Thermal Conductivity of Methylammonium Lead Halide Perovskite

Dec 1, 2017 - Here the thermal conductivity of methylammonium lead halide perovskite (CH3NH3PbX3 X= I, Br, and Cl) single crystals and thin films is a...
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Thermal Conductivity of Methylammonium Lead Halide Perovskite Single Crystals and Thin Films – A Comparative Study Ralf Heiderhoff, Tobias Haeger, Neda Pourdavoud, Ting Hu, Mine AlKhafaji, Andre Mayer, Yiwang Chen, Hella-Christin Scheer, and Thomas Riedl J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b11495 • Publication Date (Web): 01 Dec 2017 Downloaded from http://pubs.acs.org on December 2, 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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Thermal Conductivity of Methylammonium Lead Halide Perovskite Single Crystals and Thin Films – A Comparative Study Ralf Heiderhoffa,,c,*, Tobias Haegera,c, Neda Pourdavouda,c, Ting Hua,d Mine Al-Khafajia,c, Andre Mayerb,c, Yiwang Chend, Hella-Christin Scheerb,c, and Thomas Riedla,c a

Institute of Electronic Devices, University of Wuppertal, Rainer-Gruenter-Str. 21, 42119 Wuppertal, Germany b Microstructure Engineering, University of Wuppertal, Rainer-Gruenter-Str. 21, 42119 Wuppertal, Germany c Institute of Polymer Technology, University of Wuppertal, Rainer-Gruenter-Str. 21, 42119 Wuppertal, Germany d College of Chemistry/Institute of Polymers, Nanchang University, 999 Xuefu Avenue, Nanchang 330031, China ABSTRACT Thermal management in devices like solar cells, light emitting diodes and lasers based on hybrid halide perovskite thin films is expected to be of paramount importance for optimal performance and reliability. As of yet, experimental data of thermal properties of non-Iodine based hybrid halide perovskites is very scarce. Here, the thermal conductivity of methylammonium lead halide perovskite (CH3NH3PbX3 X= I, Br, and Cl) single crystals and thin films is analyzed by Scanning Near-field Thermal Microscopy. The thermal conductivity of CH3NH3PbX3 single crystals with X= I, Br, and Cl is found to be 0.34±0.12W/(mK), 0.44±0.08W/(mK), and 0.50±0.05W/(mK) at room temperature, respectively. Strikingly, similar thermal conductivities are determined for the corresponding thin-film samples. The thermal conductivity of MAPbI3 in the cubic phase (T > 55°C) increases to (1.1±0.1) W/(mK). In addition, the temperature-dependence of the thermal conductivities and of thermal expansion coefficients of MAPbI3 around the phase transition from the tetragonal to cubic phase are presented. INTRODUCTION Power conversion efficiencies of organic–inorganic hybrid perovskite solar cells (PSCs) have rapidly increased to a level of 22.1% 1. Aside from photovoltaics, this family of materials also holds great promise for future applications in light emitting diodes (LEDs)

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and lasers

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.

Despite their relatively simple processing technology their optoelectronic properties are almost comparable with the most successful inorganic semiconductors 5–7. In a wide range of device applications, thermal management becomes more and more important, because both 1 ACS Paragon Plus Environment

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lifetime and performance are influenced by temperature or temperature gradients 8. This is especially true for future perovskite based lasers, whose prospects of electrical or continuous wave operation are intimately linked to a sophisticated thermal management

9,10

. Aside from

intrinsic thermally induced loss mechanisms, thermally activated decomposition of hybrid halide perovskites may become an issue

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. Intrinsic thermal properties should be

investigated to further explore the potential in photovoltaics, as they influence electronic properties (for instance carrier mobility) as well as carrier dynamics 15. Thus, the analysis of thermal transport in these organic–inorganic hybrid perovskites is indispensable. In the past, only a few studies have been devoted to the thermal properties of hybrid 16–21

perovskites

. The heat capacity of methylammonium (MA) lead halide (CH3NH3PbX3;

X= I, Br, and Cl) perovskites has been measured by Onoda-Yamamuro et al. 16. Heating the MAPbI3 samples above room temperature, a tetragonal to pseudocubic polymorph transition (330 K) was observed while the orthorhombic to tetragonal phase change and the transitions of all other CH3NH3PbX3 perovskites occur at substantially lower temperatures (149-236 K). Pisoni et al. performed the first thermal conductivity measurements of MAPbI3 for temperatures below 300K

19

. The analysis was performed by a steady state bar technique

gluing heater and thermocouples to the sample and to the reference. The room temperature thermal conductivity λ was determined to be 0.5 W/(mK) and 0.3 W/(mK) for a single crystal and a polycrystalline sample, respectively. The same thermal conductivity was found for a polycrystalline sample measuring the effect of the methylammonium ion on phonon scattering 22

. The low thermal conductivity of the MAPbI3 single crystal was mainly attributed to

rotational motion of the MA cations thermoelectric (TE) applications

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and has attracted intense attention for possible

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. A high Seebeck coefficient of 0.82 mV/K

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, a high

charge carrier mobility in the range of 5–10 cm2/(Vs) for electrons and within 1–5 cm2/(Vs) for holes

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, as well as a high carrier diffusion length >1 µm

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were reported. The thermal

conductivity of a MAPbI3 perovskite single crystal was also evaluated in the temperature range of 299 K to 424 K using a commercial laser flash system

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. Values of around 0.30–

0.42 W/(mK) were observed. Similar values were found for films using the time-domain thermos-reflectance (TDTR) method in dependent on the modulation frequency 28. In stiking contrast to all other reports, an extremly high thermal conductivity of densely packed MAPbI3 films of 11.2 ± 0.8W/(mK) at room temperature has been claimed by Chen et al. 29. Equilibrium molecular dynamics simulations of the temperature-dependent lattice thermal conductivity of MAPbI3 have been conducted

30,31

. A very low thermal conductivity of

0.59 W/(mK) was found in the tetragonal phase at room temperature attributed to a low group 2 ACS Paragon Plus Environment

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velocity of acoustic phonons due to their low elastic stiffness and the strong anharmonicity 30. This value is in good agreement with the experimental results, whereas a much higher thermal conductivity has been predicted for the pseudocubic phase (1.80 W/(mK) at 330 K). It has been argued that the lower thermal conductivity in the tetragonal phase was due to the shorter lifetimes of the optical phonons and the smaller group velocity of acoustic phonons than that in the pseudocubic phase. On the other hand the thermal conductivity of all MAPbI3 polymorphs has been predicted to be below 1 W/(mK), and as low as 0.31 W/(mK) at room temperature

31

. Such ultralow thermal conductivity as a result of enhanced phonon–phonon

scattering from highly-overlapped phonon branches hints to short phonon lifetimes ( 57°C attributed to higher group velocities of acoustic phonons in the pseudocubic structure as predicted earlier 30. In addition the thermal expansion coefficient has been measured simultaneously by AFM within the same range of temperatures (see Figure 5). The resulting linear thermal expansion coefficients are αc-tet = (-5.1±0.4)·10-4 K-1 and αa-cub: (4.6±0.4)·10-4 K-1 for the tetragonal and pseudocubic phase, respectively.

Figure 5: Determination of linear thermal expansion coefficients and the temperature-induced phase transition from a 2mm thick MAPbI3 single crystal. While αc-tet, determined for T60°C, is one order of magnitude higher in our single crystals compared

to numbers determined for thin films. The phase transition occurs at (52±1)°C and agrees well with earlier reports of (54±1)°C 53. A somewhat higher temperature for the phase transition of (57±1)°C has been determined from calorimetric and IR analysis

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as well as from using

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equilibrium molecular dynamics simulations . CONCLUSIONS In summary, for the first time a comparative experimental assessment of the thermal conductivity of methylammonium lead halide perovskite (CH3NH3PbX3 X= I, Br, and Cl) single crystals and thin films has been performed. Similar thermal conductivities are measured for crystals and densely packed thin perovskite films. We could experimentally verify, that perovskites with decreasing atomic number of the halide possess a slight higher average thermal conductivity. For the first time, the temperature-dependent thermal conductivity as well as the change of the thermal expansion coefficients of a cleaved MAPbI3 single crystal are detected around the temperature-induced phase transition. The thermal conductivity of MAPbI3 single crystals in 9 ACS Paragon Plus Environment

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the cubic structure increases to (1.1±0.1) W/(mK). The absolute value of the negative linear thermal expansion coefficients in the tetragonal structure of (-5.1±0.4)·10-4 K-1 is found be quite similar to the positive linear thermal expansion coefficients in the cubic structure of (4.6±0.4)·10-4 K-1. We believe that our insights will be of great general importance for the thermal design of thin-film devices based on hybrid halide perovskites. ASSOCIATED CONTENT

Supporting Information preparation of single crystals by the inverse temperature crystallization procedure; thin film deposition and PHP; optical and SEM micrographs of single crystal; SEM micrographs of MAPbI3 perovskite thin film surfaces; experimental details on quantitative thermal conductivity measurements using SThM; results after irradiating the sample with varied electron beam doses AUTHOR INFORMATION *

Corresponding Author

Ralf Heiderhoff Institute of Electronic Devices, University of Wuppertal Rainer-Gruenter-Str. 21, 42119 Wuppertal, Germany e-mail: [email protected] telephone number: +49-202-439 1966

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT This work was supported by the Deutsche Forschungsgemeinschaft (DFG) under the project number HE2698/7-1. We acknowledge the German Federal Ministry for Education and Research (Grant No. 13N13819) for financial support. REFERENCES (1)

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