Effect of oligothiophene π-bridge length on the photovoltaic properties of D-A copolymers based on carbazole and quinoxalinoporphyrin

Article abstract of DOI:10.1021/ma3014367

A series of low-bandgap donor-acceptor (D-A) copolymers, P(C-T-QP), P(C-BT-QP), P(C-TT-QP), and P(C-TT-QP-Zn), using 2,7-carbazole (C) as an electron-rich unit and quinoxalino[2,3-b]porphyrins (QP) or quinoxalino[2,3-b] porphyrinatozinc(QP-Zn) as an electron-deficient unit with different length of oligothiophene π-bridges, were designed and synthesized via a Pd-catalyzed Stille-coupling method. The π-bridge between the C donor unit and the QP acceptor unit is thiophene (T) in P(C-T-QP), bithiophene (BT) in P(C-BT-QP), and terthiophene (TT) in P(C-TT-QP) or P(C-TT-QP-Zn). These copolymers possess good solubility, high thermal stability, broad absorption, and low bandgap ranging from 1.66 to 1.73 eV. The influence of the π-bridge and the central Zn ion on the electronic and photovoltaic properties was investigated and discussed in detail. It was found that the π-bridge played an important role in tuning the effective conjugation length and therefore significantly affected the molecular architecture and optoelectronic properties of the copolymers. With the π-bridge varying from thiophene to bithiophene, then to terthiophene, the hole mobility of the copolymers increased gradually, and the absorption was broadened in turn. Zn ion in the porphyrin ring also had a significant influence on the physicochemical and photovoltaic properties. Bulk heterojunction solar cells with the polymers as donor and PC₇₁BM as acceptor demonstrated PCEs of 0.97% for P(C-T-QP), 1.97% for P(C-BT-QP), 2.53% for P(C-TT-QP), and 1.45% for P(C-TT-QP-Zn). All of them are among the highest PCE values of PSCs based on porphyrin polymers. Among the four polymers, although the P(C-TT-QP-Zn) shows the highest hole mobility and the widest absorption, the corresponding PSC demonstrated the lowest PCE because the morphology of P(C-TT-QP-Zn)/PC ₇₁BM blend film is not beneficial to the exciton dissociation and charge carriers transport. This study provides a new insight toward the design and future development of quinoxalinoporphyrin-based conjugated polymers.

Full text of DOI:10.1021/ma3014367

Article
pubs.acs.org/Macromolecules
Effect of Oligothiophene π‑Bridge Length on the Photovoltaic
Properties of D−A Copolymers Based on Carbazole and
Quinoxalinoporphyrin
Shaowei Shi,† Pei Jiang,†,‡ Song Chen,§ Yeping Sun,† Xiaochen Wang,† Kai Wang,† Suling Shen,‡
Xiaoyu Li,*,† Yongfang Li,*,‡ and Haiqiao Wang*,†
†State Key Laboratory of Organic−Inorganic Composite, Key Laboratory of Carbon Fiber and Functional Polymers, Ministry of
Education, Beijing University of Chemical Technology, Beijing 100029, China
‡CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
§China Textile Academy, Beijing 100025, China
S
* Supporting Information
ABSTRACT: A series of low-bandgap donor−acceptor (D−A)
copolymers, P(C-T-QP), P(C-BT-QP), P(C-TT-QP), and P(C-
TT-QP-Zn), using 2,7-carbazole (C) as an electron-rich unit and
quinoxalino[2,3-b′]porphyrins (QP) or quinoxalino[2,3-b′]-
porphyrinatozinc(QP-Zn) as an electron-deficient unit with
different length of oligothiophene π-bridges, were designed and
synthesized via a Pd-catalyzed Stille-coupling method. The π-bridge
between the C donor unit and the QP acceptor unit is thiophene
(T) in P(C-T-QP), bithiophene (BT) in P(C-BT-QP), and
terthiophene (TT) in P(C-TT-QP) or P(C-TT-QP-Zn). These copolymers possess good solubility, high thermal stability,
broad absorption, and low bandgap ranging from 1.66 to 1.73 eV. The influence of the π-bridge and the central Zn ion on the
electronic and photovoltaic properties was investigated and discussed in detail. It was found that the π-bridge played an
important role in tuning the effective conjugation length and therefore significantly affected the molecular architecture and
optoelectronic properties of the copolymers. With the π-bridge varying from thiophene to bithiophene, then to terthiophene, the
hole mobility of the copolymers increased gradually, and the absorption was broadened in turn. Zn ion in the porphyrin ring also
had a significant influence on the physicochemical and photovoltaic properties. Bulk heterojunction solar cells with the polymers
as donor and PC71BM as acceptor demonstrated PCEs of 0.97% for P(C-T-QP), 1.97% for P(C-BT-QP), 2.53% for P(C-TT-
QP), and 1.45% for P(C-TT-QP-Zn). All of them are among the highest PCE values of PSCs based on porphyrin polymers.
Among the four polymers, although the P(C-TT-QP-Zn) shows the highest hole mobility and the widest absorption, the
corresponding PSC demonstrated the lowest PCE because the morphology of P(C-TT-QP-Zn)/PC71BM blend film is not
beneficial to the exciton dissociation and charge carriers transport. This study provides a new insight toward the design and
future development of quinoxalinoporphyrin-based conjugated polymers.
Porphyrins and their derivatives have been intensively
studied over many years due to their importance in the
photochemistry and biochemical processes. Porphyrins are
preferable photosensitizers for utilizing solar energy for the
reason that they play an important role in the photosynthesis of
plants. Their large π-conjugation systems as well as their good
thermal stabilities also make porphyrins to be promising
materials for organic photonic and electronic applications.14 Up
to now, porphyrin-based small molecules BHJ solar cells and
dye-sensitized solar cells (DSSCs) have reached power
conversion efficiencies (PCEs) of more than 5% and 12%,
respectively.15,16 However, when applying porphyrin polymers
in solar cells, poor PCEs were obtained (less than 1%).17−23
INTRODUCTION
■
Bulk heterojunction (BHJ) polymer solar cells (PSCs) which
are based on solution-processable conjugated polymer donor
and fullerene derivative acceptor materials have been widely
studied due to their low-cost solution fabrication process,
lightweight, and device mechanical flexibility as well as potential
contribution to clean and renewable energy. To achieve high-
performance PSCs, the electron-donating conjugated polymers
need to have appropriate band gaps to maximize solar radiation
capture, good hole mobility, and reasonable highest occupied
molecular orbital levels (HOMO) to maximize the short circuit
current (Jsc) and open-circuit voltage (Voc), respectively.1−11 At
present, conjugated polymers with electron donor−acceptor
(D−A) architecture are particularly attractive due to the facile
tunability of their electronic structure of the main chain by
controlling the intramolecular charge transfer (ICT) from the
donor units to the acceptor units.12,13
Received: July 11, 2012
Revised: September 10, 2012
Published: September 19, 2012
© 2012 American Chemical Society
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Scheme 1. Synthetic Route of the Monomers and Copolymers
The relatively narrow absorption spectrum and less efficient
conjugation in the main chain are the critical factors that limit
the improvement of photoelectric properties. The typical
absorption spectra of porphyrin units exhibit a sharp and
strong Soret band (410−430 nm) and weak Q-bands (530−
540 nm) without absorption between them, and the conven-
tional meso-aryl linkage in porphyrin polymers cannot broaden
the absorption effectively by intramolecular charge transfer
(ICT) due to the large aryl−porphyrin dihedral angles.23−25
However, quinoxalino[2,3-b′]porphyrins are more π-expanded
systems and were proposed as “molecular wires”.26 By
incorporation of different central metal ions or selective
functionalization at β-pyrrolic positions, it enable “fine-tuning”
of electronic properties of compounds by altering relative
energy levels of the HOMO and lowest unoccupied molecular
orbital (LUMO). But, the studies about quinoxalinoporphyrin
are still at the level of small molecules or large multiporphyrin
arrays, and it has not yet been used in polymers.27 Recently, we
designed and reported a new D-π-A copolymer P(C-TT-QP)
with quinoxalinoporphyrin as acceptor unit, 2,7-carbazole as
donor unit, and terthiophene as a π-bridge. P(C-TT-QP)
exhibited an unusually broadened and enhanced Q-band
absorbance compared to other porphyrin-based conjugated
polymers reported in the literature. More interestingly, the
absorption blank between the Soret band and Q-bands has
been covered perfectly for the first time. As a result, PCE of
solar cell based on P(C-TT-QP) reached 2.5%.28 These results
indicate that the photovoltaic performance of the porphyrin-
based conjugated polymers could be greatly improved by using
quinoxalinoporphyrin. However, to achieve higher efficiency
porphyrin polymers, it is needed to further study the effect of
the molecular structure on the optical, electrochemical, and
photovoltaic properties.29
Herein, two novel quinoxalinoporphyrin-based copolymers
P(C-T-QP) and P(C-BT-QP) were synthesized. Combining
with our previously reported P(C-TT-QP), the three
copolymers were used to systematically study the effects of
the bridges on absorption, energy levels, and charge transport
properties of the copolymers.
On the other hand, it is noted that nearly in all the literatures
reported for the porphyrin-based photovoltaic polymers so far,
zinc(II) porphyrins rather than free-base porphyrins were
chosen as building blocks without discussion on their difference
in photovoltaic properties. But Wong et al. reported that by
blending free-base porphyrin additives in the 9,9-dioctylfluor-
enyl hexa-peri-hexabenzocoronene (FHBC) moiety, the
performance of BHJ devices functioned better than the zinc(II)
porphyrin additives.30 Thus, it is of prime importance to fully
understand the effect of the central Zn ion on the
physicochemical and photovoltaic properties in the design of
new porphyrin polymers for efficient PSCs. Herein, P(C-TT-
QP-Zn) with Zn ion in the porphyrin ring of P(C-TT-QP) was
also synthesized to comparably study the effects of the central
metal ion with P(C-TT-QP). We found that conjugated bridges
and central Zn ion in porphyrin ring crucially influence the
electronic structure of polymer main chain and the interaction
between donor and acceptor units and therefore remarkably
affect coplanarity and optical, electrochemical, charge transport,
and photovoltaic properties of the D-π-A conjugated
copolymers.
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RESULTS AND DISCUSSION
■
Synthesis and Thermal Stability. The synthesis routes of
monomers and corresponding polymers are outlined in Scheme
1. Monomer QP was reacted with zinc acetate to give
compound QP-Zn in a good yield. All polymers were
synthesized by palladium-catalyzed Stille-coupling polymer-
ization and purified by sequential Soxhlet extraction with
methanol, hexanes, and CHCl3. The CHCl3 fraction was then
reduced in volume, precipitated into methanol, and collected by
filtration, yielding black or dark blue solids. P(C-T-QP), P(C-
BT-QP), and P(C-TT-QP) can be readily dissolved in common
solvents, such as chloroform, toluene, tetrahydrofuran (THF),
and chlorobenzene, and processed to form smooth and
pinhole-free films upon spin-coating. P(C-TT-QP-Zn) shows
slight poor solubility but also can be dissolved in hot solvents.
The molecular weight of the polymer was determined by GPC
in THF solution relative to polystyrene standards, and the
detailed GPC data are listed in Table 1. The thermal properties
Figure 2. UV−vis spectra of copolymers and reference monomers in
CHCl3 solutions and in thin films.
Table 1. Molecular Weights and Thermal Properties of the
Copolymers
(see Figure 2a,c). All the polymers show absorptions in the
regions of Soret band and Q-bands. Compared to the reference
QP and QP-Zn absorption, the Soret band and Q-bands of the
polymers are both broadened while the Q-bands enhance
observably. The blank between the Soret and Q-bands is filled
completely. And the absorption areas between the Soret and Q-
bands increase gradually with the oligothiophene bridges
varying from thiophene to bithiophene and then to
terthiophene due to more extended π-conjugation resulting in
a more effective ICT effect. The results demonstrate that the
effective ICT effect along the main chain of polymers is the key
factor to broaden and enhance Q-band absorbance and fill
blank absorption. The absorption spectra of the four polymers
in the solid state are all broadened compared to their
corresponding solution spectra. However, only slight red-shifts
(ca. 1−5 nm) of their absorption maxima are obtained, which is
probably due to the bulky 4-tert-butylphenyl substituents on the
porphyrins that prevent aggregation in the solid state.30 In
Figure 2a,b, it is clear that the absorption spectra of P(C-T-
QP), P(C-BT-QP), and P(C-TT-QP) vary with the number of
the conjugated thiophene rings. A comparison of the polymers
reveals that P(C-TT-QP) exhibits the most bathochromic shift
of the absorption maximum at 438 nm in the chloroform
solution, whereas P(C-T-QP) exhibited the most hypsochromic
shift absorption maximum at 427 nm. The absorption edges for
solid films of P(C-T-QP), P(C-BT-QP), and P(C-TT-QP)
increase from 679, 736, to 742 nm, corresponding to optical
band gaps (Egopt) decreasing from 1.83, 1.68, to 1.67 eV,
respectively. The decreasing in Egopt from P(C-T-QP) to P(C-
BT-QP) and to P(C-TT-QP) can attribute to the presence of a
more extended and delocalized π-electron system (in both
solutions and solid films) which extends the conjugation
lengths of the copolymers and reduces the band gap. Also,
increasing in the length of the oligothiophene bridges is
beneficial to decrease steric hindrance between QP and its
neighboring units.
a
b
a
b
polymer
Mn
Mw
PDI (Mw/Mn)
Td (°C)
P(C-T-QP)
3.4K
51.5K
61.1K
66.4K
5.2K
73.2K
1.52
1.42
2.13
2.08
423
435
440
456
P(C-BT-QP)
P(C-TT-QP)
P(C-TT-QP-Zn)
130.3K
138.1K
a
Mn, Mw, and PDI of the polymers were determined by gel permeation
b
chromatography using polystyrene standards in THF. The 5%
weight-loss temperatures in the air.
of the polymers were determined by thermogravimetric analysis
(TGA) under a nitrogen atmosphere at a heating rate of 10 °C/
min, as shown in Figure 1. All polymers have good thermal
Figure 1. TGA plots of the polymers with a heating rate of 10 °C/min
under an inert atmosphere.
stability with onset decomposition temperatures with 5%
weight loss at 426−456 °C and P(C-TT-QP-Zn) processes
the highest. Obviously, the thermal stability of these polymers is
adequate for their applications in PSCs and other optoelec-
tronic devices.
Optical Properties. The absorption spectra of all studied
polymers were measured in both dilute chloroform and the thin
films (Figure 2a−d), and the correlated optical parameters are
summarized in Table 2, including the absorption peak
wavelengths (λabs), the absorption edge wavelengths (λonset),
and the optical band gap (Egopt). For the convenience of study,
we also measured the absorption spectrum of monomers TPP,
QP, and QP-Zn as references. It is noted that when edge-fusing
porphyrin with quinoxaline, the Soret bands of QP and QP-Zn
are significantly broadened due to the extension π-conjugation
In Figure 2c,d, although the absorption maxima of P(C-TT-
QP) and QP slightly red-shift more than P(C-TT-QP-Zn) and
QP-Zn both in solution and in solid film, P(C-TT-QP-Zn) and
QP-Zn possess more bathochromic Q-bands, and the
absorption edges of P(C-TT-QP-Zn) red-shift ca. 7 nm
compared to P(C-TT-QP). It can be noticed that there are
individually identifiable two components of the Soret band in
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Table 2. Optical and Electrochemical Properties of the Polymers
HOMO/Eox(eV)/
a
b
a
c
polymer
λabs in solution (nm) λabs in film (nm) λemi in solution (nm) λonset in film (nm) Egopt (eV)
(V)
LUMO (eV)
P(C-T-QP)
427
435
438
431
431
434
439
430
708
734
744
755
717
736
742
749
1.73
1.69
1.67
1.66
−5.39/0.68
−5.36/0.65
−5.33/0.62
−5.24/0.53
−3.66
−3.68
−3.66
−3.58
P(C-BT-QP)
P(C-TT-QP)
P(C-TT-QP-Zn)
a
b
c
Measured in chloroform solution. Cast from chlorobenzene solution. Bandgap estimated from the onset wavelength (λonset) of the optical
absorption: Egopt = 1240 nm·eV/λonset
.
QP-Zn with one of these moving to lower energy compared to
the free-base analogue, and it is clear that the lower-energy
Soret component shows low intensity. This feature is retained
in the spectrum of P(C-TT-QP-Zn) with a shoulder at ca. 500
nm. The reason for this phenomenon is somewhat complicated,
and further study is ongoing. But at least we can infer that QP-
Zn shows more extended π-conjugation than the free-base
analogue.
The normalized photoluminescence (PL) spectra of the
polymers in CHCl3 solution (at concentration of 1 mg/100
mL) are shown in Figure 3. The solution PL spectra of all
Figure 4. (a) Cyclic voltammograms of the polymer films on Pt
electrode in 0.1 mol/L Bu4NPF6, CH3CN solution with a scan rate of
100 mV/s. (b) Energy level diagrams for P(C-T-QP), P(C-BT-QP),
P(C-TT-QP), and P(C-TT-QP-Zn).
mined to be 0.68, 0.65, and 0.62 V vs Ag/Ag+, respectively,
corresponding with the HOMO levels to be −5.39, −5.36, and
−5.33 eV. It was worthwhile to note that the HOMO energy
levels of the three polymers increase with the increasing
conjugation length. The copolymers possess comparable
LUMO levels between −3.66 and −3.68 eV, demonstrating
that the LUMO levels of the QP-based polymers are mainly
dominated by the QP unit.31b Compared to P(C-TT-QP),
P(C-TT-QP-Zn) shows higher HOMO/LUMO energy levels,
which can be attributed to the increased electron density, and it
can be inferred that the electron-accepting ability of QP-Zn is
weaker than QP.30 Since the open circuit voltage Voc of BHJ
PSCs is correlated to the difference between the LUMO energy
level of the fullerene and HOMO energy level of the donor
polymer, therefore a reasonable Voc for QP or QP-Zn-based
PSCs is anticipated due to the low-lying HOMO energy level.
On the other hand, the LUMO energy levels of these polymers
are located at reasonable positions from −3.66 to −3.68 eV
which offer enough driving force for charge separation and
transfer without too much energy loss.13
Theoretical Calculations. To provide further insight into
the fundamentals of molecular architecture, molecular simu-
lation was carried out for P(C-TT-QP) and P(C-TT-QP-Zn).
The molecular geometries of the two polymers were optimized
in a trans conformation, with a chain length of n = 1 at the
B3LYP/6-31G(d,p) level with the GAMESS program pack-
age.33 The alkyl chain of carbazole was replaced by methyl
group, and 4-tert-butylphenyl at 10 and 15 positions of QP or
QP-Zn was replaced by phenyl group in the calculation to avoid
excessive computation demand. To better study the effects of
substituents on the structure of the main chain, 4-tert-
butylphenyl at 5 and 20 positions was retained (as shown in
Figure 5).
Figure 3. Photoluminescence spectra of the polymers in chloroform
solution (excited at 430 nm) at concentration of 1 mg/100 mL.
polymers reveal both emission from donor segment (450−600
nm) and from the porphyrin moiety (650−800 nm), which is
indicative of incomplete energy transfer from the polymer
backbone to QP or QP-Zn unit. Also, it is clear that the PL
intensity of P(C-TT-QP-Zn) is lower than those of other three
polymers which means weaker energy transfer when incorpo-
rate Zn ion in QP. The emission peak wavelengths λemi of P(C-
T-QP), P(C-BT-QP), and P(C-TT-QP) are consistent with the
trend of λabs in their corresponding absorption spectra. P(C-
TT-QP-Zn) shows the largest bathochromic-shifted spectrum
with maximum emission at 755 nm compared to P(C-TT-QP),
which is corresponding to the red-shifted Soret band.
Electrochemical Properties. Cyclic voltammetry (CV)
has been employed and considered as an effective tool in
investigating electrochemical properties of conjugated
oligomers and polymers.31 Cyclic voltammograms of the
polymer films are shown in Figure 4. The energy levels of
the HOMO and LUMO of the polymers were calculated
according to the equations32
HOMO = −IP = −e(Eox + 4.71) (eV)
(1)
opt
The wave functions of the frontier molecular orbital are
depicted in Figure 6. As can be observed, the HOMO is
delocalized along the whole π-conjugated backbone while the
LUMO is mostly concentrated on the quinoxalinoporphyrin-
based acceptor groups. These images provide further evidence
of the formation of well-defined D-π-A structure and the ICT
LUMO = Eg + HOMO (eV)
(2)
From the value of onset oxidation potential (Eox) and Egopt of
the polymers, the HOMO and the LUMO were calculated and
are listed in Table 2. The moderate onset oxidation potentials
of P(C-T-QP), P(C-BT-QP), and P(C-TT-QP) are deter-
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the steric hindrance from the 4-tert-butylphenyls of meso
positions, and this can partly explain the lower molecule weight
of P(C-T-QP) relative to that of P(C-BT-QP) and P(C-TT-
QP). The difference of the dihedral angles between P(C-TT-
QP) and P(C-TT-QP-Zn) is slight, which demonstrates the
complexing of zinc has not a remarkable influence on the
architecture of the polymers.
Photovoltaic Properties. To investigate the effects of π-
conjugated bridges and zinc on the photovoltaic properties of
the copolymers, BHJ PSC devices with a configuration of ITO/
PEDOT:PSS/polymer:PC71BM/Ca/Al were fabricated. Figure
8 shows the current density−potential characteristic of PSCs
Figure 5. Molecular geometry sketch used for the computational data.
Figure 8. Current density−potential characteristic of PSCs based on
polymer:PC71BM under illumination of AM1.5G, 100 mW/cm2.
based on the blends of polymer:PC71BM under illumination of
AM1.5G, 100 mW/cm2. To further study the performance of
the PSCs devices, the hole mobilities in the photosensitive
layers were measured by the space charge limited current
(SCLC) method using devices with structure of ITO/
PEDOT:PSS/polymer:PC71BM/Au. For unipolar transport in
a trap-free semiconductor with an ohmic injecting contact, the
SCLC can be approximated by the Mott−Gurney equation:
Figure 6. Frontier molecular orbital (LUMO, top; HOMO, bottom)
obtained from density functional theory (DFT) calculations on the
polymers with a chain length n = 1 at B3LYP/6-31G(d,p) level of
theory.
behavior of the material (i.e., the HOMO to LUMO transition
is a donor to acceptor intramolecular charge transfer).34 The
optimized geometries are shown in Figure 7, and relevant
2
⎛
⎞
9
8
V
L ⎠
V
L
J ≅ ε ε μ exp 0.891γ
⎜
⎟
r
0
0
3
⎝
(3)
where J is the current density, εr is the dielectric constant of the
polymer, ε0 is the free-space permittivity (8.85 × 10−12 F/m),
μ0 is the charge mobility at zero field, γ is a constant, L is the
thickness of the blended film layer, V = Vappl − Vbi, Vappl is the
applied potential, and Vbi is the built-in potential which results
from the difference in the work function of the anode and the
cathode (in this device structure, Vbi = 0.2 V). Figure 9 displays
ln(JL3/V2) versus (V/L)0.5 curve for the measurement of the
hole mobility of the copolymers by the SCLC method. As
summarized in Table 4, the hole mobilities of the polymers
calculated using eq 3 are 3.1 × 10−6, 9.6 × 10−6, 3.3 × 10−5, and
1.3 × 10−4 cm2 V−1 s−1 for P(C-T-QP), P(C-BT-QP), P(C-TT-
Figure 7. Top view (top) and side view (bottom) of optimized
structures of the copolymers backbone units with a chain length n = 1.
dihedral angles are listed in Table 3. All the dihedral angles are
relatively small which is benefical to form a better planar
conjugated polymer backbone. Also, we can notice that
terthiophene and bithiophene are more helpful to decrease
Table 3. Calculated Dihedral Angles of Polymers
polymer
θ1
θ2
θ3
θ1′
θ2′
θ3′
P(C-TT-QP)
−20.03
−19.07
7.73
6.67
−14.84
−13.41
18.84
18.09
6.98
6.08
−16.41
−17.03
Figure 9. Plots of ln(JL3/V2) versus (V/L)0.5 for the measurement of
the hole mobility in polymer/PC71BM devices by the SCLC method.
P(C-TT-QP-
Zn)
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Table 4. Photovoltaic Performances of the PSCs Based on Polymer/PC71BM under the Illumination of AM1.5, 100 mW/cm2
polymer
polymer/PC71BM (w/w)
Voc (V)
Jsc (mA/cm2)
FF
PCE (%)
mobility (cm2 V−1 s−1
)
P(C-T-QP)
1:2
1:3
1:3
1:3
0.69
0.68
0.68
0.70
4.15
6.80
8.32
5.79
0.34
0.43
0.45
0.36
0.97
1.97
2.53
1.45
3.1 × 10−6
9.6 × 10−6
3.3 × 10−5
1.3 × 10−4
P(C-BT-QP)
P(C-TT-QP)
P(C-TT-QP-Zn)
QP), and P(C-TT-QP-Zn), respectively. It can be noticed that
P(C-T-QP) and P(C-BT-QP) show lower hole mobilities than
P(C-TT-QP). This variation in device behavior can be
attributed to the change of the coplanarity in the polymer
backbone by the introduction of various oligothiophene
bridges, and we can conclude that the change of the π-bridge
from thiophene to bithiophene, and then to terthiophene,
results in gradully increase of the intrachain mobility because of
more extended π-conjugation and less steric hindrance. P(C-
TT-QP-Zn) shows the highest hole mobility of the order of
10−4 cm2 V−1 s−1, indicating that the incorporation of zinc may
be in favor of enhancing the packing structure in the blends.
The Voc, Jsc, fill factor (FF), PCE, and hole mobility of the
PSCs are summarized in Table 4. An obvious increase in the Jsc
and PCE can be observed from P(C-T-QP) to P(C-TT-QP)
(i.e., P(C-T-QP) < P(C-BT-QP) < P(C-TT-QP)). The Jsc of
the polymers increased from 4.15, 6.80, to 8.32 mA/cm2, which
is consistent with the lowered band gaps (from 1.73, 1.68, to
1.67 eV). The optimum photovoltaic performance with the
maximum PCE value of 2.53% (Voc = 0.68 V, Jsc= 8.32 mA/
cm2, FF = 0.45) was obtained in the PSC device having a
weight ratio of P(C-TT-QP):PC71BM = 1:3. When complexing
zinc in the center, P(C-TT-QP-Zn) shows lower FF(0.36) and
Jsc (5.79) compared to P(C-TT-QP) and a PCE of 1.45% is
obtained. Figure 10 shows the external quantum efficiencies
maximum absorption with increasing m from M1 to M3,
resulting in higher PCE for the polymers.29a−e P(C-TT-QP-Zn)
showed lower EQE than P(C-TT-QP) with maximum EQE
value of 44% at ∼480 nm. Additionally, a less than 10%
difference between the Jsc and the integral of the EQE is
observed. Since the EQE was tested in the air, this discrepancy
should be mainly ascribed to the degradation caused by the
oxidation of the Ca electrode.
We noted that Wong et al. also found the zinc porphyrin
derivative functioned worse in solar cells than the free-base
porphyrin derivatives,30 and they attributed the poor perform-
ance of solar cell to the less stable of zinc porphyrin. However,
in this work, we observed that the zinc porphyrin polymer
possesses better thermal stability than their free-base analogues.
Considering the fact that the P(C-TT-QP-Zn) possesses the
highest hole mobility and the strongest and the widest
absorption among the four polymers, herein, we conjectured
the possible reason for the worst performance of P(C-TT-QP-
Zn)-based PSC is the film morphology of the P(C-TT-QP-
Zn)/PC71BM blend.
In order to understand the effect of mophology of the
photoactive layers on the photovoltaic performance of the
polymer solar cells, the mophological structures of the blend
films of polymer/PC71BM were analyzed by tapping mode
atom force microscopy (AFM) measurements. Figure 11 shows
Figure 10. EQE spectra of the PSCs based on polymer:PC71BM
blends.
(EQE) curves of the devices incorporating the poly-
mer:PC71BM blends. It is apparent that all devices exhibited
a broad response range, covering from 300 to 750 nm with
maximum EQE values of 37% at ∼465 nm, 48% at ∼475 nm,
and 51% at ∼460 nm for the PSCs based on P(C-T-QP), P(C-
BT-QP), and P(C-TT-QP), respectively. Compared to the
absorption spectra of pristine polymers, the significantly
broadened EQE responses in the visible region can be
attributed to both the intrinsic absorptions of the polymers
and PC71BM. P(C-TT-QP)-based device improved the EQE
upon that of P(C-T-QP) and P(C-BT-QP) markedly, especially
in the range of 400−500 nm. It is remarkable to see that the
light-harvesting ability of P(C-TT-QP) (with its commensur-
ately high charge mobilities and favorable light absorption
characterisitics) can be increased by introducing terthiophene
in the main chain. This is in agreement with increased
Figure 11. AFM topography (left) and phase (right) images of the
blend films of polymer/PC71BM; the size of the image is 2 μm × 2 μm.
the AFM images of the blend films. The blend films based on
P(C-TT-QP) and P(C-TT-QP-Zn) both demonstrated low
roughness of 1.05 and 1.29 nm, respectively. The P(C-TT-
QP)/PC71BM blend possesses higher FF due to its smoother
surface. For the P(C-TT-QP)/PC71BM blend film, the bright
patterns of polymer domains showed a good interpenetrating
network which is beneficial to the exciton dissociation and
charge carriers transport. Obviously, the suitable morphology of
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filtration and washed with methanol. Then the solid was subjected to
Soxhlet extraction with methanol, hexane, and chloroform. Sub-
sequently, the fraction that was extracted by chloroform was
evaporated under reduced pressure and then precipitated in methanol,
filtered, and finally dried under vacuum to obtain a black solid (60 mg,
the blend films are consistent with the higher photovoltaic
performance of the PSCs based on P(C-TT-QP).
CONCLUSION
■
1
In conclusion, a series of quinoxalinoporphyrin-based D−A
copolymers with carbazole as donor unit and oligothiophene as
π-bridge have been synthesized and well characterized. These
copolymers possess good solubility and high thermal stability as
well as lower bandgap. We demonstrated that their electronic
and photovoltaic properties can be easily tuned via the variation
of oligothienyl chain π-bridge length, and we carefully studied
the difference between zinc(II) porphyrin and free-base
porphyrin for the first time. By changing the type of π-bridge,
the absorption edges of P(C-T-QP), P(C-BT-QP), and P(C-
TT-QP) in films red-shift from 717, 736, to 742 nm, and the
band gaps of the polymers are tuned from 1.73, 1.68, to 1.67 eV
by upshifting the HOMO from −5.39, −5.36, to −5.33 eV.
When incorporating zinc in center of porphyrin, the HOMO/
LUMO energy levels are upshifted and 1.66 eV of the band gap
is obtained. The hole mobilities of P(C-T-QP), P(C-BT-QP),
and P(C-TT-QP) increase from 3.1 × 10−6, 9.6 × 10−6, to 3.3
× 10−5 cm2 V−1 s−1 while P(C-TT-QP-Zn) shows the highest
hole mobility of 1.3 × 10−4 cm2 V−1 s−1, indicating that the
incorporation of zinc may be propitious to enhancing the
packing structure in the blends. The PCEs of the PSCs
incorporating the polymers and PC71BM reach to 0.97% (P(C-
T-QP)), 1.97% (P(C-BT-QP)), 2.53% (P(C-TT-QP)), and
1.45% (P(C-TT-QP-Zn)). The hole mobilities and PCEs
increase with the number of thiophene rings, showing that
terthiophene is more beneficial for decreasing the degree of
curvature in the polymer backbone and increasing the effective
conjugation length. Preliminary research show that the
complexing of zinc may be a disadvantage to the improvement
of PCEs due to decreasing both short-circuit current and fill
factor although zinc(II) porphyrin possesses higher hole
mobility and better thermal stability, which could provide a
new insight into the design and future development of
porphyrin-based conjugated polymers.
yield 41%, Mn = 3.4 kDa, Mw = 5.2 kDa, PDI = 1.52). H NMR
(CDCl3, 600 MHz) δ (ppm): δ −2.34 to −2.24 (br, NH), 0.75−2.62
(br, alkyl-H), 3.75−4.52 (br, alkyl-H), 7.30−8.55 (br, Ar−H), 8.67−
9.10 (br, pyrrolic-H). Elemental analysis: Calcd for C98H99N7S2: C,
81.80; H, 6.93; N, 6.81; S, 4.46. Found: C, 79.77; H, 6.836; N, 6.530;
S, 4.245.
P(C-BT-QP). To a 25 mL two-necked flask were added monomer
CZ (49 mg, 0.10 mmol), QP(110 mg 0.10 mmol), M2 (98 mg, 0.2
mmol), and chlorobenzene (6 mL). The mixture was purged with
nitrogen for 15 min, and then Pd2(dba)3 (10 mg, 0.01 mmol) and P(o-
tol)3 (25 mg, 0.08 mmol) were added. After being purged for 15 min,
the reaction mixture was heated at 140 °C for 72 h. After being cooled
to room temperature, the reaction mixture was added dropwise to 200
mL of methanol and then collected by filtration and washed with
methanol. Then the solid was subjected to Soxhlet extraction with
methanol, hexane, and chloroform. Subsequently, the fraction that was
extracted by chloroform was evaporated under reduced pressure and
then precipitated in methanol, filtered, and finally dried under vacuum
to obtain a black solid (107 mg, yield 73%, Mn = 51.5 kDa, Mw = 73.2
kDa, PDI = 1.42). 1H NMR (CDCl3, 600 MHz) δ (ppm): δ −2.37 to
−2.24 (br, NH), 0.53−2.35 (br, alkyl-H), 3.65−3.85 (br, alkyl-H),
7.10−8.45 (br, Ar−H), 8.70−9.23 (br, pyrrolic-H). According to the
theoretical p/q ratio of 1:1. Elemental analysis: Calcd for
C106H103N7S4: C, 79.41; H, 6.48; N, 6.12; S, 8.00. Found: C, 75.94;
H, 6.414; N, 5.502; S, 7.747.
P(C-TT-QP-Zn). To a 25 mL two-necked flask were added monomer
CZ (25 mg, 0.05 mmol), QP-Zn (58 mg 0.05 mmol), M3 (57 mg, 0.1
mmol), and chlorobenzene (5 mL). The mixture was purged with
nitrogen for 15 min, and then Pd2(dba)3 (5 mg, 0.005 mmol) and P(o-
tol)3 (13 mg, 0.04 mmol) were added. After being purged for 15 min,
the reaction mixture was heated at 140 °C for 24 h. After being cooled
to room temperature, the reaction mixture was added dropwise to 200
mL of methanol and then collected by filtration and washed with
methanol. Then the solid was subjected to Soxhlet extraction with
methanol, hexane, and chloroform. Subsequently, the fraction that was
extracted by chloroform was evaporated under reduced pressure and
then precipitated in methanol, filtered, and finally dried under vacuum
to obtain a black solid (29 mg, yield 32%, Mn = 66.4 kDa, Mw = 138.1
1
kDa, PDI = 2.08). H NMR (CDCl3, 600 MHz) δ (ppm): 0.43−2.25
EXPERIMENTAL SECTION
(br, alkyl-H), 3.60−3.79 (br, alkyl-H), 7.10−8.37 (br, Ar−H), 8.65−
9.09 (br, pyrrolic-H). Elemental analysis: Calcd for C114H105N7S6Zn:
C, 74.79; H, 5.78; N, 5.36; S, 10.51. Found: C, 71.50; H, 5.763; N,
4.922; S, 10.210.
■
Synthesis. General. 2,7-Dibromo-9-dodecyl-9H-carbazole (CZ),
2,5-bis(trimethylstannyl)thiophene (M1), 2,2′-bithiophene-5,5′-bis-
(trimethylstannane) (M2), 2,2′:5′,2″-terthiophene-5,5″-bis-
(trimethylstannane) (M3), and 5,10,15,20-tetrakis(4-tert-butylphen-
yl)-5′,8′-dibromoquinoxalino[2,3-b′]porphyrin (QP) were synthesized
as reported in the literature.28,34 Other reagents and solvents were
commercial grade and used as received without further purification.
5,10,15,20-Tetrakis(4-tert-butylphenyl)quinoxalino[2,3-b′]-5′,8′-
dibromoporphyrinatozinc (QP-Zn). A mixture of QP (495 mg, 0.45
mmol), zinc(II) acetate dihydrate (296 mg, 1.35 mmol), chloroform
(100 mL), and methanol (30 mL) was heated at reflux for 3 h. The
reaction mixture was allowed to cool and the solvent removed.
Column chromatography on silica gel (hexane/CH2Cl2 = 1:1)
afforded QP-Zn as a dark blue solid (492 mg, 94%). 1H NMR
(CDCl3, 600 MHz) δ (ppm): δ1.61−1.65 (m, 36H), 7.76 (d, 4H),
7.83 (d, 4H), 7.99 (s, 2H), 8.12 (m, 8H), 8.72−8.93 (m, 6H). MS
(MALDI-TOF) m/z 1162.5 (M + H+).
Measurements and Characterization. The molecular weight of the
polymer was measured using gel permeation chromatography (GPC).
The GPC measurements were performed on Waters 515-2410 with
polystyrenes as reference standard and THF as an eluent. All new
compounds were characterized by nuclear magnetic resonance (NMR)
spectra. The NMRs were recorded on a Bruker AV 600 spectrometer
1
in CDCl3 at room temperature. Chemical shifts of H NMR were
reported in ppm. Splitting patterns were designated as s (singlet), t
(triplet), d (doublet), m (multiplet), and br (broaden). Matrix-assisted
laser desorption/ionization time-of-flight (MALDI-TOF) mass spectra
were made on a Shimadzu KOMPACT MALDI II using CHCA as a
matrix. Elemental analyses were performed on a Flash EA 1112
analyzer or Elementar vario EL III. Thermal gravimetric analysis
(TGA, NetzschTG209C) was performed under a nitrogen atmosphere
at a heating rate of 10 °C/min. UV−vis absorption spectra were
recorded on a Shimadzu spectrometer model UV-3150. Absorption
spectra measurements of the polymer solutions were performed in
chloroform (analytical reagent) at 25 °C. Absorption spectra
measurements of the polymer films were performed on the quartz
plates with the polymer films spin-coated from the polymer solutions
in chlorobenzene (analytical reagent) at 25 °C. The electrochemical
cyclic voltammetry was conducted on a Zahner IM6e electrochemical
Synthesis of Copolymers. P(C-T-QP). To a 25 mL two-necked
flask were added monomer CZ (49 mg, 0.10 mmol), QP (110 mg 0.10
mmol), M1 (82 mg, 0.2 mmol), and chlorobenzene (6 mL). The
mixture was purged with nitrogen for 15 min, and then Pd2(dba)3 (10
mg, 0.01 mmol) and P(o-tol)3 (25 mg, 0.08 mmol) were added. After
being purged for 15 min, the reaction mixture was heated at 140 °C for
72 h. After being cooled to room temperature, the reaction mixture
was added dropwise to 200 mL of methanol and then collected by
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workstation with Pt disk coated with the polymer film, Pt plate, and
Ag/Ag+ electrode as working electrode, counter electrode, and
reference electrode, respectively, in a 0.1 mol/L tetrabutylammonium
hexafluorophosphate (Bu4NPF6)−acetonitrile solution.
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Device Fabrication and Characterization of PSCs. PSCs were
fabricated with ITO glass as a positive electrode, Ca/Al as a negative
electrode, and the blend film of the polymer/PC71BM between them
as a photosensitive layer. The ITO glass was precleaned and modified
by a thin layer of PEDOT:PSS which was spin-cast from a
PEDOT:PSS aqueous solution (Clevious P VP AI 4083 H.C. Stark,
Germany) on the ITO substrate, and the thickness of the PEDOT:PSS
layer is about 35 nm. The photosensitive layer was prepared by spin-
coating a blend solution of polymers and PC71BM in o-
dichlorobenzene on the ITO/PEDOT:PSS electrode. Then the Ca/
Al cathode was deposited on the active layer by vacuum evaporation
under 3 × 10−5 Pa. The effective area of one cell is 4 mm2, and the
thickness of the photosensitive layer is ca. 90 nm. The current−voltage
(J−V) measurement of the devices was conducted on a computer-
controlled Keithley 236 Source Measure Unit in drybox under inert
atmosphere. A xenon lamp with AM 1.5 filter was used as the white
light source, and the optical power at the sample was 100 mW/cm2.
The external quantum efficiency (EQE) was measured using a
Stanford Research Systems model SR830 DSP lock-in amplifier
coupled with a WDG3 monochromator and 500 W xenon lamp. The
light intensity at each wavelength was calibrated with a standard single-
crystal Si photovoltaic cell. All the measurements were performed
under ambient atmosphere at room temperature.
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ASSOCIATED CONTENT
* Supporting Information
Figures S1 and S2. This material is available free of charge via
the Internet at http://pubs.acs.org.
■
S
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AUTHOR INFORMATION
Corresponding Author
*E-mail: wanghaiqiao@mail.buct.edu.cn (H.W.); liyf@iccas.ac.
cn (Y.L.); lixy@mail.buct.edu.cn (X.L.).
Notes
■
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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This work was financially supported by Beijing Natural Science
Foundation (2122047).
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