Electron affinities of small-molecule organic semiconductors: Comparison among cyclic voltammetry, conventional inverse photoelectron spectroscopy, and low-energy inverse photoelectron spectroscopy

The electron affinity (A) of an organic semiconductor is an energy parameter representing the electron transport level. The precise value of A in a solid is indispensable for examining and designing organic semiconductor devices. In principle, A in a solid can be determined by inverse photoelectron spectroscopy (IPES). However, because IPES is normally available only for specialists of IPES, A is often estimated from the reduction potential Ered measured in solution using a more easily available method, cyclic voltammetry (CV). Thus, the conversion relation from Ered into A is practically important in the research of organic semiconductors. On the other hand, previous IPES data may have errors of about 0.3–0.5 eV owing to sample damage and low energy resolution. In 2012, one of the authors developed low-energy inverse photoelectron spectroscopy (LEIPS), which enables the determination of A with the precision of 0.1 eV. In this work, we establish the correlation between CV data Ered and solid-state A based on LEIPS data and suggest the relation A = (1.24 ± 0.07) × e Ered + (5.06 ± 0.15) eV. We also discuss the reliability of the A values determined by the previous IPES and the present LEIPS in terms of the LUMO spectral onset and the vacuum level.