Development of novel naphtho[1,2-b:5,6-b′]dithiophene and thieno[3,4-c]pyrrole-4,6-dione based small molecules for bulk-heterojunction organic solar cells

Article abstract of DOI:10.1016/j.dyepig.2016.10.007

Two new small molecules, composed of naphthodithiophene (NDT) donor core and thienopyrroledione (TPD) group acceptor group end-capped with and without an alkyl-bithiophene, defined as NDT(TPD)₂ and NDT(TPDTT)₂ were designed and synthesized by stille coupling reactions. The thermal and electrochemical analyses carried out for both the small molecules revealed good thermal stability along with high decomposition temperature (>350?°C). NDT(TPD)₂ showed a deep HOMO level (?5.38?eV), compared to slightly upshifted HOMO (?5.26?eV) of NDT(TPDTT)₂. While X-ray diffractometry suggests crystalline and amorphous nature of NDT(TPD)₂ and NDT(TPDTT)₂ respectively, the space charge limited current analysis revealed high hole mobility in the former and appreciable charge balance in the later. The conventional organic solar cell (OSC) devices fabricated using NDT(TPD)₂ and NDT(TPDTT)₂ as donor show power conversion efficiency (PCE) of 0.26% and 0.8% respectively. While NDT(TPDTT)₂ device after blending with additive, owing to the improved D-A heterojunction yielded maximum PCE of 1.31% resulting from enhanced J_sc 3.32?mA/cm², V_oc 0.75?V and FF of 52.44.

Full text of DOI:10.1016/j.dyepig.2016.10.007

Dyes and Pigments 137 (2017) 117e125
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
0
Development of novel naphtho[1,2-b:5,6-b ]dithiophene and thieno
[3,4-c]pyrrole-4,6-dione based small molecules for bulk-
heterojunction organic solar cells
a
a
a
a
a
Sushil S. Bagde , Hanok Park , Jang-Gun Han , Yinji Li , Rohan B. Ambade ,
a
b
a, *
Swapnil B. Ambade , ByeongCheol Kim , Soo-Hyoung Lee
School of Semiconductor and Chemical Engineering, Chonbuk National University, Duckjin-dong 664-14, Jeonju 561-756, Republic of Korea
Dason Co., Ltd., Bucheon Technopark 345, Bucheon 303-601, Republic of Korea
a
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 2 July 2016
Received in revised form
Two new small molecules, composed of naphthodithiophene (NDT) donor core and thienopyrroledione
(
TPD) group acceptor group end-capped with and without an alkyl-bithiophene, defined as NDT(TPD)
2
and NDT(TPDTT) were designed and synthesized by stille coupling reactions. The thermal and elec-
2
7
October 2016
trochemical analyses carried out for both the small molecules revealed good thermal stability along with
Accepted 8 October 2016
Available online 11 October 2016
ꢀ
high decomposition temperature (>350 C). NDT(TPD)
2
showed a deep HOMO level (ꢁ5.38 eV),
compared to slightly upshifted HOMO (ꢁ5.26 eV) of NDT(TPDTT)
2
. While X-ray diffractometry suggests
2
respectively, the space charge limited
crystalline and amorphous nature of NDT(TPD) and NDT(TPDTT)
2
Keywords:
current analysis revealed high hole mobility in the former and appreciable charge balance in the later.
The conventional organic solar cell (OSC) devices fabricated using NDT(TPD) and NDT(TPDTT) as donor
show power conversion efficiency (PCE) of 0.26% and 0.8% respectively. While NDT(TPDTT) device after
Organic solar cell
Small molecule
Solution-processed
2
2
2
blending with additive, owing to the improved D-A heterojunction yielded maximum PCE of 1.31%
2
resulting from enhanced Jsc 3.32 mA/cm , Voc 0.75 V and FF of 52.44.
©
2016 Elsevier Ltd. All rights reserved.
1
. Introduction
potential advantages such as well-defined structures, mono-
disperse molecular weight, convenient purification and better
reproducibility, have stimulated great interest as an emerging
alternative to the polymer counterparts [4,5]. These features allow
for a clear understanding of structureeproperty relationships of
small molecules, which play a crucial role in developing high per-
formance new SMOSC materials. Thus far, PCE upto ~10% has been
reported for a solution-processed SMOSC device [6]. The rationale
for achieving high PCE in SMOSCs is typically influenced by inno-
vative molecular designs and their meticulous integration in a
photoactive layer [4]. Various considerations into development of
molecule design involve its solar light absorption capability,
appropriate energy levels [highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital (LUMO)], effec-
tive charge mobility, film forming capability and the ability to form
a desirable photoactive nano-morphology [7]. As a common trend
in developing small molecules, designs typically involve extended
The field of bulk-heterojunction (BHJ) organic solar cells (OSCs)
has witnessed a surge in the development of high power conver-
sion efficiency (PCE) devices, which have currently recorded
promising PCE as high as 10% [1]. The ease of solution processing,
and fascinating mechanical properties complemented by judicious
designs of semiconducting organic materials such as polymers and
small molecules, gained enormous attention as promising candi-
dates for the development of large area and flexible OSCs [2].
Moreover, the possibility of tailoring their interfaces using various
interfacial modifiers (IMs), makes OSCs interesting entities of
modern research [3]. However, critical issues in polymer based
OSCs, such as complex purification during polymer synthesis, poor
yield, and less batch-to-batch reproducibility, pose serious con-
straints for the development of polymer based OSCs.
Solution processed small-molecule OSCs (SMOSC), which offer
donor-acceptor (D-A) molecular architecture consisting of
p-con-
jugated and fused thiophene rings on a conjugated backbone. Since
fused thiophene rings can make the molecular backbone more rigid
*
Corresponding author.
E-mail address: shlee66@jbnu.ac.kr (S.-H. Lee).
http://dx.doi.org/10.1016/j.dyepig.2016.10.007
143-7208/© 2016 Elsevier Ltd. All rights reserved.
0

118
S.S. Bagde et al. / Dyes and Pigments 137 (2017) 117e125
and coplanar, and improve face to face
p
-
p
overlap, this makes
thiophene attached at 2, 7 positions of the NDT unit have been
small molecules enhancing effective -conjugation, extending ab-
p
studied systematically in polymer/small molecule organic solar
cells due to its desirable OSC properties. For instance, Takimiya et al.
evaluated the solar cell performance of PNNT-DT copolymer,
sorption, and facilitating charge transport [8e10]. Various donor
cores which fulfil the above criteria consist of small molecules such
0
0
0
as dithieno[3,2-b:2 ,3 -d]thiophene (DTT) [8], dithieno (3,2-
comprised of naphtho[1,2-b:5,6-b ]dithiophene (NDT3) and naph-
0
0
b;2 ,3 d) silole (DTS) [11], benzo[1,2-b:4,5-b] dithiophene (BDT)
tho[1,2-c:5,6-c]-bis [1,2,5]thiadiazole (NTz) as the donor and
acceptor units respectively [16]. Further they have also developed
(NDT3) based organic semiconducting polymers, PNDT3BT [17]
and PNDTBT [18], which showed promising charge mobilities for
OFETs devices. Marks and coworkers have developed 4,9-bis(2-
0
0
0
[
12], dithieno[3,2b:2 ,3 d] pyrroles (DTP) [13], naphtha[1,2-b:5,6-b ]
dithiophene (NDT) [14] and Indacenodithiophene (IDT) [15]
(Scheme 1).
Among the rigidly fused polycyclic system, NDT derivatives with
Scheme 1. Different donor core for used for small molecules.

S.S. Bagde et al. / Dyes and Pigments 137 (2017) 117e125
119
0
ethylhexyloxy)naphtho[1,2-b:5,6-b ]dithiophene (zNDT) building
134.20, 126.58, 113.39, 42.43, 38.08, 30.41, 28.41, 23.72, 22.87, 13.96,
ꢀ
block for SMOSCs, which shows encouraging photovoltaic proper-
ties [14]. In continuation to wide research on NDT, we also recently
demonstrated the OSC capabilities of NDT based novel small mol-
ecules by combining with benzothiadiazole (BNB) or triphenyl-
amine capped benzothiadiazole conjugated units (TBNBT) [19] and
thiophene(3-decanyl)-bithiazole (M3) or triphenylamine flanked
thiophene(3-decanyl)-bithiazole (M4) [20].
Among the acceptor units, thieno[3,4-c]pyrrole-4,6-dione (TPD)
is considered to be one of the promising acceptor moieties due to
its planar geometry and smaller resonance energy, which mini-
mizes its steric hindrance and promotes quinoidal characteristics.
Such properties enhance the backbone co-planarity and electron
delocalization, which eventually helps to lower the band gap. These
unique properties thus make TPD-based compounds as one of the
most promising materials for OSCs [21]. TDP was first incorporated
in BDT-based copolymers by Leclerc and co-workers, where the
resultant materials showed high Voc and an appreciable Jsc due to
lower HOMO and small band gap [22]. Many TPD based polymers
that include PCPDTTPD-Oc [23], PTPDSi-C8 [24], PDTTG-TPD [25]
and PBDTTPD [26] have shown promising performance in OSCs
by demonstrating PCEs ~5%.
Motivated by the success of TPD as an acceptor with different
donor moieties and continuing our development of NDT based
small molecules, we coupled TPD acceptor unit with the NDT
Scheme 2). Moreover, incorporation of TPD group with NDT core
unit as SMOSC material is really rare. To extend the -electron
delocalization and promote solution processability, 3-
decanylthiophene units were installed with NDT donor and 2-
ethyl-hexyl group on TPD acceptor, respectively. The chemical
structures consisting of 3-decanylthiophene NDT core flanked with
TPD unit and that end capped with hexyl bithiophene moiety are
10.27. Mp:102e103 C. MALDI-TOF: calculated for C14
H
18BrNO
2
S
þ
[MþH] 344.5476.
1,3-Dibromo-5-(2-ethylhexyl)-4H-thieno[3,4-c]pyrrole-
4,6(5H)-dione (10): 5-(2-ethylhexyl)-4,H-thieno[3,4-c]pyrrole-
4,6(5H)-dione (0.27 g, 1.01 mmol) was dissolved in concentrated
sulfuric acid (1.6 mL) and trifluoroacetic acid (3.4 mL). NBS (0.543 g,
3.05 mmol) was added in one portion and the reaction mixture was
stirred at room temperature overnight. The resultant solution was
then diluted with water (100 mL) and extracted with dichloro-
methane. The organic phase was washed with KOH solution,
further dried over anhydrous magnesium sulfate and evaporated to
afford the crude product as orange crystals. Purification by column
chromatography using silica gel and hexane/chloroform (3:2) gave
1,3-dibromo-5-(2-ethylhexyl)-4H-thieno[3,4-c]pyrrole-4,6(5H)-
1
dione (0.34 g, 79%) as pink solid. H NMR (400 MHz, CDCl
3
):
d
3.49
(d, J ¼ 7.3 Hz, 2H), 1.81e1.76 (m, 1H), 1.35e1.27 (m, 8H), 0.93e0.88
1
3
(t, 6H). C (100 MHz, CDCl
3
):
d
160.46, 134.46, 112.65, 42.26, 37.80,
ꢀ
30.13, 28.13, 23.43, 22.54, 13.66, 9.95. Mp: 110.5e112 C. MALDI-
TOF: calculated for C14
1-Bromo-5-(2-ethylhexyl)-3-(5′-hexyl-2,2′-bithiophen-5-yl)-
4H-thieno[3,4-c]pyrrole- 4,6(5H)-dione (11): In a flame dried
flask, Compounds 10 (0.3 g, 0.70 mmol) and 2-(5 -hexyl-2,2 -
þ
H17Br
2
NO
2
S [MþH] 424.5344.
0
0
bithiophen-5-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (0.29 g,
0.77 mmol) were dissolved in dry toluene (10 mL) and 2 M K
(4 mL), and the solution was degassed with N for 15 min. Then,
Pd(PPh (0.04 g, 0.035 mmol) was added. The mixture was stirred
2 3
CO
(
2
p
3 4
)
ꢀ
at 110 C overnight under a nitrogen atmosphere. The reaction
mixture was poured into water and extracted three times with
chloroform. The organic phase was combined and dried with
anhydrous magnesium sulfate. After removal of the solvent, the
crude product was purified by column chromatography hexane/
0
0
identified as NDT(TPD)
2
and NDT(TPDTT)
2
,
respectively
chloroform (1:4) gave 1-bromo-5-(2-ethylhexyl)-3-(5 -hexyl-2,2 -
(
Scheme 2). End capping with hexyl bithiophene ensures the
bithiophen-5-yl)-4H-thieno[3,4-c]pyrrole- 4,6(5H)-dione (0.38 g,
1
extension in
p-conjugation with narrowing of band gap, and self-
90%) as red solid. H NMR (400 MHz, CDCl
3
):
d
7.84 (d, J ¼ 4 Hz, 1H),
assembly of molecule [27,28]. In this contribution, we report the
synthesis, characterization, and photovoltaic properties of two
novel
NDT(TPDTT)
based on these small molecules has also been carried out.
7.10e7.07 (dd, J ¼ 3.6, 9 Hz, 2H), 6.72 (d, J ¼ 3.8 Hz, 1H), 3.52 (d,
J ¼ 7.3 Hz, 2H), 2.81 (t, J ¼ 7.2 Hz, 2H), 1.71e1.69 (m, 2H), 1.41e1.26
13
p-conjugated
small
molecules,
NDT(TPD)
2
and
3
(m, 14H), 0.94e0.88 (t, 6H). C (100 MHz, CDCl ): d 162.05, 147.44,
2
. A comparison between the solution-processed OSCs
142.06, 140.84, 134.88, 133.51, 131.04, 129.58, 125.25, 123.92, 109.49,
42.51, 38.16, 31.43, 30.17, 28.50, 23.78, 22.92, 22.48, 13.99, 10.32.
ꢀ
[MþH]þ
Mp: 81e82 C. MALDI-TOF: calculated for C28
593.8495.
H34BrNO S
2 3
2
. Experimental section
NDT(TPD)
compound 9 (0.22 g, 0.69 mmol), Pd
P(o-Tolyl) (0.008 g, 10 mol%) were added to a 50 mL flame-dried
2
: A mixture of compound 8 (0.25 g, 0.24 mmol),
2.1. Materials & synthesis
2
(dba) (0.014 g, 5 mol%) and
3
3
All reactions were carried out under nitrogen (N
2
) atmosphere
two-neck flask and subjected to three vacuum/argon fill cycles.
Argon-degassed dry chlorobenzene (10 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
product was adsorbed on silica gel and purified by column using a
with the use of standard Schlenk tube techniques. All starting
materials were used as purchased from commercial sources unless
stated otherwise. Compounds 8 and TPD were synthesized ac-
cording to the literature [19,29].
1
-Bromo-5-(2-ethylhexyl)-4H-thieno[3,4-c]pyrrole-4,6(5H)-
dione (9): 5-(2-ethylhexyl)-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione
hexane/chloroform (4:6) solvent mixture to yield the target small
1
(
(
2 g, 7.53 mmol) was dissolved in concentrated sulfuric acid
4.6 mL) and of trifluoroacetic acid (15.4 mL). NBS (1.47 g,
molecule NDT(TPD)
(400 MHz, CDCl ):
2
d
(0.165 g, 55%) as orange solid. H NMR
7.98 (d, J ¼ 8.5 Hz, 2H), 7.9 (s, 2H), 7.89 (d,
3
8
.29 mmol) was added in portion and the reaction mixture was
J ¼ 8.7 Hz, 2H), 7.62 (s, 2H), 7.57 (s, 2H). 3.57 (d, J ¼ 7.3 Hz, 4H), 2.94
stirred at room temperature overnight. The brown solution was
then diluted with water (100 mL) and extracted with dichloro-
methane. The organic phase was washed with KOH solution,
further dried over anhydrous magnesium sulfate and evaporated to
afford the crude product as orange crystals. Purification by column
chromatography using silica gel and hexane/chloroform (4:1) gave
(t, J ¼ 7.6 Hz, 4H), 1.89e1.87 (m, 2H), 1.79e1.75 (m, 4H), 1.45e1.26
13
3
(m, 44H), 0.95e0.85 (t, 18H). C (100 MHz, CDCl ): d 163.32, 162.69,
141.94, 139.57, 138.78, 137.61, 137.47, 134.63, 134.09, 132.82, 131.22,
127.84, 125.73, 124.30, 122.88, 122.71, 122.29, 121.60, 42.57, 38.20,
31.92, 30.59, 29.98, 29.64, 29.54, 29.45, 29.35, 28.90, 28.53, 23.87,
ꢀ
23.06, 22.06, 14.11, 10.46. Mp: 271e272 C. MALDI-TOF: calculated
þ
1
-bromo-5-(2-ethylhexyl)-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione
for C70
NDT(TPDTT)
compound 11 (0.21 g, 0.37 mmol), Pd
P(o-Tolyl) (0.005 g, 10 mol%) were added to a 50 mL flame-dried
H
86
N
2
O
4
S
6
[MþH] 1211.4055.
1
(
0.85 gm, 33%) as white solid. H NMR (400 MHz, CDCl
3
):
H), 3.50 (d, J ¼ 7.3 Hz, 2H), 1.81e1.78 (m, 1H), 1.37e1.25 (m, 8H),
.92e0.87 (t, 6H). 13C (100 MHz, CDCl
): 162, 161.66, 136.73,
d
7.73 (s,
2
: A mixture of compound 8 (0.15 g, 0.14 mmol),
(dba) (0.007 g, 5 mol%) and
1
0
2
3
3
d
3

120
S.S. Bagde et al. / Dyes and Pigments 137 (2017) 117e125
Scheme 2. Synthesis route for obtaining NDT(TPD)
2
2
and NDT(TPDTT) .
ꢀ
two-neck flask and subjected to three vacuum/argon fill cycles.
Argon-degassed dry chlorobenzene (8 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
product was adsorbed on silica gel and purified by column using a
10.50. Mp: 230e232 C. MALDI-TOF: calculated for C98
[MþH]þ 1707.6972.
118 2 4 10
H N O S
2
.2. General instrumentation
Chemical shifts were reported as
internal standard tetramethylsilane (TMS). The UVevis absorption
spectra of films or solutions were obtained using a Shimadzu UV-
d values (ppm) relative to the
hexane/chloroform (7:3) solvent mixture to yield the target small
1
molecule NDT(TPDTT)
2
(0.095 g, 38%) as red solid. H NMR
(
(
6
(
(
1
1
3
400 MHz, CDCl
dd, J ¼ 4, 8 Hz, 4H), 6.79 (d, J ¼ 4 Hz, 2H), 6.65 (dd, J ¼ 4, 8 Hz, 4H),
.49 (d, J ¼ 4 Hz, 2H), 3.41(d, J ¼ 8 Hz, 4H), 2.65 (t, J ¼ 8 Hz, 4H), 2.41
3
):
d
7.54 (d, J ¼ 4 Hz, 2H), 7.21 (d, J ¼ 4 Hz, 2H), 7.05
2
550 spectrophotometer. Photoluminescence (PL) spectra of films
were obtained using an FP-6500 (JASCO). Thermogravimetric
analysis (TGA) was carried out with a TA Instrument Q-50 at a
1
3
t, J ¼ 8 Hz, 4H), 1.85 (m, 2H), 1.31e1.26 (m, 64H), 0.96 (t, 24).
100 MHz, CDCl ): 162.33, 162.25, 146.39, 140.77, 139.83, 137.33,
36.23, 135.24,135.04,134.87,134.32,133.86, 130.56, 130.24,129.50,
27.54, 127.48, 124.71, 124.37, 124.11, 42.40, 38.23, 32.09, 31.62,
1.46, 31.30, 30.65, 30.18, 30.00, 29.89, 29.80, 29.71, 29.57, 29.37,
8.97, 28.63, 23.85, 23.28, 23.12, 22.82, 22.64, 14.23, 14.21, 14.11,
C
ꢀ
ꢁ1
scanning rate of 10 C min under a N
2
atmosphere. Differential
3
d
scanning calorimetry (DSC) experiments were performed on a TA
Instrument (DSC 2910) at a heating rate of 100 C min under a N
atmosphere. Cyclic voltammetry (CV) measurements were made
using a VersaSTAT3 (METEK) Electrochemical Analyzer under argon
at a scan rate of 50 mV s at room temperature, where a Pt wire
ꢀ
ꢁ1
2
2
ꢁ
1

S.S. Bagde et al. / Dyes and Pigments 137 (2017) 117e125
121
ꢀ
ꢀ
and Ag/AgCl were used as counter and reference electrodes,
respectively. The reference electrode was calibrated with the
ferrocene/ferrocenium (Fc/Fc ) redox couple (4.4 eV below the
decomposition temperatures of 420 C and 352 C respectively
[Fig. S1a]. DSC analysis performed at a temperature ramp rate of
þ
ꢀ
10 C/min (Fig. S1b) indicated a sharp crystalline feature of
vacuum level) as an external standard. The samples were prepared
in chloroform solution with 0.10 M tetrabutylammonium hexa-
NDT(TPD)
2
. A distinguishable melting temperature (T
m
) and crys-
ꢀ
talline transition (T
c
) were observed respectively at 261 C and
ꢀ
fluorophosphate (n-Bu
4
NPF
6
) as the electrolyte. The surface
214 C for NDT(TPD)
for NDT(TPDTT)2, during heating and cooling cycle. This is ascribed
to amorphous nature of NDT(TPDTT) (described further in XRD).
The difference in the nature of NDT(TPDTT) compared to
NDT(TPD) is suggestive of different packing mode in the solid
state. Thus, as a result, peaks corresponding to melting and crys-
2
, whereas no prominent peaks were observed
morphology was measured using a Digital Instruments Multimode
atomic force microscope (AFM) controlled by a Nanoscope IIIa
scanning probe microscope 20 controller.
2
2
2
2.3. Photovoltaic device fabrication and characterization
2
tallization were not observed in NDT(TPDTT) .
OSCs comprising a BHJ photoactive layer of small molecules:
,6- phenyl-C71-butyric acid methyl ester (PC71BM) were prepared
on a commercial indium-doped tin oxide (ITO)-coated glass sub-
strate with a sandwiched structure of glass/ITO/PEDOT:PSS/
6
3.2. Optical and electrochemical properties
NDT(TPD)
2
or NDT(TPDTT)
2
:PC71BM/LiF/Al. Prior to use, the
The solution and thin film absorption spectra of NDT(TPD)
2
and
patterned ITO-coated glass substrates were cleaned with deionized
water, acetone, and isopropyl alcohol by ultrasonication, followed
by treatment with UV ozone. Poly(3,4-ethylenedioxythiophene):
poly(styrene sulfonate) (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 and then
transferred into a glove box filled with N . Blends of NDT(TPD) or
NDT(TPDTT) and PC71BM (Nano-C, USA) with different weight
ratios (from 1: 1 to 1: 4 w/w) were solubilized overnight in chlo-
NDT(TPDTT) are presented in Fig. 1 and summarized in Table 1.
2
Solution spectra of both molecules exhibit broad absorption with a
distinct low energy band assigned to the internal charge transfer
(ICT) between the D- and the A-unit [30]. NDT(TPD)
absorption profile from 350 to 500 nm, with a maximum absorp-
tion peak ( max) at 438 nm. Whereas, NDT(TPDTT) shows broad
absorption from 350 to 560 nm, with max at 480 nm. An approx-
imately 42 nm red shift in the absorption maxima of NDT(TPDTT)
compared to that of NDT(TPD) , corresponds to increased conju-
2
shows an
ꢀ
l
2
l
2
2
2
2
2
ꢁ1
robenzene (20 mg mL ), filtered through a 0.45 mm poly(tetra-
fluoroethylene) (PTFE) filter and subsequently spin-coated
gation length afforded by additional bithiophene moieties [31]. A
consistent 37 nm and 35 nm red shifts in absorbances for thin films
of NDT(TPD) and NDT(TPDTT) respectively, compared to solution,
2 2
along with the emergence of more than one pronounce vibronic
(
thickness 60e70 nm) onto the PEDOT:PSS layer of the ITO. The
resulting films were dried at room temperature for 20 min under N
2
and then under vacuum for 12 h. The device fabrication was
completed by deposition of a 0.5 nm layer of LiF and a 120 nm Al
layer. These layers were thermally evaporated at room temperature
peaks, indicates well-ordered packing in as cast-film state. The
longer and broader absorbance for NDT(TPDTT)
increased rigidity and better conjugated backbones. From the
the optical bandgaps were determined to be 2.2 and 1.9 eV for
NDT(TPD) and NDT(TPDTT) respectively. To investigate the elec-
2
suggests an
l
onset
,
ꢁ
6
under high vacuum at 1 ꢂ 10 Torr. The active area of every device
2
was 11 mm . The current densityevoltage (JeV) characteristics of
2
2
the photovoltaic devices were measured in the dark and under 1
sun illumination at AM 1.5G using a solar simulator (Newport) at
trochemical properties of small molecules, cyclic voltammetry (CV)
measurements were carried out using Ag/AgCl reference electrode
ꢁ
2
2
100 mW cm adjusted with a standard PV reference (2 ꢂ 2 cm ), a
4 6
in a 0.1 M solutions of Bu NPF in acetonitrile solution at room
ꢁ1
mono-crystalline silicon solar cell (calibrated at NREL, Colorado,
USA) with a Keithley 2400 source measure unit. The external
quantum efficiency (EQE) was determined using a Polaronix K3100
spectrometer.
temperature under argon with scan rate of 50 mV Fig. 2a. The CV
instrument was calibrated using the ferrocene/ferrocenium (Fc/
þ
Fc ) redox couple as an external standard. The onset potential of
þ
the Fc/Fc redox couple was found to be 0.4 V relative to the Ag/
AgCl reference electrode. HOMO/LUMO were calculated from the
3
. Results & discussion
onset oxidation and reduction potential according to the equations
HOMO ¼ ꢁe(E onset þ 4.4) and LUMO ¼ ꢁe(E onset þ 4.4)
ox
red
3.1. Synthesis and thermal properties
The synthesis of small molecules NDT(TPD)
2
and NDT(TPDTT)
2
is outlined in (Scheme 2). TPD was selectively brominated using N-
bromosuccinimide (NBS) to produce bromine functionalized de-
rivatives,
,6(5H)-dione (9) and 1,3-dibromo-5-(2-ethylhexyl)-4H-thieno
3,4-c]pyrrole-4,6(5H)-dione (10). Pd-catalysed Suzuki coupling
1-bromo-5-(2-ethylhexyl)-4H-thieno[3,4-c]pyrrole-
4
[
0
0
reaction between compound 10 and 2-(5 -hexyl-2,2 -bithiophen-5-
yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane afforded compound 11.
The target small molecule NDT(TPD)
cross-coupling reaction of compound 8 and compound 9 using
Pd (dba) /P(o-tolyl) as the catalyst. Similarly, under identical
conditions, compound 8 was coupled with compound 11 to obtain
2
was synthesized via Stille
2
3
3
NDT(TPDTT)
NDT(TPDTT)
2
.
The molecular structure of NDT(TPD)
was confirmed by H and C NMR (Fig. S2~6 in SI).
2
and
1
13
2
The thermal properties of small molecules were investigated by
thermogravimetric analysis (TGA) and differential scanning calo-
rimetry (DSC). TGA analysis suggested high thermal stability for
Fig. 1. UVeVis absorption spectra of NDT(TPD)
solution and thin films.
2 2
and NDT(TPDTT) in chlorobenzene
2 2
both the small molecules. NDT(TPD) and NDT(TPDTT) show

122
S.S. Bagde et al. / Dyes and Pigments 137 (2017) 117e125
Table 1
The optical and electrochemical characteristics and hole/electron mobilities of small molecules.
Molecule
NDT(TPD)
almax (nm)
blmax (nm)
cE
(Opt)
(eV)
dEoxonset/HOMO (eV)
eEredonset/LUMO(eV)
fm
h
(cm2
V
ꢁ1 ꢁ1
s
)
fm
e
(cm2
ꢁ1 ꢁ1
V s )
2.5 ꢂ 10ꢁ
9.8 ꢂ 10ꢁ
5
8.6 ꢂ 10
7.4 ꢂ 10
ꢁ9
ꢁ6
2
438
480
475
515
2.22
1.96
0.98/ꢁ5.38
0.86/ꢁ5.26
ꢁ0.94/ꢁ3.46
ꢁ0.85/ꢁ3.55
6
NDT(TPDTT)
2
a
Measured in chloroform solution.
b
c
d
e
f
Spin-coated film from chloroform solution.
opt
Optical band gap, E
g
¼ 1240/(
lonset) film.
ox
HOMO ¼ ꢁe(E onset þ 4.4).
red
LUMO ¼ ꢁe(E onset þ 4.4).
h e
Hole mobility (m ) and electron mobility (m ) measured by the SCLC.
2 2 2 2
Fig. 2. a) Cyclic voltammograms of NDT(TPD) and NDT(TPDTT) b) energy levels showing the HOMO and LUMO energy levels of NDT(TPD) and NDT(TPDTT) .
as ꢁ5.38 eV/ꢁ3.46 eV for NDT(TPD)
NDT(TPDTT) . Upshifted HOMO and downshifted LUMO levels of
NDT(TPDTT)2, could be the result of increased -conjugation length
afforded by the electron donating bithiophene moiety [32]. The
HOMO energy level of NDT(TPD)
is slightly deeper (ꢁ0.12 eV) than
that of NDT(TPDTT) , indicating more oxidative stability and is
beneficial to achieve higher Voc in the OSC devices. Table 1 sum-
marizes the corresponding electrochemical characteristics for the
2 2
small molecules. The energy levels of NDT(TPD) and NDT(TPDTT)
were further compared with that of PC71BM using the energy level
diagram Fig. 2b. The LUMO energy levels of both small molecules
are sufficiently higher than the LUMO level of PC71BM to overcome
the exciton binding energy and thus ensure an efficient exciton
splitting and charge transfer [33].
2
and ꢁ5.26 eV/-3.55 eV for
Table 2
Photovoltaic performance of devices based on NDT(TPD)
2
and NDT(TPDTT)
2
.
2
Device
Jsc (mA/cm2) Voc (V) FF (%) PCE (%)
p
NDT(TPD)
NDT(TPD)
NDT(TPD)
NDT(TPD)
NDT(TPDTT)
NDT(TPDTT)
NDT(TPDTT)
2
2
2
2
:PC71BM (1:1)
:PC71BM (1:2)
:PC71BM (1:3)
:PC71BM (1:4)
0.74
1.21
0.82
0.10
1.76
2.28
2.86
2.39
0.75
0.79
0.77
0.42
0.64
0.64
0.64
0.64
0.75
0.72
27.15 0.15
27.31 0.26
24.32 0.15
21.73 0.01
31.30 0.35
36.36 0.53
42.94 0.80
43.79 0.67
52.44 1.31
44.15 0.73
2
2
2
:PC71BM (1:1)
2
:PC71BM (1:2)
2
:PC71BM (1:3)
2
NDT(TPDTT) :PC71BM (1:4)
1.0% DIO, NDT(TPDTT)
.0% DIO, NDT(TPDTT)
2
2
:PC71BM (1:3) 3.32
:PC71BM (1:3) 2.29
2
2
reduced to 0.15% owing to reduced Jsc of 0.82 mA/cm , Voc of 0.77 V
and FF of 24.32%. Moreover, for a much higher concentration of
fullerene to 1:4, device parameters were worse and the PCE drop-
ped to 0.01%.
3.3. Photovoltaic performance
Photovoltaic characteristics were investigated with the con-
Conversely, better photovoltaic performance were obtained for
ventional architecture of ITO/PEDOT: PSS/NDT(TPD)
NDT(TPD) :PC71BM/LiF/Al. The photoactive layers were prepared
by spin-coating from small molecule solutions in chlorobenzene
2
or
NDT(TPDTT)
ratios of NDT(TPDTT)
2
:PC71BM devices. For devices with 1:1 and 1:2 blend
:PC71BM, PCEs of 0.35% and 0.53% were ob-
2
2
tained respectively. A 1:2 blend ratio device comparing with a 1:1
2
(
20 mg/mL) with an active area of 11 mm . Different small mole-
ratio device shows better PCE as a result of improved Jsc (2.28 mA/
cules: PC71BM blend ratios were tested from 1:1 to 1:4 to obtain the
optimal blend ratio for each small molecule, followed by adding of
processing additive 1,8-diiodooctane (DIO) in order to evaluate the
device performance. Detailed device performance parameters are
summarized in Table 2. Current densityevoltage (JeV) character-
istics under one sun (simulated AM 1.5G irradiation at
2
2
cm vs 1.76 mA/cm ) and FF (36.36 vs 31.30) with similar Voc of
2
0.64 V. For NDT(TPDTT) :PC71BM devices, optimize blend ratio was
1
:3, which shows the highest PCE of 0.80% with increased Jsc
2
(2.86 mA/cm ) and FF (42.94), while Voc (0.64 V) remains un-
changed. Whereas a 1:4 blend ratio device shows slightly lower PCE
of 0.67%, which is ascribed to the reduced Jsc (2.39 mA/cm ), even
though highest FF of 43.79 and unchanged Voc of 0.64. A 1:3 blend
ratio device showing the best performances were further impro-
2
ꢁ
1
00 mW cm 2
NDT(TPD) :PC71BM photoactive blend ratio of 1:1, a PCE of 0.15% is
achieved resulting from Voc of 0.75 V, Jsc of 0.74 mA/cm and FF of
7.15%. The best photovoltaic performance in NDT(TPD) based
devices is observed for OSCs with the NDT(TPD) :PC71BM blend
) are shown in Fig. 3a. For devices with
2
2
2
vised by treating the NDT(TPDTT) :PC71BM photoactive layer with
2
2
the different amount of processing additive DIO (1.0 and 2.0%).
When using 1.0% DIO, device results in the maximum PCE of 1.31%
2
2
ratio of 1:2 (Jsc ¼ 1.21 mA/cm , Voc ¼ 0.79 V, FF ¼ 27.31%, and
2
with enhanced Jsc of 3.32 mA/cm , FF of 52.44 and a comparable Voc
PCE ¼ 0.26%). With further increase in ratio to 1:3, PCE is drastically

S.S. Bagde et al. / Dyes and Pigments 137 (2017) 117e125
123
2 2
Fig. 3. J-V curves (a) and EQE spectra (b) of OSC devices based on NDT(TPD) :PC71BM (1:2) and NDT(TPDTT) :PC71BM (1:3) blends.
of 0.75 V. Further addition of DIO to 2.0%, the device performance
was decreased to 0.73% as a consequence of reduced Jsc of 2.29 mA/
2
cm , FF of 44.15 and a Voc of 0.72 V. Adding high boiling point ad-
ditives, such as DIO leads to the formation of D-A interfaces with
more phase separation, as a result of selectively solubilisation of the
PCBM by DIO and eventually promoting the formation of donor
aggregation during the film drying process. Generally, in this pro-
cess, Jsc is dramatically increased due to the increased D-A in-
terfaces but Voc is usually decreased owing to the upshifted HOMO
value by aggregation of donor material. However, surprisingly in
our result, Voc is also increased on addition of DIO, especially in the
device with 1.0% DIO addition, resulting in dramatic enhancement
of PCEs. Similar observations are also observed in some other recent
works [34e36]. The comparison of dark currents measured for
devices with and without DIO reveals suppression in dark current
for devices containing DIO as an additive (Fig. S7 in SI). This is
clearly evident from nearly fourfold enhancement in shunt resis-
2 2
Fig. 4. XRD patterns of NDT(TPD) and NDT(TPDTT) films.
2
2
tance (Rsh) from 9800
U cm (OSCs with no DIO) to 28700 U cm
(
OSCs with 1% DIO). This indicates reduced recombination, which is
possibly effected by a meticulous control over aggregations along
with the formation of appropriate phase separation on addition of
DIO. The high Rsh is expected to reduce the charge recombination by
decreasing the leakage current at negative bias, which eventually
peaks in the small angled region are attributed to the distance
between the planes of the main conjugated chains of these small
molecules segregated by alkyl chains. The d spacing calculated by
the Bragg equation for NDT(TPD)
NDT(TPDTT) (34.74 Å), this indicates that NDT(TPD)
more interdigitated packing than NDT(TPDTT) as additional
bithiophene group is present in conjugated backbone of
NDT(TPDTT) . These observations are well-matched with DSC re-
sults shown in Fig. S1b.
2
(26.59 Å) is smaller than
resulted in the improvement of Voc
.
2
2
should form
To investigate the photoresponses of the devices, external
quantum efficiency (EQE) spectra of the devices based on
NDT(TPD) :PC71BM and NDT(TPDTT) :PC71BM were collected un-
2 2
der monochromatic light Fig. 3b. Both devices exhibited broad
photon response extending from 300 to 700 nm, ensuring that the
absorbed photons contributed to the photovoltaic conversion. The
2
2
3.4. Morphology
maximum EQE of 24% is observed for NDT(TPDTT)
devices with 1% DIO, which explains higher Jsc in these devices,
while a lower Jsc values in NDT(TPD) devices is justified by its
2
:PC71BM (1:3)
The morphology of selective small molecular photoactive layers
2
was investigated by atomic force microscopy (AFM) and shown in
Fig. 5. The NDT(TPD) :PC71BM (1:2) blend film shows large aggre-
minimum EQE throughout the absorption spectrum.
To gain insight into the molecular packing of these small mol-
ecules in solid state, thin film X-ray diffraction (XRD) analysis was
2
gated domains (width ~150 nm) of rod like structure with higher
root mean square roughness (RMS) of 5.575 nm Fig. 5a, which could
performed and shown in Fig. 4. NDT(TPD)
two low angled sharp diffraction peaks at 2
arising from the (100) and (200) planes, corresponding to the d-
2
pristine film displays
be the result of high crystallinity of NDT(TPD)
XRD study. The NDT(TPDTT) :PC71BM (1:3) film appears as inti-
mately mixed donor and acceptor blend with low RMS of 0.186 nm
Fig. 5b). The smaller domain size and lower RMS of
NDT(TPDTT) :PC71BM (1:3) film, comparing with
NDT(TPD) :PC71BM (1:2) film, explains better charge generation
2
as observed in the
ꢀ
ꢀ
q
¼ 3.32 and 5.12 ,
2
spacing of 26.59 Å and 17.23 Å, respectively. An additional less
(
intense peak related to
2
p-p overlap in NDT(TPD) is observed at
2
ꢀ
2
q
¼ 9.96 (with d-spacing of 8.87 Å). In comparison, the (100) and
2
ꢀ
(
200) peaks in NDT(TPDTT)
2
show a low angle shift to 2.54 and
and collection towards respective electrode, which has contributed
to improved Jsc and FF of these device. Further, the nanoscale
morphology of NDT(TPDTT) :PC71BM (1:3) film with 1% DIO shows
2
optimal RMS of 1.04 nm with nanoscale morphology and proper
phase separation (Fig. 5c), which is comparable to exciton diffusion
ꢀ
4
.33 respectively. (d-spacing of 34.74 & 20.38 Å respectively). It is
noteworthy that the much stronger peaks for NDT(TPD)
high crystallinity and ordered molecular structure, whereas less
intense peaks in NDT(TPDTT) indicates its amorphous nature. The
2
suggest
2

124
S.S. Bagde et al. / Dyes and Pigments 137 (2017) 117e125
Fig. 5. AFM images (5
mm ꢂ 5
2 2 2
mm) of (a) NDT(TPD) :PC71BM (1:2) and (b) NDT(TPDTT) :PC71BM (1:3), (c) NDT(TPDTT) :PC71BM (1:3) with 1% DIO.
length. The increased roughness of surface is a signature of self-
organization of blend (small molecule & PCBM), induced by ag-
(1:2) device, may responsible for low Jsc and FF of these devices. For
NDT(TPDTT) :PC71BM (1:3) device, well-balanced (
was observed and thus resulted into the high PCE.
2
m /m
h e
¼ 1.32)
gregation of NDT(TPDTT)
2
in the active blend promoted by addition
of DIO, which in turn enhances ordered structure formation in the
thin film, which is essential for promoting exciton dissociation and
charge separation. Thus, this desired morphology improved the Jsc
and FF of NDT(TPDTT)
2
:PC71BM (1:3) (1%DIO), based device and
4. Conclusion
resulted in the highest PCE [37].
Two new small molecules, NDT(TPD)
2 2
and NDT(TPDTT)
composed of NDT donor and TPD acceptor motif were designed and
synthesized. The molecules show good thermal stability, strong
spectral coverage with moderate HOMO energy levels.
3.5. Charge mobility
To understand the influence of charge carrier mobility on
NDT(TPD)
2
:PC71BM active blend (1:2 w/w) shows PCE of 0.26%
small molecule
photovoltaic performance, the hole and electron mobility of
NDT(TPD) :PC71BM (1:2) and NDT(TPDTT) :PC71BM (1:3) based
without any post treatment. Although NDT(TPD)
2
2
2
shows deep HOMO level, high crystallinity and high hole mobility,
the PCE is strongly effected by low Jsc (large band gap), poor FF (high
charge recombination) and unbalanced charge mobility, which are
important features for BHJ devices for obtaining good efficiency.
devices were measured by space-charge limited current (SCLC)
method, using the device structure ITO/PEDOT:PSS/active layer/
MoO /Ag for holes and ITO/ZnO/active layer/LiF/Al for electrons,
3
respectively. J-V characteristics of hole-only and electron-only de-
2
Hence effective structural modulation in NDT(TPD) is required to
vices are shown in Fig. 6 and results are summarized in Table 1. The
improve low device parameters while maintaining crystallinity and
effective charge mobility. On the other hand, relatively high PCE
NDT(TPD)
2 h
:PC71BM (1:2) device exhibits higher hole mobility (m )
ꢁ5
2
ꢁ1 ꢁ1
of 2.5 ꢂ 10 cm
V
S
and lower electron mobility (
, comparing with NDT(TPDTT) :PC71BM
). The higher
:PC71BM device could be the result of
m
e
) of
(0.8%) was obtained for NDT(TPDTT)
quence of improved film morphology, along with well-balanced
charge mobility comparing with NDT(TPD) based device. Further
treating with 1% DIO of NDT(TPDTT) :PC71BM (1:3 w/w) device
improved all the device characteristics, and the highest PCE of 1.31%
was obtained. Interestingly, in NDT(TPDTT) devices, high Voc
2
:PC71BM (1:3) as a conse-
ꢁ
9
cm2
ꢁ1 ꢁ1
V S
8
(
.6 ꢂ 10
2
ꢁ
6
ꢁ6
2
ꢁ1 ꢁ1
1:3) device (9.8 ꢂ 10 and 7.4 ꢂ 10 cm V
S
2
hole mobility of NDT(TPD)
2
2
enhanced molecular packing and highly crystalline nature of
NDT(TPD) molecule in the photoactive layer [38]. However, un-
balanced hole and electron ratio (
another important factor for device performance [39], due to much
lower electron mobility than hole mobility of NDT(TPD) :PC71BM
2
2
3
m
h e
/m
¼ 2.91 ꢂ 10 ), which is
(~0.75 V) has been resulted after incorporation of DIO additive.
Thus, further optimization to improve the device performance may
be possible, the progress of which is underway.
2
2 2
Fig. 6. Current densityevoltage (JeV) characteristics of (a) hole- and (b) electron-only devices for NDT(TPD) :PC71BM (1:2) and NDT(TPDTT) :PC71BM (1:3).

S.S. Bagde et al. / Dyes and Pigments 137 (2017) 117e125
performance. J Mater Chem A 2016;4:2252e62.
125
Acknowledgments
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