Supramolecular engineering of oligothiophene nanorods without insulators: Hierarchical association of rosettes and photovoltaic properties

Article abstract of DOI:10.1002/chem.201404428

Supramolecular rosettes of oligothiophenes that do not bear long aliphatic tails have been designed as semiconducting nanomaterials for solution-processable bulk het-erojunction solar cells. The rosettes consist of six barbiturat-ed thienyl[oligo(hexylthiophene)] units (Bar-T-hT_n; n = 3,4,5) aggregated by multiple hydrogen bonds, which have been directly visualized by scanning tunneling microscopy (STM) at a solid-liquid interface. ¹H NMR spectroscopy in [D₈]toluene showed that Bar-T-hT_n exists as a mixture of monomers and small hydrogen-bonded aggregates. Hierarchical organization of the hydrogen-bonded aggregates took place through π - π stacking interactions upon casting their toluene solutions, resulting in the growth of highly ordered nanorods whose widths are consistent with the diameters of the rosettes. The nanorods could be generated in the presence of soluble fullerene derivatives via solution casting or the annealing of the resulting thin films. The solar cells fabricated based on these bulk heterojunction films showed power conversion efficiencies of 1-3%, which are far higher than those of the non-hydrogen-bonded reference oligothiophene and the derivative that possesses long aliphatic tails.

Full text of DOI:10.1002/chem.201404428

DOI: 10.1002/chem.201404428
Full Paper
&
Supramolecular Nanomaterials
Supramolecular Engineering of Oligothiophene Nanorods without
Insulators: Hierarchical Association of Rosettes and Photovoltaic
Properties
[
a, e]
[a]
[a]
[a]
[a]
Shiki Yagai,*
Mika Suzuki, Xu Lin, Marina Gushiken, Takuya Noguchi,
[a]
[a]
[b, e]
[c]
[c]
[d]
Takashi Karatsu, Akihide Kitamura, Akinori Saeki, Shu Seki, Yoshihiro Kikkawa,
Yuki Tani, and Ken-ichi Nakayama
[b]
Abstract: Supramolecular rosettes of oligothiophenes that
do not bear long aliphatic tails have been designed as semi-
conducting nanomaterials for solution-processable bulk het-
erojunction solar cells. The rosettes consist of six barbiturat-
took place through p–p stacking interactions upon casting
their toluene solutions, resulting in the growth of highly or-
dered nanorods whose widths are consistent with the diam-
eters of the rosettes. The nanorods could be generated in
the presence of soluble fullerene derivatives via solution
casting or the annealing of the resulting thin films. The solar
cells fabricated based on these bulk heterojunction films
showed power conversion efficiencies of 1–3%, which are
far higher than those of the non-hydrogen-bonded refer-
ence oligothiophene and the derivative that possesses long
aliphatic tails.
ed thienyl[oligo(hexylthiophene)] units (Bar-T-hT ; n=3,4,5)
n
aggregated by multiple hydrogen bonds, which have been
directly visualized by scanning tunneling microscopy (STM)
1
at
a
solid–liquid interface. H NMR spectroscopy in
[
D ]toluene showed that Bar-T-hT exists as a mixture of
8
n
monomers and small hydrogen-bonded aggregates. Hier-
archical organization of the hydrogen-bonded aggregates
Introduction
plored the construction of such columnar nanostructures by
supramolecular macrocyclization of heterocyclic small mole-
[2]
Hierarchical formation of well-defined nanostructures from syn-
thetically accessible small molecules through programmed
self-assembly would be a cost-effective approach toward the
fabrication of printable, large-area molecular devices. Columnar
nanostructures spontaneously formed by discoid or star-
shaped p-conjugated small molecules are particularly attractive
nanostructures as media for the quasi-one-dimensional trans-
cules directed by multiple hydrogen bonds. However, despite
vast examples of successful construction of such hierarchically
organized columnar nanostructures, little progress has been
made toward their application in organic devices, especially in
[3–5]
solution-processable bulk heterojunction (BHJ) solar cells.
This lack of progress is partly due to the fact that the forma-
tion of thermodynamically stable supramolecular macrocyclic
[
1]
[6]
port of charge carriers. Supramolecular chemists have ex-
assemblies (rosettes) requires “electronically inactive” long ali-
phatic chains to avoid irreversible (phase-separated) aggrega-
[7]
[8]
[
a] Prof. Dr. S. Yagai, M. Suzuki, Dr. X. Lin, M. Gushiken, T. Noguchi,
Prof. Dr. T. Karatsu, Prof. Dr. A. Kitamura
tion, which can generate kinetically trapped assemblies.
Such long aliphatic chains covering the semiconducting inter-
nal structure may hamper efficient charge separation with the
n-type semiconductor and charge transportation between the
nanostructures in BHJ films. Thus, the proper design of rosette-
forming p-conjugated small molecules with sufficient solubility
in organic media is necessary to afford highly organized nano-
structures applicable to BHJ solar cells, while minimizing the
unfavorable effects of the solubilizing chains on the electronic
properties of the devices.
Department of Applied Chemistry and Biotechnology
Graduate School of Engineering, Chiba University
1
-33 Yayoi-cho, Inage-ku, Chiba 263-8522 (Japan)
Fax: (+81)43-290-3039
E-mail: yagai@faculty.chiba-u.jp
[
b] Y. Tani, Prof. Dr. K.-i. Nakayama
Graduate School of Science and Engineering, Yamagata University
1
-1-1 Higashi, Tsukuba, Ibaraki 305-8562 (Japan)
[
c] Prof. Dr. A. Saeki, Prof. Dr. S. Seki
Department of Applied Chemistry, Graduate School of Engineering
Osaka University, 2-1, Yamadaoka, Suita, Osaka 565-0871 (Japan)
We have previously demonstrated that the barbiturate unit
is a powerful functional group for the organization of various
p-conjugated units into unique nanostructures. A variety of
[
d] Dr. Y. Kikkawa
National Institute of Advanced Industrial Science and Technology (AIST)
1
-1-1 Higashi, Tsukuba, Ibaraki 305-8562 (Japan)
[
9]
[9b,10]
[10]
nanostructures such as rods, rings,
coils,
and inter-
[
e] Prof. Dr. S. Yagai, Prof. Dr. K.-i. Nakayama
CREST, JST, Chiyoda-ku, Tokyo 102-0075 (Japan)
[11]
locked rings (catenanes) have been obtained upon the cou-
pling of a barbiturate group with p-conjugated units whose
one end is equipped with a tridodecyloxyphenyl aliphatic tail
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201404428.
Chem. Eur. J. 2014, 20, 16128 – 16137
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n n 2 4
Figure 1. Chemical structures of Bar-T -C12, Bar-T-hT and Me Bar-T-hT .
(
for example, Bar-T -C12 in Figure 1). For all of the prepared
4
molecules, the formation of hexameric rosettes was proposed
on the basis of X-ray diffraction analyses of their hexagonal or
rectangular columnar mesophases, even though the literature
contains no reports regarding the formation of supramolecular
[
12]
rosettes by barbiturates.
Such a strong hydrogen-bond-
mediated self-organization capability of barbiturate-conjugated
p-systems motivated us to design supramolecular rosettes that
can stack to form highly ordered nanorods applicable to solu-
tion-processable solar cells without the use of the long aliphat-
ic tails.
Figure 2. a) Design concept of oligothiophene rosettes in this study. CCW ro-
settes of Bar-T
4
-C12 (left) and Bar-T-hT
n
are shown (see text). b) Schematic
(right).
representation of nanorods of Bar-T
4
-C12 (left) and Bar-T-hT
n
Herein we report the synthesis, self-organization and photo-
[
14]
voltaic
properties
of
barbiturate-conjugated
thienyl-
bithiophene 1,
which reacted with bromothiophene 2 or
[13]
[
oligo(hexylthiophene)]s (Bar-T-hT ; n=3,4,5 in Figure 1). In
bromobithiophene 6 under Stille coupling conditions to give
terthiophene 3 and quaterthiophene 7, respectively. After se-
n
these molecules, solubilizing alkyl chains are grafted onto the
p-conjugated backbone and thereby good solubilities of the
resulting rosettes are guaranteed by the short alkyl chains em-
bedded inside the rosettes (Figure 2a). These rosettes without
exterior aliphatic tails are anticipated to self-assemble through
stacking into nanorods with electronically active surfaces (Fig-
ure 2b). We demonstrate the hierarchical self-assembly process
of these compounds from rosettes to highly ordered nanorods.
By comparing the photovoltaic properties of the BHJ solar cells
[15]
lective monobromination of 3 and 7 using N-bromosuccini-
mide (NBS), the resulting species, 4 and 8, underwent Suzuki
coupling reactions with 5-formylthiophene-2-boronic acid to
give 5 and 9, respectively. For 8, further thienylation was ach-
ieved by coupling with 10 to give 11. After monobromination
of 11 with NBS, the resulting 12 was also coupled with 5-for-
mylthiophene-2-boronic acid to give 13. The formyloligothio-
phenes 5, 9, and, 13 were treated with barbituric acid under
neutral condition to give Bar-T-hT , Bar-T-hT , and Bar-T-hT , re-
using Bar-T-hT and soluble fullerene derivatives, we disclose
n
3
4
5
that the particular self-organization behavior of Bar-T-hT upon
spectively. The non-hydrogen-bonding derivative Me Bar-T-hT
2 4
4
thermal annealing of its BHJ film plays an important role in
achieving power conversion efficiency (PCE) greater than 3%.
We further demonstrate the importance of the present molec-
ular design to achieve high-performance supramolecular solar
cells by comparison with Bar-T -C , which possesses long ali-
was also prepared by the reaction of 9 with N,N’-dimethylbar-
bituric acid. These compounds were purified by recycling gel-
1
permeation chromatography, and characterized by H and
13
C NMR spectroscopy and high-resolution mass spectrometry.
5
12
phatic tails, and the non-hydrogen-bonding derivative Me Bar-
2
Characterization of Rosettes by STM
T-hT (Figure 1).
4
Our previous attempt to verify the formation of barbiturate ro-
settes by scanning tunneling microscopy (STM) analysis of Bar-
[
16]
Results and Discussion
T4-C12 at the liquid-solid interface failed because the rosette
architecture of Bar-T -C whose exterior is decorated with
4
12
Synthesis
bulky aliphatic tails led to a considerable free interior volume
(Figure 2a), thus lowering its two-dimensional packing efficien-
Bar-T-hT systems were synthesized as outlined in Scheme 1.
n
The starting material for the synthesis of Bar-T-hT was stannyl-
cy. However, Bar-T-hT , designed in this study, can fill the free
n
n
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peating the measurements (see the Supporting Infor-
mation, Figure S2). The center-to-center distance be-
tween neighboring rosettes (corresponding to lattice
parameters a and b) are in the range of 4–5 nm. This
result indicates that rosettes of Bar-T-hT have intrins-
4
ically low two-dimensional packing efficiencies, which
could intuitively be related to its shorter p-conjuga-
tion length compared to that of Bar-T-hT . However,
5
Bar-T-hT , for which the p-conjugation is further re-
3
duced, gave well-resolved images of two-dimension-
ally organized rosettes in a hexagonal motif (a=
4
.4ꢀ0.1 nm, b=4.2ꢀ0.1 nm, g=63ꢀ38; see the
Supporting Information, Figure S3). These observa-
tions could be explained using molecular modeling
to determine the two-dimensional packing of the
CCW rosettes of Bar-T-hT and Bar-T-hT (see the Sup-
4
3
porting Information, Figure S4). Whereas the rosettes
of Bar-T-hT3 can be packed with a minimum void
space, those of Bar-T-hT leave notable void spaces
4
surrounded by three rosettes, which suggests that
fewer intermolecular and molecule–substrate interac-
tions occur per unit area in the case of Bar-T-hT4.
Thus, the STM studies of Bar-T-hT imply that wheth-
n
Scheme 1. Synthesis of barbiturate-conjugated oligo(hexylthiophene) structures Bar-T-
hT . Reagents and conditions: i) [Pd(PPh ], DMF, microwave, 1408C; ii) NBS, CHCl /acetic
acid, 08C!RT; iii) 5-formylthiophene-2-boronic acid, K CO , [Pd(dppf)Cl ], toluene/MeOH,
08C; iv) barbituric acid, EtOH, reflux; v) 10, K CO , [Pd(PPh ], toluene/H O, reflux.
er there is an odd or even number of hexylthiophene
units strongly affects the two-dimensional organiza-
n
3
)
4
3
2
3
2
[18]
8
2
3
3
)
4
2
tion of the rosettes.
volume upon rosette formation (Figure 2a), and stabilization of
the resulting rosettes at the liquid-solid interface is therefore
anticipated. The two-dimensional self-assembly of Bar-T-hTn
was thus studied by STM at the solid-liquid interface between
highly oriented pyrolytic graphite (HOPG) and 1-phenyloctane.
Figure 3a shows a typical STM image of Bar-T-hT . Although
5
the individual thiophene rings could not be resolved, the aver-
age length of the bright rods [2.85ꢀ0.1 nm (mean ꢀSD)] is
close to the extended molecular length (calculated to be
2.87 nm using the CPK model). The windmill-shaped arrange-
ment of the bright rods demonstrates the formation of hex-
americ rosette of Bar-T-hT . The unit-cell parameters, a=5.1ꢀ
5
0
.2 nm, b=5.5ꢀ0.1 nm, and g =60ꢀ58, indicate the forma-
tion of a pseudohexagonal lattice of rosettes. The oppositely
located bright rods in a rosette are not on the same line across
the center of the rosette, indicating the formation of screw-
shaped chiral rosettes with CW (clockwise; Figure 3c) and CCW
(
counterclockwise; Figure 3d) rotations, as was observed for
[
16a,17]
melamine-functionalized oligo(p-phenylenevinylene)s.
Mo-
lecular modelling showed that both CW and CCW rosettes of
5
Figure 3. a) STM images of Bar-T-hT (domain of CCW rosettes) at 1-phenyl-
Bar-T-hT could be packed into the hexagonal lattice of the
5
octane-HOPG interface. The tunneling conditions: I=1.2 pA, V=ꢁ800 mV.
b) Packing model of the CCW rosette of Bar-T-hT with the P/trans combina-
STM images through interdigitation of the outer hexyl chains
5
(
Figure 3b). Either CW or CCW domains were observed ran-
tion (see the Supporting Information). c,d) Magnification of STM images of
c) CW and d) CCW rosettes.
2
domly for each 25ꢁ25 nm scan, and we could not find
a boundary between the two domains. More detailed discus-
sion of hydrogen-bonding motif is given in the Supporting In-
formation (Figure S1).
1
H NMR Spectroscopy
1
For Bar-T-hT , we could only obtain poorly resolved STM
Aggregation of Bar-T-hT was investigated by H NMR spectros-
4
n
1
images of rosettes, even after varying the parameters and re-
copy in [D ]toluene. Figure 4a shows the change of H NMR
8
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Bar-T-hT also exhibited downfield shift of the barbiturate
3
protons H and H upon increasing concentration (see the Sup-
a
b
porting Information, Figure S5). On increasing the concentra-
ꢁ
4
ꢁ3
tion from 1ꢁ10 m to 5ꢁ10 m, the Dd values of H and H
a
b
are 1.29 and 1.59 ppm (Figure 4c), respectively, which are com-
parable to those of Bar-T-hT . Bar-T-hT , in contrast, exhibited
4
5
downfield shift of the barbiturate protons only up to 3ꢁ
ꢁ
3
1
0 m, above which no significant shift was detected (see the
Supporting Information, Figure S6). This was at first a puzzling
observation, but we noticed that toluene solutions of Bar-T-hT5
became viscous at this concentration range, and at above 4ꢁ
ꢁ
3
1
0 m the solution immediately turned into a gel upon the
heating–cooling procedure (Figure 4d). Scanning electron mi-
croscopy (SEM) of the dried gel showed the formation of en-
tangled fibrous microstructures (see the Supporting Informa-
tion, Figure S7). From these results, the absence of downfield
ꢁ3
shift above 3ꢁ10 m can be attributed to the saturation of
Bar-T-hT in toluene. With this result in hand, we revisited tolu-
5
ene solutions of Bar-T-hT and Bar-T-hT , and found that they
3
4
ꢁ
3
turned into gels within 6 h at 5ꢁ10 m at room temperature
Figure 4d). These results demonstrate that Bar-T-hT dissolved
(
n
in toluene slowly aggregate to form microscopic fibers.
Formation of Nanorods
To study the hierarchical aggregation of Bar-T-hT into rodlike
n
structures through the formation of rosettes, their diluted tolu-
ꢁ
4
1
ene solutions (c <1ꢁ10 m), the H NMR spectroscopy of
which showed that the molecules are predominantly in molec-
ularly-dissolved state, were deposited onto HOPG. After the
solvent was slowly removed by evaporation, leading to the for-
mation of aggregates, the resulting thin films were imaged by
atomic force microscopy (AFM). When the samples were pre-
ꢁ
5
pared from the solutions of c=5ꢁ10 m, bundled nanorods
1
Figure 4. a) H NMR spectra of Bar-T-hT
tions. b) Assignments of proton signals a–e of Bar-T-hT
trations versus the chemical shift of a barbiturate proton (H
4
in [D
8
]toluene at various concentra-
. c) Plots of concen-
) for Bar-T-hT
). d) Toluene gels of Bar-T-hT3
were observed for Bar-T-hT and Bar-T-hT , whereas granular
3
5
4
nanoaggregates were observed for Bar-T-hT4 (Figures 5a–c).
These results demonstrate a weaker aggregation propensity of
Bar-T-hT in comparison to Bar-T-hT and Bar-T-hT .
a
3
(
(
*), Bar-T-hT
4
(&) and Bar-T-hT
c=5ꢁ10 m), Bar-T-hT (c=5ꢁ10 m), and Bar-T-hT
5
(~
ꢁ
3
ꢁ3
ꢁ3
4
5
(c=4ꢁ10 m), ob-
4
3
5
tained after 6 h on cooling homogeneous hot solutions.
Upon increasing the concentrations of the solutions to 1ꢁ
ꢁ
4
1
0 m, however, nanorods became observable for Bar-T-hT4.
Figures 5d–f show typical high-resolution AFM images of bun-
spectra of Bar-T-hT upon increasing the concentration from
dled nanorods of Bar-T-hT with cross-sectional analyses along
4
n
ꢁ
4
ꢁ3
1
ꢁ10 to 5ꢁ10 m. At the lowest concentration, the signals
the white lines. The cross sectional analyses showed the zigzag
patterns on the surface of bundled rods, and average top-to-
top distances between neighboring rods were calculated as
4.1 nm for Bar-T-hT , 4.3 nm for Bar-T-hT , and 5.0 nm for Bar-T-
of protons H and H of the barbiturate unit appeared at 6.58
a
b
and 6.80 ppm, respectively. On increasing concentration, these
signals shifted downfield, which indicates the formation of in-
termolecular hydrogen bonds (Figure 4b). The Dd values of Ha
and Hb reached 1.34 ppm (6.58!7.92 ppm) and 1.54 ppm
3
4
hT , upon dividing the total top-to-top distance of the bundles
5
by the number of aligned rods. Because the calculated distan-
ces for Bar-T-hT3 and Bar-T-hT5 are well matched with the
shorter lattice parameters of the two-dimensionally organized
rosettes, the lateral interaction (i.e., interdigitation) between
rosettes would play an important role in the formation of
nanorods. The average heights of the nanorods also increased
incrementally with increasing number of the thienyl units,
from Bar-T-hT (2.8ꢀ0.1 nm) to Bar-T-hT (3.0ꢀ0.2 nm), and to
ꢁ
3
(
6.80!8.34 ppm) at c=5ꢁ10 m, respectively. The signal of
the olefinic proton (H ) also showed a small downfield shift
c
(
Dd =0.12) due to deshielding by neighboring molecules. In
sharp contrast, the protons of thiophene rings are all insensi-
tive to the concentration change. These observations indicate
that the molecules are in rapid equilibrium between the mo-
nomer and the hydrogen-bonded aggregates, and no hierarch-
ical organization of the aggregates through p–p stacking takes
place under these conditions.
3
4
Bar-T-hT (3.7ꢀ0.1 nm), which is consistent with an edge-on
5
arrangement of the rosette nanorods. Although no helical mor-
Chem. Eur. J. 2014, 20, 16128 – 16137
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n
Table 1. Optical and electronic properties of Bar-T-hT .
Compound
l
max(sol)
l
max(film)
l
onset
E
g
E
HOMO
E
LUMO
[
nm]
[nm]
[nm]
[eV] [eV]
[eV]
Bar-T-hT
Bar-T-hT
Bar-T-hT
3
4
5
367, 508
385, 520
398, 522
386, 526
418, 542
538, 560
655
674
696
1.89 ꢁ5.18
1.84 ꢁ5.06
1.78 ꢁ4.96
ꢁ3.29
ꢁ3.22
ꢁ3.18
our oligothiophenes should have charge-transfer character in
their excited state, owing to the strong electron-withdrawing
effect of the barbiturate unit. The absorption maxima of Bar-T-
hT4 (lmax =520 nm) and Bar-T-hT5 (lmax =522 nm) are slightly
red-shifted compared to that of Bar-T-hT (l =508 nm). The
3
max
absorption bands of the solid films of Bar-T-hT displayed red-
n
shifts of absorption maxima and emergence of new absorption
shoulder in the longer wavelength region, and the red-shifted
values increased upon increasing conjugation length (Bar-T-
hT3: Dl=18 nm; Bar-T-hT4: Dl=21 nm; Bar-T-hT5: Dl=
39 nm). The red-shift and the absorption shoulder can be at-
tributed to the coplanarization and p–p stacked aggregation
[22]
of p-conjugated systems, respectively. The optical band gaps
of the films were determined from the onset of absorption in
the range from 1.78 to 1.89 eV (Table 1).
Figure 5. AFM images of thin films prepared by drop-casting toluene solu-
tions of a,d) Bar-T-hT
3
, b,e) Bar-T-hT
4
and c,f) Bar-T-hT
5
onto HOPG. Concen-
ꢁ
5
ꢁ4
Cyclic voltammetry (CV) measurements of the thin film sam-
trations of the solutions are 5ꢁ10 m for images (a)–(c) and 1ꢁ10 m for
images (d)–(f). For images (d)–(f), cross-sectional analysis along the white
lines are shown below the images.
ples showed that Bar-T-hT , Bar-T-hT4 and Bar-T-hT5 have
3
HOMO energy levels of ꢁ5.18, ꢁ5.06 and ꢁ4.96 eV, respective-
ly. From the optical band gaps and these HOMO energy levels,
LUMO energy levels of Bar-T-hT , Bar-T-hT and Bar-T-hT were
phology could be imaged for these nanorods by AFM, helical
stacking of rosettes (Figure 2b) would be anticipated because
offset stacking of oligothiophene p-conjugated moieties is fa-
vored by repulsive p–p interactions that is compensated by at-
3
4
5
estimated to be ꢁ3.29, ꢁ3.22, and ꢁ3.18 eV, respectively
(Table 1). These energy levels demonstrate that Bar-T-hT spe-
n
cies can be used as electron donor materials for BHJ solar cells
with PC BM ([6,6]-phenyl-C -butyric acid methyl ester) or
[
19,20]
tractive p–s interactions.
6
1
61
23]
[
PC BM as electron acceptors.
71
Optical and Electronic Properties
Photovoltaic Properties
Photovoltaic properties of Bar-T-hT were investigated by fabri-
Monomeric Bar-T-hT in toluene gave rise to broad absorption
n
n
bands in the range of 300–650 nm (Figure 6, Table 1), which
are largely red-shifted by 100 nm compared to quinque-
cating BHJ solar cells with
a structure of Al/Ca/Bar-T-
hT :PC BM/PEDOT:PSS/ITO (Figure 7a) (PEDOT:PSS, poly(3,4-
n
61
[
21]
(hexyl)thiophene (lmax =391 nm).
We thus anticipated that
ethylenedioxythiophene)-poly(styrenesulfonate), as the hole in-
jection layer; ITO, indium tin oxide, as the transparent elec-
trode). Because Bar-T-hTn species form gel-like materials in
ꢁ
1
pure toluene at concentrations above 5 mgmL , as described
previously, we employed the 1:1 (v/v) mixture of chloroform
and toluene as a solvent for the preparation of active layers
(
denoted as, e.g., Bar-T-hT :PC BM). The active layers were pre-
3 61
pared by spin-coating the 1:1 (w/w) blends of Bar-T-hT and
n
PC BM dissolved in the mixed solvents. Optimum thicknesses
61
of the active layers were found to be in the range of 60–
0 nm, which could be made with the total concentration of
8
ꢁ
1
the materials of 10 mgmL . The devices were fabricated using
as-cast or thermally annealed (50, 80 and 1108C) active layers
to study the impact of thermal organization of nanorods.
Table 2 summarizes short-circuit current density (J ), open-
ꢁ
4
sc
Figure 6. Absorption spectra of toluene solutions (c=1ꢁ10 m, solid line)
ꢁ
3
circuit voltage (V ), fill factor (FF) and PCE of the BHJ solar
and thin films (c=1ꢁ10 m, dotted line) of Bar-T-hT
Bar-T-hT
(~).
3
(*), Bar-T-hT
4
(&) and
oc
5
cells fabricated using Bar-T-hT :PC BM, Bar-T-hT :PC BM and
3
61
4
61
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ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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ed the best annealing-free device, giving a PCE of 1.49% with
ꢁ
2
a Jsc of 4.13 mAcm , a V of 0.64 V, and a FF of 56.2%. Al-
oc
though the annealing-free device of Bar-T-hT showed a higher
3
ꢁ
2
Jsc (4.68 mAcm ) and V (0.95 V) compared to those of Bar-T-
oc
hT , its lower FF of 31% resulted in a lower PCE of 1.38%. Re-
5
markably, Bar-T-hT , which has a p-conjugation length between
4
those of Bar-T-hT and Bar-T-hT , showed even lower J
sc
3
5
ꢁ
2
(
3.47 mAcm ) and FF (30.1%) values than those of Bar-T-hT ,
3
thus giving the lowest PCE (0.97%) among the annealing-free
devices.
In Figure 7b, PCEs of the solar cells of Bar-T-hT are plotted
n
versus annealing temperature applied for the active layers. The
devices of Bar-T-hT and Bar-T-hT showed only a small increase
3
5
or decrease of PCEs upon increasing annealing temperature
mainly due to the decrease of J . Interestingly, the devices of
sc
Bar-T-hT4 showed a pronounced improvement of the per-
formance upon increasing the annealing temperature from 50
ꢁ
2
to 808C: Jsc increased from 3.58 to 5.83 mAcm , FF increased
from 30.8 to 47.1%, and PCE increased from 0.83 to 2.1%. Fur-
ther elevating annealing temperature to 1108C reduced the
device performance. In most small-molecular solar cells using
PC BM, thermal annealing at a temperature in the range of
61
Figure 7. a) Current–voltage (J–V) characteristics of BHJ solar cells using 1:1
w/w) blend films of Bar-T-hT :PC61BM (black lines), Bar-T-hT :PC61BM (red
lines) and Bar-T-hT :PC61BM (blue lines) before (dotted lines) and after an-
nealing at 808C (solid lines). Inset: schematic representation of the device
structure of the BHJ solar cells applied in this study. b) Plots of annealing
100–1308C has had a notable effect of on the device per-
[24–26]
(
3
4
formance,
because the glass transition of PC BM promot-
61
5
ed favorable phase separation to form semiconducting chan-
[27]
nels. The peculiar increase of PCE in the devices of Bar-T-hT4
on applying a relatively moderate annealing temperature is
temperatures versus PCE of the devices of Bar-T-hT
3
(black line), Bar-T-hT
(blue line). c) EQE spectra of the devices of Bar-T-hT
(red line) and Bar-T-hT (blue line) before (dotted lines)
4
(
(
red line) and Bar-T-hT
black line), Bar-T-hT
5
3
a characteristic feature of the organization of Bar-T-hT , which
4
4
5
is revealed by the AFM studies described in the next section.
Further improvements in the device performance were ach-
ieved for Bar-T-hT by using PC BM as an acceptor (Table 2,
and after annealing at 808C (solid lines).
n
71
gray lines), whose reduced symmetry leads to a stronger ab-
[
28]
Table 2. Photovoltaic properties of the BHJ solar cells based on Bar-T-hT
as donor and PC61BM (annealing at as-cast, 50, 80, and 1108C) or
n
sorption in the visible region compared to that of PC BM.
61
Also in this case the highest performance was that with Bar-T-
[
a]
PC71BM (annealing at only 808C) as acceptor under the illumination of
ꢁ2
ꢁ
2
hT , for which the PCE was 3.01% (J =7.00 mAcm , V =
4
sc
oc
AM 1.5 G (100 mWcm ).
0
.79 V, FF=0.54). To knowledge, this PCE is the highest value
BHJ films
Annealing T
8C]
V
[V]
oc
J
sc
FF
[%]
PCE
[%]
reported for a small-molecular material that organizes through
ꢁ
2
[
[mAcm
]
[29]
specific hydrogen-bonding interactions, and is comparable
Bar-T-hT
3
:PC61BM as-cast
0.95 4.68
0.82 4.23
0.82 3.72
0.77 3.66
0.75 5.04
0.93 3.47
0.75 3.58
0.76 5.83
0.70 5.72
0.79 7.00
0.64 4.13
0.61 4.19
0.66 3.84
0.66 3.75
0.69 3.50
31.0 1.38
30.6 1.06
31.4 0.95
37.3 1.06
34.5 1.31
30.1 0.97
30.8 0.83
47.2 2.09
47.1 1.85
54.4 3.01
56.2 1.49
50.3 1.29
60.2 1.53
62.4 1.53
57.3 1.38
to PCEs of BHJ solar cells constructed with discotic small mole-
5
8
0
0
[30]
cules.
External quantum efficiency (EQE) spectra of the solar cells
1
10
80
:PC61BM as-cast
[
a]
Bar-T-hT
Bar-T-hT
3
:PC71BM
using as-cast and 808C-annealed films of Bar-T-hT :PC BM,
3
61
4
Bar-T-hT :PC BM and Bar-T-hT :PC BM are shown in Figure 7c.
4
61
5
61
5
8
0
0
The device of Bar-T-hT showed a negative impact of annealing
3
on EQE in the region between 500–600 nm (40.6!32.4% at
1
10
80
:PC61BM as-cast
[
a]
Bar-T-hT
Bar-T-hT
4
:PC71BM
545 nm), which is the major absorption region of Bar-T-hT .
3
5
The device of Bar-T-hT showed a small increase of EQE (26.7!
5
5
8
0
0
28.3% at 550 nm) upon annealing. In striking contrast, a pro-
nounced increase of EQE was observed for the device of Bar-T-
1
80
10
[
a]
Bar-T-hT
5
:PC71BM
hT (34.1!43.4% at 540 nm). These observations imply that
4
contrasting thermal reorganization did occur in Bar-T-
[a] A 9:1 (v/v) mixture of chloroform and toluene was used for the prepa-
hT :PC BM and Bar-T-hT :PC BM, whereas in Bar-T-hT :PC BM
ration of active layers.
3
61
4
61
5
61
a substantial phase separation would have already occurred
even before annealing due to the strong aggregation propen-
Bar-T-hT :PC BM. Figure 7a represents the J–V curves of the
sity of Bar-T-hT , thus resulting in the negligible impact of an-
5
61
5
devices using the as-cast and 808C-annealed active layers. All
nealing. In contrast, the opposite effect of the thermal anneal-
ing on photocurrent generation in Bar-T-hT and Bar-T-hT is
the devices displayed typical diode behaviors. Bar-T-hT provid-
5
3
4
Chem. Eur. J. 2014, 20, 16128 – 16137
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ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
consistent with the temperature dependence of the PCEs of
their devices (Figure 7b), which will be correlated with the
morphology change of the BHJ films as described below.
The most pronounced morphology change was observed
for Bar-T-hT :PC BM. Whereas no well-defined nanostructure
4
61
was observed in the as-cast film (Figure 8c), the formation of
nanorods, which are not extensively bundled in comparison to
Bar-T-hT :PC BM and Bar-T-hT :PC BM, was observed after an-
3
61
5
61
Nanorod Formation in BHJ films
nealing at 808C (Figure 8d). The nanorods were approximately
100 nm in length and 5–10 nm in width, which corresponds to
the formation of single-rosette columns. The dispersed nature
of these nanorods was also confirmed by the absence of dif-
fraction peaks derived from the two-dimensional ordering of
nanorods in the powder X-ray diffraction measurement of the
annealed film. Increasing the annealing temperature to 1108C
caused bundling (widthꢂ20 nm) and elongation (length
ꢂ200 nm) of nanorods (see the Supporting Information, Fig-
ure S8). This observation is consistent with the drop in the
device performance at this annealing temperature.
The morphology change of the BHJ films upon annealing was
studied by AFM. Figure 8 shows the AFM topography images
Impact of Nanorod Formation on Photovoltaic Properties
Because the performance of BHJ solar cells depends strongly
on the charge-separation efficiency and transport of generated
charge carriers, we studied the impact of supramolecular or-
ganization of Bar-T-hT on the photovoltaic properties by flash-
4
photolysis time-resolved microwave conductivity (FP-TRMC), in-
formative for the motion of photogenerated charge carriers
before complete deactivation by charge recombination and/or
[31]
trapping.
To cancel the change in the absorbance of the
films upon thermal treatment, the film thicknesses were kept
more than 1 mm in order to absorb most of the incident pho-
31c
tons.
Upon excitation of the as-cast BHJ films of Bar-T-
hT :PC BM with a laser pulse at 355 nm, a pronounced transi-
4
61
ent conductivity, given by fSm (f=photocarrier generation
yield; Sm=sum of the mobilities of photogenerated charge
carriers) was observed (Figure 9a). Upon annealing the films at
5
0, 80, and 1108C, the maximum fSm decreased gradually
ꢁ
4
2
ꢁ1 ꢁ1
ꢁ4
2
ꢁ1 ꢁ1
from 1.3ꢁ10 cm V
s
to 0.8ꢁ10 cm V s . The decrease
of fSm is attributable to a reduction in the charge carrier gen-
eration efficiency by the growth of the nanorods, which
should lead to a decrease in the p–n heterojunction interface.
In contrast, the half-decay-time (t ) of the TRMC signals in-
Figure 8. AFM topography images of the 1:1 (w/w) blend films of a,b) Bar-T-
hT :PC61BM, c,d) Bar-T-hT :PC61BM and e,f) Bar-T-hT :PC61BM before (a,c,e)
3 4 5
and after annealing at 808C (b,d,f). The values at the bottom-left corner are
root-mean-square (RMS) roughness.
1/2
ꢁ
6
ꢁ6
creased from 2.4ꢁ10 s to 10.1ꢁ10 s upon annealing at
higher temperatures (Figure 9a, inset). These results demon-
strate that neither insufficient nor excessive thermal annealing
simultaneously guarantees efficient charge separation and
transport (Figure 9b). Owing to this trade-off relationship of
the nanorod growth for optimizing OPV output, the best per-
formance was achieved when the BHJ films of Bar-T-
hT :PC BM were annealed at 808C.
of the as-cast and annealed films of Bar-T-hT :PC BM, Bar-T-
3
61
hT :PC BM and Bar-T-hT :PC BM with the values of root-
4
61
5
61
mean-square (RMS) roughness. The AFM images of Bar-T-
hT :PC BM revealed that bundled nanorods are already
3
61
formed in the as-cast state (Figure 8a), which upon annealing
resulted in a further agglomeration of nanorods (Figure 8b).
The agglomeration would cause a decrease in the BHJ inter-
face, thus reducing the photocurrent generation. For Bar-T-
hT :PC BM, the substantial phase separation is suggested
4
61
Effect of Aliphatic Tails and Rosette Formation
To shed more light on our design concept toward supramolec-
ular photovoltaic materials, we evaluated the photovoltaic
properties of Bar-T -C functionalized with long aliphatic tails,
5
61
from a higher RMS roughness recorded already before anneal-
ing, and the situation could not be improved even after an-
nealing (Figure 8e,f). This result reflects the trivial impact of
thermal annealing on the improvement of the solar cell per-
formance.
5
12
and Me Bar-T-hT , hydrogen-bonding sites of which are
2
4
capped with methyl groups. The 1:1 (w/w) blend of Bar-T -C
5
12
and PC BM gave good BHJ films owing to the presence of ali-
61
Chem. Eur. J. 2014, 20, 16128 – 16137
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ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
(
RMS roughness=0.5 nm) without the formation of well-de-
fined nanostructures (see the Supporting Information, Fig-
ure S9a). However, its solar cells showed PCEs of only 0.1%
ꢁ
2
with J =2.89 mAcm , V =0.14 V, FF=24.7 (Table 3). Thermal
sc
oc
annealing at 808C was effective but it increased the PCE up to
only 0.3%. The annealed film retained the structureless feature
of the surface (RMS roughness=1.0 nm, Figure S9b). The poor
device performance and the structureless feature of the
Me Bar-T-hT :PC BM system even after annealing thus un-
2
4
61
equivocally demonstrate that our supramolecular materials are
effective for organizing small molecular semiconductors into
well-defined hierarchical nanostructures without introducing
long aliphatic tails.
Conclusion
The supramolecular design strategy applied herein enabled
the construction of well-defined one-dimensional nanostruc-
tures through the hydrogen-bond-mediated supramolecular
macrocyclization (rosette formation) of oligothiophene semi-
conductors without the use of long aliphatic tails. Due to the
absence of exterior long aliphatic tails, the rosettes’ two-di-
mensional packing efficiency at the liquid-solid interface was
markedly affected by having an odd or even number of hex-
ylthienyl groups. Interestingly, Bar-T-hT , which showed a lower
4
two-dimensional packing efficiency of the rosette compared to
Bar-T-hT and Bar-T-hT , exhibited poorer organization propen-
Figure 9. a) Conductivity transients (fSm) of the as-cast and annealed (50, 80
and 1108C) blend films of Bar-T-hT :PC61BM (1:1, w/w), observed upon exci-
tation with a 355 nm laser pulse (excitation photon density=9.1ꢁ10 pho-
tons cm ). Inset: plot of fSm and semi-log plot of t1/2 versus the annealing
temperatures. b) Schematic illustration of nanorod growth in the active layer
and facilitation of charge carrier transportation.
3
5
4
1
5
sity into nanorods. These results indicate that rosette–rosette
interaction is important for the lateral interaction between the
nanorods that promotes their growth. This property of Bar-T-
ꢁ
2
hT ensures good miscibility with the fullerene acceptor, and
4
unbundled nanorods were formed in the BHJ films upon an-
nealing, giving rise to a reasonable nanophase-separated struc-
ture. The resulting BHJ structures of Bar-T-hT : PC BM exhibit-
phatic tails. However, its solar cells exhibited quite poor per-
formance, as PCEs could be increased up to only 0.04%
4
61
ed photovoltaic properties comparable to those of the most
commonly used standard polymer materials (P3HT).
ꢁ
2
(Table 3). A significantly lower Jsc value (0.21 mAcm ) illus-
trates that the aliphatic tails hamper the charge separation
with the acceptor material and charge carrier migration be-
tween nanorods.
Supramolecular macrocycles formed by multiple hydrogen-
bonding interactions have long been explored as archetypal
building modules for hierarchically organized columnar (rod-
like) nanoarchitectures. Most soluble building blocks that
afford these types of supermacrocylic assemblies require long
aliphatic tails to achieve sufficient solubility in organic solvents,
which is important for avoiding the unfavorable precipitation
of competing open-ended oligomers that are always in equilib-
Me Bar-T-hT featuring a high solubility and optical and elec-
2
4
trochemical properties almost comparable to those of Bar-T-
hT (see the Supporting Information, Table S2), also showed
4
a poor solar cell performance. The 1:1 (w/w) blend of Me Bar-
2
T-hT and PC BM afforded BHJ films with very smooth surface
4
61
[7]
rium with supermacrocyclic species to a small extent. Such
long aliphatic tails, however, cover the resulting columnar
nanostructures, thus diminish the photovoltaic activity of the
materials. In our molecular design, however, the good solubili-
ty is imparted by short alkyl chains embedded inside the
supramolecular macrocyclic structures. Thus, our hydrogen-
bonded oligothiophenes can form well-defined nanorods that
show, to our knowledge, the highest solar cell performance
among hydrogen-bonded materials. Because the similar strat-
egy might be applied not only to hydrogen-bonded building
blocks but also to large star-shaped molecules, we hope that
this study may open a new avenue for the design of supra-
molecular materials with not only well-defined nanoarchitec-
Table 3. Comparison of photovoltaic properties of the BHJ Solar Cells
based on Bar-T-hT or Bar-T -C12 or Me Bar-T-hT as donor and PC61BM as
acceptor under the illumination of AM 1.5 G (100 mWcm ).
4
5
2
4
ꢁ
2
BHJ films
Annealing T
8C]
V
[V]
oc
J
sc
FF
[%]
PCE
[%]
ꢁ2
[
[mAcm
]
as-cast
0.93
3.47
5.83
0.21
0.14
2.89
7.89
30.1
0.97
4
Bar-ThT :PC61BM
8
0
0.76
0.68
0.78
0.14
0.16
47.2
28.5
27.1
24.7
24.9
2.09
0.04
0.03
0.10
0.32
as-cast
Bar-T
Me Bar-T-
hT :PC61BM
5
C12:PC61BM
8
0
2
as-cat
80
4
Chem. Eur. J. 2014, 20, 16128 – 16137
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16135
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
tures and solution processability but also practical application
in devices. Current work in our group is directed toward fur-
ther improvement of device performance by engineering elec-
tronic and optical properties of molecular components, as well
as controlling over the orientation of nanorods in the device.
S. V. Kolotuchin, S. C. Zimmerman, J. Am. Chem. Soc. 2002, 124, 13757–
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Keywords: nanostructures · oligothiophene · photovoltaics ·
self-assembly · supramolecular chemistry
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Received: July 15, 2014
Published online on October 3, 2014
Chem. Eur. J. 2014, 20, 16128 – 16137
www.chemeurj.org
16137
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim