Bi–Sb Co‐Alloying Tunes Photoexcited State Dynamics in (4FPEA) ₄ AgBi _1−x Sb _x I ₈ Layered Double Perovskites through Structural Confinement

Abstract Layered double perovskites incorporating large organic cation spacers offer low toxicity alongside controllable and intriguing excited state carrier dynamics, often characterized by self‐trapped exciton (STE)‐mediated recombination. Here, it is demonstrated that alloying Bi with Sb in (4‐Fluorophenethylammonium) 4 AgBi 1−x Sb x I 8 ( x = 0–1) provides a structural handle to finely control STE recombination dynamics. Elliott model fits yield exciton binding energies ( E b ) of 205, 126, and 50 meV for x = 0, 0.5, and 1, respectively. Counterintuitively, time‐resolved photoluminescence reveals increasing radiative lifetimes (1.4 ns for x = 0, 2.4 ns for x = 0.5, and 2.5 ns for x = 1) as E b decreases. A four‐stage relaxation model is proposed, supported by ultrafast transient absorption spectroscopy measurements, wherein Sb‐rich films exhibit notably prolonged STE depopulation (150 ps) compared to Bi‐rich films (45 ps). Rietveld refinement attributes this enhanced confinement to reduced Bi/Sb–I–Ag dihedral angles and significantly increased octahedral distortions. Thus, it is found that geometric confinement, rather than coulombic binding alone, governs exciton longevity and broadband emission in these layered double perovskites, offering a useful design principle for stable, lead‐free emissive materials.