Direct Al cathode layer sputtering on LiF∕Alq3 using facing target sputtering with a mixture of Ar and Kr

Using facing target sputtering (FTS) with a mixture of Ar and Kr, direct Al cathode sputtering on $\mathrm{LiF}∕{\mathrm{Alq}}_3$ layers was accomplished without the need for a protective layer against plasma damage. Organic light emitting diodes (OLEDs) with a directly sputtered Al cathode in a mixture of Ar and Kr showed a much lower leakage current density ($\sim 1\times {10}^{-5}\mathrm{mA}∕{\mathrm{cm}}^2$ at $-6\mathrm{V}$ ) than those ($\sim 1\times {10}^{-1}\mathrm{mA}∕{\mathrm{cm}}^2$ at $-6\mathrm{V}$ ) of OLEDs with an Al cathode prepared by FTS or dc sputtering in a pure Ar ambient. This indicates that the bombardment of energetic particles is effectively restricted by mixing a heavy noble gas. Based on the current-voltage curve for the OLED, a possible mechanism is proposed to explain the effect of a heavy noble gas mixture on electrical properties of OLEDs for direct Al cathode sputtering by FTS.