Spiro-fluorene based 3D donor towards efficient organic photovoltaics

Article abstract of DOI:10.1039/c2cc36301a

A novel 3D donor material (SF8TBT) based on spiro-fluorene has been developed. Compared with the corresponding 1D linear molecule, the OPVs of this 3D donor exhibited power conversion efficiencies of 4.82%, much higher than that of 1D small molecules (1.69%).

Full text of DOI:10.1039/c2cc36301a

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Spiro-fluorene based 3D donor towards efficient organic photovoltaicsw
Shuying Ma,a Yingying Fu,b Debin Ni,a Jian Mao,a Zhiyuan Xie*b and Guoli Tu*a
Received 30th August 2012, Accepted 24th October 2012
DOI: 10.1039/c2cc36301a
A novel 3D donor material (SF8TBT) based on spiro-fluorene
has been developed. Compared with the corresponding 1D linear
molecule, the OPVs of this 3D donor exhibited power conversion
efficiencies of 4.82%, much higher than that of 1D small
molecules (1.69%).
simplicity of production, several strategies have been employed to
optimize the morphology in functional blends, including regulation
of the solvent evaporation rate, solution concentration or material
ratio during thin film formation. Particularly promising strategies,
involving thermal treatment of the cast films and high boiling
temperature solvent additives, have shown some success in
manipulating phase separation.5 Thermal annealing can accelerate
the self-assembly in the active layer and the solvent additives are
understood to alter the phase separation of blends by changing the
relative solubility, hence drying times of components during
casting. All these efforts have been responsible for improving
the morphologies of unremarkable devices based on conjugated
polymer donor materials, to produce some of the most efficient
systems to date.6 However, considering that small molecules
have better crystalline ability than polymers, which leads to
over-sized phase separation and a large amount inefficient
exciton diffusion, it is extremely important to develop new
technologies to optimize the phase separation in small molecular
donor based OPVs, instead of apply the methods for polymer
OPVs mechanically. Compared with polymers, material designs
including tuning the isomer and increasing dimensionality, have
unique advantages for small molecular designs for controlling
the morphology in OPVs due to their precise structures.
Driven by the urgent need for a next generation clean energy
source, organic photovoltaic (OPV) technologies, especially
the blended bulk hetero-junction approach, have been developed
very fast since the mid-1990s.1 When fabricated at room
temperature by printing technology, OPVs exhibit several
unique advantages such as fast fabrication, and in addition,
they are light weight and flexible. These advantages are
pushing OPVs to the leading position in low-cost flexible thin
film solar cells.2 Recently, small molecule donors based on
solution processes, with higher solubility than polymeric
analogues, have become a competitive alternative to the
conjugated polymer donor materials and these small molecule
donors can be finely purified using standard organic chemistry
protocols.3 Compared to narrow band gap conjugated polymers,
small molecule donors have been expected to show completely
reproducible properties due to their mono-disperse characteristics
and lack of end-group variations. Another major advantage of
small molecules over polymers is their tendency to self-assemble
into ordered domains, resulting in superior levels of crystallinity
and high charge-carrier mobility. The active layer in the bulk
hetero-junction architecture consists of a blended film of two
compounds, a donor materials and a fullerene-derived acceptor.
It is increasingly evident that the active layers must fulfil very
stringent morphology requirements to optimize the device
performance. Specifically, ideal active layers in bulk hetero-
junction solar cells require phase separation on a scale comparable
to the exciton diffusion length (10–20 nm) for charge separation, as
well as percolation pathways to each electrode to allow uninhibited
charge transport.4 These specifications cannot easily be controlled
during facile device fabrication processes, such as spin-coating and
printing. To enhance the OPVs efficiency, whilst maintaining
Most of the donor materials are 1D linear structures, which
rely on the intermolecular stacking and steric hindrance of side
chains to push out the PCBM to a certain distance from the
conjugated backbone toward favourable nanoscale phase
separation. This strategy relies on influencing the complicated
interplay of thermodynamic and kinetic factors, aiming to
control the non-equilibrium blend morphology. Donors with 3D
structures, including small molecular and dendritic macro
molecular structures, have been expected to start a completely
new self-assembly approach and increase the amount of stable
morphologies by increasing the dimensions of the donor
molecules.7 Inspired by this, a novel 3D mono disperse spiro
macromolecular donor (SF8TBT, Scheme 1), which was
constructed by a non-conjugated connection of two linear
small molecular donors and can be purified as small molecule,
has been developed in this contribution to investigate the
influence of 3D structure on device performances. The resulting
OPVs based on SF8TBT exhibited much higher fill factors (FFs)
and short circuit current density (Jsc) than the corresponding 1D
linear small molecule (LF8TBT), leading to dramatically improved
power conversion efficiencies (PCEs) from 1.69% to 4.82%.
SF8TBT, a spiro-fluorene with four rigid arms, was
obtained by a standard Pd-mediated Suzuki condensation
a Wuhan National Laboratory for Optoelectronics, Huazhong
University of Science and Technology, Wuhan 430074, P. R. China.
E-mail: tgl@mail.hust.edu.cn
b State Key Laboratory of Polymer Physics and Chemistry,
Changchun Institute of Applied Chemistry, Chinese Academy of
Sciences, Changchun 130022, P. R. China.
E-mail: xiezy_n@ciac.jl.cn
w Electronic supplementary information (ESI) available. See DOI:
10.1039/c2cc36301a
c
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Chem. Commun., 2012, 48, 11847–11849 11847

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Scheme 1 Chemical structure and ball-and-stick model of SF8TBT
and LF8TBT.
reaction with a yield of 58%. The resulting material, exhibiting
excellent solubility in organic solvents such as chloroform and
toluene, can be easily purified by column chromatography as
small molecules. The molecular weight of SF8TBT was 3060,
determined by mass spectrometry, which was the same as the
calculated results. LF8TBT, a 1D linear small molecule
corresponding to SF8TBT, was also synthesized by a standard
Pd-mediated Suzuki condensation reaction with a yield of
45%. Compared with LF8TBT, SF8TBT was expected to give
high-quality films by the spin-coating technique due to its
much higher molecular weight. Its straight forward synthesis
procedure and variable low bandgap structures provided a
possibility towards further increasing device performances.
The stick-with-ball model of SF8TBT and LF8TBT are
depicted in Scheme 1. SF8TBT has a spiro-fluorene centered
X-shaped 3D system, fixing a cruciform arrangement of two
mutually orthogonal conjugated backbones by covalent bonding.
Compared with LF8TBT, SF8TBT has increased dimensionality
due to the non-conjugated connection, providing a much larger
intrinsic 3D character than PC71BM, which was used as the
acceptor in the OPVs. No obvious melting point was observed in
the DSC results when raising the temperature from RT (room
temperature) to 250 1C (Fig. SI 2, ESIw). The glass transition
temperatures of SF8TBT and LF8TBT were determined to be
140 1C and 110 1C, respectively.
The SF8TBT optical spectra in toluene reveal lmax = 527 nm
and a spun cast film from chloroform (20 mg mLÀ1) exhibits a
red-shifted lmax of 7.5 nm, suggesting molecular aggregation in
the solid state. The absorption onset for the spun cast film is
630 nm, corresponding to an optical band gap of B1.97 eV.
Circular voltammetry (CV) curves were recorded referenced
to an Ag/Ag+ (0.01 M of AgNO3 in acetonitrile) electrode,
which was calibrated against the ferrocene/ferrocenium
(Fc/Fc+) redox couple below the vacuum level (À4.74 eV).
Estimated from the onsets of oxidation and reduction, SF8TBT
showed a HOMO energy of À5.34 eV and a LUMO energy
of À3.55 eV, in keeping with the calculated bandgap. The
low-lying HOMO and LUMO of SF8TBT, which are the same
as its corresponding conjugated polymer, provide the necessary
characteristics for large open circuit voltages (Voc) in BHJ
blends with the standard electron acceptor PC71BM.8 As
showed in Fig. 1, the absorption spectrum of LF8TBT in
toluene is almost the same as that of SF8TBT, which shows a
maximum absorption at 524.5 nm. However, the maximum
absorption of LF8TBT’s film is 554 nm, which is 19.5 nm more
red-shifted than that of SF8TBT, which can be attributed to the
much stronger intermolecular stacking in LF8TBT’s film
Fig. 1 UV-Vis absorption spectra of LF8TBT and SF8TBT in
toluene solution and as spin-coated films at 298 K.
(Table 1). The absorption onset for the spun cast film is
660 nm, corresponding to an optical band gap of B1.87 eV.
The HOMO and LUMO energies of LF8TBT were estimated
to be À5.35 eV and À3.60 eV, respectively, agreeing with the
UV calculations. According to the above results, the designed
spiro-fluorene structure can be applied to optimize the inter-
molecular stacking of conjugated organic molecules.
When LF8TBT and PC71BM were used as the electron
donor and the electron acceptor respectively, OPVs with a
conventional structure of ITO/PEDOT:PSS(40 nm)/
LF8TBT:PC71BM(1 : 2)/Ca(10 nm)/Al(150 nm) were fabricated
without any thermal annealing and high boiling temperature
additive in the solutions. Measured under simulated solar
illumination AM 1.5G (100 mW cmÀ2) with an optimized
thickness of 55 nm, the resulting devices based on as-cast
LF8TBT:PC71BM films showed an Voc of 0.95 V, a Jsc of
5.49 mA cmÀ2, a FF of 0.32 and a PCE of 1.69% (Fig. 2a).
The IPCE curves of LF8TBT exhibited a broad response covering
350–750 nm, with a monochromatic IPCE maximum of 44.6%
at 511 nm, as shown in Fig. 2b. Following the same device
fabrication process as LF8TBT, and with a thickness of 65 nm,
the OPVs based on a SF8TBT:PC71BM (1 : 2, W/W) blend
exhibited a maximum PCE of 3.50%, combined with an Voc of
0.97 V, a Jsc of 6.37 mA cmÀ2 and a FF of 57%. Compared to
that of the LF8TBT, the dramatically increased PCE of SF8TBT-
based devices originated from its much better FF. The Jsc and
incident photon to current efficiency (IPCE) of the SF8TBT-
based on device were also slightly higher than that of LF8TBT,
which may be attributed to the slightly thicker active layer.
Further optimization of SF8TBT-based devices could be achieved
with a thickness of 105 nm, corresponding to an IPCE maximum
of 60% at 512 nm. A PCE of 4.82% can be obtained due to the
dramatically improved Jsc (8.7 mA cmÀ2) after increasing the
active layer thickness from 65 nm to 105 nm (Fig. 2 and Table 2).
Table 1 Physical properties of SF8TBT and LF8TBT in films
Ms lmax (nm) lmax (nm) HOMOc (eV) LUMOc (eV)
a
b
SF8TBT 3064 527
LF8TBT 1764 524.5
534.5
554
À5.34
À5.35
À3.55
À3.60
a
b
c
UV-vis in solution. UV-vis in films. Obtained from the onset of
oxidation (reduction) and referenced to Fc/Fc+
.
c
11848 Chem. Commun., 2012, 48, 11847–11849
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Fig. 2 Current density–voltage (J–V) curves (a) and IPCE spectrum
(b) of solar cells based on SF8TBT:PC71BM (1 : 2, w/w) and
LF8TBT:PC71BM (1 : 2, w/w) under AM 1.5G simulated solar
illumination (100 mW cmÀ2).
Table 2 OPV performances of SF8TBT and LF8TBT
Thickness (nm) Voc (V) Jsc (mA cmÀ2) PCE (%) FF (%)
SF8TBT 105
65
LF8TBT 55
0.98
0.97
0.95
8.70
6.37
5.49
4.82
3.50
1.69
0.56
0.57
0.32
J–V characteristics of OPVs based on SF8TBT/PC71BM (1/2, w/w)
and LF8TBT/ PC71BM (1/2, w/w).
In summary, a new strategy for electron donor materials in
OPVs and a 3D mono disperse macromolecule (SF8TBT) based
on spiro-fluorene has been developed. Compared with the corres-
ponding linear small molecule, the 3D spiro structure can be used
to optimize intermolecular stacking of the donors in the active
layers. The resulting OPVs based on solution processes exhibited
dramatically higher FF than the 1D linear small molecule, as well
as much larger Jsc due to the possibility of using a thicker active
layer, resulting in an improved PCE from 1.69% to 4.82%.
We thank the National Natural Science Foundation of China
(nos.51073063, 21274048 and 20834005) for financial support.
Notes and references
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This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 11847–11849 11849