Prospects and challenges of mini-LED and micro-LED displays

Wu T., Sher C., Lin Y., Lee C., Liang S., Lu Y., Huang Chen S., Guo W., Kuo H., Chen Z.

Displays based on inorganic light-emitting diodes (LED) are considered as the most promising one among the display technologies for the next-generation. The chip for LED display bears similar features to those currently in use for general lighting, but it size is shrunk to below 200 microns. Thus, the advantages of high efficiency and long life span of conventional LED chips are inherited by miniaturized ones. As the size gets smaller, the resolution enhances, but at the expense of elevating the complexity of fabrication. In this review, we introduce two sorts of inorganic LED displays, namely relatively large and small varieties. The mini-LEDs with chip sizes ranging from 100 to 200 μm have already been commercialized for backlight sources in consumer electronics applications. The realized local diming can greatly improve the contrast ratio at relatively low energy consumptions. The micro-LEDs with chip size less than 100 μm, still remain in the laboratory. The full-color solution, one of the key technologies along with its three main components, red, green, and blue chips, as well color conversion, and optical lens synthesis, are introduced in detail. Moreover, this review provides an account for contemporary technologies as well as a clear view of inorganic and miniaturized LED displays for the display community.

Wong M.S., Hwang D., Alhassan A.I., Lee C., Ley R., Nakamura S., DenBaars S.P.

Optoelectronic effects of sidewall passivation on micro-sized light-emitting diodes (µLEDs) using atomic-layer deposition (ALD) were investigated. Moreover, significant enhancements of the optical and electrical effects by using ALD were compared with conventional sidewall passivation method, namely plasma-enhanced chemical vapor deposition (PECVD). ALD yielded uniform light emission and the lowest amount of leakage current for all µLED sizes. The importance of sidewall passivation was also demonstrated by comparing leakage current and external quantum efficiency (EQE). The peak EQEs of 20 × 20 µm2 µLEDs with ALD sidewall passivation and without sidewall passivation were 33% and 24%, respectively. The results from ALD sidewall passivation revealed that the size-dependent influences on peak EQE can be minimized by proper sidewall treatment.

Sadeghi S., Ganesh Kumar B., Melikov R., Mohammadi Aria M., Bahmani Jalali H., Nizamoglu S.

Colloidal quantum dots (QDs) have attracted significant attention in the last three decades due to high quantum yield (QY) and tunable electronic properties via quantum confinement effect and material composition. However, their utilization for efficient solid-state lighting sources has remained a challenge due to the decrease of QY from the synthesis batch in the liquid state to the host matrix in the solid state, which is also known as the host material effect. Here, we suppress the host material effect by simple liquid-state integration in light-emitting diodes (LEDs) that lead to a luminous efficiency of 64 lm/W for red, green, blue (RGB)-based and 105 lm/W for green, blue (GB)-based white light generation. For that, we maximized the QY of red- and green-emitting QDs by optimizing synthesis parameters and integrated efficient QDs with QY up to 84% on blue LED dies in liquid form at appropriate injection amounts for high-efficiency white lighting. Liquid-state integration showed two-fold and six-fold enhancement of efficiency in comparison with incorporation of QDs in polydimethylsiloxane film and close-packed formation, respectively. Our theoretical calculations predicted that the luminous efficiency of liquid QD-LEDs can reach over 200 lm/W. Therefore, this study paves the way toward ultra-high-efficiency QD-based lighting.

Tan G., Huang Y., Li M., Lee S., Wu S.

We analyze the performance of high dynamic range liquid crystal displays (LCDs) using a two-dimensional local dimming mini-LED backlight. The halo effect of such a HDR display system is investigated by both numerical simulation and human visual perception experiment. The halo effect is mainly governed by two factors: intrinsic LCD contrast ratio (CR) and dimming zone number. Based on our results, to suppress the halo effect to indistinguishable level, a LCD with CR≈5000:1 requires about 200 local dimming zones, while for a LCD with CR≈2000:1 the required dimming zone number is over 3000. Our model provides useful guidelines to optimize the mini-LED backlit LCDs for achieving dynamic contrast ratio comparable to organic LED displays.

Beckers A., Fahle D., Mauder C., Kruecken T., Boyd A.R., Heuken M.

Deng Z., Zheng B., Zheng J., Wu L., Yang W., Lin Z., Shen P., Li J.

Paranjpe A., Montgomery J., Lee S.M., Morath C.

Woodgate G.J., Harrold J.

Chen G., Wei B., Lee C., Lee H.

In this letter, monolithic red, green, and blue (RGB) micro light-emitting diodes (LEDs) were fabricated using gallium nitride based blue micro LEDs and quantum dots (QDs). Red and green QDs were sprayed onto individual region surrounded by patterned black matrix photoresist on the blue micro LEDs to form color conversion layers. Owing to its light-blocking capability, the patterned black matrix photoresist improved the contrast ratio of the micro LEDs from 11 to 22. To enhance the color conversion efficiency and the light output intensity, a hybrid Bragg reflector (HBR) was deposited on the bottom side of the monolithic RGB micro LEDs, thus reflecting the RGB light emitted to the substrate. To further improve the color purity of the red and green light, a distributed Bragg reflector (DBR) with high reflection for the blue light was deposited on the top side of the QDs/micro LEDs. The red and green light output intensities of the micro LEDs with HBR and DBR were enhanced by about 27%.

Chen H., Tan G., Wu S.

We systematically analyze the ambient contrast ratio (ACR) of liquid crystal displays (LCDs) and organic light-emitting diode (OLED) displays for smartphones, TVs, and public displays. The influencing factors such as display brightness, ambient light illuminance, and surface reflection are investigated in detail. At low ambient light conditions, high static contrast ratio plays a key role for ACR. As the ambient light increases, high brightness gradually takes over. These quantitative results set important guidelines for future display optimization. Meanwhile, to improve an OLED’s ACR at large oblique angles, we propose a new broadband and wide-view circular polarizer consisting of one linear polarizer and two biaxial films. Good performance is realized.

Chen H., Lee J., Lin B., Chen S., Wu S.

Recently, ‘Liquid crystal display (LCD) vs. organic light-emitting diode (OLED) display: who wins?’ has become a topic of heated debate. In this review, we perform a systematic and comparative study of these two flat panel display technologies. First, we review recent advances in LCDs and OLEDs, including material development, device configuration and system integration. Next we analyze and compare their performances by six key display metrics: response time, contrast ratio, color gamut, lifetime, power efficiency, and panel flexibility. In this section, we focus on two key parameters: motion picture response time (MPRT) and ambient contrast ratio (ACR), which dramatically affect image quality in practical application scenarios. MPRT determines the image blur of a moving picture, and ACR governs the perceived image contrast under ambient lighting conditions. It is intriguing that LCD can achieve comparable or even slightly better MPRT and ACR than OLED, although its response time and contrast ratio are generally perceived to be much inferior to those of OLED. Finally, three future trends are highlighted, including high dynamic range, virtual reality/augmented reality and smart displays with versatile functions. The two leading flat-panel display technologies—liquid crystal displays and organic light-emitting diode displays—have been compared. Liquid crystal displays (LCDs) currently have the upper hand, but organic light-emitting diode (OLED) technology is rapidly catching up. Shin-Tson Wu of the University of Central Florida and colleagues have documented recent material and design advances in these two technologies and analyzed display performance with respect to six key metrics: response time, contrast ratio, color gamut, lifetime, power efficiency, and panel flexibility. They concluded that LCDs are superior in terms of cost, lifetime and brightness, whereas OLED displays offer better black states, flexibility, and faster response times. The technologies have similar ambient contrast ratio, image motion blur, color gamut, viewing angle and power consumption. Emerging applications include virtual and augmented reality wearable displays as well as displays with high dynamic ranges.

Shih Y., Shi F.G.

The feasibility of quantum dot (QD)-based silicone composites as suitable candidates, in terms of down conversion efficiency, for the replacement or the enhancement of color filters in high-brightness liquid crystal displays is investigated for the first time. It is demonstrated that the replacement of color filters (CFs) with a full color conversion QD composites can lead to an 86.16% enhancement in energy efficiency over the conventional LCD with phosphor-converted white light-emitting diodes and RGB CFs as backlighting. A potential second-type technology utilizing QD-on-CF for color generation is also investigated, and it is found that the energy efficiency enhancement can reach 86.86%. In addition to QDs, the state-of-art red and green phosphors are also investigated for the purpose of enhancement of color filters in LCD displays in terms of down conversion efficiency, and the result shows a 105.78% enhancement in overall down conversion efficiency. The energy efficiency of QD-based color conversion composites can be further improved by optimization of QD dispersion in the resin, which makes them more suitable replacement for color filters in LCDs due to some intrinsic limitations in phosphor materials.

Lin H., Sher C., Hsieh D., Chen X., Chen H.P., Chen T., Lau K., Chen C., Lin C., Kuo H.

In this study, a full-color emission red–green–blue (RGB) quantum-dot (QD)-based micro-light-emitting-diode (micro-LED) array with the reduced optical cross-talk effect by a photoresist mold has been demonstrated. The UV micro-LED array is used as an efficient excitation source for the QDs. The aerosol jet technique provides a narrow linewidth on the micrometer scale for a precise jet of QDs on the micro-LEDs. To reduce the optical cross-talk effect, a simple lithography method and photoresist are used to fabricate the mold, which consists of a window for QD jetting and a blocking wall for cross-talk reduction. The cross-talk effect of the well-confined QDs in the window is confirmed by a fluorescence microscope, which shows clear separation between QD pixels. A distributed Bragg reflector is covered on the micro-LED array and the QDs’ jetted mold to further increase the reuse of UV light. The enhanced light emission of the QDs is 5%, 32%, and 23% for blue, green, and red QDs, respectively.

Olivier F., Daami A., Licitra C., Templier F.

GaN-based micro light-emitting diode (μLED) arrays are very promising devices for display applications. In these arrays, each μLED works as a single pixel of a whole image. The electro-optical performance of these μLEDs is an important subject to study. Here, we investigate the influence of LED size on the radiative and non-radiative recombination. The standard ABC model has been widely used to describe the efficiency of GaN based LEDs. Using this model, we extract A, B, and C coefficients for various LED sizes, showing how the competition between radiative and non-radiative recombination processes varies with the LED geometry. Time-resolved photoluminescence allows us to determine coefficient B, related to radiative recombination. Through current-voltage-luminance characterizations, we determine parameters A and C related to Shockley-Read-Hall and Auger recombination. We find that coefficient A is strongly dependent on LED size, indicating a drastic effect of sidewall defects on the performance of LEDs. On the other hand, coefficient C is independent of LED size. This latter result demonstrates that efficiency droop does not depend on LED size.

He J., Chen H., Chen H., Wang Y., Wu S., Dong Y.

We propose to use a hybrid downconverter system comprising low-cost green perovskite-polymer composite films for liquid crystal display (LCD) backlight unit (BLU) to realize wide color gamut. Recently, ultrastable, highly luminescent CH3NH3PbBr3 (MAPbBr3) organic-inorganic perovskite-polymer composite films have been developed. These films exhibit outstanding color quality with a full-width-at-half-maximum (FWHM) of only 18 nm and a peak wavelength of 530 nm, which makes them promising candidates as green downconverters. Two configurations to hybridize these green films with state-of-the-art red emitting downconverters, including CdSe-based quantum dots (QDs) and narrow peak phosphors, are proposed. Color and efficiency analyses indicate that the hybridization of green perovskite-polymer films with red K2SiF6:Mn4+ (KSF) phosphor could lead to wide color gamut coverage of nearly 90% Rec. 2020 and high total light efficiency (TLE) of around 20 lm/W while maintaining low cost.

Xi K., Chen Z., Wang P., An F., Zhou W., Li X., Wu T., Qin F., Peng X.

Furusawa Y., Cai W., Cheong H., Honda Y., Amano H.

An all‐GaN‐based monolithic active‐matrix micro‐LED system that integrates metal‐insulator‐semiconductor high‐electron‐mobility transistors (MIS HEMTs) with light‐emitting diodes (LEDs) is demonstrated. The proposed structure employs direct electron injection from the 2D electron gas (2DEG) in a HEMT, serving as the n‐type layer, into the quantum wells of the LEDs. A 2‐HEMT‐1‐LED pixel configuration is fabricated with one epitaxial growth, enabling the precise control of LED light output through a combination of select and drive HEMTs. The fabricated pixel achieved a maximum optical output density of 0.5 Wcm−2. A 2 × 2 pixel matrix is constructed with row and column lines connected via select HEMTs, demonstrating the capability of the 2‐HEMT‐1‐LED pixel configuration for individual LED control.

Kulkarni M.A., Ryu H., Park S., Kim T.K., Min S., Shin S.H., Thaalbi H., Tariq F., Lee S.H., Jang H.W., Kwak J.S., Shin D., Kim P.G., Ryu S.

AbstractIn this work, an innovative method is introduced for fabricating large‐area micro‐LEDs (µLEDs) specifically for green and blue emission through a combination of GaN thin film and GaN nanorods (NRs) on the orientation‐controlled template designed for its integration into display systems. Herein, an advanced lithography technique is employed to construct the template, specifically designed with the pattern to feature the regions of GaN thin film and GaN NRs for facilitating the wavelength control by utilizing indium incorporation efficiency of the different crystallographic orientations of GaN. Through metal–organic chemical vapor deposition (MOCVD) grown active region of InGaN/GaN heterostructure on the orientation‐controlled template, tunable emission in the visible region is realized and confirmed through comprehensive optical and metrological characterization. Furthermore, the feasibility of fabricating the green and blue µLED pixels on the orientation‐controlled template is affirmed. Lastly, the orientation‐controlled template of green and blue µLED pixels are successfully integrated to demonstrate a prototype full color display module. Overall, the approach of combining advanced lithographic techniques to prepare the orientation‐controlled template and MOCVD growth of the InGaN/GaN heterostructure showcased a path for fabricating full‐color pixel arrays for display technologies.

Wu X., Zhu X., Wang S., Tang X., Lang T., Belyaev V., Abduev A., Kazak A., Lin C., Yan Q., Sun J.

Micro Light Emitting Diode (Micro-LED) technology, characterized by exceptional brightness, low power consumption, fast response, and long lifespan, holds significant potential for next-generation displays, yet its commercialization hinges on resolving challenges in high-density interconnect fabrication, particularly micrometer-scale bump formation. Traditional fabrication approaches such as evaporation enable precise bump control but face scalability and cost limitations, while electroplating offers lower costs and higher throughput but suffers from substrate conductivity requirements and uneven current density distributions that compromise bump-height uniformity. Emerging alternatives include electroless plating, which achieves uniform metal deposition on non-conductive substrates through autocatalytic reactions albeit with slower deposition rates; ball mounting and dip soldering, which streamline processes via automated solder jetting or alloy immersion but struggle with bump miniaturization and low yield; and photosensitive conductive polymers that simplify fabrication via photolithography-patterned composites but lack validated long-term stability. Persistent challenges in achieving micrometer-scale uniformity, thermomechanical stability, and environmental compatibility underscore the need for integrated hybrid processes, eco-friendly manufacturing protocols, and novel material innovations to enable ultra-high-resolution and flexible Micro-LED implementations. This review systematically compares conventional and emerging methodologies, identifies critical technological bottlenecks, and proposes strategic guidelines for industrial-scale production of high-density Micro-LED displays.

Xia Z., Wang Y., Zhang Y., Zhang Y.

He N., Zhang Y., Hu C., Liu J., Shen Z., Wang W., Weng Y., wang B.

The continuous upgrading of partition backlight technology has brought new competitive advantages to LCD, making it comparable to OLED in display performance, such as high contrast. Because the LCD does not have an ideal turn-off characteristic, there will be a halo phenomenon when local dimming is turned on. Due to the low luminance value of the halo and glare interference of the luminance meter, the actual light distribution is difficult to measure, so previous studies on halo effect characterization were mostly based on simulation or qualitative analysis. This paper first gives a method to accurately measure the halo distribution by using a mask to cover the center luminous region to avoid glare interference generated by the normal luminance meter. Based on the measured halo distribution and its correlation with the subjective assessment on halo visibility, a halo visibility estimation model was established, which quantifies the impact of display luminance, panel transmittance, and backlight unit (BLU) size. The predicted value of this model was highly correlated with the subjective experiment data, with a good fit of R2=0.92. With this model, the halo visibility with any combination of panel sizes, initial contrast ratios, and BLU numbers can be easily calculated.

Huang C., Wang B., Tang H., Liao Z., Lin Z., Wang R.

Tang X., Huang X., Lang T., Xie Y., Lin X., Li Y., Zhou Y., Yan Q., Zhang K., Lin C., Sun J.

Wang F., Zhang S., Wang J., He S., Lu X., Wang J., Zhong S., Zeng D., Zhou L., Chen L., OuYang X.

Converting radiation into optical signals is a fundamental method for nuclear radiation detection. However, traditional scintillators encounter a trade-off between efficiency and response speed. This research proposes a radiation-photon converter constructed from multi-quantum-well (MQW) structures integrated into radiation-sensitive materials, providing a unique solution to this challenge. The prototype was fabricated using a homogeneous epitaxial layer of GaN on a semi-insulating substrate. The radiation-photon conversion process was facilitated by directing charge carriers generated from radiation energy deposited in the semi-insulating substrate to the MQW layer via an external electric field. The converter exhibited a sensitive and rapid response to x-ray irradiation, enabling modulation of the excited photon wavelength through the MQW layers. Luminescence spectrum tests demonstrated that the net luminescence intensity increased with rising device voltage. Imaging experiments revealed that the grayscale values of device photographs, under the combined influence of electric fields and x rays, correlated with the trend in net current variation. These findings confirmed the effective conversion of radiation into optical signals through the modulation mechanism of the electric field, highlighting significant implications for the development of advanced radiation detection methodologies.

Kumar M., Kumar P.

White light emitting materials play a pivotal role in various technological applications, spanning from lighting and displays to advanced optical technologies. This abstract provides a concise overview of the diverse applications and recent advancements in the field of white light emitting materials.

Lei J., Zhu H., Huang X., Lin J., Zheng Y., Lu Y., Chen Z., Guo W.

Miniaturized-light-emitting diode (mini-LED) backlights have emerged as the state-of-the-art technology for liquid crystal display (LCD), facilitating the improvement in a high dynamic range (HDR) and power saving. The local dimming technology divides the backlight into several dimming zones. Employing mini-LEDs, whose size ranges from 100 to 200 μm, as the light sources can enlarge the number of zones in the local dimming backlight, fulfilling the requirement for HDR. However, the halo effect still acts as one of the primary technological bottlenecks for mini-LED backlights. In this review, packaging technology of LEDs, color conversion, and the driving scheme of mini-LED backlights have been discussed. The strategies to reduce optical crosstalk in adjacent areas by various improved optical structures or to suppress the halo effect of LCDs by mini-LED backlights are summarized. The development trends of mini-LED backlights are also discussed.

Zhanghu M., Liu Y., Hyun B., Li Y., Liu Z.

Ren T., Ruan Y., Yan L., Shan X., Shen D., Tan C., Cui X., Tian P.

Abstract Red-green-blue (RGB) micro light-emitting diodes (micro-LEDs) without distributed Bragg reflector (DBR), with air-separating DBR, and with integrated DBR, were demonstrated. The effect of the DBRs as reflectors on the external quantum efficiency (EQE) and electroluminescence (EL) spectra enhancement of RGB micro-LEDs was systematically investigated for realizing higher-performance micro-LEDs for display applications. At 5 A/cm2, the EQEs of the RGB micro-LEDs with integrated DBR were improved by 38%, 33%, and 32%, respectively, with comparison to the RGB DBR free micro-LEDs. Further, the full width at half maximum (FWHM) of the red micro-LEDs was reduced by 4.3 nm at 50 A/cm2 with the integrated DBR due to the higher enhancement of the central wavelength spectrum. The green and blue micro-LEDs with integrated DBR had higher EQE and the red micro-LEDs with integrated DBR had narrower FWHM compared to those with air-separating DBR. However, the peak wavelength of the RGB micro-LEDs with integrated DBR shifted, resulting in a lower color gamut in CIE 1931. The above work provides guidance for future full-color micro-display applications based on RGB InGaN micro-LED technology.

Son B., Kim H., Lee Y., Bommireddy P.R., Park S.

Tsai T., Lin S.