Aluminum-Silicon Contact Formation Through Narrow Dielectric Openings : Application To Industrial High Efficiency Rear Passivated Solar Cells

Abstract

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.