Mini-LED, Micro-LED and OLED Displays: Present Status and Future Perspectives Yuge Huang1, En-Lin Hsiang1, Ming-Yang Deng1 and Shin-Tson Wu 1
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Huang et al. Light: Science & Applications (2020) 9:105 Official journal of the CIOMP 2047-7538 https://doi.org/10.1038/s41377-020-0341-9 www.nature.com/lsa REVIEW ARTICLE Open Access Mini-LED, Micro-LED and OLED displays: present status and future perspectives Yuge Huang1, En-Lin Hsiang1, Ming-Yang Deng1 and Shin-Tson Wu 1 Abstract Presently, liquid crystal displays (LCDs) and organic light-emitting diode (OLED) displays are two dominant flat panel display technologies. Recently, inorganic mini-LEDs (mLEDs) and micro-LEDs (μLEDs) have emerged by significantly enhancing the dynamic range of LCDs or as sunlight readable emissive displays. “mLED, OLED, or μLED: who wins?” is a heated debatable question. In this review, we conduct a comprehensive analysis on the material properties, device structures, and performance of mLED/μLED/OLED emissive displays and mLED backlit LCDs. We evaluate the power consumption and ambient contrast ratio of each display in depth and systematically compare the motion picture response time, dynamic range, and adaptability to flexible/transparent displays. The pros and cons of mLED, OLED, and μLED displays are analysed, and their future perspectives are discussed. Introduction lifetime, still need to be improved. Recently, micro-LEDs – Display technology has become ubiquitous in our daily life; (μLEDs)18 27 and mini-LEDs (mLEDs)24,25,28 have emerged its widespread applications cover smartphones, tablets, as next-generation displays; the former is particularly 19,29–31 1234567890():,; 1234567890():,; 1234567890():,; 1234567890():,; desktop monitors, TVs, data projectors and augmented attractive for transparent displays and high luminance – reality/virtual reality devices. The liquid crystal display (LCD) displays21 23, while the latter can serve either as a locally – was invented in the late 1960s and early 1970s1 4.Sincethe dimmable backlight for high dynamic range (HDR) LCDs24,28 – 2000s, LCDs have gradually displaced bulky and heavy or as emissive displays21 24.BothmLEDsandμLEDs offer cathode ray tubes (CRTs) and have become the dominant ultrahigh luminance and long lifetimes. These features are technology5,6.However,anLCDisnonemissiveandrequires highly desirable for sunlight readable displays, such as a backlight unit (BLU), which not only increases the panel smartphones, public information displays, and vehicle dis- thickness but also limits its flexibility and form factor. plays. Nevertheless, the largest challenges that remain are the – Meanwhile, after 30 years of intensive material7 14 and device mass transfer yield and defect repair, which will definitely development and heavy investment in advanced manu- affect the cost. “LCD, OLED or μLED: who wins?” has facturing technologies, organic light-emitting diode (OLED) become a topic of heated debate11. – displays7,14 17 have grown rapidly, enabling foldable smart- To compare different displays, the following are phones and rollable TVs. In the past few years, emissive important performance metrics: (1) a HDR and a high OLED displays have gained momentum and have competed ambient contrast ratio (ACR)32, (2) high resolution or a fiercely with LCDs in TVs and smartphones because of their high resolution density for virtual reality to minimize the – superior unprecedented dark state, thin profile, and freeform screen-door effect, (3) a wide colour gamut33 35, (4) a factor. However, some critical issues, such as burn-in and wide viewing angle and an unnoticeable angular colour – shift6,36 40, (5) a fast motion picture response time (MPRT) to suppress image blur41,42, (6) low power con- Correspondence: Shin-Tson Wu ([email protected]) sumption, which is particularly important for battery- 1 College of Optics and Photonics, University of Central Florida, Orlando, FL powered mobile displays, (7) a thin profile, freeform, and 32816, USA These authors contributed equally: Yuge Huang, En-Lin Hsiang, Ming- lightweight system, and (8) low cost. Yang Deng © The Author(s) 2020 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a linktotheCreativeCommons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/. Huang et al. Light: Science & Applications (2020) 9:105 Page 2 of 16 In this review paper, we compare the performance of generated differently in these three types. In Fig. 1a, RGB mLEDs, OLEDs and μLEDs according to the above- LED chips are adopted. Each LED will emit light in both mentioned criteria. In particular, we evaluate the power the upward and downward directions. To utilize down- consumption and ACR of each display in depth and sys- ward light, a reflective electrode is commonly deposited at tematically compare the dynamic range, MPRT, and the bottom of each LED chip. However, such a reflector adaptability to flexible and transparent displays. The pros also reflects the incident ambient light, which could and cons of mLED, μLED, and OLED displays are ana- degrade the ACR32. One solution is to adopt tiny chips to lysed, and their future perspectives are discussed. reduce the aperture ratio and cover the nonemitting area with a black matrix to absorb the incident ambient light26. Device configurations This strategy works well for inorganic LEDs. However, for Both mLED, μLED and OLED chips can be used as OLED displays, a large chip size helps to achieve a long emissive displays, while mLEDs can also serve as a BLU lifetime and high luminance43. Under such conditions, to for LCDs. Figure 1 illustrates three commonly used device suppress the ambient light reflection from bottom elec- configurations: red, green and blue (RGB)-chip emissive trodes, a circular polarizer (CP) is commonly laminated displays26,27 (Fig. 1a), colour conversion (CC) emissive on top of the OLED panel to block the reflected ambient displays25 (Fig. 1b), and mLED-backlit LCDs24,28 (Fig. 1c). light from the bottom electrodes. In emissive displays (Fig. 1a, b), mLED/μLED/OLED chips In Fig. 1b, each blue LED chip pumps a subpixel in the serve as subpixels. In a nonemissive LCD (Fig. 1c), an patterned CC layer (quantum dots or phosphors)44.An mLED backlight is segmented into a zone structure; each absorptive colour filter (CF) array is registered above to absorb zone contains several mLED chips to control the panel unconverted blue light44,45 and suppress ambient excitations. luminance, and each zone can be turned on and off This filter also enhances the ACR so that no CP is required. In selectively. The LC panel consists of M and N pixels, and some designs, a distributed Bragg reflector (DBR) is inserted to each RGB subpixel, addressed independently by a thin- selectively recycle the unconverted blue light46 or to enhance film transistor (TFT), regulates the luminance transmit- the red/green output efficiency47.InFig.1c, blue mLED chips tance from the backlight. The full-colour images are pump a yellow CC layer48 to generate white backlight. a CP RGB LEDs b Absorptive CF Patterned CC layer Blue LEDs c Analyzer Absorptive CF LC LCD TFT array Polarizer DBEF BEF II BEF I mLED BLU Yellow CC layer Blue LEDs Fig. 1 Display system configurations. a RGB-chip mLED/μLED/OLED emissive displays. b CC mLED/μLED/OLED emissive displays. c mini-LED backlit LCDs Huang et al. Light: Science & Applications (2020) 9:105 Page 3 of 16 Additionally, a DBR could be optionally applied. In such a achieve the same effective luminance, the instant luminance BLU, the mLED zones do not need to register with the sub- in the PM should be N times higher than that of the AM. pixels so that a larger LED chip can be used. Because the CC layer scatters light, up to two brightness enhancement films Power evaluation model of full-colour LED displays (BEFs) can be employed to collimate light onto the on-axis Our evaluation model is an improvement over those direction. A dual brightness enhancement film (DBEF)49 can models reported by Lu51 and Zhou52. From the circuits in be inserted to transmit the preferred polarization, which is Fig. 2, the static power on each subpixel is mainly com- parallel to the transmission axis of the first polarizer and to prised of the LED’s power (PLED) and the driving TFT’s recycle the orthogonal polarization. The transmitted light is power (PTFT) as: modulated by the LCD with an absorptive CF array. In some designs, RGBW CFs instead of RGB CFs are employed to ¼ þ ¼ ðÞÁþ ð Þ enhance the optical efficiency. Pstatic PLED PTFT VF VDS I 1 Power consumption The power consumption of mLED/μLED/OLED displays is where I is the current through the TD and LED, VF is the primarily determined by the driving circuitry designs, LED LED forward voltage, and VDS is the drain-to-source quantum efficiency and optical system efficiency. In this voltage of the TD. In operation, the LEDs are current- section, we describe a power consumption evaluation model driven devices, and TD serves as a current source. The and give exemplary calculations on each display technology. gate-to-source voltage (VGS) of the TD controls I, and I determines the LED emittance. In the TFT part50, each Pulse amplitude modulation (PAM) driving schemes solid black line in Fig. 3 denotes the I-VDS curve at a given PAM50, which is also called analogue driving, is commonly VGS.