Highly efficient polymer solar cells based on poly(carbazole-alt-thiophene- benzofurazan)

Article abstract of DOI:10.1039/c2nj40309a

An octyloxy substituted benzofurazan based poly(carbazole-alt-thiophene- benzofurazan) (PCzDTBF) was synthesized by Suzuki polycondensation. The absorption spectra of the polymer were peaked at 385 and 540 nm, and the HOMO and LUMO energy levels were -5.3

Full text of DOI:10.1039/c2nj40309a

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Paper
Highly Efficient Polymer Solar Cells Based on Poly(Cabarzole-alt-
Thiophene-Benzofurazan)
a
a
b
a
a,c
*a
Bin Zhang, Xiaowen Hu, Minquan Wang, Huiping Xiao, Xiong Gong, Wei Yang, and Yong Cao
a
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
Octyloxy substituted benzofurazan based poly(carbazole-alt-thiophene-Benzofurazan) (PCzDTBF) was
synthesized by Suzuki polycondensation. The absorption spectra of the polymer were peaked at 385 and
5
40 nm, and the HOMO and LUMO energy levels were -5.34 and -3.46 eV, respectively. The hole
-
mobility was found to be 4.8×10 cm V S by the space charge limited current (SCLC) method, and
4
2
-1 -1
2
the surface energy (E ) was calculated to be 35.8 mJ/m from the contact-angle measurement. Polymer
1
0
s
solar cells (PSCs) based on the blend of PCzDTBF and PCBM with a weight ratio of 1:2 were fabricated.
The devices were found to have a power conversion efficiency (PCE) of 4.2% in the device configuration
of ITO/PSS:PEDOT/PCzDTBF:PC BM/Al. By utilizing poly[(9,9-dioctyl-2,7-fluorene)-alt-(9,9- bis(3′-
7
0
(N,N-dimethylamino)propyl)-2,7-fluorene)] (PFN) as the cathode interfacial layer, the PCE was enhanced
1
5
to 5.48%, with an open-circuit voltage (V ) of 0.90 V, a short-circuit current density (J ) of 7.73
oc
sc
2
mA/cm , and a fill factor (FF) of 67%. The high efficiencies could be ascribed to the good morphology,
resulted from the matched surface energies of PCzDTBF and PCBM components, and the distinct
enhancement on Voc and FF by PFN layer where it possibly existed a built-in field potential between the
active layer and cathode.
level. However, the solubility of BT and BF based D-A-D
monomers is poor in organic solvents (e.g. toluene) due to their
high polarity and planarity. Therefore, the side chain substituted
2
0
Introduction
Polymer solar cells (PSCs) were becoming increasingly more
and more attractive as the renewable energy resources, because of
their versatile merits in low cost, flexibility, light weight, and
5
5
6
6
0
5
0
5
20
21-23
BT and BF
units can improve the solubility tremendously in
common solvents, and make it more feasible to synthesize the
polymers with high molecular weight and good solubility.
1
,2
large-scale fabrication. Some PSCs with high power conversion
2
3
3
4
4
5
0
5
0
5
efficiency (PCE) overwhelming 8% were achieved, which were
3
potentially applicable in commercial power generation.
Previously, the alcohol soluble poly[(9,9-dioctyl-2,7-fluorene)-
alt-(9,9-bis(3′-(N,N-dimethylamino )propyl)-2,7-fluorene)] (PFN)
was utilized as the cathode interfacial layer in photovoltaic device
where the photovoltaic performances were improved evidently,
Bulk-heterojunction (BHJ) was one kind of the widely utilized
device structure, based on the blend of electron-donating
materials (donor) and electron-accepting moieties (acceptor). So
far, fullerene derivatives were the best electron acceptor
candidates, such as [6,6]-phenyl-C -butyric acid methyl ester
possibly because of the existence of built-in electric field between
24,25
the active layer and cathode.
Therefore, the insertion of an
6
1
alcohol soluble PFN layer as the cathode interfacial layer is an
attractive strategy to achieve highly efficient PSCs.
(PC BM) and [6,6]-phenyl-C -butyric acid methyl ester
60 71
(
PC BM). Therefore, many works were focused on the
70
Here, an alternative conjugated polymer containing octyloxy
substituted BF unit and 9-octyl-2,7-carbazole was synthesized,
and used as a donor material in BHJ solar cells. Moreover, the
PFN was used as the cathode interfacial layer, which was
ultimately contributable to the large enhancement on the
photovoltaic efficiency because of the possibly distinct
development of electron donors, for instance, polythiophenes
4
- 7
8 , 9
(PThs),
polybenzodithiophenes (PBDTs),
11
polyfluorenes
10
(
PFs), and polysilafluorenes (PSiFs), Also, polycarbazoles
PCzs) were star polymers for PSCs, because of the fantastic
(
characters of carbazole on high hole mobility, deep highest
occupied molecular orbit (HOMO) energy level, high thermal
improvement in FF and Voc.
1
2,13
stability and feasible molecular structure tuning.
In the donor
polymers, mostly used molecular architecture was donor-
acceptor-donor (D-A-D), in which donor was electron-rich unit
Experimental section
7
0
Characterization and instrumentation
(such as thiophene and furan), and acceptor referred to the
1
13
1
2, 13
Both H nuclear magnetic resonance (NMR) and C NMR
measurements were carried out on a Bruker 300 MHz DRX
spectrometer with tetramethylsilane (TMS) as the internal
reference. Mass Spectrometry was characterized on a Bruker
electron-deficient unit (such as, benzothiadiazole (BT),
1
4
15
16-19
benzoselenodizole and quinoxaline ). Benzofurazan (BF),
containing the higher electronegativity oxygen, instead of the
sulfur atom in BT unit, can induce the deeper HOMO energy
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(6.5 g, 100 mmol) and TBAB (1.29 g, 4 mmol) were
dissolved in 100 ml toluene and refluxed for 5 hours under the
(6.29 g, 24 mmol) was added in
was added completely, the reaction
Esquire HCT PLUS. The number-average molecular weight (Mn)
NaN
3
and weight-average molecular weight (M ) were determined on a
w
Waters gel permeation chromatography (GPC) system with linear 60 argon protection. Then, PPh
3
polystyrene as standards and chloroform containing 0.25 v/v%
triethylamine as eluent. Cyclic voltammetry (CV) was
characterized on a CHI600D electrochemical workstation with a
standard three electrodes cell based on a platinum (Pt) working
small portions; when PPh
3
5
0
5
continued to refluxing for additional 2 hours. Cooled the reaction
system to room temperature (r. t.) and filtered through a short
silica plug; the solvent was removed by a low pressure
electrode and
a
Pt wire counter electrode, against 65 evaporation and the gray solid was obtained. The final solid was
recrystallized in ethanol, and yielded 3.33 g targeting product.
saturated calomel electrode (SCE) as reference electrode at a scan
-
1
1
1
rate of 50 mV s within a nitrogen-saturated anhydrous solution
H-NMR (300 MHz, CDCl
1.89(m, 4H, CH ), 1.51(m, 4H, CH
0.89(t, 6H, CH ). C-NMR (75 MHz, CDCl
as the internal reference. UV-vis absorption spectra were 70 90.74, 69.41, 31.80, 29.28, 29.24, 28.63, 26.00, 22.67, 14.08. MS
3
), 6.80(s, 2H, ArH), 4.07(t, 4H, CH
2
),
-
1
of 0.1 mol L tetrabutylammonium
hexafluorophosphates
2
2
), 1.40~1.30(br, 16H, CH
2
),
13
(
Bu NPF ) in acetonitrile, versus ferrocene/ferrocenium (Fc/Fc+)
3
), 155.25, 146.86,
3
4
6
+
performed on a HP 8453 spectrophotometer. Tapping-mode
atomic force microscopy (AFM) was carried out for the
identification of topography and blend film phase images on a
Veeco Nanoscope V scanning probe microscope. Surface energy
was measured on Dataphysics OCA40 Micro by the calculation
from the surface tension data of diiodomethane and water.
(APCI): Calcd, 376.5; found (M+1) , 377.5.
,7-Dibromo-5,6-bis(octyloxy)benzofurazan 4
,6-Bis(octyloxy)benzofurazan (3.33g, 9.67mmol) was
dissolved in 100 ml dichloromethane and 100 ml AcOH at room
temperature. Br (10.83 g, 67.66 mmol) was added dropwise, and
4
1
5
3
7
8
8
9
9
5
0
5
0
5
2
then reacted at r. t. for 40 hours. The excess Br was quenched by
2
NaHSO3 and extracted by dichloromethane for three times,
washed by NaCl aqueous solution and water respectively, and
20
Device fabrication
All devices were fabricated on ITO-coated glass substrates. A
thin-layer (40 nm) of PEDOT:PSS (Clevios 4083) was spin-
coated onto UV-ozone treated ITO substrates at 4000 rpm for 40 s
and then baked at 140 °C for 15 min in air. The solution
containing the polymer (10 mg/ml) and fullerene (20 mg/ml)
dried in MgSO . The solvent was removed by evaporation under
4
a low pressure, and the resulting product was recrystallized in
1
methanol, and yielded 2.78 g white solid. H-NMR (300 MHz,
CDCl , δ), 4.14(t, 4H, CH ), 1.86(m, 4H, CH ), 1.51(m,4H, CH ),
3
2
2
13
2
25
30
35
40
45
1
.38~1.29(br, 16H, CH ), 0.89(t, 6H, CH ). C-NMR (75 MHz,
2 3
(
polymer: fullerene =1:2, wt/wt) in o-dichlorobenzene (ODCB)
CDCl ), 155.67, 147.45, 99.57, 58.48, 31.80, 30.19, 29.33, 25.89,
3
+
22.65, 18.43, 14.09. MS (APCI): Calcd, 534.3; found (M+1) ,
were spin-coated inside a glove-box with nigtrogen at 1200 rpm
for 40 sec to form the active layers (～80 nm). For the device
structure of ITO/PEDOT-PSS/active layer/PFN/Al, the PFN
layer (5 nm) was prepared by spin-casting a mixed solution (0.2
mg/ml) with the methanol to acetic acid volume ratio of 100:1
before the cathode evaporation. Aluminum cathode (80 nm) was
5
4
35.3.
,7-Di(thiophen-2-yl)-5,6-bis(octyloxy)benzofurazan 5
4
,7-Dibromo-5,6-bis(octyloxy)benzofurazan 4 (2.06 g, 4.1
mmol), tributyl(thiophen-2-yl)stannane (4.6 g, 12.3 mmol) and
Pd(PPh ) Cl (287.7 mg, 0.41 mmol) were dissolved in 41 ml
3
2
2
-
6
then thermally evaporated under vacuum (~10 torr) through a
2
shadow mask defining an active device area of 0.16 cm . The
THF and refluxed overnight. THF was evaporated, and the
resulting mixture was purified by column chromatography in
PE:DCM=10:1 as mixed eluent, and yielded 2.0 g yellow solid.
current-voltage (J-V) curves were measured by using a Keithley
2
400 multimeter under AM 1.5 G solar illumination at 87 mW
1
H-NMR (300 MHz, CDCl3 ), 8.46(dd, J=3.84Hz, 2H, ArH),
-
2
cm and a Thermal-Oriel 150 W solar simulator. Incident photon-
to-current conversion efficiency (IPCE) values were obtained
with a monochromator and calibrated with a silicon photodiode.
Hole mobility data of neat polymer and PCzDTBF:PCBM blend
films were measured by space charge limited current (SCLC)
method with the device configuration of ITO/PEDOT:
7.50(dd, J=5.16Hz, 2H, ArH),7.22(t, 2H, ArH), 4.15(t, 4H, CH2),
1
13
.98(m, 4H, CH ), 1.47~1.30(br, 20H, CH ), 0.90(t, 6H, CH ).
2 2 3
C-NMR (75 MHz, CDCl , δ), 151.73, 146.77, 132.92, 130.85,
3
1
2
27.98, 127.12, 113.06, 74.51, 31.79, 30.26, 29.48, 29.23, 25.87,
+
2.63, 14.05. MS (APCI): Calcd, 540.8; found (M+1) , 541.8.
1
1
1
1
00 4,7-Bis(5-bromothiophen-2-yl)-5,6-bis(octyloxy)benzofurazan 6
To solution of 4,7-di(thiophen-2-yl)-5,6-bis(octyloxy)
PSS (40 nm)/active layer (80-100 nm)/MoO (10 nm)/Al (90 nm).
3
a
Preliminarily, PEDOT:PSS was spin casted on the ITO coated
glass substrate, then, active layers were prepared from ODCB
solution with the 1% concentration by spin coating. Finally,
benzofurazan 5 (2 g, 3.7 mmol) in 42 ml chloroform and 42 ml
AcOH in the absence of light, NBS (1.45 g, 8.14 mmol) was
added in small portions. Then, the reaction continued for 48 hours.
05 Water was added and extracted by DCM for three times, washed
by NaCl aqueous solution and water respectively, dried in MgSO4.
The solvent was removed by evaporation, and the resulting
product was purified by column chromatography in petroleum as
MoO and Al layers were deposited by thermal evaporation under
3
high vacuum, respectively.
Synthesis of monomers and polymer
50
The synthetic routes were displayed in Scheme 1. 1,2-
1
2
bis(octyloxy) benzene, 1,2-dinitro-4,5-bis(octyloxy)benzene and
6
eluent, and yielded 2.2 g orange solid. H-NMR (300 MHz,
10 CDCl3 ), 8.24(d, J=4.14Hz, 2H, ArH), 7.17(d, J=4.14Hz, 2H,
ArH), 4.15(t, 4H, CH ), 1.99(m, 4H, CH ), 1.55~1.31(br, 20H,
2
,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxa
2
borolan-2-yl)-9-octyl-
0
2
2
carbazole
were synthesized according to the published
1
3
CH ), 0.91(t, 6H, CH ). C-NMR (75 MHz, CDCl , δ), 151.27,
2
3
3
procedures. All the other monomers and polymer were obtained
by the following synthetic methodologies.
1
3
46.23, 134.29, 131.17, 130.05, 116.33, 112.69, 74.81, 31.78,
55
0.16, 29.42, 29.23, 25.81, 22.65, 14.07. MS (APCI): Calcd,
+
5
,6-Bis(octyloxy)benzofurazan 3
15 698.6; found (M+1) , 699.6.
1
,2-Dinitro-4,5-bis(octyloxy)benzene 2 (8.49 g, 20 mmol),
2
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Poly(2,7-(9-octyl-carbazole)-alt-4,7-di(thiophen-2-yl)-5,6-bis
octyloxy)benzofu razan) PCzDTBF
,7-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9-octyl-
carbazole (0.266 g, 0.5 mmol), 4,7-bis(5-bromothiophen-2-yl)-
5,6-bis(octyloxy)benzofurazan 6 (0.349 g, 0.5 mmol), Pd(PPh3)4 50 in solution, the absorption spectrum in film displays a little
11.6 mg, 0.01 mmol) were dissolved in 10 ml toluene under the broadened, which implicates the amorphous film and no further
argon protection. Then, tetraethylammonium hydroxide (2 ml,
0 wt/wt% in water) was added in one portion at room
film are presented in Figure 1a. The absorption spectra peaked at
385 and 540 nm are attributed to the localized π–π* transition and
the internal charge transfer (ICT) interaction between the donor
(
2
2
2
and acceptor units, respectively. Compared with the absorption
5
(
22
molecular stacking. The onset of absorption spectrum in film is
644 nm, which is corresponding to the optical band gap of 1.93
eV (Table 1).
2
temperature. Heated the reaction to 85 °C and reacted at
this temperature for 48 hours. Phenylboronic acid (0.609 g, 5
mmol) was added and reacted for 12 hours at 85°C; then,
bromobenzene (1.570 g, 10 mmol) was added in one portion and
reacted for additional 12 hours. Cooled it to room temperature
and precipitated in methanol. The crude polymer was purified by
1
1
2
0
5
0
55
The cyclic voltammogram (CV) characterization (Figure 1b)
presents the reversibly oxidative and reductive curves, in which
the onsets of oxidation and reduction potentials are located at
0.93 and -0.95 V versus SCE, respectively. The redox potential of
the Fc/Fc+ internal reference is 0.39 V vs. SCE. The highest
Soxlet extraction in methanol, acetone, hexane and chloroform 60 occupied molecular orbit (HOMO) and the lowest unoccupied
respectively; the final chloroform solution was precipitated in
methanol. Filtered off and dried at 50 °C in the vacuum oven
molecular orbit (LUMO) energy levels, determined by calculating
from the empirical formula of EHOMO= -e(E +4.8-E1/2, (Fc/Fc+)) and
ox
1
for 10 hours, and yielded 0.183g red solid. H-NMR (300
ELUMO
=
-e(E +4.8-E
re
(Fc/Fc+)), are -5.34 and -3.46 eV,
1/2,
5
respectively. The energy band gap of PCzDTBF from the CV
MHz, d-ODCB, δ), 8.53~8.19(br, 2H, ArH), 7.98~7.79(br, 4H,
ArH), 7.50~7.29(br, 4H, ArH), 4.51~3.72(br, 6H, CH2), 65 data is 1.88 eV, which is approximate to the optical band gap of
2
.48~1.83(br, 6H, CH ), 1.48~0.89(br, 39H, CH , CH ). GPC
2
1.93 eV (Table 1).
2
3
(CHCl ): M =6000, M = 12500, PDI=2.1.
3 n w
Results and discussion
Synthesis
2
3
3
4
5
0
5
0
Detailed synthetic routes of monomers and polymer are shown
in Scheme 1. The polymer PCzDTBF was synthesized by Suzuki
polycondesation in toluene, and purified by Soxlet extraction in
methanol, acetone, hexane and chloroform with 100 ml for 12 h
each, then, precipitated in methanol. The solubility of polymer is
good in organic solvents (such as tetrahydrofuran, chloroform,
chlorobenzene, dichlorobenzene), because of the octyloxy
substituted on BF unit. The gel permeation chromatography (GPC)
characterization shows that the number-average molecular weight
(M ) and weight-average molecular weight (M ) are 6000 and
n w
12500, respectively, with a polydispersity index (PDI) of 2.1
Table 1). The PCzDTBF doesn’t show a higher molecular
(
weight, which is possibly because the octyl side chain in
carbazole unit makes the polymer solubility limited and
ultimately inhibits the increment of molecular weight during
1
9
polymerization.
70
Figure 1. Uv-vis absorption spectra of PCzDTBF in chloroform and in
film(a); cyclic voltammogram of PCzDTBF in film( b) (inserted graph of
the ferrocene oxidative curve).
Scheme 1. Synthetic routes of monomers and polymer
Absorption and electrochemical analysis
4
5
The Uv-vis absorption spectra of PCzDTBF in solution and in
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Table 1. Physical properties of PCzDTBF
a
b
polymer
Mn
PDI
2.1
λmax
nm)
385, 540
Eg
Eox
(V)
Ere
EHOMO
(eV)
ELUMO
(eV)
Es
2
(mJ/m )
(
(eV)
(V)
PCzDTBF
6000
1.93
0.93
-0.95
-5.34
-3.46
35.8
a: maximal absorption in solution; b: optical band gap
5
Photovoltaic properties
PSCs were fabricated with the configurations of
ITO/PEDOT:PSS/PCzDTBF:PCBM/Al and ITO/PEDOT: PSS/
PCzDTBF:PCBM/PFN/Al, respectively. The current density-
voltage (J-V) curves are shown in Figure 2a. We can see that the
short circuit current density (J ) and the open circuit voltage (V )
10
15
20
sc
oc
of PC BM based device are larger than the ones of PC BM as
60
70
acceptor. Furthermore, the insertion of PFN as the cathode
interfacial layer made the Voc and fill factor (FF) enhanced
distinctly, and therefore, resulted in the power conversion
efficiency (PCE) increased ultimately. In Figure 2b, the incident
photon-to-current conversion efficiency (IPCE) of PC BM
70
based devices are broadened and exhibits better photo-to-current
conversion in the visible region with approximate 10%
improvement compared with the PC BM device, which is
6
0
possibly ascribed to the intensified and broadened absorption of
the PCzDTBF:PC BM based blend film.
7
0
Table 2 shows the distinctive increment in J s and PCEs from
sc
2
2
6
.61 mA/cm and 3.63% for PC BM to 7.36 mA/cm and 4.20%
60
Figure 2. Current density-voltage (J-V) characteristics of PCzDTBF:
for PC BM device, respectively. Compared with the devices
70
-
2
PCBM under AM 1.5 G, 87 mW cm (a); incident photon-to-current
conversion efficiency (IPCE) and Uv-vis absorption spectra of
PCzDTBF:PCBM blend films (b).
2
5
without PFN layer, the performances with PFN as a cathode
interfacial layer were enhanced dramatically. Even though the Jsc
increased only a little, the Voc and FF acquired distinct
enhancement, where, the Voc increased with maximal 0.1V, and
FF realized a 10% enhancement. Ultimately, the PCE reached to
5.48%, which had more than 30% increment both in PC BM and
40
-2
Table 2. Photovoltaic performances under AM 1.5 G, 87 mW cm
active layer
1:2, w/w)
PCzDTBF:PC BM
cathode
Jsc
(mA/cm ) (V) (%) (%)
Voc
FF PCE
30
2
60
(
PC BM based photovoltaic devices.
70
Al
6.61
6.67
7.36
7.73
0.80 60
0.90 69
0.85 57
0.90 67
3.63
4.73
4.20
5.48
60
PFN/Al
Al
PFN/Al
PCzDTBF:PC BM
70
45
Hole mobility and film morphology
The hole mobility both in neat polymer and blend films of
PCzDTBF:PCBM were measured by the space charge limited
current (SCLC) method with the device configuration of
ITO/PEDOT:PSS/active layer/MoO /Al. Figure 3 shows the J-V
3
50
curves of PCzDTBF and blend films from SCLC devices. The
-4
-
hole mobilities (Table 3) were 4.8×10 , 2.5×10 and 2.1×10
4
-4
2
cm
-1
-1
V
S
for PCzDTBF, PCzDTBF:PC BM and
60
PCzDTBF:PC BM, respectively, which were comparable to that
1
70
6, 20
of the polycarbazole based polymers.
Compared with the neat
55
polymer, the hole mobilities of blend films decreased by about a
half, but still sustained at the same order of magnitude, which
would provide a sufficient hole transporting ability in the
2
7
photovoltaic devices and pay extensive contribution to the PCE.
35
4
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photovoltaic performances will be escaladed to a higher level;
35
and the prospective research works are under the process.
Figure 4. AFM images of blend films (1:2 weight ratio), topography of
PCzDTBF:PC60BM (a) and PCzDTBF:PC70BM (b); phase images of
0 PCzDTBF:PC60BM (c) and PCzDTBF:PC70BM (d).
Figure 3. J-V characteristics of PCzDTBF and blend films from SCLC
devices.
4
5
Table 3. Hole mobility of PCzDTBF and blend films
Conclusions
Octyloxy substituted BF based polycarbazole (PCzDTBF) was
synthesized by Suzuki polycondensation and utilized as donor
material in PSCs. The photovoltaic performance gave a maximal
PCE of 4.2% under the device configuration of
ITO/PSS:PEDOT/PCzDTBF:PC BM/Al. By using PFN as the
active layer
thickness
nm)
µ
-
4
2
-1 -1
(
(×10 cm V S )
45
PCzDTBF
100
92
4.8
2.5
2.1
70
PCzDTBF:PC BM
60
cathode interfacial layer, the PCE was enhanced to 5.48%, with a
2
Voc of 0.90 V, a Jsc of 7.73 mA/cm , and a FF of 67 %. The high
PCzDTBF:PC BM
82
70
efficiencies were possibly attributed to the good nano-scale phase
seperation between PCzDTBF and PCBM components, resulted
from their matchable surface energies, and the distinct
improvement of Voc and FF by PFN layer where it possibly
existed a built-in field potential between the active layer and
cathode.
AFM images of the blend films are shown in Figure 4. The
root-mean-square (RMS) roughness of PC BM and PC BM
5
0
6
0
70
1
1
2
2
3
0
5
0
5
0
based films are 0.58 and 0.72 nm, respectively, which shows that
both films are very smooth. Moreover, the phase images (Figure
[
28 ]
4
c-d) display the nano-scale phase separation,
support the effective holes and electrons transporting channels
and inhibit the charge carriers recombination, and ultimately 55 Acknowledgement
which can
affect the photovoltaic properties. On the other hand, the
miscibility decides the film morphology in blend systems directly.
This work was financially supported by the National Nature
Science Foundation of China (Nos. 50990065, 51010003 and
21074038), and the State Key Basic Research Project of China
(Nos.2009CB623600 and 2009CB930604).
From Table 1, we can see that the surface energy (E ) of
s
2
PCzDTBF is 35.8 mJ/m , which is similar to that of PC BM of
60
2
29
3
4.2 mJ/m . The similar E in the blend components caused
s
well-mixed films, resulting from optimal phase separation of the
PCBM domains. This scenario can also depict the evidences for
the formation of nano-scale phase separation between PCzDTBF
60
Notes and references
a
State Key Laboratory of Luminescent Materials and Devices, Institute of
Polymer Optoelectronic Materials and Devices, South China University
of Technology, Guangzhou 510640, China. Fax: 86-020-87110606; E-
mail: pswyang@scut.edu.cn
b
65 School of Material Science & Engineering, South China University of
and PC BM or PC BM greatly.
70
6
0
Compared with the decyloxy substituted BF unit based
2
1
polycarbazole (PC-DTBX), PCzDTBF had a shorter octyloxy
side chain and higher molecular weight, which may lead to the
better device performances. On the other side, the high FF and
nano-scale phase separation were also effectively contributable to
the high photovoltaic properties.
Technology, Guangzhou 510640, China
c
College of Polymer Science and Polymer Engineering, The University of
Akron, 250 South Forge Street, Akron, Ohio 44325-0301, United States
Nevertheless, the weaker absorption of PCzDTBF after 650
1
Yongfang Li, Acc. Chem. Res. 2012, 45, 723.
2 P. M. Beaujuge, J. M. J. Fréchet, J. Am. Chem. Soc., 2011, 133, 20009.
Z. C. He, C. M. Zhong, X. Huang, W. Y. Wong, H.B. Wu, L. W. Chen, S.
nm resulted in the smaller Jsc value than other PSCs, in which J
2
sc
7
reached to more than 12mA/cm . Therefore, if the absorption of
this polymer can be extended to the longer wavelength, the
3
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Table of Contents
Octyloxy substituted benzofurazan based narrow band-gap
conjugated polymer PCzDTBF gives the maximal power
conversion efficiency of 5.48% in bulk-heterojunction polymer
solar cells.
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