Synthesis and characterization of a new phenanthrenequinoxaline-based polymer for organic solar cells

Article abstract of DOI:10.1002/pola.28166

Two donor–acceptor (D-A) conjugated polymers, PQx and PphQx, composed of alkylthienyl-substituted benzo[1,2-b:4,5-b']dithiophene (BDTT) as the electron donor and the new electron acceptors quinoxaline (Qx) or phenanthrenequinoxaline (phQx), were synthesized with Stille cross-coupling reactions. The number-averaged molecular weights (M_n) of PQx and PphQx were found to be 25.1 and 23.2 kDa, respectively, with a dispersity of 2.6. The band-gap energies of PQx and PphQx are 1.82 and 1.75 eV, respectively. These results indicate that, because phQx units have highly planar structures, their inclusion in D-A polymers will be a very effective method for increasing the polymers' effective conjugation lengths. The hole mobilities of PQx and PphQx were determined to be 5.0 × 10^?5 and 2.2 × 10^?4 cm² V^?1 s^?1, respectively. A polymer solar cell device prepared with PphQx as the active layer was found to exhibit a power conversion efficiency (PCE) of 5.03%; thus, the introduction of phQx units enhanced both the short circuit current density and PCE of the device.

Full text of DOI:10.1002/pola.28166

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Synthesis and Characterization of a New Phenanthrenequinoxaline-Based
Polymer for Organic Solar Cells
Eunsol Jo,1 Jong Baek Park,2 Woo-Hyung Lee,1 Ji-Hoon Kim,2 In Hwan Jung,3
Do-Hoon Hwang,2 In-Nam Kang1
1Department of Chemistry, the Catholic University of Korea, Bucheon, Gyeonggi-Do 14662, Republic of Korea
2Department of Chemistry, and Chemistry Institute for Functional Materials, Pusan National University, Busan 46241,
Republic of Korea
3Division of Advanced Materials, Korea Research Institute of Chemical Technology (KRICT), Daejeon 34114, Republic of Korea
Correspondence to: I.-N. Kang (E-mail: inamkang@catholic.ac.kr)
Received 12 April 2016; accepted 9 May 2016; published online 00 Month 2016
DOI: 10.1002/pola.28166
ABSTRACT: Two donor–acceptor (D-A) conjugated polymers,
PQx and PphQx, composed of alkylthienyl-substituted
benzo[1,2-b:4,5-b’]dithiophene (BDTT) as the electron donor
and the new electron acceptors quinoxaline (Qx) or phenan-
threnequinoxaline (phQx), were synthesized with Stille cross-
coupling reactions. The number-averaged molecular weights
(Mn) of PQx and PphQx were found to be 25.1 and 23.2 kDa,
respectively, with a dispersity of 2.6. The band-gap energies of
PQx and PphQx are 1.82 and 1.75 eV, respectively. These
results indicate that, because phQx units have highly planar
structures, their inclusion in D-A polymers will be a very effec-
tive method for increasing the polymers’ effective conjugation
lengths. The hole mobilities of PQx and PphQx were deter-
mined to be 5.0 3 1025 and 2.2 3 1024 cm2 V21 s21, respec-
tively. A polymer solar cell device prepared with PphQx as the
active layer was found to exhibit a power conversion efficiency
(PCE) of 5.03%; thus, the introduction of phQx units enhanced
both the short circuit current density and PCE of the device.
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Chem. 2016, 00, 000–000
KEYWORDS: charge transport; conjugated polymers; functionali-
zation of polymers; heteroatom-containing polymers; organic
solar cell
possibilities remain, so the search for new electron donor
and electron acceptor molecules continues.
INTRODUCTION The development of polymer semiconduc-
tors is exciting because they enable the fabrication with
printing techniques of polymer solar cells (PSCs).1,2 Recently,
significant progress has been made in this field, and the
power conversion efficiencies (PCEs) of PSCs have reached
approximately 10% as a result of the development of new
materials, device engineering, and the use of new processing
techniques.3–6 Currently, most research is focused on the
design and synthesis of new materials with low band gaps
to enable efficient absorption over a broad range of the solar
spectrum; other factors that must be considered in the
molecular design include the requirements of a deeper high-
est occupied molecular orbital (HOMO) to produce a high
open circuit voltage (Voc) and a high hole mobility for effi-
cient charge transportation.7,8 The polymerization of donor
(D) and acceptor (A) molecules results in a low band-gap
polymer due to the formation of an intramolecular charge
transfer (ICT) complex.9,10 High efficiency polymers require
the appropriate donor and acceptor units, but unexplored
Quinoxaline units (Qx) are electron-withdrawing molecules
because of the electron-withdrawing properties of the two
sp2 hybridized nitrogen atoms in the Qx units.11–14 Further-
more, the phenyl rings in Qx units can easily be connected
through an oxidative coupling reaction to form fused aro-
matic structures, the so-called phenanthrenequinoxaline
(phQx) units. These phQx molecules exhibit higher planarity
and better intermolecular packing due to their fused struc-
ture and extended conjugation length, which enhances the
backbone planarity and p2p stacking of the corresponding
polymers.15–20 Therefore, phQx was thought to be a highly
promising unit for the construction of donor-acceptor poly-
mers. However, the highly fused rigid structure of phQx units
leads to very poor solubility, which limits their applications
in conjugated polymers. Recent work has shown that the
rigid phQx unit can be copolymerized with a highly soluble
Additional Supporting Information may be found in the online version of this article.
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SCHEME 1 Synthetic routes for the monomers.
donor counterpart, which improves its solubility.17–19 Liu
et al. reported the improvement of the solubility of a phQx-
based polymer by introducing alkyl side chains into the
thiophene-linker units between the phQx acceptor and the
benzo[1,2-b:4,5-b’]dithiophene (BDT) donor unit.21 In this
study, we synthesized a new soluble phQx unit by introduc-
ing four branched alkoxy side chains into the two fused phe-
nyl rings of the phQx unit, then polymerized this unit with a
alkylthienyl benzo[1,2-b:4,5-b’]dithiophene (BDTT) donor to
produce the D-A polymer PphQx. The addition of highly pla-
nar phQx groups to the polymer was expected to broaden
the absorption band and enhance the carrier mobility and
thus to improve the short circuit current (Jsc) without signifi-
cant phase separation with PC70BM ([6,6]-phenyl C71-butyric
acid methyl ester). The optical, electrochemical, and photo-
voltaic properties of these polymers were investigated. For
comparison, the tetra-alkoxy-substituted Qx-based BDTT
polymer (PQx) was also synthesized with the same polymer-
ization method.
50 mV/s. Polymer film coatings on ITO anode electrode
were formed by spin-coating method.
Fabrication of the Organic Solar Cells
The devices were fabricated with the structure ITO/
PEDOT:PSS/polymer:PC70BM/Ca/Al. The PEDOT:PSS layer
was spin-coated onto each ITO anode from a solution pur-
chased from Heraeus (CleviosTM P VP AI4083) and baked
for 20 min at 140 8C in glove box. Each polymer:PC70BM
solution was then spin-coated onto the PEDOT:PSS layer. The
polymer solution for spin-coating was prepared by dissolving
the polymer (1 wt %) in chlorobenzene. Preannealing was
not carried out. LiF and aluminum contacts were formed by
vacuum deposition at pressures below 3 3 1026 torr, pro-
viding an active area of 0.09 cm2. Solar cell efficiencies were
characterized under simulated 100 mW/cm2 AM 1.5G irradi-
ation from a Xe arc lamp with an AM 1.5 global filter. Simu-
lator irradiance was characterized using
a calibrated
spectrometer, and the illumination intensity was set using an
NREL certified silicon diode with an integrated KG1 optical
filter. The EQE was measured by underfilling the device are
a using a reflective microscope objective to focus the light
output from a 100 W halogen lamp outfitted with a mono-
chromator and optical chopper; the photocurrent was meas-
ured using a lock-in amplifier, and the absolute photon flux
was determined using a calibrated silicon photodiode. All
device fabrication procedures and measurements were car-
ried out in air at room temperature.
EXPERIMENTAL
Instrumentations
1H and 13C NMR spectra were recorded on Bruker ADVANCE
300 spectrometer, with tetramethylsilane as an internal ref-
erence. Elemental analysis was done by using PerkinElmer
2400 Series II CHNS/O Analyzer. The optical absorption
spectra were measured by a Shimadzu UV-3100 UV–vis–NIR
spectrometer. The number- and weight-average molecular
weights of polymers were determined by gel permeation
chromatography (GPC) on Viscotek T60A instrument, using
chloroform as eluent and polystyrene as standard. The differ-
ential scanning calorimetry (DSC) was measured by using a
TA Q100 instrument and operated under nitrogen atmos-
phere. Cyclic voltammetry was performed on an AUTOLAB/
PG-STAT12 model system with a three-electrode cell in a
solution of Bu4NPF6 (0.10 M) in acetonitrile at a scan rate of
Fabrication of Hole-Only Devices
The hole mobilities of the PQx1 and PQx2 films were deter-
mined by employing the space-charge limited current (SCLC)
model to the J–V measurement of the device. The hole only
devices were fabricated with the structure ITO/PEDOT:PSS/
polymers/Au. The hole mobilities of the fabricated devices
were calculated from the SCLC by the following equation:
2
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SCHEME 2 Synthetic routes for PQx and PphQx.
J
SCLC5 ð9=8Þere0lðV2=L3Þ
(Mw) of PQx and PphQx were determined by performing gel
permeation chromatography (GPC) with a polystyrene stand-
ard in chloroform eluent, and were found to be 66.9 kDa
where er is the dielectric constant of the polymer layer, e0 is
permittivity of free space, l is the SCLC mobility, L is the dis-
tance between the cathode and anode, which is equivalent to
the film thickness, and V is the applied voltage.
-
-
(D 5 2.6) and 66.1 kDa (D 5 2.6), respectively. The synthe-
sized polymers readily dissolve in chlorinated organic sol-
vents such as chloroform and chlorobenzene and form good
uniform thin films. The introduction of the 3,7-dimethyloctyl
substituent was found to be sufficient to impart the required
solubility to the polymers in these solvents. Thermal analysis
of the polymers was conducted with thermogravimetric anal-
ysis (TGA), as shown in Supporting Information Figure S1.
PQx and PphQx were found to exhibit sufficiently high
decomposition temperatures (Td(5%)), 402 and 416 8C,
respectively; PphQx is more thermally stable than PQx. The
polymerization results for the synthesized polymers are
summarized in Table 1.
Materials
See Supporting Information.
RESULTS AND DISCUSSION
Synthesis and Characterization of the Monomers
and Polymers
The synthetic routes for the monomers and polymers are
shown in Schemes 1 and 2. As shown in Scheme 1, a
branched alkyl group (3,7-dimethyloctyloxy) were introduced
at the 3,4-positions of the two phenyl rings in the phQx unit
to enhance the solubility of the D-A polymers. PQx and
PphQx were obtained via Stille coupling reactions between
compounds 12 and 11 and compounds 12 and 8, respec-
tively (Scheme 2). The weight average molecular weights
Optical and Electrochemical Properties
The UV–visible absorption spectra of PQx and PphQx in
dilute chloroform solutions and in thin films are shown in
Figure 1; the absorption data for the polymers are summar-
ized in Table 2. The UV-visible spectrum of the PQx thin film
has two kmax at 436 and 585 nm, and those of the PphQx
thin film are at 443 and 593 nm, which is a common feature
of donor–acceptor-type polymers.22,23
TABLE 1 Molecular Weights and Thermal Properties of the
Synthesized Polymers
Polymers
Mn (g/mol)
Mw (g/mol)
Ð
Td(5%) (8C)a
The absorption peaks at the shorter wavelengths probably
originate from the p2p* transitions of the polymer back-
bones, whereas the absorption bands at the longer wave-
lengths are probably due to ICT between the donor–acceptor
molecules.24,25 The absorption bands of PphQx are red-
PQx
25.1K
23.2K
66.9K
66.1K
2.6
2.6
402
416
PphQx
a
Decomposition temperature at 5% weight loss.
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FIGURE 1 Normalized UV–visible absorption spectra of PQx and PphQx in (a) CHCl3 solution and (b) thin films. [Color figure can
be viewed in the online issue, which is available at wileyonlinelibrary.com.]
shifted with respect to those of PQx for both the solution
and thin film states because PphQx has a more planar geom-
etry than PQx. The effect of the planar structure of the phQx
units on the effective conjugation length of PphQx is greater
than that of the Qx units on PQx, and this planarity also
improves the strong interchain interactions of the PphQx
backbone, and thus leads to a red-shift in its absorption
bands. This result is consistent with those of other studies of
phQx-based polymers.15–17 Further, the absorption intensity
of PQx at 580 nm is higher than that of the short wavelength
band (420 nm) in the thin film state, whereas the opposite
trend was found for the absorption intensities of PphQx.
This result suggests that the phQx accepting groups interfere
more with the ICT between the D-A units in PphQx than is
the case for the Qx groups in PQx. This difference is not yet
well understood, but we suggest that it originates in the dif-
ference between the electron pulling abilities of the Qx and
phQx groups.
intermolecular interactions resulting from the introduction
of the planar phQx units increase the PL quenching.26,27
The HOMO levels of the polymer films were determined
from cyclic voltammetry (CV) measurements by using the
equation EHOMO 5 –(Eox 1 4.8) eV, where Eox is the onset oxi-
dation potential relative to the ferrocene external standard
(Supporting Information Fig. S2).28 The HOMO energy levels
of PQx and PphQx are 25.40 and 25.40 eV, respectively. The
optical band-gaps of the polymers were calculated from the
absorption onsets of the polymers: PQx and PphQx have
medium band-gaps of 1.82 and 1.75 eV, respectively. The cal-
culated lowest unoccupied molecular orbital (LUMO) levels
of the PQx and PphQx thin films were found to be 23.58
and 23.64 eV, respectively, which are higher (by ꢀ0.4 eV)
than the value obtained for the acceptor PC70BM ([6,6]-phe-
nyl C71-butyric acid methyl ester) (24.30 eV) and suggest
that the charge transfer from the two polymers to PC70BM
should be facile. The optical and electrochemical properties
of the polymers are summarized in Table 2.
The UV–visible absorption coefficients and photolumines-
cence spectra of the polymers are shown in Figure 2. The
absorption coefficients and photoluminescence measure-
ments were obtained from the same solution samples (chlo-
roform solvent, 5 3 1024 M), and the polymer solutions
were excited at the kmax of PQx and PphQx (559 nm for PQx,
570 nm for PphQx). The values of the absorption coefficients
(a) of PQx and PphQx are similar because they have similar
chromophore densities. The photoluminescence (PL) emis-
sion peaks of the PQx and PphQx solutions were found to be
at 654 and 691 nm, respectively, as expected from their UV–
visible spectra. Note that the PL intensity of PphQx is signifi-
cantly lower than that of PQx; it is possible that the strong
Theoretical Calculations
The molecular structures of PQx and PphQx were optimized
by using density functional theory (DFT) calculations. The
electronic structures were modeled with density functional
theory and the Gaussian 09 program at the B3LYP/6-31(G)
level of theory.29 To simplify the calculations, the 2-
ethylhexyl and 3,7-dimethyloctyl groups were replaced with
methyl groups, and one repeating unit of each polymer was
subjected to the calculations. Figure 3 shows the electron
density distributions across the HOMO and LUMO, which
were estimated from the results for one repeat unit. The
TABLE 2 Optical and Electrochemical Properties of PQx and PphQx
Polymers
Solution (kmax, nm)
Film (kmax, nm)
Eg (eV)a
EHOMO (eV)
ELUMO (eV)b
PQx
426, 559
435, 570
436, 587
443, 594
1.82
1.75
25.40
25.40
23.58
23.64
PphQx
a
b
Estimated from the onset of the absorption of the thin solid film
Calculated from the optical band-gap energy. Values of the PphQx
solar cell devices are higher.
(Eg 5 1240/konset).
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FIGURE 2 UV–visible absorption coefficients and PL intensities
of polymers in chloroform solution at 5 3 1024 M. [Color figure
can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
FIGURE 4 J–V characteristics of the (a) PQx/PC70BM and (b)
PphQx/PC70BM devices. [Color figure can be viewed in the
online issue, which is available at wileyonlinelibrary.com.]
electron density distributions of the HOMO orbitals for the
two polymers are delocalized along the polymer backbone,
and the electron density of the LUMO is mainly located in
the electron-accepting units (Qx for PQx and phQx for
PphQx), which indicates that strong ICT states are possible
in these D-A polymers. Furthermore, PQx is estimated to be
a twisted structure from high dihedral angles (A1 5 40.68,
A2 5 36.68) between phenyl rings and Qx. This distorted con-
formation of PQx is expected to disrupt the close-packing of
the polymer chains, leading the blue-shift of absorption peak
for PQx with respect to PphQx.
Space-Charge Limited Current Characteristics
We measured the hole mobilities of the polymer thin films
by using the space-charge limited current (SCLC) method to
characterize the vertical carrier transport properties of the
organic photovoltaic (OPV) devices. To obtain the SCLC
FIGURE 3 Theoretical results for the model compounds: (a) chemical structures and (b) electron distributions of the HOMO/LUMO
frontier orbitals. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
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TABLE 3 Photovoltaic Properties of the Polymer/PC70BM
Devices
Blend
ratio
VOC
(V)
JSC
FF
PCE
(%)
Polymer
PQx
(mA/cm2)
(%)
1: 1
1: 2
1: 1
1: 2
0.84
0.85
0.82
0.80
8.59
0.47
0.52
0.49
0.58
3.40 (3.36)a
3.97 (3.92)a
4.62 (4.59)a
5.03 (4.98)a
8.96
PphQx
11.38
10.85
a
Average PCE value for five devices.
curves, hole-only devices were fabricated with the structure
ITO/PEDOT:PSS/polymer/Au and the mobilities were calcu-
lated with the Mott-Gurney equation.30 The voltage–current
density curves of the fabricated hole-only devices are shown
in Supporting Information Figure S3. The measured SCLC
mobilities of the PQx and PphQx films were found to be 5.0
3 1025 and 2.2 3 1024 cm2/Vs, respectively.
FIGURE 6 External quantum efficiencies of the (a) PQx/PC70BM
and (b) PphQx/PC70BM blend films. [Color figure can be viewed
in the online issue, which is available at wileyonlinelibrary.
com.]
The mobility of PphQx is slightly higher than that of PQx,
which suggests that the polymer chain packing in PphQx is
better than that in PQx.
rier mobility, and the active layer film formation properties.
Given the optical band-gaps and hole mobilities of the poly-
mers, the PphQx devices were expected to exhibit higher Jsc
values than the PQx devices. The light absorption bandwidth
and the hole mobility of PphQx (1.75 eV, 2.2 3 1024
Properties of the Photovoltaic Devices
cm2V21 21
5.0 3 1025 cm2V21 21
s
) are slightly larger than those of PQx (1.82 eV,
), which means that the Jsc and PCE.
PSCs were fabricated with the layered structure ITO/
PEDOT:PSS (60 nm)/polymer:PC70BM (80 nm)/Ca/Al
(100 nm). Figure 4 shows the J–V curves of the PSCs, and
their photovoltaic parameters are summarized in Table 3.
s
To better understand the differences between the photovol-
taic performances of the PSCs, AFM images of the surface
morphologies of each solar cell device were obtained (Fig.
5). In general, nanoscale phase separation is required to
overcome the short exciton diffusion lengths (ꢀ10 nm) of
donor polymers and provide efficient charge separation.31
The root-mean-square (RMS) roughnesses of the
PQx:PC70BM (1:2) and PphQx: PC70BM (1:2) films were
found to be 6.2 and 1.2 nm, respectively. Moreover, the phase
separation in the PphQx:PC70BM film is less than that in the
The PCEs of the PQx and PphQx devices were found to be
3.97 and 5.03%, respectively, for a polymer:PC70BM blend
ratio of 1:2. The main difference between the PQx and
PphQx devices was found in the short circuit current density
(Jsc). Moreover, regardless of the blend ratio, the PphQx
devices always exhibit higher Jsc values than the PQx devices.
The Jsc values of PSCs are affected by many factors, including
the light absorption of the active layer, the band-gap, the car-
FIGURE 5 AFM tapping-mode height images of (a) PQx/PC70BM (1:2) and PphQx/PC70BM (1:2). Scan size is 2 3 2 lm.
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the total area for charge separation between the donor poly-
mer and the PCBM acceptor, and leads to a higher photocur-
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separation are evident in the PQx:PC70BM blend film, which
is likely to decrease the photocurrent density.
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were measured to evaluate their photo-responses as a func-
tion of wavelength, and are shown in Figure 6. The PQx and
PphQx devices yield EQE plots that are similar to their UV–
visible absorption spectra, which indicate that their excitons
are mainly generated in the polymer phase (Fig. 1). The Jsc
values calculated by integrating the EQE curves were found
to be within 10% of the corresponding Jsc values obtained
from the J–V curves. The spectral response of the PQx:PCBM
(1:2) device shows that photons with wavelengths in the
range 350–800 nm contribute significantly to the EQE, with
a maximum EQE of 50% at 500 nm. The PphQx:PCBM (1:2)
device exhibits a similar spectral response in the range 350–
800 nm; however, a maximum EQE of 62% was obtained at
400 nm, which results in a much higher Jsc.
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spectra, energy levels, charge transport, blend film morphol-
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