Patent Publication Number: US-2022231230-A1

Title: Organic electroluminescent materials and devices

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 16/420,555, filed May 23, 2019, which is a continuation of U.S. application Ser. No. 14/863,887, filed on Sep. 24, 2015, now U.S. Pat. No. 10,361,375, which claims priority under 35 U.S.C. § 119(e) to U.S. Provisional Applications No. 62/075,333, filed on Nov. 5, 2014, and No. 62/060,192, filed on Oct. 6, 2014, the entire contents of which are incorporated herein by reference. 
    
    
     PARTIES TO A JOINT RESEARCH AGREEMENT 
     The claimed invention was made by, on behalf of, and/or in connection with one or more of the following parties to a joint university corporation research agreement: The Regents of the University of Michigan, Princeton University, University of Southern California, and Universal Display Corporation. The agreement was in effect on and before the date the claimed invention was made, and the claimed invention was made as a result of activities undertaken within the scope of the agreement. 
     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 novel composition comprising a mixture of a first compound and a second compound that is useful as a stable co-evaporation source. In the mixture, the first compound has a different chemical structure than the second compound. The first compound and the second compound are both organic compounds. At least one of the first compound and the second compound contains at least one less abundant stable isotope atom. The first compound has an evaporation temperature T1 and the second compound has an evaporation temperature T2 where both T1 and T2 are between 100 to 400° C. and the absolute value of T1−T2 is less than 20° C. The first compound has a concentration C1 in said mixture and a concentration C2 in a film formed by evaporating said mixture 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 predefined distance away from the mixture being evaporated, wherein the absolute value of (C1−C2)/C1 is less than 5%. 
     According to an embodiment, a first device comprising a first organic light emitting device is 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 first organic composition comprising a mixture of a first compound and a second compound. The first compound has a different chemical structure than the second compound. The first compound and the second compound are both organic compounds. At least one of the first compound and the second compound contains at least one less abundant stable isotope atom. The first compound has an evaporation temperature T1 and the second compound has an evaporation temperature T2 where both T1 and T2 are between 100 to 400° C. and the absolute value of T1−T2 is less than 20° C. The first compound has a concentration C1 in said mixture and a concentration C2 in a film formed by evaporating said mixture 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 predefined distance away from the mixture being evaporated, wherein the absolute value of (C1−C2)/C1 is less than 5%. 
     According to an embodiment of the present disclosure, a method for fabricating an organic light emitting device is disclosed. The method comprises providing a substrate having a first electrode disposed thereon; depositing a first organic layer over the first electrode by evaporating a mixture of a first compound and a second compound 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 position at a predefined distance away from the mixture being evaporated; and depositing a second electrode over the first organic layer, 
     wherein the first compound has a different chemical structure than the second compound, 
     wherein the first compound and the second compound are both organic compounds, wherein at least one of the first compound and the second compound contains at least one less abundant stable isotope atom, 
     wherein the first compound has an evaporation temperature T1 of 150 to 350° C., 
     wherein the second compound has an evaporation temperature T2 of 150 to 350° C., 
     wherein absolute value of T1−T2 is less than 20° C., 
     wherein the first compound has a concentration C1 in said mixture and a concentration C2 in the first organic layer; and wherein absolute value of (C1−C2)/C1 is less than 5%. 
    
    
     
       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. 
     
    
    
     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, cycloalkenyl, 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 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 evaporated film should remain constant during the manufacturing process. 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 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 composition comprising a mixture of two or more organic compounds that can be used as a stable co-evaporation source in vacuum deposition processes is disclosed. 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 two 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. The present disclosure describes a novel approach to modulate compound evaporation temperature by replacing one or more atoms in the compound with their isotopes. Some of the isotopes are, for example,  2 H (deuterium, D) of hydrogen atom,  13 C of carbon atom, etc. The impact of isotopes on compound evaporation properties is profound. For example, D 2 O has boiling point of 101° C. compared to 100° C. of H 2 O, while benzene-d6 has boiling point of 79° C. compared to 80° C. of benzene. Depending on the concentrations of isotopes in these compounds, their evaporation temperatures could be adjusted to a different extent. Isotopes such as deuterium have been introduced to organic electronic materials to improve device performance due to the stronger chemical bonding. It is expected that the introduction of heavy isotopes will not affect the devices adversely. Therefore, this approach is expected to fine-tune the evaporation temperature for stable single-source co-evaporation while maintaining or improving the device performance. 
     In one embodiment, the stable single-source co-evaporation mixture of two or more components is used for evaporation depositing emissive layers and can be a mixture of two or more host materials, a mixture of a host material and a dopant material, a mixture of two or more host materials and a dopant material, a mixture of two or more host materials and two or more dopant materials, and a mixture of two or more dopant materials, for example. One or more of the materials would contain a stable, less abundant isotope. 
     In other embodiments, the stable single-source co-evaporation mixture of two or more components is used for evaporation depositing charge transport layers or blocking layers. 
     According to an embodiment of the present disclosure, a novel two-component composition comprising a mixture of a first compound and a second compound that is a stable co-evaporation mixture is disclosed. In the mixture, the first compound has a different chemical structure than the second compound. The first compound and the second compound are both organic compounds. At least one of the first compound and the second compound contains at least one less abundant stable isotope atom. Less abundant stable isotope(s) of an element have one or two additional neutrons than protons, and thus are heavier than the more common stable isotope for those elements. Unlike the unstable radioactive isotopes, stable isotopes do not decay into other elements. The term “organic compound” as used herein refers to chemical compounds whose molecules contain at least one carbon-carbon bond or carbon-hydrogen bond. However in this application, it excludes any compounds containing metal atoms, such as those inorganic coordination compounds. 
     The first compound can have an evaporation temperature T1 of 100 to 400° C. The second compound can have an evaporation temperature T2 of 100 to 400° C. In order to form the inventive composition comprising a mixture of the first compound and the second compound, the absolute value of T1−T2, the difference between T1 and T2, should be less than 20° C. Preferably, the absolute value of T1−T2 is less than 10° C. and more preferably less than 5° C. 
     Furthermore, the first compound has a concentration C1 in the mixture, and the first compound has a concentration C2 in a film formed by evaporating the mixture in a high vacuum deposition tool, such as a VTE 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 predefined distance away from the mixture being evaporated; and wherein the absolute value of (C1−C2)/C1 is less than 5%. The concentrations C1 and C2 are relative concentrations of the first compound. Therefore, the conditional requirement for the two compounds forming the mixture described above means that the relative concentration (C2) of the first compound in the as-deposited film should be as close to the original relative concentration of the first compound (C1) in the evaporation source mixture. 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 a 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 composition disclosed herein, the first compound has an evaporation temperature T1 of 150 to 350° C. and the second compound has an evaporation temperature T2 of 150 to 350° C. In another embodiment, the first compound has an evaporation temperature T1 of 200 to 350° C. and the second compound has an evaporation temperature T2 of 200 to 350° C. 
     Preferably, the absolute value of (C1−C2)/C1 is less than 3%. 
     In one embodiment of the composition, the first compound has a vapor pressure of P1 at T1 at 1 atm, the second compound has vapor pressure of P2 at T2 at 1 atm, and the ratio of P1/P2 is within the range of 0.90 to 1.10. 
     According to an embodiment of the composition, only one of the first compound and the second compound contains at least one less abundant stable isotope atom. In some embodiments, both of the first compound and the second compound each contains at least one less abundant stable isotope atom. In some embodiments, the less abundant stable isotope atom is deuterium. In some embodiments, the less abundant stable isotope atom is  13  C. In some embodiments, the less abundant stable isotope atom is deuterium, and wherein any carbon atom in the first compound or the second compound having deuterium atom is a non-conjugated carbon. 
     In some embodiments, at least one of the first compound and the second compound comprises at least one chemical group selected from the group consisting of triphenylene, carbazole, dibenzothiophene, dibenzofuran, dibenzoselenophene, aza-triphenylene, aza-carbazole, aza-dibenzothiophene, aza-dibenzofuran, and aza-dibenzoselenophen. 
     In some embodiments, the first compound and the second compound each independently comprises at least one chemical group selected from the group consisting of triphenylene, carbazole, dibenzothiophene, dibenzofuran, dibenzoselenophene, aza-triphenylene, aza-carbazole, aza-dibenzothiophene, aza-dibenzofuran, and aza-dibenzoselenophen. 
     In some embodiments, the first compound is a hole transporting compound, and the second compound is an electron transporting compound. 
     In some embodiments, the first compound comprises 3,3′-bicarbazole, and the second compound comprises triazine. 
     In some embodiments, the first compound and the second compound each independently comprises dibenzothiophene or dibenzofuran. 
     In some embodiments, the first compound is a host compound, and the second compound is a fluorescent or delayed fluorescent emitter. 
     Preferably, the first compound and the second compound each has a purity in excess of 99% as determined by high pressure liquid chromatography. 
     According to one embodiment, the composition further comprises a third compound, wherein the third compound has a different chemical structure than the first and second compounds, wherein the third compound has an evaporation temperature T3 of 150 to 350° C., and wherein absolute value of T1−T3 is less than 20° C. 
     According to one embodiment, the composition is in liquid form at a temperature lower than T1 and T2. In other words, where T1 and T2 are not equal, the composition is in liquid form at at temperature lower than the lower of T1 and T2. 
     The absolute value of (C1−C2)/C1 is less than that obtained from the same composition provided that all of the less abundant stable isotope atoms in the composition are replaced by corresponding more common stable isotope atoms. 
     According to another aspect of the present disclosure, a method for fabricating an organic light emitting device is disclosed. The method comprises: providing a substrate having a first electrode disposed thereon; depositing a first organic layer over the first electrode by evaporating a mixture of a first compound and a second compound 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 position at a predefined distance away from the mixture being evaporated; and depositing a second electrode over the first organic layer, 
     wherein the first compound has a different chemical structure than the second compound; 
     wherein the first compound and the second compound are both organic compounds; 
     wherein at least one of the first compound and the second compound contains at least one stable and less abundant isotope atom; 
     wherein the first compound has an evaporation temperature T1 of 150 to 350° C.; 
     wherein the second compound has an evaporation temperature T2 of 150 to 350° C.; 
     wherein absolute value of T1−T2 is less than 20° C.; 
     wherein the first compound has a concentration C1 in said mixture and a concentration C2 in the first organic layer; and 
     wherein absolute value of (C1−C2)/C1 is less than 5%. 
     In one embodiment of the method, the composition is deposited in a high vacuum thermal evaporation system having a pressure level in the range of 1×10 −8  Torr to 1×10 −12  Torr. In one embodiment, the composition leaves a residue corresponding to less than 5 wt % of the composition&#39;s original charge in the vacuum thermal evaporation system&#39;s sublimation crucible after the composition is depleted during the deposition of the composition over the first electrode. 
     According to another aspect of the present disclosure, a first device comprising a phosphorescent organic light-emitting device is disclosed. The phosphorescent organic light-emitting device comprises: an anode; a cathode; and an organic layer, disposed between the anode and the cathode, comprising a first organic composition comprising a mixture of a first compound and a second compound, wherein the first compound has different chemical structure than the second compound; 
     wherein the first compound and the second compounds are both organic compounds; 
     wherein at least one of the first compound and the second compound contains at least one less abundant stable isotope atom; 
     wherein the first compound has an evaporation temperature T1 of 100 to 400° C.; 
     wherein the second compound has an evaporation temperature T2 of 100 to 400° C.; 
     wherein the absolute value of T1−T2 is less than 20° C.; 
     wherein the first compound has a concentration C1 in said mixture, and the first compound has a concentration C2 in a film formed by evaporating the mixture 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 at a predefined distance away from the evaporation source of the mixture being evaporated; and wherein the absolute value of (C1−C2)/C1 is less 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 first organic composition 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 comprises the first organic composition. 
     In one embodiment of the first device, the organic layer is a hole transporting layer. 
     In one embodiment of the first device, the organic layer is a blocking layer and the first organic composition is a blocking material in the organic layer. In another embodiment, the organic layer is an electron transporting layer and the first organic composition is an electron transporting material in the organic 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. 
     SYNTHESIS 
     Synthesis of Compound 1 
     
       
         
         
             
             
         
       
     
     
       
         
         
             
             
         
       
     
     A Grignard reagent made by refluxing bromobenzene-d 5  (9.75 ml, 93 mmol) with Mg (2.36 g, 97 mmol) in THF (93 ml) was added dropwise to a solution of 2,4,6-trichloro-1,3,5-triazine (5.63 g, 30.5 mmol) in THF (60 ml) at 0° C. The reaction mixture was gradually warmed to room temperature, stirred for 16 hours, diluted with toluene and poured into a cold 12% HCl aqueous solution. The organic layer was isolated, washed with brine and dried over Na 2 SO 4 . After evaporation of the solvent, the residue was dissolved in dichloromethane and passed through a short plug of silica gel. The crude product was recrystallized from heptane to yield 2-chloro-4,6-bis(phenyl-d 5 )-1,3,5-triazine (4.1 g, 49%) as a white solid. 
     
       
         
         
             
             
         
       
     
     A solution of (3-chlorophenyl)boronic acid (2.36 g, 15.12 mmol), 2-chloro-4,6-bis(phenyl-d 5 )-1,3,5-triazine (4.0 g, 14.40 mmol), Pd(PPh 3 ) 4  (0.50 g, 0.43 mmol) and K 2 CO 3  (5.96 g, 43.2 mmol) in DME (127 ml) and Water (16.94 ml) was refluxed under nitrogen for 16 hours. After cooling to room temperature, the white solid was collected by filtration, washed with ethanol, and purified by column chromatography on silica gel with heptane/DCM (9/1, v/v) as eluent to yield the white solid of 2-(3-chlorophenyl)-4,6-bis(phenyl-d 5 )-1,3,5-triazine (2.35 g, 47%). 
     
       
         
         
             
             
         
       
     
     A solution of 2-(6-([1,1′-biphenyl]-4-yl)dibenzo[b,d]thiophen-4-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (2.92 g, 6.31 mmol), 2-(3-chlorophenyl)-4,6-bis(phenyl-d 5 )-1,3,5-triazine (2.35 g, 6.64 mmol), Pd 2 (dba) 3  (0.152 g, 0.166 mmol), SPhos (0.273 g, 0.664 mmol), K 3 PO 4  (4.23 g, 19.92 mmol) in DME (40 ml), toluene (13 ml) and water (13.28 ml) was refluxed under nitrogen for 16 hours. After cooling to room temperature, the mixture was diluted with water. The solid was collected by filtration, washed with water and ethanol, dissolved in boiling toluene and passed through a short plug of silica gel. The filtrate was concentrated and recrystallized from toluene providing Compound 1 (3.2 g, 74%) as a white solid. 
     Synthesis of Compound 2 
     
       
         
         
             
             
         
       
     
     
       
         
         
             
             
         
       
     
     A solution of dibenzo[b,d]thiophen-4-ylboronic acid (22.17 g, 97 mmol), bromobenzene-d 5  (15 g, 93 mmol), Pd(PPh 3 ) 4  (2.14 g, 1.85 mmol) and K 2 CO 3  (29.4 g, 278 mmol) in toluene (300 ml) and water (50 ml) was refluxed under nitrogen for 16 hours. After cooling to room temperature, the organic layer was isolated, dried over MgSO 4  and purified by column chromatography on silica gel with heptane/DCM (95/5 to 70/30,v/v) as eluent to yield 4-(phenyl-d 5 ) dibenzo[b,d]thiophene (22 g, 90%) as a white solid. 
     
       
         
         
             
             
         
       
     
     A solution of sec-BuLi in cyclohexane (40 ml, 56 mmol) was added dropwise into a solution of 4-(phenyl-d5) dibenzo[b,d]thiophene (9.0 g, 33.9 mmol) in THF (200 ml) at −78° C. The reaction mixture was stirred at this temperature for 4 hrs before quenching with 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (12.83 ml, 62.7 mmol). The reaction mixture was gradually warmed to room temperature, stirred for 16 hours and quenched with aqueous NH 4 Cl solution. The organic layer was isolated, dried over MgSO 4  and the solvent was evaporated. The crude product was dissolved in toluene, filtered through a short plug of silica gel and recrystallized from heptane to yield 2-(6-(phenyl-d 5 )dibenzo[b,d]thiophen-4-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (6.5 g, 49%) as a white solid. 
     
       
         
         
             
             
         
       
     
     A solution of 2-(6-(phenyl-d 5 )dibenzo[b,d]thiophen-4-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (4 g, 10.22 mmol), 3-(triphenylen-2-yl)phenyl trifluoromethanesulfonate (4.95 g, 10.94 mmol), Pd 2 (dba) 3  (0.28 g, 0.31 mmol), SPhos (0.50 g, 1.23 mmol) and K 3 PO 4  (7.05 g, 30.7 mmol) in DME (250 ml) and water (30 ml) was refluxed under nitrogen for 16 hours. After cooling to room temperature, it was diluted with water and the solid was collected by filtration. The solid was purified by column chromatography on silica gel with heptane/toluene (4/1 to 55/45, v/v) as eluent and recrystallization from toluene to yield Compound 2 (3.2 g, 55%) as a white solid. 
     Synthesis of Compound 3 
     
       
         
         
             
             
         
       
     
     
       
         
         
             
             
         
       
     
     A solution of bromine (3.80 ml, 73.8 mmol) in chloroform (50 ml) was added dropwise into a solution of dibenzo[b,d]thiophene-d 8  (5.4 g, 28.1 mmol) in chloroform (95 ml) at 0° C. The reaction mixture was gradually warmed to room temperature and stirred for 5 days. It was quenched with saturated aqueous solution of Na 2 S 2 O 3 . The organic layer was isolated, dried over Na 2 SO 4  and the solvent was evaporated in vacuo. The crude product was purified by recrystallization from ethyl acetate to yield 2,8-dibromodibenzo[b,d]thiophene-d 6  (5.4 g, 57%) as a white solid. 
     
       
         
         
             
             
         
       
     
     A solution of 2,8-dibromodibenzo[b,d]thiophene-d 6  (4 g, 11.49 mmol), carbazole-d 8  (4.43 g, 25.3 mmol), Pd 2 (dba) 3  (0.42 g, 0.46 mmol), SPhos (0.75 g, 1.84 mmol) and tert-BuONa (6.63 g, 68.9 mmol) in o-xylene (100 ml) was refluxed under nitrogen for 17 hours. After cooling to room temperature, it was diluted with ethyl acetate. The organic layer was isolated, washed with brine and dried over Na 2 SO 4 . After evaporation off the solvent, the residue was purified by column chromatography on silica gel with heptanes/toluene (7/3 to 6/4, v/v) as eluent and trituration with ethyl acetate to yield Compound 3 (3.69 g, 60%) as a white solid. 
     Synthesis of Compound 4 
     
       
         
         
             
             
         
       
     
     
       
         
         
             
             
         
       
     
     Into a solution of 2-bromo-9H-fluorene (5.0 g, 20.40 mmol) and THF (20.40 ml) was added tert-BuOK (5.04 g, 44.9 mmol) in one portion at 0° C. It was stirred for 10 min before iodomethane-d 3  (2.54 ml, 40.8 mmol) was added dropwise. The reaction mixture was allowed to warm gradually to room temeprature and stirred for 16 h. The precipitation was removed by filtration, the filtrate was washed with water and brine, dried over Na 2 SO 4  and the solvent was evaporated. The crude product was purified by column chromatography on silica gel with heptane/DCM (100/0 to 95/5, v/v) as eluent to yield 2-bromo-9,9-bis(methyl-d 3 )-9H-fluorene (5.14 g, 90%) as a hazy white oil. 
     
       
         
         
             
             
         
       
     
     Into a solution of 2-bromo-9,9-bis(methyl-d 3 )-9H-fluorene (5.14 g, 18.41 mmol) in THF (46 ml) was added dropwise a 2.5M solution of BuLi in hexane (7.73 ml, 19.33 mmol) under nitrogen at −78° C. It was stirred at this temperature for 1 h before quenching with 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (5.63 ml, 27.6 mmol) added in one portion. The reaction mixture was warmed gradually to room temperature and stirred overnight before quenching with methanol. The solvent was removed in vacuo and the crude product was purified by column chromatography on silica gel with heptane/ethyl acetate (99/1 to 95/1, v/v) as eluent to yield 2-(9,9-bis(methyl-d 3 )-9H-fluoren-2-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (5.0 g, 83%). 
     
       
         
         
             
             
         
       
     
     A solution of 2-(9,9-bis(methyl-d 3 )-9H-fluoren-2-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (3.55 g, 10.87 mmol), 4-bromodibenzo[b,d]thiophene (2.6 g, 9.88 mmol), Pd(PPh 3 ) 4  (0.228 g, 0.198 mmol), K 2 CO 3  (4.10 g, 29.6 mmol) in DME (40 ml) and water (10 ml) was refluxed under nitrogen for 16 h. After cooling to room temperature, the reaction mixture was extracted with toluene. The combined extracts were washed with brine and dried over Na 2 SO 4 . Upon evaporation off the solvent, the crude product was purified by column chromatography on silica gel with heptane/DCM (9/1, v/v) as eluent to yield 4-(9,9-bis(methyl-d 3 )-9H-fluoren-2-yl)dibenzo[b,d]thiophene (3.8 g, 100%) as a viscous, colorless oil. 
     
       
         
         
             
             
         
       
     
     Into a solution of 4-(9,9-bis(methyl-d 3 )-9H-fluoren-2-yl)dibenzo[b,d]thiophene (3.8 g, 9.93 mmol) in THF (62.1 ml) was added dropwise a 1.4 M solution of sec-butyllithium (9.58 ml, 13.41 mmol) under nitrogen at −78° C. It was stirred at this temperature for 2 h before 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (3.04 ml, 14.90 mmol) was added quickly via syringe. The reaction mixture was allowed to warm to room temperature and stirred for 16 h before quenching with methanol. The solvent was evaporated and the residue was purified by column chromatography on silica gel with heptane/DCM (9/1 to 0/1, v/v) as eluent to yield 2-(6-(9,9-bis(methyl-d 3 )-9H-fluoren-2-yl)dibenzo[b,d]thiophen-4-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (2.54 g, 50%) as a white solid. 
     
       
         
         
             
             
         
       
     
     A solution of 2-(6-(9,9-bis(methyl-d 3 )-9H-fluoren-2-yl)dibenzo[b,d]thiophen-4-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (2.54 g, 5.00 mmol), 2-(3-bromophenyl)-4,6-diphenyl-1,3,5-triazine (1.85 g, 4.76 mmol), Pd(PPh 3 ) 4  (0.165 g, 0.143 mmol), K 2 CO 3  (1.976 g, 14.29 mmol) in DME (28.6 ml), toluene (9.53 ml) and water (9.53 ml) was refluxed under nitrogen for 16 h. After cooling to room temperature, the reaction mixture was extracted with toluene. The combined extracts were washed with brine, dried over Na 2 SO 4  and the solvent was evaporated. The crude product was purified by column chromatography on silica gel with heptane/DCM (10/1 to 4/1, v/v) as eluent to yield Compound 4 (2.1 g, 64%) as a white solid. 
     The feasibility of modulating materials evaporation property through replacing atoms with their corresponding less abundant stable isotopes was demonstrated by compositional analysis of films fabricated by single-source co-evaporation of the premixtures containing these components. 
     Comparative Premixture Example: A bicomponent premixture (PM1) was prepared by physically mixing and grinding of Compound H1 and Compound C1 at a weight ratio of 1:1, and loaded into an evaporation source. The premixed compositions were thermally co-evaporated at a rate of 2 Å/s in a vacuum chamber under a pressure 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. The compositions of films were analyzed by high-performance liquid chromatography (HPLC) and the results are collected in Table 1. 
     
       
         
         
             
             
         
       
     
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 HPLC composition (%) of sequentially deposited films from premixture (PM1). For 
               
               
                 the HPLC experiments, a C18 column was used with acetonitrile as eluent, and the detecting 
               
               
                 wavlength is 254 nm. (Due to different absorption coefficients, the HPLC composition may 
               
               
                 or may not agree with the weight ratio.) 
               
            
           
           
               
               
               
            
               
                 Films 
                 Compound H1 
                 Compound C1 
               
               
                   
               
               
                 Plate1 
                 43.0 
                 57.0 
               
               
                 Plate2 
                 41.6 
                 58.4 
               
               
                 Plate3 
                 42.2 
                 57.8 
               
               
                 Plate4 
                 43.1 
                 56.9 
               
               
                 Plate5 
                 44.0 
                 56.0 
               
               
                 Plate6 
                 45.1 
                 54.9 
               
               
                 Plate7 
                 46.1 
                 53.9 
               
               
                 Plate8 
                 47.4 
                 52.6 
               
               
                 Plate9 
                 48.9 
                 51.1 
               
               
                   
               
            
           
         
       
     
     Premixture Example: A bicomponent premixture (PM2) was prepared by physically mixing and grinding of Compound H1 and Compound 1 at a weight ratio of 1:1, and loaded into an evaporation source. The premixed compositions were thermally co-evaporated at a rate of 2 Å/s in a vacuum chamber under a pressure 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. The compositions of films were analyzed by high-performance liquid chromatography (HPLC) and the results are collected in Table 2. 
     
       
         
         
             
             
         
       
     
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 HPLC composition (%) of sequentially deposited films from premixture (PM2). For 
               
               
                 the HPLC experiments, a C18 column was used with acetonitrile as eluent, and the detecting 
               
               
                 wavlength is 254 nm. (Due to different absorption coefficients, the HPLC composition may 
               
               
                 or may not agree with the weight ratio.) 
               
            
           
           
               
               
               
            
               
                 Films 
                 Compound H1  
                 Compound 1 
               
               
                   
               
               
                 Platel 
                 46.9 
                 53.1 
               
               
                 Plate2 
                 45.2 
                 54.8 
               
               
                 Plate3 
                 45.1 
                 54.9 
               
               
                 Plate4 
                 44.9 
                 55.1 
               
               
                 Plate5 
                 44.8 
                 55.2 
               
               
                 Plate6 
                 45.2 
                 54.8 
               
               
                 Plate7 
                 45.8 
                 54.2 
               
               
                 Plate8 
                 46.4 
                 53.6 
               
               
                   
               
            
           
         
       
     
     The data in Table 1 reveals a small compositional separation between Compound H1 and Compound C1 during sequential evaporation, which might suggest that Compound C1 evaporates slightly faster than Compound H1. The data in Table 2 shows that the compositions for Compound H1 and Compound 1 remain nearly constant throughout the sequential evaporation, except for minor random fluctuations. These data indicate that partial deuteration makes Compound 1 evaporates slightly slower than Compound C1, enabling nearly perfect premixing between Compound H1 and Compound 1. It is worth noting that the compositional separation between Compound H1 and Compound C1 is very minor, seeking perfect premixing between these two classes of compounds through alternative methods, such as derivatization and isomerization, might rely heavily on fortuitous outcomes. On the other hand, the fine-tuning capability of isotopes, herein demonstrated by deuteration, proves to be a superior option to pursue perfect premixing. 
     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-Hexaazatriphenylenehexacarbonitrile; 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. Fc + /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 A below. Table A lists non-limiting classes of materials, non-limiting examples of compounds for each class, and references that disclose the materials. 
     
       
         
           
               
               
               
             
               
                 TABLE A 
               
               
                   
               
               
                 MATERIAL 
                 EXAMPLES OF MATERIAL 
                 PUBLICATIONS 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
            
               
                 Hole injection materials 
               
            
           
           
               
               
               
            
               
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                 CF x   Fluorohydrocarbon  polymer 
                 
                   
                     
                     
                         
                         
                     
                   
                 
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                 Conducting polymers (e.g., PEDOT:PSS, polyaniline, polypthiophene) 
                 
                   
                     
                     
                         
                         
                     
                   
                 
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                 Phosphonic acid and  sliane SAMs 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 US20030162053 
               
               
                   
               
               
                 Triarylamine or  polythiophene  polymers with conductivity dopants  
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 EP1725079A1  
               
               
                   
               
               
                   
                 
                   
                     
                     
                         
                         
                     
                   
                 
                   
               
               
                   
               
               
                   
                 
                   
                     
                     
                         
                         
                     
                   
                 
                   
               
               
                   
               
               
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                 organic complexes 
                   
                   
               
               
                 Metal organometallic  
                   
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                 complexes 
                   
                   
               
               
                 Cross-linkable  
                   
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                 compounds 
                   
                   
               
               
                   
               
               
                 Polythiophene based  polymers and  copolymers 
                 
                   
                     
                     
                         
                         
                     
                   
                 
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                 Zinc complexes  
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 WO2010056066  
               
               
                   
               
               
                 Chrysene based  compounds  
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 WO2011086863  
               
               
                   
               
            
           
           
               
            
               
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                 WO2008056746  
               
               
                   
               
               
                   
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 WO2010107244  
               
               
                   
               
               
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                 JP2008074939  
               
               
                   
               
               
                   
                 
                   
                     
                     
                         
                         
                     
                   
                 
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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.