Simple and low-cost thiophene and benzene-conjugated triaryamines as hole-transporting materials for perovskite solar cells

Article abstract of DOI:10.1039/c7ra08052b

Two novel electron-rich linear small-molecules, containing benzene and thiophene as the cores with arylamine side groups, named HTM1 and HTM2, respectively, were synthesized via short, easy and efficient synthetic routes. The influence of the π-linkers of the two materials on photophysical, electrochemical, and thermal properties, and hole mobility and photovoltaic performance was investigated. The compound with thiophene as π-linker (HTM2) shows better solubility and higher hole-transporting mobility than the compound with benzene as π-linker (HTM1). When these two materials were incorporated into perovskite solar cells as hole transporting materials (HTMs), short circuit photocurrent densities (J_scs) of 15.83 mA cm^-2 and 21.1 mA cm^-2, open circuit voltages (V_ocs) of 0.79 V and 1.09 V, and fill factors (FFs) of 0.46 and 0.62, were obtained. These factors contributed to average overall power conversion efficiencies (PCEs) of 6.4% and 13.9% with the best PCEs of 7.5% and 14.7%, respectively. The performance of HTM2 is comparable to the PCE obtained using the current state-of-the-art HTM of 2,2′,7,7′-tetrakis(N,N′-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD) with the best PCE of 17.4% using a similar device preparation method and measurement conditions. These results showed that selecting a suitable π-linker is important for the performance of HTMs. And the simple HTM2 material is a promising HTM with the potential to replace the expensive spiro-OMeTAD due to its comparable performance with a much simpler synthesis route and much reduced cost (10 times less than that of spiro-OMeTAD). This study demonstrates that a compound with a suitable π-linker could be a low-cost and high performance HTM to replace spiro-OMeTAD.

Full text of DOI:10.1039/c7ra08052b

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PAPER
Simple and low-cost thiophene and benzene-
conjugated triaryamines as hole-transporting
materials for perovskite solar cells†
ab
Cite this: RSC Adv., 2017, 7, 45478
Jianchang Wu,
‡
Chang Liu, ‡b Xiang Deng,b Luozheng Zhang,b Manman Hu,b
a
b
b
Jun Tang,ab Wenchang Tan, Yanqing Tian
and Baomin Xu
*
*
*
Two novel electron-rich linear small-molecules, containing benzene and thiophene as the cores with
arylamine side groups, named HTM1 and HTM2, respectively, were synthesized via short, easy and
efficient synthetic routes. The influence of the p-linkers of the two materials on photophysical,
electrochemical, and thermal properties, and hole mobility and photovoltaic performance was
investigated. The compound with thiophene as p-linker (HTM2) shows better solubility and higher hole-
transporting mobility than the compound with benzene as p-linker (HTM1). When these two materials
were incorporated into perovskite solar cells as hole transporting materials (HTMs), short circuit
photocurrent densities (Jscs) of 15.83 mA cmꢀ2 and 21.1 mA cmꢀ2, open circuit voltages (Vocs) of 0.79 V
and 1.09 V, and fill factors (FFs) of 0.46 and 0.62, were obtained. These factors contributed to average
overall power conversion efficiencies (PCEs) of 6.4% and 13.9% with the best PCEs of 7.5% and 14.7%,
respectively. The performance of HTM2 is comparable to the PCE obtained using the current state-of-
the-art HTM of 2,20,7,70-tetrakis(N,N0-di-p-methoxyphenylamine)-9,90-spirobifluorene (spiro-OMeTAD)
with the best PCE of 17.4% using a similar device preparation method and measurement conditions.
These results showed that selecting a suitable p-linker is important for the performance of HTMs. And
the simple HTM2 material is a promising HTM with the potential to replace the expensive spiro-OMeTAD
due to its comparable performance with a much simpler synthesis route and much reduced cost (10
times less than that of spiro-OMeTAD). This study demonstrates that a compound with a suitable p-
linker could be a low-cost and high performance HTM to replace spiro-OMeTAD.
Received 21st July 2017
Accepted 20th September 2017
DOI: 10.1039/c7ra08052b
rsc.li/rsc-advances
past few years.1–3,7–9 A typical PSC device comprises a TiO2
anode, a perovskite absorbing layer, a hole transporting layer
Introduction
and a metallic counter electrode. In addition to perovskite
absorbing materials, charge transporting layers also play an
important role in charge extraction and interface modication,
thus improving the performance of PSCs device.10,11 To date,
2,20,7,70-tetrakis(N,N0-di-p-methoxyphenylamine)-9,90-spirobi-
uorene (spiro-OMeTAD) is the most extensively studied and
applied HTM for perovskite devices.12–16 Many device charac-
terization and optimization works have been centered on spiro-
OMeTAD.9,15,17 However, in generally, the complex synthetic
route and purication of spiro-OMeTAD render it expensive,
which signicantly limits potential large-scale application and
commercialization of PSC devices in the future. The synthesis of
the core of spiro-OMeTAD needs four steps, including a Suzuki–
Miyaura cross-coupling reaction, a Grignard reaction, a cycliza-
tion reaction and a bromination reaction from commercially
available raw materials, which limited its total yield (<30%) and
cost (about 94 $ per g).18,19 Thus, signicant research effort has
been spent on designing and synthesizing alternative HTMs,
which are more economical and are able to show device
performance comparable to or even better than that of
In recent years, the efficiency of organic–inorganic metal halide
perovskite-based solar cells (PSCs) has improved rapidly, and is
already comparable with current commercial technologies.1,2
Those perovskite materials, typically methylammonium lead
iodides (CH3NH3PbI3)s, have excellent properties, including
their absorption throughout the UV-vis-NIR spectrum, high
extinction coefficients, large exciton diffusion lengths and
ambipolar mobility. These characteristics make them excellent
light harvesting materials.3–6 Their outstanding performance by
showing an increase of the power conversion efficiency (PCE)
from 3% to a certied 22% has been demonstrated during the
aSchool of Advanced Materials, Peking University Shenzhen Graduate School,
Shenzhen, Guangdong 518055, P. R. China. E-mail: tanwch@pku.edu.cn
bDepartment of Materials Science and Engineering, Southern University of Science and
Technology, No. 1088, Xueyuan Rd, Shenzhen, Guangdong 518055, P. R. China.
E-mail: tianyq@sustc.edu.cn; xubm@sustc.edu.cn
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c7ra08052b
‡ These authors contributed equally.
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spiro-OMeTAD. Several conjugated polymers including poly(3- being useful for high efficient devices and a candidate to replace
hexylthiophene-2,5-diyl) (P3HT)20 and poly[bis(4-phenyl)(2,4,6- spiro-OMeTAD. However, in our studies, we achieved much
trimethylphenyl)amine] (PTAA)8 have been employed as HTMs higher PCE of 14.7%. This result may be caused by the different
in perovskite solar cells. However their device results to date components of perovskite layer. FAMAPbBrI3 was used in this
have been inferior to that of spiro-OMeTAD. What's more, the article and MAPbI3 was used in the ref. 32. It is obvious that the
intrinsic batch to batch variation issues of polymers due to device based on FAMAPbBrI3 could achieve higher perfor-
different polymerization degrees and polydispersity could mance. (Devices based on spiro-OMeTAD achieve an efficiency
hamper the performance reproducibility for possible commer- of 17.4% in this article; it was only 11.9% in the reference).
cialization of the device. On the other hand, a number of small Moreover, FAMAPbBrI3 has a deeper HOMO level compared
molecule HTMs have also been reported, including spirobi- with MAPbI3, which could match HTM2 better. Further, we
uorenes derivatives,19,21–23 thiophene derivatives,24–26 triphe- compared the cost of the synthesis of HTM2 with spiro-OMe-
nylamines,27,28 and triazatruxenes.29–31
TAD. Because of the much shorter synthesis route of HTM2
Most of them were namely decorated with diarylamines, than that of spiro-OMeTAD, the cost of HTM2 is about 1/10 fold
triarylamines and/or carbazole derivatives. The best perfor- of spiro-OMeTAD. Thus, our results demonstrated that the
mance of those HTMs show similar or superior performance to HTM2 has a great potential to replace the spiro-OMeTAD.
spiro-OMeTAD in some aspects but no material has yet emerged
clearly superior in all aspects of device performance, particu-
larly in terms of cost, device efficiency and stability. Hence it
Results and discussions
remains a great challenge to develop new, easily synthesized, The synthetic route (Scheme S1†) towards HTM1 and HTM2
low cost and high performance HTMs for PSC devices.
only requires two steps and uses commercially available raw
An efficient way to simplify the synthesis of the HTMs is to materials, which are much simpler than that of the synthesis of
change the spiro-core of the spiro-OMeTAD to a planar unit, spiro-OMeTAD. Pd-catalyzed boronation of 4-bromo-N,N-bis(4-
such as diphenyl or terphenyl. In this way, only one or two methoxyphenyl)aniline (compound 1, ESI Scheme S1†) with
synthetic steps are needed from the commercially available raw bis(pinacolato)diboron produced the key building block OMT
materials, this could reduce its cost signicantly. In this article, BPIN (compound 2, Scheme S1†) in a yield of 80%. The two nal
2,5-bis(4,40-bis(methoxyphenyl)aminophen-400-yl)-benzene
HTMs were synthesized by Suzuki coupling reaction using OMT
(HTM1, its structure is given in Fig. 1) was designed and BPIN with dibromobenzene (compound 3, Scheme S1†) and
synthesized using terphenyl to replace the spiro-core of the dibromothiophene (compound 4, Scheme S1†), respectively.
spiro-OMeTAD. It only needs two synthetic steps to get the nal HTM2 has a good solubility in common solvents, such as
product. However, when the material changes from spiro- chlorobenzene (200 mg mLꢀ1); while the solubility of HTM1 is
structure to a planar structure, a great decrease in solubility relatively low, about 60 mg mLꢀ1 in chlorobenzene. The good
and slight decrease in mobility occur. Those factors affect the solubility of HTM2 will benet the smooth lm formation.
process of the HTM lm, as a result the PCE of the devices based Differential scanning calorimetry (DSC) measurements show
on HTM1 were much lower than that of spiro-OMeTAD. To solve that HTM1 and HTM2 have a glass transition temperature (Tg)
the low mobility of HTM1, 2,5-bis(4,40-bis(methoxyphenyl)ami- at 80 ꢁC and 76 ꢁC, respectively. Both are much lower than that
nophen-400-yl)-thiophene (HTM2) was designed and synthesized of spiro-OMeTAD (around 125 ꢁC).33 The lower Tg and higher
using thiophene unit to replace the center unit (benzene) of the solubility could increase the pore-lling of the HTM in meso-
HTM1. Thiophene moiety has been widely used in organic scopic architecture device, because the pore lling procedure
electronics because of its excellent properties due to its high begins by spin-coating a solution of the HTM onto the m-TiO2/
charge mobility. Unfortunately, during the manuscript prepa- perovskite lm.33 As solvent evaporates and the solution
ration, a recent paper reported the use of HTM2 as hole trans- concentration increases, HTM diffuses into the pores allowing
porting layer in PSCs device32 with an extremely low PCE for a higher degree of pore lling than that would be obtained
(4.31%). For this performance, it seems that HTM2 is far from from the initial concentration of the solution alone. When the
concentration increased to a certain degree beyond the solu-
bility of HTM, the diffusion stops, thus higher solubility
contributes to better pore-lling of the device. And a lower Tg
could help to increase the pore-lling through a melt inltra-
tion process. The device with a high pore-lling may have a high
coverage of perovskite with HTM, thus yields a higher charge
injection efficiency and lower electron–hole recombination.33
The absorption spectra (Fig. 2a and Table 1) indicate that
HTM1 starts to absorb light (lonset) from l at 405 nm, which is
8 nm blueshied comparing to that of spiro-OMeTAD. While
the lonset value of HTM2 is 451 nm, which is 38 nm redshied
from that of spiro-OMeTAD (46 nm redshied from that of
HTM1). This is attributed to the higher electron density of the
Fig. 1 Chemical structures of spiro-OMeTAD, HTM1 and HTM2.
backbone for HTM2 compared with HTM1.
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HTM1 and 9.0 ꢂ 10ꢀ5 cm2 Vꢀ1 Sꢀ1 for HTM2). However, HTM1
and HTM2 both have a lower mobility than spiro-OMeTAD (3.7 ꢂ
10ꢀ4 cm2 Vꢀ1 Sꢀ1), which suggest that HTM1 and HTM2 based
devices should show slightly lower ll factors.
The new semiconductors HTM1 and HTM2 were tested as
HTMs in perovskite solar cells. The device structure can be seen
from the scanning electron microscope (SEM) image of the
device cross-section (Fig. 3a and b). At the anode, a compact and
mesoporous layer of TiO2 was deposited on top of a uorine-
doped tin oxide (FTO) coated glass and used as the electron
collector, while the cathode consists of a gold electrode ther-
mally evaporated onto the HTM layer. The photoactive material
FAMAPbBrI3 is sandwiched between the anode and the cathode.
The current–voltage (J–V) characteristics of perovskite solar cells
that employ HTM1, HTM2, and spiro-OMeTAD, respectively, as
the HTMs are shown in Fig. 3d and summarized in Table 2. The
PCEs of the best performing devices with HTM1, HTM2, and
spiro-OMeTAD as HTMs are 7.5%, 14.7% and 17.4%, respec-
tively. Correspondingly, the short-circuit current densities (Jsc)
of the cells are 15.8, 21.1 and 23.5 mA cmꢀ2, respectively. It is
clear that Jsc value of HTM2 is as high as that spiro-OMeTAD,
while Jsc of HTM1 is relatively low. The open-circuit voltage (Voc)
of HTM2 (1.09 V) is also higher than that of spiro-OMeTAD. The
only different parameters are the ll factors (FFs). For the device
based on spiro-OMeTAD the FF could achieve 0.72, while FF is
only 0.62 for HTM2, which contributes to the slightly low PCEs
for HTM2. HTM1 has a deeper HOMO energy level, but the Voc
Fig. 2 (a) Normalized UV-vis absorption spectra of HTM1, HTM2 and
spiro-OMeTAD in THF solution; (b) cyclic voltammograms of HTM1,
HTM2 and spiro-OMeTAD in dry dichloromethane–TBAPF6 (0.1 M);
scan speed: 100 mV sꢀ1, potentials vs. Fc/Fc+.
Table 1 Spectroscopic and electrochemical data of HTM1, HTM2 and
spiro-OMeTAD
b
lonset
EHOMO
[eV]
c
HTMs
Tg [ꢁC]
[nm]
Ega [V]
ELUMO [eV]
HTM1
HTM2
Spiro-OMeTAD
80
76
125
405
451
413
3.06
2.75
3.0
ꢀ5.21
ꢀ5.13
ꢀ5.01
ꢀ2.15
ꢀ2.38
ꢀ2.01
a
Estimated from the absorption onset wavelength, Eg (eV) ¼ 1240/lonset
b
ox
onset
c
(nm). Calculated from EHOMO ¼ ꢀ(E
+ 5.10) (eV).34 ELUMO
calculated by ELUMO ¼ EHOMO + Eg.
Cyclic voltammetric measurements were carried out to has not been improved, only 0.79 V. And its FF is also relatively
analyze and compare the energy levels of the two new HTMs and low, 0.46. Those may be caused by its poor mobility, limiting the
spiro-OMeTAD for the favourable hole injection process from the charge transport in the lm.
perovskite layer. Fig. 2b depicts the cyclic voltammograms (CV) in
To investigate the charge transfer at the interface between
dichloromethane solution (1.0 ꢂ 10ꢀ3 mol Lꢀ1) with tetrabuty- the perovskite and the hole-transport materials, steady-state
lammonium hexauorophosphate (TBAPF6, 0.1 mol Lꢀ1) at and time-resolved photoluminescence (PL) spectra were recor-
a scan rate of 100 mV sꢀ1. As can be seen from this gure, the ded and these are shown in Fig. 4. From the steady-state PL
HOMO levels of HTM1 and HTM2 are ꢀ5.21 and ꢀ5.13 eV, spectra, the perovskite lm without HTMs shows a strong PL
respectively. Both are 0.10–0.20 eV deeper than that of spiro- signal due to the recombination between the photo-generated
OMeTAD (ꢀ5.01 eV). The HOMO level of HTM1 is 0.08 eV deeper electrons and holes. Aer deposition of HTMs, a dramatic
than that of HTM2, which is attributed to the higher electron decrease of PL intensity is observed, especially for the HTM2
density of the backbone for HTM2. The deeper HOMO of the new
HTMs could benet the higher open-circuit voltage (Voc) of the
device. In principle, Voc of a conventional solid-state dye sensi-
tized solar cell (DSSC) is determined by the difference between
the quasi-Fermi levels of the electrons in TiO2 and the holes in
the HTM.35 And this is the same for PSCs devices. So from the
electrochemical experiments data, we can expect that the device
using those new HTMs would have a higher Voc than that of spiro-
OMeTAD.
To investigate the charge-carrier mobility of HTM1, HTM2 and
spiro-OMeTAD, hole-only devices were fabricated with device
structure of uorine-doped tin oxide (FTO) glasses/(poly(3,4-
ethylenedioxythiophene):polystyrene, PEDOT:PSS)/doped HTM1
or doped HTM2 (or doped spiro-OMeTAD)/Au. Hole mobilities of
HTM1 and HTM2 were measured via space charge limitation of
current (SCLC) under dark conditions, as showed in Fig. S9.† The
results calculated from the Mott–Gurney law shows that by
Fig. 3 Cross-sectional SEM pictures of PSCs devices based on HTM1
(a) and HTM2 (b); and the relative energy levels in the PSCs (c) and
replacing benzene unit with thiophene unit, the mobility of the
material increased about three times (3.1 ꢂ 10ꢀ5 cm2 Vꢀ1 Sꢀ1 for current–voltage curves (d) of PSCs based on different HTMs.
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Table 2 Summary of device parameters; open-circuit voltage (Voc),
short-circuit current density (Jsc), fill factor (FF) and average efficiency
(PCE)
synthesis of 1 g of HTM1 and HTM2 are 9.3 $ and 8.6 $,
respectively, which are more than 10 times lower than that of
spiro-OMeTAD. The main reason lies in the low cost starting
materials and the simpler synthesis route. Hence it has a great
potential to be commercialized for HTM2.
HTM
Jsc [mA cmꢀ2
]
Voc [V]
FF
PCE [%]
HTM1
HTM2
Spiro-
15.83 ꢃ 1.46
21.13 ꢃ 0.85
22.22 ꢃ 0.53
0.79 ꢃ 0.06 0.46 ꢃ 0.04 6.33 ꢃ 1.21%
1.05 ꢃ 0.04 0.61 ꢃ 0.02 13.93 ꢃ 0.77%
1.04 ꢃ 0.02 0.69 ꢃ 0.05 17.02 ꢃ 0.38%
Conclusions
MeOTAD
In conclusion, two simple triphenylamine-based HTMs were
developed by varying the core structure. It is found that thio-
phene unit played an important role not only in increasing
solubility, but also in enhancing hole mobility and charge
extraction, thus improving the PSCs performance. The PSCs
device based on HTM with thiophene (HTM2) achieved higher
Voc, Jsc and FF values than that of HTM with benzene (HTM1).
These results showed that selecting a proper p-linker is
important for the performance of the HTMs. In addition, both
the two HTMs could be synthesized simply with high yields by
using two steps from commercially available starting materials,
making those HTMs very promising for large-scale industrial
production.
Fig. 4 (a) Photoluminescence (PL) spectra of perovskite film, perov-
skite/spiro-OMeTAD film, perovskite/HTM1 film, perovskite/HTM2
film; (b) time-resolved PL spectra of perovskite/spiro-OMeTAD film,
perovskite/HTM1 film, perovskite/HTM2 film.
Experimental section
Materials
based devices. A more efficient quenching of the photo-
generated charges can lead to faster interface charge separa-
tion in HTM2 based devices than that of HTM1 based devices,
contributing to a better Jsc. However, the lower SCLC hole
mobility limited its charge transporting compared to spiro-
OMeTAD. Time-resolved PL measurements were carried out
with excitation at 530 nm and monitoring the entire emission
spectral range (Fig. 4b). Perovskite/spiro-OMeTAD samples
showed a decay time of 3.5 ns, whereas perovskite/HTM1 and
perovskite/HTM2 samples showed a longer decay lifetime of 4.3
ns and 4.2 ns, respectively. This indicates that HTM2 has
slightly better hole extraction ability as compared to HTM1. The
lower interface charge separation and poorer hole extraction
ability of HTM1 could explain the low performance of the device
based on HTM1 to some extent.
All reagents and solvents, unless otherwise specied, were
purchased from Aladdin, Alfa, Aldrich and J&K Scientic Ltd.
and were used without further purication. 4-Bromo-N,N0-bis(4-
methoxyphenyl)aniline was purchased from Sdyano ne
chemical Co. Ltd. mesoporous TiO2 paste (Dyesol, 30NR-D), CsI,
FAI, PbI2, MABr and PbBr2 were purchased from Xi'an Polymer
Light Technology Corp., and 2,20,7,70-tetrakis(N,N-di-p-methox-
yphenylamine)-9,90-spirobiuorene
purchased from Feiming Chemical Limited.
(spiro-OMeTAD)
was
Measurement and characterization
Routine nuclear magnetic resonance (NMR) spectra were
recorded on a Bruker Avance III 400 MHz (400 and 100 MHz for
1H and 13C NMR, respectively). Chemical shis were reported as
d values (ppm) with tetramethylsilane (TMS) as the internal
standard. The splitting patterns are designated as follows:
singlet (s), doublet (d), triplet (t), and multiplet (m). UV-vis
In order to compare the cost of the synthesized compound
HTM1 and HTM2 with that of spiro-OMeTAD, estimations were
made using a method published by Docampo et al.36 (see the
Table S1†). As shown in Table 3, the cost of materials (including
regents, solvents and other consumables) required for the
absorption spectra were recorded on
a
PerkinElmer
Lambda650 spectrophotometer from dilute solutions in THF.
Cyclic voltammetry (CV) was obtained in a tetrabutylammo-
nium hexauorophosphate (TBAPF6, 0.1 mol Lꢀ1) supported
CH2Cl2 solution at room temperature using a CHI600E elec-
trochemical workstation (CH Instrument Ins.) operated at
a scanning rate of 100 mV sꢀ1. A Pt wire (f ¼ 1.0 mm) embedded
in Teon column was used as the working electrode, and a Pt
sheet and Ag/AgCl electrodes were served as the counter and
reference electrodes, respectively. Ferrocene/ferrocenium was
used as the internal reference to calibrate the redox potentials.
Thermo-gravimetric analysis (TGA) and differential scanning
calorimetry (DSC) were carried out with a Mettler-Toledo TGA
STARe/DSC1 thermal analyzer under puried nitrogen gas ow
Table 3 Comparison of the estimated material costs
Material costa
($ per g)
Commercial price
($ per g)
HTM
HTM1
HTM2
Spiro-MeOTAD
9.3
8.6
—
—
93.6b
173.5–484.9
a
b
Estimated material cost for the synthesis of 1 g of product. Taken
from ref. 36. The cost unit was in dollar ($).
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with a 10 ꢁC minꢀ1 heating rate. For time resolved uorescence reaction process was monitored by TLC. It was found that the
spectroscopy, a time-correlated single photon counting (TCSPC) starting material was consumed aer 20 h. Aer cooled to room
system was used. The samples were excited with a picosecond temperature, the reaction was poured in to CH2Cl2 (100 mL),
diode laser (Edinburgh Instruments, EPL900) at 530 nm. extracted with CH2Cl2, washed with water. The crude product
Photocurrent density–voltage (J–V) curves were measured under was puried by recrystallization with methanol to give product
1
AM 1.5G one sun illumination (100 mW cmꢀ2) with a solar (1.8 g, 80%) as yellow solid. HNMR (400 MHz, CDCl3, d): 7.60
simulator (Enlitech SS-F7-3A) equipped with 300 W xenon lamp (br, 2H), 7.05 (br, 4H), 6.83 (d, J ¼ 8.0 Hz, 6H), 3.80 (s, 6H), 1.32
and a Keithley 2400 source meter. The light intensity was (s, 12H).
adjusted by an NREL-calibrated Si solar cell. The device active
2,5-Bis(4,40-bis(methoxyphenyl)aminophen-400-yl)-benzene
area was 0.08 cmꢀ2. The J–V curves were recorded from ꢀ0.2 V to (HTM1). OMT BPIN (0.97 g, 2.2 mmol), 1,4-dibromobenzene
1.2 V with 0.01 V steps, integrating the signal for 10 ms aer a 10 (0.24 g, 1.0 mmol), Pd(PPh3)4 (0.13 g, 0.12 mmol) was mixed in
ms delay. The average Voc, Jsc and FF were based on 10 devices. degassed THF (10 mL), degassed K2CO3 (2.0 mol Lꢀ1 in H2O,
2.6 mL) aqueous solution was added at the room temperature.
Then the reaction mixture was stirred at reux for 20 h. The
reaction mixture was cooled to RT and poured into CH2Cl2,
extracted with CH2Cl2, washed with water. The CH2Cl2 layer was
dried over sodium sulfate and ltered. The organic solvent was
removed by rotary evaporation, and the residue was puried by
column chromatography (silica gel, petroleum ether/CH2Cl2 ¼ 3/
2, Rf ¼ 0.3) to give product as yellow solid (0.51 g, 75%). 1H NMR
(400 MHz, CD2Cl2) d ¼ 7.63 (s, 4H), 7.50 (d, J ¼ 8.9 Hz, 4H), 7.12
(d, J ¼ 8.8 Hz, 8H), 7.00 (d, J ¼ 8.5 Hz, 4H), 6.89 (d, J ¼ 8.8 Hz,
8H), 3.83 (s, 12H). 13C NMR (100 MHz, CD2Cl2) d ¼ 156.1, 148.2,
140.8, 138.8, 132.3, 127.1, 126.7, 126.5, 120.6, 114.7, 55.4. MS
(HRESI): m/z calcd for C46H40N2O4: 684.2983; found: 684.2985.
2,5-Bis(4,40-bis(methoxyphenyl)aminophen-400-yl)-thiophene
(HTM2). OMT BPIN (0.97 g, 2.2 mmol), 2,5-dibromothiophene
(0.25 g, 1.0 mmol), Pd(PPh3)4 (0.06 g, 0.05 mmol) was mixed in
degassed THF (10 mL), degassed K2CO3 (2.0 mol Lꢀ1 in H2O,
2.6 mL) aqueous solution was added at the room temperature.
Then the reaction mixture was stirred at reux for 12 h. The
reaction mixture was cooled to RT and poured into CH2Cl2,
extracted with CH2Cl2, and then washed with water. The CH2Cl2
layer was dried over sodium sulfate and ltered. The organic
solvent was removed by rotary evaporation, and the residue was
puried by column chromatography (silica gel, petroleum
ether/CH2Cl2 ¼ 3/2, Rf ¼ 0.3) to give product as yellow powder
Device fabrication
Fluorine-doped tin oxide (FTO) glasses (Nippon Sheet Glass)
were cleaned with detergent, deionized water, and acetone
under sonication with ethanol in an ultrasonic bath for 30
minutes. Aer that, FTO glasses were treated in a UV cleaner for
30 minutes. The cleaned FTO glasses were coated with 0.15 M
titanium diisopropoxide bis(acetylacetonate) in 1-butanol solu-
tion by the spin-coating method at 2000 rpm for 60 s, followed by
heating at 125 ꢁC for 5 minutes. The lms were cooled down to
room temperature and a 0.15 M titanium diisopropoxide bis(a-
cetylacetonate) solution in 1-butanol was spin coated again to
make a pin hole free dense TiO2 lm. And then the substrates
were calcined in a box furnace at 450 ꢁC for 30 minutes. Meso-
porous TiO2 (mp-TiO2) layer was deposited on the dense TiO2
layer by spin-coating the TiO2 ethanol solution containing
14.3 wt% TiO2 paste at 4000 rpm for 20 seconds, which was then
calcined at 500 ꢁC for 0.5 hour. CsI (0.05 m), FAI (1 M), PbI2
(1.1 M), MABr (0.2 M) and PbBr2 (0.2 M) was dissolved in
anhydrous DMF:DMSO 9 : 1 (v/v), which was heated at 70 ꢁC for
2 hours under magnetic stirring, and the mixture was then spin
coated on the as-prepared TiO2 lm at 4000 rpm for 40 seconds
a nitrogen atmosphere glove box. During the spin coating step,
100 mL of chlorobenzene was poured on the spinning substrate
15 s prior to the end of the program. Aerward, the as-prepared
1
(0.59 g, 85%). H NMR (400 MHz, DMSO-d6): d ¼ 7.45 (d, J ¼
ꢁ
lms were heated at 100 C for around 60 min until the color
8.6 Hz, 4H), 7.28 (s, 2H), 7.04 (d, J ¼ 8.8 Hz, 8H), 6.92 (d, J ¼
8.9 Hz, 8H), 6.77 (d, J ¼ 9.4 Hz, 4H), 3.74 (s, 13H). 13C NMR
(100 MHz, DMSO-d6): d ¼ 156.3, 148.2, 141.9, 140.3, 127.2,
126.4, 126.0, 123.5, 119.9, 115.4, 55.7. MS (HRESI): m/z calcd for
changed to dark red. The hole-transporting layer (HTL) was
prepared by spin-coating an HTM solution at 4000 rpm for 20 s.
The HTM solution were prepared by dissolving spiro-OMeTAD,
HTM1 and HTM2 in chlorobenzene (60 mM) with 28.8 mL tBP
solution and 17.5 mL Li-bis(triuoromethanesulfonyl) imide
(Li-TFSI)/acetonitrile (520 mg/1 mL). Devices were then le
overnight in air. Finally, a 100 nm thick gold (Au) layer was
thermally evaporated under vacuum of 4 ꢂ 10ꢀ5 Torr, at a rate of
ꢄ0.05 nm sꢀ1, to nish the device fabrication.
C
44H38N2O4S: 690.2547; found: 690.2549.
Conflicts of interest
There are no conicts to declare.
Synthesis
Acknowledgements
N,N-Bis(4-methoxyphenyl)-4-(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)benzenamine (OMT BPIN, 2, Scheme S1†). This work was supported by the National Key Research and
OMT BPIN was synthesized according to the reported litera- Development Project funding from the Ministry of Science and
ture.37 A solution of 4-bromo-N,N-bis(4-methoxyphenyl)aniline Technology of China (Grants No. 2016YFA0202400 and
(2.0 g, 5.2 mmol), bis(pinacolato)diboron (2.0 g, 7.9 mmol), 2016YFA0202404), and the Peacock Team Project funding from
Pd(dppf)Cl2 (0.23 g, 0.31 mmol), and KOAc (1.5 g, 15.6 mmol) in Shenzhen Science and Technology Innovation Committee
dioxane (30 mL) was stirred at 70 ꢁC for 20 h under N2. The (Grant No. KQTD2015033110182370).
45482 | RSC Adv., 2017, 7, 45478–45483
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21 D. Q. Bi, B. Xu, P. Gao, L. C. Sun, M. Graetzel and A. Hagfeldt,
Nano Energy, 2016, 23, 138–144.
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