Heterojunctions between organic semiconductors are central to the operation of light-emitting and photovoltaic diodes, providing respectively for electron-hole capture and separation. However, relatively little is known about the character of electronic excitations stable at the heterojunction. We have developed molecular models to study such interfacial excited electronic excitations that form at the heterojunction between model polymer donor and polymer acceptor systems: poly( 9,9-dioctylfluorene-co-bisN, N-(4-butylphenyl)-bis-N, N-phenyl-1,4-phenylenediamine) (PFB) with poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT), and poly(9,9-dioctylfluorene-co-N-(4-butylphenyl)diphenylamine) (TFB) with F8BT. We find that for stable ground-state geometries the excited state has a strong charge-transfer character. Furthermore, when partly covalent, modelled radiative lifetimes ( similar to 10(-7) s) and off-chain axis polarization (30 degrees) match observed 'exciplex' emission. Additionally for the PFB: F8BT blend, geometries with fully ionic character are also found, thus accounting for the low electroluminescence effciency of this system.