Hole Injection Enhancement by a WO₃ Interlayer in Inverted Organic Light-Emitting Diodes and Their Interfacial Electronic Structures

다국어 초록 (Multilingual Abstract)

<P>The interfacial energy level alignment of hole injection layers with WO<SUB>3</SUB> 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 <I>N</I>,<I>N′</I>-bis(1-naphthyl)-<I>N</I>,<I>N′</I>-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<SUB>3</SUB> 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<SUB>3</SUB> conduction band close to the Fermi level form a charge generation layer without an interfacial chemical reaction. This is why the WO<SUB>3</SUB> 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<SUB>3</SUB> interlayer.</P><P><B>Graphic Abstract</B>
<IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/jpccck/2011/jpccck.2011.115.issue-14/jp111128k/production/images/medium/jp-2010-11128k_0007.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/jp111128k'>ACS Electronic Supporting Info</A></P>