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Perspective Cite This: J. Phys. Chem. Lett. 2019, 10, 3035−3042

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The Rise of Perovskite Light-Emitting Diodes Zhanhua Wei*,† and Jun Xing*,‡ †

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Engineering Research Center of Environment-Friendly Functional Materials, Ministry of Education, College of Materials Science & Technology, Huaqiao University, Xiamen 361021, China ‡ Key Laboratory of Eco-Chemical Engineering, Ministry of Education, College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao 266042 China ABSTRACT: Recently, metal halide perovskite materials have attracted great interest in both photovoltaic and electroluminescent devices. The external quantum efficiency of the perovskite light-emitting diodes (pero-LEDs) has grown to over 20% within four years, and the operational lifetime has been improved to tens of hours. These achievements make pero-LEDs very promising technology in solid lighting and displays. In this Perspective, we first give a general introduction of the pero-LEDs; we then discuss various pero-LEDs with different cell configurations and perovskite emitting structures; to conclude, we propose some efficiency improvement strategies and development opportunities of pero-LEDs. Provided that we can continually improve efficiency and operational lifetime of pero-LEDs, we can make them more and more compatible with other mature technologies, like organic LEDs and inorganic LEDs, and then finally realize their practical application in daily life.

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quantum yield (PLQY). (3) The raw materials and equipment for preparing perovskites are cheap, not nearly as costly as the organic emitting materials used in OLEDs. (4) The perovskites are ambipolar in charge transfer, meaning that perovskites can transport holes and electrons both, and the corresponding transport capability is relatively better than that of OLED materials. (5) The perovskites and pero-LEDs can be readily prepared using solution-based methods at low temperature; this is very different from the conventional method for preparing OLEDs (all-vacuum thermal evaporation method). All in all, the pero-LEDs may be an alternative technology to OLEDs but with lower equipment and materials cost. There are many metrics for evaluating device performance of LEDs, such as external quantum efficiency (EQE), power conversion efficiency, wall-plug efficiency, internal quantum efficiency, lifetime, etc. In the field of pero-LEDs, EQE is a well-accepted and commonly used metric to evaluate device performance, and the EQE is defined as the ratio of the number of photons emitted from the device to the number of electrons passing through the device. In 2014, Friend et al. reported the first pero-LEDs operating at room temperature with a decent maximum EQE (EQEmax) of 0.76% in the infrared region and 0.1% in the green region. These EQE values are quite below that of inorganic LEDs and OLEDs, but this work strongly encourages the research community to search for ways to further improve device performance.7−10 After only four years, in October and November 2018, four different groups reported almost at the same time that they had achieved EQEmax over 20% of pero-LEDs in the green,11 red,12

alide perovskites are regarded as the next generation of optoelectronic materials with a general formula of ABX3, where the A site is an organic or inorganic monovalent cation (e.g., CH3NH3+ (MA), CH(HN2)2+ (FA), and Cs+) and the B site represents a divalent cation (e.g., Pb2+ and Sn2+); the X site is a halogen anion (e.g., Cl−, Br− and I−). Because perovskite materials have the advantages of solution-processed capability, tunable band gap, high absorbance coefficient, balanced charges transport capability (for holes and electrons both), etc., perovskite materials have been intensely investigated as a photovoltaic active layer since 2009.1 After a decade of development, perovskite materials have proved themselves to be a legend in the field of solar cells, with an incredible and impressive power conversion efficiency over 23%.2 Beside their photovoltaic application, perovskite materials also see great potential in many other optoelectronic applications, such as electroluminescence (EL), photoluminescence (PL),3 X-ray detectors,4,5 etc. In this Perspective, we will focus on application of perovskite light-emitting diodes (pero-LEDs), summarize their origin and development, propose some general strategies for performance improvement, and discuss their challenges and opportunities. Considering that there are already mature and commercially available EL technologies, such as inorganic LEDs and organic LEDs (OLEDs), why is there a need to develop new EL technology? The perovskite materials are regarded as ideal LEDs materials because of their unique merits, including but not limited to the following: (1) The perovskites are direct band gap semiconductors and the bandwidth can be easily tuned by compositional engineering; as a result, the emission of perovskites is capable of covering the whole visible light region and even some part of the infrared light region.6 (2) The perovskites can tolerate defects and easily achieve high PL © 2019 American Chemical Society

Received: January 30, 2019 Accepted: May 17, 2019 Published: May 22, 2019 3035

DOI: 10.1021/acs.jpclett.9b00277 J. Phys. Chem. Lett. 2019, 10, 3035−3042

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and infrared13,14 light regions. The rapid efficiency growth of pero-LEDs has attracted intense interest from not only the research community but also industrial companies, even though there are still some tough issues (e.g., color stability, operation lifetime, and safety concerns) that need to be overcome; it is believed that the pero-LEDs may see great potential in lighting and display applications in the future.

thickness over 100 nm.9−11 In contrast, to prepare a normalstructured pero-LEDs (p-i-n), one needs to prepare HTL, EML, and ETL layer-by-layer. In 2015, Lee et al. achieved highly bright and efficient green pero-LEDs with EQE of 8.53%, where a self-organized conducting polymer (PEDOT:PSS and PFI) served as HTL, a modified MAPbBr3 perovskite layer as EML, and TPBi as ETL.15 The choice of proper HTL and ETL materials is dependent on, but not limited to the following aspects: (1) The energy levels of the charge transporter layers need to match well with that of perovskites. (2) The preparation method of the charge transporter layers needs to be compatible with perovskite, that is cause no damage to perovskite layers. (3) The charge transport mobility of the HTL and ETL materials had better to be close to each other. For now, scientists have achieved high performance in both cell configurations. Wei et al. adopted a pi-n device structure, where PEDOT:PSS served as HTL and B3PYMPM as ETL;11 they have achieved an EQEmax of 20.3% in green pero-LEDs. Huang et al. adopted an n-i-p device structure, where ZnO/PEIE served as ETL and TFB polymer as HTL, and they were capable of achieving an EQEmax of 20.7% in infrared pero-LEDs. ZnO or polymer-modified ZnO nanocrystals can be synthesized readily and widely used in n-ip structures, and the most well-known advantage of ZnO is the high charge mobility. However, the ZnO colloid solution needs to be freshly prepared and cannot be stored for too long, and the stability of the ZnO/perovskite interface remains unclear especially in a humid environment. On the other hand, most of the materials used in p-i-n structure are commercially available, which makes it more popular and easier for device fabrication. However, the work function of the PEDOT:PSS is around 5.0 eV, which is quite unmatched to the valence bands (normally >5.2 eV) of perovskite EMLs. There are other HTL materials available with lower energy levels, like PTAA, PVK, poly-TPD, TCTA, and others; however, their hydrophobic surface nature make it difficult to deposit high-quality perovskite film on top of them, limiting their application in fabrication of highperformance devices. It is noted that sandwich structures with three layers (p-i-n and n-i-p) are widely adopted and reported by many groups; however, to pursue higher EQE, we believe that adding more functional layers is required, such as hole injection layer, hole-blocking layer, electron injection layer, electron-blocking layer, and others. In other words, developing new cell configurations, with better charger transfer, charge confinement, and higher radiative recombination efficiency, is one of the most important research topics in pero-LEDs. Perovskite Nanocrystals or Quantum Dots. Perovskite colloidal nanocrystals or quantum dots (QDs) are one class of perovskites with attractive properties for light emission. In 2014, Pérez-Prieto et al. pioneered the ligand-assisted reprecipitation (LARP) synthesis of colloidal hybrid perovskite MAPbBr3 QDs with average size of 6 nm.16 The corresponding

The rapid efficiency growth of pero-LEDs has attracted intense interest from not only the research community but also industrial companies, even though there are still some tough issues (e.g., color stability, operation lifetime, and safety concerns) that need to be overcome. In the following section, we will first present different cell configurations of pero-LEDs; we then will classify and introduce different kinds of perovskite emitting materials (e.g., quantum dots, quantum wells, and bulk films); to conclude, we will discuss challenges and opportunities for developing pero-LEDs. Pero-LED Device Conf iguration. In a typical pero-LED device, the perovskite layer serves as the light-emitting layer (EML), and it is sandwiched by the hole transporter layer (HTL) and electron transporter layer (ETL). According to the fabrication sequence, the device structures can be further classified into normal and inverted structures (Figure 1a,b, fabricated from bottom to top layers). As shown in Figure 1c, the electrons are injected through the ETL; holes are injected through HTL, and radiative recombination of holes and electrons happens in the perovskite EML, where the photons are formed and emitted outside the device. In 2014, Friend et al. fabricated the first pero-LEDs operating at room temperature using a cell structure similar to that of a solar cell, where a 15 nm CH3NH3PbI3−xClx perovskite emitter was sandwiched between TiO2 (ETL) and poly(9,9′-dioctylfluorene) (F8, HTL). This is also the first paper about inverted structured (n-i-p) pero-LEDs. As is common sense in OLED, the EML layer needs to be very thin because of the poor carrier mobility of the organic materials. This may be the reason why a very thin (15 nm) perovskite layer was purposely adopted in the very first paper. However, as the charge mobility of perovskite materials is significantly higher than that of the pure organic materials, it seems that the corresponding pero-LEDs are not so sensitive to EML layer thickness. There are many highperformance pero-LEDs reported with perovskite EML layer

Figure 1. Cell configuration diagram of (a) the inverted LED structure of ETL/EML/HTL (n-i-p), (b) normal LED structure of HTL/EML/ETL (p-i-n), and (c) operation mechanism diagram of the both structures. 3036

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Developing new cell configurations, with better charger transfer, charge confinement, and higher radiative recombination efficiency, is one of the most important research topics in pero-LEDs. PLQY was about 20%. Although they demonstrated the first perovskite QD LED, the maximum luminance (Lmax) is less than 1 cd m−2, which is too low for practical application in lighting. Zhong et al. optimized the strategy to fabricate colortunable colloidal MAPbX3 (X = Br, I, Cl) QDs with PLQY up to 70%.17 They conducted comprehensive composition and surface characterizations and measured the time- and temperature-dependent photoluminescence spectra, from which they concluded that the intense increased PLQY originates from the increase of exciton binding energy due to size reduction as well as proper chemical passivation of the Br-rich surface. Xing et al. synthesized a series of colloidal halide perovskite MAPbX3 QDs with amorphous structure, which exhibit a high PLQY of more than 70%.18 A high-efficiency green (EL 512 nm) LED based on amorphous MAPbBr3 nanoparticles was demonstrated, and the LEDs showed a Lmax of 11 830 cd m−2, a maximum power efficiency (PEmax) of 7.84 lm W−1, and an EQEmax of 3.8%. Following this work, they optimized the QDs via a room-temperature aging treatment.19 Because of the increased crystallization of the perovskite QDs after aging for 1 week, the corresponding PLQY was significantly enhanced from 70% in solution and 35% in film to 89% in solution and 71% in film. Combined with the LED structure optimization, they obtained green (EL 524 nm) pero-LEDs with EQEmax of 12.9%, Lmax of 43 440 cd m−2, and PEmax of 30.3 lm W−1. Rand et al. demonstrated in situ LARP formation of selfassembled, nanosized perovskite crystallites by using butylamine halide as ligand.8 The long-chain ammonium halides added to the perovskite precursor solution act as a surfactant that dramatically constrains the growth of 3D perovskite grains during film forming, producing crystallites with dimensions as small as 10 nm. These nanosized perovskite grains with a longchain organic cation coating yields highly efficient emitters, resulting in pero-LEDs with EQEmax of 10.4% and Lmax of ca. 20 cd m−2 for the MAPbI3 system (EL 748 nm) and EQEmax of 9.3% and Lmax of 1200 cd m−2 for the MAPbBr3 system (EL 513 nm). Zhong et al. reported the in situ LARP fabrication of highly luminescent FAPbBr3 nanocrystal thin films by using 3,3-diphenylpropylamine bromide as ligand.20 The resulting films are uniform and composed of 5−20 nm FAPbBr3 perovskite nanocrystals, and the PLQY was improved to 78%. As a result, they achieved highly efficient pure green (EL 526 nm) pero-LEDs with EQEmax of 16.3% and Lmax of 13 970 cd m−2. In 2015, Kovalenko et al. demonstrated a new avenue for halide perovskites by designing all-inorganic perovskite colloidal QDs.21 They synthesized monodisperse colloidal nanocubes (4−15 nm) of cesium lead halide perovskites (CsPbX3) by a hot injection method (Figure 2a,b). The CsPbX3 QDs displayed narrow emission line-widths of 12−42 nm and high PLQY of 50−90%. The compelling combination of enhanced optical properties and chemical robustness makes

Figure 2. (a) CsPbX3 colloidal QD solutions under UV irradiation (λ = 365 nm) and (b) typical transmission electron microscopy image of CsPbBr3 QDs.21 Reproduced from ref 21. Copyright 2015 American Chemical Society. (c) EL (straight line) and PL (dashed line) spectra of QD LEDs and QDs solutions, respectively. Reproduced with permission from ref 22. Copyright 2015 Wiley-VCH. (d) Schematic illustration of the control of ligand density on CsPbBr3 QDs surfaces and the corresponding changes of ink stability, PLQY, and carrier injection. Reproduced with permission from ref 23. Copyright 2017 Wiley-VCH.

CsPbX3 QDs appealing for optoelectronic applications. Zeng et al. first reported the pero-LEDs based on all inorganic perovskite CsPbX3 QDs (Figure 2c).22 In the blue, green, and orange regions, they achieved Lmax of 742, 946, and 528 cd m−2, with EQEmax of 0.07%, 0.12%, and 0.09%, respectively. The limited device performance is due to the capping ligands like oleylamine (OAm) and oleic acid (OA) acting as electrically insulating layers on the QDs’ surface, which hinder significantly charge carrier injection and transport at the interface of the ensuing device. Thus, Zeng et al. optimized the CsPbBr3 QD LEDs through balancing surface passivation and carrier injection via ligand density control (Figure 2d).23 They proposed to control the surface ligand density via recyclable treatment on QDs using a hexane/ethyl acetate mixed solvent and achieved good balance between surface passivation and carrier injection after two cycles of treatment. Consequently, Lmax of 15 185 cd m−2 and EQEmax of 6.27% were obtained for green CsPbBr3 LEDs (EL 512 nm). Recently, they developed a general organic−inorganic hybrid ligand (OIHL) strategy to passivate CsPbX3 QDs for highly efficient pero-LEDs.24 Various metal bromides, such as ZnBr2, MnBr2, GaBr3, and InBr3, could feasibly be used to passivate nonradiative defects in perovskite QDs. The introduced metal bromide inorganic ligands could enhance the luminescent features and effective carrier injection originating from increase of radiative recombination and their intrinsic high conductivity. As a result, the OIHL strategy enhanced pero-LEDs performance with a nearly 40% improvement. The best EQEmax of 16.48% and Lmax of 100 080 cd m−2 were achieved based on ZnBr2 and MnBr2 passivation; this is also the highest EQE reported in 3037

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Figure 3. (a) Unit cell structure of PEA2(MA)n−1PbnI3n+1 perovskites with different ⟨n⟩ values, showing the evolution of QWs. (b) EQE−J and (c) radiance−V characteristics of PEA2(MA)n−1PbnI3n+1 QWs with different ⟨n⟩ values. Reproduced with permission from ref 26. Copyright 2016 Nature Publishing Group. (d) Schematic representation of the structures of the perovskites QWs with n = 1, n = 2 and n = ∞. (e) Schematic of cascade energy transfer in QWs. Excitation energy is transferred downstream from smaller-n QWs to larger-n QWs, and the emission is mainly from larger-n QWs. (f) EQE and energy conversion efficiency versus current density of LEDs devices based on NMA2(FA)n−1Pbn(I/Br)3n+1 QWs. Reproduced with permission from ref 7. Copyright 2016 Nature Publishing Group.

mechanism providing carrier concentration, enabling more efficient radiative recombination and thereby overcoming the trap-mediated nonradiative recombination at a much more practical fluence. As a result, the efficient pero-LEDs operating at near-infrared wavelength (ca. 760 nm) were produced with EQEmax of 8.8% and maximum radiance of 80 W sr−1 m−2 (Figure 3b, c). At the same time, Huang et al. also explored perovskite multiple QWs from the perovskite precursor containing 1-naphthylmethylamine iodide (NMAI) and FAPbI3 (Figure 3d, e).7 The perovskite multiple QWs films present high PLQY of 60%, and the perovskite QW-based near-IR LED (EL 763 nm) exhibited a high EQEmax of up to 11.7% and maximum radiance of 82 W sr−1 m−2 (Figure 3f). Sargent et al. found that only when energy is transferring from high band gap domains into a small subpopulation of lower band gap domains do radiative recombination outcompete nonradiative recombination.27 Thus, they developed composition and solvent engineering to control the domain distribution in perovskite QWs, PEA2(MA)n−1PbnBr3n+1. The resulting material exhibits 60% PLQY, and their corresponding green pero-LEDs (EL 526 nm) were fabricated with high EQEmax of 7.4% and Lmax of 8400 cd m−2. You et al. carried out surface passivation by coating organic molecule trioctylphosphine oxide (TPPO) on the perovskite QWs film, which reduces the nonradiative recombination on the perovskite surface or grain boundaries.9 As a result, the PLQY of the passivated perovskite film increased from 57% to 73%. Accordingly, green LEDs (EL 532 nm) based on the passivated perovskite QWs reached an EQEmax of 14.36% and Lmax of 9120 cd m−2. Sun et al. introduced 1,4,7,10,13,16-hexaoxacyclooctadecane in the PEA2Csn−1PbnBr3n+1 perovskite QWs to suppress the crystallization of the PEABr phase and achieve an improved domain size distribution and more controlled phase separation between the organic and perovskite phase.28 This led to a significant improvement of PLQY up to 70% and allowed achieving

QD pero-LEDs with green emission (EL 518 nm). Osman et al. explored a bidentate ligand to passivate the CsPbI3 QD surface.25 Their study suggested that the bidentate ligand can bind firmly to perovskite surfaces through dual carboxyl groups, which can reduce the surface traps and inject extra electrons into QDs. The passivated QDs exhibited near unity PLQY and substantially improved stability. The red perovskite QD LEDs (EL 688 nm) showed EQEmax of 5.02% and Lmax of 748 cd m−2. Kido et al. synthesized red perovskite QDs via anion-exchange from pristine CsPbBr3 using oleylammonium iodide (OAM-I) and aryl-based aniline hydoroiodide (AnHI).12 The anion-exchange red perovskite QDs exhibited higher PLQYs of 80 and 69% for OAM-I and An-HI, respectively, while the PLQY of the pristine CsPbBr3 QDs was only 38%. The OAM-I-based anion-exchange red perovskite QD LED (EL 653 nm) showed EQEmax of 21.3% and Lmax of 500 cd m−2; the An-HI-based QD LED (EL 645 nm) showed EQEmax of 14.1% and Lmax of 794 cd m−2. Moreover, the An-HI-based QD LED exhibited higher operational stability compared to the OAM-I-based QD LED as a result of its improved electrical properties. Perovskite Quantum Wells. Perovskite quantum wells (QWs) are a subclass of perovskites truncated and confined along one axis of the 3D perovskite. We can imagine that the 3D perovskite is cut into one-layer-thick slices along the ⟨100⟩ or ⟨110⟩ direction, and the small organic molecules or ions are substituted by long-chain organic amines. Therefore, the monolayer or multilayer perovskite slabs are sandwiched by the long-chain organic amine molecules, and we call this structure perovskite QWs. In 2016, Sargent et al. developed multilayered perovskite QWs by incorporating the bulky cation phenylethylammonium into MAPbI3, noted as PEA2(MA)n−1PbnI3n+1 (Figure 3a).26 The perovskites QWs showed PLQY of around 10% under low excitation, and the PLQYs rise as a function of the excitation intensity. That is attributed to the conveyance of excited carriers in a funneling 3038

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Figure 4. (a) Schematic illustrations of single-layered CsPbBr3, bilayered CsPbBr3/MABr, and quasi-core/shell CsPbBr3/MABr structures, all fabricated on ITO substrates; (b) cross-sectional TEM image of the quasi-core/shell CsPbBr3/MABr structure on PEDOT:PSS; and (c) EQE−L characteristics of the best-performing perovskite LEDs. Reproduced with permission from ref 11. Copyright 2018 Nature Publihing Group. (d) Schematic illustrations of the device, which represent light trapped in devices with a continuous emitting layer, can be extracted by the submicrometer structure; (e) EQE and ECE plotted against current density; (f) dependence of current density and radiance on the voltage. Reproduced with permission from ref 13. Copyright 2018 Nature Publishing Group.

Table 1. Performance Summaries of Some Selected Pero-LEDs Emitting materials MAPbBr3 QDs MAPbBr3 QDs MAPbBr3 QDs MAPbI3 QDs FAPbBr3 QDs CsPbBr3 QDs CsPbBr3 QDs CsPbI3 QDs CsPbI3 QDs PEA2(MA)n−1 PbnI3n+1 QWs NMA2(FA)n−1 Pbn(I/Br)3n+1 QWs PEA2(MA)n−1 PbnBr3n+1 QWs PEA2(FA)n−1 PbnBr3n+1 QWs PEA2(Cs)n−1 PbnBr3n+1 QWs EA2(MA)n−1 PbnBr3n+1 QWs (PEA/IPA)2(MA/Cs)n−1PbnBr3n+1 QWs CsPbBr3/MABr bulk FAPbI3 bulk a

−1

The unit is W sr

LED device architecture ITO/PEDOT:PSS/pero/TPBi/Cs2CO3/Al ITO/PEDOT:PSS/pero/TPBi/Cs2CO3/Al ITO/PVK/pero/TPBi/LiF/Al ITO/poly-TPD/pero/TPBi/LiF/Al ITO/PEDOT:PSS/TFB/pero/TPBi/LiF/Al ITO/PEDOT:PSS/poly-TPD/pero/TPBi/LiF/Al ITO/PEDOT:PSS/PTAA/pero/TPBi/LiF/Al ITO/PEDOT:PSS/poly-TPD/pero/TPBi/LiF/Al ITO/PEDOT:PSS/poly-TPD/pero/TPBi/LiF/Al ITO/TiO2/pero/F8/MoO3/Au ITO/ZnO/PEIE/pero/TFB/MoOx/Au ITO/PEDOT:PSS/pero/TPBi/LiF/Al ITO/PEDOT:PSS/pero/TPPO/TPBi/LiF/Al ITO/poly-TPD/pero/TPBi/LiF/Al ITO/PEDOT:PSS/pero /TmPyPB/CsF/Al ITO/PEDOT:PSS/pero/TPBi/LiF/Al ITO/PEDOT:PSS/pero PMMA/B3PYMPM/LiF/ Al ITO/ZnO/PEIE/pero/TFB/MoOx/Au

PLQY [%]

EL peak [nm]

Lmax [cd m−2]

EQEmax [%]

ref

77 89 7 1.9 78 90 76 95 80 10 60 60 73 70 88 80

512 524 513 748 526 512 518 688 653 760 763 526 532 514 473, 485 490 525

11830 43440 1200 ∼20 13970 15185 76940 748 500 80a 82a 8400 9120 ∼20000 200 2480 14000

3.8 12.9 9.3 10.4 16.3 6.27 16.48 5.02 21.3 8.8 11.7 7.4 14.36 15.5 2.6 1.5 20.3

18 19 8 8 20 23 24 25 12 26 7 27 9 28 29 30 11

70

803

390a

20.7

13

−2

m .

efficient pero-LEDs with Lmax of ca. 20 000 cd m−2 and EQEmax of 15.5% (EL 514 nm). Because of the quantum confinement effect, perovskite QWs provide a pathway to achieve blue perovskite LEDs. By reducing the number of inorganic layers (n), one may tune the band gap of bromide perovskite QWs from ca. 2.6 eV (n = 4) to 2.7 eV (n = 3), 2.9 eV (n = 2), and 3.1 eV (n = 1). Chih-Jen Shih et al. demonstrated LEDs based on the colloidal perovskite QWs (OLA)2(MA)n−1PbnBr3n+1 (OLA, oleylamine). The electroluminescence wavelength could cover pure green (n = 7−10), sky blue (n = 5), pure blue (n = 3), and deep blue (n = 1). However, their EQE and luminance are very low. Hin-Lap Yip et al. systematically studied the structural change and structure−property relationship of

MAPbBr3 from bulk film to QWs (POEA)2(MA)n−1PbnBr3n+1 (POEA, 2-phenoxyethylamine). They also fabricated perovskite QWs sky-blue LEDs with EQEmax of 1.1% and Lmax of 19.25 cd m−2 and blue LED with EQEmax of 0.06% and Lmax of 0.07 cd m−2. Yang et al. constructed the blue perovskite LEDs based on (EA)2(MA)n−1PbnBr3n+1 (EA, ethylamine),29 they showed an EQEmax of 2.6% and Lmax of 200 cd m−2. However, the EL spectrum of the above perovskite LEDs have a nonGaussian profile, with multiple emission peaks as well as much larger fwhm values, indicating multiemission domains with different n values. Xing et al. deliberately selected long-chain cation combinations to modulate the crystallization of perovskites QWs.30 The resulting perovskite film contained QWs with reduced n-value distribution and exhibited a single 3039

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electrons and holes injection rate in pero-LEDs. This can be realized by proper selection or modification of HTL and ETL layers,19 developing a new cell configuration with electronblocking layer or hole-blocking layer, or adding blocking layer.11 (3) Improve ηout of pero-LEDs. This topic has not yet been well-investigated and requires more systematic research; however, some strategies used in OLED may be applicable, such as substrate modification, use of microlens array, nanopatterned and nano porous films.

emission peak (EL 477 nm) with a record PLQY of 88%. They fabricated the blue perovskite QW LEDs with an Lmax of 2480 cd m−2 and EQEmax of 1.5%, and the EL wavelength could be tuned from 478 to 490 nm as different HTL materials were used. Perovskite Bulk. A general way to form a bulk perovskite film (3D structures) is to deposit a perovskite layer by spin-coating a 3D precursor solution, such as CsPbBr3, MAPbBr3, etc. However, the corresponding pero-LEDs’ performance is limited, possibly because of the excess nonradiative defects, low PLQY,15 poor surface morphology,31 unbalanced charges injection rate, and so on. Recently, Wei et al. developed a new strategy for managing the compositional distribution in the device, which could simultaneously provide high luminescence and balanced charge injection.11 Specifically, they mixed a CsPbBr3 perovskite with a MABr additive; their differing solubilities yield sequential crystallization into a micrometersized cuboids CsPbBr3/MABr with quasi-core/shell structure (Figure 4a,b). The MABr shell passivates the nonradiative defects that would otherwise be present in CsPbBr3 crystals, boosting the PLQY, while the MABr capping layer enables balanced charge injection. These effects together allowed achieving green pero-LEDs (EL 525 nm) with a record EQEmax of 20.3% and Lmax of 14 000 cd m−2 (Figure 4c). At the same time, Huang et al. reported near-infrared pero-LEDs (EL 803 nm) with a record EQE of 20.7%. They synthesized perovskites by introducing amino-acid additives into the perovskite precursor solutions.13 The additives could effectively passivate perovskite surface defects and reduce nonradiative recombination. The formed perovskite films were composed of submicrometer-scale particles, which could efficiently extract light from the device and retain wavelengthand viewing-angle-independent electroluminescence (Figure 4d). Finally, pero-LEDs with an EQEmax of 20.7% and high brightness of up to 390 W sr−1 m−2 (Figure 4e, f) were achieved. These high EQEs are now on par with those of more mature technologies such as inorganic LED or OLEDs (Table 1). Challenges and Opportunities of Pero-LEDs. For now, the reported highest EQE of green, red, and infrared pero-LEDs are 20.3%, 20.7%, and 21.3%, respectively. The EQE values are quite impressive now, especially considering that these improvements were achieved in only four years; however, the EQEs of pero-LEDs are still far below those of OLED and inorganic LED (∼30%). Thus, more efforts need to be continually put into further improving device performance of pero-LEDs. The EQE of the pero-LEDs can be determined by a general equation as follows:

The state-of-art operational lifetime is still far from the practical requirements (>10 000 h); hence, we believe that improving the long-term stability is one of the most challenging obstacles in this field. Apart from further increase of EQE, there are also some other important and interesting topics, such as developing other color emission pero-LEDs (e.g., blue, pink, yellow, and orange), developing white pero-LEDs, fabricating foldable or flexible pero-LEDs, printing pero-LEDs arrays, and so on. It is well-known that blue GaN-based inorganic LEDs are quite important, and they have already been widely used to excite phosphor and emit colorful light and white light. Similarly, fabrication of high-performance blue pero-LEDs is quite attractive. There are now two approaches for achieving blue emission pero-LEDs, namely, composition engineering (anion mixing) and quantum-confinement engineering (QDs/QWs). Unfortunately, for composition engineering, the migration of the halogen ions enables phase segregation into Cl-rich and Brrich phases under voltage bias, resulting in the EL wavelength undesirably shifting from blue to green during device operation. For quantum-confinement engineering, multiemission domains and low conductivity ligands would cause color instability and poor performance of blue pero-LEDs. Therefore, developing color-stable and efficient blue pero-LEDs remains quite challenging, which requires vast research effort. In our opinion, quantum-confinement engineering may be more promising for producing highly efficient and stable blue pero-LEDs; however, one may need to put effort into resolving the problems of the charge injection barrier caused by low conductive ligands and balancing injection rate of electrons and holes in the device. Last but not least, it is worth noting that it is still very challenging to achieve highly stable pero-LEDs with long operation lifetime. The operation lifetime represents how long the pero-LEDs can work steadily; for example, T50 is defined as the time when the luminance decreases to 50% of the initial value under a specific driving current. The operation lifetime is usually measured with accelerated aging tests, i.e., applying a very large constant current to pero-LEDs and driving the devices to supply luminance as high as thousands or even tens of thousands of candela per square meter, monitoring the luminance degradation process as a function of time, and then calculating the practical operation lifetime of a lower luminance (hundreds of cd m −2 ) with some proper assumptions and equations. For now, the best operation lifetime of the reported pero-LEDs is around tens of hours,11,14 which has already been dramatically improved, as the lifetime

EQE = ηQEγηout

where ηQE is the fraction of energy released from the perovskite EML materials as light and also called PLQY; γ is a fraction of injected charges producing excitons and also called balance factor; ηout is the extraction or out-coupling efficiency representing the proportion of photons generated in the active region that escape from the device. Accordingly, there are basically three ways to improve EQE performance: (1) Increase ηQE of the perovskite EML layer. This can be realized by proper ligand exchanging in QDs solution,32,33 defects passivation in films though incorporation of polymer,14 additives,13,15 and 2D perovskites,27 or managing the spatial distribution of the composite.11 (2) Improve γ, i.e., balancing 3040

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was reported to be only tens of seconds in the very first papers. As the pero-LEDs are normally driven in a relatively high voltage (like 3−6 V), which may induce ion migration of perovskites, this is widely thought to be responsible for the poor long-term stability. Moreover, the joule heat, produced during device operation, may induce material decomposition as some perovskites (like MA-based perovskites, organic ligands in QDs and DWs) are poor in thermal stability. In our opinion, the all inorganic perovskites may perform better in long-term stability as they have been proven to be more thermally stable. However, the state-of-art operational lifetime is still far from the practical requirements (>10 000 h); hence, we believe that improving the long-term stability is one of the most challenging obstacles in this field, which requires great effort and contribution from the research community.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Zhanhua Wei: 0000-0003-2687-0293 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by National Natural Science Foundation of China (51802102) and Scientific Research Funds of Huaqiao University (600005-Z16J0038). J.X. acknowledges financial support from the start-up fund of the Qingdao University of Science and Technology (010029046).

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