Solution-processable star-shaped photovoltaic organic molecules based on triphenylamine and benzothiadiazole with longer pi-bridge

Article abstract of DOI:10.1016/j.orgel.2011.10.017

Two solution-processable star-shaped D-π-A organic molecules with triphenylamine (TPA) as donor unit, benzothiadiazole (BT) as acceptor unit and 4-hexyl-thienylenevinylene as pi conjugated bridge, S(TPA-TBTT) and S(TPA-TBTT-TPA), have been designed and sy

Full text of DOI:10.1016/j.orgel.2011.10.017

Organic Electronics 13 (2012) 166–172
Contents lists available at SciVerse ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Solution-processable star-shaped photovoltaic organic molecules based
on triphenylamine and benzothiadiazole with longer pi-bridge
⇑
Jing Zhang, Jintao Yu, Chang He , Dan Deng, Zhi-Guo Zhang, Miaojie Zhang, Zhibo Li,
Yongfang Li
⇑
Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 24 August 2011
Received in revised form 27 October 2011
Accepted 30 October 2011
Available online 15 November 2011
Two solution-processable star-shaped D–p–A organic molecules with triphenylamine (TPA)
as donor unit, benzothiadiazole (BT) as acceptor unit and 4-hexyl-thienylenevinylene as pi
conjugated bridge, S(TPA-TBTT) and S(TPA-TBTT-TPA), have been designed and synthesized
for the application as donor materials in bulk-heterojunction organic solar cells (OSCs). The
two molecules possess broader absorption from 350 to 700 nm benefitted from the longer pi-
bridge in the molecules but weaker absorbance and poorer solubility in comparison with
their corresponding organic molecules with shorter vinylene pi-bridge. The OSC based on
S(TPA-TBTT): PC70BM (1:3, w/w) exhibited Jsc of 6.41 mA/cm2, Voc of 0.75 V, FF of 39.0% and
power conversion efficiency of 1.90%, under the illumination of AM 1.5, 100 mW/cm2.
Ó 2011 Elsevier B.V. All rights reserved.
Keywords:
Organic solar cells
Star-shaped photovoltaic organic molecules
Solution-processability
1. Introduction
tron-withdrawing indanedione, dicyanovinyl (DCN), benzo-
thiadiazole (BT), 2-{2,6-bis-[2-(4-styryl)-vinyl]-pyran-4-
Bulk heterojunction organic solar cells (OSCs) based on
solution-processable molecular donor materials have
drawn much attention in recent years, owing to the com-
bined advantages of the polymer solar cells (PSCs) with
low cost solution-processing and the small molecule solar
cells benefitted from the high purity and definite molecular
weight of the organic molecules [1–24]. Among the soluble
organic photovoltaic molecular materials, the star-shaped
three-dimensional (3D) conjugated molecules have drawn
intensive interests due to their stronger absorption and bet-
ter charge-transport properties [15]. Many 3D molecules
have been synthesized by attaching several conjugated
chains onto a central node for the OSCs application, for
examples, tetrahedral conjugated molecules based on the
fixation of four conjugated thiophene chains onto a silicon
node; [4] a hybrid conjugated systems consisting of a TPA
core or a bithiophene node carrying thiophene chains [2];
a series of star-shaped molecules based on TPA derivative
with thienylenevinylene conjugated branches and elec-
ylidene}malononitrile (DCM) or perylene imide groups
[1,5,15–20,23,24]. The power conversion efficiencies (PCE)
of the OSCs based on these donor materials have been
reached in the range of 1.33–4.3%.
TPA-containing organic molecules have been widely
investigated, in particular by Shirota, for the application
as hole-transporting or electroluminescent materials [25].
Owing to its good solubility benefitted from its propeller
structure, and high hole mobility, TPA is a good choice as
the central core for a 3D conjugated system. As we all
known, benzothiadiazole (BT) is a typical electron-acceptor
unit, which can link with an electron-rich unit to form low-
bandgap D–A structured functional organic semiconductor
materials [26]. Our group have designed and synthesized a
series of D–p–A structured molecules containing TPA as
the donor unit and BT as the acceptor unit for the applica-
tion as solution-processable organic photovoltaic donor
materials. For expanding the family of the TPA-containing
star-shaped molecules, here we designed and synthesized
two new TPA- and BT-containing star-shaped D–p–A
molecules, S(TPA-TBTT) and S(TPA-TBTT-TPA) as shown
in Scheme 1, with a longer conjugated bridge, 4-hexyl-
⇑
Corresponding authors. Fax: +86 10 62559373.
E-mail addresses: hechang@iccas.ac.cn (C. He), liyf@iccas.ac.cn (Y. Li).
1566-1199/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.orgel.2011.10.017

J. Zhang et al. / Organic Electronics 13 (2012) 166–172
167
thienylvinylene, between the TPA donor and the BT accep-
tor units. The aim of adding 4-hexyl-thienylvinylene pi-
bridge is to extend the conjugation length of the branches
of the molecules for broader absorption. The two mole-
cules do show broad absorption from 350 to 700 nm. The
OSC based on S(TPA-TBTT): PC70BM (1:3, w/w) exhibited
power conversion efficiency of 1.90%, under the illumina-
tion of AM 1.5, 100 mW/cm2.
was dried over Na/benzophenoneketyl and freshly distilled
prior to use. Compound (1) was synthesized as reported in
the literature [27]. Other chemicals were common commer-
cial level and were used as received.
2.2. Synthesis
Scheme 2 shows the synthetic routes of the monomers
and the two star-shaped molecules. The detailed synthetic
processes are described in the following.
2. Experimental
2.1. Chemicals
2.2.1. 4-(5-bromo-4-hexylthiophen-2-yl)-7-(4-hexylthiophen
-2-yl)-2,1,3-benzothiadiazole (2)
N-bromosuccinimide (NBS), n-butyllithium (2.5 mol/L in
hexane), tetra-n-butylammonium bromide, sodium acetate,
palladium acetate and N,N-dimethyl formamide (DMF)
were obtained from Acros Organics. Tetrahydrofuran (THF)
To a solution of 4,7-di(4-hexyl-2-thienyl)-2,1,3-benzo-
thiadiazole (3 g, 6.4 mmol) in 100 mL anhydrous chloro-
form was added NBS (1.14 g, 6.4 mmol) in one portion at
room temperature and stirred for 12 h. The reaction
C6H13
N
S
S
S
N
C6H13
S
S
N
N
N
N
N
C6H13
C6H13
S
S
S
S
C6H13
C6H13
S(TPA-TBTT)
N
C6H13
N
S
S
S
N
C6H13
S
S
N
N
N
N
N
C6H13
C6H13
S
S
S
S
C6H13
C6H13
N
N
S(TPA-TBTT-TPA)
Scheme 1. Chemical structure of S(TPA-TBTT) and S(TPA-TBTT-TPA).

168
J. Zhang et al. / Organic Electronics 13 (2012) 166–172
mixture was washed with brine and dried over anhydrous
magnesium sulfate. The crude product was purified by col-
umn chromatography (petroleum ether (bp: 60–90 °C)) to
give compound 2 (2 g) as dark red powder. Yield: 57%. 1H
NMR (CDCl3, 400 MHz, d/ppm): 7.98 (s, 1H), 7.83 (d, 1H),
7.76 (t, 2H), 7.05 (s, 1H), 2.71–2.62 (m, 4H), 1.73–1.63
(m, 4H), 1.45–1.33 (m, 12H), 0.95 (t, 6H). MALDI-TOF MS:
548.2, Calc. for C26H31N2S3Br 547.6. 13C NMR (CDCl3,
100 MHz, d/ppm): 152.60, 152.43, 144.53, 143.10, 138.98,
138.79, 129.30, 127.99, 126.43, 125.44, 125.04, 121.84,
111.47, 31.84, 31.78, 30.78, 30.60, 29.88, 29.83, 29.19,
29.11, 22.76, 14.24. Anal. Calc. for C26H31N2S3Br: C, 56.97;
H, 5.66; N, 5.11. Found: C, 57.05; H, 5.68; N, 5.04%.
2.2.3. 4-(5-bromo-4-hexylthiophen-2-yl)-7-{5-[2-(4-diphenyl
amino-phenyl)-vinyl]-4-hexylthionphene-2-yl}-2,1,3-benzoth
iadiazole (4)
4,7-di(5-bromo-4-hexylthiophen-2-yl)-2,1,3-benzothia
diazole (1 g, 1.6 mmol), N,N-diphenyl-N-(4-vinylphenyl)
amine (432 mg, 1.6 mmol), palladium (II) acetate (10 mg),
tetra-n-butylammonium bromide (82 mg, 0.25 mmol), and
sodium acetate anhydrous (1.31 g, 16 mmol) were dissolved
and kept in degassed DMF (30 ml), under argon at 80 °C for
12 h. The mixture was poured into water (30 ml). The pre-
cipitate was filtered, washed with water and dissolved in
dichloromethane, dried over anhydrous sodium sulfate.
After evaporation of the solvent, the residue was purified
by column chromatography (petroleum/dichloromethane,
1:1) to get 812 mg compound 4. Yield: 62.1%. 1H NMR
(CDCl3, 400 MHz, d/ppm): 7.94 (s, 1H), 7.81 (d, 1H), 7.76 (d,
2H), 7.39 (d, 4H), 7.27 (t, 6H), 7.13 (d, 4H), 7.04 (t, 4H),
6.99 (d, 2H), 2.74 (t, 2H), 2.64 (t, 2H), 1.72–1.63 (m, 4H),
1.42–1.34 (m, 12H), 0.90 (d, 6H). MALDI-TOF MS: 817.6,
Calc. for C46H46N3S3Br 816.9. 13C NMR (CDCl3, 100 MHz, d/
ppm): 152.60, 152.53, 147.64, 147.56, 143.15, 142.11,
138.89, 138.84, 135.90, 131.53, 130.87, 129.46, 128.35,
127.96, 127.39, 126.11, 125.15, 125.11, 124.89, 124.74,
123.62, 123.30, 118.43, 111.53, 31.88, 31.81, 31.14, 29.91,
29.87, 29.33, 29.14, 28.77, 22.81, 22.79, 14.28. Anal. Calc.
for C46H46N3S3Br: C, 67.57; H, 5.63; N, 5.14. Found: C,
67.46; H, 5.71; N, 5.13%.
2.2.2. 4,7-di(5-bromo-4-hexylthiophen-2-yl)-2,1,3-benzothia
diazole (3)
To a solution of 4,7-di(4-hexyl-2-thienyl)-2,1,3-benzo-
thiadiazole (3 g, 6.4 mmol) in 100 mL anhydrous chloro-
form was added NBS (2.39 g, 13.5 mmol) in one portion at
room temperature and stirred for 12 h. The reaction mix-
ture was washed with brine and dried by anhydrous mag-
nesium sulfate. The crude product was purified by
column chromatography (petroleum ether (bp: 60–90 °C))
to give compound 3 (2.5 g) as dark red powder. Yield:
62%. 1H NMR (CDCl3, 400 MHz, d/ppm): 7.76 (d, 4H), 2.65
(t, 4H), 1.66 (m, 4H), 1.42–1.32 (m, 12H), 0.90 (t, 6H). MAL-
DI-TOF MS: 626.1, Calc. for C26H30N2S3Br2 626.5.
S
N
N
C6H13
Br
S
S
S
N
N
C6H13
C6H13
I
S
(2)
S
C6H13
S
N
N
(1)
C6H13
Br
Br
S
S
C6H13
(3)
S
S
N
N
N
N
C6H13
Br
Br
C6H13
Br
C6H13
II
S
S
S
S
C6H13
N
(4)
(2)
S(TPA-TBTT)
N
III
+
S(TPA-TBTT-TPA)
(4)
Scheme 2. Synthetic route of S(TPA-TBTT) and S(TPA-TBTT-TPA): I NBS, chloroform, room temperature for 12 h; II, III Pd(OAc)2, NaOAc, n-Bu4NBr, DMF, N2,
100 °C for 24 h.

J. Zhang et al. / Organic Electronics 13 (2012) 166–172
169
2.2.4. S(TPA-TBTT)
tetrabutylammonium hexafluorophosphate (Bu4NPF6) ace-
tonitrile solution. A Pt electrode coated with the sample
film was used as the working electrode, a Pt wire and Ag/
Ag+ (0.01 M AgNO3 in acetonitrile) were used as the coun-
ter and reference electrodes respectively.
Compound 2 (1 g, 1.8 mmol), tris-(4-vinylphenyl)amine
(161 mg, 0.5 mmol), palladium (II) acetate (10 mg), tetra-
n-butylammonium bromide (77 mg, 0.24 mmol), and so-
dium acetate anhydrous (1.23 g, 15 mmol) were dissolved
and kept in degassed DMF (30 ml), under argon at 100 °C
for 24 h. The mixture was poured into water (30 ml). The
precipitate was filtered, washed with water and dissolved
in dichloromethane, dried over anhydrous sodium sulfate.
After evaporation of the solvent, the residue was purified
by column chromatography (petroleum/dichloromethane,
1:1) to get 205 mg S(TPA-TBTT). Yield: 23.8%. 1H NMR
(CDCl3, 400 MHz, d/ppm): 7.98–7.95 (d, 6H), 7.85–7.82 (d,
6H), 7.44–7.42 (d, 6H), 7.20–7.09 (m, 9H), 7.04–6.98 (t,
6H), 2.76–2.67 (m, 12H), 1.72–1.69 (t, 12H), 1.42–1.34
(m, 36H), 0.90–0.86 (t, 18H). MALDI-TOF MS: 1722.0, Calc.
for C102H111N7S9 1722.6. 13C NMR (CDCl3, 100 MHz, d/
ppm): 152.76, 146.72, 144.56, 142.31, 139.23, 138.50,
136.29, 132.49, 130.70, 129.17, 128.03, 127.53, 126.06,
125.79, 125.69, 125.41, 124.51, 121.74, 118.95, 31.90,
31.89, 31.19, 30.84, 30.65, 29.87, 29.35, 29.23, 28.82,
22.81, 14.30. Anal. Calc. for C102H111N7S9: C, 69.07; H,
6.45; N, 5.53. Found: C, 70.00; H, 6.97; N, 4.99%.
2.4. Fabrication and characterization of organic solar cells
Organic solar cells (OSCs) were fabricated in the configu-
ration of the traditional sandwich structure with an ITO po-
sitive electrode and a metal negative electrode. Patterned
ITO glass with a sheet resistance of ca. 10
X
sqꢀ1 was pur-
chased from CSG Holding Co., Ltd. (China). The ITO glass
was cleaned in an ultrasonic bath of acetone and isopropa-
nol, and treated by UVO (ultraviolet ozone cleaner, Jelight
Company, USA). Then a thin layer (30 nm) of PEDOT: PSS
(poly(3,4-ethylenedioxythiophene)-poly(styrene
sulfo-
nate)) (Baytron PVP AI 4083, Germany) was spin-coated on
the ITO glass. Subsequently, the photosensitive layer was
prepared by spin-coating the blend chlorobenzene solution
of S(TPA-TBTT): PC70BM (1:3 w/w) or S(TPA-TBTT-TPA):
PC70BM (1:3 w/w) on the top of the PEDOT:PSS layer and
baked at 80 °C for 0.5 h. The thickness of the photoactive
layer was measured using an Ambios Technology XP-2 pro-
filometer. Finally, a metal electrode layer of Al (ca. 120 nm)
was vacuum evaporated on the photoactive layer under a
shadow mask in the vacuum of ca. 10ꢀ4 Pa. The current–
voltage (I–V) measurement of the devices was conducted
on a computer-controlled Keithley 236 Source Measure
Unit. A xenon lamp coupled with A.M. 1.5 solar spectrum fil-
ters was used as light source, and the optical power at the
sample was 100 mW/cm2.
2.2.5. S(TPA-TBTT-TPA)
Compound 4 (1 g, 1.2 mmol), tris-(4-vinylphenyl)amine
(119 mg, 0.37 mmol), palladium (II) acetate (15 mg), tetra-
n-butylammonium bromide (58 mg, 0.18 mmol), and so-
dium acetate anhydrous (910 mg, 11 mmol) were dis-
solved and kept in degassed DMF (30 ml), under argon at
100 °C for 24 h. The mixture was poured into water
(30 ml). The precipitate was filtered, washed with water
and dissolved in dichloromethane, dried over anhydrous
sodium sulfate. After evaporation of the solvent, the resi-
due was purified by column chromatography (petroleum/
dichloromethane, 1:1) to get 96 mg S(TPA-TBTT-TPA).
Yield: 10.3%. 1H NMR (CDCl3, 400 MHz, d/ppm): 7.95–
7.81 (d, 12H), 7.44–7.26 (m, 30H), 7.20–7.15 (d, 6H),
7.13–7.11 (12H), 7.06–6.95 (m, 24H), 2.74 (t, 12H), 1.69
(t, 12H), 1.42–1.34 (m, 36H), 0.90 (t, 18H). MALDI-TOF
MS: 2530.2, Calc. for C162H156N10S9 2530. 13C NMR (CDCl3,
100 MHz, d/ppm): 152.65, 147.50, 145.91, 145.10, 143.41,
133.40, 131.90, 131.44, 130.54, 129.75, 129.31, 127.22,
126.63, 125.22, 124.58, 123.50, 123.13, 31.73, 31.00,
3. Results and discussion
3.1. Synthesis and thermal stability of the molecules
S(TPA-TBTT) and S(TPA-TBTT-TPA) were synthesized
according to the synthetic routes as shown in Scheme 2.
The yields of S(TPA-TBTT) and S(TPA-TBTT-TPA) are 23.8%
and 10.3%, respectively. The low yields could be due to
the side-products of one armed and two armed molecules.
The two materials are soluble in common organic solvents,
such as CHCl3, THF, chlorobenzene and toluene. But S(TPA-
TBTT-TPA) exhibits poorer solubility than S(TPA-TBTT). The
thermal stability of the compounds was investigated by
thermogravimetric analysis (TGA). The temperatures with
5% weight loss are at 345 and 308 °C for S(TPA-TBTT) and
S(TPA-TBTT-TPA), respectively, as shown in Fig. 1. The sta-
bility of the materials is good enough for the application in
optoelectronic devices. S(TPA-TBTT-TPA) shows lower 5%-
weight-loss-temperature than S(TPA-TBTT). The reason
may be more content of ethylene unit in S(TPA-TBTT-TPA).
29.71, 29.18, 28.64, 22.65, 14.13. Anal.
Calc. for
C162H156N10S9: C, 76.83; H, 6.17; N, 5.53. Found: C, 76.05;
H, 6.10; N, 5.25%.
2.3. Measurements
Nuclear magnetic resonance (NMR) spectra were taken
on a Bruker DMX-400 spectrometer. MALDI-TOF spectra
were recorded on a Bruker BIFLEXIII. Elemental analyses
were carried out on a flash EA 1112 elemental analyzer.
Absorption spectra were taken on
a
Hitachi U-3010
3.2. Absorption spectra
UV–Vis spectrophotometer. The film on quartz used for
UV measurements was prepared by spin-coating with
chloroform solution. The TGA measurement was per-
formed on a Perkin–Elmer TGA-7 apparatus. The electro-
chemical cyclic voltammogram was obtained using a
Zahner IM6e electrochemical workstation in a 0.1 mol/L
Fig. 2 shows the UV–Vis absorption spectra of S(TPA-
TBTT) and S(TPA-TBTT-TPA) in chloroform solution and film
state. The optical absorption maxima (k) were summarized
in Table 1. Benefited from their D–A molecular structure
connected with longer conjugated bridge, the molecules

170
J. Zhang et al. / Organic Electronics 13 (2012) 166–172
Table 1
100
80
60
40
20
0
Solution (CHCl3) and thin-film (glass substrate) optical absorption maxima
(k), optical energy gap (Egopt), onset oxidation potential (Eooxnset ), HOMO and
LUMO energy levels of S(TPA-TBTT) and S(TPA-TBTT-TPA).
Eox
LUMOa
opt
Compounds ksolution
kfilm
(nm)
(Eg
)
film
onset
(nm)
(eV)
(V vs Ag/Ag+) (eV)
/HOMO (eV)
S(TPA-TBTT) 402, 544 414, 552 1.88
S(TPA- 410, 560 416, 576 1.76
TBTT-
TPA)
0.59/ꢀ5.30
0.43/ꢀ5.14
ꢀ3.42
ꢀ3.38
S(TPA-TBTT)
S(TPA-TBTT-TPA)
a
LUMO energy level was calculated from the HOMO level and optical
0
100 200 300 400 500
Temperature oC
energy gap: LUMO ¼ HOMO þ Egopt
Fig. 1. TGA plots of S(TPA-TBTT) and S(TPA-TBTT-TPA).
in S(TPA-TBTT) and S(TPA-TBTT-TPA) broadened the
absorption. However, the absorptivity of the two com-
pounds decreased in comparison with the corresponding
molecules with vinylene pi-bridge. The molar absorptivity
of S(TPA-TBTT) and S(TPA-TBTT-TPA) is 7.69 ꢁ 104 and
9.16 ꢁ 104 respectively, in comparison with 12.4 ꢁ 104 for
S(TPA-BT-4HT) and 9.51 ꢁ 104 for S(TPA-BT). Probably,
the longer pi-bridge between the TPA donor and BT accep-
tor units in S(TPA-TBTT) and S(TPA-TBTT-TPA) reduced the
ICT absorption band, which results in the lower absorptiv-
ity of the two compounds. The absorption spectra of the
two molecule films are red-shifted a little than their solu-
tions, which indicate some aggregation of the molecules
in the film state existed. Consistent with the absorption
spectra of the solutions, the absorption maxima of S(TPA-
TBTT) (552 nm) film is also red-shifted by 23 nm than that
(529 nm) of S(TPA-BT-4HT) film [18] and that (576 nm) of
S(TPA-TBTT-TPA) film is red-shifted by 35 nm than that
(541 nm) of S(TPA-BT) film [15]. The absorption edge of
S(TPA-TBTT) and S(TPA-TBTT-TPA) films are at ca. 659
and 706 nm, corresponding to the band gap of 1.88 and
1.76 eV, respectively.
(a) 1.6
S(TPA-BT-4HT)
1.4
S(TPA-TBTT)
S(TPA-BT)
S(TPA-TBTT-TPA)
1.2
1.0
0.8
0.6
0.4
0.2
0.0
300
400
500
600
700
800
Wavelength (nm)
(b)
1.0
0.5
0.0
3.3. Electrochemical properties
S(TPA-TBTT) film
S(TPA-TBTT-TPA) film
Electrochemical cyclic voltammetry was carried out for
S(TPA-TBTT) and S(TPA-TBTT-TPA) films on Pt electrode, in
order to measure the HOMO energy levels of them. Fig. 3
shows the cyclic voltammograms of S(TPA-TBTT) and
300
400
500
600
700
800
Wavelength (nm)
S(TPA-TBTT-TPA) films. The values of the onset oxidation
Fig. 2. (a) absorption spectra of S(TPA-BT-4HT), S(TPA-TBTT), S(TPA-BT)
ox
potential ðE
Þ and the HOMO of the compounds are listed
and S(TPA-TBTT-TPA) in chloroform solution at
a
concentration of
onset
10ꢀ5 mol/L; (b) normalized absorption spectra of S(TPA-TBTT) and
S(TPA-TBTT-TPA) films.
in Table 1. The HOMO and LUMO energy levels of S(TPA-
TBTT) were estimated to be ꢀ5.30 and ꢀ3.42 eV, respec-
tively, from the onset oxidation and optical energy gap
ox
according to the following equations: EHOMO ¼ ꢀeðE
onset
in chloroform solution exhibit strong absorption in the
wavelength range from 350 to 650 nm. The absorption in
the wavelength range of 300–450 nm corresponds to the
þ4:71ÞðeVÞ [28], ELUMO ¼ HOMO þ EogptðeVÞ. The HOMO and
LUMO energy levels were ꢀ5.14 and ꢀ3.38 eV for S(TPA-
TBTT-TPA). Compared with the HOMO and LUMO energy
levels for PC70BM, the two compounds are suitable to be
used as the photovoltaic donors.
p–
p⁄ absorption of the molecules, and the visible absorp-
tion peak between 450 and 650 nm could be assigned to
the intramolecular charge-transfer (ICT) absorption
between the TPA moiety and BT moiety. Compared with
the absorption peak at 509 nm of S(TPA-BT-4HT) [18] and
531 nm of S(TPA-BT) [15] where the pi-bridge is vinylene
unit, the absorption peak of S(TPA-TBTT) (544 nm) and
S(TPA-TBTT-TPA) (560) is red-shifted by ca. 30–40 nm,
which implies that the longer thienylenevinylene pi-bridge
3.4. AFM morphology
Morphology of the photoactive layer is very important
for the photovoltaic performance of OSCs [29,30]. We used
atomic force microscopy (AFM) to investigate the

J. Zhang et al. / Organic Electronics 13 (2012) 166–172
171
S(TPA-TBTT)
S(TPA-TBTT-TPA)
0
1
+
Potential V vs Ag/Ag
Fig. 3. Cyclic voltammogram of S(TPA-TBTT) and S(TPA-TBTT-TPA) film
on Pt electrode in an acetonitrile solution of 0.1 mol/L n-Bu4NPF6 (n-
Bu=butyl) with a scan rate of 100 mV/s.
morphology of the star-shaped molecules: PC70BM (1:3, w/
w) blend films, as shown in Fig. 4. The AFM images of the
blend films exhibit typical amorphous morphology with-
out any crystalline domains. The surface roughness (RMS)
of the blend film of S(TPA-TBTT)/PC70BM was 5.28 nm,
while RMS of the film of S(TPA-TBTT-TPA)/PC70BM was
16.4 nm. The rougher morphology of S(TPA-TBTT-TPA)
could result from its poorer solubility.
3.5. Photovoltaic properties
Fig. 4. AFM images of star molecules: PC70BM films spin-coated from the
blend solution: (a) S(TPA-TBTT)/PC70BM. (b) S(TPA-TBTT-TPA)/PC70BM.
Both of the films were spin-coated on ITO/PEDOT: PSS substrates and the
Bulk-heterojunction OSCs were fabricated by using
S(TPA-TBTT) or S(TPA-TBTT-TPA) as donor materials,
PC70BM as acceptor material and Al as the negative elec-
trode, respectively. The device structure is ITO/PEDOT:
PSS/photoactive layer (60 nm)/Al. Fig. 5 show the typical
current–voltage curves of the OSCs under the illumination
of AM 1.5, 100 mW/cm2. The photovoltaic performance
data of the OSCs, including Voc, Jsc, FF and PCE values, are
summarized in Table 2. The best photovoltaic performance
of the OSCs based on blends of S(TPA-TBTT): PC70BM (1:3,
w/w) exhibited Jsc of 6.41 mA/cm2, Voc = 0.75, FF = 39.0%,
PCE = 1.90%, under the illumination of AM 1.5, 100 mW/
cm2. The OSC based on S(TPA-TBTT-TPA): PC70BM (1:3,
w/w) shows a Jsc = 5.41 mA/cm2, a Voc = 0.71, FF = 32.0%,
PCE = 1.23%. S(TPA-TBTT-TPA) showed lower Jsc and poorer
photovoltaic performance, due probably to its poorer solu-
bility and rough morphology mentioned above. In addition,
hole mobility of S(TPA-TBTT-TPA) could be low because of
its poorer solubility, which should also result in the lower
Jsc. In comparison with the vinylene pi-bridged molecules
S(TPA-BT-4HT) and S(TPA-BT), the longer thienylenevinyl-
ene pi-bridged molecules S(TPA-TBTT) and S(TPA-TBTT-
TPA) did not exhibit better photovoltaic performance,
although they possess broader absorption. The reason
may be as fellows: firstly, the absorbance of S(TPA-TBTT)
and S(TPA-TBTT-TPA) dropped significantly than that of
S(TPA-BT-4HT) and S(TPA-BT), which will result in lower
Jsc of the OSCs; secondly, along with the increase of the size
(longer pi-bridge) for S(TPA-TBTT) and S(TPA-TBTT-TPA),
their solubility decreased which will influence the
scan size for the images is 5 ꢁ 5
lm.
0
-5
S(TPA-TBTT)
S(TPA-TBTT-TPA)
0
1
Votage(V)
Fig. 5. Current density–voltage characteristics of the OSCs based on the
blend of S(TPA-TBTT) or S(TPA-TBTT-TPA)/PC70BM (1:3, w/w) with Al as
the negative electrode under the illumination of AM 1.5, 100 mW/cm2.
film-forming properties of the materials. As we all know,
the morphology of the photoactive layer is very important
for the photovoltaic performance of OSCs. The result indi-
cated that broad absorption, strong absorbance and good
solubility all are important for the solution-processable

172
J. Zhang et al. / Organic Electronics 13 (2012) 166–172
Table 2
References
Photovoltaic performance of ITO/PEDOT: PSS/S(TPA-TBTT) or S(TPA-TBTT-
TPA): PC70BM (1:3, w/w)/Al under the illumination of AM 1.5, 100 mW/
cm2.
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This work was supported by NSFC (Nos. 50803071,
20874106, 50933003 and 21021091), Ministry of Science
and Technology of China, and the Chinese Academy of
Sciences.