Molecular Orientation Change in Naphthalene Diimide Thin Films Induced by Removal of Thermally Cleavable Substituents

Article abstract of DOI:10.1021/acs.chemmater.8b05237

The potential of naphthalene-1,8:4,5-tetracarboxylic diimide (NDI-H) as a transparent electron-transporting material was examined. A soluble precursor was designed and synthesized having two tert-butoxycarbonyl solubilizing substituents at the imide moieties. This precursor molecule, NDI-Boc, is converted to the hydrogen-substituted NDI-H by heating the spin-coated precursor films. The molecular orientation during the thermal conversion of NDI-Boc to NDI-H was examined using two-dimensional grazing incidence X-ray diffraction (2D-GIXD) and p-polarized multiple-angle incidence resolution spectrometry. It was revealed that, driven by the formation of intermolecular hydrogen bonds, the molecular orientation changes from tilted edge-on to face-on orientation. In situ 2D-GIXD measurements confirmed that the change of molecular orientation is simultaneously caused by the cleaving of the Boc substituents. Time-resolved microwave conductivity measurements were used to show that the resulting NDI-H film has anisotropic charge-carrier transport with preferential mobility in the direction perpendicular to the film plane. We fabricated perovskite solar cells to demonstrate that the NDI-H film effectively functions as the bottom electron-transporting layer in these devices.

Full text of DOI:10.1021/acs.chemmater.8b05237

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Article
Molecular Orientation Change in Naphthalene Diimide Thin
Films Induced by Removal of Thermally Cleavable Substituents
Tomoya Nakamura, Nobutaka Shioya, Takafumi Shimoaka, Ryosuke Nishikubo, Takeshi
Hasegawa, Akinori Saeki, Yasujiro Murata, Richard Murdey, and Atsushi Wakamiya
Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b05237 • Publication Date (Web): 15 Jan 2019
Downloaded from http://pubs.acs.org on January 15, 2019
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Chemistry of Materials
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Molecular Orientation Change in Naphthalene Diimide Thin Films
Induced by Removal of Thermally Cleavable Substituents
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Tomoya Nakamura, Nobutaka Shioya, Takafumi Shimoaka, Ryosuke Nishikubo, Takeshi
,
†
‡
†
†
,†
Hasegawa,* Akinori Saeki, Yasujiro Murata, Richard Murdey, Atsushi Wakamiya*
†Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan
‡Department of Applied Chemistry, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka
65-0871, Japan.
5
ABSTRACT: The potential of naphthalene-1,8:4,5-tetracarboxylic diimide (NDI-H) as a transparent electron-transporting
material was examined. A soluble precursor was designed and synthesized having two tert-butoxycarbonyl solubilizing
substituents at the imide moieties. This precursor molecule, NDI-Boc, is converted to the hydrogen substituted NDI-H by
heating the spin-coated precursor films. The molecular orientation during the thermal conversion of NDI-Boc to NDI-H
was examined using two-dimensional grazing incidence X-ray diffraction (2D-GIXD) and p-polarized multiple-angle
incidence resolution spectrometry (pMAIRS). It was revealed that, driven by the formation of intermolecular hydrogen
bonds, the molecular orientation changes from tilted edge-on to face-on orientation. In situ 2D-GIXD measurements
confirmed that the change of molecular orientation is simultaneously caused by the cleaving of the Boc substituents.
Time-resolved microwave conductivity (TRMC) measurements were used to show that the resulting NDI-H film has
anisotropic charge-carrier transport with preferential mobility in the direction perpendicular to the film plane. We
fabricated perovskite solar cells to demonstrate that the NDI-H film effectively functions as the bottom electron-
transporting layer in these devices.
the molecular plane should be aligned flat or close to flat
on the substrate, namely, face-on orientation.
INTRODUCTION
12–15
The development of charge-transporting materials is a
crucial issue for the achievement of high-performance
organic electronic devices such as organic field-effect
In optoelectronic devices, the transparency of the
materials in the visible light region is also important to
allow efficient light into or out from devices such as
1
–3
transistors (OFETs),
organic light-emitting diodes
1
6
displays, lighting, solar cells and photodetectors. A new
class of high-performance solution-processable
4
,5
6–9
(OLEDs),
and organic photovoltaics (OPVs).
According to Marcus theory, charge mobility in organic
thin films is determined not only by reorganization
energy (), but also by electronic coupling (V) between π-
photovoltaics, perovskite solar cells (PSCs), have recently
emerged as one of the hottest topics in optoelectronic
materials research. Over the past few years, the power
conversion efficiencies (PCEs) of PSCs have rapidly
1
0,11
conjugated molecules.
The electronic coupling
represents the strength of the interactions between
frontier orbitals, and is at a maximum when the
neighboring molecules stack together in a cofacial
manner. The efficiency of charge transport materials may
therefore be optimized by ordering molecules so as to
maximize the electronic coupling in the direction of the
current flow in the target device. In lateral organic
transistors where the current flows in the plane of the
film between source and drain electrodes, optimizing the
molecular stacking generally requires the molecules to be
oriented with the π plane perpendicular to the substrate,
namely, edge-on orientation. For sandwich device
structures typically used for OLEDs, OPVs, and perovskite
solar cells (PSCs), the current flows through the film.
Therefore, for good π-stacking in this vertical direction,
17
18
increased from 3.8% to above 22%. PSCs are typically
composed of perovskite light-absorbing layer
a
sandwiched by an electron-transporting layer (ETL) and a
hole-transporting layer (HTL) with “regular” (a substrate
coated with an electron extracting layer) or “inverted” (a
substrate coated with
a
hole extracting layer)
configurations. As for the ETL, which is needed for
efficient electron extraction and hole blocking from the
perovskite layer, titanium dioxide (TiO
for the regular device configuration. Fabrication of the
TiO layer requires high temperature sintering (> 450 °C)
2
) is widely utilized
2
to obtain high crystallinity and high charge carrier
mobility, however, limiting low-cost mass production and
19
application to flexible devices. Organic semiconductors
are promising candidates to replace conventional TiO
2
,
and would facilitate the manufacture of PSCs at low
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temperature process. The development of transparent formation of carbon dioxide and isobutene, hence NDI-H
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organic ETLs with high mobilities is challenging, however,
as in addition to a wide optical gap the following
requirements should also be met:
thin films can be obtained by first spin coating the
precursor, designated NDI-Boc, and then heating the
precursor film (Figure 1b). Whether the orientation of
NDI-Boc precursor influences the orientation of the
annealed NDI-H film is an interesting question which has
not been explored. In order to study the NDI molecular
packing structure before and after removal of the
solubilizing groups, the molecular orientation during the
thermal conversion of NDI-Boc to NDI-H in thin films
was examined using two complementary techniques; two-
dimensional grazing incidence X-ray diffraction (2D-
1
.
The lowest unoccupied molecular orbital (LUMO)
energy level of the ETL should match the
conduction band of the perovskite.
2.
The material should be thermally and chemically
stable.
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.
The ETL layer should be uniform, flat, and free of
pinholes.
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A final and important consideration for the ETL,
specific to “regular” PSC cell geometries, is chemical
resistance with respect to the solvent used for the spin
coating of the perovskite layer on top of the ETL.
Consequently, organic n-type semiconductors showing
GIXD), which is sensitive to crystalline regions only, and
p-polarized multiple-angle incidence resolution
spectrometry (pMAIRS), which returns information about
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the ensemble average including amorphous regions.
To our surprise, 2D-GIXD and pMAIRS revealed that
the molecular orientation of NDI-Boc thin film changes
from tilted edge-on to face-on orientation during the
thermal conversion process. Time-resolved microwave
20–25
high-performance in PSC applications are still limited.
Rylene tetracarboxylic diimides, represented by
naphthalene diimides (NDIs), perylene diimides (PDIs),
and terrylene diimides (TDIs), have been extensively
5
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conductivity (TRMC) measurements
resulting NDI-H film has anisotropic charge-carrier
transport with preferential mobility in the
confirm that the
26
investigated as n-type materials.
They have high
electron affinities, high electron mobilities, and excellent
chemical and thermal stabilities. Among the many rylene
diimide molecules, we decided to focus on NDIs for the
applications to the transparent ETL in perovskite solar
cells. NDIs are the smallest rylene diimide systems and
have largest optical bandgap. In thin films of NDI with
alkyl substituent groups at the imide moieties, the
molecules stand vertically with respect to the substrate
plane and show high in-plane electron mobility in OFET
a
perpendicular direction to the film plane. Finally, we
fabricated perovskite solar cells to demonstrate that the
NDI-H film can be used as the bottom electron-
transporting layer in these devices.
27,28
devices.
Kido, Yokoyama, Sasabe and co-workers have
reported that the weak CH···N hydrogen bonds between
pyridine rings can enhance the face-on orientation in the
29–31
vacuum-deposited film for OLED applications.
Encouraged by these findings, we decided to examine
NDI with hydrogen atoms at the imide moieties (NDI-H).
In this molecule, the imide group can work as N–H···O
32,33
intermolecular hydrogen bond sites.
NDI-H has not
been investigated as an electron-transporting material
after a single early report indicating moderate in-plane
3
4
mobility. If directed face-on orientation could be
induced by two-dimensional sheet-like structures linked
by an intermolecular hydrogen bonds network, then we
can expect that NDI-H may show high out-of-plane
electron mobility and thus function as an effective ETL in
planar device structures such as PSCs. (Figure 1a).
Without solubilizing groups, NDI-H is expected to have
very low solubility. While this is a desirable trait in terms
of solvent resistance in the solid state, it significantly
hinders the solution processability. Though the
introduction of long alkyl chains is a common approach
to gain solubility, it often inhibits the dense packing of
Figure 1. (a) Schematic images of the two-dimensional
hydrogen bonds network of NDI-H, and (b) deposition of
NDI-H thin film by spin-coating of precursor material and
thermal conversion.
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molecules.
To obtain solution processable films,
RESULTS AND DISCUSSION
therefore, thermally cleavable solubilizing groups are
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Boc-protected naphthalene diimide (NDI-Boc) was
synthesized in two steps from commercially available
naphthalene tetracarboxylic dianhydride (NTCDA)
introduced.
An NDI precursor was synthesized
bearing tert-butoxycarbonyl (Boc) as removable
solubilizing groups. Boc can be thermally cleaved with the
(
Scheme 1). The treatment of NTCDA with aqueous
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Chemistry of Materials
ammonia gave hydrogen substituted naphthalene diimide
NDI-H) in 86% yield.53 Boc groups were introduced by
the reaction of NDI-H with di-tert-butyl dicarbonate
(Boc O) using N,N-dimethyl-4-aminopyridine (DMAP) as
a catalyst, giving the Boc-protected naphthalene diimide
(NDI-Boc) in 66% yield. Single crystals of NDI-Boc were
obtained from slow diffusion of ethyl acetate into a
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(
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i
solution in chloroform. The NDI-Boc has a C symmetry
where the tert-butyl groups in Boc moiety exist in the
opposite side of the NDI plane, and exhibited
herringbone packing structure (Figure S1).
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Scheme
Reagents/conditions: i) NH
3.0 equiv), DMAP (20 mol%), CH
1.
Synthesis
of
aq., rt, 8 h, 86%; ii) Boc
Cl , rt, 8 h, 66%.
Boc-protected
NDI.
2
O
3
(
2
2
Figure 2. (a) Thermogravimetric analysis of NDI-Boc
–
1
under nitrogen atmosphere with the rate of 2 °C min ,
and (b) absorption spectra of the spin-coated film of NDI-
Boc on a quartz substrate before and after thermal
conversion to NDI-H.
The temperature at which the Boc groups are cleaved in
the bulk material was investigated by thermogravimetric
analysis (TGA). Upon heating, a weight loss of 42.7% was
observed at the onset of 187 °C and the end point of 194 °C
(
Figure 2a). The weight loss corresponds to the
theoretical value for two Boc groups (42.9%), indicating
the simultaneous elimination of both protecting groups.
The Boc groups start to eliminate at ~180 °C, suggesting
that the thermal conversion of NDI-Boc to NDI-H in the
film state would be possible at this temperature given
sufficient time (Figure 2a, inset). NDI-H meanwhile has
high thermal stability with a decomposition temperature
of 374 °C.
Information about the molecular orientation was
obtained by analyzing the crystal orientation with two-
dimensional grazing incidence X-ray diffraction (2D-
47
GIXD). We first checked the molecular orientation on a
Si(100) substrate since it has low roughness and shows no
background diffractions. Spin-coated films of NDI-Boc on
the Si(100) substrate show diffraction patterns which can
be assigned to the single crystal structure of NDI-Boc
(Figure 3a, top). 100 and 200 peaks were observed along
The effect of the thermal conversion on the film
properties was characterized by several methods. For
NDI-Boc films prepared by the spin-coating of NDI-Boc
solution in chloroform on a quartz substrate, UV-Vis
absorption spectra were measured before and after
annealing the substrate at 180 °C for 30 minutes. The
absorption maximum was slightly red-shifted from 390
nm to 401 mm (Figure 2b). It was also noted that the film
thickness estimated by a stylus-based surface profiler
decreased from ca. 80 nm to ca. 40 nm. Full conversion to
NDI-H at this condition could also be confirmed by IR
spectrometry, as will be discussed later.
z
the ~q axis, indicating that the NDI-Boc molecules align
with the long axis perpendicular to the substrate with a
tilt angle of ca. 32° from the normal, namely, tilted edge-
on orientation (Figure 3b, top). The estimated d-spacing
of 14.9 Å is comparable to the value in the crystal
structure (15.1 Å).
After the thermal cleavage of Boc groups in the film by
annealing at 180 °C for 30 minutes, surprisingly, the
molecular orientation changed drastically. The peak at q
z
_
–1
~
2.0 Å is assignable to the 11 peak from the π–π stacking
2
structure (Figure 3a, middle), suggesting the face-on
orientation of NDI-H (Figure 3b, middle). The estimated
π–π stacking distance of 3.1 Å corresponds to the value in
3
2
the crystal structure (3.1 Å). We also prepared the film of
NDI-H by vacuum deposition for comparison. The
vacuum-deposited NDI-H film shows the similar
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diffraction patterns, suggesting that here again the face- and face-on oriented NDI-H and did not contain edge-on
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on orientation is predominant (Figure 3a, bottom). The
NDI-H film prepared by thermal conversion was found to
have higher crystallinity than the vacuum-deposited film,
judging from the more distinct 002 and 110 diffractions.
oriented NDI-H (Figure 4c and 4d). These observations
clearly refute the first and third possibilities, indicating
instead that the change of molecular orientation from
tilted edge-on to face-on directly coincides with the
conversion process from NDI-Boc to NDI-H.
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Figure 3. Crystal orientation analysis on the thin film of
NDI-Boc, NDI-H by thermal conversion, and NDI-H by
vacuum deposition on Si(100) substrates. (a) 2D-GIXD
images and (b) corresponding crystal structures.
Figure 4. In situ 2D-GIXD measurements during the thermal
conversion of NDI-Boc to NDI-H. 2D-GIXD images (a) before
annealing, after annealing at (b) 120 °C for 10 min, (c) 140 °C
for 10 min, (d) 160 °C for 10 min, and (e) 180 °C for 10 min.
A question arises with respect to the relative timing of
the molecular rearrangement and the bond cleavage.
Three possibilities are considered; (i) molecular
orientation of NDI-Boc changes before the cleavage of
Boc, (ii) face-on orientation forms immediately after the
cleavage of Boc group, or (iii) edge-on oriented NDI-H
forms as an intermediate state and it changes to face-on
orientation only during the final stages of the annealing
process. To address this question, we carried out in situ
The molecular orientation in the thin film can be
affected by the substrate. To confirm that molecular
orientation in actual devices conforms to the films grown
on silicon substrates, 2D-GIXD measurements were also
taken for NDI-Boc and NDI-H films on ITO and
ITO/polyethylenimine ethoxylated (PEIE). (See Figure 8
for the device structure.) Though the peak intensity was
weakened, similar molecular orientation was observed on
ITO or ITO/PEIE (Figure S2).
2D-GIXD measurement by sequentially raising the
temperature of the NDI-Boc thin film from room
temperature to 120 °C, 140 °C, 160 °C, and finally 180 °C for
every 10 minutes (Figure 4). At 120 °C, no change of
molecular orientation was observed for NDI-Boc films
(Figure 4b). Samples after annealing at 140 °C and 160 °C
were found to be composed of edge-on oriented NDI-Boc
The molecular orientation of the thin films was further
investigated by p-polarized multiple-angle incidence
resolution spectrometry (pMAIRS), a powerful technique
to discuss the molecular structure regardless of the
4
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crystallinity of the films.
In the spin-coated film of
NDI-Boc on the Si(100) substrate, three bands are
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Chemistry of Materials
observed in the carbonyl region (Figure 5, top). The C=O
stretching vibration band of the Boc group appears at the
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–1
highest wavenumber of 1771 cm , whereas the bands at
–
1
–1
1719 cm and 1681 cm can be assigned to the symmetric
and antisymmetric C=O stretching vibration modes of the
3
3
imide group, respectively.
The symmetric C=O
stretching vibration ( (C=O)) band appears stronger in
s
the OP (out-of-plane) spectrum than that in the IP (in-
plane) spectrum. Considering that the transition moment
of  (C=O) mode is in the direction of the long axis, this
s
indicates that the NDI-Boc molecules stand on the
substrate in the direction of the long axis. This
observation is in good general agreement with the 2D-
GIXD results.
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After annealing the substrate at 180 °C for 30 minutes,
the IR spectra changed dramatically. The disappearance
–
1
of the peak originating from the Boc group (1771 cm )
confirms the complete conversion to NDI-H (Figure 5,
middle). The 
wavenumber in the IP spectrum (1700 cm ) and OP
s
(C=O) band appears at a different
–
1
–
1
spectrum (1716 cm ). This can be explained by transverse
optic (TO)-longitudinal optic (LO) splitting caused by the
anomalous dispersion of the refractive index, which
makes this band unsuitable for structural characterization
Figure 5. pMAIRS spectra of the thin film of NDI-Boc, NDI-
H by thermal conversion, and NDI-H by vacuum deposition
on Si(100) substrates.
5
5,56
(
Figure S3).
Instead, another useful band of C–H out-
of-plane bending vibration ((C–H)) was used. The (C–
–
1
H) band at 768 cm appears stronger in the OP spectrum
than that in the IP spectrum. As the transition moment of
the (C–H) mode is perpendicular to the molecular plane,
this suggests that NDI-H adopts face-on orientation to
the substrate (Figure 5, middle). Similarly, the vacuum-
deposited film of NDI-H showed the predominant face-on
orientation (Figure 5, bottom).
The effect of the N–H···O hydrogen bonds on the IR
spectrum of NDI-H powder samples was discussed
3
3
previously in a report by Seydou and coworkers. They
found that the N–H out-of-plane bending mode ((N–H))
–
1
of 864 cm was broadened and blue-shifted relative to the
–
1
isolated molecule (710 cm ) and that this shift could be
attributed to the formation of intermolecular hydrogen
bonds. In our case, the broad (N–H) band appears at 872
–
1
cm for NDI-H thin film by thermal conversion and 875
–
1
cm for vacuum-deposited NDI-H. This indicates that the
N–H···O hydrogen bonds were also formed in these films,
and we can reasonably deduce that the formation of
hydrogen bonds is the driving force for the change of thin
film molecular orientation from tilted edge-on to face-on.
Figure 6. Schematic illustration of the molecular orientation
change in naphthalene diimide thin films induced by the
removal of thermally cleavable Boc groups.
To gain insight into the relationship between packing
structures and charge transport properties, we conducted
theoretical calculations at the PW91/DZP level and
evaluated the electronic coupling (V) for the crystal of
In Figure 6, the molecular orientation change of
naphthalene diimide induced by the removal of thermally
cleavable Boc groups is illustrated schematically. The
removal of bulky Boc groups gives some space for
molecules to change their orientation, and the
preferential face-on orientation is induced by
intermolecular hydrogen bonds accompanied by the
decrease of thickness. This would provide an explanation
for how the molecules could change their orientation in
thin films, where the molecules are thought to be
immobile under the glass transition temperature.
3
2
NDI-H. Large V values (38–61 meV) were obtained for
the electron transfer along the direction of π-π stacks,
whereas small values were observed in other directions
(1–22 meV) (Table S1). Therefore, the face-on oriented
film of NDI-H is expected to show higher electron
mobility in out-of-plane direction than that in in-plane
direction. Time-resolved microwave conductivity (TRMC)
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measurements51,52 were carried out to experimentally
examine the charge transport anisotropy. TRMC measures
the pseudo-photoconductivity () under an oscillating
microwave electric field in the absence of contact
between the semiconductors and the metal electrodes,
where  is the charge carrier generation efficiency and 
is the sum of hole and electron mobilities. Anisotropy of
the carrier mobility (OP/IP) can be estimated by the
ratio of out-of-plane to in-plane mobility, in which  is
canceled. The thin film of NDI-H on quartz substrate
prepared by thermal conversion showed larger OP
cm V s ) is comparable to that of NDI-based n-type
2
–1 –1
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polymers.
One concern about the thermal conversion method is
that the removal of Boc groups may lead to the formation
of
a
non-uniform film with pinholes or other
morphological defects that could lead to high charge
recombination rates when the material is used as the ETL
in solar cell devices. The film morphology was verified by
atomic force microscopy (AFM). We found that the spin-
coated film of NDI-Boc was composed of large grains with
dimensions up to 500 nm. The root-mean-square (RMS)
surface roughness was 20 nm (Figure 7a). After removing
the Boc groups, the grain size decreased to ~150 nm, and
the RMS roughness of the film decreased to 15 nm (Figure
7b). The thermal removal of Boc groups did not cause an
increase of the surface roughness, instead the relatively
large roughness of the NDI-H film (compared to 1.3 nm
for the vacuum-deposited NDI-H film, see Figure 7c) is
simply due to the roughness of the precursor NDI-Boc
film. A precursor molecule with lower tendency to form
large grains may therefore be a possible route to more
uniform films.
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–
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–1 –1
value of (5.8 ± 0.6) × 10 cm V s compared to IP of
–
4
2
–1 –1
(2.0 ± 0.2) × 10 cm V s , from which the anisotropy was
estimated to be 2.9 ± 0.4 (Figure S4). Similarly, the
vacuum-deposited NDI-H shows an anisotropy of 3.4 ±
–
4
2
–1 –1
0.5 (OP = (2.0 ± 0.2) × 10 cm V s , IP = (0.59 ±
–
4
2
–1 –1
0.06) × 10 cm V s ). The higher absolute value of
pseudo-photoconductivity in the film prepared by
thermal conversion is consistent with the higher
crystallinity of the film (Figure 3).5
7,58
In addition, the
electron mobility of vacuum-deposited NDI-H film was
examined by space-charge-limited current (SCLC)
–
5
method (Figure S5). The mobility of NDI-H (5.1 × 10
Figure 7. AFM images of the thin film of (a) NDI-Boc, (b) NDI-H by thermal conversion, and (c) NDI-H by vacuum deposition
on Si substrate.
Finally, the performance of NDI-H in the role of an
electron-transporting layer was investigated. To this end,
regular-type perovskite solar cells were fabricated having
a configuration of ITO/polyethylenimine ethoxylated
for Spiro-OMeTAD HTL. Vacuum-deposited NDI-H has
higher uniformity compared to NDI-H prepared by
thermal conversion, as was previously observed in the
thin film AFM images.
(
3
PEIE)/NDI-H/MAPbI /Spiro-OMeTAD/Au (Figure 8a). A
Figure 8d shows the forward scan current density-
thin layer of PEIE (< 5 nm) was added as a work function
modifier to reduce the resistance at ITO/NDI-H interface
and facilitate ohmic contact.
reference device with fullerene C60 was also fabricated
under the same condition. Figure 8b shows the
corresponding energy level diagram of the PSC. The
LUMO and HOMO energy levels of NDI-H were
estimated from the onset of the first reduction wave in
cyclic voltammetry and absorption edge, respectively
voltage (J–V) characteristics of representative devices
–2
under AM 1.5 G irradiation (100 mW cm ). The NDI-H
60
For comparison, a
layer prepared by thermal conversion functioned as the
electron-transporting layer. The device exhibited the
power conversion efficiency (PCE) of 4.8% with short
–2
circuit current density (JSC) of 13.6 mA cm , open circuit
voltage (VOC) of 0.83 V, and fill factor (FF) of 0.43 (Table
1
). The device using a vacuum-deposited NDI-H layer
–2
showed a higher PCE of 10.7% with JSC = 18.9 mA cm , VOC
(
Figure S6). The energy of the LUMO level of NDI-H (–4.0
=
0.88 V, and FF = 0.64, which was comparable to the
eV) is close to the conduction band of MAPbI (–3.9 eV).
3
performance obtained with a C60 ETL in the same device
structure (9.3%). The variation in the PCE between the
NDI-H layers fabricated by thermal conversion and
vacuum deposition arises primarily from the difference in
Figure 8c compares the cross-sectional scanning electron
microscope (SEM) images of the devices with NDI-H from
thermal conversion and vacuum deposition. The thickness
of the constituent layers can be estimated to be ~40 nm
–2
–2
the short circuit current (13.6 mA cm vs. 18.9 mA cm )
for NDI-H ETL, ~450 nm for MAPbI
3
layer, and ~200 nm
2
2
and shunt resistance (Rsh) (120  cm vs. 314  cm ),
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Chemistry of Materials
Figure 8e shows the incident photon to current
estimated from the slope of J–V curves near zero applied
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bias. Given the high surface roughness, some regions of
the thermally converted NDI-H layer can be expected to
be significantly thinner than the average thickness of 40
nm. A possible explanation for the smaller JSC and Rsh is
that these very thin regions have insufficient hole-
blocking ability, leading to an increase in the charge
recombination rates. This is corroborated by the
observation that the device using vacuum-deposited NDI-
H with a smaller thickness of 20 nm showed decreased JSC
and Rsh compared to that with 40 nm layer (Figure S7,
Table S2).
conversion efficiency (IPCE) spectra. The PSC device
using vacuum-deposited NDI-H showed relatively high
IPCE of 0.7 to 0.8 in the visible region, though a drop of
quantum efficiency was observed at the near UV region.
Referring back to the UV-Vis absorption spectrum of NDI-
H (Figure 2b), we can see that this drop of IPCE is due to
the absorption of incident light by the NDI-H layer. JSC
could be increased further, therefore, by using an ETL
with a shorter absorption wavelength.
0
1
2
3
4
5
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7
8
9
0
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0
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0
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0
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2
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7
8
9
0
Figure 8. Fabrication of perovskite solar cell using NDI-H as an electron transport layer. (a) Device structure, (b) energy level
diagram, (c) cross-sectional SEM images of solar cells, (d) forward scan J–V characteristics, and (e) incident photon to current
conversion efficiency (IPCE) spectra.
Table 1. Photovoltaic Parameters of PSCs using NDI-H or C60 as an Electron Transport Layer, Derived from Forward Scan
J–V Characteristics
–2
Rsh [ cm2]
J
SC [mA cm ]
13.6
V
OC [V]
0.83
FF
PCE [%]
4.8
NDI-H (thermal conversion)
NDI-H (vacuum deposition)
0.43
0.64
0.58
120
311
18.9
0.88
0.95
10.7
C
60
16.9
9.3
152
In summary, naphthalene diimide (NDI-H) films with
face-on orientation to the substrate were successfully
prepared by heating the spin-coated film of the NDI-Boc
CONCLUSIONS
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precursor compound. The initial edge-on orientation of
Page 8 of 11
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Wang, C.; Dong, H.; Hu, W.; Liu, Y.; Zhu, D.
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Semiconducting π-Conjugated Systems in Field-Effect
Transistors: A Material Odyssey of Organic Electronics.
Chem. Rev. 2012, 112, 2208–2267.
NDI-Boc changed to a face-on orientation during thermal
cleavage of the Boc groups. Anisotropic charge-carrier-
transport up to 3:1 was observed in the NDI-H film. The
NDI-H layers prepared by thermal conversion were
demonstrated to function as the transparent electron
transport layer in regular-type perovskite solar cells, while
the device using vacuum-deposited NDI-H showed
comparable performance to a C60-based device. Near-UV
absorption by NDI-H reduces the amount of light
absorbed by the perovskite layer, causing a proportionate
loss in conversion efficiency. The development of electron
transport materials having shorter absorption wavelength
is currently in progress in our laboratory, and results will
be reported in due course.
(
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Shirota, Y.; Kageyama, H. Charge Carrier Transporting
Molecular Materials and Their Applications in Devices.
Chem. Rev. 2007, 107, 953–1010.
Xiao, L.; Chen, Z.; Qu, B.; Luo, J.; Kong, S.; Gong, Q.; Kido, J.
Recent Progresses on Materials for Electrophosphorescent
Organic Light-Emitting Devices. Adv. Mater. 2011, 23, 926–
952.
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Walker, B.; Kim, C.; Nguyen, T. Q. Small Molecule Solution-
Processed Bulk Heterojunction Solar Cells. Chem. Mater.
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ASSOCIATED CONTENT
Supporting Information
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Kularatne, R. S.; Magurudeniya, H. D.; Sista, P.; Biewer, M.
C.; Stefan, M. C. Donor-Acceptor Semiconducting Polymers
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The Supporting Information is available free of charge on the
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and Biology. Biochem. Biophys. Acta 1985, 811, 265–322.
Osaka, I.; Takimiya, K. Backbone Orientation in
Semiconducting Polymers. Polymer 2015, 59, A1–A15.
Yokoyama, D. Molecular Orientation in Small-Molecule
Organic Light-Emitting Diodes. J. Mater. Chem. 2011, 21,
Experimental details and characterization data (PDF)
(
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AUTHOR INFORMATION
Corresponding Authors
12)
(13)
*
*
E-mail: htakeshi@scl.kyoto-u.ac.jp
E-mail: wakamiya@scl.kyoto-u.ac.jp
1
9187–19202.
(
14)
Wakamiya, A.; Nishimura, H.; Fukushima, T.; Suzuki, F.;
Saeki, A.; Seki, S.; Osaka, I.; Sasamori, T.; Murata, M.;
Murata, Y.; Kaji, H. On-Top π-Stacking of Quasiplanar
Molecules in Hole-Transporting Materials: Inducing
Anisotropic Carrier Mobility in Amorphous Films. Angew.
Chem., Int. Ed. 2014, 53, 5800–5804.
Nishimura, H.; Ishida, N.; Shimazaki, A.; Wakamiya, A.;
Saeki, A.; Scott, L. T.; Murata, Y. Hole-Transporting
Materials with a Two-Dimensionally Expanded π-System
around an Azulene Core for Efficient Perovskite Solar Cells.
J. Am. Chem. Soc. 2015, 137, 15656–15659.
Nishimura, H.; Hasegawa, Y.; Wakamiya, A.; Murata, Y.
Development of Transparent Organic Hole-Transporting
Materials Using Partially Oxygen-Bridged Triphenylamine
Skeletons. Chem. Lett. 2017, 46, 817–820.
Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T.
Organometal Halide Perovskites as Visible-Light Sensitizers
for Photovoltaic Cells. J. Am. Chem. Soc. 2009, 131, 6050–
Author Contributions
All authors have given approval to the final version of the
manuscript.
ORCID
Takeshi Hasegawa: 0000-0001-5574-9869
Akinori Saeki: 0000-0001-7429-2200
Yasujiro Murata: 0000-0003-0287-0299
Richard Murdey: 0000-0001-7621-9664
Atsushi Wakamiya: 0000-0003-1430-0947
(15)
(
(
16)
17)
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
This work was partially supported by the ALCA (JPMJAL
6051.
1
603), ERATO (JPMJER 1302), and COI programs from the
(18)
Green, M. A.; Hishikawa, Y.; Dunlop, E. D.; Levi, D. H.;
Hohl-Ebinger, J.; Ho-Baillie, A. W. Y. Solar Cell Efficiency
Tables (Version 52). Prog. Photovoltaics Res. Appl. 2018, 26,
Japanese Science and Technology Agency (JST), and NEDO.
T.N. thanks the JSPS for a Research Fellowship for Young
Scientists. 2D–GIXD experiments were performed at the
BL46XU of Spring-8 with the approval of the Japan
Synchrotron Radiation Research Institute (JASRI; Proposal
No. 2018B1617). We thank Dr. T. Koganezawa (JASRI) and Dr.
M. Wakioka for advice on the 2D–GIXD measurements.
4
27–436.
(
(
19)
Wu, W.-Q.; Chen, D.; Caruso, R. A.; Cheng, Y.-B. Recent
Progress in Hybrid Perovskite Solar Cells Based on n-Type
Materials. J. Mater. Chem. A 2017, 5, 10092–10109.
Kim, J. H.; Chueh, C. C.; Williams, S. T.; Jen, A. K. Y. Room-
Temperature, Solution-Processable Organic Electron
20)
Extraction
Layer
for
High-Performance
Planar
Heterojunction Perovskite Solar Cells. Nanoscale 2015, 7,
17343–17349.
Yoon, H.; Kang, S. M.; Lee, J. K.; Choi, M. Hysteresis-Free
Low-Temperature-Processed Planar Perovskite Solar Cells
with 19.1% Efficiency. Energy Environ. Sci. 2016, 9, 2262–
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