Insights into the increased degradation rate of CH3NH3PbI3 solar cells in combined water and O2 environments

Article abstract of DOI:10.1039/c7ta06841g

Halide perovskites offer low cost and high efficiency solar cell materials but serious issues related to air and moisture stability remain. In this study we show, using UV-vis, fluorescence and time of flight secondary ion mass spectrometry (ToF-SIMS) tec

Full text of DOI:10.1039/c7ta06841g

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Insights into the increased degradation rate of
CH NH PbI solar cells in combined water and O
3
3
3
2
Cite this: DOI: 10.1039/c7ta06841g
environments†
Nick Aristidou, ‡ Christopher Eames, ‡ M. Saiful Islam *b
a
b
a
and Saif A. Haque
*
Halide perovskites offer low cost and high efficiency solar cell materials but serious issues related to air and
moisture stability remain. In this study we show, using UV-vis, fluorescence and time of flight secondary ion
mass spectrometry (ToF-SIMS) techniques, that the degradation of methylammonium lead iodide solar cells
is significantly accelerated when both air and moisture are present in comparison to when just air or
moisture is present alone. Using ab initio computational techniques we identify the thermodynamic
driving force for the enhanced reactivity and highlight the regions of the photoexcited material that are
the most likely reaction centres. We suggest that water catalyses the reaction by stabilising the reactive
superoxide species, enabling them to react with the methylammonium cation.
Received 3rd August 2017
Accepted 15th November 2017
DOI: 10.1039/c7ta06841g
rsc.li/materials-a
exposed to water concentrations below 85% a reversible phase
change into a monohydrate phase occurs, but higher concen-
1
Introduction
Organic lead halide perovskite-based solar cells are currently trations or prolonged exposure causes an irreversible phase
a subject of intense research interest due to their combination change into a dihydrate phase. Moreover, IR spectroscopy
of low-cost production methods with high power conversion studies have shown that CH NH PbI lms are hygroscopic and
3 3 3
efficiencies, which have shown rapid increases. Despite these water uptake is rapid where the resulting interaction of water is
promising features, there are fundamental issues of operational with the methyl ammonium cation component in the crystal
1–4
and material stability, along with the associated toxicity of structure. In addition, water has been shown to permeate across
lead, that restrict the commercial possibilities of devices perovskite structures to form partly hydrated phases. These
employing these materials. A greater understanding of the factors demonstrate the potency of water in determining the
30,34
conditions that impact material life-time is crucial if commer- performance and stability of CH
3 3 3
NH PbI lms.
cial employment is to be realised.
Separate studies have shown oxygen to be an even more
A number of studies have reported the effects of moisture, aggressive degradation agent, where exposure to oxygen and
UV and temperature on the archetypal material, methyl- light leads to rapid and permanent degradation of device
5
–29
35,36
ammonium lead iodide (CH
3
NH
3
PbI
3
).
Moisture has been performances.
We recently reported that the combined
identied as a particular problem for this material leading to its action of oxygen and light can signicantly affect the device
permanent degradation aer prolonged exposure. However, efficiencies within a matter of hours, and identied the active
ꢀ
previous studies have also shown that when lms of CH NH - species, superoxide O
2
, as the key component in the oxygen
NH PbI . The generation
annealing an enhancement in its optoelectronic properties is of this reactive oxygen species is the origin of the degradation
3
3
PbI3 are exposed to water during crystallisation and and light mediated degradation of CH
3
3
3
19,30–33
observed.
This is partly due to a phase change that occurs process that initiates the breakdown of the crystal structure by
in which hydrated phases are formed and the fact that humidity an acid–base reaction with the methylammonium cation, as
controls the crystal growth, where low humidity leads to smaller shown in our previous work. More recently, the understanding
crystallites with large gaps between them. When lms are of the degradation mechanism has evolved to indicate that
37
33
iodide vacancies are key to the generation of superoxide species
and that oxygen diffusion into perovskite lms is rapid, which
a
Department of Chemistry, Imperial College London, London SW7 2AZ, UK. E-mail: s.a. contributes to the fast degradation noted in lms and devices.36
haque@imperial.ac.uk
b
In technologically relevant conditions the ambient environ-
ment may contain both oxygen and moisture as degradation
agents. Whilst it is true that in many applications device
encapsulation will be commonplace, this will restrict the use of
Department of Chemistry, University of Bath, Bath, BA2 7AY, UK. E-mail: m.s.islam@
bath.ac.uk
†
1
‡
Electronic supplementary information (ESI) available. See DOI:
0.1039/c7ta06841g

exible covers. Moreover, the oxygen diffusion into lms has
These authors contributed equally to this work.
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been shown to be extremely rapid and any barrier must be images were acquired on a LEO 1525 Field Emission Scanning
guaranteed to hold an inert atmosphere, otherwise the system Microscope operated at 5 kV using an in-lens detector.
will rapidly fail.
Superoxide probe measurements: (i) standard superoxide
As discussed above, we previously identied superoxide as probe testing was achieved by dissolving 10 ml of stock solution
the active species causing degradation. Previous research from (31.7 mmol) of the hydroethidine (HE) probe in 10 ml dry
38
Sawyer et al. investigated the mechanisms of superoxide as toluene. Sonication was used to facilitate miscibility. Perovskite
a reactive species and showed that in protic solvents or in the lms were submerged in this solution. Photoluminescence
presence of water, the solvation is very strong and accelerates spectra were recorded with an excitation wavelength of 520 nm
hydrolysis and disproportionation reactions. As such, water and a slit width of 10 mm on a Horiba Jobin-Yvon Fluorolog-3
may facilitate the degradation pathway as the reaction between spectrouorometer. The lm was illuminated by visible
the superoxide and the methylammonium cation becomes even light through a 400 nm long pass optical lter throughout the
more favourable. Consequently, we aim to test this hypothesis experiment. The illumination was provided by a tungsten halogen
ꢀ2
and identify the impact water has on the light and oxygen lamp. The light intensity was approximately 1.5 mW cm
mediated degradation in an attempt to explore the effect of real (ii) Superoxide measurements, conducted with water present in
world conditions on solar cells that employ CH NH PbI as the solution, were carried out using the same system setup as
.
3
3
3
photoactive layer. To achieve this, UV-vis spectrometry will be described above. However, the moisture was introduced by
employed to monitor the time taken for degradation to occur in generating a moisture loaded toluene solution via liquid–liquid
combination with our previously reported superoxide tests that extraction. Combining the water containing toluene with the dry
will reveal differences in the production of superoxide species toluene in the desired ratios then made the corresponding
between samples. Additionally, ab initio simulations will samples of 25% and 85% water solutions. (iii) To generate
examine reaction energetics and transient absorption spec- nitrobenzoic acid and benzoic acid containing toluene solutions
troscopy (TAS) will demonstrate if there could be any impact on the respective acid was taken and dissolved in the toluene solu-
36,37
ꢀ4
the device functionality.
tion to yield a concentration of 1 ꢁ 10 M. The solutions were
then used as in the standard protocol and the superoxide yields
were measured.
2
Methods
2
.2 Density functional theory (DFT) calculations
2.1 Experimental
39
The CASTEP plane wave DFT code, version 16.11, was used for
all calculations. The plane wave cutoff energy was 500 eV and
core electrons were represented by ultraso pseudopotentials.
Electronic exchange and correlation were modelled with the
Full details of all the experimental synthesis and character-
isation methodology can be found in the ESI.† Films of CH3-
NH PbI were prepared by spin coating. For transient
3
3
absorption spectroscopy a spiro-OMeTAD hole conducting layer
was also spin coated. During ageing all samples were sealed in
a controlled environment where dry air was gassed through
from a cylinder (<2 ppm of water) and illumination was
40
PBEsol exchange–correlation functional. A Monkhorst–Pack
41
ꢀ1
˚
grid with a density of at least 0.04 A was used for k-point
sampling. For all compounds the structure was modelled with
a supercell with minimum dimensions of 10 A˚ and the geometry
ꢀ
2
provided by a 25 mW cm lamp. For moisture control glycerol–
water solutions were employed. UV-vis measurements were
carried out in a wavelength range from 400 to 900 nm with
a scan speed of 480 nm per minute with 1 nm data intervals and
a cycle time of 1 second. Its slit width was set at 1.0 nm.
TAS measurements were performed with a 567 nm dye and
a GL-301 dye laser through an oscilloscope (TG330 function
generator with 3 MHz-generator, 120 MHz counter per sweep).
The frequency of the laser pulse was 4 Hz and it was generated
with a Bentham IL 1 illuminator and a voltage of approximately
was optimised until the forces were converged to better than
ꢀ4
ꢀ1
˚
10
eV A . For oxygen adsorption and reduction calculations,
the charge state of the O
bond length and spin constraints to the O and O molecules,
2
molecule was controlled by applying
ꢀ
2
2
using bond lengths determined for isolated molecules in the
appropriate charge state.
3
Results and discussion
3
.1 Effect of H
monochromator and an optical shielding lter. Each measure- We rst consider the inuence of moisture on light and oxygen
ment was measured at 10 ms time scales for 64 averages 10 induced degradation of CH NH PbI lms. Stability studies
PbI lms pre-soaked with water in
obtained using an IONTOF TOF: SIMS-Qtac LEIS spectrometer humid environments. Thin lms of CH NH PbI on cleaned
employing an argon sputter gun for D O detection. The D O was plain glass substrates were soaked in the dark for two hours by
2
O on light and oxygen induced degradation
12.3 V. The signal size was ltered with a long pass lter
3
3
3
ꢀ2
times with a pulse power of 25 mJ cm . ToF-SIMS data were were performed on CH
3
NH
3
3
3
3
3
2
2
introduced by substituting water in the water/glycerol mixes bubbling dry nitrogen through a specic water/glycerol mix to
used to create the humid environments. X-ray diffraction obtain a selected humidity of 0%, 25% or 85% RH (sample 1,
patterns were obtained from a PANalytical X'Pert ProMRD sample 2 and sample 3, respectively). The exact protocol for
42
diffractometer using Ni ltered Cu Ka radiation at 40 keV and sample preparation is described in the Experimental section.
4
6
0 mA. SEM-EDX measurements were performed on a JEOL As a result of this pre-treatment the CH
400 scanning electron microscope operated at 20 kV. SEM hydrated with different amounts of water contained within the
3 3 3
NH PbI crystal is
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9

lm. X-ray diffraction was employed to show that there was no of the lm into the degradation product lead iodide. The data
resulting impact on the crystal structure among all three (Fig. 2) show that the combination of moisture and light under
samples, as shown in ESI Fig. S1.† To image the dispersion of nitrogen (i.e. no oxygen) has little impact on the material
water through the crystal arising from the humid environments, stability over the period of investigation (85 h). As such, the
isotopic labelling combined with ToF-SIMS measurements was effect of moisture in the lms can then be ruled out from
employed. Isotopically labelled perovskite lms were prepared causing any alternative degradation pathways independent
by replacing H
pre-soaking step. Slices of the CH
mately halfway through the crystal (200 nm) and the 3D plots to occur. This reinforces our previous ndings of the critical
2
O with deuterated-water (D
2
O) in the moisture from oxygen. In contrast, as soon as the pre-soaked lms are
3
NH PbI
3
3
lms at approxi- subjected to dry air ux under illumination, degradation begins
36
obtained from the experiment are displayed in Fig. 1. The data importance of oxygen in the degradation of CH NH PbI . It is
3
3
3
indicate that increasing the humidity of the soaking environ- apparent from the data in Fig. 2 that, for samples exposed to
ment leads to a greater water content within the thin-lm oxygen, the rate of degradation is higher when the concentra-
structure. In addition, it is worth noting that the apparent tion of water is increased. For example, the rate of light and
diffusion of water into lms is facile, akin to oxygen diffusion oxygen induced degradation in the 85% RH sample is higher
into the perovskite structure and in agreement with previous than that in the 50% RH sample, which is in turn higher than
30,36
reports of the hygroscopic nature of CH
3
NH
3
PbI
3
lms.
that observed in the 25% RH sample. In particular, the
First, as a control experiment, we isolated the resulting complete material degradation time reduces from around 85
moisture loaded lms and subjected them to illumination hours for the sample with no water to approximately 60 h for the
under an inert nitrogen atmosphere. UV-vis measurements sample that has been exposed to 85% RH, and therefore the
were employed to monitor the degradation of the perovskite
material upon exposure to light. We have prepared CH
3 3 3
NH PbI
samples with a higher ratio of lead iodide, as iodide defects
36
have been shown to increase degradation rates. The increased
iodide content allows for both enhanced stability and a more
observable trend between samples on a detectable time-scale.
In these experiments, we selected the absorbance value at
700 nm and normalised it with respect to the starting value,
where subsequent values lead towards the complete conversion
Fig. 2 (top) Photo images of the degradation process of films with no
treatment and pre-treated films at relative humidity levels of 25% and
8
5%. (bottom) Normalised absorbance decay of CH
O at an approximately 150 nm nm. Where conversion to lead iodide is tracked under a range of
depth in thin films and 3D depth profile plots of glass/CH NH PbI conditions, including (i) no moisture, N and light; (ii) no moisture, O
films pre-treated with (top) 25% and (bottom) 85% deuterated water in and light; (iii) pre-treated under 25% RH, N and light; (iv) pre-treated
humid environments. MC is the maximum count and TC is the total under 25% RH, O and light; (v) pre-treated under 85% RH, N and light;
number of D O counts. and (vi) pre-treated under 85% RH, O and light.
3 3 3
NH PbI at 700
Fig. 1 ToF-SIMS surface imaging of D
2
3
3
3
2
2
2
2
2
2
2
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sample with highest water content. Moreover, this result drying some toluene over activated molecular sieves and a high
conrms that moisture present in the hydrated crystals leads to moisture toluene solution saturated with water by simple
faster, more facile oxygen and light mediated degradation.
liquid–liquid extraction; different ratios of these two toluene
samples were then mixed when preparing the probe solution.
The perovskite lms were then exposed to the probe solution
(1) with the superoxide yield and the subsequent degradation was
monitored.
light
ðsÞ ƒƒƒ ƒƒ ! PbI
CH
3
NH
3
PbI
3
2
ðsÞ þ CH
3
NH
3
IðsÞ
1
ꢀ
DE ¼ ꢀ54 kJ mol
light
ðsÞ ƒƒƒ ƒƒ ! PbI
The data (Fig. 3) clearly show that the yield of superoxide
increases when water is added to the probe solution (Fig. 3b)
but it decreases (Fig. 3a) when water is pre-loaded into the lms.
Moreover, these ndings support the second of the two possible
effects of water; i.e. water serves to enhance the reactivity, since
CH
3
NH
3
PbI
3
2
ðsÞ þ CH
DE ¼ þ153 kJ mol
3
NH
2
ðgÞ þ HIðgÞ
1
ꢀ
(2)
CH
3
NH
3
PbI
3
(s) + H
2
O(g)!CH
3 3 3 2
NH PbI $H O(s),
ꢀ
1
DE ¼ ꢀ16 kJ mol
(3)
within these samples the superoxide is more likely to start
4
CH NH PbI (s) + 2H O(g) /
3 3 3 2
ꢀ
1
3
PbI (s) + (CH NH ) PbI $2H O(s), DE ¼ ꢀ49 kJ mol (4)
2
3
3 4
6
2
light
ðgÞ ƒƒƒ ƒƒ !
4
CH
3
NH
3
PbI
3
ðsÞ þ O
2
4PbI
2
ðsÞ þ 4CH
3
NH
2
ðgÞ þ 2H
2
OðgÞ þ 2I ðgÞ
2
(5)
ꢀ
1
DE ¼ ꢀ74 kJ mol
3
To further quantify the degradation of photo-excited CH -
NH PbI3 we used DFT methods to calculate the enthalpy of
3
reaction for degradation pathways in a number of different
ambient environments and these are displayed in eqn (1)–(5). A
few features are apparent in these reaction energies. First,
degradation reactions which involve oxygen and light are the
most favourable in accord with observation. For example,
ꢀ
1
degradation under light alone (eqn (1)) releases 54 kJ mol
ꢀ1
whereas reaction with O
2
and light (eqn (5)) releases 74 kJ mol
.
ꢀ
Such reactions involve the highly reactive O species and allow
the deprotonation of CH NH . Second, reaction with water (eqn
2
+
3
3
(
4)) is slightly less favourable than reaction with light (eqn (1)).
This may explain the reported observations that sometimes
CH NH PbI exposed to moisture and light does not form
hydrated phases and instead directly decomposes to PbI
3
3
3
2
.
Preliminary work on reactions involving light, water and oxygen
indicate very similar energies to reaction (eqn (5)), which are
consistent with the experimental observations that indicate
enhanced reactivity when both water and oxygen are present.
However, we have not examined catalytic intermediate states
involving water, which is an area for future investigation.
3
.2 Effect of H O on superoxide formation
2
Next, we consider the effect of moisture on the yield of super-
oxide formation. For this purpose, we employed a uorescent
probe, namely hydroethidine to detect the generation of
37
superoxide as previously reported, in which superoxide can
then be more readily formed. We tested this hypothesis by
monitoring the superoxide generated in lms pre-soaked with
water. The second possibility is that water may stabilise super- Fig. 3 Superoxide yield plots for CH NH PbI where (a) demonstrates
3 3 3
the effect on the yield via pre-treating the film under no moisture, 25%
RH and 85% RH, (b) demonstrates the effect of increasing water
content in the toluene solution that houses the superoxide reactive
probe and (c) shows the impact of acid strength on the superoxide
yield, where benzoic acid and nitrobenzoic acid are doped into the
oxide aer it is formed, increasing its yield and reactivity. To
test this hypothesis we exposed the perovskite lms to O and
2
water at the same time. To do this we dissolved the superoxide
probe in a solvent (toluene) to form a probe solution. To vary the
water content, we prepared a low moisture toluene sample by toluene solution.
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reacting with the methyl ammonium cation in the crystal rather perovskite structures is to enhance the reactivity of the super-
than be extracted from the lm and react with the probe. oxide species and hence increases the degradation rate of
To further understand the observation that the degradation perovskite solar cells. The energetics and kinetics of the sepa-
rate is enhanced when water is added at the same time as O2 rate steps and pathways of the overall degradation reactions
than when water is added rst, we have calculated the require further investigation; such processes would include the
enthalpies of O
in the monohydrate CH
NH PbI $2H O phases; the results are shown in Table 1. The species to generate hydroxyl ions.
enthalpies reveal that it is favourable for O to be adsorbed into
2
adsorption and reduction in CH
3
NH
3
PbI
3
, and deprotonation of the methylammonium cation by the super-
3
NH PbI $H O and dihydrate (CH
3
3
2
3
-
oxide species and the reaction of water with the superoxide
3
)
4
6
2
2
all phases (apart from the dihydrate). However, it is only in the
photoreduced (electron polaron) regions of CH NH PbI that it
3.3 Implications for device performance
3
3
3
is favourable for superoxide to form. It is noteworthy that the Finally, we consider the impact of these ndings by exploring
band gap of the hydrated phases is much larger (>3 eV) than the role of water combined with oxygen and light in the yield of
that of CH
generate free carriers which can reduce O
and helps to explain why the superoxide yield is reduced if the absorption spectroscopy (TAS) has been previously employed to
lms are pre-soaked with water since superoxide cannot be show the impact of oxygen and light, where it was shown that
generated in any regions of the perovskite lm that become CH NH PbI fabricated on mesoporous-Al suffered signi-
hydrated.
cant losses in charge separation within 5–10 minutes. Corre-
An important question is then raised; why does the presence spondingly, CH NH PbI lms were fabricated onto this
3
NH
3
PbI
3
(1.6 eV), and hence visible light cannot charge separation between the perovskite phase and the hole
. This is a key result transporting layer, spiro-OMeTAD. Time resolved transient
2

3
3
3
2 3
O
37
3
3
3
of water make the superoxide yield and reactivity increase? mesoporous scaffold and again subjected to the same pre-
When superoxide acts as a base, the overall reaction can be
divided into two steps, deprotonation and disproportionation,
38
of which the rst one is an equilibrium process. Therefore,
increasing the proton concentration in the medium should
shi this equilibrium towards the products. This would quickly
reduce the concentration of superoxide near the surface of the
crystals and leave the sites where it is formed free for another
oxygen molecule to react with the photo-excited electrons on the
perovskite, thus resulting in a larger increase of the uores-
cence of the probe.
A further consideration arises from the fact that the super-
oxide species itself acts in an acid–base reaction with the probe
and thus a protic environment in the medium would lead to
a more facile reaction between the two species. Hence, a greater

uorescence would be observed. To further demonstrate this
point, we performed the same experiment in dry toluene adding
other protic sources such as benzoic and nitrobenzoic acid, as
shown in Fig. 3. In both cases, the superoxide generation yield
increases, and it does so in such a way that it correlates with the
a
pK ; more acidic species give rise to more superoxide. Conse-
quently, this points to the fact that the role of moisture within
2
Table 1 Calculated energies for O adsorption and reduction in
methylammonium lead iodide solar cell materials in their ground state,
photoexcited state and hydrated phases (in the excited state the
weakly bound exciton is considered as being decomposed into two
components, a hole polaron and an electron polaron)
O
2
adsorption
2
O reduction
energy (kJ mol
ꢀ
1
ꢀ1
Compound
energy (kJ mol
)
)
Fig. 4 (a) Normalised transient absorption spectroscopy yield values
obtained at 1 ms highlighting the degradation rate differences between
CH
CH
CH
CH
3
3
3
3
NH
NH
NH
NH
3
3
3
3
PbI
PbI
PbI
PbI
3
3
3
3
ꢀ14.5
ꢀ35.5
ꢀ88.2
ꢀ31.8
+65.1
+8.7
+9.6
ꢀ32.8
+15.6
—
3 3 3
CH NH PbI pre-treated under no moisture, 25% RH and 85% RH
+
+ h
upon exposure to both oxygen and light at t ¼ 0. (b) Kinetic decays
obtained after five minutes of exposure to oxygen and light for all three
samples. All films were excited with a pump wavelength of 567 nm and
ꢀ
+ e
$H
2
O
$2H
ꢀ2
(CH
3
NH
3
)
4
PbI
6
2
O
an average power of 23 mJ cm and a probe wavelength of 1600 nm
was employed to probe the resulting holes in spiro-OMeTAD.
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soaking treatment with the same selected humidity levels of separation was found to rapidly decline in combined air and
%, 25% and 85% RH. Aer soaking the hole transporting layer moisture environments.
spiro-OMeTAD was deposited on top by spin coating. Exposing In general, these ndings suggest that other hybrid perov-
0
the samples to humidity prior to the HTL being deposited skites that are shown to yield superoxide species through the
aimed to eliminate moisture affecting the HTL phase, which action of light and oxygen would also undergo accelerated
could be encountered if the layer was present during the treat- degradation upon increased moisture content. Consequently,
ment, and ensure that the lm is homogeneously hydrated the ability to prevent not only oxygen but also moisture from
throughout the whole surface and the bulk. The resulting thin- entering these systems will be of utmost importance in

lm architectures with varying moisture content of glass/mp- designing optoelectronics with sufficient stability for real world
Al O /CH NH PbI /spiro-OMeTAD were then probed at 1600 function.
2
3
3
3
3
nm to obtain the yield of charge separation. Decays were
collected at regular time intervals over a 20 minute period with
exposure to a dry air ow and ambient illumination. Measure-
Conflicts of interest
ments were obtained under a nitrogen atmosphere. The value of There are no conicts to declare.
DOD at 1 ms was taken as indicative of the yield of charge
separation. These values obtained were then normalised with
respect to the starting value (the yield of charge separation at
Acknowledgements
exposure ¼ 0). The normalised decay of the charge separation S. A. H. acknowledges nancial support from the EPRSC via EP/
yield is presented in Fig. 4.
M023532/1, EP/K010298/1 and EP/K030671/1 grants. M. S. I.
In accordance with the UV-vis data presented in Fig. 2, we acknowledges support from the EPSRC for the Energy Materials
observe that lms loaded with increasing moisture content Programme grant (EP/K016288) and Archer HPC facilities
show an increased rate of loss in the yield of charge separation. through the Materials Chemistry Consortium (EP/L000202)
When no moisture is present (black trace in Fig. 2a) it takes
approximately 13 minutes of exposure to oxygen and light for
the yield to drop by 50%, and in comparison it takes approxi-
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soaked in the 85% RH environment (blue trace in Fig. 2a) prior
to exposure to oxygen and light. In real terms this means that
the number of holes being transferred to the hole transporting
layer is critically hampered by oxygen and light, but even more
so when water is introduced into the system. It is generally
accepted that a good yield of charge separation is a pre-requisite
for optimal device functionality. Consequently these ndings
would suggest that introducing water into functioning devices
could lead to signicant performance losses. In the event of
exposure to ambient conditions devices would fail rapidly due
to the fast diffusion kinetics of oxygen and the presence of water
that accelerates the degradation mechanism. Overall, this
further suggests that water enhances and accelerates the mode
of superoxide degradation leading to faster decays in optoelec-
tronic properties and ultimately device efficiencies.
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Conclusions
A strong mix of photoluminescence, SIMS and ab initio simu-
lation techniques has provided insights into combined
humidity, oxygen and light-induced degradation of perovskite
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increased when both oxygen and moisture are present in
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showed that the yield of the reactive superoxide species is
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cannot occur in the hydrated phases of CH
3
NH
3
PbI
3
. All of this
8 Y. Han, S. Meyer, Y. Dkhissi, K. Weber, J. M. Pringle, U. Bach,
L. Spiccia, Y.-B. Cheng, P. Morvillo, E. a. Katz, A. Elschner,
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their reduction via protonation, which enhances the overall
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device function was also assessed and the yield of charge
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