Orientation of Ferroelectric Domains and Disappearance upon

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Orientation of ferroelectric domains and disappearance upon heating methylammonium lead triiodide perovskite from tetragonal to cubic phase Sarah M Vorpahl, Rajiv Giridharagopal, Giles E. Eperon, Ilka M. Hermes, Stefan A. L. Weber, and David S Ginger ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00330 • Publication Date (Web): 14 Mar 2018 Downloaded from http://pubs.acs.org on March 14, 2018

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ACS Applied Energy Materials

Orientation of ferroelectric domains and disappearance upon heating methylammonium lead triiodide perovskite from tetragonal to cubic phase Sarah M. Vorpahl†, Rajiv Giridharagopal†, Giles E. Eperon†±, Ilka M. Hermes†‡, Stefan A. L. Weber‡, David S. Ginger†* †

Department of Chemistry, University of Washington, Seattle, Washington 98105, United States

± ‡

Cavendish Laboratory, JJ Thomson Avenue, Cambridge CB3 0HE, UK

Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany

Corresponding Author *[email protected]

ABSTRACT

We study the spontaneous polarization of the archetypal semiconducting halide perovskite methylammonium lead triiodide (CH3NH3PbI3 or MAPI) that is currently being investigated for use in thin film solar cells and light emitting diodes. Using both lateral and vertical piezoresponse force microscopy (PFM) to image polycrystalline thin films, we observed domains in the piezoresponse that reversibly appear and disappear below and above the tetragonal-tocubic phase transition temperature. Importantly, we observe these domains to exhibit a

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piezoresponse that is predominantly in-plane for films with the (110) plane oriented parallel to the substrate, providing a measure of the polarization associated with specific crystal planes. We characterize the polarization and its temporal response using both local switching spectroscopy and time-dependent PFM spectra. These data show hysteresis loops with the polarization switching with bias, but relaxing back on time scales of several minutes. Our results suggest the existence of ferroelectric behavior due to off-center displacement of the Pb2+ cation, although the local polarization response is complicated by the presence of local ionic and electronic conductivity. Understanding the nature of these domains paves the way for further optimization of optoelectronic devices using CH3NH3PbI3 perovskite material.

Keywords: ferroelectricity, perovskite, piezoresponse force microscopy, hysteresis loop, methylammonium lead triiodide, solar cells

INTRODUCTION

Methylammonium lead triiodide (CH3NH3PbI3) is the archetypal halide perovskite for photovoltaic applications. The role of ferroelectric order, or lack thereof has been debated since the early days of perovskite photovoltaics.1,2 Understanding the nature of polarization and ferroelectric response in CH3NH3PbI3 is thus important for understanding the applied physics of this material, and could also inform studies of new devices structures, such as diodes to explore the bulk photoelectric effect.3 Almost since the first reports of high efficiency photovoltaics based on CH3NH3PbI3, the importance, and even existence, of ferroic order in these materials has been a topic of interest and controversy.4-11 Early theoretical work proposed that ferroelectric


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order could (favorably) help reduce carrier recombination,1,5,6 or (unfavorably) lead to hysteresis in the current voltage (J-V) curves of devices with ferroelectric domains.3,7,8 Further theoretical work has investigated the connection between carrier dynamics and Rashba splitting in CH3NH3PbI3, which arises from the breaking of inversion symmetry often associated with ferroelectricity. Rappe and co-workers suggested that long carrier lifetimes arise from locally polarized domains consistent with ferroelectric domains.9 De Angelis and co-workers similarly suggested that reduced carrier combination in perovskites could arise from Rashba splitting, which shifts the valence and conduction band populations.10 Notably, both of these groups suggested the chemical original of polarized domains as arising from both the Pb-I bond and organic cation rotation. In contrast, other theoretical work has predicted that ferroelectric order only exists lower than 50 K and arises solely from the polarity of the organic cation.11 While initial experimental studies failed to find evidence for ferroic order,2 and current thinking attributes the J-V hysteresis primarily to ion motion,12 other researchers have found evidence for ferroelastic domains using transmission electron microscopy,13 photothermal induced resonance,14 and piezoresponse force microscopy.15 Recently, Cahen and co-workers have argued that even bulk single crystals exhibit true ferroelectric domains, based on the dissipative part of the dielectric response.16 Given these often contradictory experimental and theoretical reports, it appears that currently the nature and origin of this ferroic order remains unclear. Although early studies suggest ferroic ordering of the dipolar methylammonium cation could occur,17, 18 the relatively weak interactions, and rapid rotations of the methylammonium cation, 19,17 suggest such ordering is unlikely. Even without dipolar ordering, it is possible that the chemical origin of polarized domain formation can arise from structural distortions of the inorganic lattice.20 Understanding the orientation and chemical origin of these domains could


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help realize multiple functionalities for optoelectronic devices that can be enabled by combing both ferroelectricity and semiconducting properties in CH3NH3PbI3, including the possibility of exploiting polar order to enhance photovoltaic efficiency. 21 To clarify the nature and origin of ferroic response in these materials, we study perovskite films using piezoresponse force microscopy (PFM).22 We study both vertical and lateral PFM signatures, and measure temperature-dependent PFM, as we heat a CH3NH3PbI3 sample through the tetragonal to cubic phase transition that occurs around 55 °C. Finally, we investigate the dynamic nature of domain polarization via local electrical switching and time-resolved polarization experiments.

Figure 1. Piezoresponse force microscopy (PFM) schematic and data. a) Experimental set up for vertical PFM. Micrographs from PFM including b) topography, c) out of plane (DART-mode) amplitude d) out of plane phase; e) experimental schematic for lateral PFM, f) topography, g) lateral amplitude and h) lateral phase taken on the same 3x3 um2 location of solvent annealed CH3NH3PbI3films on titania.


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We probe the ferroic nature of polycrystalline CH3NH3PbI3 samples deposited on compact titania.23 Figure 1 shows our experimental schematic and images for both vertical and lateral PFM. Briefly, PFM is a scanning probe microscopy technique that scans a conductive tip in contact with the surface of a film to apply a voltage and map the local piezoelectric response to this bias, tracking lattice deformation.24 In particular, PFM exploits the high Q of the AFM cantilever at contact resonance to amplify the piezoresponse, where the contact resonance mode is excited electrically via an AC voltage applied between the tip and the substrate.25,26 For vertical PFM (Figure 1a-d), the out-of-plane deformation of the sample is detected through the vertical bending motion of the tip. For lateral PFM (Figure 1e-h), in-plane vibrations from shear strain are detected in a similar manner using the torsional motion of the cantilever.27,28 A number of recent reviews provide further detailed discussion of these techniques.29,22 In this work, we primarily employ dual amplitude resonance tracking (DART) for better decoupling of topography from piezoresponse.30 DART reduces this crosstalk by tracking the difference in amplitude at two different frequencies around the resonance frequency. In PFM images, the average amplitude and phase response of the cantilever is due to the response of the material to the AC stimulus. For many ferroelectric materials, the vertical PFM phase response will exhibit a 180° difference in phase response between oppositely oriented vertical domains.31 Figure 1b shows the AFM topography for a typical CH3NH3PbI3 sample fabricated using a one-step deposition and post-processing solvent annealing. This annealing process has been previously shown to increase crystallinity.15 Grain sizes vary from 500-1100 nm and the samples have an RMS roughness of 24.1 ± 0.8 nm. Larger images of the films show distinct grain boundaries and minimal holes (Figure S1). Figure 1c shows the vertical amplitude response in PFM for the same area of film. The amplitude response in vertical PFM is a measure of the


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vertical surface displacement of the tip due to the AC bias. Figure 1 shows well-ordered parallel domains in almost every grain of our CH3NH3PbI3 films. These images are consistent with recent reports of ferroeleastic domain structure, revealing domains with similar morphology.13, 14 Figure 1d shows the out-of-plane PFM phase response for the CH3NH3PbI3 films. The out-ofplane PFM phase is nearly constant across the entire image of Figure 1d. We attribute the deviations at the sharp crystal edges to topographic crosstalk. One would expect that vertically oriented ferroelectric domains would show phase contrast of ~180° as the film either expands or contracts in response to the applied field. The absence of such phase contrast implies the absence of vertically oriented domains. Our PFM maps showed no vertical phase contrast over many samples, indicating that any spontaneous polarization is not out-of-plane in nature in films as we have prepared them. Previous studies have used vertical PFM and observed similar results, interpreting the apparent ferroic domain structure in the vertical amplitude signal as arising from cross-talk with an inplane piezoresponse.32 In order to explicitly test this hypothesis, we acquired lateral-mode PFM images on the same region of the sample where we had acquired vertical PFM images. Figure 1f shows the topography from the exact same area as the vertical measurements. Figure 1g shows the resulting lateral mode PFM amplitude image, and Figure 1h the corresponding lateral mode PFM phase image. The lateral amplitude image in Figure 1g shows similar banding and domain structure to the vertical PFM image shown in Figure 1c. However, while the vertical PFM phase image in Figure 1d shows a nearly constant phase across the entire image, the lateral PFM phase image in Figure 1h reveals striking domains with nearly 180°


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(here, ~160°) offsets (see line scans, Figure S2). We conclude from our lateral PFM maps that the spontaneous polarization lies parallel to the surface of the film. We also used X-ray diffraction (XRD) to characterize our films, and thus correlate crystallographic orientation with the direction of the PFM signal. We find that the perovskite films prepared as described herein, are predominantly oriented with the (110) plane parallel to the surface of the film (Figure S3). Our observation that the PFM response is parallel to the surface, and thus parallel to the (110) plane, would be consistent with a structural distortion of the Pb2+ along the c-axis in the tetragonal I4cm space group, as identified by Kanatzidis and coworkers33, as well as Ratika et al.16 Other work has suggested that a combination of the displacement of the Pb cation in the inorganic cage and an organic cation rotation are the chemical origin of ferroelectricity.34

We also characterized a sample of the all-inorganic

perovskite, CsPbI3. Figure S4 shows the same vertical and lateral PFM analysis for the Cs-based film as performed for the CH3NH3PbI3 films. We find that, as in the CH3NH3PbI3 films, the CsPbI3 shows no signal in the vertical phase maps but does show domain-like structure in the lateral phase maps. From these data, we conclude that the MA+ cation is not necessary for domain ordering observed in PFM.

Figure 2. Out-of-plane PFM (DART-mode) amplitude images for same area of solvent annealed


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CH3NH3PbI3 film at (a,c) room temperature (25°C) tetragonal phase and (b) high temperature (65°C) cubic phase. For line scans see Figure S5. Having observed ordered domains in our samples, and having confirmed lateral piezoresponse when the (110) plane is parallel to the surface of our sample, we next turn to investigate the microscopic origins of the domain order in more detail. Perovskites in the tetragonal phase can exhibit spontaneous polarization due to a loss of centrosymmetry.35 Thus, one test to distinguish between the effects of ordering of A-site cations15 and alternative explanations like off-center distortion of the tetragonal phase36 is to probe the piezoresponse in the sample as it is heated through the tetragonal-to-cubic phase transition, which occurs around 55° C for CH3NH3PbI3 films. Because of the ease of imaging with vertical PFM (and having verified that the domain structure observed in vertical PFM corresponds directly to the lateral PFM signature), we used vertical PFM to follow changes in the ordered domain structure as our samples were heated through the phase transition. Figure 2a shows the vertical DART PFM signals on the CH3NH3PbI3 film in the dark, at 25° C under a nitrogen atmosphere. We again see the wellordered domains. We slowly heated (0.2 °C/s) the sample to 65°C and reimaged the film in the exact same area. We calibrated temperatures for the stage using a thermocouple at all temperatures between 0-70 °C. Figure 2b shows the amplitude signal at 65°C, which is above the tetragonal-to-cubic transition at ~55°C. Strikingly, above the tetragonal-to-cubic phase transition temperature, we no longer observed ordered domains in the PFM image (see Figure S6 for further images, including images taken just above and just below the transition temperature). The transition temperature data also imply that the vertical PFM is not a result of geometric crosstalk via cantilever beam effects.37


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We next lowered the temperature of the sample temperature back to 25° C at the same rate. Figure 2c shows PFM image of the sample after returning to 25° C, with domains reappearing in the same location as before heating. We observed similar behavior on many samples, with the domains reappearing at their original positions. This observation of the perovskite PFM signal changing at elevated temperature is consistent with that of Seol et al., which showed similar behavior.38 Since the PFM signal disappears suddenly above the transition temperature, and reappears suddenly upon cooling (rather than gradually fading out and then back in) we conclude that the disappearance of the signal is associated with the passage of the sample through the noncentrosymmetric tetragonal to centrosymmetric cubic phase transition. Because of the importance of tetragonal symmetry in ferroic order, we conclude that the disappearance of the domains imaged in PFM when passing into the centrosymmetric cubic phase is an observation consistent with the assignment of ferroic domains.


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Figure 3. Phase maps and electrical switching loops on solvent annealed CH3NH3PbI3 films from DART mode PFM. a) lateral phase b) lateral hysteresis loop and c) lateral butterfly curve; d) vertical phase, e) vertical hysteresis loop and f) vertical butterfly curve. All measurements are taken between -5 and +5 volts DC bias and 0.05 second pulse width of 50% duty cycle (0.05 s on, 0.05 s off). One of the defining characteristics of a ferroelectric material is the ability to permanently switch between different metastable states with the application of an electric field.22 As such, the hallmark of ferroelectric order in PFM imaging is the observation of hysteresis loops that retain a


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remnant polarization after removal of the applied bias, and that disappear when probed with electric fields higher than the coercive field.31 Therefore, we next turn to explore the response of our PFM signals at a variety of time and voltage scales using piezoresponse force microscopy switching spectroscopy.26 In switching spectroscopy, hysteretic loops in the phase signal and butterfly loops in the amplitude response from PFM as a function of applied DC voltage reflect the bias-dependent nature of oriented polarization with increasing bias in a ferroelectric material. (See Figure S7 for more details). Similarly, butterfly loops plot the amplitude response from PFM as a function of the same back and forth applied DC bias. For a ferroelectric material both measurements reflect the bias-dependent nature of oriented polarization with increasing bias. Figure 3 shows 2D maps and electrical switching loops from both lateral (Figure 3a-c) and vertical (Figure 3d-f) PFM. We start by looking at the lateral phase on a CH3NH3PbI3 film (Figure 3a). Figure 3b and 3c show the representative hysteresis loop and butterfly curves from this sample, respectively. We find that the hysteresis loops in the PFM phase open to ~100°, while the butterfly curve in the PFM amplitude in Figure 3c shows an asymmetric response for positive and negative bias. In a classic ferroelectric material, we would expect to see hysteresis loops closer to 180° difference in phase and a symmetric response for both polarities in the butterfly curve. It is possible that friction forces between the tip and the surface may experimentally limit lateral electrical switching.32 However, we also consider how other – non-ferroelectric – dynamics in the system may contribute to the observed electrical switching response. We turn to vertical hysteresis to further probe the nature of polarization in our films. Figure 3d shows the vertical phase map taken on the same area of CH3NH3PbI3 film. We see no out-of-plane domains in this image and therefore would anticipate no electrical switching


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loops from vertical poling. Figure 3e and 3f show the hysteresis loop and butterfly curve obtained for this sample. Interestingly, we do see a very strong response for both measurements. Therefore, we consider the time constants probed in the imaging vs. the hysteresis and butterfly loops and ask whether the hysteresis seen in electrical switching is permanent on a longer time scale. We first analyze the frequency dependence of our hysteresis loops. Frequency dependent hysteresis loops have been used to argue that electrical switching comes from ionic motion in halide perovskite films.12 Figure S7 shows that at slower frequencies of 0.05 Hz or lower, where the polarization may have a chance to decay in time, there is a slight closing of the hysteresis loop. We next look to even longer time scales to characterize the temporal nature of depolarization for switched domains. Figure S8 shows the phase before and after polarization with a +2.8 V applied DC bias (above the coercive bias). We find that after six minutes the phase has flipped back to its original state, showing that electrical switching is only semi-permanent. Taken together with the in-plane polarization we see in CH3NH3PbI3, and the clear disappearance of PFM signal above the tetragonal-to-cubic transition, we propose that films are ferroelectric. We believe that the relatively long time scale of the phase flip suggests a virtually permanent switch compared to the timescale of ferroelectric switching.

However,

unambiguously determining ferroelectricity is complicated by the many other dynamics and depolarizing fields in the film. Therefore, we suggest that given the transient behavior and frequency dependence of hysteresis loops, local electrical switching is heavily influenced by ionic motion. Figure S9 shows temperature-dependent hysteresis loops, taken between 0-68° C. Consistent with the image data, we observe that above the phase transition the loops collapse and the hysteretic behavior disappears. These data help further support the proposal that while ionic


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motion may contribute to electrical switching response and depolarization, there is also a contribution from ferroelectric, domains. In conclusion, we have studied the ferroic properties of CH3NH3PbI3 perovskite using both vertical and lateral mode PFM, and as a function of temperature. We find that for films with the (110) plane oriented predominantly parallel to the film surface that the PFM signal is dominated by an in-plane response. We also observe that the domain structure in the PFM images disappears as the sample is heated above the tetragonal-to-cubic phase transition temperature, and that this structure reappears upon cooling. The transition to the cubic phase of CH3NH3PbI3 perovskite could release strain within the lattice, consistent with earlier assignments of ferroelastic behavior.13-15 However, ferroelastic behavior in the absence of spontaneous electric polarization would not give rise to a PFM signal. We thus interpret our results as evidence for the role of the loss of symmetry in the tetragonal phase underpinning the ferroelectric order in the CH3NH3PbI3 perovskite. In particular, we suggest that an off-center displacement of the Pb2+ due to a distortion of the PbI6 octahedra could be consistent with our observations.39 Visualizing these domains helps open the door for further correlation with device performance, as has already been tackled in theoretical work, both in terms of reducing recombination, and, perhaps, in achieving open circuit voltages larger than the bandgap.5 Future studies may find correlation of PFM response with local crystal orientation, as well as the PFM response from single crystals, useful in further illuminating this relationship. We obtained hysteresis and butterfly loops that are consistent with ferroelectric behavior given their relatively permanent nature. However, the remnant polarization disappeared on timescales of minutes at room temperature, and furthermore the hysteresis loops do not close when the AC readout signal is larger than the DC poling (coercive) bias (Figure S8), a sure sign that non-ferroelectric effects are also at play.40 Given that


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the halide perovskites are both ionic and electronic conductors, we propose that our data are consistent with CH3NH3PbI3 being a ferroelectric perovskite due to structural distortions of the tetragonal lattice.

ASSOCIATED CONTENT Supporting Information. Materials and methods, topography from AFM image, 2D-XRD, lines scans from PFM phase and amplitude images, temperature dependent PFM amplitude images, PFM for CsPbI3, frequency and temperature dependent hysteresis loops from DART switching spectroscopy, time resolved phase decays after polarization, AUTHOR INFORMATION [email protected], https://depts.washington.edu/gingerlb/ Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We thank Professor Jiangyu Li for helpful discussions. We thank Liam Bradshaw for help with XRD. We also thank Jake Precht for help with XRD and data analysis. This manuscript is based primarily on work supported by the DOE (DE-SC0013957) and includes support from NSF (DMR-1337173). We acknowledge additional support from the Alvin L. and Verla R. Kwiram Endowment from the Department of Chemistry at the University of Washington. G.E.E. is supported by the European Union’s Framework Programme for Research and Innovation Horizon 2020 (2014-2020) under the Marie Skłodowska-Curie Grant Agreement No. 699935.


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Part of this work was conducted at the Molecular Analysis Facility, a National Nanotechnology Coordinated Infrastructure site at the University of Washington, which is supported in part by the National Science Foundation (Grant ECC-1542101), the University of Washington, the Molecular Engineering & Sciences Institute, the Clean Energy Institute, and the National Institutes of Health.

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Orientation of Ferroelectric Domains and Disappearance upon

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