TiO₂-Nanoparticle-Embedded Thin-Film Encapsulation for Blue Thermally Activated Delayed Fluorescence Top-Emitting Organic Light-Emitting Devices

Blue thermally activated delayed fluorescence (TADF) top-emitting organic light-emitting diodes (TEOLEDs) have garnered widespread interest because of their 100% internal quantum efficiency (η_int) and high intensity due to microcavity effects. However, their external quantum efficiency (η_ext) is limited by total internal reflection from the thin-film encapsulation (TFE) required by TEOLEDs and their susceptibility to moisture and oxygen. This paper introduces a novel approach in which TiO₂ nanoparticles (NPs) are incorporated into a negative photoresist, and Al₂O₃ is applied to form a nanolaminated TFE layer. This TFE layer decreases the water vapor transmission rate (WVTR), enhances light extraction efficiency and viewing angle characteristics. Moreover, the optical property can be modulated by adjusting the TiO₂ NP content and a planarization layer, maintaining a total transmittance of approximately 70%. The barrier properties assessed via electrical Ca tests show that TFE achieves WVTRs below 10^-5 g m^-2 day^-1 under ambient conditions. When applied to blue TADF TEOLEDs, the TFE improves electroluminescence by 23.78% and external quantum efficiency by 32.31%. As the viewing angle shifts from 0° to 45°, the CIE coordinate and peak wavelength shifts decrease from 0.067 to 0.044 and 14 to 8 nm, respectively. These results can be theoretically explained based on Mie scattering efficiency and Snell's law, as analyzed by finite-difference time-domain simulations.