EP2862211A1

EP2862211A1 - P-i-n-type organic light-emitting diode

Info

Publication number: EP2862211A1
Application number: EP13728418.8A
Authority: EP
Inventors: Mohamed Khalifa; Hakim CHOUKRI; Hélène CLOAREC; Bruno Dussert-Vidalet; Philippe TAILLEPIERRE
Original assignee: Astron Fiamm Safety SARL
Current assignee: Astron Fiamm Safety SARL
Priority date: 2012-06-18
Filing date: 2013-06-14
Publication date: 2015-04-22
Also published as: KR20150020706A; FR2992097A1; US9419238B2; US20150171361A1; WO2013189850A1; FR2992097B1

Abstract

The invention relates to a p-i-n-type organic light-emitting diode (OLED) including a stack that includes the following ordered series: a first electrode (2, 8) of a first electrode type, a first transport layer (3, 7) of a first charge carrier type, a transmission layer (5), a second transport layer (3, 7) of a second charge carrier type, and a second electrode (2, 8) of a second electrode type. At least one of the transport layers (3, 7) includes at least two basic transport layers (3l-3n, 7l-7n). According to the invention, each basic transport layer (3l-3n, 7l-7n) of a single transport layer (3, 7) has, relative to an adjacent basic transport layer, a charge carrier mobility corresponding to said decreasing transport layer, and consequently, a conductivity that decreases as the basic transport layer (3l-3n, 7l-7n) progresses away from the adjacent electrode (2, 8), the basic transport layers of a single transport layer having the same dopant concentration.

Description

PIN-type organic light-emitting diode

The technical field of the invention is that of organic electroluminescent diodes, also called OLEs.

In such an organic light-emitting diode, an injection of charge carriers from the electrodes, electrons from a cathode or positive holes from an anode, is produced under the effect of the application of an electrical voltage between these electrodes. Charge carriers of different types are an electron and a hole, paired in an active layer or emission layer comprising organic emitters, to form excitons. A radiative recombination allows the emission of light.

For organic light-emitting diodes, operating as previously indicated, an improvement in energy efficiency is sought, both to reduce the electrical power consumed and to improve the power output.

In order to improve the energy efficiency of a OLED, it is necessary on the one hand to improve the injection of the charge carriers. This rises to an energy barrier at the interface with the electrodes.

On the other hand, it is necessary to improve the transport of these charge carriers up to the emission layer. This is hampered by the low electrical conductivity of organic materials.

US 7074500 teaches that a use of a correctly doped transport layer, disposed between an electrode and a transmission layer, solves both the injection problem and the transport problem. Such an OLE is called PI N. The letter P designates the transport layer of the holes doped with an electron acceptor. The letter N denotes the electron transport layer doped with an electron donor. The letter I designates the emission layer.

However, such doping results in an increase in the electrical conductivities of the doped transport layers. In such a structure, the resistivity of the OLED decreases sharply when the OLED becomes busy. The resistivity of the transparent electrode used in the OLEDs is always higher than that of the pass-through DELO PIN, which introduces an injection imbalance over a large area of the OLED greater than 1 cm ² .

Indeed, the increase of the surface of the transparent electrode causes a voltage drop in the LO LO as one moves away from the electrical contact of this electrode. This voltage drop on the OLED surface causes poor luminance uniformity over the entire OLED surface. This uniformity is even lower than the surface of the DE LO is large. This poor dispersion of the voltage over a large area of the OLED causes poor thermal dispersion over the entire surface, which reduces the life of the OLED.

It is known, in order to solve the problem of uniformity, to achieve a tandem structure which superimposes at least two DELOs. This reduces the current and increases the light output. However, such a tandem structure leads to excessive costs because of material and manufacturing costs, at least doubled.

WO-A1-2007071450 shows an electronic device with a plurality of stacked OLEDs.

It is still known to solve the problem of uniformity of producing an undoped or type III transport layer. Due to the low conductivity, this requires a very high operating voltage. This results in a low luminous efficiency.

Document DE 102008 051 132 A1 describes a DELO PIN with at least one transport layer having several consecutive elementary transport layers with a decrease of conductivity in the elementary layers, the further away the electrode associated with the transport. This is obtained in reducing the dopant concentration and / or modifying the dopant in the elementary transport layers.

The use of such a method is, however, limited because it achieves a rapid saturation of the electrical conductivity beyond a certain, relatively low threshold of doping. Thus, for optimal control of the conductivity by varying the concentration of the dopant in the elementary layers of the same transport layer, it would be necessary to be able to precisely adjust the dopant concentration in the elementary layers while remaining below a concentration. maximum corresponding to dopant saturation.

However, this maximum concentration is relatively low and frequently between 10 and 25% depending on the dopants. Thus, the concentration variation range of the dopant is relatively narrow and corresponds to low concentrations that are difficult to control in an OLED production system.

The problem of the present invention is to obtain for a DELO type PI N a decrease of the electrical conductivity of at least one transport layer in direction of the em issive layer by avoiding the disadvantages mentioned above.

The subject of the invention is an organic light-emitting diode, DELO, of PIN type, comprising a stack comprising the following ordered succession:

a first electrode, of a first type of electrode, among anode or cathode, able to inject a first type of charge carrier, among electron or hole,

a first transport layer of a first type of charge carrier, doped with a first type of dopant, of P or N, adapted to the type of charge carrier,

an emission layer,

a second transport layer of a second type of charge carrier, different from the first type of charge carrier, doped with a second type of dopant, different from the first type of dopant,

a second electrode, of a second type of electrode, different from the first type of electrode, capable of injecting a second type of charge carrier, at least one of the transport layers comprising at least two elementary transport layers , characterized in that each elementary transport layer of the same transport layer has, relative to an adjacent elementary transport layer, a mobility of the charge carriers corresponding to said decreasing transport layer and consequently a decreasing conductivity with the removal of the elementary transport layer relative to the electrode next, the elementary transport layers of the same transport layer having the same dopant concentration.

The technical effect of the present invention is to obtain the decay of conductivity of one or both transport layers only by acting on the mobility of the charge carriers.

Indeed, the conductivity of the charge carriers is the product of the mobility of charge carriers and the density of the charge carriers. Doping introduces an increase in the density of the charge carriers while their mobility remains unchanged.

According to the state of the art, the decay of conductivity was obtained by modifying the density of the charge carriers, which entailed the disadvantages mentioned above.

WO-A1 -2007071450 describes meanwhile, in a plurality of stacked OLEDs, a selection of materials for which the conductivity is increasing starting from an electrode, which goes against the solution of the invention. .

The present invention adopts the opposite approach which is not to act on the density of the charge carriers by decreasing it but an elementary transport layer is removed from the associated electrode but to act on the mobility of the charge carriers.

A variation in the charge carrier mobility of the decreasing elementary transport layers according to the distance of said layer relative to the associated electrode is much easier to implement than the change in the concentration of the dopant in a reduced range of concentration as is the case for a variation in the number of charge carriers according to the state of the art. This variation in the mobility of the charge carriers can, for example, be obtained by changing the base material of a transport elementary layer relative to the material of an adjacent layer, the material of the layer farthest from the electrode having the lowest charge carrier mobility.

This is done without having to modify the dopant concentration in the elementary transport layers, which is a constraining manufacturing operation. The present invention proposes on the contrary the use of different transport materials having different load carrier mobilities and doped with the same dopant concentration.

The term "transport layer" means a layer capable of carrying out a transport function of a type of load. When the layer comprises several elementary transport layers, they follow each other, being in contact, depending on the thickness OLED. The transport layer is thus a stack of elementary layers.

Advantageously, a P-type dopant used to dope an elementary transport layer is an organic or inorganic dopant and has a LUMO level or a conduction band greater than 4 eV.

Advantageously, said LUMO level or conduction band is greater than 4.8 eV.

Advantageously, an N-type dopant used to dopate an elementary transport layer is an organic or inorganic dopant and has a HOMO level or a valence band of less than 4 eV.

Advantageously, said HOMO or valence band level is less than 3 eV.

The LED light-emitting diode further comprises:

a first blocking layer of a second type of charge carrier, disposed between said first transport layer and said transmission layer,

a second blocking layer of a first type of charge carrier disposed between said transmission layer and said second transport layer.

Advantageously, the mobility of an elementary transport layer closest to the neighboring electrode is at least equal to 10 ^-3 cm ² A s.

Advantageously, the conductivity of an elementary transport layer closest to the neighboring electrode is at least equal to 10 ^-5 S / cm.

Advantageously, the decrease in mobility between an elementary transport layer and an adjacent elementary transport layer further from the neighboring electrode is a factor of at least ten.

Advantageously, the total thickness of a transport layer is at least equal to 100 nm.

Advantageously, the total thickness of a transport layer is at least 200 nm.

Advantageously, the thickness of an elementary transport layer closest to the neighboring electrode is less than or equal to 30 nm.

Advantageously, the thickness of an elementary transport layer furthest from the neighboring electrode is maximum, in order to contribute predominantly to achieve the total thickness of the transport layer.

Advantageously, the first transport layer and the second transport layer comprise the same number of elementary transport layers and the decay of conductivity between an elementary transport layer and an adjacent elementary transport layer, both belonging to the first transport layer. transport layer is substantially equal to the decay of conductivity between symmetrical elementary transport layers belonging to the second transport layer.

Advantageously, the PIN light-emitting diode has a hole transport layer comprising at least two elementary transport layers selected from the following three layers:

an elementary transport layer made of TPD,

an elementary TPB transport layer,

an elementary transport layer in MTDATA.

Advantageously, the electroluminescent diode PIN has an electron transport layer comprising at least two elementary transport layers chosen from the following four layers:

a basic TPBI transport layer,

an elementary transport layer in Bphen,

an elementary transport layer in BAIq,

an elementary transport layer in Alq3.

Other characteristics, details and advantages of the invention will emerge more clearly from the detailed description given below as an indication in relation to drawings in which:

FIG. 1 shows a sectional view of a DELO according to the invention,

FIG. 2 presents a diagram of the relative energy levels of a DELO according to the invention,

FIG. 3a shows a curve of mobility through the elementary transport layers,

- Figure 3b shows a curve of the conductivity through the elementary transport layers.

FIG. 4 shows a comparative current / voltage diagram for embodiments of an OLED according to the invention with different elementary transport layer numbers.

Figure 1 shows an organic light-emitting diode, OLED, PIN type. Such OLED is characterized by a stack, comprising a transmission layer 5 or active layer, disposed centrally between two electrodes 2, 8. This emitting layer 5 is still called EML. The application of an electrical voltage between the two electrodes, a cathode 8 and an anode 2, performs an injection of respective charge carriers in the OLED. The charge carriers are of two types: electrons of negative electric charge coming from the cathode 8 and positive holes of positive electrical charge coming from the anode 2. These respective charge carriers migrate in opposite directions in the OLED, until joining, ideally in said emission layer 5. They pair in pairs, between different charge carriers that is to say an electron with a hole, to form an exciton. Said exciton, when it is formed in the active layer 5, because of the particular chemical composition of said active layer 5, performs a radiative recombination which produces a photon and thus allows a light emission 9. This light emission 9 diffuses through the anode 2 and the substrate 1, both advantageously translucent, in the case of a downward emission OLED.

Said stack is typically disposed on a substrate 1 and the components of said stack are generally protected from environmental stress, including oxidation by air, moisture, etc., by encapsulation, not shown in the figures.

An active layer 5 capable of emitting a red light may, in particular, be produced by means of a layer of Alq3 doped at a concentration of 1% with a red organic emitter, such as 4- (dicyanomethylene) -2- t-butyl-6 (1,1,7,7-tetramethyljulolidyl-9-enyl) -4H-pyran (DCJTB). Such emission layer 5 has a typical thickness of 20 nm. This layer, as well as all the layers described in the present application, can typically be carried out, in known manner, by thermal evaporation under a vacuum of less than 10 ^-5 mtorr.

As previously mentioned, the invention relates more particularly to a PI type NLO. A PI N type denotes a DELO made by framing an insulating layer, the PIN layer, here the light emitting layer EML 5 by a transport layer. 3 P type doped, the P PIN, and a N-type doped transport layer 7, the N PIN.

A PIN type OLE typically comprises stacked in successive and orderly fashion:

an EA 2 anode,

an HTL 3 hole transport layer,

an emitting layer EML 5,

an ETL electron transport layer 7,

an EK cathode 8.

Thus, this stack comprises symmetrically an electrode 2, 8, a first transport layer 3, 7 for charge carriers of a first type, a transmission layer 5, a second transport layer 3, 7 for carriers of charge of a second type and a second electrode 2, 8.

Each electrode 2, 8 is able to inject a different type of charge carrier. Anode 2 injects holes. A cathode 8 injects electrons. An anode 2 is typically made of indium tin oxide, otherwise known as ITO, at a thickness of 120 nm. A cathode 8 is typically made of aluminum with a thickness of 100 nm.

A transport layer 3, 7 is close to an electrode 2, 8 and is doped with a dopant which may be P-type or N-type. A transport layer 3, 7 is doped in order to promote the transport of a type of charge carrier. This type of charge carrier is the type of charge carrier injected by the electrode 2, 8 which is close to it. P-type doping comprises an electron acceptor dopant and thus promotes the transport of holes. N-type doping comprises an electron donor dopant and thus promotes the transport of electrons. The transport layer 3 adjacent to the anode 2, which injects holes, is a hole-conveying layer 3 and is p-type doped. The hole transport layer 3 is also called HTL of the English Hole Transport Layer. . The transport layer 7 adjacent to the cathode 8, which injects electrons, is an electron transport layer 7 and is N-doped. The electron transport layer 7 is also called ETL of the English Electron Transport. Layer.

Referring to the figures of the present application, according to an important feature of the invention, at least one transport layer 3, 7 is made by means of at least two layers. Each of the layers thus constituting a transport layer 3, 7 is hereinafter referred to as the elementary transport layer 31, 32, 33, 3i, 3n, 71, 72, 73, 7i, 7n. Such an elementary transport layer 31 -3n, 71 -7n advantageously has, with respect to an adjacent elementary transport layer, a decreasing mobility as the elementary transport layer 31 -3n, 71 -7n moves away from the Thus, the elementary transport layer 31, 71 closest to the neighboring electrode 2, 8 of the "parent" transport layer 3, 7 has the highest mobility. Then the mobility decreases elementary transport layer 31 -3n, 71 -7n in elementary transport layer 31 -3n, 71 -7n, as one moves away from the electrode 2, 8 and that one is approaching the emission layer 5.

At least one of the two transport layers 3, 7 is thus produced by means of elementary transport layers 31 -3n, 71 -7n. The other transport layer 3, 7 can be made according to the prior art in a single thick layer. However, the embodiment with decreasing mobility with the distance from the electrode 2, 8 is advantageously applied to the two transport layers 3, 7. The dopant used for an elementary transport layer 31 -3n, 71 -7n is not different from an elementary transport layer to another elemental transport layer. However, all the elementary transport layers 31 -3n, 71 -7n of the same type of transport layer 3, 7 advantageously favor the transport of the same type of charge carrier. Also, all the elementary transport layers of the same "parent" transport layer 3, 7 are advantageously doped with a dopant of the same type, from P or N, as the type of dopant that would have been used to produce a layer. single transport. Dopant concentration is constant in all transport layers.

By way of example, an elementary transport layer 31 -3n of hole can be carried out by means of 2,7-Bis [N, N-bis (4-methoxy-phenyl) amino] -9,9-spirobifluorene (MeO- spiro-TPD); Phthalocyanine (CuPc); 4,4 ', 4 "-tris- (3-methylphenylphenylamino) triphenylamine (m-MTDATA); 2,2', 7,7'-tetra (N, N-di-tolyl) amino-spiro-bifluorene (spiro TTB); 4,4'-bis- [N- (naphthyl) -N-phenylamino] biphenyl (σ-NPD); N, N'-bis (Inaphthyl) N, N'-diphenyl-1,1 4'-biphenyl-4'-diamine (N-PB); N, N'-diphenyl-N, N'-bis (3-methylphenyl) -1,4-diphenyl-4'-diamine (TPD) , in the case of a P-doped HTL hole transport layer.

By way of example, an elementary transport layer 71 -7n of electron can be carried out using / V-arylbenzimidazoles trimer (TPBI); 4,7-Diphenyl-1,10-phenanthroline (Bphen); bis (2-methyl-8-quinolinate) -4-phenylphenolate aluminum (BAIq); tris- (8-hydroxyquinoline) aluminum (Alq3).

According to an advantageous embodiment in order to simplify the industrial production, each elementary transport layer 31 -3n, 71 -7n is doped with the same dopant concentration. This greatly simplifies the process of making an elementary transport layer 31 -3n, 71 -7n.

There is thus obtained a transport layer 3, 7 having a variable electrical conductivity as a function of the depth dimension of this transport layer. The electrical conductivity curve then typically has a stepped shape as illustrated in FIG. 3b which shows a curve representing ordinate the electrical conductivity, expressed in Siemens / centimeter or S / cm, as a function of the elementary transport layer. 31 -3n, 71 -7n, represented on the abscissa.

For a given doping chemical component, the conductivity of the elementary transport layer 31 -3n, 71 -7n is an increasing function of mobility.

A P-type dopant promotes the displacement of holes and is used to dope an elemental transport layer 31 -3n belonging to an HTL 3 hole transport layer. This component may be organic, inorganic or metallic. Advantageously according to the invention, any organic dopant of the P type used for doping such an elementary transport layer 31 -3n has a LUMO level greater than 4 eV. This advantageously allows the transfer of electrons from the transport layer to the P dopant.

Ideally said LUMO level is greater than 4.8 eV.

Similarly, an N-type dopant promotes electron displacement and is used to dope an elemental transport layer 71 -7n belonging to an ETL electron transport layer 7. This component may be organic, inorganic or metallic. Advantageously according to the invention, any N-type organic dopant used to dope such an elementary transport layer 71 -7 n has a HOMO level of less than 4 eV. This advantageously allows the transfer of electrons from dopant N to the transport layer.

Ideally said HOMO level is less than 3 eV.

The LUMO and HOMO levels apply only to organic dopants. In the case of an inorganic dopant, the term LUMO should be replaced by "conduction band", and replace the term HOMO by "valence band".

In the case of a P-type metal dopant, the metal output work is advantageously greater than 4 eV. Ideally the output work is greater than 4.7 eV. In the case of an N-type metal dopant, the metal output work is advantageously less than 4 eV. Ideally the output work is less than 3 eV.

The literature provides many indications of HUMO and LUMO values. The values can also be measured by cyclic voltammetry and material absorption measurement.

According to an optional embodiment, it is possible to add at least one blocking layer 4, 6 to a PIN-type OLE. A blocking layer 4, 6 is a layer capable of blocking / slowing down a type of charge carrier. The type of charge carrier is determined by the type of dopant used. A blocking layer 4, 6 for a type of charge carrier is advantageously arranged, adjacent to the emission layer 5, on the side opposite to the electrode 2, 8 which injects said type of charge carrier.

Thus, a hole-locking layer 6 is advantageously disposed adjacent to the emitting layer EML 5, on the opposite side, relative to the emitting layer EML 5, to the anode 2 which injects said holes. A hole blocking layer 6 is also called HBL of the English Hole Blocking Layer. A hole blocking layer 6 is thus advantageously arranged between the ETL electron transport layer 7 and the EML 5 emission layer. Thus, an electron-blocking layer 4 is advantageously disposed adjacent to the emitting layer EML 5, on the opposite side, relative to the emitting layer EML 5, to the cathode 8 which injects said electrons. An electron blocking layer 4 is also called EBL of the English Electron Blocking Layer. An electron blocking layer 4 is thus advantageously arranged between the HTL 3 hole transport layer and the EML 5 emission layer.

Thus arranged, on the side opposite to an electrode 2, 8 which injects a type of charge carrier, a blocking layer 4, 6 able to block this type of charge carrier, prevents said charge carriers from leaving the transmission layer EML 5. The blocking layer or layers 4, 6 thus produce a confinement effect of the charge carriers, and therefore excitons, in the emitting layer EML 5. This has the effect of improving the external light output, by producing more photons for the same number of charge carriers injected.

The blocking layer (s) 4, 6 have appropriate energy levels with the neighboring EML 5 emission layer.

By way of example, an EBL 4 electron-blocking layer can be produced by means of N, N'-diphenyl-N, N'-bis (3-methyl-phenyl) -I, 4,4-biphenyl. diamine (TPD), according to a thickness indicative of 10 nm.

By way of example, a HBL 6 hole-blocking layer may be produced using benzimidazolylbenzene (TPBi), with a thickness indicative of 10 nm.

Within a transport layer 3, 7 are several elementary transport layers 31 -3n, 71 -7n. One of these elementary transport layers, the elementary transport layer 31, 71 closest to the electrode 2, 8 neighbor plays a special role. This first layer of elemental carriage 31, 71 has a mobility of at least 10 ^"3 cm ² A s. This mobility introduced with a high doping conductivity at least equal to ^{10" 5} S / cm and is effective at the contact interface between the elementary transport layer 31, 71 and the electrode 2, 8, to greatly reduce the energy barrier. This reduction advantageously makes it possible to efficiently inject the charge carriers. This advantageously makes it possible to maintain a threshold voltage of the same order as the interval of the emission layer 5. This is particularly visible on the energy diagram of FIG. 2.

The mobility of the following elementary transport layers 32-3n, 72-7n is determined by being lower, while being such that each subsequent elementary transport layer 32-3n, 72-7n makes it possible to ensure a satisfactory transport of the carriers of charge.

Advantageously, in order to produce an optimal result, the decay the mobility and therefore the conductivity between an elementary transport layer 31 -3n, 71 -7n and an adjacent elementary transport layer further away from the adjacent electrode 2,8 is by a factor of at least 10.

Thus, the dopant concentration in all the transport layers is advantageously constant. In the case of an organic dopant the dopant concentration is greater than 1%, ideally 6%. In the case of an inorganic dopant the dopant concentration is greater than 5%, ideally 15%. In the case of a metal dopant the dopant concentration is greater than 10%, ideally 50%.

Thus, the mobility and consequently the conductivity decreases by an order of magnitude of an elementary transport layer 31 -3n, 71 -7n to its more central neighbor. Referring to FIG. 3a, if the elementary transport layer 31, 71 closest to the electrode 2, 8 has an electrical mobility of 10 ^-3 cm ² A s, the following elemental transport layer 32, 72 presents an electric mobility of 10 ^"4 cm ² A s.

With reference to FIG. 3b, if the elementary transport layer 31, 71 closest to the electrode 2, 8 has an electrical conductivity of 10 ^-5 S / cm, the following elemental transport layer 32, 72 has a electrical conductivity of 10 ^"6 S / cm.

A transport layer 3, 7 also has the function of planarizing the electrode 2, 8 which is adjacent thereto. This is applicable at least to the anode 2.

Indeed, when producing an OLED, the base substrate 1 must be perfectly cleaned of any dust. Despite all the efforts to work in clean zone, such a goal is difficult to reach. In addition, the electrode 2, 8 by its very embodiment, inevitably has a steep topography with significant peaks compared to the relative thicknesses envisaged for the different layers. In order to overcome these asperities, dust and peaks, it is appropriate that the transport layer 3, 7 as a whole has a thickness such that it totally immerses the said asperities, thus achieving a planarization. In order to achieve this planarization, the total thickness of the transport layer 3, 7 should be at least equal to 100 nm.

With the increase in size caused by a realization of a matrix of OLEs, the average height of said asperities increases. A thickness of 150 nm for the transport layer 3, 7 makes it possible to produce a matrix of larger size. With the increase of the matrix surface, the average height of said asperities continues to increase, until reaching a limit value where saturation occurs. It thus appears that a total thickness of the transport 3, 7 ideally equal to 200 nm allows a satisfactory planarization, including for matrices of large dimensions.

It is remarkable that the planarization is obtained by means of the total thickness of the transport layer 3, 7, that is to say the sum of the thicknesses of the constituent elementary transport layers 31 -3n or 71 -7n.

Thus, the prior art required a planarizing transport layer of great thickness, 100 to 200 nm, and having a high conductivity, greater than 10 ^-6 S / cm, which requires a high mobility of the charge carriers. electric mobility device according to the invention makes it possible to obtain separately, on the one hand the high conductivity required, in contact with the electrode 2, 8, and on the other hand a total thickness of the transport layer 3, 7 sufficiently large for carrying out planarization by means of other elementary transport layers 32-3n, 72-7n Because of their lower mobility, the progressive decay of the electrical conductivity within the elementary transport layers 31 -3n or 71- Nevertheless, it is possible to guarantee electrical and light performance, such as the level of energy efficiency, very close to those obtained with a single transport layer according to the prior art. and good homogeneity on OLEs with large areas.

In order to perform its injection function by having a high mobility and therefore a high electrical conductivity, the first elementary transport layer 31, 71, adjacent to the electrode 2, 8, requires only a thickness of at least 10nm. Ideally such a first elementary transport layer 31, 71 has a maximum thickness of 30 nm. The thicknesses of the other elementary transport layers 32-3n, 72-7n are arbitrary. They depend on the number of elementary transport layers, the total thickness of the transport layer 3, 7 and the desired electrical mobility variation with the next transport elementary layer.

Advantageously and additionally, the realization of the total thickness of the transport layer 3, 7, in order to perform the planarization function, can be obtained by means of a majority contribution of the last elementary transport layer 3n, 7n or elementary transport layer which is the furthest layer of the electrode 2, 8 and also the one closest to the emission layer 5.

Thus, a hole transport layer 3, shown in an illustrative but nonlimiting manner, may comprise the following 3 elementary transport layers 31 -33: a first elementary transport layer 31 made of TPD doped with 3% F4-TCNQ dopant with a thickness of 30 nm,

a second TPB transport elementary layer 32 doped with 3% F4-TCNQ dopant with a thickness of 20 nm,

a third elementary transport layer 33 in MTDATA doped with 3% F4-TCNQ dopant with a thickness of 1 10 nm.

Thus, it appears that the last elementary transport layer 33 contributes most of the 150 nm thickness of the hole transport layer.

The intrinsic mobility of the TPD, TPB and MTDATA layer is of the order of 10 ^"3 cm ² A s, 10 ^{" 4} A / s, 10 ^"5 cm ² A / s, respectively, because by doping these materials with 3% F4TCNQ, the conductivity of the TPD, TPB and MTDATA layer is of the order of 10 ^"5 cm ² / Vs, 10 ^{" 6} cm ² A / s, 10 ^"7 cm ² A / s, respectively.

The conductivity curve 10 of FIG. 3b, decreasing from an elementary transport layer 31, 71 adjacent to an electrode 2, 8 to an elementary transport layer furthest from the electrode 2, 8 and adjacent to the layer 5 or a blocking layer 4, 6, may have any shape. It appears, however, that a regular decay, either log-linear or, which is equivalent, according to a regular geometric progression such as that of the illustrated curve, favors the transport of the charge carriers and proves to be optimal.

Likewise, the numbers of respective elementary transport layers of the HTL 3 hole transport layer and the ETL electron transport layer 7 are different. With the same number of elementary transport layers, it may still be possible to have a different symmetry of conductivity variation on both sides of the transmission layer.

However, it appears that a symmetry of the electrical conductivity curve is beneficial in that it improves the performance of the OLED. Thus, it is advantageous to produce the first transport layer 3, 7 and the second transport layer 3, 7, each subdivided by the same number of elementary transport layers.

In addition, it is advantageous that a decrease in conductivity between an elementary transport layer and an adjacent elementary transport layer, both belonging to the first transport layer, is substantially equal to the conductivity decrease between the symmetrical elementary transport layers. belonging to the second transport layer.

With reference to FIG. 4, the influence of the number of elementary transport layers 31 -3n or 71 -7n is shown. Figure 4 shows a density diagram of current in mA / cm ² as a function of the applied voltage for three embodiments of a OLED. Curve 11 presents the characteristic curve of a PIN type OLEP whose transport layers 3, 7 comprise a single elementary transport layer. Curve 12 shows the characteristic curve of a PIN-type OLO whose transport layers 3, 7 comprise two elementary transport layers. Curve 13 presents the characteristic curve of a PI N type OLE 2 whose transport layers 3, 7 comprise three elementary transport layers. In each embodiment, the elementary transport layer 31, 71, adjacent to the electrode 2, 8 has a conductivity of 10 ^-5 S / cm, the following possible elementary transport layers 32-3n, 72-7n present respectively and, in the order of approximation of the emission layer 5, conductivities of 10 ^-6 and 10 ^-7 S / cm.

The comparison of the curves 1 1 to 13 shows that all the OLEDs have the same threshold voltage around 2.5 V. This is directly related to the doping of the first elementary transport layers 31, 71, since the threshold voltage is mainly controlled. by the injection barriers located at the interface between this first elementary transport layer 31, 71 and the electrode 2, 8.

The low resistance of a PIN type stack, under a positive bias, added to a limited conductivity of the electrode 2, 8, typically made of indium tin oxide, called ITO, for anode 2 cause a strong voltage drop across the electrode 2, 8. This leads to strong luminous disparities from one OLE to another in a large matrix arrangement. According to the prior art it is possible to obtain a uniformity, defined as the ratio of the minimum luminance to the maximum luminance over the entire surface, not exceeding 70% for a very limited surface of 1 cm ² . For larger areas, uniformity drops rapidly around 40%.

The comparison of the curves 1 1 to 13 shows again and especially that the slope of the currents 1 1 to 1 3 decreases substantially with the number of elementary transport layers. This decrease promotes the uniformity of luminance over an extended area greater than 50 cm ² . The uniformity is only 40% for a device with an elementary transport layer of the curve 1 1 illustrating the prior art. The two-layer device of the curve 12 makes it possible to observe a uniformity of 55%. The three-layer device of the curve 13 makes it possible to observe a uniformity of 80%.

Although it is described herein one or more preferred embodiments of the invention, it should be understood that the invention is not limited to these modes and that variations may be made within the scope of the following claims.

Claims

An organic light-emitting diode, OLED, PIN type, comprising a stack comprising the following ordered succession:

a first electrode (2, 8) of a first type of electrode, among anode

(2) or cathode (8), able to inject a first type of charge carrier, among electron or hole,

a first transport layer (3, 7) of a first type of charge carrier, doped with a first type of dopant, of P or N, adapted to the type of charge carrier,

an emission layer (5),

a second transport layer (3, 7) of a second type of charge carrier, different from the first type of charge carrier, doped with a second type of dopant, different from the first type of dopant,

a second electrode (2, 8) of a second type of electrode, different from the first type of electrode, capable of injecting a second type of charge carrier,

at least one of the transport layers (3, 7) comprising at least two elementary transport layers (31 -3n, 71 -7n),

characterized in that each elementary transport layer (31 -3n, 71 -7n) of the same transport layer (3, 7) has, relative to an adjacent elementary transport layer and in contact with it, a mobility of the carriers of charge corresponding to said decreasing transport layer and therefore a decreasing conductivity with the removal of the elementary transport layer (31 -3n, 71 -7n) from an elementary transport layer (31, 71) adjacent to the electrode ( 2, 8), the elementary transport layers of the same transport layer having the same dopant concentration.

2. An organic electroluminescent diode, OLED, PIN type according to the preceding claim, wherein a P-type dopant used to dope an elementary transport layer (31 -3n) is an organic or inorganic dopant and has a LUMO or band level. conduction greater than 4 eV.

3. organic light-emitting diode, OLED, PIN type according to the preceding claim, wherein said LUMO level or conduction band is greater than 4.8 eV.

The organic light emitting diode, PIN type OLED, according to any one of the preceding claims, wherein an N-type dopant used to dope an elementary transport layer (71 -7n) is an organic or inorganic dopant and has a HOMO level. or valence band less than 4 eV.

5. organic light emitting diode, OLED, PIN type according to the preceding claim, wherein said HOMO level or valence band is less than 3 eV.

The organic light-emitting diode, OLED, PIN type according to any of the preceding claims, further comprising:

a first blocking layer (4, 6) of a second type of charge carrier disposed between said first transport layer (3, 7) and said transmission layer (5),

a second blocking layer (4, 6) of a first type of charge carrier disposed between said transmission layer (5) and said second transport layer (3, 7).

The organic light-emitting diode, OLED, PIN type according to any one of the preceding claims, wherein the mobility of an elementary transport layer (31, 71) closest to the neighboring electrode (2, 8) is at less than 10 ^"3 cm ² / Vs.

An organic light-emitting diode, OLED, PI N type according to any one of the preceding claims, wherein the conductivity of an elementary transport layer (31, 71) closest to the adjacent electrode (2, 8) is at least 10 ^"5 S / cm.

An organic LED, PIN type OLED according to any one of the preceding claims, wherein the decay of mobility between an elementary transport layer (31 -3n, 71 -7n) and an adjacent elementary transport layer (31 -3n, 71 -7n) further away from the neighboring electrode (2, 8) is a factor of at least ten.

An organic light-emitting diode, OLED, PIN type according to any one of the preceding claims, wherein the total thickness of a layer of transport (3, 7) is at least 100 nm.

11. organic light-emitting diode, OLED, PIN type according to the preceding claim, wherein the total thickness of a transport layer (3, 7) is at least 200 nm.

An organic light-emitting diode, OLED, PIN type according to any one of the preceding claims, wherein the thickness of an elementary transport layer (31, 71) closest to the neighboring electrode (2, 8) is less than or equal to 30 nm.

An organic light emitting diode, OLED, PIN type according to any one of the preceding claims, wherein the thickness of an elementary transport layer (3n, 7n) furthest from the neighboring electrode (2, 8) is maximum, in order to contribute mainly to achieve the total thickness of the transport layer (3, 7).

Organic LED, PIN type OLED according to one of the preceding claims, wherein the first transport layer (3, 7) and the second transport layer (3, 7) comprise the same number of transport layers. elementary (31 -3n, 71 -7n) and where the decay of conductivity between an elementary transport layer (31 -3n, 71 -7n) and an adjacent elementary transport layer both belonging to the first transport layer (3, 7) is substantially equal to the decay of conductivity between symmetrical elementary transport layers belonging to the second transport layer (3, 7).

The organic light emitting diode, PIN type OLED according to any one of the preceding claims, which has a hole transport layer comprising at least two elementary transport layers (31 -3n) chosen from the following three layers:

an elementary transport layer (31 -3n) made of TPD,

an elementary transport layer (31 -3n) in TPB,

an elementary transport layer (31 -3n) in MTDATA.

16. Organic light-emitting diode, OLED, PIN type according to the preceding claim, which has an electron transport layer comprising at least minus two elementary layers of transport (71 -7n) chosen from the following four layers:

an elementary transport layer (71 -7n) in TPBI,

an elementary transport layer (71 -7n) in Bphen,

an elementary transport layer (71 -7n) in BAIq,

an elementary transport layer (71 -7n) in Alq3.