Studies of dark conductivity and photoconductivity are reported for ultrahigh-vacuum-prepared, highly purified thin films of the trivalent metal phthalocyanine (GaPc-Cl) on gold interdigitated array microcircuit (MC) electrodes. These GaPc-Cl/MC assemblies were subsequently exposed to O2, NO2, and NH3 from ca. 10-8 Torr to atmospheric pressure and/or ultrathin films of TCNQ. It was found that the dark current-voltage properties, the rise time to steady-state photocurrents upon first illumination, and the ratio of photoconductivity/ dark conductivity (σph/σdk) are strongly dependent upon the initial purification of the Pc, the surface cleanliness of the microcircuit, and the extent of exposure to gases like O2. Ohmic current-voltage behavior is seen for less pure GaPc-Cl films or in those cases where trace contiminants are left on the MC surface. Space charge limited current-voltage behavior was seen at low applied fields in all other cases. Light intensity dependencies of the photocurrent response in the Ohmic region indicate that trap levels are distributed uniformly as a function of energy in the "purified" Pc thin films but that this distribution is made strongly inhomogeneous after extensive exposure to electron acceptors such as O2 and NO2. The photocurrent generation process appears to be assisted by the formation of charged species during exposure to O2 and is strongly enhanced by illumination during those O2 exposures. These results suggest that doping of GaPc-Cl thin films with near atmospheric pressures of O2 occurs by means of a photoassisted charge-transfer process, Pc* + O2 ⇄ Pc•+ + O2•-. A similar, but smaller, enhancement in photoconductivity is observed following formation of ultrathin films of TCNQ on the GaPc-Cl/MC surface. Exposure to NH3 can eliminate some of the traps present in the less pure Pc films, decreasing both dark conductivity and photoconductivity but raising the σph/σdk ratio. These results are useful in rationalizing some of the chemical sensor and photoelectrochemical studies of GaPc-Cl and related Pc thin films, where the presence of traps and impurities can control both photoconductivity and the photopotentials observed in barriertype cells.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry