DOI: 10.1002/cphc.201501168
Articles
Stabilization of Organic–Inorganic Perovskite Layers by
Partial Substitution of Iodide by Bromide in
Methylammonium Lead Iodide
Raffael Ruess, Felix Benfer, Felix Bçcher, Martina Stumpp, and Derck Schlettwein*[a]
Thin films of the methylammonium lead halides
CH3NH3Pb(I1ÀxBrx)3 are prepared on fluorine-doped tin oxide
substrates and exposed to humid air in the dark and under il-
lumination. To characterize the stability of the materials, UV/Vis
spectra are acquired at fixed intervals, accompanied by XRD,
energy-dispersive X-ray spectroscopy, SEM, and confocal laser
scanning microscopy. Different degradation mechanisms are
observed depending on the environmental conditions. It is
found that bromide can successfully suppress the transforma-
tion of the perovskite into the monohydrate, presumably
owing to stronger hydrogen-bonding interactions with the or-
ganic cation. However, under illumination in humid air, rather
rapid decomposition of the perovskites was still observed,
which is due to phase segregation. The use of increased bro-
mide content in methylammonium lead halide absorbers is dis-
cussed in terms of their application in perovskite solar cells.
1. Introduction
Organic–inorganic lead halide perovskites have received much
attention in the last few years, due to their suitable properties
for application as absorber materials in solar cells, such as high
absorption throughout the whole visible spectrum, a suitable
bandgap, and high charge-carrier diffusion lengths.[1–3] The
most commonly used perovskites are methylammonium lead
halides CH3NH3PbX3 and formamidinium lead halides
CH5N2PbX3 with X=I, Br.[1,2,4–10] When these perovskite solar
cells were introduced in 2009, they had a power conversion ef-
ficiency of 3.8%,[7] which was improved until today even to
above 20%.[9] Not only the improvements in efficiency make
these solar cells interesting for technical use, but also the large
variety of preparation methods to attain such absorber layers
and the overall low cost of the cells.[4,6,9]
CH3NH3PbI3 decomposed and lead was mainly found in the
form of PbI2.[16] On the other hand, for pure CH3NH3PbI3 films
on mesoporous Al2O3 substrates exposed to air with 90% RH
at room temperature in the dark, no PbI2 was observed even
after 14 days of exposure, but formation of a hydrate phase
was seen,[13] which was recently reported to be of particular
significance in decomposition under an applied electric field.[17]
Independent of the type of degradation, both pathways would
lead to a decrease in absorbance in the whole visible spec-
trum, and therefore a huge decrease in solar-cell device per-
formance can be expected.[13,16] To prevent degradation, vari-
ous strategies have been proposed to protect the perovskite
layer in solar cells by additional layers or to prepare stable per-
ovskite materials. A protective Al2O3 layer between the perov-
skite and the hole-transport material (HTM), as well as replac-
ing the HTM with polymer-functionalized single-walled carbon
nanotubes, has been used to increase the long-term stability
of solar-cell devices.[16,18] Also, various encapsulation tech-
niques have been tested to protect the device from humid
air.[12] Another attempt at increasing the stability was the
use of two-dimensional layered perovskites, such as
[C6H5(CH2)2NH3]2(CH3NH3)2Pb3I10.[19] Cross-linking the perovskite
crystals with w-ammonium chlorides was demonstrated to pas-
sivate their surface and increase the stability.[20] Furthermore,
(HOOC5H7NH3)x(CH3NH3)1ÀxPbI3 was shown to be stable under
outdoor conditions for 7 days or at elevated temperature for
three months when embedded in a porous ZrO2/TiO2 layer in
a triple-layer solar cell.[21] Noh et al. demonstrated that the per-
ovskite CH3NH3Pb(I1ÀxBrx)3 showed enhanced stability to hu-
midity.[22] It was also observed that CH3NH3PbBr3 is not degrad-
ed even in concentrated sunlight at temperatures up to
558C,[14] but due to its weak absorption at wavelengths longer
than 550 nm, pure CH3NH3PbBr3 is not well suited for use in
perovskite solar cells.[22] However, mixing CH3NH3PbBr3 and
Despite the advances in efficiency and preparation, there are
still problems to be solved concerning the properties of the
perovskite absorber in solar cells. One of these is the long-
term stability of the perovskite layer. Even in sealed cells, there
is potential for exposure of the films to components of the am-
bient atmosphere, such as oxygen and water vapor, which
may have a serious impact, in particular under conditions of in-
creased temperature and illumination, as are typical in solar
cells.[5,11] Exposure of CH3NH3PbI3 to humid air leads to rapid
degradation.[5,10–17] Depending on the circumstances, two main
degradation pathways have recently been identified. When
films of TiO2 sensitized by CH3NH3PbI3 were exposed to air
with 60% relative humidity (RH) at 358C under sunlight,
[a] R. Ruess, F. Benfer, F. Bçcher, M. Stumpp, Prof. Dr. D. Schlettwein
Institut für Angewandte Physik
Justus-Liebig-Universität Giessen
Heinrich-Buff-Ring 16, 35392 Giessen (Germany)
Supporting Information for this article can be found under http://
ChemPhysChem 2016, 17, 1505 – 1511
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