title: Systematic Investigation of Benzodithiophene-Benzothiadiazole Isomers for Organic Photovoltaics.
abstract: Two new donor-acceptor small molecules based on benzo[1,2-b:4,5-b']dithiophene (BDT) and benzo[c][1,2,5]thiadiazole (BT) were designed and synthesized. Small molecules 4,4'-[(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene-2,6-diyl)bis(2,2'-bithiophene)-5,5'-diyl]bis(benzo[c][1,2,5]thiadiazole) (BDT-TT-BT) and 4,4'-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene-2,6-diyl)bis[7-(2,2'-bithiophene-5-yl)benzo[c][1,2,5]thiadiazole] (BDT-BT-TT) are structural isomers with the 2,2-bithiophene unit placed either between the BDT and BT units or at the end of the BT units. This work is targeted toward finding the effect of structural variation on optoelectronic properties, morphology, and photovoltaic performance. On the basis of theoretical calculations, the molecular geometry and energy levels are different for these two molecules when the position of the 2,2-bithiophene unit is changed. Optical and electrochemical properties of these two small molecules were characterized using UV-vis and cyclic voltammetry. The results showed that BDT-BT-TT has broader absorption and an elevated HOMO energy level when compared with those of BDT-TT-BT. The performance of these two isomers in solar cell devices was tested by blending with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM). Power conversion efficiencies as high as 3.22 and 3.71% were obtained in conventional solar cell structures for BDT-TT-BT and BDT-BT-TT, respectively. The morphology was studied using grazing incident wide-angle X-ray scattering and transmission electron microscopy, which revealed different phase separations of these two molecules when blended with PC71BM.
authors.affiliation: Department of Chemistry and Biochemistry, University of Texas at Dallas , Richardson, Texas 75080, United States.
authors.forename: Jia
authors.initials: J
authors.lastname: Du
authors.affiliation: Department of Chemistry, Carnegie Mellon University , Pittsburgh, Pennsylvania 15213, United States.
authors.forename: Andria
authors.initials: A
authors.lastname: Fortney
authors.affiliation: Department of Chemistry and Biochemistry, University of Texas at Dallas , Richardson, Texas 75080, United States.
authors.forename: Katherine E
authors.initials: KE
authors.lastname: Washington
authors.affiliation: Department of Chemistry and Biochemistry, University of Texas at Dallas , Richardson, Texas 75080, United States.
authors.forename: Chandima
authors.initials: C
authors.lastname: Bulumulla
authors.affiliation: Department of Chemistry and Biochemistry, University of Texas at Dallas , Richardson, Texas 75080, United States.
authors.forename: Peishen
authors.initials: P
authors.lastname: Huang
authors.affiliation: Department of Chemistry and Biochemistry, University of Texas at Dallas , Richardson, Texas 75080, United States.
authors.forename: Dushanthi
authors.initials: D
authors.lastname: Dissanayake
authors.affiliation: Department of Chemistry and Biochemistry, University of Texas at Dallas , Richardson, Texas 75080, United States.
authors.forename: Michael C
authors.initials: MC
authors.lastname: Biewer
authors.affiliation: Department of Chemistry, Carnegie Mellon University , Pittsburgh, Pennsylvania 15213, United States.
authors.forename: Tomasz
authors.initials: T
authors.lastname: Kowalewski
authors.affiliation: Department of Chemistry and Biochemistry, University of Texas at Dallas , Richardson, Texas 75080, United States.
authors.forename: Mihaela C
authors.initials: MC
authors.lastname: Stefan
keywords: benzodithiophene; benzothiadiazole; donor−acceptor molecules; morphology; organic solar cells