Synthetic
Metals
196
(2014)
110–116
Contents
lists
available
at
ScienceDirect
Synthetic
Metals
jo
ur
nal
homep
age:
www.elsevier.com/locate/synmet
n-Type
pyromellitic
diimide-benzodithiophene-containing
conjugated
polymers
for
all-polymer
solar
cells
with
high
open-circuit
voltage
Chunhua
Luo
a
,
Xiangjian
Meng
a
,
Li
Han
a
,
Shuo
Sun
a
,
Tie
Lin
a
,
Jinglan
Sun
a
,
Hui
Peng
b,c,∗
,
Junhao
Chu
a
a
National
Laboratory
for
Infrared
Physics,
Shanghai
Institute
of
Technical
Physics,
Chinese
Academy
of
Science,
Shanghai
200083,
China
b
Key
Laboratory
of
Polarized
Materials
and
Devices,
Ministry
of
Education,
East
China
Normal
University,
Shanghai
200241,
China
c
Polymer
Electronic
Research
Centre,
The
University
of
Auckland,
Private
Bag
92019,
Auckland,
New
Zealand
a
r
t
i
c
l
e
i
n
f
o
Article
history:
Received
22
April
2014
Received
in
revised
form
13
July
2014
Accepted
21
July
2014
Available
online
16
August
2014
Keywords:
Pyromellitic
diimide
Benzodithiophene
n-Type
polymer
Synthesis
Polymer
solar
cells
a
b
s
t
r
a
c
t
n-Type
conjugated
polymers
were
synthesized
by
using
pyromellitic
diimide
monomers
with
two
differ-
ent
alkyl
side
chain
substitutions
as
electron-accepting
units
and
benzodithiophene
as
electron-donating
unit
via
Stille
coupling
reaction.
The
obtained
polymers
were
characterized
by
UV–vis
spectroscopy
and
cyclic
voltammetry.
The
results
illustrate
that
the
substituted
alkyl
side
chains
2-ethylhexyl
and
2-
octyldodecyl
have
negligible
effect
on
the
optical
properties
of
the
resulting
polymers.
The
LUMO
levels
of
the
polymers
located
at
−3.5
to
−3.6
eV.
All-polymer
bulk
heterojunction
solar
cell
with
“P3HT”
as
p-
type
polymer
in
standard
architecture
were
fabricated,
the
cell
based
on
the
polymer
with
2-octyldodecyl
side
chain
(P2)
shows
good
photovoltaic
performance
with
power
conversion
efficiency
up
to
0.47%
and
a
high
open
circuit
voltage
of
0.96
V.
©
2014
Elsevier
B.V.
All
rights
reserved.
1.
Introduction
Polymer
solar
cells
(PSCs)
are
very
promising
for
photovoltaic
applications
because
they
can
be
fabricated
on
flexible,
large
area
substrates
using
solution-processing
techniques,
and
are
low
cost
and
light
weight
[1,2].
So
far,
the
most
successful
construction
for
PSCs
is
the
bulk
heterojunction
(BHJ)
configuration
by
blending
a
p-type
conjugated
polymer
as
the
electron
donor
and
an
n-type
fullerene
derivative
as
the
electron
acceptor.
The
power
conversion
efficiencies
for
BHJ
solar
cells
with
fullerene
derivatives
as
accep-
tors
have
exceeded
10%
[3],
which
are
much
higher
compared
to
PSCs
based
on
other
n-type
conjugated
polymer
acceptors.
The
superior
charge-transporting
ability
and
high
electron
affinity
make
fullerenes
the
best
acceptor
component
currently
available
for
PSCs
[4].
Although
fullerene
derivatives
have
many
advan-
tages
as
acceptors
in
PSCs,
their
relatively
weak
absorption
in
the
visible
region
greatly
restricts
the
further
improvement
of
device
performance.
Compared
to
fullerene
derivatives,
the
energy
∗
Corresponding
author
at:
Polymer
Electronic
Research
Centre,
Chemistry
Department,
The
University
of
Auckland,
Private
Bag
92019,
Auckland,
New
Zealand.
Tel.:
+86
54342721.
E-mail
address:
h.peng@auckland.ac.nz
(H.
Peng).
levels,
band
gaps
and
optical
properties
of
n-type
polymers
can
be
fine
tuned
to
match
the
solar
spectrum
and
energy
levels
of
donors,
resulting
in
the
improvement
of
PSCs
performance.
For
example,
the
development
of
n-type
polymers
with
high-lying
LUMO
(lowest
unoccupied
molecular
orbital)
energy
levels
may
lead
to
PSCs
with
large
open
circuit
voltages
(V
oc
).
The
V
oc
of
PSCs
using
fullerene
acceptors
are
limited
to
ca.
1.1
eV
due
to
their
relatively
low-lying
LUMO
levels
(∼4.0
eV)
[5,6].
Nevertheless,
the
design
and
synthesis
of
effective
n-type
poly-
mers
for
photovoltaic
applications
are
still
a
challenge.
The
power
conversion
efficiencies
(PCEs)
based
on
polymer
acceptors
are
rel-
atively
low
(up
to
3.3%)
[7]
compared
to
those
based
on
fullerene
acceptors.
More
work
needs
to
be
done
on
materials
design,
synthesis
and
device
performance
study
for
n-type
polymer
accep-
tors.
One
successful
approach
for
the
design
of
n-type
polymers
is
to
introduce
an
electron-withdrawing
group
in
the
polymer
backbone.
Imide
functioned
rylene
based
polymers
have
recently
emerged
as
promising
n-type
materials
for
PSCs
due
to
their
good
chemical,
thermal
and
photochemical
stabilities,
high
elec-
tron
affinities
and
mobilities
[8,9].
There
have
been
extensively
study
on
n-type
rylene
diimide
based
polymers
in
PSCs,
espe-
cially
on
perylene
diimide
(PDI)
[10–15]
and
naphthalene
diimide
(NDI)
[7,13,16,17],
which
are
demonstrated
to
have
good
photo-
http://dx.doi.org/10.1016/j.synthmet.2014.07.016
0379-6779/©
2014
Elsevier
B.V.
All
rights
reserved.