Enhanced performance in bulk heterojunction polymer solar cell using water soluble conjugated polymer

Article abstract of DOI:10.1166/jnn.2015.9305

We have synthesized water-soluble polymer, poly[(9,9-bis((6a?2-(N,N,N-trimethylammonium) hexyl)-2,7-fluorene))-alt-bisphenylfumaronitrile]dibromide (AHF-alt-PFN), the polymer typically obtained by the Suzuki type of polymerization reaction and shows good solubility in methanol. Bulk heterojunction polymer solar cells (BHJ-PSCs) fabricated by using water soluble conjugated polymer and positive (Cs⁺) and negative (F^-, CO^2-₃) charge ions doping as an interfacial layer for poly(3-hexylthiophene):phenyl-C61 butyric acid methyl ester (P3HT: PCBM). We have achieved an enhancement of the short circuit density and power conversion efficiency in solar cell by introducing poly(AHF-alt-PFN) layer between the active layer and the cathode metal. The device with poly(AHF-alt-PRN) layer containing F-, CO²3^- showed a short circuit current density more 1.3, 2.3 times higher than those of the device without poly (AHF-alt-PFN) + ion layer. We explain the better performance in solar cell with poly (AHF-alt-PFN) + ion layer was due not only to the increase of electron mobility in poly (AHF-alt-PFN) layer but also to the decrease of the electron barrier near cathode by the addition of the negative ions.

Full text of DOI:10.1166/jnn.2015.9305

Article
Journal of
Nanoscience and Nanotechnology
Vol. 15, 1683–1686, 2015
Copyright © 2015 American Scientific Publishers
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Enhanced Performance in Bulk Heterojunction Polymer
Solar Cell Using Water Soluble Conjugated Polymer
1
1
1
1
Kyung-Yong Park , Jun-Sung Lee , Han-Sol Namkung , Min-Sik Koo ,
2
3
5
5
Seong-Jin Cho , Byung-Woo Yoon , Young-Mo Kim , Youn-Sik Lee ,
6
1ꢀ∗
4ꢀ∗
Seong-Hwan Song , Dong-Kyu Park , and Chung-Gi Kim
1
Department of Chemistry, Kyungsung University, Busan 608-736, Korea
Department of Physics, Kyungsung University, Busan 608-736, Korea
Department of Electronic Engineering, Kyungsung University, Busan 608-736, Korea
Department of Advanced Materials Engineering, Kyungsung University, Busan 608-736, Korea
2
3
4
5
Department of Chemical Engineering, Chonbuk National University, Jeon-Ju, Jeonbuk 561-756, Korea
6
Division of Energy and Bio-Engineering, Dongseo University, Busan 617-716, Korea
ꢀ
We have synthesized water-soluble polymer, poly[(9,9-bis((6 -(N,N,N-trimethylammonium)hexyl)-
2
,7-fluorene))-alt-bisphenylfumaronitrile]dibromide (AHF-alt-PFN), the polymer typically obtained by
the Suzuki type of polymerization reaction and shows good solubility in methanol. Bulk het-
erojunction polymer solar cells (BHJ-PSCs) fabricated by using water soluble conjugated poly-
Delivere
+
d by Publishing T
−
echno2 −l ogy to: Imperial College London
mer and positive (Cs ꢀ and negative (F , CO ) charge ions doping as an interfacial layer for
3
IP: 117.30.114.129 On: Fri, 01 Apr 2016 14:15:14
poly(3-hexylthiophene):phenyl-C61 butyric acid methyl ester (P3HT:PCBM). We have achieved an
Copyright: American Scientific Publishers
enhancement of the short circuit density and power conversion efficiency in solar cell by intro-
ducing poly(AHF-alt-PFN) layer between the active layer and the cathode metal. The device with
−
2−
poly(AHF-alt-PRN) layer containing F , CO showed a short circuit current density more 1.3, 2.3
3
times higher than those of the device without poly(AHF-alt-PFN)+ion layer. We explain the better
performance in solar cell with poly(AHF-alt-PFN) + ion layer was due not only to the increase of
electron mobility in poly(AHF-alt-PFN) layer but also to the decrease of the electron barrier near
cathode by the addition of the negative ions.
Keywords: Water-Soluble Polymer, Polymer Solar Cell, Bulk Hetero Junction.
1
. INTRODUCTION
units for a better internal charge transfer (ICT).5ꢀ6 This
ICT causes broadening of the light-absorbing wavelength
range (i.e., lowering the band gap) and increase of short
circuit current (Jsc) and thus the PCE as well. Recently,
PCE of over 7% have been achieved by using low band
gap copolymers with controlled morphology of the active
Organic and polymeric materials have generated great
interest over the past decade for their potential application
in flat panel display and photovoltaic cell as well. Among
the organic photovoltaic cells, the bulkheterojuction poly-
mer solar cell (BHJ-SC) where the light absorbing active
layer is consists of a blend of electron donating conjugated
polymer (p-type) and electron accepting fullerene (n-type)
is one of the promising candidates for next generation of
solar cell due to its simple solution processing with large-
7
ꢀ8
polymer blends.
Water-soluble polymers could also
be promising candidates as efficient interfacial layer
materials for highly efficient inverted PSCs because they
can provide solution processing capabilities and avoid
intermixing between the underlying water/alcohol-soluble
interlayer and the subsequently deposited organic solu-
ble active layer, which is very beneficial for the future
development of all-solution processed or roll-to-roll pro-
1
−4
area, low cost, and flexible possibility.
Many efforts have been made in BHJ-SC for higher
power conversion efficiency (PCE), mostly trying to syn-
thesize the low-band gap donor polymers by designing
novel copolymers with various electron rich and poor
9
cessed PSCs. In this work, we have fabricated highly
efficiency polymer solar cells from water soluble conju-
gated polymer (poly[(9,9-bis(6-trimetylammonomhexyl)
∗
Authors to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2015, Vol. 15, No. 2
1533-4880/2015/15/1683/004
doi:10.1166/jnn.2015.9305
1683

Enhanced Performance in Bulk Heterojunction Polymer Solar Cell Using Water Soluble Conjugated Polymer
Park et al.
1
fluorine-alt-diphenylfumaronitrile)]) (Poly(AHF-alt-PFN))
by using it as a ultra thin electron transport layer on the
top of commercially available active layer, poly(3-hexyl-
thiophene)-2,5-diyl (P3HT):{6}-1-(3-(methoxycarbonyl)
propyl)-{5}-1-phenyl-[6,6]-C61 (PCBM). For the second
time, we have fabricated highly efficiency polymer solar
cell from ions dispersed water soluble conjugated polymer,
chloride as a white solid (Yield 80%). H NMR (DMSO,
400 MHz): ꢃ7.47 (d, 4 H), 7.32 (d, 4 H).
2
.1.4. Synthesis of Polyꢄ2-ꢁ4-ꢁ9,9-bisꢁ6-bromohexyl)-
7-methyl-9H-fluoren-2-ylꢂphenylꢂ-
3
-p-Tolylfumaronitrile
ꢀꢀ
2
,7-Dibromo-9,9-bis(6 -bromohexyl)-fluorene (0.74 g),
(Poly(AHF-alt-PFN)).
Bis(4-bromophenyl)fumaronitrile (0.38 g), Pd(PPh3)4
10 mg) were dissolved in 40 mL of THF under nitro-
gen and 20 mL 2 M Na CO was added. The mixture
was refluxed at 80 C for 72 h, and then precipitated
into methanol. The polymer was filtered and washed with
methanol and acetone, and then dried under vacuum. H-
NMR (CDCl , 400 MHz): ꢃ7.91–7.64 (m, 14 H), 3.20 (m,
H), 1.98–1.62 (m, 8 H), 1.33 (s, 4 H), 1.20 (m, 4H.),
.15 (m, 4 H), 0.55 (m, 4 H).
(
2
. EXPERIMENTAL DESCRIPTION
2
3
ꢁ
2
.1. Synthesis of Water Soluble Polymer
2
.1.1. Synthesis of 2,7-Dibromo-9,9-Bis
1
ꢀ
ꢀ
ꢁ
6 -bromohexylꢂ-Fluorene
3
9
.72 g of 2,7-dibromofluorene and 0.96 g of tetrabuty-
4
1
lammoniumbromide were dissolved in 100 mL of 1,6-
dibromohexane and 30 mL of a 45% KOH solution was
then injected under nitrogen. The solution was stirred
ꢀ
ꢁ
2.1.5. Synthesis of Polyꢄꢁ9,9-bisꢁꢁ6 -(N,N,N-
trimethylammoniumꢂhexylꢂ-2,7-fluoreneꢂꢂ-alt-
bisphenylfumaronitrileꢂꢅDibromide
at 75 C for 1 h. The reaction mixture was extracted
with 300 mL dichloromethane and the resulting solu-
tion was evaporated. The reaction mixture was distilled
under vacuum, to remove the 1,6-dibromohexane. The
reaction mixture was purified by column chromatography
3
0% aqueous solution of trimethylamine (5 mL) was
added dropwise to a solution of the Polymer (60 mg)
in THF (50 mL) at room temperature. The mixture was
stirred at room temperature for 24 h. After removing most
of the solvent, acetone was added to precipitate poly-
mer (Poly(AHF-alt-PFN). H-NMR (CD OD, 400 MHz):
1
using hexane and dichloromethane.(Yield 70%) H-NMR
(
CDCl , 400 MHz): ꢃ7.56–7.43 (m, 6 H), 3.30 (t, 4 H),
3
1
0
.92 (t, 4 H), 1.67 (t, 4 H), 1.20 (m, 4 H), 1.08 (m, 4 H),
.58 (m, 4 H).
1
3
Delivered by Publishing Technolog ꢃy 7 t. o9 8: I m( mp ,e r8 i Ha l) ,C 7o .l 7l e9 g e( mL,o 6n Hd o) ,n3.18 (m, 4 H), 3.00 (m,
IP: 117.30.114.129 On: Fri, 01 Apr 2016 14:15:14
1
(
8 H), 2.21 (m, 4 H), 1.62 (m, 4 H), 1.33 (s, 24 H), 1.20
m, 4 H), 1.15 (m, 4 H), 0.56 (m, 4 H).
2
.1.2. Synthesis of 2,7-Bisꢁ4,4,5,5-tetramethyl-
Copyright: American Scientific Publishers
1
,3,2-dioxaborolan-3-ylꢂ-9,9-
ꢁ
6-dibromohexylꢂ-Fluroene
ꢀꢀ
2.2. Fabrication of the Polymer Solar Cells
2
,7-Dibromo-9,9-bis(6 -bromohexyl)-fluorene and bis
(
pinacolato)diboron (3 g) were dissolved in 1,4-dioxane
PEDOT:PSS (∼ 40 nm) was used for all the devices as
and PdCl (dppf) was then injected under nitrogen. The
solution was stirred at 85 C for 16 h. The reaction
a hole transport layer which was spun on the cleaned
2
ꢁ
ꢁ
indium tin oxide (ITO) anode and annealed at 100 C
mixture was extracted with dichloromethane and water.
for 10 min. The light-absorbing active layer (∼ 80 nm),
The organic layer was dried with MgSO and concen-
also used for all the test devices, was prepared by spin
4
trated under reduced pressure. The reaction mixture was
purified by column chromatography using hexane and
coating onto the PEDOT:PSS film from a blend of P HT
3
and PC BM (P3HT:PC60BM = 1:0.7 by weight) in a co-
6
0
ꢁ
dichloromethane. The final product was a white solid,
solvent (CB:DCB = 1:1) and then annealed at 120 C for
10 min. As an electron transport layer, the synthesized
poly(AHF-alt-PFN) were dissolved in methanol spun with
2000 rpm for 40 sec onto the active polymer blend film
1
produced in 40% yield. H NMR (CDCl , 400 MHz):
3
ꢃ7.82 (d, 2 H), 7.74 (s, 2 H), 7.72 (d, 2 H), 3.27 (t, 4 H),
1
4
.98 (m, 4 H), 1.62 (m, 4 H), 1.33 (s, 24 H), 1.20 (m,
H), 1.15 (m, 4 H), 0.55 (m, 4 H).
ꢁ
followed by annealing at 100 C for 10 min. This leads
to ultrathin layers for poly(AHF-alt-PFN) with a thick-
ness of around 5 nm that was estimated from extrapolation
2
.1.3. Synthesis of Bisꢁ4-bromophenylꢂFumaronitrile
1
0
A two-neck flask was charged with 4-bromophenyl-
acetonitrile (2.45) and iodine (3.172) in dry ether (50 mL)
approximation. Finally, Al was vacuum evaporatied with
an evaporation rate of 0.1 nm/sec. A vacuum pressure of
ꢁ
−6
and cooled to −78 C. A solution of sodium methoxide
2 × 10 Torr was maintained during all of the vacuum
(1.38 g) in methanol (10 mL) was added into the reac-
deposition. Their thickness was 150 nm. The PSCs device
structures in this study are summarized as follows:
Device 1: A/active layer/C,
ꢁ
tion solution at −78 C under a nitrogen atmosphere. The
ꢁ
reaction mixture was gradually warmed to 0 C for 4 h.
The reaction was quenched with a 3% aqueous solution of
hydrochloric acid. The solution was filtered to isolate the
solid, which was rinsed with methanol to remove the ionic
residues. The product was recrystallized from methylene
Device 2: A/active layer/poly(AHF-alt-PFN)/C
+
Device 3: A/active layer/poly(AHF-alt-PFN)+Cs / C
−
Device 4: A/active layer/poly(AHF-alt-PFN)+F /C
2
−
Device 5: A/active layer/poly(AHF-alt-PFN)+CO /C
3
1
684
J. Nanosci. Nanotechnol. 15, 1683–1686, 2015

Park et al.
Enhanced Performance in Bulk Heterojunction Polymer Solar Cell Using Water Soluble Conjugated Polymer
Where, the A and B denote anode (ITO/PEDOT:PSS) and
cathode (Al), respectively.
(a)
3
. RESULTS AND DISCUSSION
The molecular structures of water soluble polymer
poly(AHF-alt-PRN)) are shown in Figure 1 and the
(
polymer typically obtained by the Suzuki type of
polymerization reaction and shows good solubility in
methanol. IP of poly(AHF-alt-PFN) was measured by
cyclic voltammogram and the band gaps (E ) were esti-
g
mated from absorbance measurement (Shimadzu UV-160
PC). Figure 2 shows the UV absorption for poly(AHF-alt-
PFN) as a solid film. The band gap energy of the polymer
was estimated from the absorption spectrum and was found
to be 496 nm (2.5 eV). The electrochemical behavior of the
polymer was investigated by cyclic voltammetry (CV) and
outlined in Figure 3. The onset potential for oxidation was
(
b)
observed to be 0.6 V. On the other hand, E was estimated
g
from UVonset and calculated from E = I −E . According
A
P
g
ox
ox
to the equation, I = ꢁꢄE ꢅ +4ꢆ8) eV, where ꢄE
ꢅ
onset
P
onset
was the onset potential for the oxidation of polymer, the
HOMO and LUMO of the polymer were estimated to be
5
.4 and 3.7 eV.
J–V characteristics of the fabricated BHJ-SC were
recorded from Keithley 2400 source meter under
2
1
00 mW/cm white light illumination. All devices have
Delivered by Publishing Technology to: Imperial College London
shown the characteristics of short circuit current density
Jsc), open circuit voltage (VOC), fill factor (FF) and power
IP: 117.30.114.129 On: Fri, 01 Apr 2016 14:15:14
(
Copyright: American Scientific Publishers
conversion efficiency (PCE). Figure 3 shows J–V charac-
teristics of these solar cells and the results are summa-
rized in Table I. As can be seen from Table I, the short
circuit currents and efficiencies are significantly increased
when poly(AHF-alt-PFN)+ions are introduced in the solar
cells (device 2–5) compared to that of the device with-
Figure 2. Optical and electro chemical properties. (a) UV-Vis absorp-
tion spectra, (b) cyclic voltammogram.
proposed that the counter ion attracts the electron from the
active layer and that the water soluble polymer layer is
thin enough for electron tunneling. As a result, the better
performance in solar cells with poly(AHF-alt-PFN) + ion
layer was due not only to the increase of electron mobility
out poly(AHF-alt-PFN) (device 1). J of device 5 is
sc
2
8
.49 mA/cm which is corresponding to 32% increase
2
compared to that of the device 1 (6.41 mA/cm ꢂ while
PCE of device 5 (2.3%) increases about 100% than the
device 1 (1.01%).
The improvement of photovoltaic performance with
water soluble polymer implies that poly(AHF-alt-PFN)
ultrathin layers (∼ 5 nm) help for electron injection and
transport from the active layer to cathode. We have
1
ITO/PEDOT:PSS/P3HT:PCBM/Al
ITO/PEDOT:PSS/P3HT:PCBM/Poly(AHF-alt-PFN)/Al
ITO/PEDOT:PSS/P3HT:PCBM/Poly(AHF-alt-PFN)+Cs /Al
0
+
–1
–2
–
ITO/PEDOT:PSS/P3HT:PCBM/Poly(AHF-alt-PFN)+F /Al
2–
ITO/PEDOT:PSS/P3HT:PCBM/Poly(AHF-alt-PFN)+CO3 /Al
–3
CN
–4
–
–
–
–
–
5
6
7
8
9
NC
n
–10
N
N
0.0
0.1
0.2
0.3
0.4
0.5
Br
Br
Voltage [V]
Figure 1. Molecular structure of poly(AHF-alt-PFN).
J. Nanosci. Nanotechnol. 15, 1683–1686, 2015
Figure 3. J–V characteristics of polymer solar cells.
1685

Enhanced Performance in Bulk Heterojunction Polymer Solar Cell Using Water Soluble Conjugated Polymer
Park et al.
Table I. Summary performance of the PSCs devices.
soluble polymer and ions could be useful for the enhance-
ment of polymer solar cell performance.
Jsc (mA/cm2ꢂ
Voc (V)
FF (%)
PEC (%)
Device 1
Device 2
Device 3
Device 4
Device 5
6.41
6.27
7.00
7.89
8.49
0.36
0.42
0.46
0.49
0.49
39
47
54
51
52
1.01
1.48
1.50
2.00
2.30
Acknowledgment: This work was supported by Basic
Science Research Program through the National Research
Foundation of Korea (NRF) funded by the Ministry of
Education, Science and Technology (2010-0025568).
in poly(AHF-alt-PFN) but also to the decrease of electron
barrier near cathode by the addition of the negative ions.
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We have fabricated efficiency solar cell from metal ions
dispersed water soluble conjugated polymer, Poly(AHF-
alt-PFN), which is used as an electron transport layer
on the top of commercially available active layer,
P3HT:PCBM. The device with Poly(AHF-alt-PFN) layer
(
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containing F and CO (device 4, 5) showed a better per-
3
33, 4250 (2011).
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compared to that of the solar cell without water soluble
polymer. We explain the better performance in solar cell
with Poly(AHF-alt-PFN) + ion layer was due not only to
the increase of electron mobility in Poly(AHF-alt-PFN)
layer but also to the decrease of electron barrier near cath-
ode by the addition of the negative ions. This result implies
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Delivered by Publishing Technology to: Imperial College London
IP: 117.30.114.129 On: Fri, 01 Apr 2016 14:15:14
Copyright: American Scientific Publishers
Received: 22 June 2013. Accepted: 12 October 2013.
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J. Nanosci. Nanotechnol. 15, 1683–1686, 2015