Chemically doped and cross-linked hole-transporting materials as an efficient anode buffer layer for polymer solar cells

Article abstract of DOI:10.1021/cm2024235

A series of cross-linkable hole-transporting materials (X-HTMs) consisting of indacenodithiophene, bithiophene, and thiophene units bookended by two triarylamine groups have been designed and synthesized to investigate their suitability as new anode buffe

Full text of DOI:10.1021/cm2024235

Article
pubs.acs.org/cm
Chemically Doped and Cross-linked Hole-Transporting Materials as
an Efficient Anode Buffer Layer for Polymer Solar Cells
Ying Sun,†,‡ Shang-Chieh Chien,†,§ Hin-Lap Yip,† Yong Zhang,† Kung-Shih Chen,† David F. Zeigler,⊥
Fang-Chung Chen,§ Baoping Lin,*,‡ and Alex K.-Y. Jen*,†
†Department of Materials Science and Engineering, University of Washington, Seattle, Washington, United States
‡School of Chemistry and Chemical Engineering, Southeast University, Jiangning District, Nanjing, Jiangsu Province, P.R. China
§Department of Photonics & Display Institute, National Chiao Tung University, Hsinchu 30010, Taiwan
⊥Department of Chemistry, University of Washington, P.O. Box 351700, Seattle, Washington 98195, United States
S
* Supporting Information
ABSTRACT: A series of cross-linkable hole-transporting
materials (X-HTMs) consisting of indacenodithiophene,
bithiophene, and thiophene units bookended by two triaryl-
amine groups have been designed and synthesized to
investigate their suitability as new anode buffer layer for bulk
heterojunction polymer solar cells (PSCs). These X-HTMs
can be thermally cross-linked at temperature between 150 and
180 °C to form robust, solvent-resistant films for subsequent
spin-coating of another upper layer. Energy levels of these cross-linked materials were measured by cyclic voltammetry, and the
data suggest that these X-HTMs have desirable hole-collecting and electron-blocking abilities to function as an anode buffer layer
for PSCs. In addition, by incorporating thiophene or fused ring units into the X-HTM backbone, it effectively improved the hole-
carrier motilities. To further improve the conductivity and optical transparency for PSCs, the X-HTM films were p-doped with
nitrosonium hexafluoroantimonate (NOSbF6). The doped X-HTM layers showed remarkably enhanced hole-current densities
compared to neutral X-HTM under the same electric field bias. The properties of the doped X-HTM film as anode buffer layer
has been investigated in PSCs. The resulting devices showed similar performance compared to those made using conducting
polymer, poly(3,4-ethylene- dioxylenethiophene):poly(styrenesulfonate) (PEDOT:PSS), as the anode buffer layer. Moreover, a
novel bilayer HTM structure consisting of a doped and a neutral layer was employed to exploit the feasibility of combining high
conductivity from the doped X-HTM and good electron-blocking ability from the neutral X-HTM together. Interestingly, PSC
devices based on this bilayer structure showed enhanced Voc, Jsc, and FF compared to the devices with only a single-layer doped
X-HTM. These results indicate that such X-HTMs are promising alternative materials to PEDOT:PSS as an anode buffer layer
for polymer solar cells.
KEYWORDS: hole-transporting materials, cross-linking, PEDOT:PSS, anode buffer layer, doping, polymer solar cells
ability of PSCs.13 However, the relatively high acidity of
1. INTRODUCTION
PEDOT:PSS (pH ∼ 1) tends to corrode ITO and cause the
Polymeric solar cells (PSCs) have emerged as a promising
loss of indium, resulting in device degradation.14,15 In addition,
source of alternative energy because of their flexibility, light
the electrical inhomogeneity and varying morphology of the
weight, and the potential of using roll-to-roll processing to
PEDOT:PSS film16,17 combining with its poor electron-
produce low cost solar cells. In general, a PSC consists of a bulk
heterojunction (BHJ) photoactive layer in which the p-type
blocking capability18 limit its application in PSCs.19 For these
reasons, several interfacial layer materials have been explored to
polymer donor and n-type electron acceptor can form a
replace PEDOT:PSS.14,20−26 Recently, a copolymer based on
grafting poly(styrenesulfonate) on polyaniline (PSSA-g-PANI)
has been employed as the hole-collecting anode buffer layer in
PSCs combining both good electrochemical stability and
excellent water solubility on the same material.23 The PSCs
using this copolymer as the interfacial layer exhibited higher Jsc
and PCE compared to those using PEDOT:PSS because of its
bicontinuous interpenetrating network.1−5 Currently, the
record of power conversion efficiency (PCE) of a single-
junction PSC has exceeded 7%.6−10
In most PSCs, the conducting polymer, poly(3,4-ethylene-
dioxylenethiophene):poly(styrenesulfonate) (PEDOT:PSS) is
often used as an anode buffer layer between the indium tin
oxide (ITO) substrate and the BHJ layer to facilitate hole-
collection/extraction11 and improve the surface quality of ITO
to prevent short circuits.12 Due to its high work function (5.0−
5.2 eV), PEDOT:PSS helps form Ohmic contact with the BHJ
to improve the open-circuit voltage (Voc) and charge-collecting
Received: August 15, 2011
Revised: October 9, 2011
Published: October 25, 2011
© 2011 American Chemical Society
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improved optical transparency and electrical conductivity.
Hains et al.20,21 have recently also tried to replace PEDOT:PSS
by cross-linking the blend of poly[(9,9-dioctylfluorene)-co-N-
[(4-(3-methylpropyl)] diphenyl amine] (TFB) and 4,4-bis[(p-
trichlorosilylpropylphenyl) phenylamino] biphenyl (TPDSi2).
Solar cells made with this anode buffer layer also showed
improved performance and thermal stability over those using
PEDOT:PSS in the devices. These encouraging results showed
that new interfacial materials deserve further investigation in
order to improve their properties.
simultaneously. A significantly improved device performance
can be achieved from using this hybrid bilayer structure.
2. RESULTS AND DISCUSSION
2.1. Synthesis. The molecular structures of X-HTMs
studied in this paper are shown in Scheme 1. All the HTMs
Scheme 1. Chemical Structures of HTMs
To be a suitable anode buffer layer for PSC, a material must
meet several requirements. First, it should have adequate
solvent resistance to allow subsequent solution processing of
the upper BHJ layer. Second, its highest occupied molecular
orbital (HOMO) level should be at or slightly above that of the
donor material in the BHJ layer to facilitate hole collection.
Third, its lowest unoccupied molecular orbital (LUMO) level
should be high enough to block electrons.22 Finally, it should
have good electrical conductivity and optical transparency
throughout the visible region of the spectrum.
Triarylamine (TAA) derivatives, such as N,N′-bis(tolyl)-N,N′-
diphenyl-1,1′-biphenyl-4,4′-diamine (TPD), are commonly used
as efficient hole-transporting materials (HTMs) due to their low
ionization potential and good electron-blocking character-
istics.27−30 However, these materials possess relatively low
hole-mobilities31 due to their short conjugation length and
amorphous nature. Conversely, thiophene derivatives have been
incorporated into p-type polymer to improve hole-mobility due
to their better planarity and electron richness.32−35 Therefore, it
is expected that the incorporation of thiophene analogues
should increase mobility of the TAA-based HTMs. Further-
more, if these materials can be doped, it may simultaneously
improve their conductivity and Ohmic contact as has been
successful demonstrated for organic light-emitting diodes.36−42
Motivated by these results, we have designed and prepared a
series of cross-linkable hole-transporting materials (X-HTMs)
by combining triphenylamine with indacenodithiophene
(IDT), bithiophene, and thiophene units. Bearing styryl groups,
the resulting HTMs can be thermally cross-linked at temper-
ature around 150−180 °C to form robust layers for the sub-
sequent spin-coating of active layer. The mobility of X-HTM
derived from cross-linking these materials were determined by
the space-charge-limited-current (SCLC) method in the hole-
only device configuration. It is found that the introduction of
thiophene derivatives into the main chain of X-HTM effectively
increases their carrier mobility.
In order to improve their conductivity and optical trans-
parency, the X-HTLs were doped with a strong oxidant,
NOSbF6. After doping, the absorption spectra of the X-HTLs
red-shifted to the near infrared (NIR) region to provide better
optical transparency in the visible region, while the doped
X-HTLs still maintained good solvent resistance. Hole-only
devices derived from these doped X-HTLs showed significantly
enhanced hole-current density under the same electric field
compared to those derived from neutral X-HTL. With these
combined desirable properties, the doped X-HTLs have been
applied as the anode buffer layer in PSCs. The derived devices
gave significantly enhanced Voc, Jsc, and FF values than those
without a HTL. Furthermore, the PSC with a bilayer device
structure consisting of a doped X-HTL and a neutral X-HTL
underneath has been utilized to combine Ohmic contact and
efficient hole-transporting and electron-blocking properties
are designed to have the linker of dimethylene ether group
(CH2−O−CH2) to provide adequate flexibility for the styryl
groups to react with each other in solid-state. The bulky
n-butyl-phenyl chains were introduced to the IDT unit of IDT-
BTPA to help reduce rigidity of the backbone and lower the
cross-linking temperature.
The synthetic routes for HTMs are depicted in Scheme 2.
Initially, 4-methyltriphenylamine was prepared via a palladium-
Scheme 2. Synthetic Routes of Backbone Compounds for
HTMs
catalyzed Buchwald−Hartwig cross-coupling reaction between
diphenylamine and 4-iodotoluene. Boronic ester compound 3
was obtained by monobrominating 4-methyltriphenylamine in a
one-pot synthesis to afford 2 in 58% yield. Compound 4 was
synthesized by following the reported procedure43 through the
addition of 1-hexylbenzene lithium into the solution containing
the ester to give the corresponding alcohol and then cyclized
subsequently through the acid-mediated reaction. The dibromo
IDT compound 1a was synthesized by bromonating compound
4 using N-bromo succinic imide (NBS).
Scheme 3 illustrates the procedures used for preparing the
X-HTMs. The Suzuki coupling reaction between the boronic
ester 3 and the corresponding dibromo compounds (1a, 1b,
1c) afforded compounds 2a, 2b, and 2c. The compounds
were then diformylated through the Vilsmeier reaction using
POCl3/DMF in dicholoethane to give compounds 3a, 3b, and
3c in ∼70% yield. Reduction of 3a, 3b, and 3c with sodium
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Scheme 3. General Synthetic Routes for HTMs
borohydride in ethanol/THF yielded the corresponding hydroxyl
compounds (4a, 4b, 4c), which were then treated with NaH and
4-vinylbenzyl chloride to afford the final products, IDT-BTPA,
BT-BTPA, and T-BTPA. The structure and purity of these HTM
1
precursors were confirmed by H NMR, 13C NMR, and HRMS
(ESI).
The resulting X-HTMs precursors were very soluble in
common organic solvents including THF, CHCl3, acetone,
chlorobenzene, and dichlorobenzene at room temperature. The
good solubility allows the HTMs to be easily solution processed
to obtain films with tunable thicknesses.
Figure 1. DSC curves of HTMs in the first and the second heating
circle and UV−vis absorption spectrum of X-HTM before rinsing and
after rinsing with chlorobenzene.
2.2. Thermal Cross-Linking and Optical Properties.
The styrene groups on the HTM precursors was used for cross-
linking to facilitate thermal-induced polymerization without
using any initiator.44 To investigate thermal cross-linking of the
HTMs, differential scanning calorimetry (DSC) experiments
were conducted, and the results are shown in Figure 1. The
broad exothermic peaks between 90 and 190 °C in the first scan
indicated the thermal-initiated polymerization of the styrene
groups. No obvious exothermic peaks can be observed in the
second heating of each HTM, indicating the completeness
of the cross-linking reaction after the first heating cycle. In
addition, there is no Tg or melting point that can be observed
during the second heating, which suggests that these cross-
linked HTMs possess good morphological stability.
glovebox. The annealing temperature and time for each HTM
are listed in Table 1. After curing at 180 °C for 1 h, the IDT-
Table 1. Thermal Cross-Linking Conditions and Electrical
Properties for HTMs (X-T: Cross-Linking Temperature;
X-time: Cross-Linking Time)
X-T
X-
λonset
Eg
HOMO
(eV)
LUMO
(eV)
HTM
1-TPD
(°C) time(min)
(nm)
(eV)
150
160
180
180
30
30
30
60
409.4
453.5
497.7
499.7
3.03
2.73
2.49
2.48
−5.25
−5.15
−5.08
−5.06
−2.22
−2.42
−2.59
−2.58
T-BTPA
BT-BTPA
IDT-BTPA
The higher Tmax valeus of IDT-BTPA (∼172 °C) and BT-BTPA
(∼166 °C) are consistent with their more rigid backbones
compared to T-BTPA (∼158 °C) and 1-TPD (∼136 °C). With
increased backbone rigidity, it becomes more difficult for the
styrene groups to encounter each other. Thus, cross-linking
reaction of the more rigid systems needs higher temperatures and
longer time to complete.
Solvent resistance of these HTM was studied by monitoring
the UV/vis spectrum of each cross-linked film before and after
rinsing with chlorobenzene (CB), which is the commonly
used solvent for device processing (Figure 1). The X-HTM film
was spin-coated from its chloroform solution (5 mg/mL) onto
an ITO substrate and thermally cross-linked in a nitrogen-filled
BTPA film exhibited a slight drop-off in the intensity of its
absorption λmax after rinsing with CB. However, the cross-
linked BT-BTPA and T-BTPA films showed excellent solvent
resistance evidenced by identical absorption spectra before
and after rinsing. This good solvent resistance allows the
subsequent spin-coating to be performed on top of the HTL
without eroding it. All the X-HTMs showed absorption in the
range of below 500 nm, suggesting that they have minimal
interference with the absorbance of many low bandgap
polymers (Figure S1 of the Supporting Information). The
main peak at the longer wavelength can be ascribed to the
π−π* transition derived from the conjugation chain. So red-
shift in the absorption peak can be observed with the increasing
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number of thiophene units in the X-HTMs due to the extended
conjugation length from 406 nm for T-BTPA to 431 nm for
BT- BTPA and 445 nm for IDT-BTPA. A shoulder peak
appearing in the absorption of IDT-BTPA indicates good
packing of the polymer chain in the film. The bandgaps of the
HTMs were calculated from the absorption onset (λ onset) and
listed in Table 1.
2.3. Electrochemical Properties. The electrochemical
properties of cross-linked HTMs were investigated by using
cyclic voltammetry (CV) (Figure 2). The CV potentials were
electrochemical stability. The energy levels of HTMs are
summarized in Table 1. The highest occupied molecular
orbital (HOMO) energy level was calculated from the onset
of oxidation potentials (Eox) using the equation of HOMO =
−[Eox + 4.80] eV. The lowest unoccupied molecular orbital
(LUMO) energy level of each HTM was obtained from the
difference between the bandgap and the HOMO energy level
of the HTM. The HOMO energy levels of the X-HTMs are
ranging from −5.06 to −5.15 eV, which is suitable for hole-
collection for most of the donor polymers in PSCs. In
addition, the higher LUMO energy level (−2.42 to −2.58 eV)
of the X-HTMs suggests that they can provide good electron
blocking as the anode buffer layer in the PSCs.
2.4. Doping Properties. To improve conductivity of
X-HTMs, we have performed p-doping on X-HTLs using a
strong oxidant, NOSbF6, as dopant in the solvent of acetonitrile.
Upon doping, electrons were transferred from the HTM to
reduce NO+ and form NO as gas, which can be subsequently
released. The positive charge carriers will be formed on the
−
HTM chain with SbF6 as the counterion.
Different UV−vis absorption spectra of the HTMs under
different doping states were monitored and shown in Figure 3.
Figure 2. Cyclic voltammograms of HTMs (scan rate: 100 mV/s).
measured using ITO substrates as the working electrode and
a Ag/AgCl reference electrode in acetonitrile with 0.1 M of
tetrabutylammonium hexafluorophosphate. The obtained results
were calibrated using a ferrocene/ferrocenium redox couple.
There were two reversible oxidation redox couples with cathodic
peak potentials (Epc) of 0.47 and 0.69 V for IDT-BTPA and 0.51
and 0.81 V for BT-BTPA but only one couple at 0.45 V (Epc)
for T-BTPA. For comparison, the electrochemical behavior of
1-TPD was also investigated. For 1-TPD, two peaks always
appeared during the cathodic sweep while the first and second
peaks in the anodic sweep overlapped with each other.
There have been numerous studies for the electrochemical
properties of triphenylamine derivatives due to their applica-
tions in organic electronics.45−48 It was found that multiple
oxidation peaks appeared when the coupling between the redox
centers was strong and electrons were delocalized throughout
the entire system. On the contrary, these oxidative peaks
overlapped completely when the interactions between localized
redox centers were weak.49,50 Therefore, the single oxidation
redox couple for T-BTPA may be due to the weaker electronic
coupling between two redox amino centers as a result of the
insertion of a thiophene spacer compared to 1-TPD. However,
for IDT-BTPA and BT-BTPA, the other amine probably
cannot be readily oxidized after the formation of one amino
redox center due to the increased spacer, more rigid
bithiophene, IDT units, and the two oxidative peaks probably
due to the two successive electron removal processes.50
Figure 3. UV−vis absorption of the HTMs at different doping stages.
Dopant concentration: 1-TPD 0.5 mg/mL, IDT-BTPA 0.2 mg/mL,
BT-BTPA 0.5 mg/mL, T-BTPA 0.5 mg/mL; rinsing solvent:
chlorobenzene.
The HTMs exhibited absorption between 300 and 500 nm and
were transparent in the NIR region in their neutral form as
mentioned before. Take 1-TPD as an example, in the initial
stage of the doping experiment, the intensity of the original
absorption peak decreased with the formation of new absorption
peaks at about 500 nm and another broad absorption band
(1000−2000 nm) appeared in the NIR region. The formation of
a monocation radical (polaron charge carrier) leads to the new
red-shifted absorption peak of ∼500 nm while the new
absorption band in the NIR region is due to the intervalence
charge transfer (IV-CT) between nonequivalent states related to
the positive charges in different amino centers.51 Upon further
doping, the polaron and IV-CT absorption band decreased
gradually while another new broad band grow up at about 600−
1000 nm which is due to the formation of a dication (bipolaron
charge carrier).52 At the fully doped state, the previous π−π*
peak (350 nm) disappeared while the main absorption is from
the bipolaron band. For the other three HTMs, the similar trend
The first oxidation potentials of IDT-BTPA and BT-BTPA
were lower than that of T-BTPA, which may be attributed to
the increased electron density in their backbones as more
electron-rich IDT and bithiphene units were introduced.
The cyclic voltammograms showed minimal changes after
ten cycles, indicating all these X-HTMs have very good
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of the UV−vis absorption change can be observed at different
doping stages with obvious color change that can be observed
by naked eye.
One of the critical factors for material to be used as an anode
buffer layer in OPV is the electrical conductivity, which will
affect both the Ohmic contact at the electrode and the electrical
resistance across the device. To investigate the doping effect on
the conductivity, hole-only devices incorporating doped HTLs
were fabricated. As shown in Figure 4c and d, the same trend
can be observed for four different doped HTLs. Significantly
higher current density can be obtained via doping compared to
the neutral HTLs. At the electrical field of 1 V/m, the current
density increased from 10−5−10−4 mA/cm2 for the neutral
HTLs to the range of 10−2−10−1 mA/cm2 for the doped HTLs.
These results revealed the pronounced effects of doping on
hole-transporting properties of HTLs due to increased charge
carrier number density and conductivity. The charge mobility
of HTMs were extracted by fitting the log−log current
density−voltage (J−V) curves and employing the SCLC
equation of J = 9ε0εrμV2/8L3, where J is the current density
(mA/cm2), ε0εr is the permittivity of HTM, μ is the mobility,
and L is the thickness of the HTL. The mobility of the
X-HTMs increased with more thiophene or fused rings
introduced onto the backbone of the X-HTMs. The hole-
mobility of IDT-BTPA was determined to be 1.26 ± 0.14 ×
10−4 cm2/V·s which is much higher than that of 1-TPD (6.35 ±
0.32 × 10−6 cm2/V·s). BT-BTPA also exhibited a higher
mobility (6.29 ± 0.73 × 10−5 cm2/V·s) compared to T-BTPA
(2.03 ± 0.25 × 10−5 cm2/(V·s). The increase of mobility can be
explained by the more rigid and planar structure as well as
strong intermolecular π−π interactions.
However, for different HTMs to be fully doped, the required
dopant concentration and time varied due to the possibility for
charge transfer and the diffusivity of the dopant in the matrix
films. The lower dopant concentration required to fully dope
the IDT-BTPA film may be related to its stronger electron-
donating ability and better diffusion of NOSbF6 due to the
relatively loosely packed film resulting from the branched alkyl
chains on the backbone.
For IDT-BTPA and BT-BTPA, the main absorption bands
were located in the NIR region when they were fully doped,
which provided good optical transparency with minimal
absorption loss of the active light-harvesting layer. Moreover,
significant increase in charge carrier density in the fully doped
form can result in high conductivity, which is important to the
reduction of Ohmic loss at the hole-collection contact. It is
worthy to mention that the absorption spectra of the doped
HTM films remained similar after washing several times with
CB. This indicated that the doped X-HTLs possess good solvent
resistance which is critical to multilayer solution processing of
PSCs.
2.5. Charge-Transporting Properties. In order to
investigate the hole-transporting properties of the HTMs,
hole-only devices were fabricated with the configuration of
ITO/PEDOT:PSS(40 nm)/HTM/MoO3/Al.53 Figure 4 shows
2.6. Interfacial Properties as the Anode Buffer Layer
in Photovoltaic Cells. To evaluate the suitability of these
newly developed HTMs to function as an anode buffer layer,
PSCs solar cells were first fabricated with the configuration of
ITO/HTL/polymer:PC71BM/Ca/Al, where a low bandgap
copolymer PIDTPhTQ was selected as the p-type donor and
PC71BM was chosen as the acceptor (Figure 5).59 IDT-BTPA
Figure 4. Current density−electric field (J−E) curves of the hole-only
devices incorporating various HTMs without doping (a and b) and
with doping (c and d) in linear (a and c) and log−log (b and d) plots.
the current density−electric field (J−E) curves of these hole-
only devices in linear-representing and log−log plots measured
in a dark environment. As observed in J−E curves, these newly
developed HTMs (IDT-BTPA, BT-BTPA, and T-BTPA)
exhibit comparatively higher current density compared to that
of 1-TPD under the same electric field bias. From earlier
studies, the IDT-based alternating copolymers were reported
to exhibit remarkable hole-mobility (up to 1.0 cm2/V·s) due
to their coplanar structure and strong intermolecular π−π
interactions.54−58 As expect, IDT-BTPA showed the best hole-
transporting property while BT-BTPA incorporating a bithio-
phene unit had better hole-transporting properties than T-BTPA,
which has only a single thiophene ring.
Figure 5. Chemical structure of the active polymer and OPV device
architecture with different anode buffer layer (without buffer layer;
PEDOT:PSS; single doped layer; bilayer).
and BT-BTPA were selected for study due to their superior
charge-transporting abilities over T-BTPA and 1-TPD. For
comparison, typical devices with bare ITO and PEDOT:PSS
were also fabricated in parallel. The detailed fabrication process
is depicted in the Experimental Section. The detailed device
results based on various anode buffer layers with different
thicknesses are listed in Table 2, and the representative J−V
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Table 2. Photovoltaic Properties of Polymer Solar Cells Incorporating Different Anode Buffer Layers with Different
a
Thicknesses
Active Polymer
Anode Buffer Layer
bare ITO
Voc (V)
Jsc (mA/cm2)
FF (%)
PCE (%)
SD (%)
PIDTPhTQ
PIDTPhTQ
PIDTPhTQ
PIDTPhTQ
PIDTPhTQ
PIDTPhTQ
PIDTPhTQ
PIDTPhTQ
PIDTPhTQ
PIDTPhanQ
PIDTPhanQ
0.42
0.81
0.80
0.75
0.57
0.77
0.79
0.79
0.81
0.85
0.85
8.27
10.09
9.40
45.0
56.0
49.2
47.2
52.6
52.6
49.1
50.3
55.5
61.0
66.0
1.56
4.58
3.70
2.65
0.86
3.94
3.73
3.80
4.53
5.55
5.32
0.21
0.15
0.31
0.25
0.22
0.37
0.32
0.32
0.15
0.16
0.17
PEDOT:PSS
doped BT-BTPA (7 nm)
doped BT-BTPA (15 nm)
doped BT-BTPA (20 nm)
doped IDT-BTPA (7 nm)
doped IDT-BTPA (15 nm)
doped IDT-BTPA (20 nm)
bilayer
7.48
2.89
9.74
9.66
9.51
10.08
10.74
9.53
PEDOT:PSS
bilayer
a
Bilayer: BT-BTPA (7 nm)/doped IDT-BTPA (20 nm). SD standard deviation.
curves of the optimized devices for each HTL are shown in
Figure 6. The device without any buffer layer showed poor
Figure 6. (a) Current density−voltage (J−V) curves. (b) Dark current
characteristics of the photovoltaic devices incorporating various anode
buffer layers. (Device configuration): ITO/HTL/PIDTPhTQ:PC71BM
(1:3)/Ca/Al.
performance with relatively low Voc (0.42 V), FF (45%), and
PCE (1.56%). In this case, the inferior Voc may be determined
by the anode−cathode work function differential instead of the
difference between the HOMO level of the polymer and the
LUMO level of PC71BM, which indicated the poor contact
between the bare ITO and BHJ layers. Upon addition of the
doped HTL (IDT-BTPA or BT-BTPA), the devices gave rise to
significantly improved Voc as well as higher Jsc and FF. The
obviously enhanced performance of the device after incorporat-
ing doped HTLs represented that the X-HTLs can effectively
create Ohmic contact at the anode side for achieving higher Voc
and facilitating charge collection to give higher Jsc. Moreover,
the doping processes potentially induce the gradual shift of the
HTM Fermi level toward its HOMO level and therefore reduce
interface energy barrier.40
Since the film morphology of the anode buffer layer is very
crucial for PSC application because slight defects such as
aggregates or pinholes can easily diminish device performance,
atomic force microscopy (AFM) was employed to investigate
the surface morphology of the X-HTLs film. As shown in
Figure 7, the HTMs form smooth and uniform films on ITO
substrates with the rms roughness of 0.67 and 0.48 nm for IDT-
BTPA and BT-BTPA, respectively. Therefore, the X-HTL can
effectively suppress the formation of uneven morphology due
to the rough ITO surface (rms: 2.41 nm)60 and enable the
active layer to achieve optimal morphology. The morphology of
the blend of polymer and PC71BM on the doped HTL was also
Figure 7. AFM images (5 μm × 5 μm) for the X-IDT-BTPA (a), X-
BT-BTPA (b) layer and the active layer of PIDTPhTQ: PC71BM
(1:3) on doped X-IDT-BTPA (c) and X-BT-BTPA (d).
investigated by AFM. The photoactive layer film exhibited a
smooth surface with roughness of ca. 1.29 and 1.28 nm on the
doped IDT-BTPA and BT-BTPA layer, respectively, and no
obvious phase separation can be observed.
Thickness is another critical factor for functioning as buffer
layers since it can influence the electrical characteristics of
OPVs. Therefore, we also investigated the dependence of
doped HTL thickness on device performance. The summarized
device performance is shown in Table 2. It can be seen that
IDT-BTPA showed minor effect of thickness variation on the
device performance while thickness has a huge effect on the
device characters for BT-BTPA. Thicker HTL induced poorer
device performance. Since equal dopant concentration and time
were used to dope HTL with different thicknesses, these
phenomena can be attributed to the comparatively different
doping degree in the HTL applied to the device. As previously
mentioned, IDT-BTPA can be readily doped; therefore, even
thicker film can be fully doped to reach the required
conductivity for efficient charge transport. However, BT-
BTPA is more difficult to fully dope; thereby a thick film
may remain with low conductivity at the interface between the
active layer and HTL to inhibit the efficient charge transport. In
addition, the inferior hole-transporting ability of the BT-BTPA
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Chemistry of Materials
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may cause it to be more dependent on the thickness of the
HTL compared to the IDT-BTPA.
these X-HTMs. Hole-transporting properties of these X-HTMs
were investigated in PSCs. A bilayer hybrid HTL structure with
one doped layer on top of the neutral layer has been employed
to provide both efficient hole-transporting and electron-blocking
properties. As expected, the device based on the bilayer hybrid
HTL structure gave much increased Jsc and FF than the device
using only the single layer X-HTL. The performance of the
devices reaches almost the same as the devices using
PEDOT:PSS as the HTL. The development of these X-HTMs
provides an excellent opportunity to achieve simultaneously good
solvent resistance, hole-transport, interfacial contact with
minimized energy barrier, conductivity, and electron-blocking
ability which are all very important for the future development of
highly efficient anode buffer layer for PSCs.
In addition to device performance, a more detailed
understanding of the function of HTMs was realized by
investigating the J−V curves of the devices measured in the
dark (Figure 6b). Surprisingly, the device incorporating doped
HTMs featured serious leakage current under reverse-bias and
small forward-bias, although the injection (forward-bias from
1 to 2 V) showed comparable carrier injection ability compared
to the device based on PEDOT:PSS as a buffer layer. This
leakage current causes the device to have lower FF and Jsc. We
speculate that the thin, doped HTLs behave more metallic-like
property than the semiconducting property. The semiconduct-
ing, neutral form HTL is critical for providing better-defined
energy bands to improve charge blocking effect.61 In order to
further improve the electron-blocking properties of these
HTMs, a bilayer structure with the doped IDT-BTPA layer
on top of the neutral BT-BTPA layer was introduced as the
hybrid HTL system in the devices. Since BT-BTPA requires
much higher dopant concentration than that for IDT-BTPA,
the IDT-BTPA can be fully doped while controlling BT-BTPA
to remain neutral through the manipulation of the dopant
concentration. As shown in Figure S2 (in the Supporting
Information), the original absorption peak of the bottom BT-
BTPA in the bilayer structure remained the same, while the
upper IDT-BTPA layer was fully doped. The fully doping of
IDT-BTPA was confirmed by comparing its NIR absorption
with that of the doped single layer IDT-BTPA. Therefore, the
doped IDT-BTPA can provide the needed Ohmic contact as we
discussed before while the neutral thin BT-BTPA layer can
provide electron-blocking due to its higher LUMO energy level.
In other words, this design strategy takes advantage of various
doping degree between IDT-BTPA and BT-BTPA.
Employing this bilayer structure, higher Voc, Jsc, and FF could
be achieved and consequently the efficiency improved to be
almost the same as the device using PEDOT:PSS as an anode
buffer layer. The increased electron-blocking ability can also be
verified by the dark current measurements. Figure 6b showed
that the bilayer structure exhibited decreased current density
under the same bias compared to single layer doped HTL. In
order to realize the full potential of using these novel X-HTMs
for PSCs, another high-performance BHJ system consisted of
PIDTPhanQ (Figure 5) and PC71BM was used as the BHJ
active layer in the devices with the configuration of either ITO/
Bilayer or PEDOT:PSS/PIDTPhanQ:PC71BM (1:3)/Ca/Al.58
The J−V curves of the devices under illumination were shown in
Figure S3 of the Supporting Information, and the detailed device
performance parameters were listed in Table 2. Consistently, the
same Voc (0.85 V) as well as higher FF (66%) and comparatively
similar Jsc (ca. 10 mA/cm2) compared to the typical
PEDOT:PSS device strongly indicated that the bilayer HTLs
can form good Ohmic contact and provide efficient hole-
transporting ability, thereby achieve the high PCE of 5.3%.
EXPERIMENTAL SECTION
■
Materials. All chemicals, unless otherwise specified, were
purchased from Aldrich and used as received. Toluene and THF
were freshly distilled from sodium and benzophenone ketyl under
nitrogen prior to use. Diethyl 1,4-bis(thiophen-2-yl)-2,5-benzenedi-
carboxylate43 and 5,5′-dibromo-2,2′-bithiophene62 were prepared as
reported.
Characterization. 1H and 13C NMR spectra were recorded using
Bruker AV 300 or 500 spectrometers operating at 300 or 125 MHz in
deuterated chloroform solution with TMS as reference. ESI-MS
spectra were carried out on a Bruker APEX Qe 47e Fourier transform
(ion cyclotron resonance). UV−vis spectra were studied using a
Perkin-Elmer Lambda-9 spectrophotometer. The differential scanning
calorimetry (DSC) was performed using DSC2010 (TA Instruments)
under a heating rate of 10 °C min−1 and a nitrogen flow of 50 mL
min−1. Cyclic voltammetry (CV) measurements of the cross-linked
HTMs films were conducted on a BAS CV-50W voltammetric system
with a three-electrode cell in acetonitrile with 0.1 M of
tetrabutylammonium hexafluorophosphate using a scan rate of 100
mV·s−1. ITO, Ag/AgCl, and Pt mesh were used as working electrode,
reference electrode, and counter electrode, respectively. The AFM
images under tapping mode were recorded on a Veeco multimode
AFM with a Nanoscope III controller.
Synthesis of Backbone Compounds. 4-Methyltriphenyl-
amine (Compound 1). A solution of diphenylamine (8.46 g,
50 mmol), 4-iodotoluene (11.99 g, 55 mmol), Pd2(dba)3 (0.92 g,
1 mmol), 1,1′-bis(diphenylphosphino) ferrocene (0.83 g, 1.5 mmol),
and t-BuONa (7.21 g, 75 mmol) in toluene (350 mL) was stirred at
110 °C for 24 h. After cooling to room temperature, the resulting
mixture was quenched with water (500 mL) and extracted with
CH2Cl2. The combined organic solution was dried over Na2SO4, and
the solvent was removed in vacuo. The residue was purified by column
chromatography on silica gel (eluent: hexane/ethyl acetate = 10/1) to
1
give a light yellow solid (9.23 g, 75%). H NMR (CDCl3, 300 MHz,
ppm): δ 7.30−7.22 (m, 4H), 7.12−7.07 (m, 6H), 7.05−6.97 (m, 4H),
2.33 (s, 3H). 13CNMR (CDCl3, 125 MHz, ppm): δ 148.0, 145.2,
132.7, 129.9, 129.1, 124.9, 123.6, 122.2, 20.8.
N-(4-Bromophenyl)-N-(4-methylphenyl)aniline (Compound 2). Com-
pound 1 (7.77 g, 30 mmol) and NBS (5.87 g, 33 mmol) were
dissolved in chloroform (50 mL). The mixture was then stirred under
dark at 0 °C for 24 h. The resulting mixture was washed by brine and
dried over Na2SO4. Solvent was evaporated under reduced pressure to
give the crude product which was purified by silica gel chromatography
to afford a liquid (9.11 g, 90%). 1H NMR (CDCl3, 300 MHz, ppm): δ
7.37−7.23(m, 4H), 7.15−7.02(m, 6H), 7.01−6.92(m, 3H), 2.34 (s,
3H). 13C NMR (CDCl3, 125 MHz, ppm): δ 132.98, 132.67, 132.47,
132.34, 131.49, 130.68, 130.52, 130.43, 130.35, 129.98, 129.71, 126.06,
125.84, 125.63, 125.53, 125.35, 125.19, 124.98, 124.88, 124.40, 21.52.
N-(4-Methylphenyl)-N-phenyl-4-(4,4,5,5-tetramethyl-1,3,2-dioxa-
borolan-2-yl)benzenamine (Compound 3). Compound 2 (7.00 g,
20.8 mmol) was dissolved in THF (100 mL) in a flame-dried 250 mL
flask at −78 °C. n-Butyllithium (8.31 mL, 2.5 M in hexane) was then
added dropwise to the resulting solution. The mixture was stirred at
CONCLUSION
■
A new class of thermally cross-linkable hole-transporting
materials have been designed and synthesized. The cross-linking
condition, optical, electrical, and electrochemical properties of
these X-HTMs were studied in details to investigate their
suitability to function as anode buffer layer for PSCs. The hole-
mobilities of the newly developed HTMs have been tuned
systematically through rational molecular design. p-Doping was
employed to improve conductivity and optical transparency of
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Chemistry of Materials
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−78 °C for 1 h and 2-iso-propoxy-4,4,5,5-tetramethyl-1,3:2-dioxaboro-
lane (5.1 mL, 24.9 mmol) was added into the solution. Then, the
resulting mixture was slowly warmed to room temperature and stirred
overnight. The mixture was poured into water, extracted with CH2Cl2,
and dried over Na2SO4. Solvent was removed under reduced pressure,
and the residue was purified by silica gel chromatography to afford a
2,7-Bis[4-(4′-methyl-4″-formyltriphenylamine)]-4,9-dihydro-
4,4,9,9-tetrakis(4-butylphenyl)-s-indaceno[1,2-b:5,6-b′]dithiophene
(Compound 3a). To a solution of 6 (500 mg, 0.38 mmol) and
DMF (0.1 mL) in 1,2-dichloroethane (8 mL) was slowly added POCl3
(0.12 mL). The mixture was stirred at 90 °C for 24 h. After cooling to
room temperature, the solution was poured to 0.25 M NaOAc solution
(10 mL) and stirred for 30 min. The mixture was extracted with
CH2Cl2 and dried over Na2SO4. After the removal of organic solvent
under reduced pressure, the residue was directly purified by column
chromatography on silica gel (eluent: hexane/ethyl acetate = 5/1) to
obtain product 7 (350 mg, 68%). 1H NMR (CDCl3, 300 MHz, ppm):
δ9.83 (s, 2H), 7.69 (d, J = 8.4 Hz, 4H), 7.54 (d, J = 9.0 Hz, 4H), 7.44
(s, 2H), 7.27−7.19 (m, 12H), 7.19−7.13 (m, 6H), 7.13−7.00 (m,
16H), 2.58 (t, 8H), 2.38 (s, 6H), 1.62 (m, 8H), 1.40 (m, 8H), 0.96 (t,
12H). 13C NMR (CDCl3, 125 MHz, ppm): δ190.44, 156.83, 153.21,
153.13, 146.14, 145.35, 143.26, 141.85, 141.47, 140.36, 135.42, 135.25,
131.40, 131.35, 130.52, 129.14, 128.40, 127.94, 126.57, 126.44, 125.91,
119.42, 118.95, 117.23, 63.08, 35.26, 33.56, 29.73, 22.51, 21.02, 14.00.
HRMS (ESI) (M+, C96H88N2O2S2): calcd 1364.6287; found
1364.6282.
2,7-Bis[4-(4′-methyl-4″-hydroxymethyl triphenylamine)]-4,9-di-
hydro-4,4,9,9-tetrakis(4-butylphenyl)-s-indaceno[1,2-b:5,6-b′]-
dithiophene (Compound 4a). To a solution of 7 (350 mg, 0.28
mmol) and methanol (2 mL) in THF (10 mL) was added NaBH4
(77.4 mg, 2.05 mmol). The mixture was refluxed under N2 for 24 h.
Water was added carefully to quench the reaction, and the mixture was
extracted with chloroform. After the removal of the solvent, the
resulting solid (319 mg, 91%) was dried and directly used for next
reaction without further purification. 1H NMR (CDCl3, 300 MHz,
ppm): δ 7.46 (m, 6H), 7.25 (m, 10H), 7.18 (s, 2H), 7.12 (m, 18H),
7.05 (m, 10H), 4.68 (s, 4H), 2.58 (t, 8H), 2.38 (s, 6H), 1.62 (m,
10H), 1.40 (m, 8H), 0.96 (t, 12H).
2,7-Bis[4-[4′-methyl-4″-[[(4-ethenylphenyl) methoxy]methyl] tri-
phenylamine]]-4,9-dihydro-4,4,9,9-tetrakis(4-butylphenyl)-s-
indaceno[1,2-b:5,6-b′]dithiophene (IDT-BTPA). To a solution of 8
(319 mg, 0.23 mmol) in dry DMF (5 mL) was added NaH (19 mg,
0.78 mmol). The mixture was stirred at room temperature for 1 h. And
4-vinylbenzyl chloride (0.1 mL, 0.61 mmol) was added to above
solution by syringe. The mixture was heated at 60 °C for 24 h.
And water was added to quench the reaction and extracted with
CH2Cl2. The organic layer was dried over Na2SO4 after washing with
water. After the removal of CH2Cl2, the residue was directly puri-
fied by column chromatography on silica gel (eluent: hexane/ethyl
acetate =9/1) to give the product 9 (170 mg, 68%) as a yellow solid.
1H NMR (CDCl3, 300 MHz, ppm): δ7.42 (br, 8H), 7.35 (br, 8H),
1
light yellow solid (4.64 g, 58%). H NMR (CDCl3, 300 MHz, ppm):
7.66 (d, J = 8 Hz, 2H), 7.26 (m, 4H), 7.01−7.12 (m, 7H), 2.34 (s,
3H), 1.33 (s, 12H). 13C NMR (CDCl3, 125 MHz, ppm): δ150.8,
147.3, 135.8, 129.3, 125.4, 123.7, 122.1, 83.8, 23.6, 20.0. HRMS (ESI)
(M+, C25H28BNO2): calcd 385.2213; found 385.2208.
4,4,9,9-Tetrakis(4-butylphenyl)-4,9-dihydro-s-indaceno[1,2-b:5,6-
b′]dithiophene (Compound 4). To a solution of 1-bromo-4-
butylbenzene (2.41 g, 11.3 mmol) in THF (50 mL) was added
dropwise n-BuLi (4.54 mL, 2.5 M in hexane, 11.3 mmol) at −78 °C.
The resulting solution was stirred for 1 h and then quenched with a
solution of diethyl 1,4-bis(thiophen-2-yl)-2,5-benzenedicarboxylate
(0.88 g, 2.27 mmol) in THF (15 mL). The reaction was kept at
−78 °C for 1 h and slowly warmed to room temperature for stirring
overnight. Water was added to the solution, and the mixture was
extracted with ethyl acetate. The combined organic layer was dried
with Na2SO4, and then, solvent was removed by rotary evaporation.
The resulting white solids were added to acetic acid (50 mL), and the
mixture was heated up to reflux and was treated with two drops of
concentrated H2SO4. The reaction was allowed to reflux for 5 h and
then quenched with water (150 mL). The mixture was extracted with
CH2Cl2, and the combined organic layer was washed with water 3
times. The solution was dried over Na2SO4, and the solvent was
removed under reduced pressure. The residue was purified by silica gel
chromatography to afford a white solid (1.10 g, 61%). 1H NMR
(CDCl3, 300 MHz, ppm): δ 7.45 (s, 2H), 7.25 (d, J = 4.89 Hz, 2H),
7.17 (d, J = 8.28 Hz, 8H), 7.06 (d, J = 8.31 Hz, 8H), 7.01 (d, J = 4.89
Hz, 2H), 2.58 (t, 8H), 1.65 (m, 8H), 1.42 (m, 8H), 0.98 (t, 12H). 13C
NMR (CDCl3, 125 MHz, ppm): 156.05, 153.60, 142.27, 141.57,
141.47, 135.30, 128.48, 128.10, 127.59, 123.33, 117.69, 62.85, 35.78,
31.94, 31.57, 29.37, 22.82, 14.32.
2,7-Dibromo-4,9-dihydro-4,4,9,9-tetrakis(4-butylphenyl)-s-
indaceno[1,2-b:5,6-b′]dithiophene (Compound 5). Compound 4
(794 mg, 1 mmol) and NBS (392 mg, 2.2 mmol) were dissolved in
chloroform (30 mL). The reaction mixture was stirred in the dark for
24 h. The solution was washed with brine and dried over Na2SO4. The
resulting solid by removing chloroform was purified by silica gel
chromatography (eluent: hexane) to afford 5 as a white solid (865 mg,
1
91%). H NMR (CDCl3, 300 MHz, ppm): δ 7.34 (s, 2H), 7.07−7.12
(m, 16H), 7.00 (s, 2H), 2.58 (t, 8H), 1.57 (m, 8H), 1.38 (m, 8H), 0.96
(t, 12H). 13C NMR (CDCl3, 125 MHz, ppm): 157.72, 153.77, 147.55,
142.66, 141.35, 141.15, 134.93, 130.81, 128.39, 128.20, 117.96, 62.33,
35.79, 31.95, 31.55, 29.39, 22.82, 14.32. HRMS (ESI) (M+,
C55H56Br2S2): calcd 950.2190; found 950.2185.
7.21 (br, 12H), 7.08 (m, 24H), 6.74 (m, 2H), 5.76 (d, J = 17.6 Hz,
2H), 5.26 (d, J = 8.67 Hz, 2H), 4.59 (s, 4H), 4.50 (s, 4H), 2.58
(t, 8H), 2.35 (s, 6H), 1.60 (m, 8H), 1.38 (m, 8H), 0.96 (t, 12H). 13C
NMR (CDCl3, 125 MHz, ppm): δ 156.73, 153.11, 147.28, 147.21,
146.87, 144.81, 142.06, 141.38, 139.65, 138.00, 137.06, 136.62, 135.26,
133.25, 132.42, 130.08, 129.13, 128.89, 128.40, 128.10, 128.02, 126.33,
126.12, 125.16, 123.80, 123.31, 118.20, 117.10, 113.86, 71.99, 71.88,
63.08, 35.31, 33.59, 22.55, 20.93, 14.06. HRMS (ESI) (M+,
Synthesis of IDT-BTPA. 2,7-Bis[4-(4′-methyltriphenylamine)]-
4,9-dihydro-4,4,9,9-tetrakis(4-butylphenyl)-s-indaceno[1,2-b:5,6-
b′]dithiophene (Compound 2a). Compound 3 (532 mg, 1.38 mmol)
and 1a (570 mg, 0.6 mmol) were dissolved in dry toluene (15 mL)
under N2. And then 2 M K2CO3 (4 mL) solution was added, and the
mixture was bubbled with N2 for 30 min before the addition of
Pd(PPh3)4 (20 mg). After stirring overnight at 110 °C, the reaction
mixture was cooled to room temperature and extracted with CH2Cl2.
The combined organic layer was dried over Na2SO4, concentrated
under reduced pressure, and purified by silica gel chromatography
(eluent: hexane/ethyl acetate = 10/1) to afford a yellow solid (550 mg,
C
114H108N2O2S2): calcd 1600.79; found 1600.79.
Synthesis of BT-BTPA. Compound 2b. Compound 2b was
prepared from compound 3 and 5,5′-dibromo-2,2′-bithiophene using
1
the same method as 2a with a yield of 75%. H NMR (CDCl3, 300
MHz, ppm): δ7.46 (d, J = 8.52 Hz, 4H), 7.28 (m, 4H) 7.13 (m, 12H)
7.08−7.01(m, 10H) 2.36(s, 6H). 13C NMR (CDCl3, 125 MHz, ppm):
147.60, 147.54, 144.85, 142.94, 135.91, 133.27, 130.08, 129.28, 127.67,
126.33, 125.25, 124.26, 124.13, 123.05, 122.81, 122.76, 20.91. HRMS
(ESI) (M+, C46H36N2S2): calcd 680.2320; found 680.2314.
1
70%). HNMR (CDCl3, 300 MHz, ppm): δ 7.47 (d, J = 90 Hz, 4H),
7.44 (s, 2H), 7.30−7.22 (m, 10H), 7.18 (s, 2H), 7.15−7.08 (m, 18H),
7.08−7.02 (m, 10H), 2.58 (t, 8H), 2.38 (s, 6H), 1.62 (m, 8H), 1.40
(m, 8H), 0.96 (t, 12H). 13C NMR (CDCl3, 125 MHz, ppm): δ 156.68,
153.07, 147.63, 147.35, 146.84, 144.88, 142.04, 141.35, 139.59, 135.23,
133.16, 130.04, 129.25, 128.78, 128.37, 128.00, 126.66, 126.10, 125.13,
124.00, 123.22, 122.70, 118.16, 117.07, 63.05, 35.28, 33.57, 22.53,
20.90, 14.03. HRMS (ESI) (M+, C94H88N2S2): calcd 1308.6388; found
1308.6383.
Compound 3b. Compound 3b was prepared from compound 2b
using the same method as 3a with a yield of 68%. H NMR (CDCl3,
1
300 MHz, ppm): δ 9.84 (s, 2H), 7.71 (d, J = 8.61 Hz, 4H), 7.56 (d, J =
8.49 Hz, 4H), 7.18 (m, 12H), 7.10 (m, 8H), 2.39 (s, 6H). HRMS
(ESI) (M+, C48H36N2O2S2): calcd 736.2218; found 736.2213.
Compound 4b. Compound 4b was prepared from compound 3b
1
using the same method as 4a with a yield (72%). H NMR (CDCl3,
300 MHz, ppm): δ 7.46 (d, J = 8.60 Hz, 4H), 7.25 (d, J = 8.46 Hz,
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Chemistry of Materials
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4H), 7.12 (m, 12H) 7.06 (m, 8H), 4.63 (s, 4H), 2.36 (s, 6H), 1.56 (m,
2H). 13C NMR (CDCl3, 125 MHz, ppm): 147.60, 147.54, 144.85,
142.94, 135.91, 133.27, 130.08, 129.28, 127.67, 126.33, 125.25, 124.26,
124.13, 123.05, 122.81, 122.76, 20.91.
ASSOCIATED CONTENT
■
S
* Supporting Information
UV−vis absorption of the cross-linked HTMs in film and
controlled doped bilayer, current density−voltage curves of the
PSCs devices of ITO/PEDOT:PSS or bilayer/PIDT-
PhanQ:PC71BM (1:3)/Ca/Al. This material is available free
of charge via the Internet at http://pubs.acs.org.
Compound BT-BTPA. Compound BT-BTPA was prepared from
compound 4b using the same method as 5a with a yield (59%)
1H NMR (CDCl3, 300 MHz, ppm): δ 7.44 (m, 8H), 7.36 (m, 4H),
7.26 (m, 4H), 7.08 (m, 20H), 6.74 (m, 2H), 5.77 (d, J = 17.46 Hz,
2H), 5.26 (d, J = 27.63 Hz, 2H), 4.60 (s, 4H), 4.51 (s, 4H), 2.35 (s,
6H). 13C NMR (CDCl3, 125 MHz, ppm): 147.45, 147.15, 144.76,
142.91, 137.95, 137.03, 136.58, 135.92, 133.33, 132.50,130.07, 129.12,
128.07, 127.74,126.29, 125.24, 124.26, 123.90, 123.12, 122.78, 113.83,
71.98, 71.85, 60.42, 21.09, 20.90, 14.23. HRMS (ESI) (M+,
C66H56N2O2S2): calcd 972.38; found 972.38.
AUTHOR INFORMATION
■
Corresponding Author
*Fax: +1 206 543 3100. Tel.: +1 206 543 2626. E-mail: ajen@
u.washington.edu (A.K.-Y.J.). Fax: 86-25-52090616. Tel.: 86-25-
52090616. E-mail: lbp@seu.edu.cn (B.-P.L.).
Synthesis of T-BTPA. Compound 2c. Compound 2c was
prepared from compound 3 and 2,5-dibromothiophene using the
ACKNOWLEDGMENTS
1
■
same method as 2a with a yield of 79%. H NMR (CDCl3, 300 MHz,
This work is supported by the National Science Foundation’s
NSF-STC program under Grant No. DMR-0120967, the
AFOSR (FA9550-09-1-0426), the Office of Naval Research
(N00014-11-1-0300), and the World Class University (WCU)
program through the National Research Foundation of Korea
under the Ministry of Education, Science and Technology
(R31-21410035). A.K.-Y.J. thanks the Boeing−Johnson Foun-
dation for financial support. Y.S. thanks the State-Sponsored
Scholarship for Graduate Students from China Scholarship
Council. S.C.C. thanks the National Science Council of Taiwan
(NSC98-2917-I-009-112) for supporting the Graduate Stu-
dents Study Abroad Program.
ppm): δ 7.40 (d, J = 8.49 Hz, 4H), 7.22 (m, 4H), 7.10 (m, 12H),
6.98−7.05 (m, 8H), 2.40 (s, 6H).
Compound 3c. Compound 3c was prepared from compound 2c
1
using the same method as 3a with a yield of 73%. H NMR (CDCl3,
300 MHz, ppm): δ 9.86 (s, 2H), 7.73 (d, J = 8.7 Hz, 4H), 7.61 (d, J =
8.4 Hz, 4H), 7.29 (d, J = 4.8 Hz, 4H), 7.22 (m, 8H), 7.15 (s, 2H), 7.11
(m, 6H), 2.41 (s, 6H). 13C NMR (CDCl3, 125 MHz, ppm): δ 190.45,
153.14, 145.55, 143.28, 142.73, 135.48, 131.35, 130.55, 129.19, 126.68,
125.90, 123.90, 119.46, 21.03.
Compound 4c. Compound 4c was prepared from compound 3c
1
using the same method as 4a with a yield of 71%. H NMR (CDCl3,
300 MHz, ppm): δ 7.44 (d, J = 8.4 Hz, 4H), 7.24 (d = 9.0 Hz, 4H),
7.09 (m = 18H), 4.63 (s, 4H), 2.33, (s, 6H), 1.56 (m, 2H).
Compound T-BTPA. Compound T-BTPA was prepared from
REFERENCES
■
1
compound 4c using the same method as 5a with a yield (78%). H
(1) Lenes, M.; Morana, M.; Brabec, C. J.; Blom, P. W. M. Adv. Funct.
Mater. 2009, 19, 1106.
NMR (CDCl3, 300 MHz, ppm): δ 7.43 (m, 6H), 7.36 (m, 4H), 7.26
(m, 4H), 7.08 (m, 20H), 6.74 (m, 2H), 5.77 (d, J = 17.1 Hz, 2H), 5.26
(m, 2H), 4.60 (s, 4H), 4.51 (s, 4H), 2.35 (s, 6H). 13C NMR (CDCl3,
125 MHz, ppm): δ 147.25, 144.86, 142.65, 138.00, 137.05, 136.61,
133.24, 132.41, 130.09, 129.13, 128.26, 128.09, 126.33, 126.29, 125.19,
123.81, 123.66, 123.32, 123.08, 113.86, 171.99, 171.89, 60.45, 20.94,
14.28. HRMS (ESI) (M+, C62H54N2O2S): calcd 890.3906; found
890.3879.
Preparation of the X-HTM Film and Doping Processes. The
HTM film was prepared by spin-coating the chloroform solution of
HTM on the precleaned and O2 plasma-treated ITO-covered glass
substrate. And then, the substrate with the HTL was transferred to a
glovebox and thermally cross-linked on a hot plate at elevated
temperature for a certain time. The dopant solution was prepared by
dissolving NOSbF6 in acetonitrile in glovebox. For the doping, the X-
HTL was fully covered by the dopant solution with a certain
concentration for the corresponding time and then spin-rinsed with
acetonitrile.
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