The recombination of an electron and hole injected into a nondegencrate pi-conjugated polymer is simulated as a relaxation in the excited electronic state space. The rates of the spontaneous one-photon and one-phonon processes are derived within a diabatic model adjusted to the electron-vibrational spectroscopy of PPV. It is shown that the formation of the emissive singlet exciton is accelerated in a regular acceptor-donor copolymer because of the removal of the charge-transfer states. However, the acceptor-donor junction in a diblock heterostructure gives rise to a low-lying polar state that furnishes a nonemissive decay channel. An applied electric field is shown to reduce the luminescence efficiency in the homopolymer and copolymer by polarizing the exciton. In contrast, the quantum yield for the diblock polymer increases with forward bias because of the mixing of the polar exciplex-like state and the exciton.