AN IMPROVED ELECTRODE STRUCTURE FOR ORGANIC LIGHT EMITTING DIODE DEVICES

Abstract: An improved organic light emitting diode device (20) is disclosed. The organic light emitting diode device (20) includes a first electrode (25), a second electrode (22), and an organic stack (23) interposed between the first electrode (25) and the second electrode (22). The organic stack (23) may include hole transport materials located on one side of the stack and electron transport materials located on another side of the stack. The improvement includes a thin layer (24) of high work function material interposed between the first electrode (25) and the organic stack (23). The organic light emitting diode device (20) according to the present invention has improved stability.

Description: AN IMPROVED ELECTRODE STRUCTURE FOR ORGANIC LIGHT EMITTING DIODE DEVICES Field of the Invention The present invention relates to electrode structures for an organic light emitting diode (OLED) device. In particular, the present invention relates to an improved anode structure for an OLED device. Background of the Invention Fig. 1 discloses a known OLED device 10. The OLED device 10 includes a glass substrate 11. A transparent hole-injecting anode or electrode 12 formed of indium-tin oxide (ITO) is located on the glass substrate 11. An organic stack 13 is located on the anode 12. The anode 12 forms the first or bottom layer of the stack 13. A metal cathode 14 is located on top of the organic stack 13. In the device 10, light is emitted in a downward direction through the electrode 12 and the glass substrate 11. OLED require two electrodes, an anode 12 and a cathode 14, at least one of which must be transparent to allow light to escape. The anode 12, which must supply holes to the emitting region, is usually formed from a material with high work function, since this will lead to a low energy for hole injection. The cathode 14, which must supply electrons to the emitting region, is usually formed from a material with low work function, which will facilitate electron injection. Indium tin oxide (ITO), which is a good transparent conductor has long been used as an anode material, because it has a high work function, about 5.0 eV. The injection efficiency of ITO, and consequently the luminescent efficiency of the OLED, is enhanced by pretreating the ITO with an oxygen plasma, which ensures that there is an excess of oxygen at the interface with the organic stack 13. The excess of oxygen appears to contribute to the instability of such OLEDs because the oxygen can migrate and react with the organic materials. ITO has also been used in conjunction with thin metal interlayers to form cathode contacts, where the thin metal is of a low work function material to provide electron injection. Objects of the Invention It is an object of the present invention to provide an improved electrode structure for an OLED device. It is another object of the present invention to provide an improved anode structure for an OLED device. It is another object of the present invention to provide an OLED device having improved stability. It is another object of the present invention to provide an anode with a thin interface layer to improve OLED device stability. It is another object of the present invention to improve stability of an OLED device by reducing oxygen migration. It is another object of the present invention to isolate the ITO layer from the organic stack by interposing a thin metal layer of a high work function material. Summary of the Invention The present invention is directed to an improved organic light emitting diode device. The organic light emitting diode device includes a first electrode, a second electrode, and an organic stack interposed between the first electrode and the second electrode. The organic stack may include hole transport materials located on one side of the stack and electron transport materials located on another side of the stack. The improvement includes a thin layer of high work function material interposed between the first electrode and the organic stack. The organic light emitting diode device according to the present invention has improved stability. The thin layer of high work function material may be formed from a material selected from the group consisting of Mo and alloys of Mo. The thin layer of high work function material may be formed from a material selected from the group consisting of W and alloys of W. Alternatively, the thin layer of high work function material may be formed from a material selected from the group consisting of Nb, Zr, Co, Zn, Tc, Hf, Ta, Cr, Au, Pt, Pd, Se, Ni and alloys of at least one of Nb, Zr, Co, Zn, Tc, Hf, Ta, Cr, Au, Pt, Pd, Se, and Ni. The thin layer of the high work function material may have a thickness of less than 100 A. The first electrode may be a transparent electrode and the second electrode may be a cathode. The second electrode may be formed from a low work function material. The present invention is also directed to an improved electrode structure for an organic light emitting device. The electrode may comprise an electrode layer, and a thin layer of a high work function material located adjacent the electrode layer. The thin layer of high work function material may be formed from a material selected from the group consisting of Mo and alloys of Mo. The thin layer of high work function material may be formed from a material selected from the group consisting of W and alloys of W. The thin layer of high work function material may be formed from a material selected from the group consisting of Nb, Zr, Co, Zn, Tc, Hf, Ta, Cr, Au, Pt, Pd, Se, Ni and alloys of at least one of Nb, Zr, Co, Zn, Tc, Hf, Ta, Cr, Au, Pt, Pd, Se, and Ni. The electrode structure may be an anode. The anode may be a transparent anode. The thin layer of the high work function material has a thickness of less than 100 A. Alternatively, the electrode structure may be a cathode. The present invention is also directed to an organic light emitting diode device having improved stability. The device includes an anode layer formed from a high work function material, an organic stack having hole transport materials located on one side of the stack and electron transport materials located on another side of the organic stack, a transport electrode layer, and a thin layer of a low work function material located between the organic stack and the transport layer. The anode layer may be formed from a material selected from the group consisting of Mo and alloys of Mo. The anode layer may be formed from a material selected from the group consisting of W and alloys of W. The anode layer may be formed from a material selected from the group consisting of Nb, Zr, Co, Zn, Tc, Hf, Ta, Cr, Au, Pt, Pd, Se, Ni and alloys of at least one of Nb, Zr, Co, Zn, Tc, Hf, Ta, Cr, Au, Pt, Pd, Se, and Ni. Brief Description of the Drawings The invention will be described in conjunction with the following drawings in which like reference numerals designate like elements and wherein: Fig. 1 is a schematic view of a known organic light emitting diode device; and Fig. 2 is a schematic view of an organic light emitting diode device in accordance with an embodiment of the present invention. Detailed Description of the Preferred Embodiment Fig.2 discloses an organic light emitting diode device 20 according to an embodiment of the present invention. The OLED device 20 includes a substrate 21. A first electrode 22, which functions as a cathode, is located on the substrate 21. An organic stack 23 is located on the first electrode 22. An intermediate layer 24 is located on the organic stack 23. A second electrode 25 is located on top of the intermediate layer 24. The second electrode 25 is an anode and is formed from ITO. It, however, is contemplated that other suitable materials including but not limited to indium zinc oxide (IZO) may be used to form the second electrode 25. The intermediate layer 24 is provided to isolate the second electrode 25 from the organic stack 23 to improve the stability of the OLED device 20. The layer 24 reduces oxygen migration from the second electrode 25 and reaction with the organic materials. The intermediate layer 24 is preferably a thin layer of high work function material. The layer 24 is preferably formed from Mo or alloys of Mo. The present invention, however, is not limited to thin layers formed from Mo; rather, it is contemplated that other materials having high work function properties may be used to increase the stability of the OLED device 20. These materials include, but are not limited to W, Nb, Zr, Co, Zn, Tc, Hf, Ta, Cr, Au, Pt, Pd, Se, and Ni. Oxides and nitrides of these materials may also be used to improve stability so long as che material has a high work function. With this arrangement, the second electrode 25 formed from ITO is used only for its transparency and not for its work function and injection properties. When the second electrode 25 is a transparent anode, the intermediate layer 24 must be sufficiently thin (e.g. a thickness of than 100 A) to permit light to escape through the second electrode and for conductivity. Oxides and nitrides are often insulating so the thickness must be controlled so that the device 20 may operate effectively. The first electrode 22 may then be opaque. Alternatively, the high work function materials, discussed above, may be substituted for the second electrode 25. The second electrode 25 is not transparent. Accordingly, the first electrode 21, which functions as a cathode, must be transparent or semitransparent to permit light to escape. While this invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, the preferred embodiments of the invention as set forth herein are intended to be illustrative, not limiting. Various changes may be made without departing from the spirit and scope of the invention as defined in the following claims.

Claims: WHAT IS CLAIMED IS: 1. In an organic light emitting diode device having a first electrode, a second electrode, and an organic stack interposed between said first electrode and said second electrode, said organic stack including hole transport materials located on one side and electron transport materials located on another side, the improvement comprising: a thin layer of high work function material interposed between said first electrode and said organic stack. 2. The organic light emitting diode device according to Claim 1 , wherein said thin layer of high work function material is formed from a material selected from the group consisting of Mo and alloys of Mo. 3. The organic light emitting diode device according to Claim 1 , wherein said thin layer of high work function material is formed from a material selected from the group consisting of W and alloys of W. 4. The organic light emitting diode device according to Claim 1 , wherein said thin layer of high work function material is formed from a material selected from the group consisting of Nb, Zr, Co, Zn, Tc, Hf, Ta, Cr, Au, Pt, Pd, Se, Ni and alloys of at least one of Nb, Zr, Co, Zn, Tc, Hf, Ta, Cr, Au, Pt, Pd, Se, and Ni. 5. The organic light emitting diode device according to Claim 1 , wherein said thin layer of said high work function material has a thickness of less than 100 A. 6. The organic light emitting diode device according to Claim 1 , wherein said first electrode is a transparent electrode and said second electrode is a cathode. 7. The organic light emitting diode device according to Claim 6, wherein said second electrode is formed from a low work function material. 8. The organic light emitting diode device according to Claim 1 , wherein said device has improved stability. 9. An improved electrode structure for an organic light emitting device, said electrode comprising: an electrode layer; and a thin layer of a high work function material located adjacent said electrode layer. 10. The electrode structure according to Claim 9, wherein said thin layer of high work function material is formed from a material selected from the group consisting of Mo and alloys of Mo. 1 1. The electrode structure according to Claim 9, wherein said thin layer of high work function material is formed from a material selected from the group consisting of W and alloys of W. 12. The electrode structure according to Claim 9, wherein said thin layer of high work function material is formed from a material selected from the group consisting of Nb, Zr, Co, Zn, Tc, Hf, Ta, Cr, Au, Pt, Pd, Se, Ni and alloys of at least one of Nb, Zr, Co, Zn, Tc, Hf, Ta, Cr, Au, Pt, Pd, Se and Ni. 13. The electrode structure according to Claim 9, wherein said electrode structure is an anode. 14. The electrode structure according to Claim 13, wherein said anode is a transparent anode. 15. The electrode structure according to Claim 14, wherein said thin layer of said high work function material has a thickness of less than 100 A. 16. The electrode structure according to Claim 9, wherein said electrode structure is a cathode. 17. The electrode structure according to Claim 9, wherein said electrode layer is transparent. 18. The electrode structure according to Claim 9, wherein said thin layer of said high work function material has a thickness of less than 100 A. 19. An organic light emitting diode device having improved stability comprising: an anode layer formed from a high work function material; an organic stack having hole transport materials located on one side of said stack and electron transport materials located on another side of said organic stack; a transport electrode layer; and a thin layer of a low work function material located between said organic stack and said transport layer. 20. The organic light emitting diode device according to Claim 19, wherein said anode layer is formed from a material selected from the group consisting of Mo and alloys of Mo. 21. The organic light emitting diode device according to Claim 19, wherein said anode layer is formed from a material selected from the group consisting of W and alloys of W. 22. The organic light emitting diode device according to Claim 19, wherein said anode layer is formed from a material selected from the group consisting of Nb, Zr, Co, Zn, Tc, Hf, Ta, Cr, Au, Pt, Pd, Se, Ni and alloys of at least one of Nb, Zr, Co, Zn, Tc, Hf, Ta, Cr, Au, Pt, Pd, Se and Ni.

Inventors: HOWARD WEBSTER E, JONES GARY W, HOWARD, WEBSTER, E., JONES, GARY, W.
Applicants: FED CORP, HOWARD WEBSTER E, JONES GARY W, FED CORPORATION, HOWARD, WEBSTER, E., JONES, GARY, W.
Application year: 1999
Main classification: H01J1
IPC codes: H01L51, H05B33