Title: Side-Chain-Bulk Effects on the Molecular Packing and Photovoltaic Performance of Benzotrithiophene-Benzooxadiazole Conjugated Copolymers
Authors: Lan, Shang-Che
Chang, Chiao-Kai
Wang, Yi-Chien
Wei, Kung-Hwa
Department of Materials Science and Engineering
Keywords: bulk heterojunctions;donor-acceptor systems;conjugated polymers;solar cells;side-chain engineering
Issue Date: 27-Apr-2015
Abstract: Two alternating donor-acceptor conjugated polymers, PBTTBO-C13C11 and PBTTBO-C13C8, comprising 5-alkylbenzo[1,2-b:3,4-b:5,6-d]trithiophene (BTT) as the donor and 4,7-bis(4-dodecylthien-2-yl)benzo[1,2,5]oxadiazole (BO) as the acceptor, with different alkyl side-chain architectures on their BTT units are synthesized, and their bulk heterojunction photovoltaic properties when blended with the fullerene PC71BM are characterized. Even a slight change in the length of the alkyl chain of the BTT units influences the steric bulk to such a degree that it substantially affects the molecular packing of the polymers and the performance of their photovoltaic devices. The bulkier side chains of the polymer PBTTBO-C13C11 not only prevent its crystallization, but also suppress its light-absorption coefficient relative to that of PBTTBO-C13C8, as evidenced by X-ray diffraction and UV/Vis absorption studies, presumably because of weakened intermolecular interactions. Moreover, the polymer bearing bulkier side chains, PBTTBO-C13C11, is less miscible with PC71BM than PBTTBO-C13C8, and this characteristic determines the morphology of their annealed blended films, as shown by TEM studies. The best efficiency is obtained with a device containing an annealed PBTTBO-C13C8/PC71BM (1/2 w/w) active layer that was maintained at 120 degrees C for 10 min, which shows a power conversion efficiency of 6.2%, an open-circuit voltage of 0.75 V, a short-circuit current density of 12.6 mAcm(-2), and a fill factor of 66%.
URI: http://dx.doi.org/10.1002/cphc.201402735
ISSN: 1439-4235
DOI: 10.1002/cphc.201402735
Volume: 16
Begin Page: 1268
End Page: 1274
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