Person: Urrejola, Elias
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Aluminum-Silicon Contact Formation Through Narrow Dielectric Openings : Application To Industrial High Efficiency Rear Passivated Solar Cells
The research presented in this thesis addresses several insights into a deeper understanding of local contact formation during sintering of screen printed aluminum pastes with p-type silicon substrates. The physical observations showed that the contact resistivity of thin aluminum fingers depends on the dielectric opening areas where the Al-Si alloy is formed. Contrary to our expectations, the shallowest dielectric barrier opening resulted in the lowest contact resistivity of 8 mOhmcm2. On solar cell level a reduction of the contact area for screen printed Al fingers led to a reduction of the series resistance losses. At the same time, narrow Al-Si alloy formations minimized the impact of the openings on the optical properties of the rear side, increasing the dielectric passivated area below the contacts, and reducing the short circuit current and open circuit voltage losses.
The study presented in this thesis contributed to the understanding of diffusion of silicon through dielectric openings into the aluminum thick layer. For the first time, the spread limit of silicon in a screen printed aluminum layer was determined, and it was found that its value on each side of the dielectric opening does not depend on the contact area size but rather on the firing temperature. The spread limit of silicon in the screen printed thick aluminum layer is thus predicted to 75, 225, and 375 µm for temperatures of 750, 850, and 950 °C, respectively. Additionally, the formation of voids instead of an Al-Si eutectic layer was explained by the Kirkendall effect (diffusivity of Si is higher than diffusivity of Al in Al-Si alloys), and also depends on the contact spacing, aluminum paste amount, temperature and cooling rate, factors that limit the diffusion of silicon during the sintering in this type of alloys. It was also shown that gravity may strongly affect the local Al-Si eutectic morphology. The presence of voids was partially avoided by changing the gravity field orientation parallel to the normal solidification direction of the solid/liquid phase (i.e. by sintering the solar cells front side down). Several suggestions to minimize the presence of voids in the alloy were presented which should lead to a better formation of the local back surface field (extremely important for the solar cell performance). The results presented may be applied to the sintering of screen-printed pastes on solar cells in order to reduce series resistance losses due to a better local back surface field formation. Thus, the understanding and avoiding of the well known problem of voids was thoroughly analyzed. Furthermore, simple low cost industrial processes with optimized rear local contacts for the fabrication of PERC solar cells for industrial application were presented, leading to high efficiency gains. By the end of the thesis, an all PECVD-based rear surface passivation was used as an alternative to thermal oxidation, saving processing costs and minimizing the thermal budget for the multicrystalline substrate. The use of industrial accessible equipment and processing, such as screen printing for metallization and PECVD deposition for antireflection coatings, showed the high potential of those concepts to be incorporated into existing industrial cell lines. The results presented in the field of contact formation are supported by the high efficiency results that were achieved. This thesis presents advancement in applying the rear passivated solar cell concept in industrial production.
High Efficiency Industrial PERC Solar Cells with all PECVD-Based Rear Surface Passivation
In the photovoltaic industry both the reduction of the silicon material thickness and the increase of the solar cells efficiency are critical topics for cost reduction. This work presents an innovative and industrially applicable Si-based passivation layer deposited by PECVD, with high passivation quality for the lowly doped p-type rear surface of PERC solar cells. Applying only high-throughput PECVD processes for surface passivation, the PERC process as presented in this work, might be completely feasible for industrial applications. We achieved efficiencies of 19% on monocrystalline p-type Cz-Si and above 17.2% on multicrystalline p-type wafers. The processing of the wafers is developed at a standard industrial level, and no special equipment or processing was required for achieving this efficiencies.
Influence of the Al-Si Alloy Formation in narrow dielectric barrier openings on the specific contact resistance
A full area Al-alloyed back surface field layer usually forms the rear side of standard p-type Si solar cells. However, a dielectric rear surface passivation with only small local contact openings has significant advantages over the standard fully covered Al back contact, and enables higher efficiencies on thinner wafers. This article presents a specific analysis of the formation of small localized contacts between Al and Si. We observe that the contact resistivity of screen printed Al fingers depends on the homogeneity of the Al-Si alloy formation below the contacts. The contact resistivity decreases when reducing the contact area, due to a more homogeneous alloy formation. The optimal contact formation is achieved with contact areas smaller than 50 – 80 mm in diameter.
Silicon diffusion in aluminum for rear passivated solar cells
We show that the lateral spread of silicon in a screen-printed aluminum layer increases by (1.50 +- 0.06) µm/°C, when increasing the peak firing temperature within an industrially applicable range. In this way, the maximum spread limit of diffused silicon in aluminum is predictable and does not depend on the contact area size but on the firing temperature. Therefore, the geometry of the rear side pattern can influence not only series resistance losses within the solar cell but the process of contact formation itself. In addition, too fast cooling lead to Kirkendall void formations instead of an eutectic layer
Al–Si alloy formation in narrow p-type Si contact areas for rear passivated solar cells
For high efficiency silicon solar cells, the rear surface passivation by a dielectric layer has significant advantages compared to the standard fully covered Al back-contact structure. In this work the rear contact formation of the passivated emitter and rear cell device structure is analyzed. Contrary to expected views, we found that the contact resistivity of fine screen printed Al fingers alloyed on narrow p-type Si areas depends on the geometry of the Al–Si alloy formation below the contacts, and decreases by reducing the contact area, while the contact resistance remains constant. At the solar cell level, the reduction in the contact resistivity leads to a minimization of the fill factor losses. At the same time, narrow Al–Si alloy formations increased the passivated area below the contacts, improving the optical properties of the rear side, reducing the short-circuit current and open-circuit voltage losses. Scanning electron microscopy analysis of the Al–Si alloy geometry is performed, in order to understand its influence on the contact resistivity. The analysis presented in this article has application in Al–Si alloying processes and advanced solar cells concepts, like back-contact and rear passivated solar cells.
POCl3 diffusion with in-situ SiO2 barrier for selective emitter multicrystalline solar grade silicon solar cells
We present an innovative process for the formation of a selective emitter by using an advanced phosphorous glass as a barrier layer against subsequent diffusion. The advanced barrier glass was achieved by the formation of a standard phosphorous glass treated with additional thermal oxidation immediately after deposition in the same process tube. The resistant layer is used as a barrier for the second diffusion after selective opening of the finger contact areas by screen printing of a SiO2 etching paste. The process was applied for multicrystalline Elkem Solar SoG-Si wafers as well as for references from standard EG-Si. The achieved cell parameters were compared with cell results from a homogenous emitter process. While the efficiency was not enhanced so far due to the relatively high shadow loss of the selective emitter solar cells, the open collector voltage could be increased by up to 9 mV and the short wavelength spectral response increased slightly with this selective emitter. The aim of this work was to combine the SiO2 barrier with the shallow POCl3 diffusion in one process step and the optimization of the screen
printing process for selective emitter solar cells.
Effect of gravity on the microstructure of Al-Si alloy for rear-passivated solar cells
We study the influence of the gravity field orientation on the microstructure of Al–Si forming alloy. Due to the difference between the diffusivity of aluminum and silicon, Kirkendall void formations are normally found at the back of rear-passivated solar cells instead of an eutectic layer. We show that the voids may partially be avoided by sintering the samples with the solid/liquid interface oriented opposite to the direction of the gravity field. A local back-surface-field is found underneath the voids using this approach. This phenomenon strongly applies to rear-passivated so- lar cells, which exhibited strong fill factor losses.
Proceedings of the 2nd Workshop on Metallization for Crystalline Silicon Solar Cells : Status, trends and new directions
Die Metallisierung von kristallinen Siliziumsolarzellen ist einer der Schlüsselprozesse in der Produktion von Hochleistungssolarzellen bei möglichst geringen Kosten. Obwohl rund 85% der weltweit hergestellten Solarzellen das etablierte Siebdruckverfahren nutzen, wird derzeit mit Hochdruck an der Weiterentwicklung desselben und an Alternativen geforscht. Diese Entwicklung wird zum Einen davon getrieben, dass die Herstellungskosten sinken, die Effizienz von Solarzellen aber steigen soll. Zum Anderen verlangt die Weiterentwicklung von etablierten als auch die Einführung von neuen Solarzellkonzepten neue Ansätze in der Metallisierung.
Um eine Plattform für Wissenschaftler aus Universitäten, Instituten und der Industrie für einen intensiven Austausch über dieses Thema zu schaffen, wurde mit Erfolg 2008 in Utrecht in den Niederlanden ein erster internationaler Workshop zum Thema Metallisierung von kristallinen Siliziumsolarzellen organisiert. Der zweite Workshop dieser Art fand am 14. und 15. April 2010 in Konstanz statt. 190 Spezialisten und Wissenschaftler aus der ganzen Welt trafen sich im Konzil, einem historischen Gebäude direkt am Bodensee, um die aktuellsten Entwicklungen auf diesem Gebiet vorzustellen und zu diskutieren.
In diesem Tagungsband werden ausgewählte Beiträge zum zweiten Metallisierungsworkshop in Form wissenschaftlicher Artikel veröffentlicht. Diese Beiträge wurden vom wissenschaftlichen Komitee ausgewählt und begutachtet. Das wissenschaftliche Komitee bestand aus international bekannten und etablierten Experten auf dem Gebiet der Metallisierung von kristallinen Siliziumsolarzellen.
Auf dem Workshop wurde deutlich, dass die etablierte Technik der Siebdruckmetallisierung durch aktuelle Innovationen es neuen Ansätzen nach wie vor schwer macht, eine gewichtige Rolle in der industriellen Produktion von kristalline Siliziumsolarzellen zu spielen. Kurzfristig könnte eine Hybridtechnologie aus Silber-Saatschicht Aufbringung und Silber-Plattierung an Bedeutung gewinnen, allerdings hat dieser Ansatz in den letzten zwei Jahren aufgrund der Fortschritte im Siebdruck etwas an Attraktivität verloren. Auf Kupfer basierende Metallisierungstechniken haben die größten Potentiale bezüglich Strukturbreiten, Solarzellenperformance und Kosten, allerdings sind noch einige wichtige Hürden zu nehmen, bevor diese Techniken im großen Maßstab eingesetzt werden können.