The origin of changes in electrical properties of organic films fabricated at various vacuum-deposition rates

Abstract

We investigated the dependence of electrical properties on vacuum-deposition rate for films of N,N՛-diphenyl-N,N՛-bis(1-naphthyl)-1,1՛-biphenyl-4,4՛-diamine (α-NPD) and tris-(8-hydroxyquinoline)aluminium (Alq3), hole- and electron-transport materials widely used in organic light-emitting diodes (OLEDs), respectively. Hole-only devices (HODs) of α-NPD showed an increase of hole current when α-NPD layers were fabricated at high deposition rates, while the tendency was opposite for electron-only devices (EODs) of Alq3. We found that the increased hole current at high deposition rates for HODs was caused by a horizontal orientation of α-NPD molecules relative to a substrate plane which facilitates hole transport through the films. On the other hand, the decreased electron current at high deposition rates for EODs could be ascribed to the increased surface roughness of Alq3 films which decreases electron injection. Additionally, we demonstrated that long-term operational stability was enhanced for HODs fabricated at high deposition rates and operated at a constant current. Use of a high deposition rate means a reduction of deposition time which is helpful in OLED manufacturing. The improved device performance when high deposition rates were used and the detailed understanding of its origins as we demonstrated here would lead to fabrication of high-performance OLEDs at a low cost in the future.