Chinese researchers develop 28-µm silicon solar cell with 20% efficiency, 0% breakage rate

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A research team from China's Shanghai Jiao Tong University and Sun Yat-sen University has demonstrated that applying a novel thin-silicon wafer reinforced ring (TSRR) at the edge of thin wafers makes the PV material much less prone to breakage. The TSRR was demonstrated on silicon wafers with thicknesses of 4.7 μm, solar cells measuring 28 µm thick, and larger wafers with 60 μm thickness.

“Our team developed a new technique compatible with industrial manufacturing processes to enable the use of ultrathin crystalline silicon wafers that are less than 60 μm, and can be as thin as 20 μm. The technology makes the cells radically less brittle”, researcher Taojian Wu told pv magazine.

“We were surprised that a 4-inch [100 mm] wafer with a thickness of less than 5 µm could be obtained by a simple reinforced ring method, and that we can easily prepare 28 µm silicon solar cells with an efficiency of more than 20% by this method,” said Wu, adding that subtle improvements in the “structure of the thin silicon” has led to such “major advances.”

In the research, the team first prepared a free-standing 4.7 μm thick 4-inch (100 mm) wafer using TSRR. Then it made dopant-free 28-μm silicon cells with interdigitated back contacts (IBC) using TSRR, resulting in a breakage rate of around 0%. The cell efficiency achieved was 20.33%, which was certified at 20.05%.

It was the highest efficiency reported for silicon solar cells with a thickness of less than 35 μm, according to more than a decade of performance data in the literature that was gathered by the team.

The researchers noted that the TSRR method requires only three more manufacturing steps than conventional PV mass production processing. It first deposits a 70 nm silicon nitride (SiNx) layer on both sides of conventionally thick silicon wafers by plasma-enhanced chemical vapor deposition (PECVD) or low-pressure chemical vapor deposition (LPCVD), then the SiNx is removed from the central region of one side using a die, laser, or photolithography, to make an opening, and followed by an alkaline solution etching step to the desired thickness, according to the team.

The SiNx layer provides protection and enables the silicon in the edge region of the wafer to maintain its original thickness, explained the research team. “The difficult part is the process of depositing a high-quality SiNx protective film, which has to withstand prolonged etching with an alkaline solution,” said Wu. “The easy part is the fabrication process of thin silicon solar cells because we can process thin silicon wafers with reinforced rings in the same way as we process the normal silicon wafers.”

The team asserted that the TSRR structure is applicable to “any silicon technology”, such as passivated emitter and rear cell (PERC), silicon heterojunction (SHJ), tunnel oxide passivating contact (TOPCon), as well as dopant-free passivating contact xBC cell technologies.

To understand better the optoelectrical performance of the experimental solar cells, the team performed TCAD numerical simulation of front and back contacts (FBC) solar cells.  The experiments and simulations of mechanical properties comparing TSRR and conventional thin silicon structures confirmed the supporting role of the TSRR, according to the research group.

The team also prepared 50 μm to 60 μm textured TSRR wafers, measuring 182 mm ×182 mm, and performed key manufacturing processes to confirm the industrial compatibility of the TSRR method. “We have received feedback from our partner company that the breakage rate during the fabrication of 60 μm-SHJ solar cells has been greatly reduced,” said Wu.

The research is detailed in “Free-standing ultrathin silicon wafers and solar cells through edges reinforcement,” published by nature communications.

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