Impact of the Hexagonal Phase on Multiphoton‐Absorption Properties of Mixed‐Cation Halide Perovskite: FA 0.8 MA 0.2 PbI 3

Recently, mixed‐cation halide perovskites have gained significant attention for developing stable, high‐performance solar cells. However, their nonlinear optical (NLO) properties are much less explored. Herein, this study reports on intriguing NLO properties observed from FA 0.8 MA 0.2 PbI 3 single crystals, having an ideal bandgap of 1.52 eV for photovoltaic performance. Contrary to a simple prediction, the perovskite exhibits higher‐order multiphoton absorption (MPA) over a broad wavelength range from 1300 to 1710 nm as evidenced by wavelength‐ and intensity‐dependent photoluminescence measurements. The corresponding MPA coefficients are very large to dominate lower‐order ordinary MPA across the bandgap. The hexagonal phase present in the perovskite is responsible for the observed higher‐order transition as confirmed by the recovery of ordinary MPA in the absence of the hexagonal phase. Although the perovskite is known to crystallize into the cubic phase at room temperature, these results indicate that minor inclusion of the hexagonal phase (≈4.0% at room temperature) can drastically alter the nature of the multiphoton process by double resonance uniquely offered by the mixed phases.