Novel Lead-Free Cs2Ag0.75Bi0.75Ba0.5I6 Double Perovskite: Synthesis and Application as High-Performance Photodetectors

Perovskite photodetectors face two persistent bottlenecks in practical deployment: the toxicity of Pb-based systems and the poor oxidative stability of Sn-based alternatives. Among lead-free and tin-free Cs2AgBiX6 double perovskites, Cs2AgBiI6 offers a narrower bandgap (∼2.1 eV) than Cs2AgBiBr6 (∼2.2 eV), extending optical absorption further into the visible range; however, distortion of the [AgI6]5- and [BiI6]3- octahedra limits its phase stability. Here, we synthesize Cs2Ag0.75Bi0.75Ba0.5I6 via a solvent-free mechanochemical ball-milling route. ICP-MS confirms successful B-site substitution of Ba2+ at the target 0.5 site occupancy. XRD and stability tests show enhanced crystallinity and phase stability. The optimized device delivers a 250-fold reduction in dark current, a peak specific detectivity of 2.17 × 1010 Jones, and a sub-millisecond response time; it retains 95% of its initial performance after 30 days of storage. The device further exhibits a sub-bandgap photoresponse at 785 and 1064 nm, attributed to strong electron-phonon coupling in the iodide framework. As a representative application, the device serves as the receiver, recovering 500 kHz modulated signals without distortion; the receiver has a 3-dB bandwidth of 1.493 MHz.