The effect of H- and J-aggregation on the photophysical and
photovoltaic properties of small Thiophene–Pyridine–DPP
molecules for bulk-heterojunction solar cells
Citation for published version (APA):
Más-Montoya, M., & Janssen, R. A. J. (2017). The effect of H- and J-aggregation on the photophysical and
photovoltaic properties of small Thiophene–Pyridine–DPP molecules for bulk-heterojunction solar cells.
Advanced Functional Materials
,
27
(16), [1605779]. https://doi.org/10.1002/adfm.201605779
DOI:
10.1002/adfm.201605779
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(1 of 12) 1605779
structure, and the perfectly reproducible
synthetic procedures that largely avoid the
variability between different batches seen
in polymerizations. Recently, successful
small-molecule donor systems have been
reported that reach high power conversion
efficiencies (PCEs) in organic solar cells.
[2]
The combination of a judicious molecular
design with the optimal material pro-
cessing conditions and device engineering
remains a critical point for the success
in the photoenergy conversion process.
Rational molecular design is of great
importance for further improving the pho-
tovoltaic performance of small-molecule
bulk-heterojunction organic solar cells.
However, achieving a high performance
employing newly designed small-molecule
materials remains a challenge because it is
difficult to predict in sufficient detail how mole cules assemble
in thin films and how this affects photovoltaic performance.
When small-molecule chromophores assemble in the solid
state, they often form H-type or J-type aggregates, depending
on the relative alignment of the transition dipole moments on
adjacent molecules. In an H-aggregate, molecules stack pre-
dominantly face-to-face, while J-aggregates form when mol-
ecules primarily stack in a head-to-tail arrangement. The for-
mation of such aggregates has important consequences for the
energies of the excited states and the oscillator strengths of the
transitions to these states from the ground state. Consequently,
H- and J-aggregation can strongly modify optical absorption
and the photoluminescence spectra. To contribute to our under-
standing of the effect of molecular packing on photovoltaic per-
formance, it is of interest to assemble one type of molecule in
different packing modes.
In this contribution, we do that by using molecules based
on diketopyrrolopyrrole (DPP) flanked by two 5-(thiophen-2-yl)
pyridin-2-yl units in which solubilizing hexyl side chains are
introduced on the free positions of the peripheral thiophene
units (Scheme 1). The DPP fragment has been extensively
explored for organic semiconductors, because of its strong
optical absorption and excellent charge transport properties,
[3]
especially for the design of materials for organic photovoltaic
applications.
[4]
The electron density of the DPP core can be
modified by employing different (hetero)aromatic flanking
groups. The combination of DPP with two flanking pyridin-
2-yl moieties leads to the simultaneous decrease of the fron-
tier energy levels due to the electron-withdrawing nature of
The Effect of H- and J-Aggregation on the Photophysical
and Photovoltaic Properties of Small Thiophene–Pyridine–
DPP Molecules for Bulk-Heterojunction Solar Cells
Miriam Más-Montoya and René A. J. Janssen*
The performance of organic semiconductors in optoelectronic devices
depends on the functional properties of the individual molecules and their
mutual orientations when they are in the solid state. The effect of H- and
J-aggregation on the photophysical properties and photovoltaic behavior of
four electronically identical but structurally different thiophene–pyridine–
diketopyrrolopyrrole molecules is studied. By introducing and changing the
position of two hexyl side chains on the two peripheral thiophene units of
these molecules, their aggregation in thin films between H-type and J-type is
effectively tuned, as evidenced from the characteristics of optical absorption,
fluorescence, and excited state lifetime. The two derivatives that assemble
into J-type aggregates exhibit a significantly enhanced photovoltaic perfor-
mance, up to an order of magnitude, compared to the two molecules that
form H-type aggregates. The reasons for this remarkably different behavior
are discussed.
Dr. M. Más-Montoya, Prof. R. A. J. Janssen
Molecular Materials and Nanosystems
Institute for Complex Molecular Systems
Eindhoven University of Technology
P.O. Box 513, 5600 MB, Eindhoven, The Netherlands
E-mail: r.a.j.janssen@tue.nl
Prof. R. A. J. Janssen
Dutch Institute for Fundamental Energy Research
De Zaale 20, 5612 AJ, Eindhoven, The Netherlands
DOI: 10.1002/adfm.201605779
1. Introduction
The progress of organic photovoltaic technology has been
closely related to the development of new organic materials
with excellent semiconducting properties and optical absorp-
tion. After a decade of intense research, chemists and mate-
rial scientists have designed many original building blocks and
have developed elegant novel strategies to combine different
units, creating highly efficient materials, both small molecules
and polymers, exhibiting the desired properties for photovol-
taic applications.
[1]
In comparison with polymers, the interest
for small-molecule materials is motivated by their more easy
purification and characterization, the well-defined molecular
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the pyridine ring, enhancing the chances for a high open-cir-
cuit voltage in an organic solar cell.
[5]
This feature makes the
pyridine-flanked DPP fragment an interesting building block
for organic photovoltaics. Although their potential has been
explored much less than for thiophene-flanked DPP derivatives,
the power conversion efficiencies reached in polymer solar cells
with pyridine-flanked DPP are similar to those of thiophene-
flanked DPP polymers.
[6]
DFT calculations have shown that the
reduced steric hindrance of the nitrogen atom in the 1-position
of the pyridin-2-yl rings with the alkyl substituents on the DPP
core, as compared to phenyl, effectively reduces the dihedral
angle to a fully coplanar geometry that maximizes the possi-
bility for intermolecular
π
–
π
interactions.
[5m]
The position,
[7]
length,
[8]
and stereoisomerism
[9]
of the ali-
phatic chains attached to the aromatic backbone of the semi-
conductor small molecules is known to affect the morphology
in the solid state, and thereby the optoelectronic properties and
the photovoltaic performance. By introducing hexyl side chains
on different positions of the terminal thiophene rings, we find
that it is possible to effectively tune the packing in thin films
between H-like and J-like aggregates. The H-type and J-type
aggregates exhibit distinctly different photophysical properties
in absorption, fluorescence, and excited state lifetime. Remark-
ably, we find that the two derivatives that assemble into J-type
aggregates exhibit a significantly enhanced photovoltaic per-
formance compared to the two molecules that form H-type
aggregates. The difference in PCE is up to an order of magni-
tude. We study charge generation, charge recombination, and
charge transport to obtain a deeper insight in the causes of the
behavior and how subtle structural changes induce large effects
in the properties of small-molecule donor materials.
2. Results and Discussion
2.1. Synthesis
The synthetic route for the four pyridine–DPP derivatives is
outlined in Scheme 1. 3,6-Bis(5-bromopyridin-2-yl)-2,5-bis(2-
ethylhexyl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione was
synthesized according to a previously reported procedure,
[5n]
using a traditional pseudo-Stobbe condensation reaction
between 5-bromo-2-pyridinecarbonitrile and diethyl succi-
nate to yield the pyridine–DPP core, followed by its alkylation
employing 2-ethylhexyl bromide under basic conditions. Sub-
sequently, a double Suzuki–Miyaura cross-coupling reaction
between the alkylated dibromo pyridine–DPP derivative and the
corresponding thiophene boronic ester afforded the desired
thiophene capped pyridine–DPP compounds, bearing a
hexyl chain at positions 3 (3-TDPPPy), 4 (4-TDPPPy), and 5
(5-TDPPPy) of the thiophene ring. Additionally, the derivative
without hexyl substituent attached to the peripheral thiophene
rings (H-TDPPPy) was synthesized to assess the effect of the
presence of aliphatic chains on the optoelectronic properties.
The structure and purity of all the compounds were confirmed
by
1
H-NMR and
13
C-NMR spectroscopy (Figures S1–S4, Sup-
porting Information) and matrix-assisted laser desorption/ioni-
zation time-of-flight (MALDI-TOF) mass spectrometry.
The thermal properties of the TDPPPy derivatives were
characterized by differential scanning calorimetry (DSC) meas-
urements (Figure S5, Supporting Information). In the second
heating run, sharp endothermic peaks corresponding to the
melting process were observed. Due to the presence of the hexyl
side chains in the peripheral thiophenes, 3-TDPPPy (T
m
= 119 °C),
4-TDPPPy (T
m
= 148 °C), and 5-TDPPPy (T
m
= 110 °C) displayed
lower melting temperatures than H-TDPPPy (T
m
= 197 °C).
Additionally, a cold crystallization appeared in the heating scan
of 3-TDPPPy (T
cc
= 41 °C) and a glass transition at ≈37 °C for
4-TDPPPy. Besides, weaker transitions occurred for 5-TDPPPy
likely due to the formation of different mesophases which have
already been observed in other DPP derivatives.
[10]
The cooling
scan featured distinct exothermic peaks corresponding to the
crystallization process for H-TDPPPy (T
c
= 132 °C), 4-TDPPPy
(T
c
= 106 °C), and 5-TDPPPy (T
c
= 68 °C). In contrast, a broad
crystallization peak was observed for 3-TDPPPy which might
indicate a lower crystallinity and explain the presence of the cold
crystallization peak which results from the crystallization of the
amorphous domains on heating.
2.2. Electrochemical Properties
The electrochemical properties of the four TDPPPy molecules
were evaluated by cyclic voltammetry in dry dichloromethane
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Scheme 1. Synthetic pathway of the thiophene end-capped pyridine–DPP derivatives and structures of the synthesized compounds bearing an alkyl
chain at different positions in the thiophene ring.
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solution under inert atmosphere (Figure 1a). The corresponding
redox potentials are summarized in Table 1. All four compounds
show two quasireversible oxidation and reduction waves. The
highest occupied molecular orbital (HOMO) and lowest unoc-
cupied molecular orbital (LUMO) energy levels were estimated
from the onset of the first oxidation and reduction waves,
respectively (Table 1 and Figure 1b).
[11]
The presence of a hexyl
chain or its specific position on the peripheral thiophene has
no remarkable influence on the redox potentials and hence the
four TDPPPy derivatives possess virtually identical HOMO and
LUMO energies. The relatively low-lying HOMO energies are
due to the synergistic electron-withdrawing character of the DPP
core and the pyridine moieties. The low HOMO level is expected
to give a high open-circuit voltage in photovoltaic devices.
2.3. Photophyiscal Properties of TDPPPy in Solution and Film
The optical properties of the thiophene capped pyridine–DPP
derivatives were determined by measuring the absorption and
fluorescence spectra in dichloromethane solution and as spin-
coated thin films (Figure 2). The spectral characteristics are
summarized in Table 1. The absorption spectra of the four mol-
ecules in solution are virtually identical. The spectra represent
the optical transitions among the
π
and
π
* orbitals of the conju-
gated backbone. The more intense and lowest-energy absorption
bands (450–620 nm) are assigned to
π
–
π
* transitions with intra-
molecular charge transfer character between the electron-rich
and electron-deficient moieties, while the weaker high energy
bands (300–450 nm) are ascribed to localized electronic transi-
tions of the aromatic rings. The main absorption of 3-TDPPPy
is hypsochromically shifted relative to the other derivatives, pre-
sumably due to a slightly reduced
π
-conjugation as a result of a
non-coplanar conformation of the thiophene and pyridine rings
caused by steric hindrance between the hexyl chain pointing
toward the center of the molecule and the pyridine ring (Table 1).
The absorption spectra of all four compounds shift
bathochromically when going from solution to thin films on
quartz substrates. This shift is a consequence of the intermolec-
ular interactions that occur in the solid state and the increased
molecular order. The shift is accompanied by the appearance
of more-resolved vibronic structure and an enhanced relative
intensity of the localized
π
–
π
* absorption bands. The optical
band gaps (E
g
) in the solid state are in the range of 1.86–1.94 eV,
as determined from the onsets of the absorption at low energy.
There is a generally good correlation between the optical and
the electrochemical gaps (Table 1), with slightly higher values
estimated for the electrochemical gaps. This could be related to
the exciton binding energy which lowers the optical gap relative
to the HOMO–LUMO difference,
[12]
the exchange energy, and
the difference in solvation energy for ions in solution compared
to the neutral molecules.
The differences observed in the intensity ratio of the first
two vibronic peaks, commonly referred to as the 0–0 and 0–1
transitions, in the absorption spectra allow the identification
of two different types of aggregates, when the strength of the
intermolecular (excitonic) coupling is similar to the electron-
vibrational (vibronic) coupling. In such cases, the well-known
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-2.1 -1.8 -1.5-1.2 0.30.6 0.91.2
Current
Potential vs Fc/Fc
+
/ V
H-TDPPPy
3-TDPPPy
4-TDPPPy
5-TDPPPy
(a)
-6.5
-6.0
-5.5
-5.0
-4.5
-4.0
-3.5
-5.86
-5.85
-5.81
-5.80
-6.17
-3.83
-3.82
-3.83 -3.83
-4.18
5
-
TDPP
P
y
4-TDPP
P
y
3
-
T
DP
P
Py
H-
TDPP
P
y
Energy / eV
[7
0]P
CBM
(b)
Figure 1. a) Cyclic voltammograms of TDPPPy derivatives in 10
−3
M CH
2
Cl
2
solution containing 0.1 M tetrabutylammonium hexafluorophosphate
(TBAPF
6
) as supporting electrolyte. b) Energy levels determined by cyclic voltammetry, including those of phenyl-C
71
-butyric acid methyl ester
([70]PCBM) for comparison.
Table 1. Optoelectronic properties.
E
ox
1/2
a)
[V]
E
red
1/2
a)
[V]
E
g
CVb)
[eV]
E
HOMO
c)
[eV]
E
LUMO
d)
[eV]
λ
max
abs,sol
[nm]
λ
max
abs,film
[nm]
ε
sol
e)
[
M
−1
cm
−1
]
E
g
opt,filmf)
[eV]
H-TDPPPy 0.73, 0.98
−1.47, −2.00
2.03
−5.86 −3.83
560 593 39 700 1.94
3-TDPPPy 0.73, 1.00
−1.48, −2.05
2.03
−5.85 −3.82
549 597 31 600 1.90
4-TDPPPy 0.66, 0.96
−1.47, −2.01
1.98
−5.81 −3.83
563 622 41 600 1.87
5-TDPPPy 0.64, 0.93
−1.48, −2.03
1.97
−5.80 −3.83
566 613 43 800 1.86
a)
E
1/2
= ½(E
pa
+ E
pc
);
b)
Estimated as the difference between the onsets of the oxidation and reduction waves in the cyclic voltammogram;
c)
E
HOMO
= −(5.23 + E
ox
onset
);
d)
E
LUMO
= −(5.23 + E
red
onset
);
e)
Calculated from the slope of the absorbance versus concentration calibration plot using five dichloromethane solutions of different concen-
trations;
f)
Estimated from the onset of the lower energy band of the absorption spectrum E
g
[eV] = 1240/
λ
onset
[nm].
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result from Kasha’s theory that H-aggregates exhibit blue-
shifted absorption spectra while J-aggregates exhibit the red-
shifted absorption has to be modified.
[13]
Spano has shown that
under these conditions, the two aggregate types can be dis-
tinguished by the fact that in H(J)-aggregates the ratio of the
first two vibronic peak intensities in the absorption spectrum
decreases (increases) with increasing excitonic coupling.
[14]
As
can be seen in Figure 2, the 0–0/0–1 vibronic peak ratio for
3-TDPPPy is smaller than unity, consistent with a predomi-
nant contribution of H-aggregates. In contrast, compounds
4-TDPPPy and 5-TDPPPy form J-type aggregates, given the
0–0/0–1 intensity ratios larger than unity. A slightly different
behavior is seen for H-TDPPPy where the intensities of the 0–0
and 0–1 transitions are virtually the same. This is tentatively
attributed to the coexistence of domains with both types of
aggregates in the as-cast films.
The fluorescence spectra of the four derivatives in
solution show a mirror-image relationship with the absorption
spectrum, but with a more intense high-energy emission
band and a weaker vibronic shoulder at longer wavelengths
(Figure 2b). The identification of H- and J-aggregates in thin
films was further evidenced by the characteristic features of the
fluorescence spectra and the measurement of the photolumi-
nescence lifetimes by time-correlated single photon counting.
In all cases, the emission of the films was red-shifted com-
pared to the spectra acquired in solution. However, derivatives
H-TDPPPy and 3-TDPPPy exhibited a broad, structureless
emission band, while 4-TDPPPy and 5-TDPPPy exhibit vibroni-
cally structured fluorescence spectra. The characteristics are
in agreement with the fact that the lowest-energy emission is
strongly dipole-allowed for J-type aggregation, but forbidden
in H-aggregates. The assignment is further supported by the
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400 600 800
0.0
0.2
0.4
0.6
0.8
1.0
400600 800 400600 800 400600 800
5-TDPPPy4-TDPPPy3-TDPPPy
Normalized Absorbance
Wavelength / nm
H-TDPPPy
(a)
600 700 800
0.0
0.2
0.4
0.6
0.8
1.0
600700 800 600700 800 600700 800
Normalized Emission Intensity
Wavelength / nm
(b)
H-TDPPPy 3-TDPPPy 4-TDPPPy5-TDPPPy
Figure 2. a) Normalized absorption and b) fluorescence spectra for the TDPPPy derivatives in dichloromethane solution (dashed line), as-cast thin
films (continuous line), and after solvent vapor annealing treatment of the thin films (dotted line) on quartz substrates.