Comparison Of Alq_3/alkali-metal Fluoride/ai Cathodes For Organic Electroluminescent Devices

摘要

The tris-(8-hydroxy-quinolinato) aluminum (Alq_3)/LiF/Al cathode is one of the most studied interfaces in organic electronics. Charge injection properties of the other alkali-metal fluorides (i.e., NaF, KF, RbF and CsF) at the Alq_3/electron injection layer/Al cathode interface of organic light emitting diodes were studied. It is found, in contrast to most literature, that the alkali-metal fluorides all yield equivalent device performance, despite the significant difference in the physiochemical properties of these metal fluorides. The observed phenomena are explained by the comparable strong intrinsic molecular dipole moment of these fluorides, which result in strong interfacial dipoles at the Alq_3/Al interface. This is consistent with ultraviolet photoelectron spectroscopy results and recent theory in literature. Difference in thickness dependence was also observed. LiF is found to be unique amongst the alkali-metal lluorides in terms of the thickness dependence of the device performance. This trend is found to strongly correlate with the growth mode of the metal fluoride on the Alq_3 surface; LiF quickly transitions to layer-by-layer growth after the initial island nucleation, while the other alkali-metal fluorides tend to grow as clustered islands on top of a surface wetting layer.