Patent Publication Number: US-2020295307-A1

Title: Purcell enhancement via tamm plasmon stack in oled structures

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a non-provisional, and claims the priority benefit, of U.S. patent application Ser. No. 62/817,409, filed Mar. 12, 2019, the entire contents of which are incorporated herein by reference. 
    
    
     FIELD 
     The present invention relates to structures and arrangements for use in devices such as organic light emitting diodes, and devices including the same. 
     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 diodes/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. Alternatively the OLED can be designed to emit white light. In conventional liquid crystal displays emission from a white backlight is filtered using absorption filters to produce red, green and blue emission. The same technique can also be used with OLEDs. The white OLED can be either a single EML device or a stack structure. Color may be measured using CIE coordinates, which are well known to the art. 
     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 
     According to an embodiment, an organic light emitting diode/device (OLED) is also provided. The OLED can include an anode, a cathode, and an organic layer, disposed between the anode and the cathode. According to an embodiment, the organic light emitting device is incorporated into one or more device selected from a consumer product, an electronic component module, and/or a lighting panel. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an organic light emitting device. 
         FIG. 2  shows an inverted organic light emitting device that does not have a separate electron transport layer. 
         FIG. 3A  shows a schematic representation of a reflective layer disposed in a stack with a plasmonically-active metal thin film to form a Tamm plasmon stack as disclosed herein. 
         FIG. 3B  shows an example of a stacked arrangement that includes an OLED and a Tamm plasmon stack as disclosed herein, in which the OLED stack is located within the Tamm plasmon stack. 
         FIG. 3C  shows an example of a stacked arrangement that includes an OLED and a Tamm plasmon stack as disclosed herein, in which the OLED stack is located outside of the Tamm plasmon stack. 
         FIG. 4A  shows a simulated reflectance spectrum for a DBR at normal incidence with no adjacent metal film, resulting in a photonic band gap as disclosed herein. 
         FIG. 4B  shows a reflectance minimum at normal incidence for an example device as shown in  FIG. 3B , which occurs within the photonic band gap as disclosed herein. 
         FIG. 5A  schematically depicts the expected trend that (a) device stability increases with increasing number of DBR layers while external quantum efficiency decreases. 
         FIG. 5B  schematically depicts the an optimal number of DBR layers resulting in a longest luminance-adjusted lifetime for the OLED as a result of the data in  FIG. 5A . 
         FIG. 6  shows the electric field within an example device such as the device of  FIG. 3B , where the OLED is arranged such that the peak in the electric field occurs at the same position as the EML to maximize the Purcell enhancement as disclosed herein. 
         FIG. 7  shows an electric field within two comparative example devices in which the OLED thickness varies by 10 nm, resulting in an altered electric field peak position and intensity as disclosed herein. 
         FIGS. 8A and 8B  schematically depict device structures similar to those shown in  FIGS. 3B and 3C  which further incorporate an additional Tamm plasmon stack as 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”), 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. 
     In some embodiments disclosed herein, emissive layers or materials, such as emissive layer  135  and emissive layer  220  shown in  FIGS. 1-2 , respectively, may include quantum dots. An “emissive layer” or “emissive material” as disclosed herein may include an organic emissive material and/or an emissive material that contains quantum dots or equivalent structures, unless indicated to the contrary explicitly or by context according to the understanding of one of skill in the art. Such an emissive layer may include only a quantum dot material which converts light emitted by a separate emissive material or other emitter, or it may also include the separate emissive material or other emitter, or it may emit light itself directly from the application of an electric current. Similarly, a color altering layer, color filter, upconversion, or downconversion layer or structure may include a material containing quantum dots, though such layer may not be considered an “emissive layer” as disclosed herein. In general, an “emissive layer” or material is one that emits an initial light, which may be altered by another layer such as a color filter or other color altering layer that does not itself emit an initial light within the device, but may re-emit altered light of a different spectra content based upon initial light emitted by the emissive layer. 
     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 can be incorporated into a wide variety of electronic component modules (or units) that can be incorporated into a variety of electronic products or intermediate components. Examples of such electronic products or intermediate components include display screens, lighting devices such as discrete light source devices or lighting panels, etc. that can be utilized by the end-user product manufacturers. Such electronic component modules can optionally include the driving electronics and/or power source(s). Devices fabricated in accordance with embodiments of the invention can be incorporated into a wide variety of consumer products that have one or more of the electronic component modules (or units) incorporated therein. A consumer product comprising an OLED that includes the compound of the present disclosure in the organic layer in the OLED is disclosed. Such consumer products would include any kind of products that include one or more light source(s) and/or one or more of some type of visual displays. Some examples of such consumer products include flat panel displays, computer monitors, medical monitors, televisions, billboards, lights for interior or exterior illumination and/or signaling, heads-up displays, fully or partially transparent displays, flexible displays, laser printers, telephones, mobile phones, tablets, phablets, personal digital assistants (PDAs), wearable devices, laptop computers, digital cameras, camcorders, viewfinders, micro-displays (displays that are less than 2 inches diagonal), 3-D displays, virtual reality or augmented reality displays, vehicles, video walls comprising multiple displays tiled together, theater or stadium screen, and 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. 
     In some embodiments, the OLED has one or more characteristics selected from the group consisting of being flexible, being rollable, being foldable, being stretchable, and being curved. In some embodiments, the OLED is transparent or semi-transparent. In some embodiments, the OLED further comprises a layer comprising carbon nanotubes. 
     In some embodiments, the OLED further comprises a layer comprising a delayed fluorescent emitter. In some embodiments, the OLED comprises a RGB pixel arrangement or white plus color filter pixel arrangement. In some embodiments, the OLED is a mobile device, a hand held device, or a wearable device. In some embodiments, the OLED is a display panel having less than 10 inch diagonal or 50 square inch area. In some embodiments, the OLED is a display panel having at least 10 inch diagonal or 50 square inch area. In some embodiments, the OLED is a lighting panel. 
     In some embodiments of the emissive region, the emissive region further comprises a host. 
     In some embodiments, the compound can be an emissive dopant. In some embodiments, the compound can produce emissions via phosphorescence, fluorescence, thermally activated delayed fluorescence, i.e., TADF (also referred to as E-type delayed fluorescence), triplet-triplet annihilation, or combinations of these processes. 
     The OLED disclosed herein can be incorporated into one or more of a consumer product, an electronic component module, and a lighting panel. The organic layer can be an emissive layer and the compound can be an emissive dopant in some embodiments, while the compound can be a non-emissive dopant in other embodiments. 
     The organic layer can also include a host. In some embodiments, two or more hosts are preferred. In some embodiments, the hosts used maybe a) bipolar, b) electron transporting, c) hole transporting or d) wide band gap materials that play little role in charge transport. In some embodiments, the host can include a metal complex. The host can be an inorganic compound. 
     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. 
     Various materials may be used for the various emissive and non-emissive layers and arrangements disclosed herein. Examples of suitable materials are disclosed in U.S. Patent Application Publication No. 2017/0229663, which is incorporated by reference in its entirety. 
     Conductivity Dopants 
     A charge transport layer can be doped with conductivity dopants to substantially alter its density of charge carriers, which will in turn alter its conductivity. The conductivity is increased by generating charge carriers in the matrix material, and depending on the type of dopant, a change in the Fermi level of the semiconductor may also be achieved. Hole-transporting layer can be doped by p-type conductivity dopants and n-type conductivity dopants are used in the electron-transporting layer. 
     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. 
     EBL 
     An electron blocking layer (EBL) may be used to reduce the number of electrons and/or excitons that leave the emissive layer. The presence of such a blocking layer in a device may result in substantially higher efficiencies, and or longer lifetime, 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 some embodiments, the EBL material has a higher LUMO (closer to the vacuum level) and/or higher triplet energy than the emitter closest to the EBL interface. In some embodiments, the EBL material has a higher LUMO (closer to the vacuum level) and or higher triplet energy than one or more of the hosts closest to the EBL interface. In one aspect, the compound used in EBL contains the same molecule or the same functional groups used as one of the hosts described below. 
     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. Any host material may be used with any dopant so long as the triplet criteria is satisfied. 
     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 and/or longer lifetime 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 some embodiments, the HBL material has a lower HOMO (further from the vacuum level) and or higher triplet energy than the emitter closest to the HBL interface. In some embodiments, the HBL material has a lower HOMO (further from the vacuum level) and or higher triplet energy than one or more of the hosts closest to the HBL interface. 
     ETL 
     An electron transport layer (ETL) may include a material capable of transporting electrons. The 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. 
     Charge Generation Layer (CGL) 
     In tandem or stacked OLEDs, the CGL plays an essential role in the performance, which is composed of an n-doped layer and a p-doped layer for injection of electrons and holes, respectively. Electrons and holes are supplied from the CGL and electrodes. The consumed electrons and holes in the CGL are refilled by the electrons and holes injected from the cathode and anode, respectively; then, the bipolar currents reach a steady state gradually. Typical CGL materials include n and p conductivity dopants used in the transport layers. 
     When an emitter, such as the organic emissive materials and layers disclosed herein, is placed in an environment with an increased density of photonic states relative to vacuum, the emission rate of the emitter increases. This is known as the Purcell effect. This enhanced emissive and non-radiative rates decrease the length of time the emitter stays in the excited state, thereby stabilizing the emitter and reducing the aging rate of a device such as an OLED that incorporates the emitter. Some OLED architectures take advantage of this effect by using an enhancement layer having a plasmonic material that exhibits surface plasmon resonance that non-radiatively couples to an emissive material, thereby allowing for transfer of excited state energy from the emissive material to a non-radiative mode of surface plasmon polaritons. Such devices are described, for example, in U.S. Pat. No. 9,960,386, the disclosure of which is incorporated by reference in its entirety. 
     The present disclosure provides device architectures that allow for coupling emitter excited state energy into a Tamm plasmon mode, which is an emissive plasmon state (i.e. it lies above the light line). 
     Tamm plasmon-polaritons (TPPs), also known as optical Tamm states, are surface electromagnetic states that are confined to the interface or region between two reflective photonic structures. Examples of suitable reflective structures include plasmonically-active metal thin films, typically, but not limited to, Ag, Al, Au, Ir, Pt, Ni, Cu, W, Ta, Fe, Cr, Mg, Ga, Rh, Ti, Ru, Pd, In, Bi, Ca and/or stacks or alloys of these metals, from 10-1000 nm thick, a distributed Bragg reflector (DBR), which includes alternating layers of high- and low-refractive index dielectric materials with the thickness of each layer set by the condition λ/4 n, where n is the refractive index of the layer and λ is the desired wavelength of the DBR, and/or a photonic crystal layer. More generally, a layer is “reflective” as used herein if it is sufficiently reflective to form a Tamm plasmon stack with another reflective layer as described in further detail below. That is, two layers are “reflective” as used herein if, when placed in a stacked configuration with each other, Tamm plasmon-polaritons are formed. In contrast to surface plasmon polaritons (SPPs), TPPs are emissive states that can couple energy to the far field without the aid of a prism or similar structure. It has been found that incorporating a structure that creates TPPs within an OLED may provide enhanced device stability while preserving an acceptable external quantum efficiency (EQE). 
       FIG. 3A  shows an example of such a structure, referred to herein as a “Tamm plasmon stack,” in which this effect is observed, i.e., in which TPPs are formed. The stack includes two outer reflective layers  310 ,  320  as previously disclosed. Each reflective layer may be a metal thin film or stacks of metal thin films, a Bragg reflector, or an equivalent structure. In the example shown in  FIG. 3A , a first reflective layer  310  is shown as a single layer, such as a metal thin film, and the second reflective layer  320  is shown as a multi-layer structure, such as a set of stacked thin metal films or a distributed Bragg reflector. However, these specific reflective layers are not required and may be inverted, or both layers may be the same structure. Similarly, a reflective metal thin film layer may include multiple sub-layers, which may be the same or different materials. The two layers  310 ,  320  may be described as forming a Tamm plasmon stack  300  as disclosed herein. Notably, TPPs may be created by a Tamm plasmon stack even where the stack is arranged in a stack with another structure. In some cases, other layers may be arranged between the reflective layers  310 ,  320  that define the Tamm plasmon stack without adversely affecting operation of the Tamm plasmon stack. 
       FIGS. 3B and 3C  show examples of an OLED device that includes a Tamm plasmon stack. As previously disclosed, the Tamm plasmon stack  300  may be formed by two outer reflective layers  310 ,  320 . An OLED stack  350  is disposed in a stack with the Tamm plasmon stack  300 . As used herein, the Tamm plasmon stack and the OLED stack may be described as being in a “stack” with one another and/or the devices as a whole may be described as being arranged in a “stacked configuration” since they at least partially overlap one another vertically. A particular type of stack, such as an OLED stack or a Tamm plasmon stack, may be referred to by the outermost layers that form the stack, whether or not they are also shared by other stacks or components. For example, referring to  FIG. 3B , the OLED stack is formed by the reflective layer  310  (which operates as an electrode for the OLED  350 ) and the electrode  352 , and the Tamm plasmon stack  300  is formed by the reflective layer  310  and the lower reflective layer  320 . Since the same layer may operate both as a reflective layer for a Tamm plasmon stack and as an electrode for an OLED in a stacked configuration with the Tamm plasmon stack, the same materials and configurations described with respect to the Tamm plasmon stack reflective layers may be used as electrode layers in any configurations disclosed herein. For example, electrodes that form OLED stacks as disclosed herein may include one or more layers of Ag, Al, Au, Ir, Pt, Ni, Cu, W, Ta, Fe, Cr, Mg, Ga, Rh, Ti, Ru, Pd, In, Bi, Ca, or stacks or alloys thereof. Similarly, where a Tamm plasmon stack uses a DBR as a reflective layer, one or more layers of the DBR may function as an electrode for an OLED arranged in a stack with the Tamm plasmon stack. 
     The OLED stack may include one or more of the layers described with respect to  FIGS. 1 and 2 , including but not limited to emissive layers, transport layers, blocking layers, charge generation layers, and the like. The OLED stack  350  may be disposed within the Tamm plasmon stack  300 , as in  FIG. 3B , or outside the Tamm plasmon stack  300  as shown in  FIG. 3C . It may be preferred for a thin metal film in the Tamm plasmon stack to be less than 50 nm thick, in which case the electric field may pass through the metal and reach the EML of the OLED stack  351 . This thickness may be used regardless of whether the thin film also operates as an electrode for an OLED stack or not. Depending on the thickness of the metal, the OLED may emit from either side of the stack. 
     In the arrangement shown in  FIG. 3B , the OLED stack is formed by two electrodes  310 ,  352 , one or more of which may also function as a reflective layer for the Tamm plasmon stack, with one or more emissive layers  351  disposed between the electrodes  310 ,  352 . In this example, the first reflective layer  310  is also an upper electrode for the OLED stack  350 . Alternatively or in addition, the electrode  352  may be the second reflective layer  320 , or may be provided by one layer of a multi-layer reflective layer structure  320 . The electrode  352  may be ITO or any other suitable material or combination of materials. 
     The example structure shown in  FIG. 3C  has many of the same components as  FIG. 3B . In this example, the OLED stack  350  is formed by two electrodes  352 ,  353 . As with the arrangement shown in  FIG. 3B , one of the electrodes  352  also provides a reflective layer for the Tamm plasmon stack  300 . However, in this arrangement the lower electrode  352  provides the second layer for the Tamm plasmon stack  300  instead of the upper electrode  353 . In this arrangement the second electrode  353  may be reflective and may be identical to the reflective layer  310  in  FIG. 3B , or it may be different. 
     As previously noted, one or more of the reflective layers that forms a Tamm plasmon stack as disclosed herein may be a distributed Bragg reflector (DBR). In such a configuration, the DBR component may create a photonic band gap, i.e. a wavelength range over which little or no light is transmitted through the DBR stack.  FIG. 4A  shows a simulated reflectance spectrum for a DBR at normal incidence with no adjacent metal film, which results in the photonic band gap  410 .  FIG. 4B  shows the reflectance minimum (highlighted by a dashed box) at normal incidence for an example device as illustrated in  FIG. 3B , which occurs within the photonic band gap. Notably, the addition of the metal layer results in the characteristic sharp dip in the reflection spectrum within the photonic band gap, indicating the presence of the TPP mode. 
     While the TPP mode is an emissive state, the EQE of the OLED may still be limited due to a relatively narrow spectral response of the TPP mode. The select wavelengths of light of the TPP mode may not overlap well with a broad spectrum emitter, such as one having a 50-100 nm or more FWHM emission spectrum. For TPP modes, the larger the number of layers in the DBR the less spectral width it has, while the electric field intensity increases. Thus, there may be an optimal number of DBR layers to maximize the luminance-adjusted OLED lifetime, that balances both brightness (i.e., EQE) and stability. For example, it may be desirable for the DBR to have 2-1000 layers.  FIG. 5A  schematically depicts the expected trend that device stability increases with increasing number of DBR layers while external quantum efficiency decreases. This results in an optimal number of DBR layers providing the longest luminance-adjusted lifetime for the OLED, as shown in  FIG. 5B . 
     It may be desirable to incorporate these phenomena to enhance the OLED emission rate, and hence emitter stability, by arranging the peak of the electric field representing the TPP mode to spatially overlap the emissive layer emissive layer in the OLED stack.  FIG. 6  shows the electric field strength and refractive index as functions of position within a stacked device such as shown in  FIG. 3B . Notably, the peak in the electric  610  field occurs in the OLED stack at the same position as the emissive layer to maximize the Purcell enhancement. The thickness of the OLED stack also may affect the position of peaks in the electric field, as shown in  FIG. 7 , which shows that a 10 nm difference in OLED thickness can substantially alter the electric field peak location(s) and intensity. Such structural fine-tuning may be achieved via transfer matrix simulations as well as empirical optimization of the physical device structure. 
     In some cases, it may be desirable to include multiple Tamm plasmon stacks in a single stacked device.  FIGS. 8A and 8B  show examples of such devices.  FIG. 8A  shows a device having the same structure as the device in  FIG. 3B , with an additional Tamm plasmon stack  800  formed by the upper electrode and an additional reflective layer  810  which may be, for example, a DBR, multi-layer metal thin film, or any other suitable layer as disclosed herein. As previously disclosed, the OLED  350  may be disposed within a Tamm plasmon stack, in this case the stack  300 . Alternatively, the OLED may be disposed within the Tamm plasmon stack  800 , such as between the reflective layers  810 ,  310 , in which case one or more of the layers may function as an electrode for the OLED stack. 
     Alternatively, the OLED stack  350  may be disposed outside of and in a stack with the two Tamm plasmon stacks, as shown in  FIG. 8B . In this configuration, the layer  310  operates as an electrode for the OLED stack  350  and a reflective layer for the Tamm plasmon stack  800 . In other configurations, the reflective layer and electrode may be provided by two separate and distinct, but immediately adjacent, layers. Similarly, the layer  352  operates as an electrode for the OLED stack  350  and a reflective layer for the Tamm plasmon stack  300 . 
     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.