Enhanced device performance of organic solar cells via reduction of the crystallinity in the donor polymer

Article abstract of DOI:10.1039/c0jm00664e

A new π-conjugated polymer, poly(2,5-dioctyloxyphenylene vinylene-alt-3,3-dioctylquater thiophene) (PPVQT-C8), consisting of alternating p-divinylene phenylene and 3,3-dialkylquater thiophene units, was synthesized for use in organic solar cell devices. The crystallinity of poly(quaterthiophenes) (PQTs) was reduced by introducing p-divinylene phenylene units between pairs of quaterthiophene units. A well-mixed nanoscale morphology of PPVQT-C8:PCBM photoactive layer resulted from this modification, which increased the power conversion efficiency relative to devices based on highly crystalline PQTs. Bulk heterojunction solar cells based on PPVQT-C8:PC ₆₀BM blends with a 1:3 ratio presented the best photovoltaic performances, with a short-circuit current density (J_sc) of 6.7 mA cm^-2, an open-circuit voltage (V_oc) of 0.67 V, a fill factor (FF) of 0.62 and a power conversion efficiency (PCE) of 2.8% under illumination of AM 1.5 with light intensity of 100 mW cm^-2. The charge carrier mobility and morphology studies indicated that an optimized interpenetration network composed of PPVQT-C8: PCBM could be achieved in a blend ratio of 1:3. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/c0jm00664e

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Enhanced device performance of organic solar cells via reduction of the
crystallinity in the donor polymer†
Jong Hwan Park,a Jeong-Il Park,b Do Hwan Kim,b Joo-Hyun Kim,a Jong Soo Kim,a Ji Hwang Lee,a
a
Myungsun Sim, Sang Yoon Lee and Kilwon Cho
b
a
*
Received 11th March 2010, Accepted 15th April 2010
First published as an Advance Article on the web 10th June 2010
DOI: 10.1039/c0jm00664e
A new p-conjugated polymer, poly(2,5-dioctyloxyphenylene vinylene-alt-3,3-dioctylquater thiophene)
(PPVQT-C8), consisting of alternating p-divinylene phenylene and 3,3-dialkylquater thiophene units,
was synthesized for use in organic solar cell devices. The crystallinity of poly(quaterthiophenes) (PQTs)
was reduced by introducing p-divinylene phenylene units between pairs of quaterthiophene units. A
well-mixed nanoscale morphology of PPVQT-C8:PCBM photoactive layer resulted from this
modification, which increased the power conversion efficiency relative to devices based on highly
crystalline PQTs. Bulk heterojunction solar cells based on PPVQT-C8:PC60BM blends with a 1 : 3 ratio
presented the best photovoltaic performances, with a short-circuit current density (Jsc) of 6.7 mA cmꢀ2
,
an open-circuit voltage (Voc) of 0.67 V, a fill factor (FF) of 0.62 and a power conversion efficiency
(PCE) of 2.8% under illumination of AM 1.5 with light intensity of 100 mW cmꢀ2. The charge carrier
mobility and morphology studies indicated that an optimized interpenetration network composed of
PPVQT-C8: PCBM could be achieved in a blend ratio of 1 : 3.
properties and a high hole mobility.10 However, the photovoltaic
1. Introduction
performances of PQT and PC60BM-based devices are very poor
(<1%).14,15 Even if PQT:PC60BM-based devices are thermally
treated under mild conditions (ꢁ50 ꢂC), macrophase separation
of the active layers, which results in low photocurrent and a poor
FF, occurs due to the PQTs’ high crystallinity.14 In this paper, we
describe a new p-type conjugated copolymer (PPVQT-C8)
composed of alternating p-divinylene phenylene and 3,3-dia-
lkylquaterthiophene units for use in organic solar cell applica-
tions. To minimize the complexity of device processing and to
maintain good mixing of the donor and acceptor materials in the
bulk heterojunction photoactive layers during operation, we
reduced the crystallinity of PQTs by introducing p-divinylene
phenylene units between each pair of quaterthiophene units.
Bulk heterojunction solar cells based on the PPVQT-C8:PC60BM
blend were fabricated, and their performances were optimized.
The nanophase separation of copolymer:PCBM blend layers was
found to be correlated with reduced crystallinity in the copol-
ymer. The dependence of solar cell performance on composition
ratio and thermal treatment was systematically studied.
The development of highly efficient electron donor and acceptors
materials is a key issue in developing high efficiency organic bulk
heterojunction solar cells.1–6 For efficient light absorption, donor
and acceptor materials should have a high absorption coefficient
and a low optical band gap.7 High carrier mobilities are also
needed to minimize recombination and trapping of charge
carriers during transport to the electrodes. Donor and acceptor
molecules must be well-mixed in bulk heterojunctions on the
length scale of 10–20 nm in each domain because exciton
recombination significantly increases when the domain sizes of
the donor or acceptor materials exceed the exciton diffusion
length.8,9 Crystalline organic semiconductors show higher carrier
mobilities10–13 than amorphous organic semiconductors because
the strong p–p interactions enable fast in-plane charge trans-
port. However, the photovoltaic properties of conjugated
polymers with higher carrier mobilities are not always good due
to macrophase separation of the active layers during post-
fabrication treatment process.12–15 Furthermore, the perfor-
mances of solar cell devices based on highly crystalline materials
depend strongly on the processing conditions, such as the
residual solvents,16 additives,17 and post-fabrication treatment
methods,18,19 which increase the complexity of device processing.
Poly(quater thiophenes) (PQTs) are promising candidates for
high efficiency organic solar cells due to good light absorption
2. Experimental
Synthesis
The structures and synthetic route to the PPVQT-C8 are shown
in Scheme 1. 3-Octylthiophene was converted to a boronate
(compound (1)) via lithiation with n-butyllithium at ꢀ78 ꢂC,
followed by treatment with 2-isopropoxy-4,4,5,5-tetramethyl-
1,3,2-dioxaborolane. The 3,3-dioctylquater thiophene monomer
(compound (2)) was synthesized by Suzuki coupling between
compound (1) and 5,50-dibromo-2,20-bithiophene in toluene
using Pd(PPh3)4 as a catalyst. Compound (2) was subsequently
converted to compound (3) using BuLi. Compound (4) was
aDepartment of Chemical Engineering, School of Environmental Science
and Engineering, Pohang University of Science and Technology, Pohang,
790-784, South Korea. E-mail: kwcho@postech.ac.kr
bDisplay Laboratory, Samsung Advanced Institute of Technology, Yongin,
446-712, South Korea
† Electronic supplementary information (ESI) available: Cyclic
voltammetry data, dark-current characteristics of hole and electron
only device and detailed synthetic scheme. See DOI: 10.1039/c0jm00664e
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solar simulator) with respect to a reference cell PVM 132 (cali-
brated at the National Renewable Energy Laboratory, NREL, at
an intensity of 100 mW cmꢀ2).
3. Results and discussion
The polymer possessed a molecular weight (Mn ¼ 20kDa) with
a narrow size distribution (polydispersity index, PDI ¼ 1.57) and
was soluble in common organic solvents, such as chloroform,
chlorobenzene and 1,2-dichlorobenzene. UV-Vis absorption
spectra of the PPVQT-C8 and reference polymers (PQT-12 and
MEH-PPV) were recorded in both solution and film states. The
absorption spectrum of PPVQT-C8 in chlorobenzene solution
displayed one peak at 520 nm that extended to 600 nm (Fig. 1(a)).
The absorption edge was slightly red-shifted, by 50 nm, relative
to the reference polymers. We speculated that the higher optical
band edge of PPVQT-C8 may have been due to an internal
electron donor–acceptor (D–A) interaction.22 A donor–acceptor
structure can reduce the optical band gap, extending absorption
down to the near infrared, and may raise the power conversion
efficiency (PCE) in solar cells through increased photocurrent. In
general, a low band gap polymer could result from the combi-
nation of the donor and acceptor having close HOMO level of
donor and the LUMO level of acceptor.22 However, in this case,
the shift of the absorption edge was relatively small, suggesting
that the difference in HOMO and LUMO energy levels between
p-divinylene phenylene and 3,3-dialkylquaterthiophene units
was not significant. In the case of MEH-PPV and PPVQT-C8,
the absorption edges of thin films were almost identical to those
of solutions (see Fig. 1). Meanwhile, the absorption shoulder
clearly appeared and the overall absorption spectra of PQT-12
films were red-shifted. This phenomenon could be interpreted
Scheme 1 Synthetic scheme for the semiconducting copolymer, PPVQT-
C8, composed of quaterthiophene and phenylene vinylene units.
prepared via a modified previously reported method.20 PPVQT-
C8 was polymerized by a modified Wittig reaction between
compound (3) and compound (4).
Characterization methods
1H and 13C-NMR spectra (Fig. S1–S8†) were measured using
Bruker NMR 300 MHz (DPX 300) spectrometers. Gel perme-
ation chromatography (GPC) analysis was performed using a PS
standard column connected to a UV/refractive index detector.
Cyclic voltammetry measurements were performed using
a PowerLab/4SP (AD instrument) electrochemical analyzer with
a glassy carbon working electrode (d ¼ 4 mm), a Pt wire counter
electrode, and a Ag/Ag+ reference electrode cell in a solution of
0.1 mol Lꢀ1 tetrabutylammonium hexafluorophosphate
(Bu4NPF6) in acetonitrile at a scan rate of 100 mV sꢀ1. The
potential was calibrated against a ferrocene/ferrocenium couple.
The energy levels were estimated using the equations: HOMO ¼
ꢀ(4.80 + Eonset, ox), and LUMO ¼ ꢀ(4.80 + Eonset, red).21
The HOMO level of the copolymer was estimated by ultravi-
olet photoelectron spectroscopy (AC-2, Riken Keiki Co.) UV-
Visible absorption spectra were investigated using a UV-Vis
spectrophotometer (Varian, CARY-5000). Grazing-incident X-
ray diffraction (GI-XRD) measurements were performed using
ꢀ
the 10C1 beamlines (l ¼ 1.54 A) at the Pohang Accelerator
Laboratory (PAL). The angle between the film surface and the
incident beam was fixed at 0.18ꢂ. The measurements were
obtained at scanning intervals of 2q between 3ꢂ and 26ꢂ.
Solar cell device fabrication and testing
The PPVQT-C8:PCBM blend solutions were prepared in chlo-
robenzene at total concentrations of 10 mg mLꢀ1. After cleaning
the ITO surface, PEDOT:PSS (Baytron P TP AI 4083, Bayer
AG) was spin-coated to a thickness of 30–50 nm and baked at
120 ꢂC for 60 min. The 80–120 nm thick active layers were spin-
coated on the PEDOT:PSS buffer layer. LiF (0.6 nm)/Al
(150 nm) cathodes were formed by vacuum deposition. The J–V
characteristics were measured using Keithley 4200 source/
measurement units under AM 1.5 solar illumination (Oriel 1 kW
Fig. 1 UV-Vis absorption spectra of (a) solutions and (b) thin films of
PPVQT-C8 and reference materials. PQT-12 (,), MEH-PPV (B) and
PPVQT-C8 (:). The film thickness was 30 ꢃ 5nm.
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Table 1 Summary of the photovoltaic properties of PPVQT-C8:PCBM
organic solar cell devices with various blend ratios
assuming that the red-shifted spectrum results from a lower
energy state of delocalized excitons in the highly p-conjugated
PQT-12 domains.23–25 As shown in Fig. 1(b), PPVQT-C8 films
had a relatively high absorption coefficient compared to the
reference polymers. Excellent absorption properties in p-type
polymers are essential for efficient photon harvesting, which
yields large photocurrents. Because the typical thickness of
organic solar cells is 100–200 nm, conjugated polymers with light
absorption coefficients that exceed 1 ꢄ 105/cm in the visible and
NIR regions are recommended.26
Blend ratio (PPVQT-C8:PCBM) Voc/V Jsc/mA cmꢀ2 FF PCE (%)
20% PCBM (4 : 1)
50% PCBM (1 : 1)
60% PCBM (2 : 3)
75% PCBM (1 : 3)
90% PCBM (1 : 9)
0.63
0.66
0.64
0.67
0.53
0.53
3.46
3.68
6.69
1.23
0.31 0.1
0.51 1.2
0.57 1.8
0.62 2.8
0.44 0.3
The HOMO and LUMO levels of PPVQT-C8 were evaluated
by cyclic voltammetry (Fig. S1†) and ultraviolet photoelectron
spectroscopy (AC2 equipment). The oxidation onset (Eonset,ox) of
PPVQT-C8 was found to be 0.33 V and the reduction onset
(Eonset,red) was ꢀ1.52 V vs. Fc/Fc+. Therefore, the LUMO and
HOMO levels of PPVQT-C8, estimated by cyclic voltammetry,
were 5.13 and 3.28 eV, respectively, with a HOMO–LUMO gap
of 1.87 eV. The HOMO level of PPVQT-C8, evaluated by
ultraviolet photoelectron spectroscopy, was 5.19 eV. In the case
of PQT-12, the literature values of the HOMO and LUMO levels
were 5.24 and 2.97 eV.23 Therefore, we concluded that PPVQT-
C8 and PQT-12 did not differ significantly in light absorption
properties and energy band structure.
device resulted from increased direct metal-insulator-metal
(MIM) contact between PCBM domains and the anode.28 As
shown in Fig. 3(a), the external quantum efficiency (EQE)
values of the PPVQT-C8:PCBM 1 : 3 blend, in the wavelength
range of 350–650 nm, were much higher than in other devices
with different blend ratios. A Jsc of 6.69mA cmꢀ2 for the 1 : 3
blend device was reasonable because the short circuit current
estimated from the EQE values29 of the 1 : 3 blend device was
6.0 mA cmꢀ2. When the weight composition of PCBM exceeded
60%, the shapes of the EQE spectra resembled the absorption
spectra (see Fig. 3(b)), indicating that almost all light absorption
by the donor polymer contributed to the generation of photo-
current. Therefore, the 1 : 3 blend devices appeared to have
well-mixed donor–acceptor interpenetrating three-dimensional
(3D) networks, and the carrier mobilities appeared to be well-
balanced. The low EQE and Jsc in the 1 : 9 blend device was
likely due to the low absorption properties of the blend film. On
the other hand, the relatively low Jsc and FF of the 4 : 1 blend
device may have resulted from unoptimized bulk heterojunction
morphology.
Fig. 2 shows the J–V characteristics of ITO/PEDOT/PPVQT-
C8:PCBM/LiF/Al devices under AM 1.5 illumination with an
intensity of 100 mW cmꢀ2. The PPVQT-C8:PCBM 1 : 3 (w/w)
blend device with a thickness of 100 nm showed the best
performance, with a Voc ¼ 0.67 V, a Jsc ¼ 6.69 mA cmꢀ2, a
FF ¼ 0.62, and a PCE ¼ 2.8%. The best photovoltaic properties
for devices prepared with different blend ratios and film thick-
nesses are listed in Table 1. With the exception of the 1 : 9
blend, the Voc values of all devices were similar. Although some
disagreement surrounds the origin of Voc, it is widely accepted
that the energy difference between the HOMO of the donor and
the LUMO of the acceptor determines the Voc value (0.55–
0.65 V) of the P3HT:PCBM bulk heterojunction solar cell.27
The HOMO level of PPVQT-C8 evaluated by CV and ultravi-
olet photoelectron spectroscopy was similar to that of P3HT.
Therefore, the Voc values for the PPVQT-C8:PCBM device
could be explained by the similar HOMO levels of PPVQT-C8
and P3HT. We speculated that the Voc drop for the 1 : 9 blend
Fig. 2 J–V characteristics (AM 1.5, 100 mW cmꢀ2) of copolymer:PCBM
photovoltaic (PV) cells with various blend ratios. 4 : 1 (-), 1 : 1 (,),
2 : 3 (C), 1 : 3 (B) and 1 : 9 (:).
Fig. 3 (a) The EQEs and (b) UV-Vis absorption spectra of the solar cell
devices with various blend ratios. 4 : 1 (-), 1 : 1 (,), 2 : 3 (C), 1 : 3 (B)
and 1 : 9 (:).
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The correlation between photovoltaic performance and phase
separation in the blend films was assessed by measuring the
carrier mobilities (Table 2 and Fig. 4) and morphologies (Fig. 5)
of PPVQT-C8:PCBM blend films. The charge–carrier mobilities
of blend films were measured by means of space-charge-limited
current (SCLC) measurements.30,31 The dark-current character-
istics of ITO/PEDOT:PSS/P3HT:PCBM/Pd devices (Fig. 4(a))
were measured as a function of the bias corrected by the built-in
voltage determined from the difference between the work func-
tion of the Pd and the HOMO level of PPVQT-C8. Similarly, we
fabricated electron-only devices (Al/P3HT:PCBM/Al) and
investigated their J–V characteristics in the dark. The SCLC
mobilities were estimated using the Mott–Gurney square law,32
J ¼ (9/8)3m(V2 Lꢀ3
)
where 3 is the static dielectric constant of the medium and m is the
carrier mobility. If the space charge remained in the bulk heter-
ojunction due to low charge mobilities or to an imbalance
Table 2 Hole and electron mobilties of PPVQT-C8:PCBM films eval-
uated using the Mott–Gurney law (3 ¼ 330, estimated by ellipsometry)
Fig. 5 Transmission electron microscopy (TEM) images of PPVQT-
C8:PCBM films with various blend ratios. (a) 4 : 1, (b) 1 : 1, (c) 2 : 3 and
(d) 1 : 3. The scale bar indicates 200 nm.
Blend ratio
(PPVQT-C8:PCBM)
m
h (cm2/Vs)
me (cm2/Vs)
me/mh
20% PCBM (4 : 1)
50% PCBM (1 : 1)
60% PCBM (2 : 3)
75% PCBM (1 : 3)
90% PCBM (1 : 9)
5.0 ꢄ 10ꢀ6
3.5 ꢄ 10ꢀ5
5.1 ꢄ 10ꢀ5
1.3 ꢄ 10ꢀ4
2.3 ꢄ 10ꢀ4
1.0 ꢄ 10ꢀ7
1.3 ꢄ 10ꢀ4
1.2 ꢄ 10ꢀ4
3.0 ꢄ 10ꢀ4
6.6 ꢄ 10ꢀ4
50
between the electron and hole mobilities, the photo-induced
charge carriers could not be fully extracted to produce a current
under the applied voltage, and the FF did not exceed 40%.30–34 As
shown in Fig. 5(a), bright copolymer-rich lamellar domains35,36
developed on a length scale of 20–30 nm in 4 : 1 blend films. The
presence of large lamellar copolymer domains suggested that the
lack of PCBM molecules in bulk heterojunctions resulted in low
electron mobility. As a result, the electron mobility of the 4 : 1
blend device extracted from the dark current was 50 times lower
than the hole mobility (see Table 2). Thus, low Jsc and FF in the
4 : 1 blend were explained by the effects of space charge. In
addition to the space charge effects, the low value of Jsc in the
4 : 1 blend was attributed to exciton recombination within the
large copolymer domains, the size of which exceeded the typical
exciton diffusion length (LD ¼ 10 nm) in semiconducting poly-
mers. We also found that the hole mobility was lower in the 4 : 1
blend film than in the optimal blend (1 : 3). The large domains of
either p-type copolymer or n-type PCBM produced dead zones
for transport of the opposite carriers, and the SCLC mobility was
necessarily lower than in the well-mixed case. On the other hand,
the hole and electron mobilities increased up to ꢁ10ꢀ4 cm2/Vs,
comparable to the SCLC mobility of P3HT,31 and the mobility
ratio became quite small when the composition of PCBM was
60–75% (Table 2). Moreover, the PPVQT-C8 lamellar domains
that induced exciton recombination disappeared if a well-mixed
morphology was achieved in the 1 : 3 blend films (Fig. 5(d)).
Therefore, the morphological differences indicated that the
optimal composition for constructing an interpenetrating 3D
network was 60–75% PCBM.
3.7
2.4
2.3
2.9
We investigated the dependence of solar cell performance on
thermal treatment. Fig. 6 shows the photovoltaic parameters of
thermally annealed 1 : 3 blend devices as a function of annealing
temperature. Devices annealed at low temperatures (<100 ꢂC)
Fig.
4 Dark J–V characteristics for (a) hole-only devices (ITO/
PEDOT:PSS/P3HT:PCBM/Pd) with a thickness L ¼ 100 ꢃ 10nm and (b)
electron-only devices (Al/P3HT:PCBM/Al) with a thickness L ¼ 100 ꢃ
10nm.
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(<100 ꢂC). Very poor performances of thermally annealed
devices at high temperatures could be attributed to the aggre-
gation of PCBM molecules. As shown in Fig. 7, micro-domains
of PCBM clusters37 in films annealed above 120 ꢂC could be
observed by optical microscopy. Recent reports38 that diffusion
and aggregation in PCBM were faster than in crystalline P3HT at
high temperatures (150 ꢂC) lend support to this explanation.
Increased exciton recombination and electron/hole mobility
imbalance due to microphase separation resulted in poor values
of FF and Jsc in devices annealed at high temperatures. Mean-
while, we confirmed that the PPVQT-C8 and PCBM molecules
were stable at low annealing temperatures and maintained
optimized nanoscale interpenetrating networks formed during
the spin-coating process. Self-organization of the copolymer in
homopolymeric and blend films during thermal annealing was
investigated by GI-XRD measurements. Fig. 8(a) shows the out-
of-plane diffraction patterns of as-cast and thermally annealed
homopolymeric films with thicknesses of 30 ꢃ 5nm. Intense (100)
reflections (2q ¼ 5.1ꢂ) due to the highly oriented edge-on lamellar
layer structure of PQT-1210 (d spacing, 17.3 A) were observed
ꢀ
Fig. 6 The effects of thermal annealing temperature on the photovoltaic
performance of the 1 : 3 blend solar cells. The annealing time was 5 min.
(a) FF, (b) Jsc, (c) Voc, and (d) power conversion efficiency.
after thermal annealinꢂg at 120 ꢂC. The intensities of the (100)
ꢀ
reflections at 2q ¼ 3.4 (d spacing, 25.9 A) due to the lamellar
showed insignificant changes in the photovoltaic performance. In
contrast, the performances of solar cell devices based on crys-
talline PQT-12 or P3HT were significantly affected by thermal
annealing, even at low temperatures. Solar cells based on PQT-
12, underwent macrophase separation in the active layer, and the
device performance dropped when the active films were annealed
at high temperatures (>90 ꢂC).14,15 Application of thermal
annealing dramatically changed the performance of
P3HT:PCBM solar cells even at low temperatures (<100 ꢂC).18,19
However, annealed devices based on PPVQT-C8 yielded high
efficiencies until the annealing temperature reached 120 ꢂC. The
result of this systematic thermal treatment study demonstrated
that PPVQT-C8:PCBM active layers were more thermostable
than a crystalline PQT-12 or P3HT case at low temperatures
Fig. 7 Optical microscopy images of PPVQT-C8:PCBM (1 : 3) blends
before and after thermal annealing at various temperatures. (a) As-
prepared, (b) 120 ꢂC, (c) 150 ꢂC, and (d) 180 ꢂC. The annealing time was
5 min and the scale bar indicates 50 mm.
Fig. 8 Out-of-plane GI-XRD intensities for (a) PPVQT-C8 and PQT-
12, (b) PQT-12:PCBM (1 : 3), and (c) PPVQT-C8:PCBM (1 : 3) films.
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layer structure of the intercalated PQT-12:PCBM39 dramatically
increased after thermal annealing (Fig. 8(b)). However, relatively
7 G. Dennler, M. C. Scharber and C. J. Brabec, Adv. Funct. Mater.,
2009, 21, 1.
8 U. Zhokhavets, T. Erb, H. Hoppe, G. Gobsch and N. S. Sariciftci,
Thin Solid Films, 2006, 496, 679.
9 J. S. Kim, Y. Lee, J. H. Lee, J. H. Park, J. K. Kim and K. Cho, Adv.
Mater., 2010, 22, 1355.
10 B. S. Ong, Y. Wu, P. Liu and S. Gardner, J. Am. Chem. Soc., 2004,
126, 3378.
11 D. H. Kim, Y. D. Park, Y. Jang, H. Yang, Y. H. Kim, J. I. Han,
D. G. Moon, S. Park, T. Chang, C. Chang, M. Joo, C. Y. Ryu and
K. Cho, Adv. Funct. Mater., 2005, 15, 77.
12 H. A. Becerril, N. Miyaki, M. L. Tang, R. Mondal, Y.-S. Sun,
A. C. Mayer, J. E. Parmer, M. D. McGehee and Z. Bao, J. Mater.
Chem., 2009, 19, 591.
weak diffraction peaks at 2q ¼ 6.6ꢂ (d spacing, 13.4 A) were
ꢀ
observed in the PPVQT-C8 films, and thermal treatment induced
only very small increases in the diffraction peak intensities
(Fig. 8(a)). If PPVQT-C8 was mixed with PCBM in the active
layer, the polymer rarely self-assembled in an edge-on orienta-
tion, even though blend films were annealed at 180 ꢂC (Fig. 8(c)).
These results support the assertion that the side chains and p–p
stacking of 3,3-dialkylquaterthiophene units were significantly
hindered by coupled p-divinylene phenylene units.
13 M. T. Lloyd, A. C. Mayer, S. Subramanian, D. A. Mourey,
D. J. Herman, A. V. Bapat, J. E. Anthony and G. G. Malliaras,
J. Am. Chem. Soc., 2007, 129, 9144.
4. Conclusions
14 B. C. Thompson, B. J. Kim, D. F. Kavulak, K. Sivula, C. Mauldin
ꢁ
15 G. Wantz, F. Lefevre, M. T. Dang, D. Laliberte, P. L. Brunner and
O. J. Dautel, Sol. Energy Mater. Sol. Cells, 2008, 92, 558.
16 G. Li, Y. Yao, H. Yang, V. Shrotriya, G. Yang and Y. Yang, Adv.
Funct. Mater., 2007, 17, 1636.
17 J. Peet, J. Y. Kim, N. E. Coates, W. L. Ma, D. Moses and
A. J. Heeger, Nat. Mater., 2007, 6, 497.
18 W. Ma, C. Yang, X. Gong, K. Lee and A. J. Heeger, Adv. Funct.
Mater., 2005, 15, 1617.
In conclusion, we successfully demonstrated good device
performance of organic solar cells through control of the
crystallinity in p-type conducting polymers. We reduced the
crystallinity of PQTs without significantly changing the light
absorption properties or energy band structure by introducing
a p-divinylene phenylene unit between each pair of 3,3-dia-
lkylquaterthiophene units. By optimizing the nanophase sepa-
ration in the copolymer:PCBM photoactive layers, efficient
photovoltaic performances with PCEs of 2.8% were obtained
without further post-fabrication treatments. The highest device
performances were achieved prior to PCBM aggregation at high
temperatures. The present findings should assist in highly effi-
cient organic solar cell devices with easy fabrication.
and J. M. J. Frechet, Macromolecules, 2007, 40, 7425.
ꢁ
19 M. R. Reyes, K. Kim and D. Carroll, Appl. Phys. Lett., 2005, 87,
083506.
20 J. Hou, C. Yang, J. Qiao and Y. Li, Synth. Met., 2005, 150, 297.
21 A. J. Bard and L. A. Faulkner, Electrochemical Methods-
Fundamentals and Applications, Wiley, New York, 1984.
22 E. Bundgaard and F. C. Krebs, Sol. Energy Mater. Sol. Cells, 2007,
91, 954.
23 B. S. Ong, Y. Wu, P. Liu and S. Gardner, Adv. Mater., 2005, 17, 1141.
€
24 P. J. Brown, D. S. Thomas, A. Kohler, J. S. Wilson, J. S. Kim,
C. M. Ramsdale, H. Sirringhaus and R. H. Friend, Phys. Rev. B:
Condens. Matter Mater. Phys., 2003, 67, 064203.
Acknowledgements
J. H. Park and J.-I. Park contributed equally to this work. This
work was supported by the National Research Foundation of
Korea grant (No. 2009-0093485), Manpower Development
Program for Energy and Resource (MKE, 20094020100050-11-
1-000) and Samsung Advanced Institute of Technology (SAIT).
The authors thank the Pohang Accelerator Laboratory for
providing the synchrotron radiation sources at 4C2, 8C1 and
10C1 beam lines used in this study.
€
€
25 M. Sundberg, O. Inganas, S. Stafstrom, G. Gustafsson and
€
B. Sjogren, Solid State Commun., 1989, 71, 435.
26 K. M. Coakley and M. D. McGehee, Chem. Mater., 2004, 16, 4533.
27 M. C. Scharber, D. Muhlbacher, M. Koppe, P. Denk, C. Waldauf,
A. J. Heeger and C. J. Brabec, Adv. Mater., 2006, 18, 789.
28 W. Ma, J. Y. Kim, K. Lee and A. J. Heeger, Macromol. Rapid
Commun., 2007, 28, 1776.
29 G. Dennler, M. C. Scharber and C. J. Brabec, Adv. Mater., 2006, 21,
1.
€
30 C. Melzer, E. J. Koop, V. D. Mihailetchi and P. W. M. Blom, Adv.
Funct. Mater., 2004, 14, 865.
31 V. D. Mihailetchi, H. Xie, B. de Boer, L. J. A. Koster and
P. W. M. Blom, Adv. Funct. Mater., 2006, 16, 699.
32 M. A. Lampert and P. Mark, Current Injection in Solids, Academic
Press, New York, 1970.
33 A. M. Goodman and A. Rose, J. Appl. Phys., 1971, 42, 2823.
34 V. D. Mihailetchi, J. Wildeman and P. W. M. Blom, Phys. Rev. Lett.,
2005, 94, 126602.
References and notes
1 S. E. Shaheen, C. J. Brabec, N. S. Sariciftci, F. Padinger, T. Fromherz
and J. C. Hummelen, Appl. Phys. Lett., 2001, 78, 841.
2 F. Padinger, R. S. Rittberger and N. S. Sariciftci, Adv. Funct. Mater.,
2003, 13, 85.
3 M. Svensson, F. Zhang, S. C. Veenstra, W. J. H. Verhees,
€
J. C. Hummelen, J. M. Kroon, O. Inganas and M. R. Andersson,
Adv. Mater., 2003, 15, 988.
4 M. M. Wienk, J. M. Kroon, W. J. H. Verhees, J. Knol,
J. C. Hummelen, P. A. van Hal and R. A. J. Janssen, Angew.
Chem., Int. Ed., 2003, 42, 3371.
35 W. Ma, C. Yang and A. J. Heeger, Adv. Mater., 2007, 19, 1387.
36 J. S. Moon, J. K. Lee, S. Cho, J. Byun and A. J. Heeger, Nano Lett.,
2009, 9, 230.
37 J. S. Kim, J. H. Park, J. H. Lee, J. Jo, D.-Y. Kim and K. Cho, Appl.
Phys. Lett., 2007, 91, 112111.
5 F. B. Kooistra, V. D. Mihailetchi, L. M. Popescu, D. Kronholm,
P. W. M. Blom and J. C. Hummelen, Chem. Mater., 2006, 18, 3068.
6 F. B. Kooistra, J. Knol, F. Kastenberg, L. M. Popescu,
W. J. H. Verhees, J. M. Kroon and J. C. Hummelen, Org. Lett.,
2007, 9, 551.
38 J. Jo, S. Kim, S. Na, B. Yu and D.-Y. Kim, Adv. Funct. Mater., 2009,
19, 866.
39 A. C. Mayer, M. F. Toney, S. R. Scully, J. Rivnay, C. J. Brabec,
M. Scharber, M. Koppe, M. Heeney, I. McCulloch and
M. D. McGehee, Adv. Funct. Mater., 2009, 19, 1173.
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