# Negative Photoconductivity of Ag-Poly(heptazine imide)

## TL;DR

This paper reports a new material, Ag-PHI, that shows rare negative photoconductivity due to silver nanoparticles formed under light.

## Contribution

The first PHI material exhibiting negative photoconductivity, with a novel mechanism involving Ag nanoparticles.

## Key findings

- Ag-PHI shows negative photoconductivity due to Ag-NPs trapping photogenerated holes.
- Ag-NPs form Ag-PHI black, which absorbs light via surface plasmon resonance.
- Release of trapped holes after light stops increases current gradually.

## Abstract

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.