Patent Publication Number: US-11641774-B2

Title: Organic electroluminescent materials and devices

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of co-pending U.S. patent application Ser. No. 14/863,768, filed Sep. 24, 2015, which claims priority under 35 U.S.C. § 119(e) to U.S. Provisional Applications No. 62/056,940, filed on Sep. 29, 2014, the entire contents of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to organic light emitting devices (OLEDs), and more specifically to organic materials used in such devices. More specifically, the present invention relates to a novel evaporation source comprising a mixture of two organic compounds that allows stable co-evaporation of the two organic compounds in fabrication of various layers in phosphorescent organic light emitting devices (PHOLEDs). 
     BACKGROUND 
     Opto-electronic devices that make use of organic materials are becoming increasingly desirable for a number of reasons. Many of the materials used to make such devices are relatively inexpensive, so organic opto-electronic devices have the potential for cost advantages over inorganic devices. In addition, the inherent properties of organic materials, such as their flexibility, may make them well suited for particular applications such as fabrication on a flexible substrate. Examples of organic opto-electronic devices include organic light emitting devices (OLEDs), organic phototransistors, organic photovoltaic cells, and organic photodetectors. For OLEDs, the organic materials may have performance advantages over conventional materials. For example, the wavelength at which an organic emissive layer emits light may generally be readily tuned with appropriate dopants. 
     OLEDs make use of thin organic films that emit light when voltage is applied across the device. OLEDs are becoming an increasingly interesting technology for use in applications such as flat panel displays, illumination, and backlighting. Several OLED materials and configurations are described in U.S. Pat. Nos. 5,844,363, 6,303,238, and 5,707,745, which are incorporated herein by reference in their entirety. 
     One application for phosphorescent emissive molecules is a full color display. Industry standards for such a display call for pixels adapted to emit particular colors, referred to as “saturated” colors. In particular, these standards call for saturated red, green, and blue pixels. Color may be measured using CIE coordinates, which are well known to the art. 
     One example of a green emissive molecule is tris(2-phenylpyridine) iridium, denoted Ir(ppy) 3 , which has the following structure: 
     
       
         
         
             
             
         
       
     
     In this, and later figures herein, we depict the dative bond from nitrogen to metal (here, Ir) as a straight line. 
     As used herein, the term “organic” includes polymeric materials as well as small molecule organic materials that may be used to fabricate organic opto-electronic devices. “Small molecule” refers to any organic material that is not a polymer, and “small molecules” may actually be quite large. Small molecules may include repeat units in some circumstances. For example, using a long chain alkyl group as a substituent does not remove a molecule from the “small molecule” class. Small molecules may also be incorporated into polymers, for example as a pendent group on a polymer backbone or as a part of the backbone. Small molecules may also serve as the core moiety of a dendrimer, which consists of a series of chemical shells built on the core moiety. The core moiety of a dendrimer may be a fluorescent or phosphorescent small molecule emitter. A dendrimer may be a “small molecule,” and it is believed that all dendrimers currently used in the field of OLEDs are small molecules. 
     As used herein, “top” means furthest away from the substrate, while “bottom” means closest to the substrate. Where a first layer is described as “disposed over” a second layer, the first layer is disposed further away from substrate. There may be other layers between the first and second layer, unless it is specified that the first layer is “in contact with” the second layer. For example, a cathode may be described as “disposed over” an anode, even though there are various organic layers in between. 
     As used herein, “solution processible” means capable of being dissolved, dispersed, or transported in and/or deposited from a liquid medium, either in solution or suspension form. 
     A ligand may be referred to as “photoactive” when it is believed that the ligand directly contributes to the photoactive properties of an emissive material. A ligand may be referred to as “ancillary” when it is believed that the ligand does not contribute to the photoactive properties of an emissive material, although an ancillary ligand may alter the properties of a photoactive ligand. 
     As used herein, and as would be generally understood by one skilled in the art, a first “Highest Occupied Molecular Orbital” (HOMO) or “Lowest Unoccupied Molecular Orbital” (LUMO) energy level is “greater than” or “higher than” a second HOMO or LUMO energy level if the first energy level is closer to the vacuum energy level. Since ionization potentials (IP) are measured as a negative energy relative to a vacuum level, a higher HOMO energy level corresponds to an IP having a smaller absolute value (an IP that is less negative). Similarly, a higher LUMO energy level corresponds to an electron affinity (EA) having a smaller absolute value (an EA that is less negative). On a conventional energy level diagram, with the vacuum level at the top, the LUMO energy level of a material is higher than the HOMO energy level of the same material. A “higher” HOMO or LUMO energy level appears closer to the top of such a diagram than a “lower” HOMO or LUMO energy level. 
     As used herein, and as would be generally understood by one skilled in the art, a first work function is “greater than” or “higher than” a second work function if the first work function has a higher absolute value. Because work functions are generally measured as negative numbers relative to vacuum level, this means that a “higher” work function is more negative. On a conventional energy level diagram, with the vacuum level at the top, a “higher” work function is illustrated as further away from the vacuum level in the downward direction. Thus, the definitions of HOMO and LUMO energy levels follow a different convention than work functions. 
     More details on OLEDs, and the definitions described above, can be found in U.S. Pat. No. 7,279,704, which is incorporated herein by reference in its entirety. 
     SUMMARY OF THE INVENTION 
     The present disclosure provides a first mixture containing three different compounds that is useful as a stable co-evaporation source material for a vacuum deposition tool. The first mixture comprises: a first compound; a second compound; and a third compound. The first compound, the second compound, and the third compound are all organic compounds and have different chemical structures from each other. The first compound, the second compound, and the third compound each has an evaporation temperature T1, T2, and T3, respectively, and is in the range of 150 to 350° C., wherein T1, T2, and T3 differ from each other by less than 20° C. The first compound has a concentration C1 in the first mixture and a concentration C2 in a film deposited by evaporating the first mixture in a high vacuum deposition tool with a chamber base pressure between 1×10 −6  Torr to 1×10 −9  Torr under a first deposition condition which is defined as depositing at a 2 Å/sec deposition rate onto a surface positioned at a predefined distance from the first mixture evaporation source, wherein |C1-C2)/C1| is less than 5%. The first compound has a concentration C1′ in a second mixture of the first and second compounds or has a concentration C1″ in a third mixture of the first and third compounds, and the first compound has a concentration C2′ in a film formed by evaporating the second mixture under the first deposition condition or has a concentration C2″ in a film formed by evaporating the third mixture under the first deposition condition, and at least one of |(C1′-C2′)/C1′| and |(C1″-C2″)/C1″| is greater than 5%. 
     According to an embodiment, a method of fabricating a first device is disclosed. The method comprises: providing a first container that contains a first mixture, the first mixture comprising: a first compound; a second compound; and a third compound, wherein the first compound, the second compound, and the third compound are all organic compounds and have different chemical structures from each other, wherein the first compound, the second compound, and the third compound each has an evaporation temperature T1, T2, and T3, respectively, and is in the range of 150 to 350° C., wherein the T1, T2, and T3 differ from each other by less than 20° C.; providing a substrate having a first electrode disposed thereon; depositing an organic layer over the first electrode by evaporating the first mixture in the first container in a high vacuum deposition tool under a first deposition condition which is defined as depositing at a 2 Å/sec deposition rate with a chamber base pressure between 1×10 −6  Torr to 1×10 −9  Torr onto a surface positioned at a predefined distance from the first mixture, wherein the first compound has a concentration C1 in the first mixture and a concentration C2 in the emissive layer and |(C1-C2)/C1| is less than 5%; and depositing a second electrode over the emissive layer. 
     According to an embodiment of the present disclosure, a first device comprising a first organic light emitting device is also disclosed. The first organic light emitting device comprises: an anode; a cathode; and an organic layer, disposed between the anode and the cathode, comprising a mixture of a first compound, a second compound, and a third compound, 
     wherein the first compound, the second compound, and the third compound are all organic compounds and have different chemical structures from each other, 
     wherein the first compound, the second compound, and the third compound each has an evaporation temperature T1, T2, and T3, respectively, and is in the range of 150 to 350° C., 
     wherein the T1, T2, and T3 differ from each other by less than 20° C., 
     wherein the first compound has a concentration C1 in the first mixture and a concentration C2 in a film deposited by evaporating the first mixture in a high vacuum deposition tool under a first deposition condition which is defined as depositing at a 2 Å/sec deposition rate with a chamber base pressure between 1×10 −6  Torr to 1×10 −9  Torr onto a surface positioned at a predefined distance from the first mixture, 
     wherein |(C1-C2)/C1| is less than 5%, 
     wherein the first compound has a concentration C1′ in a second mixture of the first and second compounds or has a concentration C1″ in a third mixture of the first and third compounds, and the first compound has a concentration C2′ in a film formed by evaporating the second mixture under the first deposition condition or has a concentration C2″ in a film formed by evaporating the third mixture under the first deposition condition, and 
     wherein at least one of |(C1′-C2′)/C1′| and |(C1″-C2″)/C1″| is greater than 5%. 
     In fabricating OLEDs, the disclosed first mixture can be deposited as a thin film by thermal vapor deposition where the first mixture is used as a single-source co-evaporation material. This allows for a simpler OLED device fabrication process. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    shows an organic light emitting device that can incorporate the inventive host material disclosed herein. 
         FIG.  2    shows an inverted organic light emitting device that can incorporate the inventive host material disclosed herein. 
         FIG.  3    shows HPLC composition (%) evolution of Compound C74 in sequentially deposited films from premixture BPM1. 
         FIG.  4    shows HPLC composition (%) evolution of Compound C74 in sequentially deposited films from premixture TPM1. 
         FIG.  5    shows HPLC composition (%) evolution of Compound E2 in sequentially deposited films from premixture BPM2. 
         FIG.  6    shows HPLC composition (%) evolution of Compound E2 in sequentially deposited films from premixture TPM2. 
     
    
    
     DETAILED DESCRIPTION 
     Generally, an OLED comprises at least one organic layer disposed between and electrically connected to an anode and a cathode. When a current is applied, the anode injects holes and the cathode injects electrons into the organic layer(s). The injected holes and electrons each migrate toward the oppositely charged electrode. When an electron and hole localize on the same molecule, an “exciton,” which is a localized electron-hole pair having an excited energy state, is formed. Light is emitted when the exciton relaxes via a photoemissive mechanism. In some cases, the exciton may be localized on an excimer or an exciplex. Non-radiative mechanisms, such as thermal relaxation, may also occur, but are generally considered undesirable. 
     The initial OLEDs used emissive molecules that emitted light from their singlet states (“fluorescence”) as disclosed, for example, in U.S. Pat. No. 4,769,292, which is incorporated by reference in its entirety. Fluorescent emission generally occurs in a time frame of less than 10 nanoseconds. 
     More recently, OLEDs having emissive materials that emit light from triplet states (“phosphorescence”) have been demonstrated. Baldo et al., “Highly Efficient Phosphorescent Emission from Organic Electroluminescent Devices,” Nature, vol. 395, 151-154, 1998; (“Baldo-I”) and Baldo et al., “Very high-efficiency green organic light-emitting devices based on electrophosphorescence,” Appl. Phys. Lett., vol. 75, No. 3, 4-6 (1999) (“Baldo-II”), which are incorporated by reference in their entireties. Phosphorescence is described in more detail in U.S. Pat. No. 7,279,704 at cols. 5-6, which are incorporated by reference. 
       FIG.  1    shows an organic light emitting device  100 . The figures are not necessarily drawn to scale. Device  100  may include a substrate  110 , an anode  115 , a hole injection layer  120 , a hole transport layer  125 , an electron blocking layer  130 , an emissive layer  135 , a hole blocking layer  140 , an electron transport layer  145 , an electron injection layer  150 , a protective layer  155 , a cathode  160 , and a barrier layer  170 . Cathode  160  is a compound cathode having a first conductive layer  162  and a second conductive layer  164 . Device  100  may be fabricated by depositing the layers described, in order. The properties and functions of these various layers, as well as example materials, are described in more detail in U.S. Pat. No. 7,279,704 at cols. 6-10, which are incorporated by reference. 
     More examples for each of these layers are available. For example, a flexible and transparent substrate-anode combination is disclosed in U.S. Pat. No. 5,844,363, which is incorporated by reference in its entirety. An example of a p-doped hole transport layer is m-MTDATA doped with F 4 -TCNQ at a molar ratio of 50:1, as disclosed in U.S. Patent Application Publication No. 2003/0230980, which is incorporated by reference in its entirety. Examples of emissive and host materials are disclosed in U.S. Pat. No. 6,303,238 to Thompson et al., which is incorporated by reference in its entirety. An example of an n-doped electron transport layer is BPhen doped with Li at a molar ratio of 1:1, as disclosed in U.S. Patent Application Publication No. 2003/0230980, which is incorporated by reference in its entirety. U.S. Pat. Nos. 5,703,436 and 5,707,745, which are incorporated by reference in their entireties, disclose examples of cathodes including compound cathodes having a thin layer of metal such as Mg:Ag with an overlying transparent, electrically-conductive, sputter-deposited ITO layer. The theory and use of blocking layers is described in more detail in U.S. Pat. No. 6,097,147 and U.S. Patent Application Publication No. 2003/0230980, which are incorporated by reference in their entireties. Examples of injection layers are provided in U.S. Patent Application Publication No. 2004/0174116, which is incorporated by reference in its entirety. A description of protective layers may be found in U.S. Patent Application Publication No. 2004/0174116, which is incorporated by reference in its entirety. 
       FIG.  2    shows an inverted OLED  200 . The device includes a substrate  210 , a cathode  215 , an emissive layer  220 , a hole transport layer  225 , and an anode  230 . Device  200  may be fabricated by depositing the layers described, in order. Because the most common OLED configuration has a cathode disposed over the anode, and device  200  has cathode  215  disposed under anode  230 , device  200  may be referred to as an “inverted” OLED. Materials similar to those described with respect to device  100  may be used in the corresponding layers of device  200 .  FIG.  2    provides one example of how some layers may be omitted from the structure of device  100 . 
     The simple layered structure illustrated in  FIGS.  1  and  2    is provided by way of non-limiting example, and it is understood that embodiments of the invention may be used in connection with a wide variety of other structures. The specific materials and structures described are exemplary in nature, and other materials and structures may be used. Functional OLEDs may be achieved by combining the various layers described in different ways, or layers may be omitted entirely, based on design, performance, and cost factors. Other layers not specifically described may also be included. Materials other than those specifically described may be used. Although many of the examples provided herein describe various layers as comprising a single material, it is understood that combinations of materials, such as a mixture of host and dopant, or more generally a mixture, may be used. Also, the layers may have various sublayers. The names given to the various layers herein are not intended to be strictly limiting. For example, in device  200 , hole transport layer  225  transports holes and injects holes into emissive layer  220 , and may be described as a hole transport layer or a hole injection layer. In one embodiment, an OLED may be described as having an “organic layer” disposed between a cathode and an anode. This organic layer may comprise a single layer, or may further comprise multiple layers of different organic materials as described, for example, with respect to  FIGS.  1  and  2   . 
     Structures and materials not specifically described may also be used, such as OLEDs comprised of polymeric materials (PLEDs) such as disclosed in U.S. Pat. No. 5,247,190 to Friend et al., which is incorporated by reference in its entirety. By way of further example, OLEDs having a single organic layer may be used. OLEDs may be stacked, for example as described in U.S. Pat. No. 5,707,745 to Forrest et al, which is incorporated by reference in its entirety. The OLED structure may deviate from the simple layered structure illustrated in  FIGS.  1  and  2   . For example, the substrate may include an angled reflective surface to improve out-coupling, such as a mesa structure as described in U.S. Pat. No. 6,091,195 to Forrest et al., and/or a pit structure as described in U.S. Pat. No. 5,834,893 to Bulovic et al., which are incorporated by reference in their entireties. 
     Unless otherwise specified, any of the layers of the various embodiments may be deposited by any suitable method. For the organic layers, preferred methods include thermal evaporation, ink-jet, such as described in U.S. Pat. Nos. 6,013,982 and 6,087,196, which are incorporated by reference in their entireties, organic vapor phase deposition (OVPD), such as described in U.S. Pat. No. 6,337,102 to Forrest et al., which is incorporated by reference in its entirety, and deposition by organic vapor jet printing (OVJP), such as described in U.S. Pat. No. 7,431,968, which is incorporated by reference in its entirety. Other suitable deposition methods include spin coating and other solution based processes. Solution based processes are preferably carried out in nitrogen or an inert atmosphere. For the other layers, preferred methods include thermal evaporation. Preferred patterning methods include deposition through a mask, cold welding such as described in U.S. Pat. Nos. 6,294,398 and 6,468,819, which are incorporated by reference in their entireties, and patterning associated with some of the deposition methods such as ink jet and OVJD. Other methods may also be used. The materials to be deposited may be modified to make them compatible with a particular deposition method. For example, substituents such as alkyl and aryl groups, branched or unbranched, and preferably containing at least 3 carbons, may be used in small molecules to enhance their ability to undergo solution processing. Substituents having 20 carbons or more may be used, and 3-20 carbons is a preferred range. Materials with asymmetric structures may have better solution processibility than those having symmetric structures, because asymmetric materials may have a lower tendency to recrystallize. Dendrimer substituents may be used to enhance the ability of small molecules to undergo solution processing. 
     Devices fabricated in accordance with embodiments of the present invention may further optionally comprise a barrier layer. One purpose of the barrier layer is to protect the electrodes and organic layers from damaging exposure to harmful species in the environment including moisture, vapor and/or gases, etc. The barrier layer may be deposited over, under or next to a substrate, an electrode, or over any other parts of a device including an edge. The barrier layer may comprise a single layer, or multiple layers. The barrier layer may be formed by various known chemical vapor deposition techniques and may include compositions having a single phase as well as compositions having multiple phases. Any suitable material or combination of materials may be used for the barrier layer. The barrier layer may incorporate an inorganic or an organic compound or both. The preferred barrier layer comprises a mixture of a polymeric material and a non-polymeric material as described in U.S. Pat. No. 7,968,146, PCT Pat. Application Nos. PCT/US2007/023098 and PCT/US2009/042829, which are herein incorporated by reference in their entireties. To be considered a “mixture”, the aforesaid polymeric and non-polymeric materials comprising the barrier layer should be deposited under the same reaction conditions and/or at the same time. The weight ratio of polymeric to non-polymeric material may be in the range of 95:5 to 5:95. The polymeric material and the non-polymeric material may be created from the same precursor material. In one example, the mixture of a polymeric material and a non-polymeric material consists essentially of polymeric silicon and inorganic silicon. 
     Devices fabricated in accordance with embodiments of the invention may be incorporated into a wide variety of consumer products, including flat panel displays, computer monitors, medical monitors, televisions, billboards, lights for interior or exterior illumination and/or signaling, heads up displays, fully transparent displays, flexible displays, laser printers, telephones, cell phones, personal digital assistants (PDAs), laptop computers, digital cameras, camcorders, viewfinders, micro-displays, 3-D displays, vehicles, a large area wall, theater or stadium screen, or a sign. Various control mechanisms may be used to control devices fabricated in accordance with the present invention, including passive matrix and active matrix. Many of the devices are intended for use in a temperature range comfortable to humans, such as 18° C. to 30° C., and more preferably at room temperature (20-25° C.), but could be used outside this temperature range, for example, from −40° C. to +80° C. 
     The materials and structures described herein may have applications in devices other than OLEDs. For example, other optoelectronic devices such as organic solar cells and organic photodetectors may employ the materials and structures. More generally, organic devices, such as organic transistors, may employ the materials and structures. 
     The term “halo” or “halogen” as used herein includes fluorine, chlorine, bromine, and iodine. 
     The term “alkyl” as used herein contemplates both straight and branched chain alkyl radicals. Preferred alkyl groups are those containing from one to fifteen carbon atoms and includes methyl, ethyl, propyl, isopropyl, butyl, isobutyl, tert-butyl, and the like. Additionally, the alkyl group may be optionally substituted. 
     The term “cycloalkyl” as used herein contemplates cyclic alkyl radicals. Preferred cycloalkyl groups are those containing 3 to 7 carbon atoms and includes cyclopropyl, cyclopentyl, cyclohexyl, and the like. Additionally, the cycloalkyl group may be optionally substituted. 
     The term “alkenyl” as used herein contemplates both straight and branched chain alkene radicals. Preferred alkenyl groups are those containing two to fifteen carbon atoms. Additionally, the alkenyl group may be optionally substituted. 
     The term “alkynyl” as used herein contemplates both straight and branched chain alkyne radicals. Preferred alkynyl groups are those containing two to fifteen carbon atoms. Additionally, the alkynyl group may be optionally substituted. 
     The terms “aralkyl” or “arylalkyl” as used herein are used interchangeably and contemplate an alkyl group that has as a substituent an aromatic group. Additionally, the aralkyl group may be optionally substituted. 
     The term “heterocyclic group” as used herein contemplates aromatic and non-aromatic cyclic radicals. Hetero-aromatic cyclic radicals also refer to heteroaryl. Preferred hetero-non-aromatic cyclic groups are those containing 3 or 7 ring atoms which includes at least one hetero atom, and includes cyclic amines such as morpholino, piperidino, pyrrolidino, and the like, and cyclic ethers, such as tetrahydrofuran, tetrahydropyran, and the like. Additionally, the heterocyclic group may be optionally substituted. 
     The term “aryl” or “aromatic group” as used herein contemplates single-ring groups and polycyclic ring systems. The polycyclic rings may have two or more rings in which two carbons are common to two adjoining rings (the rings are “fused”) wherein at least one of the rings is aromatic, e.g., the other rings can be cycloalkyls, cycloalkenyls, aryl, heterocycles, and/or heteroaryls. Additionally, the aryl group may be optionally substituted. 
     The term “heteroaryl” as used herein contemplates single-ring hetero-aromatic groups that may include from one to three heteroatoms, for example, pyrrole, furan, thiophene, imidazole, oxazole, thiazole, triazole, pyrazole, pyridine, pyrazine and pyrimidine, and the like. The term heteroaryl also includes polycyclic hetero-aromatic systems having two or more rings in which two atoms are common to two adjoining rings (the rings are “fused”) wherein at least one of the rings is a heteroaryl, e.g., the other rings can be cycloalkyls, cycloalkenyls, aryl, heterocycles, and/or heteroaryls. Additionally, the heteroaryl group may be optionally substituted. 
     The alkyl, cycloalkyl, alkenyl, alkynyl, aralkyl, heterocyclic group, aryl, and heteroaryl may be optionally substituted with one or more substituents selected from the group consisting of hydrogen, deuterium, halogen, alkyl, cycloalkyl, heteroalkyl, arylalkyl, alkoxy, aryloxy, amino, cyclic amino, silyl, alkenyl, cycloalkcnyl, heteroalkenyl, alkynyl, aryl, heteroaryl, acyl, carbonyl, carboxylic acid, ether, ester, nitrile, isonitrile, sulfanyl, sulfinyl, sulfonyl, phosphino, and combinations thereof. 
     As used herein, “substituted” indicates that a substituent other than H is bonded to the relevant position, such as carbon. Thus, for example, where R 1  is mono-substituted, then one R 1  must be other than H. Similarly, where R 1  is di-substituted, then two of R 1  must be other than H. Similarly, where R 1  is unsubstituted, R 1  is hydrogen for all available positions. 
     The “aza” designation in the fragments described herein, i.e. aza-dibenzofuran, aza-dibenzothiophene, etc. means that one or more of the C—H groups in the respective fragment can be replaced by a nitrogen atom, for example, and without any limitation, azatriphenylene encompasses both dibenzo[f,h]quinoxaline and dibenzo[f,h]quinoline. One of ordinary skill in the art can readily envision other nitrogen analogs of the aza-derivatives described above, and all such analogs are intended to be encompassed by the terms as set forth herein. 
     It is to be understood that when a molecular fragment is described as being a substituent or otherwise attached to another moiety, its name may be written as if it were a fragment (e.g. naphthyl, dibenzofuryl) or as if it were the whole molecule (e.g. naphthalene, dibenzofuran). As used herein, these different ways of designating a substituent or attached fragment are considered to be equivalent. 
     Often, the emissive layer (EML) of OLED devices exhibiting good lifetime and efficiency requires more than two components (e.g. 3 or 4 components). For example, an OLED emissive layer can require a hole-transporting co-host (h-host), an electron-transporting co-host (e-host), and an emissive dopant. For this purpose, 3 or 4 source materials are required to fabricate such an EML, which is very complicated and costly compared to a standard two-component EML with a single host and an emitter, which requires only two sources. Conventionally, in order to fabricate such EML requiring two or more components, a separate evaporation source for each component is required. Because the relative concentrations of the components of the EML is important for the device performance, the rate of deposition of each component is measured individually during the deposition in order to monitor the relative concentrations. This makes the fabrication process complicated and costly. Thus, it is desirable to premix the materials for the two or more components and evaporate them from a single source in order to reduce the complexity of the fabrication process. 
     However, the co-evaporation must be stable, i.e. the composition of the deposited film should remain constant throughout the manufacturing process, as any composition change may affect the device performance adversely. In order to obtain a stable co-evaporation from a mixture of compounds under vacuum, one would assume that the materials must have the same evaporation temperature under the same condition. 
     However, this may not be the only parameter one has to consider. When the two or more compounds are mixed together, they may interact with each other and their evaporation properties may differ from their individual properties. On the other hand, materials with slightly different evaporation temperatures may form a stable co-evaporation mixture. Therefore, it is extremely difficult to achieve a stable co-evaporation mixture. “Evaporation temperature” of a material is measured in a high vacuum deposition tool with a chamber base pressure, between 1×10 −6  Torr to 1×10 −9  Torr, at a 2 Å/sec deposition rate on a surface positioned at a set distance away from the evaporation source of the material being evaporated, e.g. sublimation crucible in a VTE tool. The various measured values such as temperature, pressure, deposition rate, etc. disclosed herein are expected to have nominal variations because of the expected tolerances in the measurements that produced these quantitative values as understood by one of ordinary skill in the art. 
     This disclosure describes a novel mixture of two or more organic compounds, particularly a mixture of three compounds, that can be used as a stable co-evaporation source in vacuum deposition processes. Many factors other than temperatures can contribute to the evaporation, such as miscibility of different materials, different phase transition. The inventors found that when two or more materials have similar evaporation temperature, and similar mass loss rate or similar vapor pressure, the two or more materials can co-evaporate consistently. Mass loss rate is defined as percentage of mass lost over time (minute) and is determined by measuring the time it takes to lose the first 10% of the mass as measured by thermal gravity analysis (TGA) under same experimental condition at a same constant given temperature for each compound after the composition reach a steady evaporation state. The constant given temperature is one temperature point that is chosen so that the value of mass loss rate is between about 0.05 to 0.50 percentage/min. Skilled person in this field should appreciate that in order to compare two parameters, the experimental condition should be consistent. The method of measuring mass loss rate and vapor pressure is well known in the art and can be found, for example, in Bull. et al. Mater. Sci. 2011, 34, 7. 
     Searching for a high-performance mixture for stable single-source co-evaporation could be a tedious process. A process of searching for a stable mixture would include identifying compounds with similar evaporation temperatures and monitoring the composition of the evaporated mixture. It is often the case that the mixture materials show slight separation as evaporation goes on. Adjusting the evaporation temperature by changing the chemical structure often, unfortunately, lead to much reduced device performance due to the change in chemical, electrical and/or optical properties. Chemical structure modifications also impact the evaporation temperature much more significantly than needed, resulting in unstable mixtures. 
     To address these difficulties, the present disclosure describes a method where a mixture of three compounds is used as a single source for evaporation. We envision two scenarios as detailed below. 
     In one scenario, two of the three component compounds have their concentrations changing in the opposite directions during evaporation, i.e. the concentration of the first component increases while the second component decreases, but the overall concentration of these two components, and consequently, the concentration of the third component, remain constant. Therefore, the constant overall concentration of the first two components together with the constant concentration of the third component are expected to ensure that the device performance remain unchanged throughout the manufacturing process. For an EML requiring h-host, c-host and dopant, the first two components with changing concentrations could be two h-hosts, two e-hosts or two dopants. 
     In another scenario, the introduction of the third component compound assists the co-evaporation of the first and the second components. This third component could be called a carrier compound or co-evaporation assisting compound. Some of the mechanisms to realize this co-evaporation are intermolecular interaction between the first and second components by, for instance, van der Waals force, electrostatic force, hydrogen bond, chemical bond. An analogy in nature is the oil-water-surfactant system, where surfactant as the third component greatly facilitates the intermixing between oil and water. The third component is also an essential component for EML, and could be an h-host, an e-host or a dopant. 
     According to an embodiment, a first mixture useful as a stable single-source co-evaporation mixture of three compounds is disclosed. The first mixture comprises: a first compound; a second compound; and a third compound, 
     wherein the first compound, the second compound, and the third compound are all organic compounds and have different chemical structures from each other, 
     wherein the first compound, the second compound, and the third compound each has an evaporation temperature T1, T2, and T3, respectively, and is in the range of 150 to 350° C., 
     wherein the T1, T2, and T3 differ from each other by less than 20° C. 
     Furthermore, the first compound has a concentration C1 in the first mixture and a concentration C2 in a film deposited by evaporating the first mixture in a high vacuum deposition tool under a first deposition condition which is defined as depositing at a 2 Å/sec deposition rate with a chamber base pressure between 1×10 −6  Torr to 1×10 −9  Torr onto a surface positioned at a predefined distance from the first mixture. The absolute value of (C1-C2)/C1, represented herein as |(C1-C2)/C1|, is less than 5%, the first compound has a concentration C1′ in a second mixture of the first and second compounds or has a concentration C1″ in a third mixture of the first and third compounds, and the first compound has a concentration C2′ in a film formed by evaporating the second mixture under the first deposition condition or has a concentration C2″ in a film formed by evaporating the third mixture under the first deposition condition, and at least one of |(C1′-C2′)/C1′| and |(C1″-C2″)/C1″| is greater than 5%. 
     In some embodiments of the disclosed mixture, both of |(C1′-C2′)/C1′| and |(C″-C2″)/C1″| are larger than 5%. 
     In a preferred embodiment, |(C1-C2)/C1| is less than 3%. 
     One of ordinary skill in this field should realize that the concentration of each component is expressed as a relative percentage. The concentration of each component in the mixture can be measured by suitable analytical methods such as high pressure liquid chromatography (HPLC) and nuclear magnetic resonance spectroscopy (NMR). 
     The inventors used HPLC and the percentage was calculated by dividing the integration area under the HPLC trace of each component by the total integration area. HPLC can use different detectors such as UV-vis, photo diode array detector, refractive index detector, fluorescence detector, and light scattering detector. Due to different materials properties, each component in the mixture may respond differently. Therefore, the measured concentration may differ from their real concentration in the mixture, however the relative ratio value of (C1-C2)/C1 is independent of these variables as long as the experimental condition is kept consistent, for example, all concentrations should be calculated under the exact same HPLC parameters for each component. It is sometimes preferred to select a measurement condition that gives calculated concentration close to the real concentration. However, it is not necessary. It is important to select a detecting condition that accurately detects each component. For example, fluorescence detector should not be used if one of the components does not fluoresce. 
     In another embodiment of the mixture disclosed herein, T1, T2, and T3 are in the range of 200 to 350° C. 
     In another embodiment, the second compound has a concentration C3 in the first mixture, and the second compound has a concentration C4 in a film formed by evaporating the first mixture under the first condition, wherein |(C3-C4)/C3| is less than 5%. 
     In other embodiments, the first compound, the second compound, and the third compound are each independently selected from the group consisting of a h-host, an e-host, and an emitter. The emitter can be a phosphorescent emitter or a fluorescent emitter. 
     The e-host material can be selected from the group consisting of a compound having a structure of 
                         
and a compound having a structure of
 
     
       
         
         
             
             
         
       
     
     wherein G 1  is selected from the group consisting of dibenzofuran, dibenzothiophene, dibenzosclenophene, and fluorene; 
     wherein L 1 , L 2  and L 3  are each independently selected from the group consisting of direct bond, phenyl, biphenyl, terphenyl, pyridine, pyrimidine, and combinations thereof; 
     wherein G 4  is selected from the group consisting of phenyl, biphenyl, terphenyl, naphthalene, phenanthrene, pyridine, pyrimidine, pyrazine, quinoline, isoquinoline, phenanthroline, fluorene, and combinations thereof; 
     wherein G 2 , G 3 , and G 5  are each independently selected from the group consisting of phenyl, biphenyl, terphenyl, fluorene, naphthalene, phenanthrene, pyridine, pyrimidine, pyrazine, quinoline, isoquinoline, phenanthroline, aza-fluorene, and combinations thereof; 
     wherein G 2 , G 3 , G 4 , and G 5  are each optionally further substituted with one or more unfused substituents selected from the group consisting of deuterium, alkyl, alkoxyl, cycloalkyl, cycloalkoxyl, halogen, nitro, nitrile, silyl, phenyl, biphenyl, terphenyl, pyridine, and combinations thereof; 
     wherein m is an integer from 0 to 7, 
     wherein n is an integer from 0 to 4; 
     wherein, when m or n is larger than 1, each G 4  or G 5  can be same or different; 
     wherein when n is 0, m is equal to or greater than 1, and each G 4  is selected from the group consisting of phenyl, and biphenyl; 
     wherein when n is equal to or greater than 1, L 1  is not a direct bond; 
     wherein when m and n are both 0, L 1  is biphenyl; 
     wherein when G 4  is present and is fluorene, L 1  is not a direct bond; 
     wherein Z 0  is selected from the group consisting of O, S, Se, NR 1  and CR 2 R 3 ; 
     wherein Z 1  to Z 8  are each independently selected from the group consisting of N and CR 4 , and at least one of Z 1  to Z 8  is N; and 
     wherein R 1 , R 2 , R 3  and R 4  are each independently selected from the group consisting of hydrogen, deuterium, alkyl, alkoxyl, cycloalkyl, cycloalkoxyl, halogen, nitro, nitrile, silyl, aryl, heteroaryl and combinations thereof. 
     In some embodiments, the e-host is selected from the group consisting of: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     In some embodiments, the h-host material can be selected from the group consisting of a compound having a structure of 
                         
and a compound having a structure of
 
     
       
         
         
             
             
         
       
     
     wherein Ar 1  is selected from the group consisting of triphenylene, tetraphenylene, pyrene, naphthalene, fluoranthene, chrysene, phenanthrene, and combinations thereof; 
     wherein L is selected from the group consisting of a direct bond, phenyl, biphenyl, terphenyl, naphthalene, pyridine, dibenzofuran, dibenzothiophene, dibenzoselenophene, and combinations thereof; 
     wherein Ar 2  is selected from the group consisting of benzene, biphenyl, terphenyl, naphthalene, pyridine, dibcnzofuran, dibenzothiophene, dibenzoselenophene, fluorene, carbazole, and combinations thereof; 
     wherein A 1 , Ar 2  and L are each independently and optionally further substituted with one or more substitutions selected from the group consisting of deuterium, halogen, alkyl, aryl, non-aza-heteroaryl, and combinations thereof; 
     wherein R 5  and R 8  each independently represent mono, di, tri, or tetra substitution, or no substitution; 
     wherein R 6  and R 7  each independently represent mono, di, or tri substitution, or no substitution; 
     wherein R 5 , R 6 , R 7 , R 8 , Ar3, and Ar 4  are each independently selected from the group consisting of hydrogen, deuterium, halogen, alkyl, cycloalkyl, heteroalkyl, arylalkyl, alkoxy, aryloxy, amino, silyl, alkenyl, cycloalkenyl, heteroalkenyl, alkynyl, benzene, furan, thiophene, selenophene, pyrole, biphenyl, terphenyl, naphthalene, triphenylene, anthracene, phenanthracene, tetraphenylene, pyrene, fluoranthene, chrysene, fluorene, carbazole, benzofuran, benzothiophene, benzoselenophene, dibenzofuran, dibenzothiophene, dibenzoselenophene, indole, carbazole, and combinations thereof; and 
     wherein any two adjacent substituents are optionally joined or fused into a ring. 
     The h-host material can be selected from the group consisting of: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     The emitter material can be a transition metal complex having at least one ligand selected from the group consisting of: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     wherein each X 1  to X 13  are independently selected from the group consisting of carbon and nitrogen; 
     wherein X is selected from the group consisting of BR′, NR′, PR′, O, S, Se, C═O, S═O, SO 2 , CR′R″, SiR′R″, and GeR′R″; 
     wherein R′ and R″ are optionally fused or joined to form a ring; 
     wherein each R a , R b , R c , and R d  may represent from mono substitution to the possible maximum number of substitution, or no substitution; 
     wherein R′, R″, R a , R b , R c , and R d  are each independently selected from the group consisting of hydrogen, deuterium, halide, alkyl, cycloalkyl, heteroalkyl, arylalkyl, alkoxy, aryloxy, amino, silyl, alkenyl, cycloalkenyl, heteroalkenyl, alkynyl, aryl, heteroaryl, acyl, carbonyl, carboxylic acids, ester, nitrile, isonitrile, sulfanyl, sulfinyl, sulfonyl, phosphino, and combinations thereof; and 
     wherein any two adjacent substituents of R a , R b , R c , and R d  are optionally fused or joined to form a ring or form a multidentate ligand. 
     In other embodiments, the emitter is a transition metal complex having at least one ligand selected from the group consisting of: 
     
       
         
         
             
             
         
       
     
     In other embodiments, the emitter is selected from the group consisting of: 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     According to some embodiments, the first mixture comprises a h-host, an e-host, and an emitter. In other embodiments, the first mixture comprises a first h-host, a second h-host, and an e-host. The possible materials for the h-host, the e-host, and the emitter are as defined above. 
     In some embodiments, the first mixture is selected from the following group of three-component mixtures consisting of (Compound A11, Compound A14, and Compound H26), (Compound A11, Compound C74, and Compound H17), (Compound A14, Compound C65, and Compound H5), (Compound C74, Compound H8, and Compound H17), (Compound C83, Compound H17, and Emitter 2), (Compound C83, Compound F20, and Compound F18), (Compound 83, Compound G2, and Compound G26), (Compound A5, Compound C239, and Emitter 65), and (Compound E2, Compound H5, and Emitter 25). The chemical structures of the specific compounds in this list are as defined above. 
     According to another aspect of the present disclosure, a method for fabricating a device using the disclosed first mixture is disclosed. The method comprises: providing a first container that contains a first mixture, the first mixture comprising:
         a first compound;   a second compound; and   a third compound,   wherein the first compound, the second compound, and the third compound are all organic compounds and have different chemical structures from each other,   wherein the first compound, the second compound, and the third compound each has an evaporation temperature T1, T2, and T3, respectively, and is in the range of 150 to 350° C.,   wherein the T1, T2, and T3 differ from each other by less than 20° C.;       

     providing a substrate having a first electrode disposed thereon; 
     depositing an organic layer over the first electrode by evaporating the first mixture in the first container in a high vacuum deposition tool under a first deposition condition which is defined as depositing at a 2 Å/sec deposition rate with a chamber base pressure between 1×10 −6  Torr to 1×10 −9  Torr onto a surface positioned at a predefined distance from the first mixture,
         wherein the first compound has a concentration C1 in the first mixture and a concentration C2 in the emissive layer and |(C1-C2)/C1| is less than 5%; and       

     depositing a second electrode over the emissive layer. All of the variations for the first mixture described above are applicable to this method. 
     According to another aspect of the present disclosure, a first device comprising a first organic light emitting device is disclosed. The organic light emitting device comprises: 
     an anode; 
     a cathode; and 
     an organic layer, disposed between the anode and the cathode, comprising a first mixture of a first compound, a second compound, and a third compound, 
     wherein the first compound, the second compound, and the third compound are all organic compounds and have different chemical structures from each other, 
     wherein the first compound, the second compound, and the third compound each has an evaporation temperature T1, T2, and T3, respectively, and is in the range of 150 to 350° C., 
     wherein the T1, T2, and T3 differ from each other by less than 20° C., 
     wherein the first compound has a concentration C1 in the first mixture and a concentration C2 in a film deposited by evaporating the first mixture in a high vacuum deposition tool under a first deposition condition which is defined as depositing at a 2 Å/sec deposition rate with a chamber base pressure between 1×10 −6  Torr to 1×10 −9  Torr onto a surface positioned at a predefined distance from the first mixture, 
     wherein |(C1-C2)/C1| is less than 5%, 
     wherein the first compound has a concentration C1′ in a second mixture of the first and second compounds or has a concentration C1″ in a third mixture of the first and third compounds, and the first compound has a concentration C2′ in a film formed by evaporating the second mixture under the first deposition condition or has a concentration C2″ in a film formed by evaporating the third mixture under the first deposition condition, and 
     wherein at least one of |(C1′-C2′)/C1′| and |(C1″-C2″)/C1″| is greater than 5%. 
     In one embodiment of the first device, the organic layer is an emissive layer. In another embodiment of the first device, the organic layer is a non-emissive layer. 
     In one embodiment of the first device, the organic layer further comprises a phosphorescent emitting material. 
     In one embodiment of the first device, the organic layer further comprises a host material. 
     In one embodiment of the first device, the first compound functions as a phosphorescent emitting material at room temperature. 
     In one embodiment of the first device, the first compound functions as a host material at room temperature. 
     In one embodiment of the first device, the first device further comprises a second organic light emitting device separate from the first organic light emitting device. 
     In one embodiment of the first device, the first organic light emitting device comprises a first emissive layer and a second emissive layer, wherein the first emissive layer is deposited by evaporating the first mixture. 
     In one embodiment of the first device, the organic layer is a hole transporting layer. 
     In one embodiment of the first device, the first device is a consumer product. In another embodiment, the first device is an organic light-emitting device. In another embodiment, the first device can comprise a lighting panel. 
     Examples 
     The feasibility of manufacturing multicomponent films with stable compositions was demonstrated by compositional analysis of films fabricated by single-source co-evaporation of the premixture of these components. 
     Comparative Premixture Example 1-A bi-component premixture (BPM1) was prepared by physically mixing and grinding of Compound H8 and Compound C74 at a weight ratio of 2:1, and loaded into an evaporation source. The premixed compositions were thermally co-evaporated at a rate of 2 Å/s in a high vacuum chamber with a base pressure of less than 10 −7  Torr, and deposited onto glass substrates. The substrates were replaced continuously after deposition of 500 Å of film without stopping the deposition and cooling the source till the depletion of the evaporation source. The compositions of films were analyzed by high-performance liquid chromatography (HPLC) and the results are collected in Table 1. The concentrations of Compound C74 in each film were plotted in  FIG.  1   . 
     
       
         
         
             
             
         
       
     
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 HPLC composition (%) of sequentially deposited films from premixture 
               
               
                 (BPM1) (HPLC Conditions C18, 100 45 min, Detected wavelength 
               
               
                 254 nm) (Due to different absorption coefficients, the HPLC composition 
               
               
                 may or may not agree with the weight ratio.) 
               
            
           
           
               
               
               
               
            
               
                   
                 Films 
                 Compound H8 
                 Compound C74 
               
               
                   
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Plate1 
                 68.4 
                 31.6 
               
               
                   
                 Plate2 
                 68.2 
                 31.8 
               
               
                   
                 Plate3 
                 68.2 
                 31.8 
               
               
                   
                 Plate4 
                 68.4 
                 31.6 
               
               
                   
                 Plate5 
                 69.3 
                 30.7 
               
               
                   
                 Plate6 
                 70.6 
                 29.4 
               
               
                   
                 Plate7 
                 71.7 
                 28.3 
               
               
                   
                 Plate8 
                 73.0 
                 27.0 
               
               
                   
                   
               
            
           
         
       
     
       FIG.  3    shows HPLC composition (%) evolution of Compound C74 in sequentially deposited films from premixture BPM1. The dashed line in the plot of  FIG.  3    represents a linear fit of the data presented in solid line, which shows a slope of −0.68. 
     Premixture Example 1-A tri-component premixture (TPM1) was prepared by physically mixing and grinding of Compound H8, Compound C74 and Compound H17 at a weight ratio of 1:1:1, and loaded into an evaporation source. The film preparation and concentration evaluation follow the same procedures as in BPM1. The compositions of films are collected in Table 2 and the concentrations of Compound C74 in each film were plotted in  FIG.  2   . 
     
       
         
         
             
             
         
       
     
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 HPLC composition (%) of sequentially deposited films from premixture 
               
               
                 (TPM1) (HPLC Conditions C18, 100 45 min, Detected wavelength 
               
               
                 254 nm) (Due to different absorption coefficients, the HPLC composition 
               
               
                 may or may not agree with the weight ratio.) 
               
            
           
           
               
               
               
               
            
               
                 Films 
                 Compound H8 
                 Compound H17 
                 Compound C74 
               
               
                   
               
            
           
           
               
               
               
               
            
               
                 Plate1 
                 36.9 
                 37.4 
                 25.7 
               
               
                 Plate2 
                 35.2 
                 38.2 
                 26.6 
               
               
                 Plate3 
                 34.3 
                 38.1 
                 27.6 
               
               
                 Plate4 
                 33.0 
                 38.7 
                 28.3 
               
               
                 Plate5 
                 31.3 
                 40.1 
                 28.6 
               
               
                 Plate6 
                 30.3 
                 41.2 
                 28.5 
               
               
                 Plate7 
                 30.0 
                 41.7 
                 28.3 
               
               
                 Plate8 
                 29.0 
                 43.5 
                 27.5 
               
               
                   
               
            
           
         
       
     
       FIG.  4    shows HPLC composition (%) evolution of Compound C74 in sequentially deposited films from premixture TPM1. The dashed line in the plot of  FIG.  4    is a linear fit of the data presented in solid line, which shows a slope of 0.29. 
     The absolute value of slope in the concentration plot indicates the extent of concentration separation during sequential deposition of films from a premixture. The data in  FIGS.  3  and  4    suggest that TPM1 has less concentration separation for Compound C74 than BPM1. This evaporation stability in TPM1 was achieved through the introduction of Compound H17, which shows opposite trend of concentration evolution against Compound H8 during sequential evaporation as revealed in Table 2. 
     Comparative Premixture Example 2-A bi-component premixture (BPM2) was prepared by physically mixing and grinding of Compound H5 and Compound E2 at a weight ratio of 1:1, and loaded into an evaporation source. The film preparation and concentration evaluation follow the same procedures as in BPM1. The compositions of films are collected in Table 3 and the concentrations of Compound E2 in each film were plotted in  FIG.  5   . 
     
       
         
         
             
             
         
       
     
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 HPLC composition (%) of sequentially deposited films from premixture 
               
               
                 (BPM2) (HPLC Conditions C18, 100 45 min, Detected wavelength 
               
               
                 254 nm) (Due to different absorption coefficients, the HPLC composition 
               
               
                 may or may not agree with the weight ratio.) 
               
            
           
           
               
               
               
               
            
               
                   
                 Films 
                 Compound H5 
                 Compound E2 
               
               
                   
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Plate1 
                 63.6 
                 36.4 
               
               
                   
                 Plate2 
                 64.8 
                 35.2 
               
               
                   
                 Plate3 
                 64.3 
                 35.7 
               
               
                   
                 Plate4 
                 62.2 
                 37.8 
               
               
                   
                 Plate5 
                 59.0 
                 41.0 
               
               
                   
                 Plate6 
                 53.8 
                 46.2 
               
               
                   
                   
               
            
           
         
       
     
       FIG.  5    shows HPLC composition (%) evolution of Compound E2 in sequentially deposited films from premixture BPM2. The dashed line in the plot of  FIG.  5    is a linear fit of the data presented in solid line, which shows a slope of 1.96. 
     Premixture Example 2-A tri-component premixture (TPM2) was prepared by physically mixing and grinding of Compound H5, Compound E2 and Emitter 25 at a weight ratio of 2:2:1, and loaded into an evaporation source. The film preparation and concentration evaluation follow the same procedures as in BPM1. The compositions of films are collected in Table 4 and the concentrations of Emitter 25 in each film were plotted in  FIG.  6   . 
     
       
         
         
             
             
         
       
     
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 HPLC composition (%) of sequentially deposited films from premixture 
               
               
                 (TPM2) (HPLC Conditions C18, 100 45 min, Detected wavelength 
               
               
                 254 nm) (Due to different absorption coefficients, the HPLC composition 
               
               
                 may or may not agree with the weight ratio.) 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Films 
                 Compound H5 
                 Emitter 25 
                 Compound E2 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 Plate1 
                 54.4 
                 11.4 
                 34.2 
               
               
                   
                 Plate2 
                 55.5 
                 11.0 
                 33.5 
               
               
                   
                 Plate3 
                 56.7 
                 10.2 
                 33.1 
               
               
                   
                 Plate4 
                 57.5 
                 9.5 
                 33.0 
               
               
                   
                 Plate5 
                 55.2 
                 9.4 
                 35.4 
               
               
                   
                   
               
            
           
         
       
     
       FIG.  6    shows HPLC composition (%) evolution of Compound E2 in sequentially deposited films from premixture TPM2. The dashed line in the plot of  FIG.  6    is a linear fit of the data presented in solid line, which shows a slope of 0.19. 
     The data in  FIGS.  5  and  6    suggest that TPM2 has less concentration separation and is a more stable premixture than BPM2. This evaporation stability in TPM2 was achieved through the introduction of Emitter 25, which assists the co-evaporation of Compound H5 and Compound E2. Indeed, a comparison of data in Tables 3 and 4 suggests that there is much less concentration separation for both Compounds H5 and E2 in TPM2. 
     Combination with Other Materials 
     The materials described herein as useful for a particular layer in an organic light emitting device may be used in combination with a wide variety of other materials present in the device. For example, emissive dopants disclosed herein may be used in conjunction with a wide variety of hosts, transport layers, blocking layers, injection layers, electrodes and other layers that may be present. The materials described or referred to below are non-limiting examples of materials that may be useful in combination with the compounds disclosed herein, and one of skill in the art can readily consult the literature to identify other materials that may be useful in combination. 
     HIL/HTL: 
     A hole injecting/transporting material to be used in the present invention is not particularly limited, and any compound may be used as long as the compound is typically used as a hole injecting/transporting material. Examples of the material include, but not limit to: a phthalocyanine or porphyrin derivative; an aromatic amine derivative; an indolocarbazole derivative; a polymer containing fluorohydrocarbon; a polymer with conductivity dopants; a conducting polymer, such as PEDOT/PSS; a self-assembly monomer derived from compounds such as phosphonic acid and silane derivatives; a metal oxide derivative, such as MoO x ; a p-type semiconducting organic compound, such as 1,4,5,8,9,12-Hexaazatriphenylenchexacarbonitrile; a metal complex, and a cross-linkable compounds. 
     Examples of aromatic amine derivatives used in HIL or HTL include, but not limit to the following general structures: 
     
       
         
         
             
             
         
       
     
     Each of Ar 1  to Ar 9  is selected from the group consisting aromatic hydrocarbon cyclic compounds such as benzene, biphenyl, triphenyl, triphenylene, naphthalene, anthracene, phenalene, phenanthrene, fluorene, pyrene, chrysene, perylene, azulene; group consisting aromatic heterocyclic compounds such as dibenzothiophene, dibenzofuran, dibenzoselenophene, furan, thiophene, benzofuran, benzothiophene, benzoselenophene, carbazole, indolocarbazole, pyridylindole, pyrrolodipyridine, pyrazole, imidazole, triazole, oxazole, thiazole, oxadiazole, oxatriazole, dioxazole, thiadiazole, pyridine, pyridazine, pyrimidine, pyrazine, triazine, oxazine, oxathiazine, oxadiazine, indole, benzimidazole, indazole, indoxazine, benzoxazole, benzisoxazole, benzothiazole, quinoline, isoquinoline, cinnoline, quinazoline, quinoxaline, naphthyridine, phthalazine, pteridine, xanthene, acridine, phenazine, phenothiazine, phenoxazine, benzofuropyridine, furodipyridine, benzothienopyridine, thienodipyridine, benzoselenophenopyridine, and selenophenodipyridine; and group consisting 2 to 10 cyclic structural units which are groups of the same type or different types selected from the aromatic hydrocarbon cyclic group and the aromatic heterocyclic group and are bonded to each other directly or via at least one of oxygen atom, nitrogen atom, sulfur atom, silicon atom, phosphorus atom, boron atom, chain structural unit and the aliphatic cyclic group. Wherein each Ar is further substituted by a substituent selected from the group consisting of hydrogen, deuterium, halide, alkyl, cycloalkyl, heteroalkyl, arylalkyl, alkoxy, aryloxy, amino, silyl, alkenyl, cycloalkenyl, heteroalkenyl, alkynyl, aryl, heteroaryl, acyl, carbonyl, carboxylic acids, ester, nitrile, isonitrile, sulfanyl, sulfinyl, sulfonyl, phosphino, and combinations thereof. 
     In one aspect, Ar 1  to Ar 9  is independently selected from the group consisting of: 
                         
wherein k is an integer from 1 to 20; X 101  to X 108  is C (including CH) or N; Z 101  is NAr 1 , O, or S; Ar 1  has the same group defined above.
 
     Examples of metal complexes used in HIL or HTL include, but not limit to the following general formula: 
                         
wherein Met is a metal, which can have an atomic weight greater than 40; (Y 101 -Y 102 ) is a bidentate ligand, Y 101  and Y 102  are independently selected from C, N, O, P, and S; L 101  is an ancillary ligand; k′ is an integer value from 1 to the maximum number of ligands that may be attached to the metal; and k′+k″ is the maximum number of ligands that may be attached to the metal.
 
     In one aspect, (Y 101 -Y 102 ) is a 2-phenylpyridine derivative. In another aspect, (Y 101 -Y 102 ) is a carbene ligand. In another aspect, Met is selected from Ir, Pt, Os, and Zn. In a further aspect, the metal complex has a smallest oxidation potential in solution vs. Fe + /Fc couple less than about 0.6 V. 
     Host: 
     The light emitting layer of the organic EL device of the present invention preferably contains at least a metal complex as light emitting material, and may contain a host material using the metal complex as a dopant material. Examples of the host material are not particularly limited, and any metal complexes or organic compounds may be used as long as the triplet energy of the host is larger than that of the dopant. While the Table below categorizes host materials as preferred for devices that emit various colors, any host material may be used with any dopant so long as the triplet criteria is satisfied. 
     Examples of metal complexes used as host are preferred to have the following general formula: 
                         
wherein Met is a metal; (Y 103 -Y 104 ) is a bidentate ligand, Y 103  and Y 104  are independently selected from C, N, O, P, and S; L 101  is an another ligand; k′ is an integer value from 1 to the maximum number of ligands that may be attached to the metal; and k′+k″ is the maximum number of ligands that may be attached to the metal.
 
     In one aspect, the metal complexes are: 
                         
wherein (O—N) is a bidentate ligand, having metal coordinated to atoms O and N.
 
     In another aspect, Met is selected from Ir and Pt. In a further aspect, (Y 103 -Y 104 ) is a carbene ligand. 
     Examples of organic compounds used as host are selected from the group consisting aromatic hydrocarbon cyclic compounds such as benzene, biphenyl, triphenyl, triphenylene, naphthalene, anthracene, phenalene, phenanthrene, fluorene, pyrene, chrysene, perylene, azulene; group consisting aromatic heterocyclic compounds such as dibenzothiophene, dibenzofuran, dibenzoselenophene, furan, thiophene, benzofuran, benzothiophene, benzoselenophene, carbazole, indolocarbazole, pyridylindole, pyrrolodipyridine, pyrazole, imidazole, triazole, oxazole, thiazole, oxadiazole, oxatriazole, dioxazole, thiadiazole, pyridine, pyridazine, pyrimidine, pyrazine, triazine, oxazine, oxathiazine, oxadiazine, indole, benzimidazole, indazole, indoxazine, benzoxazole, benzisoxazole, benzothiazole, quinoline, isoquinoline, cinnoline, quinazoline, quinoxaline, naphthyridine, phthalazine, pteridine, xanthene, acridine, phenazine, phenothiazine, phenoxazine, benzofuropyridine, furodipyridine, benzothienopyridine, thienodipyridine, benzoselenophenopyridine, and selenophenodipyridine; and group consisting 2 to 10 cyclic structural units which are groups of the same type or different types selected from the aromatic hydrocarbon cyclic group and the aromatic heterocyclic group and are bonded to each other directly or via at least one of oxygen atom, nitrogen atom, sulfur atom, silicon atom, phosphorus atom, boron atom, chain structural unit and the aliphatic cyclic group. Wherein each group is further substituted by a substituent selected from the group consisting of hydrogen, deuterium, halide, alkyl, cycloalkyl, heteroalkyl, arylalkyl, alkoxy, aryloxy, amino, silyl, alkenyl, cycloalkenyl, heteroalkenyl, alkynyl, aryl, heteroaryl, acyl, carbonyl, carboxylic acids, ester, nitrile, isonitrile, sulfanyl, sulfinyl, sulfonyl, phosphino, and combinations thereof. 
     In one aspect, host compound contains at least one of the following groups in the molecule: 
                                           
wherein R 101  to R 107  is independently selected from the group consisting of hydrogen, deuterium, halide, alkyl, cycloalkyl, heteroalkyl, arylalkyl, alkoxy, aryloxy, amino, silyl, alkenyl, cycloalkenyl, heteroalkenyl, alkynyl, aryl, heteroaryl, acyl, carbonyl, carboxylic acids, ester, nitrile, isonitrile, sulfanyl, sulfinyl, sulfonyl, phosphino, and combinations thereof, when it is aryl or heteroaryl, it has the similar definition as Ar&#39;s mentioned above. k is an integer from 0 to 20 or 1 to 20; k′″ is an integer from 0 to 20.
 
X 101  to X 108  is selected from C (including CH) or N.
 
Z 101  and Z 102  is selected from NR 101 , O, or S.
 
HBL:
 
     A hole blocking layer (HBL) may be used to reduce the number of holes and/or excitons that leave the emissive layer. The presence of such a blocking layer in a device may result in substantially higher efficiencies as compared to a similar device lacking a blocking layer. Also, a blocking layer may be used to confine emission to a desired region of an OLED. 
     In one aspect, compound used in HBL contains the same molecule or the same functional groups used as host described above. 
     In another aspect, compound used in HBL contains at least one of the following groups in the molecule: 
                         
wherein k is an integer from 1 to 20; L 101  is an another ligand, k′ is an integer from 1 to 3.
 
ETL:
 
     Electron transport layer (ETL) may include a material capable of transporting electrons. Electron transport layer may be intrinsic (undoped), or doped. Doping may be used to enhance conductivity. Examples of the ETL material are not particularly limited, and any metal complexes or organic compounds may be used as long as they are typically used to transport electrons. 
     In one aspect, compound used in ETL contains at least one of the following groups in the molecule: 
                         
wherein R 101  is selected from the group consisting of hydrogen, deuterium, halide, alkyl, cycloalkyl, heteroalkyl, arylalkyl, alkoxy, aryloxy, amino, silyl, alkenyl, cycloalkenyl, heteroalkenyl, alkynyl, aryl, heteroaryl, acyl, carbonyl, carboxylic acids, ester, nitrile, isonitrile, sulfanyl, sulfinyl, sulfonyl, phosphino, and combinations thereof, when it is aryl or heteroaryl, it has the similar definition as Ar&#39;s mentioned above. Ar 1  to Ar 3  has the similar definition as Ar&#39;s mentioned above. k is an integer from 1 to 20. X 101  to X 108  is selected from C (including CH) or N.
 
     In another aspect, the metal complexes used in ETL contains, but not limit to the following general formula: 
                         
wherein (O—N) or (N—N) is a bidentate ligand, having metal coordinated to atoms O, N or N, N; L 101  is another ligand; k′ is an integer value from 1 to the maximum number of ligands that may be attached to the metal.
 
     In any above-mentioned compounds used in each layer of the OLED device, the hydrogen atoms can be partially or fully deuterated. Thus, any specifically listed substituent, such as, without limitation, methyl, phenyl, pyridyl, etc. encompasses undeuterated, partially deuterated, and fully deuterated versions thereof. Similarly, classes of substituents such as, without limitation, alkyl, aryl, cycloalkyl, heteroaryl, etc. also encompass undeuterated, partially deuterated, and fully deuterated versions thereof. 
     In addition to and/or in combination with the materials disclosed herein, many hole injection materials, hole transporting materials, host materials, dopant materials, exciton/hole blocking layer materials, electron transporting and electron injecting materials may be used in an OLED. Non-limiting examples of the materials that may be used in an OLED in combination with materials disclosed herein are listed in Table 4 below. Table 4 lists non-limiting classes of materials, non-limiting examples of compounds for each class, and references that disclose the materials. 
     
       
         
           
               
               
               
             
               
                 TABLE 4 
               
               
                   
               
               
                 MATERIAL 
                 EXAMPLES OF MATERIAL 
                 PUBLICATIONS 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
            
               
                 Hole injection materials 
               
            
           
           
               
               
               
            
               
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                 Conducting polymers (e.g., PEDOT:PSS, polyaniline, polypthiophene) 
                 
                   
                     
                     
                         
                         
                     
                   
                 
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                 Phosphonic acid and silane SAMs 
                 
                   
                     
                     
                         
                         
                     
                   
                 
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     It is understood that the various embodiments described herein are by way of example only, and are not intended to limit the scope of the invention. For example, many of the materials and structures described herein may be substituted with other materials and structures without deviating from the spirit of the invention. The present invention as claimed may therefore include variations from the particular examples and preferred embodiments described herein, as will be apparent to one of skill in the art. It is understood that various theories as to why the invention works are not intended to be limiting.