Efficient inverted bulk heterojunction photovoltaic devices using a transparent polymeric interfacial buffer layer with C60 pendant and UV curable groups

Article abstract of DOI:10.1166/jnn.2012.5900

We demonstrate the synthesis of a transparent, polymeric n-type material (M1) consisting of C60 pendant and UV curable groups in side chains. This material (M1) is employed as a polymeric n-type interfacial buffer layer for an efficient inverted bulk heterojunction (BHJ) photovoltaic device based on regioregular poly(3-hexylthiophene):[6,6]-phenyl C₆₁ butyric acid methyl ester (P3HT:PC₆₁BM) active layer. Under simulated solar illumination of AM 1.5G (100 mW/cm²), the highest efficient devices fabricated with a configuration of ITO/interfacial buffer layer (M1, 10 nm)/P3HT:PC₆₁BM (1:0.9 w:w) (120 nm)/PEDOT:PSS (30 nm)/Ag (100 nm) achieve an average power conversion efficiency PCE of 2.16%, with short-circuit current J_sc = 6.70 mA/cm², fill factor FF = 54.2%, and open-circuit voltage V_oc = 0.60 V. This result is comparable to the inverted BHJ photovoltaic devices fabricated with Cs₂CO₃, one of widely used as a buffer layer. The synthesized M1 have thus proven to be promising polymeric interfacial buffer layer for high efficient BHJ photovoltaic devices. Copyright

Full text of DOI:10.1166/jnn.2012.5900

Copyright © 2012 American Scientific Publishers
All rights reserved
Printed in the United States of America
Journal of
Nanoscience and Nanotechnology
Vol. 12, 4233–4237, 2012
Efficient Inverted Bulk Heterojunction Photovoltaic
Devices Using a Transparent Polymeric Interfacial
Buffer Layer with C60 Pendant and UV Curable Groups
Younghwan Shin1ꢀ2, Seonju Jeong2, Hyeok Yong Kwon2ꢀ3, Yoon Soo Han2ꢀ∗, and Younghwan Kwon1ꢀ∗
1Department of Chemical Engineering, Daegu University, Gyeongbuk 712-714, Korea
2Department of Advanced Energy Material Science and Engineering, Catholic University of Daegu, Gyeongbuk 712-702, Korea
3Department of Polymer Engineering, Kyungpook National University, Daegu 702-701, Korea
We demonstrate the synthesis of a transparent, polymeric n-type material (M1) consisting of C60
pendant and UV curable groups in side chains. This material (M1) is employed as a polymeric n-type
interfacial buffer layer for an efficient inverted bulk heterojunction (BHJ) photovoltaic device based
on regioregular poly(3-hexylthiophene):[6,6]-phenyl C61 butyric acid methyl ester (P3HT:PC61BM)
active layer. Under simulated solar illumination of AM 1.5G (100 mW/cm2ꢀ, the highest efficient
devices fabricated with a configuration of ITO/interfacial buffer layer (M1, 10 nm)/P3HT:PC61BM
(1:0.9 w:w) (120 nm)/PEDOT:PSS (30 nm)/Ag (100 nm) achieve an average power conversion
efficiency PCE of 2.16%, with short-circuit current Jsc = 6ꢁ70 mA/cm2, fill factor FF = 54.2%, and
open-circuit voltage Voc = 0ꢁ60 V. This result is comparable to the inverted BHJ photovoltaic devices
fabricated with Cs2CO3, one of widely used as a buffer layer. The synthesized M1 have thus proven
Delivered by Publishing Technology to: McMaster University
to be promising polymeric interfacial buffer layer for high efficient BHJ photovoltaic devices.
IP: 219.140.219.164 On: Wed, 24 Feb 2016 17:45:43
Keywords: Interfacial Buffer Layer, Photovoltaic Device, Inverted Structure.
Copyright: American Scientific Publishers
1. INTRODUCTION
PVs in efficiency is still required, although this structure
has a merit of being able to remove a vacuum process
for metal electrode. One important key to highly efficient
inverted PVs is the interfacial buffer layer being inserted
between P3HT:PC61BM active layer and ITO electrode for
collecting electrons effectively. One of the advantages in
organic/polymeric buffer layer has been reported to have
better interfacial compatibility with the active layer in PVs,
facilitating the control of charge carrier transport by lower-
ing the energy barrier than those at the heterointerfaces of
inorganic oxide/organic active layers. However, the inter-
facial buffer layers are required to be compatible with
successive solution casting of active layer, which could
detract from the advantage of the interfacial compatibility
of organic/polymer buffer layers.
In this study, we demonstrate a highly efficient inverted
BHJ PV based on regioregular P3HT:PC61BM active layer
with an interfacial n-type buffer layer. For this pur-
pose, we design and synthesize a transparent polymeric
anodic buffer material with C60 pendant and UV curable
groups, in which the photosensitive group is introduced
to prevent the interfacial buffer layer from successive
solution casting of active layer. Inverted BHJ PVs were
Much attention has been paid to organic/polymer bulk het-
erojunction (BHJ) photovoltaics (PVs) due to low cost,
solution processability, and environment-friendly energy
conversion system. The low power conversion efficiency
(PCE) of these photovoltaic devices is still considered a
major obstacle for commercialization,1–3 although recent
concepts on BHJ films in which the active layers are com-
prised of conjugated polymers and fullerene derivatives
have brought about dramatic increases in the PCE.4ꢀ5 PVs
with power conversion efficiency (PCE) of 5∼6% have
been developed by several groups.6ꢀ7
In addition, several BHJ PVs using an inverted struc-
ture have also been demonstrated.8ꢀ9 Yang et al. demon-
strated inverted PVs with PCE of 2.25% by using cesium
carbonate (Cs2CO3ꢁ as the anodic buffer layer.10ꢀ11 White
et al. adapted a solution-processed ZnO buffer layer on
ITO, and achieved PCE of 2.58%.12 Waldauf et al. utilized
a solution-processed titanium oxide buffer layer on ITO
as the cathode and PEDOT:PSS as the anode to obtain
PCE of ∼3%.13 However, further improvement of inverted
∗Authors to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2012, Vol. 12, No. 5
1533-4880/2012/12/4233/005
doi:10.1166/jnn.2012.5900
4233

Efficient Inverted BHJ Photovoltaic Devices Using a Transparent Polymeric Interfacial Buffer Layer
Shin et al.
fabricated with the structure of ITO/n-type buffer layer/
2.2.3. Polymeric n-Type Interfacial Material (M1)
P3HT:PC61BM/PEDOT:PSS/Ag.
Poly(HEMA-ran-CEMA) (60:40 mole ratio) copolymer
(23.0 mg, 2.608 ꢃmol) and PPh3 (15.7 mg, 60 ꢃmol) was
added into PCBA (53.8 mg, 60 ꢃmol) solution dissolved
in 24 mL of dried THF, and stirred under nitrogen atmo-
sphere at room temperature. After complete dissolution,
DEAD (27 ꢃl, 60 ꢃmol) was slowly added into the reac-
tion mixture, and stirred for 12 hr. After removing the sol-
vent, the crude product was purified by using flash silica
column with DMC, and then precipitated into toluene after
condensing the solution. After filtration and vacuum dry-
ing, polymeric n-type interfacial material (M1, 20.1 mg)
was obtained in 23.3% of yield.
2. EXPERIMENTAL DETAILS
2.1. Materials
Cinnamoyl chloride (98%), hydroxyethyl methacrylate
(HEMA, 98%), and triphenylphosphine (PPh3, 99%)
were purchased from Aldrich Chemical Co. Triethy-
lamine (TEA, 99%) and 2,2-azobisisobutyronitrile (AIBN,
98%) were obtained from Junsei Co. Diethyl azodicar-
boxylate (DEAD, 2.2 M in toluene) was supplied from
Tokyo Chemical Industry, and [6,6]-phenyl-C61-butyric
acid (PC61BA) was purchased from Solenne Co. Sol-
vents such as tetrahydrofuran (THF) and dichloromethane
(DCM) were purified prior to use. P3HT (98% regioreg-
ularity, Mw = 6ꢂ4 × 104 g/mole) and [6,6]-phenyl-C61-
butyric acid methyl ester PC61BM) were purchased
from Rieke Metals and Nano-C, respectively. Poly-3,4-
ethyleneoxythiophene:poly-4-styrenesulfonate (PEDOT:
PSS, Baytron P VP Al 4083) was used as received from
P. H. Stark GmbH.
2.3. Device Fabrication
The inverted BHJ photovoltaic devices were fabricated
with a device configuration of ITO/interfacial buffer
layer/P3HT:PC61BM (1:0.9 w:w) (120 nm)/PEDOT:PSS
(30 nm)/Ag (100 nm). A detailed device fabrication pro-
cess is described as follows. The photovoltaic devices were
fabricated on indium tin oxide (ITO) glass (Sunic Ltd.,
15 ꢄ/sq). ITO glasses were sequentially cleaned by ultra-
sonic treatment in acetone, a detergent, deionized water
and isopropyl alcohol, and then dried in a vacuum oven.
2.2. Synthesis
2.2.1. Cinnamoyloxyethyl Methacylate (CEMA)
The ITO glass was treated with O2 plasma for 3 min, and
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transferred to a glove box filled with N gas for remain-
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Cinnamoyl chloride (4.936 g, 3ꢂ08 × 10−2 mol) dissolved
in 60 mL of DCM was slowly added into HEMA (4.000 g,
3ꢂ08 × 10−2 mol) and TEA (3.104 g, 3ꢂ08 × 10−2 mol)
dissolved in 40 mL of DCM under nitrogen atmosphere
with an ice bath. After removing an ice bath, the reac-
tion was carried out at room temperature for 12 hrs.
CEMA was obtained through silicagel column chromatog-
raphy (n-hexane:ethyl acetate = 9:1 v:v). (Yield = 77%)
1H-NMR (CDCl3-d1ꢁ ppm: 7.71 (d, 1 H, –CH ), 7.54
(s, 2 H, Ar CH) 7.39 (s, 3 H, Ar CH), 6.46 (d, 1 H, CH–
Ar), 6.16 (s, 1 H, –CH–,), 5.60 (s, 1 H, −CH–), 4.46
(t, 4 H, 2-CH2–), 1.96 (s, 3 H, –CH3ꢁ.
2
Copyright: American Scientific Publishers
ing fabrication steps. The n-type interfacial material (M1)
solution (0.5 wt%) in piperidine was spin-cast with a thick-
ness of 10 nm on the ITO glass, followed by immediate
UV exposure at 10 mW for 100 s. An active layer consist-
ing of P3HT:PC61BM (1:0.9 w:w) solution was spin-cast
on the top of the interfacial buffer layer, and dried on a hot
ꢀ
plate in a covered Petri dish for 40 min at 50 C to give
a thickness of 120 nm. PEDOT:PSS was spin-cast with
ꢀ
a thickness of 30 nm, and baked for 20 min at 150 C.
Then, Ag anode layer (100 nm) was deposited by thermal
evaporation under vacuum of ∼5 × 10−8 Torr. The fabri-
cated devices were encapsulated with a glass cap to protect
them from the air. The active area for all the devices was
defined as 9 mm2 by a shadow mask. Thermal annealing
was performed on complete devices on top of a hot plate
inside the glove box.
2.2.2. Poly(2-hydroxyethyl methacrylate-ran-
(cinnamoyloxy)ethyl methacrylate)
[poly(HEMA-ran-CEMA)] Copolymers
HEMA (1.3336 g, 1ꢂ04×10−2 mol) and CEMA (0.6664 g,
0ꢂ13 × 10−2 mol) were dissolved by stirring in 18 mL
of THF:MeOH (3.6:14.4, v:v) mixture under nitrogen
2.4. Instrumental Analysis
ꢀ
NMR spectra were measured by using Varian 300MHz
spectroscopy (Mercury 300). UV-Visible absorption spec-
tra were obtained using a Perkin Elmer Lambda 750
UV/VIS spectrometer. Photoluminescence (PL) spectra
excited by a 150 W xenon (Xe) lamp were taken using a
JASCO FP-6500 spectrofluorometer. Photocurrent–voltage
(J–V ) curves were measured using a Keithley model 2400
Source Meter and a Newport 91192 solar simulator system
atmosphere, and then temperature was set to 60 C. At
60 ꢀC of reaction temperature, AIBN (6 mole%) was
added. The polymerization was carried out with stirring
for 12 h. The resulting mixture was poured into petroleum
ether, filtered, and then dried in vacuum for 12 h at
50 ꢀC. Different compositions of poly(HEMA-ran-CEMA)
copolymers were prepared with the same procedure men-
tioned above.14
4234
J. Nanosci. Nanotechnol. 12, 4233–4237, 2012

Shin et al.
Efficient Inverted BHJ Photovoltaic Devices Using a Transparent Polymeric Interfacial Buffer Layer
equipped with 1 kW Xe arc lamp (Oriel). The molecu-
lar weights and molecular weight distributions (Mw/Mn)
were measured with a Waters GPC with THF as an eluent
against PS standards at room temperature.
moieties as UV curable groups and PCBM groups as side
chains. A series of poly(HEMA-ran-CEMA) copolymers
were first prepared by free radical copolymerization of
HEMA and CEMA, in which CEMA monomer was syn-
thesized by the reaction of HEMA and cinnamoyl chlo-
ride in the presence of TEA for 12 h at room temperature
before the polymerization. Results for molecular weights,
Mw/Mn, and compositions of UV curable random copoly-
mers are summarized in Table I. The number average
3. RESULTS AND DISCUSSION
Figure 1 presents the synthetic route to the polymeric
n-type interfacial material (M1), containing cinnamoyl
Cl
CH3
C
CH3
C
O
CH2
TEA
CH2
C
C
O
Dichloromathane
+
CH
CH
C
O
O
O
25¡É,12h
CH2
CH2
OH
CH2
CH2
O
C
O
CH
CH
CH3
CH3
CH3
CH3
C
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CH
C
C
C
O
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CH2
AIBN, MeOH/THF
CH2
C
C
O
CH2
2
y
x
O
Copyright: American Scientific Publishers
+
C
O
O
O
C
O
O
65¡É,12h
CH2
CH2
O
CH2
CH2
OH
CH2
CH2
OH
CH2
CH2
O
C
O
C
O
CH
CH
CH
CH
CH3
CH3
CH3
CH3
CH3
C
C
C
O
CH2
C
C
O
CH2
C
C
O
OH
CH2
CH2
C
x
CH2
m
O
n
O
y
y
DEAD/PPh3
r.t,inTHF24h
O
C
O
C
O
O
O
+
O
CH2
CH2
O
CH2
CH2
O
CH2
CH2
OH
CH2
CH2
OH
CH2
CH2
O
C
O
C
O
O
CH
CH
CH
CH
Fig. 1. Synthetic route to polymeric n-type interfacial material (M1).
J. Nanosci. Nanotechnol. 12, 4233–4237, 2012
4235

Efficient Inverted BHJ Photovoltaic Devices Using a Transparent Polymeric Interfacial Buffer Layer
Shin et al.
Table I. Characteristics of poly(HEMA-ran-CEMA) copolymers.
Composition of HEMA:
CEMA (mole ratio)
GPC
Mn (g/mole)
Exp.
Monomer
Polymer
Mw/Mn
1
2
3
4
80:20
60:40
40:60
20:80
- - - : - - -
61.8:38.2
39.0:61.0
19.3:80.7
6ꢀ950
6ꢀ840
8ꢀ990
1.59
1.83
1.76
1.78
10ꢀ510
molecular weights (Mn’s) of random copolymers deter-
mined by GPC with THF as an eluent were in the range of
6,840∼10,510 g/mol with molecular weight distributions
of 1.59∼1.83. Compositions of poly(HEMA-ran-CEMA)
Fig. 3. Device structure of the inverted BHJ photovoltaic device.
suggests that the synthesized M1 copolymer is suitable for
use as n-type materials in photovoltaic devices.
1
were determined from H NMR spectra using the inte-
The inverted BHJ photovoltaic devices fabricated
with a configuration of ITO/M1 with different thick-
ness/P3HT:PC61BM (1:0.9 w:w) (120 nm)/PEDOT: PSS
(30 nm)/Ag (100 nm) have been elaborated as shown in
Figure 3 and optimized to investigate the performance of
M1 as polymeric n-type interfacial layer in the devices.
Figure 4 shows the J–V curves of these devices under AM
1.5 condition with 100 mW/cm2, and Table II summarizes
the device performance. The control device (Fig. 4(a)),
without any interfacial buffer layer, presented relatively
poor photovoltaic characteristics, with an average PCE of
0.74%, Jsc = 6ꢂ50 mA/cm2, Voc = 0ꢂ38 V, and FF = 30.0%.
The device (Fig. 4(b)) made with M1 (thickness = 10 nm)
as the interfacial buffer layer showed greatly enhanced
device performance, exhibiting an average PCE of 2.16%
under 100 mW/cm2, Jsc = 6ꢂ70 mA/cm2, Voc = 0ꢂ60 V,
and FF = 54.2%. For comparison, a reference device was
also prepared with the same structure, but Cs2CO3 inter-
facial buffer layer, since the Cs2CO3 has been employed
as the interfacial buffer layer in the inverted PVs.10ꢀ11
The inverted devices (Fig. 4(c)) fabricated with Cs2CO3
gration values of the methylene peak (–CH2–OH, ꢅ =
3ꢂ65 ppm) in polyHEMA and the methane peak (–CH
ꢅ = 6ꢂ46 ppm) in polyCEMA.
,
For the synthesis of the polymeric n-type interfacial
material (M1), poly(HEMA-ran-CEMA) (60:40 mole%)
was selected among the synthesized copolymers for Mit-
sunobu reaction with PC61BA, due to the solubility in THF.
As the degree of substitution of PC61BA on the hydroxyl
groups of HEMA units increased, the methylene peak
(−CH2–OH, ꢅ = 3ꢂ65 ppm). The degree of substitution
of PC61CA by Mitsunobu reaction was calculated to be
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53.1% based on HEMA units, and hence the composition
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of each unit in the final product (M1) was determined as
Copyright: American Scientific Publishers
follows; CEMA:HEMA:HEMA-PC61BA = 39.1:28.7:32.4
mole ratio). Photoinduced electron transfer behavior from
P3HT to M1 with several blend ratios was studied by using
PL spectroscopy. The PL spectra of neat films of P3HT
and M1, respectively, were also shown for comparison in
Figure 2. Efficient PL quenching in blends of P3HT and
M1 was observed with increasing the content of M1. This
(a)
(b)
(c)
(d)
(e)
550
600
650
700
750
Wavelength (nm)
Fig. 2. Photoluminescence spectra of P3HT:M1 films blended with dif-
ferent compositions. The spectra of neat films of P3HT and M1, respec-
tively, are included for comparison; (a) P3HT, (b) P3HT:M1 (2:1 w:w),
(c) P3HT:M1 (1:1 w:w), (d) P3HT:M1 (1:2 w:w), and (e) M1.
Fig. 4. Current (J)–voltage (V ) characteristics of the inverted BHJ pho-
tovoltaic devices under an illumination of 100 mW/cm2 (AM 1.5G)
with different interfacial buffer layers; (a) no buffer layer, (b) M1, and
(c) Cs2CO3.
4236
J. Nanosci. Nanotechnol. 12, 4233–4237, 2012

Shin et al.
Efficient Inverted BHJ Photovoltaic Devices Using a Transparent Polymeric Interfacial Buffer Layer
2.16%, with Jsc = 6ꢂ70 mA/cm2, FF = 54.2%, and Voc
=
Table II. Photovoltaic properties of the inverted BHJ photo-
voltaic devices with the device configuration of ITO/anodic buffer
layer/P3HT:PC61BM (1:0.9 w:w)/PEDOT:PSS/Ag.
0ꢂ60 V. This result is comparable to the inverted BHJ pho-
tovoltaic devices fabricated with Cs2CO3, one of widely
used as a buffer layer. Therefore, this M1 material could
be applied to design a tandem polymer PVs for further
improving the efficiency of inverted BHJ PVs.
Anodic buffer Voc
layer
Jsc
FF
PCE
Rs
Rsh
(V) (mA/cm2ꢁ (%) (%) (ꢄ·cm2ꢁ (ꢄ·cm2ꢁ
No buffer
layer
Cs2CO3
(10 nm)
M1
0.38
0.57
0.60
6.50
7.57
6.70
30.0 0.74
49.9 2.17
54.2 2.16
60.4
23.8
23.5
150
747
742
References and Notes
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7ꢂ57 mA/cm2, Voc = 0ꢂ57 V, and FF = 49.9%. This result
is comparable to the inverted BHJ photovoltaic devices
fabricated with Cs2CO3, one of widely used as a buffer
layer. The synthesized M1 copolymer has thus proven to
be promising polymeric n-type interfacial buffer layer for
high efficient inverted BHJ photovoltaic devices.
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The transparent, polymeric n-type material (M1) consist-
ing of C60 pendant and UV curable groups in side chains
was synthesized, where the photosensitive group is intro-
duced to prevent the buffer layer from successive solution
casting of active layer. Under AM 1.5G (100 mW/cm2ꢁ,
12. M. S. White, D. C. Olson, S. E. Shaheen, N. Kopidakis, and D. S.
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(1:0.9 w:w) (120 nm)/PEDOT:PSS (30 nm)/Ag (100 nm)
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Received: 10 May 2010. Accepted: 20 June 2011.
J. Nanosci. Nanotechnol. 12, 4233–4237, 2012
4237