The effects of exposure to oxygen and annealing on the conductivity of copper phthalocyanine thin films

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Abstract
It is well known that oxygen has a profound effect on the conductivity of the phthalocyanines. In the present work the conductivity of copper phthalocyanine thin films has been measured both after exposure to oxygen for several weeks and after annealing at temperatures up to 150 degrees C. At low voltages the conductivity was ohmic with hole concentrations varying in the range 1014-1020 m-3. Higher values of hole concentration were derived for samples stored for longer periods, indicating that oxygen acts as an acceptor level, while annealed samples showed lower values consistent with the desorption of oxygen. At higher voltages samples stored for two weeks showed space-charge-limited conductivity dominated by a discrete trap level of concentration 7.3*1018 m-3 and activation energy 0.65 eV, which was attributed to oxygen incorporation within the films. Conversely, after annealing, an exponential trap distribution of concentration 1.5*1024 m-3 was detected which was similar to those observed in samples which had not undergone exposure to oxygen and thus confirmed the removal of oxygen during the annealing process.