Strong interfacial dipole formation with thermal evaporation of lithium cobalt oxide for efficient electron injections

We investigated the electronic structures at the interface of Al/lithium cobalt oxide (LiCoO2)/tris(8-hydoxyquinoline) aluminum (Alq3) to elucidate the origin of the electron injection enhancement with the insertion of the LiCoO2 layer in organic light-emitting devices using in situ photoelectron spectroscopy experiments. We discovered that LiCoO2 was decomposed into lithium oxide (Li2O) by thermal evaporation, and only Li2O was deposited on the desired substrate. Li2O forms a strong interfacial dipole, which reduces the surface potential on Alq3 due to its extremely low work function. As a result, the electron injection barrier was dramatically decreased by the Li2O layer. Furthermore, there is no strong chemical interaction at the interface of Al/Li2O/Alq3; hence, this would contribute to extend the device lifetime.

[1]  Yu-Hung Chen,et al.  Electronic structures and electron-injection mechanisms of cesium-carbonate-incorporated cathode structures for organic light-emitting devices , 2006 .

[2]  S. Krause,et al.  Determination of transport levels of organic semiconductors by UPS and IPS , 2008 .

[3]  Chun-Sing Lee,et al.  Electronic structures of organic/organic heterojunctions: From vacuum level alignment to Fermi level pinning , 2007 .

[4]  Yang Yang,et al.  Low‐Work‐Function Surface Formed by Solution‐Processed and Thermally Deposited Nanoscale Layers of Cesium Carbonate , 2007 .

[5]  L. W. James,et al.  Behavior of Cesium Oxide as a Low Work‐Function Coating , 1970 .

[6]  M. Tsuchida,et al.  Organic EL cells using alkaline metal compounds as electron injection materials , 1997 .

[7]  Y. Yi,et al.  Deposition sequence dependent variation in interfacial chemical reactions between 8-hydroxyquinolatolithium and Al , 2008 .

[8]  A. Kahn,et al.  Charge generation layers comprising transition metal-oxide/organic interfaces: Electronic structure and charge generation mechanism , 2010 .

[9]  W. R. Salaneck,et al.  Electronic structure of tris(8-hydroxyquinoline) aluminum thin films in the pristine and reduced states , 1999 .

[10]  W. Klein.  Photoemission from Cesium Oxide Covered GaInAs , 1969 .

[11]  C. Tang,et al.  Enhanced electron injection in organic electroluminescence devices using an Al/LiF electrode , 1997 .

[12]  Liduo Wang,et al.  Lithium cobalt oxide as electron injection material for high performance organic light-emitting diodes , 2008 .

[13]  The reduction of effective doping with extra dopant: n-Type doping of tris(8-hydroxyquinoline) aluminum with K , 2012 .

[14]  Tae Gun Kim,et al.  Highly enhanced electron injection in organic light-emitting diodes with an n-type semiconducting MnO2 layer , 2012 .

[15]  S. W. Cho,et al.  Photoemission study of the electronic structures of tris-(8-hydroxyquinoline) aluminum/Li2O∕Al interfaces , 2007 .

[16]  D. Milliron,et al.  Organic semiconductor interfaces: electronic structure and transport properties , 2000 .

[17]  A. Winnacker,et al.  Performance and stability of poly(phenylene vinylene) based polymer light emitting diodes with caesium carbonate cathode , 2009 .

[18]  Direct evidence of n-type doping in organic light-emitting devices: N free Cs doping from CsN3 , 2012 .

[19]  C. Adachi,et al.  Efficient Electron Injection Mechanism in Organic Light-Emitting Diodes Using an Ultra Thin Layer of Low-Work-Function Metals , 2003 .

[20]  Shui-Tong Lee,et al.  Mechanism of Cs2CO3 as an n-type dopant in organic electron-transport film , 2011 .

[21]  David J. Giesen,et al.  Photoemission study of aluminum/tris-(8-hydroxyquinoline) aluminum and aluminum/LiF/tris-(8-hydroxyquinoline) aluminum interfaces , 2000 .

[22]  M. Itoh,et al.  Optical Spectra of Excitons in Lithium Oxide , 1999 .