Abstract
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.
Highlights
There are many examples in the literature where in situ techniques helped to scrutinize the mechanism of electrochemical processes.[1,2,3,4,5] Combined methods provide independent information on either the redox process itself, or about the accompanied spectral, mass, or electronic changes
We demonstrated that the optical and electronic changes are caused by the redox transformation of Nickel oxide (NiO) together with the reversible population and depopulation of trap states
The bandgap, which was determined from diffuse reflectance spectrum using Tauc conversion for an indirect transition, gave a value of 3.50 eV (Figure S1), in close agreement with values reported in the literature.[24]
Summary
There are many examples in the literature where in situ techniques helped to scrutinize the mechanism of electrochemical processes.[1,2,3,4,5] Combined methods provide independent information on either the redox process itself, or about the accompanied spectral-, mass-, or electronic changes. We present how in situ spectroelectrochemistry and electrochemical impedance spectroscopy can contribute to better understanding the optoelectronic features of nanoporous NiO electrodes Using these methods, we demonstrated that the optical and electronic changes are caused by the redox transformation of NiO together with the reversible population and depopulation of trap states. We demonstrated that the optical and electronic changes are caused by the redox transformation of NiO together with the reversible population and depopulation of trap states These results might provide insights which point beyond NiO, and trigger similar studies with other semiconductor electrodes. The formed nickel oxide hydroxide films were heat treated at 300°C for an hour in air, which resulted in NiO on the electrode surface.[25,26]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
