Photoinduced Anion Segregation in Mixed Halide Perovskites

Ion migration represents an intrinsic perovskite instability, which degrades solar cell performance. Suppressing ion migration is hence key to improving the stability of perovskite solar cells.Halide segregation is a reversible process whereby illumination induces the formation of narrow bandgap I-rich and wide bandgap Br-rich domains within the parent composite. Subsequent emission occurs primarily from I-rich inclusions. Alloyed lead halide perovskites have taken a dominant role in the quest for third-generation solar cells. This is due to optimal light-harvesting properties, which can be tuned across the visible spectrum by mixing halide (X = Cl–, Br–, and I–) anions and A+ cations (A+ = FA+, MA+, and Cs+). Durability issues related to ion movement within the perovskite lattice, however, impede large-scale commercialization. Uniformly mixed halide perovskites [e.g., APb(I1–xBrx)3] reversibly segregate into narrow bandgap I-rich and wide bandgap Br-rich domains during continuous visible illumination. Subsequent I-rich domains reduce local open circuit voltages and decrease mixed halide perovskite solar cell power conversion efficiencies. In this review, we assess the known effects of halide segregation on the structural and optical properties of mixed halide materials, discuss ongoing research to suppress the phenomenon, and provide a mechanistic overview of its underlying origins. Alloyed lead halide perovskites have taken a dominant role in the quest for third-generation solar cells. This is due to optimal light-harvesting properties, which can be tuned across the visible spectrum by mixing halide (X = Cl–, Br–, and I–) anions and A+ cations (A+ = FA+, MA+, and Cs+). Durability issues related to ion movement within the perovskite lattice, however, impede large-scale commercialization. Uniformly mixed halide perovskites [e.g., APb(I1–xBrx)3] reversibly segregate into narrow bandgap I-rich and wide bandgap Br-rich domains during continuous visible illumination. Subsequent I-rich domains reduce local open circuit voltages and decrease mixed halide perovskite solar cell power conversion efficiencies. In this review, we assess the known effects of halide segregation on the structural and optical properties of mixed halide materials, discuss ongoing research to suppress the phenomenon, and provide a mechanistic overview of its underlying origins. APbX3-type lead halide perovskites [A+ = Cs+, CH3NH3+ (MA+), and CH(NH2)2+ (FA+); X = I–, Br–, and Cl–] were discovered over a century ago [1.Wells H.L. Über die cäsium- und kalium-bleihalogenide.Z. Anorg. Allg. Chemie. 1893; 3 (in German): 195-210Crossref Scopus (144) Google Scholar]. 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When coupled to fully solution-processable fabrication [5.Sun S. et al.The origin of high efficiency in low-temperature solution-processable bilayer organometal halide hybrid solar cells.Energy Environ. Sci. 2014; 7: 399-407Crossref Google Scholar], perovskites represent an inexpensive and efficient light absorbing material for fabricating third-generation solar cells. While MAPbI3 was originally responsible for initiating interest in perovskite photovoltaics [9.Manser J.S. et al.Intriguing optoelectronic properties of metal halide perovskites.Chem. Rev. 2016; 21: 12956-13008Crossref Scopus (1087) Google Scholar], state-of-the-art devices today often employ binary/ternary cation, double-halide perovskite [e.g., FAyMAzCs1–y–zPb(I1–xBrx)3] thin films [10.McMeekin D.P. et al.A mixed-cation lead mixed-halide perovskite absorber for tandem solar cells.Science. 2016; 351: 151Crossref PubMed Scopus (2167) Google Scholar, 11.Saliba M. et al.Cesium-containing triple cation perovskite solar cells: improved stability, reproducibility, and high efficiency.Energy Environ. Sci. 2016; 9: 1989-1997Crossref PubMed Google Scholar, 12.Ono K.L. et al.Progress on perovskite materials and solar cells with mixed cations and halide anions.ACS Appl. Mater. Interfaces. 2017; 9: 30197-30246Crossref PubMed Scopus (368) Google Scholar, 13.Li Z. et al.Stabilizing perovskite structures by tuning tolerance factor: formation of formamidinium lead iodide solid state alloys.Chem. Mater. 2016; 28: 284-292Crossref Scopus (1279) Google Scholar, 14.Christians J.A. et al.Stability in perovskite photovoltaics: a paradigm for newfangled technologies.ACS Energy Lett. 2018; 3: 2136-2143Crossref Scopus (101) Google Scholar]. Motivating this is that mixing I– and Br– anions [15.Noh J.H. et al.Chemical management for colorful, efficient, and stable inorganic–organic hybrid nanostructured solar cells.Nano Lett. 2013; 4: 1764-1769Crossref Scopus (3752) Google Scholar, 16.Eperon G.E. et al.Formamidinium lead trihalide: a broadly tunable perovskite for efficient planar heterojunction solar cells.Energy Environ. Sci. 2014; 7: 982-988Crossref Scopus (2932) Google Scholar, 17.Beal R.E. et al.Cesium lead halide perovskites with improved stability for tandem solar cells.J. Phys. Chem. Lett. 2016; 7: 746Crossref PubMed Scopus (855) Google Scholar] permits direct bandgap (Eg) control between pure APbI3 (Eg ~1.5 eV) and APbBr3 (Eg ~2.4 eV) limits [15.Noh J.H. et al.Chemical management for colorful, efficient, and stable inorganic–organic hybrid nanostructured solar cells.Nano Lett. 2013; 4: 1764-1769Crossref Scopus (3752) Google Scholar, 16.Eperon G.E. et al.Formamidinium lead trihalide: a broadly tunable perovskite for efficient planar heterojunction solar cells.Energy Environ. Sci. 2014; 7: 982-988Crossref Scopus (2932) Google Scholar, 17.Beal R.E. et al.Cesium lead halide perovskites with improved stability for tandem solar cells.J. Phys. Chem. Lett. 2016; 7: 746Crossref PubMed Scopus (855) Google Scholar]. A+ alloying additionally permits Eg tunability between MAPbX3, FAPbX3, and CsPbX3 bandgap limits [10.McMeekin D.P. et al.A mixed-cation lead mixed-halide perovskite absorber for tandem solar cells.Science. 2016; 351: 151Crossref PubMed Scopus (2167) Google Scholar, 11.Saliba M. et al.Cesium-containing triple cation perovskite solar cells: improved stability, reproducibility, and high efficiency.Energy Environ. Sci. 2016; 9: 1989-1997Crossref PubMed Google Scholar, 12.Ono K.L. et al.Progress on perovskite materials and solar cells with mixed cations and halide anions.ACS Appl. Mater. Interfaces. 2017; 9: 30197-30246Crossref PubMed Scopus (368) Google Scholar, 13.Li Z. et al.Stabilizing perovskite structures by tuning tolerance factor: formation of formamidinium lead iodide solid state alloys.Chem. Mater. 2016; 28: 284-292Crossref Scopus (1279) Google Scholar, 14.Christians J.A. et al.Stability in perovskite photovoltaics: a paradigm for newfangled technologies.ACS Energy Lett. 2018; 3: 2136-2143Crossref Scopus (101) Google Scholar]. Alloying A+ cations simultaneously improves film moisture/heat stabilities since unstable MA+ moieties [18.Manser J.S. et al.Making and breaking of lead halide perovskites.Acc. Chem. Res. 2016; 49: 330-338Crossref PubMed Scopus (505) Google Scholar] are partially or fully replaced with less volatile FA+ and Cs+ cations. This composition-based bandgap tunability has therefore spawned significant interest in exploiting wide gap FAyMAzCs1–y–zPb(I1–xBrx)3 materials in multijunction devices [12.Ono K.L. et al.Progress on perovskite materials and solar cells with mixed cations and halide anions.ACS Appl. Mater. Interfaces. 2017; 9: 30197-30246Crossref PubMed Scopus (368) Google Scholar,19.Kamat P.V. Hybrid perovskites for multijunction tandem solar cells and solar fuels. A virtual issue.ACS Energy Lett. 2018; 3: 28-29Crossref Scopus (33) Google Scholar]. Highlighting this has been the use of a suitably engineered, perovskite top cell layer in a record setting, 29% PCE, tandem solar cell [2.Kojima A. et al.Organometal halide perovskites as visible-light sensitizers for photovoltaic cells.J. Am. Chem. Soc. 2009; 131: 6050-6051Crossref PubMed Scopus (15167) Google Scholar]. Box 1, Box 2 provide overviews of the perovskite crystal and electronic structure, as well as the effects of halide and A+ cation alloying on each.Box 1Lead Halide Perovskite Crystal StructureLead halide perovskites are constructed by 3D networks of corner-sharing PbX64– octahedra. A+ cations sterically stabilize interstitial voids. Perovskite lattices typically exist in either orthorhombic (γ-phase), tetragonal (β-phase), or cubic (α-phase) crystal symmetries with increasing temperature. Archetypical α-phases have 180° inter-octahedra bond angles (i.e., Pb-X-Pb). Distorted β- or γ-phases have inter-octahedral bond angles, ranging from ~179° to ~160° due to octahedral tilting. Beyond α-, β-, and γ-phases, FAPbI3 and CsPbI3 thermodynamically stabilize in non-perovskite phases with face-sharing (hexagonal, δhex-phase) and edge-sharing (orthorhombic, δortho-phase) PbX64– octahedra, respectively.Figures IA–F illustrate thermodynamically preferred room and high-temperature (i.e., α-phase) crystal structures for relevant APbX3 compositions. Apparent are structural dependencies on halide and A+ composition. MAPbI3 (Figure IA) adopts a β-phase at 300 K and transitions to its α-phase at ~330 K. MAPbBr3 prefers its α-phase above ~250 K (Figure IB) [74.Lehmann F. et al.The phase diagram of a mixed halide (Br, I) hybrid perovskite obtained by synchrotron X-ray diffraction.RSC Adv. 2019; 9: 11151-11159Crossref Google Scholar]. FAPbI3 (Figure IC) assumes its α-phase above ~440 K but prefers its non-perovskite δhex-phase at ~300 K [13.Li Z. et al.Stabilizing perovskite structures by tuning tolerance factor: formation of formamidinium lead iodide solid state alloys.Chem. Mater. 2016; 28: 284-292Crossref Scopus (1279) Google Scholar]. FAPbBr3, by contrast (Figure ID), adopts its α-phase at temperatures >275 K [75.Schueller E.C. et al.Crystal structure evolution and notable thermal expansion in hybrid perovskite formamidinium tin iodide and formamidinium lead bromide.Inorg. Chem. 2018; 57: 695-701Crossref PubMed Scopus (93) Google Scholar]. CsPbI3 (Figure IE) assumes its non-perovskite δortho-phase with its α-phase favored above ~590 K [13.Li Z. et al.Stabilizing perovskite structures by tuning tolerance factor: formation of formamidinium lead iodide solid state alloys.Chem. Mater. 2016; 28: 284-292Crossref Scopus (1279) Google Scholar]. Finally, CsPbBr3 prefers its γ-phase at room temperature and transitions to its α-phase at ~400 K [76.Stoumpos C.C. et al.Crystal growth of the perovskite semiconductor CsPbBr3: a new material for high-energy radiation detection.Cryst. Growth Des. 2013; 13: 2722-2727Crossref Scopus (990) Google Scholar].Figure IIA demonstrates the evolution of MAPb(I1–xBrx)3 (200) pXRD reflections across its compositional range. Figure IIB summarizes the structural tunability of MAPb(I1–xBrx)3 [15.Noh J.H. et al.Chemical management for colorful, efficient, and stable inorganic–organic hybrid nanostructured solar cells.Nano Lett. 2013; 4: 1764-1769Crossref Scopus (3752) Google Scholar], FAPb(I1–xBrx)3 [16.Eperon G.E. et al.Formamidinium lead trihalide: a broadly tunable perovskite for efficient planar heterojunction solar cells.Energy Environ. Sci. 2014; 7: 982-988Crossref Scopus (2932) Google Scholar], CsPb(I1–xBrx)3 [17.Beal R.E. et al.Cesium lead halide perovskites with improved stability for tandem solar cells.J. Phys. Chem. Lett. 2016; 7: 746Crossref PubMed Scopus (855) Google Scholar], and FA0.83Cs0.17Pb(I1–xBrx)3 [70.Rehman W. et al.Photovoltaic mixed cation lead mixed halide perovskites: links between crystallinity, photostability and electronic properties.Energy Environ. Sci. 2017; 10: 361Crossref Google Scholar] thin films by plotting pseudocubic lattice parameters (a) versus x. Broken lines are fits to the data using Vegard’s law [Eg,mixed = Eg,I(1 – x) + Eg,Brx – bx(1 – x), where b is a bowing parameter accounting for deviations from linearity (b = 0.091 [MAPb(I1–xBrx)3], b = 0.032 [FAPb(I1–xBrx)3], b = 0.230 Å [CsPb(I1–xBrx)3], and b = 0.083 Å [FA0.83Cs0.17Pb(I1–xBrx)3]). Increasing (decreasing) x induces lattice contraction (expansion), causing a to range from ~6.29 to 5.93 Å (MA+) [15.Noh J.H. et al.Chemical management for colorful, efficient, and stable inorganic–organic hybrid nanostructured solar cells.Nano Lett. 2013; 4: 1764-1769Crossref Scopus (3752) Google Scholar], ~6.35 to 5.98 Å (FA+) [16.Eperon G.E. et al.Formamidinium lead trihalide: a broadly tunable perovskite for efficient planar heterojunction solar cells.Energy Environ. Sci. 2014; 7: 982-988Crossref Scopus (2932) Google Scholar], 6.19 to 5.85 Å (Cs+) [17.Beal R.E. et al.Cesium lead halide perovskites with improved stability for tandem solar cells.J. Phys. Chem. Lett. 2016; 7: 746Crossref PubMed Scopus (855) Google Scholar], and ~6.31 to 5.95 Å [FA+/Cs+] between x = 0 and 1.0. This, in turn, shifts Bragg reflections to higher (lower) degrees 2θ. These differences nominally permit resolution of photosegregated I-rich and Br-rich domains via pXRD.Figure IIHalide-Dependent APb(I1–xBrx)3 Perovskite Thin Film Lattice Parameters.Show full caption(A) (200) Bragg reflections from pXRD patterns of x = 0–1.0 MAPb(I1–xBrx)3. Adapted, with permission, from [20.Hoke E.T. et al.Reversible photo-induced trap formation in mixed-halide hybrid perovskites for photovoltaics.Chem. Sci. 2015; 6: 613-617Crossref PubMed Google Scholar]. (B) Pseudocubic lattice constants versus x for MAPb(I1–xBrx)3 (blue x) [15.Noh J.H. et al.Chemical management for colorful, efficient, and stable inorganic–organic hybrid nanostructured solar cells.Nano Lett. 2013; 4: 1764-1769Crossref Scopus (3752) Google Scholar], FAPb(I1–xBrx)3 (red *) [16.Eperon G.E. et al.Formamidinium lead trihalide: a broadly tunable perovskite for efficient planar heterojunction solar cells.Energy Environ. Sci. 2014; 7: 982-988Crossref Scopus (2932) Google Scholar], CsPb(I1–xBrx)3 (green +) [17.Beal R.E. et al.Cesium lead halide perovskites with improved stability for tandem solar cells.J. Phys. Chem. Lett. 2016; 7: 746Crossref PubMed Scopus (855) Google Scholar], and FA0.83Cs0.17(I1–xBrx)3 (black diamonds) [70.Rehman W. et al.Photovoltaic mixed cation lead mixed halide perovskites: links between crystallinity, photostability and electronic properties.Energy Environ. Sci. 2017; 10: 361Crossref Google Scholar]. Figure IIA reprinted, with permission, from [20.Hoke E.T. et al.Reversible photo-induced trap formation in mixed-halide hybrid perovskites for photovoltaics.Chem. Sci. 2015; 6: 613-617Crossref PubMed Google Scholar]. Data in (B) extracted using WebPlotDigitalizerii.View Large Image Figure ViewerDownload (PPT)Box 2Lead Halide Perovskite Electronic Structure and Bandgap TunabilityFigure IA summarizes optical absorption spectra of MAPb(I1–xBrx)3 thin films across their compositional range [15.Noh J.H. et al.Chemical management for colorful, efficient, and stable inorganic–organic hybrid nanostructured solar cells.Nano Lett. 2013; 4: 1764-1769Crossref Scopus (3752) Google Scholar]. Bandgap tunability stems from physical changes to Pb–X bond distances such that shorter Pb–X equals larger bandgaps (Box 1). Figure IB plots pseudocubic mixed halide perovskite thin film Eg-values from x = 0 to 1.0 for MAPb(I1–xBrx)3 [15.Noh J.H. et al.Chemical management for colorful, efficient, and stable inorganic–organic hybrid nanostructured solar cells.Nano Lett. 2013; 4: 1764-1769Crossref Scopus (3752) Google Scholar], FAPb(I1–xBrx)3 [16.Eperon G.E. et al.Formamidinium lead trihalide: a broadly tunable perovskite for efficient planar heterojunction solar cells.Energy Environ. Sci. 2014; 7: 982-988Crossref Scopus (2932) Google Scholar], CsPb(I1–xBrx)3 [17.Beal R.E. et al.Cesium lead halide perovskites with improved stability for tandem solar cells.J. Phys. Chem. Lett. 2016; 7: 746Crossref PubMed Scopus (855) Google Scholar], and FA0.83Cs0.17Pb(I1–xBrx)3 [70.Rehman W. et al.Photovoltaic mixed cation lead mixed halide perovskites: links between crystallinity, photostability and electronic properties.Energy Environ. Sci. 2017; 10: 361Crossref Google Scholar]. Bandgaps range from 1.58 to 2.28 eV (785 to 544 nm), 1.48 to 2.23 eV (840 to 556 nm), 1.80 to 2.35 eV (690 to 528 nm), and 1.58 to 2.26 eV (785 to 548 nm) for A = MA+, FA+, Cs+, and FA+/Cs+, respectively. Data are fit using a Vegard’s law expression of the form Eg,mix = Eg,I(1 – x) + Eg,Brx – bx(1 – x) where b is a bowing parameter to account for deviations from linearity (b = 0.33 eV [MAPb(I1–xBrx)3], b = 0.15 eV [FAPb(I1–xBrx)3], b = 0.35 eV [CsPb(I1–xBrx)3], and b = 0.35 eV [FA0.83Cs0.17Pb(I1–xBrx)3]). Differences between pure Br and I perovskites bandgaps are of order ~0.5 eV, enabling I-rich and Br-rich spectral features to be resolved following halide photosegregation. Moreover, Figure IC shows a band level diagram for APbI3 and APbBr3 (A = MA+, FA+, and Cs+), which illustrates differences in the absolute band edges of each material [37.Tao S. et al.Absolute energy level positions in tin- and lead-based halide perovskites.Nat. Commun. 2019; 102560Crossref PubMed Scopus (233) Google Scholar]. Favorable band offsets between I– and Br– materials will lead to charge flow into I-rich domains during photoinduced halide segregation.Figure IBandgap Tunability in APb(I1–xBrx)3 Perovskite Thin Films.Show full caption(A) Absorption coefficients for MAPb(I1–xBrx)3 thin films from diffuse spectral reflection and transmission measurements. (B) APb(I1–xBrx)3 Eg versus x for thin films where A = MA+ (blue x) [15.Noh J.H. et al.Chemical management for colorful, efficient, and stable inorganic–organic hybrid nanostructured solar cells.Nano Lett. 2013; 4: 1764-1769Crossref Scopus (3752) Google Scholar], FA+ (red *) [16.Eperon G.E. et al.Formamidinium lead trihalide: a broadly tunable perovskite for efficient planar heterojunction solar cells.Energy Environ. Sci. 2014; 7: 982-988Crossref Scopus (2932) Google Scholar], Cs+ (green +) [17.Beal R.E. et al.Cesium lead halide perovskites with improved stability for tandem solar cells.J. Phys. Chem. Lett. 2016; 7: 746Crossref PubMed Scopus (855) Google Scholar], and FA+/Cs+ (black diamonds) [70.Rehman W. et al.Photovoltaic mixed cation lead mixed halide perovskites: links between crystallinity, photostability and electronic properties.Energy Environ. Sci. 2017; 10: 361Crossref Google Scholar]. (A) Reprinted, with permission, from [20.Hoke E.T. et al.Reversible photo-induced trap formation in mixed-halide hybrid perovskites for photovoltaics.Chem. Sci. 2015; 6: 613-617Crossref PubMed Google Scholar]. Data in (B) extractedii from [15.Noh J.H. et al.Chemical management for colorful, efficient, and stable inorganic–organic hybrid nanostructured solar cells.Nano Lett. 2013; 4: 1764-1769Crossref Scopus (3752) Google Scholar, 16.Eperon G.E. et al.Formamidinium lead trihalide: a broadly tunable perovskite for efficient planar heterojunction solar cells.Energy Environ. Sci. 2014; 7: 982-988Crossref Scopus (2932) Google Scholar, 17.Beal R.E. et al.Cesium lead halide perovskites with improved stability for tandem solar cells.J. Phys. Chem. Lett. 2016; 7: 746Crossref PubMed Scopus (855) Google Scholar,70.Rehman W. et al.Photovoltaic mixed cation lead mixed halide perovskites: links between crystallinity, photostability and electronic properties.Energy Environ. Sci. 2017; 10: 361Crossref Google Scholar]. (C) Band offset diagram for APbI3 and APbBr3 (A = MA+, FA+, and Cs+). Data extractedii from [37.Tao S. et al.Absolute energy level positions in tin- and lead-based halide perovskites.Nat. Commun. 2019; 102560Crossref PubMed Scopus (233) Google Scholar].View Large Image Figure ViewerDownload (PPT) Lead halide perovskites are constructed by 3D networks of corner-sharing PbX64– octahedra. A+ cations sterically stabilize interstitial voids. Perovskite lattices typically exist in either orthorhombic (γ-phase), tetragonal (β-phase), or cubic (α-phase) crystal symmetries with increasing temperature. Archetypical α-phases have 180° inter-octahedra bond angles (i.e., Pb-X-Pb). Distorted β- or γ-phases have inter-octahedral bond angles, ranging from ~179° to ~160° due to octahedral tilting. Beyond α-, β-, and γ-phases, FAPbI3 and CsPbI3 thermodynamically stabilize in non-perovskite phases with face-sharing (hexagonal, δhex-phase) and edge-sharing (orthorhombic, δortho-phase) PbX64– octahedra, respectively. Figures IA–F illustrate thermodynamically preferred room and high-temperature (i.e., α-phase) crystal structures for relevant APbX3 compositions. Apparent are structural dependencies on halide and A+ composition. MAPbI3 (Figure IA) adopts a β-phase at 300 K and transitions to its α-phase at ~330 K. MAPbBr3 prefers its α-phase above ~250 K (Figure IB) [74.Lehmann F. et al.The phase diagram of a mixed halide (Br, I) hybrid perovskite obtained by synchrotron X-ray diffraction.RSC Adv. 2019; 9: 11151-11159Crossref Google Scholar]. FAPbI3 (Figure IC) assumes its α-phase above ~440 K but prefers its non-perovskite δhex-phase at ~300 K [13.Li Z. et al.Stabilizing perovskite structures by tuning tolerance factor: formation of formamidinium lead iodide solid state alloys.Chem. Mater. 2016; 28: 284-292Crossref Scopus (1279) Google Scholar]. FAPbBr3, by contrast (Figure ID), adopts its α-phase at temperatures >275 K [75.Schueller E.C. et al.Crystal structure evolution and notable thermal expansion in hybrid perovskite formamidinium tin iodide and formamidinium lead bromide.Inorg. Chem. 2018; 57: 695-701Crossref PubMed Scopus (93) Google Scholar]. CsPbI3 (Figure IE) assumes its non-perovskite δortho-phase with its α-phase favored above ~590 K [13.Li Z. et al.Stabilizing perovskite structures by tuning tolerance factor: formation of formamidinium lead iodide solid state alloys.Chem. Mater. 2016; 28: 284-292Crossref Scopus (1279) Google Scholar]. Finally, CsPbBr3 prefers its γ-phase at room temperature and transitions to its α-phase at ~400 K [76.Stoumpos C.C. et al.Crystal growth of the perovskite semiconductor CsPbBr3: a new material for high-energy radiation detection.Cryst. Growth Des. 2013; 13: 2722-2727Crossref Scopus (990) Google Scholar]. Figure IIA demonstrates the evolution of MAPb(I1–xBrx)3 (200) pXRD reflections across its compositional range. Figure IIB summarizes the structural tunability of MAPb(I1–xBrx)3 [15.Noh J.H. et al.Chemical management for colorful, efficient, and stable inorganic–organic hybrid nanostructured solar cells.Nano Lett. 2013; 4: 1764-1769Crossref Scopus (3752) Google Scholar], FAPb(I1–xBrx)3 [16.Eperon G.E. et al.Formamidinium lead trihalide: a broadly tunable perovskite for efficient planar heterojunction solar cells.Energy Environ. Sci. 2014; 7: 982-988Crossref Scopus (2932) Google Scholar], CsPb(I1–xBrx)3 [17.Beal R.E. et al.Cesium lead halide perovskites with improved stability for tandem solar cells.J. Phys. Chem. Lett. 2016; 7: 746Crossref PubMed Scopus (855) Google Scholar], and FA0.83Cs0.17Pb(I1–xBrx)3 [70.Rehman W. et al.Photovoltaic mixed cation lead mixed halide perovskites: links between crystallinity, photostability and electronic properties.Energy Environ. Sci. 2017; 10: 361Crossref Google Scholar] thin films by plotting pseudocubic lattice parameters (a) versus x. Broken lines are fits to the data using Vegard’s law [Eg,mixed = Eg,I(1 – x) + Eg,Brx – bx(1 – x), where b is a bowing parameter accounting for deviations from linearity (b = 0.091 [MAPb(I1–xBrx)3], b = 0.032 [FAPb(I1–xBrx)3], b = 0.230 Å [CsPb(I1–xBrx)3], and b = 0.083 Å [FA0.83Cs0.17Pb(I1–xBrx)3]). Increasing (decreasing) x induces lattice contraction (expansion), causing a to range from ~6.29 to 5.93 Å (MA+) [15.Noh J.H. et al.Chemical management for colorful, efficient, and stable inorganic–organic hybrid nanostructured solar cells.Nano Lett. 2013; 4: 1764-1769Crossref Scopus (3752) Google Scholar], ~6.35 to 5.98 Å (FA+) [16.Eperon G.E. et al.Formamidinium lead trihalide: a broadly tunable perovskite for efficient planar heterojunction solar cells.Energy Environ. Sci. 2014; 7: 982-988Crossref Scopus (2932) Google Scholar], 6.19 to 5.85 Å (Cs+) [17.Beal R.E. et al.Cesium lead halide perovskites with improved stability for tandem solar cells.J. Phys. Chem. Lett. 2016; 7: 746Crossref PubMed Scopus (855) Google Scholar], and ~6.31 to 5.95 Å [FA+/Cs+] between x = 0 and 1.0. This, in turn, shifts Bragg reflections to higher (lower) degrees 2θ. These differences nominally permit resolution of photosegregated I-rich and Br-rich domains via pXRD. Figure IA summarizes optical absorption spectra of MAPb(I1–xBrx)3 thin films across their compositional range [15.Noh J.H. et al.Chemical management for colorful, efficient, and stable inorganic–organic hybrid nanostructured solar cells.Nano Lett. 2013; 4: 1764-1769Crossref Scopus (3752) Google Scholar]. Bandgap tunability stems from physical changes to Pb–X bond distances such that shorter Pb–X equals larger bandgaps (Box 1). Figure IB plots pseudocubic mixed halide perovskite thin film Eg-values from x = 0 to 1.0 for MAPb(I1–xBrx)3 [15.Noh J.H. et al.Chemical management for colorful, efficient, and stable inorganic–organic hybrid nanostructured solar cells.Nano Lett. 2013; 4: 1764-1769Crossref Scopus (3752) Google Scholar], FAPb(I1–xBrx)3 [16.Eperon G.E. et al.Formamidinium lead trihalide: a broadly tunable perovskite for efficient planar heterojunction solar cells.Energy Environ. Sci. 2014; 7: 982-988Crossref Scopus (2932) Google Scholar], CsPb(I1–xBrx)3 [17.Beal R.E. et al.Cesium lead halide perovskites with improved stability for tandem solar cells.J. Phys. Chem. Lett. 2016; 7: 746Crossref PubMed Scopus (855) Google Scholar], and FA0.83Cs0.17Pb(I1–xBrx)3 [70.Rehman W. et al.Photovoltaic mixed cation lead mixed halide perovskites: links between crystallinity, photostability and electronic properties.Energy Environ. Sci. 2017; 10: 361Crossref Google Scholar]. Bandgaps range from 1.58 to 2.28 eV (785 to 544 nm), 1.48 to 2.23 eV (840 to 556 nm), 1.80 to 2.35 eV (690 to 528 nm), and 1.58 to 2.26 eV (785 to 548 nm) for A = MA+, FA+, Cs+, and FA+/Cs+, respectively. Data are fit using a Vegard’s law expression of the form Eg,mix = Eg,I(1 – x) + Eg,Brx – bx(1 – x) where b is a bowing parameter to account for deviations from linearity (b = 0.33 eV [MAPb(I1–xBrx)3], b = 0.15 eV [FAPb(I1–xBrx)3], b = 0.35 eV [CsPb(I1–xBrx)3], and b = 0.35 eV [FA0.83Cs0.17Pb(I1–xBrx)3]). Differences between pure Br and I perovskites bandgaps are of order ~0.5 eV, enabling I-rich and Br-rich spectral features to be resolved following halide photosegregation. Moreover, Figure IC shows a band level diagram for APbI3 and APbBr3 (A = MA+, FA+, and Cs+), which illustrates differences in the absolute band edges of each material [37.Tao S. et al.Absolute energy level positions in tin- and lead-based halide perovskites.Nat. Commun. 2019; 102560Crossref PubMed Scopus (233) Google Scholar]. Favorable band offsets between I– and Br– materials will lead to charge flow into I-rich domains during photoinduced halide segregation. Despite the earlier-mentioned stability and efficiency gains, alloyed perovskite photovoltaics still lack suitable long-term durability under standard solar cell operating conditions [14.Christians J.A. et al.Stability in perovskite photovoltaics: a parad