Achieving Balanced Charge Transport and Favorable Blend Morphology in Non-Fullerene Solar Cells via Acceptor End Group Modification

Article abstract of DOI:10.1021/acs.chemmater.8b05327

Generally, the electron-withdrawing substitution on the end-capping group of the acceptor-donor-acceptor type small-molecule acceptor (SMA) narrows the optical bandgap, and the electron-donating group lifts the lowest unoccupied molecular orbital (LUMO) e

Full text of DOI:10.1021/acs.chemmater.8b05327

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Article
Achieving Balanced Charge Transport and Favorable Blend Morphology
in Non-Fullerene Solar Cells via Acceptor End Group Modification
Minghui Hao, Tao Liu, Yiqun Xiao, Lik-Kuen Ma, Guangye Zhang, Cheng Zhong,
Zhanxiang Chen, Zhenghui Luo, Xinhui Lu, He Yan, Lei Wang, and Chuluo Yang
Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b05327 • Publication Date (Web): 04 Feb 2019
Downloaded from http://pubs.acs.org on February 5, 2019
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Page 1 of 11
Chemistry of Materials
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Achieving Balanced Charge Transport and Favorable Blend
Morphology in Non-Fullerene Solar Cells via Acceptor End Group
Modification
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Minghui Hao,†,‡,# Tao Liu,*,Δ,†,# Yiqun Xiao,|| Lik-Kuen Ma,Δ Guangye Zhang,Δ Cheng Zhong,§
Zhanxiang Chen,§ Zhenghui Luo,§ Xinhui Lu,|| He Yan,*,Δ Lei Wang,*,† and Chuluo Yang*,†,§
†Shenzhen Key Laboratory of Polymer Science and Technology, College of Materials Science and Engineering,
Shenzhen University, Shenzhen 518060, China.
‡Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, College of
Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China
ΔDepartment of Chemistry 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, China
||Department of Physics, Chinese University of Hong Kong, New Territories, Hong Kong, China
§Hubei Key Lab on Organic and Polymeric Optoelectronic Materials, Department of Chemistry, Wuhan University,
Wuhan 430072, China
ABSTRACT: Generally, the electron-withdrawing substitution on the end-capping group of the acceptor-donor-acceptor
type small-molecule acceptor (SMA) narrows the optical bandgap and that the electron-donating group lifts the lowest
unoccupied molecular orbital (LUMO) energy level of non-fullerene SMA, which increase the short-circuit current density
(JSC) and open circuit voltage (VOC) of the organic solar cells (OSCs), respectively, however their synergistic effect on the
properties of SMA has remained elusive. Here, we first report a new end-capping group (EG), namely, CFDCI, that
concurrently possesses electron-withdrawing fluorine substitute and electron-donating methyl group. A prototype SMA
(namely ITCF) based on CFDCI and its two control counterparts were prepared to fully understand the structure-property
relationship that the new EG exerts on the resultant SMA. The ITCF demonstrated a moderately crystalline morphology in
pristine film and more balanced charge transport properties as well as a reduced amount of bimolecular recombination in
blend film in comparison with its counterparts. The ITCF-based devices demonstrated a high power conversion efficiency
(PCE) of 13.25% with an outstanding fill factor (FF) of 78.8%, which significantly outperformed their counterparts. Our
study provides an important strategy to judiciously tune the properties of the SMAs for improving the performance of the
OSCs.
INTRODUCTION
13% in OSCs with appropriate donor materials and
systematic device optimization.17-29
Organic solar cells (OSCs) have attracted much attention
due to their virtues of low cost, light-weight, flexibility and
facile solution processability.1-3 Over the past two decades,
fullerene derivatives have been the dominant choice for
electron acceptors,4 leading to OSCs with power
conversion efficiencies (PCEs) of ca. 11%.5-7 However, the
intrinsic drawbacks of fullerene materials, such as the
high-cost production, limited absorption and poor
chemical and electronic adjustability, have impeded the
further development of OSCs.8 To overcome these
Typically, A-D-A-type SMAs contain two parts: a ladder-
type donor core with outstretched side chains19-23, 30-44 and
electron-withdrawing end-capping groups (EGs),45-52
wherein the EGs can facilitate the tuning of optical
bandgaps, energy levels and crystallization behavior of the
molecules. Thus, the modification of EGs to construct
electron acceptors in view of further increasing the PCEs is
of great importance. Recently, electron-withdrawing
functionalities, such as fluorine atom and chlorine atom,18,
26, 31, 41, 53-56
problems,
a
range of non-fullerene small-molecule
have been introduced onto the classic EG, i.e.,
acceptors (SMAs) have been developed and achieved great
success.9-16 In this respect, SMAs with a planar acceptor-
donor-acceptor (A-D-A) configuration have been the
intriguing representative which exhibited PCEs of over
1,1-dicyanomethylene-3-indanone (DCI, Scheme 1), to
narrow the optical bandgaps of the electron acceptors via
enhancing the intramolecular charge transfer (ICT) effect,
which lead to the increase of the short-circuit current
density (JSC). Moreover, the resulting electron acceptors
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showed improved charge transport due to the noncovalent
electron acceptors is an effective way to boost the PCE of
the OSCs.
bonding.31 However, such acceptors have a reduced optical
1
2
3
bandgap generally due to the downshifted lowest
53-55
unoccupied molecular orbital (LUMO) levels.31,
As a
CN
CN
CN
CN
CN
CN
CN
R1 = methyl , R2 = F
or
O
O
O
O
consequence, OSCs based on such acceptors suffered from
a loss of the open circuit voltage (VOC), which in turn may
be detrimental to the PCE improvement. Besides the
introduction of electron-withdrawing groups, electron-
donating groups were also introduced onto the DCI
moiety.57 It has been demonstrated that the electron-
donating groups attached on the EGs can elevate the LUMO
CN
4
5
6
7
8
9
R1 = F , R2 = methyl
H3C
CH3
F
F
R1 R2
CFDCI
DCI
DCI-DM
EG-2F
Scheme 1. Structures of the EGs
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RESULTS AND DISCUSSION
levels of the acceptors, leading to the increase of VOCs in
57, 58
OSC devices.46,
Moreover, such groups can induce
Material Synthesis. The synthetic routes of CFDCI and
the prototype electron acceptor ITCF are shown in Scheme
3. The nitration of 4-methylphthalic anhydride
accompanied with a hydrolysis afforded acid 1, which then
underwent an esterification to give compound 2 in 43%
yield. Nitro compound 2 was reduced to the amine
derivative 3 in 95% yield. The fluorination of compound 3
via Sandmeyer reaction produced compound 4 in 79%
yield. The phthalic esters of compound 4 were converted
to 1,3-indandiones by Claisen condensation reaction,
affording compound 5 in 39% yield. The EG CFDCI was
obtained by Knoevenagel condensation of compound 5
with malononitrile in 76% yield. The target acceptor ITCF
was prepared with a yield of 92% by the Knoevenagel
condensation of CFDCI and the dialdehyde compound
IDTT-CHO. Note that due to the asymmetric structure and
the similar reactivity of the two keto-carbonyls of the
precursor, the corresponding product CFDCI is composed
of two isomers, with molar ratio of one to another of 1.7
according to the 1H NMR spectra in the Supporting
Information (Figure S26). Hence, the ITCF is a mixture
comprised of three isomers, which cannot be separated
due to their similar chemical structures. The control
acceptors, IT-DM and IT-4F, were prepared according to
the literature methods.18, 57 The chemical structure of ITCF
was fully characterized using 1H, 19F and 13C NMR
spectroscopy, and high-resolution mass spectrometry. At
room temperature, ITCF is readily soluble in common
organic solvents, such as chloroform, chlorobenzene, and
ortho-dichlorobenzene. The thermal stability of ITCF was
determined by thermogravimetric analysis (TGA). The
decomposition temperature of ITCF is 368 oC with 5%
weight loss (Figure S1), indicating its good thermal
stability.
higher phase purities and more ordered donor aggregates,
and thus are beneficial for charge transport.46, 57 However,
electron acceptors modified by EGs with electron-donating
58
groups exhibited blue-shifted absorption spectra,57,
which is theoretically harmful to the JSC. Note that the
previous modification of the DCI is through the
introduction of electron-withdrawing or electron-donating
groups individually. However, how the electron-
withdrawing
and
the
electron-donating
groups
concurrently attached onto the EGs affect the properties of
the resultant electron acceptors has remained non-existent
and unclear. One possible reason is the synthetic
inconvenience of such EGs, which, along with other
possibilities, needs to be further explored.
Herein, we report the design and synthesis of a novel EG,
namely, 5-methyl-6-fluoro-3-dicycanovinylindan-1-one (as
well as its isomer 5-fluoro-6-methyl-3-dicycanovinylindan-
1-one, abbreviated as CFDCI, Scheme 1), that concurrently
possesses the electron-withdrawing and the electron-
donating groups (fluorine atom and methyl group,
respectively) to study how the combination of electron-
withdrawing and electron-donating groups affect the
properties of the resulting molecules and the
performances of the corresponding OSC devices.
prototype SMA ITCF (Scheme 2) based on CFDCI was
synthesized with the electron donor unit
A
indacenodithieno[3,2-b]thiophene (IDTT) as the central
core. Additionally, we prepared its counterparts—IT-DM
and IT-4F—as the control molecules, which were
synthesized by Knoevenagel condensation reaction
between IDTT and DCI modified with two methyl groups
or two fluorine atoms (DCI-DM and EG-2F, respectively,
Scheme 1) according to the literature, to explore the
influences of the newly established EG on the resultant
electron acceptor. ITCF shows an absorption edge and a
LUMO level between those of IT-DM and IT-4F. Thus, the
ITCF-based device shows a VOC of 0.91 V and a JSC of 18.48
mA cm−2, which are between those of the IT-DM and IT-4F
devices. Importantly, due to the balanced charge transport
as well as the favorable morphology of ITCF, which are
induced by the EG moiety, the ITCF-based device
DFT Calculation. Density functional theory (DFT)
calculations at the B3LYP/def2-SVP level with Grimme’s
D3 dispersion correction were performed to obtain the
optimized geometries of the EGs and acceptors. Then the
orbital levels were obtained at the -tuned LC-
PBE*/def2-SVP level60 using Gaussian09 program.
Regarding the EGs (Figure S2), the calculated LUMO and
highest occupied molecular orbital (HOMO) levels of
CFDCI are between
demonstrated a fill factor (FF) as high as 78.8%, which is
29, 38, 39, 48, 59
one of the highest FFs for OSCs.27,
More
importantly, the ITCF-based OSC device shows the best
PCE of 13.25%, which is clearly higher than the IT-DM
(12.05%) and the IT-4F-based (11.66%) devices. Our
study demonstrates the strategy that concurrently
introducing the electron-withdrawing and the electron-
donating groups to delicately tune the properties of the
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Chemistry of Materials
transport.61 The HOMO of ITCF is mainly distributed along
C6H13
CH3
C6H13
H3
C
S
NC
O
S
1
the conjugated central core, while the LUMO delocalizes
over the entire molecule (Figure S4). This is in line with
the results obtained for IT-DM and IT-4F. The calculated
LUMO energy level of ITCF (−2.61 eV, −2.62 eV, and −2.63
eV for the three isomers, respectively) is between that of
IT-DM (LUMO: −2.50 eV) and IT-4F (LUMO: −2.75 eV).
This result could be attributed to the electron-donating
nature of the methyl unit that lifts the LUMO level and the
electron-withdrawing property of the fluorine atom that
CN
S
NC
O
C6H13
2
3
CN
S
F
C6H13
H3
C
CH3
C6H13
IT-DM
CH3
S
NC
F
C6H13
O
S
CN
S
NC
4
5
O
C6H13
CN
F
F
S
C6H13
CH3
C6H13
S
NC
F
C6H13
O
S
CN
F
S
6
NC
CN
ITCF
O
S
7
C6H13
IT-4F
C6H13
8
9
Scheme 2. Molecular Structures of the Acceptor Molecules
(ITCF is represented by one of its three isomers)
57
lowers the LUMO level when attached onto the EGs.31,
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Indeed, methyl and the fluorine atom show different
electron distributions according to the electrostatic
potential
those of DCI-DM and EG-2F. ITCF possesses a highly
planar backbone (Figure S3), which would be beneficial to
intermolecular π-π interactions, facilitating charge
O O
O O
O O
O
O
O
O
O
O
O
HO
OH
HCl, Zn powder
HNO3, H2SO4
110 o
SOCl2, MeOH
ethyl acetate, 25 o
95%
C
reflux
C
O2N
CH3
H2N
CH3
O2N
CH3
CH3
two steps, 43%
3
1
2
CN
CN
O O
O
O
O
O
O
NaH, ethyl acetate
HBF4, NaNO2
ice water
R1 = methyl , R2 = F
or
R1 = F , R2 = methyl
AcOK
HCl/H2O
NC CN
80 o
C
toluene, 100 o
79%
C
100 o
C
ethanol, 25 o
76%
C
F
CH3
F
CH3
R1 R2
CFDCI
39%
4
5
C6H13
C6H13
R2
C6H13
O
R1
C6H13
S
S
NC
R1 = R3 = methyl , R2 = R4 = F
or
R1 = R3 = F , R2 = R4 = methyl
or
R1 = R4 = methyl , R2 = R3 = F
pyridine
CHCl3, 65 o
92%
CHO
CN
R3
S
OHC
S
S
S
S
CFDCI
+
NC
C
O
CN
S
C6H13
C6H13
R4
C6H13
C6H13
ITCF
IDTT-CHO
Scheme 3. Synthetic Route of CFDCI and ITCF
map (Figure S5). In addition, to calculate the dimer binding
energies, dimer structures with intermolecular π-π
stacking formed between the terminal acceptor units were
of ~ 400−800 nm (Figure S7). Compared to the solution
state absorption, the absorption spectra of the three SMAs
in the solid state displayed a redshift due to aggregation.
Interestingly, the absorption profiles of ITCF in both
solution and film are essentially identical to those of IT-
DM and IT-4F yet with the maximum absorption peaks
and absorption onsets between those of IT-DM and IT-4F.
The calculated optical bandgaps are 1.63, 1.57, and 1.54 eV
for IT-DM, ITCF, and IT-4F (Table 1), respectively. This
experimental result implies that the methyl group
blueshifts the absorption while fluorine atom redshifts the
absorption, which is in agreement with literature results.31,
built based on studies of molecular dynamic and crystal
63
structure.62,
The geometries were then optimized at
BLYP/def2-SVP level with the resolution-of-identity (RI)
approximation64 using ORCA 4.0 program65 (Figure S6).
BSSE correction were considered by Grimme’s gCP
method.66 The dispersion items were conducted by
Grimme's D3 version with BJ damping function.67 The ITCF
dimer has a binding energy of −37.36 kcal mol−1, which is
close to that of the IT-DM dimer (−37.81 kcal mol−1), and
larger than that of the IT-4F dimer (−35.34 kcal mol−1).
The binding energy calculations indicated that ITCF and
57
Notably, when one of the methyl groups of the EG in IT-
DM is replaced by a fluorine atom, the absorption of the
resultant molecule (i.e., ITCF) exhibits a relatively large
redshift (31 nm). When the second one was further
replaced by another fluorine atom, the resulting molecule
(IT-4F) demonstrates a further redshift by only 11 nm.
The max molar extinction coefficient of ITCF (Figure 1a,
Table1), which is 1.93 × 105 M−1 cm−1, is close to that of IT-
DM (1.82 × 105 M−1 cm−1) and IT-4F (2.05 × 105 M−1 cm−1).
IT-DM have
a stronger tendency for intermolecular
packing than IT-4F.
Optical and Electrochemical Properties. J71 (Scheme
S2) was selected as the donor polymer to pair with the
SMAs for assessing their photovoltaic performances. As
shown in Figure 1b, the absorption spectra of ITCF, IT-DM
and IT-4F are spectrally complementary to that of J71. The
absorption of the blend films cover the wavelength range
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To elucidate the combined effects of the methyl and three acceptors were measured by atomic force
1
2
3
4
5
6
7
8
fluorine atom on the electrochemical properties of ITCF,
its electronic energy levels were measured by
electrochemical cyclic voltammetry (CV) and compared
with those of IT-DM and IT-4F (Figure 1c, Table 1). The
three acceptors exhibited irreversible oxidation and
reduction waves in the CV measurements. Based on the
onset reduction potentials in the CV measurements, the
LUMO levels of IT-DM, ITCF, and IT-4F were estimated to
be −3.82, −3.95, and −4.06 eV, respectively. The energy
levels of
microscopy (AFM) (Figure S8). From the height images, the
pristine IT-DM film exhibited a relatively rough top
surface with a root-mean-square (RMS) roughness of 1.43
nm. The ITCF film demonstrated a smooth surface, with an
RMS roughness of 0.731 nm. The IT-4F film showed a
smoother surface, with an RMS roughness of 0.409 nm.
These results imply that the crystallization propensity of
ITCF is between those of IT-DM and IT-4F. This trend is
also reflected in the phase images: with the reducing
number of methyl or the increasing number of fluorine
atom in the EG of the acceptor, i.e., from IT-DM to ITCF
then to IT-4F, the films evolved gradually from the
granular texture to a fine-grained texture. In addition, the
J71 film exhibited a similar top surface to that of ITCF with
an RMS roughness of 0.615 nm.
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(b)
(a)
2.0x105
1.5x105
1.0x105
5.0x104
0.0
IT-DM
ITCF
IT-4F
1.0
0.8
0.6
0.4
0.2
0.0
J71
IT-DM
ITCF
IT-4F
The packing and orientation of acceptors and J71 were
studied by grazing incidence wide-angle X-ray scattering
(GIWAXS) (Figure S9).69 The detailed structure parameters
are summarized in Table S1. J71 shows distinct (100)
diffraction peaks in both the in-plane (IP) direction and the
out-of-plane (OOP) direction while the π-π stacking (010)
shows at 1.62 Å−1 in the OOP direction, corresponding to a
preferential face-on orientation. The ITCF neat film shows
a predominant face-on orientation with the π-π stacking
diffraction peak position and d-spacing close to those of
IT-DM. It is known that IT-DM has a strong aggregation
tendency due to its low solubility,57 therefore, the much
smaller coherence length of ITCF neat film (5.59 Å−1) than
that of IT-DM (20.15 Å−1) and the decreased π-π stacking
diffraction peak intensity of ITCF relative to that of IT-DM
indicated that the excessive aggregation of IT-DM was
substantially alleviated for ITCF. ITCF achieves a slightly
higher electron mobility (μe) of 7.44 × 10−4 cm2 V−1 s−1
(measured using the space-charge-limited current (SCLC)
method)70 than IT-DM (6.97 × 10−4 cm2 V−1 s−1, Figure S10
and Table S2). This result suggests that the combination of
methyl and fluorine atom in the EG endows ITCF with
morphology which is more favorable for charge transport.
The IT-4F neat film exhibits an amorphous feature
according to its feature from the GIWAXS results (Figure
S9), which is in good agreement with the literature
result.24 Note that IT-4F shows the highest electron
mobility among the three acceptors (8.07 × 10−4 cm2 V−1
s−1), which may be associate with the noncovalent F∙∙∙S and
F∙∙∙H bonds that facilitate charge transport.7
400
500
600
700
800
400
500
600
700
800
Wavelength (nm)
Wavelength (nm)
-3.0
-3.5
-4.0
-4.5
-5.0
-5.5
-6.0
(c)
(d)
-3.24
1.0x10-4
0.0
-3.82
-3.95
-4.06
IT-4F
J71
-1.0x10-4
-2.0x10-4
-3.0x10-4
IT-DM
ITCF
IT-4F
Fc/Fc+
IT-DM
ITCF
-5.40
2
-5.58
6
-5.59
-5.63
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5
Potential (V vs. Ag/AgCl)
0
4
8
10 12 14 16
molecules
Figure 1. UV-vis spectra of (a) acceptors in chloroform
solution (10−5 M) and (b) acceptors and J71 in thin film. (c)
cyclic voltammograms of IT-DM, ITCF, and IT-4F at a scan
rate of 100 mV s−1. (d) energy diagram relative to the
vacuum level.
J71 (HOMO = −5.40 eV; LUMO = −3.24 eV) are compatible
with those of the acceptors (Figure 1d). The LUMO level of
ITCF showed a decline of ca. 0.13 eV compared to that of
IT-DM, whereas a rise of ca. 0.11 eV compared to that of
IT-4F. Methyl, the electron-donating unit, lifts the LUMO
levels while fluorine, the electron-accepting unit, lowers
the LUMO levels. The trends in the LUMO levels are in good
agreement with the results acquired from the DFT
calculations and the results previously reported.31, 41, 53, 57
These results imply that the LUMO level of ITCF is the
consequence of synergistic effects of the methyl and
fluorine atom. The HOMO level of ITCF was estimated to be
−5.59 eV, which is comparable to that of IT-DM (−5.58 eV)
and IT-4F (−5.63 eV), indicating that the EGs have little
effect on the HOMO levels of the electron acceptor
molecules relative to their roles in tuning the LUMO levels,
Photovoltaic
Properties
and
Recombination
Mechanisms. Photoluminescence (PL) spectra of the
pristine donor and acceptor films and the blend films were
measured to investigate the exciton dissociation efficiency.
As shown in Figure S11, compared to the pure films, all
blend films show significant PL quenching, suggesting that
the exciton dissociation in the blend films are efficient. To
evaluate the photovoltaic properties of ITCF, we fabricated
OSC devices based on ITCF as acceptor and J71 as donor
with a conventional device architecture of ITO (indium tin
57
which is consistent with literature results.31, The deep
HOMO levels would be a positive factor for obtaining a
large driving force for exciton dissociation.68
Morphology and Charge Transport of Pure Films.
The surface morphology of the pristine films based on the
Table 1. Optical and Electrochemical Data of the IT-DM, ITCF, and IT-4F
acceptor λmaxsol (nm) λmaxfilm (nm) λonsetfilm (nm) εmax (M−1 cm−1)a Egopt (eV)b EHOMOCV (eV)c
ELUMOCV (eV)d
Egcv (eV)
IT-DM
669
694
759
1.82 × 105
1.63
−5.58
−3.82
1.76
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Chemistry of Materials
ITCF
IT-4F
683
690
718
726
792
803
1.93 × 105
2.05 × 105
1.57
−5.59
−5.63
−3.95
−4.06
1.64
1.57
1
2
3
4
1.54
opt
aMolar extinction coefficient at λmax in solution. Calculated from Eg = 1240/λonsetfilm. HOMO energy level estimated from
b
c
the onset oxidation potential. dLUMO energy level estimated from the onset reduction potential.
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oxide)/PEDOT:PSS
ethylenedioxythiophene):poly(styrene-
sulfonate))/J71:ITCF/ZrAcac
(poly(3,4-
using the formula Eloss = Eg − eVOC. The Eloss values of the
OSCs are 0.61-0.66 eV, which are relatively small.71 A high
JSC of 18.48 mA cm−2 was achieved for the ITCF-based
device, which is between those of the IT-DM- and IT-4F-
based devices. This result agrees well with the incident
photon-to-current conversion efficiency (IPCE) spectra
shown in Figure 2b. The IPCE (Figure S12b) onset of ITCF
is between those of IT-DM and IT-4F, which is in
accordance with the absorptions of the blend (Figure S7).
Additionally, the ITCF device outperforms the IT-DM and
IT-4F device in terms of FF. This can be ascribed to its
more balanced electron and hole mobility and the
favorable morphology of the active blend as discussed
below. As a consequence, the optimized ITCF-based device
demonstrated the best PCE of 13.25%, which is clearly
higher than those of the IT-DM- (12.05%) and IT-4F-
(11.66%) based devices. In addition, the average PCE
obtained from 20 devices for the ITCF device
(12.81±0.24%) is clearly higher than that of the IT-DM
(11.91±0.15%) and IT-4F (11.42±0.13%) devices. The
detailed photovoltaic parameters of the typical 20 devices
for ITCF are shown in Figure S13.Notably, the high FF
(78.8%) that the ITCF-based device achieved is one of the
highest values reported in all OSC devices.
(zirconium
acetylacetonate)/Al, where ZrAcac was chosen as the
cathode interlayer for lowering the work function of Al.
The weight ratio of J71:ITCF is 1:1. The active layers were
prepared by spin-coating at 2000 r.p.m. from the J71:ITCF
blend solution with a total blend concentration of 16 mg
mL−1 in chloroform. For comparison, the IT-DM-/IT-4F-
based devices were fabricated under the same conditions.
The devices were optimized by thermal annealing (TA) at
100 oC for 5 minutes. The annealed devices showed
improved device performances with the increase of JSCs
and FFs as well as a negligible decrease of VOCs when
compared with the as-prepared devices (Figure S12a and
Table S3). Figure 2a shows the optimized current density-
voltage (J-V) curves of the OSCs, and the corresponding
photovoltaic parameters are listed in Table 2.
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The optimized ITCF-based devices demonstrated a VOC
of 0.91 V, which is between those of the IT-DM (1.015 V)
and IT-4F (0.803 V) devices. This result is fairly
reasonable considering that the VOC is correlated with the
differences between the HOMO of the donor and the LUMO
of the acceptor. The photon energy loss (Eloss) is calculated
Table 2. Photovoltaic Parameters of OSCs under AM 1.5G Illumination, 100 mW cm−2
Blends
VOC (V)a
JSC (mA cm−2)a
Jcalc. (mA cm−2)b
FFa
PCE (%)a
IT-DM:J71
ITCF:J71
IT-4F:J71
1.015 (1.014±0.004) 16.74 (16.62±0.14) 16.338
0.910 (0.903±0.006) 18.48 (18.27±0.10) 18.101
0.803 (0.801±0.003) 19.58 (19.36±0.18) 19.012
0.709 (0.706±0.004) 12.05 (11.91±0.15)
0.788 (0.777±0.008) 13.25 (12.81±0.24)
0.742 (0.736±0.004) 11.66 (11.42±0.13)
aThe values in parentheses are the average values with standard deviations obtained from 20 devices. JSC integrated from
the IPCE for the optimized devices.
b
80
4
0
(d)
(a)
(b)
(c)
J71:IT-DM
J71:ITCF
J71:IT-4F
10
1
60
40
20
0
J71:IT-DM
J71:ITCF
J71:IT-4F
10
J71:IT-DM
J71:ITCF
J71:IT-4F
-4
-8
J71:IT-DM
J71:ITCF
J71:IT-4F
-12
-16
-20
1
300
400
500
600
700
800
0.0
0.2
0.4
0.6
0.8
1.0
0.1
1
1
10
Light Intensity (mW cm
100
-2
Veff (V)
Wavelength (nm)
)
Voltage (V)
Figure 2. (a) J-V characteristics of the optimized devices; (b) IPCE spectra of the optimized devices under illumination of an
AM 1.5 G at 100 mW cm−2; (c) Jph versus Veff characteristics of the optimized devices; and (d) JSC versus light intensity of the
optimized devices.
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Figure 3. AFM height and phase images of the optimized blend films based on J71:IT-DM (a, d), J71:ITCF (b, e), and J71:IT-
4F (c, f). In-plane (dashed lines) and out-of-plane (solid lines) GIWAXS intensity profiles of blend films (g).
On the other hand, the higher FF of ITCF with respect to
IT-DM and IT-4F, suggests that our methodology—fine
tuning the EG—is also beneficial for reducing the exciton
recombination of the blend device. This is further
supported by the analysis of photocurrent density (Jph)
versus effective voltage (Veff) (Figure 2c), where Jph=JL − JD
(JL and JD are the current density under illumination and in
the dark, respectively) and Veff = V0 − Va (V0 is the voltage at
Jph = 0 and Va is the applied bias). The summary of results is
presented in Table S4 (Supporting Information). The ratios
of Jph/Jph,sat are used to assess the overall efficiency of
exciton dissociation and charge collection. The exciton
dissociation and charge collection probabilities of the
optimized devices were enhanced compared with those of
the as-cast devices (Table S2). Under optimized conditions,
both the exciton dissociation and charge collection
probabilities of ITCF (97.8% and 91.1%) are higher than
those of IT-DM (95.0% and 80.5%) and IT-4F (96.3% and
88.1%) (Table S2). The effective exciton dissociation and
charge collection efficiencies in ITCF-based devices
partially contribute to its high FF. To gain deeper insight
into the relationship between light absorption and
photocurrent generation in the devices, the maximum
exciton generation rate (Gmax) was calculated. The Gmax
could be described follow the equation, Jsat = qLGmax, where
q is the elementary charge, L is the thickness of the active
layer, and Jsat stands for the saturated photocurrent
based devices show relatively lower α values of 0.94 and
0.95, respectively. This suggests that less bimolecular
recombination occurred in the ITCF device, supporting its
high FF.76
The charge transport behaviors of the active layers were
investigated by the SCLC method (Figure S14). The hole-
and electron-only devices were fabricated with the
structures of ITO/MoOX/active layer/MoOX/Al and
ITO/ZnO/active layer/ZrAcac/Al, respectively. The
detailed parameters are summarized in Table S2. The
charge transport properties of the annealed blend films
were improved compared with the as-prepared blend
films, as evidenced by the increased hole and electron
mobilities and the lowered μh/μes. This result is one of the
reasons that the annealed devices demonstrated better
performances than those of the as-prepared devices. As for
the annealed devices, a hole mobility of 8.38 × 10−4 cm2 V−1
s−1 was achieved for the ITCF-based device, which is
between those of the IT-DM- (8.07 × 10−4 cm2 V−1 s−1) and
IT-4F- (8.81 × 10−4 cm2 V−1 s−1) based device, whereas the
electron mobility (5.32 × 10−4 cm2 V−1 s−1) for the ITCF-
based device is higher than those of the IT-DM- (4.13 ×
10−4 cm2 V−1 s−1) and IT-4F- (4.89 × 10−4 cm2 V−1 s−1) based
device. Notably, the ITCF-based device demonstrated a
more balanced charge transport with a μh/μe of 1.58, which
is lower than those of the IT-DM- (1.95) and IT-4F- (1.80)
based device, accounting for its high FF.
density.72 The Gmax
s of the optimized devices were
Morphology of Blend Films. Figure 3a-f and Figure S15
shows the surface morphologies of the blend films. The
morphologies of the as-cast blend films (Figure S15) are
similar to the morphologies of the thermally annealed
blend films, with an increase in the RMS roughness for the
latter. The slight increase in the RMS implies that the
crystallinity of the film is enhanced, which is favorable for
charge transport. This result is consistent with the
enhanced JSC and FF in the corresponding OSC device. The
ITCF blend film has a nice surface morphology with fine
nanoscale phase separation, and the RMS of ITCF blend
film is roughness between that of IT-DM and IT-4F blend
film, whose surface morphology is rougher or smoother
than that of the ITCF blend film.
evaluated to be 1.10 × 1028 m−3 s−1 for IT-DM, 1.18 × 1028
m−3 s−1 for ITCF, and 1.27 × 1028 m−3 s−1 for IT-4F,
respectively. The Gmax of ITCF is between that of IT-DM
and IT-4F, which can be explained by that the absorption
onset of the ITCF blend film is between that of the IT-DM
and IT-4F blend films.73
In addition, we also measured the JSC of each optimized
device under different light intensities (Plight) to study the
charge recombination behavior (Figure 2d). The
relationship between JSC and Plight can be described as JSC
∝
74, 75
Pα
,
where the power-law exponential factor α
light
implies the extent of bimolecular recombination. The α of
the ITCF-based device is 0.98, while the IT-DM- and IT-4F-
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Chemistry of Materials
*T.L.: liutaozhx@ust.hk.
The molecular packing and orientation of the blend films
were investigated by GIWAXS (Figure 3g and Figure S16).69
The lamellar packing peaks of all blend films are at qr ~
0.30 Å−1, originated from the J71 component (Figure S9).
The as-cast blend films showed weak (010) π-π diffraction
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Author Contributions
#M.H. and T.L. contributed equally to this work.
peaks at qz
~
1.70 Å−1, which were significantly
Notes
o
strengthened after being annealed at 100 C, indicative of
the predominant face-on oriented π-π stacking in the
optimized blends. These characteristics support that the
annealed blend films achieved higher charge carrier
mobilities than the as-cast blend films did. By Scherrer
equation,77 the π-π stacking coherence length of annealed
ITCF blend film was estimated to be 22.86 Å, larger than
that of the annealed IT-DM (17.66 Å) and IT-4F (17.52 Å)
blend films, in agreement with the observed higher charge
carrier mobility and FF.
The authors declare no competing financial interests.
ACKNOWLEDGMENT
9
This study was financially supported by the National Natural
Science Foundation of China (No.21572171, No.51773118,
and No.51873160) and the Research Grant Council of Hong
Kong (General Research Fund No.14314216 and Theme-based
Research Scheme No. T23-407/13-N).
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