Improved performance of uncapped Al<inf>2</inf>O<inf>3</inf> and local firing-through Al-BSF in Bi-facial solar cells

Abstract

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 Al 2 O 3 layer on the rear, through which Al paste is fired for contact and local aluminum BSF formation. The Al 2 O 3 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 sc xV 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 Al 2 O 3 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.