Effects of the electronic structure of polymer/polymer interfaces on the electroluminescence efficiency and tunable multicolor emission of polymer heterojunction light-emitting diodes were explored by a series of 16 n-type conjugated polymers with varying electron affinities and ionization potentials in conjunction with poly(p-phenylenevinylene). Efficiency and luminance of diodes of the type indium-tin oxide/poly(p-phenylenevinylene)/n-type polymer/aluminum were maximized and were as high as 3% photons/electron and 820 cd/m(2), respectively, when the energetics at the polymer/polymer interface favored electron transfer while disfavoring hole transfer. Energetic barrier to electron transfer at the polymer/polymer interface was more important to electroluminescence efficiency and diode luminance than injection barrier at the cathode/polymer interface. By a judicious choice of the relative layer thicknesses and the components of the bilayer heterojunctions, the rate of both electron and hole transfer across the polymer/ polymer interface can be regulated by the applied voltage, resulting in continuous voltage tunability of emission colors. The voltage tunable multicolor emission is exemplified by red (5 V) <----> yellow (9 V) <----> green (12 V) and other intermediate color switching in poly(p-phenylenevinylene)/poly(2,6-(4-phenyl)-quinoline) (PPQ) diodes. The multicolors obtained from a single heterojunction diode by varying the applied voltage originated from the mixing of the component emission spectra in varying proportions facilitated by interfacial charge transfer and finite size effects. Electroluminescence microscopy was used to directly image the multicolor diodes. These results suggest that the electronic structure of polymer/polymer interfaces and finite size effects dominate the emission features and performance of light-emitting devices based on multicomponent polymers such as multilayered thin films, phase-separated blends, and block copolymers. The results also have implications for photovoltaic cells and other optoelectronic devices using conjugated polymers.