CPOS Seminar: "Understanding the Optoelectronic properties of Organic Semiconductor: Exciton Binding Energy and Reorganization Energies"
Speaker: Somayeh Kashani
Postdoctoral Research Fellow, Department of Electrical and Computer Engineering, North Carolina State University (P. Gkoupidenis Lab, Organic Neuromorphic Bioelectronics)
Ph.D., Department of Physics, North Carolina State University, Raleigh, USA (H. Ade Lab, Organic Optoelectronics and Organic Photovoltaics
Current organic solar cells (OSCs) employ bulk-heterojunction active layers composed of polymer donors and non-fullerene small molecule acceptors (NF-SMAs). Key optoelectronic properties, such as exciton binding energy (Eb) and reorganization energy, are essential for minimizing voltage losses and enhancing device performance. While reorganization energies are often measured using sophisticated photocurrent techniques or simulations, we investigate correlations between NF-SMA absorption spectra, molecular structure, and reorganization energy through multi-parameter Franck-Condon analysis.
Additionally, measuring Eb in OSCs is challenging, with reported values varying widely. Using two methods—UPS-IPES and solid-state cyclic voltammetry (CV)—we evaluated Eb for variants of the PBnDT-FTAZ polymer. Our results show that while CV-derived Eb values differ based on molecular structure, UPS-IPES yields lower values with no clear structural correlation. We discuss this discrepancy in terms of each method's limitations, as well as the influence of environmental factors, particularly high-dielectric surroundings that can impact Eb. These findings suggest that OSCs with single-material layers could achieve higher efficiencies if high-dielectric materials are developed.
