Charge carrier transport properties of thin films of an intrinsic molecular semiconductor: Lutetium bisphthalocyanine

Lutetium bisphthalocyanine (Pc2Lu) is the first example of intrinsic molecular semiconductor . Due to a small intermolecular interaction energy, simple band theory cannot be used to rationalize the electrical conduction properties of its condensed phases. A localized state model is used to describe the charge carrier transport of the material. Microlithography techniques allowed us to prepare coplanar Al/Pc2Lu/Al devices with various interelectrode spacings (2-50 mu m). Current-voltage characteristics of these devices were compared to those obtained with gold electrodes. For the former, at low voltages, the insulating Al2O3 layer present on aluminum, plays a major role in the contact properties. These contact effects become negligible above 10V. The thermal activation energies of conduction have also been determined in the ohmic and space charge limited current (SCLC) domains. These measurements were performed for both Al and Au electrodes. From the trap-filled limit voltage (U-TFL), a density of traps (N-t,) was estimated. N-t, is shown to be of the same order of magnitude as the density of equilibrium free charge carriers at room temperature, viz. 10(17) per cm(3). The possible mechanisms of charge carrier injection at the metal/molecular material interface have also been examined.