hung-chia wang, zhen bao, hsin-yu tsai, an-cih tang, and ru-shi...

review

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Perovskite Quantum Dots and Their Application in Light-Emitting Diodes

Hung-Chia Wang, Zhen Bao, Hsin-Yu Tsai, An-Cih Tang, and Ru-Shi Liu*

H. C. Wang, Z. Bao, H. Y. Tsai, A. C. Tang, Prof. R. S. LiuDepartment of ChemistryNational Taiwan UniversityTaipei 106, TaiwanE-mail: [email protected]. R. S. LiuDepartment of Mechanical Engineering and GraduateInstitute of Manufacturing TechnologyNational Taipei University of TechnologyTaipei 106, Taiwan

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/smll.201702433.

DOI: 10.1002/smll.201702433

1. Introduction

Most research[1–7] centered on colloidal quantum dots (QDs) because of their high photoluminescence (PLQY), tunable wavelength, and narrow emission wavelength. With quantum confinement effect,[2,6,8–10] emission color of QDs can be controlled according to size[11,12] and content.[6] Given these advantages, QDs are useful in solar cells,[13–16] lasers,[17–19] light-emitting diodes (LEDs),[20–25] and bioimaging.[26,27] High color purity and color-tunable wavelength of QDs make them prom-ising candidates for next-generation displays. QD-based LEDs can be separated into light converter (white-light LEDs)[28,29] and active-mode QD-LEDs (QLEDs).[30] We introduce these two types in the following sections. (I) White-light QD-LEDs fea-ture QD-based light converters that rely on InGaN blue chips

Perovskite quantum dots (PQDs) attract significant interest in recent years because of their unique optical properties, such as tunable wavelength, narrow emission, and high photoluminescence quantum efficiency (PLQY). Recent studies report new types of formamidinium (FA) PbBr3 PQDs, PQDs with organic–inorganic mixed cations, divalent cation doped colloidal CsPb1−xMxBr3 PQDs (M = Sn2+, Cd2+, Zn2+, Mn2+) featuring partial cation exchange, and heterovalent cation doped into PQDs (Bi3+). These PQD ana-logs open new possibilities for optoelectronic devices. For commercial appli-cations in lighting and backlight displays, stability of PQDs requires further improvement to prevent their degradation by temperature, oxygen, moisture, and light. Oxygen and moisture-facilitated ion migration may easily etch unstable PQDs. Easy ion migration may result in crystal growth, which lowers PLQY of PQDs. Surface coating and treatment are important procedures for overcoming such factors. In this study, new types of PQDs and a strategy of improving their stabilities are introduced. Finally, this paper discusses future applications of PQDs in light-emitting diodes.

Perovskite Quantum Dots

as a backlight source. A white light passes through color filters to generate three pri-mary colors and detects color coordinates to calculate color gamut. Wider color gamut results in improved high-quality display. White-light QD-LEDs feature three display modes of operation.[29] (i) In on-surface mode, QDs can found in a thin film over the entire display area. (ii) In on-edge mode, QDs are placed between the LED package and light guide plate. (iii) In on-chip mode, QDs in matrix are coated on a blue chip. On-chip QD package poses a challenging task for the display industry. Owing to operating temperature and high blue flux intensities of LEDs, resulting disadvantages of on-chip package include thermal quenching, photodegradation, and moisture effect. Nevertheless, on-chip white-light LEDs are preferable in lighting and display applications because of their low cost and easy production and pack-

aging. For lighting applications, using theoretical modeling, Shimizu et al.[31] reported influence of peak full width at half maximum (FWHM) for red and green emitters on luminous efficiency (LE) of white-light LEDs (Figure 1a). The model pre-dicts maximum LE upon reducing FWHM of red phosphor. Compared with commercial red phosphor-based LEDs, QD-based LEDs with narrow FWHM can improve LED conver-sion efficiency from 5% to 15% at correlated color temperature (CCT) range of 5000–2700 K.[31] For backlight applications, color gamut of phosphor-based white-light LEDs reaches 86% of the National Television System Committee (NTSC) standard. QD-based white-light LEDs can reach 104% improvement of the NTSC standard.[32] (II) In comparison with commercial organic light-emitting diodes (OLEDs), active matrix QLED devices received significant interest in recent years because of their high color purity, cost-effectiveness, and low energy consumption. As shown in Figure 1b, the color gamut from previously reported high-efficiency QLEDs exceeds 140% of the NTSC standard, surpassing that of commercial OLED dis-plays.[30] High color purity of QLED devices eliminates use of color filters, which increase power consumption. Performance of QLED devices is affected by numerous factors, such as PLQY of QDs films, carrier injection efficiency from transport layer, and band alignment of device. To date, external quantum effi-ciency (EQE) of Cd-based red QLEDs reaches 20%,[33] which is comparable with that of commercial OLEDs. Perovskite QD (PQD)-based QLEDs recently drew significant attention given their extra-narrow emission wavelength, which measures lower

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than 20 nm. QDs can be mainly separated into three types, namely, Cd-based QDs (CdSe,[34] CdTe,[35] and PbS[36]), Cd-free QDs (InP,[37] CuInS2

[25]), and PQDs (methylammonium (MA)PbBr3,[7] CsPbBr3

[6]). PQDs feature the typical chemical compo-sition of ABX3, where A represents cesium (Cs), MA, or forma-midinium (FA); B represents a metal cation (Pb, Sn); and X is a halide anion (Cl, Br, and I)). PQDs exhibit favorable optical properties because of their defect-tolerance system, in which intrinsic defects do not act as trap states.[38,39] Recent studies reported new types of PQDs, such as FAPbBr3,[40,41] PQDs with organic–inorganic mixed cations,[42,43] divalent cation doped colloidal CsPb1−xMxBr3 PQDs (M = Sn2+, Cd2+, Zn2+, Mn2+) featuring partial cation exchange,[44–46] and heterovalent cation doped into PQDs (Bi3+).[47] Swarnkar et al.[39] used the peri-odic table to illustrate different functions in PQDs system, as shown in Figure 1c, and separated elements into four kinds: (i) dopant ion, (ii) lead-free, (iii) defect-tolerant, and (iv) plas-monic-coupled. New type FA0.75Cs0.25Sn0.5Pb0.5I3 perovskite material shows outstanding photovoltaic performance, which is characterized by power conversion efficiencies exceeding 20% in solar cells.[48,49] These PQD analogs will open new pos-sibilities for optoelectronic devices. In this review, we focus on PQDs and their applications in display industry. We separate this paper into three parts. The first part introduces new PQD types. Then, we discuss formation mechanism of PQDs. The second section discusses current issues on improving PQD sta-bility by surface passivation and shell-coating. The third section presents application of PQDs in white-light LEDs and QLEDs. Lastly, we provide ideas for possible practical application of PQDs in LEDs.

2. Different Types of PQDs

General QDs feature unique emission wavelengths, which are tunable by size based on quantum confinement effect. In quantum confinement effect,[30] quantization of energy occurs when wavefunctions of electrons and holes are spatially restricted to dimensions smaller than Bohr radius. In PQDs, particle size and anion composition control emission wave-length. For CsPbBr3 PQDs, strong quantum confinement only applies for the smallest NCs (≈4 nm edge length).[10] Corre-sponding bandgap increases from 2.37 to 2.50 eV as a result of decreasing particle size from 7.3 to 4.1 nm. Figure 2 shows experimental versus theoretical size dependence of bandgap energy.[10] As shown in Figure 2, quantum confinement regime was separated into two parts. When edge length of CsPbBr3 PQDs was longer than Bohr radius (7 nm), quantum con-finement effect was unremarkable. Anionic composition dominantly contributed to valence band maximum (VBM) and conduction band maximum (CBM). Bandgap is defined between Pb 6s−Br 4p hybridized orbitals and Pb 6p levels in CsPbBr3 PQDs.[38,50] Hence, in the entire visible spectral region of 410–700 nm,[6] emission is easily tunable by anion exchange.[51,52] In comparison with general QDs, tunable wavelength mechanism presents more complexity. Thus far, synthesis methods for PQD comprise many types. Nuclea-tion growth mechanism of PQDs can be separated into LaMer nucleation and oriented attachment growth mechanism.[53]

During LaMer nucleation and growth mechanism, QDs exhibit low critical size for nucleation at high temperatures; this prop-erty prevents nuclei from transforming into large crystals.[54–57] Large crystals lower PLQY as a result of quantum confinement effect.[10] In nanocrystal synthesis, oleic acid (OA) plays a key role in growth orientation of nanoparticles and self-assembly of CsPbBr3 nanosheets. Li et al.[53] explained synthesis mechanism of CsPbBr3 QDs in low-polarity solvents. As depicted in Figure 3, intermediate products form based on LaMer nucleation. Under low-polarity solvents, intermediate products present more sta-bility because of their strong bonding with ligands. When tem-perature reaches lower than 100 °C, then intermediate products aggregate to CsPbBr3, which measures ≈3.2 nm and coexists with intermediate products. Temperatures higher than 100 °C trigger oriented attachment growth, which promotes formation of CsPbBr3 at 10 nm. On the contrary, intermediate products in

Hung-Chia Wang received his bachelor degree in Applied Chemistry from National Chi Nan University in 2016. Now, he is a Ph.D. candidate in Prof. Liu’s group at National Taiwan University. His current research interests include all-inorganic perovskite quantum dots and their application in light-emitting diodes.

Zhen Bao received his bachelor degree in Functional Material from Northeastern University in 2016. Now, he is a Ph.D. candidate in Prof. Liu’s group. His current research interests include all-inorganic perovskites quantum dots and their application in light-emitting diodes.

Ru-Shi Liu is currently a professor at the Department of Chemistry, National Taiwan University. He received his Bachelor degree in Chemistry from Shoochow University (Taiwan) in 1981. He received his Master’s degree in nuclear science from the National Tsing Hua University (Taiwan) in 1983. He obtained two Ph.D. degrees in chemistry—

one from National TsingHua University in 1990 and one from the University of Cambridge in 1992. His research concerns the field of Materials Chemistry.

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highly polar solvents are unstable and highly reactive, forming CsPbBr3 PQDs at 3.2 nm when temperature reaches lower than 100 °C. When temperature measures above 100 °C, CsPbBr3 PQDs aggregate to CsPbBr3 bulk crystallites, which feature lower PLQY. Given the different nucleation reactions, synthesis under room temperature favors addition of polar solvents, such as N,N-dimethylformamide (DMF) or dimethyl sulfoxide (DMSO), and hot-injection method without polar solvents. The term “perovskite” originated from a similar crystal structure

of CaTiO3. In ABX6, divalent metal B, which is surrounded by six halogen atoms, is an octahedral structure, and cation A is located in the center of eight BX6 octahedral frameworks. Perovskite structure depends on Goldschmidt’s tolerance factor (t), which is an empirical index for predicting stable crystal structures of perovskite materials. A t value between 0.8 and 1.0 favors cubic perovskite structures, whereas higher (>1) or lower (<0.8) t values usually result in nonperovskite structures.[58] In the following section, we introduce detailed synthesis and each type of PQDs.

2.1. Synthesis of PQDs

2.1.1. Synthesis of Hybrid Organic–Inorganic Perovskite QDs

Room-Temperature Method: Hybrid organic–inorganic perov-skite (HOIP) QDs are normally formed by reprecipitation under room temperature. Zhang et al.[59] firstly synthesized CH3NH3PbBr3 PQDs by ligand-assisted reprecipitation (LARP) through solvent mixing. First, a mixture of PbBr2, CH3NH3Br, n-octylamine, and OA was dissolved in DMF solution, and toluene was dropped to resulting mixture under vigorous stir-ring to obtain a yellow-green colloidal solution. This colloidal solution was centrifuged at 7000 rpm and stored in toluene. However, CH3NH3 PQDs are unstable in polar solvents, such as DMF, methanol, and ethanol. Polar solvents may destroy QDs and induce PLQY quenching during purification. Huang et al.[60] used two immiscible solvents (DMF, n-hexane) as emulsion systems (Figure 4a). This method results in better crystallization, and PQDs are better purified by demulsion. In this method, CH3NH3PbBr3 features a tunable size of 2–8 nm and maximum PLQY of 90%. QLEDs applications critically

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Figure 1. a) Simulated FWHM dependence for luminescence at 3000 K and 90 CRI (Reproduced with permission.[31] Copyright 2017, The Optical Society). b) CIE chromaticity diagram showing color gamut of liquid crystal display (LCD), OLED, QD-LCD, and QLED (Reproduced with permission.[30] Copyright 2017, Wiley-VCH). c) The periodic table shows four different types of elements in the perovskite system (Reproduced with permission.[39] Copyright 2017, American Chemical Society).

Figure 2. Experimental versus theoretical (effective mass approximation) size dependence of bandgap energy with quantum confinement regimes noted in relation to Bohr diameter (db = 7 nm) (Reproduced with permis-sion.[10] Copyright 2017, American Chemical Society).

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require purification of PQDs. The application section discusses in detail purification step. Synthesis of all-inorganic CsPbBr3 PQDs can be separated into four types.

2.1.2. Synthesis of All-Inorganic Perovskite QDs

Hot-Injection Method: In synthesis of monodispersed CsPbBr3 PQDs in high-boiling-point solvents, PbX and OA, oleylamine (OAm), and octadecene are completely dissolved in crude solu-tion, which is injected with Cs-oleate solution and is purified by centrifugation and stored in hexane.[6]

Room-Temperature Method: CsBr and PbBr2, OA, and OAm are dissolved in DMF or DMSO, and toluene is dropped into resulting mixture. Given the lower solubility of ions in toluene than in DMF, which induces rapid recrystallization of PQDs, small-molecule surface ligands control size and morphology of PQDs.[59,61–63]

Microfluidic Reaction System: Lignos et al.[64] used a microflu-idic reactor in synthesis of PQDs along with in situ PLQY and absorption measurement. In a microfluidic reactor, Cs-oleate precursor, PbX2, and OA were loaded onto a syringe pump. Precursor was mixed in a cross-mixing junction and heated to desired reaction temperature. This system easily optimized reaction parameters, such as molar ratio of Cs, Pb, and halide precursor, reaction time, and reaction temperature. Optimized parameters can be scaled-up for production.

Micelle Method: Hou et al.[65] used a novel and low-cost copolymer (polystyrene-block-poly-2-vinylprydime (PS-b-P2VP)) to synthesize PQDs, as shown in Figure 4b. First, PS-b-P2VP was dissolved in toluene to self-assemble inverse micelles. Next, PbBr2 precursors were dissolved in toluene and absorbed by PS-b-P2VP micelles. CsBr was dissolved in methanol and injected into micelles, followed by rapid crys-tallization within 5 min. After purification, CsPbBr3 perov-skite was encapsulated into polymer micelles. CsPbBr3 PQDs exhibit moderate PLQY and high photostability. Replacing small-molecule surface ligands (OA, Oam) with polymers increases PQD stability.

2.2. Hybrid Organic–Inorganic PQDs

There are many types of HOIPs such as CH3NH3(MA)PbBr3, CH2CH3(EA)PbBr3, CH(NH2)2(FA)PbBr3, and PQDs with organic-inorganic mixed cations. We classify these HOIPs into analogous metal halides and analogous metal halides with organic–inorganic mixed cations.

2.2.1. Analogous Metal Halides

Schmidt et al.[7] were the first to synthesize MAPbBr3 PQDs. In MAPbBr3 PQDs, MA cation serves as a capping agent that pre-vents 3D growth of PQD extensions. MAPbBr3 PQDs feature a spherical morphology with average particle size of 6 nm, and its X-ray diffraction (XRD) pattern shows a cubic structure. MAPbBr3 exhibits an emission wavelength of 525 nm with FWHM of 21 nm and is a better candidate for LED application upon improving its PLQY. Zhang et al.[59] reported a brightly lumi-nescent and color-tunable colloidal mixed halide, MAPbX3. By tuning halide ratio, emission wavelength reached 407–734 nm, and average MAPbBr3 particle size measured 3.3 nm. According to quantum confinement effect, particle size of this PQD exhib-ited high PLQY. In this case, PLQY of MAPbBr3 totaled 70%.

Instead of MA cation, ethylammonium (EA) cation is used to synthesize EAPbBr3 PQDs under one-pot room tempera-ture method. Size difference in organic cations can be used in tuning the bandgap of PQDs. Radii of MA and FA atom measure 2.70 and 2.79 Å, respectively. MA/EAPbBr3 exhibits a blueshift in emission wavelength upon increasing molar ratio of EA+. Figure 5a shows detailed band structure of MAPbBr3 and EAPbBr3. As indicated in Figure 5a, Pb 6s, Pb 6p, and Br 4p states contribute to the frontier band. Bandgap is defined between Pb 6s-Br 4p hybridized orbital and Pb 6p orbital, and organic cations do not affect the frontier band. However, inser-tion of larger organic cations (EA) increases distance between Pb atoms and PbBr bond length, decreasing orbital overlap between Pb 6s and Br 4p orbitals. Therefore, bandgap of MAPbBr3 increases upon insertion of EA cations.[50]

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Figure 3. Growth processes of CsPbBr3 QDs with low-polarity solvents at different temperatures (Reproduced with permission.[53] Copyright 2016, Wiley-VCH).

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Levchuk et al.[40] were the first to synthesize FAPbBr3 PQDs. Mixed halides of FAPbX3 (X = Cl, Br, I) PQDs were synthesized through ligand-assisted reprecipitation. In this method, PbX2, FAX, OA, and OAm were mixed in DMF, and the precursor was quickly injected into chloroform under vigorous stirring at room temperature. FAPbX3 possesses a cubic and plate-like morphology with average particle size of 15–25 nm. By tuning the halide ratio, emission wavelength measures 415–750 nm with FWHM at 20–44 nm and PLQY of up to 85%. In compar-ison with analogous MA or Cs PQDs, given its larger size, FA features stronger ionic interaction with halide ions than MA. Thus, FAPbX3 presents higher thermal and moisture stability. Recently, Kim et al.[41] developed an emulsion method for syn-thesis of FAPbBr3 PQDs for QLED applications. In this work, precursors with OA and shorter alkylamine (n-butylamine, n-hexylamine, or n-octylamine) were prepared as emulsion in DMF/hexane. Next, the demulsifier (tert-butanol) was dropped to emulsion, and precursors were recrystallized. Figure 5b shows details of this synthesis method.

2.2.2. Analogous Metal Halides with Organic–Inorganic Mixed Cations

Xu et al.[42] were the first to synthesize mixed cation MA/CsPbBr3 PQDs. In this work, the method was similar to synthesis of FAPbBr3. Addition of tert-butanol to emulsion

produced a turbid solution. Suspension was precipitated by centrifugation and finally dissolved in hexane solution. Lat-tice finger of MA/CsPbBr3 was lower because of smaller ionic radius of Cs ions. MA/CsPbBr3 exhibits high PLQY and sta-bility as a result of partial substitution of Cs ion.

Zhang et al.[43] were the first to synthesize mixed cation FA/CsPbBr3 PQDs. Mixed cations in HOIP show significant enhancement in power conversion efficiency and stability in perovskite solar cells. In FA/CsPbBr3 PQDs, ionic radius of Cs (1.81 Å) is smaller than that of FA (2.79 Å), facilitating crys-tallization of FA-based perovskite material through entropic stabilization effect. As shown in Figure 5c, diffraction pat-terns show a peak shift (from 15.01° to 15.39°) with increasing degree because of smaller Cs cations. XRD can identify effects of Cs cations on crystallization of FA/CsPbBr3 PQDs. These FA/CsPbBr3 PQDs also present remarkable performance in QLEDs.

2.3. All-Inorganic PQDs (AIPs)

The major problems in NCs include surface defects and trap states, which confine charge carriers and decrease efficiency of optoeletronic devices. AIPs, such as CsPbBr3, exhibit sig-nificant optical properties in optoelectronic devices because of their defect-tolerance system. Toxicity of Pb2+ poses negative effects on actual applications. Hence, Pb2+ is completely or

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Figure 4. a) Schematic of QD emulsion synthesis, (i) formation of emulsion, (ii) demulsion by adding demulsifier redispersion into colloidal solution, and (iii) purification into solid-state powder (Reproduced with permission.[60] Copyright 2015, American Chemical Society). b) Synthesis of CsPbBr3 PQDs by amphiphilic block copolymer micelles as nanoreactor) (Reproduced with permission.[65] Copyright 2017, American Chemical Society).

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partially replaced. At present, the most suitable substitute ele-ments include the less toxic Sn(II), Sn(IV), Bi(III), and Mn(II) ions. In these sections, we divide AIPs into two types, namely, analogous metal halides with divalent ion doping and analo-gous metal halides with heterovalent cation doping.

2.3.1. Analogous Metal Halides with Divalent Ions Doping (CsPb1−xMxX3)

Jellicoe et al.[66] used hot-injection method to fabricate lead-free mixed halides of CsSnX3 (X = Cl, Br, I) PQDs. However, Sn(II)-based CsSnX3 is unstable and requires storage in inert atmos-phere. In this case, Sn(II) easily oxidizes to Sn(IV), resulting in low PLQY. To overcome this oxidation problem, Wang et al.[67] replaced divalent lead cations (Pb(II)) with tetravalent tin (Sn(IV)) and reported stable cubic structure of Cs2SnI6 PQDs. Sn(IV) is generally more stable against oxidation than Sn(II). However, PLQY of Sn(IV) reached only 0.48%, which is extremely low for practical application. Zhang et al.[45] proposed mixed metal cations of PQDs by synthesizing CsPb1−xSnxBr3 QDs with Sn(II) substitution under low temperatures (110 °C). However, com-plete replacement of Pb(II) with Sn(II) or Sn(IV) poses difficulty given the low conductivity of Sn ion. Low ionic conductivity in PQDs results in surface defects and instability of material.

CsPb1−xMxBr3 with divalent ion (Sn2+, Cd2+, and Zn2+) substitution through cation exchange. van der Stam et al.[44]

reported a partial method of performing cation exchange in CsPbBr3 PQDs. In this method, CsPbBr3 PQDs were synthe-sized through hot-injection. After purification, CsPbBr3 PQDs dispersed in hexane. In cation exchange procedure, cation exchange precursor was prepared by dissolving MBr2 (M = Sn2+, Zn2+, and Cd2+) and OAm in toluene. Next, PQDs and cation exchange precursor were mixed and stirred at room tempera-ture for 16 h. Figure 6 shows cation exchange mechanism in CsPbBr3 PQDs. Smaller divalent cations are incorporated into CsPbBr3, resulting in contraction of atomic lattice. According to Vegard’s law, bandgap in semiconductor is approximately a linear function of lattice parameter. Small lattice parameter

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Figure 6. Schematic of partially cation exchange in CsPbBr3 PQDs. Repro-duced with permission.[44] Copyright 2017, American Chemical Society.

Figure 5. a) Schematic of variation of energy levels of MAPbBr3 in Pb 6p and (Pb 6s–Br 4p)* orbitals on insertion of EA cation (Reproduced with permission.[50] Copyright 2016, American Chemical Society). b) Synthesis schematic of FAPbBr3 QDs by using emulsion system at room temperature (Reproduced with permission.[41] Copyright 2017, Elsevier). c) XRD patterns of FA(1−x)CsxPbBr3 (x = 0–0.6) (Reproduced with permission.[43] Copyright 2017, Wiley-VCH).

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results in a blue-shift because of strong ligands within the Pb-halide octahedral. CsPb1−xMxBr3 shows high PLQY above 50% in the blue emission range of CsPbBr3. In a previous report, blue-emitting CsPb(Br/Cl)3 yielded low PLQY. This work opens new possibilities for full-color PQD–QLEDs in the future.

Mn2+:CsPbCl3: Doping ion into host material shows pos-sibility of creating charge–size imbalance in the center of host lattice. Imbalance causes host material to lose their original crystal structure and emission. Diffusion-doping is the common doping mechanism for Mn(II) ion, and it entails adding ion dopants to host lattice. Mn doping in high bandgap host material transfers excitation energy to Mn state and results in yellow-orange d–d emission. Mn doping in inor-ganic CsPbX3 PQDs becomes more favorable when X = Cl. Liu et al.[68] synthesized impure Mn2+-doped CsPbBr3 by hot-injection. In this synthesis, PbCl2 and MnCl2 were dissolved in a mixture, and nucleation and growth initially occurred upon injection of Cs-oleate precursor. After purification, precipitates redispersed in hexane. Mn: CsPbCl3 (with 9.6% doping) fea-tures a cubic morphology with average size of 11 nm. XRD peak monotonically shifted to higher angles as a result of lattice con-traction. Mn:CsPbCl3 retained its crystal structure for several months and presented relative stability in air. Mn:CsPbCl3 (with 9.6% doping) exhibited two emission peaks at 402 and 586 nm. Figure 7a shows energy transfer mechanism of Mn:CsPbCl3. Several energy transfer processes, such as electron–hole recom-bination (keh), Mn ion d–d transition (kMn), forward energy transfer (kET), and back energy transfer (kBET), control intensi-ties of these emission peaks. In Mn:CsPbCl3 PQDs, energy dif-ference (ΔE) between band edge and Mn ion d–d transition is large and positive, indicating effective energy transfer between CsPbCl3 and Mn ion dopant. Liu et al.[69] recently reported high doping ratio of Mn:CsPbCl3 PQDs by increasing reaction tem-perature. In this work, doping ratio of Mn reached 46% and increased PLQY up to 54%. Recently, Xu et al.[70] reported a facile method for synthesis of Mn:CsPbCl3. Equal molar PbAc and CsAc, OA, and OAm with different amounts of MnAc2

were mixed in toluene at room temperature under N2 atmos-phere. Finally, a white suspension formed upon addition of aqueous HCl to the solution. After purification, Mn:CsPbCl3 PQDs were dispersed in toluene. The following step involved shell growth of undoped CsPbCl3 shell. Coating precursor solu-tion was prepared by mixing CsAc and PbAc in toluene with OA and OAm, and a coating precursor was slowly added to core solution. Thicker shell coating of CsPbCl3 in Mn:CsPbCl3 PQDs results in higher thermal stability. This strategy for shell growth provides a new method for improving stability of PQDs.

2.3.2. Analogous Metal Halides with Heterovalent Doping

Bi3+:CsPbBr3: Begum et al.[47] developed an in situ doping approach for CsPbBr3 PQDs with heterovalent Bi3+ ion by hot-injection under different concentrations of bismuth bro-mide. With increasing doping ratio of Bi3+, emission wave-length partially blue-shifted from 517 to 512 nm. Crystal lattice did not show any remarkable change from Bi3+ doping. This Bi3+:CsPbBr3 showed significant improvement in charge transfer from PQDs to other molecular acceptors. To prove this phenomenon, the authors designed a mechanism for electron injection from CsPbBr3 and Bi3+:CsPbBr3 to tetracyanoethylene (TCNE) as molecular acceptor. Figure 7b shows band alignment between PQDs and TCNE. For Bi3+:CsPbBr3, higher energy offset was noted between conduction band of PQDs and lowest energy unoccupied molecular orbital (LUMO) of molecular acceptors. Therefore, Bi3+:CsPbBr3 can accelerate electron injec-tion. This novel finding in Bi-doped CsPbBr3 provides a tool in controlling charge transfer in optoelectronic applications.

Au–CsPbBr3: Balakrishnan and Kamat[71] reported growth of Au nanoparticles on corners of CsPbBr3. CsPbBr3 was treated with Au(III) salts such as AuBr3. OAm, a strong reducing agent, reduced Au3+ to Au0. Figure 8 shows detailed synthesis of Au–CsPbBr3. Au–CsPbBr3 exhibited lower PLQY as a result of charge transfer between Au nanoparticles and CsPbBr3,

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Figure 7. a) Evolution of energy level diagram of Mn-doped CsPbCl3 NCs during forward and reverse anion exchange. Relative intensities of two PL features of Mn:CsPbCl3 PQDs are established by various competing processes, including band-edge electron−hole recombination (keh) and deactivation of Mn2+-based d–d transition (kMn) and forward (kET) and back (kBET) energy transfers between PQDs and impurities. Competition between the latter two processes is also strongly influenced by energy difference, Δ, between band edge and Mn2+-based transitions (Reproduced with permission.[68] Copyright 2017, American Chemical Society). b) Bandgap structure of undoped and Bi-doped CsPbBr3 with respect to LUMO of TCNE. (Reproduced with permission.[47] Copyright 2017, American Chemical Society).

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and average lifetime also decreased from 12.5 to 9.1 ns. Au–CsPbBr3 is applicable in photocatalysis owing to its charge transfer effect. Table 1 summarizes optical properties of dif-ferent PQD types.

3. Stability of PQDs

3.1. Degradation Mechanism of PQDs

Studies[72–76] focused on colloidal PQDs because of their good optoelectronic properties and high PLQY in nonpolar solvents.

Stability of PQDs poses a major problem for practical applications in LEDs. In HOIPs, organic cations (MA, EA, and FA) trigger degradation. Under illumination, HOIPs generate electrons and react with and convert O2 or CO2 to free radicals. Free radicals react with MA+ to form volatile CH3NH2, which then evaporates. The framework of HOIPs deteriorates under strong light. In some studies, MA–Pb perovskite released an iodine atom as a result of nucleophilic substitution by water. Decomposition of HOIPs occurs from lower activation energy of surface atoms. Upon illumination of LEDs, CsPbBr3 PQDs quickly change from green to yellow with PLQY loss. Huang et al.[77] elucidated degradation in CsPbBr3 by varying reaction factors, such as oxygen, moisture, light, and temperature. In a photostability test, CsPbBr3 was mixed with a Norland optical adhesive (NOA), and resulting CsPbBr3/NOA film was dropped onto a LED chip. Photoactivation[78] occurred at the beginning

of test. Photoactivation represented enhanced luminescence upon irradiation of QD with light. Under irradiation with light (under N2 condition), increase in temperature induces recon-struction of surface atoms and surface ligands, causing initial reduction of surface defects and enhancement of PLQY. How-ever, CsPbBr3 PQDs form large crystals under long periods of illumination with blue light. Large CsPbBr3 crystal loses PLQY as a result of quantum confinement effect[10] and sur-face trap state. Oxygen and moisture also contribute to deg-radation of CsPbBr3. Oxygen acts as etching agent and etches off unstable nuclei and enables formation of large crystals. Dissociated ions from damaged CsPbBr3 continually exhibit

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Figure 8. Schematic of surface ligands leading to reduction of Au(III) at CsPbBr3 PQDs to form Au–CsPbBr3 hybrid structures (Reproduced with permission.[71] Copyright 2017, American Chemical Society).

Table 1. Summary of optical properties of different types PQDs.

QD-type Lattice structure Synthesis PLQY peak [nm] FWHM [nm] PLQY Refs.

HOIP QDs MAPbX3 Cubic RT 407–800 20–50 50–93 [59,60,83,135]

FAPbX3 Cubic RT 415–740 20–44 21–85 [40]

MA/EAPbBr3 Cubic/tetragonal RT 432–530 19–32 5–85 [50]

FA/CsPbBr3 Cubic RT 519–531 19–23 34–73 [43]

MA/CsPbX3 Cubic RT 533–539 <25 80 [42]

AIP QDs CsPbX3 Cubic Hot injection 410–700 12–42 50–90 [6]

CsPbX3 Cubic RT 440–682 12–34 50–95 [61–63]

CsPbBr3 Cubic Microfluidic 470–690 20–45 = [64]

CsSnX3 Cubic Hot injection 470–930 = <1 [66]

Cs2SnI6 Cubic Hot Injection 620 49 <1 [67]

CsSn/PbX3 Cubic Hot Injection 496–520 = 37–71 [45]

CsPb/MnX3 Orthorhombic/tetragonal Hot injection Dual-color emission 402/586 = 0–54 [46,68–70,136]

CsPb/MX3

M = Cd, Sn, Zn

Cubic Cation exchange 452–512 = >50 [44]

CsPb/BiX3 Cubic Hot injection 512–517 18–21 8–55 [47]

Au–CsPbX3 Cubic RT 472 60 [71]

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crystal growth, which results from Ostwald ripening. Hydrated CsPbBr3 is hardly removed from pristine CsPbBr3. Moisture[66] then merges surface grains and enhances ion diffusion length, assisting in crystal growth. Hence, water-resistant protection layer is the most important feature of CsPbBr3 PQDs, which are extremely unstable in high-moisture environments. Sur-face ligands play an important role in stability of perovskite. One important consideration is preventing detachment of surface ligands under environmental effects. Under blue light illumination, surface ligands are easily removed by absorp-tion of photons. Hydration may also remove surface ligands, whereas CsPbBr3 PQDs easily merge with large crystals. With the exception of crystal growth, surface detachment causes sur-face decomposition of CsPbBr3 PQDs, resulting in surface trap state. Figure 9 summarizes possible degradation pathways of CsPbBr3. To address stability problem of PQDs, we focus on surface-passivated procedure and surface shell-coating method in the following sections.

3.2. Surface-Passivated Procedure

High density of surface trap states results in nonradioactive recombination, which is necessary to passivate PDQ surface using appropriate capping ligands. Appropriate surface cap-ping ligands not only reduce surface defects but also suppress surface decomposition induced by oxygen and moisture. Pas-sivation of surface ligands and surface defects involves many procedures. In the following section, we introduce passivation as an effective method of improving stability of PQDs.

Polyhedral Oligomeric Silsesquioxane (POSS) Surface Treat-ment: Huang et al.[79] presented an approach that solves poor resistance and anion exchange of CsPbBr3 PQDs. In his work, CsPbBr3 was capped with thiol-functional POSS protective matrix. POSS is a cage-like structure of inorganic siloxane core with eight surrounding organic corner groups. POSS features high optical transparency and chemical stability under UV and visible light. Thiol stabilizers are often used in general

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Figure 9. Schematic of possible degradation pathways of CsPbBr3 PQDs under different illumination of power densities (Reproduced with permis-sion.[77] Copyright 2017, American Chemical Society).

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QDs. CsPbBr3 PQDs with POSS treatment show good water resistance.

NH2–POSS Surface Treatment: Recently, Luo et al.[80] replaced common OA ligands with branch-capping ligands, (3-ami-nopropyl)triethoxysilane) APTES, and PSS-[3-(2-aminoethyl)-amino]propylheptaisobutyl-substituted POSS (NH2–POSS). Figure 10a shows structures of APTES and NH2–POSS. APTES-capped CsPbBr3 presents higher stability because of strong steric hindrance and hydrolytic properties of APTES, which prevents protic solvents from reacting with the core of MAPbBr3. NH2–POSS capped MAPbBr3 shows lower PLQY as straight ligands can passivate more defects per unit area. How-ever, branched capping ligands provide better protection from solvent molecules that degrade PQDs.

Didodecyl Dimethylammonium Sulfide (S2− DDA+) Surface Treatment: Pan et al.[81] demonstrated air-stable and photostable CsPbBr3 PQDs by using an inorganic–organic hybrid ion pair as capping ligands. This ion-pair ligand, called S2− DDA+, was used to passivate surface defects. After passivation, PLQY of S2− DDA+-capped CsPbBr3 improved from 49% to 70% due to loss in surface defects. S2− DDA+-capped CsPbBr3 film also showed

significant photostability for at least 34 h under continuous pulse laser. Figure 10b shows details of treatment process.

Thiocyanate Surface Treatment: Recently, Koscher et al.[82] reported surface repair of CsPbBr3 PQDs by thiocyanate treatment. Thiocyanate salts, such as ammonium thiocyanate (NH4SCN) or sodium thiocyanate (NaSCN), were dispersed in anhydrous hexane or toluene. For thiocyanate treatment, thio-cyanate salt was homogeneously mixed with CsPbBr3 PQDs at room temperature. The remaining thiocyanate was removed by a PTFE syringe filter. After thiocyanate treatment, PLQY of CsPbBr3 rose to 90% with a small spectral shift. Thiocyanate salt can repair excess damage of lead surface, which narrows down electron level of CsPbBr3 and introduces structural changes. After thiocyanate treatment, lead-rich surface was repaired and decreased the nonradiative pathway (Figure 10c). This treatment effectively improves PLQY and stability.

Noncoordinated Solvents: Stability of red CH3NH3PbI3 PQDs poses a significant challenge because iodine ions are sen-sitive to moisture from air. Coordinating solvents, such as DMF, DMSO, and tetrahydrofuran (THF), cause instability of CH3NH3PbI3 because of intrinsic inner iodine vacancies that

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Figure 10. a) Molecular structure of branched capping ligand APTES and NH2–POSS (Reproduced with permission.[80] Copyright 2016, Wiley-VCH). b) Schematic of inorganic–organic ion-pair surface treatment on CsPbBr3 PQDs. c) Schematic of thiocyanate surface treatment on CsPbBr3 PQDs (Reproduced with permission.[82] Copyright 2017, American Chemical Society). d) Schematic of transformation from precursor to MAPbI3 PQDs in coordinated solvents and noncoordinated solvents (Reproduced with permission.[83] Copyright 2017, American Chemical Society).

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cause coordination effect. Zhang et al.[83] discovered coordi-nated effect in LARP synthesis and selected five polar solvents, DMF, DMSO, THF, γ-butyrolactone, and acetonitrile to explore this effect. Precursors were dissolved in coordinated solvent to form PbI2–solvent intermediate, which underwent precipita-tion. Given the strong bonding between PbI2 and coordinate solvent, CH3NH3PbI3 contained residual solvent molecules on its surface. Intrinsic iodine vacancies resulted from evaporation of coordinated solvent. Surface defects easily reacted with water and transformed into CH3NH3PbI3–H2O. By contrast, PbI2 precursor dissolved in noncoordinated solvent crystallized into defect-free CH3NH3PbI3 with improved stability. Figure 10d shows in detail the related mechanism.

3.3. Surface Coating

PQDs present many challenges, such as thermal effect, particle aggregation, surface ligand detachment, and anion exchange during LED fabrication. To solve these problems, outer shell coating serves as the most effective method. We introduce six coating procedures in the following sections.

Polymer Coating: Zhang et al.[59] encapsulated green MAPbBr3 PQDs into poly(methylmethacrylate) (PMMA) polymer and casted capsules on silicone resin mixed with KSF red phosphor. These authors provided opportunities for wide-color gamut display. However, PMMA features high oxygen diffusion coef-ficient (3.3 × 10−9 cm2 S−1) at 22 °C. Pathak et al.[84] success-fully fabricated blue, green, and red PQD films for tunable lighting applications. MAPbX3 PQDs were blended with poly-mers (PS:PMMA) to solve problems of anion exchange and sta-bility under long-term light illumination. Raja et al.[85] mixed CsPbBr3 PQDs and polymers in toluene and compared stability and water/oxygen diffusion between three polymers, (PS, poly(styrene-ethylene-butylene-styrene) (SEBS), and poly(lauryl methacrylate) (PLMA)). Trend of water resistance followed the order PS > SEBS > PLMA; oxygen diffusion rate resulted in the order PLMA > SEBS > PS. SEBS manifests good water resis-tance and low oxygen diffusion rate because of its microphase-separated structure. Therefore, oxygen and water were blocked by both PS and ethylene-butylene blocks and maintained long-term stability. Zhou et al.[86] use polyvinylidene fluoride (PVDF) as polymer matrix to synthesize MAPbBr3 PQDs. In this syn-thesis, PVDF is initially crystallized into colorless films. Next, nucleation and growth reactions occur on perovskite NC. Finally, the mixture contains 75.5 wt% PVDF, 8.5 wt% MAPbBr3, and 16 wt% DMF. Resulting from the interaction between CF2 and MA+, MAPbBr3–PVDF thin film showed uniform size and spatial distribution. This MAPbBr3-PVDF thin film yielded high PLQY of 94.6% ± 1%, was highly water resistant, and exhibited photostability. Recently, Gomez et al.[87] successfully encapsu-lated CsPbBr3 PQDs into solid–lipid structure. These nanocom-posites exhibited water resistance for more than 2 months.

Mesoporous Silica: To improve stability of PQDs, silica coating is considered traditionally effective. Silica coating is widely used in NCs, such as Au nanoparticles, Cd-based QDs, Cd-free QDs, and iron oxides. Silica shell is a transparent material that does not affect optical properties of luminescent material but dramatically improves stability. Popular silica-coating procedures

include Stöber et al.[88] and reverse microemulsion[89] methods. In both methods, the first step involves surface ligand exchange and reaction with tetraethyl orthosilicate (TEOS) under alkaline conditions. However, PQDs easily decompose due to solvation of PbBr2 in polar solvents. Wang et al.[90] first used a facial method to encapsulate CsPbBr3 PQDs into mesoporous silica matrix. MP–CsPbBr3 nanocomposites showed enhanced thermal sta-bility compared with pristine CsPbBr3 PQDs. Dirin et al.[38] demonstrated template-assisted formation of MAPbBr3 PQDs. MAPbBr3 PQDs grew on mesoporous silica matrix as template. In previous research, PQDs became unstable when particle size reached 15–20 nm. Difficulty arises from synthesizing CsPbBr3 smaller than 7–8 nm. However, template-assisted method pro-vides a strong quantum confinement system. High-concentra-tion precursor can be inserted into pores. CsPbI3 was also suc-cessfully impregnated with meso-SiO2 when using the smallest pore (2.5 nm) as template. Using mesoporous silica template provides scalable preparation and highly stable perovskite nanocomposites.

Silica Shell Coating: To overcome decomposition of PQDs by water during silica coating, Huang et al.[91] replaced TEOS precursor with tetramethyl orthosilicate, which features higher hydrolysis rate during silica coating. MAPbBr3/SiO2 nanocom-posites were successfully synthesized under waterless toluene solution. For photostability test, MAPbBr3 and MAPbBr3/SiO2 exhibited enhanced emission during the first 1–2 h of illumi-nation due to photoactivation effect. After a 49 h illumination, PLQY intensity of MAPbBr3/SiO2 nanocomposite measured 61.03%, whereas that of MAPbBr3 PQDs reached 7%. In addi-tion to improvement of photostability, PQDs with thicker silica shell can also prevent oxygen and moisture penetration. Sun et al.[92] used APTES as capping agent and precursor of silica matrix. In silica coating, as-prepared precursor was mixed with APTES under waterless condition. Next, the amino group of APTES passivated surface defects, including Oam ligands, and three silyl ether (SiOC2H5) groups of APTES were hydrolyzed to form a silica matrix. Silyl ether groups reacted with cata-lyst (H2O) from air and then transformed into SiOH group. SiOH reacted with another SiOH group to form cross-linked SiOSi coating on PQDs. This CsPbBr3@SiO2 exhib-ited no decay of PLQY after 3 months, whereas red CsPb(Br/I)3 presented a 5% decrease after 3 months. Silica coating is an outstanding method for protecting unstable PQDs. Both silica-coating procedures do not use harsh reagents, such as ammonia and water, and are universal procedures compared with Stöber and reverse microemulsion methods.

QD/Salt Nanocomposite: In addition to silica coating on QDs, encapsulating QDs into ionic matrix may also improve stability. Adam et al.[93] proposed liquid–liquid diffusion-assisted crystal-lization to synthesize QDs/NaCl nanocomposites. This method allowed incorporation of oil-based QDs into ionic matrix without further ligand exchange. Liquid–liquid diffusion-assisted crystallization method can be separated into two steps; the first step involves formation of QD/NaCl seed solution and absorption of QDs on surface of NaCl. Next, seed solution is injected into NaCl solution and maintained for 20 h. This QD/NaCl nanocomposite shows high stability even in the pres-ence of strong oxidizing agents. Yang et al.[94] selected NaNO3 as ionic matrix, which is transparent and nontoxic and used

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one-step precipitation to synthesize MAPbBr3/NaNO3 nano-composites. This MAPbBr3/NaNO3 nanocomposite showed higher photostability and thermal stability compared with pris-tine MAPbBr3. For reliability test, LEDs exhibited 50% degrada-tion after 4 h at 20 mA. Although practical application of this work remains distant, it provides opportunities for new ion matrices in improving stability of PQDs.

CsPbBr3/AlOx: Loiudice et al.[95] used atomic layer deposition (ALD) to synthesize CsPbBr3/AlOx nanocomposites. To prevent degradation of PQDs by moisture and heat, these scientists devel-oped low-temperature and short water-pulse ALD program. PQDs underwent spin-coating on glass. Then, trimethylaluminum (TMA) and H2O were used as precursors and were deposited on surface of PQDs. Given the highly compact protective layer, these CsPbBr3/AlOx nanocomposites exhibited high stability against immersion in water and thermal treatment. In the future, ALD treatment, which retains optical properties of PQDs and improves water resistance, will become a relatively significant technique for practical applications in solar cells and LEDs.

Other Methods: Stability of PQDs can be further improved by incorporation of a previous stability-enhanced method. Zhang et al.[96] reported three-step treatment on CsPbBr3. First, surface passivation was conducted on CsPbBr3 PQDs by S2− DDA+ ion-pair ligands, which can enhance PLQY of PQDs. Next, CsPbBr3–SDDA solution was mixed with mesoporous silica powder, which can enhance thermal stability. Finally, the obtained powder mixed with PMMA polymer functioned as barrier layer to prevent diffusion of oxygen and water. These multifunc-tional nanocomposites exhibited high stability toward moisture, oxygen, and heat. Recently, Li et al.[97] prepared CsPbBr3-silica/alumina (SAM) monolith by sol–gel method. Combination of SiO2 and Al2O3 coating layers can decrease pinhole defects and permeabilities of water and oxygen. First, CsPbBr3 was reacted with didodecyl dimethyl ammonium bromide to improve its brightness. Next, di-sec-butoxyaluminoxytriethoxysilane was used as single molecular precursor for SiO2/Al2O3 matrix to protect CsPbBr3 PQDs. CsPbBr3–SiO2/Al2O3 nanocomposites were blended in polydimethylsiloxane (PDMS) and poured onto a blue chip for stability test. CsPbBr3–SiO2/Al2O3 nanocom-posite maintained PLQY intensity after 96 h under blue-chip illumination (455 nm, 5 mA, 2.7 V). Figure 11 displays detailed classification of stability improvement methods.

4. Optical Properties and Application in LEDs

PQDs are outstanding candidate materials for lighting and backlight display due to their unique optical properties. Com-paring with traditional QDs (CdSe, InP), researchers discov-ered that PQDs exhibit much superior brightness and narrower emission wavelength. In this section, we discuss outstanding optoelectronic properties of PQDs.

4.1. Optical Properties

4.1.1. Defect Tolerance System

One of the major problems of colloidal semiconductor NCs is considerable density of surface defects and trap state, which

both decrease optoelectronic performance. Surface defects are difficult to remove. Thus, defect-free concept serves as an important idea in resolving this issue. Numerous studies focused on creating defect-free or defect-tolerant systems. Con-ventional CdSe/ZnS QDs use wide-bandgap protective shells (ZnS) to prevent formation of structure defects that decrease PLQY. PQDs show better optical properties due to their defect-tolerance system, wherein intrinsic defects do not act as trap states. Thus, in this case, electronic surface passivation shells are unnecessary. Figure 12a shows schematic representa-tion of defect-intolerant and defect-tolerant systems.[38,39] In defect-tolerant systems, defect states located at valence band or conduction band are nearly delocalized in nature. This phe-nomenon indicates that defect states do not form an efficient localized trap state. In perovskite systems, VBM is antibonding in nature, whereas CBM is stabilized by spin–orbit coupling. MAPbI3 as an example: VBM was formed by hybridization between Pb (6s) and I (5p) orbitals. Halide vacancies and others formed intraconduction band states. Therefore, defect states failed to affect the bandgap between Pb (6p) orbital and Pb (6s)–I (5p) hybridization orbital.

4.1.2. Quantum Confinement (Optical)

Perovskite materials in nanosize can effectively minimize exciton dissociation and enhance radiative recombination. All these research indicated that perovskite material in nanosize is an effective way to improve the device performance of optoelec-tronic device.

4.1.3. Stokes Shift (Optical)

Stokes shift refers to the difference between positions of band maxima of absorption and emission spectra. Stokes shift values feature a strong relationship with reabsorption effect. Reabsorp-tion effect is self-quenching, which decreases PLQY of QDs. In conventional QDs, average Stokes shift of Cd QDs measures 40 meV. Recently, Brennan et al.[98] reported size-dependent Stokes shift in CsPbBr3 QDs. In this research, Stokes shift ranged from 100 to 30 meV for CsPbBr3 QDs with an edge length between 4 mm and 12 nm. Figure 12b illustrates Stokes shift versus edge length. Larger PQDs exhibiting lower PLQY were demonstrated.

4.1.4. Auger Recombination

In QDs, nonradiative Auger recombination of biexcitons and trions (charged excitons) usually reduce PLQY quantum efficiency. Auger lifetimes are much shorter than single exciton lifetime. In Auger recombination, exciton energy is nonradiatively transferred to an adjacent charge carrier instead of being converted to a single photon. Limited survival time of multiexcitons in QDs poses difficulty in achieving high power efficiencies in solar cells, photodetectors, and QLED devices. Using combinations of different optical tech-niques, we can easily understand PLQY properties and decay

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dynamics of excitons. Hu et al.[99] studied exciton decay dynamics, such as Auger recombination and PLQY blinking, using transient absorption (TA) and PLQY measurement. Makarov et al.[100] used time-resolved PLQY spectroscopies (TRPL) and TA spectroscopies to discuss PQD characteristics, such as absorption cross-sections, radiative lifetimes, and nonradiative Auger decay. Recently, Yarita et al.[101] clarified efficient formation of trions in PQDs and the role of trions in luminescence of CsPbBr3 PQDs using TA, TRPL, and single-dot PLQY spectroscopies. Thus, application of QDs in LEDs requires thorough understanding of single exciton, trions, biexcitons, and multiple-exciton behavior. Schematic of charged excitons and biexcitons in CsPbBr3 PQDs, TA

second-order correlation function g(2) of CsPbBr3 PQDs were shown in Figure 12c,d.

4.2. White-Light QD-LEDs (Lighting)

White-light LEDs continually serve as alternative to traditional incandescent light because of their excellent characteristics, such as high brightness, efficiency in energy consumption, easy production, and long lifetime. For indoor lighting appli-cations, light sources are required to possess similar Com-mission Internationale de l’Eclairage (CIE) coordinates to the black body radiator, CCT in the range of 2500–6500 K, and

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Figure 11. Common PQD nanocomposites. a) Image of R/G/B perovskite crystal/polymer composite films emitting light under a UV lamp (Repro-duced with permission.[84] Copyright 2015, American Chemical Society). b) Schematic of MP-CsPbBr3 nanocomposites (Reproduced with permis-sion.[90] Copyright 2016, Wiley-VCH). c) Synthesis schematic of silica coating on MAPbBr3 PQDs (Reproduced with permission.[91] Copyright 2016, American Chemical Society). d) Schematic of one-step reprecipitation for synthesis of MAPbBr3/NaNO3 nanocomposite (Reproduced with permis-sion.[94] Copyright 2016, The Royal Society of Chemistry). e) Transmission electron microscopy image of CsPbBr3/AlOx nanocomposite and photograph of CsPbBr3/AlOx film on glass soaked in water (Reproduced with permission.[95] Copyright 2017, Wiley-VCH). f) Optical image of QDs-SAM powder (Reproduced with permission.[97] Copyright 2017, Wiley-VCH).

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color rendering index (CRI) value over 80.[102,103] Figure 13 presents development approaches for the first through fifth generations of general white-light LED lighting devices.[104] In the first gene ration, the most common type is based on InGaN blue chip combined with yellow-emitting phosphors (i.e., Ce3+:YAG). However, YAG-based white light LEDs only achieve cool white light with a CRI of 70–80.[105,106] To increase CRI values, red nitride phosphor-based LEDs were developed. However, these red materials exhibit lower LE due to a broader FWHM and additional scattering loss to color converter. In the third generation, white-light LEDs succeeded as backlight dis-play. Red QDs and narrow-band phosphor (KSF) were used in this high-quality display. Devices showed wide color gamut in comparison with a traditional phosphor device. Red QDs com-bined with YAG phosphor formed warm white-light LEDs with higher CRI.[107] For example, Wang et al.[108] incorporated red Cu:CdS–ZnS QDs with YAG phosphor; the device exhibited high CRI (Ra = 90) and CCT (4927 K) values. Chuang et al.[109] used Cd-free CuInS2/ZnS QDs to replace Cd-based QDs to cooperate with green Eu2+:BaSO4 green phosphor. This white-light LED device also exhibited high CRI (Ra = 90) and CCT (6552 K). Zhou et al.[110] first used red CsPb(Br/I)3 PQDs in a modified YAG-based white-light LED. Red CsPb(Br/I)3 PQDs were mixed with PMMA in chloroform and spin-coated on yellow-emitting phosphor in a glass disk. These remote-type PQD white-light LEDs exhibited CIE chromaticity coordinates

of (x = 0.3248, y = 0.3162), CCT of 5907 K, CRI of 90, and luminescence of 58 lm w−1 at 20 mA. In comparison with YAG-based white-light LEDs, CRI value increased from 74 to 90 and showed higher R9 value of 97. Figure 14a–c displays electroluminescence (EL) spectra, photographs, and chro-maticity coordinates, respectively, in CIE diagrams with and without red CsPb(Br/I)3 PQD device. Li et al.[53] fabricated CsPbBr3 PQDs with red (Ba, Ca, Sr)3SiO5:Eu (BCS) phosphors in homogeneous epoxy silicon and then cured the material in vacuum oven at 40 °C for 60 min and at 150 °C for 1 h. The fabricated white-light LEDs yielded a CRI value of 93.2 and CCT of 5447 K at 20 mA current. This device showed excel-lent color rendering properties even after storage in ambient environment for 1 month. Yang et al.[94] used reprecipitation to synthesize MAPbBr3/NaNO3 nanocomposites, which showed high stability in white-light LEDs. The emitted light exhib-ited a luminescence of 22.39 lm w−1, a CRI value of 86.6, and CIE chromaticity coordinates of (0.28, 0.32). EL spectra of this white-light device presented ≈50% degradation after 4 h at a current of 20 mA; these results are much improved in com-parison with those for noncoating devices. The Illuminating Engineering Society of North America developed a new meas-urement system, TM-30-2015, with a color fidelity index (CFI, Rf) and color gamut index (CGI, Rg) for fourth-generation white-light LEDs.[104] Obtaining an Rf score over 90 poses dif-ficulty for three-color (blue, green, and red) warm white-light

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Figure 12. a) Schematic of two cases of band-structure: defect-intolerant system (left) and the ideal hypothetical defect-tolerant system (right) (Repro-duced with permission.[38] Copyright 2016, American Chemical Society). b) Schematic of size-dependent Stokes shift spectra (Reproduced with per-mission.[98] Copyright 2017, American Chemical Society). c) Schematic of charged excitons and biexcitons in CsPbBr3 PQDs. d) Transient absorption spectra of CsPbBr3 PQDs and second-order correlation function g(2) of CsPbBr3 PQDs (Reproduced with permission.[101] Copyright 2017, American Chemical Society).

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LED lighting source with red narrow- or broadband phosphor due to overestimation of Ra score over 90. This phenomenon indicates that warm white-light LEDs with Ra score exceeding 90 cannot be correlated with an Rf score over 90. High-quality white colors must be produced in the range of warm to cool white-light LEDs. Recently, color-reproducible and healthy white-light LEDs were developed for next-generation lighting. This smart white-light LEDs show tunable capability for circa-dian effect, which is used for controlling melatonin secretion. For bio- and medical-related applications, spectrum of QD-containing white-light LEDs should match daily variations in sunlight under natural circadian rhythm. Circadian-controlled white-light LEDs may serve as human-centered lighting in the near future. Therefore, PQDs with tunable wavelength, high PLQY, and narrow emission wavelength are good can-didates for new types of multicolor white-light LEDs. Yoon et al.[111] developed PQD-based six-color white-light LEDs. In this research, six colors of CsPbX3 PQDs included cyan, green, yellowish green, amber, orange, and red. This multipackage white-light LEDs show high circadian tunable range and high levels of color quality. The optimized device exhibits a LE of 58.8 lm w−1, CRI value of 95, CFI with Rf = 91.4, and CGI with Rg = 102 at CCT of 6459 K. Circadian luminescence is based on the 24 h rhythmic change of natural-light enviroments. Color icons of six-color white-light LEDs, as shown in Figure 14d, can be used to systematically explain variation in trends for both hue and saturation with regard to how 16 colors change with increasing number of multipackage white-light LEDs (WLEDs) incorporated with narrow-band colored PQDs. Bluish-green and violet-red colors and hues of tri-package WLED are con-siderably oversaturated and shifted. Otherwise, the six-package WLEDs indicate that hue shift and saturation change of the 16 colors yield minimal changes with CCT in the white CCT range between 10 000 and 2700 K. These figures also indicate that color and hue distortion decrease with increasing number of monochromatic packages in multipackage WLEDs. In the future, new types of multicolor white-light LEDs with tunable

circadian and high CFI and CGI can be used in bio- and med-ical-related research.

4.3. White-Light QD-LEDs (Backlight)

Currently, the development of white-light LEDs was trans-formed into the display industry. The schematic of liquid-crystal display (LCD) module[112] was shown in Figure 15a, the white-light pass through the first polarization filter, TFT matrix with liquid crystals, the color filter, and second polarization filter. We can tune the orientation of light by the electricity. For the best performance displays, the most important factor is color gamut, which is determined by the position in the CIE diagram of R/G/B three colors. The value of color gamut was regu-lated by National Television Standard Committee (NTSC, CIE 1931). As a result, the color gamut can be increased by using the narrow band green emitter. The CIE diagram with different color gamut was shown in Figure 15b. The black dotted lines mark the NTSC color gamut. The gray triangle color gamut was formed by the commercial broad band green-yellow YAG: Ce3+ phosphor white-light LEDs. The blue triangle based on the narrow-band phosphor emitter, green β-SiAlON: Eu2+, and red KSF phosphor. The wider color gamut (red triangle) is Cd quantum dots based white-light LEDs. Recently, the new types of perovskite were demonstrated. Perovskite quantum dots show the outstanding optical properties such as high PLQY, extra narrow emission wavelength, and tunable wavelength. The NTSC space of MAPbX3 PQDs can up to the 130% (no passed through the color filter) and CsPbBr3 PQDs can up to 150% (no passed through the color filter.[8] They both showed the better color purity and color gamut than Cd-based quantum dots. Wang et al.[90] first used all-inorganic perovskite quantum dots in white-light LEDs for backlight application. The color coordinated of optimized white-light LEDs is (0.24, 0.28) and LE of 30 lm w−1. The RGB color coordinates were (0.69, 0.30), (0.19, 0.73), and (0.14, 0.04) after the PQDs were passed through

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Figure 13. Schematic of development approaches for the first through fifth generations for white-light LEDs (luminous efficiency (LE), luminescence efficiency of radiation (LER), color rendering index (CRI), color gamut index (CGI), circadian LE (CLE), and circadian luminous efficacy of radiation (CER)) (Reproduced with permission.[104] Copyright 2017, Nature Publishing Group).

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the color filter. The color gamut overlap of the NTSC space was ≈113%, which is higher than previously phosphor and Cd QDs based white-light LEDs. The other color gamut regulation is commonly known for Rec. 2020 or BT. 2020. The Rec. 2020 can be used to defined the color gamut for two higher resolutions of 3840 × 2160 (4K) and 7680 × 4320 (8K). The overlap area of the Rec 2020 of this PQD-white LED was ≈85%. Zhou et al.[86] reported in situ MAPbBr3 PQDs polymer composite films for backlight application. The configuration of the device was shown in Figure 15d, the KSF phosphor was encapsulated into UV cure adhesive film and MAPbBr3/PVDF composite film was put on the top of the red emissive layer. The color-coordinated of obtained white-light LEDs is (0.272, 0.278), LE of 109 lm w−1 and the color gamut up to 121% (no passed through the color filter) at 20 mA current. Zhang et al.[96] compared the green CsPbBr3 PQDs with narrow band green β-SiAlON: Eu2+ phosphor. The color coordinates of phosphor and PQDs based white-light LEDs were optimized at (0.264, 0.232) and (0.271, 0.232), respectively, in CIE 1931. The color coordinate of both devices was nearly the same, when the white-light pass through the color filter, the RGB color coordinate can generate a triangle area. The NTSC space of green phosphor based white-light LEDs is 89% and the NTSC space of green PQDs nanocom-posite based white-light LEDs is 102%. Among the previous

research, perovskite quantum dots with extra narrow emission wavelength was a good candidate for backlight display. Table 2 summarizes different white-light devices.

4.4. PQDs–QLEDs (Backlight)

In the past few years, the electroluminescence based on quantum dots attract many attentions due to its wide color gamut, cost effective, energy saving, and flexible.[113] For back-light display such as TV, cell phone and watch, using active matrix (AM) mode is the best choice. Active matrix Cd-based QLEDs display consists of array pixels with RGB QLEDs, thin film transistor (TFT) backplane. AM-QLED display is self-emis-sive device using the current-driving mode. Compared to the tradition white-light LEDs, AM-QLEDs display shows many advantages such as simplified structure, lower power consump-tion, shorter response time, high contrast, and wider view angle.[30] AM-QLED display eliminates the liquid crystals and color filters. Each RGB pixels are individual controlled, which can significantly decrease the power consumption. The color gamut of high-efficiency QLEDs reported in the literature[114–117] can exceeds 140% of the NTSC standard. Comparing with Cd-based QLEDs, researchers noted better color quality and wider

Small 2018, 14, 1702433

Figure 14. a) EL spectra. b) Photographic image. c) Chromaticity coordinates in CIE diagram (Reproduced with permission.[110] Copyright 2016, The Royal Society of Chemistry). d) Color icon of six-color white-light LEDs at CCT 2000–10 000 K (Reproduced with permission.[104] Copyright 2017, Nature Publishing Group).

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© 2017 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimSmall 2018, 14, 1702433

color gamut of QLEDs. However, development of PQDs started 2 years ago. Many problems were encountered in practical application of PQDs–QLEDs; such problems include mor-phology and PLQY of PQDs film, carrier injection efficiency from transport layer, band alignment of energy level, injection balance, and radiative recombination in emitting layer. In the following section, we introduce recent evolution of all types PQD-based QLEDs.

MAPbBr3 QLEDs: Huang et al.[60] used size-tunable MAPbBr3 PQDs emitting layer, which achieved maximum

brightness of 2503 cd m−2, CE of 4.5 cd A−1, and EQE of 1.1%. However, compared to the achievements in OLEDs or Cd-based QLEDs, there is still great potential for PQD–QLEDs improvement. Xing et al.[118] report the high-effi-ciency amorphous MAPbBr3 PQDs based QLEDs. In PQD-QLEDs, the common anode is indium tin oxide (ITO), hole transport layer is Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS), electron transport layer is 2,2,2-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi). The device is fabricated with the configuration of

Figure 15. a) Schematic of LCD display and thin-film transistor b) CIE (1931) diagram with different color gamuts. Gray triangle was made by broader band green–yellow-emitting materials. Blue and red triangles based on narrow green-emitting materials. Inset in black curve displays pass-band of green color filter. Gray, blue, and red curves show emission profiles of corresponding green-emitting components displayed in CIE diagram (Repro-duced with permission.[112] Copyright 2017, Nature Publishing Group). c) Color gamut of three different types of white-light LEDs (Reproduced with permission.[90] Copyright 2016, Wiley-VCH). d) Schematic diagram of configuration of LED device and images of WLED at 20 mA (Reproduced with permission.[86] Copyright 2016, Wiley-VCH).

Table 2. Summary of various types of white-light QD-LEDs.

White-light LED (blue chip) Green (yellow)/red

Type (x, y) Luminous efficacy [lm W−1]

CCT [K]

CRI R9 NTSC [%]

Refs.

YAG/CsPb(Br/I)3 Lighting (0.3248, 0.3162) 58 5907 90 98 = [110]

CsPbBr3–POSS/CsPb(Br/I)3–POSS Lighting (0.349, 0.383) 14 = 81 = = [79]

YAG/CsPb(Br/I)3 Lighting (0.4030, 0.3654) 19 3328 84 96 = [137]

CsPbBr3–NaNO3/KSF Lighting (0.28, 0.32) 22 = 86 = = [94]

CsPbBr3/BCS red phosphor Lighting (0.3339, 0.3617) = 5447 93 = = [53]

CsPbX3 Lighting (0.31467, 0.34251) = 6317 = = = [138]

Six color CsPbX3 Lighting (0.313, 0.334) 58.8 6459 91 = = [104]

MPCsPbBr3/CsPb(Br/I)3 Backlight (0.24, 0.28) 30 = = = 113 [90]

MP-CsPbBr3-SDDA@PMMA/KSF Backlight (0.271, 0.232) 37 = = = 102 [96]

Green and red QD@SiO2 Backlight (0.33, 0.33) 61 = = = 120 [92]

MAPbBr3/KSF Backlight (0.33, 0.27) 48 = = = 130 [59]

MAPbBr3–PVDF/KSF Backlight (0.272, 0.278) 109 = = = 121 [86]

Six color CsPbX3 Backlight (0.312, 0.333) 62 = = = 145 [111]

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ITO/PEDOT:PSS/QDs/TPBi/Cs2CO3/Al. This perovskite amorphous nanoparticle-based light-emitting diode shows a maximum luminous efficiency of 11.49 cd A−1, a power effi-ciency of 7.84 lm W−1, and an external quantum efficiency of 3.8%.

FAPbBr3 and Mixed Cation QLEDs: CH(NH2)2 (FA) cation ion based perovskite is rarely studied, especially FA-based QLEDs. Recently, Kim et al.[41] used FA lead bromide PQDs (FAPbBr3) as emitter due to their high stability in both film and solution states. Two immiscible solvents were used to synthe-size FAPbBr3 with different lengths of amine ligand (n-butyl-amine (C4N9NH2) or n-hexylamine (C6N13NH2) or n-octylamine (C8N17NH2)). To investigate effects of ligand length of carboxylic acids, short carboxylic acids were used to synthesize FAPbBr3. FAPbBr3 with short carboxylic acids showed a dramatically reduced PLQY because short-length ligand failed to prevent re-aggregation of PQDs and formed bulk materials. FAPbBr3 PQDs with short amine ligands (n-propylamine) also exhib-ited low PLQY. To characterize charge injection and transport capabilities, hole-only and electron-only devices were used to measure hole current density and electron density, respectively. For both devices, current density remarkably increased when amine ligand length decreased. Therefore, the use of short-length amine ligand can increase efficiency of FAPbBr3 QLEDs. FAPbBr3-n-butylamine QLEDs yield CE of 9.16 cd A−1, PE of 6.4 lm w−1, and EQE of 2.5%, which are thus far, the highest values for FAPbBr3-based QLEDs. Although PQDs show high PLQY in solution, realization of high PLQY film remains dif-ficult. Ligand engineering is another important factor affecting performance of QLED devices. Surface ligands show double side effects on QLEDs. First, high number of surface ligands can remove surface defects that decrease PLQY. However, surface-passivated ligands, such as OA and OLAM, may form

an insulating layer to block charge injection inside QLEDs. Therefore, controlling by wash method and appropriate surface ligands can facilitate charge transport of electrons and holes.

CsPbBr3 QLEDs: CsPbBr3 perovskite PQD–QLEDs had been first time reported by Zeng and co-workers[119] exhibiting EQE of 0.12% and luminance of 946 cd m−2. Li et al.[120] demon-strated high-performance QLEDs by balancing surface pas-sivation and carrier injection efficiency using ligand density control. Figure 16b illustrates ligand density control. By using hexane/ethyl acetate mixed solvent, surface ligands presented remarkable loss. Excessive ligand may cause poor charge injection efficiency of PQDs film, and insufficient ligand films exhibit low PLQY and low stability. Researchers discovered that the most suitable wash ratio comprises 3:1 ethyl acetate:hexane. Washing is repeated, and final precipitate redisperses in octane solution as the emitting layer. Resulting device showed the highest EQE of 6.27%, which is 50-fold enhanced compared with the first CsPbBr3 QLEDs. Recently, Liu et al.[121] discov-ered that CsPbBr3 PQDs showed less electron trap state with the increasing Br content. The schematic of Br-rich and Br-poor condition for synthesis was shown in Figure 16c, the CsPbBr3 PQDs were surrounded by oleylammonium bromide as a pas-sivation layer. The Br-rich self-passivation layer of CsPbBr3 can improve the durability of PQDs during the purification process. The optimized Br-rich CsPbBr3 QLEDs exhibit the maximum brightness of 12 090 cd m−2, CE of 3.1 cd A−1, and EQE of 1.194%.

Mixed Cation QLEDs: Aside from OIHP and AIP based QLEDs, Zhang et al.[43] reported the hybrid perovskite QLEDs based on organic–inorganic mixed cation ion FA/CsPbBr3 PQDs. FA1−xCsxPbBr3 is significantly dependent on the substi-tution content of the Cs cation. All inorganic Cs cation doping in hybrid PQDs cause the bandgap variation. The optimized

Figure 16. a) Schematic of LE and charge injection/transport capabilities in FAPbBr3 PQDs with short ligand (left) and long ligand (right) (Reproduced with permission.[41] Copyright 2016, Elsevier Ltd). b) Schematic of controlling ligand density on CsPbBr3 QD surfaces and corresponding changes in ink stability, PLQY, and carrier injection (Reproduced with permission.[120] Copyright 2016, Wiley-VCH). c) Schematic of halide-poor and halide-rich conditions for synthesis of CsPbBr3 PQDs (Reproduced with permission.[121] Copyright 2017, American Chemical Society).

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composition FA0.8Cs0.2PbBr3 QLEDs exhibit luminescence of 55 055 cd m−2, CE of 10.09 cd A−1, and EQE of 2.80%. Xu et al.[122] first time demonstrate MA1−xCsxPbBr3 PQDs based QLEDs. The optimized composition of MA0.7Cs0.3PbBr3 QLEDs exhibits brightness of 24 500 cd m−2, CE of 4.1 cd A−1, and EQE of 1.3%.

Others: Surface plasmon resonance (SPR) is the strong inter-action between metallic nanostructure and resonant photons. SPR can promote the effective energy transfer and improve the QLED performance by enhancing radiative recombina-tion rate.[123–127] Zhang et al.[128] report the CsPbBr3 QLEDs with the structure ITO/PEDOT:PSS/Ag/CsPbBr3/TPBi/LiF/Al. The maximum SPR effect is related to the overlap of absorp-tion spectra of metallic nanostructure and emission spectra of PQDs, which will cause an effective energy transfer and therefore enhanced the emission intensity. The emission wavelength of CsPbBr3 was 527 nm, so the suitable metallic nanostructure was Ag rod which the absorption wavelength is located in 526 nm. The maximum luminescence and efficiency of Ag–CsPbBr3 had been increase up to 42% and 43.3% due to the Ag-induced plasmonic near-field effect. In addition to PQDs based QLEDs, LEDs were also fabricated with polycrys-talline films. Cho et al.[20] report the MAPbBr3 polycrystalline bulk film based LEDs, which exhibit CE of 42.9 cd A−1 and EQE of 8.53%. The MAPbBr3 films were prepared with the

MABr and PbBr2 precursor solutions and the particle size of bulk MAPbBr3 was about 99.7 nm. In comparison with PQDs based QLEDs, the bulk MAPbBr3 film based LEDs show the higher efficiency due to the loss of insulating ligand, which will cause the inefficient charge injection and transport.[41] Li et al.[129] reported the single-layer halide perovskite LEDs. The layer consist of CsPbBr3, poly(ethylene oxide) (PEO), and poly(vinylpyrrolidone) (PVP). The single-layer LEDs were fab-ricated with ITO anode, CsPbBr3–PEO–PVP emitting layer and indium–gallium eutectic (In–Ga) cathode without hole injec-tion layer and electron injection layer. The optimized device exhibits luminescence of 593,178, CE of 21.5 cd A−1, EQE of 5.7% and PE of 14.1 lm w−1. Figure 17a–c presents the highest performances of R/G/B PQDs based QLEDs devices. The green device[120] yielded the highest EQE of 6.27%, which is 50-fold enhanced in comparison with the first CsPbBr3 QLEDs, and CE of 13.3 cd A−1, which was obtained by correct washing. The red CsPb(Br/I)3 PQDs based QLEDs[130] with the configuration of inverted ITO/ZnO/PEI/QDs/CBP/TCTA/MoOx/Au struc-ture and well-passivated perovskite NC film. By optimizing charge balance, the device exhibit CE of 3.4 cd A−1

, corre-sponding to an EQE of 6.3%, the highest value reported among red perovskite QLEDs thus far. The blue device[131] exhibited a maximum luminance of 2673 cd m−2, CE of 4.01, and EQE

Figure 17. a) Normalized EL and PLQY spectra of CsPbBr3 QLEDs and an image of a working device at 5 V (Reproduced with permission.[120] Copyright 2016, Wiley-VCH). b) Normalized EL spectra and PLQY spectra of CsPb(Br/I)3 QLEDs and an image of a working device (Reproduced with permis-sion.[130] Copyright 2016, American Chemical Society). c) Normalized EL spectra of CsPb(Br/Cl)3 QLEDs device and an image of a working device (Reproduced with permission.[131] Copyright 2016, Wiley-VCH). d) EL spectra of red-, orange-, green-, and blue-emitting perovskite NC LEDs and images of perovskite NC LEDs in operation (Reproduced with permission.[132] Copyright 2016, Wiley-VCH).

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of 1.38%, which are much better than those of most LEDs based on MAPbBr3 films. These QLEDs showed long-term sta-bility in air (humidity ≈ 50%) for at least 7 d. Li et al.[132] used a trimethylaluminum (TMA) vapor to render the nanocrystal insoluble and then deposited the charge-injection layers (TFB) without the orthogonal solvents. Figure 17d displays the full-color device by the TMA crosslinking method.

Lead-Free Device: PQDs are potential emitters for electrolu-minescent display. However, the toxicity of Pb2+ has a negative impact on the environment. Hence, total and partial replace-ment of Pb2+ is expected. At present, the most suitable substitute elements are the less toxic Sn(II) ions. Sn-based perovskite semiconductors are widely developed and applied in solar cells. However, few studies reported Sn-based LEDs. Lai et al.[133] demonstrated QLEDs based on MASn(Br1−xIx)3 thin film with

structure of ITO/PEDOT:PSS/MASn(Br1−xIx)3/F8/Ca/Ag. The device showed near-infrared EL spectrum with emission wavelength of 945 nm and maximum EQE of 0.72%. Hong et al.[134] used two methods to fabricate CsSnI3 perovskite film and its application in LEDs. This perovskite thin film exhi bited compact grains without pinhole and less cracks. Recombina-tion of electron and hole in emitting layer without decay is allowed because of surface defects of the film. QLED devices with configuration of ITO/PEDOT:PSS/CsSnI3/PBD/LiF/Al show near-infrared EL spectrum with emission wavelength of 950 nm and maximum EQE of 3.5%. These Sn-based QLEDs are applicable for sensing and medical purposes. Zhang et al.[45] also reported efficient LEDs based on CsPb1−xSnxBr3 PQDs with Sn(II) ion substitution and adopted ITO/PEDOT:PSS/poly-TPD/CsPb1−xSnxBr3/TPBi/LiF/Al normal-type structure.

Table 3. Recent progress of PQD-based LEDs.

PQDs Device structure EL peak [nm]

Von [V]

Max. L [cd m−2]

Max. CE [cd A−1]

Max. EQE [%]

Refs.

HOIP QDs–QLEDs

MAPbBr3 ITO/PEDOT:PSS/QDs/TPBi/CsF/Al 524 2.9 2503 4.5 1.1 [60]

MAPbBr3 ITO/PEDOT:PSS/QDs/TPBi/TPBi:Cs2CO3/Al 512 3.1 3515 11.49 3.8 [118]

MAPbBr3 ITO/PEDOT:PSS/PVK:QDs/TPBi/Cs2CO3/Al 534 2.8 7263 9.45 2.28 [139]

FAPbBr3 ITO/Buf–HIL/QDs/TPBi/LiF/Al 530 = = 9.16 2.05 [41]

MA/CsPbBr3 ITO/PEDOT:PSS/TFB/QDs/TPBi/LiF/Al 523 3.0 24 500 4.1 1.3 [42]

FA/CsPbBr3 ITO/PEDOT:PSS/TFB/QDs/TPBi/LiF/Al = 3.5 55 005 10.09 2.80 [43]

AIP QDs-QLEDs

Ag−CsPbBr3 ITO/PEDOT:PSS/Ag/QDs/TPBi/LiF/Al = 3.8 8911 1.42 0.43 [128]

CsPbBr3 ITO/PEDOT:PSS/QDs/TPBi/Al 527 4.6 3853 8.98 2.21 [140]

CsPbBr3 ITO/PEDOT:PSS/poly-TPD/QDs/TPBi/LiF/Al 512 2.6 1660 18.8 8.73 [141]

CsPbBr3 ITO/PEDOT:PSS/poly-TPD/QDs/TPBi/LiF/Al 512 3.4 15 185 13.3 6.27 [120]

CsSn/PbBr3 ITO/PEDOT:PSS/poly-TPD/QDs/TPBi/LiF/Al 508 5.0 5,495 3.6 = [45]

Br-rich CsPbBr3 ITO/PEDOT:PSS/poly-TPD/QDs/TPBi/LiF/Al 515 4.6 12 090 3.1 1.2 [121]

R/G/B PQDs-QLEDs

MAPb(BrCl)3 (ITO)/PEDOT:PSS/PVK/QDs/TPBi/LiF/Al 445 = 2673 4.01 1.38 [131]

MAPbBr3 (ITO)/PEDOT:PSS/PVK/QDs/TPBi/LiF/Al 525 7.8 2398 3.72 1.06

MAPb(BrI)3 (ITO)/PEDOT:PSS/PVK/QDs/TPBi/LiF/Al 640 = 986 1.52 0.53

CsPbBr1.5Cl1.5 ITO/ZnO/QDs/TFB/MoO3/Ag 480 = 8.7 = 0.0074 [132]

CsPbBr3 ITO/ZnO/QDs/TFB/MoO3/Ag 523 = 2335 = 0.19

CsPbI3 ITO/ZnO/QDs/TFB/MoO3/Ag 698 = 206 = 5.7

CsPb(Cl/Br)3 ITO/PEDOT:PSS/PVK/QDs/TPBi/LiF/Al 455 5.1 742 0.14 0.07 [119]

CsPbBr3 ITO/PEDOT:PSS/PVK/QDs/TPBi/LiF/Al 516 4.2 946 0.43 0.12

CsPb(Br/I)3 ITO/PEDOT:PSS/PVK/QDs/TPBi/LiF/Al 586 4.6 528 0.08 0.09

CsPbCl3 ITO/ZnO/PEI/QDs/CBP/TCTA/MoOx/Au 404 3.4 11 0.0049 0.61 [130]

CsPbBr3 ITO/ZnO/PEI/QDs/CBP/TCTA/MoOx/Au 516 2.4 3019 1.32 0.4

CsPb(Br/I)3 ITO/ZnO/PEI/QDs/CBP/TCTA/MoOx/Au 648 1.9 2216 3.42 6.3

Perovskite-LEDs (film types)

MASn(Br1−xIx)3 ITO/PEDOT:PSS/film/F8/Ca/Ag 945 = = = 0.72 [133]

CsSnI3 ITO/PEDOT:PSS/film/PBD/LiF/Al 950 = = = 3.8 [134]

CsPbBr3–PEO–PVP ITO/QDs:PEO:PVP/In−Ga 522 1.9 593 178 21.5 5.7 [129]

MAPbBr3 Glass/SOCP/film/TPBi/LiF/Al = = = 42.9 8.53 [20]

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This device can reach maximum luminescence and CE of 5495 cd m−2 and 3.60 cd A−1, respectively. Table 3 summarizes different QLED devices.

5. Conclusions and Outlooks

Perovskite material opens the new opportunity in solid-state solar cell since 2012. In addition to perovskite thin film, perovskite quantum dots have been extensively investigated because of their unique optical properties, such as narrow emission wavenlength, tunable wavelength, and high PLQY. Recent studies reported new types of FAPbBr3 PQDs, PQDs with organic–inorganic mixed cations, divalent cation doped colloidal CsPb1−xMxBr3 PQDs and heterovalent cation doped into PQDs. These PQD analogs opened new possibilities for optoelectronic devices. We summarized different PQD types, including synthetic, structural features, optical properties, and related LEDs applications. For the practical application, it is important to understand the degradation mechanism of PQDs in LEDs. Oxygen and moisture-facilitated ion migration may easily etch unstable PQDs. Easy ion migration may result in crystal growth, which will lower PLQY of PQDs. Different strategies were developed to prevent the degradation of PQDs such as surface treatment and coating. However, the QDs based LEDs stability with 20 W cm−2 of incident blue light and 110 °C should passed at least 1000 h without degradation for practical application. It is also a big obstacle in PQD-based LEDs. There is also increasing impact on medical-related lighting. New types of multicolor PQD-based white-light LEDs with tunable circadian rhythm and high CFI and CGI can be used in bio- and medical-related research. For instance, determining the outstanding optical properties in LEDs using single dot tech-niques is another important good research area. In the future, the AM-QLEDs have potential for next generation backlight dis-play because of the large-area, energy saving, wide color gamut and flexible. The AM-QLEDs is consisted of high-resolution RGB pixels, so the lifetime and efficiency of each color is so important. However, the blue color PQD-QLEDs also should be further improved for the application in next generation display.

AcknowledgementsThe authors thank the Ministry of Science and Technology of Taiwan (Contract No. MOST 104-2113-M-002-012-MY3).

Conflict of InterestThe authors declare no conflict of interest.

KeywordsAuger recombination, perovskite quantum dots, QLEDs, stability, white-light LEDs

Received: July 17, 2017Revised: September 26, 2017

Published online: November 30, 2017

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