How accurate are energies of HOMO and LUMO levels in small-molecule organic semiconductors determined from cyclic voltammetry or optical spectroscopy?

Abstract

Examined are relations between the energies of the transport gaps (i.e., the gaps determined by UPS/IPES measurements and taken for ‘true’ HOMO-LUMO gaps), and the electrochemical gaps (determined from CV experiments) and optical gaps (determined from UV-VIS optical spectroscopy). The statistics, based on literature data for several small (non-polymeric) molecules, shows that, on average, the electrochemical measurements underestimate the gap energies by ca. 16%, and the optical gap is smaller than the transport gap by ca. 37%. These differences seriously affect the accuracy of determination of energies of HOMO and LUMO levels by indirect methods. The concordance with the results obtained from the UPS/IPES measurements can be improved by using correction factors put forward in the present paper.