Organic light-emitting diodes with improved hole–electron balance and tunable light emission with aromatic diamine/bathocuproine multiple hole-trapping-layer

Organic light-emitting diodes (OLEDs) with multiple hole-trapping-layer (HTL) structures, which consist of N,N′-bis-(1-naphthyl)-N, N′-diphenyl-1,1′-biphenyl 4,4′-diamine (NPB), bathocuproine (BCP) and tris(8-hydroxy-quinoline)aluminum (Alq3) have been fabricated. The emitting zone of the device with the structure of ITO/[NPB/BCP]n/Alq3/LiF/Al can be changed by the number n of HTL. The effect of the multiple HTL structure upon the performance of OLEDs with tris(8-hydroxyquinoline) aluminum as the emitting material has been investigated. Compared with the brightness of the conventional green emitting diode without the multiple HTL structure, that of the diode with periodic number of five has been increased from 2512.8 cd/m2 to 8661.0 cd/m2. Such an improvement in the device performance was attributed to the improved hole–electron balance, which can be further attributed to the introduction of the multiple HTL structure.