Room‐Temperature, Photo‐Densified High Refractive Index Titanium Oxide Thin Films for Solution‐Processed Flexible Bragg Mirrors and Microcavity OLEDs

Abstract Photonic structures such as Bragg mirrors are essential for modern optical devices. Traditional fabrication methods like sputtering are limited by high temperatures and incompatibility with flexible substrates. While solution‐based processes are cost effective for large area production, they typically require high‐temperature annealing to produce dense, high‐refractive‐index films. To address this, a groundbreaking, room‐temperature solution process for fabricating high refractive index titanium oxide (TiO _X ) thin films is developed using vacuum ultraviolet (VUV) light photo‐densification. The VUV process, unlike conventional sol–gel reactions, facilitates a unique densification mechanism involving an O─C─O intermediate. This distinct pathway creates smaller pore in the film, enabling efficient densification and achieving record‐high refractive index of n = 2.01 at room‐temperature for solution‐processed films. Bragg mirrors are fabricated on PET substrates, exhibiting flexibility and curl‐free properties, by alternately layering the photo‐densified TiO _X films with low refractive index silicon oxide (SiO _X ) films. The resulting 12‐layer mirrors achieve a high reflectance of 96% at λ = 450 nm. Significant improvements in color purity ( FWHM reduced from 93 to 35 nm), directionality, and front light intensity are demonstrated by integrating these Bragg mirrors into microcavity OLEDs. This work presents a new paradigm for fabricating high‐performance optical devices on flexible and heat‐sensitive substrates.