Utilizing transparent ZnO thin film as permeation barrier to improve light outcoupling and longevity of top-emission polymer light-emitting devices

The polymer light-emitting diodes (PLEDs) encapsulated with a ZnO UV-cured epoxy resin passivation layer were investigated. The ZnO film not only serves as an encapsulation layer but also acts as a UV blocking layer during UV cured to protect the organic layer from damage from UV light because ZnO has a high transmittance of up to 95% in the 490-540 nm wavelength range and a low transmittance in the UV region. The index of refraction (n = 2) for ZnO acting as a capping layer facilitates more light outcoupling from the top side of the PLED. The PLEDs encapsulated with ZnO film were fabricated under oxygen-free conditions and the substrate was maintained at room temperature to avoid damaging the PLEDs and to promote the formation of amorphous ZnO which prevents moisture and oxygen penetration. Because the ZnO film was deposited after a transparent conducting oxide cathode, the novel passivation method did not influence the electrical behavior, for instance, turn-on voltage, leakage current, and the EL characteristic of the device. PLEDs encapsulated with ZnO/UVcured epoxy resin and glass (reference device) both exhibited a similar lifetime.