With polymer-based organic LED's, patterning of the polymer to achieve color is a critical issue. A locally dye-doped polymer film is attractive for realization of multi-color devices on a single substrate. Different dyes may be locally added either by ink-jet-printing or by masked dye diffusion onto a previously spin-coated polymer film to get R,G,B subpixels. However, high leakage and low efficiency of these single-layer doped polymer LEDs limit their applications in passive matrix displays. Both of these problems can be solved by adding an electron transport layer (ETL) over the polymer layer after dyes have been applied. The ETL Alq suppresses hole tunneling to reduce reverse leakage current, moves the cathode away from the emissive dyes to reduce cathode-quenching of the excitons via dipole interaction, and balances electron and hole transport to raise efficiency. However, due to the low exciton energy in the Alq (E{sub}(exciton)=2.3eV), excitons formed in the polymer layer will migrate to the Alq layer, so that the dye in the polymer does not control the emission color (Figure 1,2(a)). In this paper, we introduce a thin exciton blocking layer (EBL) between the doped-polymer layer and the Em, to confine the excitons within the polymer layer, yet still allow electrons to pass through, so that multi-color polymer-based devices can be realized (Figure 2(b)). The fabricated red, green, blue polymer LEDs using this novel tri-layer structure have higher efficiency and reverse leakage current two orders of magnitude lower than the single-layer devices.
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