Interface controlled current injection in hydrazone doped polyesters

The current injection properties of gold contacts fitted to hydrazone doped polyesters (HDPs) have been investigated using a series of thin-film Au/HDP/Al diode structures. Current voltage characteristics of the HDP diodes recorded over a range of temperatures suggest that the measured injection is controlled by interface states which assist in the transfer of hole charge between the Au electrode and highest occupied molecular orbital (HOMO) of the hydrazone. Injection assistance occurs due to the energetic location of the interface states which are estimated to lie between the Fermi energy of the Au and the HOMO of the hydrazone. The density of the interface states is found to be independent of the hydrazone doping concentration used in the diodes and is calculated to be about 3 x 10(16) cm(-3). The available density of interface states which participate in the injection process may be progressively reduced through exposure of the Au electrode to increasing amounts of ultraviolet radiation. The diminished current injection following ultraviolet exposure is fully restored after a sufficiently long recovery period of several hundred days at room temperature. A charge transfer model that involves depopulation of the interface states to nearby hydrazone sites is proposed to explain the ultraviolet phenomena. (C) 2010 Elsevier B.V. All rights reserved.