Highly Oriented Low-Dimensional Tin Halide Perovskites with Enhanced Stability and Photovoltaic Performance

Article abstract of DOI:10.1021/jacs.7b01815

The low toxicity and a near-ideal choice of bandgap make tin perovskite an attractive alternative to lead perovskite in low cost solar cells. However, the development of Sn perovskite solar cells has been impeded by their extremely poor stability when exposed to oxygen. We report low-dimensional Sn perovskites that exhibit markedly enhanced air stability in comparison with their 3D counterparts. The reduced degradation under air exposure is attributed to the improved thermodynamic stability after dimensional reduction, the encapsulating organic ligands, and the compact perovskite film preventing oxygen ingress. We then explore these highly oriented low-dimensional Sn perovskite films in solar cells. The perpendicular growth of the perovskite domains between electrodes allows efficient charge carrier transport, leading to power conversion efficiencies of 5.94% without the requirement of further device structure engineering. We tracked the performance of unencapsulated devices over 100 h and found no appreciable decay in efficiency. These findings raise the prospects of pure Sn perovskites for solar cells application.

Full text of DOI:10.1021/jacs.7b01815

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Article
Highly-oriented low-dimensional tin halide perovskites
with enhanced stability and photovoltaic performance
Yuqin Liao, Hefei Liu, Wenjia Zhou, Dongwen Yang, Yuequn Shang, Zhifang
Shi, Binghan Li, Xianyuan Jiang, Lijun Zhang, Li Na Quan, Rafael Quintero-
Bermudez, Brandon R Sutherland, Qixi Mi, Edward H. Sargent, and Zhijun Ning
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 24 Apr 2017
Downloaded from http://pubs.acs.org on April 25, 2017
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Highly-oriented low-dimensional tin halide perovskites with en-
hanced stability and photovoltaic performance
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Yuqin Liao †ÈHefei Liu †ÈWenjia Zhou ÈDongwen Yang , Yuequn Shang ÈZhifang Shi È
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Binghan Li , Xianyuan Jiang , Lijun Zhang *, Li Na Quan , Rafael Quintero-Bermudez , Brandon R.
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Sutherland , Qixi Mi , Edward H. Sargent , and Zhijun Ning *
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Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences.
School of Physical Science and Technology, ShanghaiTech University, 100 Haike Road, Shanghai 201210, China
Key Laboratory of Automobile Materials of MOE, State Key Laboratory of Superhard Materials, and College of Materials
Science, Jilin University, Changchun 130012, China
4 Department of Electrical and Computer Engineering, University of Toronto, 10 King’s College Road, Toronto, Ontario
M5S 3G4, Canada
†
These authors contributed equally to this work
Supporting Information Placeholder
ABSTRACT: The low toxicity and a near-ideal choice of
bandgap make tin (Sn) perovskite an attractive alternative to lead
perovskite in low cost solar cells. However, the development of
Sn perovskite solar cells has been impeded by their extremely
poor stability when exposed to oxygen. We report low-
dimensional Sn perovskites that exhibit markedly-enhanced air
stability in comparison with their 3D counterparts. The reduced
degradation under air exposure is attributed to the improved ther-
modynamic stability after dimensional reduction, the encapsulat-
ing organic ligands, and the compact perovskite film preventing
oxygen ingress. We then explore these highly-oriented low-
dimensional Sn perovskite films in solar cells. The perpendicular
growth of the perovskite domains between electrodes allows effi-
cient charge carrier transport, leading to power conversion effi-
ciencies of 5.94% without the requirement of further device struc-
ture engineering. We tracked the performance of unencapsulated
devices over 100 hours and found no appreciable decay in effi-
ciency. These findings raise the prospects of pure Sn perovskites
for solar cells application.
addition, the bandgap tunability of some of these promising lead-
free halide perovskites enables extending the light harvesting
range deeper into the infrared (IR), an important feature for many
solar, optical communications, and light-sensing applications.
Hybrid Sn perovskites such as methylammonium tin iodide
(
MASnI ) and formamidinium tin iodide (FASnI ), have absorp-
3
3
tion onsets at longer IR wavelengths compared to their Pb-based
14,18-22
counterparts, and have been studied for solar cells
. Combin-
ing Sn and Pb, graded-structure solar cells were fabricated that
2
show a large photocurrent of 40mA/cm and PCE close to
23
21.7% . To date, solar cells based on pure Sn perovskites have
yet to realize the high-efficiency achieved in Pb-based devices.
MASnI
mately 6%
perovskite solar cells (PSCs) showed PCE of approxi-
3
18,19
, and FASnI PSCs demonstrated efficiencies up to
3
14,21,22
6
.2%
.
Most tin PSCs suffer from extremely poor stability and repro-
2+
4+
ducibility due to the instantaneous oxidation of Sn to Sn (even
in nitrogen atmosphere) and uncontrollable crystal growth. The
20
use of tin fluoride (SnF ) additives and the introduction of a tin
2
iodide-dimethyl sulfoxide (SnI -DMSO) complex during the crys-
2
tallization process relieved the oxidation and decreased the crys-
24
tallization rate . However, the films degraded in minutes when
exposed to ambient atmosphere, and encapsulation is necessary
for stability tracking in nitrogen environment. The development of
more effective strategies is therefore imperative to improve the
device stability.
INTRODUCTION
Organic-inorganic hybrid halide perovskite materials have been
extensively investigated owing to their impressive optical and
electronic attributes such as an extremely low density of defects,
excellent carrier transport, strong light harvesting capability and
luminescence intensity, as well as facile solution fabrication pro-
The adsorption of water or oxygen molecules onto perovskites
is generally regarded as the source of crystal decomposition
The separation of perovskite from moisture and oxygen via en-
2
5-29
.
1-8
cessing . As perovskite film fabrication and device structure
design have progressed, the power conversion efficiency (PCE) of
2
9
capsulation can significantly improve the device stability . Re-
cently, the inclusion of large ammonium cations, such as bu-
tylammonium (BA) and phenylethylammonium (PEA), have been
shown to block moisture ingress at the boundaries of Pb-based
perovskite nanolayers, giving rise to unprecedented high stability
solar cells based on lead perovskites has rapidly ascended from
9-13
3
.8% to 22.1% in recent years . However, lead-based perov-
skites are environmentally toxic, and are subject to international
waste disposal regulations. The impressive progress of lead per-
ovskite photovoltaics prompted the search for novel Pb-free hal-
ide perovskite materials. Perovskites based on tin (Sn), bismuth
30-37
of films and devices
. Similar to Pb perovskites, two-
dimensional (2D) Sn perovskites show much better stability than
30,31
(
Bi), and copper (Cu) metals have been intensively explored re-
three-dimensional (3D) films
. However, there is no report to
14-17
cently
. These materials show promise for reduced environ-
date demonstrating low-dimensional Sn perovskites in solar cells.
This is likely ascribed to the poor carrier transport properties of
mental toxicity, especially for wearable and disposable devices. In
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31,33,34
these films as a result of their quantum well structure
. We
(ꢇ0 ꢇꢯ ꢋ’ꢆꢆpeak sharing the same angle with the (100) peak in quasi-
hypothesized that orientated perovskite films that are vertically
grown would form a continuous carrier transport pathway free of
confinement barrier impediment, enabling improved photovoltaic
cubic FASnI is contributed by the multi-layer 2D component
3
3
0
(n>5), whose structure evolves to its 3D analog as n increases .
The emerging (ꢇ0 ꢇꢯ ꢋꢆpeak at a lower angle originates from the
37
performance .
distortion of in-plane lattice parameters (a and c) as n decreases.
As increasing amounts of PEA is incorporated into the film, the
relative change in intensity of the two peaks reflects the increas-
ing ratio of few-nanolayers (n=1-5) to multi-layer (n>5) ones.
Films with over 40% PEA exhibit peaks corresponding to the
In this work, by using appropriate ratio of PEA as encapsulat-
ing molecules, we realize highly orientated growth of low-
dimensional FASnI perovskite films on nickel oxide (NiO ) sub-
strates, leading to greatly enhanced air stability. We subsequently
fabricated planar inverted solar cells using [6,6]-phenyl-C -
butyric acid methyl ester (PCBM) as an electron transporting
layer, achieving solar cells efficiency up to 5.94%. The device
retains its performance for over 100 hours without encapsulation.
3
x
(
020) facet of (PEA) SnI , which indicates the presence of single-
2 4
61
layer 2D compounds. Small-angle XRD patterns for FASnI and
3
2
4
0% PEA sample are shown in Figure S1. The diffraction peak at
for 20% PEA film features the (010) facet of two-dimensional
o
perovskite. This peak is not observed for the film without the
addition of PEA. Such result provides direct evidence for the for-
mation of low-dimensional structure. In sum, the PEA-FA mixed
Sn perovskite films are mixtures of 2D compounds with different
layers, and the composition of these films completes the transition
through pure 3D, 2D-3D mixture, to pure 2D as the PEA ratio
increases from 0 to 100%.
RESULTS AND DISCUSSION
We grew Sn perovskite films using a one-step spin coating
method. Typically, a mixture of formamidinium iodide (FAI),
SnI , SnF , and a varying amount of phenylethylammonium io-
dide (PEAI) dissolved in a N, N-dimethylformamide (DMF) and
DMSO mixed solvent was spin cast onto a NiO substrate. This
was followed by the addition of toluene as an anti-solvent (for
details, see Experimental Details). With the addition of PEA, the
FASnI3 bulk perovskite is separated into two-dimensional
nanolayers (Fig. 1a). Layers of metal-halide octahedra are sliced
2
2
x
The change in composition with increasing PEA ratio was con-
firmed by analysis of the photoluminescence (PL) and absorbance
spectra (UV-vis), as shown in Fig. 1c. The continuous blue shift
(890 to 625 nm) of the PL peak with increasing PEA ratio demon-
strates the formation of a low-dimensional perovskite phase and a
38
along the (100) direction in the original quasi-cubic FASnI , and
decreasing thickness of the perovskite nanolayers . The PL spec-
tra was pumped from both top (perovskite/air interface) and bot-
tom surfaces (perovskite/substrate interface) (Figure S2 and Fig-
ure S3) and collected from the opposite side. When pumped from
the bottom interface, PL peaks are observed at 625 nm, 693 nm,
and 762 nm for samples with over 40% PEA. These are signatures
of 2D perovskites of 1, 2, and 3 layers respectively, similar to
3
the residual structures between every two cut layers are isolated
by bilayers of PEA. The layered perovskites have an orthorhom-
bic structure with b-axis perpendicular to the planes of the PEA
30
bilayer . The structure is given as (PEA) (FA) Sn I , where n
2
n-1
n 3n+1
represents the number of tin iodide layers in the structural unit,
which is tuned by changing the stoichiometric ratio of PEA to FA
from 0% to 100%. Mixed FA-PEA samples exhibit two peaks
near 14º in X-ray diffraction (XRD) spectra (Fig. 1b). The
33
what has been observed in 2D lead perovskites . The multiple PL
peaks observed from films with 40% and 80% PEA provides
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ꢗ
ꢓꢁꢥꢏꢆꢉꢦꢋꢆꢬꢁꢐꢎꢆꢮꢁꢥꢥꢅꢄꢆꢁꢕꢘꢁꢍꢅꢑꢆꢉꢒꢄꢐꢎꢒꢄꢎꢒꢏꢦꢁꢍꢆꢑꢧꢑꢐꢅꢏꢋꢆꢒꢓꢆꢠꢄꢒꢏꢁꢟ
ꢕꢅꢕꢐꢆꢠꢅꢊꢢꢑꢈꢆꢉꢍꢋꢆꢌꢍꢎꢅꢏꢊꢐꢁꢍꢆꢁꢥꢥꢃꢑꢐꢄꢊꢐꢁꢒꢕꢆꢒꢓꢆꢐꢎꢅꢆꢉꢇ0ꢇꢋꢆꢠꢥꢊꢕꢅꢆꢒꢓꢆꢊꢆ
ꢉꢙꢚꢔꢋ
ꢣꢎꢅꢆꢥꢊꢐꢐꢅꢄꢆꢒꢕꢅꢆꢘꢁꢑꢠꢥꢊꢧꢑꢆꢥꢒꢬꢅꢄꢆꢠꢁꢕꢎꢒꢥꢅꢆꢘꢅꢕꢑꢁꢐꢧꢝꢆꢏꢒꢄꢅꢆꢍꢒꢏꢠꢊꢍꢐꢆ
ꢊꢕꢘꢆ ꢑꢏꢒꢒꢐꢎꢅꢄꢆ ꢑꢃꢄꢓꢊꢍꢅꢆ ꢏꢒꢄꢠꢎꢒꢥꢒꢂꢧꢝꢆ ꢊꢕꢘꢆ ꢏꢒꢄꢅꢆ ꢎꢒꢏꢒꢂꢅꢕꢅꢒꢃꢑꢆ
ꢂꢄꢊꢁꢕꢆꢑꢁꢷꢅꢈꢆꢆ
ꢛ
ꢉꢀꢔꢋ
ꢺ
ꢻ
ꢌꢕ ꢖꢛꢺꢆꢉꢕꢼꢻꢋꢆꢛꢫꢆꢠꢅꢄꢒꢡꢑꢢꢁꢐꢅꢆꢍꢄꢧꢑꢐꢊꢥꢈꢆ
evidence that these films are a mixture of phases. Compared to the
spectra obtained from bottom interface excitation, the top inter-
face measurements do not show emission peaks at 625 nm or
693nm in the 40% PEA film. This suggests that small n value 2D
scanning electron microscopy (SEM) image (Fig. 3). In contrast,
film 3D FASnI films are quite rough due to uncontrolled crystal
3
growth. Smooth and dense films are beneficial for protecting the
film from oxygen infiltration, which mitigates perovskite oxida-
tion. In addition, in the 20% PEA film with PEA bilayers perpen-
dicular to substrate, the pure inorganic framework similar to 3D
36
nanolayers prefer to locate at the bottom of these films . Emer-
gence of such nonhomogeneous compositional gradients under the
higher PEA concentration may be attributed to the di� erence in
solubility of PEAI and FAI precursors in the anti-solvent we used.
Similar behavior has also been reported in Pb-based 2D perov-
31
FASnI3 is sufficiently extended between the two electrodes. This
suggests promise for carrier transport that is comparable to bulk
perovskites. All these characters make the 20% PEA films prom-
ising for fabrication of planar solar cells with high efficiency and
36
skites . The absorption spectra further confirm the formation of
low- dimensional perovskite phases. A pure FASnI sample shows
3
stability.
2
an absorption edge near 950 nm while single layer 2D (PEA) SnI4
4+
We compared the Sn content in the FASnI and 20% PEA
3
samples display a sharp excitonic absorption peak at 603 nm; and
films using X-ray photoelectron spectroscopy (XPS). To exclude
the effect of surface oxidation, samples were etched before the
measurement (Figure S6 and Table S1). Fig. 4a shows the XPS
4
0% and 80% PEA samples exhibit an additional peak at 670 nm.
The position and intensity of these absorption peaks are consistent
with the PL peaks, confirming the formation of a 2D perovskite
phase.
spectra of the Sn 3d bands of FASnI and 20% PEA films with a
3
3
0 s etching time. The two peaks deconvoluted from the measured
From XRD (Fig. 1b), the (110), (210), and (220) peaks in FAS-
2+
spectra at 486.7 eV and 487.4 eV are associated with Sn and
Sn , respectively
nI are absent from the 20% PEA sample, implying the preferen-
4+
39-41
4+
3
. The Sn amount in the 20% PEA film is
tial orientation of the crystal grains. We noted that 2D perovskites
layers can be oriented along the direction either parallel or per-
much lower than that in FASnI , implying the low-dimensional
3
2+
structure prevents Sn from oxidation during film fabrication.
3
4, 36, 37
pendicular to the surface of substrate
. To further confirm
To investigate the oxidation resistance of the films, an in-situ
UV-vis experiment in ambient environment was conducted to
monitor the evolution of absorption spectra of thin films with
time, as shown in Fig. 4b. A significant decrease of absorbance
over the whole spectrum region within a few minutes was ob-
the orientation of the 2D tin perovskite, we performed a grazing-
incidence wide-angle X-ray scattering (GIWAXS) analysis using
synchrotron radiation. The scattering patterns of 0% and 20%
PEA films are shown in Fig. 2a, b. The 0% PEA sample (Fig. 2a)
displays Debye-Scherrer rings with an isotropical intensity distri-
bution, indicating complete randomness in the orientation of the
crystal grains. By contrast, in the 20% sample (Fig. 2b), sharp and
discrete Bragg spots are observed. This indicates that crystal
served for the FASnI film, and the curve constantly shifts down-
3
ward over a duration of 30 minutes. This indicates fast degrada-
tion of the sample, consistent with a previous report that 3D hy-
brid Sn perovskite films turned transparent within minutes of air
ꢯ
grains are highly oriented, with their (ꢇ0ꢇꢋ planes parallel to the
20
exposure . By contrast, the absorption spectra of the 20% PEA
substrate surface (Fig. 2c). Films with higher PEA ratios exhibit
film did not change over a span of 30 minutes, indicating im-
proved stability over the pure FA samples. Compared with the
case of bulk 3D Sn perovskites, clearly the air stability of low-
dimensional Sn perovskite films is substantially improved.
diffraction rings with stronger intensity in the q direction (Figure
z
S4). This indicates considerable randomness in the orientation of
the crystal domains. The spots closer to the central point in the qz
direction, compared to those in the image of 20% film, feature the
preferential orientation of PEA bilayers parallel to the substrate.
We assume that the orientation growth perpendicular to the sub-
strate is thermodynamically favorable only in the 20% PEA con-
dition.
To probe the origin of the enhanced air stability of low-
dimensional Sn perovskites (PEA) (FA) SnnI3n+1, we performed
2
n-1
first-principles density functional theory (DFT) calculations of
materials thermodynamic stability with respect to possible dispro-
portionation channels. The stability of one material can be indi-
cated by the decomposition enthalpy ꢉ∆ꢽꢾꢿꣀꢋ that is defined as
The perpendicular orientation growth of perovskite facilitates
the growth of compact and smooth films, as indicated from the
3
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 4 of 8
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ꢀꢁꢂꢃꢄꢅꢆꢱꢈꢆꢉꢊꢋꢆꢫꢅꢡꢁꢍꢅꢆꢊꢄꢍꢎꢁꢐꢅꢍꢐꢃꢄꢅꢆꢊꢕꢘꢆꢍꢃꢄꢄꢅꢕꢐꢆꢘꢅꢕꢑꢁꢐꢧꢟꢡꢒꢥꢐꢊꢂꢅꢆ
ꢉ꣕ꢟꣃꢋꢆ ꢍꢎꢊꢄꢊꢍꢐꢅꢄꢁꢑꢐꢁꢍꢑꢆ ꢒꢓꢆ ꢐꢎꢅꢆ ꢎꢁꢂꢎꢅꢑꢐꢆ ꢠꢅꢄꢓꢒꢄꢏꢊꢕꢍꢅꢆ ꢘꢅꢡꢁꢍꢅꢈꢆ
ꢉꢦꢋꢚ꣖ꢚꢆꢑꢠꢅꢍꢐꢄꢃꢏꢆꢊꢕꢘꢆꢚ꣖ꢚꢟꢦꢊꢑꢅꢘꢆꢁꢕꢐꢅꢂꢄꢊꢐꢅꢘꢆ ꢑꢍꢆꢒꢓꢆꢐꢎꢅꢆꢎꢁꢂꢎꢅꢑꢐꢆ
ꢠꢅꢄꢓꢒꢄꢏꢊꢕꢍꢅꢆꢘꢅꢡꢁꢍꢅꢈꢆꢉꢍꢋꢆꢌꢐꢊꢐꢁꢑꢐꢁꢍꢑꢆꢒꢓꢆꢐꢎꢅꢆꢙ꣗ꢚꢆꢘꢁꢑꢐꢄꢁꢦꢃꢐꢁꢒꢕꢈꢆꢵꢄꢟ
ꢊꢕꢂꢅꢆ ꢑꢒꢥꢁꢘꢆ ꢥꢁꢕꢅꢆ ꢘꢅꢕꢒꢐꢅꢑꢆ ꢐꢎꢅꢆ ꢸꢊꢃꢑꢑꢁꢊꢕꢆ ꢘꢁꢑꢐꢄꢁꢦꢃꢐꢁꢒꢕꢆ ꢓꢁꢐꢐꢁꢕꢂꢈꢆ ꢉꢘꢋꢆ
ꢀ
ꢁꢂꢃꢄꢅꢆꢜꢈꢆꢉꢊꢋꢆꢩꢙꢌꢆꢌꢕꢆꢗꢘꢆꢑꢠꢅꢍꢐꢄꢊꢆꢒꢓꢆꢓꢁꢥꢏꢑꢆꢬꢁꢐꢎꢒꢃꢐꢆꢙꢚꢔꢆꢊꢕꢘꢆꢬꢁꢐꢎꢆ
ꢛ0ꢤꢆ ꢙꢚꢔꢆ ꢉꢒꢄꢊꢕꢂꢅꢋꢈꢆ ꢣꢎꢅꢆ ꢐꢬꢒꢆ ꢠꢅꢊꢢꢑꢆ ꢘꢅꢍꢒꢕꢡꢒꢥꢃꢐꢅꢘꢆ ꢓꢄꢒꢏꢆ ꢐꢎꢅꢆ
ꢏꢅꢊꢑꢃꢄꢅꢘꢆꢑꢠꢅꢍꢐꢄꢊꢆꢊꢐꢆꢜꢺꣁꢈꣂꢅꣃꢆꢊꢕꢘꢆꢜꢺꣂꢈꢜꢅꣃꢆꢊꢄꢅꢆꢊꢑꢑꢒꢍꢁꢊꢐꢅꢘꢆꢬꢁꢐꢎꢆ
꣕
ꢛ꣄
ꢜ꣄
ꢌꢕ ꢆꢉꢂꢄꢅꢅꢕꢋꢆꢊꢕꢘꢆꢌꢕ ꢆꢉꢠꢃꢄꢠꢥꢅꢋꢝꢆꢄꢅꢑꢠꢅꢍꢐꢁꢡꢅꢥꢧꢈꢆꢣꢎꢅꢆꢅꢐꢍꢎꢁꢕꢂꢆꢐꢁꢏꢅꢆ
ꢑꢆ ꢗ0ꢆ ꢑꢈꢆ ꢉꢦꢋꢆ ꢚꢡꢒꢥꢃꢐꢁꢒꢕꢆ ꢒꢓꢆ ꢐꢎꢅꢆ ꢐꢄꢊꢕꢑꢏꢁꢐꢐꢊꢕꢍꢅꢆ ꢑꢠꢅꢍꢐꢄꢊꢆ ꢒꢓꢆ ꢓꢁꢥꢏꢑꢆ
ꢬꢁꢐꢎꢒꢃꢐꢆ ꢙꢚꢔꢆ ꢊꢕꢘꢆ ꢬꢁꢐꢎꢆ ꢛ0ꢤꢆꢙꢚꢔꢆ ꢅꢞꢠꢒꢑꢅꢘꢆ ꢐꢒꢆ ꢊꢏꢦꢁꢅꢕꢐꢆ ꢊꢐꢏꢒꢑꢟ
ꢠꢎꢅꢄꢅꢆ ꢁꢕꢆ ꢗ0ꢆ ꢏꢁꢕꢃꢐꢅꢑꢈꢆ ꢉꢍꢋꢆ ꢌꢐꢊꢦꢁꢥꢁꢐꢧꢆ ꢒꢓꢆ ꢉꢙꢚꢔꢋ ꢉꢀꢔꢋ
ꢕꢘꢁꢍꢊꢐꢅꢘꢆꢦꢧꢆꢐꢎꢅꢆꢍꢊꢥꢍꢃꢥꢊꢐꢅꢘꢆꢘꢅꢍꢒꢏꢠꢒꢑꢁꢐꢁꢒꢕꢆꢅꢕꢐꢎꢊꢥꢠꢧꢆ∆ꢽꢾꢿꣀꢆꢉꢁꢕꢆ
ꢅꣃꢆꢠꢅꢄꢆꢌꢕꢆꢊꢐꢒꢏꢋꢈꢆꢣꢎꢅꢆꢄꢅꢑꢃꢥꢐꢑꢆꢍꢒꢄꢄꢅꢑꢠꢒꢕꢘꢁꢕꢂꢆꢐꢒꢆꢐꢎꢅꢆꢘꢅꢍꢒꢏꢠꢒꢑꢁꢟ
ꢁ
ꢶ
ꢒꢄꢏꢊꢥꢁꢷꢅꢘꢆꢙ꣗ꢚꢆꢒꢓꢆꢐꢎꢅꢆꢃꢕꢅꢕꢍꢊꢠꢑꢃꢥꢊꢐꢅꢘꢆꢘꢅꢡꢁꢍꢅꢆꢦꢊꢑꢅꢘꢆꢒꢕꢆꢀꢔꢌꢟ
ꢕꢖ ꢆꢊꢕꢘꢆꢛ0ꢤꢆꢙꢚꢔꢟꢘꢒꢠꢅꢘꢆꢠꢅꢄꢒꢡꢑꢢꢁꢐꢅꢆꢓꢁꢥꢏꢆꢑꢐꢒꢄꢅꢘꢆꢁꢕꢆꢊꢆꢂꢥꢒꢡꢅꢦꢒꢞꢆ
ꢗ
ꢛ
ꢕ
ꢟꢇꢌꢕ
ꢕ
ꢖ
ꢗ
ꢕ
꣄ꢇ
ꢆ
ꢓꢒꢄꢆꢒꢡꢅꢄꢆꢇ00ꢆꢎꢒꢃꢄꢑꢈꢆ
ꢁ
tion resistance. Additionally, the encapsulating organic PEA lig-
ꢛ꣄
ꢐꢁꢒꢕꢆꢠꢊꢐꢎꢬꢊꢧꢑꢆꢁꢕꢡꢒꢥꢡꢁꢕꢂꢆꢏꢅꢄꢅꢥꢧꢆꢐꢎꢅꢆꢌꢕ ꢆꢍꢒꢏꢠꢒꢃꢕꢘꢑꢆꢊꢕꢘꢆꢐꢎꢅꢆ
ꢌꢕꢜ꣄ꢆ ꢍꢒꢏꢠꢒꢃꢕꢘꢑꢆ ꢊꢄꢅꢆ ꢑꢎꢒꢬꢕꢆ ꢬꢁꢐꢎꢆ ꢄꢅꢘꢆ ꢊꢕꢘꢆ ꢦꢥꢃꢅꢆ ꢥꢁꢕꢅꢑꢝꢆ ꢄꢅꢑꢠꢅꢍꢟ
ꢐꢁꢡꢅꢥꢧꢈꢆꢫꢁꢓꢓꢅꢄꢅꢕꢐꢆꢒꢄꢁꢅꢕꢐꢊꢐꢁꢒꢕꢑꢆꢒꢓꢆꢀꢔꢆꢏꢒꢥꢅꢍꢃꢥꢅꢑꢆꢊꢥꢒꢕꢂꢆꢉ00ꢇꢋꢆꢉꢍꢁꢄꢟ
ꢍꢥꢅꢑꢋꢆ ꢊꢕꢘꢆ ꢉꢇꢇꢇꢋꢆ ꢉꢑꢭꢃꢊꢄꢅꢑꢋꢆ ꢘꢁꢄꢅꢍꢐꢁꢒꢕꢑꢆ ꢒꢓꢆ ꢐꢎꢅꢆ ꢗꢫꢆ ꢭꢃꢊꢑꢁꢟꢍꢃꢦꢁꢍꢆ
ꢥꢊꢐꢐꢁꢍꢅꢆ ꢊꢄꢅꢆ ꢍꢒꢕꢑꢁꢘꢅꢄꢅꢘꢝꢆ ꢂꢁꢡꢁꢕꢂꢆ ꢭꢃꢊꢥꢁꢐꢊꢐꢁꢡꢅꢥꢧꢆ ꢍꢒꢕꢑꢁꢑꢐꢅꢕꢐꢆ ꢄꢅꢑꢃꢥꢐꢑꢈꢆ
ꢣꢎꢅꢆ ꢘꢅꢍꢄꢅꢊꢑꢁꢕꢂꢆ ꢐꢄꢅꢕꢘꢆ ꢒꢓꢆ∆ꢽꢾꢿꣀꢆꢓꢒꢄꢆ ꢦꢒꢐꢎꢆ ꢘꢅꢍꢒꢏꢠꢒꢑꢁꢐꢁꢒꢕꢆ ꢠꢊꢐꢎꢟ
ꢬꢊꢧꢑꢆꢊꢑꢆꢕꢆꢘꢅꢍꢄꢅꢊꢑꢅꢑꢆꢁꢕꢘꢁꢍꢊꢐꢅꢑꢆꢐꢎꢅꢆꢅꢕꢎꢊꢕꢍꢅꢘꢆꢑꢐꢊꢦꢁꢥꢁꢐꢧꢆꢒꢓꢆꢛꢫꢆꢌꢕꢆ
ꢦꢊꢑꢅꢘꢆꢠꢅꢄꢒꢡꢑꢢꢁꢐꢅꢑꢆꢬꢁꢐꢎꢆꢁꢕꢍꢄꢅꢊꢑꢁꢕꢂꢆꢙꢚꢔꢆꢍꢒꢕꢐꢅꢕꢐꢝꢆꢬꢎꢁꢍꢎꢆꢊꢂꢄꢅꢅꢑꢆ
ꢬꢁꢐꢎꢆꢅꢞꢠꢅꢄꢁꢏꢅꢕꢐꢊꢥꢆꢒꢦꢑꢅꢄꢡꢊꢐꢁꢒꢕꢈꢆꢆ
2+
ands and compact perovskite film also protect Sn from contact
with oxygen, further augmenting the oxidation resistance of 2D
(
PEA) (FA) Sn I
.
2
n-1
n 3n+1
We further investigated the carrier transport properties of 20%
PEA films. The carrier concentration of the 20% PEA film was
measured by capacitance–voltage (C–V) studies (Figure S8, see
details in Supporting Information). The C-V curve demonstrates
that 20% PEA films are p-type semiconductors, which is ascribed
42
to a self-doping effect . The carrier concentration is calculated to
17
-3
14
the free energy difference between the target material to be evalu-
ated and the possible decomposed products. Negative value of
be 2.27×10 cm , remarkably lower than pure FASnI . The
3
underlying mechanism is the 2D PEA-containing perovskites with
enhanced thermodynamic stability suppressing the formation of
∆
ꢽꢾꢿꣀ corresponds to thermodynamically stable condition. Two
2+
4+
decomposition pathways are considered: one involves only Sn
Sn related defects that invokes self-doping process. This is sup-
4+
compounds,
ported by the low Sn content in the 2D PEA-containing perov-
skite film as indicated by XPS measurement.
ꣅ
꣆꣇ ꣉꣇ ꣍꣎ ꣏
→ ꢛꣅ꣆꣇꣏ ꣄ ꢉ꣎ ꣒ ꢇꢋ꣉꣇꣏ ꣄ ꣎꣍꣎꣏ ꢆꢉꢇꢋ,
꣈
꣊꣋꣌
꣊ ꣐꣊꣑꣌
꣈
4+
Space charge limited current (SCLC) studies were conducted to
measure the carrier mobility of the perovskite films (see Support-
ing Information). The electron mobility was smaller than the hole
mobility (Figure S9). To estimate the carrier lifetime, we per-
formed fluorescence decay measurements using time-resolved
fluorescence (TRF) spectroscopy by depositing 20% PEA perov-
and the other involves only Sn compounds to take into account
the oxidation process,
꣊
ꣅ
꣊
꣈
꣆꣇ ꣉꣇ ꣍꣎ ꣏
→ ꢛꣅ꣆꣇꣏ ꣄ ꢉ꣎ ꣒ ꢇꢋ꣉꣇꣏ ꣄ ꣈ ꣍꣎꣏ ꣄
꣈
꣊꣋꣌
꣊
꣊ ꣐꣊꣑꣌
꣓
꣍
꣎꣔ ꣒ ꣔꣈ꢆꢉꢛꢋ.
꣈
꣈
The calculated ∆ꢽꢾꢿꣀ as a function of n for (PEA) (FA) Sn I
is shown in Fig. 4c and Table S2. Since the small FA molecules
are randomly orientated at room-temperature because of the low
rotation barrier, we considered the (PEA) (FA) Sn I
with two different orientations of FA, i.e., along (001) and (111)
directions of the 3D quasi-cubic lattice. Two cases give qualita-
2
n-1
n 3n+1
skite layers onto ITO, NiO (hole quenching layer), and PCBM
x
(electron quenching layer) (Figure S11). The carrier lifetime of
the films on ITO is estimated to be 6.3 ns. In presence of the
structure
2
n-1
n 3n+1
quenching layer (such as NiO ), one of the carriers (hole for Ni-
x
O ) in active (PEA) (FA) Sn I layer can be quickly extracted
x
2
n-1
n 3n+1
by the quenching layer. This leads to decrease in the lifetime of
this carrier, which is indicated by the reduced PL decay lifetime.
Based on the PL decay measurement for the perovskite films on
tively consistent results. One observes that the (PEA) (FA)
2
n-
Sn I
perovskites are stable with respect to the decomposition
1
n 3n+1
pathway (1) (with negative ∆ꢽꢾꢿꣀ), but unstable with respect to
the decomposition pathway (2) (with positive ∆ꢽꢾꢿꣀ). This is in
agreement with the experimentally observed tendency of the Sn
NiO and PCBM (Figure S11), the deduced carriers lifetime are
x
4.97 ns and 3.12 ns for hole and electron, respectively. The re-
duced carriers lifetime indicate efficient carrier transport in the
23
based perovskites being prone to oxidation in air atmosphere . As
(
PEA) (FA) Sn I
films.
2
n-1
n 3n+1
n decreases, the ∆ꢽꢾꢿꣀ with respect to the oxidation decomposi-
tion pathway (2) shows a substantial decrease. This suggests that
the instability against the oxidation disproportionation process is
suppressed. It is therefore the improved thermodynamic stability
of 2D Sn perovskites that is responsible for their superior oxide-
We explored 20% PEA tin perovskite films as light absorber
for solar cells with an inverted planar architecture of
ITO/NiO /Perovskite/PCBM/Al (Fig. 5a). Here, NiO and PCBM
x
x
serve as the hole and electron transport layers, respectively. The
fabrication details can be found in Experimental Details. Accord-
4
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ing to the cross-sectional SEM image of the device (Figure S12),
structed pure Sn perovskite solar cells with a high efficiency of
5.94% and enhanced stability sustained over 100 hours. The strat-
egies used in this work to fabricate highly oriented low-
dimensional films are promising for the development of high per-
formance stable perovskite solar cells and other optoelectronic
devices, especially those based on tin perovskites. We anticipate
that, with effective encapsulation, the long-term stability of Sn
perovskite solar cells can be realized.
1
2
3
4
5
6
7
8
9
the thicknesses of the perovskite, NiO , and PCBM layers are
x
approximately 200 nm, 50 nm, and 100 nm, respectively. Mean-
while, we systematically studied the impact of SnF concentration
2
on films morphology and device performance. High density of
pinholes were found in the films with too much or too little SnF2.
When 10% SnF2 is included, compact and smooth films are
formed, which yields the highest efficiency (Figure S13). We
tested the device performance under simulated AM1.5G irradia-
tion. The highest PCE reaches 5.94% (Fig. 5a), with an open cir-
cuit voltage (V ) of 0.59 V, a short-circuit current density (J ) of
EXPERIMENTAL DETAILS
Materials were obtained as follows: Triton X-100 (Sig-
ma/VETEC, RG), deionized water (purite dispenser, >18 MΩ),
oc
sc
2
1
4.44 mA/cm , and fill factor (FF) of 69%. The Voc is much high-
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
er than that of the best previously-reported 3D FASnI3 PSCs,
14,21,22
which vary from 0.262 V to 0.465 V
. The enhanced Voc can
ethylene
glycol
(Sigma-Aldrich,
99.5%),
N,
N-
be ascribed to the suppressed defect concentration induced by
oxidation, as a result of the low-dimensional structure. The high
FF value can be ascribed to suppressed defect-mediated carrier
dimethylformamide (DMF) (Sigma-Aldrich, anhydrous, 99.8%),
dimethyl sulfoxide (DMSO) (Sigma-Aldrich, anhydrous, 99.9%),
tin fluoride(SnF ) (Macklin, 99.9%), toluene (Sigma-Aldrich,
2
4
3-46
recombination, as well as balanced carrier transport
. The rela-
HPLC, 99.9%), chlorobenzene (Sigma-Aldrich, anhydrous,
99.8%), PCBM (Nano-C, 99%). Other chemicals were purchased
from Sinopharm chemical reagent Co. Ltd.
47
tively low Jsc is likely related to the low carrier mobility, as well
as charge recombination at the interface between the active layer
and carrier transport layers. The external quantum efficiency
Tin Iodide (SnI ). SnI2 was prepared according to literature .
2
(EQE) spectrum (Fig. 5b) shows broad light harvesting, ranging
Formamidinium Iodide (CH(NH ) I, FAI). 9.3ml of hydriodic acid
2
2
out to 900 nm, and is consistent with the absorption spectrum. The
was added to 50ml round-bottom flask which contained 5.2 mg of
formamidine acetate and then was stirred for 20 min at room tem-
perature. The white precipitate was recovered by evaporating the
reaction mixture at 90 °C for 10 min. The product was dissolved
in ethanol and recrystallized by ethyl acetate. After recrystallized
twice, the resulting FAI was collected by filtration and dried at 60
2
integrated Jsc calculated from the EQE spectrum (14.57 mA/cm )
agrees well with the value measured from the J-V curve. Only
slight hysteresis is observed for these devices, as shown in the J-V
curves with voltage scanned in both reverse and forward direc-
tions (Figure S14). These devices exhibit high reproducibility.
From a sample set of 30 devices, the photovoltaic figures of merit
°
C overnight in a vacuum oven. Phenylethylammonium Iodide
PEAI). PEAI was synthesized according to literature . PEAI was
2
±
one standard deviation are: J = 14.18 ± 1.29 mA/cm , V =
35
sc
oc
(
0.583 ± 0.009 V, FF = 62.14 ± 4.77%, and PCE = 5.05 ± 0.56%
Fig. 5c).
prepared by modifying this detailed procedure for the synthesis of
FAI by replacing formamidine acetate with phenethylamine and
dissolving in ethanol before adding hydriodic acid. The reaction
was proceeded at 0°C. Diethyl ether was chosen for recrystalliza-
tion.
(
We tracked the device performance in the N atmosphere glove-
2
box without encapsulation. The PSCs based on the 3D FASnI3
were fabricated for comparison (Figure S17 and Figure 5d). As
clearly seen, while the efficiency of the 3D FASnI PSC decays to
3
NiO Film Fabrication. NiO precursor solution was prepared
x
x
2
2
3% of its original value within 48 hours, the efficiency of the 2D
0% PEA-containing perovskites PSC retains 96% of its initial
48
according to literature . Nickel (II) nitrate hexahydrate
Ni(NO ) ·6H O) and ethylenediamine were dissolved in ethylene
(
3
2
2
value for over 100 hours (Figure 5d). Possible explanation for the
glycol solution in 1:1 molar ratio and stirred at 70 °C for 3 h in oil
bath to attain a 1 M precursor solution. The solution was spin-
coated onto an ITO substrate at 2,000 rpm for 90 s and then baked
fast degradation of the 3D FASnI PSC could be that a certain
3
amount of oxygen and moisture exists in the glovebox filled with
N , therefore the fast oxidation process still occurs to FASnI .
2
3
at 300 °C for 1 h in a muffle furnace. All the NiO substrates were
x
This results ambiguously demonstrate that the dimensional reduc-
tion from the 3D FASnI to the 2D (PEA) (FA) Sn I signifi-
treated in an UVO machine for 9 min and transferred into the
glovebox immediately for perovskite film fabrication.
3
2
n-1
n 3n+1
cantly improves materials stability and efficiently suppresses oxi-
dation disproportionation, resulting in stable device performance.
Perovskite Film Fabrication. SnI , FAI, PEAI, and SnF were
2 2
dissolved in mixed solvent (DMF: DMSO=4:1) in the molar ratio
of 1: (1-x): x: 0.1 (0≤x≤1) and stirred at 70 °C for 1 h to yield a
0.6 M precursor solution. For the film fabrication, the resulting
solution was filtered with a 0.22µm PTFE filter and coated onto
the substrate via a one-step spin coating process, 1,000 rpm and
CONCLUSION
In summary, we fabricated low-dimensional tin halide perov-
skite thin films by utilizing PEA as an organic separating interlay-
er. The orientation of the perovskite domains can be modified
through manipulating the PEA ratio, and a highly-oriented perov-
skite film perpendicular to the substrate was realized with 20%
PEA. The low-dimensional tin perovskites exhibit significantly
enhanced stability in air atmosphere compared to their three-
dimensional counterparts. First principles calculations indicate the
higher intrinsic thermodynamic stability of low-dimensional tin
perovskites with respect to the oxidation disproportionation chan-
nel. The existence of large PEA molecules at the boundary of
perovskite nanolayers, and the compact pinhole-free films
achieved by manipulating the film composition, can block oxygen
diffusion into the perovskite lattice. These factors jointly contrib-
ute to the improved oxidation resistance, resulting in improved
materials air stability. In addition to the enhanced stability, the
perpendicular growth of the highly-oriented film enabled im-
proved carrier transport. Based on these advancements, we con-
5
,000 rpm for 10 s and 30 s, respectively. During the second pro-
cess, 800µl toluene was dropped in the middle of the substrate.
The substrate was annealed at 100 °C for 30 min. All the process-
es were performed in a glovebox (O ≤ 1 ppm, H O ≤ 0.1 ppm).
2 2
Device Fabrication. Indium tin oxide (ITO) coated glasses were
held in a slides holder and cleaned by ultrasonication in a deion-
ized water bath with 4% Triton X-100 by volume, followed by an
isopropanol bath and finally a deionized water bath. Each step
takes 30 min. The cleaned ITO glasses were treated by oxygen
plasma for 5 min before depositing the NiO layer. All the pre-
x
pared NiO films were transferred into the glovebox immediately
x
for the following fabrication processes. After the fabrication of
perovskite layer, PCBM was spin cast at 2,000 rpm for 60s from a
10 mg/ml chlorobenzene solution, followed by annealing at 80°C
for 5 min. Finally, Al (100 nm) was deposited through a mask via
2
thermal evaporation at a rate of 0.2-0.8 Å/s to produce 0.05 cm
5
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Page 6 of 8
2
pixels. The large area (0.13 cm ) device is also fabricated, show-
ing the PCE deviation of less than 0.5%. Device Characterization
can be found in Supporting Information.
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9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
TOC
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
8
ACS Paragon Plus Environment