Ultra-high Seebeck coefficient and low thermal conductivity of a centimeter-sized perovskite single crystal acquired by a modified fast growth method

Article abstract of DOI:10.1039/c6tc04594d

A centimeter-sized organic-inorganic hybrid lead-based perovskite CH₃NH₃PbI₃ (MAPbI₃) single crystal was obtained by using a modified fast and inverse-temperature growth method. The optical properties of this single crystal at room and low temperatures were studied in terms of optical absorption and photoluminescence measurements. The single crystal exhibited optical properties with a band-gap of 1.53 eV, which is comparable to a reported value. The temperature-dependent UV-vis spectra of this perovskite single crystal showed a unique structural phase transition as the temperatures varied. The thermoelectric properties of this MAPbI₃ single crystal were studied, showing that the Seebeck coefficient of 920 ± 91 μV K^?1 almost remained unchanged from room temperature to 330 K and it progressively increased with the increase in temperature and reached 1693 ± 146 μV K^?1 at 351 K. In contrast, there was no very clear trend for thermal conductivities with changes in temperature. The thermal conductivities were maintained between 0.30 and 0.42 W m K^?1 in the temperature range of 298-425 K. These thermoelectric characteristics would be useful for potential thermoelectric applications if the electrical conductivity of this crystal is improved by tuning its composition.

Full text of DOI:10.1039/c6tc04594d

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ISSN 2050-7526
PAPER
~
Nguyên T. K. Thanh, Xiaodi Su et al.
Fine-tuning of gold nanorod dimensions and plasmonic properties using
the Hofmeister effects
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Ultra-High Seebeck Coefficient and Low Thermal Conductivity of
Centimeter-sized Perovskite Single Crystal Acquired by a Modified
Fast Growth Method
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Tao Ye,a,b† Xizu Wang,a† Xianqiang Li,c Alex Qingyu Yan,c Seeram Ramakrishna,b* Jianwei Xua*
A centimeter-sized organic–inorganic hybrid lead-based perovskite CH3NH3PbI3 (MAPbI3) single crystal was obtained by
using a modified fast and inverse-temperature growth method. The optical properties of this single crystal at room and
low temperatures were studied in terms of optical absorption and photoluminescence measurements. The single crystal
exhibited optical properties with a band-gap of 1.53 eV, which is comparable to a reported value. Temperature-dependent
UV-vis spectra of this perovskite single crystal showed a unique structural phase transition as the temperatures varied. The
thermoelectric properties of this MAPbI3 single crystal were studied, showing that the Seebeck coefficient of 920 ± 91μV/K
almost remained unchanged from room temperature to 330 K and it progressively increased with the increase in
temperature and reached to 1693 ± 146 μV/K at 351 K. In contrast, there was no very clear trend for thermal
conductivities with the temperature changes. The thermal conductivities were maintained between 0.30
during the temperature range of 298 425 K. These thermoelectric characteristics would be of useful for the potential
thermoelectric applications if its electrical conductivity is improved by tuning its composition.
̶ 0.42 Wm/K
̶
conjugation with the enhancement of the thermal Seebeck
coefficient and power factor.17 In addition, the enhanced density of
states of energy levels also led to increase in Seebeck coefficient in
the inorganic-organic material interface.15
Introduction
The organic–inorganic hybrid lead-based perovskite CH3NH3PbI3
(MAPbI3) has recently attracted considerable attention for
optoelectronics applications in solar cells and photo-detector due
to its superior characteristics including high absorption coefficient,
direct band-gap, long carrier lifetime, and high balanced hole and
electron mobility, etc.1,2 These perovskites materials have emerged
as a strong candidate in photovoltaic and solar cells. In particular,
lead halide perovskite APbB3 (A = methylammonium (MA) and
formamidium (FA); B = Cl, Br and I) solar cells have reached a record
high efficiency of 22.1% in the past 5 years.3-10 Based on unique
properties of these hybrid lead-based perovskites such as high
charge carrier mobility and high diffusion length, they have
attracted intense attention for possible thermoelectric (TE)
applications.11-13 Low thermal conductivity, high carrier mobility and
Seebeck coefficient are desirable for promising thermoelectric
materials.14 Compared with inorganic-based thermoelectric
material, polymer and organic-inorganic perovskite materials have
attracted more attention for potential thermoelectric application
due to their lower thermal conductivity and density.15,16 The high
scattering parameter at the inorganic-organic interface provided
the solid evidence for the energy filtering effect, which is in
Thermoelectric materials play a vital role in energy harvesting
applications due to their great potential in transforming
a
temperature gradient into thermoelectricity.18,19 The oxide-based
thermoelectric materials such as SrTiO3, ZnO, TiO2, CaMnO3, and
Ca3Co4O9 were fully studied as alternatives to expensive or toxic
tellurium-based and selenium-based traditional thermoelectric
compounds.20 La doped SrTiO3 inorganic perovskites have been
developed and their optimized figure-of-merit value of 0.41 (at 973
K) for an n-type Sr1−3x/2LaxTiO3 δ ceramics with 0.125 ≤ x ≤0.17521
has been achieved. With the help of advanced ab initio calculations,
researchers reported that the MAPbI3 exhibits a large value of
carrier mobility, which is ascribed to a small effective mass and a
long diffusion length, while a large value of the Seebeck coefficients
is due to the multi-degenerated conduction and valence bands.11,12
This intense interest in the lead halide perovskite APbB3 is mostly
due to their long electron and hole transport length, tunable
optoelectronic properties and high absorption coefficients.
Meanwhile, these organometallic halide perovskites are low-cost
and can be fabricated through facile deposition techniques.22-24
Moreover, perovskite single crystals have shown to possess various
merits when compared with their thin film counterparts.25-27 More
precisely, a remarkably low trap-state density of perovskite single
crystals is comparable to that of the best quality silicon in the
photovoltaic field and it is five orders of magnitude lower than that
of microcrystalline perovskites. Firstly, the carrier mobility in single
crystalline perovskites increases with carrier diffusion length.
Secondly, the light absorption onset of it is red-shifted, resulting in
a large theoretic current density for the crystalline perovskites. As a
result, the electrical conductivity and stability of single-crystalline
perovskite are expected to be improved. Recently, it was reported
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that MAPbI3 single crystal has a low thermal conductivity, which is
mainly attributed to the MA rotations in the system.11,12 Moreover,
the ultra-low thermal conductivity of ∼ 0.4 Wm/K and high Seebeck
coefficient of 820 μV/K were also obtained for 3 − 5 mm sized
MAPbI3 crystal drusy facets at a low temperature region (from 200
to 300 K).13 Polycrystals and amorphous perovskite structures,
however, cannot be reliably characterized because of their
geometries. For instance, the presence of many grain boundaries in
polycrystals and amorphous perovskites cannot result in accurate
measurement of the thermal conductivity. Therefore, large single
crystal perovskites may be necessary to explore their detailed
thermoelectric properties.
an oil bath undisturbed at 120 ℃ for crystal formation, N2 air flow
(or using a vacuum pump to keep the pressure in the inner vial is
0.7 atm.) is utilized to increase the solvent evaporation and take
away the solvent vapour. All the crystal synthesis procedures were
carried out under ambient conditions. The crystals used for
measurements were grown within 2-3 h. Then two 100 nm-gold
electrodes were deposited on the same surface of perovskite single
crystal with a space distance of 1 cm through a thermal evaporator
within a N2 filled glove box.
Characterization
A conventional four-point technique was employed to measure the
resistivity (MCP-T610, MITSUBISHI CHEMICAL ANALYTECH). The
The perovskite single crystals growth by using solution
crystallization processes always suffers from a slow growth rate.26-31
A fast crystallization technique-inverse temperature crystallization
(ITC) has been introduced to address the need for fast and massive
fabrication of perovskite single crystals.25 In our study, we optimized
a similar MAPbI3 crystal growth method to obtain a perfect
centimeter-sized organic–inorganic hybrid perovskite (MAPbI3)
single crystal. The growth rate of the resulting single crystal can be
further enhanced without compromising its quality. Herein, the
physical properties of this single crystal in a wide temperature
range have been studied by low-temperature optical absorption
and photoluminescence measurements. The thermoelectric
behaviour of a MAPbI3 single crystal grown by a fast and solution-
based method was examined at above room temperature for the
first time. The results exhibited a high and positive Seebeck
coefficient in the above room temperature region, thus suggesting
holes are the majority carriers in this kind of MAPbI3 single crystal.
In addition, the thermal conductivity of the single crystal was
measured and it had a very lower thermal conductivity than
reported inorganic perovskites. Apart from the superior
photovoltaic characteristics of centimeter-sized perovskite crystals,
it revealed unexpected thermoelectric characteristics including a
high Seebeck coefficient and low thermal conductivity at room
temperature. In addition, it was found that the increase in
temperature led to significant increase in the Seebeck coefficient.
optical absorption spectra were measured on
a PerkinElmer
Lambda 950UV/VIS/NIR spectrometer. PL spectra were obtained
from a triple-grating micro-Raman spectrometer (Horiba-JY T64000).
Liquid nitrogen was utilized for the cooling the samples. X-ray
diffraction experiments were conducted by a Bruker AXS (D8
ADVANCE GADDS) X-ray diffractometer with Cu Kα radiation (λ =
1.54 Å), the beam diameter for this instrument can range from 0.05
to 0.8 mm. Carrier concentration and mobility of the perovskite
single crystal was determined by the Hall measurement (Bio Rad
HL5500) with a four-point-probe using van der Pauw geometry.
Four 100 nm-gold electrodes were deposited on the same surface
of perovskite single crystal through a thermal evaporator within a
N2 filled glove box. The measurements of the thermal diffusivity (κ)
were carried out with a Netzsch Microflash system (LFA 457), the
density (ρ) were calculated by the measured volume and mass of
materials, and the specific heats (Cp) was measured using a
Differential Scanning Calorimetry (DSC, Mettler Toledo) at room
temperature. The TE properties were measured through a custom-
made setup. More precisely, the sample was put on two substrates
(as shown in Figure 3a) with a built-in resistive heater, and the
setting temperature can be adjusted by the resistive heater that
were driven by direct current power supply (PWS2721, Tetronix).
The temperature differences of measuring points were documented
by two thermometers (RS 1319A, Taiwan) and the voltage
differences at same points were collected by a Keithley 2400 source
meter.
EXPERIMENTAL SECTION
Results and discussion
Materials
The solubility behaviour of MA-based perovskite in N,N-
dimethylformamide (DMF) is different from common organic-
inorganic crystals. It has a high solubility at room temperature, but
its solubility decreases with increase in temperature.25 This inverse
solubility effect can be used to crystallize MAPbI3 rapidly by heating
the precursor solution as illustrated in Figure 1a. A temperature-
controllable hot plate was used to heat the oil bath and only a few
crystals were formed within the first 10 min. By using the N2 air flow
(or vacuum) to remove the evaporated DMF solvent promptly, the
crystal growth rate can be increased to ~ 5 mm3/h for the first hour
and the rate would reach ~20 mm3/h for the following hours. The
crystal used for thermoelectric measurement with a suitable size
(more than 1 cm in the length direction) can be fabricated within 2
~ 3 h (Figure 1b). By carefully removing the small crystals from the
old solution and placing them in a newly prepared perovskite
precursor solution, the crystals can grow further with a large
dimension. X-ray diffraction of the perovskite crystal demonstrates
high purity as evidenced by similar diffractions of perovskite MAPbI3
Unless specified otherwise, all materials were purchased from
either Alfa Aesar or Sigma-Aldrich. All chemicals were used as
purchased without any further purification.
Synthesis of MAI
Methylammonium iodide was prepared by reacting methylamine,
33 wt% in ethanol, with hydroiodic acid (HI) 57 wt% in water, at 0 ℃
for 4 h. HI was added dropwise while stirring. Upon drying at 100°C
on a hotplate to remove the solvent and make methylammonium
iodide crystallize, the precipitate was washed with diethyl ether and
ethanol three times. The resulting white powder was dried on a
hotplate at 65°C for 6 h in air.
Synthesis of MAPbI3 single crystal and TE sample. One molar
solution containing PbI2 and MAI was prepared in DMF. The
solution was filtered using PTFE filters with 0.2 µm pore size. Five
milliliters of the filtrate were placed in a vial and the vial was kept in
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(Figure 1c) to some previous results.25-27 High-resolution XRD was More precisely, there is a significant shift of the absorption edge
used to examine the quality of the CH3NH3PbI3 single crystal. Figure through 170 - 210 K in a single crystal. This behaviour can be
S1 shows the 2θ scan profile with only two very sharp diffractions regarded as a crystal phase transition of the perovskite single
corresponding to the 200 and 400 planes, indicating the single- crystal from a tetragonal to an orthorhombic structure at ~170 K,
crystalline nature of the sample.27 The quality of crystal grown inducing a blue shift in the MAPbI3 material absorption edge of ~ 34
within a few hours is comparable to these which were grown within meV. As the temperature decreases, the absorption band edges of
several weeks.24 The optical absorption and photoluminescence the sample show blue shifts called ‘Varshni’ trend (a positive
properties of the MAPbI3 single crystal were investigated at room thermal expansion coefficient), which has been observed in many
temperature. According to the steady state optical absorption lead composite semiconductors.32-34 Different from the perovskite
spectrum, it is easy to find a sharp band edge in the spectrum thin film, no typical exciton peak was observed in the wide
(Figure 1d). Based on the UV-vis absorption of MAPbI3, its band-gap temperature range (Figure 2a), thus implying that the exciton
is estimated to be 1.53 eV. The magnitude of band-gap of the binding energy of this single crystal is much less than that in the film,
MAPbI3 single crystal grown by using the N2 flow-assisted ITC resulting in a high free carrier generation ratio within the crystal.
method is in good agreement with that of the single crystals grown
We also present a temperature-dependent study of PL emission
of the perovskite single crystal to explore the nature of
recombination channels within the MAPbI3 single crystal at a wide
temperature range.35,36 Figure 2b shows the PL spectra at a wide
temperature range from 77 K to 340 K with a laser excitation at 532
nm. The PL spectral profile is independent on the excited laser
intensity and photon energy of the excitation in this study. At room
temperature, a broad peak at ~ 1.62 eV can be seen in the PL
spectrum. The PL intensity shows a rapid increase by more than one
order of magnitude when temperature decreases to 200 K and the
peak position red-shifts to ~ 1.61 eV (P1 of Figure 2c). According to
a previous work, the estimated MAPbI3 band-gap energy is 1.61
eV,35 only a very small Stokes shift occurs here, and thus the PL of
the single crystal is generated from the radiative recombination
by an ITC or anti-solvent crystallization method at room
temperature.25,28 The narrow and blue-shifted PL peak (~ 65 nm)
indicates a low trap density in MAPbI3 crystal and the PL peak width
was determined by the intensity difference crossing the first
derivative of the PL spectrum. The excellent overlapping of the PL
spectrum with the optical absorption spectrum of the MAPbI3 single
crystal allows photon recycling in the thick crystal by reabsorbing
the emission, which is beneficial to the solar thermoelectric
generation, an emerging technology combining concentrated solar
power and thermoelectric effect (the corresponding optical
properties of MAPbI3 thin film can be found in Figure S2).13
[conduction band (electrons) to valence band (holes)]. At T
≤ 160 K,
a new blue-shifted PL peak appears at ~ 1.64 eV, it grows rapidly
with temperature decreases (one order of magnitude in a 20 K
temperature range) and it becomes the main feature at 77 K (P2 of
Figure 2c). The evolution of PL spectra in the single crystal with
decreasing temperature is due to a tetragonal-to-orthorhombic
phase transition, in good agreement with the optical absorption
result. The tetragonal phase at the high temperature with the PL
peak at 1.61-1.64 eV is ascribed to the collective rotation (around
the c-axis) of each PbI6 octahedron from its symmetric position in
the cubic structure. In contrast, the low temperature phase
(orthorhombic) with PL peak at 1.59-1.64 eV is derived from the
tilting of the PbI6 octahedral out of the ab plane.36-38 Since the
orthorhombic MAPbI3 is a direct-band-gap crystal,39-41 multiple PL
peaks indicate the coexistence of the tetragonal and orthorhombic
phases of MAPbI3 in the temperature range of 200-160 K. When the
temperature continues to go down to 110 K, a new PL peak appears
at 1.67 eV, which may originate from free excitons and donor-
acceptor-pair transition. A peak intensity decrease can be observed
in this period (PE of Figure 2c) as the new peak enhances with
decrease in temperature. Further modelling and experiments are
needed to understand the inter-conversion among the thermal
activation of the trap states into band-gap excitons, the broad
distribution of trap states, and competing for radiative and non-
radiative recombination rates from these states.36,42
Figure 1. (a) Schematic graph showing the single crystal growth
setup in this study. (b) Image of 1.3 cm-sized single crystal. (c) X-ray
diffraction of the MAPbI3 crystal. (d) Optical absorption (red),
smoothed curve (black) and PL (blue) of a 2 mm-sized MAPbI3
crystal at room temperature.
Original single crystals instead of powder (mechanically
25-28
fabricated from single crystals)
were used for this experiment.
To study the ratio of free charges over the total photo-excitation in
this single crystal, temperature-dependent optical absorption
spectra are recorded from 340 K to 77 K.32 The absorption edge
(valance band maximum) of MAPbI3 single crystal is 1.53 eV.
Generally, the tetragonal phase of MAPbI3, in which the
methylammonium moiety is disordered, exists from 160 K to room
temperature. In contrast, the orthorhombic phase, in which the
methylammonium unit is ordered, exists below 160 K.29,32 As can be
seen from Figure 2a, the spectra of the MAPbI3 single crystal show a
remarkable structural phase transition during the cooling period.
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2400 (Figure 3a). Plots of thermal voltage vs ΔT measured at room
temperature (25 ) and the Seebeck coefficients obtained at
℃
different average temperatures are shown in Figure 3b-c,
respectively. The positive Seebeck coefficient indicates free holes
are the majority carrier type in the MAPbI3 single crystal, which is
consistent with the Hall effect measurement, but the non-
monotonic temperature dependency of the Seebeck coefficient
indicates that electrons also participate in the charge transport, and
temperature-dependent mobility of electrons is different from the
one of the holes.13,29 Moreover, the measured Seebeck coefficient
value of 920 ± 91 μV/K for this perovskite single crystal at room
temperature (297 K) is higher than that (820 μV/K and or ~300 μV/K)
of the crystal facets made with
a slow solvent evaporating
method.13,29 The trend in Figure 3c suggests that it could reach even
a larger value of 1693 ± 146 μV/K at 351 K, which is 5.4 ~ 33.9 times
of that in the pure and La-doped SrTiO3 inorganic perovskites.20 This
results seemed to conclude that with the temperature rise, the
enhanced high energy carriers transport and increased density of
state of energy levels within the single crystal perovskite lead to the
improved Seebeck coefficient.15,17,43,44 The electrical conductivity of
the single crystal was also measured at a temperature range of 296
to 350 K as shown in Figure 3d. The conductivity increases with the
increase of temperature, which is consistent with a temperature-
activated transport thermal conductivity ~ exp(Ea/T) with an energy
barrier of 128 meV (Figure S3).45,46
.
Figure 2. Optical results of the MAPbI3 single crystal. (a)
Temperature-dependent optical absorption spectra of the single
crystal at different temperatures; (b) Temperature-dependent PL
spectra of the single crystal at different temperatures; (c) The PL
intensity of MAPbI3 single crystal vs. temperature. P: Process, PT:
Phase transition and PE: Process with abnormal exciton behaviour.
Figure 3. Thermoelectric property characterization of the perovskite
single crystal. (a) Schematic graph of the thermoelectric property
measurement. (b) Thermal voltages at different applied
temperature differences. The Seebeck coefficient can be obtained
from the linear fit (dashed lines); (c) Seebeck coefficients at
different average temperatures; (d) Conductivity at different
temperatures and the dashed line is the linear fit.
A low thermal conductivity of around 0.30
̶ 0.42 Wm/K in a
The Hall effect is another important method to examine the
quality of the perovskite single crystal.26,27 It was found that the
MAPbI3 single crystal shows a high carrier mobility of 59 cm2 V-1 s-1
temperature range from 299 to 424 K was observed for this
perovskite single crystal (Figure 4), and this value was much smaller
than that of the pure or La doped SrTiO3 perovskites (4.5 – 7
Wm/K).20 Apart from the electrical conductivity, the ultrahigh
Seebeck coefficient and low thermal conductivity were observed in
this centimeter-sized perovskite single crystal, suggesting that the
enhancement of electrical conductivity would be the key factor to
achieve high thermoelectric performance of perovskite materials.
Further increases in electrical conductivity can be achieved by
doping some other metallic elements that can form more covalent
and
a
free hole concentration of 4.72
×
1011 cm-3. The
thermoelectric properties of the perovskite single crystal were
measured with a custom-made setup. Specifically, using a voltage
controllable DC power supply and electric resistance elements to
create a temperature gradient, and two thermometers together
with two copper points (wires) were used to measure the
temperature gradient. The voltage difference at the same points
was collected by another two copper points (wires) and a Keithley
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bonds with the iodine ions in the perovskite crystal.47-49 It is
noteworthy that the electrical conductivity is increased by 10 orders
of magnitude in MASnI3 with a value of 105 S/m.13 It shows a path
towards significant improvements in MA based perovskites
electrical conductivity. Holes are the majority carrier type for
MAPbI3 single crystal, but electrons also participate in the charge
transport and thus influence the overall thermoelectric
performance. This phenomenon is also called amphoterism,50 which
should be avoided in thermoelectric materials/applications.
Therefore, improving the thermoelectric performance of the
MAPbI3 single crystal could be achieved by suppressing the
amphoterism effect through heavy p-doping of this material.
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Conclusions
In conclusion, the large MAPbI3 single crystal was grown using a
modified fast growth method, and the Seebeck coefficient,
electrical conductivity and thermal conductivity of the MAPbI3
single crystals were measured. The Seebeck coefficients remained
roughly constant in the temperature range of 298 - 330 K, and then
significantly increased with the increase in temperature. A large and
positive Seebeck coefficient of 920 ± 91 μV/K at room temperature
and an ultra-high value of 1693 ± 146 μV/K at 351 K were observed.
5
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A
low thermal conductivity of 0.30 ̶ 0.42 Wm/K, which is
comparable to that of polymer materials, is obtained. It is
envisioned that the MAPbI3 single crystal could be considered as a
new class of thermoelectric materials if proper elemental doping
can be incorporated to improve the electrical conductivity and in
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Acknowledgements
The authors acknowledge financial support from the A*STAR,
Commun., 2015, 6, 7586.
Industry Alignment Fund, Pharos "Hybrid thermoelectric materials 26 Q. Dong, Y. Fang, Y. Shao, P. Mulligan, J. Qiu, L. Cao and J.
for ambient applications" Programme (Grant No.: 1527200021).
Huang, Science, 2015, 347, 967.
This work is also supported by the Grantor Lloyd's Register 27 Y. Liu, Z. Yang, D. Cui, X. Ren, J. Sun, X. Liu, J. Zhang, Q. Wei, H.
Foundation (R-265-000-553-597) and T. Y. acknowledges the
National University of Singapore for the research scholarship. The
Fan, F. Yu, X. Zhang, C. Zhao and S. Liu, Adv. Mater., 2015, 27,
5176.
authors thank Mr. Xingzhi Wang from Nanyang Technological 28 D. Shi, V. Adinolfi, R. Comin, M. Yuan, E. Alarousu, A. Buin, Y.
University for experiment assistance.
Chen, S. Hoogland, A. Rothenberger, K. Katsiev, Y. Losovyj, X.
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