Stacked metal cathode for high-contrast-ratio polymeric light-emitting devices.

An extremely high optical absorbing film made of alternating aluminum–silver layers was used as cathode in polymeric light-emitting devices (PLEDs). Physical properties of the cathodes were characterized by I–V measurement, atomic force microscopy, and x-ray photoemission spectroscopy. As a result of the slow evaporation rate, each pair of the aluminum–silver layer was shown to be in the form of aluminum–aluminum oxide nanoclusters embedded in an amorphous charge conducting network of silver. The nanoclusters helped to absorb and scatter the ambient light effectively. The use of four alternating layers structure in conventional PLEDs demonstrated 126% enhancement of contrast under 1000 lx ambient illumination. The I–V characteristics of the black cathode PLEDs remained intact when compared with reference PLEDs. This technology offers precise control of the cathode quality in terms of its reflectivity and conductivity. [ABSTRACT FROM AUTHOR]