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Super-Resolution Luminescence Micro-Spectroscopy Reveals Mechanism of Photo-Induced Degradation in CHNHPbI Perovskite Nano-Crystals 3

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Aboma Merdasa, Monojit Bag, Yuxi Tian, Elin Källman, Alexander Dobrovolsky, and Ivan G. Scheblykin J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b03512 • Publication Date (Web): 22 Apr 2016 Downloaded from http://pubs.acs.org on April 26, 2016

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The Journal of Physical Chemistry

Super-Resolution Luminescence Micro-Spectroscopy Reveals Mechanism of Photo-Induced Degradation in CH3NH3PbI3 Perovskite Nano-Crystals Aboma Merdasa1, Monojit Bag1,2, Yuxi Tian3, Elin Källman1, Alexander Dobrovolsky1, and Ivan G. Scheblykin1,*

1

Chemical Physics, Lund University, PO Box 124, 22100 Lund, Sweden

2

Department of Physics, Indian Institute of Technology Roorkee, Roorkee 247667, Uttarakhand,

India 3

School of Chemistry & Chemical Engineering, Nanjing University, 22 Hankou Rd, Nanjing

210023, China *Corresponding author telephone: +46462224848

Abstract Photo-induced degradation of individual methylammonium lead triiodide (MAPbI3) perovskite nano-crystals were studied using super-resolution luminescence micro-spectroscopy under intense light excitation. Photoluminescence (PL) intensity decrease and blue-shift of the PL spectrum up to 60 nm together with spatial shifts of the emission localization position up to a few 100 nm were visualized in real time. PL blinking was found to temporarily suspend the degradation process, indicating that the degradation needs a high concentration of mobile photogenerated charges to occur. We propose that the mechanistic process of degradation occurs as the 3-D MAPbI3 crystal structure smoothly collapses to the 2-D layered PbI2 structure. The degradation starts locally and then spreads over the whole crystal. The structural collapse is

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primarily due to migration of methylammonium ions (MA+) which distorts the lattice structure causing alterations to the Pb-I-Pb bond angle, and in turn changes the effective band gap.

Introduction Methylammonium lead halide perovskites (CH3NH3PbX3, or MAPbX3, X=I, Br, Cl) have gained much attention in the past years due to the material’s potentially high impact for photovoltaic and light emitting applications.1–8 Despite very intriguing results, there are some major hurdles that need to be overcome in order for MAPbX3 solar cells to become competitors to silicon based solar cells. One of the main challenges is the stability. The degradation of MAPbX3 thin films has been reported when exposed to moisture in air9 as well as light in the presence of oxygen.10 These have sparked a series of studies on how the degradation of these materials can be reduced in order to produce more stable solar cells.11–13 For MAPbI3 there is a growing consensus that the degradation occurs as MAPbI3 breaks down into PbI2,14 but there still lacks a comprehensive understanding of the fundamental process of the light-induced degradation. To date most studies on photo-induced degradation of MAPbI3 focus on the degradation of free-standing perovskite films or solar cell devices based on absorption spectroscopy and X-ray diffraction (XRD) analysis of samples exposed to light illumination in the range of 1-100 Sun (10-1-101 W/cm2 of excitation power density).11,12,15 Raman spectroscopy has recently revealed that photo-induced structural transformations occur in degrading MAPbI3 thin films.16 The increase in PbI2 content in the degraded perovskite sample can be characterized by spectroscopic measurements, however, these bulk measurements do not reveal the nano-scale mechanistic process involved in the photo-induced degradation. Due to inhomogeneity present in these nano-


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crystalline materials,17–21 the lack of spatial resolution in these spectroscopic methods does not allow monitoring the degradation process from the initial MAPbI3 to the final PbI2 with a nanoscale resolution.

The last decade has seen great advances in the development of optical microscopy methods where a spatial resolution beyond the diffraction limit has been achieved.22 These methods have primarily been applied in biology, however, many applications in chemistry have recently emerged in order to study structure and function of materials.23–25 Here, we use optical superresolution microscopy with the added feature of obtaining spectral characteristics of the locally emitted light. We call the method Super-resolution Luminescence Micro-Spectroscopy (SuperLuMS). It was demonstrated for the first time by some of us and used to characterize energy transfer in disordered 1-D J-aggregates.26 The main advantage of SuperLuMS is that not only the location of the emitting sites with high resolution is acquired, but also information on electronic properties of the material in these local regions can be obtained via PL spectroscopy. In this work, we used SuperLuMS to study the mechanistic process of degradation in individual MAPbI3 nano-crystals in real time. SuperLuMS allowed us to correlate different parameters such as PL spectral fingerprints, intensity and spatial localization of the emission under excitation power densities varied by several orders of magnitude. We were able to conclude that the main cause for photo-induced degradation is the migration of methylammonium ions in the presence of mobile photogenerated charges causing a smooth physical change in the lattice structure.

Methods



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We prepared MAPbI3 perovskite from lead iodide and methylammonium iodide (1:1 molar ratio) dissolved in γ-butyrolactone according to the reported procedure.4 A diluted solution was deposited on a glass substrate by spin-casting for 2 minutes at 1500 rpm followed by annealing at 80°C for 30 minutes. Due to the low concentration, scarcely distributed individual MAPbI3 nanoobjects were obtained on the glass surface. In the following text we refer to these nano-objects as “crystals”, although we don’t know if the objects are single crystals, polycrystals or crystal agglomerates of MAPbI3. The sample was measured using 514 nm excitation from an Ar-ion laser providing wide-field illumination through an oil immersion objective lens (Olympus UPlanFLN 60×, NA=1.25) attached to an inverted fluorescence microscope (Olympus IX-71). The luminescence was collected by the same objective lens and imaged onto an EMCCD camera (Princeton Instruments, ProEM). A diffraction grating (150 lines/mm) was placed in front of the CCD camera providing spectral information in the first order diffraction, in addition to the spatial distribution of the emission in the zero order diffraction (see SI Section I). Movies capturing the fluorescence fluctuations were recorded at 100 ms exposure time per frame. In each movie only up to 20 objects were measured due to their spatial scarcity. For each emitting object a 7×7 pixel region (corresponding to 1.4 ×1.4 µm at the sample plane) was cropped out around its emission pattern to secure that the entire emission profile could fit and also leaving room for potential shifts in the x-y plane. A 2-D Gaussian surface was fitted to the emission profile and the mean positions of the emission profile in each frame throughout the movie were extracted. Thus, the setup allowed us to simultaneously track the spatial, spectral and intensity features of the PL over the entire time of illumination (see SI Section I for further details).


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Results & Discussion The PL intensity of MAPbI3 crystals decreased at the time-scale of seconds in conjunction with a spectral blue-shift for all studied objects (up to 60 nm in some cases). Figure 1a shows the PL intensity transient of an object under 10 W/cm2 excitation power which is equivalent to 100 Sun (note that in single molecule spectroscopy such excitation loads are considered rather low). The PL intensity for this object remained stable for about 1 s, after which it started to steadily decrease over the time span of 5 s. Figure 1b shows the spectral evolution in the same time interval where it becomes evident that the spectrum blue-shifts in correlation with the PL intensity decline (see SI Section II for more examples as well as a movie, SI-M1, where the spectral shift can be seen in real-time for multiple objects). We found this behavior typical for MAPbI3 crystals in our experiment.

Figure 1. (a) Transient intensity profile of an object under 10 W/cm2 excitation. (b) The spectral evolution of the PL recorded simultaneously shows gradual blue shift as intensity decreases. (c) PL spectrum (normalized) recorded at different time intervals (marked as 1 – 6 ). Inset 1: PL peak position at different time frame (1 – 6). Inset 2: Full width at half maximum (FWHM) at different time frames (1 – 6).



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Observing Figure 1b one may assume that there is a systematic decrease of the red edge appearing as a blue-shift of the whole PL band. To show that the spectrum actually shifts, we plot five spectra taken in different time frames (white vertical lines marked 1-6 in Figure 1b), but mainly from the region where the intensity declines (Figure 1c). Each peak is fitted using a Gaussian function. A 60 nm shift of the peak position from 760 nm (red) to 700 nm (cyan) is seen while the FWHM increases from 330 cm-1 to 500 cm-1 during the PL spectral shift (see insets 1-2). Each spectrum is normalized to its Gaussian fit and it is clear that the last spectrum is ‘outside’ the first spectrum, which rules out bleaching of the red edge of the initial PL spectrum. A decrease of the red edge would suggest that the PL band is initially inhomogeneously broadened, which is probably not the case.27 We applied the full extent of the SuperLuMS method to another object and the results are shown in Figure 2. For this object the degradation occurs over a total time period of 25 s and the correlated PL intensity decline and spectral shift are shown in Figures 2a-b. Figure 2c demonstrates how the emission localization position shifts in space. Each solid circle represents the mean position of the emission profile in each acquisition frame with respect to the position in the first frame. This shift is also correlated to the peak wavelength of the PL spectrum in the same frame (x-axis) as well as the intensity (color). Figure 2d shows the correlation between the peak wavelength and intensity. To summarize the PL behavior of this particular object the rapid initial PL-drop occurs without much change in neither the spectrum nor mean position of emission in space. We note that the observed spatial shifts are not due to a lateral drift of the sample stage since many objects were simultaneously observed in one image and only for a few of those objects large lateral shifts as shown in Figure 2c were observed. Moreover, in those cases the shifts occurred in completely different directions. 6 ACS Paragon Plus Environment

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Spectral and spatial shifts occur mainly when intensity has dropped to roughly 30% of the initial intensity. We found such “non-linear” PL behavior of the PL intensity vs. is typical for perovskite crystal degradation under the given excitation conditions. These observations will be further clarified in relation to the proposed model (see below). We note that a spatial shift of the localization >100 nm is not seen in most objects, but when it appears, it is always accompanied with similar spectral and intensity dynamics as represented in Figure 2.

Figure 2. (a) PL Intensity trace for 25 seconds is recorded. (b) PL spectral evolution of the same crystal. Black dashed line is to guide to the eyes for the PL peak position (λP). (c) Localization of mean positions shifts with respect to initial emission vs. peak wavelength (λP). (d) Correlation between peak wavelength (λP) and PL intensity shows a general trend for most molecules under degradation. Color indicates the PL intensity.

PL blinking28,29 (the temporal “on” and “off” switching of luminescence) was observed for a few objects before their complete degradation. One example is shown in Figure 3. The object blinks “off” for over 50 seconds. Then PL briefly appears again twice for about 5 seconds each time (Figure 3a). The complete quenching of PL is due to a single photo-generated trap that is 7 ACS Paragon Plus Environment


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accessible by all photogenerated charges in the crystal when trapped charges recombine nonradiatively.29 Despite of PL blinking, the correlation between PL intensity and spectrum (see inset in Figure 3b) is qualitatively the same as for the previously discussed non-blinking object (Figure 2). We again verify that there is no bleaching of the red edge by measuring the FWHM of the initial and final spectra which increases from 330 cm-1 to 405 cm-1. It is interesting to see that despite the long period when the object possesses no PL (complete quenching), the spectrum before and the spectrum after the ’dark‘ period are the same (see Figure 3a). Note that the crystal was exposed to the light irradiation for the entire time, including the PL ‘dark’ period. This suggests that the degradation process is paused when the object blinks “off”, or, in other words, that PL must be present for the efficient degradation to occur. Presence of PL means presence of mobile chargers for at least a few nanoseconds after the excitation event. Thus, we conclude that mobile charges play a key role in the degradation process and that it is primarily photo-induced rather than being due to ambient conditions or heat, which is also supported by a recent work.15 We refer the reader to SI section II for more examples of PL blinking putting the degradation on hold.


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Figure 3. (a) PL intensity trace of a crystal shows degradation accompanied with blinking. The crystal then blinks on and off for 2 times (marked with red and blue solid dots) after being in a dark state for over 50 s. We also indicate with two black arrows that the spectra before and after the long ‘off’ period are the same. (b) Spectral evolution of the crystal before (green lines) and after blinking (blue and red lines). Inset: The correlation between peak wavelength and intensity despite the object being in “off” states for over 50 s.

There are a number of processes that can cause either spectral or spatial localization shifts of the PL. The former is a typical sign of chemical changes of the material while the latter has recently been shown to involve free charges being efficiently quenched by a single trap.29 However, we need to find an explanation that can accommodate both. Spectral changes in MAPbI3 have previously been reported and attributed to the quantum confinement effect,30 non-linear effects at high injection rates (Burstein-Moss shift),31 heat,15 or structural changes in the material itself.32 All these explanations provide some insights to the behavior of MAPbI3 perovskites under certain light excitation but do not establish a framework



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of how these spectral changes are related to the degradation of the material from the mechanistic point of view.

The observation of a gradual shift of the localization position in space tells us that the degradation starts from a particular part of the crystal and gradually extends to other parts on the time scale of seconds. The fact that we observe a shift of the emission in space also suggests that the physical part which is degraded stops emitting and the detected PL comes from the remaining intact part of the crystal. The reduced emission may be due to either a decrease in quantum yield (PL quenching) or a decrease in excitation light absorption as will be discussed later. Unless the degradation process always initiates at the edges of the crystals, we should in some cases observe a spatial shift of the emission and in some not. If the process were to occur in the whole crystal simultaneously, a localization shift would never be observed. Since we do not always observe a spatial shift of the PL localization position, we conclude that the degradation may initiate at any position within a crystal.

It is known that the spectrum of quantum dots, and in particular nano-crystals, can be tuned across the entire visible region (410-700 nm) through compositional modulations and the quantum size-effect, which has also been demonstrated in perovskite quantum dots30 as well as nano-platelets.33 However, the quantum confinement effect can only induce substantial changes in the emission wavelength for particles smaller than 15 nm.30 We observed spatial localization shifts well over 100 nm together with the spectral shift, which means the initial size of the perovskite crystal responsible for PL must be even larger than 100 nm. Therefore, the quantum


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confinement effect cannot have a significant contribution to the observed blue-shift of the spectra to the extent we observe. Since we observe that the degradation is excitation power dependent, we should quantify the number of photogenerated charges in a crystal at a given time. Because we use a CW laser we need to assume a lifetime of the free charges. An MAPbI3 lifetime34 of 100 ns gives an average concentration of about 1017 cm-3 excitations at an excitation power density of 10 W/cm2 (SI Section III). Although this number is quite high, it is still below the threshold for Auger recombination and band-filling (Burstein-Moss shift).27,31,35 Even if Auger recombination does occur, it would mainly contribute to heating up the crystal36 and not cause blue-shifted emission. Also, the Burstein-Moss shift in MAPbI3 occurs at the ns time-scale31 and can therefore not be the reason for our observed spectral shift occurring over seconds. When the excitation was switched off for a few minutes and then turned on again the emission peak did not return to its original spectral position but remained unchanged. This means that light induces permanent changes in the material. Semi-permanent light-curing of PL quenching traps has recently been reported in relation to PL enhancement.17,18 In order to be able to use the similar photo-curing hypothesis, but for shallow emissive traps, to explain the blue shift, we would need to assume that the initial emission around 760 nm is trap-assisted while the actual band edge has a higher energy. This assumption is not supported by the current literature. Thus, it is the band-gap which must increase during the permanent degradation of the semiconductor. Although intense light irradiation cannot directly cause a permanent spectral-shift through non-linear effects, we propose that free charges still may facilitate a change in the material that indirectly causes a blue-shifted emission. It has been demonstrated that local electronic properties vary spatially within individual MAPbI3 crystals which affects carrier recombination



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dynamics.19,20 Since each excitation forms an electron-hole pair, the net charge in the whole crystal should be zero. However, through spatially varying concentration of defects (vacancies and interstitials), local field gradients arise due to charge build-up at the edges or grain boundaries throughout the nano-object. It is feasible that electric field is larger at a higher photocarrier rate, which is when the degradation is observed. In the presence of such gradients, ions that are bound in the lattice structure with an activation energy barrier ranging from 0.1 eV to 0.6 eV tend to move in a specific direction11,37,38 (generally in the direction of the field). Ion migration in perovskite (including I-, MA+ and Pb2+) has been widely reported where MA+ is attributed as the main migrating species.39 Migrating MA+ ions inherently create some form of instability in the lattice structure as vacancies (iodine vacancies, VI, and methylammonium vacancies, VMA) are created and/or distributed inhomogeneously.17,18 Such an instability would lead to a distortion/decrease of the Pb-I-Pb bond angle, which has recently been demonstrated to cause an increased band-gap.37 It is also known that methylammonium ion migration over hundreds of nm can be on the time scale of seconds, which fits our observations.11,37,40 Both increasing of the bandgap and formation non-radiative recombination channels due to structural defects lead to the suppression of PL at our excitation conditions. MAPbI3 degrades under prolonged exposure to light in the presence of oxygen and also when exposed to heat.11,15 and there are two major reported pathways MAPbI3 can degrade to PbI2: (i) ion migration under illumination11 and (ii) superoxide formation in presence of photo-generated free carriers.10 In both cases, MAPbI3 3-D structure is reduced to PbI2 2-D layered structure. The apical angle between the Pb-I-Pb bond in the tetragonal phase MAPbI3 is ~170.8° due to slightly smaller ionic radius of the MA+ cation (Goldschmidt tolerance factor ~0.8, which should be 1 for stable cubic perovskite structure)37,41 and the corresponding band gap is ~1.6 eV. When this


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angle is reduced to 120° in PbI2, the corresponding band gap is ~2.48 eV. If the pure phase MAPbI3 breaks down into the pure phase PbI2 without any intermediate state we should observe two non-overlapping peaks in the PL spectrum at around 770 nm and 500 nm respectively (see SI Section IV for PL spectra of PbI2 and MAPbI3). Since 514 nm light excites MAPbI3 only, such a scenario would yield a decreasing PL peak at 770 nm as the material degrades while the emission from the PbI2 structure is absent as it has a very weak absorption at the excitation laser wavelength. For these reasons we conclude that the spectrally shifting PL is from the MAPbI3 phase undergoing a gradual distortion of the Pb-I-Pb angle caused by an increasing number of defects (ionic vacancies) (Figure 4a).

Figure 4. (a) Schematic showing how structural distortions arise as MA+ ion migrate and create vacancies in the lattice structure. The ionic void causes the Pb-I-Pb angle to change, which in turn causes the bandgap to increase (indicated by a blue-shifted color). Finally, the structure collapses into PbI2 that no longer absorbs at the excitation wavelength (514 nm). (b) A cartoon of a nano-crystal under excitation (green arrows below structure) where a gradual structural collapse of MAPbI3 into PbI2 propagates from left to right throughout the crystal. The bandgap increases causing a blue shifted emission (colored arrows above structure) in the collapsing regions. The colored crosses represent the mean position of the emission and shifts from its original position (white cross) as the



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structural collapse propagates. Non-absorbing parts of the structure are indicated by black lines. We note that the horizontal lines in (b) schematically represent layers of the lattice structure and not energy levels.

Prior to excitation there may be a few ion vacancies present in the crystal which serve as viable candidates where the degradation may initiate. Where there is an increased concentration of vacancies, the lattice structure is more unstable and is more likely to collapse.37 As long as there is structural and charge equilibrium, there is no direct reason for the collapse to occur. But once the excitation induces local field gradients and ions start to migrate, the collapse is facilitated, bond angles decrease and band gap increases locally. As the unit cell finally collapses into a 2-D structure of PbI2, MA+ ions in the adjacent 3-D MAPbI3 structure are more easily disrupted which eventually collapses as well, perpetuating the structural break down. This effect explains the PL localization position shift observed in our experiments. As more unit cells collapse, an initially large and rigid lattice structure successively becomes unstable which allows for a collective bending of multiple Pb-I-Pb bond angles simultaneously due to the tension. This creates a spatial gradient of the bandgap energy with the part closest to the collapsed 2-D structure being higher. Electrons and holes migrate to their lowest possible energy in the conduction band and valence band respectively. Therefore PL occurs due to charge recombination at the smallest bandgap the charges can access, at the condition that their kvectors match. Thus, as the collapse and collective bending of bond angles initiates, absorption still occurs in large parts of the crystal where the structure hasn’t collapsed completely, but emission occurs in the parts of the crystal where the bandgap is the lowest. This explains why the intensity initially drops significantly without any spectral shift. At some point, even the lowest accessible energy state is affected by the collective bending and only then the emission starts to blue-shift. Since there may be local energy minima in space due to spatial inhomogeneity of the 14 ACS Paragon Plus Environment

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degradation process, we see a broadening of the emission peak as demonstrated in inset 2 of Figure 1c as well as Figure 3. Figure 4b gives a schematic of how the spatially progressing structural collapse induces both spectral and spatial shifts. Since we observe that the degradation is put on hold when the PL of the entire crystal is quenched, we conclude that free charges are important for the degradation to occur as they facilitate the migration of ions.11,37,42–44 When the charges are quickly quenched, a charge buildup may not occur and ions stop migrating which puts the degradation process on hold as charge equilibrium, and thereby structural equilibrium, is reached very fast. We want stress that it is not the heat which drives the degradation process. Indeed, most of the excitation light energy goes to heat even when the sample is not yet degraded because PL quantum yield is as low as ~ 5%. The ‘off’ blinking of the PL does not changes the heating conditions, but it stops the degradation according to our observations. The degradation creates more and more non-radiative channels which quickly remove free charges, however, long-lived free charges are important for driving of the degradation process.

According to our model, there should be a size dependence on how long it takes for a crystal to completely degrade from MAPbI3 into PbI2. One such example is given in Figure 5 where a single diffraction limited object exhibited two separately shifting PL spectra. This observation also suggests that some of these objects contain structurally separated nano-domains,17 which degrade independently. Initially, spectra from all nano-domains overlap as the band-gap energy is the same (Figure 5a, inset 1). After the first few seconds, one of the nano-domains degrades faster than the other domains and two peaks appear in the PL spectrum (Figure 5a, inset 2). Utilizing SuperLuMS to visualize the emission localization in space we observe that the initial



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emission is more localized in space (left side of dashed black line in Figure 5b) but after degradation initiates, shifts and becomes more scattered. The larger spread of localizations is partially due to the separation of domains and partially due to the decreased photon count (right side in Figure 5b) after degradation initiates (see SI Section II for more examples of the effect of nano-domains on degradation).

Figure 5. (a) Spectral evolution of an object where degradation of nano-domains occur at different rates. Black dashed lines are to guide to the eyes for the spectral peak position at different time intervals. Inset 1 and inset 2 are the PL spectrum at initial and after specified (white dashed line) time interval respectively. The scale indicates the PL intensity. (b) Localization of mean positions of the emission indicated by solid circles. Red circles indicate those localization acquired before the PL peak splitting. Blue circles are localizations from frames after the time interval indicated by white dashed line when the two peaks are separated.

Using SuperLuMS, we infer that the degradation occurs due to the bending of Pb-I-Pb bond caused indirectly by the absence of MA+. This is in contrast to a recent report15 where the photodegradation was attributed to the breaking of the I-Pb bond. This was yet again measured using XRD and steady-state absorption after the degradation had occurred, which therefore could not reveal the actual mechanism of whether the Pb-I-Pb bond angle bends or breaks. In a very recent study using cathodeluminescence spectroscopy45 an electron beam was used to introduce defects 16 ACS Paragon Plus Environment

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(removal of MA+ and I-) in MAPbI3 thin films where similar spectral shifts as presented here were observed in real time. The authors proposed that an intermediate phase of the material is created when methylammonium ions are removed that causes blue-shifted emission. More recent reports also point to the migration of methylammonium ions being the key cause of MAPbI3 degrading into PbI2.11,46–48 These observations fit with our conclusions that the absence of MA+ ion leads to a structural collapse of MAPbI3 which causes the degradation.

To support our model we performed an experiment on an MAPbI3 bulk sample where an RGB image (using a Nikon D5100 DSLR commercial camera) was taken from a fresh undegraded sample using 458 nm excitation to make sure that the PbI2 structure (if present) was also excited (Figure 6a). Despite the low sensitivity of the color camera in the NIR region of the perovskite emission in comparison with the green region where emission of PbI2 was expected, we still observed red emission only. The sample was then photo-degraded with 514 nm excitation and a movie showing the PL during degradation was recorded using the Nikon DSLR camera (see SI_degradation.mov). After degradation, another RGB image was captured using 458 nm excitation. The center of the illuminated spot where the laser intensity was the highest is observed as predominantly green (Figure 6b) indicating emission of PbI2, or structurally modified perovskite. We then measured SEM of the sample in the same region (Figure 6c) so that we can compare the morphology of the photodegrated and fresh regions of the sample.



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Figure 6. (a) PL image prior to the degradation of a MAPbI3 bulk sample excited at 458 nm captured by a commercial color camera where the pure MAPbI3 structures are seen as red. (b) Another image taken of the same region after the degradation where the green emission, corresponding to PbI2 emission is observed. (c) SEM image of the same sample region. (d) Composite image between SEM and PL RGB image after degradation where the insets show the different structural properties of the undegraded (d1) and degraded (d2) regions. (e) image of the sample region in transmitted light using an incandescent lamp where difference in light transmission between the degraded and undegraded regions is clearly seen.

Overlaying of the SEM and RGB PL images of the sample (Figure 6d) allows us to identify undegraded (red emission, region d1) and degraded (green emission, region d2) regions and compare their morphologies. One can observe a slight difference in the material structure where the degraded part possesses more cracks and structural irregularities. Although we cannot directly measure the different bond angles using SEM, we believe we are observing the effect of a failing structure upon degradation. Finally, we also took an optical microscopy picture of the sample in transmitted light (incandescent lamp, LP filter at 514 nm) where it becomes clear that the degraded region shows a reduced absorption compared to the undegraded region. This is 18 ACS Paragon Plus Environment

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exactly what one would expect when the bandgap of the material shifts towards higher energy upon degradation. So, it shows that our hypothesis of the degradation mechanism also holds for bulk samples, which is of interest for solar cell research. We note that in relation to ambient conditions at which all experiments discussed in this paper were carried out, we did not observe any clear spectral shift upon photodegradation of PL in nitrogen atmosphere. In nitrogen the PL intensity dropped without a spectral change which leads us to believe there is another mechanism at hand which does not produce PbI2, which it so readily does when the degradation occurs in air. More research is needed to understand the influence of the environment on the degradation process which goes beyond the scope of the current contribution.

Conclusions In summary, we have observed photo-induced degradation of MAPbI3 nano-crystals at excitation powers a few orders of magnitude larger than normal operational conditions occurring over a time span of seconds rather than days or weeks. We observed that the PL intensity gradually decreases accompanied by a spectral blue-shift and shift of the emission in space. The gradual spectral change points to the bending of the Pb-I-Pb bond angle due to increasing ion vacancies caused by the photo-induced ion migration until the crystal structure collapses into non-emissive PbI2 (for 514 nm excitation). This collapse may initiate in a specific location and propagates throughout the crystal which appears as a spatial shift of the emission localization when observed by super-resolution luminescence micro-spectroscopy. SuperLuMS is therefore especially well-suited to study real-time degradation process in MAPbI3 over a time-span of seconds since we are able to acquire nano-scale spectral and spatial information on PL.



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The fact that the degradation process is stopped when excitations are quickly funneled out of the system can be interesting if one would attempt to engineer device architecture to fabricate photo-stable solar cell devices using MAPbI3. Excitations should be separated and harvested before they interact with the materials or creating a charge build-up at the grain boundaries and interfaces that leads to ion migration. Reducing ion migration would potentially solve the problem of photo-induced degradation.

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Supporting Information This document includes a detailed description of the experimental setup, analysis procedure and calculations. Additional results can also be found in this document together with relevant spectral characteristics of the sample.

Acknowledgments This work was financially supported by Knut & Alice Wallenberg Foundation, the Swedish Research Council and The Crafoord Foundation.


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References (1)

Deschler, F.; Price, M.; Pathak, S.; Klintberg, L.; Jarausch, D. D.; Higler, R.; Huettner, S.; Leijtens, T.; Stranks, S. D.; Snaith, H. J.; et al. High Photoluminescence Efficiency and Optically-Pumped Lasing in Solution-Processed Mixed Halide Perovskite Semiconductors. J. Phys. Chem. Lett. 2014, 5, 1421–1426.

(2)

D’Innocenzo, V.; Srimath Kandada, A. R.; De Bastiani, M.; Gandini, M.; Petrozza, A. Tuning the Light Emission Properties by Band Gap Engineering in Hybrid Lead Halide Perovskite. J. Am. Chem. Soc. 2014, 136, 17730–17733.

(3)

Kao, T. S.; Chou, Y.-H.; Chou, C.-H.; Chen, F.-C.; Lu, T.-C. Lasing Behaviors upon Phase Transition in Solution-Processed Perovskite Thin Films. Appl. Phys. Lett. 2014, 105, 231108.

(4)

Kim, H.-S.; Lee, C.-R.; Im, J.-H.; Lee, K.-B.; Moehl, T.; Marchioro, A.; Moon, S.-J.; Humphry-Baker, R.; Yum, J.-H.; Moser, J. E.; et al. Lead Iodide Perovskite Sensitized All-Solid-State Submicron Thin Film Mesoscopic Solar Cell with Efficiency Exceeding 9%. Sci. Rep. 2012, 2, 591.

(5)

Trinh, M. T.; Jin, S.; Zhu, X. Lead Halide Perovskite Nanowire Lasers with Low Lasing Thresholds and High Quality Factors. Nat. Mater. 2015, 14, 636–643.

(6)

Ponseca, C. S.; Savenije, T. J.; Abdellah, M. A.; Zheng, K.; Yartsev, A. P.; Pascher, T.; Harlang, T.; Chabera, P.; Pullerits, T.; Stepanov, A.; et al. Organometal Halide Perovskite Solar Cell Materials Rationalized - Ultrafast Charge Generation, High and MicrosecondLong Balanced Mobilities and Slow Recombination. J. Am. Chem. Soc. 2014, 136, 5189– 5192.



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

(7)

Xing, G.; Mathews, N.; Sun, S.; Lim, S. S.; Lam, Y. M.; Grätzel, M.; Mhaisalkar, S.; Sum, T. C. Long-Range Balanced Electron-and Hole-Transport Lengths in OrganicInorganic CH3NH3PbI3. Science 2013, 342, 344–347.

(8)

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 1 Micrometer in an Organometal Trihalide Perovskite Absorber. Science 2013, 342, 341– 344.

(9)

Tong, C.; Geng, W.; Tang, Z.; Yam, C.; Fan, X.; Liu, J.; Lau, W.; Liu, L. Uncovering the Veil of the Degradation in Perovskite CH3NH3PbI3. J. Phys. Chem. C 2015, 6, 3289–3295.

(10)

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. Chemie Int. Ed. 2015, 54, 8208–8212.

(11)

Bag, M.; Renna, L. A.; Adhikari, R. Y.; Karak, S.; Liu, F.; Lahti, P. M.; Russell, T. P.; Tuominen, M. T.; Venkataraman, D. Kinetics of Ion Transport in Perovskite Active Layers and its Implications for Active Layer Stability. J. Am. Chem. Soc 2015, 137, 13130–13137.

(12)

O’Mahony, F. T. F.; Lee, Y. H.; Jellett, C.; Dmitrov, S.; Bryant, D. T. J.; Durrant, J. R.; O’Regan, B. C.; Graetzel, M.; Nazeeruddin, M. K.; Haque, S. A. Improved Environmental Stability of Organic Lead Trihalide Perovskite-Based Photoactive-Layers in the Presence of Mesoporous TiO2. J. Mater. Chem. A 2015, 3, 7219–7223.

(13)

Hoke, E. T.; Slotcavage, D. J.; Dohner, E. R.; Bowring, A. R.; Karunadasa, H. I.; McGehee, M. D. Reversible Photo-Induced Trap Formation in Mixed-Halide Hybrid 22 ACS Paragon Plus Environment

Page 22 of 34

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


Perovskites for Photovoltaics. Chem. Sci. 2015, 6, 613–617. (14)

Yang, J.; Siempelkamp, B. D.; Liu, D.; Kelly, T. L. Investigation of CH3NH3PbI3 Degradation Rates and Mechanisms in Controlled Humidity Environments Using in Situ Techniques. ACS Nano 2015, 2, 1955–1963.

(15)

Misra, R. K.; Aharon, S.; Li, B.; Mogilyansky, D.; Visoly-Fisher, I.; Etgar, L.; Katz, E. A. Temperature- and Component-Dependent Degradation of Perovskite Photovoltaic Material under Concentrated Sunlight. J. Phys. Chem. Lett. 2015, 6, 326–330.

(16)

Gottesman, R.; Gouda, L.; Kalanoor, B. S.; Haltzi, E.; Tirosh, S.; Rosh-Hodesh, E.; Tischler, Y.; Zaban, A.; Quarti, C.; Mosconi, E.; et al. Photoinduced Reversible Structural Transformations in Free-Standing CH3NH3PbI3 Perovskite Films. J. Phys. Chem. Lett. 2015, 6, 2332–2338.

(17)

Tian, Y.; Merdasa, A.; Unger, E.; Abdellah, M.; Zheng, K.; Pullerits, T.; Yartsev, A.; Sundström, V.; Scheblykin, I. G. Enhanced Organo-Metal Halide Perovskite Photoluminescence from Nanosized Defect-Free Crystallites and Emitting Sites. J. Phys. Chem. Lett. 2015, 6, 4171–4177.

(18)

Tian, Y.; Peter, M.; Unger, E.; Abdellah, M.; Zheng, K.; Pullerits, T.; Yartsev, A.; Sundström, V.; Scheblykin, I. G. Mechanistic Insights into Perovskite Photoluminescence Enhancement: Light Curing with Oxygen can Boost Yield Thousandfold. Phys. Chem. Chem. Phys. 2015, 17, 24978–24987.

(19)

DeQuilettes, D. W.; Vorpahl, S. M.; Stranks, S. D.; Nagaoka, H.; Eperon, G. E.; Ziffer, M. E.; Snaith, H. J.; Ginger, D. S. Impact of Microstructures on Local Carrier Lifetime in Perovskite Solar Cells. Science 2015, 348, 683–686. 23 ACS Paragon Plus Environment


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

(20)

Halder, A.; Chulliyul, R.; Subbiah, A. S.; Khan, T.; Chattoraj, S.; Chowdhury, A.; Sarkar, S. K. Pseudohalide (SCN-) - Doped MAPbI3 Perovskites: A Few Suprises. J. Phys. Chem. Lett. 2015, 6, 3483–3489.

(21)

Qiu, C.; Grey, J. K. Modulating Charge Recombination and Structural Dynamics in Isolated Organometal halide Perovskite Crystals by External Electric Fields. J. Phys. Chem. Lett. 2015, 6, 4560–4595.

(22)

Leung, B. O.; Chou, K. C. Review of Super-Resolution Fluorescence Microscopy for Biology. Appl. Spectrosc. 2011, 65, 967–980.

(23)

Kerssens, M. M.; Kubarev, A. V; Hendriks, F. C.; Dedecker, P.; Hofkens, J.; Roeffaers, M. B. J.; Weckhuysen, B. M. High-Resolution Single-Molecule Fluorescence Imaging of Zeolite Aggregates within Real-Life Fluid Catalytic Cracking Particles. Angew. Chem. Int. Ed. Engl. 2015, 54, 1836–1840.

(24)

Park, H.; Hoang, D. T.; Paeng, K.; Kaufman, L. J. Localizing Exciton Recombination Sites in Conformationally Distinct Single Conjugated Polymers by Super-Resolution Fluorescence Imaging. ACS Nano 2015, 9, 3151–3158.

(25)

Tachikawa, T.; Yonezawa, T.; Majima, T. Super-Resolution Mapping of Reactive Sites on Titania-Based Nanoparticles. Nano Lett. 2013, 7, 263–275.

(26)

Merdasa, A.; Jiménez, Á.; Camacho, R.; Meyer, M.; Wu, F.; Scheblykin, I. G. Single Lévy States - Disorder Induced Energy Funnels in Molecular Aggregates. Nano Lett. 2014, 14, 6774–6781.

(27)

Wehrenfennig, C.; Liu, M.; Snaith, H. J.; Johnston, M. B.; Herz, L. M. Homogeneous Emission Line Broadening in the Organo Lead Halide Perovskite CH3NH3PbI3-X ClX. J. 24 ACS Paragon Plus Environment

Page 24 of 34

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


Phys. Chem. Lett. 2014, 5, 1300–1306. (28)

Park, Y.-S.; Shaoujun, G.; Nikolay, M.; Klimov, V. I. Room Temperature Single-Photon Emission from Individual Perovskite Quantum Dots. ACS Nano 2015, 9, 10386–10393.

(29)

Tian, Y.; Merdasa, A.; Peter, M.; Abdellah, M.; Zheng, K.; Ponseca, C. S.; Pullerits, T.; Yartsev, A.; Sundström, V.; Scheblykin, I. G. Giant Photoluminescence Blinking of Perovskite Nanocrystals Reveals Single-Trap Control of Luminescence. Nano Lett. 2015, 15, 1603–1608.

(30)

Protesescu, L.; Yakunin, S.; Bodnarchuk, M. I.; Krieg, F.; Caputo, R.; Hendon, C. H.; Yang, R. X.; Walsh, A.; Kovalenko, M. V. Nanocrystals of Cesium Lead Halide Perovskites (CsPbX3 X=Cl, Br, and I): Novel Optoelectronic Materials Showing Bright Emission with Wide Color Gamut. Nano Lett. 2015, 15, 3692–3696.

(31)

Manser, J. S.; Kamat, P. V. Band Filling with Free Charge Carriers in Organometal Halide Perovskites. Nat. Photonics 2014, 8, 737–743.

(32)

Filip, M. R.; Eperon, G. E.; Snaith, H. J.; Giustino, F. Steric Engineering of Metal-Halide Perovskites with Tunable Optical Band Gaps. Nat. Commun. 2014, 6, 7497.

(33)

Sichert, J. A.; Tong, Y.; Mutz, N.; Vollmer, M.; Fischer, S.; Milowska, K. Z.; Garc, R.; Nickel, B.; Cardenas-daw, C.; Stolarczyk, J. K.; et al. Quantum Size Effect in Organometal Halide Perovskite Nanoplatelets. Nano Lett. 2015, 15, 6512–6527.

(34)

Li, Y.; Yan, W.; Li, Y.; Wang, S.; Wang, W.; Bian, Z.; Xiao, L. Direct Observation of Long Electron-Hole Diffusion Distance in CH3NH3PbI3 Perovskite Thin Film. Sci. Rep. 2015, 5, 14485.



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

(35)

Xing, G.; Mathews, N.; Lim, S. S.; Yantara, N.; Liu, X.; Sabba, D.; Grätzel, M.; Mhaisalkar, S.; Sum, T. C. Low-Temperature Solution-Processed Wavelength-Tunable Perovskites for Lasing. Nat. Mater. 2014, 13, 476–480.

(36)

Achermann, M.; Bartko, A. P.; Hollingsworth, J. A.; Klimov, V. I. The Effect of Auger Heating on Intraband Carrier Relaxation in Semiconductor Quantum Rods. Nat. Phys. 2006, 21, 557–561.

(37)

Azpiroz, J. M.; Mosconi, E.; Bisquert, J.; De Angelis, F. Defect Migration in Methylammonium Lead Iodide and its Role in Perovskite Solar Cell Operation. Energy Environ. Sci. 2015, 8, 2118–2127.

(38)

Haruyama, J.; Sodeyama, K.; Han, L.; Tateyama, Y. First-Principles Study of Ion Diffusion in Perovskite Solar Cell Sensitizers. J. Am. Chem. Soc 2015, 137, 10048–10051.

(39)

Yuan, Y.; Huang, J. Ion Migration in Organometal Trihalide Perovskite and its Impact to Photovoltaic Efficiency and Stability. Acc. Chem. Res. 2016, 49, 286–293.

(40)

Beilsten-Edmands J.; Eperon, G. E.; Johnson, R. D.; Snaith, H. J.; Radaelli, P. G. NonFerroelectric Nature of the Conductance Hysteresis in CH3NH3PbI3 Perovskite-Based Photovoltaic Devices. Appl. Phys. Lett. 2015, 106, 173502.

(41)

Sri, C. H. N. H.; Jacobsson, T. J.; Pazoki, M.; Hagfeldt, A.; Edvinsson, T. Goldschmidt's Rules and Strontium Replacement in Lead Halogen Perovskite Solar Cells: Theory and Preliminary Experiments on CH3NH3SrI3. J. Phys. Chem. C 2015, 119, 25673–25683.

(42)

Eames, C.; Frost, J. M.; Barnes, P. R. F.; O’Regan, B. C.; Walsh, A.; Islam, M. S. Ionic Transport in Hybrid Lead Iodide Perovskite Solar Cells. Nat. Commun. 2015, 6, 7497.


Page 26 of 34

Page 27 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


(43)

Xiao, Z.; Yuan, Y.; Shao, Y.; Wang, Q.; Dong, Q.; Bi, C.; Sharma, P.; Gruverman, A.; Huang, J. Giant Switchable Photovoltaic Effect in Organometal Trihalide Perovskite Devices. Nat. Mater. 2015, 14, 193–198.

(44)

Deng, Y.; Xiao, Z.; Huang, J. Light-Induced Self-Poling Effect on Organometal Trihalide Perovskite Solar Cells for Increased Device Efficiency and Stability. Adv. Energy Mater. 2015, 5, 1500721.

(45)

Xiao, C.; Li, Z.; Guthrey, H.; Moseley, J.; Yang, Y.; Wozny, S.; Moutinho, H.; To, B.; Berry, J. J.; Gorman, B.; et al. Mechanisms of Electron-Beam-Induced Damage in Perovskite Thin Films Revealed by Cathodoluminescence Spectroscopy. J. Phys. Chem. C 2015, 119, 26904–26911.

(46)

Yuan, Y.; Wang, Q.; Shao, Y.; Lu, H.; Li, T.; Gruverman, A. Electric-Field-Driven Reversible Conversion Between Methylammonium Lead Triiodide Perovskite and Lead Iodide at Elevated Temperatures. Adv. Energy Mater. 2015, 6, 1501803.

(47)

Leijtens, T.; Hoke, E. T.; Grancini, G.; Slotcavage, D. J.; Eperon, G. E.; Ball, J. M.; Bastiani, M. De; Bowring, A. R.; Martino, N.; Wojciechowski, K.; et al. Mapping Electric Field-Induced Switchable Poling and Structural Degradation in Hybrid Lead Halide Perovskite Thin Films. Adv. Energy Mater. 2015, 5, 1500962.

(48)

Yuan, Y.; Chae, J.; Shao, Y.; Wang, Q.; Xiao, Z. Photovoltaic Switching Mechanism in Lateral Structure Hybrid Perovskite Solar Cells. Adv. Energy Mater. 2015, 5, 1500615.



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Figure 1. (a) Transient intensity profile of an object under 10 W/cm2 excitation. (b) The spectral evolution of the PL recorded simultaneously shows gradual blue shift as intensity decreases. (c) PL spectrum (normalized) recorded at different time intervals (marked as 1 – 6 ). Inset 1: PL peak position at different time frame (1 – 6). Inset 2: Full width at half maximum (FWHM) at different time frames (1 – 6). 266x132mm (150 x 150 DPI)

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Figure 2. (a) PL Intensity trace for 25 seconds is recorded. (b) PL spectral evolution of the same crystal. Black dashed line is to guide to the eyes for the PL peak position (λP). (c) Localization of mean positions shifts with respect to initial emission vs. peak wavelength (λP). (d) Correlation between peak wavelength (λP) and PL intensity shows a general trend for most molecules under degradation. Color indicates the PL intensity. 159x132mm (150 x 150 DPI)



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Figure 3. (a) PL intensity trace of a crystal shows degradation accompanied with blinking. The crystal then blinks on and off for 2 times (marked with red and blue solid dots) after being in a dark state for over 50 s. We also indicate with two black arrows that the spectra before and after the long ‘off’ period are the same. (b) Spectral evolution of the crystal before (green lines) and after blinking (blue and red lines). Inset: The correlation between peak wavelength and intensity despite the object being in “off” states for over 50 s. 157x139mm (150 x 150 DPI)


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Figure 4. (a) Schematic showing how structural distortions arise as MA+ ion migrate and create vacancies in the lattice structure. The ionic void causes the Pb-I-Pb angle to change, which in turn causes the bandgap to increase (indicated by a blue-shifted color). Finally, the structure collapses into PbI2 that no longer absorbs at the excitation wavelength (514 nm). (b) A cartoon of a nano-crystal under excitation (green arrows below structure) where a gradual structural collapse of MAPbI3 into PbI2 propagates from left to right throughout the crystal. The bandgap increases causing a blue shifted emission (colored arrows above structure) in the collapsing regions. The colored crosses represent the mean position of the emission and shifts from its original position (white cross) as the structural collapse propagates. Non-absorbing parts of the structure are indicated by black lines. We note that the horizontal lines in (b) schematically represent layers of the lattice structure and not energy levels. 247x153mm (150 x 150 DPI)



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Figure 5. (a) Spectral evolution of an object where degradation of nano-domains occur at different rates. Black dashed lines are to guide to the eyes for the spectral peak position at different time intervals. Inset 1 and inset 2 are the PL spectrum at initial and after specified (white dashed line) time interval respectively. The scale indicates the PL intensity. (b) Localization of mean positions of the emission indicated by solid circles. Red circles indicate those localization acquired before the PL peak splitting. Blue circles are localizations from frames after the time interval indicated by white dashed line when the two peaks are separated. 211x104mm (150 x 150 DPI)


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Figure 6. (a) PL image prior to the degradation of a MAPbI3 bulk sample excited at 458 nm captured by a commercial color camera where the pure MAPbI3 structures are seen as red. (b) Another image taken of the same region after the degradation where the green emission, corresponding to PbI2 emission is observed. (c) SEM image of the same sample region. (d) Composite image between SEM and PL RGB image after degradation where the insets show the different structural properties of the undegraded (d1) and degraded (d2) regions. (e) image of the sample region in transmitted light using an incandescent lamp where difference in light transmission between the degraded and undegraded regions is clearly seen. 279x186mm (150 x 150 DPI)



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