EFFECTS OF OXYGEN AND ILLUMINATION ON THE INSITU CONDUCTIVITY OF C-60 THIN-FILMS

We report in situ measurements of dc conductivity versus temperature in polycrystalline films of C60. Films sublimed using C60 powders from different batches synthesized according to standard techniques were used in conductivity versus temperature measurements in the range 20-180-degrees-C. The films showed clear semiconductor behavior. Activation energies E(a) and room-temperature dark conductivities sigma were in the range 0.54-0.58 eV and 10(-6)-10(-7) (OMEGA cm)-1, respectively. Exposing the films to pure O2 or air at 21-degrees-C leads to a fast decrease in dark conductivity and photoconductivity by orders of magnitude, indicating that oxygen quickly permeates the whole depth of the films. This indicates that at room temperature 02 quickly diffuses into the bulk of C60 and changes the electronic properties of the material. Most of the effect of oxygen on the conductivity can be reversed in minutes by increasing the temperature of the films to 160-180-degrees-C in vacuum, but the final state has slightly larger resistivity and activation energy. Exposure of C60 films at 21-degrees-C to Ar, N2, and He gases had no effect on sigma. Illumination of the films in the presence Of 02 causes larger and faster irreversible changes in sigma and E(a). In particular, exposing a 165-nm-thick film having sigma(21-degrees-C) approximately 10(-6) (OMEGA cm)-1 and E(a) = 0. 54 eV to a white-light intensity of 60 MW/cm2 in 1 atm 02 at 21-degrees-C for 1 h yields a state, after annealing, exhibiting sigma(21 C) approximately 10(-14) (OMEGA cm)-1 and E(a) = 0.95 eV. The effect of pure 02 on the conductivity of solid C60 can be explained in terms of a disorder potential that leads to localization of the electronic states at the edges of the highest-occupied-molecular-orbital- and lowest-unoccupied-molecular-orbital-derived bands. In addition, oxygen may act as an efficient trap for electrons in the conduction band Of C60. Illumination of the samples in the presence of oxygen promotes C-O reactions that irreversibly damage the C60 molecules, producing carbon dangling bonds and other defects that possibly generate gap-defect states in the semiconductor. The results of the present study also suggest that measured optical properties Of C60 in the low-photon-energy range may be affected by oxygen contamination and strong light.