Sub-Diffraction Infrared Imaging of Mixed Cation Perovskites: Probing

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Sub-Diffraction Infrared Imaging of Mixed Cation Perovskites: Probing Local Cation Heterogeneities Rusha Chatterjee, Ilia M. Pavlovetc, Kyle Aleshire, Gregory V Hartland, and Masaru Kuno ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b01306 • Publication Date (Web): 25 Jan 2018 Downloaded from http://pubs.acs.org on January 25, 2018

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ACS Energy Letters

Sub-Diffraction Infrared Imaging of Mixed Cation Perovskites: Probing Local Cation Heterogeneities Rusha Chatterjee, Ilia M. Pavlovetc, Kyle Aleshire, Gregory V. Hartland, Masaru Kuno* Department of Chemistry and Biochemistry, University of Notre Dame, 251 Nieuwland Science Hall, Notre Dame, Indiana 46556, United States AUTHOR INFORMATION Corresponding Author *[email protected]

ABSTRACT Compositional engineering has led to dramatic improvements in hybrid perovskite-based solar cell stabilities and performance.

Mixed cation perovskites have emerged as champion

photovoltaic materials with power conversion efficiencies exceeding 22%. However, there has been relatively little work done to explore local cation-related compositional inhomogeneities in mixed cation perovskite films.

Such studies are necessary since hybrid perovskite optical

properties and consequently their photovoltaic performance strongly depend on composition. Here, we perform spatially-resolved, sub-diffraction infrared photothermal heterodyne imaging measurements to probe cation-specific compositional distributions within FAxMA1-xPbI3 perovskite films. Our measurements reveal that these perovskites possess large compositional

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spatial heterogeneities with cation distributions varying on average ~20% from expected ensemble stoichiometries. Correlated emission measurements show intrafilm emission energies differing by over 30 meV due to these compositional differences. These measurements thus reveal cation stoichiometric heterogeneities and their direct impact on local photovoltaic response-determining optical properties of mixed cation perovskites.

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Lead halide-based perovskites have been the subject of extensive research in recent years. This stems from their outstanding performance as light harvesters in photovoltaic applications.1,2,3,4,5,6

Their optimal, direct band gaps as well as long carrier diffusion

lengths2,5,7,8,9 have enabled perovskite-based solar cells to achieve power conversion efficiencies exceeding 22% in a less than a decade.10,11

Rapid improvements in device fabrication

methodologies along with simple, low-cost syntheses and processing make the commercial use of perovskite-based solar cells a very real possibility in the foreseeable future.12 Perovskites adopt an ABX3 structure where A is a monovalent cation (methyalammonium (MA = CH3NH3+), formamidinium (FA = HC(NH2)2+), Cs+, Rb+), B is a divalent cation (Pb2+, Sn2+) and X is a halide anion (Cl-, Br-, I-).

Given the ability of the perovskite lattice to

incorporate different ions, compositional engineering using combinations of A-cations and halide


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ACS Energy Letters

anions has emerged as the method of choice to fine-tune the opto-electronic response of these materials.13 This is further motivated by the enhanced stability of such mixed composition materials13 and has led to some of the highest recorded power conversion efficiencies to date.11,14,15,16,17

Initial efforts at making compositionally-complex perovskites have entailed

mixed halide compositions.18,19 More recent efforts have extended this to corresponding mixed cation systems13,20 with resulting perovskite alloys consisting of two, 17,21,22 three,23,24,25 and even four cations.11 Notable among the mixed cation systems are the FA/MA perovskites.16,17,26,27,28,29 In this regard, FAPbI3 possesses a smaller more favorable optical band gap, as well as a longer carrier diffusion length, than its MA counterpart.30,31,32

Its photoactive cubic phase however, is

thermodynamically unstable, converting to a photoinactive hexagonal phase at room temperature.33 Mixed MA/FA cations stabilize the desired quasi-cubic (black) perovskite phase and thus form stable solar cells with power conversion efficiencies exceeding 20%.16 Despite tremendous improvements to solar cell efficiencies, no study has probed local cation inhomogeneities in mixed composition perovskites. Only halide-related heterogeneities – both light-induced34,35,36 and static37,38 – have been explored. Cation-related variations, however, are equally critical in determining solar cell efficiencies. For example, cation inhomogeneities lead to different optical band gaps16,29 which, in turn, yield correspondingly different local open circuit voltages.39 What results are thermalization losses and performance variations across device active areas. Some reports have already suggested that cation non-uniformities exist in mixed cation perovskite films. Specifically, ensemble solid state NMR measurements by Kubicki et al. have shown that mixed cation films segregate into different phases depending on the overall ensemble


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cation ratio.40 Gratia et al. have likewise reported spectral and intensity differences in local Raman transitions within (FAPbI3)0.85(MAPbBr3)0.15 films and have suggested that this indicates possible intrafilm cation variations.37

However, until now, no study has spatially-resolved

cation-specific compositions, quantified their non-uniformities, and directly linked their existence to local photophysical variations. In this study, we demonstrate spatially-resolved infrared (IR) measurements of mixed cation FAxMA1-xPbI3 perovskite films and reveal local heterogeneities in cation distributions. These measurements are performed as functions of ensemble cation stoichiometry (x) to demonstrate the ubiquitousness of these inhomogeneities.

Since vibrational transitions are specific to

chemical entities, IR spectroscopy represents an optimal tool for resolving cation-specific information in mixed cation perovskites.

Furthermore, IR imaging possesses a distinct

advantage over more commonly used Raman measurements16,37 in that IR cross-sections are easily 15 orders of magnitude larger than corresponding Raman cross-sections.41 The primary limitation of IR microscopy is its poor spatial resolution. This is because the IR diffraction limit is on the order of several microns. By contrast, the visible diffraction limit ranges from ~200 – 400 nm. In this study, we achieve sub-diffraction IR imaging using infrared photothermal heterodyne imaging (IR-PHI).42,43,44,45,46,47,48 The principle of the technique rests on the absorption of intensity-modulated IR light by a material followed by non-radiative relaxation resulting in periodic photothermal changes to the sample’s local environment. Subsequent detection of IR absorption is achieved via lockin detection of the induced intensity modulation of a visible/near infrared (NIR) probe beam focused on the sample. This makes IRPHI an IR imaging technique with a spatial resolution dictated by the probe diffraction limit.42,43,44,45,46,47,48


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ACS Energy Letters

In

the

past,

IR-PHI

has

been

implemented

using

a

co-propagating

optical

geometry.42,43,44,45,46,47 Here, collinear pump and probe laser beams are focused on the sample using a single reflective objective.

However, lower numerical aperture (NA) reflective

objectives imply limited spatial resolutions. By contrast, we employ a counter-propagating pump/probe beam geometry, and exploit the use of a second, high NA visible (refractive) objective to focus the probe light. We have consequently demonstrated IR imaging of patterned photoresists, single bacterial cells, and single polymer beads with spatial resolutions as low as 300 nm.48 In this study, we perform analogous spatially-resolved IR-PHI measurements with a NIR probe on mixed FA/MA cation perovskite films to investigate the homogeneity of cation distributions in these materials. We find that FAxMA1-xPbI3 perovskites do not form uniformly alloyed films. Instead, they exhibit local cation heterogeneities. These non-uniformities, moreover, extend across a large ensemble stoichiometric composition space where x (i.e. cation precursor ratio used for perovskite synthesis) ranges from 0.1 ≤ x ≤ 0.4.

Our measurements further reveal local

stoichiometry () variations on the order of ~20% with some areas experiencing values exceeding thrice the expected ideal stoichiometric value. These variations are further corroborated by same-area photoluminescence (PL) measurements where PL peak energies (EPL), and therefore corresponding local optical band gaps (Eg) correlate with local IR-PHIderived cation compositions. These measurements thus directly reveal inhomogeneities in local cation compositions within mixed cation films and relate them to local Eg variations. FAxMA1-xPbI3 perovskite films were synthesized using an established one-step deposition method involving solvent engineering.49 Details of the thin film preparation are outlined in the Supporting Information (SI).

Scanning electron microscopy (SEM) images indicate


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polycrystalline films with grain sizes of ~250 nm and film thicknesses of ~400 nm for all compositions. Representative SEM images can be found in Figure S1. Next, we perform general structural and optical characterization of these films to ensure film quality. Figure 1(a) shows FAxMA1-xPbI3 powder x-ray diffraction (PXRD) patterns. The corresponding pattern for a pure MAPbI3 film is shown for reference purposes. While the pure MAPbI3 PXRD pattern exhibits reflections representative of its tetragonal structure, it is apparent that the introduction of FA shifts these reflections to smaller angles 2θ.16,29 Figure 1(b) is a zoomed-in view of the shaded grey region in Figure 1(a) and demonstrates the gradual shift of the perovskite (220) reflection to smaller angles 2θ with increasing FA content. The shift of the tetragonal peak results from lattice expansion to accommodate larger FA cations and is indicative of their incorporation into the MAPbI3 lattice.16,29 Notably, a characteristic PbI2 peak at 12.5° is absent in all mixed cation films, indicating their resistance to degradation.


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ACS Energy Letters

Figure 1. (a) PXRD patterns for FAxMA1-xPbI3 films with varying x. Data offset for clarity. (b) A zoomed in view of the (220) reflection highlighted by the shaded grey region in (a). The dashed vertical line is shown as a guide to the eye. (c) Band-edge UV-VIS absorption of the different FAxMA1-xPbI3 films. (d) FTIR spectra of FAxMA1-xPbI3 films with varying x. Data offset for clarity. The vertical dashed lines represent positions of individual transitions with labels denoting corresponding assignments.

Figure 1(c) shows band-edge UV-VIS absorption spectra of the FAxMA1-xPbI3 films. The corresponding spectrum of pure MAPbI3 is shown for reference purposes. With increasing FA,


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the absorption onset shifts towards longer wavelengths. Estimated band gaps, extracted using a Tauc analysis of the spectra, characteristically shift to lower energies with increasing x (Figure S2), indicating incorporation of the FA cation into the perovskite lattice.16,29 Figure 1(d) shows corresponding Fourier transform infrared (FTIR) spectra along with those for pure MAPbI3 and FAPbI3 for comparison purposes. Most prominent in the MAPbI3 spectra is a characteristic MAPbI3 symmetric NH3+ bend at 1465 cm-1 as well as a very broad and weak asymmetric NH3+ bend at 1578 cm-1.50 FAPbI3 likewise exhibits a distinctive C=N stretch at 1710 cm-1 and a weaker C–N stretch at 1330 cm-1.51

Mixed FAxMA1-xPbI3 FTIR spectra

evidently possess both MA- and FA-specific transitions. Furthermore, their relative strength is composition-dependent. In particular, the intensity of the 1710 cm-1 FA C=N stretch increases with respect to the 1465 cm-1 symmetric MA NH3+ bend with increasing x. These ensemble characterizations of FAxMA1-xPbI3 therefore establish the formation of mixed cation films with the same structural and optical properties as those reported in the literature.16,29 Next, we perform spatially-resolved IR-PHI measurements on these films to probe their local cation compositions and to see whether they are stoichiometrically uniform. Two-dimensional (2D) MA- and FA-specific IR maps are obtained by monitoring IR-PHI signals at fixed pump wavelengths of 1465 cm-1 (symmetric MA NH3+ bend) and 1710 cm-1 (FA C=N stretch) using an off-resonance (1064 nm) laser to probe the perovskite photothermal response.

A detailed

description of the measurement and the set-up can be found in the SI (Figure S3).


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ACS Energy Letters

Figure 2. False color (a, e) MA- and (b, f) FA-specific IR-PHI maps for 10×10 µm areas in FA0.1MA0.9PbI3 (x = 0.1) and FA0.2MA0.8PbI3 (x = 0.2) films. Images are normalized to the highest observed MA and FA IR-PHI values. (c, g) Corresponding false color MA/FA ratio maps for FA0.1MA0.9PbI3 and FA0.2MA0.8PbI3, indicating spatial heterogeneities in the perovskite composition. (d, h) IR-PHI spectra obtained at the labeled points in (c) and (g) and normalized to the 1465 cm-1 MA transition.

Figures 2(a, b) show representative, normalized false color MA and FA IR-PHI maps for the same 10×10 µm area of a FA0.1MA0.9PbI3 (x = 0.1) film. Immediately apparent are variations in the IR-PHI signal for each cation which we attribute to intrafilm thickness differences. Although a simple visual comparison of individual MA- and FA-specific IR-PHI maps does not immediately indicate any apparent differences in cation composition across the area [i.e. regions with a high (low) MA IR-PHI response show a correspondingly high (low) FA signal], clear MA/FA stoichiometric heterogeneities are revealed by dividing the individual cation raw IR-PHI


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intensities to create a MA/FA ratio map. This illustrated in Figure 2(c) where the bottom shows a MA-rich section (labeled region 1) of the film while the upper right (labeled region 2) is FArich.

Figure 2(d) further highlights this intrafilm cation stoichiometric heterogeneity by

showing IR-PHI spectra for regions 1 and 2. Clear differences in the strengths of MA- and FArelated transitions are apparent.

When normalized to the intensity of the 1465 cm-1 MA

symmetric NH3+ bend transition, the spectra clearly show that the 1710 cm-1 FA C=N stretch intensity differs between regions 1 and 2, indicating local MA/FA stoichiometric differences. Back of the envelope calculations (see SI) reveal that local heating due to IR absorption is not significant enough to induce cation migration. These ratio maps therefore reflect equilibrium compositional heterogeneities in the film.

Additionally, ratios are insensitive to intrafilm

thickness variations as evidenced in Figure S4, which shows no correlation between MA/FA ratio and corresponding 532 nm absorbance (absorbance being directly related to thickness) for a 10×10 µm area in a FA0.3MA0.7PbI3 film. Similar spatial variations in MA/FA composition have been observed for all mixed cation films. As an example, Figures 2(e – h) show similar MA, FA and MA/FA cation distribution maps for a FA0.2MA0.8PbI3 specimen, along with IR spectra highlighting the spatial disparity in their MA/FA distributions. Representative IR-PHI maps and spectra for x = 0.3 and x = 0.4 FAxMA1-xPbI3 films are shown in Figure S5. Notably, all observed features in IR-PHI derived spectra agree with those acquired using traditional FTIR spectroscopy [Figure 1(d)]. This is further highlighted in Figure S6 which compares the ensemble FTIR spectrum and the IR-PHI derived spectrum of FA0.4MA0.6PbI3, confirming the fidelity of the IR-PHI technique. In order to link observed MA/FA IR-PHI ratios to actual local perovskite composition, we perform large area IR-PHI scans for all four (0.1 ≤ x ≤ 0.4) FAxMA1-xPbI3 films. Figure 3(a)


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ACS Energy Letters

shows resulting average MA/FA IR-PHI ratios (r / ) over 50×50 µm areas plotted against ensemble film stoichiometry, x. As expected, r / decreases with increasing FA content. Error bars in the plot represent observed standard deviations. Since the IR-PHI response is proportional to cation concentration, r / is expected to vary inversely with x as shown below:

r / ∝

MA 1 1 ∝ FA

(1)

Observed r / values are therefore fit to a first order inverse polynomial to obtain a calibration curve that links r / and x:

r /

0.47 0.01 0.20 0.06

(2)

The errors in the fit parameters are associated standard errors. The fit equation is shown as the dashed black curve in Figure 3(a). The fidelity of the fit is then tested by predicting r / for x = 0.25 (predicted r / = 1.5) and subsequently conducting measurements on a FA0.25MA0.75PbI3 film.

The experimental result in Figure 3(a) (open blue triangle) shows

r / = 1.3, in good agreement with the prediction.

Figure 3. (a) Average MA/FA IR-PHI ratios for x = 0.1, 0.2, 0.3, 0.4 (open red circles) and the corresponding fit to the data (dashed black line). The open blue triangle represents r / for


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a FA0.25MA0.75PbI3 film. Error bars represent observed standard deviations. (b) False color r / map for a 10×10 µm area in a FA0.1MA0.9PbI3 film. (c) Corresponding false color map derived from (b). These maps show a large degree of heterogeneity within a representative area of the film.

Equation 2 is therefore used to estimate heterogeneities in cation stoichiometry for all FAxMA1-xPbI3 films. As an illustration, Figure 3(b) shows a r / map for a 10×10 µm area in a FA0.1MA0.9PbI3 film. Equation 2 is then used to relate local IR-PHI ratios to corresponding local stoichiometries, i.e. , shown in Figure 3(c). The map has an average value of = 0.14 ± 0.03. The error represents the associated standard deviation in -values, which corresponds to a ~20% deviation from the average. The mean value of 0.14 indicates a 40% deviation from the ideal value of x = 0.1 for this composition. Furthermore, -values as high as 0.3 are observed in this nominal 10×10 µm area, which is three times larger than the expected ideal value.

Similarly large composition heterogeneities are observed for all

compositions with average deviations of ~20% across films. Observed deviations for each composition are listed in Table S1.

Our IR-PHI measurements therefore establish that

significant spatial non-uniformities exist in the cation stoichiometry of FAxMA1-xPbI3 films. Given that the perovskite optical response is sensitive to composition,16,29 our IR measurements indicate that a correspondingly large degree of intrafilm band gap variations exist. We therefore probe this using correlated PL spectral measurements of the perovskite films. Figure 4(a) first shows a map for a 10×10 µm area in a FA0.3MA0.7PbI3 film. Estimated -values range from 0.08 to 0.56, indicating a large compositional heterogeneity.


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ACS Energy Letters

Figure 4. (a) False color map of a 10×10 µm area in a FA0.3MA0.7PbI3 film.

(b)

Corresponding same area false color EPL map. The calibration bar has units of electron volts. (c) A pixel correlation plot of EPL versus for the area shown in (a, b), indicating a high degree of composition-induced Eg-variations across the film.

Next, Figure 4(b) shows the corresponding spatially-resolved EPL map of the same area. Details of these PL measurements can be found in the Experimental Methods. In general, Figure 4(b) reveals a large spread in EPL-values ranging from 1.556 – 1.593 eV, corresponding to Eg differences of 37 meV within a nominal 10×10 µm area. These EPL correlate significantly with the corresponding -values for the area. Figure 4(c) highlights this by plotting all observed EPL-values against their corresponding estimated values. The large negative Pearson correlation coefficient (PCC) of -0.70 indicates that PL peak energies in regions with high (higher local FA content) are red-shifted compared to those with lower (lower local FA content).

This agrees well with observed ensemble

absorption/emission variations with stoichiometry [Figure 1(c), Figure S2] and with the literature.16,29 Note that reabsorption can also contributes to EPL shifts. However, the significant PCC indicates that -variations predominantly contribute to local Eg differences. This is


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further highlighted by a weak correlation between EPL and corresponding local absorbance at 532 nm, which is a measure of film thickness (Figure S7). Similar correlations between EPL and are observed for other FAxMA1-xPbI3 films. Representative maps and correlation plots can be found in Figure S8. Our IR-PHI measurements therefore provide direct evidence for intrafilm heterogeneities in cation composition within FAxMA1-xPbI3 films.

Mixed cation perovskites do not form

homogeneous alloys. Instead, they exhibit local MA- or FA-rich regions. Average MA/FA variations are ~20% with composition deviations as high as thrice the expected stoichiometric value. While recent work has suggested the possibility of cation inhomogeneity in mixed composition perovskites,37 this has been attributed to local halide composition fluctuations which influence local lattice interactions and, in turn, cause cation disorder. Here, however, single halide perovskites are studied. We speculate that local cation heterogeneities therefore arise due to differences in the kinetics of local MAPbI3 and FAPbI3 lattice formation. Furthermore, given that MAPbI3 and FAPbI3 have different crystal structures (tetragonal and cubic, respectively), mixed perovskite crystals likely stabilize in different compositions depending on the local environment. Correlated PL measurements further show local disorder in perovskite optical properties. Significant local stoichiometry variations result in correspondingly large EPL- and therefore Egvariations. This has direct bearing on the perovskite photovoltaic response of FA/MA-derived devices given the direct link between band gap and open circuit voltage in solar cells. Subdiffraction IR-PHI measurements therefore reveal additional new qualitative and quantitative insights into potential limitations preventing mixed composition perovskites from realizing their full photovoltaic potential.


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ACS Energy Letters

ASSOCIATED CONTENT Supporting Information available: Thin film preparation and ensemble characterization. Representative SEM images of (0.1 ≤ x ≤ 0.4) FAxMA1-xPbI3 perovskite films. Tauc analysis derived ensemble Eg versus x.

Detailed description of the spatially-resolved IR-PHI and

emission spectrum measurements and set-up. Estimates for local temperature changes due to IR illumination. Pixel correlation plot of local MA/FA ratios and corresponding local absorbance at 532 nm for x = 0.3. Representative IR-PHI maps and spectra for x = 0.3 and x = 0.4 FAxMA1xPbI3

films. Comparison of ensemble FTIR and IR-PHI derived spectra for FA0.4MA0.6PbI3.

Observed stoichiometric deviations for all (0.1 ≤ x ≤ 0.4) FAxMA1-xPbI3 films. Pixel correlation plot of local EPL and corresponding local absorbance at 532 nm for x = 0.3. Representative and EPL maps, and EPL versus correlation plots for x = 0.1, 0.3 and 0.4 films. AUTHOR INFORMATION Notes RC and IMP contributed equally to this work. The authors declare no competing financial interests. ACKNOWLEDGMENT M.K. and G.V.H. acknowledge the National Science Foundation (grant numbers CHE1563528 and CHE-1502848 respectively) for financial support. M.K. also thanks the Division of Materials Sciences and Engineering, Office of Basic Energy Sciences, U.S. Department of Energy under award DE-SC0014334 for partial financial support of this work. The IR OPO


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system was purchased through DURIP Award W911NF1410604. We thank the Notre Dame Integrated Imaging Facility and the ND Energy Materials Characterization Facility (MCF). The MCF is funded by the Sustainable Energy Initiative (SEI), which is part of the Center for Sustainable Energy at Notre Dame (ND Energy). We also thank Michael C. Brennan for assistance with PXRD measurements. REFERENCES (1) Zhang, W.; Eperon, G. E.; Snaith, H. J. Metal Halide Perovskites for Energy Applications. Nat. Energy 2016, 1, 16048. (2) Huang, J.; Yuan, Y.; Shao, Y.; Yan, Y. Understanding the Physical Properties of Hybrid Perovskites for Photovoltaic Applications. Nat. Rev. Mater. 2017, 2, 17042. (3) Zhao, Y.; Kai Zhu, K. Organic–Inorganic Hybrid Lead Halide Perovskites for Optoelectronic and Electronic Applications. Chem. Soc. Rev. 2016, 45, 655−689. (4) Stranks, S. D.; Snaith, H. J. Metal-Halide Perovskites for Photovoltaic and Light-Emitting Devices. Nat. Nanotechnol. 2015, 10, 391−402. (5) Green, M.; Ho-Baillie, A.; Snaith, H. J. The Emergence of Perovskite Solar Cells. Nat. Photonics 2014, 8, 506−514. (6) Correa-Baena, J. -P.; Abate, A.; Saliba, M.; Tress, W.; Jacobsson, T. J.; Grätzel, M.; Hagfeldt, A. The Rapid Evolution of Highly Efficient Perovskite Solar Cells. Energy Environ. Sci. 2017, 10, 710–727. (7) Yin, W. -J.; Shi, T.; Ya, Y. Unique Properties of Halide Perovskites as Possible Origins of the Superior Solar Cell Performance. Adv. Mater. 2014, 26, 4653–4658. (8) Brivio, F.; Butler, K. T.; Walsh, A.; Van Schilfgaarde, M. Relativistic Quasiparticle SelfConsistent Electronic Structure of Hybrid Halide Perovskite Photovoltaic Absorbers. Phys. Rev. B 2014, 89, 155204.


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0.2

0.4

0.6

0.8

0.2

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0.6

MA (a)

0.8

3.0

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1 2

(d)

1.0 0.8 0.6 0.4 0.2

2 µm

1

2 µm

MA (e)

2 µm

MA/FA (g)

FA (f )

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1.4 1.2 1.0 0.8 0.6

1

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PHI signal (a.u.)

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(a)

6.0

rMA/FA (b)

xlocal (c)

0.30

5.0

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4.0

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3.0

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Sub-Diffraction Infrared Imaging of Mixed Cation Perovskites: Probing

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