Impact of peripheral groups on novel asymmetric phthalocyanine-based hole-transporting materials for perovskite solar cells

Article abstract of DOI:10.1016/j.dyepig.2020.108301

Three novel asymmetrical substituted phthalocyaninecobalt with nitro and (4-butyl formate) phenoxy, (4-propenyl-2-methoxy) phenoxy or (4-methyl formate) phenoxy as different bulky peripheral groups (CoPcNO₂-OBFPh, CoPcNO₂-OPMPh, CoPcNO₂-OMFPh) are successfully developed and applied as dopant-free hole-transporting materials (HTMs) in perovskite solar cells (PSCs). The impact of the different peripheral groups on properties of these phthalocyanines is also investigated. For the modification of bulky aroxy peripheral groups, all of the three metallophthalocyanines exhibit good solubility, suitable hole mobility, high thermal stability, and appropriate HOMO and LUMO energy levels. The most effective device based on CoPcNO₂-OBFPh demonstrates an impressive power conversion efficiency (PCE) of 13.91% under AM 1.5G standard conditions, while CoPcNO₂-OPMPh and CoPcNO₂-OMFPh devices exhibit relative lower PCE of 11.81% and 9.47% respectively. In addition, CoPcNO₂-OBFPh-based PSC shows the best stability after 1008 h in air with 50% relative humidity at room temperature.

Full text of DOI:10.1016/j.dyepig.2020.108301

Journal Pre-proof
Impact of peripheral groups on novel asymmetric phthalocyanine-based hole-
transporting materials for perovskite solar cells
Junjie Guo, Mengmeng Sun, Xianfang Meng, Hongwei Zhu, Chao Ma, Shiyan Hu,
Jiaqi Shen, Qian Wang, Jinghan Gao
PII:
S0143-7208(19)32630-0
DOI:
https://doi.org/10.1016/j.dyepig.2020.108301
DYPI 108301
Reference:
To appear in: Dyes and Pigments
Received Date: 31 December 2019
Revised Date: 11 February 2020
Accepted Date: 20 February 2020
Please cite this article as: Guo J, Sun M, Meng X, Zhu H, Ma C, Hu S, Shen J, Wang Q, Gao J,
Impact of peripheral groups on novel asymmetric phthalocyanine-based hole-transporting materials for
perovskite solar cells, Dyes and Pigments (2020), doi: https://doi.org/10.1016/j.dyepig.2020.108301.
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Impact of peripheral groups on novel asymmetric phthalocyanine-based
hole-transporting materials for perovskite solar cells
a,
a
a
b,c
a
a
a
Junjie Guo *, Mengmeng Sun , Xianfang Meng , Hongwei Zhu , Chao Ma , Shiyan Hu , Jiaqi Shen , Qian
a
a
Wang , Jinghan Gao
a Tianjin Key Laboratory of Food Biotechnology, School of Biotechnology and Food Science, Tianjin University of Commerce,
Tianjin, 300134, China
b School of Chemical Engineering and Technology, Tianjin University, Tianjin, 300072, China
c Collaborative Innovation Center of Chemical Science and Engineering Tianjin, Tianjin, 300072, China
Abstract:
Three novel asymmetrical substituted phthalocyaninecobalt with nitro and (4-butyl formate) phenoxy,
(
(
4-propenyl-2-methoxy) phenoxy or (4-methyl formate) phenoxy as different bulky peripheral groups
CoPcNO -OBFPh, CoPcNO -OPMPh, CoPcNO -OMFPh) are successfully developed and applied as
2
2
2
dopant-free hole-transporting materials (HTMs) in perovskite solar cells (PSCs). The impact of the different
peripheral groups on properties of these phthalocyanines is also investigated. For the modification of bulky
aroxy peripheral groups, all of the three metallophthalocyanines exhibit good solubility, suitable hole mobility,
high thermal stability, and appropriate HOMO and LUMO energy levels. The most effective device based on
CoPcNO
standard conditions, while CoPcNO
1.81% and 9.47% respectively. In addition, CoPcNO
in air with 50% relative humidity at room temperature.
2
-OBFPh demonstrates an impressive power conversion efficiency (PCE) of 13.91% under AM 1.5G
-OPMPh and CoPcNO -OMFPh devices exhibit relative lower PCE of
-OBFPh-based PSC shows the best stability after 1008 h
2
2
1
2
Key words: Hole transporting material; Perovskite solar cells; Long-term stability; Peripheral Groups,
phthalocyanine

1
. Introduction
As the most important natural sources, solar energy is widely studied in the renewable energy field. One of
the most effective applications of solar energy is the conversion of solar energy into electricity. For the rapid
improvement of device performance [1-4], perovskite solar cells (PSCs) have exhibited as a promising
photovoltaic technology with skyrocketed photoelectric conversion efficiency and low-cost solution [5-9]. The
high absorption coefficient with adjustable band gap [10], long carrier diffusion length [11], and ambipolar
charge transport [12], make perovskite materials appropriate for light harvesting [13-14]. The best power
conversion efficiency (PCE) of PSCs has achieved 25.2% recently [15].
The durability is an important factor for the application of PSCs. So, many researchers are devoted to the
optimizing perovskite composition [16], tolerance factor [17-20], defect passivation [21] and device structures
[
22]. The hole-transporting material (HTM) of PSCs plays an important role in improving efficiency for the
hole extraction. It serves as an effective electron blocking layer to retard charge recombination at the same time
23-25]. A superior HTM should be cost-effective, high-mobility and long-term stability (such as against light,
electrical bias, thermal and moisture stress) [26-29]. Unfortunately,
2,2′,7,7′-tetrakis-(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (Spiro-OMeTAD)), the most popular
[
(
HTM of PSCs, suffers from the deficiencies of unsuitable for commercialization, because of high synthetic cost
as well as photothermal and chemical instability [30-33]. Therefore, the replacement of Spiro-OMeTAD with
more appropriate alternatives is very urgent.
Phthalocyanines, as attractive organic semiconductor materials, with prominent stability and high mobility,
have been used as PSC HTMs [34-37]. But for the planarity of molecular configuration, phthalocyanine shows
poor solubility in most organic solvents, unsubstituted phthalocyanine HTMs have to be prepared by vacuum
deposition method [38-39]. The introduction of bulky peripheral groups to the phthalocyanine macrocycle,
could enhance the solubility, reduce the aggregation, and adjust the HOMO and LUMO energies [40-42].
Subsequently, solution-processed spin coating phthalocyanine films have became possible in PSCs, meanwhile,
the performance of PSCs is expected to be improved [43-44]. For phthalocyanine materials, different kinds of
peripheral groups can lead to different π−π interaction and enhance their hole mobilities [45-46]. It is reported
that [47], an alkoxy group of phthalocyanine can strengthen the electron donating ability, and thus ensure the
HOMO level to be located above the valence band of perovskite. We have reported the cobalt (II)
phthalocyanine with phenoxy and nitro groups as an efficient HTM in PSCs recently [48]. While, the influence
of different peripheral groups on the cobalt (II) phthalocyanine based PSCs have not been studied. The
optimization of phthalocyanine properties by substituents such as the mobility and energy levels are needed to
be specified.
In this regard, the thermal, electrical and photophysical properties of cobalt (II) phthalocyanine were
optimized by introducing peripheral groups of nitro with (4-butyl formate) phenoxy, (4-propenyl-2-methoxy)
phenoxy or (4-methyl formate) phenoxy. The dopant-free PSCs with the newly synthesized phthalocyanines as

HTMs and Spiro-OMeTAD controlled device were made under the same conditions. An inspiring efficiency of
3.91% was achieved by the device with CoPcNO -OBFPh HTM. The long-term stability test of
1
2
CoPcNO -OBFPh based device retained 75% of the initial efficiency for 1008 h in air of 50% relative humidity,
2
much higher than the other two phthalocyanines. However, only 20% of the initial efficiency with
Spiro-OMeTAD device was preserved at the same conditions. Such a noticeable improvement of the device's
stability is convinced to be correlated to the extraordinary stability and moisture-proof performance of
phthalocyanines. Therefore, this work demonstrated that more bulky substituent of phthalocyanines is preferred
as the peripheral group for the design of efficient phthalocyanine HTMs.
2
. Materials and methods
.1. Materials
2
2 2
Materials of Co(OAc) ·4H O, methyl 4-hydroxybenzoate, isoeugenol, butyl 4-hydroxybenzoate,
4
-Nitrophthalonitrile and 1,5-diazabicyclo[4.3.0] non-5-ene (DBU) were provided by Aladdin; Spiro-OMeTAD
was provided by Advanced Election Technology Co., Ltd. The substance for the preparation of solar cells,
PbBr and PbI were purchased from TCI Corporation. 30NRT TiO paste, FAI and MABr were obtained from
2
2
2
Great Cell Solar. Patterned fluorine-doped tin oxide coated glass substrates (FTO) were received from
Advanced Election Technology Co., Ltd.
2
.2. Measurements
The structures of newly synthesized cobalt (II) phthalocyanines were characterized by IR, H NMR spectra
1
and MS. They were recorded on instruments of BIO-RADFIS3000 spectrophotometer, Varian 300 MHz
spectrometer and matrix assisted laser desorption Ionization Bruker Autoflex TOF (III) (MALDI-TOF,
2
,5-dihydroxybenzoicacid was used as matrix ) respectively. Nicoet Evolution 300 spectrophotometer and
Fluorolog-Horiba fluoromet were used for the measurement of optical properties in dimethylformamide (DMF)
solvent. Q500 thermo gravimetric analyse (TGA) and Q20 differential scanning calorimeter (DSC) were used
for the record of thermo properties. In the immersion-lens mode, scanning electron micrograph (SEM,
XL30S-FEG) images were performed with 3 kV electron beam voltage and 3 nA current. The cells with an
2
-2
active area of 0.12 cm were illuminated under 100 mW·cm (AM 1.5) by an Oriel solar simulator 91192, and
the current density-voltage (J-V) curve was recorded on Keithley 2400 Source meter.
2
2
.3. Preparation
.3.1. Preparation of 4-aryloxy phthalonitrile
The intermediates of 4-(4-methyl formate) phenoxy phthalonitrile and (4-(4-butyl formate) phenoxy
phthalonitrile were prepared and characterized as reported before [49]. 2-Methoxy-4-(2-propenyl) phenoxy
phthalonitrile was obtained as follows. 4-propenyl-2-methoxy phenol (isoeugenol, 8.26 mmol, 1.354 g) and
redistilled DMF (40 mL) were put in a two-neck round bottom flask. Under vigorously stirring,

4
-nitrophthalonitrile (5.9 mmol, 1.021 g) was added and dissolved. Anhydrous potassium carbonate fine
powder (8.26 mmol, 1.140 g) was added portion wise. The reaction was stirred at room temperature till
-nitrophthalonitrile disappeared completely. The crude product was obtained by disposing the mixture into ice
water and dealed with 1% sodium hydroxide for 12 h. The yellowish white product was obtained by
4
−
1
recrystallization in methanol. Yield: 1.46 g, 85.6%, m.p. 74–76 °C. IR νmax/cm (KBr pellet): 3069 (CHar),
2
228 (C N), 1592, 1510, 1481, 1410 (C=C), 1246 (Ph-O-Ar) (Fig. S1).
.3.2. Preparation of 2,9,16-tri(4-(4-butyl formate) phenoxy)-23-nitrophthalocyaninatocobalt
-OBFPh)
In a four-necked round bottom flask, A mixture of 5.0 mmol 4-(4-butyl formate) phenoxy phthalonitrile
1.600 g), 1.7 mmol 4-nitrophthalonitrile 0.294 g) and 1.7 mmol Co(OAc) ·4H O (0.423 g) with 40 mL
2
(CoPcNO
2
(
2
2
n-octanol as solvent was put in a 100 mL four-necked round bottom flask. With stirring, catalyst (DBU 10 D)
was added to the mixture under nitrogen protection. The reaction performed at the refluxing temperature of
octanol for about 30 h till all of the phthalonitriles vanished by TLC detection. Absolute ethanol was introduced
to the reaction mixture to precipitate the navy blue product. The crude product was washed by distilled water,
2 2
ethanol and ethyl acetate respectively. Ethanol and CH Cl (volume ratio: 1:30) were used as eluent for further
−
1
purification of the crude product by column chromatography method. Yield: 0.646 g, 32.5%; IR νmax/cm (KBr
pellet): 2923 (CHar), 1715 (C=O), 1605-1334 (C=C, Ph), 1269 (Ph-O-Pc), 1000-700 (C=C Pc) (Fig. S2);
1
3 3 2 2 2
HNMR (CDCl ) δppm: 0.87 (m, 9H, CH ), 1.57 (m, 6H, CH ), 1.88 (m, 6H, CH ), 3.69 (m, 6H, CH ), 6.18 (m,
6
7
2
H, Ph-H), 6.31 (m, 3H, Pc-H), 6.88 (m, 1H, Pc-H), 7.14 (d, 3H, Pc-H), 7.21 (d, 6H, Ph-H), 7.52 (d, 3H, Pc-H),
.72 (d, 1H, Pc-H), 8.04 (d, 1H, Pc-H); MS (MALDI-TOF) m/z: Calc. 1192, Found: 1192.
.3.3. Preparation of 2,9,16-tri(4-(4-propenyl-2-methoxy) phenoxy)-23-nitrophthalocyaninatocobalt
(
CoPcNO -OPMPh)
2
The black blue CoPcNO -OPMPh was synthesized and purified the same as CoPcNO -OBFPh by replacing
2
2
4
-(4-methyl formate) phenoxy phthalonitrile with 5.0 mmol 2-methoxy-4-(2-propenyl) phenoxy phthalonitrile.
−
1
Yield: 0.544 g, 29.6%; IR νmax/cm (KBr pellet): 2922 (CHar), 1598-1409 (C=C, Ph), 1238 (Ph-O-Pc),
1
1
000-700 (C=C Pc) (Fig. S3); H NMR (CDCl ) δppm: 1.62 (m, 9H, CH ), 3.63 (m, 9H, OCH ), 4.92 (m, 6H,
3
3
3
=
CH ), 6.22 (m, 6H, Ph-H), 6.40 (m, 3H, Pc-H), 6.96 (m, 1H, Pc-H), 7.21 (d, 3H, Pc-H), 7.31 (d, 6H, Ph-H),
2
7
.36 (d, 3H, Pc-H), 7.88 (d, 1H, Pc-H), 7.91(d, 1H, Pc-H); MS (MALDI-TOF) m/z: Calc. 1102, Found: 1102.
.3.4. Preparation of 2,9,16-tri(4-(4-methyl formate) phenoxy)-23-nitrophthalocyaninatocobalt
2
(
CoPcNO -OMFPh)
2
The synthetic and purified method of black blue CoPcNO -OMFPh was the same as CoPcNO -OBFPh only
2
2
by replacing 4-(4-butyl formate) phenoxy phthalonitrile with 4-(4-methyl formate) phenoxy phthalonitrile
−
1
(
1.390 g). Yield: 0.476 g, 26.8%; IR νmax/cm (KBr pellet): 2926 (CHar), 1716 (C=O), 1597-1335 (C=C, Ph),
1
1
272 (Ph-O-Pc), 1000-700 (C=C Pc) (Fig. S4); H NMR (CDCl ) δppm: 3.65 (m, 9H, CH ), 6.12 (m, 6H,
3
3
Ph-H), 6.25 (m, 3H, Pc-H), 6.91 (m, 1H, Pc-H), 7.11 (d, 3H, Pc-H), 7.31 (d, 6H, Ph-H), 7.60 (d, 3H, Pc-H),
.82 (d, 1H, Pc-H), 8.10 (d, 1H, Pc-H); MS (MALDI-TOF) m/z: Calc. 1066, Found: 1066.
7

2
.4. Perovskite Solar cell fabrications
PSCs were fabricated with the structure of FTO/TiO
2
(compact) /TiO
2
(mesoporous)/Perovskite/HTM/Au
-OBFPh, CoPcNO -OPMPh or
-OMFPh and Spiro-OMeTAD as HTM (40 nm) was spin-coated on perovskite films. Under the same
condition, Spiro-OMeTAD HTM based PSC was also fabricated for comparison.
according to the reported methods [50]. A thin layer of CoPcNO
2
2
CoPcNO
2
3
. Results and discussion
3
.1. Synthesis and characterization
Scheme 1. Synthesis route of novel 2,9,16-triaryloxy-23-nitrophthalocyaninatocobalts.
Scheme 1 shows the synthesis route of 2,9,16-triaryloxy-23-nitrophthalocyaninatocobalts. In the solvent of
redistilled DMF, 4-aryloxy phthalonitrile intermediates were obtained by the reaction of 4-nitrophthalonitrile
and aromatic phenols at room temperature. The cobalt (II) phthalocyanines with different peripheral groups
were prepared by the cyclopolymerization of 4-aryloxy phthalonitrile, 4-nitrophthalonitrile and
Co(OAc) ·4H O salt with the mole ratio of 3:1:1 under nitrogen atmosphere.
2
2
Fig. 1 The structure of cobalt phthalocyanine hole transport materials
The structure of 4-Aryloxy phthalonitrile intermediates and novel cobalt (II) phthalocyanine products (Fig.
1
1
) were confirmed by IR, H NMR and MS respectively. Generally speaking, phthalocyanines without
peripheral substituents often show poor solubility and aggregation in common solvents [26]. While, the aryloxy
substituents make the macromolecule’s solubility increased apparently. CoPcNO -OBFPh, for the largest
2

flexible butyl format, shows the best solubility in dichloromethane, tetrahydrofuran and dimethylformamide
solvents. The methyl format has the lowest steric hindrance, CoPcNO -OMFPh shows the worst solubility of
2
the three phthalocyanines. The isomers of aryloxy substituents on the different position of phthalocyanine rings
will enhance their solubility by disrupting the crystalline order. Therefore, all of the synthesized cobalt (II)
phthalocyanines were used for the following studies without further purification.
3
.2. Physicochemical properties
Fig. 2 (a) UV-visible absorption spectra in DMF solution; (b) Photoluminescence spectra in DMF solution,
excitation wavelengths: 627 nm for CoPcNO -OBFPh, 629 nm for CoPcNO -OPMPh and 620 nm for
2
2
CoPcNO -OMFPh; (c) Photoluminescence spectra of different phthalocyanines on perovskite film, excitation
2
wavelengths: 500 nm; (d) TRPL decay curves of perovskite film and perovskite films covered by
phthalocyanine HTMs; (e) Hole mobility spectra; (f) Energy level diagram of perovskite solar cells with
phthalocyanine HTMs
Table 1
The Q band UV-visible absorption data, photoluminescence quenching efficiency and hole mobility
a
λabsQ
( nm)
674
εQ
φ
µ
3
-1
-1
2
-1 -1
HTMs
( dm ·mol ·cm )
(%)
(cm ·V ·s )
4
-5
CoPcNO
CoPcNO
2
-OBFPh
2
-OPMPh
2
-OMFPh
3.88×10
91.50
59.83
56.36
9.92*10
8.21*10
5.77*10
4
-5
-5
670
6.63×10
4
CoPcNO
675
7.82×10
a
φ: Photoluminescence quenching efficiency, φ=[(Ιο − Ι )/Ιο]∗ 100%, Ιο: Initial fluorescence intensity, I:
Fluorescence intensity after quenching[51].

As shown in Fig. 2a, the appearance of intense B band at around 300 nm and Q-band at around 670 nm
together with shoulder at around 620 nm indicates the formation of novel
,9,16-triaryloxy-23-nitrophthalocyaninatocobalts. In DMF solution, CoPcNO -OBFPh, CoPcNO -OPMPh and
a
2
2
2
CoPcNO -OMFPh show excellent light absorption abilities in the NIR–infrared region. The Q band molar
2
4
3
-1
-1
extinction coefficient (
typical photoluminescence spectra with maximum emission peaks at 689, 684, 683 nm respectively (Fig. 2b,
Table 1). CoPcNO -OBFPh displays the most intense PL intensity and the largest maximum emission
wavelength for the largest (4-alkyl format) phenoxy substituents [52], while CoPcNO -OMFPh shows the
ε) is higher than 10 dm ·mol ·cm (Table 1). All of the three phthalocyanines exhibit
2
2
weakest PL intensity and the least maximum emission wavelength for the smallest (4-methyl format) phenoxy
substitutes.
Steady-state PL was employed for perovskite film and perovskite film coated by phthalocyanine HTMs.
The PL intensity of perovskite layer with phthalocyanines HTMs (Fig. 2c) is quenched significantly than that
of pristine perovskite films, indicating that phthalocyanines HTMs are effective in hole extraction.
CoPcNO -OBFPh shows the greatest fluorescence quenching efficiency (φ) of 91.50%. The excellent π-π
2
stacking of CoPcNO -OBFPh with bulky peripheral groups leads to the intimate contact of phthalocyanine with
2
perovskite film, which makes the film have good interfacial quality and thus leads to the notable fluorescence
quenching. The PSC based on CoPcNO -OBFPh as HTM will obtain excellent Jsc and FF. For the
φ
of
-OMFPh (56.36%), both of them maybe have
the similar hole capture rate and thus lead to the similar PSC parameters. The time-resolved photoluminescence
TRPL) decay curve (Fig. 2d) shows a biexponential decay. The CoPcNO -OBFPh/perovskite,
CoPcNO -OPMPh/perovskite and CoPcNO -OMFPh/perovskite film respectively show short-lived and
long-lived lifetimes (τ = 24 ns, τ = 61.1 ns), (τ = 58.6 ns, τ = 143 ns) and (τ = 61.1 ns, τ = 150 ns)
2
2 2
CoPcNO -OPMPh (59.83%) is approximate to that of CoPcNO
(
2
2
2
1
2
1
2
1
2
respectively. By contrast, the pristine perovskite film gave τ = 91.4 ns and τ = 234.6 ns. The TRPL decay
1
2
times for the device mean that all of the three phthalocyaninies can extract the holes from the perovskite
effectively. The least lifetime of CoPcNO -OBFPh/perovskite confirms the fastest hole transport in the
2
CoPcNO -OBFPh/perovskite film. While, the similar lifetimes of CoPcNO -OPMPh/perovskite and
2
2
CoPcNO -OMFPh/perovskite indicate the similar hole transport ability of them. These are in great agreement
2
with the photoluminescence quenching tests.
3
.3. Hole mobility and thermal properties
In order to learn more about the nature of phthalocyanine HTMs, space-charge limited-current (SCLC)
model [53] was used for the measurement of hole mobility (µ). As shown in Fig. 2e, hole mobility was
calculated from the J-V curves by fitting to the Mott-Gurney law [54]. For the largest steric hindrance of bulky
butyl formate group, CoPcNO -OBFPh shows the strongest electron donating effect, least molecular
2
-
5
2
-1 -1
aggregation and therefore the highest
µ of 9.92*10 cm ·V ·s . While, Phthalocyanines with

4
-(4-propenyl-2-methoxy) phenoxy groups (CoPcNO -OPMPh) and (4-methyl formate) phenoxy groups
2
-5
2
-1 -1
-5
2
-1 -1
(
CoPcNO -OMFPh) show relative lower hole motilities of 8.21*10 cm ·V ·s and 5.77*10 cm ·V ·s
2
respectively. This is due to steric effects, larger peripheral groups decrease the conformational disorder and thus
increase the hole mobility [55]. The high mobility could provide better charge transport capability and might be
2
responsible for the increased FF and Jsc in CoPcNO -OBFPh based-PSCs [56-57].
The stability and durability are important factors that influence the commercialization of PSCs. The
stability of HTM determines the durability of devices directly. Thermo gravimetric analysis (TGA, Fig. S5)
shows that CoPcNO
2 2 2
-OBFPh, CoPcNO -OPMPh and CoPcNO -OMFPh exhibit 5% weight loss (T5%) at 309
o
o
o
o
o
o
C, 390 C and 394 C respectively. Decomposition temperatures (T
d
) of them are 386 C, 405 C, 400 C
respectively, suggesting that all of the three Pcs show more excellent stability than Spiro-OMeTAD [58]. There
o
o
are 15 C and 6 C temperature difference ( Δ ) between T and Td for CoPcNO -OMFPh and
t
5%
2
o
CoPcNO -OPMPh. While, ∆ of CoPcNO -OBFPh is 77 C, indicating that the bigger substituents on benzene
2
t
2
ring will decomposed faster than the small ones in the high temperature zone.
o
In DSC spectra (Fig. S6), all of the three cobalt phthalocyanines show conspicuous peaks around 100 C,
which can be attribute to the gasification of water. Meanwhile, CoPcNO -OBFPh shows a significant
2
o
endothermic peak around 140 C, indicating that it can be kept amorphous in the process of device fabrication
and measurement [59]. The subtle interplay between the peripheral groups with phthalocyanine rings leads to
the difference of stability. The particular thermal stability shows that all of the synthesized phthalocyanines are
fit for the application of HTMs.
Table 2
Thermal and energy level data of phthalocyanines
a
o
b
c
d
HTMs
T
d
( C)
HOMO (eV)
LUMO (eV)
g
E (eV)
CoPcNO -OBFPh
386
405
400
-4.95
-4.94
-4.96
-3.27
-3.25
-3.24
1.68
1.69
1.72
2
CoPcNO -OPMPh
2
CoPcNO -OMFPh
2
a
Decomposition temperatures.
b
Measured by photoelectron and CV spectra.
c
|LUMO| = |HOMO|-|Eg|.
d
Energy gaps calculated from the absorption thresholds (λonset).
Furthermore, the well-matched energy level between HTM and perovskite film could enhance the overall
performance of PSC. Therefore, the energy level of HTMs was obtained by measurement of electrochemical
properties with cyclic voltammetry (CV) method. The HOMO levels of CoPcNO -OBFPh, CoPcNO -OPMPh
2
2
and CoPcNO -OMFPh are −4.95, −4.94 and −4.96 eV (Fig. S7 and Table 2), and the corresponding LUMO
2

levels are −3.27, −3.25 and −3.24 eV respectively (Fig. 2f ). All of the phthalocyanine HOMO levels are
somewhat higher than that of perovskite layer [60]. Thus, hole extraction of all the novel phthalocyanine HTMs
is convinced to proceed smoothly from energy level alignment analysis.
3
.4. Photovoltaic performances
Fig. 3 Schematic structure of PSCs.
Fig. 4 (a) Cross-sectional SEM image; (b-e) SEM top-image of PSCs fabricated with different HTMs; (f) SEM
top-image of perovskite
As
reported
before,
PSC
with
the
configuration
of
FTO/TiO
2
(compact)/TiO (mesoporous)/perovskite/HTM/Au was applied for the studying of the different
2
bulkiness of phthalocyanine-HTMs on the performance of devices (Fig. 3) [61]. The cross-sectional scanning
electron microscopy (SEM) image of PSC (Fig. 4a) shows a well-defined layer-by-layer structure. The

thickness of TiO compact layer, TiO mesoporous layer, perovskite capping layer, HTM, and gold electrode is
2
2
determined as 30 nm, 200 nm, 400 nm, 40 nm, 80 nm, respectively (Fig. 4a, Fig. S8 and Table S1). It is
reported that [62-63], PSC with poor coverage and rougher surfaces would increase the possibility of interfacial
charge recombination, and lead to the decrease of hole extraction, thus result in the reducing of
device-performances. Therefore, the photovoltaic performances could be improved by more efficient charge
transport efficiency induced by fine PSC morphology with smoother surfaces [64]. SEM top-images of PSCs
fabricated with different phthalocyanine and Spiro-OMeTAD HTMs are shown in Fig. 4b-f. CoPcNO
exhibits a more uniform capping layer and has fewer pinholes on perovskite films than that of
CoPcNO -OPMPh and CoPcNO -OMFPh, because bulky substituents are beneficial to molecular
self-assembly and thus increase the π-π stacking of phthalocyanine molecules. Some apparent pinholes on the
2
-OBFPh
2
2
2
surface of doped CoPcNO -OMFPh may be the reason for the worse photovoltaic performances of
corresponding devices.
Fig. 5 J-V hysteresis curves of PSCs with different HTMs (The scan direction is from backward to forward)
-2
Under simulated AM 1.5G solar irradiation at 100 mW·cm (Fig. 5), the photovoltaic performances of the
devices with phthalocyanine and Spiro-OMeTAD as HTMs were characterized by current voltage (J-V)
measurements. The results are summarized in Table 3. For the prominent hole mobility and uniform PSC
morphology, the optimized device with CoPcNO
2
-OBFPh as HTM shows a best short-circuit current density
SC) of 20.88 mA·cm , an open-circuit voltage (VOC) of 1.03 V, a fill factor (FF) of 0.65, and most of all, a
champion power conversion efficiency (PCE) of 13.91%. Compared to CoPcNO -OBFPh, the
-2
(
J
2

CoPcNO -OPMPh-based PSC exhibits a reduced PCE of 11.81% with decreased photovoltaic-parameters (Voc
2
-2
of 0.97 V, Jsc of 20.13 mA·cm , and FF of 0.60). CoPcNO -OMFPh based PSC shows the worst
2
-2
photovoltaic-parameters (Jsc of 19.57 mA·cm , V of 0.92 V, FF of 0.53, and PCE of 9.47%). The
OC
representative J-V characteristics and corresponding power conversion efficiency of Spiro-OMeTAD based
were also recorded as comparison. As shown in Table 3, Spiro-OMeTAD shows a PCE of 19.60% with Voc of
-2
1
.12 V, Jsc of 22.69 mA·cm , and FF of 0.77. The superior photovoltaic-parameters of CoPcNO
2
-OBFPh
could be attributed to the higher hole extraction efficiency, better device morphology, higher hole mobility than
2 2
that of CoPcNO -OPMPh and CoPcNO -OMFPh.
Table 3
Photovoltaic parameters of different HTM-Based devices
J
SC
V
OC
PCE
(%)
Hysteresis Index
HTMs
FF
-2
(
mA·cm )
20.77
20.88
20.06
20.13
18.9
(V)
1.03
1.03
0.95
0.97
0.9
(%)
CoPcNO -OBFPh (forward)
0.62
0.65
0.57
0.60
0.36
0.53
0.72
0.77
13.29
13.91
10.79
11.81
6.15
4.45
2
CoPcNO -OBFPh (backward)
2
CoPcNO -OPMPh (forward)
8.64
35.06
9.9
2
CoPcNO -OPMPh (backward)
2
CoPcNO -OMFPh (forward)
2
CoPcNO -OMFPh (backward)
19.57
22.73
22.69
0.92
1.08
1.12
9.47
2
Spiro-OMeTAD (forward)
Spiro-OMeTAD (backward)
17.65
19.6
It is reported that [65], the J-V hysteresis is strongly associated with the nature of the interface between
perovskite and HTM layer. J-V hysteresis of devices with phthalocyanines and Spiro-OMeTAD as HTMs was
tested both on forward and backward directions (Fig. 5). CoPcNO -OBFPh based on PSC shows the minimal
2
Hysteresis index (calculated from equation 1, Table 3) of 4.45%, while CoPcNO -OMFPh shows the maximum
2
of 35.06%. Hysteresis and hole mobility are opposite, because that higher hole mobility would be beneficial to
hole extraction by reducing the charge accumulation at the interface of the solar absorber and HTM film, and
thus lead to less hysteresis [66].
1
The incident photon-to-electron conversion efficiencies (IPCEs) of phthalocyanines and Spiro-OMeTAD
based perovskite solar cells were also measured to obtain further effects of HTMs on device performance (Fig.
6
a). The IPCE spectrum displays a wide spectra response in the range of 300 to 800 nm, yielding a broad IPCE
plateau of around 80%. The IPCE values of CoPcNO -OBFPh, CoPcNO -OPMPh, CoPcNO -OMFPh and
2
2
2
-2
Spiro-OMeTAD are 20.72, 19.23, 18.06 and 22.01 mA·cm respectively. They are in great agreement with Jsc

values of each corresponding PSC.
-2
Fig. 6 (a) IPCE spectra; (b) The stability of PSC under simulated AM 1.5G solar irradiation at 100 mW·cm
without encapsulation; (c) The stability of PSC in air with 50% relative humidity without encapsulation.
3
.5 The stability of PSCs
Fig. 7 Contact angles between perovskite/HTMs and water
As shown in Fig. 6b and 6c, long-term stability test of device based on these phthalocyanine HTMs was
evaluated. Spiro-OMeTAD HTM based PSC was conducted as the control device for comparison. The stability
-2
under simulated AM 1.5G solar irradiation at 100 mW·cm was tested every 48 h for 240 h at room
temperature without encapsulation. The efficiencies of the devices with CoPcNO -OBFPh, CoPcNO -OPMPh
and CoPcNO -OMFPh retain 85%, 70% and 64% of the initial value. While, for the device with
2
2
2
Spiro-OMeTAD HTM, only 47% of the initial efficiency is retained at the same conditions. However, the
durability tests in air with 50% humidity at room temperature (dark state, every 72 h for 1008 h ) without
encapsulation shows that the efficiencies of the four corresponding devices retain 75%, 59%, 40% and 20%
respectively. All of the phthalocyanine based devices represent more superior stability than Spiro-OMeTAD
ones. Besides more chemical and thermal stability of phthalocyanines, the largely improved efficiency stability
of phthalocyanines based PSCs are also contributed by the hydrophobic nature of phthalocyanine HTMs. As
shown in Fig. 3, hydrophobic phthalocyanines films covered well on the surface of the devices that prevented
o
the moisture-labile perovskite layer from being attacked by water. For the maximal contact angles of 91.87
(
Fig. 7), CoPcNO
2
-OBFPh shows the best hydrophobicity. The better hydrophobicity of phthalocyanines than
Spiro-OMeTAD and indicates that phthalocyanines HTMs outperform Spiro-OMeTAD as the protective layer

of perovskite perovskite films.
4
. Conclusions
In summary, three novel asymmetric cobalt (II) phthalocyanines (CoPcNO -OBFPh, CoPcNO -OPMPh and
2
2
CoPcNO -OMFPh) with different bulky peripheral groups were synthesized. All of them show good solubility
2
and high thermal stability. The suitable hole mobility, significant photoluminescence quenching on perovskite
film, appropriate HOMO and LUMO energy levels of the three materials indicate good PSC prospect. The
dopant-free PSCs based on these phthalocyanine HTMs show satisfactory solar cell performance. The most
effective device based on CoPcNO -OBFPh with bulky substituents demonstrates an impressive
2
-2
photovoltaic-parameters with PCE of 13.91%, VOC of 1.03 V, JSC of 20.88 mA·cm and FF of 0.65 under AM
.5G standard conditions. While CoPcNO -OPMPh and CoPcNO -OMFPh device exhibits relative lower PCE
1
2
2
of 11.81% and 9.47% respectively. For the least pinholes and the largest contact angle,
CoPcNO -OBFPh-based PSC shows the best efficiency stability through durability tests. These results ascribe
2
to the fact that the more bulky substitutes are beneficial to molecular self-assembly and thus increase the π-π
stacking of HTM molecules. As a result, suitable bulky substitutes are preferred for the developing
phthalocyanine-based HTMs for efficient and stable PSCs.
Acknowledgments
This Research is financial support from Natural Science Foundation of Tianjin (18JCYBJC89900,
1
8JCQNJC76400) and National Natural Science Foundation of China (31871811, 51802222).
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High lights
ò Three Novel asymmetric cobalt phthalocyanines (CoPcNO -OBFPh,
2
CoPcNO -OPMPh and CoPcNO -OMFPh) were synthesized
2
2
ò They were applied in perovskite solar cells as hole transporting
materials
ò The PCE of the dopant-free device based on CoPcNO -OBFPh HTM
2
is up to 13.91%
ò The solar cell based on CoPcNO -OBFPh shows excellent stability in
2
air at room temperature
ò Phthalocyanine HTM with bulky substituent is more beneficial for the
improvement of PSC performance

Conflict of interest
The authors declare that we do not have any
commercial or associative interest that represents a
conflict of interest in connection with the work
submitted.
Junjie Guo

Author statement
Junjie Guo designed the experiments, carried out the experimental study on device
fabrication, and performed stability tests. Mengmeng Sun and Xianfang Meng
synthesized and the purified the phthalocyanines. Hongwei Zhu, performed the MS
and Hole mobility tests. Chao Ma, Shiyan Hu and Jiaqi Shen performed the Contact
angles, SEM, PL, and CV test. Qian Wang performed the thermal perproties test and
analyzed the data. Jinghan Gao performed the IR test. Junjie Guo wrote and reviewed
the manuscript. All authors made a substantial contribution to the discussion of the
content and reviewed and edited the manuscript before submission.