Current bottlenecks for industrialization of Al2O3 deposited by Atomic Layer Deposition (ALD) for crystalline silicon solar cell applications are low growth rate and stability of thin and uncapped layers during co-firing. First results on the performance of a high throughput ALD prototype , the Levitrack, are presented. Excellent passivation properties have been obtained after firing, for 12 nm thick films deposited on p-Cz (2.3 Ω.cm) with Seff <15cm/s (∆n=3x10 15 cm-3). These layers are compatible with solar cells that operate at a maximum open-circuit voltage of 720mV. Furthermore, we report on the passivation of 20nm uncapped aluminum oxide layers on the rear of ptype mc-Si bifacial cells. LBIC measurements unveiled excellent passivation properties on areas covered by 20nm of Al2O3 characterized by an IQE of 91% at 980nm. Remarkably, these lifetime and cell results were obtained without lengthy post-treatments like forming gas anneal.
A high-throughput ALD system based on wafer transport in an atmospheric gas bearing was developed. Line-shaped TMA and H2O precursor zones are present in the direction perpendicular to transport takes place. Thin Al2O3 films of 4-24 nm were deposited at wafer speed of 160-230 mm/s (corresponding to a throughput of 2400 - 3600 wph). After annealing and firing the films show good passivation. Effective minority carrier lifetimes measured on 2.3 ohm cm p-type c-Si wafers at excess carrier density of 1E15 cm-3 reach values of 0.3-2.5 ms depending on surface preparation, annealing steps and film thickness. No degradation of films during firing was observed and the lifetime values remain stable in time. The Levitrack system presented in this paper is expected to meet the requirements of the PV industry.
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.
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