Hole doping by molecular oxygen in organic semiconductors: Band-structure calculations

The sensitive effect of ${\mathrm{O}}_2$ adsorption on the electronic properties of organic semiconductors is investigated by band structure calculation. ${\mathrm{O}}_2$ can actually $p$-dope the host materials even without illumination, i.e., a ground state property, in the circumstance of saturated coverage. Due to hybridization between ${\mathrm{O}}_2$ and polymer, Fermi level of the oxygenated system is pinned at the nearly half-filled oxygen band and overlap with host valence band. The doping depends critically on the ionization potential. Each ${\mathrm{O}}_2$ can dope more than 0.1 hole in dark and a full charge-transfer excitation around $2.3\mathrm{eV}$ photon energy is predicted.