WO2009007919A2

WO2009007919A2 - Organic light emitting diodes having improved optical out-coupling

Info

Publication number: WO2009007919A2
Application number: PCT/IB2008/052753
Authority: WO
Inventors: Cristina Tanase; Mihaela-Ioana Popovici
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2007-07-11
Filing date: 2008-07-09
Publication date: 2009-01-15
Also published as: WO2009007919A3; JP2010533358A; TW200919800A

Abstract

An organic light emitting diode comprising a first electrode layer, a second electrode layer, an organic light-emitting layer sandwiched between said first electrode layer and said second electrode layer, and a light-extraction layer is disclosed. The light-extraction layer is arranged to receive light transmitted through said first electrode layer and/or said second electrode layer, and is separated from said electrode layers by a protecting layer. The light-extraction layer has a root mean square roughness (RMS) in the range of 10-800 nm. Also disclosed is a method for manufacturing such an organic light emitting diode.

Description

Organic light emitting diodes having improved optical out-coupling

FIELD OF THE INVENTION

The present invention relates to organic light emitting diodes comprising a light-extraction layer in order to enhance the optical out-coupling. It also relates to a method for the manufacture of such organic light emitting diodes.

BACKGROUND OF THE INVENTION

Organic Light Emitting Diodes (OLEDs) typically consist of a number of organic layers (based on small organic molecules and/or polymers), each optimized for its own functionality, sandwiched between two electrodes, i.e. an anode, e.g. indium tin oxide (ITO), and a cathode, e.g. Ba/Al, LiF/Al.

The optical extraction efficiency of light from OLEDs is a major problem that appears in the fabrication of high efficiency OLEDs. This extraction efficiency is defined as the ratio of light generated within the device to light emitted into the ambient, which is typically in the range between 0.17-0.5. US 2005/0194896 discloses an organic light emitting device aiming at improving an external coupling efficiency to extract light emitted from the emissive layer to the outside. According to US 2005/0194896, this is achieved by a nano-structure layer that functions as a light extracting layer. The nano-structure layer may contain silica and titania particles, and is a structure having micro-pores therein to reduce the average refractive index. However, a light extracting layer according to US 2005/0194896 is complicated to achieve technologically, and further, it loses its effect if being applied on the outside of an OLED.

SUMMARY OF THE INVENTION One aim with the present invention is to provide OLEDs having improved optical extraction efficiency, and which are easy to produce.

This aim is achieved by an organic light emitting diode comprising a first electrode layer; a second electrode layer; an organic light-emitting layer sandwiched between said first electrode layer and said second electrode layer; and a light-extraction layer, wherein said light-extraction layer is arranged to receive light transmitted through said first electrode layer and/or said second electrode layer; said light-extraction layer being separated from said electrode layers by a protecting layer; and having a root mean square roughness (RMS) in the range of 10-800 nm. Preferably, the light-extraction layer has a root mean square roughness (RMS) in the range of 100-600 nm. Such a light-extraction layer has both high transmission and diffuse transmission, and has proven to significantly improve the efficiency of the OLEDs.

The light-extraction layer is preferably a nanocomposite layer comprising titania nanoparticles and pores in the mesoporous range, which pores are filled with silica. Suitably, the light extraction layer has a refractive index of about 1.9.

The protecting layer separating the light-extraction layer from the electrode layers may e.g. be a substrate, a dielectric layer, or an encapsulation layer.

The present invention also relates to a light emitting device comprising an organic light emitting diode as described above. Further, the invention relates to a method for the manufacture of such an organic light emitting diode.

In particular, a method for the manufacture of an organic light emitting diode according to the invention comprises providing a first electrode layer; providing a second electrode layer; providing an organic light-emitting layer sandwiched between said first electrode layer and said second electrode layer; and providing a light-extraction layer, wherein said light-extraction layer is arranged to receive light transmitted through said first electrode layer and/or said second electrode layer; and said light-extraction layer is separated from said electrode layers by a protecting layer.

The light extraction layer is formed by applying, suitably by spincoating, a film of a precursor sol gel, in particular a titania precursor gel, in a closed environment at a humidity in the range of 30-100%. Preferably the humidity ranges from 65-85 %. Thereafter, the film is dried, e.g. at a temperature in the range of 50-70 ⁰C, e.g. 60 ⁰C. The method may further comprise the step of curing said film, e.g. at a temperature in the range of 100-120 ⁰C, e.g. HO ⁰C.

The titania precursor sol gel may be obtained by providing reactive oxocomplexes from titanium alcoxide, e.g. titanium isopropoxide; adding an amount of an alkoxysilane, e.g. tetraethoxysilane; and refluxing the resulting mixture.

The simplicity of the method, and the fact that the OLEDs device already manufactured is not damaged during the supplementary processing realized at low temperature are major advantages of the invention. These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS Fig la-c show a schematic illustration of OLEDs according to the invention on bottom emission configuration (a) and dual-side emission configuration (b,c) containing a light-extraction layer according to the invention on different positions.

DETAILED DESCRIPTION OF EMBODIMENTS The present invention relates to OLEDs having improved light extraction due to a supplementary inorganic layer Of TiO₂-SiO₂ with high refractive index applied on top of the glass substrate, either on the anode or on the cathode side of the OLEDs device. The invention can be applied in solid-state area lighting, particularly in illumination and illumination lighting. Fig la-c shows various embodiments of OLEDs according to the invention. In accordance with the present invention, both LEDs based on small molecules (smOLEDs) or polymers (PLEDs) are included. The layered structure of the OLED includes a thin organic light-emitting layer 4a, 4b, 4c, which layer is arranged between two electrodes, such as an anode 3a, 3b, 3c and a cathode 5a, 5b, 5c as shown in Fig la-c, at least one of which is transparent. The organic light-emitting layer 4a, 4b, 4c may also be an organic light emitting layer stack.

The layered structure is mounted on a substrate 2a, 2b, 2c. In addition to the layered structures shown in Fig la-c, additional layers may be added, such as micro-cavity layers, layers for changing or improving colors, scattering layers and/or hole injection layers. These possible additional layers do not change anything in the way in which the basic object is achieved in accordance with the invention.

A distinction is made in this case between what are termed top and bottom emitters. With reference to Fig Ia, bottom emitters emit the light 7a from the luminescence through the substrate 2a. In this case the anode 3 a may comprise an ITO layer and the cathode 5a a layer of aluminium or a low-workfunction metal like Ba covered with a protective metal like aluminium. The layered structure may also be applied to the substrate in the reverse order. A top emitter of this kind then emits the light not through the substrate 2a in the way shown in Fig Ia but in the opposite direction. These top emitters can be created by different anode and cathode compositions, resulting in optically transparent cathodes and reflecting anodes.

Fully transparent devices can be created as well by application of both transparent anodes and cathodes as shown in Figs Ib and Ic. In this case, the cathode 5b, 5c contains a multilayer structure that is optically transmissive and contains for example Al as electron- injecting contact, Ag for sheet resistance reduction and a transparent dielectric layer 8b, 8c with high refractive index such as ZnSe or ZnS for the enhancement of optical transmission.

The present inventors have found that light extraction from OLEDs, having bottom emission configuration or dual-side emission configuration, can be enhanced using a light-extraction layer 6a, 6b, 6c. The light-extraction layer 6a, 6b, 6c is arranged to receive light transmitted through the cathode layer 5b, 5c, and/or the anode layer 3a, 3b, 3c and is separated from the electrode layers by a protecting layer, which may for example be a transparent substrate 2a, 2b, 2c or a dielectric layer 8b, 8c. In particular, the light-extraction layer 6a, 6b, 6c may be applied on the anode encapsulation glass 2a, 2b, 2c and/or on the cathode side on the dielectric layer 8b, 8c or on the thin film encapsulation or glass encapsulation (not shown). Here, thin film encapsulation is referred to an alternation of inorganic/inorganic or inorganic/organic multilayer stack that is used to protect the OLED from the different elements from the environment (e.g. water, oxygen).

The light-extraction layer 6a, 6b, 6c shall not be directly applied on the cathode 5a, 5b, 5c or anode 3a, 3b, 3c due to the risk of chemical interactions with these layers. Chemical interactions of this layer with the cathode might determine cathode degradation and generation of black spots. As the light extraction layer has a roughness of tens to few hundreds of nm (as will be further explained below) it will disturb the organic/second electrode layer formation, generating electrical shorts of the OLEDs.

In order to quantitatively measure the nanometer scale surface of the light- extraction layer 6a, 6b, 6c, Atomic Force Microscope (AFM) was used. A suitable way of characterizing a nanoscale surface is by measuring the Root Mean Square Roughness (RMS). It was found that the improved optical out-coupling induced by the light- extraction layer 6a, 6b, 6c is due to the RMS roughness, estimated by AFM to be in a range of 10-800 nm. The roughness creates a surface scattering phenomenon from the "hills" and "valleys", where the "hills" have a high refractive index of about 1.9 and the "valleys", which are gaps filled with air, have a refractive index of about 1. The difference in the refractive index (that can be estimated then to about 0.9) contributes to the diffuse transmission of the light which is estimated to be about 60-70%, giving the maximum light output. The efficiency of OLEDs in terms of optical out-coupling can be raised with 30 % up to 60 %, and high lumen power and brightness light sources can be realized. In this connection a refractive index of "about 1.9" refers to a refractive index which must not necessarily be exactly 1.9, but for example in the range of 1.8 to 2.0, or in the range of 1.85 to 1.95.

In a preferred embodiment of the invention, the light-extraction layer 6a, 6b, 6c is a nanocomposite layer comprising titania (also referred to as TiO₂) particles and having pores in the mesoporous range. As used herein, "mesoporous range" relates to pores less than 50 nm. Preferably, the pores are filled with silica (also referred to as SiO₂).

According to the invention, the titania-silica systems can be prepared in such a way that an RMS in the range of 10-800 nm, in particular 100-600 nm, covering the most part of the VIS wavelength range, can easily be obtained. It was surprisingly found that a narrow range of humidity values during manufacture of the nanocomposite layer was directly correlated to the resulting roughness.

By tuning the right condition growth of the thin film OfTiO₂-SiO₂ nanocomposite the best refractive index, crystal size and porosity for the most efficient out- coupling of light extracted from an OLED can be obtained. The present invention provides a simple manufacturing process that employs the use of a spin-coated oxide based thin film of TiO₂-SiO₂ nanocomposite with high index of refraction as optical out-coupling structure. The thin film containing titania nanoparticles with tuned size is obtained in controlled conditions of processing. The low temperature of preparation of the TiO₂-SiO₂ nanocomposite thin film, about HO⁰C, assures the integrity of the device.

An aggregate as large as layer thickness of amorphous titania clusters is formed and having a length of tens of microns (according TEM analysis). The silicon oxide is required in the system in order to reduce the porosity of the layer and to increase the adhesion to the glass substrate. The pores formed in the film are in mesoporous range (less than 50 nm). The sol used contains reactive titanium oxocomplex clusters, which hydrolyze and form titania nanoparticles during spin coating deposition in a closed environment characterized by a certain value of the humidity and temperature.

It was surprisingly found that the RMS will increase from values of 1-2 nm, in the case of low hydrolysis ratio and that corresponds to relative humidity of the reaction medium above the substrate to be spin coated of about 15-16%, to values of 160 nm for humidities of 80 % humidity or 778 nm for 100% humidity. According to the inventive manufacturing method, the humidity preferably lies in a range of 30-100 %, most preferably in a range of 65-85 %.

One advantage of this composition and synthesis route is the low temperature of processability. The titania nanoparticles are formed instantly during the spin coating deposition and they reach the required dimension at low temperature (about HO⁰C) of processing. This temperature also assures the complete removal of the solvent used and the densifϊcation of the layer, which will display a high enough value (about 1.9) of the refractive index.

Thus, the refractive index difference between the TiO₂ZSiO₂ obtained at 110° C curing temperature and air is about 0.9, and this adds to the enhanced light extraction obtained due to the geometrical configuration of the roughened surface. This difference in refractive index of about 0.9 between the light-extraction layer according to the invention and its surrounding environment, normally air, is very advantageous. In this connection a difference in refractive index of "about 0.9" refers to a difference in refractive index which must not necessarily be exactly 0.9, but for example in the range of 0.8 to 1.0, or in the range ofO.85 to O.95.

The general processing steps for depositing a thin film OfTiO₂-SiO₂ nanocomposite according to the invention on the glass substrate that protects the OLEDs devices are described below. The light extraction layer is formed by applying a film of a precursor sol gel in a closed environment at a humidity in the range of 30%- 100%, and drying said film. A "sol gel" is a colloidal suspension that can be gelled to form a solid. The sol-gel process involves the transition of a system from a liquid (the colloidal "sol") into a solid (the "gel") phase. The resulting porous gel can form high purity oxide materials at elevated temperatures. The precursor sol gel contains the ingredients necessary for forming a layer having a root mean square roughness in the range of 10-800 nm. Preferably, the precursor sol gel is a titania precursor sol gel.

As titanium alcoxide are very reactive towards water, a titania precursor sol gel is obtained using a quasi non-hydro lytic sol-gel method. Titanium (IV) isopropoxide is firstly chelated with various ligands forming reactive oxocomplexes, by using sonosynthesis. Tetraethoxysilane is subsequently added in the system and subjected to refluxing below 100⁰C. The sol gel is applied on the glass substrate that covers the OLED devices, in a close environment at a certain value of humidity and constant room temperature. At the contact with the humidity in air, titania nanoparticles are formed by hydrolysis of the titanium oxocomplexes and condensations of the hydroxyl terminated oxoclusters. During drying of the thin film on a hotplate at HO⁰C, the Ti-O-Ti network shrinks, the solid content increases and volatile components are released. The small amount of silica present in the nanocomposite fills the pores and improves the adhesion on the glass substrate through Si- OH groups. One hour of curing at HO⁰C is enough to obtain the desired properties like refractive index, transmittance and mechanical strength.

A detailed protocol for obtaining a nanocomposite layer according to the present invention is provided in the following example.

Example

All the solvents were reagent grade from Sigma- Aldrich and Across Organics. Titanium isopropoxide (TTIP) 97% (Sigma- Aldrich) is solved in diethyleneglycol monobutyl ether (EGBE), 98%, (Across Organics) in the presence of urea puriss p. a (Fluka) solved in absolute ethanol EtOH (Across Organics) at a mole ratio of TTIP: EGBE: urea: EtOH = 1 :3:1 :33. The sol (denoted TU) is ultrasonicated for 50 min at room temperature resulting in a reactive oxocluster sol that is further kept under dried air. A small amount of tetraethoxysilane TEOS (Sigma- Aldrich) is added to the half amount of the sol for a mole ratio TTIP/TEOS=6/1 and both sols were subjected to refluxing at 97 ⁰C for 90 minutes. The resulted sols were spin coated on silicon wafers and AF45 glass substrates in a closed box with controlled humidity environment. A series of samples at different value of relative humidity were obtained. It was found that the optimum range of humidities was 65%- 85%. The thin films were dried on a hot plate at 60 ⁰C and cured afterwards at 110 ⁰C for one hour.

Claims

CLAIMS:

1. An organic light emitting diode comprising: a first electrode layer; a second electrode layer; an organic light-emitting layer sandwiched between said first electrode layer and said second electrode layer; and a light-extraction layer, wherein said light-extraction layer: is arranged to receive light transmitted through said first electrode layer and/or said second electrode layer; is separated from said electrode layers by a protecting layer; and has a root mean square roughness (RMS) in the range of 10-800 nm.

2. An organic light emitting diode according to claim 1, wherein said light- extraction layer has a root mean square roughness (RMS) in the range of 100-600 nm.

3. An organic light emitting diode according to claim 1 or 2, wherein said light- extraction layer is a nanocomposite layer.

4. An organic light emitting diode according to any one of the claims 1-3, wherein said nanocomposite layer comprises titania nanoparticles.

5. An organic light emitting diode according to claim 3 or 4, wherein said nanocomposite layer comprises pores in the mesoporous range.

6. An organic light emitting diode according to claim 5, wherein said pores are filled with silica.

7. An organic light emitting diode according to any one of the claims 1-6 wherein said light-extraction layer has a refractive index of about 1.9.

8. An organic light emitting diode according to any one of the claims 1-7, wherein said protecting layer is a substrate.

9. An organic light emitting diode according to any one of the claims 1-7, wherein said protecting layer is a dielectric layer.

10. An organic light emitting diode according to any one of the claims 1-7, wherein said protecting layer is an encapsulation layer.

11. A light emitting device comprising an organic light emitting diode according to any one of the claims 1-10.

12. A method for the manufacture of an organic light emitting diode according to any one of the claims 1-10.

13. A method for the manufacture of an organic light emitting diode comprising: providing a first electrode layer; providing a second electrode layer; - providing an organic light-emitting layer sandwiched between said first electrode layer and said second electrode layer; and providing a light-extraction layer, wherein said light-extraction layer: is arranged to receive light transmitted through said first electrode layer and/or said second electrode layer; and is separated from said electrode layers by a protecting layer; wherein said light extraction layer is formed by applying a film of a precursor sol gel in a closed environment at a humidity in the range of 30%- 100%; and drying said film.

14. A method according to claim 13, wherein the humidity ranges from 65-85 %.

15. A method according to claim 13 or 14, wherein said precursor sol is a titania precursor sol gel.

16. A method according to claim 15, wherein said titania precursor sol gel is obtained by: providing reactive oxocomplexes from titanium alcoxide; - adding an amount of an alkoxysilane; and refluxing the resulting mixture.

17. A method according to claim 16, wherein said alkoxysilane is tetraethoxy silane .

18. A method according to claim 16, wherein said titanium alcoxide is titanium isopropoxide.

19. A method according to any one of the claims 13-18, further comprising the step of curing said film.

20. A method according to any one of the claims 13-19, wherein said film is applied by spincoating.