Effects of hole transport and energy barrier on the width of the exciton formation zone in organic light-emitting diodes

Phosphorescent organic light-emitting diodes (OLEDs) have been fabricated based on MoO3 p-doped N,N'-bis-(1-naphthyl)-di-phenyl-1,1' -biphenyl-4,4'-diamine (NPB:MoO3) and 4,4'-N,N'-dicarbazole-biphenyl (CBP:MoO3). The device using 20 nm NPB:MoO3/10 nm CBP:MoO3 shows increased width of the exciton formation zone and thereby relieved efficiency roll-off than the one using 30 nm CBP:MoO3. This is mostly because the NPB:MoO3/CBP:MoO3 reduces the transit time for holes from anodes to emissive layers than the CBP:MoI3. as a result of the higher hole mobility of NPB:MoO3 than that of CBP:MoO3. Although the energy barrier from NPB:MoO3 to CBP:MoO3 markedly affects the current density versus voltage characteristics of a device, it barely alters the width of the exciton formation zone. This is ascribed to the instantaneous passage of holes through the interfacial barrier, regardless of its magnitude, thereby offering little effect on the transit time for holes from the anode to the emissive layer. The current research provides novel insight into the working mechanism of OLED, hopefully pushing forward OLED technology towards high-luminance applications.