In the quest of thin-film devices for low-cost third-generation photovoltaics with higher efficiencies, the wide bandgap semiconductor ZnO acts as a highly interesting alternative to TiO2 in dye-sensitized solar cells, also known as Grätzel cells. More specifically, due to its higher electron mobility, crystalline ZnO in the form of nanowire arrays attract considerable interest for a direct use in the cell photoelectrochemical anode. It can also be used in inorganic photovoltaic devices based on core-shell nanostructures, in which ZnO nanowires constitute the core whereas the shell is defined by another II-VI semiconductor compound such as CdTe. In both cases, the ZnO nanostructures are grown on transparent conductive oxide (TCO) substrates such as fluorine-doped tin oxide or ZnO itself. These materials, which are used as front electrodes, remarkably exhibit the combination of high electrical conductivity and optical transparency to visible light.

The objectives of the present work consist in the fabrication of solar cells based on nanostructures made of the above-mentioned materials and in the understanding of the physical mechanisms that govern their properties. ZnO nanowires will be grown either by metal-organic chemical vapor deposition or by chemical bath deposition. The structural properties and the optical characteristics of the fabricated materials will be investigated using scanning and transmission electron microscopy, X-ray diffraction, ultra-violet and visible absorption and photoluminescence. Photovoltaic devices will be ultimately fabricated and tested under standard illumination conditions in order to obtain the basic cell properties (efficiency, fill factor, ...). In addition to the standard characterization methods mentioned here above, the technique of admittance spectroscopy will be developed in order to gain more details into the electrical properties of the solar cell. The purpose of this approach is to broaden the understanding of the microscopic mechanisms which take place at the various interfaces and which play a critical role on the photovoltaic properties of the fabricated solar cells. Ready-to-use representations of the material systems in terms of equivalent electrical circuits can be conveniently used in the process of performance optimization. They will also contribute to the interpretation of the experimental curves, which can be supported by numerical simulations of the solar cell structure.

This PhD thesis will be performed within a close collaboration between the LMGP (Laboratoire des Matériaux et du Génie Physique) in Grenoble (Grenoble INP), France, and the Laboratory for solid-state physics, interfaces and nanostructures of the University of Liège (Department of Physics), Belgium. The growth and the structural and optical characterization will be performed in Grenoble while the electrical analysis by admittance spectroscopy will be carried out in Liège. Imec, the industrial partner, is a world-leading research center for nano-electronics located in Leuven, Belgium. It leverages its scientific knowledge with the innovative power of its global partnerships in information and communication technology, healthcare and energy.

Project Partners

Grenoble INP (France), Laboratoire des Matériaux et du Génie Physique (LMGP)
Fabrication of nano-structured materials such as TCO (F:SnO2, ZnO), ZnO nanowires and solar cells; physical and structural characterization.

University of Liège (Belgium), Department of Physics, Solid-State Physics, Interfaces and Nanostructures
Electrical characterization by admittance spectroscopy; opto-electrical and optical characterization; numerical simulation of semiconductor heterostructures.

Imec, Leuven (Belgium)
World-leading research center in nano-electronics providing industry-relevant technology solutions in ICT, healthcare and energy.