Short and Long-Range Electron Transfer Compete to Determine Free-Charge Yield in Organic Semiconductors

Joshua Carr, Taylor Allen, Bryon Larson, Iryna Davydenko, Raghunath Dasari, Stephen Barlow, Seth Marder, Obadiah Reid, Garry Rumbles

Research output: Contribution to journalArticlepeer-review

4 Scopus Citations

Abstract

Understanding how Frenkel excitons efficiently split to form free-charges in low-dielectric constant organic semiconductors has proven challenging, with many different models proposed in recent years to explain this phenomenon. Here, we present evidence that a simple model invoking a modest amount of charge delocalization, a sum over the available microstates, and the Marcus rate constant for electron transfer can explain many seemingly contradictory phenomena reported in the literature. We use an electron-accepting fullerene host matrix dilutely sensitized with a series of electron donor molecules to test this hypothesis. The donor series enables us to tune the driving force for photoinduced electron transfer over a range of 0.7 eV, mapping out normal, optimal, and inverted regimes for free-charge generation efficiency, as measured by time-resolved microwave conductivity. However, the photoluminescence of the donor is rapidly quenched as the driving force increases, with no evidence for inverted behavior, nor the linear relationship between photoluminescence quenching and charge-generation efficiency one would expect in the absence of additional competing loss pathways. This behavior is self-consistently explained by competitive formation of bound charge-transfer states and long-range or delocalized free-charge states, where both rate constants are described by the Marcus rate equation. Moreover, the model predicts a suppression of the inverted regime for high-concentration blends and efficient ultrafast free-charge generation, providing a mechanistic explanation for why Marcus-inverted-behavior is rarely observed in device studies.

Original languageAmerican English
Pages (from-to)312-324
Number of pages13
JournalMaterials Horizons
Volume9
Issue number1
DOIs
StatePublished - 2022

Bibliographical note

Publisher Copyright:
© 2021 The Royal Society of Chemistry.

NREL Publication Number

  • NREL/JA-5900-80749

Keywords

  • Marcus theory
  • organic photovoltaics
  • organic semiconductors
  • photoinduced electron transfer
  • solar-photochemistry
  • time-resolved microwave conductivity

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