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.
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.
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