Unraveling the Origin of Long-Lifetime Emission in Low-Dimensional Copper Halides via a Magneto-optical Study

The origin of the long lifetime of self-trapped exciton emission in low-dimensional copper halides is currently the subject of extensive debate. In this study, we address this issue in a prototypical zero-dimensional copper halide, Cs₂(C₁₈)₂Cu₂I₄-DMSO, through magneto-optical studies at low temperatures down to 0.2 K. Our results exclude spin-forbidden dark states and indirect phonon-assisted recombination as the origin of the long photoluminescence lifetime. Instead, we propose that the minimal Franck-Condon factor of the radiative transition from excited states to the ground state is the decisive factor, based on the transition probability analysis. Our findings offer insights into the electronic processes in low-dimensional copper halides and have the potential to advance the application of these distinctive materials in optoelectronics.