Probing transport energies and defect states in organic semiconductors using energy resolved electrochemical impedance spectroscopy

Abstract

Determining the relative energies of transport states in organic semiconductors is critical to understanding the properties of electronic devices and in designing device stacks. Futhermore, defect states are also highly important and can greatly impact material properties and device performance. Recently, energy-resolved electrochemical impedance spectroscopy (ER-EIS) is developed to probe both the ionization energy (IE) and electron affinity (EA) as well as sub-bandgap defect states in organic semiconductors. Herein, ER-EIS is compared to cyclic voltammetry (CV) and photoemission spectroscopies for extracting IE and EA values, and to photothermal deflection spectroscopy (PDS) for probing defect states in both polymer and molecular organic semiconductors. The results show that ER-EIS determined IE and EA are in better agreement with photoemission spectroscopy measurements as compared to CV for both polymer and molecular materials. Furthermore, the defect states detected by ER-EIS agree with sub-bandgap features detected by PDS. Surprisingly, ER-EIS measurements of regiorandom and regioregular poly(3-hexylthiophene) (P3HT) show clear defect bands that occur at significantly different energies. In regioregular P3HT the defect band is near the edge of the occupied states while it is near the edge of the unoccupied states in regiorandom P3HT.