2696 Chem. Mater. 2010, 22, 2696–2698
DOI:10.1021/cm100417z
aromatic stabilization energy gained from forming the
resonance quinoidal structure of the thieno[3,4-b]-
thiophene units that significantly red-shifts the absorp-
tion of the polymers.4b-d These polymers possess a
relatively high energy level for the highest occupied
molecular orbital (HOMO) in the range of -5.2 and
-4.6 eV. This contributes to lower open circuit voltage
(Voc) that can still be optimized to further improve the
performance of the devices.
Efficient Polymer Solar Cells Based on the Copolymers
of Benzodithiophene and Thienopyrroledione
Yong Zhang, Steven K. Hau, Hin-Lap Yip, Ying Sun,
Orb Acton, and Alex K.-Y. Jen*
Department of Materials Science and Engineering,
University of Washington, Seattle, Washington 98195
Received February 9, 2010
Revised Manuscript Received March 30, 2010
To achieve this goal, we have introduced 5-octyl-
thieno[3,4-c]pyrrole-4,6-dione (TPD) as a new acceptor
in the BDT-based copolymers. Because TPD can also
gain some stabilization energy by forming a quinoidal
thiophene-maleimide structure in its excited state, it is
possible to afford a polymer with low energy band gap.
The new polymer PBDTTPD was synthesized by the
Stille coupling polymerization between 1,3-dibromo-5-
octylthieno[3,4-c] pyrrole-4,6-dione and di(2-ethylhexy-
loxy)benzodithiophene distannane monomers. Both the
conventional and inverted photovoltaic devices based
on the blends of PBDTTPD and PC71BM were investi-
gated. Under the AM 1.5G simulated solar illumination
(100 mW/cm2), maximum power conversion efficiencies
(PCE) of >4% could be achieved in both the conven-
tional and inverted devices. Both devices exhibit large
open-circuit voltage (Voc) of up to 0.87 V, short-circuit
current (Jsc) of up to 9.1 mA cm-2, and external quantum
efficiencies (EQE) exceeding 50% over the entire visible
range (with a maxima of 65% at 540 nm). It is worthy to
note that this polymer shows higher efficiency in the
inverted device structure compared to the conventional
device structure.
Scheme 1 shows the synthesis for monomers and poly-
mers. Starting from benzo[1,2-b:4,5-b0]dithiophene-4,8-
dione, the di(2-ethylhexyloxy) benzodithiophene distan-
nane (2) was synthesized in three steps. The 1,3-dibromo-
5-octylthieno[3,4-c]pyrrole-4,6-dione (5) was synthesized
from thiophene-3,4-dicarboxylic anhydride, which was
obtained from 3,4-dibromothiophene by adapting the
methods reported in the literature.5 Finally, the Stille
polymerization of 2 and 5 was carried out using Pd-
(PPh3)4 as catalyst in toluene/DMF to afford polymer
PBDTTPD as a dark-blue solid. PBDTTPD shows good
solubility in chlorinated solvents, such as chloroform,
chlorobenzene, dichlorobenzene, and so forth. The num-
ber average molecular weight (Mn) of this polymer is
measured to be 33 000 g/mol with a polydispersity (PDI)
of 2.3 by gel permeation chromatography (GPC) with
THF as the eluent. The results from differential scanning
calorimetry (DSC) measurements show no obvious ther-
mal transition between 20 and 300 °C.
Polymer solar cells (PSCs) have been recently vigo-
rously explored as a promising renewable energy source
due to their potential for low-cost, lightweight, and
large-area processabilty.1 The most successful device
structure for high performance PSCs is based on the
bulk-heterojunction (BHJ) concept which involves a thin
film blend of conjugated polymer as the electron donor
and a fullerene derivative as the electron acceptor.2
Among various types of polymers designed for PSCs,
the donor-acceptor conjugated polymers based on the
alternating electron-rich and electron-deficient units
along the polymer backbone have been shown to be able
to achieve high power convension efficiencies (PCEs).2
The band gap and energy level of these conjugated poly-
mers can be easily tuned by selecting different electron-
rich and electron-deficient units. As a result, PCEs up to
7% have been demonstrated in these conjugated polymers
through systematic optimizations of their absorption and
open circuit voltage and relevant device parameters.3
Recently, benzodithiophene (BDT)-containing conju-
gated polymers have attracted great attention for their
functions as electron donor for PSCs.4 For instance,
Yang et al. have synthesized the BDT-based copolymers
with thiophene, benzothiadiazole, thieno[3,4-b]thiophene,
and thieno[3,4-b]pyrazine units.4 Among these polymers,
poly(DBT-thieno[3,4-b]thiophene) showed the most pro-
mising photovoltaic performance (>7%) due to the
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r
2010 American Chemical Society
pubs.acs.org/cm
Published on Web 04/15/2010