Improved performance of uncapped Al2O3 and local firing-through Al-BSF in Bi-facial solar cells

Silicon solar cells that dominate today´s market are H-pattern cells based on p-type silicon wafer material with a full Al Back Surface Field (BSF) as rear contact. ECN´s rear passivated bi-facial PASHA (Passivated on all sides H- pattern) and ASPIRe (All Sides Passivated and Interconnected at the Rear, MWT) concepts answer the market pressure to decrease the euro/watt price and increase the efficiency. For optimized cells we estimate 0.5-0.8% absolute higher cell efficiencies compared to the industrial standard due to better rear passivation and reflection, while thinner wafers <;150um) can be processed with limited yield loss. In addition, Al paste consumption can be reduced by 50-70% owing to the open rear metallization. Here we report on the improved performance of PASHA cells passivated by an uncapped Al₂O₃ layer on the rear, through which Al paste is fired for contact and local aluminum BSF formation. The Al₂O₃ dielectric layer is deposited in the Levitrack, an industrial-type system for high-throughput Atomic Layer Deposition (ALD) developed by Levitech. On Cz and mc material, a gain in J_scxV_oc of 1% and 2.5% respectively is obtained compared to the reference, at a rear metal fraction of 30%. Localized IQE mapping shows that the passivation quality of the Al₂O₃ passivation layer is maintained after firing which is a major improvement as compared to our previous report. Furthermore, reliability tests on single cell laminates (Cz cells) suggest that the passivation layer remains stable during the lifetime of a module.