Rational design of pyrrolopyrrole-aza-BODIPY-based acceptor-donor-acceptor triads for organic photovoltaics application

Article abstract of DOI:10.1039/d0cc00398k

Acceptor-donor-acceptor triads consisting of diketopyrrolopyrrole (DPP) or pyrrolopyrrole aza-BODIPY (PPAB) or both as acceptors and cyclopentadithiophene as a donor were rationally designed for near infrared (NIR) photovoltaics application. Among them, the PPAB-based triad exhibited the highest power conversion efficiency of 3.88% owing to the panchromatic absorption in the UV/vis/NIR regions.

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Rational design of pyrrolopyrrole-aza-BODIPY-based acceptor-
donor-acceptor triads for organic photovoltaics application
Ru Feng,a Narumi Sato,ab Takuma Yasuda,ab Hiroyuki Furuta*ac and Soji Shimizu*ac
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Acceptor-donor-acceptor triads consisting of diketopyrrolopyrrole (PPAB).14 The absorption and fluorescence spectra of a series of
(DPP) or pyrrolopyrrole aza-BODIPY (PPAB) or both as acceptors PPAB molecules are bathochromically shifted to the NIR region
and cyclopentadithiophene as a donor were rationally designed for compared with those of the corresponding DPP. As their far-
near infrared (NIR) photovoltaics application. Among them, the red/NIR absorption and emission is beneficial for bioimaging,
PPAB-based triad exhibited the highest power conversion efficiency sensors and therapeutics, PPABs have been further
of 3.88% owing to the panchromatic absorption in the UV/vis/NIR functionalized
toward
aggregation-induced
emission
regions.
enhancement (AIEE)15 and two-photon absorption (TPA)16
materials and applied to photoacoustic imaging,17,18 cancer
therapy18 and electrogenerated chemiluminescences (ECLs) in
the NIR region.19 During our study on the aryl-substituent
effects on the optical and electrochemical properties of PPABs,
we noticed that the dimerization of PPAB by a bithienyl linkage
caused significant broadening of the absorption in the vis/NIR
region, which is suitable for the far-red/NIR photovoltaics.20
Considering that a biphenyl-linked PPAB dimer exhibited a
single, sharp absorption almost identical to that of the
corresponding PPAB monomer due to its twisted structure, the
Diketopyrrolopyrrole (DPP) is a fascinating bicyclic lactam,
which has been widely used as an industrial pigment.1,2
Recently, DPP-based donor-acceptor (D-A) small molecules and
polymers have been intensively investigated as functional
materials for organic field effect transistors (OFETs)3–5 and
organic photovoltaics (OPVs)6–10 due to their excellent optical
and electronic properties such as efficient charge transport
property and high visible absorptivity.1,2 Owing to the relatively
low-lying HOMO levels, the DPP-based materials are promising
candidates for OPV application to achieve high open circuit
voltage (Voc).6–10
Considering the solar irradiance spectrum, panchromatic
absorption in the visible and near infrared (vis/NIR) regions is
ideal for harvesting solar energy. To achieve such a wide range
of light absorption using DPP as a structural basis, various DPP-
based -extended compounds have been intensively
investigated in addition to the DPP-based D-A small molecules
and polymers.2,11,12 Recently, we developed a facile method to
extend the -conjugation of DPP using a TiCl4-mediated Schiff
base forming reaction13 to create a dimeric aza-BODIPY
analogue, which is referred to as pyrrolopyrrole aza-BODIPY
bithienyl linkage played
a crucial role in enhancing
interchromophore interactions by forming coplanar
arrangements of PPAB moieties. Although the panchromatic
absorption of the bithienyl-linked PPAB dimer was expected to
be ideal for the OPV application, the dimer showed a poor OPV
performance with low power conversion efficiency (PCE) of
0.74% and small short circuit current (Jsc) of 3.02 mA cm–2.20 This
result implied that suitable bulk heterojunction (BHJ) structures
for charge separation were not fabricated due to the low
solubility of the dimer.
Taking these results into consideration, we rationally
designed a series of triad chromophores (T1–T3) based on the
acceptor-donor-acceptor (A-D-A) structural motif. In these
triads, DPP or PPAB or both as acceptors are attached to a
cyclopentadithiophene (CPDT) donor bearing branched alkyl
chains. The CPDT moiety not only improves the solubility but
also enhances the interchromophore interactions. Despite the
small molecular size, the absorption spectrum of the PPAB-
based triads (T2 and T3) covered a wide range in the UV/vis/NIR
regions, which is comparable to those of DPP-based polymers
for the NIR photovoltaics.8–10 The effect caused by replacing
a. Department of Chemistry and Biochemistry, Graduate School of Engineering,
Kyushu University, Fukuoka 819-0395, Japan. E-mail: hfuruta@cstf.kyushu-
u.ac.jp, ssoji@cstf.kyushu-u.ac.jp
b. INAMORI Frontier Research Centre (IFRC), Kyushu University, Fukuoka 819-0395,
Japan
c. Centre for Molecular Systems (CMS), Kyushu University, Fukuoka 819-0395, Japan
† Electronic supplementary information (ESI) available: Experimental details, NMR
and fluorescence spectra, theoretical calculations and TEM images. See
DOI: 10.1039/x0xx00000x
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Scheme 1 Synthesis of triads, T1–T3, and the chemical structure of bithienyl-linked PPAB dimer.20 Reaction conditions (i): Pd(PPh3)4 (for T1) or PdCl2(PPh3)2 (for T2 and
T3), toluene, 120 ºC.
DPP with PPAB on optical properties and photovoltaic structures. T1 exhibited a single emission at 724 nm, which was
performances were systematically investigated.
a mirror-image to the absorption spectrum (Fig. S8, ESI†).
The triads (T1–T3) were synthesized by a Stille coupling
reaction21,22 of bromo-substituted DPP (DPP-Br)23,24 or PPAB
(PPAB-Br) or both and bis(trimethylstannyl)-CPDT (CPDT-
2SnMe3).25 The isolated yields were 38% for T1 from DPP-Br,
48% for T2 from DPP-Br and PPAB-Br and 49% for T3 from
PPAB-Br. All the compounds were characterized by high-
1
resolution mass spectrometry and H NMR spectroscopy (Figs.
S1–S6, ESI†).
In chloroform, both T2 and T3 exhibited broad absorption in
the UV/vis/NIR regions with a maximum at 783 and 827 nm,
respectively, whereas the absorption of T1 was retained in the
visible region with a maximum at 648 nm (Fig. 1a). In addition,
the molar absorption coefficient (ε) increased from T1 (0.60 ×
104 M–1cm–1) to T2 (1.08 x 105 M–1cm–1) and further to T3 (1.17
× 105 M–1cm–1). T2 also showed another intense band at 626 nm
originating from the DPP moiety. Compared with the absorption
spectra of the corresponding DPP and PPAB monomer species
(Fig. S7, ESI†),14 the absorption edges shifted to the red due to
the effective interchromophore interactions via the CPDT
moiety in coplanar conformations. According to our previous
study on the bithienyl-linked PPAB dimer,20 a shoulder band at
709 nm can be ascribed to the contribution of rotational
isomers with twisted orientations of the PPAB moieties, which
Fig. 1 UV/vis/NIR absorption spectra of T1 (black), T2 (red) and T3 (blue) (a) in
chloroform and (b) in a film state.
Similarly, T2 also showed a single emission originating from the
PPAB moiety at 838 nm regardless of excitation wavelengths
probably due to the fluorescence resonance energy transfer
(FRET) from the DPP moiety to the PPAB moiety. In contrast to
these single emissions, T3 exhibited bimodal emissions at 706
and 898 nm, which is similar to that of the bithienyl-linked PPAB
dimer.20 Although there is continuous distribution of rotational
isomers, this can be explained in terms of the coplanar and
twisted orientations of the PPAB moieties at the excited state,
which may exhibit a mirror-imaged emission at 898 nm to the
farthest absorption band at 828 nm and a monomer-like
emission at 706 nm, respectively. Considering that such bimodal
emission behaviour was not observed for the other triads, a
rotational barrier of T3 from the twisted conformation to the
coplanar conformation in the excited should be high so that the
conformational change may compete the radiative decay
process. Similar bimodal emission behaviours caused by the two
extreme conformers have been investigated in detail in the case
of porphyrin and corrole dimers.26,27
exhibit
monomer-like
absorption
due
to
minor
interchromophore interactions. In the drop-cast film state, the
triads exhibited slight broadening and red-shift of the main
absorption bands, denoting the formation of the π-stacked
aggregates (Fig. 1b). Despite the small molecular size, the
absorbance onset of T3 in the NIR region is almost comparable
to that of DPP-based polymers used for NIR photovoltaics.
Although the fluorescence of triads was nearly quenched in
solution (F = ~0.03) and quenched in the solid state, their
fluorescence spectra in solution were diagnostic of the
To estimate the redox properties, cyclic voltammetry
measurements were performed in dichloromethane containing
0.1 M tetra-n- hexafluorophosphate (nBu4N·PF6) as a supporting
2 | J. Name., 2012, 00, 1-3
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Table 1 Summary of optical and electrochemical properties of T1–T3
Solution
max [nm]
648
Film
Compd.
T1
E1/2 a [V]
E1/2 a [V]
EHOMOb [eV]
–4.96
ELUMOc [eV]
–3.24
Egd [eV]
1.72

10–5  [M–1 cm–1]

max [nm]
650
ox
red
0.21
0.22
0.10
–1.64
–1.18
–1.19
0.60
1.08
1.17
T2
–4.95
–3.70
1.25
626, 783
709, 827
745, 796
795, 845
T3
–4.89
–3.68
1.21
a Determined by cyclic voltammetry (conditions: 0.5 mM CH2Cl2 solutions containing 0.1 M nBu4N·PF6 as a supporting electrolyte at a scan rate of 100 mV s–1). Potentials
ox
red
c
are given relative to ferrocenium/ferrocene (Fc+/Fc) couple. b EHOMO = – (
+ 4.8) [eV]. ELUMO = – (
+ 4.8) [eV]. d Eg = ELUMO – EHOMO [eV].
퐸
퐸
onset
onset
electrolyte (Fig. 2). The triads exhibited simple reduction waves
To investigate the photovoltaic properties of the triads as a
and multiple oxidation waves. Compared with the reduction p-type material, we fabricated solution-processed BHJ-OPVs
potentials of the corresponding DPP and PPAB monomers, the with an inverted device structure of indium tin oxide (ITO)/ZnO
reduction observed for T1 and T3 (T1: –1.64 and –2.27 V vs. (30 nm)/T1–T3:PC71BM (70−110 nm)/MoOx (10 nm)/Ag (100
Fc+/Fc, T3: –1.19 and –1.74 V) can be assigned as DPP- and nm) (Fig. S13, ESI†). The BHJ active layer consisting of T1–
PPAB-centred reductions. By referring to the data of these T3:PC71BM binary blend was prepared by spin-coating from
homo-type triads, the first and third reductions of T2 at –1.18 their chloroform solutions with or without solvent additives.
and –1.79 V and the second and fourth ones at –1.65 and –2.33 After optimization of the device fabrication conditions (donor-
V can be ascribed to the PPAB- and DPP-centred reductions, acceptor (D/A) ratio, thickness of the active layer and additives
respectively. The multiple oxidations of the triads mainly (Figs. S14–S19 and Tables S3–S8, ESI†)), the T3:PC71BM BHJ-
originate from the CPDT moiety as only one or two oxidation OPVs device fabricated in the presence of 1,8-diiodooctane
waves were observed for the PPAB dimers in the previous (DIO) as an additive showed the best PCE of 3.88% among the
study.20
three triads (Fig. 3, Table 2). The PCE values of other triads
under the same conditions were 0.18% for T1 and 1.49% for T2.
It is noticeable that T3 exhibited Jsc up to 11.7 mA cm–2, which is
a reason for its high device performance. This could be
attributed to its panchromatic absorption and high absorptivity
compared with those of T1 and T2.
Fig. 2 Cyclic voltammograms of the triads in CH2Cl2 (0.5 mM) containing 0.1 M
nBu4N·PF6 as a supporting electrolyte at a scan rate of 100 mV s–1
.
The HOMO and LUMO energy levels of T1–T3 were
estimated based on the first oxidation and reduction potentials
of the triads (Table 1), and the DFT and TDDFT calculations at
the B3LYP/6-31G(d) level reproduced the trend of their energies
and transitions (Tables S1 and S2, Figs. S9–S12, ESI†). As model
structures of T3 for DFT calculations, coplanar and
perpendicular extreme conformations were used because the
continuous distribution of rotational isomers at the ground
state was inferred from the absorption spectra. The LUMO
resides shallower (–3.24 to –3.70 eV) than that of [6,6]-phenyl-
C71 butyric acid methyl ester (PC71BM) at –4.3 eV,28 implying
that T1–T3 can be used as p-type materials in OPV devices. The
LUMO energy offset between T1–T3 and PC71BM can also
facilitate efficient charge separation.29 The LUMO energy levels
of T2 and T3 became deeper than that of T1 due to the stronger
acceptor nature of the PPAB moiety than the DPP moiety. The
low-lying HOMO energy levels of these triads would also be
beneficial to attain large Voc because Voc is directly proportional
to the energy gap between the HOMO and LUMO energy levels
of the donor and acceptor, respectively.30,31
Fig. 3 (a) J-V curves and (b) EQE spectra of T1–T3:PC71BM BHJ-OPVs.
Table 2 Device characteristics of T1–T3:PC71BM BHJ-OPVs
Blend ratio a
Solvent b
Jsc
(mA cm–2)
0.98
4.20
11.7
Voc
(V)
0.43
0.73
0.67
FF
PCE
(%)
0.18
1.49
3.88
T1:PC71BM
T2:PC71BM
T3:PC71BM
CF:DIO
CF:DIO
CF:DIO
0.36
0.48
0.50
a Donor-acceptor ratio is 1:2 (w/w), and thickness of the bulk heterojunction layer
is 90 nm. b CF: chloroform, DIO: 1,8-diiodooctane and the ratio: 99.5:0.5 vol%.
The interior morphologies of the BHJ active layers were
characterized by transmission electron microscopy (TEM) (Fig.
S20, ESI†). The TEM images of the T3:PC71BM blend films
revealed homogenous and fine nanostructures with closely
mixed D/A networks. The small average domain size of 11 nm
scaled based on the D/A segregation for T3:PC71BM is
favourable for efficient charge separation, which is consistent
with its high Jsc value. It is, therefore, worth emphasizing that
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PPAB-based small molecules are highly promising as new
materials in photovoltaic applications.
In this study, we presented three panchromatic A-D-A small
molecules, using DPP or PPAB or both as acceptors and CPDT as
a donor. We found that the CPDT moiety bearing branched alkyl
chains is a suitable mediator to enhance interchromophore
interactions. The absorption spectra of T1–T3 indicated that the
replacement of DPP with PPAB from T1 to T3 broadened the
absorption in the UV/vis/NIR region and increased the molar
absorption coefficients. In the OPV studies, T3 exhibited the
highest PCE and Jsc among the triads due to its panchromatic
absorption in the UV/vis/NIR regions and high absorptivity. Our
current results demonstrate that the A-D-A structural motif
based on PPAB is an effective approach to achieve
panchromatic photovoltaics with high PCE values, which can
further be improved by optimizing the donor unit. Research
along this direction is being intensively investigated in our
laboratory.
This work was supported by Grants-in-Aids from JSPS (Nos.
JP26708003 and JP19H02703). The authors thank Ms. Natsuko
Ide for the TEM measurements.
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Conflicts of interest
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There are no conflicts to declare.
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