Atomic‐Level Interstitial Engineering Programs Sub‐Bandgap States for Energetically Matched Charge Transfer in Cs 3 Bi 2 Cl 9 Enabling Selective and Sensitive H 2 S Sensing

ABSTRACT Constructing energetically matched charge‐transfer pathways between molecules and solid‐state materials is essential for achieving selective interfacial reactions, yet remains challenging at room temperature. Here, we introduce an atomic‐level electronic‐structure modulation strategy in which Cu + interstitials are incorporated into lead‐free Cs 3 Bi 2 Cl 9 , generating Cu‐3d‐dominated sub‐bandgap states that act as efficient electronic intermediates. These tailored states narrow the bandgap, increase the local density of states, and achieve near‐resonant alignment (ΔE ≈ 0.11 eV) with the redox level of H 2 S, establishing a highly favorable charge‐transfer pathway. Using H 2 S as a model molecule, the optimized Cu 0.10 ‐CBC exhibits intrinsically selective interaction, enabling a ppt‐level theoretical limit of detection (730 ppt), an experimentally validated detection limit of 5 ppb, fast response/recovery (50/90 s), and excellent stability over 125 days. In situ DRIFTS spectroscopy, DFT calculations, and Bader charge analysis reveal that interstitial Cu simultaneously serves as a high‐affinity adsorption center and electron‐pumping site, promoting molecular activation and interfacial redox conversion. Flexible Cu 0.10 ‐CBC devices further maintain robust performance under mechanical deformation and enable accurate breath‐H 2 S quantification. This work establishes a generalizable sub‐bandgap engineering paradigm for programming charge‐transfer energetics in halide perovskites, offering a materials‐centric foundation for selective chemical interfaces and low‐power sensing technologies.