Franz-Josef Haug

Biography

I studied Physics between 1990 and 1996 at the Universities of Ulm (Germany) and Waikato (New Zealand). While specializing mostly on Theoretical Physics and Quantum Theory during my studies, I turned towards Experimental Physics for a graduate work on gas-phase epitaxy of silicon. After that, I pursued a PhD program at ETH Zürich (Switzerland), studying Cu(In,Ga)Se2 solar cells. As PostDoc, I went to EMPA (Switzerland) to work on plasma processes for hard coatings of TiN and to the Jülich Research Centre in Germany to study light scattering ZnO front contacts for solar cells. In 2005 I joined the Institute of Microengineering (IMT) at the University of Neuchatel to lead a research group on thin film silicon solar cells within the PV-Lab. In 2009 the IMT was integrated into EPFL. My main research interests are optics, charge transport in semiconductors and renewable energy in general. My current work is devoted to passivating contacts for crystalline silicon solar cells, using thin silicon films as contact layers. In 2015 I was appointed Maitre d'Enseignement et Recherche (Senior Lecturer and Scientist) and member of the Faculty of STI. From 2013 to 2015 I was member of the EFPL Teachers Council (CCE) and from 2016 and 2018 I was elected into the Council of the Engineering School (CF-STI). In 2020 I was elected to become a member of CF-STI. My main objective for the legislature is to represent the role of EPFL's intermediate staff and to raise the awareness for concerns of the satellite campuses.

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We demonstrate the beneficial effect of a pre-annealing step prior to the boron diffusion on passivation and contact resistivity of industrially LPCVD deposited poly-Si/SiOX hole-selective contacts. We investigate the influence of the pre-annealing temperature on passivation quality, measured as implied open-circuit voltage and recombination current density, and on changes in crystallinity, characterized by Raman spectroscopy. A clear increase in passivation quality is observed on planar and textured surfaces as well as for various poly-Si thicknesses (100-230 nm) and thermal SiOX growth temperatures (600-800 degrees C). On planar surfaces and without the use of atomic hydrogenation, we report an increase in iVOC of around 5 mV with every additional increase of pre-annealing temperature by 50 degrees C (>900 degrees C) leading to an iVOC of 720 mV (J0 = 9.3 fA/cm2). After atomic hydrogenation, the effect of the pre-annealing is less pronounced. Nevertheless a gain in iVOC (reduction in J0) of 5-10 mV (2-5 fA/cm2) is achieved when comparing samples without pre-annealing with samples after a pre-annealing at 1050 degrees C. On textured surfaces on the other hand, this trend is more pronounced after atomic hydrogenation, for which a pre-crystallisation at 1050 degrees C leads to an iVOC (J0) of 705 mV (16.8 fA/cm2), which is a gain (reduction) of 24 mV (21.7 fA/cm2) compared to samples without a pre-annealing step.

ELSEVIER2023

Bulk Defects and Hydrogenation Kinetics in Crystalline Silicon Solar Cells With Fired Passivating Contacts

Christophe Ballif, Franz-Josef Haug, Andrea Ingenito, Mario Joe Lehmann, Audrey Marie Isabelle Morisset, Père Roca i Cabarrocas, Philippe Wyss

In this article, the effect of the various processing steps during the fabrication of c-Si/SiOx/SiCx fired passivating contacts on the silicon bulk lifetime is studied, and the kinetics of defect deactivation by hydrogenation is investigated. It is found that the firing step at 800 degrees C induces shallow bulk defects in float-zone silicon wafers, which can subsequently be passivated with hydrogen provided by an a-SiNx:H/D reservoir layer upon annealing at 450 degrees C. Experimental results and numerical data treatment indicate a rapid passivation of the surface within less than 1 min, followed by a slower passivation of the shallow bulk defects. In situ lifetime measurements are consistent with a slow bulk lifetime improvement by showing similar lifetime evolutions for both p-type and n-type SiCx layers. The kinetics of the hydrogenation process seems to be limited by the available hydrogen supply at the c-Si/SiOx interface, rather than by its diffusion within the bulk of the wafer. Moreover, it is affected by the bulk doping as well as the SiCx layer thickness. Finally, it is shown that hydrogenation is also possible with an a-SiNx:H/D reservoir layer deposited on one side of the wafer only, although resulting in a lower passivation level (s similar to 700 mu s compared to similar to 1300 mu s for symmetrical samples), and slower kinetics (similar to 5 min compared to similar to 0.8 min).

IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC2022

Impact of rapid thermal processing on bulk and surface recombination mechanisms in FZ silicon with fired passivating contacts

Christophe Ballif, Franz-Josef Haug, Andrea Ingenito, Mario Joe Lehmann, Sofia Libraro, Audrey Marie Isabelle Morisset

We investigate bulk and interface recombination in crystalline silicon with p-type fired passivating contacts. These consist of a chemical oxide and an in-situ doped layer of SiCx with low carbon content. Following a rapid thermal annealing to activate the dopants in the SiCx layer, we observe the formation of bulk defect states close to one of the band edges at temperatures between 450 and 700 degrees C. At temperatures of 800 degrees C and above, these bulk defects are increasingly cured. At the same time, the surface passivation provided by the interfacial oxide is increasingly deteriorated at temperatures above 800 degrees C, eventually permitting the injection of a recombination current into the SiCx layer. Consequently, there is a trade-off for the effective minority carrier lifetime with an optimum between 800 and 830 degrees C. We develop a formalism that distinguishes recombination through shallow bulk defects from the recombination current at the surface in a plot of the effective lifetime tau(eff) over the ratio n/p.

ELSEVIER2022