Light Activated Charge‐Ionic Transport in Cs ₂ AgBi ₂ I ₉ for Self‐Powered Synaptic Photodetection

ABSTRACT Self‐powered photodetectors and synaptic optoelectronic systems require materials in which charge transport, interfacial energetics, and temporal response are intrinsically governed by coupled electronic‐ionic dynamics. However, the light‐activated correlation between electronic and ionic processes remains inadequately understood in lead‐free perovskite‐inspired semiconductors. In this work, solution‐processed Cs ₂ AgBi ₂ I ₉ thin films are employed as a model platform to elucidate the interplay between photogenerated carriers and mobile ions under zero bias operation. Structural, optical, and density functional theory analyses indicate that Ag incorporation enhances electronic connectivity by increasing band edge delocalization. Photoexcited impedance spectroscopy resolves electronic and interface ionic contributions under illumination, enabling quantitative assessment of light‐induced ionic transport and its coupling to electronic conduction. Time‐resolved photoresponse measurements reveal distinct fast and slow relaxation processes associated with carrier transport and ionic motion, respectively. While Kelvin probe force microscopy directly evidences illumination‐driven interfacial potential modulation arising from ionic accumulation. These coupled charge‐ion processes manifest at the device level through broadband self‐powered photodetection, efficient X‐ray response, and pulse intensity‐dependent synaptic behavior. The results demonstrate that controlled charge‐ionic interactions provide a viable pathway to regulate interfacial transport and temporal response in self‐powered optoelectronic and neuromorphic devices.