TiO₂-Nanoparticle-Embedded Thin-Film Encapsulation for Blue Thermally Activated Delayed Fluorescence Top-Emitting...

Area of Science:

• Materials Science
• Optoelectronics
• Nanotechnology

Background:

• Blue thermally activated delayed fluorescence (TADF) top-emitting organic light-emitting diodes (TEOLEDs) offer high internal quantum efficiency but suffer from limited external quantum efficiency (ηext) due to encapsulation challenges.
• Thin-film encapsulation (TFE) is crucial for TEOLEDs but can cause total internal reflection and susceptibility to moisture and oxygen, hindering performance and longevity.

Purpose of the Study:

• To develop a novel nanolaminated TFE layer incorporating titanium dioxide (TiO2) nanoparticles (NPs) and aluminum oxide (Al2O3) for blue TADF TEOLEDs.
• To enhance the light extraction efficiency, viewing angle characteristics, and environmental stability of TEOLEDs through optimized TFE.
• To investigate the impact of TiO2 NP content and planarization on optical properties and barrier performance.

Main Methods:

• Incorporation of TiO2 NPs into a negative photoresist, followed by Al2O3 deposition to create a nanolaminated TFE layer.
• Assessment of barrier properties using electrical Calcium (Ca) tests to determine water vapor transmission rate (WVTR).
• Optical characterization including transmittance measurements and analysis of viewing angle-dependent optical shifts (CIE coordinates, peak wavelength).
• Finite-difference time-domain (FDTD) simulations and theoretical analysis based on Mie scattering and Snell's law.

Main Results:

• The nanolaminated TFE achieved WVTRs below 10⁻⁵ g m⁻² day⁻¹, significantly improving moisture and oxygen resistance.
• Light extraction efficiency and viewing angle characteristics were enhanced, with total transmittance maintained around 70%.
• Application of the TFE to blue TADF TEOLEDs resulted in a 23.78% improvement in electroluminescence and a 32.31% increase in external quantum efficiency (ηext).
• Reduced shifts in CIE coordinates (0.067 to 0.044) and peak wavelength (14 to 8 nm) were observed as the viewing angle increased from 0° to 45°.

Conclusions:

• The developed TiO2 NP-based nanolaminated TFE is effective in enhancing the performance and stability of blue TADF TEOLEDs.
• This novel encapsulation strategy overcomes key limitations associated with traditional TFE, paving the way for more efficient and durable OLED devices.
• The findings provide a pathway for optimizing TEOLED performance through advanced nanostructured encapsulation materials and designs.