The development of new and high-performance light-emitting materials and devices has been the topic of significant research efforts in the past two decades, especially with the rapidly growing flexible electronics consumer market. The organic solid-state lighting devices possess the advantages of low cost, light weight, fast response time, large viewing angle and mechanical flexibility. In recent years, the flexible optoelectronic devices such as displays and lighting have been achieved by preparing organic/polymer-based light-emitting diodes (OLEDs/PLEDs) on flexible substrates such as plastic films, and some OLEDs based on flexible substrates have been actively applied for optoelectronic sensors and bioelectronics. However, the current PLEDs still have considerable room for improvement in the device efficiency and stability needed for commercial applications. In traditional light-emitting diodes, the light needs to get in and out the sandwich structured devices, which makes the transparent indium-tin oxide (ITO) electrodes indispensable. However, there is a great technological interest in finding alternatives to ITO because of the increasing cost, limited recyclability and its fragile nature.
In this Proof of concept (PoC) project, we aimed at developing ITO-free organic multi- colored LEDs supported on free-standing ultrathin films as elements for the next generation of large-area flexible lighting and full-color active-matrix displays. We have exploited a radically new device configuration based on a nanomesh scaffold as an alternative to the existing sandwich structured LEDs and proved that it could resolve most of the current solution processed PLEDs limitations. The empty nanomesh scaffold devices supported on both rigid glass substrate and flexible polyimide (PI) substrate demonstrated excellent durability and stability with a leakage current density below 10–8 A cm–2 and >98% devices yield. More importantly, the nanomesh scaffold technique is compatible with traditional photolithography, which means it could shrink a single light-emitting pixel down to hundreds of nanometers, thus offering a path towards the realization of ultra-high resolution display equipment. This technique is also compatible with spin-coating deposition and even more interestingly with ink-jet printing technique for the site-selective deposition of multiple light-emitting materials, which also represents a major step forward towards a real application of this unique technology.
