An A2-π-A1-π-A2-type small molecule donor for high-performance organic solar cells

Article abstract of DOI:10.1039/c9tc01251f

A new A₂-π-A₁-π-A₂-type small molecule denoted as BDD-IN was synthesized employing a strong electron-withdrawing unit, 3-bis(4-(2-ethylhexyl)-thiophen-2-yl)-5,7-bis(2-ethylhexyl)benzo[1,2:4,5-c′]-dithiophene-4,8-dione (BDD

Full text of DOI:10.1039/c9tc01251f

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Nguyên T. K. Thanh, Xiaodi Su et al.
Fine-tuning of gold nanorod dimensions and plasmonic properties using
the Hofmeister effects
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DOI: 10.1039/C9TC01251F
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COMMUNICATION
An A2-π-A1-π-A2-type small molecule donor for high-performance
organic solar cells
Received 00th January 20xx,
Accepted 00th January 20xx
Qian Zhanga†*, Yanna Sunb†, Xianjie Chena, Zhijing Lina, Xin Keb, Xiaoyuan Wanga, Tian Hea,
Shouchun Yina, Yongsheng Chenb, Huayu Qiua*
DOI: 10.1039/x0xx00000x
A new A2-π-A1-π-A2-type small molecule denoted as BDD-IN was which incorporated thiophene-substituted benzodithiophene as the
synthesized employing a strong electron-withdrawing unit, 3- core and fluorinated 1H-indene-1,3(2H)-dione as the end-capped
bis(4-(2-ethylhexyl)-thiophen-2-yl)-5,7-bis(2ethylhexyl)benzo[1,2-
units, among which a high PCE of 11.08% was achieved.7 In
:4,5-c’]-dithiophene-4,8-dione (BDD) as the central acceptor unit addition, since Zhan’s group reported ITIC based on a bulky fused-
(A1), thiophene-alkoxy benzene-thiophene as the π-bridge, and ring core (indacenodithieno[3,2-b]thiophene) end-capped with 2-(3-
indenedione (IN) as the end-capped unit (A2). An optimal power oxo-2,3-dihydroinden-1-ylidene)malononitrile unit, a large number
conversion efficiency (PCE) of 8.70% with a high open-circuit of chemists focused on the A-D-A or A-π-D-π-A-type non-fullerene
voltage (Voc) of 0.965 V and a high fill factor (FF) of 72.3% was acceptors, and excellent photovoltaic performance has been
achieved.
obtained.8
Then how about substituting the central electron-donating unit
Organic solar cells (OSCs) have emerged as a promising
with an electron-accepting unit, namely A2-π-A1-π-A2-type small
molecules?9 It has been demonstrated that the open-circuit voltage
(Voc) mainly depends on the energy level difference between the
highest occupied molecular orbital (HOMO) level of the donor
materials and the lowest unoccupied molecular orbital (LUMO)
level of the acceptor materials.3a, 3b Previous researches indicate
that the HOMO energy level and the LUMO energy level are
generally dependent on the electron-donating ability of the donor
units and the electron-withdrawing ability of the acceptor units,
respectively.5, 10 Therefore, it is expected that larger Voc could be
obtained via increasing the electron-accepting ability of the whole
molecule when the small molecule is used as the donor materials.
3-Bis(4-(2-ethylhexyl)-thiophen-2-yl)-5,7-bis(2ethylhexyl)benzo[1,2-
:4,5-c’]-dithiophene-4,8-dione (BDD) unit is a strong electron-
withdrawing and planar group, which is widely used for
constructing D-A alternate copolymer donors.11 Notably, the
polymer PBDB-T based on BDD unit developed by Hou’s group has
exhibited excellent performance as wide bandgap donors in non-
fullerene polymer solar cells.12
renewable energy source to address the energy crisis and
environment-related issues.1 Through great efforts devoted into the
photoactive materials innovation and device optimization, power
conversion efficiency (PCE) of over 14% has been achieved for
single-junction polymer non-fullerene solar cells.2 Compared with
polymers, small molecules have many unique advantages, such as
well-defined molecular structure and high reproducibility, simple
synthesis and so on.3 And small molecules show great potential for
high performance solar cells.4 Among various types of small
molecules containing donor-acceptor (D-A) structure, A-π-D-π-A-
type small molecules have presented outstanding photovoltaic
performance whether used as electron donors or electron
acceptors, which incorporate an electron-donating unit as the
central core, two conjugated π bridges, and two electron-accepting
terminal units.5 For example, Chen’s group reported a small
molecule DR3TSBDT with dialkylthiol substituted BDT as the core
and 3-ethylrhodanine as the end-capped unit which exhibited a
high PCE of 9.95%.6 Wei’s group reported a suit of small molecules
Incorporating noncovalent intramolecular interactions into
molecular backbone is an effective way to enhance chain
coplanarity, which could lead to enhanced molecular order
beneficial for exciton diffusion and charge carrier transport.13
a.College of Material, Chemistry and Chemical Engineering, Hangzhou Normal
University, Hangzhou, 311121, P. R. China. *E-mail: qzhang@hznu.edu.cn;
hyqiu@hznu.edu.cn.
b.The Centre of Nanoscale Science and Technology and Key Laboratory of
Functional Polymer Materials, State Key Laboratory and Institute of Elemento
Organic Chemistry, Collaborative Innovation Center of Chemical Science and
Engineering (Tianjin), College of Chemistry, Nankai University, Tianjin, 300071,
China.
Specially, noncovalent intramolecular
O…S interactions
between alkoxy substituents and thiophene rings have been
demonstrated to be effective for minimizing torsional angles
within polymer backbones.14 Herein, we designed and
synthesized a new A2-π-A1-π-A2-type small molecule BDD-IN
which incorporated BDD as the central building block (A1),
† These authors contributed equally to this paper.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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thiophene-alkoxy benzene-thiophene as the π-bridge and level indicates that larger Voc could be expectedVbieyw AurstiicnlegOntlhinee
DOI: 10.1039/C9TC01251F
indenedione (IN) as the terminal units (A2). Its chemical compound as donor materials.
structure was depicted in Fig. 1a. Bulk-heterojunction OSCs
using BDD-IN as the donor and [6,6]-phenyl-C71-butyric acid
(a)
methyl ester (PC71BM) as the acceptor were prepared and the
photovoltaic properties were optimized. A high PCE of 8.70%
was achieved along with a high Voc of 0.965 V, a short-circuit
current density (Jsc) of 12.44 mA cm-2 and a high fill factor (FF)
of 72.3%.
The target molecule could be obtained via Stile cross-
1.2x105
1.0x105
8.0x104
6.0x104
4.0x104
1.0
0.8
0.6
0.4
0.2
0.0
(b)
coupling reaction and Knoevenagel condensation within a few
steps, and the detailed synthetic procedures and
characterization data are provided in the Electronic
Supplementary Information. The chemical structure of BDD-IN
was confirmed by NMR spectroscopy and mass spectrometry
(Fig. S1-S12). The compound showed good solubility in
common organic solvents, such as chloroform, chlorobenzene.
BDD-IN shows good thermal stability with the decomposition
temperature (Td, 5% weight loss) of 351 oC under N2
atmosphere (Fig. S13a). Clear melting temperatures (Tm) of
Solution
Film
2.0x104
0.0
300
400
500
600
700
800
Wavelength (nm)
o
o
216 C upon heating and recrystallization points (Tcr) of 197 C
upon cooling for the compound could be observed on the
differential scanning calorimeter (DSC) curves, indicative of its
crystallinity (Fig. S13b).
(c)
30
20
10
0
BDD-IN
The UV-Vis absorption spectra of BDD-IN in chloroform
solution and thin film are illustrated in Fig. 1b. The absorption
of BDD-IN in diluted CHCl3 solution exhibits a peak at 532 nm.
The maximum extinction coefficient is 1.12 × 105 M-1 cm-1.
From solution to solid state, the absorption spectrum was
broadened and red-shifted with maxima at 569 nm. Besides,
-10
-20
-30
there exists
a pronounced shoulder peak at 616 nm,
-2
-1
0
1
suggesting effective π-π stacking between molecular
backbones. The optical band gap of BDD-IN is 1.83 eV
calculated from the absorption edge. Theoretical calculation
by density functional theory (DFT) at the B3LYP/6-31G(d) level
were also performed to evaluate the distribution of the
frontier molecular orbitals and the molecular geometries. As
indicated from Fig. S14, the electron densities of HOMO levels
are mainly delocalized along the entire molecular backbone
except the end-capped groups, while the electron densities of
the LUMO levels are delocalized along the entire molecular
backbone. The extended LUMO wave function results in
enhanced overlap with the HOMO wave function, which could
contribute to its higher extinction coefficients.15 From Fig. S15,
BDD-IN showed a dihedral angle of 21.62o between the BDD
unit and its adjacent thiophene unit. The indicated dihedral
angle might break the great aggregation tendency of fully
planar molecules, which would contribute to the nanoscale bi-
continuous interpenetrating network morphology.16
The electrochemical properties of the compound were
studied by cyclic voltammograms (Fig. 1c). The potentials were
internally calibrated using ferrocene/ferrocenium (Fc/Fc+)
redox couple (4.8 eV below the vacuum level). The HOMO and
LUMO levels calculated from the onset oxidation and
reduction potentials are -5.40 and -3.51 eV, respectively. The
electrochemical band gap of BDD-IN is estimated to be 1.89 eV,
which is consistent with its optical band gap. The lower HOMO
Potential (V vs Fc/Fc+)
Fig. 1 (a) Chemical structure of BDD-IN. (b) UV-vis absorption
spectra of BDD-IN in solution and thin film. (c) Cyclic
voltammogram of BDD-IN in tetrahydrofuran solution with 0.1
mol L-1 n-Bu4NPF6 at a scan rate of 100 mVs-1.
To evaluate the photovoltaic performance of BDD-IN,
conventional devices using BDD-IN as the donor material with
the configuration of ITO/PEDOT:PSS/BDD-IN:PC71BM/PrC60MA/
Al were fabricated, where PrC60MA represented
a
fulleropyrrolidinium iodide derivative material developed by
Alex K.-Y and its structure is shown in Fig. S16.17 Typical
current density-voltage (J-V) curves of the as-cast and the
optimized devices are illustrated in Fig. 2a. After a series of
optimization, it was found that solvent vapour annealing (SVA)
treatment could deliver the optimal performance (Table S1-
S4). Without any treatment, the devices delivered a very low
PCE of 1.70%, with a Voc of 1.042 of V, a Jsc of 5.75 mA cm-2 and
an FF of 28.3%. The high Voc is attributed to the low HOMO
energy level of BDD-IN. After SVA treatment, the PCE was
sharply enhanced to 8.70%, with a high Voc of 0.965 V, a Jsc of
12.44 mA cm-2 and a high FF of 72.3%. The reduction in the Voc
after SVA treatment may be due to the increasing
intermolecular interaction between the donor and acceptor.18
The strength of intermolecular interactions could be
represented by Jso. As shown in Figure S18 and Table S5, in
2 | J. Name., 2012, 00, 1-3
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Journal of Materials Chemistry C
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comparison with the as-cast devices, the devices with SVA
COMMUNICATION
To understand the effect of SVA treatment on the device
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treatment exhibit a higher Jso, which is in accordance with the performance, atomic force microscopy D(OAIF: M10.)10w39a/sC9uTCse0d125t1oF
decreased Voc. Notably, the FF is up to 76.3% when the donor characterize the morphologies of the active layers. From Fig.
concentration is 8 mg ml-1 (Table S1). The FF values are higher 4a and 4b, the surface root-mean-square (RMS) roughness for
than those of previously reported similar molecules.9a, 9b, 15 The the as-cast and SVA-treated BDD-IN:PC71BM blend films are
external quantum efficiency (EQE) spectra of the as-cast and 0.67 and 0.79 nm, respectively. The blend films under different
optimized devices are shown in Fig. 2b. From Fig. 2b, the as- conditions all showed nano-scaled phased separation, which
cast devices exhibit relatively low EQE response with may benefit from the twisted backbones of the small molecule
maximum value of 37.5%. After SVA treatment, uniform donor compared with previously reported molecules with
enhancement of the spectral response across the 300-700 nm nearly planar backbone.9b, 15 Nevertheless, for the as-cast film,
wavelength regions are clearly observed and the maximum there are many aggregates with size about 50 nm, which are
value is 76.1%. The calculated Jsc estimated from the larger than the exciton diffusion length and adverse to exciton
integration of the EQE curves are 5.51 and 11.90 mA cm-2 for dissociation, and thus lead to poor Jsc. The discontinuous phase
the as-cast and the optimized devices, respectively, which are aggregation could cause severe bimolecular recombination,
in agreement with the values got from the J-V curves within an which leads to low FF. After SVA treatment, the largely
acceptable deviation of ~5%.
segregated donor and acceptor phases became adequately
intermixed to form a good interpenetrating network, which
could be beneficial for exciton dissociation and charge
transport, and therefore Jsc and FF were greatly enhanced.
Besides, from the X-ray diffraction (XRD) patterns for the neat
BDD-IN film (Fig. S19a), BDD-IN exhibited a very weak
diffraction peak (100) at 2Ө = 5.57o. After SVA treatment, a
distinct diffraction peak (100) at 2Ө = 5.73o was observed.
Similarly, for the as-cast BDD-IN:PC71BM blend film, no obvious
diffraction peaks appeared. However, a diffraction peak at 2Ө
= 5.60o was present after SVA treatment (Fig. S19b). These
results indicate that SVA treatment enhanced the crystallinity
of BDD-IN, which are good for exciton diffusion and charge
transport.
Fig. 2 (a) Typical J-V curves and (b) EQE curves of the as-cast and
SVA-treated devices based on BDD-IN.
To investigate the charge generation, collection and
recombination behaviours, the photocurrent density (Jph) as a
function of effective voltage (Veff) is plotted in double
logarithmic coordinates shown in Fig. 3. The Jph/Jsat ratio can
be employed to evaluate the overall exciton separation and
charge collection efficiency. Under the short-circuit conditions,
the Jph/Jsat ratios of the as-cast and optimal devices are 48%
and 97%, respectively. At the maximal power output
conditions, the Jph/Jsat ratios of the as-cast and optimal devices
are 25% and 83%, respectively. These results suggest higher
exciton dissociation and charge collection efficiency, and less
bimolecular recombination for devices with SVA treatment,
thus enhanced Jsc and FF.
Fig. 4 AFM height images of BDD-IN:PC71BM blend films (a) as cast
(b) SVA treatment. The scale bar is 5.0 μm.
The hole mobility (μh) and electron mobility (μe) of the
blend films were tested by the SCLC method (Fig. S20).19 For
the as-cast devices, the hole/electron mobilities are 2.48 × 10-4
and 7.49 × 10-4 cm2 V-1 s-1, respectively. For the SVA-treated
devices, the hole/electron mobilities are 1.83 × 10-3 and 5.33 ×
10-3 cm2 V-1 s-1, respectively. After SVA treatment, higher
mobilities are good for efficient charge transport and thus
excellent Jsc and FF.
In conclusion, a new A2-π-A1-π-A2-type small molecule
BDD-IN using BDD with strong electron-accepting ability as the
central unit was designed and synthesized. This compound
exhibited strong absorption in the wavelength range of 300-
700 nm and deeper HOMO energy levels. After SVA treatment,
the BDD-IN:PC71BM based devices delivered a high PCE of
8.70%, with a high Voc of 0.965 V, a Jsc of 12.44 mA cm-2 and a
Fig. 3 Variation of Jph with Veff for the as-cast and SVA-treated BDD-
IN:PC71BM based devices.
This journal is © The Royal Society of Chemistry 20xx
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type small molecules are very promising to construct more
efficient photovoltaic devices.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (NSFC) (No. 51703046), the Pandeng Plan
Foundation of Hangzhou Normal University for Youth Scholars of
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A new A2-π-A1-π-A2-type small-molecule donor using a strong electron-withdrawing unit as thVeiew Article Online
DOI: 10.1039/C9TC01251F
central unit was synthesized and its photovoltaic performance was investigated.
An A2-π-A1-π-A2-type small molecule
Voc = 0.965 V
Jsc = 12.44 mA cm-2
FF = 72.3%
PCE = 8.70%