Design, Synthesis, and Photovoltaic Characterization of a Small Molecular Acceptor with an Ultra-Narrow Band Gap

Article abstract of DOI:10.1002/anie.201610944

The design of narrow band gap (NBG) donors or acceptors and their application in organic solar cells (OSCs) are of great importance in the conversion of solar photons to electrons. Limited by the inevitable energy loss from the optical band gap of the photovoltaic material to the open-circuit voltage of the OSC device, the improvement of the power conversion efficiency (PCE) of NBG-based OSCs faces great challenges. A novel acceptor–donor–acceptor structured non-fullerene acceptor is reported with an ultra-narrow band gap of 1.24 eV, which was achieved by an enhanced intramolecular charge transfer (ICT) effect. In the OSC device, despite a low energy loss of 0.509 eV, an impressive short-circuit current density of 25.3 mA cm^?2 is still recorded, which is the highest value for all OSC devices. The high 10.9 % PCE of the NBG-based OSC demonstrates that the design and application of ultra-narrow materials have the potential to further improve the PCE of OSC devices.

Full text of DOI:10.1002/anie.201610944

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International Edition: DOI: 10.1002/anie.201610944
German Edition: DOI: 10.1002/ange.201610944
Photovoltaic Materials
Hot Paper
Design, Synthesis, and Photovoltaic Characterization of a Small
Molecular Acceptor with an Ultra-Narrow Band Gap
Huifeng Yao, Yong Cui, Runnan Yu, Bowei Gao, Hao Zhang, and Jianhui Hou*
Abstract: The design of narrow band gap (NBG) donors or
acceptors and their application in organic solar cells (OSCs)
are of great importance in the conversion of solar photons to
electrons. Limited by the inevitable energy loss from the optical
band gap of the photovoltaic material to the open-circuit
voltage of the OSC device, the improvement of the power
conversion efficiency (PCE) of NBG-based OSCs faces great
challenges. A novel acceptor–donor–acceptor structured non-
fullerene acceptor is reported with an ultra-narrow band gap of
1.24 eV, which was achieved by an enhanced intramolecular
charge transfer (ICT) effect. In the OSC device, despite a low
energy loss of 0.509 eV, an impressive short-circuit current
density of 25.3 mAcmꢀ2 is still recorded, which is the highest
value for all OSC devices. The high 10.9% PCE of the NBG-
based OSC demonstrates that the design and application of
ultra-narrow materials have the potential to further improve
the PCE of OSC devices.
band gap (Eogpt) over 1.55 eV, and PCEs over 11% have been
demonstrated.[3] However, further extending the photo-
response range to the infrared region often causes significant
energy losses[16–19] (Eloss = EogptꢀeVOC) from the Eogpt to the
open-circuit voltage (VOC) of the device. To date, the most
efficient OSC devices with Eogpt = 1.2 ꢁ 0.1 eV only showed
a low PCE of 5–6% with a high short-circuit current density
(JSC) over 20 mAcmꢀ2 but very low VOC (0.4–0.5 V), corre-
sponding to an Eloss of approximately 0.6–0.8 eV.[20–21] There-
fore, designing novel photovoltaic materials to significantly
promote the PCE of NBG-based OSC devices is still a great
challenge in the field.
Many efficient molecular design strategies have been
developed to broaden the absorption spectra of conjugated
polymers, such as the introduction of strong electron-donating
groups,[21–23] the formation of quinoid structures and donor-
acceptor (D-A) alternating copolymerization.[10,24–25] Facile
manipulation of the intramolecular charge transfer (ICT)
from an electron-rich unit to an electron-deficient unit
confers D-A structured conjugated polymer donors with
absorption spectra that can be easily tuned to over
1000 nm.[19–21] Recently, acceptor-donor-acceptor (A-D-A)
structured small molecules, such as IEIC and ITIC, have
emerged as highly efficient non-fullerene acceptors[4,16,26–29]
with absorption spectra limited at about 800 nm. Further-
more, researchers have reported non-fullerene-based OSC
devices that show very small energy losses.[16,30–31] Therefore, it
is of great interest and importance to further extend the
absorption spectra of non-fullerene small molecular accept-
ors; this can be easily realized by enhancing the ICT effect.
To broaden the absorption spectrum of the A-D-A
structured small molecule IEIC,[26] we introduced alkoxyl
groups and fluorine atoms onto its D and A moieties,
respectively, to enhance the ICT effect. In comparison to
IEIC, the absorption spectrum of the newly designed non-
fullerene acceptor IEICO-4F (see Supporting Information)
was redshifted approximately 200 nm with an ultra-narrow
Eogpt of 1.24 eV.[31] Use of six different polymer donors in the
OSC devices all yielded impressive JSC values over
20 mAcmꢀ2 despite low energy losses of approximately
0.5 eV. Furthermore, two of the donors were selected to
fabricate the ternary OSC device for a complementary
B
ulk-heterojunction (BHJ) organic solar cells (OSCs)[1,2]
have attracted considerable attention for their great potential
in making large area flexible solar panels through low-cost
coating methods, and the power conversion efficiencies
(PCEs) of OSC devices have been boosted to over 11%.[3–6]
As a type of device that converts sunlight into electricity, the
application of photoactive materials with broad optical
absorption spectra is deemed one of the key methods to
promote PCE of solar cells.[7–11] Successful single-crystal
silicon solar cells, which possess a broad optical absorption
band covering the entire region below 1100 nm, have
demonstrated a high PCE over 25%.[12] In the field of
OSCs, therefore, it is very important to design and apply
narrow band gap (NBG) photovoltaic materials for further
improvement of photovoltaic performance.
Over the past decade, many polymers and small molecules
with absorption spectra extending to the near infrared region
have been designed and applied to OSCs.[13–15] Significant
progress has been achieved in OSC devices with photo-
response ranges within 800 nm, corresponding to an optical
[*] H. Yao, Y. Cui, R. Yu, B. Gao, H. Zhang, Prof. J. Hou
Beijing National Laboratory for Molecular Sciences, State Key
Laboratory of Polymer Physics and Chemistry, Institute of Chemistry
Chinese Academy of Sciences
absorption, and
a
further improvement of JSC to
25.3 mAcmꢀ2 was achieved, which is among the top values
of all OSC devices. The optimal ternary OSC device gave
a high PCE of 10.9%, demonstrating the great potential
application of ultra NBG small molecular acceptors in the
field of OSCs.
Beijing 100190 (P. R. China)
and
University of Chinese Academy of Sciences
Beijing 100049 (P. R. China)
E-mail: hjhzlz@iccas.ac.cn
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
http://dx.doi.org/10.1002/anie.201610944.
Theoretical calculations were conducted using density
functional theory (DFT) to understand the influence of
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alkoxyl groups and fluorine atoms
on the properties of A-D-A struc-
tured small molecules.[32] IEIC and
IEICO-4F did not show great dif-
ferences in their optimized molec-
ular conformations and the elec-
tron distributions of the frontier
molecular orbitals (Supporting
Information, Figure S1a). When
compared with IEIC, IEICO-4F
shows an up-shifted highest occu-
pied molecular orbital (HOMO)
level, a down-shifted lowest unoc-
cupied molecular orbital (LUMO)
level, and thus a decreased calcu-
lated band gap of approximately
0.22 eV. Additionally, we evalu-
ated the strength of the intra-
molecular interaction between the
electron-rich and the electron-
deficient moieties by calculating
the dipole moment of the mole-
cules. Since both of the molecules
have symmetrical structures (Sup-
porting Information, Figure S1b),
their overall dipole moments are very small. Therefore, we
incorporated the half molecular models (Figure 1a) into the
calculation, and the results show that the dipole moment
increases from 8.65 to 9.19 Debye for IEIC and IEICO-4F,
respectively, suggesting the ICT effect from the electron-rich
core unit to the electron-deficient unit is enhanced after
modification.
Figure 1b shows the synthetic procedure for IEICO-4F.
The key intermediate, compound 5 (2-(5,6-difluoro-3-oxo-
2,3-dihydro-1H-inden-1-ylidene)malononitrile), was obtained
from 4,5-dichlorophthalic acid (compound 1) in four steps.
Firstly, compound 1 was converted to its anhydride by an
intramolecular dehydration reaction, after which a halogen
exchange reaction was performed in the aprotic polar solvent
sulfolane at high temperature. Subsequently, compound 4 was
obtained by reduction from compound 3, and a dicyanoviny-
lene moiety was added to afford compound 5. Finally, the
Knoevenagel Condensation reaction was conducted between
compound 5 and compound 6 to produce IEICO-4F in high
yield. Theoretically, IEICO-4F may have some regioisomers
because of differing double bond linkages. How-
Figure 1. a) Theoretical simulation of the intramolecular dipole moments of half molecule models
using a DFT method. b) Synthetic procedure of IEICO-4F; mPEG=poly(ethylene glycol) methyl ether.
when compared with IEIC (Eogpt = 1.50 eV).[31] Since the
absorption band of IEICO-4F is located in the near-infrared
region, suitable polymer donors with complementary absorp-
tion spectra should be carefully selected to absorb more solar
photons when applying IEICO-4F in OSC devices. From the
cyclic voltammetry (CV) plots (Figure 2b), we can determine
that the HOMO level (ꢀ5.44 eV) is up-shifted and the
LUMO (ꢀ4.19 eV) is down-shifted for IEICO-4F compared
with the values for IEIC (HOMO = ꢀ5.47 eV, LUMO =
ꢀ3.90 eV),[31] which is consistent with the theoretical calcu-
lation and the absorption spectrum measurements. Further-
more, the electron mobility of IEICO-4F was evaluated by the
space-charge-limited current method, as shown in Figure S3
(Supporting Information), and an electron mobility of
1.14 ꢀ 10ꢀ4 cm2 Vꢀ1 sꢀ1 was recorded.
To investigate the photovoltaic performance of the novel
ultra NBG electron acceptor, we first blended PBDTTT-
EFT[33] (also known as PTB7-Th[34]) and J52[16] (Figure 3a,b)
as polymer donors in the OSC devices. The two polymers
show different absorption bands covering 300–800 nm, and
ever, the possible regioisomers were not found on
analysis of the nuclear magnetic resonance
(NMR) spectra (include 1H, 13C, 19F, and their
decoupled spectra; see Supporting Information).
Therefore, the configuration at the lowest energy
is adopted to represent the chemical structure of
IEICO-4F (Supporting Information, Figure S2).
As shown in Figure 2a, in chloroform solu-
tion, IEICO-4F shows a main absorption band
from 600–900 nm, and the film has a red-shift of
approximately 100 nm with an absorption onset of
Figure 2. a) Normalized absorption spectra of IEICO-4F in chloroform solution and
approximately 1000 nm, corresponding to an Eogpt
of 1.24 eV. There is a 0.26 eV decrease of Eogpt
as a thin solid film. b) CV plot of IEICO-4F film measured in Bu4NPF6 acetonitrile
solution at a scan rate of 50 mVsꢀ1
.
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their HOMO levels are located at similar positions. As shown
in the current density and voltage (J-V) curves (Figure 3c)
and the corresponding photovoltaic parameters in Table 1,
the PBDTTT-EFT:IEICO-4F device yielded a PCE of 10.0%
with an impressive JSC of 22.8 mAcmꢀ2 and a VOC of 0.739 V,
Table 1: Detailed photovoltaic parameters for the binary OSC devices.
Device
VOC
[V]
JSC
FF
PCE
[mAcmꢀ2 [a]
]
[%][b]
PBDTTT-EFT:IEICO-4F
J52:IEICO-4F
0.739
0.734
22.8 (22.4)
21.9 (21.4)
0.594
0.585
10.0 (9.7)
9.4 (9.3)
[a] Calculated integrated current density from EQE spectra. [b] Average
PCE values obtained from ten devices are shown in parentheses.
and the corresponding energy loss is as low as 0.501 eV.
Meanwhile, the J52:IEICO-4F-based OSC device demon-
strated comparable JSC and VOC values with those of the
PBDTTT-EFT:IEICO-4F-based device, and a PCE of 9.4%
was recorded. Under the same device processing conditions,
the two polymer donors yielded moderate PCEs when
PC71BM or IEICO were used as acceptor materials (Support-
ing Information, Table S1). Furthermore, we selected another
four polymers, including PBDTTT-E-T,[35] PBDTS-
DTBTO,[36] PBQ-3,[37] and PBDB-T[38] (Supporting Informa-
tion, Figure S4), as donor materials for the fabrication of
IEICO-4F-based devices because of their good applicability
in fullerene-free OSC devices. These polymers have varied
optical absorption bands (Eogpt ranging from 1.55 to 1.77 eV),
and the four OSC devices show VOC values ranging from 0.689
to 0.771 V. As presented in Table S2 (Supporting Informa-
tion), the four devices all yielded JSC values over 20 mAcmꢀ2
even though they have relatively low energy losses from 0.469
to 0.551 eV.
As shown in Figure 3d, the external quantum efficiency
(EQE) curve of the PBDTTT-EFT:IEICO-4F-based device
shows very high EQE values in the long wavelength region
from 600–900 nm, and the edge of the photoelectron response
reached near 1000 nm. Even though the calculated integrated
current density from the EQE spectrum is as high as
22.4 mAcmꢀ2 for the PBDTTT-EFT:IEICO-4F-based
device, its EQE values in the region 300–600 nm are relatively
low for their limited absorption bands of donor and acceptor.
For the EQE curve of the J52:IEICO-4F-based device,
a similar phenomenon was observed. High EQE values in
the regions of 300–600 nm and 800–900 nm were recorded.
However, the EQE values in the region 600–800 nm are quite
low because the main absorption band of J52 is located at
400–600 nm.
With the purpose of further improving the photocurrent
by using more solar photons in the whole region of
300–1000 nm, we fabricated a ternary OSC device based on
PBDTTT-EFT and J52 as donors and IEICO-4F as
acceptor.[39,40] As both of the binary OSC devices have similar
VOC and FF values, the resulting ternary OSC device is not
expected to have many changes in comparison. Different
blend ratios (J52:PBDTTT-EFT; 0.1:0.9, 0.3:0.7, and 0.5:0.5)
for the two donors were scanned to obtain an optimal
Figure 3. a) Molecular structures and energy level diagram for
PBDTTT-EFT and J52. b) Normalized absorption spectra for the donor
films. c) J-V curves of the binary OSC devices fabricated by PBDTTT-
EFT:IEICO-4F and J52:IEICO-4F. d) EQE curves of the corresponding
binary OSC devices.
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photovoltaic performance for the ternary OSC devices
(Figure 4 and Table 2). In comparison with the binary devices,
the JSC values of the ternary OSC devices show big changes
with an increase of J52 content, while VOC (0.731–0.737 V)
and FF values (0.580–0.603) display small variations. Impres-
sively, by increasing the J52 content to 30%, the JSC of the
ternary OSC device reaches 25.3 mAcmꢀ2; to the best of our
knowledge this is among the top values for all types of OSCs.
The optimal ternary OSC device was tested by the National
Institute of Metrology, China, and a similar PCE of 10.6%
was recorded (Supporting Information, Figure S5). From the
EQE curves we can clearly find that the photoelectron
response of the ternary OSC devices in the short wavelengths
are enhanced with the addition of J52. The integrated current
density first increase to 23.1 and 24.1 mAcmꢀ2 and then drops
to 22.6 mAcmꢀ2 for the ternary OSC devices with donor
ratios of 0.1:0.9, 0.3:0.7, and 0.5:0.5, respectively.
Atomic force microscopy (AFM) and transmission
electron microscopy (TEM) measurements (Figure 5) were
performed to investigate the morphology of the blend films.
Figure 5a–c displays the AFM height images of blend films,
from which we can find that J52:IEICO-4F presents a rela-
tively rough surface with a mean-square roughness (Rq) of
Figure 5. a–c) AFM height images of the binary films and the ternary
film at a donor ratio of 0.3:0.7. d–f) TEM patterns of the binary and
ternary blend films.
1.53 nm compared to PBDTTT-EFT:IEICO-4F films (Rq =
1.22 nm), and that the addition of J52 (30%) does not cause
a very significant change in the surface roughness of the
PBDTTT-EFT:IEICO-4F host blend (Rq = 1.25 nm). Further-
more, the phase separation properties of the ternary blend
(Supporting Information, Figure S6) are close to those of the
PBDTTT-EFT:IEICO-4F film. Nanoscale phase separation
morphologies with appropriate domain sizes are also
observed in the TEM images (Figure 5d–f). The favorable
nanoscale phase separation properties should benefit the
charge generation and transport processes in the devices, and
thus high JSC values could be obtained.
To obtain more insight into the exciton dissociation, and
the carrier transport and collection processes of the OSCs, we
measured the photocurrent dependence (Jph) on the light
density (Plight) and the effective voltage (Veff) of the devi-
ces.[27,41,42] The Jph versus Veff plots (Supporting Information,
Figure S7a) show that the devices obtained saturated photo-
currents (Jsat) at relatively low voltages of approximately 2 V.
The Jph/Jsat ratios under short-circuit conditions are calculated
as 0.86, 0.88, and 0.89 for the devices fabricated with
Figure 4. a) J-V curves of the ternary OSC devices at different blend
ratios (J52:PBDTTT-EFT:IEICO-4F). b) EQE curves of the correspond-
ing ternary OSC devices.
PBDTTT-EFT:IEICO-4F,
J52:PBDTTT-EFT:IEICO-4F,
and J52:IEICO-4F, respectively, suggesting the overall
charge collection processes are quite efficient.[43] Further-
Table 2: Detailed photovoltaic parameters of the ternary OSC devices at
different blend ratios.
S
more, the power-law exponents of the relationship Jph / Plight
Device
VOC
[V]
JSC
FF
PCE
(Supporting Information, Figure S7b) for the devices are all
close to 1, indicating minor bimolecular recombination occurs
in both binary and ternary devices.[27]
[mAcmꢀ2 [a]
]
[%][b]
0.1:0.9:1.5
0.3:0.7:1.5
0.5:0.5:1.5
0.737
0.731
0.732
23.7 (23.1)
25.3 (24.1)
23.9 (22.6)
0.603
0.589
0.580
10.5 (10.2)
10.9 (10.5)
10.1 (9.9)
In summary, by enhancement of the ICT effect for an
A-D-A structured small molecule, we have successfully
designed and synthesized an ultra NBG non-fullerene
acceptor, IEICO-4F, with an ultra-narrow Eogpt of 1.24 eV.
[a] Calculated integrated current density from EQE spectra. [b] Average
PCE values obtained from ten devices are shown in parentheses.
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corresponding OSC devices, despite low energy losses of
approximately 0.5 eV. Moreover, we have demonstrated that
JSC can be further improved to 25.3 mAcmꢀ2 using a ternary
OSC device; this is among the top values of all OSC devices.
The optimal ternary OSC device yields a high PCE of 10.9%,
suggesting the great potential of ultra NBG small molecular
acceptors in the field of OSCs.
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The authors acknowledge financial support from NSFC
(21325419, 91333204, and 51673201), the Chinese Academy
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Conflict of interest
The authors declare no conflict of interest.
Keywords: fluorination · intramolecular charge transfer ·
non-fullerene acceptors · organic solar cells ·
ultra-narrow band gap
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Manuscript received: November 9, 2016
Revised: December 29, 2016
Final Article published: && &&, &&&&
Angew. Chem. Int. Ed. 2017, 56, 1 – 6
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Photovoltaic Materials
Mind the gap: A non-fullerene electron
acceptor with an ultra-narrow optical
band gap of 1.24 eV was prepared.
Despite a low energy loss of 0.509 eV, the
organic solar cell (OSC) device demon-
strated a high power conversion efficiency
of 10.9% with an impressive short-circuit
current density of 25.3 mAcmꢀ2.
H. Yao, Y. Cui, R. Yu, B. Gao, H. Zhang,
J. Hou*
&&& — &&&
Design, Synthesis, and Photovoltaic
Characterization of a Small Molecular
Acceptor with an Ultra-Narrow Band Gap
6
www.angewandte.org
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2017, 56, 1 – 6
These are not the final page numbers!