D.c. electrical measurements and temperature dependence of the Schottky-barrier capacitance on thin films of β-MgPc dispersed in polycarbonate

Abstract

Measurements of the temperature dependence of ohmic and space-charge-limited (SCL) currents on thin films of polycrystalline particles of ß-magnesium phthalocyanine (MgPc) dispersed in a polymer binder in Schottky junction cells have been carried out in air ambient. These cells showed rectification when MgPc material was sandwiched between an ohmic contact (Au) and a blocking contact (Al). At low voltages, the dark current in the forward direction which corresponds to a negative potential at the Al electrode varies exponentially with voltage. At higher voltages, two distinct regions of ohmic and SCL conduction (SCLC) under forward bias were observed. The square power dependence in SCLC indicated that current conduction is limited by a discrete trapping level above the valence band edge. Under reverse bias, the conduction process is determined by Schottky emission in the lower voltage range and the Poole-Frenkel effect for higher voltage levels. Low-frequency oscillographic capacitance measurements have been employed to characterize the Schottky-barrier cells made with oxygen-doped ß-MgPc dispersed in a polymer binder. The temperature variation of the slope for the reciprocal of the square capacitance vs. voltage plots is explained in terms of a two-site model for the dopant, oxygen.