A crystalline D-π-A organic small molecule with naphtho[1,2-b:5,6- b′]dithiophene-core for solution processed organic solar cells

Article abstract of DOI:10.1016/j.orgel.2012.09.019

In this work, we have designed and synthesized a new naphtho[1,2-b:5,6- b′]dithiophene-containing enlarged π-conjugated donor-acceptor (D-A) small molecule, NDT(TTz)₂, for use in solution-processed organic photovoltaics. NDT(TTz)₂, which contains a thiophene-bridged naphtho[1,2-b:5,6-b′]dithiophene as the central fused core and triphenylamine-flanked thiophene thiazolothiazole as a spacer, was synthesized via sequential Suzuki and Stille coupling reactions. The thermal, physiochemical, and electrochemical properties of NDT(TTz)₂ have been evaluated by differential scanning calorimetry, thermogravimetry, UV-Vis spectroscopy, photoluminescence spectroscopy, X-ray diffraction, and cyclic voltammetry. As desired for photovoltaic applications, NDT(TTz)₂ possesses good solubility, thermal stability, and a well-ordered, π-π stacked, crystallinity. The optical band gap and HOMO level of NDT(TTz) ₂ were determined to be 2.0 eV and -5.23 eV, respectively. In addition to organic thin film transistor studies, application of NDT(TTz) ₂ to preliminary photovoltaic devices has also been investigated by fabricating solution-processed bulk heterojunction solar cells together with PC₇₁BM in a typical layered device structure, ITO/PEDOT:PSS/NDT(TTz) ₂:PC₇₁BM/LiF/Al. Without extensive optimization of the device, NDT(TTz)₂ in these devices shows a maximum power conversion efficiency of 1.44% under AM 1.5 illumination at a 100 mW/cm² intensity.

Full text of DOI:10.1016/j.orgel.2012.09.019

Organic Electronics 13 (2012) 3183–3194
Contents lists available at SciVerse ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
A crystalline D-
p
-A organic small molecule with
naphtho[1,2-b:5,6-b0]dithiophene-core for solution processed
organic solar cells
Pranabesh Dutta a, Hanok Park a, Woo-Hyoung Lee b, In-Nam Kang b, Soo-Hyoung Lee a,
⇑
a School of Semiconductor and Chemical Engineering, Chonbuk National University, Duckjin-dong 664-14, Jeonju 561-756, Republic of Korea
b Department of Chemistry, The Catholic University, 43-1, Yeokaok2-dong, Wonmi-gu, Buchen-si, Gyeonggi-do 420-743, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
In this work, we have designed and synthesized a new naphtho[1,2-b:5,6-b0]dithiophene-
containing enlarged -conjugated donor–acceptor (D–A) small molecule, NDT(TTz)2, for
Article history:
Received 3 August 2012
Received in revised form 20 September
2012
Accepted 23 September 2012
Available online 12 October 2012
p
use in solution-processed organic photovoltaics. NDT(TTz)2, which contains a thiophene-
bridged naphtho[1,2-b:5,6-b0]dithiophene as the central fused core and triphenylamine-
flanked thiophene thiazolothiazole as a spacer, was synthesized via sequential Suzuki
and Stille coupling reactions. The thermal, physiochemical, and electrochemical properties
of NDT(TTz)2 have been evaluated by differential scanning calorimetry, thermogravimetry,
UV–Vis spectroscopy, photoluminescence spectroscopy, X-ray diffraction, and cyclic vol-
tammetry. As desired for photovoltaic applications, NDT(TTz)2 possesses good solubility,
Keywords:
Small molecules
Naphtho[12-b:5,6-b0]dithiophene
Solution-processable
Organic solar cells
thermal stability, and a well-ordered,
p–p stacked, crystallinity. The optical band gap
and HOMO level of NDT(TTz)2 were determined to be 2.0 eV and ꢀ5.23 eV, respectively.
In addition to organic thin film transistor studies, application of NDT(TTz)2 to preliminary
photovoltaic devices has also been investigated by fabricating solution-processed bulk het-
erojunction solar cells together with PC71BM in a typical layered device structure, ITO/PED-
OT:PSS/NDT(TTz)2:PC71BM/LiF/Al. Without extensive optimization of the device,
NDT(TTz)2 in these devices shows a maximum power conversion efficiency of 1.44% under
AM 1.5 illumination at a 100 mW/cm2 intensity.
Power conversion efficiency
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
as 8.3% for conventional solar cell device structure and
9.2% for an inverted device structure [6].
Conjugated polymers with alternating donor–acceptor
structures have been the subject of continued investigation
in recent years as photoactive donor materials in bulk-
heterojunction (BHJ) polymer solar cells (PSCs). They have
received great attention due to their strong absorption
ability, tunable energy levels and good film-forming
properties [1–5]. Great progress has recently been seen
by thorough structural and device optimization, with
power conversion efficiency (PCE) of PSCs reaching as high
By virtue of being promising alternative photoactive
materials to conjugated polymers due to simple synthesis,
monodispersity, and high purity, solution-processable
p-conjugated D–A organic small molecules, particularly
dendrimers or oligomers, have also concurrently received
considerable attention in the recent literature for their
applicationsto BHJorganic solar cells (OSCs) [7–9]. In partic-
ular, effort has been dedicated to the development of new
small molecules that are solution processable over those
that are vaccum-processed because of the promise of the
more cost-effective solution-based solar cell device fabrica-
tion techniques. Although, D–A conjugated copolymers
have proven to be efficient photoactive materials in BHJ
⇑
Corresponding author. Tel.: +82 063 270 2435; fax: +82 063 270 2306.
E-mail address: shlee66@jbnu.ac.kr (S.-H. Lee).
1566-1199/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.orgel.2012.09.019

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P. Dutta et al. / Organic Electronics 13 (2012) 3183–3194
devices with PCE reaching 8%, the development of solar cells
composed of solution processeable small molecules are still
in their early stages. Typical small molecule solar cell PCEs
are much lower than those of polymers, which is the biggest
obstacle for their use as alternate photoactive materials in
commercial photovoltaic devices. To overcome this, many
research endeavors were also devoted along with conju-
gated polymers to tuning the optical, electrochemical, and
electronic properties of the small molecules via the incorpo-
ration of electron-donor or -acceptor moieties, including
well-known organic dyes such as, merocyanine (MC),
squaraine (SQ), borondipyrromethene (BODIPY), quinacri-
done (QD), diketopyrrolopyrrole (DPP), and isoindigo (ID)
[7–15]. More recently, interest has grown in the incorpora-
gated D-p-A small molecule, NDT(TTz)2 (Chart 1) consisting
of a naphtho[1,2-b:5,6-b0]dithiophene as both donor and
core, thiazolothiazole as acceptor, and triphenyl amine as
auxiliary donor and hole transporting material. As an elec-
tron acceptor, thiazolothiazole has attracted a great deal of
attention in the recent literature in the construction of D–A
conjugated polymers for application to PSCs because of its ri-
gid coplanar structure, which facilitates favorable intermo-
lecular p–p interaction interactions, thus facilitating charge
transport through intermolecular hopping [23–30].
Although the electron-withdrawing ability of thiazolothiaz-
ole unit is weaker than that of benzothiadiazole, the low-ly-
ing HOMO of molecules containing thiazolothiazole lends a
better oxidative stability to the corresponding molecule
and a higher open circuit voltage (Voc) in the final solar cell
devices [27–30]. Because of the higher Voc in the related solar
cell devices, encouraging photovoltaic device performances
have also recently been extracted from the small molecules
containing the thiazolothiazole unit with very simple molec-
ular architectures [31,32]. Additionally, we have incorpo-
rated decylthiophene into the conjugated backbone of
NDT(TTz)2, expecting to simultaneously increase the effec-
tion of multicyclic rigidly fused p-conjugated aromatic units
into small molecules because it is well-known that fused
thiophene rings can stabilize the quinodal structure and in-
crease the rigidity and coplanarity of the molecular back-
bone. This can in turn increase absorption, reduce band
gap, and improve the charge transport characteristics,
which can all contribute to enhanced device efficiency. For
instance, Chen and coworkers reported a small molecule
including a benzo[1,2-b:4,5-b0]dithiophene (BDT) fused
core unit and an alkyl cyanoacetate-capped oligothiophene
as a spacer, which exhibited a high PCE value of 5.44% [16].
Later, the BDT unit was replaced with a dithieno[3,2-b:20,
30-d]silole (DTS) unit, which improved the PCE value up to
5.84% [17]. Marks and coworkers introduced a naph-
tho[2,3-b:6,7-b0]dithiophene fused core in a D–A small mol-
ecule using diketopyrolopyrole (DPP) as arms [18]. The
small molecule derived from the naphtho[2,3-b:6,7-
b0]dithiophene unit demonstrated a high absorption coeffi-
cient (1.1 ꢁ 105 Mꢀ1 cmꢀ1), an ideal HOMO energy level
(ꢀ5.4 eV), and remarkable photovoltaic performance with
a PCE of 4.06%. Bazan and coworkers reported synthesis of
a small molecule using DTS as the core, which showed broad
absorption extending beyond 700 nm [19]. BHJ solar cell de-
vices fabricated by blending this molecule with PC71BM
showed a noticeably high short-circuit current (Jsc) of
10.9 mA/cm2 and a PCE of 3.2%. Later, they reported an anal-
ogous small molecule with the same DTS core, in which the
acceptor-core linking pattern was changed, which demon-
strated outstanding photovoltaic performance with a record
PCE of 6.7% [20]. More recently, Seo and coworkers em-
ployed the DTS unit as the rigid core and DPP as the acceptor
[21]. The solution-processed photovoltaic devices based on
this small molecule exhibited a PCE of 2.2%. Along the same
lines, we have also recently reported for the first time a no-
tive p-conjugation and improve the solution-processability
of the resultant molecule. NDT(TTz)2 was synthesized by
sequential palladium(0)-catalyzed Suzuki and Stille coupling
reactions and its physical properties have been carefully
investigated. Organic thin film transistor devices (OTFT) have
been constructed to evaluate the charge transport ability of
NDT(TTz)2. Finally, solution-processed bulk-heterojuncation
photovoltaic devices were fabricated using NDT(TTz)2 as do-
nor and (6, 6)-phenyl C71-butyric acid methyl ester (PC71BM)
as acceptor to explore its potential in OSCs.
2. Experimental section
2.1. Materials
2,6-Dihydroxy naphthalene, 4-(diphenylamino)phenyl-
boronic acid (14), anhydrous chlorobenzene (99.8%), and
tri(o-tolyl)phosphine [P(o-tol)3] and were purchased from
Aldrich. Palladium(II)acetate [Pd(OAc)2], tris(dibenzyli-
deneacetone)dipalladium(0) [Pd2(dba)3], tetrakis(triphen-
ylphosphine)palladium(0) [Pd(PPh3)4] were obtained
from Strem Chemicals. Tetrahydrofuran was distilled over
sodium/benzophenone immediately before use to main-
tain the anhydrous conditions. Chloroform was purified
by refluxing with calcium hydride and then distilled. All
other chemicals were reagent grade and obtained from
commercial sources (Fluka, Across, and TCI) and used
as-received without further purification unless stated
otherwise. 2-Bromo-3-decylthiophene [33], naphtho[1,2-
b:5,6-b0]dithiophene (5) [34], and 2,5-bis(5-bromo-3-
decylthiophen-2-yl)thiazolo[5,4-d]thiazole (13) [32] were
synthesized according to literature procedures.
vel class of
blocks, naphtho[1,2-b:5,6-b0]dithiophene, with great poten-
tial in the development of D- -A conjugated small mole-
cules in combination with 2,1,3-bezothiadiazole
p-conjugated multicyclic rigidly fused building
p
a
acceptor unit [22]. Preliminary photovoltaic studies based
on one of the small molecule containing the naphtho[1,2-
b:5,6-b0]dithiophene unit, TBNBT (Chart 1) exhibited prom-
ising photovoltaic properties with a maximum PCE of
ꢂ2.20% in preliminary device characterization.
2.2. Synthesis
To shed more light on the potential of the naphtho[1,2-
b:5,6-b0]dithiophene building block in solution-processed
small molecule organic solar cells, we designed a novel and
elongated naphtho[1,2-b:5,6-b0]dithiophene-based conju-
2.2.1. 2,7-Bis(trimethylstannyl)naphtho[1,2-b:5,6-b0]-
dithiophene (6)
To a solution of naphtho[1,2-b:5,6-b0]dithiophene (0.5 g,
2.08 mmol) in THF (20 mL), TMEDA (0.68 mL, 4.57 mmol)

P. Dutta et al. / Organic Electronics 13 (2012) 3183–3194
3185
Chart 1. Molecular structures of the small molecules containing naphtho[1,2-b:5,6-b0]dithiophene unit.
was added at ꢀ50 °C under stirring, and then 1.6 M n-butyl-
lithium in hexane (4.57 mmol, 2.85 mL) was added drop-
wise at ꢀ78 °C. The solution was stirred at ꢀ78 °C for
30 min and then refluxed for 2 h under N2 atmosphere.
The reaction mixture was then cooled to ꢀ78 °C, and
trimethyl tin chloride in THF (1.04 mL, 5.0 mmol) was added
in one portion. The reaction mixture was warmed to room
temperature and was stirred for 6 h. The reaction was
quenched with 20 mL of water and the compound was ex-
tracted with dichloromethane. The organic layer was
washed twice with 20 mL brine and dried with anhydrous
MgSO4. Finally the residue obtained after removing the sol-
vent was recrystallized from acetone to yield 0.82 g (70%) of
6 as a white solid. 1H NMR (400 MHz, CDCl3, ppm) d: 7.56 (s,
2H, ArH), 7.92 (d, 2H, ArH), 8.05 (d, 2H, ArH). 13C NMR
(100 MHz, CDCl3, ppm), 0.47 (s, 18H, Sn(CH3)). d: 143.3,
138.7, 138.5, 133.2, 125.4, 121.9, 121.4. Elemental analysis:
Calculated for C20H24S2Sn2: C, 42.44%; H, 4.27%; S, 11.33%.
Found: C, 42.51%; H, 4.19%; S, 11.39%.
29.56, 22.89, 14.32. Elemental analysis: calculated for
42H52S4, C, 73.63%; H, 7.65%; S, 18.72%. Found: C, 73.56%;
H, 7.79%; S, 18.65%.
C
2.2.3. Synthesis of 2,7-bis(3-decylthiophene-2-yl)naphtho
[1,2-b:5,6-b0]dithiophene distanane (8)
2,7-Bis(3-decylthiophene-2-yl)naphtho[1,2-b:5,6-b0]-
dithiophene distanane (8) was prepared similarly to the
method reported for compound 6. Briefly, to a solution of
7 (0.5 g, 0.88 mmol) in THF (20 mL), TMEDA (0.29 mL,
1.94 mmol) was added at ꢀ50 °C under stirring, and then
1.6 M n-butyllithium in hexane (1.94 mmol, 1.20 mL) was
added dropwise at ꢀ78 °C under N2 atmosphere. After
30 min, the solution was refluxed for 2 h. The reaction mix-
ture was cooled to ꢀ78 °C, and trimethyl tin chloride in
THF (0.44 mL, 2.4 mmol) was added in one portion. The
reaction mixture was warmed to room temperature and
stirred for 6 h. The reaction was quenched with 20 mL of
water and the compound was extracted with dichloro-
methane. The organic layer was washed twice with
20 mL brine and dried with anhydrous MgSO4. The residue
obtained after removing the solvent was recrystallized
from acetone to yield 0.45 g (62%) of 8 as a yellow solid.
1H NMR (400 MHz, CDCl3, ppm) d: 8.00 (d, 2H, ArH), 7.86
(d, 2H, ArH), 7.44 (s, 2H, ArH), 7.06 (s, 2H, ArH), 2.94 (t,
4H), 1.77-1.69 (m, 4H), 1.44–1.24 (m, 28H), 0.88 (t, 6H),
0.50 (s, 18H, Sn(CH3)). 13C NMR (CDCl3, 100 MHz, ppm)
d: 141.78, 138.52, 138.26, 137.69, 137.58, 136.21, 135.98,
125.53, 122.83, 122.42, 121.24, 31.91, 30.88, 29.67, 29.63,
29.61, 29.46, 29.35, 29.24, 22.67, 14.11. Elemental analy-
sis: calculated for C48H68S4Sn2, C, 57.04%; H, 6.78%; S,
12.69%. Found: C, 57.12%; H, 6.82%; S, 12.63%.
2.2.2. 2,7-Bis(3-decylthiophene-2-yl) naphtho[1,2-b:5,6-b0]-
dithiophene (7)
In a 50 mL flame-dried two-neck flask fitted with a
condenser,
2,7-bis(trimethylstannyl)naphtho[1,2-b:5,6-
b0]dithiophene (6) (2.0 g, 3.53 mmol), 2-bromo-3-decylthi-
ophene (2.67 g, 8.82 mmol, 2.5 eq.), and dichlorobis-(tri-
phenylphosphine)palladium(II) (118 mg, 0.17 mmol) were
added and subjected to three vacuum/argon fill cycles.
Argon-degassed chlorobenzene (20 mL) was added and
the mixture was stirred for 20 minunder argon. The reaction
mixture was heated to reflux for 24 h and monitored by TLC.
After completion of the reaction, the reaction was quenched
with 20 mL of water and the compound was extracted with
dichloromethane. The organic layer was washed twice with
20 mL brine and dried with anhydrous MgSO4. Dichloro-
methane was removed under reduced pressure. The result-
ing crude product was purified by column chromatography
eluting with hexane to afford compound 7 (1.52 g, 63%). 1H
NMR (400 MHz, CDCl3, ppm) d: 8.01 (d, 2H, ArH), 7.88 (d, 2H,
ArH), 7.46 (s, 2H, ArH), 7.25 (s, 2H, ArH), 6.99 (d, 2H, ArH),
2.89 (t, 4H), 1.75–1.65 (m, 4H), 1.42–1.27 (m, 28H), 0.85 (t,
6H). 13C NMR (CDCl3, 100 MHz, ppm) d: 140.95, 138.60,
137.77, 135.95, 130.44, 125.81, 124.89, 123.67, 122.72,
121.54, 32.13, 31.13, 30.96, 29.85, 29.83, 29.75, 29.69,
2.2.4. 4-(5-(50-(5-Bromo-3-decylthiophene-2-yl)thiazolo[5,4-
d]thiazol-2-yl)-4-decylthiophen-2-yl)-N,N-diphenylbenzen-
amine (15)
To a mixture of 2,5-bis(5-bromo-3-decylthiophen-2-yl)
thiazolo[5,4-d]thiazole (13) (300 mg, 0.40 mmol), 4-(diphen-
ylamino)phenylboronic acid 14 (116 mg, 0.40 mmol), and
Pd(PPh3)4 (23 mg, 5 mol%) under N2 atmosphere, 12.0 mL of
degassed toluene and 4.0 mL of degassed 2 M K2CO3 (aq) were
added and the resulting solution was vigorously stirred and
heated at reflux for 24 h. After being cooled to room tempera-
ture, water was added and extracted with CH2Cl2 three times.

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P. Dutta et al. / Organic Electronics 13 (2012) 3183–3194
The organic layer was washed with brine and water and then
dried over anhydrous MgSO4. After removal of the solvent un-
der reduced pressure, the crude product was purified by col-
umn chromatography using a 4:1 hexane/CH2Cl2 mixture as
eluent to afford 15 (205 mg, 56%) as an orange solid. 1H
NMR (400 MHz, CDCl3, ppm) d: 7.42-7.20 (dd, 4H), 7.18-7.06
(dd, 2H), 7.16-7.02 (dd, 8H), 7.04 (s, 1H), 6.85 (s, 1H), 2.87–
2.78 (m, 4H), 1.66–1.60 (m, 4H), 1.37–1.19 (m, 28H), 0.81–
0.78 (t, 6H). 13C NMR (CDCl3, 100 MHz, ppm) d: 161.75,
159.61, 148.04, 147.23, 145.74, 144.38, 143.06, 133.48,
133.37, 129.93, 129.36, 129.22, 127.26, 127.03, 126.54,
125.66, 124.81, 124.27, 123.42, 123.04, 122.78, 115.10,
31.88, 30.48, 30.16, 29.86, 29.81, 29.67, 29.58, 29.55, 29.46,
29.39, 22.67, 14.11. Elemental analysis: calculated for C50H58-
N3S4Br, C, 66.05%; H, 6.43%; N, 4.62%. S, 14.11% Found: C,
66.18%; H, 6.57%; N, 4.51%, S, 14.02%.
sphere. The temperature of degradation (Td) corresponded
to a 5% weight loss. Differential scanning calorimetry
(DSC) experiments were performed on a TA Instrument
(DSC 2910) at a heating rate of 10 °C minꢀ1 under a nitrogen
atmosphere. Cyclic voltammetry (CV) measurements were
performed on a VersaSTAT3 (METEK) cyclic voltammeter
under argon at a scan rate of 50 mV sꢀ1 at room tempera-
ture, in which a Pt wire and Ag/AgCl were used as the coun-
ter and reference electrodes respectively. The reference
electrode was calibrated with Fc/Fc+ as an external standard.
The samples were prepared in chloroform solution
with 0.10 M tetrabutylammonium hexafluorophosphate
(n-Bu4NPF6) as electrolyte. Thin film X-ray diffraction
(XRD) patterns were recorded using an on a PANalytical X-
Pert diffractometer operating at 30 kV and 20 mA with a
Cu K
a
radiation (k = 1.5405 Å) at a scan speed of 1o/min
and a step size of 0.04°. The measurements were obtained
in a scanning interval of 2h between 1° and 30°. The film
samples were fabricated by drop-casting on a silicon wafer,
followed by drying under vacuum (solvent: chloroform,
solution concentration: 10 mg/mL). The surface morphol-
ogy was measured using a Digital Instruments Multimode
atomic force microscope (AFM) controlled by a Nanoscope
IIIa scanning probe microscope 20 controller.
2.2.5. 4,40-(5,50-(5,50-(50,500-(naphtho[1,2-b:5,6-b0]dithioph
ene-2,7-diyl)bis(4,40-didecyl-2,20-bithiophene-50,5-diyl))
bis(thiazolo[5,4-d]thiazole-5,2-diyl))bis(4-decylthiophene-
5,2-diyl))bis(N,N-diphenylbenzenamine) [NDT(TTz)2]
A mixture of compound 8 (220 mg, 0.22 mmol), com-
pound 15 (480 mg, 0.53 mmol), Pd2(dba)3 (10 mg,
5 mol%) and P(o-Tolyl)3 (7 mg, 10 mol%) were added to a
50 mL flame-dried two-neck flask and subjected to three
vacuum/argon fill cycles. Argon-degassed chlorobenzene
(9 mL) was added and the mixture was stirred for 30 min
flushing argon. The reaction mixture was heated to reflux
for 24 h. After completion of the reaction, chlorobenzene
was removed under reduced pressure and the crude prod-
uct was adsorbed on silica gel and purified by column
using a 6:1 hexane/CH2Cl2 solvent mixture to yield the
target small molecule NDT(TTz)2 (346 mg, 68%) as a brown
solid. 1H NMR (400 MHz, CDCl3, ppm) d: 7.62 (d, 2H), 7.50
(d, 2H), 7.58 (s, 2H), 7.50 (s, 2H), 7.40 (s, 2H), 7.23–6.98 (m,
20H), 6.9 (s, 4H) 6.88 (s, 2H), 6.88–6.79 (d, 4H), 6.59-6.49
(d, 4H), 2.62–2.36 (m, 12H), 1.66–1.28 (m, 96H), 0.93–
0.87 (m, 18H). 13C NMR (100 MHz, CDCl3, ppm) d:
147.27, 142.93, 142.21, 141.72, 140.11, 137.59, 136.84,
135.73, 134.26, 133.46, 129.27, 127.23, 126.21, 125.37,
125.09, 124.97, 124.63, 123.20, 123.05, 121.72, 120.84,
32.00, 31.98, 31.93, 30.15, 30.00, 29.94, 29.81, 29.79,
29.72, 29.67, 29.61, 29.47, 29.44, 29.38, 22.74, 22.70,
14.17, 14.14. Elemental analysis: calculated for
2.4. OTFT device fabrication and characterization
OTFT devices were fabricated in
a
bottom-contact
m). The
geometry (channel length = 12 m, width = 120
l
l
source and drain contacts were gold (100 nm thick), and
the dielectric was 300 nm thick silicon oxide (SiO2). The
SiO2 surface was cleaned, dried, and pretreated with a
solution of 10 mM octyltrichlorosilane (OTS-8) in toluene
at room temperature for 2 h under nitrogen to produce a
nonpolar, smooth surface onto which the small molecule
could be spin-coated. The small molecule NDT(TTz)2 was
dissolved to a concentration of 0.5 wt% in chloroform.
Films of the organic semiconductor were spin-coated at
1500 rpm for 50 s to a thickness of 50 nm, followed by an
annealing process. The electrical characteristics of the
OTFTs were measured under ambient conditions using a
Keithley 4200 unit. The field-effect mobilities were calcu-
lated from the saturation regime using the equation,
ID = (W/2L)
l
Ci(VG ꢀ VT)2, where ID is the drain current in
C142H166N6S12: C, 72.83%; H, 7.15%; N, 3.59%; S, 16.43%;
the saturated regime, W and L are the channel width and
found: C, 72.89%; H, 7.28%; N, 3.48%; S, 16.35%.
length, respectively, l is the field-effect mobility, Ci is the
capacitance per unit area of the gate dielectric layer, and
2.3. General Instrumentation
VG and VT are the gate and threshold voltages, respectively.
1H and 13C NMR spectra were recorded on a JEOL FT-NMR
(400 MHz) spectrophotometer using CDCl3 as a solvent.
Chemical shifts were reported as d values (ppm) relative to
the internal standard tetramethylsilane (TMS). Elemental
analyses were carried out using a CE Instruments Flash EA
1112 series elemental analyzer. The UV–Vis absorption
spectra (for films and solutions) were obtained using a Shi-
madzu UV-2550 spectrophotometer. Photoluminescence
(PL) spectra of films were obtained using a FP-6500 (JASCO)
fluorometer. Thermogravimetric analysis (TGA) was carried
out with a TA Instruments Q-50 thermogravimetry analyzer
at a scanning rate of 10 °C minꢀ1 under a nitrogen atmo-
2.5. Photovoltaic device fabrication and characterization
All the organic photovoltaic cells were prepared on
commercial ITO-coated glass substrates in a sandwich
structure: glass/ITO/PEDOT:PSS/NDT(TTz)2:PC71BM/LiF/Al.
Prior to use, the patterned indium tin oxide (ITO)-covered
glass substrates were cleaned with deionized water, ace-
tone, and isopropyl alcohol using ultrasonication, followed
by treatment with UV and O3. The PEDOT:PSS (AI 4083, H.
C. Starck) was spin-coated (2600 rpm, 40 s) onto the
cleaned ITO-glass at a thickness of 40 nm and dried at
140 °C for 20 min in atmosphere and then transferred into

P. Dutta et al. / Organic Electronics 13 (2012) 3183–3194
3187
the glove box filled with N2. Blends of NDT(TTz)2 and PC71-
3.2. Optical properties
BM (Nano-C, USA) with different weight ratios (from 1:1 to
1:4 w/w) were solubilized overnight in chlorobenzene
(concentration: 20 mg/mL) or chloroform (concentration:
The absorption spectra of NDT(TTz)2 measured in dilute
chloroform solution and thin film are displayed in Fig. 2,
and the relevant data is summarized in Table 1. The solu-
tion spectrum of NDT(TTz)2 exhibited an absorption band
between 330 and 540 nm with its absorption maximum
(kmax) at 469 nm. Compared to the solution, the thin film
absorption spectrum was red-shifted ꢂ31 nm because of
10 mg/mL) solution, filtered through a 0.45-lm poly(tetra-
fluoroethylene) (PTFE) filter and subsequently spin-coated
at 700 (for chlorobenzene)/1400 (for chloroform) rpm for
60 s (thickness, 60-70 nm) onto the PEDOT:PSS layer of
the ITO. The resulting films were dried at room tempera-
ture for 20 min under nitrogen and then under vacuum
for 12 h. The devices were completed by deposition of a
0.5-nm layer of LiF and a 120 nm Al layer. These layers
were thermally evaporated at a pressure of 1 ꢁ 10ꢀ6 Torr
at room temperature. The active area of every device was
9 mm2. The current–voltage (J–V) characteristics of the
photovoltaic devices were measured in the dark and under
white light illumination at AM 1.5G using a solar simulator
(Newport) at an intensity of 100 mW/cm2, adjusted with a
standard PV reference (2 ꢁ 2 cm) mono-crystalline silicon
solar cell (calibrated at NREL, Colorado, USA) with a Keith-
ley 2400 source-measure unit. The external quantum effi-
ciency (EQE) was determined using a Polaronix K3100
spectrometer.
aggregation and enhanced
p–p interactions between the
conjugated main chains. In addition, the NDT(TTz)2 film
exhibited an additional shoulder in the long wavelength
region of ꢂ556 nm, implying the existence of a well-or-
dered assembly of NDT(TTz)2 in the solid state, which
could be potentially contribute to a higher charge carrier
mobility. The optical band gap (Eogpt) of NDT(TTz)2, esti-
mated by the onset absorption of the thin film, is 2.00 eV.
The thin film band gap thus determined for NDT(TTz)2
was 0.05 eV higher than our recently reported napthodithi-
ophene-containing small molecule, TBNBT with a triphen-
ylamine–benzothiadiazole conjugated spacer [22]. Despite
having an enlarged conjugated skeleton, the fact that
NDT(TTz)2 has a slightly higher band gap as compared to
TBNBT could possibly be attributed to the twisting of the
conjugated backbone and weakening of the effective con-
jugation length caused by the unanticipated steric hin-
drance in the vicinity of the bulky triphenylamine moiety
and its adjacent decyl-substituted thiophene.
3. Results and discussion
3.1. Synthesis and thermal stability
The photoluminescence spectra of NDT(TTz)2 and the
NDT(TTz)2:PC71BM composite films were taken to help
understand the donor–acceptor charge transfer process
from NDT(TTz)2 to PC71BM (Fig. 2). The pristine NDT(TTz)2
film shows a distinct broad emission band with emission
maxima at 637 nm. The photoluminescence of NDT(TTz)2
was significantly quenched (by more than 99%) when it
was blended with PC71BM, even at 1:1 w/w ratio, indicat-
ing an ultra-fast charge transfer from the small molecule
to PC71BM.
The synthetic route to the small molecule NDT(TTz)2 is
illustrated in Scheme 1. The core naphtho[1,2-b:5,6-b0]dithi-
ophene (5) unit was prepared from 2, 6-dihydroxy naphtha-
lene (1). A 3-decanylthiophene unit was attached at the 2-
and 7-positions of the naphtho[1,2-b:5,6-b0]dithiophene core
to improve the solubility of the final NDT(TTz)2 molecule.
Lithiation of 2,7-bis(3-decylthiophene-2-yl)naphtho[1,2-
b:5,6-b0]dithiophene (7) with n-BuLi followed by quenching
with trimethyltin chloride afforded the intermediate 2,7-
bis(3-decylthiophene-2-yl)naphtho[1,2-b:5,6-b0]dithiophene
distannane (8). 2,5-bis(5-bromo-3-decylthiophen-2-yl)thiaz-
olo[5,4-d]thiazole (13) was synthesized following a proce-
dure from the literature. The intermediate 15, protected
with triphenylamine at one end, was synthesized by a selec-
tive Suzuki coupling reaction in a yield of 58%. The target
small molecule NDT(TTz)2 was synthesized via Stille cross
coupling reaction of 8 and 15 using Pd2(dba)3/P(o-tolyl)3 as
the catalyst. The molecular structure of NDT(TTz)2 was con-
firmed by 1H and 13C NMR and elemental analysis. Incorpora-
tion of aliphatic hydrocarbon chains throughout the
conjugated molecular backbone afforded a greater solubility
to NDT(TTz)2, which facilitated device fabrication. The ther-
mal properties of NDT(TTz)2 were investigated by thermo-
gravimetric analysis (TGA) and differential scanning
calorimetry(DSC). TGA analysis suggested a high thermal sta-
bility of NDT(TTz)2 with thermal degradation temperature of
above 387 °C (Fig. 1). DSC experiment at a temperature ramp
rate of 10 °C/min (Inset of Fig. 1) indicated a sharp crystalline
feature of NDT(TTz)2. A distinguishable melting temperature
(Tm) at 168 °C and an isotropic-to-crystalline transition (Tc) at
152 °C were observed for NDT(TTz)2 during the second heat-
ing and cooling cycle.
3.3. Electrochemical properties
Cyclic voltammetry (CV) was employed to examine the
electrochemical properties of NDT(TTz)2. CV was carried
out using a Ag/AgCl reference electrode in a 0.1 M solution
of Bu4NPF6 in acetonitrile at room temperature under ar-
gon with a scan rate of 50 mV sꢀ1. Before CV measurement
of the small molecule, the instrument was calibrated using
the ferrocene/ferrocenium (Fc/Fc+) redox couple as an
external standard. The onset potential of the Fc/Fc+ redox
couple was found to be 0.4 V relative to the Ag/AgCl refer-
ence electrode. Thus, the HOMO energy level of NDT(TTz)2
was calculated according to the following equation:
ox
onset
EHOMO ¼ ꢀðE
þ 4:4Þ ðeVÞ. Fig. 3 shows the cyclic voltam-
mogram of NDT(TTz)2 film cast from chloroform.
NDT(TTz)2 exhibited a single partial oxidation peak, the
onset (Eooxnset) of which is located at 0.83 V (versus Ag/AgCl),
corresponding to a HOMO level of ꢀ5.23 eV. Because we
were unable to obtain a reproducible reduction peak, the
LUMO energy level of NDT(TTz)2 was calculated based on
the HOMO energy level and Eogpt (Eg = EHOMO ꢀ ELUMO) as
ꢀ3.23 eV. Both HOMO and LUMO energy levels thus esti-

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P. Dutta et al. / Organic Electronics 13 (2012) 3183–3194
Scheme 1. Synthetic route to NDT(TTz)2.
1.2
100
80
60
40
20
NDT(TTz)2: PC71BM
NDT(TTz)2 Solution
NDT(TTz)2 Film
400
350
300
250
200
150
100
50
1 : 0
1 : 1
1.0
0.8
0.6
0.4
0.2
0.0
0.2
0.1
Cooling
0.0
-0.1
-0.2
-0.3
Heating
0
50
100 150 200 250
Temperature (oC)
0
100
200
300
400
500
300
400
500
600
700
800
Temperature (oC)
Wavelength (nm)
Fig. 1. TGA curve of the NDT(TTz)2 with a heating rate of 10 °C/min under
nitrogen atmosphere; inset: DSC thermogram of NDT(TTz)2 at
scanning rate of 10 °C/min in the second heating and cooling cycle.
Fig. 2. UV–Vis absorption spectra of NDT(TTz)2 in chloroform solution
and thin film and PL spectra of NDT(TTz)2 and NDT(TTz)2:PC71BM-
blended thin film.
a
a
mated for NDT(TTz)2 were higher compared to the analo-
gous small molecule TBNBT (HOMO = ꢀ5.34 eV and LU-
MO = ꢀ3.34 eV) [22]. As can be seen from the molecular
structures of the small molecules (Chart 1), NDT(TTz)2

P. Dutta et al. / Organic Electronics 13 (2012) 3183–3194
3189
Table 1
The optical, electrochemical and field effect transistor (FET) properties of NDT(TTz)2.
Small molecule
Solution
akmax (nm)
469
Film
dEox/eHOMO (V/eV)
0.83/ꢀ5.23
fLUMO (eV)
gl (cm2 Vꢀ1 sꢀ1
)
gIon/Ioff
bkmax (nm)
bkonset (nm)
cEgopt (eV)
NDT(TTz)2
500
618
2.00
ꢀ3.23
2 ꢁ 10ꢀ6
5 ꢁ 102
a
b
c
d
e
f
Measured in chloroform solution.
Spin-coated film from chloroform solution.
Optical band gap, Eogpt = 1240/(konset
)
film
.
Potential determined by cyclic voltammetry in 0.10 M Bu4NPF6/CH3CN.
HOMO = ꢀ(4.4 + Eooxnset) (eV).
LUMO = Eogpt + HOMO.
g
Hole mobilities (l) and on–off ratios (Ion/Ioff) were extracted from transfer curves.
0.00030
0.00025
0.00020
0.00015
0.00010
0.00005
0.00000
-0.00005
12000
10000
8000
6000
4000
2000
-3.23
-3.91
-5.23
-5.87
5
10
15
20
25
30
35
2
θ
(degrees)
0.00 0.25 0.50 0.75 1.00 1.25 1.50
Potential (V vs Ag/Agcl)
Fig. 4. XRD pattern of as-cast NDT(TTz)2 films. Inset: schematic repre-
sentation of molecular packing of NDT(TTz)2 in the solid state.
Fig. 3. Cyclic voltammogram of NDT(TTz)2 film in a 0.1 M Bu4NPF6–
CH3CN solution with a scan rate of 50 mV/s. Inset: Energy level diagram
showing the HOMO and LUMO energy levels of NDT(TTz)2 and PC71BM.
of the conjugated backbone. In addition, a flat broad peak
at half width peak maximum ꢂ23.91° (d2) was also seen
contains thiazolothiazole, which is a weaker electron
acceptor than benzothiadiazole, present in TBNBT. Thus,
the weaker electron-accepting ability of thiazolothiazole
relative to benzothiadiazole resulted in higher LUMO level
for NDT(TTz)2 compared to TBNBT. The higher HOMO level
for NDT(TTz)2 relative to TBNBT on the other hand could
be explained for having additional electron donating sub-
stituent (decanyl thiophene attached to thazolothaizole)
in the conjugated main chain of NDT(TTz)2. Since, the
HOMO level of NDT(TTz)2 is lower than that of regioregular
poly(3-hexylthiophene) (rrP3HT, ꢀ4.75 eV) [35], a better
oxidative stability and a higher Voc could be expected of
organic solar cells made from NDT(TTz)2 as compared to
rrP3HT.
and ascribed to the interlayer
p–p stacking of the main
chain. It was noted that if NDT(TTz)2 assembled in an
end-to-end packing mode, then the interchain spacing of
NDT(TTz)2 would be 28.01 Å [36]. However, the calculated
Bragg law interchain d-spacing of NDT(TTz)2 is smaller
than the d-spacing assumed for end-to-end packing, likely
indicating that there is some interdigitation of the decyl
chain, as depicted in the inset of Fig. 4.
3.5. Field-effect transistor characteristics
To determine the hole mobility of NDT(TTz)2, thin-film
transistors (TFTs) devices were fabricated in a bottom-gate,
top-contact device configuration, built on n-doped silicon
with an octyltrichlorosilane (OTS-8)-modified silicon diox-
ide (SiO2) gate dielectric (300 nm) on top. The gold (Au)
source/drain electrode pairs (100 nm) were deposited on
top of the thin films by high vacuum thermal evaporation
through a shadow mask with a defined channel length
3.4. X-ray diffraction
An X-ray diffractogram of the NDT(TTz)2 film spin-
coated from chloroform solvent was collected to probe its
molecular ordering and packing. As revealed by the XRD
pattern in Fig. 4, NDT(TTz)2 in solid film appears to assem-
ble into an ordered arrangement, attributing its crystalline
behavior. A strong peak was observed in the small angle re-
gion at 2h = 3.96°, corresponding to a d-spacing (d1) of
22.28 Å, which can be assigned to the interchain spacing
(L = 12 lm) and width (W = 120 lm). The device fabrica-
tion process is described in more detail in the experimental
section. All other device fabrication procedures and mea-
surements were carried out at room temperature under
ambient conditions. Fig. 5 shows plots of IDS as a function

3190
P. Dutta et al. / Organic Electronics 13 (2012) 3183–3194
of drain-source voltage (VDS) at different gate voltages (VG)
and transfer curves at constant VDS. The NDT(TTz)2 TFT de-
vices exhibited p-channel FET responses; their mobility
and on/off ratios are listed in Table 1. From the transfer
curve measured at VDS = ꢀ60 V, the hole mobility of
NDT(TTz)2 was calculated as 2 ꢁ 10-6 cm2 Vꢀ1 sꢀ1 with an
Ion/Ioff ratio of 7 ꢁ 101. The mobility of small molecule
NDT(TTz)2 was found to be one order lower than the
in Fig. 6 and the corresponding short circuit current (Jsc),
open circuit voltage (Voc), fill factor (FF), and power
conversion efficiency (PCE) device data are summarized
in Table 2. The photovoltaic performance of NDT(TTz)2
was found to be slightly better when the active layers were
processed from chlorobenzene
than chloroform.
NDT(TTz)2, when it was blended with PC71BM in chloro-
form at a ratio of 1:3 (w/w), shows a Jsc of 5.50 mA/cm2,
a Voc of 0.64 V, a FF of 0.35, and PCE of 1.23%. In contrast,
at a NDT(TTz)2:PC71BM blending ratio of 1:2 (w/w) in chlo-
robenzene, a Jsc of 4.32 mA/cm2, a Voc of 0.76 V, and a FF of
0.40 were achieved, resulting in a maximum PCE of 1.33%.
It is unusual that most of the cells fabricated from chloro-
form solutions had lower Voc values than the cells cast
from chlorobenzene solutions. Although it is known that
the Voc is directly related to the energy difference between
the HOMO of the donor and the LUMO of the acceptor,
some other factors such as morphology, mobility, and dif-
ference in work function between the two electrodes also
have some influence on the Voc [37–39]. The AFM images
of NDT(TTz)2:PC71BM blend films cast from chloroform
and chlorobenzene solutions are presented in Fig. 7. We
note that the films processed from chloroform were of low-
er quality than those processed from chlorobenzene, show-
ing NDT(TTz)2 aggregation and a large domain size. The
undesirable NDT(TTz)2 aggregation in chloroform-cast
films seems to have reduced its band gap and elevated
its HOMO energy level, thus resulting in a relatively
lowered Voc. The undesirable morphology of NDT(TTz)2/
PC71BM devices from chloroform also resulted in larger-
scale phase separation; as a consequence, excitons cannot
reach the donor–acceptor interfaces efficiently, resulting
in leakage currents due to charge recombination [40–42].
Thus, in addition to the Voc, the fill factors of the devices
cast from chloroform were also found to be lower than
those of the devices processed from chlorobenzene in most
cases.
mobility of the small molecule TBNBT (
l
= 6 ꢁ 10ꢀ5 cm2 -
Vꢀ1 sꢀ1, Ion/Ioff ratio of 7 ꢁ 101) [22]. The low hole mobility
of NDT(TTz)2-OFET devices could presumably be attributed
to either the interfacial contact problems or non-ideal
packing since in addition to the interface between the or-
ganic semiconductor and electrode, the chemical struc-
tures, morphology, and molecular arrangement of the
semiconductors also play a crucial role in the ultimate per-
formance of OFETs. Even though NDT(TTz)2 exhibited a
crystalline behavior, the introduction of decylsubstituted
thiophenes along the conjugated backbone causes the
twisted structure of the conjugated main chain and reduc-
tion in backbone coplanarity, which apparently seems
responsible for limited inter chain packing, finally disfavor-
ing charge transport and hole mobility. More experiments
on OFET devices of NDT(TTz)2 are believed could address
this issue.
3.6. Photovoltaic properties
To explore the potential of this new napthodithioph-
ene-containing small molecule, NDT(TTz)2, in solution-
processed OSC applications, bulk heterojunction photovol-
taic cells using the NDT(TTz)2 small molecule as the donor
and the PC71BM fullerene derivative as the acceptor were
constructed in
a layered ITO/PEDOT:PSS/NDT(TTz)2:-
PCBM/LiF/Al device structure. We fabricated the OSCs ini-
tially by spin coating the active layer from either
chloroform or chlorobenzene solutions containing differ-
ent NDT(TTz)2 to PC71BM weight ratios (from 1:1 to 1:4),
yielding active layer thicknesses of ꢂ82 nm. All devices
were tested in the dark and under AM 1.5G illumination
at an intensity of 100 mW/cm2. Typical solar cell device
current density–voltage (J–V) characteristics are presented
To investigate if the performance of NDT(TTz)2 could be
further improved by controlling the active layer thickness,
we fabricated additional OSC devices in a 1:2 NDT(TTz)2:-
PC71BM blend ratio from chlorobenzene by varying the
thickness of the active layer between 68 and 94 nm
1E-8
VGS= 0
VGS=-10
-6.0x10-10
-5
6.00x10
VGS=-20
VGS=-30
VGS=-40
1E-9
-5
-4.0x10-10
VGS=-50
4.50x10
VGS=-60
VGS=-70
1E-10
-5
3.00x10
-2.0x10-10
0.0
-5
1E-11
1.50x10
0.00
20
1E-12
0
-20
-40
V
-60
DS (V)
-80
-100
-80
-60
-40
VGS (V)
-20
0
Fig. 5. Output characteristics of NDT(TTz)2-based TFT devices at different gate voltages (VGS) and transfer curves for OFET devices at a source-drain voltage
(VDS) of ꢀ60 V.

P. Dutta et al. / Organic Electronics 13 (2012) 3183–3194
3191
1
0
1
(a)
(b)
NDT(TTz)2: PC71BM
0
-1
-2
-3
-4
-5
1:1
1:2
1:3
1:4
-1
-2
-3
-4
-5
-6
-7
NDT(TTz)2: PC71BM
1:1
1:2
1:3
1:4
J
J
0.0
0.2
0.4
0.6
0.8
0.0
0.2
0.4
0.6
0.8
Voltage (V)
Voltage (V)
Fig. 6. (a) J–V curves of solution processed BHJ solar cell devices based on the NDT(TTz)2:PC71BM weight ratios from: (a) chloroform, and (b) chlorobenzene
solutions under AM 1.5G illumination at an intensity of 100 mW/cm2.
Table 2
Summary of photovoltaic performances of the OSCs based on NDT(TTz)2:PCBM blends with different weight ratio and different active layer thickness (68–
95 nm) processed from chlorobenzene (CB) or chloroform (CF) solutions.
Active layer
Thickness
Jsc (mA/cm2)
Voc (V)
FF
PCE (%)
NDT(TTz)2:PC71BM (1:1, CF)
NDT(TTz)2:PC71BM (1:2, CF)
NDT(TTz)2:PC71BM (1:3, CF)
NDT(TTz)2:PC71BM (1:4, CF)
NDT(TTz)2:PC71BM (1:1, CB)
NDT(TTz)2:PC71BM (1:2, CB)
NDT(TTz)2:PC71BM (1:3, CB)
NDT(TTz)2:PC71BM (1:4, CB)
NDT(TTz)2:PC71BM (1:2, CB)
NDT(TTz)2:PC71BM (1:2, CB)
NDT(TTz)2:PC71BM (1:2, CB)
NDT(TTz)2:PC71BM (1:2, CB)
NDT(TTz)2:PC61BM (1:2, CB)
NDT(TTz)2:PC61BM (1:2, CB)
NDT(TTz)2:PC61BM (1:2, CB)
NDT(TTz)2:PC61BM (1:2, CB)
82
82
82
82
82
82
82
82
95
89
74
68
94
87
76
67
2.94
4.37
5.50
4.76
3.83
4.32
3.27
3.16
2.99
4.09
5.10
5.22
2.82
3.27
3.63
3.68
0.78
0.68
0.64
0.63
0.8
0.76
0.73
0.73
0.71
0.72
0.75
0.76
0.73
0.73
0.73
0.73
0.27
0.32
0.35
0.36
0.33
0.40
0.47
0.48
0.31
0.32
0.38
0.33
0.30
0.31
0.32
0.32
0.63
0.96
1.23
1.07
1.02
1.33
1.13
1.11
0.65
0.93
1.44
1.32
0.62
0.74
0.86
0.87
(a)
(b)
RMS =2.55 nm
RMS = 1.02 nm
Fig. 7. AFM images (2.0
and (b) chlorobenzene (1:2 w/w).
lm ꢁ 2.0 lm) showing the surface morphology of the NDT(TTz)2:PC71BM blend films spin-coated from: (a) chloroform (1:3 w/w),
(Fig. 8a). Optimization of the active layer thickness resulted
in slight improvements in device performance (Table 2).
The maximum power conversion efficiency (1.44%) was
achieved with an active layer thickness of 74 nm. Although

3192
P. Dutta et al. / Organic Electronics 13 (2012) 3183–3194
1
(a)
(b)
NDT(TTz)2: PC71BM = 1:2
NDT(TTz)2: PC61BM = 1:2
0
-2
-4
-6
0
-1
-2
-3
-4
94 nm
87 nm
76 nm
67 nm
95 nm
89 nm
74 nm
68 nm
J
J
0.0
0.2
0.4
0.6
0.8
0.0
0.2
0.4
0.6
0.8
Voltage (V)
Voltage (V)
Fig. 8. (a) J–V curves of solution-processed BHJ solar cell devices for NDT(TTz)2:PCBM at a 1:2 ratio (w/w) from chlorobenzene under an illumination of AM
1.5G, 100 mW/cm2.
NDT(TTz)2: PC61BM = 1:2
94 nm
NDT(TTz)2: PC71BM= 1:2
95 nm
(a)
(b)
50
40
30
20
10
0
50
40
30
20
10
0
87 nm
76 nm
67 nm
89 nm
74 nm
68 nm
300
400
500
600
700
800
300
400
500
600
700
800
Wavelength (nm)
Wavelength (nm)
Fig. 9. EQEs of the OSCs illuminated by monochromatic light.
the Voc (0.76–0.75 V) and FF (0.40–0.38) of the devices were
not significantly affected by decreasing the active layer
thickness from 82 nm to 74 nm, the Jsc value was improved
from 4.32 to 5.10 mA/cm2 which helped to enhance the PCE
value. It is noteworthy that decreasing the active layer
thickness further to ꢂ68 nm further improves the Jsc; how-
ever, the Voc and FF both worsen, resulting in a lower overall
photovoltaic performance. The increasing Jsc with decreas-
ing active layer thickness could be ascribed to a more effec-
tive inter-penetrating network structure with better
connectivity between the NDT(TTz)2 and PC71BM phases.
As it is commonly known that for a crystalline molecule, a
thinner active layer reduces the possibility of self-aggrega-
tion and packing, thus allowing more space to accommo-
date PC71BM between the alkyl side chains of the
molecule, which could enhance the compatibility between
the small molecule donor and fullerene derivative acceptor
with improved p/n junction and result in an effective
charge transport, consequently, increase in Jsc.
In order to compare the performance of NDT(TTz)2:-
PC71BM-based active layer system with NDT(TTz)2:
PC61BM-based active layer, some additional devices were
also made for NDT(TTz)2 by fabricating devices with PC61-
BM. NDT(TTz)2 was blended with PC61BM at a ratio of 1:2
and the spin coated thin film thickness was controlled in
the range of 67–94 nm. The J–V curves for the devices are
displayed in Fig. 8b and the respective photovoltaic
parameters have been compiled in Table 2. As shown, by
replacing PC71BM with PC61BM, the PCE for NDT(TTz)2
could not be improved. Even though the Voc and FF of
NDT(TTz)2:PC61BM based devices were comparable with
that of the devices NDT(TTz)2:PC71BM, the Jsc significantly
dropped as a result of the lower visible absorption profile
of PC61BM compared to PC71BM. Thus, the maximum PCE
for NDT(TTz)2:PC61BM based devices was recorded up to
0.87% with Jsc = 3.68 mA/cm2, Voc = 0.73 V and FF = 0.32.
The performance of the small molecule NDT(TTz)2
(PCE = 2.20%, Jsc = 6.18 mA cmꢀ2
, Voc = 0.95 V, FF = 0.37)
when compared to that of our earlier reported small mol-
ecule, TBNBT (PCE = 2.20%, Jsc = 6.18 mA cmꢀ2, Voc = 0.95 V,
FF = 0.37) [22], an inferior photovoltaic response was no-
ticed in case of NDT(TTz)2. The Voc value for NDT(TTz)2
based devices were mostly ꢂ0.2 V lower than the devices
composed of TBNBT, which is consistent considering the
higher HOMO value of NDT(TTz)2 (ꢀ5.23 eV) than
TBNBT(ꢀ5.34 eV). The Jsc value for NDT(TTz)2 was also
found significantly lower with respect to TBNBT. The lower
Jsc value for NDT(TTz)2 is a result of its poor charge trans-
port hole mobility relative to TBNBT. Fig. 9 presents the
external quantum efficiencies (EQEs) of the corresponding
devices measured under monochromatic light. Among the
NDT(TTz)2:PCBM solar cell devices with different active
layer thicknesses, the devices for PC71BM with active layer
thickness of 68 and 74 nm exhibited strong and broad pho-
toresponse in the spectral range from 300 to 700 nm,
corroborating the higher Jsc of the related photovoltaic
devices. While the 74 nm-thick device exhibited a more
intense EQE response in the range of 360–515 nm with a

P. Dutta et al. / Organic Electronics 13 (2012) 3183–3194
3193
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maximum EQE of 50% at 358 nm, the 68 nm-thick device
exhibited a broader EQE response in the visible region,
resulting in its high photocurrent.
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4. Conclusions
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In conclusion, a new enlarged D-
molecule, NDT(TTz)2, incorporating naphtho[1,2-b:5,
6-b0]dithiophene as a rigidly fused
-conjugated central
p-A conjugated small
p
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as a terminal auxiliary donor, was synthesized and charac-
terized for solution-processed organic solar cells. The
above-mentioned donor and acceptor units were linked
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4619;
with 3-decylthiophene as a p-bridging unit, which was ex-
pected to increase solubility of NDT(TTz)2 to facilitate corre-
sponding photovoltaic device fabrication. As anticipated,
insertion of 3-decylthiophene into NDT(TTz)2 afforded
excellent solubility and processability; however, it also af-
fected the planarity of the molecule, causing a medium opti-
cal band gap. Nevertheless, the crystallinity of the molecule
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was demonstrated by organized self-assembly and
p–p
stacking and the energy levels of NDT(TTz)2 were also
shown to be well-aligned with those of PC71BM, allowing
its use in organic solar cell applications. Preliminary photo-
voltaic studies with PC71BM exhibited a maximum PCE of
1.44% with a Jsc, Voc, and FF of 5.10 mA/cm2, 0.75 V, and
0.38, respectively. Further improvements in the PCE by
naphtho[1,2-b:5,6-b0]dithiophene-based OSCs are expected
based on further molecular structure manipulations, which
is presently under consideration in our laboratory.
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Acknowledgements
(c) H. Bürckstümmer, N.M. Kronenberg, M. Gsänger, M. Stolte, K.
Meerholz, F. Wü rthner, J. Mater. Chem. 20 (2010) 240;
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Kronenberg, M. Gsänger, M. Stolte, K. Meerholz, F. Würthner, Angew.
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This work was supported by the New & Renewable
Energy Program of the Korea Institute of Energy Technol-
ogy Evaluation and Planning (KETEP) Grant (No.
20103020010050) funded by the Ministry of Knowledge
Economy, Republic of Korea.
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