The interfacial energy level alignment of hole injection layers with WO 3 insertion in an inverted organic light-emitting diode structure and its influence on electroluminescence have been studied using in situ X-ray, ultraviolet photoelectron spectroscopy, and device measurements. The hole injection barrier for N, N′ -bis(1-naphthyl)- N, N′ -diphenyl-1,1′-biphenyl-4,4′-diamine (NPB) upon Al deposition was estimated to be 1.37 eV, which makes an Al anode unfavorable for hole injection. With a thin WO 3 layer deposited onto the NPB, the NPB highest occupied molecular orbital (HOMO) level bends dramatically to 0.15 eV below the Fermi level. This NPB HOMO level and WO 3 conduction band close to the Fermi level form a charge generation layer without an interfacial chemical reaction. This is why the WO 3 interlayer dramatically helps charge injection even with a low work-function Al metal anode. Indeed, the electroluminescence and current efficiency from this structural device were greatly enhanced by 3 orders of magnitude compared with that without a WO 3 interlayer.
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Kim et al. (2011) studied this question.