A new carbazole-based hole-transporting material with low dopant content for perovskite solar cells

Article abstract of DOI:10.1016/j.electacta.2016.05.203

A new carbazole-based hole-transporting material incorporating a 3,3′-biphenyl central core has been synthesized via a facile route. A reference compound without carbazole moiety was also prepared. Their geometric structures, thermal stability and photovoltaic properties were investigated. This new carbazole-based material exhibits a high glass transition temperature (167.8 °C) and a suitable highest occupied molecular orbital level well-matched with CH₃NH₃PbI₃ perovskite. A low doping content of 4-tert-butylpyridine (120 mM) is observed for the best performance of perovskite solar cell in conjunction with a carbon counter electrode. The device achieves a power conversion efficiency of 4.53% under illumination of 100 mW cm^-2_. The performance is much better than that of the reference compound (0.19%), and comparable to that of spiro-OMeTAD at the same dopant level (5.10%). The results indicate the carbazole-based material is a promising class of hole-transporting materials for perovskite solar cells.

Full text of DOI:10.1016/j.electacta.2016.05.203

Accepted Manuscript
Title: A new carbazole-based hole-transporting material with
low dopant content for perovskite solar cells
Author: Jiang Wang Yu Chen Fusheng Li Xueping Zong
Jinlin Guo Zhe Sun Song Xue
PII:
S0013-4686(16)31293-2
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.electacta.2016.05.203
EA 27427
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Electrochimica Acta
Received date:
Revised date:
Accepted date:
3-3-2016
29-5-2016
29-5-2016
Please cite this article as: Jiang Wang, Yu Chen, Fusheng Li, Xueping Zong,
Jinlin Guo, Zhe Sun, Song Xue, A new carbazole-based hole-transporting
material with low dopant content for perovskite solar cells, Electrochimica Acta
http://dx.doi.org/10.1016/j.electacta.2016.05.203
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A new carbazole-based hole-transporting material with low
dopant content for perovskite solar cells
a,b
*,a
a
a
a
a
*,a
Jiang Wang, Yu Chen, Fusheng Li, Xueping Zong, Jinlin Guo, Zhe Sun, Song Xue
a
Tianjin Key Laboratory of Organic Solar Cells and Photochemical Conversion, School of
Chemistry & Chemical Engineering, Tianjin University of Technology, Tianjin 300384, P.R. China;
b
Department of Chemistry, School of Science, Tianjin University, Tianjin, 300072, P.R. China
Corresponding author: Fax: +86-22-60214252; E-mail address: cy26tj@163.com;
xuesong@ustc.edu.cn.
1

Graphical Abstract
2

Highlights



A carbazole-based hole-transporting material was synthesized via a facile route.
This new material with low-content dopant makes device display best performance.
The optimized doping concentration of 4-tert-butylpyridine was 120mM.
3

Abstract
A new carbazole-based hole-transporting material incorporating a 3,3’-biphenyl central
core has been synthesized via a facile route. A reference compound without carbazole
moiety was also prepared. Their geometric structures, thermal stability and photovoltaic
properties were investigated. This new carbazole-based material exhibits a high glass
o
transition temperature (167.8 C) and a suitable highest occupied molecular orbital
level well-matched with CH
-tert-butylpyridine (120 mM) is observed for the best performance of perovskite solar
cell in conjunction with a carbon counter electrode. The device achieves a power
3
NH
3
PbI
3
perovskite. A low doping content of
4
-2
.
conversion efficiency of 4.53% under illumination of 100 mW·cm The performance is
much better than that of the reference compound (0.19%), and comparable to that of
spiro-OMeTAD at the same dopant level (5.10%). The results indicate the
carbazole-based material is a promising class of hole-transporting materials for
perovskite solar cells.
Keywords: Carbazole; Hole-transporting material; Perovskite solar cells; Dopant;
Carbon counter electrode
4

1
. Introduction
Organic-inorganic perovskite solar cells (PSCs) have drawn great attention because
of their low cost and high performance in solar to energy conversion [1-4]. The power
conversion efficiency (PCE) of PSC has achieved more than 20% by introducing
solid-state hole-transport material (HTM) [5-8]. The state-of-the-art HTM is
2
,2’,7,7’-tetrakis-(N,N-di-p-methoxyphenylamine)-9,9’-spirobifluorene
(spiro-OMeTAD) was used as a solid HTM in dye-sensitized solar cells (DSCs) [9], and
it is the most effective and frequently reported HTM for PSCs [10,11]. Recently,
alternative low-cost and high-efficiency organic small-molecule HTMs have been
developed for PSCs [12-16]. Organic small molecules have superiority of facile
synthesis, good infiltration and excellent hydrophobic property. These organic
small-molecule
HTMs
are
often
used
by
doping
lithium
bis(trifluoromethylsulfonyl)imide (LiTFSI) and 4-tert-butylpyridine (TBP)[17].
Occasionally, the dopant of tris(2-(1H-pyrazol-1-yl)-4-tert-butylpyridine) cobalt(III)
bis(trifluoromethylsulfonyl) imide was also used [18]. The frequently reported
optimized concentrations of LiTFSI and TBP for spiro-OMeTAD are 30 mM and 200
mM, respectively [1,19].
Carbazoles are a significant class of aromatic heterocyclic compounds.
Carbazole-based derivatives have interesting photorefractive, photoconductive,
hole-transporting, and light-emitting properties, and they have been widely used as
organic photovoltaic materials [20-22]. The carbazole-diphenylamine moiety has
currently become an attractive building block of organic small-molecule HTM for PSCs,
5

owing to their low cost and good reactivity [19,23,24]. Carbazole-based oligomer [25]
and organic small molecules have been reported to be used as HTMs for PSCs. In
organic small-molecule carbazole-based HTMs, they are bearing two-arm and three-arm
type structures. Carbazole-diphenylamines or N-alkyl carbazole can be used for donor
groups
incorporating
,4-ethylenedioxythiophene [26], 1,4-phenyl, 4,4’-biphenyl, triphenyl amine and
,3,5-trisphenylbenzene [23]. The design of HTM structure has a great influence on
with
different
central
core
units
such
as
3
1
their conductivity, molecular aggregation state and solubility in nonpolar solvents [24].
Carbazole-based organic small-molecule HTMs exhibited good thermal stability,
film-forming ability and good conductivity by developing reasonable structures.
Benefiting from the introduction of carbazole moiety and addition of appropriate dopant,
the photovoltaic performances of PSCs fabricated from carbazole-based HTMs were
found comparable to that of the device with commercial spiro-OMeTAD [19].
According to previous reports, the dopant content is a key point for enhancing the
efficiency of organic HTMs used for PSCs [27]. Reduction of organic HTMs doping
content is still a challenging subject in order to prepare both efficient and more
economical PSC devices. In the literature, the appropriate amount for LiTFSI that
introduce into organic HTMs based on carbazole-diarylamine solution was 30 mM [19].
To the best of our knowledge, the research on organic HTMs based on
carbazole-diphenylamine with different doping concentration of TBP has not been
involved yet.
In this paper, we report the synthesis and characterization of a new organic
6

small-molecule HTMs containing carbazole, 9,9’- ([1,1’-bi(4-methoxyphenyl)]-
,3’-diyl) bis(N3,N3,N6,N6-tetrakis (4-methoxy-phenyl)-9H-carbazole-3,6-diamine)
3
(
(
HBZ-70). A reference HTM, ([1,1’-bi(4-methoxyphenyl)]-3,3’-diyl)-N4,N4’-bis
4-methoxyphenyl)benzenamine (HBZ-71), was prepared for a comparative study.
Their structures are shown in Fig. 1. Both compounds possess a 3,3’-biphenyl skeleton.
Thermogravimetry (TG) and differential scanning calorimetry (DSC) were tested for the
thermal stability of HBZ-70 and HBZ-71. Spectral, electrochemical characterizations
and theoretical calculations were investigated for their absorption ability, energy level
and optimized geometric structures. PSCs based on the two compounds were prepared
and measured for studying their photovoltaic performances. The results indicate that
HBZ-70 is a possible candidate for carbazole-based HTMs using low content of dopant.
Fig.1
2
. Experimental
2
.1. Instrument and materials
All reactions were carried out under an argon condition and solvents were distilled
from analytical grade reagents. Benzothiazole, Pd
2 3
(dba) , n-butyllithium, t-BuONa,
(
t-Bu) P were purchased from Energy Chemical (China). 4-tert-Pyridine (TBP) and 0.1
3
M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) were purchased from Aldrich.
The melting points of the samples were measured with a RY-1 melting point apparatus
1
13
(
Tianfen, China), and temperatures were uncorrected. H NMR and C NMR spectra
were recorded on a Bruker AM-400 spectrometer. The reported chemical shifts were
referenced against TMS. High resolution mass spectra (HRMS) were obtained with a
7

Micromass GCT-TOF mass spectrometer.
2
.2. Synthesis
The synthesis routes for HBZ-70 and HBZ-71 are shown in Scheme 1. Intermediates
from compound 1 to 5 were known compounds, and they were prepared by following
synthetic methods reported in the literatures [19,28].
Scheme 1
2
.2.1. General procedure for synthesis of HBZ-70, HBZ-71
To a 100 mL two neck round-bottom flask were added compound 5 (5.0 mmol, 1.0
equiv.), bis(4-methoxyphenyl)amine or N3,N3,N6,N6-tetrakis(4-methoxyphenyl)-
H-carbazole-3,6-diamine (2.5 equiv.), Pd (dba) (0.1 equiv.), t-BuONa (3.0 equiv.),
P(t-Bu) (0.2 equiv.) and 30 mL anhydrous toluene. The reaction mixture was refluxed
9
2
3
3
overnight under argon. After cooling down to room temperature, the mixture was
extracted with ethyl acetate and washed with brine (3 × 15 mL). The organic layer was
dried over anhydrous sodium sulphate, filtered, and concentrated in vacuum to give the
crude product, which was purified by column chromatograph packed with silica gel to
afford pure compound.
9
,9’-([1,1’-bi(4-methoxyphenyl)]-3,3’-diyl)bis(N3,N3,N6,N6-tetrakis(4-methoxy-
1
phenyl)-9H-carbazole-3,6-diamine) (HBZ-70): A green powder, yield: 59.6%. H NMR
(400 MHz, DMSO) δ 7.71 (4H, d, J = 1.9 Hz), 7.57 (2H, dd, J = 8.7, 2.5 Hz), 7.49 (2H,
s), 7.34 (2H, s), 7.32 (4H, d, J = 8.8Hz), 7.08 (4H dd, J = 8.8, 1.9 Hz), 6.85 (16H, d, J =
1
3
9
.1 Hz), 6.79 (16H, d, J = 9.1 Hz), 3.86 (6H, s), 3.68 (24H, s). C NMR (100 MHz,
CDCl
3
) δ 155.91, 154.41, 154.08, 141.84, 141.48, 130.70, 129.64, 128.63, 127.63,
8

1
22.62, 122.31, 114.32, 114.08, 111.98, 55.84, 55.27. HRMS (ESI) calcd. for
+
C
94
H
80
N
6
O
10 (M+H ): 1453.5943, found:1453.5936.
([1,1’-bi(4-methoxyphenyl)]-3,3’-diyl)-N4,N4’-bis(4-methoxyphenyl)benzenamine
1
(
HBZ-71): A light yellow powder, yield: 76%. H NMR (400 MHz, DMSO) δ 6.91 (4H,
t, J = 2.9 Hz), 6.89 (6H, d, J = 3.4 Hz), 6.84 (8H, m), 6.81 (1H, d, J = 2.8 Hz), 6.79 (1H,
13
d, J = 2.8 Hz), 6.71 (2H, d, J = 2.8 Hz), 3.71 (12H, s), 3.63 (6H, s). C NMR (100 MHz,
CDCl
3
) δ 154.95, 141.97, 141.46, 130.91, 128.86, 125.03, 123.07, 114.51, 111.94, 56.09,
+
5
5.52. HRMS (ESI) calcd. for C42
H
40
N
2
O
6
(M+H ): 669.2893, found: 669.2886.
1
13
The H NMR and C NMR of HBZ-70 and HBZ-71 are given in the supplementary
materials.
2
.2. Device fabrication
FTO glass substrates (Nippon Sheet Glass, Hyogo, Japan, TEC-14) were cleaned in
solution of detergent, deionized water, acetone and ethanol, respectively. The compact
o
layer was deposited as reference reported and then sintering at 500 C for 30 min.
Mesoporous TiO
2
2
films were deposited by spin-coating of a diluted TiO paste at 2000
o
rmp for 30s. After sintering the film at 500 C for 30 min and cooling down to room
o
temperature, the film was immersed in 0.04M aqueous TiCl
4
at 70 C for 40 min. The
film was rinsed by deionized water and ethanol, respectively. Then the film was
o
annealed at 500 C for 30 min. The CH
3
NH
I and 0.1 mmol of PbI
DMF (99.8%, Aldrich). After stirred at 70 °C for 12 h under N
CH NH PbI solution was obtained. The one-step prepared CH NH
3
PbI
3
perovskite was obtained in one step by
(99%, Aldrich) into 2.80 mL of
protection, the
dissolving 0.1 mmol of CH
3
NH
3
2
2
3
3
3
3
3
PbI
3
precursory
9

solution was dropped on the substrate film by spin-coating at 3000 rmp for 30s, and
o
dried on a hot plate at 100 C for 15 min. The external diameter of the tubular
CH
temperature, the HTM solution (48 mM in chlorobenzene) was spin-coating on top of
the CH NH PbI /TiO /FTO substrate at 2000 rpm. The thickness of TiO /CH NH PbI
3
3 3 3
NH PbI obtained by one-step deposition is 1 m [29]. After cooling down to room
3
3
3
2
2
3
3
layer is 360 nm, and that of HTM layer is 310 nm. Finally, the carbon cathode was made
by drop coating a carbon paste onto a precleaned FTO substrate with a doctor-blade
technique and then annealed at 300°C for 30 min. The carbon paste was prepared by
stirring a mixture of commercial conductive carbon powder (800 nm, Chenyang
Graphite, Qingdao, China), terpineol and ethanol (1:5:1, wt%) vigorously for 30 min at
room temperature. A fixed active area of the carbon cathode placed on the film was 0.09
2
cm .
2
. 3. Spectral, electrochemical and thermal stability measurements
The absorption spectra of HBZ-70 and HBZ-71 in solution and coated on
mesoporous TiO
2 2 3 3 3
and TiO /CH NH PbI films were measured by SHIMADZU
UV-2600 spectrophotometer. Fluorescence measurements were carried out with a
HITACHI F-4500 fluorescence spectrophotometer. The morphology and structural
properties of the films were analyzed using a ULTRA plus ZEISS field emission
scanning electron microscope (SEM).
Cyclic voltammetry (CV) measurements for HBZ-70 and HBZ-71 were performed
on a Zennium electrochemical workstation (ZAHNER, Germany) using a
4 6
three-electrode system. Tetrabutylammoniun hexafluorophosphate (n-Bu NPF ) of 0.1
1
0

M was added into a dichloromethane solution (DCM) as electrolyte. An Ag/0.01 M
AgNO electrode (acetonitrile as solvent) was used as the reference electrode. In
3
three-electrode system, a FTO conductive glass was used as the working electrode, and
a platinized carbon electrode was used as the counter electrode. The scan rates were 10
mV/s. The measurements were calibrated using ferrocene as standard. The redox
potential of ferrocene internal reference is taken as 0.63 V versus NHE.
The thermal stability was determined by thermogravimetry (TG) and differential
scanning calorimetry (DSC). TG analyses were performed on TG 209 F3 Tarsus
o
(
NETZSCH, Germany) at a heating rate of 10 C/min under nitrogen atmosphere. DSC
analyses were conducted on DSC 200 F3 Maia (NETZSCH, Germany) at a heating rate
o
of 10 C/min under a nitrogen atmosphere.
2
.4. Photovoltaic characterizations
The photocurrent density voltage curves (J–V) characteristics of the PSC devices
were carried out using a Keithley 2400 digital source meter controlled by a computer
and a standard AM1.5 solar simulator–Oriel 91160–1000 (300W) SOLAR
SIMULATOR 2 x 2 BEAM. The light intensity was calibrated by an Oriel reference
solar cell. To prevent inflated photocurrents arising from stray light, a black metal mask
2
(
0.09cm ) surrounded the active area was placed on a testing cell during all
measurements. The action spectra of monochromatic incident photon–to–current
conversion efficiency (IPCE) for solar cell were performed by using a commercial setup
(QTest Station 2000 IPCE Measurement System, CROWNTECH, USA). Intensity
modulated photovoltage spectroscopes (IMVS) was also measured by the Zennium
11

electrochemical workstation (ZAHNER, Germany), in which a green light emitting
diode (LED, 532 nm) was modulated to yield light frequency ranging from 100 mHz to
-
2
1
0 kHz and illumination intensity from 1 to 40 mW·cm . Electron densities were
measured by charge extraction technique as reported [30].
3
. Results and discussion
3
.1. Spectral characterizations
UV-visible absorption and fluorescence emission spectra of HBZ-70 and HBZ-71 in
max
dichloromethane are given in Fig. 2. The maximum absorption wavelengths (λabs ) of
HBZ-70 and HBZ-71 are 311 nm and 295 nm, respectively. The introduction of
carbazole moiety extended conjugation length of HBZ-70, resulting in a red-shifted
absorption band compared to HBZ-71. The emission spectra of HBZ-70 and HBZ-71
max
are measured by fluorescence spectrometer. The maximum emission wavelength (λfl
)
of HBZ-70 and HBZ-71 are 456 nm and 428 nm, respectively. Based on the
opt
spectroscopic data [31], the optical band gap (E
g
) of HBZ-70 and HBZ-71 are
calculated to be 2.93 and 3.29 eV, respectively.
Fig.2
Fig.3
Fig. 3 displays the absorption spectra of HTMs (HBZ-70/HBZ-71)/TiO
HBZ-70/HBZ-71)/perovskite(CH NH PbI )/TiO films. The HTMs(HBZ-70/HBZ-71)
TiO films mainly exhibit a UV absorption, and the UV absorption of HBZ-70/TiO
film is obviously stronger than that of HBZ-71/TiO
light absorber, the perovskite /TiO
2
and HTMs
(
3
3
3
2
/
2
2
2
. Perovskite (CH
3
NH
3
PbI
3
) acts as a
2
film shows long wavelength absorption extending to
1
2

8
00 nm. The presence of HTMs(HBZ-70/HBZ-71) coated on perovskite /TiO
enhances the absorption in the UV region of perovskite/TiO film. The
HTMs(HBZ-70/HBZ-71) coated on CH NH PbI /TiO films show obvious absorption
2
films
2
3
3
3
2
in the whole visible range from 400 to 750 nm, which facilitates the use of the whole
light range of sunlight source. The spectroscopic data of HBZ-70 and HBZ-71
employed in solution and in thin films are summarized in Table 1.
Table 1
3
.2. Quantum chemical calculations
To gain insight into the electronic properties and geometries of HBZ-70 and
HBZ-71, quantum chemical calculations were performed. Their geometric structures
were optimized on the basis of DFT at the B3LYP/6-31G level. All structural
optimization and energy calculations were performed using the GAUSSIAN 09 program.
The optimized structures of HBZ-70 and HBZ-71 are shown in Fig.4, and their
representative dihedral angles are marked. The introduction of a carbazole fragment
decreases the dihedral angle between two phenyl of the 3,3’-biphenyl skeleton ranging
from 51.6 to –46.2, and also decreases the dihedral angle between two methoxyphenyl
of the diarylamine group from 42.8 to –41.7. HBZ-70 exhibits a more planar
3
,3’-biphenyl skeleton than that of HBZ-71. The 3,3’-biphenyl skeleton has a C-C
linkage, and this C-C bond length of HBZ-70 (1.489 Å) is shorter than that of HBZ-71
1.491Å). The selected bond angles and bond lengths of HBZ-70 and HBZ-71 are
(
presented in Table S1 and Fig.S1 of the supplementary materials.
The highest occupied molecular orbital (HOMO) and the lowest unoccupied
1
3

molecular orbital (LUMO) of HBZ-70 and HBZ-71 were simulated using Gaussian 09
program. Fig. 5 shows the frontier orbitals of molecules. The HOMO of HBZ-70 is
localized in the carbazole-diarylamine part, and the LUMO is localized in the
3
,3’-biphenyl central core. The HOMO of HBZ-71 is mainly localized in the
diarylamine group, and the LUMO of HBZ-71 is almost delocalized over the whole
molecule. The results indicate both of compounds have a capability of photoinduced
electron transfer by the HOMO-LUMO excitation. The HOMO-LUMO energy gap
obtained from quantum chemical calculations decrease as the conjugated length
increases, a finding consistent with that obtained from UV-vis absorption measurement.
Fig.4
Fig.5
3
.3. Electrochemical measurements
We further carried out cyclic voltammograms to determine the accurate HOMO level
of HBZ-70 and HBZ-71 (Fig. 6), and the values are presented in Table 2. The HOMO
levels of HBZ-70 and HBZ-71 calculated from CV are -5.26 and -5.42 eV, respectively
[
32]. The HOMO levels of the two compounds are between that of CH
eV) [33] and the carbon counter electrode (-5.0 eV) [34]. HBZ-70 has a more
appropriate HOMO level in mediating holes from the CH NH PbI to the carbon
counter electrode. In addition, the higher LUMO levels of HBZ-70 (−2.33 eV) and
HBZ-71 (−2.13 eV) than the conduction band edge of CH NH PbI (−3.93 eV) can
block electron transportation from CH NH PbI to the carbon counter electrode [35].
Fig.6
3 3 3
NH PbI (-5.43
3
3
3
3
3
3
3
3
3
1
4

Table 2
3
.4. Thermal properties
Thermogravimetry (TG) and differential scanning calorimetry (DSC) were used to
test the thermal stability of HBZ-70 as compared with HBZ-71. The measured DSC and
TG results were plotted in Fig. 7(a) and (b), respectively. Both HBZ-70 and HBZ-71
can form amorphous in solid states. TG data show HBZ-70 exhibits better
d
thermochemical stability. HBZ-70 has a higher decomposition temperature (T ,
o
o
4
d
18.4 C) than that of HBZ-71 (T , 368.4 C). DSC data show HBZ-70 has a high glass
o
o
transition temperature (T
g
g g
) of 167.6 C, while HBZ-71 has a T of 71.8 C. The T of
o
HBZ-70 is also higher than that of spiro-OMeTAD (125 C) [36].
Fig.7
3
.5. Device performance
To evaluate the potential of studied HTMs, PSC devices based on HBZ-70 and
HBZ-71 were fabricated. Note that, a carbon counter electrode prepared by a
low-temperature-process was used to replace noble metallic materials. The photovoltaic
performance of PSCs with one-step deposition of CH
3 3 3
NH PbI was investigated.
Tubular CH NH PbI was obtained by the one-step deposition method according to our
3
3
3
previous report [29, 37]. Thin films of HTMs were obtained by spin coating, and the
morphology of HBZ-70 on perovskite surface was measured by SEM technology (Fig.
S2 of the supplementary material). Fig.S2 (a) and (b) displays the top view SEM images
of the perovskite before and after HBZ-70 deposition coated on perovskite layer,
respectively. Fig. S2 (c) gives the cross-sectional image of a representative device. In
1
5

Fig.S2, HBZ-70 shows a good and stable amorphous state without aggregation.
Fig.8
Table 3
J-V characterizations of PSC devices based on HBZ-70 with five different
concentrations of LiTFSI and TBP as dopant were conducted under illumination of 100
-2
mW·cm . The performances of these devices are shown in Fig. 8. The proportions of
LiTFSI/TBP including 30 mM/200 mM, 30 mM/120 mM, 30 mM/60 mM, 15 mM/200
mM and 15 mM/120 mM were investigated. The corresponding performance
parameters are listed in Table 3. The dopant content had an obvious influence on the
photovoltaic performances of these devices. The optimized concentration of LiTFSI and
TBP for the device fabricated from HBZ-70 is 30 mM and 120 mM, respectively. Under
the optimized doping conditions, the device shows a short-circuit current density (Jsc) of
-2
8
.47 mAcm , an open-circuit voltage (Voc) of 1.03 V, and a fill factor (FF) of 0.50,
leading to a PCE of 4.53%. This result is comparable to that of spiro-OMeTAD obtained
by our test at the same doping level (5.10%). In addition, the PCE of device decreased
when keeping the concentration of LiTFSI constant and increasing that of TBP from
1
20 mM to 200 mM (Conditions “1 and 2”, Conditions “4 and 5”, Table 3). This new
carbzaole-based HTM can exhibit the best performance with low content of TBP. In
addition, J-V measurements of PSCs based on HBZ-70 (48 mM) as well as that based
on spiro-OMeTAD (48 mM) doped by LiTFSI/TBP with the proportion of 30 mM/120
mM were repeated more than three times. Their performance displayed replicability,
possibly benefiting from the stability of the solid-state HTM films. The corresponding
1
6

photovoltaic results of these cells are listed in the supplementary material (Table S2 and
Table S3).
As shown in Fig 8, the dark current measurements were correspondingly carried out
[29]. As the concentration of TBP increased from 60 mM to 120 mM, the dark current
was greatly decreased. However, when the concentration was up to 200 mM, the dark
current was increased. This phenomenon suggests that charge recombination was
greatly reduced under the condition of HBZ-70 doped with LiTFSI of 30 mM and TBP
of 120 mM [38]. From curve “5” and “5 dark” in Fig. 8, it was indicated that the
dropped Voc value may caused by the increase of charge recombination [39]. Therefore,
it is very important to screen an appropriate proportion of additives for HTM, so as to
obtain highly efficient PSCs.
Fig.9
The device based on HBZ-71 was also fabricated under the same doping condition,
which gave a PCE of 0.19% (Table 3). The corresponding J–V characteristics of PSCs
based on HBZ-70 and HBZ-71 are shown in Fig.9 (a). In addition, the incident
photon-to-current conversion efficiency (IPCE) spectra of devices using the
HTMs(HBZ-70/HBZ-71) were recorded in Fig. 9(b). The device fabricated from
HBZ-70 shows obvious enhancement of the photocurrent in the whole visible region
from 400 to 750 nm than that of HBZ-71. The much deeper HOMO level of HBZ-71
(
-5.42 eV) is too close to the valence band of CH
may greatly accelerate charge recombination process at the CH
Therefore, Time constant of electron recombination ( ) was measured from the
3
NH
3
PbI
3
peroskite (-5.43 eV), which
3
NH PbI /HTM interface.
3
3
n
1
7

characteristic frequency at the minimum imaginary component of intensity modulated
photovoltage spectroscopy (IMVS) [40, 41]. As shown in Fig. S3 of the supplementary
n
materials, the recombination time constant  for device based on HBZ-70 is obviously
larger than that based on HBZ-71. PSC based on HBZ-70 shows a higher electron
density, suggesting the electron recombination rate was decreased accordingly.
Therefore, the device based on HBZ-70 performed much better than HBZ-71. The
introduction of carbazole moiety plays an important role in developing high-efficiency
PSC devices.
4
. Conclusion
A new carbazole-based HTM (HBZ-70) and a reference (HBZ-71) were prepared.
Both compounds possess
a
3,3’-biphenyl skeleton as central core.
Carbazole-diarylamine groups were linked at the central core in the structure of
HBZ-70, which exhibits a more planar skeleton than that of HBZ-71 with diarylamines
linked at the central core. HBZ-70 shows higher thermal stability and more suitable
HOMO energy level by the introduction of carbazole moiety. The research on this new
carbazole-diphenylamine based HTM with different doping concentration of TBP has
been investigated. The best performance of device was obtained by doping with a lower
concentration of 4-tert-butylpyridine (120 mM) than the frequent reported concentration
(200 mM). The device fabricated from HBZ-70 achieved its best performance with a
-2
power conversion efficiency of 4.53% under illumination of 100 mW·cm , which is
comparable to the performance of spiro-OMeTAD at the same dopant level (5.10%).
HBZ-71 displayed much poorer performance with a power conversion efficiency of
1
8

0
3 3 3
.19%, as a result of the increasing electron recombination at the CH NH PbI /HTM
interface according to the dark current measurements. Therefore, the new
carbazole-based material (HBZ-70) is effective for developing highly efficient
perovskite solar cells.
Acknowledgements
This work was supported by the National Natural Science Foundation of China
(
21376179, 21506164).
1
9

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6

Figure Captions
Fig. 1. Structures of HBZ-70 and HBZ-71.
Scheme 1. Synthesis route for HBZ-70 and HBZ-71.
Fig. 2. Normalized absorption and fluorescence emission spectra of HBZ-70 and
HBZ-71 in dichloromethane solution.
Fig. 3. UV-vis spectra of thin films including HBZ-70 coated on TiO
HBZ-70/TiO ), HBZ-71 coated on TiO film (HBZ-71/TiO ), perovskite (CH NH
coated on TiO film (perovskite/TiO ) and HTMs(HBZ-70/HBZ-71) coated on
perovskite/TiO film (HBZ-70/perovskite/TiO and HBZ-71/perovskite /TiO ).
2
film
(
2
2
2
3
3
PbI
3
)
2
2
2
2
2
Fig. 4. Representative dihedral angles of HBZ-70 and HBZ-71.
Fig. 5. Frontier molecular orbitals of HBZ-70 and HBZ-71.
Fig. 6. Cyclic voltammograms of HBZ-70 and HBZ-71. The experimentally
determined HOMO-LUMO energy gap values for HBZ-70 and HBZ-71 are 2.93 and
3
.29 eV, respectively. The theoretical calculated HOMO-LUMO energy gap values for
HBZ-70 and HBZ-71 are 3.36 and 3.94 eV, respectively.
Fig. 7. (a) TG and (b)DSC curves of HBZ-70 and HBZ-71.
Fig. 8. J–V characteristics of PSCs based on HBZ-70 doping with different proportions
of LiTFSi/TBP. 1: 30 mM/200 mM; 2: 30 mM/120 mM; 3: 30 mM/60 mM; 4: 15
mM/200 mM; 5: 15 mM/120 mM.
Fig. 9. (a) J–V characteristics and (b) IPCE spectra of PSC devices based on
HTMs(HBZ-70/HBZ-71).
2
7

Scheme 1
2
8

Fig.1
Fig.2
2
9