Enhancement of electroluminescence efficiency for organic light-emitting-diodes due to the introduction of a co-evaporated layer

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Abstract
The authors found that the electroluminescence (EL) efficiency of organic light-emitting-diodes (LEDs) consisting of triphenyl diamine derivative (TPD) and aluminium quinoline (Alq3) is enhanced by the introduction of a co-evaporated layer of TPD and Alq3 at the interface between the TPD and Alq3 layers. The organic LED with a best co-evaporated layer has two to four times higher EL efficiency (lm W-1) than that without. The highest enhancement was obtained by the introduction of a 20 nm thick co-evaporated layer prepared using the doping ratio of TPD:Alq3 = 1:10. Considering that the photoluminescence efficiency of the co-evaporated layer did not increase, it was concluded that the increase of active sites for carrier transport and recombination due to the enlargement of the effective contact area between TPD and Alq3 causes the improvement of the EL efficiency.