Upconversion on the Micrometer Scale: Impact of Local Heterogeneity

The properties of perovskite/naphtho[2,3-a]pyrene (NaPy) upconversion devices are investigated by a combination of atomic force microscopy and photoluminescence mapping to understand the role of microscopic heterogeneity in the ensemble device properties. The results emphasize strong microscopic inhomogeneity across the perovskite/NaPy upconversion device due to local formation of NaPy microcrystals. NaPy shows emission from three distinct states in the solid state: S₁′ emission at 520 nm, excimer emission at 560 nm, and S₁″ emission at 620 nm. Clear spatial differences in the emission spectrum under 405 nm excitation are found, highlighting that there is a strong microcrystal-to-microcrystal variation in the optical properties─emphasizing a need for multimodal measurements. Our results indicate that microcrystals with strong emission from the strongly coupled low-energy state S₁″ (J-dimer) show much higher upconversion intensity than those with dominant emission from the high-energy S₁′ state (I-aggregate). Hence, our results suggest that microcrystals with strong emission from the low-energy state S₁″ act as isolated hotspots for upconversion.