Structural Phase- and Degradation-Dependent Thermal Conductivity

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Structural Phase- and Degradation-Dependent Thermal Conductivity of CHNHPbI Perovskite Thin Films 3

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Zhi Guo, Seogjoon Yoon, Joseph S. Manser, Prashant V. Kamat, and Tengfei Luo J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b00513 • Publication Date (Web): 16 Mar 2016 Downloaded from http://pubs.acs.org on March 17, 2016

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The Journal of Physical Chemistry

Structural Phase- and Degradation-Dependent Thermal Conductivity of CH3NH3PbI3 Perovskite Thin Films Zhi Guo†§*, Seog Joon Yoon¶‡, Joseph Manser¶⌂, Prashant V. Kamat¶‡⌂, and Tengfei Luo§* § Department of Aerospace and Mechanical Engineering, University of Notre Dame, Notre Dame, Indiana, 46556, USA ¶ Radiation Laboratory, University of Notre Dame, Notre Dame, IN 46556, USA ‡ Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, IN 46556, USA ⌂ Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, IN 46556, USA

ACS Paragon Plus Environment

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ABSTRACT Organic-inorganic lead halide perovskites have shown great promise in photovoltaics and optoelectronics. In these applications, device performance and reliability can be strongly influenced by thermal transport in the materials. Through laser pump-probe experiments, different microstructures of CH3NH3PbI3 perovskite thin films are found to give rise to different phonon scattering mechanism. The thermal conductivity in CH3NH3PbI3 neat film decreases with temperature. Even though this agrees with the behavior of its bulk crystalline counterparts, an apparent thermal conductivity change near the structural phase transition temperature of this perovskite (orthorhombic vs. tetragonal) has only been observed in the spin-coated films. Analyses suggest that this may be attributed to either an energy landscape change related to organic cation disorder or the thickness change of ferroelectric domain walls formed in the neat perovskite films that affects the phonon scattering at the domain boundaries. In contrast, no thermal conductivity discontinuity has been observed in the CH3NH3PbI3/Al2O3 mesostructured films and the thermal conductivity first shows an increasing trend at low temperature (< 80 K) and then stays nearly constant. Such a trend is typical in amorphous materials and nanostructured composites where phonon scatterings are due to morphological disorder and internal interfaces play key roles in the thermal transport. When exposed to the ambient environment, humidity induced degradation is found to have a significant impact on the overall thermal conductivity of the spin-coated CH3NH3PbI3 neat film.


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1. Introduction Recently, the organic-inorganic hybrid lead halide perovskite has attracted great attention due to its

extraordinary

photovoltaic

performance

and

highly

promising

applications

in

optoelectronics.1-3 Following the unique light emission properties found in 2D lead halide perovskite, room temperature optical gain with amplified spontaneous emission from CH3NH3PbX3 (X=Cl, Br, and I) thin films4-5 and room temperature lasing in CH3NH3PbI3 films and nano-structures3, 6-7 have been demonstrated. The potential of organometallic perovskite as an optical gaining medium for laser applications urges imminent researches into other issues that can greatly affect such an application, such as the thermal management.8-10 It is known that the lead halide perovskites have a low thermal decomposition temperature,10 and the decomposition process can be further accelerated by light illumination.8 In this aspect, a high thermal conductivity is desired to effectively dissipate heat and thus mitigate the thermal decomposition problem. On the other hand, organic-inorganic perovskites, exemplified by CH3NH3PbI3, have been identified as a potential thermoelectric material due to the large Seebeck coefficient (>0.5 mV/K at room temperature).11 For such applications, a low thermal conductivity is highly desirable to improve the thermoelectric efficiency. However, the thermal conductivity of lead halide perovskite thin films has not been thoroughly studied. Pisoni et al. first reported temperature-dependent thermal conductivity of bulk single crystal and polycrystalline CH3NH3PbI3 using steady state thermal measurements.12 The thermal conductivity values of both samples were found to be low (< 0.5 W/(mK)) at room temperature. However, a large number of recent successes on lead halide perovskite-based photovoltaic and lasing effects are observed in solution-processed CH3NH3PbI3 thin films due to their facile fabrication.4, 6 The thermal conductivity in solution-processed CH3NH3PbI3 thin films has not yet


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been studied due to its technical difficulty in conventional steady state measurement approaches. Compared with perovskite single crystals, the solution-processed films usually contain more defects, and form into grains that lead to different film microstructures. Such differences could lead to a distinct thermal transport behavior compared to the bulk materials. It is also well known that CH3NH3PbI3 structure is susceptible to moisture, and this is particularly true for thin films due to its large surface to volume ratio. The lead iodide (PbI2) can be accumulated in CH3NH3PbI3 films through a degradation of CH3NH3PbI3 in the humid air, which can significantly influence the thermal transport in perovskite films. In this work, we present temperature-dependent thermal conductivity measured in a spun-cast CH3NH3PbI3 perovskite film and a composite film with CH3NH3PbI3 intercalated into an Al2O3 mesoporous scaffold. The thermal conductivity data are measured using the time-domain thermoreflectance (TDTR) pump-probe method.13-14 We found that the thermal conductivity of the spun-cast neat perovskite film decreases with temperature increase, which is similar to the behavior observed in bulk crystalline CH3NH3PbI3. Meanwhile, a thermal conductivity jump has been observed near the phase transition temperature (~167 K) in the neat CH3NH3PbI3 film, which, however, was not seen in the bulk crystalline samples.12 When perovskite films are deposited on an Al2O3 mesoporous scaffold, the temperature-dependent thermal conductivity shows an opposite trend – it first increases with temperature at low temperature and eventually reach a saturated value, which is usually seen in glass-like solids or nanocomposites.15-16 In fresh CH3NH3PbI3 neat films, varying the heat penetration depth through changing the pump laser modulation frequency has identified higher thermal conductivity at the surface, suggesting the existence of larger grains near the film surface compared with those near the substrate. With long term exposure to the ambient environment, the degradation of CH3NH3PbI3 perovskite film is


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also reflected in its thermal conductivity. Film thermal conductivity increased to ~2 W/(mK) after 48 hours’ exposure, which is close to the values of as-annealed PbI2 films. 2. Materials and Methods 2.1 Neat CH3NH3PbI3 film on silicon substrate Silicon substrates (University Wafers, 450-µm thick undoped silicon wafer) were cleaned by ultrasonication in ethanol for 15 min followed by oxygen plasma treatment for 3 min. For the perovskite precursor solution, equimolar quantities of methylammonium iodide (Dyesol) and lead iodide (99.999%, ultra-dry, Alfa) were dissolved in 7:3 (volume) γ-butyrolactone:dimethyl sulfoxide at 1.2 mol L-1. All solvents were anhydrous grade and used as-received from Sigma Aldrich. The precursor solution was stirred for 1 h at 70 °C prior to deposition. Spin casting of the precursor solution was carried out according to a previously reported procedure.17 75 µl of the precursor solution was spread evenly across a 2 × 2 cm Si substrate and spun at 1000 rpm (ramp rate 200 rpm s-1) for 10 s followed by 5000 rpm for 20 s (ramp rate 1000 rpm s-1). 750 µl of toluene was rapidly deposited on the spinning substrate with 5 s remaining in the deposition process. Treatment with an orthogonal solvent removes excess dimethyl sulfoxide (DMSO) molecules, resulting in the formation of a PbI2-CH3NH3I-DMSO solid-state composite. The film was then annealed at 100 °C for 10 min to form CH3NH3PbI3 films. All preparation steps were carried out under inert atmosphere with less than 5 ppm water.

2.2 CH3NH3PbI3 on Al2O3/glass mesoporous substrate Micro-cover glasses (glass, 22 × 22 mm, VWR) were cleaned in by ultrasonicaton in acetone and ethanol for 15 min each. Al2O3 4 wt% suspension was prepared by mixing isopropanol and Al2O3 20 wt% suspension (Al2O3 nanoparticles, < 50 nm in isopropanol, Sigma Aldrich) with


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1:4 volume ratio. The Al2O3 suspension was coated on to the glass substrate through spin-coating process with gradual increment of spin-coating rate (700 rpm for 9 s, 1000 rpm for 10 s and then 2000 rpm for 30 s). The films were dried at 80 oC for 30 min and then annealed at 500 oC for 1 hr. Under dried inert condition ( < 10 ppm H2O), 0.6 M of CH3NH3I (PN101000, Dyesol) and PbI2 (A11A026, 99.9985%, Alfa Aesar) solution in anhydrous N,N-dimethylformamide (DMF, 227056, 99.8%, Aldrich) was spin-coated on the glass or Al2O3/glass at 2000 rpm for 30s (1000 rpm/s ramp rate). After drying of the as-deposited perovskite films, the films were placed on a hot plate at 100 oC annealed for 5 min. The prepared perovskite film thickness was determined by KLA-Tencor P6 profiler.

2.3. XRD measurements XRD was measured using a Bruker D8 Advance X-ray diffractometer. Cu Kα X-ray (λ = 1.5406 Å) was irradiated to rotating neat CH3NH3PbI3 films or CH3NH3PbI3/Al2O3 films (15° min−1) with a scan rate of 0.3° min−1 over 2 θ values of 10 ~ 50°. To protect CH3NH3PbI3 against humidity, 500 g desiccant (Silica gel with moisture indicator, EMD Millipore) was put around the XRD sample stage in advance.

2.4. Metal film deposition For TDTR measurements, the perovskite film samples were coated with an aluminum (Al) film with a nominal thickness of 100 nm using electron beam evaporation. The actual Al film thickness deposited has been determined by picosecond acoustic dynamics in TDTR measurements. For the CH3NH3PbI3 film sample, the metal film thickness was measured to be 107±5 nm, and for the CH3NH3PbI3/Al2O3 film sample, the metal film thickness was 124±5 nm.


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These values were used as inputs for the heat conduction model used to extract thermal conductivity from TDTR signals.

2.5. Scanning Electron Microscopy (SEM) characterization Sample film cross-sections were prepared by ion beam etching using focused ion beams (FIB). FIB etching, SEM analysis of the film surface morphology and film cross-sections were performed with a Helios NanoLab DualBeam 600 SEM/FIB work station (FEI Inc., USA). Figure 1b and 1c show the SEM cross-sectional images of different layers in the Al/CH3NH3PbI3/Si and Al/CH3NH3PbI3/Al2O3/glass film samples.

2.6. TDTR measurements The principle for TDTR measurements has been discussed in past literature.13-14, 18 The TDTR setup used for this study has been described in our previous work.19 Here, the 1/e2 radii of the pump and probe beam on the Al film surface were 30 µm and 5 µm, respectively. The modulation frequency of the pump beam was set to values ranging from 3.0 to 11.0 MHz for modulation frequency-dependent measurements. To extract thermal conductivity from the thermoreflectance decay signal, the phase signal data were used to fit a pulse accumulation heat conduction model.20 Typical phase signals ( φ

V out , where Vin and Vout are the in-phase ) V in

= tan − 1(

and out-of-phase voltage signal demodulated from the lock-in amplifier) extracted from TDTR measurements of different samples at room temperature are shown in Figure 2. Our TDTR measurements are sensitive to the thermal effusivity of the samples and thus their thermal


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conductivity. The measurement sensitivity to the film thermal conductivity has been analyzed in Figure S1 (Supporting Information). Every data point was averaged over 20 scans and the error bar was determined from sensitivity analysis and uncertainty analysis.

3. Results and Discussion 3.1. Temperature dependent thermal conductivity in perovskite films The as-prepared CH3NH3PbI3 perovskite neat film and CH3NH3PbI3/Al2O3 mesostructured film both show crystalline feature as found in the X-ray diffraction (XRD) patterns (Figure 1). CH3NH3PbI3 neat film typically shows significantly higher crystallinity (at least one magnitude of order higher in diffraction intensity) than that of the alumina (Al2O3) supported mesostructured film (Figure 1d). An increase of the diffraction peak intensity ratio between the (110) orientation and other orientations in the CH3NH3PbI3/Al2O3 film sample compared to that in the CH3NH3PbI3 neat film sample suggests that nano-crystallites are more randomly packed in the former. Scanning electron microscopy (SEM) imaging on the surface morphology of neat CH3NH3PbI3 perovskite film reveals sub-micron to micron size grain features (Figure 1a).


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Figure 1. (a) SEM image of CH3NH3PbI3/Si surface morphology. (b,c) Cross-sectional SEM images of each layer in the (b) Al/CH3NH3PbI3/Si and (c) Al/CH3NH3PbI3/Al2O3/glass samples. (d) XRD diffraction graphs of two film samples.

We first present the temperature-dependent thermal conductivity data of CH3NH3PbI3/Si (neat CH3NH3PbI3 perovskite film on silicon substrate, see Experimental Section) thin film samples (Figure 3, red triangles). It is known that the organometallic perovskite CH3NH3PbI3 can have different crystal structures associated with phase transitions at certain temperatures.21-23 At temperatures below 159 K, the CH3NH3PbI3 structure is in the orthorhombic phase, and the structure converts to a tetragonal structure when temperature is above 159 K (see thumbnail figures in Figure 3).21-23 In our solution-processed neat perovskite film samples, we observed that the thermal conductivity values are separated into two regimes associated with two structural phases. An overall higher thermal conductivity was found in the orthorhombic phase compared with that in the tetragonal phase.


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Figure

2.

TDTR

phase

signal

measured

in

CH3NH3PbI3/Si,

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PbI2/Si,

and

CH3NH3PbI3/Al2O3/glass samples at 294 K with 3 MHz modulation. Solid lines are fitting curves using a pulse accumulation heat conduction model. Red dash lines represent the upper and lower bounds given a 10% fitting error of the film thermal conductivity, indicating the high sensitivity of our measurements to film thermal conductivity. Inset: acoustic echoes from the Al/CH3NH3PbI3 interface and CH3NH3PbI3/Si interface, which are used to determine the Al film thickness and the speed of sound in the CH3NH3PbI3 layer.

To better understand the thermal conductivity trend as a function of temperature in our CH3NH3PbI3/Si film samples, we fit the thermal conductivity using the Debye model. The thermal conductivity, κ, from a Debye model can be expressed as equation (1): θD 3

T k k  x 4e x κ = B2  B  T 3 ∫ τ ( x ) x dx 2 2π υ  h  e −1 0

(

)

(1)


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where x =

hω

k BT

and θ D =

hω D

kB

, kB is the Boltzmann constant, ħ is the reduced Planck constant,

ω is phonon angular frequency, T is temperature, θ D is Debye temperature, ωD is Debye frequency, υ denotes speed of sound, and τ is the phonon relaxation time. In this model, we assume that the phonon scattering rate is mainly from three sources: grain boundary (τB), point defects (τPD), and the Umklapp scattering (τU). The first two scattering mechanisms are independent of temperature, and the last term is temperature-dependent. Using Matthiessen’s rule, the overall relaxation time is related to the contribution from each mechanism by equation (2) and (3).

τ −1( x ) = τ B− 1( x ) + τ PD− 1( x ) + τ U− 1( x )

τ B−1 =

υ d

−1 = , τ PD

V  θ  hγ 2 2 4 −1 B ω = ω 2T exp − D  τ , PD U 2 3 4πυ Mυ θ D  3T 

(2)

(3)

ିଵ ) and In equation (3), we adopted Klemens’ expression24 for the point defect scattering term (߬௉஽

Slack’s expression25 for the Umklapp scattering term (߬௎ିଵ ), where d is the average crystallite size, V is the unit cell volume, M is average atomic mass in a unit cell (M=0.62/12/NA kg≈0.0517/NA kg, NA = 6.02×1023), and γ is the Grüneisen parameter. Even though there are five parameters (d, υ ,

θD ,

BPD, and γ) in the thermal conductivity expression, we are able to determine

three of them (d, υ , θ D ) as summarized in Table 1, leaving only two independent variables for the fitting.


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Table 1. Parameters used to fit the temperature-dependent thermal conductivity data measured for the CH3NH3PbI3/Si film sample in Figure 3. γ values in round brackets correspond to the best fitting values obtained using a Debye temperature of 90 K. Phase

Crystal size d (nm)

Sound speed υ (m/s)

θD (K)

BPD

γ

Tetragonal

73

1.2×103

120 (90)

1.6×10-4

3.6 (1.7)

Orthorhombic

50

1.2×103

120 (90)

1.6×10-4

2.0 (1.2)

It is known that grain boundaries will scatter phonons, and thus the size of the crystalline domain (d) acts as a limiting length for phonon mean free path. Average perovskite crystallite size in its tetragonal phase – the phase at room temperature – can be estimated from the XRD measurements (Figure 1d). By fitting the full width half maximum of the diffraction peak, we found that the coherent length is about 73 nm for the (110) peak and 150 nm for the high order peak at (220). We thus used the smaller size (73 nm) as the limiting size for phonon transport in the tetragonal phase. The grain size along the cross-plane direction in the orthorhombic phase is estimated according to the difference in the lattice constants of the two phases (c axis, 12.7 Å in tetragonal phase vs. 8.6 Å in orthorhombic phase), and thus a size of 50 nm is used. Next, we estimate the speed of sound ( υ ) in the CH3NH3PbI3 structure. A relative thin perovskite film (60±10 nm) were used in this study to estimate the longitudinal sound speed in the perovskite film at room temperature using picosecond acoustic dynamics (Figure 2, inset). The first acoustic echo from the CH3NH3PbI3/Si interface gave a longitudinal sound speed of 1.6±0.3×103 m/s. This value is close to the longitudinal acoustic phonon group velocity obtained from first-principles calculations on the cubic phase CH3NH3PbI3 (1.71×103 m/s)26 and layered


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PbI2 structure (1.76×103 m/s),27 as well as that calculated from the neutron scattering derived phonon dispersion of PbI2 (1.54×103 m/s).28 The transverse sound speed was estimated using the longitudinal-to-transverse acoustic branch ratio (~1.7) for CH3NH3PbI329 or PbI2.27-28 At room temperature, an average sound speed of 1.2×103 m/s obtained by averaging the group velocity of three acoustic phonon branches is in reasonable agreement with predictions from classical elasticity theory (1.2-1.7×103 m/s).29-30 We do not expect the sound speed to change significantly between tetragonal phase and orthorhombic phase. This is because according to the classical elasticity theory, similar elastic constants (423 kBar along c axis direction for the tetragonal phase and 443 kBar for the orthorhombic phase)31 and densities (unit cell volumes are 975 Å3 for the tetragonal phase and 951 Å3 for the orthorhombic phase respectively)9 should result in similar sound speeds in two structural phases. Last, we consider the Debye temperature ( θ D ). In Onoda-Yamamuro et al.’s specific heat capacity measurements,22 the acoustic phonon branches of CH3NH3PbI3 have a Debye temperature ranging from 90 K to 120 K. In our model, we present fitting results using a Debye temperature of the two boundary values, 90 K and 120 K, respectively. However, we found that using either value does not lead to much difference in the fitted curves (Figure 3).


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Figure 3. Temperature-dependent thermal conductivity values measured in CH3NH3PbI3/Si (red triangles) and CH3NH3PbI3/Al2O3/glass (blue squares) film samples. Debye model fitting using different parameter sets are shown in dash dot line (50-160 K, θD=90 K), solid line (50-160 K, θD=120 K), dot line (160-300 K, θD=90 K), and dash line (160-300 K, θD=120 K), respectively. Previously reported thermal conductivity of polycrystalline CH3NH3PbI3 samples formed by mechanical pressing is shown as green dot lines.12 In the top legend, two thumbnail figures show the lattice structures of tetragonal phase and orthorhombic phase projected on two unit cell vectors, b and c. The CH3NH3+ cations are shown in ball-stick representation.

With the above three parameters (d, υ , θ D ) determined, we use the Debye model (equation (1)) to fit the temperature-dependent thermal conductivity separately in two temperature regimes corresponding to the two phases. It should be noted that, since the point defect scattering term


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ିଵ (߬௉஽ in equation (3)) is independent of temperature and BPD does not change if the point defect

density does not change. For this reason, the BPD coefficient obtained from fitting the low temperature regime data is used for the high temperature regime as well. In the tetragonal phase, the thermal conductivity data can be fitted well simply by considering the Umklapp scattering processes. Even by dropping the point defect scattering term, the thermal conductivity values vary by less than 3%, meaning that the defect scattering is not important anyway. This indicates that Umklapp phonon scattering is the dominant scattering source in the higher temperature regime. Using the Umklapp scattering expression proposed by Slack et al.,25 the best fit results in a generally larger Grüneisen parameter in the tetragonal phase compared with that in the orthorhombic phase. This suggests that a stronger anharmonic effect is present in the tetragonal phase lattice and may contribute to a lower thermal conductivity in the high temperature phase. One possible explanation for seeing a stronger anharmonic interaction in the high temperature phase is the emergence of more complex potential surfaces associated with metastable states.32 Those metastable states may stem from the rotational degree of freedoms of the CH3NH3+ ions in the lattice, which have been suggested by neutron scattering experiments33-34 and molecular dynamics simulations.33, 35-36 In a former report of single crystal and polycrystalline CH3NH3PbI3 thermal conductivity measurement,12 even though the phase transition induced a dip in the thermal conductivity curve, a thermal conductivity jump similar to that observed in the present work was not present. In the perovskite film with mesoporous alumina support (discussed below), we also only observed a dip in the thermal conductivity change without a jump. This suggests that the abrupt thermal conductivity change is inherent to the neat perovskite film. The intrinsic differences between perovskite single crystal, mechanically compressed polycrystalline sample and a solution


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processed film may account for this discrepancy. First, micro-crystallites in mechanical compressed polycrystalline samples do not adopt any preferential orientation order compared with solution processed film. Spun-cast perovskite film usually has a preferential orientation order during its growth stage with its c-axis oriented perpendicular to the substrate plane.37-39 Second, and more importantly, solution processed and annealed perovskite film typically form certain morphology composed of crystalline grains (Figure 1a). In contrast to the fact that the ferroelectric property is absent in the CH3NH3PbI3 single crystal,40 CH3NH3PbI3 film of certain morphologies have been demonstrated to form ferroelectric domains.41-42 Domain walls can also be an important phonon scattering interface in this case, especially its thickness will be critical if it is comparable or less than the phonon mean free path, as indicated by a recent experiment on a ferroelectric crystal.43 Near the phase transition temperature, the lattice of CH3NH3PbI3 undergoes significant changes from tetragonal phase to orthorhombic phase, and the lattice constants change accordingly. The c-axis lattice constant shrinks from 12.71 Å in the tetragonal phase to 8.58 Å in the orthorhombic phase, and the b-axis lattice constant expands from 8.81 Å to 12.63 Å.23,

44

Lattice deformation at lower temperature, e.g., the one-end free contraction

along the c-axis and the lateral expansion along the b-axis in the orthorhombic phase, should lead to a more compact structure with narrower domain walls compared with that in the tetragonal phase. The reduction of phonon scattering at the domain interface can thus increase the thermal conductivity in solution processed perovskite films. Previous work in ferroelectric film has also shown that this effect can be used to effectively control the film thermal conductivity.43 To further pinpoint whether or not the ferroelectric domain interface is indeed contributing to the thermal conductivity jump near phase transition temperature of the perovskite films, follow-up experiments can be done by using a DC voltage bias to switch on and off the perovskite


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ferroelectric domains41 and simultaneously measure the temperature-dependent thermal conductivity in these two cases. We also measured the perovskite films prepared on a substrate with mesoporous alumina (Al2O3) scaffold support, which is one kind of architectures used in high efficiency perovskite solar cell devices that show low photocurrent-voltage hysteresis.45 The Al2O3 nanoparticles used here has diameters less than 50 nm. The Al2O3 nanoparticles form mesoporous scaffolds with a weight percentage of ~50 wt% in the perovskite/Al2O3 composite (Supporting Information). The heat capacity of the composite is calculated as a combination of the Al2O3 heat capacity and the perovskite heat capacity according to their mixing ratio. When the perovskite nanocrystals are embedded in the Al2O3 scaffolds, we observed that the temperature dependence of the composite thermal conductivity demonstrates a increasing trend (Figure 3) which is usually seen in amorphous structures,15,

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nanocrystalline grains47 or nanocomposites.16 Although different

synthesis routes have been shown to affect the perovskite crystallization kinetics that further result in different morphologies in a planar perovskite film48 and consequently its thermal conductivity, this factor is considered as secondary in influencing the thermal conductivity of a mesostrcutred film. This is not only because the perovskite grain size and crystalline order in the mesoporous scaffold are greatly limited by the physical space available in the scaffold, but also because the thermal conductivity of this composite is influence by each ingredient (the Al2O3 scaffolds and CH3NH3PbI3) and their interfaces. Since no XRD peak of the crystalline Al2O3 has been found in our sample (Supporting Information), the Al2O3 scaffolds should mostly exist in the amorphous form, which contributes to the increasing trend of the thermal conductivity as a function of temperature at low temperature (< 80 K). It is well known that the increase in thermal conductivity of amorphous materials is due to the increase in heat capacity.49 Another


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mechanism that can lead to the increasing thermal conductivity as a function of temperature is the phonon interface scattering in nanocomposites, where high interface density dominates thermal transport. In our perovskite film prepared in an Al2O3 scaffold, the interfaces between Al2O3 and CH3NH3PbI3 nanocrystals may dominate phonon scattering. The temperature increase can also lead to a thermal conductivity increase of a nanocomposite due to the increase in phonon heat capacity. It is worth noting that the plateau of thermal conductivity at high temperature (> 80K, Figure 3) can also be attributed to the heat capacity effect, which saturates at high temperature. Compared to the neat CH3NH3PbI3 film, an abrupt jump in the temperaturedependent thermal conductivity is absent in the Al2O3 mesoporous scaffold support perovskite film. Following the rationale discussed in the neat perovskite film above, the explanation is that when perovskite crystallites are encapsulated in the Al2O3 mesoporous scaffold, they lose the crystal orientation preference induced by the substrate surface.50 This is similar to the mechanically compressed perovskite polycrystalline material as discussed above,12 and thus their thermal conductivity behaviors are also similar: a dip, instead of an abrupt jump, is observed at phase transition temperature. 3.2. The effect of grain size distribution on the film thermal conductivity In the fresh perovskite film, we noticed that the measured thermal conductivity shows a weak dependence on the modulation frequency or the heat penetration depth (Figure 4). TDTR measurements using higher modulation frequencies report higher measured thermal conductivity (e.g., ~0.5 W/(mK) at 11 MHz). A TDTR measurement has the highest sensitivity to the thermal conductivity of a film thickness accessible by the heat penetration depth.51 The heat penetration depth L is related to the modulation frequency f via L =

κ , where κ is the thermal πCf


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conductivity of the material, and C is its volumetric heat capacity. When frequency is high, the heat can only penetrate to a shallow region of the surface, and thus the measurement detects thermal transport properties of that portion. When frequency reduces, the penetration depth becomes deeper and thus the measured thermal conductivity corresponds to that of a thicker region.

Figure 4. Modulation frequency-dependent thermal conductivity of CH3NH3PbI3 neat film and its evolution with exposure time when exposed to the ambient condition (~40% relative humidity, 294 K). The horizontal line represents the thermal conductivity value reported by photoacoustic measurements in single crystal PbI2 along the c-axis.52 The increasing trend of thermal conductivity as a function of modulation frequency implies that the surface layer of the film has higher thermal conductivity. To roughly estimate L, we choose the average (0.4 W/(mK)) thermal conductivity values measured at different modulation frequency as an estimation for κ, and C is taken as 1.19×106 J/(m3 K) (294 K).22 The thermal penetration depth L is then estimated to be ~100 nm at 11 MHz, and ~190 nm at 3 MHz. The


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penetration depths are thus always smaller than the film thickness (~ 300 nm). A plausible explanation for seeing a higher thermal conductivity near the surface than underneath is that the film surface has larger CH3NH3PbI3 grains than those near the substrate, which reduces the phonon scattering at the grain boundaries. This is consistent with recent work on the grain growth mechanism of perovskite films, which suggests that large grains on the surface are grown out of small crystallites on the substrate at the initial stages.53-54 Smaller grains can be found close to the substrate side due to surface tension from the wetting substrate.39,

53

Thus the

modulation frequency-dependent thermal conductivity measurements also reflect the grain size distribution along the cross-plane direction. 3.3. The effect of degradation on the film thermal conductivity When CH3NH3PbI3 films are exposed to the moisture environment, they gradually undergo degradations. While the degradation process of perovskite has been suggested to take a stepwise mechanism and undergo recrystallization,55-56 the ultimate product is usually PbI2. The thermal properties of the perovskite film also undergo significant change when such degradation is present. Figure 4 shows the thermal conductivity measured in perovskite films with different days of exposure in the ambient environment (~40% relative humidity, 294 K), together with those measured in pristine PbI2 spun-cast/annealed films. The thermal conductivity kept increasing as the film was exposed to the ambient environment for longer time, and eventually values similar to those of PbI2 films were observed. Annealed PbI2 films and perovskite films have also been found to share similar orientation order relative to the substrate surface in past xray scattering experiments.37-39 Thus, TDTR measurements measure the cross-plane thermal conductivity for both samples. The average cross-plane thermal conductivity values of the PbI2 films (2.0±0.5 W/(mK)) obtained from our TDTR measurements are in good agreement with


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previous photoacoustic experiments,52 which reported a value of ~2.6 W/(mK) along the c-axis in a single crystal.

4. Conclusion In conclusion, we have measured the temperature-dependent thermal conductivity of the neat CH3NH3PbI3 perovskite film and the CH3NH3PbI3/Al2O3 mesostructured film using the TDTR method. The thermal conductivity of the neat perovskite film decreases with temperature increase and shows an abrupt decrease near the phase transition temperature. We suggest that the reason for the discontinuity is likely related to the difference in lattice anharmonicity of the two phases and the reduced ferroelectric domain wall phonon scattering in the orthorhombic phase. The validity of the latter hypothesis can be tested in future work by measuring the temperaturedependent thermal conductivity with ferroelectric domains switched on and off using a DC biased voltage. The thermal conductivity of the CH3NH3PbI3/Al2O3 composite film first shows an increasing trend below 80 K and then saturate to a plateau value, which is believed to be from the amorphous Al2O3 scaffolds and the nanoscale interfaces, whose thermal transport both improve as phonon heat capacity increases with temperature at low temperature and saturates at high temperature. Modulation frequency dependent TDTR measurements suggest that the neat CH3NH3PbI3 film has different grain sizes along the film growth direction. In the end, we have also shown that moisture-induced degradation will increase the thermal conductivity of the neat CH3NH3PbI3 film by gradually decaying into PbI2. The results of this study can provide important insight to the thermal transport physics of perovskite films and offer helpful design guidance for perovskite film-based photovoltaic and optoelectronic devices.


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Supporting Information

Details on determining the weight ratio between Al2O3 scaffolds and CH3NH3PbI3 in the mesostructured film, and the sensitivity analyses of the TDTR measurements are available in the supporting information. This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgements This work was supported in part by the Sustainable Energy Initiative (SEI) and the start-up fund from University of Notre Dame, and National Science Foundation (2DARE 1433490). JM and PVK acknowledge the support the support of King Abdullah University of Science and Technology (KAUST) through the award OCRF-2014-CRG3-2268. This is a document number NDRL5096 from the Notre Dame Radiation Laboratory, which is supported by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy through award DE-FC02-04ER15533.

Corresponding Author *Email: [email protected]; [email protected] (Phone number: 574-631-9683) Present Addresses †Z.G.: Department of Chemistry, Purdue University, 560 Oval Drive West Lafayette, IN 479072084


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References 1.

Gratzel, M., The Light and Shade of Perovskite Solar Cells. Nat. Mater. 2014, 13, 838-

842. 2.

Dong, Q.; Fang, Y.; Shao, Y.; Mulligan, P.; Qiu, J.; Cao, L.; Huang, J., Electron-Hole

Diffusion Lengths> 175 Μm in Solution-Grown Ch3nh3pbi3 Single Crystals. Science 2015, 347, 967-970. 3.

Zhu, H.; Fu, Y.; Meng, F.; Wu, X.; Gong, Z.; Ding, Q.; Gustafsson, M. V.; Trinh, M. T.;

Jin, S.; Zhu, X. Y., Lead Halide Perovskite Nanowire Lasers with Low Lasing Thresholds and High Quality Factors. Nat. Mater. 2015, 14, 636-642. 4.

Stranks, S. D.; Wood, S. M.; Wojciechowski, K.; Deschler, F.; Saliba, M.; Khandelwal,

H.; Patel, J. B.; Elston, S. J.; Herz, L. M.; Johnston, M. B., et al., Enhanced Amplified Spontaneous Emission in Perovskites Using a Flexible Cholesteric Liquid Crystal Reflector. Nano Lett. 2015, 15, 4935-4941. 5.

Xing, G.; Mathews, N.; Lim, S. S.; Yantara, N.; Liu, X.; Sabba, D.; Grätzel, M.;

Mhaisalkar, S.; Sum, T. C., Low-Temperature Solution-Processed Wavelength-Tunable Perovskites for Lasing. Nat. Mater. 2014, 13, 476-480. 6.

Deschler, F.; Price, M.; Pathak, S.; Klintberg, L. E.; Jarausch, D.-D.; Higler, R.; Hüttner,

S.; Leijtens, T.; Stranks, S. D.; Snaith, H. J., et al., High Photoluminescence Efficiency and Optically Pumped Lasing in Solution-Processed Mixed Halide Perovskite Semiconductors. J. Phys. Chem. Lett. 2014, 5, 1421-1426. 7.

Zhang, Q.; Ha, S. T.; Liu, X.; Sum, T. C.; Xiong, Q., Room-Temperature near-Infrared

High-Q Perovskite Whispering-Gallery Planar Nanolasers. Nano Lett. 2014, 14, 5995-6001.


23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

8.

Page 24 of 30

Misra, R. K.; Aharon, S.; Li, B.; Mogilyansky, D.; Visoly-Fisher, I.; Etgar, L.; Katz, E.

A., Temperature- and Component-Dependent Degradation of Perovskite Photovoltaic Materials under Concentrated Sunlight. J. Phys. Chem. Lett. 2015, 6, 326-330. 9.

Baikie, T.; Fang, Y.; Kadro, J. M.; Schreyer, M.; Wei, F.; Mhaisalkar, S. G.; Graetzel,

M.; White, T. J., Synthesis and Crystal Chemistry of the Hybrid Perovskite (Ch 3 Nh 3) Pbi 3 for Solid-State Sensitised Solar Cell Applications. J. Mater. Chem. A 2013, 1, 5628-5641. 10.

Dualeh, A.; Gao, P.; Seok, S. I.; Nazeeruddin, M. K.; Grätzel, M., Thermal Behavior of

Methylammonium Lead-Trihalide Perovskite Photovoltaic Light Harvesters. Chem. Mater. 2014, 26, 6160-6164. 11.

Mettan, X.; Pisoni, R.; Matus, P.; Pisoni, A.; Jacimovic, J.; Náfrádi, B.; Spina, M.;

Pavuna, D.; Forró, L.; Horváth, E., Tuning of the Thermoelectric Figure of Merit of Ch3nh3mi3 (M= Pb, Sn) Photovoltaic Perovskites. J. Phys. Chem. C 2015, 119, 11506-11510. 12.

Pisoni, A.; Jacimovic, J.; Barisic, O. S.; Spina, M.; Gaál, R.; Forró, L.; Horváth, E.,

Ultra-Low Thermal Conductivity in Organic–Inorganic Hybrid Perovskite Ch3nh3pbi3. J. Phys. Chem. Lett. 2014, 5, 2488-2492. 13.

Cahill, D. G.; Ford, W. K.; Goodson, K. E.; Mahan, G. D.; Majumdar, A.; Maris, H. J.;

Merlin, R.; Phillpot, S. R., Nanoscale Thermal Transport. J. Appl. Phys. 2003, 93, 793-818. 14.

Cahill, D. G., Analysis of Heat Flow in Layered Structures for Time-Domain

Thermoreflectance. Rev. Sci. Instrum. 2004, 75, 5119-5122. 15.

Kittel, C., Interpretation of the Thermal Conductivity of Glasses. Phys. Rev. 1949, 75,

972-974.


24

Page 25 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


16.

Poudel, B.; Hao, Q.; Ma, Y.; Lan, Y.; Minnich, A.; Yu, B.; Yan, X.; Wang, D.; Muto, A.;

Vashaee, D., et al., High-Thermoelectric Performance of Nanostructured Bismuth Antimony Telluride Bulk Alloys. Science 2008, 320, 634-638. 17.

Jeon, N. J.; Noh, J. H.; Kim, Y. C.; Yang, W. S.; Ryu, S.; Seok, S. I., Solvent

Engineering for High-Performance Inorganic–Organic Hybrid Perovskite Solar Cells. Nat. Mater. 2014, 13, 897-903. 18.

Norris, P. M.; Caffrey, A. P.; Stevens, R. J.; Klopf, J. M.; McLeskey Jr, J. T.; Smith, A.

N., Femtosecond Pump–Probe Nondestructive Examination of Materials. Rev. Sci. Instrum. 2003, 74, 400-406. 19.

Guo, Z.; Verma, A.; Wu, X.; Sun, F.; Hickman, A.; Masui, T.; Kuramata, A.;

Higashiwaki, M.; Jena, D.; Luo, T., Anisotropic Thermal Conductivity in Single Crystal ΒGallium Oxide. Appl. Phys. Lett. 2015, 106, 111909. 20.

Schmidt, A. J.; Chen, X.; Chen, G., Pulse Accumulation, Radial Heat Conduction, and

Anisotropic Thermal Conductivity in Pump-Probe Transient Thermoreflectance. Rev. Sci. Instrum. 2008, 79, 114902. 21.

Wasylishen, R. E.; Knop, O.; Macdonald, J. B., Cation Rotation in Methylammonium

Lead Halides. Solid State Commun. 1985, 56, 581-582. 22.

Onoda-Yamamuro, N.; Matsuo, T.; Suga, H., Calorimetric and Ir Spectroscopic Studies

of Phase Transitions in Methylammonium Trihalogenoplumbates (Ii)†. J. Phys. Chem. Solids 1990, 51, 1383-1395. 23.

Poglitsch, A.; Weber, D., Dynamic Disorder in Methylammoniumtrihalogenoplumbates

(Ii) Observed by Millimeter‐Wave Spectroscopy. J. Chem. Phys. 1987, 87, 6373-6378.


25


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

24.

Page 26 of 30

Klemens, P. G., The Scattering of Low-Frequency Lattice Waves by Static Imperfections.

Proc. Phys. Soc. Sect. A 1955, 68, 1113. 25.

Slack, G. A.; Galginaitis, S., Thermal Conductivity and Phonon Scattering by Magnetic

Impurities in Cdte. Phys. Rev. 1964, 133, A253. 26.

Brivio, F.; Frost, J. M.; Skelton, J. M.; Jackson, A. J.; Weber, O. J.; Weller, M. T.; Goni,

A. R.; Leguy, A.; Barnes, P. R.; Walsh, A., Lattice Dynamics and Vibrational Spectra of the Orthorhombic, Tetragonal and Cubic Phases of Methylammonium Lead Iodide. Phys. Rev. B 2015, 92, 144308. 27.

Pedesseau, L.; Even, J.; Katan, C.; Raouafi, F.; Wei, Y.; Deleporte, E.; Jancu, J.-M.,

Vibrational Properties of 2h-Pbi 2 Semiconductors Studied Via Density Functional Theory Calculations. Thin Solid Films 2013, 541, 9-11. 28.

Dorner, B.; Ghosh, R.; Harbeke, G., Phonon Dispersion in the Layered Compound Pbi2.

Phys. Status Solidi B 1976, 73, 655-659. 29.

Feng, J., Mechanical Properties of Hybrid Organic-Inorganic Ch3nh3bx3 (B = Sn, Pb; X

= Br, I) Perovskites for Solar Cell Absorbers. APL Mat. 2014, 2, 081801. 30.

Rakita, Y.; Cohen, S. R.; Kedem, N. K.; Hodes, G.; Cahen, D., Mechanical Properties of

Apbx 3 (a = Cs or Ch3nh3; X = I or Br) Perovskite Single Crystals. MRS Commun. 2015, FirstView, 1-7. 31.

Wang, Y.; Zhang, Y.; Zhang, P.; Zhang, W., High Intrinsic Carrier Mobility and Photon

Absorption in the Perovskite Ch 3 Nh 3 Pbi 3. Phys. Chem. Chem. Phys. 2015, 17, 11516-11520. 32.

Suemune, Y., Thermal Conductivity of Some Ferroelectric Crystals with Hydrogen

Bonds. J. Phys. Soc. Jpn. 1967, 22, 735-743.


26

Page 27 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


33.

Leguy, A. M. A.; Frost, J. M.; McMahon, A. P.; Sakai, V. G.; Kockelmann, W.; Law, C.;

Li, X.; Foglia, F.; Walsh, A.; O/'Regan, B. C., et al., The Dynamics of Methylammonium Ions in Hybrid Organic-Inorganic Perovskite Solar Cells. Nat. Commun. 2015, 6. 34.

Baikie, T.; Barrow, N. S.; Fang, Y.; Keenan, P. J.; Slater, P. R.; Piltz, R. O.; Gutmann,

M.; Mhaisalkar, S. G.; White, T. J., A Combined Single Crystal Neutron/X-Ray Diffraction and Solid-State Nuclear Magnetic Resonance Study of the Hybrid Perovskites Ch3nh3pbx3 (X = I, Br and Cl). J. Mater. Chem. A 2015, 3, 9298-9307. 35.

Lee, J.-H.; Bristowe, N. C.; Bristowe, P. D.; Cheetham, A. K., Role of Hydrogen-

Bonding and Its Interplay with Octahedral Tilting in Ch3nh3pbi3. Chem. Commun. 2015, 51, 6434-6437. 36.

Mattoni, A.; Filippetti, A.; Saba, M. I.; Delugas, P., Methylammonium Rotational

Dynamics in Lead Halide Perovskite by Classical Molecular Dynamics: The Role of Temperature. J. Phys. Chem. C 2015, 119, 17421-17428. 37.

Moore, D. T.; Sai, H.; Wee Tan, K.; Estroff, L. A.; Wiesner, U., Impact of the Organic

Halide Salt on Final Perovskite Composition for Photovoltaic Applications. APL Mat. 2014, 2, 081802. 38.

Moore, D. T.; Sai, H.; Tan, K. W.; Smilgies, D.-M.; Zhang, W.; Snaith, H. J.; Wiesner,

U.; Estroff, L. A., Crystallization Kinetics of Organic–Inorganic Trihalide Perovskites and the Role of the Lead Anion in Crystal Growth. J. Am. Chem. Soc. 2015, 137, 2350-2358. 39.

Schlipf, J.; Docampo, P.; Schaffer, C. J.; Körstgens, V.; Bießmann, L.; Hanusch, F.;

Giesbrecht, N.; Bernstorff, S.; Bein, T.; Müller-Buschbaum, P., A Closer Look into Two-Step Perovskite Conversion with X-Ray Scattering. J. Phys. Chem. Lett. 2015, 6, 1265-1269.


27


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

40.

Page 28 of 30

Yamada, Y.; Yamada, T.; Phuong, L. Q.; Maruyama, N.; Nishimura, H.; Wakamiya, A.;

Murata, Y.; Kanemitsu, Y., Dynamic Optical Properties of Ch3nh3pbi3 Single Crystals as Revealed by One- and Two-Photon Excited Photoluminescence Measurements. J. Am. Chem. Soc. 2015, 137, 10456-10459. 41.

Kutes, Y.; Ye, L.; Zhou, Y.; Pang, S.; Huey, B. D.; Padture, N. P., Direct Observation of

Ferroelectric Domains in Solution-Processed Ch3nh3pbi3 Perovskite Thin Films. J. Phys. Chem. Lett. 2014, 5, 3335-3339. 42.

Fan, Z.; Xiao, J.; Sun, K.; Chen, L.; Hu, Y.; Ouyang, J.; Ong, K. P.; Zeng, K.; Wang, J.,

Ferroelectricity of Ch3nh3pbi3 Perovskite. J. Phys. Chem. Lett. 2015, 6, 1155-1161. 43.

Ihlefeld, J. F.; Foley, B. M.; Scrymgeour, D. A.; Michael, J. R.; McKenzie, B. B.; Medlin,

D. L.; Wallace, M.; Trolier-McKinstry, S.; Hopkins, P. E., Room-Temperature Voltage Tunable Phonon Thermal Conductivity Via Reconfigurable Interfaces in Ferroelectric Thin Films. Nano Lett. 2015, 15, 1791-1795. 44.

Weller, M. T.; Weber, O. J.; Henry, P. F.; Di Pumpo, A. M.; Hansen, T. C., Complete

Structure and Cation Orientation in the Perovskite Photovoltaic Methylammonium Lead Iodide between 100 and 352 K. Chem. Commun. 2015, 51, 4180-4183. 45.

Lee, M. M.; Teuscher, J.; Miyasaka, T.; Murakami, T. N.; Snaith, H. J., Efficient Hybrid

Solar Cells Based on Meso-Superstructured Organometal Halide Perovskites. Science 2012, 338, 643-647. 46.

Lee, S.-M.; Cahill, D. G.; Allen, T. H., Thermal Conductivity of Sputtered Oxide Films.

Phys. Rev. B 1995, 52, 253.


28

Page 29 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


47.

Uma, S.; McConnell, A.; Asheghi, M.; Kurabayashi, K.; Goodson, K., Temperature-

Dependent Thermal Conductivity of Undoped Polycrystalline Silicon Layers. Int. J. Thermophys. 2001, 22, 605-616. 48.

Bae, S.; Han, S. J.; Shin, T. J.; Jo, W. H., Two Different Mechanisms of Ch3nh3pbi3

Film Formation in One-Step Deposition and Its Effect on Photovoltaic Properties of Opv-Type Perovskite Solar Cells. J. Mater. Chem. A 2015, 3, 23964-23972. 49.

Allen, P. B.; Feldman, J. L., Thermal Conductivity of Disordered Harmonic Solids. Phys.

Rev. B 1993, 48, 12581. 50.

Kim, H. S.; Kim, S. K.; Kim, B. J.; Shin, K.-S.; Kumar, M. G.; Jung, H. S.; Kim, S.-W.;

Park, N.-G., Ferroelectric Polarization in Ch3nh3pbi3 Perovskite. J. Phys. Chem. Lett. 2015. 51.

Koh, Y. K.; Singer, S. L.; Kim, W.; Zide, J. M. O.; Lu, H.; Cahill, D. G.; Majumdar, A.;

Gossard, A. C., Comparison of the 3ω Method and Time-Domain Thermoreflectance for Measurements of the Cross-Plane Thermal Conductivity of Epitaxial Semiconductors. J. Appl. Phys. 2009, 105, 054303. 52.

Silva, T.; Alves, A.; Pepe, I.; Tsuzuki, H.; Nakamura, O.; d'Aguiar Neto, M.; Ferreira da

Silva, A.; Veissid, N.; An, C., Thermal Diffusivity of Lead Iodide. J. Appl. Phys. 1998, 83, 6193. 53.

Bi, C.; Wang, Q.; Shao, Y.; Yuan, Y.; Xiao, Z.; Huang, J., Non-Wetting Surface-Driven

High-Aspect-Ratio Crystalline Grain Growth for Efficient Hybrid Perovskite Solar Cells. Nat. Commun. 2015, 6. 54.

Im, J.-H.; Jang, I.-H.; Pellet, N.; Grätzel, M.; Park, N.-G., Growth of Ch3nh3pbi3

Cuboids with Controlled Size for High-Efficiency Perovskite Solar Cells. Nat. Nano. 2014, 9, 927-932.


29


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

55.

Page 30 of 30

Christians, J. A.; Miranda Herrera, P. A.; Kamat, P. V., Transformation of the Excited

State and Photovoltaic Efficiency of Ch3nh3pbi3 Perovskite Upon Controlled Exposure to Humidified Air. J. Am. Chem. Soc. 2015, 137, 1530-1538. 56.

Leguy, A. l. M.; Hu, Y.; Campoy-Quiles, M.; Alonso, M. I.; Weber, O. J.; Azarhoosh, P.;

Van Schilfgaarde, M.; Weller, M. T.; Bein, T.; Nelson, J., Reversible Hydration of Ch3nh3pbi3 in Films, Single Crystals, and Solar Cells. Chem. Mater. 2015, 27, 3397-3407.

Table of Contents


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Structural Phase- and Degradation-Dependent Thermal Conductivity

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