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

摘要

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 (E-CT) are applied to molecular photovoltaic material systems to characterize energy landscapes. The results show that IEs and E-CT values are highly dependent on structural order and phase composition. In the sexithiophene:C-60 system both the IEs of sexithiophene and C-60 shift by over 0.4 eV while E-CT shifts by 0.5 eV depending on molecular composition. By contrast, in the rubrene:C-60 system the IE of rubrene and C-60 vary by 0.11 eV and E-CT 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.