Water adsorption enhances electrical conductivity in transparent p-type CuI

TL;DR

This study demonstrates that moisture exposure significantly increases the electrical conductivity of CuI, a transparent p-type conductor, likely due to hydration effects at grain boundaries, with implications for its stability and performance.

Contribution

It reveals the impact of ambient humidity on CuI's electrical properties and suggests hydration at grain boundaries enhances hole mobility, a novel insight for transparent conductors.

Findings

Conductivity doubles after 5 hours of humidity exposure.

Work function decreases by nearly 1 eV with humidity.

Hydration of grain boundaries may improve hole mobility.

Abstract

CuI has been recently rediscovered as a p-type transparent conductor with a high figure of merit. Even though many metal iodides are hygroscopic, the effect of moisture on the electrical properties of CuI has not been clarified. In this work, we observe a two-fold increase in the conductivity of CuI after exposure to ambient humidity for 5 hours, followed by slight long-term degradation. Simultaneously, the work function of CuI decreases by almost 1 eV, which can explain the large spread in the previously reported work function values. The conductivity increase is partially reversible and is maximized at intermediate humidity levels. Based on the large intra-grain mobility measured by THz spectroscopy, we suggest that hydration of grain boundaries may be beneficial for the overall hole mobility.