Light emitting device and method of fabrication

0001The invention relates to a light-emitting component with organic layers, in particular an organic light-emitting diode, consisting of several layers between a basic contact on a substrate and a top contact, with layers formed as a polymer layer, which consists of polymer, and with layers, which are formed as a molecular layer small molecules applied in a vacuum.

0002The invention also relates to a method for producing a light-emitting component, in which a basic contact, then several layers and finally a cover contact are applied to a substrate.

0003Organic light-emitting diodes have been used since the demonstration of low working voltages by Tang et al. 1987 [<nplcit id="ncit0001" npl-type="s"><text>CW Tang et al., Appl. Phys. Lett. 51 (12), 913 (1987</text></nplcit>)] promising candidates for the realization of large-area displays and other applications, such as lighting elements. They consist of a sequence of thin (typically 1nm to 1µm) layers of organic materials, which are preferably vapor-deposited in the form of small molecules, which creates so-called OLEDs, or spun on from solution, printed or applied in another suitable form (polymers) , whereby so-called PLED are generated. By injecting charge carriers (electrons from one side, holes from the other side) from the contacts into the organic layers between them due to an external applied voltage, the subsequent formation of excitons (electron-hole pairs) in an active zone and the radiating recombination of these excitons, light is generated and emitted by the light emitting diode.

0004Organic light-emitting diodes in the form of PLED are usually based on the following layer structure:<ol id="ol0001" compact="compact"><li>1. Substrate (transparent, e.g. glass)</li><li>2nd Anode (transparent, mostly indium tin oxide (ITO))</li><li>3rd Hole transport or hole injection layer (mostly PEDOT: PSS or PANI - polyaniline with admixtures such as PSS; PEDOT = polyethyleneedioxythiophene, PSS = polystyrene sulfonate)</li><li>4th active polymer (emits light)</li><li>5. Cathode (usually a metal with low work function such as barium, calcium)</li></ol>

0005The polymeric layers, ie the hole transport or hole injection layer and the active polymer are produced from a liquid solution (in water or in solvents). The contacts (anode, cathode) typically by vacuum processes.

0006The advantages of this structure for applications, for example displays, is the variety of processes for the production of the polymer layers, including those processes that allow simple lateral structuring of the PLED, namely inkjet printing. Here, the different polymers of the three colors are printed on previously prepared areas, which create adjacent areas of different emission colors.

0007The disadvantage is, inter alia, that it is not practical to apply more than two different polymer layers, since the solvents of the polymers have to be chosen such that they do not influence one another, that is to say do not attack the material of the base. This means that the emitting polymer must also be well suited for electron transport and injection from the cathode at the same time, a requirement that places severe restrictions on the choice of material and structure optimization.

0008In addition, it is difficult to change the order of the structure for a given material system, which means that, as in the above case, the anode has to be started. This is particularly disadvantageous for the integration of the PLED on active matrix display substrates with n-channel transistors as the switching element. The use of transparent cover contacts (also as a cathode) is just as difficult, since these are mostly produced by a sputtering process (eg ITO). But this destroys organic materials. Since the top layer in a PLED is an emitting layer, the efficiency of the light generation of the organic light-emitting diode is reduced. An improvement in the stability against sputter damage can be achieved by introducing a layer of small molecules that is evaporated in a vacuum. However, electron injection from the cathode is also a problem in this case. Another disadvantage of the above structure is that efficient electron injection can only be achieved with very unstable contact materials such as barium or calcium. However, these materials are attacked by oxygen and water.

0009Organic light-emitting diodes in the form of OLEDs are made up of small molecules that are evaporated in a vacuum. If the molecules that are to form the layers of the OLED are small enough, they can usually be applied without decomposition using a thermal process. For this purpose, the molecules are evaporated in a vacuum (because of the large free path).

0010In order to improve the injection from the contacts into the organic layer and to increase the conductivity of the transport layers, the transport layers can be doped by mixed evaporation with organic or inorganic dopants, which are acceptors (for hole doping) or donors (for electron doping). The dopants need not be in their final form at the beginning of the evaporation process, as long as the alternative precursor material used in the evaporation process (which can also be modified, for example by using electron beams) forms the dopant. The mixed layers are typically produced by mixed (co) evaporation.

0011In addition to the doped transport layers, intrinsic (i.e. non-doped) intermediate layers with certain energetic properties must then be introduced (patent <patcit id="pcit0001" dnum="DE10058578"><text>DE 100 58 578</text></patcit>, <nplcit id="ncit0002" npl-type="s"><text>M. Pfeiffer et al. "Light-emitting component with organic layers", filed on November 20, 2000; X. Zhou et al., Appl. Phys. Lett. 78, 410 (2001</text></nplcit>)).

0012The structure of the OLED is then a pin heterostructure:<ol id="ol0002" compact="compact"><li>1. Carrier, substrate,</li><li>2nd Electrode, hole injecting (anode = positive pole), preferably transparent,</li><li>3rd p-doped hole injecting and transporting layer,</li><li>4th thinner block layer on the hole side made of a material whose band layers match the band layers of the layers surrounding them,</li><li>5. light emitting layer,</li><li>6. electron-side block layer (typically thinner than the layer mentioned below) made of a material whose band layers match the band layers of the layers surrounding them,</li><li>7. n-doped electron injecting and transporting layer,</li><li>8th. Electrode, usually a metal with a low work function, electron-injecting (cathode = negative pole)</li></ol>

0013The advantages of this structure are the separate optimization of the properties of the individual layers, the adjustable large distance between the emitter layer and the contacts, the very good injection of the charge carriers into the organic layers and the small thickness of the layers that are not very conductive (4; 5; 6). This means that very low operating voltages (&lt;2.6V for 100cd / m2 luminance) can be achieved with a high level of efficiency in generating light, as shown in<nplcit id="ncit0003" npl-type="s"><text>J. Huang, M. Pfeiffer, A. Werner, J. Blochwitz, Sh. Liu, K. Leo, Appl. Phys. Lett., 80, 139-141 (2002</text></nplcit>): Low-voltage organic electroluminescent devices using pin structures. Like in the<patcit id="pcit0002" dnum="DE10135513"><text>DE 101 35 513.0</text></patcit> and in <nplcit id="ncit0004" npl-type="s"><text>XQ Zhou et al., Appl. Phys. Lett. 81, 922 (2002</text></nplcit>), this structure can also be easily inverted and top-emitting or completely transparent OLEDs can be realized, as in the <patcit id="pcit0003" dnum="DE10215210"><text>DE 102 15 210.1</text></patcit> described.

0014The disadvantage of this structure is that the OLED structure can only be structured laterally in order to build up differently colored pixels in a display using shadow masks. This process has limitations regarding the smallest achievable pixel sizes (&lt;50µm subpixel). The shadow mask process is a relatively complex process in manufacturing. However, the ink jet process cannot be used on small molecules because of their insolubility.

0015In the <patcit id="pcit0004" dnum="US2003020073A1"><text>US 2003 / 020073A1</text></patcit> describes the use of vapor-deposited block layers and electron transport layers on a polymer hole transport layer. With this arrangement, it is possible to structure the polymer layer laterally in order to produce a full-color display. However, with this arrangement the injection of charge carriers (here electrons from the cathode into the molecular electron transport layer) is problematic, which increases the operating voltage of the hybrid polymer-small molecule OLED.

0016Out <patcit id="pcit0005" dnum="US2003111666A"><text>US 2003/111666</text></patcit> a light-emitting component with organic layers is known, which consists of several layers between a basic contact and a top contact. Here, polymer layers and molecular layers are provided.

0017The article "OLEDs with doped transport layers for highly efficient displays" by Pfeiffer et al. Various doping options can be found in organic light-emitting diodes. The article "Closed-shell molecules that ionize more readily than cesium" by Cotton et al. describes stable molecules with a low ionization potential.

0018In the article "Pyronin B as a donor for n-typed doping of organic thin films" by Werner et al. describes the use of pyronine B as a donor in thin organic layers.

0019The article "Low-voltage inverted transparent vacuum deposited organic light-emitting diodes using electric doping" by Zhou et al. describes the advantages of electrical doping in transparent inverted organic light emitting diodes.

0020It is therefore an object of the invention to increase the flexibility of the structure of a light-emitting component and the injection of charge carriers into the organic layers while maintaining good structurability.

0021This object is achieved by a light-emitting component according to independent patent claim 1 and a method according to independent patent claim 22. Advantageous embodiments of the invention are the subject of dependent subclaims.

0022The dopant should consist of an organic or metal-organic molecule which has a molar mass of greater than 200 g / mol, preferably greater than 400 g / mol. It is important that the dopant active in the layer has this molar mass. For example, Cs₂CO₃ (Cesium carbonate, molar mass approx. 324 g / mol) is not suitable as a donor for the n-doping of the electron transport layer in the sense of the invention. Cs₂CO₃ as such is a comparatively stable compound that is no longer able to transfer one or more electrons to another molecule (the matrix material). However, in a vaporization process above 615 ° C (decomposition temperature) molecular Cs can be released, which would be able to transfer an electron as a dopant to the matrix material. The molar mass of Cs is around 132g / mol. As a dopant, cesium has the disadvantage that it cannot be built into the matrix layer as a relatively small molecule or atom, with negative effects on the service life of the organic light-emitting component. The same applies analogously in the case of p-doping the hole transport layer with a strong acceptor (in the case of the inverted POLED structure).

0023The two molecular vapor-deposited layers are the undoped intermediate layer (reference number 5 in the exemplary embodiment described below) and the doped transport layer. Since the energy barrier of charge carrier injection from the doped transport layer into the polymer emitter layer for common emitter polymers such as poly-phenylene vinylene, PPV, (in the case of the conventionally known layer structure with polymer hole transport layer on a substrate, the barrier for the injection of the electrons) is too large undoped intermediate layer can be inserted, which is much thinner than the doped transport layer and whose LUMO energy level (LUMO: lowest unoccupied molecular orbital) in the case of the hole transport layer must be the HOMO energy level (HOMO: highest occupied molecular orbital) between the doped transport layer and the emission polymer layer. On the one hand, this has the consequence that charge carriers can be better injected into the emitter polymer layer, and on the other hand non-radiative recombination processes occur at the interface between the emitter polymer layer and the doped transport layer, which usually occur almost inevitably at high energy barriers.

0024The doping of the molecular layer is advantageously carried out in a vacuum from two separately controlled sources as a mixed evaporation.

0025The polymer layers can be applied very precisely using simple means. This structuring then serves at the same time the structuring of the later light-emitting component without the need for complex structuring steps or means. The application of molecular layers, on the other hand, prevents the modification of polymer layers from being very limited due to the presence of usually only two disjoint solvents and increases the possibility of building up a wide variety of layer combinations.

0026The invention will be explained in more detail below using an exemplary embodiment.

0027In the accompanying drawings<dl id="dl0001"><dt>Fig. 1</dt><dd>a first layer structure of an organic light-emitting diode according to the invention,</dd><dt>Fig. 2</dt><dd>a second, for <figref idref="f0001">Fig. 1</figref> electrically inverse layer structure of an organic light emitting diode.</dd></dl>

0028As in <figref idref="f0001">Fig. 1</figref> shown, a transparent base contact 2 is applied as a anode on a substrate 1. A first polymer layer is deposited on this basic contact 2 as a polymer hole transport layer 3 and a second polymer layer as a polymer emitter layer 4. This layer composite of first and second polymer layers consists of PEDOT: PSS (Baytron-P) from HCStarck, Germany. A first molecular layer is vapor-deposited thereon as an intermediate layer 5, which consists of a layer of 10 nm BPhen (batophenanthroline). There is a second molecular layer in the form of an electron transport and injection layer 6 made of BPhen: Cs (molar doping concentration approx. 10: 1 to 1: 1). Finally, the organic light emitting diode is in accordance with<figref idref="f0001">Fig. 1</figref> provided with a cover contact 7 made of aluminum.

0029In this context, molecular Cs is to be regarded as a dopant which does not give rise to any useful electrons, since Cs has a molar mass which is too small in order to be able to achieve a diffusion-stable doped layer. Doping materials with a molar mass greater than 200 g / mol, preferably greater than 400 g / mol, and a redox potential in the range of Cs are therefore provided. Cs has a standard redox potential of -2.922 V and an ionization energy of 3.88 eV. The ionization energy of the dopant is less than 4.1 eV.

0030An example of one of these dopants is Wolfram-Paddlewheel [W₂(hpp)₄] : <chemistry id="chem0001" num="0001"><img file="EP1511094B1_D0001.tif" /></chemistry>

0031Wolfram paddlewheel has an ionization potential of approx. 3.75 eV. The structure of the simply negative hpp anion is:<chemistry id="chem0002" num="0002"><img file="EP1511094B1_D0002.tif" /></chemistry>

0032From comparisons with the gas ionization potential of molecular Cs of 3.9 eV and the electron affinity of BPhen as a layer of approx.2.4 eV, it can be estimated that it is necessary that the donor dopant for OLED transport materials has an ionization potential of less than 4.1 eV.

0033The doped layer (in the above example BPhen: Cs) must have a conductivity in the range of 1E-7 S / cm to 1E-3S / cm, preferably in a range of 1E-6S / cm to 5E-5S / cm. The conductivity of the undoped intermediate layer (in the above example BPhen) must be in a range from approx. 1E-10S / cm to 5E-8S / cm. The conductivity of the undoped layer is therefore at least half an order of magnitude worse than that of the doped layer. The preferred thickness ranges of the doped layer are between 40 nm and 500 nm, preferably 50 nm to 300 nm, those of the undoped intermediate layer between 2 nm and 30 nm, preferably between 5 nm and 15 nm. Because of its low conductivity, the undoped layer must be significantly thinner than the doped layer. The considerations with regard to layer thickness and conductivity also apply mutatis mutandis to the p-doping of the hole transport layer according to embodiment 2 below.

0034This embodiment can be modified in that a single layer can occur as the polymeric hole transport layer 3 and as the polymeric emitter layer 4, which takes on both functions, that is to say that only one polymer layer can therefore be present. Furthermore, the base contact 2 can also be non-transparent (for example gold, aluminum) and then the cover contact 7 can be transparent as a cathode, for example by means of an ITO layer produced in a sputtering process. Because of the doping of layer 6, electron injection of ITO in layer 6 is still possible. Furthermore, the dopant concentration in the case of organic dopants can be between 1: 1000 and 1:20.

0035As can be seen, the organic light-emitting diode according to the invention consists of both polymer and molecular layers and can therefore also be meaningfully referred to as POLED or hybrid OLED.

0036An alternative embodiment, not part of the invention, is in <figref idref="f0001">Fig. 2</figref> shown. It shows you one<figref idref="f0001">Fig. 1</figref> electrically inverse structure. A base contact 2 is applied to a substrate 1 as a cathode. The base contact 2 is designed as a non-transparent cathode (calcium, barium or aluminum), but can also be transparent (ITO). A first polymer layer is deposited on this basic contact 2 as a polymer electron transport layer 8 and a second polymer layer as a polymer emitter layer 4. A first molecular layer is vapor-deposited thereon as an intermediate layer 9, which can consist of a layer of 10 nm TPD (tri-phenyl diamine). There is a second molecular layer in the form of a hole transport and injection layer 10 made of, for example, m-MTDATA doped with F4-TCNQ (tris (3-methylphenylphenylamino) triphenylamine doped with tetrafluorotetracyanoquinodimethane) in a molar ratio of approximately 50: 1. Finally, the organic light emitting diode is in accordance with<figref idref="f0001">Fig. 2</figref> provided with an anode as a cover contact 7 made of, for example, transparent ITO.

0037Further embodiments, not shown, do not form part of the invention in swapping the order of the polymer and molecular layers, that is, first applying a doped molecular layer 10 or 6 to the substrate 1 of the basic contact 2 and then applying the laterally structurable polymer layers 4 and 8 or 3. Furthermore, as an alternative to this, an embodiment is possible in which an active polymer emitter layer 4 is framed by organic molecular layers.

0038If an anode is applied to the substrate 1 as a base contact 2, then the subsequent sequence of molecularly doped hole injection and transport layer 10, intermediate layer 9, polymeric layer 4, intermediate layer 5 and molecularly doped electron transport layer 10 and cover contact 7 as cathode. If the cathode is applied as a base contact 2 on the substrate 1, the sequence is inverted.

<u>Reference list</u>

0039<dl id="dl0002" compact="compact"><dt>1</dt><dd>Substrate</dd><dt>2</dt><dd>Basic contact</dd><dt>3</dt><dd>polymeric hole transport layer</dd><dt>4</dt><dd>polymeric emitter layer</dd><dt>5</dt><dd>Intermediate layer (molecular layer)</dd><dt>6</dt><dd>doped electron transport and injection layer (molecular layer)</dd><dt>7</dt><dd>Deck contact</dd><dt>8</dt><dd>polymeric electron transport layer</dd><dt>9</dt><dd>Intermediate layer (molecular layer)</dd><dt>10</dt><dd>doped hole transport and injection layer (molecular layer)</dd></dl>