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.