Cs₃Cu₂I₅ and CsCu₂I₃ have been demonstrated to emit broad spectrum light with high photoluminescence quantum efficiency (PLQY) due to their low-dimensional electronic structure and soft lattice with a strong electron–phonon coupling. Owing to their advantages of non-toxicity, simple synthesis method, high air stability and thermal stability, they have become one of the most promising semiconductors for next-generation optoelectronic devices. There are two explanations for the mechanism of their broad-spectrum luminescence, that is, self-trapped exciton (STE) luminescence and excited-state structure reorganization. We sort out the halogen composition-related PL, temperature-dependent PL and theoretical calculations of Cs₃Cu₂I₅ and CsCu₂I₃ in recent years to review the mechanism of this broad spectrum luminescence from the perspectives of electronic dimensions and lattice distortion. In addition, current application status and future prospects of Cs₃Cu₂I₅ and CsCu₂I₃ in optoelectronic fields are also presented in this review.