Light-Induced Anion Phase Segregation in Mixed Halide Perovskites

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Light-Induced Anion Phase Segregation in Mixed Halide Perovskites Michael C. Brennan, Sergiu Draguta, Prashant V. Kamat, and Masaru Kuno ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b01151 • Publication Date (Web): 22 Dec 2017 Downloaded from http://pubs.acs.org on December 22, 2017

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ACS Energy Letters

Light-Induced Anion Phase Segregation in Mixed Halide Perovskites Michael C. Brennan, †,‡ Sergiu Draguta † Prashant V. Kamat, †,‡ and Masaru Kuno*†

†

Department of Chemistry and Biochemistry and ‡Notre Dame Radiation Laboratory, University

of Notre Dame, Notre Dame, Indiana 46556, United States Corresponding Author *Email: [email protected]

ABSTRACT: Hybrid lead halide perovskites such as MAPbI3 (MA = CH3NH3+) and their mixed halide analogues represent an emerging class of materials for solar energy conversion. Intriguing aspects include sizable carrier diffusion lengths, large optical absorption coefficients and certified power conversion efficiencies that now exceed 22%. Halide-composition tunable bandgaps also make MAPb(I1-xBrx)3 systems ideal candidates for tandem solar cells. Unfortunately, preventing the effective integration of MAPb(I1-xBrx)3 into working devices are intrinsic instabilities due to light-induced halide phase segregation. Namely, under illumination mixed halide perovskites reversibly segregate into low bandgap I-rich and high bandgap Br-rich domains. Under electrical bias, halide migration has also been proposed as the source of undesirable charge injection barriers that degrade photovoltaic performance. In this perspective, we review the origin of light-induced halide phase segregation, its effects on photovoltaic response and ongoing research to suppress its influence on the optical and electronic response of mixed halide perovskites.

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Since the renaissance of lead halide perovskites in 2009, certified power conversion efficiencies (ƞ) of single-junction perovskite solar cells have increased at an unprecedented rate (~2% per year) and now exceed 22%.1,2,3 Superior performance has been attributed, in large part, to long carrier diffusion lengths, large absorption coefficients, long carrier lifetimes, and an inherent tolerance to defects.4,5,6

The success of single-junction devices has generated

tremendous interest in higher bandgap lead halide perovskites [e.g. APb(I1-xBrx)3; A = MA+, FA+ or Cs+ where MA+ = CH3NH3+ and FA+ = CH(NH2)2+] as means of harnessing the full solar spectrum.7,8,9

Halide and A-cation composition-dependent bandgaps allow for precise

engineering of their optical/electronic response between ~1.5 and 2.4 eV.7-9 When coupled to their near optimal properties (above), mixed halide systems are ideal for use in multi-junction solar cells.10,11,12,13 The main drawback to hybrid perovskite photovoltaics, however, is their long-term stability.14,15 One particular concern is the detrimental role played by moisture. When exposed to water, hybrid perovskites degrade through a series of reactions involving the deprotonation of


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ACS Energy Letters

the organic cation (e.g. MA+) or by complexation to form hydrates.14 Less widely recognized, but equally important, are instabilities caused by transformations in the material itself. These instabilities stem from ion migration under the influence of an electrical potential whether externally applied or built-in. For example, well known hysteresis behavior in MAPbI3 solar cells16 is now attributed to both anion and cation (e.g. MA+) migration followed by accumulation at MAPbI3/contact interfaces.17,18 This leads to the formation of charge injection barriers as well as the screening of built-in fields, degrading device performance.19,20,21,22 In the case of mixed halide perovskites, structural instabilities predominantly stem from lightinduced halide segregation. As originally reported by Hoke et al.23, under continuous optical irradiation MAPb(I1-xBrx)3 undergoes reversible halide phase segregation into separate I- and Brrich domains within the alloyed (parent) composite.

The effect was initially observed as

apparent redshifts of both the mixed halide absorption and emission spectrum towards energies consistent with the band gap of an I-rich MAPb(I0.8Br0.2)3 phase. Light-induced halide phase segregation has since been confirmed through concerted optical and structural measurements and is now widely accepted as the origin of spectral redshifts in both the absorption and emission under continuous visible irradiation.24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41 While general aspects of light-induced halide phase segregation have been reported, outstanding questions remain. First, what are the exact ion migration channels involved in halide phase segregation? Next, an explicit account of structural changes induced by visible light irradiation is needed. In particular, what are the effects of long term irradiation on the perovskite crystal structure? Is there a permanent crystallographic phase transition, lattice expansion, or lattice contraction? Third, an exact mechanism that accounts for all experimentally-observed aspects of light-induced anion phase segregation remains absent.

To illustrate, why is an


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emission peak consistent with a x≈0.2 halide composition near universally seen following lightinduced anion segregation? Additionally, little information exists on how light-induced phase segregation affects solar cell performance.26-27,32-33,37-39

In short, what happens to device

performance during segregation and what are the long term effects on device stability? Finally, what is the most effective method to prevent this phenomenon in mixed halide perovskites? In this perspective, we therefore review and analyze the current experimental/theoretical understanding of light-induced halide phase segregation, its effect on photovoltaic performance and potential methods to mitigate it.


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Figure 1. Bandgap energy (Eg;I/Br) vs. x and (b) pseudocubic lattice parameter aI/Br and (200) peak position vs. x for MAPb(I1xBrx)3 (solid blue triangles), FAPb(I1-xBrx)3 (open orange triangles), and CsPb(I1-xBrx)3 (red circles). Fit lines come from equations in each plot where Eg;I/Br (aI/Br) is the bandgap (lattice parameter) for given x. Literature FAPb(I1-xBrx)3,43 MAPb(I125 8 42 xBrx)3, and CsPb(I1-xBrx)3 data digitalized using WebplotDigitalizer.

Halide Composition-Dependent APb(I1-xBrx)3 Optical and Structural Properties. Key to the usefulness of mixed halide perovskites are their tunable optical and structural properties. This has been established, in large part, through static absorption and powder X-ray diffraction (pXRD) measurements conducted across stoichiometry for APb(I1-xBrx) thin films. Figures 1a,b


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demonstrate halide-composition dependent properties where data has been extracted42 from References [8] (A = Cs+), [25] (A = MA+), and [43] (A = FA+). Figure 1a first illustrates the bandgap tunablity of APb(I1-xBrx)3 by plotting Eg as a function of x. In all cases, Eg increases with x. Corresponding I- to Br- Eg limits range from ~1.48 to 2.28 eV (A = FA+),43 1.58 to 2.38 eV (A = MA+),23 and 1.80 to 2.35 eV (A = Cs+).8 Maximum bandgap differences are all > 500 meV , allowing for the ready differentiation of I-rich and Brrich spectral features following light-induced halide phase segregation. The underlying origin of observed Eg differences stems from physical changes to the Pb-X bond distance, a parameter dictated by the ionic radius of each halide anion.44 Accordingly, increasing x decreases the average Pb-X bond distance.8,25,43 This, in turn, leads to lattice contraction and results in corresponding Eg increases as well as pXRD reflections which shift from lower to higher degrees 2θ. Figure 1b explicitly demonstrates this latter trend by plotting mixed halide pseudocubic lattice parameters (aI/Br) vs. x. In the plot, aI/Br-values range from aI/Br ~6.35 to 5.98 Å (A = FA+),43 6.29 to 5.93 Å (A = MA+),25 and 6.19 to 5.85 Å (A = Cs+).8 Differences in aI/Br correspond to more than 1.5° 2θ for pseudocubic (200) reflections.

Consequently, phase

segregated I-rich and Br-rich domains can, in principle, be resolved via pXRD measurements. Beyond lattice contraction and expansion, halide stoichiometry also alters the overall room temperature symmetry of the perovskite lattice.8,25,43-44 To illustrate, MAPbI3 adopts a tetragonal (I4/mcm) structure.23,25 MAPbBr3, however, exists in the cubic (Pm-3m) phase.23,25 When x >0.2 in MAPb(I1-xBrx)3, a tetragonal-to-cubic phase transition is observed.23,25 FAPb(I1-xBrx)3 [CsPb(I1-xBrx)3] thin films also reportedly undergo a tetragonal-to-cubic (cubic-to-orthorhombic) phase transition when x=0.4–0.643 (x=0.5–0.6).8 We note that pure FAPbI3 and CsPbI3 adopt


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non-perovskite structures at room temperature while their A-cation alloys adopt a perovskite structure.7,37 All complicate the study of pure FA+ and Cs+ halide composition-dependent phase transitions.8,43 Of interest to MAPb(I1-xBrx)3 light-induced halide phase segregation is that reducing crystal symmetry effectively increases migration activation energies (Ea) via changes to intra-octaheral halide migration distances.17,42 This has been established by Meloni et al.47 through molecular dynamic simulations which suggest that equatorial to axial Br- vacancy (VBr-) migration Ea-values increase from 220 meV to 460 meV following a cubic-to-tetragonal distortion. These composition-dependent bandgap (Eg;I/Br) and pseudocubic aI/Br variations are concisely summarized using Vegard’s law.

Specifically, equations at the bottom of Figures 1a,b

summarize the data with Eg;I/Eg;Br (aI/aBr) being the APbI3/APbBr3 bandgap (lattice parameter) and b a bandgap or lattice constant bowing parameter that accounts for empirical nonlinearities.45,46 Bandgap/lattice constant bowing parameters are provided for each A-cation composition in Figures 1a,b. Table 1. Activation Energies for Ion Vacancy Migration Vacancy V Pb2+ V MA+ V FA+ V Cs+ V BrV I-

Ea (eV)

Reference

0.80-2.31* 0.46-1.13* 0.57-0.61* 0.59-1.20* 0.09-0.27* 0.17-0.25 0.08-0.58* 0.23-0.43

20,47-48 20,47-49 48 47 47-48 47,50 20,47-48 39,47,51-52

*Denotes theoretical values.

Ion Migration in Single Halide Perovskites.

Light-induced instabilities in mixed halide

perovskites are the source of corresponding optical and structural changes during illumination. Supporting this are large ion mobilities within the perovskite lattice. This stems from the fact that lead halide perovskites are well known ion conductors, something first established in the


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1970s and 1980s through ion conductivity measurements on CsPbBr3 and CsPbCl3.50,52-53 Single halide perovskite (e.g. MAPbI3 and MAPbBr3) ion migration studies therefore represent good starting points for understanding light-induced halide phase segregation. In particular, the early literature reveals that lead halide perovskite ion migration arises due to the existence of ion vacancies, interstitial sites, and anti-sites in the crystal lattice.50 Of the various possible defects, halide vacancies (e.g. VI-, VBr-) possess the lowest formation energies. Consequently, they are likely to be the most common defects found in today’s mixed halide hybrid perovskites.17,54 These ionic conductivity measurements along with complementary theoretical studies have also been instrumental in establishing vacancy migration activation energies (Ea-values) for APbI3 and APbBr3.17,47-52 Table 1 lists a sampling of known literature results wherein Ea-values are found to be sensitive to factors such as ionic radius, ionic charge, crystal symmetry, migration distance and density of crystal defects.7,20,39,47-52 The compilation makes it evident that ion mobilities follow the trend Br- < I- < MA+ < FA+ < Cs+ < Pb2+ from most to least mobile.17,39,42-48 This suggests that the most likely ion migration channels in APb(I1-xBrx)3 are those related to halide vacancies. In support of this, recent studies have found that halide vacancies play a critical role in dictating halide phase segregation kinetics.34-35 These results nominally suggest that systems with higher halide vacancy densities exhibit enhanced phase segregation kinetics. Given the sensitivity of Ea to local ionic environment17 and the fact that prior Ea measurements have predominately focused on single halide perovskites, more direct measurements of vacancy activation energies in hybrid mixed halide perovskites are needed. Towards this end, Hoke et al.23 have extracted a halide phase segregation Ea estimate of 0.27 eV


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ACS Energy Letters

for MAPb(I0.6Br0.4)3 based on temperature-dependent emission growth studies. This Ea is within the range of VBr- and VI- displayed in Table 1, further supporting the likely role played by anion vacancies in APb(I1-xBrx)3 light-induced halide phase segregation. Unfortunately, since this first report we know of no other study that has attempted to quantify anion migration Ea-values in hybrid perovskite mixed halide films. Consequently, much is still unknown about how exactly halide stoichiometry, A-site cation, and crystal symmetry influence anion migration Ea-values.

Light-Induced Structural Changes in MAPb(I1-xBrx)3 Thin Films. Next, definitive evidence of halide phase segregation was first established by tracking light-induced structural changes via pXRD measurements. To date, light-induced changes to pXRD patterns have only been investigated for MAPb(I1-xBrx).23,32-34,38 As discussed earlier, halide phase segregation in this system can lead to splitting of the parent MAPb(I1-xBrx)3 reflection into separate I-rich and Brrich reflections.23 Unfortunately, the literature is replete with inconsistencies. In what follows, we therefore describe these inconsistencies, assuming x values obtained from Vegard’s law in Figure 1b.


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Figure 2. Summary of MAPb(I1-xBrx)3 literature pXRD powder patterns before (dashed blue) and after (solid red) illumination. Initial halide compositions given by x at the top right corner of each panel. Red asterisks denote postillumination peak positions for split I-rich and Br-rich domains. Data digitalized from (a) Ref. [23], (b) Ref. [38], (c) Ref. [32], (d) Ref. [33], and (e) Ref. [34] using WebplotDigitalizer.42

Figure 2a first shows the near ideal pXRD data originally acquired by Hoke et al.23 In the powder pattern, the (200) cubic MAPb(I0.40Br0.60)3 reflection at 29.38° splits into two distinct peaks, one at 28.68° degrees and another at 29.55° degrees. Post-illumination peak positions are denoted by the red asterisks in Figure 2. These reflections arise due to the formation of an I-rich [MAPb(I0.80Br0.20)3] minority and a Br-rich [MAPb(I0.30Br0.70)3] majority phase following irradiation.

An estimated 23% (77%) of the lattice is reported23 to exist in the minority

(majority) phase based on the ratio of areas derived from Gaussian fits to minority/majority peaks. Unfortunately, to the best of our knowledge, this near ideal result has not been replicated in the literature. The most comparable results have been reported by Hu et al.38 Their data, is


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illustrated in Figure 2b and shows that under illumination a ~28.80° MAPb(I0.74Br0.26)3 (200) reflection splits into separate ~28.77° and ~28.50° features. The former (latter) is characteristic of a MAPb(I0.87Br0.13)3 [MAPb(I0.75Br0.25)3] minority (17%) [majority (83%)] phase. While Hu’s data is qualitatively consistent with that of Hoke et al.23, one contradiction exists. Namely, Hu et al. show a post-illumination MAPb(I0.75Br0.25)3 majority phase at nearly the same 2θ position as the original pre-illumination reflection. This suggests little formation of Br-rich domains. Beyond these two studies, there are other major inconsistencies. For example, Duong et al.32 observe light-induced peak splitting in MAPb(I0.52Br0.48)3 (Figure 2c) where the initial ~29.20° (200) reflection separates into two peaks, one at ~29.15° and another at ~30.30°. The former (latter) corresponds to x=0.45 (x=1.0). In this case, the Br-rich (x=1.0) minority phase makes up only 10% of the lattice, a value noticeably different from the 77% and 83% Br-rich majority phases observed by Hoke et al. (Figure 2a) and Hu et al. (Figure 2b).23,38 Further, the majority phase is representative of an I-rich MAPb(I0.55Br0.45)3 phase, indicating little formation of x≈0.2 domains. In additional pXRD measurements, Yang et al.33 (Figure 2d) and Barker et al.34 (Figure 2e) do not observe any apparent MAPb(I1-xBrx)3 peak splitting. Instead, they see shifts of the (200) reflection accompanied by peak broadening. Even then contradictions exist between the two studies. Namely, Yang et al.33 find that the MAPb(I0.62Br0.38)3 (200) reflection shifts to lower degrees 2θ. Barker et al.34 oppositely observe that the MAPb(I0.35Br0.62)3 (200) reflection shifts to higher degrees 2θ. Yang’s result corresponds to an increase in lattice constant whereas Barker’s result implies the opposite. These pXRD inconsistencies may stem, in part, from different initial halide compositions in the studied films. There is some evidence for this as Hoke et al.23 did not observe a (200) peak


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splitting for x=0.4 and x=0.8 systems under the same illumination conditions used to acquire the x=0.6 data in Figure 1a. However, this cannot explain all the inconsistences observed as even in cases where x values are similar (e.g. Figures 2a,c,e), contradictions exist. Consequently, there may be other parameters such as film thickness or grain size, which differ between studies and which critically impact light-induced anion phase segregation. Illumination conditions (e.g. excitation intensity, excitation source duty cycle, time of illumination, and excitation wavelength) also differ between experiments and may likewise critically impact anion phase segregation. What is universally consistent in these studies, however, is that (200) reflections do not fully recover under dark conditions to their original degrees 2θ positions. Several groups23,25,34 have speculated that this arises from a cubic-to-tetragonal crystallographic phase transition in small regions of the mixed halide lattice. This would, in turn, favor the long-term stabilization of Irich (x ≈ 0.2) domains. However, the hypothesis has been challenging to verify via pXRD because of decreases in reflection intensities accompanied by peak broadening. In this regard, mobile ions lead to pXRD signals more akin to those from amorphous phases of a material, decreasing the signal-to-noise ratio of pXRD measurements. Lattice strain due to ion movement also results in peak broadening. Both therefore make it difficult to resolve a potential cubic-totetragonal induced peak splitting. Beyond pXRD there are additional structural measurements such as electron microscopy and X-ray photoelectron spectroscopy (XPS) which corroborate light-induced halide phase segregation and which shed further light on important underlying parameters.28,34,39,41 It should be noted though that electron microscopy techniques are complicated by well-known electronbeam sensitivities of hybrid perovskites -an effect which leads to their structural, morphological,


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and compositional degradation.55 Despite this, scanning electron microscopy (SEM)-based cathodoluminescence (CL) and transmission electron microscopy (TEM)-based energy dispersive X-ray spectroscopy (EDXS) measurements28,34,39 have found that I-rich domains primarily emerge at mixed halide perovskite grain boundaries.

XPS results on

(FA0.85MA0.15)Pb(I0.85Br0.15)3 thin films additionally show that Br-rich regions can be found at crystal surfaces.41 These studies therefore point to interfaces as the preferred regions for halide phase segregation. At this point, several questions remain to be answered regarding light-induced changes to the perovskite crystal structure. Explicitly, what is the long term effect of visible irradiation on the crystal structure? Further, what exactly is the origin of contradictory pXRD results seen in the literature?

The limited number of structural studies to date and the ambiguity of pXRD

measurements thus demand further work using local probes of structure such as Raman56 and spatially-resolved X-Ray Absorption Fine Structure (XAFS) techniques57 to fully develop a microscopic understanding of light-induced structural changes in mixed halide perovskites.

Figure 3. (a) Time evolution of MAPb(I0.5Br0.5)3 emission spectra under λexc = 405 nm CW excitation (Iexc = 20 mW

cm-2) over the course of 3 seconds. Inset: Emission spectrum between 475-600 nm indicative of Br-rich emission. (b) An illustration of post-illumination band alignment which causes preferred emission from I-rich x≈0.2 domains. (c) Corresponding time evolution of MAPb(I0.5Br0.5)3 absorption spectra under λexc = 405 nm CW excitation. Times for spectra are 0 (blue), 1, (red), and 30 (green) minutes. Figure adapted from Ref. [31].


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Effects of Halide Phase Segregation on the Optical and Electrical Response of Mixed Halide Perovskites. The most dramatic effects of light-induced halide phase segregation are seen in the absorption and emission of mixed halide perovskites.23-39 To illustrate, Figure 3a shows a representative time series of the emission from a MAPb(I0.5Br0.5)3 thin film under continuous wave (CW) λexc=405 nm irradiation (excitation intensity, Iexc=20 mW cm-2).31

The initial

emission spectrum is characterized by a single peak with a maximum at ~660 nm (1.88 eV) and an energy consistent with x=0.5 according to Vegard’s law in Figure 1a.25 Following several seconds of illumination, the parent emission peak decreases in intensity and exhibits simultaneous growth of a tail towards longer wavelengths.

As the illumination

progresses, a second feature at ~733 nm (1.69 eV), which is consistent with the emission from a halide composition of MAPb(I0.8Br0.2)3,23,31 eventually appears and is accompanied by a significantly weaker ~530 nm (2.30 eV) feature (inset, Figure 3a). These two new peaks correspond to emission from I- and Br-rich phases nucleated within the parent perovskite. Their overall intensity differences point to the preferential recombination of photogenerated carriers within lower bandgap x≈0.2 I-rich domains as illustrated in Figure 3b.31 Interestingly, Hoke et al.23 have demonstrated that the final emission feature in phase segregated MAPb(I1-xBrx)3 films always appears to be consistent with that of MAPb(I0.8Br0.2)3 (i.e. x≈0.2) irrespective of the initial halide composition. In fact, a final emission feature roughly between 760-730 nm (1.63-1.70 eV) is near universally observed across all prior light-induced halide

phase

segregation

studies.23-39

This

phenomenon

has

even

been

seen

in

electroluminescence measurements where Braly et al.37 have demonstrated halide segregation in the dark via current injection and have subsequently observed a post phase segregated emission peak position at ~750 nm (1.65 eV)37. Of note is that the Braly result suggests that it is


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ACS Energy Letters

ultimately the photoinduced or injected current density in the perovksite that is responsible for the phase segregation. This is in line with conclusions extending from the model Ref. [31]. Finally, the observation that x≈0.2 appears to be a special compositional stability point, along with the existence of a corresponding a cubic-to-tetragonal structural phase transition,23,25 suggests that the two observations are linked. However, significant work is still required to verify this. Next, halide phase segregation can be observed through corresponding changes to the MAPb(I1-xBrx)3 linear absorption.23,25,29,35 Figure 3c depicts the absorption of a MAPb(I0.5Br0.5)3 thin film before, during, and after λexc=405 nm CW irradiation (Iexc=20 mW cm-2). In contrast to the emission data in Figure 3a, light-induced absorption changes are subtle. Only a slight decrease of the band edge absorption feature at ~625 nm occurs along with growth of a tail towards lower energies. These absorption/emission spectral asymmetries originate from partial phase segregation in mixed halide films. Specifically, absorption-based measurements by Hoke et al.,23 Braly et al.,37 Yoon et al.,29 and Draguta et al.31 suggest that 1%, 2%, 18% and 8% of the parent mixed halide perovskite converts into I-rich phases consistent with the composition MAPb(I0.8Br0.2)3. Corresponding pXRD-based measurements by Hoke et al.23 and Hu et al.38 corroborate this, indicating that only a minority (23% and 17% respectively) of the lattice exists in the x≈0.2 phase. Other pXRD results32-34 do not discern clear I-rich domains after illumination (Figures 2c-d) despite redshifts of the corresponding emission to energies consistent with an x≈0.2 composition. This may stem from a phase segregated fraction below that which is resolvable by pXRD. However, the bulk of the above results, showing relatively small phase segregated fractions, along with the favorable lower energy Eg of I-rich inclusions (Figure 3b) rationalizes


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why dramatic spectral changes are seen in the emission but not in the absorption and in some cases pXRD.31 At this point, it is important to mention some key experimental factors known to influence forward anion phase segregation rates (kforward). First, CW or higher repetition rate (i.e. quasi CW) excitation sources are needed to induce phase segregation.26 This has been established previously wherein no anion phase segregation occurs if the excitation source is pulsed and has a low repetition rate [< 500 Hz for a x=0.6 MAPb(I1-xBrx) film].26 Next, kforward depends on Iexc,26,29,31 and increases (decreases) with increasing (decreasing) Iexc. At very low excitation intensities (e.g. Iexc ~40 µW cm-2) phase segregation in a MAPb(I0.5Br0.5)3 thin film can be suppressed. By contrast, when Iexc > 40 W cm-2 kforward readily increases in a non-linear fashion before saturating at kforward=0.7 s-1.31 Adding to this, Barker et al.34 have suggested that both film thickness and excitation wavelength (λexc) matter for anion phase segregation. In this regard, they find that thicker films (~300 nm) exhibit faster phase segregation rates than thinner films (~100 nm). Measurements with higher energy excitation wavelengths (i.e. λexc=450 nm) also display faster segregation rates than near band edge excitation (i.e. λexc=640 nm). Here Barker et al.34 speculate that thicknessand λexc-dependent rates result from uneven light absorption due to wavelength-dependent excitation penetration depths. Finally, Hoke et al.23 have suggested through emission measurements that higher I- content films require more illumination time in order to observe phase segregation. This may result from changes in inter-octahedral halide migration distances where higher Br content films possess shorter average Pb-X bond distances and thus shorter intra-octahedral migration distances than I-


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ACS Energy Letters

rich films. Consequently, halide migration activation energies of Br-rich (I-rich) films are more consistent with lower MAPbBr3 (larger MAPbI3) Ea-values.17,47-52 Remarkably, halide phase segregation is reversible in the dark with near complete recovery of original absorption/emission spectra. Post-illumination recovery, however, occurs on a much slower timescale (~5–30 minutes) than light-induced phase segregation (~1–10 s).23,26-29,33-35 Although recovery timescales depend on the initial excitation intensity and irradiation time,23,26,29,31 the generally substantial differences between forward and reverse phase segregation rates can be attributed to larger halide ion migration Ea-values experienced in x≈0.2 I-rich domains following illumination.23,34,42 This may ultimately stem from stabilization of a tetragonal phase23,25,34 which, as discussed previously, possesses larger halide migration Eavalues than those of the corresponding cubic lattice.42

Theoretical Descriptions for Light-Induced Halide Phase Segregation. Having described known experimental features of light-induced halide phase segregation, we now turn to its theoretical descriptions. To date, only a few models have been developed to explain light-induced halide phase segregation.28,31,40 Brivio et al.40 were the first, using first principles density functional theory (DFT) to construct a pseudocubic MAPb(I1-xBrx)3 compositional phase diagram based on Helmholtz free energy variations. Their calculations reveal a 300 K miscibility gap in the phase diagram, suggesting that MAPb(I1-xBrx)3 systems with 0.3

Light-Induced Anion Phase Segregation in Mixed Halide Perovskites

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