Optimizing Annealing Steps for Crystalline Silicon Solar Cells with Screen Printed Front Side Metallization and an Oxide-Passivated Rear Surface with Local Contacts

Silicon solar cells that feature screen printed front contacts and a passivated rear surface with local contacts allow higher efficiencies compared to present industrial solar cells that exhibit a full area rear side metallization. If thermal oxidation is used for the rear surface passivation, the final annealing step in the processing sequence is crucial. On the one hand, this post-metallization annealing (PMA) step is required for decreasing the surface recombination velocity (SRV) at the aluminum-coated oxide-passivated rear surface. On the other hand, PMA can negatively affect the screen printed front side metallization leading to a lower fill factor. This work separately analyzes the impact of PMA on both, the screen printed front metallization and the oxide-passivated rear surface. Measuring dark and illuminated IV-curves of standard industrial aluminum back surface field (Al-BSF) silicon solar cells reveals the impact of PMA on the front metallization, while measuring the effective minority carrier lifetime of symmetric lifetime samples provides information about the rear side SRV One-dimensional simulations are used for predicting the cell performance according to the contributions from both, the front metallization and the rear oxide-passivation for different PMA temperatures and durations. The simulation also includes recombination at the local rear contacts. An optimized PMA process is presented according to the simulations and is experimentally verified. The optimized process is applied to silicon solar cells with a screen printed front side metallization and an oxide-passivated rear surface. Efficiencies up to 18.1% are achieved on 148.8 cm(2) Czochralski (Cz) silicon wafers. Copyright (C) 2009 John Wiley & Sons, Ltd.