Highly Reflective Thin-Film Optimization for Full-Angle Micro-LEDs

Effect of a mesa with different sidewall angles on the spatially resolved optical properties and light extraction efficiency for GaN-based blue and green micro-LEDs

In this paper, we studied the sidewall conditions of 28 × 52 µm2 InGaN-based blue and green micro-LEDs with different sidewall angles and their effects on external quantum efficiency (EQE). Our findings indicate that steeper sidewall mesas can reduce non-radiative recombination and leakage current, which is beneficial for achieving high internal quantum efficiency (IQE). However, as the sidewall angle increases, the light output from the micro-LED tends to concentrate in the internal region, leading to a decrease in light extraction efficiency (LEE). Using microscopic hyperspectral imaging, we observed distinct chromaticity characteristic distributions in the internal area of mesas with different sidewall angles, compared to the entire micro-LEDs. Mesas with gentler sidewalls exhibited lower chromaticity stability. For both blue and green micro-LEDs, the optimal sidewall tilt angle was found to be 48°, yielding the highest EQE. This result reflects a trade-off between LEE and IQE. Notably, the improvement in IQE for green micro-LEDs was not as significant as that for blue micro-LEDs, likely due to the higher indium content in green InGaN micro-LEDs, which results in deeper localized potential wells and a higher dislocation density. This work demonstrates the variation of IQE and LEE for blue and green micro-LEDs fabricated on mesas with different sidewall angles, aimed at achieving the highest EQE.

Zero-optical-distance mini-LED backlight with light-guiding microstructure lens for extra-thin, large-area notebook LCDs

Mini-light-emitting diode (Mini-LED) backlight units (BLUs) in combination with high dynamic range technology can reduce energy and ensure high contrast and luminance. However, the number of LEDs used in mini-LED BLUs is considerably larger than the number of partitions in local dimming, resulting in low cost effectiveness. We proposed a design combining edge-light mini-LEDs and light-guiding microstructure lenses to reduce the number of light sources required in displays considerably. A 16-inch prototype was produced for experiments. The length, width, and thickness of the liquid crystal display module were 351.87, 225.75, and 1.709 mm, respectively. For edge-light mini-LEDs with a pitch of 8.6 mm, the average luminance was 18,836 nits for an input power of 22.5 watts, the uniformity was 85%, the uniformity merit function was 10.13, and the contrast ratio was 60,000:1. Thus, a zero-optical-distance (ZOD) mini-LED backlight for extra-thin, large-area notebook LCDs was produced.

Fabricating and investigating a beveled mesa with a specific inclination angle to improve electrical and optical performances for GaN-based micro-light-emitting diodes

In this Letter, beveled mesas for 30 × 30 µm2 GaN-based micro-light-emitting diodes (µLEDs) with different inclination angles are designed, fabricated, and measured. We find that µLED with a mesa inclination angle of 28° has the lowest internal quantum efficiency (IQE) and the highest injection current density at which the peak IQE is obtained. This is due to the increased quantum confined Stark effect (QCSE) at the mesa edge. The increased QCSE results from the strong electric field coupling effect. Instead of radiative recombination, more nonradiative recombination and leakage current will be generated in the sidewall regions. Besides, the smallest angle (28°) also produces the lowest light extraction efficiency (LEE), which arises from the optical loss caused by the sidewall reflection at the beveled surface sides. Therefore, the inclination angle for the beveled mesa has to be increased to 52° and 61° by using Ni and SiO2 as hard masks, respectively. Experimental and numerical results show that the external quantum efficiency (EQE) and the optical power can be enhanced for the fabricated devices. Meanwhile, the reduced surface recombination rate also decreases the leakage current.

Multiresonant TADF materials: triggering the reverse intersystem crossing to alleviate the efficiency roll-off in OLEDs

The hunt for narrow-band emissive pure organic molecules capable of harvesting both singlet and triplet excitons for light emission has garnered enormous attention to promote the advancement of organic light-emitting diodes (OLEDs). Over the past decade, organic thermally activated delayed fluorescence (TADF) materials based on donor (D)/acceptor (A) combinations have been researched for OLEDs in wide color gamut (RGB) regions. However, due to the strong intramolecular charge-transfer (CT) state, they exhibit broad emission with full-width-at-half maximum (FWHM) > 70 nm, which deviates from being detrimental to achieving high color purity for future high-end display electronics such as high-definition TVs and ultra-high-definition TVs (UHDTVs). Recently, the new development in the sub-class of TADF emitters called multi-resonant TADF (MR-TADF) emitters based on boron/nitrogen atoms has attracted much interest in ultra-high definition OLEDs. Consequently, MR-TADF emitters are appeal to their potentiality as promising candidates in fabricating the high-efficient OLEDs due to their numerous advantages such as high photoluminescence quantum yield (PLQY), unprecedented color purity, and narrow bandwidth (FWHM ≤ 40 nm). Until now many MR-TADF materials have been developed for ultra-gamut regions with different design concepts. However, most MR-TADF-OLEDs showed ruthless external quantum efficiency (EQE) roll-off characteristics at high brightness. Such EQE roll-off characteristics were derived mainly from the low reverse intersystem crossing (k_RISC) rate values. This feature article primarily focuses on the design strategies to improve k_RISC for MR-TADF materials with some supportive strategies including extending charge delocalization, heavy atom introduction, multi-donor/acceptor utilization, and a hyperfluorescence system approach. Furthermore, the outlook and prospects for future developments in MR-TADF skeletons are described.

Noninvasive direct bilirubin detection by spectral analysis of color images using a Mini-LED light source

Urine test paper is a standard, noninvasive detection method for direct bilirubin, but this method can only achieve qualitative analysis and cannot achieve quantitative analysis. This study used Mini-LEDs as the light source, and direct bilirubin was oxidized to biliverdin by an enzymatic method with ferric chloride (FeCl₃) for labeling. Images were captured with a smartphone and evaluated for red (R), green (G), and blue (B) colors to analyze the linear relationship between the spectral change of the test paper image and the direct bilirubin concentration. This method achieved noninvasive detection of bilirubin. The experimental results demonstrated that Mini-LEDs can be used as the light source to analyze the grayscale value of the image RGB. For the direct bilirubin concentration range of 0.1-2 mg/dL, the green channel had the highest coefficient of determination coefficient (R²) of 0.9313 and a limit of detection of 0.56 mg/dL. With this method, direct bilirubin concentrations higher than 1.86 mg/dL can be quantitatively analyzed with the advantage of rapid and noninvasive detection.

Application of Mini-LEDs with Microlens Arrays and Quantum Dot Film as Extra-Thin, Large-Area, and High-Luminance Backlight

The demand for extra-thin, large-area, and high-luminance flat-panel displays continues to grow, especially for portable displays such as gaming laptops and automotive displays. In this paper, we propose a design that includes a light guide layer with a microstructure above the mini-light-emitting diode light board. The light control microstructure of concave parabel-surface microlens arrays on a light-emitting surface increases the likelihood of total internal reflection occurring and improved the uniformity merit function. We used a 17 in prototype with quantum-dot and optical films to conduct our experiments, which revealed that the thickness of the module was only 1.98 mm. When the input power was 28.34 watts, the uniformity, average luminance, and CIE 1931 color space NTSC of the prototype reached 85%, 17,574 cd/m², and 105.37%, respectively. This module provided a flat light source that was extra thin and had high luminance and uniformity.