The Effect of Trap States on the Optoelectronic Properties of Nanoporous Nickel Oxide

Nickel oxide (NiO) is often used as a hole-transporter material in both photovoltaic and photoelectrochemical solar cells. As a result of the reversible nickel(II)/(III) transformation, it is also electrochromic. These potential-dependent optoelectronic properties of this intriguing material, however, are yet to be fully understood. In this article, we show that the picture is more complicated than the generally discussed nickel(II)/(III) transformation, because of the presence of trap states. We reveal that the density of states is directly influenced by the applied potential in nanoporous NiO films; and show how it manifests in the electrical properties and Raman spectral features. We demonstrated that the population/depopulation of shallow trap states has an important role in dictating these changes. The presented insights can also contribute to the better understanding of the optoelectronic properties of different semiconductor electrodes under charging conditions. (C) The Author(s) 2019. Published by ECS.