title: Side-chain-bulk effects on the molecular packing and photovoltaic performance of benzotrithiophene-benzooxadiazole conjugated copolymers.
abstract: Two alternating donor-acceptor conjugated polymers, PBTTBO-C13 C11 and PBTTBO-C13 C8 , 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 PC71 BM 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-C13 C11 not only prevent its crystallization, but also suppress its light-absorption coefficient relative to that of PBTTBO-C13 C8 , 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-C13 C11 , is less miscible with PC71 BM than PBTTBO-C13 C8 , 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-C13 C8 /PC71 BM (1/2 w/w) active layer that was maintained at 120 °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 mA cm(-2) , and a fill factor of 66 %.
authors.affiliation: Department of Materials Science and Engineering, National Chiao Tung University, 300 Hsinchu (Taiwan).
authors.forename: Shang-Che
authors.initials: SC
authors.lastname: Lan
authors.forename: Chiao-Kai
authors.initials: CK
authors.lastname: Chang
authors.forename: Yi-Chien
authors.initials: YC
authors.lastname: Wang
authors.forename: Kung-Hwa
authors.initials: KH
authors.lastname: Wei
keywords: bulk heterojunctions; conjugated polymers; donor-acceptor systems; side-chain engineering; solar cells