Crystal Growth, Structural Phase Transitions, and Optical Gap Evolution of CH3NH3Pb(Br1-xClx)3 Perovskites

Article abstract of DOI:10.1021/acs.cgd.8b01463

Chemically tuned inorganic-organic hybrid halide perovskites based on bromide and chloride anions CH₃NH₃Pb(Br_1-xCl_x)₃ have been crystallized and investigated by synchrotron X-ray diffraction (SXRD), scanning electron microscopy, and UV-vis spectroscopy. CH₃NH₃PbBr₃ and CH₃NH₃PbCl₃ experience successive phase transitions upon cooling, which are suppressed for intermediate compositions probably due to compositional disorder. For CH₃NH₃PbCl₃, a transient phase, formerly described as tetragonal, was identified at 167.5 K; the analysis of SXRD data demonstrated that it is indeed orthorhombic, with space group Pnma, and a ≈ 2a_p; b ≈ 2a_p; c ≈ 2a_p (a_p is the ideal cubic perovskite unit-cell parameter). The band gap engineering brought about by the chemical management of CH₃NH₃Pb(Br_1-xCl_x)₃ perovskites can be controllably tuned: the gap progressively increases with the concentration of Cl ions from 2.2 to 2.9 eV, and shows a concomitant variation with the unit-cell parameters of the cubic phases at 295 K. This study provides an improved understanding of the structural and optical properties of the mixed CH₃NH₃Pb(Br_1-xCl_x)₃ perovskites.

Full text of DOI:10.1021/acs.cgd.8b01463

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Article
Crystal growth, structural phase transitions and optical
gap evolution of CH3NH3Pb(Br1-xClx)3 powders
M.C. Alvarez-Galvan, J.A. Alonso, C.A. Lopez, E. Lopez-Linares,
C. Contreras, M.J. Lázaro, F. Fauth, and M.V. Martínez-Huerta
Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b01463 • Publication Date (Web): 11 Dec 2018
Downloaded from http://pubs.acs.org on December 12, 2018
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Crystal growth, structural phase transitions and optical gap evolution of
CH NH Pb(Br Cl ) powders
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M.C. Alvarez-Galván , J. A. Alonso , C.A. López , E. López-Linares , C. Contreras ,
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M.J. Lázaro , F. Fauth and M.V. Martínez-Huerta *
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Instituto de Catálisis y Petroleoquímica, CSIC, Cantoblanco 28049 Madrid, Spain
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Instituto de Ciencia de Materiales de Madrid, CSIC, Cantoblanco 28049 Madrid, Spain
Instituto de Investigaciones en Tecnología Química (INTEQUI), UNSL, CONICET and
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Facultad de Química, Bioquímica y Farmacia, UNSL, Chacabuco y Pedernera, San Luis,
700, Argentina.
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Instituto de Carboquímica, CSIC, Miguel Luesma Castán 4, E50018 Zaragoza, Spain
5CELLS–ALBA synchrotron, Cerdanyola del Valles, Barcelona, E-08290, Spain
*
ja.alonso@icmm.csic.es; mmartinez@icp.csic.es
Abstract
Chemically tuned inorganic-organic hybrid halide perovskites based on bromide and
chloride anions CH NH Pb(Br Cl ) have been crystallized and investigated by
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synchrotron X-ray diffraction (SXRD), scanning electron microscopy and UV-vis
spectroscopy. CH NH PbBr and CH NH PbCl experience successive phase transitions
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upon cooling, which are suppressed for intermediate compositions probably due to
compositional disorder. For CH NH PbCl a transient phase, formerly described as
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tetragonal, was identified at 167.5 K; the analysis of SXRD data demonstrated that it is
indeed orthorhombic, with space group Pnma, and a ≈ √2a ; b ≈ 2a ; c ≈ √2a (a is the
p
p
p
p
ideal cubic perovskite unit-cell parameter). The band gap engineering brought about by the
chemical management of CH NH Pb(Br Cl ) perovskites can be controllably tuned: the
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1-x x 3
gap progressively increases with the concentration of Cl ions from 2.2 to 2.9 eV, and shows
a concomitant variation with the unit-cell parameters of the cubic phases at 295 K. This
study provides an improved understanding of the structural and optical properties of the
mixed CH NH Pb(Br Cl ) perovskites.
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1-x x 3
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Keywords: hybrid perovskite, mixed halide, bromine, chlorine, solar cells
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1
. Introduction
Organic-inorganic hybrid halide perovskites have shown tremendous potential as the
absorber layer in third-generation photovoltaic cells technologies that can compete with the
well-established silicon solar cells in terms of power conversion efficiency (PCE) and cost.
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. Over the past years, an intensive research has been devoted to the improvement of halide
perovskite-based solar cells, where a variety of cell architectures has been developed,
increasing the power conversion efficiencies to 23 %. 2 In addition, the structural and
optoelectronic properties of these materials can be easily tuned through compositional
engineering of various organic ligands, metals, or halogens.3-4 By varying their
compositions, perovskites with different bandgap energies (E ) have been successfully
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employed for other optoelectronic applications such as wave-tunable lasing, light-emitting
diodes, photodetectors, and photocatalysts.9
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Halide perovskite compounds include the compositions MAPbX (MA = CH NH ,
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X = Cl, Br, I). CH NH PbI -based perovskite solar cells have been a primary focus due to
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their near-complete visible light absorption in films <1 μm and their fast charge extraction
rates. However, the poor stability of CH NH PbI and rapid degradation in humidity has
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remained a major obstacle for commercialization.10 CH NH PbBr is a promising
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alternative to CH NH PbI with a large band-gap of 2.2 eV, which gives rise to a high open
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circuit voltage (Voc ≈ 1.2–1.5 V) . Their long exciton diffusion length (>1.2 μm) enables
good charge transport in devices. In addition, CH NH PbBr demonstrates higher stability
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towards air and moisture due to its stable cubic phase and low ionic mobility relative to the
pseudocubic CH NH PbI , in which inherent lattice strain provides an avenue for increased
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diffusion. Maculan et al. reported that the trap-state density, charge carrier concentration,
mobility, and diffusion length of CH NH PbCl are comparable with those of the best
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quality crystals of CH NH PbI and CH NH PbBr . Furthermore, CH NH PbCl possesses
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a wide band gap of 3.11 eV and it is visibly transparent but sensitive to the ultraviolet (UV)
region. CH NH PbCl -based UV photodetectors exhibit high ON−OFF current ratio, fast
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photoresponse, and long-term photostability. CH NH PbCl can also be used in transparent
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optoelectronics, fire and missile plume detection, and optical communications. 13 However,
CH NH PbCl has been rarely investigated both theoretically and experimentally, as well as
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the mixed anion CH NH Pb(Br Cl ) perovskites. The optimal band gap and suitable
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structure of the materials are key factors for successful application of organic–inorganic
hybrid halide perovskites in optoelectronic devices.
Herein, we performed a systematic study of the structural and optical properties of
mixed organic-inorganic hybrid perovskites CH NH Pb(Br Cl ) by combining
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synchrotron X-ray diffraction (SXRD), scanning electron microscopy (SEM) and UV-vis
spectroscopic measurements.
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. Experimental
The mixed perovskites CH NH Pb(Br Cl ) (x = 0, 0.33, 0.5, 0.67 and 1) were
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synthesized from a solution of stoichiometric amounts of CH NH X and PbX (X = Cl, Br)
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in dimethyl formamide (DMF).
Synthesis of CH NH X (X: Br, Cl)
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The chemicals used for the synthesis of methyl ammonium bromide and methyl
ammonium chloride are methyl amine (CH NH , Aldrich, 33 % in ethanol) and the
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corresponding acid: HBr (Alfa Aesar, 48 % in water) and HCl (Sigma-Aldrich, 37 % in
water). The molar ratio CH NH /HX is 1.2. The reaction between both compounds is
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exothermic, being carried out at 0ºC through a refluxing process. First, CH NH is added in
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a flask and then HX is added dropwise slowly. Then, it is left under stirring for 2 h. Finally,
the synthesis is finished by rotary evaporation at 60ºC, by which a white powder is
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formed . This product is rinsed 3 times with diethyl ether (decanting in between). The
product was finally dried in a vacuum drying oven at 55ºC for 12 h and stored in a glove
box or in a glass desiccator.
Synthesis of CH NH PbX
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The synthesis of CH NH Pb(Br Cl ) (x = 0, 0.33, 0.5, 0.67 and 1) was carried out
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by reaction between stoichiometric amounts of CH NH X (X = Cl, Br) and PbX (X = Cl,
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Br); PbBr (Alfa Aesar, ultradry 99,99 %), PbCl (Alfa Aesar, ultradry 99,99 %) in N,N
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dimethyl formamide (DMF) with a molar ratio CH NH X/PbX = 1 for all the samples,
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with the exception of CH NH PbCl , for which it was 2. This higher value was chosen in
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order to shift the reaction to the formation of the perovskite, since, in this case, the
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thermodynamic equilibrium is not so favored . The synthesized perovskites are named as
follow: MAPbBr , MAPbBr Cl, MAPbBr Cl , MAPbBrCl and MAPbCl .
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A closed vial containing the suspension of both compounds in DMF (where the
weight of DMF is equal to the sum of the precursors weight) is left under stirring on a hot
plate at 60ºC for 22 h. Then, the content is poured on a crystallizer placed on a hot plate at
100ºC for 30 min16-17. An orange/yellow powder is obtained (the colour depends on the
proportion of bromine, being more orange with a higher proportion of this element).
X-ray diffraction profiles of the powder catalysts were obtained on a PANalytical
X’Pert Pro X-ray diffractometer with a Cu Kα source. The samples were additionally
studied by synchrotron X-ray powder diffraction (SXRD) in the MSPD high-resolution
diffractometer at the ALBA facility, Barcelona (Spain), selecting an incident beam with 38
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keV energy, λ= 0.3252 Å with the high angular resolution mode (MAD set-up). The
polycrystalline powders were contained in quartz capillaries of 0.3 mm diameter, which
were rotating during the acquisition time. SXRD patterns were collected at room
temperature (295 K, RT), then cooled to 120 K, and when a phase transition was detected,
measured at increasing temperatures along the sequence 150, 180, 210, 240 and 270 K. For
MAPbCl , additional patterns were collected at 160, 165, 167.5, 170, 172.5, and 175 K.
3
The refinement of the structures was performed by the Rietveld method using the Fullprof
19
software. There were no regions excluded in the refinement. The following parameters
were refined: zero-point error; scale factor; background coefficients; pseudo-Voigt shape
parameters; occupancy of the elements; atomic coordinates; and anisotropic displacements
for the metal and halogen atoms.
The Scanning Electron Microscope images were obtained on a Hitachi instrument,
model TM-1000. The optical diffuse reflectance spectra of perovskites powder were
measured at room temperature using a UV-VIS spectrophotometer Varian Cary 5000.
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. Results and Discussion
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.1. Crystal growth
The CH NH PbX (X = Cl, Br) perovskites were obtained as crystalline materials,
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exhibiting crystals of variable sizes and colors, varying from orange for CH NH PbBr to
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increases, as shown in the optical microscope images included in Fig. 1.
Fig. 1. Optical microscope images of as-grown CH NH PbX (X = Cl, Br) perovskites,
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showing the color variation as Cl is introduced.
SEM images were obtained to better understand the effect of halide composition on
the morphology and structure of crystals (Fig. 2). In all cases, the obtained perovskites
show cuboid-type microcrystals. A second morphology, approaching parallelepiped crystals
is observed for CH NH PbBrCl , whereas some shapeless polycrystalline material is
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observed for CH NH PbBr Cl. No clear trend is observed between the size of the crystals of
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the mixed perovskites and the content of chloride (x in CH NH Pb(Br Cl ) ).
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Fig. 2. SEM images of the mixed perovskites
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.2 Structural characterization
The structural investigation was performed from SXRD data. The extreme angular
resolution of the patterns is essential to accurately define the symmetry of the different
mixed phases and to determine its evolution below room temperature (RT). In all cases, the
crystal structures at RT are cubic and can be defined in the space group Pm3m. Fig. 3
shows a Le Bail fit of the SXRD data of the end member CH NH PbBr in the mentioned
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space group. A thorough structural study in combination with neutron diffraction data was
described elsewhere11 for this end member, showing a disordered configuration of
+
CH NH organic cation in the cage of the cubic perovskite. At ambient temperature, the C
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positions all aligned along [110] direction, as illustrated in Fig. 3.
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characteristic perovskite peaks and the absence of impurities. Red crosses are the experimental
points, the black full line is the calculated profile and the blue lower line is the difference. The
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green vertical marks represent the allowed Bragg positions in the Pm3m space group, with a=
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.93114(6) Å.
Fig. 4 shows the Rietveld refinements corresponding to x = 0.33 to 1 perovskites at
RT. Minor amounts of PbCl and MACl were identified and included in the refinements as
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second and third phases. Table 1 lists the main crystallographic parameters and the
reliability factors after Rietveld refinements.
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x = 0.33 S yS nXc Rh rDo ,t Tr o=n 2 X9 5R KP D
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YObs
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Bragg Positions

= 0.3252 Å
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2 (°)
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Figure 4. SXRD profiles for CH NH Pb(Br Cl ) (x = 0.33, 0.5, 0.67 and 1) at RT, after a
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Rietveld refinement of the perovskite crystal structures. Red crosses are the experimental points, the
black full line is the calculated profile and the blue lower line is the difference. The green vertical
marks represent the allowed Bragg positions in the Pm3m space group; the second and third series
of marks correspond to PbCl and MACl.
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Fig. 5 plots the unit-cell parameter variation and the anisotropic atomic displacement
parameters (ADP) of the X ion. The unit-cell parameters exhibit a reduction as the amount
of Cl increases, as expected, but this change is not linear; the slope additionally increases
beyond x= 0.5; i.e. MAPbBr Cl . The ADPs of X atoms are significantly anisotropic, as
1.5 1.5
shown in the insets of Fig. 5, showing an oblate-type consisting of flattened disks
perpendicular to the Pb-X-Pb chemical bonds. This is the usual configuration in
perovskites, where the thermal vibrations are preferentially permitted in this direction: an
oblate spheroid locks the halogen atoms at mid-lead positions with enhanced vibrations
perpendicular to the Pb-Pb distance. On the other hand, the ADPs for intermediate mixed
halide phases (x = 0.33, 0.5 and 0.67) present a non-monotonic variation in comparison
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with both end members (x = 0 and 1), although the difference does not overcome two times
the standard deviations and are less significant.
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Fig. 5. Unit-cell parameters and anisotropic atomic displacement parameters (ADP) of X site.
Anyhow, these anomalies may be due to the structural disorder introduced by the
mixture of halides, for x = 0.33, 0.5 and 0.67. This disorder may affect the ADPs and
generates a distribution of interactions between the inorganic PbX skeleton and the
6
methyl-ammonium units, which are not present in MAPbBr and MAPbCl , containing
3
3
single halide ions. This can be visualized from a statistical distribution of the halide
environment of MA groups as shown in Fig. 6. This figure plots the probability (y-axis) for
a particular halide distribution around MA units (x-axis) in the different samples. For the
end members this probability is 100%, since all MA units are coordinated to 12 Br or Cl
atoms. The insert includes illustrative schemes of the extreme situations in comparison with
an intermediate case where the MA is coordinated to eight chlorides and four bromides, y =
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. These distributions reveal the high structural disorder given in mixed situations, in
contrast to both end members. These probably induce tensions in the lattice preventing a
linear behaviour between the pure bromine and chlorine compounds.
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1
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1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Fig. 6. Statistical probability distribution of the halide environment in CH NH Pb(Cl Br ) (x = 0,
3
3
1-x
x 3
0
.33, 0.5, 0.67 and 1).
3.3. Low-temperature transitions
The low-temperature crystal structures of some members of the series (x= 0, 0.5,
.66, 1) were also investigated from SXRD data, in the 120-300 K temperature range. For
0
both end members, MAPbBr and MaPbCl , phase transitions lowering the symmetry from
3
3
cubic to tetragonal (X= Br only) and then to orthorhombic (X= Br and Cl) have been
observed, with a concomitant decrease of the delocalization of the methyl-ammonium
group as symmetry decreases11, 20-23. In contrast, for the mixed halide compositions no
phase transitions have been detected, obtaining cubic structures (Pm3m) for the specimens
cooled down to 120 K, as illustrated in Figure 7, where the thermal variation of the unit-cell
parameters are represented. For x= 0.5 and 0.67 the SXRD patterns corresponding to the
cubic structures are collected at T= 120 K, only affected by the expected thermal
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contraction. The crystallographic data and Rietveld plots at low temperatures are included
in Tables S1-S2 and Figures S1-S6, respectively (Supporting Information).
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Fig. 7. Thermal evolution of the unit-cell parameters of the MAPb(Br Cl ) perovskites in
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the 120-300 K temperature region. Only the end members experience phase transitions with
1
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symmetry lowering. The MAPbBr data taken from Ref
3
MAPbBr presents three polymorphs between 298 to 120 K: cubic (Pm3m) (298, 270
3
and 240 K), tetragonal (I4/mcm) (210 and 180 K) and orthorhombic (Pnma) (120 K).11
These phases were also reported by other authors from single crystal data.24-25 In MAPbCl3,
the cubic symmetry remains stable down to 180 K. However, the SXRD patterns at 120 and
1
50 K, exhibit conspicuous reflection splittings as illustrated in Fig. 8. Poglitsch et al.
reports that this compound presents two phase transitions: P222 ←(173 K)→ P4/mmm
1
2
1
←
(179 K)→ Pm3m. Later, Chi et al. found that the orthorhombic polymorph presents a
Pnma space group, but with a doubled unit-cell with respect to the cubic aristotype (a ≈ b ≈
2
3
c ≈ 2a ). Our patterns (see Fig 8) at 120 and 150 K present diffraction lines compatible
p
with the model reported by Chi et al.; however, the preliminary refinements were not
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satisfactory, since they showed additional lines that match with a second orthorhombic
(Pnma) phase similar to that observed in MAPbBr (a ≈ √2a ; b ≈ 2a ; c ≈ √2a ). The
3
p
p
p
coexistence of both orthorhombic phases allows an acceptable refinement in the patterns
collected at 150 and 120 K. The differences observed in both orthorhombic models reside
in the peak width; the conventional Pnma phase presents a greater broadening than the
doubled Pnma structure. This may suggest that the phase mixture is induced by
microstructural features.
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Due to the observed phase mixtures at low temperature, the mechanism or transient
state from cubic to orthorhombic symmetry has been partially known until now. As
mentioned above, a transient tetragonal phase (P4/mmm) was described in a very narrow
21
range of temperature, 172.9-178.8 K. In the search for this intermediate phase, several
patterns were collected sequentially close to this temperature range in intervals of 2.5 K, as
illustrated in Figure 8, and indeed a transitory phase was identified at 167.5 K. This pattern
was initially fitted to a tetragonal model, however, additional diffraction lines were
observed, evidencing a true orthorhombic symmetry. A good fit was achieved with the
Pnma model (a ≈ √2a ; b ≈ 2a ; c ≈ √2a ) similar to that observed at 120, 150 and 160 K,
p
p
p
but the unit-cell parameters are substantially less split than those found at lower
temperatures. The temperature evolution of the unit-cell parameters is illustrated in Figure
9
.
From these facts, is possible to deduce that our chlorine phase presents a more
complex transition than that reported previously. In summary, we observe three phases:
cubic, orthorhombic (a ≈ √2a ; b ≈ 2a ; c ≈ √2a ) and a second orthorhombic (a ≈ 2a ; b ≈
p
p
p
p
2a ; c ≈ 2a ), labelled in Figure 9 as C, O1 and O2, respectively. Additionally, the O1
p
p
phase splits in two states with different distortion degree, O1HT and O1 . In the first case,
LT
the unit-cell parameters splitting with respect to the cubic a is substantially lower than the
p
second case. O1LT is the main phase observed at 167.5 K. In this transient phase, c/√2 and
b/2 are very similar (Fig. 9), which explains the confusion with a tetragonal lattice in the
early literature. This is, indeed, a pseudo-tetragonal phase. This complex behaviour also
involves an inversion between b/2 and c/√2 parameters upon cooling, as displayed in Figure
9. The bottom panel of Figure 9 shows the percentage of each phase in this narrow
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temperature range. The goodness of the final refinement is included in the SI for all the
temperatures (Table S2; Figures S7-S14).
0
1
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3
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0
1
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3
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Fig. 8. Selected angular region of the SXRD (= 0.3252 Å) spectra of MAPbCl at increasing
3
temperatures, illustrating the evolution and splitting of some characteristic reflections.
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.74
.72
.70
.68
MAPbCl3
c/sqrt2
1
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b/2
b/2
a
5.66
c/2
5.64
5.62
5.60
5.58
C
O1HT
O1LT
O2
a/2
a/sqrt2
1
00
80
O2
60
O2LT
O1HT
C
4
0
0
0
2
1
50
155
160
165
170
175
180
Temperature (K)
Fig. 9. Thermal evolution of the unit-cell parameters of the MAPbCl perovskite in the 150-
3
1
80 K temperature region. The cubic and the two orthorhombic phases, are labelled C, O1 and O2,
respectively. O1 is split in O1HT and O1LT phases. O1HT is strongly pseudo-tetragonal. The
lower panel shows the relative abundance of these phases.
On the other hand, an important observation is the absence of phase transitions in the
mixed halides down to 120 K. This is not expected considering that the cubic polymorphs
2
1
are only stable above 179 and 237 K for MAPbCl and MAPbBr , respectively. This
3
3
unexpected behaviour can be related to the anion disorder introduced in mixed halide
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phases. As previously discussed, the organic-inorganic interactions are dramatically
different between the intermediate and the end members of the present series, as suggested
in Fig 6. Furthermore, the halide disorder may also prevent the cooperative rearrangements
needed to drive the octahedral tilt that finally leads to a phase transition.
0
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.3. Optical gap by UV-Vis spectra
Diffuse reflectance UV-Vis spectra, represented in the Fig. 10, were used to calculate
the optical absorption coefficient (α) according to the Kubelka–Munk equation:
2
F(R) = α = (1 - R) /(2R)
(1) where R is the reflectance (%).
X = 0
X = 0.33
X = 0.5
X = 0.67
X = 1
300
400
500
600
Wavelength (nm)
Fig. 10. Absorbance vs wavelength of the incident radiation for MAPb(Br Cl ) . The
1
-x x 3
gradual evolution in the absorption edge is consistent with the continuous structural change
observed by SXRD.
For instance, for CH NH PbBr powder the optical gap is determined to be 2.20 eV
3
3
3
from the extrapolation of the linear part of the transformed Kubelka–Munk spectrum with
the hν axis. This value increases with the incorporation of chloride ions into the structure
(Table 2). The increase in the band gap as Br is replaced by Cl, is generally expected due
to the reduced covalency of the lead-halide bonds within the network of Pb–X (X = Cl, Br)
6
corner-sharing octahedra.14
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It is noteworthy that the evolution of the band gap energy (E ) with the composition
g
is not linear, and the deviation from linearity recalls that observed in the unit-cell
parameters evolution at RT, see Figure 11. An additional comparison of E with the Pb-X
g
distances also exhibits a non-linear behavior (inset of Figure 11), unveiling that the
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
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0
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2
3
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2
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evolution of E is not very much related to the features of the inorganic framework. This
g
-
observation seems to confirm that the increase in the E with the amount of Cl derives from
g
the effect on the MA units of the shrinking of the lattice upon the incorporation of smaller
-
Cl anions.
3
2
2
2
.0
.8
.6
.4
6.1
6.0
5.9
3.0
2.8
2.6
2.4
2.2
2.0
2.2
2.0 1.0
.85
x =
0.67 0.5 0.33 0.0
2
2.88
2.91
2.94
2.97
<Pb-X> (Å)
5
.8
.7
5
5.6
0.0
0.2
0.4
0.6
0.8
1.0
x (Cl contents)
Fig. 11: Comparison between the band gap energy and unit-cell parameter in MAPb(Br Cl ) , as a
1
-x x 3
function of the Cl content, x. The colour in the circles corresponds to the band gap energy.
Conclusions
We have synthesized mixed inorganic-organic hybrid halide perovskites based on
chlorine and bromine halide anions CH NH Pb(Br Cl ) (x = 0, 0.33, 0.5, 0.67 and 1). The
3
3
1-x x 3
crystal structures were reported at room temperature as cubic in the space group Pm3m, and
the band gap of these perovskites can be controllably tuned. Whereas both end-members
exhibit conspicuous phase transitions, lowering the symmetry from cubic (RT) to tetragonal
and orthorhombic, down to 120 K, the mixed-halide phases remain cubic down to this
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temperature. This suggests that the halide disorder prevents the cooperative rearrangements
needed to drive the octahedral PbX tiltings, which finally lead to phase transitions with a
6
+
progressive localization of the organic CH -NH cation.
3
3
Supporting Information. Tables with the crystallographic parameters and Rietveld plots
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
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9
0
1
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2
3
4
5
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0
1
2
3
4
5
6
7
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9
0
from SXRD data for the different compositions at different temperatures.
Acknowledgments
Authors acknowledge financial support given by KIC RAW MATERIALS from
European Union (OPTNEWOPT project) and the Spanish Ministry of Economy and
Competitiveness (ENE2014-52158-C2-1-R project and MAT2017-84496-R) co-founded by
FEDER. C.A.L. acknowledges ANPCyT and UNSL for financial support (projects
PICT2014-3576 and PROICO 2-2016), Argentine. C.A.L. is a member of CONICET.
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Castellanos-Gomez, A.; Fernández-Díaz, M. T.; Fauth, F.; Alonso, J. A., Elucidating the
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Cells. Inorganic Chemistry 2017, 56, 14214-14219.
0.
Yang, J.; Siempelkamp, B. D.; Liu, D.; Kelly, T. L., Investigation of CH3NH3PbI3
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López, C. A.; Martínez-Huerta, M. V.; Alvarez-Galván, M. C.; Kayser, P.; Gant, P.;
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Kedem, N.; Brenner, T. M.; Kulbak, M.; Schaefer, N.; Levcenko, S.; Levine, I.;
Abou-Ras, D.; Hodes, G.; Cahen, D., Light-Induced Increase of Electron Diffusion Length
in a p–n Junction Type CH3NH3PbBr3 Perovskite Solar Cell. The Journal of Physical
Chemistry Letters 2015, 6, 2469-2476.
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Wang, L.; Wang, K.; Xiao, G.; Zeng, Q.; Zou, B., Pressure-Induced Structural
Evolution and Band Gap Shifts of Organometal Halide Perovskite-Based
Methylammonium Lead Chloride. The Journal of Physical Chemistry Letters 2016, 7,
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14. Comin, R.; Walters, G.; Thibau, E. S.; Voznyy, O.; Lu, Z.-H.; Sargent, E. H.,
Structural, optical, and electronic studies of wide-bandgap lead halide perovskites. Journal
of Materials Chemistry C 2015, 3, 8839-8843.
273-5279.
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Wang, C.; Tang, Y.; Hu, Y.; Huang, L.; Fu, J.; Jin, J.; Shi, W.; Wang, L.; Yang, W.,
Graphene/SrTiO3 nanocomposites used as an effective electron-transporting layer for high-
performance perovskite solar cells. RSC Advances 2015, 5, 52041-52047.
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Sun, B., Efficient planar heterojunction perovskite solar cells employing graphene oxide as
hole conductor. Nanoscale 2014, 6, 10505-10510.
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Graetzel, M.; White, T. J., Synthesis and crystal chemistry of the hybrid perovskite
CH3NH3)PbI3 for solid-state sensitised solar cell applications. Journal of Materials
Chemistry A 2013, 1, 5628-5641.
18. Fauth, F.; Boer, R.; Gil-Ortiz, F.; Popescu, C.; Vallcorba, O.; Peral, I.; Fullà, D.;
Baikie, T.; Fang, Y.; Kadro, J. M.; Schreyer, M.; Wei, F.; Mhaisalkar, S. G.;
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Rodriguez-Carvajal, J.; Roisnel, T., Recent advances in magnetic structure
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methylammoniumtrihalogenoplumbates (II) observed by millimeter‐wave spectroscopy.
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study of low temperature phase transitions in the cubic perovskite CH3NH3PbCI3. Phase
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Gutmann, M.; Mhaisalkar, S. G.; White, T. J., A combined single crystal neutron/X-ray
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Tables
Table 1: Crystallographic data for MAPb(Br Cl ) with x = 0, 1/3, 1/2, 2/3 and 1 at room
1
-x x 3
temperature from synchrotron XRPD.
MAPb(Br Cl ) x = 0 [Ref. 11] x = 1/3 x = 1/2
x = 2/3
x = 1
1
-x x 3
a (Å)
5.93076(2)
0.0236(5)
5.8745(1) 5.83502(7) 5.76205(7) 5.68532(4)
Pb (1a)
(
0,0,0)
1
1
22
33
U =U =U
0.027(1) 0.0336(7) 0.0265(4) 0.0236(4)
Br Cl (3d)
1
-x
x
(0.5,0,0)
x
0
0.28(3)
0.013(5)
0.135(7)
0.48(2)
0.016(4)
0.129(5)
0.71(1)
0.021(3)
0.135(3)
1
U11
0.021(2)
0.136(3)
0.015(5)
0.139(5)
2
2
33
U =U
C/N (6f)
(0.5,y,y)
y
0.42(1)
0.03(5)
0.597(6)
0.01(4)*
0.597(4)
0.02(3)*
0.597(2)
0.02(1)*
0.592(4)
0.04(2)*
U */U
iso
eq
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Rp(%)
Rwp(%)
χ2
12.3
16.2
2.1
15.9
20.3
1.1
13.9
11.6
14.4
1.8
13.0
16.9
1.3
17.8
1.4
RBragg(%)
7.7
15.2
8.3
5.4
7.6
Impurities :
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
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5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
MAPbCl : 12.2(4)% PbCl y 9(1)% MACl
3
2
MAPbBrCl : 0%
2
MAPbBr Cl : 20.4(4)% PbCl
1
.5 1.5
2
MAPbBr Cl: 13.4(4)% PbCl
2
2
Table 2: Band gap (E ) of the perovskites
g
Eg
Perovskite
(eV)
CH NH PbBr
3
2.20
2.25
2.35
2.70
2.90
3
3
CH NH PbBr Cl
3
3
2
CH NH PbBr Cl
1.5 1.5
3
3
CH NH PbBrCl
2
3
3
CH NH PbCl
3
3
3
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For Table of Contents Use Only
Crystal growth, structural phase transitions and optical gap evolution of
CH NH Pb(Br Cl ) powders
3
3
1-x
x 3
0
1
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0
M. C. Alvarez-Galván, J. A. Alonso, C.A. López, E. López-Linares, C. Contreras, M.J.
Lázaro, F. Fauth and M.V. Martínez-Huerta
Both end-members exhibit conspicuous phase transitions, from cubic (RT) to
tetragonal and orthorhombic, down to 120 K, whereas the mixed-halide phases remain
cubic: the halide disorder prevents the cooperative rearrangements needed to drive the
octahedral PbX tiltings. The band gap can be controllably tuned; the deviation from
6
linearity follows the unit-cell parameters evolution: the increase in the E is closely
g
-
correlated to the collapse of the lattice upon the incorporation of smaller Cl anions.
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