Structure-Dependent Electron Affinities of Perylene Diimide-Based Acceptors

Perylene dyes are a representative framework of electron transport (n-type) organic semiconductors. The energy of their electron transport level is the electron affinity (EA), which is an important parameter in selecting the electron transport materials for device application and of the material's electron-accepting ability. Recent studies show that EA may vary by as much as 1 eV depending on the molecular orientation owing to the electrostatic potential generated by the quadrupole moments. Because perylene dyes have a large quadrupole moment, it is essential to discuss EA combined with the molecular orientation in the sample film. In this work, we determine the EAs of perylene diimide derivatives in the solid phase using low-energy inverse photoelectron spectroscopy (LEIPS), which was developed by one of the authors. By changing the substrate and the alkyl-chain length, we systematically investigate the relationship between EA and molecular arrangement in the film to derive the general trend of the EA of perylene dyes. The results show that most of the perylene dyes' EAs are in the range from 3.7 to 4.0 eV.