Patent Application
20090053559
The present invention relates to broadband light-producing OLED displays.
An organic light-emitting diode device, also called an OLED, commonly includes an anode, a cathode, and an organic electroluminescent (EL) unit sandwiched between the anode and the cathode. The organic EL unit includes at least a hole-transporting layer (HTL), a light-emitting layer (LEL), and an electron-transporting layer (ETL). OLEDs are attractive because of their low drive voltage, high luminance, wide viewing-angle, and capability for full color displays and for other applications. Tang et al. described this multilayer OLED in their U.S. Pat. Nos. 4,769,292 and 4,885,211.
OLEDs can emit different colors, such as red, green, blue, or white, depending on the emitting property of its LEL. Recently, there is an increasing demand for broadband OLEDs to be incorporated into various applications, such as a solid-state lighting source, color display, or a full color display. By broadband emission, it is meant that an OLED emits sufficiently broad light throughout the visible spectrum so that such light can be used in conjunction with filters or color change modules to produce displays with at least two different colors or a full color display.
In order to achieve broadband emission from an OLED, more than one type of molecule has to be excited, because each type of molecule only emits light with a relatively narrow spectrum under normal conditions. A light-emitting layer having a host material and one or more luminescent dopant(s) can achieve light emission from both the host and the dopant(s) resulting in a broadband emission in the visible spectrum if the energy transfer from the host material to the dopant(s) is incomplete. However, to achieve a broadband OLED having a single light-emitting layer, the concentrations of light-emitting dopants must be carefully controlled, which produces manufacturing difficulties. A broadband OLED having two or more light-emitting layers can have better color and better luminance efficiency than a device with one light-emitting layer, and the variability tolerance for dopant concentration is higher. It has also been found that broadband OLEDs having two light-emitting layers are typically more stable than OLEDs having a single light-emitting layer.
A problem sometimes seen with multilayer broadband emitters is hue shift. As the current flowing through the device is varied, the relative contributions of the different emitting layers can also vary, leading to changes in hue with current changes. This is undesirable; it is desirable for a light-emitting device to give constant hue and merely change the luminance intensity with current changes.
In addition, there remains a need to improve efficiency of OLED devices while maintaining good broadband emission.
It is therefore an object of the present invention to provide a broadband OLED device with improved efficiency and hue stability with current.
This object is achieved by an OLED device having two spaced electrodes, and comprising:
(a) a first light-emitting layer that produces green emission and includes:
(b) a second light-emitting layer that produces red emission and includes a host and a red light-emitting dopant.
It is an advantage of this invention that it provides a broadband light-emitting structure with good efficiency and low voltage requirements. It is a further advantage of this invention that it also provides a broadband light-emitting structure that shows very little hue change with changing current.
Since device feature dimensions such as layer thicknesses are frequently in sub-micrometer ranges, the drawings are scaled for ease of visualization rather than dimensional accuracy.
The term “OLED device” is used in its art-recognized meaning of a display device including organic light-emitting diodes as pixels. It can mean a device having a single pixel. The term “OLED display” as used herein means an OLED device including a plurality of pixels, which can be of different colors. A color OLED device emits light of at least one color. The term “full color” is employed to describe display panels that are capable of emitting in the red, green, and blue regions of the visible spectrum and displaying images in any combination of hues. The red, green, and blue colors constitute the three primary colors from which all other colors can be generated by appropriate mixing. The term “hue” refers to the intensity profile of light emission within the visible spectrum, with different hues exhibiting visually discernible differences in color. The term “pixel” is employed in its art-recognized usage to designate an area of a display panel that is stimulated to emit light independently of other areas. It is recognized that in full color systems, several pixels of different colors will be used together to produce a wide range of colors, and a viewer can term such a group a single pixel. For the purposes of this discussion, such a group will be considered several different colored pixels. In accordance with this disclosure, broadband emission is light that has significant components in multiple portions of the visible spectrum, for example, red and green. Broadband emission can also include the situation where light is emitted in the red, green, and blue portions of the spectrum in order to produce white light. White light is that light that is perceived by a user as having a white color, or light that has an emission spectrum sufficient to be used in combination with color filters to produce a practical full color display.
Turning now to
Light-emitting layers such as those described herein produce light in response to hole-electron recombination. Desired organic light-emitting materials can be deposited by any suitable way such as evaporation, sputtering, chemical vapor deposition, electrochemical process, or radiation thermal transfer from a donor material. The light-emitting layers in this invention include one or more host materials doped with one or more light-emitting guest compounds or dopants where light emission comes primarily from the dopant. A dopant is selected to produce color light having a particular spectrum and to have other desirable properties. Dopants are typically coated as 0.01 to 15% by weight into the host material.
First light-emitting layer 50gincludes an anthracene host and a 2,6-diaminoanthracene light-emitting dopant. The anthracene host is desirably a 9,10-diarylanthracene, certain derivatives of which (Formula A) are known to constitute a class of useful host materials capable of supporting electroluminescence, and are particularly suitable for light emission of wavelengths longer than 400 nm, e.g., blue, green, yellow, orange or red
wherein R1, R2, R3, and R4 represent one or more substituents on each ring where each substituent is individually selected from the following groups:
Group 1: hydrogen, or alkyl of from 1 to 24 carbon atoms;
Group 2: aryl or substituted aryl of from 5 to 20 carbon atoms;
Group 3: carbon atoms from 4 to 24 necessary to complete a fused aromatic ring of anthracenyl, pyrenyl, or perylenyl;
Group 4: heteroaryl or substituted heteroaryl of from 5 to 24 carbon atoms as necessary to complete a fused heteroaromatic ring of furyl, thienyl, pyridyl, quinolinyl or other heterocyclic systems;
Group 5: alkoxylamino, alkylamino, or arylamino of from 1 to 24 carbon atoms; and
Group 6: fluorine or cyano.
Particularly useful are compounds wherein R1 and R2, and in some cases R3, represent additional aromatic rings. Specific examples of useful anthracene materials for use as a host in a light-emitting layer include:
In another suitable embodiment, the 9,10-diarylanthracene host (A) is desirably a 2,9,10-triarylanthracene, certain derivatives of which (Formula B) are known to constitute a class of useful host materials capable of supporting electroluminescence, and are particularly suitable for light emission of wavelengths longer than 400 nm. e g., blue, green, yellow, orange or red
wherein d1 and d3-d8 can be the same or different and each represents hydrogen or an independently selected substituent, and each g can be the same or different and each represents an independently selected substituent, provided that two substituents can combine to form a ring group and p, r, and s are independently 0-5.
Specific examples of useful 2,9,10-triarylanthracene materials for use as a host in a light-emitting layer include structures A10 and A11, above. Further examples have been described by Klubek et al. in above-cited U.S. patent application Ser. No. 11/668,515, the disclosure of which is incorporated herein by reference.
In another suitable embodiment, the 2,9,10-triarylanthracene host (B) is desirably a 2,6,9,10-tetrarylanthracene, certain derivatives of which (Formula C) are known to constitute a class of useful host materials capable of supporting electroluminescence, and are particularly suitable for light emission of wavelengths longer than 400 nm, e.g., blue, green, yellow, orange or red.
wherein d1, d3-d5 and d7-d8 can be the same or different and each represents hydrogen or an independently selected substituent, and each g can be the same or different and each represents an independently selected substituent, provided that two substituents can combine to form a ring group and p, r, s and t are independently 0-5.
Specific examples of useful 2,6,9,10-tetraarylanthracene materials for use as a host in a light-emitting layer have been described by Klubek et al in above-cited U.S. patent application Ser. No. 11/668,515, the disclosure of which is incorporated herein by reference.
Particularly useful in this invention are 2-aryl-9,10-bis(2-naphthyl)anthracenes, e.g. structures A10 and A11.
In addition to a host material as described above, first light-emitting layer 50g also includes a 2,6-diaminoanthracene light-emitting dopant, as represented by the formula below:
wherein d1, d3-d5, and d7-d10can be the same or different and each represents hydrogen or an independently selected substituent, and each h can be the same or different and each represents one or more independently selected substituents, provided that two substituents can combine to form a ring group and a-d are independently 0-5. One useful embodiment of this is structure E:
wherein i and j are independently 0-5.
In another suitable embodiment, the 2,6-diaminoanthracene light-emitting dopant is a 2,6,9-triaminoanthracene light-emitting dopant represented by formula F:
wherein d1, d3-d5, and d7-d8 can be the same or different and each represents hydrogen or an independently selected substituent, and each h can be the same or different and each represents an independently selected substituent, provided that two substituents can combine to form a ring group, and a-f and j are independently 0-5.
In another suitable embodiment, the 2,6-diamionanthracene light-emitting dopant is a 2,6,9,10-tetraaminoanthracene light-emitting dopant represented by formula G:
wherein d1, d3-d5, and d7-d8 can be the same or different and each represents hydrogen or an independently selected substituent, and each h can be the same or different and each represents an independently selected substituent, provided that two substituents can combine to form a ring group, and a-h are independently 0-5. Diaminoanthracenes of structures D through G have been described in detail by Klubek et al in above-cited U.S. patent application Ser. No. 11/668,515, the disclosure of which is incorporated herein by reference. Diaminoanthracene dopants can be present in a concentration range of from 1% to 25% by volume, desirably 2% to 15% by volume, and usefully 3% to 10% by volume.
Also useful as light-emitting dopants are 9,10-diaminoanthracene derivatives:
wherein d1-d8 can be the same or different and each represents hydrogen or an independently selected substituent and each h can be the same or different and each represents one or more independently selected substituents, provided that two substituents can combine to form a ring group and a-d are independently 0-5.
First light-emitting layer 50g can optionally include a small amount of a blue light-emitting dopant as a stabilizer. The presence of a blue light-emitting compound, which is a higher-energy dopant, provides greater luminance stability to the green emission of diaminoanthracene dopants, while maintaining good efficiency of the green light-emitting dopants. Blue light-emitting compounds can be those described below for blue light-emitting layer 50b. The blue light-emitting compound can be present in the concentration range of 0.2% to 10% by volume, and usefully from 1% to 5% by volume.
As a stabilizer, one can instead use a small amount of a second green light-emitting dopant. For example, one can use a small amount of a quinacridone compound, e.g. a compound of the following structure:
wherein substituent groups R1 and R2 are independently alkyl, alkoxyl, aryl, or heteroaryl; and substituent groups R3 through R12 are independently hydrogen, alkyl, alkoxyl, halogen, aryl, or heteroaryl, and adjacent substituent groups R3 through R10can optionally be connected to form one or more ring systems, including fused aromatic and fused heteroaromatic rings, provided that the substituents are selected to provide an emission maximum between 510 nm and 540 nm. Alkyl, alkoxyl, aryl, heteroaryl, fused aromatic ring and fused heteroaromatic ring substituent groups can be further substituted. Some examples of useful quinacridones include those disclosed in U.S. Pat. No. 5,593,788 and in U.S. Patent Application Publication 2004/0001969A1.
Examples of useful quinacridone green dopants include:
Second light-emitting layer 50r includes one or more host materials and one or more red light-emitting dopants. A red-light-emitting dopant can include a diindenoperylene compound of the following structure J:
wherein:
Illustrative examples of useful red dopants of this class are shown by Hatwar et al. in commonly-assigned U.S. Pat. No. 7,247,394, the disclosure of which is incorporated by reference.
Some other red dopants belong to the DCM class of dyes represented by Formula K:
wherein Y1-Y5 represent one or more groups independently selected from: hydro, alkyl, substituted alkyl, aryl, or substituted aryl; Y1-Y5 independently include acyclic groups or can be joined pairwise to form one or more fused rings; provided that Y3 and Y5 do not together form a fused ring. Structures of particularly useful dopants of the DCM class are shown by Ricks et al. in commonly-assigned U.S. Pat. No. 7,252,893, the disclosure of which is incorporated by reference.
The host material in second light-emitting layer 50r includes a hole-transporting material. Hole-transporting materials useful as hosts in light-emitting layers are well known to include compounds such as an aromatic tertiary amine, where the latter is understood to be a compound containing at least one trivalent nitrogen atom that is bonded only to carbon atoms, at least one of which is a member of an aromatic ring. In one form the aromatic tertiary amine can be an arylamine, such as a monoarylamine, diarylamine, triarylamine, or a polymeric arylamine. Exemplary monomeric triarylamines are illustrated by Klupfel et al. in U.S. Pat. No. 3,180,730. Other suitable triarylamines substituted with one or more vinyl radicals and/or including at least one active hydrogen-containing group are disclosed by Brantley et al. in U.S. Pat. Nos. 3,567,450 and 3,658,520.
A more preferred class of aromatic tertiary amines are those which include at least two aromatic tertiary amine moieties as described in U.S. Pat. Nos. 4,720,432 and 5,061,569. Such compounds include those represented by structural Formula L.
wherein:
Q1 and Q2 are independently selected aromatic tertiary amine moieties; and
G is a linking group such as an arylene, cycloalkylene, or alkylene group of a carbon to carbon bond.
One class of such aromatic tertiary amines are the tetraaryldiamines. Desirable tetraaryldiamines include two diarylamino groups linked through an arylene group. Useful tetraaryldiamines include those represented by Formula M.
wherein:
each Are is an independently selected arylene group, such as a phenylene or anthracene moiety;
n is an integer of from 1 to 4; and
Ar, R7, R8, and R9 are independently selected aryl groups.
The various alkyl, alkylene, aryl, and arylene moieties of the foregoing structural Formulae L and M can each in turn be substituted. Typical substituents include alkyl groups, alkoxy groups, aryl groups, aryloxy groups, and halogens such as fluoride, chloride, and bromide. The various alkyl and alkylene moieties typically contain from 1 to about 6 carbon atoms. The cycloalkyl moieties can contain from 3 to about 10 carbon atoms, but typically contain five, six, or seven carbon atoms—e.g., cyclopentyl, cyclohexyl, and cycloheptyl ring structures. The aryl and arylene moieties are usually phenyl and phenylene moieties. Usefully, the hole-transporting host material is an N,N,N′,N′-tetraarylbenzidine, wherein the Are of Formula M represents a phenylene group and n equals 2.
Second light-emitting layer 50r can further include a 9,10-diarylanthracene derivative, as described above, as a co-host. The 9,10-diarylanthracene derivative can be present in the concentration range of from 1% to 95% by volume, and usefully from 25% to 75% by volume.
Second light-emitting layer 50r can further include a yellow light-emitting dopant. A light-emitting yellow dopant can include a compound of the following structures:
wherein A1-A6 and A′1-A′6 represent one or more substituents on each ring and where each substituent is individually selected from one of the following:
Examples of particularly useful yellow dopants are shown by Ricks et al in commonly-assigned U.S. Pat. No. 7,252,893.
Other OLED device layers that can be used in this invention have been well described in the art, and OLED device 10, and other such devices described herein, can include layers commonly used for such devices. In addition to the layers and materials described below, useful materials have been described in detail elsewhere, e.g. by Klubek et al. in above-cited U.S. patent application Ser. No. 11/668,515, the contents of which are herein incorporated by reference. OLED devices are commonly formed on a substrate, e.g. OLED substrate 20. Such substrates have been well-described in the art. A bottom electrode is formed over OLED substrate 20 and is most commonly configured as an anode 30, although the practice of this invention is not limited to this configuration. When EL emission is viewed through the anode, the anode should be transparent, or substantially transparent, to the emission of interest. Common transparent anode materials used in the present invention are indium-tin oxide (ITO), indium-zinc oxide (IZO) and tin oxide, but other metal oxides can work including, but not limited to, aluminum- or indium-doped zinc oxide, magnesium-indium oxide, and nickel-tungsten oxide. In addition to these oxides, metal nitrides such as gallium nitride, and metal selenides such as zinc selenide, and metal sulfides such as zinc sulfide, can be used as the anode. For applications where EL emission is viewed only through the cathode electrode, the transmissive characteristics of the anode are immaterial and many conductive materials can be used, regardless if transparent, opaque, or reflective. Example conductors for the present invention include, but are not limited to, gold, iridium, molybdenum, palladium, and platinum. Typical anode materials, transmissive or otherwise, have a work function no less than 4.0 eV. Desired anode materials can be deposited by any suitable way such as evaporation, sputtering, chemical vapor deposition, or electrochemical process. Anode materials can be patterned using well-known photolithographic processes.
A hole-transporting layer 40 can be formed and disposed over the anode. Hole-transporting layer 40 can include any hole-transporting material useful in OLED devices, many examples of which are known to those skilled in the art. Some examples of useful hole-transporting materials were described above as hole-transporting host materials, but hole-transporting layer 40 is not limited to these. Desired hole-transporting materials can be deposited by any suitable way such as evaporation, sputtering, chemical vapor deposition, electrochemical process, thermal transfer, or laser thermal transfer from a donor material.
An electron-transporting layer 55 can include any electron-transporting material useful in OLED devices, many examples of which are known to those skilled in the art. Electron-transporting layer 55 can contain one or more metal chelated oxinoid compounds, including chelates of oxine itself, also commonly referred to as 8-quinolinol or 8-hydroxyquinoline. Other electron-transporting materials include various butadiene derivatives as disclosed in U.S. Pat. No. 4,356,429 and various heterocyclic optical brighteners as described in U.S. Pat. No. 4,539,507. Benzazoles, oxadiazoles, triazoles, pyridinethiadiazoles, triazines, phenanthroline derivatives, and some silole derivatives are also useful electron-transporting materials. Other electron-transporting materials have been described by Klubek et al in above-cited U.S. patent application Ser. No. 11/668,515, the disclosure of which is incorporated herein by reference.
An upper electrode most commonly configured as a cathode 90 is formed over the electron-transporting layer 55. If the device is top-emitting, the electrode must be transparent or nearly transparent. For such applications, metals must be thin (preferably less than 25 nm) or one must use transparent conductive oxides (e.g. indium-tin oxide, indium-zinc oxide), or a combination of these materials. Optically transparent cathodes have been described in more detail in U.S. Pat. No. 5,776,623. Cathode materials can be deposited by evaporation, sputtering, or chemical vapor deposition. When needed, patterning can be achieved through many well known methods including, but not limited to, through-mask deposition, integral shadow masking as described in U.S. Pat. No. 5,276,380 and EP 0 732 868, laser ablation, and selective chemical vapor deposition.
OLED device 10 can include other layers as well. For example, a hole-injecting layer 35 can be formed over the anode, as described in U.S. Pat. No. 4,720,432, U.S. Pat. No. 6,208,075, EP 0 891 121 A1, and EP 1029 909 A1. Also useful as disclosed by Son et al., in U.S. Pat. No. 6,720,573, is a material of the general structure:
Other hole-injecting materials have been described by Klubek et al in above-cited U.S. patent application Ser. No. 11/668,515.
An electron-injecting layer, such as alkaline or alkaline earth metals, alkali halide salts, or alkaline or alkaline earth metal doped organic layers, can also be present between cathode 90 and electron-transporting layer 55. Other electron-injecting materials have been described by Klubek et al in above-cited U.S. patent application Ser. No. 11/668,515.
Turning now to
Turning now to
wherein:
Some examples of the above class of dopants are disclosed by Ricks et al in commonly-assigned U.S. Pat. No. 7,252,893.
Another class of blue dopants is the perylene class. Particularly useful blue dopants of the perylene class include perylene and tetra-t-butylperylene (TBP).
Another class of blue dopants includes blue-emitting derivatives of such styrylarenes and distyrylarenes as distyrylbenzene, styrylbiphenyl, and distyrylbiphenyl, including compounds described in U.S. Pat. No. 5,121,029, and U.S. Patent Application Publication No. 2006/0093856 by Helber et al. Among such derivatives that provide blue luminescence, particularly useful are those substituted with diarylamino groups. Examples include bis[2-[4-[N,N-diarylamino]phenyl]vinyl]-benzenes of the general structure R1 shown below:
[N,N-diarylamino][2-[4-[N,N-diarylamino]phenyl]vinyl]biphenyls of the general structure R2 shown below:
and bis[2-[4-[N,N-diarylamino]phenyl]vinyl]biphenyls of the general structure R3 shown below:
In Formulas R1 to R3, X1-X4 can be the same or different, and individually represent one or more substituents such as alkyl, aryl, fused aryl, halo, or cyano. In a preferred embodiment, X1-X4 are individually alkyl groups, each containing from one to about ten carbon atoms. A particularly preferred blue dopant of this class is disclosed by Ricks et al in commonly-assigned U.S. Pat. No. 7,252,893.
The invention and its advantages can be better appreciated by the following comparative examples. Examples 2 to 10 are representative examples of this invention, while Example 1 is a non-inventive OLED device example for comparison purposes. The layers described as vacuum-deposited were deposited by evaporation from heated boats under a vacuum of approximately 10−6 Torr. After deposition of the OLED layers each device was then transferred to a dry box for encapsulation. The OLED has an emission area of 10 mm2. The devices were tested by applying a current of 20 mA/cm2 across electrodes, except that the fade stability was tested at 80 mA/cm2. The results from Examples 1 to 10 are given in Table 1.
An OLED device was constructed as described above for Example 1 except that Step 6 was as follows:
An OLED device was constructed as described above for Example 3 except that Step 6 was as follows:
An OLED device was constructed as described above for Example 3 except that Step 5 was as follows:
An OLED device was constructed as described above for Example 3 except that Step 5 was as follows:
An OLED device was constructed as described above for Example 6 except that 39.5% NPB and 60% NNA were used as co-hosts in Step 5.
An OLED device was constructed as described above for Example 8 except that 24% NPB and 74% NNA were used in Step 6.
An OLED device was constructed as described above for Example 8 except that 91.5% 2,9,10-tris(2-naphthyl)anthracene was used instead of PBNA as the host in Step 7.
The results of testing these examples are shown in Table 1, below. The inventive examples (2 through 10) show, relative to the comparative example, improved luminance efficiency, quantum efficiency, and resistance to hue change with varying current. They further show similar voltage requirements.
The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.
Reference is made to commonly assigned U.S. patent application Ser. No. 11/668,515 filed Jan. 30, 2007, entitled “OLEDS Having High Efficiency and Excellent Lifetime” by Kevin P. Klubek et al, the disclosure of which is incorporated herein by reference.
