Transparent organic light-emitting diodes (OLEDs) have shown the amazing applications in full-color flat panel displays and solid-state lighting due to their prominent advantages, including low power consumption, light weight, wide color gamut, fast response time and high contrast. To realize high-performance transparent OLEDs, a major research direction is to develop the alternative transparent electrodes with superior optical and electrical properties for replacing the opaque metal top electrodes that are commonly used in bottom-emission OLEDs. Various materials and structures have been proposed to function as transparent conductive electrodes. Metal-dielectric composite electrode (MDCE) has been regarded as an effective TCE for flexible devices in terms of mechanical flexibility, electrical conductivity, optical transparency, and large-area film uniformity. Whilst MDCE may be an ideal candidate to replace ITO, several technical challenges should be overcome when using MDCEs as transparent electrodes in transparent OLEDs. First, the presence of thin metal films in MDCEs will cause surface plasmonic (SP) loss at the metal-dielectric interface due to the oscillation coupling between free electrons at the metal surface and the emitting dipoles. Second, an optical microcavity effect is inevitable with the use of a planar MDCE structure, leading to the spectral and angular dependence of the emission characteristics. Herein, an effective nanostructured metal/dielectric composite electrode (NMDCE) on plastic substrate is applied to transparent OLEDs with an ultrathin metal alloy film for optimum optical transparency, electrical conduction and mechanical flexibility. By combining an light-extraction structure for broadband and angle-independent outcoupling of white emission, the waveguided light and surface plasmonic loss can be remarkably reduced in white flexible OLEDs, resulting in a substantial increase in the external quantum efficiency and power efficiency to ~70% and 160 lm/W.
To realize high-efficiency organic light-emitting diodes (OLEDs), it is essential to boost out-coupling
efficiency. Here we review our latest reports upon light manipulation for OLEDs by integrating a
dual-side bio-inspired deterministic quasi-periodic moth’s eye nanostructure with broadband
anti-reflective and quasi-omnidirectional properties. Light out-coupling efficiency of OLEDs with
stacked triple emission units is over 2 times that of a conventional device, resulting in drastic increase in
external quantum efficiency and current efficiency to 119.7% and 366 cd A-1 without introducing
spectral distortion and directionality. Theoretical calculations furthermore clarify that the improved
device performance is primarily attributed to the effective extraction of the waveguide and surface
plasmonic modes of the confined light over all the emission wavelengths and viewing-angles.