title: A roundabout approach to control morphological orientation and solar-cell performance by modulating side-chain branching position in benzodithiophene-based polymers.
abstract: To be meaningful to guide the rational design of novel high-performance conjugated semiconductors, we prepared three benzo[1,2-b:4,5-b']dithiophene (BDT)-based polymers by systematically moving the branching point of the alkyl chain. The effect of side-chain engineering was thoroughly investigated by a range of techniques. We demonstrate that a subtle change in the branching position in the BDT core can have a critical impact on polymer packing and preferential backbone orientation in thin films; copolymers made from BDT and thieno[3,4-c]pyrrole-4,6-dione units (TPD) adopt more of a face-on orientation as the branching point is shifted closer to the backbone, which can be correlated with a dramatic difference in solar-cells performance. The high short-circuit current density (11.6 mA cm(-2) ) for the copolymer with one carbon atom between the alkoxylated oxygen atom and the branching point results from its predominantly face-on orientation and smoother surface in thin films, which results in power conversion efficiencies as high as 4.56 %.
authors.affiliation: School of Energy and Chemical Engineering, Low Dimensional Carbon Materials Center, Ulsan National Institute of Science and Technology (UNIST), Ulsan 689-798 (South Korea).
authors.forename: Kyu Cheol
authors.initials: KC
authors.lastname: Lee
authors.forename: Seyeong
authors.initials: S
authors.lastname: Song
authors.forename: Junghoon
authors.initials: J
authors.lastname: Lee
authors.forename: Dong Suk
authors.initials: DS
authors.lastname: Kim
authors.forename: Jin Young
authors.initials: JY
authors.lastname: Kim
authors.forename: Changduk
authors.initials: C
authors.lastname: Yang
keywords: branching position; conducting materials; organic solar cells; polymers; structure-activity relationships