Bandgap engineering in Alq3- and NPB-based organic light-emitting diodes for efficient green, blue and white emission

Bandgap engineering by insertion of a hole-blocking layer (HBL) between the hole-injection layer (HIL) and hole-transporting layer (HTL) for green and blue-emitting organic light-emitting device is shown to improve the current efficiency by 30% and 70%, respectively. The improvement was attributed to a better electron–hole balance in the device. Two different organic materials, 2,9-dimethyl-4,7-diphenylphenanthroline and tris-(8-hydroxyqunoline) aluminum were used as HBL. Variation of HBL and HTL thickness was shown to adjust the current efficiency and operating voltage, respectively. Insertion of a thin HBL between HIL and HTL for blue-emitting devices also resulted in appearance of multicolor emission which was used to produce a pure white light.