Organic light-emitting diodes (OLEDs) are currently being investigated for applications in large-areas flat-panel displays and solid state lighting, . Polymer OLEDs, or PLEDs offer potentially lower production cost and greater mechanical flexibility including elastomeric stretchability, , , . The introduction of a phosphorescent dopant in the organic emissive material allows for harvesting both singlet and triplet exciton energy and can increase the internal quantum efficiency (ηIQE) of the OLEDs to 100%. However, the experimentally determined external quantum efficiency (ηEQE) has been limited to less than 30% without other out-coupling component, . The 70% of the produced photons is lost due to trapping inside OLEDs in the form of waveguide (WG) modes in organic/indium–tin oxide (ITO) transparent anode layers, surface plasmon–polariton (SPP) modes at the metallic electrode/organic interface, and glass substrate modes from total internal reflection at the glass substrate/air interface, , . The use of ITO and glass also affect the flexibility and fabrication cost of the OLEDs.
A number of techniques have been reported to increase the light extraction efficiency from modifying either the interface of organic/ITO layer and ITO/glass substrate (internal mechanism) or the glass substrate (external structure). The internal mechanisms use a meshed poly(3, 4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) layer, low-index grids or nanoparticles, , , micro-cavity, , surface plasmonic structures, on the ITO layer, or embed photonic crystals, high-index optical coupling layer, dielectric mirrors, or buckled structures between ITO and glass to extract the waveguide mode in organic layer and ITO layer, , , . The internal mechanisms was to modify the light transport route and adjust the refractive index difference between the different functional layers. The methods could optimize the optical field and electrical field distribution in the organic layer and ITO layer to extract light. External structures are applied on the back surface of the substrate, including structured and shaped substrates, , , , , scattering mediums, , and micro-lenses. have also been shown to have a positive effect on the extraction of light out of the glass substrate. Also some strategy applied periodic or random nanowire electrodes, , to replace the ITO conductive electrode to prevent the ITO trapping mode. These approaches increase the light extraction efficiency to various extents, but the overall benefits have been limited due to an increased complexity in device fabrication, narrowed viewing angle, altered emission spectrum and/or angular dependency of the emission spectrum. Moreover, many of these approaches are not compatible with solution-based large scale industrial processing for flexible display and lighting panels.
