Ionization in organic thin films: Electrostatic potential, electronic polarization, and dopants in pentacene films

Ionization processes in thin films are central to organic electronics. The ionization potential I(p) or electron affinity A(p) of any molecule p depends on the electronic polarization of the surrounding molecules and on electrostatic interactions W(p) that are evaluated in films using the potential Phi((g))(r) due to gas-phase charge densities. W(p) is combined with a self-consistent treatment of electronic polarization to obtain I(p) and A(p) using molecular quantum theory and the film's structure. I(p) and A(p) are not additive but contain cross terms in electronic polarization and electrostatics. The procedure accounts quantitatively for I(p) of pentacene and perfluoropentacene films with standing molecules in bilayers or lying molecules in monolayers. Surface or subsurface dopants in pentacene films are modeled as ion pairs with Coulomb interactions between a fixed anion and an adjacent cation. Variations of Phi((g))(r) due to an ion pair modulate I(p) and A(p) locally and rationalize observed changes for tunneling into occupied and unoccupied pentacene states, respectively. As in molecular exciton theory, intermolecular overlap is neglected in the computation of I(p) or A(p). Electrostatic interactions are conveniently quantified by Phi((g))(0) at the center of molecules.