New A-A-D-A-A-type electron donors for small molecule organic solar cells

Article abstract of DOI:10.1021/ol2021077

Two A-A-D-A-A-type molecules (BCNDTS and BDCDTS), where two terminal electron-withdrawing cyano or dicyanovinylene moieties are connected to a central dithienosilole core through another electron-accepting 2,1,3-benzothiadiazole block, have been synthesized, characterized, and employed as electron donors for small molecule organic solar cells. Vacuum-deposited bilayer and planar mixed heterojunction devices based on BCNDTS and fullerene acceptors (C₆₀ or C₇₀) exhibited decent power conversion efficiencies of 2.3% and 3.7%, respectively.

Full text of DOI:10.1021/ol2021077

ORGANIC
LETTERS
2011
Vol. 13, No. 18
4962–4965
New A-A-D-A-A-Type Electron Donors for
Small Molecule Organic Solar Cells
Li-Yen Lin,† Chih-Wei Lu,‡ Wei-Ching Huang,‡ Yi-Hong Chen,‡ Hao-Wu Lin,*,‡
and Ken-Tsung Wong*,†
Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan, and
Department of Materials Science and Engineering, National Tsing Hua University, Hsin
Chu 30013, Taiwan
hwlin@mx.nthu.edu.tw; kenwong@ntu.edu.tw
Received August 3, 2011
ABSTRACT
Two A-A-D-A-A-type molecules (BCNDTS and BDCDTS), where two terminal electron-withdrawing cyano or dicyanovinylene moieties
are connected to a central dithienosilole core through another electron-accepting 2,1,3-benzothiadiazole block, have been synthesized,
characterized, and employed as electron donors for small molecule organic solar cells. Vacuum-deposited bilayer and planar mixed hetero-
junction devices based on BCNDTS and fullerene acceptors (C60 or C70) exhibited decent power conversion efficiencies of 2.3% and 3.7%,
respectively.
Organic photovoltaics (OPVs) are emerging as a clean
and competitive renewable energy resource due to their
unique features including low-cost manufacturing, light
weight, and mechanical flexibility.1 Intensive interdisci-
plinary efforts have been dedicated to improving the
power conversion efficiencies (PCEs) of solution-pro-
cessed polymer bulk heterojunction (BHJ) solar cells.2
Organic solar cells employing small molecules as electron
donors have also received great attention.3 To date, small
molecule organic solar cells (SMOSCs) based on p-type
small molecules and n-type fullerene derivatives have been
realized with PCEs in excess of 5% by using either solu-
tion-processed4 or vacuum-deposited5 fabrication techni-
ques. Moreover, a PCE of up to 8.3% efficiency has been
achieved with a vacuum-deposited tandem cell.6
Regarding the recent developments of solar-absorbing
materials utilized in SMOSCs, the intrinsic properties of
commercially available fullerenes such as high electron
affinities and superior electron mobilities make them the
best acceptor components in contemporary OPV devices.2a
Thus, the search for new donor materials with appropriate
physical properties such as low bandgaps, suitable energy
levels, high crystallinity, decent solubility, etc. has taken
center stage. Along this line, a large number of donor
molecules have been extensively investigated with varying
degrees of success. Among them, the symmetrical accep-
torꢀdonorꢀacceptor (A-D-A) or donorꢀacceptorꢀ
donor (D-A-D) systems represent the most ubiquitous
molecular architectures, where a wide variety of electron-
withdrawing groups, such as squaraine,4 dicyanovinylene,5
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r
10.1021/ol2021077
Published on Web 08/23/2011
2011 American Chemical Society

alkyl cyanoacetate,7 2,1,3-benzothiadiazole,8 thiadiazolo-
[3,4-c]pyridine,9 diketopyrrolopyrrole,10 isoindigo,11 tetra-
cyanobutadiene,12 and borondipyrromethene,13 are com-
bined with electron-donating oligothiophenes and/or ar-
ylamines. Through a judicious combination of donors and
acceptors, the energy levels and bandgaps of these quad-
rupolar chromophores can be readily modulated. How-
ever, there are only a few examples exhibiting remarkable
success with this molecular design, rendering the develop-
ment of new molecular architectures to pursue the so-
called “ideal donor molecules” a main focus of SMOSCs
research.
In this letter, we report the synthesis, characterization,
and photovoltaic application of two A-A-D-A-A-type
small molecules (BCNDTS and BDCDTS, Scheme 1), in
which two terminal electron-withdrawing cyano or dicya-
novinylene moieties are connected to a central dithienosi-
lole (DTS) core through another electron-accepting 2,1,3-
benzothiadiazole (BT) block. The central dithienosilole
unit adopted here is a versatile p-type building block for
the development of various optoelectronic materials.14
Specifically, BHJ solar cells based on DTS-containing
conjugated polymers have exhibited respectable PCEs of
up to 7.3%,15 benefiting from their better packing abilities
and higher hole mobilities as compared to those of the
carbon-based homologues.16 We envisioned that the A-A-
D-A-A molecular configuration would not only possess
deep-lying highest occupied molecular orbital (HOMO)
energy levels but also exhibit better light-harvesting abil-
ities in comparison to the A-D-A counterparts due to the
extension of conjugated π-systems and the fortified qui-
noidal character of conjugated backbones.
The synthetic routes to the target compounds BCNDTS
and BDCDTS are depicted in Scheme 1, and the synthesis
of the building blocks 1 and 2 is shown in Scheme S1 in
Supporting Information. Stille coupling reaction of the
distannyl derivative 1 with 2 afforded BCNDTS in 43%
yield. BDCDTS was obtained with a yield of 38% via a
two-step process where the first involved a Stille coupling
reaction of the distannyl derivative 1 with 7-bromo-2,1,3-
benzothiadiazole-4-carbaldehyde, subsequently followed
by Knoevenagel condensation with malononitrile.
The thermal stability of BCNDTS and BDCDTS was
investigated by thermogravimetric analysis (TGA), which
showed the decomposition temperature (Td) (5% weight
loss) to be 378 and 312 °C for BCNDTS and BDCDTS,
respectively (Figure S1 in Supporting Information). The
inferior thermal stability for BDCDTS is attributed to the
presence of the fragile dicyanovinylene moieties.
The electrochemical properties of BCNDTS and
BDCDTS were probed by cyclic voltammetry (CV). As
shown in Figure 1, both compounds exhibited one quasi-
reversible oxidation wave, corresponding to the oxidation
of the central DTS donor. In the cathodic potential regime,
four quasi-reversible reduction waves were observed for
BCNDTS. The first two waves can be attributed to the
stepwise reduction of the cyano groups, whereas the third
wave can be assigned to the reduction of the BT fragments.
On the other hand, BDCDTS showed three quasi-rever-
sible reduction waves, and the first and second wave can be
ascribed to the reduction of the dicyanovinylene and BT
blocks, respectively. Moreover, it is worthy to note that all
the reduction potentials of BDCDTS are more positive
than those of BCNDTS (Table 1), reflecting the stronger
electron-withdrawing nature of the dicyanovinylene
acceptors.
Scheme 1. Synthesis of BCNDTS and BDCDTS
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Figure 1. Cyclic voltammograms of BCNDTS and BDCDTS.
All potentials were recorded versus ferrocene/ferrocenium (Fc/
Fcþ) as an external reference. Inset: the selected reduction
region of the differential pulse voltammogram of BCNDTS.
Scan rate: 100 mV/s.
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Org. Lett., Vol. 13, No. 18, 2011
4963

Table 1. Photochemical, Electrochemical, and Thermal Parameters for BCNDTS and BDCDTS
λ
abs soln
(nm)a (ε, Mꢀ1
cmꢀ1
λabs
film
(nm)b
ΔEopt
film
(eV)c
Eox
(V)d
Ered
(V)e
ΔECV
(eV)f
HOMO
(eV)g
LUMO
(eV)h
Td
(°C)i
1
1
compd
)
BCNDTS
BDCDTS
553 (49984)
643 (78466)
572
659
2.17
1.88
0.73
0.71
ꢀ1.36
ꢀ0.99
2.09
1.70
ꢀ5.40
ꢀ5.80
ꢀ3.23
ꢀ3.92
378
312
a Measured in CH2Cl2 solution (10ꢀ5 M). b Thin film on fused-silica substrate. c Estimated from λabs film. d Measured in CH2Cl2 solution with 0.1 M
tetrabutylammonium hexafluorophosphate (TBAPF6) as supporting electrolyte. e Measured in THF solution with 0.1 M tetrabutylammonium
perchlorate (TBAP) as supporting electrolyte. All potentials were recorded versus ferrocene/ferrocenium (Fc/Fcþ) as an external reference. f Estimated
g Determined by ultraviolet photoelectron spectroscopy (UPS). h LUMO = HOMO þ ΔEopt film.
1
1
from the difference between Eox and Ered
.
i Temperature corresponding to 5% weight loss obtained from TGA analysis.
Figure 2. Normalized absorption spectra of BCNDTS and
BDCDTS in CH2Cl2 and thin film.
Figure 3. Current densityꢀvoltage characteristics (under AM
1.5G, 100 mW/cm2 illumination) and EQE spectra (inset) of
bilayer devices. The device structures are ITO/MoO3 (5 nm) or
PEDOT:PSS (30 nm)/BCNDTS or BDCDTS (8ꢀ10 nm)/C60
(35 nm) or C70 (15 nm)/BCP (10 nm)/Ag.
Normalized UVꢀvis absorption spectra of BCNDTS
and BDCDTS are shown in Figure 2. In CH2Cl2 solution,
BCNDTS and BDCDTS each show a featureless absorp-
tion band with a maximum at 553 and 643 nm, respectively.
In comparison to the reported molecule composed of a DTS
donor end-capped with BT acceptors (λabs at 503 nm),17
these two tailor-made molecules exhibit evident bathochro-
mic shifts in absorption, manifesting that enhanced solar
spectral responses can be realized through our innovative
molecular design. In contrast, the thin film absorption
bands of BCNDTS and BDCDTS are broadened and
red-shifted, which is probably stemmed from intermole-
cular πꢀπ stacking of the molecules in the solid states.
Compared to the cyano end groups in BCNDTS, the
stronger electron-withdrawing dicyanovinylene end
groups endow BDCDTS with lower-lying molecular orbi-
tals (LUMO), leading to a significantly reduced optical
bandgap (Table 1).
The resulting absorption edge of the BDCDTS thin film
even extends to 800 nm. Unfortunately, the combination
of the stronger electron-withdrawing dicyanovinylene ac-
ceptors with electron-accepting BT moieties imparts
BDCDTS thin films a higher electron affinity than full-
erenes (as shown in the band diagram, Figure S2 in
Supporting Information). Therefore, BDCDTS is hardly
suitable to be paired with commonly used fullerene accep-
tors and the device data shown later also confirm this
speculation.
Bilayer heterojunction (BLHJ) solar cells were first
fabricated to evaluate the molecules for photovoltaic applica-
tions. The thicknesses of donor (BCNDTS, BDCDTS) and
acceptor (C60, C70) layers have been tuned for accomplish-
ing the best performance (Figures S3 and S4, Table S1 in
Supporting Information). The current densityꢀvoltage
(J-V) characteristics under AM 1.5G simulated solar illu-
mination at an intensity of 100 mW/cm2 and correspond-
ing external quantum efficiency (EQE) spectra are shown
in Figure 3. Note that, due to the inefficient electron
transfer between BDCDTS and C60, the BDCDTS/C60 cell
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Org. Lett., Vol. 13, No. 18, 2011

shows a low PCE of 0.07% (Table 2). In contrast, the
device based on BCNDTS/C60 shows superior perfor-
mance with a VOC of 1.02 V, JSC of 4.17 mA/cm2, and fill
factor (FF) of 0.54, thus improving the PCE to 2.3%. The
BCNDTS/C70 BLHJ cell has also been fabricated and
measured (Figures S5 and S6, Table S2, in Supporting
Information). In comparison to the BCNDTS/C60 device,
the BCNDTS/C70 cell exhibits a higher JSC due to higher
and broader extinction coefficients of C70 thin films.
However, because of the lower electron mobilities of C70
thin films,18 the device shows a lower FF of 0.41, which is
only 76% of the BCNDTS/C60 device, resulting in a slightly
lower efficiency of 1.9%. It is noteworthy that both
BCNDTS/C60 and BCNDTS/C70 devices exhibit high
VOC values (up to 1.02 V), which can be ascribed to the
deep-lying HOMO level (ꢀ5.4 eV, measured with ultra-
violet photoelectron spectroscopy) of BCNDTS thin films.
Encouraged by the promising performance of BLHJ
cells based on BCNDTS, planar mixed heterojunction
(PMHJ) solar cells consisting of donor layer-donor:accep-
tor (1:1) mixed layer-acceptor layer as active layers were
further investigated.14d (Figures S7 and S8, Table S3, in
Supporting Information). Figure 4 shows the J-V charac-
teristics and EQE spectra of PMHJ devices. Like the BLHJ
solar cells, the higher JSC value obtained in the BCNDTS:
Table 2. Photovoltaic Parameters of Bilayer and Planar Mixed
Heterojunction Solar Cells under AM 1.5 G Simulated Solar
Illumination at an Intensity of 100 mW/cm2
device type
Jsc (mA/cm2)
Voc (V)
FF
η (%)
a
BDCDTS/C60
0.73
4.17
5.11
5.80
7.81
9.79
0.43
1.02
0.93
1.05
1.03
1.01
0.24
0.54
0.41
0.38
0.34
0.38
0.07
2.3
1.9
2.3
2.8
3.7
a
BCNDTS/C60
a
BCNDTS/C70
BCNDTS:C60 (1:1)b
BCNDTS:C70 (1:1)b
BCNDTS:C70 (1:1.5)b
a Bilayer heterojunction configuration. b Planar mixed heterojunc-
tion configuration.
which couldbemanipulated eitherby post-treatmentssuch
as thermal/solvent annealing or changing the donor:ac-
ceptor ratio. Accordingly, various blend ratios (2:1, 1.5:1,
1:1, 1:1.5, and 1:2) of BCNDTS and C70 in PMHJ solar
cells have been explored (Figures S9 and S10, Table S4, in
Supporting Information). Increasing the concentration of
C70 in mixed layer has a strong impact on the JSC, while the
VOC is less dependent on the concentration of C70 as it is
primarily dictated by the nature of the components. As
shown in Figure 4, the best performance was obtained with
the BCNDTS:C70 ratio at 1:1.5, giving a PCE of up to
3.7% with a VOC of 1.01 V, JSC of 9.79 mA/cm2, and fill
factor of 0.38 (Table 2). The results may indicate that this
blend ratio yields a preferable phase separation and carrier
transportation pathway in the mixed layer.
In summary, two new molecules (BCNDTS and
BDCDTS) with an A-A-D-A-A molecular architecture
have been synthesized. The side-by-side combination of
two strong electron-withdrawing groups leads BDCDTS
to exhibit a low optical bandgap. However, the overtuned
LUMO level prohibits BDCDTS as an effective donor
material in fullerene-based solar cells. In contrast, the thin
film absorption spectrum and energy levels of BCNDTS
show promising characteristics as efficient donor materials
in SMOSCs. A PCE as high as 2.3% has been obtained
with a simple bilayer structure. By adopting planar-mixed
device structures and tuning the donor:acceptor ratio of
the mixed layer, the PCE value has been further improved
to 3.7% without extra thermal or solvent annealing pro-
cesses. We believe that the present results can open up a
new design concept to develop small molecule electron
donors for highly efficient organic photovoltaics.
C70 device is attributed to higher extinction coefficients of
C
70 in the range of 380ꢀ470 nm, which can complement
the absorption valley of BCNDTS thin films. Interestingly,
the PMHJ BCNDTS:C60 and BCNDTS:C70 cells show
similar FF values of about 0.34 (Table 2). This could be
ascribed to the similar field-dependent carrier trans-
port and recombination properties in BCNDTS:C60 and
BCNDTS:C70 mixed layers. Consequently, the BCNDTS:
C70 cells shows a superior PCE of 2.8%.
Acknowledgment. We would like to acknowledge Na-
tional Science Council, Taiwan (NSC 98-2112-M-007-028-
MY3, 98-2119-M-002-007-MY3), National Tsing Hua
University, and National Taiwan University for financial
support.
Figure 4. Current densityꢀvoltage characteristics (under AM
1.5G, 100 mW/cm2 illumination) and EQE spectra (inset) of
PMHJ devices. The device structures are ITO/MoO3 (5 nm)/
BCNDTS (7 nm)/BCNDTS:C60 or BCNDTS:C70 (40 nm)/C60
(20 nm) or C70 (7 nm)/BCP (10 nm)/Ag.
Supporting Information Available. Synthesis, charac-
1
The performance of PMHJ devices are largely affected
by BHJ properties of the donorꢀacceptor mixed layer,
terization, copies of H and 13C NMR spectra, device
fabrication, and optimization of solar cells. This material
is available free of charge via the Internet at http://pubs.
acs.org.
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4965