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Temperature-Dependent Thermal Conductivity Study of MAPbI3: Finding the Thermal Percolation Threshold for Greatly Improved Heat Transport Anton Kovalsky, Lili Wang, Xin Guo, Jeffrey S. Dyck, and Clemens Burda J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b08231 • Publication Date (Web): 13 Oct 2017 Downloaded from http://pubs.acs.org on October 15, 2017

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Temperature-Dependent

Thermal

Conductivity

Study

of

MAPbI3: Using Mild Aging to Reach a Thermal Percolation Threshold for Greatly Improved Heat Transport

Anton Kovalsky,1 Lili Wang, 1 Xin Guo,1 Jeffrey S. Dyck,*,2 and Clemens Burda*,1 1

Department of Chemistry, Case Western Reserve University, Cleveland, OH 44106

2

John Carroll University, Department of Physics, University Heights, OH 44118

ABSTRACT: We report the thermal conductivity in the range from 7K to 300K for Methylammonium lead triiodide (MAPbI3) perovskite at different stages of environmental degradation. The results showed that thermal conductivity of the degraded perovskite pellet quickly approached that of lead iodide rather than pure MAPbI3, even while x-ray diffraction data of the degraded perovskite showed mostly peaks of the perovskite phase, alongside minor peaks from impurity phases belonging to the precursors and to an intermediate phase. For reference, a sample of pure lead iodide was also prepared and analyzed in the same way, and to our knowledge thermal conductivity for lead iodide in the 7-300K temperature range has not been published yet. The combination of thermal conductivity and materials characterization provide a more detailed knowledge of the composition and structural changes of aged MAPbI3 perovskite. Moreover, we demonstrate that a thermal percolation threshold can be achieved through mild aging while preserving most of the MAPbI3 crystal structure.

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1. INTRODUCTION Methylammonium lead triiodide (MAPbI3) perovskite, a hybrid orgnic-inorganic semiconducting material, has garnered much attention in the past half-decade, since the inception of the perovskite-sensitized solar cell.1 The excitement surrounding this material stems from impressive power conversion efficiency juxtaposed against the surprising ease and low cost of processing the compound. Lead iodide and methylammonium (MA) can be combined under mild solvothermal conditions to produce the photoactive material, which crystalizes in the ABX3 motif. However, the promising cost-effectiveness of processing this material is offset by its tendency to degrade, especially under elevated temperatures or humid conditions, with the methylammonium diffusing out of the network, leaving behind the lead iodide precursor and destroying the photovoltaic action. This thermal instability is one of the most pressing issues in developing perovskite-based technology.2–6 This problem is exacerbated by the remarkably low thermal conductivity of lead halide perovskites,7–9 which leads to poor heat dissipation, trapping heat in the active layer and promoting device degradation. We report that controlled atmospheric aging can actually improve thermal transport in the material long before the majority of the crystalline MAPbI3 has been degraded into its precursors. The degradative tendency and the mild synthesis conditions both stem from the looselybound nature of the methylammonium ion within the lattice. This loosely-bound nature is related to the ease with which methylammonium can be either introduced into or removed from the lattice. The ease of intercalation of the organic molecule into the inorganic lead iodide network is the basis for many common synthesis strategies reported for perovskite-based solar cells – in short, substrates with lead iodide already deposited are then immersed in a solution containing methylammonium, which easily penetrates the lead iodide lattice and transforms it into the


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MAPbI3 compound.10,11 On the other hand, the methylammonium can easily be leached, especially in humid atmospheric conditions, where ambient humidity can carry away the organic ion so that only PbI2 remains. Thermal measurements are well positioned for characterizing perovskite purity and quality both because of its intrinsic thermal instability12 and its unique thermal behavior related to the motion of the organic molecule in this hybrid material.8,13 Additionally, it will likely be important for the industry to develop robust thermal characterization, as thermal management is a crucial aspect of photovoltaic module design,14,15 and will certainly be a particularly crucial aspect for organic-inorganic perovskite, since its stability is so sensitive to operating temperature. In this work, we use thermal conductivity measurements to track the transformation of MAPbI3 as it ages, showing the reappearance of PbI2-like thermal behavior upon degradation, and demonstrating that tracking thermal conductivity is an effective strategy for monitoring the aging of perovskite. Importantly, our analysis is highly encouraging from the perspective of nanostructural engineering, as we demonstrate that mild atmospheric aging can take a sample of MAPbI3 through a thermal percolation threshold, greatly increasing its thermal conductivity while preserving much of the MAPbI3 content. The thermal percolation threshold strategy has been shown as an excellent means to achieving better heat dissipation in electronic materials16,17 as well as in thin films.18 Our findings highlight the potential for applying this strategy to perovskite-based devices. By comparing thermal conductivity of fresh and aged perovskite and pure lead iodide, and by tracking the development of the MAPbI3-like intermediate using X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDX),


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we show that a sample that can retain the crystalline nature of perovskite can also show a significant increase in thermal conductivity. Our results allude to the possibility of engineering high-thermal-conductivity channels in perovskite-based solar cells to improve device stability and lifetime.

2. METHODS Methylammonium lead iodide (MAPbI3) was prepared by mixing equimolar methylammonium iodide (Dyesol) and lead diiodide (99.9%, Fisher) in dimethylformamide (Anhydrous, Fisher) and allowing the solution to stir overnight at 700C. The 40 wt % solution was then dropped with a pipet onto a clean glass substrate and placed in a vacuum oven at 600C. The resulting yellow powder was scraped off the glass, ground with a mortar and pestle, and then heated to 1000C for 30 minutes under argon in a tube furnace. X-ray diffraction (XRD) confirmed that the resulting black powder was fully processed perovskite with no impurities. The powder was then loaded into a steel compaction die (General Precision co, Willoughby, OH) and compressed under a 1.62 MPa load. The resulting pellet was fitted with a differential constantanchromel-constantan thermocouple and an electronic heater and then mounted onto an oxygenfree high conductivity copper stage. The sample was then cooled to 7K under 10-5 Torr and thermal conductivity was obtained via the longitudinal steady state technique in the temperature range from 7K to 300K. Another sample of MAPbI3 was synthesized and then allowed to age under ambient conditions (humidity ~50%) for ten days and this aged powder was then compacted into a pellet for thermal conductivity measurement. Thermal conductivity of a pellet pressed just from the PbI2 powder was also measured for reference. The pellets used for thermal conductivity measurements were crushed and ground and


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used to obtain high resolution XRD (Phillips Xpert) (figure S1), and these data were analyzed using MDI Jade 6.5 software. All peaks were identified and indexed to either PbI2 or MAPbI3, and in the case of the aged perovskite sample, peaks belonging exclusively to the intermediate phase were identified by ruling out perovskite or lead iodide. Additional samples of powdered perovskite, also dried on glass plates under vacuum, heated to 1000C, and aged under ambient atmosphere, but never compressed into pellet form, were analysed in order to track the transformation of the material as it aged via XRD and scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX). These latter samples were characterized on a Rigaku X-ray diffractometer and Phenom ProX SEM/EDX at 10 kV accelerating voltage.

3. RESULTS AND DISCUSSION 3.1. Tracking Formation of Perovskite Intermediate The intermediate phase has been previously identified occurring at relative humidity around 60%.19 When the precursors are mixed in solution, they form a colloidal precursor complex which is not yet fully formed MAPbI3 perovskite, but is also distinct from the separate precursor components. This yellow precursor phase must be annealed to leave only the fully-reacted black MAPbI3 phase. By the same token, the perovskite must not be over processed – excessive heating leads to degradation back through an intermediate phase and then into the separated precursors, with the methylammonium being lost to the surrounding atmosphere.20 It is important to note that thorough identification of all XRD peaks is critical, as the existence of intermediate or complexed phases, as well as the low formation energy, lead to the high probability of impurities which can have a strong effect on energy


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Figure 1. X-ray diffraction spectra of a powdered perovskite sample, tracking the aging process under ambient conditions at 2, 4, 6, 9 and 12 days after synthesis.

transport-related properties. According to our results, aging of perovskite leads to strongly modified thermal behavior, even as XRD data shows only a small change between the fresh and the aged perovskite. The apparent insensitivity of the X-ray diffraction to the change in the material may be explained by the fact that intermediate or complexed phases can have peaks at similar values of 2θ as many reflections due to the complex symmetry21 of the unit cell. Impurities may thus be obscured in XRD results, necessitating a different approach to characterizing a particular sample’s functional purity. As our thermal conductivity measurements show, even though the aged sample’s diffractogram exhibits strong perovskite features and negligible lead iodide features, aged perovskite already begins to have elevated thermal conductivity, close to that of PbI2, especially at low temperature. Figure S1 shows high resolution XRD for the samples whose thermal conductivity was measured, and it is clear from this diffractogram of the aged perovskite (figure S1C)


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Figure 2. Scanning electron micrographs of perovskite powder 2 days (a), 4 days (b), and 12 days (c) after synthesis.

that the major peaks belong to the bulk perovskite lattice and the majority of nonperovskite peaks that appear upon aging belong to complexed perovskite-like intermediate phases, while only a few low intensity peaks are ascribed to pure lead iodide domains. Another set of powdered samples were prepared also by drying precursor solution and heat-curing on glass substrates, and then allowed to age under ambient conditions for up to two weeks. XRD (figure 1) and SEM (figure 2) characterization was performed on these samples to track the progression of the aging process, with the only difference between these samples and the ones used for thermal measurement being that only the samples used for thermal measurement underwent the compaction procedure to turn them into dense pellets. According to the XRD and SEM results, the progression of samples of MAPbI3 allowed to age under ambient atmospheric conditions is as follows (an illustration of this degradation scheme is presented in figure 3): From day 2 to day 4, perovskite aging results not in degradation but rather in the formation of larger crystalline domains. In the XRD spectra this is indicated by the


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Figure 3. Schematic of perovskite aging scheme. On the left, MAPbI3 is shown in black, PbI2 is shown in yellow, and the intermediate phase is shown in orange. On the left, the fresh MAPbI3 lattice is represented by MA-rich areas, the intermediate MAPbI3 is represented by MA-poor areas, and lead iodide domains are represented by areas devoid of MA (highlighted in yellow).


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increase in peak intensities from day 2 to day 4, and in the SEM images we can see a morphological change that indicates a redistribution and melding of this relatively soft material. A close examination of widths of the (110) and (220) peaks also shows a decrease in FWHM from day 2 to day 4, indicating the presence of larger grains. From day 4 to day 6, the only change we observe is the appearance of a lead iodide peak in the XRD spectrum at roughly 2θ = 12.5°, and by day 9, this peak has disappeared. We rationalize this observation as suggested in the schematic for days 6 and 9 in figure 3: Upon initial loss of methylammonium from near the surface of the grains, we observed the formation of lead iodide impurity at the grain boundaries near the perovskite/air interface, where the loosely bound methylammonium is most quickly and easily lost. However, as the aging progresses, the methylammonium ions trapped within the bulk of the grains begin to diffuse out, establishing a non-uniform concentration of methylammonium (figure 3, day 9, lattice renderings on right side). The resulting material is thus a perovskite-like compound which no longer exhibits the same thermal or optical properties (noted by the color change), but is nevertheless crystallographically similar to the perovskite structure (as indicated by XRD). The loss of stoichiometric equivalents of methylammonium to lead results in a quasi-perovskite lattice, with the exact stoichiometry of the compound varying with the depth within a particular grain. The decrease in peak intensities from day 6 to day 9 and then further by day 12 suggests that the loss of methylammonium leads to a significant decrease in crystallographic density (illustrated in the bottom panel of figure 3). According to the SEM images, we see a significant change in morphology from the smoother powder as it is on day 4 to the small and clearly separate grains on day 12.


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Figure 4. Thermal conductivities of pure PbI2, pure MAPbI3 and degraded MAPbI3 on a linear (a) and logarithmic (b) scale.

A central finding of our investigation is that while our sample remained crystallographically similar to perovskite, with minimal pure lead iodide domains, the thermal properties undergo a stark transformation, thus indicating that the thermal conductivity reported herein for an aged perovskite sample represents the thermal behavior of the intermediate phase, rather than simply a composite of the pure perovskite and pure lead iodide.

3.2 Temperature-Dependent Thermal Conductivity Figure 4 shows the temperature-dependent thermal conductivity for MAPbI3 before and after aging, as well as the pure PbI2 pellet. It is clear, particularly in the lowtemperature region, upon aging, the thermal behavior of the degraded perovskite pellet is more similar to that of PbI2 than MAPbI3. It is important to note that the upturn in thermal conductivity near room temperature in these plots is due to radiation loss inherent to the measurement. In figure 4B these results are presented on a logarithmic scale. This shows the power law dependence of the thermal conductivity in the temperature range between


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50K and 150K. The differing slopes of thus plotted thermal conductivity for PbI2 and MAPbI3 highlight the dissimilar thermal behavior of these two materials, specifically with respect to Umklapp (phonon-phonon) scattering mechanisms which are intrinsic to each type of unit cell. Interestingly, the degraded perovskite’s thermal behavior is somewhere between the two pure materials, both in terms of magnitude, and in the power law dependence in this temperature region, as can be noted in figure 4B in the inflection of the slope around 80K in the logarithmically plotted thermal conductivity of the degraded sample. Particular care was also taken in measuring the thermal conductivity around 161K, where it is known MAPbI3 experiences a tetragonal-to-orthorhombic phase transition.22 This was done by slowing down the measurement to accurately capture the thermal conductivity as the phase transition occurs. This result is highlighted in the inset in figure 4A. The phase transition is visible in the pure perovskite sample as a small downward step. Recently, Guo et al7 reported that this discontinuity at the phase transition is evidence of ferroelectric domain-related phonon scattering, as the low temperature orthorhombic phase would have narrower domain walls. They note that a downward jump was observed only in the thermal conductivity of a thin perovskite film on Si, but not in a polycrystalline pellet or a thin perovskite film in a mesoporous Al2O3 on Si. A preferential orientation of growth for perovskite on the pristine Si substrate was believed to be responsible for ferroelectric domain-related scattering. Here we find that the polycrystalline pellet does retain the observed step down, despite not having been formed on a Si surface. Furthermore, in our degraded perovskite sample, as in our pure PbI2 sample, this transition is no longer present. The absence of the downward step in Guo et


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al.’s perovskite-on-meso-Al2O3 may be attributed to the formation of impurities similar to those in our degraded sample. The inflection of thermal conductivity at ~80K represents a saturation point when there is a sufficient phonon population to activate perovskiterelated phonon modes in a degraded sample. Above 80K, the degraded sample’s thermal behavior bares unmistakable resemblance to the pure perovskite, and below 80K, the degraded sample conducts heat more similarly to pure lead iodide. Such an inflection can be attributed to the presence of lead iodide-like impurities in our sample as well as in the perovskite-impregnated alumina film analyzed by Guo et al, in which such impurities are likely formed at the interface between the perovskite and the alumina scaffold. The presence of the inflection point at 80K and absence of the downward jump at 161K in both our degraded perovskite and Guo et al’s perovskite-on-meso-Al2O3 film indicate that both samples had taken on the thermal behavior of a MA-poor phase, whose unique thermal signature is apparent above 80K, but whose perovskite domains are likely separated by lead iodide or intermediate impurities.23,24 The loss of the phase transition in Guo et al’s work may thus also be attributed to thermal degradation rather than a loss of preferential orientation. Whereas previous work has posited that preferentially oriented growth leads to specific thermal properties, our analysis indicates that phase purity is more important in determining thermal behaviour. Importantly, our results suggest the perovskite could transmit heat along channels of impurities.

4. Conclusion We conclude that thermal conductivity is a good parameter for tracking the degradation of organo-lead iodide perovskites. Whereas previous groups have used many other methods to


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Figure 5. (Left) Illustration of percolating pathways for thermal transport through a degraded perovskite. (Right) Electron-dispersive X-ray spectroscopic elemental mapping of lead (red) and nitrogen (blue) distribution in a powdered perovskite sample before and after aging.

track this change, such as by measuring absorbtion25 or by grazing incident wide angle x-ray scattering spectroscopy (GIWAXS),26 our results show that thermal conductivity is highly sensitive to the degradative transformation. It is potentially a more robust indicator of perovskite phase integrity than X-ray diffractometry, which must be analyzed with great care, since impurities from either lead halide precursors or complexed intermediate phases can be obscured. In the case of thermal measurements, it is likely that as methylammonium diffuses out of the system, it leaves behind channels of interconnected lead iodide or intermediate perovskitelike domains which serve as percolated paths of least resistance for thermal transport, as represented in figure 5, thus affecting the behavior of the sample as a whole, as the MA-poor domains may have a much higher thermal conductivity. According to elemental mapping, upon degradation the perovskite sample acquires domains in which the nitrogen content has been reduced while the lead content remains the same. This is evidence that lead iodide domains,


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which are known to be more thermally conductive, can be found in the degraded perovskite sample, and such impurities can easily serve as percolating paths for thermal energy transport. These findings should prove useful both from the perspective of thermal management in perovskite-based photovoltaic modules, especially as perovskites are known to have relatively low thermal conductivities,27 as well as from the perspective of optimal processing conditions. Where multiple researchers have tracked the development and degradation of the photoactive phase from the precursor phase to the degraded material with respect to processing parameters such as heating28 or preparation time,29 we posit that such dependences could be robustly monitored by thermal measurements, as thermal properties of this material are highly dependent on successful and complete intercalation of methylammonium ions into the lead iodide matrix. Moreover, the appearance of some lead iodide impurities has been recently implicated in the passivation of surface trap states in MAPbI3, leading to longer excited state lifetimes23 and higher open circuit voltages30 due to reduced recombination at grain boundaries.24 Our results also point to the significance of lead iodide impurities to the excellent physical properties of MAPbI3. PbI2 intercalation can be used in conjunction with understanding passivating effects to find optimum conditions in perovskite-based device processing. Our finding suggests the possibility that nanostructuring of perovskite grains may be used to marry the benefits of lead iodide’s trap passivating behavior with its high thermal conductivity, which will greatly improve heat dissipation in perovskite-based photovoltaic devices, thus paving the way for more efficient and more stable perovskite solar cells. As we have shown, the same type of mild atmospheric aging that leads to improved photovoltaic performance in MAPbI3-based solar cells, can also be used to achieve a thermal percolation threshold that greatly improves thermal transport in the material. We hope our analysis will inspire nanoscale structuring of perovskite with MA-poor


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regions that can simultaneously provide the thermal and electronic benefits of such impurities.

AUTHOR INFORMATION Corresponding Authors Prof. Clemens Burda, [email protected], 216.368.5918. Prof. Jeffrey S. Dyck, [email protected], 216.397.4560.

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Supporting Information. High resolution X-ray diffractograms (Phillips Xpert) of (a) pure MAPbI3, (b) pure PbI2, and (c) aged MAPbI3

Acknowledgment We gratefully acknowledge support from Case Western Reserve University.


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Temperature-Dependent Thermal Conductivity ... - ACS Publications

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