A triple bond side-chained 2D-conjugated benzodithiophene based photovoltaic polymer

Article abstract of DOI:10.1039/c4ra10154e

A series of triple bond side-chained benzodithiophene copolymers, derived from 4,8-bis(1-ethynyl-3,5-bis(octyloxy)phenyl)-benzo[1,2-b:4,5-b′]dithiophene, were synthesized by Stille coupling reactions. Several electron acceptors are introduced into the pol

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Triple Bond Side-Chained 2D-Conjugated
Benzodithiophene based Photovoltaic Polymer
Cite this: DOI: 10.1039/x0xx00000x
Shuguang Wen b§, Xiao Yun a,b§, Weichao Chen b, Qian Liu a,b, Dangqiang Zhub,
Chuantao Gub, Mingliang Sun a*, Renqiang Yang b*
Received 00th January 2012,
Accepted 00th January 2012
A series of triple bond side-chained benzodithiophene copolymers, derived from 4,8-bis(1-ethynyl-3,5-
bis(octyloxy)phenyl)-benzo[1,2-b:4,5-b']dithiophene, were synthesized by Still coupling reactions.
Several electron acceptors are introduced into the polymer backbone to tune the photo-electronic
properties of the copolymers. The obtained copolymers are readily soluble in common organic solvents,
such as toluene, THF and chloroform. The number average molecular weights of these polymers were
determined by GPC using a polystyrene standard, ranging from 15,400 to 28,300 which are decent
results for polymers with rigid triple band side chain. These polymers show high decomposition
temperature (403-419 °C), narrow band gap (1.27-1.89 eV), and low-lying HOMO energy level (-5.4 to -
5.6 eV). The best polymer solar cells (PSCs) device based on the copolymer PBDTPA-TPD and
PC61BM showed a power conversion efficiency (PCE) of 0.93% with a Voc = 0.96 V, Jsc = 1.9 mA cm-2,
and FF = 51%, which is one of the highest Voc value achieved by benzodithiophene polymer.
DOI: 10.1039/x0xx00000x
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inconvenient for PSCs devices processing.
1. Introduction
In this paper, to make more soluble alkynyl substituted BDT
donor materials, we have designed and synthesized two-dimension
conjugated unit of dialkoxy-phenylacetylene substituted BDT, 4,8-
bis(1-ethynyl-3,5-bis(octyloxy)phenyl)-benzo[1,2-b:4,5-
b']dithiophene. (Scheme 1) Copolymerized with the acceptor TPD,
DPP and TT (Scheme 2), three BDT based copolymers are obtained,
and their optical properties, electrical properties and photovoltaic
properties are studied.
Polymer solar cells (PSCs) have attracted increasing interest due
to light-weight, large-area, and flexible through solution
processing.1,2,3,4 PSCs usually adopt a bulk-heterojunction (BHJ)
structure, and the photoactive layer consists of an interpenetrating
network of π-conjugated polymer donors and soluble fullerene
acceptors.5,6,7
Among
different
polymer
donor
units,
benzodithiophene (BDT) unit shows good planarity due to
symmetrical molecular structure, meanwhile BDT polymer also
exhibits high carrier mobility due to strong intermolecular π-π
stacking interaction.8,9,10 4,8-Dialkoxy substituted BDT based donor-
acceptor (D-A) type conjugated polymer, have shown excellent
maximum power conversion efficiency (PCE) of 9.35%11 which is
the highest PCE for PSCs. However, modification and optimization
of BDT based polymer is still hot topic for more efficiency PSCs.
For example, by replacing strong electron-donating alkoxy chains
with weak electron donor, the HOMO energy level of polymer is
reduced, which can increase the open circuit voltage (Voc) of PSCs
devices.12 By the concept of two dimensional (2D) conjugated BDT,
dialkyl-thiophene substituted BDT is synthesized, which to some
extents reduces the HOMO energy level and extends absorption
spectrum of polymer, comparing to dialkoxy substituted BDT.13-17
Besides thiophene side chain of BDT, other conjugated side chain,
such as thiophene[3,2-b]thiophene, benzene etc,18 have also been
tried to build more efficiency BDT photovoltaic polymer.
2. Experimental section
2.1. Materials
Unless otherwise stated, all reagents and starting materials
were used as commercially purchased without further
purification. The TPD, DPP and TT comonomers were
purchased from Derthon Optoelectronic Materials Science
Technology Co Ltd. Tetrahydrofuran (THF) and toluene were
distilled from sodium with benzophenone as indicator and N,N-
dimethylformamide (DMF) was distilled from CaH2 under
argon atmosphere before use. All air and water sensitive
reactions were performed under argon or nitrogen atmosphere.
2.2. Materials characterization
According to the sp hybrid in the hybrid orbital theory, the triple
bond in the alkynyl group is a weak electron withdrawing group,
meanwhile performs high rigidity which is good for intermolecular
stacking.19 By introducing alkynyl group into BDT side chain, the
HOMO energy level of polymer materials can be lowered, due to
enhanced π-π stacking. PCE of (tri-isopropyl-silyl) acetylene based
BDT polymer have reached 5.76%,20 but strong rigidity of triple
bond leads to poor solubility of the polymer, which make it
The nuclear magnetic resonance (NMR) spectra were recorded
on a BrukerAdvanceIII 600 (600 MHz) using CDCl3 as the solvent.
UV-vis absorption spectra were measured with a Hitachi U-4100
spectrophotometer. The molecular weight and polydispersity were
determined by gel permeation chromatography (GPC) analysis using
an ELEOS System with polystyrenes as the reference standard and
THF as an eluent. Thermogravimetric analysis (TGA) was
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performed on a TA-Q600 analyzer with a heating rate of 20 oC min-1 was cooled to -78 °C, and 1 M CH2Cl2 solution of BBr3 (100 ml,
under nitrogen. Electrochemical cyclic voltammetry (CV) 100.0 mmol) was added slowly. Then, the solution was allowed to
measurements were conducted on an electrochemistry workstation warm to ambient temperature for overnight. The stirred solution was
(CHI660D) with a glassy carbon as working electrode, a platinum cooled to 0 °C, and then quenched with H2O. The aqueous phase
wire as counter electrode and an Ag/AgCl as reference electrode was extracted with Et2O. The combined organic layers were washed
under nitrogen in a solution of Bu4NPF6 (0.1 M).
with H2O, dried over MgSO4. Solvent was removed, and the
resulting white solids were used for the next step reaction without
further purification.
2.3. Device fabrication
Photovoltaic devices were fabricated on pre-patterned indium
tin oxide (ITO) coated glass substrates with a layered structure
1-Bromo-3, 5-bis(octyloxy)phenyl (M2)
of ITO/PEDOT:PSS/donor:PC61BM/Ca(10 nm)/Al(100 nm). 1-Bromo-3, 5-di-hydroxyphenyl (3 g, 15.9 mmol) and anhydrous
The ITO coated glass substrates were cleaned in ultrasonic bath K2CO3 (16.21 g) was diluted with 90 mL of N,N-
in acetone, toluene, methanol and isopropyl alcohol sequentially. dimethylformamide, the mixture was stirred at ambient temperature
After a twenty-minute oxygen plasma treatment, a thin layer of
PEDOT:PSS (30 nm) was spin-coated onto the ITO anode and then
dried at 120 °C for 20 min. The photosensitive layer was prepared
by spin-coating a blend solution of the polymers and PC61BM
with a weight ratio of 1:1.5 in deoxygenated anhydrous ο-
dichlorobenzene on the ITO/PEDOT:PSS substrate and then
annealed at 150 °C for 10 min in a glove box. 4% of DIO was
added as processing additive to the blend solutions. The active
layer thickness was around 90 nm. Finally, Ca (10 nm) and
aluminum (100 nm) were thermally evaporated at a vacuum of
~2×10-4 Pa on top of active layer. The photovoltaic
performance was measured under illumination at 100 mWcm-2
AM 1.5 G irradiation using a Xe arc lamp in an argon
atmosphere (<0.1 ppm H2O and O2), and the current density-
voltage (J-V) curves was obtained by Keithley 2400. The
external quantum efficiency (EQE) was obtained by a source
meter, silicon photodiode and a computer-controlled light
source-monochromator-lock-in system.
for 30 min. After the addition of bromooctane (12.4 g，64.4 mmol),
solution was heated at 60 °C overnight. The cooled reaction mixture
was filtered through Celite, and concentrated to yield the crude
product which was purified by column chromatography (hexane
eluent) to obtain 1-bromo-3,5-bis(octyloxy)phenyl 4.4 g (10.5 mmol
，66.8%).1H NMR (CDCl3, 600MHz), δ (ppm): 6.63 (d, 2H), 6.36
(t, 1H), 3.89(t, 4H), 1.75 (m, 4H), 1.42 (m, 4H), 1.29 (m, 16H), 0.89
(t, 6H) (Figure S1)
1-(Trimethylsilyl)ethynyl-3,5-bis(octyloxy)phenyl (M3)
1-Bromo-3, 5-bis(octyloxy)benzene (4.3 g, 10.5 mmol), copper
iodide (27.3 mg) and bis(triphenylphosphine)dichloridepalladium
(336 mg) was dissolved in 42 ml of dry triethylamine under
nitrogen. After the mixture was heated to 50 °C,
trimethylsilylacetylene (1.1 g，11.4 mmol) was added. The solution
was heated at 80 °C for 24 h. The cooled reaction mixture was
quenched with 2 M HCl solution, washed with H2O,dried over
MgSO4, and concentrated to yield crude product, which was purified
by column chromatography (hexane eluent) to obtain 3.2 g of light
yellow powder (7.3 mmol, 70%). 1H NMR (CDCl3, 600MHz), δ
(ppm): 6.62 (d, 2H), 6.46 (t, 1H), 3.94 (t, 4H), 1.78 (m, 4H), 1.46
(m, 4H), 1.33 (m, 16H), 0.92 (t, 6H), 0.27 (s, 9H). (Figure S2)
2.4. Synthesis
1-Ethynyl-3,5-bis(octyloxy)phenyl (M4)
1-(Trimethylsilyl)ethynyl-3,5-bis(octyloxy)benzene (3.2 g, 7.3
mmol) was dissolved in 35 ml of dichloromethane, and 2.3 g of
potassium hydroxide was dissolved in 35 ml of methanol. These two
solutions were mixed and stirred at ambient temperature for 3 h. The
reactant was poured into water and extracted by dichloromethane.
The organic layer was dried over anhydrous MgSO4. After removing
solvent, the crude product was purified by column chromatography
(hexane eluent) to obtain 2.4 g of light yellow powder (7.68 mmol,
99%).1H NMR (CDCl3, 600 MHz), δ (ppm): 6.65 (d, 2H), 6.49 (t,
1H), 3.94 (t, 4H), 3.04 (s, 1H), 1.78 (m, 4H), 1.46(m, 4H), 1.33 (m,
16H), 0.92 (t, 6H). (Figure S3)
4,8-Bis(1-ethynyl-3,5-bis(octyloxy)phenyl)-benzo[1,2-b:4,5-
b']dithiophene (M5)
Reaction conditions: (a) BBr3, methylene chloride, 0 °C; (b)
Bromooctane, K2CO3, DMF, 60 °C; (c) Ethynyl-trimethyl silicon,
CuI,
trans-dichlorobis(triphenyl-phosphine)
Palladium(II),
1-Ethynyl-3, 5-bis(octyloxy)benzene (2 g, 6.4 mmol) was dissolved
in 70 ml of dry THF under nitrogen. Solution was cooled to 0 °C,
and then 4 ml of n-BuLi (6.5 mmol, 1.6 M) was added dropwise.
The mixture was stirred at 0 °C for 30 min, then benzo[1,2-b:4,5-
b']dithiophene-4,8-dione (0.67 g, 3.1 mmol) was added. The stirred
mixture was heated at reflux for 1 h, then cooled to 0 °C again. 3.3 g
of tin chloride dihydrate dissolved in 50 mL 20% HCl solution was
added, and the reaction was heated at reflux for 2 h. The cooled
reaction mixture was extracted with ether. The organic layer was
triethylamine, 80 °C; (d) KOH, methylene chloride; (e) n-BuLi,
THF, 0 ℃, SnCl2, reflux; (f) n-BuLi, Me3SnCl, THF, 0 °C.
Scheme 1 Synthetic routes of monomers
1-Bromo-3,5-di-hydroxyphenyl (M1)
3,5-Dimethoxybromobenzene (10.0 g, 46.2 mmol) was dissolved in
100 ml dry CH2Cl2. The solution of 3, 5-dimethoxybromobenzene
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washed with H2O several times, dried over MgSO4, and concentrated
to yield crude product, which was purified by column
chromatography (hexane: dichloromethane (8:1, v/v) eluent) to
obtain 1.3 g of light yellow powder (1.47 mmol, 45%).1H NMR
(CDCl3, 600 MHz), δ (ppm): 7.74 (d, 2H), 7.62 (d, 2H), 6.84 (d,
4H), 6.55 (t, 2H), 4.02 (t, 8H), 1.84 (m, 8H), 1.51 (m, 8H), 1.36 (m,
32H), 0.93 (t, 12H). (Figure S4)
PBDTPA-DPP (OD)
2,6-Bis(trimethyltin)-4,8-bis(1-ethynyl-3,5-bis(octyloxy)phenyl)-
benzo[1,2-b:4,5-b']dithiophene (61.4 mg, 0.05 mmol), 3,6-bis(5-
bromo-thiophen-2-yl)-2,5-bis(2-octyldodecyl)pyrrolo[3,4-c]pyrrole-
1,4-(2H, 5H)-dione (DPP(OH)) (50.9 mg, 0.05 mmol), tris(o-
tolyl)phosphine
(2.4
mg,
16%)
and
tris(dibenzylideneacetone)dipalladium (0.9 mg, 2%) was dissolved
in 5 ml dry toluene under nitrogen. The other procedures are just as
PBDTPA-TPD polymer. Finally, the target polymer PBDTPA-
2,6-Bis(trimethyltin-4,8-bis(1-ethynyl-3,5-bis(octyloxy)phenyl)-
benzo[1,2-b:4,5-b']dithiophene (M6)
1
DPP(OD) is obtained as a dark green solid. H NMR (CDCl3, 600
4,8-Bis(1-ethynyl-3,5-bis(octyloxy)phenyl)-benzo[1,2-b:4,5-
MHz), δ (ppm): 9.0-8.5 (m), 7.1-6.2 (m), 4.0 (s), 2.1-1.2 (m).
(Figure S9) GPC (THF, polystyrene standard): Mn= 28.3 kDa, Mw=
275.6 kDa, PDI= 9.72.
b']bithiophene (BDTPA) (0.6 g, 0.68 mmol), N, N, N, N-
tetramethylethylenediamine (0.28 ml , 1.8 mmol) was dissolved in
30 ml of dry THF under nitrogen. The mixture was cooled to 0 °C,
and 1.2 ml of n-BuLi (1.8 mmol, 1.6 M) was added dropwise. After
being stirred at 0 °C for 4 h, 2.2 ml of 1 M trimethyltin chloride was
added slowly. The stirred solution was warm up to ambient
temperature for overnight. After the reaction, the solution was
poured into water and extracted by ether. The organic layer was
dried over anhydrous MgSO4. After removing solvent under vacuum,
the residue was recrystallized from isopropanol to obtain 0.67g of
product (0.544 mmol, 80%).1H NMR (CDCl3, 600 MHz), δ (ppm):
7.76 (t, 2H), 6.86 (d, 4H), 6.55 (t, 2H), 4.03 (t, 8H), 1.84 (m, 8H),
1.51 (m, 8H), 1.36 (m, 32H), 0.93 (t, 12H), 0.52 (t, 18H). (Figure S5)
13C NMR (CDCl3, 150 MHz), δ (ppm): 160.1, 144.7, 143.6, 139.1,
130.9, 124.2, 110.2, 110.0, 102.9, 98.7, 85.6, 68.3, 31.8, 29.4, 29.2,
26.0, 22.7, 14.1, -8.2. (Figure S6) Anal. Calcd. for C64H94O4S2Sn2:
C, 62.55; H, 7.71; S, 5.22. Found: C, 62.73; H, 7.62; S, 5.15. HRMS
(APCI) m/z: calcd. 1229.4632 [M+1]+, found: 1229.4740.
PBDTPA-TPD
2,6-Bis(trimethyltin)-4,8-bis(1-ethynyl-3,5-bis(octyloxy)phenyl)-
benzo[1,2-b:4,5-b']dithiophene (184.4 mg, 0.15 mmol), 1,3-
dibromo-5-(2-ethylhexyl)-thieno[3,4-c]pyrrole-4,6-dione
(TPD)
(63.5 mg, 0.15 mmol), (o-tolyl)phosphine (7.3 mg, 16%) and
tris(dibenzylideneacetone)dipalladium (2.8 mg, 2%) was dissolved
in 5 ml dry toluene under nitrogen. The mixture was heated to 70 ℃
for 4 h. Then, the stirred solution was heated to 110 °C for 48 h.
After the reaction, the cooled mixture was dropped into methanol.
The precipitate was collected and Soxhlet-extracted in sequence with
methanol, hexane and chloroform. The product was dried under
vacuum for 1 day to obtain the target polymer PBDTPA-TPD as a
dark red solid.1H NMR(CDCl3, 600 MHz),δ(ppm): 9.5-7.5 (m), 7.2-
5.5 (m), 4.5-3.3 (m), 2.3-0.3 (m). (Figure S7) GPC (THF,
polystyrene standard): Mn= 23.9 kDa, Mw= 57.7 kDa, PDI= 2.41.
Scheme 2 Synthetic routes of polymers
3. Results and discussion
3.1. Synthesis and Characterization
The general synthetic routes toward the monomer and copolymers
are outlined in Scheme 1 and Scheme 2 respectively. All the
PBDTPA-TT
compounds are characterized by 1H NMR spectra. They are in
21
accordance with the literature
for the precursors and show
2,6-Bis(trimethyltin)-4,8-bis(1-ethynyl-3,5-bis(octyloxy)phenyl)-
benzo[1,2-b:4,5-b']dithiophene (122.9 mg, 0.1 mmol), 2-
ethylhexyl4,6-dibromo-thieno[3,4-b] thiophene-2-carboxylate (TT)
(48.6 mg, 0.1 mmol), tris(o-tolyl)phosphine (4.9 mg, 16%) and
tris(dibenzylideneacetone)dipalladium (1.8 mg, 2%) was dissolved
in 5 ml of dry toluene under nitrogen. The other procedures are just
as PBDTPA-TPD polymer. Finally, the target polymer PBDTPA-TT
is obtained as a dark solid.1H NMR (CDCl3, 600 MHz), δ (ppm):
8.5-7.5 (m), 7.2-6 (m), 4.5-3.2 (m), 2.2-0.3 (m). (Figure S8) GPC
(THF, polystyrene standard): Mn= 15.4 kDa, Mw= 81.2 kDa, PDI=
5.25.
correctly the designed structure for polymers (see supporting
information Figure S7 to Figure S9). The obtained copolymers are
readily soluble in common organic solvents, such as toluene, THF,
and chloroform. The number-average molecular weights of these
polymers were determined by GPC using a polystyrene standard,
ranging from 15,400 to 28,300 with a polydispersity index (Mw/Mn)
between 2.41 and 9.72. The molecular weights data is summarized in
Table 1.
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Table 1 Molecule weight and optical properties of the polymers
opt
Mn
(k)
Mw
(k)
λmax-S
(nm)
λmax-F
(nm)
ε
(cm-1) Eg
polymer
PDI
thin film
(eV)
PBDTPA-
TPD
PBDTPA-
TT
PBDTPA-
DPP(OD)
23.9
15.4
28.3
57.7
81.2
275.6
2.41
5.25
9.72
587
603
683
613
624
784
4.1 × 10 4 1.89
2.4 × 10 4 1.37
4.7 × 10 4 1.27
λmax-S: Absorption peak in solution; λmax-F: Absorption peak in
film;
Figure. 1 TGA plots of the polymers with a heating rate of 20 °C
opt
Eg : Optical band gap is calculated from 1240/λonset
.
min-1 under an inert atmosphere.
The thermal properties of the polymer were measured with a heating
rate of 20 °C min-1 under inert atmosphere. As shown in Figure 1,
the decomposition temperature (Td) at 5% weight loss of PBDTPA-
TPD, PBDTPA-TT and PBDTPA-DPP(OD) was 419.7 °C, 403.7 °C
and 401.7 °C respectively. The high decomposition temperature
decreases the possibility of the deformation of the polymer film
morphology and the degradation of the polymer active layer under
applied electric fields in solar cell devices.
3.3. Electrochemical characterization
3.2. Optical properties
Figure. 3 Cyclic voltammogram of the polymers
Cyclic voltammogram (CV) (Figure 3) was performed to evaluate
the HOMO and LUMO energy levels of all the polymers. HOMO
and ELUMO were estimated from the onset oxidation potential and
reduction potential of the polymer film.23 The HOMO and LUMO
levels are calculated according to the empirical formula EHOMO
=
−e(Eoxonset+ 4.4) eV and ELUMO= −e(Eredonset+ 4.4) eV. The data
are listed in Table 2. The polymers show deep HOMO level around
-5.6 and -5.4 eV, which may result in high open circuit voltage
(Voc) in OSC device. Meanwhile, the LUMO levels were calculated
from optical band gap. The materials of PBDTPA-TT and PBDTPA-
DPP(OD) exhibit low LUMOopt value of below -4.0 eV, which may
lead to insufficient exciton dissociation at donor/acceptor interfaces
Figure. 2 Normalized absorption spectra of the polymers in
chloroform solution and in thin film
The absorption spectra of the polymers were measured both in
chloroform solution and in thin films (Figure 2, Table 1). Since the
different electron acceptors of copolymers lead to various absorption
spectral features, PBDTPA-TPD, PBDTPA-TT and PBDTPA-
DPP(OD) exhibit absorption peaks at ~587, 603 and 683 nm
respectively. Compared to the absorption in solution, the thin films
show red-shifted UV-vis absorption spectrum, with absorption peaks
at ~ 613, 624 and 784 nm, respectively. Especially for PBDTPA-
DPP(OD), the absorption spectrum becomes broader and shows a
101 nm red-shifted in thin film, which can be attributed to the planar
polymer chain structure and effective inter-chain π-π stacking in the
solid state. The introduction of alkynyl group enhances
intermolecular interactions by extending the 2D conjugation of
polymers. The absorption coefficients were tested in thin film and
middle value (< 5 × 10 4 cm-1) were obtained for all three polymers,
24
.
Table 2 Electrochemical properties of the polymers
cv
LUMOopt
a (eV)
Eox
(V)
Ered
V)
(
Eg
HOMO LUMO
polymer
(eV)
-5.65
-5.44
(eV)
-3.24
-3.57
b(eV)
-1.17
-0.85
PBDTPA-TPD
PBDTPA-TT
1.24
1.05
-3.76
-4.07
2.41
1.87
PBDTPA-
DPP(OD)
-1.02
1.01
-5.42
-3.39
-4.15
2.03
which may cause limited photocurrent in solar cells 22
.
a LUMOopt = HOMO - Egopt b Egcv = LUMO - HOMO.
;
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The external quantum efficiency (EQE) spectra of the devices are
shown in Figure 5. EQE value of < 20% were obtained for
polymers PBDTPA-TPD and PBDTPA-DPP(OD). The EQE
spectrum of PBDTPA-DPP(OD) shows relatively low value beyond
500 nm, while its maximum absorption in film is at about 784 nm.
This mismatch reflect insignificant contribution from the major
absorption of PBDTPA-DPP(OD) to the photocurrent of solar cell25.
That is to say, the conversion of the low-energy excitons (from the
polymer’s absorption) to harvestable free charges is not efficient.
This is most probably due to the poor dissociation efficiency of the
low-energy excitons as the polymer-PC61BM LUMO energy
difference is too small (<0.3 eV)26. This induces less-effective
utilization of the absorption and low Jsc is obtained as a result.
3.4. Photovoltaic performance
Table 3 Solar cells devices performance of PBDTPA-TPD and
PBDTPA-DPP(OD)
Figure. 4 J-V curves of the PSCs based on the blend of PBDTPA-
TPD or PBDTPA-DPP(OD)/PC61BM under the illumination of AM
1.5, 100 mWcm-2
Active layer
Voc (V)
0.96
Jsc (mA cm-2)
1.90
FF (%)
51
PCE (%)
0.93
PBDTPA-
TPD/PC61BM
PBDTPA-
0.45
2.26
39
0.39
DPP(OD)/PC61BM
4. Conclusions
In summary, a series of triple bond side-chained benzodithiophene
copolymer, derived from 4,8-bis(1-ethynyl-3,5-
bis(octyloxy)phenyl)-benzo[1,2-b:4,5-b']dithiophene, were
synthesized. By introducing soluble bis(octyloxy)phenyl substituted
triple band as side chain of DBT polymer, decent molecule weight
(15.4 k to 28.3 k) is achieved for polymers with rigid triple band side
chain. These polymers show high decomposition temperature and
low-lying HOMO energy level. The best PSCs device based on the
copolymer PBDTPA-TPD and PC61BM showed a nearly 1V (0.96
V) Voc. Further work to make more efficiency triple bond side-
chained benzodithiophene polymer is underway.
Figure. 5 EQE curves of the PSCs based on the blend of
PBDTPA-TPD or PBDTPA-DPP(OD)/PC61BM and optical
absorptions for the corresponding polymer/PC61BM blend film.
Polymer solar cells (PSCs) devices were fabricated with the resultant
polymer as the donor and PC61BM as the acceptor to investigate the
photovoltaic properties of the polymer. The device structure is
Acknowledgments
ITO/PEDOT:PSS/polymer:PC61BM/Ca/Al.
To
improve
the
photovoltaic performance of the device, 1, 8-diiodooctane (DIO) was
used as the additive and the optimized D/A ratio of polymer:
PC61BM was 1:1.5 (Figure S10). Figure 4 shows the current
density-voltage curves (J-V) of the devices under the illumination of
AM 1.5G (100 mW cm-2). The PBDTPA-TPD/PC61BM device
exhibited the best PCE of 0.93%, Voc of 0.96 V. Due to the low-
lying HOMO energy level of PBDTPA-TPD (-5.4 eV), the PSCs
based on PBDTPA-TPD show nearly 1 V (0.96 V) Voc which is one
of the highest Voc value achieved by benzodithiophene polymer. For
PBDTPA-DPP(OD)/PC61BM based PSCs devices, the best result
was obtained with a PCE = 0.39% and a Voc of 0.45 V. PBDTPA-TT
The authors are deeply grateful to the National Natural Science
Foundation of China (Project no. 21274134, 21202181, 51173199
and 61405209), New Century Excellent Talents in University
(NCET-11-0473), and Qingdao Municipal Science and Technology
Program (13-1-4-200-jch) for financial support.
Notes
aInstitute of Material Science and Engineering, Ocean University of China,
Qingdao 266100, People’s Republic of China. Fax: 86-532- 66781927; Tel:
86-532-66781690; E-mail: mlsun@ouc.edu.cn
based device performances wasn’t listed in Table 3, because it does bCAS Key Laboratory of Bio-based Materials, Qingdao Institute of Bioenergy
and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101,
People’s Republic of China. Fax: 86-532-80662778; Tel: 86-532-80662700;
E-mail: yangrq@qibebt.ac.cn
not
show
photovoltaic
response
in
the
ITO/PEDOT:PSS/polymer:PC61BM/Ca/Al device structure. For
polymers PBDTPA-DPP(OD) and PBDTPA-TT, the deep LUMO
level (< -4.0 eV) may be a detrimental factor for the low PCE value,
because the LUMO-LUMO offset with PC61BM acceptor is less than
0.3 eV and exciton dissociation could not be sufficiently efficient.
Furthermore, low value of absorption coefficient would also reduce
the PCE of OSC (Table 1). The dark J-V curves were tested for
polymers of PBDTPA-TPD and PBDTPA-DPP(OD) (Figure 4). It
shows good diode character.
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