Non-fullerene acceptor engineering with three-dimensional thiophene/selenophene-annulated perylene diimides for high performance polymer solar cells

Article abstract of DOI:10.1039/c8tc04926b

We report two new propeller-shaped perylene diimide (PDI) acceptors based on tetraphenylethylene (TPE) as the core and flanked with S/Se-fused PDIs, named as TPE-PDI₄-S and TPE-PDI₄-Se. Compared to the sulfur-annulated counterpart, T

Full text of DOI:10.1039/c8tc04926b

Journal of
Materials Chemistry C
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PAPER
Non-fullerene acceptor engineering with three-
dimensional thiophene/selenophene-annulated
perylene diimides for high performance polymer
solar cells†
Cite this: J. Mater. Chem. C, 2018,
, 12601
6
a
a
b
a
a
c
Gang Li,‡ Shuaihua Wang,‡ Tao Liu,‡* Pin Hao, Zhenhua Liu, Fengting Li,
c
a
a
a
d
Lian-Ming Yang,
Jiewei Li,* He Yan * and Bo Tang
Yu Zhang, Dandan Li, Shufan Yang, Jianfeng Zhao,
d
b
a
*
We report two new propeller-shaped perylene diimide (PDI) acceptors based on tetraphenylethylene
TPE) as the core and flanked with S/Se-fused PDIs, named as TPE-PDI -S and TPE-PDI -Se. Compared
to the sulfur-annulated counterpart, TPE-PDI -Se exhibits a blue-shifted absorption spectrum, stronger
absorption in the region of 400–600 nm, higher LUMO energy level and twisted molecular geometry.
(
4
4
4
The TPE-PDI
4
-Se based device with PBDB-T1 as the donor achieved a power conversion efficiency
À2
(PCE) of 7.63% along with a high VOC of 1.08 V, a JSC of 10.28 mA cm and a high fill factor of 68.8%, a
4
11.4% boost in PCE with respect to the TPE-PDI -S based control device (6.85%). The high photovoltaic
Received 29th September 2018,
Accepted 25th October 2018
performance of the TPE-PDI
4
-Se based device can be attributed to its relatively high-lying LUMO level,
complementary absorption spectra with the donor material, relatively favorable morphology and
balanced carrier transport. To the best of our knowledge, a PCE of 7.63% is among the highest values
for TPE core-based non-fullerene organic solar cells. Overall, our work provides a simple and effective
molecule-designing method to promote the PCE of non-fullerene OSCs.
DOI: 10.1039/c8tc04926b
rsc.li/materials-c
1
1–17
a polymer donor and an acceptor.
Fullerene derivatives have
Introduction
been the most investigated acceptors in the past few years and
Bulk heterojunction organic solar cells (OSCs) are believed to be a the state-of-the-art single-junction devices based on fullerene
promising choice for next generation green energy platforms to derivatives have been reported to reach over 11% power conver-
1
8–20
address the energy crisis and environmental pollution, due to sion efficiencies (PCEs).
their attractive features such as low cost, flexibility and large area made great contributions to the development of OSCs,
printing production.
Although fullerenes derivatives have
2
1,22
they
1
–10
The OSC active layer typically consists of are not the ideal candidates for practical applications because the
production of fullerenes is very expensive and the chemical
College of Chemistry, Chemical Engineering and Materials Science, Key Laboratory modification of fullerenes remains challenging. Therefore, non-
a
of Molecular and Nano Probes, Ministry of Education, Collaborative Innovation
fullerene acceptors (NFAs) composed of large conjugate struc-
tures with electron-deficient building blocks to ensure facile
charge delocalization and good charge transport have recently
Center of Functionalized Probes for Chemical Imaging in Universities of Shandong,
Institute of Materials and Clean Energy, Shandong Provincial Key Laboratory of
Clean Production of Fine Chemicals, Shandong Normal University, Jinan 250014,
emerged as a hot topic in the field of OSCs. They demonstrate
lots of advantages, such as structural versatility, adjustable
P. R. China. E-mail: tangb@sdnu.edu.cn
b
c
Department of Chemistry and Energy Institute, The Hong Kong University of
Science and Technology, Clear Water Bay, Hong Kong. E-mail: Liutaozhx@ust.hk,
hyan@ust.hk
absorption, easy tunability of the energy levels, excellent mor-
3,23–28
phological stability and self-assembly characteristics.
In
Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of
Green Printing, Institute of Chemistry, Chinese Academy of Sciences,
Beijing 100190, China
conjunction with device engineering and morphology optimiza-
tion, these new materials have been intensively studied, for
d
20,33–36
37–39
Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials
example,
ITIC
derivatives,
rhodanines,
and perylene dii-
Some NFA based OSCs have achieved over
40
41,42
(
(
IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials
SICAM), Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing,
diketopyrrolopyrroles, naphthalimides
28,43
mides (PDIs).
3% PCE in recent years.
2
11816, P. R. China. E-mail: iamjwli@njtech.edu.cn
2
9–32
1
Of the myriad of materials
†
Electronic supplementary information (ESI) available: Details of synthesis and
explored, those based upon the PDI molecules have been inten-
sely studied in the past few years and the PCEs based on PDI
characterization. See DOI: 10.1039/c8tc04926b
These authors contributed equally to the work.
‡
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43
acceptors have exceeded 10%. PDIs demonstrate several excellent
properties, such as easy functionalization, strong electron-acceptor
character, finely tunable solubility, intense light absorption
capability, energy level tunability, and self-assembling
2
4–26,28,44–51
properties.
Despite these favorable advantages, the
strong tendency for p–p stacking between PDI molecules leads
to the formation of excessively large crystalline domains, which
may result in large phase separation in the active layers, thereby
5
2–59
limiting the OSC performance.
Many strategies have been adopted to suppress the aggrega-
tion tendency including substitution at the imide position,
bay position, or non-bay position of the PDI units with
different functional groups to construct twisted PDI dimers
and three-dimensional (3D)-structure oligomers, for example,
5
5
60
61
triphenylamine, spirobifluorene, benzodithiophene, tetra-
4
5
46
50,62,63
phenylethylene (TPE) and porphyrin. Furthermore, S,
2
5,48,64
53
Se
and N heteroatoms had been incorporated at the bay
4
Scheme 1 Synthetic routes of TPE-Pin , PDI-S-Br and PDI-Se-Br.
positions of PDI units to obtain more twisted structures and
higher lowest unoccupied molecular orbital (LUMO) levels.
Despite the great progress in conjugated multidimensional
PDIs, the development of high-performance OSC acceptors based
on PDI dyes and a deep understanding of their optoelectronic
properties and structure–property relationships remain the key
issues that the chemistry and materials science communities face.
In this study, we report two new propeller-shaped small-
4 4
molecule acceptors, named TPE-PDI -S and TPE-PDI -Se based
on tetraphenylethylene (TPE) as the core and flanked with
S/Se-fused PDIs. The introduction of S and Se atoms to the
PDIs results in the obviously increased lowest unoccupied
molecular orbital (LUMO) level of À3.81 eV and À3.77 eV,
respectively. By incorporating the as-synthesized crystallographic
polymer donor, PBDB-T1 (Fig. S4, ESI†), organic solar cells based
on TPE-PDI
4
-Se show a PCE of 7.63%, much higher than those of
45
TPE-PDI -S (6.85%) and TPE-PDI
4
4
(5.53%), with an open-circuit
À2
voltage (VOC) of 1.078 V, a JSC of 10.287 mA cm and a high FF of
68.8%. To the best of our knowledge, the resulting PCE of 7.63%
is among the highest values of TPE as core based non-fullerene
organic solar cells so far.
Scheme 2 Synthetic routes of the two acceptors.
Results and discussion
Synthesis and characterization
excellent solubility in common organic solvents, such as chloro-
form, toluene and o-dichlorobenzene (O-DCB) at room tempera-
ture. These conjugated molecules also exhibit excellent thermal
The synthesis of TPE-PDI -S and TPE-PDI -Se is illustrated
4
4
stability with a decomposition temperature (T , 5% weight loss) of
d
in Schemes 1 and 2. The precursor PDI-S-Br was synthesized
4400 1C for both acceptors in a nitrogen atmosphere, while
6
2
starting from a previously reported method. Compound
PDI-NO reacted with S in NMP solvent to give compound
PDI-S, which was then treated with liquid Br at room tempera-
ture to produce monomer PDI-S-Br (Scheme 1). Afterwards,
compound PDI-S-Br was reacted with TPE-PDI (Scheme 2) via
heating the solids in air at 200 1C for two hours did not lead to a
2
1
detectable change in the H NMR spectra.
2
Optical and electrochemical properties
4
a Suzuki coupling reaction to yield target compound TPE-PDI -S. The optical absorption spectra of TPE-PDI -S and TPE-PDI -Se
4
4
4
The synthetic route of TPE-PDI -Se was similar to TPE-PDI -S in solution and thin-films were obtained (Fig. 1) and the
4
4
(
Scheme 2). The as-synthesized materials were fully characterized corresponding data are summarized in Table 1. In CH Cl2
2
1
13
by H and C NMR, matrix assisted laser desorption ionization solution, both acceptors exhibited similar absorption profiles
time of flight mass spectrometry (MALDI-TOF-MS), and the and showed very high absorption in the range of 400–600 nm,
4
original data are provided in the ESI.† Both acceptors exhibited which is characteristic of PDI chromophores. TPE-PDI -S
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Table 1). No significant change in the redox properties of the two
acceptors was observed. Both acceptors showed no obvious
oxidation waves within the measurement range. In the negative
potential region, an irreversible reduction wave of À0.51 V and
À0.55 V was observed, which is assigned to the reduction of the
PDI groups. The LUMO energy levels were calculated according to
43
the equation ELUMO = À[q(Ered À EFc/Fc+) + 4.8] eV. Since S and Se
annulated polycyclic aromatics increase the electron-donating
ability and electron density, TPE-PDI
higher LUMO levels and larger bandgaps than TPE-PDI
4
-S and TPE-PDI
4
-Se exhibited
. The
4
highest occupied molecular orbital (HOMO) energy levels for
TPE-PDI -S and TPE-PDI -Se were calculated to be À5.97 and
4
4
À5.94 eV, respectively, according to the equation E
=
HOMO
opt
g
43
À(ELUMO + E
of 1.63 eV (LUMOÀLUMO) for TPE-PDI
those of TPE-PDI -S and PBDB-T1 (1.59 eV) can produce a higher
OC for TPE-PDI -Se based PSCs.
) eV. As shown in Fig. 2b, the larger energy offset
4
-Se and PBDB-T1 than
4
V
4
Theoretical analysis
To gain deep insights into the properties of TPE-PDI -S and
4
TPE-PDI -Se at the molecular level, density functional theory
4
6
5
(
DFT) calculations were performed at the B3LYP/6-31G* level,
Fig. 1 (a) The normalized absorption spectra of TPE-PDI
4
-S and TPE- where the long alkyl chain was simplified to isopropyl groups.
PDI
4
-Se in 10 mM dichloromethane solutions. (b) The normalized absorp-
-S and TPE-PDI -Se blend films.
4 4
The LUMO energy levels of TPE-PDI -S and TPE-PDI -Se are
tion spectra of TPE-PDI
4
4
mainly located on the PDI units. However, the HOMO energy
levels of both acceptors are mainly distributed on the tetra-
phenylethene core along with the PDI unit (Fig. 3). This may
hint that photoinduced charge transfer occurred from the TPE
core to the PDI units. Moreover, the dihedral angles between
4
showed a maximum molar extinction coefficient of 9.08 Â 10
À1
À1
M
cm at 553 nm, which is higher than that of TPE-PDI
4
-Se
4
À1
À1
(6.21 Â 10
M
cm
at 557 nm) most likely due to the
synergistic effect of p–p interactions and multiple heteroatom–
heteroatom interactions. The absorption spectra of the two
acceptors in films were similar to those in solution, which
manifests weak intermolecular aggregations in the solid state.
PDIs with linked benzene for TPE-PDI
85.11 and 76.51 respectively (Fig. S3, ESI†). The dihedral angles
between PDIs with linked benzene for TPE-PDI -Se are 86.11,
0.11, 79.61 and 69.41, respectively (Fig. S3, ESI†). This indicates
4
-S are 72.11 and 81.91,
6
1
4
8
that the introduction of S/Se-fused PDIs in the TPE core
reduced the obvious intramolecular twisting between the PDIs,
which may influence the p–p stacking and lead to different
aggregation behaviors. The LUMO/HOMO energy levels of
4 4
Compounds TPE-PDI -S and TPE-PDI -Se exhibit optical
absorption onsets of 585 and 596 nm, respectively. This red-
shift can be attributed to a less twisted conjugated structure
opt
afforded by four TPE-Se subunits. The optical bandgaps (E
of TPE-PDI -S and TPE-PDI
.77 eV, respectively, from the empirical formula: E = 1240/
g
)
TPE-PDI
4
-S and TPE-PDI
4
-Se are À3.79/À5.91 eV and À3.81/
4
4
-Se were estimated to be 1.69 and
opt
g
À5.89 eV through DFT calculations (Table S1, ESI†), respectively.
The calculated values are consistent with the values by CV
measurements.
1
lonset. In addition, the absorption spectra of TPE-PDI -S and
4
TPE-PDI -Se with PBDB-T1 are well complemented to ensure
4
full harvesting of light in the visible spectrum (Fig. 1b and
Fig. S1, ESI†).
Photovoltaic properties
Redox properties were determined by cyclic voltammetry (CV) Photovoltaic properties of TPE-PDI -S and TPE-PDI -Se as
4
4
6
in dry dichloromethane with TBAPF (tetrabutylammonium hexa- acceptors were investigated in OSC devices with a structure of
+
fluorophosphate) as a supporting electrolyte (0.1 M) and Ag/Ag as indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene):poly-
+
a reference and Fc/Fc (0.259 V) as a standard (Fig. S2 (ESI†) and (styrenesulfonate) (PEDOT:PSS)/PBDB-T1:acceptor/zirconium
4 4
Table 1 Basic properties of TPE-PDI -S and TPE-PDI -Se
a
a
À1
À1
a
b
b
opt c
g
d
e
2
À1 À1
Acceptors
l
max (nm)
e
max (M cm
)
l
onset (nm)
l
max (nm)
l
onset (nm)
E
(eV) HOMO (eV) LUMO (eV)
m
e
(cm V
s )
5
À4
TPE-PDI
TPE-PDI
4
-S 502
-Se 510
1.73 Â 10
1.85 Â 10
540
553
508
517
585
596
2.12
2.08
À5.91
À5.89
À3.79
À3.81
4.53 Â 10
5
À4
4
4.82 Â 10
a
b
c
opt
g
d
In a dichloromethane solution.e In a neat film. Calculated from the empirical formula: E
= 1240/lonset
.
Cyclic voltammetry (CV) method by
measuring in dichloromethane. Measured by the space charge limited current (SCLC) method.
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Fig. 2 (a) Device configuration of the studied OSCs. (b) Estimated energy
4 4
levels of PBDB-T1, TPE-PDI -S and TPE-PDI -Se from electrochemical CV.
Fig. 4 (a) J–V curves and (b) EQE spectra.
response from 300 to 700 nm with a maximum value of 63%,
indicating a relatively efficient photoelectron conversion process.
Morphology characterization
Fig. 3 Wave functions of compounds TPE-PDI
4
-S and TPE-PDI
4
-Se.
The surface morphology of the OSC active layer was investigated
using atomic force microscopy (AFM). The as-cast blend films of
4 4
PBDB-T1:TPE-PDI -S and PBDB-T1:TPE-PDI -Se displayed smooth
acetylacetonate (ZrAcAc)/Al. The wide-bandgap polymer of and uniform morphologies with root mean square (RMS) surface
PBDB-T1 (Fig. S4, ESI†) was used as the donor due to its roughness of 0.95 and 0.93 nm (Fig. 5b and d), respectively.
complementary absorption and compatible energy levels with No strong molecule aggregation between PBDB-T1 and the two
TPE-PDI -S and TPE-PDI -Se. Dichlorobenzene was employed acceptors occurred. Moreover, both blend films exhibited clear
4
4
as the processing solvent and the total concentration of donor nanofibrillar structures, which is beneficial for the charge
À1
and acceptor was 20 mg ml with a donor : acceptor weight transport.
ratio of 1 : 1. The optimal annealed temperature was 100 1C. The
Active layer charge transport
current density–voltage (J–V) curves of the TPE-PDI
PDI
meters are summarized in Table 2. The PBDB-T1:TPE-PDI4-Se formed to determine the hole and electron mobilities of TPE-
4
-S and TPE-
-Se based PSCs are shown in Fig. 4a and the cell para- Space charge limited current (SCLC) measurements were per-
4
based devices achieved a high PCE of 7.63% along with a V of PDI
4
-S and TPE-PDI
4
-Se neat and blend films (Fig. 6). The
OC
À2
1
.078V, a JSC of 10.287 mA cm and a FF of 68.8%, which electron mobilities of both acceptor neat films were found to
À4
À4
2
À1 À1
surpass those of PBDB-T1:TPE-PDI
4
-S based PSCs (PCE = be 4.53 Â 10 and 4.82 Â 10 cm
V
s
, respectively
À2
6
.85%, VOC = 1.051 V, JSC = 9.839 mA cm , FF = 66.3%). The (Table 1). After annealing at 100 1C, the hole/electron mobility
enhancements of FF and JSC for TPE-PDI
TPE-PDI
4
-Se compared with of PBDB-T1:TPE-PDI
4
-S and PBDB-T1:TPE-PDI
4
-Se blend films
À4
2
À1 À1
À4
2
À1 À1
-S, illustrated lower loss of FF and JSC with selenization. were 3.01 Â 10 cm V
s
/1.15 Â 10 cm V
s
and
/m of
2.61 and 2.34, respectively (Table S2, ESI†). Thermal annealing
The external quantum efficiency (EQE) spectra of both increases hole and electron mobility and reduces the ratio of
4
À4
2
À1 À1
À4
2
À1 À1
This indicated that selenophene-fused PDIs perform better than 3.21 Â 10 cm V
s
/1.37 Â 10 cm V
s
with m
h
e
the thiophene-fused PDIs.
acceptor based optimized OSCs are shown in Fig. 4b. The JSC
values obtained by integrating the EQE curves with an AM1.5 G
reference spectrum are in accordance with those obtained
m
h
/m
for the TPE-PDI -S and TPE-PDI -Se based blend films.
e 4 4
Exciton dissociation and charge extraction
from J–V measurements (within 5% mismatch). The optimized The exciton dissociation and charge extraction of PBDB-T1:
TPE-PDI -Se based device showed a broad photo-to-current TPE-PDI -S and PBDB-T1:TPE-PDI -Se based OSCs were
4
4
4
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Table 2 Performance of the optimized OSC devices based on PBDB-T1 : acceptor (1 : 1, w/w)
À2
Active layer
Annealing (1C)
VOC (V)
JSC (mA cm
)
FF (%)
PCEmax (%) PCE (%)
PBDB-T1 : TPE-PDI4-S
PBDB-T1 : TPE-PDI4-Se 100
100
1.051 (1.052 Æ 0.007) 9.839 (9.632 Æ 0.161)
0.663 (0.660 Æ 0.004) 6.85
6.688 Æ 0.192
7.305 Æ 0.214
1.078 (1.070 Æ 0.006) 10.287 (10.011 Æ 0.201) 0.688 (0.682 Æ 0.007) 7.63
Fig. 5 The AFM height images for: (a) a TPE-PDI
T1:TPE-PDI -S blend film; (c) a TPE-PDI -Se neat film; (d) a PBDB-T1:
TPE-PDI -Se blend film.
4
-S neat film; (b) a PBDB-
4
4
4
Fig. 7 (a) Jph versus Veff curves. (b) Light intensity dependence of JSC
.
and Veff = V
0
À V
a 0 a
, (V is the voltage at Jph = 0 and V is the
20
measured voltage under different current densities). As shown
in Fig. 7a and Table S3 (ESI†), the Jphs of two OSCs reached
saturation at voltages approaching 3 V, and the J
after annealing were 10.508 mA cm and 10.981 mA cm for
values
sat
À2
À2
4 4
PBDB-T1:TPE-PDI -S and PBDB-T1:TPE-PDI -Se based OSCs,
respectively. After annealing treatment, the probabilities of
exciton dissociation calculated from Jph/Jsat under short circuit
conditions and probabilities of charge collection calculated from
J
ph/Jsat at the maximal power output condition were 93.6%
and 72.5% for PBDB-T1:TPE-PDI -S based OSCs, and 93.7% and
8.0% for PBDB-T1:TPE-PDI -Se, respectively. Obviously, the
4
7
4
PBDB-T1:TPE-PDI -Se based OSCs showed higher exciton disso-
4
ciation and charge collection efficiencies than those of PBDB-T1:
TPE-PDI -S, which contributed to a large JSC and FF.
4
Charge recombination reduces the amount of photogenerated
electrons and leads to low PCEs. In order to understand charge
recombination probabilities in PSCs, the current densities under
different light intensities were measured on their optimized
Fig. 6 Dark current density–voltage characteristics for (a) hole-only and
b) electron-only devices with optimized PBDB-T1:TPE-PDI4-S and
PBDB-T1:TPE-PDI4-Se OSC films.
(
devices. The linear relationship between J and the light intensity
SC
S
in logarithmic coordinates (ln(J ) p ln(P) ) is depicted in Fig. 7b,
SC
where S is the recombination constant reflecting the bimolecular
investigated by performing the photocurrent density versus recombination degree. It is generally believed that if more free
effective voltage. Where Jph = J (J and J are the charges can be effectively collected at the electrodes prior to
À J
current density under illumination and dark, respectively) recombination, the slope of S is closer to 1. As shown in
L
D
L
D
6
6
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Journal of Materials Chemistry C
Fig. 7b, the S values for PBDB-T1:TPE-PDI
to be 0.91, which is larger than that of PBDB-T1:TPE-PDI
4
-Se were calculated
-S
9 G. Zhang, J. Zhao, P. C. Y. Chow, K. Jiang, J. Zhang, Z. Zhu,
J. Zhang, F. Huang and H. Yan, Chem. Rev., 2018, 118, 3447.
4
(
0.90). The higher S value indicates that the PBDB-T1:TPE-PDI -Se 10 C. B. Nielsen, S. Holliday, H.-Y. Chen, S. J. Cryer and
4
based device has less bimolecular recombination and enhances
I. McCulloch, Chem. Res., 2015, 48, 2803.
the utilization rate of excitons and improves the transport proper- 11 M. C. Scharber, D. M u¨ hlbacher, M. Koppe, P. Denk,
ties of carriers, facilitating the enhancement of JSC and FF.
C. Waldauf, A. J. Heeger and C. J. Brabec, Adv. Mater., 2006,
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In summary, two new non-fullerence acceptors of TPE-PDI -S
4
and TPE-PDI
tetraphenylethene as the central core unit and PDI-S and PDI-Se
as the end-groups. TPE-PDI -S and TPE-PDI -Se show wide
optical bandgaps of 2.12 eV and 2.08 eV, respectively, with
relatively high-lying LUMO levels. The TPE-PDI -Se based OSCs
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with PBDB-T1 as the donor achieved high PCEs of 7.63%, with a
VOC of 1.078 V, a JSC of 10.287 mA cm and a FF of 68.8%, a
À2
11.4% boost in PCE with respect to the TPE-PDI -S based
4
device. Our results demonstrate that using suitable 3D core
and Se-annulated PDI end groups to construct SMAs is a simple
and efficient approach to achieve high performance OSCs.
Taking these excellent advantages into consideration, we expect
that the combination of the Se-annulated PDI ending group
with other 3D conformation cores will create more promising
and practical acceptors.
1
2
9 J. Zhao, Y. Li, G. Yang, K. Jiang, H. Lin, H. Ade, W. Ma and
H. Yan, Nat. Energy, 2016, 1, 15027.
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Conflicts of interest
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, 13740.
2
1 B. M. Savoie, A. Rao, A. A. Bakulin, S. Gelinas, B. Movaghar,
R. H. Friend, T. J. Marks and M. A. Ratner, J. Am. Chem. Soc.,
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B. Tang acknowledges the National Natural Science Foundation
of China (No. 21390411, 21535004, and 91753111). G. Li
acknowledges the financial support from the start-up grant of
Shandong Normal University and Shandong Province Natural
Science Foundation (ZR2016BM24).
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