Ultrathin Metal Film Transparent Conductor for Efficient Light Coupling in Organic Light Emitting Diode

Researchers have intensively investigated alternative transparent conductors (TC) as a substitute for ITO due to its limitation in mechanical brittleness for use in flexible OLED applications. New types of oxide-based TCs like IZO or AZO intrinsically have low resistivity, which needs sufficiently thick layer to meet the low enough sheet resistance. Including ITO, such oxide-based TCs with thick dielectric layers have higher index than substrate and so are prone to light trapping as a waveguided-mode, thereby compromising the outcoupling efficiency. Emerging candidates like ultrathin metal-based transparent conductors can guarantee high transmittance, low sheet resistance, as well as flexibility. The key is to control the metal layer thickness well below the skin depth for sufficient light transmission. Silver (Ag) is known to have superior conductivity and low optical loss at visible wavelength. However, Ag films tend to grow in 3D island-like (Volmer-Weber) modes on dielectric substrates [1] which makes the film electrically insulating. With appropriate seed-layer, Ag growth can be controlled to have 2D-like growth forming an ultra-smooth continuous film with low resistance [2]. Such ultrathin metal films have been stacked with top and bottom anti-reflection (AR) coatings to show high transmittance and low sheet resistance which have been demonstrated for optoelectronic applications [3]. However, the existence of AR coating in these layers makes them prone to light trapping by waveguide modes, not much different from oxide-based TC like ITO.