Poly-SiOx solar cell for 4T, 2T perovskite-silicon tandem devices

A research group led by Delft University of Technology in the Netherlands has fabricated a crystalline silicon solar cell based on carrier-selective passivating contacts (CSPCs) made of polysilicon and silicon monoxide (poly-SiO_x). They claim the tech could be used for four-terminal (4T) and two-terminal (2T) tandem perovskite silicon solar cells.

“Such CSPCs consist of doped poly-Si, alloyed with carbon or oxygen, which are deposited on an ultra-thin SiO_x layer, prepared by a wet-chemical process, thermal oxidation, UV/O₃ process, or low-temperature plasma oxidation,” the scientists said, in reference to the new passivating contacts. “The optoelectronic properties of poly-SiO_x depend on the oxygen content.”

The scientists built the cell with a silicon dioxide (SiO₂) layer deposited via plasma-enhanced chemical vapor deposition (PECVD). They also used n-type and p-type poly-SiO_x passivating contacts, which they deposited on the thermal oxide with a dual-stack layer of 10-nm thick intrinsic amorphous silicon (a-Si) layer, via a low-pressure chemical vapor deposition (LPCVD) process, and a 20-nm thick hydrogenated a-SiO_x layer from the PECVD process.

“Silane (SiH₄), carbon dioxide (CO₂), and hydrogen (H₂) gases are used as the sources to deposit these poly-SiO_x passivating contacts,” they said, in reference to the manufacturing process. “Phosphine (PH₃) and diborane (B₂H₆) gases are used as doping sources for n-type and p-type poly-SiO_x passivating contacts, respectively.”

The team also used screen printing to gear the cell with low-temperature front and rear Ag-based metallic contacts. The best solar cell built with this configuration and a p-type wafer achieved a power conversion efficiency of 20.47%, an open-circuit voltage of 95 mV, a short-circuit current of 36.68 mA/cm², and a fill factor of 80.33%. The diffusivity of hydrogen in p-type poly-Si, on the other hand. is lower than in n-type, which prevents the accumulation of excess hydrogen around the oxide which deteriorates the passivation quality.

When tested in tandem with a previously developed perovskite solar cell with an efficiency of 19.7%, the new cell respectively contributed to efficiencies of 28.1% and 23.2% in 4T and 2T cells.

The research group introduced its findings in “Crystalline silicon solar cells with thin poly-SiO_x carrier-selective passivating contacts for perovskite/c-Si tandem applications,” which was recently published in Progress in Photovoltaics. The group includes academics from the Eindhoven University of Technology and the Netherlands Organisation for Applied Scientific Research (TNO).

The TNO is also now developing a four-junction (4T) semi-transparent perovskite-silicon tandem solar cell. It is working in partnership with the Solliance consortium, which includes Delft University of Technology, Eindhoven University of Technology, and Belgian research institute Imec. The device achieved a 30.1% power conversion efficiency in September.

A Dutch-German consortium led by the TNO is also trying to commercialize 2T perovskite-silicon tandem solar cell tech under FIT4Market, a four-year research project.