Defect-Driven Origins of Intrinsic and Dopant-Induced Photoluminescence in Zero-Dimensional Cs₂ZnX₄ (X = Cl, Br)

Zero-dimensional (0D) lead-free metal halides are promising luminescent materials, yet their emission origins remain unclear. Using hybrid-functional first-principles calculations, we clarify the photophysical mechanisms in pristine and ns²/nd¹⁰-doped Cs₂ZnX₄ (X = Cl, Br). We reveal that experimentally observed emissions stem not from self-trapped excitons or isolated dopants but from intrinsic point defects and strongly interacting defect-dopant complexes. In pristine hosts, intrinsic luminescence arises from ligand-to-metal charge-transfer transitions involving halogen vacancies (V_Cl^•, and V_Br^•). For high-valent ns² dopants, emissions originate from localized s ↔ p transitions within highly coordinated dopant-interstitial complexes, such as (Sb_Zn + Cl_i)^×. Notably, isovalent Sn²⁺ exhibits a flat, dual-minima excited-state adiabatic potential energy surface, explaining its anomalous cooling-induced red shift. For nd¹⁰ dopants, emissive centers include simple substitutional defects and vacancy-assisted complexes, specifically, the (Cu_Zn + V_Br)^× complex in Cu-doped systems and the (Ag_Zn + 2V_Br)^• complex responsible for thermochromic luminescence in Ag-doped systems. Ultimately, this defect-chemistry-driven model demonstrates that abundant intrinsic defects and their coupling with dopants govern the luminescence of 0D zinc-based halides, offering insights for designing high-performance, stable lead-free materials.