Electroluminescence of Multicomponent Conjugated Polymers. 1. Roles of Polymer/Polymer Interfaces in Emission Enhancement and Voltage-Tunable Multicolor Emission in Semiconducting Polymer/Polymer Heterojunctions

Effects of the electronic structure of polymerpolymer 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 polyp-phenylene vinylene. Efficiency and luminance of diodes of the type indium-tin-oxidepolyp-phenylene vinylenen-type polymeraluminum were maximized and were as high as 3 photonselectron and 820 cdsq m, respectively, when the energetics at the polymerpolymer interface favored electron transfer while disfavoring hole transfer. Energetic harrier to electron transfer at the polymerpolymer interface was more important to electroluminescence efficiency and diode luminance than injection barrier at the cathodepolymer 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 polymerpolymer 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 left right arrow yellow 9 V left right arrow green 12 V and other intermediate color switching in polyp-phenylene vinylenepoly2,6-4-phenylquinoline 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.