Influence of the molecular orientation of oligothiophene derivatives in vacuum-evaporated thin films on photovoltaic properties

Article abstract of DOI:10.1016/j.dyepig.2013.09.010

To understand the relationship between molecular orientation in organic thin films and organic photovoltaic properties the donor-acceptor type oligothiophene derivatives 5,5″-dicyano-2,2′:5′,2″- terthiophene and 5,5?-dicyano-2,2′:5′,2″:5″, 2?-quaterthiophene were synthesized. Polarizing optical microscopy showed that the oligothiophene derivatives had liquid crystalline properties. The crystalline phases of the oligothiophene derivatives showed molecular orientation. Atomic force microscopy also showed that the vacuum-evaporated oligothiophene derivative thin films had molecular orientation. Using the oligothiophene derivative thin films as donor materials and a 3,4,9,10-perylenetetracarboxylic dianhydride thin film as the acceptor material organic photovoltaic devices were fabricated. The structure of these devices is glass-indium tin oxide/donor materials/acceptor material/aluminum. The molecular orientation of the vacuum-evaporated oligothiophene derivative thin films was found to improve the organic photovoltaic performance of these devices.

Full text of DOI:10.1016/j.dyepig.2013.09.010

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Dyes and Pigments 100 (2014) 158e161
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Influence of the molecular orientation of oligothiophene derivatives in
vacuum-evaporated thin films on photovoltaic properties
,
,
c
Jianhua Hu a b, Lusheng Liang a, Tuhua Chen a, Ping Liu a , Wenji Deng
*
a State Key Laboratory of Luminescent Materials and Devices, Research Institute of Materials Science, South China University of Technology, Guangzhou
510640, China
b Guangdong TGPM Automotive Industry Group Co., Ltd., Foshan 528222, China
c Department of Applied Physics, South China University of Technology, Guangzhou 510640, China
a r t i c l e i n f o
a b s t r a c t
Article history:
To understand the relationship between molecular orientation in organic thin films and organic
photovoltaic properties the donoreacceptor type oligothiophene derivatives 5,500-dicyano-2,20:50,200-
terthiophene and 5,5000-dicyano-2,20:50,200:500,2000-quaterthiophene were synthesized. Polarizing optical
microscopy showed that the oligothiophene derivatives had liquid crystalline properties. The crystalline
phases of the oligothiophene derivatives showed molecular orientation. Atomic force microscopy also
showed that the vacuum-evaporated oligothiophene derivative thin films had molecular orientation.
Using the oligothiophene derivative thin films as donor materials and a 3,4,9,10-perylenetetracarboxylic
dianhydride thin film as the acceptor material organic photovoltaic devices were fabricated. The struc-
ture of these devices is glass-indium tin oxide/donor materials/acceptor material/aluminum. The mo-
lecular orientation of the vacuum-evaporated oligothiophene derivative thin films was found to improve
the organic photovoltaic performance of these devices.
Received 26 July 2013
Received in revised form
7 September 2013
Accepted 10 September 2013
Available online 22 September 2013
Keywords:
Oligothiophene derivative
Liquid crystalline
Vacuum-evaporated thin film
Molecular orientation
Organic photovoltaic device
Organic photovoltaic performance
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
organic photovoltaic property is important to establish guidelines
for the molecular design of the donor (or acceptor) materials. In our
Organic photovoltaic devices have attracted a great deal of
attention because of their light weight, processability and the ease
of material design on the molecular level. The main focus of recent
research in the field of organic photovoltaic devices has been to
increase their power conversion efficiency (PCE). The PCE of organic
photovoltaic devices composed of polymer-fullerene hetero-
junctions has improved greatly by variations in processing tech-
niques [1e3]. However, the PCE of current organic photovoltaic
devices is still lower than the target PCE of 10e15% for
commercialization.
The PCE of organic photovoltaic devices is mainly dependent on
the short circuit current density (Isc) and the open circuit voltage
(Voc). The Isc and Voc are, in turn, mainly dependent on the organic
molecular structure and morphology of the donor (or acceptor)
materials. Therefore, an understanding of the relationship between
the morphology of the donor (or acceptor) materials and the
previous work we found that intermolecular hydrogen bonding
between the donor and acceptor contributed to an increase in the
Voc and PCE [4]. We also found that donor materials with a meso-
genic property or crystalline morphology could promote forward
interfacial electron transfer and transport, and improve organic
photovoltaic performance. On the contrary, donor materials with
amorphous morphology did not improve organic photovoltaic
performance [5,6].
In this paper, we discuss the relationship between the molecular
orientation of a vacuum-evaporated oligothiophene derivative thin
film and its organic photovoltaic properties. We report the syn-
thesis of the donoreacceptor type oligothiophene derivatives 5,500-
dicyano-2,20:50,200-terthiophene (3T-2CN) and 5,5000-dicyano-
2,20:50,200:500,2000-quaterthiophene (4T-2CN) (Scheme 1). Using the
3T-2CN and 4T-2CN thin films as donor materials and a 3,4,9,10-
perylenetetracarboxylic dianhydride (PTCDA) thin film as an
acceptor material organic photovoltaic devices were fabricated. The
relationship between the molecular orientation of the vacuum-
evaporated 3T-2CN and 4T-2CN thin films and organic photovol-
taic performance was investigated.
* Corresponding author. Tel./fax: þ86 20 87111686.
E-mail address: mcpliu@scut.edu.cn (P. Liu).
0143-7208/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2013.09.010

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2.3. Synthesis
2.3.1. Synthesis of 3T-2CN
3T-2CN was prepared by the reaction between 3T (0.5 g,
2.0 mmol) and CSI (1.5 g,10.6 mmol) in CH2Cl2 at room temperature
under a N2 atmosphere for 5 h and then DMF (50 mL) was added.
The solution was stirred for 16 h and then hydrolysed with water.
The aqueous solution was extracted with CH2Cl2 and washed with
brine and water. The solvent was removed under reduced pressure
to give 3T-2CN. The product was purified by silica gel column
chromatography using toluene as the eluent, to give a pale yellow
solid (0.24 g, yield 40%). mp 219e220 ꢀC; IR (KBr, cmꢁ1): 2213 (nCN);
Scheme 1. Molecular structures of 3T-2CN, 4T-2CN and PTCDA.
2. Experimental
1H NMR (400 MHz, CDCl3):
ArH), 7.18 (d, 2H, ArH); 13C NMR (400 MHz, CDCl3):
d
(ppm) ¼ 7.57 (d, 2H, ArH), 7.24 (s, 2H,
d
(ppm) ¼ 148,
2.1. Materials
145, 139, 128, 127, 113, 110; MS (EI): m/z 298.3, Found: 297.9; Anal.
Calcd. for C14H6N2S3: C, 56.34; H, 2.03; N, 9.39; S, 32.24; found: C,
56.61; H, 2.33; N, 9.12; S, 31.98.
All solvents were purified and dried bystandard methods. 2,20:50,200-
Terthiophene (3T) was prepared by a Grignard coupling reaction
between 2,5-dibromothiophene and 2-bromomagnesiothiophene in
THF. The 2,20:50,200:500,2000-quaterthiophene (4T) was also prepared by a
Grignard coupling reaction between 2,50-dibromo-2,20-dithiophene
and 2-bromomagnesiothiophene in THF. N-chlorosulfonylisocyanate
(CSI), N,N-dimethylformamide (DMF) and 3,4,9,10-perylenete-
tracarboxylic dianhydride (PTCDA) are commercially available.
2.3.2. Synthesis of 4T-2CN
4T-2CN was prepared by the reaction between 4T (0.5 g,
1.5 mmol) and CSI (2.0 g, 14.1 mmol) in CH2Cl2 at room temperature
under a N2 atmosphere for 5 h and then DMF (50 mL) was added.
The solution was stirred for 16 h and then hydrolyzed with water.
The aqueous solution was extracted with CH2Cl2 and washed with
brine and water. The solvent was removed under reduced pressure
to give 3T-2CN. The product was purified by silica gel column
chromatography using toluene as the eluent, to give a yellow solid
(0.20 g, yield 35%). mp 264e265 ꢀC; IR (KBr, cmꢁ1): 2216 (nCN); 1H
2.2. Characterization
1H NMR spectra were recorded on a Bruker Avance 400 spec-
trometer in CDCl3. Mass spectra (MS) were recorded on a Shimadzu
GCMS-QP-2010. FT-IR spectra were recorded on a Bruker Tensor 27.
Elementary analyses were performed by using Vario ELIII Analyzer.
Polarising optical microscopy (POM) was carried out with an OPTI-
PHOT X2 (Nikon) microscope, fitted with a TH-600PM hot stage
(Linkam) and crossed polarizers. Atomic force microscopy (AFM)
was carried out with Veeco MultiMode 8. The electronic absorption
NMR (400 MHz, CDCl3):
d
(ppm) ¼ 7.55 (d, 2H, ArH), 7.22 (d, 1H,
ArH), 7.16 (d, 2H, ArH), 7.14 (d, 2H, ArH); 13C NMR (400 MHz, CDCl3):
d
(ppm) ¼ 147, 143, 140, 138, 130, 128, 127, 113, 110; MS (EI): m/z
380.5, Found: 379.3 [M ꢁ H]þ; Anal. Calcd. for C18H8N2S4: C, 56.81;
H, 2.12; N, 7.36; S, 33.71; found: C, 56.93; H, 2.41; N, 8.97; S, 31.89.
3. Results and discussion
spectra were recorded on a Thermo spectronic (model HelOs g).
Cyclic voltammetry was carried out on a CHI-750A voltammetric
analyzer (Zhenhua Apparatus Co., Ltd, Shanghai, China). The IeV
characteristic curve of the device was measured with a Keithley
electrometer (model 2400) SourceMeter.
Unsubstituted oligothiophenes are crystalline in nature because
of their planar molecular structures. Vacuum-evaporated thin films
of unsubstituted oligothiophenes have been reported to be in the
crystalline state [7,8]. We have previously synthesized a series of
Fig. 1. POM images of the liquid crystalline phase and the crystalline phase of 3T-2CN and 4T-2CN obtained upon cooling the melt. (a): Liquid crystalline phase of 3T-2CN; (b): liquid
crystalline phase of 4T-2CN; (c): crystalline phase of 3T-2CN; (d): crystalline phase of 4T-2CN (magnificationꢂ 200).

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J. Hu et al. / Dyes and Pigments 100 (2014) 158e161
Fig. 4. Current densityevoltage characteristics of ITO/3T-2CN/PTCDA/Al (red) and ITO/
4T-2CN/PTCDA/Al (blue) under 100 mW/cm2 simulated sunlight through an ITO
electrode. (For interpretation of the references to colour in this figure legend, the
reader is referred to the web version of this article.)
Fig. 2. Electronic absorption spectra of the vacuum-deposited 3T-2CN, 4T-2CN and
PTCDA films.
substituted oligothiophene derivatives [9] and found that these
substituted oligothiophene derivatives could form a smectic A
phase. The ꢁCN group is an electron-withdrawing group and
whether the ꢁCN group affects the morphologies of 3T-2CN and 4T-
2CN is of interest. We observed the morphologies of the vacuum-
evaporated 3T-2CN and 4T-2CN thin films by POM using a hot
stage. When the isotropic liquids of 3T-2CN and 4T-2CN were
cooled, a smectic A phase was observed at approximately 218 ꢀC
and 262 ꢀC, respectively. Upon further cooling the crystalline
phases were observed at approximately 206 ꢀC and 248 ꢀC,
respectively. Of interest is that the crystalline phases of the 3T-2CN
and 4T-2CN thin films have molecular orientation. Fig. 1 shows
POM images of the liquid crystalline phase and the crystalline
phase for 3T-2CN and 4T-2CN upon cooling the melt.
For most applications of conjugated compounds such as in
photovoltaic devices and in photodetectors the conjugated com-
pound thin film absorbance is an important parameter particularly
in the visible region. Fig. 2 shows the electronic absorption spectra
of the vacuum-deposited 3T-2CN, 4T-2CN and PTCDA thin films.
The 3T-2CN thin film absorbed light in a wavelength range from
280 to 450 nm, the 4T-2CN thin film absorbed light in a wavelength
range from 250 to 500 nm and the PTCDA thin film absorbed light
in a wavelength range from 280 to 620 nm. Therefore, it is expected
that devices consisting of 3T-2CN, 4T-2CN and PTCDA might
respond to ultravioletevisible light over a wide wavelength region.
Electrochemical cyclic voltammetry was carried out to deter-
mine the energy levels of the highest occupied molecular orbital
(HOMO) and the lowest unoccupied molecular orbital (LUMO) of
the conjugated compounds. Cyclic voltammograms of 3T-2CN and
4T-2CN were obtained in a solution of tetra-n-butylammonium
perchlorate (0.1 mol/L) in CH2Cl2 and an Ag/AgCl electrode was
used as the reference electrode. Both 3T-2CN and 4T-2CN gave one
anodic oxidation and one cathodic reduction wave. The first onset
of the oxidation potentials for 3T-2CN and 4T-2CN were 1.08 and
1.06 V vs. Ag/AgCl, respectively. The first onset of the reduction
potentials for 3T-2CN and 4T-2CN were ꢁ1.31 and ꢁ0.95 V vs. Ag/
AgCl, respectively. From the literature [10] PTCDA has one anodic
oxidation and one cathodic reduction wave. The first oxidation
potential and the first reduction potential of PTCDA were at 2.29 V
and ꢁ0.05 V vs. Ag/AgCl, respectively. HOMO and LUMO energy
levels were calculated according to the following equations [11]:
HOMO ¼ ꢁe (Eox þ 4.71) (eV),
LUMO ¼ ꢁe (Ered þ 4.71) (eV),
where the unit of potential is V vs. Ag/AgCl. The HOMO energy
levels of 3T-2CN, 4T-2CN and PTCDA were ꢁ5.79, ꢁ5.77
and ꢁ7.00 eV, respectively. The LUMO energy levels of 3T-2CN, 4T-
2CN and PTCDA were ꢁ3.40, ꢁ3.76 and ꢁ4.66 eV, respectively.
Fig. 3 clearly shows that the LUMO of PTCDA is less than the LUMO
of both 3T-2CN and 4T-2CN. If the 3T-2CN and 4T-2CN were can-
didates as electron-donor materials and PTCDA was a candidate as
an electron-acceptor material they would match well and might
have photovoltaic properties.
To understand the relationship between the molecular orien-
tation of the 3T-2CN and 4T-2CN thin films and organic photovol-
taic performance, organic photovoltaic devices were fabricated. The
structure of the devices was Glass-ITO/3T-2CN (or 4T-2CN)/PTCDA/
Al. Organic layers of 3T-2CN, 4T-2CN and PTCDA were deposited by
vacuum deposition (vacuum pressure 4 ꢂ 10ꢁ3 Pa). A solar simu-
lator (CMH-250, Aodite Photoelectronic Technology Ltd., Beijing)
with a light intensity of 100 mW/cm2 was used as the light source.
Fig. 4 shows the IeV characteristic curves of the devices based
on 3T-2CN and 4T-2CN under 100 mW/cm2 simulated sunlight
illumination. The photovoltaic performances are listed in Table 1.
The Glass-ITO/3T-2CN/PTCDA/Al and Glass-ITO/4T-2CN/PTCDA/Al
Table 1
Performance characteristics of the devices under 100 mW/cm2 simulated sunlight
through an ITO electrode.
Voc/V
Isc/mA/cm2
F.F./%
ƞ/%
Glass-ITO/3T-2CN
Glass-ITO/4T-2CN
0.86
0.88
9.9
10.4
37.9
43.0
3.23
3.94
Fig. 3. Energy levels of 3T-2CN, 4T-2CN and PTCDA.

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161
Fig. 5. AFM images of vacuum-evaporated PTCDA (A), 3T-2CN (B) and 4T-2CN (C) thin films.
devices had an Isc of 9.9 mA/cm2 and 10.4 mA/cm2, a Voc of 0.86 V
and 4T-2CN thin films benefits the organic photovoltaic properties
and 0.88 V as well as a PCE of 3.23% and 3.94%, respectively.
of the fabricated devices.
4T does not contain a substituted group at the a and a
0 positions,
and it is not liquid crystalline. We have previously determined the
photovoltaic performance of the Glass-ITO/4T/PTCDA/Al device
[12]. The devices based on 4T had an Isc of 0.46 mA/cm2, a Voc of
0.90 V and a PCE of 0.23%. A comparison of the Isc and PCE pa-
rameters of the 4T devices shows that the Isc and PCE of the devices
based on 3T-2CN and 4T-2CN were superior. To clarify this result we
obtained AFM images of the vacuum-evaporated PTCDA, 3T-2CN
and 4T-2CN thin films (Fig. 5). We found that the vacuum-
evaporated 3T-2CN and 4T-2CN thin films had molecular orienta-
tion and thus the AFM result and the POM result is consistent.
Therefore, the molecular orientation of vacuum-evaporated 3T-2CN
and 4T-2CN thin films promotes exciton dissociation and charge
transport at the donoreacceptor interface and subconducting band
trap sites are eliminated. Upon vacuum deposition of the 3T-2CN
and 4T-2CN thin films, molecular orientation was apparent because
of the liquid crystalline nature of 3T-2CN and 4T-2CN. Under illu-
mination the vacuum-evaporated 3T-2CN and 4T-2CN thin films
were excited and the electrons were promoted from the HOMO to
the LUMO of the donor. Because the LUMO of the acceptor was
significantly lower than the LUMO of the donor the excited elec-
trons relaxed into the acceptor LUMO and thus separated from the
hole. Charge separation was far more efficient at the donore
acceptor interface because of the molecular orientation of the
vacuum-evaporated 3T-2CN and 4T-2CN thin films that possess
liquid crystalline properties.
Acknowledgements
This work was supported by the NSFC (nos. 20674022, 20774031
and 21074039). The authors also thank the Natural Science Foun-
dation of Guangdong (nos. 2006A10702003, 2009B090300025 and
2010A090100001) and The Ministry of Education of the People’s
Republic of China (no. 20090172110011) for financial support.
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To understand the relationship between the molecular orien-
tation of organic thin films and organic photovoltaic properties the
donoreacceptor type oligothiophene derivatives 3T-2CN and 4T-
2CN were synthesized. POM observations indicated that 3T-2CN
and 4T-2CN had liquid crystalline properties. Atomic force micro-
scopy showed that the vacuum-evaporated 3T-2CN and 4T-2CN
thin films had molecular orientation. Using the 3T-2CN and 4T-2CN
thin films as donor materials and a PTCDA thin film as an acceptor
material organic photovoltaic devices were fabricated. The results
show that the molecular orientation of vacuum-evaporated 3T-2CN
[12] Liu P, Pan WZ, Huang MS, Deng WJ. Organic photovoltaic properties based on
oligothiophenes. Chin Chem Lett 2006;17:903e6.