Improving the Power Efficiency of Micro-LED Displays with Optimized

crystals

Article Improving the Power Eﬃciency of Micro-LED Displays with Optimized LED Chip Sizes

En-Lin Hsiang 1, Ziqian He 1, Yuge Huang 1, Fangwang Gou 1 , Yi-Fen Lan 2 and Shin-Tson Wu 1,*

1 College of Optics and Photonics, University of Central Florida, Orlando, FL 32816, USA; [email protected] (E.-L.H.); [email protected] (Z.H.); [email protected] (Y.H.); [email protected] (F.G.) 2 AU Optronics Corp. Hsinchu Science Park, Hsinchu 300, Taiwan; [email protected] * Correspondence: [email protected]

Received: 22 May 2020; Accepted: 4 June 2020; Published: 8 June 2020

Abstract: Micro-LED (light-emitting diode) is a potentially disruptive display technology, while power consumption is a critical issue for all display devices. In this paper, we develop a physical model to evaluate the power consumption of micro-LED displays under different ambient lighting conditions. Both power efficiency and ambient reflectance are investigated in two types of full color display structures: red/green/blue (RGB) micro-LEDs, and blue-LED pumped quantum dots color-conversion. For each type of display with uniform RGB chip size, our simulation results indicate that there exists an optimal LED chip size, which leads to 30–40% power saving. We then extend our model to analyze different RGB chip sizes, and find that with optimized chip sizes an additional 12% average power saving can be achieved over that with uniform chip size.

Keywords: micro-LED display; color-conversion; power consumption; ambient contrast ratio

1. Introduction Micro-LED displays with high peak luminance, true dark state, high resolution, wide color gamut and long lifetime are emerging as next-generation displays [1–4]. Two approaches are commonly used to achieve full color: red/green/blue (RGB) subpixels and blue-LED pumped color-conversion. In the ﬁrst scheme, RGB micro-LED chips are fabricated from diﬀerent semiconductor materials, according to their lattice constants and energy band gaps. For examples, red LEDs are typically fabricated by growing AlGaInP epilayers on GaAs substrates, while green and blue LEDs are produced by depositing InGaN epilayers on sapphire substrates. In such a RGB display panel, millions of micro-LED chips are transferred from the corresponding semiconductor wafers to the display substrate through mass transfer processes [5–7]. In the color-conversion scheme, we can use UV or blue LEDs to excite the down-conversion materials, such as quantum dots (QDs) or phosphors [8–11]. These color- conversion materials can be fabricated by inject printing or photolithography [12,13]. These micro- LED displays have found potential applications in ultra-large size and seamlessly tiled video walls [14,15], 75-inch modular TVs, medium-size sunlight readable vehicle displays, smart watches, and high-resolution-density microdisplays [16–21] for augmented reality and virtual reality, just to name a few. In addition to above-mentioned properties, low power consumption is always desirable for micro-LED display to compete with its counterparts such as liquid crystal displays (LCDs) and organic LED (OLED) displays [22–24]. Especially for a mobile display, its operating time is governed by the battery capacity. Thus, power consumption is a critical issue for all mobile display devices. Although

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Crystals 2020, 10, x FOR PEER REVIEW 2 of 15 TVs and desktop monitors are usually connected to wall plugs, low power consumption helps to save the ecosystem.Generally, LED’s efficiency decreases as the chip size shrinks due to sidewall defects [25–30]. To improveGenerally, LED’s LED’sefficiency, efficiency several decreases approaches as the have chip been size investigated, shrinks due such to sidewall as enhancing defects the [25 light–30]. Toextraction improve efficiency LED’s effi andciency, boosting several the approaches internal have quantum been efficiinvestigated,ency [31,32 such]. However,as enhancing an the obvious light extractiontradeoff is the effi ciencyincreased and fabrication boosting thecomplicity. internal In quantum addition, e thefficiency external [31 quantum,32]. However, efficiency an (EQE) obvious of tradeoLEDs dependsff is the increased on the driving fabrication current complicity. density. In In order addition, to keep the externaldriving at quantum peak EQE, efficiency pulse (EQE)width ofmodulation LEDs depends (PWM) on is the a c drivingommon current method density. for power In order saving to [14,33,34 keep driving]. In PWM, at peak the EQE, driving pulse current width modulationdensity stays (PWM) at peak is EQE, a common while method the luminance for power is modulated saving [14, 33by,34 changing]. In PWM, the the duty driving ratio currentin each densityframe. stays at peak EQE, while the luminance is modulated by changing the duty ratio in each frame. InIn this this paper, paper, we we evaluate evaluate the power the power consumption consumption of both of types both of types micro-LED of micro displays,-LED including displays, RGBincluding chips RGBand blue chips LEDs and pumped blue LEDs color pumped conversion, color under conversion, different under ambient different lighting ambient conditions. lighting First, weconditions. evaluate First, the power we evaluate consumption the power of uniform consumption LED chip of uniform size, i.e. LED RGB chip subpixels size, i.e. having RGB subpixels the same chiphaving size, the as the same baseline chip for size, comparison. as the baseline The optimum for comparison. LED chip sizes The corresponding optimum LED to chipthe lowest sizes powercorresponding consumption to the for lowest three applications: power consumption smartphones, for laptopthree applications: computers, and smartphones, TVs, are analyzed. laptop Forcomputers, TV applications, and TVs, the are LED analyzed. chip size For studied TV applications, ranges from the 5 LEDµm to chip 50 µsizem. Thestudied optimal ranges LED from chip 5 sizeµm isto found50 µm. to The be 16optimalµm for LED the chip RGB size type is and found 20 µtom be for 16 the µm color for the conversion RGB type type. and At20 theµm optimalfor the color chip size,conversion the power type. saving At the can optimal reach 30–40%.chip size, Next, the power we extend saving our can model reach to 30 evaluate–40%. Next, RGB subpixelswe extend with our dimodelfferent to chip evaluate sizes. Through RGB subpixels the same optimizationwith different procedures, chip sizes. our Through proposed themicro-LED same optimization display with diprocedures,fferent RGB our chip proposed sizes further micro reduces-LED display ~ 12% with average different power RGB consumption chip sizes than further that reduces with uniform ~ 12% LEDaverage chip power size. consumption than that with uniform LED chip size.

2.2. Device ModelingModeling

2.1. RGB Micro-LEDMicro-LED Display

Figure1 1depicts depicts the the device device structure structure of ourof our proposed proposed micro-LED micro-LED display, display, where where W R,W WRG, ,W andG, and WB representWB represent the chipthe chip size ofsize RGB of RGB LEDs, LEDs, respectively. respectively. The gapThe betweengap between micro micro LEDs LEDs is filled is withfilled blackwith matrixblack matrix to reduce to reduce ambient ambient light reflection. light reflection. Because Because of the small of the aperture small aperture ratio of micro-LED ratio of micro displays,-LED thedisplays, circular the polarizer circular polarizer normally no usedrmally in OLEDused in displays OLED displays to reduce to reduce the ambient the ambient light reflection light reflection is not requiredis not required here. here.

Figure 1. Device structure of our proposed RGB micro-LEDmicro-LED display.display.

ItIt isis rarerare that that a a display display device device is usedis used under under a completely a completely dark dark room. room Thus,. Thus, ambient ambient contrast contrast ratio (ratACRio )(ACR) is a more is a more realistic realistic way toway compare to compare the performance the performance of di ffoferent different display display devices. devices. The TheACR ACRof a displayof a display is defined is defined as [35 as]: [35]: Lon + L R ACR = ambient × , (1) LoLLR fon f +L ambientambient  R ACR  × , (1) LLRoff ambient  where Lon (L 0) represents the on (off)-state luminance of the display, L is the ambient luminance, off ≈ ambient andwhere R is L theon ( ambientLoff ≈ 0) light represents reflectance the onwhich (off) depends-state luminance on the surface of the reflectivity. display, FromLambient Equation is the ambient (1), in a 6 darkluminance room, (andLambient R is= the0), anambient emissive light display reflectance can easily which achieve depends over on10 the: 1surface contrast reflectivity. ratio. However, From asEquation the ambient 1, in a luminance dark room increases, (Lambient = the 0), ACRan emissive declines display sharply can [36 ].easily achieve over 10 : 1 contrast ratio.From However, Figure as1, the incidentambient ambientluminanc lighte increases, will be partially the ACR reflected declines by sharply the LED [3 chips,6]. and absorbed by theFrom black Figure matrix. 1, the Thus, incident the total ambient ambient light light will reflection be partially of the reflected display by panel the dependsLED chips, on and the micro-LEDabsorbed by chip the size.black To matrix. investigate Thus, thethe ambienttotal ambient light reflectionlight reflec oftion RGB of micro-LEDthe display displays,panel depends we build on athe ray-tracing micro-LED simulation chip size. model To investigate based on Lightthe ambient Tools. Thelight device reflection structure of RGB of flip-chipmicro-LED RGB displays, micro-LED we build a ray-tracing simulation model based on Light Tools. The device structure of flip-chip RGB micro-LED is similar to that reported in [37]. The device material characteristics of the flip-chip RGB

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micro-LEDs are summarized in Table 1, where n and k represent the real part and imaginary part of the refractive index of the corresponding material [38,39].

Table 1. Material parameters used in RGB micro-LED display simulations.

626 nm 529 nm 465 nm Material Parameters n k n k n k Molding layer 1.48 0 1.49 0 1.50 0 Red LED chip 3.30 0 3.56 0.16 3.76 0.28

Blue/Green LED chip 2.35 4 × 10−5 2.34 4 × 10−5 2.42 4 × 10−5 Crystals 2020, 10, 494 3 of 15 Bounding metal 0.15 3.52 0.44 2.29 1.43 1.85 Glass substrate 1.5 0 1.5 0 1.5 0 is similar to that reported in [37]. The device material characteristics of the ﬂip-chip RGB micro-LEDs are summarized in Table1, where n and k represent the real part and imaginary part of the refractive Let us assume the ambient light is a standard illuminant D65 white light. The intensity spectrum indexof ambient of the correspondinglight and reflected material light [by38 ,RGB39]. micro-LEDs are shown in Figure 2a. In the 400 nm to 560 nm blue-green region, the absorption of AlGaInP-based red micro-LED is much stronger than that of Table 1. Material parameters used in RGB micro-LED display simulations. InGaN-based green/blue micro-LEDs, as Table 1 shows. The luminance ambient reflectance626 nmcan be calculated 529 from nm 465 nm Material Parameters nI k()() K  n  d  k n k  reflect Molding layer 1.48RL  0 1.49, 0 1.50 0 (2) I()() K   d  Red LED chip 3.30 ambient 0 3.56 0.16 3.76 0.28 Blue/Green LED chip 2.35 4 10 5 2.34 4 10 5 2.42 4 10 5 × − × − × − where I() isBounding the radiant metal flux and 0.15 K() represents 3.52 the 0.44photopic human 2.29 eye 1.43 sensitivity 1.85 function. From Equation 2, Glasswe find substrate the luminance 1.5 ambient reflectance 0 1.5 of the AlGaInP 0-based 1.5 red micro 0-LED is 29.94% and the InGaN-based blue/green micro-LEDs is 66.07%. As mentioned above, the ambient light is partiallyLet us reflected assume theat the ambient LED lightchip isarea a standard but is absorbed illuminant in D65the whiteblack light.matrix The region. intensity Therefore, spectrum the ofluminance ambient light ambient and reﬂectedreflectance light is proportional by RGB micro-LEDs to the aperture are shown ratio in of Figure the micro2a. In-LED the 400 display, nm to as 560plotted nm blue-green in Figure region,2b. At a the given absorption aperture of ratio, AlGaInP-based the lumina rednce micro-LEDambient reflectance is much stronger of blue and than green that ofLEDs InGaN-based is about 2.2x green stronger/blue micro-LEDs, than that of asred Table LED.1 shows.

FigureFigure 2. 2.( a()a) The The intensity intensity spectrum spectrum of of ambient ambient light, light, ambient ambient light light reflected reflected by by AlGaInP AlGaInP based based LED, LED, andand ambient ambient light light reflected reflected by by InGaN InGaN based based LED. LED. ( b ()b) The The luminance luminance ambient ambient reflectance reflectance of of RGB RGB micro-LEDsmicro-LEDs at at di differentfferent LED LED chip chip aperture aperture ratio. ratio.

TheFrom luminance Equation ambient 1, if the reflectance off-state can luminance be calculated of a display from is zero (Loff = 0), then the on-state luminance can be expressed as: R Ire f lect(λ) K(λ) dλ RL = R · · , (2) I (λ) K(λ) dλ ambient · · whereCrystalsI 2020(λ), is10, the x; doi: radiant FOR PEER flux REVIEW and K (λ) represents the photopic humanwww.mdpi.com/journal/ eye sensitivity function.crystals From Equation (2), we find the luminance ambient reflectance of the AlGaInP-based red micro-LED is 29.94% and the InGaN-based blue/green micro-LEDs is 66.07%. As mentioned above, the ambient light is partially reflected at the LED chip area but is absorbed in the black matrix region. Therefore, the luminance ambient reflectance is proportional to the aperture ratio of the micro-LED display, as plotted in Figure2b. At a given aperture ratio, the luminance ambient reflectance of blue and green LEDs is about 2.2x stronger than that of red LED. From Equation (1), if the off-state luminance of a display is zero (Loff = 0), then the on-state luminance can be expressed as:    X  L = L Rs + (1 Rs) R AP  (ACR 1), (3) display ambient ×  − × L(i) × (i) × − i=RGB Crystals 2020, 10, x FOR PEER REVIEW 4 of 15

  LLRRRAPACRdisplay ambient  s (1  s )  L()() i  i   (  1), (3) i RGB  where RCrystalsS is the2020 , 10surface, 494 ambient reflectance from the cover glass, RL(RGB) is the luminance4 of 15 ambient reflectance from RGB LEDs respectively when the aperture ratio is 1, and AP(RGB) is the aperture ratio of RGB LEDs. For touch-panel smartphones and laptop computers, the cover glass usually does not where RS is the surface ambient reflectance from the cover glass, RL(RGB) is the luminance ambient have antireflectance-reflection from (AR) RGB LEDscoating. respectively Thus, we when assume the aperture their ratio surface is 1, and reflectionAP(RGB) is is the around aperture 4%. ratio However, most ofof TVs RGB use LEDs. remote For touch-panel control so smartphonesthat we can and apply laptop AR computers, coating to the reduce cover glassthe surface usually doesreflection. not Here, we assumehave their anti-reflection surface (AR)reflection coating. is Thus, 1.2%. we Besides, assume theirthe ambient surface reflection light illuminance is around 4%. could However, vary wildly most of TVs use remote control so that we can apply AR coating to reduce the surface reflection. depending on the environment lighting conditions, e.g. direct sunlight, office light, and living room Here, we assume their surface reflection is 1.2%. Besides, the ambient light illuminance could vary light. Fromwildly Equation depending 3,on theto achieveenvironment the lighting same conditions, ACR, the e.g., display direct sunlight, luminance office should light, and increase living as the ambientroom reflectance light. From increases, Equation (3),which to achieve in turn the is same proportional ACR, the display to the luminance aperture should ratio. increase That asis to say, a smallerthe aperture ambient ratio reflectance in the increases, RGB micro which-LED in turn disp islay proportional and a lower to the ambient aperture ratio.reflectance That is towould say, lead to a lowera display smaller aperture luminance ratio infor the the RGB same micro-LED ACR. displayThe display and a lower luminance ambient reflectanceas a function would of lead aperture to ratio a lower display luminance for the same ACR. The display luminance as a function of aperture ratio under three different ambient illuminances: 2000 lux for outdoor, 450 lux for offices, and 200 lux for under three different ambient illuminances: 2000 lux for outdoor, 450 lux for offices, and 200 lux for living rooms,living rooms, is plotted is plotted in Figure in Figure 3.3 .

Figure Figure3. The 3.requiredThe required luminance luminance of of display display,, asas a a function function of apertureof aperture ratio, ratio for achieving, for achieving the listed the listed ACR underACR underthree threeambient ambient light light illuminances: illuminances: ((aa)) 2000 2000 lux, lux, (b) ( 450b) 450 lux, andlux, ( cand) 200 ( lux.c) 200 lux. Meanwhile, the power consumption of the display is also affected by the efficiency of LED chips. MeanwhileThe power, e thefficiency power of LEDs consumption is defined as of the the ratio display of luminance is also intensity affecte overd by the the power efficiency consumption of LED chips. The power(cd/W) efficiency as: of LEDs is defined as the ratio of luminance intensity over the power ΦRGB/αRGB EQERGB Tsys hνRGB KRGB consumption (cd/W) as: ηRGB = = × × × . (4) PLED q VRGB αRGB EQE×  T×  h  K In Equation (4), η stands forRGB the/  luminance RGB effiRGBciency, sysΦ the RGB luminance RGB flux, P the power RGB   . (4) consumption of LED, q the elementaryPLED charge, hv the q photon V RGB  energy, RGB K the luminance efficacy, α conversion efficiency from luminance intensity [unit: cd] to luminous flux Φ [unit:lm], and V In the Equation driving voltage 4, η stands of LED. for Since the PWM luminance driving scheme efficiency, is employed  the in our luminance simulations, flux, the P EQE the power consumptionin Equation of LED, (4) represents q the elementary the peak EQE charge, of the hv LED the whilephoton the energy, voltage isK fixed the atluminance the optimal efficacy, α conversiondriving efficiency condition. from luminance intensity [unit: cd] to luminous flux Φ [unit : lm], and V the Generally, the peak EQE of LED in Equation (4) depends on the chip size. Therefore, to investigate drivingthe voltage power of effi LED.ciency Since of diff erent PWM LED driving chip sizes, scheme we have is employed to take this in peak our EQE simulations, variation into the EQE in Equationconsideration. 4 represents The the peak peak EQE EQE deceases of the as theLED micro-LED while the chip voltage size decreases, is fixed resulting at the fromoptimal the driving condition.nonradiative recombination at the etched sidewall. The ratio of sidewall surface to the total surface Generally,increases the as the peak LED EQE chip of size LED shrinks. in Equation As a result, 4 depends the sidewall on the eff ectchip becomes size. Therefore, more significant to investigate µ the powerwhen efficiency the LED chip of size different is smaller LED than chip ~ 100 sizes,m. weThe chiphave size to dependent take this e peakfficiency EQE of InGaN variation into based micro-LEDs has been widely discussed in [25–28]. Because AlGaInP exhibits a higher surface consideration.recombination The velocity peak EQE than InGaN, deceases the e asfficiency the micro drop of-LED red micro-LED chip size is more decreases, serious than resulting the blue from the nonradiativeand green recombination ones as the chip at size the decreases etched [sidewall.29]. Detailed The theoretical ratio of analyses sidewall and surface experimental to the results total surface increaseshave as beenthe LED reported chip in size [30]. s Usinghrinks. these As published a result, results, the sidewall we plot theeffect peak becomes EQE as a functionmore significant of LED when the LEDchip chip size size for is RGB smaller LEDs inthan Figure ~ 1004. µm. The chip size dependent efficiency of InGaN based micro- LEDs has been widely discussed in [25–28]. Because AlGaInP exhibits a higher surface recombination velocity than InGaN, the efficiency drop of red micro-LED is more serious than the blue and green ones as the chip size decreases [29]. Detailed theoretical analyses and experimental results have been reported in [30]. Using these published results, we plot the peak EQE as a function of LED chip size for RGB LEDs in Figure 4.

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0.5 0.5

0.4 0.4

0.3 Red LED 0.3 Red LED Green LED Green LED Blue LED 0.20.2 Blue LED

0.10.1

00 00 2020 4040 6060 8080 100100 Chip size ( m) Chip size ( m) FigureFigureFigure 4. 4. 4. TheThe The peakpeak peak EQEEQE EQE ofof of RGB RGB micro-LEDmicro micro-LED-LED as as a afunction function of of LED LED LED chip chip chip size. size. size.

2.2.2.2.2.2. Color-Conversion ColorColor--CConversiononversion Micro-LEDMicro Micro-LED-LED DisplayD Displayisplay FigureFigureFigure5 55illustrates illustratesillustrates thethe the device device structure structurestructure of of color color conversion conversion conversion micro micro micro-LED-LED-LED displays. displays. displays. The The QD The QD color QD color colorconversionconversion conversion layerlayer layer isis depositeddeposited is deposited above above abovethe the blue blue the micro bluemicro- micro-LEDLED-LED array array in array in green green in and green and red red and subpixels. subpixels. red subpixels. These These Thesequantumquantum quantum dotsdots have dotshave have isotropicisotropic isotropic scattering. scattering. scattering. Thus, Thus, Thus, to to match match to match the the same thesame same scattering scattering scattering property, property, property, we we also we also alsofill fill fillscatteringscattering scattering particleparticle particlesss in in the inthe theblue blue blue subpixels subpixels subpixels to to make tomake make the the angular the angular angular spectrum spectrum spectrum of of RGB RGB of RGBlights lights lights consistent. consistent. consistent. The The Thecolorcolor color filterfilter filter arrayarray array isis alignedaligned is aligned with with with the the QD theQD layer QD layer layer to to prevent prevent to prevent the the ambient theambient ambient light light lightexcitation excitation excitation and and the and the blue theblue bluelightlight light leakage.leakage. leakage. The The black Theblack black matrix matrix matrix filled filled filled between between between LEDs LEDsLEDs helps helps helpssuppress suppress suppress ambie ambie ambientntnt light light reflection light reflection reflection and and color andcolor colorcrosstalkcrosstalk crosstalk [4[400].]. [TheThe40]. reflected reflected The reflected ambient ambient ambient light light consists lightconsists consists of of two two of major major two majorparts. parts. (1) parts. (1) QD QD (1)excitation excitation QD excitation (I emit(Iemit). When). ( IWhenemit). Whenthethe ambientambient the ambient lightlight lightis is incident incident is incident on on the the on QD theQD layer, QDlayer, layer, a aportion portion a portion is is absorbed absorbed is absorbed and and anddown down down-converted-converted-converted to togreen to green green or or orredred red lightlight light byby by thethe the QDQD QD m m materials.aterials.aterials. The The The down down down-converted-converted-converted light light escapes escapes from fromfrom the thethe QD QDQD layer layer layer and and and enters enters enters the the the air.air.air. From FromFrom Figure FigureFigure5 ,5, 5, the the the ambient ambient ambient light light light traverses traverses traverses through through through thethe the QDQD QD layerlayer layer twice.twice. twice. Therefore,Therefore, Therefore, the the excitationexcitatio excitation n cancancan occur occuroccur in in in both both both traveling traveling traveling routes. routes. routes. (2)(2) (2) ReflectionReflection Reflection fromfrom from LEDLED LED chipschips chips ((II RR(I).R ).As As As Fig Figure Figureure 5 depicts,5 depicts, the the incident incident incident ambientambientambient light, light,light, which whichwhich is isis not notnot completelycompletely completely absorbedabsorbed absorbed by by the the QD QD material, material, is is isreflected reflected reflected back back back by by by the the the blue blue blue micro-LEDmicromicro--LEDLED chip. chip.chip.

Figure 5. Device structure of our color-converted micro-LED display. FigureFigure 5. 5. Device Device structure structure of of our our color color-converted-converted micro micro-LED-LED display display. . As Equation (3) illustrates, in order to define the luminance of display at different ambient AsAs Equation Equation 3 3 illustrates, illustrates, in in order order to to define define the the luminance luminance of of display display at at different different ambient ambient conditions, we have to know the luminance ambient reflectance of display. Therefore, we build a conditions,conditions, wewe havehave toto knowknow the the luminance luminance ambient ambient reflectance reflectance of of display. display. Therefore, Therefore, we we build build a a LightLight Tools Tools raytracing raytracing model model to to quantitativelyquantitatively analyzeanalyze thethe luminance ambient reflectance reflectance [4 [411].]. In In the the Light Tools raytracing model to quantitatively analyze the luminance ambient reflectance [41]. In the model,model, the the mean mean path path methodmethod isis usedused toto simulatesimulate thethe photo-luminancephoto-luminance of QD materials. The The mean mean model, the mean path method is used to simulate the photo-luminance of QD materials. The mean path represents the average distance that a ray propagates before it hits the QD particle [42,43]. In the path represents the average distance that a ray propagates before it hits the QD particle [42,43]. In the Crystals 2020, 10, x; doi: FOR PEER REVIEW www.mdpi.com/journal/crystals Crystals 2020, 10, x; doi: FOR PEER REVIEW www.mdpi.com/journal/crystals Crystals 2020, 10, 494 6 of 15 Crystals 2020, 10, x FOR PEER REVIEW 6 of 15 Crystals 2020, 10, x FOR PEER REVIEW 6 of 15 pathsimulations, represents we the define average the mean distance path that value, a ray wh propagatesich can totally before absorb it hits thethe QDblue particle light emitted [42,43]. by In the simulations,simulations,micro-LED. weBecause we define define the the the QDs mean mean are path pathnanometer value, value, -whsized whichich particles,can can totally totally the absorb absorb ray does the the bluenot blue change light light emitted emitteddirection by by afterthe the micromicro-LED.hitting-LED. the QDBecause Because particle. the the QDsIn QDs addition, are are nanometer nanometer-sized the down-sized-converted particles, particles, light the the has ray ray isotropic does does not not scattering. change change direction direction The material after after hittinghittingproperties the the QD of QD blue particle. particle. LED Inare In addition, addition,listed in Tablethe the down down-converted 1. We-converted plot the spectralight light has has of isotropicRGB isotropic color scattering. scattering. filters in FigureThe The material material 6a. The propropertiesemissionperties andof of blue blueabsorption LED LED are are spectra listed listed in of inTable QD Table materials 1. 1We. We plot are plot the depicted thespectra spectra inof solidRGB of RGB andcolor colordashed filters filters inlines Figure in in FigureFigure 6a. Th6 6b.a.e emissionThe emissionphotoluminescence and absorption and absorption spectraquantum spectra of yieldQD of QDmaterials (PLQY) materials isare 88% aredepicted for depicted green in solid inQD solid and and and 90% dashed dashed for redlines lines QD in in[4Figure Figure4]. 6b.6b. TheThe photoluminescence photoluminescence quantum quantum yield yield (PLQY) (PLQY) is is 88% 88% for for green green QD QD and and 90% 90% for for red red QD QD [4 [444].].

Figure 6. (a) The transmittance spectra of color filters and (b) the emission and absorption spectra of FigureFiguregreen 6.and 6. (a( )ared )The The quantum transmittance transmittance dots. spectra spectra of of color color filters filters and and (b (b) )the the emission emission and and absorption absorption spectra spectra of of greengreen and and red red quantum quantum dots. dots. Figure 7 shows the calculated intensity of ambient light and reflected ambient light from RGB subpixels.FigureFigure The77 showsshows reflected the the calculated calculatedlight is separated intensity intensity into of of twoambient ambient categories: light light and and QD reflected reflectedexcitation ambient ambient (Iemit) and light light reflected f fromrom RGB RGBlight subpixels.subpixels.without QD The The absorption reflected reflected light (IR light). Withoutis separated is separated luminescent into into two two material,categories: categories: the QD Iemit excitation QD is zero excitation in (I blueemit) ( andsubpixels.Iemit )reflected and reflectedWe light only withoutlightshow withoutIR QD in Figure absorption QD 7a. absorption For (IR green). Without (I Rsubpix). Without luminescentels, because luminescent material, the cutoff material, the wavelength Iemit is the zeroIemit inof isbluegreen zero subpixels. color in blue filter subpixels.We is only~ 450 showWenm, only someIR inshow Figure of theIR 7a.bluein For Figure and green most7a. subpix Forof the greenels, green because subpixels, light thecan becausecutoff transmit wavelength the through cuto ff theofwavelength green green color color offilter filters. green is ~ Fromcolor 450 nm,filterFigure some is 6b, ~450 of the the nm, transmitted blue some and of most thegreen blue of light the and greencan most still light of excite the can green thetransmit green light throughQD can materials. transmit the green throughTherefore, color the filters. the green reflected From color Figurefilters.light is 6b, Fromdominated the Figuretransmitted by6b, Iemit the green as transmitted Figure light 7b can greendepicts. still lightexcite For can redthe still greensubpixels, excite QD the materials.the green cutoff QD Therefore,wavelength materials. the of Therefore, reflected red color lightthefilter reflectedis is dominated ~ 570 light nm, isbythus, dominated Iemit the as Figuretransmitted by I7bemit depicts.as light Figure canFor7b redhardly depicts. subpixels, excite For red thethe subpixels, QDcutoff materials. wavelength the cuto Therefore,ff ofwavelength red colorIemit is filterofsmall red is and color~ 570 IR filter dominatesnm, isthus, ~570 the nm, reflectransmitted thus,ted the light transmitted light as Figurecan hardly light 7c shows. can excite hardly the excite QD materials. the QD materials. Therefore, Therefore, Iemit is smallIemit is and small IR dominates and IR dominates the reflec theted reflected light as lightFigure as 7c Figure shows.7c shows.

Figure 7. Calculated intensity spectrum of ambient light, light, reflected reflected light for ( (a)) blue, blue, ( (b)) green green and and ( c) Figurered subpixels.subpixel 7. Calculateds. intensity spectrum of ambient light, reflected light for (a) blue, (b) green and (c) red subpixels. From Equation (2), we find the luminance ambient reflectance of red, green and blue subpixels From Equation 2, we find the luminance ambient reflectance of red, green and blue subpixels is is 6.83%, 16.73%, and 2.88%, respectively. Because the ambient light is reflected in the emission area 6.83%,From 16.73%, Equation and 2.88%,2, we find respectively. the luminance Because ambient the ambient reflectance light ofis reflectedred, green in and the blueemission subpixels area but is but is absorbed in the black matrix region, the luminance ambient reflectance is proportional to the 6.83%,is absorbed 16.73%, in and the 2.88%, black respectively. matrix region, Because the luminance the ambient ambient light is reflectancereflected in isthe proportional emission area to but the aperture ratio of the micro-LED display. The relationship between aperture ratio and luminance isaperture absorbed ratio in the of the black micro matrix-LED region, display. the The luminance relationsh ambientip between reflectance aperture is ratioproportional and luminance to the ambient reflectance of color conversion micro-LED display is shown in Figure8. apertureambient ratioreflectance of the of micro color-LED conversion display. micro The- LED relationsh displayip is between shown aperturein Figure ratio8. and luminance ambient reflectance of color conversion micro-LED display is shown in Figure 8.

Crystals 2020, 10, x; doi: FOR PEER REVIEW www.mdpi.com/journal/crystals Crystals 2020, 10, x; doi: FOR PEER REVIEW www.mdpi.com/journal/crystals Crystals 2020, 10, x FOR PEER REVIEW 7 of 15

Crystals 2020, 10, 494 7 of 15 Crystals 2020, 10, x FOR PEER REVIEW 7 of 15

. . Figure 8. The luminance ambient reflectance of RGB subpixels in color-conversion micro-LED display at differentFigureFigure 8. aperture 8.The The luminance luminance ratio. ambient ambient reflectance reflectance of of RGB RGB subpixels subpixels in in color-conversion color-conversion micro-LED micro-LED display display atat di ffdifferenterent aperture aperture ratio. ratio. By substituting the luminance ambient reflectance from Figure 8 into Equation 3, we calculate By substituting the luminance ambient reflectance from Figure8 into Equation (3), we calculate the the displayBy luminance substituting as thea function luminance of apertureambient reflectance ratio under from three Fig differenture 8 into ambient Equation illuminances: 3, we calculate 200 0 displaythe display luminance luminance as a function as a function of aperture of aperture ratio underratio under three dithreefferent different ambient ambient illuminances: illuminances: 2000 lux2000 lux for outdoor, 450 lux for offices and 200 lux for living rooms. Results are plotted in Figure 9. forlux outdoor, for outdoor, 450 lux 450 for lux offi forces offices and 200 and lux 200 for lux living for living rooms. ro Resultsoms. Results are plotted are plotted in Figure in Fig9. ure 9.

Figure 9. The required luminance of color-conversion micro-LED display as a function of aperture FigureFigure 9. The 9. The required required luminance luminance of of color-conversion color-conversion micro-LED micro- displayLED display as a function as a function of aperture of ratio aperture ratio for achieving the listed ACR under three ambient light illuminances: (a) 2000 lux, (b) 450 lux and for achieving the listed ACR under three ambient light illuminances: (a) 2000 lux, (b) 450 lux and (c) ratio for(c) achieving200 lux. the listed ACR under three ambient light illuminances: (a) 2000 lux, (b) 450 lux and (c) 200200 lux.

TheThe power power effi efficiencyciency of of RGB RGB subpixels subpixels in in color-conversion color-conversion micro-LED micro-LED display display is is defined defined as as the the The power efficiency of RGB subpixels in color-conversion micro-LED display is defined as the ratioratio of of luminance luminance intensity intensity over over the the power power consumption consumption (cd (cd/W/W), as:), as: ratio of luminance intensity over the power consumption (cd/W), as:  RGB/  RGB EQEB PLQY RG  LEE RG  T sys  h RGB  K RGB RGB EQE  PLQY LEE Tsys hν K . (5) ΦRGB/αRGBPB RG q V RG  RGB RGB ηRGB =  RGB/ LED RGB= EQE×B PLQY RG×  LEE RGB RG×  RGB T sys×  h RGB × K RGB . (5) RGB PLED  q VRGB αRGB . (5) P q× V ×  In Equation 5, PLQY isLED the photoluminescence quantum RGB yield RGB of QD materials, and LEE is the PLQY lightIn extraction Equation (5), efficiencyis of the QD photoluminescence film. The definition quantum of other yieldparameters of QD are materials, the same and as LEE Equation is the 4. In Equation 5, PLQY is the photoluminescence quantum yield of QD materials, and LEE is the lightUnlike extraction a RGB e ffi microciency-LED of QD display, film. The which definition uses different of other parameters LED semiconductor are the same materials, as Equation the color(4). light extraction efficiency of QD film. The definition of other parameters are the same as Equation 4. Unlikeconversion a RGB micro micro-LED-LED display display, generates which uses full di colorfferent by LED using semiconductor blue LEDs to materials, pump QD the materials. color UnlikeconversionBesides, a RGB the micro-LED micro color- LEDconversion display display, generates efficiency which full of usescolorQD material by different using remains blue LED LEDs semiconductorthe to same pump and QD is materials. independent materials, Besides, theof the color conversiontheemission color micro conversion area.-LED As a e ffi displayresult,ciency in of generatesa QDcolor material conversion full remains color micro by the- LED using same display, and blue is independent LEDsRGB subpixels to pump of thehave QD emission the materials. same Besides,area.peak the As EQE acolor result, decreasing conversion in a color trend, conversion efficiency which is micro-LED ofdetermined QD material display, by the remains RGBblue subpixelsLED the propertysame have and shown the is sameindependent in peakFigure EQE 4. ofIn the emissiondecreasingcontrast, area. in trend, As a RGBa whichresult, micro is in- determinedLED a color display, conversion by as the the blue LED micro LED chip property- sizeLED decreases, display, shown inRGB Figure subpixelssubpixels4. In contrast, have have different inthe a same peakRGB EQEdecreasing micro-LED decreasing trends display, intrend, peak as whichEQE. the LED is chipdetermined size decreases, by the RGB blue subpixels LED property have di shownfferent decreasingin Figure 4. In trends in peak EQE. contrast, in a RGB micro-LED display, as the LED chip size decreases, RGB subpixels have different decreasing3. Simulation trends inResults peak andEQE. Discussion

3.1. Optimization Process of Uniform LED Chip Size in RGB Subpixels 3. Simulation Results and Discussion Crystals 2020, 10, x; doi: FOR PEER REVIEW www.mdpi.com/journal/crystals 3.1. Optimization Process of Uniform LED Chip Size in RGB Subpixels

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3. Simulation Results and Discussion

3.1. Optimization Process of Uniform LED Chip Size in RGB Subpixels From Figure4, the power e fficiency of micro-LED display decreases as the chip size decreases. Therefore, a larger LED chip size is helpful to enhance the power efficiency. However, as shown in Equation (3), the micro-LED display with a larger LED chip size needs to deliver a higher luminance to maintain the same ACR because of its higher reflectance. In order to find the optimal LED chip size with minimum power consumption for white (D65) image content, in the following, we define two functions f (x) and g(x) to describe the power efficiency and luminance intensity of micro-LED display at different LED chip sizes, respectively. Because the power consumption is defined as the luminance intensity divided by power efficiency, we can find the optimal LED chip size when the ratio of g(x) to f (x) has a minimum. Detail of this function is discussed as follows. The luminance intensity of a display is the product of display luminance (Equation (3)) and display area. We define g(x), to describe the luminance intensity of display at different LED chip sizes, as:   X x 2  (i)  2 g(x) = L A = L Rs + (1 Rs) R ( )  (ACR 1) p N, (6) display × display ambient ×  − × L i × p2  × − × × i=RGB where xRGB is the LED chip size of RGB subpixels, p is the pixel width, Rs is the surface ambient reflectance of the cover glass, RL(RGB) is the luminance ambient reflectance from RGB subpixels respectively when aperture ratio is 1, Lambient is the ambient luminance, and N is the number of pixels. In order to define the function g(x) for RGB micro-LED display and color conversion micro-LED display, we have to substitute the corresponding RL to Equation (6). Because the power efficiency of RGB subpixels is different, thus the efficiency of display strongly depends on the image contents. Moreover, different colors can be obtained by mixing the ratios of RGB primaries. Therefore, the power efficiency of a mixed color can be determined by

1 1 γR γG γB = = + + , (7) f (x) ηpixel ηR ηG ηB where η is the power eﬃciency and γ represents the luminance intensity ratio of RGB primaries.

3.1.1. RGB Micro-LED Display In our RGB micro-LED model, the D65 white light consists of around 25% red, 68% green, and 7% blue. Based on Equations (4) and (7), we depict the power efficiency of each RGB micro-LED and the power efficiency of display at white image content as a function of LED chip size in Figure 10. As Figure 10 shows, the power efficiency of micro-LED display with different LED chip sizes displaying white image content can be represented by the function f (x) (black curve). Here, the variable x represents the LED chip size. In the following, we investigate the chip size effect on the power consumption of RGB micro-LED display. In this study, we assume the RGB subpixels have the same chip size. By varying the chip size from 5 µm to 50 µm, we hope to find an optimal chip size for the lowest power consumption. Three kinds of applications are evaluated: smartphone, laptop computer, and TV. Table2 lists the specifications of each application. For smartphone at outdoor sunlight (2000 lux), laptop at office (450 lux), and TV at family room (200 lux) lighting conditions, the specified ACR=30:1, 120:1, and 800:1 should give excellent readability. Here, we take the specific ambient conditions listed in Table2 as examples. In practice, the optimized results are still valid in all ambient conditions. This is because, in Equation (6), the parameters outside the square brackets including ambient light intensity and ACR can be treated as a constant and be normalized. As a result, the normalized illuminance function g(x) is still the same for different ambient conditions. CrystalsCrystals2020 2020, 10, ,10 494, x FOR PEER REVIEW 9 of9 15of 15 Normalized Power efficiency(cd/W)

FigureFigure 10. 10.Chip Chip size-dependent size-dependent power power eﬃ efficiencyciency of of RGB RGB micro-LED micro-LED display. display.

In the following,Table we 2. The investigate specifications the chip of smartphone, size effect laptop on the and power TV evaluated. consumption of RGB micro- LED display. In this study, we assume the RGB subpixels have the same chip size. By varying the chip size from 5 µmApplications to 50 µm, we hope to Smartphone find an optimal chip Laptop size for the lowest TV power consumption. Resolution 2688 1242 3840 2160 3840 2160 Three kinds of applications are evaluated: smartphone,× laptop× computer, and× TV. Table 2 lists the specifications of eachPixel application. size For smartphone 55.45 µm at outdoor 90 µsunlightm (2000 373 lux),µm laptop at office (450 lux), and TV atAmbient familyilluminance room (200 lux) lighting 2000 lux conditions, 450 the lux specified ACR=30:1, 200 lux 120:1, and 800:1 should give excellent readability. Here, we take the specific ambient conditions listed in Table 2 as ACR 30 120 800 examples. In practice, the optimized results are still valid in all ambient conditions. This is because, Display surface reflection 4% 4% 1.2% in Equation 6, the parameters outside the square brackets including ambient light intensity and ACR can be treated as a constant and be normalized. As a result, the normalized illuminance function g(x) is stillFigure the 11same depicts for different the function ambient of f (conditions.x), g(x), and g(x)/f (x) as micro-LED chip size increases from 5 µm to 50 µm for TV applications. As the micro-LED chip size increases, the power consumption [g(x)/f (x)] decreases firstTable and 2. The then specifications bounces back. of smartpho The optimalne, laptop LED and chip TV sizeevaluated. with minimum power consumption takes place at 16 µm (Figure 11c). The power saving at this optimum LED chip size over Applications Smartphone laptop TV that at 50 µm chip size is 48% and over that at 5 µm is 32%. These results manifest the advantage of using optimized LED chip size. With the same analysis process, we find the optimal LED chip size for Resolution 2688 × 1242 3840 × 2160 3840 × 2160 smartphoneCrystals 2020, 10 is, 6x FORµm, PEER and REVIEW for laptop is 8 µm. 10 of 15 Pixel size 55.45 µm 90 µm 373 µm Ambient illuminance 2000 lux 450 lux 200 lux ACR 30 120 800

Display surface reflection 4% 4% 1.2%

Figure 11 depicts the function of f(x), g(x), and g(x)/f(x) as micro-LED chip size increases from 5 µm to 50 µm for TV applications. As the micro-LED chip size increases, the power consumption [g(x)/f(x)] decreases first and then bounces back. The optimal LED chip size with minimum power consumption takes place at 16 µm (Figure 11(c)). The power saving at this optimum LED chip size FigureFigure 11. 11.Normalized Normalized ( a(a)) power power e efficiencyfficiency fitting fitting function functionf f(x),(x), ( (bb)) luminance luminance intensity intensity functionfunctiong g(x),(x), over that at 50 µm chip size is 48% and over that at 5 µm is 32%. These results manifest the advantage andand ( c()c) power power consumption consumption function functiong (g(x)/f(x)x)/f (x) at at di differentfferent LED LED chip chip sizes. sizes. of using optimized LED chip size. With the same analysis process, we find the optimal LED chip size 3.1.2.3.1.2for smartphone. Color-Conversion Color-Conversion is 6 µm, Micro-LED Micro and for-LED laptop Display Display is 8 µm. InIn our our colorcolor conversion micro micro-LED-LED display, display, the the D65 D65 white white light light consists consists of around of around 30% 30% red, red,61% 61%green, green, and and 9% 9%blue. blue. These These ratios ratios are are slightly slightly different different from from those those for for RGB micro-LEDsmicro-LEDs becausebecause thethe emission spectra of red and green QD materials are different from those of red and green micro- LEDs.Crystals From 2020, 10Equations, x; doi: FOR 5 PEERand 7,REVIEW the power efficiency of each individual RGBwww.mdpi.com/jou subpixel and thernal/crystals power efficiency of the display at white image content as a function of LED chip size is shown in Figure 12. Because only blue LED is applied in color conversion micro-LED and the color conversion efficiency of QD is independent of the emission area; the RGB subpixels have the same decreasing trend in power efficiency at different LED chip sizes. From Equation 7, the same decreasing trend in RGB subpixels leads to the same power-efficiency decreasing trend for all the colors. The power efficiency of color conversion micro-LED display with different LED chip sizes displaying white image content can be described by the function f(x) (black curve) shown in Figure 12. Here, the variable x represents the LED chip size.

Figure 12. LED chip size-dependent power efficiency of color-conversion micro-LED display.

We also evaluate the applications listed in Table 2 in our color conversion micro-LED model. We depicts the function of f(x), g(x), and g(x)/f(x) as micro-LED chip size increases from 5 µm to 50 µm for TV applications in Figure 13. As the micro-LED chip size increases, the power consumption [g(x)/f(x)] decreases first and then bounces back. The optimal LED chip size with minimum power consumption takes place at 20 µm. The power saving at this optimum LED chip size over that of 50

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Figure 11. Normalized (a) power efficiency fitting function f(x), (b) luminance intensity function g(x), and (c) power consumption function g(x)/f(x) at different LED chip sizes.

3.1.2. Color-Conversion Micro-LED Display

Crystals 2020In our, 10, color 494 conversion micro-LED display, the D65 white light consists of around 30% red,10 of 61% 15 green, and 9% blue. These ratios are slightly different from those for RGB micro-LEDs because the emission spectra of red and green QD materials are different from those of red and green micro- emissionLEDs. From spectra Equations of red and 5 and green 7, the QD power materials efficiency are diff oferent each from individual those of RGB red andsubpixel green and micro-LEDs. the power Fromefficiency Equations of the (5) display and (7), at white the power image e fficontentciency as of a eachfunction individual of LED RGBchip subpixelsize is shown and in the Figure power 12. effiBecauseciency ofonly the blue display LED at is white applied image in color content conversion as a function micro of-LED LED and chip the size color is shownconversion in Figure efficiency 12. Becauseof QD onlyis independent blue LED is of applied the emission in color conversionarea; the RGB micro-LED subpixels and have the colorthe same conversion decreasing efficiency trend of in QDpower is independent efficiency ofat thedifferent emission LED area; chip the sizes. RGB Fr subpixelsom Equation have 7, the the same same decreasing decreasing trend trend in powerin RGB effisubpixelsciency at leads different to the LED same chip power sizes.-efficiency From Equation decreasing (7), the trend same for decreasing all the colors. trend The in power RGB subpixels efficiency leadsof color to the conversion same power-e microffi-LEDciency display decreasing with different trend for LED all the chip colors. sizes displaying The power white efficiency image of content color conversioncan be described micro-LED by the display function with f(x di) (blackfferent curve) LED chip shown sizes in displayingFigure 12. Here, white the image variable content x represents can be describedthe LED bychip the size. function f (x) (black curve) shown in Figure 12. Here, the variable x represents the LED chip size.

FigureFigure 12. 12.LED LED chip chip size-dependent size-dependent power power eﬃ efficiencyciency of of color-conversion color-conversion micro-LED micro-LED display. display.

WeWe also also evaluate evaluate the the applications applications listedlisted in Table 2 2 inin our color color conversion conversion micro micro-LED-LED model. model. We Wedepicts depicts the the function function of off(x),f ( xg(),xg),( xand), and g(x)/f(x)g(x)/f as(x) micro as micro-LED-LED chip chip size sizeincreases increases from from 5 µm 5 toµ m50 toµm 50forµm TV for applications TV applications in Figure in Figure 13. 13 As. As the the micro micro-LED-LED chip chip size increases, increases, the the power power consumption consumption [gCrystals([g(x)/xf)/f((x 20)]x)]20 decreases, 10decreases, x FOR PEER ﬁrst first andREVIEW and then then bounces bounces back. back. The The optimal optimal LED LED chip chip size size with with minimum minimum power11 power of 15 consumptionconsumption takes takes place place at 20at µ20m. µm. The The power power saving saving at this at this optimum optimum LED LED chip sizechip oversize thatover of that 50 µofm 50 µm chip size is 16% and over that of 5 µm is 17%. With the same analysis process, we find the optimal chip size is 16% and over that of 5 µm is 17%. With the same analysis process, we ﬁnd the optimal LED LED chip size for smartphone is 7 µm, and for laptop is 11 µm. chipCrystals size 2020 for, smartphone10, x; doi: FOR isPEER 7 µ REVIEWm, and for laptop is 11 µm. www.mdpi.com/journal/crystals

FigureFigure 13. 13.Normalized Normalized ((aa)) powerpower eefficiencyfficiency fitting fitting function functionf f((x),x), ((bb)) luminanceluminance intensityintensity functionfunction gg(x),(x), andand ( c()c) power power consumption consumption function functiong g((x)x/)/ff (x(x)) at at di differentfferent LED LED chip chip sizes. sizes.

3.2. Optimization Process of Different LED Chip Sizes in RGB Subpixels In this section, we analyze the power consumption of RGB micro-LED displays with different LED chip sizes. To do so, we have to consider following two important factors: (1) The chip-size dependent power efficiency is different for the RGB micro-LEDs, and (2) the ambient light reflectance is dependent on the chip size. Therefore, we need to optimize the LED chip sizes in RGB subpixels to minimize the power consumption for achieving the same ACR. In RGB micro-LED display, when the chip size decreases, the power efficiency declines for RGB micro-LEDs, but at different rates as shown in Figure 10. The decreasing rate of red micro-LED is more significant than that of green and blue, due to its faster surface recombination rate. For example, as the LED chip size decreases from 15 µm to 5 µm, the power efficiency of red, green, and blue LEDs drops 46.23%, 41.52%, and 18.69%, respectively. Therefore, using different chip sizes (with red being the largest) could improve the overall power efficiency. On the other hand, in color conversion micro-LED displays, the chip-size dependent power efficiency is the same in RGB subpixels and the slop of chip-size dependent power efficiency, as shown in Figure 12, is smaller than the RGB micro-LED displays. Therefore, applying different LED chip size in RGB subpixels is not help for reducing power consumption in color conversion micro-LED displays. In the following paragraph, we focus on the optimization process for the RGB micro-LED displays. As mentioned above, increasing the LED chip size leads to enhanced display luminance for maintaining the same ACR. However, from Figure 2b, the slope of red micro-LED is the smallest. Therefore, among the RGB primaries, the enhancement of display luminance originated from enlarging the chip size is the smallest for the red micro-LED. By lifting the restrictions on micro-LED chip size, we conducted a systematic optimization for achieving the lowest power consumption. The optimal LED chip size in RGB subpixels is (10,5,5) µm, respectively, for the smartphone, (14,7,5) µm for the laptop, and (26,13,8) µm for the TV studied. Next, we compare these power consumption results with that of uniform chip size. The power saving at different chromaticity coordinates in DCI-P3 color space is shown in Figure 14. Overall, the power saving covers about 94.46% of DCI-P3 color space. More specifically, the power saving over 10% covers 68% area of the DCI-P3 color space.

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3.2. Optimization Process of Different LED Chip Sizes in RGB Subpixels In this section, we analyze the power consumption of RGB micro-LED displays with different LED chip sizes. To do so, we have to consider following two important factors: (1) The chip-size dependent power efficiency is different for the RGB micro-LEDs, and (2) the ambient light reflectance is dependent on the chip size. Therefore, we need to optimize the LED chip sizes in RGB subpixels to minimize the power consumption for achieving the same ACR. In RGB micro-LED display, when the chip size decreases, the power efficiency declines for RGB micro-LEDs, but at different rates as shown in Figure 10. The decreasing rate of red micro-LED is more significant than that of green and blue, due to its faster surface recombination rate. For example, as the LED chip size decreases from 15 µm to 5 µm, the power efficiency of red, green, and blue LEDs drops 46.23%, 41.52%, and 18.69%, respectively. Therefore, using different chip sizes (with red being the largest) could improve the overall power efficiency. On the other hand, in color conversion micro-LED displays, the chip-size dependent power efficiency is the same in RGB subpixels and the slop of chip-size dependent power efficiency, as shown in Figure 12, is smaller than the RGB micro-LED displays. Therefore, applying different LED chip size in RGB subpixels is not help for reducing power consumption in color conversion micro-LED displays. In the following paragraph, we focus on the optimization process for the RGB micro-LED displays. As mentioned above, increasing the LED chip size leads to enhanced display luminance for maintaining the same ACR. However, from Figure2b, the slope of red micro-LED is the smallest. Therefore, among the RGB primaries, the enhancement of display luminance originated from enlarging the chip size is the smallest for the red micro-LED. By lifting the restrictions on micro-LED chip size, we conducted a systematic optimization for achieving the lowest power consumption. The optimal LED chip size in RGB subpixels is (10,5,5) µm, respectively, for the smartphone, (14,7,5) µm for the laptop, and (26,13,8) µm for the TV studied. Next, we compare these power consumption results with that of uniform chip size. The power saving at different chromaticity coordinates in DCI-P3 color space is shown in Figure 14. Overall, the power saving covers about 94.46% of DCI-P3 color space. More specifically, the power saving over 10% covers Crystals 2020, 10, x FOR PEER REVIEW 12 of 15 68% area of the DCI-P3 color space.

FigureFigure 14. 14.The The decreased decreased power power consumption consumption (unit: (unit: %) of %) RGB of micro-LEDRGB micro displays-LED displays with di ﬀwitherent different chip sizeschip and sizes uniform and uniform chip size. chip size.

WhenWhen we we compare compare the powerthe power saving saving between between uniform uniform LED chipLED size chip and size di ﬀanderent different LED chip LED sizes, chip wesizes, have towe consider have to theconsider image the contents. image Fromcontents. Figure From 14, Figure the power 14, savingthe power varies saving with varies color contentswith color andcontents the maximum and the powermaximum saving powe (>20%)r saving occurs (>20%) at red occurs color. at red Therefore, color. Therefore, if the image if the content image is content rich in red,is rich the in power red, the saving power will saving be more will be obvious. more obvious. In the following,In the following, we use we two use test two images test images shown show in n Figurein Figure 15 to compare15 to compare the power the power consumptions consumptions of micro-LED of micro- TVs.LED TVs.

Figure 15. Two tested images for micro-LED TVs: (a) Christmas, and (b) Maldives beach.

The calculated power saving of red-dominated Christmas image and blue/green-dominated Maldives beach is around 20% and 7%, respectively. To explain the power saving difference between these two images, we plot the color histograms of these images in Figure 16. As Figure 16a shows, in comparison with the blue/green color histogram, the red color histogram in the Christmas image mainly covers the higher gray level region. This indicates the image content is rich in red. From Fig. 8, the red color exhibits a larger power saving. Therefore, the Christmas image leads to a significant power saving (~20%). On the other hand, from Figure 16b, the blue histogram in the Maldives beach covers mostly the higher gray levels and many red pixels are at zero gray level (black). As a result, the power saving is only ~ 7%.

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Figure 14. The decreased power consumption (unit: %) of RGB micro-LED displays with different chip sizes and uniform chip size.

When we compare the power saving between uniform LED chip size and different LED chip sizes, we have to consider the image contents. From Figure 14, the power saving varies with color contents and the maximum power saving (>20%) occurs at red color. Therefore, if the image content Crystalsis rich 2020in red,, 10, the 494 power saving will be more obvious. In the following, we use two test images show12 of 15n in Figure 15 to compare the power consumptions of micro-LED TVs.

Figure 15. TTwowo testedtested imagesimages for micro-LEDmicro-LED TVs: ( a) Christmas Christmas,, and (b) Maldives beach.beach.

The calculated power saving of red-dominatedred-dominated Christmas image and blueblue/green/green-dominated-dominated Maldives beach is around 20% and 7%, respectively. To explain the power saving difference diﬀerence between these two images, images, we we plot plot the the color color histograms histograms of of these these images images in in Figure Figure 16 16. As. As Figure Figure 16a 16 shows,a shows, in incomparison comparison with with the the blue/green blue/green color color histogram, histogram, the the red red color color histogram histogram in in the the Christmas image mainly coverscovers thethe higher higher gray gray level level region. region. This This indicates indicates the the image image content content is rich is rich in red. in red. From From Figure Fig.8, the8, the red red color color exhibits exhibits a largera larger power power saving. saving. Therefore, Therefore, the the ChristmasChristmas imageimage leadsleads toto a signiﬁcantsignificant power saving (~20%). On On the the other other hand, hand, from from Figure 16b,16b, the blue histogram in the Maldives beach covers mostl mostlyy the higher gray levels and many red pixels are at zero gray level (black). As As a result, theCrystals power 2020 saving, 10, x FOR is onlyPEER ~7%.~REVIEW 7%. 13 of 15

Crystals 2020, 10, x; doi: FOR PEER REVIEW www.mdpi.com/journal/crystals

FigureFigure 16. 16.The The color color histogram histogram of of ( a(a)) Christmas Christmas and and ( b(b)) Maldives Maldives beach beach images. images.

WeWe also also evaluated evaluated some some frequently frequently displayed displayed images images for smartphones,for smartphones, laptop laptop computers, computers, and TVs. and ForTVs. smartphones, For smartphones, we compared we compared the image the contentsimage contents of Facebook of Facebook homepage, homepage, Google map,Google Google map, search,Google YouTubesearch, YouTube homepage, homepage, and iPhone and homepage iPhone homepage with app icons.with app Due icons. to copyright Due to issue,copyright we doissue, not showwe do thesenot show images these here. images The average here. The power average saving power is 12.9%. saving For is laptop12.9%. computers,For laptop computers, we evaluated we the evaluated image contentsthe image of Amazon contents homepage, of Amazon Gmail, homepage, Facebook Gmail, homepage, Facebook YouTube homepage, homepage, YouTube and computer homepage, game and PUBG,computer and game the average PUBG, powerand the saving average is 13.2%. power For saving TVs, is we 13.2%. evaluated For TVs, the imagewe evaluated contents the of CNNimage news,contents NBA ofgame, CNN footballnews, NBA game, game, TV show,football and game, weather TV forecast,show, and and weather the average forecast, power and reduction the average is 11.7%.power Therefore, reduction byis 11.7%. employing Therefore, various by RGB employing micro-LED various chip RGB sizes, micro we can-LED obtain chip aboutsizes, we 12% can average obtain powerabout saving12% average in all the power three saving intended in all applications the three intended applications

4.4. Conclusions Conclusion WeWe developed developed a a model model for for evaluating evaluating the the power power consumption consumption of of RGB RGB and and color-conversion color-conversion basedbased micro-LED micro-LED displays. displays. In In the the model, model, we we investigate investigate the the power power e efficiencyfficiency and and luminance luminance ambient ambient reflectionreflection of of RGB RGB subpixels subpixels in in each each type type of of micro-LED micro-LED display. display. The The optimal optimal chip chip sizes sizes corresponding corresponding toto the the lowest lowest power power consumption consumption are are found found in in three three application application scenarios: scenarios: smartphones, smartphones, laptop laptop computers,computers, andand TVs.TVs. The major major findings findings are are twofold: twofold: (1) (1) For For TV TV applications, applications, the the optimized optimized chip chip size (16 m) leads to 48% and 32% power saving, as compared to uniform LED chip size at 50 µm and 5 µm, respectively. Similar analysis shows 16% and 17% power reduction in color-conversion based micro-LED display with uniform chip size at 50 µm and 5 µm, respectively. (2) Our proposed micro- LED display employing different RGB LED chip sizes further reduces ~ 12% average power consumption over the optimized RGB micro-LED display with uniform LED chip size.

Author Contributions: Conceptualization, E.-L.H., Y.H., S.-T.W.; methodology, E.-L.H., F.G., Z.H.; software, E.-L.H, F.G.; writing—original draft preparation, E.-L.H.; writing—review and editing, S.-T.W.; supervision, Y.- F.L, S.-T.W. All authors have read and agreed to the published version of the manuscript.

Funding: This research is funded by a.u.Vista, Inc.

Conflicts of Interest: The authors declare no conflict of interest.

References

1. Lin, J.Y.; Jiang, H.X. Development of microLED. Appl. Phys. Lett. 2020, 116, 100502. 2. Jiang, H.X.; Lin, J.Y. Nitride micro-LEDs and beyond-a decade progress review. Opt. Express. 2013, 21, A475–A484. 3. Huang, Y.; Tan, G.; Gou, F.; Li, M.C.; Lee, S. L.; Wu, S. T. Prospects and challenges of mini-LED and micro- LED displays. J. Soc. Inf. Disp. 2019, 27, 387–401. 4. Huang, Y.; Hsiang, E.L.; Deng, M.Y.; Lin, C.L.; Wu, S.T. Mini-LED, Micro-LED and OLED displays: Present status and future perspectives. Light: Sci. Appl. 2020, in press.

Crystals 2020, 10, x; doi: FOR PEER REVIEW www.mdpi.com/journal/crystals Crystals 2020, 10, 494 13 of 15 size (16 µm) leads to 48% and 32% power saving, as compared to uniform LED chip size at 50 µm and 5 µm, respectively. Similar analysis shows 16% and 17% power reduction in color-conversion based micro-LED display with uniform chip size at 50 µm and 5 µm, respectively. (2) Our proposed micro-LED display employing diﬀerent RGB LED chip sizes further reduces ~ 12% average power consumption over the optimized RGB micro-LED display with uniform LED chip size.

Author Contributions: Conceptualization, E.-L.H., Y.H., S.-T.W.; methodology, E.-L.H., F.G., Z.H.; software, E.-L.H., F.G.; writing—original draft preparation, E.-L.H.; writing—review and editing, S.-T.W.; supervision, Y.-F.L., S.-T.W. All authors have read and agreed to the published version of the manuscript. Funding: This research is funded by a.u.Vista, Inc. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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