Morphology-Controlled Synthesis of Organometal Halide Perovskite Inverse Opals

Article abstract of DOI:10.1002/anie.201506367

The booming development of organometal halide perovskites in recent years has prompted the exploration of morphology-control strategies to improve their performance in photovoltaic, photonic, and optoelectronic applications. However, the preparation of organometal halide perovskites with high hierarchical architecture is still highly challenging and a general morphology-control method for various organometal halide perovskites has not been achieved. A mild and scalable method to prepare organometal halide perovskites in inverse opal morphology is presented that uses a polystyrene-based artificial opal as hard template. Our method is flexible and compatible with different halides and organic ammonium compositions. Thus, the perovskite inverse opal maintains the advantage of straightforward structure and band gap engineering. Furthermore, optoelectronic investigations reveal that morphology exerted influence on the conducting nature of organometal halide perovskites.

Full text of DOI:10.1002/anie.201506367

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Angewandte
Communications
International Edition: DOI: 10.1002/anie.201506367
German Edition: DOI: 10.1002/ange.201506367
Perovskites
Morphology-Controlled Synthesis of Organometal Halide Perovskite
Inverse Opals
Kun Chen and Harun Tüysüz*
[
11]
Abstract: The booming development of organometal halide
perovskites in recent years has prompted the exploration of
morphology-control strategies to improve their performance in
photovoltaic, photonic, and optoelectronic applications. How-
ever, the preparation of organometal halide perovskites with
high hierarchical architecture is still highly challenging and
a general morphology-control method for various organo-
metal halide perovskites has not been achieved. A mild and
scalable method to prepare organometal halide perovskites in
inverse opal morphology is presented that uses a polystyrene-
based artificial opal as hard template. Our method is flexible
and compatible with different halides and organic ammonium
compositions. Thus, the perovskite inverse opal maintains the
advantage of straightforward structure and band gap engineer-
ing. Furthermore, optoelectronic investigations reveal that
morphology exerted influence on the conducting nature of
organometal halide perovskites.
have been reported, OHPs with high hierarchical architec-
ture prepared by a universal and scalable method has yet to be
addressed.
Inverse opals, the replicas of opals, consist of an ordered
array of regular interspace surrounded by solid materials
accompanied by a uniform alternation of low and high
refractive index area. This kind of material can be fabricated
by colloidal crystal templating using well-defined particles as
hard templates. Subsequent to assembly and infiltration of the
voids of the three-dimensional(3D) template, drying and
template removal steps, inverse opal structure can be
obtained. The inverse opal structure provides several benefits
for different applications. On one hand, the porous structure
of the inverse opal provides a larger surface area than the bulk
counterpart. On the other hand, the periodicity of the
refractive index affects the propagation of electromagnetic
waves in the material and results in a photonic band gap
[
12]
owing to the Bragg reflections on the lattice.
Thus, the
O
rganometal halide perovskites (OHP) have attracted
unique structure of the inverse opal increases the active sites
and optical path as well as enhances the light scattering and
intense attention in the field of photovoltaics, with continuous
progress on improvements to their solar-to-electric power
[13]
absorption.
Utilization of the inverse opal structure has
[
1]
[14]
conversion efficiency. The low cost and facile solution
processing of this hybrid semiconductor has opened up bright
prospects for clean and sustainable energy production using
OHPs. Meanwhile, the inherent advantages of OHPs, such as
band gap engineering capabilities, high fluorescence quan-
tum yield, long carrier lifetimes, and low non-radiative
recombination rates, make it promising for applications in
optical and optoelectronic fields. Investigations on the
properties of OHP demonstrated that the morphology and
crystallinity play a central role in the device performance.
For instance, the crystallinity and grain size have an effect on
the carrier mobility and thus the performance of the solar
cell. Consequently, exploration of novel morphologies and
the development of a universal method for structure and
morphology control will pave the way not only for improving
the OHP performance in the aforementioned applications,
but equally for exploring unprecedented applications. How-
ever, the low thermal stability of OHPs arising from its
organic composition, its rapid reaction between the lead(II)
halide and ammonium salts, and the restructuring in the
presence of trace water,
displayed enhanced performance in solar cells, photocatal-
[15]
[16]
[17]
ysis, and sensors and energy storage. Moreover, the
inverse opal morphology may also increase the modulation
[18]
speeds of semiconductor laser and enhance the light out-
coupling in OLED device. Fabricating OHP in inverse opal
[
2]
[19]
[
3]
morphology may improve the performance of OHP material
in solar cells, lasers, and OLEDs.
[
4]
[
5]
Herein, we report a method for preparing OHP materials
in the form of inverse opals using a colloidal crystal
templating route (Scheme 1; for experimental details, see
the Supporting Information). Briefly, polystyrene (PS) micro-
spheres were centrifuged to fabricate the artificial opal
template. After drying, the template was immersed into
a dimethyl sulfoxide (DMSO) solution containing a 1:1
stoichiometric ratio between CH NH Br and PbBr . The
[6]
[
7]
3
3
2
[8]
[
9]
[10]
make control of the OHP
morphology highly challenging. Although a few examples
[
*] Dr. K. Chen, Dr. H. Tüysüz
Max-Planck-Institut für Kohlenforschung
Kaiser-Wilhelm-Platz 1, 45470 Mülheim an der Ruhr (Germany)
E-mail: tueysuez@kofo.mpg.de
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201506367.
Scheme 1. Preparation route of OHP inverse opals.
1
3806
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DMSO was evaporated under vacuum and the obtained light-
orange color composition indicated formation of
CH NH PbBr . Finally, the polystyrene was selectively
indicating the formation of OHP. After removing the
template, the perovskite inverse opal replicate with an
ordered structure was obtained (Figure 1b–d). The cross-
section images confirmed the topological interconnection of
the inner voids (Figure 1e,f). Moreover, the morphology
directly correlated with the nature of the artificial template.
As the diameter of the microsphere decreased to 375 nm, the
void sizes and the wall thickness also decrease. In the
infiltration process, the different concentrations of perovskite
precursor (0.5, 0.9, 1.4, and 1.8m) in DMSO solution were
screened to optimize the templating process. An inverse opal
structure could not be obtained using the low concentration of
0.5m, while higher concentrations of 1.4 and 1.8m increased
the fraction of non-structured solid. The best inverse opal
structure was obtained with the solution containing 0.9m
3
3
3
removed with toluene and the perovskite inverse opal was
collected by centrifugation. The method is very flexible and
can be applied to the other perovskite inverse opals, for
example the methylammonium (MA) cation can be replaced
by formamidine (FA) or cesium (Cs) while the bromide anion
can be replaced by other single halide anions or halide
mixture. As a result, the method maintains the straightfor-
ward band gap engineering properties of the OHP. Moreover,
the halide perovskite opal shows a sensitive and stable
photoresponse, which demonstrates its potential for applica-
tions in photovoltaic and optoelectronic devices.
The assembly of the colloidal particles and formation of
the artificial opal template was accomplished with centrifu-
PbBr and MABr (Supporting Information, Figure S3). The
2
[
20]
gation using PS microspheres with different diameters (515,
75, and 125 nm). As seen in Figure 1a (see the Supporting
colloidal crystal templating process for obtaining the perov-
skite structure has fast infiltration and crystallization steps,
which probably results in polycrystalline structures since the
construction of single crystalline materials needs much longer
3
Information, Figure S1 at lower magnification for larger
area), typical scanning electron microscopy (SEM) images
of assembled PS spheres show 3D ordered structure and the
[22]
drying time. To check the polycrystallinity, the material was
further investigated with TEM, where the well-organized
inverse opal structure could be observed at lower magnifica-
tion (Supporting Information, Figure S4). However, using
higher magnification to determinate the crystallinity caused
beam damage and the inverse opal structure was partly
destroyed. The 3D porous structure increased the sensitivity
of the perovskite structure to decomposition by the higher
applied voltage. Energy-dispersive X-ray (EDX) spectra
confirmed the 1:3 stoichiometric ratios between lead and
bromide (Supporting Information, Figure S5). Compared
[
21]
facets of a face-centered cubic structure. In the infiltration
process, DMSO was selected owing to its incompatibility with
polystyrene but good compatibility with the halide perovskite
at low temperatures. Because DMSO can also dissolve
polystyrene and destroy the template at high temperatures,
the removal of DMSO was conducted under vacuum at 608C
for 3 h to obtain the crystallized OHP. Upon DMSO removal,
a color change from white to light-orange was observed,
with the bulk material, the MAPbBr inverse opal showed
3
a circa 0.1 eV enhancement of the band gap, which can be
attributed to the well-known limitation of the 3D length
[23]
(Supporting Information, Figure S6).
The band gap of the halide perovskite can be easily
manipulated by adjusting the halide composition, which is
very important for using OHP as light absorber and emitter
materials. To test the compatibility of our method with this
advantage, the perovskite materials with single halides or
halide mixture were prepared. Because lead iodide can form
[
9]
a stable complex with DMSO, the perovskite with iodide
composition needed to be annealed at 1008C to obtain the
crystalline material. Typical SEM images provided direct
evidence for the feasibility of preparing OHP inverse opal
with other halides and mixtures of halides (Supporting
Information, Figures S7,S8). EDX spectra confirmed the
composition of OHP with different halides (Supporting
Information, Figure S9). X-ray powder diffraction (XRD)
patterns (Figure 2a,b) confirmed the cubic phase of
MAPbCl , MAPbBr , MAPbBr Cl , and MAPbBr I
3
3
1.5
1.5
1.5 1.5.
MAPbI showed a mixture of cubic and tetragonal phases.
3
For MAPbBr Cl and MAPbBr I , the diffraction peaks
1.5
1.5
1.5 1.5
were in the middle of the peaks observed for the two end
members MAPbCl3 and MAPbBr3, or MAPbI3 and
Figure 1. a) SEM images of artificial opal template of polystyrene (PS;
5
15 nm). Scale bar: 2 mm. b)–d) HR SEM images of MAPbBr inverse
3
MAPbBr , respectively, indicating the solid-solution forma-
3
opal with b) 515, c) 375, and d) 125 nm PS microspheres as a template.
Scale bars: b) 400 nm, c) 200 nm, d) 100 nm. e),f) HR cross-section
tion. However, the diffraction peaks of MAPbI Cl dis-
1.5
1.5
played a superposition of the individual MAPbCl3 and
images of MAPbBr inverse opal with e) 515 nm and f) 375 nm PS
3
microspheres as template. Scale bars: 200 nm in both (e) and (f).
MAPbI patterns, indicating this composition was a mixture
3
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One of the main bottlenecks of OHP materials is their
instabilities. The stability of the OHP inverse opal was also
investigated. After their preparation, the samples were stored
in covered glass vials for one month at room temperature
under ambient conditions and characterized using several
analytic techniques. The SEM image and diffuse reflectance
spectra of MAPbBr3 inverse opal did not display any
significant change in morphology and absorption properties.
However, XRD patterns of fresh and stored samples dis-
played a slight change between different peak intensity ratios
(Supporting Information, Figures S15,S16). The iodide coun-
terpart showed a more pronounced change in morphology,
diffuse reflectance spectrum, and XRD patterns (Supporting
Information, Figures S17,S18). Our observation is consistent
with that of the OHP material with high content of bromide
showing higher stability than the iodide counterpart both
[28]
under ambient atmosphere and in the presence of moisture.
The sample should be kept in dry inert atmosphere for longer
Figure 2. a) XRD patterns of OHP inverse opals with different composition
of halides; b) The enlarged XRD pattern between 138 and 178; c) diffuse
reflectance spectra and Tauc plots of OHP inverse opals with different
compositions of halides.
storage times. The different stability between MAPbBr and
3
MAPbI is related to the difference in bond strength and
3
[29]
crystalline forms.
Photoconductivity is one of the most crucial properties of
OHP materials for the applications in solar cells, lasers, light-
emitting diodes, photodetectors, and other optoelectronic
devices. Thus, the optoelectronic performance of the
MAPbBr3 and MAPbI3 inverse opals were investigated.
Upon illumination with a solar simulator lamp, the MAPbI3
inverse opal displayed a linear relationship under scanning
from À2 V to 2 V. Clear enhancements in current with light
intensity are observed (Figure 3a). The slope of the I-V curve
of these phases, rather than a solid solution. To further
confirm the coexistence of different phases, the perovskite
inverse opal with different Cl/I composition ratios were
prepared and XRD patterns exhibited that the intensity of the
same lattice plane depends on the Cl/I composition (Support-
ing Information, Figure S10). This observation can be attrib-
uted to the unstable structure when chloride replaces the
4À
[24]
equatorial iodine in the PbI6 block. Moreover, the diffuse
reflectance spectra also corroborated the XRD observations.
The spectra of MAPbI Cl showed two humps near to
[30]
represents the transconductance (g ) of the OHP layer.
m
The ratios of transconductance obtained at different light
1.5
1.5
À2
MAPbCl and MAPbI , respectively, confirming the mixture
intensities (25.5, 17.1, 13.8, and 6.9 mWcm ) and that in
3
3
of different phases (Figure 2c). Using the Kubelka–Munk
equation to determine the band gap, the band gaps for
MAPbBr Cl and MAPbBr1.5I1.5 were in between the two
darkness for MAPbI inverse opals was 57.8, 47.8, 45.0, and
3
1
.5
1.5
individual species respectively, which demonstrates that the
morphology-control method is compatible with the straight-
forward engineering of the OHP band gap (Figure 2d).
To demonstrate the universality of the preparation
method, other perovskite inverse opals were also prepared.
Formamidine perovskites have high thermal stability and
[8]
have the optimum band gap for single-junction solar cells.
Cesium perovskites were recently exhibited as novel opto-
electronic materials owing to their high quantum yield and
[25]
wide color spectrum. The SEM images and EDX spectra
exhibited that our method was feasible when formamidine or
cesium replaced methylammonium (Supporting Information,
Figures S9 and S11). XRD patterns confirmed the cubic phase
of FAPbBr and orthorhombic phase of CsPbI (Supporting
3
3
[26]
Information, Figure S12).
The band gap of FAPbBr3 is
smaller than MAPbBr owing to the larger size of formami-
3
dine than methylammonium (Supporting Information, Fig-
ure S13). The yellow color of CsPbI3 also indicated the
[27]
presence of the orthorhombic phase.
Meanwhile the
Figure 3. a) Typical I-V curves and b) photocurrent of MAPbI inverse opal
3
absorption onset of the Tauc plot (Supporting Information,
Figure S14) was at 1.70 eV, which is consistent with the known
value.
under different light intensities with 1 V applied bias. c) Typical I-V curves of
crushed inverse opal and d) the plot of transconductance ratio (g /g )
m
d
[26]
versus light intensity. g , transconductance under the light intensity m; g ,
m
d
transconductance in darkness.
1
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3
1.8. The I-V curves of MAPbBr inverse opal also displayed
Keywords: inverse opals · morphology control ·
organometal halide perovskites · photoconductivity ·
template synthesis
3
a linear relationship (Supporting Information, Figure S19).
The transconductance ratios for MAPbBr inverse opal were
3
1
7.8, 13.6, 11.2, 4.2, indicating a less sensitive to illumination
How to cite: Angew. Chem. Int. Ed. 2015, 54, 13806–13810
Angew. Chem. 2015, 127, 14011–14015
than MAPbI (Figure 3d).
3
Both MAPbBr3 and MAPbI3 generated stable photo-
currents under light illumination (Figure 3b; Supporting
Information, Figure S19b). When the cycles were repeated
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5
0 times, the photocurrents displayed excellent reproducibil-
ity (Supporting Information, Figure S20). Even when the time
was extended to 3 h, the OHP inverse opal still showed
a stable response (Figure S21), which demonstrated the good
photostability of the perovskite inverse opals. To test the
morphology effect on the optoelectronic properties, the OHP
inverse opal was pressed to crush the 3D ordered porous
structure (Figure S22). The I-V curves of crushed samples
displayed the nonlinear relationship of semiconductor diode
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Figure 3c; Supporting Information, Figure S23). The non-
linear relationship was even more pronounced in the bulk
counterpart (Supporting Information, Figure S24). OHPs
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[31]
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In summary, a facile colloidal crystal templating method
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porting Information, Figure S25). In principle, the method
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for the perovskite solar cell is in progress.
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