The interpretation of current density–voltage and activation energy measurements on freshly prepared and heat-treated nickel phthalocyanine thin films

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Abstract
Current density-voltage characteristics of nickel phthalocyanine thin films sandwiched between ohmic gold electrodes have been investigated. Ohmic conduction was predominant at low applied fields, with the free hole concentration dependent on subsequent sample treatment. Freshly prepared samples show a room-temperature free hole concentration p 07.25× 1018m−3, whereas samples heat-treated at 500 K. have a much lower value, of approximately 2.35 × 1014m−3. The high-field conductivity was associated with space-charge-limited conductivity (SCLC); different trapping modes were found to dominate this type of conduction, depending on post-deposition sample treatment. For freshly prepared samples the SCLC was dominated by a single trapping level situated at E t0.58eV above the valence band edge. Traps distributed exponentially with energy in the forbidden energy gap of the material dominate the J-V characteristics of the heat-treated samples, as indicated by a J∝V n relationship, with n>2. Trap concentrations associated with the single dominant level and with the exponential distribution were approximately 1 × 1019m−3 and 9.26 × 1026 m−3 respectively. These different trapping modes may be ascribed to adsorbed gaseous species in the former case and to a high concentration of structural defects resulting from a possible phase transformation in the latter.