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A highly crystalline non-fullerene acceptor
enabling efficient indoor organic
photovoltaics with high EQE and fill factor
Fujin Bai, Jianquan Zhang,
Anping Zeng, ..., Jiaen Liang,
Wei Ma, He Yan
msewma@xjtu.edu.cn (W.M.)
hyan@ust.hk (H.Y.)
Highlights
Materials with rationally tailored
properties are crucial for indoor
OPV applications
FCC-Cl achieves a record indoor
efficiency of 28.8% under a 2,600
K LED at 500 lux
A high tolerance of active-layer
thickness is observed for FCC-Clbased indoor devices
A new high-performance indoor non-fullerene acceptor with a desirable optical
band gap, named FCC-Cl, was developed by combining a weak electron-donating
core and a moderate electron-withdrawing end group. The FCC-Cl-based devices
achieved a record indoor power conversion efficiency of 28.8% at 2600K LED at
500 lux due to the well-matched absorption spectrum, high crystallinity, and high
absorption coefficient of FCC-Cl. Our work provides guidelines for efficient
materials toward high-performance indoor OPVs and proves the feasibility of
practical applications of indoor OPVs.
Bai et al., Joule 5, 1231–1245
May 19, 2021 ª 2021 Elsevier Inc.
https://doi.org/10.1016/j.joule.2021.03.020
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A highly crystalline non-fullerene acceptor
enabling efficient indoor organic photovoltaics
with high EQE and fill factor
Fujin Bai,1,3,4,6 Jianquan Zhang,1,6 Anping Zeng,1 Heng Zhao,2 Ke Duan,5 Han Yu,1 Kui Cheng,1
Gaoda Chai,1 Yuzhong Chen,1 Jiaen Liang,1 Wei Ma,2,* and He Yan1,7,*
SUMMARY
Context & scale
The growth of the internet of things (IoT) is creating a demand for
convenient energy sources, like organic photovoltaics, to power
various small IoT devices. Here, we report a highly crystalline small
molecular acceptor (named FCC-Cl) with an optical band gap of
1.71 eV suitable for indoor applications. The important design rationale of FCC-Cl is the combination of a weak electron-donating core
and a moderate electron-withdrawing end group, which leads to
needed band gap and high crystallinity. The OPVs based on
D18:FCC-Cl achieved a high external quantum efficiency up to
85% and a high fill factor of 80% due to the high absorption coefficient and strong crystallinity of FCC-Cl. Consequently, an impressive
power conversion efficiency of 28.8% was achieved under a 2,600 K
LED lamp at 500 lux. It was also demonstrated that PM6:FCC-Clbased devices can achieve high efficiencies over a wide range of
active-layer thicknesses, which is a feature necessary for large-scale
roll-to-roll printing processes.
The rapid development of the
internet of things (IoT) has
motivated researchers to invent
convenient energy sources to
power various small IoTs. Among
different types of photovoltaics,
organic photovoltaics (OPVs) are
promising candidates for this
application due to the mechanical
flexibility, printability, and tunable
light-absorption properties of
OPV panels. But most indoor OPV
materials suffer from several
disadvantages, such as
absorption spectra mismatch,
large voltage losses, and low
external quantum efficiencies.
Here, we developed a highperformance indoor non-fullerene
acceptor, named FCC-Cl, which
exhibits a suitable band gap, high
crystallinity, and high absorption
coefficient. As a result, a record
indoor power conversion
efficiency of 28.8% was achieved
by D18:FCC-Cl devices at 2,600 K
LED at 500 lux. Also, the high
thickness tolerance of PM6:FCCCl devices, which is a desirable
feature for roll-to-roll large-area
printing productions, was
demonstrated in this work.
INTRODUCTION
In the past several years, our community has seen the rapid development of the
internet of things (IoT), which creates a large demand for off-grid electricity sources
to power various indoor electronic devices, including sensors, blue-tooth devices,
and other wearable or smart electronic devices.1–3 Most IoT devices are used in
the indoor environment and only need electric power in the range of 1–100 mW.4
Photovoltaic (PV) devices can be good candidates for off-grid energy sources as
they can convert indoor luminous energy into electricity reasonably efficiently.5–11
Among different types of PV devices, organic photovoltaics (OPVs) offer several
attractive advantages such as low-cost and environmentally friendly production,
easy tunability of light-absorption properties, and compatibility with the roll-toroll printing process, making them an ideal choice for indoor PV applications.12–18
Despite a large number of research papers published for outdoor OPVs,19–31 limited
research efforts have concentrated on the molecular design and device optimization
of indoor OPV devices.32–34 State-of-the-art OPV materials are not necessarily suitable for indoor operations, as the design rules of photoactive materials and best-device fabrication conditions for indoor OPVs can be much different from those of outdoor OPVs. First, typical emission spectra of indoor-light sources range from 450 to
750 nm,35 which is much different from the global AM1.5G spectrum. In order to
achieve high-performance indoor OPVs, the active layer of OPV devices need to
have an absorption spectrum matching indoor-light sources. Second, OPV devices
Joule 5, 1231–1245, May 19, 2021 ª 2021 Elsevier Inc. 1231
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also need to minimize the leakage current and trap-assisted recombination in order
to achieve good efficiencies.33,36 The charge-carrier density in OPV devices is much
lower under indoor illumination owing to the low incident-light intensity, so the
leakage current and trap-assisted recombination can have more significant impacts
on indoor device performance. Finally, a high external quantum efficiency (EQE) and
low voltage loss are also necessary (for both indoor and outdoor applications) to
convert incident photons to electrons and minimize the energy loss of incident
photons.23,37,38
Most of the reported indoor OPV systems can only achieve open-circuit voltages
(VOC) of below 0.85 V under indoor illumination at 1,000 lux, which limits the indoor
power-conversion efficiency (PCE). The low VOC of those systems is attributed to the
absorption spectrum mismatch or the large voltage loss of the OPV materials.39–42
Recently, several indoor OPV material systems with high VOC were reported based
on the PDI-series acceptors, IO-4Cl, and ITCC.32–34,43,44 However, these materials
have low EQEs, possibly due to suboptimal morphology and less than perfect
charge separation at the D/A interface. Therefore, it is important for researchers
to develop high-performance indoor OPV materials that can achieve a low voltage
loss, a high and matching EQE spectrum, a low leakage current, and low trap-assisted recombination.
In this paper, we report an acceptor-donor-acceptor (A-D-A)-type non-fullerene
acceptor (NFA) named FCC-Cl (Figure 1A), using a fluorenedicyclopentathiophene
core with TIC-Cl end groups (2-(2-chloro-6-oxo-5,6-dihydro-4Hcyclopenta[b]thiophen-4-ylidene)-malononitrile). This molecule exhibits an absorption onset of
725 nm with an optical band gap (Egopt) of 1.71 eV. The FCC-Cl acceptor can be
combined with two reported donor polymers (D18 and PM6) and achieve efficiencies
of above 13% under one-sun conditions. Furthermore, D18:FCC-Cl blends display
several benefits: a high EQE of >85%, a matching absorption spectrum, a relatively
low voltage loss, and a low extent of charge recombination. As a result, an impressive indoor PCE of 28.8% was achieved under a 2,600 K LED at 500 lux, which is one
of the highest reported efficiencies for indoor OPVs. We also demonstrate that the
performance of PM6:FCC-Cl-based devices is insensitive to active-layer thicknesses.
The PCEs of the PM6:FCC-Cl-based devices were only reduced by 5% (from 27.9%
to 26.5%) when the thickness of the active layer was increased from 100 to 300 nm.
This is an imoprtant feature for the large-scale production of OPV devices. Our work
provides effective OPV material-design guidelines for developing high-performance
indoor OPV devices and proves the feasibility of practical applications for indoor
OPVs.
RESULTS AND DISCUSSION
Design and synthesis of the large-band-gap non-fullerene acceptor
State-of-art NFAs (such as IT4F and Y6) exhibit relatively small optical band gaps and
are not suitable for indoor applications.27,45,46 To tune the band gap of NFAs, it is
important to pay attention to the intramolecular charge transfer (ICT) between the
central core and terminal groups, which has been demonstrated to be vital in determining the absorption properties of the A-D-A-type NFAs. To achieve matching absorption spectra, the rational selection of the electron-donating central core and
electron-withdrawing terminal groups is key to obtaining suitable ICT effects. In
this study, an FDCT-C8 unit, which is an excellent weak electron-donating unit for
efficient large-gap NFAs, was used as the central core.13 In terms of terminal groups,
numerous studies have presented that introducing halogen atoms is significant for
1232 Joule 5, 1231–1245, May 19, 2021
1Department
of Chemistry, Guangdong-Hong
Kong-Macao Joint Laboratory of Optoelectronic
and Magnetic Functional Materials, Energy
Institute and Hong Kong Branch of Chinese
National Engineering Research Center for Tissue
Restoration & Reconstruction, Hong Kong
University of Science and Technology, Clear
Water Bay, Kowloon, Hong Kong
2State
Key Laboratory for Mechanical Behavior of
Materials, Xi’an Jiaotong University, Xi’an 710049,
P.R. China
3Hong
Kong University of Science and
Technology, Shenzhen Research Institute, No. 9
Yuexing first RD, Hi-tech Park, Nanshan,
Shenzhen 518057, P.R. China
4Institute
of Polymer Optoelectronic Materials
and Devices, State Key Laboratory of
Luminescent Materials and Devices, South China
University of Technology (SCUT), Guangzhou
510640, P.R. China
5MOE
Key Laboratory of Macromolecular
Synthesis and Functionalization, Department of
Polymer Science and Engineering, Zhejiang
University, Hangzhou 310027, China
6These
7Lead
authors contributed equally
contact
*Correspondence: msewma@xjtu.edu.cn (W.M.),
hyan@ust.hk (H.Y.)
https://doi.org/10.1016/j.joule.2021.03.020
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Figure 1. Molecular structures and ESP distribution of the materials
(A) Molecular structures of D18, PM6, and FCC-Cl.
(B) ESP distribution of D18, PM6, and FCC-Cl.
strengthening the ICT effects, enhancing intermolecular aggregation, and
improving film morphology of NFAs.47,48 We choose a chlorinated end-group,
TIC-Cl, which exhibits moderate electron-withdrawing properties and strong solidstate packing.49 Based on these units, we construct a large-band-gap NFA named
FCC-Cl. Figure S1 displays the chemical structure and the density-functional theory
(DFT) calculations results of FCC-Cl, F-4F, and ITCC-Cl. From DFT calculations, FCCCl is expected to have a larger band gap compared with its analogs, F-4F and ITCCCl. In addition, FCC-Cl has a neutral electrostatic potential (ESP), which is different
from the negative ESP of two large band-gap donor polymers, PM6 and D18 (Figure 1B). From previous reports, this could be beneficial for good charge transfer
from PM6 and D18 to FCC-Cl.32 UV-vis absorption measurement was carried out
to study the absorption properties of FCC-Cl. In spite of the relatively weak ICT,
FCC-Cl exhibits excellent absorption coefficients (2.00 3 105 M1 cm1 in solution
and 1.36 3 105 cm1 in films) (Figure 2B). The FCC-Cl neat film shows an intense absorption in the range of 450–720 nm with an Egopt of 1.71 eV, and a 41-nm bathochromic shift of absorption onset from solution to the film was observed, implying
the strong intermolecular p-p stacking in the solid state. The energy levels of the
materials were studied by cyclic voltammetry (CV) using ferrocene/ferrocenium
(Fc/Fc+, 4.80 eV) as the external standard. The LUMO and HOMO levels of FCCCl in the film state were estimated to be 3.71 and 5.73 eV, respectively, from
the onset reduction and oxidation potentials of the CV curves (Figures 2A and S2).
PV performance under one-sun conditions
High-performance donor polymers (D18 and PM6) were chosen to match with FCCCl, as they have deep HOMO levels and complementary absorption (Figure 2C). The
D/A blends based on FCC-Cl exhibit high photoluminescence (PL) quenching efficiencies (95.5% for D18:FCC-Cl and 98.3% for PM6:FCC-Cl) (Figure S3), revealing
the good charge transfer between the donors and acceptor. The device parameters
of D18:FCC-Cl and PM6:FCC-Cl are summarized in Table 1. As presented in the current-density-voltage (J-V) curves in Figure 2D, the D18:FCC-Cl devices show a
Joule 5, 1231–1245, May 19, 2021 1233
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Figure 2. Energy levels, absorption spectra, and photovoltaic properties of the materials
(A) Energy levels of the material systems.
(B) The absorption coefficients (a) versus wavelength spectra of FCC-Cl in solution and film.
(C) Normalized absorption spectra of D18, PM6 and FCC-Cl films.
(D) The J-V characteristic curves of D18:FCC-Cl and PM6:FCC-Cl under one-sun conditions.
(E) The EQE spectra of D18:FCC-Cl and PM6:FCC-Cl.
(F) The normalized emission spectra of a typical indoor-light source. LED, light-emitting diode; FL, fluorescent lamp.
slightly lower fill factor (FF) of 0.760 and short-circuit current density (JSC) of 16.00
mA/cm2 than those of PM6:FCC-Cl (FF = 0.781, JSC = 16.22 mA/cm2), while the
VOC of D18:FCC-Cl (1.08 V) is 0.06 V higher than that of PM6:FCC-Cl (1.02 V). To understand the VOC difference between D18:FCC-Cl and PM6:FCC-Cl, ultraviolet
photoelectron spectroscopy (UPS) was performed to accurately measure the
HOMO levels of D18 and PM6, which are determined to be 5.15 and 5.10 eV for
D18 and PM6, respectively (Figure S5). The deeper HOMO of D18 should be one
of the reasons for the higher VOC of the D18:FCC-Cl-based devices. In addition,
FCC-Cl may adopt different molecular stacking in the D18:FCC-Cl and PM6:FCCCl blend films, which may also influence the VOC of the devices.
In the EQE spectra (Figure 2E), both D18:FCC-Cl and PM6:FCC-Cl show a very high
spectral response in the range of 450–700 nm with the highest EQE values of around
85%. This broad and high spectral response covers almost all the emission spectra of
typical LED sources (Figure 2F), which satisfies the requirement of indoor applications. Owing to the high FF, EQE response, and relatively low VOC losses of both systems, high PCEs of 13% were achieved under one-sun illumination, which are the
highest reported PCEs for the NFAs with the Egopt of >1.7 eV.
Morphology study of pure and blend films
To understand the molecular packing of the FCC-Cl molecule in the neat and blend
films, grazing incidence wide-angle X-ray scattering (GIWAXS) measurements were
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Table 1. The optimal photovoltaic parameters of D18:FCC-Cl and PM6:FCC-Cl under AM1.5G
Active layer
VOC [V]
JSC [mA cm2]
FF [%]
PCEa [%]
JEQEb [mA cm2]
D18:FCC-Cl
1.08
16.04
76.0
13.1
15.90
(1.07 G 0.006)
(15.68 G 0.39)
(75.6 G 0.5)
(12.7 G 0.3)
1.02
16.22
78.1
13.0
(1.01 G 0.002)
(16.11 G 0.16)
(77.0 G 1.1)
(12.6 G 0.2)
PM6:FCC-Cl
a
16.00
The average device parameters and the standard deviations in parentheses are based on the measurement of over ten independent devices from two batches.
Calculated JSC values (JEQE) by the corresponding EQE spectrum.
b
performed. The two-dimensional GIWAXS patterns of the neat FCC-Cl, neat PM6,
and PM6:FCC-Cl blend films, and the corresponding scattering profiles in the inplane (IP) and out-of-plane (OOP) directions are shown in Figure 3, while the extracted morphology parameters are summarized in Table 2. The FCC-Cl neat film
(Figure 3A) present a predominant face-on orientation with an apparent p-p stacking peak located at 1.813 Å1 in the OOP direction (corresponding to a d-spacing of
3.47 Å), which reveals a close p-p stacking of the FCC-Cl molecules in solid state. In
the IP direction, a sharp (100) peak is observed at 0.360 Å1 (d-spacing = 17.48 Å),
which can be assigned to the lamellar stacking of FCC-Cl. It is noteworthy that the
crystal coherence lengths (CCLs) of the (010) and (100) peaks are 73.23 Å and
287.6 Å, respectively, which are larger than most of the reported NFAs.
The exceptionally large crystal size of FCC-Cl can be attributed to the linear alkyl
side chains and the chlorinated thiophene terminal groups that can enhance intermolecular packing significantly. In addition, the neat PM6 film (Figure 3B) displays
bimodal lamellar peaks in both IP and OOP directions at q = 0.292 Å1 (d-spacing =
21.54 Å) and a weaker p-p stacking in the OOP direction at q = 1.685 Å1 (dspacing = 3.73 Å). The PM6:FCC-Cl blend film (Figure 3C) shows two distinct diffraction peaks in the OOP direction at qz = 1.708 and 1.811 Å1, corresponding to the
(010) peaks of PM6 and FCC-Cl, respectively. This suggests that the donor and
acceptor adopt a preferred face-on orientation in the bulk heterojunction film, which
is beneficial for charge transport in the vertical direction across the electrodes. In the
IP direction, two prominent peaks at qxy = 0.297 and 0.353 Å1 are also ascribed to
the lamellar peaks of PM6 and FCC-Cl, respectively. The large CCLs of the p-p stacking (59.13 Å) and lamellar peaks (133.7 Å) of FCC-Cl maintained in the blend film
again indicate the strong aggregation property of FCC-Cl even in the blend film.
As a result, the strong aggregation property of FCC-Cl and the desirable blend
morphology lead to the high electron mobility of 8.0 3 104 cm2 V1 s1 (Figure S6)
and the FF of up to 78% of the devices.
The morphology of the D18:FCC-Cl and PM6:FCC-Cl blend films were further investigated by atomic-force microscopy (AFM). As shown in the height images in Figures
S7A and S7C, both films have smooth surfaces. The root-mean-square surface
roughness (Rq) are 1.32 and 1.55 nm for D18:FCC-Cl and PM6:FCC-Cl blend films,
respectively. The AFM phase images (Figures S7B and S7D) suggest that both films
form nanofibrillar morphologies, indicating the ideal phase segregation in the blend
films. All of these morphology features result in the high performance of the FCC-Clbased devices.
Photovoltaic performance under indoor-light conditions
For indoor OPV device testing, we used a similar procedure described in previous
work using white LED lamps (2,600, 3,000, 4,000, 6,500 K) as light sources.32 The device performance under the 2,600 K LED lamp was studied in detail, and the device
results under the other three lamps were shown in Figure S8; Table S4. In addition,
Joule 5, 1231–1245, May 19, 2021 1235
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Figure 3. Molecular packing behaviors of different films
(A–C) 2D GIWAXS patterns of FCC-Cl neat film (A), PM6 neat film(B), and PM6:FCC-Cl blend film (C).
(D) 1D profiles of different films.
the photovoltaic performance of the devices was investigated at 100, 500, 1,000,
1,600, 2,000 lux, which cover almost all the illumination conditions for indoor applications. The light intensity and emission spectra of the 2,600 K lamp (Figure 4C) were
measured by the fiber optics spectrometer, and the integrated light power density
for this 2,600 K LED lamp at 100, 500, 1,000, 1,600, 2,000 lux are 0.032, 0.159, 0.318,
0.509, 0.637 mW cm2, respectively.
Figures 4A and 4B shows the J-V characteristic curves of the D18:FCC-Cl and
PM6:FCC-Cl devices. Table 3 summarizes the device parameters of these two systems under different indoor-light conditions. It is noticed that although these two
systems yield similar PCEs under one-sun conditions, D18:FCC-Cl presents better
performance than PM6:FCC-Cl under indoor conditions. At an illumination of 500
lux, the D18:FCC-Cl devices show a PCE of 28.8% with a VOC of 0.936 V, a JSC of
61.6 mA cm2, and an FF of 0.795. Although the PM6:FCC-Cl devices have a similar
JSC of 61.1 mA cm2 and a slightly higher FF of 0.808, the lower VOC of 0.874 V limited
the PCE to 27.1%. As shown in Table 3, the PCEs of the OPV devices gradually increase with the increasing light intensities. This is because that the increased carrier
density at a higher light intensity reduces the effect of leakage current and trap-assisted recombination, as well as enlarged the difference between electron and hole
quasi-Fermi levels (see Note S1 for details).
In addition, the stability of D18:FCC-Cl devices was also tested under continuous
illumination by LED lamps at 500 lux (Figure 4F). The device was encapsulated under
N2 atmosphere before the stability test to block water and oxygen. The PCEs
1236 Joule 5, 1231–1245, May 19, 2021
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Table 2. Morphological parameters obtained from GIWAXS
(010) Peak
1
(100) Peak
Samples
qz [Å ]
d-spacing [Å]
FWHM
CL [Å]
qxy [Å1]
d-spacinga [Å]
FWHM
CLb [Å]
FCC-Cl
1.813
3.47
0.095
73.23
0.360
17.48
0.024
287.6
PM6
1.685
3.73
0.225
31.02
0.292
21.54
0.079
88.1
PM6:FCC-Cl
1.708
3.68
0.171
40.80
0.297
21.13
0.048
146.8
1.811
3.47
0.188
59.13
0.353
17.79
0.052
133.7
a
b
a
Obtained by the equation of d = 2p/q, in which q is the corresponding x-coordinate of diffraction peak.
b
Calculated using the equation: CL = 2pK/w, in which w is the full width at half maxima and K is a form factor.
maintained around 95% of the original values after 500-h indoor-light soaking,
demonstrating the feasibility of practical indoor applications. At such a low light intensity compared with one-sun illuminations, the decrease of the indoor PCE may
come from the imperfect encapsulation which may lead to oxygen and water ingress
or the contact problems when testing the devices.
Furthermore, the dependence of JSC on the light intensity was measured, and integrated current densities were calculated to verify our measurement results. First, the
bimolecular recombination in these OPV devices was studied by the dependence of
JSC on the light intensity, and the fitting slopes of JSC versus light intensity are 0.992
and 0.998 for D18:FCC-Cl and PM6:FCC-Cl, respectively (Figure S9), indicating the
negligible bimolecular recombination in these systems and ensures photon-to-electron conversion ratios keep constant with the decrease of light intensity. Then, the
integrated current densities can be calculated by EQE curves (Figure 2E) and photon
flux spectra at different light conditions. The details of the calculations are provided
in Note S2. From Table 3 and Figures 4D and 4E, it is found that the integrated current densities are roughly equivalent to the JSC values from the J-V characteristic
curves, which confirms the reliability of our indoor PV performance measurement.
Large-area devices were also fabricated, and the device results are shown in Figure S10 and Table S5. It is found that the efficiencies showed only a slight reduction
from the small-area (D18:FCC-Cl: 29.4%, 5.9 mm2; PM6:FCC-Cl: 27.9%, 5.9 mm2) to
large-area devices (D18:FCC-Cl: 28.5%, 85 mm2; PM6:FCC-Cl: 27.4%, 5.9 mm2) due
to the series-resistance insensitivity of device performance under indoor
conditions.32
Next, we investigated the reasons for the high indoor performance of FCC-Clbased devices. Our previous work has demonstrated that the leakage current
and the trap-assisted recombination have significant impacts on the VOC and FF
of indoor OPV devices since the charge-carrier density is extremely low under indoor conditions.33 For FCC-Cl-based devices, the leakage current and the trap-assisted recombination were investigated by dark current and the dependence of
VOC on light intensities. Figures 4G and 4H display the J-V characteristics of these
two blends under different light conditions, including the dark condition. Both systems exhibit low dark current, and the calculated shunt resistances were 5.8 3 106
and 6.2 3 106 U cm2 for D18:FCC-Cl and PM6:FCC-Cl, respectively, which should
be high enough and have negligible impacts on the VOC and FF of indoor OPV devices. The dependence of VOC on light intensities of these two blends was
measured under indoor conditions at different light intensities (Figure S11). The
ideal factors, n, calculated from the slope of the graph, were 1.23 and 1.09 for
D18:FCC-Cl and PM6:FCC-Cl, respectively. These two material systems have lower
ideal factors than two reported systems (1.26 for PM6: Y6-O and 1.69 for P3TEA:
FTTB-PDI4),33 which indicates the lower extent of trap-assisted recombination for
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Figure 4. Photovoltaic performances of D18:FCC-Cl and PM6:FCC-Cl under indoor illuminations
(A and B) The J-V characteristic curves of D18:FCC-Cl (A) and PM6:FCC-Cl (B) under indoor conditions with different light intensity.
(C) The emission power spectrum and integrated power density curve of the 2,600 K LED at 2000 lux.
(D and E) The photon flux spectrum of 2,600 K LED lamp at 2,000 lux and the integral current density of D18:FCC-Cl and (D) PM6:FCC-Cl (E) under 2,600 K
LED lamp at 2,000 lux.
(F) The normalized device parameters for D18:FCC-Cl versus time. The device was encapsulated in N 2 atmosphere before the stability test and stored at
a temperature of 25 C–30 C and a relative humidity around 60% under 500 lux LED lamp.
(G and H) The current density (log scale) as a function of the voltage for (G) D18:FCC-Cl and (H) PM6:FCC-Cl under different light conditions.
(I) The predication of PV performance under a 2,600 K LED at 1,000 lux.
D18:FCC-Cl and PM6:FCC-Cl. The low leakage current, high shunt resistance, and
the low trap-assisted recombination of these two systems ensure that even at a
dim light condition (100 lux), the devices can still perform decently and maintain
the PCEs of above 25%. In addition, we simulated the PCEs of indoor PV cells
with different band gap (Eg) and voltage loss (Figure 4I) based on the emission
spectrum of the 2600K LED lamp. The EQE curves are assumed to be a step function of 90%, and FF was fixed as 0.8 (most of the high-performance indoor PVs’ FFs
are around 0.80). The VOC can be calculated from Eg and voltage loss, and the minimum value of voltage loss was set to be 0.5 V according to the previous literature.32 As shown in Figure 4I, the optimal band gap of the active layer is 1.76 V
under this 2600K LED lamp, which is close to that of FCC-Cl. The suitable optical
band gap of FCC-Cl should also be one of the factors that lead to the high indoor
performance of FCC-Cl-based devices.
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Table 3. Device parameters of the OPVs under a 2,600 K LED lamp at different light intensities
Active
layer
Light
intensity
[lux]
Pin
[mW cm2]
D18:FCC-Cl
100
0.032
500
1,000
1,600
2,000
PM6:FCCCl
100
500
1,000
1,600
2,000
a
0.159
0.318
0.509
0.637
0.032
0.159
0.318
0.509
0.637
JSC [uA cm2]
Jcala
[uA cm2]
0.880
12.3
12.1
(0.871
G 0.008)
(12.1 G 0.4)
VOC [V]
0.936
61.6
(0.932
G 0.004)
(60.8 G 0.9)
0.955
123.0
(0.951
G 0.004)
(121.3 G 1.8)
0.969
196.5
(0.965
G 0.004)
(193.3 G 2.8)
0.975
245.4
(0.970
G 0.006)
(241.2 G 3.7)
0.829
12.2
(0.828
G 0.002)
(12.1 G 0.2)
0.874
61.1
(0.874
G 0.001)
(60.3 G 0.9)
0.895
122.4
(0.895
G 0.001)
(120.6 G 1.8)
0.906
195.3
(0.907
G 0.002)
(194.2 G 1.54)
0.914
244.1
(0.914
G 0.001)
(243.2 G 2.49)
60.4
120.8
193.2
241.6
12.1
60.4
120.9
193.4
241.7
FF [%]
Pout
[uW cm2]
PCEb [%]
75.3
8.2
25.5
(74.1 G 1.9)
(7.8 G 0.4)
(24.5 G 1.1)
79.5
45.8
28.8
(78.7 G 0.7)
(44.5 G 0.8)
(28.0 G 0.5)
79.8
93.5
29.4
(79.4 G 0.4)
(91.6 G 1.6)
(28.8 G 0.5)
80.1
152.2
29.9
(79.7 G 0.4)
(148.6 G 2.5)
(29.2 G 0.5)
80.1
191.7
30.1
(79.7 G 0.4)
(187.3 G 3.2)
(29.4 G 0.5)
79.8
8.1
25.3
(78.6 G 0.1)
(7.9 G 0.2)
(24.7 G 0.5)
80.8
43.1
27.1
(80.1 G 0.5)
(42.1 G 0.8)
(26.5 G 0.5)
81.1
88.7
27.9
(80.6 G 0.2)
(86.8 G 1.3)
(27.3 G 0.4)
81.1
143.0
28.1
(80.8 G 0.2)
(142.0 G 1.0)
(27.9 G 0.2)
81.2
181.5
28.5
(81.0 G 0.1)
(180.2 G 1.9)
(28.3 G 0.3)
Jcal was obtained by integrating the EQE spectrum over the light source.
The average device parameters and the standard deviations in parentheses are based on the measurement of over ten independent devices from two batches.
b
Photovoltaic performance of thick-film devices
We also study the influence of device thickness on OPV performance for the
PM6:FCC-Cl system. Figures 5B and 5C show the J-V characteristic curves and
EQE spectra of the devices with different film thicknesses, and Table 4 and Figures
5D and 5E summarize their corresponding device parameters. It is found that PCE
dropped significantly from 13.0% to only 9.0% under one-sun conditions when the
thickness increased from 100 to 400 nm. However, under indoor conditions, the efficiencies only exhibited a small decrease from 27.9% to 25.8% for the same thickness change. Among the three main device parameters, the FF dropped significantly
from 0.781 to 0.521 under one-sun conditions but displayed a much smaller
decrease from 0.811 to 0.775 under indoor conditions. On the other hand, the
VOC and JSC only displayed a small difference under both indoor and outdoor
conditions.
To better understand the large difference in FF between one-sun and indoor
condition, we performed the calculation of FF based on empirical expressions
within the context of the equivalent circuit model. According to previous literature,50 for the case where shunt resistance (Rsh) is so large as to be negligible,
but series resistance (Rs) is essential, the FF can be simplified into the following
expressions:
Joule 5, 1231–1245, May 19, 2021 1239
ll
Article
Figure 5. Photovoltaic performances of PM6:FCC-Cl with different thicknesses under AM1.5G condition and 2,600 K LED at 1,000 lux
(A) The device architecture of PM6:FCC-Cl.
(B) The J-V characteristic curves of D18:FCC-Cl and PM6:FCC-Cl with different thicknesses under solar simulator and 2,600 K LED at 1,000 lux.
(C) The EQE spectra of PM6:FCC-Cl with different thicknesses.
(D) The FF and normalized PCE versus thickness plots under AM1.5G condition and 2,600 K LED at 1,000 lux.
(E) The V OC and normalized J SC versus thickness plots under AM1.5G condition and 2,600 K LED at 1,000 lux.
(F) The normalized predicted FF and FF from J-V curves versus the series resistance.
FF0 =
gOC lnðgOC + 0:72Þ
goc + 1
FFS = FF0 ð1 gs Þ
gs =
Rch =
(Equation 1)
(Equation 2)
Rs
Rch
(Equation 3)
VOC
AJSC
(Equation 4)
where gOC is the VOC normalized to the thermal voltage (gOC = e VOC/nkT, n being
the diode ideality factor), gs is the normalized series resistance, Rch is the characteristic resistance for the device, and A is the effective area of the device. From
the dark current curves (Figure S12), the Rsh of PM6:FCC-Cl at 100 and 400 nm
was calculated to be 5.6 3 106 and 7.6 3 106 U cm2, respectively, which are
very large and have a negligible influence on FF either in one-sun condition or in
indoor-light condition at 1,000 lux, so the FF of PM6:FCC-Cl can be calculated
based on these equations. The calculated FF versus Rs is shown in Figure 5F. It
is found that the FF remains almost constant under indoor-light conditions but
was reduced significantly under one-sun conditions in the range from 1 to 10 U
cm2, where the Rs of PM6:FCC-Cl with different thicknesses is located. Besides,
the calculated FF values shows a similar descending trend to the experimental results, which reveals that the large series resistance is one of the main reasons for
the decrease of FF under one-sun conditions yet does not have a significant impact
on FF under indoor conditions.
1240 Joule 5, 1231–1245, May 19, 2021
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Article
Table 4. Device parameters of the PM6:FCC-Cl with different thicknesses under AM1.5G condition and a 2,600 K LED at 1,000 lux
Illumination
condition
Thickness
[nm]
Pin
[mW cm2]
AM1.5G
100
100
200
300
400
2,600 K at
1,000 lux
100
0.318
200
300
400
a
JSC
[mA cm2]
Jcala
[mA cm2]
1.022
16.22
16.00
(1.015 G
0.002)
(16.11
G 0.16)
VOC [V]
1.010
16.84
(1.011 G
0.001)
(16.46
G 0.41)
1.004
17.24
(1.000 G
0.003)
(17.01
G 0.21)
0.995
17.35
(0.993 G
0.003)
(17.03
G 0.32)
0.895
0.1224
(0.895 G
0.001)
(0.1206 G
0.0018)
0.885
0.1199
(0.885 G
0.003)
(0.1188 G
0.0017)
0.882
0.1210
(0.881 G
0.003)
(0.1203 G
0.0011)
0.878
0.1206
(0.876 G
0.003)
(0.1195 G
0.0021)
FF [%]
Pout [uW cm2]
78.1
–
(77.0 G 1.1)
16.75
17.23
17.36
0.1209
0.1185
0.1200
0.1196
PCEb [%]
13.0
(12.6 G 0.2)
72.1
12.3
(73.1 G 1.0)
(12.2 G 0.1)
62.8
10.9
(60.9G2.4)
(10.4 G 0.4)
52.1
9.0
(52.0 G 2.2)
(8.7 G 0.4)
81.1
88.7
27.9
(80.6 G 0.2)
(86.8 G 1.3)
(27.3 G 0.4)
80.0
84.6
26.6
(77.9 G 1.7)
(81.7 G 1.9)
(25.7 G 0.6)
79.2
84.3
26.5
(77.0 G 1.4)
(81.4 G 1.6)
(25.6 G 0.5)
77.5
82.0
25.8
(75.5 G 1.9)
(78.9 G 2.5)
(24.8 G 0.8)
Jcal was obtained by integrating the EQE spectrum over the light source.
The average device parameters and the standard deviations in parentheses are based on the measurement of over ten independent devices from two batches.
b
Conclusion
In this work, we designed and synthesized a NFA named FCC-Cl with an Egopt of 1.71
eV. FCC-Cl was blended with two widely used donor polymers (D18 and PM6), and
both systems showed PCEs over 25% under indoor illumination simulated by a 2,600
K LED at 100–2,000 lux. The D18:FCC-Cl device exhibits a high EQE of up to 85%
and a high FF of 80% due to the high absorption coefficient and strong crystallinity
of FCC-Cl. As a result, an impressive PCE of 28.8% was achieved by D18:FCC-Cl at
500 lux, which is one of the highest reported indoor performances for OPVs. The
thick-film devices based on PM6:FCC-Cl were also fabricated, and the devices
with a thickness of 300 nm can still maintain a PCE of 26.5% under indoor illumination
at 1,000 lux. This high thickness tolerance of indoor OPV devices is a desirable
feature for roll-to-roll large-area printing productions. Overall, our study presents
an effective indoor OPV material design guideline for developing high-performance
indoor OPV devices and proves the feasibility of indoor OPVs’ practical applications.
EXPERIMENTAL PROCEDURES
Resource availability
Lead contact
Further information and requests for resources and materials should be directed to
and will be fulfilled by the lead contact, He Yan (hyan@ust.hk)
Materials availability
All chemicals, unless otherwise specified, were purchased from commercial resources and used as received. The donor polymer PM6, and the interlayer PDI-NO
was purchased from Solarmer Material Inc. Diethyl zinc solution, chloroform and molybdenum trioxide was purchased from Sigma-Aldrich. The donor polymer D18 was
purchased from eFlexPV and used directly. Toluene and THF were distilled from
Joule 5, 1231–1245, May 19, 2021 1241
ll
sodium benzophenone under nitrogen before use. The detailed synthetic procedure
of FCC-Cl is provided in the Supplemental materials. ITO was purchased from
Shaanxi Fangdecheng Construction Engineering Company. The sheet resistance is
15 U and the transmission is around 93%.
Data and code availability
There is no dataset and/or code associated with the paper.
Device fabrication
The patterned ITO-coated glass was scrubbed by detergent and then cleaned inside
an ultrasonic bath by using deionized water, acetone, and isopropyl alcohol sequentially and dried overnight in an oven. Before use, the glass substrates were treated in
a UV-Ozone Cleaner for 20 min to improve its work function and clearance. For D18
based devices, a thin PEDOT: PSS (Heraeus Clevios P VPA 4083) layer with a thickness of about 40 nm was spin-coat onto the ITO substrates at 4,000 rpm for 40 s,
and then dried at 150 C for 10 min in air. The PEDOT: PSS coated ITO substrates
were transferred to a N2-filled glove box for further processing. D18 and FCC-Cl
with a ratio of 1:1.5 was dissolved in chloroform with a total concentration of
7.5 mg/ml. Then the solution was stirred at 60 C for 1 h in a nitrogen-filled glove
box to completely dissolve the materials. The blend solution was spin-cast on the
top of PEDOT: PSS layer at 600–1,000 rpm for 40 s. Then it was treated with solvent
vapor annealing by chloroform for 5 min to elaborately tune the morphology of the
blend films. The PDI-NO in alcohol with a concentration of 1 mg/ml was then spincoated on the top of the active layer at 3,000 rpm. Then, the active layer coated substrates were quickly transferred to a glove-box integrated thermal evaporator for
electrode deposition. Al layer (100 nm) were sequentially evaporated under the vacuum of 5 3 105 Pa through a shadow mask. The optimal blend thickness measured
on a Woollam Alpha-SE ellipsometer was about 100 nm. For PM6 based devices, a
ZnO electron transport layer was prepared by spin coating at 5,000 rpm from a ZnO
precursor solution (diethyl zinc). Active layer solutions (D/A ratio 1:1 by weight) were
prepared in chloroform with 0.5% DIO. To completely dissolve the polymer, the
active layer solution should be stirred on a hot plate at 60 C for at least 1 h. Active
layers were spin coated on the ZnO substrates in a N2 glovebox at 600–3,000 rpm
with polymer concentration range from 7 to 10 mg/ml to obtain different thicknesses. The blend films were then annealed at 100 C for 5 min before being transferred to the vacuum chamber of a thermal evaporator inside the same glovebox.
At a vacuum level of 5 3 10–5 Pa, a thin layer (10 nm) of MoO3 was deposited as
the anode interlayer, followed by deposition of 100 nm of Ag as the top electrode.
The blend thickness measured by a Woollam Alpha-SE ellipsometer. All cells were
encapsulated using epoxy inside the glovebox.
Device performance characterization
The current-voltage (J-V) characteristic curves of all packaged devices were
measured by using a Keithley 2400 Source Meter in air and were measured in the forward direction from 0.2 to 1.2 V, with a scan step of 10 mV and a dwell time of 5 ms.
For the one-sun performance, the photocurrent was measured under AM 1.5G (100
mW cm2) using a Newport solar simulator in an air. The light intensity was calibrated
using a standard Si diode (with KG5 filter, purchased from PV Measurement) to bring
spectral mismatch to unity. For the indoor performance, J-V characteristic curves
were measured under intensity adjustable LEDs (2,600, 3,000, 4,000, 6,500 K) in
dark room at room temperature. The emission power spectrum of this LED at
different light intensity was measured by a fiber optics spectrometer. All the cells
are measured with a 5.9 mm2 shadow mask and the effective areas of the cells
1242 Joule 5, 1231–1245, May 19, 2021
Article
ll
Article
were 7.0 mm2. For the large-area devices, the cells are measured with an 85 mm2
shadow mask and the effective areas of the cells were 100 mm2. The area of the cells
and masks was determined by an optical microscope. The epoxy was coated on the
devices and were solidified by 5 min UV-light illumination to do the encapsulation.
And then the cells were measured under a temperature of 25 C–30 C in the air condition after encapsulation. EQEs were measured using an Enlitech QE-S EQE system
equipped with a standard Si diode. Monochromatic light was generated from a
Newport 300 W lamp source.
SUPPLEMENTAL INFORMATION
Supplemental information can be found online at https://doi.org/10.1016/j.joule.
2021.03.020.
ACKNOWLEDGMENTS
The work described in this paper was partially supported by the National Key
Research and Development Program of China (no. 2019YFA0705900) funded by
MOST, the Basic and Applied Basic Research Major Program of Guangdong Province (no. 2019B030302007), Guangdong-Hong Kong-Macao Joint Laboratory of
Optoelectronic and Magnetic Functional Materials (project number
2019B121205002), the Shen Zhen Technology and Innovation Commission (project
number JCYJ20170413173814007 and JCYJ20170818113905024), the Hong Kong
Research Grants Council (Research Impact Fund R6021-18, collaborative research
fund C6023-19G, project numbers 16309218, 16310019, and 16303917), Hong
Kong Innovation and Technology Commission for the support through projects
ITC-CNERC14SC01 and ITS/471/18), National Natural Science Foundation of China
(NSFC, no. 91433202).
AUTHOR CONTRIBUTIONS
F.B. and H.Y. conceived the idea and designed the experiments; F.B. synthesized
the acceptor material FCC-Cl, fabricated the solar cells, and carried out the device-performance measurements; J.Z. designed the synthesis route of FCC-Cl;
A.Z. synthesized the polymer donors; H.Z. performed the morphology analysis supervised by W.M.; and K.D. and H.Y. measured the emission spectra of LED lamps.
F.B., J.Z., and H.Y. wrote the paper. All the authors discussed the results and substantially contributed to the preparation of the manuscript.
DECLARATION OF INTERESTS
The authors declare no competing interests.
Received: December 30, 2020
Revised: February 15, 2021
Accepted: March 26, 2021
Published: April 28, 2021
REFERENCES
1. Gubbi, J., Buyya, R., Marusic, S., and
Palaniswami, M. (2013). Internet of things (IoT):
a vision, architectural elements, and future
directions. Future Gener. Comput. Syst. 29,
1645–1660.
2. Al-Fuqaha, A., Guizani, M., Mohammadi,
M., Aledhari, M., and Ayyash, M. (2015).
Internet of things: a survey on enabling
technologies, protocols, and applications.
IEEE Commun. Surv. Tutorials 17, 2347–
2376.
3. Jeon, K.E., She, J., Soonsawad, P., and Ng, P.C.
(2018). BLE beacons for Internet of things
applications: survey, challenges, and
opportunities. IEEE Internet Things J 5,
811–828.
4. Klinefelter, A., Roberts, N.E., Shakhsheer, Y.,
Gonzalez, P., Shrivastava, A., Roy, A., Craig,
K., Faisal, M., Boley, J., Oh, S., et al. (2015).
21.3 A 6.45mW self-powered IoT SoC with
integrated energy-harvesting power
management and ULP asymmetric radios.
2015 IEEE International Solid-State Circuits
Conference - (ISSCC) Digest of Technical
Papers, pp. 1–3.
Joule 5, 1231–1245, May 19, 2021 1243
ll
5. Yue, X., Kauer, M., Bellanger, M., Beard, O.,
Brownlow, M., Gibson, D., Clark, C.,
MacGregor, C., and Song, S. (2017).
Development of an indoor photovoltaic energy
harvesting module for autonomous sensors in
building air quality applications. IEEE Internet
Things J 4, 2092–2103.
6. Foti, M., Tringali, C., Battaglia, A., Sparta, N.,
Lombardo, S., and Gerardi, C. (2014). Efficient
flexible thin film silicon module on plastics for
indoor energy harvesting. Sol. Energy Mater.
Sol. Cells 130, 490–494.
7. Mathews, I., King, P.J., Stafford, F., and Frizzell,
R. (2016). Performance of III–V solar cells as
indoor light energy harvesters. IEEE J.
Photovoltaics 6, 230–235.
8. Mathews, I., Kantareddy, S.N., Buonassisi, T.,
and Peters, I.M. (2019). Technology and market
perspective for indoor photovoltaic cells. Joule
3, 1415–1426.
9. Freitag, M., Teuscher, J., Saygili, Y., Zhang, X.,
Giordano, F., Liska, P., Hua, J., Zakeeruddin,
S.M., Moser, J.-E., Grätzel, M., and Hagfeldt, A.
(2017). Dye-sensitized solar cells for efficient
power generation under ambient lighting.
Nature Photon 11, 372–378.
10. Cui, M., Li, D., Du, X., Li, N., Rong, Q., Li, N.,
Shui, L., Zhou, G., Wang, X., Brabec, C.J., and
Nian, L. (2020). A cost-effective,
aqueous-solution-processed cathode
interlayer based on organosilica nanodots for
highly efficient and stable organic solar cells.
Adv. Mater. 32, e2002973.
11. Bai, F., Zhang, J., Yuan, Y., Liu, H., Li, X., Chueh,
C.C., Yan, H., Zhu, Z., and Jen, A.K. (2019). A
0D/3D heterostructured all-inorganic halide
perovskite solar cell with high performance and
enhanced phase stability. Adv. Mater. 31,
e1904735.
12. Liu, J., Chen, S., Qian, D., Gautam, B., Yang, G.,
Zhao, J., Bergqvist, J., Zhang, F., Ma, W., Ade,
H., et al. (2016). Fast charge separation in a
non-fullerene organic solar cell with a small
driving force. Nat. Energy 1, 16089.
13. Meng, L., Zhang, Y., Wan, X., Li, C., Zhang, X.,
Wang, Y., Ke, X., Xiao, Z., Ding, L., Xia, R., et al.
(2018). Organic and solution-processed
tandem solar cells with 17.3% efficiency.
Science 361, 1094–1098.
14. Zhao, J., Li, Y., Yang, G., Jiang, K., Lin, H., Ade,
H., Ma, W., and Yan, H. (2016). Efficient organic
solar cells processed from hydrocarbon
solvents. Nat. Energy 1, 15027.
15. Kim, S.M., Kim, C.H., Kim, Y., Kim, N., Lee, W.J.,
Lee, E.H., Kim, D., Park, S., Lee, K., Rivnay, J.,
and Yoon, M.H. (2018). Influence of PEDOT:PSS
crystallinity and composition on
electrochemical transistor performance and
long-term stability. Nat. Commun. 9, 3858.
16. Fan, B., Du, X., Liu, F., Zhong, W., Ying, L., Xie,
R., Tang, X., An, K., Xin, J., Li, N., et al. (2018).
Fine-tuning of the chemical structure of
photoactive materials for highly efficient
organic photovoltaics. Nat. Energy 3, 1051–
1058.
17. Zhang, J., Tan, H.S., Guo, X., Facchetti, A., and
Yan, H. (2018). Material insights and challenges
for non-fullerene organic solar cells based on
small molecular acceptors. Nat. Energy 3,
720–731.
1244 Joule 5, 1231–1245, May 19, 2021
Article
18. Liao, C.-Y., Chen, Y., Lee, C.-C., Wang, G.,
Teng, N.-W., Lee, C.-H., Li, W.-L., Chen, Y.-K.,
Li, C.-H., Ho, H.-L., et al. (2020). Processing
strategies for an organic photovoltaic module
with over 10% efficiency. Joule 4, 189–206.
19. Burlingame, Q., Huang, X., Liu, X., Jeong, C.,
Coburn, C., and Forrest, S.R. (2019). Intrinsically
stable organic solar cells under high-intensity
illumination. Nature 573, 394–397.
20. Che, X., Li, Y., Qu, Y., and Forrest, S.R. (2018).
High fabrication yield organic tandem
photovoltaics combining vacuum- and
solution-processed subcells with 15%
efficiency. Nat. Energy 3, 422–427.
21. Zhou, Z., Xu, S., Song, J., Jin, Y., Yue, Q., Qian,
Y., Liu, F., Zhang, F., and Zhu, X. (2018). Highefficiency small-molecule ternary solar cells
with a hierarchical morphology enabled by
synergizing fullerene and non-fullerene
acceptors. Nat. Energy 3, 952–959.
22. He, Z., Zhong, C., Su, S., Xu, M., Wu, H., and
Cao, Y. (2012). Enhanced power-conversion
efficiency in polymer solar cells using an
inverted device structure. Nature Photo 6,
591–595.
23. Liu, S., Yuan, J., Deng, W., Luo, M., Xie, Y.,
Liang, Q., Zou, Y., He, Z., Wu, H., and Cao, Y.
(2020). High-efficiency organic solar cells with
low non-radiative recombination loss and low
energetic disorder. Nat. Photonics 14, 300–305.
24. Zuo, G., Linares, M., Upreti, T., and Kemerink,
M. (2019). General rule for the energy of waterinduced traps in organic semiconductors. Nat.
Mater. 18, 588–593.
25. Qian, D., Zheng, Z., Yao, H., Tress, W., Hopper,
T.R., Chen, S., Li, S., Liu, J., Chen, S., Zhang, J.,
et al. (2018). Design rules for minimizing
voltage losses in high-efficiency organic solar
cells. Nat. Mater. 17, 703–709.
26. Ye, L., Hu, H., Ghasemi, M., Wang, T., Collins,
B.A., Kim, J.H., Jiang, K., Carpenter, J.H., Li, H.,
Li, Z., et al. (2018). Quantitative relations
between interaction parameter, miscibility and
function in organic solar cells. Nat. Mater. 17,
253–260.
27. Yuan, J., Zhang, Y., Zhou, L., Zhang, G., Yip,
H.-L., Lau, T.-K., Lu, X., Zhu, C., Peng, H.,
Johnson, P.A., et al. (2019). Single-junction
organic solar cell with over 15% efficiency using
fused-ring acceptor with electron-deficient
core. Joule 3, 1140–1151.
28. Yao, H., Bai, F., Hu, H., Arunagiri, L., Zhang, J.,
Chen, Y., Yu, H., Chen, S., Liu, T., Lai, J.Y.L.,
et al. (2019). Efficient all-polymer solar cells
based on a new polymer acceptor achieving
10.3% power conversion efficiency. ACS
Energy Lett 4, 417–422.
29. Yu, H., Qi, Z., Zhang, J., Wang, Z., Sun, R.,
Chang, Y., Sun, H., Zhou, W., Min, J., Ade, H.,
and Yan, H. (2020). Tailoring non-fullerene
acceptors using selenium-incorporated
heterocycles for organic solar cells with over
16% efficiency. J. Mater. Chem. A 8, 23756–
23765.
30. Zhang, J., Li, Y., Huang, J., Hu, H., Zhang, G.,
Ma, T., Chow, P.C.Y., Ade, H., Pan, D., and Yan,
H. (2017). Ring-fusion of perylene diimide
acceptor enabling efficient nonfullerene
organic solar cells with a small voltage loss.
J. Am. Chem. Soc. 139, 16092–16095.
31. Swick, S.M., Gebraad, T., Jones, L., Fu, B.,
Aldrich, T.J., Kohlstedt, K.L., Schatz, G.C.,
Facchetti, A., and Marks, T.J. (2019). Building
blocks for high-efficiency organic
photovoltaics: interplay of molecular, crystal,
and electronic properties in post-fullerene ITIC
ensembles. ChemPhysChem 20, 2608–2626.
32. Cui, Y., Wang, Y., Bergqvist, J., Yao, H., Xu, Y.,
Gao, B., Yang, C., Zhang, S., Inganäs, O., Gao,
F., and Hou, J. (2019). Wide-gap non-fullerene
acceptor enabling high-performance organic
photovoltaic cells for indoor applications. Nat.
Energy 4, 768–775.
33. Ma, L.-K., Chen, Y., Chow, P.C.Y., Zhang, G.,
Huang, J., Ma, C., Zhang, J., Yin, H., Hong
Cheung, A.M., Wong, K.S., et al. (2020). Highefficiency indoor organic photovoltaics with a
band-aligned interlayer. Joule 4, 1486–1500.
34. Cui, Y., Yao, H., Zhang, T., Hong, L., Gao, B.,
Xian, K., Qin, J., and Hou, J. (2019). 1 cm2
organic photovoltaic cells for indoor
application with over 20% efficiency. Adv.
Mater. 31, e1904512.
35. Mori, S., Gotanda, T., Nakano, Y., Saito, M.,
Todori, K., and Hosoya, M. (2015). Investigation
of the organic solar cell characteristics for
indoor LED light applications. Jpn. J. Appl.
Phys. 54, 071602.
36. Gupta, V., Kyaw, A.K., Wang, D.H., Chand, S.,
Bazan, G.C., and Heeger, A.J. (2013). Barium:
an efficient cathode layer for bulkheterojunction solar cells. Sci. Rep. 3, 1965.
37. Cui, Y., Yao, H., Zhang, J., Zhang, T., Wang, Y.,
Hong, L., Xian, K., Xu, B., Zhang, S., Peng, J.,
et al. (2019). Over 16% efficiency organic
photovoltaic cells enabled by a chlorinated
acceptor with increased open-circuit voltages.
Nat. Commun. 10, 2515.
38. Zhang, G., Chen, X.K., Xiao, J., Chow, P.C.Y.,
Ren, M., Kupgan, G., Jiao, X., Chan, C.C.S., Du,
X., Xia, R., et al. (2020). Delocalization of exciton
and electron wavefunction in non-fullerene
acceptor molecules enables efficient organic
solar cells. Nat. Commun. 11, 3943.
39. Lechêne, B.P., Cowell, M., Pierre, A., Evans,
J.W., Wright, P.K., and Arias, A.C. (2016).
Organic solar cells and fully printed supercapacitors optimized for indoor light energy
harvesting. Nano Energy 26, 631–640.
40. Ann, M.H., Kim, J., Kim, M., Alosaimi, G., Kim,
D., Ha, N.Y., Seidel, J., Park, N., Yun, J.S., and
Kim, J.H. (2020). Device design rules and
operation principles of high-power perovskite
solar cells for indoor applications. Nano Energy
68, 104321.
41. Lee, H.K.H., Wu, J., Barbé, J., Jain, S.M., Wood,
S., Speller, E.M., Li, Z., Castro, F.A., Durrant,
J.R., and Tsoi, W.C. (2018). Organic
photovoltaic cells – promising indoor light
harvesters for self-sustainable electronics.
J. Mater. Chem. A 6, 5618–5626.
42. Singh, R., Chochos, C.L., Gregoriou, V.G.,
Nega, A.D., Kim, M., Kumar, M., Shin, S.C., Kim,
S.H., Shim, J.W., and Lee, J.J. (2019). Highly
efficient indoor organic solar cells by voltage
loss minimization through fine-tuning of
polymer structures. ACS Appl. Mater.
Interfaces 11, 36905–36916.
43. Yin, H., Ma, L.K., Yan, J., Zhang, Z.,
Cheung, A.M.H., Zhang, J., Yan, H., and
ll
Article
So, S.K. (2020). Thick-film low
driving-force indoor light harvesters. Sol.
RRL 4, 2000291.
44. Ding, Z., Zhao, R., Yu, Y., and Liu, J. (2019). Allpolymer indoor photovoltaics with high opencircuit voltage. J. Mater. Chem. A 7, 26533–
26539.
45. Lai, H., Zhao, Q., Chen, Z., Chen, H., Chao, P.,
Zhu, Y., Lang, Y., Zhen, N., Mo, D., Zhang, Y.,
and He, F. (2020). Trifluoromethylation enables
a 3D interpenetrated low-band-gap acceptor
for efficient organic solar cells. Joule 4,
688–700.
46. Zhao, W., Li, S., Yao, H., Zhang, S., Zhang, Y.,
Yang, B., and Hou, J. (2017). Molecular
optimization enables over 13% efficiency in
organic solar cells. J. Am. Chem. Soc. 139,
7148–7151.
48. Zhang, H., Yao, H., Hou, J., Zhu, J., Zhang, J., Li,
W., Yu, R., Gao, B., Zhang, S., and Hou, J.
(2018). Over 14% efficiency in organic solar cells
enabled by chlorinated nonfullerene smallmolecule acceptors. Adv. Mater. 30, e1800613.
47. Aldrich, T.J., Matta, M., Zhu, W., Swick, S.M.,
Stern, C.L., Schatz, G.C., Facchetti, A.,
Melkonyan, F.S., and Marks, T.J. (2019).
Fluorination effects on
Indacenodithienothiophene acceptor
packing and electronic structure, endgroup redistribution, and solar cell
photovoltaic response. J. Am. Chem. Soc. 141,
3274–3287.
49. Zhang, J., Li, Y., Hu, H., Zhang, G., Ade, H., and
Yan, H. (2019). Chlorinated thiophene end
groups for highly crystalline alkylated nonfullerene acceptors toward efficient organic
solar cells. Chem. Mater. 31, 6672–6676.
50. Green, M.A. (1981). Solar cell fill factors:
general graph and empirical expressions. Solid
State Electron 24, 788–789.
Joule 5, 1231–1245, May 19, 2021 1245
Joule, Volume 5
Supplemental information
A highly crystalline non-fullerene acceptor
enabling efficient indoor organic photovoltaics
with high EQE and fill factor
Fujin Bai, Jianquan Zhang, Anping Zeng, Heng Zhao, Ke Duan, Han Yu, Kui Cheng, Gaoda
Chai, Yuzhong Chen, Jiaen Liang, Wei Ma, and He Yan
Experimental Procedures
Optical Characterization
Film UV-vis absorption spectra were acquired on a PerkinElmer Lambda 20 UV/VIS
Spectrophotometer. All film samples were spin-cast on ITO substrates. The UV-Vis absorption
spectrum of solution was collected from the solution of FCC-Cl with a concentration of 1×105
M in chloroform. A cuvette with a stopper (Sigma Z600628) was used to avoid volatilization
during the measurement.
GIWAXS Characterization
GIWAXS measurements were performed at SAXS/WAXS beamline, Australian Synchrotron
ANSTO. Samples were prepared on Si substrates using identical blend solutions as those used
in devices. The 15.2 keV X-ray beam was incident at a grazing angle of 0.08°-0.12°, selected
to maximize the scattering intensity from the samples. The scattered x-rays were detected using
a Dectris Pilatus 2M photon counting detector. In-plane and out-of-plane sector averages were
calculated using the Nika software package. The uncertainty for the peak fitting of the
GIWAXS data is 0.3 Å. The coherence length was calculated using the Scherrer equation: CL
= 2πK/Δq, where Δq is the full width at half-maximum of the peak and K is a shape factor
(1.11 was used here).
Electrochemical Characterizations
Cyclic voltammetry was carried out on a CHI610E electrochemical workstation with three
electrodes configuration, using Ag/AgCl as the reference electrode, a Pt plate as the counter
electrode, and a glassy carbon as the working electrode. 0.1 mol/L tetrabutylammonium
hexafluorophosphate in acetonitrile was used as the supporting electrolyte. FCC-Cl were dropcast onto the glassy carbon electrode from chloroform solutions (5 mg/mL) to form thin films.
Potentials were referenced to the ferrocenium/ferrocene couple by using ferrocene as external
standards in acetonitrile solutions. The scan rate is 100 mV/s. The HOMO energy levels were
determined by EHOMO = - [q (Ere – Eferrocene) + 4.8 eV], while the LUMO energy levels were
determined by ELUMO = - [q (Eox – Eferrocene) + 4.8 eV]. The LUMO of D18 and PM6 are
1
calculated by the difference between the optical bandgap of the films and the HOMO of the
materials.
SCLC Measurements
The electron and hole mobility of FCC-Cl blend films were measured by using the method of
space-charge limited current (SCLC). The electron-only SCLC device was a stack of
ITO/ZnO/active layer/ PDINO/Al, and the hole-only SCLC device was a stack of
ITO/PEDOT/active layer/MoO3/Al. The electron-only and hole-only SCLC devices fabricating
methods were the same as those for OSCs. The charge carrier mobility was determined by
fitting the dark current to the model of a single carrier SCLC according to the equation:
J=9ε0εrμV2/8d3, where J is the current density, d is the film thickness of the active layer, μ is
the charge carrier mobility, εr is the relative dielectric constant of the transport medium, and ε0
is the permittivity of free space. V= Vapp-Vbi, where Vapp is the applied voltage, Vbi is the offset
voltage. The carrier mobility can be calculated from the slope of the J1/2~V curves.
Ultraviolet Photoelectron Spectroscopy Measurements
Ultraviolet photoelectron spectroscopy (UPS) measurements were performed with an
unfiltered HeI (21.22 eV) gas discharge lamp. All film samples were spin-cast on ITO
substrates. The HOMOs of D18 and PM6 was calculated to be 5.15 eV and 5.10 eV,
respectively, by subtracting the width of the He I UPS spectra (Figure S5) from the excitation
energy (21.22 eV).
Shunt Resistance Measurements
The shunt resistance of the FCC-Cl-based devices were obtained from the dark J-V curves.
These curves can be divided into three regions according to the change of the slope: parallel
resistance region, injection region and series resistance region. The behavior of the curve in
these regions is dominated by the shunt resistance, diode ideality, and series resistance,
respectively. The differential resistance in the parallel resistance region is equal to the
corresponding shunt resistance [Figure S12(C-D)].1
2
Materials and Synthesis
TIC-Cl was synthesized according to our previous work and the isomer was further purified by
column. Compound 1 was synthesized according to the literature.2
FCC-Cl was synthesized according to the same method with our previous work (Figure S14).
Compound 1 (155 mg, 0.116 mmol) and TIC-Cl (135.7 mg, 0.578 mmol) were dissolved in
anhydrous CHCl3 (25 mL) under N2. Anhydrous pyridine (2.5 mL) was then added and the
mixture was stirred and refluxed overnight. The resulting mixture was poured into water (50ml)
and then extracted with chloroform (20ml) twice. The combined organic layer was washed with
brine (30ml) and dried over MgSO4. After removing the solvent by rotary evaporator, the crude
product was purified on a silica-gel column chromatography (eluent: hexane: dichloromethane
= 1:2) to afford FCC-Cl as a blue solid (151 mg, 86%). 1H NMR (400 MHz, CDCl3) δ 8.72 (s,
2H), 7.85 (s, 2H), 7.68 (d, J = 4.6 Hz, 4H), 7.59 (s, 2H), 2.13 – 1.90 (m, 12H), 1.10 (d, J = 25.8
Hz, 66H), 0.83 – 0.65 (m, 24H). 13C NMR (101 MHz, CDCl3) δ 180.25 (s), 160.72 (s), 156.41
(d, J = 2.4 Hz), 156.02 (s), 152.15 (s), 150.12 (s), 145.31 (s), 144.96 (s), 142.19 (s), 139.13 (s),
137.53 (s), 136.73 (d, J = 30.3 Hz), 136.57 – 136.48 (m), 122.72 (s), 121.67 (s), 115.90 (s),
114.30 (d, J = 17.7 Hz), 113.86 (s), 68.95 (s), 54.71 (s), 54.17 (s), 40.42 (s), 39.15 (s), 31.75
(d, J = 3.2 Hz), 29.94 (d, J = 5.6 Hz), 29.38 – 29.09 (m), 24.45 (s), 23.88 (s), 22.58 (d, J = 2.0
Hz), 14.04 (s). MS: calcd for C93H112Cl2N4O2S4 (M+): 1514.70, Found: 1514.76.
3
Energy levels (eV)
-3.0
-3.31 eV
-3.5
-3.34 eV
-3.46 eV
FCC-Cl LUMO
F-4F LUMO
ITCC-Cl LUMO
-4.0
-4.5
-5.0
-5.51 eV
-5.5
-5.61 eV
-6.0
-5.70 eV
FCC-Cl HOMO
F-4F HOMO
FCC-Cl
F-4F
ITCC-Cl HOMO
ITCC-Cl
Figure S1. Frontier molecular orbitals (HOMO, LUMO) and chemical structures of FCC-Cl,
F-4F and ITCC-Cl.
1
Current
Fc/Fc+
FCC-Cl
D18
PM6
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
Voltage (V)
Figure S2. Cyclic voltammograms of FCC-Cl, D18, PM6 in solid states. The LUMO of D18
and PM6 are calculated by the difference between the optical bandgap of the films and the
HOMO of the materials.
4
A
B
FCC-Cl @ 633nm
D18: FCC-Cl @ 633nm
6.0k
3.0k
6.0k
3.0k
0.0
650
FCC-Cl @ 633nm
PM6: FCC-Cl @ 633nm
9.0k
Counts
Counts
9.0k
0.0
700
750
800
850
900
650
700
Wavelength (nm)
750
800
850
900
Wavelength (nm)
Figure S3. Photoluminescence (PL) quenching spectra of (A) D18: FCC-Cl and (B) PM6:
FCC-Cl. The PL quenching efficiencies for D18: FCC-Cl and PM6: FCC-Cl are 95.5% and
Normalized absorbance (a.u.)
98.3% respectively.
0
1.0
D18: FCC-Cl
PM6: FCC-Cl
0.8
0.6
0.4
0.2
0.0
400
500
600
700
800
Wavelength (nm)
Figure S4. Normalized absorption spectra of D18: FCC-Cl and PM6: FCC-Cl blend films.
5
A 5x10
B
5
D18
4000
Intensity(cps)
Intensity(cps)
4x105
5000
5
3x10
2x105
1x105
D18
3000
2000
1000
16.94
0.87
0
0
0
C
5
10
15
20
0
D
Binding energy (eV)
5x105
2
3
4
Binding energy (eV)
5000
PM6
4000
Intensity(cps)
4x105
Intensity(cps)
1
3x105
2x105
1x105
PM6
3000
2000
1000
0.84
16.96
0
0
0
5
10
15
20
0
Binding energy (eV)
1
2
3
4
Binding energy (eV)
Figure S5. UPS spectra of (A-B) D18 and (C-D) PM6 pure films. The HOMO levels were
determined by subtracting the width of the He I UPS spectra from the excitation energy (21.22
eV).
6
Table S1. Optimization of the conventional devices D-A ratio and Solvent vapor (SVP)
annealing time for D18:FCC-Cl blend films under illumination of AM 1.5G,100 mW cm-2
D18:FCC-Cl (w/w)
SVP time (min)
VOC (V)
JSC (mA cm-2)
FF (%)
PCE (%)
1:1
5
1.07
15.50
0.75
12.5
1:1.5
5
1.08
16.04
0.76
13.1
1:2
5
1.07
15.69
0.76
12.8
1:1.5
0
1.07
15.40
0.70
11.4
1:1.5
10
1.08
15.93
0.75
12.9
Table S2. Optimization of the inverted devices D-A ratio, annealing temperature and additive
concentration for PM6:FCC-Cl blend films under illumination of AM 1.5G,100 mW cm-2
PM6:FCC-Cl
(w/w)
a)
Additive (%)a)
Annealing
temperature
VOC (V)
JSC (mA cm-2)
FF (%)
PCE (%)
1:1
0.5
100
1.02
16.22
0.781
13.0
1:1.5
0.5
100
1.02
16.10
0.78
12.8
1:0.7
0.5
100
1.01
15.08
0.76
11.6
1:1
0
100
1.04
15.02
0.68
10.6
1:1
1
100
1.00
15.54
0.79
12.3
1:1
0.5
-
1.02
14.98
0.70
10.6
1:1
0.5
150
1.02
16.07
0.79
12.9
Additive: 1,8-Diiodooctane.
7
Table S3. Device parameters of D18:FCC-Cl and PM6:FCC-Cl blend films under different
architecture.
Active layer
Device architecture a)
VOC (V)
JSC (mA cm-2)
FF(%)
PCE(%)
D18:FCC-Cl
Inverted
1.07
15.90
0.70
11.7
D18:FCC-Cl
Conventional
1.08
16.04
0.76
13.1
PM6:FCC-Cl
Inverted
1.02
16.22
0.78
13.0
PM6:FCC-Cl
Conventional
1.02
15.94
0.80
13.0
a)
Inverted architecture: Glass/ITO/ZnO/Active layer/MoO3/Ag; Conventional architecture:
Glass/ITO/PEDOT:PSS/Active layer/PDI-NO/Al.
1
J1/2 (A1/2 m-1)
150
PM6: FCC-Cl h-only
h-mobility: 9.7×10-4 (cm2 V-1 s-1)
PM6: FCC-Cl e-only
e-mobility: 8.0×10-4 (cm2 V-1 s-1)
100
50
0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Vappl-Vbi-Vs (V)
Figure S6. J1/2~V characteristics of the hole-only device and the electron-only device
of the PM6: FCC-Cl blend film.
8
A
RMS 1.32nm
B
20 nm
20 o
0 nm
0o
200 nm
200 nm
D
C
RMS 1.55nm
20 nm
20 o
0 nm
0o
200 nm
200 nm
Figure S7. AFM images of (A, B) D18:FCC-Cl and (C, D) PM6:FCC-Cl blend films.
9
250
2.0
200
1.5
150
1.0
100
0.5
50
0.0
400
500
600
700
0
800
(B)
6.0x1012
3000K LED
4000K LED
6500K LED
100
4.0x1012
50
2.0x1012
0.0
400
500
Wavelength (nm)
600
700
0
800
(C)
Current density (mA*cm-2)
2.5
300
Integrated Current Density (uA*cm-2)
3000K LED
4000K LED
6500K LED
Photo Flux (s-1*cm-2*nm-1)
350
3.0
Integrated Power Density (uW*cm-2)
Power Density (uW*cm-2*nm-1)
(A)
0.0
D18: FCC-Cl
3000K
4000K
6500K
-0.1
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
Voltage (V)
Wavelength (nm)
Figure S8. Photovoltaic Performances of D18:FCC-Cl under white LEDs with different
colour temperature at 1000 lux.
(A) The emission power spectrum and integrated power density curve of white LEDs at 1000
lux.
(B) The photon flux spectrum of white LEDs at 1000 lux and the integral current density of
D18:FCC-Cl under these white LEDs.
(C) The J-V characteristic curves of D18:FCC-Cl under white LEDs at 1000 lux.
Table S4. Device parameters of D18:FCC-Cl under white LEDs with different colour
temperature at 1000 lux.
Active layer
D18:FCC-Cl
a)
Color
Pin
VOC
-2
Jcal a)
JSC
-2
-2
FF
PCE b)
Temperature
[mW cm ]
[V]
[uA cm ]
[uA cm ]
[%]
[%]
3000K
0.290
0.953
108.6
106.7
79.8
28.5 (28.0±0.5)
4000K
0.289
0.951
103.9
102.0
79.6
27.3 (26.8±0.4)
6500K
0.306
0.952
104.8
103.2
79.7
26.0 (25.6±0.4)
Jcal was obtained by integrating the EQE spectrum over the light source b) The average device
parameters and the standard deviations in parentheses are based on the measurement of over
ten independent devices.
10
B
10
Current Density (mA cm-2)
Current Density (mA cm-2)
A
D18:FCC-Cl S=0.992
1
10
100
10
PM6: FCC-Cl 100nm S=0.998
PM6: FCC-Cl 400nm S=0.996
1
10
-2
100
-2
Light Intensity (mW cm )
Light Intensity (mW cm )
Figure S9. light-intensity-dependent JSC experiments on (A) D18: FCC-Cl and (B) PM6: FCCCl with different thicknesses.
11
(B)
10
D18:FCC-Cl
PM6:FCC-Cl
AM 1.5G
0
-5
-10
-15
0.2
0.4
0.6
0.8
1.0
1.2
-0.1
-0.2
0.0
0.2
Voltage (V)
(C)
0.4
(D)
150
Photo Flux (s-1*cm-2*nm-1)
EQE(%)
80
60
40
20
D18:FCC-Cl
PM6:FCC-Cl
0
400
500
600
700
Wavelength (nm)
800
6.0x1012
D18: FCC-Cl
100
4.0x1012
50
2.0x1012
0.0
400
0.6
0.8
1.0
Voltage (V)
500
600
700
0
800
(E)
150
6.0x1012
PM6: FCC-Cl
100
4.0x1012
50
2.0x1012
0.0
400
Wavelength (nm)
500
600
700
0
800
Integrated Current Density (uA*cm-2)
0.0
0.0
Photo Flux (s-1*cm-2*nm-1)
-20
-0.2
D18: FCC-Cl
2600 K LED at 1000 LUX
Integrated Current Density (uA*cm-2)
5
Current density (mA*cm-2)
Current density (mA*cm-2)
(A)
Wavelength (nm)
Figure S10. Photovoltaic performances of large-area (85 mm2) D18:FCC-Cl and
PM6:FCC-Cl devices under AM1.5 and 2600 K LED at 1000 lux
(A) The J-V characteristic curves of large-area D18:FCC-Cl and PM6:FCC-Cl devices under
AM1.5.
(B) The J-V characteristic curves of large-area D18:FCC-Cl and PM6:FCC-Cl devices under
2600 K LED at 1000 lux.
(C) The EQE spectra of large-area D18:FCC-Cl and PM6:FCC-Cl devices.
(D-E) The photon flux spectrum of 2600K LED lamp at 1000lux and the integral current
density of large-area (D) D18:FCC-Cl and (E) PM6:FCC-Cl devices under 2600K LED lamp
at 1000lux.
12
Table S5. Device parameters of large-area (85 mm2) D18:FCC-Cl and PM6:FCC-Cl under
2600 K LED at 1000 Lux.
Active layer
Illumination
D18:FCC-Cl
AM1.5G
Pin
VOC
JSC
Jcal a)
FF
Pout
PCE b)
[mW cm-2]
[V]
[mA cm-2]
[mA cm-2]
[%]
[uW cm-2]
[%]
100
1.08
15.9
15.9
68.0
11.7
/
a)
PM6:FCC-Cl
AM1.5G
100
1.03
16.2
16.1
71.0
D18:FCC-Cl
2600K at 1000lux
0.318
0.955
0.1200
0.1191
79.0
90.6
PM6:FCC-Cl
2600K at 1000lux
0.318
0.890
0.1218
0.1211
80.3
87.1
(11.3±0.3)
11.9
(11.7±0.2)
28.5
(28.0±0.5)
27.4
(27.0±0.4)
Jcal was obtained by integrating the EQE spectrum over the light source b) The average device
parameters and the standard deviations in parentheses are based on the measurement of five
independent devices from one batch.
13
1.00
1
D18: FCC-Cl
PM6: FCC-Cl
0.95
VOC (V)
Slope=1.23kT/q
0.90
0.85
Slope=1.09kT/q
0.80
e-4
e-3
e-2
e-1
-2
Light Intensity (mW cm )
Figure S11. The low-light-intensity-dependent VOC experiments on D18: FCC-Cl and PM6:
FCC-Cl.
B
Current density (mA cm-2)
Current density (mA cm-2)
A
10
D18:FCC-Cl
0.1
0.001
1E-5
1E-7
-1.0
-0.5
0.0
0.5
10
Injection
region
1E-5
1E-7
-1.0
-0.5
D
0.0
0.5
1.0
1.5
Voltage (V)
106
Rdiff (Ω cm-2)
Rdiff (Ω cm-2)
Series
Resistance
region
Parallel Resistance region
0.001
106
D18:FCC-Cl
104
Parallel Resistance region
PM6:FCC-Cl 100nm
PM6:FCC-Cl 400nm
104
-0.5
0.0
0.5
100
-1.0
1.0
Series
Resistance
region
Parallel Resistance region
102
102
100
-1.0
PM6: FCC-Cl 100nm
PM6: FCC-Cl 400nm
0.1
1.0
Voltage (V)
C
1
Injection
region
-0.5
0.0
0.5
1.0
1.5
Voltage (V)
Voltage (V)
Figure S12. The dark J–V Curves of (A) D18:FCC-Cl and (B) PM6: FCC-Cl devices with
different thicknesses; The differential resistances derived from the dark J–V curves of (C)
D18:FCC-Cl and (D) PM6: FCC-Cl devices with different thicknesses.
14
Figure S13. The image of the thick D18: FCC-Cl film (around 300nm). Due to the poor
solubility of the commercial D18 material, the polymer cannot completely dissolve in
chloroform when the concentration of D18 is increased to make thick films, and lots of particles
formed in the active layers during the spin-coating process. These particles may increase the
trap-assist recombination, which may be the reason of the poor indoor performance of thickfilm (around 300nm) D18: FCC-Cl devices.
Table S6. Device parameters of the thick D18: FCC-Cl devices (around 300nm) under
AM1.5G and 2600K LED at 1000 lux.
ACTIVE LAYER
D18:FCC-Cl
100nm
D18:FCC-Cl
300nm
Illumination
VOC (V)
JSC (mA cm-2)
FF (%)
PCE (%)
AM1.5G
1.08
16.04
76.0
13.1%
2600K LED at 1000 lux
0.955
0.123
79.8
29.4%
AM1.5G
1.06
17.07
55.0
10.0%
2600K LED at 1000 lux
0.915
0.123
64.1
22.6%
Figure S14. The synthesis route of FCC-Cl.
15
Figure S15. 1H NMR spectrum of FCC-Cl.
Figure S16. 13C NMR spectrum of FCC-Cl.
16
Supplementary Note 1: The effect of current density on VOC
According to previous literature3-4, VOC is determined by the split of the electron and hole
quasi-Fermi energy levels:
𝑉𝑂𝐶 =
𝐸𝐹𝑛 − 𝐸𝐹𝑝
𝑞
where EFn and EFp are the electron and hole quasi-Fermi levels; q is the elementary charge.
As shown in Figure S17, under low illumination, the devices exhibit a down-shifted EFn and
a up-shifted EFp compared with the devices under strong illumination. This is one of the
reasons why the devices exhibit a lower VOC at low light intensity.
EFn1
Acceptor
Energy (eV)
EFn2
qVOC1
qVOC2
Excited
EFp2
EFp1
Donor
Low illumination
Strong illumination
Figure S17. The schematic diagram of the density of states in both the donor and acceptor
materials under strong and low illumination. The red star dots represent the thermalized
electrons and holes.
Supplementary Note 2: The procedure to calculate calculated Jsc
The integrated current density (Jcal) was obtained based on following equations:
𝐽𝑠𝑐 (𝑐𝑎𝑙𝑐𝑢𝑙𝑎𝑡𝑒𝑑) = ∫ 𝐸𝑄𝐸(𝜆) ∗ Φ(λ) ∗ q
Η(𝜆) = Φ(λ) ∗ q ∗ E(eV)
where Φ(λ), Η(λ) are the photon flux and the Power density of the indoor light sources. For
example, the Jcal of D18:FCC-Cl under 2600K at 2000lux were obtained through the EQE
spectra (Figure 2E), the photon flux (Figure 4C) and the Power density (Figure 4D). The
calculated result is 245.4 uA cm-2.
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Reference
1. Chang, Y., Lau, T.-K., Chow, P.C.Y., Wu, N., Su, D., Zhang, W., Meng, H., Ma, C., Liu, T.,
Li, K., et al. (2020). A 16.4% efficiency organic photovoltaic cell enabled using two donor
polymers with their side-chains oriented differently by a ternary strategy. J Mater Chem A 8,
3676-3685.
2. Zhang, J., Li, Y., Hu, H., Zhang, G., Ade, H., and Yan, H. (2019). Chlorinated Thiophene
End Groups for Highly Crystalline Alkylated Non-Fullerene Acceptors toward Efficient
Organic Solar Cells. Chem Mater 31, 6672-6676.
3. Vandewal, K., Tvingstedt, K., Gadisa, A., Inganas, O., and Manca, J.V. (2009). On the origin
of the open-circuit voltage of polymer-fullerene solar cells. Nat Mater 8, 904-909.
4. Cui, Y., Wang, Y., Bergqvist, J., Yao, H., Xu, Y., Gao, B., Yang, C., Zhang, S., Inganäs, O.,
Gao, F., et al. (2019). Wide-gap non-fullerene acceptor enabling high-performance organic
photovoltaic cells for indoor applications. Nat Energy 4, 768–775.
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