Organic electro-optical device, light source, display device and solar cell

FIELD OF THE INVENTION:

The invention relates to an organic electro-optical device. The invention also relates to a light source, a display device and a solar cell comprising the organic electro -optical device.

BACKGROUND OF THE INVENTION:

Organic electro-optical devices are well known and comprising an organic electro-optical compound sandwiched between two electrodes. Such organic electro-optical device may be configured as an organic light emitting diode (also known as OLED) which emits light when a potential difference is applied across the two electrodes. Alternatively, these organic electro-optical devices may be used as solar cells which generate a potential difference across the two electrodes when light comprising a certain part of the light spectrum impinges on the organic electro-optical compound. Such organic electro-optical devices are relatively sensitive to moisture. To protect the organic electro-optical devices against moisture, the organic electro-optical devices are sealed and protected from the environment.

Conventionally the sealing of the electro-optical devices is being done by applying a free-standing cover over the electro-optical device. When a getter is applied within the cavity between cover and device, degradation by external water can be avoided for many years. Common practice is to use a getter at the inside of the cover. Alternatively, a getter layer directly on top of the cathode can be applied as is shown in the international patent application WO 2007/070529.

A serious drawback of the application of these covers is their considerable increase of the thickness of the device. Moreover, the free-standing cover is relatively difficult to apply onto large-area electro-optical devices. So the industry is looking for a thin- film sealing solution for electro-optical devices.

SUMMARY OF THE INVENTION:

It is an object of the invention to provide an organic electro-optical device having reduced thickness while being protected against degradation of the cathode by oxidizing agents, such as water.

According to a first aspect of the invention the object is achieved with an organic electro -optical device comprising an organic electro-optical compound sandwiched between an anode layer and a cathode layer, the cathode layer comprising an electron- injecting metal being a metal having a relatively low work-function for injecting electrons into the organic electro-optical compound, the cathode layer being sealed from moisture via a sealing layer deposited on the cathode layer, the sealing layer comprising: a getter layer for consuming water, an organic planarization layer for encapsulating contamination particles being present in or on the cathode layer, the getter layer being sandwiched between the cathode layer and the organic planarization layer, and - an inorganic barrier layer.

The effect of the measures according to the invention is that the getter layer protects the cathode layer from moisture by consuming water which migrates towards the cathode layer. The organic planarization layer encapsulates any contamination particles which may be present in or on the cathode layer and which may form water migration paths to the interface between the cathode layer and the organic electro -optical compound. Finally the inorganic barrier layer seals the stack of getter layer and organic planarization layer from moisture resulting from the environment.

The cathode layer of an organic electro-optical device is relatively sensitive to water contamination and thus the cathode layer must be sealed to prevent water to contaminate the cathode layer. In the known organic electroactive device, a (metal) capping layer is used to protect the electron injection layer. Often the capping layer comprises contamination particles which may form water migration paths to the electron injection layer and may contaminate or react with the electron injection layer, reducing the quality of the organic electroactive device. To further protect the cathode, the known organic electroactive device further comprises a further capping layer (inorganic barrier layer) on top of the metal capping layer on the electron injecting layer. The inventors have found that the further capping layer as defined in the known organic electroactive device does not block all water migration paths via contamination particles to the electron injection layer, thus still limiting the life-time of the known organic electroactive device. In the organic electro -optical device

according to the invention, the organic planarization layer is deposited on the getter layer. This organic planarization layer encapsulates any contamination particles which may be present in or on the cathode layer and planarizes the current stack of layers which may have become un-even due to the contamination particles. The organic planarization layer is sealed using the inorganic barrier layer as mentioned above which blocks substantially all moisture from the environment to enter the sealing layer. Without being held to any scientific explanation the inventors have found that the efficiency of the inorganic barrier layer for blocking water from penetrating the sealing layer is substantially improved due to the presence of the organic planarization layer which substantially reduces the degradation of the cathode.

Organic electro-optical devices are devices which comprise an anode, a cathode and an organic electro-optical compound sandwiched between the anode and cathode. The organic electro-optical compound may, for example, emit light when a potential difference is applied across the anode and the cathode. Such an organic electro-optical device is also known as organic light emitting diode (also known as OLED). Currently there are two different types of organic electro-optical compounds which may be used in organic light emitting diodes. A first organic electro-optical compound is constituted of polymer molecules. These polymer molecules are soluble and may be applied, for example, using spin-coat techniques, ink-jet printing or other liquid based deposition techniques. A second organic electro -optical compound, also known as small molecules organic light emitting compound, is constituted of relatively small molecules. These small molecules may be applied using vacuum deposition at relatively high temperatures or may be applied using liquid based deposition techniques.

Alternatively, the organic electro-optical compound may, for example, absorb light impinging on the organic electro-optical compound and may generate a potential difference across the anode and cathode. In such a configuration, the organic electro-optical device may be used as a solar cell, converting impinging light into electric energy.

A further benefit of the electro-optical device according to the invention is it eases the production of a flexible organic electro -optical device. In the known organic electroactive device, the sealing is performed using the free-standing cover. Often this freestanding cover is constituted of a glass coverlid. The applying of such a moisture-proof glass coverlid generally reduces the ability of the known electro -optical device to be bent and thus prevents the known electro-optical devices from being used in flexible display devices. The applying of the sealing layer comprising the getter layer, the organic planarization layer and

the inorganic barrier layer on the cathode layer results in a relatively thin and flexible sealed organic electro -optical device which may be used to produce a flexible display device.

In an embodiment of the organic electro -optical device, the inorganic barrier layer is deposited on the organic planarization layer for sealing the organic planarization layer. A benefit of this embodiment is that applying the inorganic barrier layer directly on the organic planarization layer results in a relatively simply three-layer sealing layer which fully seals the cathode layer of the organic electro-optical device from environmental influences and from water. This substantially reduces the complexity of the production process of the organic electro -optical device according to the invention. In an embodiment of the organic electro -optical device, the getter layer fully covers the cathode layer. A benefit of this embodiment is that the getter layer forms a barrier consuming water for substantially the whole cathode layer. Water which may migrate through or from the organic planarization layer to the cathode layer is consumed by the getter layer before damaging the cathode layer. Although the organic planarization layer preferably should be free from water, there may still be residual amounts of water in the organic planarization layer. These residual amounts of water may still contaminate the cathode layer and degrade the cathode layer. By applying the getter layer such that it fully covers the cathode layer, the getter layer forms a water consuming barrier between the organic planarization layer and the cathode layer, blocking any residual water from the organic planarization layer from reaching the cathode layer.

In an embodiment of the organic electro -optical device, the getter layer comprises a further metal having a relatively low work-function. Metals having a relatively low work- function not only may be beneficially applied as electron-injection material for injecting electrons into the organic electro-optical compound, but may also be beneficially applied as getter material. These metals which have a relatively low work-function relatively easily react with water, and as such efficiently consume the water which migrates towards the cathode layer thus protecting the cathode layer from migrating water. A further benefit of the use of metals which have a relatively low work- function is that the applying of such a layer requires similar processing as the applying of the cathode layer which further reduces the complexity of the production of the organic electro-optical device according to the invention.

In an embodiment of the organic electro -optical device, the cathode layer comprising a first metal layer applied to the organic electro-optical compound and a second metal layer applied on the first metal layer, the first metal layer comprising the electron- injecting metal, and the second metal layer comprising a conductive metal sealing layer. A

benefit of this embodiment is that the second metal layer provides a further seal against environmental influences to the first layer.

In an embodiment of the organic electro -optical device, the getter layer comprises the same electron- injecting metal as the first metal layer of the cathode layer. A benefit of this embodiment is that the use of the same metal in the getter layer as is used as electron- injecting metal in the first metal layer of the cathode layer is that the layered structure comprising the cathode layer and the getter layer may be produced in a same vacuum deposition system (e.g. thermal evaporator) in which the first metal layer, the second metal layer and the getter layer are deposited sequentially. This substantially simplifies the production method of the stack of metal layers even further. Only the organic planarization layer and the inorganic barrier layer need to be applied in subsequent production steps for generating the organic electro -optical device which comprises a cathode layer which is substantially sealed against moisture.

In an embodiment of the organic electro -optical device, the getter layer is selected from a list containing: the alkaline earth metals and their oxides, for example,

Calcium, Calcium-Oxide, Barium and Barium-Oxide. A benefit when using any of the listed materials is that these materials have sufficiently strong reactivity towards water to protect the cathode layer against moisture while omitting the need for relatively expensive processing steps when, for example, alkaline metals such as Sodium would be used as getter layer.

In an embodiment of the organic electro -optical device, the organic planarization layer is constituted of any organic compound that may be applied by liquid deposition methods or evaporation techniques. Preferably the organic material should be hydrophobic in order to avoid water uptake during processing. In an embodiment of the organic electro -optical device, the inorganic barrier layer is constituted of a material selected from a list containing: silicon-nitride (Si _x N _y H _z ), silicon-oxinitride (Si _x O _y N _z ), aluminum oxide (Al _x Oy) and silicon carbide (Si _x Cy). The suffixes x, y and z may be any number in a range between 0 and 4, i.e. 0 < x, y, z < 4. A benefit when using any of the listed inorganic materials as inorganic barrier layer is that they are relatively common in the IC industry and that they can be applied using well known deposition techniques, such as plasma enhanced chemical vapor deposition PECVD. For example, the inorganic barrier layer may comprise silicon-nitride (S13N4), silicon-oxinitride (in which, for example, Si = 37%, O = 54% and N = 9%), aluminum oxide (AI2O3) or silicon carbide (SiC).

The invention also relates to a method of producing a sealing layer for sealing a cathode layer of an organic electro-optical device from moisture according to claim 8 and 9, to a light source according to claim 10, to a display device according to claim 11 and to a solar cell according to claim 12.

BRIEF DESCRIPTION OF THE DRAWINGS:

These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.

In the drawings: Fig. IA shows a simplified cross-sectional view of an organic electro-optical device according to the invention, and Fig. IB shows a detail of the cathode layer of the organic electro -optical device,

Figs. 2A and 2B show the electroluminescence from an organic electro -optical device, in which Fig. 2A shows the organic electro-optical device without the sealing layer, and Fig. 2B shows the organic electro-optical device with the sealing layer according to the invention, and

Figs. 3 A, 3B and 3C show a simplified plan view of a light source, a display device and a solar cell comprising the organic electro-optical device according to the invention, respectively. The figures are purely diagrammatic and not drawn to scale. Particularly for clarity, some dimensions are exaggerated strongly. Similar components in the figures are denoted by the same reference numerals as much as possible.

DETAILED DESCRIPTION OF EMBODIMENTS: Fig. IA shows a simplified cross-sectional view of an organic electro-optical device 10 according to the invention. The electro -optical device 10 according to the invention comprises an organic electro-optical compound 40 sandwiched between an anode layer 30 and a cathode layer 50. The anode layer 30 may be transparent and may be applied to a transparent substrate 20 such as glass or quartz. The organic electro-optical device 10 according to the invention further comprises a sealing layer 60 for protecting the cathode layer 50 against environmental influences such as moisture. The sealing layer 60 comprises a getter layer 70 applied to the cathode layer 50. The sealing layer 60 further comprises an organic planarization layer 80. The getter layer 70 is a water consuming layer, absorbing any water which may migrate towards the cathode layer 50. The getter layer 70 is applied on the

cathode layer 50 such that it absorbs the water before it reaches the cathode layer 50. The organic planarization layer 80 is a planarization layer which encapsulates any particle 100 which may contaminate the organic electro -optical device and which may penetrate the cathode layer 50. The preferred embodiment shown in Fig. IA the sealing layer 60 further comprises an inorganic barrier layer 90 directly applied to the organic planarization layer 80 and sealing the organic planarization layer 80. This preferred embodiment results in a sealing layer 60 having only a few layers which results in a cost-effective sealing solution. Alternatively, additional layers may be present between the inorganic barrier layer 90 and the organic planarization layer 80 (not shown). The inorganic barrier layer 90 is substantially impermeable for water and may be constituted of silicon-nitride (Si _x N _y H _z ), silicon-oxinitride (Si _x O _y N _z ), aluminum oxide (Al _x Oy) and silicon carbide (Si _x Cy). The suffixes x, y and z may be any number in a range between 0 and 4. For example, the inorganic barrier layer may comprise silicon-nitride (S13N4), silicon-oxinitride (in which, for example, Si = 37%, O = 54% and N = 9%), aluminum oxide (AI ₂ O ₃ ) or silicon carbide (SiC). Generally the applying of an inorganic barrier layer 90 may have been sufficient for sealing the cathode layer 50 from moisture. However, sometimes dust particles 100 may be present during the applying of the cathode layer 50 and may protrude from the cathode layer 50 (as is indicated in Fig. IA). These protruding particles 100 may not be fully covered by the inorganic barrier layer 90 causing minute openings in the inorganic barrier layer 90 through which residual amounts of water may migrate towards the cathode layer 50 degrading the cathode layer 50 and causing black spots to appear in the organic electro-optical device (see Fig. 2A). A solution to the occurrence of the minute openings, an organic planarization layer is applied on the cathode layer 50 before applying the inorganic barrier layer 90. The inorganic barrier layer 90 encapsulates the particles 100 such that the inorganic barrier layer 90 forms a tight and closed barrier for any water from the environment migrating towards the cathode layer 50. However, this still does not provide an organic electro-optical device which is substantially free from black-spots and thus free from contaminating water. The inventors have found that residual water which typically is present in the organic planarization layer still migrates towards the cathode layer 50 and causes degradation of the cathode layer 50. To consume any water which may still migrate from the organic planarization layer towards the cathode layer 50, a getter layer 70 is applied on the cathode layer 50. The inorganic barrier layer 90 prevents water from the environment to enter the sealing layer 60, and the getter layer 70 consumes

any residual water which may be present in the organic planarization layer 80 and which may migrate towards the cathode layer 50.

Between the organic planarization layer 80 and the inorganic barrier layer 90 additional sealing or getter layers (not shown) may be present. The getter layer 70 may, for example, comprises a metal having a relatively low work-function, such as the alkaline earth metals (e.g. Calcium, Barium) or their oxides (e.g. calcium oxide, barium oxide). Such a getter layer 70 reacts readily with any water which may migrate through the inorganic barrier layer 90 or through the organic planarization layer 80 towards the cathode layer 50. The metal of the getter layer 70 which has a relatively low work- function may be the same metal as used in the first metal layer 52 of the cathode layer 50 for injecting electrons into the organic electro-optical compound 40. A benefit of choosing the same metal in the getter layer 70 and in the first metal layer 52 is that the applying of the cathode layer 50 together with the getter layer 60 may be done in a single vacuum deposition machine (e.g. Thermal evaporator). This reduces the production complexity of the organic electro-optical device 10 according to the invention. The getter layer 70 may have a thickness within a range from 1 nanometer to 100 nanometers. The choice of the thickness of the getter layer 70 is a compromise between longer deposition time when applying a relatively thick getter layer 70 and sufficient getter capacity when applying a relatively thin getter layer 70. On top of the getter layer 70 the organic planarization layer 80 is applied. The organic planarization layer 80 may be constituted of any organic compound that can be applied by liquid deposition methods or evaporation. The organic planarization layer may have a thickness within a range from 1 micron to 10 micron. The choice of the thickness of the organic planarization layer 80 may depend on the size of the particles 100 which may be expected in the cathode layer 50 or may be chosen via experiments. The organic planarization layer 80 may, for example, be applied via spin-coating, ink-jet printing, or, for example, via other deposition techniques.

On top of the organic planarization layer 80 the inorganic barrier layer 90 is applied. The inorganic barrier layer 90 may be constituted of Silicon-Nitride and may have a thickness of approximately 100 nanometers such that the inorganic barrier layer 90 is substantially impermeable to water. When choosing a different inorganic barrier material, the inorganic barrier layer 90 may have a different thickness to ensure that the inorganic barrier layer 90 is substantially impermeable to water. The inorganic barrier layer 90 may, for example, be applied via chemical vapor deposition (CVD) techniques.

Generally, the cathode layer 50 is constituted of a first metal layer 52 and a second metal layer 54 as indicated in Fig. IB which shows a detail of the cathode layer 50 of the organic electro-optical device 10. The first metal layer 52 is constituted of electron- injecting metal for injecting electrons into the organic electro-optical compound 40. Preferably the first metal layer 52 is directly applied on the organic electro-optical compound 40. The first metal layer 52 may be constituted of, for example, Calcium, Barium, Ytterbium, Lithium Fluoride, and typically has a thickness of a few nanometers. On top of the first metal layer 52 a second metal layer 54 is applied. The second metal layer 54 may, for example, be constituted of Aluminum or Silver and forms a metal sealing layer for protecting the first metal layer 52 from being contaminated by water. The second metal, as applied in layer 54 has excellent intrinsic barrier properties with respect to contaminating water which may migrate towards the first metal layer 52 of the cathode layer 50. Particles 100 which may result from the environment during the processing of the second metal layer 54 may create pinholes in this second metal layer 54, thus destroying the barrier properties of the second metal layer 54.

Figs. 2A and 2B show a photograph of an operating organic electro-optical device 10 (see Fig. IA), in which Fig. 2A shows the organic electro-optical device without the sealing layer 60 (see Fig. IA), and Fig. 2B shows the organic electro-optical device with the sealing layer 60 (see Fig. IA) according to the invention. As can clearly be seen from the two pictures, the organic electro-optical device without the sealing layer shows black spots as a result of water contamination of the cathode layer. The organic electro-optical device shown in Fig. 2B is substantially clean from contaminations due to the applying of the sealing layer 60 according to the invention.

Fig. 3A shows a simplified plan view of a light source 12 comprising the organic electro -optical device 10 (see Fig. IA) according to the invention. Fig. 3B shows a simplified plan view of a display device 14 comprising the organic electro-optical device 10 (see Fig. IA) according to the invention. And Fig. 3C shows a simplified plan view of a solar cell 16 comprising the organic electro-optical device 10 (see Fig. IA) according to the invention. It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims.

In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb "comprise" and its conjugations does not

exclude the presence of elements or steps other than those stated in a claim. The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. In the device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.