Ultralow Dark Current (6 fA at 1 V) in Quasi-Two-Dimensional CsGaGeSe 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 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 A cm) 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. 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.