CN1373908A - Long-lifetime polymer light-emitting devices with improved luminous efficiency and radiance - Google Patents

Abstract

The luminous efficiency and radiance of light emitting diodes (LEDs) fabricated from organic emissive materials can be increased by using a multilayer cathode including a low work function layer and a high work function high reflectivity layer, in combination with a high work function, high reflectivity anode material in the device.

Description

Long-lived polymer light-emitting devices with improved luminous efficiency and radiance

Field of Invention

The present invention relates to organic polymer light emitting diodes with improved luminous efficiency and improved emissivity.

Related technical description

Diodes, and especially light-emitting diodes (LEDs), fabricated from conjugated organic polymer layers have attracted much attention due to their application potential in display technology. A standard polymer LED structure consists of the following sequential layers in contact: a substrate with an indium-tin oxide (ITO) coating, a passivation layer, an emissive polymer, and then a cathode monolayer. In the field of organic polymer-based LEDs, higher work function metals are commonly used as anodes for hole injection in otherwise filled π-bands of electroluminescent polymers with semiconducting properties. Lower work function metals are preferred as cathode materials for injecting electrons in otherwise empty π ^* -bands of electroluminescent polymers with semiconducting properties. The holes injected at the anode and the electrons injected at the cathode recombine radiatively within the active layer and emit light. Typical higher work function materials used as anode materials include transparent indium/tin-oxide conductive films. Alternatively, polyaniline films in the form of conductive emeraldine salts can be used. Indium/tin-oxide films and polyaniline films in the form of conductive emeraldine salts are generally preferred because, as transparent electrodes, both allow light emitted from the LED to exit the device at useful levels.

Typical lower work function metals suitable for use as cathode materials are metals such as calcium, magnesium and barium. Alkali metals tend to be too mobile and can get incorporated into emissive layers (eg, electroluminescent polymers), thereby causing short circuits and unacceptably short device lifetimes.

Low work function metal ultrathin layers [see Cao, Y.; PCT WO98/57381 and Pichler, K., International Patent Application WO 98/10621] or low work function metal oxide ultrathin layers are known in the art [See Cao, Y.; PCT Application 99 US/23775] Cathode-generated LEDs that provide equal or better initial performance (e.g., brightness and efficiency) and extended lifetime compared to similar LEDs using conventional thick-film cathodes. operating life.

Although the methods of making polymer LEDs have improved, there are still problems that need to be solved. For example, polymer LEDs are bright and efficient enough to be used in some display applications. However, in battery-operated devices, luminous efficiency is a key parameter. Higher luminous efficacy directly translates to longer lifetime without recharging the battery. More generally, higher luminous efficiencies enable a wider range of display applications. Therefore, there is still a demand for LEDs with higher luminous efficiency. In particular applications, the light output is preferably a narrow cone along the forward direction. In this application, high emissivity is especially important.

Invention Summary

The present invention relates to a light-emitting diode comprising an anode comprising a translucent layer with high reflectivity and high work function and a cathode comprising at least one element selected from the group consisting of metals, metal oxides and combinations thereof. A first cathode layer of low work function material and at least one second cathode layer having high reflectivity and high work function.

The invention achieves improved luminous efficiency and improved emissivity. In a first embodiment, the translucent layer or the second cathode layer has a reflectivity of at least 91.4% and a work function of greater than about 4 eV. In a second embodiment, the translucent layer and/or the second cathode layer has a reflectivity of at least 86% in the emission wavelength range of 400-500 nm. In a preferred embodiment, both the translucent layer and the second cathode layer are silver.

The phrase "adjacent" as used herein does not necessarily mean that one layer is immediately adjacent to another layer. There may be one or more intermediate layers between said adjacent layers.

As used herein, the phrase "reflectivity over the emission wavelength range..." refers to the reflectivity of a layer at a particular wavelength of light. The wavelength at which reflectance is quoted is the peak wavelength emitted from the device. Reflectance values can be read from the standard tutorial tables of J.H. Weaver, H.P.R. Frederikse, "Optical Properties of Metals and Semiconductors", CRC Handbook, pages 12-117.

The phrase "translucent" as used herein is defined as capable of transmitting at least some light, preferably between about 4% and 25% of the amount of light of the particular wavelength of interest.

Brief description of attached drawings

FIG. 1 is a schematic diagram of the construction of a polymer LED device used in the present invention. Not drawn to scale.

Figure 2 shows the sensitivity of the human eye to light as a function of wavelength.

Figure 3 shows the electroluminescence spectrum of a comparative polymer LED fabricated from ITO electrodes and Ba/Al electrodes using Covion PDO 122 (i.e. Comparative Example A).

Figure 4 shows the electroluminescence spectrum of a polymer LED fabricated from a 300 Å silver anode and a Ba/Ag electrode of the present invention (ie Example 3).

Figure 5 shows the electroluminescence spectrum of a comparative polymer LED fabricated from ITO electrodes and Ba/Al electrodes using Covion PDY 131 (ie comparative example C).

Figure 6 shows the electroluminescence spectrum of a polymer LED of the present invention fabricated using Covion PDY 131 from a 300 Å silver anode and a Ba/Ag cathode (ie Example 4).

Figure 7 shows luminance versus voltage curves for Example 4 and Comparative Example C devices.

            Preferred Implementation Scheme Description

As best seen in FIG. 1 , LED device 100 includes substrate 110 and anode 120 . Anode 120 includes a translucent layer 122 of a highly reflective metal and a selective passivation layer 128 . The translucent layer 122 has a first surface 124 adjacent to the substrate 110 and an opposite second surface 126 . At least one emissive layer 130 is placed between the anode 120 and the cathode 140 . Cathode 140 includes a first cathode layer 142 of low work function material and a second cathode layer 144 of high reflectivity metal. Light is emitted from substrate 110 as indicated by arrow 150 . Substrate

Suitable materials that may be used as substrate 110 include, for example, glass and polymer films. anode

Although transparent electrodes such as ITO are usually used on the light-emitting side of the light-emitting device, in order to minimize the loss of light transmission through the electrode, the present invention replaces or adds a transparent electrode with a high-reflectivity metal thin layer 126 to improve the efficiency of the device. As best seen in FIG. 1 , the anode 120 may be a composite layer consisting of a translucent layer 126 and a passivation layer 128 of conductive polymer coated on the second surface 124 of the translucent layer 126 .

In a first alternative embodiment (not shown), the anode includes only a conductive current-carrying layer, which can serve as a hole-injecting layer, and no passivation layer. In a second alternative embodiment (not shown), the anode includes a transparent conductive layer such as ITO adjacent to the first surface 124 of the translucent layer 126 and a passivation layer 128 . In a third alternative embodiment (not shown), the anode includes a transparent conductive layer such as ITO adjacent the first surface 124 of the translucent layer 126 and does not include a passivation layer. The translucent layer 126 or alternative single anode layer (not shown) of composite anode 120 is made of an anode material selected from high reflectivity metals with high work function (typically greater than about 4.0 eV). Examples of suitable metals include silver, gold, aluminum and copper. In a preferred embodiment, translucent layer 126 has a reflectivity of at least 91.4% at the emission wavelength, is a good electrical conductor (conductivity of about 10 ² to about 10 ⁸ Ω ⁻¹ cm ⁻¹ , and capable of forming a smooth contact film). In a second preferred embodiment, the translucent layer has a reflectivity of greater than about 92% at the emission wavelength. In a third preferred embodiment, the translucent layer has a reflectivity of from about 92% to about 96.5% at the emission wavelength. In a fourth preferred embodiment, the translucent layer has a reflectivity of from about 94% to about 96.5% at the emission wavelength. In a fifth preferred embodiment, translucent layer 126 has a reflectivity of greater than about 96% at the emission wavelength. In another preferred embodiment, the translucent layer 126 has a reflectivity of at least 86% over the emission wavelength range of 400-500 nm. Examples of such anode materials include silver, aluminum, gold and copper and alloys of these metals.

Translucent layer 126 may generally be fabricated using any technique known in the thin film deposition art, using, for example, pure metals or alloys or other thin film precursors, including, for example, vacuum evaporation, sputter deposition, electron beam deposition, or chemical vapor deposition. . The thickness of the metal layer can be controlled by the rate and time of evaporation/deposition. Typical evaporation/deposition rates are about 0.5-10 Å/sec. The thickness of the translucent layer should be thin enough to transmit at least some light (so as to be translucent), and thick enough to provide a continuous layer. Typically, the translucent metal layer 126 has a thickness of about 100 to about 500 Å. In a first preferred embodiment, the translucent layer has a thickness of about 250 to about 400 Å. In a second preferred embodiment, the translucent layer has a thickness of from about 275 to about 350 Å. In a third preferred embodiment, the translucent layer has a thickness of about 275 to about 325A.

The selective passivation layer 128 of conductive material allows the use of highly reflective metals with work functions that cannot be precisely matched to the emissive polymer utilized. The precise form of conductive material used in the present invention can vary widely and is not critical. Examples of suitable conductive materials include, but are not limited to, polyaniline, polyaniline blends, polythiophenes, and polythiophene blends. Useful conductive polyanilines include homopolymers, derivatives, and blends with bulk polymers. Examples of useful polyanilines include those disclosed in U.S. Patent 5,232,631 and U.S. Patent 5,723,873. Useful conductive polythiophenes include homopolymers, derivatives, and blends with bulk polymers. Examples of useful polythiophenes include poly(ethylenedioxythiophene) (PEDT), such as poly(3,4-ethylenedioxythiophene), and those disclosed in U.S. Patent 5,766,515 and U.S. Patent 5,035,926. The terms "polyaniline" and "polythiophene" are used herein generally to include both substituted and unsubstituted materials. Also included in this manner are any incidental dopants, especially acidic materials used to impart conductivity to the material. cathode

The first cathode layer 142 is selected from low work function metals or low work function metal oxides (typically less than about 3.5 eV). Examples of suitable low work function materials include alkali metals, alkaline earth metals and lanthanide metals and oxides of alkali metals, alkaline earth metals and lanthanide metals. The term "alkali metal" is used herein in the conventional sense to denote elements of Group IA of the Periodic Table. The term "alkali metal oxide" is used herein in the conventional sense to denote compounds of alkali metals and oxygen. For convenience, alkali metal oxides are represented here by the chemical formulas of the corresponding simple oxides (e.g., _Li2O , _Na2O , _K2O , _Rb2O , and _Cs2O ); Indications are intended to include other oxides, including mixed oxides and non-stoichiometric oxides (e.g., Li _x O, Na _x O, K _x O, Rb _x O, and Cs _x O, where x is from about 0.1 to about 2) .

The term "alkaline earth metal" is used herein in the conventional sense to denote elements of Group IIA of the Periodic Table. Preferred alkaline earth metals include magnesium (ie Mg), calcium (ie Ca), strontium (ie Sr) and barium (ie Ba). The term "alkaline earth metal oxide" is used herein in the conventional sense to denote compounds of alkaline earth metals and oxygen. For convenience, alkaline earth metal oxides are represented here by the formulas of the corresponding simple oxides (e.g., MgO, BaO, CaO, SrO, and BaO); however, this representation by simple oxides is intended to include other oxides, including mixed Oxides and non-stoichiometric oxides (eg, _MgxO , _BaxO , _CaxO , _SrxO , and _BaxO , where x is from about 0.1 to about 1).

The term "lanthanide metals" is used herein in the conventional sense to denote the elements of the lanthanide series of the periodic table, from cerium (ie, Ce) to lutetium (ie, Lu). Preferred lanthanide metals include samarium (ie, Sm), yttrium (ie, Yb), and neodymium (ie, Nd). The term "lanthanide metal oxide" is used herein in the conventional sense to denote compounds of lanthanide metals and oxygen. For convenience, lanthanide metal oxides are represented here by the chemical formulas of the corresponding simple oxides in the +3 valence state (such as Sm ₂ O ₃ , Yb ₂ O ₃ and Nd ₂ O ₃ ); however, this use of simple oxides The reference to is intended to include other oxides, including mixed oxides and non-stoichiometric oxides (eg, Sm _x O, Yb _x O, and Nd _x O, where x is from about 0.1 to about 1.5).

In a preferred embodiment, first cathode layer 142 includes a low work function metal oxide. The first cathode layer 142 may generally be deposited by thermal vacuum evaporation. Typically the first cathode layer 142 has a thickness of about 10-200 Å. Typical evaporation/deposition rates are from about 0.2 to about 4 Å per second.

Like the translucent layer 126, the second cathode layer 144 has a high reflectivity and high work function, and is made of a material that can form a smooth, contacting film. Generally, the second cathode layer 144 has a work function greater than 4eV. In a preferred embodiment, the second cathode layer 144 has a reflectivity of at least 91.4% at the emission wavelength. In a second preferred embodiment, the second cathode layer has a reflectivity of from about 92% to about 96.5% at the emission wavelength. In a third preferred embodiment, the second cathode layer has a reflectivity of from about 94% to about 96.5% at the emission wavelength. In a fourth preferred embodiment, the reflectivity of the second cathode layer at the emission wavelength is greater than 96%. In another preferred embodiment, a metal having a reflectivity of at least 86% in the emission wavelength range of 400-500 nm is used as the second cathode layer 144 . Like the translucent layer 126, the second cathode layer 144 includes a cathode material selected from metals and metal alloys. Examples of suitable high work function metals include aluminum, silver, copper, gold, etc. and alloys of these metals.

A preferred embodiment uses a metal or metal alloy having a reflectivity of at least 91.4% at the emission wavelength for both the translucent layer 126 and the second cathode layer 144 . In another preferred embodiment, a metal having a reflectivity of at least 86% in the emission wavelength range of 400-500 nm is used as both the semi-transparent layer 126 and the second cathode layer 144 .

In general, the second cathode layer 144 need not be the same material as that used for the translucent layer 126 . For example, gold can be used as a high work function semitransparent anode, while silver can be used as a high reflectivity metal layer in a double-layer cathode. In a preferred embodiment, the highly reflective layer 142 contains a metal having a reflectivity of at least 91.4% or a reflectivity of at least 86% in the emission wavelength range of 400-500 nm as both the second cathode layer 144 and the semi-transparent layer 126. In a more preferred embodiment, silver (Ag) is used as the high reflectivity metal layer and semi-transparent anode in the dual layer cathode.

Optionally a multilayer cathode system (not shown) can be used. For example, a first high reflectivity cathode layer (preferably thick enough to be opaque) may be covered by another high reflectivity cathode layer (which may have a higher or lower reflectivity than the first high reflectivity cathode layer). In a three-layer cathode cap structure, the uppermost metal can be any stable metal capable of forming a smooth, contacting film, such as aluminum or an aluminum alloy. Subsequent layers can be added for special functions, eg for passivation and sealing of the device. Examples of layers used to seal the device include air-stable cover layers. The term "air stable" refers to the ability to protect the layers below the cover layer from ambient oxygen and moisture that may be present around the device. Materials suitable for the air-stable cover include metals or metal alloys.

Like the translucent layer 126, the second cathode layer 144 can be fabricated using known deposition techniques. Typical evaporation/deposition rates are about 1-20 Å/sec. The thickness of the second cathode layer 144 should be thick enough to cover the first cathode layer and opaque enough to give high reflectivity at wavelengths of interest. Typically the thickness of the second cathode layer is at least about 800 Å. emission layer

In the LEDs of the present invention, said at least one emissive layer 130 (also referred to as a light emitting layer or an electroluminescent layer) comprises an electroluminescent semiconducting organic material. Materials typically used as emissive layers in LEDs include polymeric or molecular materials that exhibit electroluminescence, and more specifically, materials that exhibit electroluminescence and are soluble and can be processed from solution to form uniform thin films.

Examples of useful molecular emissive materials include simple organic molecules known to exhibit electroluminescence such as anthracene, thiadiazole derivatives and coumarin derivatives. In addition, complexes such as 8-quinolinate with trivalent metal ions, especially aluminum, are also suitable emissive materials, as described, for example, in US Pat. No. 5,552,678 to Tang et al.

Examples of useful polymeric emissive materials include semiconducting conjugated polymers. Examples of suitable semiconducting conjugated polymers include poly(phenylene vinylene), PPV and soluble derivatives of PPV such as poly(2-methoxy-5-(2'-ethylhexyloxy)-1 , 4-phenylene vinylene), MEH-PPV, a semiconducting polymer with an energy gap _Eg of ~2.1eV. This material is described in more detail in Wudl, F., Hoger, S., Zhang, C., Pakbaz, K., Heeger, AJ, Polymer Preprints, 1993, 34(no. 1), 197. Another useful material described in this application is poly(2,5-dicholestalkoxy-1,4-phenylene vinylene) BCHA-PPV, an energy gap _Eg of ~2.2 eV semiconducting polymers. This material is described in more detail in US Patent 5,189,136. Other suitable polymers include, for example, Braun, D., Gustafsson, G., McBranch D. and Heeger, AJ in "Electroluminescence and electrotransport in poly(3-thiophene) diodes", J. Appl. Phys., Poly(3-alkylthiophenes) described in 1992, 72, 564; Grem, G., Leditzky, G., Ullrich, B. and Leising, G. in "Blue light emission using poly(p-phenylene) Realization of the device", Adv.Mater., 1992, 4, 36 poly(p-phenylene) and Yang, Z., Sokolik, I and Karasz FE in "Soluble blue light-emitting polymers", Macromolecules, 1993 , 26, its soluble derivatives described in 1188; and Parker, ID, Pei, Q., Marrocco, M in "Effective blue photoluminescence from fluoropolyquinolines", Appl.Phys.Lett., 1994 , 65, the polyquinoline described in 1272. Blends of conjugated semiconducting polymers with non-conjugated host or carrier polymers can also be used as active layers in polymer LEDs, as Zhang, C., von Seggern, H., Pakbaz, K., Kraabel, B. ., Schmidt, HW and Heeger, AJ in "Blue light-emitting diodes utilizing blends of poly(p-phenylenevinylene) in poly(9-vinylcarbazole), Synthetic Metals, 1994, 62, 35 described. Also useful are blends comprising two or more conjugated polymers, as in Yu, G. and Heeger, AJ in "Efficient Light-Emitting Devices Prepared from Semiconducting Polymers", Synthetic Metals, 1997, 85, 1183 mentioned.

In one embodiment, the electroluminescent organic material is an electroluminescent semiconducting organic polymer that is a π-conjugated polymer or a copolymer containing segments of π-conjugated moieties. Conjugated polymers are well known in the art. Examples of suitable electroluminescent semiconducting organic polymers include, but are not limited to:

(i) poly(p-phenylene vinylene) and derivatives thereof substituted at various positions on the phenylene moiety;

(ii) poly(p-phenylene vinylene) and derivatives thereof substituted at various positions on the vinylene moiety;

(iii) poly(p-phenylenevinylene) and derivatives thereof substituted at various positions on the phenylene moiety and substituted at various positions on the vinylene moiety;

(iv) poly(arylenevinylene), wherein the arylene group can be a moiety such as naphthylene, anthracenylene, furylene, thienylene, oxadiazole, etc.;

(v) poly(arylene vinylene) derivatives, wherein the arylene group can be the same as (iv) above, and additionally have a substituent at each position of the arylene group;

(vi) poly(arylene vinylene) derivatives, wherein the arylene group can be the same as (iv) above, and additionally have a substituent at each position of the vinylene group;

(vii) poly(arylene vinylene) derivatives, wherein the arylene group can be the same as (iv) above, and additionally have substituents at each position of the arylene group and at each position of the vinylene group;

(viii) copolymers of arylenevinylene oligomers, such as those described in (iv), (v), (vi) and (vii), with non-conjugated oligomers;

(ix) Polyparaphenylene and its derivatives substituted at various positions on the phenylene moiety, including ladder polymer derivatives such as poly(9,9-dialkylfluorene);

(x) Polyarylene, wherein the arylene can be moieties such as naphthylene, anthracenylene, furylene, thienylene, oxadiazole, etc.; and substituted at each position of the arylene derivative;

(xi) copolymers of oligoarylenes as described in (x) with non-conjugated oligomers;

(xii) polyquinoline and its derivatives;

(xiii) Copolymers of polyquinoline and p-phenylene substituted on the phenylene with, for example, alkyl or alkoxy groups to provide solubility;

(xiv) Rigid rod polymers such as poly(p-phenylene-2,6-benzobithiazole), poly(p-phenylene-2,6-benzobisoxazole), poly(p-phenylene- 2,6-benzimidazole) and its derivatives; and so on.

Also useful are combinations of semiconducting conjugated polymers with discrete molecules, with discrete molecular compounds blended with or covalently attached to the semiconducting conjugated polymers. Also useful are polyfluorene derivatives. See, eg, US Patent 5,777,070; US Patent 5,708,130; and US Patent 5,900,327.

In one embodiment, the electroluminescent semiconducting organic material is an electroluminescent semiconducting organic polymer. In a preferred embodiment, the electroluminescent semiconducting organic material is selected from poly(p-phenylene vinylene), poly(arylene vinylene), poly(p-phenylene) and polyarylene.

The emissive layer may also include other materials such as carrier polymers and additives. Typically the emissive layer has a thickness of about 600 to about 1100 Å, depending on the desired emission wavelength and hole size.

The emissive layer can generally be fabricated using any technique known in the art, particularly in the art of organic molecular and organic polymer LEDs, including, for example, casting directly from solution, and casting polymer precursors followed by reaction The desired polymer is formed (eg by heating).

We have found that excellent electron injection, high reflectivity, high Q in the microporous structure and thus improved luminous efficiency and improved emissivity can be obtained using a multilayer cathode comprising at least one layer covered with Low work function metal or metal oxide ultrathin layer of high reflectivity metal (provides efficient electron injection), the anode includes a translucent high reflectivity metal layer. We believe that the microhole effect improves the luminous efficiency and brightness. The higher reflectivity of the semi-transparent metal anode and cathode double layer of the device forms a high-performance polymer LED with a microporous structure. Microhole effect narrows the luminescent band. As a result of the narrowing, the dominant wavelength of emitted photons is shifted to a region where the human eye is more sensitive (see FIG. 2 ) and thus significantly improves the luminous efficiency of the light-emitting structure. The broad electroluminescence spectrum of a polymer LED fabricated in a conventional configuration with the same light-emitting polymer is shown in Figure 3 for comparison. encapsulation

It is generally preferred to encapsulate the LEDs of the invention to prevent long-term degradation. Encapsulation methods are well known in the art. For example, the device can be sealed between glass plates, or between layers of isolating polymers.

Example

The following examples illustrate certain features and advantages of the invention. They are intended to illustrate the invention, not to limit it.

In the following examples and comparative examples, the following measurements are carried out according to the following methods: Efficiency:

Efficiency was determined using a UDT S370 vision meter (supplied by UDT, Gamma Scientific Division, San Diego, CA) that included a photodiode calibrated using the following procedure: NIST calibrated light source that emits light. A mask is used so that only the active area of the pixel emits the beam. Place the photodiode at a given distance from the light and record the voltage value. From this, the voltage value corresponding to the special optical density (340 cd/m ² ) is obtained. Emissivity:

Emissivity was measured using a Newport photodiode (supplied by Newport Corporation of Irvine, CA). life:

For operational lifetime tests, the LEDs were sealed with epoxy and a glass cover. Life tests were performed in air on a single pixel in the device at constant current, 0.5 msec pulse, 0.5% cycle time, 5 mA per pixel. The time required for the pixel to decay to zero light output was measured using a UDT S370 vision meter with a calibrated photodiode.

Comparative Example A

Polymer LED devices were fabricated by spin-coating a solution of the polyaniline blend in air at 6,000 rpm on a partially ITO-coated glass substrate (the general method of preparation is described in US Patent 5,626,795). The resulting film was dried on a hot plate at 50°C for 30 minutes and then at 70°C overnight under vacuum. A toluene solution of Covion PDO 122 supplied by Covion Organic Semiconductors GmbH (Frankfurt, Germany) was spin-coated on the pAni film (in a nitrogen glove box) at 1,800 rpm. The film was vacuum dried at room temperature for 1 hour. A barium cathode was vapor deposited on Covion PDO 122 polymer film to a thickness of 30 Angstroms. On top of the barium layer a layer of aluminum was vapor deposited to a thickness of 3,000 Å.

Comparative Example B

A polymer LED device was fabricated as in Comparative Example A, except that the aluminum layer was replaced by a vapor-deposited silver layer having a thickness of 3,000 Å.

Example 1

A polymer LED device was fabricated as in Comparative Example A, except that the ITO was replaced by a vapor-deposited silver layer having a thickness of 300 Å.

Example 2

A polymer LED device was fabricated as in Comparative Example A, but with a 300 Å thick layer of silver vapor deposited on top of the ITO. The performance of the device was similar to that of Example 1 and Example 3 described below.

Example 3

A polymer LED device was fabricated as in Example 1 or Example 2, except that the aluminum layer was replaced by a vapor-deposited silver layer having a thickness of 3,000 Å. The efficiency of these devices was determined.

The properties of these devices are summarized in Table 1:

Table 1 Efficiency (cd/A) and operating voltage of the device at 0.3mA

Example Example Efficiency at 0.3mA Voltage (V)

(cd/A)

 Comparative Example A                                                                                                                           ,

 Comparative example B                                                                                

Example 1 5.9 10.2

Example 2 9.5 10.4

Table 1 above shows that replacing the ITO with a 300 Å silver layer (but leaving the aluminum in place) improves the light output somewhat, with a 12% increase in brightness (ie Comparative Example A vs. Example 1). However, the most significant improvement was obtained in the structural device described in Example 3, where silver anodes were used and silver was the high reflectivity metal used in the dual layer cathode structure. The device of Example 3 is more than 80% brighter than the device of Comparative Example A. Table 1 also shows that simply substituting silver for aluminum on the cathode side (Comparative Example A vs. B) while keeping the anode unchanged does not improve the efficiency of the device. In fact, the light output is also reduced.

The emission of the devices of Examples 1 and 3 was also measured in radiometric units (W/Sr/m ² ), which ignores human eye response effects and measures light output in absolute terms. The results are summarized in Table 2 below. Note that the radiation from devices fabricated according to the present invention (Example 3) was 2.5 times higher than that from devices fabricated with the same light-emitting polymer in a conventional polymer LED configuration.

Figure 4 shows the electroluminescence spectrum of the device from Example 3. Note the narrowing of the electroluminescence emission spectrum relative to the spectrum shown in Figure 3, even though the same light-emitting polymer was used in both devices. We believe that Covion PDO 122 is confined in the micropores of Example 3 resulting in narrowing of the emission spectrum.

        Table 2 Radiation rate of the device

Example Example Radiation rate at 0.3mA

(W/Sr/m ² )

     Comparative Example A                                        

Comparative Example B 28

Example 1 41

Example 3 102

Comparative Example C

A polymer LED device was fabricated as in Comparative Example A, but the semiconducting conjugated polymer used was Covion PDY 131 supplied by Covion Organic Semiconductors GmbH (Frankfurt, Germany), and the Covion PDY 131 film was spin-coated at 3,000 rpm. The thickness of the barium layer was 15 Å.

The electroluminescence spectrum of the device is shown in FIG. 5 .

Example 4

A polymer LED device was fabricated as in Example 3, but the semiconducting conjugated polymer used was Covion PDY 131, and the Covion PDY 131 film was spin-coated at 3,000 rpm. The thickness of the Ba layer was 15 Å.

The electroluminescence spectrum of the device is shown in FIG. 6 .

Efficiency (cd/A) and operating voltage of the device at 0.3mA in the embodiment of table 3

Example Example Efficiency (cd/A) Voltage (V)

Comparative Example C 10.8 8.9

Example 4 27.4 10

       Table 4 The emissivity of the device in the embodiment

Example Example Radiation rate at 0.3mA

(W/Sr/m ² )

Comparative Example C 36

Example 4 83

Tables 3 and 4 above show the results for the devices in Comparative Example C and Example 4. It is clear that the invention is not limited to one semiconducting polymer, since a significant improvement in light output was also achieved with another semiconducting polymer when the cathode cover metal was changed from aluminum to silver, and on the anode side ITO was replaced by silver. The device from Example 4 showed 2.5 times higher luminous efficiency than the device in Comparative Example C (Table 3).

The light output versus voltage (L-V) curves of the devices of Example 4 and Comparative Example C were determined, and the measured curves are shown in FIG. 7 . The data in Figure 7 demonstrates that the brightness of the device fabricated in Example 4 was significantly improved.

Table 5 below shows that the devices of Examples 1 and 3 had longer lifetimes and were more stable than the comparative device comprising an ITO anode layer.

Table 5 Device life under high current (stress) conditions

Example Example Time to zero light output (h)

     Comparative Example A                                                                       

    Example 1   Did not reach (15h) when the test was terminated

    Example 3   Did not reach (15h) when the test was terminated

The devices of Examples 1 and 3 and Comparative Example A were tested under what are considered to be very high current conditions (0.5 msec pulse, 0.5% cycle time, 5 mA per pixel) in order to accelerate the aging process and thus allow testing of many devices . Of the three devices, the device of Comparative Example A emitted the least light and dropped to zero the fastest (no light emission after 2.5 hours). The devices of Examples 1 and 3 show very different behavior during their lifetime. In each case, the brightness dropped to about 50% of the initial value and then remained at this level. In the case of the Example 1 and 3 devices, the point of failure could not be reached before the test was considered complete.

Thus, while ITO is generally preferred over silver as the anode layer, the present invention achieves a longer operating life than ITO devices with a silver structure.

Claims (10)

1. a light-emitting device (100), this device comprises an anode (120) and a negative electrode (140), described anode (120) comprises a semitransparent layer (122) with high reflectance and high work function, and described negative electrode (140) comprises first cathode layer (142) of the low work function material that at least one is selected from metal, metal oxide and bond thereof and second cathode layer (144) that at least one has high reflectance and high work function.

2. the device of claim 1, wherein semitransparent layer has the work function greater than 4eV.

3. the device of claim 1, wherein semitransparent layer comprises a kind of anode material that is selected from metal and metal alloy.

4. the device of claim 1, wherein second cathode layer has the work function greater than 4eV.

5. the device of claim 1, wherein second cathode layer comprises a kind of cathode material that is selected from metal and metal alloy.

6. the device of claim 1, wherein at least one device assembly that is selected from semitransparent layer and at least one second cathode layer has in the transmitted wave strong point and is at least 91.4% reflectivity.

7. the device of claim 1, wherein at least one device assembly that is selected from semitransparent layer and at least one second cathode layer has in 400-500nm transmitted wave strong point and is at least 86% reflectivity.

8. the device of claim 1, wherein at least one device assembly that is selected from semitransparent layer and at least one second cathode layer comprises silver.

9. the device of claim 1, wherein semitransparent layer has first surface adjacent with negative electrode (124) and opposing second surface (126), and anode further comprises a passivation layer (128) adjacent with first surface, and this passivation layer comprises a kind of passivating material that is selected from polyaniline, polyaniline blend, polythiophene and polythiophene blend.

10. the device of claim 1, wherein semitransparent layer has first surface and the opposing second surface adjacent with negative electrode, and anode further comprises an indium/tin-oxide hyaline layer adjacent with the second surface of semitransparent layer.

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