Rapid Capillary-Assisted Solution Printing of Perovskite Nanowire Arrays Enables Scalable Production of Photodetectors

Article abstract of DOI:10.1002/anie.202004912

Despite recent progress in producing perovskite nanowires (NWs) for optoelectronics, it remains challenging to solution-print an array of NWs with precisely controlled position and orientation. Herein, we report a robust capillary-assisted solution printing (CASP) strategy to rapidly access aligned and highly crystalline perovskite NW arrays. The key to the CASP approach lies in the integration of capillary-directed assembly through periodic nanochannels and solution printing through the programmably moving substrate to rapidly guide the deposition of perovskite NWs. The growth kinetics of perovskite NWs was closely examined by in situ optical microscopy. Intriguingly, the as-printed perovskite NWs array exhibit excellent optical and optoelectronic properties and can be conveniently implemented for the scalable fabrication of photodetectors.

Full text of DOI:10.1002/anie.202004912

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
International Edition Chemie
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Accepted Article
Title: Rapid Capillary-Assisted Solution Printing of Perovskite Nanowire
Arrays Endables Scalable Production of Photodectors
Authors: Zhiqun Lin, Shuang Pan, Haiyang Zou, Aurelia Wang, Zewei
Wang, Jiwoo Yu, Chuntao Lan, Qiliang Liu, Zhong Lin Wang,
Tianquan Lian, and Juan Peng
This manuscript has been accepted after peer review and appears as an
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of the final Version of Record (VoR). This work is currently citable by
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202004912
Link to VoR: https://doi.org/10.1002/anie.202004912

Angewandte Chemie International Edition
10.1002/anie.202004912
RESEARCH ARTICLE
Rapid Capillary-Assisted Solution Printing of Perovskite Nanowire Arrays
Enables Scalable Production of Photodectors
Shuang Pan, Haiyang Zou, Aurelia C. Wang, Zewei Wang, Jiwoo Yu, Chuntao Lan, Qiliang Liu, Zhong
*
*
Lin Wang, Tianquan Lian, Juan Peng, and Zhiqun Lin
Abstract: The ability to deliberately solution-print large-scale
perovskite-based nanostructures represents a significant endeavor
towards the convenient creation of a rich variety of optoelectronic
materials and devices. Despite recent impressive progress in
producing perovskite nanowires (NWs) for optoelectronics, it
remains challenging to solution-print an array of NWs with precisely
controlled position and orientation. Herein, we report a robust
capillary-assisted solution printing (CASP) strategy to rapidly access
aligned and highly crystalline perovskite NWs array. The key to the
CASP approach lies in the judicious integration of capillary-directed
assembly via periodic nanochannels and solution printing via the
programmably-moved substrate to fast guide the deposition of
perovskite NWs. The growth kinetics of perovskite NWs is closely
examined by in-situ optical microscopy to scrutinize the perovskite
crystallization. Intriguingly, as-printed perovskite NWs array exhibit
excellent optical and optoelectronic properties, and can be
conveniently implemented for scalable fabrication of photodetectors.
perovskite polycrystalline films, including doctor-blade coating,
[
12]
inkjet printing, spray-coating, and so on.
However, it remains
challenging to achieve one-dimensional perovskite nanowires (NWs)
array with exquisite control of the crystal growth, alignment, and
location via solution-printing due primarily to stochastic nucleation
nature and lack of control over the drying dynamics and the associated
[
13-16]
convective flows.
Moreover, the alignment of perovskite NWs
into ordered arrays at well-defined locations can enhance the current
output and active area in conjunction with better uniformity and
reproducibility than randomly distributed NWs. Notably, the
implementation of solution-printed perovskite one-dimensional
nanostructures may contribute to efficient light management and
impart new functionalities due to their anisotropic geometry, large
surface-to-volume ratio, and charge carrier confinement in two
[
17-19]
dimension.
For instance, perovskite NWs with high aspect ratios
[
19, 20]
can function as polarization-sensitive photodetectors.
Moreover, the use of micro- and nano-patterns to assist the solution-
printing of perovskites and facilitate the positioning of the resulting
perovskite nanostructures may render their advanced applications in
[21]
fields of photodetectors, FETs, and lasers.
Recently, several approaches based on lithographically
patterned substrate have been demonstrated to direct the perovskite
Introduction
[
22, 23]
crystal growth into different size and shape.
For example,
Over the past decade, metal halide perovskites
experienced significant progress in synthesis and fundamental
understanding of their photophysics that underpins the performance
have
microfluidic technology has been used to manipulate the fluid flow
and mass-transport perovskite precursor solutes within
[
24]
microchannels to yield perovskite structures. It is notable that the
key to the template-guided methodologies noted above is the design
and engineering of capillary force-directed fluid flow to ensure the
controlled alignment and patterning of perovskite structures.
However, the most of reported techniques for patterning perovskite
crystals invokes the use of multi-step lithography with high operation
cost, thereby limiting them from mass-production and industrial scale
[
1-3]
of perovskite-based optoelectronic devices, including solar cells,
[
4]
[5]
[6]
light-emitting diodes, photodetectors, and lasers. Perovskites
carry a set of appealing attributes such as large optical absorption
coefficient, long carrier diffusion length, and outstanding carrier
[
7-9]
mobility.
They can also be readily solution-processed or printed
[
10]
on various substrates due to their ionic crystal nature. In the latter
context, the ability to solution-print the perovskite solution enables a
large-area, cost-efficient and high throughput production of
perovskite films for optoelectronic applications. To date, several
solution-printing methods have been proven effective in depositing
[
25]
roll-to-roll fabrication. Moreover, creating lithographical patterns
rely greatly on microfabrication and the associated etching process,
which unavoidably involves the use of corrosive chemicals and
[
11]
[
26]
complex processes that may not be readily and widely accessible.
In this context, harnessing colloidal cracks formation via controlled
evaporation of the colloid aqueous suspension to one-step form
[
27, 28]
micro- or nanochannels with high regularity over a large area
may provide an environmentally friendly platform for large-scale
patterning of perovskite materials in a simple and controllable
manner. Thus, it may be highly desirable to integrate low-cost
patterning approach based on controlled evaporation with facile
solution-printing technique for structuring the perovskite crystals
growth. This, however, has yet to be explored.
Herein, we report a rapidly and highly effective capillary-
assisted solution printing (CASP) strategy to craft highly aligned,
crystalline perovskite NWs array for optoelectronics. Nonetheless,
the CASP approach integrates a nanochannel-enabled, capillary-
directed assembly with a moving substrate-facilitated printing to yield
perovskite NWs. Specifically, a liquid bridge functioned as the
reservoir is formed by placing the perovskite precursor solution (i.e.,
ink) in a restricted geometry composed of a lower patterned substrate
containing the periodic nanochannels and an upper flat featureless
plate. Subsequently, the perovskite ink quickly flow through the
nanochannels at the early stage driven by capillarity. Afterwards, the
[
]
Dr. S. Pan, Dr. H. Zou, A. Wang, Dr. Z. Wang, J. Yu, C. Lan,
Prof. Z. Wang and Prof. Z. Lin*
School of Materials Science and Engineering, Georgia
Institute of Technology, Atlanta, GA 30332, USA
E-mail: zhiqun.lin@mse.gatech.edu
Homepage: http://nanofm.mse.gatech.edu/
Dr. S. Pan and Prof. J. Peng
State Key Laboratory of Molecular Engineering of Polymers,
Department of Macromolecular Science, Fudan University,
Shanghai 200438, China
E-mail: juanpeng@fudan.edu.cn
Q. Liu and Prof. T. Lian
Department of Chemistry, Emory University, 1515 Dickey
drive, NE, Atlanta, Georgia 30322, United State
Supporting information for this article is given via a link at the
end of the document.
This article is protected by copyright. All rights reserved.
1

Angewandte Chemie International Edition
10.1002/anie.202004912
RESEARCH ARTICLE
solvent evaporation drives the nucleation and growth of the deposited
perovskite inks and converts into the NW within the nanochannels,
and more importantly propagates along the nanochannels toward the
perovskite ink reservoir. Notably, as the lower nanochannel-patterned
substrate is placed on a computer-controlled translation stage that can
be programmably moved, the receding meniscus of the perovskite ink
progressively sweep across the entire lower nanochannel-patterned
substrate, thereby resulting in the constant growth rate of perovskite
NWs and printing a continuous perovskite NWs array on the
substrate. Moreover, the growth kinetics of perovskite NWs are
scrutinized by in-situ optical microscopy, providing insights into the
capability of CASP strategy for precise control over the morphology
and crystallinity of perovskite NWs array. Upon the removal of the
sacrificial polymer colloidal nanoparticles, the optoelectronic
properties of the resulting spatially arranged perovskite NWs are
investigated. Finally, perovskite NWs array-based photodetectors are
constructed to demonstrate their utility for optoelectronic devices.
The CASP technique is rapid and scalable, and thus may open up a
new avenue for solution-printing semiconductors of interest with a
variety of desirable patterns as building blocks for use in
microelectronics, optoelectronics, microfluidic devices, etc.
meniscus, thereby triggering PS nanoparticles to transport to the
[
29]
meniscus front.
Notably, the competition between the stress
increase owing to the loss of residual water from the formed PS
nanoparticles film and the stress relaxation via nanoscale crack
opening led to the formation of PS colloidal thin film with periodic
[
30]
nanochannel-like cracks. These nanochannels are perpendicular to
[
31]
the drying front. It is noteworthy that the creation of nanochannels
pattern was rapid (i.e., 1cm wide × 0.2 cm long within 2.56 minutes;
the width of PS colloidal thin film is only determined by the width of
upper plate), environmentally friendly (water as solvent), and highly
reproducible in one-step, thereby dispensing with the need for a
multi-step lithographic technique and the use of toxic organic
solvents associated with lithography.
The surface morphology and dimension of nanochannels were
measured by optical microscope (OM) and scanning electron
microscope (SEM). The average spacing between two adjacent
nanochannels is 4.58±0.19 μm (Figures 1b-1c). A typical inverted
trapezoid shape of nanochannels is clearly evident by cross-sectional
[
30]
SEM (Figure 1d). Generally, these nanochannels have an average
depth of 0.91±0.08 μm, an upper width of 0.67±0.04 μm, and a lower
width of 0.44±0.03μm. Moreover, the cross-sectional analysis of 3D
atomic force microscopy (AFM) height image also shows the
nanochannel of a depth of approximately 900 nm (Figures 1e-f). To
Results and Discussion
directionally grow perovskite NWs, all inorganic perovskite CsPbBr
was selected because of its good stability at ambient condition. First,
the pure orthorhombic phase CsPbBr powder was produced
according to the literature as it is difficult to prepare a well-dissolved
precursor solution by simply dissolving PbBr and CsBr in DMSO or
DMF solutions due to the limited solubility of CsBr. The XRD data
of as-prepared CsPbBr powder suggested a standard orthorhombic
phase of CsPbBr (Figure S2b), and the SEM image (Figure S2c-d)
and energy dispersive X-ray spectroscopy (EDS) elemental mapping
Figure S3) confirmed that the CsPbBr powder has micrometers in
size with an atomic ratio of Cs: Pb: Br ≈ 1 : 1 : 3.
3
Figure. S1 depicts the preparative route to perovskite NWs
array by capillary-assisted solution printing (CASP) strategy. Briefly,
the meniscus-assisted solution-printing of polystyrene (PS) latex
3
nanoparticles aqueous suspension yields aligned, periodic cracks (i.e.,
2
[
32]
nanochannels separated by PS stripes) over a large area [27] ( regarded
as the first solution-printing process), as will be discussed below.
Afterwards, the formed nanochannels was utilized as the lower
substrate for subsequent CSAP of the perovskite precursor solution
containing CsBr and PbBr at 1:1 stoichiometric ratio to deposit
2
perovskite NWs within the nanochannels (regarded as the second
solution-printing), displaying yellow appearance that signifies the
3
3
(
3
[
33]
After the meniscus-assisted solution-printing to form
nanochannels on the Si substrate (referred to as nanochannel
substrate), the CASP was performed to craft large-area CsPbBr NWs
3
3
formation of CsPbBr (Figures S1b-c). Finally, the PS stripes were
selectively dissolved with toluene and the resulting perovskite NWs
array was annealed at 250C for a few mins to fully remove residual
toluene (Figure S1d).
Specifically, the PS latex nanoparticles (diameter, D = 20 nm)
were selected as nonvolatile solute to prepare nanochannels. As
shown in Figure 1a, a droplet of
array driven by nanochannel-induced capillary that directs the
printing of NWs on the moving nanochannel substrate, as illustrated
st
in Figure 2a and Figure S4a-ii (right panel). Similar to the 1 solution-
3
printing, the metal halide CsPbBr in DMSO was loaded in a two-
PS
nanoparticle
aqueous
suspension was loaded in a
restricted two-plate geometry
with an upper fixed glass blade
(upper plate) placed over a
lower flat SiO -coated Si
2
substrate (referred to Si
substrate; lower plate) mounted
on a translational stage that
programmably moved the Si
substrate. A meniscus was thus
created at the edge of the two
[
27]
plates (Figure 1a).
The
moving speed of the lower Si
substrate was set at 13 μm/s and
the substrate was heated at 65 ℃
during the course of solution
st
printing (i.e.,
1
solution-
printing). During the solution-
printing process via continuous
moving of the lower Si
substrate, the evaporative loss of
water at the three-phase contact
line was fastest at the edge of the
Figure 1. (a) Scheme of meniscus-assisted solution-printing of PS latex nanoparticle aqueous suspension. (b)
Representative optical micrograph of nanochannels during the solution-printing process. (c) SEM image of nanochannels
(
(
top view). (d) Representative SEM of nanochannels (side view). Side-view nanochannels is illustrated in a lower panel.
e) Representative 3D AFM height images and (f) the corresponding line scan of a nanochannel. The inset in (e) is the
corresponding SEM image.
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RESEARCH ARTICLE
Figure 2. (a) Schematic illustration of the formation of perovskite NWs in nanochannels (left panel) by integrating capillary-directed assembly
with the moving substrate-facilitated printing (i.e., a CASP strategy). (b) Optical micrograph of the perovskite precursor ink reservoir confined in
a two-plate geometry comprising a lower nanochannel substrate and an upper stationary plate. (c) Optical micrographs of the growth of perovskite
NWs as a function of time, where the white arrow marks the progressive growth of the NW. (d) Plot of the length of the tracked CsPbBr
3
NW
grown as a function of time. (e) Temperature and moving speed-dependent formation of CsPbBr NWs. Red dot: continuous NWs; Blue triangle:
3
discontinued NWs; Green cube: Widened NWs. Orange line: nanochannels formed by solution-printing of PS nanoparticles cannot be used at
temperature higher than 85 C due to the instability of PS stripes. (f) Representative optical image of perovskite NWs array. (g-i) Representative
AFM height images. (g) and (h) 2D and 3D AFM height images, respectively, and (i) the corresponding line scan of nanochannels filled with
3
CsPbBr NWs. The lower inset shows the schematic of nanochannels with NWs deposited (side view).
plate geometry where the lower plate used is the nanochannel
substrate as described above. The nanochannel substrate continuously
translated against the upper stationary glass blade. A representative
side-view optical micrograph of the confined perovskite precursor ink
that bridges two plates with a concave meniscus at the edge is shown
in Figure 2b. It is interesting to note that the capillary-driven flow of
the perovskite precursor solution within the nanochannels (Figure
S4b) spontaneously occurred upon the loading of perovskite
precursor ink in the two-plate geometry, thereby shaping the ink into
than that produced by lithography-based micro/nanofluidic method
(k = 0.74 μm/s), demonstrating the advantageous characteristic of
g
[
24]
rapid nanofabrication of perovskite NWs using this strategy. In sharp
contrast, when the lower nanochannel substrate remained stationary,
the deposition front was found to quickly reach the edge of the
meniscus and formed large crystals, and moreover the speed of the
deposition front approaching the perovskite precursor ink reservoir
[
37]
was seen to decrease (see Supplementary Movie S2). Thus, it is
clear that the synergy of capillary-directed assembly enabled by the
implementation of nanochannels and the moving-substrate-facilitated
printing renders the exquisite control over the growth of perovskite
NWs. Moreover, it is important to note that the perovskite NWs array
can be rapidly attained (i.e., 1cm wide × 0.2 cm long; 2.56 minutes).
As perovskite crystallization rate depends heavily on the
convective flow caused by solvent evaporation, the temperature of
nanochannel substrate and solution-printing speed represent the two
[
34]
extended liquid filament in each open nanochannel, as depicted in
Figure S4a-ii. As the nanochannel substrate was heated at 75 C, the
liquid perovskite precursor filaments resided in the nanochannels
[
35]
gradually dried to form perovskite NWs,
forming the deposition
front, and more interestingly, propagating toward to the perovskite
precursor ink reservoir as illustrated in the right panel of Figure S4a-
[
36, 37]
iii.
Notably, as the lower nanochannel substrate progressively
[
38]
and unidirectionally moved forward at a fixed speed v of 20 μm/s, in
the direction opposite to the receding meniscus, the distance between
the deposition front and the meniscus edge (i.e., the length of liquid
filaments (LFs), dLF) remained nearly constant. A real-time tracking
of the formation of perovskite NWs array using optical microscopy is
shown in Figure 2c (also see Supplementary Movie S1), displaying
key variables in the CASP process.
According to in-situ optical
microscopy observation, an uniform and continuous NW can only be
achieved when the moving speed v of the lower substrate at a certain
substrate temperature (i.e., v = 20 μm/s at 75 ℃) was comparable to
the receding velocity of the meniscus edge in the opposite direction
(~ 20 μm/s), at which the optimum distance between the deposition
front and the meniscus edge of the ink reservoir, that is, dLF, was
the 1D dimensional growth of CsPbBr
3
crystals into aligned
[
35]
continuous wire-like structures along the moving direction of the
lower nanochannel substrate. A nearly linear growth with a rate
reached (Figure 2c). When v (e.g., 13 μm/s at 75 ℃; Figure S5)
was slower than the receding velocity of the meniscus edge (~ 20
μm/s) or the substrate temperature was higher (e.g., 80 ℃), the
deposition front was overly close to the meniscus edge (i.e., reduced
g
constant k of 26.6 μm/s was observed (Figure 2d). It is worth noting
that the growth rate of NW using the CASP technique is much higher
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Angewandte Chemie International Edition
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RESEARCH ARTICLE
Figure 3. (a-b) SEM images of the perovskite NWs array after the removal of PS stripes. (c) The corresponding EDS elemental mapping of a single
CsPbBr NW. (d) Cross-sectional SEM images of the NWs array, showing inverted trapezoid shape with convex round top (upper: high-magnification;
3
lower: low-magnification). (e) Typical 2D and 3D AFM height images of NWs and the corresponding line scan. (f) Histograms of the width and height
distributions of NWs from the SEM images in (a) and (d). Representative (g) TEM image, (h) the corresponding HRTEM image and (i) SAED pattern
of the NW shown in (a). (j) XRD profile of the perovskite NWs.
d
LF), leading to the formation of the widened NWs (Figure 2e and
the NWs determined from the SEM (Figures 3a and 3d) and AFM
measurements (Figure 3e) are 0.87±0.09 μm and 0.98±0.07 μm,
respectively. Figure 3f displays the histograms of the width and
height of NWs. Figure 3g shows a typical TEM image of a single
Figure S5). On the other hand, when v (e.g., 32.5 μm/s at 75 ℃) was
faster than the meniscus receding velocity or the substrate
temperature was rather low (65 ℃; Figure S6), the liquid filament
was found to easily dewet into segmented perovskite NWs (Figure
3
CsPbBr NW with a uniform width of approximately 900 nm. The
2
e). Intriguingly, some much smaller NWs aligned along the bottom
high-resolution TEM (HRTEM) image has a lattice fringe of
approximately 0.42 nm, corresponding to the (002) planes of
corner of nanochannels on both sides were witnessed as marked with
white arrows in the inset of Figure S6c. This can be ascribed largely
to the fact that the ink solution gradually dewetted from the
nanochannel, leaving behind smaller fine NWs that wetted the corners
of nanochannels upon solvent evaporation.[39] To quantify the
dimension of perovskite NWs within nanochannels (Figure 2f), the
detailed NWs structure before the removal of PS stripes was
performed by AFM (Figures 2g-h). Clearly, the NWs formed within
the nanochannels have a convex shape, driven by the minimization of
the surface area, during the crystallization of NWs. The NWs fully
covered the nanochannel with an average 175±35 nm higher than that
of nanochannels (Figure 2i).
[
33]
orthorhombic perovskite CsPbBr
3
(Figure 3h). The selected-area
electron diffraction (SAED) pattern presents sharp diffraction spots
corresponding to the (002) and (020) zone axes, reflecting that the
[40]
NWs are of high crystallinity (Figure 3i). The crystallography of
NWs array was also confirmed by the X-ray diffraction (XRD) with
intense diffraction peaks that can all be indexed to orthorhombic
[41]
phase CsPbBr
3
with no other impurity peaks present (Figure 3j).
These results collectively corroborate that aligned and highly-
crystalline CsPbBr
strategy.
3
NWs array were attained via the rapid CASP
The optical properties of as-crafted perovskite NWs array after
the removal of PS stripes were then examined by UV-Vis,
photoluminescence (PL) and time-resolved photoluminescence
(TRPL) measurements. The UV-vis spectrum of perovskite NWs
array presents a clear absorption onset at 512 nm (Figure 4a) and the
corresponding optical bandgap can be calculated to be 2.31 eV by
fitting the Tauc plot (Figure 4b). The perovskite NWs array exhibits
a PL peak centered at 521 nm. The narrow PL peak also suggests a
lower trap density in the perovskite single-crystal NWs (Figure
Subsequently, the PS stripes were removed by toluene, leaving
3
behind aligned CsPbBr NWs array as revealed by SEM (Figure 3a).
The close-up SEM image shows that the NWs are crack-free and
smooth. The EDS element mapping analysis on the NW suggested a
homogenous spatial distribution of Cs, Pb and Br elements with a Cs:
Pb: Br ratio of ∼1:1:3, which is in good agreement with the
stoichiometry of feeding ratio (Figure S7). The XPS spectra of
3
CsPbBr NWs show the strong signals of Cs, Pb and Br elements,
[
42]
demonstrating the presence of these elements (Figures S8). The cross-
sectional SEM image of NWs array showed that the NWs have an
inverted trapezoid shape with a convex round top and well-shaped
side walls, conforming well with the dimension of the used
nanochannels. The width of NW in its broadest region and height of
4a). The charge carrier lifetime was measured by TRPL to evaluate
the carrier dynamics in the aligned perovskite NWs showing two time
1 2
components, that is, a fast (τ = 3.95 ns) and a slow (τ = 25.72 ns)
dynamics (Figure 4c). Furthermore, the carrier transport property of
the NWs array was investigated, from which the defect density of the
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Angewandte Chemie International Edition
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RESEARCH ARTICLE
the precipitation rates of PbBr
2
and CsBr varied. Taken together, the
application of nanochannels is of key importance in achieving
periodic, aligned perovskite NWs with outstanding optoelectronic
properties and pure phase (i.e., orthorhombic phase).
Moreover, it is also worth noting that the wettability of substrate
should be deliberately tailored to promote the adhesion of PS
nanoparticles-containing stripes to the Si substrate (i.e., SiO -coated
2
Si as noted above). We found that when the smaller-sized PS latex
nanoparticles with less surface carboxyl groups were used (i.e., D =
2
0 nm with 5.9 carboxyl groups per nanoparticle), less hydrophilic
SiO -coated Si substrate should be used (Figure S14). As shown in
2
Figure S14, the nanochannels fabricated on less hydrophilic Si
o
substrate with a DMSO contact angle of 70.2 exhibited an excellent
adhesion to the Si substrate, and thus retained its well-defined
geometry during the CASP of the perovskite precursor solution
Figure 4. (a) UV–vis absorption and photoluminescence (PL) spectra of
the perovskite NWs array. (b) The Tauc plot of the perovskite NWs array.
(
2
Figure S14c). In stark contrast, on more hydrophilic O -plasma-
o
treated Si substrate with a DMSO contact angle of 25.3 ,
nanochannels on such a Si substrate displayed a poor adhesion of PS
stripes and thus delaminated from the Si substrate during the CASP
of the perovskite precursor solution (Figure S14d). Interestingly,
when the larger-sized PS latex nanoparticles with more surface
carboxyl groups were employed (i.e., D = 80 nm with 5100 carboxyl
(c) Time-resolved photoluminescence (TRPL) decay spectrum of the
perovskite NWs array. (d) Current-voltage curve of the perovskite NWs
array for space charge limited current (SCLC) analysis.
sample was calculated according to the space charge limited current
theory. By fitting the I−V curves obtained under the dark condition
n
2
groups per nanoparticle), more hydrophilic O -plasma-treated Si
with the double logarithm, a linear ohmic region (I ~ V ; n = 1)
substrate can be applied. Therefore, it is clear that the surface polarity
should be tailored to match with that of PS latex nanoparticles to
ensure the adhesion of PS stripes for the subsequent solution-printing
followed by a trap-filled region (n > 3) was obtained (Figure 4d),
where the trap-filled limit (TFL) voltage (VTFL) was found to be 1.45
V. Thus, the charge-trap density ntrap calculated according to the
[
48]
2
12
−3
of NWs within nanochannels.
Notably, utilizing larger-diameter
equation ntrap = 2εε
0
VTFL/(ew ) is 1.06×10 cm , where ε is the
[
43]
PS latex nanoparticles (D = 80 nm) allowed for the production of
larger channels (i.e., microchannels) (Figure S15). Correspondingly,
dielectric constant of CsPbBr
3
(≈ 16.46),
0
ε is the vacuum
permittivity, e is the elemental charge, and w is the device channel
width. Notably, the calculated ntrap is comparable to that of the
the CsPbBr
3
NWs with a broader width of ~ 2 μm and a thickness of
~
2 μm were obtained. More interestingly, radially aligned
CsPbBr
NWs array.
We note that the key to create unidirectional, long CsPbBr
3
single crystals, revealing the low charge traps of formed
[
44]
microchannels created from freely evaporating colloidal suspension
were also exploited for capillary-directed solution printing using the
two-plate geometry (Figure S16). As illustrated in Figure S16b, well-
ordered spoke-like patterns of microchannels over a large area can be
applied to solution-print the perovskite ink, producing radially
aligned yet local parallel and continued NWs. In comparison, spin-
coating of perovskite ink on this patterned substrate only yielded
discontinued NWs. The microchannels with branches (i.e., secondary
cracks) between cracks and 2D grids were formed from freely
evaporating PS latex nanoparticles aqueous suspension at room
3
NWs relies on the judicious implementation of nanochannels to guide
the flow of perovskite precursor ink driven by capillary force. In this
context, the flat Si substrate without nanochannels pattern was
employed for comparison. It is not surprising that due to the absence
of nanochannel substrate in the restricted two-plate geometry and the
associated capillary force, only randomly arranged nonuniform NWs
were yielded (Figure S9). The resulting structure comprising
herribone-like stripes at a concentration of 0.125 M were obtained,[45]
[
49]
temperature (Figure S17).
3
suggesting that the CsPbBr crystals tend to grow uniformly along the
Finally, the solution-printed perovskite NWs array were
exploited for photodetector. A lateral device was fabricated by
thermal evaporation of Au electrodes through a shadow mask with a
channel width of 50 μm (Figure 5a). As shown in Figure 5b, an
three major crystallographic directions, which is in consistence with
crystals formed by drop-casting (Figure S10). With a further
decreased concentration of 0.0325 M, wire-like structure can be
achieved (Figure S11). However, the EDS elemental mapping
revealed that nonstoichiometric ratio of Cs: Pb: Br = 1:1.21:3.3
−10
ultralow dark current of 2.20× 10 A was found owing to the low
[
19]
intrinsic carrier concentration of perovskite NWs array.
The
(Figure S12). Specifically, Pb-rich (with Pb further concentrated at
photosensitivity of the photodetector is over three order of magnitude
higher than the dark current under 445-nm laser illumination at 27
the NW edge) crystals were witnessed by the line scan of the EDS
spectrum (Figure S13a-b). The existence of Pb-rich region was also
proved by XRD pattern, clearly showing a peak at 11.7° (Figure
−2
3
mW cm . The normalized current~time curve of the CsPbBr NWs
[
46]
shows the response time with the rise and fall times of 95 and 40 ms,
respectively (Figure S18). The photocurrent increased linearly as the
S13c) that may be assigned to the phase of CsPb
suggests that the printing of CsPbBr precursor solution without the
use of nanochannels lacks good control over the concurrent
2
Br
5
or PbBr
2
.
This
3
2
incident light intensity I increased from 0.014 to 27.03 mW/cm as
[47]
the number of photogenerated carriers is proportional to the absorbed
photon flux (Figure 5d). The transient photocurrent displays a good
on/off light-switching behavior with the on-and-off switch of the
illumination controlled by a light shutter (Figure 5c). We note that the
responsivity of a photodetector increases with the decreasing I during
the course of illumination. The responsivity of the photodetector from
2
precipitation of two precursor components (i.e., CsBr and PbBr ).
The PL emission spectrum of Pb-rich perovskite structure shows a
peak at 542 nm (Figure S13d). The TRPL study on perovskite crystals
solution-printed on the flat, featureless substrate showed the shorter
lifetime of τ = 1.83 and τ = 6.33 ns than the NWs array discussed
1 2
above. This may be due to the fact that a spreading liquid film on the
featureless substrate can easily undergo a dewetting process which
2
2
0
.19 A/W at I = 27.03 mW/cm to 5.49 A/W at I = 0.014 mW/cm
was obtained (Figure S19). As an advantage of unidirectional 1D
NWs array, the polarization-dependent light detection of
3
ruptures the wet film of CsPbBr precursors. The nano-size domains
along the NWs structure were produced during drying of the dewetted
region (i.e.,Pb-rich crystals along the NWs). Owing to different
evaporation rates along the dewetted regions and NWs drying front,
[
50, 51]
photodetector was then measured.
The incident polarized-light
was controlled using a polarizer and half-wave plate (Figure S20a).
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5

Angewandte Chemie International Edition
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RESEARCH ARTICLE
Figure S20b demonstrates the photocurrent dependence on the
polarization angle. Clearly, the photocurrent reaches the maximum
with the initial incident light polarized parallel to the direction of
aligned perovskite NWs array (i.e., at polarization angle  = 0° and
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3
60°), while the minimum photocurrent emerges at  = 90° and 270°.
[
On the basis of these results, a peak-to-valley ratio Imax/Imin of ~1.24
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[
optoelectronic devices.
[
Conclusion
[
In summary, we developed a convenient, rapid and robust
capillary-assisted solution printing (CASP) strategy that judiciously
combining a nanochannel-enabled, capillary-guided assembly with a
moving substrate-imparted printing to effectively create perovskite
NWs array with excellent optoelectronic properties for perovskite
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architectures, with high yield and reproducibility for use in NWs
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This article is protected by copyright. All rights reserved.
7

Angewandte Chemie International Edition
10.1002/anie.202004912
RESEARCH ARTICLE
RESEARCH ARTICLE
A convenient, rapid and robust capillary-assisted solution printing (CASP) strategy that combining a nanochannel-enabled,
capillary-guided assembly with a moving substrate-imparted printing realizes the creation of perovskite NWs array with
excellent optoelectronic properties for perovskite photodetector.
8
This article is protected by copyright. All rights reserved.