Defect Origin of Emission in CsCu₂I₃ and Pressure-Induced Anomalous Enhancement

Lead-free metal halide perovskites CsCu₂X₃ (X = Cl, Br, I) with a high photoluminescence quantum yield are promising materials for optoelectronic devices. However, the origin of photoluminescence (PL) emission is still under debate, and the anomalous dependence of PL on pressure is unclear. Here, we systemically study the effects of high pressure on the structural, electronic, and optical properties of CsCu₂I₃ using a diamond anvil cell (DAC) and first-principles calculations. We argue that the ground state structure of CsCu₂I₃ belongs to the pnma phase rather than the cmcm phase under ambient conditions. There is a structural phase transition from the pnma to the cmcm phase for CsCu₂I₃ at ∼5 GPa. The optical band gap derivative from absorption spectra increases from 3.57 to 3.62 eV within a pressure range of 0 to 4.03 GPa, and it then decreases over 4.03 GPa. There are two major PL emissions peaks at 2.11 and 2.32 eV, which are attributed to the intrinsic defect related trap states in CsCu₂I₃. Interestingly, there is an anomalous dependence of both PL emissions on pressure, such that PL peaks show a blueshift and the PL intensity is enhanced from 0 to ∼4 GPa, with redshifting and decreasing at pressures above ∼4 GPa. The anomalous evolution of the two PL emissions also suggests a defect origin of emissions.