Evidence on Enhanced Exciton Polarizability in Donor/Acceptor Bulk Heterojunction Organic Photovoltaics

Using electroabsorption spectroscopy, we explore the polarizability of Frenkel excitons in both pristine donor and D/A blend films. We observe for the first time that the polarizability of excitonic states in pristine donors can be dramatically increased by adding an n-type acceptor. By investigating the dielectric effect in different organic semiconductor systems, we find that the polarizability of Frenkel excitons has direct correlation with the measured dielectric constant of the bulk heterojunction thin films. Our results disclose the difference in the nature of Frenkel excitons in pristine donor and D/A blend systems, revealing an important role of excitonic states in charge separation process of organic photovoltaic devices.