The Role of Oxygen in the Degradation of Methylammonium Lead Trihalide Perovskite Photoactive Layers

Article abstract of DOI:10.1002/anie.201503153

In this paper we report on the influence of light and oxygen on the stability of CH₃NH₃PbI₃ perovskite-based photoactive layers. When exposed to both light and dry air the mp-Al₂O₃/CH₃NH₃PbI₃ photoactive layers rapidly decompose yielding methylamine, PbI₂, and I₂ as products. We show that this degradation is initiated by the reaction of superoxide (O₂^-) with the methylammonium moiety of the perovskite absorber. Fluorescent molecular probe studies indicate that the O₂^- species is generated by the reaction of photoexcited electrons in the perovskite and molecular oxygen. We show that the yield of O₂^- generation is significantly reduced when the mp-Al₂O₃ film is replaced with an mp-TiO₂ electron extraction and transport layer. The present findings suggest that replacing the methylammonium component in CH₃NH₃PbI₃ to a species without acid protons could improve tolerance to oxygen and enhance stability.

Full text of DOI:10.1002/anie.201503153

.
Angewandte
Communications
International Edition: DOI: 10.1002/anie.201503153
German Edition: DOI: 10.1002/ange.201503153
Perovskites
The Role of Oxygen in the Degradation of Methylammonium Lead
Trihalide Perovskite Photoactive Layers**
Nicholas Aristidou, Irene Sanchez-Molina, Thana Chotchuangchutchaval, Michael Brown,
Luis Martinez, Thomas Rath, and Saif A. Haque*
Abstract: In this paper we report on the influence of light and
oxygen on the stability of CH3NH3PbI3 perovskite-based
photoactive layers. When exposed to both light and dry air
the mp-Al2O3/CH3NH3PbI3 photoactive layers rapidly decom-
pose yielding methylamine, PbI2, and I2 as products. We show
that this degradation is initiated by the reaction of superoxide
molecular oxygen and light.[10] More specifically, in our
previous work we employed transient absorption spectrosco-
py to monitor the yield of long-lived charge separation in
mesoporous TiO2/CH3NH3PbI3/spiro-OMeTAD and Al2O3/
CH3NH3PbI3/spiro-OMeTAD photoactive layers before and
after exposure to light and oxygen. After the ageing process,
the mesoporous Al2O3-based samples showed a rapid and
substantial drop in the yield of long-lived charge separation.
We attributed this drop in yield to degradation of the
CH3NH3PbI3 absorber resulting from a reaction between
oxygen and the photoexcited electrons in CH3NH3PbI3.
Moreover, this degradation pathway was apparently hindered
in the mesoporous TiO2-based samples; here electron injec-
tion from the photoexcited state of CH3NH3PbI3 precludes
this parasitic decomposition reaction. In this paper, we build
on our previous work and report on the mechanism of this
photodegradation reaction. Specifically, we show that the
combined action of light and molecular oxygen on
CH3NH3PbI3 photoactive layers results in the decomposition
of the perovskite leading to the formation of PbI2, I2, and
methylamine. Importantly, we show that this photodegrada-
tion reaction is triggered by the action of a reactive oxygen
À
(O2 ) with the methylammonium moiety of the perovskite
absorber. Fluorescent molecular probe studies indicate that the
À
O2 species is generated by the reaction of photoexcited
electrons in the perovskite and molecular oxygen. We show that
À
the yield of O2 generation is significantly reduced when the
mp-Al2O3 film is replaced with an mp-TiO2 electron extraction
and transport layer. The present findings suggest that replacing
the methylammonium component in CH3NH3PbI3 to a species
without acid protons could improve tolerance to oxygen and
enhance stability.
O
ver the last few years, organic lead halide perovskites
have aroused enormous interest with respect to their appli-
cation in low-cost, solution-processable photovoltaics. A wide
range of device architectures employing such perovskite
absorbers have been reported so far, with efficiencies
approaching 20%.[1–4] In spite of this tremendous progress,
a number of key issues must be overcome before wide-spread
commercialization is possible, mainly, those associated with
toxicity of lead, understanding phase behavior,[5] and long-
term material stability. Regarding the latter, UV irradiation[6]
and water, are well known to affect the stability of methyl-
ammonium lead triiodide (CH3NH3PbI3, MAPbI3,
MAPI).[1,7,8] Recently, a few mechanistic studies on the
action of water on CH3NH3PbI3 have been reported, identi-
fying lead(II) iodide as one of the final products, although
different degradation pathways were proposed.[9,10] More-
over, we have recently reported that decomposition of
CH3NH3PbI3 can also be triggered by the combination of
À
species (superoxide; O2 ) on the organic (i.e., methylammo-
nium) component of the CH3NH3PbI3 perovskite absorber.
The present findings indicate that the identification of
+
alternative cations to methylammonium (CH3NH3 ) may be
of crucial importance to the design of environmentally stable
organometal trihalide perovskite photovoltaic devices.
To investigate whether superoxide generation contributes
to the degradation of CH3NH3PbI3 photoactive layers and
leads to their reduced photochemical stability under illumi-
nation in a moisture-free atmosphere, a molecular fluorescent
probe, hydroethidine (HE) was employed. This compound is
known to show a characteristic increase in emission at 610 nm
upon exposure to the superoxide radical anion.[12]
CH3NH3PbI3 films were fabricated onto two glass substrates
coated either with mp-TiO2 or mp-Al2O3. The resulting mp-
TiO2/CH3NH3PbI3 and mp-Al2O3/CH3NH3PbI3 films were
then immersed into a 0.317 mm solution of the HE probe in
dry toluene. The photodegradation conditions consisted of
ageing the samples under continuous illumination from
a tungsten halogen lamp and dry air flow (21% oxygen
content). In the current experiment, the fluorescence emis-
sion of the HE probe was monitored at regular time intervals
over the course of one hour with the mp-TiO2/CH3NH3PbI3
and mp-Al2O3/CH3NH3PbI3 films exposed to light and dry air.
Figure 1a shows the emission characteristics of HE (fluores-
cence intensity versus wavelength) as a function of ageing
time for both mp-TiO2/CH3NH3PbI3 and mp-Al2O3/
[*] N. Aristidou, Dr. I. Sanchez-Molina, T. Chotchuangchutchaval,
Dr. M. Brown, Dr. L. Martinez, Dr. T. Rath, Prof. S. A. Haque
Department of Chemistry, Imperial College London
South Kensington Campus, London SW7 2AZ (United Kingdom)
E-mail: s.a.haque@imperial.ac.uk
[**] S.A.H. acknowledges financial support from the Engineering and
Physical Sciences Research Council (EPSRC) through EP/H040218/
2 and EP/K010298/1 projects. S.A.H. acknowledges financial
support from the European Community’s Seventh Framework
Programme (Nanomatcell, grant agreement number 308997). T.R.
acknowledges financial support from the Austrian Science Fund
(FWF) under the grant number J3515-N20.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201503153.
8208
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 8208 –8212

Angewandte
Chemie
sample. A key difference between using mesoporous TiO2
and Al2O3 films is that the TiO2 film can accept an electron
from the photoexcited perovskite, due to a favorable energy
offset at the heterojunction,[13] as shown in Figure 2. This, in
Figure 2. Schematic model showing the electron transfer of the photo-
excited electrons in the MAPbI3 layers to oxygen resulting in the
formation of superoxide. A higher yield of superoxide generation is
seen in the Al2O3/MAPbI3 system as compared to the TiO2/MAPbI3
system. In the Al2O3/MAPbI3 system, the electrons remain on the
perovskite and can react with oxygen to form superoxide. If the
perovskite is prepared on mp-TiO2, the photoexcited electrons can be
injected into the mesoporous TiO2. In the TiO2/MAPbI3 system
electron transfer from the perovskite to the mp-TiO2 can successfully
compete with electron transfer from the perovskite to oxygen resulting
in a reduced yield of superoxide production.
turn, would reduce the number of photoexcited electrons in
the perovskite available for electron transfer to oxygen,
thereby reducing the yield of superoxide generation, which is
consistent with the data presented in Figure 1. We hypothe-
size that the relatively low yield of superoxide generation in
the mp-TiO2/CH3NH3PbI3 film (as compared to the Al2O3-
based sample) is most likely due to the presence of
a CH3NH3PbI3 overlayer as revealed by scanning electron
microscopy (SEM) studies. Support for this assertion comes
from cross-sectional scanning electron microscopy studies
which revealed the presence of an approximately 50 nm thick
overlayer of perovskite in both mp-Al2O3/CH3NH3PbI3 and
mp-TiO2/CH3NH3PbI3 films (see the Supporting Information,
Figure S1). As such, the reduced rate of production of
superoxide in the TiO2-based samples compared to that of
the Al2O3-based films could thus arise as a consequence of the
photoinduced electron transfer process from CH3NH3PbI3 to
the TiO2 layer being in direct competition with the reaction of
the photogenerated electrons with oxygen. Furthermore, it is
pertinent to note that our observation of increased superoxide
production in Al2O3 samples is in good agreement with our
previous work showing a rapid and profound degradation in
the yield of photoinduced charge separation in mp-Al2O3/
CH3NH3PbI3/spiro-OMeTAD films upon exposure to light
and dry air.[11] Moreover, the present findings suggest that the
yield of superoxide generation can be significantly reduced by
ensuring that the photogenerated electrons in the perovskite
are rapidly extracted before they can react with oxygen.
We now consider the compositional changes in the
CH3NH3PbI3 perovskite film resulting from the ageing
process. To elucidate the products of the degradation that
occurs as a result of the combination of light and dry air, we
Figure 1. a) Raw emission data collected from the probe solution,
highlighting the increasing intensity observed with ageing under
illumination and dry air flux. b) Normalized fluorescence intensity at
610 nm of the superoxide probe solution, with excitation at 520 nm.
IF(t) is the fluorescence intensity maximum at time t, whilst IF(t0) is
the background fluorescence intensity of the probe at t=0. The ratio
IF(t)/IF(t0) is a measure of the yield of superoxide generation.
CH3NH3PbI3. The rate of increase in emission ([IF(t)/[IF(t0)] at
610 nm versus ageing time), and therefore superoxide gen-
eration is shown in Figure 1b. Control measurements per-
formed in the absence of light (green curve, Figure 1b)
showed no increase in HE emission at 610 nm, indicating that
the process is photoinduced. Similarly, when the dry air
stream was replaced with nitrogen and illumination was
introduced, there was no detectable increase in fluorescence
intensity (red curve, Figure 1b). In contrast, when the mp-
TiO2/CH3NH3PbI3 and mp-Al2O3/CH3NH3PbI3 samples were
exposed to both dry air and light, the expected increase of
fluorescence of the probe was observed, which is consistent
with the presence of superoxide in the solution. However, it is
apparent from the data presented in Figure 1b that a higher
yield of superoxide generation is observed in the mp-Al2O3/
CH3NH3PbI3 film as compared to the analogous mp-TiO2/
CH3NH3PbI3 sample. The next question that arises relates to
why the mp-Al2O3/CH3NH3PbI3 film exhibits a greater yield
of superoxide generation than the mp-TiO2/CH3NH3PbI3
Angew. Chem. Int. Ed. 2015, 54, 8208 –8212
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 8209

.
Angewandte
Communications
immersed the substrates in dry toluene under a constant flow
of dry oxygen and illumination, thus excluding moisture.
Similar to the HE probe experiments, mp-Al2O3-based films
were immersed in dry toluene and then a constant flow of dry
oxygen and illumination were used to create the controlled
degradation conditions. It was noted that after 72 h exposure
to both light and oxygen the CH3NH3PbI3 perovskite films
changed color from dark brown to yellow and the toluene
solution turned light pink, most likely due to the formation of
iodine. UV/Vis spectroscopy was used to monitor the
evolution of iodine in the solution (Figure 3a). The change
in the absorption spectra is consistent with the production of
iodine, as the detected absorption spectrum resembles the
absorption of iodine.[14] The addition of sodium thiosulfate,
which can reduce iodine to iodide, turned the solution
colorless, which further supports the identification of the
product as iodine.
To further analyze the compositional change in the aged
films, scanning electron microscopy energy-dispersive X-ray
spectroscopy (SEM-EDX) measurements were performed.
EDX analysis showed the Pb/I ratio to deplete from 1:3 to 1:2
in the fresh and degraded samples respectively, as illustrated
in Figure 3b.
An X-ray diffraction analysis of the samples shows the
characteristic peaks of methylammonium lead iodide in the
fresh sample (Figure 4).[15,16] These peaks are largely dimin-
Figure 4. X-ray diffraction patterns of a fresh and an aged (for 72 h)
sample showing the formation of PbI2 as the result of degradation in
a moisture-free environment. Dotted vertical lines in the graph indicate
the peak positions of methylammonium lead iodide according to
literature data.[15,16] The additional peaks in the degraded sample can
be assigned to PbI2 (reference patterns: PDF 01-080-1000 for hexago-
nal and PDF 01-073-1753 for rhombohedral PbI2).
ished in the degraded sample and distinct additional peaks
stemming from PbI2 are detected. The formation of PbI2 was
additionally confirmed by Raman spectroscopy, which clearly
shows the characteristic peaks of PbI2 that coincide with
a PbI2 reference sample (Figure S2).
Having identified two of the degradation products, the
next question that arises relates to how the superoxide species
causes degradation of the perovskite. One possibility is that
the superoxide reacts with the organic component of the
perovskite (i.e. methylammonium cation). To test this
hypothesis and to better understand the degradation mech-
anism we investigated whether the methylammonium cations
are undergoing any transformation. For this purpose, a model
reaction was set up and NMR was employed to monitor the
transformation. The model reaction consisted of a sample of
methyl ammonium iodide (MAI) dissolved in dry ethanol, to
which potassium superoxide (KO2) was added. The sample
was then transferred to an NMR tube to which some
Figure 3. a) UV/Vis spectra showing the production of iodine over
a 72-hour time period, during which the mp-Al2O3/MAPI films were
continuously subjected to the degradation conditions. b) SEM-EDX
spectra and elemental composition analysis showing that on ageing
(for 72 h) the molar ratio of Pb/I in the sample changes from 1:3 to
1:2. Peaks in the spectra not assigned to Pb or I stem from the glass
substrate or the chromium coating of the samples to prevent charging
during the SEM-EDX analysis.
8210
www.angewandte.org
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 8208 –8212

Angewandte
Chemie
deuterated acetone was added for calibration of the spectrum.
The reaction proceeded with a color change, from colorless to
orange. Proton NMR spectra of the sample and of a control
containing only methylammonium iodide are shown in
+
Figure 5. Both the signal for the NH3 and the methyl group
Figure 6. Potential route for degradation caused by superoxide as
predicted from the obtained data reported herein. Optical excitation of
MAPbI3 results in the formation of MAPbI3*. MAPbI3* is s species that
carries both photoinduced electrons and holes. Superoxide is gener-
ated through electron transfer from MAPbI3* to O2. The resulting
superoxide attacks the perovskite absorber leading to the formation of
methylamine (MeNH2), lead iodide, iodine, and water as degradation
products.
a water-free environment, degradation can be triggered by the
presence of oxygen and light. The data presented in this paper
and reported in our previous work[11] show that the
CH3NH3PbI3 films rapidly degrade (e.g., on a timescale of
a few hours to days) when exposed to both light and oxygen.
As such, this light- and oxygen-induced degradation pathway
could have a significant impact on perovskite solar cell device
performance and long-term device stability. It is therefore
crucial to ensure that CH3NH3PbI3 perovskite layers, when
incorporated in photovoltaic devices, are isolated from both
water and oxygen. The data presented in this paper suggests
that the superoxide-mediated degradation pathway can be
significantly reduced or even prevented by ensuring that the
photogenerated electrons on the perovskite are extracted
before they can react with oxygen to form superoxide. Herein,
we have shown that the implementation of a mesoporous
electron-extracting layer (e.g., mp-TiO2) can facilitate the
deactivation of the electron transfer process to oxygen,
resulting in reduced superoxide generation. Obtaining more
efficient electron extraction should lead to enhanced stability
in ambient and moisture-free environments. Alternatively
material design aimed at replacing the methylammonium
cation to a species without acid protons could improve
stability and may alleviate the need for oxygen barriers.
Figure 5. NMR Spectra of a reference sample containing methylammo-
nium iodide (bottom) and the resulting spectra after addition of
potassium superoxide. Inset shows an expansion of the methyl signal.
protons shift up-field upon addition of potassium superoxide,
which is consistent with the deprotonation of the ammonium
group to form methylamine (MeNH2). The presence of iodine
in the sample was again determined by the addition of sodium
thiosulfate, which turned the solution from orange to color-
less. The pH of the solution was also estimated with pH-paper
to be around 10. This could be accounted for by the potential
formation of KOH due to the potassium salt being used in the
model reaction. It is therefore plausible that water can be
formed in the oxygen- and light-assisted degradation of
CH3NH3PbI3.
Herein we have shown that the degradation of the mp-
Al2O3/CH3NH3PbI3 sample in a moisture-free environment
can be accounted for by the generation of superoxide through
electron transfer from the photoexcited perovskite phase to
molecular oxygen. The present findings suggest that the
superoxide acts to break down the perovskite by first
deprotonating the methylammonium cation as shown by
NMR spectroscopy. Moreover, the deprotonation of the
methylammonium cation ultimately leads to the formation of
PbI2, as confirmed by XRD and Raman spectroscopy, and
iodine, detected by UV/Vis spectroscopy and confirmed by
addition of sodium thiosulfate. Based on these observations
we propose a possible reaction scheme for the degradation
(Figure 6). It is pertinent that water is expected to be another
by-product of decomposition as discussed above. The result-
ing water could then participate in further degradation
pathways, perhaps similar to the ones reported in the
literature, in which water would now act as an active species
in the degradation or act to hydrate the perovskite.[9,10] The
overall effect after a first action of superoxide on the
perovskite will be an increase of the decomposition rate,
because, even if the CH3NH3PbI3 is carefully encapsulated in
Experimental Methods
Materials: All chemicals were purchased from Sigma–Aldrich and
used as received, except TiO2 nanoparticles from Dyesol and
methylammonium iodide, which was synthesized in the lab. Anhy-
drous toluene was filtered through silica gel prior to use.
Synthesis of methylammonium iodide: Methylamine solution
(33 wt%) in ethanol (6.2 mL, 0.046 mol) was cooled down in an ice
bath. Hydroiodic acid solution (55 wt%) in water (10 mL, 0.073 mol)
was then added dropwise under vigorous stirring. The reaction was
stirred for 1 h at 08C. The product precipitated from the solution as
a white-yellowish solid. Ethanol (5 mL) was added to ensure full
precipitation of the solid, which was filtered and washed with cold
ethanol. Recrystallization of the product in chloroform afforded the
pure compound as white crystalline solid (6.4 g, 87%).
Film Fabrication: All films were deposited onto clean glass
substrates with a size of ca. 1 ” 1 cm. The glass substrates were washed
sequentially in acetone, deionized water and isopropanol (IPA) under
sonication for 10 min during each washing cycle. A Laurell Tech-
nologies WS-650MZ-23NPP Spin Coater was used to fabricate the
films. A 1m solution of CH3NH3PbI3 was formed by adding PbI2 in
Angew. Chem. Int. Ed. 2015, 54, 8208 –8212
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
8211

.
Angewandte
Communications
a 1:1 molar ratio with methylammonium iodide in a solvent mixture of
4:1 gamma-butyrolactone (GBL) to DMSO. This solution was then
spin-coated onto the substrates using a consecutive two-step spin
program under a nitrogen atmosphere in a glovebox. The first
spinning cycle is performed at 1000 rpm for 10 s followed by 5000 rpm
for 20 s, as reported by Jeon et al.[17] During the second phase, the
substrate was treated with toluene ( ꢀ 350 mL) drop-casting. The films
were then annealed at 1008C for 10 min. The perovskite layers were
deposited onto mesoporous Al2O3 and mesoporous TiO2 films
prepared as follows:
a) Mesoporous Al2O3 layers were fabricated on cleaned glass
substrates by spin coating (4500 rpm for 45 s) an Al2O3 dispersion
prepared in a 2:1 vol. ratio of Al2O3 (< 50 nm particle size in IPA
solution) and IPA. After spin coating, the films were placed on
a hotplate at 1508C for 30 min.
How to cite: Angew. Chem. Int. Ed. 2015, 54, 8208–8212
Angew. Chem. 2015, 127, 8326–8330
[1] J. Burschka, N. Pellet, S.-J. Moon, R. Humphry-Baker, P. Gao,
M. K. Nazeeruddin, M. Graetzel, Nature 2013, 499, 316 – 319.
[2] D. Bi, S.-J. Moon, L. Haggman, G. Boschloo, L. Yang, E. M. J.
Johansson, M. K. Nazeeruddin, M. Graetzel, A. Hagfeldt, RSC
Adv. 2013, 3, 18762 – 18766.
[3] M. M. Lee, J. Teuscher, T. Miyasaka, T. N. Murakami, H. J.
Snaith, Science 2012, 338, 643 – 647.
[4] O. Malinkiewicz, A. Yella, Y. H. Lee, G. Minguez Espallargas,
M. Graetzel, M. K. Nazeeruddin, H. J. Bolink, Nat. Photonics
2014, 8, 128 – 132.
[5] T.-B. Song, Q. Chen, H. Zhou, S. Luo, Y. Yang, J. You, Y. Yang,
Nano Energy 2015, 12, 494 – 500.
[6] T. Leijtens, G. E. Eperon, S. Pathak, A. Abate, M. M. Lee, H. J.
Snaith, Nat. Commun. 2013, 4, 2885.
[7] H. P. Zhou, Q. Chen, G. Li, S. Luo, T. B. Song, H. S. Duan, Z. R.
Hong, J. B. You, Y. S. Liu, Y. Yang, Science 2014, 345, 542 – 546.
[8] J. A. Christians, P. A. Miranda Herrera, P. V. Kamat, J. Am.
Chem. Soc. 2015, 137, 1530 – 1538.
[9] G. Niu, W. Li, F. Meng, L. Wang, H. Dong, Y. Qiu, J. Mater.
Chem. A 2014, 2, 705 – 710.
b) Mesoporous TiO2-based films were fabricated using a paste of
20 nm TiO2 nanoparticles in a 1:5 weight ratio with dry ethanol. This
was spin-coated onto glass substrates at 6000 rpm for 30 s. Once spun,
the films were sintered at 4508C for 1 h in a furnace.
Probe Testing: A stock solution (31.7 mm) of the hydroethidine
(HE) probe was prepared by dissolving 10 mg HE in 10 mL of dry
toluene; sonication was used to facilitate miscibility. Films were then
added to 10 mL of 0.317 mm solution created from the stock solution.
Photoluminescence spectra were recorded using an excitation wave-
length of 520 nm and a slit width of 10 mm on a Horiba Jobin–Yvon
Fluorolog-3 spectrofluorometer. In the experiments with oxygen
involved, dry oxygen was bubbled through the toluene. For the
control using N2, a Schlenk line was used to keep a continuous flow of
nitrogen through the system.
Ageing Conditions: Films were submerged in 10 mL dry toluene.
Toluene (Sigma–Aldrich 99.8%) was dried over molecular sieves,
with a pore size of ca. 3 Š. Dry air was gassed through for the duration
of the degradation. Illumination was provided by a tungsten halogen
lamp of approximately 1.5 mWcmÀ2 power.
Superoxide Model Reaction: Potassium superoxide and methyl-
ammonium iodide were dissolved in a 1:1 molar ratio in dry ethanol.
The reaction resulted in a color change from colorless to orange.
Characterization: 1H NMR spectra were recorded on a 400 MHz
Bruker spectrometer using TopSpin software. UV/Vis spectra were
acquired on a PerkinElmer UV/VIS Spectrometer Lambda 25. X-ray
diffraction patterns were measured on a PANalytical X’Pert Pro
MRD diffractometer using Ni filtered Cu Ka radiation at 40 kV and
40 mA. SEM-EDX measurements were carried out on a JEOL 6400
scanning electron microscope operated at 20 kV.
[10] J. Yang, B. D. Siempelkamp, D. Liu, T. L. Kelly, ACS Nano 2015,
2, 1955 – 1963.
[11] cF. T. F. OꢀMahony, Y. H. Lee, C. Jellett, S. Dmitrov, D. T.
Bryant, J. R. Durrant, B. C. OꢀRegan, M. Graetzel, M. K.
Nazeeruddin, S. A. Haque, J. Mater. Chem. A 2015, 3, 7219 –
7223.
[12] A. Gomes, E. Fernandes, J. Lima, J. Biochem. Biophys. Methods
2005, 65, 45 – 80.
[13] H. J. Snaith, J. Phys. Chem. Lett. 2013, 4, 3623 – 3630.
[14] O. J. Walker, Trans. Faraday. Soc. 1935, 31, 1432 – 1438.
[15] T. Baikie, Y. Fang, J. M. Kadro, M. Schreyer, F. Wei, S. G.
Mhaisalkar, M. Graetzel, T. J. White, J. Mater. Chem. A 2013, 1,
5628 – 5641.
[16] D. S. Bhachu, D. O. Scanlon, E. J. Saban, H. Bronstein, I. P.
Parkin, C. J. Carmalt, R. G. Palgrave, J. Mater. Chem. A 2015, 3,
9071 – 9073.
[17] N. J. Jeon, J. H. Noh, Y. C. Kim, W. S. Yang, S. Ryul, S. I. Seok,
Nat. Mater. 2014, 13, 897 – 903.
Keywords: methylammonium lead triiodide · perovskites ·
Received: April 6, 2015
solar cells · spectroscopy · stability
Published online: May 26, 2015
8212
www.angewandte.org
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 8208 –8212