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Space-charge-limited conductivity in evaporated α-form metal-free phthalocyanine thin films

Publication Date
DOI: 10.1016/s0042-207x(98)00014-1
  • Thin Films
  • Structure And Basic Physical Properties
  • Mathematics


Abstract The electrical properties of thermally evaporated thin films of metal-free phthalocyanine (H 2Pc), having gold electrodes, have been investigated. Capacitance measurements as a function of film thickness at a standard frequency of 1 kHz showed the usual geometric dependence, with a relative permittivity value of ε r = 2.9. Room temperature dc current density-voltage ( J-V) measurements showed a linear ohmic dependence at low voltages, followed by a power-law dependence, of exponent 4, at higher voltage levels. These results confirm that gold acts as an ohmic contact to H 2Pc, and indicate space-charge-limited conductivity (SCLC) at higher voltages. Measurements of current density as a function of inverse temperature at constant applied voltage yielded a hole mobility value μ = 6.25 × 10 −7 m 2 V −1 s −1. In conjunction with the room temperature J-V data this gave a value of p 0 = 8 × 10 18 m −3 for the hole concentration. The SCLC at higher voltages was dominated by an exponential distribution of traps, P( E) = P 0 exp(− E/ KT t , where P 0 = 1.29 × 10 46 J −1 m −3, T t = 900 K and total trap concentration N t = 1.6 × 10 26 m −3. The results suggested desorption of oxygen, regrowth of the phthalocyanine and the appearance of crystal defects during heat treatment.

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