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Dalton
Transactions
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Published on 26 October 2017. Downloaded by University of Newcastle on 27/10/2017 01:24:40.
PERSPECTIVE
Cite this: DOI: 10.1039/c7dt02421e
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A chemical approach to perovskite solar cells:
control of electron-transporting mesoporous TiO2
and utilization of nanocarbon materials
Tomokazu Umeyama
*a and Hiroshi Imahori
*a,b
Over the past several years, organometal halide perovskite solar cells (PSCs) have attracted considerable
interest from the scientific research community because of their potential as promising photovoltaic
devices for use in renewable energy production. To date, high power conversion efficiencies (PCEs) of
more than 20% have been primarily achieved with mesoscopic-structured PSCs, where a mesoporous
TiO2 (mTiO2) layer is incorporated as an electron-transporting mesoporous scaffold into the perovskite
crystal, in addition to a compact TiO2 (cTiO2) as an electron-transporting layer (ETL). In this Perspective,
we first summarize recent research on the preparation strategies of the mTiO2 layer with a high electron
transport capability by facile sol–gel methods instead of the conventional nanoparticle approach. The
importance of the control of the pore size and grain boundaries of the mTiO2 in achieving high PCEs for
PSCs is discussed. In addition, an alternative method to improve the electron transport in the mTiO2 layer
Received 5th July 2017,
Accepted 11th October 2017
via the incorporation of highly conductive nanocarbon materials, such as two-dimensional (2D) graphene
DOI: 10.1039/c7dt02421e
and one-dimensional (1D) carbon nanotubes, is also summarized. Finally, we highlight the utilization of
zero-dimensional (0D) nanocarbon, i.e., fullerenes, as an n-type semiconducting material in mesostruc-
rsc.li/dalton
ture-free planar PSCs, which avoids high-temperature sintering during the fabrication of an ETL.
1.
a
Introduction
Department of Molecular Engineering, Graduate School of Engineering,
Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan.
E-mail: umeyama@scl.kyoto-u.ac.jp, imahori@scl.kyoto-u.ac.jp
b
Institute for Integrated Cell-Material Sciences (WPI-iCeMS), Kyoto University,
Sakyo-ku, Kyoto 606-8501, Japan
Photovoltaic devices directly convert unlimited solar power
into electricity and so are regarded as a promising means to
provide a sustainable source of electric power. Since the
pioneering work on photovoltaic devices based on organic–
Tomokazu Umeyama was born in
1976 and studied chemistry at
Kyoto University. He received his
BS (1999), MS (2001), and PhD
(2004) in polymer chemistry
under
the
guidance
of
Prof. Y. Chujo. He was also a
fellow of the Japan Society for
the Promotion of Science (JSPS)
in 2003–2004. He moved to the
Department
of
Molecular
Engineering in the same institute
as an Assistant Professor
Tomokazu Umeyama
(2004–2013) and is presently an
Associate Professor (2013–to date) in the group of Prof. Imahori.
His current interests involve development of photofunctional
organic and nanocarbon materials.
Hiroshi Imahori completed his
doctorate in organic chemistry at
Kyoto University. In 1992, he
became an Assistant Professor,
ISIR, Osaka University and then
moved to the Graduate School of
Engineering, Osaka University,
as an Associate Professor. Since
2002, he has been a Professor of
Chemistry, Graduate School of
Engineering, Kyoto University.
He has received the JSPS Prize
(2006), CSJ Award for Creative
Hiroshi Imahori
Work (2006), Osaka Science
Prize (2007), NISTEP Researcher Award (2007), and the ECS
Fellow (2016). His current interests involve artificial photosynthesis, organic solar cells, and organic functional materials.
This journal is © The Royal Society of Chemistry 2017
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Perspective
inorganic halide perovskites (CH3NH3PbX3, X = I, Br, or Cl)
reported by Miyasaka and coworkers in 2009,1 perovskite solar
cells (PSCs) have stimulated worldwide attention as a nextgeneration solar cell because of their multiple advantages over
inorganic-based cells, including light weight, low cost, and
ease of fabrication.2–4 The increasing attention on PSCs is also
due to unprecedented rapid progress in improving their power
conversion efficiencies (PCEs). Within less than 7 years, PCEs
have been increased from 3.8%1 to 22%.5–11 In PSCs, perovskite layers are typically sandwiched between two electrodes,
with charge collection facilitated by respective intermediate
layers, one being an electron-transporting layer (ETL), which
transports electrons but blocks holes, while the other is a holetransporting layer (HTL), which transfers holes but blocks electrons. Planar PSCs can be divided into n–i–p (conventional) and
p–i–n (inverted) structures, in which the ETL and HTL are
respectively adjacent to bottom transparent conducting electrodes such as fluorine-doped tin oxide (FTO) and indium-tin oxide
(ITO) (Fig. 1a and b). Conversion of light energy to electricity
generally involves the following fundamental steps: (i) absorption
of incident photons by the perovskite, (ii) rapid charge dissociation to form free carriers,12 (iii) electron and hole diffusion
in the perovskite toward the ETL and HTL, (iv) their diffusion in
the ETL and HTL toward the respective electrodes, and (v) charge
collection at the electrodes. Ideal materials for the ETL and HTL
require high charge mobilities and suitable energy levels of the
conduction and valence bands (CB and VB) that are wellmatched to those of the perovskite to facilitate charge injection
and reduce energy loss. Compact structures that are free from
defects and pinholes are also essential for the ETL and HTL to
suppress charge recombination (CR).
Fig. 1 Schematic illustrations of typical (a) planar n–i–p (conventional),
(b) planar p–i–n (inverted), and (c) mesoscopic structures for PSCs. ETL:
electron-transporting layer, HTL: hole-transporting layer, ETMS: electron-transporting mesoporous scaffold.
Dalton Trans.
Dalton Transactions
The diffusion length for holes is longer than that for electrons in a typical perovskite material, CH3NH3PbI3.13 To compensate for the shorter electron diffusion length, an electrontransporting mesoporous scaffold (ETMS) for the perovskite
crystalline is often employed (Fig. 1c). Mesoporous TiO2
(mTiO2) is widely employed as the ETMS in combination with
compact TiO2 (cTiO2) as an ETL, where electrons are transported not only by the perovskite but also through the mTiO2
to the FTO/cTiO2 electrode, leading to better charge collection.14 Crystalline TiO2 has a suitable CB edge position to
extract a photogenerated electron in a perovskite material, a
long electron lifetime, and a high transparency.15,16 Although
other metal oxide semiconductors including SnO2 and ZnO
with relatively high electron mobilities have also been investigated as materials for use as an ETL and ETMS and have
achieved high PCE values,17–25 most of the PSCs with PCEs of
more than 20% have utilized compact and mesoscopic TiO2
materials so far.10,26–29 Spin-coating a paste containing TiO2
nanoparticles with sizes of 20–50 nm onto the cTiO2 layer followed by subsequent sintering is a typical procedure to
prepare a uniform mTiO2 layer with a three-dimensional (3D)
mesoporous network. However, the preparation of the TiO2
nanoparticle paste is time-consuming and tedious, thereby
impeding the advantages of cost-effective PSCs. More importantly, a large number of grain boundaries in the mesoporous
network of nanoparticles lead to rapid CR. The rather homogeneous network of nanoparticles also involves random electron transit paths and thus limits the net electron transport
rate, deteriorating the device performance. It should be
emphasized that the uniform and complete infiltration of the
perovskite7 into a 3D mesoporous structure is desirable for the
formation of a highly crystalline perovskite structure exhibiting
excellent device performance.30–32 The morphology, pore size,
and electron transport ability of mTiO2 also play important
roles in determining the device performance of mesoscopic
PSCs. TiO2 nanorods and nanowires with a one-dimensional
(1D) alignment have been utilized as an ETMS to improve
charge transport properties and suppress CR in PSCs,33–37 but
device performance has been generally inferior to those using
TiO2 nanoparticles, as a consequence of the loss of a large
surface area and incomplete formation of perovskite crystals.38
In addition, poor adhesion of nanowires to the electrode often
causes a problem for device fabrication, and a good electrical
contact between nanowires and electrodes should be carefully
addressed.39–45 As a result, some recent research has focused
on developing facile strategies to fabricate an ETMS layer with
superior charge transport properties and controlled mesoscopic structures for the PSC applications.
In this Perspective, focusing on our work46–48 and related
studies, we highlight recent strategies for the facile preparation
of high-performance electron-transporting mTiO2 layers. First,
polymer material-assisted sol–gel methods are systematically
introduced. The importance of controlling pore size (Fig. 2a)
and reducing grain boundaries (Fig. 2b) in the mTiO2 layer by
judicious selection of the employed polymer materials for
high-performance PSCs is described in detail. In response to
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Perspective
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Fig. 4 Schematic illustration of a planar n–i–p PSC device with a fullerene-based ETL.
However, such values frequently depend in a large part on
device fabrication techniques and conditions that are off-topic
for this manuscript, and therefore direct comparisons of the
PCE values from different reports are not scientifically meaningful in many cases. Therefore, we emphasise the comparisons of PCE values from reference devices described in each
paper.
Fig. 2 (a) Effect of pore size in mTiO2 on perovskite formation. (b)
Effect of TiO2 nanoparticle boundaries on the electron transportation
ability.
2. Polymer-assisted structure control
of mTiO2 prepared by sol–gel
reactions
2.1.
the explosive growth of PSC research, several excellent
reviews discussing the design of compact and mesoporous
electron-transporting materials in PSCs have recently
appeared,15,16,49–52 but a comprehensive review on this topic
has not been published. Then, we summarize the incorporation of nanocarbon materials such as two-dimensional (2D)
graphene and 1D carbon nanotubes (CNTs), as a method to
improve electron transport in a mTiO2 layer (Fig. 3). The highly
conductive nature and unique structure of these nanocarbon
materials can enhance electron transport in mTiO2 by bridging
the TiO2 nanoparticle boundaries, as has been observed in
organic–inorganic hybrids with nanocarbon.53–56 Furthermore,
we also focus on the utilization of zero-dimensional (0D) nanocarbon, i.e., fullerenes, as an n-type semiconducting material
in mesostructure-free planar n–i–p PSCs, to explore the feasibility of relatively low PCE, but low-cost and flexible PSC
devices with low hysteresis that can be processed at low temperature (Fig. 4). Finally, solubility control of fullerene molecules for stacked-layer device fabrication is presented in detail.
There are significant differences in the PCE values of PSC
devices with respect to each report in this Perspective.
Fig. 3
Smooth electron transport by graphene in an mTiO2 layer.
This journal is © The Royal Society of Chemistry 2017
Block copolymer template
To fabricate a mTiO2 film with a high specific surface area by
the standard nanoparticle method, particle size should be
reduced. In such a case, the electron transport is undesirably
slowed down owing to the increase in the particle boundaries
(Fig. 2b). In addition, the pore size in mTiO2 films would
become small, preventing the infiltration of the perovskite into
them. Li and coworkers utilized a mesoporous film made from
a block copolymer, polystyrene-block-poly(2-vinyl pyridine), as
a template for producing a boundary-less mTiO2 layer by combining atomic layer deposition (ALD) and calcination.57 The
mesoscopic structure and thickness of the mTiO2 layer were
tunable by changing the conditions of film formation of the
block copolymer. The PSC device based on the optimized
mTiO2 layer with a configuration of FTO/mTiO2CH3NH3PbI3−xClx/P3HT/Ag (P3HT; poly(3-hexylthiophene))
showed a higher PCE (12.5%) than a planar PSC device
without the mTiO2 layer (9.8%). However, this method requires
the use of a costly ALD system as well as high-temperature calcination, and so other cost-effective techniques are desirable.
A simple approach to obtain boundary-less mesoporous structures of metal oxides is based on sol–gel reactions assisted by
self-assembling amphiphilic block copolymers.58–63 Metal
oxide precursors are usually embedded in a hydrophilic
domain of a phase-separated block copolymer, and then sintered to remove the organic materials and form the mesoporous metal oxides, reflecting the phase separation architecture. The normal nanoparticle approach yields mTiO2 with an
“inverse mesospace”, but such self-assembly methods generally provide a “mesospace” structure (Fig. 5a). As sacrificial
templates, poly(alkylene oxide)-based block copolymers, e.g.,
poly(ethylene glycol)-block-poly( propylene glycol)-block-poly
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Dalton Transactions
diameter could be tuned by varying the weight ratio of TMB/
F127. On the other hand, the TiO2 film prepared in the same
manner without TMB exhibited high-density crystalline TiO2
nanopillars surrounded by the inverse mesospace,64,65 indicating that the presence of the swelling agent played a pivotal role
in directing the mesophase of the TiO2 films as well as in
increasing the pore size. The mTiO2 layers obtained by the sol–
gel reaction were used as the ETMS in PSC devices with a configuration of FTO/cTiO2/mTiO2-CH3NH3Pb(I0.9Br0.1)3/PTAA/Au
(PTAA; poly(triarylamine)).65 The device based on the mTiO2
with an average pore size of 15 nm showed a higher PCE of
12.8% (short-circuit current density ( JSC) = 18.8 mA cm−2,
open-circuit voltage (VOC) = 1.04 V, and fill factor (FF) = 0.660)
than that with the average pore size of 10 nm (PCE = 11.7%,
JSC = 18.6 mA cm−2, VOC = 1.00 V, and FF = 0.626). The enlargement of the mesospace by TMB exerted a positive effect on
device performance, which may be attributed to the improved
infiltration of the perovskite formed throughout the mTiO2
layer (Fig. 2a). However, the PCE was still comparable to that
of the device based on mTiO2 with an average pore size of
15 nm prepared by the conventional nanoparticle method
(PCE = 12.7%, JSC = 19.6 mA cm−2, VOC = 0.987 V, and FF =
0.658). This result indicates that further optimization of sol–
gel reaction conditions using block copolymer templates to
make suitable pore sizes is necessary to take full advantage of
the mesospace-structured mTiO2.
Fig. 5 (a) Structures of inverse mesospace and mesospace in mesoporous films. (b) Preparation of an mTiO2 layer with a mesospace structure by an F127-assisted sol–gel reaction with a swelling agent TMB. (c)
Structure of amphiphilic graft copolymer, PVC-g-POEM.
(ethylene glycol) (named Pluronic P123, F127 etc., depending
on the molecular weight and composition ratio), are often
used to define the phase separation structure.58–64 mTiO2
films prepared with such poly(alkylene oxide)-based block
copolymers typically have seamless mesospace structures, but
their pore sizes are generally lower than 10 nm, which would
inhibit the desirable infiltration of the perovskite into the
mTiO2 films.
To overcome the pore size limitation of mesospace TiO2
films, Seok et al. utilized a hydrophobic solvent, 1,3,5-trimethylbenzene (TMB), as a swelling agent in combination
with a sacrificial block copolymer template, F127.65 A mixed
solution of F127 and TMB in ethanol was stirred to form template micelles with expanded sizes, and then a titania precursor (Ti(O-Bu)4 and HCl) for the sol–gel reaction was added and
stirred for 24 h (Fig. 5b). TMB is miscible with the hydrophobic part of the surfactant micelle and is therefore able to
increase the size of the hydrophobic core. After spin-coating
onto a cTiO2 layer on a FTO substrate (denoted as FTO/cTiO2),
the as-prepared thin film was aged for 3 days at room temperature and then sintered (Fig. 5b). The TiO2 film obtained possessed well-ordered body-centered cubic mesospace with diameters of 10–15 nm throughout the entire film thickness. The
Dalton Trans.
2.2.
Graft copolymer template
Amphiphilic graft copolymers, an alternative to block copolymers, are also available as a template for the mTiO2 structures.
Graft copolymers are more attractive than block copolymers
due to their low cost and ease of synthesis.66 A mTiO2 layer
with a uniform and interconnected mesospace structure was
prepared by a sol–gel process with Ti(O-iPr)4 and a graft copolymer, PVC-g-POEM. The copolymer consisted of hydrophilic
poly(oxyethylene methacrylate) (POEM) side chains that could
interact with inorganic TiO2 precursors and a hydrophobic
poly(vinyl chloride) (PVC) backbone that could produce a
mesopore upon calcination (Fig. 5c).66,67 In contrast to the
block-copolymer-based sol–gel reaction, the pore size was
increased up to 70 nm, the pore becoming larger with an
increase in the ratio of the PVC hydrophobic domain in the
copolymer composition. Moreover, the thickness of the mesoporous layer was adjusted to be 300–800 nm by varying the
concentration of the solution. The PSC device with a configuration of FTO/cTiO2/mTiO2-CH3NH3PbI3/spiro-OMeTAD/Ag
(spiro-OMeTAD;
2,2′,7,7′-tetrakis(N,N-bis(p-methoxyphenyl)
amino)-9,9′-spirobifluorene) exhibited the highest PCE of
11.9% ( JSC = 20.21 mA cm−2, VOC = 0.982 V, and FF = 0.599)
when the pore size and the thickness of the mTiO2 were set to
be 70 nm and 300 nm, respectively.67 The pore size also
affected the photovoltaic properties, with both the JSC and VOC
values increasing when the pore size of the mTiO2 increased
from 30 to 70 nm, probably because of the more efficient formation of large and defect-free perovskite crystals in the pore
as the pore size increased (Fig. 2a). Additionally, its photovol-
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taic performance was superior to that of the PSC device based
on the mTiO2 prepared by the conventional nanoparticle ( particle size: ca. 50 nm) method (PCE = 8.39%, JSC = 16.54
mA cm−2, VOC = 0.822 V, and FF = 0.617). Unfortunately, the
electron transport abilities of the mTiO2 layers prepared by the
graft copolymer-assisted sol–gel reaction and the nanoparticle
method in this study were not compared. Nevertheless, these
results demonstrate the potential of the graft copolymer-templated mTiO2 nanostructures as an ETMS to fabricate costeffective and highly efficient mesoscopic PSCs.
2.3.
Polymer sphere template
Amphiphilic block and graft copolymers can be regarded as
“soft templates” that have flexible structures, but they
cause self-assembly under specific conditions to form
nanostructures through hydrophilic and hydrophobic
interactions. H. G. Yang and coworkers have used “hard
templated” polymer materials, such as polystyrene (PS)
spheres, to direct the porous structures into mTiO2 scaffolds.68
As illustrated in Fig. 6a, two kinds of titanium precursor solutions, aqueous Ti(SO4)2 solution and Ti(SO4)2 in PS (diameter:
∼100 nm) emulsion, were spin-coated on an FTO substrate in
this order. After sintering and rinsing to remove the templates,
a uniform inverse opal-like TiO2 (ioTiO2) film was obtained.
The diameter of the pore was ∼100 nm, reflecting well the
sizes of the PS sphere templates. The pores were connected to
each other by the TiO2 wall with a thickness of 6 nm.
Interestingly, the FTO/cTiO2/ioTiO2 substrate showed an excellent transmittance of visible light in comparison with the bare
FTO, while the FTO/cTiO2/mTiO2 substrate that was prepared
Perspective
with P25 (TiO2 nanoparticles with diameters of 20–30 nm)
exhibited lower transmittance than the bare FTO in the wavelength region from 300 to 600 nm.68 This antireflection property of the ioTiO2 film enabled the incident light to arrive at
the perovskite layer more efficiently. The best performance
PSC with a configuration of FTO/cTiO2/ioTiO2-CH3NH3PbI3/
spiro-OMeTAD/Ag demonstrated a PCE value of 13% ( JSC =
21.93 mA cm−2, VOC = 0.973 V, and FF = 0.61). These JSC and
PCE values are significantly higher than those of the device
with the conventional P25 mesoporous layer ( JSC = 19.90
mA cm−2 and PCE = 11%) as a consequence of the excellent
light manipulation ability of the ioTiO2 layer. Therefore, the
hard template methodology could pave the way for introducing
photonic structures into a TiO2 scaffold for perovskite crystals
for use in high-performance and low-cost mesoscopic PSCs.
S. Yang et al. have investigated the size effect of PS spheres
on PSC device performance with an HTL-free structure of FTO/
cTiO2/ioTiO2-CH3NH3PbI3/carbon paste.69 The ioTiO2 layers
with pore diameters of 160 nm, 200 nm, and 470 nm and wall
thicknesses of 25–50 nm were prepared by a PS-sphere templated sol–gel reaction. The antireflection property was most
enhanced in the ioTiO2 layer with a 200 nm pore, and therefore the PCE (12.02%) and JSC (22.67 mA cm−2) of the device
based on the ioTiO2 layer with the 200 nm pores were higher
than those of the others. The authors also demonstrated by
photoluminescence decay and electrical impedance spectroscopy (EIS) measurements that the charge extraction and
charge transport capabilities of the ioTiO2-based PSC devices
were superior to those of the planar n–i–p structure-based
ones.
2.4.
Fig. 6 Preparation of (a) an ioTiO2 layer with a mesospace structure by
a PS sphere-assisted sol–gel reaction and (b) an mTiO2 layer with an
inverse mesospace structure by a PMMA-assisted sol–gel reaction.
This journal is © The Royal Society of Chemistry 2017
Mesoscopic phase separation
We have recently established a methodology to obtain a mTiO2
layer by a facile sol–gel technique assisted by a general copolymer, poly(methyl methacrylate) (PMMA).46 In contrast to the
amphiphilic block and graft copolymers, PMMA does not form
nanometer-sized structures on its own. Nevertheless, mesoscopic phase separation was caused during the sol–gel reaction
of titanium precursors in the presence of PMMA. For instance,
by spin-coating a precursor solution containing 3.1% Ti(OiPr)4, 1.6% TiCl4, and 1.0% PMMA in chloroform onto an FTO/
cTiO2 substrate and with subsequent calcination, a TiO2 film
with a crack-free inverse mesoscopic structure was obtained,
where nanoparticles with a size of ∼30 nm were interconnected seamlessly (Fig. 6b). The pores were sufficiently large
and the TiO2 domains were well-connected, seamless structures in comparison with the mTiO2 film made by using the
conventional TiO2 nanoparticle (diameter: ∼20 nm) paste.
Furthermore, control of the mesoporous structure of TiO2 was
attainable by changing the loaded PMMA amount.46 The
higher loading of up to 4.0% PMMA rendered the TiO2 particle
sizes smaller (diameter: ∼20 nm) and the grain boundary
more distinct. This point is highly beneficial because changing
the loading amount of PMMA is much easier than varying the
structures and component ratios of block and graft copolymers. Conversely, the same sol–gel method with 1% PS instead
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of PMMA gave a compact TiO2 film without forming the
porous structures.46 This result corroborates the importance of
additive polymer structures in assisting mesopore formation
in TiO2. PMMA has polar ester groups on the side chains,
whereas PS consists only of nonpolar hydrocarbons. The high
polarity of the esters may improve their miscibility with titanium reagents, inducing meso-sized phase separation.
We constructed and evaluated a PSC device based
on mTiO2 prepared by the sol–gel reaction with 1% PMMA
with a configuration of FTO/cTiO2/mTiO2-CH3NH3PbI3/spiroOMeTAD/Au (PCE = 14.0%, JSC = 18.7 mA cm−2, VOC = 0.979 V,
and FF = 0.763). This device outperformed the reference
devices with the mTiO2 prepared by the conventional nanoparticle (diameter: ∼20 nm) approach (PCE = 13.1%, JSC =
18.2 mA cm−2, VOC = 0.961 V, and FF = 0.749).46 The electron
transport behavior of the PSC devices based on the mTiO2
layers prepared by the PMMA-assisted sol–gel method and the
nanoparticle approach was examined by measuring transient
photocurrent in a short circuit, revealing that the electron
diffusion coefficient (De) of the former was much higher than
the latter. In mesoscopic PSCs, the generated electrons travel
across the mTiO2 layers before reaching the FTO/cTiO2 electrode.14 Fast electron transport in the mTiO2 layer in the
former device is attributed to the lack of an obvious boundary
between the TiO2 particles (Fig. 2b). This enhanced De value
resulted in the superior photovoltaic parameters in the former.
Consistently, the PSC devices based on mTiO2 with a higher
PMMA loading (e.g., the device based on mTiO2 prepared by
the 4% PMMA-assisted sol–gel method; PCE = 8.59%, JSC =
13.4 mA cm−2, VOC = 0.900 V, and FF = 0.712) showed lower
device performance, reflecting the smaller sizes and more distinct grain boundary of the TiO2 particles (Fig. 2b). These
results exemplify the great potential of the facile sol–gel technique with a simple polymer additive to provide mesostructure-tuned TiO2 layers with no clear boundaries and thereby
contribute significantly toward the development of low-cost
mesoscopic PSCs.
Dalton Transactions
efficiencies and thereby PCEs of dye-sensitized solar cells.76–79
In 2014, Snaith and coworkers reported the utilization of graphene in PSCs based on cTiO2 and mesoporous Al2O3 layers,
suggesting that graphene incorporation into the cTiO2 layer
lowered interfacial resistance between the cTiO2 layer and the
FTO, resulting in the promotion of electron transport.80
Meanwhile, Yang et al. inserted an ultrathin layer of graphene
quantum dots (GQDs) between CH3NH3PbI3 and mTiO2.81
GQDs have a small size of only several nanometers with
special quantum-confinement effects and edge effects, making
them distinct from conventional large graphene.82 They found
that the GQDs facilitated the electron injection from
CH3NH3PbI3 to mTiO2 rather than improving the electron
transportation, boosting the JSC and PCE values of the PSC
devices from 15.43 mA cm−2 and 8.81% to 17.06 mA cm−2 and
10.2%, respectively.81
The first example of incorporating large graphene materials
into mTiO2 and cTiO2 layers was reported by our group in 2015
(Fig. 7a).47 Compact TiO2 was deposited on an FTO substrate
by spin-coating an ethanol solution of titanium diisopropoxide
bis(acetylacetonate) with 0.15 wt% GO under an ambient
atmosphere and with subsequent thermal treatment at 500 °C
under a nitrogen flow to cause simultaneous transformation of
amorphous TiO2 to crystal, and facile GO reduction to RGO,
providing FTO/cTiO2(RGO) (cTiO2(RGO); an RGO-embedded
cTiO2 layer). The RGO-incorporated mTiO2 (mTiO2(RGO)) was
then fabricated on the cTiO2(RGO) layer by spin-coating a TiO2
nanoparticle paste with 0.015 wt% GO and after subsequent
calcination at 500 °C under nitrogen to form FTO/cTiO2(RGO)/
mTiO2(RGO). Both the cTiO2 and mTiO2 layers were crack-free
even with the inclusion of RGO. We fabricated and evaluated
the PSCs with a configuration of FTO/cTiO2(RGO)/
mTiO2(RGO)-CH3NH3PbI3/spiro-OMeTAD/Au, establishing the
cascading energetic sequences (Fig. 7b).47 The cross-sectional
view of the device obtained using scanning electron
microscopy (SEM) revealed the uniformity of the cTiO2(RGO)
3. Nanocarbon embedment into
mTiO2
3.1.
Graphene embedment
Graphene, a 2D nanocarbon material, possesses remarkable
properties including excellent electron mobility, good durability, and high transparency.70 Reduced graphene oxide
(RGO), which is obtained by the intensive oxidation of graphite
to yield graphene oxide (GO) and the subsequent reduction of
GO to remove the oxygen-containing functional groups, has
some defects, but is functionally similar to graphene and
easier to be processed.71,72 Therefore, as a strategy to facilitate
electron transport in TiO2 materials, graphene and RGO have
been widely embedded to improve the performance of TiO2based devices such as photocatalysts and solar cells.73–80
Several studies have demonstrated that the incorporation of
RGO into nanostructured TiO2 can boost charge collection
Dalton Trans.
Fig. 7 (a) Schematic illustration and (b) energy diagram of an RGOembedded PSC device with a mesoscopic structure.
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and mTiO2(RGO)-CH3NH3PbI3 layers, indicating that RGO was
seamlessly and homogeneously integrated into the composite.
Averaged JSC, VOC, and FF values of the RGO-embedded PSC
device were 16.5 mA cm−2, 0.835 V, and 0.674, respectively,
yielding a PCE value of 9.29%. The photovoltaic parameters
were higher than those of the device with the RGO-embedded
cTiO2 and RGO-free mTiO2 layers (FTO/cTiO2(RGO)/mTiO2CH3NH3PbI3/spiro-OMeTAD/Au; PCE = 8.14%, JSC = 16.1
mA cm−2, VOC = 0.799 V, and FF = 0.633) and RGO-free TiO2
layers (FTO/cTiO2/mTiO2-CH3NH3PbI3/spiro-OMeTAD/Au; PCE =
6.61%, JSC = 14.9 mA cm−2, VOC = 0.761 V, and FF = 0.583).
These results unambiguously corroborate the positive effect of
RGO-incorporation into the mTiO2 and cTiO2 layers on photovoltaic properties. EIS measurements revealed the remarkably
lower series resistance (Rs) of the PSC with RGO-embedded
TiO2 layers than that with RGO-free TiO2 layers.47 The addition
of RGO facilitated electron transport in the TiO2 layers by a
bridging effect and reduced the contact resistance at the perovskite/mTiO2 and cTiO2/FTO interfaces (Fig. 3). Furthermore,
most of the electron density is on RGO, and the resultant TiO2
in contact with the perovskite accelerated the charge injection
from the perovskite to the TiO2 layers.
Following our work, several groups reported the utilization
of RGO– or graphene–mTiO2 nanocomposites as an ETMS in
PSCs.83–87 Jung and coworkers fabricated a mTiO2(RGO) layer
on the FTO/cTiO2 substrate by spin-coating a slurry containing
TiO2 nanoparticles and RGO following sintering.83 Note here
that GO was chemically reduced by hydrazine before mixing
with the TiO2 nanoparticles, whereas the crystallization of
TiO2 and the reduction of GO occurred simultaneously during
sintering in our method.47 The optimized PSC device with a
configuration of FTO/cTiO2/mTiO2(RGO)-CH3NH3PbI3/spiroOMeTAD/Ag achieved a PCE of 15% ( JSC = 22.0 mA cm−2, VOC =
0.93 V, and FF = 0.707), which was higher than the RGO-free
mesoscopic PSC device (PCE = 12%, JSC = 19.6 mA cm−2, VOC =
0.86 V, and FF = 0.668). An EIS analysis was conducted, revealing that the mTiO2(RGO) film reduced the internal resistance
and enhanced the charge collection efficiency relative to RGOfree mTiO2 (Fig. 3). Despite the absence of RGO in the cTiO2
layer, the electron flow from the RGO in the mTiO2(RGO) to
the cTiO2 remained sufficiently efficient. On the one hand,
Nazeeruddin et al. attained an excellent PCE value of 20%
( JSC = 21.98 mA cm−2, VOC = 1.11 V, and FF = 0.80) for a
PSC device with a configuration of FTO/cTiO2/mTiO2(RGO)(FAPbI3)0.85(CH3NH3PbI3)0.15/spiro-OMeTAD/Au (FA; formamidinium), where the mTiO2 surface was treated with bistrifluoromethanesulfonimidate (Li-TFSI) to facilitate interfacial electron injection.84 They also incorporated RGO into the perovskite and spiro-OMeTAD layers, but unwanted shunt resistance
pathways were generated, deteriorating the overall device performance. Carlo and coworkers embedded graphene prepared
by ultrasonic-assisted liquid-phase exfoliation of graphite into
the mTiO2 layer.86 The maximum PCE reached up to 16.3%
( JSC = 22.95 mA cm−2, VOC = 1.03 V, and FF = 0.689) in a
PSC with a configuration of FTO/cTiO2/mTiO2(graphene)CH3NH3PbI3/spiro-OMeTAD/Au. More importantly, the
This journal is © The Royal Society of Chemistry 2017
Perspective
addition of graphene enhanced the stability, with the graphene-embedded PSC device retaining more than 88% of its
initial PCE after 16 h of prolonged 1-sun illumination at the
maximum power point, while the PCE of the graphene-free
PSC decreased by ∼40% under the same conditions. The
results listed here suggest that the RGO- or grapheneembedded TiO2 layers can be considered as cost-effective, critical components in achieving a significant improvement in the
PCEs and stabilities of PSCs.
3.2.
Carbon nanotube embedment
CNTs, a 1D nanocarbon material, also exhibit excellent conductivity originating from their unique structure.88 As a result,
the integration of CNTs into an ETMS is also expected to
provide an ultrafast electron transport pathway to enhance
photovoltaic performance.89–92 Recently, Shapter et al.
embedded single-walled carbon nanotubes (SWNT) into
mTiO2 for PSC applications for the first time by adding surfactant-assisted SWNT aqueous dispersion into a TiO2 nanoparticle paste, spin-coating the paste onto an FTO/cTiO2 substrate, and sintering at 450 °C.93 As expected, the PSC device
based on the SWNT-embedded mTiO2 (denoted as
mTiO2(SWNT)) with a configuration of FTO/cTiO2/
mTiO2(SWNT)-CH3NH3PbI3/spiro-OMeTAD/Au outperformed
(PCE = 16%, JSC = 21.964 mA cm−2, VOC = 1.041 V, and FF =
0.70) the PSC with SWNT-free mTiO2 (PCE = 14%, JSC =
19.485 mA cm−2, VOC = 0.988 V, and FF = 0.70). Theoretical
studies showed that the interaction between SWNT and TiO2
increases the CB minimum of TiO2, leading to an improvement in the VOC value. The EIS and dark J–V measurements
revealed reduced CR and low series resistance compared to a
SWNT-free device. Furthermore, the SWNT-embedded PSC
showed enhanced resistance to light and long-term storage
stabilities, demonstrating the excellent potential of the SWNT
as an additive into the mTiO2.
4. Planar n–i–p PSCs with solution
processable fullerene-based ETL
4.1.
Fullerene-based ETL
Despite the high performance of mesoscopic PSCs based on
mTiO2 and cTiO2, the preparation of highly crystalline TiO2
layers requires high-temperature sintering (>450 °C). This
requirement causes a rise in production cost and precludes its
application to flexible plastic substrates. Although low-temperature-processed TiO2, SnO2, and ZnO compact layers have
been applied to planar n–i–p PSCs to circumvent this
problem,23,25,80,94 synthesis of these materials often involves
long reaction times and a solvent washing process. Moreover,
a hysteresis in the J–V curve measured in the forward and
reverse bias scans is more severe in the planar PSCs than in
the mesoscopic PSCs.95
Compared to metal oxide ETLs, the facile, low-cost solution-based processes of soluble n-type organic semiconductors
are advantageous.16,52 Phenyl-C61-butyric acid methyl ester
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(PCBM, Fig. 4) and other fullerene derivatives have been recognized as promising candidates for the n-type material in ETLs
and have been widely used in planar p–i–n PSCs.96–106
Fullerene-based ETLs reduce the density of trap states and passivate the grain boundaries of the perovskite-absorbing layer,
suppressing hysteresis in the J–V curves.107,108 In such planar
p–i–n PSCs, however, poly(3,4-ethylenedioxythiophene):poly
(styrenesulfonic acid) (PEDOT:PSS) is predominantly used as
the HTL material on a transparent electrode, and its acidic
and hygroscopic nature impairs the long-term stability and
PCE of the device.109–111 Therefore, placing a fullerene-based
material on the transparent electrode as an ETL in planar n–i–p
PSCs is highly desirable. It is noteworthy that the attachment
of solubilizing groups, as seen in PCBM, is necessary for good
solution processability, but it simultaneously deteriorates the
resistance to attack by N,N-dimethylformamide (DMF), which
is commonly used in fabricating the perovskite layers on the
ETL.2–4 Partial dissolution of the ETL would create shunting
paths and thereby impair the device performance. In the following sections, recent examples of achieving orthogonal solubility to fabricate planar n–i–p PSCs with fullerene-based ETLs
are introduced.
4.2.
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method was extended to a flexible ITO/PEN (PEN; poly(ethylene naphthalate)) substrate and achieved a PCE of 11.1%,
suggesting its potential for flexible photovoltaic devices that
can be processed at low temperatures.
4.3.
Self-assembled monolayer (SAM)
A SAM of fullerene molecules on a transparent electrode has
been demonstrated to act as an efficient and reliable ETL in
planar n–i–p PSCs.113 First, a bare FTO surface was activated
by oxygen plasma to increase the amount of hydroxyl surfaceterminated groups (Fig. 8a). Then, the substrate was dipped
into a solution of N-[3-(triethoxysilyl)propyl]-2-carbomethoxy3,4-fulleropyrrolidine (Sil-C60) in anhydrous toluene to form a
covalent bond between the FTO and Sil-C60 (Fig. 8a). Dry conditions and the activated FTO promoted the self-assembly and
Orthogonal solvent processing
Seok et al. successfully utilized PCBM as an ETL material in a
planar n–i–p PSC in a configuration of FTO/PEI/PCBM/
CH3NH3PbI3/PTAA/Au (PEI; polyethyleneimine) using an
orthogonal solvent process.112 PEI was used here as an interfacial material that modifies the work function of an FTO and
facilitates electron extraction. For orthogonal solvent processing, robust solvent resistance of a bottom layer against the
spin-coating solvent of an upper layer is necessary. The poor
solubility of PEI in chlorobenzene enabled the deposition of
the PCBM layer onto the FTO/PEI substrate. In addition,
dimethyl sulfoxide (DMSO)/γ-butyrolactone (GBL) (3 : 7, v/v)
was used instead of DMF as a spin-coating solvent to form the
perovskite layer because GBL is a marginal solvent for PCBM.
However, perovskite crystallization was retarded by the interaction with solvent molecules and thereby the perovskite layer
prepared from the mixed solvent formed a textile-like inhomogeneous structure that did not fully cover the substrate.95,112
To quickly remove the residual solvent and make the perovskite film smoother, diethyl ether was dropped onto the substrate during the spinning process. Diethyl ether was selected
as the dripping solvent because it causes no damage to the
underside PEI and PCBM layers due to its low solubility. Then,
a PTAA HTL was prepared by spin-coating from toluene, a poor
solvent for CH3NH3PbI3, and the Au electrode was deposited
by thermal evaporation. The PSC device showed a high PCE
value of 15% ( JSC = 21.8 mA cm−2, VOC = 0.98 V, and FF = 0.72)
in the reverse bias scan and a steady-state PCE of 14% by
measuring the stabilized photocurrent held at a forward bias
of 0.82 V.112 The difference in the PCE values between the
reverse bias scan and the steady-state was significantly small
when compared to other planar n–i–p PSCs with the cTiO2 as
the ETL. Furthermore, this orthogonal solvent processing
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Fig. 8 Preparation of (a) FTO/Sil-C60-SAM, (b) FTO/Sil-C60-crosslink,
(c) FTO/PCBCB-crosslink, and (d) FTO/C60 by post-thermal treatment of
FTO/C60(9MA).
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avoided the cross-linking of Sil-C60, resulting in a dense fullerene monolayer on the FTO (denoted as FTO/Sil-C60-SAM). As a
result of the stiff covalent bonding, the fullerene monolayer was
tolerant to the subsequent perovskite and HTL formations by
solution processes. The fabricated PSC device with a configuration of FTO/Sil-C60-SAM/(FAPbBr3)0.1(FAPbI3)0.65(CsPbI3)0.05
(CH3NH3PbI3)0.2/spiro-OMeTAD/Au attained a PCE of 15% ( JSC =
19.4 mA cm−2, VOC = 1.04 V, and FF = 0.74) in the reverse bias
scan and a comparable steady-state PCE of 15%.113 In contrast
to the spin-coating method, this solution deposition technique
minimizes the consumption of fullerene materials and thereby
reduces the overall cost of large-scale device fabrication.
4.4.
Post-treatment
Another strategy to achieve orthogonal solubilities for the fabrication of stacked-layer devices is a post-treatment
approach.114,115 Snaith and coworkers employed two soluble
fullerene-based precursor molecules to generate insolubilized
films as ETLs in planar n–i–p PSCs.116 One precursor molecule
is Sil-C60 that can not only form a SAM on the activated FTO
(Fig. 8a), but also can cause cross-linking by a sol–gel reaction
with acid-treatment (Fig. 8b). The other is phenyl-C61-butyric
acid benzocyclobutene ester (PCBCB), which has a structure
similar to PCBM, but possesses a benzocyclobutene group that
can cause thermally induced ring-opening reactions (Fig. 8c).
By using post-treatments, namely trifluoroacetic acid (TFA)
vapor exposure and heating at 200 °C, Sil-C60 and PCBCB
yielded insoluble films on the FTO, which can function as the
ETLs. These substrates are denoted as FTO/Sil-C60-crosslink
and FTO/PCBCB-crosslink, respectively (Fig. 8b and c). In contrast to the SAM method, the sol–gel reaction of Sil-C60 on the
non-activated FTO yielded the robust fullerene-based film with
a thickness of a few tens of nanometers. The optimized PSC
devices with the architectures of both FTO/Sil-C60-crosslink/
CH3NH3PbI3/spiro-OMeTAD/Au and FTO/PCBCB-crosslink/
CH3NH3PbI3/spiro-OMeTAD/Au exhibited high PCE values of
18% ( JSC = 23.0 mA cm−2, VOC = 1.07 V, and FF = 0.73 for the
former and JSC = 22.4 mA cm−2, VOC = 1.11 V, and FF = 0.73 for
the latter) in the reverse bias scans and steady-state PCEs of
17% and 15%, respectively.116 These results demonstrate that
highly soluble Sil-C60 and PCBCB can act as precursors for
producing efficient and reliable ETL in the planar n–i–p PSCs.
Although the attachment of a solubilizing group on a fullerene core is necessary for the deposition of compact fullerenebased layers with controlled film thickness, pristine C60 with
no bulky substituents is expected to be packed more densely to
facilitate intermolecular charge transport.117–122 Indeed,
planar p–i–n PSC devices (ITO/PEDOT:PSS/CH3NH3PbI3/fullerene/Ag) with a C60 ETL outperforms devices with the PCBM
ETL.101 To use pristine C60 molecules as an ETL in planar n–i–
p PSCs with sufficient solution processability, we recently
applied the post-thermal-treatment methodology.48 A film of a
highly soluble precursor compound, i.e., C60–9-methylanthracene adduct (C60(9MA)), was formed on an FTO substrate by spin-coating, and then the C60(9MA) film was heated
at 140 °C to cause the retro-Diels–Alder reaction, yielding a
This journal is © The Royal Society of Chemistry 2017
Perspective
pristine C60 film with a thickness less than 10 nm (Fig. 8d).
C60(9MA) possesses a superior film-forming property relative
to pristine fullerene C60.123 In addition, because pristine C60 is
poorly soluble in DMF, the formation of a CH3NH3PbI3 layer
on the FTO/C60 substrate caused little damage to the underlying C60 film. The best-performing PSC device with a configuration of FTO/C60/CH3NH3PbI3/spiro-OMeTAD/Au showed the
PCE values of 15.6% ( JSC = 21.1 mA cm−2, VOC = 0.988 V, and
FF = 0.748) in the reverse bias scan and 14.5% in the forward
bias scan. These values are higher than those of the planar
n–i–p PSC device with the cTiO2 as the ETL (FTO/C60/
CH3NH3PbI3/spiro-OMeTAD/Au, PCE = 14.2%, JSC = 19.6
mA cm−2, VOC = 0.974 V, and FF = 0.745 in the reverse bias
scan and PCE = 11.6% in the forward bias scan).48 In addition,
the hysteresis behavior, i.e., the difference in the PCE values
between the reverse and forward scans of the C60-based device
was impressively suppressed compared to a cTiO2-based device
owing to the enhanced electron-selective collection ability of
the pristine fullerene. These results suggest that the post-treatment concept is of significant importance for the solutionprocessed, low-cost, and flexible PSC device fabrication where
thin ETLs with high uniformity and well-controlled thickness
are required.
5. Summary and outlook
This Perspective has provided an overview of recent notable
strategies for the fabrication of mTiO2 layers with high electron-transport properties, for applications as ETMSs in PSCs;
particle boundary-less mesoporous structures by various
polymer material-assisted sol–gel reactions and nanoparticlebased mesoporous structures bridged by 1D and 2D nanocarbons. A quick charge transportation in such PSC devices is of
pivotal importance in maximizing their performance. Thus,
these strategies have contributed to improvements of the
PCEs. Furthermore, perovskite crystallization is known to be
affected by the pore size in ETMSs. Thus, especially in the
former strategy, the control and optimization of mesoporous
structures by varying the structures and amounts of templating
polymers also attained an enhancement of device performances. In addition, we have also focused on recent attempts to
utilize fullerene-based materials as compact ETLs in planar n–
i–p PSCs. Various methodologies to enable the construction of
the stacked-layer structures under low-temperature conditions
were summarized. The topics in this article are not a comprehensive list of ETMSs and ETLs in PSC research, but include
the exploration of new design and concepts for ETMSs and
ETLs to inspire further studies on PSCs.
PSC research has continued to evolve and impressively high
PSC performance has been realized, allowing their commercialization in the near future. There are two options to take
advantage of PSCs. One is based on hard glass substrates and
high-temperature-sintering processes to enable TiO2 crystallization, aiming to achieve higher PCEs than already commercialized multicrystalline silicon solar cells. Although the high-
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temperature processes raise the production cost to some
extent and close the door to the utilization of flexible plastic
substrates, PSCs with crystalline TiO2 materials still have the
advantages of cost reduction and mass production over crystalline silicon solar cells. As discussed in this Perspective, developing new fabrication methodologies for mTiO2 with precise
control over the morphology, porosity, thickness, and crystallinity will further facilitate favorable perovskite crystallization
and long-range electron transport. Additional functionalities,
such as antireflection properties, will also be realized in order
to improve the photovoltaic performance. Therefore, the
optimization of the mTiO2 structure in PSC devices will
provide a path toward high PCE values close to the theoretical
limit of 26%. Another option is based on plastic substrates
and low-temperature processes, accomplishing the flexibility
and low cost, that is inaccessible to crystalline silicon solar
cells. As also shown in this Perspective, ETLs composed of
organic fullerene molecules are suitable for this purpose.
Recently, a SnO2 layer has been fabricated by a simple, lowtemperature, solution process.17–22 The planar PSC device with
the SnO2 layer as an ETL showed excellent device performance,
with a PCE of 20.7%. The efficient, cost-effective, and flexible
characteristics22 of such PSC devices will make them economically viable for commercialization.
However, there are still significant obstacles to overcome
before PSC technologies warrant commercialization. Longterm stability, reproducibility, and large-scale fabrication are
the most desirable characteristics in addition to the high
efficiency and flexibility.9,124–127 The degradation mechanism
of perovskites need to be investigated in detail both experimentally and theoretically. The stability of all components in
the PSC devices other than the perovskite layer should also be
improved.128 In order to scale up device size with high
throughput and material saving, perovskite deposition should
move beyond spin-coating-based methods. Nucleation and
crystal growth may be significantly different depending on the
procedure used for perovskite film formation. In addition,
engineering of perovskite–ETMS/ETL and ETL–electrode interfaces requires a deep understanding of relevant charge separation and collection mechanisms. The difference in the mesoscopic structure of mTiO2 and the embedding of a dopant
such as a nanocarbon material into mTiO2 may also exert a
significant influence on the interaction of such a perovskite–
ETMS/ETL interface. Theoretical calculations based on firstprinciples density functional theory and Car–Parrinello
molecular dynamics can contribute to a precise understanding
of the electron injection and carrier recombination at such
interface,129–131 which has been already studied intensively
in dye-sensitized solar cells.132–134 This issue also overlaps
with the hysteresis behavior of the PSCs. Further design
and elaborated synthesis of new electron-transporting
materials that allow precise control of morphology, fine-tuning
of energy levels, and provide high charge mobilities with
controlled surface engineering will play a core role in the
market development of large-scale perovskite-based photovoltaic devices.
Dalton Trans.
Dalton Transactions
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by Grant-in-Aid for Scientific
Research (S) (No. JP25220501 to H. I.), Kansai Research
Foundation for Technology Promotion, Grant-in-Aid for Young
Scientists (A) (No. JP26708023 to T. U.), and Grant-in-Aid for
Scientific Research on Innovative Areas “New Polymeric
Materials Based on Element-Blocks (No. 2401)” (JP15H00737
to T. U.). The authors thank Prof. Seigo Ito (University of
Hyogo), Dr Youfeng Yue, Prof. Easan Sivaniah (Kyoto
University), and Prof. Vaidyanathan (Ravi) Subramanian
(University of Nevada) for collaborative work in the development of new electron-transporting materials in PSCs.
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