WO2006030338A1

WO2006030338A1 - Transparent electrode for leds or oleds comprising inorganic metals

Info

Publication number: WO2006030338A1
Application number: PCT/IB2005/052859
Authority: WO
Inventors: Ralph Kurt; Johannes F. M. Cillessen
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2004-09-14
Filing date: 2005-08-31
Publication date: 2006-03-23

Abstract

The invention relates to a transparent electrode comprising at least a first (10) and a second (11) inorganic material, the at least first and second inorganic materials being capable of forming a eutectic alloy (9). The at least first and second inorganic materials are selected from a group consisting of the pairs: As-Pb, Bi-Ag, Bi-Cd, Bi-Co, Bi-In, Bi-Pb, Bi-Sn, Bi-Zn, Cd-In, Cd-Pb, Cd-Sb, Cd-Sn, Cd-Tl, Cd-Zn, Ga-In, Ga-Mg, Ga-Sn, Ga-Zn, In-Sn, In-Zn, Mg-Pb, Mg-Sn, Mg-Tl, Pb-Pd, Pb-Pt, Pb-Sb, Pb-Sn, Sb-Sn, Sb-Tl (1), Se-Tl, Sn-Tl (1), and Sn-Zn. A display device having the transparent electrode according to the invention has advantageous optical properties.

Description

Transparent electrode for LEDs or OLEDs comprising inorganic metals

The present invention relates to transparent electrodes for use in light emitting devices and in particular with top-emitting organic light emitting devices, and to top-emitting organic light emitting diodes (OLEDs), arrays, pixelated lamps, large area light sources, and displays, such as active and passive matrix displays, and methods of making the devices. Typically, OLEDs are built up on a transparent substrate comprising the anode, and light is coupled out through the anode, thus by bottom emission. Often Indium- Tin-Oxide (ITO) anode on top of transparent substrates (e.g. glass) is used as such transparent electrode in combination with PEDOT (or PANI) as they provide a substantially transparent anode with a high work function and a good hole injection. The cathode on top of the structure is typically a low work function material, i.e. Ba, Ca, or LiF deposited as a very thin (a few nm thick) layer. Electron transport layers, optionally equipped with an additional hole blocking layer, as well as doped interfaces are also known in the art to act as cathode. In order to improve electrical conductivity, and to protect the low work function material from the surroundings, a metal layer (thickness of a few tens to hundreds of nm) is subsequently deposited on top. These cathodes are not transparent and, thus, cannot be used for top emission.

In bottom emission devices, the aperture, which is the ratio of the surface producing light in relation to pixel size, is limited due to the space required for electrical driving (e.g. wires, powerlines). For active matrix (AM) devices, where further functionality is provided by TFTs occupying space as well, the aperture is even smaller e.g. about 35-45% depending on the design.

There is therefore a need for development of top emitting devices both for improving the aperture and for improving the device lifetime. An aperture of up to 90% seems to be obtainable by using top emitting devices. A number of approaches for top emitting devices have been suggested.

US 2003 0107326 discloses the use of a transparent conductive film where the transmission of the film may be varied as a pixel electrode, i.e. anode electrode. The use of a film, the transmission of which may be varied by injecting a high-energy source into the transparent conductive film, is useful for increasing the contrast ratio without using e.g. a polariser or a black matrix made of Cr/CrOx.

In US 5,714,838 a transparent top electrode for OLEDs is disclosed allowing for top emission. A diffusion layer needs to be provided between the thin metallic film and the organic light emitting material to avoid electrical shorts between the anode and the cathode. The transparent top electrode is a very thin metallic film, such as 5-10 nm MgAg, such as ZnS (200 A), GaN, ITO, ZnSe or any combination of the materials. The metallic films need to be very thin in order to obtain a sufficient degree of transparency. Also in WO 98/07202 a transparent GaN cathode for OLEDs is suggested and in US 5,457,565, ITO is suggested as cathode material. However, ITO has a high work function and can thus in itself only act as anode / hole injection layer. Moreover ITO is not readily deposited on top of an active organic layer, since typical high deposition temperatures and/or UV radiation from the deposition process may damage the OLED layer during deposition. Furthermore, ITO may oxidize many low work function materials, such as e.g. Ba. Thus, shielding and protection layers etc. are needed thus making the structure more complex.

It is a disadvantage of using these very thin films as cathode electrodes because the lifetime of the OLEDs is reduced since the thin films have a limited chemical resistance and stability against e.g. O₂ and H₂O. Furthermore, even by providing a thin layer of a generic non-transparent metal, the transmission is limited to approx. 50 %.

Alternatively, the metallic electrode could be made as thin as possible. To give a few common examples (thickness for 50% transmission) Al = 3nm, Ag = 10 nm, Cu =14 nm, Ba = 23 nm.

Provision of a transparent electrode is especially advantageous in active matrix displays in order to improve the aperture, i.e. the ratio of the emitting surface to total pixel size, by using top emitting devices.

It is an object of the present invention to provide a transparent electrode that at least partly overcomes the above disadvantages.

According to a first aspect of the present invention, the above and other objects are fulfilled by a transparent electrode comprising at least a first and a second inorganic material, the at least first and second inorganic materials being capable of forming a eutectic alloy. The transparent electrode according to the invention may be also used in e.g. switchable mirrors or windows even if those devices are different in nature from LEDs or OLEDs. For instance also switchable LCD shutters, electrochrome devices or the like might comprise the transparent electrode comprising at least a first and a second inorganic material, the at least first and second inorganic materials being capable of forming a eutectic alloy.

According to a second aspect of the present invention a light emitting device is provided comprising: a substrate, an anode, a light emitting structure, and a transparent cathode the transparent cathode comprising at least a first and a second inorganic material, the at least first and second inorganic materials being capable of forming a eutectic alloy. The light emitting device may be a top-emitting light emitting device, wherein the anode is positioned on at least a part of the substrate, the light emitting structure is provided on top of the anode, and the transparent cathode is provided on top of the light emitting structure, and the transparent cathode comprising at least a first and a second inorganic material, the at least first and second inorganic materials being capable of forming a eutectic alloy.

Alternatively, the light-emitting device may be a bottom-emitting device, i.e. with a transparent substrate, a transparent cathode (according to the invention), a light emitting layer, and an anode on top. Such a structure is referred to as inverted structure.

Alternatively, the light-emitting device may be a device emitting to both sides, i.e. a top- and bottom-emission device, i.e. with a transparent substrate, a transparent anode (as known in the art), a light emitting layer, and a at least partially transparent cathode (according to the invention) on top. Such devices might be suitable for lighting applications.

In one preferred embodiment, the light emitting structure comprises an organic light emitting structure. The light emitting structure may be an organic light emitting structure.

In the present context, the term "cathode" is used when describing the electrode of a light-emitting device. However, in any other cases the term electrode is used. It is envisaged that all features and characteristics described in connection with the cathode will also apply for the electrode and vice versa. The at least first and second inorganic materials may be selected from the group consisting of the pairs: As-Pb, Bi-Ag, Bi-Cd, Bi-Co, Bi-In, Bi-Pb, Bi-Sn, Bi-Zn, Cd-In, Cd-Pb, Cd-Sb, Cd-Sn, Cd-Tl, Cd-Zn, Ga-In, Ga-Mg, Ga-Sn, Ga-Zn, In-Sn, In-Zn, Mg-Pb, Mg-Sn, Mg-Tl, Pb-Pd, Pb-Pt, Pb-Sb, Pb-Sn, Sb-Sn, Sb-Tl, Se-Tl, Sn-Tl, and Sn-Zn or any combinations thereof. In a preferred embodiment, the at least first and second inorganic materials comprise Bi-Ag, Bi-In, Bi-Sn or In-Sn.

It is envisaged that these materials may be combined in many ways to form low melting alloys, and furthermore, many combinations of 2, 3, 4, 5, 6 or even more materials to form alloys capable of forming a eutectic alloy. Examples are Bi:In:Pb:Sn and Bi:In:Pt:Sn, however further materials may be formed.

The capability of the materials to form eutectic alloys does not limit the material compositions to the strict eutectic compositions, since the material characteristics useable for forming transparent electrodes are obtainable also for material compositions outside of the strict eutectic composition.

The materials are capable of forming a eutectic alloy, thus the phase diagram of the materials has a eutectic point, being the point at the lowest temperature at which two or more constituents in a system simultaneously solidify in a binary (ternary, quarternary , etc.) system. A eutectic system is a binary, ternary, or quarternary, etc. system in which one particular alloy solidifies at a constant temperature, which is lower than the beginning of solidification in any other alloy.

It is presently preferred that the transparency of the electrode is higher than 40 %, or even higher up to 60%.

The thickness of the electrode is chosen so as to provide a suitable transparency, sufficient electrical conductivity, and at the same time a thickness providing a minimum of chemical resistance, and preferably an enhanced chemical resistance in comparison with the chemical resistance of thin metallic layers, as mentioned above. The thickness is preferably selected between 20 nm and 200 nm, such as from 25 nm to 150 nm, such as from 30 nm to 125 nm, such as from 50 nm to 100 nm, such as from 75 nm to 90 nm. In one embodiment the thickness of a BiIn layer is selected to be 100 nm, and a transparency of 40 -60 % is readily obtained.

An important feature of a transparent electrode is the conductivity of the electrode. Often there will be a trade off between good transparency, which typically requires thin layers and sufficient conductivity, i.e. low sheet resistance, which becomes better for thick layers. Using the above-described materials provides materials showing a sufficient conductivity for the electrode.

In order to improve the conductivity additional "wire grids" or "shunting" (of material with high conductivity) may be deposited on top of the transparent cathode. Again, there is a trade off between overall transparency and conductivity. Preferably the mesh is placed over the non-emitting part of the device.

The at least first and second inorganic materials may be deposited directly in the correct stoichiometry. Hereby, no post-processing of the material is needed. Alternatively, the at least first and second inorganic materials may be deposited first layer by at least second layer. The materials may then subsequently be annealed to obtain an alloy of the materials.

Preferably, the thickness of the first and second layers are selected so that an alloy formed by melting and solidifying at least a part of the first and second layers has a substantially eutectic composition. Hereby, a near eutectic alloy may be formed upon annealing. By selecting the materials composition near the material composition forming a eutectic alloy, a substantially uniform composition of the alloy may be obtained.

The thickness of the layers is thus selected so that the composition of the alloys upon annealing forms an alloy, such as a eutectic alloy. The thickness of the layers may for example be selected between 5 and 200 nm, such as between 10 and 50 nm, such as between 30 and 50 nm.

Each inorganic material may have a complex refractive index n ± ik and the second (top) inorganic material may be selected to have a real part of the refractive index lower than the real part of the refractive index of the first material (between the organic light emitting structure and second inorganic material) and an imaginary part of the refractive index higher than the imaginary part of the refractive index of the first material.

Thus, the order of the layers may be selected so that the most reflecting layer is deposited on top of the structure, hereby, the contrast of e.g. a pixel element using the electrode of the invention as cathode electrode may be significantly improved. That is, the as- deposited stack of inorganic materials may be selected to have an initial high reflectivity before any annealing procedures, and the annealed stack may then have a high transmission.

Thus, in one preferred embodiment of e.g. a matrix display, the layered structure may be annealed only locally, by e.g. mask illumination or by laser treatment, inside the emissive area or pixel making the layer there transparent. In a preferred embodiment, a polariser is further provided on top of the device, so that the remaining highly reflective edge layers will act as a "black matrix" to hereby improve the day light contrast of the device. The device may thus be covered with a polariser to absorb incident light reflected from the as- deposited first and second layers. In another preferred embodiment the as-deposited layers are annealed only locally in a periodic lateral patterned way to obtain an optical grating improving light out- coupling by cavity effects. This is obtained due to the fact that the as-deposited layers are reflective also for light generated by the device itself. One or two-dimensional patterns could be applied with typical pitches in the order of sub- wavelength, i.e. in the range between 100 nm and 1000 nm, preferably between 200 nm and 500 nm. Such structures could be obtained by scanning laser treatment or by applying optical interference pattern.

In a preferred embodiment, the first inorganic material forming the first layer is Bi and the second inorganic material forming the second layer is In or Sn, or the first inorganic material may be Sn and the second inorganic material may be In.

The reflectivity of the as-deposited first and second layers of inorganic materials may possess a high reflectivity, such as a reflectivity higher than 50 %, even up to 80%.

It is preferred that when annealing a stack of materials having an initially high reflectivity, the annealing of the first layer and at least second layer results in a drop of reflectivity of the annealed layer.

The drop in reflectivity may be 40% or more.

Given an OLED device, furthermore an electron injection layer may be provided between the light emitting structure and the transparent electrode layer. The electron injection layer may comprise a low work function material, such as e.g. Ba, Ca, Li, LiF, K, Na, Rb, Y, etc.

Alternatively the electron injection layer may comprise an organic electron transport layer such as triazoles and the like or n-type doped interfaces as known in the art. Additionally a hole-blocking layer may even further improve the efficiency of the device. The thickness of the electron injection material is preferably selected between

0,5 and 10 nm, such as about 2 nm.

Furthermore, the device may comprise a diffusion layer between the electron injection layer and the transparent electrode. The diffusion layer is preferably made of material(s), compounds and/or alloys not alloying with the electron injection layer material, and the diffusion layer may comprise Fe, Mo, Mn, Ti, V, W, Ni or chemically alike metals such as Cr, Ta, Hf, Nb, or Zr or any combination or alloys thereof.

According to a further aspect of the present invention a method of making a transparent electrode is provided, the method comprising the steps of depositing at least a first and a second inorganic material, the at least first and second inorganic materials being capable of forming a eutectic alloy.

The electrode may be deposited on any structure suitable for supporting an electrode, particularly any structure suitable for supporting a transparent electrode. According to a still further aspect of the present invention a method of making an LED is provided, the method comprising the steps of providing a substrate providing an anode providing a light emitting structure, and - depositing a transparent cathode, the transparent cathode comprising at least a first and a second inorganic material, the at least first and second inorganic materials being capable of forming a eutectic alloy.

Thus, for a method of making a top emitting LED is provided by providing a substrate - providing an anode on at least a part of the substrate, providing a light emitting structure over the anode, and depositing a transparent cathode on top of the light emitting structure, the transparent electrode comprising at least a first and a second inorganic material, the at least first and second inorganic materials being capable of forming a eutectic alloy.

Fig. 1 shows a top emitting OLED according to the present invention, Fig. 2 shows the optical density for a 50 nm/50 nm Bi/In bi- layer film versus used read-out wavelength, Fig. 3 shows the reflection/transmission of a number of bi- layer films versus annealing temperature,

Fig. 4 shows a Bi/In phase diagram, Fig. 5 shows a Bi/Sn phase diagram, Fig. 6 shows an In/Sn phase diagram, Fig. 7 shows a Ba-Fe phase diagram,

Fig. 8 shows the light intensity as a function of wavelength and output angle of a conventional OLED, and

Fig. 9 shows the light intensity of an OLED according to the invention. In figure 1, a top emitting OLED is shown. A hole injection layer 4 is formed on a substrate 2. The hole injection layer may be PEDOT or Au. Furthermore, a thin film transistor 3 is formed on the substrate partly below the hole injection layer, the anode, 4. The TFT provides further functionality to e.g. OLEDs forming part of an active matrix display. On top of the anode 4 an organic light-emitting layer is formed, such as a light emitting polymer (LEP) layer or small molecule OLED layers. Furthermore, multilayer stacks may be used comprising additional functional layers such as hole blocking layers (on that side of the stack, where the electron injection take place) or electron blocking layers. Such stacks are known in the art.

A sub-pixel separator 6 is provided for insulating one anode from the next in e.g. an array or matrix of pixel. In this preferred embodiment further an electron injector 7, such as Ba, is deposited on top of the anode 4 and the sub-pixel separator 6. Furthermore, a diffusion barrier 8, such as a layer of Fe, may be provided between the electron injector 7 and the electrode 9. The diffusion barrier is preferably made of a material, which does not alloy with the electron injector material. An example of a Ba-Fe phase diagram is shown in figure 7 from where it is evident that no alloying is formed.

A bi-metallic layer 10, 11 is formed and the bi-metallic layer is partly annealed in the anode area or the emissive pixel area to form the transparent electrode (cathode) 9.

To further protect the structure, a dielectric layer 12 is deposited on top of the bi-metallic layer 10,11 and the electrode 9.

In order to improve the contrast, a contrast foil, such as a polariser is provided on top of the entire structure. The polariser ensures that light reflected from the reflective bi- metallic layer 10, 11 is absorbed by the polariser and thus not emitted from the structure, so that a "black matrix" is formed and the contrast thus enhanced.

It is envisaged that the structure shown in figure 1 shows many features not essential for providing a transparent electrode or an OLED according to the present invention. However, for the purpose of illustration also non-essential elements are shown in the figure.

Figure 2 shows the optical density OD, that is the transmission T=10^"od, of a 50 nm/50 nm Bi/In bi-layer structure versus the used read-out length (see G. Chapman et al. SPIE 4688 (2002)). It is seen that e.g. at 650 nm, there is an OD of 3.7 observed in the as- deposited state (OW) in dependence of the layer thickness. This is also in accordance with the calculations of the inventors. As the exposure power (equivalent to temperature) is increased, the OD of the converted area reached a minimum corresponding to the energy at which all of the material of the bi- layer is converted to an alloy, preferably an alloy of eutectic composition. At e.g. 650 nm a minimum value of roughly 0.25 OD is reached corresponding to a transmission of about 60%.

Furthermore, the smooth and flat absorption/transmission spectrum over a long range of wavelength especially in the visible range is remarkably and providing the properties for a transparent cathode for top-emission OLEDs.

Different types of bi-layers have been sputter-coated onto substrates. Figure 3 shows the optical properties (reflectivity and transmission) of Bi/Sn and Sn/Bi bi-layers with single layers of 15 nm and 30 nm, respectively. The optical properties are shown as a function of the temperature during heating up (thermal tests). It is seen in curves 31, 33, 35 and 37 that a very high initial reflectivity (about 70%), is observed and at about 130 ⁰C a sharp transition is observed resulting in a drop of the reflectivity (to about 10 %) and a corresponding increase in transmission (curves 32, 34, 36 and 38) to about 40-50 %. As will be clear from below, these compositions do not correspond to the exact eutectic composition and it is envisaged that even better results, (higher transmission) may be obtained by being closer to the eutectic composition.

The thickness ratio of the two or more layers, which will form a eutectic alloy after exposure, is important for the optical performance. The binary phase diagrams for Bi/In, Bi /Sn and In/Sn are shown in Figs. 4-6. The phase diagrams provide information at which ratio, in weight %, and at which cool down temperature a eutectic alloy is formed. In table 1 examples are given of the ratio of the individual as-deposited layers in order to come to the eutectic composition. Deviations from the eutectic composition will cause that the materials are not completely alloyed and that residual reflecting parts being less transparent will be present in the material. This may effectively lower the modulation. Furthermore, the melting temperatures of the initial layers are seen. This gives an indication of which temperatures are needed for alloying. It is seen that the melting temperature is below 300 °C in all cases. Preferably, materials are selected which may be alloyed at a temperature below 100 - 200 °C.

A maximum reflectivity of the Bi - Sn system is found with a layer thickness of about 20 nm layer 10 and 20 nm layer 11. Hereby, a reflectivity of approx. 80 % is reached.

The sheet resistance of the layers are also of importance and some measurements on Bi/In films have been made. (Chapmann et al. "BiIn Bimetallic thermal resist for microfabrication, Photomasks and micromachining applications", Proc. of SPIE, vol. 4688 (2002)). It is seen from table 2 below, that actually the conductivity is almost the same for the alloy compared to the as-deposited layers of Bi and In.

For comparison figure 8 shows the light intensity of a "standard" (known in the art) top-emission OLED stack: reflective anode - transparent hole injection layer(s) - LEP (light emitting polymer) - thin metallic cathode (in this case Ba-Ag-ZnSe). Optics used for calculations include micro-cavity effects. Figure 9 shows the light intensity of a similar stack comprising one the transparent cathode according to the invention (as an example a 50nm BiSn alloy layer). Other materials as described in the invention are also promising. Light output can be improved using additional out-coupling layers and optimized thicknesses.

It is clearly seen that for the example shown in figure 9 the top-emission spectrum (in this case for a green emitter) is broader and also the angular intensity dependency is improved compared with the reference (Fig. 8).

Claims

CLAIMS:

1. A transparent electrode comprising at least a first and a second inorganic material, the at least first and second inorganic materials being capable of forming a eutectic alloy.

2. A transparent electrode according to claim 1 , wherein the at least first and second inorganic materials are selected from a group consisting of the pairs: As-Pb, Bi-Ag, Bi-Cd, Bi-Co, Bi-In, Bi-Pb, Bi-Sn, Bi-Zn, Cd-In, Cd-Pb, Cd-Sb, Cd-Sn₅ Cd-Tl, Cd-Zn, Ga¬ in, Ga-Mg, Ga-Sn, Ga-Zn, In-Sn, In-Zn, Mg-Pb, Mg-Sn, Mg-Tl, Pb-Pd, Pb-Pt, Pb-Sb, Pb-Sn, Sb-Sn, Sb-Tl, Se-Tl, Sn-Tl, and Sn-Zn.

3. A switchable mirror, a display, or an array of pixelated light sources comprising a transparent electrode according to claim 1.

4. A light emitting device comprising: - a substrate an anode a light emitting structure and a cathode comprising the transparent electrode according to claim 1.

5. A light emitting device according to claim 4, wherein the thickness of the cathode is between 20 nm and 200 nm.

6. A light emitting device according to claim 4, wherein the first and the second inorganic materials each have a complex refractive index and wherein the second inorganic material is selected to have a real part of the refractive index lower than the real part of the refractive index of the first material and an imaginary part of the refractive index higher than the imaginary part of the refractive index of the first material.

7. A light emitting device according to claim 4, wherein the first and second inorganic materials have been locally annealed to obtain a partially transparent, light emitting device.

8. A light emitting device according to claim 4, wherein the device is covered with a polariser to absorb incident light reflected from the first and second inorganic materials.

9. A light emitting device according to claim 4, wherein the first and second inorganic materials are only annealed locally in a periodic laterally patterned way to obtain an optical grating.

10. A light emitting device according to claim 4, further comprising an electron injection layer provided between the light emitting structure and the transparent cathode layer.

11. A light emitting device according to claim 10, wherein the electron injection layer comprises Ba, Ca, Li, LiF, K, Na, Rb, or Y.

12. A method of making a transparent electrode, the method comprising the steps of depositing at least a first and a second inorganic material, the at least first and second inorganic materials being capable of forming a eutectic alloy.