WO2022159949A1

WO2022159949A1 - Directional polarized light emission from thin-film light emitting diodes

Info

Publication number: WO2022159949A1
Application number: PCT/US2022/070253
Authority: WO
Inventors: Xiangyu FU; Yash Mehta; Lei LEI; Liping Zhu; Franky So; Yi-An Chen; Chih-Hao Chang
Original assignee: North Carolina State University
Priority date: 2021-01-19
Filing date: 2022-01-19
Publication date: 2022-07-28
Also published as: WO2022159949A9

Abstract

Various highly directional and polarized waveguide emission thin-film light emitting diodes (LEDs) are disclosed. The highly directional and polarized waveguide emission LEDs can include organic light emitting diodes (OLEDs) and perovskite light emitting diodes (PeLEDs). Further, the highly directional and polarized waveguide emission LEDs can be fabricated on a corrugated substrate such that the resulting gratings only extract the transverse electric (TE) waveguide mode while suppressing light emission from other optical modes. To achieve these emission characteristics, corrugation at a top cathode can be planarized by thermal evaporation of a thick organic stack. As a result, background air mode emission can be reduced, and the diffraction of surface plasmon polariton (SPP) and transverse magnetic (TM) waveguide modes can be reduced.

Description

DIRECTIONAL POLARIZED LIGHT EMISSION FROM THIN-FILM LIGHT EMITTING DIODES

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to, and the benefit of, co-pending U.S. provisional application entitled “Directional Polarized Light Emission from Thin-Film Light Emitting Diodes” having serial no. 63/138,993, filed January 19, 2021 , which is hereby incorporated by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] The present invention was made with United States government support under grant number DE-EE0008720 awarded by the U.S. Department of Energy. The United States government has certain rights in the invention.

BACKGROUND

[0003] With the rapid growth in portable displays, thin-film light emitting diodes (LEDs) have attracted great attention due to their low fabrication cost and high efficiency compared with inorganic LED and liquid crystal displays. Typical thin-film LEDs are less than two hundred nanometers in thickness, consisting of an indium tin oxide (ITO) anode, hole and electron transport layers, an emitting layer, and a metal cathode. Depending on the emitting material, thin-film LEDs can be categorized into organic LEDs (OLEDs), polymer LEDs (PLEDs), quantum dot LEDs (QLEDs) and perovskite LEDs (PeLEDs).

SUMMARY

[0004] Aspects of the present disclosure are related to directional polarized light emission from LEDs. In one aspect, among others, a waveguide emission light emitting diode, comprises an indium tin oxide anode disposed on the corrugated transparent substrate; a thin- film light emitting diode stack disposed on the indium tin oxide anode; and a metal electrode that forms a substantially planarized top layer of the waveguide emission light emitting diode. The thin-film light emitting diode stack can comprise a hole transport layer; an emitting layer disposed on the hole transport layer; and an electron transport layer disposed on the emitting layer. In one or more aspects, the waveguide emission light emitting diode can comprise a perovskite light emitting diode.

[0005] In various aspects, the waveguide emission light emitting diode can comprise a pixel size of 2 mm x 2 mm. The corrugated transparent substrate can comprise one of a 1-D grating substrate or a 2-D square grating substrate. The corrugated transparent substrate can comprise a corrugation or grating depth of about 20 nm to about 200 nm. The corrugated transparent substrate can comprise a grating period in a range from about 200 nm to about 800 nm. In one or more aspects, the 2-D square grating substrate can comprise a first grating period in a first direction and a second grating period in a second direction. The first and second grating periods can be different. The 2-D square grating substrate can comprise a 2- D array of geometric pillars. Pillars of the 2-D array of geometric pillars can have a circular or rectangular cross-section. The 1-D grating substrate can comprise a plurality of parallel ridges.

[0006] In various aspects, the indium tin oxide anode can comprise a thickness of about 100 nm or greater. The electron transport layer can comprise a thickness in a range from about 60 nm to about 170 nm. In one or more aspects, the waveguide emission light emitting diode can comprise a europium (Eu) complex based organic light emitting diode. The waveguide emission light emitting diode can comprise an iridium (Ir) complex based organic light emitting diode. The indium tin oxide anode can be directly sputtered on the corrugated transparent substrate. The metal electrode can be planarized via a thermal evaporation process.

[0007] Other systems, methods, features, and advantages of the present disclosure will be or become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the present disclosure, and be protected by the accompanying claims. In addition, all optional and preferred features and modifications of the described embodiments are usable in all aspects of the disclosure taught herein. Furthermore, the individual features of the dependent claims, as well as all optional and preferred features and modifications of the described embodiments are combinable and interchangeable with one another.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] For a more complete understanding of the embodiments and the advantages thereof, reference is now made to the following description, in conjunction with the accompanying figures briefly described as follows:

[0009] FIG. 1 illustrates an example of a simulated optical mode distribution in an organic light emitting diode (OLED) by varying the electron transport layer (ETL) thickness, in accordance with various aspects of the present disclosure.

[0010] FIG. 2 illustrates an example of an electric field (|E|²) distribution of the optical modes shown in FIG. 1 from a finite-difference time-domain (FDTD) simulation, in accordance with various aspects of the present disclosure.

[0011] FIG. 3 illustrates an example of mode dispersion of a reference OLED with 140 nm ETL, in accordance with various aspects of the present disclosure.

[0012] FIG. 4 illustrates an example of a simulated angular emission profile in a glass substrate at 60 nm ETL and 140 nm ETL, in accordance with various aspects of the present disclosure.

[0013] FIG. 5 illustrates an example of a cross-section scanning electron microscope (SEM) of a waveguide emission OLED fabricated on a 1-D grating and the corresponding electric field (|E|² distribution of the optical modes from a FDTD simulation, in accordance with various aspects of the present disclosure. [0014] FIG. 6 illustrates an example of measured mode dispersion in transverse electric

(TE) and transverse magnetic (TM) polarizations for a planar OLED with 60 nm ETL, in accordance with various aspects of the present disclosure.

[0015] FIG. 7 illustrates an example of measured mode dispersion in TE and TM polarizations for a corrugated OLED with 60 nm ETL, in accordance with various aspects of the present disclosure.

[0016] FIG. 8 illustrates an example of an angular profile at 520 nm for the planar OLED with 60 nm ETL, in accordance with various aspects of the present disclosure.

[0017] FIG. 9 illustrates an example of the angular profile at 520 nm for the planar OLED with 140 nm ETL, in accordance with various aspects of the present disclosure.

[0018] FIG. 10 illustrates an example of measured mode dispersion in TE and TM polarizations for a planar OLED with 140 nm ETL, in accordance with various aspects of the present disclosure.

[0019] FIG. 11 illustrates an example of measured mode dispersion in TE and TM polarizations for a waveguide emission OLED with 140 nm ETL, in accordance with various aspects of the present disclosure.

[0020] FIG. 12 illustrates an example of angular profile at 520 nm for the planar OLED with 140 nm ETL, in accordance with various aspects of the present disclosure.

[0021] FIG. 13 illustrates an example of angular profile at 520 nm for the waveguide emission OLED with 140 nm ETL, in accordance with various aspects of the present disclosure.

[0022] FIG. 14 illustrates an example of an extinction ratio of the waveguide emission OLED at each wavelength, in accordance with various aspects of the present disclosure.

[0023] FIG. 15 illustrates examples of external quantum efficiencies (EQEs) of the 140 nm ETL planar OLED, 140 nm ETL corrugated OLED, and 140 nm waveguide emission OLED, in accordance with various aspects of the present disclosure. [0024] FIG. 16 illustrates an example of electro luminescence (EL) spectra comparison of the 140 nm ETL planar OLED, 140 nm ETL corrugated OLED, and 140 nm waveguide emission OLED, in accordance with various aspects of the present disclosure.

[0025] FIG. 17 illustrates examples of spatial patterns from a reference OLED, a waveguide (WG) emission OLED on a 1-D grating substrate, and a waveguide emission OLED on a 2-D square grating substrate by using a narrow spectrum Eu complex emitter, in accordance with various aspects of the present disclosure.

[0026] FIG. 18 illustrates an example of dependence of optical mode percentage on the refractive index of an emitting layer in a waveguide emission thin-film LED, in accordance with various aspects of the present disclosure.

[0027] FIG. 19 illustrates examples of spatial patterns from a reference perovskite LED (PeLED) and a waveguide emission PeLED on a 1-D grating substrate, in accordance with various aspects of the present disclosure.

[0028] FIG. 20 illustrates an example of comparisons of device EQEs of the reference PeLED and the waveguide emission PeLED, in accordance with various aspects of the present disclosure.

[0029] FIG. 21 illustrates examples of current efficiencies measured in the normal direction for the reference PeLED and the waveguide emission PeLED, in accordance with various aspects of the present disclosure.

[0030] FIG. 22 illustrates examples of EL spectra measured at θ = 3° normal to the grating grooves for the reference PeLED and the waveguide emission PeLED, in accordance with various aspects of the present disclosure.

DETAILED DESCRIPTION

[0031] Light emitting diodes (LEDs) with directional and polarized light emission have many photonic applications, and beam shaping of these devices is fundamentally challenging because they are Lambertian light sources. In particular, highly directional beam shapes lead to many interesting applications in solid-state lighting, stereoscopic displays, holographic displays, optical communication, and integrated lasers.

[0032] In recent years, near-eye displays such as virtual reality (VR) and augmented reality (AR) have gained great momentum in various applications. Despite the rapid progress, VR displays are often bulky and heavy due to the light collimating refractive lenses. In contrast, AR displays use micro-displays and waveguide optical components to project the images which leads to a smaller form factor. However, the throughput from the input-output diffractive/holographic components is only 10%, resulting in an overall outcoupling efficiency less than 2%. One way to improve both AR and VR displays is to use image sources with directional light emission. This eliminates the need for light collimator or optical combiners, thus reducing the display size while improving the outcoupling efficiency.

[0033] Several beam-shaping approaches have been demonstrated in thin-film LEDs and they have their merits and limitations. Distributed Bragg reflectors (DBRs) can significantly enhance the cavity resonance to achieve directional emission and even lasing. The layered structure of DBRs is compatible with thin-film LED fabrication, but the emission direction is sensitive to the cavity length and its emission spectrum is highly angle dependent. Alternatively, thin-film LED pixels can be made into line or point sources with microlens arrays to collimate the emitted light. This approach has been used in white emitting organic LEDs (OLEDs) to demonstrate a small beam divergence angle of 9°, but due to the small pixel size, the maximum brightness per area is limited and therefore this approach is not practical.

[0034] Instead of beam-shaping the air mode, diffractive optical elements (DOEs) have also been used to extract thin-film optical modes for directional emission. Because the optical cavity length of a thin-film LED is close to the wavelength of light, it only supports the low order transverse optical modes, which have a highly quantized mode dispersion. Because the optical cavity length of a thin-film LED is close to the wavelength of light, it only supports the low order transverse optical modes, which have a highly quantized mode dispersion. By incorporating a DOE into a thin-film LED, the optical modes can be extracted to a narrow range of angles through Bragg diffraction. An OLED emitting layer with 2D square array of pillars have been patterned in the past, and the resulting corrugated silver (Ag) electrode diffracts the waveguide modes. However, with this device, the corrugated metal cathode extracts the transverse electric (TE) waveguide mode, the transverse magnetic (TM) waveguide mode, and the surface plasmon polariton (SPP) mode, resulting in a complex emission profile with the presence of the background air mode.

[0035] An alternative approach is to laterally separate the LED pixel from the DOE pixel in the device stack, such that the air mode emitted from the LED pixel can be blocked, and subsequently emitted photons are coupled into a thin-film waveguide stripe and extracted by the DOE pixel, yielding directional emission. However, due to low coupling efficiency and high waveguide loss, the device efficiency is less than 1%.

[0036] Another aspect of thin film LEDs that is often overlooked is how to achieve polarized emission. One approach is to mechanically align the emitting molecules, resulting in polarized emission from the device. Alternatively, a fine metallic grating can be used as an external polarizer to selectively transmit the TM light and reflect the TE light. However, these approaches are not practical. Uniaxial alignment inevitably induces contamination to the emission layer and damages the devices, and the metallic grating wastes the TE light and can reduce the outcoupling efficiency by half. Therefore, it is desirable to have a light source intrinsically emitting polarized light.

[0037] According to various embodiments described in the present disclosure, full-area highly directional polarized light emission from organic and perovskites LEDs on nanostructured substrates can be demonstrated to selectively extract the TE waveguide mode while suppressing the SPP, TM waveguide, and air modes. A device concept is first demonstrated using an OLED with an Ir-complex emitter. By tuning the thickness of the OLED stack, the corrugation is mostly planarized at the cathode and the diffraction of TM waveguide mode and SPP mode is highly suppressed. To further suppress the emission from the air mode, the thickness of the electron transport layer (ETL) is tuned to its valley thickness in the air mode profile, so the emission from the OLED cavity is blocked by total internal reflection from glass to air as shown in FIG. 1. The FDTD simulation results confirm that the TE waveguide mode in such a device is located at the vicinity of the ITO anode. Therefore, having a corrugated ITO anode will allow effective extraction of the TE waveguide mode by diffraction as shown in FIG. 2. Because the TE waveguide mode has a very narrow dispersion, this translates to a narrow emission angle in air as shown in FIG. 3. In detail, FIG. 1 illustrates a simulated optical mode distribution in an OLED by varying the ETL thickness. FIG. 2 illustrates the electric field (|E|²) distribution of the optical modes from FDTD simulation. FIG. 3 illustrates the mode dispersion of a reference OLED with 140 nm ETL.

[0038] The resulting waveguide emitting LED shows strong TE waveguide emitted light with a high TE to TM mode extinction ratio. Next, a similar architecture can be applied to an OLED with an Eu-complex emitter having a narrow emission spectrum to demonstrate the highly directional beam shape with a divergence angle less than 3°. Finally, the large index of perovskite materials and strong TE waveguide mode present in perovskite LEDs can be taken advantage of, and directional polarized emission from such devices with a 2.6 times enhancement in current efficiency compared with the reference planar device can be demonstrated.

Device Architecture

[0039] To achieve waveguide-only emission in a thin-film LED, the device architecture can extract the waveguide mode while suppressing emission from the air, TM waveguide, and SPP modes. The air mode emission of a thin-film LED is determined by the cavity effect. Because the reflectivity is 85% for the Al cathode but 2% for the ITO anode, the cavity effect is mostly determined by the distance between the emitting layer (EML) and the reflective electrode, which is the thickness of the ETL in most thin-film LEDs. According to various embodiments disclosed herein, a typical OLED having a structure of glass substrate/ITO/hole transport layer (HTL/EML/ETL/cathode) is used to demonstrate the waveguide emission architecture. A common Ir-complex tris [2-phenylpyridinato-C²,N]iridium(lll) (Ir(ppy)₃) is used as the emitter, which has an EL peak at 520 nm. [0040] To achieve pure TE waveguide emission, the first step includes suppressing the emission from the air mode. Based on the optical mode distribution, the air mode intensity changes periodically with the ETL thickness as shown in FIG. 1. From the peak to the valley, the air mode contribution drops from 26% to 3% while the substrate mode increases from 21 % to 42%. To understand the difference in the mode distribution, the angular emission profile inside the glass substrate was simulated at the air mode peak (60 nm ETL) and valley (140 nm ETL), respectively. FIG. 4 shows the simulated angular emission profile in the substrate at (a) 60 nm ETL (maximal air mode) and (b) 140 nm ETL (minimal air mode). The wavelength of the light is 520 nm, corresponding to the EL peak of I r(ppy)₃.

[0041] Further discussing FIG. 4, with a 60-nm-thick ETL, the emission profile is acorn- shaped with a strong distribution in the normal direction, therefore light can easily escape from the substrate, resulting in the air mode being the strongest. With a 140-nm-thick ETL, the emission profile becomes bowl-shaped, and the peak angle shifts to 61 °, above the critical angle of 41 ° from glass to air, which results in the total internal reflection of the light and a strong reduction of the air mode. When the ETL thickness increases above 140 nm, a higher order cavity mode appears, and the air mode intensity increases again. These simulation results indicate that the air mode emission from the thin film LED cavity can be suppressed by simply tuning the ETL thickness.

[0042] For OLEDs, optical simulation was used to characterize the waveguide modes. Because the refractive index of ITO is higher than that of the organic layers and the glass substrate, it forms a slab thin-film waveguide. FIG. 5 illustrates the finite-difference time- domain (FDTD) method that was used to simulate the electric field distribution of the optical modes in an OLED with a 140-nm ETL. FIG. 5 further illustrates a cross-section SEM of the waveguide emission OLED fabricated on a 1-D grating, and the corresponding electric field (| E|²) distribution of the optical modes from the FDTD simulation. The results confirm that the TE waveguide mode is located at the vicinity of the ITO anode, therefore having a corrugated ITO anode will allow effective extraction of the TE waveguide mode by diffraction. The diffraction process is described by the Bragg equation

, where

and are

the in-plane wavevectors of the diffracted and the original waveguide mode respectively, and is the grating vector defined by the periodicity of the corrugation Λ, such that G = 2π / Λ. When k is smaller than the vacuum wavevector k₀ = 2π /λ, the waveguide mode is extracted into air at the angle of θ = sin^-1(k/k₀).

[0043] Because the TE waveguide mode is confined in the low-loss ITO anode, it has a narrow dispersion peak, which translates to a small divergence angle from the waveguide emission OLED. Note that in addition to the TE waveguide mode, there is one TM waveguide mode and one SPP mode present in the simulated OLED. To achieve a strongly directional and polarized emission, the TE waveguide mode can be extracted while suppressing the emission from the air, TM waveguide, and SPP modes. To suppress the air mode, the thickness of the ETL can be 140 nm, which corresponds to the air mode valley in the optical mode profile plot. Depending on the emitter spectrum width and wavelength, the minimal air mode background is in a range from about 2% to about 4%. Deviation from the air mode valley causes higher background emission and reduces the extinction ratio. Based on FIG. 1 , a 10% deviation (about 15 nm) from the optimal ETL thickness increases air mode background by 2%.

[0044] In a conventional OLED where the air mode is optimized, the ETL thickness should be about 60 nm, which corresponds to the air mode maximum. Typically, having such a thin ETL in a corrugated OLED will also result in a corrugation in the Al electrode, which will diffract the SPP as well as the TM waveguide mode as these modes have strong distribution at the metal interface.

[0045] On the other hand, since a thick ETL is utilized to suppress the air mode, this thermally evaporated thick ETL can render the top cathode to be almost planarized (FIG. 5). As a result, the diffraction of both the TM waveguide and SPP modes can be strongly suppressed. The residual corrugation depth is not sufficient to extract the TM modes, as they have shorter propagation length due to the absorption from the metal. Further, the planarized cathode also ensures the cavity effect is preserved such that the background emission can be efficiently suppressed by increasing the ETL thickness.

Waveguide Emission OLED Based on Ir-complex Emitter

[0046] In order to validate the device design, OLED devices were fabricated to study the effects of both the substrate corrugation and the ETL thickness (e.g., 60 nm vs 140 nm). The corrugated substrates were patterned by soft imprinting using a master mold comprising 1-D gratings having a 350 nm period and 100 nm depth. The modest corrugation depth ensures good conductivity on the ITO anode, as well as to minimize its influence on the OLED cavity. The period of the grating can be chosen based on the desired waveguide emission angle Q. Bragg diffraction dictates that

, where

is the in-plane wavevector of the

TE waveguide mode,

is the vacuum wavevector, and

is the grating vector . A

typical range of the grating period is about 200 nm to about 800 nm. The grating depth influences the conductivity of the ITO anode, the scattering of the cavity emission and the waveguide emission directionality. A large grating depth can compromise the device yield and operational stability. The typical grating depth can be in a range from about 20 nm to about 200 nm, about 50 nm to about 200 nm, or about 50 nm to about 100 nm. The ITO was then sputtered and the organic/metallic layers were evaporated on the substrates to fabricate the OLED devices. Angle-resolved emission spectra measurements were used to characterize the air mode dispersion in both TE and TM polarizations. For OLEDs on the 1-D grating substrates, the measurement plane is normal to the grating grooves. To show the effect of beam-shaping, the air mode was tuned to show the angular emission profile at 520 nm, corresponding to the peak wavelength of the green emitter I r(ppy)₃.

[0047] FIG. 6 illustrates the measured mode dispersion in TE and TM polarizations for a planar OLED with 60 nm ETL, while FIG. 7 illustrates the measured mode dispersion in TE and TM polarizations for a corrugated OLED with 60 nm ETL. The emission profiles are shown in FIGS. 8 and 9 for the planar and the corrugated OLEDs, respectively, at 520 nm for each OLED. For the corrugated OLED, a faint feature corresponding to TE waveguide mode at 0° is visible because the corrugated substrate has larger surface area and an overall higher film thickness than the simulated planar OLED.

[0048] For the planar OLED with a 60 nm thick ETL, the typical broad air mode background was observed. However, with a corrugated substrate, the OLED showed additional diffraction features in addition to the featureless background from the air mode (FIG. 7). Based on the polarization and in-plane wavevector, the diffraction features were identified as the diffracted TE waveguide mode, TM waveguide mode and SPP mode. In addition, the strong TM waveguide mode and SPP mode diffraction is caused by the corrugated Al with a depth of around 60 nm. Due to the light scattering from the corrugated Al, the cavity effect is weakened, and the background emission is reduced. Since several optical modes are diffracted into the air mode, the emission profile of the corrugated OLED has multiple peaks in both TE and TM polarizations (see FIG. 10). The two TE polarized peaks come from the diffracted TE waveguide mode propagating at opposite directions, while the TM polarized peaks may be attributed to the diffracted SPP modes at ±20° and the diffracted TM waveguide modes at ±4°. The magnitude of the SPP peaks is higher than the waveguide peaks because the SPP mode percentage of a device with a 60 nm-thick ETL is higher than the waveguide modes and is strongly diffracted at the highly corrugated Al cathode.

[0049] FIG. 10 illustrates the measured mode dispersion in TE and TM polarizations for a planar OLED with 140 nm ETL, and FIG. 11 illustrates the measured mode dispersion in TE and TM polarizations for a waveguide emission OLED with 140 nm ETL. The emission profiles are shown in FIGS. 12 and 13 for the planar and the waveguide OLEDs, respectively, at 520 nm for each OLED. For the planar OLED with a 140 nm ETL, the measured mode dispersion is very different compared to the planar OLED with a 60 nm ETL. Both the TE and TM polarized air modes have almost completely vanished due to the suppression of the air mode emission (FIGS. 11 and 13). With a corrugated substrate, the OLED still shows a negligible air mode background as expected, but with distinct TE waveguide mode diffraction features in the TE light profile. Note that there is almost no TM waveguide mode or SPP mode features in the TM light profile (FIGS. 12 and 14). The vanishing of the TM waveguide and SPP features may be attributed to the 140 nm-thick ETL layer which efficiently planarized the Al cathode and suppressed the diffraction of the TM waveguide and SPP modes.

[0050] From the emission profile, a highly directional emission peak is visible that corresponds to the TE waveguide emission, with a full-width at half-maximum (FWHM) divergence angle between 3.5° to 4.1°, depending on the wavelength. Due to the effective suppression of both the air mode background and diffraction of the TM polarized modes, the emission is highly polarized. The TE/TM extinction ratio for each wavelength at the corresponding waveguide emission peak was then calculated as shown in FIG. 14. A high extinction ratio of 13 was obtained between 520 nm and 540 nm, where the air mode emission is strongly suppressed using the cavity effect. Such highly polarized light is potentially useful for 3-D displays and displays which use a circular polarizer to reduce the reflection from ambient light.

[0051] Next, the external quantum efficiencies (EQEs) of the planar OLED and the waveguide emission OLED is compared in FIG. 16. For the planar OLED having a 60 nm- thick ETL, the outcoupling efficiency is maximized and the device has an EQE of 25%. When the ETL is increased to 140 nm, the outcoupling efficiency is minimized and the EQE is reduced to 2%. By incorporating corrugation in the device to extract the TE waveguide mode, the EQE of the waveguide emission OLED is increased to 7%, indicating the TE waveguide mode emission contributes an additional 5% of the EQE. This efficiency is much lower than the 21% TE waveguide mode distribution based on the optical simulation results, which estimates to a 24% TE waveguide mode extraction efficiency. The inefficient extraction of the waveguide mode may be attributed to two factors. First, diffraction due to the grating is limited by the shallow corrugation depth of 100 nm and the small index contrast between the ITO and grating. Second, the propagation length of the TE waveguide mode is limited by the residual optical absorption from ITO, Al, and grating, which limits the chance of diffraction. Two approaches can be used to improve the extraction efficiency of the TE waveguide mode. First, the absorption can be reduced with a more reflective Ag top electrode. Second, the corrugation geometry and the index contrast of the grating can be improved, which can be achieved by further optimization of the grating design and fabrication.

[0052] In addition to the directionality and polarization, the waveguide emission OLED also shows stronger EL peak intensity and smaller FWHM at FIG. 16. At 18° viewing angle, the EL peak of the waveguide emission OLED is 1.6 times higher than the reference OLED, while the FWHM is only 20 nm, much narrower than the planar OLED which has a 65 nm wide emission peak. In comparison, the corrugated OLED with a 60 nm ETL has a similar FWHM as the planar OLED because of the strong air mode emission. The smaller FWHM in a waveguide emission OLED stems from the narrow dispersion of the TE waveguide mode which is confined in the low-loss 110-nm-thick ITO anode. The spectral width of the extracted TE waveguide mode is 18 nm, much narrower than the spectral width of lr(ppy)₃. Therefore, the FWHM of the emitted light is significantly reduced. However, this also means a large portion of the emitter spectrum does not contribute to the waveguide emission at a given angle. As a result, the luminance of the waveguide emission OLED, which is a convolution of the (electro luminescence) EL spectra with the luminosity function, is actually lower than the planar OLED.

Waveguide Emission OLED Based on Eu-complex Emitter

[0053] Due to dispersion of the air and waveguide mode, the waveguide emission angle is different for each wavelength. Therefore, the directional emission cannot be spatially realized. To visualize the directional waveguide emission, a europium (Eu) complex Tris(dibenzoylmethane) mono(1 ,10-phenanthroline)europium(lll) (Eu(dbm)₃(phen)) can be used as the emitter, which has a narrow FWHM of 4 nm. This can drastically reduce the divergence caused by dispersion and allows visualization of the spatial pattern from the waveguide emission. A 2-D square grating substrate can also be used to demonstrate the effect of the DOE on the spatial pattern. The 2-D grating can have the same lattice constant of 350 nm as the 1-D grating. The practical periods and depths of the 1-D and 2-D gratings can be the same. [0054] Eu-complex based OLEDs were fabricated on a planar substrate, a 1-D grating substrate, and a 2-D square grating substrate. All three OLEDs have the same pixel size of 2 mm x 2 mm. To minimize the background emission near the Eu complex emission peak at 612 nm, a small adjustment was made to the OLED structure by increasing the ETL thickness to 170 nm. The increased ETL thickness is to ensure the cavity effect is at the air mode valley for a red emitter. As stated before, a 10% deviation from the optimal ETL thickness would lead to about 2% additional background emission, up from about 2% to about 4% based on the emitter spectrum width. Next, the emission pattern cast from the OLEDs onto a flat surface were compared as illustrated in FIG. 17. FIG. 17 illustrates spatial patterns from a reference OLED, a waveguide (WG) emission OLED on a 1-D grating substrate, and a waveguide emission OLED on a 2-D square grating substrate by using a narrow spectrum Eu complex emitter. All devices were driven at 12.5 mA/cm².

[0055] For the planar OLED, the pattern is broad and featureless due to the Lambertian profile. For the waveguide emission OLED on a 1-D grating substrate, two bright arcs were observed, corresponding to the two counter-propagating TE waveguide modes diffracted by the grating. Different points on the arcs originate from the TE waveguide modes propagating at different in-plane directions, which can be explained using the reciprocal space. For the waveguide emission OLED on a 2-D square grating substrate, a cross pattern comprising four arcs was observed. This is because a square lattice defines two orthogonal sets of

, therefore the TE waveguide mode is diffracted in the two orthogonal directions. Note that a gap can be observed between the arcs with the 1-D grating substrate but is not observed with the 2-D grating substrate. The reason may be attributed to the 2-D grating substrate having a lower fill factor than the 1-D grating, which leads to more ITO at the corrugated interface and a larger effective refractive index of the waveguide mode (and thus the radius of the arcs), resulting in closing the gap.

[0056] The emission pattern of a waveguide emission OLED based on its tunability and robustness can also be examined. Based on the Bragg equation

, the spatial pattern of the waveguide emission OLED can be modified by tuning the waveguide mode

or the DOE pattern

. Since tuning can be achieved by changing the refractive indices of the OLED layers, it can be limited by the available materials with the suitable optical and electrical properties. A more feasible way to modify the spatial pattern is using different DOEs, as has been demonstrated with the 1-D and 2-D patterns. In addition, the emission angle can be tuned from 1 ° to 19° by changing the grating periodicity from 350 nm to 300 nm, respectively. With a 2-D patterned substrate, the emission angles can be tuned separately for each optical axis using a dual-periodicity substrate.

[0057] Once a DOE pattern is chosen, it is beneficial for the waveguide emission pattern to have high tolerance for the variability in the OLED fabrication process. For the waveguide emission design, the grating periodicity can be precisely controlled by lithography. Other parameters that can be considered are thicknesses of the layers, which can alter the optical cavity. For example, during thermal evaporation, the organic layer thickness may have a slight variation across the panel or across each pixel (due to the shadow mask). To study the extreme cases, the thicknesses of the HTL were varied by 30 nm and the ETL by 20 nm to examine the change in the waveguide emission angle. In both cases, the emission angle only changed by 1 °, which confirms the robustness of the waveguide emission OLED design.

[0058] Using a similar approach as the Ir-complex waveguide emission OLED, the spectral FWHM and the luminance profile of the Eu-complex waveguide emission OLED were examined. The spectral width of the TE waveguide mode was also 18 nm, much larger than the spectral FWHM of the Eu-complex emitter, resulting in the spectrum narrowing effect being less noticeable. On the other hand, the broader TE waveguide mode enhances all the wavelength components of an Eu-complex emitter at the peak angle. Therefore, a 2X luminance enhancement in the forward luminance with a 1-D grating was observed, and 3X luminance enhancement with a 2-D grating was observed, respectively. The stronger enhancement with a 2-D grating is because the four waveguide emission arcs perfectly align in the normal direction. Waveguide Emission Perovskite LEDs

[0059] Although Eu and other Lanthanide complex emitters have a very narrow EL peak that is suitable for highly directional emission, they often have low quantum efficiency below 10%. In comparison, quantum dot LEDs and perovskite LEDs have shown over 20% external quantum efficiency with tunable EL spectra as narrow as 20 nm and thus, are great candidates for realizing high efficiency, low chromatic dispersion waveguide emission.

[0060] Other than intrinsic EQE, it was discovered that an important factor that determines the efficiency of the waveguide emission LED is the refractive index of the emitting material. As the refractive index of the EML increases, the TE waveguide mode percentage will be higher, which can be explained by the normalized modal electric field distribution. To demonstrate the refractive index dependence, the optical mode percentage in a waveguide emission LED architecture with a 30 nm-thick EML was simulated as a function of the refractive index of the emission layer n(EML) between 1.5 and 2.5 as shown in FIG. 18. FIG. 18 shows the dependence of optical mode percentage on the refractive index of the emitting layer in a waveguide emission thin-film LED. Consequently, the result shows that the TE waveguide mode percentage increases with n(EML). Specifically, for a perovskite emitter having a high refractive index (n ~ 2.3), 42% of the emitted photons are coupled to the TE waveguide mode, which is two times higher than the OLED case. With a stronger TE waveguide coupling, a stronger directional emission in a waveguide emission PeLED can be expected.

[0061] Green PeLEDs were fabricated on a 1-D grating substrate based on quasi 2-D perovskite having a composition of (PEA)₂(FA)₃Pb₄Br₁₃. The emission peak of the perovskite material is 522 nm, and the photoluminescence (PL) spectrum FWHM is 28 nm. The period of the grating is 300 nm, which diffracts the TE₀ waveguide mode at 530 nm to the normal direction. From the spatial pattern, the planar PeLED was observed to behave like a Lambertian emitter while the waveguide emission PeLED casts two sets of arcs, originating from the diffracted TE₀ and TEi waveguide modes, respectively. Note that the TEi waveguide mode forms because the thickness of the perovskite layer is 35 nm, which can be suppressed by reducing the ITO thickness or the EML thickness.

[0062] The two diffracted TE₀ arcs intersect at the normal direction, forming a strip of bright area. At each wavelength, the FWHM divergence angles of the TE₀ waveguide mode peaks are around 4°, and the divergence angle of the integrated luminance profile is 10° near the normal direction. As a result, the normal direction current efficiency of the waveguide emission PeLED is 56 cd/A, which is 2.6 times higher than the reference PeLED, despite the similar device EQEs. Spatial patterns from a reference PeLED and a waveguide emission PeLED on a 1-D grating substrate are shown in FIG. 19. Comparisons of the device EQEs are shown in FIG. 20. Comparisons of the device current efficiencies measured in the normal direction are shown in FIG. 21 , and comparisons of the device electroluminescence (EL) spectra measured at Q = 3° normal to the grating grooves are shown in FIG. 22.

[0063] It can be noted that the modest device efficiency of the reference PeLED may be due to the perovskite film having a low PL quantum yield (PLQY) of 40%. The thin perovskite film confines the TE waveguide mode in the ITO anode, which suppresses the re-absorption from the perovskite EML and maximizes the waveguide mode extraction. To date, the highest current efficiencies reported for green PeLEDs are 62.5 cd/A (PLQY = 73.8%) in a 2-D perovskite system and 78 cd/A (PLQY = 80%) in a 3-D perovskite system. With close to 100% PLQY and a fully optimized reference green emitting PeLED, the current efficiency is expected to be higher than 80 cd/A. Using such a device to fabricate a waveguide emission PeLED, a current efficiency of 170 cd/A is possible. Such highly directional and high efficiency PeLED design paves the way for high color purity light emitting devices for display and solid-state lighting applications.

[0064] According to various embodiments, highly directional and polarized waveguide emission thin-film LEDs were designed on a corrugated substrate such that the resulting gratings only extract the TE waveguide mode while suppressing light emission from other optical modes. To achieve these emission characteristics, the corrugation at the top cathode is planarized by thermal evaporation of a thick organic stack. This not only reduces the background air mode emission, but also suppresses the diffraction of SPP and TM waveguide modes, resulting in highly directional with a small divergence angle of 3° and polarized light emission from the TE waveguide mode having a TE/TM extinction ratio of 13. In addition, perovskite emitters can be the perfect candidates for waveguide emission due to the intrinsically high refractive index and thus high TE waveguide distribution. By extracting the TE waveguide modes into the forward direction, a waveguide emission perovskite LED was demonstrated with 2.6 times enhancement in current efficiency. Because the device is simple to fabricate and can be easily scaled-up, this discovery of strong directional and polarized light emission from OLEDs and perovskite LEDs has important applications for displays, lighting, and other photonic applications.

Fabrication of 1-D and 2-D Gratings

[0065] The 1-D and 2-D grating nanostructures on silicon substrate can be patterned using, e.g., a combination of interference lithography (IL) and transferred using reactive ion etching (RIE). To begin, a silicon substrate can be spin-coated with, e.g., 100 nm antireflection coating (ARC i-con-7, Brewer Science) and 180 nm positive photoresist (PFI-88A2, Sumitomo). The antireflection coating film can be used to reduce the reflection from silicon substrate during interference lithography. The 1-D and 2-D periodic grating nanostructures in photoresist can be patterned using, e.g., 325 nm wavelength HeCd laser exposure in a Lloyd’s mirror IL setup. Two coherent laser beams can be interfered to create periodic intensity pattern in Lloyd’s mirror IL setup. Then, the periodic grating pattern can be transferred to the underlying silicon substrate using, e.g., O₂ and CI₂ RIE. After etching, an RCA cleaning process can be used to remove the organic contaminants on the substrate surface. The surfaces of the molds can then be treated with silane to mitigate adhesion for the subsequent soft-imprinting process.

Fabrication of Waveguide Emission OLEDs

[0066] For the waveguide emission OLEDs, a corrugated substrate was first fabricated through soft-imprinting. A polydimethylsiloxane (PDMS) stamp was used to replicate the pattern from the master mold. The glass substrates were cleaned with standard ultrasonication procedure in acetone and isopropyl alcohol for 15 minutes each. Then, a small amount of NOA-81 epoxy (from Norland Products Inc.) was dropcast on the glass substrate. The stamp was pressed on the epoxy to remove air gaps in between. Then, the substrate with stamp was treated under 365 nm UV light (delight UVO cleaner Model 42) for 4 minutes to cure the epoxy. Afterwards the stamp was removed to leave behind the corrugated substrate.

Fabrication of Perovskite LEDs

[0067] The materials used for the PeLEDs were formomidinium bromide (FABr), lead bromide (PbBr₂), methylammonium chloride (MACI), anhydrous N-Methyl-2-pyrrolidone (NMP) and chlorobenzene, which were purchased from Sigma-Aldrich. Phenethylammonium bromide (PEABr) was purchased from Greatcell Solar. For the preparation of the perovskite, PEABr, FABr, PbBr2 (2:3:4 molar ratio) were dissolved in 1 mL anhydrous NMP to make 0.25 M (Pb²⁺ concentration) solution, and 1 mol% MACI was added. The solution was stirred for 2 h at 60 °C in a glovebox with a nitrogen environment. For the reference and waveguide emission PeLED, the pre-patterned ITO substrates were UV-Ozone treated for 15 minutes. PEDOT:PSS (4083) was spin-coated at 4000 rpm for 40 s and annealed at 150 °C for 15 min. Thereafter, the perovskite solution was spin-coated at 3000 rpm for 2 min, during which time (at 26 s for NMP) chlorobenzene (150 pL) was dripped onto the surface, followed by annealing at 90 °C for 10 min. The as-prepared substrates were then transferred into a thermal evaporator, and 40 nm TPBi, 2 nm Cs₂CO₃, and 100 nm Al were deposited layer by layer. For the waveguide emission PeLED, the ETL is TPBi (40 nm)/Bphen: 10% Cs₂CO₃ (90 nm). Finally, the fabricated devices were sealed in glovebox by ultraviolet-curable resin before testing.

Characterization of OLEDs and PeLEDs

[0068] The device voltage - current density curves were measured using a Keithley 2400 SourceMeter. The EQE was measured in an integration sphere (Labsphere lllumia). The edge of the substrates was covered to block the substrate mode leakage. The current efficiency was measured with a LS-100 luminance meter. For the angular EL spectra (ARES) measurements, a spectral goniometer was set up using an automatic rotary stage (Griffin Motion, RTS-DD-100). Light from the operating device was collected and sent to the spectrometer (Ocean Optics HR4000) by an optical fiber (Thorlabs 0200 pm, 0.22 NA) from 20 cm away. A wire grid polarizer (Thorlabs WP25L-VIS) was used to measure TE and TM light, respectively. For full-angle (-90° to 90°) measurements, the angle step is 1°. To determine the divergence angle FWHM, a finer angle step of 0.2° was used within a smaller angle range.

[0069] It should be noted that ratios, concentrations, amounts, and other numerical data may be expressed herein in a range format. It is to be understood that such a range format is used for convenience and brevity, and thus, should be interpreted in a flexible manner to include not only the numerical values explicitly recited as the limits of the range, but also to include all the individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly recited. To illustrate, a concentration range of “about 0.1% to about 5%” should be interpreted to include not only the explicitly recited concentration of about 0.1 wt% to about 5 wt%, but also include individual concentrations (e.g., 1%, 2%, 3%, and 4%) and the sub-ranges (e.g., 0.5%, 1.1%, 2.2%, 3.3%, and 4.4%) within the indicated range. The term “about” can include traditional rounding according to significant figures of numerical values. In addition, the phrase “about ‘x’ to ‘y’” includes “about ‘x’ to about ._y,„

[0070] Disjunctive language, such as the phrase “at least one of X, Y, or Z,” unless specifically stated otherwise, is to be understood with the context as used in general to present that an item, term, etc., can be either X, Y, or Z, or any combination thereof (e.g., X, Y, and/or Z). Thus, such disjunctive language is not generally intended to, and should not, imply that certain embodiments require at least one of X, at least one of Y, or at least one of Z to be each present. [0071] It should be emphasized that the above-described embodiments of the present disclosure are merely possible examples of implementations set forth for a clear understanding of the principles of the disclosure. Many variations and modifications can be made to the above-described embodiment(s) without departing substantially from the spirit and principles of the disclosure. All such modifications and variations are intended to be included herein within the scope of this disclosure and protected by the following claims.

Claims

CLAIMS At least the following is claimed:

1. A waveguide emission light emitting diode, comprising: an indium tin oxide anode disposed on the corrugated transparent substrate; a thin-film light emitting diode stack disposed on the indium tin oxide anode, the thin- film light emitting diode stack comprising: a hole transport layer; an emitting layer disposed on the hole transport layer; and an electron transport layer disposed on the emitting layer; and a metal electrode that forms a substantially planarized top layer of the waveguide emission light emitting diode.

2. The waveguide emission light emitting diode of claim 1 , wherein the waveguide emission light emitting diode comprises a perovskite light emitting diode.

3. The waveguide emission light emitting diode of claim 2, wherein the waveguide emission light emitting diode comprises a pixel size of 2 mm x 2 mm.

4. The waveguide emission light emitting diode of one of claims 1-3, wherein the corrugated transparent substrate comprises one of a 1-D grating substrate or a 2-D square grating substrate.

5. The waveguide emission light emitting diode of claim 4, wherein the corrugated transparent substrate comprises a grating depth of about 20 nm to about 200 nm.

6. The waveguide emission light emitting diode of claim 4, wherein the corrugated transparent substrate comprises a grating period in a range from about 200 nm to about 800 nm.

7. The waveguide emission light emitting diode of claim 4, wherein the 2-D square grating substrate comprises a first grating period in a first direction and a second grating period in a second direction.

8. The waveguide emission light emitting diode of claim 7, wherein the first and second grating periods are different.

9. The waveguide emission light emitting diode of claim 4, wherein the 2-D square grating substrate comprises a 2-D array of geometric pillars.

10. The waveguide emission light emitting diode of claim 9, wherein pillars of the 2-D array of geometric pillars have a circular or rectangular cross-section.

11 . The waveguide emission light emitting diode of claim 4, wherein the 1-D grating substrate comprises a plurality of parallel ridges.

12. The waveguide emission light emitting diode of any one of claims 1-11 , wherein the indium tin oxide anode comprises a thickness of about 100 nm or greater.

13. The waveguide emission light emitting diode of any one of claims 1-12, wherein the electron transport layer comprises a thickness in a range from about 60 nm to about 170 nm.

14. The waveguide emission light emitting diode of any one of claims 1-13, wherein the waveguide emission light emitting diode comprises a europium (Eu) complex based organic light emitting diode.

15. The waveguide light emitting diode of any one of claims 1-13, wherein the waveguide emission light emitting diode comprises an iridium (Ir) complex based organic light emitting diode.

16. The waveguide light emitting diode of any one of claims 1-13, wherein the indium tin oxide anode is directly sputtered on the corrugated transparent substrate.

17. The waveguide emission light emitting diode of any one of claims 1-13, wherein the metal electrode is planarized via a thermal evaporation process.