Breaking 10% Efficiency in Semitransparent Solar Cells with Fused-Undecacyclic Electron Acceptor

Abstract

A fused-undecacyclic electron acceptor IUIC has been designed, synthesized and applied in organic solar cells (OSCs) and semitransparent organic solar cells (ST-OSCs). In comparison with its counterpart, fused-heptacyclic ITIC4, IUIC with a larger π-conjugation and a stronger electron-donating core exhibits a higher LUMO level (IUIC: −3. 87 eV vs. ITIC4: –3.97 eV), 82 nm-redshifted absorption with larger extinction coefficient and smaller optical bandgap, and higher electron mobility. Thus, IUIC-based OSCs show higher values in open-circuit voltage, short-circuit current density and thereby much higher power conversion efficiency (PCE) than those of the ITIC4-based counterpart. The as-cast OSCs based on PTB7-Th: IUIC without any extra treatment yield PCEs of up to 11.2%, higher than that of the control devices based on PTB7-Th: ITIC4 (8.18%). The as-cast ST-OSCs based on PTB7-Th: IUIC without any extra treatment afford PCEs of up to 10.2% with an average visible transmittance (AVT) of 31%, higher than those of the con-trol devices based on PTB7-Th: ITIC4 (PCE = 6.42%, AVT = 28%).

Full text

Breaking 10% Efficiency in Semitransparent Solar Cells with Fused-
Undecacyclic Electron Acceptor
Boyu Jia,
†
Shuixing Dai,
†
Zhifan Ke,
‡
Cenqi Yan,
†
Wei Ma,*
,‡
and Xiaowei Zhan*
,†
†
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
‡
State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an 710049, China
*
SSupporting Information
ABSTRACT: A fused-undecacyclic electron acceptor IUIC has been designed,
synthesized and applied in organic solar cells (OSCs) and semitransparent
organic solar cells (ST-OSCs). In comparison with its counterpart, fused-
heptacyclic ITIC4, IUIC with a larger π-conjugation and a stronger electron-
donating core exhibits a higher LUMO level (IUIC: −3. 87 eV vs ITIC4: −3.97
eV), 82 nm red-shifted absorption with larger extinction coefficient and smaller
optical bandgap, and higher electron mobility. Thus, IUIC-based OSCs show
higher values in open-circuit voltage, short-circuit current density, and thereby
much higher power conversion efficiency (PCE) than those of the ITIC4-based
counterpart. The as-cast OSCs based on PTB7-Th: IUIC without any extra
treatment yield PCEs of up to 11.2%, higher than that of the control devices
based on PTB7-Th: ITIC4 (8.18%). The as-cast ST-OSCs based on PTB7-Th:
IUIC without any extra treatment afford PCEs of up to 10.2% with an average
visible transmittance (AVT) of 31%, higher than those of the control devices
based on PTB7-Th: ITIC4 (PCE = 6.42%, AVT = 28%).
■INTRODUCTION
Organic solar cells (OSCs) have attracted much interest
because of their merits, such as low cost, light weight,
flexibilility, and semitransparency.
1−5
Traditional OSCs are
based on blends of donor materials and fullerene acceptors that
form bulk heterojunctions (BHJs) in devices. However, the
development of this field has recently shifted to organic
nonfullerene acceptors. Fullerene derivatives suffer from some
drawbacks, such as limited energy level tunability, weak
absorption in the visible region, and morphology instability,
which constrain the further development of OSCs.
6
In contrast
to the widely used fullerene acceptors, the optical properties
and energy levels of nonfullerene acceptors can be easily
adjusted.
7−14
In 2015, we reported the first fused-ring electron acceptors
(FREAs) with an acceptor−donor−acceptor (A−D−A)
structure based on fused aromatic cores with strong electron-
withdrawing end groups, exemplified by ITIC
15
and IEIC.
16
Relative to fullerene acceptors, A−D−A type FREAs exhibit
much stronger absorption in the visible region. FREA-based
OSCs can achieve higher power conversion efficiencies (PCEs),
greater thermal and photochemical stability and longer device
lifetime than their fullerene-based counterparts.
17−32
To date,
most of FREAs reported in literature are based on fused-5-ring
to fused 10-ring cores.
33−35
Most of these cores have relatively
weak electron-donating property, leading to limited intra-
molecular charge transfer (ICT) and therefore limited
absorption in near-infrared (NIR) region (generally absorption
edge <800 nm with couple exceptions >900 nm).
36−38
Semitransparent OSCs (ST-OSCs) have great potential for
building integrated photovoltaic application and power-
generating windows.
39
Most of ST-OSCs are based on polymer
donors and fullerene acceptors, and exhibit relatively low PCEs,
due to weak absorption of fullerene acceptors; the best PCEs
reported in literature are 4−6% for single-junction and 7−8%
for tandem fullerene-based devices.
39−52
There have been only
couple examples of nonfullerene ST-OSCs, in which active
layer consisted of a narrow-bandgap polymer donor PTB7-Th
and a NIR-absorbing FREA; the PCEs are 7.74%
38
and
9.77%,
36
respectively.
In this work, we designed and synthesized a new fused-
undecacyclic electron acceptor, IUIC, based on a fused-11-ring
core IU, coupled with strong electron-withdrawing 2-(5,6-
difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene)-malononitrile
(2FIC) unit (Scheme 1). IUIC is the largest FREA and exhibits
strong NIR-absorbing property. We chose IU because it
possesses highly planarity, large π-conjugation and strong
electron-donating ability, and has been used for constructing p-
type polymer semiconductors that have exhibited promising
performance in organic field-effect transistors (hole mobility as
high as 0.024 cm2V−1s−1) and as donors in OSCs (PCE as
Received: October 9, 2017
Revised: December 4, 2017
Published: December 10, 2017
Article
pubs.acs.org/cm
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© XXXX American Chemical Society ADOI: 10.1021/acs.chemmater.7b04251
Chem. Mater. XXXX, XXX, XXX−XXX

high as 6.46%).
53
Difluorination of the end groups in the case
of 2FIC can extend the absorption due to enhanced ICT
between IU and 2FIC, and can improve electron mobility due
to noncovalent F−S and F−H bonding, as we previously
reported.
27
For comparison, we also synthesized a fused
heptacyclic electron acceptor ITIC4 (Scheme 1) with a fused-
7-ring core.
18
Relative to ITIC4 with a smaller core, IUIC with
a larger core exhibits (a) higher energy levels, (b) red-shifted
absorption spectra with larger extinction coefficient, and (c)
higher electron mobility, which are beneficial to (a) increasing
open-circuit voltage (VOC), and (b) short-circuit current
density (JSC). Indeed, as-cast OSCs based on IUIC: PTB7-
Th
54
(Scheme 1) without any additional treatment yield PCEs
of up to 11.2%, which is much higher than that of the control
devices based on ITIC4: PTB7-Th (8.18%). Furthermore, the
as-cast ST-OSCs based on IUIC: PTB7-Th without any
additional treatment yield PCEs of up to 10.2% with an
average visible transmittance (AVT) of 31%, which is much
higher than that of the control devices based on ITIC4: PTB7-
Th (PCE = 6.42%, AVT = 28%). This is the first example of
ST-OSCs with PCEs breaking 10% (the reported best was
9.77%
36
).
■RESULTS AND DISCUSSION
Synthesis and Characterization. IU was lithiated by n-
butyllithium and quenched with dry dimethylformamide
(DMF) to afford aldehyde IU-CHO. Subsequent Knoevenagel
condensation between IU-CHO and 2FIC yielded the final
product IUIC (Scheme 1). All new compounds were fully
characterized by mass spectrometry, 1H NMR, 13C NMR, 19F
NMR, and elemental analysis (see the Supporting Informa-
tion). IUIC exhibits excellent thermal stability with decom-
position temperature (5% weight loss) of 343 °C in nitrogen
atmosphere by thermogravimetric analysis (Figure S1).
The normalized spectra of optical absorption of IUIC and
ITIC4 in chloroform solution (10−6M) and in solid film are
shown in Figure 1a. ITIC4 shows an absorption maximum at
690 nm with an extinction coefficient of 1.9 ×105M−1cm−1in
solution, while IUIC shows a red-shifted maximum at 772 nm
with a higher extinction coefficient of 3.2 ×105M−1cm−1in
solution. Relative to those in solution, ITIC4 and IUIC in thin
film exhibit red-shifted absorption spectra with a maximum of
730 and 788 nm, respectively. The optical bandgap of IUIC
estimated from the absorption edge of the thin film is 1.41 eV,
narrower than that for ITIC4 (1.52 eV). The larger π-
Scheme 1. Chemical Structures of IUIC, ITIC4, and PTB7-Th and the Synthetic Route of IUIC
Figure 1. (a) Absorption spectra of IUIC and ITIC4 in chloroform solution and thin film; (b) cyclic voltammograms for IUIC and ITIC4 in
CH3CN/0.1 M [Bu4N]+[PF6]−at 100 mV s−1, the horizontal scale refers to an Ag/AgCl electrode as a reference electrode.
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conjugation and stronger electron-donating ability of IU core in
IUIC is responsible for its red-shifted absorption and narrower
bandgap.
Cyclic voltammetry was employed to investigate the
electrochemical properties of ITIC4 and IUIC (Figure 1b).
IUIC and ITIC4 exhibit irreversible reduction and oxidation
waves. The highest occupied molecular orbital (HOMO) and
lowest unoccupied molecular orbital (LUMO) energy levels are
estimated from the onset oxidation and reduction potentials,
respectively, versus ferrocenium/ferrocene (FeCp2+/0), assum-
ing the absolute energy level of FeCp2+/0 is 4.8 eV below
vacuum. IUIC shows higher HOMO (−5.45 eV) and LUMO
(−3.87 eV) energy levels relative to ITIC4 (HOMO = −5.77
eV, LUMO = −3.97 eV) (Table 1), due to the larger π-
conjugation and stronger electron-donating ability of IU core in
IUIC.
The electron mobilities of IUIC and ITIC4 were measured
using the space charge limited current (SCLC) method in
electron-only devices with a structure of Al/IUIC or ITIC4/Al
(Figure S2). The electron mobility of IUIC is 1.1 ×10−3cm2
V−1s−1, higher than that of ITIC4 (8.9 ×10−4cm2V−1s−1)
(Table 1).
Photovoltaic Properties. To demonstrate potential
application of IUIC in OSCs, we chose PTB7-Th as a donor
based on the following considerations. (a) The widely used
narrow-bandgap polymer PTB7-Th exhibits strong absorption
Table 1. Basic Properties of IUIC and ITIC4
λmax (nm)
compd Td(°C) solution film Eg(eV) ε(M−1cm−1) HOMO (eV) LUMO (eV) μe(cm2V−1s−1)
IUIC 343 772 788 1.41 3.2 ×105−5.45 −3.87 1.1 ×10−3
ITIC4 332 690 730 1.52 1.9 ×105−5.77 −3.97 8.9 ×10−4
Table 2. Performance of the optimized OSC and ST-OSC as-cast devices based on PTB7-Th:acceptor (1:1.5, w/w)
acceptor VOC (V)
a
JSC (mA cm−2)
a
FF
a
PCE (%)
a
Calc JSC (mA cm−2)
IUIC 0.792 ±0.004 (0.796) 21.51 ±0.33 (21.74) 0.647 ±0.008 (0.649) 11.0 ±0.2 (11.2) 20.69
IUIC
b
0.789 ±0.004 (0.794) 18.12 ±0.23 (18.31) 0.690 ±0.009 (0.703) 9.91 ±0.10 (10.2) 18.26
ITIC4 0.685 ±0.003 (0.687) 16.47 ±0.31 (16.66) 0.705 ±0.006 (0.715) 7.91 ±0.20 (8.18) 16.84
ITIC4
b
0.682 ±0.003 (0.680) 13.05 ±0.22 (13.20) 0.711 ±0.004 (0.716) 6.31 ±0.15 (6.42) 13.47
a
Average values with standard deviation were obtained from 20 devices, the values in parentheses are the parameters of the best device.
b
ST-OSC
devices.
Figure 2. (a) J−Vcurves and (b) EQE spectra of optimized OSC and ST-OSC as-cast devices based on PTB7-Th:ITIC4 and PTB7-Th:IUIC under
the illumination of AM 1.5G, 100 mW cm−2. (c) Jph versus Veff characteristics and (d) JSC versus light intensity of optimized as-cast devices based on
PTB7-Th:ITIC4 and PTB7-Th:IUIC.
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in 550−750 nm, complemented the absorption of IUIC (Figure
S3). (b) The energy levels of PTB7-Th (HOMO = −5.20 eV,
LUMO = −3.59 eV) match well with those of IUIC, which is
favorable for efficient exciton dissociation. (c) PTB7-Th
exhibited a good hole mobility of 2.8 ×10−3cm2V−1s−1
measured by SCLC method,
55
which is similar to the electron
mobility of IUIC, ensuring balanced charge transport in the
active layer. Thus, we fabricated regular devices with a structure
of ITO/ZnO/PTB7-Th:IUIC/MoOx/Ag(90 nm) and semi-
transparent devices with a structure of ITO/ZnO/PTB7-
Th:IUIC/MoOx/Au(1 nm)/Ag(15 nm), and compared with
the control devices based on PTB7-Th: ITIC4. The ultrathin
Au (1 nm) forms dense nucleation centers, which reduce
percolation of Ag film and enhance the Ag film uniformity,
leading to optimal transmittance and low electrical resistance.
56
We optimized the donor: acceptor weight ratio (D/A), and
found the best performance was obtained at D/A = 1:1.5
(Table S1). Table 2 summarizes VOC,JSC,fill factor (FF), and
PCE of the optimized OSC and ST-OSC devices. The
optimized as-cast devices based on PTB7-Th:IUIC exhibit
VOC of 0.796 V, JSC of 21.74 mA cm−2, FF of 0.649, and PCE of
11.2% without any extra treatment, whereas the optimized as-
cast devices based on PTB7-Th:ITIC4 show VOC of 0.687 V,
JSC of 16.66 mA cm−2, FF of 0.715, and PCE of 8.18% (Figure
2a). The higher values in VOC and JSC of IUIC-based devices are
attributed to the higher LUMO energy level, red-shifted and
stronger absorption and higher electron mobility of IUIC.
When the thickness of the Ag electrode decreases from 90 to 15
nm, the ST-OSCs based on PTB7-Th:IUIC exhibit VOC of
0.794 V, JSC of 18.31 mA cm−2, FF of 0.703, and PCE of 10.2%
(Table 2), whereas the ST-OSCs based on PTB7-Th:ITIC4
exhibit VOC of 0.680 V, JSC of 13.20 mA cm−2, FF of 0.716 and
PCE of 6.42%. The transmission spectra of the optimized ST-
OSC devices in the visible region (370−740 nm) were
measured to calculate the AVT of the ST-OSCs (Figure S4).
The AVT of glass/ITO/ZnO and glass/ITO/ZnO/MoOx/
Au(1 nm)/Ag(15 nm) is 88% and 66%, respectively, while the
AVT of glass/ITO/ZnO/PTB7-Th:IUIC/MoOx/Au(1 nm)/
Ag(15 nm) and glass/ITO/ZnO/PTB7-Th: ITIC4/MoOx/
Au(1 nm)/Ag(15 nm) is 31% and 28%, respectively. The
transmittance is relatively high at 370−500 nm (>40%), but
low after 650 nm (<20%). The Comission Internationale de
I’Eclairage (CIE) 1931 color coordinate, correlated color
temperature (CCT), and color rendering index (CRI) are
calculated to evaluate the color perception and color rendering
property of the ST-OSCs.
57
ST-OSCs based on IUIC and
ITIC4 have CIE 1931 color coordinates of (0.2331, 0.2870)
and (0.2649, 0.3204) (Figure S5), CCT of 16753 and 10918 K
and CRI of 75 and 85, respectively.
The external quantum efficiency (EQE) spectra of the
optimized regular devices exhibit broad photoresponse with the
maxima of 80% for PTB7-Th:IUIC in 300−900 nm and 78%
for PTB7-Th:ITIC4 in 300−800 nm, whereas the EQE maxima
of the ST-OSCs decrease to 71% for PTB7-Th:IUIC and 62%
for PTB7-Th:ITIC4 (Figure 2b).
To gain insight into charge generation and extraction
properties, the relationship of the photocurrent density (Jph)
and the effective voltage (Veff) was measured (Figure 2c). At
high applied voltage (Veff > 2.2 V), Jph reaches saturation,
meaning that almost all the photogenerated excitons are
dissociated and free charge carriers are completely collected by
the electrodes. The ratio of Jph to the saturation photocurrent
density (Jsat) of the devices based on PTB7-Th:IUIC and
PTB7-Th:ITIC4 are over 95%, indicating excellent charge
extraction.
The charge carrier recombination in the active layers of the
optimized devices was investigated by measuring JSC under
different light intensities (Plight)(Figure 2d). The correlation
between JSC and Plight is expressed by JSC ∝Plightα, where αvalue
close to 1 indicates negligible bimolecular recombination in the
devices. The values of αin PTB7-Th:IUIC and PTB7-
Th:ITIC4 blends are 0.97 and 0.99, respectively, which suggests
very weak bimolecular recombination.
The charge transport properties of active layers were
investigated by SCLC method with the structure of
PEDOT:PSS/active layer/Au for hole mobility and Al/active
layer/Al for electron mobility (Figure S6). The blend films
based on PTB7-Th:IUIC exhibit a higher hole mobility (μh)of
7.7 ×10−4cm2V−1s−1and a higher electron mobility (μe)of
5.5 ×10−4cm2V−1s−1relitive to the PTB7-Th:ITIC4 blends
respectively, which is favorable for higher JSC, whereas the
PTB7-Th:ITIC4 blends exhibit μhof 4.9 ×10−4cm2V−1s−1
and μeof 4.2 ×10−4cm2V−1s−1(Table 2).
We investigated the preliminary light and thermal stability of
the OSC devices based on IUIC and ITIC4. Under continuous
illumination with AM 1.5G simulator at 100 mW cm−2for 180
min, the PCEs of devices based on IUIC and ITIC4 retained ca.
75 and 62% of their original values, respectively (Figure S7a).
Under heating at 100 °C for 180 min, the PCEs of devices
based on IUIC and ITIC4 retained ca. 57 and 36% of their
original values, respectively (Figure S7b).
Morphology Characterization. The morphology of the
active layers of PTB7-Th:IUIC and PTB7-Th:ITIC4 was
studied by atomic force microscopy (AFM), grazing-incidence
wide-angle X-ray scattering (GIWAXS) and resonant soft X-ray
scattering (R-SoXS). According to the AFM images, the
blended film of PTB7-Th:IUIC is slightly smoother with the
root-mean-square roughness (Rq) value of 1.0 nm relative to
that of PTB7-Th:ITIC4 (Rq= 1.2 nm) (Figure S8). The
molecular packing of PTB7-Th:IUIC and PTB7-Th:ITIC4
blended films was investigated by GIWAXS.
58
Based on the
informaiton of peaks provided in the neat films (Figure S9), the
crystallinity of both donor and acceptor in the blended films is
obtained. In the PTB7-Th:IUIC blended film, IUIC displays
weak (100) diffraction peak (q≈0.35 Å-1) and (010) π−π
stacking peak (q≈1.42 Å−1), whereas PTB7-Th exhibits much
stronger crystallinity with sharper (100) diffraction peak (q≈
0.26 Å−1) and (010) π−πstacking peak (q≈1.60 Å−1). ITIC4
hardly crystallizes in the blended films as it does not show
visible diffracttion peaks in the scattering profiles (Figure 3).
Both (100) scattering peak (q≈0.28 Å−1) and (010) π−π
stacking peak (q≈1.65 Å−1) in the PTB7-Th:ITIC4 blended
film belong to PTB7-Th. The location of PTB7-Th π−π
stacking peak shifts to higher q, indicating that the molecular
packing of PTB7-Th is more compact when blended with
ITIC4. By calculating via the Scherrer equation,
59
PTB7-Th
exhibits similar π−πstacking coherence length of ∼1.2 nm in
PTB7-Th:IUIC and PTB7-Th:ITIC4 blended films.
R-SoXS is further utilized to characterize the mode length
(domain size) and average composition variation.
60,61
Figure 4
shows the R-SoXS profiles of blend films of PTB7-Th:IUIC and
PTB7-Th:ITIC4. To obtain enhanced polymer/small molecule
contrast and avoid too high absorption, the energy of 285.2 eV
is selected. The domain size is half of characteristic mode
length (domain spacing, ξ). The domain size of PTB7-Th:
IUIC and PTB7-Th:ITIC4 is calculated to be ∼19 nm and ∼25
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nm, respectively. Because of the limited exciton diffusion length
(ca. 10−20 nm), the appropriate domain size increases
interfacial area between donor and acceptor, which facilitates
exciton dissociation and reduces geminate recombination.
Additionally, via integrating of total scattering intensity (TSI),
R-SoXS can probe the average composition variation (relative
domain purity). The higher the total scattering intensity
(integration of the scattering profiles over q), the purer the
average domain. The relative domain purity of PTB7-Th:IUIC
and PTB7-Th:ITIC4 is calculated to be 1 and 0.87, respectively.
The higher domain purity would minimize the possibility of
bimolecular recombination and promotes charge transport.
Thus, the as-cast devices based on PTB7-Th:IUIC exhibit
higher EQE, JSC, and PCE than that of PTB7-Th:ITIC4.
■CONCLUSION
We designed and synthesized an fused-undecacyclic electron
acceptor IUIC, which is the largest FREA so far. In comparison
with ITIC4, IUIC has a larger extended π-conjugation with a
stronger electron-donating ability. Thus, IUIC exhibits higher
energy levels, red-shifted and stronger absorption in 600−900
nm, narrower bandgap, and higher electron mobility. As a
result, blended with the PTB7-Th polymer donor that has
matched energy levels and complementary absorption spec-
trum, the IUIC-based OSCs show higher values in VOC,JSC, and
finally much higher PCE than the ITIC4-based OSCs. The as-
cast OSCs based on PTB7-Th:IUIC without any additional
treatment afford PCEs of up to 11.2%, much higher than that of
the control devices based on PTB7-Th:ITIC4 (8.18%). The as-
cast ST-OSCs based on PTB7-Th:IUIC exhibit a champion
PCE of 10.2%, than that of the control devices based on PTB7-
Th:ITIC4 (6.42%). The 10.2% PCE is a new record for any ST-
OSCs. These results demonstrate the great potential of the
fused-11-ring unit IU for constructing NIR-absorbing non-
fullerene acceptors used for high-performance ST-OSCs.
■ASSOCIATED CONTENT
*
SSupporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.chemma-
ter.7b04251.
Experimental details, TGA curves, SCLC data, absorp-
tion spectra and energy levels, the representation of color
coordinate of the semitransparent device on CIE 1931
xyY chromaticity diagram, visible transmission spectra,
AFM images, GIWAXS of neat films, and the
optimization and stability test of the OSC devices
(PDF)
■AUTHOR INFORMATION
Corresponding Authors
*E-mail: xwzhan@pku.edu.cn (X.Z.).
*E-mail: msewma@xjtu.edu.cn (W.M.).
ORCID
Wei Ma: 0000-0002-7239-2010
Xiaowei Zhan: 0000-0002-1006-3342
Notes
The authors declare no competing financial interest.
■ACKNOWLEDGMENTS
X.Z. thanks the NSFC (91433114 and 21734001) for support.
W.M. thanks from Ministry of Science and Technology
(2016YFA0200700) and NSFC (21504066, 21534003) for
the support. X-ray data were acquired at beamlines 7.3.3 and
11.0.1.2 at the Advanced Light Source, which is supported by
the Director, Office of Science, Office of Basic Energy Sciences,
of the U.S. Department of Energy, under Contract 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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