Synthesis and photovoltaic properties of a low-band-gap copolymer of dithieno[3,2- b:2′,3′-d]thiophene and dithienylquinoxaline

Article abstract of DOI:10.1021/ma102722d

The synthesis and photovoltaic properties of new thiophene-bridged dithienothiophene (DTT)-containing D-π-A conjugated low-band-gap and solution-processable polymers was reported. The low-band-gap copolymer of Dithieno[3,2-b:2',3'-d]thiophene and Dithieny

Full text of DOI:10.1021/ma102722d

COMMUNICATION TO THE EDITOR
pubs.acs.org/Macromolecules
Synthesis and Photovoltaic Properties of a Low-Band-Gap Copolymer
of Dithieno[3,2-b:20,30-d]thiophene and Dithienylquinoxaline
Abasaheb V. Patil,† Woo-Hyung Lee,‡ Eunwoo Lee,† Kyuri Kim,† In-Nam Kang,*,‡ and Soo-Hyoung Lee*,†
†School of Semiconductor and Chemical Engineering, Chonbuk National University, Duckjin-dong 664-14, Jeonju 561- 756, Republic of
Korea
‡Department of Chemistry, The Catholic University, 43-1, Yeokaok2-dong, Wonmi-gu, Buchen-si, Gyeonggi-do 420-743, Republic of
Korea
S Supporting Information
b
olymer solar cells (PSCs) have attracted wide attention in
recent years due to their low-cost solution fabrication pro-
Xiaowei Zhan and co-workers10a,10d reported polymers consist-
ing of alternating perylenediimides (PDI)-dithienothiophene
(DTT) unit as an acceptor and bis(thienylvinylene)-substituted
polythiophene as a donor with PCE ∼ 1% under simulated
AM1.5, 100 mW/cm2 conditions. Moreover, this group also
reported porphyrene-based DTT-π-conjugated alternating
copolymers10b and substituted-DTT alternating with thiophene
copolymers10c for photovoltaic applications. However, the de-
vice performance of these two examples was poor. Therefore, we
rationally designed and adopted a simple D-π-A structure with
thiophenes as a shorter conjugated spacer between the electron
donor and electron acceptor for facilitating the electronic cou-
pling, tuning the wavelength and absorption capability between
the donor and acceptor, resulting in a red-shifted absorption.
In this Communication, we report the synthesis and photo-
voltaic properties of new thiophene-bridged DTT-containing
D-π-A conjugated low-band-gap and solution-processable poly-
mers, with (2,6-bis(thiophen-2-yl)-3,5-didecanyldithieno[3,2-
b:20,30-d]thiophene (T-DTT-T) and 2,3-bis((4-octyloxy)phen-
yl)-5,8-dithien-2-ylquinoxaline) (DTQ) as the donor and accep-
tor units, respectively. Introduction of thiophene units on the 2-
and 6-positions of the DTT core can reduce steric hindrance,
extend conjugation, enhance absorption, and improve the charge
transport property. However, increasing the rigidity in fused
thiophenes further is known to decrease their solubility, solution
processability, and environmental stability.7f Therefore, we in-
troduce linear long chain alkyl (decanyl) groups at the 3- and
5-positions of the DTT core and long alkoxy groups in the DTQ
unit in order to improve the solubility and processability of the
resulting copolymer. Furthermore, the design of materials con-
taining thiophene-bridged DTT derivatives has led to a novel
family of polymeric semiconductors with the potential use for
optoelectronic applications as polymer backbones with good π-
π stacking combined with good solubility and stability.
P
cess, light weight, large area, and flexible panels as well as potential
contribution to clean and renewable energy.1 Among the various
types of polymer-based bulk heterojunction (BHJ) photovoltaic
devices, the most efficient cells, with a power conversion efficiency
(PCE) of ∼3-6%, are fabricated using a blend of poly(3-
hexylthiophene) (P3HT) (electron-donor p-type) and PCBM
(electron-acceptor n-type) as the active layer.2 However, P3HT only
harvests photons with wavelengths below 650 nm, while the majority
of the energy from solar photons has a longer wavelength around 700
nm.2e Therefore, polymer materials with low band gaps are needed
to harvest the longer wavelength solar photons, particularly in the
NIR region. Various design strategies have been pursued to fulfill this
requirement. One popular approach introduced by Havinga et al.3 in
macromolecular systems is to synthesize copolymers containing
alternating electron-rich donor (D) and electron-poor acceptor
(A) monomeric units on a conjugated molecular backbone. A great
deal of attention has been paid to D-A conjugated polymers whose
optical and electronic properties could be tunable through intramo-
lecular charge transfer (ICT) from the D to the A with a better power
conversion efficiency (up to around 5-6%).4,9c
In the design of D-A conjugated polymer, a useful strategy is
to introduce a monomer unit with quinoidal character into the
conjugated system, which can efficiently reduce the band gap and
enhance π-π stacking. Fused thiophene ring systems are well-
known to stabilize the quinoidal structure.5 Recently, solar cell
devices with efficiencies greater than 7% have been demonstrated
using fused thiophene (thieno[3,4-b]thiophene) conjugated
polymers through a systematic tuning of the band gap, absorp-
tion, and relevant device parameters.1b,9 In our search for new
electron-rich monomers and taking into account these recent
results, we became interested in the dithieno[3,2-b:20,30-
d]thiophene (DTT) unit,6a,6b an important building block for a
wide variety of functional organic materials. The planarity and
S-S interaction of the fused DTT structure promotes highly
ordered π-stacking7d,8 and high hole mobility,8a which are
predictors for high charge transport in devices.8b Several groups
have reported the synthesis of DTT derivatives for applications
in organic thin film transistors (OTFTs).7 It is interesting to note
that despite all of these promising features, to the best of our
knowledge, there have been quite a few reports on the photo-
voltaic properties of DTT-containing D-A type copolymers.10
Synthetic approaches to the monomers and the polymer are shown
in Scheme 1. Starting from 3,5-didecanyldithieno[3,2-b:20,30-d]thio-
phene6b (1), the (2,6-bis(thiophen-2-yl)-3,5didecanyldithieno[3,
2-b:20,30-d]thiophene distanane (4) was synthesized in four
steps. DTQ (7) was synthesized in two steps starting form 5,
Received: November 30, 2010
Revised:
January 31, 2011
Published: February 09, 2011
r
2011 American Chemical Society
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dx.doi.org/10.1021/ma102722d Macromolecules 2011, 44, 1238–1241
|

Macromolecules
COMMUNICATION TO THE EDITOR
Scheme 1. Synthetic Route to Polymer PTDTTTQX
respectively. The absorption spectrum of a thin film in CB is almost
the same as in solution, whereas in ODCB it is slightly red-shifted
from solution. The lack of spectral change from solution to solid
state indicates the polymer to be amorphous, which is also indicated
by the lack of melting transition in the DSC measurement (Figure
S9 in Supporting Information) and the absence of crystalline peak in
XRD measurement (Figure S10 in Supporting Information). The
optical band gap calculated from the absorption edge of the polymer
film in ODCB is 1.74 eV.
Cyclic voltammetry (CV) of PTDTTTQX was carried out in a
three-electrode cell (Figure 1b). From anodic and cathodic scans,
the onset oxidation and reduction potentials of the polymer
occurred at 0.57 and -0.63 eV, respectively. The highest
occupied molecular orbital (HOMO) and lowest unoccupied
molecular orbital (LUMO) levels as well as the electrochemical
band gap (Eegc) from the oxidation and reduction potentials of the
polymers were determined to be -5.66, -3.99, and 1.67 eV,
respectively.12 The deeper HOMO level at -5.66 eV for the
polymer implies that the polymer could be more stable against
oxidation, which will enhance the device stability. These optical
and electrochemical data are summarized in Table 1.
Since hole mobility of a polymer is an important parameter
that can affect the performance of solar cells, the hole mobility of
PTDTTTQX was measured using an organic thin film transistor
(OTFT) device with a bottom gate, top contact device config-
uration built on an n-doped octadecyltrichlorosilane-treated
silicon wafer. Figure S11 (Supporting Information) shows the
drain current versus drain-source voltage characteristics and
transfer characteristics of the OTFT based on PTDTTTQX
with thermal annealing at 140 °C. The output curve shows good
saturation behavior with clear saturation currents (Figure S11a).
The device performance of PTDTTTQX exhibits a hole mobility
of 3.0 ꢀ 10-4 cm2 V-1 s-1 at room temperature (Figure S11b),
whereas the thermal annealing at 140 °C for 5 min (Figure S11c)
leads to improved performance with a mobility of 5.4 ꢀ 10-4
cm2 V-1 s-1. PTDTTTQX showed good charge-carrier mobility
owing to its rigid and coplanar structure, which is similar to other
low-band-gap conjugated polymers.9c
Figure 1. (a) UV-vis absorption spectra of PTDTTTQX in solution
and in film with different solvents (CB and ODCB). (b) CV of
PTDTTTQX in 0.1 mol/L n-Bu4NPF6 in acetonitrile solution.
Photovoltaic properties of PTDTTTQX were investigated by
fabricating the PSCs based on PTDTTTQX as the electron donor
and PC71BM as the electron acceptor in a BHJ device with a general
structure of ITO/PEDOT:PSS (40 nm)/PTDTTTQX:PC71BM
(90 nm)/LiF/Al, where PEDOT:PSS and LiF were used as buffer
layers to facilitate hole and electron extractions. PC71BM was
selected as the acceptor instead of PC61BM because it has similar
electronic properties to those of PC61BM but has increased
absorption in the visible region11a and that is indicated by UV-
vis absorption spectra of the polymer: PCBM blend (1:1 wt ratio in
ODCB) (see Figure S12 in Supporting Information).
Figure 2a shows a typical current density-voltage (J-V)
curve of the devices under light illumination of AM 1.5, 100 m W
cm-2. The PSC device based on PTDTTTQX:PC71BM with a
1:1 wt ratio in ODCB demonstrated a Voc of 0.61, Jsc of 9.03 m/A
cm2, and a FF of 0.54, leading to a power conversion efficiency
(PCE) of 2.96%. In the preliminary investigation, we used
ODCB as a solvent because a high boiling point solvent such
as ODCB (∼180 °C) has been shown to promote packing of the
polymer by offering an adequate time for drying the film which
enhances charge transport between polymer chains.4d,11b The
external quantum efficiency (EQE) of the device is shown in
Figure 2b. The device shows efficient photoconversion efficiency
from 400 to 700 nm, which covers most of the visible wavelength
8-dibromo-2,3-bis(4-octyloxyphenyl)quinoxaline (5), which was
synthesized by adapting methods reported in the literature.6c
Finally, a Stille polymerization of monomers 4 and 7 was carried out
using Pd2(dba)3/P(o-tolyl)3 as catalyst in anhydrous chlorobenzene
to afford PTDTTTQX as a dark, black solid. The detailed synthetic
procedure and compound characterization can be found in the
Supporting Information (see Figures S1-S6). PTDTTTQX shows
good solubility in chlorinated solvents, such as chloroform, chlor-
obenzene, and o-dichlorobenzene in which the polymer is most
soluble and can readily be processed from solution. The number-
average molecular weight and polydispersity index were estimated
to be 11.1 ꢀ 104 and 2.33, respectively, by GPC with polystyrene
standards in tetrahydrofuran (THF) (see Figure S7 in Supporting
Information). The TGA suggested excellent thermal stability with
an onset decomposition temperature under nitrogen of 410 °C,
while the DSC did not show any transition temperatures such as Tg
which may be due to the rigid structure of the polymer (see Figures
S8 and S9 in Supporting Information).
The absorption spectra of PTDTTTQX were taken in chlor-
obenzene (CB) as well as in o-dichlorobenzene (ODCB) solu-
tion (Figure 1a), in which the spectrum has a broad absorption
band from 400 to 700 nm with maximum absorption peaks at 445
and 585 nm due to π-π* and intrachain charge-transfer transitions,
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Macromolecules
COMMUNICATION TO THE EDITOR
Table 1. Summary of Optical and Electrochemical Properties of PTDTTTQX
λoAnbset (nm), ODCB
Eoxa/HOMO (eV)
Ereda/LUMO (eV)
Eegc (eV)
Eogpt (eV)b
solution 700
film 714
0.95/-5.66
-0.72/-3.99
1.67
onset
Ab
1.74
a Potentials determined by cyclic voltammetry in 0.10 M Bu4NPF6-CH3CN vs ferrocene/ferroceneþ. b Egopt = 1240/λ
.
’ ACKNOWLEDGMENT
This work was supported by the New & Renewable Energy of
the Korea Institute of Energy Technology Evaluation and Plan-
ning (KETEP) grant funded by the Korea Government Ministry
of Knowledge Economy (2008-N-PV08-02). This work was also
supported by the New & Renewable Energy Program of the
Korea Institute of Energy Technology Evaluation and Planning
(KETEP) Grant (No. 20103020010050) funded by the Ministry
of Knowledge Economy, Republic of Korea.
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Figure 2. (a) J-V curve for PTDTTTQX:PC71BM solar cell device at
in ODCB under illumination of an AM 1.5G, at 100 mW/cm2. (b) EQE
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In conclusion, we have designed and synthesized a new low-
band-gap, solution-processable, DTT-based D-π-A copolymer,
PTDTTTQX containing 3,5-didecanyldithieno[3,2-b:20,30-
d]thiophene bridged with thiophene as donor unit and dithie-
nylquinoxaline as acceptor unit. PTDTTTQX shows a broader
absorption spectrum and a deeper HOMO energy level at -5.66
eV. The photovoltaic properties of the PTDTTTQX:PC71BM-
based BHJ polymer solar cell demonstrated PCE of ∼3% with
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to be a promising, quite simple, and useful comonomer building
block for photoactive materials in polymer photovoltaic applica-
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copolymer can further be improved from the standpoint of
device fabrication conditions and control of phase morphology
using cosolvents and processing additives. These areas of re-
search are currently being pursued in our laboratory.
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’ ASSOCIATED CONTENT
S
Supporting Information. Details of the synthesis, GPC,
b
TGA, DSC, XRD, and AFM data, TFT and solar cell fabrication.
This material is available free of charge via the Internet at http://
pubs.acs.org.
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’ AUTHOR INFORMATION
Corresponding Author
*E-mail: shlee66@jbnu.ac.kr (S.-H.L.); inamkang@catholic.ac.kr
(I.N.K.).
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