Development of naphthalene and quinoxaline-based donor-acceptor conjugated copolymers for delivering high open-circuit voltage in photovoltaic devices

Article abstract of DOI:10.1002/pola.26567

Two new quinoxaline-based polymers, poly[1,5-didecyloxynaphthalene-alt-5, 5′-(5,8-dithiophen-2-yl)-2,3-bis(4-octyloxyphenyl)quinoxaline (PNQx-p) and poly[1,5-didecyloxynaphthalene-alt-5,5′-(5,8-dithiophen-2-yl)-2,3-bis(3- octyloxyphenyl)quinoxaline (PNQx-m), were synthesized by Suzuki coupling reaction and characterized. Thermogravimetric analysis revealed that these polymers are thermally stable with degradation temperature up to 320 °C. As evident from the electrochemical and optical studies, the copolymers have comparable optical band gap (～2 eV) and nearly similar deep highest occupied molecular orbital (HOMO) energy levels of -5.59 (PNQx-p) and -5.61 eV (PNQx-p). The resulting copolymers possessed relatively low HOMO energy levels promising good air stability and high open circuit voltage (V_oc) for photovoltaic applications. The optimized photovoltaic device with a structure of ITO/PEDOT:PSS/PNQx-m:PC₇₁BM (1:2, w/w)/LiF/Al shows a power conversion efficiency up to 2.29% with a short circuit current density of 5.61 mA/cm², an V_oc of 0.93 V and a fill factor of 43.73% under an illumination of AM 1.5, 100 mW/cm². The efficiency of the PNQx-m polymer improved from 2.29 to 2.95% using 1,8-diiodoocane as an additive (0.25%).

Full text of DOI:10.1002/pola.26567

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Development of Naphthalene and Quinoxaline-Based Donor–Acceptor
Conjugated Copolymers for Delivering High Open-Circuit Voltage in
Photovoltaic Devices
Myung-Jin Baek,1 Hanok Park,1 Pranabesh Dutta,1 Woo-Hyung Lee,2
In-Nam Kang,2 Soo-Hyoung Lee1
1School of Semiconductor and Chemical Engineering, Chonbuk National University, Jeonju 561-756,
Republic of Korea
2Department of Chemistry, The Catholic University of Korea, Bucheon 420-743, South Korea
Correspondence to: S.-H. Lee (E-mail: shlee66@jbnu.ac.kr)
Received 5 December 2012; accepted 12 January 2013; published online 7 February 2013
DOI: 10.1002/pola.26567
ABSTRACT: Two new quinoxaline-based polymers, poly[1,5-
didecyloxynaphthalene-alt-5,50-(5,8-dithiophen-2-yl)-2,3-bis(4-
octyloxyphenyl)quinoxaline (PNQx-p) and poly[1,5-didecyloxy-
naphthalene-alt-5,50-(5,8-dithiophen-2-yl)-2,3-bis(3-octyloxyphe-
nyl)quinoxaline (PNQx-m), were synthesized by Suzuki
coupling reaction and characterized. Thermogravimetric analy-
sis revealed that these polymers are thermally stable with deg-
radation temperature up to 320 ꢀC. As evident from the
electrochemical and optical studies, the copolymers have com-
parable optical band gap (ꢁ2 eV) and nearly similar deep high-
est occupied molecular orbital (HOMO) energy levels of ꢂ5.59
(PNQx-p) and ꢂ5.61 eV (PNQx-p). The resulting copolymers
possessed relatively low HOMO energy levels promising good
air stability and high open circuit voltage (Voc) for photovoltaic
applications. The optimized photovoltaic device with a struc-
ture of ITO/PEDOT:PSS/PNQx-m:PC71BM (1:2, w/w)/LiF/Al
shows a power conversion efficiency up to 2.29% with a short
circuit current density of 5.61 mA/cm2, an Voc of 0.93 V and a
fill factor of 43.73% under an illumination of AM 1.5, 100 mW/
cm2. The efficiency of the PNQx-m polymer improved from
2.29 to 2.95% using 1,8-diiodoocane as an additive (0.25%).
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Chem. 2013, 51, 1843–1851
KEYWORDS: charge transfer; conjugated polymers; naphthalene; funct-
ionalization of polymers; organic solar cells; quinoxaline; synthesis
INTRODUCTION The synthesis and study of conjugated poly-
mers has been the subject of broad research in recent years
because of their utility in wide-ranging optoelectronic appli-
cations, such as polymer light emitting diodes, polymer field
effect transistors, sensors, photo detectors, and polymer so-
lar cells (PSCs).1–4 The use of conjugated polymers in or-
ganic solar cells5–12 is of interest because of the promise of
inexpensive, large area materials for energy conversion.
Among several structures of organic solar cells, the introduc-
tion of a bulk heterojunction (BHJ) structure has led to in-
tensive research and rapid development in the field of PSCs.
The BHJ is formed by blending an electron-rich polymer as a
donor and an electron-deficient fullerene derivative as an
acceptor.13–15
yield a large short circuit current (Jsc), fill factor (FF), and
relatively deep highest occupied molecular orbital (HOMO)
energies to achieve a large open circuit voltage (Voc).16–19 To
further improve the efficiency, low-bandgap polymers would
better match the solar spectrum and thereby produce higher
Jsc. However, a relatively low HOMO energy level of such a
low-bandgap polymer must be maintained to avoid Voc loss.
According to the BHJ solar cell model and experimental
results, the Voc is related to the energy difference between
the HOMO level of the donor and the lowest unoccupied mo-
lecular orbital (LUMO) level of the acceptor.20–23 Large band
gap polymers have a HOMO energy level of ꢂ5.0 to ꢂ5.2 eV,
which achieves an Voc of 0.5–0.7 V in a BHJ device. However,
the large band gap limits the short circuit current (Jsc).
Therefore, to produce a higher PCE, an ideal conjugated
polymer with a medium bandgap and deeper HOMO level is
required.
For a polymer to serve as an effective donor material in BHJ
PSCs, a number of properties are necessary, including broad
absorption from the solar spectrum for efficient light har-
vesting, efficient charge transfer to the acceptor material
(typically fullerene derivatives), effective charge transport to
Recently, several classes of conjugated polymers have been
developed to better harvest the solar spectrum with deeper
Additional Supporting Information may be found in the online version of this article.
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HOMO energies that can potentially increase the Voc.24–28 You
and coworkers proposed that a weak donor is beneficial for
maintaining a deep-lying HOMO level, while a strong acceptor
help to decrease the LUMO levels and band gaps of the resulting
polymers via intra molecular charge transfer transition.29 Dif-
ferent electron-deficient moieties can be used to tune the elec-
tronic energy gap of a semiconducting polymer, while the deep
HOMO of the electron-sufficient units leads to greater Voc val-
ues.30 Therefore, naphthalene derivatives with fusing two less
electron-rich benzene rings could be used as donor units to
form relatively deep HOMO levels that can produce high Voc val-
ues in solar cell devices and long-range intermolecular p–p
stacking arrangements.31 In addition, the introduction of decy-
loxy group can increase the stacking characteristics through
easy interdigitation and may tune the electronic properties of
naphthalene due to the flexible properties of such groups.32
Owing to their strong p-stacking and intense fluorescence prop-
erties, these derivatives are often used as mesogenic cores of
liquid crystals and fluorophores of luminescent materials.33–36
However, despite its promises as a new electron donor material,
we found that there has been very few report so far its uses in
D-A conjugated polymers for solar cell applications. Recently,
Kwon and coworkers synthesized the conjugated polymers, pol-
y(50,500-bithiophene-alt-2,6-[(1,5-didecyloxy)naphthalene])
(PBDN) and poly(4-(5-(1,5-bis (decyloxy)-naphthalen-2-yl)th-
iophen-2-yl)-7-(thiophen-2-yl) benzo[c][1,2,5]thiadiazole)
(PDNTBT), from didecyloxy naphthalene, bithiophene and
didecyloxy naphthalene, a 4,7-dithien-2-yl-2,1,3-benzothia-
diazole unit.37–38 Photovoltaic performances of 1.3%
(PBDN) and 4.12% (PDNTBT) were observed with a high Voc
of 0.83 V.37–38 Alternatively, polymers containing quinoxa-
line (Qx) as an electron-deficient moiety were successfully
developed and exhibited promising photovoltaic perform-
ance due to that very high electron-affinity N-heterocycle of
Qx can effectively tune the energy level and reduce the band
gap of the resulting copolymers.39–42
coupling reaction to yield alternating copolymers. The result-
ing polymers showed good solubility, moderate optical
bandgaps and deep HOMO energy levels to serve as donor
materials in PSC applications. More importantly, benefiting
from the desirable HOMO values, the BHJ solar cell device
based on poly[1,5-didecyloxynaphthalene-alt-5,50-(5,8-dithio-
phen-2-yl)-2,3-bis(4-octyloxyphenyl)quinoxaline
(PNQX-p)
and PNQx-m with PC71BM exhibited noticeably high Voc
ꢁ
0.94–0.96 V and comparable or larger PCE than many Qx
21,45–48
based polymers reported in literature.
EXPERIMENTAL
Materials and Instruments
All chemicals were purchased from Aldrich and used without
further purification. The following compounds were synthesized
using modified literature procedures: 4,7-Bis(5-bromothiophen-
2-yl)benzo[c][1,2,5]thiadiazole,49 2,6-bis(4,40,5,50-tetramethyl-1,3,
2-dioxaborolane)-1,5-didecyloxynaphthalene (3)50 and 5,8-bis(5-
bromothiophen-2-yl)-2,3-bis(3-octyloxyphenyl)quinoxaline (4)51
were synthesized as previously described.
NMR spectra were recorded using a JEOL FT-NMR (400
MHz) spectrometer. Molecular weights were determined by
size exclusion chromatography with a Viscotec gel permea-
tion chromatography (GPC) system (Model 350 HTGPC) in
chloroform (CHCl3) as
a solvent. Chemical shifts were
reported as d values (ppm) relative to the internal standard
tetramethylsilane. UV-visible spectra were obtained using a
V-670 (JASCO) spectrophotometer. Thermogravimetric analy-
sis (TGA) was performed on a TA instrument Q-50 at a heat-
ing rate of 20 ꢀC/min under nitrogen. The temperature of
degradation corresponded to a 5% weight loss. Differential
scanning calorimetry (DSC) experiments were performed on
a TA Instrument (DSC 2910) at a heating rate of 10 ꢀC/min
under a nitrogen atmosphere.
Cyclic voltametry (CV) was performed using a VersaSTAT3
(METEK) for polymer films deposited on ITO glass in an ace-
tonitrile solution of 0.10 M n-Bu4NPF6 under argon at a scan
rate of 50 mV/s. A Pt wire and Ag/Agþ were used as coun-
ter and reference electrodes, respectively. Atomic force mi-
croscopy (AFM) was performed using a MultiMode þ Bio-
Scope (Digital Instruments) in tapping mode. ITO/
PSS:PEDOT/polymer:PC71BM/LiF/Al photovoltaic devices
were fabricated. Patterned ITO glass substrates were cleaned
using sequential ultrasonication in detergent, water, acetone,
and 2-propanol. After drying the substrate, PEDOT:PSS (Cle-
vios P VP AI4083) was spin-coated on the ITO glass
(2800rpm for 40s) to a thickness of ca. 40 nm, and dried at
110 ꢀC for 10 min. Polymer and PC71BM in chlorobenzene
were spin-coated to the substrate in a nitrogen-filled glove-
box. LiF (ca. 1.0 nm) and Al (ca. 150 nm) layers were depos-
ited by thermal evaporation under a vacuum (10ꢂ6 Torr).
The current density-voltage (J–V) characteristics of the devi-
ces in the dark and under white light illumination were
measured using an AM 1.5G solar simulator (Newport) at
100 mW/cm2, adjusted with a standard PV reference cell (2
ꢃ 2 cm2 monocrystalline silicon solar cell, calibrated at
NREL, CO) with a Keithley 2400 source-measurement unit.
Although many efforts have focused on the development of
D-A conjugated polymers based on Qx derivatives,39–43 there
are no reports of D-A conjugated polymers containing dia-
lkoxy-substituted naphthalene and Qx for PSCs. Therefore, in
this work, we synthesized copolymers containing a coplanar
naphthalene unit as a weak donor and a Qx unit with p- and
m-alkoxy groups as a strong acceptor. Alkyl or alkoxy side
chains are commonly incorporated into the donor and
acceptor backbones of D-A polymers to ensure high solubil-
ity and allow for solution processing. Alkoxy groups have
stronger electron donating effects than alkyl groups and the
proper placement of the alkoxy group is important to induce
a pronounced effect on the Voc of the resulting PSCs.39,44
Generally, electron acceptors containing an alkoxy substitu-
ent show deeper HOMO values and more effectively tune to
a high Voc in solar cell devices.39,44 Therefore, we designed
and synthesized two polymers with a naphthalene-Qx back-
bone containing alkoxy groups but with different positions
of para and meta in Qx to ensure high solubility and achieve
deep HOMO value of the polymers. Both co-monomers were
prepared in multi-step procedures and combined via Suzuki-
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ꢀ
The external quantum efficiency (EQE) was determined
using a Polaronix K3100 spectrometer.
vigorously stirred at 115–120 C under argon for 60 h. Phe-
nylboronic acid (20 mg in 0.5 mL of CB) was added. After 2
h, 1-bromobenzene (0.3 mL) was introduced. The polymer
was purified by precipitation in methanol/HCl (10/1), fil-
tered through a thimble and washed in Soxhlet apparatus
with methanol, acetone, and chloroform. The chloroform
fraction was reduced to 3–5 mL under reduced pressure,
precipitated in methanol (200 mL), filtered through paper
and finally air-dried overnight. PNQx-p was obtained as a
dark red solid (110 mg, yield 41.9%). GPC: Mn ¼ 46,000 g/
mol, Mw ¼ 135,000 g/mol, polydispersity indexe (PDI) ¼
2.91.
Synthesis
1,2-Bis-[3-octyloxyphenyl]-ethane-1,2-dione (1)
A Grignard reagent was prepared by the drop-wise addition
of 1-bromo-3-octyloxy-benzene (7.71 g, 27.03 mmol) in tet-
rahydrofuran (THF) (15 mL) to a suspension of magnesium
(730 g, 30.03 mmol) in THF (15 mL). In a separate flask, a
solution of LiBr (4.69 g, 54.06 mmol) was added to a stirred
suspension of CuBr (3.88 g, 27.03 mmol) in THF (10 mL).
The mixture was stirred until homogeneous and was then
cooled in an ice-water bath. The Grignard reagent was added
to the LiBr/CuBr suspension. Oxalyl chloride (1.37 g, 10.81
mmol) was then added drop-wise to the cold solution, and
after 30 min the reaction was quenched with saturated
NH4Cl. The mixture was extracted with ether. The organic
solvent was dried over anhydrous MgSO4 and the solvent
was removed to produce a brownish solid. This solid was
subject to chromotography over silica gel using hexane:-
methylene chloride (MC) (5:1) as an eluent. Compound 1
was obtained as a white solid (2.21 g, 44%).
Poly[1,5-didecyloxynaphthalene-alt-5,50-(5,8-dithiophen-2-
yl)-2,3-bis(3-octyloxyphenyl)quinoxaline (PNQx-m)
A mixture of compound 3 (237 mg, 0.342 mmol), 2 (298 mg,
0.342 mmol), Pd(PPh3)4 (20 mg, 5 mol %) and tri(cyclohex-
yl)phosphine (10 mg, 10 mol %) was dissolved in 7.0 mL
degassed chlorobenzene and 1.5 mL degassed 20% aqueous
tetraethylammonium hydroxide. The reaction mixture was vig-
orously stirred at 115–120 ꢀC under argon for 64 h. Phenyl-
boronic acid (20 mg in 0.5 mL of CB) was added. After 2 h, 1-
bromobenzene (0.4 mL) was introduced. The mixture was
allowed to reflux for 3 h. The polymer was purified by precipi-
tation in methanol/HCl (10/1), filtered through a thimble and
washed in Soxhlet apparatus with methanol, acetone and chlo-
roform. The chloroform fraction was reduced to 3–5 mL under
reduced pressure, precipitated in methanol (200 mL), filtered
through paper and finally air-dried overnight. PNQx-m was
obtained as a dark red solid (248 mg, yield 63.5%). GPC: Mn
¼ 49,000 g/mol, Mw ¼ 87,000 g/mol, PDI ¼ 1.78.
1H NMR (CDCl3, 400 MHz, d): 7.53 (d, 4H), 3.90 (t, 4H), 1.74
(m, 4H), 1.57–1.30 (m, 20H), 0.90 (t, 6H); 13C NMR (CDCl3,
400 MHz, d): 194.56, 159.85, 134.19, 129.92, 122.94, 122.23,
113.54, 70.76, 39.38, 30.46, 29.04, 23.80, 23.01, 14.06, and
11.09. Anal. calcd for C30H42O4: C, 77.21; H, 9.07. Found: C
77.39; H 9.12.
5,8-Bis(5-bromothiophen-2-yl)-2,3-bis
(3-octyloxyphenyl)quinoxaline (2)
Zinc dust (571 mg, 8.73 mmol) was added to a solution of
RESULTS AND DISCUSSIONS
4,7-bis(5-bromothiophen-2-yl)benzo[1,2,5]thiadiazole
(4ꢀ00
Synthesis and Characterization
mg, 0.873 mmol) in acetic acid (20 mL) and stirred at 80 C.
After 2 h of reaction, the mixture was filtered and dione (1)
(265 mg, 0.567 mmol)ꢀwas added to the solution. The solu-
tion was stirred at 60 C overnight, whereupon orange crys-
tals precipitated. The solid material was filtered and washed
with water. Recrystallization in methanol/MC (ca. 10:1) was
performed to obtain an orange crystal product (420 mg,
55%).
4,7-Bis(5-bromothiophen-2-yl)benzo[c][1,2,5]thiadiazole, 2,6-bis(4,
40,5,50-tetramethyl-1,3,2-dioxaborolane)-1,5-didecyloxynaphthalene
(3) and 5,8-bis(5-bromothiophen-2-yl)-2,3-bis(3-octyloxyphe-
nyl)quinoxaline (4) were synthesized according to the litera-
ture.42–44 The synthetic routes to the two new polymers are
shown in Scheme 1. The monomer was satisfactorily charac-
1
terized using H NMR and 13C NMR. The alternating polymers
of the diboronic ester of naphthalene (3), the dibromo deriva-
tives of p- (4) and m-Qx (2) were prepared in degassing chlor-
obenzene using the Suzuki polymerization method with
Pd(PPh3)4 and PCy3 as catalysts. The polymers were then puri-
fied by multiple Soxhlet extractions to obtain the final products
as dark red solids. The resulting polymers exhibited good solu-
bility in most of the tested organic solvents, including tetrahy-
drofuran, chloroform, chlorobenzene, and o-dichlorobenzene.
1H NMR (400 MHz, CDCl3, d): 8.09(s, 2H), 7.57 (d, 2H), 7.55
(s, 2H), 7.22 (t, 2H), 7.12 (d, 2H), 7.10 (d, 2H), 6.98 (d, 2H),
4.04 (t, 4H), 1.82 (m, 4H), 1.54 (m, 4H), 1.34 (m, 16H) 0.99
(t, 6H); 13C NMR (CDCl3, 400 MHz, d): 159.36, 151.92,
139.47, 139.37, 136.42, 130.53, 129.03, 125.62,125.43,
122.90, 117.17, 115.17, 68.32, 31.85, 29.41, 29.33, 29.30,
26.17, 22.68, and 14.11. Anal. calcd for C44H48 Br2N2O2S2: C,
61.39; H, 5.62; N, 3.25; S, 7.45. Found: C 61.14; H 5.79; N,
3.19; S, 7.51.
The structures of the polymers were confirmed by the 1H
NMR measurement and the spectra are shown in Figure S1
(Supporting Information). The proton resonance peaks
observed at 8.2–6.8 ppm correspond to the aromatic rings.
The peaks at 4.1–3.8 ppm correspond to the AOCH2A
groups in the side chains of the polymers. The resonance
positions and shapes of these peaks are different due to the
p- and m-alkoxy groups in the quinoxaline molecules of the
polymers.
Poly[1,5-didecyloxynaphthalene-alt-5,50-(5,8-dithiophen-2-
yl)-2,3-bis(4-octyloxyphenyl)quinoxaline (PNQx-p)
A mixture of compound 3 (159 mg, 0.230 mmol), 4 (200 mg,
0.230 mmol), Pd(PPh3)4 (15 mg, 5 mol %) and tricyclohexyl-
phosphine (6.5 mg, 10 mol %) was dissolved in 6.0 mL
degassed chlorobenzene and 1.0 mL degassed 20% aqueous
tetraethylammonium hydroxide. The reaction mixture was
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SCHEME 1 Synthetic scheme of PNQx-p and PNQx-m.
The molecular weights of the copolymers were determined
using GPC in chloroform based on a calibration with monodis-
perse polystyrene standards. Using chloroform as the eluent,
the number-averaged molecular weights of PNQx-p and PNQx-
m were 46,000 and 49,000 g/mol, respectively (Table 1). The
PDIs were 2.91 and 1.78. The PDI of PNQx-p was little larger
than that of PNQx-p but both are the anticipated PDI values
for these types of polymerizations.52
Optical Properties
Figure 1(a) shows the UV–visible absorption spectra of the
polymers in a chlorobenzene solution and as thin films. Both
polymers absorbed light over ꢁ350–650 nm, however in the
solid state they exhibited red shifts by ꢁ15 nm compared to
the solution spectra. The red shift resulted from the closer
intermolecular electronic interaction in the solid state.53 The
optical band gap (Eogpt) values of PNQx-p and PNQx-m in the
solid film form were estimated from the absorption band
edge (konset) of the absorption spectra as 2.03 and 2.04 eV,
respectively. All optical properties are summarized in Table
1.
The thermal stability of the polymers was investigated with
TGA, as shown in Figure S2 (Supporting Information). The
TGA of PNQx-p and PNQx-m showed ca. 5 wt % loss on heat-
ing to 332 ꢀC and 321 ꢀC, respectively, indicating that they
were both sufficiently thermally stable for application in
PSCs. No obvious glass transition temperatures were
The PL emission of polymers in the chloroform solution was
gradually quenched when the PC71BM content increased
from 1:1 to 1:4 (not shown). As shown in Figure 1(b), the
ꢀ
observed for either polymer between 40 and 300 C.
TABLE 1 Optical, Electrochemical Properties, and Molecular Weight of Polymers
UV
CV
GPC
Sol.
Film
Polymer kmaxa [nm] Egc [eV] kmaxb [nm] Eg [eV]
E
ox[V]d/HOMOe [eV] Ered [V]/LUMOe [eV]
Mn
Mw
PDI
c
d
PNQx-p
507
2.11
2.08
524
536
2.04
2.02
1.19/ꢂ5.59
1.21/ꢂ5.61
d
ꢂ1.08/ꢂ3.32
ꢂ1.04/ꢂ3.36
46,000 135,000 2.91
49,000 87,000 1.78
PNQx-m 524
a
Measured in chlorobenzene solution.
Spin-coated film from chlorobenzene solution.
Potential determined by cyclic voltammetry in 0.10 M n-Bu4NPF6/
b
c
CH3CN.
opt
e
Optical band gap, Eg ¼ 1240/(konset).
HOMO ¼ ꢂe(4.4 þ Eox) (eV) and LUMO ¼ ꢂe(4.4 þ Ered) (eV).
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HOMO energy levels even though the alkoxy group was sub-
stituted with para and meta positions in the Qx unit, such
that the two polymers indicate that the HOMO level is delo-
calized along the polymer backbone, while the LUMO level is
primarily localized on the Qx alkoxy moiety.55 Compared to
the HOMO of P3HT (ꢂ5.0 eV)56, polymers were deeper by
ꢁ0.6 eV indicating a relatively high Voc could be expected for
the polymer-based solar cells because it is linearly correlated
with the difference of the HOMO of the donor and the LUMO
of the acceptor (Fig. 3). Additionally, the LUMO level of the
polymers are sufficient such that the energy difference
between the LUMO levels of the polymers and PCBM are
greater than 0.3–0.5 V, which are the minimum energy for
efficient charge separation of excitons formed in the
polymers.57
Mobility
As the mobility of a polymer is an important parameter that
can affect the performance of solar cells, the hole mobilities
of polymers were measured using an organic thin film tran-
sistor (OTFT) device with a bottom gate, top contact device
configuration built on an n-doped octadecyltrichlorosilane-
treated silicon wafer. Figure 4 shows the drain current
FIGURE 1 (a) Normalized UV–vis absorption spectra of PNQx-p
and PNQx-m in dilute chlorobenzene solution (S) and thin film
(F) at room temperature and (b) PL spectra of polymers and
polymers/PC71BM-blended film.
emission spectra of the polymers undergoes a significant
quenching on addition of PC71BM (1:1). This result suggests
that the photo-excited electrons of the polymers are more
effectively transferred to PC71BM in the solid state than the
solution due to closer interface interactions.
Electrochemical Properties
The electrochemical property of the polymers was examined
by CV, which was performed in a three-electrode cell with
0.10 M n-Bu4NPF6 as a supporting electrolyte in acetonitrile.
All potentials are reported versus Ag/Agþ with the ferro-
cene/ferrocenium couple used as an internal standard. The
cyclic voltammogram of the polymers are shown in Figure 2
and summarized in Table 1.
The HOMO and LUMO levels were calculated according to
the formula HOMO ¼ ꢂ(Eoxonset þ 4.4) eV and LUMO ¼
þ 4.4) eV.54 Based on the CV data, the HOMO energy
red
ꢂ(E
onset
levels of PNQx-p and PNQx-m were calculated to be ꢂ5.59
and ꢂ5.61 eV, respectively, and their LUMO levels were
found to be ꢂ3.32 and ꢂ3.36 eV. The electrochemical poten-
tial of polymers showed small differences in the LUMO and
FIGURE 2 Cyclic voltammogram of PNQx-p and PNQx-m films
deposited on ITO glass in an acetonitrile solution of 0.1 M
n-Bu4NF6 at a scan rate of 50 mV/s: (a) reduction and (b)
oxidation.
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TABLE 2 Solar Cell Characteristics of Polymers:PC71BM Devices
with Different Blend Ratios in Chlorobenzene
Jsc (mA/cm2)
Voc (V)
FF (%)
PCE (%)
Polymer:
PC71BM
PNQx-p
1:1
1:2
1:3
1:4
1:1
1:2
1:3
1:4
5.54
5.14
4.09
3.95
4.47
5.61
4.22
4.27
0.96
0.92
0.92
0.87
0.94
0.93
0.84
0.83
35.96
46.06
47.31
48.44
29.49
43.73
45.65
45.10
1.91
2.18
1.78
1.67
1.23
2.29
1.61
1.61
PNQx-m
FIGURE 3 Energy diagram of ITO, PEDOT:PSS, polymers
lene and Qx units. These mobility values are similar to other
quinoxaline containing polymers, APFOs (2–6 ꢃ 10ꢂ5 cm2/
(V s), which were reported by Andersson group.37
(P3HT, PNQx-p, PNQx-m), and PC71BM.
versus drain-source voltage characteristics and transfer char-
acteristics of the OTFT based on PNQx-p and PNQx-m. The
output curve shows good saturation behavior with clear sat-
uration currents. The device performance of PNQx-p and -m
exhibited hole mobilities of 4.5 ꢃ 10ꢂ5 and 3.0 ꢃ 10ꢂ5 cm2/
(V s), respectively, at room temperature and on/off ratio
exceeded 105 (Fig. 4). The polymers have similar mobilities
because the polymer backbones contain the same naphtha-
Photovoltaic Device
BHJ photovoltaic devices were fabricated with ITO/
PEDOT:PSS/polymer:PC71BM/LiF/Al structure using poly-
mer:PC71BM blends at a weight ratio of 1:1, 1:2, 1:3, or 1:4.
The solar cell characteristics, Jsc, Voc, FF, and PCE under a
FIGURE 4 Transfer characteristics of (a) PNQx-p and (b) PNQx-
FIGURE 5 (a) J–V and (b) EQE curves of PNQx-p and PNQx-m
m OTFTs.
solar cells.
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FIGURE 6 AFM images of (a) PNQx-p:PC71BM (1:2), (b) PNQx-m:PC71BM (1:2) blend films, (c–e) PNQx-m:PC71BM(1:2) blend films
with different contents of DIO: (c) 0.25%, (d) 0.5% (e) 1% (scan size: 2 ꢃ 2 lm).
simulated 1 sun of AM 1.5G radiation (100 mW/cm2) are
summarized in Table 2 (also in Fig. S3, Supporting Informa-
tion). The best PCE for the PNQx-m:PC71BM-based solar cells
was 2.29% (Jsc ¼ 5.61 mA/cm2, Voc ¼ 0.93 V, FF ¼ 43.73%)
while PNQx-p-based solar cells showed slightly worse results
wavelengths of the polymers contributed to photocurrent
generation.
The morphology of both polymer blend films with PC71BM was
observed using tapping-mode AFM. Figure 6(a and b) show
AFM images of PNQx-p and PNQx-m films blended with
PC71BM (1:2 ratio). The PNQx-p:PC71BM film shows a large do-
main size (ꢁ120 nm) and rougher surface morphology (rms ¼
9.03 nm) than that of PNQx-m:PC71BM film (rms ¼ 0.50 nm).
The favorable smooth surface and small domain size of PNQx-
m:PC71BM film is expected to produce a greater Jsc in the
PNQx-p:PC71BM device because a large p–n interface can
achieve significant charge generation and dissociation.58
with a maximum PCE of 2.18% (Jsc ¼ 5.14 mA/cm2, Voc
¼
0.92 V, FF ¼ 46.06%) when each polymer was mixed with a
PC71BM at a weight ratio of 1:2. Figure 5 contains the J–V
and EQE curves for these PSC devices. As expected, the pho-
tovoltaic devices showed large Voc values (PNQx-m ¼ 0.93 V
and PNQx-p ¼ 0.92 V), suggesting that they may be promis-
ing electron-donors for BHJ PSCs. The EQE of the devices are
shown in Figure 5(b). The devices show photoconversion
efficiencies from 300 to 700 nm, with EQE values greater
than 40% over 350–550 nm and a maximum EQE of ꢁ43%
at 500 nm occurred for PNQx-m and ꢁ42% at 480 nm for
PNQx-p. The shape of the EQE plots is similar to the absorp-
tion spectra of the devices, indicating that all absorption
The addition of 1,8-diiodoctane (DIO) as a processing addi-
tive to the PNQx-m:PC71BM(1:2) blend film was further
investigated to improve the device performance as it is
known to enhance device performances by inducing well-or-
dered phase separation.56 Supporting Information Figure S4
shows the J–V and EQE characteristics of the PNQx-
m:PC71BM device with different contents of DIO. By adding
0.25% DIO to the PNQx-m:PC71BM(1:2) film, the Jsc and FF
increased to approximately 13.4 and 24.8%, while the Voc
slightly decreased by approximately 7.5%, such that the PCE
was enhanced by approximately 30% compared to the devi-
ces without DIO (Table 3). However, the device performances
dramatically decreased by adding more than 0.5% DIO show-
ing a decrease in all solar cell parameters. The AFM study
indicates that this decrease is due to the large phase separa-
tion in the PNQx-m:PC71BM films by adding 0.5 and 1% DIO
TABLE 3 Solar Cell Characteristics of PNQx-m:PC71BM (1:2)
Devices with Different Contents of DIO
DIO (v/v %)
Jsc (mA/cm2)
Voc (V)
FF
PCE (%)
0
5.61
6.36
2.38
0.96
0.93
0.86
0.83
0.83
43.73
54.14
51.27
35.35
2.29
2.95
1.02
0.28
0.25
0.5
1
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[Fig. 6(c–e)]. Janssen and coworkers59 also reported PCEs of
devices dramatically decreased with increasing amounts of
DIO due to the degraded morphology with a large phase sep-
aration. As a result, the best PCE of 2.95% was obtained
from the PNQx-m:PC71BM(1:2)-based device with 0.25%
DIO.
10 N.-K. Persson, H. Arwin, O. Inganas, J. Appl. Phys. 1997,
205, 034503/1.
11 D. Gendron, M. Leclerc, Energy Environ. Sci. 2011, 4,
1225–1237.
12 C. Duan, F. Huang, Y. Cao, J. Mater. Chem. 2012, 22,
10416–10434.
13 G. Yu, J. Gao, J. C. Hummelen, F. Wudl, A. J. Heeger, Sci-
ence 1995, 270, 1789–1791.
CONCLUSIONS
14 Y. Li, Acc. Chem. Res. 2012, 45, 723–733.
High Voc copolymers, PNQx-p and PNQx-m, containing a qui-
noxaline unit as an electron acceptor and a didecyloxynaph-
thalene unit as a donor, were designed and successfully syn-
thesized through the Suzuki coupling reaction. The polymers
had a large band gap (> 2.0 eV) and their HOMO energy lev-
els were deeper (ꢁꢂ5.6 eV) than that of P3HT (ꢂ5.0 eV).
Photovoltaic devices made of PNQx-m and PNQx-p polymers
with PC71BM (1:2 w/w) demonstrated PCEs greater than 2%
with a 1:2 ratio in chlorobenzene solvent with a greater Voc
(0.91 V) than P3HT-based devices (< 0.60 V). The solar cell
device performance was further improved by adding 0.25%
DIO to a 1:2 ratio of PNQx-m:PC71BM and demonstrated a
maximum PCE of approximately 3%. More importantly, this
work shows that both naphthalene and quinoxaline with dia-
lkoxy substitution are promising, simple, and useful comono-
mer building blocks for solar cell materials in polymer pho-
tovoltaic applications. The solar cell performance of this type
of copolymer can further be improved by device fabrication
conditions and control of the phase morphology using cosol-
vents, annealing, and other processing additives. Work in
this direction is currently being performed in our laboratory
and the results will be presented in due course.
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(20103020010050) funded by the MKE, Republic of Korea.
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