Synthesis and characterization of perovskite FAPbBr3?xIx thin films for solar cells

Article abstract of DOI:10.1007/s00706-017-1958-0

Abstract: FAPbI₃, FAPbBr₃, and FAPbBr_3?xI_x perovskite thin films were produced in a single step from a solution containing a mixture of FAI, PbI₂, FABr, and PbBr₂ (FA?=?formamidinium). FAPbBr_3?xI_x perovskite thin films were deposited onto ITO-coated glass substrates by spin coating. X-ray diffraction analyses confirmed that these thin-film perovskites crystallize in the cubic phase (Pm-3?m) for all composition range 0?≤?x?≤?3. Mixed lead perovskites showed a high absorbance in the UV–Vis range. The optical band gap was estimated from spectral absorbance measurements. It was found that the onset of the absorption edge for FAPbBr_3–xI_x thin films ranges between 1.47 and 2.20?eV for x?=?0 and x?=?3, respectively. Photoluminescence emission energies for mixed halide perovskites were also dependent on their composition and presented intermediate values from 810.4?nm for FAPbI₃ to 547.3?nm for FAPbBr₃. Graphical abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s00706-017-1958-0

Monatsh Chem
DOI 10.1007/s00706-017-1958-0
ORIGINAL PAPER
Synthesis and characterization of perovskite FAPbBr32xIx thin
films for solar cells
4
B. Slimi1,2,4 M. Mollar4 I. Ben Assaker1 A. Kriaa3 R. Chtourou1 Bernabe Marı
•
•
•
•
•
´
´
Received: 11 October 2016 / Accepted: 19 March 2017
Ó Springer-Verlag Wien 2017
Abstract FAPbI3, FAPbBr3, and FAPbBr3-xIx perovskite
thin films were produced in a single step from a solution
containing a mixture of FAI, PbI2, FABr, and PbBr2
Graphical abstract
FAPbBr
3
1.0
0.8
0.6
0.4
0.2
0.0
FAPbBr
FAPbBr
FAPbBr
FAPbBr
FAPbBr
FAPbBr
FAPbBr
I
2.2 0.8
I
2 1
(FA = formamidinium). FAPbBr3-xIx perovskite thin
I
1.8 1.2
I
1.6 1.4
films were deposited onto ITO-coated glass substrates by
spin coating. X-ray diffraction analyses confirmed that
these thin-film perovskites crystallize in the cubic phase
(Pm-3 m) for all composition range 0 B x B 3. Mixed lead
perovskites showed a high absorbance in the UV–Vis
range. The optical band gap was estimated from spectral
absorbance measurements. It was found that the onset of
the absorption edge for FAPbBr3–xIx thin films ranges
between 1.47 and 2.20 eV for x = 0 and x = 3, respec-
tively. Photoluminescence emission energies for mixed
halide perovskites were also dependent on their composi-
tion and presented intermediate values from 810.4 nm for
FAPbI3 to 547.3 nm for FAPbBr3.
I
1.4 1.6
I
1.2 1.8
I
1 2
FAPbI
3
500
550
600
650
700
/ nm
750
800
850
Keywords Organic–inorganic perovskites Á
Formamidinium lead iodide Á
Formamidinium lead bromide Á X-ray diffraction Á
Optical absorption Á Photoluminescence
Introduction
Organometal halide perovskites are currently one of the
most actively researched materials. They are typically
composed of an organic group (A), a metal ion (B), and one
or two halides (X) forming the perovskite structure ABX3.
These hybrid perovskites have become one of the most
attractive and promising materials for photovoltaic over the
last 5 years. In particular, the methylammonium (MA) lead
halide and formamidinium (FA) lead halide perovskites,
namely CH3NH3PbX3 (or MAPbX3) and CH(NH2)2PbX3
(or FAPbX3) (X = Br, I, Cl) show remarkable properties
for photovoltaic applications [1–4] and light-emitting
diodes [5]. Solar power conversion efficiencies of more
than 20% have been reported [6], and tunable light-emit-
ting diodes have been also reported [7]. Their low cost and
ease of production make them potential candidates for
´
´
& Bernabe Marı
bmari@fis.upv.es
1
´
`
Laboratoire de Nanomateriaux et Systemes pour les Energies
Renouvelables (LANSER), Centre de Recherches et des
Technologies de l’Energie Technopole Borj Cedria, BP 95,
Hammam Lif 2050, Tunisie
2
3
4
´
Faculte des Sciences de Bizerte, Universite de Carthage,
Carthage, Tunisie
´
´
Laboratoire de Chimie Moleculaire Organique, 5 Avenue
Taha Houssein Monfleury, 1089 Tunis, Tunisie
´
´
`
Departament de Fısica Aplicada-IDF, Universitat Politecnica
de Valencia, Camı de Vera s/n 46022, Valencia, Spain
`
`
123

B. Slimi et al.
future technologies. Due to their great absorption coeffi-
cient and high charge carrier mobility, these groups of
perovskites are well suited for converting solar energy
[8, 9]. In fact, halide perovskites drew an important impact
in the field of thin-film photovoltaics in the last years,
boosting up the energy conversion efficiency from 3.8%,
for the first perovskite-based solar cells in 2009 [1], up to
more than 20% [6, 10–15].
(a)
FAPbI 180°C
3
-FAPbI
3
PbI2
-FAPbI
3
FAPbI 140°C
3
Among these perovskites, MAPbI3 is, by far, the most
widely studied perovskite for solar cell applications [13–15].
However, the FAPbI3 perovskite with broader light absorp-
tion has been attracting increasing interest [6, 10, 16–25].
Like for MAPbI3, typical routes for depositing FAPbI3 per-
ovskite thin films include one-step [6, 18, 21] and sequential
deposition methods [6, 16, 19] have been studied. More
recently, many studies focusing on material properties
directly related to device performance, such as carrier
mobilities, recombination lifetimes, excitonic properties,
and optical absorption have been published [26–29].
Increasing the photocurrent by expanding the absorption
spectra of ABX3 perovskites through chemical modification
has been proposed as a method for further improving solar
cell efficiency. For example, replacing the MA cation in
MAPbI3with a FA cation, which has a larger ionic radius,
results in an ABX3 perovskite with a smaller band gap for
broader spectrum light harvesting.
10
20
30
40
50
60
70
2
/ deg
(b)
FAPbI3 140°C
FAPbI3 150°C
FAPbI3 170°C
*
+ ITO
*
+
+
+
*
*
*
+
*
*
+
*
FAPbI3, MAPbI3 perovskites as well as their mixed halide
analogues ((FA,MA)Pb(I1-xBrx)3, (FA,MA)Pb(I1-xClx)3)
have been already reported [3, 30–33]. In addition, most
10
15
20
25
2
30
/ deg
35
40
45
50
fractional mixtures of FAPb(BrxI1–x)3 were found to be
Fig. 1 a Typical XRD pattern of the FAPbI3 powder at 140 and
180 °C; b XRD patterns of FAPbI3 thin films at 140, 150, and 170 °C
crystalline, with the exception of the region between x\0.3
and x[0.5 where the crystal structure changed [19, 34, 35].
By increasing the [Br/I] ratio in the mixed halide perovskite
film, it is possible to tune the band gap from 2.43 to 1.48 eV
with a continuous red shift [34].
powder diffractograms correspond to the same cubic
structure corresponding to the spatial group labeled as Pm-
˚
3 m (a = 6.357 A). The most intense diffraction peak
located below 14° (13.86°) corresponds to (001) diffraction
planes and the peaks located at about 20, 24, 28, 31, and
40° are related to (110), (111), (002), (210), and (220)
diffraction planes, respectively. This corresponds to the a-
FAPbI3 phase of our perovskite and not to the d-FAPbI3
phase characterized by a peak at 11.8° (100). The sharp
diffraction peaks indicate the high crystallinity of the as-
synthesized FAPbI3 compound. Rietveld refinement anal-
ysis of the X-ray powder diffraction pattern reveals that the
synthesized FAPbI3 is a pure phase with the space group of
Pm-3 m, which was in good agreement with recent reports
[3, 29, 30, 39, 40].
Such mixed halide films also offer tunability of the
emission following band gap excitation [36]. Such tunable
and coherent light emission properties have been utilized in
lasing applications [37, 38].
In this paper, we present a study of the crystalline
structure, absorption and emission properties of pure
(FAPbI3, FAPbBr3) and mixed halide perovskites
(FAPbBr3–xIx) synthesized in a single step via drying in
DMF solution at a different temperature on ITO substrates
as a function of the Br/I ratio.
Figure 1b shows the X-ray diffractograms for FAPbI3
thin films annealed at 140, 150, and 170 °C in air for
30 min. The same cubic structure corresponding to the
spatial group-labeled Pm-3 m was observed for all sam-
ples. The reflection at 2h = 11.8° is correlated to d-FAPbI3
(yellow phase), while the peak at 2h & 14° represents the
Results and discussion
XRD results
Figure 1a shows the X-ray diffractograms for FAPbI3
powder annealed at 140 and 180 °C, respectively. Both
123

Synthesis and characterization of perovskite FAPbBr3-xIx thin films for solar cells
a-FAPbI3 (black phase). As shown, the yellow d-phase of
(a)
FAPbI
3
FAPbI
FAPbBr
3
FAPbI3 was produced at annealing temperatures below
150 °C, as indicated by the peak at 11.8°. After annealing
at 170 °C the secondary phase, d-FAPbI3, almost disap-
pears. In this figure, the diffraction peaks corresponding to
ITO substrates have been also labeled.
3
FAPbBr
3
*
13.0
13.5
14.0
2
14.5
(deg)
15.0
15.5
*
*
Hybrid perovskite materials in the form of FAPbBr3–xIx
(x = 0–3) were synthesized via spin-coating deposition.
Figure 2a, b shows XRD patterns of FAPbBr3–xIx prepared
with different molar ratios of Br to I at room temperature.
In this case, all the films with different iodide contents have
been annealed at 150 °C in air for 30 min. After increasing
iodide contents, the XRD measurements show a shift
+
+
+
*
*
*
+
*
*
+
10
15
20
25
30
35
40
45
50
toward low angles for all diffraction planes (labeled as ‘*’
in Fig. 2b). This shift is related to an increase in the crystal
lattice [19, 27]. The diffraction angle decreases with the
increase in iodide contents, in agreement with the bigger
size of iodide relative to bromide, which expands the
crystal lattice. FAPbBr3–xIx adopts a cubic structure with a
space group of Pm-3 m. The gradual shift in the diffraction
angle (with the presence of single intense peaks) is a strong
indication that a mixed phase of FAPbBr3–xIx is formed in
which the two anions are both inserted in the same lattice
frame.
2 / deg
*
(b)
*
*
*
FAPbBr
3
*
*
FAPbBr
I
2.8 0.2
FAPbBr
FAPbBr
I
2.6 0.4
I
2.4 0.6
FAPbBr
I
2.2 0.8
FAPbBr I
2 1
FAPbBr
I
1.8 1.2
With the increase of iodide concentration, the position
of diffraction peaks shifts to lower angles meaning larger
interplanar distances. For all samples, as shown in Fig. 2c,
strong peaks are detected at 14° and 28°, corresponding to
the (001) and (002) planes, respectively. Such observation
confirms the formation of a cubic perovskite structure.
The (001) peak of the a-phase (2h & 14°) consistently
shifts to higher angles with higher Br ratio, corresponding
to smaller lattice constants. The lattice constants were
calculated by fitting the whole pattern using JADE soft-
ware. For the FAPbBr3–xIx perovskite, the material is in a
single phase throughout the entire composition range. The
monotonic shift of the (001) reflection that we observed
from 2h & 14.7° to 13.9° (Table 3) is consistent with a
FAPbBr I
1 2
FAPbI
3
10
15
20
25
30
35
40
45
50
2
/ deg
(c)
* ITO
*
FAPbBr
3
FAPbBr
FAPbBr
FAPbBr
FAPbBr
FAPbBr I
FAPbBr
I
2.8 0.2
I
2.6 0.4
I
2.4 0.6
I
2.2 0.8
2 1
I
1.8 1.2
FAPbBr I
FAPbI
˚
1 2
shift of the cubic lattice constant from 6.306 to 5.955 A as
3
the material incorporates a larger fraction of the smaller
halide Br [19, 27]. It was found that the lattice constant
follows the so-called Vegard’s law which states that lattice
constants vary linearly with the ratio between the two
halide components of the thin films [32, 41].
13
14
28
30
2
/ deg
Fig. 2 a X-ray diffractograms of the FAPbI3 and FAPbBr3 thin films;
b X-ray diffractograms of FAPbBr3-xIx thin-film perovskites with Br/
I molar ratios from 0 to 3; c X-ray diffractograms for FAPbBr3-xIx
For the pure FAPbI3 film, the intensity of the peak near
12° corresponding to the d-phase decreases with increasing
the annealing temperature from 140 to 170 °C. This phase
can disappear totally for an annealing temperature higher
than 170 °C. Since perovskites with higher bromide con-
tent have a smaller lattice constant than those with higher
iodide contents, this splitting is consistent with the pres-
ence of a minority phase with significantly enhanced
(x [ 1) iodide contents and a majority phase with slightly
thin films. Magnified view of regions 13°–16° and 27°–30°
enhanced bromide contents (x \ 1) compared to the orig-
inal material. If we compare the magnitude of XRD
intensity from the two phases, we estimate that the
minority phase represents 30% of the material (Fig. 2a),
after taking into account the structural differences between
the two synthesized phases.
123

B. Slimi et al.
It should be noted that the diffractograms indicate the
presence of a small peak at 12.64° corresponding to PbI2
phase. This peak has been often identified in thin-film
X-ray diffraction and is an indicator of a slight decompo-
sition of the samples. These results suggest that the
presence of the impurity phase PbI2 is due to the solution
interaction FAPbBr3/FAPbI3 that leaves a small amount of
FAPbI3 (150 °C) unreacted, thus leaving a corresponding
amount to PbI2 formation. However, as one can notice in
Fig. 2b, the formation of PbI2 was negligible and the per-
ovskite peaks were dominant. The additional peak
observed previously at 12.6° disappears with decreasing of
iodide contents. The disappearance of PbI2 phase in the
diagrams proves that our mixed perovskite becomes more
stable with the addition of bromide.
The amount of Br and I in thin films were determined by
elemental analysis using EDX. The space group (Pm-3 m
˚
for both materials) and unit cell parameters (a = 5.9944 A
˚
for FAPbBr3 and a = 6.3573 A for a-FAPbI3) measured by
XRD were found to be consistent with those from previous
reports [39, 45].
Table 1 also displays the crystallite size obtained from
the Scherrer formula [46] applied to the (001) peak. As can
be seen the crystallite size decreases as Br concentration
˚
decreases and ranges from 1020 to 280 A for FAPbBr3 and
FAPbI3, respectively.
Figure 3 shows the evolution of the interplanar spacing
as a function of Br/I ratio. This distance is proportional to
the amount of bromide. In other words, it increases with the
increase of iodide quantity. This behavior can be explained
by the ionic radius of both elements. In fact, the ionic
The high crystallinity of the synthesized FAPbI3,
FAPbBr3, and FAPbBr3–xIx was highlighted by the sharp
diffraction peaks. This finding is fairly coherent with the
literature [41].
˚
radius of Br and I are 1.96 and 2.20 A, respectively. We
can clearly see that the ionic radius of I is greater than Br
and as a result influences the interplanar distance.
The diffraction angle shift indicated that the two ions Br
and I are both inserted in the same lattice, meanwhile, the
shift of the peak maximum toward lower angle for
(x = 0–3) indicates an increase in lattice parameter. The
changes in the lattice parameter are probably due to the
incorporation of the larger I anion instead of the smaller Br
SEM and EDX analyses
Figure 4 shows the scanning electron micrographs of
FAPbBr1–xIx thin films deposited onto ITO-glass substrate
with different [Br]/[In] ratios. As can be seen from this
figure, the effect of [Br]/[In] ratios is visible on the aspect
of the surface film. All micro-structures consist of grains
with certain porosity. As illustrated in Fig. 4a, films pre-
pared with only Br element show rough organization with
some holes on the surface. For 0 B x B 0.8, the micro-
graphs of thin films reveal non-uniform grain sizes and
non-homogeneous surface with mixture of smaller and
larger clusters.
˚
anion (the ionic radii of Br and I are 1.96 and 2.20 A,
respectively) [42, 43].
The lattice parameters of FAPbBr3–xIx shown in
Table 1 are extracted from the (001) peak. The contrac-
tion of the lattice with increasing iodide contents (Fig. 2a)
confirms the substitution of the bromide anion by iodide.
The calculated lattice parameters of FAPbBr3 and FAPbI3
˚
˚
are a = 5.993 A and a = 6.352 A, respectively, which is
in good agreement with the previously reported phase of
FAPbI3 and FAPbBr3 [27, 44]. In addition, for different
iodide contents the FAPbBr3–xIx keeps the same cubic
phase. The lattice parameter of FAPbBr3–xIx (0 B x C 3),
which exhibits a linear relationship with the [Br/I] con-
tents in each region, increases with the increase of iodide
contents. Therefore, the linear trend indicates the forma-
tion of the FAPbBr3–xIx with a change of the lattice
parameter rather than the phase of FAPbI3 and FAPbBr3.
This illustrates that it is not trivial to determine the (Br, I)
contents from lattice parameters deduced from XRD
measurements.
FESEM images show the dependence of the grain size
on the iodide contents. The obtained films with different
iodide contents have a different shape, morphology, and
size. Indeed, as the composition x increases, the aggrega-
tion of gains increased [47, 48]. This effect can be related
to the electron beam while the films are observed by SEM.
This behavior could indicate fragile grain boundaries,
which potentially lead to a problem in charge carrier
transport through the grain boundaries. This result was also
obtained by Anand et al. [49], who have synthesized
FAPbBr3 by spin coating technique. When the percentage
of iodide increases (Fig. 4 for x = 0.8, 1.4, and 2), we
Table 1 Lattice parameter (a) of FAPbBr3–xIx (x = 0–3) film and crystallite size for the (001) diffraction planes
FAPbBr3–XIX
0
0.4
0.8
1.2
1.6
2
3
˚
a/A
5.993
1020
6.037
810
6.089
620
6.112
450
6.206
410
6.247
320
6.352
280
˚
Crystallite size/A
123

Synthesis and characterization of perovskite FAPbBr3-xIx thin films for solar cells
6.40
6.35
6.30
6.25
6.20
6.15
6.10
6.05
6.00
Table 2 presents the EDX results for all synthesized
FAPbBr3–xIx samples. These results indicate that FA, Pb,
Br, and I are homogeneously distributed in the perovskite
crystal, suggesting that Pb and Br/I are uniformly incor-
porated in the FAPbI3, FAPbBr3, and FAPbBr3-xIx films in
the grain perovskite rather than in separate chemical pha-
ses. The results of EDX analysis indicate that all films
present a deficit of Pb. The measured ratio for Pb:I, Pb:Br,
and Pb:I ? Br is in the range of 21–24%, which is slightly
lower than the expected theoretical value (25%).
2.3
2.2
2.1
2.0
1.9
1.8
1.7
1.6
1.5
d ( )
E (eV)
d
E
0.0
0.5
1.0
1.5
2.0
2.5
3.0
UV–Vis analyses
[ I ] ion content
The differences in the structural and morphological fea-
tures have a dramatic influence on the optical properties of
the prepared materials. In Fig. 5, the absorbance for
FAPbBr3-xIx (x = 0–3) thin films is shown.
Fig. 3 Interplanar spacing (d) and optical gap (Eg) as a function of
[Br/I] ratio
notice a reduction of clusters size giving a densely packed
and nearly homogeneous surface consists of small grains.
This reduction reinforces the densification of the FAPbBr1–
xIx films and reduces the leakage current due to grain
boundaries. The same micrograph was obtained by
Vanessa et al. who have synthesized pure FaPbI3 [50].
It is worth to highlight that the crystallite sizes obtained by
the Scherrer equation are smaller than those observed from
SEM. This indicates that the FAPbBr3–xIx grains as observed
by SEM images are composed of several crystallites.
It is important to notice that for absorption measure-
ments we used an integrating sphere to collect both
specular and diffuse transmittance to remove the effect of
light scattering originating from refraction and reflection
phenomena inside the perovskite crystals. Comparing to
both compounds presented in Fig. 5a, we can note that the
FAPbI3 present a wider range absorbance than FAPbBr3. In
fact, the absorption in the visible range is attributed to the
difference between the conduction and the valence band,
Fig. 4 FESEM images of
FAPbBr3-xIx thin films
prepared on glass/ITO
substrates
FAPbBr2.6I0.4
FAPbBr3
10 m
10 m
FAPbBr2.2I0.8
FAPbBr1.6I1.4
10 m
10 m
10 m
10 m
FAPbBr1I2
FAPbI3
123

B. Slimi et al.
0.35
0.30
0.25
0.20
0.15
0.10
Table 2 EDX data of the variation of the atomic percentage between
the iodine, lead, and bromide for FAPbBr3-xIx thin films (x = 0–3)
(a)
FAPbBr
3
FAPbI
3
Atomic % Pb
Atomic % Br
Atomic % I
FAPbBr3
23.64
22.96
21.96
22.03
20.81
23.11
22.20
23.25
76.36
64.74
55.40
51.41
42.67
31.36
28.73
0
0
FAPbBr2.6I0.4
FAPbBr2.2I0.8
FAPbBr2I1
FAPbBr1.6I1.4
FAPbBr1.2I1.8
FAPbBr1I2
FAPbI3
12.30
22.64
26.56
36.52
45.54
49.07
76.75
450
500 550
600 650
700 750
800 850
900
/ nm
1.0
0.8
0.6
0.4
0.2
(b)
FAPbBr
3
which indicates that the substitution of Br by I leads to a
decrease of band gap energy. The values of Eg as a func-
tion of anions’ composition are summarized in Table 3.
These materials exhibit optical band gaps that match
with the different colors of the films. For the FAPbX3
(X = I, Br), the data at wavelengths below 841 nm for
X = I and 558 nm for X = Br have been omitted due to
dispersion and diffusion effects. A comparison between
Fig. 5a and b reveals that the absorption properties of
perovskite films differ significantly from the precursor (Br–
Pb–I) confirming further the transformation into perovskite
structure Br/I. Hence, the presence of a single emission
peak close to the absorption edge is a proof of a single
phase of the perovskite structure.
FAPbBr
FAPbBr
FAPbBr
I
I
I
2.8 0.2
2.6 0.4
2.2 0.8
FAPbBr I
2 1
FAPbBr
FAPbBr
FAPbI
3
I
I
1.6 1.4
1.2 1.8
450 500 550 600 650 700 750
/ nm
800 850 900
FAPbBr
FAPbBr
FAPbBr
FAPbBr
FAPbBr
FAPbBr
FAPbBr
1.2
0.8
0.4
0.0
3
(c)
I
I
I
2.8 0.2
2.6 0.4
2.2 0.8
2 1
1.6 1.4
1.2 1.8
These results confirm that the observed emission arises
from charge carrier recombination within the band struc-
ture of mixed halide perovskites films. It is interesting to
note that the difference absorption spectra of all mixed
halide perovskites films (Fig. 5b) exhibit similar spectral
prints of induced absorption and bleaching of the absorp-
tion band. The only difference is the position of the
maximum, which shifts to red as the iodide contents
increases. The position of the bleaching maximum agrees
well with the absorption edge for the corresponding per-
ovskite film. This spectral feature, in turn, indicates a
common semiconducting behavior of mixed halide per-
ovskites despite the difference in the halide composition.
For formamidinium lead bromide iodide mixed halide
perovskites FAPbBr3–xIx, the absorption of photons with
energy similar to that of the band gap, Eg % hm, leads to an
optical transition producing an electron in the conduction
band and a hole in the valence band (exciton) to explore the
range of band gap tunability of the formamidinium lead
trihalide system [19]. Indeed, strong size confinement
effects can be observed in nanocrystalline systems when
their crystal size becomes smaller than the exciton Bohr
diameter.
I
I
I
FAPbI
3
1.25
1.50
1.75
2.00
2.25
2.50
photon energy/eV
Fig. 5 a Absorbance of FAPbI3 and FAPbBr3 films deposited on ITO
substrates; b absorbance spectra of FAPbBr3-x 3 (x = 0–3) films
deposited on ITO substrates; c estimation of the optical band gap c of
FAPbBr3-xI3 (x = 0–3) films on ITO substrates
I
increasing the ionic radius of the anion in the FAPbX3
systems can be observed. For X = I, the band gap has a
value of 1.48 eV, which is consistent with the data reported
in the literature [19]. These authors also found that Pb-
based phases have optical band gaps that are independent
of the preparation method. By comparing to the values
obtained from Fig. 5c, the extrapolation gave an Eg of
1.48 eV for FAPbI3 and an Eg of 2.3 eV for FAPbBr3,
which are confirmed by the literature [19, 51]. The shift of
the band gaps might be due to the presence of PbI2
impurity phase in the prepared samples for (x = 1.2–1.8)
According to the optical absorption measurements
shown in Fig. 5a, a decrease of the band gap energy by
123

Synthesis and characterization of perovskite FAPbBr3-xIx thin films for solar cells
Table 3 Changes in the peak positions (001) and wavelength in PL
and absorption with the variation of Br and I
FAPbI 140°C
1.0
3
(a)
FAPbI 150°C
3
FAPbI 170°C
3
(001)
k
Abs/nm
EAbs/eV
k
PL/nm
EPL/eV
0.8
0.6
0.4
0.2
0.0
FAPbBr3
14.78
14.68
14.60
14.52
14.36
14.19
14.12
13.92
558.6
604.3
632.7
664.8
715.6
770.1
792.2
840.3
2.22
2.05
1.96
1.86
1.73
1.61
1.56
1.47
547.3
589.1
687.9
692.4
731.4
766.1
787.1
810.5
2.26
2.10
1.80
1.79
1.69
1.62
1.57
1.53
FAPbBr2.6I0.4
FAPbBr2.2I0.8
FAPbBr2I1
FAPbBr1.6I1.4
FAPbBr1.2I1.8
FAPbBr1I2
FAPbI3
700
725
750
775
800
825
850
875
/ nm
proved by XRD. The absorption spectra in Fig. 5b show
that the band gap increases with bromide fraction from
about 1.48 to 2.22 eV. These results are in good agreement
with theory, which predicts that the larger the ion size (Br)
the smaller the Eg is. The fact that the mixture shows peaks
from both samples (FAPbBr3, FAPbI3, and FAPbBr3–xIx)
indicates that a mixture of the two samples was formed. In
addition, the Eg of the mixture is between the band gap
energies of FAPbBr3 and FAPbI3 which is a further support
claiming that a new phase has been synthesized.
FAPbBr
3
(b) 1.0
FAPbBr
I
2.2 0.8
FAPbBr I
2 1
0.8
0.6
0.4
0.2
0.0
FAPbBr
FAPbBr
FAPbBr
FAPbBr
I
1.8 1.2
I
1.6 1.4
I
1.4 1.6
I
1.2 1.8
FAPbBr I
1 2
FAPbI
3
The systematic shift in the position of the absorption
edge of FAPbBr3–xIx thin films to lower wavelengths,
observed as the concentration of Br decreased, is illustrated
in Table 3. As the iodide contents decreases, the absor-
bance edge shifts to short wavelength values, indicating the
increase of the band gap energy of the prepared perovskite
thin films. The onset band gap of mixed (Br, I) perovskite
thin films are located in intermediate values between
1.48 eV (FAPbI3) and 2.3 eV (FAPbBr3) meaning that the
band gap can be tuned by varying the composition of the
ratio Br/I.
500
550
600
650
700
/ nm
750
800
850
Fig. 6 a Normalized PL spectra of FAPbI3 thin films at different
annealing temperatures; b normalized PL spectra at room temperature
for FAPbBr3-xIx (x = 0–3) thin films
We compare the PL results shown in Fig. 6b of
FAPbBr3-xIx formed with a different fraction of Br/I. Inten-
sities are normalized to clarify the shift of the emission peaks.
A systematic shift in the position of the peaks to lower
wavelengths is observed as the concentration of bromide
increases. This indicates that the recombination centers come
probably from the presence of the bromide and iodide in the
perovskite lattice, which is in good agreement with the XRD
analyses and absorbance spectra. These observations are
consistent with the literature [24], suggesting a clean band gap
and one photo-active species from which emission arises in
the Br/I lead perovskites films and indicating that the transi-
tion between perovskites occurs via mixed halide.
Luminescence properties
Organometal halide perovskites also emit strong room
temperature photoluminescence (PL) that arises from
excitons (radiative recombination of charge carriers) in the
inorganic sheets [52]. PL measurements of films prepared
by the one-step deposition method were complicated by the
appearance of the yellow d-phase of the formamidinium
iodide, which optically manifests low only the prepared
FAPbI3 thin films. The absence of the a-phase explains the
blue-shifted emission in FAPbI3 prepared by deposition
method with different temperature annealing (Fig. 6a).
These results are confirmed by the evolution of the wave-
length for FAPbI3 from k = 791 nm when T = 140 °C to
k = 810 nm when T = 150 °C, and thus it involves the
passage to the most stable phase the a–d, which is con-
firmed by the results of XRD [53].
The PL spectra of the films displayed in Fig. 6b show
single transitions with a monotonic shift in peak position
with increasing iodide content from pure tri-bromide per-
ovskite (2.26 eV) to pure triiodide perovskite (1.53 eV).
This finding supports our observation of a clean band gap
in these films. However, mixing the halide component with
the perovskite offers the finest tuning of the optical
123

B. Slimi et al.
properties of the perovskite film. Here, the mixed organic
lead iodide/bromide system has recently gained strong
interest for application in perovskite solar cells [19, 34]. By
changing the ratio between bromide and iodide (at the X
site anion), the band gap can be tailored between 1.53 eV
(FAPbI3) and 2.26 eV (FAPbBr3) in agreement with pre-
vious reports [54], which results in the coverage of much of
the visible spectrum and paves the way for the develop-
ment of tandem solar cell [31]. The analysis of Fig. 6b
indicates that the substitution of Br by I leads to a red shift
in the spectrum. Also, we can see that the photolumines-
cence spectra contain several peaks with low intensity,
which means that, apart from inter band transitions, other
radiative transitions between the valence and/or conduction
bands and some energy levels located inside the band gap
are involved. The component of PL spectra with the
highest energy corresponds to transitions from the con-
duction band to the valence band and the lower energy
components of the PL spectra involve energy levels inside
the band gap, which is in agreement with the sub-band gap
absorption mentioned before.
substrates by spin coating in only one step. Our results
show that changing the molar ratio Br/I in the precursor
solution has a significant effect on the morphology, optical
absorption and PL properties of FAPbBr3–xIx.
All the synthesized FAPbBr3-xIx (x = 0–3) perovskites
reported in this paper crystallized in the same cubic phase
(space group Pm-3 m) independently of the x value. The
shift of diffractions peaks towards low angles is directly
related to the increase of the iodide contents.
Absorbance measurements show that FAPbBr3–xIx per-
ovskite films exhibit a very high absorbance for a high
concentration of iodide. It was found that the onset of the
absorption edge for FAPbBr3-xIx thin films reaches inter-
mediate values ranging from 1.48 eV (FAPbI3) to 2.3 eV
(FAPbBr3). Room temperature PL emission can be tuned
from 548 nm for FAPbBr3 to 810 nm for FAPbI3 by varying
the Br/I ratio. Perovskites with high iodide contents as well
as pure bromide perovskites have a clean PL-spectrum with
emissions from a single region centered at the optical band
gap energy. These PL emissions appear to be stable over
time. Finally, the results obtained confirmed that mixed
FAPbBr3-xIx perovskites are promising materials for appli-
cations in light-emitting and photovoltaic devices.
The high intensity of PL emission suggests that most
decay transitions are radiative and nonradiative decay is
negligible. Since the radiative recombination dominates,
one can deduce that most energy levels within the band gap
correspond to shallow levels and electron–hole pairs
formed during the optical excitation recombining radia-
tively emitting photons. A number of deep levels inside the
band gap are very low and as a result, both pure and mixed
perovskites exhibit high luminescent efficiency. Theoreti-
cal calculations show that the red shift upon moving from
Br- to I- derives from the associated decrease of the
electronegativity of the halogen atom [35, 55]. Exchanging
Br- with I- changes the nature of the halide frontier orbital
contribution to the valence band, which reduces the band
gap energy [56, 57]. The observed PL peak positions are
consistent with the absorption onsets seen in the absor-
bance spectra of the examined FAPbBr3-xIx films,
confirming that the PL arises primarily from band-edge
emission rather than from minority phases or trap states.
Table 3 displays the position of the most intense PL
peak and the related wavelength at room temperature for
different FAPbBr3-xIx (x = 0–3) perovskite thin films. For
these samples, the wavelength position of the maximum of
PL emission is shifted to lower wavelengths when com-
pared with the onset of the absorption edge.
Experiment
Synthesis of formamidinium iodide
and formamidinium bromide (FAI and FABr)
To synthesize mixed halides FAPbBr3-xIx perovskites,
both formamidinium iodide (HC(NH2)2I or FAI) and for-
mamidinium bromide (HC(NH2)2Br or FABr) were first
prepared by directly mixing formamidine acetate with
hydroiodic acid (HI) or hydrobromic acid (HBr) at 0 °C.
CH(NH2)2I or CH (NH2)2Br was synthesized by slowly
dissolving formamidinium acetate powder in HI (57 wt%
in water, from Sigma-Aldrich) or HBr (48 wt% in water,
from Sigma-Aldrich) in a molar ratio of 1:1, the mixture
was stirred for 30 min in a round-bottom flask, which was
kept in an ice bath (0 °C). The solvent was evaporated
using a rotary evaporator and the solid was re-crystallized
in ethanol two times, and then dried again. The product
obtained was collected by filtration after having been
washed thoroughly using diethyl ether until a white crys-
talline solid was obtained. The solids were finally dried
under vacuum for one night before use.
Conclusion
Synthesis of formamidinium lead iodide powders
The organic–inorganic perovskites FAPbBr3-xIx
were
FAPbI3 perovskite powder was synthesized by mixing
0.08 mol hydroiodic acid (HI, 57 wt% in H2O, distilled,
stabilized, 99.95% from Sigma-Aldrich) and 3.1 cm3
prepared by mixing FAPbBr3 and FAPbI3 in the desired
proportions (x = 0–3) and deposited as thin films onto ITO
123

Synthesis and characterization of perovskite FAPbBr3-xIx thin films for solar cells
and others were annealed in vacuum at 140 and 150 °C for
30 min.
Table 4 The theoretical and experimental results of the atomic ratio
of FAI and FABr
Elements
C
1
N
H
5
I
Br
0
Characterization
FAI
2
1
Theoretical atomic ratio
6.98 16.29 2.91 73.82
0.00
The X-ray diffraction (XRD) patterns of the prepared films
were measured using a RIGAKU Ultima IV diffractometer
Experimental atomic ratio 7.30 16.46 2.83 73.41
0.00
1
FABr
1
2
5
0
˚
equipped with CuKa radiation (k = 1.5406 A). The mor-
Theoretical atomic ratio
9.61 22.42 4.00
0.00 63.97
0.00 64.82
phology of the films was observed using a field-emission
scanning electron microscope (MIRA3 LMU, Tescan). The
elemental analysis (C, H, N, and S) of the perovskite was
performed using a FISONS EA_1108 CHN analyzer, with
a high-resolution scanning electron microscope (FESEM)
at an acceleration voltage of 10 kV. Photoluminescence
(PL) spectra were recorded at room temperature using a
He-closed cryostat. The PL excitation source was a He–Cd
laser emitting at 325 nm. Photoluminescence data were
recorded by a Si-based CCD detector Hamamatsu. Optical
measurements were performed at room temperature using a
spectrometer Ocean Optics HR4000 equipped with a Si-
CCD detector. An integrating sphere was used to collect
both direct and diffuse transmittance.
Experimental atomic ratio 9.48 21.81 3.98
hypophosphorous acid (H3PO2). The mixture was stirred
for 30 min in a 250 cm3 round-bottom flask, which was
kept in an ice bath (0 °C). Then 0.075 mol of formami-
dinium acetate powder previously dissolved in 20 cm3 of
distilled water was added dropwise. Subsequently, this
mixture was heated up to 100 °C for 30 min and then
0.008 mol of lead(II) acetate trihydrate previously dis-
solved in 30 cm3 of distilled water maintained at 60 °C,
was added dropwise under vigorous stirring. The solution
was kept at 100 °C for 1 h with stirring and reflux. After
this process, a precipitate of black crystalline powder takes
place. The remaining solution was then left to cool until
50 °C and was then filtered. The crystalline powders were
washed several times with absolute ethanol, diethyl ether
and then dried under vacuum for 15 min. Finally, the
powder was annealed at different temperatures (140 and
180 °C) for 2 h.
Acknowledgements This work was supported by Ministerio de
´
Economıa y Competitividad (ENE2016-77798-C4-2-R) and Gener-
alitat valenciana (Prometeus 2014/044).
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By elemental analysis, it was found that the weight ratio
of C:N:H:(I:Br) in the as-prepared FAI and FABr was
7.30:16.46:2.83:73.41 and 9.48:21.81:3.98:64.82, respec-
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of any other impurity, as verified by XRD. It was found
that the mixture of FAI, FABr, PbI2, and PbBr2 with molar
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