A peri-Xanthenoxanthene Centered Columnar-Stacking Organic Semiconductor for Efficient, Photothermally Stable Perovskite Solar Cells

Article abstract of DOI:10.1002/chem.201806015

Modulating the structure and property of hole-transporting organic semiconductors is of paramount importance for high-efficiency and stable perovskite solar cells (PSCs). This work reports a low-cost peri-xanthenoxanthene based small-molecule P1, which is

Full text of DOI:10.1002/chem.201806015

A Journal of
Accepted Article
Title: A peri-Xanthenoxanthene Centered Columnar-Stacking Organic
Semiconductor for High-Efficiency, Photothermal Stable
Perovskite Solar Cells
Authors: Niansheng Xu, Yang Li, Ruihan Wu, Rui Zhu, Jidong Zhang,
Shaik Zakeeruddin, Hangying Li, Ze-Sheng Li, Michael
Graetzel, and Peng Wang
This manuscript has been accepted after peer review and appears as an
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using the Digital Object Identifier (DOI) given below. The VoR will be
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content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.201806015
Link to VoR: http://dx.doi.org/10.1002/chem.201806015
Supported by

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10.1002/chem.201806015
COMMUNICATION
A peri-Xanthenoxanthene Centered Columnar-Stacking Organic
Semiconductor for High-Efficiency, Photothermal Stable
Perovskite Solar Cells
Niansheng Xu, Yang Li, Ruihan Wu, Rui Zhu, Jidong Zhang, Shaik M. Zakeeruddin, Hanying Li, Ze-
Sheng Li, Michael Grätzel and Peng Wang*
Abstract: Modulating the structure and property of hole-transporting
organic semiconductors is of paramount importance for high-
efficiency and stable perovskite solar cells (PSCs). Herein we report
a low-cost peri-xanthenoxanthene based small-molecule P1, which is
prepared at a total yield of 82% via a three-step synthetic route from
the low cost starting material 2-naphthol. P1 molecules stack in one-
dimensional columnar arrangement characteristic of strong
intermolecular – interactions, contributing to the formation of a
solution-processed, semicrystalline thin-film exhibiting one order of
magnitude higher hole mobility than the amorphous one based on the
and purification cost, low hole mobility, and severe degradation of
photovoltaic performance under a thermal stress. So far only a
5
few hole-transporting candidates can achieve comparable
photovoltaic performance to spiro-OMeTAD, with PCEs close to
or exceeding 20%.1k,4f,4i,4j,6 On the other hand, unlike inorganic7
8
and polymeric hole-transporting materials, small-molecule based
hole-transporters are scarcely reported for stable perovskite solar
9
cells under thermal and light dual stress. In 2018, Shang and his
co-workers10 utilized a benzodipyrrole-based hole-conductor for a
o
17.2%-efficiency PSC exhibiting good stability at 35 C. Later, Lee
et al.1k reported a fluorene-terminated hole-conductor for a 60 C
o
state-of-the
art
hole-transporter,
2,2-7,7-tetrakis(N,N′-di-
paramethoxy-phenylamine 9,9′-spirobifluorene (spiro-OMeTAD).
PSCs employing P1 as the hole-transporting layer attain a high
efficiency of 19.8% at the standard AM1.5G conditions, and good
stable PSC stored in the dark.
o
long-term stability under continuous full sunlight soaking at 40 C.
Organic–inorganic halide PSCs have attracted great attention
owing to inexpensive precursors for photoactive layer, energy-
saving fabrication protocols, and high power conversion
1
efficiencies (PCEs). Improving the long-term operational stability
2
is crucial for future practical application of PSCs. In a typical PSC,
a hole-transporting layer is indispensable affording selective hole-
extraction from the photo-excited perovskite and their
transportation to the positive electrode. In this context, a lot of
organic semiconductors have been explored,3,4 among which the
state-of-the-art spiro-OMeTAD is very efficient. For large-scale
application, spiro-OMeTAD is however limited by high preparation
[
[
a]
b]
N. Xu, Prof. P. Wang
Center for Chemistry of Novel & High-Performance Materials
Department of Chemistry
Zhejiang University
Hangzhou 310028, China
Y. Li, Dr. S. M. Zakeeruddin, Prof. M. Grätzel
Laboratory of Photonics and Interfaces
Institute of Chemical Sciences & Engineering
ꢀ
cole Polytechnique Fꢁdꢁrale de Lausanne
CH-1015 Lausanne, Switzerland
Y. Li, Prof. J. Zhang
Changchun Institute of Applied Chemistry
Chinese Academy of Sciences, Changchun 13002, China
R. Wu, Prof. H. Li
[
c]
d]
[
Figure 1. (a) Chemical structure of P1, (b) HOMO distribution, and (c) single-
crystal optical microscopy image of P1. Crystal structures along (d) a axis view
and (e) b axis view. (f) Transfer characteristic of single-crystal field-effect
transistor (VDS = −120 V).
Department of Polymer Science and Engineering
Zhejiang University
Hangzhou 310027, China
[e]
R. Zhu, Prof. Z Li
School of Chemistry
Beijing Institute of Technology
Beijing 10081, China
Our strategy for high-performance small-molecule based hole-
conductors in PSCs consists in the functionalization of a planar
π-conjugated two-dimensional heterocycle with multiple electron-
donating bisarylamino substituents to fine-tune its orbital energy,
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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and significantly enhance hole mobility via core-to-core stacking.
bis(trifluoromethylsulfonyl)imide and 4-tert-butylpyridine as
1
1
−4
As a model example, we design a peri-xanthenoxanthene (PXX)
additive, the hole mobility of P1 film was improved to 8.55 × 10
3
3
9
9
2
−1 −1
centered
small-molecule,
N ,N ,N ,N -tetrakis(4-
cm V
s
(Figure S4), which is around four times higher than
−4
2
−1 −1
methoxyphenyl)xantheno[2,1,9,8-klmna]xanthene-3,9-diamine
that of doped spiro-OMeTAD (1.89 × 10 cm V s ).
(
abbreviated as P1, Figure 1a), for high-performance PSCs.
The synthesis of P1 was carried out in a total yield of 82%, via
three-steps of aerobic C−H/C−O cyclization,12 bromination, and
Buchwald-Hartwig cross-coupling from low-cost 2-naphthol
(
Scheme S1). Note that the overall cost of materials for 1 gram of
P1 in our lab (Table S1) is estimated to be less than one tenth of
that for spiro-OMeTAD.5j In addition, only one-time column
purification is required during the whole synthetic process, making
large-scale synthesis and application viable. Density functional
theory (DFT) calculations reveal that the highest occupied
molecular orbital (HOMO) energy level of P1 is −4.57 eV in
vacuum, which is slightly deeper than that of spiro-OMeTAD
(
−4.49 eV). As depicted in Figure 1b, the HOMO of P1 is
delocalized over the whole -conjugated skeleton, which is crucial
for intermolecular hole-hopping. Thermogravimetric analysis
(
TGA) shows that P1 exhibits excellent thermal stability, with a

370 °C decomposition temperature (Figure S1).
To fully elucidate the molecular packing and charge-transport
characteristics of P1 in solid state, we grew its single-crystals by
slowly evaporating saturated methanol/dichloromethane
a
solution. The as-grown P1 single-crystals present an overall
crack-free morphology and distinct borders (Figure 1c). X-ray
crystallographic structure analysis shows that it forms one-
dimensional π-stacked columnar motif along the b axis with an
interplanar distance of 3.71 Å (Figure 1d,e, Figure S2, Table
S2).13 Each layer is further stapled in a staggered fashion. One
column links to adjacent columns with multiple C-H/ and C-H/O
interactions to form a three-dimensional network. To determine
the hole mobility for the P1 single-crystal, a bottom-gate/top-
contact single-crystal field-effect transistor was fabricated. The
transfer curve (Figure 1f) display typical hole transport
characteristics and an impressive hole mobility of ~2.5910 cm2
−3
−1
−1
V
s . Theoretical calculations were further performed based on
the experimental single-crystal structure to gain deeper insight
into the charge transport characteristics. The hole mobility of P1
was theoretically calculated by using the Einstein relation
= eD/ kBT based on the hopping model.14 With a large charge

transfer integral along the axis of the column (~26 meV), the
calculated mobility can reach up to 1.7910−2 cm V
2
−1 −1
s
(Figure
S3 and Table S3).
We
further
executed
space-charge-limited
current
measurements with a P1 thin film spun from chlorobenzene with
a hole-only device. The average hole mobility of dopant-free P1
is 5.7310− cm V s−1 (Figure S4), over 10 times higher than
5
2
−1
that of non-doped spiro-MeOTAD (4.9510 cm2 V−1 s ).
Grazing-incidence wide-angle X-ray scattering (GIWAXS) of the
P1 thin film was measured to probe the mesoscale molecular
ordering, which is vital to the charge transport property. Figure 2a
reveals the semicrystalline nature of spin-coated P1 film. The
−6
−1
Figure 2. (a) GIWAXS maps for spin-coated thin films of P1. (b) J–V
characteristic of a typical PSC with P1 measured under AM 1.5G illumination.
The data for a control cell with spiro-OMeTAD is also included. (c) IPCE spectra
and integrated Jsc from the IPCE curves.
−1
isotropic diffraction ring at 17 nm , is related to a d-spacing of
0.37 nm, which is well consistent with the -stacking reflection (0,
2, 0) from the single-crystal data (Table S2). In view of the
amorphous feature of a spin-coated spiro-OMeTAD thin film
Figure S5),15 the observed -stacking ordering of the P1 thin film
explains its much higher hole mobility. With lithium
To gain insight into the ability of P1 to extract holes from a
perovskite layer, we carried out steady-state photoluminescence
(
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Chemistry - A European Journal
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COMMUNICATION
(
PL)
and
time-resolved
photoluminescence
(TRPL)
long-term stability towards environmental factors. We tested the
hydrophobicity of P1 through measuring the contact angle formed
by a thin film with a water droplet. The decreased wettability of the
water droplet on the doped P1 based thin film compared to the
measurements. Compared with
(
the PL intensity of a P1-coated perovskite film is significantly
a
bare perovskite film
Cs0.05(FA0.83MA0.17
)0.95Pb1.03Br0.48
I
2.52, abbreviated as Cs
5
M),16
attenuated (Figure S6a). The PL quenching yield with P1 is 98.0%, doped spiro-OMeTAD film is apparent from Figures 3e and 3f
almost the same as the sample with spiro-OMeTAD (97.8%). As
shown in Figure S6b, the bare perovskite film presents a long PL
lifetime of ~243 ns. Once coated with organic semiconductors PL
lifetimes decrease greatly, being 29.7 ns for the P1 sample and
which illustrate that the contact angle increases from 78.7 ° for the
spiro-OMeTAD film to 92.2 ° for P1. We also monitored the cell
stability in ambient atmosphere (Figure 3c and S12). The un-
encapsulated device employing P1 retained 95% of the initial
efficiency after 1000 h. In contrast, the spiro-OMeTAD control
device showed only 80% retention. In addition, the photovoltaic
performance remained almost unaltered in argon filled box
(Figure S13 and Table S6) for both devices, confirming that
moisture induced the degradation of the perovskite solar cell. The
increased VOC after 60 days could be related to the enlargement
of perovskite grains triggered by spontaneous crystal
coalescence.18
40.6 ns for the spiro-OMeTAD sample. Note that the HOMO
energy level of P1 derived from ultraviolet photoelectron
spectroscopy measurement (Figure S7) is −5.25 eV, lower than
that of spiro-OMeTAD (−5.13 eV). Interestingly, the hole
extraction kinetics and yield do not decrease even with reduced
hole extraction driving force.
We then used doped P1 as the hole-transporting layer to
fabricate PSCs with Cs M as light-absorber. The details for cell
5
fabrication are described in the Supporting Information. From
atomic force microscopy (Figure S8a) and scanning electron
microscopy (Figure S8c) images, some pinholes were observed
for the doped P1 film on the perovskite layer. We optimized the
thickness of hole-transporting layer in PSCs by use of 40, 50, 60,
or 70 mM P1 in chlorobenzene. The photovoltaic parameters were
shown in Figure S9 and Table S4. In average, the cells fabricated
with 50 mM P1 presents the highest Jsc and FF, leading to a
champion PCE. The J-V curves and incident-photon-to-collected-
electron conversion efficiency (IPCE) spectra of the best-
performed devices based on P1 and spiro-OMeTAD are shown in
Figure 2b and 2c, respectively. The device with P1 has a Jsc of
-2
2
3.184 mA cm , a VOC of 1118 mV, and an FF of 76.1%,
generating a high PCE of 19.8%, which is comparable to the
control cell with spiro-OMeTAD. Furthermore, the maximum
power point tracking under AM 1.5G illumination (Figure S10) was
conducted to eliminate the scan rate dependent device
performance, and we got the stabilized PCE of 19.3% and 19.5%
for P1 and spiro-OMeTAD based devices, respectively, which are
similar with the results derived from the J-V curves. The integrated
photocurrent densities calculated from the IPCE spectra of
devices (Figure 2c) agreed closely with those obtained from the
J-V curves, indicating that the spectrum of our simulator matches
well with AM-1.5 standard solar radiation. The distributions of the
photovoltaic parameters obtained from three batches were shown
in Figure S11 and Table S5, which indicates the good
reproducibility of the P1 based perovskite solar cells.
Photostability of PSCs (not encapsulated) was first evaluated
with maximum power point (MPP) tracking under continuous light
irradiation at ambient temperature in a nitrogen atmosphere.
Figure 3a shows that the P1 based cell retained >95% of its initial
efficiency after 300 h, better than the spiro-OMeTAD control.
Further, we investigated the cell stability under the thermal and
light dual stress (Figure 3b). After 1000 h continuous one sun
illumination at 40 ºC, the P1 cell retained over 80% of its initial
efficiency, which is much higher than that of 17% for the spiro-
OMeTAD control. The enhanced operational stability of P1 based
Figure 3. Evolution of normalized PCEs of un-encapsulated devices examined
−2
at MPP tracking under continuous light irradiation (AM 1.5ꢂG, 100ꢂmWꢂ cm ) at
o
(
a) room temperature and (b) 40 C under nitrogen. (c) Stability of un-
encapsulated devices in air (humidity: 20−40%) under indoor light and room
temperature. (d) Differential scanning calorimeter curves. Contact angle images
of water on doped (e) P1 and (f) spiro-OMeTAD films.
g
cell probably results from the improved T (Figure 3d), giving an
indication for further design of organic semiconductors for PSCs.
Moisture induced decomposition of the perovskite layer is the
critical degradation path2a,17 in PSCs. Therefore, the
hydrophobicity of the hole-transporting layer is also vital to the
To summarize, we have synthesized a peri-xanthenoxanthene
-centered small-molecule as
a remarkably cheap organic
semiconductor for efficient and stable perovskite solar cells. The
improved glass transition temperature and hydrophobicity of the
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COMMUNICATION
hole-transporting layer jointly contributed to the enhanced long-
term device stability. The planar discotic conjugated backbone
enables the formation of a one-dimensional columnar stacking via
intermolecular - interactions, contributing to the high hole
mobility of solution-processed thin films.
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Acknowledgements ((optional))
N.X. and Y.L. contributed equally to this work. P.W. express
sincere gratitude to the National 973 Program (2015CB932204),
the National Science Foundation of China (No. 51673165 and No.
91733302), the Key Technology R&D Program (BE2014147-1) of
Science and Technology Department of Jiangsu Province, and
the Fundamental Research Fund (No.2017FZA3007) for the
Central Universities for financial support. We thank beamline
BL14B1 (Shanghai Synchrotron Radiation Facility) for providing
the beam time during GIWAXS experiments.
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Keywords: polycycles • organic semiconductor • charge transport
•
solar cell • photostability
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COMMUNICATION
COMMUNICATION
Easy, high-efficiency and stable: A
Niansheng Xu, Yang Li, Ruihan Wu, Rui
Zhu, Jidong Zhang, Shaik M.
low-cost
peri-xanthenoxanthene-
centred columnar-stacking organic
semiconductor has been prepared and
used as hole-transporter in perovskite
Zakeeruddin, Hanying Li, Ze-Sheng Li,
Michael Grätzel and Peng Wang*
solar cells achieving
conversion efficiency of 19.8ꢂ% and
good long-term stability under
a
power
Page No. – Page No.
A peri-Xanthenoxanthene Centered
Columnar-Stacking Organic
Semiconductor for High-Efficiency,
Photothermal Stable Perovskite Solar
Cells
continuous full sunlight soaking at 40
o
C.
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