Quantifying Optical Disorder in CsFaPb(Br,I) ₃ Perovskites via Urbach Energy Analysis Using OPAL2 Fresnel‐Based Simulations

ABSTRACT This study examines light‐matter interactions in CsFAPb(Br, I) 3 perovskite thin films by analyzing their absorption, reflection, and emission properties using Fresnel‐based OPAL2 simulations. The optical spectra show sharp absorption edges at 1.55 – 1.60 eV for CH 3 NH 3 PbI 3 and triple‐cation perovskites, which blue‐shift to ∼1.8 eV with increasing bromide content. Urbach energies (0.082 – 0.410 eV) indicate rising energetic disorder as Br incorporation increases. Ray‐tracing simulations reveal strong optical confinement, with the CsFAPbIBr (1.62 eV) composition achieving optimal performance, where ∼ 66% of incident photons contribute to photogeneration and ∼26% are lost to reflection and parasitic absorption. Overall, the results demonstrate tunable bandgaps, efficient photon management, and a clear link between structural coherence, electronic disorder, and optoelectronic performance.