Gettering in PolySi/SiO Passivating Contacts Enables Si-Based Tandem Solar Cells with High Thermal and Contamination...

Area of Science:

• Materials Science
• Photovoltaics
• Semiconductor Physics

Background:

• Tandem solar cells combining multijunction top cells with crystalline silicon (c-Si) bottom cells promise lower electricity costs.
• Direct monolithic integration of top cells onto c-Si is challenging due to high-temperature constraints and interfacial trade-offs affecting transmittance, electrical connection, and bottom cell degradation.
• PolySi/SiO2 passivating contacts offer a potential solution to mitigate degradation in the Si bottom cell.

Purpose of the Study:

• To investigate the use of polySi/SiO2 passivating contacts to protect c-Si bottom cells during top cell fabrication in tandem solar cells.
• To evaluate the effectiveness of tuning polySi/SiO2 stack thickness for contaminant gettering and preventing Si bottom cell degradation.
• To assess the performance of low-cost chalcogenide top cells (CZTS, CGSe, AIGSe) integrated with c-Si bottom cells using this passivation strategy.

Main Methods:

• Fabrication of tandem solar cells with c-Si bottom cells and Cu2ZnSnS4 (CZTS), CuGaSe2 (CGSe), or AgInGaSe2 (AIGSe) top cells under harsh sulfur or selenium atmospheres (>550 °C).
• Tuning the thickness of heavily doped polySi layers in the polySi/SiO2 passivating contacts from 40 nm to 400 nm.
• Measurement of Si carrier lifetime, contaminant concentration in the c-Si bulk, and tandem solar cell efficiency.

Main Results:

• Increasing polySi layer thickness up to 400 nm prevented a significant reduction in Si carrier lifetime (maintained >500 μs) over large areas (up to 20 cm²).
• A 99.9% reduction in contaminant concentration within the c-Si bulk was achieved, demonstrating the gettering capability of the thick polySi layer.
• The Si bottom cell resilience varied with top cell material: AIGSe > CGSe > CZTS, correlating with copper contamination and annealing temperatures. A CZTS/Si tandem achieved up to 7% efficiency.

Conclusions:

• Thick polySi/SiO2 layers effectively act as buried gettering layers, protecting the c-Si bottom cell from degradation during high-temperature top cell synthesis without compromising passivation.
• This approach circumvents major integration challenges, making c-Si a viable bottom cell for various low-cost chalcogenide top cells.
• The developed passivation and gettering strategy enables efficient tandem solar cells where the Si bottom cell is no longer the performance bottleneck.