Ligand-Stabilized Reduced-Dimensionality Perovskites

Article abstract of DOI:10.1021/jacs.5b11740

Metal halide perovskites have rapidly advanced thin-film photovoltaic performance; as a result, the materials' observed instabilities urgently require a solution. Using density functional theory (DFT), we show that a low energy of formation, exacerbated in the presence of humidity, explains the propensity of perovskites to decompose back into their precursors. We find, also using DFT, that intercalation of phenylethylammonium between perovskite layers introduces quantitatively appreciable van der Waals interactions. These drive an increased formation energy and should therefore improve material stability. Here we report reduced-dimensionality (quasi-2D) perovskite films that exhibit improved stability while retaining the high performance of conventional three-dimensional perovskites. Continuous tuning of the dimensionality, as assessed using photophysical studies, is achieved by the choice of stoichiometry in materials synthesis. We achieve the first certified hysteresis-free solar power conversion in a planar perovskite solar cell, obtaining a 15.3% certified PCE, and observe greatly improved performance longevity.

Full text of DOI:10.1021/jacs.5b11740

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Article
Ligand-Stabilized Reduced-Dimensionality Perovskites
Li Na Quan, Mingjian Yuan, Riccardo Comin, Oleksandr Voznyy, Eric M. Beauregard, Sjoerd Hoogland,
Andrei Buin, Ahmad R. Kirmani, Kui Zhao, Aram Amassian, Dong Ha Kim, and Edward H. Sargent
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.5b11740 • Publication Date (Web): 03 Feb 2016
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Ligand-Stabilized Reduced-Dimensionality Perovskites
Li Na Quan,1,2† Mingjian Yuan,1† Riccardo Comin,1 Oleksandr Voznyy,1 Eric M. Beauregard,1 Sjoerd
Hoogland,1 Andrei Buin,1 Ahmad R. Kirmani,3 Kui Zhao,3 Aram Amassian,3 Dong Ha Kim,2* Edward H.
Sargent1*
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1 Department of Electrical and Computer Engineering, University of Toronto, 10 King’s College Road, Toronto, On-
tario, M5S 3G4, Canada.
2 Department of Chemistry and Nano Science, Ewha Womans University, 52, Ewhayeodae-gil, Seodaemun-gu,
Seoul 03760, Korea
3 Solar and Photovoltaic Engineering Research Center, King Abdullah University of Science and Technology
(KAUST), Thuwal 23955-6900, Saudi Arabia
KEYWORDS : Perovskites; Photovoltaics; Reduced hysteresis; Quasi-two dimensional crystals; Stability.
ABSTRACT: Metal halide perovskites have rapidly advanced thin film photovoltaic performance; as a result, the materi-
als’ observed instabilities urgently require a solution. Using density functional theory (DFT), we show that a low energy of
formation, exacerbated in the presence of humidity, explains the propensity of perovskites to decompose back into their
precursors. We find, also using DFT, that intercalation of phenylethylammonium between perovskite layers introduces
quantitatively appreciable van der Waals interactions; and these drive an increased formation energy and should there-
fore improve material stability. Here we report the reduced-dimensionality (quasi-2D) perovskite films that exhibit im-
proved stability while retaining the high performance of conventional three-dimensional perovskites. Continuous tuning
of the dimensionality, as assessed using photophysical studies, is achieved by the choice of stoichiometry in materials
synthesis. We achieve the first certified hysteresis-free solar power conversion in a planar perovskite solar cell, obtaining a
15.3% certified PCE, and observe greatly improved performance longevity.
The rapid advance of organic-inorganic metal halide
crafted using bulkier organic molecules in place of me-
thylammonium, that exhibit enhanced stability; however,
these suffer from poor photovoltaic performance13. The
PCE is still lower than 5% due to their low absorption
coefficient and poor carrier transport14, and this has hin-
dered their further application. Here, we develop and
investigate a platform of mixed-organic, dimensionally-
tuned quasi-2D perovskite thin films that continuously
bridge the gap between 2D and 3D materials15, 16. We find
that members of the quasi-2D family combine the en-
hanced stability of 2D perovskites with the excellent op-
toelectronic parameters, including long-range photocarri-
er diffusion, of 3D perovskites. Our approach relies on
computational methods to unravel the origins of instabil-
ity in conventional MAPbI3, while complementary studies
of the physical and optoelectronic properties of quasi-2D
perovskites allow us to identify optimized conditions to
design and fabricate chemically-stable, high-efficiency
photovoltaic devices.
perovskites represents
a
breakthrough for next-
generation thin-film optoelectronics1–6. In particular, me-
thylammonium (MA = CH3NH3) lead iodide (MAPbI3)
8
perovskites possess an appropriate bandgap energy7,
,
large absorption coefficient, and long-range ambipolar
photocarrier diffusion9. Tremendous efforts from multiple
research groups have propelled the certified power con-
version efficiency (PCE) of perovskite solar cells to
20.1%.10
Unfortunately, perovskite materials lack long-term sta-
bility under ambient operation conditions. This has been
ascribed, with the aid of theoretical studies, to the low
formation energy of MAPbI311. This challenge has motivat-
ed efforts to improve device stability, and progress in this
direction has been made by developing novel device ar-
chitectures12.
Here we focus instead on the stability of the active ma-
terial itself. We investigate, using theory and experiment,
the origins of instability in conventional MAPbI3; and we
propose and fabricate enhanced perovskites - taking ad-
vantage of the versatile physical chemistry of this materi-
als system - that constitute low-dimensional hybrid or-
ganic-inorganic crystalline thin films. There exist prior
reports of two-dimensional (2D) layered perovskites,
The intermediates between 3D and 2D are layered per-
ovskites systematically synthesized by introducing a large
organic cation, phenylethylammonium (PEA = C8H9NH3),
at a judiciously-chosen stoichiometry. This cation pos-
sesses a large ionic radius which is incompatible with a 3D
perovskite structure; therefore, perovskites based on this
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cation tend to crystallize into a layered, 2D dimensional different layer (n) values in the series PEA2(CH3NH3)n-
structure17. Consequently, we can continuously tune the
dimensionality of metal halide perovskite compounds by
mixing stoichiometric quantities of lead iodide (PbI2),
methylammonium iodide (CH3NH3I), and phenyle-
thylammonium iodide (PEAI) to yield compounds with
1PbnI3n+1 (table S1). In this notation, the limit n=∞ corre-
sponds to the cubic 3D perovskite, CH3NH3PbI3, while the
other n values describe 2D (n=1) or quasi-2D (n>1) perov-
skite structures (Figure. 1a).
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Figure 1. Energetics of perovskite formation and stability. (a) Unit cell structure of (C8H9NH3)2(CH3NH3)n-1PbnI3n+1 perovskites
with different n values, showing the evolution of dimensionality from 2D (n=1) to 3D (n=∞). (b) DFT simulation of the formation
energy
of
perovskite
with
different
n
values
in
different
atmospheres
We investigated the stability of dimensionally-tuned
perovskites using density functional theory (DFT) simula-
tions. Calculations show that the conventional 3D perov-
skite is marginally stable relative to decomposition into
bulk PbI2 and MAI constituents (see details for DFT Simu-
lations in Supporting Information Figure S1), and indeed
both components have been observed in thermal gravi-
metric analysis18. However, this process cannot happen
within the bulk spontaneously and needs to start from the
surface where molecules have sufficient freedom to rear-
range. We further analyze the formation enthalpy into
that of constituent layers, predicting ~0.55 eV per mole-
cule required to recrystallize MAI layer and ~0.6 eV gain
upon conversion of the PbI2 layer (see details on DFT
simulations and results discussion in the Supporting In-
formation). Thus, MAI serves as a protection layer, limit-
ing perovskite decomposition, while the PbI2 surface ter-
mination is inherently unstable.
Notably, we find that van der Waals interactions be-
tween the capping organic molecules are key drivers of
increased material stability21: the energy required to re-
move PEAI from the perovskite is 0.36 eV higher than
that for MAI, reducing the desorption rate by six orders of
magnitude, and slowing film decomposition one-
thousand-fold (additional details on DFT simulations and
results can be found in Supporting Information).
Motivated by these insights, and building on the previ-
ously-reported solvent-engineering fabrication process22,
we fabricated dimensionally-tuned perovskite films using
single-step spin-coating. X-ray diffraction (XRD) patterns
for the 3D and quasi-2D perovskite thin films show very
similar reflections, indicating a preferential orientation of
the crystallites relative to the substrate. Thus, thin film
samples do not exhibit Bragg reflections at low diffraction
angles 2θ°, yet these low-diffraction-angle features repre-
sent the unambiguous signatures of the layered structure,
specifically of the elongated unit cell23.
Under ambient conditions, the degradation of 3D per-
ovskites is known to result in the formation of a PbI2 yel-
low phase, and X-ray photoelectron spectroscopy (XPS)
suggests that it is accompanied by the loss of MAI into
the gas phase, with no measurable formation of perov-
skite hydrate19 even in a high-humidity environment. In
contrast, simulations predict a greater than 2 eV desorp-
tion energy for organic molecules. To reconcile these ob-
servations, we included water in our simulations,20 and
found a significant decrease in desorption energies and
decomposition lifetimes (Figure. 1b), consistent with the
We therefore obtained powders from the films that
would lack preferential orientation. These powder sam-
ples show diffraction scans that, in all but the 3D case (n =
∞), exhibit a series of Bragg peaks at low angles (2θ
<10°)24. The XRD data confirm the formation of lead io-
dide n-layer perovskite sheets (Figure. 2a), but are not
conclusive on the phase purity of quasi-2D films. Conse-
quently, we embarked on a study of the phase composi-
tion of dimensionally-tuned perovskite solids of various n.
sensitivity of perovskite to moisture18,33
.
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Figure 2. Structure and optical characterization of dimensionally-tuned perovskite films. (a) Low diffraction angle region of
XRD spectra. (b) Absorption spectra of the perovskite films with different n values. (c) Valence band photoemission. (d) XPS
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data
for
the
atomic
core-level
peaks.
As expected, decreasing the dimensionality of the per-
ovskite structure causes an increase in the electronic
bandgap30,31. The dimensional tuning of the electronic
bandgap, EG, can be followed from the absorption spectra
(Figure. 2b), which show a higher-energy absorption edge
for lower n values. The trend is in good agreement with
DFT calculations (Figure. S3). The energy levels of these
compounds have been analyzed using a combination of
ultraviolet photoemission spectroscopy (UPS) and UV-Vis
absorption spectra, as shown in Figure. 2c. The valence
band maximum (VBM) for 3D CH3NH3PbI3 perovskite is
found 5.34 eV below the vacuum level. From this and the
optical bandgap, we conclude that the conduction band
minimum (CBM) is located at 3.77 eV25. When we de-
crease n values, the CBM becomes shallower while the
VBM shows a negligible variation (Figure. 2c). The stoi-
chiometry of these perovskite films was verified using X-
ray photoelectron spectroscopy (XPS) (Figure. 2d), and
which reveals a gradual decrease of both the I:Pb and
N:Pb ratios throughout the film with increasing n values,
in good agreement with the values expected from the
nominal molar ratio of precursors (table S1). The above
studies, taken together, indicate a controlled synthesis of
mixed-stoichiometry materials in which the organic cati-
on enables continuous tuning of the dimensionality, and
resultant optical properties, of the perovskite compounds.
Figure 3. Photophysical and structural stability of 3D and
quasi-2D perovskite films. (a) Relative absorption intensity
near the absorption onset for different n values. (b) Film
XRD spectra as a function of n at high humidity level (90%)
with different aging time. (c) PL-decay data before and after
10 days storage.
We then investigated material stability as a function of
dimensionality. The perovskite films were stored for ex-
tended periods of time in environments having controlled
humidity levels. In the case of bulk MAPbI3 perovskites,
the absorbance (Figure. 3a) near the absorption edge on-
set degrades rapidly over two months when the sample is
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stored in 90% relative humidity (RH). After two months, skite structure are observed for all fresh samples. Howev-
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the absorbance of 3D perovskite films at wavelengths
longer than 500 nm is significantly suppressed, signaling
full reversion to PbI2. In contrast, the pure 2D perovskite
(n=1), PEA2PbI4, exhibits excellent stability over the same
period of time using the same measures, with no ostensi-
ble variation in the absorbance observed, which is in good
agreement with previous reports16. As expected, the quasi-
2D perovskites (n=10, 40, 60) exhibit significantly im-
proved stability compared to 3D perovskite. A slight
change in the absorbance is observed for the prominent
perovskite peak around 750 nm, suggesting that a part of
these perovskite films has converted to PbI2 (Figure. 3a).
Layered perovskites with lower n value (n=10) (i.e. closer
to 2D) are found to exhibit better stability of optical
properties compared to higher n values (n=40, 60).
er, a new peak at 2θ=12.42° emerges for 3D perovskite
film, corresponding to a Bragg reflection of PbI2 structure,
with an increasing intensity over time. In contrast, only a
very weak PbI2 reflection appears in the n=60 and n=40
perovskite films with a peak intensity nearly unchanged
over the same period of time, which indicate a significant
improvement of stability compared to 3D perovskite.
We also investigated the materials’ stability using tran-
sient photoluminescence (PL) decay measurements. We
then tracked any changes over time (Figure. 3c). Dramatic
changes are observed in the PL-decay traces for the 3D
perovskite after 10 days’ storage in air. There is no observ-
able variation in the carrier lifetime of quasi-2D (n=40,
60) perovskites. The PL-decay changes provide further
support of enhanced stability: specifically, the lower n-
valued perovskites exhibit improved stability of PL life-
time compared to higher n-values counterparts, a finding
that is in good agreement with DFT studies (Figure. 1b)
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We tracked the materials structure stability using XRD
periodically recorded on perovskite films stored at 90%
RH for more than a week (Figure. 3b). Strong (002) dif-
fraction peaks (at 2θ=13.98°) characteristic of the perov-
Figure 4. Comparison of J-V curves for different number of perovskite layers. (a) Simulation using experimental absorp-
tion spectra and band gaps, and mobility of 0.1 cm2V-1s-1. (b) Simulation with radiative recombination of 5·10-10 cm3 s-1 and
gradually decreasing mobility for lower n. Same trap density is used for all cases. (Perovskite layer was estimated to be 200
nm thickness). (c) Experimental curves.
Figure 5. Solar cell device architecture and performance. (a) Cross-sectional scanning electron microscopy (SEM) image for a
quasi-2D perovskite device with n=60. (b and c) J-V curve and PCEs for champion perovskite device with different n values. (d
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and e) J-V curve, both forward and reverse scan and external quantum efficiency (EQE) spectra of the certified device with n=60
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perovskite.
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We turned to evaluating the enhanced-stability materi-
als for photovoltaic applications. Unfortunately the pure
2D perovskites (n=1) and low n-values quasi-2D perov-
skite films exhibit appreciably reduced mobility µ and
lower lifetime21; further, they have a significantly wider
bandgap that will sacrifice absorption in the near-infrared
(NIR) portion of the solar spectrum. In contrast, our stud-
ies of carrier lifetime τ and carrier diffusion length LD9
(table S2) indicated that the quasi-2D perovskite having
higher n values (n=40, 60) films exhibit impressive τ, µ,
and LD compatible with photovoltaic performance27.
Since the 3D perovskite films are limited not by mobility
but instead by trap volume density26, we reasoned that
some sacrifice to mobility in the quasi-2D materials may
potentially be tolerable before radiative recombination
becomes a considerable loss, an idea confirmed via self-
consistent optoelectronic simulations. Fig. 4 shows a
qualitative comparison of simulated and experimental J-V
curves. Modeling assuming the same carrier diffusion
length of 350 nm for all samples and using experimental
absorption spectra and band gaps (Fig. S4a) cannot re-
produce experimental trends (Fig. S4c): (i) the fill-factor
(FF) stays the same for all samples and for low n it is
higher than experimental values; (ii) VOC is practically
unaltered, following the band gap trends, while experi-
mental VOC is higher for lower n; (iii) the changes in ab-
sorption are not sufficient to explain the drop of JSC for
lower n. The fill-factor is primarily defined by the carrier
diffusion length, which in turn depends on trap density.
However, introducing more traps will lead to deteriora-
tion of all parameters, JSC, FF, and VOC, while experimen-
tally VOC actually increases for lower n. The increase of
VOC is usually a signature of decreased mobility, since
slower carrier extraction allows for more carrier accumu-
lation, and thus, higher VOC. Higher carrier accumulation,
in turn, increases the chances for carriers to meet and
recombine radiatively. Including the radiative recombina-
tion, in combination with lower mobility, allows repro-
ducing experimental trends, i.e. decrease in JSC and FF
while increasing VOC (Fig. S4b). In sum, weak absorption
and higher bandgap are detrimental only for very-low n
devices, for n=6 and higher the main limiting factor is the
reduced mobility, leading to strong radiative recombina-
tion losses. This suggests that quasi-2D perovskites, with
high n values, may be capable of performance comparable
to 3D materials with increased stability.
sorber layer, a dense-grained uniform morphology with
grain sizes in the range of 100-500 nm was observed in all
samples. The grain size shows a slight reduction with de-
creasing n value (Figure. S4).
The perovskite with n<40 shows lower performance due
to lower mobility which leads to higher charge accumula-
tion and increased radiative recombination losses, which
negatively impacts the fill factor (FF) (Figure. 5b and c).
In addition, perovskite films with dimensionality ap-
proaching the 2D limit (n<10) have a wider bandgap and
reduced absorption across the solar spectrum. As ex-
pected, devices based on perovskite active layers showing
a very low n value (n=6) exhibited very low initial perfor-
mance in view of their low absorbed photon current and
poor carrier transport (Figure. 2b and 5b).
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The top-performing quasi-2D perovskite (n=60) devices
exhibited a hysteresis-free certified AM1.5 PCE of 15.3%
(Figure. 5d and e). Certification was obtained by an inde-
pendent photovoltaic calibration laboratory (Figure. S5).
The steady-state current-density measurement was per-
formed for 3D and n=60 perovskite device (Figure S6).
Photocurrent decay during testing of devices was ob-
served in the case of 3D perovskites; while devices with
n=60 perovskite showed constant current density over
time, consistent with quasi-2D perovskites’ low hysteresis.
To the best of our knowledge, this is the first report of
certified, hysteresis-free planar perovskite solar cells. The
introduction of these multilayered perovskite structures
might alter the mechanisms of electronic and ionic mo-
tion across the film, thus promoting the suppression of
the well-known hysteresis in the current-voltage charac-
teristic of planar 3D perovskite solar cells28.
We measured solar cell performance and its evolution
in time (Figure. 6a). The 3D perovskite devices began with
16.6% PCE but degraded over 8 weeks to less than 3% PCE
when stored in N2. Moderate n value perovskites (n=10)
showed improved performance together with good long-
term stability (Figure. 6b-e); however, the absolute device
performance is still low compared with state of the art
perovskite device due to poor carrier transport, which
limits their further application.
Most interesting were the quasi-2D perovskites, both
n=60 and n=40 (Figure. 6b-e). The performance of the
n=60 device declined to 11.3% PCE after 60 days under low
humidity atmosphere, while the n=40 device had a similar
initial PCE and declined to 13.1% after 60 days (tables S4
to S8). Longer-term stabilization is expected when a phys-
ical moisture barrier such as a carbon electrode is de-
ployed into the device architecture, combined with use of
quasi-2D perovskites (n=60, 40)12.
We therefore constructed solar cells employing the var-
ious active layers in a planar heterojunction architecture
(Figure. 5a). Cross-sectional SEM indicates the presence
of two uniform layers, a TiO2 layer (~ 50 nm) over coated
with a dense perovskite absorber (~ 450 nm). For the ab-
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Figure 6. Solar cell device stability. (a) The device performance as a function of n value, which shows increased performance
was achieved with increased n value, however, at the meantime, stability was decreased. (b) The evolution of device performance
as a function of time with different n values. Record of current density (c), fill factor (d) and open circuit voltage (e) change for
different
perovskite
device
stored
in
Nitrogen
over
two
months.
In analogous studies under humid air (55% RH), the 3D
perovskite lost the majority of its performance, decreasing
to 4.2% PCE in 3 days. In contrast, over two weeks under
high RH, the n=60 and n=40 devices decreased to about
13% PCE in each case (table S8). Statistical data on the
decay of performance were summarized in Supporting
Information (Figure. S7). The thermal stability of devices
was evaluated by annealing at 100 ºC for 36 hr (Figure.
S8). The PCE of the device with n=60 perovskite showed a
modest improvement in thermal stability compared with
the 3D perovskite devices.
low trap state density for high voltage – may be incorpo-
rated into more robust solar cells.
EXPERIMENTAL SECTION.
Phenylethylammonium Iodide Synthesis. Phenyle-
thylamine (10 mL, 80 mmol) was dissolved in 10 mL of
ethanol in an ice bath, and HI aqueous solution (55%, 20
mL, 170 mmol) was added dropwise to the flask with vig-
orous stirring. After the addition of HI, the mixture was
allowed to stand in ice bath for another 20 minutes until a
colorless precipitate appeared. The precipitate was fil-
tered under vacuum and washed thoroughly using cold
diethyl ether. The crude precipitate was collected and
recrystallized in ethanol twice to obtain thin, plate-like
white crystal (16.5g, yield 83%) EI-MS: m/z 249.2, cacld
249.0.
Film Fabrication. The series of different dimensional
perovskite [(C6H5C2H4NH3)2(CH3NH3)n-1PbnI3n+1] solution
were prepared by dissolving specific stoichiometric quan-
tities of lead iodide (PbI2), methylammonium idodide
(MAI) and phenylethylammonium iodide (PEAI) in DMF
(n<10) or γ-butyrolactone / DMSO (5:5, volume ratio)
mixture (n ≥ 10) at 60 ºC for 12 hours with vigorous stir-
ring. The resulting solution was filtered by PTFE syringe
filter (0.2 µm) before deposition. The DMF solution
(n<10) was then deposited onto the substrate by a single-
step spin-coating process at 5000 r.p.m. for 30 seconds.
In sum, the very low formation energy of 3D perovskites
accounts for their low stability relative to their precursor
phases. This problem is exacerbated by water and humidi-
ty. We show herein using that adding an organic mole-
cule that does not incorporate into the 3D lattice can be
used to produce capped-layered quasi-2D perovskite-
based materials; and that these possess much higher for-
mation energies, owing to appreciable van der Waals
forces that confer improved stability. By replacing the
methylammonium (MA) with the formamidinium
(HN=CHNH3+, FA) cation to further reduced quasi-2D
perovskites' bandgap, solar devices with even higher per-
formance without hysteresis could potentially be engi-
neered in the future10. The present work suggests that,
with further progress, the benefits of perovskites – in long
diffusion length for high current and fill factor, as well as
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solar cell (Newport). The accuracy of the power meas-
The mixed γ-butyrolactone / DMSO solution (n ≥ 10) was
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coated onto the substrate via a consecutive two-step spin
coating process at 1000 r.p.m. and 5000 r.p.m. for 10 and
60 seconds, respectively. During the second spin step, 100
µL of chlorobenzene were deposited onto the substrate.
The resulting films were then annealed at 100 ºC for 1
hour for better crystallization.
urement was estimated to be ± 5%.
UPS Measurements. UPS measurements were carried
out in an ultrahigh vacuum chamber (UHV) with a base
pressure below 5x10-9 mbar. The photon line width is ~250
meV and the minimum spot size ~1 mm. He I photons
(21.2 eV) were used to acquire the spectra at normal emis-
sion. The photoelectrons were collected by the SPHERA
U7 hemispherical energy analyzer with a 7 channel MCD
detector, in Constant Analyzer Energy (CAE) mode. The
binding energy values shown with 10 meV precision
should be rounded to the nearest 100 meV value in ac-
cordance with the overall energy resolution.
XPS Measurements. XPS measurements were carried
out in a Thermo Scientific K-Alpha system, with a 300 µm
spot size, 75 eV pass energy, and energy steps of 0.05 eV.
All signals are normalized to Pb for direct comparison
between different samples.
XRD Measurements. XRD measurements on oriented
films were conducted on a Panalytical X'Pert Pro diffrac-
tometer with a Bragg-Brentano geometry and PIXCEL 1D
detector equipped with a nickel filter. Unoriented scans
were collected on powders loaded into a 0.1-mm diameter
silicon capillary on a Bruker D8 Venture diffractometer
using monochromated Mo Kα1 (0.71073 Å) radiation.
Other Characterizations. The EQE spectrum for pho-
tovoltaic device is obtained using a previously published
process32. Samples are tested in N2 ambient. Photolumi-
nescence (PL) was measured using a Horiba Fluorolog
Time Correlated Single Photon Counting system with
photomultiplier tube detectors. For steady-state meas-
urements, the excitation source is a monochromatized Xe
lamp. For time-resolved PL, the excitation source is a la-
Photovoltaic Device Fabrication. A dense compact
TiO2 electron transport layer (50 nm) was deposited onto
a F-doped SnO2 (FTO, Pilkington, TEC 15) substrate, fol-
lowed by drying at 130 ºC for 10 minutes and calcining at
500 ºC for 1 hour. The TiO2 precursor solution was pre-
pared by dissolving 890 mg titanium isopropoxide (TTIP)
and 30 mg HCl (37%) solution in 8 mL ethanol; the mix-
ture was then stirred at room temperature for overnight
before use. The resulting TiO2 film was then immersed in
120 mM of TiCl4 aqueous solution at 70 ºC for 1 hour, then
calcined at 500 ºC for another 30 minutes. Then the per-
ovskite absorber was deposited onto the TiO2 using the
method described before. The hole-transport layer was
spin-coated onto the perovskite film at 4000 r.p.m. for 30
seconds using a chlorobenzene solution that containing
of 63 mg/ml of spiro-OMeTAD and 20 µL of tert-
butylpyridine, as well as 70 µL of bis(trifluoro-
methane)sulfonimide lithium salt (170 mg/mL in acetoni-
trile). Top electrodes were deposited using an Angstrom
Engineering deposition system in an Innovative Technol-
ogy glovebox. The contacts consisted of 100 nm electron-
beam deposited gold deposited at 0.4 Å/s.
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Relative Humidity Control. For all characterization,
the perovskite films were stored at room temperature
(measured as 25± 1 °C) in the controlled humidity desicca-
tor. The relative humidity (RH= 90 %) was achieved with
an accuracy of ± 3% by a water/glycerol solution with
careful adjustment of the weight ratio of glycerol. The
precise relative humidity was measured periodically using
a calibrated hygrometer.
ser diode with a wavelength
nm; the combined
(source + instrumental response function) time resolution
is ns.
Photovoltaic Performance Characterization. The
steady-state open-circuit voltage, VOC, is first measured
using the Keithley 2400 by fixing the current to zero and
sampling the voltage at multiple time points. The steady-
state short-circuit current, JSC, is measured by setting the
bias voltage to zero and sampling the current at multiple
time points. Instantaneous J-V curves are then measured
with a scanning rate of 0.02 V s-1. The maximum power
point (VMPP) is determined from the instantaneous J-V
curve. The steady-state power conversion efficiency, PCE,
is measured by setting the bias voltage to the estimated
VMPP. Under the bias of VMPP, the current density value is
sampled during 15 seconds to obtain JMPP. The PCE is fi-
nally obtained as the product of VMPP and JMPP. The active
area is determined by the aperture before the solar cell to
avoid overestimating the photocurrent. Through this ap-
erture (area 0.049 cm2) illumination intensity was cali-
brated using a Melles–Griot broadband power meter and
set to be 1 sun (100 mW cm− 2). The AM 1.5 solar power is
supplied by a class A (< 25% spectral mismatch) solar
simulator (Science Tech). The spectral mismatch of the
system was characterized using a calibrated reference
ASSOCIATED CONTENT
Supporting Information.
Materials and methods, additional results, discussion, and
references. This material is available free of charge via the
Internet at http://pubs.acs.org.”
AUTHOR INFORMATION
Corresponding Author
* ted.sargent@utoronto.ca (EHS); dhkim@ewha.ac.kr (DHK)
Author Contributions
†L. N. Q and M. Y contributed equally.
ACKNOWLEDGMENTS
This publication is based in part on work supported by
Award KUS-11-009-21, made by King Abdullah University of
Science and Technology (KAUST), by the Ontario Research
Fund Research Excellence Program, and by the Natural Sci-
ences and Engineering Research Council (NSERC) of Canada.
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Page 8 of 16
L. N. Quan and D. H. Kim acknowledge the financial support
by National Research Foundation of Korea Grant funded by
the Korean Government (2014R1A2A1A09005656;
2015M1A2A2058365).
（24） Kegan, C. R.; Mitzi, D. B.; Dimitakopoulos, C. D. Sci-
ence. 1999, 286, 945.
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（25） Miller, E. M.; Zhao, Y.; Mercado, C.; Saha, S.; Luther, J.;
Zhu, K.; Stevanovic, V.; Perkins, C.; Lagemaat, J. Physical
Chemistry Chemical Physics. 2014, 16, 22122.
（26） Tan, Z.; Moghaddam, R.; Lai, M.; Docampo, P.; Higler,
R.; Deschler, F.; Price, M.; Sadhanala, A.; Pazos, L.; Hanu-
sch, F.; Bein, T.; Snaith, H. J.; Friend, R. H. Nat. Nanotech-
nol. 2014, 9, 687.
The authors thank Dr. X. Lan for assistance in SEM image
acquisition, M. Adachi, B. Sutherland, F. Fan, J. Xu, A.
Janmohamed for fruitful discussions, and R. Wolowiec and
D. Kopilovic for their assistance during the course of study.
9
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Figure 1. Energetics of perovskite formation and stability. (a) Unit cell structure of (C8H9NH3)2(CH3NH3)nꢀ
1PbnI3n+1 perovꢀskites with different n values, showing the evolution of diꢀmensionality from 2D (n=1) to
3D (n=∞). (b) DFT simulation of the formation energy of perovskite with different n values in different
atmospheres.
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Figure 2. Structure and optical characterization of dimen-sionally-tuned perovskite films. (a) Low diffraction
angle region of XRD spectra. (b) Absorption spectra of the perov-skite films with different n values. (c)
Valence band photoe-mission. (d) XPS data for the atomic core-level peaks.
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Figure 3. Photophysical and structural stability of 3D and quasi-2D perovskite films. (a) Relative absorption
intensity near the absorption onset for different n values. (b) Film XRD spectra as a function of n at high
humidity level (90%) with different aging time. (c) PL decay data before and after 10 days storage
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Figure 4. Comparison of J-V curves for different number of perovskite layers. (a) Simulation using
experimental absorption spectra and band gaps, and mobility of 0.1 cm2V-1s-1. (b) Simulation with
radiative recombination of 5•10-10 cm3 s-1 and gradually decreasing mobility for lower n. Same trap
density is used for all cases. (Perovskite layer was estimated to be 200 nm thickness). (c) Experimental
curves.
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Figure 5. Solar cell device architecture and performance. (a) Cross-sectional scanning electron microscopy
(SEM) image for a quasi-2D perovskite device with n=60. (b and c) J-V curve and PCEs for champion
perovskite device with differ-ent n values. (d and e) J-V curve, both forward and reverse scan and external
quantum efficiency (EQE) spectra of the certified device with n=60 perovskite.
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Figure 6. Solar cell device stability. (a) The device perfor-mance as a function of n value, which shows
increased per-formance was achieved with increased n value, however, at the meantime, stability was
decreased. (b) The evolution of device performance as a function of time with different n values. (c) Record
of Jcs, FF and Voc change for different per-ovskite device stored in Nitrogen over two months.
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