Synthesis and Application of Perylene-Embedded Benzoazoles for Small-Molecule Organic Solar Cells

Article abstract of DOI:10.1021/acs.orglett.8b02650

Two new building blocks of perylene-embedded benzoazoles containing both rigid 2D-conjugated aromatic rings and flexible branched alkyl chains were designed and facilely synthesized in high yields for organic solar cells (OSCs). With a typical acceptor of

Full text of DOI:10.1021/acs.orglett.8b02650

Letter
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Synthesis and Application of Perylene-Embedded Benzoazoles for
Small-Molecule Organic Solar Cells
Zhicai Chen, Jing Li, Mingguang Li, Cailin Chen, Shen Xu, Xingxing Tang, Lingfeng Chen,
Runfeng Chen,* and Wei Huang*
Key Laboratory for Organic Electronics and Information Displays & Jiangsu Key Laboratory for Biosensors, Institute of Advanced
Materials (IAM), Jiangsu National Synergistic Innovation Center for Advanced Materials (SICAM), Nanjing University of Posts &
Telecommunications, 9 Wenyuan Road, Nanjing 210023, China
*
S Supporting Information
ABSTRACT: Two new building blocks of perylene-embedded benzoazoles
containing both rigid 2D-conjugated aromatic rings and flexible branched
alkyl chains were designed and facilely synthesized in high yields for organic
solar cells (OSCs). With a typical acceptor of diketopyrrolopyrrole, small
molecular OSC donors constructed in an acceptor-fused donor−acceptor
motif exhibit excellent solubility, stability, and light absorption with tunable
frontier orbitals, leading to a power conversion efficiency of up to 2.88% in
preliminary OSCs.
olution-processed bulk heterojunction (BHJ) organic solar
cells (OSCs) with a large interfacial area between the
(PDIs) are one of the most important derivatives of perylene,
which have shown extensive applications in organic elec-
S
10,11
donor and acceptor materials have drawn tremendous
attention because of their unique advantages such as light
tronics.
By introducing heteroatoms or heterocycles into
the bay-position of perylene, the S-heterocyclic annulated
perylene exhibits high hole mobility due to the significant S···S
1
weight, high flexibility, and excellent device performance.
12
Owing to the evolution of novel donor materials, including
interactions for enhanced solid-state packing arrangement.
polymers and small molecules, power conversion efficiencies
However, these bay-position modified perylenes are difficult to
introduce flexible alkyl chains on the skeletons, leading to poor
2
,3
(
PCEs) higher than 10% have been achieved. An effective
13
strategy in designing OSC donors involves employing a
conjugated framework with multiple fused aromatic rings in
a ladder-type building block; the fused donors with a planar
molecular geometry and a delocalized π-electron cloud are
beneficial to bathochromically shift photoabsorptions, restrict
rotational motions, extend effective conjugation length,
improve charge carrier transport and intermolecular π−π
stacking, as well as enhance the physical, chemical, and
solubilities. Moreover, fused aromatic perylenes often cause
excessive aggregation in the solid states because of their
extended π-surfaces, and the excessive aggregation is usually
harmful to the construction of optimal phase-separated
14,15
morphology for OSCs.
Therefore, it is urgent to develop
efficient synthetic routes to introduce flexible alkyl chains on
the rigid PAH of perylene with both high optoelectronic
properties and good solubility for processing and crystalline
domain control in blend films for high-performance OSCs.
Here, we present an efficient approach to prepare perylene-
based OSC building blocks by covalently fusing benzoazoles
with thiophene to construct an embedded 2D-conjugated
perylene core with flexible chains at the bay-positions of the
synthetically formed perylenes. This approach can fuse the
famous strong acceptors of 4,7-dithien-2-yl-2,1,3-benzothiadia-
4
mechanical stabilities. Moreover, OSC donors with the
acceptor−donor−acceptor (A−D−A) motif are also of
particular interest due to their widely tunable energy levels,
5
high device performance, and facile synthetic approaches.
Therefore, by introducing the fused donor (FD) into the A−
D−A architecture to afford A−FD−A materials has been met
with great success in either polymers or small molecules for
1
6
6
,7
zole (DTBT) and 4,7-di(thiophene-2-yl)-2H-benzo[d]-
both OSC donors and acceptors.
17
[
1,2,3]triazole (DTBTA) to afford BThP-Br and BTrP-Br
Among the various polycyclic aromatic hydrocarbons
PAHs), perylene is the most studied one, due to its unique
through the classical Suzuki−Miyaura coupling and FeCl
(
3
8
,9
oxidative cyclization. The rigidification and planarization of
chemical and photoelectronic properties.
Through 12
functionalizable positions (3,4,9,10: peri-positions; 1,6,7,12:
bay-positions; 2,5,8,11: ortho-positions), various perylenes can
be developed. Modified at the peri-positions, perylene imides
Received: August 19, 2018
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b02650
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
BThP and BTrP can restrict rotational motions, extend
effective conjugation length, and promote intermolecular
interactions, leading to broad light absorption and improved
mobilities; while, two or three branched alkyl chains on the
fused aromatic units could offer good solubility for material
purification, device fabrication and morphology control.
Considering that diketopyrrolopyrrole (DPP), as a widely
used acceptor unit, has many advantages including good
photochemical stability, strong light absorption, and highly
18
ordered molecular packing, we then attempt to construct
new A−FD−A-type OSC molecules using BThP or BTrP as
the FD and DPP as the terminal acceptor unit, respectively.
Starting from the easily prepared compds 3, 4, and 8
(
Schemes S1−S3), two building blocks of BThP-Br and
Figure 1. DFT-calculated HOMO, LUMO, bandgap (E ), and
frontier orbital isosurface (the isosurface value is 0.02) of BThP,
BThP−DPP, DPP, BTrP−DPP, and BTrP.
g
BTrP-Br with simultaneously embedded perylene, benzoa-
zoles, and alkyl chains of 2-ethylhexyl (EH) were facilely
synthesized in three steps of conventional Suzuki−Miyaura
coupling, bromination, and oxidative cyclization in high
yields of up to 85%, 95%, and 72%, respectively (Scheme 1).
1
9
20
stronger electron-donating ability of N in DTBTA. Therefore,
BTrP−DPP has a higher HOMO than that of BThP−DPP,
and both of them exhibit much lower bandgap (E ) than either
g
Scheme 1. Synthetic Route of Perylene-Embedded OSC
Donors of BThP−DPP and BTrP-DPP in A−FD−A
Structure
DPP or BTrP/BThP, demonstrating a typical feature of D−A
6
,7
materials. However, owing to the much higher LUMO of
BTrP, the corresponding BTrP−DPP also shows higher
LUMO than BThP−DPP, which could provide higher
energetic offset for the exciton dissociation into holes and
electrons at the BTrP-DPP/fullerene interfaces. The A-FD-A
molecular feature can be also distinguished by frontier orbital
isosurfaces. The dominated LUMO wave function distribution
on DPP unit suggests clearly the acceptor nature of DPP;
while, the nearly equal contributions of DPP and BTrP/BThP
on HOMO indicate that BTrP and BThP are indeed weak
donors in these small molecular solar cell materials. It should
be noted that BTrP and BThP also participate to some extent
in the LUMO isosurface, possibly due to the electron-deficient
16,17
nature of benzoazoles embedded in DTBT and DTBTA.
From the onset of the oxidation waves of BThP−DPP and
+
BTrP−DPP at −0.50 and −0.44 V vs Ag/Ag (Figure S21),
their HOMO energy levels were determined to be −5.30 and
−5.24 eV, respectively. Also, with the onset of reduction waves,
their LUMOs were found to be at −3.37 and −3.24 eV, in
good accordance with the computational results (Figure 1).
Then they were coupled with DPP boronic ester by a Suzuki−
Miyaura reaction, resulting in the corresponding target
molecules of BThP−DPP and BTrP−DPP with total yields
over 43%; these new compounds were fully characterized
The optical E s of BThP−DPP and BTrP−DPP were
g
(
Figures S1−S18). Owing to the presence of long and
measured to be 1.55 and 1.60 eV from their absorption onsets
branched EH substituents, these molecules are readily soluble
in common solvents, while the highly rigid perylene-embedded
backbone leads to high thermal stabilities with decomposition
at 801 and 776 nm in solid films (Table S1), which are close to
that of ideal OPV donor with HOMO and E of −5.4 and 1.5
g
eV based on traditional acceptor of fullerenes. These energy
levels of BThP−DPP and BTrP−DPP are promising when
they are used as donor materials in [6,6]-phenyl C -butyric
temperatures (T ) higher than 380 °C as revealed by the
d
thermogravimetric analysis (TGA) (Figure S19). These
perylene-benzoazoles with both good solubility and high
thermal stability are applausive for solution processing of
OSC devices and long-term stability upon solar luminance on
7
1
21
acid methyl ester (PC BM)-based OSCs.
7
1
High photon absorption ability in a wide range of
wavelengths covering the whole sun-light spectrum is
1
22
operation.
important for an efficient OSC material. From the absorption
The highest occupied molecular orbital (HOMO) and the
lowest unoccupied molecular orbital (LUMO) were inves-
tigated by both density functional theory (DFT) calculations
spectra BThP-Br, BThP−DPP, BTrP-Br, and BTrP−DPP in
both diluted CH Cl solutions and thin films (Figure 2a), red-
2
2
shifted absorption bands of BThP-based molecules were
(Figures 1 and S20) and experimental cyclic voltammetric
observed in both solution and film with lower E than that
g
(
CV) measurements (Figure S21). In the A−FD−A molecules,
of BTrP-Br and BTrP−DPP, which is in line with the
theoretical predictions (Figure 1). The sunlight absorption
range of the building block of BThP-Br is slightly wider than
that of BTrP-Br in dilute CH Cl solution with absorption
the acceptor of DPP has deep HOMO and LUMO energy
1
8
levels; the perylene-embedded BThP and BTrP units with
fused molecular structures show slightly higher frontier orbitals
than that of DPP, demonstrating the weak donor characteristic
of BThP and BTrP. The BTrP unit seems to be a stronger
donor than BThP from its high-lying HOMO level, due to the
2
2
peaks at 556 and 478 nm, respectively; however, BTrP−DPP
exhibits a much higher molar extinction coefficient (ε = 1.3 ×
5
−1
−1
4
−1
−1
10 M cm ) than BThP−DPP (ε = 7.3 × 10 M cm ),
B
DOI: 10.1021/acs.orglett.8b02650
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Figure 2. UV−vis absorption spectra of BThP−Br, BThP−DPP,
−
5
BTrP−Br, and BTrP−DPP in CH Cl (∼10 M) and thin film (a).
2
2
DFT-simulated absorption spectra of the intermediates and target
compounds (b).
probably owing to its higher oscillator strength for stronger
photoabsorption (Figure 2b). Moreover, the significantly red-
shifted, stronger and broader absorption bands around 500−
Figure 3. J−V characteristics (a) and incident photon to current
efficiency (IPCE) spectra (b) of DIO-processed BThP−DPP/
PC BM and TA-processed BTrP−DPP/PC BM OSCs. CV
7
1
71
7
50 nm compared to that of the building blocks demonstrate
measurements (c, d) and TEM images (e, f) of BThP−DPP/
5
PC BM (c, e) and BTrP−DPP/PC BM (d, f) films treated at
clearly the characteristic features of D−A molecules. It should
71
71
different conditions. Note that the thermal treatment (TA) is at 100
°C for 10 min.
be also noted that these molar extinction coefficients are
18
among the highest values of OSC donors.
In the solid films, very small red-shifts (<10 nm) of the
absorption peaks were observed in both the building blocks
and almost no shift in target donors, owing to the rigid
molecular structures of these compounds. Interestingly, an
additional absorption shoulder peak appears at 723 and 701
nm for BThP−DPP and BTrP−DPP, respectively, implying
efficient π−π stacking of these perylene-embedded A−FD−A
relatively larger Jsc of 9.44 mA cm− was achieved by the
2
thermal-annealing (TA) at 100 °C for 10 min (Table 1). The
Table 1. Photovoltaic Properties of BThP−DPP- and
BTrP−DPP-Based BHJ Solar Cells
Voc (V) Jsc (mA/m2) FF (%) PCE (%)
molecule
cond
6
,7
molecules in the solid states. To further study the effects of
bromination, rigid fusing, and D−A coupling of these A−FD−
A molecules, theoretical simulations of their absorption spectra
were performed. From Figure 2b, the bromination of BThP
and BTrP only leads to slightly red-shifted absorption peaks,
while the fusing of compounds 9 and 10 to prepare the
building blocks of BThP and BTrP results in much red-shifted
absorption bands (∼100 nm), since the fused rigid perylene
BThP−DPP
w/o
TA
DIO
w/o
TA
0.66
0.66
0.63
0.72
0.69
0.63
2.72
2.55
7.37
4.45
9.44
8.13
55.2
50.6
52.7
40.7
44.3
50.3
0.99
0.85
2.45
1.30
2.88
2.57
BTrP−DPP
DIO
4
skeleton favors the delocalization of π electrons. Interestingly,
incident photon to current efficiency (IPCE) spectra reveal
that the DIO-processed BThP−DPP/PC BM device has the
efficiencies over 20% throughout the 350−750 nm range with
a maximum of 27.6% at 670 nm, while BTrP−DPP/PC BM
device exhibits significantly improved efficiencies throughout
the whole visible region with the maximum IPCE up to 48% at
599 nm (Figure 3b). This higher IPCE in a wide range should
be related to high ε for sunlight absorption, leading to better
photovoltaic performance of BTrP−DPP/PC BM system,
although BThP−DPP and BTrP−DPP have similar HOMO
energy levels and molecular structures.
To understand the higher PCEs of BTrP−DPP-based OSCs
with higher V and larger J , cyclic voltammetry (CV)
BThP−DPP and BTrP−DPP show stronger absorption with
7
1
3
−4-fold higher oscillator strengths at long wavelength ranges
(
>600 nm) compared to either the acceptor unit of DPP or
7
1
fused donors of BThP and BTrP.
To evaluate the potential of these two perylene-embedded
small molecules in the A−FD−A structure as donor materials
of OSCs, preliminary BHJ cells were fabricated in a
conventional device structure of ITO/PEDOT:PSS/BThP−
DPP or BTrP−DPP:PC BM/LiF/Al, where ITO and
7
1
7
1
PEDOT:PSS are indium tin oxide and poly(3,4-
ethylenedioxythiophene):poly(styrenesulfonate), respectively
2
3
(
Figure S22). Under AM 1.5 illumination, the current
oc
sc
2
4
density−voltage (J−V) characteristics of these BHJ cells show
the highest PCEs at varied additive and thermal annealing
conditions (Figure 3a). For BThP−DPP/PC BM blend
measurements were performed. The BThP−DPP/PC BM
7
1
and BTrP−DPP/PC BM blended films show varied onset
7
1
ox
ox
onset
potential of oxidative wave (E ). The close E
of BThP−
71
onset
system, the 3 vol % diiodooctane (DIO)-processed devices
give the highest PCE of 2.45% with an open-circuit voltage
DPP/PC BM films treated at different conditions may be
71
responsible for their similar V in devices (Figure 3c), while
the quite different E
result in different Voc of their OSCs. The larger E
oc
ox
onset
(
V ) of 0.63 V, a short-circuit current density (J ) of 7.37 mA
of BTrP−DPP/PC BM films could
oc
sc
71
−
2
ox
cm , and a fill factor (FF) of 52.7%. Regarding the BTrP−
DPP/PC BM blend film, the best PCE of 2.88% with
of
onset
BTrP−DPP/PC BM should be a main reason for the higher
7
1
71
C
DOI: 10.1021/acs.orglett.8b02650
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Voc of their devices (Figure 3d), although BThP−DPP has
relatively lower HOMO than BTrP−DPP. Additionally, the
transmission electron microscopy (TEM) images show high
phase separation and severe aggregation in pristine BThP−
DPP/PC BM-blended films (Figure 3e), which may be
tions (Dingshan), and the Six Talent Plan of Jiangsu Province
(2016XCL050).
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Synthesis and characterization, thermal properties,
electrochemical properties, DFT calculations and device
fabrication, and characterization data including Schemes
S1−S3, Figures S1−S18, and Table S1 (PDF)
(
1
1
(
AUTHOR INFORMATION
Corresponding Authors
■
(
Heumuller, T.; Bartelt, J. A.; Burke, T. M.; Li, W.; You, W.; Amassian,
A.; McGehee, M. D. J. Am. Chem. Soc. 2014, 136, 14078.
*
E-mail: iamrfchen@njupt.edu.cn.
E-mail: wei-huang@njtech.edu.cn.
*
ORCID
Runfeng Chen: 0000-0003-0222-0296
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This study was supported in part by the National Natural
Science Foundation of China (21674049, 21772095 and
■
6
1704089), Science Fund for Distinguished Young Scholars of
Jiangsu Province of China (BK20150041), 1311 Talents
Program of Nanjing University of Posts and Telecommunica-
D
DOI: 10.1021/acs.orglett.8b02650
Org. Lett. XXXX, XXX, XXX−XXX