Passivation | EPFL Graph Search

vignette|Mécanisme de passivation d'un acier inoxydable : les atomes de chrome dans l'acier réagissent avec le dioxygène de l'air et forment une couche protectrice d'oxyde de chrome. La passivation ou passivité représente un état des métaux ou des alliages dans lequel leur vitesse de corrosion est notablement ralentie par la présence d'un film passif naturel ou artificiel, par rapport à ce qu'elle serait en l'absence de ce film. Dans la plupart des cas (aluminium, acier, acier inoxydable, titane…), ce film passif apparaît spontanément par oxydation, parce que l'oxyde formé sur la surface est insoluble et constitue un obstacle qui ralentit les processus ultérieurs. Dans un milieu aqueux, la formation de ce film est liée à un domaine de potentiel électrochimique ainsi qu'à un domaine de pH dans lesquels l'oxyde est stable. De ce fait, on s'attache à ce que le film passif se forme à l'air avant la mise en service de la pièce :

en fin de fabrication, on décape la pièce afin que rien ne

Evaluation of Cost-Effective Technologies for Highly Efficient Silicon-Based Solar Cells

Luca Gautero

The work underscores the feasibility of highly efficient silicon solar cell structures manufactured with high throughput machines. The main challenge consists in the implementation of more performant structures of intrinsic higher complexity. These structures are meant to be fabricated within similar constraints on time and on cost as for conventional devices of industrial manufacture. Achievements in this direction foster larger economical deployment of this kind of renewable energy. The structure chosen for the implementation of "high efficiency" concepts requires an advanced machining of the silicon substrate. Etching techniques, dielectric coating preparations, and metal to semiconductor contact formation were evaluated for their integration in a complete silicon solar cell fabrication sequence. These sequences were later tested to gain knowledge regarding the effects of the aforementioned advanced processing. Prototypes were created using the fabrication sequence. They were analysed to acquire a further understanding of the advanced processing influences. Statistical interpretation of the data obtained was used to support physical interpretation of the observed phenomena. For one particular implementation of a surface passivation (floating junction passivation) an attempt of modelling aiming to unveil its specific dynamics is reported. A resulting solar cell concept compatible with high throughput equipment achieved a certified efficiency of 18% (VOC = 635 mV, JSC = 37.3 mA/cm2, FF = 76 %) on substrate with a reduced thickness (120 µm). The resulting sequence prepares the back surface with a thermal oxidation for passivation purposes and with a laser technique to locally contact the bulk. The thickness is compatible with the intent of cost reduction through the decrease in material consumption. Other approaches performed on the same substrate achieved 17.1% efficiency (VOC = 617 mV, JSC = 36.1 mA/cm2, FF = 75 %). In this case the passivation was achieved with deposited dielectric layers on the back surface. Furthermore, the investigated dynamics of the floating junction passivation allows for better insight into its traits. The proposed solution has an interesting ratio of efficiency versus costs. The general consequence is a higher market appeal of this particular renewable source on the energy market. The study on the floating junction passivation allows a better exploitation of this particular implementation towards more performant silicon solar cells.

EPFL2010

Effects of surface oxide formation on germanium nanowire band-edge photoluminescence

Shruti Thombare

The effect of intentional surface oxide formation on band-edge photoluminescence (PL) of Ge nanowires was investigated. Thermal oxidation in molecular O-2 was used to produce a surface oxide layer on assemblies of single crystal nanowires grown by the vapor-liquid-solid method. With increasing oxidation of the wires, the band-edge PL associated with the indirect gap transition becomes more intense. X-ray photoelectron spectroscopy confirms the formation of an increasingly GeO2-like surface oxide under annealing conditions that enhance the indirect-gap PL, consistent with surface oxide passivation of nonradiative recombination centers initially present on the nanowire surface. (C) 2013 AIP Publishing LLC.

American Institute of Physics2013

3D multi-layer probe for application in neuroprosthetics

Marta Jole Ildelfonsa Airaghi Leccardi, Bastien Duckert, Vivien Gaillet, Diego Ghezzi

In neuroprosthetics, prostheses with higher and denser electrodes are needed to obtain both more precise recordings and targeted stimulations. The main limit is the routing of the active sites to the electronic control unit. A solution is to place electrodes and connecting traces on different layers. A multi-layer approach allows the upscaling of electrodes number, thus increasing also their density. Nevertheless, a drawback of the multi-layer system is the passivation layer that increases the gap from the exposed electrode to the target tissue, especially when polymeric materials are used, such as PI or PDMS. For the layers placed more at the bottom, this gap may increase to undesired values. Indeed, the electrode-cell distance is one of the most important parameter affecting the spatial resolution and the efficiency in recording and stimulation. We introduced a new 3D design and microfabrication process, allowing all of the active sites to reach or even protrude the implant surface and traces to be placed in multi-layers. Our device may be fabricated on flexible and soft substrates, with arbitrary number of layers and arbitrary height of active sites. Bi-layer polyimide neural probes with Pt electrodes reaching 6 µm above the encapsulation layer were produced; the electrode size is 100 µm. Electrochemical characterizations show no significant differences between the multi-layer 3D shaped electrodes and planar monolayer electrodes. The average impedance at 1 kHz is 80 kOhm.

2017

Evaluation of Cost-Effective Technologies for Highly Efficient Silicon-Based Solar Cells

Luca Gautero

EPFL2010