Polymerizable anions and/or cations can be used as the ionically conductive species for the formation of a p-i-n junction in conjugated polymer thin films. After the junction is formed, the ions are polymerized in situ, and the junction is locked thereafter. The resulting polymer p-i-n junction diodes could have a high current rectification ratio. Electroluminescence with high quantum efficiency and low operating voltage may be produced from this locked junction. The diodes may also be used for photovoltaic energy conversion. In a photovoltaic cell, the built-in potential helps separate electron-hole pairs and increases the open-circuit voltage.