Novel pendent thiophene side-chained benzodithiophene for polymer solar cells

Article abstract of DOI:10.1002/pola.27585

In this article, pendent thiophene (2-butyl-5-octylthiophene) side chain is used to modify the backbone of the polymers containing benzo[1,2-b:4,5-b′]dithiophene (BDT) and thieno[3,4-c]pyrrole-4,6-dione (TPD). Compared with the dodecyloxy side-chained pol

Full text of DOI:10.1002/pola.27585

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Novel Pendent Thiophene Side-Chained Benzodithiophene
for Polymer Solar Cells
1
,2
1,2
2
2
1,2
1,2
Ruiying Sheng, Qian Liu, Manjun Xiao, Chunyang Gu, Tong Hu, Junzhen Ren,
1
2
Mingliang Sun, Renqiang Yang
1
Institute of Materials Science and Engineering, Ocean University of China, Qingdao 266100, People’s Republic of China
2
CAS Key Laboratory of Bio-Based Materials, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of
Sciences, Qingdao 266101, People’s Republic of China
Correspondence to: M. L. Sun (E-mail: mlsun@ouc.edu.cn) or R. Q. Yang (E-mail: yangrq@qibebt.ac.cn)
Received 17 December 2014; accepted 28 January 2015; published online 00 Month 2015
DOI: 10.1002/pola.27585
ABSTRACT: In this article, pendent thiophene (2-butyl-5-octylth-
and nearly 1 V high open circuit voltage (VOC). The polymer
iophene) side chain is used to modify the backbone of the
solar cell devices based on three copolymers show power con-
version efficiencies from 2.01% to 4.13%. The hole mobility of
these polymers tested by space charge limited current method
0
polymers containing benzo[1,2-b:4,5-b ]dithiophene (BDT) and
thieno[3,4-c]pyrrole-4,6-dione (TPD). Compared with the dode-
cyloxy side-chained polymer (P1), pendent thiophene-based
polymers (P2 and P3) show similar number-average molecular
24
24
2 21 21
range from 3.4 3 10 to 9.2 3 10 cm V
s
. 2015 Wiley
V
C
Periodicals, Inc. J Polym Sci Part A: Polym Chem 000: 000–000,
2015
weight (M
n
), polydispersity index, thermal stability (T
d
ꢀ 334–
ꢁ
3
37 C), and optical band gaps (E go pt) (ꢀ1.8 eV). Polymer (P2)-
0
based BDT with pendent thiophene and ethylhexyl-modified
KEYWORDS: benzo[1,2-b:4,5-b ]dithiophene; pendent thiophene;
TPD shows relatively low-lying HOMO energy level (25.52 eV)
polymer solar cells; thieno[3,4-c]pyrrole-4,6-dione
INTRODUCTION Bulk-heterojunction (BHJ) polymer solar
cells (PSCs) with an active layer composed of a conjugated
Recently, researchers also find the side chains play an
1
20,21,30–33
important role in solar cell performances.
Hou et al.
polymer as the donor and a fullerene derivative as the
acceptor have been proved to be the most efficient struc-
ture, and PSCs have attracted considerable attention due to
reported that the introduction of alkylthienyl substituents
onto the BDT units of D–A polymers was found to form the
two dimensional (2D) conjugated structure, which could
enhance the hole mobility and reduce the HOMO energy lev-
els compared with the dialkoxy-substituted BDT ana-
2
their potential applications in low-cost, flexible, large-area
3
–8
photovoltaic devices via solution process.
Recently, signifi-
8
,34,35
cant progress has been made in this field. Power conversion
efficiencies (PCEs) of 10.8% have been achieved for single
logs.
Beaujuge and McGehee et al. reported by varying
the size and branching of solubilizing side chain groups onto
the BDT units, the PCE reached 8.5% with the open-circuit
voltage (VOC) of 0.97 V in BHJ devices with PC71BM as
acceptor, and they also found the substitutions of linear side
chains in PBDTTPD polymers could greatly impact the poly-
mer self-assembling properties and solar cell device effi-
9
10
junction and 11% for tandem devices. Nevertheless, this
is still not sufficient high PCEs for future commercialization,
1
1
especially compared with dye-sensitized solar cells and
1
2
perovskite solar cells. Thus, the polymer donors and fuller-
ene derivative acceptors as the key photovoltaic materials
will need to be further optimized for the better performance.
Most of recent milestones achieved in PSCs were based on
2
1,36
ciency.
To the best of our knowledge, the side chains
attached on BDT copolymers are linear (or branched) alkoxy
chains or conjugated units (thiophene, benzene, furan, and
benzo[1,2-b:4,5-b’]dithiophene
(BDT)-based
copolymers
3
7,38
which were first used as photovoltaic material in 2008 by
selenophen)
by now. BDT-based polymer with “semi-
1
3
Hou and Yang. After that, lots of BDT-based copolymers
were designed and synthesized to further optimize the solar
conjugated” side chain, we mean the side chain possesses
conjugated segment (CS) but this CS is not directly attached
to the BDT segment, is not reported. This kind of semiconju-
gated side chain definitely will influence the self-assembling
properties of the polymers in PSC active layer, and it will not
1
4–24
cell performances.
on conjugated backbone design typically achieved by copoly-
At first, the main efforts are focused
1
7–19,25–29
merizing BDT with various other building blocks.
R.S. and Q.L. contributed equally to this work.
V
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SCHEME 1 Synthetic routes of monomers and polymers.
0
cause serious polymer backbone twist as direct-attached
bulky side groups.
Compounds 4,8-dihydrobenzo[1,2-b:4,5-b ]dithiophen-4,8-dione,
0
4,8-didodecyloxybenzo[1,2-b:4,5-b ]dithiophene and 2,6-bis-
(
trimethyltin)-4,8-didodecyloxybenzo[1,2-b:4,5-b’]dithiophene
In this article, one model semiconjugated (2-butyl-5-octylthio-
phene) side-chained BDT monomer (Scheme 1) was designed
and synthesized; two polymers (P2 and P3) based on this
building block were obtained by Stille coupling polymerization.
Polymer without this building block (P1) was also synthesized
for comparison. The TPD acceptor unit is copolymerized into
BDT polymer backbone to widen the ultraviolet-visible (UV–
Vis) absorption and tune the LUMO and HOMO energy levels
of the polymer. Preliminary results do show semiconjugated
side chain can lower the HOMO energy level with the help of
some other side groups on copolymerization unit, and it can
help the polymer form homogeneous PC BM blend thin film.
13
were synthesized according to the corresponding literature.
Characterization
1
13
H and C NMR spectra were recorded on a Bruker Advance
III 600 (600 MHz). High resolution mass spectra (MS) were
recorded in Atmospheric Pressure Chemical Ionization (APCI)
mode on a Bruker Maxis UHR TOF spectrometer. The molecu-
lar weight and polydispersity were determined by gel permea-
tion chromatography (GPC) analysis using a HLC-8320GPC
EcoSEC System. Thermogravimetric analysis (TGA) measure-
ments were performed using a TA-Q600 analyzer with a heat-
ing rate of 10 C min under a nitrogen atmosphere. The UV–
Vis absorption spectra were recorded using the Hitachi U-4100
spectrophotometer. Cyclic voltammetry (CV) measurements
were performed on a CHI660D electrochemical workstation
with a glassy carbon working electrode, a platinum wire coun-
ter electrode, and an Ag/AgCl reference electrode. The CV
curves of BDTTPD-based polymers were carried out under an
argon atmosphere in a solution of Tetrabutylammonium Hexa-
fluorophosphate Bu NPF (0.1 M)/CH CN at a scan rate of 50
ꢁ
21
6
1
EXPERIMENTAL
Materials
All reagents and starting materials were obtained from com-
mercial purchase without further purification for synthesis
unless the specified requirements. Toluene and tetrahydrofu-
ran (THF) were distilled from sodium with benzophenone as
indicator, and N,N-dimethylformamide (DMF) was distilled
from CaH2 under an argon atmosphere before use for syn-
thesis. All air and water sensitive reactions were performed
under anhydrous and an argon or nitrogen atmosphere.
4
6
3
21
mV s at ambient temperature.
Device Fabrication
Photovoltaic devices were fabricated on 15 mm 3 15 mm
patterned indium tin oxide (ITO)-coated glass substrates
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with a layered structure of ITO/PEDOT:PSS/polymer:PC BM
then to orange. Then, compound 1 (3.31 g, 10 mmol) and tet-
rabutylammonium bromide (TBAB) (0.14 g, 0.44 mmol) were
added into the flask. The reactant was refluxed overnight, and
then the reactant was poured into cold water and extracted
with diethyl ether. The organic layer was dried over anhy-
6
1
blend/Ca/Al. The ITO-coated glass substrates were cleaned
in an ultrasonic bath with acetone, toluene, methanol, and
isopropyl alcohol sequentially. The substrates were then oxy-
gen plasma treated for 20 min, spin coated with PEDOT:PSS
(
5
poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) at
000 rpm, and dried under argon at 120 C for 20 min.
drous MgSO . After removing solvent, the crude product was
purified by column chromatography on silica gel with hexane/
4
ꢁ
BDTTPD-based polymers and [6,6]-phenyl-C -butyric acid
dichloromethane (5:1) as eluent to give compound 2 as white
6
1
1
methyl ester (PC61BM) were dissolved in deoxygenated anhy-
drous o-dichlorobenzene in the different weight ratios and
stirred overnight in an MBraun glove box. An active layer con-
sisting of the blend of BDTTPD-based polymers and PC BM
3
solid. Yield: 1.78 g (61.5%); H NMR (CDCl , 600 MHz), d
(ppm): 7.46 (d, 2H), 7.36 (d, 2H), 6.60 (d, 2H), 6.57 (d, 2H),
4.30 (t, 4H), 2.87 (t, 4H), 2.74 (t, 4H), 1.96 (m, 8H), 1.64 (m,
1
3
4H), 1.37–1.26 (m, 20H), 0.88 (t, 6H). C NMR (CDCl , 151
6
1
3
were then spin coated on PEDOT:PSS with a thickness of
MHz), d (ppm): 144.47, 143.63, 142.43, 131.59, 130.17,
126.09, 123.67, 123.37, 120.28, 73.49, 31.88, 31.73, 30.19,
29.94, 29.92, 29.36, 29.24, 29.19, 28.23, 22.67, 14.12. HRMS
(APCI): m/z found: 722.3303; calcd for C H O S : 722.3320.
ꢀ
80 nm. A typical concentration of the PBDTTPD/PC BM
61
2
1
blending solution was 24 mg mL
. Subsequently Ca
(
ꢀ10 nm) and Al (ꢀ100 nm) were thermally evaporated in a
4
2 58 2 4
24
vacuum of ꢀ2 3 10 Pa on top of the active layer as a cath-
ode. The current density–voltage (J–V) characteristics were
measured with a Keithley 2420 source measurement unit
2,6-Bis(trimethyltin)-4,8-bis(4-(5-octylthiophen-2-
0
yl)butoxy)benzo[1,2-b:4,5-b ]dithiophene (3)
22
under simulated 100 mW cm (AM 1.5 G) irradiation from a
Newport solar simulator. Light intensity was calibrated with a
standard silicon solar cell. The thickness of the active layer
was determined by Dektak 150 profilometer. The external
quantum efficiencies (EQEs) of solar cells were analyzed
using a certified Newport incident photon conversion effi-
ciency measurement system. Surface morphological charac-
terizations of the films were characterized by a tapping mode
atomic force microscope (AFM, Agilent 5400).
Compound 2 (1.28 g, 1.77 mmol) and 50 mL of THF were
added into a flask under an inert atmosphere. The solution
ꢁ
was cooled down to 278 C, and 2.5 mL of n-butyllithium
(4.07 mmol, 1.6 M in n-hexane) was added dropwise into the
ꢁ
solution. After being stirred at 278 C for 1 h, the reaction
mixture was warmed to ambient temperature and stirred for
2 h. Then, 4.43 mmol of trimethyltin chloride (4.43 mL, 1 M
in n-hexane) was added in one portion, and the reactant
ꢁ
turned to be clear rapidly. The mixture was stirred at 278 C
for 30 min and then slowly warmed to ambient temperature
and stirred overnight. Then, it was poured into 200 mL of
cool water and extracted by diethyl ether three times. The
organic layer was washed by water twice and then dried by
Synthesis of Monomers and Polymers
2
-(4-Bromobutyl)-5-octylthiophene (1)
Under an argon atmosphere, n-butyllithium (21.6 mmol,
3.5 mL, 1.6 M in hexane) was added dropwise to the solu-
1
anhydrous MgSO . After removing solvent under vacuum, the
4
tion of 2-octylthiophene (4.24 g, 21.6 mmol) in THF (50 mL)
residue was recrystallized by isopropyl alcohol. 1.42 g of com-
ꢁ
at 278 C. The reaction mixture was stirred for 1 h at 278
pound 3 (1.35 mmol, yield 76%) was obtained as colorless
ꢁ
1
C and then warmed to room temperature and stirred for 2 h.
needle crystal. H NMR (CDCl , 600 MHz), d (ppm): 7.51 (s,
3
1
,4-Dibromobutane (9.33 g, 43.2 mmol) was added slowly to
2H), 6.61 (d, 2H), 6.57 (d, 2H), 4.32 (t, 4H), 2.88 (t, 4H), 2.74
(t, 4H), 1.97 (m, 8H), 1.64 (m, 4H), 1.37–1.26 (m, 20H), 0.88
ꢁ
the reaction mixture at 278 C, and the mixture was elevated
to room temperature and stirred for 12 h. The reaction was
quenched with water, and product was extracted with diethyl
ether. The organic layer was washed with water and dried
over anhydrous MgSO , and then evaporated under reduced
4
pressure. The residue was purified by column chromatogra-
1
3
(t, 6H), 0.44 (s, 18H). C NMR (CDCl , 151 MHz), d (ppm):
3
143.55, 143.02, 142.59, 140.68, 134.03, 132.95, 127.93,
123.59, 123.35, 73.13, 31.88, 31.75, 30.20, 29.96, 29.36,
29.24, 29.20, 28.29, 22.67, 14.12, -8.28. HRMS (APCI): m/z
found: 1050.2674; calcd for C H O S Sn : 1050.2616.
4
8
74 2 4
2
phy on silica gel with hexane as eluent to give compound 1
1
as pale yellow oil. Yield: 6.32 g (88.3%); H NMR (CDCl , 600
4,8-Didodecyloxybenzo[1,2-b:4,5-b’]dithiophene (4)
3
MHz), d (ppm): 6.56 (d, 1H), 6.55 (d, 1H), 3.42 (t, 2H), 2.78
Compound 4 was synthesized according to the corresponding
1
3
(
t, 2H), 2.73 (t, 2H), 1.92 (m, 2H), 1.79 (m, 2H), 1.63 (m, 2H),
literature.
4,8-Dihydrobenzo[1,2-b:4,5-b’]dithiophen-4,8-
1
3
.37–1.26 (m, 10H), 0.88 (t, 3H). HRMS (APCI): m/z found:
dione (1.10 g, 5 mmol), zinc powder (0.92 g, 14 mmol), NaOH
(3.75 g), 1-bromododecane (3.75 g, 15 mmol), and TBAB
30.1073; calcd for C H BrS: 330.1017.
1
6 27
(
0.17 g, 0.55 mmol) were used. 1.95 g of compound 4 (3.49
1
4
,8-Bis(4-(5-octylthiophen-2-yl)butoxy)benzo[1,2-b:4,5-
mmol, yield 70%) was obtained as colorless solid. H NMR
0
b ]dithiophene (2)
(CDCl , 600 MHz), d (ppm): 7.47 (d, 2H), 7.36 (d, 2H), 4.27 (t,
3
0
4,8-Dihydrobenzo[1,2-b:4,5-b ]dithiophen-4,8-dione (0.88 g, 4
4H), 1.87(m, 4H), 1.56 (m, 4H), 1.39–1.27 (m, 32H), 0.88 (t, 6H).
mmol), zinc powder (0.72 g, 11 mmol), and NaOH (3 g) were
put into a 50 mL flask under an argon atmosphere; then
2,6-Bis(trimethyltin)-4,8-didodecyloxybenzo[1,2-b:4,5-
b ]dithiophene (5)
0
20 mL of pure water was added into the mixture. The mixture
was well stirred and heated to reflux for 1 h. During the reac-
tion, the color of the mixture changed from yellow to red and
Compound 5 was synthesized according to the correspond-
ing literature. Compound 4 (0.84 g, 1.5 mmol), 2.11 mL of
1
3
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TABLE 1 Molecular Weight and Thermal Data of the Polymers
Pd (dba) (1.8 mg, 0.002 mmol), and P(o-tol) (4.9 mg, 0.016
2 3 3
mmol) were used. The other processes were just as the syn-
a
a
b
a
ꢁ
T
d
Polymer
M
w
(kDa)
M
n
(kDa)
n
PDI
( C)
thesis of P1 polymer. The target polymer was obtained as a
1
dark solid, yield 74%, 150 mg. H NMR (CDCl , 600 MHz), d
3
P1
P2
P3
36.0
21.9
71.3
23.4
42.3
25.8
70.3
23.1
44.3
1.6
1.7
3.0
2.6
334
337
336
(
3
ppm): 8.98–8.04 (br, 2H), 7.11–6.47 (br, 4H), 4.15 (br, 6H),
.42–2.72 (br, 8H), 2.05–0.87 (br, 53H).
119.8
71.1
2
2
P4
Polymer P3
a
Compound 3 (209.8 mg, 0.2 mmol), 1,3-dibromo-5-octylth-
w n
The M , M , and PDI of the polymers were estimated by GPC using
polystyrene as the standard and THF as the eluent.
ieno[3,4-c]pyrrole-4,6-dione (84.6 mg, 0.2 mmol), Pd
2 3
(dba)
b
n 0 0
The n was repeating unit calculated from the equation n 5 M /M (M
is the molecular weight of the structure unit. P4 came from ref. 22).
(
1.8 mg, 0.002 mmol), and P(o-tol)3 (4.9 mg, 0.016 mmol)
were used. The other processes were just as the synthesis of
P1 polymer. The target polymer was obtained as a dark
1
n-butyllithium (3.38 mmol, 1.6 M in n-hexane), 3.75 mmol of
solid, yield 71%, 145 mg. H NMR (CDCl , 600 MHz), d
3
trimethyltin chloride (3.75 mL, 1 M in n-hexane) were used.
(ppm): 8.82–8.04 (br, 2H), 7.08–6.42 (br, 4H), 4.21 (br, 6H),
2.88 (br, 8H), 1.84–0.88 (br, 53H).
1
Yield 70% (1.05 mmol, 0.93 g). H NMR (CDCl , 600 MHz), d
3
(ppm): 7.51 (s, 2H), 4.29 (t, 4H), 1.88 (m, 4H), 1.58 (m, 4H),
1.41–1.27 (m, 32H), 0.88 (t, 6H), 0.44 (s, 18H).
RESULTS AND DISCUSSION
Polymer P1
Synthesis and Characterization
Compound 5 (176.9 mg, 0.2 mmol), 1,3-dibromo-5-(2-ethylhex-
yl)thieno[3,4-c]pyrrole-4,6-dione (84.6 mg, 0.2 mmol),
The synthetic routes of monomers and the polymers were
shown in Scheme 1. Compound 3 was prepared by the reac-
tion of compound 2, n-butyllithium, and trimethyltin chlo-
Pd (dba) (1.8 mg, 0.002 mmol), and P(o-tol) (4.9 mg, 0.016
2
3
3
mmol) were dissolved into 5 mL of toluene and 2 mL of DMF in
a flask with condensate protected by argon. The mixture was
then heated at 110 C for 24 h. After cooling to ambient temper-
ride. Compound
5 was synthesized according to the
ꢁ
ature, the mixture was poured into methanol. The precipitated
solid was collected and purified by Soxhlet thimble, which was
then subjected to Soxhlet extraction with methanol, hexane, and
chloroform. The polymer recovered from chloroform was puri-
fied by preparative gel permeation chromatography. After dry-
ing overnight under vacuum, the target polymer was obtained
1
as a dark solid, yield 85%, 145 mg. H NMR (CDCl
3
, 600 MHz), d
(ppm): 8.86–8.03 (br, 2H), 4.27 (br, 6H), 1.55–0.87 (br, 61H).
Polymer P2
Compound 3 (209.8 mg, 0.2 mmol), 1,3-dibromo-5-(2-ethyl-
hexyl)thieno[3,4-c]pyrrole-4,6-dione (84.6 mg, 0.2 mmol),
FIGURE 1 TGA plot of the polymers with a heating rate of 10
ꢁ
FIGURE 2 UV–Vis absorption spectra of the polymers in chlo-
roform solution (a) and as thin films (b). [Color figure can be
viewed in the online issue, which is available at wileyonlineli-
brary.com.]
21
C min under the nitrogen atmosphere. [Color figure can be
viewed in the online issue, which is available at wileyonlineli-
brary.com.]
4
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TABLE 2 Optical Properties and Electronic Energy Levels of the Polymers
a
b
b
opt
g
c
d
e
Polymer
k
max (nm)
k
max (nm)
k
onset (nm)
E
(eV)
E
ox (V)
E
HOMO (eV)
E
LUMO (eV)
P1
P2
P3
624
606
622
625
633
624
628
625
694
684
698
670
1.79
1.81
1.78
1.85
1.01
1.10
0.93
0.60
25.43
25.52
25.35
25.31
23.64
23.71
23.57
23.56
2
2
P4
a
d
e
Measured in chloroform solution.
Measured in thin solid films state.
Estimated from the onset of oxidation wave of CV.
LUMO = E go pt1EHOMO. P4 came from ref. 22.
b
c
E
opt
Calculated from the empirical equation Eg 5 1240/konset
.
1
3
corresponding literature.
1,3-Dibromo-5-(2-ethylhexyl)th-
and 1,3-dibromo-5-octylth-
influence the decomposition temperature of the polymers,
which may be due to the same polymer backbone.
ieno[3,4-c]pyrrole-4,6-dione
ieno[3,4-c]pyrrole-4,6-dione were purchased directly. Finally,
the D–A structured polymers were synthesized via Stille cou-
Optical Properties
pling polymerization in the presence of Pd (dba)3 catalyst
2
Figure 2 shows the UV–Vis absorption spectra of the poly-
mers in chloroform solution and as thin films. All of the
polymers show two shoulder absorption peaks at around
and tri(o-tolyl)phosphine ligand. The molecular weight of the
polymers and polydispersity index (PDI) were measured by
GPC using monodispersed polystyrene as the standard and
THF as the eluent (Table 1). The number-average molecular
550 nm and 620 nm in chloroform solution and as thin
films, which is a common feature for D–A polymers caused
by the strong intramolecular charge transfer state between
weight (M ) and PDI of the polymers are (21.9 kDa, 1.6),
n
(
71.3 kDa, 1.7), and (23.4 kDa, 3.0), respectively. The repeat-
ing units are calculated to be 25.8 for P1, 70.3 for P2, and
3.1 for P3, respectively, Compared with P1, the P2 shows
relatively high molecular weight, probably due to pendent
2
2
2
thiophene side chain. Conjugated side-chained polymer,
compared with the semiconjugated side-chained polymer (P2
and P3), shows similar but a little lower molecular weight.
Thermal Stability
The thermal properties of the polymers were measured by
TGA. As can be seen from Figure 1, TGA indicates that the
polymers exhibit similar high thermal stability and the
decomposition temperature (T ) at 5% weight loss is about
d
ꢁ ꢁ ꢁ
3
34 C, 337 C, and 336 C (Table 1), respectively, indicating
sufficient thermal stability for PSCs application. The intro-
duction of the semiconjugated side chain does not obviously
FIGURE 4 J–V characteristics of the polymers:PC61BM before
ꢁ
(
a) and after (b) thermal annealing at 100 C with a blend ratio
22
FIGURE 3 CV curves of the polymer films. [Color figure can be
viewed in the online issue, which is available at wileyonlineli-
brary.com.]
of 1:1 under the illumination of AM 1.5G, 100 mW cm . [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 PSCs based on the Blend of Polymers and PC61BM
PCE (%)
Thermal
22
Polymer
P1
Annealing
VOC (V)
JSC (mA cm
)
FF (%)
av
max
N
Y
N
Y
N
Y
0.93
0.95
0.96
0.99
0.92
0.96
7.31
6.28
3.82
3.46
4.92
4.52
60.60
56.42
55.51
57.25
44.41
47.53
3.99 6 0.14
3.24 6 0.11
1.95 6 0.08
1.88 6 0.07
1.91 6 0.10
2.00 6 0.06
4.13
3.35
2.03
1.95
2.01
2.06
P2
P3
Optimized devices with a polymer: PC61BM ratio of 1:1 (w/w) were used. All of the devices were solution-cast from o-dichlorobenzene (DCB). Devices
ꢁ
were prepared without (N) or with (Y) thermal annealing at 100 C for 10 min.
the electron-donating BDT and the electron-accepting TPD
Electrochemical Properties
3
9
units in the main chain. The introduction of TPD unit in
the BDT polymer backbone can effectively enhance the light
absorption in the 600–700 nm region, compared with BDT
The oxidation potentials of the polymers were determined
electrochemically by CV with the polymer coated on glassy
carbon electrodes (Fig. 3); the electrochemical data are sum-
marized in Table 2. The HOMO energy levels of the polymers
4
0
homopolymer. The absorption peak wavelengths (kmax) in
the solution and thin films and the absorption onset wave-
lengths (konset) are shown in Table 2. The basic optical prop-
were
calculated
using
the
empirical
formula:
EHOMO 5 2e[E 2(24.8)20.38] eV, where 24.8 eV is the
ox
2
2
1
erties of the polymers with the conjugated side chain and
the same backboned polymer (P1, P2, and P3) are also listed
in Table 2 for comparison. The absorption spectrum of P2
and P3 become broader and show the obvious red-shift in
the solid film, which indicates more effective p–p stacking in
the solid state due to pendent thiophene side chain to
reduce the steric hindrance, compared with conjugated side-
redox potential of ferrocene/ferrocenium (Fc/Fc ) internal
reference versus vacuum and 0.38 eV is the redox potential
1
of ferrocene/ferrocenium (Fc/Fc ) internal reference versus
1
Ag/Ag (Eox is the onset oxidation potential). Thus the
HOMO energy levels of P1, P2, and P3 were calculated to be
25.43, 25.52, and 25.35 eV, respectively. It demonstrates
relative low-lying HOMO energy levels of the polymers. It is
widely acknowledged that the VOC of PSCs is closely corre-
lated to the difference between the HOMO energy level of
the polymer donor and the LUMO energy level of the
2
2
chained polymer (P4) with similar backbone structures.
opt
The optical band gap (Eg ) of the polymers are 1.79 eV for
P1, 1.81 eV for P2, and 1.78 eV for P3, respectively, which
were determined from the onset of the UV–Vis absorption in
the thin solid films. In general, the four polymers with non-
conjugated side-chained polymer (P1), semiconjugated side-
chained polymers (P2 and P3), and conjugated side-chained
4
1
PC BM electron acceptor. HOMO of P3 is a bit higher than
6
1
that of P1 and P2, but P3 is still low HOMO (25.3 eV) mate-
rials reported in the literature for PSC donor. Therefore, it
is not difficult to infer that the polymers will have a higher
4
2
2
2
polymer (P4) show similar optical properties.
FIGURE 6 Hole mobility characteristics of the corresponding
devices under optimized conditions. The symbols are experimen-
tal data for the transport of holes, and the solid lines are fitted
according to the SCLC model. [Color figure can be viewed in the
online issue, which is available at wileyonlinelibrary.com.]
FIGURE 5 EQE spectra of the corresponding devices under
optimized conditions. [Color figure can be viewed in the online
issue, which is available at wileyonlinelibrary.com.]
6
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FIGURE 7 AFM of the polymers:PC61BM blend films under optimized conditions. Panels a, c, and e are height images and panels
b, d, and f are phase images, respectively. All images are 4 3 4 mm. [Color figure can be viewed in the online issue, which is avail-
able at wileyonlinelibrary.com.]
VOC due to their deeper lying HOMO. The LUMO energy lev-
tigate the photovoltaic properties of the polymers. PC BM is
61
els of the three polymers (Table 2) were calculated from the
added to the PSC active layer to form donor (polymer)–
opt
HOMO energy levels and
E
by the equation:
acceptor (PC BM) BHJ structure, which will facilitate the
g
61
ELUMO 5 EHOMO 1 (E go pt) eV. The high LUMO energy levels of
the three polymers ensure a sufficient driving force for
charge transfer from the polymer to the PC BM. In general,
electron transfer in D–A interface and then enhance the pho-
tovoltaic performance. The optimized weight ratio of the
4
3
polymers and PC BM was 1:1 in this work. The PSC devices
6
1
61
compared with the polymer without conjugated side chain
were measured under the simulated AM 1.5G illumination
condition (100 mW cm ). The P1, without pendent thio-
2
2
22
(
P1) and polymer with conjugated side chain (P4), the
semiconjugated side-chained polymers (P2 and P3) show
phene side chain, based device shows a PCE of 4.13% with a
similar low lying HOMO (25.3–5.5).
VOC of 0.93 V, a short-circuit current density (J ) of 7.31 mA
SC
2
2
cm , and a fill factor (FF) of 60.6%. The PSC devices based
on semiconjugated side-chained polymers (P2 and P3) and
PSC Performance
2
2
The solar cell devices were fabricated using the resultant
polymers as the donor and PC BM as the acceptor to inves-
conjugated side-chained polymer show similar PCE about
2–3%. Nevertheless P2 and P3 show the relative low HOMO
6
1
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and high VOC, compared with conjugated side-chained poly-
the blend films are 15.7 nm for P1/PC BM, 9.29 nm for
61
2
2
mer P4 with similar backbone structures. P1 shows higher
JSC (caused by higher EQE) and FF than that of P2 and P3,
so the PCE of P1 is highest among the P1, P2, and P3. It has
been well-recognized that thermal annealing may be helpful
in optimizing the nanoscaled phase separation of the DA
blends, thus thermal annealing (100 C for 10 min) was
used to further optimize the PSC devices. The PCE of the
thermal annealed devices do not show significantly improve-
ment compared with the pristine devices. The open circuit
voltage (VOC) show slight enhancement for all these three
polymers. Especially for P2 polymer-based device, VOC can
P2/PC61BM, and 1.85 nm for P3/PC61BM. It demonstrates a
big difference of the roughness between P3 blend film and
P1, P2 blend films, which indicates that the pendent thio-
phene segment in BDT side groups for P3 do have changed
the stacking properties of the counterpart polymers (P1 and
P2). The smooth surface of the blend film ensures a better
contact with the Ca/Al electrode, which is helpful in the
increase of the charge collection efficiency. The AFM phase
images show the status of the donor and acceptor distribu-
tion and interpenetrating network structure. As shown in
1
6
ꢁ
1
6
Figure 7(b, d, and f), the AFM image of the P3/PC BM blend
61
achieve around 1 V (0.99 V), which is the highest V
film reveals that P3 has good miscibility with PC BM and
OC
61
0
reported for benzo[1,2-b:4,5-b ]dithiophene and thieno[3,4-
an optimized nanoscale phase separation has been formed.
So P3 has the best miscible ability with PC61BM among the
three polymers. The P2 polymer, which also has pendent thi-
ophene segment in BDT side groups as P3, shows high AFM
roughness probably due to the branched side chain on TPD
2
1,44,45
c]pyrrole-4,6-dione copolymers.
All the optimized
device performances have averaged over six devices, and the
data are given in Table 3. The J–V curves are shown in Figure
4
. Our preliminary results show that the pendent thiophene
side chain can lightly enhance the VOC of the devices (Table
and Fig. 4).
segment which will make PC BM away from polymer back-
61
3
bone, and this has leaded to the aggregation of polymer or
4
7
PC61BM.
The EQEs of devices based on the polymers and PC61BM
(
1:1, w/w) blend films before (P1 and P2) and after thermal
CONCLUSIONS
ꢁ
annealing (P3) at 100 C are measured under illumination
with monochromatic light and are shown in Figure 5. The
polymer devices show a photo response between 300 and
Two novel pendent thiophene side-chained BDT- and TPD-
based polymers were successfully synthesized. BDT with
pendent thiophene and ethylhexyl side-chained TPD-based
polymer shows relatively low-lying HOMO energy level
(25.52 eV) and nearly 1 V high VOC in PSC device (0.99 V)
which is the highest VOC achieved by BDT- and TPD-based
polymers. The semiconjugated side group does have some
special influence on the PSC device performances. Further
work to make full use of semiconjugated side group concept
to build more efficiency PSC materials and devices is
underway.
7
00 nm, along with absorption spectra. To confirm the accu-
racy of the photovoltaic measurements, the JSC (7.16 mA
2
2
22
cm
.47 mA cm
integration of the EQE curves, which agree well with the J
for P1/PC61BM, 3.72 mA cm
for P2/PC61BM, and
2
2
4
for P3/PC BM) were calculated from the
6
1
SC
2
2
22
(7.31 mA cm
for P1/PC BM, 3.82 mA cm
for P2/
6
1
22
PC BM, and 4.52 mA cm for P3/PC BM) obtained from
the J–V measurements.
6
1
61
Hole Mobility
The hole mobility of the polymers:PC BM blend films meas-
6
1
ured via the space charge limited current (SCLC) method are
ACKNOWLEDGMENTS
2
4
2
21 21
24
2
21 21
24
9
.2 3 10 cm V
s
, 6.1 3 10 cm V
for the P2, P3, and P1, respectively (Fig. 6). Film
thickness of the polymers:PC BM devices are 200 nm, 140
s
, 3.4 3 10
2
21 21
The authors gratefully acknowledge financial support from the
NSFC (21274134, 21274161, and 51173199), New Century
Excellent Talents in University (NCET-11-0473), and Shandong
Province Postdoc Foundation (201301007).
cm V
s
6
1
nm, and 130 nm, respectively. Polymers with pendent thio-
phene side chains (P2 and P3) show lightly improved hole
mobility than P1, probably due to pendent thiophene side
chain, although hole mobility of all the polymers (P1, P2,
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