Quantifying thin-film nonidealities to assess the performance of low-cost, spin-coated reflectors

Solution-processed multilayer dielectric stacks can act as scalable and effective light-managing optoelectronic devices with low input cost but are less efficient than their vacuum-deposited counterparts due to layer thickness inconsistency and scattering caused by defects. To further the understanding of how solution-processed thin-film devices can be more accurately modeled and ultimately become viable in real applications, this work developed an experimentally based approach for measuring and modeling nonidealities in spin-coated one-dimensional photonic crystals. This was done by synthesizing these structures using the promising TiO₂-PMMA material system and directly quantifying the thickness variability, film roughness values, and macroscopic scattering defects via atomic force microscopy. The resulting experimental parameters were then directly incorporated into ideal transfer matrix method calculations to more accurately model the real design space for spin-coated one-dimensional photonic crystals. This exercise revealed that the layer thickness variability remains the greatest limitation for these structures as compared to scattering. Despite these challenges, strong agreement with ideal transfer matrix method curves and large reflectance peaks (R≥0.9) were achieved across the visible light domain with strong color purity. To simplify the fabrication, different iterations of the key TiO₂ sol-gel film heating steps were performed. This demonstrated a new degree of reduced processing burden: 7-layer 1D PCs were able to recover the reflectance spectra of equivalent stacks that underwent multiple interstitial heating cycles through a single, long heating step after complete synthesis. Finally, to investigate the system's readiness in more complex applications, the quantified thickness variability was simulated for three aperiodic structures with many layers. This revealed that the spin-coated TiO₂-PMMA material system has high potential in longpass filtering roles, but reducing the degree of thickness deviation is needed for high performance in shortpass filter and microcavity applications.