Light Extraction Efficiency Research Articles

Disordered metasurface-enhanced perovskite composite films with ultra-stable and wide color gamut used for backlit displays

Perovskite nanocrystals (PNCs) are promising to replace the traditional phosphor color conversion layers in backlit displays due to their excellent optical properties, which can enhance the color gamut and saturation of displays. However, poor stability and low luminescence efficiency hinder the development of perovskite-based devices. Here, inspired by the Yunnan cicada wing, a low-cost, easy-to-fabricate, and large-area (> 78 cm2) all-dielectric metasurface based on the Purcell effect is developed to tune and improve the light extraction efficiency of PNCs. All-dielectric metasurfaces composed of weakly disordered TiO2/PS spheres are embedded in the PNCs/PVDF (Poly (vinylidene fluoride)) films prepared by in-situ fabrication strategy, resulting in 3.19-fold photoluminescence (PL) maximum enhancement of these composite films. The stability of PNCs is also markedly improved after PVDF encapsulation. As an application demo, these all-dielectric metasurface-modulated perovskite composite films are applied in LCD backlit, with the color gamut of 157% of commercial LCD and 107% of NTSC, showing tremendous potential in high-end lighting and display. Furthermore, combined with the scalability of large-area fabrication, cost-effective weakly disordered metasurfaces are a potential method to bridge the gap between high-efficient laboratory demonstrations and commercialization in the field of display.

52‐4: Novel Polarizer‐Free OLED Structure with High Optical Gain

With the recent increase in OLED usage, the size and shape of devices are diversifying. In addition, portability is becoming more important along with product performance and improvement. In order to enhance portability, it is necessary to improve battery performance and increase the time available without charging. To this end, it is required to reduce display power consumption, and various methods and structures for reducing power consumption by improving the efficiency of many OLED products have been proposed.Most OLED displays have a polarizer film applied to secure black viewing in the off‐state. However, this reflectance‐reducing film has a transmittance 50% or less, thus the light extraction efficiency is very bad.This paper introduces a simulation‐based design methodology of a new display that is devoid of polarizer film. It required the task of obtaining high outdoor contrast by reducing reflectance of external light with improved light efficiency. By separating the panel structure and optimizing optical filters and OLED devices, reflection reduction, efficiency improvement, and viewing angle characteristics were secured at the same time.The simulation‐based optical design methodology is an important technology that can simultaneously optimize the optical characteristics of new type of display, and is expected to be applied to various product developments in the future.

Improving the Light Extraction Efficiency of GRIN Coatings Pillar Light Emitting Diodes

Improving the Light Extraction Efficiency of GRIN Coatings Pillar Light Emitting Diodes

A 10 × 10 deep ultraviolet light-emitting micro-LED array

In this work, we design and fabricate a deep ultraviolet (DUV) light-emitting array consisting of 10 × 10 micro-LEDs (μ-LEDs) with each device having 20 μm in diameter. Strikingly, the array demonstrates a significant enhancement of total light output power by nearly 52% at the injection current of 100 mA, in comparison to a conventional large LED chip whose emitting area is the same as the array. A much higher (~22%) peak external quantum efficiency, as well as a smaller efficiency droop for μ-LED array, was also achieved. The numerical calculation reveals that the performance boost can be attributed to the higher light extraction efficiency at the edge of each μ-LED. Additionally, the far-field pattern measurement shows that the μ-LED array possesses a better forward directionality of emission. These findings shed light on the enhancement of the DUV LEDs performance and provide new insights in controlling the light behavior of the μ-LEDs.

19‐3: Crucial Effect of Aspect Ratio of Quantum‐Dot Color‐Conversion Pixels on the Performance of High‐Resolution Full‐Color MicroLED Microdisplays

The light extraction efficiency of the QDCC‐based microLED displays is strong related to the aspect ratio (width/thickness) of the patterned QDCC pixels, because the very thin QDCC films and BM banks are required for the realization of high‐resolution microdisplays for AR and VR applications. We formulate high resolution QD photoresist, which can achieve color gamut higher than 85% BT. 2020 coverage at film feature resolution of 5 μm and the total thickness of QDCC and traditional CF less than 2.5 μm. The atomic layer deposition is also processed to protect the device, which results in prominent enhancement on the reliability performance.

Surface-emitting photonic crystal terahertz quantum cascade laser adopting uniform triangular prism photonic crystal with a double-metal waveguide

Herein, we investigate surface-emitting photonic crystal terahertz quantum cascade lasers adopting double metal waveguide for high light extraction efficiency by utilizing finite-difference time-domain and filter diagonalization method. For the ease of fabrication and scalability of beam power, only a typical uniform 2D unit cell with triangular lattice and connected metal electrodes are considered in this study. Cylinder or uniform triangular prism photonic crystals are adopted as resonators within the selected unit cell structure. Candidate -point modes for efficient surface-normal emission are searched based on the unit cell analysis as a first step. After that, far-field, field distribution, and light-extraction efficiency are calculated to verify the candidate modes’ suitability by deploying 15(16) × 15 unit cells. Extracted results show that the surface-emitting quantum cascade lasers with triangular prism photonic crystal slabs have the possibility to attain high light-extraction efficiency with Gaussian beam shape even with compact chip size and high active region occupancy.

Performance and analysis of an ultraviolet C-band light emitting diode with an emission wavelength of 268 nm

We report a high-power ultraviolet C-band light-emitting diode (LED) with peak wavelength at 268 nm delivering 199 mW optical power at 350 mA direct current. The peak wall-plug and external quantum efficiencies are greater than 10% and 13%, respectively. We developed an approach to determine light-extraction efficiency (LEE) and internal quantum efficiency (IQE) based on the carrier recombination rate equation and used it to analyze the junction-temperature-dependent optical power behavior. For this LED, we obtained a LEE of ∼16% and a peak IQE of ∼83%. The efficiency droop percentage was quantified to be ∼20% at 350 mA, owing to Auger and other higher order carrier losses.

Size dependent characteristics of AlGaN-based deep ultraviolet micro-light-emitting-diodes

AlGaN-based deep ultraviolet (DUV) micro-light-emitting diodes (μLEDs) with emission wavelengths between 277 and 304 nm with mesa dimensions down to 20 μm were fabricated. Their size-dependent electrical and optical characteristics were analyzed. At 20 A cm−2, the external quantum efficiency (EQE) increased from 2.0% to 2.3% mainly due to the improved light extraction efficiency; the forward voltage was 7.6 V in 20 μm sized μLEDs in comparison to 9.1 V in 300 μm LEDs due to better current spreading in the smaller devices. The peak EQEs of the 20 μm μLEDs were 2.5% and 4.0% for 277 and 304 nm, among the highest reported for DUV μLEDs.

Effects of electron transport layer thickness on light extraction in corrugated OLEDs.

This study reported the effects of electron transport layer (ETL) thickness on light extraction in corrugated organic light-emitting diodes (OLEDs) and each layer in OLEDs exhibited a periodical corrugated structure, which was determined by depositing thin films on a glass substrate with a nanoimprinted blazed grating structure. The insight is that light extraction in corrugated OLEDs significantly depends on the ETL thickness. Varying the ETL thickness changed the distribution of carrier recombination and led to exciton formation and optical interference, thereby resulting in different attribution of optical loss modes in OLEDs, which increased or even decreased light extraction and device efficiency. Trapped light extraction from the surface plasmon polariton (SPP) and waveguide (WG) modes was identified by splitting the light into transverse electric and transverse magnetic emissions. Thus, the contributions from the individual SPP and WG modes to the external quantum efficiency (EQE) were distinctly clarified by comparing the experimental results with the theoretical calculations. At the ETL thickness of 115 nm, the corrugated OLED exhibited a significantly enhanced (1.83-fold) EQE compared to the planar one due to the effective extraction of trapped light from the SPP and WG modes. The EQE was enhanced by 0.5%, wherein 0.39% came from the WG mode and 0.11% came from the SPP mode.

Enhanced light extraction efficiency of UV LEDs by encapsulation with UV-transparent silicone resin

Increase of light extraction efficiency (LEE) and total output power of UV light emitting diodes (LEDs) emitting at 265 and 310 nm, respectively, after encapsulation with a UV-transparent silicone are studied. Raytracing simulations suggest that a properly placed hemispherical encapsulation with a refractive index in the range from 1.4 to 1.8 enhances the LEE from 8% to up to 16% for flip-chip mounted UV LEDs with non-reflective metal contacts. The simulations also show that the absorption coefficient of the encapsulant determines the maximum LEE and optimum dome diameter and that it should be below 3 cm−1. The silicone encapsulant exhibits a refractive index of 1.47 (1.45) and an absorption coefficient of 1.3 cm−1 (0.47 cm−1) at 265 nm (310 nm). AlGaN/sapphire-based UVC and UVB LED chips were flip-chip mounted on planar AlN ceramic packages and encapsulated with a 1.5 mm-radius hemispherical silicone dome. The total output power at an operation current of 350 mA increased from 27 to 46 mW for 265 nm LEDs and from 45 to 78 mW for 310 nm LEDs. This corresponds to an enhancement of about 70%, which agrees with the simulations. Moreover, far-field measurements of encapsulated LEDs showed a narrowing of the emission cone.

Designing highly luminescent nanocrystals embedded bulk single crystals for X-ray scintillators

Metal halide perovskite nanocrystals (PNCs) have demonstrated intense luminescence and short emission lifetime, which perfectly match the requirements of X-ray fast scintillators. However, the poor stability of PNCs restricts their applications. Herein, CsPbBr3 NCs in a wide band-gap Cs4PbBr6 perovskite single crystal (PSC) matrix for efficient light extraction is reported. Benefitting from the protection of the PSC matrix and the perfect lattice matching, the PNCs embedded PSC show highly minimized nonradiative recombination and greatly improved stability. Moreover, by controlling the chemical equilibrium, the self-absorption of PNCs is reduced and the quantum yield increased to 78.5%. Furthermore, the PNCs embedded PSC demonstrates an appreciable light yield of 15,290 photons MeV−1, much higher than the commercial scintillator BGO (8200 photons MeV−1), and a decay time of 12.8 ns, significantly shorter than almost all the representative scintillators. This work demonstrates unambiguously the potential of PNCs embedded PSC for next-generation fast scintillators.

Thermal behavior of AlGaN-based deep-UV LEDs.

This study utilized thin p-GaN, indium tin oxide (ITO), and a reflective passivation layer (RPL) to improve the performance of deep ultra-violet light-emitting diodes (DUV-LEDs). RPL reflectors, which comprise HfO2/SiO2 stacks of different thickness to maintain high reflectance, were deposited on the DUV-LEDs with 40 nm-thick p-GaN and 12 nm-thick ITO thin films. Although the thin p-GaN and ITO films affect the operation voltage of DUV-LEDs, the highly reflective RPL structure improved the WPE and light extraction efficiency (LEE) of the DUV-LEDs, yielding the best WPE and LEE of 2.59% and 7.57%, respectively. The junction temperature of DUV-LEDs with thick p-GaN increased linearly with the injection current, while that of DUV-LEDs with thin p-GaN, thin ITO, and RPL was lower than that of the Ref-LED under high injection currents (> 500 mA). This influenced the temperature sensitive coefficients (dV/dT, dLOP/dT, and dWLP/dT). The thermal behavior of DUV-LEDs with p-GaN and ITO layers of different thicknesses with/without the RPL was discussed in detail.

Interplay of sidewall damage and light extraction efficiency of micro-LEDs.

This Letter describes the impact of shape on micro light-emitting diodes (µLEDs), analyzing 400 µm2 area µLEDs with various mesa shapes (circular, square, and stripes). Appropriate external quantum efficiency (EQE) can yield internal quantum efficiency (IQE) which decreases with increasing peripheral length of the mesas. However, light extraction efficiency (ηe) increased with increasing mesa periphery. We introduce analysis of Jpeak (the current at peak EQE) since it is proportional to the non-radiative recombination. Etching the sidewalls using tetramethylammonium hydroxide (TMAH) increased the peak EQE and decreased the sidewall dependency of Jpeak. Quantitatively, the TMAH etching reduced non-radiative surface recombination by a factor of four. Hence, shrinking µLEDs needs an understanding of the relationship between non-radiative recombination and ηe, where analyzing Jpeak can offer new insights.

AlGaN-Based Deep Ultraviolet Light-Emitting Diodes with Thermally Oxidized Al x Ga2-x O3 Sidewalls.

AlGaN and GaN sidewalls were turned into AlxGa2–xO3 and Ga2O3, respectively, by thermal oxidation to improve the optoelectrical characteristics of deep ultraviolet (DUV) light-emitting diodes (LEDs). The thermally oxidized Ga2O3 is a single crystal with nanosized voids homogenously distributed inside the layer. Two oxidized AlxGa2–xO3 layers were observed on the sidewall of the AlGaN layer in transmission electron microscopy images. The first oxidized AlxGa2–xO3 layer is a single crystal, while the second oxidized AlxGa2–xO3 layer is a single crystal with numerous nanosized voids inside. The composition of Al in the first oxidized AlxGa2–xO3 layer is higher than that in the second one. The thermal oxidation at high temperature degrades the quality of the p-GaN layer and increases the forward voltage from 8.18 to 11.36 V. The thermally oxidized AlxGa2–xO3 sidewall greatly enhances the light extraction efficiency of the lateral light of the DUV LEDs by combined mechanisms of holey structure, graded refractive index, high transparency, and tensile stress. Consequently, the light output power of the DUV LEDs increases from 0.69 to 0.88 mW by introducing a 420 nm thick AlxGa2–xO3 sidewall oxidized at 900 °C, in which the enhancement of light output power can reach 27.5%.

Thermal release tape-assisted semiconductor membrane transfer process for hybrid photonic devices embedding quantum emitters

The ability to combine different materials enables a combination of complementary properties and device engineering that cannot be found or exploited within a single material system. In the realm of quantum nanophotonics, one might want to increase device functionality by, for instance, combining efficient classical and quantum light emission available in III–V semiconductors, low-loss light propagation accessible in silicon-based materials, fast electro-optical properties of lithium niobate, and broadband reflectors and/or buried metallic contacts for local electric field application or electrical injection of emitters. However, combining different materials on a single wafer is challenging and may result in low reproducibility and/or low yield. For instance, direct epitaxial growth requires crystal lattice matching for producing of defect-free films, and wafer bonding requires considerable and costly process development for high bond strength and yield. We propose a transfer printing technique based on the removal of arrays of free-standing membranes and their deposition onto a host material using a thermal release adhesive tape-assisted process. This approach is versatile, in that it poses limited restrictions on the transferred and host materials. In particular, we transfer 190 nm-thick GaAs membranes that contain InAs quantum dots and which have dimensions up to about 260 μm × 80 μm onto a gold-coated silicon substrate. We show that the presence of a back reflector combined with the etching of micropillars significantly increases the extraction efficiency of quantum light from a single quantum dot line, reaching photon fluxes exceeding 8 × 105 photons per second. This flux is four times higher than the highest count rates measured from emitters outside the pillars on the same chip. Given its versatility and ease of processing, this technique provides a path to realising hybrid quantum nanophotonic devices that combine virtually any material in which free-standing membranes can be made onto any host substrate, without specific compatibility issues and/or requirements.

An Improved Model for Light Transport in the Color Conversion Element of Light-Emitting Diodes

Light transport with fluorescence in a phosphor layer based on radiative transfer theory is an efficient tool for understanding the performance of a phosphor-converted light-emitting diode. In this work, the models based on radiative transfer theory including light conversion of phosphor particles are developed for calculating the light transport in planar remote phosphor layers. The models developed include an ordinary differential approximation and an improved model based on the differential approximation and an integral formulation of the transmission light flux for a phosphor layer with Fresnel boundaries. We calculate the light extraction efficiency (LEE) of a phosphor layer as a function of various parameters, such as the thickness of the layer and the concentration of phosphor. The present models are validated by comparing the results obtained by the present methods, the double spherical harmonics method of order one and a Monte Carlo method. Comparing the results obtained by those methods, one can see that the improvement based on the differential approximation and integral formulation of transmission light flux for calculating the LEE can yield better results for optically thin cases.

Silk Fibroin‐Based Flexible Organic Light‐Emitting Diode with High Light Extraction Efficiency

AbstractIt is an essential issue for the efficiency of flexible organic light‐emitting diodes (OLEDs) to be improved in display and lighting applications. Herein, the use of silk fibroin (SF) as a flexible OLED substrate with enhanced light extraction efficiency (LEE) is reported. Regenerated SF is prepared via a casting method and then coated with conductive indium tin oxide (ITO) via magnetron sputtering. SF‐based flexible OLED outperforms its reference device in terms of turn‐on voltage, peak brightness, and LEE, where glass and polyethylene terephthalate are used as rigid and flexible substrates, respectively. The improved device performance can be ascribed to the low refractive index of SF and its lower surface roughness and more matched energy levels compared to those of other substrates, resulting in a high LEE. Notably, a molecular dipole layer can be formed between the interface of ITO and SF. The dipole moment pointing to the ITO surface effectively changes the ITO work function and reduces the potential barrier height for hole transfer. The work provides new ideas for constructing high‐performance OLEDs on biomass‐based substrates with excellent flexibilities and high LEEs.

Demonstration of Efficient Ultrathin Side-Emitting InGaN/GaN Flip-Chip Light-Emitting Diodes by Double Side Reflectors.

This work proposes an InGaN/GaN multiple-quantum-well flip-chip blue ultrathin side-emitting (USE) light-emitting diode (LED) and describes the sidewall light emission characteristics for the application of backlight units in display technology. The USE-LEDs are fabricated with top (ITO/distributed Bragg reflector) and bottom (Ag) mirrors that cause light emission from the four sidewalls in a lateral direction. The effect of light output power (LOP) on lateral direction is consistently investigated for improving the optoelectronic performances of USE-LEDs. Initially, the reference USE-LED suffers from very low LOP because of poor light extraction efficiency (LEE). Therefore, the LEE is improved by fabricating ZnO nanorods at each sidewall through hydrothermal method. The effects of ZnO nanorod lengths and diameters on LOP are systematically investigated for optimizing the dimensions of ZnO nanorods. The optimized ZnO nanorods improve the LEE of USE-LED, which thus results in increasing the LOP > 80% compared to the reference LED. In addition, the light-tools simulator is also used for elucidating the increase in LEE of ZnO nanorods USE-LED.

Enhanced Light Extraction from Organic Light-Emitting Diodes with Micro-Nano Hybrid Structure.

In this study, an external light extraction layer with a micro-nano hybrid structure was applied to improve the external light extraction efficiency of organic light-emitting diodes (OLEDs). A reactive ion-etching (RIE) process, using O2 and CHF3 plasma, was performed on the surface of the micro-scale pattern to form micro-nano hybrid structures. According to the results of this study, the nanostructures formed by the treatment of O2 and CHF3 were different, and the efficiency according to the structures was analyzed experimentally and theoretically. As a result, the OLED, to which the micro-nano hybrid structure, manufactured through a simple process, is applied, improved the external light extraction efficiency by up to 38%, and an extended viewing angle profile was obtained. Additionally, an effective method for enhancing the out-coupling efficiency of OLEDs was presented by optimizing the micro-nano hybrid structure according to process conditions.

Enhancement of the light extraction characteristics and wide-angle emissive behavior of deep-ultraviolet flip-chip light-emitting diodes by using optimized optical films.

We propose the use of optical films to enhance the light extraction efficiency (LEE) and wide-angle emission of traditional packaged deep-ultraviolet light-emitting diodes (DUV-LEDs). Total internal reflection occurs easily in DUV-LEDs because they contain sapphire, which has a high refractive index. DUV-LEDs also contain an aluminum nitride (AlN) ceramic substrate, which has high light absorption in the ultraviolet band. Photons are absorbed by the sapphire and AlN ceramic substrate, which reduces the LEE of DUV-LEDs. By adding a brightness enhancement film (BEF) on the sapphire surface and a high-reflection film (HRF) on the surface of the AlN ceramic substrate, the LEE of DUV-LEDs can be increased. Moreover, we designed a single-layer metal reflective film (SMRF) on the upper surface of the quartz glass in order to achieve wide-angle emission. Experimental results indicated that compared with traditional packaged DUV-LEDs, the light output power and external quantum efficiency of DUV-LEDs with a plated BEF, HRF, and SMRF increased by 18.3% and 18.2%, respectively. Moreover, an emission angle of 160° was achieved. In a reliability test, DUV-LEDs maintained more than 95% of the initial forward voltage and light output power after 1000 h of operation at 25°C, which indicated that the addition of an optical film can improve the light efficiency and long-term reliability of DUV-LEDs.