title: Homocoupling defects in diketopyrrolopyrrole-based copolymers and their effect on photovoltaic performance.
abstract: We study the occurrence and effect of intrachain homocoupling defects in alternating push-pull semiconducting PDPPTPT polymers based on dithienyl-diketopyrrolopyrrole (TDPPT) and phenylene (P) synthesized via a palladium-catalyzed cross-coupling polymerization. Homocoupled TDPPT-TDPPT segments are readily identified by the presence of a low-energy shoulder in the UV/vis/NIR absorption spectrum. Remarkably, the signatures of these defects are found in many diketopyrrolopyrrole (DPP)-based copolymers reported in the literature. The defects cause a reduction of the band gap, a higher highest occupied molecular orbital (HOMO) level, a lower lowest unoccupied molecular orbital (LUMO) level, and a localization of these molecular orbitals. By synthesizing copolymers with a predefined defect concentration, we demonstrate that their presence reduces the short-circuit current and open-circuit voltage of solar cells based on blends of PDPPTPT with [70]PCBM. In virtually defect-free PDPPTPT, the power conversion efficiency is as high as 7.5%, compared to 4.5-5.6% for polymers containing 20% to 5% defects.
authors.affiliation: Molecular Materials and Nanosystems, Institute for Complex Molecular Systems, Eindhoven University of Technology , P.O. Box 513, 5600 MB Eindhoven, The Netherlands.
authors.forename: Koen H
authors.initials: KH
authors.lastname: Hendriks
authors.forename: Weiwei
authors.initials: W
authors.lastname: Li
authors.forename: Gaël H L
authors.initials: GH
authors.lastname: Heintges
authors.forename: Gijs W P
authors.initials: GW
authors.lastname: van Pruissen
authors.forename: Martijn M
authors.initials: MM
authors.lastname: Wienk
authors.forename: René A J
authors.initials: RA
authors.lastname: Janssen