Highly efficient top-emitting organic light-emitting devices with aluminium electrodes

Top-emitting organic light-emitting diodes (OLEDs) using aluminium (Al) and nickel (Ni) as the anode and Al as the semitransparent cathode were developed. Hole injection was largely improved by spin coating poly(ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT) on the metal anode. For top-emitting OLEDs with a configuration of metal anode/PEDOT/NPB/Alq(3)/LiF/Al/Alq(3), both devices using Al and Al/Ni anodes showed efficient green light emission with a narrow emission peak. Current efficiency was increased from 4.32 cd A(-1) to 6.45 cd A(-1) when replacing high work function Ni with low work function At as the anode. A luminance of 20 000 cd m(-2) can be achieved for these top-emitting OLEDs, The device made with the Al/Ni anode showed a low driving voltage and high power efficiency as compared to that made with the At anode. The improvement on light-emitting efficiency was attributed to balanced charge carrier injection and utilization of the high reflective semitransparent Al cathode.