WO2007004106A1

WO2007004106A1 - Light-emitting device

Info

Publication number: WO2007004106A1
Application number: PCT/IB2006/052079
Authority: WO
Inventors: Frank A. Van Abeelen; Oscar H. Willemsen; Marc W. G. Ponjee; Johannes F. M. Cillessen; Johannes J. W. M. Rosink; Nijs C. Van Der Vaart; Herbert Lifka
Original assignee: Koninklijke Philips Electronics NV
Current assignee: Koninklijke Philips NV
Priority date: 2005-06-30
Filing date: 2006-06-26
Publication date: 2007-01-11
Anticipated expiration: 2007-12-30
Also published as: TW200711527A

Abstract

The invention relates to a light-emitting device comprising a light emissive layer (E) arranged between an at least semi-transparent first electrode layer (A; C) and an at least semi- reflective second electrode layer (C; A) . The at least semi-transparent first electrode layer is arranged between said light emissive layer and a further at least semi-reflective layer (R) . The further at least semi-reflective layer is separated from said at least semi-transparent first electrode layer by one or more light transparent intermediate layers. By controlling the thickness of the one or more light transparent intermediate layers which are located outside the electrically critical region of the light-emitting device, interference effects of the light- emitting device can be tuned without disturbing the delicate balance of the electrical properties of the device.

Description

Light-emitting device

The invention relates to a light-emitting device comprising a light emissive layer arranged between an at least semi-transparent first electrode layer and an at least semi- reflective second electrode layer.

Light-emitting devices, such as display panels, back lights for display panels and light sources for illumination purposes, have seen considerable technological developments in recent years. One of these developments relates to the use of microcavity effects, wherein a resonant cavity is formed between a first and a second reflective layer, at least one of which is semi-transparent, and comprises a light emitting layer arranged between these reflective layers. The microcavity effect is caused by coherent interference effects in the very small resonant cavity formed between the first and second reflective layers. Microcavity effects can be used to increase the external light efficiency of the display device and to obtain an improved colour purity. EP-A-I 154 676 discloses an organic electroluminescent device having a first electrode of a light reflective material, an organic layer including an organic light emitting layer, a semitransparent reflection layer, and a second electrode of a transparent material which are stacked sequentially and so configured that the organic layer functions as a cavity portion of a cavity structure. Light that resonates in a certain spectral width (wavelength λ) is extracted by configuring the device such that the optical path length L becomes minimal in a sequence satisfying (2L)/λ + Φ/(2π)= m (m is an integer). Φ designates the phase shift for light reflected by opposite ends of the cavity portion, L is the optical path length of the cavity portion, and λ is the peak wavelength of the spectrum of part of light to be extracted.

A drawback of the prior art microcavity display device is that optimisation of the microcavity effect in the light-emitting device may disturb the electrical properties of the device, in particular the delicate balance between electron and hole currents within the organic layers. It is an object of the invention to provide a light-emitting device that may employ microcavity effects with a reduced or eliminated impact on the electrical properties of the light-emitting device.

This object is accomplished by a light-emitting device comprising a light emissive layer arranged between an at least semi-transparent first electrode layer and an at least semi-reflective second electrode layer, wherein said at least semi-transparent first electrode layer is arranged between said light emissive layer and a further at least semi- reflective layer and wherein said further at least semi-reflective layer is separated from said at least semi-transparent first electrode layer by one or more light transparent intermediate layers.

In case of a substantially transparent first electrode layer, a microcavity can be formed between the at least semi-reflective second electrode layer and the further at least semi-reflective layer. In case of a partly transparent, partly reflective first electrode layer, a first microcavity can be formed between the at least semi-reflective second electrode layer and the partly reflective first electrode layer and a second microcavity can be formed between the partly reflective first electrode layer and the further at least semi-reflective layer. By controlling the thickness of the one or more substantially light transparent intermediate layers, which are located outside the electrically critical region of the light-emitting device, the interference effects of a said microcavity of the light-emitting device can be tuned without disturbing the delicate balance of the electrical properties of the device.

The invention can be advantageously applied in display devices defined in claim 2, wherein the at least semi-transparent first electrode layer is an at least semi- transparent anode layers, improving the processability of the light-emitting devices. The invention is advantageously being applied in top-emission devices defined in claim 3. The embodiment of the invention as defined in claim 4 has the advantage that a separate at least semi-reflective layer can be omitted.

It is noted that an at least semi-reflective layer or layer stack generally would have a reflectivity exceeding 10% of the incident light. A semi-transparent layer should generally be understood as a layer or stack of layers having a transparency of at least 10%, preferably at least 20% and more preferably at least 50%. A substantially light reflective layer or layer stack would generally have a reflectivity of over 60%, preferably over 70% and more preferably over 80% or 90% of the incident light. The embodiment of the invention as defined in claim 5 allows the formation of a first microcavity between the first electrode layer and the second electrode layer and a second microcavity between the first electrode layer and the reflective layer.

An advantageous embodiment of the invention is described in claim 6. The layers of the stack of intermediate layers can be designed with specific optical and/or physical-chemical properties. For instance, the stack could have planarizing properties or provide a chemical barrier in between a metallic reflector and subsequent layers. The optical properties of the multi layer stack can be such that the emitted light intensity and/or colour purity of emitted light is optimised and/or the angular distribution of the light-output is optimised. In addition, the optical properties can be chosen to minimise reflection of ambient light from the microcavity to improve the daylight contrast of a display. Moreover, outcoupling structures as defined in claim 10 may be included in the stack to enhance outcoupling of light that would otherwise be subject to waveguiding.

A particularly advantageous embodiment of the invention is defined in claim 7. An intermediate light transparent layer with a high index of refraction has been found to reduce the colour shift Δc as a function of the viewing angle to the light-emitting device, i.e. a reduced change of the chromaticity coordinates of emitted light with viewing angle. This observation is of particular relevance when the optical width of the microcavity is of the order of or larger than the wavelength λ₀ of the light emitted by the light emissive layer. Preferably, the intermediate layers comprise a further layer as defined in claim 8 with the function of a transparent tuning layer. Variation of the thickness of the layers with different refractive indices enables tuning of the effective reflection from the microcavity. The reflection can even be made higher than for a microcavity without the light transparent intermediate layer(s) of the invention. In an embodiment of the invention, the intermediate light transparent layer comprises a birefringent layer as defined in claim 9. This embodiment allows the light- emitting device to emit partly polarized light which is advantageous for several applications.

In an embodiment of the invention, a further at least semi-reflective layer as defined in claim 11 is applied. As the further at least semi-reflective layer is electrically insulated from the first and second electrode layers, the semi-reflective layer can be used for electrically shunting either one of these electrode layers or other current carrying lines such as power lines of an active matrix display.

In an embodiment of the invention, the light-emitting device is constructed as defined in claim 12. As the charge carrying injection layer can be applied in a printing process, thickness variations of this layer are obtained relatively easily. Although the light emissive layer is typically printed as well, the thickness range in which light is effectively emitted is limited.

It should be acknowledged that the above described embodiments, or aspects thereof, may be combined.

Particularly advantageous applications of the light-emitting device are display devices as defined in claims 13 and 14 and illumination devices as defined in claim 15. In case of an organic display device, the invention is applicable to both active and passive matrix display panels. The invention will be further illustrated with reference to the attached drawings, which schematically show preferred embodiments according to the invention. It will be understood that the invention is not in any way restricted to these specific and preferred embodiments.

In the drawings:

Figs. IA- IH schematically show various embodiments of layer stacks of light- emitting devices according to the invention;

Figs. 2 A and 2B show simulation results for the devices of Fig. IA with respectively a transparent and a semi-transparent first electrode layer displaying the influence on the light emission intensity as a function of the wavelength of the emitted light and the thickness of the intermediate light transparent layer for various viewing angles;

Fig. 3 shows a spectral characteristic illustrating the colour shift of a conventional light-emitting device; Fig. 4 shows a further embodiment of a layer stack of a light-emitting device according to the invention;

Fig. 5 illustrates a cross-section of an intermediate birefringent light transparent layer for a light-emitting device according to the invention, and

Figs. 6A-6C schematically show applications of light-emitting devices according to the invention. Figs. IA- IH schematically show various embodiments of layer stacks of light- emitting devices 1 according to the invention. A viewing angle θ is indicated. For a physical description of microcavity effects, reference is made to EP-A-I 154 676.

In the embodiments of Figs. IA-IH a light-emissive layer or layer stack E, which is typically a stack of organic layers, is arranged between a first electrode layer A₅C and a second layer electrode layer C₅A. Light may be emitted from the devices 1 by providing appropriate signals to at least one of the first electrode layer A₅C and the second electrode layer C₅A. The layer stacks are typically arranged on top of a substrate S.

The light emissive layer E may e.g. comprise a polymer layer, such as polyphenylenevinylene (PPV)₅ polyfluorenes and polyspirofluorenes. By engineering red, green and blue emitting layers, a full color light-emitting device can be accomplished. A polyethylenedioxythiophene (PEDOT) layer (see Fig. 4) can be used as a hole-injection layer. In Figs. 1 A-ID, an at least semi-transparent first electrode layer A, commonly referred to as the anode or anode layer, is arranged between the light emissive layer E and a further at least semi-reflective layer R, whereas the semi-reflective layer R is separated from the anode layer A by one or more substantially light transparent intermediate layers I. Figs. IA and IB illustrate top-emission devices 1, wherein light is output in a direction away from a substrate S. Figs. 1C and ID illustrate bottom-emission devices 1, wherein light is output through a light transparent substrate S. In Figs. IE- IH, an at least semi-transparent first electrode layer C, commonly referred to as the cathode or cathode layer, is arranged between the light emissive layer E and a further at least semi-reflective layer R, whereas the semi-reflective layer R is separated from the cathode layer C by one or more substantially light transparent intermediate layers I. Figs. IE and IF illustrate top-emission devices 1, wherein light is output in a direction away from a substrate S. Figs. IG and IH illustrate bottom-emission devices 1, wherein light is output through a light transparent substrate S.

It is noted that the embodiments shown in Figs. 1 A-ID are preferred over the embodiments of Figs. IE- IH for processing on at least semi-transparent cathode layers may be difficult. In Fig. IA, the cathode layer C is partly light transparent, partly light reflective for light emitted from the light emissive layer E. The cathode layer is e.g. made of a thin layer, typically a few nm, of Ba, Ca or Mg with on top a thin metal film such as Ag, Al or Pt. Also cathode layers C incorporating LiF can be applied. In one embodiment of the invention, the anode layer A is substantially transparent. Typically a layer of transparent conductive oxide (TCO), such as indium tin oxide (ITO), is employed as a transparent anode layer A. The anode layer A may have a thickness in a range of 1-200 nm, e.g. 100 nm. In another embodiment of the invention, the anode layer A is partly transparent, partly reflective. A thin metallic film of e.g. Ag or Al of a few nm may be used to obtain such an anode layer.

The intermediate light transparent layer I comprises e.g. ZnSe, ZnS, TiO2, silicon-nitride, silicon-oxide and/or silicon-oxynitride. In order to take advantage of microcavity effects, this light transparent layer I should have a thickness not larger than approximately 1 μm.

A substantially reflective layer R, e.g. made of a metal film of Mo, Cr, Al or Ag with a thickness of e.g. larger than 50 nm, e.g. 100 nm, is arranged over a substrate S. A reflective layer R with a finite transmittance (0.1-10%) may be selected in order to allow optical feedback by employing a light sensitive device (not shown) beneath the layer R.

In Fig. IA, in case of a substantially transparent anode layer A, a microcavity is formed between the partly reflective cathode layer C and the reflective layer R. By appropriate selection of the thickness of the intermediate layer I, the microcavity effect can be tuned without disturbing the electrical characteristics of the device 1 near the light emissive layer E. Microcavity effects can be diminished or eliminated by choosing the thickness of the intermediate layer I larger than approximately 1 μm in which case coherency of individual light beams is lost and/or interference effects average out.

Fig. 2A shows simulation results illustrating how the light output of the device 1 varies as a function of the wavelength λ₀ (in nm along the horizontal axis) and thickness of the intermediate film I (in nm along the vertical axis) for various viewing angles θ. Clearly, the thickness of the intermediate layer I can be selected to tune the interference effects of the microcavity. With increasing thickness, the interference maxima are located at larger wavelengths λ₀.

In Fig. IA, in case of a partly transparent, partly reflective anode layer A, a first microcavity is formed between the partly reflective cathode layer C and the partly reflective anode layer A and a second microcavity is formed between the partly reflective anode layer A and the substantially fully reflective layer R. The second microcavity can be tuned by selecting a thickness of the intermediate layer I without effecting the electrical characteristics of the device 1 near the light emissive layer E. Fig. 2B shows simulation results illustrating how the light output of the device 1 with two microcavities varies as a function of the wavelength λ₀ (in nm along the horizontal axis) and thickness of the intermediate film I (in nm along the vertical axis) for various viewing angles. With respect to Fig. 2 A it can be observed that the light emitted as a function of angle and wavelength by a device comprising two microcavities differs from that of a device comprising one cavity.

Similar effects can be obtained by the devices shown in Figs. 1B-1H. In Fig. IB an inverted top-emission light-emitting device 1 is shown. The layer stack comprises a substrate S, a light reflective cathode layer C, a light emissive layer E, an anode layer A that may be either substantially light transparent or partly light transparent, partly light reflective. The anode layer A is separated from a partly light transparent, partly light reflective layer R by one or more light transparent layers I.

In Fig. IC a bottom-emission light-emitting device 1 is shown. The layer stack comprises a light transparent substrate S over which a light reflective cathode layer C, a light emissive layer E and an anode layer A that may be either substantially light transparent or partly light transparent, partly light reflective, are applied. The anode layer A is separated from a partly light transparent, partly light reflective layer R by one or more light transparent layers I.

The cathode layer C is substantially light reflective for light emitted from the light emissive layer E. The cathode layer is e.g. made of a thick metallic layer with a thickness of e.g. 100 200nm, including a low work function film of 1-10 nm of Ba, Ca or Mg combined with a metal film such as Ag, Al or Pt. Also cathode layers C incorporating LiF can be applied.

In one embodiment of the invention, the anode layer A is substantially transparent. Typically a layer of transparent conductive oxide (TCO), such as indium tin oxide (ITO), is employed as a transparent anode layer A.

In another embodiment of the invention, the anode layer A is partly light transparent, partly light reflective. A thin metallic film of e.g. Ag or Al of a few nm may be used to obtain such an anode layer. The light transparent intermediate layer I may comprise ZnSe, ZnS, TiO2, silicon-nitride, silicon-oxide and/or silicon-oxynitride. In order to take advantage of microcavity effects, this light transparent layer I should have a thickness not larger than approximately 1 μm. A partly light transparent, partly light reflective layer R, e.g. comprising a film containing Mo, Cr, Al or Ag is arranged over a transparent substrate S.

In Fig. 1C, in case of a substantially transparent anode layer A, a microcavity is formed between the substantially reflective cathode layer C and the partly light transparent, partly light reflective layer R. By appropriate selection of the thickness of the intermediate layer I, the microcavity effect can be tuned without disturbing the electrical characteristics of the device 1 near the light emissive layer E.

In Fig. 1C, in case of a partly transparent, partly reflective anode layer A a first microcavity is formed between the substantially reflective cathode layer C and the partly reflective anode layer A and a second microcavity is formed between the partly reflective anode layer A and the partly light reflective, partly light transparent layer R. The second microcavity can be tuned by selecting a thickness of the intermediate layer I without effecting the electrical characteristics of the device 1 near the light emissive layer E.

In Fig. ID an inverted bottom-emission light-emitting device 1 is shown. The layer stack comprises a substrate S, a partly light transparent, partly light reflective cathode layer C, a light emissive layer E, an anode layer A that may be either substantially light transparent or partly light transparent, partly light reflective. The anode layer A is separated from a substantially light reflective layer R by one or more light transparent layers I.

In case the anode layer A is substantially light transparent, a microcavity is formed between the reflective layer R and the cathode layer C. In case the anode layer A is partly light transparent, partly light reflective, a first microcavity is formed between the cathode layer C and the anode layer A and a second microcavity is formed between the anode layer A and the reflective layer R. Microcavity effects in the device 1 can be controlled without or without substantially disturbing the electrical characteristics of the device 1 near the light emissive layer E.

In Fig. IE, a top emission light-emitting device 1 is shown. The layer stack comprises a substrate S, a light reflective anode layer A, a light emissive layer E, a cathode layer C that may be either substantially light transparent or partly light transparent, partly light reflective. The cathode layer C is separated from a partly light transparent, partly light reflective layer R by one or more light transparent layers I.

In Fig. IF, an inverted top emission light-emitting device 1 is shown. The layer stack comprises a light transparent substrate S over which a light transparent anode layer A, a light emissive layer E and a cathode layer C that may be either substantially light transparent or partly light transparent, partly light reflective, are applied. The cathode layer C is separated from a light reflective layer R by one or more light transparent layers I.

In Fig. IG, a bottom emission light-emitting device 1 is shown. The layer stack comprises a transparent substrate S, a partly light transparent, partly light reflective anode layer A, a light emissive layer E, a cathode layer C that may be either substantially light transparent or partly light transparent, partly light reflective. The cathode layer C is separated from a substantially light reflective layer R by one or more light transparent layers I.

In Fig. IH an inverted bottom-emission light-emitting device 1 is shown. The layer stack comprises a light transparent substrate S over which a light reflective anode layer A, a light emissive layer E and a cathode layer C that may be either substantially light transparent or partly light transparent, partly light reflective, are applied. The cathode layer C is separated from a light reflective layer R by one or more light transparent layers I.

In case the cathode layer C is substantially light transparent, a microcavity is formed between the reflective layer R and the anode layer A. In case the cathode layer C is partly light transparent, partly light reflective, a first microcavity is formed between the cathode layer C and the anode layer A and a second microcavity is formed between the cathode layer C and the reflective layer R. Microcavity effects in the device 1 can be controlled without or without substantially disturbing the electrical characteristics of the device 1 near the light emissive layer E.

It is noted that further embodiments of light-emitting device are envisaged by the applicant of the present application. In case of top-emission light-emitting devices, the substrate S may be a reflective substrate, such that the presence of a reflective layer R on or over the substrate S is unnecessary. A steel substrate may e.g. be applied. In this case, the intermediate layer or layer I may act also as a barrier against e.g. oxygen and water. Further, light emission of the light emissive layer E may leave the device 1 in both directions, i.e. a dual side emitting device, such that all layers in the stack should be at least semi-transparent.

It should further be appreciated that conventional additional layers, including encapsulation layers, light outcoupling layers on top of the cathode layer C or anode layer A, organic charge carrier injection layers adjacent to the light emissive layer E and barrier layers between the anode layer A and cathode layer C have been omitted for clarity purposes.

It should further be appreciated that the light transparent intermediate layer may comprise a plurality of layers with specific optical and/or physical-chemical properties, an example of which will be described below with reference to Fig. 4. For instance, the stack could have planarizing properties or provide a chemical barrier in between the metallic reflector and subsequent layers. The optical properties of the multilayer stack can be such that the emitted light intensity and/or color purity of emitted light is optimised and/or the angular distribution of the light-output is optimised. In addition, the optical properties can be chosen such as to minimize reflection of ambient light from the cavity to improve the daylight contrast of a display. Moreover, outcoupling structures could be included in the stack to enhance outcoupling of light that would otherwise be subject to waveguiding.

It should also be appreciated that the (semi-) reflective layer R in the above- mentioned embodiments may be a conductive reflective layer, for it is electrically insulated by the intermediate layer I from the anode layer A or cathode layer C. In the above- mentioned embodiments, the conductive reflector may be used for electrically shunting either one of the electrode layers (e.g. cathode, anode) or other current carrying lines such as power lines of an active matrix display.

Fig. 3 shows an example of resonance within a microcavity, with the grey line G designating the emission spectrum of a light emissive layer and the black lines B designating the resonance spectra at θ =0 and θ =50 degrees (dashed spectrum B). The intensity enhancement (along the vertical axis in arbitrary units) for light emitted from a light emissive layer E in a microcavity is plotted as a function of wavelength λ (along the horizontal axis in nm). The wavelength λ_max for which this enhancement reaches its maximum depends on the thicknesses of the layer(s) between the reflecting surfaces and the viewing angle θ . In the simple case with only the light emissive layer E between the reflectors of the microcavity, the resonance condition is:

φ_c +φ_a + 2π ^^cosθ = N2π (1)

^max

where φ_c and φ_a are the phase shifts (in radians) resulting from reflection at the cathode layer and the anode layer, and N is an integer. In this formula, the optical layer thickness d_ovt = <tf_en_e is used, where d_e is the physical thickness of the light emissive layer and n_e the (real part of the) refractive index of the light emissive layer. It is advantageous to have a maximum overlap between the resonance spectrum B and the emission spectrum G of the light emissive layer for a viewing angle θ = 0 as the light output normal to the device is then maximum. Let λ₀ be the resonance peak wavelength λ_max for which this occurs. Setting λ_max = λ₀ in equation (1) shows that the optimal overlap can be realised with different optical thicknesses of the cavity, corresponding to different N values. The smaller d_opt, the more slowly the deviation from the resonance condition will grow as a function of wavelength, and the wider the resonance will be. Because typically -π < φ_c, φ_a < 0, the widest resonance corresponds to N= 0, and an optical thickness of the order of λ_o/2. If other materials are present between the reflecting surfaces, in addition to the light emissive layer, equation (1) is no longer applicable, but the resonances with λ_max = λ₀ occur at similar values of the total optical thickness of the cavity. Also in this case, the resonances are numbered with N, starting with N = 0 for the widest resonance corresponding to the smallest d_ovt.

For a viewing angle θ > 0, the resonance maximum is shifted from λ₀ to lower wavelength λ_max(θ). If the cavity contains only the light emissive layer and the dependence of φ_c and φ_a on θ are neglected, it follows from equation (1) that

λ_nm (θ) = λ₀ cosθ^/ (2)

with θ' the propagation angle of light inside the cavity, given by Snell's law

sinθ = τz_e sinθ ' (3)

The resonance shift is illustrated by Δλ in Fig. 3 for θ = 50° and n_e = 1.7. A different resonance maximum means that a different part of the emission spectrum is enhanced by the cavity. Thus the observed colour will change with viewing angle θ. For a cavity containing two or more layers with different refractive indices H₁, the effects are similar, although the function λ_max(θ) is more complex. It then resembles a weighted average of terms of the form of equation (2) with a different θ' for each layer (determined by n_t), but is also influenced by reflections at the interfaces between the layers.

The colour shift Δc as a function of viewing angle θ increases with increasing JV-number of the resonance. The shift of a wide enhancement profile acting on the emission spectrum B has less effect on the observed colour than the shift of a narrow profile.

Therefore, it is advantageous to construct a cavity with an optical thickness corresponding to the N= 0 resonance, as described in EP-A-I 154 676. It can, however, be difficult to make the cavity as thin as is needed for the N= 0 resonance wherein the optical thickness d_opt equals λ_o/2 because, for a properly working device, it needs to contain several layers that each have a minimum thickness. These minima relate e.g. to the functionality of the layers or to limitations for reliable and accurate processing of these layers for the light-emitting device. This is in particular true for a device (or a part of it) that emits blue light for which λ₀ will be relatively small.

The substantially light transparent intermediate layer I between the anode layer A and the reflective layer R according to an embodiment of the invention has been found to be advantageous to cope with this problem. In particular, an intermediate layer Il of high refractive index material between the reflecting surfaces, e.g. the partly light reflective, partly light transparent cathode layer C and the reflective layer R of the light emitting devices of Fig. 1, is advantageous. A high refractive index light transparent intermediate layer Il here means that the material is transparent in the visible spectrum and has a refractive index rih with the real part higher than 1.9, preferably higher than 2.0, for at least a part of the visible spectrum. Appropriate examples of such materials are ZnSe, ZnS, silicon nitride and TiO₂. The other layers in the cavity that are needed for operation of the device typically have a refractive index less than 2. It is favourable to give these other layers a minimum thickness and to select the thickness of the high refractive index intermediate layer Il so that the total optical thickness of the cavity corresponds to a λ_max = λ₀ resonance. The advantage of the inclusion of the intermediate layer Il is a reduction of the colour shift Δc as a function of viewing angle θ. As discussed above, this is particularly important when it is not possible to create an N= 0 resonance due to limitation in the minimum thickness of the layers. The reduction of the colour shift Δc is directly related to a reduction of the resonance shift Δλ that is the result of the relatively small propagation angle θ' in the intermediate layer II, as defined by equation (3) (with n_e replaced by rih).

Inclusion of the high refractive index light transparent intermediate layer Il may reduce the effective reflection of the reflective layer R. This may be prevented by choosing for one or more of the other layers not their minimum thickness.

Otherwise, in addition to the high refractive index layer II, a transparent tuning layer 12 (shown in Fig. 4) with a refractive index different from n^ may be included between the transparent anode layer A and the reflective layer R. The transparent tuning layer 12 may be located on both sides of the high refractive index layer II. Variation of the thickness of the two layers with different refractive index allows tuning of the effective reflection. The reflection can even be made higher than it is without the additional layers of this invention.

Fig. 4 shows a portion of an embodiment of the invention of a top-emission polymer colour light-emitting device 1 with three subpixels R, G and B. The high refractive index light transparent intermediate layer Il (e.g. ZnSe, ZnS, silicon nitride, TiO₂) is located between a substantially reflective layer R (e.g. Mo, Cr, Al, Ag) and an ITO light transparent anode layer A. Optionally, there is a thin tuning layer 12 with lower refractive index (e.g. SiO₂) between the intermediate layer Il and the reflective layer R. On top of the anode layer a hole injection layer H and the light emissive layers E(R), E(G) and E(B), separated by dams D, are disposed covered by a partly transparent, partly reflective cathode layer C of e.g. Ba- Ag. Possible extra layers on top of the cathode and the encapsulation are not shown. Underneath the reflective layer R control electronics for driving the subpixels R, G and B are provided in a layer Q.

The materials of the light emissive layer E for different subpixels R, G and B have different emission spectra (typically they emit red, green and blue light). Thus λ₀ is also different for each of these materials, and the cavity between the reflective layer R and the cathode layer C must have a different optical thickness for each subpixel. This is realised by varying the thickness of the hole-injection layer H which can be applied by printing thereby enabling thickness variations per subpixel relatively easy. Although the light-emissive layers E(R), E(G) and E(B) are also applied by printing, the thickness range for which these operate properly is limited.

Variation between subpixels of the thickness of the high refractive index intermediate layer Il would give a minimal colour shift Δc for all subpixels R, G and B, but may be disadvantageous from a processing point of view. In that case, the thickness of the hole-injection layer H may be given its minimum thickness (to allow for a maximum thickness of the high-zz layer) only in the subpixel with the smallest resonant thickness, i.e. the blue subpixel B. As the human eye is most sensitive to variations of the observed spectrum in the short- wavelength range (blue side) of the visible spectrum, this approach is suitable. Simulations have been performed for the light-emitting device 1 of Fig. 4 for the intensity / and the colour shift Δc at θ = 0 and θ = 50° (with respect to θ = 0). The colour shift is defined as the length of the vector that connects the (u , v ) chromaticity coordinates.

The device 1 of Fig. 4 according to an embodiment of the invention has a ZnSe-layer as the high refractive index light transparent intermediate layer Il and a SiO2 layer as tuning layer 12. A green light emissive layer E has been used. The results are listed in Table 1.

A reference device without the high refractive index at least transparent intermediate layer Il and without a SiO2 layer as tuning layer 12 is tabulated in the first row of the table. In this reference device, an ITO anode is positioned immediately on top of a Cr reflective layer and has a larger thickness. The total optical thickness always corresponds to the N= 1 resonance.

Inclusion of the intermediate layer Il without the SiO₂ clearly reduces the colour shift Δc. It also slightly reduces the intensity/, however, due to a decrease of the effective reflection at the side of the anode layer A. Introduction of a thin SiO₂ layer as a tuning layer 12 restores the intensity, while there is still a significant reduction of the colour shift Δc. The last row in the table shows that the invention can also be used to increase the reflection at the anode side and thus increase the intensity /of the light output. The colour shift Δc is then still smaller than for the reference device.

Table 1 Simulation results of the device of Fig. 4

Embodiment 12 (nm) Il (nm) A (nm) / (a.u.) Ac(u , v ) reference 0 0 125 1 0.038 invention 0 59 30 0.93 0.023 invention 11 50 30 1 0.025 invention 55 30 30 1.35 0.031

It is envisaged that in practice, a microcavity may be designed so that the resonance for θ = 0 has a maximum λm_aX that is slightly smaller than λ₀. This may be done in order to even further reduce the colour shift Δc or to get a better chromaticity coordinate (i.e. with improved purity) of the emitted light.

In an embodiment of the invention, the substantially light transparent intermediate layer I may be made of a birefringent material, shown in top view in Fig. 5. The intermediate layer I has an index of refraction n₀ and a different index of refraction n_e in the plane of the layer I. As a result, a ray of light Ll with polarization direction Pl will see a different index of refraction than ray L2 with the orthogonal polarization direction P2. An exciton of the light emissive layer E will see a different index of refraction. By tuning the layer thickness of the intermediate layer I, it is possible to enhance the emission of light from the device in the one polarization direction and prohibit the emission of light in the orthogonal polarization direction. In this way it is possible to have an light-emitting device that emits (partially) polarized light.

The emission of polarized light can offer advantageous applications, such as a light emitting device, e.g. backlight emitting polarized light, for a LCD display device, shown in Fig. 6B.

In a direct view display device, shown in Fig. 6A, a light-emitting device according to the invention can be combined with a polarizing front plate (not shown). The contrast of the display panel may be significantly increased compared to a display panel without a polarizing plate. Compared to a display with circular polarizer (usually used for contrast enhancement), the daylight contrast decreases, but the brightness doubles.

Figs. 6A-6C schematically show applications of light-emitting devices according to the invention.

Fig. 6A shows a display device 10 comprising a display panel comprising e.g. the light-emitting device 1 of Fig. IA and a driver 11 capable of driving the light emitting device 1 by supplying driving signals to at least one of the anode layer A and the cathode layer C for displaying images on the display panel.

Fig. 6B shows a display device 10 comprising e.g. a light-emitting device 1 of Fig. IA for illuminating a display panel 12 and a driver 11 capable of driving said light- emitting device 1 by supplying driving signals to at least one of said first electrode and second electrode for displaying images on said display panel.

For display devices 10, it has been found that the light transparent intermediate layer I should not exceed a thickness of approximately 300 μm in order to avoid significant image distortion due to parallax effects. Finally, Fig. 6C depicts a light source 20 for illumination purposes employing e.g. a light-emitting device 1 of Fig. IA.

In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word "comprising" does not exclude the presence of elements or steps other than those listed in a claim. The word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. 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.

Claims

CLAIMS:

1. A light-emitting device (1) comprising a light emissive layer (E) arranged between an at least semi-transparent first electrode layer (A;C) and an at least semi-reflective second electrode layer (C;A), wherein said at least semi-transparent first electrode layer is arranged between said light emissive layer and a further at least semi-reflective layer (R) and wherein said further at least semi-reflective layer is separated from said at least semi- transparent first electrode layer by one or more light transparent intermediate layers (I).

2. The light-emitting device (1) according to claim 1, wherein said first electrode layer is an at least semi-transparent anode layer (A) and said one or more intermediate layers (I) are arranged between said anode layer and said further at least semi-reflective layer (R).

3. The light-emitting device (1) according to claim 1, wherein said light-emissive layer (E) is arranged over a substrate (S), said substrate being substantially non-transparent for light emitted from said light-emissive layer.

4. The light-emitting device (1) according to claim 1, wherein said light-emissive layer is arranged over a substrate (S), said substrate comprising a reflective surface capable of functioning as said further at least semi-reflective layer (R).

5. The light-emitting device (1) according to claim 1, wherein said first electrode layer (A;C) is a semi-reflective light transparent layer.

6. The light-emitting device (1) according to claim 1, wherein said light transparent intermediate layers (I) comprise at least a first intermediate layer (II) and a second intermediate layer (12).

7. The light-emitting device (1) according to claim 1, wherein at least one of said light transparent intermediate layers is a high-refractive index layer (II) with a real part of the refractive index higher than 1.9, preferably higher than 2.0.

8. The light-emitting device (1) according to claim 7, wherein said light transparent intermediate layers include a further intermediate layer (12) with a refractive index different from the refractive index of said high-refractive index layer.

9. The light-emitting device (1) according to claim 1, wherein at least one of said light transparent intermediate layers (I) includes a birefringent intermediate layer.

10. The light-emitting device (1) according to claim 1, wherein at least one of said light transparent intermediate layers (I) includes an optical outcoupling structure.

11. The light-emitting device (1) according to claim 1, wherein said further at least semi-reflective layer (R) comprises an electrically conductive layer contacting portions of said first electrode layer (A;C) or said second electrode layer (C;A) or other current carrying lines of the light-emitting device.

12. The light-emitting device (1) according to claim 1, wherein said light-emitting device comprises a plurality of pixels (R₅G₅B) with light-emissive layers (E(R)₅ E(G)₅ E(B)) having different light emission spectra, each of said pixels comprising a charge carrier injection layer (H) adjacent to said light emissive layer and wherein the thickness of said injection layer varies between pixels of different light emission spectra.

13. A display device (10) comprising a display panel with a light-emitting device (1) according to claim 1 and a driver (11) capable of driving said light-emitting device by supplying driving signals to at least one of said first electrode layer (A;C) and second electrode layer (C;A) for displaying images on said display panel.

14. A display device (10) comprising a display panel (12), a light-emitting device (1) according to claim 1 for illuminating said display panel and a driver (11) capable of driving said light-emitting device by supplying driving signals to at least one of said first electrode layer (A;C) and second electrode layer (C;A) for displaying images on said display panel.

15. An illumination device (20) comprising a light-emitting device (1) according to claim 1 and a driver (11) capable of driving said light-emitting device by supplying driving signals to at least one of said first electrode layer (A;C) and second electrode layer (C; A) for emitting light from said light emissive layer (E).