A low-energy-gap organic dye for high-performance small-molecule organic solar cells

Article abstract of DOI:10.1021/ja205126t

A novel donor-acceptor-acceptor (D-A-A) donor molecule, DTDCTB, in which an electron-donating ditolylaminothienyl moiety and an electron-withdrawing dicyanovinylene moiety are bridged by another electron-accepting 2,1,3-benzothiadiazole block, has been synthesized and characterized. A vacuum-deposited organic solar cell employing DTDCTB combined with the electron acceptor C₇₀ achieved a record-high power conversion efficiency (PCE) of 5.81%. The respectable PCE is attributed to the solar spectral response extending to the near-IR region and the ultracompact absorption dipole stacking of the DTDCTB thin film.

Full text of DOI:10.1021/ja205126t

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pubs.acs.org/JACS
A Low-Energy-Gap Organic Dye for High-Performance Small-Molecule
Organic Solar Cells
Li-Yen Lin,† Yi-Hong Chen,‡ Zheng-Yu Huang,‡ Hao-Wu Lin,*,‡ Shu-Hua Chou,† Francis Lin,†
Chang-Wen Chen,‡ Yi-Hung Liu,† and Ken-Tsung Wong*,†
†Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan
‡Department of Materials Science and Engineering, National Tsing Hua University, Hsin Chu 30013, Taiwan
S Supporting Information
b
Tailoring of the conjugation length of the oligothiophene core
unit as well as the length and location of pendant alkyl side chains
ABSTRACT: A novel donorÀacceptorÀacceptor (DÀAÀA)
donor molecule, DTDCTB, in which an electron-donating
ditolylaminothienyl moiety and an electron-withdrawing
dicyanovinylene moiety are bridged by another electron-
accepting 2,1,3-benzothiadiazole block, has been synthesized
and characterized. A vacuum-deposited organic solar cell
employing DTDCTB combined with the electron acceptor
C70 achieved a record-high power conversion efficiency
(PCE) of 5.81%. The respectable PCE is attributed to the
solar spectral response extending to the near-IR region
and the ultracompact absorption dipole stacking of the
DTDCTB thin film.
has enabled this class of active materials to demonstrate reliable
PCEs.7 One distinctive feature of such materials is that they
possess deep-lying highest occupied molecular orbital (HOMO)
energy levels and thus afford SMOSCs with extraordinary open-
circuit voltages (Voc) of ∼1 V. On the other hand, DÀA-type
molecules incorporating arylamines as electron-donating groups
also appear to be attractive candidates because of their effective
intramolecular charge transfer (ICT) characteristics. Moreover,
taking advantage of the versatility of structural modifications in
such systems enables the frontier orbital energy levels to be
readily tuned through judicious combinations of different elec-
tron-donating and/or -accepting functional groups. Along this
line, a series of triphenylamine-based DÀA materials have been
reported to exhibit PCEs of up to 2.2%.8 However, both of these
types of materials generally suffer from insufficient light-harvest-
ing capabilities. They usually have absorption maxima at less than
600 nm, which may be one of the main impediments to further
improvement of their efficiencies. Although a few dyes, such as
squaraine9 and merocyanine,10 have been explored to address
this issue, the progress still lags behind that for the AÀDÀA
counterparts. Therefore, it is highly desired to design new
molecular architectures that can readily allow donor materials
to extend the spectral responses to the far-red and even near-IR
regions.
In this communication, we report a novel donorÀacceptorÀ
acceptor (DÀAÀA)-typedonormolecule, DTDCTB (Scheme1),
whose thin-film absorption can extend into the near-IR region. In
this molecule, anelectron-donating ditolylaminothienyl moietyand
an electron-withdrawing dicyanovinylene moiety are bridged by
another electron-accepting 2,1,3-benzothiadiazole (BT) block.
Specifically, DTDCTB was designed with the following structural
characteristics: (i) The ditolylaminothienyl block behaves as a
stronger electron-donating moiety in comparison with the conven-
tional ditolylaminophenyl congener because of its fortified quinoi-
dal character. The quinoidal resonance structure is energetically less
stable than the aromatic form, and thus, the adoption of the
ditolylaminothienyl donor endows DTDCTB with a smaller band
gap.1c (ii) The BT moiety represents the most ubiquitous acceptor
utilized in optoelectronic materials (e.g., low-band-gap poly-
mer donors,1 nonfullerene acceptors,11 and n-type field-effect
transistors12) because of its fascinating features, including low-
rganic solar cells (OSCs) have garnered considerable
O
research interest because of their prominent merits, such
as low cost, light weight, and mechanical flexibility. At present,
solution-processed bulk heterojunction (BHJ) solar cells1 based
on bicontinuous interpenetrating networks of π-conjugated
polymers and soluble fullerene derivatives have demonstrated
remarkable achievements, with power conversion efficiencies
(PCEs) in excess of 7%.2 Beyond that, the small-molecule
counterparts, particularly p-type organic semiconductors utilized
for OSCs, have also shown exceptional promise. The competitive
nature of small molecules relative to polymeric materials can be
ascribed to the predominant advantages including well-defined
molecular structures, easier purification, and better batch-to-
batch reproducibility. Therefore, tremendous research endeavors
have been devoted to developing small-molecule OSCs
(SMOSCs),3 which exhibit appreciable PCEs of >5%, by using
either solution-processing or vacuum-deposition fabrication
techniques.4 Although solution processing is generally consid-
ered to be more cost-effective than vacuum deposition, vacuum-
deposited SMOSCs are emerging as competitive OSCs because
of the advantage of easy fabrication of multilayer tandem
architectures.5 In this regard, a tandem SMOSC device with a
PCE of up to 8.3% has been disclosed recently.6
To date, the molecular architectures of most donor materials
for SMOSCs fabricated by vacuum-evaporation processes can be
classified into two main categories: acceptorÀdonorÀacceptor
(AÀDÀA) and donorÀacceptor (DÀA) systems. AÀDÀA
systems, in which electron-rich oligothiophenes are end-capped
with various electron-withdrawing groups such as dicyanoviny-
lene, 2,1,3-benzothiadiazole, and thiadiazolo[3,4-c]pyridine, are
currently among the most successful molecular architectures.
Received: June 7, 2011
Published: September 09, 2011
r
2011 American Chemical Society
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Scheme 1. Synthetic Route to DTDCTBa
Figure 2. Cyclic voltammograms of DTDCTB in solution.
a (i) NBS, azobis(isobutyronitrile), chlorobenzene, 80 °C, 83%. (ii)
AgNO3, H2O/MeCN, reflux, 92%. (iii) Malononitrile, Al2O3, toluene,
70 °C, 67%. (iv) PdCl2(PPh3)2, toluene, 110 °C, 55%.
Figure 1. X-ray-analyzed molecular structure and crystal packing of
DTDCTB.
band-gap character, high absorption coefficient, and appropriate
energy levels. (iii) According to our previous study,13 the innovative
combination of two acceptors (BT and dicyanovinylene used here)
would effectively narrow the optical band gap of the resulting
molecule while maintaining a relatively low-lying HOMO energy
level, and thus, an enhanced short-circuit current (Jsc) and Voc value
can be concurrently anticipated. As a proof of concept, our
preliminary investigation of vacuum-deposited SMOSCs employ-
ing DTDCTB as the electron donor and C70 as the electron
acceptor revealed a remarkable PCE as high as 5.81%. To the best
of our knowledge, this efficiency is among the highest values ever
reported for vacuum-deposited single cells with organic donor
molecules. This encouraging result indicates the great potential of
such DÀAÀA systems in creating high-performance donor mate-
rials for SMOSCs.
The synthesis of DTDCTB is described in Scheme 1. Benzylic
bromination of 4-bromo-7-methyl-2,1,3-benzothiadiazole14 with
N-bromosuccinimide (NBS) afforded 1. Silver nitrate-promoted
hydrolysis of 1 gave aldehyde 2, which was then condensed with
malononitrile to give the key intermediate 3 via Kn€oevenagel
reaction in the presence of basic Al2O3. Finally, Stille coupling of
5-(N,N-ditolylamino)-2-(tri-n-butylstannyl)thiophene15 and 3
yielded DTDCTB in good yield. It is noteworthy that this
synthetic route provides a versatile method for further engineer-
ing of the molecular structure through the combination of the
unsymmetrical intermediate 3 and other electron-donating
groups.
Figure 3. (a) Normalized absorption spectra of DTDCTB in CH2Cl2
and a thin film. (b) Optical constants (refractive index, n, and extinction
coefficient, k) of DTDCTB, C60, and C70 thin-film spectra.
almost coplanar conformation between the thiophene and BT
rings with a dihedral angle of 5.5°. This coplanarity facilitates the
electronic coupling between the electron-donating and electron-
withdrawing blocks, enhancing the ICT efficiency and thus
ensuring a distinctive bathochromic shift of the spectral re-
sponses. The strong polar character and coplanar conformation
of the heteroaryl components lead DTDCTB crystals to pack in
an antiparallel manner along both molecular axis directions.
The cofacial arrangement of two neighboring BT rings with a
shortest point-to-point (N1ÀC3) distance of 3.55 Å indicates
non-negligible πÀπ interactions, which may facilitate charge-
carrier hopping in the solid state.
The electrochemical properties of DTDCTB were probed by
cyclic voltammetry in solution. As shown in Figure 2, DTDCTB
exhibits one reversible oxidation potential at 0.35 V vs ferrocene/
ferrocenium (Fc/Fc+), corresponding to oxidation of the dito-
lylaminothienyl donor. On the other hand, two reversible
reduction waves at À1.09 and À1.74 V vs Fc/Fc+ were observed
in the cathodic potential regime. The first reduction potential can
be ascribed to the reduction of the dicyanovinylene block and the
second to reduction of the BT fragment. By reference to the
Fc/Fc+ redox couple, where the HOMO of Fc is assigned to be
4.8 eV below the vacuum level, the HOMO and lowest
The molecular structure of DTDCTB was analyzed by X-ray
crystallography. As shown in Figure 1, DTDCTB displays an
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unoccupied molecular orbital (LUMO) energy levels of
DTDCTB were calculated to be À5.15 and À3.71 eV on the
basis of the oxidation potential and the first reduction potential,
respectively. In spite of the presence of the strong ditolylami-
nothienyl donor, DTDCTB shows a relatively low-lying HOMO
level because of the strong electron-withdrawing character of the
BT and dicyanovinylene blocks. Given energy level shifts due to
intermolecular interactions, the HOMO level of the DTDCTB
thin film was determined by UV photoelectron spectroscopy to
have a value of À5.30 eV. The fairly low-lying HOMO would
result in a large energy level offset to the LUMOs of fullerenes,
ensuring relatively large Voc values.
Figure 3a shows the electronic absorption spectra of DTDCTB
in a vacuum-deposited thin film and in CH2Cl2 solution.
Surprisingly, with such a short effective conjugation length, the
absorption spectrum of DTDCTB in solution shows a band at
λmax = 663 nm accompanied by a high extinction coefficient (k)
of up to 41 660 MÀ1 cmÀ1. To gain more insight into the
electronic and optical properties of DTDCTB, density functional
theory (DFT) and time-dependent DFT (TDDFT) calculations
were performed for the molecule in CH2Cl2 solution [Figure S1
and Tables S1 and S2 in the Supporting Information (SI)] and
gave a computed λmax value of 673 nm that is close to the
experimental results. On the other hand, the thin-film absorption
of DTDCTB exhibits λmax = 684 nm. The significant broadening
of the thin-film absorption spectrum is possibly due to inter-
molecular πÀπ stacking as evidenced in the crystal packing.
Apparently, the thin-film absorption of DTDCTB effectively
extends the useful photon-harvesting range down to near-IR
wavelengths (700À800 nm). Optical constants of vacuum-
deposited thin films of DTDCTB, C60, and C70 are shown in
Figure 3b. The DTDCTB thin film exhibits high k values across
the 550À800 nm wavelength range, with kmax ≈ 0.95 at λ =
670 nm, which is coincident with the absorption spectrum shown
in Figure 3a. Notably, the kmax value isamong the highestreported
for solar-absorbing organic thin films, as compared with the
commonly used nano/microcrystalline poly(3-hexylthiophene)
(P3HT) and copper phthalocyanine (CuPc), which show kmax
values of ∼0.65 and ∼0.95, respectively.16,17 This result indicates
that the efficient ICT gives the DTDCTB molecule high polar
character, which favors the formation of ultracompact absorption
dipole packing in the thin film upon vacuum deposition. In
addition, the k spectra of the commonly used acceptor molecules
C60 and C70 show complementary wavelength coverage
(<550 nm) with respect to the DTDCTB thin film. As a result,
fullerenes C60 and C70 were selected as acceptor materials to be
paired with DTDCTB for subsequent device fabrication.
In photovoltaic characterizations, we adopted the vacuum-
deposited planar mixed heterojunction (PMHJ) structure incor-
porating one layer of mixed donor/acceptor materials sandwiched
between pure donor and acceptor layers.18 The optimized device
structures were configured as follows: Device I: ITO/MoO3
(30 nm)/DTDCTB (7 nm)/1:1 (v/v) DTDCTB:C60
(40 nm)/C60 (20 nm)/ 2,9-dimethyl-4,7-diphenyl-1,10-phenan-
throline (BCP) (10 nm)/Ag (150 nm). Device II: ITO/MoO3
(5 nm)/DTDCTB (7 nm)/1:1 (v/v) DTDCTB:C70 (40 nm)/
C70 (7 nm)/BCP (10 nm)/Ag (150 nm). In these devices, BCP
was employed as both an electron-transporting layer and an
exciton-blocking layer. More importantly, MoO3 was chosen as
the hole-transporting layer because of its superior performance in
comparison with several other hole-transportingmaterials.19 Itwas
noted that BHJ polymer solar cells incorporating MoO3 usually
Figure 4. (a) JÀV characteristics and (b) EQE spectra of DTDCTB:
C
60 PMHJ (squares) and DTDCTB:C70 PMHJ (circles) solar cells.
exhibit better device stability than those fabricated with poly
(3,4- ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:
PSS).20 Optimizations to fine-tune the thicknesses of the
DTDCTB and MoO3 layers are shown in the SI. It was found
that the devices with a 7 nm DTDCTB donor layer demonstrated
superior performance. Moreover, through spectrum-response mod-
eling of the simplified bilayer cells, the exciton diffusion length in the
DTDCTB thin film was found to be ∼6 nm (Figure S4),21 which
is within the typical range of 5À10 nm for organic semiconduc-
tors. Figure 4a shows the current densityÀvoltage (JÀV) char-
acteristics of DTDCTB:fullerene PMHJ solar cells. The
DTDCTB:C60 PMHJ device gave a Voc of 0.80 V, a Jsc of
11.40 mA/cm2, and fill factor (FF) of 0.48, yielding a PCE of
4.41% under AM 1.5 G 1 sun (100 mW/cm2) simulated solar
illumination. Remarkably, the DTDCTB:C70 device delivered
higher performance, with a Voc of 0.79 V, Jsc of 14.68 mA/cm2, FF
of 0.50, and PCE of 5.81%, which is the highest one ever reported
for SMOSCs. The high Voc values of these devices are due to
the moderately low-lying HOMO level of DTDCTB. However,
the Voc value (∼0.8 V) still leaves room for future improvement. The
FF values of the DTDCTB:C60 and DTDCTB:C70 devices are
similar, suggesting similar blending layer morphologies and
charge-carrier percolation networks in the two devices. The
higher Jsc of the DTDCTB:C70 device can be attributed to the
higher extinction coefficient of C70 relative to C60, which is fully
consistent with the external quantum efficiency (EQE) spectra
shown in Figure 4b. The EQE spectrum of the DTDCTB:C70
device shows impressive high values of ∼50% throughout the
UVÀvis to near-IR range (350À770 nm), resulting in the high Jsc.
It is noteworthy that the integrated EQE values are in agreement
with the measured short-circuit currents (within 3À5% error; see
Tables S3 and S4). Although the DTDCTB:C70 films show
amorphous morphologies (see the X-ray diffractogram in Figure
S5), the donorÀacceptor phase separation can be clearly observed
in the phase image of the DTDCTB:C70 film (Figure S6). The
effective phase separation may provide carrier transportation
pathways and contribute to high quantum efficiencies.
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In conclusion, a novel DÀAÀA-type donor material,
DTDCTB, in which an electron-donating ditolylaminothienyl
moiety is connected to an electron-withdrawing dicyanovinylene
moiety through another electron-accepting 2,1,3-benzothiadia-
zole block, has been synthesized and applied in the fabrication of
vacuum-deposited SMOSCs. The innovative structural design
strategy enables DTDCTB to exhibit distinguished light-harvest-
ing abilities with spectral responses close to the near-IR region.
A vacuum-deposited SMOSC employing DTDCTB as the
electron donor and C70 as the electron acceptor demonstrated
an exceptional PCE of up to 5.81% in initial trials. This efficiency
is among the highest ever obtained for organic vacuum-deposited
single cells. The high efficiency is primarily attributed to the
broad and intensive absorption (giving high Jsc) and a reasonably
low-lying HOMO level (giving high Voc) of the DTDCTB thin
film. Our results may advance efforts to develop new organic dyes
to boost the device performance of SMOSCs.
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’ ASSOCIATED CONTENT
S
Supporting Information. Synthesis, characterization, co-
b
pies of 1H and 13C NMR spectra, CIF for DTDCTB, procedures
for fabricating devices and performing measurements, quantum-
mechanical calculations, an atomic force microscopy image, and
an X-ray diffractogram. This material is available free of charge via
the Internet at http://pubs.acs.org.
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’ AUTHOR INFORMATION
Corresponding Author
hwlin@mx.nthu.edu.tw; kenwong@ntu.edu.tw
’ ACKNOWLEDGMENT
We thank the National Science Council of Taiwan (NSC 98-
2112-M-007-028-MY3 and NSC 98-2119-M-002-007-MY3)
and the Low Carbon Energy Research Center, National Tsing-
Hua University, for financial support. We are also grateful to the
National Center for High-Performance Computing for computer
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