION IN C-60 FILMS

It is found that photoinduced absorption in the energy range 0.6-2.1 e V in C-60 films grown by the same method can differ by a factor of 100 . This change is attributable a 10(6)-fold increase in the relaxation time tau of electron-hole photoexcitations after working with the film s for several months. It is established that the bimolecular recombina tion mechanism for photoexcitations (triplet excitons and polarons), w hich is typical of as-prepared and partially aged films, is superseded in fully aged films by thermally activated tunneling of localized pho toexcitations. An investigation of the film transmission spectra in th e photon energy range 0.2-5.0 eV shows that the long-time variations o f the optical properties are associated with a decrease in the concent ration of defects forming shallow tails of the density of states. An a brupt decrease of the relaxation time tau and the photoinduced absorpt ion is observed in both types of films at T greater than or equal to 8 0 K. (C) 1997 American Institute of Physics.