Direct soldering of screen-printed Al-paste layer on back-side of silicon solar cell using SnAg solder

Direct joining of Al back surface field (Al BSF) in a polycrystalline silicon solar cell using a green Sn-3.5Ag solder by the assistance of ultrasound was investigated. SEM, peel force, electrical resistance and open-circuit voltage (Voc) tests were used to study the effect of ultrasonic action time on the performance of the solar cell. The results show that with the increasing of ultrasonic action time, more Al particles in the paste residual layer dissolved into the solder layer. The dissolved Al existed in the bond metal as forms of α-Al and Ag-Al compound phases. The solder bonded directly with the Al-Si eutectic layer under ultrasonic action time of 6 s. The resistance of the joints was 1.27 mΩ and the peel force could reach as high as ∼1.44 N/mm. The Voc of solar cell was 524 mV, which was higher than the 467 mV of solar cell soldered with Ag electrode.