Low Barrier for Exciton Self-Trapping Enables High Photoluminescence Quantum Yield in Cs3Cu2I5

The metal halide Cs3Cu2I5 displays anomalous optical properties: an optical absorption onset in the ultraviolet region (∼ 330 nm) with highly efficient luminescence in the blue region (∼ 445 nm). Although self-trapped exciton formation has been proposed as the origin of giant Stokes shift, its connection to the photoluminescence quantum yield exceeding 90% remains unknown. Here, we explore the photochemistry of Cs3Cu2I5 from first-principles and reveal a low energy barrier for exciton self-trapping associated with Cu–Cu dimerization. Kinetic analysis shows that the quantum yield of blue emission in Cs3Cu2I5 is sensitive to the excited carrier density due to the competition between exciton self-trapping and band-to-band radiative recombination.