Development of naphtho[1,2-b:5,6-b′]dithiophene based novel small molecules for efficient bulk-heterojunction organic solar cells

Article abstract of DOI:10.1039/c1cc15465f

Two new small molecules with a rigid planar naphtho[1,2-b:5,6-b′] dithiophene (NDT) unit were designed and synthesized. Solution processed bulk-hetereojunction organic solar cells based on blends of the small molecules and [6,6]-phenyl-C₇₁-buty

Full text of DOI:10.1039/c1cc15465f

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COMMUNICATION
Development of naphtho[1,2-b:5,6-b0]dithiophene based novel small
molecules for efficient bulk-heterojunction organic solar cellsw
Pranabesh Dutta,a Wooseung Yang,a Seung Hun Eom,a Woo-Hyung Lee,b In Nam Kangb
and Soo-Hyoung Lee*a
Received 2nd September 2011, Accepted 19th October 2011
DOI: 10.1039/c1cc15465f
Two new small molecules with a rigid planar naphtho[1,2-b:5,
6-b0]dithiophene (NDT) unit were designed and synthesized.
Solution processed bulk-hetereojunction organic solar cells
based on blends of the small molecules and [6,6]-phenyl-C71-
butyric acid methyl ester (PC71BM) exhibited promising photo-
voltaic device performance with a maximum power conversion
efficiency up to 2.20% under the illumination of AM 1.5G,
commercial applications, at least for generating low cost solar
cell devices, their efficiency needs to be increased. New D–A
organic small molecules are urgently needed which require
new design concepts and novel building blocks to maximize
the solar cell parameters.
To address this need, we have designed new solution proces-
sable small molecules based on a p-conjugated, rigidly fused,
symmetrical, and planar building block, naphtho[1,2-b:5,6-b0]-
dithiophene (NDT), which is a potential semiconductor for
organic field effect transistors (OFETs).6a Semiconducting poly-
mers incorporating NDT into a polythiophene backbone
exhibited field-effect mobilities as high as 0.5 cm2 Vꢀ1 sꢀ1 and
maximum current on/off ratios (Ion/Ioff) of up to 108 in OFET
devices, which are believed to be amongst the best values of
field-effect mobilities reported thus far.6b Taking its symmetrical
planar structure and higher mobility into consideration, we
envisioned that incorporating an alternating D–A conjugated
unit into the NDT unit could efficiently reduce the band gap and
enhance p–p stacking, thus improving both charge generation
and charge transport for photovoltaic applications. However, it
is worth mentioning that, despite all of these promising features,
this NDT unit has not yet been explored in OSCs.
100 mW cmꢀ2
.
Over the past decade, conjugated polymers with alternating donor–
acceptor (D–A) structures have been extensively investigated
as donor materials for bulk-heterojunction (BHJ) polymer
solar cells to meet the growing demand for inexpensive renew-
able energy sources. After a prolonged effort, impressive
progress has recently been achieved leading very close to large
scale commercialization for plastic solar cells with a power
conversion efficiency (PCE) of over 7.7%.1,2
Recently, the interest of solution processable p-conjugated
D–A organic small molecules has been accelerating for organic
solar cell (OSC) applications. These are emerging as promising
substitutes for conjugated polymers with substantial potential
for generating low cost solar cells owing to the advantages of
simple synthesis, high purity, low processing cost, and reprodu-
cible photovoltaic performance. So far, using small molecules,
the best photovoltaic performance was obtained with diketo-
pyrrolopyrrole units with benzofuran substituents giving rise to a
PCE of conventional BHJ OSCs of 4.4%.3 A variety of other
small molecules based on triphenylamine, isoindigo, merocyanine,
or squaraine derivatives with either linear, dendritic, X- or
star shaped architectures have also been investigated recently
in solution-processed OSCs.4,5 Despite the appreciable recent
advances, the device efficiencies obtained for these small
molecules are relatively low as compared to the PCEs of
polymer solar cells. Obviously, to realize small molecules for
In this communication, we report the syntheses, and photo-
voltaic performances of first NDT-containing p-conjugated
D–A organic small molecules. The small molecules, BNB and
TBNBT were synthesized with thiophene-bridged NDT as the
central donor unit and benzothiadiazole or triphenylamine-
capped benzothiadiazole as arms (see Fig. 1 for structures).
Benzothiadiazole was selected as an acceptor as it has been
established as one of the most efficient acceptors used in low
band gap materials for polymer solar cell applications.7 Moti-
vation for the introduction of a triphenyl amine donor unit at
a School of Semiconductor and Chemical Engineering, Chonbuk
National University, Duckjin-dong 664-14, Jeonju 561-756, Korea.
E-mail: shlee66@ jbnu.ac.kr; Fax: +82 63-270-2306;
Tel: +82 63-270-2435
b Department of Chemistry, The Catholic University, 43-1,
Yeokaok2-dong, Wonmi-gu, Buchen-si, Gyeonggi-do 420-743, Korea
w Electronic supplementary information (ESI) available: Experimental
procedures including structural characterizations and Fig. S1–S7. See
DOI: 10.1039/c1cc15465f
Fig. 1 Molecular structures of the small molecules.
Chem. Commun., 2012, 48, 573–575 573
c
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Fig. 2 (a) UV-vis absorption spectra of BNB and TBNBT in chloro-
form solutions (1 ꢁ 10ꢀ5 M) and in thin films. (b) Cyclic voltammo-
grams of BNB and TBNBT films in a 0.1 mol Lꢀ1 Bu4NPF6/CH3CN
solution.
transfer process between the small molecules and PCBM, which
is desired for potential solar cell applications.
The energy levels of the small molecules were investigated
by cyclic voltammetry (CV) (Fig. 2b). CV was carried out for
BNB and TBNBT films with a Pt counter electrode and an
Ag/Ag+ reference electrode in a solution of tetrabutylammonium
hexafluorophosphate (Bu4NPF6) (0.1 M) in acetonitrile with a
scan rate of 50 mV sꢀ1. The highest occupied molecular orbital
(HOMO) energy levels of BNB and TBNBT were determined
to be ꢀ5.16 and ꢀ5.34 eV, respectively, using the ferrocene
reference value of ꢀ4.41 eV below the vacuum level.9 The
LUMO levels of BNB and TBNBT were determined to be
ꢀ3.18 and ꢀ3.34 eV, respectively. The related electrochemical
data are listed in Table 1. The estimated energy levels of both
BNB and TBNBT indicate that these molecules are suitable for
use as donor materials blended with PCBM as an acceptor
in OSCs. Since the Voc of the OSC device is related to the
difference between the HOMO of the donor and the LUMO of
the acceptor, the low-lying HOMO levels of BNB and TBNBT
further ensure an increase in Voc on construction of the solar
cell devices.
Scheme 1 Synthetic route to the small molecules.
the terminal position in the TBNBT molecule was to extend
conjugation, enhance absorption and improve the charge transfer
capability of the small molecules.8
Synthetic approaches for the small molecules are shown in
Scheme 1. Starting from commercially available 2,6-dihydroxy-
naphthalene (3), the NDT (7) molecule was synthesized in four
steps following procedures described in the literature.6a The
pristine NDT molecule lacks alkyl chains in its core and thus
has limited solubility in organic solvents. In order to improve its
solubility and processability, we introduced decanyl thiophene
units at the 2- and 7-position of the NDT core by a Pd-catalyzed
stille coupling reaction between 8 and 2-bromo-3-decylthio-
phene. 4-Bromobenzo[c]-[1,2,5]thiadiazole (12) was synthesized
by selective bromination of benzothiadiazole (11) in a 21%
yield. Stille coupling reactions of 12 and 10 were carried out
using Pd2dba3/P(o-tolyl)3 as a catalyst in anhydrous chloro-
benzene resulting in a red solid, BNB, in a 68% yield. Compound
15 was obtained in a 54% yield by a Pd(PPh3)4-assisted Suzuki
coupling reaction between 13 and 14. Finally, similar to BNB, a
stille coupling reaction of 10 and 15 afforded a desired dark
brown solid, TBNBT (2) in a 72% yield. Detailed synthetic
procedures and characterizations can be found in the ESI.w
The UV-vis absorption spectra of the small molecules in the
chlorobenzene solutions and in films are shown in Fig. 2(a).
The corresponding absorption data are summarized in Table 1.
The BNB in the chlorobenzene solution exhibited a narrow
absorption band covering the wavelength range from 300 to
500 nm with a molar absorptivity of 24 880 L Mꢀ1 cmꢀ1 at 426 nm.
The absorption spectra broadened and extended to 600 nm in
the solution with a molar absorptivity of 70 445 L Mꢀ1 cmꢀ1 at
499 nm for TBNBT, due to its terminal triphenyl amine units.
In comparison with their absorption spectra in solution, the
absorption spectra for both BNB and TBNBT in thin films were
red-shifted, exhibiting absorption onsets at 572 and 632 nm,
corresponding to the optical band gap (Eogpt) of 2.16 and 1.96 eV,
respectively, and thus indicating a strong intermolecular inter-
action in the solid state. The PL spectra of BNB and TBNBT
showed strong emission bands with emission maxima at 631 nm
and 671 nm, respectively (see Fig. S4 in ESIw). Upon addition of
PCBM, the emission bands of both the small molecules were
found to be significantly quenched, indicating an effective charge
To determine the charge carrier mobilities of the small
molecules, OFET devices were fabricated with a bottom-
contact device configuration built on an n-doped octyltrichloro-
silane (OTS-8)-treated silicon wafer as described in the ESI.w
Unfortunately, despite our repeated attempts, the solution
processed BNB devices were unable to exhibit any field effect
mobility because of the poor quality of the films on Si/SiO2
substrates. However, benefiting from the terminal triphenyl-
amine units, the TBNBT-based devices showed homogeneous
thin films with p-channel FET responses. A field-effect hole
mobility of 6.0 ꢁ 10ꢀ5 cm2 Vꢀ1 sꢀ1 and a Ion/Ioff ratio of 102
were observed in TBNBT FETs (Fig. S5, ESIw) under the same
experimental conditions.
Photovoltaic performances of the small molecules were
investigated by fabricating BHJ devices in a conventional
device configuration of ITO/PEDOT : PSS (40 nm)/BNB or
TBNBT : PC71BM (1 : 1 to 1 : 4 w/w)/LiF (0.5 nm)/Al (120 nm).
PC71BM was selected as the acceptor due to its stronger light
absorption in the visible region compared to PC61BM.10
The detailed device fabrication process is described in the
ESI.w In the preliminary device investigations, chlorobenzene
(CB) was chosen as a processing solvent because it is believed
to be a better solvent in high-performance bulk-heterojunction
organic solar cells.8,11 The active layers were spin-coated from
CB at different BNB/TBNBT and PC71BM blending ratios
c
574 Chem. Commun., 2012, 48, 573–575
This journal is The Royal Society of Chemistry 2012

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Table 1 The optical and electrochemical properties of BNB and TBNBT and the photovoltaic characteristics of BHJ solar cell devices at 1 : 4 (w/w)
small molecule/PC71BM compositions blended in chlorobenzene
Small molecule
lmaxa/nm
lmaxb/nm
Eogptc (filmb)
HOMOd/eV
LUMOe/eV
Jsc/mA cmꢀ2
Voc/V
FF
PCE (%)
BNB
TBNBT
426
499
494
533
2.16
1.96
ꢀ5.16
ꢀ5.34
ꢀ3.18
ꢀ3.34
3.79
6.18
0.93
0.95
0.28
0.37
0.98
2.20
a
b
c
d
ox
Measured in chlorobenzene solution. Spin-coated film from chlorobenzene solution. Eogpt = 1240/lonset
.
HOMO = ꢀ(4.4 + E
) (eV).
onset
e
red
onset
LUMO = ꢀ(4.4 + E
).
for both the BNB : PC71BM and TBNBT : PC71BM films.
However, the TBNBT : PC71BM blend film (rms = 0.245 nm)
showed better miscibility with low PCBM grain aggregation
compared with the BNB : PC71BM blend film (rms = 0.41 nm),
which could explain the relatively higher FF of the
TBNBT : PC71BM cell.12
In conclusion, a new p-conjugated rigidly fused, symmetrical,
and planar building block naphtho[1,2-b:5,6-b0]dithiophene
(NDT) was employed for the first time to develop novel
conjugated small molecules for OSCs. As an outcome of our
sequential structural design, solution-processed BHJ OSCs
based on one of the small molecules, TBNBT, reached a PCE
of 2.20% with a noticeably high Voc of 0.95 V under preliminary
device characterizations. This work shows for the first time
that a naphtho[1,2-b:5,6-b0]dithiophene moiety bridged with a
thiophene as a conjugated spacer is a quite promising, simple,
and useful building block for designing efficient photoactive
materials.
Fig. 3 (a) J–V curves of BHJ solar cell devices based on the
BNB : PC71BM (red line) and TBNBT : PC71BM (blue line) at a
1 : 4 w/w ratio in CB under an illumination of AM 1.5G, 100 mW cmꢀ2
.
(b) EQE and absorption spectra of the corresponding devices.
(1 : 1 to 1 : 4 w/w) with active layer thicknesses ranging from
55 to 75 nm. Optimal fabrication conditions were achieved with
a small molecule/PC71BM ratio of 1 : 4 (w/w) at a 65 nm active
layer thickness. Fig. 3(a) shows the current density–voltage
(J–V) curves of the OSCs in the dark and under illumination
of AM 1.5G (100 mW cmꢀ2). The corresponding photovoltaic
properties of the devices are summarized in Table 1. The device
with a 1 : 4 weight ratio of BNB to PC71BM provided a Voc of
0.93 V, a Jsc of 3.79 mA cmꢀ2, and a fill factor (FF) of 0.28,
resulting in an estimated PCE of 0.98%. In contrast, the device
based on TBNBT and PC71BM with the same weight ratio
delivered a superior performance with a Voc of 0.95 V, a Jsc of
6.18 mA cmꢀ2, and an FF of 0.37, yielding a PCE of 2.20%.
The higher Voc values are consistent with the deep lying HOMO
values of both BNB and TBNBT. However, the slightly larger
band gap and smaller absorption coefficient of BNB translated
into a lower Jsc in the BNB/PC71BM solar cell devices.
This work was supported by the New & Renewable Energy
program of KETEP (No. 20103020010050) funded by MKE
and Pioneer Research Center Program through NRF funded
by MEST (No. 2008-05103), Korea.
Notes and references
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External quantum efficiencies (EQE) of the devices (1 : 4 w/w)
measured under the illumination of monochromatic light are
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The film morphologies of the active layers were observed by
atomic force microscopy (AFM) measurements. The surface
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films at a 1 : 4 w/w blend ratio is shown in Fig. S7 (ESIw).
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c
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Chem. Commun., 2012, 48, 573–575 575