Stéphanie Parola

Influence of the embedding matrix on optical properties of Ge nanocrystals-based nanocomposite

Germanium nanocrystals were prepared by a nanocluster source and characterized by photoluminescence and spectroscopic photometry methods. The optical measurements were carried out in order to estimate the effective bandgap of the Ge nanocrystals. Both Mie theory and the quantum confinement theory were applied to interpret the extracted absorption data. We found that the quantum confinement theory enables to explain the nanocrystal size and the host matrix dependence of the nanocrystal bandgap. On the other hand, the photoluminescence measurements have not allowed to confirm the bandgap evaluated from absorption data. This is explained as due to the dominant effect of the recombination at the nanocrystals surface.

Time-resolved photoluminescence for self-calibrated injection-dependent minority carrier lifetime measurements in silicon

Time-resolved photoluminescence (TRPL) was investigated on passivated silicon wafers under modulated square-wave laser illumination. It is shown that time-correlated single-photon counting can be used to record the transient signals on silicon wafers with doping levels commonly used for photovoltaic applications. This article reports the self calibrated evaluation of the injection-dependent effective minority carrier lifetime from the TRPL measurements. The method only requires knowing the doping level, the incident laser power, the reflection coefficient and the sample thickness. TRPL results were found to be in good agreement with photoconductance lifetime measurements. The effect of the surface recombination velocity on the generation of the PL signal was shown experimentally and discussed with PC1D calculations.

Optoelectronic properties of p-i-n heterojunctions based on germanium nanocrystals

We investigated the possibility of using physical vapour deposited Ge nanocrystals (NCs) in optoelectronic devices such as solar cells. We have prepared p-i-n heterojunctions based on p+-doped Si substrate/undoped Ge NCs/ZnO:Al layer stacks and their optoelectronic properties were characterised. Under light, the generation of photo-carriers from the Ge NCs themselves was demonstrated. The photovoltaic behaviour of the p-i-n structure was also highlighted, with a measured Voc of 224 mV compared to 580 mV in theory. The discrepancy between theory and experiment was discussed on the basis of TEM observations, optical and carrier generation measurements as well as modelling.