Unravelling the Effect of Crystal Structure on Degradation of

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Unravelling the Effect of Crystal Structure on Degradation of Methylammonium Lead Halide Perovskite Bhanu Pratap Dhamaniya, Priyanka Chhillar, Bart Roose, Viresh Dutta, and Sandeep Pathak ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b00831 • Publication Date (Web): 30 May 2019 Downloaded from http://pubs.acs.org on May 30, 2019

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ACS Applied Materials & Interfaces

Unravelling the Effect of Crystal Structure on Degradation of Methylammonium Lead Halide Perovskite Bhanu Pratap Dhamaniya1, Priyanka Chhillar1, Bart Roose2, Viresh Dutta1and Sandeep K. Pathak1* 1

Centre for Energy Studies, Indian Institute of Technology Delhi, Hauz Khas, New Delhi – 110016 India 2

Cavendish Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge, CB30HE, UK

* Corresponding Author E-mail: [email protected] Abstract Despite the remarkable efficiencies of perovskite solar cells, moisture instability has still been the major constrain in the technology deployment. Although, some research groups have discussed the possible mechanisms involved in the perovskite degradation, but still 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 behaviour and unravelled that diversity in 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 single step (lead chloride based) route undergoes at much faster degradation rate as compared to films prepared using single step (acetate based) and two-step (or sequential deposition) routes. Although, the resulting film is MAPbI3 regardless of processing via different routes, however, 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, Perovskite hydrate, Decay rate, Crystal structure, Plane orientation, Cubic symmetry, Tetragonal symmetry

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Introduction Organometal halide perovskite solar cells have emerged as a front-runner among the existing photovoltaic technologies. After the first-ever fabrication of a 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 tuneable band gap3,4, very low Urbach energy of around 15 meV,5 long ranging charge carrier diffusion lengths due to high crystallinity6 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 towards commercialization. A variety of film fabrication techniques have been explored and exploited to attain good quality perovskite films, for example vapour deposition,8 single step and two step deposition.9 Although vapour phase deposition method can produce a proficient pin hole free perovskite with a reported power conversion efficiencies (PCE) of around 15%,8 this technique is associated with a few downsides like precise control over precursor deposition rate and process-energy inefficiency, eventually leading to high processing cost.10 While the counterparts, single step & sequential routes are popular for deposition from precursor solution at low temperatures with optimum energy utilization.11,12 Apart from skyrocketing efficiencies, 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 oxygen15 and UV


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radiation16 exposure. Out of these factors, moisture has been reported as the primary culprit in achieving good device lifetime.17-18 Moisture exhibits a two-faced behaviour in the device performance. During fabrication of perovskite, a small content of moisture in air boosts the devices performance. Pathak et al. in their work reported that air (with 35-40% RH ) annealed films show superior photo-physical properties with improved power conversion efficiency (PCE) as compared to films annealed in N2.19 Snaith and co-workers also proved that 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 crystallisation of the film, deteriorating the optoelectronic properties of the perovskite.21 In order 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. While 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 mono hydrate phase (CH3NH3PbI3.H2O) as the first step of hydration.24-25 Legay 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 best of our knowledge, this is the first experimental report shedding the 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 behaviour under moisture. With the spectroscopic techniques and device data, we have shown that the film prepared



from single step lead chloride precursor route follows an intermediate path and undergoes much faster decay rate as compared to single step acetate precursor and two step routes. Perovskite films fabricated via these three routes crystallise with different crystal orientations and hold the reason for this distinct degradation behaviour. Lead chloride precursor based film have dominant (100) plane orientation with hygroscopic organic part on the exposed facet that governs the faster degradation. While in other two cases perovskite film crystallise 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 degradation behaviour with aging time has been provided through absorption spectroscopy and X-ray diffraction patterns analysis. The study also suggest the reason for intermediate phase formation during degradation and also manifests that the grain domains are not the only predominant factor to drive the moisture induced Perovskite degradation. Result & Discussion In order 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 higher degree of control over moisture or obeying in situ approaches, these samples are subjected to real-time environment (RH 7080%), 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 notable perovskite absorption edge at ~770 nm.


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From Figure 1(a), it is evident that the lead chloride route film shows a huge reduction in absorption within 4 hours 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 hours. On the other hand, absorption data of two step route and acetate precursor route films (Figure1 (b) & (c) respectively), show only a small decrease in the absorption even after 24 hours. Here, it is clear that the degradation of perovskite is quite resistant to moisture in the case of two step and acetate route films. We have plotted the area under the absorption curve against aging time (Figure1 (d)) to extrapolate the decay rates. Notably, the time taken by the two step and acetate route films for 30% decay in initial absorbance, τ0.3, is around 20-fold with respect to time taken by the lead chloride route film (see Table 1). Interestingly, the absorption profile for aged films was found rather different in case of 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 hours. An intermediate absorption edge at around 430nm was noticed which might be attributed to perovskite hydrate. After around 336 hours of environment exposure, absorption edge at around 510 was spotted and can be assigned to lead iodide (PbI2) absorption onset22. 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 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.



a Lead chloride precursor route film 1.0 Magnified view Fresh 4 hours 144 hours 432 hours

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0.2 0.0

500

600

700

Wavelength (nm)

800

04 8

20

40

60

80

Aging time (hours)

100

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.

Table 1: time taken by different fabrication routes for 30% decay in absorption


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In Figure 1, perovskite film prepared from lead chloride precursor route shows a much sharper absorption onset as compared to other two films. In the lead chloride precursor route film, presence of chlorine during crystallisation phase improves the crystal growth process and facilitates much larger grains (as shown in the SEM images in Figure 9) with preferred orientation of methyl ammonium molecules inside PbI6-4 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, influence of chlorine in crystallisation providing high degree of crystallisation and smooth surface morphology with very low grain boundary disorders, might result in such sharper absorption edge. Furthermore, we noticed a shift in the absorption onset for the lead chloride route film. The magnified image of Figure 1(a) shows the blue shift in the absorption edge after 4 hours of aging (from A to B). With extended exposure till 144 hours, 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. b Two step route

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Intensity (a.u.)

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900 650

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Wavelength (nm)

Figure 2. Emission spectras of (a) lead chloride route film (b) two step route film and (c) acetate route film Photoluminescence for lead chloride, two step and acetate precursor route films are shown in Figure 2. All the samples were aged in the same environmental conditions and measured at



room temperature with excitation wavelength of 520 nm. It can be inferred from Figure 2(a) that the photo activity of lead chloride route films dropped drastically within 4 hours of aging and vanished after 24 hours. The hydrated form of perovskite might be responsible for the immediate loss of the photo physical properties of the film. Examining the two step and acetate route films (Figure 2 (b & c)) we observed an increase in emission intensity for initial few hours. Probably moisture has helped in improving the perovskite photo activity. This improvement might be because of passivation of surface defects generated due to unreacted methylammonium iodide and dangling bonds, or coalescence of perovskite grains.28 Prolonged aging of the films leads to detrimental effects and lessens the luminescence of the film. (c) 100

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fresh 48 hours 96 hours 144 hours

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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. Formation of hydrated phase in the lead chloride based perovskite film can also be proved by Fourier Transform Infra Red (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 the films beyond fingerprint region (wave number > 1200 cm-1). In the IR spectrums of the these films, peaks at around 1420 cm-1 and 1463 cm-1 can be assigned to CH bending and peaks in 1575 - 1650cm-1 range correspond to N-H bending. 29-30 The peak in the 3100 – 3200 cm-1 range signifies N-H stretching vibrations. Notable differences has been observed between IR spectrums of lead chloride route film and Two step & acetate route


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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 N-H stretch with Hydrogen bonded O-H stretch.31 Broad shoulder in 3450 – 3550 cm-1 range can be designated to hydrogen bonding.31 Presence of this broad shoulder spectral peak in the lead chloride route film signifies an interaction of perovskite with water and proves that the CH3NH3PbI3 exists in the film along with water molecules through hydrogen bonding (in perovskite hydrated form). 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. (a)18

Acetate precursor route Lead chloride precursor route Two step route

16

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



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Performance degradation for all the three precursor routes were studied by making full devices employing TiO2 and Poly(triarylamine) (PTAA) as electron and hole collecting layers respectively. Figure 4(a-d) shows the comparison in the device performance parameters for all the three films. The original J-V curves have been supplied in supplementary Figure S2. In consistency with the previous results we observed that lead chloride based films are losing PCE much faster compared to acetate and two step films (as shown in Figure 4(a)). We noticed a boost in the performance from t=4 hours to t=21 hours 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 normalised Jsc graph (Figure 4(b)), current density for lead chloride route is dropping rapidly as compare to other two devices. This might be due to loss in the absorption of perovskite after exposing to moisture as indicated in the Figure 1(a). Normalised FF and Voc graphs are also plotted for the comparative analysis of device performance. Note that degradation of these devices is much slower than 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 is affecting the perovskite structure and why there is a discrepancy in the degradation rates of these films, X-ray diffraction patterns were recorded for all the three samples. The XRD of a fresh lead chloride route perovskite sample (Figure w

5a) e

t n di

ite e n e

nd

nd

ti n t te

new di

t

ee

idit e ti n

e

t

e, t e di

e

in ti n

i

t tte n nde

ine e

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 8

it in

ein , t e e

t

gets

strengthened and the film turns white to transparent. We attribute this low angle peak to the meta-stable phase of monohydrated perovskite. Similar low angle diffraction peaks were also


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observed by Christians et al.25 and Zhao et al. referring to diffraction from meta-stable CH3NH3PbI3.H2O.33 This peak was not e i tent te

e ti e, new e corresponding

to

n e ti e nd

i e in t e di

monohydrate

ti n

t di

tte n t

(CH3NH3PbI3.H2O)

e ed

e and

nd dihydrate

((CH3NH3)4PbI6.2H2O) phase of perovskite respectively.24,26 The dihydrated phase became more intense around

,

n wit t e

te i ti e d i dide e

t

. Further

exposure to moisture may lead to evaporation of MAI with water leading to lessening of dihydrate content and leaving lead iodide behind. With further aging, PbI2 e

t

becomes more pronounced while the dihydrate peak reduces in intensity. After 432 hours, 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 8 (A), a white to yellow tinge was observed in the film signifying the colour of lead iodide. de e

,t e e

ite

) should reduce with the ageing showing reduction in the amount of

perovskite but, at initial stages, the perovskite peak also increases confirming the enhancement in the crystallinity of the perovskite due to effect of moisture. Improved crystallinity can be justified by observing a decrease in the FWHM (Full Width at Half Maximum values) values, as plotted in Figure 5(d). On the other hand, the peak width of the hydrate t

remains almost the same despite an increased intensity.

Moving on to the XRD patterns of two step and acetate route films in Figure 5 (b) & (c), we see that no deterioration was detected in both the 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 case of the two step route film, the film significantly decomposes into PbI2. The signature (001) peak of the lead iodide hexagonal structure34 is present and persists on and after the 1st day. Eventually



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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 to 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). e

ized

di tin t de

35

d ti n

Absence of any diffraction peaks below or ar e

ni

w i

d e n’t in

de n t

e

nd d ted

perovskite. Visually investigating the films in Figure 8 (B), no whitish spots were identified in either of the cases unlike that in the lead chloride route film. This suggests that formation of a hydrated phase could be a possible reason for higher degradation rate in lead chloride precursor route film as compared to that of the other two films.


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Perovskite Hydrates

PbI2 a Lead chloride precursor route b Two step route



432 hours

 PbI2

 Perovskite

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&

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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 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.



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To examine any possibilities of hydrate formation in acetate and two step routes, these films were subjected to high humidity environment (RH 90-95%) and XRD patterns were collected (Figure 5 (e)). Surprisingly, two step route film alone indicates the hydrate formation concurrently with PbI2 residue. T e

et te

e

te didn’t e

e ien e

d te

formation as shown in the Figure S5. The comparatively larger grains of the acetate route film would have aided in preventing the degradation through hydrated phases (analysed using SEM images and discussed in the following section). Adding to this, we conducted another expe i ent t t

we e en e

idit di

nd identi ied t e

ti n e

t

n w t e en iti it ti n

e

e d ite

ide i d te

t e

; see Figure 5 (f). With these we can infer that, lead

chloride route film transforms to hydrated phase even at low humidity range indicating higher sensitivity towards water while, two step and acetate route films may form the hydrated phase only at very high humidity. Similar to the outcomes of absorption analysis, XRD data also suggests that the final decomposed product is same (lead iodide) in all the cases. Discussion From the above discussed characterisation results and device data analysis, we can say that there is clearly a difference in time taken and route adapted in the degradation of perovskite prepared from different routes. The root for this distinct degradation behaviour lies in the crystal structures of the perovskites, attained at the time of crystallisation. 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.


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It is well known that CH3NH3PbI3 crystallises into tetragonal phase at room temperature.36,37,38,39,40 Tetragonal to cubic phase transformation takes places at higher temperat e

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- favours the cubic phase of perovskite. Perovskite formation using PbCl2 precursor follows formation 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 hours in our case), retaining the cubic structure of the perovskite. 49 Presence of Clduring crystallisation phase reduced the cubic to tetragonal phase transition temperature and favours 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



(a)

(b)

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14.06

Two step precursor route Fit peak 1 Fit peak 2 Cumulative peak

Intensity (a.u.)

Intensity (a.u.)

13.91

Model Equation

Reduced Chi-S qr Adj. R-Square Peak1(DMT-I3 ( Initial)) Peak1(DMT-I3 ( Initial)) Peak1(DMT-I3 ( Initial)) Peak1(DMT-I3 ( Initial)) Peak1(DMT-I3 ( Initial)) Peak1(DMT-I3 ( Initial)) Peak1(DMT-I3 ( Initial)) Peak2(DMT-I3 ( Initial)) Peak2(DMT-I3 ( Initial)) Peak2(DMT-I3 ( Initial)) Peak2(DMT-I3 ( Initial)) Peak2(DMT-I3 ( Initial))

12

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14

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(c)

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2theta(degree) (d)

Lead chloride precursor route

(100)

Peak2(DMT-I3 ( Initial))

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2theta (degree)

Two step route Acetate precursor route

(210)

10

20

30

(224) (314)

(404)

40

50

(300)

(112) (211)

(220)

(004)

(100) (200)

(110)

(213) (114)

Intensity (a.u)

(002)

(110)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 34

40

50

60

10

20

2theta (degrees)

30

60

2theta (degree)

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 & acetate route.

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 6 (a-b). ti

e

t t in

w e e , in t e

e nte

e d

ide

t,

de

te n e

t

ne e e ti n i

e ed t

9 along with the dominant peak at

is noticed and can be resolved by the Gaussian peak fitting. These two close diffractions in the later case can be assigned to (002) and (110) reflection of tetragonal


Peak2(DMT-I3 ( Initial))

Page 17 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


symmetry as reported by Baikie et.al39 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 6 (cd). Reflections for lead chloride route film can be seen t

, 28.39 , 31.75 and 42.81

corresponding to the (100), (200), (210) and (300) lattice planes of the cubic phase.49 While e

two step and acetate route fil 28.17 ,

nd t

e

.91 , 14.06 , 19.98 , 24.34 ,

, 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

respectively designating the (002), (110), (112),

(211), (004), (220), (213), (114), (224), (314) and (404) plane orientations of tetragonal structure.30,49,50,51 The reflections at (211) and (213) in case of two step and acetate precursor route are reflection which 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 films has been calculated. Diffraction data for the lead chloride route film proves its cubic phase with lattice parameter a = 6.311 Å. And the reflections from the other two routes demonstrate the tetragonal symmetry with lattice constants a = b = 8.81 Å and c = 12.65 Å for two step route and a = b = 8.80 Å and c = 12.66 Å for acetate precursor route film. Previous reports have found that change in the structural symmetry of CH3NH3PbI3 material will lead to 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; 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 to 1.589 eV for two step route and 1.587 eV for acetate route films due to higher symmetry order in cubic phase.



Hence, all these facts described above evident that lead chloride precursor route film crystallise in cubic phase while two step and acetate precursor route film adapts tetragonal phase. Structures shown in the insets of Figure 6 represents that organic part of MAPbI3 (MA+ ions), highly prone to moisture attack, occupies the exposed (100) plane of cubic lead chloride route film. This results in faster interaction of water molecules with the film. As shown in Figure 7(A) the degradation process is initiated when the interacting H2O molecules make a hydrogen bond with the CH3NH3+ ion and incorporated in the perovskite structure to give the monohydrate phase (CH3NH3PbI3.H2O). The process leads to

(A)

H2 O

C

N

Pb

I

(B)

Figure 7. Illustration for initiation of degradation mechanism in case of (A) 100 exposed facet in lead chloride route film and (B) 110 exposed facet in acetate and two step route films deformations in the perovskite structure and causes the optical properties of the film to attenuate severely. With the aging time, monohydrated phase is transformed into dihydrated perovskite phase ((CH3NH3)4PbI6.2H2O)53 with an increased order of structure deterioration.


Page 18 of 34

Page 19 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Further moisture exposure leads to separation and subsequent evaporation of MAI with water leaving PbI2 as the final decomposed product (Figure 8(A)). Intermediate Stages (MA)4PbI6.2H2O

MAPbI3.H2O

(A)

-MAI - H2O +H2O PbI2

MAPbI3

24 Hrs

8 Hrs

6 days

Fresh

18 days

Aging at 70-80% RH

(B)

Two step route

C N

Fresh

24 Hrs

3 days

4 days

Pb I

Acetate route

as prepared

MAPbI3

O

Direct decomposition (no intermediate hydrated stage)

-MA -HI

Degraded

PbI2

Figure 8. Crystal structures of (A) MAPbI3, hydrated MAPbI3 and PbI2 along with pictures of lead chloride route aged film at different aging times (B) Crystal structure of MAPbI3 and PbI2 with picture of two step and acetate route films at different aging time In the two step and acetate precursor route films the dominant facet (110) is Pb-I exposed (as shown in the Figure7 (B)), 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. However, the Pb2+ also has a tendency to coordinate with H2O.54 When H2O attacks on the PbI64- cage, the Pb2+ coordinates with the H2O and forms the plumbate like species PbI42-



Page 20 of 34

(H2O)2, PbI3-(H2O)3, PbI2(H2O)4 and subsequently releases the I- ions.55,56 These free I- ions bind with the MA+ (via hydrogen bonding) and make a separate MAI unit. Now, the MAI which 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 PbI64octahedra, cause defects55 in the film which might be the reason for the blue shift in the emission spectra of these films during decomposition. Simulation work reported by Edoardo et al. also reported the fact that facets terminated with MAI are faster to degrade as compared to PbI2 exposed facets.54

a Lead halide precursor route

b Two step route

Contact angle - 0

Contact angle - 0

300 nm

100 nm

c Acetate precursor route Contact angle - 0

Contact angle - 42.9 100 nm

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

Figure 9 shows the morphology of the films prepared from different precursor routes. It is clear that the synthesis process have a major impact on the perovskite surface. The lead chloride route film shows quite large grains, this might be benefitted due to presence of Chloride related species in the film during crystallization.27 However, the chlorine species are


Page 21 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


not present in the final perovskite film. The EDS data shown in the Figure S4 also confirms t e

en e

t e

e

et te

e

te i

t

d e n’t

d

e

e

in

as chloride route but the grains are rather coalesced and have smooth surface. Perovskite film prepared from the two step route possess very rough surface with irregular shaped grains. Due to uneven and small sized grains, the film is associated with quite more number of grain boundaries and a few pin holes. As per the previous reports, perovskite film with less number of grain boundaries manifests slower degradation as compared to film with more number of grains.27,57 But in our case, the films are fabricated using different precursor composition and have dissimilar crystallographic properties. Hence the theory based only on grain boundaries nd

e te t e

n’t

e

in t e de

d ti n

te di e en e e

e ien ed

t ee

films. Crystal orientation related aspects also play a major role in governing the degradation mechanism.58 Although the lead chloride route film have large grains compared to other two routes, its decomposition initiates much faster as shown in absorption and XRD analysis. The arrangement of (100) plane orientation with hygroscopic organic part on the surface have been a reason for this fast degradation. But in case of other two route films, having similar (110) lattice arrangements, surface morphology has a significant role in the initiation of perovskite degradation. Two step route film, due to more grain boundaries and irregular shaped grains, starts decomposing quite faster compared to acetate route as shows in the diffraction data. Intergranular spaces with rough surface and uneven grains as shown in the cartoon in Figure 10a, provides more scope for water molecule to attack and facilitates 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 two step route degradation starts a little faster than acetate route.



Page 22 of 34

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

42.9

0

Glass Substrate (a) Two step route

Glass Substrate (b)Acetate precursor route

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


Page 23 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Experimental Section Materials: Lead iodide (PbI2, Alfa acer, anhydrous, 99.95%), lead chloride (PbCl2, Alfa acer, anhydrous, 99%), lead acetate (Pb(Ac)2.3H2O, Alfa acer, 99%), 2-Propanol (alfa acer, anhydrous), Dimethylformamide (DMF, Alfa acer, anhydrous, 99.8%), Hydroiodic acid (57% in water, Merck). All the 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. 24ml of methylamine was dissolved in ethanol under continuous stirring and 10 ml of HI was added at the interval of 10 min per ml. White colour 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 recrystallisation. Recrystallised MAI powder was collected as white shiny crystals. Perovskite film fabrication: The perovskite (CH3NH3PbI3) films used for characterisations were casted on glass slides. Glass slides were cleaned with the laboline solution followed by rinsing with DI water, acetone and sonication in IPA (Isopropyl alcohal) 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 sec followed by annealing at

C for 2.5 hours. Single precursor acetate films were

prepared keeping same spin parameters followed by drying and annealing for 10 and 5 minutes respectively. Two step CH3NH3PbI3 films were prepared by fabricating PbI2 as first e nd te

step and then coating ti n w

e

ed in

nd

i t te de

n t

e

C. MAI in IPA (10mg/ml) solution then coated on the PbI2 i


iti n,

e d i dide

nd nne ed nd nne ed t

in t C


Page 24 of 34

for 40 min to form perovskite. All the perovskite films were prepared in the dry box under moisture level of 10-15%. Device fabrication: T

ted

Ω/ q, Si

di

w

e ned

ni ti n in

2% Hellmanex solution for 15 minutes, followed by rinsing wit dei ni ed w te , in

et ne, in in wit

T e

t te we e

et ne, in in in i eq ent

n

ed n t

nd t

in

ni ti n

in in wit i

te nd e ted t

n 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 room temperature before a mesoporous TiO2 layer was deposited by spin coating TiO2 paste (150 mg/ml 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, EM Index) layer, doped with bis(trifluoromethylsulfonyl)imide lithium salt and 4-tert-butylpyridine in 0.08 and 04 ratio, was spincoated on top of the perovskite film at 4000 rpm for 20 s. Finally, a 80 nm thick Au back contact was deposited by thermal evaporation. Characterization: di

t

ete wit

-

di

ti n di ti n

e

e ent we e e e t

n

te

ed n

i

de / in nd te

ti ize

.

UV-Visible absorption data for all the films were recorded using a Perkin – Elmer lambda 1050 spectrophotometer. Photoluminescence was carried out using Cary Eclipse G9800A fluorescence spectrophotometer. SEM images are taken using Zeiss EVO 18 at operating voltage of 20kV.


Page 25 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Conclusion The study reports the crystal orientation dependent degradation of MAPbI3 perovskite films. We observed that the films fabricated through various precursors crystallises in different orientations and has a distinct degradation mechanism. The crystal orientation which is rich in (100) and (200) planes undergoes formation of a hydrated phase, which leads to a much faster decay rate. While the films rich in (110) and (220) planes follows a slow decomposition process without adapting any intermediate hydrated phase at 70-80% RH level. We also demonstrate that there is a possibility of hydrate formation in the (110) dominating plane at higher humidity levels of around 90%. Our study not just states a clear answer to why there is a difference in the degradation mechanism, but also facilitates a possible way to fabricate a more humidity resistant MAPbI3 film via controlling its orientation in the (110) crystal plane. Supporting Information Information includes XRD data for crystallinity comparison, original J-V curves for fresh and aged films, Normalized absorption spectra with tangent for band gap calculations, EDS spectra for lead chloride precursor route film and XRD data for acetate precursor route film aged at very high humidity (90-95%). Conflicts of Interest There are no conflicts of interest to declare. Acknowledgment Authors would like to acknowledge Department of Science and Technology, India for providing funding under grant number RP03396G. We would also like to acknowledge Nanoscale Research Facility (NRF) and Central Research Facility (CRF) at Indian Institute of Technology, Delhi for characterisation measurements. We are very grateful to Prof. Josemon



Jacob and Prof. Neeraj Khare, IIT Delhi for allowing using the characterisation facility available in their labs. BR would like to acknowledge to Royal Society for funding through a Newton International Fellowship. References (1)

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Unravelling the Effect of Crystal Structure on Degradation of

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