Band-gap and correlation effects in the organic semiconductorAlq3

We present a spectroscopic study of the electronic properties of a molecular organic semiconductor ${\mathrm{Alq}}_3,$ and its potassium intercalation compound ${\mathrm{KAlq}}_3.$ It is argued that both compounds are strongly correlated materials with a band gap that is to a large extent determined by electron correlation effects. In particular, the Coulomb repulsion energy for two electrons on an ${\mathrm{Alq}}_3$ molecule in the solids is about 1.4 eV that renders ${\mathrm{KAlq}}_3$ a Mott-Hubbard insulator and causes a large exciton binding energy in solid ${\mathrm{Alq}}_3.$ The implications of these results for organic devices are discussed.