FIELD OF THE INVENTION

The invention relates to an illumination device comprising a blue and/or a green emitting organic light emitting diode (OLED) and a converter layer with a luminescent material.

BACKGROUND OF THE INVENTION

Illumination devices of the aforementioned kind are for example disclosed in the US 2006/0197437 Al, which also mentions some examples of luminescent materials that can be used as converter.

SUMMARY OF THE INVENTION

Based on this background it was an object of the present invention to provide an illumination device with improved properties, wherein it is particularly desirable that the device has a high color rendering index and/or lumen equivalent. This object is achieved by an illumination device according to claim 1.

Preferred embodiments are disclosed in the dependent claims.

An illumination device according to the present invention comprises as a basic component a blue and/or a green emitting organic light emitting diode (OLED) for generating blue and/or green primary light. In this context, a "blue emitting OLED" is supposed to have its peak emission between about 420 and 500 nm, while a "green emitting OLED" shall have its peak emission between about 500 and 560 nm. If both a blue and a green emitting OLED are used, they may have any appropriate spatial arrangement, for example layered one above the other, disposed next to each other, or distributed in some matrix array. It should be noted that the singular term "the OLED" will in the following refer back to both the blue and the green OLED if both are present. The illumination device further comprises a converter layer that is at least partially disposed in the light output path of the OLED, i.e. at least a part of the primary light generated by the OLED passes the converter layer before it can leave the illumination device as output light. During this passage, the primary light may be absorbed and reemitted with a converted (longer) wavelength. The converter layer comprises at least one luminescent material that is selected from the group consisting of the following materials, which are listed here in six subgroups for clarity:

a) (Zni_ _x Cdχ)(Si_ _y Se _y ), (Ini_ _x Ga _x )(Pi_ _y As _y ) with x, y e [0, 1], wherein these materials are preferably present as particles with a size between 20 nm and 5 μm; b) uranine (i.e. the sodium salt of fluorescein), rhodamine 6G, rhodamine B; c) complexes of Ir ³⁺ , particularly [Ir(phenylpyridine)3], wherein phenylpyridine is also known as 4-Azabiphenyl, p-phenylpyridine, and 3-azabiphenyl; d) complexes of Tb ³⁺ , particularly [Tb(benzoate)3], [Tb(salicylate)3], [Tb(picolinate)3], wherein salicylate is the salt of salicylic acid and picolinate is the salt of picolinic acid; e) the complexes [Eu(acac) ₃ (X)], [Eu(ttfa) ₃ (X)], [Eu(thd) ₃ (X)], and [Eu(dibenzoylmethane)3(X)] with X = bipyridine, phenanthroline, diphenylphenathroline and substitutes thereof, wherein "acac" is acetylacetone, "ttfa" is the salt of thenoyltrifluoroacetone, and "thd" is 2,2,6,6-tetramethyl-3,5-heptanedionate; f) SrLi ₂ Si0 ₄ :Eu, (Ba,Sr) ₂ Si0 ₄ :Eu, (Ca,Sr) ₂ Si0 ₄ :Eu.

The converter layer may comprise just one or several (typically two or three) different luminescent materials of the listed group. If the illumination device comprises a green emitting OLED, the converter layer will usually not comprise (need) a luminescent material that emits in the green range. With the enumerated luminescent materials, a white-emitting illumination device with good light emitting properties can be achieved because the absorption characteristics of the converter materials fit very well to the emission spectrum of the OLED.

In a preferred embodiment of the invention, the converter layer comprises only yellow emitting luminescent materials, i.e. luminescent materials which have a peak emission in the range of about 520-600 nm. In this way an illumination device with a cool white light output can be achieved.

In another preferred embodiment, the converter layer comprises only yellow and red emitting luminescent materials, wherein the red emitting luminescent materials are assumed to have a peak emission in the range of about 590-660 nm. In this way an illumination device with a warm white light output can be achieved. In still another embodiment, the converter layer comprises only green and red emitting luminescent materials, wherein the green emitting luminescent materials are assumed to have a peak emission in the range of about 500-570 nm. In this way an illumination device with a cool and warm white light output can be achieved. The converter layer may completely consist of the luminescent material(s). Preferably, the converter layer comprises however a matrix material in which particles of the luminescent material are embedded. Using such a granular structure makes the converter layer independent of the chemical and/or physical mixing properties of the luminescent material and allows to control its optical properties (absorption and emitting characteristics) via the particle size and density. In general, the aforementioned particles of luminescent material may typically have a mean particle size between about 1 nm and 20 μm. If a translucent but not transparent converter layer is desired, the particles size should lie between 0.1 μm and 20 μm. If however a transparent converter layer is desired, the particle size should be below 100 nm, preferably below 50 nm. In another embodiment of the invention, the luminescent material may have a quantum dot type composition. This can for example be achieved by embedding particles with a size between 1 nm and 10 nm in a matrix material.

Particles of the luminescent material may be directly embedded into some suitable matrix material. In a preferred embodiment, the particles are however coated on their surface before they are embedded. With a coating, the light in- and out-coupling as well as the long-term stability of the luminescent materials can be improved. Suitable coating materials comprise for example AI2O7, MgO, MgAl ₂ O4, La ₂ O?, Y2O7, SC2O7, Ca ₂ P ₂ O ₇ , Sr ₂ P ₂ O ₇ , and Mg ₂ P ₂ O ₇ .

The matrix material in which particles of the luminescent material are embedded can preferably comprise a transparent polymer, for example PMMA (polymethylmethacrylate) or PC (polycarbonate). Such materials can readily be processed, e.g. by injection molding, and yield mechanically stable components that can conventionally be mounted in a socket.

It was already mentioned that the illumination device provides improved light generation properties. Preferably, the device has a color rendering index Ra according to the definition of the International Commission on Illumination (CIE) of more than 85.

The illumination device may optionally comprise a color filter in order to cut out undesired parts of the emission spectrum, for example infrared (IR) components.

If both a blue and a green OLED are incorporated into the illumination device, it is preferred that they are individually controllable, i.e. that the power supply to them can be set separately. The spectral composition of the light output can then selectively be adjusted.

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:

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows an illumination device with a blue OLED and a yellow converter layer; Figure 2 shows an illumination device with a blue and a green

OLED and a yellow converter layer; Figure 3 shows an illumination device with a blue OLED and a yellow and red converter layer;

Figure 4 shows an illumination device with a blue and a green OLED and a yellow and red converter layer;

Figure 5 shows an illumination device with a blue OLED and a green, yellow and red converter layer. Like reference numbers in the Figures refer to identical or similar components.

DETAILED DESCRIPTION OF EMBODIMENTS

Organic light emitting diodes (OLEDs) are presently being commercialized in small-size self-emissive displays, e.g. for cameras or cell phones. OLEDs work by the principle of electroluminescence. Positive and negative electrical charges are injected respectively from anode and cathode into a stack of organic layers of several tens of nanometers in thickness. The voltage applied is in the range of a few Volts only. When migrating through the organics, these charges eventually recombine by forming an exciton. This exciton can release its excitation energy into a photon with a certain probability. The higher this probability the more efficient the OLED device will be. Internal quantum efficiencies above 50% have already been proven in research laboratories. So-called small molecule based sm-OLEDs are manufactured by evaporating several layers of low molecular weight organic materials in vacuum processes. In another technology, polymer molecules are deposited onto the substrate using printing or other wet-deposition methods (pm-OLEDs). For other applications than displays, it would be desirable to create high- brightness, high efficiency and long-life white OLEDs for lighting, with superior light quality with a very high color rendering index (CRI > 90).

White light from OLED light sources can for example be generated by down-conversion of a blue emitting OLED by a converter layer comprising suitable converter materials. The main drawback of known white OLED light sources is however the mismatch of the converter material to the emission spectrum of the blue OLED and the resulting white spectrum, which is far from an optimum with respect to color rendering and luminous efficacy.

The present invention therefore proposes new OLED illumination devices with improved properties for general lighting purposes which use a converter layer to convert blue emitting OLEDs (420 - 500 nm) into warm or cool white light sources. This is achieved by a converter layer comprising one or several luminescent materials, e.g. a yellow phosphor, a yellow and red phosphor, or a green and a red phosphor. Alternatively a partial conversion of a green emitting OLED into red using a red phosphor can be used. Combining this with a blue emitter one can realize warm or cool white light. The phosphors are selected in a way that an emission spectrum with a high color rendering (Ra > 85) and a high lumen equivalent ( > 280 lm/W _op t) is achieved. Moreover, coated luminescent materials can be applied with a coating that improves light in- and out-coupling and long-term stability of the converter layer. Such a white OLED device then hardly shows color point shift over rated lifetime due to the higher stability of the converter layer relative to the excitation source, i.e. the OLED stack. Figures 1 to 5 schematically show different illumination devices 10-50 according to the above concepts. These devices comprise a luminescent screen serving as converter layer that is disposed next/parallel to a flat OLED substrate to enhance light outcoupling from the OLED and to achieve a white light emitting light source.

The first illumination device 10 shown in Figure 1 is useful to achieve a cool white light source with a rather high color rendering. It comprises a blue emitting OLED Ob and requires a translucent and yellow to orange emitting converter layer C y, which can comprise one or several luminescent compositions showing broad band emission in the range from 540 to 600 nm. Suitable luminescent materials may be selected from the following

Table 1 :

(Zni_ _x Cdχ)(Si_ _y Se _y ) and (Ini_ _x Ga _x )(Pi_ _y As _y ) with x, y e [0, 1] uranine, rhodamine 6G, and rhodamine B complexes of Ir , particularly [Ir(phenylpyridine) ₃ ] complexes of Tb ³⁺ , particularly [Tb(benzoate)3], [Tb(salicylate)3], or [Tb(picolinate) ₃ ] the complexes [Eu(acac) ₃ (X)], [Eu(ttfa) ₃ (X)], [Eu(thd) ₃ (X)], and [Eu(dibenzoylmethane) ₃ (X)] with X = bipyridine, phenanthroline, diphenylphenathroline and further derivatives

SrLi ₂ Si0 ₄ :Eu, (Ba,Sr) ₂ Si0 ₄ :Eu, and (Ca ₅ Sr) ₂ SiO ₄ :Eu Additional luminescent materials for the conversion of 420-500 nm OLED radiation into yellow to orange light can be selected from the following

Table 2:

Figure 2 shows a modified illumination device 20, in which an additional green emitting OLED substrate Og has been added. The residual composition of this illumination device 20 is identical to that of Figure 1.

Figure 3 shows a further embodiment of an illumination device 30 comprising a blue emitting OLED Ob and a converter layer C_yr that comprises yellow and red converting luminescent materials. Thus a warm white light output of the whole illumination device 30 can be achieved. Suitable luminescent materials with an emission in the yellow and/or red spectral range can be selected from Table 1.

Additionally, broad band emitting phosphors activated by Eu ²⁺ with a strong absorption between 400 and 560 nm can be selected from the following

Table 3:

Composition ^-max [nm]

(Ca-|. _v . _z Sr _v )S:Eu ₂ (y = 0.0 - 1 0, 0 0 < z < 0 Ά 610 - 655

(Ca-ι_ _x _ _v _ _z Sr _x Ba _v ) ₂ Si ₅ N ₈ :Eu z (X, Y = 0.0 - 1 0 0 0 < z < 0.2) 590 - 630

(Ca-ι_ _x _ _v _ _z Sr _x Ba _v ) ₂ Si ₅ - _a Al _a N 8-aO _a :Eu z (a = 0 .0 — 2.0, x, y = 0 .0 - 1.0, 0.0 < z < 0.2) 615 - 650

(Ca-,. _v . _z Sr _v )AISiN 3:Eu _z (y = 0.0 - 1 .0, 0.0 < z < 0.2) 620 - 650

(Ca-ι_ _x _ _v _ _z Sr _x Ba _v ) ₃ Si ₂ N ₂ O ₄ : Eu _z (x, y = 0.0 - 1 .0 0.0 < z < 0 .2) 620 - 640

Alternatively a red line emitting phosphor (590 - 630 nm) can be added, which is advantageous with respect to the lumen equivalent of the light source, since less light is emitted beyond 630 nm, where the human eye sensitivity is very low. Such red line emitting phosphors can be selected from the following

Table 4:

Figure 4 shows a modified illumination device 40, in which an additional green emitting OLED substrate Og has been added. The residual composition of this illumination device 40 is identical to that of Figure 3.

Figure 5 shows a further embodiment of an illumination device 50 that comprises a blue emitting OLED Ob and a conversion layer C gr with luminescent materials for converting into the green and red spectral range. Figure 5 further indicates a color filter CF that can optionally be added to remove possible undesired parts of the emission spectrum in this and the previous embodiments.

Suitable luminescent materials can be selected from Table 1. Additionally, red emitting luminescent materials can be selected from tables 3 and 4. Moreover, green emitting luminescent materials, which can be excited by 420-500 nm radiation and are activated by Eu ²⁺ or Ce ³⁺ , can also be selected from the following

Table 5:

All of the above mentioned luminescent compositions in tables 1 - 5 are preferably applied as micro scale powders (mean particle size 0.1-20 μm). This results in a scattering converter layer, which is translucent but not transparent. If a transparent converter layer is required, the applied luminescent compositions must have a mean particle size below 50 nm. Alternatively, quantum dot type luminescent compositions (mean particle size 1-10 nm), e.g. (Zni_ _x Cd _x )(Si_ySe _y ), (Ini_ _x Ga _x )(Pi_ _y As _y ), Si, Ge, or (Agi_ _x Cu _x )(Bri_ _y I _y ) with x, y = 0.0 - 1.0, could be applied for transparent converter layers, too. A further alternative to obtain transparent converter layers is the application of organic luminescent materials, which can be dissolved into a transparent organic polymer, e.g. PMMA or PC. Suitable organic luminescent materials exhibiting a strong absorption in the blue spectral range are perylene and its derivatives, coumarine and its derivatives, Ln ³⁺ (Ln = Eu and Tb), Ir ³⁺ and Ru ²⁺ -complexes. The described illumination devices according to of the invention have the following advantages: simple construction (blue/green OLED + green/yellow/red luminescent screen); suitable inorganic phosphors and blends can be optimized; - degradation of the blue OLED does not result in color point shift; the use of a narrow-band emitting phosphor for red avoids losses in the infrared that are often encountered in "broadband" white

OLEDs using a red (plus green plus blue) organic emitter; improved light outcoupling and thus enhanced wall plug efficiency.

In summary, the illumination devices according to the invention can be described as phosphor converted organic light emitting diodes (pcOLEDs) with an organic light emitting element (OLED) and a converter layer. Optionally they further have one or more of the following features:

OLED has its peak emission between 420 and 500 nm (blue OLED); the OLED has its peak emission between 510 and 560 nm (green OLED); - the converter layer comprises one or several compositions from tables 1 to 5; they comprise a color filter element. The illumination devices can be used as light sources for the application in general, advertisement, automotive, and decorative lighting or for signaling or backlighting purposes. 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 emitting their scope.