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

The authors found that the electroluminescence (EL) efficiency of organic l ight-emitting-diodes (LEDs) consisting of triphenyl diamine derivative (TPD ) and aluminium quinoline (Alq3) is enhanced by the introduction of a co-ev aporated layer of TPD and Alq3 at the interface between the TPD and Alq3 la yers. 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 w as obtained by the introduction of a 20 nm thick co-evaporated layer prepar ed using the doping ratio of TPD:Alq3 = 1:10. Considering that the photolum inescence efficiency of the co-evaporated layer did not increase, it was co ncluded that the increase of active sites for carrier transport and recombi nation due to the enlargement of the effective contact area between TPD and Alq3 causes the improvement of the EL efficiency.