2, Electronics and Telecommunications Research Institute (ETRI), Daejeon, , Korea (the Republic of)
Top-emission organic light-emitting diodes (TE-OLEDs) are popular in high-resolution displays or in aperture-controlled transparent displays as they can maximize the areal utilization of emitting devices. Semi-transparent metal layers are generally used for a top electrode in TE-OLEDs because they can be easily formed via thermal evaporation and can control a micro-cavity resonance effect, thereby achieving wide color gamut. Nevertheless, metal based top electrodes which show low transmittance and high reflectance, have serious limitations such as photon energy loss due to strong surface plasmon polariton (SPP) modes, and viewing angle color distortion.
To overcome these problems, we here report a non-metal based transparent conductive film as a top electrode, which consists of the transparent conductive electrode and the transparent UV curable polymer layer. The proposed conductive film can be uniformly transferred by a novel vacuum lamination process onto an OLED stack. The angular distribution of the electroluminescence (EL) intensity of the laminated devices shows that of nearly ideal Lambertian, which is enabled by high transmittance and spectrally neutral character of the proposed film over the whole visible range.
Furthermore, we successfully demonstrate outcoupling structures at the surface of the proposed film for enhancing the light extraction using an imprinting method. Outcoupling structures and high refractive index of the transparent polymer layer can significantly increase the external quantum efficiency (ηEQE) of TE-OLEDs as shown in our simulation results. These results indicate that the waveguided modes in the organic and top electrode layers can be extracted to the substrate-confined modes due to high refractive index of the transparent polymer layer, and the confined light in the substrate (the transparent polymer layer) can be greatly extracted through the out-coupling structures. Consistent with the simulation results, laminated devices with concave hexagonal structures (ηEQE. w/ structures = 38.6%) shows 1.7 times higher ηEQE than those with planar structures (ηEQE. w/o structures = 23.0%). In addition, while we formed out-coupling structures in the film, laminated devices are shown to exhibit little optical blurring due to thin film thickness (~ 60 µm).