The Effect of Crystal Grain Orientation on the Rate of Ionic Transport in

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The Effect of Crystal Grain Orientation on the Rate of Ionic Transport in Perovskite Polycrystalline Thin Films Paul Fassl, Simon Ternes, Vincent Lami, Yuriy Zakharko, Daniel Heimfarth, Paul Hopkinson, Fabian Paulus, Alexander Taylor, Jana Zaumseil, and Yana Vaynzof ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b16460 • Publication Date (Web): 05 Dec 2018 Downloaded from http://pubs.acs.org on December 6, 2018

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ACS Applied Materials & Interfaces

The Effect of Crystal Grain Orientation on the Rate of Ionic Transport in Perovskite Polycrystalline Thin Films

Paul Fassl#,1,2, Simon Ternes#,1,2,Vincent Lami1,2, Yuriy Zakharko3,†, Daniel Heimfarth1,2, Paul E. Hopkinson1,2, Fabian Paulus2,3, Alexander D. Taylor1,2, Jana Zaumseil2,3 and Yana Vaynzof1,2*

1

Kirchhoff Institute for Physics, Heidelberg University, 69120 Heidelberg, Germany

2

Centre for Advanced Materials, Heidelberg University, 69120 Heidelberg, Germany

3

Institute for Physical Chemistry, Heidelberg University, 69120 Heidelberg, Germany #

These authors contributed equally

Corresponding author: [email protected]

KEYWORDS MAPbI3 perovskite, microstructure, zone-casting, photoluminescence, ion migration

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ABSTRACT

In this work, we examine the effect of microstructure on ion migration induced photoluminescence (PL) quenching in methylammonium lead iodide perovskite films. Thin films were fabricated by two methods: spin-coating, which results in randomly oriented perovskite grains, and zone-casting, which results in aligned grains. As an external bias is applied to these films, migration of ions causes a quenching of the PL signal in the vicinity of the anode. The evolution of this PL-quenched zone is less uniform in the spin-coated devices than in the zone-cast ones, suggesting that the relative orientation of the crystal grains plays a significant role in the migration of ions within polycrystalline perovskite. We simulate this effect via a simple Ising model of ionic motion across grains in the perovskite thin film. The results of this simulation align closely with the observed experimental results, further solidifying the correlation between crystal grain orientation and the rate of ionic transport.


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INTRODUCTION Organic-inorganic hybrid perovskite materials have attracted considerable attention within the photovoltaic (PV) research community1, both a cause and effect of their unprecedented rise in power conversion efficiency (PCE). The most commonly studied material, methylammonium lead iodide (MAPbI3), has risen from a PCE of 3.8% in 20092 to surpassing 22% in 20173. Such impressive performance is due to their excellent photovoltaic characteristics, such as long exciton diffusion lengths and high absorption coefficients over the visible spectrum.4–7 These attributes, in combination with inexpensive material costs and simple fabrication methods8, make perovskite solar cells (PSCs) a promising new source of renewable energy. As their PCE has become competitive with other, commercially available, materials, a significant amount of the research community’s attention is now focused on the issue of stability.9–11 While many devices have shown promising lifetimes when stored “on the shelf”12 (no operation, and in the dark), devices operated under realistic operation conditions (1 sun, elevated temperatures) degrade on the order of days, making longevity a major barrier to commercialization.13,14 A specific problem related to degradation is the migration of ions.15,16 While the remarkable defect tolerance of perovskite materials (most defects only form shallow traps) enables highly efficient devices, the low formation energies of MA and I related defects (vacancies and interstitials)17–20 within the perovskite crystal lattice give rise to significant ionic movement, and in turn current-voltage (I-V) hysteresis and device decay.15,21–23 It has been proposed that ion migration can be suppressed by engineering the electric field distribution in the device by using undoped charge blocking layers.24 Experimentally, several methods have been utilized in order to minimize the effect of such ion migration, such as the improvement of perovskite crystallinity,25,26 the addition of co-doping cations,27,28 and the inclusion of ion blocking layers in the device 3 ACS Paragon Plus Environment


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stack.29–31 However, identifying the most effective solution to this problem requires the ability to precisely monitor ionic motion within the perovskite layer. Some techniques that have been used to accomplish this include x-ray photoemission spectroscopy (XPS),15,32 secondary ion mass spectroscopy (SIMS),33,34 conducting atomic force microscopy (c-AFM) and kelvin probe force microscopy (KPFM),35–38 and especially photoluminescence (PL) spectroscopy.34,39–43 Previous work has shown that the presence of dark areas in PL images is associated with the occurrence of ion migration and/or accumulation.34,40–44 PL microscopy is therefore particularly useful for characterizing this ionic movement, since it allows for the pseudo-direct visualization of ionic species in the perovskite film. For example, Qiu et al. observed PL blinking in particles of MAPbBr3 on 1-10 s timescales, which is consistent with the time scales of ion drift and thus is likely responsible for the PL behavior.40 Furthermore, Deng et al. showed a reversible decrease in PL intensity and lifetime in electrically biased MAPbI3 layers close to the anode, and attributed it to the accumulation of I- ions.41 Similarly, Li et al. employed wide-field PL imaging to spatially map the progression of PL intensity in electrically biased MAPbI3 films as a function of time.42 They also found that a quenched region forms starting at the positive electrode (anode), which travels towards the negative electrode (cathode) with increasing bias or time. The velocity of the front of this quenched region was shown to be linearly proportional to the strength of the electric field, which is consistent with a simple mobile ion model.45 In a more recent publication, the same group proposed a new model, only taking into account the motion of iodide vacancies (VI+),46 in which the formation of a reduced hole density results in increased non-radiative and reduced radiative recombination in the dark PL zone.47 Lastly, a very recent study by Birkhold et al. postulates that only the combination of charge injection and ion migration results in a quenching of PL under an applied electric field, which they attribute to the reduction of Pb2+ into Pb0 species


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which act as non-radiative recombination centers.43 Interestingly, in contrast to most other reports, they observe PL quenching starting at the cathode instead of the anode. These, to some extent contradictory, results and the various proposed models illustrate the complexity of field-induced PL quenching in MAPbI3 perovskite films. The reported values for the mobility of the fast moving ionic species in MAPbI3 at room temperature (which is typically attributed to iodide related defects)46 range, depending on the measurement method, from as high as 5.4 x 10-5 cm2V-1s-1,48 to as low as 1-2 x 10-9 cm2V-1s-1.37,38 The reported values for the mobility of MA related defects are several orders of magnitude smaller as compared to iodide related defects,48,49 in line with a higher activation energy for these defects.17,18 Further factors influencing such results include the fabrication method (resulting in different defect densities, microstructures and crystal orientations)20,50,51, measurement setup (e.g. intensity and duration of light illumination, which has a direct impact on the iodide ionic mobility)52 and surrounding atmosphere (N2, dry air, ambient) during the measurement of fieldinduced PL quenching. This points to the necessity of a critical comparison of results from different research groups and motivates more work on field induced PL quenching. One aspect of microstructure that has yet to be studied is the effect crystal grain boundaries have on ionic transport. Previous studies have examined the impact that crystal grain orientation can have on charge transport within perovskite films, but it’s effect on ion migration remains still unclear.53–55 To elucidate this relationship, we fabricate polycrystalline perovskite layers with either aligned or randomly oriented crystal grains. The aligned polycrystalline structures are obtained via zone-casting56,57 which we show to result in uniaxially aligned perovskite ribbons. These ribbons lie parallel to the casting direction, are approximately 1-10 μm in width, and are spatially separate from one another (Fig. S3). The randomly oriented samples are fabricated by



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spin-coating. By employing confocal PL microscopy, we compare the ionic motion under electrical bias between the two, and observe significantly more uniform ion transport in the aligned (ribbon) films when compared with the randomly oriented films. This shows that ionic transport is strongly dependent upon the inter grain orientation. Numerical simulations support our experimental findings, replicating the observed behavior of the two films perfectly. Previous work has conclusively shown a correlation between ion migration and nonradiative recombination pathways,41,42 and so these results help to strengthen our understanding of the dynamics of ionic transport within polycrystalline perovskite networks. This understanding is critical to our ability to engineer efficient, long lasting perovskite photovoltaic devices.

RESULTS AND DISCUSSION Characterization of zone-cast MAPbI3 films Different microstructures of MAPbI3 thin films are obtained by employing two solution deposition techniques: spin-coating and zone-casting (see Supplementary Fig. S1). As shown in Fig. 1c, zone-cast films display networks of aligned polycrystalline ribbons oriented parallel to the casting direction. The spin-coated films (Fig. 1a), based on a lead acetate recipe, are polycrystalline, compact and pinhole-free as shown in earlier publications.58–60 To probe the differences in orientation parallel to the substrate, the films were characterized using angledependent polarized optical microscopy. While no apparent correlation with polarization angle can be seen for the spin-coated films (Fig. 1b, i-iii), the zone-cast films extinguish uniformly at angles of 0° and 90° (Fig. 1d i and iii), while transmitting strongly at 45° (Fig. 1d ii), indicating a high degree of orientational uniformity parallel to the substrate and casting direction.8 Additionally, both types of films were characterized using 2D X-ray diffraction (XRD). To illustrate the 6 ACS Paragon Plus Environment

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differences between the orientation of both types of films, 2D XRD maps were collected from two directions: parallel and perpendicular to the ribbons, and similarly on the spin-coated sample. The patterns measured on the spin-coated samples confirm the presence of a highly crystalline perovskite layer with no preferential orientation between the grains, as measurements from both directions result in the same Debye ring patterns characteristic of a MAPbI3 perovskite (Fig. S2a,b). On the other hand, the zone-cast samples (Fig. S2c,d) show a preferential orientation as maps collected parallel and perpendicular to the perovskite ribbons show clear anisotropy. The intensity along the Debye rings is no longer homogenous but is rather localized, indicating a higher degree of order among the grains. This suggests that the likelihood of neighboring grains exhibiting similar orientation is significantly enhanced in the zone cast sample. Both types of films consist of multiple grains as can be seen in atomic force micrographs collected on the surface of the spincoated film and the surface of a ribbon (see Supplementary Fig. S3). We note that the average grain size is much smaller for the zone cast ribbons (200-300 nm) than for the spin-coated film (~1 m), however, as we will discuss in the following, this does not affect the results of this study. These results allowed us to construct an approximate model of each type of thin film. As illustrated in Fig. 1e, the spin-coated films are polycrystalline, consisting of grains oriented randomly with respect to each other. In other words, if one grain consists of a tetragonal perovskite unit cell oriented in a given direction, the adjacent grains have no preferred orientation; they may all consist of the same alignment, leading to a “domain” of highly aligned grains, or they may all be pointed differently, leading to a domain of complete misalignment. The zone-cast films, on the other hand, exhibit uniform alignment of the crystalline grains; each unit cell is oriented in approximately the same direction as all others within the ribbon (Fig. 1f).



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Unlike the spin-coating method employed in this study, zone-casting is not an established technique to form perovskite films from solution. We therefore employed X-ray photoemission spectroscopy (XPS) and imaging in order to verify the chemical composition of the aligned ribbons. Fig. 2 displays spatial XPS maps for four of the elements present within MAPbI3, each representing one of the primary components of MAPbI3 (Fig. 2a-d). By comparing an image with all four channels overlaid with an optical image of the same region (Fig. 2e-f), we can confirm that each component is present in the zone-cast ribbons. XPS spectra collected on the zone-casted films (Supplementary Fig. S4) reveal the following elemental composition: 55.09% I, 16.3% Pb, 15.31% N and 13.3% C, which is in agreement with the expected ratio of I:Pb:N:C of 3:1:1:1. Additionally, the films were characterized by photoluminescence spectroscopy, showing a characteristic emission peak at ~770 nm,61 further confirming that the film consists of MAPbI3 perovskite ribbons (Supplementary Fig. S5). Electrical-field induced spatial evolution of PL in spin-coated and zone-cast MAPbI3 To investigate how the film microstructure affects ion migration, PL maps were collected upon applying an external bias of increasing strength, parallel to the sample’s surface using Au electrodes with a channel width of ~100 µm. The time between the recording of a PL map (~2 min) and the next voltage step was kept as short as possible. As the charged mobile ions drift under the effect of the electric field, the PL signal is quenched in the vicinity of the positive electrode (anode), as displayed in Fig. 3. While the exact mechanism behind the observed quenching is under extensive debate,41,43,47 it can serve as indicator for the extent of the field-induced ionic movement within the film. We assume iodide interstitials and vacancies (Ii- and VI+) to be the dominant type of defects moving on the timescale of our experiment,17,20,37,47 however we cannot exclude a contribution of MA related defects.18,38


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Two observations are apparent by comparing the two types of sample’s PL maps. First, the overall bulk ionic motion is similar, but not identical. For each voltage value, the PL quenching edge, or the horizontal line between the quenched and emitting regions, is roughly at the same position for both samples. There are some differences in the overall extent of PL quenching between the individual ribbons, however due to the gradual nature of the quenching in the spincoated films a direct comparison in the overall quenching extent is impossible. For a more detailed explanation, see Supplementary Note 2 and Fig. S7. Second, and most important, the homogeneity of the ionic motion is very different between the two samples. In the zone-cast film, an extremely uniform PL signal is observed, with each ribbon’s PL edge smoothly moving vertically towards the negative electrode (cathode) in response to the increasing strength of the electric field. In contrast, the PL signal obtained from the spin-coated film is extremely inhomogeneous, displaying regions of both high and low PL intensity even without an applied field, which has been shown to be due to local variations in defect density at the surface and grain boundaries of the films.62,63 It should be noted that the somewhat reduced PL intensity in the vicinity of the electrodes even with no applied field can be explained by Fermi level equilibration with the Au electrode. As the bias is applied, the PL edge is undefined, and exhibits significant variation across the film. For example, at a medium electric field of 0.6 V/μm, parts of the quenched zone already reach the opposite electrode, while the average of the PL edge is only at two thirds of the channel’s length. As another example, while at a high electric field of 0.9 V/µm most of the film is quenched, there are still parts in the middle of the channel showing a PL signal. We note that these differences cannot arise from the previously mentioned variation in the average grain size of the zone cast and spin-coated perovskite samples. While the spin-coated films consist of ~1 m grains which are on average 3-



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5 times bigger than those in the zone cast samples, the inhomogeneity in the quenching front of the former is on the order of tens of ms, so it cannot be related to the grain size. Taken together, these observations provide insight into the energetics of ionic transport across a polycrystalline MAPbI3 film with varying defect density and crystal orientations. As the external field is applied, a force will be applied on the mobile ions, which will move until the field is compensated. To do so, they must travel through the crystal lattice and across grain boundaries. We assume that intra-grain travel is very similar in both types of samples, and thus cannot be responsible for the differences in ionic behavior observed. We believe the differences must arise from crossing the grain boundaries, which present themselves as an energetic barrier that the ions must overcome. We term this the ion grain boundary barrier (IGBB). Since the PL edge for both types of samples is located at approximately the same position for each bias value, the average height of this barrier must be similar. However, the extremely uniform PL quenching front of the zone-cast samples in contrast to the wide variation displayed by the spin-coated films suggests that the IGBB distribution is significantly broader for the latter. A quantitative illustration of this theory is displayed in Fig. 4. To illustrate the exact extent of the ion migration, each map - obtained from Fig. 3 - was analyzed via the following method in ImageJ. First, each pixel in the map was determined to be “on” or “off” by comparing it against the maximum intensity present. Anything below 1/e (~36%) of the maximum intensity value was considered off. The PL maps then were split into columns of a single pixel each, and the on/off ratio was quantified, representing the extent to which that column had been quenched. Finally, the distribution of these ratios was plotted (bin size of 2 percentage points) as a function of applied electric field.


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This analysis confirms the previously discussed observations: both films exhibit a linear relationship between field strength and quenching extent, but the zone-cast samples have a significantly sharper quenching front. We note that we observe slight variation between the averages of the quenching fronts for the spin-coated and zone-cast samples as well as between individual ribbons. These variations can be explained by a number of factors. For example, as some ribbons do not lie perfectly perpendicular to the channel, their average inter-grain orientation might be different, resulting in a different IGBB. It is also possible that the contact between the metal and the perovskite layer is slightly different, resulting in a small variation in the effective electric field in the channel between the two types of samples. Lastly, due to the gradient-like nature of the spin-coated films (contrasted with the step function-like nature of the ribbons), the choice of cutoff for the “on” vs. “off” pixels will affect the determination of the quenching front much more strongly in the spin-coated films than in the zone-cast ribbons. While these differences are noteworthy, we emphasize again that the ribbons exhibited a significantly more uniform quenching front when compared to the spin-coated films. Temporal evolution of PL quenching in zone-cast MAPbI3 ribbons The uniformity of the PL signal and evolution of the quenching front in the ribbon samples allows us to perform a time dependent single line scan measurement of the PL behavior of a single ribbon under bias, in order to obtain a value for the ionic mobility. As seen in Fig. 5, after applying the bias a quenching front forms at the bottom region, and travels non-linearly towards the top. After just under 800 seconds the signal is completely quenched. As the ions move throughout the film, they increasingly compensate for the external applied voltage. If we assume that this field screening decreases the effective electric field exponentially with respect to time, we can fit the observed behavior with the same functionality and extract an



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ionic mobility of 3.8 x 10-9 cm2V-1s-1, corresponding to a diffusion coefficient D of 9.8 x 10-11 cm2s-1 at 300 K. It is highly likely that the exact nature of determining the location of the quenching front contributes to the precision of this measurement. These estimates are in good agreement with reported mobility and diffusion coefficient values for iodide migration in MAPbI3 of 1-2 x 10-9 cm2V-1s-1 and 2.1 x 10-11 cm2s-1.35,37,41 However, we cannot exclude that the movement of MA+ defects contributes to the PL quenching in our experiments. While recent NMR experiments by Senocrate et al, have revealed that MA+ diffusion through bulk perovskite is very slow (D~10-17 cm2s-1), the authors found a second diffusing component with a lower limit of D~10-13 cm2s-1, which they attributed to MA+ migration via grain boundaries.49,64 In general, the widespread range of MA+ diffusion coefficients reported in literature (D=10-9 to 10-17 cm2s-1 obtained via different methods38,48,49) prevents us from definitively assigning the observed behavior to iodide migration, however the most comparable experiments to our own all conclude that iodide is most likely responsible. 42,46,47,49 It is sometimes argued that the movement of I- and VI defects is too fast to explain device hysteresis,18,48,65 which typically takes place on a timescale of seconds to minutes, and such studies propose that MA defect species are instead responsible. In contrast, other studies attribute hysteresis to the movement of iodide defects,66 or a combination of both species.67 In addition, device hysteresis strongly depends on the cell architecture, interfaces, and applied bias, which complicates a direct comparison of our mobility values with that derived from measurements of complete devices.65,67,68

Ising model of PL quenching in spin-coated and zone cast MAPbI3


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We hypothesize that it is the relative alignment of the crystal grains that determines the magnitude and the distribution of the IGBB in our films. That is, for highly aligned crystal grains (the relative angle between their crystallographic planes is close to zero) the ionic transport across the grain boundary is fastest. As the relative angle between the grain’s axes increases the IGBB increases in kind, and ionic transport is hindered. With this framework in mind, we can consider that ionic transport in the two types of films can be modeled by changing the width of the distribution of IGBB. The random nature of the spincoated film results in some paths being more closely aligned than others, causing easier ion migration in these regions and therefore quenched areas further into the channel. The macroscale effect of this would be a broad quenching front as the one we observe in the PL experiments. A narrow distribution of IGBB would result in a uniform progression of quenching within the film, such as the one of the zone-casted ribbons. To confirm this hypothesis, we simulated a similar situation using an Ising model. In our statistical model, isotropy in the directions perpendicular to the electric field is assumed. Hence, the situation can be represented by a two-dimensional grid of pixels (150×100), with an arbitrary first dimension of 150 µm and a second dimension representing the channel length of 100 µm. This grid was created to represent the perovskite film, with each pixel representing a crystal grain. Each pixel was assigned a certain transport time which corresponds to the value of the IGBB and is thus proportional to the inverse ionic movement resistance, resulting in a transport time matrix (TTM). The TTM values were randomly assigned to each pixel according to a Gaussian distribution. To represent the spin-coated film, the standard deviation of this distribution was chosen to be relatively large (mean value µ = 5 and standard deviation σ = 3), while the zone-cast sample was assigned a much smaller standard deviation (µ = 5, σ = 10-4). Every negative TTM



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value was set to zero to rule out negative transport times. As ions are introduced into the system, they are allowed to move based on a probability basis (see Supplementary Note 1 for detailed information), such that progression of time (or simulation steps) represents increasing electrical field across the layer. The results of these simulations are presented in Fig. 6, along with the PL maps from Fig. 3 to allow for direct comparison. The behavior between the simulated and experimental data shows an obvious similarity. The characteristic “speckled” pattern and rough PL edge is present only for the simulated spin-coated film, while the simulated zone-cast sample possesses the same smooth, slightly rounded PL edge. The striking similarity between simulated and observed PL maps further supports the idea that the energetic barrier for ionic transport between crystal grains is determined by their relative orientation, and thus plays an important role in the overall ionic behavior. As described above, ion migration is of great importance to the function and stability of a number of perovskite-based devices. Most commonly it is discussed in the context of perovskite photovoltaics, where ion diffusion has been linked to both device hysteresis and reduced device stability.16,24,28,69–71 The role of local inter-grain orientation in determining the rate of ion migration highlights the need to address not only the intra-grain properties, such as average grain size and crystallinity, but also the inter-grain order when discussing ion migration induced effects such as degradation. Furthermore, ion migration has been particularly detrimental to the development of perovskite-based field-effect transistors,72–74 and our results suggest that varying the inter-grain alignment will also impact the performance of these devices.

CONCLUSION


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In conclusion, we have demonstrated that for polycrystalline perovskite films, ionic transport through the film is affected by the relative orientation of the crystal grains. PL maps of zone-cast samples, which yield ribbon-like structures of aligned crystal grains, showed evidence of uniform ionic transport when an external bias was applied. Spin-coated films, in contrast, are made up of randomly oriented crystal grains and therefore had widely varying rates of ionic transport across the film. This conclusion is reinforced by Ising model simulations, which match the experimental findings. It is important to emphasize that zone-cast perovskite ribbons do not suppress or enhance the ion migration rate. Instead, our results further the overall understanding of the mechanics of ion drift and diffusion within polycrystalline perovskite films, which is necessary for the implementation of stable high-performance perovskite devices.

EXPERIMENTAL METHODS Materials. Methylammonium iodide was purchased from GreatCell Solar. Lead iodide (99.9%) and lead acetate (99.999%) were purchased from Sigma Aldrich. All other materials and solvents were purchased from Sigma Aldrich. Fabrication of perovskite ribbons via zone-casting. A home-built zone-casting set-up was used for fabrication of the perovskite ribbon films. The substrates were placed on a heated aluminum block and moved using stepper motors below a heated nozzle, which supplied the perovskite solution (Supplementary Fig. S1). Stoichiometric amounts of MAI and PbI2 (1:1) were dissolved in anhydrous N,N-dimethylformamide (DMF) with a concentration of 0.075 M. The solution and nozzle were held at 60° C, and the substrate temperature was 80° C during the deposition process. After formation of the meniscus, the stage was driven with a velocity of 0.05 mm/s to form the ribbon structure. Afterwards, the substrates were annealed at 80° C for another 10 min.



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Fabrication of compact perovskite films via spin-coating. The compact perovskite films were fabricated by the lead acetate trihydrate recipe: CH3NH3I and Pb(Ac)2·3(H2O) (3:1 molar ratio) were dissolved in anhydrous DMF with a concentration of 40 wt% with the addition of hypophosphorous acid solution (6 µL/1 mL DMF). The perovskite solution was spin-coated at 2000 rpm for 60 s in a drybox (RH < 0.5 %). After drying for 5 min, the samples were annealed at 100 °C for 5 min. Optical microscopy. Optical microscope images were recorded using an optical microscope in transmission mode equipped with crossed polarizers and a CCD camera. Confocal

photoluminescence

microscopy

measurements.

For

photoluminescence

measurements the spectrally separated output (λ = 550 nm) of a WhiteLase SC400 supercontinuum laser source (Fianium) operating in a pulsed (~10 ps, 20 MHz repetition rate) mode was used for excitation. Samples were mounted on a XYZ Nano-LP200 piezo-stage (Mad City Labs Inc.) and illuminated with a focused beam through a ×50, 0.65 N.A. objective (Olympus). Emitted photons were collected with the same objective and detected by a silicon single-photon avalanche diode (Micro Photon Devices) directly (for PL maps) or after wavelength separation with an Acton SpectraPro SP2358 monochromator (for PL spectra). Photon counts were recorded with a PicoHarp 300 time-correlated single photon counting system (PicoQuant). PL intensity maps were acquired for the entire detection range by a confocal raster-scanning with a piezo-stage at a step size of 0.5 µm and dwell time of 10 ms for the 2D maps (Fig. 3) and 1 µm and 10 ms for the singleribbon scan (Fig. 5). 2D X-ray diffraction Spectroscopy (2D XRD): Spin-coated and zone-cast MAPbI3 samples were measured using a Rigaku Smart Lab equipped with a 2D HyPix3000 detector (with a 0.2 mm collimator in a coupled theta-2theta scan, theta 0-450).


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X-ray photoemission spectroscopy (XPS). Perovskite ribbon samples were transferred into a UHV chamber (ThermoFisher ESCALAB 250Xi) for XPS measurements. The measurements were performed using a XR6 monochromatic Al ka source (hv = 1486.6 eV) and a pass energy of 20 eV. XPS images were collected for each element using a 2D imaging detector. Atomic force microscopy (AFM). AFM (Bruker MultiMode) was performed in peak force tapping mode in air with silicon tips to study the surface morphology of the perovskite films.



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Figures:

Figure 1: Optical microscopy characterization of perovskite films. (a) Transmission and (b) polarized microscopy images of spin-coated films, where i, ii, and iii are 00, 450, and 900 polarization angles with respect to the ribbon direction. (c) and (d)i-iii are the same, but for a zone-


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cast film. The zone-cast film transmits strongly only at a polarization angle of 45°, while the spincoated film shows no orientational uniformity. Schematic view of films deposited via e) spincoating, or f) zone-casting between Au electrodes. Note the orientation of the crystal grain’s axes with respect to its neighboring grains – uniform in the zone-cast film, while randomly oriented in the spin-coated film.



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Figure 2: XPS image maps for four of the elements (I = (a), Pb = (b), C = (c), N = (d), as well as (e) all channels mixed) that make up the perovskite MAPbI3 ribbons. (f) Optical image of the same region.


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Figure 3: PL maps of (a) spin-coated vs. (b) zone-cast MAPbI3 samples under increasing external bias, applied parallel to the sample. As negative ions (Ii-/VMA-) migrate towards the bottom of the channel and positive ions (VI+/MAi+) towards the top of the channel, the PL signal is quenched.



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Note the inhomogeneous pattern present in the spin-coated film, compared with the extremely uniform PL signal of the zone-cast ribbons.


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Figure 4: Distribution of quenching extent, as a function of applied voltage for spin-coated films vs. a single ribbon (middle in Fig. 3) from the zone-cast perovskite sample. Each PL map was split into “on” and “off” pixels according to PL intensity, and then spatially into columns. Thus, each curve corresponds to the distribution of on/off ratios amongst columns, or qualitatively the “sharpness” of the quenching front. Lines are a Gaussian fit to the data and serve as a guide for the eye.



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Fig 5: Single line time evolution of the quenching front from a single zone-cast ribbon biased at 0.9V/μm. Overlaid is the position of the quenched edge, with the line-of-best-fit to extract the ionic mobility μ. To fit the front’s position, the velocity of the ions was assumed to follow a diffusion model of v = μE, and the electric field E was assumed to decrease exponentially over time E = E o e-Rt, due to the screening effect of the accumulated ions.


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Figure 6: Simulated (right) vs. experimental (left) maps of ion diffusion for (a) spin-coated and (b) zone-cast perovskite samples.



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Associated Content: Details of numerical simulation (Ising model); Schematic image of the zone-casting setup; Atomic force microscopy of spin-coated and zone cast films; X-ray photoemission spectroscopy and PL spectrum of zone cast MAPbI3 ribbons Author information: Corresponding Author * Email: [email protected]. Present Address: †

Department of Physics, Technical University of Denmark, 2800 Kongens Lyngby, Denmark

Author Contributions: The manuscript was written through contributions from all authors. All authors have given approval to the final version of the manuscript. The authors declare no competing financial interest. Acknowledgements: We would like to kindly thank Prof. Uwe Bunz for providing access to the metal evaporation facilities and optical microscopy. We are also grateful to Prof. Dr. Annemarie Pucci for access to the atomic force microscope. P.F. thanks the HGSFP for scholarship. We thank Dr. Katelyn Goetz for help with sample preparation. Y.Z. and J.Z. acknowledge financial support by European


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Research Council under the European Union’s Seventh Framework Programme (FP/20072013)/ERC Grant Agreement No. 306298. Supporting Information: Detailed description of Ising Simulation, explanation of quenching front threshold selection, AFM micrographs of spin- and zone-casted films, 2D XRD maps of spin-coated and zone cast perovskite samples, XPS characterization of perovskite ribbons, sample PL spectrum of perovskite ribbons.



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Table of Contents figure:


The Effect of Crystal Grain Orientation on the Rate of Ionic Transport in

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