Effect of location and width of doping region on efficiency in doped organic light-emitting diodes

We systematically investigated an optimum structure (doping width and location) of organic light-emitting diodes (OLED) with Coumarin 540 (C540)-doped aluminum quinoline (Alq3) emission layer. Our OLEDs consisted of ITO/triphenyl diamine derivative (TPD) [50 nm]/Alq3 emission layer [50 nm]/AlLi. We fabricated seven kinds of partially C540-doped OLEDs. These OLEDs are classified into two types of doping device structure: the OLEDs having continuous C540-doping region with a different doping width; the OLEDs having separated C540-doping regions. When the doping width was changed from 0 nm to 50 nm, the OLEDs with a 10-nm thick doping region had the highest EL efficiency of all. The main recombination zone was 10-nm thick near the TPD interface. C540 molecules are found to prevent electrons and holes from recombining on Alq3 molecules in an Alq3 layer by regarding C540 molecules in Alq3 as carrier traps and hopping sites.