Optimization of Stable Quasi-Cubic
FA
x
MA
1−x
PbI
3
Perovskite Structure for Solar
Cells with Efficiency beyond 20%
Yi Zhang,
†,‡
Giulia Grancini,*
,†
Yaqing Feng,
‡
Abdullah M. Asiri,
§
and Mohammad Khaja Nazeeruddin*
,†
†
Group for Molecular Engineering of Functional Materials, EPFL Valais Wallis, CH-1951 Sion, Switzerland
‡
School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China
§
Center of Excellence for Advanced Materials Research (CEAMR), King Abdulaziz University, Jeddah, Saudi Arabia
*
S
Supporting Information
ABSTRACT: Complex compositional engineering of mixed
halides/mixed cations perovskites has recently fostered a rapid
progress in perovskite solar cell technology. Here we demonstrate
that when 10% of formamidinium (FA
+
) is simply added into
methylammonium lead iodide (MAPbI
3
) a highly crystalline and
compositionally uniform perovskite is formed, self-organizing into
a stable “quasi-cubic” phase at room temperature. We reached
power conversion efficiency of over 20.2%, the highest value
reported to date for FA
x
MA
1−x
PbI
3
perovskite.
H
ybrid perovskite technology with astonishing power
conversion efficiency (PCE) beyond 22% promises to
be an ideal candidate for near future solar power
generation.
1−4
Beyond their excellent optoelectronic proper-
ties
5,6
and ease, versatility, and low cost in the processing
techniques,
7
the ability of the perovskite to accommodate
different cations and anions within the lattice gives room for a
myriad of different material combinations.
2,3
The incorporation
of methylammonium (MA
+
), formamidinium (FA
+
), cesium
(Cs
+
), and rubidium (Rb
+
) along with a mixture of Cl, Br, and I
halides enabled the fabrication of high-effi ciency solar cells
with, in some cases, improved stability.
3,7
Additionally, ion
intermixing enables a fine-tuning of the material bandgap with
direct impact on tandem solar cell engineering.
2
However, as a
drawback, compositional engineering induces severe changes in
the crystal structure and film morphology associated with halide
phase segregation
7,8
and light/field-induced ion movement
9−12
altering the device function. Interestingly, the inclusion of Cs
+
has been shown to enhance device stability.
4,9
However, the
added complexity of the material structure and the difficulties in
reproducing the performances casts the doubt on the ease of
reproducibility and scale-up of this route. On the other side, a
much simpler structure in which only FA is introduced with
MA as the organic compound leaving intact the inorganic
backbone has attracted a great deal of interest.
10−13
FA-based
perovskite has shown a superior stability compared to that of
the methylammonium, longer charge diffusion length, and a
band gap closer to the ideal value.
2,7,7,9,11,11,13−16
However, the
larger radii of the FA cation disturbs the formation of a stable α
phase and high-quality FAMAPbI
3
film, especially when a large
percentage of FA is used (note that pure FAPbI
3
undergoes a
phase transition to yellow nonperovskite δ-FAPbI
3
12
).
7,10
This,
in turn, has limited to date the device performances of
FA
x
MA
1−x
PbI
3
to 15−16% because of the poor crystallization
of the resulting film.
17−19
In this study we found that by using a
small amount of FA (between 10 and 30%) in the MAPbI
3
perovskite processed by a one-step deposition method followed
by solvent engineering step, a high-quality crystal film can be
obtained. By means of structural and optical characterization we
found that the FA
+
intercalates with the MA
+
within the voids
of the PbI
6
octahedra, stabilizing the perovskite structure into a
“quasi-cubic” phase at room temperature. We fabricated solar
devices where the perovskite is sandwiched between the
2,2′,7,7′-tetrakis(N,N-dipmethoxyphenylamine)-9,9′-spirobi-
fluorene (spiro-OMeTAD) hole-transporting material and the
mesoscopic TiO
2
scaffold (m-TiO
2
) electron transporter in a
standard architecture.
3,20
The results show a significant step
Received: February 7, 2017
Accepted: March 9, 2017
Letter
http://pubs.acs.org/journal/aelccp
© XXXX American Chemical Society 802 DOI: 10.1021/acsenergylett.7b00112
ACS Energy Lett. 2017, 2, 802−806
forward in device efficiency, with champion device showing
PCE > 20.2% accompanied by a very good statistics on device
performance and hysteresis-free behavior. Such performances,
not far from that obtained with much more complicated
structures which incorporate many different compound, can be
obtained with a much simpler and highly reproducible protocol.
This is of the utmost importance for the scale-up and future
commercialization of this technology.
Figure 1a shows the absorption spectra of the mixed cation
lead iodide perovskites FA
x
MA
1−x
PbI
3
at diff ere nt FA
percentage (from 0 to 30%). When the amount of FA% is
increased, the absorption onset experiences a red-shift of
around 25 nm, as shown in the inset. The normalized
photoluminescence (PL) spectra are reported in Figure 1b. In
agreement, a significant red-shift in the emission peak of
FA
x
MA
1−x
PbI
3
is observed upon increasing x. In addition, a
reduction of the broadening of the peak is measured (see the
inset of Figure 1b). The gradual shift derives from the
intermixing of FA and MA in the perovskite lattice, while the
reduced inhomogeneous broadening can suggest a higher
crystallinity, with the increasing FA% associated with lower
energetic disorder with respect to the pure MAPbI
3
.
7,21
Figure
1c reports the time-resolved PL. The dynamical radiative decay
is here monitored to better understand the influence of the
perovskite composition, i.e. the addition of FA, on the carrier
dynamics.
Measurements are performed using time-correlated single-
photon counting (TCSPC) under low-intensity pulsed
excitation at 460 nm at <1 nJ/cm
2
, close to the typical solar
light flux.
6
At this intensity, the photoexcited carrier densities
are expected to be low and trap-mediated recombination is
likely the dominant decay pathway. In this regime we observe
that the decay processes can be well fit by monoexponential
profiles with PL lifetimes increasing from 10 to 17 ns with
Figure 1. (a) Absorption spectra of FA
x
MA
1−x
PbI
3
films (x = 0, 10%, 20%, 30%). Inset: band edge position as a function of FA%. (b) PL
spectra of FA
x
MA
1−x
PbI
3
films (x = 0, 10%, 20%, 30%) excitation at 460 nm. Insets: (top) fwhm of the PL spectra retrieved from single-peak
fitting and (bottom) peak position as a function of FA%. (c) PL decay upon excitation at 460 nm at excitation density of <1 nJ/cm
2
corresponding to an average density of absorbed photons of 10
16
photons/cm
3
. The films have been encapsulated to prevent degradation or
any oxygen- or moisture-induced effects. (d) SEM top-view images of the FA
x
MA
1−x
PbI
3
films (x = 0, 10%, 20%, 30%). Scalebar: 1 μm.
Figure 2. (a) X-ray diffraction spectra of FA
x
MA
1−x
PbI
3
films (x = 0, 10%, 20%, 30%) along with a zoomed-in view of the peak at 14°. (b)
Raman spectra of FA
x
MA
1−x
PbI
3
films (x = 0, 10%, 20%, 30%).
ACS Energy Letters Letter
DOI: 10.1021/acsenergylett.7b00112
ACS Energy Lett. 2017, 2, 802−806
803
increasing FA%. The monoexponential decay can be associated
with monomolecular and trap-mediated recombination.
7,14,22
This observed dependence of PL decay dynamics on
composition suggests that with increasing FA% the crystallinity
is enhanced, leading to reduced charge trapping or improved
carrier diffusion length, as suggested in refs 7 and 14. Note also
that a longer PL lifetime has been measured when the
perovskite organizes into a stable cubic structure.
10,14
(See also
Figure S1, which report s the calculated rate (as 1/time
constant) as a function of FA%.) Figure 1d shows the SEM
top image of the films investigated. Notably, as the FA%
increases, the average size of the grains increases (see also the
calculated averaged values in Figure S2) going from 160 nm for
0% FA to 300 nm for 30% FA. However, one could note that
when FA% > 20% the films show the presence of large darker
regions possibly due to thinner areas or pinholes resulting from
a nonuniform growth. Figure 2 shows the X-ray diffraction
(XRD) patterns of the mixed cation perovskites developed.
With respect to the pure MAPbI
3
, the introduction of the large
FA
+
cation with the smaller MA
+
decreases the tolerance factor
and induces the formation of a stable cubic perovskite
phase.
10,13,17,22−24
Depending on film morphol ogy an d
deposition conditions, adding a small amount of FA (such as
10%) might lead to a tetragonal phase as observed when the
film is formed by precipitation from aqueous hydroiodic acid.
24
In our case, we observe that the peak position of the diffraction
peaks at 14.1 (see also the zoomed-in views), 20.0, 24.4, 28.4,
and 31.8 decrease with increasing FA%, which enlarges the
crystal lattice going to a cubic or “quasi-cubic” crystal phase.
The gradual shift in the diffraction angle further confirms that
in the mixed FA
x
MA
1−x
PbI
3
the two cations are copresent in
the perovskite lattice, stabilizing the perovskite in a “quasi
cubic” phase. Figure 2a further shows the zoomed (100)
diffraction peak at 2θ ≈ 14°, which continuously shifts to
13.86° as the FA% increases. Note also that the peaks related to
pure FAPbI
3
or MAPbI
3
phases are not present. This further
supports the fine intermixing of the cations in the mixed
perovskite without inducing the phase segregation within the
film. Figure 2b shows the Raman spectrum of the mixed
FA
x
MA
1−x
PbI
3
perovskite films, here reported for the first time.
The Raman spectra consist of four main peaks (see additional
data reported in Table S1). Similarly to the pristine MAPbI
3
perovskite (see also Figure S3), the peaks below 150 cm
−1
reflect the Pb−I vibrations representing a combination of the
Pb−I stretching and bending modes along with a minor
contribution of the organic cations previously identified at 129
cm
−1
.
8,25−27
On the other side, the peak at higher wavenumber
relates to the torsion of the organic cations.
8
In particular, this
peak has been measured and calculated to be around 230 cm
−1
for the pure FA torsional mode, being on the other side shifted
to 290 cm
−1
for the pure MA torsion
8,26,27
(see also Figure S3).
From the analysis of this spectral region for the mixed
FA
x
MA
1−x
PbI
3
, we observe that only one peak is observed that
continuously red shifts with increasing the amount of FA%.
Note that in the case of halide segregation, as recently
demonstrated by a few of us,
8
two distinguishable peaks from
the FA
+
and MA
+
torsions can be identified, while in this case
only one mode is identified and is related to the cubic
perovskite structure where FA and MA are combined together
and do not form segregated perovskite phases.
Solar cells using 0−30% addition of FA have been fabricated
and optimized. The current−voltage (J−V) characteristics of
the solar cells under simulated air mass 1.5 global standard
sunlight (AM1.5G) are reported in Figure 3a along with the
device parameters. Champion devices show for the pure
MAPbI
3
power conversion efficiency of 18.59%, in line with
what has been reported in the literature for the pristine
compound.
20
Solar cells with the addition of FA generally show
Figure 3. (a) Current density−voltage (J−V) curves and parameters of FA
x
MA
1−x
PbI
3
(x = 0, 10%, 20%, 30%) devices including (b) the J−V
curve hysteresis, (c) EQE and corresponding integrated JSC of the devices comparing 0% FA with 10% FA, and (d) distribution of the device
parameters comparing 40 devices with 0% and 10% FA.
ACS Energy Letters Letter
DOI: 10.1021/acsenergylett.7b00112
ACS Energy Lett. 2017, 2, 802−806
804
a higher open-circui t voltage (V
oc
) and, for 10% FA, a
significant improvement in the device PCE. The most efficient
FA
0.1
MA
0.9
PbI
3
-based device provides a PCE of 20.26%,
outperforming the single-cation compositions MAPbI
3
. Device
statistics showing the parameters averaged on 30 devices are
reported in Figure S4 along with the photovoltaic parameters in
Table S2.Thisfurtherconfirms the statistically higher
performances we could o btain with the addition of
FA
0.1
MA
0.9
PbI
3
.InTable 1 we report the PV characteristics
of a series of cells with varying the FA%. Figure 3b shows the
J−V hysteresis of the cell (the corresponding photovoltaic
parameters are listed in Table S3). The FA
0.1
MA
0.9
PbI
3
shows
the lowest hysteresis factor, suggesting that adding 10%FA
strongly stabilizes the structure, limiting any light- or field-
induced ion movement and impeding the halide segregation.
Note also that a stable device behavior is obtained after few
seconds of illumination, upon which stable device parameters
could be registered (see details in Figure S5 and in Table S4).
Overall, this favors a stable device operation. Further details on
the long-term stability of the device are reported in Figure S6.
Similarly to the MAPbI
3
, an initial decay is observed, possibly
due to the interdiffusion of the gold top electrode.
28
However,
over longer time, an improved long-term stability is registered
at least for the first 300 h time lapse. The incident photon-to-
current conversion efficiency (IPCE), or external quantum
efficiency, across the visible spectrum is presented in Figure 3c.
The photo curren t ons et is shifte d in the red for the
FA
0.1
MA
0.9
PbI
3
device from 780 to 800 nm, providing a gain
in the photocurrent. The higher V
oc
can correlate with the
slightly larger band gap upon the introduction of FA.
Integration of the IPCE spectra (340−850 nm) over the
AM1.5G solar emission spectrum yielded short-circuit photo-
current densities of 22.57 and 21.77 mA·cm
− 2
for
FA
0.1
MA
0.9
PbI
3
and pure MAPbI
3
, respectively. The statistics
of the photovoltaic parameters obtained are reported in Figure
3d, showing that the results are statistically significant and
reproducible, providing a strong proof of the concept presented
here.
In conclusion, we have reported for the first time a simple
and highly reproducible method for obtaining highly efficient
hybrid perovskite solar cells by incorporating 10% of FA into
the MAPbI
3
structure. The formation of a stable “quasi-cubic”
phase at room temperature accompanied by improved crystal
quality is the reason behind the high performances obtained.
We envisage that, although a lot of work with promising results
is ongoing on mixed halide perovskite including mixture of Cs
+
and Rb
+
as cations and iodine and bromide as anions, a much
simpler and robust method, as the one here presented, can
sustain the realization of >20% efficient device and, more
importantly, open the way for the scale-up of the hybrid
perovskite technology.
■
ASSOCIATED CONTENT
*
S
Supporting Information
The Supporting Information is available free of charge on the
ACS Pub lications website at DOI: 10.1021/acsenergy-
lett.7b00112.
Additional data and details on the experimental methods
(PDF)
■
AUTHOR INFORMATION
Corresponding Authors
*E-mail: giulia.grancini@epfl.ch.
*E-mail: mdkhaja.nazeeruddin@epfl.ch.
ORCID
Giulia Grancini: 0000-0001-8704-4222
Mohammad Khaja Nazeeruddin: 0000-0001-5955-4786
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
G.G. is supported by the cofunded Marie Skłodowska Curie
fellowship, H2020 Grant agreement no. 665667, fund number
588072. This project was funded by the Deanship of Scientific
Research (DSR), King Abdulaziz University, Jeddah, under
Grant no. 79-130-35-HiCi. Y.Z. acknowledges the financial
support of China Scholarship Council fellowship. The authors
acknowledge Dr. Emad Oveisi of CIME-EPFL for SEM
measurement and Manuel Tschumi for stability measurement.
■
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ACS Energy Lett. 2017, 2, 802−806
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