Direct visualization of the charge transfer state dynamics in dilute-donor organic photovoltaic blends

Gareth John Moore, Florian Günther, Kaila M. Yallum, Martina Causa’, Anna Jungbluth, Julien Réhault, Moritz Riede, Frank Ortmann () and Natalie Banerji ()
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Gareth John Moore: University of Bern
Florian Günther: Universidade de São Paulo (USP)
Kaila M. Yallum: University of Bern
Martina Causa’: University of Bern
Anna Jungbluth: University of Oxford
Julien Réhault: University of Bern
Moritz Riede: University of Oxford
Frank Ortmann: Technische Universität
Natalie Banerji: University of Bern

Nature Communications, 2024, vol. 15, issue 1, 1-11

Abstract: Abstract The interconversion dynamics between charge transfer state charges (CTCs) and separated charges (SCs) is still an unresolved issue in the field of organic photovoltaics. Here, a transient absorption spectroscopy (TAS) study of a thermally evaporated small-molecule:fullerene system (α6T:C60) in different morphologies (dilute intermixed and phase separated) is presented. Spectral decomposition reveals two charge species with distinct absorption characteristics and different dynamics. Using time-dependent density functional theory, these species are identified as CTCs and SCs, where the spectral differences arise from broken symmetry in the charge transfer state that turns forbidden transitions into allowed ones. Based on this assignment, a kinetic model is formulated allowing the characterization of the charge generation, separation, and recombination mechanisms. We find that SCs are either formed directly from excitons within a few picoseconds or more slowly (~30–80 ps) from reversible splitting of CTCs. These findings constitute the first unambiguous observation of spectrally resolved CTCs and SCs.

Date: 2024
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DOI: 10.1038/s41467-024-53694-4

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