Synthesis and photovoltaic properties of donor-acceptor oligothiophene derivatives possessing mesogenic properties

Article abstract of DOI:10.1080/00397911.2010.519845

For the purpose of developing novel photovoltaic materials and organic photovoltaic devices with good performance characteristics, 5-cyano-2,2′: 5′,2″-terthiophene (3T-CN) and 5-cyano-2,2′:5′,2″: 5″,2″ ′-tetrathiophene (4T-CN) were synthesized. The 3T-CN and 4T-CN were donor-acceptor oligothiophene derivatives possessing mesogenic properties. The photovoltaic properties of 3T-CN and 4T-CN were studied. The rigid and flexible photovoltaic devices were fabricated using 3T-CN, 4T-CN, and 3,4,9,10-perylenetertracarboxylic dianhydride (PTCDA). The results showed that the -CN group played an important role in increasing short circuit current density (I_sc) and power conversion efficiency (PCE). Both rigid device glass-ITO/4T-CN/PTCDA/Al and flexible device PET-ITO (indium tin oxides coated with polyethylene terephthalate)/4T-CN/PTCDA/Al had greater I _sc and PCE compared with rigid device glass-ITO/4T/PTCDA/Al. It was possible that the -CN group, with strong electron-withdrawing character, and mesogenic properties of 4T-CN enhanced the efficiency by promoting forward interfacial electron transfer. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397911.2010.519845

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Synthesis and Photovoltaic Properties
of Donor–Acceptor Oligothiophene
Derivatives Possessing Mesogenic
Properties
Li Guan a c , Juan Wang a , Jiale Huang a , Chuanyu Jiang a , Ming La
a , Ping Liu a & Wenji Deng b
a State Key Laboratory of Luminescent Materials and Devices,
Research Institute of Materials Science, South China University of
Technology , Guangzhou , China
b Department of Applied Physics , South China University of
Technology , Guangzhou , China
c College of Pharmacy , Guangdong Pharmaceutical University ,
Guangzhou , China
Accepted author version posted online: 30 Jun 2011.Published
online: 16 Jun 2011.
To cite this article: Li Guan , Juan Wang , Jiale Huang , Chuanyu Jiang , Ming La , Ping Liu &
Wenji Deng (2011) Synthesis and Photovoltaic Properties of Donor–Acceptor Oligothiophene
Derivatives Possessing Mesogenic Properties, Synthetic Communications: An International
Journal for Rapid Communication of Synthetic Organic Chemistry, 41:24, 3662-3670, DOI:
10.1080/00397911.2010.519845
To link to this article: http://dx.doi.org/10.1080/00397911.2010.519845
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Synthetic Communications1, 41: 3662–3670, 2011
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2010.519845
SYNTHESIS AND PHOTOVOLTAIC PROPERTIES
OF DONOR–ACCEPTOR OLIGOTHIOPHENE
DERIVATIVES POSSESSING MESOGENIC
PROPERTIES
Li Guan,1,3 Juan Wang,1 Jiale Huang,1 Chuanyu Jiang,1
Ming La,1 Ping Liu,1 and Wenji Deng2
1State Key Laboratory of Luminescent Materials and Devices, Research
Institute of Materials Science, South China University of Technology,
Guangzhou, China
2Department of Applied Physics, South China University of Technology,
Guangzhou, China
3College of Pharmacy, Guangdong Pharmaceutical University, Guangzhou,
China
GRAPHICAL ABSTRACT
Abstract For the purpose of developing novel photovoltaic materials and organic photovoltaic
devices with good performance characteristics, 5-cyano-2,20:50,200-terthiophene (3T-CN)
and 5-cyano-2,20:50,200:500,2000-tetrathiophene (4T-CN) were synthesized. The 3T-CN and
4T-CN were donor–acceptor oligothiophene derivatives possessing mesogenic properties.
The photovoltaic properties of 3T-CN and 4T-CN were studied. The rigid and flexible photo-
voltaic devices were fabricated using 3T-CN, 4T-CN, and 3,4,9,10-perylenetertracarboxylic
dianhydride (PTCDA). The results showed that the -CN group played an important role in
increasing short circuit current density (Isc) and power conversion efficiency (PCE). Both
rigid device glass-ITO/4T-CN/PTCDA/Al and flexible device PET-ITO (indium tin oxides
coated with polyethylene terephthalate)/4T-CN/PTCDA/Al had greater Isc and PCE com-
pared with rigid device glass-ITO/4T/PTCDA/Al. It was possible that the -CN group, with
strong electron-withdrawing character, and mesogenic properties of 4T-CN enhanced the
efficiency by promoting forward interfacial electron transfer.
Keywords Cyano group; mesogenic property; oligothiophene derivative; photovoltaic
property; rigid and flexible photovoltaic devices
Received December 7, 2009.
Address correspondence to Ping Liu, Research Institute of Materials Science, South China
University of Technology, Guangzhou 510640, China. E-mail: mcpliu@scut.edu.cn
3662

DONOR–ACCEPTOR TYPE OLIGOTHIOPHENE DERIVATIVES
3663
1. INTRODUCTION
Organic photovoltaic devices have attracted a great deal of attention because of
their light weight, processability, and ease of material design on the molecular level.
When Tang[1] discovered that bringing a donor and an acceptor together in one cell
could dramatically increase power conversion efficiency (PCE), a major break-
through came in the field of organic photovoltaics. The concept of heterojunction
has since been widely exploited in dye–dye, polymer–dye, polymer–polymer, and
polymer–fullerene blend photovoltaic devices. Recently, organic photovoltaic devices
employing polymer–fullerene heterojunctions have been shown to have PCE
approaching 6–7.4%,[2] obtained through variations in the processing techniques.
However, all these organic photovoltaic devices were processed on rigid glass sub-
strates. Although Maher et al. reported flexible polythiophene organic photovoltaic
devices,[3] there have been few studies of flexible oligothiophe derivative organic
photovoltaic devices. As long as organic photovoltaic devices require rigid glass sub-
strates, they will offer only limited advantages compared with silicon.
It is well known that the PCE is dependent on the short circuit current density
(Isc) and the open circuit voltage (Voc). Although encouraging progress has been made
in recent years with 6–7.4% PCE, the Isc and Voc are still not high. The search for new
materials with good performance characteristics to improve the Isc and Voc has been a
subject of importance. In our previous works,[4] we found that intermolecular hydro-
gen bonding between donor and acceptor contributed to enhancement of the Voc and
PCE. Oligothiophenes with well-defined structures have recently received a great deal
of attention not only as model compounds for conducting polythiophenes but also as
a new class of functional p-electron systems. A variety of oligothiophenes have been
synthesized,[5] and their molecular and crystal structures; self-ordering; and electro-
chemical, photophysical, optical, and electrical properties have all been studied. In
addition, their potential application in field-effect transistors, photovoltaic devices,
and organic electroluminescent devices has been investigated.
In this article, we address the aim of developing new organic photovoltaic
materials and increasing the Isc based on oligothiophene derivatives. We herein report
the synthesis of 5-cyano-2,20:50,200-terthiophene (3T-CN) and 5-cyano-2,20:50,200:500,
2000-tetrathiophene (4T-CN) (Scheme 1). Because the -CN group has strong
Scheme 1. Molecular structures of 3T-CN, 4T-CN, and PTCDA.

3664
L. GUAN ET AL.
electron-withdrawing character, 3T-CN and 4T-CN are conjugated donor–acceptor
systems. It is expected that the -CN group plays an important role in increasing short
circuit current density (Isc) and PCE. The rigid and flexible photovoltaic devices were
fabricated using 3T-CN, 4T-CN, and 3,4,9,10-perylenetertracarboxylic dianhydride
(PTCDA). The performance characteristics of devices were investigated.
EXPERIMENTAL
Materials
All solvents were purified and dried by ordinary methods. Glass-ITO (indium tin
oxides coated with glass), PET-ITO (indium tin oxides coated with polyethylene ter-
ephthalate), and N-chlorosulfonyl isocyanate (CSI) are commercially available.
2,20:50,200-Terthiophene (3T) was prepared by a Grignard coupling reaction of 2,5-
dibromothiophene with 2-bromomagnesiumthiophene in tetrahydrofuran (THF).
Likewise, 2,20:50,200:500,2000-teterthiophene (4T) was prepared by Grignard coupling
reaction of 2,50-dibromo-2,20-dithiophene with 2-bromomagnesiumthiophene in THF.
Characterization
1H NMR spectra were recorded on a Bruker Avanece 400 spectrometer in
CDCl3. Mass spectra (MS) were recorded on a Shimadzu GCMS-QP-2010. Fourier
transform–infrared (FT-IR) spectra were recorded on a Bruker Tensor 27. The
electronic absorption spectra of the thin film of materials were recorded with a
Thermospectronic (model HekOs c). Cyclic voltammetry was carried out on a
CHI-750-A voltammetric analyzer (Zhenhua Apparatus Co., Ltd., Shanghai). The
I-V characteristic of the device was measured with a Keithley electrometer (model
2400) source meter.
Synthesis
3T-CN[6] was prepared by the reaction of 3T with CSI in methylene chloride at
room temperature under a nitrogen atmosphere for 4 h, and then N,N-dimethylfor-
mamide (DMF) was added. The solution was stirred for 15 h and then hydrolyzed
with water. The aqueous solution was extracted with methylene chloride and washed
with brine and water. The solvent was removed under reduced pressure to give
3T-CN. The product was purified by silica-gel column chromatography using toluene
1
as the eluent. MS (EI): m=z 274 (Mþ); H NMR (400 MHz, CDCl3): d (ppm) ¼ 7.52
(d, 1H, ArH), 7.28 (d, 1H, ArH), 7.22 (d, 1H, ArH), 7.18 (d, 1H, ArH), 7.11 (s, 1H,
ArH), 7.10 (s, 1H, ArH), 7.04 (t, 1H, ArH); IR (KBr, cmꢀ1): 2212 (nCN).
4T-CN was synthesized using analogous procedures with 4T. MS (EI): m=z 355
(Mþ); 1H NMR (400 MHz, CDCl3): d (ppm) ¼ 7.52 (d, 1H, ArH), 7.24 (d, 1H, ArH),
7.19 (s, 1H, ArH), 7.18 (s, 1H, ArH), 7.15–7.07 (m, 4H, ArH), 7.03 (t, 1H, ArH); IR
(KBr, cmꢀ1): 2216 (nCN).
RESULTS AND DISCUSSION
Electrochemical cyclic voltammetry is performed for determining the energy
levels of the highest occupied molecular orbital (HOMO) and the lowest unoccupied

DONOR–ACCEPTOR TYPE OLIGOTHIOPHENE DERIVATIVES
3665
molecular orbital (LUMO) of conjugated compounds. The cyclic voltammograms
of 3 T-CN and 4 T-CN were done in the solution of tetra-n-butylammonium
perchlorate (0.1 mol=L) in dichloromethane, and an Ag=AgCl electrode was used
as reference electrode. Both 3 T-CN and 4 T-CN exhibited one anodic oxidation
and one cathodic reduction wave. The first onset of oxidation potential and the first
onset of reduction potential were 1.51 V and ꢀ0.62 V vresus Ag=AgCl for 3 T-CN,
and 1.12 V and ꢀ0.58 V versus Ag=AgCl for 4 T-CN. According to literature,[7]
PTCDA exhibited one anodic oxidation and one cathodic reduction wave. The first
oxidation potential and the first reduction potential were 2.29 V and ꢀ0.05 V versus
Ag=AgCl for PTCDA. HOMO and LUMO energy levels were calculated according
to the following equations:[8]
HOMO ¼ ꢀe ðEox þ 4:71ÞðeVÞ
LUMO ¼ ꢀe ðEred þ 4:71ÞðeVÞ
where the unit of potential is V versus Ag=AgCl. The HOMO energy levels of
3 T-CN, 4 T-CN, and PTCDA were ꢀ6.22 eV, ꢀ5.83 eV, and ꢀ7.00 eV, respectively.
The LUMO energy levels of 3 T-CN, 4 T-CN, and PTCDA were ꢀ4.09 eV, ꢀ4.13 eV,
and ꢀ4.66 eV, respectively. Figure 1 clearly shows that the LUMO of PTCDA was
less than the LUMO of 3 T-CN and 4 T-CN. If 3 T-CN and 4 T-CN were candidates
as electron-donor materials, and PTCDA was a candidate as the electron-acceptor
material, they would match well and might exhibit good photovoltaic properties.
For most of the conjugated compounds, the absorbance in the visible region of
the thin film of the compound is an important parameter when they are used as photo-
voltaic materials. Figure 2 showed the electronic absorption spectra of the
vacuum-deposited 3 T-CN, 4 T-CN, and PTCDA films. The 3 T-CN film absorbed
light in the wavelength range of 284–570 nm, the 4 T-CN film absorbed light in the
wavelength range of 300–630 nm, and the PTCDA film absorbed light in the wave-
length range of 400–640 nm. All of the wavelength ranges were useful for photovoltaic
Figure 1. Energy levels of 3 T-CN, 4 T-CN, and PTCDA. (Figure is provided in color online.)

3666
L. GUAN ET AL.
Figure 2. Electronic absorption spectra of the vacuum-deposited 3 T-CN, 4 T-CN, and PTCDA films.
(Figure is provided in color online.)
conversion. Therefore, it is expected that the devices consisting of 3 T-CN, 4 T-CN,
and PTCDA might respond to ultraviolet–visible light over a wide wavelength region.
To check the photovoltaic properties of 3 T-CN and 4 T-CN, we have fabricated
two rigid photovoltaic devices and two flexible photovoltaic devices, glass-ITO=
3 T-CN= PTCDA=Al, glass-ITO=4 T-CN=PTCDA=Al, PET-ITO=3 T-CN=PTCDA=
Al, and PET-ITO=4 T-CN=PTCDA=Al. The organic layers of 3 T-CN, 4 T-CN, and
PTCDA were deposited by vacuum deposition (vacuum pressure 4 ꢁ 10ꢀ3 Pa). The
thicknesses of 3 T-CN, 4 T-CN, and PTCDA films were about 80–100 nm,
80–100 nm, and 80–100 nm, respectively. A solar simulator (CMH-250, Aodite
Photoelectronic Technology Ltd., Beijing) with light intensity of 78.2 mW=cm2 was
used as light source.
Figure 3 showed the I-V curves of the two rigid devices under the illumination
of 78.2 mW=cm2 simulated sunlight. Figure 4 showed the current density–voltage
characteristics for two flexible devices under 78.2 mW=cm2 simulated sunlight
through the PET-ITO electrode. The photovoltaic properties are listed in Table 1.
The glass-ITO=3 T-CN=PTCDA=Al and glass-ITO=4 T-CN=PTCDA=Al devices
had Isc of 7.60 mA=cm2 and 9.68 mA=cm2 and PCE of 1.51% and 2.10%, respect-
ively, whereas the PET-ITO=3 T-CN=PTCDA=Al and PET-ITO=4 T-CN=PTCDA=
Al devices had Isc of 4.17 mA=cm2 and 5.42 mA=cm2 and PCE of 0.86% and 1.20%,
respectively.
Electronic absorption spectra of the vacuum-deposited 3 T-CN and 4 T-CN
films showed that the conjugated length of 4 T-CN was longer than that of
3 T-CN. An increase in oligothiophene length provides stronger light absorption
and promotes better p-p stacking or aggregation of the oligomers. Therefore, the
charge transport properties of 4 T-CN were better than that of 3 T-CN, and the Isc
and PCE of the devices glass (or PET)-ITO=4 T-CN=PTCDA=Al were greater than
that of the glass (or PET)-ITO=3 T-CN=PTCDA=Al.

DONOR–ACCEPTOR TYPE OLIGOTHIOPHENE DERIVATIVES
3667
Figure 3. I-V curves of the two rigid devices under the illumination of 78.2 mW=cm2 simulated sunlight
through the glass-ITO electrode. (Figure is provided in color online.)
The 4 T does not contain a substituted group at the a position. However,
4 T-CN contains a -CN group at the a position. The -CN group can increase the
Isc and PCE of the devices. We have determined the photovoltaic properties of the
glass-ITO=4 T=PTCDA=Al device under the same condition.[9] It was found that
the Isc and PCE of the glass-ITO=4 T=PTCDA=Al device were 0.46 mA=cm2 and
0.23%. In comparison to Isc and PCE of the glass-ITO=4 T=PTCDA=Al device, the
Isc of the Glass-ITO=4 T-CN=PTCDA=Al was increased from around 0.46 mA=cm2
to 9.68 mA=cm2, and the PCE was increased from 0.23% to 2.10%. These results
confirmed that the -CN group at the a position of oligothiophene affected the photo-
voltaic device performances. That is to say, the -CN group could increase the Isc and
PCE of the devices.
Figure 4. I-V curves of the two flexible devices under the illumination of 78.2 mW=cm2 simulated sunlight
through the PET-ITO electrode. (Figure is provided in color online.)

3668
L. GUAN ET AL.
Table 1. Photovoltaic properties of the devices under the illumination of
78.2 mW=cm2 simulated sunlight
Voc (V)
Isc (mA cmꢀ2)
F.F. (%)
g (%)
Glass-ITO=3T-CN
Glass-ITO=4T-CN
PET-ITO=3T-CN
PET-ITO=4T-CN
0.65
0.83
0.62
0.80
7.60
9.68
4.17
5.42
23.9
20.3
25.1
21.7
1.51
2.10
0.86
1.20
Oligothiophenes are crystal in nature because of their planar molecular struc-
tures. Vacuum-evaporated and spin-coated films of oligothiophenes have been reported
to be crystalline.[10,11] We observed the morphologies of films of 3 T, 3 T-CN, 4 T, and
4 T-CN by polarizing microscopy with a hot stage. It was found that 3 T and 4 T
exhibited crystalline states, whereas 3 T-CN and 4 T-CN films exhibited mesogenic
properties. The -CN group played an important role in forming mesogenic properties.
It was possible that the molecular orientation of vacuum-evaporated 3 T-CN and
4 T-CN films were better than that of vacuum-evaporated 3 T and 4 T films. The
mesogenic properties of 3 T-CN and 4 T-CN enhanced the efficiency by promoting
forward interfacial electron transfer.
Under the illumination, the dissociation of excitons diffusing to the heterojunc-
tion interface was described in Scheme 2. Because the films of 4 T-CN (or 3 T-CN)
possessing mesogenic property have better molecular orientation, the electron was
promoted from HOMO to LUMO of the donor, leaving a hole behind when the
vacuum-evaporated film of donor 4 T-CN (or 3 T-CN) was excited under the illumi-
nation of simulated sunlight. The electron and hole can dissociate. Because the
LUMO of the acceptor PTCDA was sufficiently less than the donor LUMO, the
excited electron would relax into the acceptor LUMO and in this way separate from
the hole. Charge separation was much more efficient at the donor–acceptor interface
than at the electrode interface.
Scheme 2. Exciton dissociation and charge transport at the donor–acceptor interface. (Figure is provided
in color online.)

DONOR–ACCEPTOR TYPE OLIGOTHIOPHENE DERIVATIVES
3669
It was noteworthy that the I-V curves of the two rigid devices glass-ITO=
3 T-CN= PTCDA=Al and glass-ITO=4 T-CN=PTCDA=Al exhibited slope at
negative voltage. This effect could result from various factors, such as low shunt
resistance, mesogenic properties of 3 T-CN and 4 T-CN, or photoconductivity.
Further work is required to clarify this point.
CONCLUSION
In summary, for the purpose of developing novel photovoltaic materials and
organic photovoltaic devices with good performance characteristics, we have fabri-
cated rigid and flexible photovoltaic devices using 3 T-CN, 4 T-CN, and PTCDA.
It was found that 3 T-CN and 4 T-CN were not only donor–acceptor system but also
possessed mesogenic properties. Compared with devices using unsubstituted 4 T, the
4 T-CN device had greater Isc and PCE. The -CN group at the a position of oligothio-
phene could increase the Isc and PCE of the device. It was possible that mesogenic
properties of 4 T-CN enhanced the efficiency by promoting forward interfacial
electron transfer.
ACKNOWLEDGMENTS
This work was supported by the Natural Science Foundation of China (Nos.
20674022 and 20774031). The authors also thank the Natural Science Foundation
of Guangdong (Nos. 2006A10702003 and 2009B090300025) for financial support.
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