The structures of blended films composed of a light-emitting polymer and an ionic liquid were investigated, and the relationship between the emission properties of organic light-emitting diodes (OLEDs) and the structure of the blended films was discussed. We have demonstrated emission in positive and negative biases of the blended films, which contributed to the effects of the electric double layer. Moreover, we have visualized the distribution of the ionic liquid, not only in-plane but also out of-plane, by employing electroluminescence in a sandwich device. Ionic liquids form a micrometer-order phase-separation structure in the blended film and are found in higher concentration on the top of the film. Finally, we have summarized the guidelines for ideal film structure of single-layer OLEDs.