Optical Extraction Efficiency Research Articles

Enhancing light extraction efficiency and color stability for OLED by truncated micro-cone array films

A newly truncated micro-cone array film as light extraction layer for organic light emitting diodes (OLEDs) has been developed, which was fabricated by process combined with photolithography, polydimethylsiloxane (PDMS) molding, self-assembled anti-adhesion, and ultraviolet (UV) forming techniques. In this study, by varying the sidewall angle and fill factor of this film, their influences on the optical properties and light extraction efficiency for OLED are fully investigated. As a result, device performance of OLED employed truncated micro-cone array film, with a high fill factor and sidewall angle of 76.49⁰, was found to be significantly enhanced up to 52% compared with the reference OLED. Notably, the proposed truncated micro-cone array film also stabilized emissive color stability while the viewing angle changes from 0° to 70°, achieving an excellent angular shift with CIE-indices of (0.0025, 0.0066). Aforementioned findings contribute to the development in advanced OLEDs for display and lighting applications.

Enhancing the Light Extraction Efficiency in Micro‐Organic Light‐Emitting Diodes with Metalens

Due to the total internal reflection caused by the mismatch of the refractive indices between organic layers and oxide layers within an organic light‐emitting diode (OLED), most of the light energy is trapped inside the device; if the size of an OLED is further limited to a few micrometers, most of the light will be lost on the side wall, which will further reduce the light outcoupling efficiency. Herein, the mechanism of the light extraction efficiency of micro‐OLEDs with metalens is studied. The results show that the metalens has a better optical extraction efficiency for single‐reflection light rather than requiring light to be multireflected by the conventional photonic structures. As a result, it effectively improves the light‐emitting coupling efficiency and even the OLED size shrinks to 1 μm. This technique is expected to boost the efficiency in ultrahigh‐pixel‐density microdisplays required in emerging augmented reality (AR)/virtual reality (VR) and is extrapolated to other light‐emitting devices such as quantum‐dot light‐emitting diodes (QLEDs), micro‐light‐emitting diodes (μLEDs), and perovskite light‐emitting diodes (PeLEDs).

Decomposition of dimethyl methylphosphonate vapor on ultrathin-film titania photocatalytic light absorber

The decomposition of chemical warfare agent simulant, dimethyl methylphophonate (DMMP) vapor, was investigated on an ultrathin film titania (TiO2) photocatalytic light absorber. The light absorber contains an aluminum (Al) reflector and the TiO2 thin film with different thicknesses, sequentially deposited on a supportive glass substrate. The designed structure constructs a nanocavity that exhibits strong light absorption within the photocatalytic TiO2 ultrathin film. Thus, the intrinsic trade-off between optical absorption and charge carrier extraction efficiency, i.e., a light absorber should be thick enough to absorb the light allowable by its band gap but thin enough to allow charge carrier extraction for catalytic reactions, is conquered. The TiO2/Al light absorber significantly boosted TiO2 photocatalytic activity compared to the benchmark Aeroxide®P25 catalyst (i.e., up to 2013 times increase in reaction rate). The effects of reactant (i.e. DMMP, water and oxygen, respectively) partial pressure and reaction temperature on photocatalytic decomposition of DMMP by the ultrathin-film TiO2 photocatalytic light absorber were studied. Kinetic data of the DMMP decomposition can be described by the Langmuir-Hinshelwood model.

Design and optical characterization of an efficient polarized organic light emitting diode based on refractive index modulation in the emitting layer.

Luminescent liquid Crystal (LC) material is regarded as the most promising material for polarized organic light emission due to their intrinsic characteristics including orderly alignment and luminescence. Nevertheless, the optical extraction efficiency of LC based organic light emitting diodes (OLEDs) devices still requires significant effort and innovation towards real-world applications. In this paper, we propose the design of a highly linearly polarized light-emission from OLEDs with integrated refractive index nanograting in the emissive layer (EML) based on photo aligned luminescent liquid crystal material. The simulation results indicate that the geometrically optimized polarized device yields an external quantum efficiency (EQE) up to 47% with a polarized ratio up to 28 dB at a 550 nm emission wavelength. This conceptual design offers a new opportunity to achieve efficient polarized organic luminescence, and it is (to the best of our knowledge) the first approach that enhances the light extraction of OLEDs based on luminescent liquid crystal via index grating in the EML.

Broadband, wide-angle antireflection in GaAs through surface nano-structuring for solar cell applications

We demonstrate broadband and wide-angle antireflective surface nanostructuring in GaAs semiconductors using variable dose electron-beam lithography (EBL). Various designed structures are written with EBL on a positive EB-resist coated GaAs and developed followed by shallow inductively coupled plasma etching. An optimized nanostructured surface shows a reduced surface reflectivity down to less than 2.5% in the visible range of 450–700 nm and an average reflectance of less than 4% over a broad near-infrared wavelength range from 900–1400 nm. The results are obtained over a wide incidence angle of 33.3°. This study shows the potential for anti-reflective structures using a simpler reverse EBL process which can provide optical absorption or extraction efficiency enhancement in semiconductors relevant to improved performance in solar photovoltaics or light-emitting diodes.

Optical Extraction Efficiency for External Cavity Quantum Cascade Lasers

Optical Extraction Efficiency for External Cavity Quantum Cascade Lasers

Optical light polarization and light extraction efficiency of AlGaN-based LEDs emitting between 264 and 220 nm

The influence of aluminum mole fraction of AlxGa1-xN/AlyGa1-yN multiple quantum wells (MQWs) on the optical polarization, light extraction efficiency (LEE) and external quantum efficiency (EQE) of deep ultra violet light emitting diodes in the wavelength range between 264 and 220 nm is investigated. The on-wafer EQE decreases from 0.6% to 0.00013% in this wavelength range. Polarization resolved photoluminescence and electroluminescence measurements show a change from dominant transverse-electric to dominant transverse-magnetic polarized light emission with increasing aluminum mole fraction in the MQW. The quantitative agreement with k·p calculations allow to ascribe this shift to a change of the characteristic of the Γ7+ valance band. Ray tracing simulations predict a reduction of the on-wafer LEE from 4% to 1.5%. Therefore the dramatic drop of the EQE in this wavelength range cannot be attributed to a drop in LEE and is most likely dominated by charge carrier injection and radiative recombination efficiency.

Efficient performance enhancement of GaN-based vertical light-emitting diodes coated with N-doped graphene quantum dots

Novel vertical light-emitting diodes (VLEDs) decorated with N-doped graphene quantum dots (N-GQDs) have been fabricated, based on the metal organic chemical vapor deposition method (MOCVD), wafer bonding technique, laser lift-off (LLO) technique, inductively coupled plasma (ICP) etching and impregnation processing. There is an obvious luminescence enhancement and resistance reduction for VLEDs after coating N-GQDs. The results show an improvement of light output power (LOP), dependent on the improvement of optical extraction efficiency and current spreading. The improvement of external quantum efficiency (EQE) is mainly owing to the roughened surfaces. Meanwhile, an enhancement of electroluminescence (EL) intensity and photoluminescence (PL) intensity has been achieved when the VLEDs were immersed in 0.9 mg/mL of N-GQDs solution, 4 times that of bare VLEDs. After coating N-GQDs, the performance of VLEDs is greatly improved, which could be attributed to the photons recovered by extracting light from waveguide modes.

Ultrathin-Film Titania Photocatalyst on Nanocavity for CO2 Reduction with Boosted Catalytic Efficiencies.

Photocatalytic CO2 reduction with water to hydrocarbons represents a viable and sustainable process toward greenhouse gas reduction and fuel/chemical production. Development of more efficient catalysts is the key to mitigate the limits in photocatalytic processes. Here, a novel ultrathin‐film photocatalytic light absorber (UFPLA) with TiO2 films to design efficient photocatalytic CO2 conversion processes is created. The UFPLA structure conquers the intrinsic trade‐off between optical absorption and charge carrier extraction efficiency, that is, a solar absorber should be thick enough to absorb majority of the light allowable by its bandgap but thin enough to allow charge carrier extraction for reactions. The as‐obtained structures significantly improve TiO2 photocatalytic activity and selectivity to oxygenated hydrocarbons than the benchmark photocatalyst (Aeroxide P25). Remarkably, UFPLAs with 2‐nm‐thick TiO2 films result in hydrocarbon formation rates of 0.967 mmol g−1 h−1, corresponding to 1145 times higher activity than Aeroxide P25. This observation is confirmed by femtosecond transient absorption spectroscopic experiments where longer charge carrier lifetimes are recorded for the thinner films. The current work demonstrates a powerful strategy to control light absorption and catalysis in CO2 conversion and, therefore, creates new and transformative ways of converting solar energy and greenhouse gas to alcohol fuels/chemicals.

95-ps all-solid-state laser with a low-power microchip laser seed and a two-stage single-pass bounce geometry amplifier

A low-power Nd:YVO4 microchip seed laser with a pulse duration of ∼95 ps is amplified by two single-pass Nd:YVO4 bounce amplifiers. The microchip is a semiconductor saturable absorber mirror (SESAM)-based and passively Q-switched laser with 10 mW output power at a repetition rate of 100 kHz. The first Nd:YVO4 bounce amplifier delivers an average power of 12 W with pump power of 55 W, corresponding to an optical–optical extraction efficiency of 22%. The output power from the second amplifier is 32 W, corresponding to pulse peak power of ∼3.4 MW and pulse energy of ∼320 μJ. The beam quality factor M2 is 1.25 in the horizontal direction and 1.20 in the vertical direction. The investigations offer a high-peak-power master oscillator power amplifier with both high gain and high beam quality. The all-solid-state laser system is flexible, robust, and cost effective, showing attractive potential for further applications.

Enhancement of optical extraction efficiency in white LED package with quantum dot phosphors and air-gap structure

An efficient package configuration for a white-light-emitting diode based on quantum dot (QD) phosphors was proposed and fabricated with a polymer lens, remote QD polymer layers, and an air-gap structure. The optical extraction efficiency of the newly proposed air-gap structure showed enhancements of 33.0% by simulation and 29.7% by experiment as compared with that of a remote layer-by-layer QD structure without an air gap. This enhancement comes from the fact that some reemitted and scattered light from QDs can be reflected effectively by total internal reflection at the interface between the QD polymer layer and air to reduce the absorption loss in the package.

Hybrid GaN/Organic white light emitters with aggregation induced emission organic molecule

We demonstrate hybrid white light emitters by employing Gallium Nitride (GaN)-based light-emitting diodes (LEDs) as blue emitters and aggregation-induced emission (AIE) organic molecule 4,7-Bis[4-(1,2,2-triphenylvinyl)phenyl]benzo-2,1,3-thiadiazole (BTPETD) as down-converted yellow emitters. The fabricated device shows CIE coordinates of (0.32, 0.33) and 45.4% optical extraction efficiency from BTPETD layer with pump leakage of 17.3%. At driving current of 30 mA, the highest luminous efficacy of 123.8 lm/W was obtained for the hybrid devices which corresponded a luminance of 1.4 × 105 cd/m2. Our work indicates that formation of aggregates from AIE materials such as BTPETD is able to greatly enhance the device extraction efficiency and improve the hybrid white LED performance.

Thermal and Optical Performance of a Light-Emitting-Diode Metal Package With an Integrated Reflector and Heat Spreader Structure

We develop a light-emitting diode (LED) metal package consisting of a reflector integrated with a heat spreader as one body. The structure functions obtained from the thermal transient measurement reveals that the integrated structure of the metal package leads to improved thermal performance. We make significant efforts to clarify the interaction between the extracted light from the LED chip and the inside surface of the metal package. The radiant flux from the LED chip is calculated from the definition of thermal resistance and is implemented as an input parameter to predict the optical power and ray tracing. The simulated optical output power in the far-field mode is 145.3 mW, which is in good agreement with the value measured by an integrating sphere. It is shown that a large portion of the emitted light (37.12 mW) is lost by the interaction with the package surface before it reaches the far-field receiver. It is demonstrated that the angle of the first reflector cup is one of the critical design parameters in the metal package structure that we investigate. When the first reflector angle is optimized from 80° to 55° (with the second reflector angle of 50°), the optical power increases from its original value of 145.3-170.15 mW with an improved optical extraction efficiency from 75.63% to 88.57%. The ray tracing and illuminance raster charts also confirm that the improved optical performance of the metal package mainly stems from the optimization of the reflector angles.

대면적 엣지형 LED 디스플레이의 광특성 향상을 위한 도광판설계

Through the rapid development of LED TV, a large-area edge-lit backlight unit is widely researched in the display industry for enhancing the optical efficiency, luminous uniformity and slim design. In addition, an edge-lit backlight unit includes many optical sheets such as reflection sheet, prism sheet and diffuser sheet. However the function can be replaced by the carefully patterned light guide plate (LGP). Micro patterns which located on the top and bottom surface of light guide plate was designed by ray-tracing optical simulation. The designed patterns improved the light characteristics of LGP, including optical extraction efficiency and intensity distribution uniformity. These results suggest that it is very effective method to design edge-lit LGP with micro-patterns on both sides for large-area display and to reduce the fabrication cost.

Efficient copper vapor laser using metal (Cu, Ag) chlorides in thermal insulation and performance with new prism resonator configurations

A copper vapor laser based on the use of copper chloride and silver chloride mixture embedded inside the laser head thermal insulation is successfully demonstrated. The use of external HCl generator cell containing zirconium chloride normally used for its kinetically enhanced mode of operation is completely eliminated. With this new configuration laser power of ~70 W was achieved from a wide aperture ~47-50 mm bore discharge tube with input power of ~5 kW and overall high efficiency of ~1.4% without external supply of HCl vapors to the laser head. In a typical operational cycle the laser initially operates as low temperature CuCl laser with startup time of few minutes and output power of ~10 W during low tube temperature range of ~300-500 °C. Thereafter, the laser transforms itself into efficient kinetically enhanced copper vapor laser (CVL) at high temperature range of ~1200-1600 °C with maximum laser output power of ~70 W. This dual mode of operation observed in a single CVL system is unique and has not been reported so far in any high temperature copper vapor laser. New resonator configurations, namely, the prism resonator in stable and unstable form are successfully demonstrated for the first time in a copper vapor laser to achieve low divergence beam with dramatic increase in misalignment tolerance to ~25 mrad, which is an improvement of about ~50 times compared to standard CVLs with conventional spherical or plane-plane resonators. With these new resonator configurations the CVL functions almost as an "alignment free laser" system with significantly reduced beam divergence of ~0.2 mrad and high optical extraction efficiency of ~70%-80%.

Improvement of Optical Extraction Efficiency of Organic Light-Emitting Diode Using a Large Area Nano-Pattern

In order to improve optical extraction efficiencies, we propose a nano-patterned organic light-emitting diode (OLED) which doesn't rely on high precision lithography nor rigorous periodicity. The nano-pattern is fabricated by spin-coating PS (poly-styrene) spheres on the substrate and carrying out reactive ion etching and flattening using dielectric material. The verification of the optical properties of the nano patterns was carried out by fabricating OLED-like structures using quantum dot and distributed Bragg reflector (DBR). As a result, the nano-patterned structure showed a 28% increase in optical efficiency compared to the non-patterned sample. In addition, the use of a prism sheet on the backside of a glass substrate also showed a 23% increase in optical efficiency by disturbing the total internal reflection between glass and air. In this way, the large area nano-patterns can be fabricated and applied to increasing the optical extraction in OLED.

Optical anisotropy and light extraction efficiency of MBE grown GaN nanowires epilayers

The use of nanowires as active medium seems very promising for the development of high brightness LEDs. With a lower effective refractive index than bulk, semiconductor nanowire layers may lead to a high light extraction efficiency. We hereafter discuss the anisotropic properties of dense arrays of molecular beam epitaxy (MBE) grown gallium nitride (GaN) nanowires and the consequences on the optical design of nanowire based LEDs. In particular we show numerically that light extraction efficiency as high as 72% can be expected for GaN nanowires layer grown on a low cost Si substrate.

거친 표면구조를 이용한 400 nm 파장 GaN계 발광다이오드의 광 추출효율 개선

We fabricated the GaNLED emitting 400 nm wavelength and improved the optical extraction efficiency by making surface patterns on n-GaN layer and ITO layer above p-GaN. In addition, the light reflection metal under the n and p pad is made and the light reflection metal is installed on the backside of the chip. The light extraction efficiency is increased by 20 % with texturing n-GaN layer and 18% with texturing ITO layer at 20 mA. Compared to planar-surface LED, the light extraction efficiency for surface texturing both n-GaN and ITO is increased by 32% at 20mA.

Effects of Metallic Absorption and the Corrugated Layer on the Optical Extraction Efficiency of Organic Light-Emitting Diodes

The absorption of a metallic cathode in OLEDs is analyzed by using FDTD calculation. As the light propagates parallel to the layer, the intensity of E(z) polarization decreases rapidly. The intensity at 2.0 microm from the dipole is less than a quarter of that at 0.5 microm. The strong absorption by a cathode can be a critical factor when considering the increase of optical extraction by means of bending the optical layers. The calculation indicates that the corrugation of layers helps the guided light escape the guiding layer, but also increases the absorption into a metallic cathode. The final optical output power of the corrugated OLED can be smaller than that of the flat OLED. On the contrary, the corrugated structure with a non-absorptive cathode increases the optical extraction by nearly two times.

Analysis of lasing in chemical oxygen-iodine lasers with unstable resonators using a geometric-optics model

A simple geometric-optics model is developed that describes the power extraction in chemical oxygen-iodine lasers (COILs) with unstable resonators. The positive and negative branch unstable resonators with cylindrical mirrors that were recently used in COILs are studied theoretically. The optical extraction efficiency, spatial distributions of the intracavity radiation intensity in the flow direction, and the intensity in the far field are calculated for both kinds of resonator as a function of both the resonator and the COIL parameters. The optimal resonator magnifications that correspond to the maximum intensity in the far field are found.