Contact formation of LiF/Al cathodes in Alq-based organic light-emitting diodes

A systematic study has been carried out on the formation of an effective electron-injecting contact by depositing an LiF/Al bilayer on tris-(8-hydroxyquinoline) aluminium (Alq) in organic light-emitting diodes. Efficient electron injection is observed in both a LiF/Al bilayer cathode and an LiF-doped Al composite cathode. An analysis with ultraviolet photoelectron spectroscopy reveals a strong similarity in interface chemistry between LiF/Al and LiF-doped Al on Alq. Measurements with high-resolution electron energy loss spectroscopy show limited interfacial reaction of LiF on both Al and Alq, whereas a strong attenuation of the loss peak related to the Li-F stretch mode is observed after depositing an ultrathin Al film on Alq/LiF. The results indicate that the contact is formed as a consequence of chemical reaction with a reacted layer of 1 nm or less. Molecular orbital calculation suggests that the release of Li and subsequent reaction with Alq is thermodynamically allowed. The shallow-contact nature allows for much greater flexibility in the design of cathode structures and potential applications to various device configurations.

[1]  N. Peyghambarian,et al.  Photoemission spectroscopy of LiF coated Al and Pt electrodes , 1998 .

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

[3]  Shui-Tong Lee,et al.  Theory of magnesium/Alq3 interaction in organic light emitting devices , 1999 .

[4]  S. Shaheen,et al.  A model for the current–voltage characteristics and the quantum efficiency of single-layer organic light emitting diodes , 1997 .

[5]  C. Tang,et al.  Application of an ultrathin LiF/Al bilayer in organic surface-emitting diodes , 2001 .

[6]  C. Tang,et al.  Organic Electroluminescent Diodes , 1987 .

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

[8]  Shui-Tong Lee,et al.  Direct evidence for interaction of magnesium with tris(8-hydroxy-quinoline) aluminum , 2000 .

[9]  M. Matsumura,et al.  Effect of Al/LiF Cathodes on emission efficiency of organic EL devices , 1998 .

[10]  W. R. Salaneck,et al.  Interfacial chemistry of Alq3 and LiF with reactive metals , 2001 .

[11]  Yongli Gao,et al.  Investigation of the interface formation between calcium and tris-(8-hydroxy quinoline) aluminum , 1998 .

[12]  N. Peyghambarian,et al.  Aluminum based cathode structure for enhanced electron injection in electroluminescent organic devices , 1998 .

[13]  Paul Seidler,et al.  Organic–inorganic multilayer structures: a novel route to highly efficient organic light-emitting diodes , 1999 .

[14]  S. Shaheen,et al.  Bright blue organic light-emitting diode with improved color purity using a LiF/Al cathode , 1998 .

[15]  Albrecht Winnacker,et al.  Impact of the cathode metal work function on the performance of vacuum-deposited organic light emitting-devices , 1999 .

[16]  Franco Cacialli,et al.  LiF/Al cathodes and the effect of LiF thickness on the device characteristics and built-in potential of polymer light-emitting diodes , 2000 .

[17]  S. Shaheen,et al.  Highly efficient and bright organic electroluminescent devices with an aluminum cathode , 1997 .