Improved efficiency by a graded emissive region in organic light-emitting diodes

Electrical and optical properties of organic light-emitting diodes (OLEDs) with a stepwise graded bipolar transport emissive layer for a better control of charge transport and recombination are presented. The graded bipolar transport layer was formed by co-evaporating a hole-transporting material ${\mathrm{N,N}}^{\prime }$ -diphenyl-${\mathrm{N,N}}^{\prime }$ -bis${\text{(1,1}}^{\prime }$ -biphenyl)-${\text{4,4}}^{\prime }$ -diamine (NPB) and an electron-transporting/emissive material tris-(8-hydroxyquinoline) aluminum $({\mathrm{Alq}}_3)$ in steps, where each step has a different concentration ratio of NPB to ${\mathrm{Alq}}_3.$ Compared to a conventional heterojunction OLED, electroluminescence efficiency was enhanced by a factor of more than 1.5, whereas the turn-on voltage remained unchanged in the graded structure.