The Roles of Structural Order and Intermolecular Interactions in Determining Ionization Energies and Charge‐Transfer State Energies in Organic Semiconductors. Issue 22 (17th August 2016)

• Record Type: Journal Article
• Authors: Graham, Kenneth R.; Ndjawa, Guy O. Ngongang; Conron, Sarah M.; Munir, Rahim; Vandewal, Koen; Chen, John J.; Sweetnam, Sean; Thompson, Mark E.; Salleo, Alberto; McGehee, Michael D.; Amassian, Aram
• Is Part Of: Advanced energy materials. Volume 6:Issue 22(2016)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2016-08-17
• Subjects: charge‐transfer states -- ionization energies -- organic electronics -- organic photovoltaics -- ultraviolet photoelectron spectroscopy; Energy harvesting -- Materials -- Periodicals; Energy conversion -- Materials -- Periodicals; Energy storage -- Materials -- Periodicals; Photovoltaics -- Periodicals; Fuel cells -- Periodicals; Thermoelectric materials -- Periodicals; 621.31
• DOI: 10.1002/aenm.201601211
• Languages: English
• ISSNs: 1614-6832

Abstract

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 ( 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:C60 system both the IEs of sexithiophene and C60 shift by over 0.4 eV while E CT 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 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. Abstract : Ionization energies, as measured by UV photoelectron spectroscopy, and charge‐transfer state energies in blends of π‐conjugated organic molecules vary by up to 0.6 eV depending on blend composition. These energy variations are highly system dependent and have significant implications on the performance of organic electronics, such as determining the probability of charge separation and the open‐circuit voltage in organic photovoltaics.