Solution-processable tetrazine and oligothiophene based linear A-D-A small molecules: Synthesis, hierarchical structure and photovoltaic properties

Article abstract of DOI:10.1016/j.orgel.2013.02.038

A series of high coplanar alternative linear small molecules with acceptor-donor-acceptor (A-D-A) structure containing electron-accepting tetrazine (Tz) moiety and electron-donating oligothiophenes (OTs) moiety, alkylated thiophene attached to both sides of the Tz moiety were designed and synthesized. The influences of varied oligothiophene length on small molecules' optical and electrochemical properties, crystallization, self assembling morphology in blend film with (6,6)-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM), and photovoltaic properties for the application as donor materials in organic solar cells (OSCs) were studied. The optical and electrochemical properties of small molecules showed that the HOMO and LUMO energy levels were determined by the number of OTs moiety and electron-accepting ability of Tz in the alternative small molecules, respectively. Meanwhile, the varied OT moieties can significantly affect the hierarchical structures when mixed with PC₆₁BM. The molecule with intermediate conjugate moity length showed the highest ordering in its crystalline state, as revealed by differential scanning calorimetry (DSC) and X-ray diffraction experiments, and best photovoltaic properties when blended together with PC₆₁BM or (6,6)-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM) as active layer in photovoltaic devices. The results indicate that hierarchical structures controlled by adjusting the conjugate moity length of small molecules is an effective way to improve the performance of OSCs. The photovoltaic device based on TT(HTTzHT)₂:PC₇₁BM with 1% DIO additives showed the best performance, with a J_sc of 7.87 mA/cm² and a PCE of 3.24%.

Full text of DOI:10.1016/j.orgel.2013.02.038

Organic Electronics 14 (2013) 1424–1434
Contents lists available at SciVerse ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Solution-processable tetrazine and oligothiophene based linear
A–D–A small molecules: Synthesis, hierarchical structure and
photovoltaic properties
a
a
a
a,b,
⇑
a
b
a,
⇑
Yujin Chen , Chao Li , Pan Zhang , Yaowen Li
, Xiaoming Yang , Liwei Chen , Yingfeng Tu
a
Jiangsu Key Laboratory of Advanced Functional Polymer Design and Application, Department of Polymer Science and Engineering, College of Chemistry,
Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, PR China
i-LAB, Suzhou Institute of Nano-Tech and Nano-Bionics (SINANO), Chinese Academy of Sciences, Suzhou 215123, PR China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of high coplanar alternative linear small molecules with acceptor–donor–acceptor
A–D–A) structure containing electron-accepting tetrazine (Tz) moiety and electron-donat-
Received 11 January 2013
Received in revised form 19 February 2013
Accepted 23 February 2013
Available online 20 March 2013
(
ing oligothiophenes (OTs) moiety, alkylated thiophene attached to both sides of the Tz moi-
ety were designed and synthesized. The influences of varied oligothiophene length on
small molecules’ optical and electrochemical properties, crystallization, self assembling
morphology in blend film with (6,6)-phenyl-C61-butyric acid methyl ester (PC61BM), and
photovoltaic properties for the application as donor materials in organic solar cells (OSCs)
were studied. The optical and electrochemical properties of small molecules showed that
the HOMO and LUMO energy levels were determined by the number of OTs moiety and
electron-accepting ability of Tz in the alternative small molecules, respectively. Mean-
while, the varied OT moieties can significantly affect the hierarchical structures when
mixed with PC61BM. The molecule with intermediate conjugate moity length showed the
highest ordering in its crystalline state, as revealed by differential scanning calorimetry
Keywords:
Oligothiophene
A–D–A small molecule
Crystallization
Hierarchical structure
Photovoltaic cell
(DSC) and X-ray diffraction experiments, and best photovoltaic properties when blended
together with PC61BM or (6,6)-phenyl-C71-butyric acid methyl ester (PC71BM) as active
layer in photovoltaic devices. The results indicate that hierarchical structures controlled
by adjusting the conjugate moity length of small molecules is an effective way to improve
2
the performance of OSCs. The photovoltaic device based on TT(HTTzHT) :PC71BM with 1%
2
DIO additives showed the best performance, with a Jsc of 7.87 mA/cm and a PCE of 3.24%.
Ó 2013 Elsevier B.V. All rights reserved.
1
. Introduction
In recent years, small molecules (SMs) for bulk-hetero-
tribution problems as their polymer counterparts. Their
band structures could be tuned easily with much more
choices of chemical modification, and generally have high-
er charge mobility and open voltages [5–6]. However, even
with these advantages, SM-based OSCs have not been
investigated intensively due to their generally poor film
quality compared with their polymer counterparts when
using the simple solution spinning process [7]. Until re-
cently, profound progress has been achieved in the synthe-
sis of new solution processable SMs and the corresponding
photovoltaic applications, such as donor–acceptor (D–A)
molecules [8–10], oligothiophenes [11–13], star-shaped
molecules [3,14], and organic dyes [4]. To date, the highest
junction (BHJ) organic solar cells (OSCs) have attracted
more and more attention for photovoltaic applications
due to their high purity, solution processability, well-de-
fined molecular structures and definite molecular weight
[
1–4]. They do not suffer from batch-to-batch variations,
difficulty of purification, and broad molecular weight dis-
⇑
Corresponding authors. Tel./fax: +86 512 65882130.
E-mail addresses: ywli@suda.edu.cn (Y. Li), tuyingfeng@suda.edu.cn
(
Y. Tu).
1
566-1199/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.orgel.2013.02.038

Y. Chen et al. / Organic Electronics 14 (2013) 1424–1434
1425
published PCEs for solution-processed SM based BHJ OSCs
is more than 7% by carefully molecular designing to control
the hierarchical structures like crystallization and nano-
scale phase separation morphology [15–17]. It is thus ex-
pected that better PCE could be achieved if the intrinsic
poor film quality and hierarchical structures in BHJ archi-
tecture could be improved. In order to achieve this, careful
molecule design has to be carried out to address many fac-
tors simultaneously, including the materials solar light
absorption associated with its highest occupied molecu-
lar orbital (HOMO) and lowest unoccupied molecular
orbital (LUMO) positions, crystallization of donor materials
and its morphology compatibility with the acceptors, and
so on.
Donor–Acceptor (D–A) chromophores involving elec-
tron-donating and electron-accepting moieties have been
widely investigated for OSCs molecules [18]. Well-chosen
donor and acceptor groups are particularly desirable for
low band-gap materials due to a significant enhancement
of the ICT intensity and conjugated length, which lead to
a better extended absorption and higher absorption coeffi-
cient [19]. Recently, the acceptor–donor–acceptor (A–D–A)
molecules, which is a type of D–A structure molecule, have
caused more and more interests due to their outstanding
photovoltaic performance [17,20–22]. The A–D–A mole-
cules have demonstrated some advantages. First, they have
ordered structure and continuous carrier transport channel
even in composite form. On the other hand, tetrazine (Tz)
has a very high electron affinity, and would behave as a
strong electron-deficient unit in a conjugated molecule to
lower its HOMO level [29]. Tetrazines have a rare and use-
ful physical chemistry characteristic directing several
types of applications in various domains. Audebert et al.
have reported some donor-acceptor-donor s-tetrazines
deriatives possessing excellent fluorescence [30,31].When
incorporated into DꢀA conjugated materials, both HOMO
and LUMO levels of Tz based materials are generally lower
than other acceptor moiety-based materials, indicating a
stronger electron affinity of the Tz moiety. Interestingly,
the Tz moiety shows more impact on HOMO levels than
on LUMO levels, which is different from most acceptor
units [32]. This results in slightly broader band gaps of Tz
based materials when compared with other acceptor moi-
ety based materials. Thus, introducing the electron-accept-
ing Tz moiety and low dimensional OTs into A–D–A
structure SM may produce surprising and superb results
in BHJ OSCs.
Based on the above idea, a series of high coplanar linear
SMs with A–D–A structure were designed and synthesized
consisting of OTs with unalkylated as central donor moiety,
Tz moiety as acceptor on both sides and alkylated thio-
phene as bridge between oligothiophene and Tz moieties.
As expected, these linear OTh based SMs with A–D–A
structure show relatively low band gap and high efficient
6
–10 effective conjugated units and two electron-accept-
ing moieties, resulting in high conjugated length and hence
similar solar cell absorption to that for the corresponding
polymers. Furthermore, the HOMO and LUMO energy lev-
els can also be tuned through designing different central
donor moiety and two conjugated electron-accepting moi-
ety on both sides [23]. Third, the central donor moiety
based on varied conjugated backbone generally could offer
a high mobility, crystallization and improved miscibility
between donor and acceptor materials, which can contrib-
ute to formation of good film quality with high ordered
structure using spin-coating method [24]. It is thus
expected that great improvement could be made to the
photovoltaic performance for SM with A–D–A structure
through delicate chemical structure design and
modification.
sunlight harvesting, owing to their high
p conjugated back-
bone. To understand the relationship between structure
and properties, the crystallization, self-assembling mor-
phology and the photovoltaic properties of the SMs with
different thiophene unit number as building block of donor
part were studied and compared. OSCs were fabricated
using these SMs as donor and PC61BM or PC71BM as the
acceptor. A PCE of 3.24% was achieved with a Voc of
2
0.86 V, Jsc of 7.87 mA/cm and a FF of 0.48 for TT(HTTzHT)
2
.
2. Experimental section
2.1. Materials and synthesis
Oligothiophenes (OTs) have attracted comprehensive
interest and been advanced to the most frequently used
0
2,5-bis(trimethylstannyl)thiophene (5), 5,5 -bis(trim-
0
00
p-conjugated materials, in particular, as active compo-
ethylstannyl)-2,2 -bithiophene (7) and 5,5 -bis(trimethyl-
0
0
00
nents in organic optoelectronic devices, such as organic
field effect transistors [25,26] and solar cells [2,27]. This
is due to their intriguing electronic, optical, and electro-
chemical properties of OTs and the high coplanarity of thi-
stannyl)-2,2 :5 ,2 -terthiophene (9) were synthesized
according to Li et al. reported [13,28]. All reagents and
chemicals were purchased from commercial sources (Al-
drich, Across, Fluka) and used without further purification
unless stated otherwise. Toluene, tetrahydrofuran (THF),
ophene rings lead to a better p–p interaction and excellent
charge transport properties, which is one of the most cru-
cial assets for applications in organic electronics. More-
over, the OTs, which are not alkylated, can increase the
and Et
dimethylformamide (DMF) was purified by heating over
CaH followed by vacuum distillation.
2
O were distilled over Na/benzophenone. N,N-
2
conjugated length,
p
–
p
stacking, and assure the coplana-
4-hexylthiophene-2-carbaldehyde (1) 3-hexylthioph-
ene 10 g (59.5 mmol) and 100 mL THF were added into a
flask under an inert atmosphere. The solution was cooled
down to ꢀ78 °C by a liquid nitrogen-acetone bath, and
40.9 mL of n-butyllithium (65.4 mmol, 1.6 M in n-hexane)
was added dropwise. After being stirred at ꢀ78 °C for 2 h,
a great deal of white solid precipitates appeared in the
flask. DMF 5.73 g (78.4 mmol) was added in one portion,
rity of the molecules [28]. In addition, one-dimensional
OTs with pull–push structure exhibited promising OSCs
performance. For example, OTs with electron-accepting
end groups yielded very high PCEs up to 6.1% when blend-
ing with PC61BM [17]. Because low dimensional oligomer is
molecular and crystal orientation, especially the OTs based
materials with high crystallization, which can result in a

1
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Y. Chen et al. / Organic Electronics 14 (2013) 1424–1434
and the reactant turned to clear rapidly. The cooling bath
was removed, and the reactant was stirred at ambient tem-
perature for 24 h. Then, it was poured into 200 mL of cool
water and extracted by ether three times. The organic layer
was washed by water two times and then dried by anhy-
moved by a rotate evaporator and the residue was purified
by silica-gel chromatography (CHCl /Petroleum ether = 1/
5, v/v) to yield red needle-like crystal (3.11, yield:
58%). H NMR(400 MHz, CDCl ): d(ppm) 8.08 (s, 1H), 7.94
3
(s, 1H), 7.28 (s, 1H), 2.67 (t, 4H, J = 7.8 Hz), 1.65 (m, 4H),
1.35 (m, 12H), 0.89 (m, 6H).
2,5-bis{5-(6-(4-hexylthiophen-2-yl)-1,2,4,5-tetrazine-3-yl)-
2
(3-hexylthiophene-2-yl)}thiophene (T(HTTzHT) ). 2.2 mmol
compound 4, 1.0 mmol of 2,5-bis(trimethylstannyl)thio-
phene 5, and 40 mL of toluene were put into a flask with
oil bath. The solution was flushed with argon for 10 min,
3
1
drous MgSO
residue was purified with column chromatography
EtOAc/petroleum ether = 1/8 v/v) to give yellow liquid
.68 g (49.3 mmol, yield 83%). 1H NMR (400 MHz, CDCl
):
4
. After removing solvent under vacuum, the
(
9
3
d(ppm) 9.95 (s, 1H), 7.51 (d, 1H, J = 1.4 Hz), 7.19 (d, 1H,
J = 1.4 Hz), 2.66 (t, 2H, J = 8.0 Hz), 1.62 (m, 2H), 1.28 (m,
6
H), 0.88 (m, 3H).
-hexylthiophene-2-carbonitrile (2) A mixture solution of
-hexylthiophene-2-carbaldehyde (5.9 g, 30.0 mmol) and
3 4
and then 25 mg of Pd(PPh ) was added into the flask.
4
The solution was flushed again for 20 min. The oil bath
was heated to 110 °C carefully, and the reactant was stirred
for 48 h at this temperature under an argon atmosphere.
Then, the reactant was cooled to room temperature. The
solution was poured into distilled water and extracted
with chloroform. The organic phase was washed with
4
hydroxylamine hydrochloride salt (3.15 g, 45 mmol) in
pyridine/ethanol (20 mL, 1/1 v/v) was stirred at 80 °C over-
night. Then the solvent was removed using a rotary evap-
orator. The residue was dissolved in chloroform, and the
solution was washed with distilled water and dried over
4
water, dried over anhydrous MgSO . The solvent was re-
anhydrous MgSO
4
. The solvent was removed under vac-
moved by a rotate evaporator and the residue was purified
by silica-gel chromatography, the obtained material was
further purified by redissolving in chloroform and depos-
ited in 100 mL of methanol. The solid was dried under vac-
uum, and the viscous liquid residue was dissolved in acetic
anhydride containing potassium acetate (0.1 g) and then
refluxed for 3 h. The mixture was poured into distilled
water and extracted with hexane. The organic phase was
washed with 5% aqueous sodium hydroxide solution and
uum for 1 day to get the final product. The yield of the
1
coupling reaction was 72%. H NMR (400 MHz, CDCl
3
): d
then water, dried over anhydrous MgSO
4
before the sol-
(ppm) 8.06 (s, 4H), 7.27 (d, 2H), 7.23 (s, 2H), 2.87 (t, 4H,
J = 1.9 Hz), 2.68 (t, 4H, J = 1.9 Hz), 1.76 (m, 4H), 1.68 (m,
4H), 1.37 (m, 24H), 0.89 (m, 12H). MALDI-TOF MS: Calcd
vent was removed by a rotate evaporator. The yellow li-
quid residue was purified by silica-gel column
chromatograph (EtOAc/petroleum ether = 1/10, v/v) to
for C48
H
60
N
8
S
5
: 908.35; Found: 908.48.
0
yield a clear light yellow liquid product. (5.1 g, 88% yield).
5,5 -bis{5-(6-(4-hexylthiophen-2-yl)-1,2,4,5-tetrazine-3-yl)-
1
0
3
H NMR (400 MHz, CDCl ): d (ppm) 7.41 (d, 1H, J = 1.4 Hz),
(3-hexylthiophene-2-yl)}-2,2 -bithiophene (TT(HTTzHT)
2
). This
7
1
.15 (d, 1H, J = 1.4 Hz), 2.62 (t, 2H, J = 8.0 Hz), 1.60 (m, 2H),
.28 (m, 6H), 0.88 (m, 3H).
compound was prepared with the same procedure as
T(HTTzHT) . H NMR (400 MHz, CDCl ): d (ppm) 8.08 (s,
2 3
1
3
,6-bis(4-hexyl-2-thienyl)-1,2,4,5-tetrazine (3) To a mix-
ture of compound (5.0 g, 25.9 mmol) and sulfur
0.576 g, 18.2 mmol) in anhydrous ethanol (2 mL) was
slowly added fresh hydrazine monohydrate (3.91 g,
7.6 mmol) at room temperature. The solution turned into
4H), 7.27 (d, 2H), 7.23 (s, 4H), 2.88 (t, 4H, J = 1.9 Hz), 2.69
(t, 4H, J = 1.9 Hz), 1.74 (m, 4H), 1.68 (m, 4H), 1.38 (m,
2
(
24H), 0.89(m, 12H). MALDI-TOF MS: Calcd for C52
62 8 6
H N S ,
990.34; Found 990.89.
00
7
5,5 -bis{5-(6-(4-hexylthiophen-2-yl)-1,2,4,5-tetrazine-3-yl)-(3-
0
0
00
yellow and large amount of gas evolved. The solution was
heated up to reflux for 2 h. Then it was cooled down to
room temperature with crystal formed in solution. The
crystal was collected by filtration and rinsed with cold eth-
anol before dried under vacuum. To a chloroform solution
of the obtained solid, isoamylnitrite (6.06 g, 51.7 mmol)
was added and the solution was stirred at room tempera-
ture overnight. The solvent was removed and the resulting
red solid was washed with methanol twice before purified
hexylthiophene-2-yl)}-2,2 :5 ,2 -terthiophene (TTT(HTTzHT)
2
).
This compound was prepared with the same procedure
1
2 3
as T(HTTzHT) . H NMR (400 MHz, CDCl ): d (ppm) 8.08
(s, 4H), 7.27 (d, 2H), 7.21 (s, 2H),7.15 (s, 4H), 2.87 (t, 4H,
J = 1.9 Hz), 2.68 (t, 4H, J = 1.9 Hz), 1.76 (m, 4H), 1.68 (m,
4H), 1.36 (m, 24H), 0.90 (m, 12H). MALDI-TOF MS: Calcd
64 8 7
for C56H N S , 1072.33; Found 1072.87.
2.2. Measurements and characterization
3
by silica-gel column chromatography (CHCl /Petroleum
ether = 1/2, v/v) to yield red needle-like crystal (0.51 g,
Differential scanning calorimetry (DSC) was performed
under nitrogen flushing at a heating rate of 10 °C/min with
a NETZSCH (DSC-204) instrument. Thermal gravimetric
analysis (TGA) was carried out on a Perkin–Elmer Pyris 6,
with a heating rate of 10 °C/min under nitrogen flow. Elec-
trochemical measurements of these derivatives were per-
1
yield: 58%). H NMR(400 MHz, CDCl
2
1
3
): d (ppm) 8.10 (s,
H), 7.29 (s, 2H), 2.69 (t, 4H, J = 7.8 Hz), 1.67 (m, 4H),
.35 (m, 12H), 0.89 (m, 6H).
-(5-bromo-4-hexyl-2-thienyl)-1,2,4,5-tetrazine (4). To a
3
suspension solution of compound 3 (0.75 g, 1.8 mmol) in
chloroform (20 mL) and acetic acid (20 mL) was added N-
Bromosuccinimide (NBS, 0.32 g, 1.8 mmol) at room tem-
perature. The mixture was stirred at room temperature un-
der dark for 1 h before heated at 80 °C for 5 h. Then the
solution was poured into distilled water and extracted
with chloroform. The organic phase was washed with
formed with
electrochemical workstation. The cyclic voltammetry (CV)
diagrams of the SMs were obtained by using n-Bu NPF
a Bioanalytical Systems BAS100 B/W
4
6
as supporting electrolyte in acetonitrile solution with a
glass carbon working electrode, a platinum wire counter
electrode and a Ag/AgNO
3 2
reference electrode under N
4
water, dried over anhydrous MgSO . The solvent was re-
atmosphere. Ferrocene was used as the internal standard.

Y. Chen et al. / Organic Electronics 14 (2013) 1424–1434
1427
+
The redox potential of Fc/Fc which has an absolute energy
vents, including CH
2
Cl
2 3
, CHCl , THF, chlorobenzene (CB)
level of ꢀ4.8 eV relative to the vacuum level for calibration
and dichlorobenzene (DCB).
4 6
is located at 0.17 V in 0.1 M n-Bu NPF /acetonitrile solu-
1
tion [33]. H NMR spectra were measured using a Varian
Mercury-400 NMR. Time-of-flight mass spectra were re-
corded with a Kratos MALDI-TOF mass system. UV–visible
absorption spectra were measured using a Shimadzu UV-
3.2. Thermal properties
The thermal stability of all the compounds was investi-
gated by TGA at a heating rate of 10 °C/min under N . The
5% weight loss temperatures (T ) of these SMs were deter-
mined from TGA curves (Fig. S1, Supporting Information).
Each SM exhibits high thermal stability with a T greater
than 257 °C. DSC experiments were performed to investi-
gate the thermal properties of T(HTTzHT) , TT(HTTzHT)
and TTT(HTTzHT) . Fig. 1 shows the DSC curves of these
2
3
100 spectrophotometer. Atomic force Microscopy (AFM)
d
images were recorded under ambient conditions, using a
Vecco Digital Instrument Multimode NanoscopeIIIa oper-
ating in the tapping mode regime. The samples were pre-
pared by spin-coating onto silica at 800 rpm from sample
solutions in chloroform. X-ray diffraction (XRD) analysis
was performed on an X’Pert-Pro MPD diffractometer with
a Cu K radiation source at room temperature. The trans-
mission electron microscopy (TEM) measurements were
conducted on a Tecnai G2 F20 S-Twin transmission elec-
tron microscope operated at 200 kV.
d
2
2
2
molecules after they are purified through recrystallization,
and it clearly reveals these SMs are in the crystalline state
at room temperature. The effect of number of thiophene
repeating units in oligothiophene as building block of do-
nor part in these SMs on crystalline properties is studied.
With the repeating number of central thiophene increases
from T(HTTzHT)
melting temperature (T
74 °C, concomitantly with a strong tendency to crystallize
during the heating scan with the crystallization enthalpy of
5.4 kJ/mol, 55.4 kJ/mol and 37.6 kJ/mol, respectively. The
largest crystallization enthalpy of TT(HTTzHT) indicates
that it has the highest tendency to crystallize by its en-
hanced interchain interaction. It should also be noted that
the corresponding entropy at crystal melting changed from
2
to TT(HTTzHT)
2 2
and TTT(HTTzHT) , the
2
.3. Device fabrication and characterization
m
) varied from 177 to 205 and
1
The active layer contained a blend of SM as electron do-
nor and PC61BM or PC71BM as electron acceptor, which was
prepared from varied weight ratios by solution (6 mg/mL
of SMs) in chloroform. After spin coating the blend from
solution at 800–1400 rpm, the devices were completed
by evaporating a 0.8 nm LiF layer protected by 100 nm of
4
2
ꢀ4
Al at a base pressure of 4 ꢁ 10 Pa. The effective photovol-
1
02 J/mol K for T(HTTzHT)
2
to 116 J/mol K for TT(HTTzHT)
. Generally, the entropy
2
taic area defined by the geometrical overlap between the
2
and 83.5 J/mol K for TTT(HTTzHT)
2
bottom ITO electrode and the top cathode was 12 mm .
should increase with increasing thiophene repeating units
due to the similar molecules structure. The largest change
of entropy for TT(HTTzHT) indicates that molecules are
2
packed more regularly in crystalline state than the others.
This would be beneficial for applications in organic solar
cells since a highly ordered heterojunction nanostructure
enhanced by the crystallization of the SM in the active
layer is desirable for charge separation and transport [34].
Current–voltage characteristics of the solar cells in the
dark and under illumination of 100 mW/cm2 white light
from a Hg–Xe lamp filtered by a Newport 81094 Air Mass
Filter, using a GWinstek SFG-1023 source meter. Mono-
chromatic light from a Hg–Xe lamp (Newport 67005) in
combination with monochromator (Oriel, Cornerstone
2
60) was modulated with a mechanical chopper. The re-
sponse was recorded as the voltage over a 50 resistance,
X
using a lock-in amplifier (Newport 70104 Merlin). A cali-
brated Si cell was used as reference. All the measurements
were performed under ambient atmosphere at room
temperature.
3
.3. Optical properties
Among three SMs, there is a symmetrical A–D–A struc-
ture, where D represents the electron-donating OTs moiety
1T, 2T, 3T) in center, while A represents the electron-
(
3
. Results and discussion
accepting Tz moieties (bisthiophene-tetrazine) at both
sides. The normalized UV–vis absorption spectra of these
SMs in dilute chloroform solution (10 M) are shown in
Fig. 2a, and the main optical properties are listed in Table 1.
ꢀ5
3.1. Synthesis and structural characterization
Scheme 1 outlines the synthetic approach and depicts
One absorption peak is apparent in the range of 300–
ꢂ
the structures of the final products of T(HTTzHT)
TT(HTTzHT) and TTT(HTTzHT) . Compound 3 was ob-
tained through a known procedure and then reacted with
NBS in CHCl /AcOH to provide compound 4 in 58% yield.
2
,
400 nm for the
p–p
transition between OT and TTTz moi-
2
2
eties in the backbone, and the other absorption maximum
in the 450–500 nm range is for the intramolecular charge
transfer (ICT) transition [4]. As the incorporated the num-
27
3
The bis(trimethylstannyl)thiophene derivatives 5, 7 and 9
were synthesized from the corresponding oligothiophene
or their bromides after successive treatments with n-BuLi
ber of OT units increase from T(HTTzHT)
2 2
to TT(HTTzHT)
and TTT(HTTzHT) , the ICT transition red-shifted from
2
447 to 463 and 473 nm, respectively. This phenomenon
is because that the increasing OT units enhance the conju-
gated backbone [28]. Moreover, the relatively high absorp-
tion coefficients (emax) could be calculated from the Beer’s
law equation with the same dilute concentration of the
and trimethyltin chloride. The final products T(HTTzHT)
TT(HTTzHT) and TTT(HTTzHT) were produced through
Pd(0)-catalyzed Stille crosscoupling reaction of compound
with 5, 7 and 9, respectively. T(HTTzHT) , TT(HTTzHT)
and TTT(HTTzHT) were soluble in common organic sol-
2
,
2
2
4
2
2
2
SMs in chloroform, which are listed in Table 1.

1
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Y. Chen et al. / Organic Electronics 14 (2013) 1424–1434
2 2
Scheme 1. Molecular structures and synthesis of T(HTTzHT) , TT(HTTzHT) and TTT(HTTzHT)2.
Fig. 2b shows the optical absorption spectra of thin
films of the SMs, and the optical properties are summa-
rized in Table 1. The thin film absorption spectra are gen-
erally similar in shapes to those in dilute solution. The
maximum absorption peaks and absorption edges (kedge
)
in film were red shifted significantly compared with those
in solution, suggesting that intermolecular interactions ex-
isted in the solid state. Compared to T(HTTzHT)
2
, com-
pounds TT(HTTzHT) and TTT(HTTzHT) showed stronger
2
2
absorption intensity due to strong intermolecular interac-
tions and the high crystallization in the solid state, which
results in a longer effective conjugation length than in
the solution state (Fig. S2, Supporting Information). The
optical band gaps (Eg,opt) of the compounds TT(HTTzHT)
and TTT(HTTzHT) derived from the absorption edge of
the thin film spectra are around 1.85 eV (Table 1). These
2
Fig. 1. DSC traces of T(HTTzHT)
2
,
TT(HTTzHT)
2
and TTT(HTTzHT)
2
2
measured under N flow at a heating rate of 10 °C/min.
2

Y. Chen et al. / Organic Electronics 14 (2013) 1424–1434
1429
respectively, which are very similar. It indicates that alter-
nating electron-donating OTs with different number of thi-
ophene units have slight effect on the reduction potential
of the SMs with A–D–A structure, as the LUMO energy level
is mainly determined by the Tz based acceptor unit. On the
other hand, the HOMO energy levels of the three SMs be-
have quite differently. The HOMO energy levels of the
T(HTTzHT)
2
, TT(HTTzHT)
2
and TTT(HTTzHT)
2
are ꢀ5.5 eV,
ꢀ5.18 eV, and ꢀ5.02 eV, respectively. The improved HOMO
energy levels of the three SMs are caused by the enhanced
conjugated length, which indicates that the OT with increas-
ing number of thiophene possessed higher electron-donat-
ing ability can increase the conjugated length of the
molecules and result in a higher HOMO energy level [28].
The electrochemical band gaps (Eg,ec) of the three SMs,
calculated from the difference between HOMO and LUMO
values, are well consistent with the Eg,opt in trend. The var-
ied HOMO energy level and relative stable LUMO energy
level result in the reduced band gaps of the SMs, demon-
strated the significance of the intramolecular charge trans-
fer through the A–D–A structures inside the SMs. It clearly
indicates the HOMO energy level and band gap of the SM
can be controlled strictly by introducing oligothiophene
with different number of thiophene.
3.5. Photovoltaic performance
In order to investigate the photovoltaic properties of the
SMs, the BHJ photovoltaic cells, with a structure of ITO/
PEDOT:PSS/SM:PCBM/LiF/Al, were fabricated, where the
SMs were used as donors and PC61BM or PC71BM as accep-
tor. The active layers of the photovoltaic cells were fabri-
Fig. 2. Normalized UV–Vis absorption spectra of the SMs. (a) In chloro-
form solutions with the concentration of 10 mol/L and (b) films spin-
coating from a 10 mg/mL chloroform solution.
ꢀ
5
3
cated from CHCl solutions of SM and PC61BM or PC71BM
by spin-coating. Different conditions including the weight
ratio between donor and acceptor and the thickness of
the active layer were optimized. The performances were
achieved with the varied weight ratios of SM:PC61BM from
low band gaps can improve light harvesting and enhance
the photocurrent of the OSCs since they are very close to
the photon flux maximum of the solar spectrum (1.77 eV).
1
9
:1 to 1:4 (w/w). The thickness of the active layer is 60–
0 nm, which could be controlled by changing both the
3
.4. Electrochemical properties
spin-coating rate and the concentration of the solution.
Due to the high performance of TT(HTTzHT) , we also fab-
ricated the photovoltaic cell with the active layer (around
100 nm) prepared by spin-coating CHCl solutions consist-
ing of TT(HTTzHT) and PC71BM with the weight ratio at
2
Fig.
3
shows the cyclic voltammetry curves of
, TT(HTTzHT) and TTT(HTTzHT) . The results
T(HTTzHT)
2
2
2
3
of the electrochemical measurement and calculated energy
levels of the SMs are listed in Table 1. The estimated LUMO
energy levels of T(HTTzHT)
2
1:1 (w/w), and 1,8-diiodooctane (DIO, 1% by volume) was
added as a processing additive. All devices were character-
2
,
TT(HTTzHT)
2
and
2
TTT(HTTzHT)
2
are ꢀ3.54 eV, ꢀ3.55 eV and ꢀ3.41 eV,
ized in the dark and under AM 1.5 (100 mW/cm ) white
Table 1
Optical and electrochemical data of the small molecules.
SM
In solutiona
In filmb
abs
abs
max
onset
ox
ro en ds et (V)/LUMO
E
(eV)
Electrochem.
Eg,ec (eV)
Opticalc
k
(nm) (emax
cm ))
)
kedge (nm)
k
kedge
E
(V)/
max
Eg,opt (eV)c
ꢀ
1
ꢀ1
(
M
HOMO (eV)
T(HTTzHT)
TT(HTTzHT)
TTT(HTTzHT)
2
447 (84,500)
463 (96,300)
473 (82,600)
544
559
569
467
498
507
605
664
669
0.87/ꢀ5.50
0.55/ꢀ5.18
0.39/ꢀ5.02
ꢀ1.09/ꢀ3.54
ꢀ1.08/ꢀ3.55
ꢀ1.22/ꢀ3.41
1.96
1.63
1.61
2.05
1.87
1.85
2
2
a
b
c
ꢀ5
1
ꢁ 10 M in anhydrous chloroform.
Spin-coating from a 10 mg/mL chloroform solution.
The optical band gap (Eg,opt) was obtained from absorption edge.

1
430
Y. Chen et al. / Organic Electronics 14 (2013) 1424–1434
Fig. 3. Cyclic voltammetry curves of T(HTTzHT)
TTT(HTTzHT) films on platinum electrode in 0.1 mol/L n-Bu
CH CN solution, at a scan rate of 100 mV/s.
2
,
TT(HTTzHT)
2
and
in
Fig. 4. Current–voltage characteristics of optimized photovoltaic cells
based on T(HTTzHT) :PC61BM, TT(HTTzHT) :PC61BM and TTT(HTTzHT) :-
PC61BM under illumination of AM 1.5, 100 mW/cm white light.
2
4
NPF
6
2
2
2
2
3
light with simultaneous recording of their current–voltage
characteristics. The photovoltaic performance characteris-
tics of each OSCs are given in Table 2.
prepared using CHCl
3
with 1% DIO, reaches a Voc = 0.86 V,
2
Jsc = 7.87 mA/cm , FF = 0.48 and PCE = 3.24%. All the param-
eters of optimized photovoltaic devices are list in Table 2.
Since the serials of OTs based A–D–A SMs exhibit the sim-
ilar chemical structure, the greatly increased PCEs of
The optimized current–voltage curves of the three SMs
are presented in Fig. 4. The cells based on T(HTTzHT)
BM (1:1 w/w in CHCl solution) showed an open circuit
voltage (Voc) of 0.73 V, a short circuit current density (Jsc
2
:PC61-
2
TT(HTTzHT) based photovoltaic device can be attributed
3
)
to the enhanced interchain interactions, as discussed in
the thermal properties part, plus the energy level, crystal-
lization, absorption properties, the hierarchical structures
of the SM:PCBM blend films as discussed below.
It is generally considered that the Voc is mainly deter-
mined by the difference between the HOMO energy level
of the donor (SM) and LUMO energy level of the acceptor
(PCBM). However, several other parameters should also
be taken into account such as carrier recombination, resis-
tance related to thickness of the active layer, and degree of
phase separation between the components in the blend,
which can modify the energetically expected Voc value.
2
of 1.77 mA/cm , a fill factor (FF) of 0.38, giving a PCE of
.49%. Interestingly, with the increasing number of thio-
phene unit in the SM backbone, the photovoltaic cell based
on TT(HTTzHT) :PC61BM (1:1 w/w in CHCl solution)
0
2
3
2
showed Voc of 0.74 V, Jsc of 5.77 mA/cm , FF of 0.52, and
significantly increased PCE to 2.22%, which is more than
three times higher than that of T(HTTzHT)
the photovoltaic performances of TTT(HTTzHT)
2
. However,
with
2
increasing thiophene number in backbone decreased
greatly. Considering the promising photovoltaic properties
of TT(HTTzHT)
optimization on the OSCs based on TT(HTTzHT)
2
among the three SMs, we performed the
by using
Although T(HTTzHT)
(around ꢀ5.50 eV) and similar LUMO energy level com-
pared with the other two SMs TT(HTTzHT) and
TTT(HTTzHT) , the TT(HTTzHT) based devices exhibits
the highest Voc. Moreover, the Voc of the devices based on
TT(HTTzHT) can be further improved by replacing PC61BM
with PC71BM as acceptor and adding processing additive.
The relatively high Voc of TT(HTTzHT) based device could
2
showed lowest HOMO energy levels
2
PC71BM as acceptor instead of PC61BM and adding varied
volume ratios DIO as a processing additive. Fig. 5 shows
the current–voltage characteristics of the OSCs devices at
different mixed solvents device fabrication conditions.
We found that the device performance can be further im-
proved by using PC71BM as acceptor instead of PC61BM
2
2
2
2
and adding varied DIO:CHCl
3
volume ratio. The best device,
2
Table 2
Characteristic Current–voltage Parameters from Device Testing at AM 1.5G White Light Conditions.
SM/PCBM (w/w ratio)
V
oc (V)
J
sc (mA/cm2)
FF
PCE (%)
DIOc (volume %)
R
sh (O cm2)
R
s
(O cm2)
RMSd (nm)
T(HTTzHT) a
1:1
1:1
1:1
1:1
1:1
1:1
1:1
0.73
0.74
0.66
0.81
0.86
0.84
0.84
1.77
5.77
1.57
5.59
7.87
7.85
5.96
0.38
0.52
0.41
0.54
0.48
0.47
0.47
0.49
2.22
0.42
2.45
3.24
3.08
2.37
–
–
–
–
1%
3%
5%
857
29.39
5.26
40.73
8.31
10.08
8.00
11.81
2.11
5.18
3.93
–
–
–
2
a
TT(HTTzHT)
TTT(HTTzHT)
2
1510
1394
1312
632
781
879
a
2
b
TT(HTTzHT)
TT(HTTzHT)
TT(HTTzHT)
TT(HTTzHT)
2
2
2
2
b
b
b
–
a
b
c
The active layer based on the blend film of SM and PC61BM.
The active layer based on the blend film of SM and PC71BM.
The varied volume mixed ratio between DIO and CHCl .
3
d
Root mean-square (RMS) roughness from AFM measurement.

Y. Chen et al. / Organic Electronics 14 (2013) 1424–1434
1431
2
Fig. 5. Current–voltage characteristics of TT(HTTzHT) :PC71BM photo-
Fig. 6. XRD patterns of SM:PCBM blend films spin-coating onto glass
substrate: (a) chloroform solution of SM:PC61BM; (b) chloroform solution
of SM:PC71BM; (c) DIO:chloroform (1%) mixed solution of SM:PC71BM.
voltaic cell prepared from varied volume mixed solvents CHCl
3
:DIO under
2
illumination of AM 1.5, 100 mW/cm white light.
be explained by using the following theoretical equation
35]:
crystallize and form a well ordered packing structure dur-
ing the spin-coating process. This is further supported by
the XRD of the blend films of small molecules with PC61BM,
as indicated in Fig. 6, showing the corresponding similar
diffraction peaks. It should also be noted that the full width
at half maxim intensity (FWHM) of XRD peaks for SMs in-
creased in the blend film, indicates that crystalline domain
size decreased when blended with PC61BM, according to
Debye–Scherrer’s equation [37,38].
[
ꢀ
ꢁ
nkT
q
Jsc
Jso
DE
DA
V
oc
¼
ln
þ
2q
where q is the fundamental charge, n is the diode ideality
factor, Jso is a pre-exponential factor that depends on the
molecules and
DEDA is the energy difference between the
LUMO level of the acceptor material and the HOMO level
of the donor material. Considering the Materials properties
affected the magnitude of Jso including the reorganization
energy for D–A electron transfer, the intermolecular over-
lap at the D/A interface, the layer electrical conductivities,
Combining the length of SMs in the range of 28–36 Å
obtained from the B3LYP functional and employing the 6-
ꢂ
31G basis set method (see Fig. S4, Supporting Informa-
tion), the much smaller d spacing calculated from XRD of
blend films indicates that the diffraction peaks should
come from the lamella packing of SMs in the crystalline
structure [39,40]. It is very interesting to find that the la-
the area of the D/A interface. A more ordered TT(HTTzHT)
2
crystalline structure in active layer, and ideal phase sepa-
ration between donor and acceptor materials can give a
lower Jso for the optimized TT(HTTzHT)
2
based device
mella packing distances (d100
TT(HTTzHT) :PC61BM is higher than that of T(HTTzHT)
PC61BM and TTT(HTTzHT) :PC61BM blend films, although
)
in blend films of
[
36]. By combining high Jsc and low Jso, the high Voc of
2
2
:-
TT(HTTzHT)
2
based devices can be explained according to
2
the above equation.
it possesses intermediate side alkyl chain density per con-
jugated moiety. Since this molecule shows the best photo-
voltaic properties among the three studied SMs with
similar chemical structure, the different molecular packing
in the crystalline structure may play the main role. At-
tempts to grow single crystals of the above molecules to
reveal the molecular packing in the crystal are under cur-
rent investigation.
The absorption spectrum, and the morphology formed
by the blends of the SM and fullerene are generally consid-
ered as the main factors that influence the Jsc of SM/fuller-
ene BHJ solar cells. For all the optimized photovoltaic
devices,
2
a higher Jsc based on TT(HTTzHT) :PC61BM
2
(
5.77 mA/cm ) was obtained than that of the device based
2
on T(HTTzHT)
PC61BM (1.41 mA/cm ). This could be explained by the rel-
ative higher absorption coefficients of TT(HTTzHT) as
2 2
:PC61BM (1.77 mA/cm ) and TTT(HTTzHT) :
2
It is noted that the lamella distance of TT(HTTzHT)
2
2
molecules in crystalline part reduced from 23.1 Å to
21.4 Å (see in Fig. 6), when PC71BM was used as acceptor
to replace PC61BM. This means that the donor molecules
are packing more closely, and more efficient BHJ hole
transporting domains formed. As a result, the photovoltaic
performances are improved, with the PCE increased from
2.22% to 2.45%. Note in the optimized photovoltaic cell that
indicated in Table 1, and the different hierarchical struc-
tures in blend film as discussed in the following part.
The structural ordering of pristine T(HTTzHT)
TT(HTTzHT) and TTT(HTTzHT) in spin-coating films from
CHCl solutions onto glass substrates with same condition
2
,
2
2
3
was investigated by X-ray diffraction (XRD) (Fig. S3, Sup-
porting Information). The strong diffraction peaks of
replacing PC61BM with PC71BM as acceptor and adding DIO
c
2
T(HTTzHT)
2
, TT(HTTzHT)
2
, and TTT(HTTzHT)
2
at q values
(Fig. 6 TTðHTTzHTÞ Þ, can not only reduce the stacking dis-
ꢀ
1
ꢀ1
ꢀ1
of 0.328 Å , 0.272 Å , and 0.331 Å suggest that they
are in the crystalline state in the film, with corresponding
d spacing (d100) of 19.2 Å, 23.1 Å and 19.0 Å, respectively.
These results indicate these small molecules are easy to
tance (20.9 Å) but also increase the crystallization of the
blend film, which could form more closed stacking
crystalline state and improved photovoltaic performance
2
(Jsc = 7.87 mA/cm , PCE = 3.24%). Therefore, we can

1
432
Y. Chen et al. / Organic Electronics 14 (2013) 1424–1434
Fig. 7. Topography images obtained by tapping mode AFM showing the morphology of the blend films spin-coating from chloroform for: (a)
T(HTTzHT) :PC61BM (w/w, 1:1); (b) TT(HTTzHT) :PC61BM (w/w, 1:1); (c) TTT(HTTzHT) :PC61BM (w/w, 1:1) with (size: 5 m).
m ꢁ 5
2
2
2
l
l
Fig. 8. TEM images obtained by the blend films spin-coating from chloroform for: (a) T(HTTzHT)
2 2
:PC61BM (w/w, 1:1); (b) TT(HTTzHT) :PC61BM (w/w, 1:1);
(
c) TTT(HTTzHT) :PC61BM (w/w, 1:1). The insets are the corresponding electron diffraction (SAED) patterns.
2
conclude that a more closed lamella stacking and high
crystallization of donor molecules in blend film will lead
to better photovoltaic properties by adding DIO or replac-
ing PC61BM with PC71BM. The reason is that excitons and
charges are hopping easier in ordered structure and con-
tinuous transport channel, leading to significantly in-
creased Jsc [41].
As OSCs properties are linked not only to molecular
stacking and crystallization, but also to the morphology
and relative miscibility of the components, we employed
AFM and TEM to assess the correlation between spatial
dimensions of phase-separated domains and OSCs proper-
To probe further the morphology throughout the entire
film, TEM experiments were carried to obtain a real-space
image of the phase separated morphology of the SM:PCBM
blend film structure, where the domains can be distin-
guished from their different electron densities [42].
Fig. 8a shows the TEM image of T(HTTzHT) :PC61BM (w/
2
w, 1:1) blend film. PCBM grain-aggregation with the size
around 60 nm is almost homogeneously dispersed in the
2
T(HTTzHT) matrix. The large aggregation may result in a
decreased diffusional escape probability for mobile charge
carriers, and hence increased recombination. The diffused
ring from its corresponding electron diffraction pattern
(inset of Fig. 8a) shows the low crystallization of
ties. Fig. 7 shows the AFM height images of T(HTTzHT)
2
:-
PC61BM, TT(HTTzHT) :PC61BM and TTT(HTTzHT) :PC61BM
2
2
2 2
T(HTTzHT) and PCBM. As for the TTT(HTTzHT) , the TEM
blend films with the same weight ratio (1:1 w/w) spin-coat-
ing from chloroform solutions. As clearly evidenced by AFM,
image of blend film (Fig. 8c) shows weak bean-like aggre-
gation and lamella phase separation with domain size
about 15 nm. Meanwhile, the weak inner and outer reflec-
tion rings of corresponding SAED pattern is from the
the as-cast blend films of T(HTTzHT)
TTT(HTTzHT) :PC61BM show relatively low surface rough-
ness of 2.11 and 3.93 nm (Fig. 7a and c and Table 2), respec-
tively. As for the TT(HTTzHT) :PC61BM blend film, it shows a
relatively higher coarse surface with the root mean square
RMS) of 5.18 nm (Fig. 7b and Table 2), which may come
2
:PC61BM and
2
TTT(HTTzHT)
2
crystals. Although the crystallization has
2
been increased, the deteriorated morphology owning to
the relatively poor solubility gives a more important influ-
ence on the recombination of charge carrier, thus the low
FF [43]. All above are fully consistent with the relative
low short circuit current densities and high series resis-
(
from the high crystallization tendency of molecule, as re-
vealed by DSC. This will result in an ordered structure, con-
tinuous carrier transport channel, and hence increase the
distance of carrier transport and decrease recombination,
leading to the greatly improved short circuit current densi-
tance (R
mA/cm ,
(Jsc = 1.57 mA/cm ,
s
) obtained for the T(HTTzHT)
2
:PC61BM (Jsc=1.77 -
= 29.39 O cm ) and TTT(HTTzHT) :PC61BM
= 40.73 O cm ) photovoltaic cells.
The relative low shunt resistances (Rsh) of T(HTTzHT) :-
PC61BM and TTT(HTTzHT) :PC61BM based OSCs (Rsh = 857
2
2
R
s
2
2
2
R
s
ties from 1.57 mA/cm2 for TTT(HTTzHT)
2
:PC61BM to
2
2
2
5.77 mA/cm for TT(HTTzHT) :PC61BM.
2

Y. Chen et al. / Organic Electronics 14 (2013) 1424–1434
1433
formance of OSCs by using SM:PC61BM as the active layer
was obtained in the order of TT(HTTzHT) > T(HTTzHT) > -
TTT(HTTzHT) . XRD experiments of the SM:PC61BM blend
2
2
2
films showed well ordered packing structure in the crystal-
line state, and the more closed lamella stacking of donor
material in blend film lead to better photovoltaic proper-
ties. AFM and TEM experiments also revealed morphology
control by adjusting the crystallization of SM and PCBM is
an effective way to improve the performance of OSCs. OSCs
with the highest power conversion efficiency of 3.24% was
2
also observed when the TT(HTTzHT) was blended with
PC71BM and 1% DIO as additives. Our results indicated that
careful molecular design can improve the photovoltaic
properties of A–D–A based small molecules by incorpora-
tion oligothiophene moiety with proper conjugated length
and two sets of Tz moieties. This would be very useful in
designing photovoltaic cell materials with high efficiency.
2
Fig. 9. The IPCE spectra of devices based on TT(HTTzHT) :PC71BM with 1%
DIO additive.
2
Acknowledgments
and 1394 O cm ) are also observed from the I–V curve, and
the Voc (0.73 and 0.66 V) and FF (0.38 and 0.41) deterio-
The financial support from the National Nature Science
Foundation of China (Grant Nos. 21204057, 21074079 and
2
rated significantly. For blend films of TT(HTTzHT) :PC61-
BM, elongated fibrillar structure was observed (Fig. 8b),
which can greatly increase the interface area and construct
the continuous transport channel. It is notable that the
SAED pattern of TT(HTTzHT)
stronger intensity than the others, indicating a more or-
dered structure formed. This leads to the lower
2
1274099), Key Project in Science & Technology Innova-
tion Cultivation Program of Soochow University, Jiangsu
Provincial Natural Science Foundation of China (Grant No.
BK2012213), Research Fund for the Doctoral Program of
Higher Education of China (Grant No. 20113201120004)
and a Project Funded by the Priority Academic Program
Development of Jiangsu Higher Education Institutions is
gratefully acknowledged.
2
:PC61BM blend film shows
R
s
2
2
(
5.26 O cm ) and higher Rsh (1510 O cm ) of the photovol-
taic cell, and hence greatly improved Jsc to 5.77 mA/cm2
and FF to 0.52.
The incident photon-to-current conversion efficiency
(
IPCE) curve of the optimized OSCs based on
Appendix A. Supplementary material
TT(HTTzHT) :PC71BM (w:w, 1:1) with 1% DIO additive un-
2
der monochromatic light is shown in Fig. 9. The IPCE curve
exhibits monochromatic IPCE maximum of 60% at 430 nm
and a broad response covering 300–700 nm, consistent
with the optical absorption spectrum (Fig. 2b). This IPCE
value indicates that the photoresponse is very efficient
for this SM based OSCs device. The Jsc calculated from the
TGA, umnormalized UV–Vis absorption spectra of small
molecule film, XRD spectra of small molecules, calculated
optimized molecule structures of small molecules and
molecule dimension. Supplementary data associated with
this article can be found, in the online version, at http://
dx.doi.org/10.1016/j.orgel.2013.02.038.
integral of IPCE curve based on TT(HTTzHT)
AM1.5G reference spectrum were 7.87 mA cm , consis-
2
with an
ꢀ
2
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. Conclusions
[
2
We have designed and synthesized a series of high
2
coplanar A–D–A SMs with varied electron donor or accep-
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[
1
4
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