Micro and Nano LEDs

## **Chapter 1**

## Full-Color Micro-LED Devices Based on Quantum Dots

*Tingzhu Wu, Tingwei Lu, Yen-Wei Yeh, Zhong Chen and Hao-Chung Kuo*

## **Abstract**

Quantum dots (QDs) show remarkable optical and electrical characteristics. They offer the advantage of combining micro-LEDs (μLEDs) for full-color display devices due to their exceptional features. In addition, μLED used in conjunction with QDs as color-conversion layers also provide efficient white LEDs for high-speed visible light communication (VLC). In this article, we comprehensively review recent progress in QD-based μLED devices. It includes the research status of various QDs and white LEDs based on QDs' color conversion layers. The fabrication of QD-based highresolution full-color μLEDs is also discussed. Including charge-assisted layer-by-layer (LbL), aerosol jet printing, and super inkjet printing methods to fabricate QD-based μLEDs. The use of quantum dot photoresist in combination with semipolar μLEDs is also described. Finally, we discuss the research of QD-based μLEDs for visible light communication.

**Keywords:** quantum dots, micro-LEDs, full-color displays, visible light communication

## **1. Introduction**

Light-emitting diodes (LED) have been widely used in daily life due to their advantages of high efficiency, energy saving, and long working life, such as lighting sources, full-color displays, and backlight sources of liquid crystal displays [1]. Recently, mobile device displays, such as smart furniture, augmented reality (AR), virtual reality (VR), and wearable devices have piqued extensive attention from the semiconductor industries and research on micro-LEDs (μLEDs), which are referred to as chips with sizes less than 50 × 50 μm2 [2]. Due to their excellent properties in terms of brightness, lifetime, resolution, and efficiency, μLEDs have been considered the most promising next-generation display technologies [3]. The potential of μLEDs to replace conventional display technologies is owing to the combination of their self-emissive mechanism and inorganic material characteristics. Over the past decade, the number of commercially available μLED displays has grown significantly, as manufacturers seek to capitalize on the success of this technology. Sony introduced its first 55-inch full high-definition (HD) μLED TV panel with 1920 × 1080 resolution in 2012, which consists of over 6 million individual μLEDs. Samsung unveiled the world's first consumer modular μLED 146-inch TV in 2018, which is named "The

Wall." In 2022, Samsung will launch a new μLED TV with 25 million pixels, providing vivid colors,high definition, and contrast, and also supports 20-bit grayscale depth, more than 1 million levels of brightness and color. In the academic field, μLEDs have been studied for more than 10 years.

In addition to displays, μLEDs have recently been adopted as transmitters in visible light communication (VLC) systems, based on their quick response times [4]. Currently, radio frequency (RF) communication faces some challenges, such as interference, bandwidth limitations, security issues, transmission power limitations, and power inefficiency [5]. VLC is an emerging technology that addresses the crowded radio spectrum, using visible light to communicate to enable high-speed internet connections. As a light source that is harmless to human body, LED can have both lighting and communication functions when used in VLC, which can save extra power [6].

The modulation bandwidth of an LED is constrained by the carrier lifetime and the time constant consisting of capacitance of a depletion layer and junction differential resistance. Due to their small size, μLEDs can withstand higher injection current densities, thereby enabling smaller carrier lifetimes and higher modulation bandwidths. In addition, a smaller active area will reduce the geometric capacitance of the device, thereby reducing the RC time constant. In addition, μLEDs also have better current uniformity, which will also increase the modulation bandwidth featuring exclusive properties making μLEDs widely applicable for high-speed VLC system.

## **2. Full-color μLED display based on quantum dots**

## **2.1 Background of full-color μLED display**

The commonly used full-color strategy is employing the combination of red–green–blue (RGB) μLED devices in a display. However, this approach has a number of drawbacks. First, the so-called "green gap" created by green μLEDs results in low efficiency [7]. For green LEDs, a high proportion of indium is required in the active region, which requires relatively low growth temperatures, resulting in poor crystal quality of the LED epitaxial layers. In addition, high proportions of indium produce strong polarization fields in InGaN/GaN multiple quantum wells (MQWs) and lead to strong quantum-confined Stark effects, reducing recombination efficiency [7]. Also, red μLEDs are problematic. The active region of the red LED consists of AlGaInP material, which has a high surface recombination velocity (~106 cm/s) [8], coupled with a long carrier diffusion length of about a few microns, making nonradiative surface recombination much more efficient [9]. Therefore, as the device size shrinks to a few microns, the EQE degradation of red μLEDs is more severe than that of blue and green μLEDs. Another problem in the RGB μLED strategy is the mismatch of drive voltages between RGB pixels. The threshold voltage of the blue LED is about 3.3 V, while the threshold voltages of the red and green LEDs are 1.7 and 2.2 V, which complicates the driver circuit design.

To address these issues, blue μLEDs can be integrated with color converters, such as yellow-emitting phosphors or red and green-emitting nanocrystals (NCs), for higher-quality full-color displays [10]. To date, extensive research and development have been carried out on phosphor materials for PC-LEDs. Lin. Wait. Successfully fabricated high luminance efficiency and wide color gamut for NC-based solid-state and hybrid WLED devices, which exhibited higher efficiency (51 lm W<sup>−</sup><sup>1</sup> ), wide color gamut (122% of NTSC and 91% of Rec. 2020), and the efficiency decays by about 12% during the 200-h reliability test [11]. However, organic or inorganic phosphors are generally not suitable for μLED displays due to their spectral width and asymmetry, inherent instability, low red phosphor efficiency, down conversion energy loss, and low absorption cross-section in the blue/UV wavelength region [12]. In addition, the particle size of conventional phosphors may be comparable to or larger than that of μLED chips, which will affect device performance.

## **2.2 QD-based color conversion for μLEDs**

Quantum dots (QDs) are nanoscale semiconductor crystals, and whose electrical and optical properties can be tuned by changing their sizes [13, 14]. Compared with traditional fluorophores, several QD photophysical properties are distinct and unmatched. The first is the ability to tune photoluminescence emission according to the core size and quantum confinement effects of semiconductor binary combinations. This unique advantage allows one to control the emission properties of QDs by controlling the core size [1]. In addition, QDs have broad absorption spectra, starting with blue emission and then steadily increasing towards UV. As combined with the aforementioned properties, quantum dots have emerged as omnidirectional attractive fluorophores for full-color displays.

Core-type QDs, such as CdSe, CdS, or CdTe, are the most studied and commercialized QDs due to their excellent optical and electrical properties [15, 16]. Using these precursors, high fluorescence quantum yields of up to 80% have been reported [17, 18]. However, many heavy metal elements are contained in such quantum dots, and Core-type QDs will pollute the environment, which limits their wide application. To prevent the use of toxic metal elements for the persistent development of QD-based products, the fabrication of Cd/Pb-free QDs has become a subject of intense interest in recent years. In 2020, Soheyli et al. reported a novel aqueous-phase approach for the preparation of multicomponent In-based QDs. Absorption and photoluminescence emission spectra of the as-prepared QDs were tuned by alteration of QDs' composition as Zn-Ag-In -S/ZnS, Ag-In-S/ZnS, and Cu-Ag-In-S/ZnS core/ shell QDs [19]. However, such materials have extremely small color gamut and color purity and are not suitable for display applications [20].

As an environmentally benign material belonging to the III–V group semiconductor, InP is deemed as another promising candidate to replace Cd/Pb-based QDs [21]. InP QDs are typical III–V group semiconductor nanocrystals that feature large excitonic Bohr radius and high carrier mobility [22]. Owing to the merits of low toxicity, high QYs, and broad color tunability, InP QDs are particularly suitable to construct LEDs for indoor/outdoor illumination, traffic signaling, and liquid-crystal display backlighting [23]. In general, InP QDs can be applied in μLEDs either as a photoluminescent layer or an electroluminescent layer. In 2015, Zhang et al. demonstrated a single "cadmium-free" component system consisting of Cu-doped InP core/ZnS barrier/InP quantum well/ZnS shell QDs [24]. These QDs exhibit two emission peaks by controlling the barrier thickness under single-excitation wavelength, one of which is attributed to Cu-doped InP, and the other resulting from InP quantum well. Using optimal structures as color converters, the WLED was obtained with a color rending index (CRI) up to 91 and CIE color coordinates of (0.338, 0.330) by combination with blue LED chip.

The metal halide perovskites QDs (PQDs) are considered the next generation thin-film LED light emitters because of their outstanding characteristics compared to colloidal QDs, which are derived from their narrower FWHM (below 30 nm) [25]. Tunable narrow and symmetrical PL peaks in the visible spectral range can be achieved by tuning the halide composition in perovskite materials. This property yields greater color purity, higher photoluminescence quantum yield (approaching 100%), more convenient synthesis, and lower manufacturing costs than conventional QDs [26]. In addition, PQDs have short carrier recombination lifetimes, which also lays the foundation for their application in the development of white-light VLC systems with high modulation bandwidth [27]. The ability to easily handle colloidal PQDs in solution enables the production of cost-effective large-area light-emitting layers [28]. Therefore, PQDs have been extensively studied in electronic and optoelectronic applications, such as photodetection, photovoltaics, and photoemission; especially as active materials or color converters in LEDs. To date, various PQD-based white LEDs have been reported using diverse structural designs that feature different advantages and disadvantages. However, due to poor thermal resistance and instability under high energy radiation, most PQD-based white light-emitting diodes (WLEDs) show only modest luminous efficiency of approximately 50 lm/W and a short lifetime of <100 h.

In 2019, Kang et al. demonstrated a new type of PQD film called PQD paper by using cellulose nanocrystals (CNCs) [29]. **Figure 1(a)** schematically shows the fabrication process of the PQD paper. The CNC suspension and CH3NH3PbBr3 are combined in dimethylformamide, upon which the mixture is dried on the membrane to produce the PQDs paper. As shown in **Figure 1(b)**, the entangled CNC structure was clearly demonstrated by SEM. With the help of the CNC structure, the PQD paper performs the flexibility and unique mechanical strength. The size of the PQDs was ≈3<sup>−</sup><sup>8</sup> nm, which can enhance the light emission of perovskite by the provided strong quantum confinement, as shown in **Figure 1(c)**. **Figure 1(d)** illustrates the flux and current-dependent luminous efficiency of the PQD paper-based LED, the inset shows the emission of white LED. It can be clearly observed that the PQD paper-based LED has an ultrahigh luminous efficiency of 124 lm W<sup>−</sup><sup>1</sup> and still exhibits a luminous efficiency of over 100 lm W<sup>−</sup><sup>1</sup> even when the drive current increases to 50 mA. Moreover, after continuously working for 240 h, the PQD paper-based LED can maintain 87.6% luminous flux. Compared to the normal flat design, by using curved PQD paper, the viewing angle of LED was increased from 120° to 143° benefiting from the flexible nature of paper. This work also shows the fabricated white LEDs have a wide color gamut of 123% of NTSC standard and 92% of Rec. 2020, as shown in **Figure 1(e)**.

## **2.3 Fabrication of QD-based μLEDs for high-resolution displays**

QD-based display technology has proven to be ideal for next-generation displays. In order to meet the needs of achieving high-resolution display, RGB QDs with sufficiently high pixel density need to be deposited on top of the substrate surface of μLED array. The μLED device realized in this way can replace the traditional OLED or LCD display to realize an ultra-high-definition display, for example, to meet the needs of AR/VR or ultra-high-definition TV. QD deposition and patterning techniques on a selected area of a substrate remain the major bottlenecks in realizing such devices. Therefore, in recent years, researchers have been exploring facile and efficient QDs patterning techniques, such as spray coating, aerosol jet printing, super-inkjet printing, etc.

To accurately locate QDs on the μLED array, the inkjet printing (IJP) has become a key technology for realizing QD-LED display. In 2015, Han et al. combined ultraviolet (UV) μLED and colloidal QDs to achieve a full-color display. RGB QDs were deposited on an array of GaN-based UV μLEDs with the help of an aerosol inkjet

*Full-Color Micro-LED Devices Based on Quantum Dots DOI: http://dx.doi.org/10.5772/intechopen.107280*

#### **Figure 1.**

*(a) Schematic of the fabrication process of the PQD paper. (b) SEM image of the PQD paper surface. (c) TEM image of the CH3NH3PbBr3 PQDs obtained from the paper. The electron diffraction pattern in the inset reveals the high crystallinity of the PQDs. (d) Current-dependent luminous efficiency and luminous flux of the PQD paper-based LED. The inset shows the emission of white LED. (e) CIE diagram illustrating the color gamut of the NTSC standard, the Rec. 2020 standard, and the PQD paper-based LED [29]. Figure reproduced with permission from John Wiley and Sons.*

printer to ensure fine printing that is highly precise and mask-less and to enable noncontact deposition of liquids containing functional materials [30]. MQWs μLED arrays are grown on sapphire substrates. The UV micro-LED array is fabricated on a UV epitaxial wafer with a peak wavelength of 395 nm. Since pixels in the same column share an electrode of n-type GaN, the GaN is dry-etched into the sapphire substrate to create isolation trenches to isolate all micro-LED arrays. When the array is complete, the RGB QDs are sequentially sprayed onto the microLED array using

#### **Figure 2.**

*Process flow of the full-color microdisplay. (a) The structure of the micro-LED arrays. (b) Aligning the mold to the UV micro-LED array. (c)–(e) Consequently jetting the RGB QDs inside the molded window to form the full-color pixels [31]. Figures reproduced with permission from Optica Publishing Group.*

aerosol jet printing. The concentration of RGB QDs is about 5 mg/ml. Among the process parameters of aerosol jet printing, the working distance, table speed, carrier gas flow, sheath gas flow, and atomization frequency need to be adjusted to obtain a spraying line width of 35 μm. The working distance and table speed between the nozzle and the substrate were 1 mm and 10 mm/s, respectively. However, the cross-talk effect still occurred during the QD deposition due to the overflow of QDs during the solvent evaporation. To address the problem of cross-talk during QD deposition, the position of the QD must be restricted. In 2017, Lin et al. demonstrated a significant reduction in the cross-talk effect during the AJ printing process by using a photoresist (PR)-defined mold with a blocking wall to confine QDs [31]. **Figure 2** shows the fabrication process of the monolithic device. First, UV micro-LED arrays were fabricated on UV wafers with a peak wavelength of 395 nm and a pitch size of 40 μm. Isolation trenches are formed by etching GaN on a sapphire substrate. Through the dry etching process, SiO2 is used as a hard mask. Finally, p-electrode stripes are defined on top of the chip, n-electrode stripes are defined on the n-GaN layer, and all pixels in the same row are connected. By aligning the window of the mold with the micro-LED mesa, as shown in **Figure 2(b)–(e)**, AJ RGB QDs can be efficiently deposited on the micro-LED mesa area without overlapping the trench area. The printing parameters of different quantum dots need to be optimized. For blue QDs, the working distance is 1 mm, the carrier gas flow rate is 66 sccm, the sheath gas flow rate is 11 sccm, and the stage speed is 10 mm/s. For green QDs, the working distance is 1 mm, the carrier gas flow rate is 72 sccm, the sheath gas flow rate is 15 sccm, and the stage speed is 10 mm/s. For red QDs, the working distance is 1 mm, the carrier gas flow rate is 83 sccm, the sheath gas flow rate is 17 sccm, and the stage speed is 10 mm/s.

Pixels of an RGB display are demonstrated after depositing QDs on micro-LEDs using AJ on a die that has been window-confined. **Figure 3(a)** shows the microscope

#### **Figure 3.**

*(a) Microscope image of the full-color micro-LED after jetted QDs in the PR mold. (b) The RGB pixel array observed by fluorescence microscopy. (c) The fluorescence microscopy image of the jetted QD pixels without the PR mold [31]. Figures reproduced with permission from Optica Publishing Group.*

image of the full-color micro-LED after jetted QDs in the PR mold. **Figure 3(b)** shows the RGB pixel array observed by fluorescence microscopy, indicating that the luminescent regions of quantum dots have clear boundaries. Compared with the result of printing without PR window restriction as shown in **Figure 3(c)**, the fluorescence image shows no obvious separation between the printed QDs. Clearly, crosstalk is observed in the blue QD lines.

In 2019, Chen et al. proposed hybrid QD nanoring μLEDs (QD-NR-μLEDs) fabricated by QD printing and electron beam (E-beam) lithography [32]. There are three parts to this device, namely, a normal green LED, a blue NR-μLED, and a red QD-NRμLED, and each region can be regarded as a subpixel, as shown in the SEM images of **Figure 4(a)**. Besides, the nonradiative resonant energy transfer (NRET) mechanism was allowed for adjacent coupling between the exposed InGaN/GaN MQW sidewalls and QDs. During the manufacturing process, an electron beam lithography system is first used to define an area with negative photoresist, which is used as the RGB pixel. The green pixels are all shaped into rectangular mesas. The remaining red and blue regions form NR arrays with hexagonal close packing. After that, nickel is deposited by electron beam evaporation, and then the photoresist is removed by a lift-off process to form a hard mask pattern. Subsequently, by using Inductively Coupled Plasma Reactive Ion Etching (ICP-RIE), the GaN-based material is etched to define the active regions, separating the pn layers and isolating each subpixel. Next, residual nickel is removed by HCl solution. The ALD technology was used to deposit the Al2O3 passivation layer. In order to create color conversion layer, CdSe/ZnS red QDs were sprayed on a region of blue NR-μLED via the super-inkjet (SIJ) printing system. Then, spinon glass (SOG) was used to isolate the pn electrodes and protect the QD layer. After, a transparent conducting oxide (TCO) layer was orderly deposited via the hole process by SOG etching and Ni/Au metal deposition for the pn electrodes with the lift-off process. Finally, a distributed Bragg reflector (DBR) was used to recycle blue light and cover the red color region to filter out. **Figure 4(b)** showed the SEM image of NR-μLED with 30° tilt angle. Besides, it can be clearly observed from the transmission electron microscopy (TEM) images in **Figure 4(c)** that the sidewalls of MQWs were closely surrounded by QDs, which is important to the NRET mechanisms.

The overlapping relationship between the absorption spectrum of red QDs and the EL spectrum of a blue NR-μLED was demonstrated in **Figure 5(a)**. The emission wavelength of the blue NR-μLED is 467 nm, which is in the range of intense QD absorption. That indicates good spectral overlaps of MQWs with QD absorption and

#### **Figure 4.**

*(a) SEM image of RGB pixel array (top view); (b) SEM image of NR-μLED with 30° tilt angle; (c) TEM image of the contact area between MQWs and QDs [32]. Figures reproduced with permission from Optica Publishing Group.*

#### **Figure 5.**

*(a) Absorption curve of red QDs and electroluminescence (EL) spectrum of blue NR-μLEDs (inset depicts a schematic configuration of spraying red QDs on blue NR-μLEDs using the SIJ printing system). (b) EL spectra of RGB hybrid QD-NR-μLEDs; (c) color gamut of RGB hybrid QD-NR-μLEDs, NTSC, and Rec. 2020 [32]. Figures reproduced with permission from Optica Publishing Group.*

makes sure the strong coupling exists between excitons in MQWs and the absorption dipoles of QDs. Moreover, the inset of **Figure 5(a)** schematically shows the process of spraying red QDs on blue NR-μLEDs by using the SIJ printing system. **Figure 5(b)** demonstrated the EL spectra of individual RGB colors in hybrid QD-NR-μLEDs, the peak wavelengths at 467,525, and 630 nm, respectively. Due to the narrow EL spectra, the fabricated QD-NR-μLEDs can achieve a wide color gamut, which overlaps the NTSC space at approximately 104.8%, and overlaps Rec. 2020 space at about 78.2%.

In recent years, inkjet printing combined with photolithography, namely photolithography-inkjet printing (PHO-IJP) is usually used to fabricate a single pixel of conventional semiconductor quantum dots, with pixel pits on a substrate being created by photolithography followed by filling them with ink via inkjet printing [33]. In 2022, Bai et al. proposed "Interface Engineering-Inkjet Printing-Plasma Etching (IE-IJP-PE)" to fabricate large-area μPeLEDs with microscale and self-emissive pixels [34]. To achieve full-color display, CsPbCl1.56Br1.44 (blue), CsPbBr3 (green), and CsPbBrI2 (red) PeQD cyclohexylbenzene solutions were used as RGB inks. The

### *Full-Color Micro-LED Devices Based on Quantum Dots DOI: http://dx.doi.org/10.5772/intechopen.107280*

micro-PeLED arrays were constructed with a structure of glass/ITO/PEDOT: PSS/ PVK/SDS/RGB PeQD arrays/TPBi/LiF/Al. The fabricated substrates were ultrasonically cleaned sequentially in detergent, acetone, ethanol, and deionized water and dried. After the devices were dried, the ITO glass substrates were treated with UV ozone to remove residual organics for 15 min. The filtered PEDOT:PSS solution with a poly(tetrafluoroethylene) syringe filter (0.45 μm) was spin-coated onto an ITO glass substrate at 4000 rpm for 30 s and annealed at 140°C for 15 min. Dissolve PVK in chlorobenzene to form a 5 mg ml solution. The filtered PVK solution was spin-coated with a 0.45 μm poly(tetrafluoroethylene) syringe filter onto the PEDOT:PSS layer at 3000 rpm for 30 s and annealed at 130°C for 15 min in a glove box. SDS was then spin-coated at 3000 rpm for 30 s and heated at 100°C for 15 min. The RGB PeQD ink was printed on the hole transport layer in air by an IJP process and then treated by plasma cleaning for 15 s to destroy the excess hole transport layer and reduce the leakage current of the micro-PeLED array. Finally, the samples with RGB PeQD arrays were transferred to an interconnected high-vacuum deposition system through thermal evaporation to complete the device. The color gamut of RGB μPeLEDs covers 135% of NTSC, which is much wider than full-color μLEDs fabricated from red and green CdSe-based QDs. In addition, the red μPeLED has a maximum EQE of 0.832%, which is higher than its InGaN-based and AlInGaP-based counterparts.

As mentioned earlier, QD printing technology has become increasingly mature. However, inkjet printing is difficult to achieve large-area production. Colloidal quantum dots exhibit unique optical properties derived from quantum confinement effects that make them suitable for use as color-conversion layers for μLEDs [35]. PRs can be used to combine with quantum dots to form QDPRs after surface modification. By changing the parameters, QDPR can freely control thickness and size, while retaining the advantages of lithography. This method provides a cost-effective and practical solution for the development of large-area, high-resolution fabrication of full-color μLEDs for display applications.

In 2020, Chen et al. demonstrated a full-color μLED display with high color stability using semipolar (20–21) InGaN LEDs and quantum-dot photoresist [36]. To overcome the expensive and not desirable mass production of the conventional way, an innovative orientation-controlled epitaxy (OCE) process with semipolar GaN material selectively grown directly on the standard sapphire wafer was used in this work. The μLED array process started with depositing a transparent conducting oxide (TCO) layer, then, a p-type ohmic contact was formed by annealing at 450°C with the ambient atmospheric conditions. Next, in order to form a 1 μm depth mesa etch and etch the TCO film, inductively coupled plasma-reactive ion etching (ICP-RIE) and an HCl solution were used respectively in this research. Afterward, by using electron beam evaporation, Ti/Al/Ti/Au layers were deposited as the n-type electrode. After that, with the help of plasma-enhanced chemical vapor deposition, a 200 nm thick SiO2 passivation layer was deposited. Eventually, followed by ICP-RIE, the μLED array was completed by the via-hole process. After the μLED array process, as shown in **Figure 6(a)**, the Ni/Au (p-electrode metal) lines were deposited on the flattened surface to link each chip, as shown in **Figure 6(b)**. Then, by using lithography process, the gray photoresist, red QDPR, green QDPR, and transparent PR were fabricated sequentially to form a color pixel on a highly-transparent glass substrate with 0.7 mm thickness, as shown in **Figure 6(c)**. Finally, in **Figure 6(d)**, an aligner and UV resin are used to stick the color pixel array on the glass. The gray PR mold can attain a higher height and provide higher reflectivity, which can reduce the cross-talk effect among pixels and enhance output intensity by inside reflection compared to the

#### **Figure 6.**

*Process flow for the fabrication of a full-color RGB pixel array. (a) μLED array process. (b) Black PR matrices and p-electrode metal lines. (c) Red, green, and blue (transparent) pixel lithography process. (d) Color pixel bonding [36]. Figures reproduced with permission from Optica Publishing Group.*

black PR. Besides, the semipolar μLED shows a stabilizing wavelength shift of 3.2 nm, while the c-plane μLED's shift is 13.0 nm. Above all, the RGB pixels present a wide color gamut of 114.4% NTSC and 85.4% Rce.2020, showing great promise for display applications. The research also demonstrated a color-conversion layer consisting of QDPR is capable of the common lithography process, which is suitable for large-scale manufacturing.

Recently, μLED all-rounder displays based on silicon backplanes have also attracted the attention of researchers. In 2020, Kawanishi et al. reported a siliconbased full-color micro-LED display, called "Silicon Display" [37]. To fabricate the display array, a p-electrode layer is first formed on the p-GaN layer of the LED epitaxial wafer. The epitaxial layer was then etched down to the n-GaN layer by photolithographic patterning using ICP to form the mesa structure, and the n-GaN layer was exposed to form the n-electrode. Each pixel is defined by a groove down to the sapphire substrate formed by another ICP etch. **Figure 7(a)** shows a schematic diagram of a single pixel. Red and green sub-pixels are formed by exciting the QD color converters with blue LEDs. The quantum dot material is deposited on the surface of the device by a photolithography process after being mixed with a photoresist. A full-color silicon display with a resolution of 1053 ppi and 352 × 198 pixels, each of which is 24 μm in size, is realized. **Figure 7(b)** shows a photo of the overall display. As can be seen from the photo, each QD layer was successfully formed on each sub-pixel through the photolithography process.

*Full-Color Micro-LED Devices Based on Quantum Dots DOI: http://dx.doi.org/10.5772/intechopen.107280*

**Figure 7.**

*(a) Schematic cross-section of a single pixel of silicon display. (b) Optical micrograph of 1053 ppi micro-LED array during fabrication after forming red and green QDs [37]. Figures reproduced with permission from the SOCIETY FOR INFORMATION DISPLAY.*

## **2.4 VLC applications with QD-based color-conversion μLEDs**

Incorporating a color conversion layer can allow μLEDs to simultaneously achieve full-color display and high-speed modulation. However, the modulation bandwidth of this class of devices is significantly limited by the long response times of color-converting materials [38]. The traditional phosphor material used as the color conversion layer is yttrium aluminum garnet (YAG) phosphor Y3–xAl5O12:xCe3+ (YAG:Ce) [39], which has a critical limitation for VLC applications due to the slow phosphor conversion process caused by the long excited-state lifetimes [40], on the order of microseconds. The modulation bandwidth of phosphors is typically only a few MHz [41]. To overcome the bottleneck in response speed, organic materials, such as BODIPY, MEH-PPV and BBEHP-PPV have been recently used as potential candidates for color converters for VLCs due to their visible light emission, high PLQY, direct radiative recombination, and ease of integration with nitride-based semiconductors [42]. However, their excited state lifetimes are still very long. Therefore, developing light-converter phosphor materials with fast decay and high efficiency (that is, short radiative lifetime and high brightness) remains a major challenge for VLC and solid-state lighting (SSL) applications.

As previously introduced, quantum dot (QD)-based color converters have very promising applications, however, the modulation bandwidth of conventional CdSe/ZnS QDs is limited to ~3 MHz, which is much lower than the requirement of VLC [43]. Lead halide perovskite QDs exhibit high PLQY (≥70%) and relatively short PL lifetimes [27]. In 2018, Shi et al. reported an all-inorganic white light system for VLC [44]. The system uses blue GaN-based μLEDs as the excitation light source and inorganic yellow-emitting CsPbBr1.8I1.2 perovskite quantum dots (YQDs) as the color conversion layer. The maximum modulation bandwidth of the packaged 80 μm × 80 μm blue-emitting μLED is about 160 MHz, and the peak emission wavelength is about 445 nm. Maximum −3 dB E-O modulation bandwidths of ~73 and ~85 MHz were achieved for perovskite quantum dots and white light systems combining μLED and perovskite quantum dots, respectively. In addition, based on the high bandwidth white light system, the real-time data rate is 300 Mbps using no return Zero-On–Off Keying (NRZ-OOK) modulation scheme.

Most GaN-based μLEDs are typically grown on (0001) "polar" c-plane sapphire substrates, which leads to strong Stark effect QCSE effect (QCSE), which, in addition to leading to a drop in efficiency, will limit the modulation bandwidth of μLEDs [45]. The spontaneous polarization of GaN is responsible for the QCSE due to the highest symmetry compatible with its structure. Meanwhile, the strain caused by the lattice mismatch between InxGa1 − xN and GaN also produces polarization. These internal polarization fields along the c-plane will lead to band tilting, separating the wave functions of electrons and holes. QCSE also causes wavelength shift and efficiency drop with increasing injection current density. The applications of the semipolar (20–21) and (20–2–1) epitaxial structures have been proven to effectively suppress the effects of QCSE [46]. Semipolar devices enable higher modulation bandwidths due to weak polarization fields and flat energy gap distributions, which lead to larger electron–hole wavefunction overlap reducing carrier lifetimes [47]. Further, Zhao et al. revealed that faster carrier transport in semipolar devices also contributes to the weaker phase-space filling effect, which was determined for the low-droop phenomenon in semipolar LEDs because of small QCSE and short carrier lifetimes using the consistency between theoretical and experimental results [48]. The aforementioned advantages imply that a semipolar LED is capable of simultaneously achieving high modulation speed and maintaining high efficiency with increasing injected current owing to low droop performance.

In 2020, Huang Chen et al. realized long-wavelength (initial wavelength 540 nm) InGaN/GaN VLC-LEDs with high 3 dB bandwidth using semipolar epitaxy and μLED structures [49]. The epitaxial process of semipolar (20–21) GaN on a (22–43) PSS is carried out through a low-pressure metalorganic chemical vapor deposition (MOCVD). In addition, the passivation layer of aluminum oxide (Al2O3) was grown to repair sidewall defects. Some previous studies have stated that the influence of sidewall defects increases as the chip size decreases [50, 51]. In particular, when the LED device achieves a micrometer scale, traditional passivation methods, such as the PECVD process, are no longer useful owing to the large leakage current of the μLED device. ALD dielectric thin films have been regarded as an effective passivation technique in the μLED area [52]. The semipolar device had a shorter lifetime because of the weak polarization-related electric field and large overlap of the electron–hole wave function, which yielded a faster carrier recombination lifetime [53]. The QCSE reduction in the semipolar device yielded faster carrier transport and a shorter recombination lifetime, resulting in a weaker phase-space filling effect. Therefore, the semipolar μLED can achieve a high modulation bandwidth owing to its faster carrier recombination lifetime.

The outstanding performance of semipolar μLEDs in display and communication has also led researchers to combine it with QDs to make high-performance full-color display devices with VLC potential. In 2021, Singh et al. proposed a flexible white-light system for high-speed VLC applications [54]. The white-light system fabrication process is shown in **Figure 8**. The system consists of nanostructured green CsPbBr3 PQD paper, red CdSe QD paper, and semipolar blue micro-LEDs. Regarding the production of green CsPbBr3 PQD paper, firstly, a solution of CsPbBr3 quantum dots were prepared using the hot injection method, and then the solution was added to the cellulose nanocrystal (CNC) suspension, and the mixed solution was filtered through a filter membrane using a vacuum pump device. The PQD paper in nanostructured form is then separated from the filter membrane. The QD paper produced by this method has strong mechanical strength and flexibility to be used with flexible systems. In addition, the PQD paper fabricated by this method has nanostructures, which should provide a strong quantum confinement effect to increase

*Full-Color Micro-LED Devices Based on Quantum Dots DOI: http://dx.doi.org/10.5772/intechopen.107280*

#### **Figure 8.**

*Fabrication of white-light system [54]. Figures reproduced with permission from Optica Publishing Group.*

the probability of carrier recombination. The flexible μLED was fabricated using a polyimide (PI) substrate covered with copper-foil shielding tape, where the latter was subjected to photolithography and wet etching to establish electrical conduction. The μLED flip-chip was bonded on the PI substrate using a silicone-based electrically conductive anisotropic adhesive to maintain the electrical conductivity between the chip metal contact and the AuSn solder on the substrate; this increased the system flexibility. For the color converter, CsPbBr3 PQD paper and CdSe QD paper were prepared. These papers were adhered to the top of the μLED with flexible substrate using an adhesive to achieve white-light system.

Semipolar μLEDs had a narrow FWHM and was therefore responsible for the delivery of pure emitted light, matching colors, and a wide color gamut. **Figure 9(a)** shows the color performance of the white-light system created using the semipolar blue μLED with PQD paper and CdSe QD paper under driving conditions from 10 to 1200 A/cm2 . The white-light system demonstrated a wide color gamut, achieving 98.7% of the NTSC and 91.1% Rec. 2020 of the CIE 1931. The color gamut of the white-light system remained almost unchanged with increasing injection current density owing to the wavelength stability.

The average PL lifetimes calculated for the PQD paper and CdSe QD paper were 5.92, and 12.88 ns, as shown in **Figure 9(b)**. The PQD paper had a shorter carrier lifetime than those reported in other studies, while also being considerably shorter than those of phosphors microsecond to millisecond range. This shorter carrier lifetime is attributable to the quantum confinement effect, which yields faster radiative recombination. However, the CdSe QD-paper carrier recombination lifetime is insufficient to independently achieve high bandwidth for VLC applications. Therefore, semipolar μLEDs and PQDs have considerable potential for VLC applications. The PQD-film bandwidth was found to be 111 MHz, as shown in **Figure 9(c)**; the PQD-based whitelight system also displayed a frequency bandwidth of 95.5 MHz at a 113 mA injection current. Hence, the high bandwidth of the PQD paper is suitable for achieving highspeed VLC. This outcome implies that the nanostructure has a higher recombination rate than the bulk and hence a higher modulation bandwidth.

Although PQDs have significant advantages over traditional color conversion materials, they also have some drawbacks. They have, for example, exhibited vulnerability under ambient conditions, particularly in the case of red-emitting PQDs that

**Figure 9.**

*(a) Color gamut of white-light system according to CIE 1931 color space under various current densities. (b) TRPL curves for semipolar μLED and PQD and CdSe QD papers. (c) Comparison of bandwidth of PQD paper in nanostructure with that of PQD film, Inset: eye diagram for PQD paper [54]. Figures reproduced with permission from Optica Publishing Group.*

contain iodine [55]. The application bottleneck of PQDs is long-term stability. It degrades rapidly when exposed to the environment. Water vapor, oxygen, high temperature, and light irradiation cause alteration to the crystal structures of PQDs, typically resulting in photoluminescence (PL) quenching [56]. These four factors coexist in the environment, so their effects are difficult to distinguish from each other [57]. In terms of immediate optical performance, most of the crystal structure changes are negative, but a few of them are positive. Therefore, the high-stability PQDs color conversion layer made by the new encapsulation method will have good application prospects in the fields of display and communication. Embedding QDs in organic polymers can significantly extend their lifetimes, as the polymers ensure a hermetic seal from air [58, 59]. In the current study, the organic shell is still not perfect because the polymer cannot withstand UV or blue illumination from the excitation light source for long periods of time [60]. Thus, shells composed of inorganic substances have been favored since recently, including Al2O3, ZrO2, and anodized aluminum templates [61, 62]. Although these inorganic shells can withstand blue light and UV irradiation better than organic shells, the porous structures are not as water/O2-proof as the organic ones because the pore structures remain. In 2021, Lin et al. reported an inorganic encapsulation of mesoporous SiO2, in conjunction with a high-temperature sintering synthesis process under an inert atmosphere [58]. This synthesis process is compatible with various halide contents, yielding PeNCs sealed in SiO2 particles that emit PL emission covering the entire visible range from 420 to 700 nm. The PeNCs– SiO2 sample showed remarkable stability after undergoing aging tests under various exaggerated stresses as well as mitigated thermal quenching during thermal cycling. The PeNCs–SiO2 can be blended into a photoresist and remains luminous during the development procedure, which is compatible with the photolithography process; this facilitates the mass production of color conversion layers. This robust encapsulation also has great application value for full-color display or visible light communication based on quantum dot-based μLED.

In 2021, Wu et al. reported a PNC–μLED device for a full-color display that is developed using a semipolar (20–21) blue μLED array with green-emitting CsPbBr3 and red-emitting CsPbBrI2 PNCs [63]. They encapsulated the PQDs in an all-inorganic SiO2 shell, which significantly improved the stability of the color conversion layer. Regarding the fabrication of CsPbBrI2-SiO2, the precursor solution was first prepared by mixing the precursor salt with MCM-41 molecular sieves and dispersing the mixture

## *Full-Color Micro-LED Devices Based on Quantum Dots DOI: http://dx.doi.org/10.5772/intechopen.107280*

in 25 ml of purified water. Next, the precursor solution was sonicated for 20 min and vigorously stirred for 10 min to improve dispersion. The precursor solution was transferred into a crucible and placed in a tube furnace filled with high-purity Ar gas. The temperature of the tube furnace was raised to 200°C and kept for 1 h to evaporate the water. Then, sintering was performed continuously for 30 min at a temperature of 750°C with an Ar flow rate of 15 ml/min. Subsequently, the samples were cooled to room temperature under Ar protection, during which their color gradually changed and finally crystallized into PNCs. During cooling, the perovskite forms inside the SiO2, which constrains the PNCs inside that facilitate the formation of crystal phase, thereby ensuring the high stability of the red-emitting PNCs. **Figure 10(a)** illustrates

## **Figure 10.**

*(a) The fabrication process and a photo of the color conversion layer; and (b) the proposed PNC-μLED device. EDS element maps of the (a) CsPbBrI2-SiO2 and (b) CsPbBr3-SiO2 PNCs [63]. Figures reproduced with permission from Optica Publishing Group.*

**Figure 11.**

*(a) The normalized intensity of the PL properties of the red and green PNCs, as well as the solution-processed samples under blue light exposure. (b) Color gamut of the PNC-μLED under different current densities. (c) Frequency response for the PNC-μLED [63]. Figures reproduced with permission from Optica Publishing Group.*

the fabrication detail and presents a photo of the as-fabricated films under natural light, which are composed of CsPbBr3-SiO2 and CsPbBrI2-SiO2. During this process, CsPbBrI2-SiO2 and CsPbBr3-SiO2 were mixed with toluene to make a red and green mixture. The EVA polymer was dissolved into the mixture and heated to 50°C with stirring. The mixture was then spin-coated (1800 rad/s) on a glass substrate to obtain a thin film of uniform thickness. In **Figure 10(b)**, the color conversion layer is combined with the semipolar μLED, achieving the white-light PNC-μLED device for the backlight. The fine structure of both PNC samples is also revealed from the EDS element maps, as illustrated in **Figure 10(c)** and **(d)**. These maps indicate that the spatial distribution of the elements within the PNCs is highly similar; they are confined to approximately circular regions surrounded by areas of SiO2. This further confirms the encapsulation of the PNCs inside SiO2 shells.

**Figure 11(a)** shows the normalized intensity of the PL properties of the red and green PNCs, as well as the solution-processed samples under blue light exposure. The SiO2-embedded samples exhibit remarkable stabilities, showing no degradation under blue irradiation. **Figure 11(b)** shows the frequency response of the PNC-μLED, the highest 3-dB bandwidth was measured as 655 MHz, corresponding to an injection current of 200 mA. **Figure 11(c)** presents the performance of the PNC-μLED for the display backlight application under different current densities between 2.55 and 203.83 A/cm<sup>2</sup> . Because of its narrow EL spectrum, the RGB pixel assembled from the semipolar μLEDs and PNCs exhibited a wide color gamut of 127.23% of the NTSC and 95.00% of the Rec. 2020.

## **3. Conclusions**

QDs-based μLED provide superior display performance and are a promising platform for VLC systems. In this article, we comprehensively review recent progress in QD-based μLED devices. It includes the research status of various QDs and white LEDs based on QDs color conversion layers. The fabrication of QD-based high-resolution full-color μLEDs is also discussed. Including charge-assisted layerby-layer (LbL), aerosol jet printing, and super inkjet printing methods to fabricate QD-based μLEDs. The use of quantum dot photoresist in combination with semipolar μLEDs is also described. Finally, we discuss the research of QD-based μLEDs for visible light communication, which allows a single device to be used for both display

*Full-Color Micro-LED Devices Based on Quantum Dots DOI: http://dx.doi.org/10.5772/intechopen.107280*

and high-speed communication, enhancing the versatility of μLEDs. Advances in the development of QD-based μLEDs are expected to make this display technology ubiquitous in the near future. Recent breakthroughs in QDs and LEDs will provide a promising outlook for future demand in the semiconductor industry.

## **Acknowledgements**

Additionally, we thank Xiangshu Lin for their contributions to the investigation.

## **Funding**

This research was supported by the National Natural Science Foundation of China (62274138, 11904302), Science and Technology Plan Project in Fujian Province of China (2021H0011), Major Science and Technology Project of Xiamen, China (3502Z20191015).

## **Author details**

Tingzhu Wu1,2, Tingwei Lu1 , Yen-Wei Yeh3 , Zhong Chen1,2 and Hao-Chung Kuo3,4\*

1 School of Electronic Science and Engineering, Fujian Engineering Research Center for Solid-State Lighting, Xiamen University, Xiamen, China

2 Innovation Laboratory for Sciences and Technologies of Energy Materials of Fujian Province (IKKEM), Xiamen, China

3 Department of Photonics, Graduate Institute of Electro-Optical Engineering, College of Electrical and Computer Engineering, National Yang Ming Chiao Tung University, Hsinchu, Taiwan

4 Semiconductor Research Center, Hon Hai Research Institute, Taipei, Taiwan

\*Address all correspondence to: hckuo@faculty.nctu.edu.tw

© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

## **References**

[1] Alivisatos P. The use of nanocrystals in biological detection. Nature Biotechnology. 2004;**22**(1):47-52. DOI: 10.1038/nbt927

[2] Tian PF, McKendry JJD, Gu ED, Chen ZZ, Sun YJ, Zhang GY, et al. Fabrication, characterization and applications of flexible vertical InGaN micro-light emitting diode arrays. Optics Express. 2016;**24**(1):699-707. DOI: 10.1364/oe.24.000699

[3] Liu ZJ, Zhang K, Liu YB, Yan SW, Kwok HS, Deen J, et al., editors. Fully multi-functional GaN-based micro-LEDs for 2500 PPI micro-displays, temperature sensing, light energy harvesting, and light detection. In: 64th IEEE Annual International Electron Devices Meeting (IEDM); December 01-05, 2018. New York, San Francisco, CA: IEEE; 2018

[4] Singh KJ, Huang YM, Ahmed T, Liu AC, Chen SWH, Liou FJ, et al. Micro-LED as a promising candidate for high-speed visible light communication. Applied Sciences— Basel. 2020;**10**(20):7384. DOI: 10.3390/ app10207384

[5] Khan LU. Visible light communication: Applications, architecture, standardization and research challenges. Digital Communications and Networks. 2017;**3**(2):78-88. DOI: 10.1016/j. dcan.2016.07.004

[6] Chow CW, Yeh CH, Liu YF, Liu Y. Improved modulation speed of LED visible light communication system integrated to main electricity network. Electronics Letters. 2011;**47**(15):867- U1954. DOI: 10.1049/el.2011.0422

[7] Cho J, Park JH, Kim JK, Schubert EF. White light-emitting diodes: History, progress, and future. Laser &

Photonics Reviews. 2017;**11**(2):1600147. DOI: 10.1002/lpor.201600147

[8] Bulashevich KA, Karpov SY. Impact of surface recombination on efficiency of III-nitride light-emitting diodes. Physica Status Solidi-Rapid Research Letters. 2016;**10**(6):480-484. DOI: 10.1002/ pssr.201600059

[9] Hwang D, Mughal A, Pynn CD, Nakamura S, DenBaars SP. Sustained high external quantum efficiency in ultrasmall blue III-nitride micro-LEDs. Applied Physics Express. 2017;**10**(3):032101. DOI: 10.7567/apex.10.032101

[10] Wang XC, Bao Z, Chang YC, Liu RS. Perovskite quantum dots for application in high color gamut backlighting display of light-emitting diodes. ACS Energy Letters. 2020;**5**(11):3374-3396. DOI: 10.1021/acsenergylett.0c01860

[11] Lin CH, Verma A, Kang CY, Pai YM, Chen TY, Yang JJ, et al. Hybrid-type white LEDs based on inorganic halide perovskite QDs: Candidates for wide color gamut display backlights. Photonics Research. 2019;**7**(5):579-585. DOI: 10.1364/prj.7.000579

[12] Lin CC, Liu RS. Advances in phosphors for light-emitting diodes. Journal of Physical Chemistry Letters. 2011;**2**(11):1268-1277. DOI: 10.1021/ jz2002452

[13] Huang YM, Singh KJ, Hsieh TH, Langpoklakpam C, Lee TY, Lin CC, et al. Gateway towards recent developments in quantum dot-based light-emitting diodes. Nanoscale. 2022;**14**(11):4042- 4064. DOI: 10.1039/d1nr05288h

[14] Sapsford KE, Pons T, Medintz IL, Mattoussi H. Biosensing *Full-Color Micro-LED Devices Based on Quantum Dots DOI: http://dx.doi.org/10.5772/intechopen.107280*

with luminescent semiconductor quantum dots. Sensors. 2006;**6**(8):925- 953. DOI: 10.3390/s6080925

[15] Araki Y, Ohkuno K, Furukawa T, Saraie J. Green light emitting diodes with CdSe quantum dots. Journal of Crystal Growth. 2007;**301**:809-811. DOI: 10.1016/j.jcrysgro.2006.11.105

[16] Du JH, Wang CL, Xu XJ, Wang ZY, Xu SH, Cui YP. Assembly of lightemitting diode based on hydrophilic CdTe quantum dots incorporating dehydrated silica gel. Luminescence. 2016;**31**(2):419-422. DOI: 10.1002/ bio.2976

[17] Morris-Cohen AJ, Donakowski MD, Knowles KE, Weiss EA. The effect of a common purification procedure on the chemical composition of the surfaces of CdSe quantum dots synthesized with Trioctylphosphine oxide. Journal of Physical Chemistry C. 2010;**114**(2):897- 906. DOI: 10.1021/jp909492w

[18] Stan CS, Secula MS, Sibiescu D. Highly luminescent polystyrene embedded CdSe quantum dots obtained through a modified colloidal synthesis route. Electronic Materials Letters. 2012;**8**(3):275-281. DOI: 10.1007/ s13391-012-1108-0

[19] Soheyli E, Ghaemi B, Sahraei R, Sabzevari Z, Kharrazi S, Amani A. Colloidal synthesis of tunably luminescent AgInS-based/ZnS core/ shell quantum dots as biocompatible nano-probe for high-contrast fluorescence bioimaging. Materials Science & Engineering C-Materials for Biological Applications. 2020;**111**:110807. DOI: 10.1016/j.msec.2020.110807

[20] Reifsnyder DC, Ye XC, Gordon TR, Song CY, Murray CB. Three-dimensional self-assembly of chalcopyrite copper indium Diselenide nanocrystals

into oriented films. ACS Nano. 2013;**7**(5):4307-4315. DOI: 10.1021/ nn4008059

[21] Chen B, Li DY, Wang F. InP quantum dots: Synthesis and lighting applications. Small. 2020;**16**(32):2002454. DOI: 10.1002/smll.202002454

[22] Brus LE. Electron electron and electron-hole interactions in small semiconductor crystallites—The size dependence of the lowest excited electronic state. Journal of Chemical Physics. 1984;**80**(9):4403-4409. DOI: 10.1063/1.447218

[23] Chen O, Wei H, Maurice A, Bawendi M, Reiss P. Pure colors from core-shell quantum dots. MRS Bulletin. 2013;**38**(9):696-702. DOI: 10.1557/ mrs.2013.179

[24] Zhang ZL, Liu D, Li DZ, Huang KK, Zhang Y, Shi Z, et al. Dual emissive Cu:InP/ZnS/InP/ZnS nanocrystals: Single-source "greener" emitters with flexibly Tunable emission from visible to near-infrared and their application in white light-emitting diodes. Chemistry of Materials. 2015;**27**(4):1405-1411. DOI: 10.1021/cm5047269

[25] Milstein TJ, Kroupa DM, Gamelin DR. Picosecond quantum cutting generates photoluminescence quantum yields over 100% in ytterbium-doped CsPbCl3 nanocrystals. Nano Letters. 2018;**18**(6):3792-3799. DOI: 10.1021/acs.nanolett.8b01066

[26] Schmidt LC, Pertegas A, Gonzalez-Carrero S, Malinkiewicz O, Agouram S, Espallargas GM, et al. Nontemplate synthesis of CH3NH3PbBr3 perovskite nanoparticles. Journal of the American Chemical Society. 2014;**136**(3):850-853. DOI: 10.1021/ ja4109209

[27] Protesescu L, Yakunin S, Bodnarchuk MI, Krieg F, Caputo R, Hendon CH, et al. Nanocrystals of cesium Lead halide perovskites (CsPbX3, X = Cl, Br, and I): Novel optoelectronic materials showing bright emission with wide color gamut. Nano Letters. 2015;**15**(6):3692- 3696. DOI: 10.1021/nl5048779

[28] Wang HC, Bao Z, Tsai HY, Tang AC, Liu RS. Perovskite quantum dots and their application in light-emitting diodes. Small. 2018;**14**(1):1702433. DOI: 10.1002/ smll.201702433

[29] Kong CY, Lin CH, Lin CH, Li TY, Chen SWH, Tsai CL, et al. Highly efficient and stable white light-emitting diodes using perovskite quantum dot paper. Advanced. Science. 2019;**6**(24):1902230. DOI: 10.1002/advs.201902230

[30] Han HV, Lin HY, Lin CC, Chong WC, Li JR, Chen KJ, et al. Resonant-enhanced full-color emission of quantum-dotbased micro LED display technology. Optics Express. 2015;**23**(25):32504- 32515. DOI: 10.1364/oe.23.032504

[31] Lin HY, Sher CW, Hsieh DH, Chen XY, Chen HMP, Chen TM, et al. Optical cross-talk reduction in a quantum-dot-based full-color microlight-emitting-diode display by a lithographic-fabricated photoresist mold. Photonics Research. 2017;**5**(5):411-416. DOI: 10.1364/prj.5.000411

[32] Chen SWH, Shen CC, Wu TZ, Liao ZY, Chen LF, Zhou JR, et al. Full-color monolithic hybrid quantum dot nanoring micro light-emitting diodes with improved efficiency using atomic layer deposition and nonradiative resonant energy transfer. Photonics Research. 2019;**7**(4):416-422. DOI: 10.1364/prj.7.000416

[33] Xuan TT, Shi SC, Wang L, Kuo HC, Xie RJ. Inkjet-printed quantum dot color conversion films for high-resolution and

full-color micro light-emitting diode displays. Journal of Physical Chemistry Letters. 2020;**11**(13):5184-5191. DOI: 10.1021/acs.jpclett.0c01451

[34] Bai WH, Xuan TT, Zhao HY, Shi SC, Zhang XY, Zhou TL, et al. Microscale perovskite quantum dot light-emitting diodes (micro-PeLEDs) for full-color displays. Advanced Optical Materials. 2022;**10**(12):2200087 DOI: 10.1002/ adom.202200087

[35] Erdem T, Demir HV. Color science of nanocrystal quantum dots for lighting and displays. Nano. 2013;**2**(1):57-81. DOI: 10.1515/nanoph-2012-0031

[36] Chen SWH, Huang YM, Singh KJ, Hsu YC, Liou FY, Song J, et al. Full-color micro-LED display with high color stability using semipolar (20-21) InGaN LEDs and quantum-dot photoresist. Photonics Research. 2020;**8**(5):630-636. DOI: 10.1364/prj.388958

[37] Kawanishi H, Onuma H, Maegawa M, Kurisu T, Ono T, Akase S, et al. Highresolution and high-brightness full-colour "silicon display" for augmented and mixed reality. Journal of the Society for Information Display. 2021;**29**(1):57-67. DOI: 10.1002/jsid.968

[38] Xu Y, Chen J, Zhang H, Wei H, Zhou L, Wang Z, et al. White-lightemitting flexible display devices based on double network hydrogels crosslinked by YAG:Ce phosphors. Journal of Materials Chemistry C. 2020;**8**(1):247-252. DOI: 10.1039/C9TC05311E

[39] Li PP, Lu Y, Duan YM, Xu SQ, Zhang JJ. Potential application of perovskite glass material in photocatalysis field. Journal of Physical Chemistry C. 2021;**125**(4):2382-2392. DOI: 10.1021/acs.jpcc.0c11241

[40] Wang ZM, Wei ZX, Cai YT, Wang L, Li MT, Liu P, et al. Encapsulation-enabled *Full-Color Micro-LED Devices Based on Quantum Dots DOI: http://dx.doi.org/10.5772/intechopen.107280*

perovskite-PMMA films combining a Micro-LED for high-speed white-light communication. ACS Applied Materials & Interfaces. 2021;**13**(45):54143-54151. DOI: 10.1021/acsami.1c15873

[41] Gao H, Xie YY, Geng C, Xu S, Bi WG. Efficiency enhancement of quantumdot-converted LEDs by 0D-2D hybrid scatterers. ACS Photonics. 2020;**7**(12):5430-5439. DOI: 10.1021/ acsphotonics.0c01240

[42] Su CY, Wu YC, Cheng CH, Wang WC, Wang HY, Chen LY, et al. Color-converting violet laser diode with an ultrafast BEHP-PPV. ACS Applied Electronic Materials. 2020;**2**(9):3017- 3027. DOI: 10.1021/acsaelm.0c00619

[43] Xiao X, Tang H, Zhang T, Chen W, Chen W, Wu D, et al. Improving the modulation bandwidth of LED by CdSe/ZnS quantum dots for visible light communication. Optics Express. 2016;**24**(19):21577-21586. DOI: 10.1364/ OE.24.021577

[44] Mei SL, Liu XY, Zhang WL, Liu R, Zheng LR, Guo RQ, et al. Highbandwidth white-light system combining a micro-LED with perovskite quantum dots for visible light communication. ACS Applied Materials & Interfaces. 2018;**10**(6):5641-5648. DOI: 10.1021/ acsami.7b17810

[45] Piprek J. Efficiency droop in nitridebased light-emitting diodes. Physica Status Solidi A—Applications and Materials Science. 2010;**207**(10):2217- 2225. DOI: 10.1002/pssa.201026149

[46] Liu SG, Han SC, Xu CC, Xu HW, Wang XY, Wang D, et al. Enhanced photoelectric performance of GaNbased Micro-LEDs by ion implantation. Optical Materials. 2021;**121**:111579. DOI: 10.1016/j.optmat.2021.111579

[47] Zhao YJ, Fu HQ, Wang GT, Nakamura S. Toward ultimate efficiency: Progress and prospects on planar and 3D nanostructured nonpolar and semipolar InGaN light-emitting diodes. Advances in Optics and Photonics. 2018;**10**(1):246- 308. DOI: 10.1364/aop.10.000246

[48] Fu HQ, Lu ZJ, Zhao XH, Zhang YH, DenBaars SP, Nakamura S, et al. Study of low-efficiency droop in semipolar (20(2)over-bar(1)over-bar) InGaN light-emitting diodes by time-resolved photoluminescence. Journal of Display Technology. 2016;**12**(7):736-741. DOI: 10.1109/jdt.2016.2521618

[49] Chen S-WH, Huang Y-M, Chang Y-H, Lin Y, Liou F-J, Hsu Y-C, et al. High-bandwidth green semipolar (20- 21) InGaN/GaN micro light-emitting diodes for visible light communication. ACS Photonics. 2020;**7**(8):2228-2235. DOI: 10.1021/acsphotonics.0c00764

[50] Kou JQ, Shen CC, Shao H, Che JM, Hou X, Chu CS, et al. Impact of the surface recombination on InGaN/GaNbased blue micro-light emitting diodes. Optics Express. 2019;**27**(12):A643-AA53. DOI: 10.1364/oe.27.00a643

[51] Yang W, Zhang SL, McKendry JJD, Herrnsdorf J, Tian PF, Gong Z, et al. Size-dependent capacitance study on InGaN-based micro-lightemitting diodes. Journal of Applied Physics. 2014;**116**(4):044512. DOI: 10.1063/1.4891233

[52] Wong MS, Hwang D, Alhassan AI, Lee C, Ley R, Nakamura S, et al. High efficiency of III-nitride micro-lightemitting diodes by sidewall passivation using atomic layer deposition. Optics Express. 2018;**26**(16):21324-21331. DOI: 10.1364/oe.26.021324

[53] Monavarian M, Rashidi A, Aragon A, Oh SH, Nami M, Denbaars SP, et al.

Explanation of low efficiency droop in semipolar (20(21)over-bar) InGaN/ GaN LEDs through evaluation of carrier recombination coefficients. Optics Express. 2017;**25**(16):19343-19353. DOI: 10.1364/oe.25.019343

[54] Singh KJ, Fan XT, Sadhu AS, Lin CH, Liou FJ, Wu TZ, et al. CsPbBr3 perovskite quantum-dot paper exhibiting a highest 3 dB bandwidth and realizing a flexible white-light system for visible-light communication. Photonics Research. 2021;**9**(12):2341-2350. DOI: 10.1364/ prj.434270

[55] Wei Y, Cheng Z, Lin J. An overview on enhancing the stability of lead halide perovskite quantum dots and their applications in phosphor-converted LEDs. Chemical Society Reviews. 2019;**48**(1):310-350. DOI: 10.1039/ C8CS00740C

[56] Shangguan ZB, Zheng X, Zhang J, Lin WS, Guo WJ, Li C, et al. The stability of metal halide perovskite nanocrystals—A key issue for the application on quantum-dot-based micro light-emitting diodes display. Nanomaterials. 2020;**10**(7):1375. DOI: 10.3390/nano10071375

[57] Zhang L, Ju MG, Liang WZ. The effect of moisture on the structures and properties of lead halide perovskites: A first-principles theoretical investigation. Physical Chemistry Chemical Physics. 2016;**18**(33):23174-23183. DOI: 10.1039/ c6cp01994c

[58] Lin Y, Zheng X, Shangguan Z, Chen G, Huang W, Guo W, et al. Allinorganic encapsulation for remarkably stable cesium lead halide perovskite nanocrystals: Toward full-color display applications. Journal of Materials Chemistry C. 2021;**9**(36):12303-12313. DOI: 10.1039/D1TC02685B

[59] Liu LG, Deng LG, Huang S, Zhang P, Linnros J, Zhong HZ, et al. Photodegradation of organometal hybrid perovskite nanocrystals: Clarifying the role of oxygen by single-dot photoluminescence. Journal of Physical Chemistry Letters. 2019;**10**(4):864-869. DOI: 10.1021/acs.jpclett.9b00143

[60] Zhang Y, Cai GF, Fang Y, Han GJ. Relaying transmission for multiresolution M-DCSK modulation in multi-relay networks. In: 4th International Conference on Communication and Information Systems (ICCIS); 2019 December 21-23. Wuhan. New York: IEEE; 2019. p. 1-6

[61] Quan LN, Rand BP, Friend RH, Mhaisalkar SG, Lee TW, Sargent EH. Perovskites for next-generation optical sources. Chemical Reviews. 2019;**119**(12):7444-7477. DOI: 10.1021/ acs.chemrev.9b00107

[62] Rajagopal A, Yao K, Jen AKY. Toward perovskite solar cell commercialization: A perspective and research roadmap based on interfacial engineering. Advanced Materials. 2018;**30**(32):1800455. DOI: 10.1002/adma.201800455

[63] Wu T, Lin Y, Huang Y-M, Liu M, Singh KJ, Lin W, et al. Highly stable full-color display device with VLC application potential using semipolar micro-LEDs and all-inorganic encapsulated perovskite nanocrystal. Photonics Research. 2021;**9**(11):2132- 2143. DOI: 10.1364/PRJ.431095

## **Chapter 2**

## Milestone Developments and New Perspectives of Nano/Nanocrystal Light Emitting Diodes

*Jyoti Singh, Niteen P. Borane and Rajamouli Boddula*

## **Abstract**

Light emitting diode (LED) is a one type of p/n junction semiconductor device which is used in less energy consumption for numerous lighting functions. Because of their high performance and long existence, their eye-catching application is getting increasing numbers in recent times. LEDs are nowadays defined as using the "ultimate light bulb". In a previous couple of years, its efficiency has been multiplied through converting it to nano size. This new light-emitting has a nano-pixel structure and it affords high-resolution performance and the geometry of the pixel is cylindrical or conical form. Due to the fact that the previous few years, a few impurity-doped nanocrystal LEDs are varying a good deal in trend. Its performance is very excessive and consumes a smaller amount of voltage. Its monochromatic behavior and indicator excellent are shown publicly demanded in the market and in this work, it's covered evaluations of the fundamental's standards of LEDs and the specific mixed metallic and nanocrystal shape of emitters. In addition, it covers the upcoming challenges that the current trend is working to resolve to get efficient materials to fulfill the future energy crisis.

**Keywords:** nanostructure, impurity doped nanocrystal, metal mixed LEDs

## **1. Introduction**

A light-emitting diode is one of the eminent revolutionary devices where several applications are currently running such as optoelectronics, lighting, sensing, and medical applications. Light-emitting materials are gained much attention in recent days due to their capacity to reduce the consumption of global electricity. It affects the usage of fossil fuels since more than 20% of electricity is globally consumed and needs an alternative to existing lighting materials. Early stage expected that the Solid State Lighting (SSL) is a type of lightning in LEDs to saved 50% of the electricity consumption of lighting which can reduce 300 x 106 tons of annual carbon emission [1, 2]. In addition, commercially available white LEDs moving towards 20–30% efficiency via exhibits fluorescent emission as lamps in limited applications which can be realized by primary color renders (blue, green, red). However, achieving the white LEDs with high color rendering quality is quite challenging and many research groups are working to reach 70–100% efficiency [3]. There is an alternative to reach maximum efficiency by

eliminating blue LEDs where efficiency droop arises. To improve efficiency, eminent research focuses attended on nanocrystal-based materials for optoelectronics. In other ways to develop primary colors emitting materials to achieve efficient white light generation, P. H. Fu et al. developed a GaN-based multi-quantum well light emitting diode using SiO2 nano-honeycomb arrays, natural lithography, and reactive ion etching. Nano-honey comb light output power was found to be 77.8% while using a 40 mA driving current [3]. H. Perlman et al. discussed a nano light emitting device performance enhancement designed for high-resolution display [4].

Further, lighting applications are increasing interest in the field of doping-free and impurity doping materials usage. It is used for LED/Organic LEDs applications where the required resolution high and easy to implement nano-LEDs. In this area still, work is going on phase change materials between two layers. Another focusing area is InGaN/GaN quantum dots structural and optical implementation, along that focused on nano-pixel matrix [5, 6]. Single nanowire pixels are also reported with a full-color demonstration by Y. Ra et al. [7]. Single chip and multi-color nanowire LED pixels are represented in **Figure 1a** and lighting technology efficiencies are in **Figure 1b**. This nanowire single chip arranged InGaN/GaN LEDs exhibits a turn-on voltage of approx. 2 V with no leakage current. Similarly, nano-pixel matrices prepared with metal ions conjugation into polymers by S. Basak et al. Here, RGB colors are arranged in a hierarchical 3-dimensional way and a layer of high-resolution cross strips printed with Eu and Tb metal ions. It was designed at approximately 100 × 100 μm<sup>2</sup> and is ca. 0.64μm<sup>2</sup> [8].

There several other methods are emphasized to develop the lighting materials to enable high luminous efficiency. Additionally, by deliberately introducing atoms or ions of various impurity-doped elements (such as alkali metals, rare earth, lanthanide impurities, and transition metals) into host lattices or non-stoichiometry-induced self-doping, it is possible to produce diverse impurity-doped nanocrystals with desired properties and functions. Because the self-quenching and reabsorption caused by an enhanced Stokes shift may be eliminated, impurity-doped nanocrystals are significantly less sensitive to thermal, chemical, and photochemical perturbations than ones that are not. More holes (p-type doping) or electrons (n-type doping), in particular, are provided with the use of impurities, improving electrical applications. The doping levels and dopant placements are altered by the synthesis schemes (such as doping agents, reaction parameters, and operation temperatures), which also alter the dopant luminescence and electronic impurities [9, 10].

#### **Figure 1.**

*a) Single chip arranged monolithically integrated multi-color nanowire LED pixels, b) development of the lighting technology efficiencies. Image permission was taken from ref [2, 7].*

### *Milestone Developments and New Perspectives of Nano/Nanocrystal Light Emitting Diodes DOI: http://dx.doi.org/10.5772/intechopen.108907*

Recently, interest in impurity-doped nanocrystal light-emitting diodes (LEDs) has increased in both academia and industry due to their great potential to meet the rising need for lighting, display, and signaling technologies. Impedance-doped nanocrystal LEDs have been shown to have a number of advantages over their undoped counterparts, higher brightness, including higher efficiency, longer stability, and lower voltage. In addition to the inherent advantages of nanocrystals, impurity-doped nanocrystals frequently exhibit additional advantages such as enhanced chemical and thermal stability, increased photoluminescence quantum efficiency (PLQY), reduced Auger recombination, customized charge mobility, and impurity-related emission. Because of these benefits, impurity-doped nanocrystals have inspired efforts to satisfy the requirements of several optoelectronic applications [11, 12].

As with many LED emitters, impurity-doped nanocrystals have been intensively studied. Band-edge and impurity-related emissions can frequently be seen in impuritydoped nanocrystal LEDs. Impurity-doped nanocrystal LEDs consequently display three emission behaviors (i.e., LEDs exhibit only host emissions, LEDs show only impurity emissions, and LEDs possess both host and dopant emissions). This is distinct from band-edge emissions exclusively observed in undoped nanocrystal LEDs. Impuritydoped nanocrystal LEDs can also have higher efficiency and brightness compared to their undoped counterparts. For instance, impurity doping was utilized to increase the brightness of PeLEDs by nearly ten times and the external quantum efficiency (EQE) of CQW-LEDs by nine times, respectively. The stability of impurity-doped nanocrystal LEDs may also be higher than that of ones that are undoped. Impurity-doped nanocrystal LEDs, particularly for PeLEDs, CQW-LEDs, and CQD-LEDs, are very promising for upcoming lighting, display, and signaling technologies (such as improved luminance, voltage-increased stability, enhanced efficiency, and lowered power consumption) due to their unique characteristics and amazing benefits [9].

During forecast period 13, which runs from 2021 to 2026, the worldwide OLED display market is expected to grow significantly. The market is expanding steadily in 2020, and due to major players' increasing adoption of strategies, the industry is anticipated to increase during the expected time frame. Considering the influence on the global OLED display market it impacted from both global and regional perspectives. The main key players in OLEDs are Europe, North America, Japan, and China, as per the report put emphasis on the analysis of the market corresponding response policy in different regions [13]. An OLED is a carbon-based light-emitting diode that produces light when current passes through the conductors, the anode and the cathode. With OLED display technology, it delivers superior image quality compared to other display technologies such as liquid crystal displays (LCDs) and LEDs. According to a CAGR of 2021–2026, the OLED display market-generated million of USD in revenue in 2016, million of USD in 2021, and millions of USD in 2026, respectively [14].

## **2. Development of LEDs**

### **2.1 Milestone inventions**

Because of their extraordinary benefits, such as excellent brightness, high efficiency, impressive power consumption, extended lifetime, and low voltage, nanocrystal light-emitting diodes offer enormous promise in signaling, lighting, and display applications [15–17]. In 1994, Alivisatos et al. published the first nanocrystal LED with a maximum EQE of 0.01% using CdSe colloidal quantum dots (CQDs) [18]. A

widely used technique to give nanocrystals a variety of, optical, catalytic, new electrical, transporting, and magnetic capabilities is impurity doping [19, 20]. Before the 20th century, four types of semiconductor technology generations are stepping stones for the semiconductor LEDs and the development of the semiconductor technology duration wise demonstrated in **Figure 2** [21, 22].

## **2.2 Past technology**

The most prominent SSL sources are blue GaN/InGaN LEDs with yttrium aluminum garnet (YAG) phosphor. Correlated color temperatures (CCT) were found to be 4000–8000 K for white light generation by mixing this broad yellowish emission of YAG phosphor and blue LED. The color rendering indices (CRI) are recognized to be below 80. These results are somewhat recognizable however, indoor illumination applications required a CCT of less than 4000 K and a CRI value of more than 80. To overcome these limitations, several analyses are invented to improve efficiency. Among them, recently nanocrystal-based materials have expanded the possibility of new milestones. In addition, these materials are capable of tunable and narrow emission visible spectral range. Further, the small overlap between the absorption and emission spectra indicates fewer strokes shift values. Few of the reports stated CRI above 80 by using the dual hybridization of polymers and nanocrystals on LEDs [23–26]. The tunability was achieved by using layer by layer combination of CdSe/ ZnS and nanocrystals packed on polyfluorene.

## **2.3 Knowledge gap and current trend**

All the LEDs mentioned above are being used nowadays to make the hundredthlumen-per-lamp, in which Green, Blue, Red, and White LEDs are used, and red LEDs are the most effective. For this, the LEDs have been prepared with solid-phase materials. The most common application of these LEDs is being used to make displays. Red-green-blue (RGB) LEDs or white LEDs are used to backlight ultra-large video displays or LCDs. Initially, this backlighting is used for mobile displays later on it is

*Milestone Developments and New Perspectives of Nano/Nanocrystal Light Emitting Diodes DOI: http://dx.doi.org/10.5772/intechopen.108907*

used for computer and television displays. The most common use of LED is now in LCD backlighting. This backlighting usually requires more color, but not much, white point-source LEDs are employed for this. However, in the long run, the future of LED backlighting is not certain as LCD has no features. The main reason for this is organic LEDs (OLEDs) have made tremendous progress recently it has greater light emitting efficiency and long life than LEDs and the second reason is LEDs fabrication cost is too high for OLEDs [27].

Devices made of LEDs need more research in the past few decades because of the great advancement in it. In particular, we can envision a time when solid-state lighting is both intelligent and incredibly efficient. This type of intelligent, ultra-efficient solid-state lighting would enable: very high (and gt; 150%) and quot; Effective and quot; light output and usage efficiencies; a variety of new system applications (including feature-rich lighting, and integrated lights/displays), which need to be expanded to various areas of human welfare, as well as technology. In addition, nanotechnology incorporation into the LEDs era is one of the boosting points to develop efficient LEDs. It also noted hybrid light emitting materials tune the emission behavior such as multiple nanocrystals pumped by blue nitride emitters expressed tunability properties. Another preference is to use ultraviolet emitting LEDs by using the nanocrystal thinner films which overcome the blue pumping. Here, the photoemission arises from nanocrystals and direct not depend on the LED platform. It leads to fetching the property of tunability of the color. UV LEDs promising features can have the possibility to reach higher optical levels.

## **3. CIE, CCT and color rendering index parameters**

Nano-crystallized LED materials are characterized by CIE parameters; CIE is "Commission International de l' Eclairage" this method provides basic information about a specific range of light exposed by the nanomaterials (LEDs). The chromaticity variations of light sources for lighting have been offered by the Macadam ellipses' (Macadam, 1942). The CIE color area is purpose monochromatic primary colors with wavelengths of 435.8 nm for blue and 546.1 nm for green, 700 nm for red [28, 29]. **Figure 3** shows the chromaticity graph representing the x, and y-axis.

H. Orucu, et al. have synthesized tridoped Yb/Er/Tm (III) metal-metal ion gadolinium gallium garnet nanocrystal by Solgel Pechini method and its size was determined by X-ray diffraction [30]. It is nearly about 26–56 nm and its CIE values Gd3Ga5O12: 1% Er3+/1.5%Tm3+ 2%Yb3+ observed in white space = 0.03244, y = 0.3297 at ordinary temperature under 975 nm IR excitations spectrum. P. Fu and co-workers' nano honeycomb light-emitting diodes (GaN) have 77.8% efficiency [31]. Further G.M.Wu and others increased the extraction effectiveness of GaN/GaN by including small-size photonic inside the LED [32]. The lighting peak's integrated area expanded by 75%, its intensity by 91%, and its integrated area by 106%. T. Xiang, et al. prepared perovskite crystal of 12-crown-4 ether complexes of CAIG PCs, its external QE is 16% [33].

Recently modified high-power LED was prepared using in order to get a higher luminous flux requirement but it has been found that 75–90% of the input power is dissolute as heat so proper heat management is needed for the optical performance of LED for long time temperatures dropped the LED efficiency 1% when the temperature is increased 1° c. K. Yen Yong, et al. synthesized graphene-coated nanoplatelets it reduced the excess temperature and noteworthy improvements in LED cooling [34].

**Figure 3.** *Chromaticity diagram as CIE.*

#### **Figure 4.**

*Nano-pixel plots that show competence graph for cylindrical devices vs. conical devices. Image permission was taken from ref [4].*

*Milestone Developments and New Perspectives of Nano/Nanocrystal Light Emitting Diodes DOI: http://dx.doi.org/10.5772/intechopen.108907*

Microscale light-emitting diodes without sidewall emission have been created by Xinpei Hu and colleagues [35]. In comparison to traditional vertical μLED pixels, the inclined μLED pixel with ODR can boost its overall light extraction efficiency by 2.24 times while reducing sidewall emission by 99.6%. H. Perlman, et al. discussed emission spectra of GaN layer with different color ranges [4]. **Figure 4** indicate different overlapping curve of green, blue and red lines with ranges of color spectra.

## **4. Standards and design of nanocrystals LEDs**

Long-lasting and brighter nanocrystal characteristics for LEDs. The actual lightemitting LED chip is the main component of an LED diode, and it is enclosed in a clear protective domed shell or lens. While a large portion of the light generated by the chip simply travels through the shell, part of it is reflected inward to get around these issues in LEDs.

Nanocrystals are attracting noteworthy consideration for nano-electrical applications for the growth of new non-volatile, high-density appliances. Nanocrystal show many opto-chemical properties, but control of shape and size has been an interesting topic for the advanced growth of nanocrystals. Nano-crystal LEDs also known as light-emitting nano-pixel structured devices have high-resolution displays [4, 36]. It is a unit cell of many more composite structures of LENA (Light Emitting Nano-pixel Array). Nano LEDs improve the feasibility analysis with more clarity vision's these devices are made of two portions, 1st one is nano cone LEDs (**Figure 5**) and 2nd portion is a parabolic concentrator out of these two parts 1st one is very important in order for the reduced the total internal reflection and improve light extraction efficiency. **Table 1** shows a comparative analysis of nano-pixel techniques and an outline of the designed device. When stimulated with a 365 nm LED, some devices exhibit white emissions with excellent CIE coordinates (0.35, 0.31) and a very high color rendering index of 93.

#### **Figure 5.**

*Represented three-dimension structures of device (a) LENS and (b) CPC. Image permission was taken from ref [ 4] .*


**Table 1.**

*Comparative analysis of nano-pixel techniques and their specifications.*

## **5. Nanocrystals for LEDs**

### **5.1 White LEDs**

Generally, traditionally structured white light-emitting diodes (w-LEDs) be contingent on 3-way combination approaches depending on blue LEDs, yellow-red phosphors and organic resources combinations. However, it frequently suffers from difficult predicaments, including the particularly high fabrication price of traditional yellow and pink phosphors and relatively low robustness due to peripherally natural encapsulating materials. These obstacles have significantly disadvantaged the similarly commercially increasing white light emitting diode (w-LEDs). For this mono-vanadate phosphorus films that had been immediately fabricated on an organic framework at the normal room, temperatures were investigated for w-LEDs. Nanostructures have small nano size compared to other materials like the wavelength of light and are particularly perfect to decorate light interactions [37]. There are so many techniques available for defining the nanostructures' and the morphology of metallic surfaces like surface plasmon polariton (SPP) resonance is of unique interest in this particular area [38]. In 2015 a unique excessive-overall performance phosphors-loose w-LEDs, which is designed by nano-single crystal Ba2V2O7 or Sr2V2O7 quantum dots (BVQD or SVQD) at once growing on common quartz substrates throughout the deposition of polymer (PAD). As compared to the metavanadatesprimarily based phosphor thin layers, the excellent quality of BVQD or SVQD affords a broader band spectrum at 400-700 nm for white LEDs with a 95% quantum yield.

Greater prominently, for homogenous nano-mono crystals di-vanadates quantum dots had been effectively full-grown on not uncommon quartz substrates. Relying on the specific position of polymer-attached metals brings a remarkable leap forward

### *Milestone Developments and New Perspectives of Nano/Nanocrystal Light Emitting Diodes DOI: http://dx.doi.org/10.5772/intechopen.108907*

within the subject of heteroepitaxial growth [39]. In 2016 metallic nanostructures assisting plasmonic resonances is a thrilling alternative to this method because of their strong connection between light and matter be counted interplay, which simplifies regulate of light emission without necessitating outside secondary optical fragments. Colloidal semiconductor nanoplatelets (NPLs), a new class of semiconductor nanocrystals with particular structural and electrical characteristics resulting from their flat ring design, have received attention in 2018. It has been shown that Type II NPLs have enormous promise for optoelectronic devices like solar cells and lasers. Here, type II NPL-based nanocrystal light-emitting diodes have been created. These type II NPLs (CdSe/CdSe 0.8 Te 0.2 core/crown) are in use, and their photoluminescence quantum yield is near 85%. Due to their easily adjustable band gaps, strong photoluminescence quantum yield, pure color emission, and cheap cost, inorganic cesium halide perovskite nanocrystals have received a lot of attention in the same year for application in optoelectronic applications. Cesium lead bromide (CsPbBr3) is an inorganic perovskite in which all the bonded atoms are inorganic in nature. However, the structural and optical features of CsPbBr3 nanocrystals deteriorate when they are converted from colloidal solutions to solid thin films, which causes problems with device functioning [40]. This is because organic surfactants are unavoidably used throughout the synthesis. At the same time, the field of optoelectronics has seen a complete revolution owing to hybrid organic-inorganic metal halide perovskites, with exponential growth in efficiency seen for both photovoltaic and light emission applications [41].

The amount of energy consumed worldwide has been rising throughout time. The need for research into sustainable and renewable energy sources is driven by the finite availability of fossil fuels. One of the most promising research to fulfill the rising energy needs of future generations without harming the climate is the conversion of sunlight into electricity. Direct conversion of photon energy into electricity is made possible by solar cell technology, which is environmentally beneficial and renewable [42]. Unfortunately, scientists have not yet been successful in developing photovoltaic devices that are extremely efficient, affordable, and scalable. A unique type of semiconductors has recently developed, the organic-inorganic perovskite with an ABX3 structure, where X is Cl, Br, or I, and A is cesium (Cs), methyl ammonium (MA), or formamidinium (FA). Perovskites can be treated using a variety of methods, including spray coating and a nozzle that diffuses tiny liquid droplets on substrates for the perovskite layer [43].

Colloidal nanocrystals of organic-inorganic hybrid perovskites (OIHPs), which have exceptional photophysical properties, were a new class of solid-state lighting materials in 2019. An in-depth study has been put into developing high-performance light-emitting diodes based on these materials, including interface engineering, which is crucial for balancing the injection of electrons and holes in gadgets. The effective perovskite nanocrystal LEDs are based on the high electron density 9,10-bis(Nbenzimidazolyl) anthracene (BBIA), a novel electron transport material (ETM). The anthracene-based compounds might present fresh study directions for perovskite LED interface engineering [44]. In order to meet the individualized requirements of cutting-edge applications like mobile phones, wearable watches, virtual/augmented reality, micro-projectors, and ultra-high-definition TVs, micro-light-emitting diodes (m-LEDs) are regarded as the foundation of next-generation display technology in 2020. However, due to the limited absorption cross-section, standard phosphor color conversion cannot provide enough brightness and yield to enable highresolution screens as LED chip sizes decrease to below 20 m. Due to their exceptional

photoluminescence, narrow bandwidth emission, color tunability, high quantum yield, and nanoscale size, quantum dot (QD) materials are anticipated to arise and fill this gap, offering a potent full-color solution for -LED displays [45]. White lightemitting diodes (WLEDs) made of silicon nanocrystals (SiNCs) were announced as taking the place of gallium nitride (GaN)-based products that currently dominate the solid-state general lighting market by the end of 2021. Today's high-power IGBT (insulated gate bipolar transistor) devices, wireless and fiber communications, space and compound solar cells, and improved shape memory alloys all depend on Ga in an irreplaceable way. The observed SiNCs have a photoluminescence quantum yield (PLQY) of 11.4% [46].

Jonathan D. Gosnell, et al. designed a new monodispersed nano-size CdSe phosphors excited LEDs to overcome the earlier problem related to LEDs emissions [47]. It gives a large spectrum of 420–710 nm and a small value of quantum yield (10%). Due to its high potential application of nano-size receiving much concentration. White light emitting diodes have various beneficial qualities like high storage capacity and long life and low voltage uses etc. therefore these materials have very high demand [48]. Earlier prepared semiconductor hybrid materials are used for white light emitting diodes like CdSe, CdSe/ZnS, and CdSe/CdS/ZnS but it is very costly as well as associated high energy power consumptions. To overcome these problems Jie Chen, et al. [16] synthesized new silicon-based carbon dots/nanocrystals for LEDs. Carbon nanodots have fascinating properties like it as thermal stability and biocompatibility, eco-friendly materials. Further, it was fabricated by APTES by hydrothermal method. Yanqin Li and coworkers created three CdSe/ZnS QDs of various sizes and blended them in a CBP in an organic matrix to create optically stable ternary nanocrystal composites [49]. A stirring procedure of MS and with CsPbBr3 NCs blended in toluene solution was used by Xiaoxuan Di and colleagues to create CsPbBr3NCs mesoporous silica based light emitting, and the resulting NCs-MS based WLED realizes an easy chromaticity tuning [50].

Combinations of CdSe/ZnS core-shell nanocrystals hybridized on InGaN/GaN LEDs for high color rendering index are used to provide warm white light with these color-converting nanocrystal emitter combinations. Three different sets of proof-of-concept devices are created to provide high-quality warm white light, with the following specifications: 1 (x, y) = 0.37, 0.30, LE = 303 lm/W, CRI = 79.6, and CCT = 1982 K; 2 (x, y) = 0.38, 0.31, LE = 323 lm/W, CRI = 81.0, and 3190 K; and 3 (x, y) = 0.37, 0. CdSe nanocrystal for LEDs was synthesized by Michael A, et al. having excellent color properties and it is defined by CIE color coordinates (0.333, 0.333) and color temp 5461–6007 K with a higher index 96.6 [51]. A simple two-step solution synthesis method was used to create all-inorganic CsPbBr3 perovskite nanocrystals with a mean size of about 300 nm. Although a CsPb2Br5 impure phase is also present in the finished product, high-resolution transmission electron microscopy and X-ray diffraction characterizations reveal the CsPbBr3 nanoparticles are single crystals with good crystallinity as synthesized [52]. Using an electrospray (e-spray) deposition of a silazane (SZ) oligomer-decorated PeNC solution, He Cheng Yoon and co-authors synthesized CsPbBr3 perovskite nanocrystal (PeNC)-embedded inorganic polymer film by encapsulating the PeNC in a Si N/Si O-based matrix [53]. The LE and CE rates of the white LED were 71.0 lm/W and 50.8%, respectively, at a corresponding color temperature of 9334 K, with only an 8% decline in the LE during long-term operation of 100 h. This indicates that the photophysical properties of these molecules are superior compared to earlier ones. The mechanism of conversion of colors of LEDs for White LEDs is produced using blue LEDs, as seen in **Figure 6** [54].

*Milestone Developments and New Perspectives of Nano/Nanocrystal Light Emitting Diodes DOI: http://dx.doi.org/10.5772/intechopen.108907*

**Figure 6.** *Designed mechanism of conversion of colors of LEDs. Image permission was taken from ref [54].*

### **5.2 Blue LEDs**

The light-emitting diode is a type of semiconductor which play an important role in low lightening power consumption therefore there are a lot of trends of LEDs in the market, but blue LEDs are also creating a lot of attention. In the 1990s Prof Isamu Akasaki and Hiroshi and Shuji Nakamura prepared the first blue LEDs they also described the brightness of LEDs by (InGaN) systems due to less consumption of power blue LEDs have various applications. With advances in phosphor, blue LED efficiency rises from 130 to 140 lm/W to 200–210 lm/W between 2014 and 2018. Therefore, using blue LEDs will significantly reduce lighting costs and benefit the environment. In developing nations, blue light diodes are employed in electronics, solar-powered applications, and indoor and outdoor lighting. Steady enhancement in mug up with accessibility of all colors unexpectedly raised up LEDs like candidates for monochrome application [54]. Here we discussed year-wise progress in LEDs technology. **Figure 7** indicates year-wise growth in LEDs technology.

Wenjie Xu, et al. have designed new in-situ perovskite blue nanocrystal diodes by the antisolvent process. The LEDs describe all means of EL at 465 nm and EQE values of 2.4% and 2.5 cd A−1 separately, by directly adding ligands to the perovskite precursor solution. Nanocrystal halides (CsPbBr/Cl)3 nanocrystals (NCs) play a significant role in blue light emitting diodes [55], the use of dipolar ions of the NCs used in two hole-transporting layers to overcome earlier issues.

Copper-containing tertiary (I-III-VI) chalcogenide nanocrystals (NCs) are prepared by Eric C. Hansen and co-workers by the combinations of lead, and cadmium [56]. The photophysical qualities of these compounds found a PL maximum of 450 nm. En-Ping Yao and others designed inorganic M-X (metal halide) composite perovskite nanocrystal universal light emitting diodes' perovskite nanocrystals (NCs) [16]. Recently so many metals halide perovskite structure blue diodes are synthesized by univalent organic cation. Due to their superior optoelectronic characteristics, such as their narrow emission line widths, high photoluminescence quantum yield (PLQY), tuneable emission wavelength, and high color purity, perovskite-based lightemitting diodes (PLEDs) have become a promising alternative. For green, red, and near-infrared PLEDs with a high external quantum efficiency of about 20%, notable

**Figure 7.** *Wide-reaching application of LEDs in the market. Image permission was taken from ref [54].*

#### **Figure 8.**

*Effect of Mn addition on electroluminescence spectra. Image permission was taken from ref [58].*

progress has been made during the past few years. However, a number of technical challenges, such as subpar film quality, an inefficient device structure, a higher trap density, and others have restricted the development of blue PLEDs [57]. New

perovskite structures of Mn-based blue LEDs were created by Shaocong Hou et al., and a small quantity of Mn was added to improve the device's lighting [58]. Blue LEDs with extremely high efficiency and brightness are produced through mn doping. The range of produced compounds' electroluminescence is shown in **Figure 8**.

## **5.3 Green LEDs**

Organometallic compounds were used by Zhi-Kuang Tan and colleagues to create solid-state organic light emitting [57]. Using DDAX as the lone ligand in the hot-injection synthesis, Y. Shynkarenko and co-workers developed long-chain metal halide LEDs with quaternary ammonium-capped CsPb (Br1-xCy)3 NCs. Emitting a green light, which has C12-hydrocarbon chains with an EQE of 9.8% and a brightness of 34,700 Cd/m2 , produces the best results [59]. Recently lead halide colloidal nanocrystal are used as an extraordinary candidate for LEDs because it has excellent optoelectronic quality, along with very high PL yields, and small size with wide color tuneable properties. Young-Hoon and others proved highly organized light-emitting diodes constructed by the colloidal nanocrystal perovskite by using a multifunctional buffer hole layer (Buf-HIL), This Buf-HIL coupled with poly(3,4-ethylene dioxythiophene)/poly(styrene sulfonate) and per-fluorinated ionomer with these approaches they achieved excellent yields of photoluminescent quantum yield (60.5%) [60]. Normally light emitting diodes are a combination of the organic framework with inorganic metal halide, Perovskite NCs with dimensions bigger than the DB can be created using technique 10 by using a multipurpose buffer hole injection layer and (ii) (Buf-HIL) **Figure 9**.

Jun Xing, co-author of organic metal framework associated metal halide LEDs and provided a systematic route of a chain of colloidal halide perovskite preparation of CH3NH3PbX3 non-crystalline nanoparticles and this shows maximum luminous efficiency of 11.49 Cd/A of 3.8% EQE values and these values higher than previously reported colloidal quantum dot-based LEDs [61]. **Figure 10**, discussed the structure and analysis data of CH3NH3PbX3 non-crystalline nanoparticles. Bright and reliable light-emitting diodes were created by Hsinhan Tsai and colleagues using perovskite nanocrystals stabilized in metal-organic frameworks (PeMOF) [62]. Cs-PeMOF is used to further improve the compound's stability because the modification device has the property of thermal stability. Internal quantum efficiency was raised by 25% in the NVPCs reported by Yu-Lin Tsai and others, and droop behavior was decreased from 37.4% to 25.9% [63]. With a driving current of 350 mA, an increase in light output power of up to 151% is possible. Researchers have also created layers of ZnO nanocrystals capable of electron injection in blue, red, and green nanocrystals. The performance of green LEDs is lower than that of their red and blue relatives, which are much more obtrusive and need more work [64].

## **5.4 Red LEDs**

The color efficiency of LEDs depends on the size of the device blue LEDs have already completed all these demands. It was found that green LEDs has some drawback mainly suffering from an emission peak frequency which is very high and also some color purity issue [62, 65]. LEDs are semiconductors and in the case of semiconductor emissions, colors can be altered by changing the size of the crystal. In the market white, blue and green LEDs are more popular but as the demand for Red LEDs is increasing, so scientists are making Red LEDs by using different materials.

**Figure 9.**

*(a) Distribution of inorganic quantum dots and structure of nanocrystal, (b) device fabrication ways (c) structure of nanocrystal perovskites, (d) histogram of nanocrystals, (c) FTIR and TEM analysis (f) elemental mapping of perovskite NPs. Image permission was taken from ref [60].*

Parth Vashishtha and et al. have investigated that mixed halide nanocrystal shows excellent color quality's for the new family of nanocrystal light-emitting diodes. Prepared compounds produce stable red emissions in the red region (620-650 nm) [66]. Mixed halide CsPbBr, X, (X = I or Cl) peNC organic LEDs using peNC emitters with photoluminescence. **Figure 11** illustrated the construction of QD-OLEDs from CsPbX3 nanocrystals (X = Cl, Br or I) grown via a colloidal synthesis procedure. Red NCs (nanocrystals are a very significant component of perovskite light emitting devices but their stability is very low because of the stacked structure so some modification is

*Milestone Developments and New Perspectives of Nano/Nanocrystal Light Emitting Diodes DOI: http://dx.doi.org/10.5772/intechopen.108907*

**Figure 10.**

*a) XRD patterns of amorphous CH3NH3PbBr3 NPs and polycrystalline film, (b) atomic model of cubic phase CH3NH3PbBr3. Blue ball, I; pink ball, Pb; black ball, CH3; green ball, NH3. (c,d) HRTEM image, (e) SAED pattern, and (f) size distribution of CH3NH3PbBr3 NPs. Image permission was taken from ref [61].*

necessary. Some researchers modified its morphology and efficiency by using transition metal therefore luminescent red perovskite CsPbBrI2 NCs were achieved through Cu substitution and halide rich passivation strategy [67]. With a higher photoluminescence quantum yield (up to 94.8%) and enhanced stability, Cu2+-substituted CsPbBrI2 NCs have demonstrated exceptional performance. In order to create highperformance red perovskite LEDs, the very stable and luminous Cu2+-substituted CsPbBrI2 NCs can work effectively as light emitters. Using an epitaxial solution growth technique, Jibin Zhang and colleagues created new, very stable, red-emitting CsPbBrI2/PbSe heterojunction nanocrystals (h-NCs), where each CsPbBrI2 NC was covered in PbSe in the CsPbBrI2/PbSe heterodimers [66, 68].

The purification of the chemicals to create extremely luminous and stable CsPbCl3xBrx and CsPbI3 PNCs was accomplished using various crystallization techniques of PbX2 (X = Cl or I), according to Chae Hyun Lee and the co-author [69]. By using a hydrothermal (Hyd) procedure to enhance the quality of the PbCl2 as-prepared, blue-emitting PNCs are given effective ligand surface passivation, a maximum photoluminescence quantum yield (PLQY) of around 88%, and increased photocatalytic activity to oxidize benzyl alcohol, producing 40%. The creation of red-emissive PNCs with a PLQY of up to 100% was then seen as a result of the heated recrystallization of PbI2 prior to (Hyd) treatment. CsPbBrxI3-x LEDs based on nanosized - CsPbBrxI3-x crystallites have recently been manufactured primarily by the traditional colloidal technique, which includes a laborious procedure of nanocrystal synthesis, purification, ligand or anion, etc. CsPbBrxI3-x LEDs have only been able to turn on at high turn-on voltages (>2.7) while using the commonly used traditional LED device structure. Additionally, this mix-halide system may experience significant spectra shift under bias. This study describes the preparation of CsPbBrxI3-x thin films with nano-sized crystallites using a one-step spin-coating method that incorporates several ammonium ligands. The growth of CsPbBrxI3-x nanograins is restricted by

#### **Figure 11.**

*Normalized photoluminescence spectra (top) taken at 400 nm excitation of a solution of CsPbBr3-xIx, for varying values of x from 0 (green) to 2.75 (deep red). Blue NCs consist of CsPbBr3-xCIx (x = 0.75). Normalized electroluminescence spectra (bottom) from ITO/PEDOT: PSS/pTPD/peNC/TPBi/Al LEDs for these same peNCs. Image permission was taken from ref [62].*

the various ammonium ligands. Such quantum confinement-benefited CsPbBrxI3-x thin films are advantageous. The corresponding CsPbBrxI3-x LEDs emit pure-red color (CIE) coordinates of (0.709, 0.290), (0.711, 0.289), etc., which represent the highest color-purity for reported pure-red perovskite [70]. They use a conventional LED structure of indium-doped tin oxide (ITO) /poly (3,4 ethylene dioxythiophene) poly(styrene).

## **6. Future perspectives**

Here, we faced the common characteristics of all LEDs and also concentrated on the issues relating to the color characteristics and stability characteristics of various types of perovskite forms of LEDs. Here, we addressed the inherent characteristics of all LEDs, and that was our main objective. The main obstacles to obtaining desired stability and efficiency can be roughly divided into two groups: (a) a careful selection of photonic structures, and (b) an understanding of and control of intrinsic material qualities for optimum performance. Additionally, interfacial layers enable smooth integration, improved illumination, and fall avoidance for devices. We assume these approaches to be in addition changed to improve the nice of LED improvement within the destiny.

*Milestone Developments and New Perspectives of Nano/Nanocrystal Light Emitting Diodes DOI: http://dx.doi.org/10.5772/intechopen.108907*

## **7. Conclusions**

A nano light emitting device performance enhancement design of high-resolution display which is currently focusing area in lighting technology. The white light emission, blue can exempt however red is a common and essential parameter. Blue accepts by yellow is feasible to reach more efficiency and electroluminescence semiconductor light emitters usage is preferable. Which are better than the phosphors filling in the red-yellow-green gap of lighting. It expects to improve efficiency and it leads to improving productivity. Since a few years ago, nanocrystal LEDs have become very popular because of their appealing photophysical characteristics. We have covered all of the physical characteristics and production methods for white, blue, green, and red LEDs in this review post. In addition, discussed a Noble prize works by duration in terms of semiconductors. The necessity for rigorous research into these material systems and the best device configurations. Since the long run, the future of LED backlighting is not certain as LCD has no features. Additionally, there is a need for significant upgrades. In this section, we also discussed how nanocrystals are introduced and color rendering properties. The design of Nanocrystal LEDs with different dimensions is elaborated to understand the reported design structures which help to design new upcoming architectures of lighting.

## **Acknowledgements**

Authors gratefully acknowledge the financial support by the GUJCOST, Gov. of India (Project No. GUJCOST/2020-2021/2012).

## **Conflict of interest**

There are no conflicts to declare.

## **Author details**

Jyoti Singh, Niteen P. Borane and Rajamouli Boddula\* Tarsadia Institute of Chemical Science, Uka Tarsadia University, Maliba Campus, Bardoli, Gujarat, India

\*Address all correspondence to: rajamouliboddula@gmail.com

© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

## **References**

[1] Sandia National Laboratories. Sandia National Laboratories Solid-State Lighting: Better and More Effic. Published online 2008. Available frrom: http://www.sandia.gov

[2] Tsao JY, Han J, Haitz RH, Pattison PM. The blue LED Nobel prize: Historical context, current scientific understanding, human benefit. Annalen der Physik. 2015;**527**(5-6):A53-A61. DOI: 10.1002/andp.201570058

[3] Laboratories SN. Solid-State lighting Research & development at Sandia national laboratories. 2008. Available from: http://www.ssls.sandia.gov

[4] Perlman H, Eisenfeld T, Karsenty A. Performance enhancement and applications review of nano light emitting device (led). Nanomaterials 2021;**11**(1):1-27. DOI: 10.3390/ nano11010023

[5] Tsao JY, Crawford MH, Coltrin ME, et al. Toward smart and ultra-efficient solid-state lighting. Advanced Optical Materials. 2014;**2**(9):809-836. DOI: 10.1002/adom.201400131

[6] Hosseini P, Wright CD, Bhaskaran H. An optoelectronic framework enabled by low-dimensional phase-change films. Nature. 2014;**511**(7508):206-211. DOI: 10.1038/nature13487

[7] Ra YH, Wang R, Woo SY, et al. Full-color single nanowire pixels for projection displays. Nano Letters. 2016;**16**(7):4608-4615. DOI: 10.1021/acs. nanolett.6b01929

[8] Basak S, Mohiddon MA, Baumgarten M, Müllen K, Chandrasekar R. Hierarchical multicolor nano-pixel matrices formed by

coordinating luminescent metal ions to a conjugated poly(4′-octyl-2′,6′-bispyrazoyl pyridine) film via contact printing. Scientific Reports. 2015;**5**(i):8406. DOI: 10.1038/srep08406

[9] Luo D, Wang L, Qiu Y, Huang R, Liu B. Emergence of impurity-doped nanocrystal light-emitting diodes. Nanomaterials Review. 2020;**10**:1226. DOI: 10.3390/nano10061226

[10] Bryan JD, Gamelin DR. Doped semiconductor nanocrystals: Synthesis, characterization, physical properties, and applications. Progress in Inorganic Chemistry. 2005;**54**:47-126 DOI: 10.1002/0471725560.ch2

[11] Kanemitsu Y. Multiple exciton generation and recombination in carbon nanotubes and nanocrystals. Accounts of Chemical Research. 2013;**46**(6):1358- 1366. DOI: 10.1063/pt.4.0563

[12] Kriegel I, Scotognella F, Manna L. lasmonic doped semiconductor nanocrystals: Properties, fabrication, applications and perspectives. Physics Reports. 2017;**674**:1-52. DOI: 10.1016/j. physrep.2017.01.003

[13] Laricchia F. OLED panel production capacity share 2016-2025, by country. 2022. Available from: http://www. statista.com

[14] OLED display market size, COVID-19 impact analysis, regional outlook, application development potential, Price trend, Competitive Market Share & Forecast, 2022 – 2028. Glob Mark insights. Published online 2022:Report ID: GMI3827.

[15] Mondal N, De A, Samanta A. Achieving near-Unity photoluminescence *Milestone Developments and New Perspectives of Nano/Nanocrystal Light Emitting Diodes DOI: http://dx.doi.org/10.5772/intechopen.108907*

efficiency for blue-violet-emitting perovskite nanocrystals. ACS Energy Letters. 2019;**4**(1):32-39. DOI: 10.1021/ acsenergylett.8b01909

[16] Ge C, Fang Q, Lin H, Hu H. Review on blue perovskite light-emitting diodes: Recent advances and future prospects. Front in Materials. 2021;**8**(March):1-7. DOI: 10.3389/fmats.2021.635025

[17] Cui B, Feng XT, Zhang F, Wang YL, Liu XG, Yang YZ, et al. The use of carbon quantum dots as fluorescent materials in white LEDs. Xinxing Tan Cailiao/New Carbon Materials. 2017;**32**(5):385-401. DOI: 10.1016/S1872-5805(17)60130-6

[18] Tolbert SH, Alivisatos AP. Size dependence of a first order solid-solid phase transition: The wurtzite to rock salt transformation in CdSe nanocrystals. Science (80-). 1994;**265**(5170):373-376. DOI: 10.1126/science.265.5170.373

[19] Kovalenko MV, Protesescu L, Bodnarchuk MI. Properties and potential optoelectronic applications of lead halide perovskite nanocrystals. Science (80- ). 2017;**358**(6364):745-750. DOI: 10.1126/ science.aam7093

[20] Liang S, Zhang M, Biesold GM, et al. Recent advances in synthesis, properties, and applications of metal halide perovskite nanocrystals/polymer nanocomposites. Advanced Materials. 2021;**33**(50):1-36. DOI: 10.1002/ adma.202005888

[21] Mittleman DM, Schoenlein RW, Shiang JJ, Colvin VL, Alivisatos AP, Shank CV. Quantum size dependence of femtosecond electronic dephasing and vibrational dynamics in CdSe nanocrystals. Physical Review B. 1994;**49**(20):14435-14447. DOI: 10.1103/ PhysRevB.49.14435

[22] Schlamp MC, Peng X, Alivisatos AP. Improved efficiencies in light emitting

diodes made with CdSe(CdS) core/shell type nanocrystals and a semiconducting polymer. Journal of Applied Physics. 1997;**82**(11):5837-5842. DOI: 10.1063/1.366452

[23] Demir HV, Nizamoglu S, Ozel T, Nizamoglu S, Ozel T, Mutlugun E, et al. White light generation tuned by dual hybridization of nanocrystals and conjugated polymers. New Journal of Physics. 2007;**9**(10). DOI: 10.1088/ 1367-2630/9/10/362

[24] Ali M, Chattopadhyay S, Nag A, Kumar A, Sapra S, Chakraborty S, et al. White-light emission from a blend of CdSeS nanocrystals of different Se: S ratio. Nanotechnology. 2007;**18**(7):1-4. DOI: 10.1088/0957-4484/18/7/075401

[25] Nizamoglu S, Demir HV. Hybrid white light sources based on layerby-layer assembly of nanocrystals on near-UV emitting diodes. Nanotechnology. 2007;**18**(40):1-4. DOI: 10.1088/0957-4484/18/40/405702

[26] Schubert EF. Light-Emitting Diodes. Cambridge, UK: Cambridge University Press. 2006. Available from: https://archive. org/details/lightemittingdio00schu\_0

[27] Chynoweth AG. Charge multiplication phenomena. In: Semicond Semimetals. 1968;**4**:263-325. DOI: 10.1016/S0080-8784(08)60345-2

[28] Rajamouli B, Sivakumar V. Eu(III) complexes for LEDs based on Carbazole- and Fluorene-functionalized Phenanthro-imidazole ancillary ligands: Detailed Photophysical and theoretical study. ChemistrySelect. 2017;**2**(14):4138- 4149. DOI: 10.1002/slct.201700266

[29] Westland S. Review of the CIE system of colorimetry and its use in dentistry. Journal of Esthetic and Restorative Dentistry. 2003;**15**:S5-S12. DOI: 10.1111/j.1708-8240.2003.tb00313.x [30] Örücüa H, Sevcan Tabanli ME, Öztürk Y, Eryürekb G. Bright white light up-conversion luminescence from Yb3+/ Er3+/Tm3+ tridoped gadolinium gallium garnet nano-crystals for multicolor and white light-emitting diodes. Optical Materials (Amst). 2022;**131**:112613. DOI: 10.1016/j.optmat.2022.112613

[31] Fu PH, Lin GJ, Wang HP, Lai KY, He JH. Enhanced light extraction of light-emitting diodes via nanohoneycomb photonic crystals. Nano Energy. 2014;**8**:78-83. DOI: 10.1016/j. nanoen.2014.05.006

[32] Films TS. Effects of nanostructured photonic crystals on light extraction enhancement of nitride light-emitting diodes. GMWu, CCYen, HWChien, HCLu, TWChang, TENee. 2011;**519**(15):5074-5077. DOI: 10.1016/j. tsf.2011.01.131

[33] Xiang T, Li T, Wang M, Zhang W, Ahmadi M, Wu X, et al. 12-Crown-4 ether assisted in-situ grown perovskite crystals for ambient stable light emitting diodes. Nano Energy. 2022;**95**:107000. DOI: 10.1016/j.nanoen.2022.107000

[34] Yong KY, Chan YK, Von LE, Hung YM. Effective passive phasechange light-emitting diode cooling system using graphene nanoplatelets coatings. Case Studies in Thermal Engineering. 2022;**31**(January):101795. DOI: 10.1016/j.csite.2022.101795

[35] Hu X, Cai J, Liu Y, Zhao M, Chen E, Guo Y, et al. Design of inclined omnidirectional reflector for sidewallemission-free micro-scale light-emitting diodes. Optics and Laser Technology. 2022;**154**:108335. DOI: 10.1016/j. optlastec.2022.108335

[36] Pimpin A, Srituravanich W. Reviews on micro- and nanolithography techniques and their applications.

Engineering Journal. 2012;**16**(1):37-55. DOI: 10.4186/ej.2012.16.1.37

[37] Wang X, He H, Gao J, Gao J, Hu H, Tang S, et al. Effects of nanoparticle structural features on the light-matter interactions in nanocermet layers and cermet-based solar absorbers. Journal of Materials. 2021;**7**(5):1103-1111. DOI: 10.1016/j.jmat.2021.01.011

[38] Moon H, Bersin E, Chakraborty C, Lu A, Grosso G, Kong J, et al. Surface plasmon subwavelength optics. ACS Photonics. 2003;**424**:824-830. DOI: 10.1021/acsphotonics.0c00626

[39] Yang Z, Liu J, Chen LH, Zhang L, Pan H, Wu B, et al. A high-performance white-light-emitting-diodes based on nano-single crystal divanadates quantum dots. Scientific Reports. 2015;**5**(1):1-7

[40] Wang L, Liu B, Zhao X, Demir HV, Gu H, Sun H. Solvent-assisted surface engineering for high-performance allinorganic perovskite nanocrystal lightemitting diodes. ACS Applied Materials & Interfaces. 2018;**10**(23):19828-19835. DOI: 10.1021/acsami.8b06105

[41] Kar S, Jamaludin NF, Yantara N, Mhaisalkar SG, Leong WL. Recent advancements and perspectives on light management and high performance in perovskite light-emitting diodes. Nanophotonics. 2020;**10**(8):2103-2143. DOI: 10.1515/nanoph-2021-0033

[42] Shaikh MRS. A review paper on electricity generation from solar energy. International Journal of Research Applications Science Engineering and Technology. 2017;**V**(IX):1884-1889. DOI: 10.22214/ijraset.2017.9272

[43] Wang R, Mujahid M, Duan Y, Wang ZK, Xue J, Yang Y. A review of perovskites solar cell stability. Advanced Functional Materials. 2019;**29**(47):1-25. DOI: 10.1002/adfm.201808843

*Milestone Developments and New Perspectives of Nano/Nanocrystal Light Emitting Diodes DOI: http://dx.doi.org/10.5772/intechopen.108907*

[44] Kumar S, Marcato T, Vasylevskyi SI, Jagielski J, Fromm KM, Shih CJ. Efficient perovskite nanocrystal light-emitting diodes using a benzimidazole-substituted anthracene derivative as the electron transport material. Journal of Materials Chemistry C. 2019;**7**(29):8938-8945. DOI: 10.1039/c9tc02352f

[45] Liu Z, Lin CH, Hyun BR, Sher CW, Lv Z, Luo B, et al. Micro-light-emitting diodes with quantum dots in display technology. Light Science Application. 2020;**9**(1):1-23. DOI: 10.1038/ s41377-020-0268-1

[46] Zhang YC, Yu ZY, Xue XY, Wang FL, Li S, Dai XY, et al. High brightness silicon nanocrystal white light-emitting diode with luminance of 2060 cd/m2 . Optics Express. 2021;**29**(21):34126. DOI: 10.1364/oe.437737

[47] Schreuder MA, Gosnell JD, Smith NJ, Warnement MR, Weiss SM, Rosenthal SJ. Encapsulated white-light CdSe nanocrystals as nanophosphors for solid-state lighting. Journal of Materials Chemistry. 2008;**18**(9):970-975. DOI: 10.1039/b716803a

[48] Chen J, Liu W, Mao LH, Yin YJ, Wang CF, Chen S. Synthesis of silicabased carbon dot/nanocrystal hybrids toward white LEDs. Journal of Materials Science. 2014;**49**(21):7391-7398. DOI: 10.1007/s10853-014-8413-y

[49] Li Y, Rizzo A, Cingolani R, Gigli G. Bright white-light-emitting device from ternary nanocrystal composites. Advanced Materials. 2006;**18**(19):2545- 2548. DOI: 10.1002/adma.200600181

[50] Di X, Shen L, Jiang J, Shen L, Jiang J, He M, et al. Efficient white LEDs with bright green-emitting CsPbBr3 perovskite nanocrystal in mesoporous silica nanoparticles. Journal of Alloys and Compounds. 2017;**729**(Complete):526-532. DOI: 10.1016/j.jallcom.2017.09.213

[51] Schreuder MA, Xiao K, Ivanov IN, Weiss SM, Rosenthal SJ. White lightemitting diodes based on ultrasmall CdSe nanocrystal electroluminescence. Nano Letters. 2010;**10**(2):573-576. DOI: 10.1021/nl903515g

[52] Yue W, Liu Y, Heyu C, Chunyang L, Weizhen L, Haiyang X, et al. White LED based on CsPbBr3 nanocrystal phosphors via a facile two-step solution synthesis route. Materials Research Bulletin. 2018;**104**(February):48-52. DOI: 10.1016/j.materresbull.2018.03.055

[53] Yoon HC, Lee S, Song JK, Yang H, Do YR. Efficient and stable CsPbBr3 quantum-dot powders passivated and encapsulated with a mixed silicon nitride and silicon oxide inorganic polymer matrix. ACS Applied Materials & Interfaces. 2018;**10**(14):11756-11767. DOI: 10.1021/acsami.8b01014

[54] Bergh AA. Blue laser diode (LD) and light emitting diode (LED) applications. Phys Status Solidi Application Research. 2004;**201**(12):2740-2754. DOI: 10.1002/ pssa.200405124

[55] Ochsenbein ST, Krieg F, Shynkarenko Y, Rainò G, Kovalenko MV. Engineering color-stable blue lightemitting diodes with Lead halide perovskite nanocrystals. ACS Applied Materials & Interfaces. 2019;**11**(24):21655- 21660. DOI: 10.1021/acsami.9b02472

[56] Hansen EC, Liu Y, Utzat H, Bertram SN, Grossman JC, Bawendi MG. Blue light emitting defective nanocrystals composed of earth-abundant elements. Angewandte Chemie International Edition. 2020;**59**(2):860-867. DOI: 10.1002/anie.201911436

[57] Tan ZK, Moghaddam RS, Lai ML, Docampo P, Higler R, Deschler F, et al. Bright light-emitting diodes based on organometal halide perovskite. Nature Nanotechnology. 2014;**9**(9):687-692. DOI: 10.1038/nnano.2014.149

[58] Hou S, Gangishetty MK, Quan Q, Congreve DN. Efficient blue and white perovskite light-emitting diodes via manganese doping. Joule. 2018;**2**(11):2421- 2433. DOI: 10.1016/j.joule.2018.08.005

[59] Smock SR, Chen Y, Rossini AJ, Brutchey RL. The surface chemistry and structure of colloidal Lead halide perovskite nanocrystals. Accounts of Chemical Research. 2021;**54**(3):707-718. DOI: 10.1021/acs.accounts.0c00741

[60] Kim YH, Wolf C, Kim YT, Cho H, Kwon W, Do S, et al. Highly efficient light-emitting diodes of colloidal metal-halide perovskite nanocrystals beyond quantum size. ACS Nano. 2017;**11**(7):6586-6593. DOI: 10.1021/ acsnano.6b07617

[61] Xing J, Yan F, Zhao Y, Chen S, Yu H, Zhang Q, et al. High-efficiency lightemitting diodes of Organometal halide perovskite amorphous nanoparticles. ACS Nano. 2016;**10**(7):6623-6630. DOI: 10.1021/acsnano.6b01540

[62] Vashishtha P, Halpert JE. Field-driven ion migration and color instability in red-emitting mixed halide perovskite nanocrystal light-emitting diodes. Chemistry of Materials. 2017;**29**(14):5965- 5973. DOI: 10.1021/acs.chemmater.7b01609

[63] Tsai YL, Liu CY, Krishnan C, Lin DW, Chu YC, Chen TP, et al. Bridging the "green gap" of LEDs: Giant light output enhancement and directional control of LEDs via embedded nano-void photonic crystals. Nanoscale. 2016;**8**(2):1192-1199. DOI: 10.1039/c5nr05555e

[64] Janssen RAJ, Stouwdam JW. Red, green, and blue quantum dot LEDs with solution processable ZnO nanocrystal electron injection layers. Journal of Materials Chemistry. 2008;**18**(16):1889- 1894. DOI: 10.1039/b800028j

[65] Sciences M. Investigation of InGaN-based red/green micro-lightemitting diodes. Optics Letters. 2021;**46**(8):1912-1915

[66] Shen W, Zhang J, Dong R, Chen Y, Yang L, Chen S, et al. Stable and efficient red perovskite light-emitting diodes based on Ca2+-doped CsPbI3 nanocrystals. Research. 2021;**2021**:1-11. DOI: 10.34133/2021/9829374

[67] Zhang J, Zhang L, Cai P, Xue X, Wang M, Zhang J, et al. Enhancing stability of red perovskite nanocrystals through copper substitution for efficient light-emitting diodes. Nano Energy. 2019;**62**:434-441. DOI: 10.1016/j. nanoen.2019.05.027

[68] Zhang J, Liu X, Jiang P, Chen H, Wang Y, Ma J, et al. Red-emitting CsPbBrI2/PbSe heterojunction nanocrystals with high luminescent efficiency and stability for bright light-emitting diodes. Nano Energy. 2019;**66**:104142. DOI: 10.1016/j. nanoen.2019.104142

[69] Lee CH, Shin YJ, Villanueva-Antolí A, Das Adhikari S, Rodriguez-Pereira J, Macak JM, et al. Efficient and stable blue- and redemitting perovskite nanocrystals through defect engineering: PbX2Purification. Chemistry of Materials. 2021;**33**(22):8745-8757. DOI: 10.1021/acs.chemmater.1c02772

[70] Jiang M, Hu Z, Ono LK, Qi Y. CsPbBrxI3-x thin films with multiple ammonium ligands for low turn-on purered perovskite light-emitting diodes. Nano Research. 2021;**14**(1):191-197. DOI: 10.1007/s12274-020-3065-5
