Processible conjugated polymers can be used to fabricate a range of thin-film diodes which can be designed to show good characteristics both as electroluminescent diodes and also as photoconductive diodes. We consider the present understanding of the operation of light-emitting diodes which use conjugated polymers for both charge transport and emission. We highlight the improvement to the electroluminescence efficiency that can be produced by the use of two polymer layers selected so that the heterojunction between the two layers is able to confine charge and thus bring about electron-hole capture to generate excitons at this interface. Photon absorption produces excitons which are considered to be bound at room temperature. Charge penetration requires ionisation of these excitons, which can be achieved efficiently at heterojunctions between layers with different electronegativities, and we report how this can be achieved at the interfaces between interpenetrating networks of electron- and hole-accepting polymers.