Review Advances in Quantum-Dot-Based Displays

1,2, 1, 1 2, Yu-Ming Huang † , Konthoujam James Singh †, An-Chen Liu , Chien-Chung Lin * , Zhong Chen 3, Kai Wang 4, Yue Lin 3, Zhaojun Liu 4 , Tingzhu Wu 3,* and Hao-Chung Kuo 1 1 Department of Photonics & Graduate Institute of Electro-Optical Engineering, College of Electrical and Computer Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan; [email protected] (Y.-M.H.); [email protected] (K.J.S.); [email protected] (A.-C.L.); [email protected] (H.-C.K.) 2 Institute of Photonic System, National Chiao Tung University, Tainan 71150, Taiwan 3 Department of Electronic Science, Fujian Engineering Research Center for Solid-State Lighting, Xiamen University, Xiamen 361005, China; [email protected] (Z.C.); [email protected] (Y.L.) 4 Department of Electrical and Electronic Engineering, Southern University of Science and Technology, 1088 Xueyuan Blvd, Xili, Nanshan, Shenzhen 518055, China; [email protected] (K.W.); [email protected] (Z.L.) * Correspondence: [email protected] (C.-C.L.); [email protected] (T.W.); Tel.: +886-6303-2121-57754 (C.-C.L.) These authors contributed equally to this work. †  Received: 27 May 2020; Accepted: 3 July 2020; Published: 6 July 2020 

Abstract: In terms of their use in displays, quantum dots (QDs) exhibit several advantages, including high illumination efficiency and color rendering, low-cost, and capacity for mass production. Furthermore, they are environmentally friendly. Excellent luminescence and charge transport properties of QDs led to their application in QD-based -emitting diodes (LEDs), which have attracted considerable attention in display and solid-state lighting applications. In this review, we discuss the applications of QDs which are used on color conversion filter that exhibit high efficiency in white LEDs, full-color micro-LED devices, and liquid-type structure devices, among others. Furthermore, we discuss different QD printing processes and coating methods to achieve the full-color micro-LED. With the rise in popularity of wearable and see-through red, green, and blue (RGB) full-color displays, the flexible substrate is considered as a good potential candidate. The anisotropic conductive film method provides a small controllable linewidth of electrically conductive particles. Finally, we discuss the advanced application for flexible full-color and highly efficient QD micro-LEDs. The general conclusion of this study also involves the demand for a more straightforward QD deposition technique, whose breakthrough is expected.

Keywords: quantum dots; light-emitting diodes (LEDs); white LEDs; high efficiency; high polarization;

1. Introduction Quantum dots (QDs) are miniscule particles, whose sizes are on the order of a few nanometers. They exhibit unique electronic and optical properties that are different from those of the bulk semiconductor materials. QDs have discrete electronic states, like those of natural , and their electronic is somewhat analogous to that of a real . Hence, they are frequently referred to as artificial atoms [1–3]. Because of their distinctive properties, QDs have found applications in a variety of modern day technologies including solar cells [4–8], photodetectors [9], photodiodes [10], field-effect [11], biological systems [12–15], and light-emitting diodes [16–18]. QDs are used as color down-converters for light-emitting diodes (LEDs) to accomplish efficient illumination sources and high-quality displays. Both electrically and optically pumped quantum dots

Nanomaterials 2020, 10, 1327; doi:10.3390/nano10071327 www.mdpi.com/journal/nanomaterials Nanomaterials 2020, 10, 1327 2 of 26 are used with LEDs [19]. Along with the radiative energy transfer from LEDs to QDs, another mechanism involves Forster resonance energy transfer (FRET), which is responsible for the transfer of non-radiative energy from LEDs to QDs. FRET, also referred to as non-radiative resonant energy transfer (NRET) is sufficiently strong to be observed when emissive quantum wells of the LED are in close contact with the QD phosphor layer. Researchers made use of innovative ideas to build QD-LEDs with better performances. The availability of multiple material choices for QDs indicates their variability across different reports. Displays based on QD electroluminescence (EL) have higher capability than either of the QD PL-based OLED or technologies to provide the best solution of wide gamut and pure black color. The QD EL display (“QD-LED display” and also called “QLED” or “EL-QLED” display) has and holes pumped into QDs where they recombine to directly produce photons for the main red, green, or blue [20]. However, in this review, we do not categorize QDs on the basis of their material or design, as this deducts from the prime focus of our discussion, i.e., the joint use of optically excited QDs and LEDs to accomplish QD-LEDs. Illumination sources and display devices based on QD-LEDs demonstrated in the past few years are discussed separately in the following sections. Energy is transferred from active LED to QDs by radiative energy transfer and NRET. In direct radiative energy transfer, the absorption of acceptors, i.e., QDs, must correspond to the emission spectrum of donors, i.e., the active LED. In contrast, in NRET, donors and acceptors must be in close contact [21]. Both of these energy transfer mechanisms can generate -hole pairs in QDs and hence generate radiation. An efficient QD-LED device must be designed such that both of these mechanisms can contribute to the process of energy transfer with minimum losses. QDs absorbing (UV)/blue and emitting blue, green, and red colors are used with a UV or blue LED for the realization of QD-LEDs. Recent QD-LED televisions with an edge-lit lenses promoted the first solution to QDs in consumer displays. It used a dispersion of QDs in a polymer embedded in a glass tube placed at the edges of a screen, over a strip of LEDs. This technique has many disadvantages including decreased temperature production. It proved challenging for early QDs that suffered from thermal instability issues; including a very heavy hermetic tube to ensure continued reliability of operation. The current technology of choice for QD-LCD televisions is the QD films in displays. First, quantum dots based on selenide or were overlaid on a blue LED and incorporated into prototypical LCD matrices, which provided the benefits of QD’s color performance. Because of its higher performance with luminous efficiency and wide-gamut, CdSe-based quantum dots are normally used in the past few years. However, the Cd has the negative environmental effects. Consequently, QDs for InP and perovskite are considered candidates for solving the problem. The latest industry standard solution for producing QDs on screens is cadmium-free films. Yet there is still a consensus that QD color filters (QDCFs) of the next generation are still an upcoming stop on the QD research and development route. Among various types of QDs, such as CdSe and InP, perovskite quantum dots (PQDs) exhibit several remarkable optical characteristics, including high , tunable emission , high color purity, making them a possible candidate for next-generation cost-effective display technology [22–25]. Consequently, PQDs have been sought after in the field of research over the past few decades owing to their quantum confinement effect and resistance to various defects. Several synthetic routes were attempted to find a method that can reliably manufacture stable and defect-resistant perovskite quantum dots. Two major routes have now been developed for the synthesis: room-temperature synthesis and hot injection synthesis. The first one involves mixing cesium halide (CsX) and lead halide (CsX2) into a good solvent such as dimethyl sulfoxide (DMSO) or N,N-dimethylformamide (DMF) followed by the addition of capping like oleic acid and oleylamine with vigorous stirring. The mixture is then added to a vigorously stirring flask containing a poorer solvent such as toluene, so that the perovskite quantum dots begin to precipitate and can be further separated by centrifugation. The second method is the so-called “hot-injection” method involving the preparation of cesium oleate in 1-octadecene (ODE) under argon at 150 ◦C Nanomaterials 2020, 10, 1327 3 of 26 by stirring of cesium carbonate and oleic acid. Lead halide is separately dried in ODE by vacuum heating, and capping ligands including oleic acid and oleylamine are added under argon to dissolve it completely. Then the solution of cesium oleate is stirred for 5–10 s at 150 ◦C before it gets precipitated which is further separated by centrifugation. Compared to CdSe- and InP-dependent QDs, PQDs have several advantages, such as tunability of emission wavelengths from blue to red, narrow full width at half maximum (FWHM), facile fabrication, etc., [26]. However, several problems impede the execution of PQDs in terms of presenting them as a color converter for display applications, and some of the challenges are addressed in this review. The optical properties of PQDs may be easily impaired by different environmental influences, such as heat, moisture, high-energy radiation, etc., which can alter their surface properties and stability in the long term [27]. Furthermore, the QDs configuration is important to assess their stability, as the QD composition and interaction between the atoms are determining factors for the QD optical properties and stability [27]. Numerous researchers reported that inorganic PQDs exhibit superior properties for optical display compared to other types of PQDs in terms of low PL quenching and PL peak shifting at elevated temperature, as shown by Sinatra et al. [27,28]. While PQDs have excellent optical properties, their display applications are limited to green emitting CsPbBr3 PQDs. Moreover, they are not appropriate for display applications as they suffer from high thermal quenching at higher temperatures, leading to stability issues. Koscher et al. and Azpiroz et al. showed that the causes behind this issue of the PQDs are surface and bulk defects that can induce surface traps and ion migration, respectively [29,30]. Hence, these problems need to be overcome such that PQDs can be successfully introduced for display applications without stability issues. Numerous researchers explored various approaches to resolve these stability issues. Sinatra et al. have suggested three approaches to enhance PQDs reliability for display applications. The first is to reduce PQDs defects, in particular halide vacancies, through improving the synthesis condition by insertion of additional halide in the system [31]. The second is the treatment of PQDs with strong ligands. In most PQDs, oleic acid and oleylamine act as ligands for PQDs synthesis, and they are likely to be detached from the surface because of their weak binding to the surface at higher temperatures, leading to surface defects. The third is to provide protection for PQDs in a post-treatment method by supplying thin inorganic oxide precursors. Wei et al. suggested several ways of improving the stability of PQDs that are very similar to those described above, such as compositional engineering, surface engineering, matrix encapsulation, and device encapsulation [32]. Another explanation for this instability is the movement of protons between oleic acid (OA) and oleylamine (OLA), which induces significant loss. Addressing this issue, Cai et al. reported on the enhancement of stability by suppressing the inter-ligand proton transfer and applying polystyrene coating. This was achieved by replacing oleylamine (OLA) with cetyl trimethylammonium bromide, which cannot be protonated, thereby suppressing the transfer of protons between the ligands and improving the stability in PQDs. Further, to improve moisture and thermal stability, PQDs can be further composited with carboxyl-functionalized polystyrene (cPS) via chemical interactions. Further, Lv et al. proposed multiple encapsulation techniques to improve the stability of metal halide PQDs by inhibiting light-induced decomposition and concentrating on enhancing chemical and thermal stability [33]. Several encapsulation methods include the sol-gel method, template-assisted method, physical method, and microencapsulation method which can be found in the literature. However, there is still a need for further improvement in the performance of these encapsulated PQDs to meet the increasing demand for various practical applications. In this review, in order to enhance mass-transfer, several efficient and stable full-color LED devices are presented with various QDs patterning technologies. Inkjet-printed QDs, which combine the polymer and QD technologies, are low-cost, mask-free, and represent a simple and rapid technology. Researchers successfully demonstrated a fine linewidth (micrometer-scale) and uniform red and green QDs. Because of these excellent characteristics, QDs are relatively suitable for many applications of display technologies such as white LEDs, flexible systems containing sensors, actuators, etc. Meanwhile, PQDs are regarded as forefront material owing to their high efficiency and color purity. Nevertheless, they have encountered several challenges that can be easily overcome and this article Nanomaterials 2020, 10, 1327 4 of 26 discusses some of the solution approaches. Hence, PQDs have the potential to demonstrate lighting application and flexible display technology if the PQDs are well stored and if they can be stable enough to bear high energy radiation.

2. Micro-LED Display and QD Color-Conversion Technology

2.1. Background of Micro-LED Display Micro-LED (µLED) display technology emerged in the recent years as a promising display technology. This technology is advocated as the ultimate choice for future displays by several researchers across the globe. The potential of µLEDs to replace conventional display technologies is owing to the combination of their self-emissive mechanism and inorganic material characteristics. The expected (or even demonstrated at the research level) better performance of µLED displays in terms of luminance, efficiency, power consumption, contrast ratio, lifetime, and response time makes them an attractive topic of research [34,35]. µLED displays are suitable for a number of applications like wearable , mobile phones, automotive head-up displays, AR/VR, micro projectors, and high-end televisions [36]. To use µLEDs as a display, we need to have an array of them emitting all three primary colors, i.e., red, green, and blue (RGB). Full-color µLED displays may be assembled into RGB LED matrices using mass-transfer technologies [37]. The approach using µLED chips of primary colors simultaneously to obtain full colored µLEDs comes with some crucial drawbacks. First, there is no suitable material to grow efficient green emitting LEDs due to the low efficiency emission forming a green gap among other wavelengths. Second, LEDs with different colors are usually grown on various substrates and are subject to different operating conditions. Thus, a complex circuitry is required for their integration, which is costly. These concerns along with many other significant challenges such as the low transfer yield, slow transfer time, high fabrication cost, and inspection and repair difficulties, limit the use of the above-mentioned approach to realize full-color µLEDs. An alternative approach to realize full-color LEDs is to combine single-color LEDs and certain color-converting material, which is usually a phosphor. For LEDs, phosphor-assisted color down-conversion is a process through which a phosphor material absorbs short wavelengths and re-emits relatively longer wavelengths after energy down-conversion. In common practice, a blue or violet LED is used in combination with a broad, yellow emitting phosphor. Emitted colors are tunable by customizing the proportions of direct emissions from the LED and phosphor material, as well as by changing phosphor composition. Industrial white LED sources are commonly achieved by 3+ combining nitride-based single color LEDs with Y3Al5O12: Ce (YAG: Ce) yellow emitting phosphor. This kind of white LEDs (WLEDs) used for illumination purposes are dominant in the market owing to their low energy consumption. YAG: Ce yellow phosphor-based white LEDs still suffer from significant limitations, such as low color quality and low spectral efficiency, which reduces their scope of application [38–40]. Optically pumped QDs were introduced as an innovative and promising class of phosphors in past few years. QD-incorporated LEDs (QD-LEDs) find their applications in a range of technologies such as general lighting, display , and self-emissive displays. QDs have numerous potential advantages in terms of color quality, efficiency, and flexible incorporation into opto-electronic devices. The advantages of QDs as a color-converting material have been discussed in detail in the following section. By incorporating QDs as color-converter components, the output performance of LEDs can be significantly enhanced and by using the optical and electrical properties of QDs, a color-converted hybrid LED emitting at a longer wavelength can be developed. The process of color conversion with the incorporation of QDs is an equivalent method to the color conversion utilizing phosphors. However, the bandwidth emission for phosphor is very large compared to QDs that emit with the FWHM of around 40 nm which is significantly smaller than that of phosphors. In addition, a slightly smaller quantity of QDs may result in the process of color conversion, whereas very large quantities of phosphor are required to achieve full color conversion. Optical absorption is an important feature for the efficient Nanomaterials 2020, 10, 1327 5 of 26 operation of a hybrid color-converted system and hence the color-conversion materials should have decent absorption to exhibit high efficiency during the process of color conversion. QDs can absorb nearly all photons with wavelengths slightly shorter than those ’ emission wavelength and hence these superlative qualities of QDs make them very promising candidates as an efficient color converter. Two decades ago, one of the very first hybrid full-color LEDs based on GaN-LED and II-VI QDs was demonstrated by Lee et al. [41]. ZnS-coated CdSe and CdS QDs were pumped optically by means of GaN-based blue and ultraviolet LEDs to achieve emission wavelengths spanning almost the entire visible spectrum. In 2012, Zhang et al. [21] designed their own InGaN/GaN LEDs with a nanopillar structure and CdSe/CdS-based colloidal core/shell QDs to accomplish a hybrid light emitting device. QDs were mounted on the surface of nanopillar LED devices by dipping them in the QD solution, such that close proximity between the emissive quantum wells (QWs) and the QD layer was maintained. A strong non-radiative resonant energy transfer (NRET) from the QWs to QD layer was observed and described with the help of photoluminescence (PL) data. The internal of nanopillar LED was reported to be enhanced by 263% in comparison with the planar LED owing to the non-radiative resonant energy transfer (NRET) process.

2.2. History of QD Patterning Technique Displays are essential components of electronic systems and recent displays like micro-LED displays are based on small LED arrays. To integrate QDs into the array systems, we need to transfer QDs to a substrate surface and use it for display applications later on. To this end, several researchers implemented numerous techniques of QD printing or patterning, such as spray coating, aerosol jet printing, super-, etc. Hence, QDs are electrically and optically pumped to be used with light-emitting diodes. Colloidal QD-LEDs are used as color down-converters for light-emitting diodes to accomplish efficient illumination sources and high-quality displays. In 2012, Chen et al. [42] demonstrated QD-LEDs emitting all three primary colors by coating Cd-based colloidal QDs on a nitride-based UV LED. QDs were coated on the LED using a pulse spray method to ensure uniform deposition and fabricate large-area and small footprint QD-LEDs. A HfO2/SiO2 distributed Bragg reflector (DBR) was mounted on top of the device to ensure reusability of UV photons. A of polydimethyl siloxane (PDMS) was deposited between individual layers with red, green, and blue emission to avoid cross-contamination. The large color gamut of these QD-LEDs makes them a suitable choice as back light units for display applications. Figure1a shows the schematic of QD-LED and a photograph of the QD-LED during operation. Figure1b shows the electroluminescence (EL) spectra of QD-LED with and without the DBR structure. Three emission peaks are observed at 460 530, and 640 nm. Further, the EL spectrum exhibits higher visible illumination upon adding the DBR structure, as the strong reflection of the UV band successfully suppresses the 395 nm peak and thus increases the intensity of visible light. Abe et al. [43] prepared a hybrid phosphor by mixing europium-doped strontium thiogallate (STG) green phosphor with red CdSe/CdS QDs and applied it on a blue LED to realize a display backlight. Therefore, this technique of pulsed spray coating is crucial to providing consistent RGB layers. Nanomaterials 2020, 10, 1327 6 of 26 Nanomaterials 2020, 10, x FOR PEER REVIEW 6 of 29

FigureFigure 1. (a) Schematic1. (a) Schematic of QD-LED of QD-LED device device (bottom). (bottom). Photograph Photograph ofof thethe QD-LEDQD-LED during during operation operation (top). (b) EL spectra(top). (b) EL with spectra and with without and with theout DBR the DBR structure. structure. ( c(c)) ImageImage of of sprayed sprayed QDs QDs µLEDµ arrayLED under array under fluorescencefluorescence microscopy microscopy at di ffaterent different magnifications. magnifications. (d ()d) Top-view Top-view imageimage of of the the µLEDµLED layout layout with with pitch of 40.2 µpitchm defined of 40.2 µm as thedefined intended as the inte channelnded channel (left). Quantum(left). Quantum dots dots (QD) (QD) droplets droplets jetted jetted in in the the PRPR mold to mold to confine the size and resolve the cross-talk effect (right) [44]. Figure reproduced with confine the size and resolve the cross-talk effect (right) [44]. Figure reproduced with permission from permission from John Wiley and Sons. John Wiley and Sons. As mentioned earlier, QD-LED-based display technology is the ultimate choice for future Asgeneration mentioned displays earlier, because QD-LED-based of their compelling display potential technology advantages is in the terms ultimate of efficiency, choice power for future generationutilization, displays contrast because ratio, lifetime, of their and compelling response time. potential For a higher advantages resolution, inLEDs terms with ofa size effi ciency, power utilization,smaller than 100 contrast µm (known ratio, as lifetime, µLEDs) andmust responsebe used; hence, time. displays For a higherrealized resolution,utilizing QDs LEDs and with a size smallerµLEDs than are often 100 referredµm (known to as QD-µLED as µLEDs) displays. must In be2015, used; Han et hence, al. [45] displaysreported a realizedfull-color display utilizing QDs realized with the help of an ultra-violet (UV) µLED and colloidal QDs (CQDs). GaN-based UV µLEDs and µLEDs are often referred to as QD-µLED displays. In 2015, Han et al. [45] reported a full-color with a small pitch of 40 µm were combined with CQDs of all three primary colors, i.e., red, green, displayand realized blue. RGB with QDs the helpwere ofdeposited an ultra-violet on the array (UV) ofµ UVLED LEDs and with colloidal the help QDs of (CQDs).an aerosol GaN-basedjet (AJ) UV µLEDsprinter with ato small ensure pitch fine ofprinting 40 µm that were is combinedmore precise with and CQDs mask-less, of all and three to enable primary non-contact colors, i.e., red, green, anddeposition blue. RGBof liquids QDs containing were deposited functional on materials. the array The of UVAJ system LEDs comprises with the helptwo main of an parts; aerosol an jet (AJ) printerultrasonic to ensure atomizer fine printing and a thatspraying is more chamber. precise Th ande QD mask-less, suspension andis atomized to enable inside non-contact the atomizer deposition of liquidsusing containing ultrasonic functionaloscillations. The materials. resulting The aerosol AJ systemis then transferred comprises to two the mainnozzle parts; of the anspray ultrasonic chamber using the flow of nitrogen gas. The nozzle of the AJ printer then prints QDs on the substrate atomizerwith and narrow a spraying linewidth chamber. and high The resolution. QD suspension A layer of is DBR atomized was used inside on the top atomizer of the device using ultrasonicto oscillations.enhance The the resulting utilization aerosol of UV light is then by reflecting transferred leaked to UV the light nozzle back of to thethe QD spray layers. chamber The luminous using the flow of nitrogenflux of gas. blue, The green, nozzle and of red the colors AJ printer was boosted then printsby 194%, QDs 173%, on theand substrate183% in comparison with narrow to the linewidth and highsamples resolution. without A DBR. layer The of DBRluminous was efficacy used on of the radiation top of was the devicereported to to enhance be 165 lm/Watt the utilization under of UV light byvarious reflecting currents. leaked An QD- UV µLED light backarray of to 35 the × 35 QD µm layers.2 area comprised The luminous small emissive flux of sections blue, green, with a and red pitch size of 40 µm, as shown in Figure 1c. Each emissive section operated as a with three sub- colors was boosted by 194%, 173%, and 183% in comparison to the samples without DBR. The luminous emitting blue, green, and red colors. The device was further optimized in 2017, when Lin et al. efficacy[44] of radiationdeposited photoresist was reported (PR) mold to be between 165 lm layers/Watt of under each individual various currents.color to reduce An QD-opticalµ LEDcross- array of 2 35 35talkµm betweenarea comprised them. This technique small emissive proved to sections be useful with, as emission a pitch efficiencies size of 40 ofµ m,blue, as red, shown and green in Figure 1c. × Each emissive section operated as a pixel with three sub-pixels emitting blue, green, and red colors.

The device was further optimized in 2017, when Lin et al. [44] deposited photoresist (PR) mold between layers of each individual color to reduce optical cross-talk between them. This technique proved to be useful, as emission efficiencies of blue, red, and green QDs were reported to be enhanced by 5%, 23%, and 32%, respectively. PR mold was also found to be helpful in reducing the coffee-ring effect that leads to the non-uniform distribution of QDs on the LED surface shown in Figure1d. This strategy is likely to be beneficial for future generation of light-emitting devices. Nanomaterials 2020, 10, 1327 7 of 26

Nanomaterials 2020, 10, x FOR PEER REVIEW 7 of 29 For a more better QDs printing technique than the previous technique, in a study by QDs were reported to be enhanced by 5%, 23%, and 32%, respectively. PR mold was also found to be Huang Chen et al. in 2019, nanoring (NR) structures were fabricated on a green LED epitaxial wafer helpful in reducing the coffee-ring effect that leads to the non-uniform distribution of QDs on the for the purposeLED surface of tuningshown in the Figure peak 1d. This wavelength strategy is likely from to green be beneficial to blue for future through generation strain of relaxation,light- in a method referredemitting devices. to as strain-induced engineering [46]. The epitaxial layers of GaN LEDs with 525 nm emissionFor a weremore better grown QDs on printing c-plane technique pattern than sapphire the previous substrates technique, using in a study metal by organic Huang chemical Chen et al. in 2019, nanoring (NR) structures were fabricated on a green LED epitaxial wafer for the vapor deposition. The releasing strain technique was used to tune the emission wavelength from purpose of tuning the peak wavelength from green to blue through strain relaxation, in a method green to blue,referred and to redas strain-induced QDs were depositedengineering on[46]. the The blue epitaxial emission layers regionof GaN forLEDs color with down-conversion. 525 nm Strain-inducedemission engineering were grown on involves c-plane pattern the reduction sapphire substrates of LEDs using volume metal by organic etching, chemical which vapor results in the release ofdeposition. strain and The decrease releasing in strain the quantum-confinedtechnique was used to Starktune the eff emissionect. This wavelength effect induces from green the to flattening of blue, and red QDs were deposited on the blue emission region for color down-conversion. Strain- the tilted energy band, expands the overlap among distributions, and blue-shifts the induced engineering involves the reduction of LEDs volume by etching, which results in the release wavelengths.of strain The and final decrease device in comprisesthe quantum-confined an array Stark of RGB effect. pixels This effect with induces each containing the flattening a of green the subpixel, a blue subpixeltilted energy with band, nanorings, expands andthe overlap a red among subpixel quantum with state nanorings distributions, as well and asblue-shifts red QDs, the as shown in the scanningwavelengths. electron The microscopyfinal device comprises (SEM) andan array transmission of RGB pixels electron with each microscopy containing (TEM)a green images in subpixel, a blue subpixel with nanorings, and a red subpixel with nanorings as well as red QDs, as Figure2a. It is clearly observed from the SEM images that the sidewalls of the InGaN /GaN MQWs shown in the scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were closelyimages surrounded in Figure 2a.by It is QDs clearly which observed are from important the SEM images for the that nonradiative the sidewalls of resonant the InGaN/GaN energy transfer (NRET) mechanisms.MQWs were closely Alternatively, surrounded by a 1-nm-thickQDs which are Al important2O3 layer for the deposited nonradiative on resonant the NR- energyµLED sidewall is obvioustransfer from the(NRET) TEM mechanisms. image. The Alternatively, area ofeach a 1-nm-thick subpixel Al2O was3 layer limited deposited to 3on the10 NR-µLEDµm2 to maintain a sidewall is obvious from the TEM image. The area of each subpixel was limited to× 3 × 10 µm2 to sufficiently high resolution. For the deposition of a QDs in a tight space, the super inkjet (SIJ) printer maintain a sufficiently high resolution. For the deposition of a QDs in a tight space, the super inkjet was employed,(SIJ) printer which was ensuredemployed, the which linewidth ensured the of linewidth the QD layerof the QD that layer is as that thin is as as thin 1.65 as 1.65µm. µm. To passivate the sidewallsTo passivate of the nanoring-the sidewallsµ ofLED, the nanoring-µLED, a layer of Al 2aO layer3 was of Al deposited2O3 was deposited through through atomic atomic layer layer deposition, which enhanceddeposition, the which PL enhanced intensity the by PL 143.7% intensity in bycomparison 143.7% in comparison with the with reference the reference structure. structure. The color The color gamut of the final QD-NR-µLED device was reported to be 104.8% and 78.2% according to gamut of the final QD-NR-µLED device was reported to be 104.8% and 78.2% according to NTSC NTSC and Rec. 2020 standards, respectively, as depicted in Figure 2b. Thus, we can precisely spray and Rec. 2020red QDs standards, on a subpixel respectively, region of a blue as depicted NR-µLED inusing Figure the SIJ2b. printing Thus, strate we cangy to precisely ensure red-light spray red QDs on a subpixelemission. region of a blue NR-µLED using the SIJ printing strategy to ensure red-light emission.

Figure 2. Figure(a) Scanning 2. (a) Scanning electron electron microscopy microscopy (SEM) (SEM) image image of red, of green, red, and green, blue (RGB) and bluepixel array (RGB) and pixel array NR-µLED with 30° tilt angle; transmission electron microscopy (TEM) image of the contact area and NR-µLED with 30◦ tilt angle; transmission electron microscopy (TEM) image of the contact area between multi quantum wells (MQWs) and QDs. (b) Color gamut of RGB hybrid QD-NR-µLEDs, µ between multiNTSC, and quantum Rec. 2020 wells [46]. Figure (MQWs) reproduced and QDs.with permission (b) Color from gamut Optical of Society RGB of hybrid America. QD-NR- LEDs, NTSC, and Rec. 2020 [46]. Figure reproduced with permission from Optical Society of America. Changing the thickness of micro-led is a must and can be configured by doing so to enable the Changingcolor conversion the thickness mechanism of micro-led from blue to is green, a must or blue and to can red befor full-color configured displays. by doing Further so study to enable the color conversionfor inkjet mechanism printing has been from done blue by to Hu green, et al. by or demonstrating blue to red for uniform full-color quantum displays. dots printed Further in study for Inkjet as color conversion layers for full-color Organic LED displays in 2020 [47]. Figure 3a illustrates inkjet printing has been done by Hu et al. by demonstrating uniform quantum dots printed in Inkjet as color conversion layers for full-color Organic LED displays in 2020 [47]. Figure3a illustrates the measurements of produced a polymer-based QD ink with uniform green and red QDs at micrometer thickness. This indicates that the polymer-based QD inks effectively absorb blue light better than the solvent-based counterpart. The light conversion efficiency (LCE) from blue to green and red can be tuned by changing the thickness of the QD layer. The LCE of the green QD layer reached 90% at ∼ 10.2 µm thickness and 33% for red QDs at 10.5 µm thickness. The color gamut of the QD-OLED could ∼ Nanomaterials 2020, 10, x FOR PEER REVIEW 8 of 29

the measurements of produced a polymer-based QD ink with uniform green and red QDs at micrometer thickness. This indicates that the polymer-based QD inks effectively absorb blue light better than the solvent-based counterpart. The light conversion efficiency (LCE) from blue to green Nanomaterialsand2020 red, 10 can, 1327 be tuned by changing the thickness of the QD layer. The LCE of the green QD layer 8 of 26 reached ∼90% at 10.2 µm thickness and ∼33% for red QDs at 10.5 µm thickness. The color gamut of the QD-OLED could reach ∼95% according to the BT 2020 standard, as shown in Figure 3b. This inkjet reach 95%printing according process tointroduced the BT 2020in this standard,work offers asa cost-effective shown in Figureway to 3expandb. This QDs inkjet for full-color printing process ∼ introduceddisplay in this applications. work off ers a cost-effective way to expand QDs for full-color display applications.

Figure 3. Figure(a) Photoluminescence 3. (a) Photoluminescence (PL) (PL) spectra spectra andand light light co conversionnversion efficiency efficiency (LCE) of (LCE) green and of greenred and red QD layersQD at layers different at different thicknesses. thicknesses. ( b(b)) ColorColor gamut gamut of the of QD-OLED the QD-OLED display (blue display triangle (blue area), triangle in area), comparison with the color gamut of BT. 2020 (black triangle area) [47]. Figure reproduced with in comparisonpermission with from the Royal color Society gamut of Chemistry. of BT. 2020 (black triangle area) [47]. Figure reproduced with permission from Royal Society of Chemistry. Despite the progress achieved by methods employed in previous articles, a facile and efficient Despitetechnique the progress for the patterning achieved of QDs by methodsis needed for employed further development. in previous Numerous articles, researchers a facile are and efficient techniquecurrently for the studying patterning QD deposition of QDs is methods. needed The for study further of Yue development. et al. in 2018 [48] Numerous demonstrated researchers the are synthesis of QD-based photoresist (QDPR) with high stability and dispersity of QDs for the QDCF currently studying QD deposition methods. The study of Yue et al. in 2018 [48] demonstrated the application, which achieved the fine-pitch by photolithographic patterning of QD embedded PR synthesismaterials. of QD-based Because photoresist of the composition (QDPR) of scattering with high particles, stability the absorption and dispersity efficiency of QDCFs QDs for are the QDCF application,obviously which enhanced, achieved as shown the fine-pitchin Figure 4a,b. by The photolithographic absorbance of with and patterning without scattering of QD particles embedded PR materials.are Because 80% and of40% the (QDPR-Red); composition the Green of scattering QDPR increases particles, from the 28% absorption to 65%. Thus, e ffitheciency EQE of ofthe QDCFs are Red and Green QDPR increases three and five times more than without scattering particles. Figure obviously enhanced, as shown in Figure4a,b. The absorbance of with and without scattering particles 4c,d show that the red and green QDs are efficiently dispersed in QDPR formulations. are 80% and 40% (QDPR-Red); the Green QDPR increases from 28% to 65%. Thus, the EQE of the Red and GreenNanomaterials QDPR increases 2020, 10, x FOR three PEER REVIEW and five times more than without scattering particles.9 of 29 Figure 4c,d show that the red and green QDs are efficiently dispersed in QDPR formulations.

Figure 4. ComparisonFigure 4. Comparison of (a) of red (a) andred and (b )(b green) green QDCFs QDCFs spectra spectra with with and andwithout without scattering scattering particles particles under blueunder LED blue backlight. LED backlight. (c,d )(c The,d) The cross-sectional cross-sectional SEM SEM images images of red of and red green and QDPR green on QDPR glass on glass through photolithographythrough process process.[48]. [48]. Figure Figure reproduced reproduced withwith permission from from John John Wiley Wiley and and Sons. Sons.

Further, the leakage of the blue incident light and cross-talk remain challenging issues for the µLED application. In 2020, Huang Chen et al. [49] combined the CdSe/CdZnS-based thick-shell QDs and black PR with TiO2, which can return the incident light to the QDPR and effectuate re-absorption in QDs. Black PR can absorb redundant incident blue light, as shown in Figure 5a. Hence, using the bottom side black PR and the upper side gray PR which provided height wall, the cross-talk between RGB pixels is prevented. The RGB EL microscope images are shown in Figure 5b. The use of the semi- polar µLED is another key point. Comparing c-plane and semi-polar µLED as sources, semi-polar devices exhibit a small wavelength shift during the driving current from 1 A/cm2 to 200 A/cm2. Figure 5c demonstrates the performance under the 1 A/cm2 to 200 A/cm2 driving current in the CIE 1931 colors space, and the coordinates varied from (0.1572, 0.1067) to (0.1483, 0.0379) and (0.1433, 0.0388) to (0.1490, 0.0317) for the c-plane and semi-polar blue µLEDs, respectively. Semi-polar devices demonstrate a wide color gamut of 114.4% NTSC and 85.4% Rec. 2020, which is attributed to the almost unchanged wavelength peak. A color conversion layer developed by QDPR is ideal for the typical lithography process and, consequently, for large-scale output.

Nanomaterials 2020, 10, 1327 9 of 26

Further, the leakage of the blue incident light and cross-talk remain challenging issues for the µLED application. In 2020, Huang Chen et al. [49] combined the CdSe/CdZnS-based thick-shell QDs and black PR with TiO2, which can return the incident light to the QDPR and effectuate re-absorption in QDs. Black PR can absorb redundant incident blue light, as shown in Figure5a. Hence, using the bottom side black PR and the upper side gray PR which provided height wall, the cross-talk between RGB pixels is prevented. The RGB EL microscope images are shown in Figure5b. The use of the semi-polar µLED is another key point. Comparing c-plane and semi-polar µLED as sources, semi-polar devices exhibit a small wavelength shift during the driving current from 1 A/cm2 to 200 A/cm2. Figure5c demonstrates the performance under the 1 A /cm2 to 200 A/cm2 driving current in the CIE 1931 colors space, and the coordinates varied from (0.1572, 0.1067) to (0.1483, 0.0379) and (0.1433, 0.0388) to (0.1490, 0.0317) for the c-plane and semi-polar blue µLEDs, respectively. Semi-polar devices demonstrate a wide color gamut of 114.4% NTSC and 85.4% Rec. 2020, which is attributed to the almost unchanged wavelength peak. A color conversion layer developed by QDPR is ideal for the typical lithographyNanomaterials 2020 process, 10, x FOR and, PEER consequently, REVIEW for large-scale output. 10 of 29

Figure 5. Figure(a) EL 5. spectra (a) EL spectra of red of and red and green green pixels. pixels. ( b(b)) ELEL image image of of RGB RGB pixels. pixels. (c) Color (c) Colorgamut of gamut RGB of RGB pixel assemblypixel assembly from c-plane from c-planeµLED µLED and QDPRand QDPR at variousat various current current densitiesdensities in in CIE CIE 1931 1931 color color space space [49]. Figure reproduced[49]. Figure withreproduced permission with permission from Optical from Optical Society Society of America. of America.

In summary, the historical development of QD printing at Kuo’s Lab is illustrated in Figure 6. In summary, the historical development of QD printing at Kuo’s Lab is illustrated in Figure6. The overview of the developmental history of QD-based display technology with their description The overviewcan be of found the developmental in Table 1. First, historythe problem of QD-based needs to be display effectively technology solved. Thus, with we theirwill focus description on can be found infabricating Table1. the First, full-color the problem display needsby improving to be e everyffectively method. solved. In the Thus,beginning, we willthe spray focus coating on fabricating the full-colormachine display was applied by improving for large-scale every chips. method. However, In because the beginning, of the non-uniform the spray QD region coating obtained machine was applied forby large-scale this method, chips. repeatability However, is challenging because even of the under non-uniform the same printing QD region conditions. obtained Moreover, by this the method, pulsed spray coating machine cannot control the precise linewidth. Subsequently, AJ printing was repeatabilityemployed is challenging on the full-color even LED, under which the combined same printing PR mold conditions.and RGB QD pattern. Moreover, The QD the pattern pulsed spray coating machinelayer is successfully cannot control controlled the have precise thickness linewidth. under 35 Subsequently, µm. In recent years, AJ printing the super wasink-jet employed printing on the full-color(SIJ) LED, technology which combined appeared along PR moldwith the and widespread RGB QD application pattern. of The µLED. QD The pattern SIJ can layer maintain is successfully the controlledQD have pattern thickness layer below under 10 µm, 35 µandm. it In can recent further years, precisely the jet super the QD ink-jet solution printing onto the (SIJ) surface. technology Moreover, QDs need to be printed repeatedly so that its layer becomes thick in order to avoid leakage appeared along with the widespread application of µLED. The SIJ can maintain the QD pattern layer of the blue incident light, which will take a lot of time. The method effectively uses QD-based below 10 µphotoresistm, and it to can achieve further fine pitch precisely QD patterning. jet the QD The solution advantage onto involves the surface.quick, thickness-controlled, Moreover, QDs need to be printedlarge-scale repeatedly processes so that which its could layer possibly becomes become thick the in trend order of the to avoidfuture. Most leakage importantly, of theblue there incident light, whichare willsome takeissues a that lot ofneed time. to be The resolved method including effectively the high uses blue QD-based incident light photoresist leakage, cross-talk to achieve fine pitch QD patterning.effect, and so on. The Hence advantage the black involvesmatrix should quick, prevent thickness-controlled, cross-talk effects because large-scale of its high processes contrast which ratio. Additionally, the QDPR, which is a combination of QDs, photoresist, and TiO2 [49]. TiO2 can be could possiblyconsidered become a mirror the trendto reflect of incidents the future. of blue Most light importantly,. This method therecan efficiently are some reduce issues outer that blue need to be resolved includinglight significantly. the high blue incident light leakage, cross-talk effect, and so on. Hence the black

Nanomaterials 2020, 10, 1327 10 of 26

Nanomaterials 2020, 10, x FOR PEER REVIEW 11 of 29 matrix shouldNanomaterialsNanomaterials prevent 2020 2020, cross-talk10,, 10x FOR, x FOR PEER PEER e REVIEWff ectsREVIEW because of its high contrast ratio. Additionally, 11 the of 11 29ofQDPR, 29 which is a combination of QDs, photoresist, and TiO2 [49]. TiO2 can be considered a mirror to reflect incidentsNanomaterials of blue light. 2020, 10 This, x FOR method PEER REVIEW can efficiently reduce outer blue light significantly. 11 of 29

FigureFigureFigure 6. 6. Developmental Developmental6. Developmental history history history of of QD ofQD QD printing printing printing system system system at at Kuo’satKuo’s Kuo’s Lab Lab Lab [44, [44, [44, 46] 46] 46] . . Figure Figure . Figure reproduced reproduced reproduced with with with permissionpermissionpermission fromfrom from OpticalOptical Optical SocietySociety Society ofof America.ofAmerica. America. Figure 6.FigureDevelopmental 6. Developmental history history of QDof QD printing printing system at atKuo’s Kuo’s Lab Lab[44,46] [44 . Figure,46]. Figure reproduced reproduced with with Further,Further,Further, aa briefabrief brief descriptiondescription description aboutabout about thethe the developmentaldevelopmental developmental historyhistory history ofof ofQDQD QD patterningpatterning patterning technologytechnology technology isis is permission from Optical Society of America. permissiongivengivengiven from inin inthethe the Optical followingfollowing following Society TableTable Table of 1.1. America.1. Further, a brief descriptionTable 1. Development about the developmental in QD patterning history technology. of QD patterning technology is TableTableTable 1.1. Development1.Development Development inin QDinQD QD patterningpatterning patterning technology.technology. technology. given in the following Table 1. MethodsMethodsMethods Description Description Description Description Table 1. Development in QD patterning technology. TheTheThe operationoperation operation isis preciselyisprecisely precisely Methods The operation Description is precisely controlled controlledcontrolledcontrolled byby by aa computer computera computer andand and itsits its qualityby a computer of spray and was its quite quality stable of spray Pulsed-Spray qualityThequality operation of ofspray spray is was precisely was quite quite stable stable Pulsed-SprayPulsed-SprayPulsed-Spray each time.was The quite disadvantage stable each time.is the CoatingCoating Machine Machine controlledeacheach time.The time. by disadvantageThe a The computer disadvantage disadvantage and is the its is spray theis the CoatingCoating Machine Machine spray diameters which is large, qualityspraydiametersspray of diameters spray diameters which was which isquite which large, stableis whichlarge,is large, could Pulsed-Spray which could not be used to spray eachwhich time.notwhich be could The usedcould disadvantage not to not spraybe beused used precise tois tospraythe spray pattern. Coating Machine precise pattern. spray diameterspreciseprecise which pattern. pattern. is large,

which could not be used to spray TheTheThe QDsQDs QDs solutionsolution solution waswas was aerosolizedaerosolized aerosolized The QDsprecise solution pattern. was aerosolized by byby thethe ultrasonicultrasonic vibration.vibration. TheThe theby ultrasonic the ultrasonic vibration. vibration. The AerosolThe Aerosol Jet process began with a TheAerosol QDsAerosolJet solution processJet Jet process process was began aerosolizedbegan began with with awith mist a a Aerosol Jet mist generator that atomized AerosolAerosolAerosol Jet Jet Jet by themist mistgeneratorultrasonic generator generator thatvibration. that atomizedthat atomized atomized The liquid Printing System liquid materials into small PrintingPrintingPrinting System System System Aerosolmaterialsliquid liquidJet process materials materials into began small into into droplets.with small small a The droplets. The disadvantage is the Aerosol Jet mistdroplets.disadvantagedroplets. generator The The disadvantage that isdisadvantage theatomized large sprayis theis the area, largeand spray QDs area, also printedand QDs out also of the Printing System liquidlargelarge spraymaterials spray area, area, into and and small QDs QDs also also printed out ofwindows. the windows. droplets. printedprinted The outdisadvantage out of ofthe the windows. windows. is the largeTheThe Thespray pressurepressure pressure area, generated generatedand generated QDs also byby by the the the oscillating electric field is used by oscillatingprintedoscillatingThe out pressure electric of electric the windows.field generated field is usedis used by by the by this printing machine to print the Thethisoscillatingthis pressure printing printing electricgenerated machine machine field to by toprint is the usedprint the bythe this QDs.QDs.QDs.printing WithWith With better machinebetter better controlcontrol control to print ofof ofQDs-QDs- the QDs- QDs. Super-InkjetSuper-Inkjet oscillating electric field is used by Super-Inkjet inks,With it provides better control fine-linewidth of QDs-inks, Super-InkjetPrinting Printing System this inks,printinginks, it providesit machine provides fine-linewidth to fine-linewidth print the PrintingPrinting System System pattern.it provides In addition, fine-linewidth printing pattern. System [46[46]] QDs.pattern. Withpattern. better In Inaddition, control addition, ofprinting QDs-printing Super-Inkjet[46][46] In addition, printing sufficiently dense inks,sufficientlysufficientlysufficiently it provides densedense dense fine-linewidth QDsQDs QDs causedcaused caused thethe the Printing System QDs caused the higher color pattern.higherhigherhigher In colorcolor addition, color conversionconversion conversion printing ofof ofthethe the [46] conversion of the color Red and Green sufficientlycolorcolorcolor RedReddense Red andand QDsand GreenGreen Greencaused isis timeistime the time Nanomaterials 2020, 10, x FOR PEER REVIEW is time consuming. 12 of 29 consuming.consuming. higher color conversionconsuming. of the color Red and Green is time Quantum dots photoresist Quantumconsuming. dots photoresist methods by methods by photolithography is a photolithography is a fast and Quantum Dots fastconvenience and convenience method. method. This methodThis Quantum Dots Photoresist methodcan control can control the thickness the thickness of QDPR to Photoresist methods methods of QDPRprevent to prevent the leakage the leakage blue light. However,blue light. the However, disadvantage the is QD disadvantageusage is QD isusage high. is high.

For a µLED, as the array size decreases, its light-emitting efficiency improves while experiencing massive non-radiative recombination from sidewall defects; however, the far-field radiation pattern will deviate from the ideal Lambertian distribution depending on the sidewall emission, resulting in the color shift of µLED displays. The color-conversion method and mass transfer process are two common solutions to achieving the full-color display; however, manufacturing challenges remain in terms of achieving high yield, large size displays such as tablets, monitors, TVs, video walls, etc., [50, 51]. The most widely used commercial LED wafer is based on structures of multi quantum wells (MQW), i.e., GaInP/AlGaInP MQWs for red LEDs and blue and green LEDs for InGaN/GaN MQWs [52], resulting in inconsistent angular distribution between RGB µLEDs due to different epitaxy materials and structures between them. Consequently, there is a color shift of mixed colors in RGB µLEDs displays. To minimize this particular color shift, Gou et al. introduced a device layout and studied the angular color shift of RGB µLED displays from a mismatched angular distribution suggesting a simulation model to support the experimental findings [53]. The author found that the green and blue chip sidewall emissions are far larger than those of red chips due to better absorption in red MQWs. The light emission from the red chip decreases because of the Lambert’s cosine law, while that of green and blue µLEDs increases and subsequently decreases. The difference in material dynamics of RGB chips will cause an angular shift in mixed colors. Owing to the low-cavity effect within µLEDs, no color change is observed at a fixed driving current for primary colors. The color shift problem is exacerbated as the size of the µLEDs decreases because of increased sidewall emissions from green and blue chips. To mitigate the color change, green and blue µLED sidewall emissions must be removed, and the RGB radiation patterns, i.e., the Lambertian distribution, must be obtained. Hence, Gou et al. suggested a system mode shown in Figure 7a, where the system consists of a µLED array and top black matrix outside the emission region. Further, the space between the µLEDs is filled with resin with a of about 1.5, such that the sidewall emissions from green and blue chips are completely absorbed by the black matrix, resulting in emissions from the top of RGB µLEDs with matched Lambertian distributions only. Moreover, the author introduces a taper angle, α, shown in Figure 7a, to boost the light intensity from the top emission. The light intensity from the green and blue chips is observed to increase as α increases from 90° to 140°, upon which it saturates, with little influence on the red chip. The color shift is extreme when α is larger than 130°, and the wider taper angle contributes to a narrower angular distribution between green and blue emissions, with no effect on the red channel, thus inducing a severe color shift. Figure 7b illustrates the simulated color shifts of ten reference colors at viewing angles from 0° to 80° for the RGB µLED display with top black matrix and 120° taper angle. The average color shift at 80° is 0.05, and the maximum value for the magenta channel is 0.014, which is below 0.02, and thus it is acceptable for commercial applications. The simulated radiation patterns for RGB µLEDs with the top black matrix clearly illustrate that the sidewall emission completely disappears owing to the presence of the black matrix, and the color shift is significantly reduced with a matched angular distribution for both RGB colors, as shown in Figure 7c. Therefore, with a high light extraction output, this device structure is believed to significantly suppress the color shift.

Nanomaterials 2020, 10, 1327 11 of 26

Further, a brief description about the developmental history of QD patterning technology is given in the following Table1. For a µLED, as the array size decreases, its light-emitting efficiency improves while experiencing massive non-radiative recombination from sidewall defects; however, the far-field radiation pattern will deviate from the ideal Lambertian distribution depending on the sidewall emission, resulting in the color shift of µLED displays. The color-conversion method and mass transfer process are two common solutions to achieving the full-color display; however, manufacturing challenges remain in terms of achieving high yield, large size displays such as tablets, monitors, TVs, video walls, etc., [50,51]. The most widely used commercial LED epitaxy wafer is based on structures of multi quantum wells (MQW), i.e., GaInP/AlGaInP MQWs for red LEDs and blue and green LEDs for InGaN/GaN MQWs [52], resulting in inconsistent angular distribution between RGB µLEDs due to different epitaxy materials and structures between them. Consequently, there is a color shift of mixed colors in RGB µLEDs displays. To minimize this particular color shift, Gou et al. introduced a device layout and studied the angular color shift of RGB µLED displays from a mismatched angular distribution suggesting a simulation model to support the experimental findings [53]. The author found that the green and blue chip sidewall emissions are far larger than those of red chips due to better absorption in red MQWs. The light emission from the red chip decreases because of the Lambert’s cosine law, while that of green and blue µLEDs increases and subsequently decreases. The difference in material dynamics of RGB chips will cause an angular shift in mixed colors. Owing to the low-cavity effect within µLEDs, no color change is observed at a fixed driving current for primary colors. The color shift problem is exacerbated as the size of the µLEDs decreases because of increased sidewall emissions from green and blue chips. To mitigate the color change, green and blue µLED sidewall emissions must be removed, and the RGB radiation patterns, i.e., the Lambertian distribution, must be obtained. Hence, Gou et al. suggested a system mode shown in Figure7a, where the system consists of a µLED array and top black matrix outside the emission region. Further, the space between the µLEDs is filled with resin with a refractive index of about 1.5, such that the sidewall emissions from green and blue chips are completely absorbed by the black matrix, resulting in emissions from the top of RGB µLEDs with matched Lambertian distributions only. Moreover, the author introduces a taper angle, α, shown in Figure7a, to boost the light intensity from the top emission. The light intensity from the green and blue chips is observed to increase as α increases from 90◦ to 140◦, upon which it saturates, with little influence on the red chip. The color shift is extreme when α is larger than 130◦, and the wider taper angle contributes to a narrower angular distribution between green and blue emissions, with no effect on the red channel, thus inducing a severe color shift. Figure7b illustrates the simulated color shifts of ten reference colors at viewing angles from 0◦ to 80◦ for the RGB µLED display with top black matrix and 120◦ taper angle. The average color shift at 80◦ is 0.05, and the maximum value for the magenta channel is 0.014, which is below 0.02, and thus it is acceptable for commercial applications. The simulated radiation patterns for RGB µLEDs with the top black matrix clearly illustrate that the sidewall emission completely disappears owing to the presence of the black matrix, and the color shift is significantly reduced with a matched angular distribution for both RGB colors, as shown in Figure7c. Therefore, with a high light extraction output, this device structure is believed to significantly suppress the color shift. Nanomaterials 2020, 10, 1327 12 of 26 Nanomaterials 2020, 10, x FOR PEER REVIEW 13 of 29

Figure 7. (a) RGB µLED display with top black matrix. (b) Simulated color shifts of ten reference colors Figure 7. (a) RGB µLED display with top black matrix. (b) Simulated color shifts of ten reference at viewing angles from 0 to 80 for the RGB µLED display with a top black matrix and 120 taper colors at viewing angles◦ from 0°◦ to 80° for the RGB µLED display with a top black matrix and◦ 120° angle. (c) Simulated radiation patterns for RGB µLEDs with top black matrix [53]. Figure reproduced taper angle. (c) Simulated radiation patterns for RGB µLEDs with top black matrix [53]. Figure with permission from Optical Society of America. reproduced with permission from Optical Society of America. 3. White-Light-Emitting Diode 3. White-Light-Emitting Diode Despite numerous challenges, such as sidewall emissions, as discussed above, LEDs have gained Despite numerous challenges, such as sidewall emissions, as discussed above, LEDs have gained considerable attention over the past decade, both in the scientific community and in the industry, considerable attention over the past decade, both in the scientific community and in the industry, owing to their energy-saving capabilities [42,54–56]. White-light LEDs (WLEDs) have lower power owing to their energy-saving capabilities [42,54–56]. White-light LEDs (WLEDs) have lower power consumption, high durability, smaller size, and high power efficiency compared to conventional consumption, high durability, smaller size, and high power efficiency compared to conventional lighting methods, hence they are expecting to become next generation solid-state lighting devices. lighting methods, hence they are expecting to become next generation solid-state lighting devices. In3 + In 1996, Nichia commercialized white LEDs by combining a blue LED chip with yellow Y3Al5O12:Ce 1996, Nichia commercialized white LEDs by combining a blue LED chip with yellow Y3Al5O12:Ce3+ (YAG) phosphor. WLEDs were originally used for backlighting LCDs, but with improved output (YAG) phosphor. WLEDs were originally used for backlighting LCDs, but with improved output power,power, they they will will be be increasingly increasingly used used in in flashlights flashlights andand otherother lighting sources. Additionally, Additionally, to to enable enable thethe effi efficientcient use use of of white white LEDs LEDs in in general general lighting, lighting, thethe luminousluminous fluxflux of a single white white LED LED must must be be improvedimproved and and to to increase increase the the luminous luminous effi efficacycacy of of white white LEDs, LEDs, the the effi efficiencyciency of of blue blue LEDs LEDs needs needs to to be increased,be increased, as they as arethey the are source the ofsource excitation of excitation light. Several light. studiesSeveral have studies concentrated have concentrated on improving on theimproving internal quantumthe internal effi quantumciency (IQE) efficiency for the (IQE) blue for LEDs the by blue enhancing LEDs by the enhancing growth ofthe crystals growth andof researchingcrystals and to researching enhance the to device’s enhance light the device’s extraction light effi extractionciency. efficiency. Further,Further, QDs QDs are are excellent excellent materials materials for for white white LEDLED (WLED)(WLED) applications owing owing to to their their various various basicbasic properties, properties, such such as as a a wide wide absorption absorption band,band, narrownarrow emissionemission peak, wavelength-dependent wavelength-dependent size size whichwhich could could cover cover all all the the visible visible spectrum spectrum byby changingchanging thethe particle size, high high quantum quantum yields yields (QYs), (QYs), andand good good spectral spectral overlap,overlap, all all of of which which are are the the efficient efficient improvement improvement factors factors for WLEDs for WLEDs [57–59]. [57 New–59 ]. Newforms forms of WLEDs of WLEDs have have been been developed developed overover the pa thest pastfew years few years by combining by combining CdSe-based CdSe-based QDs and QDs andyellow yellow phosphor phosphor demonstrating demonstrating a higher a higher luminous luminous efficacy efficacy compared compared to to that that using using all all phosphor phosphor materials,materials, but but such such a a structure structure leads leads toto highhigh QDQD reabsorptionreabsorption loss due to high thermal thermal power power and and reducedreduced optical optical stability stability [ 60[60].]. Vertically Vertically layered layered packagingpackaging (VLP)(VLP) structures are are believed believed to to enhance enhance thethe optical optical effi ciencyefficiency of the of QD-phosphorus the QD-phosphoru (Q-P) hybrids (Q-P) WLEDs hybrid but WLEDs unfortunately but unfortunately the backscattering the issuebackscattering of phosphor issue is not of solvedphosphor in suchis not VLP solved structure in such [ 61VLP–64 structure]. [61–64].

3.1.3.1. CdSe CdSe QD-Based QD-Based WLED WLED Recently,Recently, Zongtao Zongtao Li Li et et al. al. has has demonstrated demonstrated aa configuration-basedconfiguration-based quasi- quasi-horizontalhorizontal separation separation (c-HS)(c-HS) structure structure to to increase increase the the QD QD light light extraction extraction eefficiencyfficiency ofof QD—phosphor-basedQD—phosphor-based white white LEDs LEDs (WLEDs)(WLEDs) to to resolve resolve this this issue issue of of backscattering backscattering problem problem [[65].65]. This structural arrangement arrangement is is reported reported toto eff effectivelyectively suppress suppress the the backscattered backscattered lossloss fromfrom thethe phosphorphosphor at the top region region of of the the QD QD layer layer giving excellent radiative power and luminous flux compared to the conventional vertical layered packaging structure. The fabrication process for this hybrid structure is shown in Figure 8.

Nanomaterials 2020, 10, 1327 13 of 26 giving excellent radiative power and luminous flux compared to the conventional vertical layered packagingNanomaterials structure. 2020, 10, x The FORfabrication PEER REVIEW process for this hybrid structure is shown in Figure8. 14 of 29

Figure 8. (a) Diagram of the fabrication process of WLEDs c-HS structure. Diagram of the light-extraction Figure 8. (a) Diagram of the fabrication process of WLEDs c-HS structure. Diagram of the light- mechanism in Q–P hybrid WLEDs. (b) Backscattered blue light and backward-emission yellow light extraction mechanism in Q–P hybrid WLEDs. (b) Backscattered blue light and backward-emission in the top region of reference structure; (c) backscattered red light in the top region of reference yellow light in the top region of reference structure; (c) backscattered red light in the top region of structure; (d) extraction red light with less backscattering in the top region of the c-HS structure [65]. reference structure; (d) extraction red light with less backscattering in the top region of the c-HS Figure reproduced with permission from Optical Society of America. structure [65]. Figure reproduced with permission from Optical Society of America. In Figure8a, QDs are mixed with silicone, then the mixture is dispensed on the blue chip and In Figure 8a, QDs are mixed with silicone, then the mixture is dispensed on the blue chip and subsequentlysubsequently treated treated in in the the oven oven at at 150 150◦ C.°C. AA hollowhollow semi-spherical lens lens is isinserted inserted into into the theframe frame of of the devicethe device structure structure and and phosphor phosphor slurry slurry (phosphorus (phosphorus mixed mixed with with silicone) silicone) is isthen then pumped pumped into into the the hollowhollow space space followed followed by by 3-hour 3-h heat heat treatment treatment at at 100 100 °C◦ Cin in the the oven. oven. Centrifugation Centrifugation technology technology is is commonlycommonly used used tomonitor to monitor the the phosphorus phosphorus distribution distribution in in the the device device and andthis this willwill helphelp toto increase the colorthe uniformity color uniformity of phosphor-converted of phosphor-converted LEDs [LEDs66,67 ].[66, Therefore, 67]. Therefore, centrifugation centrifugation technology technology is adopted is for thisadopted c-HS for structure this c-HS to monitorstructure the to phosphorusmonitor the ph settlementosphorus after settlement injection after of injection phosphor, of whichphosphor, is again processedwhich inis again the oven processed to complete in the oven the packaging. to complete The the authorpackaging. also The discusses author thealso ediscussesffect of centrifugation the effect speedof oncentrifugation the optical speed performance on the optical of the performance device as the of phosphor the device concentration as the phosphor is loweredconcentration on the is top regionlowered of the on structure the top region with increaseof the structure in centrifugation with increase speed. in centrifugation Because of speed. the strong Because scattering of the strong effect of scattering effect of phosphor, the backscattered blue light emitted from the blue chip and the phosphor, the backscattered blue light emitted from the blue chip and the backward emission light backward emission light from the phosphor will penetrate through the QD layer again and increase from the phosphor will penetrate through the QD layer again and increase QD’s absorption efficiency, QD’s absorption efficiency, thus emitting more red light from QD. Thus, with an increase in thus emitting more red light from QD. Thus, with an increase in centrifugation speed, the concentration centrifugation speed, the concentration of phosphorus decreases in the top region of the device, then of phosphorusthe backscattering decreases probability in the of top red region light from of the ph device,osphor thendecreases, the backscattering suppressing the probability portion of red of red lightlight from that phosphor penetrates decreases, through suppressingthe QD layer thethus portion reducing of the red reabsorption light that penetrates loss which through can be seen the QD layerfrom thus Figure reducing 8b–d. the In reabsorption this way, more loss red which light can of beQDs seen can from be extracted Figure8b–d. from In the this device way, as more the red lightcentrifugation of QDs can speed be extracted increases. from In addition, the device the ra asdiant the power centrifugation and the luminous speed increases.flux for the reference In addition, the radiantand c-HS power structure and are the compared luminous with flux the for variou the references correlated and color c-HS temperature structure are(CCT) compared values as with the variousshown in correlated Figure 9a,b color and from temperature the figure (CCT)it is very values clear that as shown the radiant in Figure power9 anda,b andthe luminous from the flux figure it isof very the clearQ-P thathybrid the WLEDs radiant with power c-HS and structure the luminous are obviously flux of higher the Q-P than hybrid that of WLEDs the reference with c-HS structurestructure are with obviously an improvement higher than of that13.6% of and the reference10.8% respectively, structure at with a standard an improvement warm white of color 13.6% of and

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10.8% respectively, at a standard warm white color of ~4000 K. The c-HS structure is therefore beneficial for lessNanomaterials backscattered 2020, 10, x red FOR light PEER andREVIEW better QD-layer light-extraction performance. 15 of 29

Figure 9. (a) Radiant power and (b) luminous flux of the reference structure and c-HS structure at Figure 9. (a) Radiant power and (b) luminous flux of the reference structure and c-HS structure at different correlated color temperature (CCT) values [65]. Figure reproduced with permission from different correlated color temperature (CCT) values [64]. Figure reproduced with permission from Optical Society of America. Optical Society of America. Commercial WLEDs are generally produced using the blue GaN LEDs to pump yellow phosphor, Commercial WLEDs are generally produced using the blue GaN LEDs to pump yellow as discussedphosphor, inas the discussed above section.in the above But section. under highBut under current high injection, current GaN-basedinjection, GaN-based LEDs confronted LEDs someconfronted efficiency some droop efficiency problems droop like problems current like droop curre (reductionnt droop (reduction of efficiency of efficiency with increase with increase in current) andin thermal current) droop and thermal (reduction droop of (reduction efficiency of with efficiency increase with in temperature)increase in temperature) [68,69]. Some [68,69]. researchers Some haveresearchers identified have the causeidentified for the this cause efficiency for this droop, efficien andcy droop, it is dueand it to is various due to various factors factors such such as built-in as fieldsbuilt-in [70], Augerfields [70], recombination Auger recombination [71], and electron[71], and leakage electron [ 72leakage]. Some [72]. researchers Some researchers have suggested have in recentsuggested developments in recent developments that these limiting that these factors limiting can factors be reduced can be reduced by employing by employing InGaN InGaN QDs for LEDsQDs in for the LEDs active in layer.the active Chunyu layer. Chunyu Zhao et Zhao al. has et al. demonstrated has demonstrated a green a green light light emitting emitting InGaN InGaN QD LEDQD structure LED structure grown ongrown c-plane on sapphirec-plane sapphire substrate su usingbstrate metal using organic metal chemicalorganic chemical vapor deposition vapor (MOCVD)deposition technique (MOCVD) showing technique a reducedshowing a e ffireducedciency effi droop,ciency highdroop, temperature high temperature stability, stability, and and strong strong internal quantum efficiency (IQE) for the LED operating in “green gap” [73]. Green gap refers internal quantum efficiency (IQE) for the LED operating in “green gap” [73]. Green gap refers to the to the reduction of efficiency of LEDs in the green-yellow range of the visible spectrum. The QD LED reduction of efficiency of LEDs in the green-yellow range of the visible spectrum. The QD LED structure structure consists of GaN capping that protects the QDs during subsequent GaN growth at high consists of GaN capping that protects the QDs during subsequent GaN growth at high temperature. temperature. FigureFigure 10a 10a represents represents the the TRPL TRPL measurement measurement showing showing the the mono-exponential mono-exponential decay decay spectrum spectrum of the cappedof the capped QDs atQDs 300K at 300K and and 18K. 18K. The The lifetime lifetime is is achieved achieved atat 480480 ps at at 300 300 and and the the radiative radiative lifetime lifetime at 18at K 18 is K 707 is 707 ps indicatingps indicating that that non-radiative non-radiative recombinationrecombination at at low low temperature temperature is fully is fully suppressed. suppressed. SinceSince the radiativethe radiative lifetime lifetime is inversely is inversely proportional proportional to the to overlap the overlap of electron of electron and hole and wave hole functions,wave thisfunctions, shorter lifetime this shorter is a lifetime solid indication is a solid indication of built-in of fieldsbuilt-in reduction. fields reduction. The light The light output output power power (LOP) of the(LOP) device of the is showndevice is in shown Figure in 10 Figureb showing 10b showing a linear aincrease linear increase withrespect with respect to the to injected the injected current densitycurrent up density to 106 Aup/cm to 2106. To A/cm investigate2. To investigate the built-in the built-in field infield the in QDs,the QDs, the the electroluminescence electroluminescence (EL) spectra(EL) of spectra the LED of the is measured LED is measured for diff erentfor differen injectiont injection currents currents at room at room temperature temperature which which are shownare in Figureshown 10 inc. Figure It is apparent10c. It is apparent that there that isthere no shiftis no shift of EL of peakEL peak wavelength wavelength as as the the injectioninjection current current is is gradually increased from 1 A/cm2 to 106 A/cm2 suggesting a fully screened built-in fields and yet gradually increased from 1 A/cm2 to 106 A/cm2 suggesting a fully screened built-in fields and yet green QD LEDs with no observed efficiency drop have not yet been identified. Figure 10d depicts the green QD LEDs with no observed efficiency drop have not yet been identified. Figure 10d depicts external quantum efficiency (EQE) of the LED as a function of the injection current density and it is the externalvery clear quantum that the efficiency efficiency without (EQE) droop of the up LED to an as injection a function current of the density injection of 106 current A/cm2 densityis due toand 2 it isfully very screened clear that built-in the e ffifieldciency thereby without inducing droop an in upcrease to an in injectionthe rate of current radiative density recombination. of 106 A The/cm is dueAuger to fully recombination screened built-in coefficient field therebyis calculated inducing from the an increasetheoretical in simulation the rate of of radiative IQE as shown recombination. in the Thefigure Auger and recombination found to havecoe a veryfficient low value is calculated which indica fromtes the that theoretical it does not simulationcontribute to of the IQE efficiency as shown in thedroop. figure Hence, and foundthere is tostrong have evidence a very lowthat valueInGaN whichQDs can indicates substantially that itenhance does not LED contribute performance to the efficiencyand solve droop. efficiency Hence, droop there problems is strong with evidence fully screened that InGaN built-in QDsfields. can substantially enhance LED performance and solve efficiency droop problems with fully screened built-in fields.

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Figure 10. (a) TRPL measurements of capped QDs; (b) LOP versus injection current density; Figure 10. (a) TRPL measurements of capped QDs; (b) LOP versus injection current density; (c) (c) electroluminescenceFigure 10. (a) TRPL measurementsspectra of QD of capped LED; QDs; (d) EQE (b) LOP as aversus function injection of current current density; density (c) [73]. electroluminescence spectra of QD LED; (d) EQE as a function of current density [72]. Figure Figureelectroluminescence reproduced with spectra permission of QD from LED; Optical (d) EQE Society as aof function America. of current density [72]. Figure reproduced with permission from Optical Society of America. reproduced with permission from Optical Society of America. 3.2. Perovskite QD-Based WLED 3.2. Perovskite QD-Based WLED 3.2. Perovskite QD-Based WLED In the recent years, halide PQDs were demonstrated as spectacular for lighting In the recent years, halide PQDs were demonstrated as spectacular semiconductors for lighting applications.In the recent As stated years, in halide the previousPQDs were sections, demonstrated PQDs as ospectacuffer significantlar semiconductors advantages for inlighting terms of applications. As stated in the previous sections, PQDs offer significant advantages in terms of photoluminescenceapplications. As stated quantum in the yield, previous high sections color purity,, PQDs more offer tolerantsignificant to defects,advantages etc., in comparedterms of to photoluminescence quantum yield, high color purity, more tolerant to defects, etc., compared to other otherphotoluminescence types of QDs, hence quantum they yield, are promising high color purity, for next-generation more tolerant to display defects, and etc., lightingcompared technologies. to other typestypes of ofQDs, QDs, hence hence they they are are promising promising for for next-gen next-generationeration displaydisplay and lighting technologies.technologies. Lin Lin et et Lin et al. demonstrated hybrid-type white LEDs based on inorganic halide PQDs in three different al.al. demonstrated demonstrated hybrid-type hybrid-type whitewhite LEDsLEDs basedbased onon inorganicinorganic halide PQDs inin threethree differentdifferent geometries including liquid, solid, and hybrid types; they presented performance improvements for geometriesgeometries including including liquid, liquid, solid, solid, and and hybrid hybrid ty types;pes; theythey presentedpresented performance improvementsimprovements for for realistic applications by addressing the issues mentioned above [74]. Figure 11a–c shows the systematic realisticrealistic applications applications by by addressing addressing thethe issuesissues mentionedmentioned aboveabove [74]. FigureFigure 11a–c11a–c showsshows thethe proceduresystematicsystematic for procedure theprocedure fabrication for for the the offabrication fabrication liquid-, solid-, of of liquid-, liquid-, and solid-, hybrid-typesolid-, andand hybrid-typehybrid-type device structures. device structures.structures.

Figure 11. Process flow of perovskite quantum dots (PQDs)-based WLEDs: (a) liquid, (b) solid, and (c) hybrid types [74]. Figure reproduced with permission from Optical Society of America. Nanomaterials 2020, 10, x FOR PEER REVIEW 17 of 29 Nanomaterials 2020, 10, 1327 16 of 26

Figure 11. Process flow of perovskite quantum dots (PQDs)-based WLEDs: (a) liquid, (b) solid, and (c) hybrid types. Figure reproduced with permission from Optical Society of America. In this study, hot-injection methods were employed to synthesize the inorganic green (CsPbBr3) and red (CsPbBr1.2I1.8) light halide PQDs, all of which were pumped by the blue LED. Figure 12a In this study, hot-injection methods were employed to synthesize the inorganic green (CsPbBr3) encompasses a wide area of NTSC (120%) and Rec. 2020 standards (90%), while Figure 12b covers the and red (CsPbBr1.2I1.8) light halide PQDs, all of which were pumped by the blue LED. Figure 12a widerencompasses areas of 122% a wide of area NTSC of andNTSC 91% (120%) of Rec. and 2020 Rec. standards. 2020 standards Although (90%), the while solid-type Figure structure12b covers system the exhibitedwider areas large of values 122% in of the NTSC color and gamut, 91% it of was Rec. not 20 as20 powerful standards. as theAlthough hybrid-type. the solid-type Hence, thestructure decrease insystem productivity exhibited depends large primarilyvalues in the on thecolor type gamut, of packaging it was not [ 75as]. powerful Aggregated as the solid-type hybrid-type. QDs Hence, enclosed inthe silicone decrease resin in can productivity cause dispersion depends andprimarily re-absorption, on the type leading of packaging to loss [75]. of effi Aggregatedciency [58, solid-type76], and it is believedQDs enclosed that the in effi siliconeciency ofresin QDs can may cause be improveddispersion byand changing re-absorption, the structure leading and to loss dispersion of efficiency of QDs. Consequently,[58,76], and it the is believed hybridtype that exhibitsthe efficiency improved of QDs performance may be improved and a by wide changing color gamut the structure compared and to bothdispersion the liquid of andQDs. solid Consequently, forms. the hybrid type exhibits improved performance and a wide color gamut compared to both the liquid and solid forms.

Figure 12. Color gamut on CIE 1931 color space for (a) solid and (b) hybrid-type WLED. Figure 12. Color gamut on CIE 1931 color space for (a) solid and (b) hybrid-type WLED. (c) Luminance (c)efficiency Luminance with effi ciencyrespectwith to current. respect to(d)current. Stability ( dof) Stabilitysolid- and of solid-hybrid-type and hybrid-type WLEDs [74]. WLEDs Figure [74 ]. Figurereproduced reproduced with permission with permission from Optical from Optical Society Society of America. of America.

TheThe emission emission spectra spectra appearappear unchanged after after 200 200 h hof ofaging, aging, leading leading to a to nearly a nearly constant constant area of area ofcolor color gamut gamut because because of constant of constant spectral spectral form. form.The stability Thestability of the fabricated of the fabricateddevices candevices be estimated can be estimatedfrom Figure from 11. Figure The 11performance. The performance based on based the current-dependent on the current-dependent efficiency effi forciency both for solid- both and solid- andhybrid-type hybrid-type WLED WLED is shown is shown in Figure in Figure 12c, 12 indicatic, indicatingng that thatthe efficiency the efficiency for the for hybrid-type the hybrid-type is 51 is 51lm/W, lm/W, whereas whereas that that for for the the solid-type solid-type is 41 is lm/W 41 lm, /demonstratingW, demonstrating a significant a significant improvement improvement in case in caseof ofthe the hybrid-type. hybrid-type. The Theself-aggregation self-aggregation of QDs of QDsin solid in solidPQD PQDfilms filmsis the ismain the maincause causebehind behind the theefficiency efficiency loss loss [77]. [77 Figure]. Figure 12d il 12lustratesd illustrates the luminance the luminance efficiency effi ofciency the devices, of the showing devices, a decrease showing a decreaseof only of12.02% only for 12.02% the hybrid-type for the hybrid-type WLEDs and WLEDs a corresponding and a corresponding decrease of decrease28% for the of solid-type 28% for the solid-typeafter being after operated being operatedfor 200 h. forFrom 200 these h. From results, these it is results, inferred it that is inferred the hybrid-type that the hybrid-typedevice exhibits device a exhibitssubstantial a substantial increase in increase stability, in aside stability, from aside a high from performance a high performance compared to compared the solid type. to the Moreover, solid type. the temperature in the solid-type device was found to be 78 °C at 250 mA. This high surface Moreover, the temperature in the solid-type device was found to be 78 ◦C at 250 mA. This high surface temperature arises from the non-radiative relaxation energy. Consequently, the system suffers from a decline in performance and reliability. In contrast, the hybrid-type unit has a low surface temperature, NanomaterialsNanomaterials2020 2020, 10, 10, 1327, x FOR PEER REVIEW 18 of17 29 of 26

temperature arises from the non-radiative relaxation energy. Consequently, the system suffers from i.e.,a 40decline◦C at in 250 performance mA, which and makes reliability. the system In contrast, more reliable the hybrid-type in terms of unit performance has a low and surface quality. Thetemperature, hybrid structure i.e., 40 is °C being at 250 proposed mA, which to solve makes the th rede system perovskite more anion reliable exchange in terms problem of performance to achieve anand ultra-high quality. gamut; The hybrid however, structure the reliability is being ofpropos this designed to solve must the be furtherred perovskite improved anion to meet exchange specific applicationproblem to requirements achieve an ultra-high in future research.gamut; however, Several attemptsthe reliability have of been this made design over must the be past further decade toimproved develop highly to meet functional specific applicat and long-lastingion requirements solutions in future for the resear use ofch. WLEDs’ Several applicationattempts have in displaybeen technologymade over and the room past lightingdecade to systems. develop highly functional and long-lasting solutions for the use of WLEDs’In another application research in for display PQDs-based, technology Kang and et room al. demonstrated lighting systems. white light emitting diodes (WLEDs) In another research for PQDs-based, Kang et al. demonstrated white light emitting diodes using PQD exhibiting an ultrahigh luminous intensity, wide color gamut, and long operational (WLEDs) using PQD paper exhibiting an ultrahigh luminous intensity, wide color gamut, and long lifetime [78]. The aim of this research was to establish a cost-effective and scalable production technique operational lifetime [78]. The aim of this research was to establish a cost-effective and scalable to resolve high-energy radiation by demonstrating a paper manufacturing process using cellulose production technique to resolve high-energy radiation by demonstrating a paper manufacturing nanocrystals (CNCs) to manufacture a new form of PQD film. Figure 13a illustrates the manufacturing process using cellulose nanocrystals (CNCs) to manufacture a new form of PQD film. Figure 13a process of PQD paper. The CNC suspension and CH NH PbBr are combined in dimethylformamide, illustrates the manufacturing process of PQD paper.3 The3 CNC3 suspension and CH3NH3PbBr3 are uponcombined which thein dimethylformamide, mixture is dried on upon the membrane which theto mi producexture is dried the PQDs on the paper. membrane In comparison to produce to the other approaches,PQDs paper. this In technique comparison is fast, to other convenient, approaches, and consumes this technique less time is fast, to aidconvenient, in the rapid and development consumes ofless PQD time paper. to aid The in the inclusion rapid development of cellulose of nanocrystals PQD paper. (CNC) The inclusion enables of the cellulose orderly nanocrystals arrangement (CNC) of the crystalenables structure, the orderly thus arrangement creating mechanical of the crystal strength struct inure, the thus paper creating and allowing mechanical the strength capping in ligands the topaper encapsulate and allowing the growth the ofcapping the QD ligands structures to enca bypsulate reprecipitation, the growth thereby of the contributing QD structures to strong by fluorescencereprecipitation, emission. thereby However, contributing the useto strong of CNC degrades theemission. quantum However, yield of the the use PQDs, of CNC as the CNCsdegrades absorb the light quantum in the yield ultraviolet of the PQDs, region. as From the CNCs the TEM absorb and light SEM in images the ultraviolet shown inregion. Figure From 13b,c, it isthe evident TEM and that SEM the images size of shown the PQDs in Figure lies within 13b,c, theit is range evident of that 3–8 nm,the size which of the facilitates PQDs lies a powerfulwithin quantumthe range confinement of 3–8 nm, which effect. facilitates This confinement a powerful eff quectantum plays confinement a major role effect. in enhancing This confinement the light emittingeffect effiplaysciency a major of the role perovskite. in enhancing the light emitting efficiency of the perovskite.

FigureFigure 13. 13.(a ()a Schematic) Schematic of of fabrication fabrication processprocess ofof PQDPQD paper. ( (bb)) TEM TEM image image of of CH CH3NH3NH3PbBr3PbBr3 PQDs.3 PQDs. (c)( SEMc) SEM of of PQD PQD paper paper surface surface [ 78[78].]. Figure Figure reproducedreproduced with permission permission from from John John Wiley Wiley and and Sons. Sons.

TheThe emission emission peak peak also also corresponds corresponds toto thethe absorptionabsorption edge, edge, and and the the PQDs PQDs paper paper can can serve serve as asa a lightlight converter converter for for the the blue blue GaN-LED GaN-LED chips,chips, asas itit exhibits high high absorption absorption characteristics characteristics in inthe the short short wavelengthwavelength region. region. In In this this study,study, thethe authorsauthors focusedfocused on on using using PQDs PQDs paper paper as as green green converter converter for for whitewhite LED, LED, which which is is achieved achieved by by placing placing thethe PQDsPQDs paper on on top top of of the the blue blue LED LED package package dispensed dispensed 4+ with a mixture of red K2SiF6: Mn (KSF) phosphor and resin. The PQDs paper-based LED Nanomaterials 2020, 10, x FOR PEER REVIEW 19 of 29 Nanomaterials 2020, 10, 1327 18 of 26

with a mixture of red K2SiF6: Mn4+ (KSF) phosphor and silicon resin. The PQDs paper-based LED displays a wide color gamut of 123% according to the NTSC standard and 92% according to the Rec. displays a wide color gamut of 123% according to the NTSC standard and 92% according to the Rec. 2020 standard, as shown in Figure 14a. This is mainly suitable for 8K4K display technologies of the 2020 standard, as shown in Figure 14a. This is mainly suitable for 8K4K display technologies of the next next decade [79]. Figure 14b depicts the current-dependent luminous efficiency of the PQD paper- decade [79]. Figure 14b depicts the current-dependent luminous efficiency of the PQD paper-based based LED, showing a high value of 124 lm/W at 6 mA. Even at different drive currents, the PQD LED, showing a high value of 124 lm/W at 6 mA. Even at different drive currents, the PQD paper-based paper-based LED exhibits high performance by consistently exhibiting a luminous efficiency above LED exhibits high performance by consistently exhibiting a luminous efficiency above 100 lm/W. 100 lm/W. Further, the reliability of the device is depicted by the red line in Figure 14c, indicating Further, the reliability of the device is depicted by the red line in Figure 14c, indicating only 12.4% only 12.4% depletion of the luminous flux after 240 h of operation. depletion of the luminous flux after 240 h of operation.

Figure 14. (a) CIE diagram illustrating the color gamut of the NTSC standard, the Rec. 2020 standard, andFigure the 14. PQD (a) CIE paper-based diagram illustrating LED. (b) Current-dependent the color gamut of the luminous NTSC standard, efficiency the and Rec. luminous 2020 standard, flux of theand PQD the PQD paper-based paper-based LED. LED. (c) Time-dependent (b) Current-dependent luminous luminous flux of effici the LEDency device and luminous under continuous flux of the operationPQD paper-based [78]. Figure LED. reproduced (c) Time-dependent with permission luminous from Johnflux Wileyof the and LED Sons. device under continuous operation [78]. Figure reproduced with permission from John Wiley and Sons. Figure 15a,b provides a summary of the PQD paper-based device and other reported LEDs usingFigure QDs as 15a,b the colorprovides filter. a summary The main of reason the PQD behind paper-based the high performancedevice and other of this reported PQD paper-based LEDs using LEDQDs as is attributedthe color filter. to the The QD main structure reason structured behind the inside high performance the cellulose of material, this PQD which paper-based is primarily LED becauseis attributed capping to the ligands QD structure play a major structured role in inside creating the a cellulose strong complex material, structure which is on primarily the PQD because surface, therebycapping enhancing ligands play the a stability major role of the in creating PQD [32 a]. strong Some ofcomplex the challenges structure faced on the by numerousPQD surface, researchers thereby areenhancing the loss the of ligands,stability asof forthe conventionalPQD [32]. Some colloidal of the PQDchallenges synthesis, faced the by longnumerous chain oleicresearchers acid (OA) are andthe loss oleylamine of ligands, (OLA) as for ligands conventional are not tightlycolloidal bound PQD to synthesis, the PQD the surface long andchain can oleic be lostacid during (OA) and the manufacturingoleylamine (OLA) process, ligands making are not them tightly the key bound factor to for the PQD PQD instability surface and [80]. can In this be paper,lost during the issue the ofmanufacturing ligand loss isprocess, completely making avoided, them the as key the PQDsfactor for produced PQD instability are in solid [80]. form, In this as paper, the growth the issue of perovskiteof ligand loss is accompanied is completely by avoided, the CNC as capping the PQDs ligands, produced leading are toin well-capped solid form, andas the stable growth PQDs. of Hence,perovskite the developedis accompanied PQD paperby the demonstrates CNC capping an li impressivegands, leading ability to to well-capped produce high-e andffi stableciency PQDs. white LEDsHence, with the adeveloped wide range PQD of colors, paper indicating demonstrates a promising an impressive display technologyability to produce material. high-efficiency white LEDs with a wide range of colors, indicating a promising display technology material.

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Figure 15. Comparison of device performance between PQD paper-based LED and other reported LEDsFigure using 15. Comparison QDs as color of converters.device performance (a) Luminous between effi PQDciency paper-based and color gamutLED and performance other reported and (LEDsb) operational using QDsdurability as color ofconverters. reported QD-based(a) Luminous LEDs efficiency [78]. Figure and reproducedcolor gamut with performance permission and from (b) Johnoperational Wiley anddurability Sons. of reported QD-based LEDs [78]. Figure reproduced with permission from John Wiley and Sons. In practical applications, however, PQDs face difficulties, such as instability,anion-exchange reactions, QD surfaceIn practical oxidation, applications, photo-bleaching, however, and PQDs aggregation face difficulties, [24,81]. As such mentioned as instability, in the previousanion-exchange section, encapsulationreactions, QD issurface one of theoxidation, methods photo-bleaching, used to increase theand stability aggregation of PQDs [24,81]. and with As thismentioned Yujun Xie in etthe al. demonstratedprevious section, encapsulated encapsulation room-temperature is one of the method synthesizeds used CsPbX to increase3 perovskite the stability quantum of PQDs dots for and display with applicationsthis Yujun Xie showing et al. demonstrated high stability andencaps wideulated color room-temperature gamut to solve these synthesized problems [CsPbX82]. The3 perovskite fabricated green-emittingquantum dots andfor display red-emitting applications QDs are showing being used hi forgh thestability application and wide of quantum color gamut dot-converted to solve whitethese LEDsproblems showing [82]. aThe wide fabricated color gamut green-emitting of 135% NTSC and andred- 101%emitting Rec. QDs 2020 are respectively. being used First, for the green application emitting CsPbBrof quantum3 and dot-converted red emitting CsPbBr white 1.2LEDsI1.8 QDsshowing are synthesized a wide color based gamut on of the 135% method NTSC of supersaturatedand 101% Rec. recrystallization.2020 respectively. The First, fabricated green emitting QDs are CsPbBr mixed with3 and mesoporous red emitting silicates CsPbBr (SBA-15)1.2I1.8 QDs in are hexane synthesized solution andbased then on stirredthe method 600 rpm of for supersaturated 1 h followed by recrystallization. centrifugation at The 4000 fabricated rpm to obtain QDs the are mixed with powdermesoporous of SBA-15 silicates encapsulated (SBA-15) in QDs hexane after drying.solution Inand addition, then stirred the encapsulated 600 rpm for QDs 1 h obtained followed were by dispersedcentrifugation in poly at 4000 (methyl rpm methacrylate) to obtain the(PMMA) nanocomposite and the powder preparation of SBA-15 process encapsulated is shown in FigureQDs after 16a anddrying. subsequently In addition, the the PMMA-SQD encapsulated composite QDs obtained films were were prepared dispersed by in casting poly (methyl on a quartz methacrylate) substrate. The(PMMA) PMMA-QD and the composite preparation films process were also is shown prepared in Figure for references. 16a and Figure subsequently 16b evaluates the PMMA-SQD the thermal stabilitycomposite of thefilms composite were prepared films, which by casting shows on that a thequartz PMMA-SQD substrate. film The is PMMA-QD more resistant composite to the thermal films effectwere also as it prepared shows a better for references. recovery of Figure relative 16b PL evaluates intensity the rising thermal back to stability 70.7% when of the the composite temperature films, is cooledwhich downshows from that 100the◦ CPMMA-SQD to room temperature. film is more The resi measuredstant to photoluminescence the thermal effect quantum as it shows yield a (PLQY) better valuerecovery for PMMA-SQDof relative PL and intensity PMMA-QD rising filmsback areto 70.7% respectively when the 77% temperature and 71.4% but is cooled after heat down treatment from 100 for 16°C h,to theroom PMMA-SQD temperature. film The still measured has a PLQY photolum value ofinescence 48% without quantum any peakyieldwavelength (PLQY) value shift for while PMMA- the PMMA-QDSQD and PMMA-QD film has only films 14.4% are and respectively the peak was 77% red-shifted and 71.4% by morebut after than 2heat nm astreatment shown in for Figure 16 h, 16 thec,d. PMMA-SQDThe photo film stability still has analysis a PLQY of value the composite of 48% without films isany evaluated peak wavelength for the films shift with while continuous the PMMA- UV irradiationQD film has at only a wavelength 14.4% and of the 365 peak nm andwas from red-shift Figureed 17bya, more it is clear than that2 nm the as PLQY shown of in the Figure PMMA-SQD 16c,d. films decreased from 77% to 48.5% after 54 h continuous irradiation, while the PMMA-QD films has only 18% PLQY. To demonstrate the display application for the perovskite composite films, WLED is being created by mounting the green and red emitting QDs films on a blue-LED chip. The color gamut of the resulting white LED is measured and it is able to achieve a wide color gamut of 135% NTSC and 101% Rec. 2020 as shown in Figure 17b. The combination of PMMA and SQD thus leads to outstanding performance of the device by suppressing unfavorable grain growth and surface trap sites.

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Figure 16. (a) A schematic diagram for the preparation process of PMMA-SQD composites; (b) the relative PL intensity as a function of heating temperature from 30 °C to 100 °C; (c) the PLQY values and (d) PL peak wavelength as a function of time when the films continuously heated at 80 °C [81]. Figure reproduced with permission from Optical Society of America.

The photo stability analysis of the composite films is evaluated for the films with continuous UV irradiation at a wavelength of 365 nm and from Figure 17a, it is clear that the PLQY of the PMMA- SQD films decreased from 77% to 48.5% after 54 h continuous irradiation, while the PMMA-QD films has only 18% PLQY. To demonstrate the display application for the perovskite composite films, WLED is being created by mounting the green and red emitting QDs films on a blue-LED chip. The colorFigure gamut 16. of the(a) resulting A schematic white diagram LED is for measured the preparation and it is process able to ofachieve PMMA-SQD a wide composites;color gamut of Figure 16. (a) A schematic diagram for the preparation process of PMMA-SQD composites; (b) the 135%(b NTSC) the relative and 101%PL intensity Rec. 2020 as aas function shown of in heating Figure temperature 17b. The combination from 30 ◦C to of 100 PMMA◦C; (c) and the PLQYSQD thus leadsvaluesrelative to outstanding and PL intensity (d) PL performance peak as a wavelengthfunction of of the he as deviceating a function temperature by suppressing of time from when 30unfavorable the°C to films 100 continuously°C; grain (c) the growth PLQY heated and values surface at and (d) PL peak wavelength as a function of time when the films continuously heated at 80 °C [81]. trap 80sites.◦C[ 82]. Figure reproduced with permission from Optical Society of America. Figure reproduced with permission from Optical Society of America.

The photo stability analysis of the composite films is evaluated for the films with continuous UV irradiation at a wavelength of 365 nm and from Figure 17a, it is clear that the PLQY of the PMMA- SQD films decreased from 77% to 48.5% after 54 h continuous irradiation, while the PMMA-QD films has only 18% PLQY. To demonstrate the display application for the perovskite composite films, WLED is being created by mounting the green and red emitting QDs films on a blue-LED chip. The color gamut of the resulting white LED is measured and it is able to achieve a wide color gamut of 135% NTSC and 101% Rec. 2020 as shown in Figure 17b. The combination of PMMA and SQD thus leads to outstanding performance of the device by suppressing unfavorable grain growth and surface trap sites.

Figure 17. (a) The PLQY values under UV irradiation; (b) color gamut of the resulting WLED [82]. Figure reproduced with permission from Optical Society of America.

3.3. Flexible WLED The WLEDs discussed above are typically brittle and hard, as the bonding strength between the QDs powders and the substrate material is inadequate, often resulting in reduced substrate extensibility for flexible display devices. The development of a stretchable and tough white-light emitting material for flexible display devices is therefore very important. Rogers et al. extensively investigated flexible devices and used transfer printing technology to achieve the goals of flexible lighting technology [83]. However, the technology proved to be too complicated to manufacture the devices. The commonly

Nanomaterials 2020, 10, x FOR PEER REVIEW 22 of 29

Figure 17. (a) The PLQY values under UV irradiation; (b) color gamut of the resulting WLED [81]. Figure reproduced with permission from Optical Society of America.

3.3. Flexible WLED The WLEDs discussed above are typically brittle and hard, as the bonding strength between the QDs powders and the substrate material is inadequate, often resulting in reduced substrate extensibility for flexible display devices. The development of a stretchable and tough white-light emitting material for flexible display devices is therefore very important. Rogers et al. extensively investigated flexible devices and used transfer printing technology to achieve the goals of flexible Nanomaterialslighting technology2020, 10, 1327 [83]. However, the technology proved to be too complicated to manufacture21 of the 26 devices. The commonly applied method is to combine the solid-state lighting and curved light guide; nevertheless, the curved light guide is so stiff that it is unsuitable to be bent numerous times, as appliedrequired method for flexible is to combinedevices [84]. the solid-stateIn 2015, Kuo lighting et al. [85] and used curved the light flip-chip, guide; silicone-based nevertheless, theanisotropic curved lightconductive guide is adhesive, so stiff that and it isphosphor unsuitable film, to be which bent numerouseasily achieved times, the as requiredfabrication for of flexible a flexible devices lighting [84]. Indevice. 2015, KuoA uniform et al. [85 ]flexible used the LED, flip-chip, high-efficiency silicone-based illumination, anisotropic and conductive a wide adhesive, range of andbending phosphor was film,achieved which without easily achieveddeterioration the fabricationof the total oflight a flexible output. lighting Figure device.18a shows A uniforman optical flexible microscope LED, high-eimageffi ofciency the LED illumination, array, and and Figure a wide 18b range shows of the bending flexible was LEDs achieved operating without under deterioration different bending of the totalcurvatures. light output. The size Figure of this 18 flexiblea shows white an optical LED is microscope ~5 × 5 cm2, image and the of total theLED thickness array, is and approximately Figure 18b shows6 mm theat the flexible cross-section LEDs operating of the underflexible di ffwhiteerent LED bending structure. curvatures. Figure The 18c size shows of this the flexible voltage white and LED is ~5 5 cm2, and the total thickness is approximately 6 mm at the cross-section of the flexible luminous ×efficiency as a function of the current, indicating little variation up to a bending diameter whiteof 3 cm. LED Figure structure. 18d Figureshows 18 thec shows top and the bottom voltage andpictures luminous before effi andciency after as bonding a function of of the the adhesive current, indicatingphosphor littlefilm and variation blue LED up toarrays, a bending respectively. diameter The of results 3 cm. indicate Figure 18 thatd shows this large the area top andflexible bottom LED picturesarray is beforesuitable and for afterflexible bonding displays of theand adhesive devices. phosphor film and blue LED arrays, respectively. The results indicate that this large area flexible LED array is suitable for flexible displays and devices.

Figure 18. (a) LED array under optical microscope (left), phosphor layers at 5000 K and 3000 K (right top andFigure bottom, 18. (a respectively).) LED array under (b) Schematic optical microscope of flexible device. (left), phosphor (c) Voltage layers and luminousat 5000 K fluxand with3000 respectK (right totop current and bottom, at different respectively). bending diameters (b) Schematic for the of flexible flexible white device. LED. (c) (dVoltage) Top and and bottom luminous photographs flux with depictrespect before to current and after at different bonding bending the adhesive diameters phosphor for the film flexible and blue white LED LED. arrays, (d) respectivelyTop and bottom [85]. Figurephotographs reproduced depict with before permission and after from bonding Optical the Society adhesive of America. phosphor film and blue LED arrays, respectively [85]. Figure reproduced with permission from Optical Society of America. A brief description and comparative study of different types of displays based on QDs, PQDs, and flexible one have been made in the following Table2 showing their performance and some of the challenges they are facing. Therefore, the future of display technology is quite bright with the advent of QDs in this field and many exciting and unpredictable things can be expected to happen in the coming years with the expectation that everyone can get a highly efficient, reliable, and low-power display technology. Nanomaterials 2020, 10, 1327 22 of 26

Table 2. Comparative study of different display types.

Types Performance Challenges Improved color and energy Reduction in EQE of the devices, efficiency, good at absorption high subjected to non- recombination energy blue light and re-emitting processes including surface QDs-based displays (CdSe) at longer wavelength. CdSe-based trapping, Auger recombination, QDs has more than 90% PLQY and etc. of Cd-based QDs less than 30 nm FWHM leading to limits their use. better color quality. Impressive ultra-narrow linewidth Stability issues making them of 15–18 nm for green, tunable susceptible to degradation from wavelengths leading to higher PQDs-based displays high temperature and light flux. efficiency and brightness. Color shift problem. Content of Emission of light strongly and lead in PQDs. efficiently in a variety of colors. Growing demand for flexible and Some of the issues include wearable displays, integration of thinness and robustness of the flexible QLEDs with wearable QDs-based Flexible displays display, reliability, high cost, sensors, micro-controllers, wireless poor optical clarity, issue of communication units for next mass production generation .

4. Conclusions This study reviews the trends in QDs-based display technology, mainly focusing on µLEDs, including array structures and other perovskite QDs-based LEDs that exhibit high efficiencies and high polarization features. Owing to their exciting potential benefits in terms of performance, power consumption, contrast ratio, lifetime, and response time, QD-LED-based display technology is considered the ultimate option for future generation displays. The development of QD printing technology has been described, including methods of pulsed spray coating, aerosol jet printing, and super inkjet printing. We moreover discuss PQDs-based white light LEDs exhibiting ultra-high luminous intensity and long operation lifetimes. Several challenges facing PQDs, such as heat, pressure, pressure, high-energy radiation, etc., were overcome using various methods developed by researchers that are explained in this study. Moreover, colloidal QD-LEDs are used as color down-converters for LEDs to achieve effective sources of illumination and displays of high quality. A breakthrough in the field of QDs-based µLEDs for use in display technology is certainly expected in the near future.

Author Contributions: Data curation, A.-C.L. and Y.L.; project administration, C.-C.L., Z.C., and K.W.; supervision, Z.L. and H.-C.K.; writing—original draft, Y.-M.H. and K.J.S.; writing—review and editing, T.W. All authors have read and agreed to the published version of the manuscript. Funding: Ministry of Science and Technology, Taiwan (107-2221-E-009-113-MY3, 108-2221-E-009-113-MY3); National Natural Science Foundation of China (11904302); Hsinchu Science Park Bureau, Ministry of Science and Technology, Taiwan (108A08B); Major Science and Technology Project of Xiamen, China (3502Z20191015). Acknowledgments: The authors would like to thank Rong Zhang of Xiamen University and Kei May Lau of Hong Kong University of Science and Technology for the helpful discussion. Conflicts of Interest: The authors declare no conflict of interest.

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