PHOTOCONDUCTIVITY DARK CONDUCTIVITY STUDIES OF CHLOROGALLIUM PHTHALOCYANINE THIN-FILMS ON INTERDIGITATED MICROCIRCUIT ARRAYS

Studies of dark conductivity and photoconductivity are reported for ul trahigh-vacuum-prepared, highly purified thin films of the trivalent m etal phthalocyanine (GaPc-Cl) on gold interdigitated array microcircui t (MC) electrodes. These GaPc-CI/MC assemblies were subsequently expos ed to O2, NO2, and NH3 from ca. 10(-8) Torr to atmospheric pressure an d/or ultrathin films of TCNQ. It was found that the dark current-volta ge properties, the rise time to steady-state photocurrents upon first illumination, and the ratio of photoconductivity/dark conductivity (si gma(ph)/sigma(dk)) are strongly dependent upon the initial purificatio n of the Pc, the surface cleanliness of the microcircuit, and the exte nt of exposure to gases like O2. Ohmic current-voltage behavior is see n for less pure GaPc-Cl films or in those cases where trace contiminan ts are left on the MC surface. Space charge limited current-voltage be havior was seen at low applied fields in all other cases. Light intens ity dependencies of the photocurrent response in the Ohmic region indi cate that trap levels are distributed uniformly as a function of energ y 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 an d is strongly enhanced by illumination during those O2 exposures. Thes e results suggest that doping of GaPc-Cl thin films with near atmosphe ric pressures of O2 occurs by means of a photoassisted charge-transfer process, Pc + O2 reversible PC.+ + O2.-. A similar, but smaller, enh ancement in photoconductivity is observed following formation of ultra thin films of TCNQ on the GaPc-Cl/MC surface. Exposure to NH3 can elim inate some of the traps present in the less pure Pc films, decreasing both dark conductivity and photoconductivity but raising the sigma(ph) /sigma(dk) ratio. These results are useful in rationalizing some of th e chemical sensor and photoelectrochemical studies of GaPc-Cl and rela ted Pc thin films, where the presence of traps and impurities can cont rol both photoconductivity and the photopotentials observed in barrier -type cells.