Monitoring a Silent Phase Transition in CH3NH3PbI3 Solar Cells via

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Letter 3

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Monitoring a Silent Phase Transition in CHNHPbI Solar Cells via Operando X-ray Diffraction

Laura T. Schelhas, Jeffrey A Christians, Joseph J. Berry, Michael F. Toney, Christopher J. Tassone, Joseph M. Luther, and Kevin H Stone ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.6b00441 • Publication Date (Web): 13 Oct 2016 Downloaded from http://pubs.acs.org on October 17, 2016

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Monitoring a Silent Phase Transition in CH3NH3PbI3 Solar Cells via Operando X-ray Diffraction

Laura T. Schelhas,1,† Jeffrey A. Christians,2,† Joseph J. Berry,2 Michael F. Toney,1 Christopher J. Tassone,1 Joseph M. Luther,2* Kevin H. Stone1*

1

SSRL Materials Science Division, SLAC National Accelerator Laboratory, Menlo Park, CA

94025, USA 2

National Renewable Energy Laboratory, Golden, CO 80401, USA

*

Author to whom correspondence should be addressed. Email: [email protected],

[email protected] †

These authors contributed equally

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Abstract: The relatively modest temperature of the tetragonal to cubic phase transition in CH3NH3PbI3 perovskite is likely to occur during real world operation of CH3NH3PbI3 solar cells. In this work, we simultaneously monitor the structural phase transition of the active layer along with solar cell performance as a function of the device operating temperature. The tetragonal to cubic phase transition is observed in the working device to occur reversibly at temperatures between 60.5 and 65.4 °C. In these operando measurements no discontinuity in the device performance is observed indicating electronic behavior that is insensitive to the structural phase transition. This decoupling of device performance from the change in long range order across the phase transition suggests the optoelectronic properties are primarily determined by the local structure in CH3NH3PbI3. That is, while the average crystal structure as probed by XRD shows a transition from tetragonal to cubic, the local structure generally remains well characterized by uncorrelated, dynamic octahedral rotations which order at elevated temperatures but are unchanged locally.

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Methylammonium lead iodide (CH3NH3PbI3) has quickly become a promising low cost alternative photovoltaic absorber material.1–4 This material crystallizes in the perovskite, AMX3 crystal structure where A is the organic cation (CH3NH3+), M is the metal cation (Pb2+), and X is the halide anion (I-). At room temperature CH3NH3PbI3 adopts a tetragonal perovskite structure with tilted PbI6 octahedra; however, at modest temperatures it transforms into a cubic perovskite structure (Pb–I–Pb bond angle of 180°).5–7 A handful of studies explore the CH3NH3PbI3 photophysics as a function of temperature; however, these reports mostly focus on the tetragonal phase.8–12 Since the phase transition occurs at a temperature range that is within the typical operation temperatures expected for real world photovoltaic installations, it is important to understand what the implications are for cell level operations. Several studies have reported discontinuities in optoelectronic properties13,14 across the phase transition; although, the reported tetragonal-to-cubic transition temperature widely varies within the literature from 37 °C, reported from photoluminescence13 to 55-57 °C reported from XRD5–7,15 and DSC.6 On the other hand, there are also experimental temperature-resolved UVvis absorption,16 photovoltaic performance17, and impedance spectroscopy18 studies that indicate little change over the tetragonal to cubic phase transition. While the later experiments do not monitor the structure directly they appear to agree with ab initio molecular dynamics simulations, by Quarti et al. that indicate that no abrupt change in optoelectronic properties should occur. This prediction is a result of the fact that while the high temperature phase displays cubic symmetry on average, the calculations, and recent pair distribution function analysis by Beecher et al., indicate large deviations from cubic symmetry at the sub-picosecond time scale which result in a tetragonal-like local structure on the time scale of electronic transitions.16,19 Given the variability in the reported tetragonal-to-cubic transition temperature, and the discrepancy in the literature as to the impact of such a transition on optoelectronic properties, it is difficult to assess what, if any, connection exists between the structural phase and device operation. We examine this directly by measuring the structure and device performance simultaneously, or operando, reporting the structural characterization of an operational CH3NH3PbI3 photovoltaic device. The operando measurement is accomplished using a sample stage developed for simultaneous, temperature dependent measurements of current-voltage curves and X-ray diffraction. This stage has allowed us to obtain X-ray diffraction data, including full structural refinements, during operation of CH3NH3PbI3 devices across relevant 3 ACS Paragon Plus Environment

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temperatures for terrestrial operation. We find, consistent with the picture that Quarti et al. have presented,16 that at high temperature the local arrangement of atoms is actually more closely described by those in the tetragonal phase despite the fact that the structure is on average cubic in nature. These results suggest that this local ordering is more important to the optoelectronic properties in the CH3NH3PbI3 system relevant to its use in photovoltaics. The CH3NH3PbI3 devices used for operando measurements have been prepared by the “two-step method” previously reported.20–22 In brief, films are prepared by spin-coating an initial PbI2 film and then converting the film to CH3NH3PbI3 using a solution of CH3NH3I in isopropanol (see the Experimental Methods in the Supporting Information for complete details). A typical device was investigated with an open-circuit voltage (VOC) of 0.93 V, shortcircuit current density (JSC) of 18.3 mA/cm2, and fill factor (FF) of 0.66, leading to a power conversion efficiency (PCE) of 11.3 % (Table S1, Figure S1).

Figure 1. A) The operando chamber used from simultaneous XRD and current-voltage (IV) characterization. B) The tetragonal and C) cubic crystal structures for CH3NH3PbI3. Lead (gray), CH3NH3 (blue), and iodine (purple), are represented by spheres, and single unit cells are identified by red lines. Iodine atoms (purple) are shown as thermal ellipsoids.


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To measure the device operando we have designed a chamber (Figure 1A) for simultaneous current-voltage measurements and XRD at the Stanford Synchrotron Radiation Lightsource (SSRL). The sample sits on the stage while the electrodes of the device are contacted by electrical pads that also serve as clips to hold the sample in place. The device is illuminated by a white LED source (intensity approximately 0.30 Sun) shone from outside of the chamber through a window in both the bottom of the chamber and sample stage. The entire chamber is enclosed by a metal cap with X-ray-transparent entrance and exit windows. Heat is applied to the sample at a rate of approximately 1°C/5 min by a heating tape attached to the chamber cap, and the temperature of the device is measured by a thermocouple attached to the surface of the sample. To prevent reaction with moisture and/or air,22 the chamber is purged with helium gas for the duration of the experiment. One of the characteristic features of the tetragonal to cubic phase transition is the disappearance of the (211)tet peak which is not allowed in the cubic symmetry. Therefore, we identify the phase transition as being in the range between identifying a measurable (211)tet peak (Q = 1.66 A-1) and its disappearance. Due to diffuse scattering above the transition temperature (see below), this represents a maximum transition temperature. Figure 2A, is a plot of XRD patterns measured operando of the (211)tet peak at a selection of temperatures. By plotting the intensity of the (211)tet peak versus temperature (Figure 2B) we identify the phase transition in this device to be between 60.5 and 65.4 °C. This is slightly higher temperature for the phase transition than the previous report of 57 °C;6 however, we note a difference in the definition of the phase transition temperature. We choose to identify the phase transition as the point at which no XRD evidence of the tetragonal phase peaks is seen (i.e., the (211)tet intensity is zero). Other reports have relied on DSC to determine the temperature of the transition.6 Baikie et al. have shown that scattering of the (211)tet peak can persist at higher temperatures and determine the phase transition to be defined as the onset of a large reduction in the (211)tet intensity which occurs at the same temperature as the DSC peak in their study.6 Due to the differences in the way the phase transition temperature is measured and identified, throughout this work we note the literature value of 57 °C as well as when we are no longer able to measure the (211)tet peak. It is also worth noting that often the phase transition is measured in CH3NH3PbI3 films or single crystals, and it is possible that the solar cell architecture alters the phase transition temperature (i.e., due to strain or heterogeneity).23 Given that we see hysteresis in the phase transition 5 ACS Paragon Plus Environment

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temperature between heating and cooling (Figure 3C), it is equally possible that there is a kinetic aspect to the phase transition such that the exact transition temperature will depend on the heating profile used.

Figure 2. A) Select XRD plots of the (211)tet reflection (X-ray energy of 12.5 keV) at (a) 26.4, (b) 36.3, (c) 55.6, (d) 65.4, and (e) 69.9 °C, and B) the integrated intensity versus temperature of the (211)tet peak upon heating of the solar cell. To better understand how the phase transition could affect the device performance, we performed Rietveld refinements on full XRD powder patterns taken on the device during heating and cooling (Figure 3). Powder XRD data was acquired by integrating 2D grazing-incidence wide angle X-ray scattering (GIWAXS) patterns over polar angles, χ, from 5° ≤ χ ≤ 90° (Figure S2). A summary of all of the refinement results can be found in Tables S2-S4 and are consistent with previous reports.5,6,24,25 One concern when heating CH3NH3PbI3 is degradation to PbI2; however, as shown in Table S4, little to no change is seen in the weight percent of PbI2, as calculated from the refinement data, when heating and cooling the sample through the phase transition (12.0 ± 0.7 %). Another question arises about possible grain growth due to heating of the device. Prior to annealing the structure of the films shows large domains by scanning electron microscopy (Figure S3); however, it is difficult to measure the structure of the films by microscopy after annealing since they are fully formed devices which may behave differently than bare perovskite films. To address the possibility of grain growth the width of the (211)tet peak can be used as an estimate of the approximate grain size. We measure the full width at half maximum of the (211)tet peak to be 0.18 ± 0.01 prior to annealing, and after annealing. This lack of change in FWHM signifies little to no change in crystalline domain size. Our Rietveld refinements show a large anisotropic thermal parameter on the iodine site in the cubic phase 6 ACS Paragon Plus Environment

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(Figure 1). This has been previously reported and indicates a tendency for the iodine atom to be poorly localized orthogonal to the Pb-I-Pb bonding direction due to uncorrelated octahedral rotations. Upon cooling, the random displacement of the iodine atoms will order, breaking the average cubic symmetry and leading to a tetragonal distortion. Structurally this leads to a tilting of the octahedra (Oh) in the tetragonal phase with alternating directions for neighboring planes along the c-axis, as illustrated in Figure 1B.

Figure 3. A) In-situ XRD patterns of the CH3NH3PbI3 solar cell taken as the device is heated and then cooled to demonstrate the reversibility of the cubic-to-tetragonal phase transition with the portion of the spectrum containing the (211)tet peak highlighted in B). Data are plotted in Q = [4π*sin(θ)]/λ or Q = 2π/d, where λ is X-ray wavelength and d is the d-spacing. C) Integrated intensity versus temperature of the (211)tet peak upon heating (red) followed by cooling (blue) of the solar cell


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For proper interpretation of the X-ray refinements it is important to note that there are two types of disorder, thermal and static disorders, which will both contribute to the disorder parameter. Traditional diffraction provides both a temporally and spatially averaged structure, making the rapid atomic motions due to thermal disorder indistinguishable from static but spatially random atomic displacements. For the CH3NH3PbI3 system, the PbI6 octahedra may rotate away from cubic on a local level, either in a rapidly fluctuating manner due to thermal motion or by having each octahedron tilt with little to no correlation between neighboring octahedra. Both effects would give a structure which is cubic on average, as observed at elevated temperature. Quarti et al. predict rapid fluctuations of the Oh-ordering at elevated temperatures suggesting that the observed cubic phase is just a time-averaged ensemble of rapid structural fluctuations. The picture of the material is thus of a structure which is tetragonal on a local, instantaneous level, while only globally (the time/space average crystal structure measured by diffraction) does the system appear cubic. This is further confirmed by pair distribution function (PDF) analysis, which unlike XRD is sensitive to the local order. On scraped powders and single crystals of CH3NH3PbI3 it has been shown that the local order of these materials most closely resembles the tetragonal structure with domains of approximately 1-3 nm in diameter.19,26 The implication is that the material’s electronic properties are dominated by the local structure which remains unchanged through the phase transition, and will thus be invisible to device performance as measured as a function of temperature.


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Figure 4. A) IV-curves at various temperatures taken at with a scan rate of 100 mV/s. Initial and final room temperature data show the stability of the device to heating and the reversibility of the measurements. The device short-circuit current (ISC), open-circuit voltage (VOC), and fill factor (FF) versus temperature are summarized in B)-D), respectively. The phase transition measured operando during sample heating and cooling is represented by the gray bar while the red bar represents the phase transition measured on the sample during heating alone. The black dashed line is the phase transition reported in ref 6.

To then explore this hypothesis operando device metrics are plotted in Figure 4B-D as a function of temperature through the structural phase transition. The maximum temperature range in which the phase transition occurs, as measured operando in this study by heating and cooling the solar cell, is represented by the gray bar, while the red bar represents the phase transition range measured on the sample during only heating (as presented in Figures 2, & 3). As explained above the phase transition reported by Baikie et al. is denoted by the black dashed line.6 To avoid artifacts due to sample conditioning,27 the measurements were performed at constant time intervals. The protocol used for the measurement is as follows: (i) light on, (ii) current-voltage curve, (iii) XRD, (iv) light off, (v) 10 minute rest period. The device was not masked during the measurement and the illumination source was not spectrally calibrated; 9 ACS Paragon Plus Environment

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therefore, the device metrics obtained in these operando experiments should be looked at as relative trends and some care should be taken when looking at the absolute numbers. To determine the effect of the structural phase transition, we examine the device metrics as a function of temperature presented in Figure 4. Most importantly, there is no large effect or discontinuity in any of the performance parameters (ISC, VOC, or FF) induced by moving through the structural phase transition. The ISC measured from the device is nearly constant across all temperatures indicating little changes in charge collection efficiency. The VOC shows, as expected, a linear decrease as a function of temperature. Furthermore, when the VOC versus temperature data is linearly extrapolated to 0 K, a value of 1.69 ± 0.01 V is calculated which is similar to the bandgap of CH3NH3PbI3 (Figure S4).28,29 The variation in the FF as a function of temperature is not as easily explained as this is the result of many competing factors,14,30–32 including the other components in the device stack. To confirm our results, we have performed complementary ex situ measurements at 1Sun illumination on two CH3NH3PbI3 devices with identical device architectures but different perovskite deposition procedures. One device was fabricated using the same two-step deposition technique to form the CH3NH3PbI3 active layer as the device measured in operando (Figure S5), while in the other device (Figure 5) the CH3NH3PbI3 layer was formed via the Lewis acid-base adduct approach described by Park and coworkers (see Supplementary Information for details).4 To support the generality of our conclusions, we find similar trends in both devices as seen in the operando experiments and confirm that no discontinuity in device performance exists in this temperature range. It has also been reported that thermal expansion and strain across the phase transition can cause delamination of the device stack resulting in interfacial defects and decreased device performance.14 In the present experiments we do not see evidence for delamination, although this is likely strongly dependent on the details of the device’s interfaces. Taken together these data suggest that, although the phase transition does occur at modest temperatures, this change of the absorber should not be a concern for terrestrial operation of CH3NH3PbI3 devices.


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Figure 5. Reverse scan, ex-situ current density-voltage (JV) curves of a high efficiency TiO2/CH3NH3PbI3/spiro-OMeTAD solar cell taken under 100 mW cm-2 illumination. The efficiency parameters before (after) the temperature experiment were: short-circuit current density (JSC) = 22.08 (21.91) mA cm-2, VOC = 1.062 (0.986) V, FF = 0.723 (0.719), efficiency = 16.96 (15.61) %.

Using operando measurements, we demonstrate the reversibility of the tetragonal to cubic phase transition in CH3NH3PbI3 solar cells. We have presented methods for monitoring the structural characteristics of full device stacks operando and believe this is a broadly useful approach that can be applied to study photovoltaic devices under a number of different stimuli (i.e., heating/cooling, current/voltage stressing, light, and atmosphere). Furthermore, in the case of CH3NH3PbI3 based devices we find no discontinuity in the photovoltaic performance of these cells across this phase transition. This lack of discontinuity, coupled with our observation of I atom disorder, is consistent with a dynamic crystal structure.16 That is, while the average crystal structure, as probed by XRD, shows a transition from tetragonal to cubic; the structure is unchanged at the local level, it being this local structure which determines the material properties.

Acknowledgements: The authors wish to thank Tim Dunn, Valery Borzenets, Samuil Belopolskiy, and Doug Van Campen for help with the chamber design and operation. Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, was supported by the U.S. Department of Energy, Ofﬁce of Basic Energy Sciences under Contract No. DE-AC0211 ACS Paragon Plus Environment

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76SF00515. Work at the National Renewable Energy Lab (NREL) was supported by U.S. Department of Energy Office of Energy Efficiency under contract No. DE-AC36-08- GO28308 under the NREL Laboratory Director’s Research and Development program (JJB and JML) as well as the Hybrid Perovskite Solar Cell program of the National Center for Photovoltaics funded by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Solar Energy Technologies Office (JAC, LTS).

Supporting Information Available: Detailed experimental information, additional device characterization, 2D WAXS data, and detailed XRD refinements provided.

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Dang, Y.; Liu, Y.; Sun, Y.; Yuan, D.; Liu, X.; Lu, W.; Liu, G.; Xia, H.; Tao, X. Bulk 14 ACS Paragon Plus Environment

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ACS Energy Letters

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Monitoring a Silent Phase Transition in CH3NH3PbI3 Solar Cells via

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