Effects of oxygen and illumination on theinsituconductivity ofC60thin films

Abstract

We report in situ measurements of dc conductivity versus temperature in polycrystalline films of ${\mathrm{C}}_{60}$. Films sublimed using ${\mathrm{C}}_{60}$ powders from different batches synthesized according to standard techniques were used in conductivity versus temperature measurements in the range 20–180 °C. The films showed clear semiconductor behavior. Activation energies ${\mathit{E}}_{\mathit{a}}$ and room-temperature dark conductivities σ were in the range 0.54–0.58 eV and ${10}^{\mathrm{-}6–}$ ${10}^{\mathrm{-}7}$ (Ω cm${)}^{\mathrm{-}1}$, respectively. Exposing the films to pure ${\mathrm{O}}_2$ or air at 21 °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 ${\mathrm{O}}_2$ quickly diffuses into the bulk of ${\mathrm{C}}_{60}$ 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 °C in vacuum, but the final state has slightly larger resistivity and activation energy. Exposure of ${\mathrm{C}}_{60}$ films at 21 °C to Ar, ${\mathrm{N}}_2$, and He gases had no effect on σ. Illumination of the films in the presence of ${\mathrm{O}}_2$ causes larger and faster irreversible changes in σ and ${\mathit{E}}_{\mathit{a}}$.