OBSERVATIONS OF SCHOTTKY AND POOLE-FRENKEL EMISSION IN LEAD PHTHALOCYANINE THIN-FILMS USING ALUMINUM INJECTING ELECTRODES

DC current density-voltage measurements have been performed on vacuum evaporated aluminium-lead phthalocyanine-gold sandwich structures for both electrical polarities. The electrical characteristics differed markedly depending on whether the positive hole-injecting electrode was gold (forward-bias) or aluminium (reverse-bias). An extensive study of such characteristics revealed two distinct regions in the forward-bias characteristics. In the first of these a power-law dependence of current density (J) on voltage (V) was observed with exponent n approximately 4, and this was identified with space-charge-limited conductivity (SCLC) controlled by an exponential distribution of trap levels of approximate concentration 10(25) m-3, in agreement with the authors' earlier work. In the second region a square-law dependence of J on V was observed, which was identified with SCLC controlled by traps of approximate concentration 10(26) m-3 and situated at a discrete energy level. Under reverse-bias, current density was considerably lower than for forward-bias and invariably showed a linear dependence of log J on V1/2, but with different slopes in the lower and higher voltage ranges. At lower voltages this behaviour was related to Schottky emission with barrier height phi(s) approximately 1 eV and width approximately 50nm, while at higher voltages Poole-Frenkel emission was identified. Measurements of capacitance C as a function of applied voltage gave a linear dependence of C-2 on V under reverse bias, confirming both the existence of a Shottky barrier and a barrier height of approximately 1 eV.