Solar Hydrogen (completed)
This project explores the possibilities of Zn(Cd)O as a photoanode material in photoelectrochemical cell (PEC). On this purpose, various photoanode structures are investigated by optical and electrochemical means with the objective of increasing the efficiency of PEC cells.
About the project
Secure and abundant energy supply is one of the most important challenges for the global society development on our planet. Hydrogen is the preferable base of an environmentally acceptible energy system, which will be able to eventually replace the fossil fuels on a time scale of 20-50 years. This is because of its advantages for use in mobile applications, energy storage and electrical net load balancing.
There is a consensus in literature that the most promising method of hydrogen generation using a renewable energy source is based on solar photoelectrochemical (PEC) water decomposition and the overall objective of our project is to acquire a better understanding of the mechanisms behind PEC water splitting focusing primarily on the band gap and doping engineering in the multilayered semiconductor electrodes. The properties of photoelectrodes should satisfy several specific requirements in terms of semiconductor, electrochemistry and chemical stability. Our idea is to develop monolithic oxide multilayered (MOM) electrode concept. In the MOM electrode different oxide components provide the necessary functions, like corrosion stability or efficient light absorbtion, simultaneously minimizing the risk associated with the formation of isolating layers at the interfaces. One approach for MOM electrodes is to combine a chemically stable protecting layer (for example TiO2 based) thin enough to exhibit carrier tunnelling properties with a CdO/ZnO/MgO multilayer light absorbing structure.
The project is a part of the SolarH2-network established on behalf of the Nordic Energy Research Association (N-INNER).
Our specific objectives are:
- to investigate the optimal profiles of Mg, Zn and Cd through the MOM structure
- to study electronic doping issues in Zn(MgCd)O
- to study the tunnelling properties of the TiO2 combined with the corrosion resistivity
Due to its outstandig properties, zinc oxide (ZnO) is a promising semiconductor for optoelectronic devices including photoelectrochemical (PEC) applications. For many devices, it is favorable to mix ZnO with cadium oxide (CdO) and magnesium oxide (MgO) in a controlled way in order to tune the absorption range and thus to cover a broad range of the solar spectrum. The performance of ZnO-based devices may be increased even further by fabricating mulitlayer (ML) structures of different ZnCdO layers which allow for a wide absorption range and thus increased efficiency. However, certain complications may arise when alloying ZnO with MgO or CdO leading to defects in the crystals.
On the course of this project, defects in ZnMgO have been investigated and a model has been proposed explaining the behavior of the defect related luminescence in these films. Understanding the defects may help to eliminate them and therewith improve the film quality. In addition, ZnCdO ML structure have been analyzed. By optical measurements the absorption of light in every of the single layers could be confirmed. Accordantly, priliminary PEC measurements show a greater photocurrent in the ML structure than in single films which is very . There is ongoing efforts to improve the performance of the devices even further by investigating the influence of the TiO2 protection layer on top of the ZnCdO films as well as by nanostructuring the films. Recent studies on the fabrication of graded ZnCdO nanowires show promising luminescence comparable to the results obtained from the ML structure.
The Research Counsil of Norway