Embedding Upconversion Nanoparticles in Polymer Films Toward Mono‐Dispersity at High Loading Factor

Lanthanide‐doped upconversion nanoparticles (UCNPs) exhibit unique luminescence properties, making them promising for applications in displays, sensors, security labels, and solar cells. Embedding UCNPs in polymer films can enhance their functionality; however, the properties of the polymer matrix significantly influence the dispersion and loading capacity of UCNPs, ultimately affecting optical performance. In this study, we investigate the incorporation of UCNPs into two distinct polymer matrices, poly(3‐hexylthiophene) (P3HT) and poly(methyl methacrylate) (PMMA), via spin coating at different speeds. Our findings demonstrate that UCNP dispersion and monodispersity are governed by polymer polarity, viscosity, and UCNP concentration in the suspension. To enhance UCNP loading, multiple spin coatings were explored. In UCNP−P3HT films, the volume fraction of UCNPs increased from 26.1% to 51.4% after three consecutive spin coatings, while maintaining a uniform distribution. In contrast, the lower miscibility and higher viscosity of PMMA restricted UCNP loading to 12.0% before significant clustering occurred. Although multiple spin coatings increased the total UCNP content in PMMA films, the volume fraction decreased to 8.0% due to film thickening. This comparative analysis highlights the critical role of polymer matrix properties in UCNP embedding and provides valuable insights for optimizing UCNP−polymer composites for advanced optical applications.