Efficient Semitransparent Organic Solar Cells with Tunable Color enabled by an Ultralow-Bandgap Nonfullerene Acceptor

Article abstract of DOI:10.1002/adma.201703080

Semitransparent organic solar cells (OSCs) show attractive potential in power-generating windows. However, the development of semitransparent OSCs is lagging behind opaque OSCs. Here, an ultralow-bandgap nonfullerene acceptor, “IEICO-4Cl”, is designed and synthesized, whose absorption spectrum is mainly located in the near-infrared region. When IEICO-4Cl is blended with different polymer donors (J52, PBDB-T, and PTB7-Th), the colors of the blend films can be tuned from purple to blue to cyan, respectively. Traditional OSCs with a nontransparent Al electrode fabricated by J52:IEICO-4Cl, PBDB-T:IEICO-4Cl, and PTB7-Th:IEICO-4Cl yield power conversion efficiencies (PCE) of 9.65 ± 0.33%, 9.43 ± 0.13%, and 10.0 ± 0.2%, respectively. By using 15 nm Au as the electrode, semitransparent OSCs based on these three blends also show PCEs of 6.37%, 6.24%, and 6.97% with high average visible transmittance (AVT) of 35.1%, 35.7%, and 33.5%, respectively. Furthermore, via changing the thickness of Au in the OSCs, the relationship between the transmittance and efficiency is studied in detail, and an impressive PCE of 8.38% with an AVT of 25.7% is obtained, which is an outstanding value in the semitransparent OSCs.

Full text of DOI:10.1002/adma.201703080

COMMUNICATION
Organic Solar Cells
www.advmat.de
Efficient Semitransparent Organic Solar Cells
with Tunable Color enabled by an Ultralow-Bandgap
Nonfullerene Acceptor
Yong Cui, Chenyi Yang, Huifeng Yao,* Jie Zhu, Yuming Wang, Guoxiao Jia, Feng Gao,
and Jianhui Hou*
of the devices should be good enough to
Semitransparent organic solar cells (OSCs) show attractive potential in
[
15]
fulfill the window applications; ii) The
color of the semitransparent OSCs could
be tuned to meet the requirements of
power-generating windows. However, the development of semitransparent
OSCs is lagging behind opaque OSCs. Here, an ultralow-bandgap non-
fullerene acceptor, “IEICO-4Cl”, is designed and synthesized, whose absorp-
tion spectrum is mainly located in the near-infrared region. When IEICO-4Cl
is blended with different polymer donors (J52, PBDB-T, and PTB7-Th), the
colors of the blend films can be tuned from purple to blue to cyan, respec-
tively. Traditional OSCs with a nontransparent Al electrode fabricated by
J52:IEICO-4Cl, PBDB-T:IEICO-4Cl, and PTB7-Th:IEICO-4Cl yield power
conversion efficiencies (PCE) of 9.65 ± 0.33%, 9.43 ± 0.13%, and 10.0 ± 0.2%,
respectively. By using 15 nm Au as the electrode, semitransparent OSCs
based on these three blends also show PCEs of 6.37%, 6.24%, and 6.97%
with high average visible transmittance (AVT) of 35.1%, 35.7%, and 33.5%,
respectively. Furthermore, via changing the thickness of Au in the OSCs, the
relationship between the transmittance and efficiency is studied in detail, and
an impressive PCE of 8.38% with an AVT of 25.7% is obtained, which is an
outstanding value in the semitransparent OSCs.
[
15–17]
varied applications.
Although the past two decades have
seen much significant progress in OSCs,
the development of semitransparent
devices is relatively lagging behind
because of the lack of suitable photoactive
[
8,18–21]
materials.
Currently, the highly
efficient OSCs with PCE over 11% were
fabricated by the photoactive materials
with optical absorption limited at around
[
22–26]
8
00 nm.
The strong absorption of
the active materials in the visible range
causes the great difficulty to make a bal-
ance between the PCE and transmittance,
which is unfavorable for their application
in semitransparent OSCs. For example,
when a wide bandgap polymer donor
PM6 and a fullerene derivative acceptor
phenyl-C -butyric acid methyl ester
71
As a clean-energy technology, organic solar cells (OSCs)[1–6]
have attracted much attention from academia and industry due
(PC BM) were used to fabricate the semitransparent OSCs,
71
the active layer thickness was only 75 nm. Although the device
possessed high transparency, the inefficient light absorption
to their advantages in making large area panels by low-cost solu-
tion processing methods.[
7–10]
Semitransparent OSCs that uti-
resulted in its low short-circuit density (JSC) of 9.4 mA cm
−2
lize transparent conductive electrodes and high-transmittance
and thus a moderate PCE of 5.7%, which was much lower
[
19]
active layer materials show an attractive potential in power-
than its opaque analog (9.2%).
Semitransparent OSCs
generating windows.[
11–14]
For the semitransparent OSCs, some
based on poly[[2,6′-4,8-di(5-ethylhexylthienyl)benzo[1,2-b;3,3-
b]dithiophene][3-fluoro-2[(2-ethylhexyl)carbonyl]thieno[3,4-b]-
critical requirements should be met: i) the power conversion
efficiencies (PCEs) of the devices should be as high as possible
[
15]
thiophenediyl]] (PTB7-Th):PC BM
showed an high JSC of
71
[
7]
−2
and also the average visible transmittance (AVT, 370–740 nm)
13.1 mA cm at 90–100 nm thickness, but its AVT was only
Y. Cui, C. Yang, Dr. H. Yao, J. Zhu, G. Jia, Prof. J. Hou
State Key Laboratory of Polymer Physics and Chemistry
Beijing National Laboratory for Molecular Sciences
CAS Research/Education Center for Excellence in Molecular Sciences
Institute of Chemistry
Chinese Academy of Sciences
Beijing 100190, China
E-mail: yaohf@iccas.ac.cn; hjhzlz@iccas.ac.cn
C. Yang, G. Jia
School of Chemistry and Biology Engineering
University of Science and Technology Beijing
Beijing 100083, China
J. Zhu
Institute of Material Science and Engineering
Ocean University of China
Qingdao 266100, P. R. China
Y. Cui, Prof. J. Hou
University of Chinese Academy of Sciences
Beijing 100049, China
Y. Wang, Prof. F. Gao
Biomolecular and Organic Electronics IFM
Linköping University
Linköping 58183, Sweden
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adma.201703080.
DOI: 10.1002/adma.201703080
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2.2%. In order to avoid strong absorption in the visible range
conventional OSCs with opaque Al as electrode, all the three
devices based on these blends showed very good photovoltaic
performance at low energy losses. Furthermore, photovoltaic
performances of the semitransparent OSCs were fully inves-
tigated and the PTB7-Th:IEICO-4Cl-based OSCs yielded an
impressive PCE of 8.38% with a high AVT of 25.7%.
and maintain relatively high absorption properties of the active
layer, the main absorption peak of the photovoltaic material
should be in the near-infrared (NIR) region, which is a feasible
approach to achieve high PCE and high AVT simultaneously in
semitransparent OSCs.
In OSCs using fullerene derivatives as acceptors, many
low-bandgap donors with absorption spectra over 900 or even
For organic semiconductors, it has been demonstrated that
[
36–38]
the enhanced intramolecular charge transfer (ICT)
from
[27–31]
1
000 nm have been developed,
whose absorption proper-
the electron-rich moiety to the electron-withdrawing groups can
largely extend the absorption spectra of the resulting materials.
ties in the visible range are very weak. However, these studies
suggested that this kind of OSC showed limited PCEs due to
Fluorination of the acceptor moiety has been proved to be an
opt
g
opt
g
[25,39,40]
the large energy losses (Eloss = E
− eVOC, where E
is the
effective method to enhance the ICT effect.
However, the
optical bandgap of the active materials, and VOC is the open-
fluorination procedure is generally tedious and costly, which is
unfavorable for practical applications. In this contribution, as
shown in the Figure 1a, we incorporate chlorination into the
NF acceptor to enhance the ICT effect and thus redshifted
its absorption spectrum. The detailed synthetic procedure of
IEICO-4Cl is provided in the Supporting Information. IEICO-
4Cl exhibits good solubility in common organic solvents, such
as dichloromethane, chloroform, and chlorobenzene (CB).
Thermogravimetric analysis suggests that IEICO-4Cl has good
thermal stability with a decomposition temperature at ≈390 °C
(Figure S1, Supporting Information).
circuit voltage) and the low external quantum efficiency (EQE)
in the NIR region.[
32,33]
In the past few years, nonfullerene-
based OSCs (NF-OSCs) have achieved many important pro-
gresses. Some results demonstrated that the absorption spectra
of the NF acceptors could be extended to NIR region and the
corresponding NF-OSCs exhibited high EQE and thus high
PCE,[
21,34,35]
offering a potential opportunity to promote the
photovoltaic performance of semitransparent OSCs.
In this work, a novel NF acceptor, 2,2′-((2Z,2′Z)-(((4,4,9-tris(4-
hexylphenyl)-9-(4-pentylphenyl)-4,9-dihydro-s-indaceno[1,2-
b:5,6-b dithiophene-2,7-diyl)bis(4-((2-ethylhexyl)oxy)thiophene-
The absorption spectra of IEICO-4Cl in diluted solution and
as thin film are depicted in Figure 1c. In chloroform solution,
IEICO-4Cl exhibits a maximum absorption peak at 802 nm,
5
,2-diyl))bis(methanylylidene))bis(5,6-dichloro-3-oxo-2,3-di-
hydro-1H-indene-2,1-diylidene))dimalononitrile (IEICO-4Cl,
was designed and synthesized, which showed an ultralow
and the film has a redshift of 85 nm with an absorption onset
opt
opt
g
bandgap (E
region (370–740 nm). When three polymers, poly[4-(4,8-bis(5-
2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophen-2-yl)-
,6-difluoro-2-(2-2hexyldecyl)-7-(thiophen-2-yl)-2H-benzo[d]
1,2,3]triazole] (J52), poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-
g
= 1.23 eV) and very weak absorption in the visible
of ≈1010 nm, corresponding to an E
of 1.23 eV. It should be
noted that the main absorption band of IEICO-4Cl film locates
at 745–945 nm, which is out of the visible region and hence
makes IEICO-4Cl a good candidate to fabricate semitrans-
parent OSCs. Furthermore, when compared to 2,2′-((2Z,2′Z)-
(((4,4,9-tris(4-hexylphenyl)-9-(4-pentylphenyl)-4,9-dihydro-s-
indaceno[1,2-b:5,6-b-dithiophene-2,7-diyl)bis(4-((2-ethylhexyl)
oxy)thiophene-5,2-diyl))bis(methanylylidene))bis(5,6-difluoro-
3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile
(IEICO-4F) in our recent work, IEICO-4Cl shows obviously
(
5
[
2
5
-yl)benzo[1,2-b:4,5-b′]dithiophene)-co-(1,3-di(5-thiophene-2-yl)-
,7-bis(2-ethylhexyl)benzo[1,2-c:4,5-c′]dithiophene-4,8-dione)]
(PBDB-T), and PTB7-Th with varied absorption properties were
used as donors to blend with IEICO-4Cl, respectively, the colors
of the blend films were determined by the polymers. In the
Figure 1. a) Synthetic routes of IEICO-4Cl. b) Device architecture of semitransparent OSCs in this work. c,d) Normalized absorption spectra (c) and
CV curve (d) of IEICO-4Cl.
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Figure 2. a) The molecular structure of J52, PBDB-T, and PTB7-Th. b) Absorption spectra of blend films. The inset is the photographs of blend and
neat films. 1: ITO-Glass, 2: IEICO-4Cl, 3: J52, 4: J52:IEICO-4Cl, 5: PBDB-T, 6: PBDB-T:IEICO-4Cl, 7: PTB7-Th, 8: PTB7-Th:IEICO-4Cl. c,d) J–V curves
(c) and EQE curves (d) of opaque OSCs based on J52:IEICO-4Cl, PBDB-T:IEICO-4Cl, and PTB7-Th:IEICO-4Cl.
redshifted absorption spectrum (Figure S2, Supporting Infor-
mation),[ implying that chlorination is more effective than
fluorination in enhancing the ICT effect. IEICO-4Cl is more
suitable for utilizing NIR solar photons than IEICO-4F in the
OSCs. As shown in Figure 1d, the molecular energy levels of
IEICO-4Cl were measured by cyclic voltammetry (CV), and the
highest occupied molecular orbital (HOMO) and the lowest
unoccupied molecular orbital (LUMO) levels calculated from
the onset of oxidation and reduction potentials are −5.56 and
and PTB7-Th:IEICO-4Cl show very similar colors with the neat
polymers (Figure 2b), demonstrating that the color of the blend
film can be easily tuned by selecting different donors.
35]
First, we investigated the photovoltaic characteristics of
IEICO-4Cl via fabricating conventional opaque OSCs on indium
tin oxide (ITO) substrate with Al as electrode, where the above-
mentioned three polymers and poly(3,4-ethylenedioxythiophene)-
polystyrenesulfonic acid (PEDOT:PSS)/poly[9,9-bis[6′-(N,N,N-
trimethylammonium)hexyl]fluorene-alt-co-1,4-phenylene]-
bromide (PFN-Br) were used as the donors and interlayers,
respectively. After careful optimization of the device fabrica-
tion conditions like donor/acceptor ratios and solvent additives
(Experimental Section), as shown in Figure 2c and Table 1, the
OSCs based on J52:IEICO-4Cl, PBDB-T:IEICO-4Cl, and PTB7-
Th:IEICO-4Cl show PCEs of 9.65 ± 0.33%, 9.43 ± 0.13%, and
10.0 ± 0.2%, respectively, which are among the top values in
−
4.23 eV, respectively.
The color of the blend active layer is determined by the absorp-
tion properties of both the donor and the acceptor. Since IEICO-
Cl has very weak absorption in the visible range, the colors of
IEICO-4Cl-based blend films are highly dependent on the donors.
4
[41]
Here, we selected three polymer donors named J52, PBDB-
[42]
[43]
T, and PTB7-Th to blend with IEICO-4Cl (Figure 2a). As pro-
vided in Figure S3 (Supporting Information), the main absorp-
tion bands of the three polymers locate at 490–620, 530–660, and
[34,44,45]
the ultralow-bandgap OSCs.
Specifically, the OSCs show
similar VOC around 0.72 V because of the similar HOMO levels
(≈−5.3 eV) for the three polymer donors, leading to very small
energy losses around 0.5 eV (the calculated energy losses of
620–745 nm, respectively, making their colors as purple, blue,
and cyan. The blend films of J52:IEICO-4Cl, PBDB-T:IEICO-4Cl,
Table 1. The photovoltaic performance parameters of the conventional OSCs based on IEICO-4Cl.
Donor
VOC
JSC
[mA cm−2]
Jcala)
[mA cm−2]
FF
PCEb)
[%]
Eloss
[eV]
Thickness
[nm]
[
V]
J52
0.700
0.744
0.727
23.8
20.8
22.8
23.3
20.4
22.2
0.607
0.625
0.620
10.1 (9.65 ± 0.33)
9.67 (9.43 ± 0.13)
10.3 (10.0 ± 0.2)
0.530
0.486
0.503
110
100
110
PBDB-T
PTB7-Th
a)Integrated Jcal from the EQE curves; b)The average parameters were calculated from more than ten independent cells.
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Figure 3. a,b) Transmission spectra (a) and J–V curves (b) of the semitransparent OSCs with Au (15 nm) illuminated from the ITO side. c) Photograph
of the three semitransparent devices, from left to right are J52:IEICO-4Cl-, PBDB-T:IEICO-4Cl-, and PTB7-Th:IEICO-4Cl-based devices. d) EQE curves
of the semitransparent OSCs.
the devices will be around 0.6 eV if the bandgap (E , 1.33 eV)
increase a little in the semitransparent devices, and the resulting
PCE values are 6.37%, 6.24%, and 6.97% for J52:IEICO-4Cl-,
PBDB-T:IEICO-4Cl-, and PTB7-Th:IEICO-4Cl-based devices.
The high PCEs along with its good transmittances of these
devices make them very promising in the practical applica-
tions. The EQE curves of semitransparent devices are shown
in Figure 3d. As Au has high transmittance in the range of
300–600 nm (Figure S5, Supporting Information), the EQE
values have an obvious decrease in this range in comparison
with that of the opaque devices. The integrated current densi-
ties from the EQE spectra are 16.1, 14.4, and 16.7 mA cm− for
J52:IEICO-4Cl-, PBDB-T:IEICO-4Cl-, and PTB7-Th:IEICO-4Cl-
based semitransparent devices, respectively.
g
of the blend is determined according to some reported litera-
opt
[46,47]
tures instead of Eg (Figure S4, Supporting Information)
).
−2
Impressively, all the three OSCs possess JSC of over 20 mA cm .
When a transparent electrode is used in the OSCs, relatively
high JSC values are still guaranteed. Furthermore, the fill factors
(FFs) of the three devices are all above 0.6.
The EQE spectra of the devices are displayed in Figure 2d,
which are consistent with the absorption spectra of the cor-
responding blend films. All the three devices show high
photoresponse in the whole absorption region, and the
maximum EQE values of J52:IEICO-4Cl- and PTB7-Th:IEICO-
2
4
Cl-based devices are even over 70%, which are outstanding
results for the OSCs with such low energy losses. The inte-
grated current densities of the three OSCs from EQE spectra
are 23.3, 20.4, and 22.2 mA cm , respectively, which are in
good agreement with those from the J–V measurements.
Since the 400–600 nm wavelength is the most sensitive region
[
48]
to human eyes,
high transmittance is specially required
−2
in this region for the preparation of semitransparent OSCs.
Among the three semitransparent OSCs, PTB7-Th:IEICO-4Cl
has the highest average transmittance (AT, 400–600 nm) of
43.6% with a maximum transmittance of 50.9% at 462 nm,
and its efficiency is the highest one. Therefore, we take PTB7-
Th:IEICO-4Cl as an example to investigate the relationship of
transmittance and efficiency of the semitransparent OSC by
varying the thickness of Au. First, as shown in Figure 4a, as
the thickness of Au increases from 10 to 30 nm, the AT of the
devices decreases from 45.2% to 33.5% (the corresponding
device pictures are provided in Figure S6 in the Supporting
Information). The increased thickness of Au contributes to
the improved light reflection and lower series resistance in
the devices, and hence, as shown in Figure 4b and Table 2,
Via replacing Al with 15 nm Au as the transparent elec-
trode in the devices, we fabricated the IEICO-4Cl-based semi-
transparent OSCs with a structure of ITO/PEDOT:PSS/active
layers/PFN-Br/Au. Figure 3a shows the transmission spectra of
the semitransparent devices, and the AVTs are 35.1%, 35.7%,
and 33.5% for J52:IEICO-4Cl, PBDB-T:IEICO-4Cl, and PTB7-
Th:IEICO-4Cl, respectively. From the optical photograph in
Figure 3c, we can find that the colorful devices have very good
transparency, and the background pattern can be clearly seen
through the three semitransparent OSCs. Figure 3b shows the
J–V curves of the devices, and the detailed photovoltaic para-
meters are collected in Table 2. Compared with their opaque
devices, the VOC and JSC suffer some decreases, while the FF
−
2
increased JSC (16.8–19.6 mA cm ) and FF (0.497–0.590) are
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Table 2. The photovoltaic performance parameters of the semitransparent OSCs based on IEICO-4Cl.
Active layer
Thickness of Au
Illuminationa)
VOC
[V]
JSC
[mA cm−2]
FF
PCEb)
[%]
AVT
[%]
AT
[%]
[
nm]
15
15
15
10
10
15
20
20
25
25
30
30
J52:IEICO-4Cl
ITO
ITO
ITO
ITO
Au
0.673
0.724
0.714
0.709
0.700
0.700
0.719
0.695
0.722
0.697
0.725
0.699
17.2
15.4
17.6
16.8
11.4
10.6
18.1
9.49
18.8
8.57
19.6
7.36
0.551
0.560
0.554
0.497
0.501
0.561
0.574
0.587
0.583
0.598
0.590
0.617
6.37 (6.22 ± 0.12)
6.24 (6.04 ± 0.14)
6.97 (6.76 ± 0.18)
5.92 (5.51 ± 0.32)
4.00 (3.78 ± 0.15)
4.16 (4.01 ± 0.11)
7.47 (7.24 ± 0.19)
3.87 (3.72 ± 0.13)
7.91 (7.71 ± 0.17)
3.57 (3.46 ± 0.09)
8.38 (8.17 ± 0.15)
3.17 (3.08 ± 0.06)
35.1
35.7
33.5
34.7
34.5
33.4
31.8
31.7
28.2
28.1
25.6
25.7
33.1
37.6
43.6
45.2
45.1
43.4
41.4
41.3
36.8
36.8
33.5
33.6
PBDB-T:IEICO-4Cl
PTB7-Th:IEICO-4Cl
PTB7-Th:IEICO-4Cl
PTB7-Th:IEICO-4Cl
PTB7-Th:IEICO-4Cl
PTB7-Th:IEICO-4Cl
PTB7-Th:IEICO-4Cl
PTB7-Th:IEICO-4Cl
PTB7-Th:IEICO-4Cl
PTB7-Th:IEICO-4Cl
PTB7-Th:IEICO-4Cl
Au
ITO
Au
ITO
Au
ITO
Au
a)Semitransparent OSCs illuminated from the ITO or the Au side; b)The average parameters were calculated from more than ten independent cells.
obtained. Consequently, the PCE of the semitransparent device
has a significant increase from 5.92% to 8.38%. To the best of
our knowledge, the PCE of 8.38% with an AT of 33.5% (AVT of
absorption of Au. Compared with the photovoltaic performance
of devices illuminated from the ITO side, the devices illumi-
nated from the Au side show relatively lower VOC and JSC but
higher FF. Consequently, the devices show an optimal PCE of
4.16%, with a VOC of 0.70 V, a JSC of 10.6 mA cm , and a FF of
0.561 when 15 nm Au was used.
In summary, we have successfully designed and synthe-
sized a new NF acceptor named IEICO-4Cl via chlorination,
and its applications in efficient semitransparent OSCs were
systematically studied. IEICO-4Cl has an ultralow bandgap of
2
5.6%) is one of the top values for semitransparent OSCs so far.
Furthermore, we investigated the photovoltaic performance
−
2
of the PTB7-Th:IEICO-4Cl-based semitransparent OSC illumi-
nated from Au electrode.[49] The J–V characteristics are shown
in Figure 4d and detailed photovoltaic parameters are collected
in Table 2. With the increase of the thickness of Au, the JSC
of the corresponding devices decreases gradually due to the
Figure 4. a,b) Transmission spectra (a) and J–V curves (b) of the semitransparent OSCs based on PTB7-Th with different Au thickness illuminated
from the ITO side. c) Relationships between the PCE and the transmittance of the semitransparent OSCs illuminated from the ITO and the Au sides.
d) J–V curves of the semitransparent OSCs based on PTB7-Th with different thicknesses of Au illuminated from the Au side.
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.23 eV with weak absorption in the visible region. The colors Keywords
of IEICO-4Cl-based blend films could be easily tuned from
purple, blue to cyan via selecting J52, PBDB-T, and PTB7-Th as
polymer donors, respectively. In the conventional opaque OSCs
with Al electrode, PCE around 10% with a low Eloss of 0.5 eV
was recorded. Furthermore, the semitransparent OSCs were
fabricated by using 15 nm Au as electrode, and J52:IEICO-4Cl-,
PBDB-T:IEICO-4Cl-, and PTB7-Th:IEICO-4Cl-based devices
yielded high PCE of 6.37%, 6.24%, and 6.97% with high AVT
of 35.1%, 35.7%, and 33.5%, respectively. Furthermore, via
changing the thickness of Au in the OSCs, the relationship
between the transmittance and efficiency was studied in detail,
and an impressive PCE of 8.38% with an AVT of 25.6% was
obtained, which is an outstanding value in the semitransparent
OSCs. These results demonstrate that design and application of
ultralow-bandgap NF acceptor is an effective strategy to improve
the photovoltaic performance of semitransparent OSCs.
nonfullerene acceptors, organic solar cells, semitransparent, tunable
color
Received: June 1, 2017
Revised: August 3, 2017
Published online:
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Adv. Mater. 2017, 1703080
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