Patent Application
20070222371
The present invention relates to a top-emitting organic light-emitting diode device having metallic electrodes with improved reliability and enhanced operational stability.
An organic electroluminescent (OEL) device, alternately known as organic light emitting diode (OLED), is useful in flat-panel display applications. This light-emissive device is attractive because it can be designed to produce red, green, and blue colors with high luminance efficiency; operable with a low driving voltage on the order of a few volts, and clearly viewable from oblique angles. These unique attributes are derived from a basic OLED structure including a multilayer stack of thin films of small-molecule organic materials sandwiched between an anode and a cathode. Tang et al in commonly-assigned U.S. Pat. Nos. 4,769,292 and 4,885,211 disclose such a structure. The common electroluminescent (EL) medium includes a bilayer structure of a hole-transport (HTL) layer and an electron-transport layer (ETL), typically on the order of a few tens of nanometer (nm) thick for each layer. When an electrical potential difference is applied at the electrodes, the injected carriers, hole at the anode and electron at the cathode, migrate towards each other through the EL medium and a fraction of them recombines in the emitting layer (EML) a region close to the HTL/ETL interface, to emit light. The intensity of electroluminescence is dependent on the EL medium, drive voltage, and charge injecting nature of the electrodes. The light viewable outside of the device is further dependent on the design of the organic stack and optical properties of the substrate, and electrodes.
Conventional OLEDs are bottom emitting (BE), meaning that the display is viewed through the substrate that supports the OLED structure. The devices normally employ transparent glass substrates having a highly transparent indium-tin-oxide (ITO) layer that also serves as the anode. The cathode is typically a reflective thin film of Mg—Ag alloy, although lithium-containing alloys are also used as an efficient electron-injecting electrode. The light generated within the device is emitted in all directions. However, only a small fraction of generated light is available for viewing, and about 80% of generated light is trapped within the device in waveguiding modes in glass, ITO and organic layers. The light emitted toward the anode at less than the critical angle passes through the anode and through the substrate to the viewer, and the light emitted in the opposite direction is reflected at the cathode and passes through the anode and the substrate, enhancing the viewing intensity. A transparent substrate, a high-transparency anode and a high-reflectivity cathode are thus required to yield high luminance efficiency devices.
The OLED display elements are typically coupled with active matrix (AM) circuitry in order to produce high performance displays. The AM display uses switching elements of thin film transistors. The transistors are fabricated on glass substrates and are not transparent. Consequently the entire display area on the substrate is not available for the light to emerge. With the application of multi-transistor and complex circuitry in the backplane the open area through which the light emerges is significantly reduced. The ratio of the open area to that of the entire display area is called the aperture ratio. Due to the reduction of the aperture ratio the display will run dim. To compensate for the reduced average brightness level the drive current has to be increased subjecting the display to increased risk of operational degradation. It follows that further improvement in back plane design cannot be readily implemented without further compromising the aperture ratio and the operational stability.
To alleviate this problem the emitted light should emerge through the top surface. In the top-emitting design the drive circuitry is fabricated on substrate as in the bottom-emitting display but the light emerges through the surface opposite to the substrate. This design permits the use of complex circuitry occupying whatever substrate space is needed and the light-emitting area and the aperture ratio is not affected. The high aperture ratio makes the display viewable consuming less power. The devices have the prospect of running at low drive current while maintaining readability and thus extending the operational life.
In devices employing opaque backplanes such as silicon, the OLED must be of the top-emitting type. The top surface, usually the cathode, needs to be at least semitransparent to permit light to exit through the top. The device should preferably include a reflector or a reflecting anode opposite to the cathode side to redirect the light that strikes the anode to the cathode side.
Any device design should be aimed at achieving highest possible efficiency. However, realizing high efficiency by reclaiming light lost to waveguiding modes can be very difficult. To recover even a fraction of light lost to the waveguiding modes the device architecture can be very complex.
An approach to enhance the efficiency without introducing such complexity is to implement a microcavity design for the device, which includes reflecting electrodes. By employing highly reflective electrodes it is possible to remarkably increase the out-coupling of generated light. In microcavity OLEDs, metallic electrodes are generally preferred to meet the requirements of electrical conductivity and light transmission. To realize high efficiency a reflective opaque anode and a low absorption reflective cathode are needed. Sony Corporation (EP 1 154 676 A1) disclosed an anode made of light-reflecting materials such as Pt, Au, Cr, W, or presumably other high-work function materials in conjunction with an optional buffer/hole-injecting layer (HIL). Sony also disclosed (EP 1 102 317 A2) an anode composed of a transparent conducting film such as ITO formed on the reflecting layer. The top electrode is a semitransparent reflecting layer of MgAg or Al:Li alloy serving as the cathode through which the light emerges. Lu et al. disclose top-emitting, highly efficient OLEDs that use reflective metals in the anode structure, a phosphorescent emissive layer, Ir(ppy)3, and a semitransparent compound cathode. (“High-efficiency top-emitting organic light-emitting devices”, M.-H. Lu, M. S. Weaver, T. X. Zhou, M. Rothman, R. C. Kwong, M. Hack, and J. J. Brown, Appl. Phys. Lett. 81, 3921 (2002)). Riel et al. (“Phosphorescent top-emitting organic light-emitting devices with improved light outcoupling”, H. Riel, S. Karg, T. Beierlein, B. Rushtaller, and W. Rieb, Appl. Phys. Lett. 82, 466 (2003) disclosed a high-efficiency top emitter, also using the Ir(ppy)3 emissive layer, high work-function metal anodes, and semitransparent metal cathodes and further employing a ZnSe capping layer over the semitransparent compound cathode for improved light outcoupling. These top-emitters demonstrated efficiencies that are higher than the equivalent bottom-emitting non-microcavity devices. Raychaudhuri et.al. disclose top- and bottom-emitting microcavity devices that are twice as efficient as the optimized bottom-emitting non-microcavity device (“Performance enhancement of top- and bottom-emitting organic light-emitting devices using microcavity structures”, P. K. Raychaudhuri, J. K. Madathil, Joel D. Shore, and Steven A. Van Slyke, J SID Dec.3 (2004) p 315.
In top-emitting OLED the top electrode, the cathode usually includes a low work function metal or a metal on an electron-injecting surface. To achieve high efficiency the transparency of the top-electrode needs to be high requiring the use of a low absorption material in the thinnest possible form. Ag is most preferred as the top electrode as it meets the requirements very appropriately. Ag films can be made highly transmissive and reflective resulting in strong microcavity effect and significantly enhancing emission out-coupling. Furthermore, Ag thin film is sufficiently electrically conductive making implementation possible in a reasonably sized display without using an overlayer of a transparent conductive oxide (TCO) or buss lines.
One of the disadvantages of an Ag cathode is the unpredictability of device yield and irreproducibility of device characteristics. Some of the devices as made do not exhibit electroluminescence. The devices that initially work often fail in operation. The modes of failure include rapid performance degradation and catastrophic failure. In the rapid performance degradation mode the luminance falls and drive voltage rises very rapidly in operation. In the catastrophic mode the luminescence vanishes unpredictably and instantly. The devices showing no EL properties, as made or that fail in operation have or developed electrical shorts. The drive current flows through the shorting paths and not through the EL medium resulting in no electroluminescence.
The exact mechanism by which Ag induces shorting is not known. However, reactivity of Ag is high and its adhesion is low. The interface between the organic and Ag can be incomplete and unstable defects can exist. With the existence of these defects paths are generated in addition to normal carrier path resulting in a leaky or short-circuited diode. The leaky diodes are believed responsible for catastrophic failures in operation.
Another problem associated with thin electrode is that the film is not robust. In order to preserve the integrity of the electrode, a capping layer is often necessary. Furthermore, the capping layer when properly selected can also enhance the out coupling of light. The capping layer must be made of highly transparent materials. Deposition methods for transparent coating generally involve sputter deposition. Sputtering permits optimization during film deposition of the film composition needed for maximization of transparency. However, the sputtering deposition of the capping layer on the thin Ag layer increases the occurrence of short circuits between the anode and the cathodes causing further reduction of device yields. Thus, an alternative to Ag electrode is desired from the manufacturing point of view where consistency of process and high yield are of prime concern.
Aluminum, being highly reflective, electrically conductive and stable, is commonly used as a cathode in bottom emitting OLEDs. Al can also be one of the preferred top-electrode materials in a top-emitting device. However Al is not generally considered suitable because of expected low efficiency due to high absorptivity of Al. The absorbance of a 10 nm Al thin film is 30-50% in the visible wavelength range (“Optical Properties of Semi-transparent Metal Cathode for Top Emission Organic Light Emitting Diode”, Chan-Jae Lee, Dae-Gyu Moon, Jeong-In Han, Sung-Ho Baek, No-Hoon Park and Seoung-Sam Ju, Proceedings of the 8th Asian Symposium on Information Display, p 693, Feb. 15-17, 2004, Nanjing, China). The absorbance of a Ag film of comparable thickness is 5-10%. Thin metallic films generally require a capping layer to preserve their integrity. A capping layer including high transparency materials is employed to limit to attenuation of the emitted light. When properly selected the capping layer can enhance the output of a top-emitting device. Mechanistically, the capping layer increases the luminance, primarily by reducing the absorption of light within the semi-transparent cathode (“Performance enhancement of top- and bottom-emitting organic light-emitting devices using microcavity structures”, P. K. Raychaudhuri, J. K. Madathil, Joel D. Shore, and Steven A. Van Slyke, J SID Dec.3 (2004) p 315). Thus the capping layer is also termed as the absorption reduction layer (ARL) or the antiabsorption layer (AAL). From an optical simulation study of a TE OLEDs with 10-nm Al cathode an increase of total emission amounting to 70% is predicted for a 60-nm thick ITO capping layer. (“Optical Simulation of Top-emitting Organic Light Emitting Diodes, H. J. Peng, C. F. Qie, Z. L. Xie, H. Y. Chen, M. Wong and H. S. Kwok, Proceedings of the 8th Asian Symposium on Information Display, p 331, Feb. 15-17, 2004, Nanjing, China). For semitransparent Ag cathode an antiabsorption layer of Alq having similar optical properties as ITO typically increases the emission by only about 20%. It is possible that an antiabsorption layer is more effective on a metal layer with higher absorbance. Even with higher enhancement of top-emission with a capping layer the semitransparent Al electrode devices may still not be as efficient as the device that uses Ag as the semitransparent electrode. However, manufacturing yield and operational stability should be superior to those of the Ag electrode devices.
The electrode structure involving Al can be unstable if the capping layer is not properly selected. One of the most commonly used capping layers is ITO. Thermodynamic data suggest that the components of ITO, In2O3 and SnO2, are reducible by Al. In operation the reduced In and Sn can accumulate at the electrode-capping layer interface degrading the device performance. In extended operation the In and Sn can diffuse through the cathode and eventually creating electrical shorts between the anode and the cathode causing complete failure of the device. As Al is very reactive to many oxides, nitrides and other compounds that included in the transparent capping layer, the electrode structure can be unstable. For long-term stability consideration, the capping materials should be selected on the basis of thermodynamic functions, particularly the free energy function. It should be recognized that the conductance of a thin semitransparent Al film is far superior to those of TCO films. Hence the capping layer is not restricted to a TCO allowing access to a wider variety of transparent materials.
It is therefore an object of the present invention to provide a top-emitting OLED device (TE-OLED) with improved long-term operational stability. Another object of the present invention is to provide a device capable of being manufactured with high yield. It is a further object of this invention to provide a cathode with increased lateral electrical conductivity for improved a real uniformity of luminance.
These objects are achieved by an OLED device capable of emitting light through the top electrode of such device comprising:
(a) a substrate;
(b) a reflective and conductive first electrode including a metal or metal alloy or both formed over the substrate;
(c) at least one organic layer formed over the first electrode and including an emissive layer having electroluminescent material; and
(d) a semitransparent, reflective and conductive second electrode provided over the organic layer, wherein the second electrode includes a first layer having material M, wherein M is a metal, and a second layer providing an anti-absorption function in contact with the first layer and having a compound M′X , wherein M′ is a ′metal and X is a non-metal, wherein M′X has a free energy of formation equal to or more negative than the free energy of formation of the compound MX.
Throughout the ensuing description acronyms are used to designate the names of the different organic layers and operating features of organic light-emitting devices. For reference they are listed in Table 1.
Turning to
According to another embodiment of the present invention as shown in
According to yet another embodiment of the present invention as shown in
In operation in the three embodiments, the first and the second electrode are connected to a voltage source and electrical current is passed through the organic layers, resulting in light generation in the emissive layer. The intensity of generated light is dependent on the luminescent and electrical characteristics of the organic layers as well as the charge-injecting natures of the electrodes and the charge-injection layers, and the magnitude of the electrical current passed through the OLED device. A part of the generated light is emitted through the top electrode in the direction shown by the arrow. The emission viewable from the top surface is further dependent on the reflectance of the bottom electrode, the layer structure of the OLED, the transmittance of the semitransparent top electrode and the antiabsorption properties of the AAL layer.
The composition and the function of the various layers constituting the OLED device are described as follows.
Substrates 101 (
The bottom electrode (
The bottom electrode (
The bottom electrode 301 (
Hole-injection layer 103 (
Hole-transport layer 104 (
wherein:
Useful selected (fused aromatic ring containing) aromatic tertiary amines are the following:
Thickness of the HTL is chosen to maximize the luminance and its selection is dependent on the optical stack including the device. Both electrodes of the device of the present invention are metallic and substantially reflective; the optical path lengths from the emissive zone to the electrodes are to be chosen following a microcavity model in reference to
Emissive layer 105 (
Preferred host materials include the class of 8-quinolinol metal chelate compounds with the chelating metals being Al, Mg, Li, Zn, for example. Another preferred class of host materials includes anthracene derivatives such as 9,10 dinaphthyl anthracene; 9,10 dianthryl anthracene; and alkyl substituted 9,10 dinaphthyl anthracene, as disclosed in Shi et al. commonly assigned U.S. Pat. No. 5,935,721.
Dopant materials include most fluorescent and phorphorescent dyes and pigments. Preferred dopant materials include coumarins such as coumarin 6, dicyanomethylenepyrans such as 4-dicyanomethylene-4H pyrans, as disclosed in Tang et al. commonly assigned U.S. Pat. No. 4,769,292 and in Chen et al. in commonly assigned U.S. Pat. No. 6,020,078.
Electron-transport layer 106 (
Electron-injection layer 107 (
The second electrode is semitransparent, reflective and conductive and includes a semitransparent metal layer as the first layer (layer 108 (
When the first layer acts as an anode it can include a metal M, Al for example, with a hole injector which includes, but not limited to CFx, ITO, IZO, Pr2O3, TeO2, CuPc, SiO2, VOX, MoOx, or Ru2O3. M can be any reflective metal such as Zn, Mg, Ca, Pd or Pt. The first layer being metallic is not fully transparent but sufficiently transmissive to permit the generated light to pass through the top surface. By transmissive it is meant that the transmittance of the film on glass is 20% or more in the visible wavelength range. The first layer should be greater than about 4 nm in thickness to be laterally conductive, but less than about 50 nm so as not to cause excessive absorption of the emitted light.
The second electrode also includes a layer (layer 109 (
The first layer generally includes reactive metals and the second layer is made up of metallic compounds. If not properly selected the two layers can chemically react causing degradation of device performance and its eventual failure. For example, Al can be used as the first layer and a widely used transparent conductive oxide (TCO) such as ITO can be used as the second layer. To determine the stability of the interface one can examine thermodynamic functions, particularly the free energy function of the reactants and the potential products of the reaction. The reactants are Al and the components of ITO which are In2O3 and SnO2, and the potential products of reaction are Al2O3, In and Sn. The standard free energy of formation of Al2O3 at 0° K., ΔG (Al2O3), is −266.6 Kcal/mole O2. The ΔG (In2O3) is −148.0 Kcal/mole O2, and the ΔG (SnO2) is −138.8 Kcal/mole O2 and those of pure elements are 0 by definition. The standard free energy of formation of a compound is an indication of its stability, the more negative the value the more stable compound is. It follows that Al2O3 is more stable than In2O3 or SnO2. Thus a reaction at the Al/ITO interface is likely; Al will be oxidized with the formation of Al2O3, and the In2O3 and SnO2 will be reduced with formations of In and Sn, respectively. Similarly on the basis of free energy functions it is predicted that if IZO is selected as the AAL layer instead of ITO the Al/IZO interface will also be thermodynamically unstable. Therefore ITO and IZO are not selected according to the present invention. The ΔG (MgO)=−286 Kcal/mole O2 which is more negative than ΔG (Al2O3). If MgO is selected as the second layer the interface with Al first layer is stable, as the Al will not reduce the MgO. Thus MgO is an appropriate AAL material according to the present invention. In accordance with the present invention Al2O3 is another appropriate AAL material. This is because Al2O3 in the second layer will not oxidize Al first layer, or Al in the first layer will not reduce Al2O3 second layer. Thus, in accordance with the invention, if the first layer includes a metal M and the second layer includes a compound M′X where M′ is a metal and X is non metal then in order for the M/M′X interface to be stable the free energy of formation of the compound M′X has to be more negative than or equal to the free energy of formation of MX. The nonmetal X can be oxygen or nitrogen or a halogen or carbon.) The AAL materials include oxides of Ac, Al, Ba, Be, Ca, Hf, Li, Mg, Sc, Sr, Th, Y, Zr or rare earth metals, halides of Ac, Al, Ba, Be, Ca, Hf, La, Li, K, Mg, Na, Sc, Sr, Th, Y, Zr or rare earth metals, nitrides of Al, Ce, Hf, Ti, or Zr, and carbides of Al, Si, Ti or Zr or mixtures thereof. The thickness of the AAL layer is selected to maximize the on-axis luminance. The thickness is dependent on the optical properties of the material and ranges from 20 nm to 150 nm.
The antiabsorption layer (AAL) 109 (
Most OLED devices are sensitive to moisture or oxygen or both, so they are commonly sealed in an inert atmosphere such as nitrogen or argon, along with a desiccant such as alumina, bauxite, calcium sulfate, clays, silica gel, zeolites, alkaline metal oxides, alkaline earth metal oxides, sulfates, or metal halides and perchlorates. Methods for encapsulation and desiccation include, but are not limited to, those described in U.S. Pat. No. 6,226,890. In addition, barrier layers such as Al2O3, SiOx, Teflon etc. and alternating inorganic/polymeric layers are known in the art for encapsulation.
OLED devices of this invention can employ various well-known optical effects in order to enhance their properties if desired. This includes optimizing layer thicknesses to yield maximum light transmission, using scattering layer to enhance light extraction, replacing reflective electrodes with light-absorbing electrodes to enhance contrast, providing anti-glare or anti-reflection coatings over the display, providing a polarizing medium over the display, or providing colored, neutral density, or color conversion filters over the display. Filters, polarizers, and anti-glare or anti-reflection coatings may be specifically provided over the cover or as part of the cover.
The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.
