Synthesis of annulated thiophene perylene bisimide analogues: Their applications to bulk heterojunction organic solar cells

Article abstract of DOI:10.1039/c0cc05448h

Annulated thiophene perylene bisimides and their triphenyl-amine based oligomers have been synthesized. One of the oligomers FPTTPA has been demonstrated to be an efficient electron donor in bulk heterojunction (BHJ) organic solar cells, giving a power co

Full text of DOI:10.1039/c0cc05448h

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COMMUNICATION
Synthesis of annulated thiophene perylene bisimide analogues: their
applications to bulk heterojunction organic solar cellsw
Hyunbong Choi, Sanghyun Paek, Juman Song, Chulwoo Kim, Nara Cho and Jaejung Ko*
Received 8th December 2010, Accepted 18th March 2011
DOI: 10.1039/c0cc05448h
Annulated thiophene perylene bisimides and their triphenyl-
amine based oligomers have been synthesized. One of the
oligomers FPTTPA has been demonstrated to be an efficient
electron donor in bulk heterojunction (BHJ) organic solar cells,
giving a power conversion efficiency of 1.42%.
electron-rich or electron-deficient chromospheres to the
annulated thiophene unit.
In this communication, we report the synthesis of the
annulated thiophene perylene bisimide, its electron-rich
derivatives and its applications for organic solar cells.
The three oligomeric molecules FPTTPA, FP3HTTPA, and
PTPA have been efficiently synthesized by the stepwise
synthetic protocol illustrated in Scheme 1. The Suzuki coupling
reaction14 of 1,7-dibromoperylenebis(dicarboximide)s with
thiophene-3-ylboronic acid led to 7. Photocyclization in
the presence of I2 yielded the thiophene annulated PBI 5.
Bromination of 5 with Br2 gave 8. FPTTPA and FP3HTTPA
were prepared from 8 with compound 9 and compound 10 by
a Suzuki reaction, respectively. For comparison, the triphenyl
Perylene bisimides (PBIs) represent a class of thermally stable
organic semiconductors with high extinction coefficients, high
photostabilities and high quantum yields.1 They have shown
wide applications such as in field-effect transistors,2 electron
transfer cascades,3 photovoltaic devices,4 and dye lasers.5 PBIs
are also potential candidates as organic solar cells since they
have been found to possess excellent electron affinities and
mobilities.6 Recently, the extension of the aromatic core of
PBI has been intensively studied for n-type semi-conductors
and NIR dyes.7 Extension of the aromatic unit along the long
molecular axis such as in 2 and 3 induced a bathochromic shift
of about 100 nm per addition of naphthalene unit.8
Annulation of PBIs with a variety of cyclic units in the bay
region has been actively investigated.9 Such core-expanded
perylene dyes include annulated dibenzocoronenebis-
(dicarboximide) 410 and diverse heterocycles such as pyridyl-,11
sulfur-,12 and N-heterocycles13 substituted PBI. However, the
above annulated PBIs hamper further applications because
they do not have specific substitution sites for any structural
modifications. Therefore, we envisioned that if from the
thiophene annulations of PBI 1 compounds such as 5 could
be synthesized, they could be utilized for the synthesis of
electronic materials through the incorporation of numerous
Scheme
1
Reagents: (i) thiophene-3-ylboronic acid, Pd(PPh3)4,
Department of New Material Chemistry, Korea University, Jochiwon,
Chungnam 339-700, Korea. E-mail: jko@korea.ac.kr
w Electronic supplementary information (ESI) available. See DOI:
10.1039/c0cc05448h
K2CO3, THF; (ii) I2, hn, dichloromethane; (iii) Br2, CH2Cl2; (iv) for
R = H, compound 9; for R = hexyl, compound 10, Pd(PPh3)4,
K2CO3, THF; (v) compound 11, Pd(PPh3)4, K2CO3, THF.
c
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interest.15 Therefore, we adopted the triphenylamine-based
oligomers incorporating an annulated perylene unit as donor
materials for organic solar cells because the compounds have
broad absorptions and high extinction coefficients as well as
the proper energy difference in the LUMO of the donor and
acceptor used as the active layer.
Bulk heterojunction (BHJ) organic solar cells were
fabricated by spin-coating perylene : PCBM (weight ratio =
1 : 2, perylene = FPTTPA, FP3HTTPA, and PTPA) blends
in chlorobenzene onto cleaned indium tin oxide (ITO),
deposited by first spin-coating with PEDOT : PSS [poly(3,4-
ethylene-dioxythiophene) : polystyrenesulfonate] (layer thickness:
40 nm). The thickness of an active layer was 40 nm. After the
cells were completely dried, vacuum deposition of Al (B 100 nm)
as the cathode was conducted.
Fig. 1 Absorption spectra of FPTTPA (black line), FP3HTTPA
(red line), and PTPA (blue line) in CH2Cl2 (solid line) and thin film
(dotted line).
amine moiety was connected at 1, 7 bay positions of perylene
bisimide to give PTPA.
Fig. 2a shows the J–V curves of devices based on the three
compounds. A summary of photovoltaic properties is listed in
Table 1. The device with a 1 : 2 weight ratio of FPTTPA to
PCBM gave the best performance with a power conversion
efficiency (Z) of 1.42% which is significantly higher than the
other two devices based on FP3HTTPA and PTPA. The
devices based on FPTTPA and FP3HTTPA exhibited a higher
Voc of 0.78–0.96 V compared to the device based on PTPA,
which is in agreement with the higher HOMO level of both
compounds. The Jsc enhancement of the FPTTPA : PCBM
blend relative to the other two blends can be related to the
charge carrier mobility. In order to confirm our results, space
charge current limited measurements in dark were carried out
on the three blend layer. The hole and electron mobilities
Optical absorption spectra of FPTTPA, FP3HTTPA, and
PTPA in CH2Cl2 and in films are shown in Fig. 1. The
absorption spectrum of PTPA showed two broad maxima at
420 and 523 nm. The short wavelength absorption at 420 nm is
associated with the characteristic perylene transition, while the
long wavelength absorption at 523 nm corresponds to an
intramolecular charge transfer between the TPA-thiophene
donor part and the acceptor perylene group. Comparison of
the spectra of the three compounds shows that the absorption
maximum of FPTTPA and FP3HTTPA in the long wave-
length is bathochromically shifted by 50 and 40 nm compared
with that of PTPA, respectively, as a result of the more planar
conformation. The FPTTPA/C61-PCBM film has three broad
absorption bands, which span a wide range of the UV and
visible spectrum from 300 to 680 nm (see S1 in the ESIw). Such
a broad spectrum makes FPTTPA a potential compound for
organic solar cells.
of the FPTTPA : PCBM device were calculated to be mh
=
364 Â 10À6 and me = 6.42 Â 10À4, whereas those of
PTPA : PCBM were sharply decreased to mh = 1.28 Â 10À6
and me = 2.74 Â 10À6 (see S4 in the ESIw). The increased
mobility of charge carriers and balanced charge transport in
Electrochemical properties of the three compounds
FPTTPA, FP3HTTPA, and PTPA were investigated by
cyclovoltammetry. All three compounds exhibited two
reversible oxidation potentials. The oxidation potentials of
PTPA are lower by about 250 mV than those of FPTTPA and
FP3HTTPA, indicating a significant HOMO-raising effect.
FPTTPA showed a large positive shift in its redox potential,
rendering oxidation more difficult due to the electron delocal-
ization on
a relative planar configuration. Compounds
FPTTPA and PTPA exhibited two reversible reduction
potentials, whereas FP3HTTPA exhibited one irreversible
reduction potential at À1.38 V. The first reduction potential
of FPTTPA is 0.21 eV lower than that of FP3HTTPA,
indicating stronger electron accepting power, which makes it
a potential p-type material for organic solar cells (see S2 in the
ESIw).
The HOMO and LUMO levels of the three compounds were
calculated to be À4.98 to À5.27 and À3.33 to À3.55 eV,
respectively. In comparison with the HOMO level of
À6.1 eV and LUMO level of À3.7 eV of PCBM, the three
compounds are suitable for use as donor materials blended
with PCBM as the acceptor in organic solar cells.
Solution processable small molecule semiconductors as
alternatives to conjugated polymers in bulk heterojunction
(BHJ) organic solar cells are subject to a considerable current
Fig. 2 J–V curves (a) and IPCE (b) spectra of BHJ solar cells based
on the FPTPA : C61-PCBM (black line), FP3HTTPA : C61-PCBM
(red line), and PTPA : C61-PCBM (blue line).
c
5510 Chem. Commun., 2011, 47, 5509–5511
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Table 1 Photovoltaic performances of the BHJ oligomer solar cells
composed of perylenes/C61-PCBM (1 : 2) blends
and the New & Renewable Energy of the Korea Institute of
Energy Technology Evaluation and Planning (KETEP) grant
funded by the Korea government Ministry of Knowledge
Economy (No. 2010T100100674).
Components
Jsc/mA cmÀ2
Voc/V
FF (%)
Z (%)
FPTTPA
FP3HTTPA
PTPA
5.94
2.53
0.83
0.776
0.960
0.575
30.80
33.31
27.24
1.42
0.81
0.13
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Chem. Commun., 2011, 47, 5509–5511 5511