Hexa-substituted benzene derivatives as hole transporting materials for efficient perovskite solar cells

Article abstract of DOI:10.1016/j.dyepig.2018.12.010

Two low cost hexa-substituted benzene derivatives, namely HFB-OMeDPA and HPB-OMeDPA, have been successfully utilized as hole transporting materials (HTMs) for efficient perovskite solar cells (PSCs). The relationships between molecular structure, electronic properties of the semiconductor and eventually the photovoltaic performance are investigated. The planar PSCs employing HPB-OMeDPA as HTM exhibit excellent power conversion efficiencies exceeding 16% under AM 1.5G illumination conditions, which are comparable to the reference cells based on spiro-OMeTAD.

Full text of DOI:10.1016/j.dyepig.2018.12.010

Accepted Manuscript
Hexa-substituted benzene derivatives as hole transporting materials for efficient
perovskite solar cells
Yuan Liu, Chen Tao, Guohua Xie, Jeroen Van Der Velden, Sergio Marras, Zhenghui
Luo, Xuan Zeng, Annamaria Petrozza, Chuluo Yang
PII:
S0143-7208(18)31992-2
https://doi.org/10.1016/j.dyepig.2018.12.010
DYPI 7215
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 8 September 2018
Revised Date: 24 October 2018
Accepted Date: 6 December 2018
Please cite this article as: Liu Y, Tao C, Xie G, Van Der Velden J, Marras S, Luo Z, Zeng X, Petrozza
A, Yang C, Hexa-substituted benzene derivatives as hole transporting materials for efficient perovskite
solar cells, Dyes and Pigments (2019), doi: https://doi.org/10.1016/j.dyepig.2018.12.010.
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ACCEPTED MANUSCRIPT
MS Title: Hexa-substituted benzene derivatives as hole transporting materials
for efficient perovskite solar cells
Authors: Yuan Liu, Chen Tao, Guohua Xie, Jeroen Van Der Velden, Sergio Marras,
Zhenghui Luo, Xuan Zeng, Annamaria Petrozza and Chuluo Yang
Graphical Abstract
Two hexa-substituted benzene compounds have been successfully utilized as
efficient hole transporting materials (HTMs) for high performance perovskite solar
cells. The relationships between molecular structure, electronic properties of the
semiconductor and eventually the photovoltaic performance are investigated. Both
compounds exhibit good solubility, excellent thermal stability, high hole mobility and
suitable energy levels. Perovskite solar cells employed these two HTMs exhibit
impressive PCEs of 15.9% and 16.7%, respectively, which are comparable to the
reference cells based on spiro-OMeTAD.

ACCEPTED MANUSCRIPT
Hexa-
s
ubstituted benzene derivatives as hole transporting materials for
efficient perovskite solar cells
a,1
c,1,*
b
c
d
Yuan Liu , Chen Tao , Guohua Xie , Jeroen Van Der Velden , Sergio Marras ,
b
b
c
b,*
Zhenghui Luo , Xuan Zeng , Annamaria Petrozza and Chuluo Yang
a
School of Chemical Engineering and Light Industry, Guangdong University of Technology,
Guangzhou 510006, China.
b
Hubei Key Lab on Organic and Polymeric Optoelectronic Materials, Department of Chemistry,
Wuhan University, Wuhan 430072, China.
c
Center for Nano Science and Technology @PoliMi, Istituto Italiano di Tecnologia, via Giovanni
Pascoli 70/3, 20133 Milan, Italy.
d
Department of Nanochemistry, Istituto Italiano di Tecnologia via Morego 30, 16163 Genova, Italy.
1
These authors contributed equally to this work.
*
Corresponding author, E-mail: clyang@whu.edu.cn (C. Yang); taochen635@whu.edu.cn (C. Tao)
Abstract
Two low cost hexa-substituted benzene derivatives, namely HFB-OMeDPA and HPB-
OMeDPA, have been successfully utilized as hole transporting materials (HTMs) for
efficient perovskite solar cells (PSCs). The relationships between molecular structure,
electronic properties of the semiconductor and eventually the photovoltaic performance
are investigated. The planar PSCs employing HPB-OMeDPA as HTM exhibit excellent
power conversion efficiencies exceeding 16% under AM 1.5G illumination conditions,
which are comparable to the reference cells based on spiro-OMeTAD.
Keywords: Hole transporting material; Hexa-substituted; Perovskite solar cell; Low
cost
1

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1
. Introduction
Organic-inorganic metal halide perovskite solar cells (PSCs) have attracted
significant attention in the field of photovoltaics due to the boosting power conversion
efficiencies (PCEs) exceeding 22% during the past few years [1-4]. The superior
photovoltaic performances of PSCs originate from the unique opto-electronic properties
of perovskite, such as direct bandgap, ambipolar charge mobilities, long carrier diffusion
lengths, high extinction coefficient in a wide absorption range [5-8]. Many efforts have
been devoted to the optimization of PSCs, including device architectures, the quality of
perovskite films, and interfacial engineering, e.g., the charge transport materials between
perovskites and electrodes [9]. The hole transporting materials (HTMs) that transfer holes
from the perovskite layer to the hole-collecting electrodes and reduce the electron-hole
recombination by blocking the electrons, play crucial roles in achieving high performance
PSCs. Ideal HTMs are required to have appropriate energy levels, high hole mobility and
conductivity, solution processibility, low cost and reproducibility. In this regard, small
molecular HTMs possessing dominant advantages over inorganic semiconductors or
polymers are deliberately designed. The state-of-the-art small molecule, (2,2’,7,7’-
tetrakis[N,N-di-p-methoxyphenylamine]-9,9’-spirobifluorene) (spiro-OMeTAD), is the
most commonly used and efficient HTM in PSCs [10-12]. However, the complicated
synthesis and purification procedures make it expensive and, therefore, limit its practical
applications in PSCs and hence the future commercialization.
Significant efforts have been dedicated towards designing and synthesizing new HTMs
[13-15]. A large number of small molecular HTMs based on ethylenedioxythiophene,
cruciform oligothiophenes, pyrene, triptycene, triazines, conjugated quinolizino acridine,
2

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planar triphenylamine, furan, silolothiophene, carbazole, tetrathiafulvalene, phenoxazine,
paracyclophane and ethene-tetraarylamine cores have been reported [13-28]. Although
the device performances based on these HTMs are similar to or even superior than those
of the devices with spiro-OMeTAD in some aspects, there still remains a concern of
commercialization when taking cost, efficiency and stability into consideration. Besides,
the relationships between molecular structures and hole-transport properties as well as the
photovoltaic performances are not fully addressed. In this regard, further exploration of
advanced HTMs is significant to both industry and academia.
Star-shaped molecules typically comprise a central core and multiple conjugated arms
as the functional groups. Owing to the well-defined structure with precise molecular
weights, star-shaped molecules combine the advantages of both small molecules and
polymers, such as good thermal stability, well film-forming property, and good
reproducibility [29-31]. The 3D structure of spiro-OMeTAD has been widely
investigated, in which four dimethoxydiphenylamine units attach to the spiro core.
Similar to the spiro-OMeTAD, the HTMs with different numbers of the attaching arms
have been discussed as well [32,33]. It is well known that their photophysical,
electrochemical, thermal properties as well as the hole mobilities strongly rely on the
central core, the linkage bridge and the numbers of arms. However, the effect of multi-
armed, e.g. hexa-substituted molecules have rarely been reported. It should be noted that
during the preparation of this manuscript, Bo and his coworkers just published their
research of one flower-shaped molecule as HTM for perovskite solar cells, which is
identical to our study [34]. The hexarylbenzene-based compounds possess sterically
3

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crowded structures, in which six aryl groups are grafted onto the central benzene core and
can be readily modified on the peripheral aryl groups.
In this work, two hexarylbenzene-based compounds with six dimethoxydiphenylamine
groups linked by fluorenyl or phenyl bridges attaching to the central benzene core,
namely
7,7',7'',7''',7'''',7'''''-(phenyl-1,2,3,4,5,6-hexakis(9,9-dihexyl)-N,N-bis(4-
methoxyphenyl)-9H-fluoren-2-amine) (HFB-OMeDPA) and 1,2,3,4,5,6- hexakis(((4-
methoxyphenyl)amine)phenyl)benzene (HPB-OMeDPA), are synthesized (Scheme 1).
The hydrotropy dimethoxydiphenylamine units and additional flexible alkyl chains allow
both compounds with extremely high solubility. Moreover, owing to the different linkage
bridges, these two compounds exhibit different photophysical, electrochemical, thermal
and hole transport properties. The PSCs employing these compounds as HTMs deliver
average conversion efficiencies of 15.4% and 16.4%, respectively, which are comparable
to the efficiency of the controlled device with spiro-OMeTAD of 17.1%. We show that
the additional flexible alkyl chains in the HFB-OMeDPA influence the packing mode in
solid state, resulting in a relatively lower hole mobility. The photoluminescence
quenching indicates that the hole transfer from perovskites to both HPB-OMeDPA and
HFB-OMeDPA is comparably efficient to the spiro-OMeTAD case. These results
demonstrate that the design of linkage bridges in the hexarylbenzene-based compounds
are particularly important, and provide a fundamental understanding for further
optimizing HTMs for PSCs.
4

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Scheme 1. Chemical structures of HFB-OMeDPA and HPB-OMeDPA.
2
. Results and discussion
2
.1 Synthesis and characterization
The synthetic routes of the two compounds are depicted in Scheme S1, and the detailed
synthetic procedures are provided in the supporting information. The key intermediate of
,2,3,4,5,6-hexakis(7-bromo-9,9-dihexyl-9H-fluoren-2-yl)benzene (1) was synthesized
1
via cyclotrimerisation of diarylethyne catalyzed by dicobaltoctacarbonyl [30]. Another
key intermediate of hexakis(4-bromophenyl)benzene (2) was readily prepared from
hexaphenylbenzene by bromination using neat bromine [35]. The final products of HFB-
OMeDPA and HPB-OMeDPA were obtained by the Buchwald-Hartwig coupling
reaction of 1 or 2 with 4,4’-dimethoxydiphenylamine with over 70% yield. The structures
1
13
of HFB-OMeDPA and HPB-OMeDPA were fully characterized by H NMR, C NMR,
mass spectrometry and elemental analysis, and the data were all in good agreement with
the designed structures. Moreover, owing to the peripheral dimethoxydiphenylamine
units and additional flexible alkyl chains, all the compounds exhibit excellent solubility
in the common solvents, such as chloroform and chlorobenzene. The lab synthesis costs
5

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-1
-1
of HFB-OMeDPA and HPB-OMeDPA are estimated at 12.95 US$ g and 10.27 US$ g ,
respectively, which are approximately a tenth of the costs of purified spiro-OMeTAD
[36]. The detailed costs are listed in Table S1.
2
.2 DFT calculation
To estimate the frontier molecular orbitals of the designed molecules, the density
functional theory (DFT) calculations were performed. The HOMO and LUMO
distributions for the optimized ground-state geometries of these molecules are shown in
Fig. 1. The HOMO of HFB-OMeDPA is uniformly distributed over the molecule, while
the HOMO of HPB-OMeDPA is located in one arm. For both molecules, the six arms are
highly twisted with respect to the benzene ring in order to alleviate the steric hindrance.
The respective dihedral angles between these two planes are 66.2º for HFB-OMeDPA
and 61.2º for HPB-OMeDPA. Owing to the relatively larger dihedral angle, HFB-
OMeDPA exhibits lower conjugation degree than HPB-OMeDPA. Besides, the flexible
alkyl chains in the fluorenyl bridge of HFB-OMeDPA are free from the molecule plane,
which inhibits the molecular packing and weakens the intermolecular interaction.
6

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Fig. 1. The HOMO and LUMO distributions of HFB-OMeDPA and HPB-OMeDPA
using DFT calculations.
2
.3 Photophysical, electrochemical and thermal properties
UV-Vis absorption spectra of HFB-OMeDPA and HPB-OMeDPA in dichloromethane
*
and neat films are illustrated in Fig. 2a. Both compounds show intense π-π absorption
bands that mainly locate in the UV range. The nearly optical transparency of these two
compounds in visible and near infrared region leads to negligible parasitic absorption in a
solar cell. The absorption maximum is 306 nm for HPB-OMeDPA. It shifts to 360 nm for
HFB-OMeDPA in that the π bridge changes from phenyl to fluorenyl unit. HFB-
OMeDPA exhibits analogical absorption spectra in solution and films. However, the
absorption maximum of HPB-OMeDPA films (314 nm) appears a slight redshift
compared to that in dichloromethane, which indicates the emergence of stronger
intermolecular interaction originated from the π-π stacking in solid state. The absorption
spectra of spiro-OMeTAD in dichloromethane and neat film are carried out for
7

ACCEPTED MANUSCRIPT
comparation. Calculated from the onsets of the optical absorption spectra, the energy
gaps (Egs) of HFB-OMeDPA, HPB-OMeDPA and spiro-OMeTAD are estimated to be
3
.05, 3.19 and 2.98 eV, respectively. The UV/Vis absorption spectra of HTMs and
dopants (4-tert-butylpyridine and lithium bis(trifluoromethylsulphonyl)imide) in films
are illustrated in Fig. S1. The doped HTMs exhibit slightly higher absorbance in the
range of 250~270 nm than the respective undoped HTMs, which might be attributed to
the absorption of 4-tert-butylpyridine.
The electrochemical properties of these compounds are investigated by cyclic
voltammetry (CV) measurements. As shown in Fig. 2b, all compounds display reversible
oxidation behavior in dichloromethane, which can be assigned to the oxidation of
peripheral dimethoxydiphenylamine moieties. HFB-OMeDPA, HPB-OMeDPA and
spiro-OMeTAD exhibit the onset of oxidation potential of 0.57, 0.60 and 0.47 V,
respectively. Considering the onset of oxidation potential (0.48 V in dichloromethane)
and vacuum energy level (-5.1 eV) of ferrocene [37], the HOMO levels are established to
be -5.19, -5.22 and -5.09 eV for HFB-OMeDPA, HPB-OMeDPA and spiro-OMeTAD,
respectively. Deduced from the optical
g
E s and HOMO levels, the respective LUMO
levels are -2.32, -2.32 and -2.11 eV. It should be noted that HFB-OMeDPA and HPB-
OMeDPA show no overlapped oxidation waves assigned to the through-space interaction
between arms. This implies that the hole wavefunction is delocalized over peripheral
dimethoxydiphenylamine moieties and the arms are independent to each other [29]. The
HOMO levels of HFB-OMeDPA and HPB-OMeDPA match well with the valence band
(
VB) of CH
3
NH
3
PbI
3
(-5.43 eV) and the work function of Au (-5.0 eV), thereby favoring
NH PbI /HTM interface. Besides, the LUMO levels of both
the hole transfer at CH
3
3
3
8

ACCEPTED MANUSCRIPT
3 3 3
compounds are higher than the conduction band (CB) of CH NH PbI (-3.92 eV), which
could block the electron transfer from perovskite to anode (Au) and hence suppress
interfacial charge recombination.
a)1.0
b)
HFB-OMeDPA
HPB-OMeDPA
HFB-OMeDPA solution
HFB-OMeDPA film
HPB-OMeDPA solution
HPB-OMeDPA film
Spiro-OMeTAD solution
Spiro-OMeTAD film
0.8
0.6
0.4
0.2
0.0
Spiro-OMeTAD
+
Fc/Fc
250
300
350
400
450
500
550
600
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
Wavelength (nm)
Potential (V)
Fig. 2. a) UV/Vis absorption spectra of HFB-OMeDPA, HPB-OMeDPA and spiro-
OMeTAD in dichloromethane and neat films, respectively. b) Cyclic voltammograms of
HFB-OMeDPA, HPB-OMeDPA and spiro-OMeTAD in dichloromethane for oxidation,
respectively.
The thermal properties of HFB-OMeDPA and HPB-OMeDPA are investigated by
thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). As
shown in Fig. S2, both compounds exhibit high decomposition temperatures (Td) of
o
o
above 420 C, which are close to that of spiro-OMeTAD (445 C) and far above
temperatures for conventional device operation [36]. The glass transition temperatures
o
(
Tgs) for HFB-OMeDPA and HPB-OMeDPA are 199 and 160 C, respectively, which are
o
both higher than that of spiro-OMeTAD (120 C) [35]. The high Tgs are mainly ascribed
to the rigidity of the hexarylbenzene architecture, in which six bulky arms grafted onto
single benzene prohibit the free rotation of the arms. The good thermal stability of HFB-
9

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OMeDPA and HPB-OMeDPA are favored for the lifetime of perovskite solar cells in
operation.
2
.4 Hole transporting property
In order to gain more insight into the hole transporting properties of HFB-OMeDPA
and HPB-OMeDPA, we carry out space-charge-limited current (SCLC) measurements
with the hole-only device configuration of ITO/PEDOT:PSS/HTM/Au. As shown in Fig.
S3, the hole mobility of each compound was determined by fitting the current density-
voltage (
J-V) curves to the Mott-Gurney law (Equation S1). The hole mobility of spiro-
OMeTAD was measured under the same experimental conditions for comparison, and
-4
2
-1 -1
evaluated to be (8.42 ± 1.44) x 10 cm V s . The hole mobility of HPB-OMeDPA is
comparable to that of spiro-OMeTAD, and one order of magnitude higher than that of
HFB-OMeDPA. The calculated hole mobilities of HFB-OMeDPA and HPB-OMeDPA
-5
-4
2
-1 -1
are (5.54 ± 0.62) x 10 and (7.38 ± 1.12) x 10 cm V s , respectively. Notably, HFB-
OMeDPA exhibits distinct light-soaking effect. Its hole mobility increases and reaches up
-4
2
-1 -1
to 1.65 x 10 cm V s upon light soaking. The higher hole mobility of HPB-OMeDPA
should be attributed to the stacking in solid state, which facilitates efficient
π-π
intermolecular charge transfer. However, the flexible alkyl chains in the fluorenyl bridge
of HFB-OMeDPA inhibit the aggregation of the molecules, resulting in relatively low
hole mobilities. The detailed parameters of HFB-OMeDPA and HPB-OMeDPA are
summarized in Table 1.
Table 1. Photophysical, electrochemical, thermal parameters and hole mobilities of HFB-
OMeDPA and HPB-OMeDPA.
10

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c
b
HOMO
e
f
a
film
abs
λ
abs
/λ
E
g
d
T
[°C]
d
/T
g
μ
h
HTM
/LUMO
eV]
2
-1 -1
[nm]
[eV]
3.05
3.19
[cm V s ]
[
HFB-
OMeDPA
-5
-4
-4
3
3
3
60/360
-5.19/-2.14
-5.22/-2.03
-5.09/-2.11
465/199 (5.54 ± 0.62) x 10
420/160 (7.38 ± 1.12) x 10
HPB-
OMeDPA
06/314
87/376
spiro-
2.98
-/-
(8.42 ± 1.44) x 10
OMeTAD
a
b
Measured in DCM solution. Calculated from the edge of the long wavelength
c
d
absorption. Determined from the onset of oxidation potentials. Deduced from HOMO
e
and Eg. Obtained from TGA and DSC measurements (Td, corresponding to 5% weight
f
loss). Measured in the space-charge limited current (SCLC) regime and fitted using the
Mott-Gurney law.
2
.5 Photoluminescence spectroscopy
3 3 3
Fig. 3a shows the normalized photoluminescence (PL) spectra of CH NH PbI films
with and without HTM deposited on glass substrates. The excitation wavelength was 460
nm that was beyond the HTM absorption spectra (shown in Fig. 2a). The pristine
perovskite sample exhibits a distinct PL emission peaking at 775 nm. After the
incorporation of different HTM layers, the PL significantly quenches. The PL quenching
degree increases as the order of HFB-OMeDPA<HPB-OMeDPA<spiro-OMeTAD
(shown in insert Fig. 3a).
The charge dynamics of the perovskite layer with and without HTMs are investigated
with time-resolved PL (TR-PL) measurements. The signals have been collected at 775
nm, where CH NH PbI films peak. The pristine CH NH PbI show a long-living signal.
As shown in Fig. 3b, the fitting lifetime for the pristine CH NH PbI film was 93.9 ns.
3
3
3
3
3
3
3
3
3
11

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3 3 3
However, the decay lifetimes of the CH NH PbI /HTM bilayer are found to be
significantly reduced. The lifetimes progressively shorten to be 10.1, 7.1 and 4.6 ns for
perovskites covered with HFB-OMeDPA, HPB-OMeDPA and spiro-OMeTAD,
respectively. This suggest that an efficient hole transfer is established at the
CH NH PbI /HTM interface.
3
3
3
0
b)10
a)1.0
P
P/HFB-OMeDPA
P/HPB-OMeDPA
P/Spiro-OMeTAD
0
.8
.6
.4
.2
.0
-
1
2
3
1
0
0
P
P/HFB-OMeDPA
P/HPB-OMeDPA
P/Spiro-OMeTAD
0
-
10
760
780
800
0
-
0
10
700 720 740 760 780 800 820 840 860 880
0
100
200
300
400
500
Wavelength (nm)
Time (ns)
Fig. 3. a) The steady-state photoluminescence (PL) spectra and b) time-resolved PL
spectra of CH NH PbI , CH NH PbI /HFB-OMeDPA, CH NH PbI /HPB-OMeDPA and
3
3
3
3
3
3
3
3
3
CH NH PbI /spiro-OMeTAD.
3
3
3
2
.6 Device performance
We use HFB-OMeDPA, HPB-OMeDPA and spiro-OMeTAD to fabricate planar
perovskite solar cells. Fig. 4 shows the cross-sectional scanning electron microscopy
SEM) images of a full device stack with MAPbI . The planar perovskite solar cells with
different HTMs were fabricated with TiO /crosslinked [6,6]-phenyl-C -butyric styryl
(
3
2
61
dendron ester (C-PCBSD) as the electron transport layer. Tao et al. had shown that the
incorporation of C-PCBSD at TiO /perovskite interface not only facilitates perovskite
2
grain growth but also significantly prohibit charge recombination [38]. The optimized
thicknesses of HFB-OMeDPA and HPB-OMeDPA are both ~100 nm, which is slightly
below the optimized thickness for spiro-OMeTAD of ~150 nm. Lithium
12

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bis(trifluoromethylsulfonyl)-imide (Li-TFSI) and 4-tert-butyl pyridine (t-BP) are the
additives for all the HTMs (detailed information can be found in supplementary
information).
Fig. 4. a) Energy level diagrams of the used materials. SEM cross section of photovoltaic
cells based on HFB-OMeDPA b), HPB-OMeDPA c) and spiro-OMeTAD d),
respectively. The scale bar is 100 nm.
Fig. 5 presents the current density versus voltage (J-V) characteristics of our champion
devices measured in air under air mass 1.5 global (AM 1.5G) conditions with a slow scan
-1
rate of 0.023 V s . The corresponding photovoltaic parameters, such as short-circuit
current density (JSC), open-circuit voltage (VOC), fill factor (FF), and PCE, are
summarized in Table 2. For comparison, the spiro-OMeTAD-based device exhibits the
best performance among them, reaching an average PCE of 17.1% with a VOC of 1.10 V,
13

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-2
a JSC of 21.2 mA cm , a FF of 73.1%. The HFB-OMeDPA-based devices yielded a V
OC
-2
of 1.09 V, a JSC of 19.9 mA cm , a FF of 71.4%, leading to an average PCE of 15.4%.
-2
The HPB-OMeDPA based device exhibited a VOC of 1.09 V, a JSC of 20.3 mA cm and a
FF of 73.5%, resulting in an average PCE of 16.4%.
The external quantum efficiency (EQE) spectra of the PSCs with different HTMs were
recorded and shown in Fig. 5d. The integrated Jscs calculated from the EQE spectra were
-2
1
9.2, 19.3, and 20.1 mA cm for HFB-OMeDPA, HPB-OMeDPA, and spiro-OMeTAD,
respectively, which is in agreement with those obtained from J-V curves. The EQE values
maintain above 70% in the range of 450-750 nm, which indicates the efficient light
harvesting and photon-to-electron conversion.
a)
b)
20
15
10
5
20
HFB-OMeTAD
Reverse Scan
15
10
5
HPB-OMeTAD
Reverse Scan
Forward Scan
Forward Scan
0
0
0
.0
0.2
0.4
0.6
Bias (V)
0.8
1.0
1.2
0.0
0.2
0.4
0.6
Bias (V)
0.8
1.0
1.2
100
d)
c)
2
1
1
0
5
0
5
80
60
40
20
0
Spiro-OMeTAD
Reverse Scan
Forward Scan
HFB-OMeDPA
HPB-OMeDPA
Spiro-OMeTAD
0
0
.0
0.2
0.4
0.6
Bias (V)
0.8
1.0
1.2
400
500
600
700
800
Wavelength (nm)
14

ACCEPTED MANUSCRIPT
Fig. 5. J-V characteristics of PSCs utilizing a) HFB-OMeDPA, b) HPB-OMeDPA and c)
-1
spiro-OMeTAD as HTMs with a scan rate of 0.023 V s , d) EQE spectra of PSCs based
on different HTMs.
Table 1. Photovoltaic parameters of the champion PSCs with different HTMs. (scanning
-1
rate: 0.023 V s )
VOC
V)
JSC
(mA cm )
FF
(%)
PCE
(%)
HTM
Scan direction
Reverse
-2
(
1.09
1.08
1.09
1.09
1.09
1.09
1.11
1.09
1.10
19.8
73.8
69.0
71.4
75.4
71.6
73.5
76.4
69.8
73.1
15.9
14.9
15.4
16.7
16.0
16.4
18.0
16.2
17.1
HFB-OMeDPA
HPB-OMeDPA
Spiro-OMeTAD
Forward
Average
Reverse
19.9
19.9
20.3
20.3
20.3
21.1
21.2
21.2
Forward
Average
Reverse
Forward
Average
We have been able to repeat these results over several batches as demonstrated by the
device statistics in Figs. S4 and S5 for HFB-OMeDPA and HPB-OMeDPA with 23
devices, respectively. The resulting device performances yield average PCEs of 14.1% ±
0
.4% and 15.2% ± 0.3% for HFB-OMeDPA and HPB-OMeDPA based devices,
respectively. The small deviations demonstrate the good reproducibility of the device
performance based on these HTMs. The PCE difference is mainly ascribed to VOC [(1.04
±
0.02) V vs (1.07 ± 0.02) V] and FF (69.0 ± 3.8% vs 73.3% ± 1.6%). Thanks to the same
15

ACCEPTED MANUSCRIPT
device structure before HTM deposition, the microstructure and defect states in MAPbI
3
are not taken into consideration for the VOC difference between devices with HFB-
OMeDPA and HPB-OMeDPA. However, the abovementioned stronger intermolecular
interaction in HPB-OMeDPA originated from the π-π stacking in solid state and hence
higher hole mobility might account for the average higher VOC in HPB-OMeDPA-based
PSCs [39]. The average higher FF in the devices with HPB-OMeDPA might be related to
the higher hole mobility of HPB-OMeDPA.
3
. Conclusions
In summary, we have successfully demonstrated two hexarylbenzene-based
compounds as efficient HTMs for high performance perovskite solar cells. These
compounds are comprised of six dimethoxydiphenylamine units and benzene core, which
are linked by fluorenyl or phenyl bridges, respectively. Both compounds exhibit good
solubility, excellent thermal stability, high hole mobility and suitable energy levels.
Perovskite solar cells employed these two HTMs exhibit impressive PCEs of 15.9% and
1
6.7%, respectively, which are comparable to the spiro-OMeTAD-based device. This
work encourages us to further optimize the HTMs based on the hexa-substituted benzene
derivatives, featuring considerably high performance and low cost.
Acknowledgements
We gratefully acknowledge financial support from the National Natural Science
Foundation of China (Nos. 51703035, 21572171 and 61575146), and the Innovative
Research Group of Hubei Province (No. 2015CFA014).
Appendix A. Supporting data
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Supplementary data associated with this article can be found in the online version at
http://dx.doi.org/
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Highlights:
►
Two low cost hexa-substituted benzene compounds are facile synthesized, in
which dimethoxydiphenylamine groups are linked by fluorenyl or phenyl bridges
attaching to the central benzene core.
►
►
The additional flexible alkyl chains in the compounds influence the packing
mode in solid state and the hole mobility.
Perovskite solar cells employed these HTMs exhibit impressive PCEs of 15.9%
and 16.7%, respectively, which are comparable to the reference cells based on
spiro-OMeTAD.