The Future Is Blue (LEDs): Why Chemistry Is the ... - ACS Publications

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Perspective

The future is blue (LEDs): why chemistry is the key to perovskite displays C-H. Angus Li, Zhicong Zhou, Parth Vashishtha, and Jonathan E. Halpert Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b01650 • Publication Date (Web): 16 Jul 2019 Downloaded from pubs.acs.org on July 17, 2019

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Chemistry of Materials

The future is blue (LEDs): why chemistry is the key to perovskite displays C-H. Angus Li1, Zhicong Zhou1, Parth Vashishtha2, Jonathan E. Halpert*1 Affiliation: Department of Chemistry, Hong Kong University of Science and Technology (HKUST), Clear Water Bay Rd, Kowloon, Hong Kong 999077, China S.A.R. 1

School of Materials Science and Engineering, Nanyang Technological University (NTU), 50 Nanyang Avenue, Singapore 639798, Republic of Singapore 2

Abstract: In this perspective, we review the recent work on metal halide perovskite light-emitting devices (peLEDs) with a focus on current challenges and the prospect of commercialization of perovskite displays. Metal halide perovskites have shown remarkable hole and electron mobilities, high photoluminescence quantum yield and color tunable, thin linewidth emission peaks making them ideal materials for the emissive layer in LEDs. Red and green emitting perovskites have already achieved very high external quantum efficiencies of greater than 20 % in the last year and may be ready to begin the commercialization process. However, blue peLEDs still lag behind in efficiency and stability. After reviewing recent attempts to improve the blue peLED efficiency, we then list the current challenges in the field and suggest possible avenues of future research, including improvements in materials chemistry and device architecture. In particular, we find that advancements are needed in color stability and lifetime, by using doping and nanostructured materials to avoid ion migration effects, and in the synthesis of novel compounds. We then compare the current state of the art in peLEDs with the competitive landscape and conclude that fixing the blue peLEDs is the key to building micro-pixel LEDs from perovskites. Without blue emitting perovskites, high PLQY perovskite films can still be useful for down-conversion color filters but may have difficulty displacing existing technologies. Thus, for perovskites, the future is indeed blue (LEDs).

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1.0 Introduction Metal halide perovskites have gained significant attention in recent years as the active layer in solar cells, transistors, LEDs, lasers and other optoelectronic devices.1-6 These materials, in particular CsPbX3 and MAPbX3, (X = Cl, Br, I) possess excellent properties for light emitting devices (LEDs), having very bright emission, with a high photoluminescence quantum yield, a thin linewidth of emission and high mobility for holes and electrons. In addition, these can be made emission tunable across the visible spectrum by changes in both composition and crystal size.7-10 Generally, metal halide perovskites have an ABX3 crystal structure where the A group is a monovalent cation, which consist of cesium (Cs), methylammonium (MA) or formamidinium (FA), the B metal group is a divalent cation, generally Pb or Sn, and the X are the halides Cl, Br, and I or mixtures thereof.6, 11, 12 LEDs made from these materials can use a bulk crystalline thin film or a nanostructured thin film, where the edges of individual nanocrystalline sheets or cubes are passivated with surface ligands.13-15 Thin films of bulk, nanostructured and nanocrystalline materials can all be produced via solution processing techniques.16 This suggests the possibility of inkjet, screen printing or roll-to-roll processing to cheaply fabricate large area displays with a full color palette, high brightness and contrast and high resolution. Very recently, nanostructured films of lead-based halide perovskites, using CsPbBr3 and CsPbI3 have been used to make high efficiency LEDs with greater than 20 % external quantum efficiency (EQE) for both green and red emitting devices.14, 17 However, three colors are needed to create displays with electroluminescence (EL) peaks in the red (630 - 650 nm), green (520 - 530 nm)18, 19 and blue (460 - 480 nm)20, 21. These colors (RGB) provide optimal points for constructing the largest possible CIE color triangle. While most of the recent research has focused on red and green LEDs, with several attempts at sky-blue (480 nm – 500 nm)22-25, pure blue emitting perovskite LEDs have so far lagged in efficiency, as was true for previous LED technologies, where red and green emitting devices tend to be easier to produce.26-29 For this reason, earlier technologies, such as quantum dot (QD) displays, use red and green QDs in a down conversion filter layer rather than in the active layer in an LED pixel.30, 31 Here we will review the recent progress in peLEDs with a view towards the eventual commercial goal of building micropixel displays. More so than other color peLEDs, blue peLEDs tend to have issues with difficult charge injection from charge transport layers, energy down-conversion, trapping and non-radiative recombination, low quantum yield, poor stability and lifetimes, as well as suffering from ion migration effects.9, 32-34 While most reported blue peLEDs displayed less than 2 % EQE, the recent publication of >2 % blue and sky-blue peLEDs (480 nm - 500 nm)22, 23 indicates that significant improvements are still possible.8, 22, 35 However, ion migration, leading to spectral instability and short lifetimes, is still a leading issue for these devices. Additionally, lead is a toxic metal and may hinder development of perovskite displays in some markets. Regardless of the challenges, achieving the potential of perovskites as solution processable micro-pixel displays will require developing high efficiency blue LEDs. If higher EQEs cannot be achieved with improvements to layer materials and device architecture then metal halide perovskites, as a class of material, may be relegated to becoming yet another down-conversion, filter layer. If so, it may fail to supplant the current existing, and soon-to-be-realized, competitive display technologies. After reviewing progress in red and green LEDs, we will contrast the recent progress blue peLEDs, then list the many challenges specific to blue peLEDs and suggest chemical strategies for further improvement of the active layer materials. Finally, we can compare perovskite LEDs to state-of-the-art and expected future LED technologies to predict the future of peLEDs, with and without blue as an effective EL device.

2.0 Perovskite LEDs: A Brief History of Red and Green peLEDs Here we briefly review the successful history of red and green perovskites. These color peLEDs showed EQE, lifetime and color purity that suggest they are much closer to the metrics needed to consider commercialization. The less successful blue peLEDs are covered in a later section although there are many parallels in their development. [FIGURE 1] Figure 1. A timeline summarizing key events in the peLED field for efficiency improvement, updated until late 2018. The events are categorized according to color: NIR (> 700 nm), red (630 - 650 nm), green (520 - 530 nm), sky-blue (480 - 500 nm) and pure blue (460 - 480 nm).

2.1 First Report and Current Record


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Back in the 1990s, halide perovskite materials had already been reported in devices displaying electroluminescence, however only under cryogenic conditions.36, 37 Since 2009, when the first efficient halide perovskite photovoltaic (PV) was reported by the Miyasaka group,4 the power conversion efficiency (PCE %) has surged from 3.8 % to over 22 %.38, 39 It was noticed at that time that the high photoluminescence quantum yield (PLQY) of perovskite halide thin films40 indicate that the primary recombination pathway is radiative.41, 42 It had been suggested, in the case of organic solar cells, that for a solar cell to approach the Shockley-Queisser limit all recombination should be radiative and in the absence of charge extraction layers, the luminescence should be maximized.43, 44 By that reasoning, lead halide perovskites should also be ideal candidates for light emitting devices.43 The first efficient room temperature halide peLED was reported by the Optoelectronics Group at the University of Cambridge in 2014 using a 3D halide perovskite material (Figure 2).5 The group successfully demonstrated infrared colored peLED of EQE 0.76 % using a thin layer emissive layer of CH3NH3PbI3-xClx, sandwiched between titanium oxide (TiO2) and poly(9,9’-dioctylfluorene) (F8) and a green colored peLED using CH3NH3PbBr3 as emissive layer with EQE 0.1 %.5 As a result, once perovskite solar cells began approaching the single junction efficiency limit, many groups pivoted from working on perovskite

[FIGURE 2]

Figure 2. First efficient room temperature peLED as proof-of-concept. The resulting device showed EQE 0.76% at infrared and 0.1% at green region. (a) Device architecture of the infrared CH3NH3PbI3xClx peLED. (b) Energy-level diagram of different layers of materials in the infrared PeLED, showing conduction and valence band levels with respect to vacuum. (c) Absorption spectrum (black) showing that onset occurs at 780 nm. Electroluminescence (green) occurs at 754 nm and photoluminescence (red) at 773 nm. Adapted with permission from ref 5. Copyright 2014 Nature Publishing Group.

solar cells to focus on perovskite LEDs instead. This shift explains the rapid rise of the field today, with most of the significant improvements having occurred in just the last three years. 2.2 Structural Classes of Lead Halide Perovskites With the rapid development in the peLED field, four classes of perovskite materials have evolved based on their structure at the nanoscale: 3D perovskites, nanostructured 3D perovskites using additives and ligands, 2D and quasi-2D perovskites (Ruddlesden-Popper phase perovskites), and 0D perovskite nanocrystals (including quantum dots and quasi-0D nanocubes). 3D bulk film perovskites have shown a long carrier lifetime45 and high charge mobility46. From spectroscopic studies it is known that exciton binding energy in bulk perovskite (CH3NH3PbI3) film is low (6-50 meV)47, 48, indicating the charge carriers are free instead of existing in a tightly-bound excitonic state,49 and the dominant non-radiative recombination pathway in 3D perovskites is trap-assisted recombination.50 These properties make bulk perovskites perfect candidates for photovoltaics. However, the increased exciton binding energy (EB) and spatial confinement of excitons in quasi-2D nanostructured perovskites, 2D nanosheets or 0D nanocrystals, appear to be needed to improve the rates of radiative recombination, as required for LEDs. 2.2.1 Bulk 3D perovskites: Since the demonstration of the first room temperature peLED,5 multiple groups have reported improvements upon those devices. Kumawat et al. reported the first blue-green peLEDs based on a 3D perovskite produced by tuning the chloride content in MAPb(BrxCl1-x)3 perovskites.10 Wang et al. refined the 3D green peLED performance to EQE 3.5 % with MAPbBr3 as the emissive layer by improving the interface between the charge transporting layer and the perovskite film.51 High quality perovskite films formed on hydrophilic polyethyleneimine (PEI) modified ZnO, were found to facilitate efficient charge injection.51 To further increase device efficiency, improvements must be made on thin-film quality, where defect sites are inevitable formed during solution processes, as well as effective recombination of charge carrier at the perovskite layer. Options have been explored, ligands and bulky cations are added to promote film quality, reduce surface roughness and decrease defect density, prompting the development of nanostructure 3D perovskites (Section 2.2.2) and quasi-2D perovskites (Section 2.2.4). Meanwhile, to effectively confine excitons, 0D nanocrystal perovskites (Section 2.2.3) are attempted given the successful experience in inorganic quantum dots such as CdSe and



[FIGURE 3]

Figure 3. Efficient peLED with nanocrystal pinning method (NCP). With excess MABr added, metallic Pb-atoms are removed and solvent-based NCP method (S-NCP) promoted nano-sized grains, resulting in stronger confinement of excitons. SEM images of MAPbBr3 layers of (a) normal MABr:PbBr2 = 1:1 without NCP. (b) 1.05:1 with S-NCP, (c) XRD patterns of MAPbBr3 nanograin layers with MABr:PbBr2 = 1:1.05, 1:1, and 1.05:1. (d) Luminance of PeLEDs based on S-NCP and MAPbBr3 nanograin emission layers with varying molar ratio of MABr:PbBr2. Adapted with permission from ref 52. Copyright 2015 The American Association for the Advancement of Science.

InP. Quasi-2D perovskites are also proven to be another pathway for effective exciton confinement. 2.2.2 Nanostructured 3D perovskites via ligands or additives: Notable advancement came from Cho et al., who, in 2015, reported a device based on “nanocrystal-pinning (NCP) method” in which excess methylammonium bromide (MABr) is added during the spin-coating process (Figure 3). The resulting device gave an EQE of 8.53 %.52 They reported that that excess MABr could prevent the formation of photoluminescence (PL) quenching by metallic lead (Pb) atoms which arise from either the unintended loss of Br atoms or an incomplete reaction between MABr and PbBr2.53 They also suggested that the NCP process which anti-solvent is added during spin-coating could change the morphology of the MAPbBr3 from scattered microscale grains to tightly-packed nano-grains, and that this helps confine excitons within the nanosized grains.52 More recently, Xiao et al. in 2017, reported the incorporation of bulk n-butylammonium halide cations (BAX, X = I, Br), thus also forcing the 3D MAPbX3 perovskite films to form nanosized-grains with improved film roughness.54 The addition of BAX improved the iodide-based peLED from EQE 1.0 % to 10.4 % while the bromide-based peLED improved from an EQE of 0.03 % to 9.3 %. Additionally this group reported an increased shelf life of 8 months storage under nitrogen without degradation.54 In October 2018, more milestone devices were reported in the green and NIR regions, respectively,

[FIGURE 4]

Figure 4. Efficient green peLED with MABr/CsPbBr3 quasi-core/shell structure. An EQE of 20.31% was achieved at green region (525nm). The bottom-layer MABr are suggested to passivate grain boundaries of perovskites while upper-layer MABr are suggested to balance charge injections. a) Schematic illustrations of single-layered CsPbBr3 (top), bi-layered CsPbBr3/MABr (middle), and quasi-core/shell CsPbBr3/MABr structures (bottom), all fabricated on ITO substrates. b) Photographs of the three asprepared perovskite films under ultraviolet light. c) EQE-L curve. Adapted with permission from ref 14. Copyright 2018 Nature Publishing Group.

using one-step processing methods.14, 55 Most notably, Lin et al. achieved an EQE of 20.3 % at green (525 nm) while using a CsPbBr3/MABr quasi-core/shell structure and poly(methyl methacrylate) (PMMA) as a charge balancing layer between the perovskite and electron transporting layer (ETL) (Figure 4).14 In their work, they suggested that the bottom-layer of MABr could passivate grain boundaries of CsPbBr3 while the top-layer of MABr could balance charge injection after a onestep deposition.14 Similarly, Cao et al. reported peLEDs with EQE 20.7 %, in the NIR spectral region, also making use of spontaneous formed nanostructures with the addition of a bulky amino acid, 5-aminovaleric acid (5AVA).55 There, it was observed that an organic layer was formed and had filled the gaps between the perovskite nanoplatelets (Figure 5). They suggested this was from a dehydration reaction which occurred during annealing of the 5AVA, which could prevent LED leakage current.55 The effect of 5AVA was also reported to be surface passivating and to promote emission quality.55 Recently, Zhao et al. reported a high performing NIR peLED with a peak EQE of 20.1 % using a novel structure, a perovskite-polymer bulk hetero-structure (PPBH) shown in Figure 6.42 The emissive layer was composed of a mixture of 3D/quasi-2D (NMAI:FAPbI3) perovskites with a wide bandgap polymer, poly(2-hydroxyethyl methacrylate) (poly-HEMA).42 It was suggested that both the 3D and quasi-2D phases were present in the emissive layer from the absorption spectrum and high-resolution transmission electron microscope (HR-TEM) images, and that the perovskite crystallites are isotropically dispersed among the polymer.42 Most recently, Xu et al. reported highly efficient NIR peLED with most optimized passivation agent 2,2’-[oxybis(ethylenoxy)] diethylamine (ODEA) with perovskites, the champion device showed a record EQE of 21.6% with stable performance at high current desity.56


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[FIGURE 5]

Figure 5. Efficient NIR peLED with spontaneous self-formed sub-micrometer structure. Bulky amino acid (5AVA) was added to promote spontaneous formed nanostructured as showed. The adding of 5AVA promoted surface passivation and emission quality was improved. (a) Device structure, rays A, B and C, which represent light trapped in devices with a continuous emitting layer, can be extracted by the sub-micrometre structure. (b) Cross-section HAADF-STEM tomography image at high magnification. The scale bar represents 100 nm. (c) SEM image of the perovskite. The scale bar represents 1 μm. Adapted with permission from ref 55. Copyright 2018 Nature Publishing Group.

The selection of ligands and additives has been evolved from excess precursor material,52 to long and bulky alkyl chains,54 to more complex ligand materials, such as 5AVA.55 With improvement of the surface morphology induced by well-packed nanograins, the efficiencies of nanostructured perovskites have surged from ~3 % EQE of early devices to more than 20%. The high performance peLEDs in this material class has set the bench mark for peLED performance.



[FIGURE 6]

Figure 6. High efficiency NIR perovskite-polymer bulk-heterojunction (PPBH) LED. (a) Cross-sectional scanning electron microscopy (SEM) image of the LED structure. Inset: Structure of poly(2-hydroxyethyl methacrylate) (poly-HEMA). (b) GIWAXS patterns of a PPBH layer deposited on silicon. It was suggested that the perovskite crystallites are isotropically dispersed among the polymer (c) Current-voltage (black curve) and radiance-voltage (red curve) characteristics. (d) Absorbance (black curve) and photoluminescence (PL, red curve) spectra of a PPBH film on fused silica. The emissive layer composited of a mixture of 3D/quasi-2D (NMAI:FAPbI3) perovskites with a wide bandgap polymer, poly(2-hydroxyethyl methacrylate) (polyHEMA). Adapted with permission from ref 42. Copyright 2018 Nature Publishing Group.


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2.2.3 Zero Dimensional (0D) Nanocrystal (NC) Perovskite LEDs: 0D nanocrystals, often called quantum dots (QDs), have been very well studied for a wide variety of materials and a full description of their properties is beyond the scope of this review.16, 57 To be 0D, a nanocrystal must be small enough to approach the bulk Bohr radius of the exciton. Two effects are then observed: 1) a quantization of the near band-edge states, instead of a continuous density of states, and 2) the confinement effect, whereby when an excited state is formed, the hole and electron are forced, by the small dimensions of the material, into very close proximity.58 The increased interaction energy increases the energetic distance between the band edge states, similar to a particle in a box model. This increase the effective band gap, giving bluer first absorption and emission peaks than would be expected from the bulk material.59, 60 In 2015, Protesescu et al. was the first to report a colloidal synthesis of all-inorganic cesium lead halide perovskite nanocrystals through a hot-injection method (CsPbX3, X = Cl, Br, I, and mixed Cl/Br and Br/I) (Figure 7a - b). The assynthesized NCs demonstrated a PLQY up to 90 % with narrow emission line-widths of 12-42 nm and up to 140 % coverage of the NTSC color standard.61 Several optical studies have found that CsPbBr3 NCs possess 2-state degeneracy at the band edge, as well as displaying the onset of intermediate stage confinement for nanocubes with edge lengths of 50,000 hours operating lifetime.241 An emerging area for GaN-based blue and green LEDs is their use in micro-pixel displays.243-245 These displays can be applied to indoor/outdoor video walls and electronic products, such as smart phones, tablets, televisions, and smart watches. Inorganic μLEDs offer potential advantages compared to OLEDs and LCDs, including higher brightness, higher transparency, longer lifetimes, lower power consumption, and shorter response times. III-nitride based μLEDs exhibit a luminance of 105 cd m-2, which is much better than that of LCDs and OLEDs.243 Additionally, the response time of μLEDs is just nanosecond scale, which is much faster than that of LCDs and OLEDs.243 Companies like Apple and Google, have invested in μLEDs in recent years, which indicates that this emerging technology has attracted significant market interest.241 5.1.5 Comparisons of Key Metrics for Various Display Technologies: Although peLEDs have emerged as a promising technology for display and research has focused on increasing the performance of peLEDs in recent years, there is still a gap between peLEDs and other display technologies. One of the biggest concerns of peLEDs is their operational stability. The half-lifetime of peLEDs at an initial luminance of 100 cd m-2 is ~ 250 h246, which is far behind that reported for the other display technologies. Moreover, the device efficiency of blue peLEDs is relatively low (6.2 %) comparing to the red peLEDs (21.3 %) and green peLEDs (20.3 %). Additionally, lead content in perovskite is still a safety concern though it has already fulfilled the RoHS standards. Lead-free perovskites will need to be developed as alternative materials for peLEDs to meet a safer application. Despite of some of the deficiencies of the current peLEDs, their wide colour gamut, high colour purity and low cost of production look to make peLEDs a competitive display technology in the near future. Table 3 compares the key metrics for various display technologies.

Table 3. Comparisons of key metrics for various display technologies Type

LCDs

OLEDs

Technology

CFL backlight

Organic light-emitting material

70 % NTSC 247

> 100,000 h

>500,000 h (LT50 @ 1000 cd m-2)

249

250

3-5 % efficiency252

>20 % (no enhanced outcoupling)

Color gamut Lifetime

EQE

peLEDs Perovskite light-emitting material 140 % NTSC

QD-enhanced backlight

110 % NTSC

QLEDs Quantum dots light-emitting material 140 % NTSC

248

27

55

247

2,300 h (LT95 @1000 cd m-2)

>250 h (LT50 @ 100 cd m-2)

>30,000 h (commercial)

251

246

221, 228

>20 %

>20 %

254-256

14, 17, 55

6-8 % efficiency (LCD / NREF)

100, 214, 253

5.2 Current Industry


QLCDs

110 % NTSC

252

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Since the National Television System Committee (NTSC) agreed on one of the first broadcast standards for color TV – the NTSC 1953 color standard, researchers have been working on improving display technology while consumers demand for ever higher quality displays has increased. The color gamut evaluation standard developed from sRGB (widely used in general high-definition Televisions) to NTSC, and now is expected to eventually conform to the Rec. 2020 standard which is twice wider than that of the sRGB (Figure 22).257, 258 To achieve this standard, superior RGB emitters are needed to replace older organic dyes and phosphors.

[FIGURE 22]

Figure 22. Commission Internationale de l’Eclairage (CIE) chromaticity coordinates. (a) CIE chromaticity coordinates showing color gamut of different color standards. (b) CIE chromaticity coordinates showing color gamut of different display technologies. (b) Adapted with permission from ref 27. Copyright 2017 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim.

Sony introduced the first quantum dot TV Sony Triluminos with QD Vision’s Color IQ technology using Cd-based QDs in 2013.225 Since then Sony, TCL, Samsung, and LG have successively listed quantum dot products, such as TVs, tablets, monitors and mobile phones.221 Continued research of quantum dots in display technology mainly includes two aspects: 1) QD backlight-based QD photoluminescence properties for quantum dot backlight unit (QD⁃BLU); 2) QLED-based QD electroluminescence properties.27 Photoluminescence down-conversion QLED display technology has already entered the commercial stage. Nanosys cooperates with QD-film manufacturing company, like 3M, LMS, Hitachi, and Nitto Denko to commercialize the QD-films, which contributes to the development of QLED TVs.259 From 2015, many TV manufacturer, like TCL, Philips, Thomson and Hisense, launched 55-inch QLED TV using CdSe QD edge optic technology, with over 100 % NTSC color gamut and over 400 nits luminance.227 Since launching its first InP-based QLED TVs in 2015, Samsung had been making progress in brightness and wider color gamut of its TV products, with most recent products which comprise InP QD color converting films and blue LEDs, showing 800 nits luminance and over 100 % color volume.259 Even though CdSe and InP QDs and their enhanced films have been successfully used in commercial products. However, there are still some disadvantages of these technologies, including the complicated manufacturing process, relatively high cost of production, and the limitations of the use of cadmium content. In recent years, perovskite QDs have emerged as a competitive and promising material for high quality display technologies. In 2016, Zhong et al. reported a method of in-situ fabrication of MAPbX3 NCs embedded in a PVDF composite film.260 The preparation of the QD-embedded film was greatly simplified, and the dispersion of perovskite QDs in the polymer was significantly improved. In addition, perovskite QDs films-based (PQDF-based) BLU applied in a LCD TV prototype was achieved by TCL in 2018.261 This green PQDF-based LCD can achieve 101 % of the NTSC color gamut and high brightness (up to 500 nits) by combining blue LED chips and a red K2SiF6: Mn4+ (KSF) phosphor. The greatest assets of QD-LEDs as electroluminescent light sources are their low cost, high efficiency production compatibility, flexible and versatile form factors, and the capacity to be a light source spread over a large area than a point light source.262 In 2017, some panel manufacturers such as BoE and TCL, started producing full-color active-matrix QLED (AMQLED) panels.263 The product uses the inkjet printing process to prepare quantum dot light-emitting devices, eliminating the need for a backlight, enabling full-color display, significantly improving the display color gamut.263 Due to the new standard Rec. 2020, QLEDs have a promising future because quantum dot display is the only one among other technologies, such as LCDs and OLEDs, that can satisfy Rec. 2020 without any optical compensation. Therefore, more and more companies are working on quantum dot research, such as Nanjing Tech, BoE and TCL in China, Samsung and LG in South Korea, Nanosys, QD vision and Apple in US, and Nanoco in the UK.27

6.0 Conclusions Although there have been significant improvements on peLED over the few years, with suitable optical engineering to enhance out-coupling efficiency194 the maximum EQE% of peLED could even reach an impressive value 28.2% very recently195, yet the overall performance of peLEDs is still inferior to established technologies such as OLED or QLCD which OLED has lifetime > 80,000 hours264-266 for LT50 @ 1000 cd m-2 and QLED has lifetime > 2,300 hours251 for LT90 @1000 cd m-2 compared to most stable peLED ever reported. In order for perovskites to become commercially developed, improved



operational stability and spectral stability are of equal importance. Ion-migration induced spectral instability remains an issue for mixed halide perovskites and other methods of color tuning are still being developed. At this time layered perovskites (quasi-2D peLED) with phase purity control (n), showing optimal confinement effects (nano-structured and nanocrystals) for color tuning, carrier conduction and PLQY have shown superior performance as the emitting layer in peLEDs, particularly in blue peLEDs. If peLEDs with an emissive active layer can be engineered with stable, high EQE, that, plus the benefit in contrast for micropixel LEDs in general, should make them an attractive replacement to existing down-conversion LCD technologies. This suggests a few possible outcomes. If the color purity can match that of quantum dots but are much better active layer materials, they could supplant QD and OLED in future micro-pixel displays. If they can be made with high brightness and more cheaply than GaN LEDs and diode lasers, then they could supplant solid state micro-pixels or at least precede them. LCDs are also likely to remain a popular technology in the future and serious attempts at down-conversion LCD filters are promising but have yet to prove superior in any comparison to QLEDs. As we have seen, perovskites may be able to overcome some of the shortcomings of existing displays technologies. However, there are many contingencies here, including questions about micropixel technology, decreasing LCD costs and market place demand, in addition to the technical issues described in this perspective. But the first hurdle for the adoption of perovskite EL devices is whether blue peLEDs can be made which match the EQE and processability of red and green perovskites, while attaining industry standards for resolution, brightness, lifetime.

Acknowledgements: C-H.A.L., Z.Z. and J.E.H. acknowledge funding from the Hong Kong University of Science and Technology (HKUST) School of Science (SSCI) and Department of Chemistry via project funds IGN17SC05 and R9398. P.V. would like to acknowledge the Presidential Postdoctoral Fellowship, NTU via grant M408070000.


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The future is blue (LEDs): why chemistry is the key to perovskite displays C-H. Angus Li, Zhicong Zhou, Parth Vashishtha, Jonathan E. Halpert*

Author Biography

C-H. Angus Li

C-H. Angus Li is now pursuing his PhD in Chemistry at the Hong Kong University of Science and Technology (HKUST) under the supervision of Prof. Jonathan E. Halpert. His current research focus is on perovskite material for efficient lightemitting devices for display applications. Before joining HKUST, he worked as a research assistant at Prof. Alex Jen’s group while obtaining his MSc at City University of Hong Kong.

Zhicong Zhou

Zhicong Zhou completed his B.Sc. degree in Materials Science and Engineering in South China Agricultural University in 2016 and his MSc in Analytical Chemistry in HKUST in 2017. Since then he has been working in Prof. Jonathan E. Halpert’s group in the department of chemistry at HKUST. His current research is focused on high performance quantum dots LEDs and perovskite materials. Parth Vashishtha


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Parth Vashishtha obtained his PhD degree at Victoria University of Wellington (VUW), New Zealand in 2018 on Nanostructured Perovskites for Optoelectronic Applications. Prior to this, He carried out his MPhil dissertation at University of South Australia (UniSA) in 2014 on synthesis of InP quantum dots for DSSC. Currently, He is a Presidential Postdoctoral Fellow at School of Materials Science & Engineering, Nanyang Technological University (NTU), Singapore. His current research area is focused on lead-free perovskite nanocrystals for optoelectronics.

Jonathan E. Halpert*

Jonathan E. Halpert received his PhD in Physical Chemistry in 2008 at the Massachusetts Institute of Technology (MIT). He was a visiting fellow at the Chinese Academy of Sciences’ Institute for Process Engineering (CAS-IPE), and a postdoctoral researcher in the Optoelectronics Group (OE) at the University of Cambridge. He was a lecturer and senior lecturer in the School of Chemical and Physical Sciences (SCPS) at VUW from 2013-2017, as well as a Rutherford Discovery Fellow and a Principal Investigator in the MacDiarmid Institute for Advanced Materials and Nanotechnology. He recently moved to the Department of Chemistry at HKUST where he has been an assistant professor since 2017. His research interests include nanocrystals, nanomaterials and quantum dots using semiconductor materials, especially perovskites, to produce functional electronic and optoelectronic devices, including memristors, energy storage devices, photodetectors, solar cells and LEDs.



TOC 63x35mm (96 x 96 DPI)


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Figure 1, see manuscript for caption 338x190mm (96 x 96 DPI)





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The Future Is Blue (LEDs): Why Chemistry Is the ... - ACS Publications

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