Effects of oxygen on the photocarrier dynamics in aC60film: Studies of transient and steady-state photoconductivity

The intrinsic dynamics of photoexcited carriers in an oxygen-free ${\mathrm{C}}_{60}$ film and their remarkable evolution as the film is exposed to oxygen are revealed by transient and steady-state photoconductivity (PC) measurements at various temperatures, light intensities, and photon energies. Exposure of ${\mathrm{C}}_{60}$ film to oxygen creates deep traps that reduce drastically the carrier lifetime and, consequently, the room-temperature steady-state photoconductivity by three to six orders of magnitude. Oxygen affects the steady-state photoconductivity excitation spectrum in a qualitatively similar way, as does decreasing the ambient temperature; in both cases, the photoconductivity decreases faster when carriers are photoexcited into the band edges. The transient and steady-state PC of a ${\mathrm{C}}_{60}$ film fully exposed to oxygen becomes temperature independent.