WO2010064165A1

WO2010064165A1 - Oled device with adjustable color appearance

Info

Publication number: WO2010064165A1
Application number: PCT/IB2009/055307
Authority: WO
Inventors: Claudia M. Goldmann
Original assignee: Philips Intellectual Property & Standards Gmbh; Koninklijke Philips Electronics N. V.
Priority date: 2008-12-01
Filing date: 2009-11-24
Publication date: 2010-06-10
Also published as: TW201031246A

Abstract

The invention relates to an OLED device(100) and a method for manufacturing such a device which has a given desired color appearance in its off-state. The OLED device (100) comprises an electroluminescent layer (12) with a first and a second electrode (11, 13). Furthermore, it comprises a coating layer (20), for example disposed on one of the electrodes (13),that has a wavelength-dependant reflectivity based on interference effects. The desired color appearance can for example be achieved by adjusting the thickness (d) of the coating layer (20).

Description

OLED DEVICE WITH ADJUSTABLE COLOR APPEARANCE

FIELD OF THE INVENTION

The invention relates to an OLED device having a given color appearance in its off-state and to a method to manufacture such a device.

BACKGROUND OF THE INVENTION

The US 20080218369 Al discloses a device with light emitting diodes disposed on a carrier that is made flexible by a grid of slits. To give the device a desired color appearance in its off-state, it may be covered with a colored fabric.

SUMMARY OF THE INVENTION

Based on this situation it was an object of the present invention to provide an alternative lighting device having a given color appearance in its off-state. Preferably, said device shall have an improved optical efficiency.

This object is achieved by an OLED device according to claim 1 and a method according to claim 2. Preferred embodiments are disclosed in the dependent claims. According to its first aspect, the invention relates to an OLED device with a given desired color appearance in its off-state, wherein said "color appearance" can for example be described by the color point of light reflected from the OLED device when it is illuminated with light of a given spectral composition (e.g. a standard white). By definition, the "off-state" is the state in which the device is not actively emitting light. The OLED device comprises the following components: a) An organic electroluminescent layer with a first electrode layer being (directly or indirectly) disposed on one side of it and a second electrode layer on the opposite side. Such a stack of an electroluminescent layer and electrodes corresponds to an organic light emitting diode (OLED) system as it is well known in the state of the art.

It should be noted that the term "layer" shall comprise also a multilayer that is composed of two or more sub-layers of a certain composition (for example a homogeneous composition, or an inhomogeneous composition with e.g. doping gradients). The electroluminescent layer will typically be such a multilayer. b) At least one further layer, which will be called "coating layer" in the following and which has a reflectivity (measured from the side exterior to the OLED device) that depends, based on interference effects, on the wavelength of the incident light in such a way that it reproduces the desired color appearance.

The invention further relates to a method for manufacturing such an OLED device with a given desired color appearance in its off-state, comprising the following steps: a) Providing an organic electroluminescent layer with first and second electrode layers on opposite sides. b) Applying at least one coating layer with a wavelength-dependent reflectivity that is based on interference effects and that reproduces the desired color appearance.

The described OLED device has the advantage that it provides a desired color appearance in its off-state with little or even without any impairment of the total light output efficiency. This is because the coating layer has a wavelength-dependent reflectivity that is based on interference effects instead on e.g. a partial absorption of wavelengths as in a colored filter material. The interference changes the spectral composition of reflected light without absorbing light energy. Consequently, the coating layer will not diminish the light output of the OLED device in its on-state (it should however be noted that the changed spectral composition may cause a changed absorption behavior of other components, e.g. of the electrodes, the electroluminescent layer, outcoupling layers etc., that may affect the total light output).

In the following, various embodiments of the invention will be described that relate both to an OLED device and a method of the kind described above. In a first preferred embodiment, the OLED device is at least partially transparent, i.e. it allows the transmission of more than about 30 %, preferably more than about 50 %, most preferably more than about 65 % of the external light intensity falling on it from a given direction (typically this given direction is perpendicular to the layers; in other directions, the transparency may have other values). To provide this transparency, the layers composing the OLED device (i.e. the organic electroluminescent layer, the electrode layers, and the coating layer) have to be transparent, too. For a transparent OLED device, interference effects in the coating layer can produce different color appearances with respect to different (opposite) directions. If the reflectivity is for example such that more blue light than red light is reflected, said excess of reflected blue light will miss in the transmitted light, yielding a corresponding red-shift in the transmission appearance. In contrast to this, colored absorbing materials would appear to have the same colors in all directions.

According to a further development of the aforementioned transparent OLED device, the transmission of the OLED device may have a given wavelength dependency (comprising the case that the transmission is constant for all wavelengths). The interference effects of the coating layer will usually cause a wavelength-dependency of the transmission that is a mirror image of the wavelength-dependent reflectivity. With further measures, for example an absorbing color filter behind the coating layer, the transmission properties of the OLED device can optionally be adjusted. A red-shift generated by the coating layer in transmitted light could for example be compensated for by a bluish color filter.

According to another preferred embodiment of the invention, the thickness of the coating layer may be adapted to achieve a desired color appearance. This is for example possible if the wavelength-dependency of the reflectivity of the coating layer is generated by the interference of light reflected at the front surface and the back surface of the coating layer, respectively. By adapting the thickness of the coating layer appropriately, the resulting interference and thus the color appearance of the coating layer can be tuned to essentially any desired color point within a given range of possible values.

According to a related embodiment, the thickness of the coating layer and/or the composition of the organic electroluminescent layer may be chosen to achieve a desired emission characteristics of the OLED device in its on-state. Thus both a desired color appearance in the off-state as well as a desired emission characteristics in the on-state can be achieved. Preferably, selections are made sequentially, i.e. first the thickness of the coating layer is determined to achieve a desired off-state characteristics, and then the composition of the organic electroluminescent layer is determined to achieve a desired emission characteristics in the on-state (taking the properties of the coating layer into account). Determining the composition of the organic electroluminescent layer may particularly comprise a selection of the amounts of materials emitting at different colors (e.g. blue, green, red). An adaptation of the active emission of the OLED device is particularly possible and desirable if the actively generated emission is not monochromatic but covers a broad spectrum (e.g. of white light).

In a particular embodiment of the OLED device, the coating layer may comprise an inorganic material, particularly an inorganic material selected from the group consisting of SiO₂, SiN, ZnSe, ZnS, and a metal (e.g. Ag, Al, Cu, Au, or Ni). Preferably, the coating layer may in this case have a thickness between 0.5 nm and 500 nm, most preferably between 5 nm and 200 nm.

In another particular embodiment of the OLED device, the coating layer may comprise an organic material, particularly an organic material selected from the group consisting of tris-(8-hydroxyquinoline) aluminum (AIq₃) and α- naphthylphenylbiphenyl diamine (α-NPD). Preferably, the coating layer may in this case have a thickness between 0.5 nm and 500 nm, most preferably between 10 nm and 100 nm.

Of course the mentioned materials can also be combined, for example if the coating layer is composed of several single sub-layers of different materials.

The coating layer may preferably be or comprise an anti-reflective coating which reduces the reflectivity of the external surface of the OLED device in the overall spectral range of incident (ambient) light. Such an anti-reflective coating may for instance be applied to a lighting device to prevent or reduce disturbing mirroring effects. According to another embodiment of the invention, the coating layer may comprise a thin- film encapsulation of the OLED device that serves as a sealing and a mechanical protection. As such a thin- film encapsulation may have optical effects, its parameters (e.g. its thickness) usually have to be taken into account when the whole coating layer is designed.

In general, the coating layer may be disposed at any position in the path of external light incident on the OLED device. Thus it may for example be a separate, standalone layer disposed at a distance from the electroluminescent layer and its electrodes. As its name indicates, the coating layer is however preferably disposed on the surface of some other layer. This other layer may particularly be one of the electrodes of the OLED device. Alternatively, there may be further layers between the coating layer and the next electrode, for example a filter layer or a luminescent layer for converting primary light emissions by the electroluminescent layer. Theoretically, it is also possible that the coating layer is disposed between an electrode layer and the electroluminescent layer (provided it has appropriate electrical characteristics). In practice, the coating layer will however usually be an outermost layer of the OLED device that is directly or indirectly (e.g. via an intermediate encapsulation) exposed to the environment.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter. These embodiments will be described by way of example with the help of the accompanying drawings in which:

Fig. 1 shows a schematic section through an OLED device according to the present invention; Fig. 2 shows exemplary transmission and reflection curves of a coating layer.

Like reference numbers in the Figures refer to identical or similar components. DETAILD DESCRIPTION OF EMBODIMENTS

Many organic light emitting diodes (OLEDs) comprise a nontransparent electrode, making the whole device nontransparent, too. In contrast to this, an OLED device can also be made at least partially transparent by providing it with transparent electrodes. Such a transparent OLED device will typically emit light through both the top side and the bottom side. When it is switched off, the optical impression generated by it is determined by the characteristics of the whole stack (comprising organic and inorganic materials), usually yielding a transparency with a shimmering color appearance.

There are a variety of applications in which it would be desirable to have a (transparent or nontransparent) OLED device with a given color appearance in its off- state. To achieve this, it is proposed here to adjust the off-state color impression of an OLED device deliberately by an additional coating layer that has a wavelength-dependent reflectivity which is based on interference effects. Preferably, this coating layer may simultaneously serve as an anti-reflective coating. Moreover, the coating layer may affect the color point of the actively emitted light in the on- state of the OLED device. A white-emitting OLED stack may for example get different colors of the emissions at the top side and the bottom side, respectively, due to the coating layer. An exemplary realization of the above general concepts will in the following be explained with reference to the section through an OLED device 100 shown in Figure 1. It should be noted that the drawing is only schematic and that the layers and their thicknesses are not to scale. The OLED device 100 as a whole is transparent and comprises, from bottom to top, the following components: - A transparent substrate 10, for example a glass plate, serving as a mechanical carrier.

A transparent first electrode 11 , for example operated as an anode, which is for example disposed directly on the substrate 10. An organic electroluminescent layer 12, which is for simplicity shown as a single layer though it will typically comprise a plurality of sub-layers (e.g. transport layers and injection layers) as it is known to a person skilled in the area of OLED devices. A transparent second electrode 13, for example operated as a cathode, which is disposed on the electroluminescent layer. In the following, it will be assumed without loss of generality that in this example the first electrode layer is an anode layer and the second electrode layer a cathode layer.

An anti-reflective coating layer 20 disposed on the cathode layer 13. This coating layer 20 may be a single homogeneous layer or a system of several sub-layers. The coating layer 20 may serve as an anti-reflective coating that reduces the reflectivity of the cathode layer 13. This is particularly favorable if the cathode layer 13 comprises a thin metal layer or sequence of such layers.

The OLED device 100 may further comprise a housing or encapsulation (not shown) that provides a sealing and mechanical protection. Of course the housing has to be transparent in regions of the top side and bottom side of the OLED device 100 through which light emission shall take place.

It should be noted that the OLED device may optionally comprise further layers, for example a luminescent conversion layer, that are not shown in the embodiment of Figure 1. Due to its transparency, the OLED device 100 emits light generated in the organic layer 12 through both the top side and bottom side (possibly with different intensities).

To make the OLED device 100 assume a given desired color impression in its off-state (i.e. when no light is actively generated in the electroluminescent layer), the coating layer 20 is adapted accordingly. More particularly, the thickness d of the coating layer can be adjusted such that, by interference effects, a desired color impression of light reflected from the coating layer is achieved. The coating layer 20 therefore serves not only for reducing reflections from the surface of the OLED device 100, but also for the adjustment of the color impression in the off-state.

Figure 2 shows in this respect curves representing the dependency of the transmission T (solid lines) and the reflection R (dashed lines), respectively, upon the wavelength λ of incident light. The data refer to an OLED device with an anti-reflective coating layer of α-NPD and an encapsulation of glass, wherein each curve corresponds to a different thickness d of the coating layer ranging between 35 nm and 85 nm.

For a coating layer with a thickness of 35 nm, a pronounced transmission maximum exists at a wavelength λ of 460-470 nm in the blue or green/blue spectral range. For thicker α-NPD coating layers, the transmission curves first become more leveled, while a transmission maximum in the range of about 650-700 nm evolves for larger thicknesses in the red spectral range.

As the reflectivity R, the transmission T, and the residual absorption together yield 100 %, the reflectivity curves for corresponding thicknesses typically show the opposite trends: for thinner coating layers, reflection is higher in the red spectral range, while for thicker coating layers reflection is highest in the blue/green spectral range (about 450-550 nm). Accordingly, the transparency and the color impression of the OLED device in its off-state changes with the thickness d of the coating layer from a reddish to a bluish color.

A concrete embodiment of an OLED device according to the invention may comprise the following particular architecture: a glass substrate 10, e.g. coated with indium-tin-oxide (ITO) as anode 11 ; a p-doped hole-injection layer (e.g. MTDATA:F₄-TCNQ (1%) with a thickness of about 40 nm); a hole-conduction layer (e.g. α-NPD with a thickness of about

10 nm); an orange emission layer (α-NPD :Ir(MDQ)₂(acac) (10%) with a thickness of about 20 nm); - an electron conduction layer (e.g. BAIq with a thickness of about

20 nm); an n-doped layer (e.g. LiF with a thickness of about 1 nm); an Al layer (with a thickness of about 1 ,5 nm) and a transparent

Ag layer (with a thickness of about 15 nm), together serving as a cathode 13; one or more coating layers 20 for an optimal out-coupling of light. The aforementioned light out-coupling coating layer(s) 20 may for example comprise the following materials: inorganic materials like SiO₂, SiN, ZnSe, or ZnS with a thickness d between 0.5 nm and several hundred nanometers

(typically 5-200 nm); organic materials like AIq₃ and α-NPD with a thickness d between 0.5 nm and several hundred nanometers (typically 10-100 nm); - multi- layer systems of the listed materials.

Furthermore, the OLED device 100 may be transparently encapsulated. To this end, different possibilities are available, for example: a transparent glass cover is glued onto the prefabricated OLED device (usually at the border of the cover only); a frame is first glued onto the prefabricated OLED device, on which then a transparent glass plate is glued; a glass plate is directly (and optionally with its total face) glued onto the prefabricated OLED device; - a thin-film encapsulation is applied onto the prefabricated OLED device, for example three double layers comprising

SiN (200 nm)/ SiO₂ (100 nm), or any other known or commercially available thin-film encapsulation; in addition to a thin- film encapsulation on the prefabricated OLED device, a glass plate is glued onto this encapsulation to increase mechanical robustness.

In general, the influence of an encapsulation on the optical impression of the OLED device has to be taken into account. If the encapsulation is achieved by a glass cover or glass plate, this influence is comparatively small. In case of a thin- film encapsulation, the thickness of this encapsulation may however require an adjustment and coordination with other parameters to achieve a desired color impression. It should be noted that the color point of emissions by the described transparent OLED device in its on-state can differ for the light emitted through the top side and the bottom side, respectively, if the transmission/reflection curves strongly depend on the wavelength. For monochromatic OLED devices, this difference may hardly be noticeable by the human eye. For white emitting OLEDs, the effect can however be visible. This can be exploited to provide an OLED device that has in its on-state different emission characteristics on opposite sides, for example a more bluish color point at one side and a more reddish color point at the opposite side.

The described OLED devices may be used in any application of transparent OLEDs. In case of interior lighting, it may for example be desirable to provide rooms with room dividers or walls with a slight color impression that depends on the usage of the associated room (e.g. a reddish impression for lounges, a neutral to reddish impression for tanning salons, a bluish impression for sports facilities like fitness studios or swimming halls etc.). Moreover, it may be desirable in malls to adapt the illumination to a particular theme, season etc. Furthermore, windows with integrated, large area OLEDs in e.g. buildings or cars may be provided with a particular color impression.

The possibly different spectral composition of white light actively emitted from opposite sides of an OLED device may be advantageous, too, for example in the area known as "atmosphere creation". Preferably, such transparent OLEDs may be arranged on turnable carriers (e.g. with a horizontal or vertical axis of rotation), thus allowing to change the emission of the two light colors arbitrarily.

Finally it is pointed out that in the present application the term "comprising" does not exclude other elements or steps, that "a" or "an" does not exclude a plurality, and that a single processor or other unit may fulfill the functions of several means. The invention resides in each and every novel characteristic feature and each and every combination of characteristic features. Moreover, reference signs in the claims shall not be construed as limiting their scope.

Claims

CLAIMS:

1. An OLED device (100) with a given desired color appearance in its off- state, comprising: a) an organic electroluminescent layer (12) with a first and a second electrode layer (11, 13) being arranged on opposite sides thereof; b) at least one coating layer (20) with a wavelength-dependant reflectivity (R) that is based on interference effects and that reproduces the desired color appearance.

2. A method for manufacturing an OLED device (100) with a given desired color appearance in its off-state, comprising: a) providing an organic electroluminescent layer (12) with a first and a second electrode layer (11, 13) being arranged on opposite sides thereof; b) applying at least one coating layer (20) with a wavelength-dependant reflectivity (R) that is based on interference effects and that reproduces the desired color appearance.

3. The OLED device (100) according to claim 1 or the method according to claim 2, characterized in that the OLED device (100) is at least partially transparent.

4. The OLED device (100) or the method according to claim 3, characterized in that the transmission (T) of the OLED device has a given wavelength- dependency.

5. The OLED device (100) according to claim 1 or the method according to claim 2, characterized in that the thickness (d) of the coating layer (20) is adapted to achieve the desired color appearance.

6. The OLED device (100) according to claim 1 or the method according to claim 2, characterized in that the thickness (d) of the coating layer (20) and/or the composition of the organic electroluminescent layer is chosen to achieve a desired emission characteristics of the OLED device (100) in its on-state.

7. The OLED device (100) according to claim 1 or the method according to claim 2, characterized in that the coating layer (20) comprises a material selected from the group consisting of Siθ2, SiN, ZnSe, ZnS, AIq₃, α-NPD, and a metal.

8. The OLED device (100) or the method according to claim 7, characterized in that the coating layer (20) has a thickness (d) between about 0.5 nm and 500 nm.

9. The OLED device (100) according to claim 1 or the method according to claim 2, characterized in that the coating layer (20) comprises an anti-reflective coating.

10. The OLED device (100) according to claim 1 or the method according to claim 2, characterized in that the coating layer (20) comprises a thin-film encapsulation.