German Edition: DOI: 10.1002/ange.201904862Lead-Free PerovskitesInternational Edition: DOI: 10.1002/anie.201904862

Lead-Free Halide Perovskite Nanocrystals: CrystalStructures, Synthesis, Stabilities, and Optical PropertiesQianqian Fan+, Gill V. Biesold-McGee+, Jianzhong Ma,* Qunna Xu, Shuang Pan,Juan Peng,* and Zhiqun Lin*

AngewandteChemie

Keywords:lead-free perovskites ·optical properties ·nanocrystals

AngewandteChemieReviews

1030 www.angewandte.org T 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2020, 59, 1030 – 1046

1. Introduction

Lead halide perovskites (LHPs), which have a generalformula of APbX3 (where A = CH3NH3

+ (MA), CH(NH2)2+

(FA), or Cs+; X = Cl, Br, or I), have been explored since themiddle of the 20th century.[1–5] Within the past decade, LHPshave received much attention owing to its set of intriguingproperties, including strong optical absorption, low excitonbinding energy, long diffusion lengths, high carrier mobility,and facile preparation.[6–12] Remarkably, within just 9 years,the power conversion efficiency (PCE) of photovoltaicdevices using LHPs has been rapidly improved from 3.8%to 24.2 %.[13–25] Additional study has already demonstrated thefeasibility of achieving over 25% PCE.[26] Recently, exten-sions of LHPs have also enriched the field of colloidalsemiconductor NCs.

First discovered in 2014, LHP NCs are of great interestbecause the bright, narrow-band photoluminescence (PL) ofthis new class of nanomaterials can be readily tuned fromultraviolet to near-infrared wavelengths simply by tailoringeither the halide composition or size of NCs.[27–33] A notableadvantage of LHP NCs over conventional semiconductorNCs (such as CdSe and InP) is their defect tolerance (i.e., theapparently benign nature of structural defects with respect tooptical and electronic properties).[34–36] Great effort has beendevoted to synthesizing various LHP NCs, investigating theirphotophysical properties, and exploiting them for use inoptoelectronic or photovoltaic devices.[37–41]

Despite the outstanding advantages described above, thepresence of lead in perovskite NCs raises obvious toxicityconcerns upon introduction of these materials into consumerelectronics. Currently, the use of hazardous heavy metals,including lead, in electronic devices is regulated in theEuropean Union, with many other countries expected to

introduce similar regulations soon.Therefore, the ability to develop lead-free halide perovskite (LFHP) NCsthat retain outstanding attributes seenin LHP NCs is of great fundamentaland practical significance. Much efforthas been centered on exploring LFHPanalogues, and all possible lead

replacement candidates have been summarized inFigure 1.[42,43] To this end, SnII, SnIV, SbIII, BiIII, PdIV, CuII,InIII, and AgI have been employed to replace Pb and yieldLFHP NCs. There have been several Reviews and perspec-tives focusing on LFHP thin films in literature.[42–48] It isnotable that only three Reviews have been publishedpertaining to LFHP NCs. The first Review concentrates onlight-emitting device applications of LFHP NCs.[49] Thesecond is a Minireview that provides a brief introduction onsynthesis, stability and application of currently studied LFHPNCs with a few insights into crystal structure, and opticalproperties.[50] The third only gives an overview of the band-gap engineering and stability of double LFHP NCs.[51]

In recent years, there have been rapid advances in the synthesis of leadhalide perovskite nanocrystals (NCs) for use in solar cells, lightemitting diodes, lasers, and photodetectors. These compounds havea set of intriguing optical, excitonic, and charge transport properties,including outstanding photoluminescence quantum yield (PLQY) andtunable optical band gap. However, the necessary inclusion of lead,a toxic element, raises a critical concern for future commercialdevelopment. To address the toxicity issue, intense recent researcheffort has been devoted to developing lead-free halide perovskite(LFHP) NCs. In this Review, we present a comprehensive overview ofcurrently explored LFHP NCs with an emphasis on their crystalstructures, synthesis, optical properties, and environmental stabilities(e.g., UV, heat, and moisture resistance). In addition, strategies forenhancing optical properties and stabilities of LFHP NCs as well asthe state-of-the-art applications are discussed. With the perspective oftheir properties and current challenges, we provide an outlook forfuture directions in this rapidly evolving field to achieve high-qualityLFHP NCs for a broader range of fundamental research and practicalapplications.

From the Contents

1. Introduction 1031

2. Crystal Structure of Lead-FreeHalide Perovskite Nanocrystals 1032

3. Lead-Free Halide PerovskiteNanocrystals: Synthesis,Stabilities to Applications 1034

4. Optical Properties of Lead-FreeHalide Perovskite Nanocrystals 1039

5. Conclusions and Outlook 1042

[*] Q. Fan,[+] Prof. J. Ma, Prof. Q. XuCollege of Bioresources Chemical and Materials EngineeringShaanxi University of Science & TechnologyXi’an 710021 (P. R. China)E-mail: majz@sust.edu.cn

Q. Fan,[+] G. V. Biesold-McGee,[+] S. Pan, Prof. Dr. Z. LinSchool of Materials Science and EngineeringGeorgia Institute of TechnologyAtlanta, GA 30332 (USA)E-mail: zhiqun.lin@mse.gatech.edu

S. Pan, Prof. J. PengState Key Laboratory of Molecular Engineering of PolymersDepartment of Macromolecular ScienceFudan UniversityShanghai 200438 (P. R. China)E-mail: juanpeng@fudan.edu.cn

[++] These authors contributed equally.

The ORCID identification number(s) for the author(s) of this articlecan be found under:https://doi.org/10.1002/anie.201904862.

AngewandteChemieReviews

1031Angew. Chem. Int. Ed. 2020, 59, 1030 – 1046 T 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

In this Review, we aim to summarize the recent advancesin widely studied LFHP NCs and delve into the progress inunderstanding their crystal structure, synthesis method,optical properties, and environmental stability (e.g., UV,heat, and moisture resistance). Moreover, strategies forcontrolling and enhancing optical and optoelectronic proper-ties as well as stabilities of LFHP NCs are discussed. Theapplications of LFHP NCs are also provided. Finally, currentand future challenges facing LFHP NCs as well as an outlookof further potential applications are present.

2. Crystal Structure of Lead-Free Halide PerovskiteNanocrystals

Typically, the APbX3 structure of LHPs is cubic, consistingof a three-dimensional (3D) network of corner-sharing[PbX6]

4@ octahedra, in which Pb2+ ions are located at thecenter of the octahedra, while the large cavity betweenadjacent octahedra is occupied by A+ ions (Figure2 a). In thecase of LFHP NCs, the crystal structure depends primarily onsubstitutions of B sites (e.g., Pb sites as in LHP), includingGroup 14 elements (Sn and Ge), Group 15 elements adjacentto Pb (Sb and Bi), and double elements (Bi combined withAg).[48] Various substitutions offer a wide diversity of crystalstructures for LFHP NCs. The crystal structures of currentlywidely studied LFHP NCs are summarized as follows.

2.1. Group 14 Elements

Among commonly studied Pb substitutions, only Sn andGe (i.e., divalent Group 14 elements) can form the traditionalperovskite structure because they both fulfill the coordinationand charge balance prerequisites.[52] Tin-based halide perov-skite (ASnX3) NCs were first explored as analogues to LHPNCs because of their comparable ionic radius of Sn2+ (1.35 cfor Sn2+ and 1.49 c for Pb2+), thus avoiding significant latticeperturbation induced by substitution.[48]

Unlike the orthorhombic phase of CsPbX3 (Figure 2b),[53]

CsSnCl3 NCs adopt the cubic perovskite phase (Pm3m), whileCsSnX3 containing bromide and iodide ions produce a lowersymmetry orthorhombic phase (Pnma), as shown in Fig-ure 2c.[54] This observation is consistent with that reported forthe parent bulk materials.[55] Some studies indicate thatCsSnBr3 quantum rods also belongs to the cubic system,

Qianqian Fan is a PhD student in theCollege of Bioresources Chemical and Mate-rials Engineering at Shaanxi University ofScience & Technology under the supervisionof Prof. Jianzhong Ma, and has been a visit-ing student in Prof. Zhiqun Lin’s group atthe Georgia Institute of Technology since2017. Her research interests focus on perov-skite quantum dots and organic–inorganicnanocomposites.

Gill Biesold-McGee is a PhD student in theMaterials Science and Engineering depart-ment at Georgia Institute of Technology. Hereceived his B.S. in Materials Science andEngineering from Clemson University in2017. His current research is focused onquantum dots and their potential applica-tions in lasing and sensing.

Jianzhong Ma is a Professor in the College ofBioresources Chemical and Materials Engi-neering at Shaanxi University of Science &Technology. He received his PhD in PolymerChemistry and Physics from Zhejiang Uni-versity in 1998. His research interests includefunctional inorganic nanoparticles andorganic–inorganic nanocomposites.

Qunna Xu is an Associate Professor in theCollege of Bioresources Chemical and Mate-rials Engineering at Shaanxi University ofScience & Technology. She received her PhDin Bioresourses Chemicals and MaterialsEngineering from the Shaanxi University ofScience & Technology in 2013. Her researchinterests include design of nanostructuralinorganic nanoparticles, organic–inorganicnanocomposites, multifunctional coatings.

Figure 1. Candidates to replace lead in perovskite-type compounds.Reproduced with permission.[42] Copyright 2016, American ChemicalSociety. Candidates for double perovskite A2B

+B3+X6 have been markedseparately. Reproduced with permission.[43] Copyright 2017, Springer.

AngewandteChemieReviews

1032 www.angewandte.org T 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2020, 59, 1030 – 1046

which conflicts with previous findings, suggesting a need forfurther exploration.[56] To date, there have been no reports on

FASnI3 (FA: formamidinium) NCs. However, the FASnI3

crystalline film is assigned the orthorhombic (Amm2) crystalstructure and MASnI3 film adopts the highest symmetryphase (a-phase).[57, 58] For SnII-based perovskites, the easyoxidation of Sn2+ to Sn4+ is a large concern for their stability.Substitution of Sn2+ with the more-stable Sn4+ has beenadopted as an attractive option, though maintaining chargebalance requires modification to the composition and crystalstructure of the SnII-based perovskite. A typical example ofthis substitution is Cs2SnCl6. The crystal structure of Cs2SnCl6

NCs is derived from the 3D perovskite CsSnCl3 by removingevery second Sn layer along (111), as shown in Figure 2d.[59]

Ge, another Group 14 metal, is a new candidate forreplacing Pb in halide perovskites. Ge-based halide perov-skites possess a formula of AGeX3. There has been only onereport on CsGeX3 NCs, that is, CsGeI3 NCs that crystallize inthe rhombohedral structure (R3m),[60] as depicted in Fig-ure 2e. CsGeCl3 and CsGeBr3 were only explored in theirbulk form, which both undergo a pressure-induced phasetransition to a primitive cubic phase at room temperature.Conversely, the CsGeI3 crystal does not encounter phasetransitions during low-temperature solution growth.[61, 62]

2.2. Neighboring Elements of Pb

Rather than being restricted to Group 14 elements,another possible solution for LFHP is to replace Pb withneighboring elements, such as Bi and Sb. Because of theirhigher trivalent oxidation state, A3B2X9 becomes the stablestoichiometry. There are two main polymorphs of A3B2X9

materials which are determined by the preparation methods,A cations, and halide compositions. The first polymorph isa hexagonal phase consisting of zero-dimensional (0D) face-sharing bioctahedral [B2X9]

3@ (dimer phase). The second isthe two-dimensional (2D) layered perovskite structure (lay-ered phase) of corrugated layers with partially corner sharing[BX6]

3@ octahedra, which can be formed by removing 1/3 ofthe B sites from an 3D parent perovskite.[63–65] Notably,introduction of smaller Rb+ and NH4

+ as A cations caneffectively stabilize the structure in its layered form.[64,67]

In the case of Bi-based halide perovskite, Cs3Bi2I9 NCsand MA3Bi2I9 NCs both exhibit a 0D structure with face-sharing bismuth iodide octahedra. Replacing Cs+ or MA+

with smaller ions (e.g., Rb+, NH4+, and K+) tends to form

layered perovskite structure with corner-sharing [BX6]3@

Dr. Shuang Pan is a currently postdoctoralfellow in the Department of MacromolecularScience at Fudan University. She has beena visiting researcher in Prof. Zhiqun Lin’sgroup at the Georgia Institute of Technologysince 2016. She received her PhD in Macro-molecular Science from Fudan University in2018. Her current research focuses on syn-thesis, self-assembly, and patterning of per-ovskite materials for use in LEDs and photo-detectors.

Juan Peng is a Professor in the Departmentof Macromolecular Science at Fudan Univer-sity. She received her PhD in State KeyLaboratory of Polymer Physics and Chemis-try from Changchun Institute of AppliedChemistry, Chinese Academy of Sciences in2005. Her research interests include conju-gated polymers, block copolymers, polymer-based nanocomposites, hierarchical struc-tures, and functional nanocrystals.

Zhiqun Lin is a Professor in the School ofMaterials Science and Engineering at theGeorgia Institute of Technology. He receivedhis PhD in Polymer Science and Engineeringfrom the University of Massachusetts,Amherst in 2002. His research interestsinclude perovskite and polymer solar cells,photocatalysis, electrocatalysis, batteries,thermoelectrics, and conjugated polymers.

Figure 2. a) The cubic LHP structure (A = MA, FA). b) The orthorhom-bic LHP structure (A = Cs). (a) and (b) both reproduced with permis-sion.[53] Copyright 2017, Science Publishing Group. c) Cubic structureof CsSnCl3 NCs. Reproduced with permission.[54] Copyright 2016,American Chemical Society. d) Crystal structure of Cs2SnCl6 NCs.Reproduced with permission.[59] Copyright 2018, Wiley-VCH. e) Rhom-bohedral structure of CsGeI3 NCs. Reproduced with permission.[60]

Copyright 2018, Wiley-VCH.

AngewandteChemieReviews

1033Angew. Chem. Int. Ed. 2020, 59, 1030 – 1046 T 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.angewandte.org

octahedra networks (Figure 3a).[69–72] Conversely, Cs3Bi2Br9

and MA3Bi2Br9 NCs both form a 2D structure, whileMA3Bi2Cl9 possess a 1D structure.[73–75] Crystal structureevolutions of parent 3D structure to 2D MA3Bi2Br9 and 1D

MA3Bi2Cl9, respectively, are shown in Figure 3b, illustratinghow MA3Bi2Br9 can be simply viewed as cutting along the(111) direction of 3D parent perovskite structure, with everythird Bi layer removed, thereby forming the 2D structure. Incontrast, MA3Bi2Cl9 is formed by removing every third Bilayer along the (110) direction of parent 3D structure, and, tokeep charge balance, the (001) planes also need to be slicedleaving two-thirds of the Bi atoms, resulting in the final 1Dstructure. Similarly, the crystal structure of the Cs3Sb2Br9 NCsexhibit 2D layered structures, consistent with its bulkmaterial, as illustrated in Figure 4a.[76] A3Sb2I9 (A = MA orCs) film crystallizes in the 0D dimer phase, but Cl incorpo-ration can induce a phase transformation from the 0D dimerphase to the 2D layered phase (Figure 4b). It is notable thatA3Sb2I9 NCs have not yet been reported.[65]

2.3. Monovalent and Trivalent Elements for Pb Substitution

The replacement of Pb by Sb or Bi can reduce thedimensionality of the perovskite structure, leading to reducedband dispersion, increased effective masses of charge carriers,and decreased carrier mobilities.[77] To create 3D LFHP andcircumvent the issues noted above, double halide perovskiteswith B-site substitution by mixed cations (B=B+, B’=B3+) hasrecently been explored to form 3D LFHPs (A2BB’X6), whichis derived from the idea of oxide double perovskites(A2BB’O6). The most popular double halide perovskite isCs2AgBiX6, which can be indexed to the standard cubicperovskite structure.[78] The corresponding crystal structure of

Cs2AgBiX6 is shown in Figure 5, in which every pair ofadjacent B2+ cations are replaced by one Ag+ and one Bi3+

cation in a 3D perovskite structure.[79] The Ag+ and Bi3+ ionslocated on the B and B’ sites are bonded with X@ to form twotypes of regular octahedra which then alternatively link intoa face-centered cubic structure, with larger Cs+ filling intotheir gaps, forming the typical cubic structure. Similar casescan be seen in Cs2AgSbCl6, Cs2AgSbBr6, and Cs2AgInCl6

double perovskite NCs.[80, 81]

3. Lead-Free Halide Perovskite Nanocrystals:Synthesis, Stabilities to Applications

3.1. Tin-Based Halide Perovskite NCs

For Sn-based halide perovskite, the first study on spatiallyconfined NCs was reported in 2016.[54] CsSnX3 perovskite NCswere synthesized via a modified hot injection method by

Figure 3. a) Crystal structure evolution of Cs3Bi2I9 (P63/mmc) andRb3Bi2I9 (P21/n) with empty squares marking the unoccupied metalpositions of the perovskite archetype. Reproduced with permission.[68]

Copyright 2015, American Chemical Society. b) Crystal structure evolu-tion of MA3Bi2Br9 and MA3Bi2Cl9. Reproduced with permission.[73]

Copyright 2018, American Chemical Society.

Figure 4. a) Removal of every third Sb layer along the (111) directionof the CsSbBr3 perovskite structure results in the 2D layered structureof Cs3Sb2Br9. Reproduced with permission.[76] Copyright 2017, Ameri-can Chemical Society. b) Schematic representation of the Cl-addition-induced transformation for A3Sb2I9 from the 0D dimer phase to the 2Dlayered phase. Reproduced with permission.[65] Copyright 2018, Ameri-can Chemical Society.

Figure 5. Polyhedral model of a) the conventional unit cell and b) theprimitive unit cell of Cs2AgBiX6 (X =Cl, Br), the red dashed lines markthe primitive lattice vectors. Reproduced with permission.[79] Copyright2017, American Chemical Society.

AngewandteChemieReviews

1034 www.angewandte.org T 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2020, 59, 1030 – 1046

dissolving SnX2 in the coordinating solvent tri-n-octylphos-phine prior to injection. The long-term stability of CsSnBr3

NCs poses a challenge. CsSnX3 samples were colloidallystable for up to 2 weeks. At longer times, however, someparticles precipitated which is most likely due to the dynamiccharacter of ligand absorption and desorption processes.[82]

Moreover, once exposed to ambient air for only 5 min,oxidation from SnII to SnIV occurred in CsSnX3 samples.[54,83]

MASnBr3@xIx quantum dots (QDs) have been synthesized viaa modified hot injection method.[84] As-synthesized QDs wereused as light harvesters in mesoscopic solar cell, achievingreasonable power conversion efficiency of 8.79% and rapidstart-up behaviors (Figure 6a).[84]

When compared to their bulk counterparts, 2D layeredperovskite solar cells have been reported to exhibit enhancedresistance to air and water exposure owing to the surface

passivation by ligands.[85,86] Similar to the bulk perovskite, thenanoplatelets (NPLs) can be recognized by the formulaL2[ABX3]n@1BX4, where L represents the ligand with + 1oxidation state and n@1 refers to the thickness of NPLs interms of the bulk unit cell, as shown in Figure 6b. TheL components enable colloidal stability and prevent verticalgrowth of NPLs because of their inability to fit within the unitcell. Sn-based perovskite NPLs were synthesized using a non-solvent crystallization method.[87] The alteration of theA cation can significantly impact the NPL stability.[88] Cur-rently, there is no in-depth analysis on the NPL stability. Analternative synthetic approach for 2D CsSnI3 NPLs was alsodeveloped by tuning the capping ligands used during catalyst-free colloidal synthesis. It was found that the combination ofoctylamine, oleylamine with a short-chain organic acid is ofkey importance in directing the formation of thin NPLs(thickness < 4 nm).[89] Ab initio calculations revealed thatalthough the intrinsic defects in CsSnI3 do not introduce deeplevels inside the band gap, their concentration can be quitehigh, thus leading to instability and rearrangement into theorthorhombic phase. To improve the stability of CsSnI3 NPLs,SnI2-complex treatment was used to increase the formationenergy of defects, similar to the role of SnF2 in spin-coatedCsSnI3.

[90,91] In addition, a new A cation, 2-phenylethan-1-aminium (C6H5(CH2)2NH3, PEA), was employed with car-boxylic acid to synthesize strongly coupled 2D PEA2SnI4 (n =

1) NPLs.[92] The presence of carboxylic acid reduces the defectdensity, and the interaction of the aromatic electron on PEAshrinks the distance between PEA2SnI4 NPLs, which furthersignificantly reduces the defect density and improves thestability (Figure 6c). PEA2SnI4 NPLs retain 75 % of theiroriginal emission intensity under continuous 406 nm(10 mW cm@2) illumination for 120 h.

To improve the stability of Sn-based LFHP NCs, CsSnBr3

cubic nanocages were synthesized through a facile colloidalhot-injection approach (Figure 7a),[93] with stannous 2-ethyl-hexanoate and commercial MgBr2 used as the precursor(instead of previous TOP-SnBr2)

[54] and the bromide source,respectively. When exposed to air, the absorption edge of theCsSnBr3 nanocage film decreased slightly with time. Aftera simple treatment with perfluorooctanoic acid (PFOA),nearly no decrease in the absorption onset (ca. 620 nm) of

Figure 6. a) TEM and HR-TEM images of MASnBr2I QDs (upper leftpanel), IPCE curves of solar cells sensitized with MASnBr3@xIx QDs(upper right panel) and schematic diagram of making a MASnBr3@xIx

QDs tailored solar cell device (lower panel). Reproduced with permis-sion.[84] Copyright 2018, Elsevier. b) TEM image (left) and structuralrepresentation of NPLs (n =1; right). Reproduced with permission.[87]

Copyright 2016, American Chemical Society. c) The proposed modelfor strongly coupled PEA2SnI4 NPLs (left), and photostability test forPEA2SnI4 under continuous UV illumination (right). Reproduced withpermission.[92] Copyright 2018, Wiley-VCH.

Figure 7. a) TEM image of CsSnBr3 nanocages (left), typical EDSmapping with STEM-HAADF image and the corresponding elementalmaps of Cs, Sn, and Br (right). b) Absorbance of the CsSnBr3 nano-cube films before and after treated with PFOA under ambientcondition. (a) and (b) both reproduced with permission.[93] Copyright2017, American Chemical Society.

AngewandteChemieReviews

1035Angew. Chem. Int. Ed. 2020, 59, 1030 – 1046 T 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.angewandte.org

CsSnBr3 nanocages film was observed in 16 h under ambientconditions. Moreover, the color of the PFOA-functionalizedfilm remained dark red (Figure 7b). The improved stabilitywas attributed to the strong interaction of F@ (in PFOA) withSn2+, resulting in the inhibition of oxidation. Steric hindranceof bulky ligands on PFOA can also prevent external speciesattack, such as O2 and H2O, thus enhancing the stability of theperovskite structure.[94, 95]

The replacement of Sn2+ to air-stable Sn4+ has beenlargely explored to yield relatively stable Sn-based halideperovskite. Cs2SnI6 NCs were obtained via a facile phosphine-free hot-injection approach.[96] The shapes of Cs2SnI6 NCs canbe tuned from spherical QDs, nanorods (NRs), nanowires(NWs), and nanobelts to NPLs by controlling the reactiontime and ligands (Figure 8a; upper panel). Oleic acid andoleylamine were found to collectively control the crystalliza-tion of Cs2SnI6 NCs. Recently, as-synthesized Cs2SnI6 nano-belts were used to fabricate FET devices, which can beprocessed in air with 60 % humidity under ambient condi-tions. The improved stability of Cs2SnI6 NCs is a greatimprovement for their potential widespread applications inoptoelectronic devices. Moreover, the Cs2SnI6 nanobelt-basedFETs displayed p-type semiconductor behavior with high

hole mobility (> 20 cm2/(Vs)) and high I-ON/I-OFF ratio(> 104) under ambient conditions (Figure 8 a; lower panel). Inaddition, Cs2SnI6 NCs with no organic capping ligands wereprepared via a hot-injection method.[97] It is interesting to notethat Cs2SnI6 NCs with no ligands are colloidally stable up to12 h and can be conveniently deposited into high-quality thinfilms using a simple drop-cast procedure, dispensing with theneed to remove the excess organic materials (e.g., ligands)that was a feature of previous approaches.[53, 97–101] There wereno large holes and cracks in these films, and they also possessa smooth substrate–film interface (Figure 8b), which areadvantageous for their applications in electronic devices.[97]

Recently, Cs2SnI6 NCs with different shapes were synthesizedby a reverse injection process.[102] They were then employed asa colloidal ink to create a stable and fast photodetector withhigh current gain.[102] The fabricated photodetector can beused repeatedly for many cycles, as depicted in Figure 9a.[102]

Impurity doping has been demonstrated to be an effectivestrategy to prepare metal halide perovskites with enhancedphysical properties and stability. This was further verified byshowing that CsPbCl3 NCs doped with Mn2+ demonstrate animproved PLQY of 54 % that is stable for severalmonths.[103–105] Additionally, Bi-doped Cs2SnCl6 NCs witha high PLQY of 78.9 % were first explored and used as theluminescent component in the LED device (Figure 9b).[59]

The stable oxidation state of Sn4+ and the formation ofa BiOCl protective layer upon the moisture exposure

Figure 8. a) Scheme of controlled synthesis of Cs2SnI6 perovskite NCs(upper panel); and schematic illustration of the FET device usingCs2SnI6 nanobelts as the channel materials (lower left panel), andlinear plot of the I–V characteristics (lower right panel) at a drain–source voltage (Vds = 10 mV) where black lines as positive scan andred lines are negative scans (inset shows the Ids–Vgs curve on thenatural logarithmic scale). Reproduced with permission.[96] Copyright2016, American Chemical Society. b) Top-down and cross-sectionalSEM images of the Cs2SnI6 NCs film. Reproduced with permission.[97]

Copyright 2017, American Chemical Society.

Figure 9. a) Schematic diagram of photoresponsive device (left) andits photoswitching behavior of different shaped Cs2SnI6 NCs at biasvoltage 5 V (right). Reproduced with permission.[102] Copyright 2018,Wiley-VCH. b) XRD patterns of undoped and Bi-doped Cs2SnCl6samples (top left), anti-water stability of Cs2SnCl6 :2.75% Bi (top right), and commission international de L’Eclarage (CIE) color coordinatescorresponding to white-LED device (lower panel). Reproduced withpermission.[59] Copyright 2018, Wiley-VCH.

AngewandteChemieReviews

1036 www.angewandte.org T 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2020, 59, 1030 – 1046

imparted good thermal and water stability of the device(Figure 9b; upper panel). The resulting white LED exhibitedhigh warm white light emission with a correlated colortemperature of 4486 K. Similar to Sn2+, Ge2+ is prone to beoxidized to Ge4+; however, investigation into the substitutionof Ge2+ with Ge4+ in halide perovskite NCs has never beenreported.

3.2. Bi-Based Halide Perovskite NCs

It is notable that Pb equivalent substitutions by Group 14elements suffer from easy oxidation. In this regard, A3Bi2X6

has been recognized as a promising candidate because of thehigher trivalent oxidation state of Bi. In 2016, MA3Bi2X9

perovskite QDs were first reported using ligand-assistedreprecipitation (LARP) method.[106] MA3Bi2Br9 QD dis-played high stability in ethanol, while the moisture stabilitywas not satisfactory, and white precipitates of BiOBr werefound in the colloidal QDs solution after one-week storage inhumid environment.[106] Under sun illumination, the PLintensity for MA3Bi2Br9 QD was observed to decrease byonly 8% after 1500 min. A Cl-passivation method wasemployed for Cl-MA3Bi2Br9 QDs which yielded a maximumPLQY of 54.1% at the wavelength of 422 nm (Figure 10 a).[73]

Moreover, Cl-passivated MA3Bi2Br9 QDs also demonstratebetter photostability and storage stability than MA3Bi2Br9

QDs, as shown in Figure 10b.

In general, perovskites containing organic cations aremore susceptible to decompose compared to inorganiccounterparts. In this context, all inorganic Cs3Bi2Br9 perov-skite NCs were synthesized in a one-step reaction: In thepreparation, CsBr and BiBr3 were dissolved in dimethylsulfoxide to form a precursor solution and then isopropanolwas used as an antisolvent to precipitate NCs.[107] As-

synthesized Cs3Bi2Br9 NCs exhibited a high air-stability forover 30 days (Figure 11a).[107] Recently, Cs3Bi2Br9 QDs werealso prepared with steady ligand shells via the bindingbetween octylammonium halide and oleic acid.[108] Notably,

after being heated to 180 88C for 1 h, only a slight reduction inPL was observed in Cs3Bi2Br9 solution, demonstratingexcellent thermal stability. The enhanced thermal stabilitywas attributed to the solid framework and chemical bonds inall inorganic perovskite, as well as the strong ligand binding.Additionally, Cs3Bi2Br9 QDs was also could be prepared viaa relatively green route, which utilized ethanol as the anti-solvent.[74] The obtained Cs3Bi2Br9 QDs showed only a 20%reduction in PL intensity after 78 h illumination (Figure 11 b;left panel), which are much better than MA3Bi2Br9 QDs.Furthermore, Cs3Bi2Br9 QDs presented better water stability,with only 10% relative reduction in PL intensity after 8 hstorage in the presence of water (Figure 11 b; middle panel).To further improve stability for more viable potentialapplications, one-pot encapsulation was employed to yieldQDs/silica composites; white LED was then fabricated bysimply placing Cs3Bi2Br9 QDs/silica and yellow-emissiveY3Al5O12 (YAG) on violet-emissive GaN chips.[74] CIE colorwith the coordinates of (0.18, 0.03), (0.40, 0.57), and (0.29,0.30) corresponding to the QD/silica, YAG, and W-LED,respectively is shown in Figure 11 b (right panel). Theremnant PL of the device was at 78 % after 365 nm UVillumination for 16 h. Notably, Cs3Bi2Br9 NCs were preparedin the presence of polar solvents, indicating a favorableperformance among LFHP NCs.[109] As-prepared Cs3Bi2Br9

NCs also demonstrated excellent stability in air for 21 days,which hold much promise in future optoelectronic devices.Recently, a hydrothermal method was used to obtain newRb7Bi3Cl16 LFHP NCs.[110] These NCs presented good mois-ture stability of over one month. This good stability may beattributed to its much higher ratio of Rb atoms and [BiCl6]

3@

octahedra on the surface of Rb7Bi3Cl16 NCs that can forma protective inorganic BiOCl shell.[59, 110]

Figure 10. a) Passivation model (left ) and photographs (right) of Cl-passivated MA3Bi2Br9 QDs under a 325 nm UV excitation. b) Airstability (left) and photostability (right) of pure and Cl-passivatedMA3Bi2Br9 QDs. (a) and (b) both reproduced with permission.[73] Copy-right 2018, American Chemical Society.

Figure 11. a) Air-stability of ligand-free Cs3Bi2Br9 NCs. Reproduced withpermission.[107] Copyright 2017, Wiley-VCH. b) Photostability (left) andwater stability (middle) of Cs3Bi2Br9 QDs, and CIE color (right)coordinates corresponding to Cs3Bi2Br9 QDs, YAG, and W-LED device,respectively. Reproduced with permission.[74] Copyright 2018, Wiley-VCH.

AngewandteChemieReviews

1037Angew. Chem. Int. Ed. 2020, 59, 1030 – 1046 T 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.angewandte.org

3.3. Sb-Based Halide Perovskite NCs

The other popular trivalent substitution of Pb is Sb. In2017, Cs3Sb2Br9 QDs were synthesized via a modified LARPmethod. This LARP method is performed at room temper-ature and the reaction takes only few seconds (Figure 12 a;upper panel).[76] In the air stability test, 70 % of originalPLQY of Cs3Sb2Br9 QDs was retained after aging for 35 dayswithout encapsulation (Figure 12 a; lower left panel). Nota-bly, during the first several days, emission intensities ofperovskite QDs were enhanced, which originated from thesmoothing of QDs and the removal of dangling bonds andother surface defects.[111] The photostability test revealed thatthe relative intensity of Cs3Sb2Br9 QDs remained 50 % of itsinitial value after 108 h irradiation (Figure 12a; lower rightpanel).

Cs3Sb2I9 NCs were prepared via the hot-injection methodfor visible-light optoelectronics.[64] The morphology tuning ofCs3Sb2I9 NCs to yield NPLs and NRs was realized by varyingthe reaction temperature, as depicted in Figure 12b. Recently,uniform colloidal Cs3Sb2Cl9 perovskite NWs (with lengths upto several microns) have been obtained by tuning theprecursors and ligands in the hot injection method.[112]

Photodetectors fabricated using these Cs3Sb2Cl9 NWs dem-onstrated excellent sensitivity, fast time response, and repeat-ability (Figure 12 c). Compared to LHP NCs, defects are morereadily found in Sb-based halide perovskite NCs. Thus,a better control over defect chemistry is needed for Sb-based NCs in the future.

3.4. Double Halide Perovskite NCs

Although a variety of LFHP NCs have been reported overthe past 3 years, their instability toward moisture, air, andillumination still largely exists. In this context, it is highlydesirable to explore LFHP NCs with good stabilities. B-sitesubstitution with mixed cations (A2BB’X6) represents a facilestrategy for synthesis of LFHP NCs. Recently, Cs2AgBiBr6

double perovskite microcrystal and single crystal werereported.[78] They exhibit good stability against thermal,light, and moisture, signifying a promising material foroptoelectronic applications.

Cs2AgBiBr6 double perovskite NCs with high crystallinitywere synthesized via hot-injection approach (Figure 13 a;upper left panel).[113] Such NCs were capable of maintainingtheir structural stability in a set of varied environments, suchas low polarity solutions up to 3 weeks, 55 % relativelyhumidity for 90 days, light soaking of 70 mWcm@2 for 500 h,and 100 88C heating for 300 h (Figure 13 a; upper right panel).It is interesting to note that Cs2AgBiBr6 NCs were recentlyexploited in photochemical conversion of CO2 into solar fuels,achieving a total electron consumption of 105 mmolg@1 forCO2 reduction reaction under simulated solar light illumina-tion for 6 h (Figure 13 a; lower panel); this clearly signifiestheir great potential as environmentally friendly photocata-lysts. Notably, a recent interesting study showed thatCs2AgBiBr6 NCs could be converted into Cs2AgBiI6 NCs bytreating with trimethylsilyl iodide (TMSI).[114] Similarly, the

conversion of Cs2AgBiCl6 into Cs2AgBiBr6 can be driven viatreatment using TMSBr. As shown in Figure 13b, the post-synthetic modification of the cation composition was alsoexplored.[114] A phosphine (e.g., PEt3) treatment ofCs2AgBiBr6 NCs in toluene resulted in their conversion into

Figure 12. a) Schematic illustration of the reaction system for m-LARPtechnique (top left), typical optical images of Cs3Sb2Br9 QD solutionwith and without 365 nm UV light excitation (top right), air stability(bottom left) and photostability (bottom right) tests of Cs3Sb2Br9 QDs.Reproduced with permission.[76] Copyright 2017, American ChemicalSociety. b) Schematic representation of synthesis of Cs3Sb2I9 NPLs andNRs. Reproduced with permission.[64] Copyright 2017, Wiley-VCH.c) Schematic representation of photodetector device (top left), CVcharacteristics of CSC NWs in dark and under illumination (top right;inset: optical image of the device), photoswitching behavior at around0.9 V revealing fast switching ON and OFF states with time interval ofapproximately 500 ms (bottom left), rise (tr) and decay (td) time ofa single ON–OFF cycle (bottom right). Reproduced with permis-sion.[112] Copyright 2018, American Chemical Society.

AngewandteChemieReviews

1038 www.angewandte.org T 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2020, 59, 1030 – 1046

Cs2Bi3Br6 due to strong binding of PEt3 to Ag+ cations.[114] Afacile conversion of Cs2AgBiX6 into Cs2Bi3X6 can thus beachieved. Stability tests show that, when exposed to air andlight for 6 weeks, films of Cs2AgBiBr6 NCs exhibit somedecomposition, while films of Cs2AgBiI6 NCs fully decom-pose within 3 days. Cs2AgBiX6 NCs were also obtained viadirect synthesis by an anti-solvent recrystallization.[77] Theobtained Cs2AgBiI6 NCs were found to be stable in nano-structures, but no further investigations are available.

Size-tunable Cs2AgInCl6 NCs and Mn-doped Cs2AgInCl6

NCs have been prepared via a new colloidal route.[81] In thisapproach, metal carboxylate precursors and ligands (oleyl-amine and oleic acid) were dissolved in diphenyl ether, thentreated with benzoyl chloride at 105 88C. The obtainedCs2AgInCl6 NCs are stable in air for several days. Interest-ingly, in thermal stability tests, the melting point of Mn-dopedNCs was found to increase from 469.4 88C to 479.6 88C byincreasing the doping level from 0.5% to 1.5%.Cs2AgInxBi1@xCl6 NCs (x = 0, 0.25, 0.5, 0.75, and 0.9) NCswere synthesized via anti-solvent recrystallization method.[115]

Cs2AgInxBi1@xCl6 NCs are the first double perovskite NCs to

have a direct band gap. Notably, the band gap ofCs2AgInxBi1@xCl6 NCs can be tuned from being an indirectband gap (x = 0, 0.25, and 0.5) to a direct band gap (x = 0.75and 0.9) by In/Bi alloying. The direct band gaps ofCs2AgIn0.9Bi0.1Cl6 NCs show enhanced PL (PLQY= 36.6%)compared to the indirect band gaps of Cs2AgBiCl6 NCs(PLQY= 6.7 %). In addition to the two families of doubleperovskite colloidal NCs described above, Cs2AgSbX6 (X =

Br or Cl) NCs have been prepared via a modified hotinjection method.[80] Notably, Cs2AgSbBr6 NCs are a newdouble perovskite that has not been reported in the bulkform, which suggest that some challenges involved in thesynthesis of bulk double perovskites may be overcome bysynthesizing their NCs counterparts. Significantly, the sameabsorption spectra were observed in the mixedCs2AgSb1@xBixBr6 (x = 0.5, 0.75) colloidal NCs and their thinfilms, suggesting that Cs2AgSb1@xBixBr6 (x = 0.5, 0.75) NCsare stable in both colloidal solution and solid phase. Thesemixed colloidal NCs exhibit superior stability in air. Theabsorption spectra of Cs2AgSb0.25Bi0.75Br6 NCs and those NCsstored in air for one month are nearly identical. The enhancedstability of the mixed double perovskite NCs may be becausethe ionic sizes match better after incorporating Sb intoCs2AgBiX6 double perovskite.[116, 117]

3.5. Most Recently Synthesized LFHP NCs

Recently, Pb2+ has been replaced in LFHP NCs by othermetals not previously mentioned. Results from these newmaterials are summarized below. All-inorganic Cs2PdBr6

perovskite NCs (with an average diameter of 2.8: 1.1 nm)were synthesized via a facile anti-solvent method.[118]

Cs2PdBr6 NCs exhibit long-term stability, owing to outstand-ing resistance to moisture, light, and heat. Moreover,Cs2PdBr6 NCs have shown sensitive photo-responsive behav-iors in photochemical testing, promising its future use inphotoelectric applications. Additionally, Cs2PdI6 NC can befabricated via anion-exchange treatment. Cs2CuX4 (X = Cl,Br, or I) QDs with highly stability in ambient condition havebeen prepared by LARP technique at room temperature.[119]

After aging for 30 days, the emission intensity of Cs2CuBr4

QDs decreased by only 8% of the initial value. The robuststability makes them good candidates for optoelectronicapplications.

4. Optical Properties of Lead-Free Halide PerovskiteNanocrystals

Gaining a deeper understanding of the nature of photo-excitations and origins of PL in perovskite NCs is paramountfor improving device efficiencies.[120] From study to study,there is discontinuity regarding the optical properties ofLFHP NCs. For example, some groups reported relativelyhigh PLQY from Bi-based perovskites NCs, while otherreports show a low PLQY. To shed light on this intriguingdisagreement, we will summarize and discuss the opticalproperties of currently explored LFHP NCs.

Figure 13. a) Schematic illustration of synthesis of Cs2AgBiBr6 NCs viahot-injection route (top left), XRD patterns of absolute ethanol washedCs2AgBiBr6 NCs stored in different environments (top right), compar-ison of photocatalytic CO2 reduction performance of as-prepared andwashed Cs2AgBiBr6 NCs (bottom left), schematic diagram of thephotoreduction of CO2 on the surface of Cs2AgBiBr6 NCs (bottomright). Reproduced with permission.[113] Copyright 2018, Wiley-VCH.b) Synthesis and reactivity of Cs2AgBiX6 NCs. Reproduced with permis-sion.[114] Copyright 2018, American Chemical Society.

AngewandteChemieReviews

1039Angew. Chem. Int. Ed. 2020, 59, 1030 – 1046 T 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.angewandte.org

4.1. Sn-Based Halide Perovskite NCs

The optical band gaps of CsSnX3 NCs can be tuned fromthe near-infrared to the visible spectral region by varyinghalide composition.[54] In addition, varying the size of theCsSnBr3 NCs can further tune the band gap from 630 to680 nm.[54] It has been demonstrated that increasing theelectronegativity of the atom occupying the B site in theABX3 perovskite will red-shift the band gap, which explainswhy Sn-based perovskite NCs are red-shifted compared totheir Pb-based analogues.[58, 121] CsSnX3 NCs exhibit a rela-tively low PLQY (< 0.14%). To understand the opticalproperties of as-synthesized Sn-based NCs, transient PLexperiments on solution-dispersed NCs were performed(Figure 14 a). The spectral red-shift of the luminescencewithin a few nanoseconds and the presence of two distinctPL decay processes can be understood as follows: the fast-decaying luminescence of CsSnX3 NCs may originate fromband-edge states, while the slower decay can originate fromrecombination from states just below the band edge. Suchshallow states within the band gap are likely to arise from theintrinsic defect sites which are common in solution-dispersed

Sn-based perovskite particles.[122] CsSnBr3 nanocages displaya broad absorbance peak at around 655 nm,[93] and PLemission peak positioned at 685 nm with full width at halfmaximum (FWHM) of 56 nm. Average PL decay lifetimes of6.52 ns are observed, which is comparable to those of Pb-containing perovskite (1–22 ns) and Sn-based perovskite thinfilms (ca. 3.9 ns).[93] The measured PLQY of CsSnBr3 nano-cages is 2.1%, which leaves much room for improvement.

Colloidal NPLs are recognized as a promising class ofsemiconductor nanomaterials, exhibiting bright luminescenceas well as tunable and spectrally narrow absorption andemission. For NPLs with the formula of L2[ABX3]n@1BX4 (asdescribed in Section 3.1), the variation of n, B, and X can beused to tune absorption and emission through the entirevisible wavelength region (Figure 14b).[87] The change on theA cation produces notably subtle variations in the absorptionand emission energy. The FWHM of the emission peaks fromthe bromide and iodide NPLs follows the trend of FA < Cs <MA as a result of the decreasing size of the cation, which maymore easily enable post-structure rearrangement within theNPLs.[87] In particular, the use of FA over MA producesa large boost in the PLQY of L2[APbBr3]PbBr4 NPLs: fromaround 6% (with MA) to 22 % (with FA). In othercompositions, FA again proved to be an excellent cation forNPLs, demonstrated by the highest PLQY of L2[FASnI3]SnI4

being 2.6%. Exploration of other cation species may bea viable pathway for increasing PLQY of NPLs.[88] In anotherstudy, the usage of a PEA cation in conjunction with branchedaliphatic carboxylic acid additives demonstrated a facilestrategy for improving the PLQY, FWHM, and tunability ofSn-based perovskite NPLs.[92] For PEA2SnX4 (n = 1) NPLs,the PL emission showed a remarkable red shift compared tothat found in the literature, which could be attributed to thedecreased band gap as a result of the strong coupling amongNPLs.[54] The emission wavelength of PEA2SnX4 (n = 1) NPLscan be tuned from 640 nm (PEA2SnI4) to 550 nm(PEA2SnBr4) by mixing with iodo- and bromo-based pre-cursors. Accordingly, PLQY and FWHM of those NPLs canbe varied between 0.16–6.40% and 36–80 nm, respectively.

The optical properties of Cs2SnI6 NCs with no ligandswere investigated.[97] The band gaps and PL peaks of Cs2SnI6

NCs depend linearly on the inverse square of diameter of theNC due to the quantum confinement effect. Transientabsorption measurements demonstrate that approximately90% of the first excitonic state of the NCs decay with a timeconstant shorter than 30 ps, which is attributed to chargetrapping at the unpassivated surface of the NCs. Owing toemissive and non-emissive trap states, the PL intensity for allthe samples was relatively low. In addition, the broad sizedistribution of NCs resulted in a large line width of 100–120 nm in the PL spectra. All observations revealed that theligand capping effect on NCs is important for enhancedoptical properties. Cs2SnI6 nanobelts shows an emission peakaround 620 nm with FWHM of 49 nm, suggesting a significantblue-shift (compared to the bulk) resulting from a quantumconfinement effect.[96] Quite different from Pb-based perov-skites, the fluorescence of Cs2SnI6 nanobelts is quicklyquenched upon removal from the stock solution. Such rapidquenching of fluorescence may be ascribed to the dominant

Figure 14. a) PL of as-synthesized CsSnX3 (X = Cl, Br, Br0.5I0.5, I) NCs insolution after an ultrashort excitation (<ps) at 400 nm. The coloredsolid lines represent the integrated PL from 0 to 5 ns for each sample.Dashed lines are the integrated PL at time-interval of 30–60 ns(CsSnCl3), 5–8 ns (CsSnBr3), 30–50 ns (CsSn(Br0.5I0.5)3), and 8–30 ns(CsSnI3) (left panel); Fast (inset) and slow PL decay kinetics of as-synthesized and aged CsSnBr3 NCs in solution (right), excited at490 nm, and detected at 590–900 nm, where the fits are indicated asdashed black lines. Reproduced with permission.[54] Copyright 2016,American Chemical Society. b) Absorbance (dotted lines) and PL (solidlines) spectra for n =1 and n = 2 NPLs with different halide (X) andmetal (B) compositions. Reproduced with permission.[92] Copyright2018, Wiley-VCH. c) PLQY of Cs2SnCl6 :x Bi (inset: digital images ofCs2SnCl6 :xBi under 365 nm UV light illumination). Reproduced withpermission.[61] Copyright 2018, Wiley-VCH.

AngewandteChemieReviews

1040 www.angewandte.org T 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2020, 59, 1030 – 1046

defects of iodine vacancies and interstitial Sn. Moreover, thePLQY of Cs2SnI6 nanobelts is very low (0.054%), and it canbe adjusted by changing the shape of NCs (0.48 % for QDs,0.11% for nanorods, 0.08% for nanowires, and 0.046 % fornanoplatelets).

A significant boost of the PLQY (78.9 %) was observedupon doping in the case of Cs2SnCl6 NCs (Figure 14 c).[59] Thisis the highest PLQY reported for all inorganic LFHP NCs,and is even comparable to the highest value of leadperovskites with blue emissions.[59] It should be noted thatthe emission spectrum of Cs2SnCl6 :Bi shows a peak around454 nm with a wide FWHM of 66 nm, and a large Stokes shiftof 90 nm. In sharp contrast, the pristine Cs2SnCl6 shows no PLwhatsoever, regardless of excitation intensity. The wideemission and large Stokes shift may be a result of self-trapped excitons that are typically found in 0D and 2D halideperovskites. Photogenerated excitons in halide perovskiteswith a polarizable lattice often cause transient deformation ofthe crystal lattice and are localized, thus forming the so-calledself-trapped excitons. This elastic lattice distortion changesthe nuclear coordinate and dissipates some energy fromexciton, so broad emission with a large Stokes shift isobserved. First-principle calculations indicate that Bi3+ isincorporated into the Cs2SnCl6 matrix at the Sn site, andwould preferably form a BiSn + VCl defect complex that isresponsible for the strong blue emission.

4.2. Bi-Based Halide Perovskite NCs

The PL peaks of MA3Bi2X9 QDs can be varied from 360 to540 nm via post-synthesis halide anion exchange.[106] ThePLQY of MA3Bi2Br9 QD still needs improvement, witha maximum value of only 12%. It was found that Cl-passivation of MA3Bi2Br9 QDs can increase the PLQY to54.1% at the wavelength of 422 nm.[73] Fundamentally, theCl@ anions mainly localize on the surface of the QDs, therebysuppressing surface defects and boosting radiative recombi-nation. It was found that typical ligand-free Cs3Bi2Br9 NCshad a low PLQY (0.2%) because of the fast trapping process,which can be passivated by adding oleic acid, which improvesthe PLQY to 4.5%.[107] The effect of ligands on PL isespecially evident in Cs3Bi2Br9 QDs capped by octylammo-nium halide and oleic acid. These QDs showed a PLQY of upto 22%, which is 110 times that of ligand-free Cs3Bi2Br9

QDs.[108] In addition, Cs3Bi2X9 NCs that were synthesizedusing ethanol as the anti-solvent (octylamine and oleic acid asligands) showed a blue emission at 410 nm with a highestPLQY of 19.4%.[74]

The band structures of Cs3Bi2I9 remain elusive. The PLspectrum of Cs3Bi2I9 was found to exhibit a dual-spectralfeature at room temperature with comparable intensities(Figure 15 a; left panel), which may be due to an excitonrecombination process involving both indirect and directtransitions simultaneously.[123] Recombination pathways forexcitons in Cs3Bi2I9 NCs are shown in Figure 15 a (rightpanel). Upon excitation, electrons in the valence band arepromoted to the conduction band. Such an absorption processhas a direct band gap transition due to a considerable intrinsic

absorption coefficient at 488 nm (1.6 X 104 cm@1). Accordingto KashaQs rule, the excited electrons relax to the conductionband minimum and recombine with holes in the valence bandmaximum. With the assistance of phonons, indirect band gaptransitions can occur, accompanied by the generation of anemission band at 605 nm.[123] It is noteworthy that the lowPLQY of Cs3Bi2I9 NCs can also be attributed to the presenceof nonradiative recombination pathways, such as nativedefects, which are induced by its indirect band gap.[26]

Similarly, the other study on Cs3Bi2I9 NCs also witnessedthe dual-spectra feature in their PL spectrum.[69] As shown inFigure 15 b, a sharp excitonic peak at 2.56 eVat 10 K is clearlyseparated from the electronic band gap at 2.86 eV for NCs,resulting in a very high excitonic binding energy, Eb

X =

300 meV. Because EbX (300 meV) is much greater than the

effective phonon energy (36 meV), the phonon-mediatedrelaxation of carriers from the conduction band minimum tothe excitonic state is suppressed to some extent.[69] Conse-quently, the two PL peaks relate to the bulk band edge andthe excitonic transitions. Conversely, dual-spectral featureswere not observed in the case of Cs3Bi2I9 obtained via the hotinjection method. In addition, it is worth noting that Cs3Bi2I9

has a 0D perovskite crystal structure at the molecular level.The 0D refers to the crystal structure irrespective of thecrystal size, which is a distinct feature compared to otherperovskite NCs.

4.3. Sb-Based Halide Perovskite NCs

Cs3Sb2Br9 QDs prepared by the reprecipitation methodshowed a sharp and narrow emission peak at 410 nm witha FWHM of 41 nm. The PL wavelengths can be tuned from

Figure 15. a) Absorption and PL spectra of Cs3Bi2I9 NCs (left), pro-posed recombination pathways in Cs3Bi2I9 NCs (right). Reproducedwith permission.[123] Copyright 2017, American Chemical Society.b) Tauc plot of Cs3Bi2I9 NCs at 10 K (left), schematic diagram showingthe probable excitation and de-excitation processes (right). Repro-duced with permission.[69] Copyright 2018, American Chemical Society.

AngewandteChemieReviews

1041Angew. Chem. Int. Ed. 2020, 59, 1030 – 1046 T 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.angewandte.org

370 to 560 nm by anion exchange reactions.[76] Compared tothat of a Cs3Sb2Br9 single crystal, the QD emission peakshowed a 120 nm blue-shift, demonstrating a strong quantumconfinement effect.[76] The Stokes shift of 280 meV observedin the case of Cs3Sb2Br9 QDs implies almost no overlapbetween absorption and PL emission. Weak self-absorption isof great importance for the use as phosphors in lightingapplications. The high PLQY value (46 %) of Cs3Sb2Br9 QDscan be ascribed to the reduced surface trapping effect formedby the Br-rich surface and the large exciton binding energy,which allows for efficient radiative recombination of excitonsat room temperature.

Colloidal Cs3Sb2I9 (NPLs and NRs) were prepared via thehot injection method.[64] The length of the Cs3Sb2I9 NRs wasfound to be larger than their excitonic Bohr diameters.However, the thickness (1.5 nm) of the Cs3Sb2I9 NPLs issufficiently small such that they still experience a quantumconfinement effect, resulting in a blue-shift in the optical gap.In the UV/Vis absorption spectrum of Cs3Sb2I9 NRs, thelowest energy peak at 574 nm (2.16 eV), with longer wave-length absorption tail extending to about 615 nm (2.02 eV)was observed, similar to bulk samples. PL spectra of bothsamples show band-edge emission with narrow FWHM of32 nm and 30 nm for Cs3Sb2I9 NPLs and NRs, respectively.However, a tail in the PL spectra to the longer wavelengthsmay be due to defect-related emission from Cs3Sb2I9 NCs. Itwas reported that compared to shallow defects in CsPbI3,deeper defects in Cs3Sb2I9 may originate from both local-ization and reduction of spin-orbit splitting of the Sb 5p-orbitals, compared to the Pb 6p-orbitals.[64] Therefore, bettercontrol of defect density is required in the future.

4.4. Double Halide Perovskite NCs

The absorption measurements of Cs2AgBiX6 NCs withdifferent halide compositions revealed the tunable excitonpeaks ranging from 367 to 500 nm with the corresponding PLpeaks varying from 395 to 575 nm.[77] As-synthesizedCs2AgBiBr6 NCs exhibited prominent sub-band gap trappingprocesses, which may result from surfaces defects of theNCs.[77] These traps can be passivated by oleic acid, which canlead to a 100-times increase in PLQY (6.7 %). The absorptiontail can also be suppressed by adding OA. In the PL spectra ofligand-free and OA-capped NCs, ligand-free NCs were foundto exhibit a double PL peak at 465 nm and 510 nm. Afteradding 1% OA, the PL at 510 nm quenched while the PL at465 nm increased. These results suggest the PL peak at465 nm should originate from the band edge emission whilethe peak at 510 nm is associated with the emission of sub-bandgap trap states. For Cs2AgBiBr6 NCs prepared via the hotinjection method,[113] an absorption onset at around 500 nmwas observed, which was in accordance with an indirect bandgap of 2.52 eV. A broad, weak PL emission at 625 nm(1.98 eV) was observed from phonon-assisted recombinationof self-trapped excitons. Cs2AgBiBr6 has an indirect band gap,so during the growth of Cs2AgBiBr6 intrinsic defects are easilyformed, such as Bi vacancies and AgBi anti-sites. Theabsorption spectra of Cs2AgBiBr6 NCs were obtained for

both the colloidal NCs and thin films.[80] The colloidal NCs ofCs2AgBiBr6 were observed to exhibit dual absorption peak at427 nm and 380 nm. In contrast, only a single absorption peakat 427 nm was seen in the thin films. The 427 nm absorptionpeak can be attributed to the direct Bi s–p transition, whilethe 380 nm peak may be assigned to the isolated octahedralBiBr6

3+ complex in colloidal solution.[113] These resultsindicate the Cs2AgBiBr6 NCs are unstable in colloidalsolutions yet stable in solid thin films. The dual absorptionpeaks can also be observed in Cs2AgSbBr6 colloidal NCs.Therefore, it can be concluded that the 360 nm absorptionpeak in Cs2AgSbBr6 colloidal NCs originates from impurities,such as the isolated octahedral SbBr6

3+ complex. In theabsorption spectra of Cs2AgSbCl6 NCs, an absorption peak at365 nm was observed for solid thin films, whereas theabsorption peak is not obvious in its NC form. These resultsindicate that Cl-containing Ag-Sb NCs are unstable incolloidal solution.

Study on the optical properties of Cs2AgInCl6 NCsrevealed that the absorption onset of Cs2AgInCl6 NCs wasobserved at around 350 nm with a strong increase in theabsorbance only below about 290 nm.[81] The sample exhib-ited a weak and broad PL emission at 560 nm (PLQY: ca.1.6: 1%). Similar to the undoped Cs2AgInCl6 NCs, theabsorption onset of Mn-doped NCs was seen to be close to350 nm, and they showed a broad PL band centered at around620 nm from the 4T1!6A1 transition of Mn2+ dopants.[124] Theintensity of the PL emission of Mn-doped Cs2AgInCl6 NCsincreased with the doping level from 0.5% to 1.5% of Mn,with the highest PLQY of about 16: 4 %. Most of thereported double perovskite NCs exhibit an indirect band gapnature, which has prominent sub-band gap absorption,corresponding primarily to indirect transitions, thereby lead-ing to a low PLQY.

Cs2AgInxBi1@xCl6 (x = 0.75 and 0.9) NCs with a directband gap show a larger absorption cross section, lower trapstates, and higher PLQYs compared to those of the indirectband gap double perovskite NCs (e.g., Cs2AgBiCl6).[115] APLQY of 36.6% for Cs2AgIn0.9Bi0.1Cl6 NCs is comparable tothose observed for Pb-based perovskite NCs in the violetregion and is presently the highest value for reported doubleperovskite NC samples. In addition, the direct band gapCs2AgIn0.9Bi0.1Cl6 NCs are orange-emitting. Taken the resultsof density functional theory (DFT) calculation, steady-stateabsorption, PL, and transient absorption spectra together,dual-color emission was found to be due to the direct band-to-band transition (violet) and the forbidden transition (orange).

Based on the above discussion, some optical properties ofLFHP NCs are summarized in Table 1. Different synthesismethods, ligands, shapes, and solvents give LFHP NCs withwildly varying optical properties. In the future, systematicstudies and comparison on optical properties of LFHP NCsneed to be pursued.

5. Conclusions and Outlook

Driven by the unique attributes of LHP NCs and toxicityconcerns of Pb, research effort to develop suitable LFHP NCs

AngewandteChemieReviews

1042 www.angewandte.org T 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2020, 59, 1030 – 1046

substitutes has progressed greatly over the past few years. Aseries of low-toxic or non-toxic metal cations, such as SnII,SnIV, GeII, BiIII, SbIII, and AgI have been exploited as Pbsubstitutes to produce LFHP NCs. In this Review, wesummarized recent advances in LFHP NCs by concentratingon their crystal structures, synthesis methods, stabilities,properties, and applications. Nonetheless, there are stillsome issues required to be addressed in the future.

To unlock the potential of LFHP NCs, a better under-standing of their crystal structure is needed. Only doublehalide perovskite (Cs2BiAgX6) and Sn-based halide perov-skite (CsSnCl3) NCs exhibit the typical cubic perovskitestructure,[54, 78] while Bi/Sb-based perovskite NCs show low-dimensional structures of 0D, 1D or 2D.[65,68, 73, 76] Notably,crystal structures of these NCs may be changed by tuningA cations, B sites, or X halide compositions, however, therehas been no systematical study on this aspect in literature. Asdiscussed in this Review, a variety of A cations allow thetailoring of stability and PLQY of LFHP NCs. More inves-tigations are needed to understand the underlying mecha-nisms for optimization of A site cations. Reduced-dimen-sional crystal structures of Bi/Sb-based perovskites typicallylead to reduced band dispersion, increased effective masses ofcharge carriers, and decreased carrier mobilities, yet thestructure–property relationship is still unclear. Investigationinto the structure and the resulting properties (e.g., optical,optoelectronic, and photovoltaic) merits a detailed, inte-grated theoretical and experimental study.

Unlike LHP NCs, synthetic routes to LFHP NCs havefocused exclusively on either recrystallization or hot injection.Control over capping ligands, A cations, and reaction temper-

ature, may allow the formation of LFHP NCs with differentmorphologies including QDs,[54] NPLs,[96] and NRs.[102] Anionexchange of LFHP NCs has been shown to be a facile way toyield LFHP NCs with different halide compositions. Moststudies rely on direct synthesis methods using different halidesources, yet better harnessing of the power of anion exchangecould lead to further advances in this field. Further inves-tigation is clearly needed to provide a better understanding asto how to create high-quality NCs and diversify theirapplications. Seeking novel or modifying traditional methodsof preparing high-quality NCs should be a priority for allworking in this field. One such idea could be using polymermicelles as templates to synthesize high-quality LFHP NCs.This could be done by drawing inspiration from one studyQssuccess in synthesizing colloidally stable LHP NCs.[125]

Polymer “hairs” on the NCs surface may promise enhancedstability and good dispersion. By judiciously selecting pre-cursors and synthetic parameters, it is reasonable to expectthat this synthetic approach can be readily extended toproduce other high-performance, lead-free, and stable per-ovskite NCs.

Poor stabilities of LFHP NCs against heat, light, oxygen,humidity, and chemical resistance arise from the ionic natureof perovskites, and is not limited to LFHPs. It remainschallenging for LHPs and even non-nanocrystalline bulkmaterials. LFHP NCs with different B cations show differentstabilities. SnII-based perovskite NCs always show poorstability because of their easy oxidation from SnII to SnIV. Incontrast, SnVI-based perovskite NCs and Bi/Sb-based perov-skite NCs exhibit enhanced stability. Tuning surface cappingligands, A cations, and shapes can improve the stability of

Table 1: Optical properties, ligands, and synthesis methods of LFHP NCs

Formula Absorptionpeak [nm]

Emission peak [nm] PLQY[%]

Ligands Method Ref.

CsSnCl3 ca. 420 ca. 490 ,0.14 Tri-n-octylphosphine, oleylamine, oleic acid Modified hot injection method [54]CsSnBr3 ca. 610 ca. 660 ,0.14 Tri-n-octylphosphine, oleylamine, oleic acid Modified hot injection method [54]CsSnl3 ca. 750 ca. 945 ,0.14 Tri-n-octylphosphine, oleylamine, oleic acid Modified hot injection method [54]Cs2SnI6 – ca. 620 ,0.48 Oleylamine, oleic acid Hot injection method [96]MA3Bi2Cl9 ca. 360 360 15 Oleylamine, oleic acid Co-LARP [106]MA3Bi2Br9 376 423 12 Oleylamine, oleic acid Co-LARP [106]MA3Bi2I9 ca. 400 540 0.03 Oleylamine, oleic acid Co-LARP [106]Cs3Bi2Br9 439 468 0.2 No ligands Recrystallization method

(Isopropanol as antisolvent)[107]

Cs3Bi2Br9 439 460 4.5 Oleic acid Recrystallization method(Isopropanol as antisolvent)

[107]

Cs3Bi2Br9 396 410 19.4 Oleylamine, oleic acid Recrystallization method(Ethanol as antisolvent)

[74]

Cs3Bi2Cl9 ca. 290 393 26.4 Oleylamine, oleic acid Recrystallization method(Ethanol as antisolvent)

[74]

Cs3Bi2I9 ca. 395 545 0.018 Oleylamine, oleic acid Recrystallization method(Ethanol as antisolvent)

[74]

Cs3Sb2Br9 ca. 375 410 46 Oleylamine or n-octylamine, and oleic acid m-LARP) [76]Cs3Sb2Cl9 ca. 360 370 11 Oleylamine or n-octylamine, and oleic acid m-LARP) [76]Cs3Sb2I9 ca. 520 560 23 Oleylamine or n-octylamine, and oleic acid m-LARP) [76]Cs2AgBiBr6 440 465 0.7 Oleic acid Recrystallization method

(Isopropanol as antisolvent)[77]

Cs2AgBiCl6 ca. 360 395 6.7 Oleic acid Recrystallization method(Isopropanol as antisolvent)

[77]

Cs2AgBiI6 ca. 500 575 0.1 Oleic acid Recrystallization method(Isopropanol as antisolvent)

[77]

AngewandteChemieReviews

1043Angew. Chem. Int. Ed. 2020, 59, 1030 – 1046 T 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.angewandte.org

NCs. Notably, doping other metal cations into the perovskitestructure was recently found to significantly improve thestability and enhance the PLQY.[59] Looking into the future,other strategies may also be pursued for preparing stableLFHPs NCs. First, coating LFHP NCs with an organic orinorganic shell is likely to enhance the stability.[125–129] Second,various defect-passivation methods should continue to beexploited for LFHP NCs with higher quality and stability. It isimportant to note that the mechanistic understanding of thedegradation of LFHP NCs is currently lacking. The ability tocomprehend the degradation mechanism of LFHP NCs willassist in preparing LFHP NCs with long-term stability, whichis of key importance for commercially viable perovskite-based products. Third, replacement with other air-stablemetals (e.g., transition metals, such as, Fe, Zn) cations maybroaden the choice of stable LFHP NCs.

Because of the discrepancy between investigations on theoptical properties of LFHP NCs, a systematic and compara-tive study of these properties is needed. Such a study mustanswer the following three questions: 1) why is there dis-agreement on the optical properties of LFHP NCs? 2) whatare the significant factors that affect the optical properties ofLFHP NCs? 3) how to control the optical properties of LFHPNCs? After addressing the above questions, a step-by-stepcharacterization and calculation should be conducted to offerclear and convincing understanding.

Currently, the properties of LFHP NCs may be inferior toLHP counterparts. However, because of their safety, they arelikely to receive much widespread use. As such, the synthesisroutes, stabilities, properties, and applications of LFHP NCsstill deserve further investigation. Moreover, more studies areneeded to better understand the nature of nonradiativerecombination processes and defects in LFHP NCs, which areknown to quench PL. Better understanding of these mech-anisms will help boost the relatively low PLQY of LFHP NCs.

Acknowledgements

We gratefully acknowledge the support from the NationalKey Research and Development Program of China(2017YFB0308602), the National Science Foundation(CBET 1803495, CMMI 1914713), China Scholarship Council(CSC), the National Natural Science Foundation of China(21674024, 21320102005), the Ministry of Science and Tech-nology of China (2016YFA0203301), and the Senior VisitingScholarship of State Key Laboratory, Fudan University.

Conflict of interest

The authors declare no conflict of interest.

How to cite: Angew. Chem. Int. Ed. 2020, 59, 1030–1046Angew. Chem. 2020, 132, 1042–1059

[1] C. K. Møller, Nature 1958, 182, 1436.[2] A. Poglitsch, D. Weber, J. Chem. Phys. 1987, 87, 6373.[3] D. Weber, Z. Naturforsch. B 1978, 33, 1443.

[4] N. Onoda-Yamamuro, T. Matsuo, H. Suga, J. Phys. Chem.Solids 1992, 53, 935.

[5] G. C. Papavassiliou, I. B. Koutselas, Synth. Met. 1995, 71, 1713.[6] W. J. Yin, T. Shi, Y. Yan, Adv. Mater. 2014, 26, 4653.[7] M. A. Green, A. Ho-Baillie, H. J. Snaith, Nat. Photonics 2014,

8, 506.[8] A. Miyata, A. Mitioglu, P. Plochocka, O. Portugall, J. T.-W.

Wang, S. D. Stranks, H. J. Snaith, R. J. Nicholas, Nat. Phys.2015, 11, 582.

[9] D. Shi, V. Adinolfi, R. Comin, M. Yuan, E. Alarousu, A. Buin,Y. Chen, S. Hoogland, A. Rothenberger, K. Katsiev, Y. Losovyj,X. Zhang, P. A. Dowben, O. F. Mohammed, E. H. Sargent,O. M. Bakr, Science 2015, 347, 519.

[10] J. M. Ball, M. M. Lee, A. Hey, H. J. Snaith, Energy Environ. Sci.2013, 6, 1739.

[11] D. Liu, T. L. Kelly, Nat. Photonics 2014, 8, 133.[12] W. Nie, H. Tsai, R. Asadpour, J.-C. Blancon, A. J. Neukirch, G.

Gupta, J. J. Crochet, M. Chhowalla, S. Tretiak, M. A. Alam, H.-L. Wang, A. D. Mohite, Science 2015, 347, 522.

[13] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, J. Am. Chem.Soc. 2009, 131, 6050.

[14] H.-S. Kim, C.-R. Lee, J.-H. Im, K.-B. Lee, T. Moehl, A.Marchioro, S.-J. Moon, R. Humphry-Baker, J.-H. Yum, J. E.Moser, M. Gr-tzel, N.-G. Park, Sci. Rep. 2012, 2, 591.

[15] M. M. Lee, J. Teuscher, T. Miyasaka, T. N. Murakami, H. J.Snaith, Science 2012, 338, 643.

[16] M. Liu, M. B. Johnston, H. J. Snaith, Nature 2013, 501, 395.[17] G. Hodes, Science 2013, 342, 317.[18] M. Saliba, T. Matsui, K. Domanski, J.-Y. Seo, A. Ummadisingu,

S. M. Zakeeruddin, J.-P. Correa-Baena, W. R. Tress, A. Abate,A. Hagfeldt, M. Gr-tzel, Science 2016, 354, 206.

[19] N. J. Jeon, H. Na, E. H. Jung, T.-Y. Yang, Y. G. Lee, G. Kim, H.-W. Shin, S. I. Seok, J. Lee, J. Seo, Nat. Energy 2018, 3, 682.

[20] M. He, B. Li, X. Cui, B. Jiang, Y. He, Y. Chen, D. OQNeil, P.Szymanski, M. A. EI-Sayed, J. Huang, Z. Lin, Nat. Commun.2017, 8, 16045.

[21] M. Ye, C. He, X. Hong, X. Liu, J. Iocozzia, X. Cui, X. Meng, M.Rager, Z. Lin, X. Liu, J. Phys. D 2017, 50, 373002.

[22] M. He, X. Pang, X. Liu, B. Jiang, Y. He, H. Snaith, Z. Lin,Angew. Chem. Int. Ed. 2016, 55, 4280; Angew. Chem. 2016, 128,4352.

[23] M. He, D. Zheng, M. Wang, C. Lin, Z. Lin, J. Mater. Chem. A2014, 2, 5994.

[24] X. Meng, X. Cui, M. Rager, S. Zhang, Z. Wang, J. Yu, Y. Harn,Z. Kang, B. K. Wagner, Y. Liu, C. Yu, J. Qiu, Z. Lin, NanoEnergy 2018, 52, 123.

[25] Https://www.nrel.gov/pv/cell-efficiency.html.[26] D. P. McMeekin, G. Sadoughi, W. Rehman, G. E. Eperon, M.

Saliba, M. T. Hçrantner, A. Haghighirad, N. Sakai, L. Korte, B.Rech, M. B. Johnston, L. M. Herz, H. J. Snaith, Science 2016,351, 151.

[27] L. Wu, H. Hu, Y. Xu, S. Jiang, M. Chen, Q. Zhong, D. Yang, Q.Liu, Y. Zhao, B. Sun, Q. Zhang, Y. Yin, Nano Lett. 2017, 17,5799.

[28] Y. Tong, E. Bladt, M. F. Ayggler, A. Manzi, K. Z. Milowska,V. A. Hintermayr, P. Docampo, S. Bals, A. S. Urban, L.Polavarapu, J. Feldmann, Angew. Chem. Int. Ed. 2016, 55,13887; Angew. Chem. 2016, 128, 14091.

[29] Y. Hassan, O. J. Ashton, J. H. Park, G. Li, N. Sakai, B. Wenger,A.-A. Haghighirad, N. K. Noel, M. H. Song, B. R. Lee, R. H.Friend, H. J. Snaith, J. Am. Chem. Soc. 2019, 141, 1269.

[30] F. Zhang, H. Zhong, C. Chen, X. Wu, X. Hu, H. Huang, J. Han,B. Zou, Y. Dong, ACS Nano 2015, 9, 4533.

[31] L. Protesescu, S. Yakunin, M. I. Bodnarchuk, F. Krieg, R.Caputo, C. H. Hendon, R. X. Yang, A. Walsh, M. V. Kovalenko,Nano Lett. 2015, 15, 3692.

AngewandteChemieReviews

1044 www.angewandte.org T 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2020, 59, 1030 – 1046

[32] Q. A. Akkerman, V. D’Innocenzo, S. Accornero, A. Scarpellini,A. Petrozza, M. Prato, L. Manna, J. Am. Chem. Soc. 2015, 137,10276.

[33] M. C. Brennan, J. Zinna, M. Kuno, ACS Energy Lett. 2017, 2,1487.

[34] D. N. Dirin, L. Protesescu, D. Trummer, I. V. Kochetygov, S.Yakunin, F. Krumeich, N. P. Stadie, M. V. Kovalenko, NanoLett. 2016, 16, 5866.

[35] H. Huang, M. I. Bodnarchuk, S. V. Kershaw, M. V. Kovalenko,A. L. Rogach, ACS Energy Lett. 2017, 2, 2071.

[36] M. Pandey, K. W. Jacobsen, K. S. Thygesen, J. Phys. Chem. Lett.2016, 7, 4346.

[37] A. Swarnkar, A. R. Marshall, E. M. Sanehira, B. D. Cherno-mordik, D. T. Moore, J. A. Christians, T. Chakrabart, J. M.Luther, Science 2016, 354, 92.

[38] G. Li, F. W. R. Rivarola, N. J. L. K. Davis, S. Bai, T. C. Jellicoe,F. D. L. PeÇa, S. Hou, C. Ducati, F. Gao, R. H. Friend, N. C.Greenham, Z.-K. Tan, Adv. Mater. 2016, 28, 3528.

[39] X. Zhang, H. Lin, H. Huang, C. Reckmeier, Y. Zhang, W. C. H.Choy, A. L. Rogach, Nano Lett. 2016, 16, 1415.

[40] J.-H. Im, C.-R. Lee, J.-W. Lee, S.-W. Park, N.-G. Park, Nano-scale 2011, 3, 4088.

[41] P. Ramasamy, D.-H. Lim, B. Kim, S.-H. Lee, M.-S. Leeb, J.-S.Lee, Chem. Commun. 2016, 52, 2067.

[42] F. Giustino, H. J. Snaith, ACS Energy Lett. 2016, 1, 1233.[43] S. F. Hoefler, G. Trimmel, T. Rath, Monatsh. Chem. 2017, 148,

795.[44] S. Chatterjee, A. J. Pal, J. Mater. Chem. A 2018, 6, 3793.[45] S. Yang, W. Fu, Z. Zhang, H. Chen, C.-Z. Li, J. Mater. Chem. A

2017, 5, 11462.[46] C. Zhang, L. Gao, S. Hayase, T. Ma, Chem. Lett. 2017, 46, 1276.[47] M. Lyu, J. H. Yun, P. Chen, M. Hao, L. Wang, Adv. Energy

Mater. 2017, 7, 1602512.[48] H. Hu, B. Dong, W. Zhang, J. Mater. Chem. A 2017, 5, 11436.[49] J. Sun, J. Yang, J. I. Lee, J. H. Cho, M. S. Kang, J. Phys. Chem.

Lett. 2018, 9, 1573.[50] S. Ghosh, B. Pradhan, ChemNanoMat 2019, 5, 300.[51] S. Khalfin, Y. Bekenstein, Nanoscale 2019, https://doi.org/10.

1039/C9NR01031A.[52] W. Ke, C. C. Stoumpos, M. Zhu, L. Mao, I. Spanopoulos, J. Liu,

O. Y. Kontsevoi, M. Chen, D. Sarma, Y. Zhang, M. R.Wasielewski, M. G. Kanatzidis, Sci. Adv. 2017, 3, e1701293.

[53] M. V. Kovalenko, L. Protesescu, M. I. Bodnarchuk, Science2017, 358, 745.

[54] T. C. Jellicoe, J. M. Richter, H. F. J. Glass, M. Tabachnyk, R.Brady, S. E. Dutton, A. Rao, R. H. Friend, D. Credgington,N. C. Greenham, M. L. Bçhm, J. Am. Chem. Soc. 2016, 138,2941.

[55] D. E. Scaife, P. F. Weller, W. G. Fisher, J. Solid State Chem.1974, 9, 308.

[56] L.-J. Chen, C.-R. Lee, Y.-J. Chuang, Z.-H. Wu, C. Chen, J. Phys.Chem. Lett. 2016, 7, 5028.

[57] J. Xi, Z. Wu, B. Jiao, H. Dong, C. Ran, C. Piao, T. Lei, T.-B.Song, W. Ke, T. Yokoyama, X. Hou, M. G. Kanatzidis, Adv.Mater. 2017, 29, 1606964.

[58] F. Hao, C. C. Stoumpos, D. H. Cao, R. P. H. Chang, M. G.Kanatzidis, Nat. Photonics 2014, 8, 489.

[59] Z. Tan, J. Li, C. Zhang, Z. Li, Q. Hu, Z. Xiao, T. Kamiya, H.Hosono, G. Niu, E. Lifshitz, Y. Cheng, J. Tang, Adv. Funct.Mater. 2018, 28, 1801131.

[60] X. Wu, W. Song, Q. Li, X. Zhao, D. He, Z. Quan, Chem. Asian J.2018, 13, 1654.

[61] D.-K. Seo, N. Gupta, M.-H. Whangbo, Inorg. Chem. 1998, 37,407.

[62] T. Krishnamoorthy, H. Ding, C. Yan, W. L. Leong, T. Baikie, Z.Zhang, M. Sherburne, S. Li, M. Asta, N. Mathews, S. G.Mhaisalkar, J. Mater. Chem. A 2015, 3, 23829.

[63] B. Saparov, F. Hong, J.-P. Sun, H.-S. Duan, W. Meng, S.Cameron, I. G. Hill, Y. Yan, D. B. Mitzi, Chem. Mater. 2015, 27,5622.

[64] J. Pal, S. Manna, A. Mondal, S. Das, K. V. Adarsh, A. Nag,Angew. Chem. Int. Ed. 2017, 56, 14187; Angew. Chem. 2017,129, 14375.

[65] F. Jiang, D. Yang, Y. Jiang, T. Liu, X. Zhao, Y. Ming, B. Luo, F.Qin, J. Fan, H. Han, L. Zhang, Y. Zhou, J. Am. Chem. Soc. 2018,140, 1019.

[66] T. Singh, A. Kulkarni, M. Ikegami, T. Miyasaka, ACS Appl.Mater. Interfaces 2016, 8, 14542.

[67] C. Zuo, L. Ding, Angew. Chem. Int. Ed. 2017, 56, 6528; Angew.Chem. 2017, 129, 6628.

[68] A. J. Lehner, D. H. Fabini, H. A. Evans, C.-A. H8bert, S. R.Smock, J. Hu, H. Wang, J. W. Zwanziger, M. L. Chabinyc, R.Seshadri, Chem. Mater. 2015, 27, 7137.

[69] J. Pal, A. Bhunia, S. Chakraborty, S. Manna, S. Das, A. Dewan,S. Datta, A. Nag, J. Phys. Chem. C 2018, 122, 10643.

[70] S. Sun, S. Tominaka, J.-H. Lee, F. Xie, P. D. Bristowe, A. K.Cheetham, APL Mater. 2016, 4, 031101.

[71] B.-W. Park, B. Philippe, X. Zhang, H. Rensmo, G. Boschloo,E. M. J. Johansson, Adv. Mater. 2015, 27, 6806.

[72] R. D. Nelson, K. Santra, Y. Wang, A. Hadi, J. W. Petrichbc,M. G. Panthani, Chem. Commun. 2018, 54, 3640.

[73] M. Leng, Y. Yang, Z. Chen, W. Gao, J. Zhang, G. Niu, D. Li, H.Song, J. Zhang, S. Jin, J. Tang, Nano Lett. 2018, 18, 6076.

[74] M. Leng, Y. Yang, K. Zeng, Z. Chen, Z. Tan, S. Li, J. Li, B. Xu,D. Li, M. P. Hautzinger, Y. Fu, T. Zhai, L. Xu, G. Niu, S. Jin, J.Tang, Adv. Funct. Mater. 2018, 28, 1704446.

[75] M.-Q. Li, Y.-Q. Hu, L.-Y. Bi, H.-L. Zhang, Y. Wang, Y.-Z.Zheng, Chem. Mater. 2017, 29, 5463.

[76] J. Zhang, Y. Yang, H. Deng, U. Farooq, X. Yang, J. Khan, J.Tang, H. Song, ACS Nano 2017, 11, 9294.

[77] B. Yang, J. Chen, S. Yang, F. Hong, L. Sun, P. Han, T. Pullerits,W. Deng, K. Han, Angew. Chem. Int. Ed. 2018, 57, 5359; Angew.Chem. 2018, 130, 5457.

[78] A. H. Slavney, T. Hu, A. M. Lindenberg, H. I. Karunadasa, J.Am. Chem. Soc. 2016, 138, 2138.

[79] F. Wei, Z. Deng, S. Sun, F. Zhang, D. M. Evans, G. Kieslich, S.Tominaka, M. A. Carpenter, J. Zhang, P. D. Bristowe, A. K.Cheetham, Chem. Mater. 2017, 29, 1089.

[80] B. Yang, F. Hong, J. Chen, Y. Tang, L. Yang, Y. Sang, X. Xia, J.Guo, H. He, S. Yang, W. Deng, K. Han, Angew. Chem. Int. Ed.2019, 58, 2278; Angew. Chem. 2019, 131, 2300.

[81] F. Locardi, M. Cirignano, D. Baranov, Z. Dang, M. Prato, F.Drago, M. Ferretti, V. Pinchetti, M. Fanciulli, S. Brovelli, L.De Trizio, L. Manna, J. Am. Chem. Soc. 2018, 140, 12989.

[82] J. De Roo, M. Ib#Çez, P. Geiregat, G. Nedelcu, W. Walravens, J.Maes, J. C. Martins, I. V. Driessche, M. V. Kovalenko, Z. Hens,ACS Nano 2016, 10, 2071.

[83] M. Kwoka, L. Ottaviano, M. Passacantando, S. Santucci, G.Czempik, J. Szuber, Thin Solid Films 2005, 490, 36.

[84] H. Xu, H. Yuan, J. Duan, Y. Zhao, Z. Jiao, Q. Tang, Electro-chim. Acta 2018, 282, 807.

[85] L. N. Quan, M. Yuan, R. Comin, O. Voznyy, E. M. Beauregard,S. Hoogland, A. Buin, A. R. Kirmani, K. Zhao, A. Amassian,D. H. Kim, E. H. Sargent, J. Am. Chem. Soc. 2016, 138, 2649.

[86] H. Tsai, W. Nie, J.-C. Blancon, C. C. Stoumpos, R. Asadpour, B.Harutyunyan, A. J. Neukirch, R. Verduzco, J. J. Crochet, S.Tretiak, L. Pedesseau, J. Even, M. A. Alam, G. Gupta, J. Lou,P. M. Ajayan, M. J. Bedzyk, M. G. Kanatzidis, A. D. Mohite,Nature 2016, 536, 312.

[87] M. C. Weidman, M. Seitz, S. D. Stranks, W. A. Tisdale, ACSNano 2016, 10, 7830.

[88] M. C. Weidman, A. J. Goodman, W. A. Tisdale, Chem. Mater.2017, 29, 5019.

AngewandteChemieReviews

1045Angew. Chem. Int. Ed. 2020, 59, 1030 – 1046 T 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.angewandte.org

[89] A. B. Wong, Y. Bekenstein, J. Kang, C. S. Kley, D. Kim, N. A.Gibson, D. Zhang, Y. Yu, S. R. Leone, L.-W. Wang, A. P.Alivisatos, P. Yang, Nano lett. 2018, 18, 2060.

[90] A. G. Kontos, A. Kaltzoglou, E. Siranidi, D. Palles, G. K.Angeli, M. K. Arfanis, V. Psychari, Y. S. Raptis, E. I. Kamitsos,P. N. Trikalitis, C. C. Stoumpos, M. G. Kanatzidis, P. Falaras,Inorg. Chem. 2017, 56, 84.

[91] S. Gupta, T. Bendikov, G. Hodes, D. Cahen, ACS Energy Lett.2016, 1, 1028.

[92] M.-Y. Chen, J.-T. Lin, C.-S. Hsu, C.-K. Chang, C.-W. Chiu,H. M. Chen, P.-T. Chou, Adv. Mater. 2018, 30, 1706592.

[93] A. Wang, Y. Guo, F. Muhammad, Z. Deng, Chem. Mater. 2017,29, 6493.

[94] G. Xing, M. H. Kumar, W. K. Chong, X. Liu, Y. Cai, H. Ding,M. Asta, M. Gr-tzel, S. Mhaisalkar, N. Mathews, T. C. Sum,Adv. Mater. 2016, 28, 8191.

[95] T. M. Koh, T. Krishnamoorthy, N. Yantara, C. Shi, W. L. Leong,P. P. Boix, A. C. Grimsdale, S. G. Mhaisalkar, N. Mathews, J.Mater. Chem. A 2015, 3, 14996.

[96] A. Wang, X. Yan, M. Zhang, S. Sun, M. Yang, W. Shen, X. Pan,P. Wang, Z. Deng, Chem. Mater. 2016, 28, 8132.

[97] D. S. Dolzhnikov, C. Wang, Y. Xu, M. G. Kanatzidis, E. A.Weiss, Chem. Mater. 2017, 29, 7901.

[98] D. V. Talapin, J.-S. Lee, M. V. Kovalenko, E. V. Shevchenko,Chem. Rev. 2010, 110, 389.

[99] A. T. Fafarman, W.-K. Koh, B. T. Diroll, D. K. Kim, D.-K. Ko,S. J. Oh, X. Ye, V. Doan-Nguyen, M. R. Crump, D. C. Reifs-nyder, C. B. Murray, C. R. Kagan, J. Am. Chem. Soc. 2011, 133,15753.

[100] D. M. Balazs, D. N. Dirin, H.-H. Fang, L. Protesescu, G. H. T.Brink, B. J. Kooi, M. V. Kovalenko, M. A. Loi, ACS Nano 2015,9, 11951.

[101] H. Katagiri, K. Saitoh, T. Washio, H. Shinohara, T. Kuruma-dani, S. Miyajima, Sol. Energy Mater. Sol. Cells 2001, 65, 141.

[102] S. Ghosh, S. Paul, S. K. De, Part. Part. Syst. Charact. 2018, 35,1800199.

[103] R. Begum, M. R. Parida, A. L. Abdelhady, B. Murali, N. M.Alyami, G. H. Ahmed, M. N. Hedhili, O. M. Bakr, O. F.Mohammed, J. Am. Chem. Soc. 2017, 139, 731.

[104] W. van der Stam, J. J. Geuchies, T. Altantzis, K. H. W. V. D.Bos, J. D. Meeldijk, S. V. Aert, S. Bals, D. Vanmaekelbergh,C. D. M. Donega, J. Am. Chem. Soc. 2017, 139, 4087.

[105] H. Liu, Z. Wu, J. Shao, D. Yao, H. Gao, Y. Liu, W. Yu, H.Zhang, B. Yang, ACS Nano 2017, 11, 2239.

[106] M. Leng, Z. Chen, Y. Yang, Z. Li, K. Zeng, K. Li, G. Niu, Y. He,Q. Zhou, J. Tang, Angew. Chem. Int. Ed. 2016, 55, 15012;Angew. Chem. 2016, 128, 15236.

[107] B. Yang, J. Chen, F. Hong, X. Mao, K. Zheng, S. Yang, Y. Li, T.Pullerits, W. Deng, K. Han, Angew. Chem. Int. Ed. 2017, 56,12471; Angew. Chem. 2017, 129, 12645.

[108] Y. Lou, M. Fang, J. Chen, Y. Zhao, Chem. Commun. 2018, 54,3779.

[109] M. Gao, C. Zhang, L. Lian, J. Guo, Y. Xia, F. Pan, X. Su, J.Zhang, H. Li, D. Zhang, J. Mater. Chem. C 2019, 7, 3688.

[110] J.-L. Xie, Z.-Q. Huang, B. Wang, W.-J. Chen, W.-X. Lu, X. Liu,J.-L. Song, Nanoscale 2019, 11, 6719.

[111] C. Carrillo-Carriln, S. C#rdenas, B. M. Simonet, M. Valc#rcel,Chem. Commun. 2009, 5214.

[112] B. Pradhan, G. S. Kumar, S. Sain, A. Dalui, U. K. Ghorai, S. K.Pradhan, S. Acharya, Chem. Mater. 2018, 30, 2135.

[113] L. Zhou, Y.-F. Xu, B.-X. Chen, D.-B. Kuang, C.-Y. Su, Small2018, 14, 1703762.

[114] S. E. Creutz, E. N. Crites, M. C. D. Siena, D. R. Gamelin, NanoLett. 2018, 18, 1118.

[115] B. Yang, X. Mao, F. Hong, W. Meng, Y. Tang, X. Xia, S. Yang,W. Deng, K. Han, J. Am. Chem. Soc. 2018, 140, 17001.

[116] M. R. Filip, X. Liu, A. Miglio, G. Hautier, F. Giustino, J. Phys.Chem. C 2018, 122, 158.

[117] R. T. Shannon, Acta Crystallogr. Sect. A 1976, 32, 751.[118] L. Zhou, J.-F. Liao, Z.-Gu. Huang, X.-D. Wang, Y.-F. Xu, H.-Y.

Chen, D.-B. Kuang, C.-Y. Su, ACS Energy Lett. 2018, 3, 2613.[119] P. Yang, G. Liu, B. Liu, X. Liu, Y. Lou, J. Chen, Y. Zhao, Chem.

Commun. 2018, 54, 11638.[120] J. Shamsi, A. S. Urban, M. Imran, L. D. Trizio, L. Manna, Chem.

Rev. 2019, 119, 3296.[121] Q. Chen, N. D. Marco, Y. (M.) Yang, T.-B. Song, C.-C. Chen, H.

Zhao, Z. Hong, H. Zhou, Y. Yang, Nano Today 2015, 10, 355.[122] N. K. Noel, S. D. Stranks, A. Abate, C. Wehrenfennig, S.

Guarnera, A.-A. Haghighirad, A. Sadhanala, G. E. Eperon,S. K. Pathak, M. B. Johnston, A. Petrozza, L. M. Herz, H. J.Snaith, Energy Environ. Sci. 2014, 7, 3061.

[123] Y. Zhang, J. Yin, M. R. Parida, G. H. Ahmed, J. Pan, O. M.Bakr, J.-L. Bredas, O. F. Mohammed, J. Phys. Chem. Lett. 2017,8, 3173.

[124] K. Xu, A. Meijerink, Chem. Mater. 2018, 30, 5346.[125] S. Hou, Y. Guo, Y. Tang, Q. Quan, ACS Appl. Mater. Interfaces

2017, 9, 18417.[126] Y. Chang, Y. Yoon, G. Li, E. Xu, S. Yu, C. Lu, Z. Wang, Y. He,

C. Lin, B. K. Wagner, V. V. Tsukruk, Z. Kang, N. Thadhani, Y.Jiang, Z. Lin, ACS Appl. Mater. Interfaces 2018, 10, 37267.

[127] Q. Fan, J. Ma, Q. Xu, J. Wang, Y. Ma, Ind. Eng. Chem. Res. 2018,57, 6171.

[128] Y. Zhang, M. Wang, Y. Zheng, H. Tan, B. Y. Hsu, Z. Yang, S. Y.Wong, A. Y. Chang, M. Choolani, X. Li, J. Wang, Chem. Mater.2013, 25, 2976.

[129] S. Huang, Z. Li, L. Kong, N. Zhu, A. Shan, L. Li, J. Am. Chem.Soc. 2016, 138, 5749.

Manuscript received: April 18, 2019Accepted manuscript online: May 14, 2019Version of record online: December 12, 2019

AngewandteChemieReviews

1046 www.angewandte.org T 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2020, 59, 1030 – 1046