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Alkyl Branching Position in Diketopyrrolopyrrole Polymers

  • Diketopyrrolopyrrole (DPP)-based donor acceptor copolymers have gained a significant amount of research interest in the organic electronics community because of their high charge carrier mobilities in organic field-effect transistors (OFETs) and their ability to harvest near-infrared (NIR) photons in solar cells. In this study, we have synthesized four DPP based donor-acceptor copolymers with variations in the donor unit and the branching point of the solubilizing alkyl chains (at the second or sixth carbon position). Grazing incidence wide-angle X-ray scattering (GIWAXS) results suggest that moving the branching point further away from the polymer backbone increases the tendency for aggregation and yields polymer phases with a higher degree of crystallinity (DoC). The polymers were blended with PC70BM and used as active layers in solar cells. A careful analysis of the energetics of the neat polymer and blend films reveals that the charge-transfer state energy (E-CT) of the blend films lies exceptionally close to the singlet energy ofDiketopyrrolopyrrole (DPP)-based donor acceptor copolymers have gained a significant amount of research interest in the organic electronics community because of their high charge carrier mobilities in organic field-effect transistors (OFETs) and their ability to harvest near-infrared (NIR) photons in solar cells. In this study, we have synthesized four DPP based donor-acceptor copolymers with variations in the donor unit and the branching point of the solubilizing alkyl chains (at the second or sixth carbon position). Grazing incidence wide-angle X-ray scattering (GIWAXS) results suggest that moving the branching point further away from the polymer backbone increases the tendency for aggregation and yields polymer phases with a higher degree of crystallinity (DoC). The polymers were blended with PC70BM and used as active layers in solar cells. A careful analysis of the energetics of the neat polymer and blend films reveals that the charge-transfer state energy (E-CT) of the blend films lies exceptionally close to the singlet energy of the donor (E-D*), indicating near zero electron transfer losses. The difference between the optical gap and open-circuit voltage (V-OC) is therefore determined to be due to rather high nonradiative 418 +/- 13 mV) and unavoidable radiative voltage losses (approximate to 255 +/- 8 mV). Even though the four materials have similar optical gaps, the short-circuit current density (J(SC)) covers a vast span from 7 to 18 mA cm(-2) for the best performing system. Using photoluminescence (PL) quenching and transient charge extraction techniques, we quantify geminate and nongeminate losses and find that fewer excitons reach the donor-acceptor interface in polymers with further away branching points due to larger aggregate sizes. In these material systems, the photogeneration is therefore mainly limited by exciton harvesting efficiency.show moreshow less

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Author details:Rishi ShivhareORCiD, Tim Erdmann, Ulrich Hoermann, Elisa Collado-FregosoORCiD, Stefan Zeiske, Johannes Benduhn, Sascha UllbrichORCiD, Rene Huebner, Mike HambschORCiD, Anton KiriyORCiD, Brigitte VoitORCiD, Dieter NeherORCiDGND, Koen VandewalORCiD, Stefan C. B. MannsfeldORCiD
DOI:https://doi.org/10.1021/acs.chemmater.8b02739
ISSN:0897-4756
ISSN:1520-5002
Title of parent work (English):Chemistry of materials : a publication of the American Chemical Society
Subtitle (English):Interplay between Fibrillar Morphology and Crystallinity and Their Effect on Photogeneration and Recombination in Bulk-Heterojunction Solar Cells
Publisher:American Chemical Society
Place of publishing:Washington
Publication type:Article
Language:English
Date of first publication:2018/09/28
Publication year:2018
Release date:2021/09/01
Volume:30
Issue:19
Number of pages:9
First page:6801
Last Page:6809
Funding institution:German Excellence Initiative via the Cluster of Excellence [EXC 1056]; German Alexander von Humboldt foundationAlexander von Humboldt Foundation; German Federal Ministry for Education and Research (BMBF) through the InnoProfile Projekt "Organische p-i-n Bauelemente 2.2" [03IPT602X]; graduate academy of the TU Dresden - excellence initiative of the German federal government; BMBF (UNVEIL) [FKZ 13N13719]; DFGGerman Research Foundation (DFG) [SFB 951]; graduate academy of the TU Dresden - excellence initiative of the German state government
Organizational units:Mathematisch-Naturwissenschaftliche Fakultät / Institut für Physik und Astronomie
DDC classification:5 Naturwissenschaften und Mathematik / 53 Physik / 530 Physik
Peer review:Referiert
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