Synthesis and characterization of a thiazolo[5,4-d]thiazole-based copolymer for high performance polymer solar cells

Article abstract of DOI:10.1039/c0cc03036h

A highly processable, new semiconducting polymer, PCDTTz, based on alternating thiazolothiazole and carbazole units was synthesized. The new polymer exhibited a field-effect carrier mobility of up to 3.8 × 10 ^-3 cm² V^-1 s^-1 and bulk heterojunction solar cells made from PCDTTz produced a power conversion efficiency of 4.88% under AM 1.5 G (100 mW cm^-2) conditions.

Full text of DOI:10.1039/c0cc03036h

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Synthesis and characterization of a thiazolo[5,4-d]thiazole-based
copolymer for high performance polymer solar cellsw
Sang Kyu Lee,a Jung Min Cho,a Youngran Goo,a Won Suk Shin,a Jong-Cheol Lee,a
Woo-Hyung Lee,b In-Nam Kang,b Hong-Ku Shim*c and Sang-Jin Moon*a
Received 4th August 2010, Accepted 12th November 2010
DOI: 10.1039/c0cc03036h
A highly processable, new semiconducting polymer, PCDTTz,
based on alternating thiazolothiazole and carbazole units was
synthesized. The new polymer exhibited a field-effect carrier
mobility of up to 3.8 ꢀ 10ꢁ3 cm2 Vꢁ1 sꢁ1 and bulk heterojunction
solar cells made from PCDTTz produced a power conversion
efficiency of 4.88% under AM 1.5 G (100 mW cmꢁ2) conditions.
backbone and, therefore, a highly extended p-electron system
with strong p-stacking.
Here, we report the application of a new carbazole-based
polymer,
poly[N-900-hepta-decanyl-2,7-carbazole-alt-2,5-bis
(3-hexylthiophene-2-yl)thiazolo[5,4-d]thiazole] (PCDTTz), in
bulk heterojunction solar cells. Because the thiazolothiazole
unit is an electron deficient fused ring, the backbone of the
thiazolothiazole-based copolymer acts as a donor–acceptor
system. The well-characterized thiazolothiazole moieties were
introduced as a second bithiophene unit to improve the
solubility of the resultant copolymer, which facilitated polymer
characterization and photovoltaic device fabrication. The new
copolymer semiconductor indeed exhibited a band gap of 2.07 eV,
a field-effect carrier mobility of up to 3.8 ꢀ 10ꢁ3 cm2 Vꢁ1 sꢁ1, and
bulk heterojunction solar cells made from PCDTTz produced
a power conversion efficiency of 4.88% under 100 mW cmꢁ2
AM 1.5 G irradiation in ambient air. Although several widely
investigated topics in PSCs research area is the development of
small band gap (o1.74 eV) conjugated polymers to match well
with the solar spectrum,17,18 organic semiconductors with a
band gap of at least 2.0 eV are also required for efficient
harvesting of solar energy over a broad spectral range in
organic tandem solar cells.19,20
Polymer solar cells (PSCs) have attracted considerable
attention over previous decades because of their unique
advantages, which include low cost, lightweight, solution
processability, and flexibility.1–9 Despite considerable
progress in this field, power conversion efficiencies (PCE) of
PSCs must be further improved for commercialization. The
decisive parameters that determine the efficiency of PSCs are
the open-circuit voltage (VOC), short-circuit current (JSC), and
fill factor (FF). VOC is limited by the difference between the
highest occupied molecular orbital (HOMO) of the donor
and the lowest unoccupied molecular orbital (LUMO) of
the acceptor.10 Among the various conjugated polymers,
polyfluorene or polycarbazole derivatives have
a deep
HOMO level that increases the VOC of the PSCs. However,
these derivatives generally exhibit poor solubility, relatively
low hole mobilities, and low molecular weights.11,12 The JSC
and FF are directly limited by the charge carrier mobility.
Higher charge carrier mobilities enable better carrier transport
within an active layer without significant photocurrent loss
due to recombination of opposite charges. High charge
carrier mobilities also lead to high fill factors.13 Recently, the
thiazolothiazole unit has aroused interest as a semiconducting
The PCDTTz polymer structure is shown in Scheme 1.
PCDTTz was synthesized through a Suzuki coupling reaction
using Pd(OAc)2 as the catalyst and tricyclohexyl phosphine
as the ligand. The properties of PCDTTz were compared to
those of poly[N-900-hepta-decanyl-2,7-carbazole-alt-5,5-(40,70-
di-2-thienyl-20,10,30-benzothiadiazole)] (PCDTBT) reported by
Blouin et al.,21 prepared via the same synthetic procedure. The
material with
a
high carrier mobility.14–16 McCullough
and coworkers reported the field-effect mobility of novel
semiconducting polymers that incorporate thiazolothiazole
fused rings into a polythiophene backbone, which showed a
high charge carrier mobility of 0.14 cm2 Vꢁ1 sꢁ1 16
.
The use of
the thiazolothiazole-fused ring ensures a rigid coplanar
a Energy Materials Research Center, Korea Research Institute of
Chemical Technology, Daejeon, Korea. E-mail: moonsj@krict.re.kr;
Tel: +82 42 860 7517
b Department of Chemistry, The Catholic University of Korea,
Gyeonggi-do, Korea
c Department of Chemistry, Korea Advanced Institute of Science and
Technology, Daejeon, Korea. E-mail: hkshim@kaist.ac.kr;
Tel: +82 42 350 2827
w Electronic supplementary information (ESI) available: Experimental
sections, physical properties, computation methodology: the charge-
density isosurface of model compounds, electrochemical properties of
PCDTTz and PCDTBT, transfer characteristics of FET fabricated using
PCDTBT, the performance of the PSCs under various conditions, and
AFM topography of films spin coated from PCDTBT/PC71BM and
PCDTTz/PC71BM. See DOI: 10.1039/c0cc03036h
Scheme 1 Synthesis of PCDTTz and molecular structure of PCDTBT.
(a) Mg, THF, 70 1C for
5 h then DMF, room temperature;
(b) dithiooxamide, DMF, reflux for 5 h; (c) NBS, chloroform, reflux
for 3 h; (d) Pd(OAc)2, tricyclohexyl phosphine, anhydrous toluene,
tetraethyl ammonium hydroxide solution, reflux.
c
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Chem. Commun., 2011, 47, 1791–1793 1791

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weight average molecular weights (Mw) of the synthesized
polymers PCDTTz and PCDTBT were determined by gel
permeation chromatography (GPC), using
a polystyrene
standard in chloroform eluent, and were found to be 58000
(PDI = 2.9) and 59000 (PDI = 1.2), respectively. PCDTTz was
readily soluble in common organic solvents, such as toluene,
chloroform, and THF, whereas PCDTBT dissolved only in
warm chloroform or warm chlorinated benzene, but remained
dissolved when the temperature returned to room temperature.
The thermal behavior of the copolymer under a nitrogen
atmosphere was characterized by thermogravimetric analysis
(TGA) and by differential scanning calorimetry (DSC).
PCDTTz showed a high glass transition temperature (Tg; ca.
120 1C) and excellent thermal stability (5% degradation at
400 1C). Detailed description of the synthesis and character-
ization is provided in the ESI.w
Fig. 1 (a) UV-Vis absorption spectra of PCDTBT (black line) and
PCDTTz (red line) in chloroform (solid line) and in the solid state
(dashed line). (b) Energy levels for the copolymers.
The UV-Vis absorption spectra of the polymers, PCDTTz
and PCDTBT, in chloroform and in thin films are shown in
Fig. 1. In chloroform solvent, the spectrum of PCDTTz
showed a lmax of 487 nm, which was significantly blue-
shifted relative to that of PCDTBT. The spectrum of
PCDTBT displayed the two distinct absorption bands, which
were characteristic of the push–pull units along the polymer
backbone introduced by the low band gap polymers via
intramolecular charge transfer (ICT) between the donor and
the acceptor.22 In contrast, PCDTTz presented a single broad
absorption peak. To obtain further information about ICT
electronic structure of the polymers, density-functional theory
(DFT) calculation for the model compound was carried out
using the DMol 3. Fig. S3 (ESIw) shows the calculated
molecular orbitals of the model compounds. As shown in
Fig. S3 (ESIw), the HOMO and LUMO of PCDTTz were
delocalized over the p-conjugation systems, whereas in the case
of PCDTBT, the HOMO was delocalized over the polymer
backbone, and the LUMO was highly localized on the
benzothiadiazole (BT) unit. Pure PCDTBT thin films
presented two absorption bands at 396 and 577 nm and an
absorption onset at 660 nm. Pure PCDTTz showed an
absorption band with a peak at 518 nm and an absorption
onset at 600 nm, which corresponded to a band gap of 2.07 eV.
The energy level of the polymer was investigated by cyclic
voltammetry (CV). The HOMO energy levels of PCDTBT and
PCDTTz were calculated to be ꢁ5.45 and ꢁ5.31 eV, by using
the ferrocene reference value of ꢁ4.8 eV below the vacuum
level.23 The LUMO levels of PCDTBT and PCDTTz were
calculated to be ꢁ3.57 and ꢁ3.24 eV, as estimated from the
optical band gaps and HOMO energy levels (Fig. 1b).
Fig. 2 (a) Output characteristics at different gate voltages for
PCDTTz. (b) Transfer characteristics in the saturated regime for
PCDTTz. (c) Current–voltage characteristics of polymer solar cells
under AM 1.5 G conditions (100 mW cmꢁ2). (d) External quantum
efficiency of PCDTBT:PC71BM and PCDTTz:PC71BM.
10ꢁ3 cm2 Vꢁ1 sꢁ1 24
and a good fill factor.
)
allowing for efficient charge extraction
Polymeric bulk heterojunction solar cells were fabricated with
the structure of ITO/PEDOT:PSS/polymer:PC71BM/LiF/Al.
The detailed device fabrication process is described in
the ESI.w The performance of PSCs is strongly affected by
processing parameters, such as the choice of solvent, blend
ratio of polymer to PC71BM, solution concentration, and
thermal annealing.25 We investigated the performance of
the PSC materials under variety of conditions (see the
ESIw). Optimal fabrication conditions were obtained from a
14 mg mLꢁ1 o-dichlorobenzene solution, spin-coated at
600 rpm, and a polymer/PC71BM ratio of 1 : 3 (w/w). The
thicknesses of the layers were PEDOT:PSS (50 nm), the active
layer (60 nm), LiF (0.7 nm) and Al (100 nm). Fig. 2(c) shows the
current–voltage characteristics of PSC devices based on the
polymer and PC71BM blends. The optimized cells provided a
The hole mobilities of the polymers were calculated from
the transfer characteristics of field effect transistors (FETs)
fabricated with bottom-contact geometry using Au electrodes,
and the results are plotted in Fig. 2. A field-effect hole
mobility of 3.8 ꢀ 10ꢁ3 cm2 Vꢁ1 sꢁ1 and an on/off ratio of
105 were observed in PCDTTz FETs. The mobility of
PCDTBT was about 1.7 ꢀ 10ꢁ4 cm2 Vꢁ1 sꢁ1 and the on/off
ratio was found to be 105 under the same experimental
conditions (Fig. S5, ESIw). The hole mobility in pure
PCDTTz was an order of magnitude higher than that
obtained in PCDTBT. The hole mobility fell within a
desirable range for PSC materials (higher than or close to
V
OC of 0.86 V, a JSC of 9.15 mA cmꢁ2, and a FF of 0.62 under
simulated AM 1.5 G (100 mW cmꢁ2) conditions, resulting in an
estimated power conversion efficiency (Z) of 4.88%. For the
purpose of direct comparison, a PCDTBT:PC71BM device was
also fabricated, and an efficiency of 3.34% was achieved
(Table 1). Fig. 2(d) compares the external quantum efficiency
(EQE) spectra of devices fabricated using PCDTBT:PC71BM
c
1792 Chem. Commun., 2011, 47, 1791–1793
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Table 1 FET and PSC performances of PCDTTz and PCDTBT
Notes and references
ma/
JSC
/
b
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Polymer cm2 Vꢁ1 sꢁ1 Ion/Ioff VOCb/V mA cmꢁ2 FFb Zb (%)
a
PCDTTz 3.8 ꢀ 10ꢁ3
105
105
0.86
0.82
9.15
8.84
0.62 4.88
0.46 3.34
PCDTBT 1.7 ꢀ 10ꢁ4
a
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(100 mW cmꢁ2) provided an estimate for the JSC under
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with a band gap above 2.0 eV.
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polymer, PCDTTz, based on alternating carbazole and
thiazolothiazole units was synthesized. Polymer solar cells
prepared from blends of this polymer with fullerene
derivatives exhibited high solar energy conversion efficiencies
of 4.88% without special treatments. Considering the field-
effect carrier mobility and photovoltaic properties of PCDTTz,
this polymer exhibits promising potential as a candidate for
next generation solar cell materials. Moreover, this polymer
constitutes a requisite large band gap material for tandem
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titanium oxide was used as the optical spacer and hole blocking
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¨
This study was supported by a grant (M2009010025) from
the Fundamental R&D Program for Core Technology of
Materials funded by the Ministry of Knowledge Economy
(MKE) and by the Converging Research Center Program
through the Ministry of Education, Science and Technology
(2010K000970), Republic of Korea.
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c
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Chem. Commun., 2011, 47, 1791–1793 1793