Solution-processable star-shaped molecules with triphenylamine core and dicyanovinyl endgroups for organic solar cells

Article abstract of DOI:10.1021/cm102077j

Two new star-shaped D-′-A molecules with triphenylamine (TPA) as core and donor unit, dicyanovinyl (DCN) as end group and acceptor unit, and 4,4′-dihexyl-2,2′-bithiophene (bT) or 4, 4′-dihexyl-2, 2′-bithiophene vinylene (bTV) as ′ bridge, S(TPA-bT-DCN) and S(TPA-bTV-DCN), were synthesized for the application as donor materials in solution-processed bulk-heterojunction organic solar cells (OSCs). The two compounds are soluble in common organic solvents, because of the three-dimensional structure of the TPA unit and the two hexyl side chains onthe bithiophene unit. S(TPA-bTV-DCN) film shows a broad absorption band from 360 to 750 nm. Absorption edge of S(TPA-bTV-DCN) film is red-shifted by ca. 78 nm than that of S(TPA-bT-DCN) film, benefitted from the vinylene bridges between TPA and bithiophene units in S(TPA-bTV-DCN). Power conversion efficiency (PCE) of the solution-processed bulk-heterojunction OSC based on a blend of S(TPA-bTV-DCN) and [6,6]-phenyl-C₇₁-butyric acid methyl ester (1:2, w/w) reached 3.0% with a short circuit current density of 7.76 mA/cm² and an open circuit voltage of 0.88 V, under the illumination of AM. 1.5, 100 mW/cm ². In comparison, PCE of the OSC based on S(TPA-bT-DCN) as donor is 1.4% under the same experimental conditions. The PCE of 3.0% for S(TPA-bTV-DCN) is among the top values for the solution-processed molecule-based OSCs reported so far.

Full text of DOI:10.1021/cm102077j

Chem. Mater. 2011, 23, 817–822 817
DOI:10.1021/cm102077j
Solution-Processable Star-Shaped Molecules with Triphenylamine Core
and Dicyanovinyl Endgroups for Organic Solar Cells†
Jing Zhang,‡,§ Dan Deng,‡,§ Chang He,*,‡ Youjun He,‡ Maojie Zhang,‡,§ Zhi-Guo Zhang,‡
Zhanjun Zhang,§ and Yongfang Li*,‡
‡Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences,
Beijing 100190, China, and Graduate University of Chinese Academy of Sciences, Beijing 100049, China
§
Received July 26, 2010. Revised Manuscript Received October 12, 2010
Two new star-shaped D-π-A molecules with triphenylamine (TPA) as core and donor unit,
dicyanovinyl (DCN) as end group and acceptor unit, and 4,40-dihexyl-2,20-bithiophene (bT) or 4,
40-dihexyl-2,20-bithiophene vinylene (bTV) as π bridge, S(TPA-bT-DCN) and S(TPA-bTV-DCN),
were synthesized for the application as donor materials in solution-processed bulk-heterojunction
organic solar cells (OSCs). The two compounds are soluble in common organic solvents, because of
the three-dimensional structure of the TPA unit and the two hexyl side chains on the bithiophene unit.
S(TPA-bTV-DCN) film shows a broad absorption band from 360 to 750 nm. Absorption edge of
S(TPA-bTV-DCN) film is red-shifted by ca. 78 nm than that of S(TPA-bT-DCN) film, benefitted
from the vinylene bridges between TPA and bithiophene units in S(TPA-bTV-DCN). Power
conversion efficiency (PCE) of the solution-processed bulk-heterojunction OSC based on a blend
of S(TPA-bTV-DCN) and [6,6]-phenyl-C71-butyric acid methyl ester (1:2, w/w) reached 3.0% with a
short circuit current density of 7.76 mA/cm2 and an open circuit voltage of 0.88 V, under the
illumination of AM.1.5, 100 mW/cm2. In comparison, PCE of the OSC based on S(TPA-bT-DCN) as
donor is 1.4% under the same experimental conditions. The PCE of 3.0% for S(TPA-bTV-DCN) is
among the top values for the solution-processed molecule-based OSCs reported so far.
1. Introduction
and capability to fabricate flexible devices. For the donor
materials in OSCs, solution-processable conjugated organic
molecules have drawn much attention in recent years2-16
because of the advantages of high purity and definite
molecular weight of the organic small molecules in compar-
ison with the lower purity and molecular weight distribution
of the polymers. Among the soluble organic photovoltaic
molecules, the donor-π conjugated bridge-acceptor
(D-π-A) structured organic compounds have attracted
considerable research interests for their broad absorption in
visible region resulted from the intramolecular charge trans-
fer (ICT) and for their lower HOMO levels tuned by the
acceptor units.6,8-16 For example, Nguyen et al.10 synthe-
sized a D-A organic molecule containing benzofunan-
thiophene as donor unit and diketopyrrolopyrrole (DPP)
Bulk heterojunction organic solar cells (OSCs)1 are
composed of a photoactive blend layer of a conjugated
polymer or organic molecule donor and a soluble fullerene
derivative acceptor sandwiched between an indium-tin
oxide (ITO) positive electrode and a low workfunction metal
negative electrode. In comparison with the traditional solar
cells based on inorganic semiconductors, the OSCs possess
the advantages of easy fabrication, low cost, light weight,
† Accepted as part of the “Special Issue on π-Functional Materials”.
*Corresponding author. E-mail: hechang@iccas.ac.cn (C.H.); liyf@iccas.
ac.cn (Y.L.). Fax: 86-10-62559373.
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2010 American Chemical Society
Published on Web 10/26/2010
pubs.acs.org/cm

818 Chem. Mater., Vol. 23, No. 3, 2011
Zhang et al.
as acceptor unit. The molecule film possesses a broad
absorption covering from 380 to 700 nm and lower HOMO
level of -5.2 eV. The power conversion efficiency (PCE) of
the bulk heterojunction OSC (device area of 17 mm2)9b
based on the molecule as donor and [6,6]-phenyl-C71-butyric
acid methyl ester (PC70BM) as acceptor reached 4.4% with a
high open circuit voltage (Voc) of 0.92 V,10 which indicates
that the organic D-π-A molecules are promising as high-
efficiency photovoltaic materials for the application as donor
in the bulk heterojunction OSCs.
Scheme 1. StructuresofS(TPA-bT-DCN) and S(TPA-bTV-DCN)
The triphenylamine (TPA) unit possesses a three-
dimensional propeller structure that makes the TPA-
containing molecules exhibit good solubility. In addition,
the TPA-containing molecules have high hole mobility and
are widely used as hole-transporting materials in organic
light-emitting diodes.17 Many D-π-A structured mole-
cules containing TPA as donor unit have been designed
and synthesized for the applications as solution-processable
organic photovoltaic donor materials, in considering its
good solubility and high hole mobility.3,8,11-16 At present,
benzothiadiazole (BT)15,16 and 2-{2,6-bis-[2-(4-styryl)-
vinyl]-pyran-4-ylidene}malononitrile (DCM)8,11,13 have
been used as acceptor units in the solution-processable
TPA-containing D-π-A molecules. The highest PCE
reached 2.39% for the bulk heterojunction OSC based on
a star-shaped organic molecule with TPA as core and
benzothiodiazole -4-hexylthiophene as arms.15
Dicyanovinyl (DCN) is a well-known acceptor group in
the organic optoelectronic materials. Roncali et al.2
synthesized the star-shaped D-π-A molecules with
TPA as core and donor unit, DCN as end group and
acceptor unit, and thiophene as the π-bridge. The mole-
cules showed promising photovoltaic performance with a
PCE of 1.17% for the bilayer OSC based on the molecules
as donor and C60 as acceptor.2 Wong et al.18 reported the
linear conjugated D-π-A organic molecules with TPA-
fluorene as donor unit, DCN as acceptor unit and oligo-
thiophene as π-bridge. The molecule films demonstrate
broad and strong absorption in the visible region from
360 to 700 nm. The bilayer OSC with the molecules as
donor and C60 as acceptor displayed a PCE as high as
2.67%.18 The results indicate that the D-π-A organic
molecules with TPA as donor unit and DCN as acceptor
unit are promising photovoltaic materials. However, poor
solubility of the molecules containing DCN hampers the
fabrication of bulk heterojunction OSCs, and the bilayer
OSCs mentioned above were fabricated by vacuum-
evaporation.1,18 Obviously, it should be interesting if we can
obtain the solution-processable D-π-A organic molecules
containing TPA and DCN units for the application as donor
materials in bulk heterojunction OSCs.
π-bridge, S(TPA-bT-DCN) and S(TPA-bTV-DCN), as
shown in Scheme 1. The two hexyl side chains on the
π-bridge of bithiophene in the molecules are used to improve
solubility and film forming properties of the molecules. The
two compounds do show good solubility in common organic
solvents as expected, and possess broad visible absorption
band. Bulk heterojunction OSCs were fabricated by spin-
coating the blend solution of the compounds (as donor) and
PC70BM (as acceptor). The photovoltaic performance of the
OSC device based on a blend of S(TPA-bTV-DCN) and
PC70BM (1:2, w/w) exhibited a PCE of 3.0% with a short
circuit current density (Jsc) of 7.76 mA/cm2, a Voc of 0.88 V,
and a fill factor (FF) of 43.9%, under the illumination of
AM.1.5, 100 mW/cm2. The PCE of 3.0% is the highest value
for the OSCs based on the solution-processable TPA-contain-
ing organic molecules, and it is among the top values in the
bulk heterojunction molecule-based OSCs reported so far.
2. Experimental Section
In this work, we designed and synthesized two new star-
shaped D-π-A molecules with TPA as core and donor unit,
DCN as end group and acceptor unit, and 4,40-Dihexyl-2,
20-bithiophene or 4,40-Dihexyl-2,20-bithiophene vinylene as
2.1. Materials. All reagents were obtained from Aldrich,
Acros, or TCI Chemical Co, and used as received. PC70BM
was bought from ADS (American Dye Source Inc.) Tetra-
hydrofuran (THF) was dried over Na/benzophenoneketyl and
freshly distilled prior to use. CHCl3 and triethylamine were
dried over a molecular sieve.
(17) Shirota, Y. J. Mater. Chem. 2005, 15, 75–93.
(18) Xia, P. F.; Feng, X. J.; Lu, J. P.; Tsang, S. W.; Movileanu, R.; Tao,
Y.; Wong, M. S. Adv. Mater. 2008, 20, 4810–4815.
2.2. Synthesis. The synthetic routes of the compounds are
shown in Scheme 2. The detailed synthesis processes are as follows.

Article
Chem. Mater., Vol. 23, No. 3, 2011 819
Scheme 2. Synthetic Routes of S(TPA-bT-DCN) and S(TPA-bTV-DCN)a
a (I) AgNO3, KF, DMSO, dark, 60 °C for 24 h; (II) n-BuLi, DMF, THF, -78 °C; (III) Pd(dac)2, K3PO4 (2 mol/L), toluene, 120 °C reflux for 24 h; (IV)
Pd(OAc)2, NaOAc, n-Bu4NBr, DMF, under N2, 100 °C for 24 h; (V, VI) malononitrile, CHCl3, triethylamine, under N2, 75 °C reflux for 24 h.
5-Bromo-50-formyl-4,40-dihexyl-2,20-bithiophene (4). 5,50-Dibromo-
4,40-dihexyl- 2,20-bithiophene (3) (13 g, 26.42 mmol) was dis-
solved in 200 mL THF in a well-dried flask under the protection
of N2 flow. N-Butyl lithium (9.2 mL, 26.96 mmol, 2.93 M in
hexane) was added dropwise by syringe at -78 °C. After the
addition, the reaction mixture was stirred at -78 °C for 2 h and
DMF (2 mL, 26.42 mmol) was added dropwise, then the mixture
was warmed to room temperature. After stirring for 12 h, the
mixture was poured into water and extracted with dichloro-
methane. The organic layer was washed with brine and dried
over Na2SO4. After solvent removal, the residue was purified by
column chromatography on silica gel (petroleum/CH2Cl2, 15:1)
to give 6.2 g of compound 4 with a yield of 53.1%. GC/MS: 441
(Mþ). 1H NMR (400 MHz, CDCl3): δ ppm 9.98 (s, 1H), 7.03 (s,
1H), 7.02 (s, 1H), 2.07 (t, 2H), 2.26 (t, 2H), 1.64-1.70 (m, 2H),
1.55-1.60 (m, 2H), 1.32-1.37 (m, 12H), 0.89 (t, 6H).
Tris{4-[5-(5-formyl-4-hexyl-2-thienyl)-3-hexyl-2-thienyl]phenyl}-
amine (5). A mixture of 5-bromo-50-formyl-2,20-bithiophene (4)
(2.68 g, 6.06 mmol), K3PO4 (24 mL, 2 M in aqueous solution),
degassed toluene (40 mL), Pd(dac)3 (21 mg), [(CH3)3C]PHBF4
(27 mg, and Monomer 1 (1.18 g, 1.89 mmol) were stirred under the
protection of N2 flow. The solution was kept at 120 °C for 24 h. The
mixture was poured into water, extracted with dichloromethane,
and washed by water, dried over Na2SO4. After evaporation of the
solvent, the residue was purified by column chromatography on
silica gel (petroleum/CH2Cl2, 1:1) to produce 1.09 g of compound 5
with a yield of 43.5%. MALDI-TOF MS: 1325.7, calcd for
C81H99NO3S6 1325.6. 1H NMR (400 MHz, CDCl3): δ ppm 10.02
(s, 3H), 7.42 (d, 3H), 7.41 (d, 3H), 7.29 (d, 3H), 7.20 (d, 3H), 7.08 (s,
3H), 7.253 (s, 3H), 2.95 (t, 6H), 2.70 (t, 6H), 1.71 (m, 12H), 1.31 (m,
36H), 0.9 (t, 18H).
Tris{4-[2-(5-(5-formyl-4,40-dihexyl-2-thienyl)-3-hexyl-2-thienyl)-
vinyl]phenyl}amine (6). A mixture of Monomer 2 (730.5 mg,
2.26 mmol), Pd(OAc)2 (31 mg), NaOAc (5.56 g, 67.84 mmol),
5-bromo-50-formyl-2,20-bithiophene (4) (3 g, 6.79 mmol), and
n-Bu4NBr (1.05 g, 3.26 mmol) was dissolved in degassed DMF
(75 mL). The solution was kept under a nitrogen atmosphere at
100 °C for 24 h. The mixture was poured into water. The precipitate
was filtered, washed with water and dissolved in dichloromethane,
and dried over Na2SO4. After evaporation of the solvent, the residue
was purified by column chromatography on silica gel (petroleum/
CH2Cl2, 1:1) to produce 2.10 g of Compound 6 with a yield of
66.2%. MALDI-TOF MS: 1403.6, calcd for C97H105NO3S6 1404.6.
1H NMR (400 MHz, CDCl3): δ ppm 9.98 (s, 3H), 7.38-7.40 (d,
6H), 7.01-7.12 (m, 12H), 6.90 (d, 6H), 2.92 (t, 6H), 2.66 (t, 6H),
1.62-1.70 (m, 12H), 1.26-1.32 (m, 36H), 0.89 (t, 18H).

820 Chem. Mater., Vol. 23, No. 3, 2011
Zhang et al.
Tris{4-[5-(5-dicyanomethylidenemethyl-4-hexyl-2-thienyl)-3-hexyl-
2-thienyl]phenyl}amine (S(TPA-bT-DCN)). To a solution of
Tris{4-[5-(5-formyl-4-hexyl-2-thienyl)-3-hexyl-2-thienyl]phenyl}-
amine (5) (1 g, 0.75 mmol) in 50 mL of chloroform were added
malonodinitrile (149 mg, 2.26 mmol) and 0.5 mL triethylamine,
and the mixture was refluxed at 75 °C for 24 h under nitrogen.
After the solution was cooled to room temperature, dichloro-
methane was added, the solution was washed with water and
dried over MgSO4. After solvent removal, the residue was
purified by column chromatography on silica gel (dichloro-
methane) to give 0.80 g S(TPA-bT-DCN) (yield: 72%) of black
red solid. MALDI-TOF MS: 1470.60, calcd for C91H103N7S6
1469.63. 1H NMR (400 MHz, CDCl3): δ ppm 7.83 (s, 3H),
7.38-7.40 (d, 6H), 7.33 (s, 3H), 7.22-7.25 (d, 6H), 7.10 (s, 3H),
2.74(t, 6H), 2.68(t, 6H), 1.64(m, 12H), 1.31(m, 36H), 0.90(t, 18H).
Tris{4-[2-(5-(5-dicyanomethylidenemethyl-4-hexyl-2-thienyl)-
3-hexyl-2-thienyl)-vinyl]phenyl}amine (S(TPA-bTV-DCN)). The
synthesis procedure for S(TPA-bTV-DCN) is similar to that of
S(TPA-bT-DCN) but using precursor 6 instead of 5. To a
solution of 6 (1.4 g, 1 mmol) in 50 mL of chloroform, malonodini-
trile (0.2 g, 3.03 mmol), and 0.5 mL of triethylamine were added, and
the mixture was refluxed at 75 °C for 24 h under nitrogen. After the
solution was cooled to room temperature, dichloromethane was
added, and the solution was washed with water and dried over
MgSO4. After solvent removal, the residue was purified by
column chromatography on silica gel (dichloromethane) to
give 1.09 g S(TPA-bTV-DCN) (yield: 70%) of a black purple
solid. MALDI-TOF MS: 1548.70, calcd for C97H109N7S6
Figure 1. TGA plots of S(TPA-bT-DCN) and S(TPA-bTV-DCN).
1
1547.68. H NMR (400 MHz, CDCl3): δ ppm 7.80 (s, 3H),
7.41 (d, 6H), 7.23 (s, 3H), 7.08 (m, 12H), 6.91 (d, 6H), 2.73 (t,
6H), 2.67 (t, 6H), 1.64 (m, 12H), 1.30 (m, 36H), 0.91 (t, 18H).
2.3. Measurements. Nuclear magnetic resonance (NMR) spec-
tra were taken on a Bruker DMX-400 spectrometer. MALDI-TOF
mass spectra were recorded on a Bruker BIFLEXIII. Absorption
spectra were taken on a Hitachi U-3010 UV-vis spectropho-
tometer. The film on quartz used for UV measurements was
prepared by spin-coating a 1% chloroform solution of the samples.
The thermogravimetric analysis (TGA) measurement was per-
formed on a Perkin-Elmer TGA-7 apparatus. The electrochemical
cyclic voltammetry was performed using a Zahner IM6e electro-
chemical workstation in a 0.1 mol/L tetrabutylammonium hexa-
fluorophosphate (Bu4NPF6) acetonitrile solution. A Pt electrode
coated with the sample film was used as the working electrode; a Pt
wire and Ag/Agþ (0.01 M AgNO3 in acetonitrile) were used as the
counter and reference electrodes, respectively.
Figure 2. UV-vis absorption spectra of S(TPA-bT-DCN) and S(TPA-
bTV-DCN) in chloroform solution and in film state.
photosensitive layer was ca. 80 nm measured by Ambios Tech-
nology XP-2 profilometer. A bilayer cathode consisted of Ca
(∼10 nm) capped with Al (∼100 nm) was thermal-evaporated
under a shadow mask in a base pressure of ca. 1 ꢀ 10-4 Pa. The
device active area of the PSCs is 4 mm2. The current-voltage
(I-V) measurement of the devices was conducted on a compu-
ter-controlled Keithley 236 Source Measure Unit. Device char-
acterization was done in glovebox under simulated AM1.5G
irradiation (100 mW cm-2) using a Xenon-lamp-based solar
simulator (from Newport Co., Ltd.).
3. Results and Discussion
2.4. Fabrication and Characterization of Organic Solar Cells.
The organic solar cells were fabricated in a sandwich geometry
consisting of a bottom indium tin oxide (ITO) glass positive
electrode, a photosensitive layer composed of a blend of the
organic molecule as donor and PC70BM as acceptor, and a top
Ca/Al negative electrode. Patterned ITO glass with a sheet
resistance of ca. 10 Ω sq-1 was purchased from CSG HOLD-
ING Co., LTD (China). The ITO glass was cleaned in an
ultrasonic bath of acetone and isopropanol, and then treated
in an ultraviolet-ozone chamber for 30 min (Ultraviolet Ozone
Cleaner, Jelight Company, USA). Then a thin layer (30 nm) of
PEDOT: PSS (poly(3,4-ethylenedioxythiophene)-poly(styrene
sulfonate)) (Baytron PVP A1 4083, Germany) was spin-coated
at 4000 rpm on the ITO glass and baked at 150 °C for 30 min in
the air. Subsequently, the photosensitive active layer was pre-
pared in a drybox by spin-coating the blend chlorobenzene
solution of S(TPA-bT-DCN) or S(TPA-bTV-DCN) and
PC70BM (1:2 w/w, 18 mg/mL) on the top of the PEDOT: PSS
layer with a spin speed of 2000 rpm. The thickness of the
3.1. Synthesis and Thermal Stability. S(TPA-bT-DCN)
and S(TPA-bTV-DCN) were synthesized by the synthetic
routes shown in Scheme 2. Monomers 1, 2 and 3 were
synthesized according to literatures.14,19 Compound 5
was prepared from Monomers 1 and 4 through Suzuki
reaction with a yield of 39.8%. Compound 6 was synthe-
sized from Monomer 2 and 4 by Heck reaction with a
yield of 62.9%. Finally, S(TPA-bT-DCN) and S(TPA-
bTV-DCN) were obtained by Knoevenagel condensation
of malononitrile with the appropriate aldehyde. Both
S(TPA-bT-DCN) and S(TPA-bTV-DCN) are soluble in
common organic solvents, such as CHCl3, THF, chloro-
benzene, and toluene.
(19) Mori, A.; Sekiguchi, A.; Masui, K.; Shimada, T.; Horie, M.;
Osakada, K.; Kawamoto, M.; Ikeda, T. J. Am. Chem. Soc. 2003,
125, 1700–1701.

Article
Chem. Mater., Vol. 23, No. 3, 2011 821
Table 1. Optical Absorption and Electrochemical Properties and Electronic Energy Levels of S(TPA-bT-DCN) and S(TPA-bTV-DCN)
compds
λsolution (nm)
λfilm (nm)
(Eg
film (eV)
Eredonset (Va) /LUMO(eV)
Eoxonset(Va) /HOMO(eV)
Egcv (eV)
opt
)
S(TPA-bT-DCN)
S(TPA-bTV-DCN)
296, 342, 504
298, 357, 518
312, 402, 563
320, 421, 585
1.83
1.65
-1.37/-3.34
-1.29/-3.42
0.51/-5.22
0.32/-5.03
1.88
1.61
a The reference electrode is Ag/Agþ
The thermal stability of S(TPA-bT-DCN) and S(TPA-
bTV-DCN) was investigated by TGA analysis. The tem-
peratures with 5% weight loss are 358 °C for S(TPA-bT-
DCN) and 347 °C for S(TPA-bTV-DCN), as shown in
Figure 1. The stability of S(TPA-bT-DCN) and S(TPA-
bTV-DCN)) is good enough for the applications in
optoelectronic devices.
3.2. Optical Properties. Figure 2 shows the UV-vis
absorption spectra of S(TPA-bT-DCN) and S(TPA-
bTV-DCN) in solutions and solid films. The absorption
peak wavelengths (λ) of the two compounds were sum-
marized in Table 1. The absorption spectrum of S(TPA-
bT-DCN) solution shows three absorption peaks at 296,
342, and 504 nm, respectively, covering a broad wave-
length range from 260 to 600 nm, which is benefited from
its D-π-A molecular structure. The two UV absorption
peaks at 296 and 342 nm correspond to the π-π*
absorption of S(TPA-bT-DCN), and the strong visible
absorption peak at ca. 504 nm is attributed to the
intramolecular charge transfer (ICT) transition between
the TPA-bithiophene donor unit and DCN acceptor unit.
The spectrum of S(TPA-bT-DCN) with two UV peaks
and a visible peak agrees with that of a similar molecule
(molecule 5) in ref 2. The absorption spectrum of the
S(TPA-bT-DCN) film red-shifted in comparison with its
solution, which results from the aggregation of the mole-
cules in the solid film state. The three absorption peaks of
the S(TPA-bT-DCN) film red-shifted to 312, 402, and
563 nm, respectively. The relatively lower intensity of the
π-π* transition and the higher intensity of the ICT band
in the absorption spectra agree with that reported by
Roncali et al.2 S(TPA-bTV-DCN) shows similar and red-
shifted absorption spectra to that of S(TPA-bT-DCN).
The three absorption peaks of S(TPA-bTV-DCN) film
red-shifted to 320, 421, and 585 nm, respectively. The
absorption spectrum of the S(TPA-bTV-DCN) film cov-
ers a broad wavelength range in the visible region from
380 to 750 nm, which is beneficial to the application as
photovoltaic materials. In comparison with S(TPA-bT-
DCN), the red-shift of the absorption spectra of S(TPA-
bTV-DCN) should result from the vinylene linkage be-
tween TPA and bithiophene in it. Probably, the vinylene
linkage in S(TPA-bTV-DCN) could eliminate torsional
interactions between TPA and bithiophene, and extend
the conjugation length of each branch in the molecule,
leading to the red-shifted and broader absorption
spectrum.20 The absorption edges of S(TPA-bT-DCN)
and S(TPA-bTV-DCN) films are at ca. 674 nm and ca.
752 nm, corresponding to the band gaps of 1.83 and
Figure 3. Cyclic voltammograms of S(TPA-bT-DCN) and S(TPA-bTV-
DCN) film on Pt electrode in an acetonitrile solution of 0.1 mol/L
Bu4NPF6 (Bu = butyl) with a scan rate of 100 mV/s.
1.65 eV, respectively. The detailed optical data of S(TPA-
bT-DCN) and S(TPA-bTV-DCN) are listed in Table 1.
3.3. Electronic Energy Levels. The electronic energy
levels of the organic semiconductors are crucial for their
photovoltaic performance,21 and the electrochemical on-
set oxidation potentials and onset reduction potentials
are corresponding to the ionization potential (IP) and
electron affinity (EA) of the organic semiconductors
respectively.22 In the simple HOMO-LUMO energy
diagram of the organic photovoltaic donor and acceptor
materials, the HOMO energy level can be estimated from
IP with HOMO = -IP and the LUMO energy level can
be estimated form EA with LUMO = -EA. To measure
the electronic energy levels of the compounds, we carried
out electrochemical cyclic voltammetry for the two com-
pounds. Figure 3 shows the cyclic voltammograms of
S(TPA-bT-DCN) and S(TPA-bTV-DCN) films on a Pt
electrode. The onset reduction potential (Eredonset), the
onset oxidation potential (Eoxonset) and the electrochemi-
cal bandgap (Egcv) of the compounds are also listed in
Table 1. The HOMO, LUMO energy levels and the
electrochemical band gap of S(TPA-bT-DCN) were esti-
mated to be -5.22, -3.34, and 1.88 eV, respectively, from
the onset oxidation and reduction potentials according to
the following equations:23 EHOMO = -IP = -e (Eox
onset
þ 4.71) (eV), ELUMO = -EA = -e (Ered
þ 4.71)
onset
(eV), and Eg = IP - EA. For S(TPA-bTV-DCN), The
HOMO and LUMO energy levels and the electrochemical
band gap were estimated to be -5.03, -3.42, and
1.61 eV, respectively. The electrochemical band gaps of
ꢀ
(21) Bredas, J.; Norton, J. E.; Cornil, J.; Coropceanu, V. Acc. Chem.
Res. 2009, 42, 1691–1699.
(22) Eckhardt, H.; Shacklette, L. W.; Jen, K. Y.; Eisenbaumer, R. L. J.
Chern. Phys. 1989, 91, 1303–1315.
(23) Sun, Q. J.; Wang, H. Q.; Yang, C. H.; Li, Y. F. J. Mater. Chem.
2003, 13, 800–806.
(20) Mei, J. G.; Heston, N. C.; Vasilyeva, S. V.; Reynolds, J. R.
Macromolecules 2009, 42, 1482–1487.

822 Chem. Mater., Vol. 23, No. 3, 2011
Zhang et al.
Table 2. Photovoltaic Performance of ITO/PEDOT:PSS/S(TPA-bT-
DCN) or S(TPA-bTV-DCN): PC70BM (1:2, w/w)/Ca/Al under the
Illumination of AM 1.5, 100 mW/cm2
active layer
Voc (V) Jsc (mA/cm2) FF (%) PCE (%)
S(TPA-bT-DCN): PC70BM 0.84
S(TPA-bTV-DCN): PC70BM 0.88
5.21
7.76
30.8
43.9
1.4
3.0
important role for the better photovoltaic performance. As
mentioned above, the absorption edge of the S(TPA-bTV-
DCN) film is ca. 78 nm red-shifted than that of S(TPA-bT-
DCN) film. The higher Jsc value of 7.66 mA/cm2 for the
OSC based on S(TPA-bTV-DCN) should be benefitted
from the broad absorption band. The HOMO energy level
(-5.22 eV) of S(TPA-bT-DCM) is 0.19 eV lower than that
(-5.03 eV) of S(TPA-bTV-DCM), which should lead to
higher Voc of the OSC basedon S(TPA-bT-DCM) as donor
because the Voc depends on the difference between the
LUMO level of the acceptor and the HOMO level of the
donor.30,31 But the Voc value of the OSC based on the
S(TPA-bT-DCM) is similar to that of S(TPA-bTV-DCM).
Probably, other factors such as morphology of the photo-
active layer, hole mobility of the molecules, etc. influenced
the Voc.
Figure 4. Current density-voltage characteristics of the OSCs based on
theblendofS(TPA-bT-DCN) orS(TPA-bTV-DCN)/PC70BM(1:2, w/w)
with Ca/Al as the negative electrode at dark and under the illumination of
AM 1.5, 100 mW/cm2.
S(TPA-bT-DCN) and S(TPA-bTV-DCN) are in agree-
ment with the optical measurements. Compared with the
HOMO (-5.87 eV) and LUMO (-3.91 eV) energy levels for
PC70BM,24 the two compounds are suitable for the applica-
tion as donor materials with PC70BM as acceptor in OSCs. In
addition, the deeper HOMO level of the two compounds is
beneficial to higher open circuit voltage (Voc) of the OSCs
with them as donor materials, because the Voc is usually
proportional to the difference between the LUMO level of the
acceptor and the HOMO level of the donor.25,26
4. Conclusion
Two star-shaped D-π-A molecules containing TPA
donor unit and DCN acceptor unit with 4,40-dihexyl-2,
20-bithiophene (bT) or 4,40-dihexyl-2,20-bithiophene viny-
lene (bTV) as π bridge, S(TPA-bT-DCN) and S(TPA-bTV-
DCN), were synthesized. Solution-processability of the two
compounds was realized for the DCN containing molecules
by the two hexyl side chains on the bithiophene bridges in the
molecules. The absorption spectrum of the S(TPA-bTV-
DCN) film covers a broad wavelength range in the visible
region from 380 to 750 nm, which is red-shifted by ca. 40 nm
than that of the S(TPA-bT-DCN) film, benefitted from the
vinylene bridges between TPA and bithiophene units in it.
The PCE of the bulk heterojunction OSC based on a blend of
S(TPA-bTV-DCN) and PC70BM (1:2, w/w) reached 3.0%
with a Jsc of 7.76 mA/cm2 and a Voc of 0.88 V, under the
illumination of AM.1.5, 100 mW/cm2. Although the PCE of
the OSC based on S(TPA-bT-DCN) as donor is 1.4% under
the same experimental conditions. The PCE of 3.0% for
S(TPA-bTV-DCN) is the highest value for the OSCs based
on the solution-processable TPA-containing organic mole-
cules, and it is among the top values for the solution-
processed molecule-based OSCs reported so far. The results
indicate that the structural modifications with the hexyl side
chains and the vinylene bridge in the DCN-containing star-
shaped molecules improve the solution-processability, ab-
sorption and photovoltaic properties of the molecules sig-
nificantly, and the star-shaped organic molecules with TPA
core and DCN end groups are promising donor materials for
high-performance bulk heterojunction OSCs.
3.4. Photovoltaic Properties. Bulk-heterojunction OSCs
were fabricated by using S(TPA-bT-DCN) or S(TPA-bTV-
DCN) as electron donor, PC70BM as electron acceptors27-29
with a donor/acceptor weight ratio of 1:2. The device
structure is ITO/PEDOT: PSS/photoactive layer/Ca/Al.
Figure 4 shows the typical current-voltage curves of the
OSCs at dark and 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 summar-
ized in Table 2. The OSC device based on S(TPA-bTV-
DCN) demonstrated a Voc of 0.88 V, a Jsc of 7.66 mA/cm2, a
FF of 43.9%, corresponding to a PCE of 3.0%, under the
illumination of AM 1.5, 100 mW/cm2. In comparison, the
photovoltaic performance of the OSC based on S(TPA-bT-
DCN) showed a Voc of 0.84 V, a Jsc of 5.21 mA/cm2, a FF of
30.8%, corresponding to a PCE of 1.4%, under the same
experimental conditions. Obviously, the broader absorp-
tion of S(TPA-bTV-DCN) with vinylene bridge plays an
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Acknowledgment. This work was supported by NSFC
(50633050, 50803071, 20874106 and 20721061) and the Chinese
Academy of Sciences.
€
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