Exciton Dynamics of Orthorhombic Phase PbI2 Embedded in PbI2-PbBr2 Mixed Crystals

Reflection, time-integrated and time-resolved luminescence spectra have been measured in PbI_2(1-x)Br_2x (x ≥ 0.5) mixed crystals in order to elucidate the excitonic properties and relaxation processes of orthorhombic phase PbI₂ embedded in the mixed crystals. The crystal structure of PbI_2(1-x)Br_2x above x = 0.5 is orthorhombic and an observed new reflection peak (~2.5 × 10⁴ cm^-1) above x = 0.5 is due to an excitonic state of orthorhombic phase PbI₂. On the excitation energy above 2.5 × 10⁴ cm^-1, a broad luminescence band with a large Stokes shift from the reflection peak was observed. This luminescence band is attributed to a self-trapped exciton state. The decay of the luminescence has two components. Based on an atomic excitation picture, the shorter decay component (≤10 ns) is attributed to the allowed transition from ³P₁ to ¹S₀. On the other hand, the longer decay component (~3 μs) might be due to the forbidden transition from ³P₀ to ¹S₀.