Tetraphenylmethane-Arylamine Hole-Transporting Materials for Perovskite Solar Cells

Article abstract of DOI:10.1002/cssc.201601683

A new class of hole-transporting materials (HTM) containing tetraphenylmethane (TPM) core have been developed. After thermal, charge carrier mobility, and contact angle tests, it was found that TPA-TPM (TPA: arylamine derivates side group) showed higher g

Full text of DOI:10.1002/cssc.201601683

Accepted Article
Title: Tetraphenylmethane-arylamine hole transporting materials for
perovskite solar cells
Authors: Xuepeng Liu, Fantai Kong, Tai Cheng, Wangchao Chen,
Zhao’ao Tan, Ting Yu, Fuling Guo, Jian Chen, Jianxi Yao, and
Songyuan Dai
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To be cited as: ChemSusChem 10.1002/cssc.201601683
Link to VoR: http://dx.doi.org/10.1002/cssc.201601683
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ChemSusChem
Tetraphenylmethane-arylamine hole transporting materials
for perovskite solar cells
Xuepeng Liu,[a,b] Fantai Kong,*[a] Tai Cheng,[c] Wangchao Chen,[a,b] Zhan’ao Tan,*[c]
Ting Yu,[a,b] Fuling Guo,[a] Jian Chen,[a] Jianxi Yao,[c] and Songyuan Dai*[c,a]
a Key Laboratory of Novel Thin-film Solar Cells, Institute of Applied Technology, Hefei
Institutes of Physical Science, Chinese Academy of Sciences, Hefei, Anhui, 230088,
P.R. China
b University of Science and Technology of China, Hefei, 230026, P.R. China
c Beijing Key Laboratory of Novel Thin-Film Solar Cells, North China Electric Power
University, Beijing, 102206, P.R. China
Abstract: A new class of hole transporting materials containing tetraphenylmethane
core have been developed. After thermal, charge carrier mobility and contact angle
tests, it is found that TPA-TPM show a higher glass transition temperature and larger
water contact angle than spiro-OMeTAD with comparable hole mobility.
Photoluminescence and impedance spectroscopy study indicate that TPA-TPM own
comparable hole extraction ability with spiro-OMeTAD. Scanning electron
microscope and atomic force microscopy results suggest that TPA-TPM show a
smooth surface. When TPA-TPM is used in mesoscopic perovskite solar cells, a
power conversion efficiency comparable to spiro-OMeTAD is achieved. Notably, the
perovskite solar cells employing TPA-TPM show better long-term stability than that
of spiro-OMeTAD. Moreover, TPA-TPM can be prepared from relatively cheap raw
materials with
a
facile synthetic route. The results demonstrate that
tetraphenylmethane-arylamines can be a new class of HTM for efficient and stable
perovskite solar cells.
Introduction
Organic–inorganic metal halide perovskites (ABX3, A = CH3NH3 or NHCHNH3, B =
Pb, X = Br, I or Cl) have drawn great attention as excellent light harvesting materials
for their direct band gap, large exciton diffusion lengths and excellent ambipolar
mobility.[1-4] The power conversion efficiency (PCE) of perovskite solar cells (PSCs)
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exceeding 20%[5-6] have been achieved. In PSCs, the hole transporting materials
(HTM) play an important role for collecting and transferring hole, and enhancing
stability of the devices.[7,8] To date, spiro-OMeTAD is the most commonly HTM used
in high-efficiency devices. However, the multi-step low yielding synthesis and
difficult purification process of spiro-OMeTAD limits its practical applications.[9]
Moreover, the stability of the PSCs based on spiro-OMeTAD is relatively poor.
Therefore, to reduce the cost of cell fabrication, lots of small-molecular organic
HTMs have been developed for PSCs. Among the reported HTMs, star-shaped
molecules have been proved to be efficient HTM in PSCs. They are usually obtained
by varying the cores and connecting arylamine derivatives. At present, the
triptycene,[10]
thiophene,[11,12]
triphenylamine,[13]
benzene,[14,15]
triazine,[16]
bifluorenylidene,[17] [2,2]paracyclophane,[18] indolocarbazole,[19] tetraarylethene,[20]
carbazole,[9] benzotrithiophene,[21] azulene,[22] truxene,[23,24] cores or their derivatives
and some new spiral cores[25-29] have been reported. However, some of those reported
HTMs suffer relatively poor stability or obvious low PCE than spiro-OMeTAD. On
the other hand, to reduce the costs of HTM in PSCs, new cheap HTM with facile
synthetic route also urgent need to be developed.
Previously, non-twist and star-burst tetraphenylmethane unit has been reported
with high thermal stability and good contact with different layer in OLEDs, organic
solar cells or single-molecule junction devices with gold electrodes.[30-34] In addition,
tetraphenylmethane can be prepared from relatively low-cost raw materials with a
facile synthetic route, which is simple illustrated in experimental section and
supporting information. For consideration of the advantage of tetraphenylmethane
unit, we try to introduce tetraphenylmethane as core to develop new stable and
efficient HTMs. Two new compounds with tetraphenylmethane as core and bulky
arylamine side groups as terminal groups (DPA-TPM and TPA-TPM) and a
reference compound with anisole as terminal groups (Ph-TPM) are synthesized, as
shown in Figure 1. Among these new compounds, TPA-TPM show good solubility in
common organic solvents, high thermal stability, good film-forming ability, large
water contact angle, high hole mobility and suitable highest occupied molecular
orbital (HOMO) level compared to CH3NH3PbI3. Due to these advantages of TPA-
TPM, the PSCs with it show comparable power conversion efficiency and better
long-term stability than that of spiro-OMeTAD. The easy synthesis of TPA-TPM and
high PCE of the device enable us to prepare other cheap and efficient HTM
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incorporating tetraphenylmethane core.
Figure 1. Chemical structures of synthesized HTMs with tetraphenylmethane core
and spiro-OMeTAD.
Results and Discussion
The UV-vis absorption and photoluminescence (PL) properties of new compounds are
investigated, as presented in Figure 2a. DPA-TPM and TPA-TPM show absorption
peaks at 308 nm and 342 nm. The spectrum of TPA-TPM is red shifted compared to
DPA-TPM and Ph-TPM, which could be resulted from the extension of π-
conjugation by inserting phenyl ring between tetraphenylmethane core and
diphenylamine arms. The absorption spectrum confirmed that DPA-TPM and TPA-
TPM do not compete significantly with CH3NH3PbI3 in devices for light harvesting.
The PL spectra of DPA-TPM and TPA-TPM show a large Stokes shift of 85 nm and
103 nm, which demonstrate that significant changes in geometrical structure of
molecules would occur in the excited state.[35] The optical band gaps (Eg) of Ph-TPM,
DPA-TPM and TPA-TPM estimated from the intersection of normalized PL and
absorption spectra are 4.03 eV, 3.36 eV and 3.13 eV, respectively.
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Table 1. Optical, electrochemical, thermal properties and hole mobility of Ph-TPM,
DPA-TPM and TA-TPM.
[a]
Molecule λabs (nm)
Eg (eV)
HOMO[b] LUMO[c](
Tg (oC)
Μ (cm2V-
1s-1)
(eV)
-5.36
-5.32
eV)
Ph-TPM
-
4.03
3.36
-1.33
-1.96
112
163
8.05×10-6
3.88×10-6
DPA-
TPM
308
TPA-
TPM
342
3.13
-5.29
-2.16
139
1.05×10-5
[a] Optical band gap (Eg) obtained from the normalized PL and absorption spectra. [b]
HOMO is calculated from EHOMO = - 5.11 - (E1/2ox vs. Fc/Fc+), where E1/2ox vs.
Fc/Fc+ is the average value of the first redox peaks with reference to ferrocene. [c]
Calculated from LUMO = HOMO + Eg.
The electrochemical properties of new compounds are performed by cyclic
voltammetry (CV) measurements in CH2Cl2 solution, as shown in Fiure 2b and Figure
S2. The pairs of redox peaks of new compounds are highly reversible and show good
reproducibility, suggesting that they have excellent electrochemcial stability.10 The
calculated HOMO levels of Ph-TPM, DPA-TPM, TPA-TPM and spiro-OMeTAD
are -5.36 eV, -5.32 eV, -5.29 eV and -5.13 eV, respectively. As the reported HOMO
level of CH3NH3PbI3 is around -5.43 eV,7 which indicates that new compounds
should have favourable energetics for hole extraction. LUMO level of Ph-TPM,
DPA-TPM and TPA-TPM calculated from CV and UV-vis data are -1.33 eV, -1.96
eV and -2.16 eV. The obviously higher LUMO levels of new compounds than the
conduction band of CH3NH3PbI3 can block the electrons from CH3NH3PbI3 to Au
counter electrode and ensure carrier transfer at the CH3NH3PbI3/HTM interface when
the HTMs are excited (Figure 5b).
The thermal properties of new compounds are studied by differential scanning
calorimetry (DSC) measurements. The glass transition temperature (Tg) of Ph-TPM,
DPA-TPM, TPA-TPM and spiro-OMeTAD shown in Figure 2c are 112, 163, 139
o
and 122 C. The higher Tg of TPA-TPM than that of spiro-OMeTAD demonstrate a
more stable amorphous state, which is beneficial to the device stability.[36]
To determine the charge carrier mobility, hole-only devices
(ITO/PEDOT:PSS/HTM/Au) are fabricated and measured from the space charge
limitation of current (SCLC) in the J-V characteristics (shown in Figure 2d). The
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thickness of Ph-TPM, DPA-TPM, TPA-TPM and spiro-OMeTAD is 26, 32, 45 and
40 nm, respectively. And the hole mobilities of Ph-TPM, DPA-TPM, TPA-TPM
and spiro-OMeTAD evaluated using the Mott-Gurney law are 8.05×10-6, 3.88×10-6,
1.05×10-5 and 1.31×10-5 cm2 V-1s-1, respectively. It is noted that the extension of π-
conjugation by inserting phenyl ring between tetraphenylmethane core and
diphenylamine arms can improve the hole mobility. The optical, electrochemical,
thermal properties and hole mobility of new compounds are summarized in Table 1.
Figure 2. (a) UV-vis absorption and PL spectra in DCM, (b) cyclic voltammogram
with ferrocene as the reference, (c) differential scanning calorimetry curves, and (d)
space-charge-limited current hole mobility measurements of spiro-OMeTAD, Ph-
TPM, DPA-TPM and TPA-TPM.
To study the hydrophobicity of new compounds and spiro-OMeTAD, we
measured the water contact angles of these compounds. As shown in Figure S1, Ph-
TPM, DPA-TPM, TPA-TPM and spiro-OMeTAD film show a water contact angle
of around 70o, 71o, 85o and 72o, respectively. The doped TPA-TPM and spiro-
OMeTAD show a water contact angle 84o and 74o. And the shadowy center of Ph-
TPM and DPA-TPM in Figure S1 can be resulted from their poor films. The higher
water contact angle of TPA-TPM than spiro-OMeTAD may be due to it’s superior
solubility in chlorobenzene and high aggregation tendency.[30]
The film morphology of these HTMs and spiro-OMeTAD are analyzed by
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scanning electron microscopy (SEM) and atomic force microscopy (AFM). Figure 3
show a top view of perovskite and covered by new compounds and spiro-OMeTAD,
and Figure 4 show AFM topographical images of DPA-TPM, TPA-TPM and spiro-
OMeTAD. It can be noted that the perovskite covered Ph-TPM, DPA-TPM show a
rough surface. The root-mean-square (RMS) roughness of films DPA-TPM, TPA-
TPM and spiro-OMeTAD is 11.9 nm, 1.2 nm and 2.13 nm, respectively. TPA-TPM
show a slight smoother surface than that of spiro-OMeTAD, which also can explain
why TPA-TPM film have a large water contact angle. And the extension of π-
conjugation by inserting phenyl ring between tetraphenylmethane core and
diphenylamine arms can improve the film-forming ability.
To investigate the performance of new compounds as HTM in perovskite solar
cells, the mesoscopic perovskite solar cells are fabricated and schematic architecture
in our devices are shown in Figure 5a. Figure 5b shows the energy level diagram of
each component in the PSCs.
Figure 3. SEM images (top view) of the perovskite layers covered by new
compounds and spiro-OMeTAD.
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Figure 4. AFM topographical images of HTM films based on DPA-TPM, TPA-
TPM and spiro-OMeTAD.
Figure 5. (a) Device architecture, and (b) energy level diagram of PSCs with two new
HTMs.
As the PSCs have exhibited a striking enhancement in devices’ performance by
increasing the conductivity of HTMs via doping 4-tertbutylpyridine (t-BP) and
lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI) into the HTM,[37] we fabricated
the device by doping t-BP and Li-TFSI into all the HTM layer. The detailed
fabrication of PSCs are shown in experimental section.
The photovoltaic performances of devices employing new compounds and spiro-
OMeTAD as HTMs are measured under AM 1.5G illuminations (100 mW cm-2).
Figure 6a shows the J-V curves obtained from the cells using these HTMs, as
summarized in Table 2. The PCEs of PSC devices with Ph-TPM and DPA-TPM are
only 4.62% and 9.33%, respectively. The bad performance of these devices with Ph-
TPM and DPA-TPM can be resulted from its inferior hole mobility and bad film-
forming ability, which can be observed from SEM images and high standard deviation
on the parameter values of the devices based on them (shown in the Supporting
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Information).
When HTM layer is replaced by well-soluble TPA-TPM, the performance of
PSC device show notably improvement, and yield a Jsc of 19.19 mA cm-2, a Voc of
1.07 V, and a FF of 0.73, leading to a PCE of 15.06%. The devices performance with
TPA-TPM are comparable with that of spiro-OMeTAD under the same conditions,
with a Jsc of 19.49 mA cm-2, a Voc of 1.06 V, and a FF of 0.75, leading a PCE of
15.49%. Moreover, the average PCE value of TPA-TPM-based devices is 13.90%,
(shown in Table S3), which indicate good reproducibility. Hysteresis behavior of the
devices with different are measured through forward and reverse scans, as shown in
Figure S3 and Table S5. The devices with TPA-TPM and spiro-OMeTAD show
similar hysteresis.
Figure 6. (a) J-V curves, and (b) IPCE spectra of the devices with different HTMs.
Table 2. Summary of device parameters with different HTMs.
HTM
Ph-TPM
Voc (V)
0.87
Jsc (mA cm-2)
FF (%)
42
PCE (%)
4.62
12.82
DPA-TPM
TPA-TPM
spiro-OMeTAD
0.92
15.81
64
9.33
1.07
19.19
73
15.06
15.49
1.06
19.49
75
The Voc of the devices based on different HTM decreases in the order of TPA-
TPM > spiro-OMeTAD > DPA-TPM > Ph-TPM, however, which is not consistent
with HOMO energy level from our CV data. Since the Voc of the devices depends on
the recombination, the splitting of the Fermi levels for photogenerated charges, and
the energetics of the devices.[10,38] Moreover, the bad film-forming properties of Ph-
TPM and DPA-TPM may lead to inferior charge extraction capability, as illustrated
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in TIPL and TRPL measurements. On the other hand, the Jsc of the PSCs with
different HTM decreases in the order spiro-OMeTAD > TPA-TPM > DPA-TPM >
Ph-TPM, which may be attributed to the different hole mobility of them.
The incident photo-to-current conversion (IPCE) spectra of the devices are
recorded to further confirm photo-current density, as shown in Figure 6b. All the
devices show remarked plateau between 400 and 750 nm due to the similar
fabrication process of these PSCs. In good agreement with those of measured
photocurrent density under AM 1.5G illuminations, the TPA-TPM based devices
enhanced the photocurrent in 400-750 nm relative to that of DPA-TPM by improving
charge collection and hole mobility. More efficient charge transfer between the HTMs
and perovskite layer indicates easier regeneration, resulting in higher IPCE values.
Since the stability is very important for solar cells, we carried out the aging test
of the CH3NH3PbI3-based devices on TPA-TPM and spiro-OMeTAD without
encapsulation under the same conditions (humidity is around 30 RH% without
illumination). After 9 days of aging, the TPA-TPM based cell give an around 28%
reduction (Figure 7), which is less than the spiro-OMeTAD based cell (41%) due to
better film-forming ability and molecular thermal stability of TPA-TPM. This result
shows that the long-term stability superiority of the devices based on TPA-TPM are
better than that of spiro-OMeTAD.
Figure 7. Stability test for CH3NH3PbI3-based devices based on TPA-TPM (●) and
spiro-OMeTAD (▲) in air at room temperature.
To further prove the potentiality of the application of TPA-TPM in PSCs, we
optimize the devices by compositional engineering of perovskite materials
((FAPbI3)0.85(MAPbBr3)0.15).[39] After optimization, a 17.42% PCE of spiro-
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OMeTAD-based device has been achieved, as shown in Figure 8 and Table 3. And
TPA-TPM-based devices show a PCE of , which is also comparable with spiro-
OMeTAD. We believe that with extensive optimization of the device fabrication and
perovskite preparation, PCE values of PSCs employing TPA-TPM close to 20%
might be attainable.
Figure 8. J-V curves of the (FAPbI3)0.85(MAPbBr3)0.15-based devices with TPA-TPM
and spiro-OMeTAD.
Table 3. Summary of device parameters with TPA-TPM and spiro-OMeTAD.
HTM
Voc (V) Jsc (mA cm-2) FF (%) PCE (%)
Reverse
Forward
Reverse
Forward
1.06
1.06
1.07
1.05
20.97
20.96
21.44
21.43
75
71
76
75
16.76
15.72
17.42
16.76
TPA-TPM
spiro-OMeTAD
The charge transfer dynamics of the CH3NH3PbI3 and the bilayer films
(HTM/CH3NH3PbI3) are investigated by time-integrated PL (TIPL) and time-resolved
PL (TRPL) measurements and the perovskite layer (around 100 nm) are deposited on
the FTO glass surface. The efficient PL quenching and the reduction of the PL
lifetime are attributed to charge extraction at the CH3NH3PbI3/HTM interface. Figure
9a shows that the bilayer films reveal dramatic quenching with respect to
CH3NH3PbI3 (DPA-TPM: 83%, TPA-TPM: 93%, spiro-OMeTAD: 95%). From
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TRPL spectra (Figure 9b), the estimated decay time are: 10.3 ns for pristine
CH3NH3PbI3, 3.4 ns for CH3NH3PbI3/DPA-TPM, 2.1 ns for CH3NH3PbI3/TPA-TPM,
and 1.5 ns for CH3NH3PbI3/spiro-OMeTAD, respectively. TPA-TPM and spiro-
OMeTAD reduced the lifetime of holes at the same level. TIPL and TRPL
measurements confirm that TPA-TPM has comparable hole extraction ability
compared with spiro-OMeTAD.
To further study charge transport and recombination in CH3NH3PbI3-based PSCs
with different HTM, impedance spectroscopy (IS) are performed. The recombination
resistance (Rrec) can be obtained from impedance spectra where the first arc at high
frequency region is related to transport and extraction of cathode/HTM.[40-42] The
second arc result from the recombination between TiO2 and HTMs.[43,44] The devices
are analyzed at forward bias between 0.4 and 0.9 V under dark condition. The
recombination resistance are fitted using a equivalent circuit model (Figure 10c). As
shown in Figure 10a and Figure 10b, Rrec for devices with TPA-TPM or spiro-
OMeTAD show lower values than that of DPA-TPM, which illustrate that
recombination in TPA-TPM or spiro-OMeTAD-based is lower than DPA-TPM-
based cells. Therefore, both of PL and IS study illustrate the better devices
performance with TPA-TPM than that of DPA-TPM. Moreover, TPA-TPM with
insertion of an extra phenyl group between the nitrogen and tetraphenylmethane than
DPA-TPM show better hole extraction ability, which result in better device
performance.
Figure 9. (a) Time-integrated photoluminescence, excitation at 473 nm, and (b) time-
resolved PL spectra of CH3NH3PbI3 (P), P/DPA-TPM, P/TPA-TPM and P/spiro-
OMeTAD. Excitation at 450 nm, monitor at 765 nm.
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Figure 10. (a) Nyquist plots of the device with different HTMs in the dark at 0.9 V
bias. (b) plots of recombination resistance (Rrec) by applying bias voltages. (c)
Equivalent circuit model.
Conclusions
In summary, we develop a series of new HTMs that incorporated
tetraphenylmethane as a core via facile synthesis process. Among these developed
compounds, TPA-TPM shows good film-forming ability, suitable band alignments
with CH3NH3PbI3, high thermal stability and comparable hole mobility. Therefore,
TPA-TPM show superior performance when applied in PSCs due to these advantages.
And the devices fabricated with TPA-TPM show a comparable performance with the
current spiro-OMeTAD. Moreover, the stability of the TPA-TPM-based PSCs are
better than that of spiro-OMeTAD. We believe that further modification of the
tetraphenylmethane core through molecular engineering of functional groups may
further improve the device performance more efficiently. Our research results also
provide a strategy for the design and development of new efficient HTMs for PSCs.
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Experimental Section
Materials
The toluene is dried by 4A molecular sieves. 4-methoxy-N-(4-methoxyphenyl)-
N-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)aniline (compound 4) is
synthesized and purified in our lab from previous literature.[45,46] tetrakis(4-
bromophenyl)methane, 4,4'-Dimethoxydiphenylamine, PbI2 are purchased from
Aldrich or TCI. All other chemicals and solvents used in this work are pure grade and
used without further purification.
Characterization
1H NMR and 13C NMR spectra are measured on a Brucker spectrometer (400
MHz). Mass spectra (MS) are performed with a LTQ Orbitrap XL Mass Spectrometer.
UV-vis absorption spectra of the HTMs are obtained on a UV-vis spectrophotometer
(U-3900H, Hitachi, Japan). The time-integrated photoluminescence (PL) spectra are
performed by a standard 450 W xenon CW lamp, and the TRPL are recorded on a
pulsed nitrogen/dye laser (QM400, Photo Technology International, USA). Cyclic
voltammetry (CV) measurement is performed on a CHI-660d electrochemical
analyzer using a typical three-electrode electrochemical cell with an SCE reference
electrode, a Pt working electrode and a Pt wire counter electrode. The supporting
electrolyte is 0.1 M tetrabutylammonium hexafluorophosphate in CH2Cl2, the scan
rate is 50 mV s-1. The monochromatic incident photon-to-current conversion
efficiency spectra are conducted using a QE/IPCE measurement kit (Newport
Corporation, USA). The IPCE measurement is conducted using a QE/IPCE
measurement kit (Newport Corporation, USA). Differential scanning calorimetry of
o
compounds is recorded with scan rate of 20 C min-1 (DSC, TA Instruments-Waters
LLC, USA, Q2000).
Synthesis of Ph-TPM, DPA-TPM and TPA-TPM
The synthetic procedure for Ph-TPM, DPA-TPM and TPA-TPM is shown in
Scheme 1. DPA-TPM and TPA-TPM are synthesized through Suzuki-Miyaura and
Buchwald-Hartwig cross coupling reaction with high yields, respectively. The detail
synthetic procedures are presented as follows.
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Scheme 1. Synthetic routes for Ph-TPM, DPA-TPM and TPA-TPM. (i) K2CO3,
Pd(PPh3)4, DMF, 90 oC; (ii) t-BuOK, Pd(OAc)2, P(t-Bu)3, toluene, reflux.
Ph-TPM. Compound 1 (0.38 g, 0.6 mmol), compound 2 (0.55 g, 3.5 mmol),
Pd(PPh3)4 (58 mg, 0.5 mmol), DMF (20 mL) and 2 M K2CO3 (5 mL) are added into a
50 mL flask, then degassed using Ar. The reaction mixture is stirred at 90 oC for 24 h.
After cooling down, the reaction mixture is poured into cold Na2SO4 aqueous solution,
and crude product precipitate out as solids. The crude product is purified by column
chromatography (DCM : PE = 1 : 1) to obtain the product as a white solid (0.33 g,
1
72%). H NMR (400 MHz, CDCl3) δ 7.58 (d, J = 2.9 Hz, 8H), 7.55 – 7.47 (m, 8H),
13
7.44 – 7.35 (m, 8H), 7.00 (d, J = 4.9 Hz, 8H), 3.88 (s, 12H). C NMR (100 MHz,
CDCl3) δ 156.05, 148.28, 140.64, 126.71, 126.24, 122.34, 120.52, 114.78, 55.52.
HRMS (MALDI-TOF) m/z: [M-H] calcd 744.32; found 744.34.
DPA-TPM. Compound 1 (0.64 g, 1 mmol), compound 3 (1.15 g, 5 mmol), t-BuONa
(0.77 g, 8 mmol), P(t-Bu)3 (0.1 M in toluene, 0.5 mL), Pd(OAc)2 (90 mg, 0.4 mmol)
are added into a 50 mL flask and degassed using Ar. Then 20 mL dry toluene is
degassed using Ar and injected in to the flask. The reaction mixture is kept with
stirring at 100 oC for 2 days. After cooling down, the reaction mixture is diluted by 40
mL dichloromethane (DCM) and ished with 80 mL water for 3 times. The organic
phase is dried over NaSO4, then removed solvent using a rotary evaporator. The crude
product is purified by column chromatography (DCM : PE = 2 : 1) to obtain the
1
product as white solid (0.92 g, 75%). H NMR (400 MHz, CDCl3) δ 7.06 (d, J = 4.4
Hz, 16H), 6.99 (d, J = 3.3 Hz, 8H), 6.83 (d, J = 4.6 Hz, 24H), 3.81 (s, 24H). 13C NMR
(100 MHz, CDCl3) δ 156.16, 148.53, 140.54, 126.83, 126.55, 126.31, 120.40, 114.82,
55.52. HRMS (MALDI-TOF) m/z: [M-H] calcd 1228.46; found 1228.44.
14
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10.1002/cssc.201601683
ChemSusChem
TPA-TPM. Compound 1 (0.51 g, 0.8 mmol), compound 4 (1.72 g, 4 mmol),
Pd(PPh3)4 (70 mg, 0.6 mmol), DMF (20 mL) and 2 M K2CO3 (5 mL) are added into a
o
50 mL flask, then degassed using Ar. The reaction mixture is stirred at 90 C for 2
days. After cooling down, the reaction mixture is poured into cold Na2SO4 aqueous
solution, and crude product precipitate out as solids. The crude product is purified by
column chromatography (DCM : PE = 3 : 2) to obtain the product as a white solid
1
(0.88 g, 72%). H NMR (400 MHz, DMSO) δ 7.55 (d, J = 5.7 Hz, 8H), 7.48 (d, J =
5.9 Hz, 8H), 7.26 (d, J = 7.0 Hz, 8H), 7.05 (d, J = 5.6 Hz, 16H), 6.92 (d, J = 7.4 Hz,
16H), 6.80 (d, J = 6.0 Hz, 8H), 3.74 (s, 24H). 13C NMR (100 MHz, CDCl3) δ 155.87,
148.07 145.21, 140.98, 138.15, 132.56, 131.52, 127.36, 126.56, 125.41, 120.86,
114.73, 55.51. HRMS (MALDI-TOF) m/z: [M-H] calcd 1532.84; found 1532.57.
Device fabrication
F-doped SnO2 glass substrates are subsequently cleaned by sonicating in detergent for
15 min, followed by deionized water and ethanol. Then TiO2 compact layer is
deposited on substrate by spray pyrolysis deposition method using dry air as carrier
gas, from a precursor solution of titaniumdiisopropoxide bis(acetylacetonate) in
isopropanol solution at 450 oC. Then the substrates are annealed at 450 oC for 40 min,
then cooling down to room temperature. The mesoporous TiO2 layer are deposited by
spin coating at 4000 rpm for 20 s using 30 nm TiO2 (Dyesol 30 NRD) paste diluted in
o
isopropanol (1:5, weight ratio). Then the TiO2 films are sintered at 510 C for 1 h.
o
After cooling down to 120 C, the substrates are immediately transferred in dry air
atmosphere. The CH3NH3PbI3 films are deposited from a precursor solution
containing 1.15 M PbI2 and 1.10 MAI in DMF:DMSO (v:v = 1.5:8.5). The precursor
solution is spin coated on the substrates by a two-step program at 1000 rpm for 10 s
and 4000 rpm for 30 s. During second step, 150 μL chlorobenzene is dripping on the
substrates at 15 s after starting. Then the substrates are put in hot plate immediately
and annealed for 1 h. After the substrates are cooled down to r. t., The HTM solution
(Ph-TPM: 10 mg in 1 mL chlorobenzene, DPA-TPM: 20 mg in 1 mL chlorobenzene,
TPA-TPM and spiro-OMeTAD: 60 mg in 1 mL chlorobenzene) containing
bis(trifluoromethylsulfonyl)imide lithium salt and 4-tert-butyl pyridine are deposited
by spin coating at 4000 rpm for 30 s. Finally, 60 nm gold electrode is deposited by
thermal evaporation under high vacuum.
15
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10.1002/cssc.201601683
ChemSusChem
Acknowledgements
This work is supported by the National Basic Research Program of China (No.
2015CB932200), CAS-Iranian Vice Presidency for Science and Technology Joint
Research Project, 116134KYSB20160130 and Natural Science Foundation of Anhui
Province (No.1508085SMF224). We acknowledge the Steady High Magnetic Field
Facility in High Magnetic Field Laboratory, Chinese Academy of Sciences for the
NMR measurements.
Keywords: tetraphenylmethane • hole-transporting materials • perovskite solar cell •
efficient • stability
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Graphical Abstract
TPA-TPM achieved a power conversion efficiency comparable to spiro-OMeTAD in
perovskite solar cells, which also show better long-term stability.
20
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