Thermal and Structural Characterization of Methylammonium- and Formamidinium-Halide Salts

Article abstract of DOI:10.1002/pssa.202100246

Four organic halide salts of interest to alloyed perovskite solar cell fabrication are characterized using attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), powder X-ray diffraction (XRD), and thermogravimetric analysis. The chemical and crystal structures of methylammonium iodide (MAI), methylammonium bromide (MABr), and formamidinium iodide (FAI) are confirmed, and the experimental ATR-FTIR spectrum and XRD pattern of formamidinium bromide (FABr) are presented. The enthalpy, ΔH _vap, and entropy, ΔS _vap, of vaporization are quantified for each salt and are used to estimate their vapor pressures in the temperature range of 150–300 °C. MAI, MABr, and FAI have similar vapor pressures in this temperature range, whereas FABr has a higher vapor pressure in the temperature range of 150–240 °C. These data provide a foundation for achieving effective control of vapor phase concentrations for vapor processing of alloyed perovskite solar cells.

Full text of DOI:10.1002/pssa.202100246

Thermal and Structural Characterization of Methylammonium- and
Formamidinium-Halide Salts
Alexander J. Harding Kevin D. Dobson Babatunde A. Ogunnaike William N. Shafarman*
A. J. Harding, Dr. K. D. Dobson, Prof. W. N. Shafarman
University of Delaware, Institute of Energy Conversion
Email Address: wns@udel.edu
A. J. Harding, Prof. W. N. Shafarman
University of Delaware, Department of Materials Science and Engineering
Prof. B. A. Ogunnaike
University of Delaware, Department of Chemical and Biomolecular Engineering
Keywords: lead halide perovskites, vapor pressure, methylammonium halides,
formamidinium halides, crystal structure, vacuum deposition
Four organic halide salts of interest to alloyed perovskite solar cell fabrication are
characterized using attenuated total reflectance Fourier transform infrared spectroscopy
(ATR-FTIR), powder x-ray diffraction (XRD), and thermogravimetric analysis. The chemical and
crystal structures of methylammonium iodide (MAI), methylammonium bromide (MABr), and
formamidinium iodide (FAI) are confirmed, and the experimental ATR-FTIR spectrum and XRD
pattern of formamidinium bromide (FABr) are presented. The enthalpy, Hvap , and entropy,
Svap , of vaporization are quantified for each salt and are used to estimate their vapor pressures in
the temperature range of 150–300
temperature range, while FABr has a higher vapor pressure in the temperature range of 150–240
. This data provides a foundation for achieving effective control of vapor phase concentrations
for vapor processing of alloyed perovskite solar cells.
C
. MAI, MABr, and FAI have similar vapor pressures in this
C
1
Introduction
Hybrid perovskite materials have attracted substantial interest over the past decade due to
[1, 2, 3]
their impressive solar cell power conversion efficiency and increasingly promising stability.
Many improvements have been attributed to the effects of alloying methylammonium lead
triiodide ( MAPbI3 ) with formamidinium (FA), cesium, and bromide. Incorporating these
components into the material allows the perovskite’s electronic, optical, and structural properties
to be tuned, resulting in improved stability and increased stabilized power output of the solar
[
4, 5, 6]
cell.
This in turn increases the commercial viability of perovskite solar cells, even though
some questions regarding manufacturing remain unanswered. While a variety of viable solution
and vapor processes exist for high-throughput perovskite production, some unresolved issues
remain with producing and validating complex stoichiometries that contain multiple components,
[5]
such as Cs (MA FA y) Pb(I yBry )3
.
x
y
1
1
x
1
This article has been accepted for publication and undergone full peer review but has not been
through the copyediting, typesetting, pagination and proofreading process, which may lead to
differences between this version and the Version of Record. Please cite this article as doi:
1
0.1002/pssa.202100246.
This article is protected by copyright. All rights reserved

Here we focus on the challenge of incorporating alloys in vapor processes. While there are
many vapor deposition processes, most fall into one of two categories: single-step processes such
[
7, 8, 9]
[10, 11, 12, 13]
as co-evaporation,
and sequential, deposition-plus-reaction processes.
In
co-evaporation systems, it has proven difficult to control the flux of volatile organic reactants such
as methylammonium iodide (MAI) using, for example, quartz crystal microbalance (QCM)
[
11, 14, 15, 16]
technology.
This is due to the high vapor pressure and low sticking coefficient of MAI,
such that it does not obey line-of-sight mass transport nor does it deposit uniformly on the QCM
[
17]
surface. Some have tried to circumvent this problem by depositing the perovskite at a fixed
system pressure and assuming that the measured pressure is equal to the volatile organic salt’s
[
14, 15, 16]
partial pressure.
While this approach might suffice to produce simple perovskite
chemistries, such as MAPbI3 , for complex stoichiometries containing MA, FA, Cs, and Br, its
usefulness is limited due to a lack of control over each species’ flux. Similar problems arise in
sequential, deposition-plus-reaction processes, where a PbI2 precursor film is reacted with
[
10, 13]
ambient MAI vapor at a fixed temperature over a period of time.
Here, there is limited
understanding of the rate of the MAI delivery as well as intrinsic reaction kinetics. Again, while
this approach may be used to produce perovskites with simple stoichiometries, it is difficult to
determine the composition of alloyed perovskite films without an understanding of
species-specific fluxes to the sample surface. Without this knowledge, scaled-up vacuum
deposition processes are significantly harder to design and implement.
A central problem with these two approaches is a lack of information on the vapor
pressures ( pvap ) of the organic precursor salts used to alloy the perovskite film (MABr, FAI, and
FABr). With known vapor pressures, the flux of each species may be evaluated from measured
temperatures, using standard transport phenomena calculations, and the final composition may be
[
18, 19, 20]
estimated from the flux ratios.
vaporization for MAI, MABr, FAI, and FABr are evaluated using thermogravimetric analysis
TGA). Using a Clausius-Clapeyron fit, the vapor pressure of each species is quantified over the
temperature range = 150–300 . The syntheses of MAI, MABr, and FAI are confirmed using
In this article, the enthalpy ( Hvap ) and entropy ( Svap ) of
(
T
C
attenuated total reflectance spectroscopy Fourier transform infrared spectroscopy (ATR-FTIR),
and powder X-ray diffraction patterns (XRD) are presented for each salt. To the best of our
knowledge, as of the time of this writing, no X-ray diffraction standards for FAI or FABr exist in
the International Centre for Diffraction Data (ICDD) database. We validate our methods by
[
21]
[22]
comparing ATR-FTIR spectra, XRD patterns, and thermodynamic properties of MAI with
literature and then apply them to MABr, FAI, and FABr.
2
Results
2
.1 ATR-FTIR spectroscopy
ATR-FTIR spectroscopy was carried out on powders of each MA- and FA-salt, and the
spectra are shown in Figure 1. Peak positions and assignments for MAI and MABr are provided in
Table S1. The measured peaks for each CH3 , C–N, and NH3 mode are consistent with previous
[
21, 23]
reports and confirm the synthesis of MAI and MABr.
Notably, the MABr peaks are shifted
toward higher wavenumber than the MAI peaks, likely due to different hydrogen bonding
[
24]
interactions between the halides.
This article is protected by copyright. All rights reserved

Figure 1: ATR-FTIR spectra for MAI, MABr, FAI, and FABr powders.
Figure 2: Powder XRD patterns of a) MAI and MABr, and b) FAI, and FABr.
Peak positions and assignments for FAI and FABr are listed in Table S2, and are consistent
[
23, 24]
with previous reports.
>
The appearance of asymmetric NH2 stretching vibrations 3000
1
1
cm , and the NCN stretch at
1690cm , confirms the formation of the formamidinium salt.
The FABr spectrum is nearly identical to that of FAI with strong absorptions of NH2
>
3000
1
1
cm and NCN
1700cm which confirm its structure. As with the MAI salts, many of the
[
24]
FABr absorption peaks appear to be shifted to higher wavenumber.
NH4I or NH Br may be present as a degradation side product of FAI or FABr, respectively,
It is possible that some
[
25,
4
2
6, 27]
1
1
but, due to the overlapping absorption of NH vibrational modes at 1390cm , 3050cm
,
1
[28]
and 3100cm , their presence cannot be confirmed using ATR-FTIR spectroscopy.
This article is protected by copyright. All rights reserved

2
.2 X-ray diffraction analysis
Figure 2 shows powder XRD scans carried out on each of the MA- and FA-salts. The
MAI and MABr patterns have a near-perfect match to their reference patterns in the ICDD
(00-010-0737 and 00-010-0699, respectively). This corroborates the match to published
ATR-FTIR spectra and confirms each species was synthesized successfully.
In the absence of FAI or FABr standards in the ICDD, it is difficult to confirm the structure
[29, 24]
of these materials. However, several published patterns for FAI can be used for comparison.
Previous work on theoretical and single crystal diffraction patterns suggests that FAI can exist in
up to three different phases depending upon the temperature of the sample, although it is thought to
[
29]
exist in the monoclinic space group P2 / c at room temperature.
Several characteristic
1
diffraction peaks for FAI were observed to be in agreement with literature and are listed in Table
[29]
S3. However, several of these peaks are difficult to distinguish from the background due to the
strong reflection at 2 = 25.7 . It is possible that a spurious orientation effect may have been
introduced during the powder pressing process due to strong hydrogen bonding within the FAI
[
24]
crystals, resulting in the strong reflection at 2 = 25.7
.
As for the FABr sample, assigning a crystal structure is beyond the scope of this work;
nevertheless, for completeness, the experimental data are provided. One might expect a similar
structure to FAI, with shifted peak positions due to the smaller Br atoms, however, due to the
[
24, 29]
temperature dependence of the FAI structure,
this might not be the case. It is worth
mentioning that several of the measured diffraction peaks of the FABr sample align with the XRD
pattern of cubic NH Br (ICDD 00-001-0974) and are marked in Table S3 of the Supplementary
4
Information. These NH Br peaks have lower intensities than the FABr peaks and likely indicate
4
that a trace amount of NH Br is present in the sample purchased from Sigma Aldrich.
4
2
.3 Thermogravimetric analysis
Single-pass TGA scans on each MA- and FA- salt are shown in Figure 3. All four salts
lose mass in a single, broad step, although there are small kinks in the FAI and FABr curves at T
[
29]
2
C
50–255 , which are thought to correspond to the melting point of these substances. Here,
FABr begins evaporating at the lowest temperature with MAI, MABr, and FAI starting to
evaporate around the same temperature. MAI appears to finish evaporating first, at a temperature
around 340
C C C
, followed by MABr at 360 , and both FAI and FABr around 400 . While trace
NH Br was observed in the XRD pattern of FABr, it does not appear to affect the rate of mass
4
loss likely due to its high vapor pressure[30] and low concentration.
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Figure 3: Single-pass TGA curves for each MA- and FA-salt. Scans take place from 25
C
to 500
1
1
C
at 10 Cmin under a N2 flow at 50 mLmin
.
Since mass loss occurs in a single step, the thermodynamic properties of vaporization (
Hvap and Svap ) can be determined using the method described in section 5. This method was
applied to each MA and FA salt, and the rates of mass loss are shown in Figure S1 of the
Supplementary Information. In general, the data indicates a linear rate of mass loss for each salt.
While there is some curvature to the measured mass of MAI at 300
C
, this divergence from a
linear rate of mass loss can be attributed to depletion of the MAI powder and small deviations in
the holding temperature during the experiment.
The average rate of mass loss, msub , was evaluated from the mass loss profiles at each
temperature for each salt and used for further analysis. Equations (S1)–(S4) were applied to
produce the mass loss plots shown in Figure S2 and calculate Hvap and Svap . The fitting
2
2
parameters are shown in Table S4 with
R
-values > 0.986 and -values > 0.983 indicating
R
adj
reasonably good agreement between the model and the data without over-constraining the model.
This justifies the use of the slope and the intercept to extract the thermodynamic data for each salt.
Values of Hvap and Svap determined from Figure S2 are presented in Table 1. These
results show that the volatility of the salts increase in order of MAI, MABr, FAI, and FABr, with
[
22]
FABr being significantly more volatile than the others. In previous work on MAI,
the
extrapolated thermodynamic parameters were Hvap =105 5 kJ mol-1 and Svap = 210 14
J mol-1 (Tsub = 247 26 C). There is good agreement between Hvap presented here and from
literature, but Svap differs by 60 J mol-1K-1. This deviations may be due to differences in
measurement protocol, such as total mass of the MAI sample or different N2 flow rates.
Nevertheless, the agreement in Hvap confirms the validity of this method and suggests that the
values of Hvap for MABr, FAI, and FABr are accurate.
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Table 1: Enthalpy and entropy of vaporization for each MA- and FA-salt, where the error bars
represent the 95% confidence interval for each value.
-1
-1 -1
Salt
Hvap [kJ mol ]
Svap [J mol K ]
MAI
MABr
FAI
96 , 6
153 , 12
89 , 1
86 , 1
47 , 3
141 , 1
136 , 3
70 , 5
FABr
The calculated vapor pressure of each MA- and FA-salt is plotted vs 1/T in Figure 4.
There is significant overlap between MAI, MABr, and FAI, indicating that their vapor pressures
are approximately equal across the range of temperatures tested. FABr deviates from the other
salts at lower temperature, with a higher vapor pressure in correspondence with its higher
volatility.
Figure 4: Vapor pressure fits for each salt. The error bars represent the 95% confidence intervals
of each sample.
3
Discussion
The absence of secondary mass loss events in the single-pass TGA scans indicates that the
powders do not undergo decomposition prior to evaporation. We note that both MAI and FAI have
[
17, 31, 32]
been documented to decompose on evaporation.
Therefore, the vapor pressure can be used
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to quantify the partial pressure of each decomposition product. For example, in the case of MAI,
the proposed degradation pathways are
CH NH I(s) >[ ]CH NH (g) HI(g)
(1)
3
3
3
2
CH NH I(s) >[ ]CH I(g) NH (g)
(2)
For the case
3
3
3
3
[
17, 31]
where equation 2 is thought to be favored at temperatures below 350
C
.
[
31, 32]
of FAI, the proposed degradation pathways are
CH(NH )2I(s) >[ ]1/3(HCN) (g) NH (g) HI(g)
(3)
(4)
(5)
2
3
3
CH(NH )2I(s) >[ ]HCN(g) NH (g) HI(g)
2
3
CH(NH )2I(s) >[ ]CN H (g) HI
2
2
5
[
31]
where equation 3 is thought to be favored below 350
C
.
In either case, the products of a
given degradation pathway form according to the stoichiometry of the decomposition process,
with the sum of the partial pressures of each degradation product equal to the vapor pressure of the
salt in question multiplied by its stoichiometric coefficient. However, while the degradation
[
17, 31, 32]
pathways of MAI and FAI are well-documented,
those of MABr and FABr are not as
well-established. It is likely that these bromide salts have similar degradation pathways to their
iodine counterparts.
The data presented here allows researchers to quantify the vapor pressure of MAI, MABr,
FAI, and FABr and calculate the partial pressure of each degradation product for vapor phase
perovskite processing. By controlling source temperatures, an appropriate amount of each species
can be delivered to the substrate in both batch and continuous processes. This should improve the
understanding of vapor phase deposition and reaction processes and allow more precise control
over the deposited perovskite’s stoichiometry. The concentration data and models developed from
this data will serve as a cornerstone for the design of scaled-up vapor-phase perovskite processing
systems.
4
Conclusion
The organic halide salts MAI, MABr, FAI, and FABr were characterized using
ATR-FTIR, XRD, and TGA to confirm their identities, provide experimental diffraction patterns,
and to evaluate the equilibrium vapor pressure parameters Hvap and Svap . MAI, MABr, and
FAI were found to have near-identical vapor pressures over the window of temperatures tested.
FABr was found to have a significantly higher vapor pressure than the other salts from 150–240
C
C
but begins to overlap from 240–300 . With this data, the temperature-dependent vapor
pressure can be evaluated for any vapor processing system of interest, allowing more precise
control over the stoichiometry of alloyed perovskite thin films.
This article is protected by copyright. All rights reserved

5
Methods
Synthesis of MAX and FAX
MAI and MABr were synthesized based on procedures established in literature.[33, 34] 40
mL hydroiodic acid (57 wt.% in water, Sigma Aldrich) or 31 mL hydrobromic acid (48 wt.% in
water, Sigma Aldrich) were added drop-wise to a 38 mL methylamine solution (33 wt.% in
absolute ethanol 98%, Sigma Aldrich) in argon and stirred at room temperature for 2 hours. The
resulting solution was dried until a white precipitate formed, which was collected and rinsed with
diethyl ether (Fisher). The powder was recrystallized twice from ethanol (Decon), rinsed well with
diethyl ether, dried, and stored in a dry N2 desiccator.
[
23]
For FAI synthesis,
beaker of 50 mL HI at 0
. The resulting white solids were recrystallized from ethanol twice at T
well with diethyl ether. Moderate temperatures were used during the purification steps to avoid
19.7 g formamidinium acetate (Sigma Aldrich), was added to a
under argon and stirred for 2 hours. Solutions were then dried at T
50 and rinsed
C
<
5
0
C
<
C
[25, 26, 27]
decomposition of the FA+ ion to NH /NH .
3
4
Due to difficulties in preventing the decomposition of the FA cation during FABr synthesis
and in separating synthesized FABr from the side product of NH Br, FABr was purchased from
4
Sigma Aldrich (98%), and the ATR-FTIR, XRD, and TGA analyses were performed on the
commercial salt.
Measurement procedures
Prior to measurement, all samples were ground to a fine powder using a mortar and pestle
in an inert N2 glovebox.
ATR-FTIR spectra were recorded using a Thermo Scientific Nicolet 6700 FTIR
spectrometer equipped with an ATR element accessory. The ATR attachment was fitted with a
ZnSe crystal and powder samples were held in direct contact with the crystal using an attached
clamp.
Powder samples were prepared for XRD by pressing the ground powder sample into a
platen using a clean glass slide. Powder XRD patterns were obtained using a Rigaku D/Max 2200
scanning x-ray diffractometer with a CuK X-ray source operating at 40 kV and 40 mA in
Bragg-Brentano parafocusing geometry. Scans were carried out from 5–90 at a rate of 0.5
1
min . The XRD sample chamber was filled with Ar to prevent the hygroscopic FABr powder
from absorbing moisture.
Thermal analysis was carried out using a TA Instruments Discovery TGA. The TGA pans
were scrubbed using ethanol and heated with a blowtorch prior to weight calibration and sample
1
measurement. Scans were performed under a nitrogen flow of 50 mLmin . Single pass TGA
1
scans were carried out from 25
C
to 500
C
at a heating rate of 10 Cmin . Incremental
isothermal scans were performed by heating a sample from 25
Cmin , holding for a 10 min period, and ramping to the following temperature setpoint. The
C
to 150
C
at a rate of 25
1
isothermal temperature setpoints of 150, 200, 225, 250, 275, and 300
C
were used for each
sample.
Determining vapor pressure
The following equations are used to determine the enthalpy of vaporization, Hvap , and
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the entropy of vaporization, Svap , for each species from the TGA measurements and to evaluate
their vapor pressures, pvap , with an associated 95% confidence interval. First, according to an
expression from Langmuir, pvap , is a function of the rate of mass loss due to sublimation, msub
,
[
35]
and temperature,
T
.
msub 2 RT
pvap
=
(6)
A
Mw
Here,
Mw is the molecular weight.
Next, pvap is related to Hvap through the Clausius-Clapeyron equation.
is the exposed area of the evaporating species, is the ideal gas constant, and
A R
dln pvap
dT
Hvap
=
(7)
(8)
2
RT
When evaluated as an indefinite integral, the Clausius-Clapeyron equation becomes
Hvap
ln p =
C,
RT
where
C
is a constant of integration.
Equation (6) and (8) can then be combined to relate msub with Hvap
.
msub
2
RT
Hvap
RT
ln
=
C
(9)
A
Mw
For TGA measurements of powder samples, the exposed area,
the 1 cm diameter of the TGA pan. By substituting the appropriate values for Mw
the left-hand-side can be plotted vs. 1/T to give a straight line whose slope , and intercept
A
, is approximated from
,
msub , and ,
T
m
C
[36]
,
are used to determine, respectively, Hvap and Svap .
Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements
This material is supported in part by the Engineering Research Center Program of the
National Science Foundation and the Office of Energy Efficiency and Renewable Energy of the
Department of Energy under NSF Cooperative Agreement No. EEC-1041895. The authors wish to
acknowledge Brian McCandless for assistance with the powder x-ray diffraction measurements
and Gerald Poirier for assistance with phase identification.
Conflict of Interest
The authors have no conflict of interest to declare.
This article is protected by copyright. All rights reserved

Data Availability Statement The data that support the findings of this study are openly
[
37]
available in Mendeley Data at http://doi.org/10.17632/647z57d26b.2, reference number
.
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[
36] Abrefah J., Olander D. R., Balooch M., Siekhaus W. J., Appl. Phys. Lett. 1992, 60,
1
313 1313.
[
37] Harding A., Dobson D., Ogunnaike B., Shafarman W., ATR-FTIR, XRD, and
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Table of Contents
Four organic halide salts of interest to perovskites solar cells (methylammonium iodide,
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methylammonium bromide, formamidinium iodide, and formamidinium bromide) are structurally
and thermally characterized. Their ATR-FTIR spectra and x-ray diffraction patterns are presented.
In addition, their sublimation enthalpies and entropies are determined and used to calculate each
salt’s vapor pressure in the temperature range 150
< <
C C
T
300 .
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