Dopant-free and low-cost molecular bee hole-transporting materials for efficient and stable perovskite solar cells

Article abstract of DOI:10.1039/c7tc03931j

With the dramatic development of the power conversion efficiency (PCE) of perovskite solar cells (PSCs), the device lifetime has attracted extensive research interest and concern. To enhance device durability, developing dopant-free hole-transporting materials (HTMs) with a high performance is a promising strategy. Herein, three new HTMs with a N,N′-diphenyl-N,N′-di(m-tolyl)benzidine (TPD) core: TPD-4MeTPA, TPD-4MeOTPA and TPD-4EtCz are designed and synthesized, showing suitable energy levels and excellent film-formation properties. PCEs of 15.28% were achieved based on pristine TPD-4MeOTPA as the HTM, which is a little lower than that of the p-doped 2,2′,7,7′-tetrakis-(N,N-di-4-methoxyphenylamino)-9,9′-spirobifluorene (spiro-OMeTAD)-based device (17.26%). Importantly, the devices based on the new HTMs show relatively improved stability compared to devices based on spiro-OMeTAD when aged under ambient air with 30% relative humidity in the dark.

Full text of DOI:10.1039/c7tc03931j

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Nguyên T. K. Thanh, Xiaodi Su et al.
Fine-tuning of gold nanorod dimensions and plasmonic properties using
the Hofmeister effects
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Dopant-Free and Low-cost molecular “Bee” Hole-Transporting
Materials for Efficient and Stable Perovskite Solar Cells
†
Received 00th January 20xx,
Accepted 00th January 20xx
Xicheng Liua,+, Fei Zhangb,c,+, Zhe Liua, Yin Xiaob,c, Shirong Wangb,c, Xianggao Lib,c
*
DOI: 10.1039/x0xx00000x
With the dramatic development of the power conversion efficiency (PCE) of perovskite solar cells (PSCs), device lifetime
has become one of the extensive research interests and concerns. To enhance the device durability, developing dopant-
free hole-transporting materials (HTMs) with high performance is a promising strategy. Herein, three new HTMs with N,N'-
diphenyl-N,N'-di(m-tolyl)benzidine (TPD) core: TPD-4MeTPA, TPD-4MeOTPA and TPD-4EtCz are designed and synthesized,
showing suitable energy levels and excellent film-formation property. PCEs of 15.28% was achieved based on pristine TPD-
www.rsc.org/
4MeOTPA as HTM, which is
a little lower than p-doped 2,2',7,7'-tetrakis-(N,N-di-4-methoxyphenylamino)-9,9'-
spirobifluorene (spiro-OMeTAD)-based device (17.26%). Importantly, the devices based on new HTMs show relatively
improved stability compared to devices based on spiro-OMeTAD when aged under ambient air with 30% relative humidity
in
the
dark.
field. However, unbefitting energy level, poor solubility and thermal
stability limit its application.31-33 In this paper, we designeed and
synthesized three novel TPD-based HTMs as shown in Fig. 1. The
energy levels of the HTMs was tuned by substituting the TPD-core
with different electron-donating groups through olefinic bonds.
Owing to structure readjustment and increasing molecular weight,
solubility and thermal stability were effective improved. The devices
based on [(FAI)0.85(PbI2)0.85(MABr)0.15(PbBr2)0.15], fabricated with
doped-free HTMs, achieve the highest PCE of 15.28 % under AM 1.5
G (100 mW cm-2) illumination. This result is a little lower than the
well-known p-doped spiro-OMeTAD (17.26%) obtained under the
same conditions. Moreover, the devices based on the as-
synthesized HTMs present improved stability compared with the
device based on spiro-OMeTAD under ambient air with 30% relative
humidity without encapsulation after 600 h in the dark.
1. Introduction
Recently, a new class of solid-state heterojunction solar cells based
on solution-processable organometal halide perovskite absorbers
have caused the extensive concern.1-8 Owing to the unique
characteristics, such as high charge carrier mobility, large absorption
coefficient, broad spectral absorption range, and long diffusion
length,9-11 the power conversion efficiency (PCE) of solid-state
perovskite solar cells (PSCs) quickly increased to over 20%.12-14
Due to the advantages of promoting hole migration, preventing
internal charge recombination and enhancing the stability of cells,
hole-transporting materials (HTMs) play a key role in PSCs.15-18
Many kinds of HTMs have been applied in PSCs, such as inorganic p-
type semiconductors, conducting polymers, and small molecule
hole conductors.19-30 Owing to the advantage of convenient
purification, controllable molecular structures and relatively high
efficiency,31-33 small molecular HTMs were widely used in PSCs.
Among these, 2,2',7,7'-tetrakis-(N,N-di-4-methoxyphenylamino)-
9,9'-spirobifluore-ne (spiro-OMeTAD) was the most studied. Due to
the relatively tedious synthesis and fancy price, numerous
alternative HTMs have been explored to replace sprio-OMeTAD.34-37
However, the PCE of dopant-free PSCs based on which are
consistently lying between 10% and 13%, few are over 15%.38-42
N,N'-diphenyl-N,N'-di(m-tolyl)benzidine (TPD) is a promising
hole-transporting material for application in organic photoelectric
Fig. 1 Molecular structures of as-synthesized TPD-based HTMs.
2. Results and Discussions
2.1 Normal Information
HTMs were obtained by Wittig reaction with cheap starting
materials and simple synthesis process. These three new HTMs
were fully characterized by 1H NMR spectroscopy and mass
spectrum. All the analytical data are consistent with the proposed
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structures (ESI Fig. S1). We roughly estimated the synthetic cost of 1 S1, S2 and S3). The
g as-synthesized HTMs according to previous report43-45 (ESI Table
Table 1 The optical, electrochemical, thermal properties and energy levels of HTMs.
λonset
[nm]a
λabs
[nmb]as
Eg
[eV]b
HOMO LUMO
Tg
Td
[eV]c
[eV]d [°C]e [°C]f
TPD-4MeTPA
TPD-4MeOTPA
TPD-4EtCz
454
459
442
305, 407 / 309, 408# 2.68
299, 408 / 303, 423# 2.59
301, 394 / 304, 397# 2.76
-5.33
-5.28
-5.34
-2.65
-2.69
-2.58
145 432
150 416
138 452
b
a Absorption spectra in the 1.0×10-5 mol L-1 THF solution. Optical energy gaps calculated by the absorption thresholds (λonset) from UV-Vis
c
absorption spectra of films. Measured by photoelectron yield spectroscopy (PYS, ESI Fig. S3). d |LUMO|=|HOMO|-|Eg|. e Decomposition
f
temperatures, measured by TGA at a heating rate of 10 °C min-1 under N2. Measured by DSC at a heating rate of 10 °C min-1 under N2
according to the heat-cool-heat procedure. # Absorption peaks of solid films.
estimated synthesis cost are 83.4 $ g-1, 96.7 $ g-1 and 44.5 $ g-1 for mainly distribute on the TPD branches. As shown in Fig. 3, similar to
TPD-4MeTPA, TPD-4MeOTPA and TPD-4EtCz, respectively, which spiro-OMeTAD, the TPD center of as-synthesized HTMs adopt a spiro
were much cheaper than that of spiro-OMeTAD (581.52 $ g-1).46 The conformation because of the larger terminal branches. The
glass transition temperatures (Tg) and decomposition temperatures measured angles between two terminal branches are 62.72°, 64.87°
(Td) of HTMs were determined by differential scanning calorimetry and 61.77° for TPD-4MeTPA, TPD-4MeOTPA and TPD-4EtCz,
(DSC) and thermogravimetric analysis (TGA), respectively (ESI Fig. respectively. Compared to 89.94° between the two fluorine rings in
S2). As shown in Tabel 1, outstanding thermal stability of as- spiro-OMeTAD,50 the spiro conformation of as-synthesized HTMs
synthesized HTMs were confirmed by high Tg and Td.
will prevent stronger intermolecular stacking and form an
Ultraviolet-Visible (UV-Vis) absorption spectra of as- amorphous solid film. The energy levels agreed well with the
synthesized HTMs in dilute tetrahydrofuran (THF) solutions (1.0×10-5 tendency determined by photoelectron yield spectroscopy (PYS, ESI
mol L-1) and solid films are shown in Fig. 2. The peaks at short Fig. S3) and optical measurements (Table 1). The HOMO levels of as-
wavelength (~300 nm) can be assign to n-π* transition of the TPA synthesized HTMs matched well with the valence band level of
moiety, while peaks at longer wavelength (~400 nm) are organometal halide perovskite (-5.5 eV) to favor the hole migration.
intramolecular charge transfer (ICT) of π–π*.47 These three The LUMO levels are much higher than the conduction band level (-
compounds showed similar n-π* transition peaks, while the ICT 3.9 eV) to block the electron back-transfer from perovskite to Au
peaks showed an bathochromic shift from TPD-4EtCz to TPD- electrode (Fig. 4a).15-18
4MeTPA and TPD-4MeOTPA due to extend of π-π conjugation.48,49
No obvious change in absorption spectra for thin films indicate the
amorphous structure of spin-coated films, which were further
identified by XRD measurements (ESI Fig. S4).50
A slight
bathochromic shift (1 nm, 15 nm and 3 nm for TPD-4MeTPA TPD-
4MeOTPA and TPD-4EtCz, respectively) and broadening of ICT bands
in the solid films can be observed which indicate the existing of
slight intermolecular interactions in the solid state.51
Fig. 3 Distributions of HOMO and LUMO and optimized molecular
structures of the new HTMs.
Fig. 2 Normalized UV-Vis absorption spectra of the new HTMs (a)
THF solution (c = 1.0×10-5 mol L-1); (b) solid films.
The time-of-flight (TOF, ESI Fig. S5) transient hole-current
measurement was applied to measure the hole mobilities of
HTMs.52 At room temperature, the hole mobilities are 2.14×10-
4, 4.92×10-4 and 1.27×10-4 cm2 V-1 s-1 for TPD-4MeTPA, TPD-
4MeOTPA and TPD-4EtCz, respectively, compared to 1.42×10-4
The optimized molecular structures and frontier molecular
orbital distribution of the new HTMs were studied by DFT
calculation.31 The highest occupied molecular orbital (HOMO) levels,
lowest unoccupied molecular orbital (LUMO) energy levels and the
energy gap (Eg) are shown in Fig. 3. From Fig. 3, we found that the
HOMO distribute on the whole molecular skeleton. The LUMO
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cm2 V-1 s-1 for spiro-OMeTAD at applied electric fields of lower than spiro-OMeTAD (17.26%) doped with Lithium
1.5×105 V cm-1, respectively.
bis(trifluoromethyl-sulfonyl)imide (LiTFSI) and 4-tert-butylpyridine
(tBP). However, devices based the new HTMs doped with LiTFSI and
2.2 Application in the mesoporous structured PSCs
The HOMO levels of as-synthesized HTMs (-5.28
~
-5.34 ev) tBP exhibit lower photovoltaic performance, especially in terms of
matched well with the valence band level of organometal FF and JSC (ESI Table S4). This is partly because the additives and
halide perovskite (-5.5 eV) to favour the hole migration. The dopants which work well with spiro-OMeTAD may not be suitable
LUMO levels (-2.69~
-2.58 ev) are much higher than the for the new HTMs.55,56 Moreover, the dopants seem to have a
conduction band level (-3.9 eV) to block the electron back- negative impact on film morphology, which are further confirmed
transfer from perovskite to Au electrode (Fig. 4a).15-18 The by the SEM image in ESI Fig. S6. Similar behaviour was observed in
perovskite layer was obtained by using the anti-solvent other reports.42,54,57-60 In the absence of additives and dopants,
method as described in literature.53 The device structure can spiro-MeOTAD-based devices generated a PCE of only 5.43% owing
be clearly seen from a high-resolution cross-sectional scanning to significant lowering of the VOC and FF compared to the doped
electron microscopy (SEM) image (Fig. 4b). As can be seen devices.61,62 Only small hysteresis was observed in the J–V curves.
from the image, the perovskite penetrates into the mp-TiO2 The measured PCE differences [(PCEbackward - PCEforward) / PCEaverage
×
and forms an overlayer. Similarly, the HTMs blend into the 100%] are 1%, 9%, 3% and 1% and the stabilized power outputs are
pores in the TiO2/perovskite layer and form a thin capping 14.31%, 15.02%, 11.28% and 17.09% for devices based on TPD-
layer on the top.
4MeTPA, TPD-4MeOTPA, TPD-4EtCz and spiro-OMeTAD, respectively
(ESI Fig. S7), consistent with the obtained PCE.54
The incident photon-to-electron conversion efficiency (IPCE)
spectrums of the cell with the four different HTMs are presented in
Fig. 4d. The integrated current densities estimated from the IPCE
spectra (19.90 mA cm-2, 20.30 mA cm-2, 19.45 mA cm-2, and 20.83
mA cm-2 for TPD-4MeTPA, TPD-4MeOTPA, TPD-4EtCz, and spiro-
OMeTAD,
respectively) are in good agreement with the JSC values obtained
from the J–V curves. We fabricated batches of 10 cells each using
different HTMs and demonstrate in Fig. 5 excellent reproducibility
by the narrow statistical distribution of the photovoltaic metrics.
Fig. 4 (a) Energy level diagram of the perovskite solar cells based on
different HTMs; (b) Cross-sectional SEM image of the PSC with
configuration of FTO/compact TiO2 (40 nm)/mesoporous TiO2 (200
nm)/perovskite (480 nm)/HTM (160 nm)/Au (80 nm), the scale bar
is 200 nm; (c) J–V hysteresis curves of PSCs comprising champion
devices with HTMs measured starting with backward scan and
continuing with forward scan, and (d) IPCE spectra of the devices
based on the new HTMs without additives/dopants and spiro-
OMeTAD with additives/ dopants.
Fig. 5 Statistical deviation of the photovoltaic parameters for 10
different solar cells using different HTM: (a) Voc, (b) Jsc, (c) FF, and
We evaluated the photovoltaic performance of PSCs based on
the three new HTMs and spiro-OMeTAD with or without additives
and dopants. The photocurrent densitiy–voltage (J–V) curves under (d) PCE.
AM 1.5 G irradiation of 100 mW cm-2 are presented in Fig. 4c, and
the photovoltaic parameters are summarized in Table 2. The lower
performance by TPD-4EtCz is mainly related to the lower FF caused
by the poor film-forming ability shown in ESI Fig. S6.54 As expected,
TPD-4MeTPA and TPD-4MeOTPA give higher open-circuit voltage
than spiro-OMeTAD for the device, which are correspond with their
lower HOMO levels. The best device based on TPD-4MeOTPA
affords a VOC of 1.099 V, a short-circuit current density (JSC) of 20.84
mA cm-2, and a fill factor (FF) of 0.667, leading to a PCE of 15.28%
under AM 1.5 G (100 mW cm-2) illumination. This result is a little
The time-resolved photoluminescence (PL) spectra are shown in
Fig. 6, which are obtained by fitting the data with a bi-exponential
decay function (ESI Table S5). For the TiO2/CH3NH3PbI3 film without
a HTM layer, the pristine perovskite film gave short-lived lifetime τ1
= 22.14 ns and long-lived lifetime τ2 = 171.70 ns. When coated with
a thin HTM layer on the surface of perovskite, the lifetime
significantly decreases to (τ1 = 8.50 ns, τ2 = 74.53 ns), (τ1 = 7.55 ns,
τ2 = 63.70 ns), (τ1 = 12.26 ns, τ2 = 92.58 ns) and (τ1 = 3.59 ns , τ2 =
30.26 ns) for TPD-4MeTPA, TPD-4MeOTPA, TPD-4EtCz and sipro-
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OMeTAD based film, respectively. The PL decay lifetime for new efficiently.
HTMs are somewhat longer compared with doped spiro-OMeTAD,
but significantly shorter than the HTM-free device, which means
that the new HTMs can extract the holes from the perovskite
Table 2 J-V curves of HTMs and spiro-OMeTAD based device under different scan directions [a]
HTMs [b]
Jsc(mA cm-2)
20.601
20.502
20.835
20.800
20.310
20.271
21.612
21.593
Voc(V)
1.089
1.099
1.099
1.118
1.104
1.134
1.074
1.089
FF
PCE (%)
backward
forward
0.637
0.625
0.667
0.599
0.523
0.493
0.714
0.696
14.33
14.13
15.28
13.94
11.73
11.34
17.26
17.04
TPD-4MeTPA
backward
forward
TPD-4MeOTPA
TPD-4EtCz
backward
forward
backward
forward
spiro-OMeTAD ++
[a] Bias step of 5 mV. [b]+ + = samples include LiTFSI and tBP additives
3. Experimental Section
Fig. 6 Normalized time-resolved PL spectra of the devices based on
as-synthesized without additives/dopants and spiro-OMeTAD with
additives/dopants.
2.3. Stability and lifetime of PSCs
Fig. 7 The stability of the new HTMs and spiro-OMeTAD-based PSCs
in ambient air without encapsulation ( TPD-4MeTPA; TPD-
4MeOTPA; TPD-4EtCAZ; spiro-OMeTAD).
The stability of PSCs is a difficulty for application, especially in
arbitrary environment. We compared the stability of the PSCs with
as-synthesized HTMs and spiro-OMeTAD by exposing them to
ambient air at 30% relative humidity without encapsulation. The
operation stability was investigated in following way: The J-V curves
were measured after storage under atmospheric environment per
24 hours. The time evolution of the photovoltaic metrics is shown in
Fig. 7.
■
●
▲
▼
3.1 Materials
N,N'-Diphenyl-N,N'-di(m-tolyl)benzidine (TPD) was purchased from
Heowns Biochemical Technology CO., LTD., Tianjin, China. PbI2 and
PbBr2was purchased from Tokyo Chemical Industry CO., LTD, N,N′-
dimethylformide (DMF) from Alfar Aesar, hydroiodic acid (AR, 45
wt.% in water) and methylamine (AR, 27% in methanol) from
Sinopharm Chemical Reagent Co. Ltd. 2,2′,7,7′-tetrakis (N,N-di-p-
methoxy-phenylamine)-9,9′-spirobifluorene (spiro-OMeTAD) was
from Luminescence Technology Corp., Taiwan, China.
Tetrahydrofuran was distilled before using, all the other agents were
directly used without further purification. Substrates were FTO
conducting glass (Pilkington, thickness: 2.2 mm, sheet resistance: 14
Ω/square). Patterned FTO glass was first cleaned with mild
detergent, rinsed several times with distilled water and
subsequently with ethanol in an ultrasonic bath, finally dried under
air stream.
The PCE based on three HTMs only decreased < 8% after 600 h
storage in ambient air without encapsulation. (Fig. 7). In the first
300 h, a slight increase of Voc can be observed in non-doped PSCs
with as-synthesized HTMs. For the long storage time (300-600 h),
the decreased of Jsc in PSCs based on doped spiro-OMeTAD are
more obvious. This indicated the non-doped PSCs based on as-
synthesized HTMs have promising long-term stability at room
temperature. Which was mainly attributed to their lack of doping
additives and less pinholes in the HTM layer.12,63 The contact angles
(ESI Fig. S8) of TPD-4MeTPA (89.4°), TPD-4MeOTPA (94.8°), and TPD-
4EtCz (89.1°) are larger than for doped spiro-OMeTAD (66.9°)
reported by our recent paper.60 Larger contact angles indicate
higher hydrophobicity of the film. The more hydrophobic nature of
as-synthesized HTMs help to expel moisture away from the
perovskite film, which could also lead to enhanced device stability.
3.2 Measurements
The as-synthesized compounds were identified by nuclear magnetic
resonance (NMR) spectrum and high-resolution mass spectroscopy
(HRMS). The NMR was obtained on a Bruker AVANCE III 400 MHz
spectrometer, with the chemical shifts reported in ppm using
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tetramethylsilane (TMS) as an internal standard. HRMS was a two neck 250 mL round bottom flask, followed by 90 mL of
recorded on a SolariX maldi-FTMS mass spectrometer. Ultraviolet- acetonitrile. Trifluoroacetic anhydride (17.3 mL, 123.0 mmol) was
Visible absorption (UV-Vis) spectroscopy was obtained by the droped under nitrogen atmosphere (N2). Then above mixture
Thermo Evolution 300 UV-Visible spectrometer. Decomposition refluxed until TPD consumed completely (monitored by thin-layer
temperature (Td) and glass transition temperature (Tg) were chromatography). The reaction solution was poured into 1 L water
determined by the thermo gravimetric analysis (TGA) and to dissolve out yellow powder. The filter cake washed with water
differential scanning calorimeter (DSC) on a TA Q500 thermo until the filtrate became colorless, compound (1) obtained: yellow
gravimetric analysis and TA Q20 thermal analysis under a nitrogen solid (15.0 g, 99.1%): mp 243-245°C,1H NMR (400 MHz, CDCl3) δ
atmosphere. The photoelectron yield spectroscopy (PYS) was (ppm): 2.63 (s, 6H), 6.89 (s, 4H), 7.04-7.24 (m, 14H), 7.53 (d, J = 8.4
carried out on the Sumitomo PYS-202 ionization energy detection Hz, 4H), 7.38 (d, J = 8.5 Hz, 4H), 6.74 (d, J = 10.1 Hz, 8H), 7.27 (s, 4H);
system. X-ray diffraction (XRD) was measured with a Rigaku Miniflex ESI-MS (m/z): [M–] Calcd for C66H40F24N10O8, 1556.3; Found 1556.9.
600 X-Ray diffraction. Scanning electron microscope (SEM) was
measured with Hitachi S-4800.
The polyimidazoline product ((1), 10 g, 6.4mmol) was dissolved
in 200 mL THF. Then pumped HCl gas (100 mL, 2.5 mol L-1 - prepared
Current-voltage characteristics (J–V) were measured on a by adding 21.0 mL of concentrated HCl to 79.0 mL H2O). The
Keithley 2602 SourceMeter under AM 1.5 irradiation (100 mW cm-2) reaction solution reflux for 12 h. The reaction solution was cooled to
from an Oriel Solar Simulator 91192. A mask with a window of 0.16 room temperature, then an orange solid formed. The reaction
cm2 was clipped on the TiO2 side to define the photoactive area of mixture was filtered and recrystallized from diethyl ether,
the cells. Incident-photon-to-current conversion efficiency (IPCE) compound (2) obtained (3.8 g, 93.0%): mp 197-199 °C, IR: 2803.99,
1
was measured by the direct current (DC) method using a lab-made 2722.76, 1693.86; H NMR (400 MHz, CDCl3) δ (ppm): 10.15 (s, 2H),
IPCE setup under 0.3-0.9 mW cm-2 monochromic light illumination 9.91 (s, 2H), 7.80 (d, J = 8.5 Hz, 4H), 7.74 (d, J = 8.4 Hz, 2H), 7.62 (d, J
without bias illumination. Time resolved PL spectra was recorded on = 8.4 Hz, 4H), 7.32-7.21 (m, 8H), 7.19-7.04 (m, 4H), 2.60 (s, 6H); ESI-
PL spectrometer, Edinburgh Instruments, FLS 900, excited with a MS (m/z): [M+H]+ Calcd for C42H32N2O4, 628.2; Found, 629.5.
picosecond pulsed diode laser (EPL-445), and measured at 775 nm
Wittig regents (3)
after excitation at 445 nm. The time-of-flight (TOF) measurement
Wittig reagents (3) were synthesized according to literature.31
was record on a TOF401 measurement system, Sumitomo Heavy
Industries. Ltd. Samples were prepared through spin-coated with a N,N-di(phenyl)-N′,N′-di(4-(4-N,N-di(4-(4-methoxy-phenyl))amino)
structure of ITO/as-synthesized compounds (~1 μm)/Al (100 nm) phenyl)ethenyl) -1,1′-biphenyl-4,4′-diamine (TPD-4MeTPA)
with a working area of 3×3 mm2.
4-[N,N-di(p-tolyl)amino]benzyl(triphenyl)phosphonium
bromide
3.3 Synthesis
(W1, 2.64 g, 4 mmol) and compound (2) (0.31 g, 0.5 mmol) were
The designed synthetic routes for as-synthesized compounds are added into a 100 mL round-bottom flask under N2. Anhydrous THF
depicted in Scheme 1, which were synthesized by Wittig reaction (40 mL) was added to above flask, cooled down to 0 °C. The THF
using formyl replaced TPD (2) and Wittig reagents (3) in three steps solution of t-BuOK (16 mmol, 0.8 mol L-1) was added dropwise to
from commercially available and relatively inexpensive starting above flask, stirred for 30 min at 0 °C, followed with stirred at room
reagents. The structure of as-synthesized compounds were temperature until compound (2) was consumed completely
confirmed via 1H NMR and MS, which agreed well with the (monitored by thin-layer chromatography). The reaction was
proposed molecular structure (See ESI Fig. S1).
terminate with ice water. The crude product was heated under
reflux for 8 h in THF with a catalytic amount of iodine. Then the
remaining iodine was removed by sodium hydroxide (NaOH)
solution (Wt = 10%, 100 mL) by stirring for 2 h. After that, the
product was purified by chromatographed on a silica gel column
(petroleum ether: ethyl acetate =50:1 as eluent) to give the title
COCF3
F3COC
N
N
N
N
N
F3COC
F3COC
COCF3
COCF3
C3H4N3/(CF3CO)2
THF
O
N
N
HCl
N
N
THF
N
TPD
OHC
1
N
N
COCF3
F3COC
(1)
compound as a pure E stereoisomer TPD-4MeTPA (0.59 g, 69%): H
CHO
NMR (400 MHz, CDCl3) δ (ppm): 7.67 (dd, J = 11.9, 7.9 Hz, 2H), 7.58-
7.42 (m, 6H), 7.40-7.28 (m, 10H), 7.20-6.76 (m, 60H), 2.30 (q, J = 5.0
Hz, 30H). HRMS (m/z): Calcd for C124H108N6, 1705.86690; Found
1705.86589.
Wittig Regents
(3)
I2/THF
N
N
TM
t-BuOK/THF
(2)
OHC
CHO
TPD-4MeOTPA and TPD-4EtCz were synthesized by W2 (4-[N,N-
di(p-
O
O
N
PPh3Br
N
methoxyphenyl)amino]benzyl(triphenyl)phosphoniumbromide), W3
Wittig Regents:
(3)
N
(3-[(9-ethyl)-carbazole]methyl(triphenyl)phosphonium
bromide)
PPh3Br
PPh3Br
and compound 2 using the same method.
W1
W3
W2
TPD-4MeOTPA: (0.63 g, 73%): 1H NMR (400 MHz, CDCl3) δ
(ppm): 7.53-7.43 (m, 6H), 7.37 (d, J = 8.3 Hz, 4H), 7.31 (t, J = 7.2 Hz,
8H), 7.10 (dd, J = 35.0, 7.7 Hz, 26H), 6.91 (s, 16H), 6.83 (d, J = 8.6 Hz,
18H), 3.79 (s, 24H), 2.33 (d, J = 11.3 Hz, 6H). HRMS (m/z): Calcd for
C126H108N6O8, 1833.82622; Found 1833.82536.
Scheme 1 Synthetic route for the three new HTMs.
4,4'-([1,1'-biphenyl]-4,4'-diylbis((4-formylphenyl) azanediyl))bis(2-
methylbenzaldehyde) (2)
TPD (5.0 g, 9.7 mmol), imidazole (5.1 g, 61.5 mmol) were added into
TPD-4EtCz: (0.51 g, 65%): 1H NMR (400 MHz, CDCl3) δ (ppm):
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DOI: 10.1039/C7TC03931J
ARTICLE
Journal Name
5 F. Zhang, S. R. Wang, X. G. Li and Y. Xiao, Synth. Met., 2016,
220, 187-193.
6 Y. X. Zhao and K. Zhu, Chem. Soc. Rev., 2016, 45, 655-689.
7 Y. K. Song, S. T. Lv, X. C. Liu, X. G. Li, S. R. Wang, H. Wei, D. M.
Li, Y. Xiao and Q. B. Meng, Chem. Commun., 2014, 50, 15239-
15242.
8.15 (dd, J = 24.0, 17.1 Hz, 6H), 7.73-7.63 (m, 4H), 7.59 (t, J = 9.7 Hz,
2H), 7.56-7.27 (m, 22H), 7.22-6.92 (m, 14H), 4.34 (t, J = 12.2 Hz, 8H),
2.39 (d, J = 25.3 Hz, 6H), 1.50-1.35 (m, 12H). HRMS (m/z): Calcd for
C102H84N6, 1393.67910; Found 1393.67831.
3.4 Quantum chemical calculation
8 P. M. Da and G. F. Zheng, Nano Res., 2017, 10, 1471-1497.
9 T.Ye, S. Y. Ma, X. Jiang, M. Petrovic, C. Vijila, S. Ramakrishna
and L. Wei, Nanoscale, 2017, 9, 412-420.
10 J. Y. Liu, J. J. Shi, D. M. Li, F. Zhang, X. G. Li, Y. Xiao, S. R. Wang,
Synth. Met., 2016, 215, 56-63.
11 Y. Li, Z. Xu, S. L. Zhao, B. Qiao, D. Huang, L. Zhao, J. Zhao, P.
Quantum chemical calculation was performed on a Gaussian 03
program with the Beck’s three-parameter exchange functional and
the Lee-Yang-Parr’s correlation functional (B3LYP) using 6-31G (d)
basis sets.64
3.5 Solar cell fabrication details
Wang, Y. Q. Zhu, X. G. Li, X. X. Liu and X. R. Xu, Small, 2016, 12
4902-4908.
12 F. Zhang, W. D.Shi, J. S. Luo, N. Pellet, C. Y. Yi, X. Li, X. M.
Zhao, T. J. S. Dennis, X. G. Li, S. R. Wang, Y. Xiao, S. M.
Zakeeruddin, D. Q. Bi and M. Grätzel, Adv. Mater., 2017, 29
1606806.
13 F. Zhang, Z. Q. Wang, H. W. Zhu, N.-P. Pellet, J. S. Luo, C. Y. Yi,
X. C. Liu, H. L. Liu, S. R. Wang, X. G. Li, Y. Xiao, S. M.
Zakeeruddin, D. Q. Bi and M. Grätzel, Nano Energy, 2017, DOI:
10.1016/j. nanoen.2017.09.035.
,
A 30 nm-thickness compact TiO2 layer, 200-300 nm mesoporous
TiO2
layers
and
perovskite
layer
([(FAI)0.85(PbI2)0.85(MABr)0.15(PbBr2)0.15]) were prepared according to
the reported methods.53 The HTMs layers were spin-coated on the
,
top of TiO2/perovskite films at 3000 rpm for 20
concentration of 20 mg mL-1. For comparison, perovskite solar cells
based on spiro-OMeTAD were also fabricated by using
chlorobenzene solution doped with Lithium
s with a
a
bis(trifluoromethylsulfonyl)imide (LiTFSI) and 4-tert-butylpyridine
(tBP) under the same conditions. All the above fabrication
processes were carried out in air. Finally, 80 nm-thickness Au
photocathode was deposited by thermal evaporation.
14 D. Q. Bi, C. Y. Yi, J. S. Luo, J. D. Décoppet, F. Zhang, S. M.
Zakeeruddin, X. Li, A. Hagfeldt and M. Grätzel, Nat. Energy,
2016, 1, 16142.
15 T. Swetha adn S. P. Singh, J. Mater. Chem. A, 2015, 3, 18329-
18344.
16 S. Ameen, M. A. Rub, S. A. Kosa, K. A. Alamry, M. S. Akhtar, H.
S. Shin, H. K. Seo, A. M. Asiri and M. K. Nazeeruddin,
ChemSusChem, 2016, 9, 10-27.
4. Conclusions
17 X. M. Zhao, F. Zhang, C. Y. Yi, D. Q. Bi, X. D. Bi, P. Wei, J. S.
Luo, X. C. Liu, S. R. Wang, X. G. Li, S. M. Zakeeruddin and M.
Grätzel, J. Mater. Chem. A, 2016, 4, 16330-16334.
We synthesized three TPD-core HTMs (TPD-4MeTPA, TPD-4MeOTPA
and TPD-4EtCz) with simple synthetic procedures and low cost. The
PSC based on dopant-free TPD-4MeOTPA as the HTM affords an
18 S. Shi, Y. Li, X. Li and H. Wang, Mater. Horiz., 2015,
2, 378-
impressive PCE of 15.28%, which is a little lower than that obtained 405.
19 J. J. Wang, S. R.Wang, X. G. Li, L. F. Zhu, Q. B. Meng, Y. Xiao
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Hu, Nano Res. 2017, 10.1007/s12274-017-1618-z.
21 W. B. Yan, Y. Li, Y. L. Li, S. Y. Ye, Z. W. Liu, S. F. Wang, Z. Q.
Bian and C. H. Huang, Nano Res., 2015, 8, 2474-2480.
22 Y. S. Kwon, J. Lim, H. J. Yun, Y. H. Kim and T. Park, Energy
Environ. Sci., 2014, 7, 1454-1460.
23 J. H. Heo, S. H. Im, J. H. Noh, T. N. Mandal, C. S. Lim, J. A.
Chang, Y. H. Lee, H. J. Kim, A. Sarkar, M. K. Nazeeruddin, M.
Grätzel and S.ⅡSeok, Nat. Photonics, 2013, 7, 486-491.
24 P. Qin, S. Tanaka, S. Ito, N. Tetreault, K. Manabe, H. Nishino,
M. K. Nazeeruddin and M. Grätzel, Nat. Commun., 2014, 5,
3834.
25 K. T. Cho, O. Trukhina, C. Roldán-Carmona, M. Ince, P. Gratia,
G. Grancini, P. Gao, T. Marzalek, W. Pisula, P. Y. Reddy, T.
Torres and M. K. Nazeeruddin, Adv. Energy Mater., 2017, 7,
employing the well-known p-doped spiro-OMeTAD. The devices
based on as-synthesized HTMs obtained a higher stability than the
device based on spiro-OMeTAD afer 600 h at room temperature in
ambient air with 30% relative humidity without encapsulation. The
introduction of these three novel HTMs with improved synthesis
and excellent performance highlight their potential use in the future
deployment of PSCs.
Acknowledgements
The authors gratefully acknowledge the financial support from
the National Key R&D Program of China (2016YFB0401303),
the National Natural Science Foundation of China (21676188)
and Key Projects in Natural Science Foundation of Tianjin
(16JCZDJC37100). The calculation in this work was supported
by high performance computing center of Tianjin University,
China.
1601733.
26 X. Q. Jiang, Z. Yu, J. B. Lai, Y. C. Zhang, M. W. Hu, N. Lei, D. P.
Wang, X. C. Yang and L. C. Sun, ChemSusChem, 2017, 10
1838–1845.
,
27 X. Q. Jiang, Z. Yu, H.-B. Li, Y. W. Zhao, J. S. Qu, J. B. Lai, W. Y.
Ma, D. P. Wang, X. C. Yang and L. C. Sun, J. Mater. Chem. A,
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DOI: 10.1039/C7TC03931J
TPD based molecular “bee” as dopant-free HTMs for PSCs exhibit the
PCE of 15.28% which is comparable to doped spiro-OMeTAD.