Interfacial energetics of NaCl–organic composite layer at an OLED anode

Abstract

Although low work function alkaline halides are widely used as a cathode interlayer for organic light-emitting diodes (OLEDs), NaCl–organic composites are shown to be an efficient anodic buffer. Here we suggest a mechanistic origin of the improved OLED performance upon the use of a NaCl-containing organic buffer layer between an indium tin oxide (ITO) anode and N, N′-bis(naphthalene-1-yl)-N, N′-bis(phenyl)benzidine (NPB), based on the studies with ultraviolet photoelectron spectroscopy and atomic force microscopy. While a pure NaCl interlayer has a high hole-injection barrier (1.40 eV), the NPB : NaCl composite layer exhibits a substantially lower barrier (0.84 eV), which is comparable to the value at a bare ITO/NPB interface. Furthermore, the wettability of the composite onto ITO is enhanced due to significant adhesive interactions of NaCl with both ITO and NPB, leading to effective electrical contacts. The two key factors, i.e. the plausible hole-injection barrier and better wettability of the NPB : NaCl composite, contribute to the improved hole injection efficiency and lifetime.