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Lionel Gérard

II-VI semiconductor nanostructures with type-II band alignment for photovoltaics

Published on 17 December 2013
Thesis presented December 17, 2013

This work focuses on the study of II-VI semiconductor heterostructures with type II band alignments, especially in the form of superlattices. This is a system that can be promising for photovoltaic applications, and my work is presented in this perspective. Thus the first part deals with a conceptual reflection on the contribution of type II interfaces for photovoltaics. In a second step I present a study on the growth of CdSe and ZnTe by molecular beam epitaxy on various substrates. These materials are particularly interesting and suitable for this application because they have a direct bandgap, are almost lattice-matched, present a type II band alignment, and CdSe shows a bandgap compatible with the solar spectrum. In addition, a structure based on a very short-period superlattice of these materials allows to create a solar absorber with adjustable properties which can approach the ideal absorber, and with intrinsically fitted contacts. But in return we have to deal with binary semiconductors which have no atoms in common, so that the growth of samples with specific thicknesses close to the monolayer is challenging. For this reason we conducted a detailed study at the interfaces through analysis of X-ray diffraction and transmission electron microscopy, which allows us to conclude on the chemical nature of the atoms near the interfaces. This is followed by a detailed spectroscopy study on the effects of type II interfaces on the charge carriers through their energy and recombination dynamics. We have developed an analytical model that allows to precisely adjust all the features observed in relation to these interfaces, and shows a very efficient charge separation mechanism. We show later that these effects are inherent characteristics of all interfaces of type II, regardless of materials and structures, and that they allow us to accurately extract the values of band offsets between different materials with type II band alignments. The last chapter finally deals with the development of photovoltaic cells based on the concepts studied in this thesis. It addresses in particular the device structures we thought of and our first results of efficiency measurements from solar cells formed by semiconductor heterostructures with this type of band alignment.

nanophysics, semiconductors, photovoltaic energy, growth, molecular beam epitaxy, time-resolved spectroscopy

On-line thesis.