New ferrocenyl-containing organic hole-transporting materials for perovskite solar cells in regular (n-i-p) and inverted (p-i-n) architectures

Article abstract of DOI:10.1039/C8RA08946A

Three triphenylamine derivatives containing ferrocenyl groups (JW6, JW7 and JW8) were synthesized by facile syntheses. Their HOMO levels match the valence band energy of CH₃NH₃PbI₃. The introduction of ferrocenyl was aimed to obtain hole transporting materials with high mobility for perovskite solar cells. JW7 shows higher hole mobility (4.2 × 10^?4 cm² V^?1 s^?1) than JW6 (1.3 × 10^?4 cm² V^?1 s^?1) and JW8 (1.5 × 10^?4 cm² V^?1 s^?1). Their film-forming properties are affected by their molecule structures. The methoxyl and N,N-dimethyl terminal substituents of JW7 and JW8 are beneficial for having better solubility than JW6. The regular mesoporous TiO₂-based perovskite solar cells (n-i-p) and the inverted planar heterojunction perovskite solar cells (p-i-n) fabricated using JW7 show the highest power conversion efficiency of 9.36% and 11.43% under 100 mW cm^?2 AM1.5G solar illumination. For p-i-n cells, the standard HTM PEDOT-based cell reaches an efficiency of 12.86% under the same conditions.

Full text of DOI:10.1039/C8RA08946A

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PAPER
New ferrocenyl-containing organic hole-
transporting materials for perovskite solar cells in
Cite this: RSC Adv., 2019, 9, 216
regular (n-i-p) and inverted (p-i-n) architectures†
Jingwen Jia, Liangsheng Duan, Yu Chen, * Xueping Zong, Zhe Sun,
Quanping Wu and Song Xue*
Three triphenylamine derivatives containing ferrocenyl groups (JW6, JW7 and JW8) were synthesized by
facile syntheses. Their HOMO levels match the valence band energy of CH
3 3 3
NH PbI . The introduction of
ferrocenyl was aimed to obtain hole transporting materials with high mobility for perovskite solar cells.
ꢁ
4
2
ꢁ1 ꢁ1
ꢁ4
2
ꢁ1 ꢁ1
V s
JW7 shows higher hole mobility (4.2 ꢀ 10 cm
V
s
) than JW6 (1.3 ꢀ 10 cm
) and JW8
ꢁ
4
2
ꢁ1 ꢁ1
(
1.5 ꢀ 10 cm
V s ). Their film-forming properties are affected by their molecule structures. The
methoxyl and N,N-dimethyl terminal substituents of JW7 and JW8 are beneficial for having better
solubility than JW6. The regular mesoporous TiO -based perovskite solar cells (n-i-p) and the inverted
Received 29th October 2018
Accepted 8th December 2018
2
planar heterojunction perovskite solar cells (p-i-n) fabricated using JW7 show the highest power
DOI: 10.1039/c8ra08946a
ꢁ2
conversion efficiency of 9.36% and 11.43% under 100 mW cm AM1.5G solar illumination. For p-i-n
rsc.li/rsc-advances
cells, the standard HTM PEDOT-based cell reaches an efficiency of 12.86% under the same conditions.
possessing tunable energy levels, suitable solubility in organic
solvents, and good lm-forming properties.
Ferrocene is an organometallic compound with a sandwich
1
Introduction
23–26
Perovskite solar cells have attracted much attention due to the
1–7
high photovoltaic performance of the organolead halide. The structure. Ferrocene and its derivatives are widely used in elec-
organolead halide perovskite was used as the light absorber in trochemical sensors, biomedicine, chemical catalysts and photo-
27–30
liquid-based dye-sensitized solar cells (DSSCs), and showed voltaic materials due to their good redox abilities.
Ferrocene
a power conversion efficiency (PCE) of 3.8% in 2009. The initial can also act as a suitable electron-donating unit and can be easily
8
,9
PCE of the solid-state perovskite solar cells was 9.7% in 2012.
functionalized, due to which it has been reacted with small
Recently, PCE of perovskite solar cells has been over 22%. The molecules for conjugation. Ferrocene-containing triphenylamine
commonly reported CH NH PbI
was used in the rst solid- derivatives have been reported because of their good light-
state PSC, and a solid hole-transport layer was made of harvesting ability and fast charge regeneration.
,20,7,70-tetrakis-(N,N-di-p-methoxyphenylamine)-9,90-
substituted triphenylamines are used as sensitizers in DCCCs,
spirobiuorene (spiro-OMeTAD), which shows good charge and an inserted p-linker between ferrocene and triphenylamine is
10
3
3
3
3
1,32
Ferrocenyl-
2
33–35
extraction and improves the long-term stability of the benecial for charge transport in the molecule.
Structural
It is necessary to place the perovskite between an n- changes have a signicant inuence on electron delocalization
type semiconductor layer and a p-type semiconductor layer, though the entire molecule. Ferrocenyl-substituted triphenyl-
9,11–15
device.
16
according to the structure of traditional PSCs, for charge amines have also been used as p-type redox promoters in electro-
separation and transport in a PSC device. Organic small- chromic display devices, and they can form a lm layer by spin-
36
molecule HTMs such as spiro-OMeTAD are commonly used as coating. Therefore, ferrocene is a promising candidate for
p-type semiconductor materials for PSCs. Compared with photovoltaic materials. In our research, new ferrocenyl-substituted
inorganic HTMs and organic polymer HTMs, organic small- triphenylamine derivatives were synthesized, and they were used
molecule HTMs have the advantages of facile synthesis, ex- as organic small-molecule HTMs to construct PSCs. To the best of
17–22
ible modication of structure, and easy purication.
Organic small-molecule HTMs can reach high hole mobility by containing ferrocene have been reported in PSCs.
In this study, three new ferrocene-containing compounds (JW6,
our knowledge, this is the rst time that organic small molecules
JW7 and JW8) were prepared, and their structures are shown in
Scheme 1. In JW6, JW7 and JW8 molecules, biferrocene groups
were connected with triphenylamines via triple bonds, and
a double bond was also introduced to link a moiety of triphenyl-
Tianjin Key Laboratory of Organic Solar Cells and Photochemical Conversion, School
of Chemistry & Chemical Engineering, Tianjin University of Technology, Tianjin
3
†
1
00384, PR China. E-mail: cytjut@163.com; xuesong@ustc.edu.cn
Electronic supplementary information (ESI) available. See DOI:
0.1039/c8ra08946a
0
amine, bis(4-methoxyphenyl)methane and 4,4 -methylenebis(N,N-
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dimethylaniline), which can adjust the solubility, energy levels and 2.2 Fabrication of PSCs
charge transport. Theoretical calculations were performed to
In this study, two architectures of PSCs were fabricated. The
conventional n-i-p type PSCs (FTO/TiO /perovskite/HTM/Ag)
observe the optimized geometric structures and frontier molecular
orbitals of the three compounds. Spectral and electrochemical
measurements were applied for the determination of light
absorption and energy levels. Differential scanning calorimetry
2
37
were fabricated according to the published method. The
main study was focussed on the inverted p-i-n type of PSCs (ITO/
HTM/perovskite/PCBM/BCP/Ag). In brief, ITO glasses were
successively washed using a special cleaner, acetone, water and
(DSC) was performed for analysing the thermal stability of JW6,
JW7 and JW8. PSCs based on JW6, JW7 and JW8 were fabricated,
and we found that the concentration of HTM solution signicantly
inuenced the device performance. In the inverted planar heter-
ojunction perovskite solar cells (p-i-n), the concentration of HTMs
ꢁ1
ethanol, and then covered with HTM layer (5 mg mL ) by
spinning at 4000 rpm for 40 s. This rst layer was sintered at
ꢂ
1
00 C for 10 min. By the sequential deposition method,
a perovskite layer was covered on the HTM layer. Aer the
perovskite precursor solution (461 mg PbI , 159 mg MAI, 70.9
mL DMSO and 634.92 mL DMF) was spin-coated at 5000 rpm for
ꢁ1
was 5 mg mL without any dopant. The PCE of p-i-n cells based
on the studied HTMs reached 89% of the PCE of those cells based
on the standard HTM (poly(3,4-ethylenedioxythiophene), PEDOT).
2
2
1
5 s in a nitrogen lled glovebox, the device was annealed at
2
In devices of the regular mesoporous TiO -based PSCs (n-i-p),
ꢂ
ꢁ1
00 C for 10 min. Then, PCBM (20 mg mL ) in CB and BCP
different concentrations of HTM solutions were spin-coated by
adding only 4-tert-butylpyridine (TBP). A low concentration of
ꢁ1
(0.5 mg mL ) in isopropanol were spin-coated on the perov-
skite layer at 4000 rpm for 40 s. The PCBM layer was annealed at
ꢁ1
1
0 mg mL was good for the studied HTM to establish excellent
ꢂ
8
0 C for 10 min, while BCP was only spin-coated on the PCBM
contact with the perovskite layer. Among the three compounds,
JW7 showed better photovoltaic performances than JW6 and JW8.
The structure–efficiency relationship of these ferrocene-containing
HTMs was discussed based on the measurements of J–V charac-
layer without any heating. Finally, Ag was vacuum-evaporated
on the top of the device as the counter electrode.
terizations and steady-state and time-resolved photoluminescence 2.3 Characterizations
PL) spectra. The hole mobility of the new HTMs was measured by
(
1
13
H NMR and C NMR spectra were recorded on a Bruker AM-400
space-charge-limited-current (SCLC), and charge recombination
was studied by electrochemical impedance spectroscopy (EIS).
Through this research we hope to provide some experimental basis
for designing new HTMs for PSCs.
spectrometer. The chemical shis were reported against TMS.
High resolution mass spectra were obtained on a Micromass
GCT-TOF mass spectrometer. UV-vis absorption spectra were
recorded on a SHIMADZU UV-2600 spectrophotometer. Photo-
luminescence (PL) measurements were recorded on a HITACHI
F-4500 uorescence spectrophotometer at 500 nm excitation
wavelength. Cyclic voltammetry (CV) measurements were recor-
ded using a Zennium electrochemical workstation (ZAHNER,
2
Experimental
2.1 Materials
All the starting materials used in this study were purchased from Germany) under a three-electrode system, platinum was used as
J&K Chemical, Energy Chemical or Sigma-Aldrich and used the working electrode, Ag/AgCl electrode was used as the refer-
without further purication, unless otherwise stated. N,N-Dime- ence electrode, and Pt-wires were used as the counter electrode.
thylformamide (DMF) and tetrahydrofuran (THF) were dried using Electrochemical impedance spectroscopy (EIS) in the frequency
200 mesh molecular sieve and sodium metal, respectively.
range from 200 mHz to 100 kHz was performed with a Zennium
electrochemical workstation (ZAHNER, Germany) in the dark
with the alternate current amplitude set at 10 mV. Current–
voltage (J–V) characteristics of PSCs were measured using
a Keithley 2400 digital SourceMeter controlled by a computer
under a standard AM 1.5 solar simulator (Oriel 91160-1000 (300
W) Solar Simulator). Incident photon-to-current conversion effi-
ciency (IPCE) for PSCs was recorded on the QTest Station 2000
IPCE measurement system, CROWNTECH, USA. The space-
charge-limited-current (SCLC) method was used to measure the
37
hole mobility of HTMs using the equation:
2
9
J ¼ m3
8
V
0 r
3
d3
2.4 Synthesis of JW6, JW7 and JW8
The synthesis routes for JW6, JW7 and JW8 are shown in
Scheme 2. The intermediates of compounds 1–7 have been re-
34,38–41
Scheme 1 Chemical structures of JW6, JW7 and JW8.
ported in literature reports.
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General procedure for the synthesis of compounds JW6, JW7 electrochemical measurements. As shown in Fig. 1(a), UV-
and JW8. Compounds 7a–c (0.677 mmol) were dissolved using visible absorptions of JW6, JW7 and JW8 were measured in
anhydrous THF in a 100 mL two-neck round-bottom ask under lm and in dichloromethane solution. The energy gap (E0–0
)
ꢂ
N
2
. Aer the temperature dropped to 0 C, the THF solution of t- values of JW6, JW7 and JW8 were calculated from the onset
BuOK (0.880 mmol) was added dropwise, and the solution wavelengths of the absorption spectra in solution, which were
ꢂ
turned yellow. The reaction was kept at 0 C for 1 hour. Then, 2.92 eV, 3.01 eV and 2.85 eV, respectively. JW8 showed the
THF solution of 4 (0.564 mmol) was added slowly. The mixture smallest E0–0 value among the three HTMs, suggesting that JW8
was then placed at room temperature and stirred overnight. possesses larger p-conjugation than JW6 and JW7. This was
Then, saturated NH Cl was added to stop the reaction and the
4
product was extracted with dichloromethane. The combined
organic layer was washed with brine and dried over anhydrous
2 4
Na SO . The solvent was evaporated, and the remaining crude
product was puried by column chromatography to obtain the
desired compounds JW6, JW7 and JW8.
Characterization details for the synthesis of JW6, JW7 and
JW8 are provided in ESI.†
3
Results and discussion
3.1 Spectral, electrochemical and thermal stability
measurements
Suitable HOMO and LUMO levels are important for HTMs to
achieve hole transport in a PSC device. The HOMO levels
matching the valence band energy of CH NH PbI are suitable
3
3
3
for fabricating efficient PSCs. In this study, the energy levels of
the three HTMs were determined by spectroscopic and
Scheme 2 The synthesis routes for JW6, JW7 and JW8. Conditions: (i)
ꢂ
POCl
1
3
ꢂ
, DMF, 0 C, 0.5 h then RT, 5 h; (ii) NIS, CHCl
3
/AcOH(4 : 1),
N/THF (1 : 1), N
2
, 0 C, 0.5 h then RT, 4 h; (v) and dichloromethane solution (dash line); (b) cyclic voltammetry of
00 C, 0.5 h; (iii) ethynyl ferrocene, Pd(PPh
reflux, 6 h; (iv) NaBH
3
)
4
, Et
3
2
,
Fig. 1 (a) Absorption spectra of JW6, JW7 and JW8 in film (solid line)
ꢂ
4
, EtOH, NaOH, N
, reflux, 6 h; (vi) P(OEt)
NaBH
4
, i-PrOH, H
2
O, N
2
3
, I
2
, RT, 5 h; (vii) 4, t- JW6, JW7 and JW8; (c) energy level scheme of JW6, JW7 and JW8 in
BuOK, THF, RT, 10 h.
a p-i-n type PSC.
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Table 1 Spectral and electrochemical properties of JW6, JW7 and JW8
sol
a
lmb
max /nm
2
ꢁ1 ꢁ1
c
d
e
HTMs
l
max /nm
l
l
onset/nm
Mobility/cm V
s
E
0–0 /eV
E
HOMO /eV
E
LUMO /eV
ꢁ
ꢁ
ꢁ
4
4
4
JW6
JW7
JW8
372
372
378
378
378
382
425
412
435
1.3 ꢀ 10
4.3 ꢀ 10
1.5 ꢀ 10
2.92
3.01
2.85
ꢁ5.40
ꢁ5.41
ꢁ5.42
ꢁ2.48
ꢁ2.40
ꢁ2.57
a
ꢁ6
b
c
UV-vis spectra were measured in dichloromethane solution (5 ꢀ 10 M).
lonset is the onset wavelength of absorption spectrum. Optical band
HOMO was calculated from the equation EHOMO ¼ ꢁ4.7 ꢁ Eox. Eox was standardized with ferrocene (0.63 V vs.
d
gap was calculated from 1240/lonset
.
E
e
NHE).
E
LUMO ¼ EHOMO + E0–0
.
ꢂ
consistent with the maximum absorption peak of JW8 display-
C). The DCS result shows JW6, JW7 and JW8 to have good
ing a red shi in comparison with that of JW6 and JW7 both in stability in the amorphous state.
dichloromethane solution and in solid-state lm. The absorp-
tion of the studied HTMs bearing biferrocene was extended to
around 450 nm. Cyclic voltammetry (CV) was used to measure 3.2 Theoretical calculations
the oxidation and reduction peaks of JW6, JW7 and JW8 in
The substituents have an important inuence on the electron
dichloromethane solution, and the CV curves are shown in
Fig. 1(b). The HOMO levels of JW6, JW7 and JW8 were calculated
from the equation EHOMO ¼ ꢁ4.7 ꢁ Eox. The corresponding
distribution and conjugated construction of HTM molecules.
Theoretical calculations on the basis of DFT at the B3LYP/6-
31G(g,d) level were used to optimize the geometric structures
E
HOMO values of JW6, JW7 and JW8 were ꢁ5.40 eV, ꢁ5.41 eV and
of the new HTMs. The optimized structures of JW6, JW7 and
JW8 are given in Fig. S1,† and the representative bond lengths
and dihedral angles are listed in Table S1 (seen in ESI†). The
frontier molecular orbitals of the three compounds are shown
in Fig. 3. The highest occupied molecular orbitals (HOMO) of
the three compounds are almost delocalized over the whole
molecule. The lowest unoccupied molecular orbitals (LUMO)
are localized in the double bond part. The LUMO of JW7 is
transferred from the triphenylamine part to the (4-methox-
yphenyl)methane section. As shown in Table S1,† the C–C bond
lengths of ferrocenylethynyl and triphenylamine in the three
ꢁ
5.42 eV, respectively. The EHOMO values of the studied HTMs
were very close and slightly higher than the valence band energy
of CH NH PbI . Thus, it was easy for the new HTMs to accept
holes transferred from CH NH PbI . The LUMO levels of JW6,
3
3
3
3
3
3
JW7 and JW8 were calculated from the equation ELUMO ¼ E
HOMO
+
E0–0, and the corresponding ELUMO values were ꢁ2.48 eV,
2.40 eV and ꢁ2.57 eV, respectively. The higher ELUMO value of
ꢁ
JW7 can more effectively prevent the electron transfer from
3 3 3
CH NH PbI to HTMs, and thus reduce charge recombination
in the device. Moreover, the energy level scheme for a p-i-n cell
is represented in Fig. 1(c), in which the difference in HOMO and
LUMO energy levels of the three compounds can be clearly seen.
The related spectral and electrochemical results are listed in
Table 1.
˚
compounds are the same (1.423 A), and the C–N bond lengths of
˚
˚
this triphenylamine are similar, which are 1.419 A, 1.418 A and
˚
1
.416 A for JW6, JW7 and JW8, respectively. JW7 and JW8 have
a shorter C–N bond length, suggesting a longer p-conjugated
Differential scanning calorimetry (DSC) was used to test the
g
glass transition temperature (T ) of JW6, JW7 and JW8. DSC
curves of these new HTMs are displayed in Fig. 2. JW8 pos-
0
sessing 4,4 -methylenebis(N,N-dimethylaniline) and JW7 pos-
sessing bis(4-methoxyphenyl)methane have higher T
g
values
ꢂ
ꢂ
(129 C and 120 C) than JW6 possessing triphenylamine (116
Fig. 2 DSC curves of JW6, JW7 and JW8.
Fig. 3 Frontier molecular orbitals of JW6, JW7 and JW8.
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length. Moreover, JW7 and JW8 have similar C]C bond lengths
˚
˚
of 1.468 A and 1.467 A, respectively. The C–C bonds of double
bond and diphenylmethane group for JW7 and JW8 are 1.491 A˚
˚
and 1.489 A, respectively. Overall, JW8 has the shortest bond
length among the three HTMs. The dihedral angles of the two
phenyl rings of the triphenylamine connecting ferrocenyle-
ꢂ
ꢂ
ꢂ
thynyl in JW6, JW7 and JW8 are 40.9 , 39.4 and 37.9 , respec-
tively. The dihedral angles between the double bond and the
phenyl ring of diphenylmethane group in JW7 and JW8 are
ꢁ
ꢂ
ꢂ
26.9 and ꢁ25.2 , respectively. The data of dihedral angles
also indicate that JW7 and JW8 have a more planar conjugated
plane than JW6. Therefore, the results suggest that JW7 and
JW8 may have a trend for faster charge transport than JW6. The
device performances of the three compounds will be discussed
in detail in Section 3.3.
3.3 Device performance
HTM solutions were prepared to be spin-coated for fabricating
PSC devices, and two architectures of PSCs were constructed to
evaluate the photovoltaic performances of the new HTMs-based
cells. In the inverted planar heterojunction perovskite solar
cells (p-i-n), the studied HTMs are used without any additives.
ꢁ
1
The concentration of HTM solutions was 5 mg mL . The p-i-n
cells were based on the structure of ITO/HTM/perovskite/PCBM/
BCP/Ag. Fig. 4 shows a cross-section SEM image of the p-i-n-type
cell. The structure of the device is dense and neat. The ll factor
(FF) of p-i-n cells fabricated by using JW6, JW7 and JW8 is
higher than that of their n-i-p cells. The PCE of p-i-n cells based
on JW6, JW7 and JW8 are 10.23%, 11.43% and 10.42%,
respectively. The PCE of PSCs fabricated using the standard
PEDOT is 12.86%. The corresponding J–V curves of p-i-n cells
are shown in Fig. 5(a). In addition, device performances of p-i-n
cells display replicability and stability. The photovoltaic
parameters of eight cells of p-i-n-type PSCs fabricated using JW7
Fig. 5 (a) J–V characteristic curves and (b) IPCE spectra for p-i-n
PSCs.
ꢁ
1
through reverse and forward bias at a scan rate of 0.1 V s . The
data of hysteresis test for p-i-n type PSCs fabricated using the
standard PEDOT are also provided in Table S3† as a reference.
Results of the incident photon-to-current conversion effi-
ciency (IPCE) spectra of p-i-n cells fabricated using JW6, JW7
and JW8 are shown in Fig. 5(b). The three HTMs have a wide
range from 450 nm to 700 nm. The difference in JSC is in
accordance with that in the measured values in the order of JW7
are presented in Table S2.† They are kept at room temperature
ꢂ
(
25 C) and in ambient air of 30% RH without encapsulation.
Hysteresis test for p-i-n type PSCs based on the new HTMs was
conducted, and the results are summarized in Table S3.†
Fig. S2† exhibits the J–V plots of p-i-n type PSCs based on JW7
>
JW8 > JW6 (Table 2).
Furthermore, data based on the space-charge-limited-
current (SCLC) method was used to study the hole mobility of
JW6, JW7 and JW8. Fig. 6(a) shows the tting current density–
voltage (J–V) curves. The calculated values of the hole mobility of
ꢁ
4
2
ꢁ1 ꢁ1
ꢁ4
2
JW6, JW7 and JW8 were 1.3 ꢀ 10 cm V
s
, 4.2 ꢀ 10 cm
ꢁ1
ꢁ1
ꢁ4
2
ꢁ1 ꢁ1
V
s
and 1.5 ꢀ 10 cm V
s . JW7 showed higher hole
mobility than JW6 and JW8, indicating that different substitutes
Table 2 J–V parameters of PSCs (p-i-n type) with JW6, JW7 and JW8
ꢁ2
HTMs
VOC/mV
JSC/mA cm
FF
PCE/%
JW6
JW7
850
833
820
867
19.73
22.13
20.17
22.26
0.61
0.62
0.63
0.69
10.23
11.43
10.42
12.86
Fig. 4 Cross-section SEM image of the p-i-n type cell fabricated using JW8
JW7.
PEDOT
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had signicant inuence on the intermolecular charge trans- Table 3 J–V parameters of n-i-p type PSCs with different concen-
trations of JW7
port of the studied HTMs. Photoluminescence (PL) measure-
ments were recorded to study the charge extraction capability of
JW6, JW7 and JW8, and those for PEDOT were also recorded as
a standard. Fig. 6(b) displays the steady-state PL spectra of the
ꢁ2
Conditions
V
OC/mV
J
SC/mA cm
FF
PCE/%
a
1
825
881
865
836
870
864
842
825
847
9.64
8.87
0.28
0.26
0.52
0.47
0.47
0.46
0.57
0.56
0.53
2.23
2.03
8.65
8.22
7.77
7.53
9.36
8.88
8.28
a
CH
3
NH
3
PbI
3
perovskite lms without or with capping different
2
b
3
19.24
20.92
18.99
18.95
19.50
19.21
18.45
HTMs. The PL band of the perovskite lm is observed to be
centered at 756 nm by excitation at 500 nm. All the bilayers
b
4
b
5
b
6
c
7
c
8
c
9
a
ꢁ1
b
ꢁ1
PSC covered with 30 mg mL HTM. PSC covered with 20 mg mL
c
ꢁ1
HTM. PSC covered with 10 mg mL HTM.
show a drastic quenching of PL with respect to the pristine
perovskite, following the order of perovskite/JW7 > perovskite/
JW8 > perovskite/JW6. This suggests that JW7 has a more
positive effect on device performances than JW6 and JW8,
beneting from the higher hole mobility and hole extraction
42
capability. Electrochemical impedance spectroscopy (EIS) was
performed to further understand charge recombination in PSCs
fabricated using JW6, JW7 and JW8. Fig. 6(c) displays the
Nyquist plots. The recombination resistance (Rrec) is repre-
sented by the large arc at low frequency. The Rrec value increases
in the sequence of JW8, JW7 and JW6, indicating that the device
performances based on JW8 are affected by faster charge
recombination. Larger Rrec of JW6 and JW7 are benecial for
37
reducing charge recombination. However, the device perfor-
mances based on JW6 are limited by relatively lower hole
mobility, according to data of PL measurements. Therefore,
0
JW7 with the 4,4 -methylenebis(N,N-dimethylaniline) substit-
uent shows a good performance in the inverted planar hetero-
junction perovskite solar cells.
In addition, different concentrations of JW7 were tested in
2
the regular mesoporous TiO -based perovskite solar cells (n-i-p).
The structure of the n-i-p-type cells is FTO/TiO /perovskite/
2
HTM/Ag. The photovoltaic parameters of the representative
devices based on n-i-p-type PSCs are listed in Table 3. All the n-i-
p cells are based on HTM solutions prepared by adding only
TBP and without adding expensive additives such as LiTFSI.
Fig. 6 (a) The space-charge-limited-current (SCLC) measurements of
hole-only devices of ITO/HTM/PCBM/BCP/Ag based on JW6, JW7
and JW8. (b) Steady-state and time-resolved photoluminescence (PL)
spectra of JW6, JW7 and JW8. (c) EIS for PSCs based on JW6, JW7 and
JW8 measured in the dark under 0.9 V bias displayed in the form of
Nyquist plots.
Fig. 7 J–V characteristic curves for PSCs (n-i-p type).
RSC Adv., 2019, 9, 216–223 | 221
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Fig. 7 shows the effect of different concentrations on n-i-p
device performances determined by J–V measurements. When
6 F. Yang, D.-W. Kang and Y. –S. Kim, RSC Adv., 2017, 7, 19030–
19038.
7 A. Yusoff, P. Gao and M. K. Nazeeruddin, Coord. Chem. Rev.,
2018, 373, 258–294.
8 A. Kojima, K. Teshima, Y. Shirai and T. Miyasaka, J. Am.
Chem. Soc., 2009, 131, 6050–6051.
9 H. Kim, C. Lee, J. Im, K. Lee, T. Moehl, A. Marchioro,
S. Moon, R. Humphry-Baker, J. Yum, J. E. Moser,
M. Gr ¨a tzel and N. Park, Sci. Rep., 2012, 2, 591.
ꢁ1
using JW7 of 30 mg mL , the value of photocurrent (JSC) drops
sharply, and the highest PCE is only 2.23%. The PCE increases
to 8.65% when the concentration of JW7 is reduced to 20 mg
ꢁ1
mL . Furthermore, the device performances get even better
when the concentration of HTM solution decreases to 10 mg
ꢁ1
mL , and the PCE can reach 9.36%.
1
0 W. Yang, J. H. Noh, N. J. Jeon, Y. C. Kim, S. I. Ryu, J. Seo and
S. Seok, Science, 2015, 348, 1234–1237.
4
Conclusions
1
1 F. Santiago, J. Bisquert, L. Cevey, P. Chen, M. Wang,
S. M. Zakeeruddin and M. Gr ¨a tzel, J. Am. Chem. Soc., 2009,
In this study, three triphenylamine derivatives containing
ferrocene were synthesized and used as organic HTMs for
fabricating efficient PSCs. The substitutes on their structures
are different, which are triphenylamine for JW6, bis(4-
131, 558–562.
1
1
2 N. J. Jeon, H. G. Lee, Y. C. Kim, J. Seo, J. H. Noh, J. Lee and
S. Seok, J. Am. Chem. Soc., 2014, 136, 7837–7840.
0
methoxyphenyl)methane for JW7 and 4,4 -methylenebis(N,N-
3 C. S. Ponseca, E. M. Hutter, P. Piatkowski, B. Cohen,
T. Pascher, A. Douhal, A. Yartsev, V. Sundstr ¨o m and
T. J. Savenije, J. Am. Chem. Soc., 2015, 137, 16043–16048.
4 M. Jung, S. R. Raga, L. K. Ono and Y. Qi, Sci. Rep., 2015, 5,
dimethylaniline) for JW8. The maximum absorption wave-
lengths of the three compounds are around 370–380 nm and
that of JW8 (378 nm) has a red shi when compared with JW6
1
(372 nm) and JW7 (372 nm). Theoretical calculations show that
9863.
JW7 and JW8 have a more planar conjugated structure. The
ꢂ
15 A. Marchioro, J. Teuscher, D. Friedrich, M. Kunst and
three HTMs display T
g
values beyond 110 C. SCLC results show
R. Krol, Nat. Photonics, 2014, 8, 250–255.
that JW7 exhibits the highest hole mobility among the studied
HTMs. In the inverted p-i-n-type of PSCs (ITO/HTM/perovskite/
PCBM/BCP/Ag), JW7 shows better photovoltaic performance
than JW6 and JW8 under the same conditions, leading to a PCE
of 11.43% (standard PEDOT, 12.86%). In the conventional n-i-p
1
1
1
6 X. Zhao and M. Wang, Materials Today Energy, 2018, 7, 208–
20.
7 Y. Wang, T. Su, H. Tsai, T. Wei and Y. Chi, Sci. Rep., 2017, 7,
859.
2
7
8 F. Zhang, S. Wang, H. Zhu, X. Liu, H. Liu, X. Li, Y. Xiao,
S. M. Zakeeruddin and M. Gr ¨a tzel, ACS Energy Lett., 2018,
2
device of FTO/TiO /perovskite/HTM/Ag, the optimal concen-
ꢁ1
tration of HTM is 10 mg mL . The corresponding highest PCE
for these n-i-p cells was 9.36%, which were fabricated using JW7
and adding TBP as the dopant. Therefore, ferrocene-containing
materials have a great potential in perovskite solar cells, and
there is a lot of work to be done in the development of these
ferrocene-based HTMs for fabricating high-efficiency PSCs.
3
, 1145–1152.
1
9 Y. Wu, Z. Wang, M. Liang, H. Cheng, M. Li, L. Liu, B. Wang
and J. Wu, ACS Appl. Mater. Interfaces, 2018, 10, 17883–
17895.
2
0 K. Rakstys, A. Abate, M. I. Dar, P. Gao, V. Jankauskas,
G. Jacopin, E. Kamarauskas, S. Kazim, S. Ahmad,
M. Gr ¨a tzel and M. K. Nazeeruddin, J. Am. Chem. Soc., 2015,
Conflicts of interest
1
37, 16172–16178.
2
2
1 R. Grisorio, B. Roose, S. Colella, A. Listorti, G. P. Suranna
and A. Abate, ACS Energy Lett., 2017, 2, 1029–1034.
2 M. S. Kang, S. D. Sung, I. T. Choi, H. Kim, M. Hong, J. Kim,
W. Lee and H. K. Kim, ACS Appl. Mater. Interfaces, 2015, 7,
There are no conicts to declare.
Acknowledgements
2
2213–22217.
This study was supported by the National Natural Science
Foundation of China (21506164, 21671148, and 21576215).
2
3 N. J. Jeon, H. Na, E. H. Jung, T. Y. Yang, Y. G. Lee, G. Kim,
H. Shin, S. Seok, J. Lee and J. Seo, Nat. Energy, 2018, 3,
682–689.
2
4 N. J. Jeon, J. Lee, J. H. Noh, M. K. Nazeeruddin and
M. Gr ¨a tzel, J. Am. Chem. Soc., 2013, 135, 19087–19090.
Notes and references
1
N. Arora, M. Ibrahim Dar, A. Hinderhofer, N. Pellet, 25 H. Nishimura, N. Ishida, A. Shimazaki, A. Wakamiya,
F. Schreiber, S. M. Zakeeruddin and M. Gr ¨a tzel, Science,
017, 358, 768–771.
A. Saeki, L. T. Scott and Y. Murata, J. Am. Chem. Soc., 2015,
137, 15656–15659.
2
2
3
H. J. Snaith, Nat. Mater., 2018, 17, 372–376.
K. Mahmood, S. Sarwar and M. T. Mehran, RSC Adv., 2017, 7,
26 W. Ke, P. Priyanka, S. Vegiraju, C. C. Stoumpos,
I. Spanopoulos, C. Soe, T. J. Marks, M. C. Chen and
M. G. Kanatzidis, J. Am. Chem. Soc., 2018, 140, 388–393.
17044–17062.
4
5
T. Salim, S. Sun, Y. Abe, A. Krishna, A. C. Grimsdalea and 27 A. Ghosh, S. Mishra, S. Giri, S. M. Mobin, A. Bera and
Y. M. Lam, J. Mater. Chem. A, 2015, 3, 8943–8969.
S. Chatterjee, Organometallics, 2018, 37, 1999–2002.
S. Wang, W. Yuan and Y. S. Meng, ACS Appl. Mater. Interfaces,
2015, 7, 24791–24798.
222 | RSC Adv., 2019, 9, 216–223
This journal is © The Royal Society of Chemistry 2019

View Article Online
Paper
RSC Advances
28 R. J. Durand, S. Achelle, S. Gauthier, N. Cabon, M. Ducamp, 36 C. H. Noh, S. H. Cho, K. Y. Song, T. R. Choi and T. Byk, US
S. Kahlal, J. Saillard, A. Barsella and F. R. Guen, Dyes Pigm.,
018, 155, 68–74.
9 N. Dwadnia, J. Roger, N. Pirio, H. Cattey and J. C. Hierso,
pat. 20080186564, 2008-08-07.
2
37 J. Zhang, B. Xu, L. Yang, C. Ruan, L. Wang, P. Liu, W. Zhang,
N. Vlachopoulos, L. Kloo, G. Boschloo, L. Sun, A. Hagfeldt
and E. Johansson, Adv. Energy Mater., 2018, 8, 1701209.
2
3
Coord. Chem. Rev., 2018, 355, 74–100.
0 J. C. Spiteri, A. D. Johnson, S. A. Denisov, G. Jonusauskas, 38 H. J. Snaith and M. Gr ¨a tzel, Appl. Phys. Lett., 2006, 89,
N. D. McClenaghan and D. C. Magri, Dyes Pigm., 2018, 157,
78–283.
1 C. Su, Y. Ye, L. Xu and C. Zhang, J. Mater. Chem., 2012, 22,
2658–22662.
262114.
2
39 W. Zhang, P. Liu, A. Sadollahkhani, Y. Li, B. Zhang, F. Zhang,
M. Safdari, Y. Hao, Y. Hua and L. Kloo, ACS Omega, 2017, 2,
9231–9240.
3
2
3
3
2 R. Maragani and R. Misra, Tetrahedron, 2014, 70, 3390–3399. 40 H. Shang, H. Fan, Q. Shi, S. Li, Y. Li and X. Zhan, Sol. Energy
3 R. Maragani, R. Misra, M. S. Roy, M. K. Singh and Mater. Sol. Cells, 2010, 94, 457–464.
G. D. Sharma, Phys. Chem. Chem. Phys., 2017, 19, 8925–8933. 41 L. Zhang, Y. Liu, Z. Wang, M. Liang, Z. Sun and S. Xue,
3
3
4 R. Misra, R. Maragani, K. R. Patel and G. D. Sharma, RSC
Adv., 2014, 4, 34904–34911.
5 A. M. El-Zohry, J. Cong, M. Karlsson, L. Kloo and B. Zietz,
Dyes Pigm., 2016, 132, 360–368.
Tetrahedron, 2010, 66, 3318–3325.
42 C. Huang, W. Fu, C. Li, Z. Zhang, W. Qiu, M. Shi,
P. Heremans, A. Jen and H. Chen, J. Am. Chem. Soc., 2016,
138, 2528–2531.
This journal is © The Royal Society of Chemistry 2019
RSC Adv., 2019, 9, 216–223 | 223