Researchers at Yangzhou University in China have fabricated a solar cell based on tunnel oxide passivated contact (TOPCon) technology by reducing the use of expensive silver paste with a screen-printed aluminum (Al) paste.
“We are still adjusting further process and Al paste to improve efficiency,” the research' corresponding author, Jianning Ding, told pv magazine. “We are testing this innovation together with Chinese solar module manufacturer JA Solar and paste manufacturer Ruxing Technology Development Co. Ltd.”
The group built the 182 mm x 182 mm cell on a 140 μm n-type Czochralski wafer. The rear side passivation contact structure was based on a 1.6 nm silicon monoxide (SiOx) layer and a phosphorus-doped polycrystalline silicon (250 nm) layer, which were both formed by low-pressure chemical vapor deposition (LPCVD). The device also features an aluminum oxide (AlOx) layer fabricated by atomic layer deposition (ALD).
The cell was then screen printed with a mixed paste made of silver (Ag) and Al on the front side and a full Al paste on the rear side. “The addition of silicon in Al paste can inhabit the violent reaction between Al paste and n+-poly-Si, but also decreased the grids conductivity,” the researchers specified. “Thus, the Al paste with different weight percentages of silicon was applied in metallization.”
“Adding silicon to aluminum paste can suppress the reaction between the aluminum paste and poly-Si, reducing metal recombination and improving open-circuit voltage,” research co-author, Qinqin Wang, added, noting that ohmic contact was established with the formation of Al-Si alloy. “Al-Si alloy thickness decreased with the weight percentage of silicon in Al paste increase.”
The scientists explained that the solar cell sample built with the highest percentage of silicon achieved the best performance. Tested under standard illumination conditions, this device achieved a power conversion efficiency of 22.56%, an open-circuit voltage of 663.6 mV, a short-circuit current density of 40.73 mA/cm2, and a fill factor of 83.48%.
“Although the efficiency was 9.40% relative lower than that of TOPCon cells printed with Ag paste, but the cost of Al paste only accounted for 10% of Ag paste;” they further explained. “In the future, the highest efficiency of TOPCon solar cells printed with Al paste on the rear side are expected to reach the commercial TOPCon based on Ag paste, which applies laser doping selective emitter technology (SE) and bifacial poly-Si passivation structure.”
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