Ultralow Thermal Conductivity and Ultrahigh Thermal Expansion of

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C: Energy Conversion and Storage; Energy and Charge Transport

Ultralow Thermal Conductivity and Ultrahigh Thermal Expansion of Single Crystal Organic-Inorganic Hybrid Perovskite CHNHPbX (X=Cl, Br, I) 3

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Chunyu Ge, Mingyu Hu, Peng Wu, Qi Tan, Zhizhong Chen, Yiping Wang, Jian Shi, and Jing Feng J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b05919 • Publication Date (Web): 02 Jul 2018 Downloaded from http://pubs.acs.org on July 2, 2018

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Ultralow Thermal Conductivity and Ultrahigh Thermal Expansion of Single Crystal OrganicInorganic Hybrid Perovskite CH3NH3PbX3 (X=Cl, Br, I) Chunyu Ge,†,§ Mingyu Hu,†,§ Peng Wu,† Qi Tan,† Zhizhong Chen,‡ Yiping Wang,‡ Jian Shi,*,‡ and Jing Feng*,† †

Faculty of Material Science and Engineering, Kunming University of Science and Technology, Kunming 650093, China



Department of Materials Science and Engineering, Rensselaer Polytechnic Institute Troy, NY 12180, USA

Corresponding Author *

E-mail: [email protected]

*

E-mail: [email protected]

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ABSTRACT

Improving device lifetime and stability remains the stumbling-block of the commercialization of hybrid perovskite based devices (HPDs). While extensive efforts have been paid, thermal property, one of the most crucial parameters in conventional solid-state electronic devices, has rarely been studied for HPDs. Here, we investigate the temperature-dependent ultralow thermal conductivity and ultrahigh thermal expansion of single-crystalline MAPbX3 (MA=CH3NH3), which are found distinct from traditional thin-film solar cells materials. Particularly, for MAPbI3, thermal conductivity is observed being only 0.3 W·m-1·K-1 and linear thermal expansion coefficient along [100] direction is as high as 57.8×10-6 K-1 (tetragonal) and much higher at the structural phase transition point. We attribute the ultralow thermal conductivity and ultrahigh thermal expansion to the weak chemical bonds associated with the soft perovskite materials. These unique properties can be very challenging for the multilayer devices design, but their ultralow thermal conductivity may unveil a new thermoelectric material concept.

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Introduction Organic-inorganic hybrid perovskite MAPbX3 is attractive for the development of highperformance optoelectronic devices such as solar cells,1-3 lasers,4, 5 light-emitting diodes,6, 7 and photodetectors.8, 9 Recently MAPbI3 solar cells have reached more than 22% power conversion efficiency.10 Nevertheless, devices’ stability and lifetime have been hindrances to their commercialization. Since advanced encapsulation technology has been keeping solar cells from moisture, oxygen and ultraviolet light,11-14 the intrinsic stability of the material and structure itself become extremely important. Especially, HPDs are generally of multilayer-structure, the accumulation of heat in working period can reduce device’s useful lifetime, transmission efficiency, and the following mismatching of thermal expansion between each layer may lead to the decoupling of components even degradation of perovskite layer.11 Thermal expansion (α) and thermal conductivity (κ) are valuable references for evaluating the thermal stress and thermal diffusion that the material will undergo upon temperature cycling. Using temperature-dependent X-ray diffraction11,

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and atomic force microscopy,16 linear

thermal expansion coefficient of MAPbI3 have been measured indirectly. It was found the α (MAPbI3) is large. Particularly around the phase transition temperature, values are even over 400×10-6 K-1.16 To minimize thermal stress in HPDs, the accurate value for α (MAPbX3) is yet to be determined. However, the non-macro measurements so far give rather dispersed values (see Table S1). In this regard, without the noise from grain boundaries, secondary phases and tiny grain size etc., single-crystal-based macro measurements are needed. With the mature of MAPbX3 single crystal growth technology,17-19 it is promising to use single crystals to study these intrinsic properties. Lots of researches have showed that perovskite single crystals exhibit

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long carries diffusion length (175±25 µm), remarkably low trap-state densities, while their carrier mobility reaches 164±25 cm2·V-1·S-1.19 The high power conversion efficiency may root in the above superior properties. In addition to α, κ can also be obtained from the single crystal. To our knowledge, there are some measurements of κ (MAPbI3).16, 20-22 However, these measurements give dispersed results as well (see Table S1) and the temperature dependence of thermal conductivity has not been studied yet. Therefore, to shed light on fundamental properties as well as device design, it’s necessary to measure the α and κ using bulk single crystals of MAPbX3. In our work, resorting to anti-solvent vapor-assisted crystallization (AVC)17 and inverse temperature crystallization (ITC)18 method respectively, high-quality, centimeter-scale single crystals of MAPbCl3, MAPbBr3 and MAPbI3 were grown. Using these single crystals, we first investigated the thermal properties of MAPbX3 systematically.

Results and discussion

Figure 1. Single crystal photographs: a: MAPbCl3 (AVC); b: MAPbBr3 (AVC); c: MAPbI3 (ITC) Centimeter-sized MAPbX3 single crystals with crack-free, smooth surfaces, distinguishable facets (Figure 1a, b, c) were successfully grown. The MAPbCl3 and MAPbBr3 crystals showed

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uniform color and high transparency, consistent with their bandgaps. The shapes of all the crystals obeyed Wulff construction and were consistent with previous reports.18,

19, 23-25

It is

worth mentioning that MAPbI3 single crystals were obtained by ITC, because we cannot get large enough samples by AVC. We identified these crystals by X-ray diffraction and used Tauc plots to get their optical band gaps in Figure S1, these parameters are very close to the values in earlier reports.17-19, 24

Figure 2. Thermal properties of MAPbX3 (X = Cl, Br, I) single crystals: a: Temperaturedependent heat capacity Cp (* is the work by Onoda-Yamamuro et al.26); b: The temperature dependence of the thermal diffusion coefficient D; c: The thermal conductivity κ as a function of temperature; d-f: Theoretical thermal conductivity of MAPbX3 (X = Cl, Br, I). By merit of the high-quality bulk single crystals we have grown, the thermal properties of MAPbX3 such as heat capacity, heat transport properties, and thermal expansion properties were

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accurately measured, shown in Figure 2. We complemented the Cp data above room temperature by differential scanning calorimetry (DSC) (Figure 2a), which are in good connection with the highly reliable Cp data below room temperature tested by Onoda-Yamamuro et al.26 The Cp of MAPbX3 become lower in accordance with the order of Cl, Br and I. Meanwhile, there are obviously peaks in the heat capacity curves for MAPbX3, which are related to the phase transformation (orthorhombic-tetragonal-cubic) from low temperature to high temperature. Especially for MAPbI3, its phase transmission from tetragonal to cubic near 58.9 ℃ may have a great impact on practical application. Thermal diffusion coefficient in Figure 2b were measured by the laser flash method commonly recognized by the international thermophysics community to have higher precision.27 As the κ and D follow eq 1 ߢ = ‫ܥߩܦ‬௣

(1)

where ρ is density (measured by Archimedes method in Table S3), κ was calculated and shown in Figure 2c. At room temperature, κ are 0.52±0.04 W·m-1·K-1, 0.37±0.04 W·m-1·K-1, and 0.30±0.01 W·m-1·K-1 for MAPbX3 (X=Cl, Br, I) respectively. These values are ultra-low and closer to the previous reports.16 Similar to the Cp, heat transport properties have the same decreased regularity with the order of Cl, Br and I on account of the gradually reduced trend of electronegativity. The thermal transport properties of MAPbI3 are also affected by the phase transition. When the two phases co-exist, the D of MAPbI3 decreases dramatically due to the disordered structure, but κ does not change significantly due to the increase of Cp. By using the model developed by Cahill et al., the amorphous limit of thermal conductivity (κmin) was estimated. As shown in Figure 2d-f, thermal conductivities of MAPbX3 are close to κmin at the high temperature. With density-functional theory (DFT) and Slack model28, 29(details

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in Supporting Information), the lattice thermal conductivities of polycrystalline MAPbX3 were calculated. Based on the Wiedemann-Franz law, electronic contribution to the thermal conductivity is negligible within MAPbX3.21 The theoretical thermal conductivities agree well with experimental thermal conductivities measured by the single crystals. Simultaneously Table S5 showed the low Debye temperature and low acoustic velocity, which represent the weak strength of the average chemical bonds and are also the reason of low thermal conductivity.

Figure 3. a: The temperature dependence of thermal expansion dL/L0; b: linear thermal expansion coefficient α; Insets: complete plot of α for MAPbI3. The temperature-dependent thermal expansion dL/L0 and linear thermal expansion coefficient α were directly measured by thermomechanical analysis (TMA) reflecting thermal expansion

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properties macroscopically. (Figure 3) As temperature goes up, dL/L0 of MAPbX3 (X=Cl, Br) is increased, indicating the regular expansion of samples. The stable value for thermal expansion coefficient of MAPbCl3 is 29.3×10-6 K-1 (cubic phase in [100] direction) and MAPbBr3 is 31.9×10-6 K-1 (cubic phase in [100] direction). Whereas at 56 ℃,due to the phase transition, there is a sudden expansion for MAPbI3, concurrently thermal expansion coefficient dramatically grows from 57.8×10-6 K-1 (tetragonal phase in [100] direction) to 263.5×10-6 K-1 and finally decrease to approximately 39.0×10-6 K-1 (cubic phase in [100] direction). It can be found that the phase transition temperatures are a little different between thermal expansion (56 ℃) which is closer to literature value30-32 and heat capacity (58.9 ℃). Here it can be ascribed to the higher temperature increasing rate of heat capacity (see Supporting Information) and the dynamic thermal expansion test is more sensitive to phase transitions. Table 1. Grüneisen constant and thermal conductivity in different directions. 150℃

Direction

γtheory

γexperiment

(100)

1.29

0.91

(110)

1.85

0.32

(111)

1.94

0.42

Polycrystalline

1.67

0.39

(100)

1.22

(110)

1.87

0.25

(111)

1.99

0.40

Polycrystalline

1.67

0.33

(100)

1.14

0.35

MAPbI3

(110)

1.84

(cubic)

(111)

1.96

0.36

Polycrystalline

1.61

0.28

MAPbCl3

MAPbBr3

1.02

1.17

κtheory

κexperience

0.47

0.45

0.40

0.20

0.35

0.28

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The anharmonicity of lattice vibrations can be characterized by Grüneisen constant γ. γ (Table 1) was calculated by the thermal expansion coefficient.33 It was close to the values calculated from the DFT elastic constant. Utilizing the γ and acoustic velocity in different directions, we got the κ in different directions consistent with the experimental values. Although the γ derived from the elastic constant has obvious anisotropy, the anisotropy of the thermal conductivity is not salient owing to the anisotropy of acoustic velocity. The γ of MAPbX3 is not large, which means the non-harmonic vibration does not contribute much to its low κ. We think this may be connected to the high symmetry of the cubic perovskite structure. In other words, the ultra-low κ may not be directly related to the perovskite structure.

Figure 4. 2D map: Thermal expansion coefficient vs. thermal conductivity of MAPbX3 (X = Cl, Br, I) and other traditional transport layer and absorption layer materials. The data can be found in Table S1.

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To further discuss the thermal properties of MAPbX3, thermal expansion coefficient vs. thermal conductivity of MAPbX3 (X = Cl, Br, I), the materials of electron transport layer, and other absorption layer materials are summarized in Figure 4. Obviously, MAPbX3 possess ultralow κ and ultrahigh α. Some articles ascribe the low thermal conductivity to the organic ions and the perovskite structure.21, 34, 35 But apparently in Figure 4 the κ for CsPbBr3 and CsSnI3 are analogously very low which are around 0.4 W·m-1·K-136 close to MAPbX3, while the thermal conductivity of traditional oxide perovskites materials are not low.37-39 This further suggests that the low κ of the halogen perovskite is mainly caused by the weak interaction in Pb/Sn-X framework. On the other hand, compared with CsPbX3, κ of MAPbX3 are lower. The more complicated composition and larger number of atoms in the organic cations help to further decrease κ whereas it was not the dominant reason to low κ. Moreover, since α is closely related to the strength and stiffness of lattice, we also attribute the high α (MAPbX3) to the weak interaction in Pb/Sn-X framework, which echoes its ultra-low κ. As shown in Figure 4, their prominent ultralow κ and high α of MAPbX3 locate them at the upper-left corner, indicating major incompatibility between their thermal properties with commercial transport layer materials and substrates. For HPDs, due to the ultralow κ, accumulation of heat is inevitable in working period. the mismatching between every layers may generate strain and cause instability of device.11 It should be noted that the aforementioned thermal degradation can be partially solved by the anisotropy of thermal expansion of MAPbX3. The α (MAPbX3) along [001] is negative.15, 16 If the perovskite layer can be grown such that its [001] is parallel to substrate, the mismatch in interlayer α can be reduced significantly. Luckily, there are some methods to crystallize perovskite films as described above.40,

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In addition, it is also helpful to develop new

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transportation layer and substrates materials whose thermal properties have a better matching with MAPbX3. Dialectically thinking, although the ultralow κ of halogen perovskite is a hidden threat for device stability, it also makes halogen perovskite a promising potential thermoelectric material. Their thermal conductivities are lower than the typical thermoelectric materials SnSe (0.7),42 Bi2Te3 (0.77),43 Bi2S3 (0.64).44 Surprisingly, a low κ and relatively high electrical conductivity 282 S·cm-1 materials CsSnI3 was found.36 Furthermore, the insights obtained from this work suggest materials with Pb/Sn-X but of non-perovskite structure might have low thermal conductivity and hold promise for thermoelectric application. This may unveil a new thermoelectric material system.

Conclusions In this work, centimeter-sized and high quality MAPbX3 single crystals were grown. Using laser flash method, we find the ultralow thermal conductivity of MAPbX3 ranges from 0.3 to 0.52 W·m-1·K-1, being attributed to the weak lattice framework and phonon scattering from organic cation. The thermal expansion (MAPbX3) along [100] direction measured by thermomechanical analysis is found higher than 30×10-6 K-1. Ultralow thermal conductivity and ultrahigh thermal expansion might be a challenge for device stability under environments with severe thermal fluctuation. The discovery and understanding suggests that extra consideration such as phase transition-type thermal stress is needed in designing photovoltaic module based on halide perovskite. ASSOCIATED CONTENT Supporting Information

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Experimental methods describing synthesis and characterization, calculation methods details, X-ray diffraction (XRD), crystal structure, optical band gaps, DSC, transmission spectra, thermal conductivity and thermal expansion coefficient data of MAPbX3 and traditional absorption layer or transport layer materials in solar cells, Rietveld profile refinement details, density of MAPbX3, bond dissociation energy of Pb-X, calculated acoustic velocity, Debye temperature, Young’s modulus and bulk modulus in different directions. AUTHOR INFORMATION Corresponding Author *

E-mail: [email protected] Tel: +86-015925200330

*

E-mail: [email protected] Tel: 573-239-8872

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. §Chunyu Ge and Mingyu Hu contributed equally to this work. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was funded by the National Natural Science Foundation of China under Grant No. 11504146. REFERENCES

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(27) Parker, W. J.; Jenkins, R. J.; Butler, C. P.; Abbott, G. L. Flash method of determining thermal diffusivity, heat capacity, and thermal conductivity. J. Appl. Phys. 1961, 32, 1679-1684. (28) Tian, Z.; Zheng, L.; Wang, J.; Wan, P.; Li, J.; Wang, J. Theoretical and experimental determination of the major thermo-mechanical properties of RE2SiO5 (RE = Tb, Dy, Ho, Er, Tm, Yb, Lu, and Y) for environmental and thermal barrier coating applications. J. Eur. Ceram. Soc. 2016, 36, 189-202. (29) Zhao, M.; Ren, X.; Yang, J.; Pan, W. Thermo-mechanical properties of ThO2-doped Y2O3 stabilized ZrO2 for thermal barrier coatings. Ceramics International 2016, 42, 501-508. (30) Kawamura, Y.; Mashiyama, H.; Hasebe, K. Structural study on cubic-tetragonal transition of CH3NH3PbI3. J. Phys. Soc. Jpn. 2002, 71, 1694-1697. (31) Stoumpos, C. C.; Malliakas, C. D.; Kanatzidis, M. G. Semiconducting tin and lead iodide perovskites with organic cations: phase transitions, high mobilities, and near-infrared photoluminescent properties. Inorg. Chem. 2013, 52, 9019-9038. (32) Poglitsch, A.; Weber, D. Dynamic disorder in methylammoniumtrihalogenoplumbates (II) observed by millimeter-wave spectroscopy. J. Chem. Phys. 1987, 87, 6373-6378. (33) N., B. V. The acoustical Grüneisen constants of solids. Tech. Phys. Lett. 2004, 30, 91-93. (34) Hata, T.; Giorgi, G.; Yamashita, K. The effects of the organic-inorganic interactions on the thermal transport properties of CH3NH3PbI3. Nano Lett. 2016, 16, 2749-2753. (35) Yue, S.-Y.; Zhang, X.; Qin, G.; Yang, J.; Hu, M. Insight into the collective vibrational modes driving ultralow thermal conductivity of perovskite solar cells. Phys. Rev. B 2016, 94, 115427.

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(36) Lee, W.; Li, H.; Wong, A. B.; Zhang, D.; Lai, M.; Yu, Y.; Kong, Q.; Lin, E.; Urban, J. J.; Grossman, J. C. et al. Ultralow thermal conductivity in all-inorganic halide perovskites. Proc. Natl. Acad. Sci. U.S.A. 2017, 114, 8693-8697. (37) Yu, C.; Scullin, M. L.; Huijben, M.; Ramesh, R.; Majumdar, A. Thermal conductivity reduction in oxygen-deficient strontium titanates. Appl. Phys. Lett. 2008, 92, 191911. (38) Ohta, K.; Yagi, T.; Taketoshi, N.; Hirose, K.; Komabayashi, T.; Baba, T.; Ohishi, Y.; Hernlund, J. Lattice thermal conductivity of MgSiO3 perovskite and post-perovskite at the coremantle boundary. Earth Planet. Sci. Lett. 2012, 349-350, 109-115. (39) Chen, L.; Jiang, Y.; Chong, X.; Feng, J. Synthesis and thermophysical properties of RETa3O9 (RE = Ce, Nd, Sm, Eu, Gd, Dy, Er) as promising thermal barrier coatings. J. Am. Ceram. Soc. 2018, 101, 1266-1278. (40) Deng, Y. H.; Peng, E.; Shao, Y. C.; Xiao, Z. G.; Dong, Q. F.; Huang, J. S. Scalable fabrication of efficient organolead trihalide perovskite solar cells with doctor-bladed active layers. Energ. Environ. Sci. 2015, 8, 1544-1550. (41) Dong, Q.; Yuan, Y.; Shao, Y.; Fang, Y.; Wang, Q.; Huang, J. Abnormal crystal growth in CH3NH3PbI3−xClx using a multi-cycle solution coating process. Energ. Environ. Sci. 2015, 8, 2464-2470. (42) Zhao, L. D.; Lo, S. H.; Zhang, Y.; Sun, H.; Tan, G.; Uher, C.; Wolverton, C.; Dravid, V. P.; Kanatzidis, M. G. Ultralow thermal conductivity and high thermoelectric figure of merit in SnSe crystals. Nature 2014, 508, 373-377.

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(43) Zhang, Q.; Ai, X.; Wang, L.; Chang, Y.; Luo, W.; Jiang, W.; Chen, L. Improved thermoelectric performance of silver nanoparticles-dispersed Bi2Te3 composites deriving from hierarchical two-phased heterostructure. Adv. Funct. Mater. 2015, 25, 966-976. (44) Pei, J.; Zhang, L.-J.; Zhang, B.-P.; Shang, P.-P.; Liu, Y.-C. Enhancing the thermoelectric performance of CexBi2S3 by optimizing the carrier concentration combined with band engineering. J. Mater. Chem. C 2017, 5, 12492-12499.

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Figure 1. Single crystal photographs: a: MAPbCl3 (AVC); b: MAPbBr3 (AVC); c: MAPbI3 (ITC) 177x54mm (300 x 300 DPI)

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The Journal of Physical Chemistry

Figure 2. Thermal properties of MAPbX3 (X = Cl, Br, I) single crystals: a: Temperature-dependent heat capacity Cp (* is the work by Onoda-Yamamuro et al.26); b: The temperature dependence of the thermal diffusion coefficient D; c: The thermal conductivity κ as a function of temperature; d-f: Theoretical thermal conductivity of MAPbX3 (X = Cl, Br, I). 283x172mm (300 x 300 DPI)

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Figure 3. a: The temperature dependence of thermal expansion dL/L0; b: linear thermal expansion coefficient α; Insets: complete plot of α for MAPbI3. 139x201mm (300 x 300 DPI)

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The Journal of Physical Chemistry

Figure 4. 2D map: Thermal expansion coefficient vs. thermal conductivity of MAPbX3 (X = Cl, Br, I) and other traditional transport layer and absorption layer materials. The data can be found in Table S1. 177x135mm (300 x 300 DPI)

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TOC Graphic 82x44mm (300 x 300 DPI)

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