Chem. Mater. 2010, 22, 5617–5624 5617
DOI:10.1021/cm1019537
Influence of the Annealing Temperature on the Photovoltaic Performance
and Film Morphology Applying Novel Thermocleavable Materials
Martin Helgesen,*,† Morten Bjerring,‡ Niels Chr. Nielsen,‡ and Frederik C. Krebs†
†Risø National Laboratory for Sustainable Energy, Technical University of Denmark,
Frederiksborgvej 399, DK-4000 Roskilde, Denmark, and ‡Center for Insoluble Protein Structures (inSPIN),
Interdisciplinary Nanoscience Center (iNANO) and Department of Chemistry, Aarhus University,
DK-8000 Aarhus C, Denmark
Received July 14, 2010. Revised Manuscript Received August 25, 2010
Di-2-thienyl-2,1,3-benzothiadiazole (DTBT) bearing thermally cleavable ester groups in different
positions were prepared and copolymerized with alkylsubstituted cyclopentadithiophene (CPDT). The
polymers were found to have band gaps in the range of 1.66-2.03 eV and were explored in polymer
photovoltaic devices as mixtures with soluble methanofullerenes. The positioning of the ester groups
proved to be very significant despite the identical conjugated backbone of 2-methyl-2-hexyl 5-(4,4-
bis(2-ethylhexyl)-4H-cyclopenta[1,2-b:5,4-b0]dithiophen-2-yl)-2-(7-(3-(((2-methylhexan-2-yl)oxy)-car-
bonyl)thiophen-2-yl)benzo[c][1,2,5]thiadiazol-4-yl)thiophene-3-carboxylate (T1) and 2-methyl-2-hexyl
2-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[1,2-b:5,4-b0]dithiophen-2-yl)-5-(7-(4-(((2-methylhexan-2-yl)oxy)-
carbonyl)thiophen-2-yl)benzo[c][1,2,5]thiadiazol-4-yl)thiophene-3-carboxylate (T2). Power conversion
efficiencies of up to 1.92% were observed for polymers bearing ester groups on the 4-positions of the
thienyl groups (T2), but shifting them to the 3-positions (T1) reduced the efficiency significantly to 0.18%.
The thermal behavior of the polymers was studied with thermogravimetric analysis (TGA) that showed a
weight loss around 200 °C corresponding to elimination of the ester side chains followed by a second
weight loss around 300 °C corresponding to loss of CO2 via decarboxylation. The temperature of
thermocleavage of the active layer films was optimized to 265 °C whereby the T2:PCBM solar cells
maintained a significant performance giving efficiencies up to 1.49%.
Introduction
material which offers significant advantages, both in terms of
conjugated polymer synthesis,7-11 device processing,3,12,13
and operational stability.14-17 To overcome the fact that
the softness/photochemistry provided by solubilizing chains
has been linked to the instability of polymer solar cells,18-25
the thermocleavable materials exploit a thermally labile
bond in the molecule that functions as the linker between
From a materials point of view the state-of-the-art in the
field of organic photovoltaics is currently represented by
bulk heterojunction solar cells based on a polymer/copolymer
and a methanofullerene ([60]PCBM and [70]PCBM) where
power conversion efficiencies are approaching encourag-
ing 8% for small area devices.1 The steady increase in
performance during the past 10 years reveals a great poten-
tial for polymer solar cells as a low-cost renewable energy
source,2-6 but aside from the efficiency, processing and
stability are two other important aspects that have to be
addressed with equal intensity to fully realize that potential.
To combine all three parameters into a useful material and
device further research in device science and new materials is
needed. A recent approach is to utilize a thermocleavable
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*To whom correspondence should be addressed. E-mail: manp@risoe.
dtu.dk.
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r
2010 American Chemical Society
Published on Web 09/08/2010
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