Excitonic Effect in Organic Semiconductors Studied by Electromodulation Spectroscopy
Student thesis: Doctoral Thesis
Related Research Unit(s)
Electromodulation (EM) is a powerful characterization method used to determine material’s properties such as polarizability change and dipole moment change, which are key factors in determining exciton properties. These parameters rely on a specific fitting procedure which produces a large variation in fitting value. Specifically, the following work is included in this thesis: (1) a systematic EM study is conducted of reflection and transmission configurations, and the results are compared with three different fitting methods. Our analysis reveals that reliable fitting methods should be based on optical simulations; this has been overlooked in previous literature. (2) The benchmark method for the above study has been applied to probe different materials’ excitonic properties. We found that the first excitonic energy level of high dielectric pristine materials demonstrates a degenerate character with a continuum state. (3) For blended materials, EM’s subgap signal emerges and a systematic research study is carried out to explore DC bias response of those subgap features. It was found that those subgap signals show charge modulation features which are evidence of charge accumulation at the organic heterojunction and the subgap signal can be tuned from a bleaching character to an absorption one, pending injected electron-hole balance.