Stabilization of highly efficient and stable phase-pure FAPbI
3
Perovskite Solar Cells by
Molecularly Tailored 2D-Overlayers
Yuhang Liu
1†
, Seckin Akin
1,4†
, Alexander Hinderhofer
2
, Felix T. Eickemeyer
1
, Hongwei Zhu
1
, Ji-
Youn Seo
1
, Jiahuan Zhang
3
, Frank Schreiber
2
, Hong
Zhang
1
, Shaik M. Zakeeruddin
1
, Anders
Hagfeldt
3
, M. Ibrahim Dar
1*
,
Michael Grätzel
1*
1
Laboratory of Photonics and Interfaces, Department of Chemistry and Chemical Engineering,
École Polytechnique Fédérale de Lausanne, Lausanne CH-1015, Switzerland.
2
Institut für Angewandte Physik, Universität Tübingen, 72076 Tübingen, Germany.
3
Laboratory of Photomolecular Science, École Polytechnique Fédérale de Lausanne, Station 6, CH-
1015 Lausanne, Switzerland.
4
Department of Metallurgical and Materials Engineering, Karamanoglu Mehmetbey
University, Karaman, Turkey.
Correspondence to: ibrahim.dar@epfl.ch (M.I.D.); michael.graetzel@epfl.ch (M.G.)
†
These authors contributed equally to the work.
Abstract
Due to their attractive optolectronic properties, metal halide APbI3 perovskites employing
formamidinum (FA
+
) as A cation are presently the focus of intense research, The superior chemical
and thermal stability of FA
+
cations
renders α-FAPbI
3
more suitable for solar cell applications than
methylammonium lead iodide (MAPbI
3
). However, its spontaneous conversion to the yellow non-
perovskite phase (δ-FAPbI
3
) under ambient conditions poses a serious challenge for practical
applications. Here, we report on the stabilization of the desired α-FAPbI
3
perovskite phase by
protecting it with a two-dimensional (2D) IBA
2
FAPb
2
I
7
(IBA = iso-butylammonium overlayer,
formed via stepwise annealing. Remarkably, the α-FAPbI
3
/IBA
2
FAPb
2
I
7
based perovskite solar cell
(PSC) reached a high power conversion efficiency (PCE) of close to 23 %. In addition, it showed
excellent operational stability, retaining ~85% of its initial efficiency under severe combined heat
and light stress, i.e. simultaneous exposure with maximum power tracking to full simulated sunlight
at 80 °C over a period of 500 h.
Introduction
Due to their high power conversion efficiency (PCE) exceeding already 25 % and ease of production,
hybrid organic metal halide perovskite solar cells (PSCs) are presently attracting wide attention both
from the scientific community and photovoltaic industry (1-7). Most PSCs employ formulations
containing methylammonium (MA
+
) cations which however are unstable decomposing to
methylamine upon exposure to heat, moisture, and light (8-13). Therefore, for the large-scale
deployment of highly-efficient PSCs, the intrinsic instability issues associated with the loss of MA
+
cations need to be mitigated (7, 14-20). In this regard, formamidinium (FA
+
) cations offers an
attractive alternative to MA
+
as pristine FAPbI
3
features a lower volatility and close to optimal
Goldschmidt tolerance factor (21). as well as an absorption edge extending into the near IR to ~840
nm. rendering pristine FAPbI
3
a more efficient solar light than MAPbI3 (4, 22, 23). However, the
spontaneous formation of the photovoltaically inactive, non-perovskite yellow phase (δ-FAPbI
3
)
below the α to δ phase transition at 170
o
C (16, 24) causes a serious predicament. introducing Cs
+
,
or MA
+
into the FAPbI
3
lattice prevents the formation of its delta phase, However, the incorporation
of these ions affects the spectral response and operational stability of the resulting PSCs under heat
stress and in humid conditions (4, 25-27).
Here we achieve stabilizeation of the α-FAPbI
3
phase by introducing a stepwise annealing process
and covering it with a novel two-dimensional IBA
2
FAPb
2
I
7
(n = 2) perovskite layer containing iso-
butylammonium (IBA) as a spacer layer. The confluence of stepwise annealing and IBA
2
FAPb
2
I
7
lamination allowed the formation of pristine α-FAPbI
3
films containing long-lasting charge-carriers
with lifetime exceeding 1.5 µs. These desired structural and spectral features translated into high
photovoltage of 1113 mV as compared to 1053 mV for reference, leading to the realization of a PCE
close to 23%, a record for FAPbI
3
–based PSCs. Remarkably we achieved excellent operational
stability, the PSCs retaining ~95% of their initial efficiency at the maximum power point (MPP)
under full-sun illumination over 700 h. The superior robustness of the FAPbI
3
protected by the 2D
overlayer was further confirmed by long term testing under harsh simultaneous heat and light stress.
These devices showed only small degradation under full-sun illumination for 500 hours at 80°C,
while unprotected FAPbI
3
films lost rapidly over 60% of their initial performance.
Results and discussion
Stepwise annealing for high-quality FAPbI
3
perovskite film
We employed methylammonium chloride (MACl) (28) as a crystallization aid to form films of pure
a-FAPbI
3
containing only trace amounts of yellow δ-FAPbI
3
phase as established by grazing
incidence wide angle synchrotron X-Ray diffraction (GIWAX), and absorption and
photoluminescence analysis
(28). Details are presented in Supplementary Note 1 and Figure S2
– S5. Top-view scanning electron microscopy (SEM) pictures of the FAPbI
3
perovskite films with
and without MACl treatment are shown in Figure 1a and Figure S1, respectively and reveal that
the MACl treatment reduces their surface roughness. To probe the formation of δ-FAPbI
3
phase, x-
ray diffraction (XRD) patterns were recorded, and the results are shown in Figure 1b. The FAPbI
3
films formed in the absence of MACl exhibit a peak at 2θ = 11.6
o
, resulting from the presence of δ-
phase. By contrast, MACl-treated FAPbI
3
did not show these δ-FAPbI
3
reflections. However, the
formation of large pinholes is apparent presumably related to the rapid release of gaseous MACl.
Such pinholes mostly cause shunts resulting in low PCEs and poor stability (Figure S6). To prevent
their formation we applied a stepwise annealing procedure as shown in Figure S7. An intermediate
annealing step at 120
o
C for a duration of 5 min substantially reduced the amount of pin holes in the
FAPbI
3
perovskite film as shown in Figure 1c. Finally, we obtained smooth and conformal FAPbI
3
perovskite films composed of micron-sized grains as shown in Figure 1d by optimizing a three-step
annealing procedure.
Power conversion efficiencies (PCE) obtained with such films
using the FTO/TiO
2
/perovskite/spiro-OMeTAD/Au architecture are shown in Table 2 We achieved
a maximum PCE of ~ 20.4% with such MACl-treated and stepwise annealed FAPbI
3
perovskite
films. In contrast, PSCs made without the stepwise annealing procedure yielded a PCE of only 16.5%
due to a significantly lower open circuit voltage (V
OC
) and fill factor (FF) ascribed to the poor film
quality.
Figure 1a, Morphological and structural analysis: Single-step annealing treatment, b, XRD patterns
of FAPbI
3
perovskite with and without MACl treatment. c, two-step annealing treatment and d,
three-step annealing treatment. Representative pinholes are highlighted in red circle, and the scale
bars represent 500 nm in length.
Surface treatment and structural characterization of FAPbI
3
–based perovskite
Although the as-prepared FAPbI
3
perovskite films were phase pure (See supplementary note 1)
and yielded PCEs of up to 20.4%, prolonged exposure to ambient air led to the formation of photo-
inactive phases, i.e. PbI
2
and δ-FAPbI
3
as shown in Figure S8. To avoid this degradation and further
improve the PCE, we introduced iso-butylammonium iodide (IBAI) as to form a 2D-protective layer
on top of the 3D a- FAPbI
3
phase (Figure 2a). Our treatment involved spin-coating a 30 mM
solution of IBAI in isopropanol on the surface of as-prepared FAPbI
3
perovskite films followed by
annealing at 110 °C for 10 min. We ascertained by SEM that this treatment did not alter the
morphology of the underneath photoactive layer (Figure S9).
We investigated the structure of the overlayer formed by the IBA surface treatment using
grazing incidence wide-angle X-ray scattering (GIWAXS) and present data in Figure 2b. The
GIWAXS reflections show distinct diffraction patterns, which we indexed to the structure of the 2D-
perovskite IBA
2
PbI
4
. Table 1. summarizes the orthorhombic unit cell dimensions derived from
fitting the GIWAXS data. For comparison, we show also the unit cell parameters of the previously
reported structural analogous butylammonium lead iodide (BA
2
PbI
4
) (29). Compared to the
BA
2
PbI
4,
the unit cell of IBA
2
PbI
4
is smaller, mainly because the long unit cell axis is shortened by
1.5 Å, which is consistent with the alkyl chain length difference in the two organic spacer molecules.
Tab le 1, unit cell parameters of BA
2
PbI
4
(29) and IBA
2
PbI
4
based 2D perovskites.
2D Perovskite
a
b
c
Volume
2D-BA
2
PbI
4
(29)
8.87
8.69
27.6
2129
2D-IBA
2
PbI
4
8.9
8.7
26.2
2029
170
o
C - 30’
80
o
C – 3’ ; 120
o
C – 5’ ; 170
o
C - 22’
120
o
C – 5’ ; 170
o
C - 25’
a b
d
c
10 20 30 40
*
*
a
a
*
a
a
d
a
Intensity (a.u.)
2 theta (degree)
FAPbI
3
with MACl
FAPbI
3
without MACl
Figure 2. a, Chemical structure of IBA
+
and schematic illustration of pure 2D perovskite based on
IBAI. b, GIWAXS data of pure 2D-IBA
2
PbI
4
(angle of incidence = 0.14°). c, XRD data of a bare
FAPbI
3
film, and FAPbI
3
/2D-IBA
2
FAPb
2
I
7
prepared by the stepwise annealing method.
XRD data of pristine and IBAI treated FAPbI
3
films are shown in Figure 2c. Each film exhibits
structural features corresponding to the α-FAPbI
3
perovskite phase with a lattice spacing of 6.36 Å
(30), the hexagonal FAPbI
3
(non-perovskite phase) (31) and PbI
2
. The IBAI treated FAPbI
3
films
revealed the formation of IBA
2
FAPb
2
I
7
overlayer exhibiting reflections of the 2D-structure with a
lattice spacing of 38.56 Å (Table 1). We assume some FA of the underlying 3D structure to be used
up in the formation of the IBA
2
FAPb
2
I
7
(n = 2) layer. The lattice spacing is slightly smaller than the
lattice spacing of the BA
2
FAPb
2
I
7
(n = 2) structure of 39.35 Å (32), which is consistent with the
length difference in the spacer molecules BAI and IBAI. To the best of our knowledge, this is the
first investigation of iso-butanol as spacer layer in 2D metal halide perovskites. Cho et.al (33) used
mixtures of n-butyl and iso-butylammonium cations for surface passivation treatments yielding a
PCE of 21.7% mixed cation.
GIWAXS data of pristine and IBAI treated FAPbI
3
films (Figure 3a and 3b) confirm the
formation of a-FAPbI
3
perovskite-phase in agreement with the XRD measurements. The additional
2D reflections (Figure 3b) arising from the 2D-IBA
2
FAPb
2
I
7
overlayer are along the q
z
axis
indicating their orientation to be parallel to the substrate. With the radially integrated data over the
measured q-space, (Figure 3c) we can qualitatively estimate the amount of PbI
2
and hexagonal
phase FAPbI
3
in the scattering volume. In the sample with 2D-IBA
2
FAPb
2
I
7
overlayer a negligible
content of the hexagonal polymorph and a smaller amount of excess PbI
2
compared to the non-
covered sample is observed.
a
b
= C
= N
= H
= R-NH
3
+
= IBA
+
c
0.5 1.0 1.5
10
2
10
4
10
6
10
8
10
10
10
12
(111)
(110)
FaPbI
3
a-phase
(001)
d-phase
hexagonal
2D
2D
2D
PbI
2
Intensity (a.u.)
q
z
(Å
-1
)
FAPbI
3
+ IBA
2
FAPb
2
I
7
FAPbI
3
2D
PbI
2
Figure 3: Structural and spectroscopic characterization. GIWAXS data of (a) FAPbI
3
film (b)
FAPbI
3
/2D IBA
2
FAPb
2
I
7
. Angle of incidence was 0.14°. c Radially integrated GIWAXS data from
Figures a)-b) and d), TRPL results of neat FAPbI
3
, and FAPbI
3
/IBA
2
FAPb
2
I
7
– based perovskites.
The blue and green curves are fits to the kinetic model published previously. (34). Both cases (k =
0 and S = 0 for FBAI treated films) result in the same fit.
Charge-carrier dynamics
We further investigated the impact of the IBA
2
FAPb
2
I
7
layer on the optoelectronic properties of the
α-FAPbI
3
perovskite films using time-resolved photoluminescence (TRPL) as shown in Fig. 3d.
Both the untreated and treated films show two distinct decay regimes. A very fast decay within the
first 10 ns is followed by a slow monexponential decay. The fast decay at early times is caused by
carrier diffusion due to the initial exponential excitation profile in combination with charge carrier
trapping very likely caused by shallow traps on or near the surface (PbI
2
, δ-FAPbI
3
non-perovskite
phase or surface states). The IBAI treated film shows a slower decay by more than one order of
magnitude within the first 10 ns compared to the untreated film. Since the carrier diffusion rate is
the same in both films, we infer that the IBAI treatment alters the defect chemistry of the surface
which leads to a significantly lower trap density. Also the first order decay kinetics at times > 10 ns
are much slower for the treated than the untreated films. We applied numerical simulations to
analyze the different decay kinetis . Details for this simulation can be found in Zhu et al (34). The
single exponential decay can be caused by either pseudo first SRH recombination with a
recombination constant k
1
or by surface recombinations with a surface recombination velocity S.
Since both films have the same bulk material and, hence, the same k
1
, the only difference lies in S.
We considered two extreme cases for the IBAI treated film, where in the first case the film exhibits
no bulk recombinations (k
1
= 0), i.e. the monoexponential decay is exclusively caused by surface
recombination. Conversely, in the second case we assume that the film exhibits no surface
recombination (S = 0), i.e. the decay is solely caused by SRH type bulk recombination.. From this
a
b
c
0.50 0.75 1.00 1.25 1.50
FAPbI
3
+ IBA
2
FAPb
2
I
7
FAPbI
3
(110)
2D
FaPbI
3
a-phase
(001)
2D
PbI
2
q
xy
(Å
-1
)
d-phase
hexagonal
Intensity (a.u.)
d
IBAI treated FAPbI
3
Pristine FAPbI
3
0 500 1000 1500
1E-4
1E-3
0.01
0.1
1
PL (counts)
Time (ns)
