EP1836737A2

EP1836737A2 - Organic double-sided light-emitting diode with a light extraction dielectric layer

Info

Publication number: EP1836737A2
Application number: EP05823775A
Authority: EP
Inventors: Christophe Fery; Eric Marcellin-Dibon
Original assignee: Thomson Licensing SAS
Current assignee: Thomson Licensing SAS
Priority date: 2004-12-21
Filing date: 2005-12-21
Publication date: 2007-09-26
Also published as: CN100533810C; WO2006067200A2; KR101321956B1; CN101080827A; JP2008524819A; KR20070087616A; JP5762667B2; US20080084159A1; WO2006067200A3

Abstract

A diode comprising an organic light-emitting layer (4) which is arranged between a transparent lower electrode (5) and a transparent upper electrode (3); a dielectric layer (6,2) which is placed in contact with each electrode (5,3) opposite the organic light-emitting layer (4) and adapted in such a way as to obtain maximum reflectivity of emitted light when combined with the electrode (6, 2), Said structure optimizes light extraction and thus light output. Lighting panels or display panels comprising a network of said diodes.

Description

ORGANIC ELECTROLUMINESCENT DIODE WITH DOUBLE-SIDED TRANSMISSION, DIELECTRIC LAYER OF EXTRACTION

FROM LIGHT.

The invention relates to an organic electroluminescent diode capable of emitting light by two opposite faces, comprising:

a transparent or semi-transparent substrate,

an organic electroluminescent layer capable of emitting light deposited on this substrate, interposed between a lower electrode and an upper electrode, each electrode being transparent or semi-transparent.

[002] So it is a so-called diode upward and downward transmission. Such diodes may be of conventional structure, in which case the upper electrode is a cathode, or so-called reverse structure, in which case the upper electrode is an anode.

[003] The invention also relates to the networks of these diodes, including those included in the lighting or display panels, including video images.

[004] US6762436 discloses a diode and a panel of this type.

[005] The documents EP1076368, EP1439589 and EP1443572 describe diodes emitting by a single face, where is added a dielectric layer to the lower transparent electrode or the upper transparent electrode (see Figure 4d in EP1439589); this dielectric layer (referenced 22, made of ZnS material: 20% SiO) has a function of reducing the absorption of the light emitted through the transparent or semi-transparent electrode to which it is attached.

According to these documents, this dielectric absorption reduction layer is adapted as indicated below to the metal layer of the electrode to which it is attached, index and thickness to improve the extraction of light. issued.

[007] In document EP1076368, it is indicated (see in particular § 17) that the addition of a dielectric layer on a relatively thick metal layer (20 nm) allows to double the transmittance of the electrode (which goes from 30% at 60%).

[008] Examples and tables of data also illustrate this point in EP1439589:

- example 2 and table 2 for a downward emission diode: transparent silver anode: with an almost identical thickness of silver (17.5 nm and 18.5 nm), the addition of a dielectric layer (ITO ) of absorption reduction makes it possible to increase the luminance of the diode by 6.5%.

- example 4 and table 4 for an upward emission diode: upper cathode transparent silver: although the thickness of the silver is increased by 50% (20.3 nm against 13.7 nm in the absence of a dielectric layer), the luminance is increased by 14% thanks to a dielectric layer in ZnS - 20% SiO of 61.4 nm.

An object of the invention is to improve the light output of organic diodes emitting by two opposite faces. According to the invention, a dielectric layer is added to each of the electrodes and each stack of an electrode is fitted with its dielectric layer so as not to obtain a minimum of absorption, but a maximum of reflectivity, while maintaining sufficiently transparent electrodes to limit absorption losses. Thanks to this high reflectivity, one can benefit from an optical cavity effect without absorption losses between the two electrodes, while they are nevertheless transparent or semi-transparent. Note that the US6124024 document, if it disclaims a number of conditions concerning the thickness of the layers, does not teach the maximum reflectivity combined with the intrinsic transparency of the electrodes. Note that US5652067 teaches well to insert a lower dielectric layer between the substrate and the lower electrode, but that this layer is transparent to ultraviolet radiation and not to the light emitted by the diode, and that its thickness n ' is not adapted to obtain, in combination with the lower electrode, a maximum of reflectivity.

[010] More precisely, the subject of the invention is an organic electroluminescent diode capable of emitting light by two opposite faces, comprising:

a transparent or semi-transparent substrate,

an electroluminescent organic layer capable of emitting light deposited on this substrate, interposed between a lower electrode and an upper electrode, each electrode being transparent or semi-transparent,

a lower dielectric layer interposed between the substrate and said lower electrode and an upper dielectric layer covering said upper electrode.

The upper dielectric layer thus covers the upper electrode opposite the electroluminescent layer; it can therefore be used as an interface with air or other ambient medium, in which case it also preferably serves as an encapsulation layer and protection against the risks of degradation of the organic layer by oxygen or steam. water from the air. The dielectric layers, both bottom and top, are not diffusing layers as in EP 1406474, but transparent layers preferably having an intrinsic transmittance greater than or equal to 85% for the emitted light.

When an electric current flows between the lower electrode and the upper electrode through the electroluminescent layer, this layer emits light.

[013] Preferably, the material of the lower dielectric layer and that of the layer higher dielectric have an index greater than 1.6.

[014] Preferably, the lower electrode comprises a lower conductive layer which is in contact with the lower dielectric layer and the upper electrode comprises an upper conductive layer which is in contact with the upper dielectric layer.

[015] Preferably, the material and the thickness d of said upper dielectric layer, and the material and the thickness d of said upper conductive layer are adapted so that the reflectivity of said emitted light evaluated on this stack of layers is approximately maximum.

[016] Preferably, the material and the thickness d of said lower dielectric layer

6 and the material and the thickness d of said lower conductive layer are adapted so that the reflectivity of said emitted light evaluated on this stack of layers is approximately maximum.

[017] The reflectivity of the stack of layers involves an interferential effect between these layers, however intrinsically transparent or semi-transparent, this interference effect being adapted to obtain a high reflectivity. Thanks to the transparency, there is therefore little loss by absorption, thanks to this high reflectivity obtained by interference, optimizes the optical cavity between the electrodes and improves the extraction of light.

[018] The material and the thickness (d and / or d) of the conductive layer (lower and / or higher) being for example fixed, in particular from criteria of low resistivity, the curve which gives the evolution of the reflectivity on this stack of light emitted as a function of the thickness (d 6 and / or d 2) of the corresponding dielectric layer

(lower or upper) has minima and maxima, which reflect the phenomena of interference at the interfaces. According to the invention, a dielectric layer thickness (d and / or d) corresponding to a maximum of this curve is chosen.

6 2

[019] By optimizing the two stacks, we obtain an optical cavity between the two electrodes, which is optimal for the extraction of light. [020] Preferably:

the intrinsic transmittance of said light emitted from said lower conductive layer is greater than or equal to 85%, which corresponds, for an ITO layer, to a limiting thickness of 150 nm;

the intrinsic transmittance of said light emitted from said upper conductive layer is greater than or equal to 85%, which corresponds, for an ITO layer, to a limiting thickness of 150 nm.

[021] Intrinsic transmittance means the transmittance evaluated independently of interference effects, the layer itself or neighboring layers. [022] In summary, the organic electroluminescent diode according to the invention comprises: an electroluminescent organic layer capable of emitting light, interposed between a transparent or semi-transparent lower electrode and a transparent or semi-transparent upper electrode,

a dielectric layer placed in contact with each electrode opposite the organic electroluminescent layer, which is adapted to obtain, in combination with said electrode, the maximum reflectivity of said emitted light.

[023] Preferably, the material of the upper conductive layer is identical to the material of the lower conductive layer.

[024] According to another variant, preferably, said organic electroluminescent layer comprises an organic emissive sub-layer and at least one non-emissive upper organic sub-layer which is interposed between the upper electrode and said emissive sub-layer, and the thickness of the non-emissive upper organic sub-layer (s) is / are adapted so that the distance z sup approximately separating the medium, in the thickness, from said organic sublayer emissive from said electrode superior approximately satisfies the relation:

- where r is any integer,

where i is said wavelength λ close to a maximum of emittance of the emitted light, and n 4 is the average index of the organic electroluminescent layer at this wavelength,

where I sup is the phase shift of a ray of light emitted, after reflection by the upper electrode.

[025] This equation reflects constructive interferences between the light emitted and the light reflected on the upper electrode.

According to this variant, the organic electroluminescent layer preferably comprises an organic emissive sub-layer and at least one non-emissive lower organic sub-layer which is interposed between the lower electrode and said emissive sub-layer, and the thickness of the non-emissive lower organic sub-layer (s) is / are adapted so that the distance z inf approximately separating the medium, in the thickness, from said organic sub-layer emissive from said lower electrode approximately satisfies the relationship:

In ₄ \ 2π)

where q is any integer,

where λ is said wavelength close to a maximum of emittance of light emitted, and n 4 is the average index of the organic electroluminescent layer at this wavelength,

where h int is the phase shift of a ray of emitted light, after reflection by the lower electrode. [027] This equation reflects constructive interferences between the light emitted and the light reflected on the lower electrode. [028] Generally, the non-emissive lower organic sublayer (s) are adapted for the injection and / or transport of carriers of a first kind and the non-emissive organic sublayer (s) are adapted for injection and / or the transport of carriers of a second kind; carrier types correspond to electron and hole. [029] Preferably, the material of said upper dielectric layer is identical to the material of said lower dielectric layer. [030] Preferably, the thickness d 4 of said organic electroluminescent layer is adapted to obtain constructive interference of the light emitted between the lower electrode and the upper electrode. These constructive interferences advantageously favor the extraction of the light emitted through the two electrodes, which improves the luminous efficiency of the diode. [031] The invention also relates to an image display panel or lighting comprising a plurality of diodes according to the invention, characterized in that these diodes are supported by the same substrate. [032] Preferably, said plurality forms a two-dimensional array of diodes whose diagonal is less than 40 cm. As the size of the panel is reduced, good uniformity of display is obtained over the entire width and height of this panel. [033] Preferably, said upper electrode is common to the plurality of said diodes. [034] The invention will be better understood on reading the description which follows, given by way of non-limiting example, and with reference to the appended figures in which:

FIG. 1 is a diagrammatic section of the assembly of a diode according to one embodiment of the invention,

FIG. 2 describes the evolution of the light reflectivity in the stack of each electrode with its dielectric layer according to the embodiment of FIG. 1, as a function of the thickness (nm) of this dielectric layer;

[035] We will now describe an embodiment of a diode or a network of diodes according to the invention, with some non-limiting variants, and some stages of its manufacture, with reference to Figure 1.

[036] Starting from a substrate 7, for example a transparent glass plate or a transparent or semi-transparent active matrix comprising control circuits for the diodes. US2004-1555846 discloses an example of a transparent active matrix of the prior art. This transparent or semi-transparent substrate is provided with a transparent or semi-transparent electrode or network of lower electrodes intended to serve as cathodes, each electrode being connected, where appropriate, to an output of a circuit of substrate control. The lower electrodes are here formed by a conductive lower layer 5 made of ITO ("Indium Tin Oxide" in English) with a thickness of d = 150 nm. Before the deposition of this layer of ITO, a dielectric layer 6 is deposited in zinc selenide (ZnSe) whose thickness d is

6 determined as follows.

[037] Due to a reduced thickness of ITO, here of 150 nm, for the lower conductive layer 5, the transmittance of the lower electrodes is greater than or equal to 85% for the emitted light; these ITO transmittance data are given in the prior art, for example in the thesis of David Vaufrey published in July 2003, and supported at the Laboratory of electronics, optoelectronics, microsystems, the central school of Lyon, France. There is also ITO-related transmittance data in the paper "The improvement of ITO film with high work function on OLED applications" by Ping-Wei Tzeng and Allii, published as part of IDMC 2005 (pages 711). at 713 of the annals).

[038] Using the curve of FIG. 2 which is referenced "d 6" and which gives the evolution of the reflectivity of the stack of layers 5 and 6 as a function of the thickness d (in

6 nm) of the layer 6 and a value of d = 50 nm corresponding to approximately

6 at a maximum of this curve. The reflectivity of the stack of these layers is evaluated at a wavelength of 550 nm corresponding approximately to a maximum of emittance of the organic electroluminescent layer which will be deposited to form the diode.

[039] On layer 5 of ITO is deposited in a known manner in itself an organic electroluminescent layer 4 formed of the following stack:

an underlayer 12 of cesium-doped 4,7-diphenyl-l, 10-phenanthroline (BPhen) for the injection and transport of the electrons;

an underlayer 13 of 4,7-diphenyl-l, 10-phenanthroline (BPhen) approximately 10 nm thick, for blocking the holes;

an emissive sub-layer 11 approximately 20 nm thick, adapted to emit green light when a current flows through it; the emittance of this sublayer has a maximum for a wavelength? = 550 nm;

an underlayer 14 of spiro-bifluorene of 2,2 ', 1,1'-tetrakis (N, N-di-phenyl-amine) -9,9' (Spiro-TAD) of approximately 10 nm of thickness, for blocking electrons; and a sub-layer 15 of spiro-bifluorene of

2,2 ', 7,7'-Tetrakis (N, N-di-m-methyl-phenylamine) -9,9' (Spiro m-TTB), F4-TCNQ-doped for injection and transport Holes.

[040] On the electroluminescent organic layer thus obtained, is then deposited a conductive top layer of ITO 3, also of thickness d = 150 nm, for forming the upper electrode of the diode. Due to a reduced thickness of ITO, here of 150 nm, for the upper conductive layer 3, the transmittance of the upper electrodes is greater than or equal to 85% for the emitted light.

[041] Then a dielectric upper layer 2 is deposited in zinc selenide (ZnSe) whose thickness d is determined as follows. Zinc selenide has an index of 2.6, well above 1.6.

[042] Using the curve of FIG. 2 which is referenced "d" and which gives the evolution of the reflectivity of the stack of layers 3 and 2 as a function of the thickness d (in nm) of layer 2 and a value of d = 50 nm corresponding to a maximum of this curve is chosen. The reflectivity of the stack of these layers is evaluated at a wavelength of 550 nm approximately corresponding to a maximum of emittance of the organic electroluminescent layer which has just been deposited to form the diode.

[043] The stack formed on the substrate 7 by the dielectric lower layer 6, the conductive lower layer 5, the organic electroluminescent layer 4, the conductive upper layer 3 and the dielectric upper layer 2, then forms a diode or a network of electrodes. organic electroluminescent diodes according to one embodiment of the invention. In the case of a diode array, the conductive upper layer 3 and the dielectric upper layer 2 preferably cover all the diodes; the upper electrode is therefore common to all the diodes and the manufacture is facilitated.

[044] As the stacks of the layers 5 and 6 on the one hand, 3 and 2 on the other hand, are adjusted for a maximum of reflection for the light emitted by the diode, the space between the lower electrode and the the upper electrode of the diode (s) (s) then forms an optical cavity and provides a technical effect likely to improve the extraction of the emitted light, provided that certain geometric criteria are respected; these criteria will now be specified.

[045] To obtain and optimize this cavity effect, we will now establish the relations defining the distance z approximately separating the medium, in the thickness, of the emissive organic sub-layer 11 of the conductive lower layer 5, the distance z sup approximately separating the medium, in the thickness, of the emissive organic sub-layer of the conductive upper layer 3. It will be deduced the total thickness d of the organic electroluminescent layer 4. [046] We consider:

At the wavelength close to the emittance maximum of the light emitted previously defined, and n, the average index of the organic electroluminescent layer at this wavelength,

- ιt inf, the phase shift of a ray of light emitted at this wavelength, after reflection by the lower electrode, and 4 sup, the phase shift of a ray of light emitted at this wavelength, after reflection by the upper electrode.

[047] The thickness of the infeed and / or hole transport sub-layer 12 is approximately chosen so that the distance z inf is approximately equal to: ^λ ( _q - * "

, where q is any integer. This equation reflects constructive interferences between the light emitted and the light reflected on the lower electrode.

[048] The thickness of the injection and / or electron transport sub-layer 15 is chosen approximately so that the distance z sup is approximately equal to:

where r is any integer. This equation reflects constructive interferences between the light emitted and the light reflected on the upper electrode. [049] In lieu of the arithmetic method for calculating z inf and z sup, a graphical optimization method can be used without departing from the invention. To determine z inf, we then use a three-dimensional chart giving the light intensity emitted from the bottom of the diode, through the lower electrode, as a function of d 4 and z inf: this abacus makes it possible to determine z inf = 70 nm. To determine z sup, we then use a three-dimensional abacus giving the luminous intensity emitted from the top of the diode, through the upper electrode, as a function of d 4 and z sup: this abacus makes it possible to determine z sup = 70 nm. Note that d 4 = z inf + z sup = 140 nm allows to obtain the maximum emission of the diode, ie the maximum extraction.

[050] All constructive interference described above advantageously promote the extraction of the light emitted through the two electrodes of the diode, which improves the light output of the diode.

[051] Values of z and z, we deduce: inf sup

the thickness of the cesium-doped BPhen underlayer 12 for the injection and transport of the electrons: 70 nm (z inf) -10 nm (thickness of the underlayer 13) - 10 nm

(half thickness of the emissive sub-layer 11) = 50 nm, the thickness of the sub-layer 15 of Spiro m-TTB, for injection and transport of the holes: 70 nm (z sup) - 10 nm (thickness of the underlayer 14) - 10 nm (half thickness of the emissive sublayer 11) = 50 nm.

[052] An electroluminescent diode or an array of light emitting diodes with upward emission is obtained, exhibiting an excellent luminous efficiency by virtue of the combination of characteristics specific to the invention which have just been described.

[053] The present invention also applies to a diode or organic electroluminescent panel where the injection of the charges is by doped organic layers; it is obvious to those skilled in the art that it can be applied to other types of diodes, lighting panels or display without departing from the scope of the claims below.

Claims

claims

[001] Organic electroluminescent diode capable of emitting light by two opposite faces, comprising:

a transparent or semi-transparent substrate (7),

an organic electroluminescent layer (4) capable of emitting light deposited on this substrate, interposed between a lower electrode and an upper electrode, each electrode being transparent or semi-transparent,

a lower dielectric layer (6) interposed between the substrate (7) and said lower electrode, which is in contact with a lower conductive layer (5) of said lower electrode and an upper dielectric layer (2) covering said upper electrode, which is in contact with an upper conductive layer (3) of said upper electrode, characterized in that:

the material and the thickness d of said upper dielectric layer (2) and the material and the thickness d of said upper conductive layer (3) are adapted so that the reflectivity of said emitted light evaluated on this stack of layers (3) , 2) is approximately maximum.

the material and the thickness d of said lower dielectric layer (6) and the

6 material and the thickness d of said lower conductive layer (5) are adapted so that the reflectivity of said emitted light evaluated on this stack of layers (6, 5) is approximately maximum.

Diode according to claim 1 characterized in that the intrinsic transmittance of said light emitted from said lower conductive layer (6) is greater than or equal to 85%.

[003] Diode according to claim 2 characterized in that the intrinsic transmittance of said light emitted from said upper conductive layer (3) is greater than or equal to 85%.

[004] Diode according to any one of the preceding claims, characterized in that the material of the lower dielectric layer (6) and that of the upper dielectric layer (2) have an index greater than 1.6.

Diode according to claim 4 characterized in that the lower dielectric layer (6) and the upper dielectric layer (2) have an intrinsic transmittance of the emitted light greater than or equal to 85%.

Diode according to any one of the preceding claims characterized in that the material of said upper conductive layer (3) is identical to the material of said lower conductive layer (5).

[007] Diode according to any one of the preceding claims characterized in that the material of said upper dielectric layer (2) is identical to the material of said lower dielectric layer (6).

Diode according to any one of the preceding claims, characterized in that said organic electroluminescent layer (6) comprises an organic emissive under-layer (11) and at least one non-emissive upper organic sublayer (14, 15). which is interposed between the upper electrode (3) and said emissive sub-layer (1 1), and in that the thickness of the non-emissive upper organic sub-layer (s) is / are adapted (s) so that the distance z sup approximately separating the medium, in the thickness, of said emissive organic sub-layer (11) from said upper electrode (3) approximately satisfies the relation:

- where r is any integer,

where l is said wavelength l close to a maximum of emittance of the emitted light, and n 4 is the average index of the organic electroluminescent layer at this wavelength,

where (| sup is the phase shift of a ray of light emitted after reflection by the upper electrode. [009] Diode according to claim 8, characterized in that said organic electroluminescent layer (6) comprises an organic underlayer. emissivity (11) and at least one non-emissive lower organic sublayer (12, 13) interposed between the lower electrode (5) and said emissive underlayer (11), and that the thickness of the the non-emissive lower organic sub-layer (s) is / are adapted so that the distance z inf approximately separating the medium, in the thickness, of said emissive organic sub-layer (11) from said lower electrode (5) approximately satisfies the relation:

where q is any integer,

where) is said wavelength close to a maximum of emittance of the emitted light, and n 4 is the average index of the organic electroluminescent layer at this wavelength,

where (| inf is the phase shift of a ray of light emitted, after reflection by the lower electrode.

[010] Diode according to any one of the preceding claims, characterized in that the thickness d 4 of said organic electroluminescent layer (4) is adapted to obtain constructive interferences of the light emitted between the lower electrode and the upper electrode. An image display or illumination panel comprising a plurality of diodes according to any one of the preceding claims characterized in that these diodes are supported by the same substrate.