applied sciences

Review Mini-LED and Micro-LED: Promising Candidates for the Next Generation Display Technology

Tingzhu Wu 1 ID , Chin-Wei Sher 2,3, Yue Lin 1, Chun-Fu Lee 2, Shijie Liang 1, Yijun Lu 1, Sung-Wen Huang Chen 2, Weijie Guo 1 ID , Hao-Chung Kuo 2,4,* and Zhong Chen 1,*

1 Department of Electronic Science, Fujian Engineering Research Center for Solid-State Lighting, Xiamen University, Xiamen 361005, China; [email protected] (T.W.); [email protected] (Y.L.); [email protected] (S.L.); [email protected] (Y.L.); [email protected] (W.G.) 2 Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan; [email protected] (C.-W.S.); [email protected] (C.-F.L.); [email protected] (S.-W.H.C.) 3 HKUST Fok Ying Tung Research Institute, Nansha District, Guangzhou 511458, China 4 Department of Electronic Engineering, Xiamen University, Xiamen 361005, China * Correspondence: [email protected] (H.-C.K.); [email protected] (Z.C.); Tel.: +86-592-2181712 (H.-C.K. & Z.C.)  Received: 31 July 2018; Accepted: 30 August 2018; Published: 5 September 2018 

Abstract: 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.

Keywords: mini-LED; micro-LED; full-color display; quantum dot

1. Introduction The traditional display technology features a cathode ray tube (CRT) based on the principle of a steered beam excitation of a fluorescent screen [1]. The structure of CRT is basically a vacuum tube with one or more built-in electron guns, which produce the to be accelerated and steered. The steered electron beam excites one or more of the on the screen designed to emit red, green, and blue (RGB), primary colors. By appropriate scanning, an image is produced on the pixelated screen. Since the invention of the first color CRT (TV) in 1950, the CRT TV has dominated the display market for many decades because of its outstanding characteristics, such as excellent visual depth of field and high response rate. This dominance of CRT displays remained for a remarkably long time until the year of 2000, when two new display technologies, liquid-crystal display (LCD) and display panel, were demonstrated [2–4]. Because of portability and power efficiency

Appl. Sci. 2018, 8, 1557; doi:10.3390/app8091557 www.mdpi.com/journal/applsci Appl. Sci. 2018, 8, 1557 2 of 17 features, they are very popular with consumers. Later, owing to the continuous improvement in reducing the cost and performance improvements in the LCD technology, the shortly thereafter became uncompetitive. However, because LCD displays have major disadvantages, such as slow response time, poor conversion efficiency and low color saturation, the technology had been repeatedly criticized by consumers [5]. Consequently, the LCD manufacturers have taken steps to improve LCD displays, such as replacing the common liquid crystal materials with high response materials, using relatively larger conversion efficiency backlight modules, and utilizing high color saturation fluorescent materials. As a result, some high performance LCDs possess extremely short response times and thus are used in several virtual reality (VR) devices [6,7]. In recent years, new display technologies have become more mature, such as organic light-emitting diode (OLED) display and light-emitting diode (LED) displays [8]. The OLED display technology was developed in the 1990s. Compared with LCD displays, OLED displays have advantages, among which are self-luminous, wide viewing angle, high contrast, power saving, fast response, etc. [9,10]. However, due to limitations in material science and mass production capabilities, are not as widely used in consumer electronics market as LCDs [11]. The LED- based displays are mainly applied to large outdoor screens with the advantages of power saving, high color saturation and high brightness [12]. If LEDs are used as pixels of the display, the size of LEDs would need to be reduced according to the desired resolution. Because a growing number of manufacturers regard the LED display as the next- generation display technology, the onset of mini-LED and micro-LED has been triggered [13]. The comparison between mini-LED and micro-LED is shown in Table1. The size of mini-LEDs is about 100~200 µm, which is between the size of conventional LEDs (>200 µm) and micro-LEDs (<100 µm).

Table 1. Comparison between mini-LED and micro-LED.

Mini-LED Micro-LED Size (µm) 100~200 <100 Purpose Backlight for LCD Self-emitting display Features High dynamic range, power saving, thin High contrast, high efficiency, high resolution, high response time Yield >80% Hard to estimate Application LCD backlight—From small to large LCD panel Micro-projection display, display from small to large size

According to the Research and Markets, a market research institution, the global micro-LED display market is predicted to soar from USD 0.6 billion in 2019 to USD 20.5 billion in 2025, with a compound annual growth rate of about 80% [14]. The main reason for the market outbreak is the sharp increase in demand for brighter and more energy-efficient display panels, needed for surging devices such as smart watches, mobile phones, TVs, laptops, augmented reality (AR) and VR. According to Yole’s optimistic estimate, the market of micro-LED display will reach 330 million units by 2025 (Figure1)[ 15]. Although the prospects of market are highly optimistic at present, micro-LED displays still face technological challenges, especially in the cases for which some key technologies and process equipment have not yet been made sufficiently developed. Therefore, the relatively mature mini-LED is expected to be the first commercialized variety while the micro-LED display technology is still in its nascent state. Appl. Sci. 2018, 8, 1557 3 of 17 Appl. Sci. 2018, 8, x FOR PEER REVIEW 3 of 17

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Figure 1. Forecast about the development of micro-LED displays. Figure 1. Forecast about the development of micro-LED displays.

2. Mini-LED 2. Mini-LED Figure 1. Forecast about the development of micro-LED displays. High dynamic range (HDR) is one of the important features for next generation displays [16]. 2.ToHigh Mini-LED achieve dynamic HDR rangewith contrast (HDR) ratio is one (CR) of higher the important than 100000:1, features high for peak next brightness generation and displays excellent [ 16]. To achievedark state HDR of the with system ratio are (CR)simultaneously higher than required 100000:1, [17]. highAlthough peak the brightness best requirement and excellent for High dynamic range (HDR) is one of the important features for next generation displays [16]. darkHDR state is of pixel the displaylevel dimming, system which are simultaneously is described as required micro-LED [17 ].technology, Although there the bestare still requirement some To achieve HDR with contrast ratio (CR) higher than 100000:1, high peak brightness and excellent for HDRtechnological is pixel bottlenecks level dimming, that make which micro-LED is described harder as to micro-LEDrealize quick technology,commercialization. there areTherefore, still some dark state of the display system are simultaneously required [17]. Although the best requirement for technologicala compromising bottlenecks way to that realize make multi-zone micro-LED local harder dimming to realize for LCD quick is direct-lit commercialization. mini-LED backlight. Therefore, HDR is pixel level dimming, which is described as micro-LED technology, there are still some Mini-LED technology has much smaller size of LED, which means it can divide more dimming blocks a compromisingtechnological bottlenecks way to realize that make multi-zone micro-LED local hard dimminger to realize for LCDquick iscommercialization. direct-lit mini-LED Therefore, backlight. in a certain size LED backlight. Recently, LED manufacturers have turned to research and develop Mini-LEDa compromising technology way has to much realize smaller multi-zone size oflocal LED, dimming which for means LCD itis can direct-lit divide mini-LED more dimming backlight. blocks mini-LED. Most of existing processes and equipment for conventional LED can be used continuously in aMini-LED certain size technology LED backlight. has much Recently, smaller size LED of LED, manufacturers which means have it can turned divide tomore research dimming and blocks develop for fabrication of mini-LED. mini-LED.in a certain Most size of existingLED backlight. processes Recently, and equipment LED manufacturers for conventional have turned LED to can research be used and continuously develop Tan et al. discussed the system modeling and performance evaluation of LCDs with mini-LED for fabricationmini-LED. Most of mini-LED. of existing processes and equipment for conventional LED can be used continuously backlight [18]. First, a model of LCD system with a direct-lit mini-LED backlight is set up for for fabrication of mini-LED. simulationTan et al. discussed(Figure 2). The the systembacklight modeling unit consists and of performance square-shaped evaluation mini-LED ofarray. LCDs A diffuser with mini-LED plate Tan et al. discussed the system modeling and performance evaluation of LCDs with mini-LED backlightis utilized [18]. to First, widen a model both spatial of LCD and system angular with distributions, a direct-lit mini-LEDand a liquid backlight crystal (LC) is set panel up for is applied simulation backlight [18]. First, a model of LCD system with a direct-lit mini-LED backlight is set up for (Figureto control2). The the backlight output light. unit consistsThe parameters of square-shaped of the model, mini-LED such as p array., s, H1, Aand diffuser H2, are platebased is on utilized the simulation (Figure 2). The backlight unit consists of square-shaped mini-LED array. A diffuser plate to widendevice both configuration spatial and reported angular in distributions, Reference [19]. and To avalidate liquid crystalthe model, (LC) four panel patterns is applied are used to control to is utilized to widen both spatial and angular distributions, and a liquid crystal (LC) panel is applied the outputsimulate light. the dynamic The parameters CR of the model, of the and model, the si suchmulated as presults, s, H1 can, and agreeH2 ,with are the based measured on the data device to control the output light. The parameters of the model, such as p, s, H1, and H2, are based on the configurationfrom Reference reported [19] reasonably in Reference well. [19 ]. To validate the model, four patterns are used to simulate device configuration reported in Reference [19]. To validate the model, four patterns are used to the dynamic CR of the model, and the simulated results can agree with the measured data from simulate the dynamic CR of the model, and the simulated results can agree with the measured data Referencefrom Reference [19] reasonably [19] reasonably well. well.

Figure 2. Schematic diagram of the model for the LCD with a mini-LED backlight [18]. Figure reproduced with permission from Optical Society of America.

FigureFigureNext, 2. Schematic the2. Schematic proved diagram model diagram of is the utilizedof model the model forto find the for LCDout the the withLCD relationship a with mini-LED a mini-LED backlightbetween backlight [the18]. device Figure [18]. reproducedstructureFigure to thewith finalreproduced permission HDR display with from permission Optical performance, Society from Optical ofespecially America. Society the of halo America. effect (Figure 3). Final HDR performance of displayed images are calculated via independently adjusting two key parameters of the device Next, the proved model is utilized to find out the relationship between the device structure to structure, the local dimming zone number and the LCD contrast ratio. According to simulation theNext, final the HDR proved display model performance, is utilized especially to find the out halo the relationshipeffect (Figure between3). Final HDR the deviceperformance structure of to the finaldisplayed HDR images display are performance, calculated via especially independently the halo adjusting effect two (Figure key3 parameters). Final HDR of the performance device of displayedstructure, imagesthe local are dimming calculated zone via number independently and the LCD adjusting contrast two ratio. key According parameters to simulation of the device Appl. Sci. 2018, 8, 1557 4 of 17 structure,Appl. the Sci. local2018, 8 dimming, x FOR PEER zone REVIEW number and the LCD contrast ratio. According to simulation results,4 of 17 the dimming zone number mainly affects the halo area, while LCD contrast ratio influences the local Appl.results, Sci. 2018 the, 8, x dimming FOR PEER REVIEWzone number mainly affects the halo area, while LCD contrast ratio influences4 of 17 image distortion,the local image and distortion, more local and dimming more local zones dimming and higher zones and LC higher contrast LC ratio contrast can ratio reduce can the reduce halo effectresults, andthe improvehalo the dimmingeffect the anddisplay zone improve number performance. the displaymainly performance.affects the halo area, while LCD contrast ratio influences the local image distortion, and more local dimming zones and higher LC contrast ratio can reduce the halo effect and improve the display performance.

Figure 3. Displayed image simulation: (a) Mini-LED backlight modulation; (b) luminance distribution Figure 3. Displayed image simulation: (a) Mini-LED backlight modulation; (b) luminance distribution of the light incident on LC layer; and (c) displayed image after LCD modulation [18]. Figure of the light incident on LC layer; and (c) displayed image after LCD modulation [18]. Figure reproduced Figurereproduced 3. Displayed with image permission simulation: from Optical(a) Mini-LED Society backlight of America. modulation; (b) luminance distribution withof permission the light incident from Optical on LC Society layer; ofand America. (c) displayed image after LCD modulation [18]. Figure reproducedThen, a with subjective permission experiment from Optical is Societydesigned of America.and carried out to determine the human visual Then,perception a subjective limit of experimenthalo effect. The is designedLabPSNR, andan evaluation carried outmetric to used determine to quantify the humanthe difference visual Then, a subjective experiment is designed and carried out to determine the human visual perceptionbetween limit displayed of halo effect.image and The targetLabPSNR image,, an should evaluation be larger metric than used47.4 dB. to quantifyBased on thethis differencelimit, the perception limit of halo effect. The LabPSNR, an evaluation metric used to quantify the difference betweenrequirement displayed of image local dimming and target zone image, number should can be be proposed: larger than over 47.4200 local dB. Baseddimming on zones this limit, for high the betweenCR ≈ 5000:1displayed LCD image panels and and target more image,than 3000 should dimming be larger zones than for CR47.4 ≈ dB. 2000:1 Based LCDs on (Figurethis limit, 4). the requirementrequirement of localof local dimming dimming zone zone number number cancan bebe proposed: over over 200 200 local local dimming dimming zones zones for high for high CR ≈CR5000:1 ≈ 5000:1 LCD LCD panels panels and and more more thanthan 3000 dimming zones zones for for CR CR ≈ 2000:1≈ 2000:1 LCDs LCDs (Figure (Figure 4). 4).

Figure 4. Simulated LabPSNR for different HDR display systems with various local dimming zone

numbers and LC contrast ratios [18]. Figure reproduced with permission from Optical Society of Figure 4. Simulated LabPSNR for different HDR display systems with various local dimming zone Figure 4.America.Simulated LabPSNR for different HDR display systems with various local dimming zone numbers and LC contrast ratios [18]. Figure reproduced with permission from Optical Society of numbers and LC contrast ratios [18]. Figure reproduced with permission from Optical Society America.Although the above simulations and experiments are all based on the small-size of America. displays with viewing distance at 25 cm, the analysis and conclusion can also be applied to display devicesAlthough with the different above sizessimulations and resolutions and experiments via converting are all the based results on from the small-size spatial domain smartphone to angular displaysAlthoughdomain with (Figure the viewing above 5). In distance simulations summary, at 25 Tan cm, and et the al. experiments demonstranalysis andated are conclusion the all required based can on local also the dimming be small-size applied zone to smartphone displaynumber to displaysdevicesexhibit with with comparable viewing different distance sizes HDR and performance at resolutions 25 cm, the with via analysis convOLED,erting and the the conclusion results HDR performance from can spatial also becoulddomain applied not to be angular to achieved display devicesdomainby with conventional (Figure different 5). In sizessegmented summary, and resolutions LED Tan etbacklight. al. demonstr via convertingThe atedsimulation the the required resultsmodel local can from dimmingprovide spatial useful zone domain numberguidelines to angular to to domainexhibittheoretically (Figure comparable5). Inoptimize summary, HDR performancethe mini-LED Tan et al. with backlit demonstrated OLED, LCDs and for the theachieving HDR required performance excellent local dimmingHDR could display. not zone be achieved number to by conventional segmented LED backlight. The simulation model can provide useful guidelines to exhibit comparable HDR performance with OLED, and the HDR performance could not be achieved theoretically optimize the mini-LED backlit LCDs for achieving excellent HDR display. Appl. Sci. 2018, 8, 1557 5 of 17 by conventional segmented LED backlight. The simulation model can provide useful guidelines to theoreticallyAppl. Sci. 2018 optimize, 8, x FOR PEER the mini-LEDREVIEW backlit LCDs for achieving excellent HDR display. 5 of 17

Appl. Sci. 2018, 8, x FOR PEER REVIEW 5 of 17

Figure 5. Conceptual diagram of scaling up display size based on same angular size [18]. Figure Figure 5. Conceptual diagram of scaling up display size based on same angular size [18]. reproduced with permission from Optical Society of America. FigureFigure reproduced 5. Conceptual with permissiondiagram of fromscaling Optical up display Society size of based America. on same angular size [18]. Figure reproduced with permission from Optical Society of America. While mini-LED, an ideal backlight candidate for local dimming LCDs, is ready to be produced inWhile volumeWhile mini-LED, mini-LED,at present, an an idealmicro-LED ideal backlight backlight stillcandidate has candidate to be for processedfor local local dimmingdimming inside laboratories LCDs, is is ready ready and to toneeds be be produced produced further in volumepreparationin volume at present, at before present, micro-LED entering micro-LED stillthe hasmass still to production. be has processed to be processedMany inside companies laboratories inside laboratorieshave and joined needs theand further competitionneedspreparation further of beforemini-LEDpreparation entering and before the tried mass entering to production.develop the massthe Manymini-LED production. companies back Manylight have technologycompanies joined thehaveto replace competition joined the the traditional competition of mini-LED LCD of and triedbacklightmini-LED to develop recently. and the tried mini-LED to develop backlight the mini-LED technology backlight to replace technology the traditional to replace LCDthe traditional backlight LCD recently. backlightAUAU Optronics Optronicsrecently. Corporation Corporation (AUO) (AUO) demonstrated demonstrated several several high-end high-end mini-LED mini-LED backlit backlit LCD displays,LCD includingdisplays,AU a 27”includingOptronics 4K 144 aCorporation 27” Hz 4K gaming 144 Hz(AUO) monitorgaming demonstrated monitor and a 1000and several a PPI1000 2” PPIhigh-end LTPS 2” LTPS VR mini-LED VR display display backlit (Figure (Figure LCD6)[ 6) 20]. [20]. The gaming monitor used a straight down type mini-LED backlight to provide accurate local Thedisplays, gaming monitorincluding used a 27” a straight4K 144 Hz down gaming type monitor mini-LED and backlight a 1000 PPI to 2” provide LTPS VR accurate display local (Figure dimming 6) dimming with ultra-high brightness, creating a more realistic visual enjoyment for customer. with[20]. ultra-high The gaming brightness, monitor creatingused a straight a more down realistic type visualmini-LED enjoyment backlight for to customer.provide accurate However, local the However,dimming thewith cost ultra-high of mini-LEDs brightne is stillss, severalcreating ti mesa more higher realistic than traditional visual enjoyment backlight fortechnology customer. at cost of mini-LEDs is still several times higher than traditional backlight technology at present. present.However, For the head-mounted cost of mini-LEDs VR display, is still several AUO shows times highera 2-inch than panel traditional equipped backlight with an technologyactive matrix at For head-mounted VR display, AUO shows a 2-inch panel equipped with an active matrix (AM) (AM)present. driver For circuithead-mounted which can VR achieve display, 1024 AUO local shows dimming a 2-inch zones panel for equippedvivid images. with an active matrix driver circuit which can achieve 1024 local dimming zones for vivid images. (AM) driver circuit which can achieve 1024 local dimming zones for vivid images.

Figure 6. The 27” gaming monitor and the 2” VR display of AUO. Figure 6. The 27” gaming monitor and the 2” VR display of AUO. JDI, a JapaneseFigure manufacturer, 6. The 27” demonstrated gaming monitor an andautomotive the 2” VR central display control of AUO. panel based on a 16.7 inch JDI,curved a Japanese screen with manufacturer, a straight downdemonstrated type mini-LED an automotive backlight central at 2018 control SID Display panel based Week, on shown a 16.7 ininchJDI, Figure curved a Japanese 7a screen[21]. The manufacturer, with contrast a straight and down demonstrated color type of the mini-LED screen an automotivecan backlight be presented at central2018 perfectlySID control Display even panel Week, in complex based shown on a situations because of the local dimming with 104 dimming zones in the screen. BOE, one of the largest 16.7in inch Figure curved 7a [21]. screen The withcontrast a straight and color down of the type screen mini-LED can be presented backlight perfectly at 2018 even SID Displayin complex Week, display makers in China, exhibited a 27-inch ultra-high-definition (UHD) panel, which used a mini- shownsituations in Figure because7a [ of21 the]. Thelocal contrastdimming andwith color104 dimming of the screenzones in can the screen. be presented BOE, one perfectly of the largest even in LED backlight with 1000 local dimming zones. Its brightness is up to 600 nits and its contrast is up to complexdisplay situations makers in because China, exhibited of the local a 27-inch dimming ultra-high-definition with 104 dimming (UHD) zones panel, in the which screen. used BOE, a mini- one of 1,000,000:1.LED backlight In addition,with 1000 BOE local also dimming showed zones. a 5.9-inch Its brightness mobile phoneis up to panel 600 nits which and was its contrast only 1.4 is mm up into the largest display makers in China, exhibited a 27-inch ultra-high-definition (UHD) panel, which used thickness,1,000,000:1. shown In addition, in Figure BOE 7b also [21]. showed a 5.9-inch mobile phone panel which was only 1.4 mm in a mini-LED backlight with 1000 local dimming zones. Its brightness is up to 600 nits and its contrast is thickness, shown in Figure 7b [21]. up to 1,000,000:1. In addition, BOE also showed a 5.9-inch mobile phone panel which was only 1.4 mm in thickness, shown in Figure7b [21]. Appl. Sci. 2018, 8, 1557 6 of 17 Appl. Sci. 2018, 8, x FOR PEER REVIEW 6 of 17 Appl.Appl.Appl. Sci. Sci. Sci.2018 2018 2018, 8, ,8 x,, 8xFOR, FORx FOR PEER PEER PEER REVIEW REVIEW REVIEW 6 6of of 617 of17 17

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OLED contrast, head head[30] [30]head [30].-. -. - Therefore,uplowupTherefore, Therefore,updisplays, displays, power displays, micro consumption, microAR/VR, micro-LEDAR/VR, AR/VR,-LED-LED micro microdisplays displaysmicro displays high projectors, projectors, projectors, brightness,have have have the theand andthe potential andpotential and high potentialhigh high especially-end-end to -toend televisionsmatchto televisionsmatch matchtelevisions the or highor exceedor exceed[29] [29]exceed life-span [29] ( Table(todayTable today( Tabletoday’s [231’ s2)’ .)s]. OLED.Additionally, 2 OLEDAdditionally,) .OLED Additionally, displays, displays, displays, micro micro with microwith with- - - Table 2. Requirements for mini-LED and micro-LED in typical applications. itsLEDLEDits highitsLED high can highcan contrast,can becontrast, be contrast, combinedbe combined combined low low low power powerwith withpower with aconsumption, a consumption,f lexible faconsumlexible flexible substrate substrateption, substrate high high high tobrightness, tobrightness, realize brightness,torealize realize the the and andtheflexible andflexible especially flexibleespecially espe characteristics characteristicscially characteristics the the the high high high life likelife like life-span- spanlike- spanOLED OLED OLED[31] [31] [31].[30] . [30]. [30]. . . Therefore,Therefore,Therefore, micro micro microTable-LED-LED-LED 2.displays displaysRequirements displaysAuto have have have Display forthe the mini-LEDthepotential potential potential and to to match micro-LEDtomatch match TV or or exceed or inexceed typicalexceed today today applications. Digitaltoday’s’ sOLED ’OLED sDisplay OLED displays, displays, displays, with with with itsits highits high high contrast, contrast, contrast,Table lowTable lowTable low power2. power2. Requirements 2.powerRequirements Requirements consumption, consumption, consumption, for for formin min highmini-LED ihigh-LEDi- LEDhigh brightness, brightness,and andbrightness, and micro micro micro-LED- LEDand- LEDand and inespecially especiallyin typical inespeciallytypical typical applications. applications.the theapplications. thehigh high high life life -lifespan -span-span [31] [31] [31]. . . Auto Display TV Digital Display Application AutoAutoAuto Display Display Display TVTV TV DigitalDigital Digital Display Display Display TableTableTable 2. 2. Requirements Requirements2. Requirements for for minfor min mini-iLED-LEDi-LED and and and micro micro micro-LED-LED-LED in in typical intypical typical applications. applications. applications. Application ApplicationApplicationApplication AutoAutoAuto Display Display Display TVTV TV DigitalDigitalDigital Display Display Display Panel Size (inch) 6~12 32~100 150~220 PPI 150~250 40~80 20~30 ApplicationApplicationApplication Chip volume (M) 4.1 24.9 24.9 PanelPanelChipPanel Size Size Size Size (inch) (inch)(μ (inch)m) 50~1006~6~12 6~1212 32~ 50~8032~ 32~100100100 80~100150~150~ 150~220220 220 Panel Size (inch) 6~12 32~100 150~220 PPIPPI 150~150~250250 40~40~8080 20~20~3030 PPI AR150~250 Watch 40~80 Mobile 20~30 ChipPanelChipPanelChipPanel volume Sizevolume Size volume Size PPI(inch) (inch) (M) (inch)(M) (M) 6~4.16~ 150~250124.1126~ 4.1 12 32~32~24.9 40~8024.910032~100 24.9 100 150~ 20~30150~24.924.9150~220 24.9220 220 ChipChipChipChip SizePPI PPISize volumeSize PPI (μm) (μm) (μ m) (M) 150~50~150~50~150~50~100100250100 4.1250 250 40~50~40~50~ 24.9 50~808040~8080 80 80~20~80~ 24.920~ 80~1001003020~10030 30 Application ARAR WatchWatch MobileMobile ChipChipChip Chipvolume volume volume Size (M) (M) (µ (M) m) 4.1 50~1004.1 AR4.1 24.9 50~8024.9Watch24.9 80~10024.924.9Mobile24.9 ChipChipChip Size Size Size (μm) (μm) (μm) 50~50~10050~100 100 50~50~8050~80 80 80~80~10080~100 100 AR Watch Mobile ARAR AR WatchWatchWatch MobileMobileMobile ApplicationApplication PanelApplication Size (inch) 0.5~1 1~1.5 4~6 ApplicationPPI 450~2000 200~300 300~800 ApplicationApplicationApplication Chip volume (M) 49.8 0.4 6.2

PanelPanelChip Size Size Size (inch) (inch)(μm) 0.5~0.5~1~51 1 10~301~1~1.51.5 30~504~4~6 6 Panel Size (inch) 0.5~1 1~1.5 4~6 PPIPPI 450~450~20002000 200~200~300300 300~300~800800 PPI 450~2000 200~300 300~800 AtChip present, volume the (M) micro-LED display49.8 technology still faces0.4 some challenges, like6.2 the transfer ChipPanelPanelChipPanel volume Size Size volume Size (inch) (inch) (M) (inch) (M) 0.5~49.80.5~0.5~ 49.81 1 1 1~0.41~1.51.51~ 0.4 1.5 4~6.24~6 64~6.2 6 printingChipChip Panelof Size Sizemicro-LED Size(μm) (μm) (inch) chips for 1~mass1~ 0.5~15 5 production [32–34]10~10~ 1~1.5 30and30 the full-color method30~ 4~630~5050 for display ChipPPIPPI Size PPI (μm) 450~450~450~200020001~52000 200~200~200~ 10~30300300 300 300~300~300~ 30~50800800 800 applicationsChipChipChip volume volume [35–37].volumePPI (M) (M) (M) With the rapid49.8 450~200049.8 development49.8 of some 200~300 transfer0.40.4 0.4 printing approaches 300~8006.26.2 6.2 summarized in AtTableAt present, present, 3, the firstthe the problemmicro micro-LED- LEDis expected display display to technology technologybe effectively still still solved. faces faces Thus,some some we challenges, challenges, will focus likeon like methods the the transfer transfer of ChipAtChipChip Chippresent, Size Size volumeSize (μm) (μm) (μm)the (M) micro-LED1~1~ 5display 49.8 51~ 5 technology 10~st10~ill 0.43010~30 faces 30 some challenges,30~30~ 6.25030~50 50like the transfer printingprintingfabricating of of micro the micro full-color-LED-LED chips chipsdisplay for for formass mass micro-LEDs. production production [32 [32–34–34] ]and and the the full full-color-color method method for for display display printing Chipof micro-LED Size (µm) chips for 1~5mass production [32–34] 10~30 and the full-color 30~50 method for display applicationsapplicationsAtAt At present, present, present, [35 [35– 37–the 37the] . ]theWith.micro Withmicro micro the -theLED-LED rapid- LEDrapid display display development displaydevelopment technology technology technology of of some somestill still still transferfaces transferfaces faces some some printing printingsome challenges, challenges, challenges, approaches approaches like like like summarizedthe summarizedthe thetransfer transfer transfer applications [35–37]. With the rapid development of some transfer printing approaches summarized inprintingprintingin Tableprinting Table 3of ,3of the , microofthe micro firstmicro first-LED -problemLED problem-LED chips chips chips is is for expectedfor expected formass mass mass productionto productionto be productionbe effectively effectively [32 [32 –[32 –34solved.34 solved.–] 34]and and] and Thusthe Thusthe thefull , fullwe, wefull-color- willcolor will-color focusmethod focusmethod method on on methodsfor formethods for display display display of of in Table 3, the first problem is expected to be effectively solved. Thus, we will focus on methods of fabricatingapplicationsapplicationsfabricatingapplications the the[35 [35 full – [35full–3737-–]color-.37] color.With With]. Withdisplay displaythe the therapid rapid for rapid for developmentmicro developmentmicro development-LEDs.-LEDs. of of some ofsome some transfer transfer transfer printing printing printing approaches approaches approaches summarized summarized summarized fabricating the full-color display for micro-LEDs. inin Table inTable Table 3 3, ,the 3the, thefirst first first problem problem problem is is expected isexpected expected to to be tobe effectivelybe effectively effectively solved. solved. solved. Thus Thus Thus, ,we we, wewill will will focus focus focus on on methodson methods methods of of of fabricatingfabricatingfabricating the the thefull full full-color-color-color display display display for for formicro micro micro-LEDs.-LEDs.-LEDs. Appl. Sci. 2018, 8, 1557 7 of 17

At present, the micro-LED display technology still faces some challenges, like the transfer printing of micro-LED chips for mass production [32–34] and the full-color method for display applications [35–37]. With the rapid development of some transfer printing approaches summarized in Table3, the first problem is expected to be effectively solved. Thus, we will focus on methods of fabricating the full-color display for micro-LEDs. Appl. Sci. 2018, 8, x FOR PEER REVIEW 7 of 17 Appl. Sci. 2018, 8, x FOR PEERTable REVIEW 3. Massively selective transfer printing methods. 7 of 17 Appl. Sci. 2018, 8, x FOR PEER REVIEW 7 of 17 Appl. Sci. 2018, 8, x FOR PEER REVIEW 7 of 17 Appl. Sci. 2018, 8, x FOR PEER REVIEWTable 3. Massively selective transfer printing methods. 7 of 17 CompanyTable Principle3. Massively selective transfer printing Description methods. CompanyTable Table 3. Massively 3.Principle Massively selective selective transfer transfer printing printing methods. methods.Description Company Table Principle3. Massively selective transfer printing methods.Description Company Principle The transfer heads are dividedDescription by the dielectric layer CompanyCompany PrinciplePrinciple The transfer heads are Description dividedDescription by the dielectric layer to form a ElectrostaticElectrostatic Theto formtransfer a pair heads of are silicon divided electrodes, by the dielectric which layer are to form a Electrostatic LuxVueLuxVue pair of silicon electrodes, which are positively and negatively array array LuxVue pairThepositively transferof silicon andheads electrodes, negatively are divided which charged, by are the positively dielectric respectively, and layer negatively to form a Electrostaticarray The transferThecharged, transfer heads respectively, headsare divided are before divided by the picking by dielectric the up dielectric the layer target tolayer form LED. to a form a ElectrostaticElectrostatic LuxVue charged,pairbefore of silicon pickingrespectively, electrodes, up the before targetwhich picking are LED. positively up the target and LED.negatively array LuxVueLuxVue pair ofpair silicon of silicon electrodes, electrodes, which which are positively are positively and negatively and negatively array array charged, respectively, before picking up the target LED. charged, charged, respectively, respectively, before before picking picking up the up target the targetLED. LED.

Magnetic Micro-LEDs are adsorbed and placed by the electromagnetic MagneticMagnetic ITRI MicroMicro-LEDs-LEDs are are adsorbed adsorbed and andplaced placed by the by electromagnetic the array ITRIITRI force generated by the coil. Magneticarray forceMicro generated-LEDs are byadsorbed the coil. and placed by the electromagnetic Magneticarray ITRI Micro-LEDselectromagnetic are adsorbed force and placed generated by the by electromagnetic the coil. Magneticarray ITRI forceMicro generated-LEDs are byadsorbed the coil. and placed by the electromagnetic array ITRI force generated by the coil. array force generated by the coil.

The pick-up and transfer processes are aided by the Van der Elastomer X- The pick-up and transfer processes are aided by the Van der Elastomer X- Waals forces between the viscoelastic elastomer stamp and the stamp Celeprint WaalsTheThe pick pick-up forces-up andbetween and transfer transfer the processesviscoelastic processes are elastomer aided are aidedby stampthe byVan and the der the ElastomerElastomerstamp CeleprintX- The pick-upThesolid pick micro and-up transfer-LEDs. and transfer processes processes are aided are aidedby the by Van the der Van der ElastomerElastomer X-CeleprintX- X- solidWaalsVan micro der forces Waals-LEDs. between forces the between viscoelastic the elastomer viscoelastic stamp and the stampstamp Celeprint Waals Waalsforces forcesbetween between the viscoelastic the viscoelastic elastomer elastomer stamp stamp and the and the stampstamp CeleprintCeleprint solidelastomer micro-LEDs. stamp and the solid micro-LEDs. solid micro-LEDs.solid micro- LEDs.

A roll-based transfer technology for transferring nanoscale

Roll to plate KIMM Aobjects roll-based from transfera donor technologysubstrate to for a target transferring substrate nanoscale with high Roll to plate KIMM objectsA roll-based from transfera donor technologysubstrate to for a target transferring substrate nanoscale with high A roll-basedAyields roll- basedandtransfer productivity. transfer technology technology for transferring for transferring nanoscale nanoscale Roll to plate KIMM yieldsobjectsA roll-based and from productivity. a donor transfer substrate technology to a target for substrate transferring with high RollRoll toRoll plate to plate KIMM KIMMKIMM objects yieldsobjects nanoscalefrom anda from donor productivity. objectsa donor substrate fromsubstrate to aa donortarget to a targetsubstrate substrate substrate with to ahigh with target high yields yieldsandsubstrate productivity. and productivity. with high yields and productivity.

3.1. RGB Micro-LED Full-Color Display 3.1. RGB Micro-LED Full-Color Display 3.1. RGBThe Miprinciplecro-LED of Full RGB-Color full -Displaycolor display is based on the law of three primary colors that can be 3.1. RGB3.1. RGBMicro-LED Micro- LEDFull-Color Full-Color Display Display 3.1. RGB Micro-LEDThe principle Full-Color of RGB Display full-color display is based on the law of three primary colors that can be combinedThe principle to create of all RGB colors full in- colornature display via a certain is based ratio on settingthe law (Figure of three 8) primary. Therefore colors, for RGBthat canLEDs, be Thecombined principleThe principle to create of RGB ofall RGB full-colorcolors full in- color naturedisplay display via is abased certain is based on ratio the on law settingthe oflaw three (Figure of three primary 8) primary. Therefore colors colors that, for canRGBthat be canLEDs, be combinedThedifferent principle currents to create of RGB are all applied full-colorcolors in to naturecontrol display via the is a basedbrightness certain on ratio the of laweachsetting of LED three (Figure to primaryrealize 8). Therefore the colors combination that, for canRGB of be LEDs, three combinedcombineddifferent to create currents to create all colorsare all applied colors in nature in to naturecontrol via a via certaithe a brightness certainn ratio ratiosetting of eachsetting (Figure LED (Figure to8). realize Therefore, 8). Therefore the combination for RGB, for RGBLEDs, of LEDs, three combineddifferentprimary to create colorscurrents alland colorsare achieve applied in naturethe to full control via-color a certainthe display. brightness ratio It is setting the of eachmethod (Figure LED that 8to). realizeis Therefore, usually the employedcombination for RGB LEDs,in outdoor of three differentdifferentprimary currents colors currents are and applied are achieve applied to controlthe to full control -thecolor brightne the display. brightnessss Itof is each the of eachmethodLED LEDto realize that to realizeis theusually combination the employedcombination of in three outdoor of three differentprimaryLED currents large colors screens. are and applied achieve to controlthe full- thecolor brightness display. It of is each the method LED torealize that is theusually combination employed of in three outdoor primaryprimaryLED colors large colors andscreens. achieve and achieve the full-color the full-color display. display. It is the It is method the method that is that usually is usually employed employed in outdoor in outdoor primaryLED colors large andscreens. achieve the full-color display. It is the method that is usually employed in outdoor LED LEDlarge large screens. screens. LED large screens.

Figure 8. Mechanism of RGB micro-LED full-color display. Figure 8. Mechanism of RGB micro-LED full-color display. Figure 8. Mechanism of RGB micro-LED full-color display. In the RGB fullFigure-colorFigure 8. display Mechanism 8. Mechanism method, of RGB ofeach micro-LEDRGB pixel micro contains -full-colorLED full a- color display.set of display. RGB micro -LEDs. Generally, In the RGB fullFigure-color 8. displayMechanism method, of RGB each micro-LED pixel contains full-color a display.set of RGB micro-LEDs. Generally, the P and N electrodes of the three-color micro-LEDs are connected to the circuit substrate by means the PIn and the N RGB electrodes full-color of the display three -method,color micro each-LEDs pixel are contains connected a set to of the RGB circuit micro substrate-LEDs. Generally, by means Inof bondingtheIn RGB the RGBorfull-color flip full-chip-color display. display method, method, each eachpixe lpixel contains contains a set aof set RG ofB RGBmicro-LEDs micro-LEDs.. Generally, Generally, oftheIn bonding the P and RGB N full-coloror electrodes flip-chip display .of the method, three-color each micro pixel-LEDs contains are aconnected set of RGB to micro-LEDs. the circuit substrate Generally, by the means the Pthe and P After Nand electrodes N that, electrodes a dedicatedof the of three-color the ful threel-color-color micro-LED driver micro chip-sLEDs are is connectedused are connected to drive to the each to circuit the color circuit substrate of microsubstrate by-LEDs means by viameans the P andof N bonding electrodesAfter that, or flip of a the-dedicatedchip three-color. full-color micro-LEDs driver chip are connected is used to todrive the circuiteach color substrate of micro by means-LEDs via of the of bondingofpulse bonding widthor flip-chip. or modulationflip-chip . (PWM) current. The PWM current drive method can achieve digital bondingpulse orAfter widthflip-chip. that, modulation a dedicated (PWM) full-color current. driver Thechip PWM is used current to drive drive each methodcolor of canmicro achieve-LEDs via digital the AfterdimmingAfter that, by that,a dedicatedsetting a dedicated the full-color duty ful cyclel-color driver of thedriver chip current. chipis used isFor usedto example, drive to drive each an eachcolor8-bit color PWMof micro-LEDs of full micro-color-LEDs viadriver the via chip the dimmingpulseAfter that, width by a dedicatedsetting modulation the full-colorduty (PWM) cycle driver current.of the chip current. The is used PWM For to example, drive current each an drive color8-bit method PWM of micro-LEDs full can-color achieve viadriver the digital chip pulsepulsecan width achieve width modulation 2 8modulation = 256 kinds (PWM) of (PWM) dimming current. current. effectsThe ThePW of a M PWMsingle current - currentcolor drive micro drive method-LED, method then can forachieve can a pixel achieve digital containing digital pulsedimming width modulation by setting8 (PWM) the duty current. cycle of The the PWM current. current For drive example, method an 8 can-bit achieve PWM full digital-color dimming driver chip can achieve 2 = 256 kinds of dimming effects 3of a single-color micro-LED, then for a pixel containing dimmingdimmingthree by-color setting by LED setting8 the theoretically duty the dutycycle cancycle of theachieve of current. the 256current. Fo = 16,777,216r example, For example, kindsan 8-bit an of 8dimmingPWM-bit PWM full-color effect full- colorwhich driver driver means chip chip the by settingcan achieve the duty 2 = cycle 256 kinds of the of current. dimming For effects example, 3of a an single 8-bit-color PWM micro full-color-LED, driver then for chip a canpixel achieve containing can achievethree-color 28 = 256LED8 kinds theoretically of dimming can achieveeffects of 256 a single-c = 16,777,216olor micro-LED, kinds of dimming then for effecta pixel which containing means the threecansame achieve- numbercolor LED 2 of= 256 theoreticallycolors kinds can of be dimming candisplayed. achieve effects Figure 256 3of = 9a16,777,216 demonstratsingle-color kindses micro the of specificdimming-LED, then full effect -forcolor a which pixel display containingmeans driving the three-colorsame number LED theoretically of colors can can be achieve displayed. 2563 Figure = 16,777,2163 9 demonstrat kinds ofes dimming the specific effect full which-color displaymeans the driving samethreeprinciple- numbercolor [38] LED. of theoreticallycolors can be candisplayed. achieve Figure 256 = 916,777,216 demonstrat kindses the of specificdimming full effect-color which display means driving the sameprinciple number of[38] colors. can be displayed. Figure 9 demonstrates the specific full-color display driving principlesame number [38]. of colors can be displayed. Figure 9 demonstrates the specific full-color display driving principleprinciple [38]. [38]. Appl. Sci. 2018, 8, 1557 8 of 17

28 = 256 kinds of dimming effects of a single-color micro-LED, then for a pixel containing three-color LED theoretically can achieve 2563 = 16,777,216 kinds of dimming effect which means the same number ofAppl. colors Sci. 2018 can, 8 be, x FOR displayed. PEER REVIEW Figure 9 demonstrates the specific full-color display driving principle8 [ of38 17]. Appl. Sci. 2018, 8, x FOR PEER REVIEW 8 of 17

Brightness adjustment Brightness adjustment … & constant current driving unit … & constant current driving unit … Process & control … Image data Process & control Image data unit … … … unit … … Array … Array Addressing unit … Addressing unit …

Figure 9. Substituted scheme of the driving circuit for micro-LED display. FigureFigure 9. 9. SubstitutedSubstituted scheme scheme of of the the driving driving circuit circuit for for micro-LED micro-LED display. display. However, the RGB micro-LED based technology suffers from a severe disadvantage in mass However, the RGB micro-LED based technology suffers from a severe disadvantage in mass production.However, For the example, RGB micro-LED in order to based manufacture technology a 4K suffersresolution from display, a severe nearly disadvantage 25 million in micro- mass production. For example, in order to manufacture a 4K resolution display, nearly 25 million micro- production.LEDs are needed For example, to be assembled in order toand manufacture connected awithout 4K resolution a single display, error in nearly an economical 25 million and micro-LEDs efficient LEDs are needed to be assembled and connected without a single error in an economical and efficient areway, needed with placement to be assembled accuracy and of connected 1 μm or less. without Obviously, a single it erroris very in difficult an economical to transfer and or efficient grow such way, way, with placement accuracy of 1 μm or less. Obviously, it is very difficult to transfer or grow such witha huge placement number accuracyof three different of 1 µm ormicro-LEDs less. Obviously, on the it same is very substrate. difficult to transfer or grow such a huge a huge number of three different micro-LEDs on the same substrate. numberAs an of threeideal differentsolution, micro-LEDs all RGB micro-LEDs on the same would substrate. be composed of the same material with the As an ideal solution, all RGB micro-LEDs would be composed of the same material with the sameAs behaviors an ideal and solution, driving all RGBconditions. micro-LEDs The challenge would be is composed to find one of material the same capable material of with spanning the same the same behaviors and driving conditions. The challenge is to find one material capable of spanning the behaviorsblue to red and spectrum. driving conditions.Theoretically, The the challenge InGaN isallo toy find can onecover material the entire capable visible of spanning range by the adjusting blue to blue to red spectrum. Theoretically, the InGaN alloy can cover the entire visible range by adjusting redthe spectrum.indium content Theoretically, to fine tune theInGaN the peak alloy emission can cover wavelength. the entire visibleUnfortunately, range by high adjusting indium the content indium the indium content to fine tune the peak emission wavelength. Unfortunately, high indium content contentin GaN-based to fine tuneLEDs the results peak in emission poor quality wavelength. becaus Unfortunately,e of lattice mismatches high indium between content the in GaN GaN-based buffer in GaN-based LEDs results in poor quality because of lattice mismatches between the GaN buffer LEDslayers resultsand the in InGaN poor qualityquantum because wells. of lattice mismatches between the GaN buffer layers and the layers and the InGaN quantum wells. InGaNTo quantumaddress wells.these issues, A. Even et al. developed an innovative substrate called InGaNOS To address these issues, A. Even et al. developed an innovative substrate called InGaNOS (InGaNTo addresspseudo-substrates) these issues, that A. Even overcomes et al. developed lattice mismatch an innovative [39]. substrateThe substrate called has InGaNOS a top (InGaNrelaxed (InGaN pseudo-substrates) that overcomes lattice mismatch [39]. The substrate has a top relaxed pseudo-substrates)InGaN layer that can that be overcomesused as a seed lattice layer mismatch for full [InGaN39]. The LED substrate growth, has as ashown top relaxed in Figure InGaN 10a. layer The InGaN layer that can be used as a seed layer for full InGaN LED growth, as shown in Figure 10a. The thatexperimental can be used results as a seedillustrate layer forthat full InGaN InGaN structures LED growth, grown as shownon InGaNOS in Figure substrates 10a. The can experimental span the experimental results illustrate that InGaN structures grown on InGaNOS substrates can span the resultsspectrum illustrate from blue that (482nm) InGaN structuresto red (617nm) grown [40], on as InGaNOS shown in substrates Figure 10b. can InGaNOS span the technology spectrum from can spectrum from blue (482nm) to red (617nm) [40], as shown in Figure 10b. InGaNOS technology can bluebe used (482nm) to create to redsubstrates (617nm) with [40 ],mixed as shown lattice in parameters, Figure 10b. enabling InGaNOS growth technology of different can color be used LEDs to be used to create substrates with mixed lattice parameters, enabling growth of different color LEDs createon the substratessame substrate. with mixedThis could lattice drastically parameters, reduce enabling the cost growth of micro-LED of different mass color transfer LEDs for on full-color the same on the same substrate. This could drastically reduce the cost of micro-LED mass transfer for full-color substrate.display fabrication This could in the drastically future. reduce the cost of micro-LED mass transfer for full-color display displayfabrication fabrication in the future. in the future.

Figure 10. (a) Schematic diagram of InGaNOS substrate, (b) photoluminescence spectra at room Figure 10. (a) Schematic diagram of InGaNOS substrate, (b) photoluminescence spectra at room Figuretemperature 10. ( aof) Schematicfull InGaN diagramstructures of grown InGaNOS on InGaNOS. substrate, (b) photoluminescence spectra at room temperaturetemperature of of full full InGaN InGaN structures structures grown grown on on InGaNOS. InGaNOS. 3.2. Color Conversion Full-Color Display 3.2. Color Conversion Full-Color Display The full-color display can be achieved by employing excitation sources, such as (UV) The full-color display can be achieved by employing excitation sources, such as ultraviolet (UV) micro-LED or blue micro-LED, with color-conversion materials. RGB color-conversion materials are micro-LED or blue micro-LED, with color-conversion materials. RGB color-conversion materials are needed to achieve RGB three primary colors if UV micro-LEDs are used, while only red and green needed to achieve RGB three primary colors if UV micro-LEDs are used, while only red and green color-conversion materials are required if blue micro-LEDs are used (Figure 11). Generally, color- color-conversion materials are required if blue micro-LEDs are used (Figure 11). Generally, color- conversion materials can be divided into the phosphor and the quantum dots (QDs). conversion materials can be divided into the phosphor and the quantum dots (QDs). Appl. Sci. 2018, 8, 1557 9 of 17

3.2. Color Conversion Full-Color Display The full-color display can be achieved by employing excitation sources, such as ultraviolet (UV) micro-LED or blue micro-LED, with color-conversion materials. RGB color-conversion materials are needed to achieve RGB three primary colors if UV micro-LEDs are used, while only red and green color-conversion materials are required if blue micro-LEDs are used (Figure 11). Generally, Appl.color-conversion Sci. 2018, 8, x FOR materials PEER REVIEW can be divided into the phosphor and the quantum dots (QDs). 9 of 17 Appl. Sci. 2018, 8, x FOR PEER REVIEW 9 of 17

Figure 11. Mechanism of color conversion full-color display. FigureFigure 11. 11.Mechanism Mechanism of of color color conversion conversion full-color full-color display. display. 3.2.1. Phosphor 3.2.1.3.2.1. Phosphor Phosphor The phosphor can emit the light of a specific wavelength under the excitation of blue or UV TheThe phosphor phosphor can can emit emit the the light light of aof specific a specific wavelength wavelength under under the excitation the excitation of blue of or blue UV LEDs.or UV LEDs. The color of the phosphor is determined by its material, and the production method is simple TheLEDs. color The of color the phosphor of the phosphor is determined is determined by its material,by its material, and the and production the production method method is simple is simple and and easy to implement. After LEDs are integrated with the substrate of the drive circuit, the phosphor easyand toeasy implement. to implement. After After LEDs LEDs are integrated are integrated with with the substratethe substrate of the of drivethe drive circuit, circuit, the the phosphor phosphor is is deposited on the surface of LEDs via spin coating or pulse-spray coating [41]. Figure 12 shows an depositedis deposited on on the the surface surface of LEDsof LEDs via via spin spin coating coatin org or pulse-spray pulse-spray coating coating [41 [4].1]. Figure Figure 12 12 shows shows an an example of the deposition of . exampleexample of of the the deposition deposition of of phosphors. phosphors.

Figure 12. (a) One colorful pixel includes four sub-pixels in combination of RGGB and (b) a full-color Figure 12. (a) One colorful pixel includes four sub-pixels in combination of RGGB and (b) a full-color displayFigure 12.by (excitationa) One colorful of RGB pixel phos includesphors by four UV sub-pixels LEDs [41]. in Figure combination reproduced of RGGB with and permission (b) a full-color from display by excitation of RGB phosphors by UV LEDs [41]. Figure reproduced with permission from Johndisplay Wiley by excitationand Sons. of RGB phosphors by UV LEDs [41]. Figure reproduced with permission from John Wiley and Sons. John Wiley and Sons. However, this method still has some shortcomings. The layer of phosphor will absorb part of However, this method still has some shortcomings. The layer of phosphor will absorb part of the energy and reduce the conversion efficiency. Moreover, the optimized size of the phosphor the However,energy and this reduce method the still conversion has some shortcomings.efficiency. Moreover, The layer the of phosphoroptimized will size absorb of the part phosphor of the particles for lighting and display, which is about 1–10 micrometers, is still relatively large [42,43]. As energyparticles and for reduce lighting the and conversion display, efficiency.which is about Moreover, 1–10 micrometers, the optimized is sizestill ofrelatively the phosphor large [42,43]. particles As sizes of micro-LED pixels continually decrease, the layer of phosphor will become uneven and reduce forsizes lighting of micro-LED and display, pixels which continually is about decrease, 1–10 micrometers, the layer of isphosphor still relatively will become large [ 42uneven,43]. As and sizes reduce of the luminance homogeneity. The disadvantages mentioned above give the technology of QDs a micro-LEDthe luminance pixels homogeneity. continually decrease,The disadvantages the layer of mentioned phosphor above will become give the uneven technology and reduce of QDs the a chance to shine. luminancechance to shine. homogeneity. The disadvantages mentioned above give the technology of QDs a chance to shine. 3.2.2. Quantum Dots 3.2.2. Quantum Dots 3.2.2.To Quantum realize Dotsfull-color display, colloidal QDs can be a great choice. The QDs are usually To realize full-color display, colloidal QDs can be a great choice. The QDs are usually synthesizedTo realize byfull-color chemical display,solution colloidal process QDs[44] canand bepossess a great unique choice. properties The QDs aresuch usually as high synthesized quantum synthesized by chemical solution process [44] and possess unique properties such as high quantum yield,by chemical size-dependent solution emission process wavelength, [44] and possess and narrow unique emission properties linewidth such as [45,46]. high quantum yield, yield, size-dependent emission wavelength, and narrow emission linewidth [45,46]. size-dependentCombining emissionUV micro-LEDs wavelength, and RGB and narrowQDs together emission, Prof. linewidth Kuo et [al.45 ,demonstrated46]. a method to Combining UV micro-LEDs and RGB QDs together, Prof. Kuo et al. demonstrated a method to achieveCombining full-color UV high-quality micro-LEDs micro-LED and RGB QDs displays together, via Prof.the aerosol Kuo et al.jet demonstrated(AJ) technique a method[47,48]. achieve full-color high-quality micro-LED displays via the aerosol jet (AJ) technique [47,48]. Additionally,to achieve full-color a photoresist high-quality (PR) mold micro-LED is fabricated displays to limit via the the optical aerosol cross-talk jet (AJ) effect, technique and the [47 QDs,48]. Additionally, a photoresist (PR) mold is fabricated to limit the optical cross-talk effect, and the QDs Additionally,are deposited ainto photoresist the PR mold (PR) with mold clear is fabricated separation to between limit the each optical other. cross-talk Furthermore, effect, a and distributed the QDs are deposited into the PR mold with clear separation between each other. Furthermore, a distributed Braggare deposited reflector into (DBR) the PRcovered mold above with clear the PR separation mold ca betweenn significantly each other. enhance Furthermore, both the utilization a distributed of Bragg reflector (DBR) covered above the PR mold can significantly enhance both the utilization of UV light and the emission intensity of RGB QDs. UV light and the emission intensity of RGB QDs. As the core-shell type with ligands attached to the outer shells, RGB QDs in the experiment have As the core-shell type with ligands attached to the outer shells, RGB QDs in the experiment have the average particle sizes about 9.3 nm, 6.2 nm and 2.5 nm, and the emission spectra of them are 630 the average particle sizes about 9.3 nm, 6.2 nm and 2.5 nm, and the emission spectra of them are 630 nm, 520 nm and 450 nm, respectively. Figure 13 shows the absorption and emission spectra as well nm, 520 nm and 450 nm, respectively. Figure 13 shows the absorption and emission spectra as well as images of blue, green, and red QDs solution, respectively. Moreover, due to the narrow emission as images of blue, green, and red QDs solution, respectively. Moreover, due to the narrow emission linewidths of RGB QDs, large color can be readily achieved. linewidths of RGB QDs, large color gamut can be readily achieved. Appl. Sci. 2018, 8, 1557 10 of 17

Bragg reflector (DBR) covered above the PR mold can significantly enhance both the utilization of UV light and the emission intensity of RGB QDs. As the core-shell type with ligands attached to the outer shells, RGB QDs in the experiment have the average particle sizes about 9.3 nm, 6.2 nm and 2.5 nm, and the emission spectra of them are 630 nm, 520 nm and 450 nm, respectively. Figure 13 shows the absorption and emission spectra as well as images of blue, green, and red QDs solution, respectively. Moreover, due to the narrow emission linewidthsAppl. Sci. 2018 of, 8,RGB x FOR QDs, PEER largeREVIEW color gamut can be readily achieved. 10 of 17

FigureFigure 13.13. AbsorptionAbsorption and emissionemission spectra of (a)) blue,blue, ((c)) greengreen andand ((ee)) redred QDsQDs asas wellwell asas solutionsolution photosphotos ofof ((bb)) blue,blue, ((dd)) greengreen andand ((ff)) redred QDsQDs [[47].47]. FigureFigure reproducedreproduced withwith permissionpermission fromfrom OpticalOptical SocietySociety ofof America.America.

In order to fully excite RGB QDs and greatly raise color quality, an UV micro-LED array is In order to fully excite RGB QDs and greatly raise color quality, an UV micro-LED array is prepared prepared as an excitation source on an UV epitaxial wafer. The emission wavelength of the excitation as an excitation source on an UV epitaxial wafer. The emission wavelength of the excitation source is source is 395 nm, while the size of micro-LED chip is 35 μm × 35 μm, as shown in Figure 14a. For 395 nm, while the size of micro-LED chip is 35 µm × 35 µm, as shown in Figure 14a. For reducing reducing the cross-talk effect of QDs, a PR mold with open windows and blocking walls is fabricated the cross-talk effect of QDs, a PR mold with open windows and blocking walls is fabricated via the via the simple lithography technique. The definition of the mask for the PR mold fabrication is simple lithography technique. The definition of the mask for the PR mold fabrication is adopted with adopted with the pitch size of 40 μm as the same as the micro-LED array. By aligning the mold the pitch size of 40 µm as the same as the micro-LED array. By aligning the mold windows to the windows to the micro-LED mesa, as shown in Figure 14b–e, UV micro-LEDs can accurately excite micro-LED mesa, as shown in Figure 14b–e, UV micro-LEDs can accurately excite RGB QDs without RGB QDs without the cross-talk effect. the cross-talk effect. The AJ system consists primarily of an ultrasonic atomizer and a spraying chamber (Figure 15). Inside the ultrasonic atomizer, ultrasonic vibrations first atomize the RGB QD suspension. Then, a nitrogen gas flow transfers the AJ to the nozzle to becoming small drops. Ultimately, QDs are deposited on the substrate with high resolution and narrow linewidth. Table4 lists the optimized parameters of the AJ system. The working distance and stage speed are fixed according to the experimental experience. Different sizes of the QDs will affect the deposition conditions. Therefore, different flow rates are required to control the AJ system. Generally, as the particle size of QDs increases, both the carrier gas flow rate and the sheath gas flow rate need to be increased to maintain the linewidth of the deposited QDs, which is about 35 µm in the study.

Figure 14. Process flow of the full-color micro-LED display. (a) The structure of the UV micro-LED array. (b) Aligning the PR mold to the micro-LED array. (c–e) Consequently jetting the RGB QDs inside the PR mold window to form full-color pixels [48]. Figure reproduced with permission from Optical Society of America.

The AJ system consists primarily of an ultrasonic atomizer and a spraying chamber (Figure 15). Inside the ultrasonic atomizer, ultrasonic vibrations first atomize the RGB QD suspension. Then, a nitrogen gas flow transfers the AJ to the nozzle to becoming small drops. Ultimately, QDs are deposited on the substrate with high resolution and narrow linewidth. Appl. Sci. 2018, 8, x FOR PEER REVIEW 10 of 17

Figure 13. Absorption and emission spectra of (a) blue, (c) green and (e) red QDs as well as solution photos of (b) blue, (d) green and (f) red QDs [47]. Figure reproduced with permission from Optical Society of America.

In order to fully excite RGB QDs and greatly raise color quality, an UV micro-LED array is prepared as an excitation source on an UV epitaxial wafer. The emission wavelength of the excitation source is 395 nm, while the size of micro-LED chip is 35 μm × 35 μm, as shown in Figure 14a. For reducing the cross-talk effect of QDs, a PR mold with open windows and blocking walls is fabricated via the simple lithography technique. The definition of the mask for the PR mold fabrication is adopted with the pitch size of 40 μm as the same as the micro-LED array. By aligning the mold windowsAppl. Sci. 2018 to, 8the, 1557 micro-LED mesa, as shown in Figure 14b–e, UV micro-LEDs can accurately excite11 of 17 RGB QDs without the cross-talk effect.

FigureFigure 14. 14. ProcessProcess flow flow of of the the full-color full-color micro-LED micro-LED display. display. ( (aa)) The The structure structure of of the the UV UV micro-LED micro-LED array.array. ( b) AligningAligning thethe PR PR mold mold to to the the micro-LED micro-LED array. array. (c– e()c Consequently–e) Consequently jetting jetting the RGB the QDsRGB insideQDs insidethe PR the mold PR windowmold window to form to full-color form full-color pixels [pixe48].ls Figure [48]. Figure reproduced reproduced with permission with permission from Optical from OpticalSociety Society of America. of America. Appl. Sci. 2018, 8, x FOR PEER REVIEW 11 of 17 The AJ system consists primarily of an ultrasonic atomizer and a spraying chamber (Figure 15). Inside the ultrasonic atomizer, ultrasonic vibrations first atomize the RGB QD suspension. Then, a nitrogen gas flow transfers the AJ to the nozzle to becoming small drops. Ultimately, QDs are deposited on the substrate with high resolution and narrow linewidth.

Figure 15. The image and the schematic illustrationillustration of the AJ system [[47,48].47,48]. Figure reproduced with permission from Optical SocietySociety ofof America.America.

Table 4.4 Parameterslists the optimized of the AJ system parameters for RGB of QDs the [ 48AJ]. system. Table reproduced The working with permission distance and from stage Optical speed are fixedSociety according of America. to the experimental experience. Different sizes of the QDs will affect the deposition conditions. Therefore, different flow rates are required to control the AJ system. Generally, as the particle size Workingof QDs increases, Distance bothCarrier the Gascarrier Flow gas Rate flow Sheathrate and Gas the Flow sheath Rate gas Stageflow rate Speed need Color of QD to be increased to maintain(mm) the linewidth of the(sccm) deposited QDs, which(sccm) is about 35 μm in (mm/s)the study. Red 1 83 17 10 TableGreen 4. Parameters of 1the AJ system for RGB 72QDs [48]. Table reproduced 15 with permission 10from OpticalBlue Society of America. 1 66 11 10

Carrier Gas Sheath Gas Working Stage Speed ColorFigure of 16 QDa demonstrates the RGB pixel ofFlow the micro-LEDRate displayFlow afterRate the deposition of QDs on Distance (mm) (mm/s) the PR mold lying above micro-LEDs. In this manner,(sccm) RGB pixels have(sccm) very clear boundaries between one anotherRed Figure 16b, compared1 with the previous 83 case where cross-talk17 effect is observed10 without any PR moldGreen Figure 16c. Therefore,1 the blocking 72walls in the PR mold15 can efficiently confine10 QDs to avoid theBlue cross-talk effect. 1 66 11 10

Figure 16a demonstrates the RGB pixel of the micro-LED display after the deposition of QDs on the PR mold lying above micro-LEDs. In this manner, RGB pixels have very clear boundaries between one another Figure 16b, compared with the previous case where cross-talk effect is observed without any PR mold Figure 16c. Therefore, the blocking walls in the PR mold can efficiently confine QDs to avoid the cross-talk effect.

Figure 16. (a) Microscope image and (b) fluorescence microscopy image of the RGB QDs on the PR mold that is above micro-LEDs. (c) The cross-talk effect of the QD pixels without the PR mold [48]. Figure reproduced with permission from Optical Society of America.

Moreover, the coffee ring effect Figure 17a [49] that the suspended particles are driven to the edge and deposited in a ring-like pattern can also be resolved by the PR mold. Figure 17c Appl. Sci. 2018, 8, x FOR PEER REVIEW 11 of 17

Figure 15. The image and the schematic illustration of the AJ system [47,48]. Figure reproduced with permission from Optical Society of America.

Table 4 lists the optimized parameters of the AJ system. The working distance and stage speed are fixed according to the experimental experience. Different sizes of the QDs will affect the deposition conditions. Therefore, different flow rates are required to control the AJ system. Generally, as the particle size of QDs increases, both the carrier gas flow rate and the sheath gas flow rate need to be increased to maintain the linewidth of the deposited QDs, which is about 35 μm in the study.

Table 4. Parameters of the AJ system for RGB QDs [48]. Table reproduced with permission from Optical Society of America.

Carrier Gas Sheath Gas Working Stage Speed Color of QD Flow Rate Flow Rate Distance (mm) (mm/s) (sccm) (sccm) Red 1 83 17 10 Green 1 72 15 10 Blue 1 66 11 10

Figure 16a demonstrates the RGB pixel of the micro-LED display after the deposition of QDs on the PR mold lying above micro-LEDs. In this manner, RGB pixels have very clear boundaries between one another Figure 16b, compared with the previous case where cross-talk effect is observed without anyAppl. PR Sci. mold2018, 8 ,Figure 1557 16c. Therefore, the blocking walls in the PR mold can efficiently confine QDs12 of to 17 avoid the cross-talk effect.

Figure 16. (a) Microscope image and (b) fluorescence microscopy image of the RGB QDs on the PR Figure 16. (a) Microscope image and (b) fluorescence microscopy image of the RGB QDs on the PR mold that is above micro-LEDs. (c) The cross-talk effect of the QD pixels without the PR mold [48]. mold that is above micro-LEDs. (c) The cross-talk effect of the QD pixels without the PR mold [48]. Figure reproduced with permission from Optical Society of America. Figure reproduced with permission from Optical Society of America.

Appl. Sci.Moreover, 2018, 8, x FORthe PEERcoffee REVIEW ring effect Figure 17a [49] that the suspended particles are driven 12to ofthe 17 edge Moreover,and deposited the coffee in ringa ring-like effect Figure pattern 17a can [49] thatalso thebe suspendedresolved by particles the PR are mold. driven Figure to the edge17c demonstratesand deposited the in aimage ring-like of patternthe coffee can ring also effect be resolved when byQD the drops PR mold. directly Figure deposited 17c demonstrates on a GaN substratethe image without of the coffee the PR ring mold. effect The when principle QD drops and directlythe image deposited of the PR on mold a GaN to substrate eliminate without the coffee the ringPR mold. effect Theare shown principle in andFigure the 17b,d, image respectively. of the PR mold With to the eliminate help of the the coffee PR mold, ring the effect thickness are shown of the in liquidFigure within17b,d, respectively.a cell would With be theequalized, help of thewhic PRh mold,leads theto a thickness similar speed of the liquidat which within the aliquid cell would will evaporate,be equalized, preventing which leads the toQD a similarparticles speed from at bein whichg driven the liquid to the will perimeter evaporate, or preventingmiddle site theby QDthe solution.particles from being driven to the perimeter or middle site by the solution.

Figure 17. (a) Principle of the coffee ring effect. ( (b)) Principle of of reducing reducing the coffee ring effect via the PR mold. ( (cc)) The The coffee coffee ring ring effect effect can can be be observed observed without without the the PR PR mold. mold. (d)The (d)The PR PRmold mold confines confines the QDthe QDdrops drops and and resolve resolve the the coffee coffee ring ring effect effect [48]. [48 ].Figure Figure reproduced reproduced with with permission permission from from Optical Society of America.

The UV micro-LED array emits the 395 nm light which can excite the RGB QDs deposited on the The UV micro-LED array emits the 395 nm light which can excite the RGB QDs deposited on the PR mold. However, due to the thin thickness of the QD layer, the UV photons from the underneath PR mold. However, due to the thin thickness of the QD layer, the UV photons from the underneath LED cannot be absorbed completely [50]. Therefore, in order to increase the UV light utilization, and LED cannot be absorbed completely [50]. Therefore, in order to increase the UV light utilization, and avoid bio-logical damage, a DBR structure has been designed for the micro-display. As shown in avoid bio-logical damage, a DBR structure has been designed for the micro-display. As shown in Figure 18a, the DBR provides 90% of reflectance at 395 nm and high transmission (around 90%) at Figure 18a, the DBR provides 90% of reflectance at 395 nm and high transmission (around 90%) at RGB bands. as shown in Figure 18b, a pronounced UV peak (at 395 nm) is an indicator of less-efficient RGB bands. as shown in Figure 18b, a pronounced UV peak (at 395 nm) is an indicator of less-efficient pumping, while the RGB signals are weak without the DBR. After the DBR is added, the strong pumping, while the RGB signals are weak without the DBR. After the DBR is added, the strong reflection at UV band successfully suppressed the 395 nm peak and increase the visible intensities by 194% (blue), 173% (green) and 183% (red).

Figure 18. (a) Measured reflectance spectrum of the DBR. (b) The emission spectra with and without the DBR structure [47]. Figure reproduced with permission from Optical Society of America. Appl. Sci. 2018, 8, x FOR PEER REVIEW 12 of 17

demonstrates the image of the coffee ring effect when QD drops directly deposited on a GaN substrate without the PR mold. The principle and the image of the PR mold to eliminate the coffee ring effect are shown in Figure 17b,d, respectively. With the help of the PR mold, the thickness of the liquid within a cell would be equalized, which leads to a similar speed at which the liquid will evaporate, preventing the QD particles from being driven to the perimeter or middle site by the solution.

Figure 17. (a) Principle of the coffee ring effect. (b) Principle of reducing the coffee ring effect via the PR mold. (c) The coffee ring effect can be observed without the PR mold. (d)The PR mold confines the QD drops and resolve the coffee ring effect [48]. Figure reproduced with permission from Optical Society of America.

The UV micro-LED array emits the 395 nm light which can excite the RGB QDs deposited on the PR mold. However, due to the thin thickness of the QD layer, the UV photons from the underneath LED cannot be absorbed completely [50]. Therefore, in order to increase the UV light utilization, and avoid bio-logical damage, a DBR structure has been designed for the micro-display. As shown in Appl.Figure Sci. 18a,2018, 8the, 1557 DBR provides 90% of reflectance at 395 nm and high transmission (around 90%)13 of 17at RGB bands. as shown in Figure 18b, a pronounced UV peak (at 395 nm) is an indicator of less-efficient pumping, while the RGB signals are weak without the DBR. After the DBR is added, the strong reflectionreflection atat UVUV bandband successfullysuccessfully suppressedsuppressed thethe 395395 nmnm peakpeak andand increaseincrease thethe visiblevisible intensitiesintensities byby 194%194% (blue),(blue), 173%173% (green)(green) andand 183%183% (red).(red).

Figure 18. (a) Measured reflectance spectrum of the DBR. (b) The emission spectra with and without Figure 18. (a) Measured reflectance spectrum of the DBR. (b) The emission spectra with and without the DBR structure [47]. Figure reproduced with permission from Optical Society of America. the DBR structure [47]. Figure reproduced with permission from Optical Society of America.

In addition to exciting QDs for full-color display via UV micro-LEDs, Chen G.S. et al. fabricated monolithic RGB micro-LEDs, using GaN-based 451 nm blue micro-LEDs to individually excite red and green QDs [50]. A black matrix photoresist with light-blocking capability is spun onto the micro-LEDs to improve the pixel CR and color purity by suppressing the crosstalk among the RGB light. A hybrid Bragg reflector (HBR) is deposited on the bottom side of the substrate to reflect the RGB light and the light output intensity, while a DBR with high reflection for the blue light is deposited on the top side of the QDs to further improve the color purity of the red and green light. Finally, the output intensities of red and green lights of the micro LEDs with HBR and DBR can be enhanced by about 27%. These researches above achieve remarkable performance in full-color displays based on photoluminescence performance of QDs. Moreover, fast development and great enhancement in electroluminescence performance have been obtained for red, green and blue QD-LED whose external quantum efficiencies reach up to 20.5%, 23.68% and 19.8%, respectively [51–53]. In these applications, QDs are inserted between the p- and n-type of semiconductor layers, functioning as the active region, which excited by electrons and holes instead of photons. Therefore, QD-LEDs have become one of the most interesting research hotspots in high-end display fields [54]. However, some problems in the QD technology still require in-depth research currently, such as high cost, toxicity of certain types, high requirements for the heat dissipation as well as the encapsulation to ensure the stability.

3.3. Optical Lens Synthesis Full-Color Display Optical lens synthesis refers to a method of synthesizing full-color display from RGB micro-LEDs using a trichroic prism. Liu et al. did a lot of comprehensive studies on fabricating a novel backlight-unit (BLU)-free full-color micro-LED based projector via this method [55,56]. After flip-chip bonding of the micro-LED arrays onto the AM panels, the RGB LED on silicon (LEDoS) chips are die-attached and wire-bonded onto individual packaging boards, which are connected to a control board. Then the packaging boards are mounted to a trichroic prism to form a full-color projection source, as shown in Figure 19a,b. Finally, the result of the 3-LEDoS projector prototype is illustrated in Figure 19c,d. Via the exiting projector lens, a 15-inch HKUST full-color logo is projected on a wall 2 m away. Appl. Sci. 2018, 8, x FOR PEER REVIEW 13 of 17

In addition to exciting QDs for full-color display via UV micro-LEDs, Chen G.S. et al. fabricated monolithic RGB micro-LEDs, using GaN-based 451 nm blue micro-LEDs to individually excite red and green QDs [50]. A black matrix photoresist with light-blocking capability is spun onto the micro- LEDs to improve the pixel CR and color purity by suppressing the crosstalk among the RGB light. A hybrid Bragg reflector (HBR) is deposited on the bottom side of the substrate to reflect the RGB light and the light output intensity, while a DBR with high reflection for the blue light is deposited on the top side of the QDs to further improve the color purity of the red and green light. Finally, the output intensities of red and green lights of the micro LEDs with HBR and DBR can be enhanced by about 27%. These researches above achieve remarkable performance in full-color displays based on photoluminescence performance of QDs. Moreover, fast development and great enhancement in electroluminescence performance have been obtained for red, green and blue QD-LED whose external quantum efficiencies reach up to 20.5%, 23.68% and 19.8%, respectively [51–53]. In these applications, QDs are inserted between the p- and n-type of semiconductor layers, functioning as the active region, which excited by electrons and holes instead of photons. Therefore, QD-LEDs have become one of the most interesting research hotspots in high-end display fields [54]. However, some problems in the QD technology still require in-depth research currently, such as high cost, toxicity of certain types, high requirements for the heat dissipation as well as the encapsulation to ensure the stability.

3.3. Optical Lens Synthesis Full-Color Display Optical lens synthesis refers to a method of synthesizing full-color display from RGB micro- LEDs using a trichroic prism. Liu et al. did a lot of comprehensive studies on fabricating a novel backlight-unit (BLU)-free full-color micro-LED based projector via this method [55,56]. After flip-chip bonding of the micro-LED arrays onto the AM panels, the RGB LED on silicon (LEDoS) chips are die- attached and wire-bonded onto individual packaging boards, which are connected to a control board. Then the packaging boards are mounted to a trichroic prism to form a full-color projection source, as shownAppl. Sci. in2018 Figure, 8, 1557 19a,b. Finally, the result of the 3-LEDoS projector prototype is illustrated in Figure14 of 17 19c,d. Via the exiting projector lens, a 15-inch HKUST full-color logo is projected on a wall 2 m away.

Figure 19. (a) The schematic of the optical architecture, (b) the 3-LEDoS projector prototype, (c) the Figure 19. (a) The schematic of the optical architecture, (b) the 3-LEDoS projector prototype, (c) the HKUST logo from the lens of the projector (d) A 15-inch HKUST full-color logo is projected on a wall HKUST logo from the lens of the projector (d) A 15-inch HKUST full-color logo is projected on a [56].wall Figure [56]. Figure reproduced reproduced with withpermission permission from fromJohn JohnWiley Wiley and Sons. and Sons.

4. Conclusions 4. Conclusions This paper reviews the developments in mini-LEDs and micro-LEDs, while mainly focusing on This paper reviews the developments in mini-LEDs and micro-LEDs, while mainly focusing the colorization method of the micro-LED displays. In general, micro-LEDs have the potential to on the colorization method of the micro-LED displays. In general, micro-LEDs have the potential to improve the properties of miniature display systems such as LCDs and OLEDs displays, but the imperative mass production technology for micro-LEDs has still not yet been fully developed. A Mini-LED backlight significantly improves the performance of present LED backlight and the cost of the mini-LED is relatively easy to control. Due to the above advantages, the mini-LED runs ahead on the road towards commercialization, compared with micro-LEDs. For the latter, micro-LED, there are two major challenges before insertion in the market, the mass transfer printing, and the full-color solution, which, as introduced in this article, which have been under extensive research. It is reasonable to expect breakthroughs in these areas within a few years, as well as a bright future for micro-LED displays.

Author Contributions: Supervision, H.-C.K. and Z.C.; Writing-Original Draft Preparation, T.W. and S.L.; Project Administration, C.-W.S.; Writing-Review & Editing, Y.L. and C.-F.L.; Resources, Y.L., S.-W.H.C. and W.G. Funding: This research was funded by the Strait Postdoctoral Foundation of Fujian Province, the National Natural Science Foundation of China (61504112, 11604285, and 51605404), the Science and Technology Project between University–Industry Cooperation in Fujian Province (2018H6022), the Technological Innovation Project of Economic and Information Commission of Fujian Province, and the Natural Science Foundation of Fujian Province (2018J01103). Acknowledgments: The authors would like to thank Kei May Lau from HKUST and Zhaojun Liu from SUSTC for the technical support on micro-LED array. Conflicts of Interest: The authors declare no conflict of interest.

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