Hole injection improvement using ultrathin Li-TFSI layer in organic light-emitting diodes

A high device performance of organic light-emitting diodes (OLEDs) can be achieved by minimizing the hole injection barrier (HIB) at the anode interface. However, the work function of indium tin oxide (ITO) is not sufficiently high; thus, the use of an appropriate hole injection layer (HIL) is necessary. In this study, we demonstrated that bis(trifluoromethane)sulfonimide lithium salt (Li-TFSI) is an efficient HIL to decrease the HIB in OLEDs. The device performances of hole-only devices (HODs) and OLEDs were significantly enhanced by the insertion of the ultrathin Li-TFSI HIL between the N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB) hole transport layer and ITO. In situ ultraviolet photoelectron spectroscopy measurements revealed that the work function of ITO was considerably increased by the deposition of the Li-TFSI HIL. Accordingly, the HIB from ITO to NPB was remarkably reduced. This is the origin of the hole injection improvements in the HODs and OLEDs with the Li-TFSI HIL. Thus, Li-TFSI would be an efficient interface modifier to increase the work function of the electrode and decrease the HIB in various electronic devices.