The halogen chemistry of halide perovskites

In analogy to the role of the oxide anion in oxide perovskites, the halogen chemistry of halide perovskites significantly contributes to the chemical, physical, and optoelectronic properties of this class of materials.Investigating and manipulating the halide site has improved our understanding of the electronic structure, reactivity, defect chemistry, and ionic conductivity of halide perovskites.A clear understanding of the chemistry of the halide site is expected to (i) yield novel perovskites with targeted optoelectronic properties and controlled defect chemistry, (ii) improve the efficiency and stability of perovskite optoelectronic devices, and (iii) expand the scope of postsynthetic reactions between perovskites and small molecules. The oxygen chemistry of oxide perovskites has been studied for decades, revealing reactivity, transport, and electronic properties enabled by the oxide anion. Similarly, the halide anion dictates the properties and unusual phenomena observed in halide perovskites. Manipulating the halide site has improved the performance and stability of halide perovskites in a range of optoelectronic technologies, most commonly as components in solar cells and light-emitting diodes. Further, the redox activity of the halides allows for controlled electronic doping and halide exchange, as well as for the capture and separation of halogen gases. Herein, we summarize the key aspects of the chemistry of the perovskite halide site. We close with a discussion of pseudohalide perovskites, which hold commonalities with the halide perovskites. The oxygen chemistry of oxide perovskites has been studied for decades, revealing reactivity, transport, and electronic properties enabled by the oxide anion. Similarly, the halide anion dictates the properties and unusual phenomena observed in halide perovskites. Manipulating the halide site has improved the performance and stability of halide perovskites in a range of optoelectronic technologies, most commonly as components in solar cells and light-emitting diodes. Further, the redox activity of the halides allows for controlled electronic doping and halide exchange, as well as for the capture and separation of halogen gases. Herein, we summarize the key aspects of the chemistry of the perovskite halide site. We close with a discussion of pseudohalide perovskites, which hold commonalities with the halide perovskites. Perovskites are crystalline solids with the general formula ABX3, where X is an anionic ligand such as oxide, chalcogenide, or halide. Historically, oxide perovskites have received the most attention in this family of materials. Although halide perovskites are also not new―dating back to the 1800s [1.Berzelius J.J. Untersuchung über die eigenschaften des tellurs.Ann. Phys. 1834; 108: 577-627Google Scholar, 2.Cross W. Hillebrand W.F. Contributions to the mineralogy of the rocky mountains.in: Bulletin of the United States Geological Survey No. 20. Government Printing Office, 1885: 57Google Scholar, 3.Wells H.L. Über die cäsium- und kalium-bleihalogenide.Z. Anorg. Allg. Chem. 1893; 3: 195-210Google Scholar, 4.Elliott N. Pauling L. The crystal structure of cesium aurous auric chloride, Cs2AuAuCl6, and cesium argentous auric chloride, Cs2AgAuCl6.J. Am. Chem. Soc. 1938; 60: 1846-1851Google Scholar]―their study has recently been invigorated through the discovery of their outstanding semiconducting properties. The lead-halide perovskites have emerged as candidates for high-efficiency and low-cost solar absorbers [5.Kojima A. et al.Organometal halide perovskites as visible-light sensitizers for photovoltaic cells.J. Am. Chem. Soc. 2009; 131: 6050-6051Google Scholar, 6.Kim H.-S. et al.Lead iodide perovskite sensitized all-solid-state submicron thin film mesoscopic solar cell with efficiency exceeding 9%.Sci. Rep. 2012; 2: 591Google Scholar, 7.Lee M.M. et al.Efficient hybrid solar cells based on meso-superstructured organometal halide perovskites.Science. 2012; 338: 643Google Scholar], while other applications such as phosphors [8.Protesescu L. et al.Nanocrystals of cesium lead halide perovskites (CsPbX3, X = Cl, Br, and I): novel optoelectronic materials showing bright emission with wide color gamut.Nano Lett. 2015; 15: 3692-3696Google Scholar,9.Smith M.D. et al.Tuning the luminescence of layered halide perovskites.Chem. Rev. 2019; 119: 3104-3139Google Scholar], light-emitting diodes [10.Chondroudis K. Mitzi D.B. Electroluminescence from an organic−inorganic perovskite incorporating a quaterthiophene dye within lead halide perovskite layers.Chem. Mater. 1999; 11: 3028-3030Google Scholar,11.Lin K. et al.Perovskite light-emitting diodes with external quantum efficiency exceeding 20 per cent.Nature. 2018; 562: 245-248Google Scholar], and scintillators [12.Chen Q. et al.All-inorganic perovskite nanocrystal scintillators.Nature. 2018; 561: 88-93Google Scholar] have also shown promise. The halide site dictates many of the defining characteristics of halide perovskites: solution-state self-assembly, stabilization of low-valent metals, soft lattices with bulk moduli (see Glossary) that can be as small as those of organic molecules, structures that exhibit fast and slow dynamics, and electronic structures that commonly yield shallow trap states. Indeed, manipulating the halide site has enabled key optoelectronic properties to be optimized [e.g., optical bandgap and photoluminescence (PL) color, electronic transport, and defect chemistry], has explained unusual phenomena observed in perovskite-based devices (e.g., halide conductivity and light-induced halide segregation in mixed-halide compositions), and has resulted in new materials and applications (e.g., radioactive I2 capture, halogen gas separation). Here, we compile our understanding of the halogen chemistry of halide perovskites. We describe the role of the halogen in dictating perovskite structure, optical absorption and emission, electronic and ionic transport, defect chemistry, and reactivity with small molecules. We conclude by briefly reviewing the closely related pseudohalide perovskites. We refer interested readers to recent reviews on halide conductivity [13.Senocrate A. Maier J. Solid-state ionics of hybrid halide perovskites.J. Am. Chem. 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Substitutions at the A- and B-sites of ABO3 oxide perovskites have led to important applications in electroceramics, including dielectrics and piezoelectrics [18.Bhalla A.S. et al.The perovskite structure—a review of its role in ceramic science and technology.Mater. Res. Innov. 2000; 4: 3-26Google Scholar]. Non-stoichiometry at the oxide site (e.g., ABO3±δ) and oxygen point defect reactions, however, are appreciated for their role in modern discoveries, including high-temperature cuprate superconductors [19.Bednorz J.G. Müller K.A. Possible high Tc superconductivity in the Ba−La−Cu−O system.Z. Phys. B. 1986; 64: 189-193Google Scholar]. A detailed treatment of the chemistry of the oxide site has afforded a good understanding of the bonding and electronic structure, electronic and ionic transport, and reactivity of oxide perovskites. As members of the same materials family, parallels in the role of the X-site can be made between the oxide and halide perovskites. The critical temperature (Tc) for superconductivity in YBa2Cu3O7–δ is sensitive to the local Cu–O coordination changes induced by oxygen non-stoichiometry [20.Cava R.J. Synthesis and crystal chemistry of high-Tc oxide superconductor.in: Jin S. Processing and Properties of High-TC Superconductors. World Scientific, 1993: 492Google Scholar]; further, interactions between electronic charge carriers and oxygen defects influence the Tc upon aliovalent doping of La2CuO4 (i.e., La2–xMxCuO4, M = Ba2+, Sr2+) [21.Rao C.N.R. Perovskite oxides and high-temperature superconductivity.Ferroelectrics. 1990; 102: 297-308Google Scholar,22.Maier J. Physical Chemistry of Ionic Materials: Ions and Electrons in Solids. John Wiley & Sons, 2004Google Scholar]. The proposed mechanism for superconductivity in these two perovskites involves holes forming O– (O2− + h+) or O22– peroxo dimers [21.Rao C.N.R. Perovskite oxides and high-temperature superconductivity.Ferroelectrics. 1990; 102: 297-308Google Scholar]. Conversely, at elevated temperatures, oxygen-vacancy–mediated transport enables oxide conductivity in solid electrolytes such as alloyed and doped LaMO3 (M = Ga3+, Al3+, In3+, Sc3+, Y3+) perovskites [23.Kharton V.V. et al.Transport properties of solid oxide electrolyte ceramics: a brief review.Solid State Ion. 2004; 174: 135-149Google Scholar]. The electronic and ionic transport properties of halide perovskites are similarly sensitive to halogen point defects and halide site occupancy. Oxygen exchange is a critically important defect reaction that describes the equilibrium between the oxide lattice, oxygen vacancies, electrons, and gaseous O22OOX⇌2VO••+4e′+O2g[1] where OOX is the pristine oxygen lattice site, VO•• is a dicationic oxygen vacancy, and e′ is an electron (given in the Kröger–Vink notation). Manipulating this equilibrium through the O2 partial pressure and temperature (generally >700 K) has improved our understanding of oxide diffusion and the thermodynamics of ionic and electronic conductivity [22.Maier J. Physical Chemistry of Ionic Materials: Ions and Electrons in Solids. John Wiley & Sons, 2004Google Scholar]. The rational design of complex oxide perovskites, for instance to promote or resist oxygen exchange, has been informed by composition-dependent diffusion kinetics and thermodynamic modeling of dopants. Halide perovskites exhibit analogous point defect reactions, with particularly low activation barriers for the formation of halogen vacancies, calling for similar chemical approaches to improve stability. Thus, uncovering the parallels between the chemistry of oxide perovskites and that of their halide congeners should promote a deeper understanding of the latter (see Outstanding questions). Halide perovskites that adopt the 3D ABX3 structure feature B-sites with octahedral coordination to six halides (X); each halide bridges two B-sites to afford an anionic BX3– framework of corner-sharing octahedra. Organic and/or inorganic A-site monocations reside in the cuboctahedral voids, providing charge compensation (Figure 1A ). The B site is occupied by a 2+ metal in the ABX3 single perovskites (e.g., Pb2+, Hg2+, Mn2+ [9.Smith M.D. et al.Tuning the luminescence of layered halide perovskites.Chem. Rev. 2019; 119: 3104-3139Google Scholar]), whereas two distinct B and B′ sites alternate in the A2BB′X6 double perovskites, affording an average charge of 2+ (e.g., Ag+ and Bi3+ or Sn4+ and a vacancy; Figure 1B) [24.Wolf N.R. et al.Doubling the stakes: the promise of halide double perovskites.Angew. Chem. Int. Ed. 2021; 60: 16264-16278Google Scholar]. The 2D perovskites are related to the 3D structure through the formal addition of an AX salt (ABX3 + AX ➔ A2BX4), resulting in both bridging and terminal halides as the inorganic sheets are separated by larger A-site cations (Figure 1C) [25.Saparov B. Mitzi D.B. Organic–inorganic perovskites: structural versatility for functional materials design.Chem. Rev. 2016; 116: 4558-4596Google Scholar,26.Smith M.D. et al.The diversity of layered halide perovskites.Annu. Rev. Mater. Res. 2018; 48: 111-136Google Scholar]. Linus Pauling's radius ratio rules dictate the size of the cation that can support octahedral coordination of a given halide. Thus, as the anion/cation radius ratio increases, lower coordination numbers are preferred. For example, we are not aware of iodide perovskites with B-site cations as small as Mn2+ or Cd2+. However, CsCdX3 (X = Cl−, Br−) and KMnCl3 crystallize as perovskites with the smaller halides [27.Natta G. Passerini L. Isomorphism, polymorphism and morphotropy, I. 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Compositions that deviate from the tolerance factor for an ideal, undistorted perovskite, with 180° B–X–B angles (Figure 1B), will either adopt a lower-symmetry perovskite structure (e.g., with tilted octahedra) (Figure 1A) or a non-perovskite structure [33.Steele J.A. et al.Trojans that flip the black phase: impurity-driven stabilization and spontaneous strain suppression in γ-CsPbI3 perovskite.J. Am. Chem. Soc. 2021; 143: 10500-10508Google Scholar]. In analogy to the effects of hydrostatic pressure or temperature, the halide composition can also direct preferential crystallization of desired crystallographic phases. For example, Cs-substitution has been shown to induce a cubic-to-tetragonal phase transition in [CH(NH2)2]yCs1–yPb(BrxI1–x)3 perovskite thin films [34.Beal R.E. et al.Structural origins of light-induced phase segregation in organic-inorganic halide perovskite photovoltaic materials.Matter. 2020; 2: 207-219Google Scholar]. Further increasing the effective radius of the A-site cation separates the metal–halide octahedra. This is often achieved with organoammonium cations and their effective size, charge, and shape determines the dimensionality (0D, 1D, 2D) and connectivity (corner-, edge-, face-sharing) of the metal–halide octahedra [26.Smith M.D. et al.The diversity of layered halide perovskites.Annu. Rev. Mater. Res. 2018; 48: 111-136Google Scholar]. Halogen bonding can also template the crystallization of 2D perovskites. For example, interactions between organohalogens and inorganic halides direct the assembly of some 2D perovskites [35.Connor B.A. et al.Dimensional reduction of the small-bandgap double perovskite Cs2AgTlBr6.Chem. Sci. 2020; 11: 7708-7715Google Scholar,36.Lemmerer A. Billing D.G. Effect of heteroatoms in the inorganic–organic layered perovskite-type hybrids [(ZCnH2nNH3)2PbI4], n = 2, 3, 4, 5, 6; Z = OH, Br and I; and [(H3NC2H4S2C2H4NH3)PbI4].CrystEngComm. 2010; 12: 1290-1301Google Scholar]. Lighter halides are reported to preferentially occupy the bridging sites in mixed-halide 2D perovskites, whereas heavier halides tend to occupy the terminal sites [37.Suzuki Y. Kubo H. Distribution of Cl- and Br- ions in mixed crystals (CH3NH3)2Cu(Cl1-xBrx)4.J. Phys. Soc. Jpn. 1983; 52: 1420-1426Google Scholar,38.Sourisseau S. et al.Hybrid perovskite resulting from the solid-state reaction between the organic cations and perovskite layers of α1-(Br-(CH2)2-NH3)2PbI4.Inorg. Chem. 2007; 46: 6148-6154Google Scholar]. The electronic structure of halide perovskites is mainly set by the B-site metal and the halide. In lead-halide perovskites, lead s-orbitals and halide p-orbitals compose the valence band, whereas the conduction band primarily comprises lead p-orbitals (Figure 2A ) [39.Umebayashi T. et al.Electronic structures of lead iodide based low-dimensional crystals.Phys. Rev. B. 2003; 67155405Google Scholar,40.Goesten M.G. Hoffmann R. Mirrors of bonding in metal halide perovskites.J. Am. Chem. Soc. 2018; 140: 12996-13010Google Scholar]. The presence of lead, iodine, and other high-atomic-number elements in the perovskite structure necessitates treatment of spin–orbit coupling effects for accurate band structures and has motivated the study of various relativistic phenomena, including Rashba–Dresselhaus spin-splitting at the band edges [41.Kepenekian M. Even J. Rashba and Dresselhaus couplings in halide perovskites: accomplishments and opportunities for spintronics and spin–orbitronics.J. Phys. Chem. Lett. 2017; 8: 3362-3370Google Scholar]. Alloying at the halide site in 3D perovskites produces solid solutions of Cl−/Br− perovskites and Br−/I− perovskites, evidenced by powder X-ray diffraction measurements showing a continuous lattice expansion as larger halides are alloyed. Further, the absorption spectra of mixed-halide perovskites show a continuous shift in absorption onset with halide ratio for most compositions, although a discontinuity has been observed for x = 0.5 in (CH3NH3)Pb(BrxI1–x)3 films, attributed to imperfect mixing (Figure 2B) [42.Hoke E.T. et al.Reversible photo-induced trap formation in mixed-halide hybrid perovskites for photovoltaics.Chem. Sci. 2015; 6: 613-617Google Scholar]. Because of the larger halide contribution to the valence band, the bandgap can be increased by substituting more electronegative halides (Figure 2A). For example, the bandgaps of 3D lead-halide perovskites can vary between 1.5 (for iodides) and 3.0 eV (for chlorides), while mixed-halide compositions show intermediate gaps (Figure 2B) [26.Smith M.D. et al.The diversity of layered halide perovskites.Annu. Rev. Mater. Res. 2018; 48: 111-136Google Scholar]. The importance of halide–halide interactions is evident in double perovskites with vacant B sites (□), such as A2Sn□X6. Although these materials are structurally 0D, they display small bandgaps and large band dispersion. Their 3D electronic structure arises from 90° interactions between halides across vacant B sites [43.Slavney A.H. et al.A pencil-and-paper method for elucidating halide double perovskite band structures.Chem. Sci. 2019; 10: 11041-11053Google Scholar]. For 3D perovskites, monovalent cations that fit the A-site cavity [e.g., CH3NH3+, Cs+, CH(NH2)2+] are few and only contribute to the electronic structure by inducing structural distortions in the inorganic framework. Varying the A-site composition can either increase the optical bandgap by tilting the BX6n− octahedra, or decrease the bandgap upon lattice contraction [44.Yang T.-Y. et al.The significance of ion conduction in a hybrid organic–inorganic lead-iodide-based perovskite photosensitizer.Angew. Chem. Int. Ed. 2015; 54: 7905-7910Google Scholar]. The 2D perovskites exhibit similar orbital contributions to the band edges as their 3D counterparts [39.Umebayashi T. et al.Electronic structures of lead iodide based low-dimensional crystals.Phys. Rev. B. 2003; 67155405Google Scholar], but due to electronic confinement effects, photoexcitation produces excitons (bound electron–hole pairs) instead of free charge carriers. Excitons manifest as sharp peaks below the bandgap absorption onset. With halide alloying, the excitonic absorption peak also shows a continuous blueshift from ~2.3 eV for the pure Pb−I to ~3.6 eV for the pure Pb−Cl 2D perovskites and the excitonic PL tracks similarly [45.Lanty G. et al.Room-temperature optical tunability and inhomogeneous broadening in 2D-layered organic–inorganic perovskite pseudobinary alloys.J. Phys. Chem. Lett. 2014; 5: 3958-3963Google Scholar]. The width of the excitonic absorption peak has been used as an indicator of local halide variation in (PEA)2PbClxBr4−x and (PEA)2PbBrxI4−x (PEA = phenethylammonium) [45.Lanty G. et al.Room-temperature optical tunability and inhomogeneous broadening in 2D-layered organic–inorganic perovskite pseudobinary alloys.J. Phys. Chem. Lett. 2014; 5: 3958-3963Google Scholar,46.Kitazawa N. Excitons in two-dimensional layered perovskite compounds: (C6H5C2H4NH3)2Pb(Br,I)4 and (C6H5C2H4NH3)2Pb(Cl,Br)4.Mater. Sci. Eng. B. 1997; 49: 233-238Google Scholar]. Here, the pure halides show narrow peaks, and the broader peaks seen in mixed-halide compositions have been attributed to locally disordered halides that change the potential energy landscape sampled by the exciton. Most 2D lead-halide perovskites show intense, narrow excitonic PL with high color purity, promising for phosphor applications, with the halides modulating the emission color [45.Lanty G. et al.Room-temperature optical tunability and inhomogeneous broadening in 2D-layered organic–inorganic perovskite pseudobinary alloys.J. Phys. Chem. Lett. 2014; 5: 3958-3963Google Scholar]. For example, in (PEA)2PbX4, X = I− produces green light, whereas X = Br− yields blue light. Certain 2D perovskites also emit broadband white light [47.Smith M.D. Karunadasa H.I. White-light emission from layered halide perovskites.Acc. Chem. Res. 2018; 51: 619-627Google Scholar]. Here too, halide mixing can tune the color rendering index (CRI) and correlated color temperature (CCT) of the emission. For example, in the prototypical white-light–emitting perovskite (N-MEDA)PbClxBr4–x (N-MEDA = N1-methylethane-1,2-diammonium), the CRI was improved from 82 (x = 0) to 85 (x = 0.5) with halide mixing in isostructural analogs [48.Dohner E.R. et al.Self-assembly of broadband white-light emitters.J. Am. Chem. Soc. 2014; 136: 1718-1721Google Scholar]. Likewise, the chloride and bromide derivatives of (EDBE)PbX4 (Figure 1C) afford "cold" (CCT = 5509 K) and "warm" (CCT = 3990 K) white light, respectively, although here the chloride perovskite adopts the flat (001) perovskite sheets and the bromide perovskite adopts the corrugated (110) perovskite sheets [49.Dohner E.R. et al.Intrinsic white-light emission from layered hybrid perovskites.J. Am. Chem. Soc. 2014; 136: 13154-13157Google Scholar]. Many of the phenomena highlighted in this review require significant halide conductivity, particularly for bulk reactions in 3D perovskites where, unlike in many 2D perovskites, there is no organic layer to facilitate ion transport or volume expansion. Indeed, halide conductivity is invoked to explain anomalous behavior observed in 3D perovskite-based devices including current–voltage hysteresis [50.Snaith H.J. et al.Anomalous hysteresis in perovskite solar cells.J. Phys. Chem. Lett. 2014; 5: 1511-1515Google Scholar,51.Meloni S. et al.Ionic polarization-induced current–voltage hysteresis in CH3NH3PbX3 perovskite solar cells.Nat. Commun. 2016; 7: 10334Google Scholar], switchable photovoltaic effects [52.Xiao Z. et al.Giant switchable photovoltaic effect in organometal trihalide perovskite devices.Nat. Mater. 2015; 14: 193-198Google Scholar], large low-frequency dielectric responses [44.Yang T.-Y. et al.The significance of ion conduction in a hybrid organic–inorganic lead-iodide-based perovskite photosensitizer.Angew. Chem. Int. Ed. 2015; 54: 7905-7910Google Scholar,53.Juarez-Perez E.J. et al.Photoinduced giant dielectric constant in lead halide perovskite solar cells.J. Phys. Chem. Lett. 2014; 5: 2390-2394Google Scholar,54.Wilson J.N. et al.Dielectric and ferroic properties of metal halide perovskites.APL Mater. 2019; 7: 5079633Google Scholar], and band bending and alignment at interfaces [55.Bertoluzzi L. et al.Mobile ion concentration measurement and open-access band diagram simulation platform for halide perovskite solar cells.Joule. 2020; 4: 109-127Google Scholar,56.Lopez-Varo P. et al.Effects of ion distributions on charge collection in perovskite solar cells.ACS Energy Lett. 2017; 2: 1450-1453Google Scholar]. Ionic semiconductors exhibit varying degrees of ionic conductivity owing to structural disorder and the mobility of ions and point defects (see Figure IA,B in Box 1). The mixed ionic-electronic conductivity of inorganic halide perovskites was established in the 1980s [57.Mizusaki J. et al.Ionic conduction of the perovskite-type halides.Solid State Ion. 1983; 11: 203-211Google Scholar]. The initial work was motivated by oxide perovskite and antiperovskite analogs exhibiting oxide conductivity, with structural data suggesting large displacements of the oxide anion. Mizusaki and colleagues [57.Mizusaki J. et al.Ionic conduction of the perovskite-type halides.Solid State Ion. 1983; 11: 203-211Google Scholar] reported significant halide conductivity for CsPbX3 (X = Cl−, Br−) and KMnCl3 and proposed halogen vacancies as the mobile species based on gravimetric changes of coulometric reaction cells (see Figure IC,D in Box 1). The activation energy (Ea) of halogen vacancy migration was relatively small for CsPbX3 (X = Cl−, Br−; 0.25-0.29 eV) and for KMnCl3 (0.39 eV) [57.Mizusaki J. et al.Ionic conduction of the perovskite-type halides.Solid State Ion. 1983; 11: 203-211Google Scholar]. Halide conductivity was later suggested in the hybrid perovskites: (CH3NH3)MCl3 (M = Ge2+, Sn2+) [58.Yamada K. et al.Chloride ion conductor CH3NH3GeCl3 studied by Rietveld analysis of X-ray diffraction and 35Cl NMR.Solid State Ion. 1995; 79: 152-157Google Scholar,59.Yamada K. et al.Phase transition and electric conductivity of ASnCl3 (A = Cs and CH3NH3).Bull. Chem. Soc. Jpn. 1998; 71: 127-134Google Scholar] and (CH3NH3)PbX3 (X = Cl−, Br−) [60.Maeda M. et al.Dielectric studies on CH3NH3PbX3 (X = Cl and Br) single crystals.J. Phys. Soc. Jpn. 1997; 66: 1508-1511Google Scholar]. Halide conductivity in (CH3NH3)PbI3 had been extensively discussed [50.Snaith H.J. et al.Anomalous hysteresis in perovskite solar cells.J. Phys. Chem. Lett. 2014; 5: 1511-1515Google Scholar,52.Xiao Z. et al.Giant switchable photovoltaic effect in organometal trihalide perovskite devices.Nat. Mater. 2015; 14: 193-198Google Scholar,61.Azpiroz J.M. et al.Defect migration in methylammonium lead iodide and its role in perovskite solar cell operation.Energy Environ. Sci. 2015; 8: 2118-2127Google Scholar]; however, bulk ionic conductivity measurements were not directly revisited until 2015 by Maier, Gregori, and colleagues [44.Yang T.-Y. et al.The significance of ion conduction in a hybrid organic–inorganic lead-iodide-based perovskite photosensitizer.Angew. Chem. Int. Ed. 2015; 54: 7905-7910Google Scholar]. Polycrystalline (CH3NH3)PbI3 exhibits significant ionic conductivity in the dark and the iodine vacancy was established as the dominant mobile defect from a reaction cell (see Figure ID in Box 1) [12.Chen Q. et al.All-inorganic perovskite nanocrystal scintillators.Nature. 2018; 561: 88-93Google Scholar,44.Yang T.-Y. et al.The significance of ion conduction in a hybrid organic–inorganic lead-iodide-based perovskite photosensitizer.Angew. Chem. Int. Ed. 2015; 54: 7905-7910Google Scholar]. Iodine vacancies were confirmed as the dominant mobile defect in (CH3NH3)PbI3 based on the dependencies of the ionic conductivity on I2 partial pressure [p(I2)] and acceptor doping, as well as various solid-state NMR measurements [12.Chen Q. et al.All-inorganic perovskite nanocrystal scintillators.Nature. 2018; 561: 88-93Google Scholar,62.Senocrate A. et al.The nature of ion conduction in methylammonium lead iodide: a multimethod approach.Angew. Chem. Int. Ed. 2017; 56: 7755-7759Google Scholar,63.Senocrate A. et al.Slow CH3NH3+ diffusion in CH3NH3PbI3 under light measured by solid-state NMR and tracer diffusion.J. Phys. Chem. C. 2018; 122: 21803-21806Google Scholar].Box 1Halogen point defects and mixed ionic-electronic conductivity in halide perovskitesSeveral halide perovskites are mixed ionic–electronic conductors. These perovskite semiconductors exhibit bandgap energies greater than 1 eV (see Figure 2A in main text); thus, the dark electrical conductivity derives from intrinsic carrier concentrations (Figure IA, left) or carrier concentrations modified through doping. The addition of donors generates an excess electron concentration (n-type doping; Figure IA, middle), whereas acceptors generate an excess hole concentration (p-type doping; Figure IA, right). Bulk ionic conductivity in the halide perovskites is attributed to mobile point defects, particularly halogen vacancies, in analogy to the well-established relationship between oxygen vacancies and oxide mobility in oxide perovskites. Halide (X–) loss results in a positively charged vacancy or point defect, which must be compensated by another charged defect or charge carrier (holes or electrons). Among the possible halogen vacancy–based defect reactions, halogen off-gassing (positively charged halogen vacancies compensated by electrons; Figure IB, top left), Schottky disorder (positively charged halogen vacancies compensated by cation vacancies; Figure IB, top right), and anti-Frenkel disorder (positively charged halogen vacancies compensated by negatively charged halogen interstitials; Figure IB, bottom) have been the focus of investigation in the defect chemistry of the halide perovskites. Bulk halide conductivity occurs due to the presence of halogen vacancies and has been proposed to follow a vacancy-assisted hopping mechanism (Figure IC). Ionic conductivity was established using solid-state electrochemistry techniques, particularly electrolysis of coulometric reaction cells based on CsPbBr3 (Figure ID, top) [57.Mizusaki J. et al.Ionic conduction of the perovskite-type halides.Solid State Ion. 1983; 11: 203-211Google Scholar], CsPbCl3 (identical construction and measurement as CsPbBr3; not show