ENHANCED PURCELL EFFECT USING RED-SHIFTED SURFACE PLASMON MODES AND HIGH INDEX MATERIAL IN ORGANIC LIGHT EMITTING DEVICES

An OLED comprises a first electrode, an emissive layer positioned over the first electrode, a charge transport layer positioned over the emissive layer, and a second electrode positioned over the charge transport layer, wherein the charge transport layer is in direct contact with the second electrode, and wherein the charge transport layer has an index of refraction of at least 1.7. An OLED comprises a cavity formed between first and second metal electrodes, an organic light emitting element positioned within the cavity, and an efficiency enhancement layer positioned between the organic light emitting element and the second silver electrode, the efficiency enhancement layer having a refractive index of at least 1.7.

BACKGROUND OF THE INVENTION

Organic light emitting devices (OLEDs), especially phosphorescent OLEDs, suffer from triplet-triplet annihilation (TTA) and triplet-polaron annihilation (TPA) due to the long lifetime of triplet excitons. Therefore, increasing the radiative decay rate of a triplet exciton by introducing a stronger Purcell effect, can reduce triplet exciton density in the emission layer and achieve a longer device operational lifetime. Some previous cavity designs utilize multiple silver surfaces in the near-field region to maximize the radiative coupling between the exciton energy and silver surface plasmon polariton (SPP) modes. Simulation results show a Purcell factor of around 5 is achieved, which means the radiative lifetime can reach at minimum one-fifth of that in vacuum or in the infinite host material environment. However, these multilayers bring new difficulties to the fabrication process. Thus there is a need in the art for improved devices, systems and methods for enhancing exciton decay rate in OLEDs.

SUMMARY OF THE INVENTION

Some embodiments of the invention disclosed herein are set forth below, and any combination of these embodiments (or portions thereof) may be made to define another embodiment.

In one aspect, an OLED comprises a first electrode, an emissive layer positioned over the first electrode, a charge transport layer positioned over the emissive layer, and a second electrode positioned over the charge transport layer, wherein the charge transport layer is in direct contact with the second electrode, and wherein the charge transport layer has an index of refraction of at least 1.7.

In one embodiment, the charge transport layer comprises a material selected from Alq3, BPyTP2, or ZnS. In one embodiment, the charge transport layer has a thickness of between 10 nm and 30 nm. In one embodiment, the charge transport layer has a thickness of about 15 nm. In one embodiment, the second electrode is a cathode and the charge transport layer is an electron transport layer. In one embodiment, the first electrode and the second electrode both comprise silver. In one embodiment, the first electrode has a thickness of about 30 nm, and the second electrode has a thickness of about 100 nm. In one embodiment, the OLED further comprises a first Indium Tin oxide (ITO) layer between the first electrode and the emissive layer. In one embodiment, the first ITO layer has a thickness of about 15 nm. In one embodiment, the OLED further comprises a second ITO layer below the first electrode. In one embodiment, the charge transport layer has a refractive index of at least 1.9. In one embodiment, the charge transport layer has a refractive index of at least 2.0.

In another aspect, an OLED comprises a cavity formed between first and second metal electrodes, an organic light emitting element positioned within the cavity, and an efficiency enhancement layer positioned between the organic light emitting element and the second silver electrode, the efficiency enhancement layer having a refractive index of at least 1.7.

In one embodiment, the efficiency enhancement layer has a refractive index of at least 1.9. In one embodiment, the efficiency enhancement layer has a refractive index of at least 2.0. In one embodiment, the efficiency enhancement layer comprises an electron transporting material. In one embodiment, the cavity has a Purcell factor of at least 5. In one embodiment, the OLED further comprises an ITO layer positioned within the cavity. In one embodiment, wherein the ITO layer is positioned between the first metal electrode and the organic light emitting element. In one embodiment, the organic light emitting element is a white organic light emitting element.

DETAILED DESCRIPTION OF THE INVENTION

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, devices and methods for enhancing exciton decay rate in (OLEDs. 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.

Referring now in detail to the drawings, in which like reference numerals indicate like parts or elements throughout the several views, in various embodiments, presented herein are systems, devices and methods for enhancing exciton decay rate in OLEDs. In previous work, Purcell-effect-enhanced cavity OLEDs have been demonstrated by using a pair of silver mirror electrodes with metal-dielectric alternating layers. In this disclosure, a redshift in surface plasmon polariton (SPP) modes induced by high refractive index material is utilized for a higher Purcell factor, achieving a larger radiative decay rate and a longer device operational lifetime. In an embodiment, the high refractive index comes from the anomalous dispersion brought by the coupling between the SPP and the resonant exciton of the charge transport layer (i.e., plasmon exciton polariton). In some embodiments, a strong coupling is formed between the SPP and the plasmon exciton polarity. In alternative embodiment, a weak coupling is formed between the SPP and the plasmon exciton polarity. This provides the degrees of freedom for balancing plasmon coupling to the outcoupling.

As used herein, and as would be understood by one skilled in the art, “HATCN” (referred to interchangeably as HAT-CN) refers to 1,4,5,8,9,11-Hexaazatriphenylenehexacarbonitrile.

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.

Certain embodiments of the disclosure relate to a light emitting device comprising an emissive layer (EML) spaced far from a cathode as described herein. Conventional organic light emitting devices typically place the EML near a metal cathode which incurs plasmon losses due to near field coupling. To avoid exciting these lossy modes it is necessary to space the EML far from the cathode. However, utilizing a thick electron transport layer (ETL) can be problematic due to changes in charge balance and increased resistivity. These problems can be overcome by utilizing a charge generation layer, for example a charge generation layer comprising at least one electron transport layer and at least one hole transport layer, to convert electron into hole current. This allows the use of higher mobility hole transporting materials and maintains the charge balance of the device. In some embodiments, the charge generation layer may be replaced or combined with any other layer capable of conducting electrons.

Devices of the present disclosure may comprise one or more electrodes, some of which may be fully or partially transparent or translucent. In some embodiments, one or more electrodes comprise indium tin oxide (ITO) or other transparent conductive materials. In some embodiments, one or more electrodes may comprise flexible transparent and/or conductive polymers.

Layers may include one or more electrodes, organic emissive layers, electron- or hole-blocking layers, electron- or hole-transport layers, buffer layers, or any other suitable layers known in the art. In some embodiments, one or more of the electrode layers may comprise a transparent flexible material. In some embodiments, both electrodes may comprise a flexible material and one electrode may comprise a transparent flexible material.

An OLED fabricated using devices and techniques disclosed herein may have one or more characteristics selected from the group consisting of being flexible, being rollable, being foldable, being stretchable, and being curved, and may be transparent or semi-transparent. In some embodiments, the OLED further comprises a layer comprising carbon nanotubes.

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 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.

Referring now toFIGS.3A and3B, the structure of a previous cavity OLED design and a simulation of Purcell factor and outcoupling efficiency at wavelength of 466 nm are shown, respectively. At an ideal metal/dielectric interface, the SPP dispersion is defined by:

with an asymptotic behavior at kx→∞, where ω→ωSP, the surface plasma frequency. The surface plasma frequency is given by the plasma frequency of the metal and the optical constant of the dielectric material:

where by increasing the dielectric function Ed, the SPP dispersion and its density of states are both easily tunable, providing another optical degree of freedom for optoelectronic devices.

The fabrication process by which the device ofFIG.3Ais produced can be simplified by changing the dielectric material near the silver (Ag) surface to material with a higher refractive index, one can use a redshifted SPP coupling for a better enhanced Purcell effect. Furthermore, thin Ag or other metal mirrors can be utilized as one or both electrodes for a phosphorescent OLED to enhance cavity effects. In some embodiments, the electron transport layer (ETL) is replaced with a high-n material, for the purpose of both providing a red-shifted dispersion and transporting electrons, where high-n is defined by an index of refraction of at least 1.8. By fine-tuning the thicknesses (ranging from 5 nm to 40 nm) of the high-n ETL material, one can red-shift the surface plasmon frequency of the SPP modes, obtain a higher density of states (DoS) due to a flat dispersion, and optimize the charge balance in the exciton emission later (EML).

With reference toFIG.5, in some embodiments, the OLED300comprises a first metal electrode layer307, an OLED stack309positioned over the first metal electrode layer307, a charge transport layer310positioned over the OLED stack309, and a second metal electrode layer311positioned over the charge transport layer310. In some embodiments, the OLED stack309comprises any suitable layers including but not limited to an emissive layer, an HTL, an HBL, an EML, an EBL, an interfacial layer, and in any suitable quantity and arrangement.

In some embodiments, the charge transport layer310is in direct contact with the second metal electrode layer311. In some embodiments, a charge injection layer is positioned between the charge transport layer310and the second metal electrode layer311. In some embodiments, the charge injection layer has a thickness of less than 20 nm, less than 10 nm, less than 5 nm, less than 2.5 nm, or any other suitable thickness. In some embodiments, the charge transport layer310is configured to enhance efficiency of the OLED300. In some embodiments, the charge transport layer310has an index of refraction of at least 1.7. In some embodiments, the charge transport layer310comprises Alq3, BPyTP2, and/or N-type ZnS. In some embodiments, the charge transport layer310has a thickness between 10 nm and 30 nm, about 15 nm, or any other suitable thickness. In some embodiments, the charge transport layer310is configured as an electron transport layer (ETL). In some embodiments, the charge transport layer310is configured as a hole transport layer (HTL).

In some embodiments, the OLED300further comprises a substrate layer305positioned under the first metal electrode layer307. In some embodiments, the substrate layer305comprises glass or other suitable materials, for example a material that is transparent to at least a portion of the emissive spectrum of the OLED stack309.

In some embodiments, the first metal electrode layer307comprises an anode. In some embodiments, the first metal electrode layer307comprises a cathode. In some embodiments, the first metal electrode layer307comprises silver. In some embodiments, the first metal electrode layer307has a thickness of about 30 nm.

In some embodiments, the second metal electrode layer311comprises an anode. In some embodiments, the second metal electrode layer311comprises a cathode. In some embodiments, the second metal electrode layer311comprises silver. In some embodiments, the second metal electrode layer311has a thickness of about 100 nm, or at least greater than 80 nm.

In some embodiments, the second metal electrode layer311comprises a cathode and the charge transport layer110comprises an electron transport layer. In some embodiments, both the first and second metal electrode layers (307,311) comprise silver.

In some embodiments, the OLED300further comprises an ITO layer308(top) between the first metal electrode layer307and the OLED stack309. In some embodiments, the OLED300further comprises an ITO layer306(bottom) below the first metal electrode layer307. In some embodiments, a first electrode is defined by the ITO/first metal electrode/ITO layer stack (306,307,308).

In some embodiments, the OLED300includes one or more ITO layers positioned between the charge transport layer310and the second metal electrode layer311, and/or over the second metal electrode layer311.

In some embodiments, the OLED stack309comprises any suitable emissive structure and thickness. In one exemplary embodiment, the structure of the OLED stack309can comprise 5 nm (3,3′-Di(9H-carbazol-9-yl)-1,1′-biphenyl) mCBP/30-50 nm mCBP: iridium (III) tris[3-(2,6-dimethylphenyl)-7-methylimidazo[1,2-f] phenanthridine] [Ir(dmp)3] (graded doping from 18%-8%)/10 nm dipyrazino[2,3,-f:2′,3′-h]quinoxaline 2,3,6,7,10,11-hexacarbonitrile (HATCN).

In some embodiments, the cavity312of OLED300has a Purcell factor of at least 5.

In some embodiments, the OLED300comprises a cavity312formed between first metal electrode307and second metal electrode layer311, an OLED stack309positioned within the cavity312, and a charge transport layer310positioned between the OLED stack309and the second metal electrode layer311. In some embodiments, the charge transport layer310has a refractive index of at least 1.7. In some embodiments, the charge transport layer310has a refractive index of at least 1.9. In some embodiments, the charge transport layer310has a refractive index of at least 2.0.

In some embodiments, the charge transport layer310comprises an electron transporting material such as Alq3, BPyTP2, or ZnS, for example. In some embodiments, the cavity312has a Purcell factor of at least 5.

In some embodiments, the OLED300further comprises an ITO layer308positioned within the cavity312. In some embodiments, the ITO layer308is positioned between the first metal electrode307and the OLED stack309. In some embodiments titanium (Ti) interfacial layers are layered between the ITO and Ag layers in the form of ITO/Ti/Ag/Ti/ITO. In some embodiments, the top ITO/Ti layer stops Ag diffusion into the organics, and the bottom Ti/ITO layer provides a wetting surface for Ag growth. In some embodiments, the thickness range of the top ITO layer308, first metal electrode layer307, and bottom ITO layer306is 15-30 nm, 16-30 nm, and 20-100 nm, respectively. In some embodiments, the Ti interfacial layers between 307/308, and 306/307 are each 2-3 nm thick. In some embodiments, the OLED stack309is a white OLED stack.

FIGS.6A-6Ddescribe the radiative power dissipated at each wavelength and propagation wavevector. The color plot indicates the density of states of the dissipation channel. The total radiation power, or the normalized Purcell factor, is then described as:

where μ is the transition dipole moment. In the region kx<1, the radiative power is outcoupled or trapped in the waveguided mode in the device planar structure, while in the region kx>1, the radiative power is mainly coupled to the SPP modes of the Ag/charge transport layer surface, and the asymptote surface plasma frequency of the three devices follows: D3(ZnS) 103<D2(BPyPT2) 102<D1(Alq3) 101. Here the impact of surface roughness is neglected. For exciton emission in the blue region, for example, peaked at 470 nm, a more redshifted SPP dispersion means that there is more radiative power coupled to the flat curve of density of states (DoS). Combined with previous methods, by coupling to multiple SPP modes at different Ag surfaces as inFIG.6D, this method provides another degree of freedom for enhancing the Purcell effect.

However, the balance between the Purcell effect and the EQE of an OLED device must be considered for optimizing the device lifetime. For a commercialized blue phosphor with internal quantum efficiency of around 50%, and exciton emission peak at 466 nm, one can define the figure of merit of a long-lived blue phosphorescent device as:

where the figure of merit of a conventional device with Al/LiQ/Organics/ITO structure usually ranges from 1 to 1.5, while the disclosed optimized OLED design100is peaked at around 9 to 10.

From the simulation results inFIGS.7A-7D, the optimized SPP coupling is between the scenario of D2102and D3103, in which the Purcell effect is prominent without losing too much outcoupling efficiency. This also indicates the SPP modes should be detuned from the outcoupling peak frequency.

Fabrication and Characterization:

In some embodiments, cavity OLED devices are deposited using vapor thermal evaporation (VTE) in high vacuum. Thin Ag layers are doped with Cu, Al or wetted by sputtering thin Ti or NiCr. ITO is deposited by sputtering.

External quantum efficiency (EQE) and J-V curve is measured by a parameter analyzer with precision down to fA. Exciton lifetime is measured via transient photoluminescence using a chirped pulse amplifier (CPA) laser, an optical parametric amplifier (OPA) and a streak camera/single photon counter. Device lifetime and voltage rise is measured using Si photodiodes under continuous, steady current supply.

Combination with Other Materials

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 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 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)

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.