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Lookup NU author(s): Professor Thomas Penfold
This work is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License (CC BY-NC 4.0).
Efficient charge photogeneration in conjugated polymers typically requires the presence of a second component to act as electron acceptor. Here, we report a novel low band-gap conjugated polymer with a donor / orthogonal acceptor motif: poly-2,6-(4,4-dihexadecyl-4H-cyclopenta [2,1-b:3,4-b']dithiophene) -alt-2,6-spiro [cyclopenta[2,1-b:3,4-b']dithiophene-4,9'- fluorene]-2',7'-dicarbonitrile, referred to as PCPDT-sFCN. The role of the orthogonal acceptor is to spatially isolate the LUMO from the HOMO, allowing for negligible exchange energy between electrons in these orbitals and minimising the energy gap between singlet and triplet charge transfer states. We employ ultrafast and microsecond transient absorption spectroscopy to demonstrate that, even in the absence of a separate electron acceptor, PCPDT-sFCN shows efficient charge photogeneration in both pristine solution and film. This efficient charge generation is a result of an isoenergetic singlet/triplet charge transfer state equilibrium acting as a reservoir for charge carrier formation. Furthermore, clear evidence of enhanced triplet populations, which form in less than 1 ps, is observed. Using group theory, we show that this ultrafast triplet formation is due to highly efficient, quantum mechanically allowed intersystem crossing between the bright, initially photoexcited local singlet state and the triplet charge transfer state. Remarkably, the free charges that form via the charge transfer state are extraordinarily long-lived with millisecond lifetimes, due to the stabilisation imparted by the spatial separation of PCPDT-sFCN’s donor and orthogonal acceptor motifs. The efficient generation of long-lived charge carriers in a pristine polymer paves the way for single-material applications such as organic photovoltaics and photodetectors.
Author(s): Shaikh J, Congrave D, Forster A, Minotto A, Cacialli F, Hele T, Penfold T, Bronstein H, Clarke T
Publication type: Article
Publication status: Published
Journal: Chemical Science
Print publication date: 10/05/2021
Online publication date: 10/05/2021
Acceptance date: 08/05/2021
Date deposited: 25/06/2021
ISSN (print): 2041-6520
ISSN (electronic): 2041-6539
Publisher: Royal Society of Chemistry
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