Design, synthesis, and characterization of a novel c-donor-nc-bridge-c- acceptor type block copolymer for optoelectronic applications

Article abstract of DOI:10.1002/pola.27098

A novel conjugated block copolymer system containing a donor-type conjugated block (c-D) covalently connected to an acceptor type conjugated block (c-A) via a nonconjugated and flexible bridge chain (nc-B), also called a DBA type block copolymer, has been designed, synthesized, and characterized for potential cost-effective and high-efficiency optoelectronic applications such as solar cells. Specifically, D is a regio-regular para-2-ethylhexyloxy- substituted polyphenylenevinylene (or EH-RO-PPV), A is a regio-regular polyphenylenevinylene with sulfone (SO₂) acceptor moiety and a linear oxydecane (-OC₁₀H₂₁) group substituted on every phenylene unit, and B contains an aliphatic chain with two or four methylene units. The size of each block can be controlled via synthetic feed ratio of the monomer and the terminator. The measured average molecular weights of D, A, and DBA based on gel permission chromatography are in good agreements with the molecular weights calculated using the monomer:terminator synthetic feed ratios. Preliminary optoelectronic device studies revealed an order of magnitude better improvement in photoelectric power conversion efficiency of DBA over the corresponding D/A blend under identical fabrication and testing conditions. Such improvements could be attributed to more efficient photo induced charge separation and charge transport in DBA versus in D/A blends. 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 1149-1160 A novel conjugated block copolymer system, containing a donor type conjugated block (c-D) covalently connected to an acceptor type conjugated block (c-A) via a non-conjugated and flexible bridge chain (nc-B), is designed, synthesized, and characterized for potential cost-effective and high-efficiency optoelectronic applications. Preliminary optoelectronic device studies reveal improvement that is an order of magnitude better in the photoelectric power conversion efficiency of DBA over the corresponding D/A blend under identical fabrication and testing conditions. Copyright

Full text of DOI:10.1002/pola.27098

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Design, Synthesis, and Characterization of a Novel c-Donor-nc-Bridge-c-
Acceptor Type Block Copolymer for Optoelectronic Applications
Sam-Shajing Sun,1,2 Jaleesa Brooks,1 Thuong Nguyen,1 Cheng Zhang1,2*
1Center for Materials Research, Norfolk State University, Norfolk, VA
2Chemistry Department, Norfolk State University, Norfolk, VA
Correspondence to: S.-S. Sun (Email: ssun@nsu.edu) or C. Zhang (Email: cheng.zhang@sdstate.edu)
Received 14 November 2013; accepted 29 December 2013; published online 4 February 2014
DOI: 10.1002/pola.27098
ABSTRACT: A novel conjugated block copolymer system contain-
ing a donor-type conjugated block (c-D) covalently connected to
an acceptor type conjugated block (c-A) via a nonconjugated
and flexible bridge chain (nc-B), also called a DBA type block
copolymer, has been designed, synthesized, and characterized
for potential cost-effective and high-efficiency optoelectronic
applications such as solar cells. Specifically, D is a regio-regular
para-2-ethylhexyloxy-substituted polyphenylenevinylene (or EH-
RO-PPV), A is a regio-regular polyphenylenevinylene with sul-
lated using the monomer:terminator synthetic feed ratios.
Preliminary optoelectronic device studies revealed an order of
magnitude better improvement in photoelectric power conver-
sion efficiency of DBA over the corresponding D/A blend under
identical fabrication and testing conditions. Such improvements
could be attributed to more efficient photo induced charge sepa-
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ration and charge transport in DBA versus in D/A blends. 2014
Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem.
2014, 52, 1149–1160
fone (SO2) acceptor moiety and a linear oxydecane (-OC10H21
)
group substituted on every phenylene unit, and B contains an
aliphatic chain with two or four methylene units. The size of
each block can be controlled via synthetic feed ratio of the
monomer and the terminator. The measured average molecular
weights of D, A, and DBA based on gel permission chromatog-
raphy are in good agreements with the molecular weights calcu-
KEYWORDS: block copolymers; charge transfer; conjugated
acceptor block; conjugated block copolymers; conjugated
donor block; donor-bridge-acceptor (DBA); electron transport;
light harvesting; nano morphology; optoelectronic; photovol-
taic; polyphenylenevinylenes (PPV); solar energy
binary system solid-state morphologies are desirably con-
sisted of two continuous phases or bi-continuous ordered
nanostructures (BONS) with at least one dimension around
20 nm (the average exciton diffusion length for PPVs6–11) in
each phase. The BONS type morphologies appear ideal for
both exciton diffusion and charge transport.6–11
INTRODUCTION Polymer-based optoelectronic materials and
devices are very attractive for future optoelectronic applica-
tions due to advantages such as cost effectiveness, light-
weight, and flexibility in terms of both device shape and
fabrications.1–10 Development of organic and polymer based
solar cells have advanced tremendously since the initial dem-
onstration of organic donor/acceptor binary phase concept,1
the ultrafast photo induced charge transfer between donor
and acceptor,2 the donor/acceptor large interface, or bulk
heterojunction BHJ concept.3–5 Though the photoelectric
power conversion efficiencies of the donor/acceptor conju-
gated polymer blend systems (also called “bulk hetero-
junction” systems) have reached about 10%,5 the blend
systems, however, exhibit inherent disadvantages. For
instance, due to the statistic distribution of domain sizes, it
is hard to avoid the formation of very large or small
domains, which are not desirable for organic or Frenkel type
exciton dissociation as well as charge transport. To further
minimize exciton and carrier losses, the donor/acceptor
Block copolymers are known to self-assemble into a variety of
BONS type morphologies at nanoscale,12–15 making it an
attractive candidate for solar cell applications. There have
been a number of efforts on the development of various block
copolymer systems containing conjugated donor and acceptor
blocks, since chemically different donor and acceptor blocks
may undergo phase separation on the nanometer
scale.7,10,11,16–28 When two different conjugated blocks are
covalently connected directly, charge separated states appear
not stable.20,27,28 Two popular approaches include, for exam-
ple, (1) block copolymers where a conjugated electron-
donating block is covalently linked to a nonconjugated flexible
*Present address: Cheng Zhang, Department of Chemistry, South Dakota State University, Brookings, South Dakota
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2014 Wiley Periodicals, Inc.
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UV–vis absorption data were collected on a Perkin-Elmer
Lambda-1050 spectrophotometer. Luminescence spectra
were obtained from a Horiba Fluoromax-3 spectrofluorome-
ter. Thermal analysis was performed on a Perkin-Elmer
TGA6/DSC6 system. Polymer film thicknesses were meas-
ured with a Dektak-6M profilometer.
Gel Permeation Chromatography (GPC)
The polymer molecular weights were measured on a Visco-
tek T60A/LR40 triple-detector gel permeation chromatogra-
phy (GPC) system at ambient temperature. Polystyrene
standards were used for universal/conventional calibration.
Although CHCl3 is a better solvent than tetrahydrofuran
(THF) for PPVs, its refractive index (RI 5 1.446) is too high,
leading to very weak RI detector signal. THF, with a RI of
1.405 is used for all materials measured by GPC.
SCHEME 1 Scheme of potential solid-state stacking patterns of
(DBAB)n and DBA type block copolymers. [Color figure can be
viewed in the online issue, which is available at wileyonlineli-
brary.com.]
block with an electron acceptor as side chains,7,23,24 and (2)
donor/acceptor block copolymers where D/A-conjugated
blocks are separated by nonconjugated bridge units
(B).7,10,11,16–22
Cyclovoltammetry
Electrochemical studies were performed on a Bioanalytical
(BAS) Epsilon-100W tri-electrode cell system. Three electro-
des include a Pt working electrode, an ancillary Pt electrode,
and a silver reference electrode in a CH3CN solution of 0.01
M AgNO3 and 0.1 M tetrabutylammonium hexafluorophosph-
onate (TBA-HFP). The polymer samples were dissolved in
hot o-dichlorobenzene and then coated onto the Pt working
electrode. The measurements were performed in a N2-
purged 0.1 M TBA-HFP/acetonitrile solution at a scan rate of
100 mV/s. Between the experiments, the surface of the elec-
trodes were cleaned or polished. Ferrocene (2 mM in 0.10 M
TBA-HFP/CH3CN solution) was used as an internal reference
standard and its HOMO level of–4.80 eV was used in
calculations.
We have been working on the second approach and have
earlier developed a (DBAB)n type block copolymer earlier
(shown in Scheme 1, top), where D is a conjugated donor
block, A is a conjugated acceptor block, and B is a nonconju-
gated and flexible bridge chain.7,10,11,16–22 However, as
Scheme 1 shows, the (DBAB)n type block copolymer could
suffer from poor solid-state phase separation when the sizes
of the donor or acceptor blocks are not uniform. In contrary,
as shown in Schemes 1 and 2, a c-Donor-nc-Bridge-c-
Acceptor or DBA type block copolymer has a potential to
form D/A phase separated nanostructures even if the poly-
dispersity of the D or A are broad.21 Furthermore, as Scheme
1 bottom shows, the DBA phase separation may be further
enhanced with addition of D and A as plasticizers.
Optoelectronic Device Fabrication and Testing
Preliminary polymer solar cell devices are fabricated and
tested inside a custom built MBraum inert gas glove box sys-
tem coupled with a vacuum thermal deposition chamber
(vacuum up to 1 3 1027 mbar), a solar simulator (providing
a one-Sun or 100 mW/cm2, 1.5 AM visible light radiation),
and a current–voltage source-measure unit (Keithley SMU-
237), and a data processing PC.
Due to the rational discussed above, a DBA type block copol-
ymer (Scheme 2) containing a conjugated donor block c-D
covalently linked to a conjugated acceptor block c-A via a
non-conjugated and flexible bridge unit nc-B was thus
designed and developed. Photo induced charge separation in
small molecular DBA system are known to be very effec-
tive.29 As demonstrated in our earlier work, the sizes of the
donor and acceptor blocks can be chemically controlled to be
on the ranges of exciton diffusion lengths between 5 and 20
nm depending on the number of polymer repeat units. How-
ever, the synthesis of DBA turned out to be more challenging
than (DBAB)n mainly due to a need for unsymmetrically func-
tionalized monomers and one-end (mono-end) functionalized
D and A blocks as exhibited in Schemes (3 and 5).
Donor Synthesis
1,4-Bis(2-ethyl-hexyloxy)benzene (2)
In an argon environment, a mixture of DMSO (300 mL),
hydroquinone 1 (20 g, 182 mmol, 1 mol equiv), and 2-
ethylhexyl bromide (97 g, 454 mmol, 2.5 mol equiv) were
stirred and heated in a hot oil bath at 90 ꢀC, afterwards a
EXPERIMENTAL
Starting Materials and Instrumentation
All starting materials, reagents, and solvents were purchased
from commercial sources (mostly from Sigma-Aldrich and
Fisher-Scientific) and used as received unless noted other-
wise. NMR spectra were obtained from a Bruker Advance
300 MHz spectrometer with TMS as the internal reference.
SCHEME 2 Scheme of a DBA block copolymer, where D is a
conjugated electron-donating block, A is a conjugated electron
accepting block, and B is a non-conjugated and flexible bridge
unit. [Color figure can be viewed in the online issue, which is
available at wileyonlinelibrary.com.]
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SCHEME 3 Design and synthesis of a mono-end terminal functionalized donor block C8-RO-PPV (7, D).
SCHEME 4 Design and synthesis of a mono-end terminal functionalized acceptor block C10-SF-PPV (17, A).
SCHEME 5 Design and synthesis of an example DBA block copolymer. [Color figure can be viewed in the online issue, which is
available at wileyonlinelibrary.com.]
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FIGURE 1 Donor Monomer 5 (a) 1H NMR and (b) 13C NMR Spectra in CDCl3. [Color figure can be viewed in the online issue, which
is available at wileyonlinelibrary.com.]
solution of 1:1 solution of NaOH (23 g) and water (23 mL)
was slowly added dropwise to the stirred mixture over 10
min. The reaction mixture was diluted and neutralized with
100 mL of water and 100 mL of 10% NaHCO3 solution, then
the product was extracted with 250 mL of hexane, dried in
MgSO4, and condensed to give 44.15 g of a crude oil product
to be used in the next step.
1H NMR: d (ppm) 6.82 (s, 1H); 6.76 (s, 1H); 4.13–3.93 (m,
8H); 3.79 (t, J 5 6 Hz); 3.27 (d, J 5 22 Hz); 1.72 (m, 2H);
1.6–1.2 (m, 16H), 0.92 (t, J 5 7 Hz, 18H). Anal. Calcd.: C
66.91; H 10.19. Found: C 67.18; H 10.37.
[2,5-Bis(2-ethyl-hexyloxy)24-formyl-benzyl]phosphonic
acid diethyl ester (5, donor monomer)
To an ice bath cooled reaction mixture containing 4 (4.85 g,
10 mmol, 1 mol equiv), methylene chloride (33 mL), and a,a-
dichloromethyl ether (1.39 g, 12 mmol), titanium tetrachlor-
ide (7 mL, 35mmol) was added dropwise over 2 min. Imme-
diately, the reaction mixture color changed to red. An hour
later, the reaction mixture was treated with a stirred mixture
of K2CO3 (8.42 g), H2O (200 mL), and ice (50 g) in a 1 L
beaker. The reaction mixture was diluted/neutralized with
10% NaHCO3 solution and water, and then the product was
extracted with hexane. Dried in MgSO4 and condensed and
purified in a silica gel column (2:1 hexane:ethylacetate elu-
ent) to give 3.84 g (75% yield) of pure product.
2-Bromomethyl-1,4-bis(2-ethyl-hexyloxy)benzene (3)
In an argon environment, a mixture of 2 (39.68 g, 118.5 mmol,
1 mol equiv), paraformaldehyde (5.4 g, 1.5 mol equiv), glacial
acetic acid (50 mL, 7 mol equiv), and 30% HBr/HOAc (22 mL,
3 mol equiv) (11 mL added dropwise at start of rxn, the other
11 mL added in 2–3 mL increments ever 2 h over a 6-h period)
was heated at 60 ꢀC for 6.5 h. The heat was stopped and reac-
tion allowed to continue for another 3 h. A mixture of 2 (minor-
ity component) and 3 (majority component) were obtained.
The reaction mixture was diluted and neutralized with 10%
NaHCO3 solution, and then the product was extracted with hex-
ane, dried in MgSO4, and condensed into a crude product mix-
ture to be used in the next step without purification.
1H NMR in CDCl3 [also shown in Fig. 1(a)]: d (ppm) 10.45
(s, 1H); 7.29 (s, 1H); 7.10 (d, due to phosphorus, 1H); 4.10–
3.92 (m, 8H); 3.33 (d, J 5 22 Hz, 2H); 1.72 (m, 2H); 1.6–1.2
(m, 16H), 0.92 (t, J 5 7 Hz, 18H). 13C NMR in CDCl3 [also
shown in Fig. 1(b)]: d 11.25, 14.16, 16.43, 23.12, 24.05,
26.23, 28.06, 29.20, 30.71, 39.61, 62.17, 71.45, 77.33 (t, chlo-
roform), 108.94, 116.15, 123.92, 129.47, 150.94, 156.06,
189.33. Anal. Calcd.: C 65.60; H 9.63. Found: C 65.60; H 9.76.
[2,5-Bis(2-ethyl-hexyloxy)benzyl]phosphonic acid diethyl
ester(4)
In an argon environment, a crude product of 3 (63.21 g, 1
mol equiv) was added with triethyl phosphite (40 mL, 1.5
mol equiv) in 2 mL 3 20 times increments within a 5-min
period, and the reaction was then continued to be stirred in
a hot oil bath at 140 ꢀC for 3 h. The unreacted triethyl
phosphite was removed using a vacuum pump. The remain-
ing crude product residue was purified in a silica gel col-
umn to afford 15.5 g colorless oil pure product 4 (44%
yield).
(4-Fluoro-benzyl)phosphonic acid diethyl ester (6, the
terminator)
The terminator 6 was synthesized following a literature pro-
cedure reported before.30
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TABLE 1 Gel Permeation Chromotography data of D and A
The Donor Block C8-RO-PPV (7, D)
In a nitrogen glove box, and at room temperature, to a vig-
orously stirred mixture of donor monomer 5 (2.30 g, 4.49
mmol, 1 mol equiv) and terminator (4-fluoro-benzyl)-phos-
phonic acid diethyl ester 6 (45 mg, 0.183 mmol, 1/25 mol
equiv), 55 mL THF was added in 10 mL increments until
everything dissolved, t-BuOK (5.5 mL, 1.0 M in THF, 5.5
mmol) was added drop wise over 5 min, and the reaction
was allowed to continue for another 5 min. The reaction
mixture was taken out from the glove box and 150 mL of
methanol was added to precipitate out the reddish polymer
product. The suspension mixture was placed in the refrig-
erator for 1 h to catalyze precipitation of the polymer.
Finally, the suspension was slowly filtered via a Buchner
fine glass funnel, rinsed twice with MeOH and left to dry
overnight to give 1.345 g (58% yield) of donor polymer
block (D) or 7.
D, 7
A, 17
Mn
9,300 Daltons
21,700 Daltons
2.34
7,700 Daltons
19,300 Daltons
2.51
Mw
PDI
Repeat unit weight
358 g/mol
25.2
462 g/mol
16.1
# of Repeat units
(DP, measured)
# of Repeat units
(DP, targeted/
calculated)
25
15
(2-Decyloxy-5-decylsulfanyl-benzyl)phosphonic Acid
Diethyl Ester (11)
1H NMR in CDCl3 (also shown in Fig. 3): d (ppm) 7.7–7.4 (m,
br, PPV aromatic); 7.3–7.1 (m, br, PPV vinylenene); 3.9–4.1
(s, trans-OCH2), 3.5 (s, cis-OCH2), 2.0–1.1 (m, aliphatic CH
and CH2), 1.1–0.7 (m, aliphatic CH3). GPC data are tabulated
in Table 1. Other physical properties are listed in Discussion
section.
In an enclosed environment, 25.85 g of crude 10 (containing
about 20.41 g or 40.9 mmol 10) and triethyl phosphite (20
mL, 19.38 g, 116.6 mmol, 2.9 mol equiv) were vigorously
stirred and heated in a hot oil bath at 140 ꢀC for 18 h.
Triethyl phosphite was removed by high vacuum at 100
ꢀ
mbar at 160 C, the remaining product was purified using a
silica gel column (1:2 hexane/ethyl acetate solution) to give
a colorless oil pure product. Yield: ꢁ40%.
Acceptor Synthesis
1-Decyloxy-4-decylsulfanyl-benzene (9)
1H NMR: d (ppm) 7.36 (s, 1H); 7.23 (s, 1H); 6.78 (s, 1H);
4.10–3.9 (m, 6H); 3.27 (d, J 5 2 2Hz, 2H); 2.81 (t, J 5 6 Hz,
2H); 1.85–1.15 (m, 32H); 0.88 (t, J 5 4Hz, 12H). Anal. Calcd.:
C 66.87; H 10.32. Found: C, 67.10; H, 10.58.
A solution of 4-mercaptophenol 8 (12.62 g, 0.10 mol, 1 mol
equiv), potassium hydroxide (12.9 g, 0.23 mol, 2.3 mol
equiv), and deionized water (15 mL) in acetonitrile (400
mL) was heated to 90 ꢀC. Next, 1-bromodecane (44.24 g,
0.20 mol, 2 mol equiv) was added. After reacting for 48 h,
the reaction mixture was poured into deionized water
(ꢁ800 mL). The white colored solid was collected by filtra-
tion and then dissolved in a small amount of chloroform
(ꢁ20 mL), and then re-precipitated in cold methanol (ꢁ800
mL). Solid was filtered again and dried yielding 36 g (yield:
88.5%) 9.
(4-Bromomethyl-2-decyloxy-5-decylsulfanyl-
benzyl)phosphonic Acid Diethyl Ester (12)
In an enclosed environment, a stirred solution containing 11
(23.87 g, 42.87 mmol, 1 mol equiv), paraformaldehyde (7.72
g, 257.22 mmol, 6 mol equiv), trifluoroacetic acid (33 mL,
428.7 mmol, 10 mol equiv), trifluoro acetic anhydride (12
mL, 18.13 g, 86.3 mmol, 2 mol equiv), and 30% HBr/HOAc
(23 mL, 31.14 g, 115.5 mmol, 2.7 mol equiv) was stirred and
1H NMR (CDCl3): d 0.88 (t, 6H, J 5 6.1 Hz), 1.27 (m, 28H),
1.57 (quintet, 2H, J 5 7.0 Hz), 1.76 (quintet, 2H, J 5 7.0 Hz),
2.80 (t, 2H, J 5 7.3 Hz), 3.92 (t, 2H, J 5 6.5 Hz), 6.82 (d, 2H,
J 5 8.3 Hz), 7.31 (d, 2H, J 5 8.3 Hz). Anal. Calcd. for
ꢀ
heated in a hot oil bath at 60 C for 5.5 h. The reaction mix-
ture was diluted/neutralized with water and 10% NaHCO3
solution, then the product was extracted with hexane. Dried
in MgSO4 and condensed to give 29.6 g of crude oil product
directly used in the next step without purification.
C26H46OS: C, 76.78; H, 11.40. Found: C, 76.76; H, 11.43.
2-Bromomethyl-1-decyloxy-4-decylsulfanyl-benzene (10)
In an inert gas environment, a mixture of 9 (38.5 g, 94.66
mmol, 1 mol equiv), paraformaldehyde (6.06 g, 201.8
mmol, 2.1 mol equiv), glacial acetic acid (22 mL, 23.1 g,
385 mmol, 4.07 mol equiv), acetic anhydride (3.5 mL, 3.78
g, 37 mmol, 0.39 mol equiv), and 30% HBr/HOAc (25 mL,
33.85 g, 126 mmol, 1.3ꢀ3 mol equiv) was stirred and heated
in a hot oil bath at 90 C for 3 h. The reaction mixture was
diluted/neutralized with water and 10% NaHCO3 solution,
then the product was extracted with hexane, dried in
MgSO4, and condensed to give a crude oil product 10 to be
used directly in the next step. Product weight: 25.84 g
(ꢁ80% mono-CH2Br based on 1H NMR analysis. Yield of
mono-CH2Br: ꢁ43%).
(2-Decyloxy-5-decylsulfanyl-4-hydroxymethyl-
benzyl)phosphonic Acid Diethyl Ester (13)
In an enclosed environment, a mixture of crude 12 (29.6 g,
45.56 mmol, 1 mol equiv), sodium bicarbonate (11.5 g,
136.7 mmol, 3 mol equiv), water (62 mL, 3417 mmol, 75
equiv), and 1-methyl-pyrolidinone (549 mL, 5695 mmol, 125
ꢀ
mol equiv) was stirred and heated at 90 C in a hot oil bath
for 2.75 h. The reaction mixture was diluted/neutralized
with water and 10% NaHCO3 solution, then the product was
extracted with hexane. Dried in MgSO4 and condensed to
give a crude oil product that was purified using a silica gel
column (1:1 hexane/ethyl acetate solution) to give 15.92 g
(ꢁ60% yield) of pure product 13.
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FIGURE 2 Acceptor Monomer 16 (a) 1H NMR and (b) 13C NMR Spectra in CDCl3. [Color figure can be viewed in the online issue,
which is available at wileyonlinelibrary.com.]
1H NMR: d (ppm) 7.4 (s, 1H); 6.93 (s, 1H); 4.76 (s, 1H);
4.10–3.9 (m, 6H); 3.25 (d, J 5 22 Hz, 2H); 2.80 (t, J 5 6 Hz,
2H); 1.85–1.15 (m, 32H); 0.88 (t, J 5 6 Hz, 12H).
PCC (6.52 g, 30.2 mmol, 1.2 mol equiv) was stirred at room
temperature for 18 h. The reaction mixture was diluted with
water and 10% NaHCO3 solution, and the product was
extracted with hexane, dried in MgSO4, and condensed to
give a sticky dark brown solution. The sticky solution was
placed in the freezer for recrystallization overnight. The
white crystal formed was filtered and dried to give 4.68 g of
pure product 16 (45% yield).
[5-(Decane-1-sulfonyl)22-decyloxy-4-hydroxymethyl-
benzyl]phosphonic Acid Diethyl Ester (15)
In an enclosed environment, a mixture of 13 (15.92 g, 27.13
mmol, 1 mol equiv), 50% H2O2 (4.5 mL, 4.5 g, 66 mmol, 2.4
mol equiv), and acetic acid (22 mL, 352.69 mmol, 13 mol
equiv) were stirred at room temperature for 25 min then
1H NMR in CDCl3 [also shown in Fig. 2(a)]: d (ppm) 10.8 (s,
1H); 7.99 (d, due to phosphonate group, 1H); 7.54 (s, 1H);
4.19–4.01 (m, 6H), 3.38 (d, J 5 22 Hz, 2H); 3.18 (t, J 5 8 Hz,
2H); 1.92–1.15 (m, 32H); 0.88 (t, J 5 4 Hz, 12H). 13C NMR in
CDCl3 [also shown in Fig. 2(b)]: d 14.09, 14.11, 16.36, 16.44,
22.65, 22.67, 25.36, 26.23, 28.07, 28.23, 28.99, 29.21, 29.24,
29.31, 29.33, 29.41, 29.54, 31.83, 31.89, 58.61, 62.18, 62.26,
69.49, 111.11 (d, 2.7 Hz), 127.38 (d, 9.4 Hz), 131.69 (d, 3.9
Hz), 133.78 (d, 4.9 Hz), 135.31 (d, 3.3 Hz), 160.71 (d, 6.1
Hz), 189.46. Anal. Calcd: C, 62.31; H, 9.31; S, 5.20. Found: C,
62.09; H, 9.25; S, 5.22.
ꢀ
stirred and heated in a hot oil bath at 90 C for 35 min. The
reaction mixture was allowed to cool to room temperature,
diluted/neutralized with water and 10% NaHCO3 solution,
then the product was extracted with hexane. Dried in MgSO4
and condensed to give a crude oil product mixture (14115).
This crude product mixture was treated with a mixture of
KOH (12 g), water (50 mL), and EtOH (81 mL) at room tem-
perature for 5 min. After 5 min, 14 g of NaHCO3 was added
and the mixture was condensed. The residue was extracted
with ether or hexane, and the extract was dried in MgSO4
and condensed to give 15.61 g crude product 15 to be used
directly in the next step (60.8% yield).
The Acceptor Block (17, A)
In a glove box, and at room temperature, to a vigorously
stirred mixture of acceptor monomer 16 (1.52 g, 2.46 mmol,
ꢁ15 mol equiv) and terminator (4-fluoro-benzyl)-phosphonic
acid diethyl ester 6 (41 mg, 0.167 mmol, 1 mol equiv), 45
mL of anhydrous THF were added in 10 mL increments until
everything dissolved. t-BuOK (3.2 mL, 1.0 M, 3.2 mmol, ꢁ19
equiv) was added drop wise over 5 min, and the reaction
was allowed to continue for another 5 min. The reaction
mixture was taken out from the glove box and 150 mL of
methanol was added to precipitate out the reddish polymer
product. The polymer suspension solution was placed in the
1H NMR: d (ppm) 7.93 (s, 1H); 7.07 (s, 1H); 4.89 (s, 1H);
3.85–4.05 (m, 6H); 3.25 (d, J 5 22 Hz, 2H); 3.13(t, J 5 8 Hz,
2H); 1.9–1.18 (m, 32H); 0.88 (t, J 5 4 Hz, 12H).
[5-(Decane-1-sulfonyl)22-decyloxy-4-formyl-
benzyl]phosphonic acid diethyl ester (16, acceptor
monomer)
In an enclosed environment, a mixture of 15 (15.47 g, 25
mmol, 1 mol equiv), methylene chloride (72 mL, 95.4 g,
1123 mmol, 45 mol equiv), and pyridinium chlorochromate
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(dissolved in 1 g THF) was added dropwisely. After being
stirred for 5 min after the addition, the reaction mixture was
dropped into a stirred 100-mL methanol MeOH.ꢀ Polymer
product was collected by filtration and dried at 60 C in vac-
uum for two days.
Yield: 684 mg (98%). 1H NMR in CDCl3 (also shown in Fig.
3): d (ppm) 8.5–8.1 (m, acceptor PPV aromatic); 7.9 (small,
bridge aromatic), 7.7–7.4 (m, donor PPV aromatic); 7.4–7.2
(m, br, donor and acceptor PPV vinylenene); 4.3–4.1 (m,
acceptor trans-OCH2), 4.1–3.9 (s, donor trans-OCH2), 3.5 (s,
donor cis-OCH2), 3.2–3.1 (acceptor cis-OCH2), 2.1–1.1 (m, ali-
phatic CH and CH2), 1.1–0.7 (m, aliphatic CH3). GPC data are
tabulated in Table 2, and RI signal curves of the GPC are
shown in Figure 4. Other physical property data are listed in
Discussion section.
refrigerator for 1 h to catalyze the precipitation of polymer.
Finally, the polymer suspension was slowly filtered through
a Buchner fine glass funnel, rinsed twice with methanol and
left to dry overnight to give 0.63 g (40% yield) of acceptor
polymer 17.
Optoelectronic Solar Cell Device Fabrications and
Characterizations
Polymer solar cells are fabricated and tested using following
general protocols and procedures: (1) Polymer solution
preparations: For DBA cells, 15 mg of synthesized DBA poly-
mer sample are mixed with 15 mg PC60BM (from Nano-C,
batch # BJ100507) in 1 mL dichlorobenzene (DCB). For D/A
blend cells, 8 mg donor block (D, 7) mixed with 7 mg
acceptor block (A, 17) and 15 mg PC60BM, also dissolved in
1 mL DCB. The sꢀample solutions are gently heated to
between 60 and 90 C for at least 1 h and filtered through a
0.2-mm PTFE filter before spin coated on pretreated and pat-
terned ITO glass slides. (2) ITO glass slides (from Delta Tech-
nologies, part # CG-50IN-S107, 25 3 75 3 0.7 mm, 8–12 X)
patterning and treatments: Each ITO glass slide was cut in
half and each cut slide was patterned into eight solar cells
using a pattern mask, where each patterned solar cell has an
effective size of 0.45 3 0.45 cm2 5 0.2025 cm2. Along the
two long edges of the ITO side, about 0.5-cm ITO coating
layer from the edge was etched away (for later electrode
clamping positions) using hydrochloric acid (6N HCl for 15
min. Scotch tape is used to protect the nonetched part). The
patterned ITO slides are then cleaned by detergent, DI water,
acetone, and finally isopropanol each for 5 min in ultra-
sound. (3) Polymer film depositions: First PEDOT:PSS aque-
ous solution (Clevious PVPAI 4083, from H.C. Starck) was
dropped onto the cut ITO slide using a 1-mL syringe and a
0.2-mm PTFE filter. The spin coating protocol include a key
step of 5000 RPM for 30 s, tyically yielding a 30–40-nm
thick PEDOT:PSS film. After PEDOT:PSS films are dried (60
ꢀC under vacuum pump for at least 2 h, or 110 ꢀC at room
pressure for at least 30 min), the photovoltaic polymer DCB
1H NMR in CDCl3 (also shown in Fig. 3): d (ppm) 8.5–8.1 (m,
br, PPV aromatic); 7.3–7.1 (m, br, PPV vinylenene); 4.3–4.0
(m, br, trans-OCH2), 3.2–3.1 (cis-OCH2), 2.1–1.1 (m, aliphatic
CH and CH2), 1.1–0.7 (m, aliphatic CH3). GPC data are tabu-
lated in Table 1.
The Bridge Unit (18)
The synthesis of the bridge unit (in Scheme 5, where t 5 2,
4) follows a literature procedure.31 Both two and four
carbon-based bridge units were synthesized. Since properties
of the polymer samples from either two or four carbon
bridge unites are almost the same or similar, DBA data based
mainly on the two carbon bridge unit are reported here
unless noted otherwise.
Block Copolymer Synthesis
Phosphonate-Terminated C8-RO-PPV 1 Bridge (19, DB)
One example: In a glove box, a mixture of the donor polymer
D (0.43g, 1 mol equiv of terminal phosphonate group), the
bridge compound B (t 5 2, 0.164 g, about 21 mol equiv alde-
hyde groups) and THF (7.5 mL) was heated until the poly-
mer was fully dissolved. After being cooled for 5 min, t-
BuOK (38 mg) in 1 mL THF was added over 10 s. After being
stirred for 30 s, the reaction was quenched with 0.1 mL
30% acetic acid HOAc. The mixture was dropped into metha-
nol MeOH and the polymer product was collected by filtra-
tion, yielding 0.415g (95% yield) DB.
1H NMR in CDCl3 (also shown in Fig. 3): d (ppm) 9.9 (s,
CHO), 7.9 (small, bridge aromatic), 7.7–7.4 (m, br, PPV aro-
matic); 7.4–7.2 (m, br, PPV vinylenene); 3.9–4.1 (s, br, trans-
OCH2), 3.5 (s, cis-OCH2), 2.0–1.1 (m, aliphatic CH and CH2),
1.1–0.7 (m, aliphatic CH3).
TABLE 2 Gel Permeation Chromotography data of DBA
DBA, 20
Mn (measured)
16,700 Daltons
38,900 Daltons
2.32
Block Copolymer (20, DBA)
Mw
One example: In a glove box, to a stirred mixture of the DB
(300 mg, 1 mol equiv), the acceptor polymer A (400 mg,
about 1.6 mol equiv), and THF (13 g THF), 65.8 mg t-BuOK
PDI
Mn (targeted/calculated)
16,700 Daltons
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FIGURE 4 Normalized RI signal curves of D, A, and DBA from
GPC. [Color figure can be viewed in the online issue, which is
available at wileyonlinelibrary.com.]
FIGURE 6 Thermal gravimetric analysis (TGA) curves of D, A,
and DBA. [Color figure can be viewed in the online issue,
which is available at wileyonlinelibrary.com.]
solutions were spun coated on top of dried PEDOT:PSS layer
at a spin rate of 500–3000 RPM for 160 s (desired film
thickness are between 100 and 200 nm). After the films are
dried under vacuum, they are taken into the nitrogen-filled
glove box for device fabrications. (4) Final solar cell device
fabrications: Inside a high vacuum deposition chamber inside
the glove box, polymer coated ITO slides are each covered
and clamped with an eight-cell pattern mask, and are
mounted facing downward on a rotary disk on top of the
vacuum deposition chamber. At about 1 3 1027 mbar vac-
uum level, Aluminum negative electrodes are thermally
evaporated onto the mask covered cell slides to a thickness
of about 100 nm. ITO positive electrode was exposed by
scratching off a small area of polymer/PEDOT:PSS film at
one terminal of the slide using a solvent such as THF. (5)
Solar cell measurements: The light/photo incurred current–
voltage (IV) curves are measured by exposing the thin film
cells in a measurement holder to a solar simulator generated
one Sun (100 mW/cm2) 1.5 AM radiation from the top of
the glove box. A voltage–current Source-Measure Unit (Keith-
ley SMU-237) coupled with a PC were used to monitor and
measure IV data on each cell.
unit that can connect the donor and acceptor blocks (Scheme
5 exhibits a specific DBA synthesis example). Horner–Emmons
reaction is used to build individual donor or acceptor PPV
blocks. Phosphonate instead of aldehyde is chosen as the ter-
minal functional group of donor and acceptor blocks because
it is more stable than the aldehyde group to the strong base
(e.g., t-BuOK), and it is used in excess in the polymerization
reaction. The stability of the aldehyde group in the bridge unit
is not a major concern as the coupling reaction time is very
short and that the bridge compound is used in large excess.
Monomer Synthesis
The example synthesis of a PPV-based donor and acceptor
monomers (5 and 16) and corresponding polymer blocks (7
and 17) are shown in Schemes 3 and 4. Both donor and
acceptor monomers (5 and 16) are asymmetrically di-
functionalized with an aldehyde and a phosphonate groups
which can be polymerized into PPV via the Horner-Emmons
reaction. In donor monomer synthesis,
a branched 2-
ethylhexyl (C8) is used instead of a straight or linear alkyl
group because a linear alkyl group substituent regio-regular
PPV is more difficulty to dissolve. Interestingly, the acceptor
block with linear side chains does not appear to have solubil-
ity problem. Presumably, the polar repeat units interact more
strongly with the polar solvents used. The use of different
side chains also facilitates donor and acceptor block phase
separation. In the synthesis of 2-CH2Br (methylene bromide)
substituted 3 and 10, control of reaction temperature and
RESULTS AND DISCUSSION
Materials Synthesis
General
DBA synthesis requires the synthesis of a mono-end function-
alized conjugated donor block (Scheme 3 exhibits an example),
a mono-end functionalized conjugated acceptor block (Scheme
4 exhibits an example), and a double-end functionalized bridge
FIGURE 5 Differential scanning calorimetry (DSC) curves of D,
A and DBA. [Color figure can be viewed in the online issue,
which is available at wileyonlinelibrary.com.]
FIGURE 7 UV–vis absorption spectra of D, A, and DBA in THF.
[Color figure can be viewed in the online issue, which is avail-
able at wileyonlinelibrary.com.]
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FIGURE 8 Photoluminescence PL emission spectra of D, A, and
DBA in 1 3 1027 M THF solutions of corresponding repeat
units (in DBA case, concentration refers to acceptor repeat unit
concentration). The excitation is at 400 nm which is optimal for
the acceptor A block but not optimal for the donor D block
(Donor block excitation is optimal at about 500 nm). The small
peak spike at around 453 nm comes from the THF solvent
used. [Color figure can be viewed in the online issue, which is
available at wileyonlinelibrary.com.]
FIGURE 10 Cyclic voltammetry curves of Ferrocene (dotted
line), D (solid line), and A (dashed line). [Color figure can be
viewed in the online issue, which is available at wileyonlineli-
brary.com.]
fore, a terminator with higher P-CH2 reactivity (or acidity) is
required. When terminator 6 (with an electron-withdrawing
fluorine substitution at the para position) was used, the size
control in both donor and acceptor block synthesis worked
well. For instance, since each PPV repeat unit has a straight
length of about 0.65 nm along the conjugation chain, a donor
monomer/terminator feed molar ratio of 25:1 is supposed to
yield a chain length of 25 repeat unit (degree of polymeriza-
tion, DP 5 25).32 and it actually yielded a donor block of
around 25.2 repeat units as measured from GPC (see Table 1
and Fig. 4). An acceptor monomer/terminator feed molar
ratio of 15:1 is supposed to yield a chain size of 15 repeat
units (DP 5 15), and it actually yielded an acceptor block
size of about 16.1 repeat units (see Table 1). 1H NMR spec-
trum of the D, DB, A, and DBA are shown in Figure 3, where
the monomer’s characteristic aldehyde peak disappeared
completely when the D or A blocks or DBA formed.
time are very critical to avoid the undesirable 2,5-di-CH2Br
substitution. The light yellowish oil donor monomer 5 and
the whitish solid acceptor monomer 16 were fully character-
ized by NMR (also shown in Figs. 1 and 2) and elemental
analysis. The monomers can be further modified with differ-
ent substituents for a variety of optoelectronic application
needs.
Conjugated Block Synthesis
Synthesis of both donor and acceptor blocks requires the
use of a block terminator (compound 6 in Scheme 3) to ter-
minate one end of the block as well as to control the size of
the blocks (Schemes 3 and 4). The choice of the terminator
is very critical. For instance, compound 3 was initially tested
as the terminator. However, when t-BuOK was added to the
mixture of donor or acceptor monomer and compound 3 in
THF, a high molecular weight PPV (judged by the way it pre-
cipitated in MeOH) was obtained. Presumably, the P-CH2
group in 3 are not reactive (or acidic) enough to compete
with the P-CH2 group in monomer for t-BuOK added, and
therefore, 3 does not participate in polymerization. There-
DBA Block Copolymer Synthesis
As shown in Scheme 5, when synthesizing donor-bridge or
DB unit, the bridge B is in a large access amount (more than
20 mol equiv) of donor block D to minimize the chance of
FIGURE 9 Photoluminescence PL emission spectra of D and
DBA in 1 3 1027 M donor repeat unit solutions. The excitation
is at 500 nm which is optimal for the donor D block but it can-
not excite the acceptor block (Acceptor block excitation needs
400 nm). [Color figure can be viewed in the online issue, which
is available at wileyonlinelibrary.com.]
FIGURE 11 Comparison of light JV curves of solar cells made
of a D/A blend (dashed curve) versus a DBA block copolymer
(solid curve). DBA cell exhibits an order of magnitude higher
power conversion efficiency (PCE) then the D/A blend cell.
[Color figure can be viewed in the online issue, which is avail-
able at wileyonlinelibrary.com.]
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exist certain difference between polystyrene determined
molecular weights versus actual PPV molecular weights due
to polymer shape differences. If this indeed occurs, it would
be a systematic error, that is, it would apply to all measured
PPV samples including D, A and DBA blocks, but such sys-
tematic error would not affect summation relationship of
D1A5DBA. Details of the synthetic procedures and data are
described in the Experimental section.
Physical Properties
Thermal Properties
Differential scanning calorimmetry (DSC) and thermal gravi-
metric analysis (TGA) scans of D, A, and DBA are shown in
Figures 5 and 6. Thermal study results indicate that all three
polymer samples exhibit no significant thermal induced tran-
ꢀ
sitions and/or degradations to about 300 C.
Optoelectronic Properties
UV–vis absorption spectra of the D, A, and DBA in THF are
shown in Figure 7. Donor block exhibits an absorption maxi-
mum at around 500 nm, and acceptor block exhibits an
absorption maximum at around 420, the DBA exhibits a
maximum absorption peak around 500 nm (reflecting the
donor absorption maximum) and a relatively small but
noticeable absorption peak at around 420 nm (reflecting the
acceptor absorption maximum), confirming (1) DBA contains
both D and A; and (2) D and A still maintain their electronic
structures separately after connected by the bridge unit, and
that there is no evidence of ground state charge transfer
between D and A in DBA which is necessary for optoelec-
tronic applications. The optical excitation absorption cut off
(corresponding to optical excitation energy gaps) are 560
nm (2.21 eV) for the donor (D) and 530 nm (2.34 eV) for
the acceptor (A) blocks. Due to an intramolecular electronic
push–pull interaction within the acceptor block repeat
units,22 the absorption peak maximum of the acceptor block
is at higher energy (around 420 nm) and has lower intensity
compared to the donor block (peak maximum around 500
nm).
FIGURE 12 Taping mode topographic AFM images for spin-
coated thin films of (a) D/A blend and (b) DBA block copolymer.
Image dimensions are at 500 3 500 nm. [Color figure can be
viewed in the online issue, which is available at wileyonlinelibrary.
com.]
The photoluminescence (PL) emission spectra of the D, A,
and DBA in THF are shown in Figure 8. All three polymer
samples were dissolved in THF at the same molar concentra-
tion of 1.0 3 1027 M corresponding to their repeat units (in
DBA case, concentration corresponds to acceptor repeat
unit). Similar to what was observed in absorption, DBA
emission spectrum exhibits two emission peaks correspond-
ing to the emission peaks of the donor block (around 550
nm) and the acceptor block (around 500 nm). The excitation
wavelength is set at 400 nm, which is optimal for the
acceptor block but is not optimal for the donor block (Donor
block excitation is at 500 nm). This explains why the donor
emission intensity appears much weaker than the acceptor
even if their concentrations are the same. The significant
reduction of the emission peak of A at around 500 nm in
DBA as compared to pristine A can be attributed to both
charge transfer and energy transfer between the donor and
the acceptor in DBA. Energy transfer is evidenced by the
forming DBD species, and to ensure that resulting polymer
has one aldehyde group still available (see small aldehyde
peak of DB in its NMR spectrum in Fig. 3). The available
aldehyde group can further couple with the acceptor block
forming the final DBA as indicated by the disappearance of
the DB aldehyde peak and appearance of the NMR peaks
associated with acceptor block (Fig. 3). The measured num-
ber average molecular weight of 16,700 Daltons matches
nicely with the calculated molecular weight of 16,700 Dal-
tons based on the individual donor and acceptor block aver-
age molecular weights (see Table 2 and Fig. 4). There could
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donor emission peak increase at around 550 nm from the
pristine D to DBA in Figure 8. The small peak spike at
around 453 nm comes from the THF solvent used. Charge
transfer is evidenced by the donor block emission peak at
550 nm dropped significantly from the pristine D to DBA
when the sample solutions were excited at 500 nm (Fig. 9).
This is also another evidence of a close proximity of D block
to the A block in the DBA.
circuit current density of 0.92 mA/cm2, an open circuit volt-
age of 0.65 volt, a fill factor of 26%, and a power conversion
efficiency of about 0.2%. At the same cell fabrication and
testing setup, the reference P3HT/PCBM best cell yielded a
short circuit current density of 3.59 mA/cm2, an open circuit
voltage of 0.56 volt, a fill factor of 40%, and a power conver-
sion efficiency of about 0.8%. The relatively lower efficiency
for the new PPV based cells as compared to the reference
P3HT/PCBM cells can be contributed to several obvious fac-
tors: (1) Energy gaps for these PPVs are typically over 2.2
eV, while P3HT has an energy gap of about 1.8 eV. Since
most abundant sunlight photons are between 1 and 2 eV at
AM 1.5, photon losses are expected to be more severe in
PPV as in P3HT. (2) Regio-regular P3HT are known for its
highly ordered solid-state morphology and corresponding
high charge mobility, while the typical solid-state morpholo-
gies and charge transport in PPV are relatively poorer, thus,
charge carrier losses are expected to be more severe in PPV
as compared to P3HT. (3) The PPV thin film solar cell device
fabrication protocol used here are from optimizations for the
P3HT/PCBM reference cells, and that the optimal processing
or morphologies of P3HT/PCBM cells are not necessarily the
same or optimal for the PPV based solar cells. Even for our
best P3HT/PCBM reference cell, a 0.8% PCE efficiency
achieved is still much lower than the best or typical litera-
ture reported value of around 5%,5 that is, our device fabri-
cation protocol still has room for further improvements.
However, and most importantly, an order of magnitude bet-
ter PCE improvement of the DBA cell over the D/A blend
cell was clearly observed and demonstrated. Due to similar-
ities of the frontier orbitals of DBA versus D/A blend, the
significant PCE improvement should be attributed to the
improvement on materials spatial regimes and not on energy
regimes. For instance, there should be much less exciton and
carrier transport losses in DBA than in D/A blend, as every
exciton generated in DBA can easily reach its D/A interface
within its life time, and every newly generated electron or
hole can diffuse to their respective electrodes in their own
domain or phases with much less interruptions in DBA than
in D/A blend. Figure 12 exhibits preliminary atomic force
microscopy (AFM) taping mode topographic images for spin
coated thin films of (a) D/A blend and (b) DBA block copol-
ymer. Image sizes are at 500 3 500 nm. It appears certain
phase domains on the 10–20 nm scales can be seen in the
DBA block copolymer film but not in the D/A blend film.
Systematic studies on polymer solid-state morphological
optimizations and their effects to optoelectronic device per-
formance are under way and will be reported in future.
The frontier orbitals (HOMOs and LUMOs) of the donor and
acceptor blocks were analyzed using electrochemical cyclic
voltammetry (CV) (shown in Fig. 10. A small bump at about
20.4 volt is from Ferrocene measurement and is therefore
excluded in molecular frontier orbital determinations) com-
bined with UV–vis absorption spectra (Fig. 7). Due to poten-
tial irreversible electrochemically incurred changes to
polymer samples in CV negative (reduction) scan measure-
ments, only the first oxidation onset potentials are used here
to estimate HOMO values. Optical excitation energy gap data
are used to estimate the LUMO values. Specifically, the
HOMO and LUMO levels of donor and acceptor blocks are
calculated as follows:
Frontier Orbital Calculations via Cyclic Voltammetry
Measurements
D2HOMO ! 24:8eV – ð1:05–0ÞeV 5 25:85eV
A2HOMO ! 24:8eV – ð1:2220ÞeV 5 26:02eV
Frontier Orbital Calculations Using Optical Energy Gaps
Combined with HOMOs
Optical excitation energy gap (Eg) 5 (1240)/ (UV–vis absorp-
tion cut off)
Donor optical energy gap 5 1240/560 5 2.21eV (LUMO/
HOMO5 23.64eV/25.85eV)
Acceptor optical energy gap 5 1240/530 5 2.34 (LUMO/
HOMO523.68eV/26.02eV)
The frontier orbital data indicate that the LUMO orbital off-
set (0.04 eV) is relatively small compared to the most
desired or popular values (i.e., 0.3eV), this may explain the
relatively low efficiency of photo induced charge transfer
between D and A here (and therefore the potential optoelec-
tronic device performance). Further optimizations of the
frontier orbitals are necessary.
Preliminary Optoelectronic Device Studies
Figure 11 exhibit preliminary comparison between several
cells fabricated from DBA (blue curve represents a best DBA
cell in a particular measurement) versus several cells fabri-
cated from D/A blends (red dashed curve represents a best
D/A cell in a particular measurement). Based on measured
JV data at AM 1.5 and one Sun intensity, the best D/A blend
cell yielded a short circuit current density of 0.22 mA/cm2,
an open circuit voltage of 0.73 volt, a fill factor of 13%, and
a power conversion efficiency of about 0.02%. The best DBA
cell (not yet optimized), on the other hand, yielded a short
CONCLUSIONS
A novel class of conjugated block copolymer system contain-
ing a donor type conjugated block c-D covalently connected
to an acceptor type conjugated block c-A via a nonconju-
gated and flexible bridge unit nc-B, also called a DBA type
conjugated block copolymer system, has been designed, syn-
thesized, and characterized for potential cost effective and
high efficiency optoelectronic applications. The D and A
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6 C. Brabec, V. Dyakonov, J. Parisi, N. S. Sariciftci, Organic
Photovoltaics: Concepts and Realization; Springer: Berlin, 2003.
block synthesis involve development of unsymmetrical di-
functionalized donor or acceptor type monomers and a ter-
minator unit, for instance, an aldehyde and a phosphonate
functional groups are needed for Horner-Emmons coupling
reaction, and that the size of each conjugated block can be
controlled via synthetic feed ratio of the monomer and the
terminator. The measured average molecular weights of D, A,
and DBA based on gel permission chromatography (GPC)
measurements are in good agreements with calculated val-
ues based on synthetic monomer feed ratios. Preliminary
optoelectronic device studies revealed an order of magnitude
better solar cell power conversion (PCE) efficiency improve-
ment of DBA over the corresponding D/A simple blend
under identical device fabrication and testing conditions.
Such improvements are attributed to inherent improvement
of DBA solid-state nanomorphology over the D/A blend
resulting in more efficient photo induced charge separation
and charge transport in DBA versus in D/A blend. Further
improvement of DBA device efficiencies can be achieved via
better matched frontier orbitals between the donor and
acceptor blocks to enhance charge separation and transfer
and minimize charge recombinations, better excitation
energy gaps (to match the intended radiation energy range),
optimal bridge length between the donor and the acceptor,
better DBA solid-state morphologies (e.g., more ordered and
self-assembled solid-state packing via stereo - or regio-
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and improved device fabrications (e.g., optimal film thickness
and post-treatments, improved electron, or hole blocking/
collecting layers and electrodes).
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This material is based on work supported in part, by research
and educational grant awards from a number of sponsors
including the Department of Energy (DOE Award # DE-EE-
0004002), Army Research Office (ARO Award #W911NF-11-1-
0158), and the National Science Foundation (NSF Award
#1036494).
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