A new donor-acceptor molecule with uniaxial anisotropy for efficient vacuum-deposited organic solar cells

Article abstract of DOI:10.1039/c1cc12139a

A new D-π-A molecule (TPDCDTS) adopting coplanar diphenyl-substituted dithienosilole as a central π-bridge of triphenylamine (donor) and dicyanovinylene (acceptor) has been synthesized as donor material for small molecule organic solar cells (SMOSC) incor

Full text of DOI:10.1039/c1cc12139a

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COMMUNICATION
A new donor–acceptor molecule with uniaxial anisotropy for efficient
vacuum-deposited organic solar cellsw
Hao-Wu Lin,*a Li-Yen Lin,b Yi-Hong Chen,a Chang-Wen Chen,a Yu-Ting Lin,a
Shi-Wen Chiua and Ken-Tsung Wong*b
Received 14th April 2011, Accepted 16th May 2011
DOI: 10.1039/c1cc12139a
A new D–p–A molecule (TPDCDTS) adopting coplanar diphenyl-
substituted dithienosilole as a central p-bridge of triphenylamine
(donor) and dicyanovinylene (acceptor) has been synthesized as
donor material for small molecule organic solar cells (SMOSC)
incorporating C70 as an acceptor showed an appreciable power
conversion efficiency of 3.82%.
can be extended toward longer wavelength by effective intra-
molecular charge-transfer (ICT) between donor and acceptor
moieties; (ii) the energy level can be readily tuned by incorporating
different electron-donating and/or -accepting groups and even
the p-bridges; (iii) the intermolecular p–p interaction can be
manipulated by the judicious selection of central p-bridges.
Triphenylamine (TPA) has been widely utilized as optoelectronic
materials owing to its good electron-donating nature and high
hole-transporting capabilities.8 Furthermore, the three-
dimensional structural features of TPA-based molecules usually
lead to amorphous materials with isotropic optical and hole-
transporting characteristics. In this aspect, it has been reported
that vacuum-deposited SMOSCs utilizing TPA-based materials,
in which TPA behaves as a core and electron-donating unit,
exhibit PCEs of up to 2.2%.9 On the other hand, various
electron-withdrawing groups, such as nitro,10a dimesitylboryl,10a
dicyanovinyl,10b,9a,b trifluoroacetyl,10c benzothiadiazole,10d–g and
thiadiazolopyridine10d–g have been introduced as electron-
accepting subunits, giving the resulting donor molecules
low-lying HOMO energy levels and thus consequently rendering
cells with large Voc values.
Organic thin-film photovoltaics (OPVs) have received much
attention as the next-generation solar cells due to their potential
mechanical flexibility and low cost. Since the pioneering work
on bilayer heterojunction solar cells,1 the power conversion
efficiencies (PCEs) of solution-processed polymer solar cells
have reached over 7%.2 On the other hand, small molecule
organic solar cells (SMOSC) fabricated by solution and
vacuum processes reach PCEs of up to 4.4%3 and 5.2%,4
respectively. Although the state-of-the-art polymer solar cells
show higher efficiencies than low-molecular-weight counter-
parts, an increasing number of studies have been devoted to
developing SMOSCs.5 The main reasons are because of several
unique advantages of small molecules over polymeric materials
such as well-defined molecular structures, better batch-to-
batch reproducibility, and possibilities of purification by
sublimation. Moreover, vacuum-deposited SMOSCs show
remarkable promise due to the advantages of easy fabrication
of multilayer tandem architectures.6 For instance, Heliatek
GmbH released a certified PCE of 8.3% for a tandem device
prepared by the combination of two bulk heterojunction
devices,7 rendering SMOSCs competitive to contemporary
solar energy technologies.
In this communication, we report a new organic donor
material TPDCDTS (Scheme 1) that contains triphenylamine
as an electron-donating moiety and dicyanovinylene as an
electron-withdrawing moiety separated by a coplanar diphenyl-
substituted dithienosilole (DTS) p-conjugated spacer. The
adoption of this D–p–A structure with a coplanar bridge
facilitates the electronic coupling between the donor and
acceptor blocks, ensuring an extension of the spectral response
to the red portion of the solar spectrum. In addition, the
introduction of the diphenyl-substituted DTS as a central
bridge possesses several key advantages required for SMOSCs:
(i) the HOMO–LUMO gap of DTS is smaller than that of
2,20-bithiophene and cyclopentadithiophene due to its lower
lying LUMO (s*–p* conjugation). The smaller band gap of
DTS is favorable for harvesting sunlight;11 (ii) the HOMO of
DTS is lower than that of cyclopentadithiophene, which is
With regard to the molecular architectures of donor
materials for SMOSCs fabricated by vacuum-evaporation
processes, donor–(p-bridge)–acceptor (D–p–A) systems are
emerging as good candidates due to their prominent merits,
such as (i) the absorption spectrum of the D–p–A molecules
a Department of Materials Science and Engineering,
National Tsing Hua University, Hsin Chu 30013, Taiwan.
E-mail: hwlin@mx.nthu.edu.tw
b Department of Chemistry, National Taiwan University,
Taipei 10617, Taiwan. E-mail: kenwong@ntu.edu.tw
w Electronic supplementary information (ESI) available: Synthesis,
characterization, cyclic voltammogram of TPDCDTS, energy levels
alignments of active layers, AFM surface morphology analysis, copies
of 1H and 13C NMR spectra, devices and measurements. See DOI:
10.1039/c1cc12139a
Scheme 1 Synthesis of TPDCDTS.
c
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This journal is The Royal Society of Chemistry 2011

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critical for increasing the Voc;11 (iii) it was reported that
DTS-containing polymers show higher hole mobility (3-fold
higher) than that of cyclopentadithiophene-containing
polymers.12 In addition, the Csp2–Si bond of DTS is significantly
longer than the Csp2–Csp3 bond of cyclopentadithiophene,
allowing more efficient p–p stacking of conjugated backbones
due to the reduced steric hindrance between the pendant
groups and thiophene rings13; (iv) the tetrahedral silicon center
of diphenyl-substituted DTS does not distort the coplanarity
of the p-conjugated system. Instead, the rigid and bulky
substituents lead to strongly anisotropic molecular geometries
(e.g. rod-like shapes) and the molecules can be assembled into
oriented configurations in thin films. Usually, this kind of
rod-like molecules tend to align their molecular axes and p–p*
transition dipole moments along the substrate surface upon
vacuum deposition, resulting in higher in-plane extinction
coefficients, which is also beneficial for light absorption.14
The synthetic route for the compound TPDCDTS is shown
in Scheme 1. 5-[N,N-bis(phenylamino)phenyl]-50-formyl-3,30-
diphenylsilylene-2,20-bithiophene (1)15 was condensed with
malononitrile to give the target compound TPDCDTS in
72% yield via the Knoevenagel reaction in the presence of
basic Al2O3.
Fig. 2 Ordinary (in-plane) and extraordinary (out-of-plane) optical
constants of TPDCDTS thin film spectra.
of horizontally polarized incident light. Moreover, a very high
in-plane refractive index (42.0) in the wavelength of
570–700 nm improves the constructive interferences in the
active layers, which reinforces the optical field and thus results
in higher photon harvest in the relatively weak absorption
wavelength region. In addition to preferable optical effects,
such anisotropic molecular orientation may also lead to more
efficient vertical carrier transport and hence a higher carrier
extraction efficiency.14
The UV-vis absorption spectra of TPDCDTS in thin-film
and solution (in CH2Cl2) are shown in Fig. 1. The absorption
spectrum in solution shows a maximum (lmax) centered at
537 nm (e = 45 800 Mꢀ1 cmꢀ1) with a full-width-at-half-
maximum (FWHM) at 106 nm, while the thin-film absorption
exhibits lmax peaked at 542 nm with a FWHM at 140 nm. The
red-shifted and broadened absorption spectrum in the thin-film is
presumably due to intermolecular p–p stacking in the solid
state. The cut-off absorption wavelength is extended from
B600 nm (in solution) to B650 nm (in thin-film), which
may benefit for harvesting more photons.
The electrochemical characteristics of TPDCDTS were
probed by cyclic voltammetry (CV) in CH2Cl2 with Bu4NPF6
as a supporting electrolyte. TPDCDTS exhibits two quasi-
reversible oxidation potentials at 0.43 and 0.84 V (vs. Fc/Fc+),
respectively, as shown in Fig. S1 of ESI.w The first oxidation
wave at lower potential can be attributed to the oxidation of
the triphenylamine donor, whereas the second oxidation can
be assigned to the DTS core according to the previous reports
on electrochemical behaviours of DTS derivatives.16
The potential utilization of TPDCDTS in vacuum-deposited
SMOSCs was then explored. Planar-mixed heterojunction
(PMHJ) TPDCDTS : fullerenes solar cells were fabricated with
a configuration of ITO/hole transport layer (HTL)/TPDCDTS
(3 nm)/TPDCDTS:C60 or C70 (1 : 1, 35 nm)/C60 or C70
(10–20 nm)/BCP (10 nm)/Ag (120 nm), in which BCP is
adopted as an exciton blocking layer. For initial characterization,
several hole-transport materials including PEDOT : PSS
(poly(3,4-ethylenedioxythiophene) : poly(styrenesulfonate)),
HAT(CN)6 (hexaazatriphenylenehexacarbonitrile), PEDOT :
PSS/HAT(CN)6, PEDOT : PSS/MoO3, and MoO3 were tested
to optimize the cell performances (See Table S1 in ESIw). It is
found that devices with MoO3 as HTL exhibit superior
performances to other HTLs. Accordingly, MoO3 was chosen
as hole transport material for further device fabrication.
Note that the device structures and fabrication conditions in
this study are comparatively simple without complicated
conductive doping in transporting layers and thermal annealing
treatments.2–4
Variable-angle spectroscopic ellipsometry (VASE) was used
to study the optical constants and molecular orientation of
TPDCDTS thin films. As shown in Fig. 2, vacuum-deposited
films of TPDCDTS exhibit significant uniaxial anisotropy
with the optical axis along the surface normal in both the
refractive index and absorption. The results indicate that
TPDCDTS molecules are prone to align their molecular axes
along the substrate surface. As a result, the thin-film shows
high in-plane extinction coefficients up to 0.75 at the peak of
the absorption. Since in common photovoltaic applications,
the incident light is in normal direction; the high in-plane
extinction coefficients directly contribute to higher absorption
Fig. 3(a) shows the current density–voltage (J–V) characteristics
of TPDCDTS : fullerene PMHJ solar cells with MoO3 (30 nm)
as HTL. The TPDCDTS : C60 PMHJ device gave an open
circuit voltage (Voc) of 0.88 V, a short circuit current density
(Jsc) of 6.56 mA cmꢀ2, and a fill factor (FF) of 0.46, yielding a
PCE of 2.69% under AM 1.5 G, 1 sun (100 mW cmꢀ2
)
simulated solar illumination. Furthermore, the TPDCDTS : C70
Fig. 1 Absorption spectra of TPDCDTS in CH2Cl2 and thin film.
PMHJ device delivered a higher performance with Voc of 0.83 V,
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and red-shifted absorption in the visible region, and uniaxial
optical anisotropy along the surface normal. These promising
physical characteristics of TPDCDTS in conjunction with the
complementary absorption of the C70 acceptor in the range of
380–500 nm give appreciable SMOSCs performances with
a PCE up to 3.82%. Further improvement of the device
performance could be achieved by optimizing the donor/
acceptor ratio in the mixed layer, incorporating conductive
doped buffer layers as well as fine-tuning of processing
conditions such as post-annealing and/or substrate heating.
These results will be disclosed in due course.
The authors would like to acknowledge the financial
support from National Science Council of Taiwan (NSC 98-
2112-M-007-028-MY3, 98-2119-M-002-007-MY3).
Notes and references
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Fig. 3 (a) J–V characteristics and (b) external quantum efficiency of
TPDCDTS:C60 PMHJ (square) and TPDCDTS:C70 PMHJ (circle)
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Jsc of 9.53 mA cmꢀ2, FF of 0.48, and PCE of 3.82%. The
HOMO level of TPDCDTS thin film is as low as ꢀ5.4 eV
determined by photoelectron spectroscopy. The low-lying
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The difference in Jsc between two cells can be explained by
the wider absorption range of C70 relative to that of C60, which
is also reflected in the external quantum efficiency (EQE)
spectra shown in Fig. 3(b). Both cells show a high and broad
range of EQE around 500–650 nm closely corresponding to
the absorption spectrum of TPDCDTS. However, due to the
supplementary absorption of the C70 in the range of 380–500 nm,
which complementarily covers the spectral range where the
contribution of TPDCDTS thin films is less efficient. Thus, the
EQE spectrum of TPDCDTS : C70 PMHJ device can effectively
cover the entire visible region. The matching of absorption
spectrum range of TPDCDTS and C70 leads to a broader and
higher EQE range, resulting in a higher Jsc value. In addition,
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TPDCDTS and TPDCDTS : C70 thin film with root-mean-
square roughness of 0.95 and 1.04 nm, respectively (see Fig. S4
in ESIw). A smooth morphology of the organic film may result
in more stable device performance.
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In conclusion, we have synthesized a new small-molecule
TPDCDTS as donor material for efficient SMOSC. Due to its
intrinsic structural features, thin films of TPDCDTS show
intriguing physical properties such as low-lying HOMO, broad
c
7874 Chem. Commun., 2011, 47, 7872–7874
This journal is The Royal Society of Chemistry 2011