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Probing Perovskite Inhomogeneity Beyond the Surface: TOFSIMS Analysis of Halide Perovskite Photovoltaic Devices Steven Harvey, Zhen Li, Jeffrey A Christians, Kai Zhu, Joseph M. Luther, and Joseph J. Berry ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b07937 • Publication Date (Web): 19 Jul 2018 Downloaded from http://pubs.acs.org on July 19, 2018

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ACS Applied Materials & Interfaces

Probing Perovskite Inhomogeneity Beyond the Surface: TOF-SIMS Analysis of Halide Perovskite Photovoltaic Devices Steven P. Harvey*, Zhen Li, Jeffrey A. Christians, Kai Zhu, Joseph M. Luther, Joseph J. Berry National Renewable Energy Laboratory, Golden, CO, 80401, USA email: [email protected] Abstract — Understanding the origins and evolution of inhomogeneity in halide perovskite solar cells appears to be a key to advancing the technology. Time-of-flight secondary-ion mass spectrometry (TOF-SIMS) is one of the few techniques that can obtain chemical information from all components of halide organic-inorganic perovskite photovoltaics in one dimension (standard depth profiling), two dimensions (high-resolution 100-nm imaging), as well as three dimensions (tomography combining high-resolution imaging with depth profiling). TOF-SIMS has been used to analyze perovskite photovoltaics made by a variety of methods, and the breadth of insight that can be gained from the technique is illustrated here including: cation uniformity (depth and lateral), changes in chemistry upon alternate processing, changes in chemistry upon degradation (including at interfaces), and lateral distribution of passivating additives. Using TOF-SIMS on multiple perovskite compositions, we show information regarding halide perovskite formation as well as inhomogeneity critical to device performance can be extracted providing one of the best proxies for understanding compositional changes resulting from degradation. We also describe in detail the measurement artifacts and recommended best practices that enable unique insight regarding halide perovskite solar cell materials and devices. Keywords: TOF-SIMS, HPSC, cation migration, interface, interface chemistry, passivating additive, degradation, tomography.

I. INTRODUCTION By now, almost everyone in the photovoltaic (PV) community knows of the impressive growth in efficiency that halide perovskite solar cells (HPSCs) have experienced over the past few years.1-3 Perovskite semiconductors have a unique crystal structure in the ABX3 stoichiometry where A is a large +1 cation— either cesium or often a polyatomic organic molecule such as methylammonium+ or formamidinium+. The BX3- anion is typically PbI3 but could also include Sn2+ on the B site and Br- or Cl- on the X site. The

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materials function well in many device configurations with complex compositions on all three lattice sites. Furthermore, quantum dot solar cells or traditional planar films, both with specific grain-passivation chemistries, are being actively researched.4-5 Although HPSCs have made dramatic strides in power conversion efficiency, the long-term stability of the devices remains a key barrier to widespread commercialization of the technology. Recent developments have demonstrated minimal efficiency loss in an unencapsulated device after >1,000 hours of operation in ambient air with interface and device stack modification6; however, improvement in device and active layer stability are still paramount. Although complex alloys across the A, B, and X site have proven useful in improving HPSC active-layer stability, the mobility of ions7 and material heterogeneity8 make detailed materials characterization critical. To continue to improve performance and reliability, the community is now turning its focus to the interfaces in HPSC devices and examining in detail the critical role they play in performance.9-10 A variety of techniques have been employed to investigate interface properties, but time-of-flight secondary-ion mass spectrometry (TOF-SIMS) is one of the few techniques that can provide depth and lateral information about both the inorganic and organic components of the HPSC material as well as other device components (organic and inorganic) and at interfaces. Thus, it can provide deep insight into the development of more efficient and reliable halide perovskite devices. It also plays a role complementary to other more-widely used techniques in the field such as photoemission spectroscopy (PES). For example, PES can elucidate both chemistry and electronic structure at an interface, whereas TOF-SIMS has much greater sensitivity than photoemission and can more easily detect complex organic species. The utility of TOF-SIMS in elucidating the important role of interfaces was shown in our recent work demonstrating >1,000 hours of operation in ambient air with little degradation as a result of interface and device stack modification.6 TOF-SIMS was used to highlight the role that interfaces play in device performance by revealing that cation migration within the HPSC absorber layer was suppressed when the electron transport layer was changed from TiO2 to SnO2. Another area where TOF-SIMS can provide critical insight is in the role played by 1) variations in material processing (e.g., in formulation, annealing conditions) on establishing materials performance, and 2) changes in processing that result in changes in chemical gradients in the HPSC material. After first presenting a brief overview of the SIMS technique, we will highlight the breadth of unique information that can be obtained for HPSC films and devices with TOF-SIMS by showing some examples of 1-D profiling, 2-D imaging, and 3-D tomography. We will discuss the importance of understanding cation gradients in the material and how they are not captured by the common practice of simply using the bath composition as a proxy for film composition. We will cover beam-damage artifacts as they relate to the organic component of HPSC materials and methods to combat it. Finally, we will discuss known pitfalls and recommended best


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practices when collecting and analyzing the data, such as how to limit the effects of beam damage and other relevant information critical to informing HPSC researchers as these data become more prevalent. II. EXPERIMENTAL Secondary-ion mass spectrometry is a powerful analytical technique for determining elemental and isotopic distributions in solids.11-12 HPSCs and closely related halide perovskite colloidal quantum dot solar cells have been fabricated by a number of methods. Indeed, the processing methods for these materials and devices are changing rapidly, and thus, they continue to require detailed characterization to gain understanding.13 In the study presented here, we have chosen several methods including; a one-step method to form a triple cation HPSC utilizing DMF:DMSO 4:1 (v:v), a two-step process involving exposure of a PbI2 film to a solution of CH(NH2)2I, a solvent engineering method involving a lewis-base adduct of lead(II)iodide, and CsPbI3 colloidal quantum dots (QD) synthesized using slightly modified hot injection reactions from Kovalenko et al.14-15, where the QD films are spin-cast from octane with solution treatments to remove ligands and electronically couple the QDs. The details for all synthesis methods utilized are provided in Refs.4,

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TOF-SIMS profiling, imaging, and 3-D tomography were used to measure the

lateral and depth distribution of HPSC absorbers and completed devices. The samples were analyzed by TOF-SIMS using a hybrid ION-TOF TOF SIMS V instrument. The hybrid system is a TOF-SIMS V basic configuration instrument (including a 30-keV 3-lens BiMn primary ion gun), with a TOF-SIMS generation IV sputter gun and detector. Measurements were also conducted by ION-TOF and were completed on an ION-TOF TOF SIMS V instrument that also has several non-standard hardware upgrades, including an extended dynamic range detector and a gas cluster ion-source sputter gun. Secondary ions for analysis were created by a 3-lens 30-keV BiMn ion gun. Standard depth profiling was accomplished with a Bi3+ primary ion-beam cluster (21-ns pulse width, 0.7–0.8-pA pulsed beam current), and 3-D tomography was completed using a Bi3++ primary ion-beam cluster (100-ns pulse width, 0.1-pA pulsed beam current); this measurement mode is capable of lateral resolution of better than 100 nm. Measurements used a cesium- or oxygen-ion beam for sputtering, with energy varying from 600 eV to 3 keV (sputtering current 1–25 nA). Standard profiling was completed with a 50×50-µm primary-beam area (128:128 raster, one shot/pixel), and a 200×200-µm sputter-beam raster. Measurements were collected in non-interlaced mode to limit beam damage from the primary beam. The sputter time per cycle varied depending on the film thickness, but 5 or 10 seconds are typical sputter intervals chosen for profiling just a HPSC absorber. The sample was not rotated during profiling. At least two profiles per sample were



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collected to check for uniformity of the film. Further experimental details on the tomography measurements are given in the section discussing imaging and tomography. Measurements completed for us by ION-TOF USA Inc. also used a 3-lens 30-keV BiMn primary ion gun and a Bi3+ primary ion-beam cluster; however, sputtering was accomplished with an argon gas cluster ion source (1,500+-atom argon gas cluster, 20 keV, 1-nA current), as well as oxygen (1–3 keV, 100–700 nA) sputter sources. Other relevant details of the measurement conditions will be covered when discussing the results. III. RESULTS Secondary-ion mass spectrometry is based on a sputtering process due to an incident high-energy ion beam causing a collision cascade when it hits the sample, as illustrated in Fig. 1. The collision cascade is the result of an elastic transfer of energy from the primary ion to atoms in the sample through a series of elastic collisions. This causes some damage to the sample surface within the collision cascade region, with the volume of this region changing depending on the energy of the incident ion. The collision cascade can cause the ejection of sample material from the first few monolayers of the sample surface to vacuum. Charged secondary ions are a small fraction of the ejected material, typically less than 1%, and they can be collected for SIMS analysis. When depth profiling, the same principles related to the collision cascade apply for the sputter beam, except the incident ion flux onto the sample surface is much greater. This would be similar to increasing the amount of ejected material shown in Fig. 1 by two or three orders of magnitude. This vastly increased primary-ion flux causes bulk sputtering to occur because such a large amount of material is ejected from the sample.


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Figure 1: The SIMS primary-ion collision cascade results in sample material being ejected from the first few monolayers of the sample surface. A small amount of the ejected material is charged, and we can then analyze the mass of these charged secondary ions. SIMS is a versatile and powerful technique, but it is not without its limitations. One significant limitation is the complex relationship between intensity and concentration, which makes quantification difficult. The SIMS intensity equation is: ,

(1)

where the measured SIMS intensity depends on the primary-ion intensity ( ), sputter yield (Y) of the ion, ionization probability ( ) of that ion, transmission efficiency ( ) of the detection system, the species isotopic abundance ( ), as well as its fractional concentration in the material (XA). Both the sputter yield and ionization probability are affected by the matrix, which means that the intensity of any secondary ion measured could be different when present in the same concentration in two different matrixes. The complicated relationship between intensity and concentration manifests itself in HPSC data in many ways. One common artifact, often referred to as the “matrix effect,” is that the ionization probability for a species depends strongly on the elements or molecules that surround that species in the solid material being probed.



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This matrix effect could result in a Pb intensity that is quite different for Pb in a CsPbI3 film compared to Pb in a CH3NH3PbI3 (MAPbI3) or CH(NH2)2PbI3 (FAPbI3) film, even though Pb is present at the same concentration in the films. The same matrix effect could mean that if both films contained the same amount of bromine (e.g., 30% Br in CsPbI2.7Br0.3 and CH(NH2)2PbI2.7Br0.3), the bromine signal measured by TOFSIMS for each sample could differ by several orders of magnitude. Another example of the complex relationship between intensity and concentration is that the profile of the FA signal in a CH(NH2)2PbI3 film prepared via a standard 2-step deposition on a flat ITO/glass substrate would differ from the profile of an FA signal for a CH(NH2)2PbI3 film deposited on a mesoporous layer of ZnO or TiO2, for example. Oxygen is very electronegative, so it increases the probability of positive secondary-ion formation and enhances positive secondary-ion signals. For the example given (FA film on mesoporous ZnO or TiO2), having an increasing amount of oxygen in the matrix as the profile goes into the mesoporous oxide would enhance all signals; furthermore, the FA signal would increase, even though the actual FA concentration is not increasing. SIMS excels at detecting small quantities of an element or species in a matrix of differing composition. But quantification of SIMS data at matrix-level compositions is difficult, and other standardless techniques such as X-ray photoelectron spectroscopy or Auger depth profiling may be more appropriate for matrix-level quantification of the non-organic components of a HPSC. However, with these other techniques, it would be difficult to distinguish the organic species MA and FA from other carbon signals, which is not an issue with TOF-SIMS.

Section: Depth profiling TOF-SIMS can provide information about the uniformity of both the inorganic and organic components of the HPSC made by a variety of methods through the depth of the film or device. This becomes quite important for understanding the energetics in perovskite solar cells—in particular, as the field trends toward greater complexity in the composition of HPSC films—and TOF-SIMS can step in to fill a critical information gap. Unfortunately, common practice in the HPSC field is to simply report the ratio of precursors used in the solution from which a film is deposited, often with the assumption that the deposited film has the same stoichiometry as the solution. This practice is insufficient, and—even if the inherent assumption that the resultant annealed film is composed of these components from the liquid phase in the same exact ratios were correct—it does not address the uniformity of the components through the thickness of the film; this issue may become even more important as more-complex HPSC compositions are investigated where heterogeneities are known to exist.8 The problem with this practice of reporting bath chemistries is illustrated in Fig. 2A, which shows the A-site organic cation CH(NH2)2 (FA) distribution for


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two films, Cs0.25FA0.75PbI3 and Cs0.5FA0.5PbI3, subjected to similar post-deposition processing. Although the bulk Cs/FA content in the film is slightly different based on the bath chemistry, the bigger difference between the two resulting films is the vastly different FA cation distribution through the thickness of the film. It is possible (or even likely) that the FA cation distribution through the film is more important to the performance than the small difference in film composition, and one could erroneously correlate this composition change to a performance metric that is actually a result of the different FA gradient. Such a gradient in the as deposited device could have significant implications to the device operation and evolution of the performance under power production.

TOF-SIMS depth profiling can provide the needed

information on the distribution of all the cations, including the A-site organic cations, through the film thickness, thus yielding great insight into the film chemistry and how it relates to performance. An important point when discussing SIMS data is that the signal is measured as a function of the mass-tocharge ratio (m/z); thus, to be observed at the correct mass, only singly charged secondary ions are followed. For example, for the main isotope of Pb, a Pb+ positive secondary-ion signal is seen at 208 on the m/z spectrum, but a doubly charged secondary-ion signal (Pb2+) would show up at 104 on the m/z spectrum. Thus, it is incorrect to label the secondary-ion species with the charge that they possess in the crystal structure (so, Pb2+ in this example). For this reason and to avoid confusion, no charge superscripts will be shown in the SIMS data; as is common for SIMS data, all secondary-ion species followed were singly charged.



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Figure 2. A) TOF-SIMS depth profile of formamidinium for films of slightly different composition, subjected to identical post-deposition processing. This illustrates how simply using the precursor bath chemistry as a proxy for film composition fails to capture actual film properties because they would relate to the cation distribution in the film. B) Depth profiles of FA (CH(NH2)2) and the carbon-13 FA isotope (13CH(NH2)2) normalized to total counts for both 1-step and 2-step prepared films. The FA profiles use the left-hand y-axis and the carbon-13 FA profiles use the right-hand y-axis. In addition to a single deposition step, there are advantages to separating the deposition of the perovskite into two steps. Early work by Mitzi et al. in the 1990s on halide metal halide perovskites (not necessarily for photovoltaic applications) patented a 2-step method to process perovskite and halide metal halide layered compounds where the metal halide is deposited first and essentially infused with AX species to convert the compound into the corner-sharing octahedral crystal structure. PbI2 can be deposited in vapor phase or sublimation in addition to solution methods and may have process advantages. TOF-SIMS depth profiling can probe the question of the A-site organic cation distribution through the film depth. A nonuniform A-site cation distribution could result in a gradient within the device and at lead iodide-rich substrate interfaces at the back of the device, which would in turn affect device stability. This question of uniform A-site organic cation distribution in 1-step vs 2-step fabricated films is wellsuited to be investigated by TOF-SIMS. One-step and 2-step films were made by methods described elsewhere; the films had similar but not identical composition (~FAPbI3), and they were not aged and were profiled shortly after fabrication.17, 20 Both films were investigated with a gas cluster ion source used for sputtering by ION-TOF USA to investigate this apparent difference between the two fabrication methods


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without beam damage affecting the profiles. These data, along with data collected on our system using the 1-keV oxygen ion sputter source, are presented in Fig. 2B. The depth-profile data in Fig. 2B show both the FA (CH(NH2)2) signal and the signal for FA with the carbon-13 isotope (13CH(NH2)2). TOF-SIMS is sensitive to stable isotopes, so this carbon-13 FA signal is often followed if the main FA signal is saturating the detector. There is a slight difference in both the FA (CH(NH2)2) and 13FA (13CH(NH2)2) intensities measured between the 1-step and 2-step films. But this is to be expected because the films were of slightly different composition, which likely affected the total ion yield through the SIMS matrix effect discussed earlier.12 Beyond these observed differences in total intensity, only subtle differences in the FA and 13FA profiles were noted when comparing the 1-step and 2step prepared films. There is clearly not drastically less FA present in the 2-step film at the back of the device when compared to the 1-step film. The HPSC layer is ~200 nm thick, so other than the expected drop in intensity past 200 nm as the profile continues into the back contact, both films show a similar slight gradient in the A-site organic cation through the film thickness. This illustrates the power of the TOF-SIMS data to directly reveal how changes in processing result in changes in the film chemistry and cation gradients (or lack thereof). The data in Fig. 2B show that the FA gradient through the film thickness for these two films does not differ significantly when they are made via a 1-step or 2-step deposition method. Of course, this does not mean that these two films would behave identically electrically when made into a device. The electrical properties and band alignment of the front interface are highly critical to device performance. So these data also exemplify the need for complementary characterization where a technique such as photoemission can step in and examine the difference in the films’ electrical properties at the front interface to provide a more complete picture of the comparison between these two films.13 The results shown in Figs. 2A and 2B illustrate radical differences in the resulting films due to small changes in composition under identical process conditions.

Questions regarding how modification of

process conditions coupled to changes in vertical, and lateral compositional homogeneity provide key critical insight into the physics of device operation.

The type of insight into the impact of process and

resulting chemistry of HPSC and film formation that can be gained from TOF-SIMS depth profiling when appropriate measurement conditions and data handling are applied can enable more rapid and rational development of these systems for PV. This chemical information can be collected with sub-nanometer depth resolution through the film thickness by careful tuning of the TOF-SIMS measurement parameters. These methods were recently used to show the change in uniformity of the cation distributions through a device and how it can then be related to changes in the device stability when comparing devices with little efficiency loss after >1,000 hours of operation in air and those with more rapid degradation.6



The care required to extract meaningful differences in the profiles by tuning the TOF-SIMS measurement parameters is shown in Fig. 3 for an early 1-D depth profile taken of a methylammonium+ (MA) lead-iodide perovskite on a mesoporous ZnO scaffold. A 600-eV oxygen-ion beam was used as the sputter source, which is the lowest-energy sputter conditions at which the system is capable of operating. In Fig. 3A, a large gradient in the MA profile is observed; in addition, one can see a smaller gradient of about one order of magnitude through the film thickness for Pb and I, as well as the total counts (which is a summation of the total measured intensity detected at each data point in the profile). The gradient in total counts implies there is some amount of beam damage affecting the total ion yield of the HPSC material during profiling. By plotting the data as point-to-point normalized to total counts, as shown in Fig. 3B, the intensity at each data point is then referenced to the total counts measured at that particular data point. Figure 3B shows that the slight change in ion yield of the matrix when profiling now appears to be mitigated, because the lead and iodine profiles in Fig. 3B now appear completely uniform through the film thickness.

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One of the greatest concerns when performing depth profiling of organic-containing materials is sputterinduced damage by both the primary ion beam and the sputter beam, which leads to decreased yield of the


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organic upon profiling.21 As an example, we found in our early TOF-SIMS work on MA or FA films that there was no difference in the MA or FA profiles when the sputter-beam energy was increased from 600 eV to 1 keV. Our standard profiling conditions now use a 1-keV oxygen sputter beam (5–7 nA sputter current) when only profiling through a HPSC absorber layer. For profiling through complete device stacks including gold contacts, a 3-keV sputter energy has been used in the past because the slow sputtering of the gold contacts makes using a 1-keV 5-nA beam too time-intensive to profile through a completed device. As shown in Fig. 3C, when the sputter-beam energy is increased to 3 keV, there is a drastic change in the shape of the FA profile compared to profiles taken at lower sputter energy, whereas there is no change in the inorganic species (only Pb shown in Fig. 3C). This change in the FA profile when using the higher-energy 3-keV sputter beam is a beam-damage artifact. If a true representation of the organic A-site cation is desired, then such high-energy sputter sources should be avoided or detailed efforts to understand and account for the impact of the beam damage should be undertaken. This also illustrates the need to consider these factors in HPSC depth profiles to ensure appropriate normalization of the data to mitigate this type of measurement artifact with our preferred method of performing a point-to-point normalized to total counts. Figure 3C showed beam damage manifested itself when using the 3-keV oxygen-ion beam for profiling as a large initial drop in FA signal, followed by an FA signal with little change through the rest of the film thickness. This is contrary to what would be expected from the sputter damage to the FA in the underlying layers leading to a continually decreasing ion yield (and thus signal) for FA as the subsurface FA is increasingly damaged through the profile. Low-energy (1-keV, 0.6-keV) oxygen-ion sputtering sources are typically used when profiling, and as Fig. 3C showed, these two profiles were identical and showed a typical FA gradient through the film thickness. One can also observe in Fig. 3C how the non-organic components (only Pb is shown) are unaffected by the change of sputter-gun energy. This illustrates the key issue of beam damage when profiling and how it is related to the MA/FA gradient using standard sputter hardware (as would be present on most SIMS and X-ray photoelectron spectroscopy or Auger instruments). Higher sputter energies may erroneously result in a profile that appears to show a much different FA distribution than is actually present. To address this issue, we worked in collaboration with ION-TOF USA Inc. to investigate the same films on our system and with state-of-the-art hardware at ION-TOF including a gas-cluster ion source, which is not available on our system. The gas-cluster sputter guns use a cluster of up to 7,500+ argon atoms at 2–20keV cluster energy; thus, the energy per incident argon ion can be lower than 1 eV. This allows for profiling of even very large molecular-weight organic species without fragmentation artifacts due to beam damage.2224



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The depth profiles for formamidinium collected by ION-TOF USA Inc. are shown in Fig. 4A. This instrument is also equipped with an extended dynamic-range detector so the FA profile itself is followed without detector saturation issues, which will be covered later. Similar to what was observed for the 3-keV sputter-gun energy for data taken on our system in Fig. 3C, ion-beam damage is noted for all but the gascluster ion-source profile; even with the 1-keV beam energy, the newer-generation sputter source is capable of much higher current densities and those employed here result in significant beam damage to the A-site organic cation of HPSC material. Similar to what was observed in Fig. 3C, as expected, the inorganic signals are unchanged by altering the sputter-gun energy (not shown). The depth profiles for the 13

CH(NH2)2 signal (carbon 13FA analog, which is followed to avoid detector-saturation issues with FA)

collected on the NREL system for the same material are shown in Fig. 4B, where the 13CH(NH2)2 (13FA) trace from the gas-cluster profile is also overlaid for reference. It appears that the low-current 0.6- and 1keV profiles track the

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currents, using the 3-keV oxygen beam results in beam-damage artifacts in the FA profile. The standard primary beam used when depth profiling is the 30-keV Bi3+ beam because the 30-keV Bi3+ beam leads to less beam damage compared to the 30-keV Bi+ beam. Since the beam is actually a cluster of three bismuth atoms at 30 keV, the ion energy for each incident bismuth primary ion in the Bi3+ beam is 10 keV, which is significantly less than the 30 keV when using only the 30-keV Bi+ beam. This reduced energy for each primary-ion impact decreases the depth at which the bismuth primary ion is implanted into the material, which of course causes the collision cascade and broken bonds generating the SIMS signal as shown in Fig. 1. This decrease depth of the primary beam collision cascade limits beam damage.


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Figure 4. A) FA profiles (CH(NH2)2) of 1-step HPSC film taken on ION-TOF system with different sputter sources. Beam damage is noted in all but the gas-cluster ion source profile. B) Carbon-13 FA (13CH(NH2)2) profiles of the same film taken on the NREL system. The ION-TOF gas-cluster 13FA data are also shown for direct comparison. A similar profile shape indicates that beam damage is not noted in the 0.6-keV and 1keV oxygen profiles. Beam damage in the data taken on the NREL system is noted when a 3-keV oxygen sputter beam is used.

Section: Imaging and tomography High-resolution imaging can be very useful in investigating any phase segregation or additive accumulation in perovskite films. The topic of grain-boundary (GB) and interface passivation is currently at the forefront of HPSC research because GBs have been shown to be a high concentration of trap states, and passivation of these states by a passivating additive can lead to significantly increased efficiency

25-26

.

Researchers have had success with a variety of passivating additives including potassium, phenethylammonium, Lewis acid or base functional groups, fullerenes, and F4TCNQ.27-31 A key question arises whether such passivating additives concentrate specifically at GBs or interfaces, or rather, are they more uniformly distributed through the HPSC material while providing a passivating benefit at functional interfaces? TOF-SIMS imaging (with a lateral resolution of 100 nm or better) can answer these questions for both organic or inorganic passivating species. Figure 5 shows two images revealing the segregation of an additive used to increase grain size in the HPSC material; in this case, the additive is lead thiocyanate (Pb(SCN)2).18 When imaging additives or impurities with a low signal, long integration times may be needed, which may push the primary-beam dosage beyond the static SIMS limit of 1×1012 ions/cm2. Figure 5A shows an image of the annealed sample after what would typically be considered a long (30-min)



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imaging integration time, at a primary-ion beam dosage of 1×1012 ions/cm2. Although it may appear as if SCN is rich at some GBs, it is not extremely clear. Another measurement was then performed at the same location with an integration time of 2 h (3×1014 ions/cm2 dose density), at which point Fig. 5B was obtained; the enrichment of SCN at the GBs due to the Pb(SCN)2 post-deposition treatment is now very clear. This long integration is, of course, not always possible; but sometimes it may be necessary depending on the counting statistics of the species of interest. Of course, when surpassing the static SIMS limit, a nonnegligible amount of material is removed by the primary beam, which may be critical in some cases. In this case, the sputtered depth from the long imaging with the primary beam was on the order of 10 nm. These same methods can be applied to imaging interfaces within the HPSC device; segregation of additives or impurities to any of the interfaces in a device could be imaged with TOF-SIMS.

A

B -3

·10

24

20

3

·10-3

24

20

1.6

16

16

1.2

2

12

12

8

8

0.8

1

0

0

µm

0.4

4

4

0

10

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µm 0

10

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CSN- normalized to total

CSN- normalized to total of a passivating additive (Pb(SCN) ) at the grain boundaries after a highFigure 5: TOF-SIMS 2-D image 2 temperature process step (SCN signal shown). The color scale for each image is normalized intensity per pixel (Ix/ITOT) A) Taken after 30 min of imaging (1×1012 1/cm2 dose density); although some enrichment at boundaries is visible, it is not strikingly clear. B) The same spot as in A, after 2 h of imaging (3×1014 1/cm2 dose density). An enrichment of sulfur and cyanide at the GBs due to the (Pb(SCN)2) post-deposition treatment is now very clear. Data was taken after publication of a study on this material by Ke et. al.18 and samples were provided by the authors of said study.

By combining high-resolution imaging with sputtering, 3-D tomography at 100-nm lateral resolution can be accomplished, which can yield useful insight into the local microstructure of the device or absorber. One could use TOF-SIMS tomography data to probe the cations of a mixed-cation perovskite through the depth of the film. This approach can show differences in cation distribution due to processing or how changes in


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these distributions affect device stability. Figure 6 shows the tomography results for a two-cation-containing HPSC absorber, where one cation is an organic species and the other is an inorganic species, subjected to a post-deposition anneal at different temperatures. The results clearly show a gradient in the inorganic A-site cation where it is rich at the back of the device. In addition, there are the small domains that are rich in the inorganic cation in the film processed at low temperature—an indication of phase segregation. These domains begin to coalesce at higher temperatures (middle image), and a uniform absorber is seen after the highest-temperature anneal (right image). Such tomography can be quite enlightening and used to reveal changes in the device chemistry that result in improved performance or stability.

Figure 6: TOF-SIMS 3-D tomography results for a two-cation perovskite, where one cation is an organic species and the other is inorganic, processed at different temperatures. Each 3-D reconstruction is 50×50×0.5 µm. The organic cation is red and inorganic cation is blue. A gradient of the inorganic cation from back to front is observed in all samples. The temperature increased from left to right, and the lateral segregation of the inorganic cation is shown to be mitigated with the higher processing temperatures. With a lateral resolution of 100 nm, such TOF-SIMS tomography data can be used to investigate phase segregation and cation distributions in detail through the depth of the film.



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Section: Known artifacts, instrumental limitations, and recommended best practices

Subsection: Issues during high-resolution imaging and tomography It is important to note that both the mass resolution and lateral resolution of TOF-SIMS data depend on the primary-ion beam conditions used. Although TOF-SIMS is known for high mass resolution, this only occurs when the beam is operated in bunched mode. This means that the primary-ion beam pulses, on the order of 100 ns in length, are compressed to about a picosecond in length, which allows for more accurately determining the flight time of the primary ions, which yields excellent mass resolution. This is illustrated in Fig. 7A for a silicon surface, where several other species close to a nominal mass of 28 can also be seen and differentiated from the

28

Si peak. Recently, we demonstrated the utility of this high mass resolution in

HPSCs where dimethyl ammonium, a solvent degradation product incorporated into the perovskite films, can be clearly differentiated from carbon 13-FA (13CH(NH2)2) despite their near identical m/z ratios (46.066 and 46.046, respectively).32 However, this bunching process spreads out the beam laterally, so the imaging resolution in this measurement mode is on the order of 3–5 microns, as seen in Fig. 7B. When performing high-resolution imaging, the beam is not bunched, which allows for excellent focusing of the beam and chemical imaging at ~100-nm lateral resolution, as shown in Fig. 7D. However, with the un-bunched beam, the pulse length of the primary ions is on the order of 100 ns; thus, the mass spectrum has only unit mass resolution, as show in Fig. 7C. All the additional peaks seen in Fig. 7A are lumped together at a nominal mass of 28. Thus, if chemical imaging or 3-D tomography is going to be performed, it is imperative that a profile or spectra be collected with high mass resolution before attempting imaging so that any potential mass interferences can be noted and mitigated. If mass interference is an issue, then following isotopes of different abundance of secondary-ion clusters can sometimes circumvent the issue. In some cases, highresolution imaging is simply not possible without mass interference issues.


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Figure 7. A) High mass-resolution spectra of a silicon surface taken with a bunched primary-ion beam, and B) a corresponding image of a copper grid in spectroscopy mode. C) High lateral-resolution spectra, where the mass resolution is poor, and D) a corresponding image of a copper grid in imaging mode, where lateral resolution is good. Due to the drastic change in mass resolution when performing imaging, it is important to first collect a spectrum with high mass resolution to note any mass interferences that may be problematic when imaging. Fortunately, many of the species of interest in HPSC materials do not have serious mass interference issues when imaging, although as new passivating additives are being employed, this may not always be the case. If 3-D tomography is to be performed, we believe it is useful to first collect a profile on the sample of interest in spectroscopy mode to: 1) determine the sputter time needed to profile through the HPSC film to accurately calculate the sputter time steps and yield the desired data density through the film thickness, and 2) have high mass-resolution spectral data to assess which peaks of interest will be suitable to use in imaging mode where mass interferences will be common. When setting up tomography measurements, the total measurement time will depend on the integration time per image and the number of data points through



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the film thickness. Our standard conditions are typically a 50×50-micron analysis area, 512×512-pixel raster, 1 shot/pixel, and 4 frames/cycle. This takes about 2 min per imaging cycle for tomography when tracking only major species in the HPSC material (the conditions will likely not yield enough intensity in the images to elucidate the lateral distribution of additives at very low concentration). For a 50×50-µm analysis area, a sputtered area of 200×200 µm is used. The sputter interval is typically chosen to yield 20– 30 images through the HPSC film for a film thickness of about 500 nm, resulting in typical measurement times of 1.5–2 h per tomography profile. One problem when working with samples with greater surface roughness (generally >50 nm rms roughness as a rule-of-thumb) is that the roughness itself can impart a change in the total ion yield—and thus, the measured intensity—when imaging. By performing a point-to-point normalization to total counts for images, the intensity at every pixel is divided by the total intensity measured at that specific pixel, which can mitigate some amount of surface roughness artifacts if the roughness is not too extreme. This is illustrated in Fig. 8, where 8A shows the intensity image for the FA signal, with some apparent lateral variation noted. Figure 8B shows the total counts measured for the measurement. Figure 8C then shows the intensity normalized to total counts, which is a ratio of IFA/Itotal for every pixel in the image. The lateral distribution of FA now appears much more uniform because the intensity variation was strongly influenced by the roughness of the sample surface. ION-TOF TOF SIMS V generation detectors (not present on the NREL system) can collect data in a “delayed extraction” measurement mode, which significantly lessens the effect of topology on ion yield, which could mitigate the need for this normalization. It is suggested that this delayed extraction measurement mode be used for rough samples, if available.


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Figure 8. A) TOF-SIMS 2-D intensity image of formamidinium. B) The total counts image for the same image, where slight fluctuations of intensity due to the samples topography are observed. C) The same image as in A, normalized at each pixel to the total intensity image shown in B. The formamidinium signal now appears more laterally uniform. All images are 50×50 microns, and the color scale represents intensity in A and B and normalized intensity in C. Subsection: Detector Saturation Detector saturation can be an issue when depth profiling perovskite materials because some of the species present (e.g., cesium, Pb, iodine) readily ionize and result in high intensities that can easily lead to detector saturation. Detector-saturation levels when referencing just a counts threshold depend on the measurement conditions used. Our standard conditions for depth profiling is a 50×50-micron analysis area, 128×128 primary ion beam pixel raster within this area, and one primary ion beam shot per pixel. This results in a matrix of 16,384 primary ion beam shots and their resulting mass spectra over the analysis area. Under these conditions for the ION-TOF TOF SIMS IV generation detector on our system, detector saturation is reached when the counts for a secondary ion is above 100,000 counts/s (equivalent to 7 counts/pixel or greater). This means that the profile for a saturated species would appear as a straight line at 1×105 counts. Above 10,000 counts (equivalent to 0.6 counts/pixel), the onset of detector saturation affects the peak shape and integrated peak area. So some level of Poisson correction is applied to try to maintain the correct peak shape without saturation issues affecting the peak area. This Poisson correction functions well up to values of 65,000 counts (equivalent to 4 counts/pixel). Between values of 65,000 and 1×105 counts (between 4 and 7 counts/pixel) counts, the Poisson correction is possible but with significant errors; this means that within this range, the absolute intensity values are likely unreliable, but one can still follow trends in intensity. Poisson correction is helpful for following signals of high intensity, but whenever possible, it is recommended to follow secondary-ion species below the 4 counts/pixel limit to avoid the need for Poisson correction and detector saturation. Of course, if quantification methods are being employed, it is best to



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target a count rate of 0.5 counts/shot or less for the reference signals. Newer instruments with extended dynamic-range detectors can extend the dynamic range by a factor 10 or 100 for selected species. Thus, under the same measurement conditions described above, detector saturation would not be reached until a level of 1×106 or 1×107 counts. When collecting TOF-SIMS data, one has the choice of polarity for which secondary ions will be collected. We have found positive-polarity secondary ions to be most useful, because almost all the species of interest in HPSC absorber layers and completed devices have good ion yields in this polarity (e.g., formamidinium, methylammonium, Pb, cesium, lithium, indium, tin, metal contacts, and more). The exceptions, of course, are those highly electronegative species that are present only at doping levels (e.g., chlorine, oxygen). Even iodine, which is very electronegative, is usually observed with acceptable statistics in positive polarity due to its high concentration in HPSCs. When profiling is completed in negative polarity, iodine often saturates the detector. Because iodine has no lower-abundance stable isotopes to track, the I2+ or I3+ secondary-ion cluster can be followed to avoid detector saturation. To track the I+ signal directly, one has three options: 1) lower the primary ion beam current so that all signal intensities are lowered, 2) purchase a new detector with extended dynamic-range capability, or 3) use burst-mode profiling.33 Even when employing burst-mode profiling, the primary ion beam current may need to be lowered significantly to reach the 0.5 counts/shot threshold for quantification. Unfortunately, this often makes the intensities of all other signals in the profile too low to be useful. Detector saturation is also an issue when profiling in positive polarity. In positive polarity, the main lead isotope (208Pb) is often above the intensity levels where Poisson correction is applied. Thus, instead of following 208Pb, we recommend following some of the lower natural abundance Pb isotopes such as 204Pb or 206

Pb. Similarly, formamidinium (CH(NH2)2) often saturates the detector, so the carbon-13 FA analog

(13CH(NH2)2) can be followed with decent statistics and typically no detector saturation issues. Cesium is very electropositive and thus shows up with high intensity, and detector saturation issues often occur if cesium is present at moderate concentrations in the HPSC. In this case, as proposed for iodine in negative polarity where only one stable isotope exists, we recommend avoiding detector saturation by following the signal for the Cs2+ or Cs3+ secondary-ion cluster. We do not recommend following a Cs+X secondary-ion signal (e.g., CsPb+ or CsI+) because for the CsPb+ example, this signal results from the ejection of a cesium and a Pb atom from the HPSC surface due to the primary ion beam impact and immediate recombination into a singly charged secondary-ion complex. In depth profiling, this secondary-ion complex could thus reflect either the cesium or Pb profile through the thickness of the film; due to this ambiguity, we do not recommend following the Cs+X secondary-ion signal.


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As seen in Eq. (1), the SIMS intensity is related to the primary ion beam current incident on the sample surface. Detector saturation is an issue on our system when doing standard depth profiling in spectroscopy mode under the measurement conditions listed previously (50-micron analysis area, 128×128 pixel raster, one shot/pixel), and the pulsed primary ion beam currents (Bi3+ beam) are on the order of 0.7–0.8 pA. When conducting high-resolution imaging or 3-D tomography, the pulsed primary ion beam currents are much lower—on the order of 0.05–0.1 pA. The significantly lower primary ion beam currents when imaging mean that the signals measured are much lower and detector saturation is rarely an issue. If detector saturation is an issue when imaging, then all the methods suggested to avoid saturation when doing standard depth profiling can also be applied when doing imaging or 3-D tomography.

Subsection: Recommended Best Practices The recommended instrumental conditions used for profiling are covered in other areas of the manuscript. Here, we will summarize some important points that we hope can serve as a guideline for a SIMS operator new to the HPSC field. In addition, we will highlight some of the experimental details that should be presented in publications, keeping in mind how the data have been collected and analyzed to minimize artifacts and improve data quality. Absence of some of this information in publications unfortunately leaves in question the quality of the data and the results related to the SIMS data. 1. When profiling, it is important to use the Bi3+ primary beam, because the 10-KeV/incident Bi primary ion limits the beam damage in the organic components of the HPSC material and device compared to the 30-KeV/incident ion of a Bi+ beam. If a publication only states that “data were collected on a TOFSIMS instrument from Manufacturer X using a 30-KeV bismuth beam for the analysis,” this says nothing about the primary beam conditions or the sputter-beam conditions and does not inform the reader if the data have the minimum amount of beam-damage artifacts; so, unfortunately, it leaves the quality of the data in question. 2. Reporting the primary-beam and sputter-beam energies, as well as the sputter currents is preferred. Data collected with a gas-cluster ion source for sputtering are preferred, but often this hardware is not available. If an oxygen or cesium sputter beam was used, an energy of 1 keV or less should be used; otherwise, significant beam damage to the organic component of the HPSC material results, as shown in Fig. 3C. Beam damage can also occur at these low sputter energies if the sputter current is too high (as shown in Fig. 4A). Although it is assumed that data are collected in a manner so as to remain under that static SIMS limit of 1×1012 ions/cm2, if this is not stated explicitly, one can calculate the primary ion dosage if the analysis area, beam current, and pixel raster are given.



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3. Although it is not always stated in experimental sections, standard profiling should be completed in non-interlaced mode, because this limits the total primary ion dose on the sample and the resultant beam damage when profiling. This measurement mode is inherent to 3-D tomography measurements due to the long imaging time per cycle. Our commonly used measurement conditions in tomography are covered in detail in the subsection, “Issues during high-resolution imaging and tomography.” 4. As outlined in the discussion around Fig. 7, mass resolution is very poor during imaging and 3-D tomography. The SIMS operator should collect spectra or an entire depth profiles in spectroscopy mode to ensure minimal mass interference with the peak of interest when imaging. 5. Detector saturation should be avoided. This saturation can usually be avoided by following secondaryion clusters (e.g., I2-) or lower-abundance isotopes (e.g., 204Pb+). IV. CONCLUSIONS TOF-SIMS remains one of the few techniques that can obtain chemical information from all components of halide perovskite photovoltaics, and it can do this in up to in three dimensions with 100-nm lateral resolution and sub-nm depth resolution. This allows for deep insight into cation distributions and how they relate to performance and stability, both within the absorber layer and at interfaces, which is key to advancing the technology. We illustrated how films with slightly different bath chemistries and similar processing can have vastly different FA profiles through the film thickness—a critical insight that is unique to TOF-SIMS depth profiling. We also showed that films made by 1-step and 2-step deposition methods did not have drastically different FA distributions, illustrating the need for further characterization with complementary techniques such as photoemission, which can probe the electrical and chemical properties of those films at the surface and at interfaces. We discussed measurement methods and recommended best practices to limit beam damage and other artifacts when profiling and imaging. TOF-SIMS 2-D imaging was shown to directly elucidate the distribution of additives that passivate grain boundaries within the material, and 3-D tomography was shown to provide insight into the cation gradients in the film in three dimensions, which helped to relate changes in film processing and degradation to changes in film chemistry.

IV. ACKNOWLEDGMENTS This work was authored by Alliance for Sustainable Energy, LLC, the manager and operator of the National Renewable Energy Laboratory for the U.S. Department of Energy (DOE) under Contract No. DE-AC36-


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08GO28308. Funding provided by U.S. Department of Energy Office of Energy Efficiency and Renewable Energy Solar Energy Technologies Office, Halide Perovskite Solar Cell Program. The views expressed in the article do not necessarily represent the views of the DOE or the U.S. Government. The U.S. Government retains and the publisher, by accepting the article for publication, acknowledges that the U.S. Government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this work, or allow others to do so, for U.S. Government purposes. J.A.C. was supported by the Department of Energy (DOE) Office of Energy Efficiency and Renewable Energy (EERE) Postdoctoral Research Award under the EERE Solar Energy Technologies Office administered by the Oak Ridge Institute for Science and Education (ORISE) for the DOE under DOE Contract Number DE-SC00014664. The authors are grateful to Nathan Havercroft of ION-TOF USA, and Paula Clark of Tascon USA for coordinating and performing the depth profile measurements on the state-of-the-art TOF SIMS hardware. The authors would also like to thank Birgit Hagenhoff from Tascon Gmbh for detailed discussions on detector saturation limits. The authors would also like to thank Yanfa Yan for providing samples with the lead-thiocyanate additive as detailed in the publication by Ke et al.18 V. REFERENCES

(1) Green, M. A.; Ho-Baillie, A. Perovskite solar cells: The Birth of a New Era in Photovoltaics. ACS Energy Letters 2017, 2 (4), 822-830, DOI: 10.1021/acsenergylett.7b00137. (2) Green, M. A.; Ho-Baillie, A.; Snaith, H. J. The Emergence of Perovskite Solar Cells. Nat Photon 2014, 8 (7), 506-514, DOI: 10.1038/nphoton.2014.134. (3) Zhang, W.; Eperon, G. E.; Snaith, H. J. Metal Halide Perovskites for Energy Applications. Nature Energy 2016, 1, 16048, DOI: 10.1038/nenergy.2016.48. (4) Sanehira, E. M.; Marshall, A. R.; Christians, J. A.; Harvey, S. P.; Ciesielski, P. N.; Wheeler, L. M.; Schulz, P.; Lin, L. Y.; Beard, M. C.; Luther, J. M. Enhanced mobility CsPbI3 Quantum Dot Arrays for Record-Efficiency, High-Voltage Photovoltaic Cells. Science Advances 2017, 3 (10), DOI: 10.1126/sciadv.aao4204. (5) Yang, W. S.; Park, B.-W.; Jung, E. H.; Jeon, N. J.; Kim, Y. C.; Lee, D. U.; Shin, S. S.; Seo, J.; Kim, E. K.; Noh, J. H.; Seok, S. I. Iodide Management in Formamidinium-Lead-Halide–Based Perovskite Layers for Efficient Solar Cells. Science 2017, 356 (6345), 1376-1379, DOI: 10.1126/science.aan2301. (6) Christians, J. A.; Schulz, P.; Tinkham, J. S.; Schloemer, T. H.; Harvey, S. P.; Tremolet de Villers, B. J.; Sellinger, A.; Berry, J. J.; Luther, J. M. Tailored Interfaces of Unencapsulated Perovskite Solar Cells for >1,000 Hour Operational Stability. Nature Energy 2018, 3 (1), 68-74, DOI: 10.1038/s41560-017-0067-y. (7) Yuan, Y.; Huang, J. Ion Migration in Organometal Trihalide Perovskite and Its Impact on Photovoltaic Efficiency and Stability. Accounts of Chemical Research 2016, 49 (2), 286-293, DOI: 10.1021/acs.accounts.5b00420. (8) Chatterjee, R.; Pavlovetc, I. M.; Aleshire, K.; Hartland, G. V.; Kuno, M. Subdiffraction Infrared Imaging of Mixed Cation Perovskites: Probing Local Cation Heterogeneities. ACS Energy Letters 2018, 3 (2), 469-475, DOI: 10.1021/acsenergylett.7b01306. (9) Azhar, F.; Lukas, S.-M.; Germà, G.-B.; Rajan, J.; Ivan, M.-S. Interfaces in Perovskite Solar Cells. Advanced Energy Materials 2017, 7 (22), 1700623, DOI: doi:10.1002/aenm.201700623. (10) Schulz, P. Interface Design for Metal Halide Perovskite Solar Cells. ACS Energy Letters 2018, 3 (6), 1287-1293, DOI: 10.1021/acsenergylett.8b00404. (11) De Souza, R. A.; Martin, M. Secondary Ion Mass Spectrometry (SIMS) – A Powerful Tool for Studying Mass Transport over Various Length Scales. physica status solidi (c) 2007, 4 (6), 1785-1801, DOI: 10.1002/pssc.200675227.



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(12) Stevie, F. Secondary Ion Mass Spectrometry: Applications for Depth Profiling and Surface Characterization. , Momentum Press: 2015. (13) Hoye, R. L. Z.; Schulz, P.; Schelhas, L. T.; Holder, A. M.; Stone, K. H.; Perkins, J. D.; Vigil-Fowler, D.; Siol, S.; Scanlon, D. O.; Zakutayev, A.; Walsh, A.; Smith, I. C.; Melot, B. C.; Kurchin, R. C.; Wang, Y.; Shi, J.; Marques, F. C.; Berry, J. J.; Tumas, W.; Lany, S.; Stevanović, V.; Toney, M. F.; Buonassisi, T. Perovskite-Inspired Photovoltaic Materials: Toward Best Practices in Materials Characterization and Calculations. Chemistry of Materials 2017, 29 (5), 1964-1988, DOI: 10.1021/acs.chemmater.6b03852. (14) Swarnkar, A.; Marshall, A. R.; Sanehira, E. M.; Chernomordik, B. D.; Moore, D. T.; Christians, J. A.; Chakrabarti, T.; Luther, J. M. Quantum Dot–Induced Phase Stabilization of α-CsPbI3 Perovskite for High-Efficiency Photovoltaics. Science 2016, 354 (6308), 92-95, DOI: 10.1126/science.aag2700. (15) 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 Letters 2015, 15 (6), 3692-3696, DOI: 10.1021/nl5048779. (16) Li, Z.; Xiao, C.; Yang, Y.; Harvey, S. P.; Kim, D. H.; Christians, J. A.; Yang, M.; Schulz, P.; Nanayakkara, S. U.; Jiang, C.S.; Luther, J. M.; Berry, J. J.; Beard, M. C.; Al-Jassim, M. M.; Zhu, K. Extrinsic Ion Migration in Perovskite Solar Cells. Energy & Environmental Science 2017, 10 (5), 1234-1242, DOI: 10.1039/c7ee00358g. (17) Burschka, J.; Pellet, N.; Moon, S.-J.; Humphry-Baker, R.; Gao, P.; Nazeeruddin, M. K.; Grätzel, M. Sequential Deposition as a Route to High-Performance Perovskite-Sensitized Solar Cells. Nature 2013, 499, 316, DOI: 10.1038/nature12340 https://www.nature.com/articles/nature12340#supplementary-information. (18) Ke, W.; Xiao, C.; Wang, C.; Saparov, B.; Duan, H.-S.; Zhao, D.; Xiao, Z.; Schulz, P.; Harvey, S. P.; Liao, W.; Meng, W.; Yu, Y.; Cimaroli, A. J.; Jiang, C.-S.; Zhu, K.; Al-Jassim, M.; Fang, G.; Mitzi, D. B.; Yan, Y. Employing Lead Thiocyanate Additive to Reduce the Hysteresis and Boost the Fill Factor of Planar Perovskite Solar Cells. Adv. Mater. 2016, 28 (26), 52145221, DOI: 10.1002/adma.201600594. (19) Ahn, N.; Son, D.-Y.; Jang, I.-H.; Kang, S. M.; Choi, M.; Park, N.-G. Highly Reproducible Perovskite Solar Cells with Average Efficiency of 18.3% and Best Efficiency of 19.7% Fabricated via Lewis Base Adduct of Lead(II) Iodide. Journal of the American Chemical Society 2015, 137 (27), 8696-8699, DOI: 10.1021/jacs.5b04930. (20) Li, Z.; Yang, M.; Park, J.-S.; Wei, S.-H.; Berry, J. J.; Zhu, K. Stabilizing Perovskite Structures by Tuning Tolerance Factor: Formation of Formamidinium and Cesium Lead Iodide Solid-State Alloys. Chemistry of Materials 2016, 28 (1), 284-292, DOI: 10.1021/acs.chemmater.5b04107. (21) Brison, J.; Muramoto, S.; Castner, D. G. ToF-SIMS Depth Profiling of Organic Films: A Comparison Between Single-Beam and Dual-Beam Analysis. Journal of Physical Chemistry C 2010, 114 (12), 5565-5573, DOI: 10.1021/jp9066179. (22) Shard, A. G.; Havelund, R.; Seah, M. P.; Spencer, S. J.; Gilmore, I. S.; Winograd, N.; Mao, D.; Miyayama, T.; Niehuis, E.; Rading, D.; Moellers, R. Argon Cluster Ion Beams for Organic Depth Profiling: Results from a VAMAS Interlaboratory Study. Analytical Chemistry 2012, 84 (18), 7865-7873, DOI: 10.1021/ac301567t. (23) Bailey, J.; Havelund, R.; Shard, A. G.; Gilmore, I. S.; Alexander, M. R.; Sharp, J. S.; Scurr, D. J. 3D ToF-SIMS Imaging of Polymer Multilayer Films Using Argon Cluster Sputter Depth Profiling. ACS Applied Materials & Interfaces 2015, 7 (4), 26542659, DOI: 10.1021/am507663v. (24) Rading, D.; Moellers, R.; Cramer, H. G.; Niehuis, E. Dual Beam Depth Profiling of Polymer Materials: Comparison of C60 and Ar Cluster Ion Beams for Sputtering. Surf. Interface Anal. 2013, 45 (1), 171-174, DOI: 10.1002/sia.5122. (25) Lee, J. W.; Bae, S. H.; De Marco, N.; Hsieh, Y. T.; Dai, Z.; Yang, Y. The Role of Grain Boundaries in Perovskite Solar Cells. Materials Today Energy 2018, 7, 149-160, DOI: 10.1016/j.mtener.2017.07.014. (26) Zhao, P.; Kim, B. J.; Jung, H. S. Passivation in Perovskite Solar Cells: A Review. Materials Today Energy 2018, 7, 267-286, DOI: 10.1016/j.mtener.2018.01.004. (27) Abdi-Jalebi, M.; Andaji-Garmaroudi, Z.; Cacovich, S.; Stavrakas, C.; Philippe, B.; Richter, J. M.; Alsari, M.; Booker, E. P.; Hutter, E. M.; Pearson, A. J.; Lilliu, S.; Savenije, T. J.; Rensmo, H.; Divitini, G.; Ducati, C.; Friend, R. H.; Stranks, S. D. Maximizing and Stabilizing Luminescence from Halide Perovskites with Potassium Passivation. Nature 2018, 555 (7697), 497501, DOI: 10.1038/nature25989. (28) Lee, D. S.; Yun, J. S.; Kim, J.; Soufiani, A. M.; Chen, S.; Cho, Y.; Deng, X.; Seidel, J.; Lim, S.; Huang, S.; Ho-Baillie, A. W. Y. Passivation of Grain Boundaries by Phenethylammonium in Formamidinium-Methylammonium Lead Halide Perovskite Solar Cells. ACS Energy Letters 2018, 3 (3), 647-654, DOI: 10.1021/acsenergylett.8b00121. (29) Niu, T.; Lu, J.; Munir, R.; Li, J.; Barrit, D.; Zhang, X.; Hu, H.; Yang, Z.; Amassian, A.; Zhao, K.; Liu, S. F. Stable HighPerformance Perovskite Solar Cells via Grain Boundary Passivation. Adv. Mater. 2018, DOI: 10.1002/adma.201706576. (30) Shao, Y.; Xiao, Z.; Bi, C.; Yuan, Y.; Huang, J. Origin and Elimination of Photocurrent Hysteresis by Fullerene Passivation in CH3NH3PbI3 Planar Heterojunction Solar Cells. Nature Communications 2014, 5, DOI: 10.1038/ncomms6784. (31) Wu, W.-Q.; Wang, Q.; Fang, Y.; Shao, Y.; Tang, S.; Deng, Y.; Lu, H.; Liu, Y.; Li, T.; Yang, Z.; Gruverman, A.; Huang, J. Molecular Doping Enabled Scalable Blading of Efficient Hole-Transport-Layer-Free Perovskite Solar Cells. Nature Communications 2018, 9 (1), 1625, DOI: 10.1038/s41467-018-04028-8.


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(32) Dou, B.; Wheeler, L. M.; Christians, J. A.; Moore, D. T.; Harvey, S. P.; Berry, J. J.; Barnes, F. S.; Shaheen, S. E.; van Hest, M. F. A. M. Degradation of Highly Alloyed Metal Halide Perovskite Precursor Inks: Mechanism and Storage Solutions. ACS Energy Letters 2018, 3 (4), 979-985, DOI: 10.1021/acsenergylett.8b00305. (33) De Souza, R. A.; Zehnpfenning, J.; Martin, M.; Maier, J. Determining Oxygen Isotope Profiles in Oxides with Time-ofFlight SIMS. Solid State Ionics 2005, 176 (15), 1465-1471, DOI: https://doi.org/10.1016/j.ssi.2005.03.012.

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SIMS Principle

1-D profiling of organic cations in HPSC materials


3-D tomography of HPSC materials and devices

Probing Perovskite Inhomogeneity Beyond the ... - ACS Publications

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