Enhancing performance of electron acceptor ITIC-Th via tailoring end groups

Abstract

We choose the high-performance nonfullerene acceptor ITIC-Th as an example, and incorporate electron-donating methoxy and electron-withdrawing F groups onto the terminal group 1,1-dicyanomethylene-3-indanone (IC) to construct a small library of four fused-ring electron acceptors. With this series, we systematically investigate the effects of the substituents on end-groups on electronic properties, charge transport, film morphology, and photovoltaic properties in ITIC-Th series. Electron-withdrawing ability increases from methoxylated to unsubstituted, fluorinated, and difluorinated IC, leading to downshift of energy levels and redshift of absorption spectra. The optimized organic solar cells based on ITIC-Th series show power conversion efficiencies ranging from 8.88% to 12.1%.

Full text

This journal is ©The Royal Society of Chemistry and the Chinese Chemical Society 201 8 Mater. Chem. Front.
Cite this: DOI: 10.1039/c7qm00547d
Enhancing the performance of the electron
acceptor ITIC-Th via tailoring its end groups†
Zeyuan Li,‡
a
Shuixing Dai,‡
a
Jingming Xin,
b
Lin Zhang,
b
Yang Wu,
b
Jeromy Rech,
c
Fuwen Zhao,
d
Tengfei Li,
a
Kuan Liu,
a
Qiao Liu,
a
Wei Ma,
b
Wei You,
c
Chunru Wang
d
and Xiaowei Zhan *
a
We choose the high-performance nonfullerene acceptor ITIC-Th as an example, and incorporate
electron-donating methoxy and electron-withdrawing F groups onto the terminal group 1,1-dicyanomethylene-
3-indanone (IC) to construct a small library of four fused-ring electron acceptors. With this series, we
systematically investigate the effects of the substituents on the end-groups on the electronic properties, charge
transport, film morphology, and photovoltaic properties of the ITIC-Th series. The electron-withdrawing
ability increases from methoxylated to unsubstituted, fluorinated, and difluorinated IC, leading to a
downshift of energy levels and a redshift of absorption spectra. Optimized organic solar cells based on
the ITIC-Th series show power conversion efficiencies ranging from 8.88% to 12.1%.
Introduction
Organic solar cells (OSCs) are a promising technology for clean
and renewable energy conversion. OSCs possess some advantages,
such as low cost, semi-transparency, flexibility, and light weight.
1–5
Fullerene derivatives (e.g.,PC
61
BM and PC
71
BM) are the classical
electron acceptors used in OSCs for a long period of time.
6,7
Blends
of electron donating polymers or small molecules and fullerene
derivatives exhibit power conversion efficiencies (PCEs) over
11%. However, their shortcomings, such as weak absorption
in the visible region and limited tunability in energy levels,
restrict the further development of OSCs.
Nonfullerene acceptors present several advantages, such as
enhanced absorption in the visible and even near-infrared (NIR)
regions, adjustable energy levels and good device stability.
8
For
example, perylene diimide and naphthalene diimide small
molecules and polymers are one class of high-performance non-
fullerene acceptors, and have exhibited PCEs as high as 9–10%.
9–29
Since 2015, we have developed original fused-ring electron
acceptors (FREAs) with an acceptor–donor–acceptor structure
based on indacenodithiophene and indacenodithieno[3,2-b]-
thiophene, end-capped with two electron-withdrawing terminal
groups, 1,1-dicyanomethylene-3-indanone (IC).
30–40
These FREAs
exhibit broad and strong light absorption, and their LUMO
and HOMO energy levels can be readily tuned. OSCs based
on blends of FREAs and some high-performance donors have
exhibited PCEs over 12%.
41–47
For instance, we reported a high-
performance nonfullerene acceptor, ITIC-Th,
34
which showed
a PCE of 9.6%, when blended with a wide-bandgap polymer,
PDBT-T1. ITIC-Th was also used by other groups and afforded
PCEs of 10–11%.
42,48
IC is the most commonly used electron-withdrawing terminal
group in FREAs, and chemical modification on IC was employed
to adjust molecular energy levels. For instance, the benzene on
IC could be replaced with thiophene
49,50
and naphthalene;
51
the benzene on IC could be decorated by fluorine,
52,53
chlorine,
54
methyl
55
and methoxy groups.
56
However, there
have been rare systematic studies on the effects of substituents
on IC.
57,58
In this work, we choose the widely used ITIC-Th as an
example, and incorporate electron-donating methoxy and electron-
withdrawing F groups onto IC to construct a small library of four
FREAs (ITIC-Th series, Fig. 1). With this series, we are able to
systematically investigate the effects of the substituents on the
end-groups on the electronic properties, charge transport, film
morphology, and photovoltaic properties of the ITIC-Th series.
The electron-withdrawing ability increases from methoxylated
to unsubstituted, fluorinated, and difluorinated IC, leading to
a downshift of energy levels and a redshift of absorption.
Furthermore, optimized OSCs based on the ITIC-Th series show
PCEs ranging from 8.88% to 12.1%.
a
Department of Materials Science and Engineering, College of Engineering,
Key Laboratory of Polymer Chemistry and Physics of Ministry of Education,
Peking University, Beijing 100871, China. E-mail: xwzhan@pku.edu.cn
b
State Key Laboratory for Mechanical Behavior of Materials,
Xi’an Jiaotong University, Xi’an 710049, China
c
Department of Chemistry, University of North Carolina at Chapel Hill,
Chapel Hill, North Carolina 27599-3290, USA
d
Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, China
†Electronic supplementary information (ESI) available: Synthesis and character-
ization of ITIC-Th2 and ITIC-Th3, device fabrication procedures, TGA and DSC
curves, SCLC data, and AFM images. See DOI: 10.1039/c7qm00547d
‡Zeyuan Li and Shuixing Dai contributed equally.
Received 30th November 2017,
Accepted 10th January 2018
DOI: 10.1039/c7qm00547d
rsc.li/frontiers-materials
MATERIALS CHEMISTRY
FRONTIERS
RESEARCH ARTICLE
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Results and discussion
Synthesis and characterization
Two new FERAs, ITIC-Th2 and ITIC-Th3, were synthesized via
the Knoevenagel condensation reaction (see Scheme S1 in the
ESI†). ITIC-Th2 and ITIC-Th3 were fully characterized by spectro-
scopic methods and elemental analysis. Both ITIC-Th2 and
ITIC-Th3 show good solubility in common organic solvents, such
as chloroform and dichloromethane. The thermal stability of
ITIC-Th2 and ITIC-Th3 was investigated using thermogravimetric
analysis (TGA) and differential scanning calorimetry (DSC)
(Fig. S1 in the ESI†). ITIC-Th2 and ITIC-Th3 show decomposition
temperatures (T
d
, 5% weight loss) at 299 1C and 333 1C, respectively.
The UV-vis absorption spectra of the ITIC-Th series in
chloroform solution and as thin films were measured (Fig. 2);
4 molecules in solution show absorption peaks in the 664 to
680 nm range with molar extinction coefficients varying from
1.4 10
5
to 3.6 10
5
M
1
cm
1
(Table 1). All molecules in thin
films show broader and redshifted absorption relative to their
solutions. The optical bandgaps of the ITIC-Th series are
calculated to be 1.63 to 1.54 eV from the absorption edge
(Table 1). Relative to the parent ITIC-Th, fluorinated ITIC-Th1
and ITIC-Th2 have reduced bandgaps, while methoxylated ITIC-Th3
has a slightly larger bandgap.
The electrochemical properties of the four FERAs were
investigated by cyclic voltammetry (Fig. 3a). Assuming the
Fig. 1 Chemical structures of the ITIC-Th series acceptors and FTAZ donor.
Fig. 2 Absorption spectra of the ITIC-Th series in chloroform solution (a) and as thin films (b).
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absolute energy level of FeCp
2+/0
to be 4.8 eV below vacuum,
the HOMO and LUMO energy levels were calculated from the
onset oxidation and reduction potentials, respectively. The
HOMO energy levels of the 4 molecules range from 5.66 eV
to 5.75 eV, while the LUMO energy levels range from 3.73 eV
to 4.07 eV (Table 1). Relative to the parent ITIC-Th, methoxylated
ITIC-Th3 exhibits a similar HOMO but a significantly upshifted
LUMO, due to the electron-donating effect of the methoxy group.
Relative to the parent ITIC-Th, fluorinated ITIC-Th1 and ITIC-Th2
exhibit a downshifted HOMO and a downshifted LUMO, due to
the electron-withdrawing effect of the fluorine atom (Fig. 3b).
The electron mobilities of the 4 compounds were measured
using the space charge-limited current (SCLC) method (Fig. S2,
ESI†). The electron mobilities of ITIC-Th, ITIC-Th1, ITIC-Th2
and ITIC-Th3 are 2 10
4
,510
4
,210
4
, and 5 
10
4
cm
2
V
1
s
1
, respectively (Table 1). Monofluorinated ITIC-
Th1 and methoxylated ITIC-Th3 exhibit higher mobilities than
the parent ITIC-Th and difluorinated ITIC-Th2.
Photovoltaic properties
Our previously reported wide-bandgap polymer donor FTAZ
(Fig. 1) exhibits strong absorption at 400–620 nm with a molar
extinction coefficient of 9.8 10
4
M
1
cm
1
,
59
which complements
the absorption spectra of the ITIC-Th series. The energy levels of
FTAZ match with those of the ITIC-Th series (Fig. 3b). Moreover,
FTAZ exhibits a hole mobility as high as 1.2 10
3
cm
2
V
1
s
1
.
60
Thus, we used FTAZ as the donor and the ITIC-Th series as the
acceptors to fabricate bulk heterojunction (BHJ) OSCs with an
inverted device structure of indium tin oxide (ITO)/ZnO/FTAZ:
acceptor/MoOx/Ag. The optimized FTAZ/acceptor weight ratio is
1 : 1.5, and the optimized content for the processing additive,
1,8-diiodooctane (DIO), is 0.25% (v/v) when using chloroform as
processing solvent. The J–V curves of the best devices based on
blends of the FTAZ:ITIC-Th series are shown in Fig. 4a.
Relative to the ITIC-Th-based devices (0.915 V), the ITIC-Th1
and ITIC-Th2-based OSCs exhibit lower open-circuit voltages
(V
OC
) (0.849 V and 0.751 V, respectively), due to the lower
LUMOs of ITIC-Th1 and ITIC-Th2 (Table 2); while the ITIC-
Th3-based OSCs exhibit higher V
OC
(0.962 V), due to the
elevated LUMO of ITIC-Th3. Compared to the ITIC-Th and
ITIC-Th3-based devices, the ITIC-Th1 and ITIC-Th2-based
devices show higher short-circuit current density ( J
SC
) and
higher fill factors (FF), which are partially caused by redshifted
and stronger absorption and the stronger intermolecular inter-
action caused by the fluorine atoms. The best PCE of the OSCs
based on the parent ITIC-Th is 8.88%, while the best PCEs of
the OSCs based on monofluorinated ITIC-Th1 and difluorinated
ITIC-Th2 are 12.1% and 9.06%, respectively, and the best PCE of the
OSCs based on methoxylated ITIC-Th3 is 10.7%. Fluorination and
methoxylation of the IC unit indeed enhance device performance; in
particular, the monofluorinated ITIC-Th1 performs best.
The external quantum efficiency (EQE) spectra of the OSCs
based on blends of the FTAZ:ITIC-Th series are shown in
Fig. 4b. The OSCs based on these four ITIC-Th series acceptors
show a broad photoresponse extending from 300 to 850 nm. In
the NIR region, the EQE spectra are broadened and enhanced
from ITIC-Th and ITIC-Th3 to ITIC-Th1 and ITIC-Th2, resem-
bling their absorption profiles in the NIR region (Fig. 2b).
Table 1 Properties of the ITIC-Th series
Compound T
da
(1C) l
s,maxb
(nm) l
f,maxc
(nm) e
maxd
(M
1
cm
1
)E
ge
(eV) HOMO
f
(eV) LUMO
g
(eV) m
eh
(cm
2
V
1
s
1
)
ITIC-Th 310
i
668 706 1.5 10
5
1.60 5.66 3.93 2 10
4
ITIC-Th1 271
j
677 728 1.8 10
5
1.55 5.74 4.01 5 10
4
ITIC-Th2 299 680 735 3.6 10
5
1.54 5.75 4.07 2 10
4
ITIC-Th3 333 664 698 1.4 10
5
1.63 5.67 3.73 5 10
4
a
Decomposition temperature measured from TGA.
b
Absorption maximum in solution.
c
Absorption maximum in films.
d
Molar extinction
coefficient at l
max
in solution.
e
Optical bandgap calculated from the absorption edge of the thin film.
f
Estimated from the onset oxidation
potential.
g
Estimated from the onset reduction potential.
h
Electron mobility measured by the SCLC method.
i
Taken from ref. 34.
j
Taken from
ref. 53.
Fig. 3 Cyclic voltammograms for the ITIC-Th series in CH
3
CN/0.1 M NBu
4
PF
6
at 100 mV s
1
(a). The onset oxidation or reduction potentials are
determined by the intersection of the tangent lines of the oxidation or reduction waves and flat sections, respectively. Energy level diagram of FTAZ and
the ITIC-Th series (b).
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We measured J
SC
as a function of incident light intensity (P)
and the data were fitted to the power law: J
SC
pP
a
, to study
charge recombination in the devices (Fig. 5a).
61
The exponent a
for ITIC-Th series-based cells is 0.98 to 0.99, indicating
very weak bimolecular recombination under short circuit
conditions in the active layers of all devices. We also measured
the photocurrent density ( J
ph
)versus the effective voltage (V
eff
)
to investigate their charge generation, dissociation and extrac-
tion properties (Fig. 5b). It is assumed that all the photogene-
rated excitons are dissociated into free charge carriers and
collected by electrodes at a high V
eff
(that is, V
eff
= 2 V), so the
saturation photocurrent density ( J
sat
) is only limited by the
total amount of absorbed incident photons.
62
The J
sat
values of
the ITIC-Th, ITIC-Th1, ITIC-Th2 and ITIC-Th3-based solar cells
are 16.37, 20.51, 17.93, and 16.52 mA cm
2
, respectively.
Fluorinated ITIC-Th1 and ITIC-Th2 show relatively higher J
sat
,
partially due to better light harvesting and exciton generation.
The J
SC
/J
sat
values for the ITIC-Th series are 495%, indicating
excellent charge extraction in all devices.
The hole mobilities and electron mobilities of the blended
films were measured using the SCLC method (Fig. S3 and Table
S1, ESI†). The blended films based on FTAZ:modified ITIC-Th
exhibit higher electron mobility and therefore more balanced
charge transport relative to the one based on FTAZ:parent
ITIC-Th, which is responsible for the higher FFs of their devices.
Film morphology
Atomic force microscopy (AFM) images of FTAZ:ITIC-Th series
blends are shown in Fig. S4 (ESI†). The root-mean-square
roughnesses of ITIC-Th, ITIC-Th1, ITIC-Th2 and ITIC-Th3-based
blended films are 0.80, 0.76, 0.99 and 0.82 nm, respectively. All films
have a smooth and uniform surface.
Grazing incidence wide angle X-ray scattering (GIWAXS)
measurements were employed to characterize the molecular
Fig. 4 Current density versus voltage characteristics (a) and EQE curves of devices based on FTAZ:ITIC-Th series blends (b).
Table 2 Photovoltaic performance of OSCs based on the ITIC-Th series
a
Device V
OC
(V) J
SC
(mA cm
2
) FF (%) PCE (%)
FTAZ:ITIC-Th 0.915 (0.914 0.003) 15.84 (15.67 0.23) 61.26 (61.14 0.86) 8.88 (8.67 0.15)
FTAZ:ITIC-Th1 0.849 (0.847 0.002) 19.33 (19.22 0.18) 73.73 (72.56 0.29) 12.1 (11.9 0.1)
FTAZ:ITIC-Th2 0.751 (0.748 0.004) 17.19 (16.97 0.25) 70.07 (69.34 0.77) 9.06 (8.93 0.15)
FTAZ:ITIC-Th3 0.962 (0.960 0.003) 16.34 (16.26 0.13) 68.33 (68.12 0.25) 10.7 (10.6 0.15)
a
FTAZ/acceptor = 1 : 1.5 (w/w), 0.25% DIO (v/v); average data in brackets are obtained from 20 devices.
Fig. 5 Dependence of J
SC
on light intensity (a); and photocurrent density versus effective voltage curves (b).
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packing and crystallinity of the neat and blended films
(Fig. 6).
63
Overall, the neat films of fluorinated ITIC-Th1 and
ITIC-Th2 show decreased crystallinity, while methoxyl-modified
ITIC-Th3 exhibits strong ordered packing with three sharp
lamellar stacking peaks. After being blended with FTAZ, almost
all the peaks of FTAZ:ITIC-Th1 are stronger/sharper, compared
with FTAZ:ITIC-Th. The (010) peaks for FTAZ:ITIC-Th, FTAZ:
ITIC-Th1 and FTAZ:ITIC-Th2 locate at B1.8 Å with coherence
lengths of 3.7, 4.0 and 2.0 nm, respectively. Similar to the neat
film, the FTAZ:ITIC-Th3 blend shows a sharp (010) peak at
1.78 Å
1
with a coherence length of 4.3 nm. The improved
molecular packing is known to benefit the charge transport.
Thus, FTAZ:modified ITIC-Th blends show higher mobility, which
leads to higher FFs, relative to the FTAZ:parent ITIC-Th blend.
Resonant soft X-ray scattering (R-SoXS) was utilized to
characterize the phase separation in the active layer of four
blends.
64
The photon energy of 286.8 eV is selected to enhance
the material contrasts. The phase separation length scale x,
so-called domain spacing, can be obtained from the equation
x=2p/qand the domain size is half of x. The scattering profiles
are fitted by log normal distributions (Fig. 7). The mode
Fig. 6 2D GIWAXS patterns and scattering profiles (in-plane and out-of-plane) for ITIC-Th series neat and blended films.
Fig. 7 Normalized R-SoXS profiles on a log scale for FTAZ:ITIC-Th series
blended films.
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domain sizes of ITIC-Th, ITIC-Th1, ITIC-Th2 and ITIC-Th3
based blend films are calculated to be 15, 13, 15 and 20 nm,
respectively. As the exciton diffusion length is limited to
10–20 nm, smaller domain size is favorable for the charge
separation. Therefore, FTAZ:ITIC-Th1 achieves the highest J
SC
.
Conclusions
We incorporate electron-donating methoxy and electron-
withdrawing F groups onto the terminal group IC in the widely
used nonfullerene acceptor ITIC-Th, and investigate the effects
of the substituents on the end-groups on the electronic properties,
charge transport, film morphology, and photovoltaic properties of
the ITIC-Th series. Compared with the parent ITIC-Th, fluorinated
ITIC-Th1 and ITIC-Th2 show lower HOMO and lower LUMO
energy levels, red-shifted absorption, and smaller bandgaps, while
ITIC-Th3 exhibits a similar HOMO but a higher LUMO level,
slightly blue-shifted absorption and a slightly larger bandgap.
Monofluorinated ITIC-Th1 and methoxylated ITIC-Th3 exhibit
higher mobility than the parent ITIC-Th and difluorinated
ITIC-Th2. FTAZ:modified ITIC-Th blended films exhibit stronger
molecular packing and longer coherence length, leading to
higher electron mobilities and balanced charge transport, which
are responsible for the higher FFs of their devices relative to
FTAZ:parent ITIC-Th. Moreover, FTAZ:ITIC-Th1 films present
relatively smaller domain sizes to afford more D/A interfaces
for exciton dissociation. OSCs based on FTAZ:ITIC-Th1 exhibit
PCEs as high as 12.1%, higher than those based on FTAZ:
ITIC-Th (8.88%), FTAZ:ITIC-Th2 (9.06%) and FTAZ:ITIC-Th3
(10.7%). Our results demonstrate that subtle tailoring on end-
groups can significantly boost the performance of the non-
fullerene acceptor.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
X. Z. wish to thank NSFC (No. 21734001 and 51761165023). W. Y.
thanks NSF (DMR-1507249 and CBET-1639429). W. M. thanks the
Ministry of Science and Technology of China (2016YFA0200700)
and NSFC (21504066 and 21534003). X-ray data was acquired at
beamlines 7.3.3 and 11.0.1.2 at the Advanced Light Source, which
was supported by the Director, Office of Science, Office of Basic
Energy Sciences, of the U. S. Department of Energy under
Contract No. DE-AC02-05CH11231. The authors thank Chenhui
Zhu at beamline 7.3.3, and Cheng Wang at beamline 11.0.1.2 for
assistance with data acquisition.
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