Efficiency gain for Bi-facial multi-crystalline solar cell With uncapped Al2O3 and local firing-through Al-BSF

The p-type bi-facial cell concept, p-PASHA (Passivated on all sides H-pattern), is developed at ECN and employs an uncapped Al₂O₃ passivation layer on the rear through which a screen printed H-pattern of aluminium contacts is fired. Here we report a net gain in cell efficiency of 0.2% absolute for the p-PASHA cell vs. industrial reference with the addition of a clean and an ALD step. Even higher gains up to 0.5% abs. are expected after optimization of the cell design and process. Apart from the efficiency gain, the bi-facial cell concept allows for 50-80% reduction in Al paste consumption, the use of thinner wafers, and consists of less processing steps compared to prevalent PERC concepts. The Al₂O₃ dielectric layer is deposited in the Levitrack, an industrial-type system for high-throughput Atomic Layer Deposition (ALD) developed by Levitech. The efficiency gain is obtained on multi-crystalline wafers, at a rear metal fraction of 40%. Localized IQE mapping, cross-sectional SEM investigation, resistance measurements and 2D simulation relate the efficiency improvement compared to our conventional process to better eutectic and BSF formation at the Al contact edges.