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Angewandte
A Journal of the Gesellschaft Deutscher Chemiker
International Edition
Chemie
www.angewandte.org
Accepted Article
Title: Methylammonium Iodide Effect on the Supersaturation and
Interfacial Energy of the Crystallization of Methylammonium Lead
Triiodide Single Crystals
Authors: Bichen Li, Furkan Isikgor, Hikmet Coskun, and Jianyong
Ouyang
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201710234
Angew. Chem. 10.1002/ange.201710234
Link to VoR: http://dx.doi.org/10.1002/anie.201710234
http://dx.doi.org/10.1002/ange.201710234
10.1002/anie.201710234
Angewandte Chemie International Edition
COMMUNICATION
Methylammonium Iodide Effect on the Supersaturation and
Interfacial Energy of the Crystallization of Methylammonium Lead
Triiodide Single Crystals
Abstract: It is very important to study the crystallization of hybrid
organic-inorganic perovskites because their thin films are usually
prepared from solution. But the investigation on the growth of
perovskite films is limited by their polycrystallinity. In this work,
methylammonium lead triiodide single crystals grown from solutions
with different methylammonium iodide (MAI):lead iodide (PbI2) ratios
were investigated. We observed a V-shaped dependence of the
crystallization onset temperature on the MAI:PbI2 ratio. This is
attributed to the MAI effects on the supersaturation of precursors and
the interfacial energy of the crystal growth. At low MAI:PbI2 ratio (<1.7),
more MAI leads to the supersaturation of the precursors at lower
temperature. At high MAI:PbI2 ratio, the crystal growing plans change
from (100)-plane dominated to (001)-plane dominated. The latter have
higher interfacial energy than the former, leading to higher
crystallization onset temperature.
Hybrid organic-inorganic perovskites (HOIPs) have attracted
great attention due to their interesting structure and electronic
properties. Film-based devices like perovskite solar cells (PSCs)
show excellent performance.The power conversion efficiencies
(PCEs) rapidly climbed from 3.8% to over 22% in just 6 years.[1–4]
The performance of PSCs strongly depends on the quality of the
HOIP thin film. The popular way to prepare a perovskite thin film
is through the solution processing of precursors. The composition
and properties of the precursor solution including the ratio of the
precursors are critical for the quality of the perovskite film. For
example, a small amount of excess PbI2 can improve the PCE
because it can act as a passivation layer between the grain
boundaries to inhibit recombination centers.[5–7] In contrast,
excess MAI helps the formation of smooth, continuous and
pinhole free HOIP films.[8,9] In addition, excess MAI can lead to
better crystal quality and higher PL yields at the grains and grain
boundaries, giving rise to high open circuit voltage. [10,11] However,
the study on the precursor stoichiometry effect has been focused
on the HOIP polycrystalline film. Coordination complexes in
precursor solution may remain in the HOIP films due to rapid
evaporation of solvent.[12-15] They may change the crystallization
of HOIP by means of formation of unfavourable intermediate
phases at the early stage.
[*]
B. Li, F. Isikgor, H. Coskun, Prof. J. Ouyang
Materials Science and Engineering
National University of Singapore
7 Engineering Drive 1, Singapore, 117574
E-mail: mseoj@nus.edu.sg
These problems can be addressed by studying HOIP single
crystals, because the perfect crystallographic structure of single
crystals can exclude the effect of any residual such as excess
precursor or intermediate phase on its properties. In addition,
HOIP single crystals have much better stability than HOIP thin
films in ambient conditions. Herein, MAPbI3 single crystals were
grown from precursor solutions with different MAI:PbI 2 ratios. The
MAI:PbI2 ratio affects the crystallization onset temperature and
surface planes of the MAPbI3 single crystals. These are ascribed
to the MAI effects on the supersaturation of the precursors and
the interfacial energy of the growing planes.
MAPbI3 single crystals were grown by the inverse temperature
crystallization (ITC) method.[16,17] When the precursors become
oversaturated at a certain temperature, crystallization starts and
a black tiny crystal can be observed. The temperature for the
observation of the first tiny crystal is defined as the crystallization
onset temperature in this paper. As shown in Figure 1, the onset
temperature depends on the MAI:PbI2 molar ratio. It is 159 oC for
the solution with equimolar MAI and PbI2, and it decreases to 112
o
C with the increasing MAI concentration until the ratio reaches
1.7. The temperature then increases to 138 oC at the ratio of 2.5.
At the MAI:PbI2 ratio of 3, no crystals could be observed even
when the temperature was raised to 170 oC. These results
suggest the formation of perovskites needs to overcome higher
energy barrier at the presence of much excess of MAI. This
observation is consistent with the activation energy for the MAPbI 3
film formation as calculated by Khlyabich et al.[18] Their
calculations indicated a higher activation energy for the MAI:PbI 2
ratio of 3 than for the MAI:PbI2 ratio of 1. Apart from growing single
crystal with excess MAI, we also tried to grow single crystals in
solutions with excess PbI2. At the PbI2:MAI molar ratio of 1.2,
yellow precipitates instead of black MAPbI 3 crystals were
observed due to the poor solubility of PbI2 in γbutyrolactone (GBL). Therefore, we focus on the precursor
solutions with excess MAI.
The V-shaped dependence of the crystallization onset
temperature on the MAI:PbI2 ratio indicates a complicated
nucleation and growth process of the crystals. In terms of
thermodynamics, the onset temperature is related to the
nucleation process of the single crystal, which is dependent on
the change of the total free-energy (G) of precursor solution
together with the nucleus,[19]
∆G(r) = (−
4πr3
3VM
) RTln(S) + 4πr 2 γCL (1)
where CL is the interfacial energy between liquid and nucleus, VM
is the molar volume of the nucleus, R is the gas constant, T is
absolute temperature and S is the supersaturation ratio defined
as S=C/Cs with C being the solute concentration and Cs the
solubility limit (C>Cs).[19] The supersaturation causes the nucleus
This article is protected by copyright. All rights reserved.
Accepted Manuscript
Bichen Li, Furkan H. Isikgor, Hikmet Coskun and Jianyong Ouyang*
10.1002/anie.201710234
Angewandte Chemie International Edition
COMMUNICATION
20
1:1-25 oC
1:1-50 oC
1:1-75 oC
1:1-90 oC
Onset temperature
Intensity (%)
160
150
140
130
15
1.6
10
1:1
1.2:1
1.5:1
1.7:1
2:1
1.4
1.3
1.2
1.1
5
120
1.0
2.0
1.5
2.0
2.5
MAI:PbI2 molar ratio
Figure 1. Variation of the crystallization onset temperature of MAPbI3 with the
MAI:PbI2 molar ratio
formation, while the interfacial energy acts as energy barrier for
the nucleation.
In order to understand the MAI effect on the supersaturation,
the precursor solutions with different MAI:PbI2 ratios were studied
by the absorption spectroscopy. As reported by Yan et al, the
HOIP precursor solutions are colloidal dispersions made of lead
polyhalide frameworks, such as iodoplumbates PbIn(n-2)-.[9] The
iodoplumbates with different n values have absorption bands in
different wavelengths. As shown in Figure S1, the three
absorption bands at 276, 320 and 370 nm are due to PbI 2, PbI3and PbI42-, respectively.[20] At the presence of more MAI, more
iodoplumbates with higher n values can be observed.
The colloids in the precursor solution were studied by and
dynamic light scattering (DLS) and absorption spectroscopy
(Figure 2 and Figure S2). The solutions were gradually heated
from 25 to 90 oC. It is worth to note that there is no nucleation in
this temperature range because the temperatures are lower than
the lowest onset temperature of nucleation. This helps
understand the change in the precursor species prior to the
nucleus formation. In all the precursor solutions, there are mainly
two DLS peaks at around 1 nm and several hundreds of
nanometers. This is consistent with the observations by Tsai et
al.[21] The colloidal size changes with the MAI:PbI 2 ratio and the
temperature. The size of the larger colloids increases from ~400
to ~1100 nm when the MAI:PbI2 ratio increases from 1 to 2, that
is, more MAI may cause the formation of high-order iodoplumates.
When the temperature is raised, the size of the large colloids
decreases, which indicates the dissolution of colloids. The small
colloids with the size of around 1 nm become smaller at high
temperature as well. Although size change of the colloids in HOIP
precursor solution was observed during aging,[21] its fundamental
mechanism is different from ours. The size change in our
experiments is caused by temperature. The colloids sizes change
to the original values when the solution at 90 oC is cooled back to
25 oC (Figure S2(e)), which also indicates the size change is
reversible.
The two types of colloids are much larger than the size of
individual I- or Pb2+ ion whose ionic radii are 0.2 and 0.13 nm,
respectively. It is interesting to note these ion sizes are quite close
to the temperature-caused size change of the colloid of around 1
nm as shown in Figure 2(b). Thus, we can infer that the decrease
in the colloid size arises from the release of free ions into the
solution. This is consistent with a recent study that proposed the
release of free ions occurs prior to the HOIP crystallization.[22]
Absorbance (a.u.)
1.0
1.5
0.9
1
10
100
1000
20
Size (d.nm)
(c)
RT
70 oC
100 oC
130 oC
PbI2
1.0
PbI-3
PbI24
0.5
0.0
300
350
400
450
30
40
4
50
60
70
Temperature (oC)
80
(d)
90
100
RT
70 oC
100 oC
130 oC
3
PbI2
2
PbI3-
PbI24
1
0
300
Wavelength (nm)
350
400
450
Wavelength (nm)
Figure 2. (a) Size distribution of colloids in perovskite precursor solutions with
the MAI:PbI2 ratio of 1 from 25 to 90 oC. (b) Variation of the colloids of ~1 nm
with temperature. (c) Absorption spectra of a precursor solution with the
MAI:PbI2 ratio of 1.7 at different temperatures and its normalized spectra with
the absorbance normalized to that at 370 nm (d).
The conversion from high-order to low-order iodoplumbates and
release of free ions are further verified by the absorption
spectroscopy (Figure 2(c)). It is evidently shown that the
iodoplumbates decrease with the elevating temperature,
indicating the conversion of iodoplumbates to free ions. In addition,
the normalized spectra show that the PbI 3- and PbI2 amounts
relative to PbI42- increase as well. This evidences the release of
free ions from the iodoplumbates. This process is reversible as
revealed by the recovery of the absorption spectrum (Figure S3).
The chemical reactions in equation (2) present the conversion
from iodoplumbates to I- and Pb2+.
PbI42− ↔ PbI3− + I−
PbI3− ↔ PbI2 + I−
PbI2 ↔ PbI+ + I−
PbI+ ↔ Pb2+ + I− (2)
The iodoplumbate formation and the ion release shown above
depend on the interaction between iodoplumbates and solvent
molecule as well. The Gibbs free energy of formation of the hexacoordinated lead complex with GBL is much higher than that with
DMSO or DMF and slightly higher than some iodoplumbates like
PbIM5+ or PbI2M4 with M representing solvent molecule. Free ions
are favoured at high temperature in GBL but not in DMF or
DMSO.[23] Hence, the dissociation of GBL-based plumbates at
elevated temperature is necessary to commence retrograde
solubility in the ITC method.[24] As a result of the release of free
ions, the solution becomes supersaturated, and the nucleation
commences. This nucleation can be described by the La Mer
mode.[25,26] The concentration of monomers increases above the
saturation concentration when nucleation starts. Surface growth
occurs by ions diffusion towards the clusters followed by reaction
of the ions at the surface. In the case of excess MAI in the
This article is protected by copyright. All rights reserved.
Accepted Manuscript
0
110
100
(b)
1.5
Normalized absorbance (a.u.)
Onset temperature (oC)
1.7
(a)
Size (d.nm)
170
10.1002/anie.201710234
Angewandte Chemie International Edition
Figure 3. Photos of MAPbI3 crystals grown from precursor solutions of different
MAI:PbI2 ratios annotated as (a) type-I, (b) type-II, and (c) type-III.
precursors, more concentrated colloids and high-order
iodoplumbates accelerate the dissolution of colloids and release
of free ions, which is understandable in terms of the equilibrium of
the chemical reactions. The accelerated process of reaching
super-saturation leads to higher S in equation (1), and thus lowers
the onset temperature of the nucleation.
As shown in the Figure 3, the MAI:PbI2 ratio affects the shape
of the crystals and the crystallographic planes of the surfaces. The
samples are single crystals rather than polycrystalline materials,
which is supported by the SEM images (Figure S4). Apart from
the surface, the crystals were cleaved, and no boundary was
observed on the cleaved faces. The crystals can be categorized
into three types in terms of their shapes. Crystals grown from the
precursor solutions with the ratios of 1, 1.2 and 1.5 have the same
shape, and they are annotated as type-I. The crystal shape
becomes very different for the ratios of 1.7 and 2. They are
annotated as type-II and type-III, respectively. The face indexes
of the crystal surfaces were determined by the powder XRD
(Figure S5 and Table S1).
The shapes of the crystals are consistent with the face
indexing results. The type-II crystal has three obvious surfaces
that are perpendicular to each other, which should be the faces
perpendicular to the crystallographic a, b, and c axes. But such
three perpendicular faces cannot be found in type-I and type-III
crystals. The large surface of the type-III crystal can be identified
as the (001) plane.
The crystal surfaces can be considered as the growing planes
of the crystals. Thus, the main growing plane is (100) plane for
the type-I crystal, which is the largest surface on top of the crystal
and consistent with the single crystals reported by other
groups.[27,28] This is evidenced by the strong diffraction from (200)
and (400) as well. But the (001) plane becomes an important
plane for the type-II crystal as shown by the large lateral surface
in Figure 3(b). It is interesting to note that the (001) plane
becomes the dominant growing plane at high MAI:PbI 2 ratio.
We try to understand the interfacial energy through the
estimation of the surface energy. Figure S6 and Figure S7 shows
the atoms on the different crystal planes and the creation of
corresponding planes, respectively. The surface energies are
mainly due to the dangling bonds at the surfaces. The numbers
of the broken bonds, surface areas and estimated surface energy
are listed in Table S2. The estimated surface energies were close
to previous report on the calculation of surface energy in
orthorhombic crystal.[29] Among the crystal planes, the (100) plane
has the lowest surface energy. This plane is the main growth
plane for the type-I crystal. With the increasing amount of MAI in
the precursor solution, (001) planes can be observed for the typeII crystal. Since the (001) plane has higher surface energy than
(100), the interfacial energy contribution to the change of the
Gibbs free energy increases. This becomes more significant
when the MAI:PbI2 ratio increases to 2.
It is worthy to mention that the type-III crystal is not perfect
single crystal even though no grain boundaries were found.
Faceting occurs on the surface of the crystal. This may be
attributed to the very high interfacial energy between the nuclei
and the solution. To reduce the energy of the system,
decomposition of the boundary plane into planes with low
energies is expected to create small areas of connecting planes.
This is consistent with the existence of relatively low-energy plane
such as (110).
The interfacial energy does not change too much when the
MAI:PbI2 ratio is less than 1.7, which is supported by the same
shape of the crystals. In terms of equation (1), when the interfacial
energy remains unchanged, higher S leads to lower onset
temperature of crystallization. As a result, the onset temperature
will decrease until the MAI:PbI2 ratio reaches 1.7. In this case, the
supersaturation process is the dominant factor for the
crystallization. However, when the ratio is higher than 1.7, S may
further increase but the interfacial energy becomes the dominant
factor. This is evidenced by the shape change of the crystals and
the appearance of planes with higher surface energy. As surface
energy is related to interfacial energy, the increase in the surface
energy will elevate the onset temperature.
The results on single crystal growth can help understand the
formation of HOIP thin films. However, the temperature for film
formation is usually different from the crystallization onset
temperature, and the crystallization is caused mainly by the
solvent vaporization. In order to understand whether the
temperature and solvent vaporization affect the growing planes of
the single crystals, we prepare samples of numerous tiny crystals
by drop casting method. Two temperatures, the crystallization
onset temperature and 100 oC, were investigated for each
precursor solution. The SEM images of the samples are shown in
Figure S8. The crystals grown at the two temperatures exhibit the
same shapes. Thus, the temperature during the crystallization
may have negligible effect on the surface planes of the crystals.
The surface planes depend primarily on the MAI:PbI2 ratio in the
precursor solution. In addition, the shapes of the tiny crystals
prepared by drop casting are consistent with the single crystals
by the ITC method, though their ways to the crystallization are
different. Therefore, our study on the effect of excess MAI on the
crystallization of perovskite single crystal helps understand the
crystallization of perovskite thin films.
In conclusion, we have reported the effects of MAI on the
crystallization onset temperature, the growing planes and the
shape of perovskite single crystals . When the MAI:PbI 2 ratio is
low, the MAI effect on the supersaturation of the perovskite
precursor is the dominant factor for the crystallization. More MAI
results in crystal growth at lower temperature. At high MAI:PbI 2
ratio, the interfacial energy becomes the dominant factor. More
MAI leads to the increase in the interfacial energy. Accordingly,
the crystallization onset temperature increases with the
increasing MAI:PbI2 ratio.
Acknowledgements
This article is protected by copyright. All rights reserved.
Accepted Manuscript
COMMUNICATION
10.1002/anie.201710234
Angewandte Chemie International Edition
This research work was financially supported by a research grant
from the Ministry of Education, Singapore (R284-000-147-112).
[13]
[14]
Conflict of interest
The authors declare no conflict of interest.
Keywords: crystallization • interfacial energy • perovskites •
HOIP • growing plane
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This article is protected by copyright. All rights reserved.
Accepted Manuscript
COMMUNICATION
10.1002/anie.201710234
Angewandte Chemie International Edition
COMMUNICATION
Entry for the Table of Contents (Please choose one layout)
Layout 1:
COMMUNICATION
Onset temperature
160
150
Page No. – Page No.
140
Methylammonium Iodide Effect on the
Supersaturation and Interfacial
Energy of the Crystallization of
Methylammonium Lead Triiodide
Single Crystals
130
120
110
100
Bichen Li, Furkan H. Isikgor, Hikmet
Coskun, Jianyong Ouyang*
1.0
1.5
2.0
2.5
MAI:PbI2 molar ratio
This article is protected by copyright. All rights reserved.
Accepted Manuscript
170
Onset temperature (oC)
The methylammonium iodide
(MAI):PbI2 ratio significantly affect the
supersaturation of precursors and the
interfacial energy of the crystal
growth. As a result, it has effect on
both the crystallization onset
temperature and the shape of the
perovskite single crystal.
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