Star-shaped and linear TT-conjugated oligomers consisting of a tetrathienoanthracene core and multiple diketopyrrolopyrrole arms for organic solar cells

Article abstract of DOI:10.3762/bjoc.12.142

Solution-processable star-shaped and linear π-conjugated oligomers consisting of an electron-donating tetrathienoanthracene (TTA) core and electron-accepting diketopyrrolopyrrole (DPP) arms, namely, TTA-DPP4 and TTA-DPP2, were designed and synthesized. Ba

Full text of DOI:10.3762/bjoc.12.142

Star-shaped and linear π-conjugated oligomers consisting of
a tetrathienoanthracene core and multiple
diketopyrrolopyrrole arms for organic solar cells
Hideaki Komiyama*1,2, Chihaya Adachi1,3 and Takuma Yasuda*2
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2016, 12, 1459–1466.
1International Institute for Carbon Neutral Energy Research
(WPI-I2CNER), Kyushu University, 744 Motooka, Nishi-ku, Fukuoka
819-0395, Japan, 2INAMORI Frontier Research Center (IFRC),
Kyushu University and 3Center for Organic Photonics and Electronics
Research (OPERA), Kyushu University
doi:10.3762/bjoc.12.142
Received: 18 April 2016
Accepted: 01 July 2016
Published: 14 July 2016
Email:
This article is part of the Thematic Series "Organo photovoltaics".
Guest Editor: D. J. Jones
Hideaki Komiyama* - komiyama@ifrc.kyushu-u.ac.jp;
Takuma Yasuda* - yasuda@ifrc.kyushu-u.ac.jp
* Corresponding author
© 2016 Komiyama et al.; licensee Beilstein-Institut.
License and terms: see end of document.
Keywords:
bulk heterojunction; diketopyrrolopyrrole; organic solar cells;
star-shaped oligomers; tetrathienoanthracene
Abstract
Solution-processable star-shaped and linear π-conjugated oligomers consisting of an electron-donating tetrathienoanthracene (TTA)
core and electron-accepting diketopyrrolopyrrole (DPP) arms, namely, TTA-DPP4 and TTA-DPP2, were designed and synthesized.
Based on density functional theory calculations, the star-shaped TTA-DPP4 has a larger oscillator strength than the linear TTA-
DPP2, and consequently, better photoabsorption property over a wide range of visible wavelengths. The photovoltaic properties of
organic solar cells based on TTA-DPP4 and TTA-DPP2 with a fullerene derivative were evaluated by varying the thickness of the
bulk heterojunction active layer. As a result of the enhanced visible absorption properties of the star-shaped π-conjugated structure,
better photovoltaic performances were obtained with relatively thin active layers (40–60 nm).
Introduction
Solution-processable organic semiconductors have been inten- tectures have rapidly increased to approximately 10% [5],
sively studied as key materials for low-cost, flexible, and large- approaching those of state-of-the-art polymer-based OSCs [6,7].
area optoelectronic devices, including organic solar cells To produce high-performance OSCs, the donor materials are re-
(OSCs) [1-4]. OSCs have drawn much attention as promising quired to possess suitable frontier orbital energy levels, high
next-generation renewable energy sources because abundant carrier mobility, excellent film-forming ability, and good misci-
sun-light energy can be directly converted into electricity. bility with a fullerene derivative as an acceptor. Moreover,
Recently, the power conversion efficiencies (PCEs) of OSCs strong visible light photoabsorption ability for visible light is
based on small molecules with bulk heterojunction (BHJ) archi- vital for the donor materials.
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Beilstein J. Org. Chem. 2016, 12, 1459–1466.
Star-shaped molecules tethering multiple π-conjugated arms are logue, TTA-DPP4 and TTA-DPP2 (Figure 1). TTA can be
capable of harvesting incident light effectively owing to their regarded as a promising central core unit for star-shaped
extended dimensionality, in comparison to their linear ana- π-conjugated oligomers, and has previously been utilized as a
logues. Star-shaped π-conjugated molecules consisting of hexa- building block of semiconducting polymers for OSCs [24] and
peri-hexabenzocoronene [8-14], pyrene [15-17], and triphenyl- organic field-effect transistors [25-27]. To the best of our know-
amine [18-23] central cores have been developed as donor ma- ledge, however, there has been no precedent on TTA-based
terials for BHJ-OSCs. The design of star-shaped π-conjugated star-shaped and linear π-conjugated oligomers that can be
molecules offers possibilities for not only further enhancement applied to OSCs.
of the photoabsorption ability but also for the improvement of
Results and Discussion
molecular packing, solubility, and film-forming ability. Howev-
er, the impact of star-shaped molecular structures on optoelec- Molecular design and synthesis
tronic functionality has not been fully characterized because of To develop star-shaped and linear π-conjugated oligomers with
the small variation in the central core units researched so far. acceptor–donor–acceptor electronic structures, an electron-do-
Herein, we report the design and synthesis of a new star-shaped nating TTA unit was coupled with electron-accepting diketo-
π-conjugated oligomer composed of an electron-donating tetra- pyrrolopyrrole (DPP)-based chromophoric units, which are
thienoanthracene (TTA) core coupled with multiple electron- well-known building blocks used in OSCs [28-31]. Such molec-
accepting diketopyrrolopyrrole (DPP) arms, and its linear ana- ular structures facilitate intramolecular charge transfer,
Figure 1: Chemical structures of TTA-DPP4 and TTA-DPP2.
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Beilstein J. Org. Chem. 2016, 12, 1459–1466.
rendering the molecules with narrower bandgaps, and broad and gravimetric analysis (TGA). As shown in Figure 3, the
strong light absorptions. To gain insight into the geometric and 5% weight-loss temperature of TTA-DPP4 and TTA-DPP2,
electronic structures of TTA-DPP4 and TTA-DPP2, density under N2 atmosphere, was determined to be 378 and 380 °C, re-
functional theory (DFT) calculations were performed at the spectively, suggesting the high thermal stability for these com-
B3LYP/6-31G(d,p) level. The calculated energy levels and the pounds as donor materials.
respective frontier orbital distributions for these molecules are
displayed in Figure 2. The HOMOs and LUMOs of TTA-DPP4 Optical properties
and TTA-DPP2 are well-expanded across the entire π-conju- The UV–vis absorption spectra of TTA-DPP4 and TTA-DPP2
gated structure. Both molecules possess similar HOMO and in chloroform solutions and as-spun thin films are depicted in
LUMO energy levels of approximately −4.7 and −2.7 eV, re- Figure 4, and the corresponding photophysical data are summa-
spectively. The star-shaped structure has an apparent effect on rized in Table 1. In chloroform solution, TTA-DPP4 and TTA-
its oscillator strength (f) such that a larger f value leads to a DPP2 exhibit strong and broad absorptions with two character-
larger absorption coefficient. Therefore, the four-armed TTA- istic bands: a shorter-wavelength absorption (300–400 nm)
DPP4 (f = 3.02) is expected to show better photoabsorption arising from π–π* transitions and a longer-wavelength absorp-
ability than the two-armed TTA-DPP2 (f = 2.60) because of the tion (500–700 nm) originating from intramolecular charge
two-dimensionally extended π-conjugated structure.
transfer (ICT) transitions between the electron-donating TTA
and the electron-accepting DPP units. Evidently, the maximum
The synthetic routes to TTA-DPP4 and TTA-DPP2 are outlined absorption coefficient (ε) of TTA-DPP4 (22.3 × 104 M−1 cm−1)
in Scheme 1 (see also Experimental section and Supporting is more than twice that of TTA-DPP2 (9.6 × 104 M−1 cm−1),
Information File 1 for details). TTA-DPP4 and TTA-DPP2 which is consistent with the increment in the oscillator strength
were synthesized in 44% and 49% yields, respectively, via the revealed by the DFT calculation results. The ICT absorption
palladium-catalyzed Stille cross-coupling reaction. Both TTA- peaks are slightly red-shifted and broadened in the solid thin
DPP4 and TTA-DPP2 were soluble in common organic sol- films as compared to those in the chloroform solutions, which
vents, such as chloroform, toluene, and chlorobenzene. The can be ascribed to molecular aggregation in the condensed solid
thermal stability of these compounds was analyzed by thermo- state. The optical band gaps of TTA-DPP4 and TTA-DPP2 are
Figure 2: HOMO and LUMO distributions, calculated energy levels, and associated oscillator strengths (f) for TTA-DPP4 and TTA-DPP2 calculated at
the B3LYP/6-31G(d,p) level. The arrows indicate the first singlet–excited state (S1) transition. The alkyl chains are modified to methyl groups to
simplify the calculations.
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Scheme 1: Synthesis of TTA-DPP4 and TTA-DPP2.
armed TTA-DPP4 in the visible region. The HOMO energy
levels of TTA-DPP4 and TTA-DPP2 in the thin films were de-
termined to be –5.36 and –5.40 eV, respectively, via photoelec-
tron yield spectroscopy (Figure S1 in Supporting Information
File 1). The LUMO energy levels were calculated to be –3.72
and –3.75 eV for TTA-DPP4 and TTA-DPP2, respectively.
Because of their deep-lying HOMO levels and sufficient
LUMO offsets (>0.5 eV), both TTA-DPP4 and TTA-DPP2 can
serve as electron-donor materials in combination with [6,6]-
phenyl-C71-butyric acid methyl ester (PC71BM) as an acceptor
materials, which has a LUMO level of –4.3 eV.
Photovoltaic properties
The photovoltaic properties of TTA-DPP4 and TTA-DPP2 were
also evaluated. BHJ-OSCs were fabricated using TTA-DPP4
and TTA-DPP2 as electron-donor materials and PC71BM as an
electron-acceptor material, with an inverted device configura-
Figure 3: TGA curves of TTA-DPP4 and TTA-DPP2 at a heating rate
of 10 °C min-1 under N2.
estimated to be 1.64 and 1.65 eV, respectively, from the onset tion of glass/ITO/ZnO/donor:PC71BM(1:1.5, w/w)/MoO3/Ag.
positions of their absorption bands. The absorption coefficient A 30 nm thick ZnO layer was deposited on the ITO-coated
(α) of TTA-DPP4 (16.7 × 104 cm−1) in the thin film is still glass via the sol–gel method (see Experimental section). The
considerably larger than that of TTA-DPP2 (11.5 × 104 cm−1), active layer was then spin-coated from a solution of the donor
indicating the enhanced photoabsorption property of the four- material and PC71BM in a mixed solvent of chloroform/1,8-
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Figure 4: UV–vis absorption spectra of TTA-DPP4 (red) and TTA-DPP2 (black) in (a) chloroform solutions and (b) thin films.
Table 1: Photophysical data of TTA-based molecules.
Solutiona
Thin filmb
HOMOc [eV]
LUMOd [eV]
Ege [eV]
λmax [nm]
ε [104 M−1 cm−1]
λmax [nm]
α [104 cm−1]
TTA-DPP4
TTA-DPP2
616
623
22.3
9.6
625, 676
626, 677
16.7
11.5
−5.36
−5.40
−3.72
−3.75
1.64
1.65
aMeasured in chloroform solutions (10−5 M); bthin films spin-coated from chloroform solution onto quartz substrate; cdetermined by photoelectron yield
spectroscopy in spin-coated thin films; ddetermined from the HOMO and optical energy gap (Eg): LUMO = HOMO + Eg; ederived from the absorption
onset of the thin films.
diiodooctane (DIO) (98:2, v/v). The thickness of the active ated under simulated AM 1.5G illumination at an intensity of
layer was controlled within the range of ca. 40–90 nm by 100 mW cm−2 (1 sun).
varying the rotation speed during spin-coating. A 6 nm thick
MoO3 layer as a hole extraction layer and a 100 nm thick Figure 5 shows the current density–voltage (J–V) characteris-
Ag anode were vacuum-deposited on the active layer. The tics of the BHJ-OSCs based on TTA-DPP4 and TTA-DPP2
photovoltaic properties of the fabricated BHJ-OSCs were evalu- with different active layer thicknesses. The dependence of the
Figure 5: J–V characteristics of BHJ-OSCs based on (a) TTA-DPP4:PC71BM (1:1.5, w/w) and (b) TTA-DPP2:PC71BM (1:1.5, w/w) with different
active layer thicknesses, measured under simulated AM 1.5G, 100 mW cm−1 illumination.
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Beilstein J. Org. Chem. 2016, 12, 1459–1466.
active layer thickness on PCE is presented in Figure 6, and the ported, the best photovoltaic performances were generally
representative photovoltaic parameters are listed in Table 2. For achieved with an active layer thickness of around 100 nm.
the TTA-DPP4-based devices, the photovoltaic properties were However, TTA-DPP4 and TTA-DPP2 presented the highest
not influenced much when changing the thickness of the active PCEs with much thinner active layers of approximately
layer in the range of ca. 40–90 nm, providing PCEs of 40–60 nm. Moreover, the PCEs of TTA-DPP4-based devices
1.7–2.0%. Apparently, the TTA-DPP4-based OSCs showed were higher than those of TTA-DPP2, presumably because of
better photovoltaic performance with a thinner active layer its larger absorption coefficient resulting from its star-shaped
(40–80 nm), as compared to conventional poly(3-hexylthio- molecular structure with a two-dimensionally expanded
phene) (P3HT)-based devices. A similar effect of active layer π-conjugated backbone.
thickness on PCE was also observed in TTA-DPP2-based
devices, where a PCE of 1.4% was obtained with an active layer Conclusion
thickness of 47 nm. In contrast, as presented in Figure 6 and In this study, the first attempt to introduce a TTA unit as central
Table 2, the PCEs of P3HT-based devices steeply decreased electron-donating core into star-shaped and linear π-conjugated
with decreasing active layer thickness in the range of 40–80 nm, oligomers was demonstrated. Multiple electron-accepting DPP
agreeing with the previously reported results [32]. For most of arms were attached to the electron-donating TTA core to form
the donor materials, including polymers and small molecules re- star-shaped TTA-DPP4 and linear TTA-DPP2 that have
acceptor–donor–acceptor electronic structures. TTA-DPP4
showed a better photoabsorption property than TTA-DPP2
because of its larger oscillator strength, as expected from the
DFT calculations. The BHJ-OSCs based on TTA-DPP4 and
TTA-DPP2 showed good photovoltaic properties even with thin
active layers of 40–60 nm. This behavior was highly different
from that of the reported general polymer- and small-molecule-
based OSCs. A star-shaped molecular structure containing a
two-dimensionally extended π-conjugated system is a promis-
ing electronic system for designing photovoltaic organic materi-
als, as a result of its excellent photoabsorption properties.
Experimental
General methods
Matrix-assisted laser desorption ionization time-of-flight
(MALDI–TOF) mass spectra were collected on a Bruker
Daltonics Autoflex III spectrometer using dithranol as the
Figure 6: Relationship between active layer thickness and power
conversion efficiency (PCE) for TTA-DPP4, TTA-DPP2, and P3HT-
matrix. Elemental analysis was carried out using a YANACO
based BHJ-OSCs (see also Figures S2–S4 in the Supporting Informa-
tion File 1 for detailed photovoltaic data).
CHN coder MT-6. Thermogravimetric analysis (TGA) was per-
formed using a Hitachi High-Tech Science TG/DTA7300 with a
Table 2: Photovoltaic parameters for inverted BHJ-OSCs based on TTA-DPP4 or TTA-DPP2:PC71BM and P3HT:PC61BM under simulated AM 1.5 G,
100 mW cm−1 illumination.
Donor
Thickness [nm]
JSC [mA/cm2]
VOC [V]
FF [%]
PCE [%]
TTA-DPP4
TTA-DPP4
TTA-DPP4
76 ± 3
59 ± 1
40 ± 2
5.84
6.02
5.95
0.70
0.70
0.66
42.9
45.6
46.7
1.76
1.93
1.84
TTA-DPP2
TTA-DPP2
TTA-DPP2
87 ± 1
59 ± 1
47 ± 2
3.26
3.89
4.25
0.76
0.76
0.74
44.9
46.4
44.7
0.95
1.37
1.40
P3HT
P3HT
P3HT
76 ± 1
55 ± 1
39 ± 1
7.32
5.60
2.97
0.56
0.43
0.32
62.8
52.4
50.0
2.58
1.28
0.47
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heating rate of 10 °C min−1 under N2 atmosphere. UV–vis The product was purified by silica gel column chromatography
absorption spectra were recorded on a JASCO V-670Y spec- (eluent: chloroform/hexane 1:1, v/v) and GPC to give TTA-
trometer. Photoelectron yield spectra were recorded on a Riken- DPP2 as a dark purple solid (yield = 0.32 g, 49%). MS
Keiki AC-2 ultraviolet photoelectron spectrometer. The thick- (MALDI–TOF) m/z: [M]+ calcd for 1991.95; found 1991.67;
ness of photoactive layers was measured using a Bruker anal. calcd (%) for C122H150N4O4S8: C, 73.52; H, 7.59; N,
DektakXT system.
2.81; found: C, 73.33; H, 7.53; N, 2.84.
Fabrication and evaluation of organic solar
Synthesis
All reactions were carried out under N2 atmosphere using stan- cells
dard Schlenk techniques. All starting materials, unless other- The OSC devices were fabricated and tested by using similar
wise specified, were purchased from commercial suppliers and procedures described in reference [30]. Pre-patterned ITO-
used without further purification. 2,5,9,12-Tetrabromoanthra- coated glass substrates were cleansed sequentially by soni-
[1,2-b:4,3-b':5,6-b'':8,7-b''']tetrathiophene (1) [25], 2,5-bis(2- cating in detergent solution, deionized water, acetone, and
ethylhexyl)-3-(5-(4-hexylphenyl)thiophen-2-yl)-6-(5-(trimethyl- isopropanol for 15 min each, and then subjected to UV/ozone
stannyl)thiophen-2-yl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4- treatment for 30 min. A thin layer (~30 nm) of ZnO was pre-
dione (2) [30,33], (5,12-bis(2-ethylhexyl)anthra[1,2-b:4,3- pared by spin-coating (at 5000 rpm) a precursor solution of zinc
b':5,6-b'':8,7-b''']tetrathiophene-2,9-diyl)bis(trimethylstannane) acetate (0.50 g) and ethanolamine (0.14 g) in 2-methoxyethanol
(3) [24,27], and 3-(5-bromothiophen-2-yl)-2,5-bis(2- (5 mL), followed by baking at 200 °C for 10 min under air. The
ethylhexyl)-6-(5-(4-hexylphenyl)thiophen-2-yl)-2,5-dihydro- photoactive layer was then prepared by spin-coating from a
pyrrolo[3,4-c]pyrrole-1,4-dione (4) [30] were synthesized ac- chloroform solution containing the donor material and
cording to the reported procedures. Detailed synthetic schemes PC71BM, after passing through a 0.45 μm PTFE membrane
for these compounds are provided in the Supporting Informa- filter. Finally, a 6 nm thick MoO3 layer and a 100 nm-thick Ag
tion File 1.
layer were thermally evaporated on top of the active layer under
high vacuum through a shadow mask, defining an active area of
Synthesis of TTA-DPP4: To a mixture of 1 (0.19 g, 0.04 cm2 for each device. Current density–voltage (J–V) mea-
0.27 mmol) and 2 (2.29 g, 2.70 mmol) in a mixture of dry DMF surements for the fabricated OSCs were conducted on a com-
(10 mL) and dry toluene (20 mL) was added Pd(PPh3)4 puter-controlled Keithley 2400 source measure unit in air, under
(0.016 g, 0.014 mmol). The mixture was stirred for 38 h at simulated AM 1.5G solar illumination at 100 mW cm−2 (1 sun),
120 °C. After cooling to room temperature, the reaction mix- using a Xe lamp-based Bunko-Keiki SRO-25 GD solar simu-
ture was poured into water and then extracted with chloroform. lator. The light intensity was calibrated using a standard silicon
The combined organic layers were washed with water and dried photovoltaic reference cell.
over anhydrous Na2SO4. After filtration and evaporation, the
product was purified by silica gel column chromatography
Supporting Information
(eluent: chloroform/hexane 4:1, v/v) to provide TTA-DPP4 as a
dark purple solid. This compound was further purified by recy-
Supporting Information File 1
cling preparative gel permeation chromatography (GPC; eluent:
chloroform) prior to use (yield = 0.36 g, 44%). MS
(MALDI–TOF) m/z: [M]+ calcd for 3133.42; found 3133.51;
anal. calcd (%) for C190H226N8O8S12: C, 72.80; H, 7.27; N,
3.57; found: C, 71.29; H, 7.15; N, 3.53. Well-resolved NMR
signals could not be obtained for both TTA-DPP4 and TTA-
DPP2 in CDCl3 or DMSO-d6 even at elevated temperatures due
to the macromolecular nature of the compounds (Figure S5 in
Supporting Information File 1). The experimental results of
isotope pattern deconvolution in mass spectra of TTA-DPP4
and TTA-DPP2 showed good agreements with the theoretical
isotope patterns, supporting their chemical structures.
Synthesis of compounds 1–13, evaluation of HOMO levels
for TTA-DPP4 and TTA-DPP2, photovoltaic properties of
TTA-DPP4, TTA-DPP2, and P3HT-based OSCs and
1H NMR spectra of TTA-DPP4 and TTA-DPP2.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-12-142-S1.pdf]
Acknowledgements
This work was partially supported by the ACCEL project from
Japan Science and Technology Agency (JST), and Grants-in-
Aid for Scientific Research (Nos. 16K21218 (H.K.) and
15H01049 (T.Y.)) from JSPS, the Cooperative Research
Program of “Network Joint Research Center for Materials and
Devices”, the Yoshida Education and Foundation (H.K.), the
Yashima Environment Technology Foundation (T.Y.), and the
Synthesis of TTA-DPP2: This compound was prepared in a
similar fashion to TTA-DPP4, using 3 (0.31 g, 0.33 mmol) and
4 (0.53 g, 0.69 mmol), and Pd(PPh3)4 (0.015 g, 0.013 mmol).
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