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 Crystals 2020, 10, 494; doi:10.3390/cryst10060494 www.mdpi.com/journal/crystals Crystals 2020, 10, 494 2 of 15 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 havequantum been effici investigated,ency [31,32 such]. However, as enhancing an theobvious 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 thisthis paper, paper, we we evaluate evaluate the powerthe power consumption consumption of both of types both of types micro-LED of micro displays,-LED includingdisplays, RGBincluding chips RGB and 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 thesame baseline chip forsize, comparison. as the baseline The optimum for comparison. LED chip sizesThe correspondingoptimum LED to chip the lowest sizes powercorresponding consumption to the for lowest three applications: power consumption smartphones, for laptopthree applications: computers, andsmartphones, 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. ThroughRGB subpixels the same optimizationwith different procedures, chip sizes. our Through proposed the micro-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 isL theon ( ambientLoff ≈ 0) lightrepresents reflectance the on which (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 Crystals 2020, 10, x; doi: FOR PEER REVIEW www.mdpi.com/journal/crystals Crystals 2020, 10, x FOR PEER REVIEW 3 of 15 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].