Novel donor-acceptor polymers based on 7-perfluorophenyl-6H-[1,2,5]thiadiazole[3,4-g]benzoimidazole for bulk heterojunction solar cells

Article abstract of DOI:10.1039/c5ra05849j

Three new fluorinated D-A type conjugated polymers based on a novel building unit 7-perfluorophenyl-6H-[1,2,5]thiadiazole[3,4-g]benzoimidazole have been synthesized through a Sitlle coupling reaction. The as-prepared polymers exhibit a narrow band gap (from 1.31 to 1.34 eV) and low lying energy levels with lowest occupied molecular orbital (LUMO) energy levels of -3.95, -3.97 and -4.15 eV, respectively. These polymers exhibit excellent solubility in common organic solvents due to the introduction of perfluorophenyl and long alkyl sidechains. The power conversion efficiency (PCE) of solar cells based on these as-prepared polymers and PC₇₁BM could reach as high as 1.92%. Our results could provide a simple strategy for designing high performance D-A polymers based on this unit and a potential to further improve their performance.

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Journal Name
ARTICLE
Novel Donor-Acceptor Polymers based on 7-Perfluorophenyl-6H-
1,2,5]thiadiazole[3,4-g]benzoimidazole for Bulk Heterojunction
Solar Cells
[
Received 00th January 20xx,
Accepted 00th January 20xx
‡
a
‡b
a
b
b
a,c
DOI: 10.1039/x0xx00000x
Benlin Hu, Miaomiao Li, Wangqiao Chen, Xiangjian Wan, Yongsheng Chen,* Qichun Zhang*
www.rsc.org/
Three new fluorinated D–A type conjugated polymers based on a novel building unit 7-perfluorophenyl-6H-
1,2,5]thiadiazole[3,4-g]benzoimidazole have been synthesized through Sitlle coupling reaction. The as-prepared polymers
exhibit a narrow band gap (from 1.31 to 1.34 eV) and low lying energy levels with the lowest occupied molecular orbital
LUMO) energy levels of -3.95, -3.97 and -4.15 eV, respectively. These polymers exhibit excellent solubility in common
organic solvents due to the introduction of perfluorophenyl and long alkyl sidechains. The power conversion efficiency
PCE) of solar cells based on these as-prepared polymers and PC71BM could reach as high as 1.92 %. Our results could
[
(
(
provide a simple strategy for designing high performance D-A polymers based on this unit and a potential to further
improve their performance.
1
2-24
functional substituents can also have strong influence.
Introduction
Under these empirical rules, elaborate design of donors and
acceptors can help maintain the upward trend of the device
efficiency towards the predicted PCE of 15% for BHJ polymer
Since the conception of bulk-heterojunction (BHJ) was
proposed in 1992, BHJ solar cells have attracted numerous
1
7,25,26
1
-8
solar cells.
attentions in the past two decades. Especially, the power
conversion efficiencies (PCEs) of BHJ solar cells based on
polymer/fullerene blends are approaching 10% for single-
junction cells, which can be attributed to the rapid progress of
Fused aromatics, typically via the annulation of different
small aromatic units (e.g., benzene, thiophene, pyrrole,
pyridine, etc.), are the most frequently used structural units as
donors or acceptors in the D-A polymers for organic
9
-19
novel donor-acceptor (D-A) polymers.
The most charming
1
6-18,25,27,28
electronics.
Among the reported fused aromatics, D-
feature of D-A polymers for BHJ solar cells is the high flexibility
in tuning the absorption and frontier molecular orbital energy
levels of the objective D-A polymers. Generally, the highest
occupied molecular orbital (HOMO) and the lowest occupied
molecular orbital (LUMO) energy levels of D-A polymers can be
estimated via the HOMO level of the “donor” (D) structural
unit and the LUMO level of the “acceptor” (A) unit,
respectively. This important feature enables scientists to
design and synthesize new polymers with a suitable LUMO
level through using specialized acceptors and in coupling with
a low bandgap to obtain a high open circuit voltage (Voc) and a
A polymers based on 2,1,3-benzothiadiazole (BT) have been
1
1,13-18, 29-32
found with a PCE as high as 10%.
Although BT is a
strong electron-withdrawing unit, it is still not strong enough
to pull LUMO level down to ~ -4.0 eV in D-A polymer, which
could have a broad bandgap, an inferior absorption spectrum,
and a lower PCE of the resulted solar cells. Therefore,
introducing more electron-withdrawing groups into BT units is
highly desirable to further increase the PCE of solar cells. Most
of recently-reported BT derivatives are summarized in Chart 1
.
Generally, incorporating strong electron-withdrawing units
onto BT unit is the most popular strategy. For example,
quinoxaline unit has been employed as an electron-
1
7-20
high short circuit current (Jsc), simultaneously.
Besides that
the aromatic units in D-A polymers can largely determine the
photovoltaic performance of the BHJ solar cells, other
structural parameters including the peripheral side chains and
32-35
withdrawing block to form BT-M1 (Chart 1) through
a
simple synthesis; however, the PCE did not show too much
enhancement. Then fluorine atoms were introduced into BT
systems as strong electron-withdrawing groups to improve
the interaction between polymers:PCBM and electrodes, and
the resulted BT-M2 and BT-M3 have become the mostly used
2
1-24
units to improve the PCE.
In order to further increase the
electron-withdrawing ability of BT unit, another strategy is to
fuse two conjugated five-member-ring heterocycles onto one
building unit to generate a novel exceptionally strong
acceptor, which can be further employed as construction
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1-24
blocks to form low band gap polymers. For example, D-A current density (Jsc) and high open-circuit voltage (Voc).
In
polymers based on BT-M4 (X = S or Se) show longer this paper, we designed and synthesized a novel monomers
wavelength absorptions and deeply LUMO energy levels, and including
3
7-Perfluorophenyl-6H-[1,2,5]thiadiazole[3,4-
6-38
the enhancing PCE.
If M5 was used, conjugated polymers g]benzoimidazole (PFIBT) units. Three new D-A polymers
with BT-M5 have been demonstrated to show good PCE based on PFIBT show good solubility, excellent absorption
3
9-41
values.
However, the synthetic procedures of all the spectra in the range of 300-800 nm, and deeply LUMO energy
above-mentioned monomers are very long and tedious. levels. In addition, the BHJ solar cells with these polymers as
Therefore, it is urgent to explore other novel strong acceptors, donors have been fabricated and the performance of the as-
which could be incorporated into BT unit to enhance the PCE prepared devices were carefully investigated.
of solar cells.
Chart 1. The chemical structures of 2,1,3-benzothiadiazole (BT) Experimental section
and its derivates (X= S or Se)
Materials and methods
Compound
1 and M2-M4 (Scheme 1) were prepared according
1
2,42-47
to the reported procedures.
All starting chemicals, unless
otherwise specified, were purchased from Alfa Aesar or Sigma-
Aldrich and used as received. [6,6]-Phenyl-C71-butyric acid
methyl ester (PC71BM, >99%, EL device grade) was purchased
from Solenne. All solvents were purified by normal procedure
before use.
1
1
H Nuclear Magnetic Resonance ( H NMR) spectra were
measured at 300 MHz on a Bruker 300 AVANCE
spectrometer with chloroform (CDCl ),
Dimethylmethanamide (DMF-d
) as solvents. UV-Vis absorption spectra were recorded on a
Ⅲ
3
N,N-
7
) or dimethyl sulfoxide (DMSO-
d
6
Shimadzu 2501 PC spectrophotometer at room temperature.
Thermogravimetric analysis (TGA) was performed on a TA
Instrument Q500 instrument under a nitrogen atmosphere
with a gas-flow rate of 50 mL/min and a heating rate of 10
℃/min. Gel permeation chromatography (GPC) traces of the
samples were monitored on an Agilent 1260 GPC-MDS system
fitted with differential refractive index (DRI) and Ultraviolet
(
UV) detectors using THF as the eluent and linear polystyrenes
as the molecular weight standards. Cyclic voltammetry (CV)
measurements were carried out with CHI 604E
a
Electrochemical Workstation (CH Instruments, Shanghai
Chenhua Instrument Corporation, China) under an argon
atmosphere. The polymer film coated on a Pt plate (working
Herein, we report novel D-A polymers based on 5H- electrode) was scanned at 10 mV/s in an anhydrous
imidazo[4,5-f]-2,1,3-benzothiadiazole (IBT) as an acceptor unit, dichloromethane
where the imidazole unit is employed to improve the electron hexafluorophosphate (n-Bu
solution
NPF
of
tetrabutylammonium
4
6
, 0.1 M), with a Pt wire and
affinity. Obviously, IBT unit is a stronger acceptor than BT unit. platinum gauze as the reference and counter electrodes,
Moreover, the N-H···F intramolecular interactions in the respectively. The thickness of the polymer film was measured
monomer of PFIBT can hinder the rotation of the adjacent by a KLA Tencor Alpha-Step Surface Profiler.
thiophene ring and increase the coplanarity and conjugated
region in the D-A copolymers, and these interactions can also Fabrication and characterization of polymer BHJs
lower the aromaticity of
π-systems to give better charge
The devices were fabricated with
a
structure of
transport and higher PCEs. Furthermore, another strong
electron-withdrawing unit pentafluorophenyl is introduced
into benzimidazole unit to further increase the electron affinity
and produce a larger conjugated backbone than that of BT
unit. The introduction of fluorine atoms is generally believed
to improve the solubility and the adhesion to electrodes, and
then lower the energy barrier between active layer and
electrodes. In addition, the using of fluorine atoms in D-A
polymers can also obtain high fill factor (FF), large short-circuit
glass/ITO/PEDOT:PSS/donor:acceptor/PFN/Al. The indium tin
oxide coated glass substrates were cleaned sequentially with
detergent, DI water, acetone, and isopropyl alcohol and then
dried by a nitrogen flow. Before use, the substrate was treated
with oxygen plasma.
A
thin layer of poly(3,4-
ethylenedioxthiophene):poly(styrene sulfonate) (PEDOT:PSS)
(Baytron P VP AI 4083, filtered at 0.45 μm) was spin-coated
(
3000 rpm, ca. 40 nm thick) onto ITO surface. After being
baked at 150 °C for 20 min, the substrates were transferred
2
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into an argon-filled glove box. The active layers were prepared and N-bromosuccinimide (1.60 g, 9 mmol) was added to the
by dissolving polymers and PC71BM at weight ratios of 1:1 solution in one portion. The mixture was kept at 60 °C for about 12
-
1
(w/w) with
a
total concentration (20 mg mL ) in h until TLC analysis showed no starting material. Then, the mixture
chlorobenzene, and then an active layer was deposited by cooled to room temperature and was acidified with 2 M HCl
spin-coating from the solution at a speed of 1500 rpm onto the solution (300 mL). The as-obtained solution was extracted with
ITO/ PETDOT:PSS substrates for 40 s. The thickness of each ethyl ether for three times. The organic layers were combined and
active layer was
2 4
∼90-100 nm. Finally, a thin layer of PFN were dried Na SO , and then all solvents were evaporated. The as-
spin-casted on top of active layers and then Al (80 nm) was resulted mixture was treated with 30 mL hexane and frozen at -15
evaporated to form the top electrode. The effective area of °C for two days. Orange red crystals were collected, washed with
2
1
each cell was 4 mm defined by shallow masks. The current methanol, and dried under a vacuum in a yield of 90% (1.29 g). H
density-voltage (J-V) curves of photovoltaic devices were NMR (300 MHz, CDCl ): δ (ppm) d 7.17 (s, 2H), 2.62 (t, 4H, J = 7.2
obtained by a Keithley 2400 source-measure unit. The devices Hz), 1.65-1.55 (m, 4H), 1.37-1.25 (m, 12H), 0.90 (t, 6H, J = 6.9 Hz).
3
−
2
13
were measured under AM 1.5 radiation (100 mW cm ) using
C NMR (75 MHz, CDCl ): δ (ppm), 151.76, 143.42, 138.29, 131.63,
3
an Oriel 96000 solar simulator. The spectral mismatch factor 129.09, 120.37, 116.88, 31.56, 29.47, 29.43, 28.75, 22.57, 14.06.
was calculated by comparison of the solar simulator spectrum
4
,7-Bis(5-bromo-4-hexylthiophen-2-yl)-benzo[1,2,5]thiadiazole-
and the AM 1.5 spectrum at room temperature. External
quantum efficiency (EQE) values of the encapsulated devices
were measured using a lock-in amplifier (SR810, Stanford
Research Systems). The devices were illuminated by
monochromatic light from a 150 W xenon lamp passing
through an optical chopper and a monochromatic. Photon flux
was determined by a calibrated standard silicon photodiode.
5,6-diamine (3). Fine iron powder (0.56 g) was added to a
stirred solution of 4,7-Bis-(5-bromo-4-hexylthiophen-2-yl)-5,6-
dinitro-benzo[1,2,5]thiadiazole (0.72 g, 1 mmol) in acetic acid
(
25 mL). The mixture was heated to 80 ◦C, stirred for 4 h and
then cooled to room temperature. The cooled reaction
mixture was poured carefully into 2 N NaOH aqueous solution
at 0 °C. The aqueous solution was extracted with diethyl ether
three times (3× 50 mL). The combined organic solution was
washed with brine, dried over anhydrous Na SO and then
Synthesis
2
4
4
,7-Bis(5-bromo-4-hexylthiophen-2-yl)-5,6-dinitro-
evaporated to give a dark brown shiny stuff. Hexane (20 mL)
benzo[1,2,5]thiadiazole (2). Compound 1 (1.12 g, 2 mmol), was was added into this shiny stuff and kept to refrigerator two
dissolved in 30 mL DMF. The resulted solution was heated to 60 °C , days. A reddish powder was obtained after dried under
Scheme 1. The synthesis of monomer and polymers
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1
Figure 1. The H NMR of compound
2
3
(top), 3 (middle) and M1 (bottom) in CDCl .
1
vacuum to afford compound 3 (0.59 g, 90 %). H NMR (300 under reduced pressure. The crude product was purified by column
MHz, CDCl ): δ (ppm) 7.05 (s, 2H), 4.40 (s, 4H), 2.64 (t, 4H, j = chromatography (eluent: DCM/hexane = 1:3) to yield M1 as a
3
1
.5 Hz), 1.70-1.60 (m, 4H), 1.42-1.25 (m, 12H), 0.90 (t, J = 6.9 purple solid (0.45 g, 85 %). H NMR (300 MHz, CDCl ): δ (ppm) 9.43
7
3
1
Hz). C NMR (75 MHz, CDCl ): δ (ppm), 150.44, 142.38, (s, 1H), 8.44 (s, 1H), 7.41 (s, 1H), 2.65-2.53 (m, 4H), 1.71-1.64 (m,
3
3
1
9
39.24, 134.79, 129.59, 110.83, 106.87, 31.62, 29.71, 29.69, 4H), 1.40-1.26 (m, 12H), 0.95-0.89 (m, 6H). F NMR (282 MHz,
1
2
9.03, 22.60, 14.09.
,10-Bis(5-bromo-4-hexylthiophene-2-yl)-7-(2,3,4,5,6-
pentafluorophenyl)-6H-[1,2,5]thiadiazole[3,4-g]benzoimidazole
M1). 2,3,4,5,6-Pentafluorobenzaldehyde (196 mg, 1 mmol) and
compound 3 (410 mg, 0.63 mmol) were thoroughly mixed in DMF
10 mL), and p-methylbenzenesulfonic acid (100 mg) was added to
the solution, and the mixture was stirred overnight. Then
FeCl ·6H O (500 mg) was added in to the solution, and this mixture
CDCl ): δ (ppm) -137.25 (d, 2F, J = 19.46 Hz), -148.10 (t, 1F, J = 21.7
3
4
Hz), -159.35 (dd, 2F, J = 21.1 Hz, 20.8 Hz).
General procedure of copolymerization by Stille coupling.
(
3 4
The equimolecular amount of M1 and M2-M4, and Pd(PPh ) (∼1.6
mol%) were dissolved in anhydrous toluene under argon. Argon was
bubbled through the solution for 30 min and then the mixture was
vigorously stirred at 110 °C under an argon atmosphere for 48
hours. After cooling to room temperature, the mixture was poured
into methanol. After washed with methanol, oligomers and catalyst
residues were removed by hexane through a Soxhlet apparatus for
(
3
2
was heated to 80 °C for 3h. After the solution cooled to room
temperature, the reaction mixture was poured into water and
extracted with EtOAc (3 ×50 mL), and then washed with brine (3
2
4 hours. Then, the product was extracted, by tetrahydrofuran
4
×50 mL). After dried by MgSO , the organic solvent was evaporated
4
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(
THF) for another 24 hours. The THF extract was concentrated and prepared through the condensation reaction between 3 and
the product was precipitated by methanol, and collected by 2,3,4,5,6-pentafluorobenzaldehyde using methylbenzenesulfonic
filtration and dried in vacuum overnight.
3 2
acid and FeCl ·6H O as catalysts and air as an oxidation reagent.
1
Figure 1 shows the H NMR spectra of compound 2, 3 and M1. The
protons on the thiophene group in compound 1 show a single peak
with a chemical shift of 7.17 ppm, while the corresponding peak in
compound 2 shows a little shift (0.12 ppm) to high field. The peak of
amino groups in compound 2 shows a broad peak at 4.40 ppm.
Because of the asymmetry from the benzimidazole group in M1,
two different peaks of the protons on the thiophene are observed
at 7.40 and 8.44 ppm; and the proton of benzimidazole ring is
observed at 9.43. In addition, the chemical structure of M1 can also
P1: M1 (208.2 mg, 0.25 mmol), M2 (256.2 mg, 0.25 mmol),
Pd(PPh
3
)
4
(4.5 mg), toluene (3 mL). Black powder, 200 mg, yield
1
5
8%. H NMR (300 MHz, CDCl ): δ (ppm): 10.38 (s), 9.08 (s), 8.10-
3
8
2
.01 (m), 7.66-7.60 (m), 7.47 (d), 6.90 (s), 3.07 (s), 2.80-2.75 (m),
.17 (s), 1.86-1.74 (m), 1.25 (br), 0.91-0.82 (m).
P2: M1 (208.2 mg, 0.25 mmol), M3 (193.0 mg, 0.25 mmol),
Pd(PPh ) (4.4 mg), toluene (3 mL). Black powder, 208 mg, yield
3
4
1
6
8
2
7%. H NMR (300 MHz, CDCl ): δ (ppm): 10.27 (s), 9.06 (s), 8.12-
3
.03 (m), 7.68-7.60 (m), 7.43 (d), 6.90 (s), 3.03 (s), 2.80-2.71 (m),
.16 (s), 1.89-1.72 (m), 1.25 (br), 0.91-0.88 (m).
19
be confirmed by F NMR with three group peaks at -137.25, -
1
48.10 and -159.35 ppm. D-A polymers P1-P3 were synthesized
P3: M1 (208.2 mg, 0.25 mmol), M4 (226.1 mg, 0.25 mmol), through Stille coupling polymerization between M1 and M2-M4.
Pd(PPh ) (4.7 mg), toluene (3 mL). Black powder, 210 mg, yield The chemical structures of all as-obtained polymers can be
3
1
4
1
6
8
6.4%. H NMR (300 MHz, CDCl ): δ (ppm): 10.28 (s), 9.01-8.83 (m), confirmed by H NMR spectroscopy. Table 1 lists molecular weights
3
.06-7.14 (m), 6.94 (d), 3.79 (t), 2.92-2.59 (m), 1.86-1.27 (m), 0.94 and thermal decomposition temperature of P1-P3. The molecular
weight and polydispersity index (PDI) were characterized by GPC
(
t)
with calibration against polystyrene standards and THF as an eluant.
Table 1. Molecular weights and thermal properties of P1-P3
The number average molecular weights (M ) of P1-P3 were found
n
to be 14.1, 14.3 and 16.5 kDa, with PDIs of 1.76, 1.64 and 1.85,
M
n
/kDa
M
w
/kDa
PDI
1.76
1.64
1.85
T
d10% /°C
309
respectively. The polymers show good thermal stability (See Figure
P1
P2
P3
14.1
14.3
16.5
24.8
23.4
30.3
2
), as evidenced by its 10% weight-loss temperature (Td10%). P1-P3
360
show good solubility in common organic solvents such as THF,
chloroform and DMF.
461
Figure 2. TGA curves of P1-P3 under argon atmosphere at a
–1
heating rate of 10 °C min .
Results and discussion
Synthesis of monomers and polymers
Scheme 1 shows the synthetic routes that were used to prepare the
monomer M1 and the polymers P1-P3. Compound
1
was
After treated with NBS,
compound 1 was converted into 4,7-Bis(5-bromo-4-hexylthiophen-
-yl)-5,6-dinitro-benzo[1,2,5]thiadiazole (2), which was further Figure 3. UV-Vis spectra of 6F-BAHP-PC PI in THF solution (a)
6
,36-38
synthesized according to literature.
2
reduced by iron to form 4,7-Bis(5-bromo-4-hexylthiophen-2-yl)- and films (b).
benzo[1,2,5]thiadiazole-5,6-diamine (3). Subsequently, M1 was
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Table 2. Optical and Electrochemical Properties of Polymers P1-P3
ꢀ
ꢃ
ꢁꢂ
ꢄꢅ
ꢀꢆꢇꢈꢂ
1.21
1.04
1.24
ꢉꢈꢊ
ꢀꢆꢇꢈꢂ
-0.51
-0.29
-0.53
ꢈꢋ
ꢃ
λ
max (nm)
λ
max (nm) film
HOMO
(eV)
-5.69
-5.52
-5.72
LUMO
(eV)
-3.97
-4.19
-3.95
(eV)a
E
(V)
E
(V)
E
(eV)
b
E
solution
P1 352, 615
P2 352, 576
P3 352, 580
360, 437, 671
366, 441, 677
363, 454, 689
932
942
925
1.33
1.31
1.34
ꢈꢋ
ꢃ
a
Calculated from the edge of the absorption spectrum of the film. b
E
= HOMO – LUMO.
Optical and electrochemical properties
0.1 M solution of tetrabutylammonium tetrafluoroborate (TBATFB)
-1
-
5
at 10 mV·s are shown in Figure 4, and the CV data are summarized
in Table 2. All the polymers show obvious reduction peaks at -0.74,
The absorption spectra of all polymers in dilute THF solution (1×10
M of each repeat unit) and as solid films on glass are shown in
Figure 3, and the data are summarized in Table 2. Two main
absorption bands at 300-490 nm and 510-800 nm are observed in
THF solution of P1-P3. The bands in the range of 300-490 nm can be
-
0.66 and -0.84 V for P1-P3, respectively, which are resulted from
the strong electron-withdrawing acceptor of IBT unit. And the
corresponding reduction onsets were located at -0.51, -0.29 and -
0
.53 V, respectively. Polymers P1-P3 show gradual oxidation peaks
attributed to the π-π* transition delocalized along the π-electronic
and the responding oxidation onsets were observed at 1.21, 1.04
and 1.24 V, respectively. The HOMO and LUMO levels of P1-P3 can
be calculated from the oxidation and reduction onset according to
the energy level of the ferrocene reference (-4.8 eV versus vacuum
level). The deep HOMO levels of these polymers suggest that they
can transport holes. These results suggest that these polymers have
great potential for the application in solar cells.
system, whereas the bands in the range of 510-800 nm can be
attributed to the strong charge transfer between IBT acceptors and
BDT donors. Generally, the THF solutions of P1-P3 have strong
absorptions in the whole range of 300-800 nm, even at the gap of
~500 nm. Compared with the absorption in solution, the absorption
peaks of P1-P3 thin films are red-shifted, especially the bands from
the charge transfer between donors and acceptors shifted from
~600 nm to ~680 nm, suggesting stronger intermolecular
interactions and aggregation in solid states than those in solution.
In addition, the polymer films show no obvious absorption edge to
~1300 nm. The optical band gaps of P1-P3, calculated form the
absorption edge in THF solution, are 1.31-1.34 eV. The polymer
films show exceptional absorption in the range of 300-900 nm,
indicating that the polymers may be good candidates for solar cell
applications.
Figure 4. Cyclic voltammograms of P1-P3 films coated on ITO
glasses
.
Figure 5. (a) Current density-voltage characteristics of
-2
illuminated (AM 1.5G, 100 mW cm ) polymer solar cells
incorporating active layers of polymers/PC71BM; (b) Spectra of
EQE spectra of active layers based on polymers/PCBM.
Electrochemical Properties
Cyclic voltammetry (CV) was used to investigate the redox
behaviour of all polymers and to estimate their HOMO and LUMO
energy levels. The CV curves of P1-P3 in dichloromethane using a
6
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which show less 5% mismatch compared with the Jsc value obtained
from the J-V curves.
The relationship of photocurrent density (Jph) versus effective
voltage (Veff) is employed to investigate the causes of the low
device performance of our devices. As shown in Figure 6a, for P1-P3
based devices, the Jph shows a strong field dependence in a large
bias and has not saturated even at Veff = 3.8 V. This suggests the
devices based on the polymers have low excition dissociation
efficiency and low charge transport and collection efficiency with
significant geminate and/or bimolecular recombination and/or less
efficient interfacial contact, thus low FF and J , which is attributed
sc
the poor morphology of the photoactive layers, as discussed below.
The device based on P3 show more possible tendency to be
saturated than that based on P2 and P1. Furthermore, light and
dark current (Figure 6b and6c) are measured to confirm the origin
of the poor device performance. As shown in Figure 6c, the light
current cannot be saturated even at -3V, which suggests the
collection of electrons and holes is not effective and the
recombination is significant. Combining these measurements and
AFM morphology study, we can conclude that the poor morphology
leads generation problem and then low PCE.
Table 2. PSC characteristics of the polymers: PCBM BHJ PSCs
2
Polymers
V
oc (V)
J
sc (mA/cm )
FF (%)
36.0
36.5
38.6
PCE (%)
0.82
P1
P2
P3
0.74
0.75
0.76
2.52
4.97
6.76
1.35
1.92
Figure 6. a) Photocurrent density versus effective voltage (Jph-Veff
)
characteristics for both devices under constant incident light Morphology study
intensity (AM 1.5G, 100 mW cm−2). Dark b) and light c) J-V to -3V
The morphology of blend films of the polymers with fullerene
derivatives plays a vital part on the charge transfer and the
efficiency of bulk heterojunction solar cells. Herein, the atomic
force microscopy (AFM) was applied to investigate the morphology
of the polymer:PC71BM blend films, as shown in Figure 7. The scan
size is 5 μm × 5 μm. The root-mean-square (rms) roughness values
are 6.64, 0.961 and 0.532 nm for the blend films of P1-P3 with
PC71BM, respectively, which suggests that the contact between the
active layer and the cathode becomes better from P1 to P3. The
blend films of P1-P3 exhibit large phase separation and poor
interpenetrating network, which is unfavourable for exciton
diffusion/dissociation and charge transport. Moreover, compared
with those of P1 and P2, the blend film of P3 exhibits better
interpenetrating networks. As a result, the Jsc also becomes larger
from P1 to P3. Tuning morphology of active blend films is complex
and needs further work focus on it in the future.
Photovoltaic Properties
BHJ photovoltaic devices with P1-P3 as donor materials and PC71BM
as an acceptor material have been fabricated with a typical
configuration of ITO/PEDOT:PSS/polymers:PC71BM/PFN/Al. The
weight ratio of polymers vs. PCBM71 (1:1) with a total concentration
−1
of 20 mg mL in chlorobenzene solution was employed to fabricate
the devices, and the thickness of the BHJ blend was about ~90 nm.
As expected, relatively high Voc in the range of 0.74-0.76 V were
observed in all the three polymer:PCBM based BHJ solar cells,
consistent with the low-lying HOMO energy levels of P1-P3. The Jsc
-2
values for P1-P3 are 2.52, 4.97 and 6.76 mA·cm , respectively. The
corresponding PCEs of devices based on P1-P3 are 0.82%, 1.35%
and 1.92%, respectively. The external quantum efficiency (EQE)
spectra of the devices with P1-P3 are shown in Figure 5b. From the
EQE spectra, these polymers-based devices exhibit very broad
photo-to-current response from 300 to about 900 nm with the
highest EQE values of 17%, 38% and 45%, respectively. For the P1-,
P2- and P3-based devices, the calculated Jsc obtained by the
-2
integration of the EQE curves are 2.38, 4.88 and 6.67 mA·cm ,
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Figure 7. AFM height images of polymers:PC BM, a) P1, b) P2 and c) P3. The areas are 5 µm × 5 µm
7
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acknowledges the financial support AcRF Tier 1 (RG 16/12 and
R/G 133/14), Tier 2 (ARC 20/12 and ARC 2/13) from MOE, and
CREATE program (Nanomaterials for Energy and Water
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TOC page
Novel Donor-Acceptor Polymers based on 7-Perfluorophenyl-6H-
[1,2,5]thiadiazole[3,4-g]benzoimidazole for Bulk Heterojunction Solar Cells
a
Benlin Hu, Miaomiao Li, Wangqiao Chen, Xiangjian Wan, Yongsheng Chen, Qichun Zhang
Three polymers with 7-Perfluorophenyl-6H-[1,2,5]thiadiazole[3,4-g]benzoimidazole as a building
unit were synthesized for BHJ solar cells, and a PCE of 1.92% was obtained.
1
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