Low-Cost Carbazole-Based Hole-Transport Material for Highly Efficient Perovskite Solar Cells

Article abstract of DOI:10.1002/cssc.201700678

A low-cost carbazole-based small-molecule material, 1,3,6,8-tetra(N,N-p-dimethoxyphenylamino)-9-ethylcarbazole, was designed and synthesized through a facile three-step synthetic route. The material was characterized and applied as a hole-transport materi

Full text of DOI:10.1002/cssc.201700678

Accepted Article
Title: Low-cost Carbazole-based Hole Transport Material for Highly
Efficient Perovskite Solar Cells
Authors: Zhiliang Chen, Hui Li, Xiaolu Zheng, Qi Zhang, Zhanfeng Li,
Yuying Hao, and Guojia Fang
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To be cited as: ChemSusChem 10.1002/cssc.201700678
Link to VoR: http://dx.doi.org/10.1002/cssc.201700678
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10.1002/cssc.201700678
ChemSusChem
Low-cost Carbazole-based Hole Transport Material for
Highly Efficient Perovskite Solar Cells
Zhiliang Chen[a], Hui Li[b], Xiaolu Zheng[a], Qi Zhang[a], Zhanfeng Li*[b], Yuying
Hao[b], Guojia Fang*[a]
[a] Dr. Z. Chen, Dr. X. Zheng, Dr. Q. Zhang, Prof. Dr. G. Fang
Key Laboratory of Artificial Micro- and Nano-structures of Ministry of Education of
China, School of Physics and Technology, Wuhan University, Wuhan 430072,
People’s Republic of China.
E-mail: gjfang@whu.edu.cn
[b] Dr. H. Li, Prof. Dr. Z. Li, Prof. Dr. Y. Hao
Key Laboratory of Advanced Transducers and Intelligent Control System, Ministry of
Education and Shanxi Province, College of Physics and Optoelectronics, Taiyuan
University of Technology, Taiyuan 030024, People’s Republic of China.
E-mail: lizhanfeng@tyut.edu.cn
Zhiliang Chen and Hui Li contribute equally to this work.
A
low
cost
carbazole
based
small
molecule
material
1,3,6,8-tetra(N,N-p-dimethoxyphenylamino)-9-ethyl-carbazole was designed and
synthesized using a facile three-step synthetic route. The novel material was fully
characterized and further applied as hole transport material (HTM) for low
temperature processed planar perovskite solar cells (PSCs). Devices based on this new
HTM exhibit a high power conversion efficiency (PCE) of 17.8%, which is
comparable
to
that
(PCE
of
18.6%)
of
the
costly
2,2',7,7'-Tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9'-spirobifluorene
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10.1002/cssc.201700678
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(Spiro-OMeTAD) based PSCs.
Introduction
In the last few years, organic-inorganic metal halide perovskite materials (ABX3,
+
+
A=CH3NH3 , HC-(NH2)2 , B=Pb, Sn, X=Cl, Br, I) have attracted considerable
attention as light absorber for third generation solar cells. Due to their excellent
opto-electronic properties, such as high absorption coefficient,[1] ambipolar charge
transportation,[2] and long carrier diffusion length,[3, 4] the power conversion efficiency
(PCE) of perovskite solar cells (PSCs) has been improved from 3.8% to 22.1% in just
8 years (2009-2017).[5-10] In efficient PSCs, the perovskite layer is generally
sandwiched between electron transport material (ETM) and hole transport material
(HTM), which play a crucial role in transporting photo-generated charges from
perovskite absorber to electrodes. The most common HTM used in high performance
PSCs
is
2,2',7,7'-Tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9'-spirobifluorene
(Spiro-OMeTAD), which was first introduced in PSCs by Kim five years ago[6] and is
still indispensable for PSCs to achieve high efficiencies.[11-13] To date, the record
PCEs of >20% was achieved using this organic HTM combined with state-of-the-art
perovskite materials as light absorber.[14] However, during the period of its wide
application in laboratory research, Spiro-OMeTAD still has some weakness, such as
complex synthetic route and the high cost it causes, as well as humidity instability,
leading to a setback for the commercial process of PSCs. Given this, plenty of efforts
were made to explore substitutes for this imperfect HTM.[15-18] Liu and co-workers
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reported Spiro[fluorene-9,9’-xanthene]-based HTMs for PSCs, in which not only high
PCE but also better stability were achieved.[19] And Xu and Bi et al. achieved high
PCEs of over 19% based on similar structure small molecule material (X59 and
X60).[20, 21] However, the materials reported above and Spiro-OMeTAD all comprise
fluorine group, which is expensive and responsible for a large proportion of the final
cost of ultimate product. Replacing fluorine with a cheaper substitute seems to be an
effective way to cut the cost of the HTMs. Carbazole is commercially available
material and the price is temptingly low. Because of its special rigid structure and
large conjugated system, carbazole has excellent photoelectrical property.[22] Since 1,
3, 6, 8 reactive sites of carbazole can be easily substituted with functional groups to
meet a variety of requirements, carbazole-based derivatives are widely used in
electroluminescence and photovoltaic devices. Recently, carbazole derivatives with
various molecular structures have been designed for HTMs in PSCs.[23-32] In the
molecular structure of many HTMs, carbazole together with some atypical hole
transport groups form the core of the molecular, with phenylamine structure
outside.[23, 24, 26-29] These atypical hole transport groups such as benzene and biphenyl
can transport both holes and electrons, which may impair the hole-mobility of these
HTMs. Taking away these moieties and using a single carbazole group as a molecular
core may be an effective way to produce high mobility HTM. In the present work, we
report on the synthesis of a structure-simple carbazole based small molecule material
1,3,6,8-tetra(N,N-p-dimethoxyphenylamino)-9-ethyl-carbazole (Cz-OMeTAD) as
HTM for PSCs. The Cz-OMeTAD compound was synthesized using a facile
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10.1002/cssc.201700678
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three-step synthetic route and shows high solubility in organic solvents (e.g.
chlorobenzene, toluene), which is applicable for fabricating thin film devices via
solution processing. Photoelectrical measurements (UV/Vis, cyclic voltammetry (CV),
thin-film conductivity and mobility) suggest that Cz-OMeTAD exhibits excellent
properties, such as suitable energy levels, high mobility and conductivity, making it an
ideal candidate for HTM in efficient PSCs. We prepared planar PSCs with a device
structure of FTO/SnO2/CH3NH3PbI3/HTM/Au. The Cz-OMeTAD based PSCs achieve
a high PCE of 17.81% with open circuit voltage (VOC) of 1.14 V, short circuit current
density (JSC) of 22.26 mA cm−2, and fill factor (FF) of 0.71, which is comparable to
that of devices using the costly Spiro-OMeTAD as HTM, suggesting that the novel
material we synthesize is very promising for replacing the costly Spiro-OMeTAD in
commercial application of PSCs in the future.
Results and Discussion
The synthetic route towards Cz-OMeTAD is illustrated in Scheme 1 and the detailed
information can be found in the experimental section. As shown, the synthesis of
Cz-OMeTAD requires only three-step reaction process and is much simple than that
of Spiro-OMeTAD, which typically need five steps to obtain the ultimate product.[33]
Firstly, commercially available material carbazole was used as the starting material to
synthetize 1,3,6,8-TetrabroMocarbazole via a bromine substitute reaction in a 91%
yield, which was in preparation for the final cross-coupling reaction to add
diphenylamine group on the carbazole core. The second reaction process (yield 98%)
of substituting the active hydrogen on N-H with ethyl aimed at improving the stability
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10.1002/cssc.201700678
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of the final product. On the other hand, the ethyl-substituent was believed to be
beneficial for the solubility of the material. The synthesis of Cz-OMeTAD was
finished by palladium-catalyzed C-N cross-coupling reaction with
a
4,4’-dimethoxydiphenylamine and 9-ethyl-1,3,6,8-tetrabromocarbazole (yield 80%).
The overall yield of this synthetic route is >70%, and no air sensitive, corrosive or
toxic reagents were used during the synthesis process. Most importantly, the synthesis
cost of Cz-OMeTAD is much lower than that of Spiro-OMeTAD. We compared the
synthesis cost of these two compounds using the cost model described by Pablo et
al.[33] and Osedach et al.[34] The quotes for the necessary raw material were mainly
collected from the official websites (China Mainland region) of Sigma-Aldrich and
Alfa. Since the only difference in molecule structure between Cz-OMeTAD and
Spiro-OMeTAD is the center part (9-ethyl-carbazole for Cz-OMeTAD and
spirobifluorene for Spiro-OMeTAD, Figure 1), we estimated the synthesis costs of 1
gram
2,2’,7,7’-tetrabromo-9,9’-spirobi[fluorene] to compare the cost of these two materials.
As presented in Figure S4 and Table 1, the synthesis cost of 1 gram
9-ethyl-1,3,6,8-tetrabromocarbazole
and
9-ethyl-1,3,6,8-tetrabromocarbazole is estimated to be 21.94 $ g-1, which is almost 20
times lower than that of its counterpart (Figure S3 and Table S1).
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Scheme 1. The synthetic route of Cz-OMeTAD.
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10.1002/cssc.201700678
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Figure 1. Molecular structure of the Cz-OMeTAD synthesized in this work. The
molecular weight is given in g mol-1.
Table
1.
Materials
quantities
and
cost
for
the
synthesis
of
9-ethyl-1,3,6,8-tetrabromocarbazole.
Chemical name
Weight
reagent
(g)
Weight
solvent
(g)
Weight Price of Chemical Material cost
Cost per step
workup
(g)
($/kg)
($/g product)
($/step)
Carbazole
N,N-DiMethylfor
MaMide
1.57
476.54
170.46
0.75
0.76
8.71
4.47
N-Bromosuccinimi
de
6.70
197.18
170.46
1.32
5.08
N,N-DiMethylfor
29.78
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10.1002/cssc.201700678
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MaMide
Water
62.88
15.71
-
-
CH2Cl2
NaH
50.89
0.80
3.55
1.29
0.96
0.80
3696.11
170.46
13.23
N,N-DiMethylfor
MaMide
Bromoethane
water
7.57
150
2110.92
-
1.69
-
CH2Cl2
125.88
1
50.89
165.88
31.07
-
6.41
0.17
0.12
-
Na2SO4
Methanol
Total
3.96
21.94
Thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC)
were then carried out to measure the thermal stability of the product obtained above.
As presented in Figure 2a, the decomposition temperature corresponding to a 5%
weight loss (Td) is about 423℃, indicating good thermal stability of Cz-OMeTAD.
The concave point at around 262℃ (Figure 2b) represents the melting point of
Cz-OMeTAD, and the glass transition temperature (Tg) of Cz-OMeTAD is evaluated
to be 114.9℃.
Figure 2. TGA (a) and DSC (b) curves of Cz-OMeTAD with a scan rate of 10 °C
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10.1002/cssc.201700678
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min-1 under a nitrogen atmosphere.
To evaluate the value of the novel material we synthesize for application in
photovoltaic devices, we conducted a series of measurements to characterize the
photoelectrical properties of Cz-OMeTAD, with the commonly used Spiro-OMeTAD
as a reference. Figure 3a shows the UV-vis absorption spectra of Cz-OMeTAD and
Spiro-OMeTAD in CH2Cl2. Similar to other work reported,[35] Spiro-OMeTAD
exhibits characteristic absorption peaks at about 310 nm and 390 nm, which were
attributed to the aromatic ring and conjugated system composed of triarylamine,
respectively. Compared to Spiro-OMeTAD, one more absorption peak was observed
at 420 nm for Cz-OMeTAD, this weaker characteristic peak may be ascribed to the
ethyl substituent on carbazole. Figure 3b depicts the corresponding Tauc plot of the
absorption spectra, from which the optical band gap (Eg) can be evaluated by
extending the linear portion of the plots to the binding energy axis. As labeled in
Figure 3b, the obtained Eg values are 3.02 eV for Spiro-OMeTAD and 2.75 eV for
Cz-OMeTAD, respectively.
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10.1002/cssc.201700678
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Figure 3. (a) UV-vis absorption spectra of Cz-OMeTAD and Spiro-OMeTAD tested
in CH2Cl2 and (b) the corresponding Tauc plot of the absorption spectra. (c) Cyclic
voltammogram of Cz-OMeTAD in acetonitrile with ferrocene/ferrocenium as an
internal standard. (d) Energy level diagram of the corresponding materials applied in
PSCs in this work.
For HTMs used in photovoltaic devices, suitable energy levels can facilitate the
transporting of photoexcited holes from light absorber layer to the anode. Here we
conducted CV characterization to determine the highest occupied molecular orbital
(HOMO) energy level of Cz-OMeTAD, in which ferrocene/ferrocenium were used as
an internal standard reference. Figure 3c shows the CV curves of Cz-OMeTAD.
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According to the method reported,[36] the HOMO energy levels is obtained at -5.27 eV
for Cz-OMeTAD, which is well-aligned with the valence band of CH3NH3PbI3 (-5.43
eV). And the HOMO energy level of Cz-OMeTAD is slightly lower than that of
Spiro-OMeTAD (5.22 eV),[8] which allows photoexcited holes cascade into the HTM
with less energy loss and this will further increase the VOC value of devices.
Combined with the band gaps obtained from the Tauc plot of the absorption spectrum,
the lowest unoccupied molecular orbital (LUMO) energy levels of Spiro-OMeTAD
and Cz-OMeTAD were determined to be at -2.20 eV and -2.52 eV, respectively (Table
2). The energy barrier (1.41 eV) existing between the LUMO of Cz-OMeTAD and the
conduction band edge of CH3NH3PbI3 would effectively block electron carrier
transporting (Figure 3d), which should avoid charge recombination.
Figure 4. (a) Current density-voltage and (b) current-voltage characteristics of the
doped Cz-OMeTAD and Spiro-OMeTAD films.
Subsequently, The hole mobility of Cz-OMeTAD and Spiro-OMeTAD films were
measured using space-chargelimited currents model,[37, 38] in which a hole-only device
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10.1002/cssc.201700678
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with a structure of FTO/PEDOT:PSS/HTMs/Au was used. By fitting the current
density-voltage (J-V) curves (Figure S5), the mobility values were obtained according
to the modified Mott-Gurney equation.[39]
9
V 2
d3
J  0r 
(1)
8
where J is the current density, ε0 is the permittivity of free space, εr is the relative
permittivity, µ is the zero-field mobility, V is the applied voltage and d is the thickness
of active layer. As listed in Table 2, the doped Cz-OMeTAD film exhibits a mobility
of 1.82x10-3 cm2 V-1 s-1, which is at the same order of magnitude and slightly lower
than that of Spiro-OMeTAD based film (3.08x10-3 cm2 V-1 s-1). We also tested the
conductivities of the two HTMs with a two-contact electrical conductivity set-up as
literature.[23] The conductivities of these HTMs were determined from equation (2),[37]
where σ is the conductivity, L is the channel length, w is the channel width, t is the
film thickness, and R is the resistivity calculated from the gradients of the curves
(Figure 4b). The obtained conductivity of Spiro-OMeTAD is also a little higher than
that of Cz-OMeTAD (Table 2).
L
 
(2)
R w t
Table 2. Summary of optical, electrochemical and electrical data of Cz-OMeTAD and
Spiro-OMeTAD.
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[a]
HTM
λabs
[nm]
Egap HOMO LUMO
Hole mobility
[cm2 V-1 s-1]
Conductivity
[S cm-1]
[eV]
[eV]
[eV]
Cz-OMeTAD
450.9 2.75
410.6 3.02
-5.27
-5.22
-2.52
-2.20
1.82x10-3
3.08x10-3
1.20x10-4
4.41x10-4
Spiro-OMeTAD
[a] Absorption onset determined at UV-vis spectra.
The excellent photoelectrical properties of Cz-OMeTAD, such as suitable energy
levels and high hole mobility (after Li-doping), make it promising for HTM in PSCs.
We prepared PSCs applying Cz-OMeTAD as HTM with Spiro-OMeTAD as a
reference. The schematic view in Figure 5a shows of the device structure used in this
work, wherein a thin layer (ca.40 nm) of SnO2 nanocrystals was used as ETM on
fluorine-doped SnO2 (FTO) glass, a thickness of 360 nm CH3NH3PbI3 film as light
absorber, and ~50 nm gold as electrode. The thickness of HTMs layer here is about
150 nm. All the thickness data were obtained from the cross-section scanning electron
microscopy (SEM) image shown in Figure S6. The photovoltaic performance of the
best-performing device was presented in Figure 5b. The champion PCE obtained is
17.81% with a VOC of 1.14 V, a JSC of 22.26 mA cm−2, and a FF of 0.71 for
Cz-OMeTAD based devices, which is comparable to that of Spiro-OMeTAD based
PSCs. Devices based on these two HTMs both exhibit good long-term stability, the
average PCE of Li-TFSI doped Cz-OMeTAD and Spiro-OMeTAD based PSCs retains
around 90% of the initial PCEs after 1000 h storage at 30 ± 5% RH and room
temperature (Figure S7). However, PSCs based on Cz-OMeTAD and Spiro-OMeTAD
exhibit J-V hysteresis (Figure S8), which may originate from the ferroelectric nature
of the perovskite material or accumulation of charges in defects at its
surface/interfaces.[40] Further work on preparing hysteresis free Cz-OMeTAD based
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PSCs is on the way in our laboratory. Figure 5c displays the corresponding Incident
photon-to-current conversion efficiency (IPCE) spectrum, the value of JSC integrated
from its IPCE spectrum is 20.54 mA cm−2, which closely agree with the value
obtained from the J−V curve. The IPCE spectrum exhibits a broad plateau from 400 to
800 nm, suggesting a high photon-charge conversion efficiency of the PSC. The
statistics data of the PCEs are presented with histogram in Figure 5d. The average
PCE of the 40 devices is 16.22±0.80%, and 60% of the fabricated devices exhibit a
PCE over 16%. These results indicate that Cz-OMeTAD is an excellent HTM and
very promising for replacing Spiro-OMeTAD in commercially application of PSCs.
Figure 5. (a) Schematic view of the structure of device used in this work. (b) J-V
curves of PSCs based on Cz-OMeTAD and Spiro-OMeTAD, with a table of
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10.1002/cssc.201700678
ChemSusChem
performance data inserted. (c) IPCE spectrum of the champion Cz-OMeTAD based
PSCs. (d) Histogram of efficiency of PSCs with a structure of FTO/SnO2/
Perovskite/Cz-OMeTAD/Au.
Conclusions
In
summary,
we
have
developed
a
novel
small
molecule
1,3,6,8-tetra(N,N-p-dimethoxyphenylamino)-9-ethyl-carbazole (termed Cz-OMeTAD)
as HTM for efficient PSCs. The Cz-OMeTAD was synthesized via a simple three-step
synthetic route using inexpensive starting material without air sensitive, corrosive or
toxic reagents in the synthetic process, making it cost effective and environmental
friendly for photovoltaic devices. PSCs based on Cz-OMeTAD HTM exhibited high
performance (PCE of 17.81%, VOC of 1.14 V, JSC of 22.26 mA cm−2, and FF of 0.71),
which is competitive with the costly Spiro-OMeTAD based PSCs. These results
suggest that this novel HTM is very promising for commercial application in
photovoltaic devices in the future.
During revision of the manuscript upon editorial request, we synthesized a series
of 9-ethyl-carbazole core based HTMs, with only the outside part different from the
Cz-OMeTAD (Figure S9). But devices based on these HTMs exhibit poor
performance (PCE of nearly 0), this result suggests the important role of OMeTAD
groups outside the carbazoles core to achieve high performance as HTM in PSCs.
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Experimental Section
Materials
Tin oxide (SnO2) nanocrystals were prepared as reported and used with a mole
concentration of 0.1M. 617.1 mg of lead iodide (PbI2) (Aladdin reagent, 99.99%),
219.5 mg methylammonium iodide (CH3NH3I) (Aladdin reagent, 99.99%) and were
dissolved in 1 mL solution of 800μL N,N-dimethylformamide (DMF) (Alfa-aesar)
and 200μL dimethyl sulfoxide (DMSO) (Alfa-aesar). The obtained mixture was
stirred at 60 °C overnight. 68 mM Spiro-OMeTAD (Shenzhen Feiming Science and
Technology Co., Ltd., 99.0%), 55 mM TBP (Aladdin reagent), and 26 mg of Li-TFSI
(Aladdin reagent) were dissolved in acetonitrile and chlorobenzene (1:10 in volume
ratio). Cz-OMeTAD powder was synthesized as detailed below and used to prepare
precursor. The doping concentrations of Cz-OMeTAD were the same as those in
Spiro-OMeTAD.
Synthesis
Synthesis of 1,3,6,8-Tetrabromocarbazole. Carbazole (5g, 29.94mM) and 15mL
DMF were mixed in a three-necked flask and stirred in the ice bath under nitrogen
atmosphere for 1h to prepared solution A. N-Bromosuccinimide (21.317g, 119.76mM)
and 100mL DMF were mixed and stirred to form solution B. Then solution B was
added dropwise into the cooling solution A. Then the reaction mixture was kept at
room temperature. After 10h, 200mL deionized water was added in to the mixture and
a precipitate was appeared. Preliminary product was obtained by filtering, washing
and drying the precipitate. Then highly purified 1,3,6,8-Tetrabromocarbazole was
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10.1002/cssc.201700678
ChemSusChem
recrystallization by dichloromethane to afford a final product in 91% yield (13.04g).
Synthesis of 9-ethyl-1,3,6,8-tetrabromocarbazole. 1,3,6,8-Tetrabromocarbazole
(11.95g, 25.0mM) and NaH (12g, 50.0mM) ) were dissolved in 100mL DMF, then the
solution was added dropwise into bromoethane (10g, 50mM). The obtained mixture
was stirred at 40℃ under a nitrogen atmosphere for 4h. When the reaction stopped,
the mixture was extracted with CH2Cl2. The organic phases were washed with
deionized water and dried with anhydrous MgSO4. After the solvent was evaporated,
the residue was recrystallization by CH2Cl2 and CH3OH to afford a white solid in
98% yield (12.51g).
Synthesis of 1,3,6,8-tetra(N,N-p-dimethoxyphenylamino)-9-ethyl-carbazole. The
synthesis of 1,3,6,8-tetra(N,N-p-dimethoxyphenylamino)-9-ethyl-carbazole powder
was
achieved
by
Buchwald–Hartwig
reaction
in
which
9-ethyl-1,3,6,8-tetrabromocarbazole (0.51g, 1.0mM), 4,4′-dimethoxydiphenylamine
(1.15g, 5.0mM), Nat-Bu (0.57g, 6.0mM)， tris(dibenzylideneacetone)dipalladium
(0.037g, 0.04mM) and tritertbutylphosphine (0.012g, 0.06mM) were added to 20mL
of dry toluene under nitrogen atmosphere. The mixture solution was heated to 110℃
and refluxed overnight. After cooling to room temperature, the reaction mixture was
extracted with dichloromethane and the organic phases were dried with anhydrous
MgSO4. After the solvent was evaporated, the residue was purified by column
chromatography (petroleum ether/ethyl acetate =10/1) to afford 0.88g (yield 80%) of
1
Cz-OMeTAD. H NMR (400 MHz, CDCl3) δ: 7.21-6.55 (m, 32H), 3.92-3.62 (m,
24H), 1.26 (d, J = 3.1 Hz, 3H), 1.20 (s, 2H), 1.18 (d, J = 1.6 Hz, 2H), 1.16 (s, 2H). 13C
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10.1002/cssc.201700678
ChemSusChem
NMR (101 MHz, CDCl3) δ: 154.6, 154.4, 142.0, 141.6, 134.2, 133.2, 127.4, 126.2,
124.2, 122.6, 114.4, 114.2, 113.0, 80.4, 55.4, 55.4, 28.5, 15.4. HR-MS (ESI) m/z:
[M+1]+ calcd for 1103.4827; found, 1104.4874. Corresponding spectrums can be
found in Figure S1 and Figure S2.
Solar Cell Fabrication
FTO substrates (OPVtech, 13 Ohm/square) were sequentially rinsed by sonication in
detergent, deionized water, acetone, and ethanol and finally dried in air. The SnO2
nanoparticles were spin-coated on the cleaned FTO substrates at 3000 rpm for 30 s,
followed by an annealing process at 200℃ for 30 mins. Perovskite absorber layer
was then deposited on the samples using a solvent-engineering technique reported as
literature. The perovskite solution was deposited on the SnO2 films by spin-coating
first at 1000 for 10 s and then at 4000 rpm for 40 s. During the spin-coating process,
400μL chlorobenzene was dripped on the substrate 10 s prior to the end of the step,
the as-deposited films were then annealed at 100℃ for 10 mins to achieve a uniform
perovskite film. The Cz-OMeTAD and Spiro-OMeTAD films were spin-coated on
perovskite film at 3000 rpm for 30 s. The devices were completed by thermal
evaporation of a thin layer of gold as back contact.
Characterization
1H NMR was recorded on a Bruker 400 MHz NMR spectrometer. TGA and DSC
were carried on a STA 409 PC instrument at a heating rate of 10 °C min-1 under
nitrogen. Absorption spectra were recorded by a Unico UV-2600 PCS
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10.1002/cssc.201700678
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spectrophotometer. CV was performed using a CHI 660E electrochemical workstation
at a scan rate of 100 mV s-1. All experiments were carried out in a three-electrode
compartment cell with a Pt-sheet counter electrode, a glassy carbon working electrode,
and a Pt-wire reference electrode. The supporting electrolyte used was 0.1 M
tetrabutylammonium hexafluorophosphate (Bu4NPF6) solution in dry acetonitrile. The
cell containing the solution of the sample (1 mM) and the supporting electrolyte was
purged with a nitrogen gas thoroughly before scanning for its oxidation and reduction
properties. Ferrocene was used for potential calibration in each measurement. All the
potentials were reported relative to ferrocene-ferrocenium couple, whose oxidation
potential was +0.13 V relative to the reference electrode. Density functional theory
(DFT) calculations (B3LYP) were performed for the target compound with the 6-31G
(d,p) basis set by using the Gaussian 09 program. The cross section image of the PSCs
was observed by a high-resolution field emission SEM (JSM 6700F, Japan). J-V
characteristics of devices were performed on a CHI 660D electrochemical workstation
(Shanghai Chenhua Instruments, China) with a standard ABET Sun 2000 Solar
Simulator. The area of the Au electrode was 0.09 cm2. IPCE was measured by a
QE/IPCE system (Enli Technology Co. Ltd.) in 300-800 nm wavelength range.
Acknowledgements
This work was financially supported by the National High Technology Research and
Development Program (2015AA050601), the National Natural Science Foundation of
China (11674245, 61376013, and 91433203).
Keywords: carbazole • small molecule • hole transport material • perovskite solar
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10.1002/cssc.201700678
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