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ABSTRACT We present the use of polymer/polymer interfaces to control light-emitting polymer devices. Bilayer devices utilizing poly(9-vinyl carbazole) (PVK) as a hole transporting/electron blocking polymer together with a pyridine containing electron transporting layer show dramatically improved efficiency and brightness as compared to single layer devices. This is attributed to charge confinement and exciplex emission at the PVK/emitting polymer interface. The introduction of emeraldine base (EB) form of polyaniline (PAN) on both side of the emitting layer enables the device to work under both forward and reverse bias, as well as in AC modes. Interfaces play an important role in the operation of these devices. Furthermore, when the EB is replaced by sulfonated polyaniline (SPAN) on the cathode side and the emitting layer is properly modified to balance electron and hole transport, the device generates different colors of light, red under forward bias and green under reverse bias. INTRODUCTION Electroluminescence (EL) combined with other unique properties of polymers, such as solution processibility, band gap tunability and mechanical flexibility, make conjugated polymers excellent candidates for low cost large area display applications [1-4]. Most "conventional" polymer light-emitting devices have been shown to be tunneling diodes and can be operated only under forward DC bias [5]. Among the most important limitations associated with many of the "conventional" polymer light-emitting diodes (LEDs) are poor stability and shelf lifetime. The double charge injection mechanism of the "conventional" polymer LEDs requires the matching of the cathode (anode) work function to the corresponding LUMO (HOMO) level of the polymer in order to achieve efficient charge injection. The relatively low electron affinity of most conjugated polymers requires metals with very low work functions to achieve efficient electron injection. Pyridine-containing conjugated polymers are promising candidates for light-emitting devices [6]. The pyridine containing polymers are highly luminescent, especially the copolymers. The high electron affinity of pyridine based polymers enables the use of relatively stable metals such as Al, Au, or ITO and doped polyaniline as electrodes. Taking advantages of the better electron transport properties of the pyridine-containing polymers, we fabricated bilayer devices utilizing poly(9-vinyl carbazole) (PVK) as hole transporting/electron blocking polymer, which improves the device efficiency and brightness significantly due to the charge confinement and exciplex emission at the PVK/emitting polymer interface [7]. By inserting a layer of emeraldine base (EB) form of polyaniline on both side of the emitting polymer, we fabricated symmetrically configured AC light-emitting (SCALE) devices [8] which work under both forward and reverse DC bias as well as in AC modes. When we replace the EB layer on the cathode side of the SCALE device with sulfonated polyaniline (SPAN) and modify the emitting layer to appropriately b