9,9′-Bifluorenylidene-Core Perylene Diimide Acceptors for As-Cast Non-Fullerene Organic Solar Cells: The Isomeric Effect on Optoelectronic Properties

Article abstract of DOI:10.1002/chem.201705480

Two different non-fullerene small-molecule acceptors, m-PIB and p-PIB, based on 9,9′-bifluorenylidene (BF) and perylene diimide (PDI) were designed and synthesized. Four β-substituted PDIs were linked to BF in different positions. Based on DFT analysis, derivative p-PIB exhibited reduced intramolecular twisting between the PDI moieties, more delocalized wave function, and sufficiently wider π-electron delocalization than that of m-PIB. The absorption ability of p-PIB was enhanced due to increased intermolecular interactions. By blending p-PIB with poly{4,8-bis[5-(2ethylhexyl)thiophen-2-yl]benzo[1,2-b:4,5-b′]dithiophene-co-3-fluorothieno[3,4-b]-thiophene-2-carboxylate} (PTB7-Th), organic solar cells (OSCs) based on p-PIB obtained a maximum power conversion efficiency of 5.95 % without any treatments. Due to the improved and balanced hole and electron mobilities, the short-circuit current and fill factor of OSCs based on PTB7-Th and p-PIB were significantly increased. The AFM and TEM results revealed that the PTB7-Th:p-PIB film had favorable nanoscale phase separation and formed a bicontinuous interpenetrating network.

Full text of DOI:10.1002/chem.201705480

A Journal of
Accepted Article
Title: 9,9'-Bifluorenylidene Cored Perylene Diimide Acceptors for As-
cast Non-fullerene Organic Solar Cells: the Isomeric Effect on
Optoelectronic Properties
Authors: Yuan Zhao, Huan Wang, Shengpeng Xia, Feng Zhou,
Zhenghui Luo, Jiajia Luo, Feng He, and Chuluo Yang
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.201705480
Link to VoR: http://dx.doi.org/10.1002/chem.201705480
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9,9'-Bifluorenylidene Cored Perylene Diimide Acceptors for As-
cast Non-fullerene Organic Solar Cells: the Isomeric Effect on
Optoelectronic Properties
Yuan Zhao[a], Huan Wang[b], Shengpeng Xia[a], Feng Zhou[a], Zhenghui Luo[a], Jiajia Luo[a], Feng He*[b],
Chuluo Yang*[a]
Abstract: Two different non-fullerene small molecule acceptors，
named m-PIB and p-PIB based on 9,9'-bifluorenylidene (BF) and
perylene diimide (PDI) were designed and synthesized. Four β-
substituted PDIs were linked to 9,9'-bifluorenylidene in different
positions. Based on the analysis of density function theory (DFT), p-
PIB exhibited reduced intramolecular twisting between the perylene
diimide moieties, more delocalized wave function and sufficiently
wider π-electron delocalization compared to m-PIB. The absorption
ability of p-PIB was enhanced due to the increased intermolecular
interaction. By blending p-PIB with PTB7-Th, organic solar cells
(OSCs) based on p-PIB obtained a maximum PCE of 5.95% without
any treatments. Due to the improved and balanced hole and electron
mobilities, the short-circuit current and fill factor of OSCs based on
PTB7-Th and p-PIB were significantly increased. The AFM and TEM
results revealed that the PTB7-Th: p-PIB film had favorable
developed as one of the excellent units of non-fullerene
acceptors, extensive efforts have been devoted to design all
kinds of molecules to suppress the intermolecular aggregation of
the PDI units.[8] Generally used methods include substitution the
imide position, bay position or non-bay position of the PDI units
with different functional groups to construct a twisted three-
dimensional (3D) geometry.[9], [10] Such modifications for the PDIs
based acceptors can reduce the intermolecular interaction and
boost the power conversion efficiency (PCE) of the
corresponding OSCs to over 6%.[11] Meanwhile, the introduction
of intramolecular twisting has negative effects on the
intermolecular π-π stacking and charge transport. So how to
adjust intramolecular twisting plays a vital role in photovoltaic
performance for PDI based acceptors.[12]
Choi et al synthesized three structural isomers based on
cyclohexane as sp3-σ core, in which the nitrogen atoms of the
PDI unit were linked to different positions of the cyclohexane.
The three isomers exhibited different photovoltaic performances
due to the change in the geometries and crystalline
properties.[13] Wu and coworkers developed a molecule in which
four PDIs were linked with a 4,8-di(thiophen-2-yl)benzo[1,2-
b:4,5-b']dithiophene (BDT-Th) core by the functionalization of the
α-position. The α-substituted PDI derivative displayed a high
PCE of 8.47% while the PCE of the β-substituted one was lower
because the molecule based on the α-substituted PDI exhibited
better planarity.[14] However, less attention has been paid to the
effect of different functionalization of the core and β-substituted
PDI on the photovoltaic performances. In fact, the isomerization
lead by the alternation of the link position has a great influence
on the intramolecular twisting in OSCs. Thus, the π-π stacking in
thin films and the distances between backbones will be changed.
So the photovoltaic performance may be enhanced by isomeric
effect.
9,9'-bifluorenylidene (BF) contains two fluorenyl bridged by a
C=C bond. The C=C bond restricts the free rotation of the two
arms, which results in a good planarity. There has been lots of
research on 9,9'-bifluorenylidene owing to its easily synthetic
route.[15] Recently, 9,9'-bifluorenylidene was employed as a core
of the hole-transporting materials in perovskite solar cells and
reached a PCE of 17.8%.[16] Considering the steric hindrance
between the hydrogen atoms of BF and PDI units may
successfully suppress the aggregation of molecules based on
PDIs, we designed two different isomers, namely m-PIB and p-
PIB, respectively (Scheme 1). The β-substituted PDIs were
linked to the core in different positions and then the
intramolecular twisting may be different. As a result, p-PIB has
reduced intramolecular twisting between PDI and BF moieties,
more delocalized wave function and sufficiently wider π-electron
delocalization. The decreased intramolecular twisting of p-PIB
results in stronger absorption, higher electron mobilities and fill
factor. Finally, the p-PIB achieved high PCE (5.95%) when
nanoscale
phase
separation
and
formed
bicontinuous
interpenetrating network.
Introduction
In the recent years, organic solar cells (OSCs) have experienced
great progress with the development in non-fullerene acceptor
materials, device engineering and the deep research of the
working mechanism.[1] The power conversion efficiencies (PCEs)
based on non-fullerene acceptor materials have achieved
13.1%.[2] Non-fullerene acceptors have attracted wide spread
attention because of the advantages in structural versatility，
adjustable absorption， tunable energy levels and the excellent
morphological stability of the active layers.[3] Among the non-
fullerene acceptors, perylene diimides (PDIs) derivatives were
one of the most commonly used owing to their high electron
mobility, good electron-accepting ability and low energy
levels.[4],[5] However, the small molecule acceptors based on
PDIs have a great tendency to form self-aggregation and severe
phase separation.[6] This will lead to strong geminate or non-
geminate recombination in active layers, which will limit the
performance of organic solar cells.[7] Since the PDIs were
[a]
Y. Zhao, S. Xia, F. Zhou, Z. Luo, J. Luo and Prof. C. Yang
Hubei Collaborative Innovation Center for Advanced Organic
Chemical Materials
Hubei Key Lab on Organic and Polymeric Optoelectronic Materials
Department of Chemistry, Wuhan University, Wuhan 430072,
People’s Republic of China
E-mail: clyang@whu.edu.cn
[b]
H. Wang, Prof. F. He
Department of Chemistry, Southern University of Science and
Technology, Shenzhen 518055, People’s Republic of China
E-mail: hef@sustc.edu.cn
Supporting information for this article is given via a link at the end of
the document.
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Scheme 2. The synthesis routes of the two compounds (i) lawesson’s reagent,
toluene, 110 oC (ii) Pd(dppf)Cl2, KOAc, bis(pinacolato)diboron, 1,4-dioxane, 85
oC (iii) Pd(PPh3)4, K2CO3, THF/H2O, 80 oC.
Scheme 1. Chemical structures of PTB7-Th, m-PIB and p-PIB.
(DFT) was performed at the level of B3LYP/6-31G(d,p). The
molecular geometries and frontier molecular orbitals of m-PIB
and p-PIB were shown in Figure 1. To simply the calculation,
the long alkyl chains of the two molecules were replaced with
methyl. The dihedral angles between the core and its
neighboring PDI units exhibit a big difference for m-PIB and p-
PIB. The four dihedral angles in m-PIB are 66.94o, 68.54o,
67.42o, and 50.92o, while smaller dihedral angles of 55.36o,
53.21o, 54.81o and 45.38o are observed in p-PIB. The results
indicate the two molecules have twisted geometries, which may
efficiently suppress self-aggregation of the PDI units. The larger
blended
with
poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-
benzo[1,2-b:4,5-b′]dithiophene-co-3-fluorothieno[3,4-b]-
thiophene-2-carboxylate] (PTB7-Th) for the as-cast films with
favorable nanoscale phase separation.
Results and Discussion
The two compounds were synthesized via Suzuki coupling
reaction
between
the
corresponding
borate
and
monobrominated perylene diimide unit, respectively (Scheme 2). dihedral angles in m-PIB may due to larger steric hindrance
The detailed synthesis procedures and the characterization of
the two compounds were displayed in the experimental section.
The structures of m-PIB and p-PIB were fully characterized by
1H NMR, 13C NMR, MALDI-TOF MS and elemental analysis. m-
PIB and p-PIB exhibited excellent solubility in common solvents
between the hydrogen of the core and PDI units. The
geometries of the two molecules indicate that p-PIB has
reduced intramolecular twisting of PDI moieties, which is helpful
for intermolecular π-π stacking and charge transport.
In addition to the molecular geometry rigidity, the distributions
of the electron density were also observed for the two molecules.
For m-PIB, the lowest unoccupied molecular orbital (LUMO)
levels mainly locate at two PDI units, while the LUMO levels of
p-PIB focus on the core and a PDI unit. For m-PIB, the highest
occupied molecular orbital (HOMO) levels localize in the ladder
type of the core PDI-BF-PDI. However, the HOMO levels of p-
PIB are well delocalized along the core and three PDI units. The
more delocalized wave function of p-PIB may promote a larger
intermolecular electron-hole separation, which will benefit for the
short circuit current (Jsc). The calculated HOMO/LUMO energy
levels of m-PIB and p-PIB are -5.77/-3.49 eV and -5.66/-3.57 eV,
respectively.
such
as
chloroform,
dichloromethane,
toluene,
and
chlorobenzene due to the long alkyl chains attached on the
nitrogen atoms. Both of the compounds showed good thermal
stability with 5% weight-loss at 379 oC and 316 oC for m-PIB and
p-PIB, respectively, by thermogravimetric analysis (TGA) under
N2 atmosphere (Figure S1). The melting points or glass
transitions of the two acceptors can not be observed in the
differential scanning calorimetry (DSC). (Figure S3).The results
demonstrated that the two compounds have good thermal
stability and can be applied to practical use in OSCs.
In order to deeply understand the aggregation-structure
relationships of the two molecules, density functional theory
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Figure 2. (a) Absorption spectra of m-PIB and p-PIB in solution. (b)
Absorption spectra of PTB7-Th, m-PIB and p-PIB in thin films. (c) Absorption
spectra of the blended films. (The D:A ratios of PTB7-Th: acceptor are 1:1.5.)
(d) CV diagram of PTB7-Th, m-PIB and p-PIB.
In order to evaluate the electronic properties of the two
compounds, the HOMO and LUMO levels of m-PIB and p-PIB
were also characterized by the cyclic voltammetry experiments
(Figure S2). As depicted in the energy diagram in Figure 2(d),
the HOMO levels of m-PIB and p-PIB are -5.41 and -5.42 eV
calculated from the onset oxidation potential, respectively. The
LUMO levels of m-PIB and p-PIB are -3.72 and -3.78 eV
determined from the onset reduction potential, respectively. The
similar HOMO and LUMO levels of m-PIB and p-PIB
demonstrate that the linkage position has little influence on the
electrochemical properties of the PDI based acceptors. Both of
the energy offsets of HOMO levels between PTB7-Th and
acceptor are lower than 0.2 eV, which is enough for the effective
exciton dissociation to overcome the binding energy. The slightly
higher LUMO of m-PIB may lead to improved open circuit
voltage (Voc) in organic solar cells. The trend of the energy levels
is consistent with the calculated results.
To investigate the photovoltaic performance of the two novel
compounds as acceptor, OSCs with an inverted configuration of
indium tin oxide (ITO)/ZnO/active layer/MoO3/Ag were prepared.
PTB7-Th was chosen as donor because of the complementary
absorption with the two acceptors. The optimal conditions for
preparing the devices were obtained by choosing the spin-
coating speed, annealing temperatures and additives carefully.
The active layers were spin-coated from chlorobenzene solution.
The optimized weight ratio of donor and acceptor was 1:1.5. The
J-V curves of the OSCs with optimized devices under AM 1.5G,
100 mW cm-2 were exhibited in Figure 3(a) and the
corresponding photovoltaic parameters, Voc, Jsc, fill factor (FF)
and PCE of the devices were summarized in Table 2.
Figure 1. Simulated molecular geometries obtained by DFT calculations for
simplified molecules of m-PIB and p-PIB
Uv-vis absorption spectra of m-PIB and p-PIB in solution and
thin films were measured and showed in Figure 2(a)-(b). Table
1 summarized the related data. m-PIB displays vibronic band
from 400 to 600 nm with two similar absorption peaks in solution.
The absorption peaks of m-PIB are located at 497 and 531 nm,
respectively, which can be assigned to the π-π* transitions of
the aromatic backbone. The highest molar extinction coefficient
of m-PIB is 8.71 × 104 M-1cm-1. However, p-PIB shows one
obvious absorption peak at 501 nm, which illustrates the
existence of strong intermolecular interaction. The maximum
molar extinction coefficient of 9.35 × 104 M-1cm-1 is recorded for
p-PIB. The stronger absorption ability may benefit for absorbing
more photons and generating high photocurrent, which is a
strong advantage of PDI derivatives. Compared with the
absorption in solution, the absorption peak of the two
compounds in films show no obvious red-shift. This reveals that
the aggregation of the two molecules in the solid films may be
suppressed due to the torsion between the central core and the
PDI units. The optical bandgaps of m-PIB and p-PIB calculated
from the absorption onsets of thin films are 2.02 and 1.89 eV,
respectively. The absorption spectra of blended films are
showed in Figure 2(c). The spectral features of the two
compounds display good complementary with the absorption of
donor material, PTB7-Th. In the blended films, PTB7-Th:p-PIB
also exhibits stronger absorption than PTB7-Th:m-PIB, which is
critical for achieving a higher Jsc in OSCs.
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Table 1. The optical data of m-PIB and p-PIB.
Compound
Td (oC)[a]
εmax (M-1cm-1)
λmaxsol (nm)[b]
λmaxfilm (nm)[c]
Egopt (eV)[d]
HOMO (eV)[e]
LUMO (eV)[f]
Egcv (eV)[g]
m-PIB
p-PIB
379
316
8.71 × 104
9.35 × 104
497, 531
501
533
500
2.02
1.89
-5.41
-5.42
-3.72
-3.78
1.69
1.64
[a] Decomposition temperature with 5% weight loss. [b] Solution absorption spectra (0.01 mg ml-1 in chloroform). [c] Absorption spectra of film on quartz glass.
[d] Optical bandgap derived from the absorption onset in thin films: Egopt= 1240/λonset [e] Determined from onset oxidation potential. [f] Determined from onset
reduction potential. [g] The bandgap obtained from cyclic voltammetry experiments.
As shown in Table 2, the PTB7-Th: m-PIB based devices
show Voc of 0.87 V, Jsc of 10.38 mA cm-2, FF of 46.72% and PCE
of 4.27% without any treatments. Turning to p-PIB, the 3,6-
substituted counterpart p-PIB yields a higher PCE of 5.95% with
increased Jsc (12.32 mA cm-2) and FF (58.72%). The different
photovoltaic performances of the two isomers may result from
the different linkage position. The improvements in Jsc and FF in
the device of p-PIB may stem from wide light-harvesting range
and balanced charge carrier transport.
given by the equation: Eloss = Eg – q Voc, where q is the
elementary charge and Eg is the optical gap of the main
absorber. The energy loss can be caused by the non-radiative
and radiative recombination. Since radiative recombination is
unavoidable in OSCs, the higher energy losses compared to
other non-fullerene systems mainly come from the non-radiative
recombination.
Table 2. Summary of photovoltaic parameters of PTB7-Th: m-PIB and PTB7-
Th: p-PIB based OSCs under simulated AM 1.5 G (100 mW cm−2) illumination.
Voc
(V)
Jsc
FF
(%)
PCEmax
(%)
PCEave
(%)
Thickness
(nm)
Acceptor
(mA cm-2)
m-PIB
0.87
10.38
46.72
4.27
4.19
88
m-PIB[a]
p-PIB
0.87
0.82
0.82
9.32
12.32
10.82
48.90
58.72
56.86
3.95
5.95
5.06
3.78
5.83
4.87
85
71
73
p-PIB[a]
[a] with 3% DIO and thermal annealing at 100 oC for ten minutes.
Figure 3. (a) J-V characteristics of PTB7-Th: m-PIB and PTB7-Th: p-PIB
based solar cells without/with additives. (b) External quantum efficiency
spectra of PTB7-Th: m-PIB and PTB7-Th: p-PIB based solar cells without/with
additives.
It is worth pointing out that a similar non-fullerene acceptor
BF-PDI4 based on 9,9'- bifluorenylidene was synthesized by Fan
et al.[17] However, the PCE of the device using BF-PDI4 as
acceptor was only 3.64%. The higher PCE of m-PIB may come
from the better solubility due to the longer alky chains used on
the N,N'-alkyl substituents.
As usual, different annealing temperatures and solvent
additives, 1,8-diiodooctane (DIO) and 1-chloronaphthalene (CN)
were employed to modify the morphology of the active layer and
improve the performance. However, the results went contrary to
most cases reported in literatures. As shown in Supporting
information (Table S1 and S2), the OPV devices using m-PIB
as acceptor with DIO or CN exhibit inferior performance with
decreased Jsc. The PCEs of organic solar cells based on m-PIB
with additives are slightly enhanced (3.95%) after thermal
annealing (TA) at 100 oC for ten minutes, which are still lower
than the devices without any treatments. The device based on
PTB7-Th: p-PIB shows a PCE of 5.06%, with a Voc of 0.82 V
and a Jsc of 10.82 mA cm-2 under the same conditions. The poor
performance after using mixing solvent additives indicates that
the molecular packing and phase separation become worse
during slow volatilization of solvent additives. Generally
speaking, additive deteriorates the reproducibility of the device
performance and will accelerate the photo-oxidative degradation,
which will limit the lifetime of device. So the simple process of
m-PIB and p-PIB are favorable for practical applications, which
is very meaningful for OSCs.
Figure 4. (a) Hole mobilities of the blended films. (b) Electron mobilities of the
blended films.
The corresponding external quantum efficiency (EQE) spectra
of the OSCs made from PTB7-Th as donor and m-PIB or p-PIB
as acceptor are overlaid in Figure 3(b). The devices based on
PTB7-Th: m-PIB display broad photoresponse in the range of
500 - 700 nm. A high maximum EQE value of 58% is obtained
for PTB7-Th and p-PIB blended films. The EQE values are
decreased for both of m-PIB and p-PIB after adding 3% DIO,
It should be noted that the energy loss (Eloss) of the organic
solar cells based on m-PIB and p-PIB were the same of 0.82 eV
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Figure 5. AFM height (a, b, c, d), phase (e, f, g, h) and TEM (i, j, k, l) images of blended films (PTB7-Th: m-PIB without DIO (a, e, i); PTB7-Th:m-PIB with 3% DIO
(b, f, j); PTB7-Th: p-PIB without DIO (c, g, k); PTB7-Th: p-PIB with 3% DIO(d, h, l)).
especially from 400 - 600 nm, which show the same trend with
the Jsc of the J-V curves. The integrated photocurrents
calculated from the EQE spectra are 9.81, 11.98, 8.89, 10.37
calculated to be 11.81. This agrees well with the slightly
increased FF after adding DIO as additives and thermal
annealing. The μh and μe of PTB7-Th: p-PIB are found to be
mA cm-2 for m-PIB, p-PIB, m-PIB (3% DIO) and p-PIB (3% DIO), about 3.20 × 10-4 and 3.30 × 10-5 cm2 V-1 s-1, respectively. The
respectively, which are consistent (± 5%) with the Jsc obtained
from the J-V measurements.
ratio μh/μe is 9.70 for PTB7-Th: p-PIB. The higher and more
balanced charge carrier mobilities of PTB7-Th and p-PIB blend
film should be responsible to the improved FF, which may hint
better interchain and intrachain electronic coupling in the active
layer.
The different link position may also have a great effect on the
charge carrier transport properties of the two compounds, which
is critical for OSC performance. In order to assess the charge
transport properties and the recombination of opposite charges
of the two compounds, space-charge-limited-current (SCLC)
were carried out to measure the hole and electron mobility (μh
and μe ). The μh and μe were obtained from the device structure
of ITO/PEDOT: PSS/active layer/MoO3/Ag and ITO/ZnO/active
layer/Ca/Al, respectively. As shown in Table 3 and Figure 4, the
as-cast blend films of PTB7-Th and m-PIB show a μh of 3.40 ×
10-4 cm2 V-1 s-1 and a μe of 2.15 × 10-5 cm2 V-1 s-1, while the
PTB7-Th: p-PIB exhibits slightly higher μh of 4.03 × 10-4 cm2 V-1
s-1 and μe of 4.55 × 10-5 cm2 V-1 s-1. In addition, the more
balanced hole/electron mobility (μh/μe) of PTB7-Th: p-PIB blend
film (8.86) is achieved than that of PTB7-Th: m-PIB blended film
(15.81). Moreover, the μh and μe with DIO and TA were also
estimated in the similar method. The blend films of PTB7-Th and
m-PIB possess minor increased mobilities (μh of 4.77 × 10-4 cm2
V-1 s-1 and μe of 4.04 × 10-5 cm2 V-1 s-1) and the ratio μh/μe are
Tapping-mode atomic force microscopy (AFM) and
transmission electron microscopy (TEM) were employed to
investigate the surface morphologies of active layer in PTB7-Th
and m-PIB/p-PIB blend films with and without DIO and TA. As
can be seen from the height and phase images in Figure 5a-5h,
the root-mean-square (RMS) values for PTB7-Th: m-PIB
(without DIO), PTB7-Th: m-PIB (3% DIO), PTB7-Th: p-PIB
(without DIO) and PTB7-Th: p-PIB (3% DIO) are measured as
0.637, 0.862, 0.642 and 0.624 nm, respectively. This difference
in morphology results from the different aggregation behaviors of
m-PIB and p-PIB. The addition of DIO increases the RMS value
of PTB7-Th: m-PIB blend film and decreases the RMS value of
PTB7-Th: p-PIB blend film, which either makes the surface
rougher or smoother. The appropriate RMS values of the as-cast
PTB7-Th: p-PIB films indicate the excellent miscibility of donor
and acceptor, which could lead to efficient bicontinuous
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Synthesis of compound 5:
A mixture of 4 (0.26 g, 0.40 mmol),
interpenetrating networks. The results of TEM are consistent
with the AFM observations as shown in Figure 5i-5l. The films
of PTB7-Th and p-PIB without any treatment exhibit uniform
surface morphology with appropriate domain size, thereby
leading to better exciton dissociation and charge transport,
consequently, better photovoltaic performance. With 3% DIO in
the active layer and thermal annealing, the smaller domain size
of m-PIB cannot ensure the contact areas of donor and acceptor,
while the larger domain size of p-PIB can not favor the exciton
dissociation path.
bis(pinacolato)diboron (0.42 g, 1.65 mmol), Pd(dppf)Cl2 (30 mg, 0.05
mmol) and KOAc (0.32 g, 3.30 mmol) in 1,4-dioxane (8 mL) was heated
at 85 oC for 48 h under N2. The reaction mixture was then cooled and
dissolved in chloroform. The organic layer was washed with brine and
water, dried over Na2SO4, filtered, and concentrated under vacuum. The
residue
was
purified
using
column
chromatography
(dichloromethane/hexane, v/v: 3:1) to afford 5 as orange solid (0.24 g,
71%). 1H NMR (400 MHz, CDCl3) δ (ppm): 8.33 (d, J = 8.0 Hz, 4H), 8.22
(s, 4H), 7.61 (dd, J = 4.0, 8.0 Hz, 4H), 1.38 (s, 48H). 13C NMR (100 MHz,
CDCl3) δ (ppm): 142.62, 140.69, 140.66, 133.21, 130.14, 126.52, 125.95,
83.96, 24.92. HRMS: Calcd: 833.4739 [M+H]+; Found: 833.4765.
Table 3．Hole and electron mobilities for the blended films.
Synthesis of compound m-PIB: To a mixture of 2 (100 mg, 0.12 mmol), 3
(600 mg, 0.72 mmol), K2CO3 (2.76 g, 0.02 mol), and Pd(PPh3)4 (14 mg,
0.012 mmol) were added into 30 ml of degassed tetrahydrofuran/water
(v/v: 2:1). The mixture was refluxed for 48h under argon. After the
solution was cooled, H2O (25 ml) was added into the solution, followed by
extraction with chloroform (30 ml × 3). The organic layer was collected
and dried with anhydrous Na2SO4. The solvent was removed in vacuum.
The pure product was obtained by silica gel chromatography using a
mixture of methylene chloride and n-hexane (v/v: 1:3) as an eluent to
give dark red solids (305 mg, yield 76%). 1H NMR (400 MHz, CDCl3) δ
(ppm): 9.53-9.23 (m, 3H), 8.91-8.00 (m, 30H), 7.84-7.55 (m, 5H), 7.45-
7.36 (m, 2H), 5.16 (m, 6H), 4.77 (m, 1H), 4.54 (m, 1H), 2.37-1.93 (m,
16H), 1.82 (m, 12H), 1.50-1.01 (m, 116H), 0.92-0.40 (m, 64H). 13C NMR
(100 MHz, CDCl3): δ (ppm): 164.62, 163.94, 163.63, 162.92, 162.05,
142.46, 142.06, 141.91, 141.58, 141.33, 141.16, 140.95, 140.80, 140.63,
135.86, 134.72, 134.56, 134.48, 134.35, 133.96, 133.21, 132.86, 131.66,
130.78, 130.53, 129.26, 129.10, 128.49, 128.25, 128.17, 127.59, 127.34,
126.62, 126.16, 123.84, 123.44, 123.11, 122.65, 122.39, 121.97, 121.76,
54.69, 32.37, 31.95, 31.79, 31.75, 29.73, 29.40, 29.26, 29.11, 26.94,
22.62, 22.48, 14.10, 14.05. MS (MALDI-TOF) m/z: 3337.95; Found:
μh
μe
μh/μe
Active layer
(cm2 V-1 s-1)
(cm2 V-1 s-1)
PTB7-Th:m-PIB
PTB7-Th:m-PIB[a]
PTB7-Th:p-PIB
PTB7-Th:p-PIB[a]
3.40 × 10-4
4.77 × 10-4
4.03 × 10-4
3.20 × 10-4
2.15 × 10-5
4.04 × 10-5
4.55 × 10-5
3.30 × 10-5
15.81
11.81
8.86
9.70
[a] with 3% DIO and thermal annealing at 100 oC for ten minutes.
Conclusions
In summary, two novel small molecule non-fullerene acceptor,
m-PIB and p-PIB, consisting of a bifluorenylidene core and four
PDI units, have been synthesized and characterized
successfully. The 3D rigid structures of the two molecules
effectively suppressed intermolecular aggregation of perylene
diimide and reduced charge recombination. Compared with m-
PIB, p-PIB has reduced intramolecular twisting of PDI moieties
and more delocalized wave function, which lead to stonger
absorption ability. Organic solar cells based on PTB7-Th: p-PIB
obtained a highest PCE of 5.95% with a Voc of 0.82 V, a Jsc of
12.32 mA cm-2, and a FF of 58.72% without any treatment. The
results offer an effective strategy to design high-efficient non-
fullerene small molecule acceptors based on perylene diimides.
3361.34 (M+Na) Elemental Analysis: Anal. Calcd (%) for C226H256N8O16
:
C, 81.26; H, 7.72; N, 3.35. Found: C, 81.40; H, 7.71; N, 3.26.
Synthesis of compound p-PIB: The synthesis of compound p-PIB used
the same method to the synthesis of m-PIB by replacing 2 with 5. 1H
NMR (400 MHz,CDCl3) δ (ppm): 8.84-8.51 (m, 26H), 8.38-7.95 (m, 12H),
7.52 (m, 2H), 5.17 (m, 8H), 2.34-2.10 (m, 14H), 1.82 (s, 12H), 1.35-1.09
(m, 128H), 0.81 (s, 52H). 13C NMR (100 MHz, CDCl3) δ (ppm): 164.82,
163.71, 162.93, 143.13, 141.22, 139.93, 138.53, 134.99, 131.60, 130.82,
129.32, 128.73, 128.15, 127.71, 127.02, 123.65, 122.85, 122.00, 120.50,
120.16, 54.79, 54.54, 50.95, 32.35, 31.95, 31.75, 31.66, 29.73, 29.22,
29.09, 26.89, 26.80, 22.58, 14.28, 14.06. MS (MALDI-TOF) m/z: 3337.95;
Found: 3359.76 (M+Na) Elemental Analysis: Anal. Calcd (%) for
C226H256N8O16: C, 81.26; H, 7.72; N, 3.35. Found: C, 81.15; H, 7.86; N,
3.61.
Experimental Section
Commercially available chemicals were purchased from commercial
sources and used without further purification unless otherwise specified.
All solvents were dried over sodium and freshly distilled prior to use.
Acknowledgements
Compounds 1, 2,
3 were synthesized according to the reported
literatures.[17]
This work was supported by the National Natural Science
Foundation of China (No. 21572171), the Innovative Research
Group of Hubei Province (No. 2015CFA014), Shenzhen
fundamental research program (JCYJ20150630145302237) and
“Peacock Plan” (KQTD20140630110339343).
Synthesis of compound 4: A mixture of 3,6-dibromo-9H-fluoren-9-one
(1.0 g, 3.0 mmol) and Lawesson’s reagent (0.6 g, 1.5 mmol) were added
into toluene (30 mL) and refluxed overnight. The reaction mixture was
cooled to room temperature. Then the precipitate was collected by
filtration, washed with acetone and dried to obtain product as orange
solids (770 mg, yield: 44%). The product was directly involved into the
next step without any further purification.
Keywords: organic solar cells • power conversion efficiency •
perylene diimide
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10.1002/chem.201705480
Chemistry - A European Journal
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10.1002/chem.201705480
Chemistry - A European Journal
FULL PAPER
FULL PAPER
Isomeric effect on optoelectronic
properties of two novel small molecule
non-fullerene acceptors based on
bifluorenylidene and perylene diimide
were investigated. Organic solar cells
based on these materials achieved
the highest PCE of 5.95% without any
treatments.
Yuan Zhao, Huan Wang, Shengpeng
Xia, Feng Zhou, Zhenghui Luo, Jiajia
Luo, Feng He*, Chuluo Yang*
Page No. – Page No.
9,9'-Bifluorenylidene Cored Perylene
Diimide Acceptors for As-cast Non-
fullerene Organic Solar Cells: the
Isomeric Effect on Optoelectronic
Properties
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