Jw. Pankow et al., PHOTOCONDUCTIVITY DARK CONDUCTIVITY STUDIES OF CHLOROGALLIUM PHTHALOCYANINE THIN-FILMS ON INTERDIGITATED MICROCIRCUIT ARRAYS, Journal of physical chemistry, 97(32), 1993, pp. 8485-8494
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.