Ultralow Dark Current (6 fA at 1 V) in Quasi-Two-Dimensional CsGaGeSe 4 Single Crystals for High Signal-to-Noise Ratio Photodetectors under Strong Electron Localization.

To satisfy the demands of photodetectors for weak-light detection, materials selected for device fabrication should have an extremely low background carrier concentration to suppress the dark current of devices. In this work, a new quasi-two-dimensional CsGaGeSe 4 single crystal with an extremely low background carrier concentration was synthesized by a co-solvent reaction based on which a photoconductive detector was prepared with an ultralow dark current density (6 fA at 1 V and ∼10 -10 A cm -2 ) and a high response speed (∼0.74 s) was achieved, presenting a great potential of being applied to the field of weak-light detection. The ultralow dark current density originates from both the good crystal quality and the strongly asymmetric band structure of CsGaGeSe 4 . In the darkness, electrons locally bound in the valence band bring an ultralow dark current density; after illumination, the electrons transiting to the conduction band will participate in the conduction in a non-localized state, resulting in a high signal-to-noise ratio. This work not only provides a new choice of potential materials for weak-light detection but also proposes an effective strategy for material selection.