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Effect of Halide Composition on the Photochemical Stability of Perovskite Photovoltaic Materials

Photochemical stability of encapsulated films of mixed halide perovskites with a range of MAPb(I1-xBrx)3 compositions (solid solutions) was investigated under accelerated stressing using concentrated sunlight. Evolution of light absorption and the corresponding structural modifications in the films were recorded by UV-vis spectroscopy and X-ray diffraction (XRD), respectively. MAPbBr3 film exhibited no degradation. In MAPbI3 and mixed halide MAPb(I1-xBrx)3 films, decomposition of the perovskite material with crystallization of inorganic PbI2, and the corresponding degradation of light absorption, were recorded. Introduction of bromine into the solid solution was found to stress its structure and accelerate the degradation. The relevance of accelerated testing to standard operational conditions of solar cells was confirmed by comparison to degradation experiments under outdoor sunlight exposure. Reasons for the reduced stability of the mixed halide compositions are discussed.

Organic−inorganic hybrid perovskites have recently achieved a breakthrough in the field of thin film photovoltaics (PV), with extremely high power conversion efficiencies (PCE) of over 20% for wet-processed devices. 1 The structure of such perovskites is AMX3, where M is a divalent cation, X are halides, and A is an organic cation. Such perovskites have excellent properties for PV sunlight conversion, including direct band gaps, large absorption coefficients and high carrier mobility. 2 The ability to tune the energy band gap by changing the halide anion composition is another important advantage of these materials. 3 Seok et al. showed colorful, efficient perovskite solar cells using different I/Br ratios with PCE of up to 12.3%. 4 Etgar et al. used a two-step deposition technique to synthesize CH3NH3Pb(I1-xBrx)3 (MAPb(I1-xBrx)3, 0 ≤ x ≤ 1) perovskites and utilized them in hole conductor free PV cells that combined the perovskites' good hole conduction and tunable absorption spectrum. 5 A significant challenge en route to commercialization of perovskite-based PV is the development of devices combining high PCE and operational stability. Accelerated stability tests 6 can rapidly screen different PV materials and enhance the development of stable and efficient devices. Recently we demonstrated such accelerated stability study of perovskite thin films using concentrated sunlight. 7 In particular, encapsulated MAPbBr3 films were shown to be more stable than MAPbI3 films following exposure to concentrated sunlight of 100 suns (1 sun = 100 mW/cm 2 ), with the latter decomposing into PbI2 and organic components. We then postulated that combining the better photovoltaic performance of MAPbI3 with the added stability of its bromide counterpart, by using mixed compositions of MAPb(I1-xBrx)3, would yield an optimized PV material. 7 Seok et al. demonstrated enhanced "shelf-life" stability of the PCE of cells based on MAPb(I1-xBrx)3 with x ranging between 0.2 and 0.29, after cell storage in the dark in controlled humidity atmosphere. 4 Han et al., in their review article, attributed this improved stability to the more compact and dense crystal structure of the mixed halide perovskites. 8 On a different note, reversible instability phenomena were demonstrated in relatively Br-rich compositions: McGehee et al. 9 noted a change in photoluminescence in films with x ≥ 0.2 under visible light soaking, which was attributed to segregation into iodide-rich and bromide-rich domains. This change was fully reversible after few minutes in the dark, hence it cannot be related to long-term degradation. Friend et al. noticed the possible existence of two phases in fresh mixed halide films with x = 0.4 and 0.6, which evolved into a single phase after a few days of storage at room temperature, with no further changes. 10 Though both phenomena were short-termed, they may point to a certain instability of mixed halide films at specific compositions.
Herein we compare the photochemical stability of encapsulated films of mixed halide perovskites with a range of MAPb(I1-xBrx)3 compositions under accelerated stressing tests using concentrated natural sunlight at 100 suns. The results are compared to degradation under outdoor sunlight exposure to establish the relevance of the accelerated testing to standard operational conditions of PV cells. The evolution of light absorption and the corresponding structural modifications in the films following stressing were investigated. Contrary to our initial presumption, we found that MAPbI3 and MAPbBr3 are more stable than mixed halide compositions, under both concentrated and standard solar exposures. The reasons for the reduced stability of the mixed halide compositions are discussed.
The film structure and composition were studied using UV-vis absorbance spectroscopy and X-ray diffraction (XRD). Figures 1 and 2 show the XRD patterns of MAPb(I1-xBrx)3 films of various compositions, and their properties are summarized in Table 1. XRD patterns of MAPbI3 and MAPbBr3 films for the entire 2θ ranges (Figures 1a, e) indicate their tetragonal I4/mcm and 3 ̅ cubic perovskite structures, respectively. 4,11 For pure MAPbI3 (bottom curve in Figure   2), two diffraction peaks located at 28.11 o and 28.36° (bottom curve in Figure 2) are associated with the (004) and (220) planes in the tetragonal I4/mcm phase. 12 In accordance with a recent report, 4 increasing x was found to result in: (a) a pattern shift to larger scattering angles ( Figure   2) and the corresponding decrease of the lattice parameters (Table 1); (b) merging of the (004) and (220) peaks to a single peak at 28.   Table 1.    Table 1.
The actual halide composition of the films was determined and compared to that in the precursor solutions. According to Vegard's law, the variation of the lattice parameter in a homogeneous alloy (solid solution) is linear with composition. 13 Seok et al. 4 suggested the relevance of this rule for their MAPb(I1−xBrx)3 films at the entire range of x (0 ≤ x ≤ 1).
Applying Vegard's law to our XRD results showed a considerable difference between the bromine content between the expected and actual composition (  Figure 3). 14 3 films are the same as those in the solutions since a stoichiometric precursor composition in a single deposition step was used (PbI2/MAI(Br) = 1). 10 However, in other studies, 18 where an excessive amount of methylamonium salts was involved in the film preparation, a discrepancy between actual halide compositions in solid perovskites and initial precursor solutions was reported. In our case, this result can be related to the use of the two-step deposition process where PbI2 or PbBr2 films were dipped into isopropanol solutions containing MAI and MABr at various ratios. 5 Different reaction rates of the lead halides with MAI vs MABr control the actual halide composition of the films.
The evolution of light absorption and the corresponding structural modifications in the films following stressing were investigated using UV-vis absorbance spectroscopy and XRD, respectively. Figure 3a illustrates the evolution of UV-vis light absorbance spectra for the encapsulated MAPbI3 film due to its exposure to concentrated sunlight of 100 suns. Concentrated sunlight accelerates stressing via increased light intensity which can be accompanied by sample heating (see Experimental Methods section). We have previously shown that the decomposition mechanism of perovskites such as MAPbI3 is light-and thermally enhanced or initiated, while heat alone did not induce decomposition. 7 Following the first 20 minutes of exposure the absorption decreased in the range of ~520-620 nm forming a hump around 520 nm. With increased exposure time, this decrease expands to a wide range of 520-800 nm and the hump becomes more prominent at ~500 nm. The overall degradation in absorption of this film was quantified by calculating the Absorption Degradation State (ADS), i. e., the ratio of the number of absorbed solar photons (from the AM1.5G spectrum) by the exposed film to that by the fresh sample 7 (for the calculation method see Supporting Information). This ratio showed a slight gradual decrease with the exposure time (by ~2% for 60 minutes, maximum dose of 100 sun*hours, black line in Figure 4a). The evolution of light absorption of the mixed halide MAPb(I1-xBrx)3 films showed similar trends ( Figure S1) but at considerably higher degradation rates (green, blue and purple curves in Figure 4a). The largest decrease in ADS (by ~ 13 %) was shown by sample number 4, the most Br-rich mixed halide film. The same message is provided by the black curve in Figure 5 showing the normalized difference in absorption following degradation (1-ADS) as a function of Br content in the film, x, calculated using the XRD analysis (Table 1). At the same time UV-vis absorbance spectrum of pure MAPbBr3 did not exhibit any degradation following exposure (Fig.   3b), but rather a slight improvement in the absorption (see also increase in ADS for this film in Figure 4 and negative final "degradation parameter" (1-ADS) = -0.1 in Figure 5). We have verified that this increased absorption cannot be explained by degradation, as PbBr2 has negligible absorption in the range of 400-800 nm (Fig. S2). The slight increase in absorption might be explained by enhanced conversion of the PbBr2/ MABr precursors to the final MAPbBr3 perovskite in the film. This conversion was not complete in the pristine film, as indicated by the presence of PbBr2 detected by XRD (Fig. 1e). However, this conversion was too small to be reflected in the XRD after sunlight exposure. This effect should be further investigated.
Joint analysis of the light absorbance (Figures 3a and S1) and XRD data ( Figure 1) suggests that decomposition of the perovskite via crystallization of PbI2 occurs during exposure of MAPbI3 film and mixed halide MAPb(I1-xBrx)3 films, in accordance with our recent results. 7 Decomposition of MAPbI3 to PbI2 and MAI was previously suggested as the result of photolysis, with further dissociation of MAI to methyl amine, iodine and other small molecules (hydrogen, HI, water). [19][20][21] PbI2 crystallization was directly confirmed herein by XRD measurements ( Figure  1). The observed shift of the XRD perovskite-related peaks towards higher 2θ values following exposure ( Figure 2) indicated an increased Br content in the remaining perovskites following PbI2 release. The lack of splitting of these perovskite-related XRD peak suggests no phase separation to the MAPbI3 and MAPbBr3 perovskites. FTIR measurements showed a decrease in organic (carbon-and nitrogen-related) groups' absorption, with no alternative peaks rising (Fig.   S3), indicating their elimination from the solid film. Electron paramagnetic resonance spectroscopy of our perovskite samples illuminated by 100 suns in a closed ampoule showed the involvement of MAI dissociation in the degradation process (to be published elsewhere). We therefore suggest a photolysis decomposition of the mixed halide perovskite to MAI, PbI2 and a Br-richer mixed halide perovskite as a possible first step, which is followed by further MAI decomposition into volatile species and iodine. [19][20][21] To estimate the degree of the degradation, the ratio of {002} peak intensity of PbI2 (indicated by arrows in Figure 1) to that of the {100} peak (2θ =14º) of I-containing MAperovskites was calculated for the fresh and the exposed films. The ratio of the exposed films was then normalized by that of the fresh film, and the resulting values are presented in Figure 5 (blue curve). This value was found to increase with increasing x. While this value was almost 1 after exposure of samples 1 (MAPbI3) and 2 (MAPb (I1-xBrx Table 1.

(a) (b)
We conclude that, though MAPbBr3 is the most stable composition, increased bromine content in the mixed halide perovskite films resulted in accelerating their photochemical degradation. We relate this finding with the observed increase in the Full Width at Half Maximum (FWHM) of the XRD peaks for mixed halide perovskites compared to MAPbI3 and MAPbBr3 ( Fig. 5 and Table 1). Similar behavior was previously observed for CH3NH3Sn(I1-xBrx)3 solid solutions. 22 The FWHM of XRD peaks is inversely related to the crystal coherence

Synthesis of MAPb(I1-xBrx)3 perovskites and film preparation: CH3NH3I (MAI) and CH3NH3Br
(MABr) were synthesized as described earlier, 5  Film exposure to concentrated sunlight. Sunlight collected and concentrated outdoors was focused into a transmissive optical fiber and delivered indoors onto the sample (Fig. S5a). 25,26 A 2.5 cm long, 0.25 cm 2 square cross-section kaleidoscope placed between the distal fiber tip and the sample was used to achieve flux uniformity (Fig. S5b). 26  where the solar spectrum was found to be very close to the standard AM1.5G solar spectrum. 27 The spectrum of light delivered to the sample was also close to the AM1.5 standard. 25 The concentration of incident sunlight on the sample was measured using a spectrally blind pyranometer (thermopile) of 5% accuracy. During the exposure, the samples were thermally Optical and crystallographic characterization. The UV-vis absorbance spectra of the films before and after exposure were recorded in direct transmission mode using a Cary 5000 UV-vis-NIR spectrophotometer (Agilent technologies). To verify that diffused scattering was not responsible for the observed changes in the measured absorbance, the diffuse scattering intensity spectra were measured after exposure and were found to be almost entirely independent of the sample's light exposure (Fig. S6).

Notes
The authors declare no competing financial interests.  . FTIR spectra of the MAPbI 3 film (sample 1) before and after exposure to 100 suns*hours. Measurements were performed using a Thermo Scientific Nicolet 8700 FTIR spectrometer with a deuterated triglycine sulfate (DTGS) detector. The samples were prepared by grinding the film and mixing it with KBr powder at a concentration of 0.5% w/w, followed by pressing into a pellet. Peak identification was based on previously published data [1].

S5
a b c Figure S4. SEM images of perovskite films: (a) sample 1; (b) sample 3; (c) sample 5. The images show that MAPbI 3 (sample 1) has the largest grains, MAPb(I 1-x Br x ) 3 (x=0.2 according to XRD analysis, sample 3) has the smallest grains, and MAPbBr 3 (sample 5) has a mixture of small and large grains. Images were taken using a JEOL 7400F field emission gun high resolution scanning electron microscope (HRSEM) in SE mode using an in-lens detector.  A t (λ) -the measured absorbance at a given wavelength λ and after exposure time t, (λ) -the incident photon flux, λ1=400 nm and λ2=800 nm. A t (λ) was directly extracted from the UV-vis absorbance spectra after the corresponding exposure time, and the photonic flux was taken from S8 the ASTMG173 standard, which was used as the AM1.5G reference spectrum [4]. To follow the evolution of the degradation of the perovskite, was normalized by the number of photons absorbed by the fresh sample N o tot , to provide the ratio indicated as "absorption degradation state" (ADS).