Patent Application
20230091551
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, on a conventional energy level diagram, with the vacuum level at the top, a “shallower” energy level appears higher, or closer to the top, of such a diagram than a “deeper” energy level, which appears lower, or closer to the bottom.
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.
Phosphorescent white organic light emitting devices (WOLEDs) are attractive solid state lighting sources due to their high-power efficiency, tunable spectrum, and potentially long lifetime. White light emission can be obtained from multilayer WOLED structures, in which different layers emit different parts of the visible spectrum. However, the color spectrum of WOLEDs often changes with the operating current density because exciton formation zone shifts in emission layers when the injected charges are not balanced. Excitation of phosphors occurs via charge trapping: holes are transported on the phosphors, electrons on the EML host that ultimately transfer to the phosphors as well.
There is a need in the art for a WOLED structure employing a hole transport barrier layer that impedes the hole conduction and improves the charge balance. The present invention satisfies that need; this design reduces the WOLED spectrum color change at different operating current densities and potentially increases WOLED lifetime without decreasing quantum efficiency.
In one aspect, an organic light emitting device comprises an anode, a first hole transport layer positioned over the anode, a barrier transport layer positioned over the first hole transport layer, a second hole transport layer positioned over the barrier transport layer, at least one emissive layer positioned over the second hole transport layer, and a cathode positioned over the at least one emissive layer. In one embodiment, the barrier transport layer is positioned in direct contact with the first and second hole transport layers.
In one embodiment, the first and second hole transport layers have the same composition. In one embodiment, the first hole transport layer comprises a material not present in the second hole transport layer. In one embodiment, the barrier transport layer comprises a hole blocking material with a highest occupied molecular orbital energy that is lower than the that of a material included in the first and second hole transport layers. In one embodiment, the hole blocking material possesses high carrier mobilities. In one embodiment, the barrier transport layer is configured to reduce a shift of a location of exciton formation with changes in current.
In one embodiment, the at least one emissive layer comprises a plurality of emissive layers. In one embodiment, the device is configured to emit white light. In one embodiment, the emissive layer has a thickness between 5 nm and 100 nm. In one embodiment, the device further comprises an electron transport layer positioned between the cathode and the emissive layer. In one embodiment, the electron transport layer comprises at least first and second electron transport sublayers, wherein the first electron transport sublayer comprises a material different from the second electron transport sublayer.
In one aspect, an organic light emitting device comprises an anode and a cathode, an emissive layer having an anode-facing surface and a cathode-facing surface, positioned between the anode and the cathode, a first hole transport layer positioned between the anode and the emissive layer, and a barrier transport layer positioned between the anode and the emissive layer, at a distance from the anode-facing surface of the emissive layer of between 1 nm and 50 nm.
In one embodiment, the device further comprises a second hole transport layer positioned between the anode and the emissive layer. In one embodiment, the barrier transport layer is positioned between the first and second hole transport layers. In one embodiment, the barrier transport layer is positioned between the anode and the emissive layer at a distance from the anode-facing surface of the emissive layer of between 1 nm and 10 nm. In one embodiment, the device further comprises an electron transport layer positioned between the emissive layer and the cathode.
In one embodiment, the electron transport layer comprises at least a first electron transport sublayer and a second electron transport sublayer having a material different from the first electron transport sublayer. In one embodiment, emissive layer comprises a plurality of emissive sublayers. In one embodiment, the plurality of emissive sublayers comprise red, green, and blue sublayers.
The foregoing purposes and features, as well as other purposes and features, will become apparent with reference to the description and accompanying figures below, which are included to provide an understanding of the invention and constitute a part of the specification, in which like numerals represent like elements, and in which:
It is to be understood that the figures and descriptions of the present invention have been simplified to illustrate elements that are relevant for a clearer comprehension of the present invention, while eliminating, for the purpose of clarity, many other elements found in systems and methods of a white organic light emitting device with stable spectrum employing transport barrier layers. Those of ordinary skill in the art may recognize that other elements and/or steps are desirable and/or required in implementing the present invention. However, because such elements and steps are well known in the art, and because they do not facilitate a better understanding of the present invention, a discussion of such elements and steps is not provided herein. The disclosure herein is directed to all such variations and modifications to such elements and methods known to those skilled in the art.
Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, the preferred methods and materials are described.
As used herein, each of the following terms has the meaning associated with it in this section.
The articles “a” and “an” are used herein to refer to one or to more than one (i.e., to at least one) of the grammatical object of the article. By way of example, “an element” means one element or more than one element.
“About” as used herein when referring to a measurable value such as an amount, a temporal duration, and the like, is meant to encompass variations of ±20%, ±10%, ±5%, ±1%, and ±0.1% from the specified value, as such variations are appropriate.
Ranges: throughout this disclosure, various aspects of the invention can be presented in a range format. It should be understood that the description in range format is merely for convenience and brevity and should not be construed as an inflexible limitation on the scope of the invention. Where appropriate, the description of a range should be considered to have specifically disclosed all the possible subranges as well as individual numerical values within that range. For example, description of a range such as from 1 to 6 should be considered to have specifically disclosed subranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6 etc., as well as individual numbers within that range, for example, 1, 2, 2.7, 3, 4, 5, 5.3, and 6. This applies regardless of the breadth of the range.
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.
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 F4-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.
The simple layered structure illustrated in
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
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 processability 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 disclosure 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 disclosure 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 disclosure 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, curved 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, rollable displays, foldable displays, stretchable 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, theaters or stadium screens, light therapy devices, and signs. Various control mechanisms may be used to control devices fabricated in accordance with the present disclosure, 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.
Although certain embodiments of the disclosure are discussed in relation to one particular device or type of device (for example OLEDs) it is understood that the disclosed improvements to light outcoupling properties of a substrate may be equally applied to other devices, including but not limited to PLEDs, OPVs, charge-coupled devices (CCDs), photosensors, or the like.
Although exemplary embodiments described herein may be presented as methods for producing particular circuits or devices, for example OLEDs, it is understood that 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, or other organic electronic circuits or components, 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, at least one of the anode, the cathode, or a new layer disposed over the organic emissive layer functions as an enhancement layer. The enhancement layer comprises a plasmonic material exhibiting surface plasmon resonance that non-radiatively couples to the emitter material and transfers excited state energy from the emitter material to non-radiative mode of surface plasmon polariton. The enhancement layer is provided no more than a threshold distance away from the organic emissive layer, wherein the emitter material has a total non-radiative decay rate constant and a total radiative decay rate constant due to the presence of the enhancement layer and the threshold distance is where the total non-radiative decay rate constant is equal to the total radiative decay rate constant. In some embodiments, the OLED further comprises an outcoupling layer. In some embodiments, the outcoupling layer is disposed over the enhancement layer on the opposite side of the organic emissive layer. In some embodiments, the outcoupling layer is disposed on opposite side of the emissive layer from the enhancement layer but still outcouples energy from the surface plasmon mode of the enhancement layer. The outcoupling layer scatters the energy from the surface plasmon polaritons. In some embodiments this energy is scattered as photons to free space. In other embodiments, the energy is scattered from the surface plasmon mode into other modes of the device such as but not limited to the organic waveguide mode, the substrate mode, or another waveguiding mode. If energy is scattered to the non-free space mode of the OLED other outcoupling schemes could be incorporated to extract that energy to free space. In some embodiments, one or more intervening layer can be disposed between the enhancement layer and the outcoupling layer. The examples for interventing layer(s) can be dielectric materials, including organic, inorganic, perovskites, oxides, and may include stacks and/or mixtures of these materials.
The enhancement layer modifies the effective properties of the medium in which the emitter material resides resulting in any or all of the following: a decreased rate of emission, a modification of emission line-shape, a change in emission intensity with angle, a change in the stability of the emitter material, a change in the efficiency of the OLED, and reduced efficiency roll-off of the OLED device. Placement of the enhancement layer on the cathode side, anode side, or on both sides results in OLED devices which take advantage of any of the above-mentioned effects. In addition to the specific functional layers mentioned herein and illustrated in the various OLED examples shown in the figures, the OLEDs according to the present disclosure may include any of the other functional layers often found in OLEDs.
The enhancement layer can be comprised of plasmonic materials, optically active metamaterials, or hyperbolic metamaterials. As used herein, a plasmonic material is a material in which the real part of the dielectric constant crosses zero in the visible or ultraviolet region of the electromagnetic spectrum. In some embodiments, the plasmonic material includes at least one metal. In such embodiments the metal may include at least one of Ag, Al, Au, Ir, Pt, Ni, Cu, W, Ta, Fe, Cr, Mg, Ga, Rh, Ti, Ru, Pd, In, Bi, Ca alloys or mixtures of these materials, and stacks of these materials. In general, a metamaterial is a medium composed of different materials where the medium as a whole acts differently than the sum of its material parts. In particular, we define optically active metamaterials as materials which have both negative permittivity and negative permeability. Hyperbolic metamaterials, on the other hand, are anisotropic media in which the permittivity or permeability are of different sign for different spatial directions. Optically active metamaterials and hyperbolic metamaterials are strictly distinguished from many other photonic structures such as Distributed Bragg Reflectors (“DBRs”) in that the medium should appear uniform in the direction of propagation on the length scale of the wavelength of light. Using terminology that one skilled in the art can understand: the dielectric constant of the metamaterials in the direction of propagation can be described with the effective medium approximation. Plasmonic materials and metamaterials provide methods for controlling the propagation of light that can enhance OLED performance in a number of ways.
In some embodiments, the enhancement layer is provided as a planar layer. In other embodiments, the enhancement layer has wavelength-sized features that are arranged periodically, quasi-periodically, or randomly, or sub-wavelength-sized features that are arranged periodically, quasi-periodically, or randomly. In some embodiments, the wavelength-sized features and the sub-wavelength-sized features have sharp edges.
In some embodiments, the outcoupling layer has wavelength-sized features that are arranged periodically, quasi-periodically, or randomly, or sub-wavelength-sized features that are arranged periodically, quasi-periodically, or randomly. In some embodiments, the outcoupling layer may be composed of a plurality of nanoparticles and in other embodiments the outcoupling layer is composed of a plurality of nanoparticles disposed over a material. In these embodiments the outcoupling may be tunable by at least one of varying a size of the plurality of nanoparticles, varying a shape of the plurality of nanoparticles, changing a material of the plurality of nanoparticles, adjusting a thickness of the material, changing the refractive index of the material or an additional layer disposed on the plurality of nanoparticles, varying a thickness of the enhancement layer, and/or varying the material of the enhancement layer. The plurality of nanoparticles of the device may be formed from at least one of metal, dielectric material, semiconductor materials, an alloy of metal, a mixture of dielectric materials, a stack or layering of one or more materials, and/or a core of one type of material and that is coated with a shell of a different type of material. In some embodiments, the outcoupling layer is composed of at least metal nanoparticles wherein the metal is selected from the group consisting of Ag, Al, Au, Ir, Pt, Ni, Cu, W, Ta, Fe, Cr, Mg, Ga, Rh, Ti, Ru, Pd, In, Bi, Ca, alloys or mixtures of these materials, and stacks of these materials. The plurality of nanoparticles may have additional layer disposed over them. In some embodiments, the polarization of the emission can be tuned using the outcoupling layer. Varying the dimensionality and periodicity of the outcoupling layer can select a type of polarization that is preferentially outcoupled to air. In some embodiments the outcoupling layer also acts as an electrode of the device.
WOLEDs for general lighting require stable spectra that are largely independent of brightness. Described herein, the phosphorescent WOLED spectra are stabilized using a barrier transport layer (BTL) inserted between two hole transport layers. As used herein, a barrier transport layer refers to a hole blocking layer positioned on the anode side of the emissive layer, adjacent to or between one or more hole transport layers.
The spectral improvement induced by the BTL is investigated in three-color WOLEDs and two-color OLEDs. For a WOLED with a peak external quantum efficiency of 15±1% at 0.1 mA/cm2, the color difference (ΔEab*) in the CIELab 1976 color space is noticeably reduced from 12.8±0.2 to 8.6±0.1 as the current density is increased from 1 to 100 mA/cm2. A numerical model is derived to systematically understand the role of charge blocking in tailoring the exciton distribution in the complex WOLED layer structure. The model is validated by direct measurements of the exciton distribution within an OLED emission layer. The color shift with current in conventional WOLEDs is attributed to unbalanced charge injection due to asymmetrical energy barriers for electrons and holes between the emission layer and the surrounding transport and injection layers. Addition of a barrier transport layer balances the charge conduction, generates a uniform exciton profile, and reduces the spectral shift.
Phosphorescent white organic light emitting diodes (WOLEDs) are promising solid state lighting sources given their high-power efficiencies, color rendering indexes (CRI) and long operating lifetimes (Coburn, C., et al, ACS Photonics 2018, 5, 630-635. Kato, K., et al., J. Photopolym. Sci. Technol. 2015, 28 (3), 335-340. Qu, B., et al., ACS Nano 2020, 14 (10), 14157-14163. Sasabe, H. and Kido, J., J. Mater. Chem. C 2013, 1 (9), 1699-1707.) White light emission is typically obtained by combining several emitting organic molecules that span the visible spectrum. Often, WOLEDs rely on arranging multiple color emitting elements within a united emitting layer (EML).
However, this deceptively simple architecture can exhibit unwanted intensity-dependent color-shifts (Andrade, B. B. W. D. and Thompson, M. E., Adv. Mater. 2002, 2, 147; Sun, Y., et al., Nature 2006, 440, 908; Zhao, C., et al., J. Mater. Chem. C 2018, 6 (35), 9510-9516. Sun, Q., et al., Appl. Phys. Lett. 2008, 92 (25), 251108). Progress has been made in realizing stable white spectra by using additional chromatic layers to enhance the color balance, or transport layers in the middle of the emission zone to alter the charge balance (Li, J., et al., J. Phys. D. Appl. Phys. 2021, 54 (16), 165105; Gather, M. C., et al., Adv. Mater. 2007, 19 (24), 4460-4465; Wang, Q., et al., J. Phys. D. Appl. Phys. 2013, 46 (15), 155102; Yang, F., et al., Opt. Mater. (Amst). 2018, 82, 130-134). These interlayers adjust the charge transport by introducing energy barriers or low mobility regions within, or directly adjacent to the EMLs, and can sometimes improve device efficiency (Wang, Q., et al., J. Phys. D. Appl. Phys. 2013, 46 (15), 155102; Loeser, F., et al., J. Photonics Energy. 2012, 2, 021207). However, such interlayers add fabrication complexity when they are used in high quality lighting sources, whose EML consists of at least three color elements to generate the full spectrum. Reliable transport materials with energy levels compatible with all EML elements are also limited. Moreover, it can be challenging to predict the exciton distribution across broad emission zones common to WOLEDs, making their optimal design accessible only through the fabrication and measurement of numerous, complex designs that iterate through different materials, layer thicknesses and their positions within the device stack.
The invention described herein is focused on stabilizing the WOLED spectrum using a single charge blocking layer that is external to the three-color EML, and the effects that charge balance has on determining the location of exciton formation are investigated. The charge distribution and balance is studied through the fabrication of single carrier devices combined with a charge drift-diffusion model that accurately simulates the exciton recombination dynamics within the layers, and across the interfaces (Altazin, S., et al., J. Appl. Phys. 2018, 124 (13), 135501; Ruhstaller, B., et al., J. Appl. Phys. 2001, 89, 4575; Arkhipov, V. I., et al., J. Appl. Phys. 2001, 90, 2352; Lu, F., et al., J. Semicond. 2014, 35, 044005.) Simulation results were verified by direct measurements of the exciton distribution in multilayer OLEDs (Staudigel, J., et al., J. Appl. Phys. 1999, 86 (7), 3895-3910).
Specifically, WOLEDs employing three phosphorescent emitters in the red, green and blue, are fabricated. The spectral change with current originates from the shift of the exciton formation zone between different doped layers within the EML. Inserting materials to create energy barriers within the hole transport layers (HTLs) balances the charge transport and reduces the shift of the location of exciton formation with current. Including a barrier transport layer (BTL) within the HTL noticeably stabilizes the WOLED Commission Internationale de L'Eclairage (CIE) 1931 chromaticity coordinate drift (ΔCIE) from (0.032, 0.025) to (0.021, 0.017) as the current density is increased from 1 to 100 mA/cm2, whereas the peak external quantum efficiency of EQE=15±1% is unchanged. The associated color difference (ΔEab*) in CIELab 1976 color space is reduced from 12.8±0.2 to 8.6±0.1.
Described herein is a WOLED structure wherein a barrier transport layer (BTL) is inserted at the interface between the hole transport layer (HTL) and the EML, or in the middle of the HTL to intentionally introduce an energy barrier in hole conduction. The BTL materials possess high carrier mobilities and HOMOs that are lower than both the adjacent or surrounding HTLs and dopants in EMLs. Exemplary advantages of employing a BTL in WOLEDs include, 1. balancing the hole and electron charge population in EMLs at different applied voltages by forcing the hole conduction regime to be similar to electron conduction, 2. reducing the spectrum color change and retaining high CRI at different operating current densities, and 3. potentially increasing WOLED lifetime by avoiding unnecessary high energy triplet formation due to shifts in the excition formation zone. Furthermore, since BTLs do not interfere with EMLs or decrease WOLED quantum efficiencies, this architecture can be applied to previously existing WOLED designs.
Conventional multilayer WOLED structures consist of electrodes, hole/electron injection layers (H/EILs), hole/electron transport layers (H/ETLs), and EMLs made of multiple sub-layers for emitting light in different parts of the spectrum. The WOLED employing a BTL is shown in
Example devices W1, W2, W3, and W4 shown in
In various configurations, a device as contemplated herein could include a barrier transport layer (BTL) positioned between the emissive layer and the hole transport layer (as shown in device W2) or between two hole transport layers each comprising the same or a different hole transport material, as shown in device W4. The BTL could be positioned at a distance from the nearest surface of the emissive layer of about 5 nm, as shown in device W4, or between 1 nm and 50 nm, or between 1 nm and 40 nm, or between 1 nm and 30 nm, or between 1 nm and 20 nm, or between 1 nm and 10 nm, or between 1 nm and 5 nm, or at a distance of less than 50 nm, less than 40 nm, less than 30 nm, less than 20 nm, less than 10 nm, or less than 5 nm. In some embodiments, a distance between the BTL and the nearest surface of the EML may vary across the surface area of the device.
The depicted devices all additionally include a hole injection layer 307, which in the depicted example comprises HATCN, an ITO anode 308, and a 1.5 nm thick Liq electron injection layer (on the bottom surface of Aluminum cathode 310). Devices W3 and W4 include a different electron transport layer comprising two sublayers of different materials, the first sublayer 306 as a 45 nm thick BP4mPy, and the second sublayer 309 as a 10 nm thick Alq3 sublayer.
The schematic energy levels of the layers involved in the WOLED design are shown in
In WOLEDs employing BTLs, the hole current is impeded by the transport barrier 323 and increases at a comparable rate as the electron current with voltages so that the exciton formation zone does not shift within the EML.
Charges injected from the electrodes 331 and 332 are transported into the EML 324 where they eventually recombine to form excitons, or are transported into adjacent layers to nonradiatively recombine. Charge drift-diffusion with thermionic emission over energy barriers between layers describes the electron distribution, yielding the following expressions (Staudigel, J., et al., J. Appl. Phys. 1999, 86 (7), 3895-3910; Erickson, N. C. and Holmes, R. J., Adv. Funct. Mater. 2013, 23 (41), 5190-5198; Coburn, C., et al., Adv. Opt. Mater. 2016, 4 (6), 889-895):
Here, jn is the electron current density, and E(x,t) is the electric field at position x and time t. Here, x=0 is at the EML/HTL interface. Also, q is the unit charge, n is the electron density, k is Boltzmann's constant, Tis the temperature, and μn is the local electron carrier mobility. The electron diffusion constant Dn is related to μn at equilibrium by the Einstein relation Dn=kTμn/q. Also, E is the dielectric constant of the material and p is the hole density. Charge transport across an interface between layers whose frontier orbital energies differ by c is based on the Miller-Abrahams model with Poole-Frenkel type field dependence in Equation 3 (Horowitz, G., Adv. Mater. 1998, 10, 365; Forrest, S. R., Organic Electronics: Foundations to Applications. Oxford University Press, Oxford, 2020). Finally, E0 is the reference electric field for the reference mobility, μ0, and ΔΦ=qEd is the energy difference induced by the potential drop across a layer of thickness d adjacent to the interface. The current density for holes, jp, is found by an analogous set of expressions.
By calculating the spatial distribution of electrons and holes, exciton generation in the EML is found assuming Langevin recombination at rate (Malliaras, G. G., J. Appl. Phys. 1999, 85 (10), 7426):
Here, it is assumed that all singlet excited states are transferred to triplets in phosphorescent emitters via intersystem crossing. They can either radiatively decay as governed by their lifetime τ or leave the generation site by diffusion. Neglecting second order processes such as exciton annihilation, the electron, hole, and exciton densities n(x,t), p(x,t), and N(x,t), respectively, are found using:
The exciton diffusion constant, DN, in the EML is calculated based on the average dopant molecular spacing (Forrest, S. R. Organic Electronics: Foundations to Applications. Oxford University Press, Oxford, 2020). A finite difference method is used to simulate the time-dependent solutions to Equations 5 and 6 until the steady state is reached (Coburn, C., et al., Adv. Opt. Mater. 2016, 4 (6), 889-895). The simulation is applied to the entire organic stack (>5 layers) within the OLED, including the role of the multiple energy barriers to both charge carriers. The boundary conditions are potential V=0 at the ETL/cathode interface, and V=Va−Vbi at HTL/EML interface, where Va denotes the applied voltage and Vbi is the built-in voltage. Charge neutrality is assumed across the transport layers and the EML. Ohmic contacts of the electrodes on the organic layers are assumed. The time-evolving iteration begins with the calculation of E(x,0) from the initial charge distribution, then sequentially calculates the local charge mobility and drift-diffusion currents and terminates by updating the charge density distribution. The exciton density at x is calculated after each iteration as the product of the electron and hole densities based on Equation 6.
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.
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.
A hole injecting/transporting material to be used in the present disclosure is not particularly limited, and any compound may be used as long as the compound is typically used as a hole injecting/transporting material.
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.
The light emitting layer of the organic EL device of the present disclosure 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.
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.
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.
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.
As previously disclosed, OLEDs and other similar devices may be fabricated using a variety of techniques and devices. For example, in OVJP and similar techniques, one or more jets of material is directed at a substrate to form the various layers of the OLED.
The invention is now described with reference to the following Examples. These Examples are provided for the purpose of illustration only and the invention should in no way be construed as being limited to these Examples, but rather should be construed to encompass any and all variations which become evident as a result of the teaching provided herein.
Without further description, it is believed that one of ordinary skill in the art can, using the preceding description and the following illustrative examples, make and utilize the present invention and practice the claimed methods. The following working examples therefore, specifically point out the preferred embodiments of the present invention, and are not to be construed as limiting in any way the remainder of the disclosure.
Compared herein are 4 WOLEDs (W1-W4) whose structures are shown schematically in
The normalized emission spectra of the four devices at current density (j)=1 to 100 mA/cm2 are shown in
White spectra present red, green and blue emission peaks originating from the phosphor dopants, iridium (III) bis(2-phenyl quinolyl-N,C20) acetylacetonate (PQIr), Tris(2-phenylpyridine)iridium(III) (Ir(ppy)3), and fac-tris[(2,6-diisopropylphenyl)-2-phenyl-1H-imidazol[e]iridium(III) (Ir(iprpmi)3), respectively. As j increases, the WOLED spectra blue-shift in W1 and W2, and red-shift in W3 and W4, which corresponds to a positive and negative change in correlated color temperature, ACCT, respectively. The change of CCT is smaller in devices with the BTLs. The color-shift, ΔCIE, in devices with the BTLs is reduced from (−0.036, 0.026) and (0.032, 0.025), to (−0.024, 0.020) and (0.021, 0.017) in devices W1 and W2, and W3 and W4, respectively, due to BTLs inserted in W2 and W4. The measure of change in visual perception of two colors, ΔEab*, defined as Euclidian distance between two points in CIELab 1976 color space, is also reduced from 18.0±0.3 to 13.1±0.2 (W1 to W2), and 12.8±0.2 to 8.6±0.1 (W3 to W4) by the BTLs.
The current density-Voltage-luminance (j-V-L) and efficiency characteristics of the devices are shown in
With reference to
To understand the dependence of the exciton density, N, on the BTL, the charge and exciton dynamics using Equations. 1-6 were simulated. The numerical model was applied to 2 multilayer systems (G1 and G3) that are similar to RG1 and RG3, where all transport and blocking layers in RG1 and RG3 are included, yet the emission zones were replaced by only a single, 30 nm-thick green-emitting Ir(ppy)3 doped mCBP (vol. 8%) EML. The calculations employ one-dimensional 0.5 nm mesh grids, with the origin at the EML/HTL interface. The calculated n and p distribution within G1 and G3 at j=0.1 and 100 mA/cm2 are shown in
The calculated exciton profile in the EML of G1 is shown in graph 901 in
The spectral shift is primarily controlled by charge injection barriers at the periphery of the EML. In devices W1-W4, the phosphor-doped layers were arranged such that charge transport barriers were absent within the EMLs themselves (see
An BTL with a low HOMO energy creates an injection barrier to holes, analogous to that for electrons at the EML/ETL interface. The similar conduction regimes for holes and electrons provide improved charge balance, leading to broader exciton formation zones, and hence spectra that are largely independent of current. Thus, the CBP BTL in device W4 yields the smallest color shift with ΔEab*=8.6±0.1 as j is varied from 1 to 100 mA/cm2, which is more than a 50% improvement over device W1. For reference, in the CIELab 1976 color space, ΔEab*=1 is considered as one unit of “just noticeable difference” (JND), which is equivalent to the color range within a 1-step MacAdam ellipse (Abeyta, R. N., Embry-Riddle Aeronautical University 2001).
Careful control of the position and thickness of the BTL in an OLED is essential to avoid improving spectral stability at the expense of the quantum efficiency, as in device W2. In device W4, the CBP BTL is placed between NPD and Tris-PCz so that Tris-PCz (triplet energy=2.8 eV) layer blocks triplet diffusion while preventing electron leakage from the EML to the CBP BTL. The EQE of device W4 is hence similar to W3, as shown in
The parameters used in the simulations to calculate n, p, and Nin
The hole mobility in the EMLs of G1 and G3 is 5.0±0.1×10−7 cm2/Vs, obtained by fitting the measured N profile of device G1 to the calculated results in graph 901 in
The spectral shift of a WOLED is reduced in terms of ΔCIE and ΔEab* from (0.032, 0.025) to (0.021, 0.017), and from 12.8±0.2 to 8.6±0.1, respectively by inserting a CBP BTL. This design concept reduces the original color shift to a “9-step MacAdam ellipse”, which is closer to the recommended chromaticity range for general solid state lighting products, whose color space should lie within a “7-step MacAdam ellipse” (American National Standard for Electric Lamps—Specifications for the Chromaticity of Solid State Lighting (SSL) Products. ANSI 2017). The resulting device shows EQE=15±1% at 0.1 mA/cm2 and comparable j-V characteristics to the device without a BTL. However, the introduction of the HOMO barrier associated with the BTL can induce a luminous efficacy drop, unless a larger LUMO barrier (Alq3/BP4mPy barrier in W3) exists previously in the device. A numerical model is introduced to describe the charge transport and exciton distribution in OLEDs with or without BTLs, which is validated by measuring emission of phosphor sensors embedded across the device. Unbalanced charge injection from asymmetrical energy barriers for electrons and holes is found to be the origin of intensity-dependent color shift. The disclosed study suggests that the introduction of BTLs leads to a uniform exciton distribution profile in EML that stabilizes WOLED spectra as current is varied over two orders of magnitude. The BTL design and detailed knowledge of the current-dependent exciton profile from our numerical model provide an opportunity to engineer high quality WOLED lighting sources.
All layers were grown by vacuum thermal evaporation at a base pressure of 10−7 Torr on glass substrates with pre-patterned, 1 mm wide indium tin oxide (ITO) anode strips giving device areas of 1 mm2. The device structures are 70 nm ITO anode/5 nm hexaazatriphe-nylene hexacarbonitrile (HATCN) hole injection layer/20 nm HTL/ETL/55 nm ETL/1.5 nm thick 8-hydroxyquinolinato lithium (Liq) electron injection layer/100 nm thick Al cathode. The emission zone for WOLEDs are 5 nm Ir(ppy)3:mCBP (8% vol.)/10 nm PQIr:mCBP (4% vol.)/10 nm Ir(iprpmi)3:mCBP (13% vol.). The emission zones for two-color OLEDs are 15 nm Ir(ppy)3:mCBP (8% vol.)/15 nm PQIr:mCBP (8% vol.). The EML for devices G1 and G3 is 30 nm Ir(ppy)3:mCBP (8% vol.). The detailed structures for the HTLs and ETLs are summarized in Table 3:
The 0.1 nm thick neat PQIr sensing layers were located at x=—7.5, 0.0, 7.5, 15.0, 22.5, and 30.0±0.1 nm in device G1, and x=—7.5±0.1 nm in device G3. The electroluminescent spectra of all devices were collected via a spectrometer using a lens-coupled fiber. The j-V-EQE characteristics of the PHOLEDs were measured using a parameter analyzer and a calibrated photodiode that collected all light exiting the substrates in the viewing direction.
The parameters used in the simulations were determined as follows: DN was calculated to be 4.1±0.1×10−7 cm2/s based on the average dopant molecular spacing of 2.3 nm (8% doping vol.) and the exciton Bohr radius of 1.6 nm.38 The Ir(ppy)3 triplet lifetime is τ=615±7 ns measured from its time-resolved photoluminescence (Zhang, Y., et al., Chem. Phys. Lett. 2013, 590, 106-110). The electron mobility of the ETLs and mCBP, and the hole mobility of the HTLs and CBP at E0=1.7×106 V/cm were obtained from the literature (Nguyen, N. D., et al., Phys. Rev. B. 2007, 75 (7), 075307; Liu, N., et al., Micromachines 2019, 10 (5); Parshin, M. A., et al., SPIE Photonics Europe. 2006, 6192, 419). The hole mobility in the ETLs was assumed to be significantly lower (by a factor of 10−3) than the electron mobility for simplicity. The electron mobility in the HTLs was assumed to be less than that of holes by the same factor. Also, c was based on the energy diagram from
The following publications are incorporated herein by reference in their entireties:
The disclosures of each and every patent, patent application, and publication cited herein are hereby incorporated herein by reference in their entirety. While this invention has been disclosed with reference to specific embodiments, it is apparent that other embodiments and variations of this invention may be devised by others skilled in the art without departing from the true spirit and scope of the invention.
This application claims priority to U.S. provisional application No. 63/247,485 filed on Sep. 23, 2021, incorporated herein by reference in its entirety.
This invention was made with government support under DE-EE0008723 awarded by the U.S. Department of Energy. The government has certain rights in the invention.
| Number | Date | Country | |
|---|---|---|---|
| 63247485 | Sep 2021 | US |
