SERIS researchers developed a 16 cm² perovskite-silicon tandem solar cell with a double-sided TOPCon bottom cell, which improves passivation, reduces recombination and increases the voltage and filling factor.
Researchers at the Solar Energy Research Institute of Singapore (SERIS) in Singapore have fabricated a perovskite-silicon tandem solar cell based on a bottom TOPCon device that relies on double-sided (DS) thin polysilicon (poly-Si) layers.
In standard single-sided TOPCon (SS-TOPCon) solar cells, charge carrier selectivity is achieved using a heavily boron-doped surface layer on the front and a back stack made of a thin layer of silicon oxide (SiOx) plus phosphorus-doped polysilicon (poly-Si). In double-sided TOPCon (DS-TOPCon), the front boron-doped emitter is replaced by a layer of silicon oxide (SiOx) combined with boron-doped polysilicon (poly-Si).
Compared to SS-TOPCon, DS-TOPCon reduces recombination losses. This lower recombination allows for a higher open-circuit voltage and can improve overall efficiency. Carrier-selective contacts on both sides also improve charge extraction, potentially increasing the fill factor. The symmetrical and fully passivated structure has the potential to improve the mechanical stability of the cell and makes it particularly suitable for tandem and high-efficiency cell concepts.
Despite these advantages, DS-TOPCon is more complex and expensive to produce, which is why SS-TOPCon remains dominant in industrial production today. “At this stage our priority was to establish the technical potential of the approach. Although we have not yet engaged with manufacturers, our process has been developed with industrial compatibility in mind, and we have demonstrated this with a fully screen-printed 16cm2 proof-of-concept device,” said the study’s corresponding author, Erik Spaans pv magazine.
In the newspaper “Heated ITO deposits on poly-Si passivated contacts enable efficient 16 cm2 perovskite/silicon tandem solar cells”, published in Solar energy materials and solar cellsthe scientists explained that the DS poly-Si layers were deposited using indium tin oxide (ITO) sputtering and low-pressure chemical vapor deposition (LPCVD), including a on site annealing step after the heated sputter deposition.
They also highlighted that DS-TOPCon cell precursors showed better performance when the ITO was deposited at higher temperatures, but excessive heating can be detrimental. This negative effect may be due to hydrogen effusion, deactivation of dopants, and the formation of a thin resistive layer between the poly-Si and ITO films.
The 16.7% efficient bottom TOPCon cell was fabricated with an M2-size Cz silicon wafer etched by saw damage, a 1.5 nm thermally grown SiOx layer, and on site doped hydrogenated amorphous silicon (a-Si:H). Solar cell precursors received p-doped a-Si:H at the back and n-doped a-Si:H at the front, followed by annealing at 850 C to form poly-Si layers.
After annealing, excess SiOx was removed and the samples were hydrogenated using silicon nitride (SiNx) deposition and baking at 700 °C. The SiNx was then etched away and approximately 75 nm of ITO was sputtered on both sides using industrial DC microwave tools. For solar cell precursors, Ag contacts were screen printed and cured at 200 C, completing the final structures.
By combining this TOPCon cell with a topperovskite device, the research team built a 16 cm2 tandem cell that achieved an energy conversion efficiency of 22.1%, an open-circuit voltage of 1.727 V, a short-circuit density of 17.9 mA/cm2 and a fill factor of 71.7%.
“To our knowledge, this is one of the largest DS-TOPCon-based PSTs reported in the literature, and the only one using both industrial Cz wafers and fully screen-printed metal contact,” the academics pointed out. “In this proof-of-concept device, the use of heated ITO and low-temperature metallization on both contacts of the Si cell enables direct integration into perovskite-silicon tandems, with the added freedom to deposit the perovskite in a nip or pin architecture.”
Looking ahead, the research group plans to use burn-through back contacts with a thicker, more conductive poly-Si layer. “Our current focus is on urban applications, where the higher power density of perovskite/silicon tandem solar cells is particularly valuable given space limitations,” concludes Spaans, referring to the solar cell’s potential applications.
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