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Quasi two-dimensional (2D) layered organic–inorganic perovskite materials (e.g., (BA)2(MA)n−1PbnI3n+1; BA = butylamine; MA = methylamine) have recently attracted wide attention because of their superior moisture stability as compared with three-dimensional counterparts. Inevitably, hydrophobic yet insulating long-chained organic cations improve the stability at the cost of hindering charge transport, leading to the unsatisfied performance of subsequently fabricated devices. Here, we reported the
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Novel Series of Quasi-2D Ruddlesden−Popper Perovskites Based on
Short-Chained Spacer Cation for Enhanced Photodetection
Ruoting Dong,
†
Changyong Lan,
†,∥
Xiuwen Xu,
†
Xiaoguang Liang,
†,⊥
Xiaoying Hu,
∥
Dapan Li,
†,⊥
Ziyao Zhou,
†,⊥
Lei Shu,
†,‡,⊥
SenPo Yip,
†,‡,⊥
Chun Li,
∥
Sai-Wing Tsang,
†
and Johnny C. Ho*
,†,‡,§,⊥
†
Department of Materials Science and Engineering,
‡
State Key Laboratory of Millimeter Waves, and
§
Centre for Functional
Photonics, City University of Hong Kong, Kowloon 999077, Hong Kong
∥
School of Optoelectronic Science and Engineering, University of Electronic Science and Technology of China, Chengdu 610054, P.
R. China
⊥
Shenzhen Research Institute, City University of Hong Kong, Shenzhen 518057, P. R. China
*
S
Supporting Information
ABSTRACT: Quasi two-dimensional (2D) layered organic−inorganic perovskite materials (e.g., (BA)
2
(MA)
n−1
Pb
n
I
3n+1
;BA=
butylamine; MA = methylamine) have recently attracted wide attention because of their superior moisture stability as compared
with three-dimensional counterparts. Inevitably, hydrophobic yet insulating long-chained organic cations improve the stability at
the cost of hindering charge transport, leading to the unsatisfied performance of subsequently fabricated devices. Here, we
reported the synthesis of quasi-2D (iBA)
2
(MA)
n−1
Pb
n
I
3n+1
perovskites, where the relatively pure-phase (iBA)
2
PbI
4
and
(iBA)
2
MA
3
Pb
4
I
13
films can be obtained. Because of the shorter-branched chain of iBA as compared with that of its linear
equivalent (n-butylamine, BA), the resulting (iBA)
2
(MA)
n−1
Pb
n
I
3n+1
perovskites exhibit much enhanced photodetection
properties without sacrificing their excellent stability. Through hot-casting, the optimized (iBA)
2
(MA)
n−1
Pb
n
I
3n+1
perovskite films
with n = 4 give the significantly improved crystallinity, demonstrating the high responsivity of 117.09 mA/W, large on−off ratio
of 4.0 × 10
2
, and fast response speed (rise and decay time of 16 and 15 ms, respectively). These figure-of-merits are comparable
or even better than those of state-of-the-art quasi-2D perovskite-based photodetectors reported to date. Our work not only paves
a practical way for future perovskite photodetector fabrication via modulation of their intrinsic material properties but also
provides a direction for further performance enhancement of other perovskite optoelectronics.
KEYWORDS: quasi-2D, Ruddlesden−Popper perovskite, thin film, short-chained spacer, hot-cast, photodetection
■
INTRODUCTION
In recent years, three-dimensional (3D) organic−inorganic
halide perovskite materials, such as MAPbI
3
(MA = CH
3
NH
3
+
),
have attracted wide attention because of the fast development
of solar cells based on them.
1−4
Particularly, the power
conversion efficiency of these 3D hybrid halide perovskites
has been increased from 3.81 to 22.1% in just a few years.
5−8
Owing to the excellent light absorption coefficients, long charge
diffusion lengths, high carrier mobility, direct band gap, and low
rates of nonradiative charge recombination,
9, 10
organic−
inorganic halide perovskites also find extensive applications in
light-emitting diodes (LEDs),
11−13
photodetectors (PDs),
14,15
nanolasers,
16
transistors,
17
etc. However, these perov skite
materials still inevitably suffer from the inherent instability
over moisture, heat, and light, which seriously hampers their
practical utilizations.
18,19
At the same time, quasi two-dimensional (quasi-2D) layered
perovskite materials (also known as Ruddlesden−Popper, RP,
phases) have the crystal structure consisting of quasi-2D
perovskite slabs interleaved with cations,
20
in which they
generally adopt a chemical formula of L
2
A
n−1
B
n
X
3n+1
, where L is
a large size or long-chain organic cation, A is a regular cation, B
is a divalent metal cation, and X is a halide.
21−23
The variable n
is an integer, indicating the number of metal halide octahedral
Received: March 1, 2018
Accepted: May 9, 2018
Published: May 9, 2018
Research Article
www.acsami.org
Cite This: ACS Appl. Mater. Interfaces 2018, 10, 19019−19026
© 2018 American Chemical Society 19019 DOI: 10.1021/acsami.8b03517
ACS Appl. Mater. Interfaces 2018, 10, 19019−19026
layers between the two L cation layers.
24−26
For example, for
(BA)
2
(MA)
n−1
Pb
n
I
3n+1
(BA = CH
3
(CH
2
)
3
NH
3
+
,MA=
CH
3
NH
3
+
) materials, the (MA)
n−1
Pb
n
I
3n+1
layer is sandwiched
by two BA layers. These basic layers would then stack along the
c axis via van der Waals interaction, forming the bulk quasi-2D
layered perovskite material. If n becomes infinite, the material
would become MAPbI
3
, the conventional 3D perovski te
material. Lately, it has been reported that these quasi-2D
layered perovskite materials are more stable as compared with
their 3D counterparts.
27,28
Smith et al. demonstrated that
(PEA)
2
(MA)
2
Pb
3
I
10
(PEA = C
6
H
5
(CH
2
)
2
NH
3
+
) can be simply
deposited by spin-coating and subsequent high-temperature
annealing processes as high-quality quasi-2D layered perovskite
films with the good moisture stability, where the fabricated
perovskite solar cells exhibit a power conversion effi ciency of
4.73%.
29
In addition, Cao et al. also prepared a series of
(BA)
2
(MA)
n−1
Pb
n
I
3n+1
with n = 1, 2, 3, 4 and found that these
quasi-2D perovskites are surprisingly stable, in which the
correspondingly fabricated solar cells can retain their photo-
voltaic performance even after long-duration exposure in
humidity environments.
30
Thereafter, the widespread inves-
tigation on exploring the fundamental properties of quasi-2D
perovskites and their exploitations in high-performance
optoelectronic devices have been stimulated. Guo and his
group thoroughly studied the electron-phonon scattering of
atomically thin (BA)
2
(MA)
n−1
Pb
n
I
3n+1
layers,
31
while Kammin-
ga et al. systematically investigated the quantum confinement
phenomena in these low-dimensional lead iodide perovskite
hybrids.
32
Dou and his team also prepared (BA)
2
PbBr
4
flakes
with the atomic thickness and reve aled the ir thickness-
dependent photoluminescence (PL), being similar to the
graphene-like 2D materials.
33
Notably, Liang et al. fabricated
LEDs based on (PEA)
2
PbBr
4
(PEA = C
6
H
5
CH
2
CH
2
NH
3
+
),
which yield the efficient room-temperature violet electro-
luminescence at 410 nm with a narrow bandwidth.
47
All these
findings evidently indicate their great potentials as the active
materials in state-of-the-art optoelectronics.
Among many quasi-2D layered perovskite materials,
(BA)
2
(MA)
n−1
Pb
n
I
3n+1
is one of the most intensively studied
materials because of the ease of its synthesis and unique
properties. Interestingly, the fabricated fi lm of
(BA)
2
(MA)
n−1
Pb
n
I
3n+1
is shown w ith the n-dependent
orientation. When n is below 4, the film maintains a preferential
orientation of the basal plane (i.e., the sandwiched plane). For n
= 4, the film does not display any more the basal plane
orientation. In this case, because the BA layer is insulating,
being detrimental for the carrier transport, the resulting film
would yield the poor photoelectric properties.
34
Taking it into
consideration, Tsai et al. have made use of this character and
optimized to obtain (BA)
2
(MA)
3
Pb
4
I
13
-based solar cells, which
exhibit the much enhanced photovoltaic efficiency of 12.5%.
27
Inevitably, as the large BA spacer cation possesses the long
linear chain hindering the collection of charge carriers, the
corresponding carrier mobility would get degraded, particularly
in the deep layer of the perovskites, and hence significantly
undermine their optoelectronic performances. It is noted that
when the linear chain BA cation is substituted by the branched
chain iBA counterpart, the obtained film crystallinity as well as
the air stability would get improved, which can be attributed to
the more eff ective packing of branched alkane chains.
35
The
solar cells based on hot-casted (iBA)
2
(MA)
3
Pb
4
I
13
films can
then yield a respectable power conversion efficiency of
10.63%.
35
In any case, there is still very limited investigation
on the systematic synthesis and the effect of crystal orientation
on the optoelectronic properties of (iBA)
2
(MA)
n−1
Pb
n
I
3n+1
layered perovskites with varied values of n.
In this work, iso-butylamine (iBA), an isomer to linear n-
butylamine (BA), has short-branched chain and is deliberately
chosen as the spacer cation to synthesize a set of novel quasi-
2D RP perovskites. In specific, we have systematically
investigated the synthesis of (iBA)
2
(MA)
n−1
Pb
n
I
3n+1
(n =1,2,
3, and 4) layered perovskite films and found that relatively pure
phases can be obtained for n = 1 and 4. Mixed phases are
obtained for n = 2 and 3 with the main phase of (iBA)
2
PbI
4
and
(iBA)
2
MAPb
2
I
7
, respectively. All these films exhibit the
excellent stability in air, without any noticeable degradation
even after the exposure in controlled humidity environments
for 60 days. Importantly, PDs made from these layered
perovskite films are demonstrated with the impressive photo-
sensing performance. Through a hot-casting method, the 2D
perovskite with n = 4, namely, (iBA)
2
(MA)
3
Pb
4
I
13
, reveals the
improved crystallinity and the PD performance, which are
comparable with or even better than those of state-of-the-art
quasi-2D perovskite PDs reported to date. All these results not
only provide important guidelines for the future quasi-2D RP
perovskite-based device construction from the viewpoint of
tailoring intrinsic material properties via their synthesis but also
offer a direction for the further performance enhancement of
other quasi-2D perovskite optoelectronic devices.
■
EXPERIMENTAL SECTION
Perovskite Precursor Synthesis. Precursor solutions were
prepared by dissolving PbI
2
,C
4
H
9
NH
2
, HI, and CH
3
NH
3
Iata
molar ratio of n:2:2:n − 1 in dimethyl formamide (DMF). The total
Pb
2+
molar concentration is 2 M in the solutions. The solutions were
then stirred at room temperature overnight.
Device Fabrication. The fabrication of PDs started with the one-
step spin-coating method of perovskite precursor solution in a
nitrogen-filled glovebox, where the oxygen and moisture concentration
are well-controlled at the ppm level. Specifically, the glass substrates
were first ultrasonically washed by acetone and ethanol and deionized
water for 15 min in succession, followed by a mild oxygen plasma
treatment for 5 min (0.26 Torr, 30 W). After that, 50 μL of precursor
solution was spin-coated on the glass substrate at 3000 rpm for 30 s,
subsequently with a thermal annealing at 100 °C for 15 min for the full
crystallization of samples. The samples for n = 1, 2, 3, and 4 are labeled
as #1, #2, #3, and #4, respectively. The thickness of the samples was
determined by atomic force microscopy (AFM) and is found to be
665, 405, 540, and 419 nm for #1, #2, #3, and #4, respectively. In the
case of employing hot-casting during the fabrication, prior to spin-
coating, the precursor solution and substrate were preheated at 120
°C. Then, the hot-casted sample #4 has a thickness of 529 nm. Finally,
with the assistance of a shadow mask, gold (100 nm) was thermally
evaporated on the films as electrodes. The channel length of the
devices is 10 μm.
Film and Device Characterization. Surface morphologies of all
the samples were characterized with scanning electron microscopy
(SEM, Quanta FEG450) and AFM (diMultimode V, Veeco). X-ray
diffraction (XRD, D2 PHASER with Cu Kα radiation, Bruker) was
used to evaluate the crystallinity and crystal structure of the products.
UV−vis absorption spectra were recorded using a PerkinElmer model
Lambda 2S UV−vis spectrometer. The PL spectra were acquired by
iHR320 photoluminescence spectroscopy with an excitation wave-
length of 425 nm. Time-resolved photoluminescence (TRPL)
measurement was performed on a time-correlated single-photon
counting spectrometer from Edinburgh Instruments (LifeSpec II). The
electrical performance of the fabricated device was characterized with a
standard electrical probe station and an Agilent 4155C semiconductor
analyzer (Agilent Technologies, California, USA). A 532 nm laser
diode was used as the light source for the photodetection
ACS Applied Materials & Interfaces Research Article
DOI: 10.1021/acsami.8b03517
ACS Appl. Mater. Interfaces 2018, 10, 19019−19026
19020
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