Negative Photoconductivity of Ag-Poly(heptazine imide)

Poly-(heptazine imide) (PHI), a covalent organic framework containing metal ions, is a novel, visible-light-driven photocatalyst. The physical properties of PHI vary depending on the type of metal ion. The photoresponse current of Ag-PHI exhibits negative photoconductivity (NPC), in contrast to PHIs containing many other metals. This is the first PHI material exhibiting NPC, which is extremely rare among organic semiconductors. Ag nanoparticles (Ag-NPs) produced by white-light irradiation of Ag-PHI are essential in the NPC mechanism. Ag⁺ released from the PHI structure aggregates into Ag-NPs under photoirradiation, with the formation of Ag-PHI black. The color of Ag-PHI black is black due to the light absorption of the surface plasmon resonance of the Ag-NPs it contains. The Ag-NPs in Ag-PHI black trap the photogenerated holes, reducing the number of carriers and forming an electron injection barrier at the electrode interface, thereby decreasing the current. Upon discontinuing photoirradiation, the holes trapped in the Ag-NPs were released, and the current increased gradually. The proposed mechanism not only explains the NPC exhibited by Ag-PHI but also provides important insights for developing organic semiconductors that exhibit NPC.