Improved electron injection in organic LED with lithium quinolate/aluminium cathode

Lithium quinolate (Liq), covered with aluminium, was used as an electron injection layer in a double layer organic light emitting diode consisting of NPD as the hole transport layer and Alq as the emitting layer resulting in lower turn on voltage and increased power efficiency. The driving voltage required to achieve a luminance of 100 cd/m2 decreased from 5.8 V for the Ca/Al to 4.2 V when Liq/Al was used, improving device power efficiency from 2.3 to 4.1 lm/W. The performance tolerance to layer thickness of Liq devices is also better than that of the devices with lithium fluoride (LiF). Due to the highly insulating nature of LiF, it can only be used when deposited as an ultra-thin layer, while the Liq can be deposited into layers as thick as 5 nm without significantly affecting the EL properties. An Liq electron injecting layer has also been tried in combination with Ca, Mg and Ag cathodes. Our experiments support the assumption that free lithium is released from lithium quinolate, as in the case of lithium fluoride, when Liq is over coated with active metals such as Al.