Unraveling the Effect of Crystal Structure on Degradation of

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Cite This: ACS Appl. Mater. Interfaces 2019, 11, 22228−22239

Unraveling the Effect of Crystal Structure on Degradation of Methylammonium Lead Halide Perovskite Bhanu Pratap Dhamaniya,† Priyanka Chhillar,† Bart Roose,‡ Viresh Dutta,† and Sandeep K. Pathak*,† †

Centre for Energy Studies, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India Cavendish Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge CB30HE, U.K.

‡

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S Supporting Information *

ABSTRACT: Despite the remarkable efficiencies of perovskite solar cells, moisture instability has still been the major constraint in the technology deployment. Although, some research groups have discussed the possible mechanisms involved in the perovskite degradation, no broader understanding has been developed so far. Here, we demonstrate that the crystal orientation of perovskite film plays a major role in its degradation. We observed that the films fabricated via different routes led to different degradation behaviors and unraveled that diversity in the degradation rate arises due to the difference in crystallographic characteristics of the films. Using optical and electrical measurements, we show that the film prepared via a single-step (lead chloride precursor based) route undergoes a much faster degradation rate as compared with films prepared using single step (acetate precursor based) and two-step (or sequential deposition) routes. Although the resulting film is methylammonium lead iodide (MAPbI3) regardless of processing via different routes, their respective crystal orientation is different. In this manuscript, we correlate crystal orientation of MAPbI3 with their degradation pattern. Our studies also suggest a possible way to make stable perovskite film. KEYWORDS: fabrication routes, perovskite degradation mechanism, perovskite hydrate, decay rate, crystal structure, plane orientation, cubic symmetry, tetragonal symmetry

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eventually leading to high processing cost,10 whereas the counterparts, single step and sequential route, are popular for deposition from precursor solution at low temperatures with optimum energy utilization.11,12 Apart from skyrocketing efficiencies, the robustness of the device in outdoor environment is critical for economic and commercial viability. The biggest obstacle in the deployment of this technology is the instability of perovskite against moisture,13 heat,14 oxygen,15 and UV radiation16 exposure. Out of these factors, moisture instability has been reported as the primary reason obstructing to achieve good device lifetime.17,18 Moisture exhibits a two-faced behavior in the device performance. During fabrication of perovskite, a small content of moisture in air boosts the device performance. Pathak et al. in their work reported that air (with 35−40% RH)-annealed films show superior photophysical properties with improved power conversion efficiency (PCE) as compared with films annealed in N2.19 Snaith and co-workers also proved that a controlled amount of water in the precursor solution aids in quick film formation and improved quantum yield.20 Conversely, moisture shows an adverse effect after complete

INTRODUCTION Organometal halide perovskite solar cells have emerged as a front-runner among the existing photovoltaic technologies. After the first-ever fabrication of a methylammonium lead iodide (MAPbI3) (MA: methyl ammonium) perovskite-based solar cell device by Miyaska and co-workers in 2009,1 a tremendous rise in the efficiency, from 3.9% to as high as 23.3% have been reported.2 These impressive efficiencies are attributes of superior optoelectronic properties of this material, including broad absorption of the solar spectrum complemented with a tuneable band gap,3,4 very low Urbach energy of around 15 meV,5 long-ranging charge carrier diffusion lengths due to high crystallinity,6 and ambipolar charge-carrying capabilities.7 Also, the ease of synthesis adding to its low cost and variety of processing methods has made perovskite the rising star in the photovoltaic community. The solution processability of the perovskite is a strong selling point toward commercialization. A variety of film fabrication techniques have been explored and exploited to attain good quality perovskite films, for example, vapor deposition8 and single-step and two-step deposition.9 Although the vapor-phase deposition method can produce a proficient pin hole-free perovskite with reported power conversion efficiencies (PCEs) of around 15%,8 this technique is associated with a few downsides, like precise control over the precursor deposition rate and process-energy inefficiency, © 2019 American Chemical Society

Received: January 16, 2019 Accepted: May 30, 2019 Published: May 30, 2019 22228

DOI: 10.1021/acsami.9b00831 ACS Appl. Mater. Interfaces 2019, 11, 22228−22239

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ACS Applied Materials & Interfaces

Figure 1. UV−vis analysis of fresh and aged films for (a) lead chloride route film (b), two-step route film (c), acetate precursor route film, and (d) rate of degradation for all three films.

crystallization of the film, deteriorating the optoelectronic properties of the perovskite.21 To suggest a better way for attaining stable devices, an in depth study of moisture-induced degradation mechanism is necessary. Several groups have reported degradation mechanisms, but the outcomes are often contradictory. Whereas some claimed that the hydrolysis of MAPbI3 leads to direct decomposition into CH3NH2 (aq), HI (aq), and PbI2 (s),22,23 Yang et al. and Christians et al. have found the formation of intermediate monohydrate phase (CH3NH3PbI3·H2O) as the first step of hydration.24,25 Leguy and co-workers also observed the formation of dihydrated perovskite ((CH3NH3)4PbI6· 2H2O) upon elongated moisture exposures.26 Hence, an in depth study is required to clarify the ambiguities and to suggest a singular theory for MAPbI3 degradation. In this work, we aim to expand the research on perovskite degradation and propose its dependence on the crystal structure of the perovskite film. To the best of our knowledge, this is the first experimental report shedding light on the crystal structure-related aspects of perovskite degradation and reports that perovskite films prepared from various fabrication routes can have distinct orientations and thus different degradation behaviors under moisture. With the spectroscopic techniques and device data, we have shown that the film prepared from the single-step lead chloride precursor route follows an intermediate path and undergoes a much faster decay rate as compared with the single-step acetate precursor and two-step routes. Perovskite films fabricated via these three routes crystallize with different crystal orientations and are the reason for this distinct degradation behavior. Lead chloride precursorbased film has dominant (100) plane orientation with the hygroscopic organic part on the exposed facet that governs the faster degradation, whereas in the other two cases, perovskite film crystallizes in tetragonal phase with (110) plane

orientation dominant. In this plane arrangement, hygroscopic methyl ammonium ions are caged inside the inorganic part and facilitate comparatively slower degradation. A detailed understanding on the degradation behavior with aging time has been provided through absorption spectroscopy and X-ray diffraction (XRD) patterns analysis. The study also suggests the reason for intermediate phase formation during degradation and also manifests that the grain domains are not the only predominant factor driving the moisture-induced perovskite degradation.

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RESULTS AND DISCUSSION To conduct the analysis, three MAPbI3 films were fabricated using (i) single-step lead chloride precursor route, (ii) two-step lead iodide route, and (iii) single-step lead acetate precursor route. Unlike the other degradation studies carried out with a higher degree of control over moisture or obeying in situ approaches, these samples are subjected to real-time environment (RH 70−80%), which might vary depending upon the experimental platforms. Figure 1 shows the UV−Vis absorption spectra of MAPbI3, recorded for fresh (not exposed to moisture) and aged films. Fresh films, in all three cases, exhibited a wide absorption, covering almost the entire visible range with a notable perovskite absorption edge at ∼770 nm. From Figure 1a, it is evident that the lead chloride route film shows a huge reduction in absorption within 4 h of exposure. This decay in absorption endures with ageing of the film over the entire visible spectrum, indicating nearly 80% loss in the optical absorption within 24 h. On the other hand, absorption data of the two-step route and acetate precursor route films (Figure 1b,c respectively) show only a small decrease in the absorption even after 24 h. Here, it is clear that the degradation 22229


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Furthermore, we noticed a shift in the absorption onset for the lead chloride route film. The magnified image of Figure 1a shows the blue shift in the absorption edge after 4 h of aging (from A to B). With extended exposure till 144 h, the absorption onset at 430 nm was noted, probably indicating the transformation of perovskite into perovskite double hydrate. However, we didn’t observe any band edge shift for two-step route and acetate-route films. Photoluminescence for lead chloride, two-step, and acetate precursor route films is shown in Figure 2. All samples were aged under the same environmental conditions and measured at room temperature with an excitation wavelength of 520 nm. It can be inferred from Figure 2a that the photoactivity of lead chloride route films dropped drastically within 4 h of aging and vanished after 24 h. The hydrated form of perovskite might be responsible for the immediate loss of the photophysical properties of the film. Examining the two-step and acetateroute films (Figure 2b,c), we observed an increase in the emission intensity for initial few hours. Probably, moisture has helped in improving the perovskite photoactivity. This improvement might be because of passivation of surface defects generated due to unreacted methylammonium iodide (MAI) and dangling bonds or due to coalescence of perovskite grains.28 Prolonged aging of the films leads to detrimental effects and lessens the luminescence of the film. Formation of the hydrated phase in the lead chloride-based perovskite film can also be proved by Fourier transform infrared (FTIR) spectroscopy. Figure 3 shows the FTIR spectra of fresh and aged films for all three fabrication routes. A clear difference can be spotted in the FTIR spectrum of all films beyond the fingerprint region (wave number > 1200 cm−1). In the IR spectra of the these films, peaks at around 1420 and 1463 cm−1 can be assigned to C−H bending and peaks in the 1575−1650 cm−1 range correspond to N−H bending.29,30 The peaks in the 3100−3200 cm−1 range signify N−H stretching vibrations. Notable differences have been observed between IR spectra of the lead chloride route film and two-step and acetate-route films in the 3100−3500 cm−1 region. The lead chloride route perovskite shows very strong and broad vibrations in this region. This is because of merging of the N−H stretch with hydrogen-bonded O−H stretch.31 A broad shoulder in the 3450−3550 cm−1 range can be designated to hydrogen bonding.31 The presence of this broad shoulder spectral peak in the lead chloride route film signifies interaction of perovskite with water and proves that the CH3NH3PbI3 exists in the film along with water molecules through hydrogen bonding (in a perovskite hydrated form).

of perovskite is quite resistant to moisture in the case of twostep and acetate-route films. We have plotted the area under the absorption curve against aging time (Figure 1d) to extrapolate the decay rates. Notably, the time taken by the twostep and acetate-route films for 30% decay in initial absorbance, τ0.3, is around 20-fold with respect to the time taken by the lead chloride route film (see Table 1). Table 1. Time Taken by Different Fabrication Routes for 30% Decay in Absorption fabrication route

τ0.3 (h)

lead chloride precursor route two-step route acetate precursor route

2.06 46.38 39.13

Interestingly, the absorption profile for aged films was found rather different in the case of the lead chloride route as compared to the other two routes. For lead chloride route film, the absorption reduced uniformly throughout the visible region and almost vanished after 24 h. An intermediate absorption edge at around 430 nm was noticed, which might be attributed to perovskite hydrate. After around 336 h of environment exposure, the absorption edge at around 510 was spotted and can be assigned to lead iodide (PbI2) absorption onset.22 In the case of acetate and two-step route films, the perovskite absorption edge almost disappeared within 3−4 days of aging and absorption only at wavelength below 510 nm was visible, signifying PbI2 as the only degraded product. This observation led to the fact that the final degraded product is PbI2 in all the cases but the degradation kinetics is different. This statement is well validated in the discussion of XRD. In Figure 1, perovskite film prepared from the lead chloride precursor route shows a much sharper absorption onset as compared with other two films. In the lead chloride precursor route film, presence of chlorine during the crystallization phase improves the crystal growth process and facilitates much larger grains (as shown in the scanning electron microscope (SEM) images in Figure 9) with preferred orientation of methyl ammonium molecules inside PbI64− octahedral.27 The lead chloride route film shows much higher order of crystallinity, as indicated by the XRD patterns of fresh film shown in the Figure S1. Thus, the influence of chlorine in crystallization providing a high degree of crystallization and smooth surface morphology with very low grain boundary disorders might result in such a sharp absorption edge.

Figure 2. Emission spectra of (a) lead chloride route film, (b) two-step route film, and (c) acetate-route film. 22230


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Figure 3. FTIR spectra for fresh and aged films fabricated from (a) lead chloride route film, (b) two-step route film, and (c) acetate-route film.

Figure 4. Degradation rate comparison of the device prepared with different precursor routes. Comparison of (a) PCE, (b) normalized Jsc, (c) normalized fill factor (FF), and (d) normalized Voc.

On the other hand, no such broad vibrations were encountered in the two-step and acetate-route films and only strong peaks of N−H stretching can be seen stating the absence of any intermediate hydrate in the process. Degradation of the film can be monitored by observing the decrease in the peak intensity. Performance degradation for all three precursor routes was studied by making full devices, employing TiO2 and poly(triarylamine) (PTAA) as electron and hole collecting layers, respectively. Figure 4a−d shows the comparison in the device performance parameters for all three films. The original J−V curves have been supplied in Supporting Information Figure S2. In consistency with the previous results, we observed that lead chloride-based films are losing PCE much faster compared with acetate and two-step films (as shown in Figure 4a). We noticed a boost in the performance from t = 4 to 21 h that might be the effect of variation in the humidity and temperature. Research groups have already reported the possibility of recovery after moisture exposure.32 Comparing from the normalized Jsc graph (Figure 4b), current density for the lead chloride route drops rapidly as compared with other two devices. This might be due to loss in the absorption of

perovskite after exposing to moisture, as indicated in Figure 1a. Normalized FF and Voc graphs are also plotted for the comparative analysis of device performance. Note that degradation of these devices is much slower than that of neat films due to the presence of a PTAA capping layer, protecting the underlying perovskite film from moisture. To get a better insight of how humidity affects the perovskite structure and why there is a discrepancy in the degradation rates of these films, X-ray diffraction patterns were recorded for all three samples. The XRD of a fresh lead chloride route perovskite sample (Figure 5a) shows strong diffraction at 2θ = 14.02 and 28.34°, resembling highly crystalline perovskite peaks. After just 4 h of humidity exposure, the diffraction pattern undergoes changes and a new diffraction peak at a 2θ value of 7.34° was observed along with the perovskite peaks. At the same time, visual inspection shows that the film started fading off marking transparent spots (see Figure 8A). Within 8 h of ageing, the peak at 7.34° gets strengthened and the film turns white to transparent. We attribute this low angle peak to the metastable phase of monohydrated perovskite. Similar low angle diffraction peaks were also observed by Christians et al.25 and Zhao et al., 22231


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Figure 5. XRD data of fresh and aged perovskite films fabricated via (a) lead chloride precursor route, (b) two-step route, and (c) acetate precursor route; (d) FWHM values at the dominant perovskite peak for all three films (e) two-step route perovskite film aged under high humidity (around 90−95%) conditions, and (f) lead chloride precursor perovskite film aged under low humidity (40−45% RH) conditions.

referring to diffraction from metastable CH3NH3PbI3·H2O.33 This peak was not persistent for a longer time and almost disappeared after 24 h. Over time, new peaks arise in the diffraction pattern at 2θ values of 8.6 and 10.6° corresponding to monohydrate (CH 3 NH 3 PbI 3 ·H 2 O) and dihydrate ((CH 3 NH 3 ) 4 PbI 6 ·2H 2 O) phases of perovskite, respectively.24,26 The dihydrated phase became more intense around 336 h, along with the characteristic lead iodide peak at 12.6°. Further exposure to moisture may lead to evaporation of MAI with water, leading to lessening of the dihydrate content and leaving lead iodide behind. With further aging, the PbI2 peak at 12.6° becomes more pronounced whereas the dihydrate peak reduces in intensity. After 432 h, only the PbI2 peak was visible and no traces of any form of perovskite were seen. This proves that the ultimate degraded product was PbI2. Visually, from Figure 8A, a white to yellow tinge was observed in the film, signifying the color of lead iodide. Ideally, the perovskite peak

(2θ = 14.02°) should reduce with the ageing, showing reduction in the amount of perovskite but in the initial stages, the perovskite peak also increases, confirming the enhancement in the crystallinity of the perovskite due to the effect of moisture. Improved crystallinity can be justified by observing a decrease in the full width at half-maximum (FWHM) values, as plotted in Figure 5d. On the other hand, the peak width of the hydrate at 7.34° remains almost the same despite increased intensity. Moving on to the XRD patterns of two-step and acetateroute films in Figure 5b,c, we see that no deterioration was detected in both films for the first few hours. Further, after a day, the diffraction data suggested an improved crystallinity and the effect of moisture explained in the case of the lead chloride route film could be extrapolated to these films too. Although there is a considerable increase in the crystallinity (of perovskite) in the case of the two-step route film, the film 22232


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Figure 6. Magnified XRD peak for dominant reflection in perovskite made from (a) lead chloride precursor route (b) two step route and XRD patters of fresh films with plane designation for (c) lead chloride precursor route and (d) two step and acetate route.

data also suggests that the final decomposed product is same (lead iodide) in all the cases.

significantly decomposes into PbI2. The signature (001) peak of the lead iodide hexagonal structure34 is present and persists on and after the first day. Eventually, the films degraded to PbI2 within about a period of 3 days. Moreover, it took a little less time to initiate the degradation in the two-step film as compared with the acetate-route film; the reason might be the small grain size of the two-step film associated with pin holes, as shown in the SEM images (Figure 9).35 Absence of any diffraction peaks below or around 10° emphasized a distinct degradation mechanism, which does not include any trace of hydrated perovskite. On visually investigating the films in Figure 8B, no whitish spots were identified in either of the cases unlike that in the lead chloride route film. This suggests that formation of hydrated phase could be a possible reason for higher degradation rate in the lead chloride precursor route film as compared to other two route films. To examine any possibilities of hydrate formation in acetate and two-step routes, these films were subjected to a highhumidity environment (RH 90−95%) and XRD patterns were collected (Figure 5e). Surprisingly, the two-step route film alone indicates the hydrate formation concurrently with PbI2 residue. The acetate precursor route film did not experience hydrate formation, as shown in Figure S5. The comparatively larger grains of the acetate-route film would have aided in preventing the degradation through hydrated phases (analyzed using SEM images and discussed in the following section). Adding to this, we conducted another experiment to know the sensitivity of lead chloride film at lower humidity (40−45% RH) and identified the formation of perovskite hydrate by the presence of a diffraction peak at 7.34°; see Figure 5f. With these, we can infer that the lead chloride route film transforms to a hydrated phase even at a low humidity range, indicating higher sensitivity toward water, while two step and acetateroute films may form the hydrated phase only at very high humidity. Similar to the outcomes of absorption analysis, XRD

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DISCUSSION From above discussed characterization results and device data analysis, we can say that there is clearly a difference in the time taken and route adapted in the degradation of perovskite prepared from different routes. The root for this distinct degradation behavior lies in the crystal structures of the perovskites attained at the time of crystallization. Looking into the crystallographic diffraction data of the films we identified that they possess distinct crystal orientations. Films prepared from the lead chloride route attain a cubic phase with the dominating plane of (100) while the two step and acetate-route films possesses tetragonal phase with (110) facet as dominant. It is well known that CH3NH3PbI3 crystallizes into tetragonal phase at room temperature.36−40 Tetragonal to cubic phase transformation takes places at higher temperatures (>54.4 °C) and when it returns to room temperature it attains tetragonal form which is the most stable phase of the perovskite at ambient.41 The polar nature of CH3NH3+ ion leads to a disordered cubic phase that converts into stable tetragonal phase at room temperature.42,43 Perovskite films fabricated from acetate and two step route follow this approach and crystallizes into tetragonal phase. But, in the typical synthesis process of MAPbI3 via the lead chloride route (using MAI and PbCl2 as precursor) it is likely that the cubic phase can be attained after annealing at higher temperatures for longer duration.44,42 In case of lead chloride route, it is the chlorine that is playing major role in the crystal formation of perovskite. As suggested by Pistor et al.,45 Noh et al.,46 and Wang et al.42 in their reports, incorporation of smaller size halides like Cl− and Br− favors the cubic phase of perovskite. Perovskite formation using PbCl2 precursor follows formation 22233


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Figure 7. Illustration for initiation of the degradation mechanism in the case of (A) 100 exposed facet in the lead chloride route film and (B) 110 exposed facet in acetate and two-step route films.

(112), (211), (004), (220), (213), (114), (224), (314), and (404) plane orientations of the tetragonal structure.30,49−51 The reflections at (211) and (213) in the case of two-step and acetate precursor route are reflections that are only present in the tetragonal symmetry and are not expected in the cubic symmetry. Any change in the symmetry can be justified by examining these reflections.39,49,52 Lattice parameter with respect to diffraction data of all the three films has been calculated. Diffraction data for the lead chloride route film proves its cubic phase with lattice parameter a = 6.311 Å, and reflections from the other two routes demonstrate the tetragonal symmetry with lattice constants a = b = 8.81 and c = 12.65 Å for the two-step route and a = b = 8.80 and c = 12.66 Å for the acetate precursor route film. Previous reports have found that change in the structural symmetry of CH3NH3PbI3 material will lead to a change in the band gap. It was observed that a lower symmetry structure shows a little higher band gap with respect to higher symmetry structures. These findings were also experienced in our case, as shown in the Figure S3; the perovskite film fabricated from lead chloride route shows a red-shift in the absorption onset and imparts improved optical absorption. It exhibits a smaller band gap of 1.560 eV compared with 1.589 eV for two-step route and 1.587 eV for acetate route films due to a higher symmetry order in the cubic phase. Hence, all of these facts described above make it evident that the lead chloride precursor route film crystallizes in the cubic phase whereas the two-step and acetate precursor route film adapts the tetragonal phase. Structures shown in the insets of Figure 6 represent that the organic part of MAPbI3 (MA+ ions), highly prone to moisture attack, occupies the exposed (100) plane of the cubic lead chloride route film. This results in faster interaction of water molecules with the film. As shown in Figure 7A, the

of MAPbCl3 as an intermediate stage having cubic (100) plane orientation.47,48 This cubic orientation of MAPbCl3 perovskite acts as a template for MAPbI3 formation by substituting the Cl− ions with I− ions. This substitution is very slow process and occur during annealing of film at 100 °C for very long time (2.5 h in our case), retaining the cubic structure of the perovskite.49 Presence of Cl− during crystallization phase reduced the cubic to tetragonal phase transition temperature and favors the stable cubic structure of perovskite at room temperature.45 Luo and co-workers, also reported retention of the cubic phase of lead chloride route perovskite annealed at high temperatures.44 To supports the fact that lead chloride perovskite shows a cubic symmetry and other two routes have a tetragonal symmetry, a closer magnified look of dominant plane reflection of lead chloride route film and two step route film has been plotted in the Figure 6a,b. It is clear that in case of lead chloride route only one reflection is observed at 2θ = 14.02° whereas, in the counterpart, a shoulder peak at 2θ = 13.89° along with the dominant peak at 2θ = 14.06° is noticed and can be resolved by the Gaussian peak fitting. These two close diffractions in the latter case can be assigned to (002) and (110) reflection of tetragonal symmetry as reported by Baikie et al.39 in their studies. This peak splitting is inconsistent with the cubic phase.39 XRD patterns of pristine films along with all designated planes are shown in the Figure 6c,d. Reflections for lead chloride route film can be seen at 14.02, 28.39, 31.75, and 42.81°, corresponding to the (100), (200), (210), and (300) lattice planes of the cubic phase,49 whereas two-step and acetate-route films correspond to 2θ values of 13.91, 14.06, 19.98, 24.34, 28.17, 28.38, 31.53, 31.76, 40.48, 43.13, 50.15, and 13.97°, 14.06, 19.85, 24.34, 28.11, 28.26, 31.54, 31.70, 40.49, and 43.03°, respectively, designating the (002), (110), 22234


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Figure 8. Crystal structures of (A) MAPbI3, hydrated MAPbI3, and PbI2 along with pictures of lead chloride route aged film at different aging times and (B) crystal structure of MAPbI3 and PbI2 with pictures of two-step and acetate-route films at different aging times.

Figure 9. Contact angle measurement and SEM images of (a) lead chloride route film, (b) two-step route film, and (c) acetate-route film.

exposure leads to separation and subsequent evaporation of MAI with water, leaving PbI2 as the final decomposed product (Figure 8A). In the two-step and acetate precursor route films, the dominant facet (110) is Pb-I exposed (as shown in the Figure 7B), which is less sensitive to moisture. The more hygroscopic inorganic part is protected by PbI64− cage and governs the slower degradation in two-step and acetate route films.

degradation process is initiated when the interacting H2O molecules make a hydrogen bond with the CH3NH3+ ion and are incorporated in the perovskite structure to give the monohydrate phase (CH3NH3PbI3·H2O). The process leads to deformations in the perovskite structure and causes the optical properties of the film to attenuate severely. With the aging time, the monohydrated phase is transformed into the dihydrated perovskite phase ((CH3NH3)4PbI6·2H2O)53 with an increased order of structure deterioration. Further moisture 22235


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ACS Applied Materials & Interfaces However, Pb2+ also has a tendency to coordinate with H2O.54 When H2O attacks the PbI64− cage, Pb2+ coordinates with H2O and forms the plumbate-like species PbI42−(H2O)2, PbI3−(H2O)3, and PbI2(H2O)4 and subsequently releases I− ions.55,56 These free I− ions bind with MA+ (via hydrogen bonding) and make a separate MAI unit. Now, the MAI that is a separate entity might get solvated and sublimated in the presence of water. Hence, there is a very low probability of making a hydrated form of perovskite in (110) terminated phases. The plumbate species resulting from water molecule incorporation in the PbI64− octahedra cause defects55 in the film, which might be the reason for the blue-shift in the emission spectra of these films during decomposition. The simulation work conducted by Mosconi et al. also reported this observation that the facets terminated with MAI are faster to degrade as compared with PbI2-exposed facets.54 Figure 9 shows the morphology of the films prepared from different precursor routes. It is clear that the synthesis process has a major impact on the perovskite surface. The lead chloride route film shows quite large grains; this might be the benefit from the presence of chloride-related species in the film during crystallization.27 However, the chlorine species are not present in the final perovskite film. The energy-dispersive X-ray spectroscopy (EDS) data shown in Figure S4 also confirm the absence of the same. Although the acetate precursor route film does not hold very large grains like chloride route film but the grains are rather coalesced and have a smooth surface. The perovskite film prepared from two-step route possesses a very rough surface with irregular-shaped grains. Due to uneven and small-sized grains, the film is associated with quite a more number of grain boundaries and a few pin holes. As per previous reports, perovskite film with less number of grain boundaries manifests slower degradation as compared with the film having more number of grains.27,57 But in our case, the films are fabricated using different precursor compositions and have dissimilar crystallographic properties. Hence, the theory based only on grain boundaries and surface texture cannot fully explain the degradation rate differences experienced by these films and the role of crystal orientationrelated aspects becomes dominant in governing the degradation mechanism.58 Although the lead chloride route film has large grains compared with the other two routes, its decomposition is initiated much faster, as shown in the absorption and XRD analysis. The arrangement of (100) plane orientation with the hygroscopic organic part on the surface has been a reason for this fast degradation. But in the case of the other two route films, having similar (110) lattice arrangements, surface morphology has a significant role in the initiation of perovskite degradation. The two-step route film, due to more grain boundaries and irregular-shaped grains, starts decomposing quite faster compared with the acetate route as shown in the diffraction data. The intergranular spaces and the rough surface with uneven grains, as shown in the cartoon in Figure 10a, provide more scope for water molecules to attack and facilitate rapid penetration of water molecules into the perovskite layer. In acetate route film, mending of perovskite grains provides compact and more continuous film that allows little more time for moisture to attack on perovskite. Hence, in a two-step route, degradation starts a little faster than in acetate route. Water contact angles (presented in Figure 9 (top)) were measured for these films terminated with different crystal facets. It was observed that in the case of lead chloride and

Figure 10. Cartoon depiction of the surface morphology and contact angle in (a) two-step and (b) acetate route films.

two-step route films, the water droplet gets flattened, referring to a contact angle of 0°. Only acetate precursor route film exhibited an angle of 42.9°, showing the least affinity to water among all films. However, the flattening of water drop in the case of a two-step route film might be because of immediate penetration of water into the intergranuals of very small and irregular shaped grains, as shown in SEM images above. A cartoon picture for the same has also been depicted in Figure 10.

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EXPERIMENTAL SECTION

Materials. Lead iodide (PbI2, Alfa Aesar, anhydrous, 99.95%), lead chloride (PbCl2, Alfa Aesar, anhydrous, 99%), lead acetate (Pb(Ac)2· 3H2O, Alfa Aesar, 99%), 2-propanol (Alfa Aesar, anhydrous), dimethylformamide (DMF, Alfa Aesar, anhydrous, 99.8%), and hydroiodic acid (57% in water, Merck) were used. All chemicals were used as received without further purification. Methylammoniumiodide (MAI) was synthesized in our laboratory. MAI Preparation. Methylamine (33 wt % in absolute ethanol, Sigma-Aldrich), ethanol (AR grade), and hydroiodic acid were used for synthesis of MAI. Twenty-four milliliters of methylamine was dissolved in ethanol under continuous stirring and 10 mL of HI was added at the interval of 10 min mL−1. White color powder (MAI) was obtained as a product of drying the solution. This MAI powder was washed several times with diethylether and dissolved in ethanol for recrystallization. Recrystallized MAI powder was collected as white shiny crystals. Perovskite Film Fabrication. The perovskite (CH3NH3PbI3) films used for characterizations were casted on glass slides. Glass slides were cleaned with the laboline solution, followed by rinsing with deionized water, acetone, and sonication in isopropyl alcohol (IPA) for 30 min. Precursor solutions for single-step lead chloride- and single-step acetate route films were made by dissolving PbCl2 and Pb(Ac)2 in DMF with MAI in the molar ratios of 1:3, respectively. Lead chloride route films were made by spin coating the precursor at 2000 rpm for 30 s followed by annealing at 100 °C for 2.5 h. Singleprecursor acetate films were prepared keeping the same spin parameters followed by drying and annealing for 10 and 5 min, respectively. Two-step CH3NH3PbI3 films were prepared by fabricating PbI2 as the first step and then coating of MAI as the second step. For the first-step deposition, 0.7 M lead iodide solution was prepared in DMF and spun at 3000 rpm for 30 s and annealed for 30 min at 70 °C. MAI in IPA (10 mg mL−1) solution was then coated on the PbI2 films and annealed at 100 °C for 40 min to form perovskite. All perovskite films were prepared in the dry box under a moisture level of 10−15%. Device Fabrication. Fluorine-doped tin oxide-coated glass (7 Ω sq−1, Sigma-Aldrich) was cleaned by sonication in 2% Hellmanex solution for 15 min, followed by rinsing with deionized water, sonication in acetone, rinsing with acetone, rinsing in isopropanol, and a final rinsing with isopropanol. The substrates were subsequently placed on to a hotplate and heated to 450 °C. A TiO2 compact layer was deposited by spray pyrolysis from a precursor containing titanium diisopropoxide bis(acetylacetonate) (0.6 mL) and acetylacetone (0.4 mL) in 9 mL of EtOH. The substrates were allowed to cool down to 22236


Research Article

ACS Applied Materials & Interfaces room temperature before a mesoporous TiO2 layer was deposited by spin-coating TiO2 paste (150 mg mL of Dyesol 30NR-D in EtOH) at 4000 rpm for 10 s. The samples were then annealed at 450 °C for 30 min and allowed to cool down to 150 °C before being transferred to a N2-filled glovebox. Perovskite films were deposited as described above. After being cooled to room temperature, a PTAA (10 mg mL−1, EM Index) layer, doped with bis(trifluoromethylsulfonyl)imide lithium salt and 4-tert-butylpyridine in 0.08 and 0.04 ratios, was spincoated on top of the perovskite film at 4000 rpm for 20 s. Finally, an 80 nm thick Au back contact was deposited by thermal evaporation. Characterization. X-ray diffraction measurements were performed on a Rigaku ultima IV diffractometer with a Cu Kα radiation source at a scan rate of 8° min−1 and step size of 0.02°. UV−Visible absorption data for all films were recorded using a Perkin-Elmer Lambda 1050 spectrophotometer. Photoluminescence was carried out using a Cary Eclipse G9800A fluorescence spectrophotometer. SEM images are taken using Zeiss EVO 18 at operating voltage of 20 kV.

using the characterization facility available in their labs. B.R. would like to acknowledge the Royal Society for funding through a Newton International Fellowship.

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(1) Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. Organometal Halide Perovskites as Visible- Light Sensitizers for Photovoltaic Cells. J. Am. Chem. Soc. 2009, 131, 6050−6051. (2) National Renewable Energy Laboratory NREL. Best ResearchCell Efficiencies Chart. https://www.nrel.gov/pv/assets/images/ efficiency-chart-20180716.jpg, accessed Aug 08, 2018. (3) Kulkarni, S. A.; Baikie, T.; Boix, P. P.; Yantara, N.; Mathews, N.; Mhaisalkar, S. Band-Gap Tuning of Lead Halide Perovskites Using a Sequential Deposition Process. J. Mater. Chem. A 2014, 2, 9221− 9225. (4) Kumawat, N. K.; Dey, A.; Kumar, A.; Gopinathan, S. P.; Narasimhan, K. L.; Kabra, D. Band Gap Tuning of CH3NH3Pb(Br1‑XClx)3 Hybrid Perovskite for Blue Electroluminescence. ACS Appl. Mater. Interfaces 2015, 7, 13119−13124. (5) De Wolf, S.; Holovsky, J.; Moon, S. J.; Löper, P.; Niesen, B.; Ledinsky, M.; Haug, F. J.; Yum, J. H.; Ballif, C. Organometallic Halide Perovskites: Sharp Optical Absorption Edge and Its Relation to Photovoltaic Performance. J. Phys. Chem. Lett. 2014, 5, 1035−1039. (6) Stranks, S. D.; Stranks, S. D.; Eperon, G. E.; Grancini, G.; Menelaou, C.; Alcocer, M. J. P.; Leijtens, T.; Herz, L. M.; Petrozza, A.; Snaith, H. J. Electron-Hole Diffusion Lengths Exceeding. Science 2014, 342, 341−344. (7) Heo, J. H.; Im, S. H.; Noh, J. H.; Mandal, T. N.; Lim, C. S.; Chang, J. A.; Lee, Y. H.; Kim, H. J.; Sarkar, A.; Nazeeruddin, M. K.; Gratzel, M.; Seok, S. I. Efficient Inorganic-Organic Hybrid Heterojunction Solar Cells Containing Perovskite Compound and Polymeric Hole Conductors. Nat. Photonics 2013, 7, 486−491. (8) Liu, M.; Johnston, M. B.; Snaith, H. J. Efficient Planar Heterojunction Perovskite Solar Cells by Vapour Deposition. Nature 2013, 501, 395−398. (9) Burschka, J.; Pellet, N.; Moon, S. J.; Humphry-Baker, R.; Gao, P.; Nazeeruddin, M. K.; Grätzel, M. Sequential Deposition as a Route to High-Performance Perovskite-Sensitized Solar Cells. Nature 2013, 499, 316−319. (10) Fan, P.; Gu, D.; Liang, G.-X.; Luo, J.-T.; Chen, J.-L.; Zheng, Z.H.; Zhang, D.-P. High-Performance Perovskite CH3NH3PbI3 Thin Films for Solar Cells Prepared by Single-Source Physical Vapour Deposition. Sci. Rep. 2016, 6, No. 29910. (11) Carnie, M. J.; Charbonneau, C.; Davies, M. L.; Troughton, J.; Watson, T. M.; Wojciechowski, K.; Snaith, H.; Worsley, D. A. A OneStep Low Temperature Processing Route for Organolead Halide Perovskite Solar Cells. Chem. Commun. 2013, 49, 7893. (12) Nie, W.; Tsai, H.; Asadpour, R.; Blancon, J.-C.; Neukirch, A. J.; Gupta, G.; Crochet, J. J.; Chhowalla, M.; Tretiak, S.; Alam, M. A.; Wang, H.-S.; Mohite, A. D. High-Efficiency Solution-Processed Perovskite Solar Cells with Millimeter-Scale Grains. Science 2015, 347, 522−525. (13) Han, Y.; Meyer, S.; Dkhissi, Y.; Weber, K.; Pringle, J. M.; Bach, U.; Spiccia, L.; Cheng, Y.-B. Degradation Observations of Encapsulated Planar CH 3 NH 3 PbI 3 Perovskite Solar Cells at High Temperatures and Humidity. J. Mater. Chem. A 2015, 3, 8139− 8147. (14) Conings, B.; Drijkoningen, J.; Gauquelin, N.; Babayigit, A.; D’Haen, J.; D’Olieslaeger, L.; Ethirajan, A.; Verbeeck, J.; Manca, J.; Mosconi, E.; Angelis, F. D.; Boyen, H.-G. Intrinsic Thermal Instability of Methylammonium Lead Trihalide Perovskite. Adv. Energy Mater. 2015, 5, No. 1500477. (15) Aristidou, N.; Sanchez-Molina, I.; Chotchuangchutchaval, T.; Brown, M.; Martinez, L.; Rath, T.; Haque, S. A. The Role of Oxygen in the Degradation of Methylammonium Lead Trihalide Perovskite Photoactive Layers. Angew. Chem., Int. Ed. 2015, 54, 8208−8212. (16) Leijtens, T.; Eperon, G. E.; Pathak, S.; Abate, A.; Lee, M. M.; Snaith, H. J. Overcoming Ultraviolet Light Instability of Sensitized

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CONCLUSIONS The study reports the crystal orientation-dependent degradation of MAPbI3 perovskite films. We observed that the films fabricated through various precursors crystallize in different orientations and have a distinct degradation mechanism. The crystal orientation that is rich in (100) and (200) planes undergoes formation of a hydrated phase, which leads to a much faster decay rate, whereas the films rich in (110) and (220) planes follow a slow decomposition process without adapting any intermediate hydrated phase at 70−80% RH level. We also demonstrate that there is possibility of hydrate formation in the (110) dominating plane at higher humidity levels of around 90%. Our study not just states a clear answer as to why there is a difference in the degradation mechanism but also facilitates a possible way to fabricate a more humidityresistant MAPbI3 film via controlling its orientation in the (110) crystal plane.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b00831. XRD patterns and J−V curves for fresh films; normalized absorption spectra for band gap calculations; EDS spectra of lead chloride; XRD pattern of acetate precursor route film (PDF) (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Bhanu Pratap Dhamaniya: 0000-0003-1935-3597 Priyanka Chhillar: 0000-0003-0334-5605 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Authors would like to acknowledge the Department of Science and Technology, India for providing funding under grant number RP03396G. We would also like to acknowledge the Nanoscale Research Facility (NRF) and Central Research Facility (CRF) at Indian Institute of Technology, Delhi for characterization measurements. We are very grateful to Prof. Josemon Jacob and Prof. Neeraj Khare, IIT Delhi for allowing 22237


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Unraveling the Effect of Crystal Structure on Degradation of

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