Enhancement of electron injection in organic light-emitting devices using an Ag/LiF cathode

A LiF-buffered silver cathode has been used in organic light-emitting devices (OLEDs) with structure indium–tin–oxide/N,N^′-bis-(1-naphthl)-diphenyl-1,1^′-biphenyl-4,4^′-diamine $\text{(50\hspace{0.17em}}\mathrm{nm}\mathrm{)/}{\mathrm{Alq}}_3$ (100 nm)/cathode. The efficiency of electron injection from the cathode is strongly dependent on the thickness of the LiF buffer layer. While a LiF layer thinner than 1.0 nm leads to higher turn-on voltage and decreased electroluminescent (EL) efficiency, a LiF layer of 3.0 nm significantly enhances the electron injection and results in lower turn-on voltage and increased EL efficiency. A brightness of 16 000 cd/m² and EL efficiency of 4.8 cd/A can be achieved with an Ag/LiF cathode. This dependence of electron injection on the LiF thickness is quite different from that reported for OLEDs with a Al/LiF cathode, but can be well understood using the tunneling model.