Flexible organic light-emitting diodes (OLEDs) have potential application in next-generation display technology such as smart windows, mobile devices, virtual reality and wearable electronics. Typically, flexible OLEDs are based on flexible plastic substrates. However, these plastic substrates cause Fresnel reflection (10.4 %) and internal reflection (89%), so generated light from an emissive layer remains inside the OLED and substrate. To enhance the efficiency of the OLEDs without haze, it is known that subwavelength structures (< 250 nm, SWS) are essential. To obtain SWS on the plastic film, nanoimprint lithography (NIL) has been proposed due to the thermal and chemical weakness of polymer. However, the NIL requires master mold and complex surface treatment of the mold and substrate. And the master mold may deform after repeated use. Such problems could be solved by exposing chlorine (Cl2) plasma treatment onto a thin Ag-coated plastic substrate. The low ionization energy of Ag can react with Cl2 easily, producing subwavelength-scale AgCl nanostructure quickly. The SWS AgCl nanostructure can be directly formed on the surface of the flexible plastic film without surface treatment and additional etching mask.
In this work, we design the optimal SWS structure for highly transparent and haze-free plastic film, and demonstrate the AgCl SWS on the plastic substrate by a sequential process of Ag deposition and Cl2 plasma treatment. To design the optimal SWS structure, rigorous coupled wave analysis (RCWA) and finite-difference time-domain (FDTD) simulation were systematically calculated with respect to the various geometry of the nanostructure such as period, height, and gap. From the scanning electron microscopy (SEM) and UV-Vis spectrophotometer, we determined the optimal AgCl nanostructure. And X-ray diffraction and X-ray photoelectron spectroscopy were examined to understand forming mechanism of AgCl SWS. The Ag film reacts with Cl radicals in Cl2 plasma to form AgCl. AgCl has larger molar volume (25.7 cm3 mol-1) than that of Ag (10.2 cm3 mol-1), so surface morphology drastically changed from a two-dimensional (2D) flat Ag surface to three-dimensional (3D) SWS to release volume expansion during the plasma treatment. Consequently, the optimized AgCl nanostructure drastically enhanced the current efficiency of the OLEDs up to 10.8% without modifying a shape of angular emission patterns.