Understanding the stability of mixed A-cation lead iodide perovskites

Article abstract of DOI:10.1039/c7ta08617b

The routes and kinetics of the degradation of thin films of methylammonium (MA)/formamidinium (FA) lead iodide perovskites (MA_1-xFA_xPbI₃, 0 ≤ x ≤ 1) under dry atmospheric conditions have been investigated. MA-rich phases decompose to the precursor iodide salts and PbI₂, while FA-rich phases convert mainly to the yellow hexagonal phase. The reactivity is strongly inhibited for mixed cation phases of MA_1-xFA_xPbI₃, for x = 0.4 to 0.6, where the decomposition routes available to end member phases become less favourable. It is shown that for pristine films with x = 0.6, PbI₂ formation can be completely suppressed for up to 10 days. Kinetic analysis reveals that the rate of PbI₂ formation decays exponentially with increasing FA content until x = 0.7, beyond which the FA containing perovskite transforms rapidly to the hexagonal phase. Ab initio simulations of the decomposition reaction energies fully support the increased kinetic stability found experimentally for the mixed A-cation perovskites.

Full text of DOI:10.1039/c7ta08617b

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January 2016 Pages 1–330
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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS nanocages as a lithium ion battery anode material
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Understanding the Stability of Mixed A-Cation Lead Iodide
Perovskites
3
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Bethan Charles, Jessica Dillon, Oliver J. Weber, M. Saiful Islam* and Mark T. Weller*
The routes and kinetics of the degradation of thin films of properties and, most pertinently, the stability of the resultant
1
2-14
methylammonium(MA)
perovskites (MA1-xFA PbI
conditions have been investigated. MA-rich phases decompose to iodide (CH
the precursor iodide salts and PbI , while FA-rich phases convert group I4/mcm at room temperature. Formamidinium (FA)
mainly to the yellow hexagonal phase. The reactivity is strongly lead iodide (CH(NH PbI ), FAPI, by contrast preferentially
inhibited for mixed cation phases of MA1-xFA PbI , for x = 0.4 to crystallises in the yellow, 2H hexagonal perovskite type
.6, where the decomposition routes available to end member polymorph at room temperature, which can be driven to
phases become less favourable. It is shown that for pristine films transform the black cubic perovskite polymorph (required for
with x = 0.6, PbI formation can be completely surpressed for up photovoltaic applications) by heating above room
to 10 days. Kinetic analysis reveals that the rate of PbI
/
formamidinium(FA) lead iodide material.
, 0 ≤ x ≤ 1) under dry atmospheric The archetypal hybrid perovskite methylammonium (MA) lead
NH PbI ), MAPI, crystallises in the tetragonal space
x
3
3
3
3
1
5
2
2
)
2
3
x
3
0
2
1
6-19
2
formation temperature.
Such varying phase behaviour can be
decays exponentially with increasing FA content until x = 0.7, accounted for by the different sizes and hydrogen bonding
beyond which the FA containing perovskite transforms rapidly to mechanisms of these dissimilar organic cations, MA and FA.
the hexagonal phase. Ab initio simulations of the decomposition
Both MA and FA cations display directional hydrogen
reaction energies fully support the increased kinetic stability bonding, a factor of increasing importance in controlling phase
found experimentally for the mixed A-cation perovskites.
behaviour as the system temperature is reduced and also for
2
defining the relative stability of the two polymorphs of FAPI.
0
Hybrid lead halide perovskites have recently risen to
prominence as highly versatile materials for optoelectronic
technologies, particularly high efficiency, solution processed
Stabilisation of mixed A site compounds relative to the end
members of the series can be rationalised using geometrical
arguments in which the perovskite tolerance factor is tuned
using differently sized cations, as well as an entropic factor due
1
-4
solar cells. They display near optimal band gap for solar light
absorption, high absorption coefficients, steep absorption
onsets and significant charge carrier lifetimes (and thus
2
1
to cation mixing.
It has been established that by combining the MA and FA
cations in solid solutions, the cubic perovskite phase is
2
,5
diffusion lengths). These properties, combined with the
6
processability of these materials, have resulted in solar cell
,7
2
2-25
stabilised versus decomposition.
We have previously
8
device efficiencies rising rapidly to over 22%. However, the
shown that for compositions MA1-xFA PbI 0 < x < 0.2 the
x
3
commercial applicability of these materials is hampered by
their relative lack of stability compared to established
structures are best described by a tetragonal unit cell from X-
ray diffraction, while the rest of the composition range, 0.2 ≤ x
2
9
-11
6
inorganic and organic semiconductors.
By chemical site-
≤
1, forms with a cubic cell, space group Pm-3m.
substitution at any of the A, B or X sites of the ABX perovskite
3
structure, it has been found that it is possible to tune the
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
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The compositional space for A site substitution between FA
Journal Name
2
7,28
23
and MA has been explored in the literature.
studied the charge transport characteristics of single
PbI single crystals over the composition ranges 0 < x
0.2 and 0.8 < x < 1 in increments of x = 0.05. High carrier
Huang et al.
MA1−xFA
x
3
<
mobilities and lifetimes were obtained for x = 0.1 and 0.15.
Promising performance has also been demonstrated recently
2
2,29
in photovoltaic devices,
3
where MA0.9FA0.1PbI delivered an
efficiency improvement to 20.2%, versus 18.6% for pure MAPI.
Links between crystallinity, crystal phase stability, good
optoelectronic properties and operational stability have been
≥
0.4 showed that the presence of increasing levels of FA
cation supress the formation of PbI
of the (001) PbI reflection decreasing with increasing x, for the
same time period of exposure to air. For the film composition x
0.6, no PbI is observed after 7 days by PXD, with only trace
2
, with the relative intensity
2
4
2
drawn, indicating that highly crystalline compositions also
suppress effects such as mixed halide phase segregation.
Furthermore for the synthesis and efficient operation of
devices the perovskite materials need to be stabilised with
respect to the phase transition to the undesirable hexagonal
=
2
amounts present after 10 days.
For compositions x ≥ 0.7, the hexagonal δ-phase is
observed immediately after film fabrication. As previously
observed for FAPI, increasing the annealing temperature to
2
H-perovskite polymorph, as occurs for FAPI and FA-rich solid
23,25
solutions.
The phase stability of MA1-xFA PbI has been
x 3
1
80° achieves full conversion to the perovskite α-phase for
32
studied for samples stored under a nitrogen atmosphere,
indicating that for FA-rich compositions with x > 0.76, partial
transition to the hexagonal phase is observed within days,
while for compositions with more MA, the hexagonal phase is
MA1-xFA PbI , x ≥ 0.8. However, these films convert back to
x
3
Fig.1 Powder X-Ray patterns of MA1-xFA
Patterns were measured immediately after synthesis (bottom trace) then through days
, 3, 5, 7 and 10 (top trace).
x 3
PbI thin films where x = 0, 0.4 and 0.6.
1
3
0
not observed by X-ray diffraction. The propensity for hybrid
3
1
the hexagonal δ-phase within 30 minutes of exposure to air.
perovskites to react with water has also been reported.
While high photovoltaic performance for MA1-xFA PbI has
Suppression of δ-phase formation was observed for x = 0.7.
x
3
2
The presence of small amounts of PbI in the as-made films
been demonstrated, the extended stability lifetime and
decomposition of these compounds under moderately dry
atmospheric conditions,, likely to be similar to those used in
device processing, has yet to be fully reported and are not
completely understood.
was found, for several values of x, to cause markedly increased
rates of degradation. This was investigated further through the
synthesis of x = 0.6 films that contained varying amounts of
PbI
a 10 day period following exposure to air showed the intensity
of the PbI (001) reflection grew most rapidly for samples that
contained observable levels of PbI in the as-made sample (see
ESI S3). It is apparent from the data that phase pure perovskite
films are required to suppress the growth rates of PbI , as even
trace amounts of PbI present in as made films markedly
catalyse the rate of further PbI formation. At the low moisture
2
in the as-made thin films. PXD analysis of these films over
In this work, for the first time, the degradation of
MA1−xFA PbI thin films over the full compositional range 0 < x
2
x
3
2
<
1 in increments of x = 0.1 in air has been systematically
investigated. The results provide a full understanding of the
processes, thermodynamics and kinetics involved in the
degradation of mixed A-cation lead perovskites.
2
2
2
X-ray powder diffraction (PXD) data from MA1-xFA PbI
x
3
level used in this study neither sunlight nor laboratory lighting
had a discernible effect on the rate of material degradation.
The kinetics of the decomposition of thin films MA1-
were initially collected on as-made pristine films (see ESI S1
and S2). Further diffraction patterns were subsequently
obtained from the same samples after 1, 3, 5, 7 and 10 days.
The films were stored below 30% relative humidity in the dark
in order to control exposure to high levels of moisture and
x
FA
rate of PbI
following Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation,
x
PbI
3
0.0 ≤ x ≤ 0.6 were investigated through analysis of the
2
phase formation. This was modelled through the
3
1
intense light, which are already known to cause degradation .
Raw diffraction data were corrected for sample height
displacement and analysed using the EVA diffraction suite.
PXD patterns for samples with 0 ≤ x ≤ 1 displayed the
emergence of a peak centred at 2θ = 12.7°, representing the
ꢀꢁ 1 ꢂ exp ꢀꢃ ꢂꢃꢄꢅꢆꢇꢆ
(1)
∝
where α is the phase fraction of the product, k is the rate
constant, t is time and m is the Avrami exponent, the value of
2
(001) reflection for PbI (Fig. 1). For 0 ≤ x ≤ 0.3, this reflection
3
3
which depends on the crystallisation mechanism. Phase
fractions in the degrading films were initially estimated using
grew in intensity over time, surpassing the relative intensity of
the (100) perovskite reflection centred at 2θ = 14° after 10
days. However, consideration of PXD data from samples with x
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x
FA
x
PbI
3
film degradation to PbI
2
as x increases from 0.0 and
Fig. 2 Example Avrami plot for MA0.5 FA0.5PbI
3
. A linear relationship is observed when
plotting ln(t) in hours and ln ln(1/1-A), where the A is peak area under the PbI
2
(001)
reflections in the X-ray diffraction patterns and t is time in hours. Similar plots were
produced for each value of x.
2
Fig. 3 Rate constant equivalents for PbI growth fitted using an exponential decay
function, calculated by extracting the intercept and gradient of linear models in the
JMAK kinetic plots (Fig. 2), plotted using the peak area of the (001) PbI reflection as
an approximation for the phase fraction of PbI transformed.
2
-20
2
-
40
60
80
100
Rietveld profile fitting of the PXD data. However, refinements
for these highly textured orientated films resulted in phase
fraction values with large errors. Therefore, the peak area (A)
-
-
-
of the (001) PbI reflection was used to approximate the extent
2
of
-120
-140
phase formation (α) as required for this analysis of the
kinetics.
-
160
180
200
-220
From the JMAK equation a linear relationship is expected
when plotting ln(t) with ln ln(1/1-A) (Fig. 2). For x = 0.0 - 0.3
this linear relationship was observed up to 5 days. After this
-
-
time, with very high levels of PbI present, the change in the
2
0.0
0.2
0.4
0.6
0.8
1.0
peak area of the (001) PbI2 reflection is unlikely to be
representative of the true phase fraction of PbI present in the
2
Composition MA1-xFA PbI
x 3
surface of the film. For x ≥ 0.4, with the lower levels of PbI2 Fig.5 Calculated energies for the decomposition reaction (2) for MA1-xFA PbI3 as a
x
formed, the linearity was observed for the full duration of the function of FA content (x). For consistent comparison, starting structural
configurations based on pseudo-cubic, high order structures are presented here (with
1
0 day study. Extracting the gradient and intercept of the
full data showing the same trends found in ESI S4). The shaded region indicates that in
linear model for each composition, x, a trend in rate constant
the preparation of compositions x > 0.7 the yellow hexagonal (non-perovskite)
modelled to a simple exponential decay can be observed (Fig.
polymorph is observed.
approaches 0.6.
3). It should be noted these results do not give absolute values
The degradation routes observed for MA1-xFA
x 3
PbI are
for the rate constant of PbI formation, due to the peak area
2
summarised in Fig. 4. High rates of PbI formation are found
2
approximation employed to yield the level of PbI2.
Nevertheless, the exponential decay seen in Fig.3
demonstrates a clear trend of increasing resistance in MA1-
for x ≤ 0.3, although increasing the FA content strongly
supresses this decomposition pathway. For x ≥ 0.7 this
decomposition route competes with a phase transformation to
the hexagonal δ-phase.
Morphology and crystallite size within the thin films are
factors that might affect the degradation rate. Therefore SEM
images were acquired for the as-prepared MA1-xFA
x 3
PbI films
(ESI S5). For films with x increasing from 0.0 to 0.6 the
morphology was very similar and the average crystallite size (a
few microns) only increased slightly with increasing FA
content. For films with x = 0.7, which coincides with the
appearance of a portion of the hexagonal δ-phase, a very
different crystallite morphology and much larger crystallite
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
Fig. 4 Schematic of the experimentally determined degradation routes of MA1-xFA
0 ≤ x ≤ 1). MA-rich phases decompose to PbI and the precursor iodide salts, while FA-
rich phases convert to the yellow δ-phase. The routes are less favourable for the more
x 3
PbI
(
2
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stable x = 0.4 to 0.6 mixed cation phases.

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size was observed. Therefore it seems likely that the decomposition reaction energy for perovskite-structured alpha-
thermodynamics, as opposed to film morphology, is a major FAPbI we find highly exothermic energy suggesting
factor contributing to the decomposition kinetics. thermodynamically unstable structure. Indeed, it is known that in
3
a
a
To complement our experimental diffraction and kinetic the preparation of compositions x ≥ 0.7 the yellow hexagonal (non-
measurements, we performed ab initio simulations on pure perovskite) δ-phase is observed at room temperature.
MAPI and a range of mixed MA/FA compositions to obtain
The kinetic and thermodynamic stability results are further
insights into the trends in energetics of the degradation supported through the theoretical optimum A-cation size
process. Such density functional theory (DFT) methods have suggested by the Goldschmidt and octahedral tolerance
been applied successfully to other recent studies on hybrid factors. Fig.6 shows the effect of A site mixing on the
3
4-36
perovskites.
Using direct calorimetric techniques, Navrotsky et.al.
Goldschmidt tolerance factor and octahedral factor, indicating
which compositions are geometrically likely to adopt a cubic
3
7
have shown MAPI to be thermodynamically unstable with perovskite unit cell. Other well-known hybrid / inorganic halide
respect to decomposition to PbI2 and MAI. Here, the same perovskite compounds substituted at the A, B, or X sites are
intrinsic decomposition reaction was investigated involving the plotted for comparison. The octahedral factor is defined as:
formation of PbI and organic iodides as well as decomposition
2
to the δ-phase:
ꢖꢗ
(
3)
ꢕ ꢁꢀ
,
ꢖ
ꢘ
and the Goldschmidt tolerance factor as:
ꢖꢙ ꢓ ꢖ
ꢘ
ꢅ ꢁꢀ
(4)
√
2ꢃꢖ ꢓ ꢖ ꢆ
ꢗ ꢘ
where r
A
, r
B
and r
X
are the ionic radii of the ABX compound.
3
39
Effective radii for organic cations taken from G. Kieslich et.al
Fig.
compositions stabilised in the cubic perovskite structure for hybrid lead halide
perovskites. Compositions MA1-xFA PbI , 0.3 < x < 0.8, which showed the greatest
6
Octahedral factor (ꢁ) versus tolerance factor (t) showing the range of
and revised ionic radii for iodide and bromide compounds are
x
3
40
taken from W. Travis et.al.
greatest stability towards decomposition, MA1-xFA
0.8, lie close to the centre of the cubic perovskite stability
field.
In conclusion, thin films of the mixed organic cation
perovskite system MA1-xFA PbI undergo degradation in air to
Phases with the observed
stability against degradation, are observed to lie close to the centre of the cubic
perovskite structure stability field.
x
PbI , 0.3 < x
3
<
ꢈꢉꢊꢋꢌꢍꢉ ꢎꢏꢐ ꢀ→ ꢃ1 ꢂ ꢒꢆꢈꢉꢐ ꢓ ꢀꢒꢍꢉꢐ ꢓ ꢎꢏꢐ
ꢔ
ꢌ
ꢑꢀ
Other decomposition routes and the effect of the cubic to
tetragonal phase transition for MA-rich phases are discussed
further in the ESI S4. The reaction energies were calculated for a
range of compositions (x) and the values are presented in Fig. 5. The
results reveal three main features.
x
3
non-perovskite phases at a much slower rate than the end-
member MAPI and FAPI phases. For example, the composition
3 2
MA0.4FA0.6PbI synthesised with no contaminating PbI phase is
stable in air for up to 10 days. Ab initio simulations of the
decomposition reaction energies fully support the observed
increased stability of the mixed A-cation perovskites. The
reduced reactivity of these phases is likely to be related to
suppression of the two possible degradation mechanisms of (i)
First, pure MAPI shows
indicating thermodynamic instability with respect to the
reaction products PbI and the organic iodide, which agrees
well with experimental calorimetric measurements. Second,
all the mixed MA1-xFA PbI compositions (0.0 ≤ x ≤ 0.6) show
less favourable energetics, and hence a greater degree of
stability, than pure MAPbI . These results are consistent with
our observation that formation of PbI was suppress e( 2 for
increasing FA content with only trace amounts of PbI found
a highly exothermic energy
2
3
7
decomposition to PbI
and (ii) phase change as occurs for FA-rich phases (see also ESI
S4). As FAPI does not degrade readily to FAI and PbI , and
2
and MAI, as found for MA-rich phases,
x
3
2
3
MAPI adopts a single structure type, then any decomposition
route of a mixed cation phase, such as MA0.4FA0.6PbI3, requires
the simultaneous formation of three new structure types,
2
d
)
2
3
8
after 10 days for x = 0.6. We note that Brivio et. al find very
small entropy terms for halide ion mixing in MAPI-based
perovskites, which would further increase
2
namely yellow FAPI, PbI and MAI. We have shown that such a
degradation route is less thermodynamically favourable and
also has much slower kinetics. This combined experimental-
computational study provides a greater understanding of the
the stability of mixed cation compositions. Third, when we consider
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degradation routes of mixed A-cation perovskite halides,
which is crucial for controlling and enhancing the long-term
stability of perovskite solar cell materials.
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Journal of Materials Chemistry A
Page 6 of 6
View Article Online
DOI: 10.1039/C7TA08617B
Thin films of the mixed A-cation phase
MA FA PbI have been shown experimentally
1
−x
x
3
and computationally to have greater kinetic and
thermodynamic stability towards
decomposition.