Optically pumped white light-emitting diodes based on metal halide perovskites and perovskite-related materials

Optically pumped white light-emitting diodes (WLEDs), consisting of blue/ultraviolet LED chips and down conversion phosphors, have a wide range of applications in our daily life, such as full color display and solid-state lighting. While remarkable progress in light quality, device efficiency, and lifetime has been achieved during the last two decades, many challenges remain in optically pumped WLEDs, and searching for low cost high performance down conversion phosphors is still of great interest. Recently, metal halide perovskites have emerged as a highly promising new generation of light emitters for their exceptional optical properties with high quantum efficiencies and color tunability, which have also inspired researchers to investigate their derivatives. In this perspective, we briefly review the progress during the last few years in the development of metal halide perovskites and perovskite-related materials as down conversion phosphors for optically pumped WLEDs. We also highlight some major issues and challenges that need to be addressed to enable perovskite-based light emitters to possibly replace currently used rare-earth doped inorganic phosphors and quantum dots.


I. INTRODUCTION
In 2014, the Nobel Prize for physics was awarded to a trio of scientists in Japan and the US for the invention of efficient blue light-emitting diodes (LEDs), which, together with down conversion phosphors, have enabled bright and energy-saving solid-state white light sources. During the last two decades, these LED-based light sources have revolutionized the display and lighting industries with their numerous advantages (e.g., energy efficient, long lifetime, ecofriendly, and color tunable) over conventional technologies, such as incandescent and fluorescent lighting. 1,2 The performances of representative optically pumped white LEDs (WLEDs) are summarized in Table I. The three major device characteristics are color rendering index (CRI), correlated color temperature (CCT), and luminous efficiency. While optically pumped WLEDs have steadily become the mainstream, there is still plenty of room for further improvement before they can really be considered as perfect light sources.
Recently, metal halide perovskites and their low-dimensional derivatives have emerged as highly promising light emitters that could serve as down conversion phosphors for optically pumped WLEDs. Electrically driven green, 32 red, 33 and NIR 34,35 metal halide perovskite light-emitting diodes with efficiencies near the theoretical limit have also been demonstrated. The remarkable photoluminescence properties of ABX 3 (A = Cs, CH 3 NH 3 , or NH 2 CHNH 2 , B = Pb or Sn, and X = Cl, Br, I, or their mixtures) perovskites, in particular, their nanocrystals (NCs), include narrow emission covering the whole of the visible region with a small full width at half maximum (FWHM) of less than 20 nm, high photoluminescence quantum efficiencies (PLQEs) of up to near-unity, and exceptional color tunability via structural and compositional controls. Since the first report of using this class of hybrid materials as down conversion phosphors for WLEDs in 2015, 36 tremendous efforts in developing highly luminescent and stable perovskite NCs, as well as novel encapsulation techniques, have led to remarkable improvements of device efficiency and stability. 37 For their low-dimensional derivatives, in which metal halides form two-dimensional (2D), onedimensional (1D), or zero-dimensional (0D) structures at the molecular level, highly efficient broadband emissions with PLQEs of up to near-unity could be realized. 38 Of particular interest are corrugated-2D and -1D organic metal halide hybrids that exhibit intrinsic white light emissions, making them highly promising single component white phosphors for optically pumped WLEDs.
In this perspective, we provide a brief review on the development of optically pumped WLEDs based on metal halide perovskite NCs and low-dimensional organic metal halide hybrids. Notable achievements during the last few years will be presented, followed by discussions on the issues and challenges for these materials. The outlook will be provided in the end, with proposed research to eventually make metal halide perovskites and perovskite-based materials commercially viable phosphors for optically pumped WLEDs.

II. METAL HALIDE PEROVSKITE NCs FOR OPTICALLY PUMPED WLEDS
Metal halide perovskite NCs, such as CsPbX 3 , have attracted great research attention with potential applications in various types of optoelectronic devices, [39][40][41] owing to their exceptional optical properties, i.e., highly tunable narrow emissions over the entire visible region with PLQEs of up to near-unity. 37,[42][43][44][45][46] Since the first report of perovskite NC synthesis by Perez-Prieto and colleagues in 2014, 47 a variety of approaches have been developed to prepare metal halide perovskite NCs, including hot-injection, 37 ligand-assisted reprecipitation (LARP), 48 microwave irradiation, 49 in situ formation, [50][51][52] and template-assisted methods [Figs. 1(a) and 1(b)]. [53][54][55][56] Typical electron transmission micrographs of colloidal NCs synthesized using the hot injection method are shown in Figs. 1(c) and 1(d). For photoluminescence color tuning, two major strategies have been developed to achieve bandgap control of perovskite NCs, one relying on the manipulation of halide compositions 37 and the other through quantum-size effects 57 [Figs. 1(e) and 1(f)]. The excellent photoluminescence properties of perovskite NCs make them highly promising down conversion phosphors for optically pumped WLEDs. After the first demonstration of WLEDs using perovskite NCs in 2015, with a luminous efficiency of 48 lm/W, 48 the device characteristics of WLEDs as well as their stabilities have improved steadily during the last few years, thanks to continuous efforts on material development and device optimization with several notable achievements summarized below.
In 2015, Protesescu et al. reported the colloidal synthesis of CsPbX 3 NCs with narrow emissions (FWHM of 12-42 nm), high PLQEs (50%-90%), and tunable emission color spanning the entire visible region (410-700 nm) through halide mixing and substitution. 37 Following that, perovskite NCs have been widely investigated as light emitters for various optoelectronic devices. [58][59][60][61][62][63][64] Optically pumped WLEDs [schematics shown in Fig. 2(a)] have been a natural application for perovskite NCs, with the utilization of their exceptional photoluminescence properties. For instance, Dong and colleagues reported the preparation of brightly luminescent and color-tunable colloidal CH 3 NH 3 PbX 3 NCs with high PLQEs via a LARP strategy. Optically pumped WLEDs using green emitting CH 3 NH 3 PbBr 3 quantum dots and red emitting K 2 SiF 6 :Mn 4+ (KSF:Mn) phosphors as color converters showed an optimized luminous efficiency of 48 lm/W at a driving current of 4.9 mA. 48 Due to the limited thermal stability of organic-inorganic perovskite NCs, all-inorganic CsPbX 3 perovskites have been widely considered to be more suitable and promising for lighting applications. Meyns and co-workers have used poly(maleic anhydride-alt-1-octadecene) (PMA) coated CsPbX 3 NCs to demonstrate WLEDs with 4.5 lm/W luminous efficiency, a CRI of 72.4, and a CCT of 3665 K [ Fig. 2(b)]. 59 Because of the narrow emissions of perovskite NCs with low spectral coverage, the white emission had a less-than-optimal CRI. Moreover, the red emitting NCs based on iodide anions experienced photo-induced phase transition from a highly emissive cubic phase to less emissive tetragonal and orthorhombic phases. This, in turn, led to spectral instability in the fabricated WLEDs and significant changes in CRI and CCT values. To address the limited spectral coverage and low CRI values, six-color multipackage WLEDs were demonstrated with a luminous efficiency of 62 lm/W, a CRI of 96, and a CCT of 6500 K [ Fig. 2(c)]. 60 Zhou and co-workers showed WLEDs based on pure all-inorganic red emitting perovskite NCs combined with YAG:Ce, exhibiting warm white light with a CCT of 3328 K and a CRI of 84.7. 62 Similarly, Zhang et al. used green perovskite NCs along with KSF:Mn to demonstrate WLEDs with color coordinates of (0.271,0.232). 61 In the above discussed cases, non-perovskite phosphors have often been used together with perovskite NCs to achieve white emission. Alternatively, dual emission can be realized by doping other metals in perovskite NCs, such as Mn 2+ , Bi 3+ , and Eu 2+ . [65][66][67][68][69][70][71] Chen et al. exploited the exciton energy transfer from a CsPb(Cl/Br) 3 host to a Mn 2+ dopant to synthesize blue LED excitable SiO 2 coated green and orange NCs, and subsequently fabricated WLEDs with a tunable CCT (5942 K-6636 K) and CRI (82.7-84.6) and a maximum luminous efficiency of 40 lm/W [ Fig. 2(d)]. 72  (Cl/Br) perovskite nanoplatelets. 66 68 Although great strides have been made in incorporating perovskite NCs into WLEDs, there are still many issues and challenges, including thermal instability, particle aggregation, anion exchange, ligand dissociation, and so on. To address these issues, various surface treatments have been developed for perovskite NCs. Wang and colleagues introduced the small molecule 1-tetradecylphosphonic acid (TDPA) as a coating agent to improve the stability of CsPbBr 3 NCs. The resulting NCs not only retained a high PLQE but also exhibited better moisture and thermal stability. The WLEDs, using green-emitting CsPbBr 3 /TDPA NCs and red-emitting KSF:Mn phosphors as down converters, showed a high luminous efficiency of 63 lm/W. 73 In addition, the WLEDs showed high device stability, where 90% of the luminous efficiency was retained for 15 h operating under a relative humidity (RH) of 60% [ Fig. 2 74 Huang's group reported water resistant perovskite NC powders via surface protection of CsPbX 3 with a polyhedral oligomeric silsesquioxane (POSS), which prevents anion exchange between perovskite NCs of different compositions, preserving their distinct emission. 75 The resulting WLEDs based on mixtures of green emitting POSS-CsPbBr 3 and red-emitting POSS-CsPb(Br/I) 3 nanocrystal powders showed CIE coordinates of (0.349, 0.383), a CRI value of 81, and a luminous efficiency of 14.1 lm/W. The use of other polymeric coatings, such as poly(maleic anhydride-alt-1-octadecene) (PMA) 59 and poly(maleic anhydridealt-1-octadecene) (PMAO), 76 has also been reported. Interestingly, by using perovskite NCs as photointiators for polymerization of high molecular weight polymers, Wong et al. reported highly PERSPECTIVE scitation.org/journal/apm efficient perovskite-polymer nanocomposites as down-conversion layers. 77 Silica coating has also been found to be an effective way to improve the stability of perovskite NCs against moisture ingress. Liu and co-workers synthesized silica-coated perovskite NCs, which showed increased stability under ambient conditions. 78 By using those nanocomposites as down converters, WLEDs with CIE coordinates of (0.31, 0.34) and a CRI of 85 were fabricated. He and colleagues demonstrated stable CsPbX 3 nanocomposites by the in situ anchoring of CsPbX 3 NCs onto octadecyl-/propylamine-capped silica nanoparticles (SiO 2 NPs). WLEDs fabricated using this strategy featured CIE color coordinates of (0.3225, 0.3468) and a luminous efficiency of 26.3 lm/W. 79

III. LOW-DIMENSIONAL ORGANIC METAL HALIDE HYBRIDS: BROADBAND EMITTERS AS DOWN-CONVERSION PHOSPHORS
Materials that exhibit intrinsic white emissions are of great interest for optically pumped WLEDs. The use of such materials has the potential to overcome problems observed in the conventional optically pumped WLEDs based on phosphor mixtures, such as efficiency losses due to cross and self-absorption and the change of the white light quality over time due to differential aging of mixed phosphors. 9,84 Recently, low-dimensional metal halide hybrids have been demonstrated to show broadband white emission suitable for WLED applications. Low-dimensional metal halide hybrids are bulk assemblies of metal halide polyhedrons into sheets, wires, and molecular/cluster species isolated from each other by large organic cations at the molecular level. 41 Single crystalline low-dimensional metal halide hybrids can show the intrinsic properties of individual metal halide sheets, chains, or polyhedra, as a result of strong quantum confinement and site isolation.  85 This 2D organic metal halide hybrid displayed broadband white emission with CIE coordinates of (0.33, 0.39), a CCT of 5509 K, and a CRI of 81. The emission profile displayed a broad spectrum (FWHM = 208 nm) attributed to emission from self-trapped states and a small shoulder peak at higher energy coming from free exciton recombination. Although the white light quality was promising, this material suffered from a low PLQE (2%). Similarly, a white light emitting 2D organic metal halide hybrid, composed of [PbBr 4 ] 2− sheets and cis-1,3bis(methylaminohydrobromide)cyclohexane (CyBMA 2+ ) cations, was reported by Neogi and colleagues. 86 The authors speculated the "softness" and flexibility of organic spacer cations lead to strong exciton-phonon interactions to yield self-trapped excitons. Radiative decays of self-trapped excitons as well as free excitons were believed to be responsible for the broad white emission at room temperature. The PLQE of this white emission was reported to be around 1.5%. Although phenethylammonium (C 6 H5C 2 H 4 NH 3 + ) has been used to form [PbBr 4 ] 2− and [PbI 4 ] 2− inorganic sheets with narrow emissions, surprisingly, its combination with [PbCl 4 ] 2− sheets has been shown to produce a broad white emission. 87 The broad emission was attributed to self-trapping caused by distortion in the organic spacers. Similarly, Wang et al. demonstrated an ultra-high CRI (≈91) white emission and a higher PLQE of 3.37% by using (1, 2mept = 2-methyl-1,5-diaminopentane) (2mept 2+ ) to create 2D metal halide sheets of [PbBr 4 ] 2− . 88 <100> oriented 2D organic metal halide hybrids were also obtained by varying the position of methoxy groups on methoxy benzylammonium at ortho, meta, and para positions. 89 Large lattice distortion and exciton self-trapping were demonstrated by using the ortho isomer of methoxy benzylammonium to form [PbBr 4 ] 2− 2D sheets, which led to broad white emission. Optically pumped WLEDs fabricated using this material as a phosphor showed a high color rendering index (CRI, Ra = 86) and a luminous efficiency of 16 lm/W.

Corrugated 2D (<110 > oriented) metal halide hybrids
Recently, corrugated-2D lead halide perovskites have emerged as another highly promising class of single component white emitters. These materials are achieved by slicing the 3D cubic ABX 3 perovskite structure along the (110) crystallographic plane. In 2014, Dohner et al. 90 reported corrugated-2D lead halides (N-MEDA) PbBr 4 (N-MEDA = N-methylethane-1,2-diammonium) that displayed a broadband emission with a maximum around 560 nm from self-trapped excitons and a shoulder at higher energy from free excitons upon UV-excitation. Substituting a small amount of Br with Cl in (N-MEDA)PbBr 4−x Clx tuned the color of the emission to achieve "warm" and "cold" white light with PLQEs between 0.5% and 1.5% and improved the CRI from 82 to 85. Later, the same group reported another corrugated-2D structure (EDBE)PbBr 4 (EDBE = 2,2 ′ (ethylenedioxy)bis(ethylammonium)) that also shows a white light emission but with a higher PLQE of 9% and a CRI of 84 [ Figs. 3(a) and 3(b)]. 85 Our group employed a facile one pot synthesis to prepare microcrystalline (EDBE)PbBr 4 that possesses a PLQE of 18%, much higher than that of bulk crystal counterparts. The microscale crystals that can be suspended in solutions also have much better film processability than bulk crystals. 91 A UVpumped WLED based on these microcrystals was demonstrated, which showed a cold white light with a CIE of (0. 30 The intriguing optical properties of 2D and corrugated-2D metal halide hybrids have been investigated using structural, spectroscopic, and theoretical studies to reveal the origin of the broad white emission. [94][95][96][97] Because of the soft and deformable nature of the inorganic lattices, strong exciton-phonon interactions are present in these materials during photoexcitation. 94 This exciton-phonon coupling leads to the formation of self-trapped states and multiple color centers in the form of polaron-excitons localized in the inorganic lattice. 96 Radiative decay of the excited state via these selftrapped intra-bandgap emissive states leads to the observed large Stokes shift and emission broadening. 98 The excited state dynamics of these systems is illustrated in the configurational coordinate diagram in Fig. 3(c). Moreover, the increase in photoluminescence intensity with excitation power without saturation indicates the transient nature of these self-trapped states. The presence of emissive intra-gap states was also confirmed through transient absorption spectroscopy which showed a photo-induced absorption band covering the whole of the visible spectrum. 96 In-depth treatment of the excited state dynamics in these materials is given elsewhere. 98,99 More detailed reviews on layered halides with white emission can also be found in pervious reviews. 99,100 Further investigations into this class of materials are needed to develop effective means to control the emissions and improve the PLQEs and stabilities, with which optically pumped WLEDs with a higher performance would be possible.

B. One-dimensional (1D) organic metal halide hybrids
One-dimensional (1D) organic metal halide hybrids are bulk assemblies of metal halide wires/chains/tubes made up of corner, edge, or face sharing metal halide octahedra. In these materials, the 1D metal halides are isolated from each other by bulky organic cations, leading to strong quantum confinement in two dimensions. Our group reported the first luminescent 1D organic metal halide hybrid, C 4 N 2 H 14 PbBr 4 , in 2017, 101 104 More recently, Gautier and colleagues reported two broadband white emitting 1D organic metal halide hybrids based on PbBr 6 4− octahedrons and different piperazine based organic cations. 105 Although the 1,4-bis(3-aminopropyl)piperazine (BAPP) based corrugated (110) 1D organic metal halide hybrid has a high CRI of 87, the PLQE is limited to 1.5%. In contrast, the trans-2,5dimethylpiperazine (TDMP) based post-perovskite type 1D organic metal halide hybrid has a relatively low CRI of 75, but a record high PLQE of 45% [ Fig. 3(i)].
The broadband emissions from 1D organic metal halide hybrids are attributed to self-trapped excitons due to strong excitonphonon interactions, similar to those of 2D and corrugated-2D structures. Self-trapping in 1D organic metal halide hybrids is considered to be easier, as compared to 2D and corrugated 2D metal halide hybrids, because of the stronger quantum confinement and lower deformation energy required. 106 The choice of organic cation, halide anion, and connectivity of the octahedrons is also believed to impact the self-trapping depth and PLQEs of 1D organic metal halide hybrids. While the recent developments of white emitting 1D organic metal halide hybrids are encouraging, rational design principles are still not available to guide the preparation of materials with high PLQEs needed for WLED applications. Moreover, device integration has not received much attention using these materials, which requires further exploration.
C. Zero-dimensional (0D) organic metal halide hybrids with broadband emissions 0D organic metal halide hybrids are ionically bonded materials, in which isolated metal halide photoactive species are periodically embedded in an organic matrix to form a perfect "hostguest" structure [ Fig. 4(a)]. 107 Since 2017, our group has developed a series of highly luminescent 0D organic metal halide hybrids with tunable emission colors, by using various metal halides and organic cations. [108][109][110][111] For instance, 0D (C 4 N 2 H 14 Br) 4 SnBr 6 , 109 (Ph 4 P) 2 SbCl5, 111 and (C 9 NH 20 ) 2 SnBr 4 110 have been prepared to exhibit yellow, orange, and deep red emissions with PLQEs of up to near-unity [ Fig. 4(b)]. The photoluminescence mechanism of these materials, with no electronic band formation between individual metal halide species, could be explained in Fig. 4(c), i.e., strongly Stokes shifted emissions are observed from low energy distorted excited states after excited state structural reorganization. 109,112 Through structural and compositional control of these 0D organic metal halide hybrids, we have demonstrated emission color tuning spanning the whole visible spectrum [ Fig. 4(d)]. Allinorganic 0D metal halides that display a highly Stokes-shifted broadband emission have also been reported recently. For instance, Benin's group synthesized Cs 4 SnBr 6 all-inorganic 0D metal halide crystals with broadband room-temperature emission centered at 540 nm. 113 By varying both the A-site cation and the halide component, they were able to demonstrate tunable emission with peaks in the range of 500 nm-600 nm. Similarly, Zhou et al. reported a red emitting Cs 2 InBr5⋅H 2 O inorganic 0D metal halide with highly Stokes-shifted broadband emission and a PLQE of 33%. 114 Besides 0D organic metal halide hybrids containing mononuclear polyhedrons, our group has recently extended this 0D organic metal halide phase space to bulk assemblies of metal halide clusters, in which multiple metal halide polyhedrons are fused together to form (bmpy: 1-butyl-1-methylpyrrolidinium, C 9 NH 20 + ) to green (peaked at 512 nm with a near-unity PLQE) in (bmpy) 9 117 The unique photophysical properties with highly efficient broadband emissions and little-to-no self-absorption, as well as good photostability of these 0D organic metal halide hybrids, make them of great interest for optically pumped WLEDs. 41 The first demonstration of WLEDs using 0D organic metal halide hybrids involved yellow emitting (C 4 N 2 H 14 Br) 4 SnBr 6 blended with a commercial blue emitting europium-doped barium magnesium aluminate (BaMgAl 10 O 17 :Eu 2+ ) in a polydimethylsiloxane (PDMS) matrix. By varying the relative concentration of the yellow and blue phosphors, the white emissions can be tuned with CCTs ranging between warm and cold. Balanced emissions between (C 4 N 2 H 14 Br) 4 SnBr 6 and BaMgAl 10 O 17 :Eu 2+ generated a white emission with CIE coordinates of (0.35, 0.39), a CCT of 4946 K, and a CRI of 70. These WLEDs exhibited decent stability under varying operating current and ambient conditions. Replacing (C 4 N 2 H 14 Br) 4 SnBr 6 by (C 4 N 2 H 14 Br) 4 SnBrxI 6−x (x = 3), WLEDs with high CRIs of up to 85 have been demonstrated, as a result of increased coverage in the red spectral region. 118 Our group has also demonstrated a proofof-concept WLED by combining red, yellow, and green emitting 0D organic metal halide hybrids with the blue emitting commercial phosphor BaMgAl 10 O 17 :Eu 2+ . 119 Several devices were fabricated by varying the ratio of the different phosphors to obtain a white emission with CCT ranging from 3000 K to 6000 K [ Fig. 4(e)]. Because of the broadband emission from the 0D organic metal halide hybrids, full coverage of the visible spectrum was easily achieved to mimic emission from a blackbody radiator. The champion device had CIE coordinates of (0.3782, 0.3722) with an ultra-high CRI (99), color quality scale (99), and R9 (99) 120 0D organic metal halide hybrids are becoming the new rivals to the established phosphors because of their near unity PLQEs, color tunability, little to no self-absorption, simple wet-chemistry synthesis, and broadband emissions with a large FWHM. Moreover, most of the 0D metal halide hybrids reported to date are lead-free and based on earth abundant components. These easily prepared phosphors have the potential to enable the development of WLEDs with performances comparable or superior to commercially available lighting sources. 38

IV. DOUBLE PEROVSKITES
In search of lead-free, stable, and efficient perovskite emitters, isovalent substitution of Pb 2+ by Sn 2+ and Ge 2+ has been explored. However, poor electronic and optical performances of lead-free isovalent metal ion substituted perovskites leave a lot to be desired. Instead, double perovskites, obtained through the heterovalent substitution of Pb 2+ by two metal ions [ Fig. 4(g)], have captured attention as an alternative route to lead-free perovskites. This class of materials has a general formula of A 2 BB'X 6 (where A is a monovalent cation, B and B' are heterovalent metal ions, and X is a halide ion) and can display a broad emission from self-trapped excited states.
White emission based on lead-free perovskites was recently reported by Luo et al., where they prepared a lead-free double perovskite which exhibits an efficient and stable white-light emission resulting from the Jahn-Teller distortion of the AgCl 6 octahedron in the excited state. 121 By optimizing the Na + content, the authors showed the highest ever reported PLQE (86% ± 5%) for a single white light emitting material [ Fig. 4(h)]. The WLED based on optimally alloyed Cs 2 (Ag 0.6 Na 0.4 )InCl 6 with 0.04% bismuth doping has CIE coordinates of (0.396, 0.448) with a CCT of 4054 K, and negligible degradation when operated at about 5000 cd m −2 for over 1000 h in air [ Fig. 4(i)]. The same group also demonstrated the colloidal synthesis of Cs 2 Ag 1−x NaxIn 1−y BiyCl 6 NCs. The resulting NCs showed emission color temperatures that can be tuned from 9759.7 K to 4429.2 K by Na + and Bi 3+ incorporation. 122,123 Although white emitting double perovskites present a promising alternative to inorganic phosphors, their lack of absorption within the visible region limits their application to only UV LED based WLEDs, which show much lower device performance compared to those based on blue LEDs. Thorough studies by using double perovskites in optically pumped WLEDs are still lacking to date.

V. SUMMARY AND OUTLOOK
Metal halide perovskites and low-dimensional metal halide hybrids have shown great promise as down conversion phosphors for optically pumped WLEDs. Thus far, perovskite NCs, double perovskites, and various types of low-dimensional metal halide hybrids have been demonstrated to possess desirable traits of ideal phosphors, such as high PLQE, good white light quality, and facile processability (Table II). However, there still remain many challenges to overcome before we can replace commercially successful inorganic phosphors with these emerging materials to achieve highly efficient optically pumped WLEDs with long lifetimes. Further research and development are needed to achieve ideal phosphors to meet all the following requirements. The raw materials used to synthesize the phosphors should be earth abundant, low-cost, and eco-friendly; preparation of phosphors in high yield and low-cost energy efficient ways should be possible; the PLQEs should be as high as near-unity and the emissions should be highly tunable; the phosphors should have environmental, thermal, and photostability as good as conventional inorganic phosphors to ensure durability; and the integration of new phosphors into optically pumped WLEDs should be compatible with existing mass production processes. Moving forward, we expect that future efforts could be directed toward addressing the following issues.
• Environmental, thermal, and photostability: The stability of metal halide perovskites and low-dimensional metal halide hybrids still leaves a lot to be desired. This is partially because of the ionic nature of the crystal structures that has a much lower formation energy and softer lattice than conventional covalently bonded inorganic materials. The use of various passivation and encapsulation techniques has been shown to reduce their degradation in the ambient atmosphere. Moreover, using all-inorganic perovskites has PERSPECTIVE scitation.org/journal/apm also proven to increase their thermal stability. Photostability of mixed halide perovskite NCs is another concern, as photoinduced ion migration has been shown to result in phase segregation and spectral instability. Further investigations are needed in the areas of nanocrystal passivation and ion migration suppression to make stable perovskite NCs suitable for optically pumped WLED applications. One promising approach to improving the stability is inorganic surface passivation of perovskite NCs. The success of inorganic core-shell quantum dots is a good example of how inorganic passivation can increase thermal and environmental stabilities. Moreover, unique device architectures such as remote phosphor WLEDs can mitigate thermal stress on perovskite phosphors. • Efficient lead-free perovskite NCs: Thus far, perovskite NCs with a near-unity PLQE have all been based on lead halide systems. However, the toxicity of these materials is a major concern that must be overcome before commercialization. The success in obtaining highly efficient lead-free double perovskites and 0D metal halide hybrids is encouraging. Besides traditional trial and error approaches, searching for lead-free metal halide perovskites with direct band narrow emissions might be facilitated by computation supported combinatorial chemistry, in which screening of a large number of possible element combinations with tunable band structures could be realized. [124][125][126] • Emission tuning for better control of CRI and CCT: One obstacle in using highly efficient perovskite NCs for optically pumped WLEDs stems from their narrow emissions. Although the combination of blue, green, and red emitting perovskite NCs can yield white emissions, the lack of full coverage of the visible spectrum leads to poor white light quality for lighting applications. The inclusion of multiple emitters to obtain a good white light quality is also cumbersome. Broadband emissions from low-dimensional metal halide hybrids could possibly be more favorable for this reason. On the other hand, the gain in color quality through the use of broadband emitters presents efficiency tradeoffs due to spillover of light into the deep-red region. For this reason, a combination of a broad emission in the greenyellow region and a narrow red emission might be the best compromise to obtain a good color quality as well as high luminous efficiency, when blue LEDs are used as the optical pump source. • Absorption of phosphors: Because of the strong absorbance of metal halide perovskites and low-dimensional organic metal halide hybrids in the UV region, UV LEDs (365 nm) can serve as the common pump sources for downconversion WLEDs. However, the low efficiency of UV LEDs, compared to blue LEDs, results in great efficiency loss. The use of more efficient near-UV and deep blue LEDs (for instance 405 nm) would address this problem but requires materials to have appropriate Stokes shifts and high absorption in this spectral region. So far, neither metal halide perovskites nor low dimensional metal halide hybrids have ideal Stokes shifts, i.e., the former too small with strong overlaps between absorption and emission resulting in cross-/selfabsorption energy losses and the latter too large with absorption band edges within the UV region. Future efforts could be directed toward visible light excitable materials with a reasonable Stokes shift. • PLQE of low-dimensional organic metal halide hybrid white emitters: Although low-dimensional organic metal halide hybrids with intrinsic white emissions show many advantages, such as a large Stokes shift reducing reabsorption losses and size-independent optical properties, for use in optically pumped WLEDs, their PLQEs are still too low. There is still a lack of fundamental understanding of the radiative and non-radiative processes in these materials. Also, no rational design principles are available yet to increase the PLQEs or control the broadband emissions of this class of materials. Detailed studies of the excited state structural and optical dynamics are therefore needed, which could be achieved by using ultrafast X-ray and optical spectroscopies. • Device engineering: With the above issues on the material side being addressed, comprehensive device engineering studies with focuses on the suitability of various phosphor encapsulation techniques and device architectures would be necessary to bring the luminous efficiency of metal halide perovskite and low-dimensional organic metal halide hybrid based WLEDs up to par with the state-of-the-art WLEDs.