Strategic end-halogenation of π-conjugated small molecules enabling fine morphological control and enhanced performance of organic solar cells

Article abstract of DOI:10.1039/c9ta03869h

Organic solar cells based on π-conjugated small molecules (SM-OSCs) have developed rapidly in the past few years. Nevertheless, the design strategies of small-molecule (SM) donors for high-efficiency SM-OSCs require further improvement. Halogenation is an effective way to modulate their electronic properties, and to date, fluorination has been most widely used for the design of organic photovoltaic materials. However, the feasibility and utility of photovoltaic materials incorporating other halogens, especially heavier bromine and iodine, have not been fully explored. Here, a novel family of SM donor materials, having the same π-conjugated backbone but different terminal halogen groups (F, Cl, Br, and I), are systematically designed and developed. Furthermore, the structure-property-function relationships stemming from the variations in the substituted halogen atoms are discussed. Among these end-halogenated SM donors, the I-containing material shows remarkably high photovoltaic performance in the fullerene-based SM-OSCs, demonstrating power conversion efficiencies of up to 9.2% without any processing additives and post-treatment processes. Detailed morphological analyses reveal that end-halogenation with heavier halogen atoms, typified by I and Br, effectively modulated the interfacial free energy of the blend with a fullerene acceptor and facilitate the formation of fine interpenetrating networks in the active layer. This study establishes a new design paradigm featuring end-halogenation for producing high-performance photovoltaic materials for SM-OSCs.

Full text of DOI:10.1039/c9ta03869h

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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS nanocages as a lithium ion battery anode material
2
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DOI: 10.1039/C9TA03869H
ARTICLE
Strategic end-halogenation of -conjugated small molecules
enabling fine morphological control and enhanced performance
of organic solar cells
Received 00th January 20xx,
Accepted 00th January 20xx
ab
ab
Seiichi Furukawa and Takuma Yasuda*
DOI: 10.1039/x0xx00000x
Organic solar cells based on -conjugated small molecules (SM-OSCs) have developed rapidly in the past few years.
Nevertheless, the design strategies of small-molecule (SM) donors for high-efficiency SM-OSCs require further improvement.
Halogenation is an effective way to modulate their electronic properties, and to date, fluorination has been most widely
used for the design of organic photovoltaic materials. However, the feasibility and utility of photovoltaic materials
incorporating other halogens, especially heavier bromine and iodine, have not been fully explored. Here, a novel family of
SM donor materials, having the same -conjugated backbone but different terminal halogen groups (F, Cl, Br, and I), are
systematically designed and developed. Furthermore, the structure–property–function relationships stemming from the
variations in the substituted halogen atoms are discussed. Among these end-halogenated SM donors, the I-containing
material shows remarkably high photovoltaic performance in the fullerene-based SM-OSCs, demonstrating power
conversion efficiencies of up to 9.2% without any processing additives and post-treatment processes. Detailed
morphological analyses reveal that end-halogenation with heavier halogen atoms, typified by I and Br, effectively modulated
the interfacial free energy of the blend with a fullerene acceptor and facilitate the formation of fine interpenetrating
networks in the active layer. This study establishes a new design paradigm featuring end-halogenation for producing high-
performance photovoltaic materials for SM-OSCs.

-Conjugated small-molecule (SM) donors,6,7 although less
Introduction
studied than their polymer counterparts thus far, have drawn
increasing research interest because of their inherent
advantages such as well-defined chemical structures without
molecular weight distribution, precise tunability of energy
levels, and excellent reproducibility in photovoltaic
performance. These features motivated us to study the
structure–property–function relationships in SM donors, as
their performance strongly depends on the component
chemical structures. For the design of SM donors, the use of π-
conjugated acceptor–donor–acceptor (A–D–A) electronic
systems, combining a central electron-donating core (D) and
two terminal electron-withdrawing units (A), is currently
With increasing renewable energy demands for a sustainable
future, organic solar cells (OSCs) are an emerging energy
conversion technology owing to their potential advantages such
as low cost, light weight, mechanical flexibility, solution
processability, and facile large-area manufacturing.1,2 Advances
in synthetic chemistry for producing diverse and useful
-
conjugated molecular scaffolds have led to a sharp increase in
the power conversion efficiencies (PCEs) of OSCs in the last
decade. Solution-processed OSCs are typically based on bulk-
heterojunction (BHJ) architectures comprising a blend of p-type
(electron donor) and n-type (electron acceptor) organic
7–21
considered as one of the most promising structural motifs.
semiconductors. The PCEs of single-junction polymer:fullerene-
based OSCs have exceeded 11%, and nowadays, those of
polymer:nonfullerene-based devices have caught up and
reached over 14%. In addition to recent innovations in such
nonfullerene acceptor materials, a major breakthrough in the
design of donor materials is of urgent need to further enhance
the PCEs of OSCs and accelerate their practical applications.
8
–19
For instance, benzo[1,2-b:4,5-b']dithiophene (BDT)
and
3
2
0
oligothiophene have been extensively utilized as the central D
unit and rhodanine derivatives as the terminal A units in high-
performing SM donors. With proper device engineering and
optimizations, the PCEs of state-of-the-art single-junction OSCs
employing these SM donors have progressed beyond
4
5
11,14,16,17
1
1%,
closing upon those of polymer-based devices.
While the design and combination of the D and A components
are crucial, effective building units that are eligible for SM
donors are very limited, and versatile design guidelines are still
lacking.
Halogenation is a powerful strategy to modulate the energy
levels, BHJ morphologies, and eventually photovoltaic
performances in OSCs. In particular, replacing hydrogen atoms
a.Department of Applied Chemistry, Graduate School of Engineering, Kyushu
University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan.
INAMORI Frontier Research Center (IFRC), Kyushu University, 744 Motooka, Nishi-
b.
ku, Fukuoka 819-0395, Japan. *E-mail: yasuda@ifrc.kyushu-u.ac.jp
22
†
Electronic Supplementary Information (ESI) available: Detailed experimental
procedures and characterization data, and additional data (NMR, TGA, DSC, OSC,
XRD, and contact angles). See DOI: 10.1039/x0xx00000x
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ARTICLE
with fluorine (F) atoms, the most electronegative element, has nonhalogenated
Journal Name
1
. Our results demonstrated that incorporating
View Article Online
3
,4,23–
DOI: 10.1039/C9TA03869H
been applied to a wide variety of semiconducting polymers
unexplored heavy halogen atoms (e.g., Br and I) is a viable and
25
11,26
and recently a few SM donors.
At present, record-high effective strategy for developing high-performance
PCEs of polymer-based OSCs have been achieved by using photovoltaic materials.
3
,4
fluorinated polymer donors. This backbone fluorination can
down-shift the energy levels of both the highest occupied
molecular orbital (HOMO) and lowest unoccupied molecular
orbital (LUMO) of polymer donors without causing large steric
Results and discussion
Materials synthesis and characterization
hindrance, and thereby increase the open-circuit voltages (Voc
of the resulting devices. Additionally, due to noncovalent F∙∙∙H,
F∙∙∙S, and F∙∙∙ interactions, fluorination can enhance the
)
The synthetic routes to the new SM donors 1–5 are described in
Scheme 1, and the detailed synthetic procedures including
characterization data are provided in the ESI†. Compound 7 was
prepared from 3,4'-dihexyl-2,2'-bithiophene in two steps

intermolecular interactions, which contribute to the
improvements in crystallinity, film morphology, and charge-
transport properties. Meanwhile, some research groups have
recently begun to investigate whether halogenation (mostly
focused on fluorination or chlorination) on the nonfullerene
ladder-type acceptors can further improve the photovoltaic
through sequential bromination and Vilsmeier–Haack
1
9
formylation, according to our recent report. Two-fold Stille
coupling reactions between and the distannyl-BDT ( ) using
Pd(PPh as the catalyst afforded dialdehyde as the key
precursor in 87% yield. Finally, Knoevenagel condensation
reactions between and ID (9-H) or 5-halo-IDs (9-Xs; X = F, Cl,
7
6
3
)
4
8
4b,27
properties,
as it does for the foregoing polymer donors. To
8
date, however, systematic studies on photovoltaic materials
incorporating different halogen substituents, apart from F
Br, I) yielded 1–5 as black powdery solids. This synthetic
approach is effective for the introduction of various halogen
groups, including reactive Br and I groups, into
molecules. All these target compounds were highly soluble in
chlorinated solvents such as chloroform and chlorobenzene by
virtue of six side chains, and successively purified by column
chromatography and recycling preparative gel permeation
chromatography (GPC) with 35–93% isolation yields. Note that,
4c,26a,27d,e
atoms, in the -conjugated backbones are really rare.
-extended
Although considerably lesser attention has been given to the
introduction of heavier/larger bromine (Br) or iodine (I) atoms,
it is of great interest to investigate the variations in the
physicochemical and photovoltaic properties of SM donors
substituted with a series of the halogen atoms.
unlike 1, each material of end-halogenated 2–5 was obtained as
a mixture of three inseparable isomers since the terminal halo-
ID units were attached so as to form two local isomeric
structures in the final condensation step. The chemical
1
13
structures of 1–5 were verified by H and C NMR spectroscopy,
mass spectrometry, and elemental analysis (ESI†).
Fig. 1 Chemical structures of BDT-ID-Xs, 1–5, with different end-groups.
In this work, to reveal how the systematic halogenation
affects the resultant material properties and device
performance, we designed and synthesized a novel family of SM
donor materials, namely, BDT-ID-Xs (X = H, F, Cl, Br, and I for 1–
5
; Fig. 1). These molecules had identical
backbones, consisting of a central BDT core and two peripheral
,3-indandione (ID) units tethered by -conjugated bithiophene
-conjugated
1

1
9
Scheme 1 Synthetic routes for 1–5.
linkers with alkyl side chains, but different terminal halogen
substituents in the ID units. With this unique SM donor family,
the halogenation effect on the photovoltaic performances of 1–
The thermal properties of 1–5 were evaluated by
thermogravimetric analysis (TGA) and differential scanning
calorimetry (DSC) under a nitrogen atmosphere (ESI†). The
decomposition temperatures with 5% weight loss were found
5
blended with [6,6]-phenyl-C71-butyric acid methyl ester
(PC71BM) as the acceptor, as well as the derived morphological
and charge-transport properties were systematically
investigated and compared. Importantly, it was found that the
to decrease in the order of
C) > (363 °C) > (346 °C); the observed trend is related to the
lowering of the associated C–X bond dissociation energies.
d
1 (T = 384 °C) > 2 (378 °C) > 3 (375
°
4
5
SM-OSCs based on iodinated
.2%, which is markedly higher than those of the devices
fabricated using the other end-halogenated 2–4 and
5 achieved the highest PCE of
28
9
Nevertheless, the thermal stabilities of all these end-
2
| J. Name., 2012, 00, 1-3
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halogenated molecules were sufficiently high for OSC
applications. It is also noted that the melting temperatures and
View Article Online
DOI: 10.1039/C9TA03869H
Table 1 Optical properties of 1–5
associated transition enthalpies of 2–5 (T
m
= 221–227 °C and

H
a

max (nm)
EHOMO
(eV)
d
ELUMO
(eV)
e
Eg
f
−
1
compound
=
25.4–28.7 kJ mol ) were much higher than those of
solb
542
548
554
554
556
—g
filmc
(eV)
−
1
nonhalogenated
m
1 (T = 204 °C and H = 23.2 kJ mol ) in spite
1
659
659
665
665
666
—g
−5.10
−5.20
−5.22
−5.22
−5.23
−6.1
−3.40
−3.50
−3.53
−3.52
−3.53
−4.3
1.70
1.70
1.69
1.70
1.70
of having only subtle variations in the two atoms at the
molecular termini. This result is indicative of stronger
intermolecular interactions in end-halogenated 2–5
2
3
.
4
5
Optical properties and electronic structures
PC71BM
1.8
The UV–vis absorption spectra of 1–5 in chloroform solutions
and in solid thin films are depicted in Fig. 2a,b, and the relevant
photophysical data are compiled in Table 1. In dilute solutions
a
b
−5
Absorption peak wavelength. Measured in chloroform solution (10 M) at
00 K. Measured in a neat film spin-coated from a chloroform solution onto
a quartz substrate. HOMO energy level determined by photoelectron yield
spectroscopy in a neat film. LUMO energy level calculated using ELUMO = EHOMO
c
3
d
(Fig. 2a), all of these molecules showed very similar broad
e
absorption bands below 650 nm, which were attributed to the
f
g
+ E . Optical bandgap derived from the absorption onset of the neat film. Not
g
intramolecular
charge
transfer
(ICT)
transitions.
determined.
Nonhalogenated
1
exhibited an absorption peak (max) at 542
nm in chloroform. Upon introducing halogen atoms, solutions
of 2–5 presented slightly red-shifted max in the range of 548–
56 nm. Relative to their solutions, the absorption spectra of 1–
in the thin films showed obvious red-shifts of
appearance of pronounced vibronic peaks (Fig. 2b), suggesting
enhanced intermolecular interactions in the solid state.
Consequently, 1–5 showed nearly identical film absorption
In contrast, it was found that end-halogenation affected the
molecular energy levels. We performed photoelectron yield
spectroscopy to determine the HOMO energy levels (EHOMO; or
ionization potentials) of 1–5 in the thin films (Fig. 2c). As listed
in Table 1, the EHOMO values of end-halogenated 2–5 were
measured to be −5.20 to −5.23 eV, which were over 0.1 eV lower

5
5
max with an
than that of
1 (EHOMO = −5.10 eV). The low-lying EHOMO of all
spectra, with
extending over 730 nm, corresponding to the optical bandgaps
) of approximately 1.7 eV. The similar absorption profiles,
max of 659–666 nm and absorption edges
these SM donors would be beneficial to obtain higher Voc in the
SM-OSCs because Voc is directly proportional to the energy
offset between the EHOMO of the donor and the LUMO energy
level (ELUMO; or electron affinity) of the acceptor. Likewise, the
(E
g
maximum extinction coefficients, and bandgap energies of 1–5
imply that the variation in the halogen substituents had minimal
effect on the electronic transition characteristics in this system.
ELUMO of 2–5 showed a decline of ~0.1 eV as compared to that
of because of the inductive electron-withdrawing effect
1
and/or high polarizability of the halogen groups. Overall, end-
halogenation simultaneously stabilized the EHOMO and ELUMO to
nearly the same degree, thus having a lesser impact on the
g
optical E values.
To further investigate the molecular geometries and
electronic properties, density functional theory (DFT)
calculations were performed for 1–5 at the B3LYP/3-21G* level.
As can be seen from Fig. 3a, the five molecules exhibited very
similar frontier orbital distributions together with comparable
high-planar geometries owing to their identical -conjugated
backbone. For all molecules, the HOMO wave functions were
delocalized over the BDT core and the neighboring bithiophene
moieties; whereas the LUMO wave functions were primarily
distributed around the ID units along with the bithiophene
moieties and extend to the edge of the ID units. Despite the
similar frontier orbital distributions, end-halogenated 2–5
presented slightly lower calculated EHOMO and ELUMO values as
compared to
foregoing optical experimental results. Moreover, the lowest
excited singlet (S ) states of 1–5, which are dominated by the
HOMO LUMO transition, exhibited comparably large
1. This trend is reasonably consistent with the
1
→
oscillator strengths (f = 2.06–2.14), leading to large extinction
coefficients that are beneficial for the effective utilization of
sunlight. For further comparison, we also calculated the electric
dipole moments for each 2-thienylmethylene-ID fragment of 1–
Fig. 2 UV–vis absorption spectra of 1–5 in (a) chloroform solutions and
5
(Fig. 3b). As expected, the end-halogenated ID units possessed
(
b) as-spun thin films. (c) Photoelectron yield spectra of 1–5 in thin films.
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Fig. 3 (a) Frontier molecular orbital distributions and energy levels for 1–5 calculated at the B3LYP/3-21G* level. All alkyl chains were replaced by methyl
groups to simplify the calculations. The black arrows in (a) indicate the transition to the first excited singlet (S ) state with the oscillator strengths (f). (b)
1
Schematics of the dipole moments (μ, denoted by the blue arrows) simulated for the terminal 2-thienylmethylene-ID fragments of 1–5.
Fig. 4 (a) J–V characteristics under one sun illumination at 100 mW cm−2 (inset: dark J–V curves), (b) external quantum efficiency (EQE) spectra, (c)
photocurrent density (Jph) versus effective voltage (Veff) characteristics, and light intensity dependence of (d) Jsc and (e) Voc for the optimized SM-OSCs
based on 1–5:PC71BM (2:1, w/w) BHJ blends. The straight lines in (d,e) are fittings of each of the data plots.
2
–3-fold larger local dipole moments (μ = 2.08–2.55 D)29 than Solution-processed BHJ OSCs were fabricated employing 1–5 as
the parent ID unit (μ = 0.85 D) along the
backbones. Inducing large permanent dipoles can lead to a structure of indium tin oxide (ITO)/ZnO (30 nm)/PFN-
decrease in miscibility with nonhalogenated PC71BM, and Br/donor:PC71BM (ca. 70–80 nm)/MoO
(10 nm)/Ag (100 nm).30
-conjugated SM donors and PC71BM as an acceptor, with an inverted device
x
simultaneously increase the domain purity within the D/A The ZnO electron extraction layer was deposited on the ITO-
segregated BHJ morphologies when mixed with the PC71BM coated substrates via a sol–gel method, in which the surface
acceptor.
was treated with a conjugated polyelectrolyte, PFN-Br, as an
31
interfacial layer to facilitate electron extraction. The BHJ
active layer consisting of a donor:PC BM binary blend was
Photovoltaic performance
71
formed by spin-coating from their chloroform solutions. A thin
4
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Table 2 Optimized photovoltaic parameters for SM-OSCs based on 1–5:PC71BM
View Article Online
DOI: 10.1039/C9TA03869H
Jb
V
(V)
FF
(%)
PCEc
(%)
μ
d
μ
d
thickness
J
sc
oc
D/A
D
donora
−
2
−2
2
−1 −1
2
−1 −1
(nm)
(mA cm )
(mA cm )
(cm V s )
(cm V s )
1
2
3
4
5
77
77
80
74
78
14.0 ± 0.2
11.6 ± 0.1
11.4 ± 0.2
12.5 ± 0.1
14.2 ± 0.1
16.0
12.8
12.4
13.3
15.1
0.94 ± 0.01
0.95 ± 0.01
0.94 ± 0.01
0.94 ± 0.01
0.94 ± 0.01
44 ± 1
67 ± 1
68 ± 1
68 ± 1
68 ± 1
5.8 ± 0.1 (5.9)
7.4 ± 0.1 (7.5)
7.2 ± 0.1 (7.4)
8.0 ± 0.1 (8.2)
9.0 ± 0.1 (9.2)
3.5 × 10−3
3.0 × 10−2
4.6 × 10−2
4.1 × 10−2
5.1 × 10−2
9.5 × 10−2
4.3 × 10−2
4.6 × 10−2
4.0 × 10−2
5.1 × 10−2
a
c
b
Device structure: ITO/ZnO (30 nm)/PFN-Br/donor:PC71BM (2:1, w/w; 70–80 nm)/MoO
x
(10 nm)/Ag (100 nm). Calculated by integrating the EQE spectra.
, where P is the incident light intensity (100
Average power conversion efficiency (PCE) calculated using four individual devices by PCE = (Jsc × Voc × FF)/P
0
0
−
2
d
mW cm ); the best PCEs are given in parentheses. Hole mobilities for the donor:PC71BM (2:1, w/w) blend film (μD/A) and pristine donor neat film (μ )
D
evaluated using the SCLC technique.
layer of MoO
x
as a hole extraction layer and an Ag anode were
1-based device, the Jsc value was lower than the current density
subsequently vacuum-deposited on the active layer. In most estimated from the integration of the EQE spectrum (Table 2).
reported OSCs, their BHJ active layer morphologies were This can be attributed to inferior exciton dissociation and
generally optimized by using solvent additives such as 1,8- charge collection characteristics in the nonhalogenated
diiodooctane to enhance the photovoltaic performance. In the
present systems, however, we have prepared the BHJ active
1:PC71BM BHJ system, as will be discussed later.
layers without using any solvent additives or post-treatments Charge generation, recombination, and transport characteristics
such as thermal annealing and solvent vapor annealing, in order Considering the nearly same photoabsorption coverage among
to elucidate the intrinsic properties and potential abilities of 1– the five donor materials (Fig. 2b), the difference of exciton
5
as photovoltaic materials. This feature can offer significant dissociation and charge collection behaviors should be
advantages in terms of the simplicity of the fabrication responsible for the large variation in the J and FF of the SM-
sc
processes and morphological stability of the OSC devices.
71
OSCs based on 1–5:PC BM. To better understand these
To attain the best photovoltaic performance, we first characteristics, we analyzed the dependence of the
screened the donor:acceptor (D/A) blend ratios from 1:1 to 4:1; photocurrent density (J ) on the effective voltage (V ), as
ph
eff
the optimal D/A blend ratios were found to be around 2:1 (w/w) presented in Fig. 4c. Here, Jph is defined as J − J , where J and
L
D
L
for the five donor materials (ESI†). Fig. 4a shows the current JD are the current densities of the devices under illumination
density–voltage (J–V) curves for thus optimized SM-OSCs based and in the dark, respectively; Veff that governs the internal
on 1–5:PC71BM (2:1, w/w) under simulated AM 1.5G electric field within the device, is defined as Veff = V – V, where
0
illumination. The detailed photovoltaic parameters are given in V0 is the voltage at which Jph is fully compensated (i.e., J = 0)
ph
3
2
Table 2 for a clear comparison. The nonhalogenated
1-based and V is the external applied voltage. Upon increasing Veff up
devices showed a relatively low maximum PCE of 5.9% in spite to ~1.5 V, the Jph increased and reached saturation (to give Jsat),
−
2
of its high short-circuit current density (Jsc) of ~14 mA cm , indicating that almost all photo-generated excitons were
which primarily originated from a low fill factor (FF) of ~44%. dissociated into free charge carriers and collected by the
Intriguingly, however, the FF values of the SM-OSCs featuring electrodes with minimal bimolecular recombination. The ratio
end-halogenated donors (2–5) were found to drastically of Jph to Jsat (J /J ) is a useful measure of the overall efficiency
ph sat
increase (by more than 20%) at the slight expense of Jsc. Besides, (or probability) for exciton dissociation and charge collection in
the Voc remained unchanged and retained high values of 0.94– OSCs. Under short-circuit conditions, J /J ratios as large as
ph sat
0
.95 V, regardless of the variation of the halogen substituents. 0.96–0.97 were obtained for the end-halogenated 2–5-based
As a result, the highest PCEs of up to 9.2% (9.0% on average) devices, while it was 0.88 for the
-based device. Even at the
were achieved for the iodinated -based devices, with Jsc of 14.2 maximum power output conditions, the 2–5-based devices
mA cm , Voc of 0.94 V, and FF of 68%. However, the likewise exhibited much larger J /J ratios of 0.77–0.81 as
1
5
−
2
ph sat
introduction of Br, Cl, and F atoms instead of I atoms in the compared to the
terminal ID units led to a gradual decrease in the Jsc values with unambiguously indicate that the end-halogenation of the SM
respect to that of , resulting in lowering the maximum PCEs in donors leads to more efficient exciton dissociation and charge
the order of iodinated (9.2%) > brominated (8.2%) > transport and collection, contributing to enhanced FF of the
chlorinated (7.4%) ≈ fluorinated (7.5%). The external devices.
quantum efficiency (EQE) spectra for these SM-OSCs are shown
As shown in Fig. 4d,e, we also measured the J and V for
1-based device (0.58). These results
5
5
4
3
2
sc
oc
in Fig. 4b. In comparison with the 2–4-based devices, the
5
1
- and each optimized device under different incident light intensities
-based ones apparently exhibited better photon-to-current (P) in the range of 1–100 mW cm− (i.e., 0.01–1 sun) to further
2
responses over the entire wavelength range between 350 and analyze the charge recombination behavior. In general, the
α 19,33
7
00 nm, thereby contributing to their improved Jsc values. For relationship between J and P can be described as J
∝ P ,
sc sc
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where the exponential factor α is a figure-of-merit denoting the
degree of bimolecular charge recombination. If more
dissociated free charge carriers can be swept out and collected
by the electrodes prior to recombination, α is closer to 1 (α ≤ 1).
By fitting the double logarithmic plots of Jsc versus P (Fig. 4d),
similar α values of 0.94–0.96 were obtained for the SM-OSCs
made with 1–5:PC71BM, confirming that none of these devices
suffered from serious bimolecular recombination losses under
short-circuit conditions. The dependence of Voc on P can be
used to probe the charge recombination behavior under open-
circuit conditions, in which all photo-generated charge carriers
should recombine within the device (Fig. 4e). In principle,
bimolecular (Langevin) recombination is the sole dominant
View Article Online
DOI: 10.1039/C9TA03869H
mechanism when the slope of Voc versus ln(P) is equal to 1kT/q Fig. 5 Double logarithmic plots of J–V characteristics of hole-only
(
= 25.86 mV at 300 K); whereas trap-assisted (Shockley–Read– devices with the structures of ITO/PEDOT-PSS (30 nm)/(a) 1–5 neat films
or (b) 1–5:PC71BM blend films (ca. 130–180 nm)/MoO (10 nm)/Ag (100
x
Hall) recombination should be much more pronounced when
the slope approaches 2kT/q (where k is the Boltzmann constant,
T is absolute temperature, and q is the elementary charge).33,34
While the slopes of the fitted lines for the 2–5-based devices
nm). In the SCLC region at high voltages, the log(J) versus log(V) is ≈2.
Film morphology and nanostructures
The nanoscale structural order and crystallinity of 1–5 in the
neat films and BHJ blend films were investigated by X-ray
diffraction (XRD) measurements, as presented in Fig. 6.
were in the range of 1.33–1.43kT/q, the
exhibited the smallest slope close to 1kT/q. It is thus anticipated
that the :PC71BM blend can reduce the trap densities at the
1-based device
1
Nonhalogenated
1
exhibited a relatively sharper (100)
D/A interfaces, thereby suppressing the monomolecular or
trap-assisted recombination.
−1
diffraction peak located at 0.33 Å (d = 1.9 nm) together with
higher-order diffractions of up to (300), originating from the
ordered lamellar packing in the thin films. For both neat and
blend films of 2–5, somewhat broader (100) diffraction peaks
The charge transport properties are another important factor
in determining the photovoltaic device performance, especially
FF. As demonstrated in Fig. 5, the hole mobilities of 1–5 in both
neat films and blend films were measured using the hole-only
devices by fitting their J–V data to a space-charge-limited
−1
were observed at 0.32–0.34 Å , corresponding to similar
interlamellar distances of d = 1.8–2.0 nm. Using the Scherrer
36
equation, the crystal coherence lengths (or crystallite sizes)
35
current (SCLC) model. As listed in Table 2, all as-spun neat
for 1–5 were estimated by correlating with the full-width at
half-maximum of the respective (100) peak in the XRD patterns.
films exhibited rather high hole mobilities (μ
D
) that exceeded
−
2
2
−1 −1
2
1
0
cm V s . Notably, the μ
D
of pristine
1
reached ~0.1 cm
The lamellar coherence length (L) for
1 in the blend film was
−
1
−1
V
s , which was approximately twice as high as those of end-
estimated to be 18 nm (corresponding to ~10 molecular
lamellar stacks), much smaller than that in the neat film (L = 30
nm), which was likely responsible for the lower hole mobility
halogenated 2–5 in the neat films, owing to its higher degree of
molecular ordering (see below). However, as a consequence of
excessive material mixing with the PC71BM acceptor, the hole
(
Table 2). In contrast, the calculated lamellar coherence lengths
for end-halogenated 2–5 in their neat and blend films were all
2–14 nm, and remained almost unchanged even after mixing
mobility of the
1:PC71BM blend film (μD/A) was decreased
sharply by more than one order of magnitude, lowering to 3.5 ×
1
−
3
2
−1 −1
1
0
cm V s . In contrast, the μD/A values of the 2–5:PC71BM
with PC71BM.
blend films were found to be unchanged even after mixing with
PC71BM, retaining the high values in the order of 10−2 cm V
2
−1
−
1
s . This feature is attributable to suppressed material mixing
between the end-halogenated SM donors and PC71BM acceptor
at the molecular level.
Fig. 6 XRD patterns of (a) neat films of 1–5 and (b) blend films of 1–
5
:PC71BM (2:1, w/w).
6
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The interior morphologies of the BHJ active layers were also
characterized by transmission electron microscopy (TEM). As
can be seen from Fig. 7, the TEM images of the 1–5:PC71BM
blend films revealed markedly different nanoscale phase
segregation for the five analogous SM donors, depending only
on the variation in the terminal substituents. The bright and
dark regions in each TEM image correspond to the SM donor-
rich and PC71BM-rich domains, respectively, as PC71BM has a
higher electron density than the SM donors. The
View Article Online
DOI: 10.1039/C9TA03869H
nonhalogenated
1:PC71BM blend film had relatively
homogeneous and fine nanostructures with closely mixed D/A
networks (Fig. 7a). Because the Jsc of the BHJ OSCs is closely
related with the interfacial domain area, it is envisioned that
this smaller size scaled D/A segregation in 1:PC71BM is beneficial
for photocurrent generation. Upon fluorination, however, the
length scale of the D/A phase segregation evidently increased
and distinct agglomerates (over 100 nm in size) existed within
the 2:PC71BM blend film (Fig. 7b), presumably because of the
low entropy of mixing. More importantly, when the terminal
halogen groups were replaced by larger Cl, Br, and I atoms, the
domain sizes became gradually smaller (Fig. 7c–e), and much
finer bicontinuous D/A interpenetrating networks were
eventually formed with the 5:PC71BM blend film.
For more quantitative comparison of these nanoscale BHJ
morphologies, mean domain sizes (D), which correspond to the
periodicity of the phase-segregated structures, were estimated
using power spectral densities (PSDs) obtained from two-
Fig. 7 TEM images of BHJ active layers composed of (a)
1:PC71BM, (b)
2
:PC71BM, (c) :PC71BM, (d) :PC71BM, and (e) :PC71BM (2:1, w/w)
3
4
5
blends. The D values represent the average domain sizes. (f) PSD profiles
of the blend films obtained from radially averaged 2D-FFT analysis of
the TEM images.
37
dimensional fast Fourier transform (2D-FFT) analysis (Fig. 7f).
From the PSD peak positions, the average domain sizes in the Interfacial free energy analysis
BHJ blend films were found to decrease in the order fluorinated
The surface free energy of a material is defined as the excess of
energy at its surface as compared to the bulk, and is influenced
by its polarity, crystallinity, and noncovalent intermolecular
interactions. To quantify the miscibility (compatibility) between
the donor and acceptor materials and unveil the mechanism of
the interior morphological changes in the 1–5:PC71BM BHJ
systems, we evaluated the surface free energies of 1–5 as well
as PC71BM from the contact angle measurements (ESI†). The
2
(D = 56 nm) ≈ chlorinated
3
(D = 55 nm) > brominated
4 (D =
4
4 nm) > iodinated (D = 32 nm) > nonhalogenated 1
5
(D = 11
nm). Considering the limited exciton diffusion length of organic
semiconductors, which is usually less than 10 nm, finely nano-
segregated domains without excessive agglomeration would be
more favorable for charge generation at the D/A interfaces,
though each domain needs to have proper size and purity for
ensuring the effective percolation channels of charge transport.
It seemed that the interior morphologies obtained for the
correlation between the contact angle (θ) and surface free
energies of a solid surface (γ
S
) and a probe liquid (γ
L
) is given by
iodinated 5:PC71BM blend best matched these criteria, and
hence, resulted in superior performance in the SM-OSCs.
the Young equation:
훾S = 훾SL + 훾 cos 휃
L
(1)
where γSL denotes the interfacial free energy between the solid
S) and liquid (L) interface. Here, the total solid surface free
energy γ is defined as the sum of the surface dispersive and
polar components (γ
(
S
d
p
S
and γ
S
, respectively) as follows.
훾S = 훾Sd + 훾Sp
According to the Owens–Wendt–Kaelble method,38
represented using the following geometric mean function:
훾SL = 훾 + 훾 − 2 (√훾Sd훾Ld + √훾Sp훾Lp)
(3)
are the dispersive and polar components of
(2)
γSL is
S
L
where γ d
the liquid surface free energy (γ
and γ p
L
L
d
p
L
L L
= γ + γ ). By combining eq 1
and eq 3, the following formula can be derived.
ꢌ
ꢀ
ꢁ
ꢂ1ꢃꢄꢅꢆ ꢇꢈ
ꢊ
ꢀ
ꢁ
√훾Sd + √훾Spꢋꢀ
ꢁ
=
(4)
ꢊ
ꢉ√ꢀ
ꢁ
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:PC71BM and
d
p
Accordingly, if the
liquids are given, γ
θ
, γ
L
, and γ
S
L
values for different probe TEM images (Fig. 7e). In contrast, the fluorinated 2
View Article Online
d
p
DOI: 10.1039/C9TA03869H
S
and γ
of each material can be calculated chlorinated
3
:PC71BM systems with larger γD/A values tended to
by using eq 4.
form less homogeneous, aggregated morphologies originating
from the suppressed D/A interface formation during the spin-
coating process (Fig. 7b,c). Correspondingly, the resultant Jsc
values of the OSC devices were found to have a close correlation
with the γD/A. It is noteworthy that as for the end-halogenated
2
–5:PC71BM BHJ systems, a smaller γD/A and mean domain size
led to a higher Jsc in the actual OSC devices (Fig. 8b).
Table 3 Surface and interfacial free energy data
γd a
mJ m )
γp a
(mJ m−2)
γ a
(mJ m−2)
b
γ
D/A
compound
−
2
−2
(
(mJ m )
1
2
3
4
5
20.6
26.8
27.6
28.6
28.5
0.76
0.24
0.18
0.17
0.27
0.77
21.4
27.0
27.8
28.8
28.8
35.4
1.80
0.65
0.60
0.51
0.42
—
PC71BM
34.6
a
d
p
Dispersive (γ ) and polar (γ ) components of surface free energy of each
compound; γ = γ + γ (eq 2). Interfacial free energy between each donor
d
p
b
and PC71BM calculated using eq 5.
Fig. 8 (a) Owens–Wendt plots for the neat films of 1–5 and PC71BM. The
straight lines are the fitting of the plots using eq 4. (b) Correlations of
mean domain size (D) in the 1–5:PC71BM blend films (upper plots) and
J
sc of the corresponding OSCs (lower plots) with D/A interfacial free Conclusions
energy (γD/A).
In this paper, we presented a new family of photovoltaic SM
donors, BDT-ID-Xs (X = H, F, Cl, Br, and I), having the same -
conjugated backbone but different terminal halogen groups.
Employing this series of analogous materials, we systematically
investigated the impact of end-halogenation on the
physicochemical, charge-transport, and photovoltaic properties
as well as film morphological characteristics. All these
molecules demonstrated a very similar strong photoabsorption
over a wide visible range, with comparable large extinction
Fig. 8a depicts the Owens–Wendt plots for the as-spun neat
films of 1–5 and PC71BM wetted by different probe liquids (i.e.,
water, glycerol, ethylene glycol, N,N-dimethylformamide, and
octane) based on the contact angle measurements. From linear
regression for these plots using eq 4, the γ and γ values of each
material could be extracted, and are listed in Table 3. The
PC71BM film had a relatively large total surface free energy (γ)
d
p
5
−1
−
2
39
coefficients (>10 cm ) and narrow bandgaps (~1.7 eV) in the
solid state. Importantly, it was found that the subtle change in
the terminal halogen groups drastically influenced the
photovoltaic properties and film morphologies. Among these
end-halogenated SM donors, the iodinated and brominated
materials indeed achieved remarkable photovoltaic
performance with PCEs of up to 9.2% and 8.2%, respectively, by
significant increment of Jsc and FF in the fullerene-based SM-
OSCs, without using any solvent additives. Further, the
morphological analyses revealed that end-halogenation with
heavy halogen atoms, typified by I and Br, allowed effective
modulation of the interfacial free energy of the blend with
PC71BM and facilitated the spontaneous formation of finer D/A
interpenetrating networks within the active layer. Although
attention has never been paid to such I- and Br-containing
functional materials so far (except for the synthetic precursors
and monomers for the cross-coupling reactions), our results
clearly demonstrate that the strategic halogenation with I and
Br groups is an effective and versatile approach for producing
sophisticated organic semiconducting materials. We thus
anticipate that these useful iodinated and brominated building
blocks will be widely applied in future studies of OSCs, organic
of 35.4 mJ m , which is close to the literature values. Among
the four end-halogenated SM donors, iodinated and
exhibited the largest γ values (28.8 mJ m ), which
5
−
2
brominated
4
primarily originated from the increase of the dispersive
component via iodination and bromination. It is anticipated that
the closer γ values of the individual components in the BHJ
binary blends improved the miscibility of the materials,
resulting in well-mixed D/A domains. To obtain more direct
information about the morphological differences, the D/A
interfacial free energies (γD/A) between these SM donors and
PC71BM acceptor were assessed using the following equation
(cf. eq 3):
훾D/A = 훾 + 훾 − 2 (√훾Dd훾Dd + √훾 p훾Ap) (5)
D
A
D
where subscripts D and A denote 1–5 and PC71BM, respectively.
As presented in Fig. 8b and Table 3, the subtle difference in the
terminal halogen atoms resulted in significant changes of the
γ
D/A values, which were consistently reduced from
Therefore, for the iodinated :PC71BM system with the smallest
D/A value, the energy penalty required to form a large D/A
2 through 5.
5
γ
interfacial area (or fine D/A interpenetrating networks with a
small mean domain size) can be minimized, as observed in the
8
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12 X. Yin, Q. An, J. Yu, F. Guo, Y. Geng, L. Bian, Z. XVuie,wBA.rtZichleoOun,linLe.
energy-harvesting devices,40 and other organic electronic
devices.
Xie, F. Zhang and W. Tang, Sci. Rep., 2016, 6, 25355.
DOI: 10.1039/C9TA03869H
1
1
1
3 L. Yang, S. Zhang, C. He, J. Zhang, H. Yao, Y. Yang, Y. Zhang, W.
Zhao and J. Hou, J. Am. Chem. Soc., 2017, 139, 1958.
4 J. Wan, X. Xu, G. Zhang, Y. Li, K. Feng and Q. Peng, Energy
Environ. Sci., 2017, 10, 1739.
Conflicts of interest
There are no conflicts to declare.
5 B. Qiu, L. Xue, Y. Yang, H. Bin, Y. Zhang, C. Zhang, M. Xiao, K.
Park, W. Morrison, Z.-G. Zhang and Y. Li, Chem. Mater., 2017,
2
9
, 7543.
1
1
6 Z. Zhou, S. Xu, J. Song, Y. Jin, Q. Yue, Y. Qian, F. Liu, F. Zhang
Acknowledgements
This work was supported in part by a Grant-in-Aid for JSPS
KAKENHI Grant No. JP18H02048 (T.Y.) and Amano Institute of
and X. Zhu, Nat. Energy, 2018, 3, 952.
7 L. Yang, S. Zhang, C. He, J. Zhang, Y. Yang, J. Zhu, Y. Cui, W.
Zhao, H. Zhang, Y. Zhang, Z. Wei and J. Hou, Chem. Mater.,
2
018, 30, 2129.
Technology (T.Y.). The computations were performed using the 18 Y. Hou, X.-T. Gong, T.-K. Lau, T. Xiao, C. Yan, X. Lu, G. Lu, X.
Zhan and H.-L. Zhang, Chem. Mater., 2018, 30, 8661.
9 H. Komiyama, T. To, S. Furukawa, Y. Hidaka, W. Shin, T.
Ichikawa, R. Arai and T. Yasuda, ACS Appl. Mater. Interfaces,
computer facilities at the Research Institute for Information
Technology, Kyushu University. The authors are grateful for the
support of the Cooperative Research Program "Network Joint
Research Center for Materials and Devices". S.F. acknowledges
the support from the Leading Graduate Schools Program of
1
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