The Roles of Structural Order and Intermolecular Interactions in Determining Ionization Energies and Charge-Transfer State Energies in Organic Semiconductors

Kenneth R. Graham, Guy O.Ngongang Ndjawa, Sarah M. Conron, Rahim Munir, Koen Vandewal, John J. Chen, Sean Sweetnam, Mark E. Thompson, Alberto Salleo, Michael D. McGehee, Aram Amassian

Research output: Contribution to journalArticlepeer-review

43 Scopus citations

Abstract

The energy landscape in organic semiconducting materials greatly influences charge and exciton behavior, which are both critical to the operation of organic electronic devices. These energy landscapes can change dramatically depending on the phases of material present, including pure phases of one molecule or polymer and mixed phases exhibiting different degrees of order and composition. In this work, ultraviolet photoelectron spectroscopy measurements of ionization energies (IEs) and external quantum efficiency measurements of charge-transfer (CT) state energies (ECT) are applied to molecular photovoltaic material systems to characterize energy landscapes. The results show that IEs and ECT values are highly dependent on structural order and phase composition. In the sexithiophene:C60 system both the IEs of sexithiophene and C60 shift by over 0.4 eV while ECT shifts by 0.5 eV depending on molecular composition. By contrast, in the rubrene:C60 system the IE of rubrene and C60 vary by ≤0.11 eV and ECT varies by ≤0.04 eV as the material composition varies. These results suggest that energy landscapes can exist whereby the binding energies of the CT states are overcome by energy offsets between charges in CT states in mixed regions and free charges in pure phases.

Original languageEnglish
Article number1601211
JournalAdvanced Energy Materials
Volume6
Issue number22
DOIs
StatePublished - Nov 23 2016

Bibliographical note

Publisher Copyright:
© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Keywords

  • charge-transfer states
  • ionization energies
  • organic electronics
  • organic photovoltaics
  • ultraviolet photoelectron spectroscopy

ASJC Scopus subject areas

  • Renewable Energy, Sustainability and the Environment
  • Materials Science (all)

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