Tuning Excitonic Properties and Charge Carrier Dynamics by Halide Alloying in Cs ₃ Bi ₂ (Br ₁− x I x ) ₉ Semiconductors

The perovskite‐inspired bismuth halide semiconductor Cs 3 Bi 2 Br 9 is widely investigated as a photoactive material for light‐conversion applications. However, charge generation and separation are inherently limited by its modest sunlight absorption and strong exciton binding energy, respectively. Here, we demonstrate that both the light absorption and exciton dissociation are improved by controlled substitution of Br − with I − via mechanochemical synthesis of Cs 3 Bi 2 (Br 1− x I x ) 9 . X‐ray diffraction and Raman analyses confirm atomic‐level halide mixing and reveal a crystallographic phase transition near x = 0.8. From absorption measurements on thin films, we determine the absorption coefficient, Urbach tail, and exciton binding energy for several Cs 3 Bi 2 (Br 1− x I x ) 9 compositions. From here, we find that the bandgap can be tuned from 2.59 to 1.93 eV (for x = 0.9), while exciton binding energies reach a minimum at x = 0.6. Finally, transient absorption spectroscopy measurements suggest a weak correlation between recombination lifetime and Urbach energy, where the longest lifetimes are observed for the materials with the lowest disorder. These results offer valuable insights for designing stable bismuth halide semiconductors with favorable light absorption properties and charge carrier dynamics.