In situ investigation of degradation at organometal halide perovskite surfaces by X-ray photoelectron spectroscopy at realistic water vapour pressure

Article abstract of DOI:10.1039/c7cc01538k

Near-ambient-pressure X-ray photoelectron spectroscopy enables the study of the reaction of in situ-prepared methylammonium lead iodide (MAPI) perovskite at realistic water vapour pressures for the first time. We show that MAPI decomposes directly to PbI<

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc
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In Situ Investigation of Degradation at Organometal Halide
Perovskite Surfaces by X-ray Photoelectron Spectroscopy at
Realistic Water Vapour Pressure†
Received 00th Feburary 2017,
Accepted 00th March 2017
a,b
b,c
d
c
c
DOI: 10.1039/x0xx00000x
Jack Chun-Ren Ke , Alex S. Walton , David J. Lewis , Aleksander Tedstone , Paul O’Brien ,
b,d,
a,b,
Andrew G. Thomas * and Wendy R. Flavell
*
www.rsc.org/
1
7, 19-21
Near-ambient-pressure X-ray photoelectron spectroscopy enables (UHV) environment,
so that degradation of the films
before measurement is ruled out. Several XPS studies have
been carried out to determine the surface composition or film
the study of the reaction of in-situ-prepared methylammonium
lead iodide (MAPI) perovskite at realistic water vapour pressures
for the first time. We show that MAPI decomposes directly to
1
9-27
28-30
growth mechanism,
and band alignment.
Despite these studies, the mechanism of degradation of
pristine films at the atomic and nanometre scale remains
imprecisely defined. Currently, two models of OHP
degradation have been proposed. The first involves the water-
catalysed loss of nitrogen, in the form of ammonia, and
2 3
PbI , HI and NH without formation of methylammonium iodide,
allowing us to distinguish between alternative mechanisms for the
atmospheric degradation reaction.
Organometal halide perovskite (OHP) absorbers have rapidly
emerged as one of the most promising photovoltaic materials
for use in solar cells due to their high visible to near-infrared
17
hydrogen iodide via the reaction (Equation 1):
2
H O
CH NH PbI ꢀsꢁ ꢂꢃ ꢀ-CH -ꢁ ꢀsꢁ + NH ꢀgꢁ + HI ꢀgꢁ + PbI (s)
Equation 1
3
3
3
2
3
2
1
-4
5
light absorptivity, remarkably low exciton binding energy,
and extraordinarily long diffusion length for electrons and
holes. As a result, perovskite solar cells (PSCs) have achieved
Thermogravimetric analysis of bulk MAPI by Grätzel and co-
workers suggests that the organic component of the
perovskite material is lost via volatilisation of HI and CH NH
6
, 7
8
3
2
certified power conversion efficiencies of more than 20%, in
just over 7 years from the initial discovery of the photovoltaic
effect in TiO
3
1
with the latter species lost at higher temperatures. The
second proposal involves decomposition to methyl ammonium
iodide (MAI) and lead iodide in the presence of liquid water
9
coated with nanocrystalline perovskite. These
2
efficiencies are now comparable to those of commercial
polycrystalline silicon solar cells. However, OHPs show poor
long-term stability, particularly when exposed to humid
atmospheres which means that there are questions about the
1
6
(Equation 2):
2
H O
CH NH PbI ꢀsꢁ ꢂꢃ CH NH I (aq) + PbI (s)
Equation 2
3
3
3
3
3
2
1
0
commercial exploitation of PSCs in solar panels. Improving Key difficulties in understanding the degradation mechanism of
the stability of OHP absorbers is of paramount importance, but MAPI are the difficulty in determining the surface chemistry of the
only with insight into the degradation mechanisms at the first few atomic layers of the pristine material without any exposure
atomic level solutions can this be realised.
to ambient atmospheric conditions, and, conversely, the difficulty in
Numerous characterisation techniques have been applied measuring this same surface chemistry in real time under exposure
to investigate the degradation mechanism, such as ultraviolet- to realistic environments. In this work we address both problems
1
1-13
11, 13,
visible (UV-Vis) spectroscopy,
X-ray diffraction (XRD),
and X-ray photoelectron investigate the real-time degradation of an in-situ-deposited MAPI
This latter technique is useful since it film, exposed to water vapour pressures equivalent to atmospheric
simultaneously by utilising Near-Ambient Pressure XPS (NAPXPS) to
1
4
15
Raman spectroscopy,
1
6-18
spectroscopy (XPS).
allows in situ deposition of the films in an ultrahigh vacuum relative humidity (RH) of c.a. 30% at a standard temperature of 25
o
C, conditions which could be realistically be observed under
ambient atmospheric environments. This yields an unparalleled
mechanistic insight. The data unambiguously indicate a degradation
pathway by reaction with water which involves complete loss of
nitrogen from the film without the formation of MAI.
The MAPI film was prepared in UHV by vacuum
2 3
deposition of PbCl and MAI onto a SrTiO (100) single crystal
substrate, using the two-step route of Liu et al. (see Electronic
Supplementary Information (ESI)†) for details of preparation
3
2
and characterisation). Fig. 1a shows high-resolution NAPXPS
spectra of the Sr 3d and Pb 4f core levels recorded from the
MAPI film before water exposure, during water exposure at
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H
2
O vapour pressures of 3 mbar (RH 10%) and 9 mbar (RH
0%), and after the water vapour has been removed and the
system returned to UHV conditions. The intensity is normalised
to the integrated area of the Sr 3d5/2 peak from the SrTiO
substrate, which allows comparison of the intensity of the Pb
f peaks as a function of water exposure. All photoelectron
binding energies are quoted to a precision of ± 0.05 eV. Prior
to exposure to H O vapour and under UHV, the Pb 4f7/2 peak of
the perovskite film is located at a binding energy (BE) of
38.60 eV, which is in excellent agreement with previous
reports for the Pb in MAPI perovskite materials (denoted 'Pb-I'
3
3
4
2
1
1
6, 22, 30, 33
in Fig. 1).
at 136.80 eV, which is attributed to metallic lead.
has been observed for several perovskite materials, such as
We note that there is a weaker Pb 4f7/2 peak
1
6, 22, 30, 33
This
1
6, 22, 27, 33
34
33
.
MAPI,
observed in MAPI perovskite films, both for the standard
MAPbI
2
Cl, MAPbBr
3
Metallic lead is often
3
5, 36
22
solution-processed films
study presented here the metallic Pb probably arises due to
decomposition of the PbCl precursor vapour during the initial
and CVD-deposited films . In the
2
2
1
deposition step; some beam-induced decomposition cannot
be ruled out, but is not the major source of elemental Pb (as
we discuss further below). No Cl was observed in survey scans
(see Fig. S1). The Pb:I:N stoichiometry of the film determined
from XPS, excluding the metallic Pb component, was found to
be 1.0 ± 0.1 : 2.7 ± 0.1 : 0.9 ± 0.2 (Table 1), in good agreement
with the nominal stoichiometry of MAPI. Quantification of the
XPS signals is discussed further below and in the ESI (Tables S1-
3).
Fig. 1 NAPXPS spectra of (a) Pb 4f & Sr 3d, and (b) I 3d
Fig.1(a) shows Pb 4f and Sr 3d spectra recorded during
exposure to H O vapour. The Pb-I features decrease in
intensity, whilst the metallic Pb features increase in intensity.
The BE of the Pb 4f7/2 Pb-I feature shifts upwards slightly to
core levels under various conditions as noted
2
(before/during/after water exposure). All the spectra are
normalised to the corresponding integrated area of the Sr
3d5/2 peak.
138.70 eV on exposure to 3 mbar H
the water exposure is increased to 9 mbar. This may be due to
2
O and to 138.80 eV when
Table 1
Stoichiometries of the film determined from UHV
1
6
the formation of lead iodide (Pb 4f7/2 BE =138.90 eV);
however the peak cannot be resolved into individual
components arising from MAPI and PbI . The BE of the
MAPI/PbI peak recorded after water exposure, when the
XPS at different stages, before and after water exposure. The
metallic lead content is excluded from these quantifications. A
full description of the quantification is given in the ESI.
2
2
water vapour is pumped away and the sample returned to
ultra-high vacuum conditions (Fig.1(a)iv), is not shifted relative
to the spectrum recorded at a water vapour pressure of 9
mbar (Fig.1(a)iii). This suggests that the presence of water
vapour is not responsible for the observed binding energy
shifts, i.e. the shifts are due to changes in the nature of the
material. The data recorded following removal of the water
vapour indicate that the changes induced by exposure to
water vapour are not reversible. We observe an additional
increase in the intensity of the metallic-lead-derived peak in
the UHV spectrum taken following water exposure. In
Status ID
I/Pb
N/Pb
UHV before water exposure
UHV after water exposure
2.7 ± 0.1
2.4 ± 0.1
0.9 ± 0.2
0.0 ± 0.2
shifts towards higher BE, from 619.50 eV (UHV before) to
19.60 eV at 3 mbar water exposure and reaches 619.65 eV
6
at 9 mbar water exposure. The change in BE matches the
changes in the Pb 4f spectra. The I signal decreases
significantly upon water exposure, suggesting it is lost from the
surface, which is not consistent with a degradation mechanism
resulting in the formation of MAI. We note that the intensity
increases slightly when the water vapour is removed. This is a
consequence of the lower kinetic energy of the I 3d electrons
relative to Pb 4f. Lower kinetic energy electrons are less likely
to be detected by the analyser than high energy ones as the
gas pressure in the NAPXPS cell is increased.
1
6
synchrotron studies this was attributed to beam damage, and
it is well known that PbI slowly decomposes to Pb and I
2
2
3
7
under illumination with visible light. However, spectra taken
at this stage from fresh points on the sample surface (see ESI,
Fig. S3) show a similar surface stoichiometry to that in
Fig.1(a)iv, and in particular, similar amounts of metallic Pb. We
conclude that the decomposition of PbI
be attributed solely to beam damage.
2 2
into Pb and I cannot
Fig.1b shows I 3d core-level spectra, also normalised to
the integrated area of the Sr 3d5/2 peak. The BE of the I 3d5/2
2
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water vapour, the N 1s spectrum of the pristine sample lies at
a binding energy of 402.45 eV, in good agreement with the
1
6, 21
methylammonium group in MAPI.
During and after
exposure to water the N 1s peak is seen to dramatically
decrease in intensity during water exposure, and after water
exposure the N 1s signal cannot be reliably distinguished from
the spectral noise. This confirms that at least some of the
intensity in the C 1s peak at 286.75 eV arises from other
carbon species (such as C=O/C-OH), since there is some
remnant intensity, even after all of the N 1s signal is removed.
During and after exposure to 9 mbar water vapour there is an
increase in the relative intensity of the lower binding energy
feature which may be due to conversion of C-N to C-H/C-C,
consistent with the mechanism proposed in equation 1.
The stoichiometry of the film before and after water exposure is
calculated from the integrated peak areas, using CASAXPS
39
sensitivity factors (Table 1). These calculations are only performed
for the sample under UHV conditions, since RSFs are not available
for the analyser under near-ambient conditions. As noted above,
the Pb:I:N: ratio of the as-deposited film is close to the expected
2
stoichiometry. Following H O vapour exposure the Pb/I ratio
decreases and N is completely lost from the surface. Although
obtaining information on the stoichiometry of the C part of the
material is difficult because of the hydrocarbon contamination,
there are some observations which support the decomposition of
3
MAPI to form NH . The decrease in intensity of the higher binding
energy component of the C 1s spectrum is consistent with the
overall loss of N (see ESI, Table S2). In addition, we find that the
total amount C at the surface remains constant, suggesting that C-N
Fig. 2 NAPXPS spectra of (a) C 1s & Sr 3p, and (b) N 1s core
levels before/during/after water exposure. All spectra are
normalised to the corresponding integrated area of the Sr
is converted to –CH
2
– in accordance with Equation 1.
3d5/2 peak.
The evidence from NAPXPS strongly supports
decomposition mechanism similar to that suggested by Li et al.
a
Fig. 2 shows the C 1s and Sr 3p spectra. The C 1s spectrum
allows us to investigate the behaviour of the organic part of
the perovskite under water vapour exposure. The Sr 3p3/2
peaks are located at 269.20 eV, consistent with Sr–O
17
(
Equation 1), where nitrogen is lost from the perovskite via
reaction with water to give NH (g). The results are consistent
with conversion of lead in the MAPI film to PbI (I/Pb is
3
3
8
2
3
bonding. The clean SrTiO spectrum shows a very weak signal
significantly reduced, Table 1), some of which is then further
decomposed as shown in Equation 3.
associated with a small amount of residual adventitious carbon.
In the freshly-prepared MAPI film we observe two C species in
the C 1s spectrum. The larger C 1s peak is located at a BE of
2
H O, hv
PbI (s) ꢂꢄꢄꢄꢃ Pb (s) + I (g)
ꢅꢅꢅ
Equation 3
2
1
2
2
2
85.35 eV. This peak has been assigned both to CH
3
I, and to
C-C bonds (hydrocarbon). Ng et al. linked the formation of
2
1
The observed changes in the C 1s spectra induced by water
exposure are in agreement with the work of Li et al. suggesting
that the formation of MAI (via Equation 2) is not involved in
the decomposition mechanism in the presence of water
2
2
2
the C-C species to reaction with PbI , but it is perhaps more
likely to occur due to decomposition of the precursors or
simply due to outgassing of the sources during evaporation of
the films. A shoulder is also observed at 286.85 eV, 1.5 eV
higher than the main peak, corresponding to C–N bonding in
1
7
vapour. This is further confirmed by the complete loss of the
N 1s feature, and a commensurate decrease in the intensity of
the feature assigned to C-N in the C 1s spectrum. If MAI were
formed we would expect simply to observe a shift to lower BE
1
6, 17, 21, 34
the MAPI perovskite.
However, as discussed below,
we believe this feature also contains contributions from
C=O/C-OH groups in the hydrocarbon contamination. We do
not believe the MAI precursor contributes significantly to the
higher binding energy feature here, since the core level shift in
2
1
relative to the MAPI N 1s signal. The release of nitrogen must
therefore occur directly from the MAPI film as stated in
Equation 1, probably in the form of ammonia gas. We cannot
rule out the formation of some CH
during the degradation process, as its C 1s binding energy
cannot be distinguished from that of MAPI. Indeed, this is
suggested by the final Pb/I ratio of 2.4 ± 0.1, higher than the
3 3
PbI by loss of ammonia
the C 1s feature between C-C/CH
1
3
I and C–N in MAI is larger at
.9 eV, than the 1.5 eV measured here. Upon exposure to
2
2
2
1
water vapour, the relative intensities of the C 1s peaks change
substantially. The higher energy peak, which contains
contributions from C-N in the MAPI, is seen to decrease in
intensity. During and after exposure to 9 mbar water vapour
there is an increase in the relative intensity of the lower
binding energy feature possibly due to the presence of further
hydrocarbon contamination over time in the NAP cell.
2
2.0 expected if only PbI is produced.
In conclusion, XPS analysis suggests a stoichiometric MAPI
thin film was successfully fabricated via vapour deposition, in
40, 41
good agreement with previous studies.
The data clearly
demonstrate the pathway of water-induced degradation of the
MAPI film at relative humidity similar to those found in
ambient air, using NAPXPS. It is shown that MAI is not formed
Stronger evidence for the cleavage of the C-N bond is seen
in the corresponding N 1s spectra in Fig.2. Before exposure to
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The authors thank the University of Manchester and EPSRC
(UK) (grant number EP/K009710) for funding. C.R. Ke thanks
the University of Manchester for the award of a President’s
Doctoral Scholarship.
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