Highly Enhanced Light-Outcoupling Efficiency in ITO-Free Organic Light-Emitting Diodes Using Surface Nanostructure Embedded High-Refractive Index Polymers

We develop the high-performance internal light-outcoupling (HRLOC) system based on the high-refractive index polyimide (PI) and metal oxide nanoparticles for organic light-emitting diodes (OLEDs) with silver nanowires (AgNWs). The spontaneously formed nanobump structures, high refractive index, and light-scattering properties of HRLOC significantly enhance the light-extraction efficiency of OLEDs. Not only do the outcoupling structures improve the light-extraction efficiency, but also remarkably enhance the electrical properties of OLEDs. HRLOC leads to the regular and smooth formation of AgNWs, resulting in the improvement of the electrical properties of devices by preventing electrical shorts and leakage currents. The power efficiency of the AgNW-based OLEDs with PI is improved by a factor of 1.31 compared to the reference device with indium tin oxide (ITO) transparent electrode at a luminance of 20 000 cd/m2. The efficiency is further improved by incorporating TiO2 nanoparticles into the PI matrix by a factor of 1.69. To our knowledge, the optically and electrically enhanced OLEDs show one of the highest enhancement factors reported for ITO-free OLEDs with internal outcoupling structures. In addition, the outcoupling structures are solution processable, thermally stable, and can be scaled up to 200 × 200 mm2 for large-area applications. We believe that the light-outcoupling structures developed here have great potential for efficient, low-cost, and flexible ITO-free OLEDs.