Researchers from Fraunhofer ISE have developed a perovskite-silicon tandem solar cell using a TOPCon bottom cell with standard structured front surfaces. Their results show that TOPCon bottom cells can perform comparably to heterojunction cells in tandem devices in terms of shunt resistance, supporting scalable, cost-effective industrial production.
Researchers from Germany’s Fraunhofer Institute for Solar Energy Systems (Fraunhofer ISE) have fabricated a perovskite-silicon tandem solar cell that relies on a bottom-mounted PV device with a TOPCon design and high passivation quality.
“We have built the first perovskite-silicon tandem device that integrates a TOPCon bottom cell with an industry-standard textured front end, achieving an efficiency of more than 30%,” said Mario Hanser, the study’s lead author. pv magazine. “Demonstrating that the TOPCon2 cell design and streamlined process flow are compatible with perovskite/silicon tandem integration marks an important step toward cost-efficient industrial tandem solar cell manufacturing.”
In the newspaper “Fully textured perovskite/silicon tandem solar cells with an efficiency of over 30% on both sides tunnel oxide passivating contacted bottom cells”, which was recently published in RRL solar energyHanser and his colleagues explained that the proposed bottom cell relies on two full-surface tunnel oxide passivating contacts developed by single-sided in situ doped amorphous silicon layer deposits.
This cell configuration has a conventional micrometer-scale random pyramid structure on the front, which the scientists described as standard in silicon production lines and which offers higher shunt resistance. They also highlighted that local shunts in the TOPCon emitter are identified as one of the key challenges for the TOPCon2 solar cell design, compared to the heterojunction (HJT) architecture, as they are more common in n-type wafers where the p-TOPCon layer on the back surface forms the p-n junction.
The scientists used plasma enhanced chemical vapor deposition (PECVD) for the hydrogenated deposition of silicon nitride (SiNj) in wafers with a flat p-TOPCon backside and the deposition of aluminum oxide and silicon nitride (AlOx-SiNj), where both cell sides are annealed in an oven at 650 C.
“On wafers with a textured back, the back metallization consists of a stack of layers made with indium tin oxide (ITO) and silver (Ag), while the flat p-TOPCon backs come into direct contact with Ag,” they specified, noting that a curing annealing plate was used after the sputter deposition of ITO to prevent damage.
Image: Fraunhofer ISE
The bottom TOPCon cell was then integrated into a tandem device with an upper perovskite device built with an ITO substrate, a hole transport layer (HTL) made of a phosphonic acid called methyl substmodified carbazole (Me-4PACz)a perovskite absorber, an electron transport layer (ETL) that relies on buckminsterfullerene (C60), a tin oxide (SnOx) buffer layer, an anti-reflective coating based on magnesium fluoride (MgF2) and an Ag-metal contact.
Tested under standard lighting conditions, the tandem cell achieved an energy conversion efficiency of 30.6%, an open-circuit voltage of 1,930 mV and a short-circuit current density of 19.8 mA cm.2and a fill factor of 80%, where the tests showed that the cell achieved performance comparable to that of a benchmark tandem cell based on a bottom-end HJT device.
“This clearly indicates that top cell formation and top cell performance are not affected by the type of silicon substrate, SHJ or TOPCon,” the scientists emphasized. “This result is expected to be transferable to other soil cell concepts with a structured n-TOPCon front contact, for example interdigitated TOPCon (iTOPcon) solar cells.”
Looking ahead, the scientists plan to transfer the proposed cell design to industrially relevant Czochralski (CZ) material.
Other researchers from the Fraunhofer ISE have recently applied the so-called mask-and-plate front metallization approach to the development of perovskite-silicon tandem solar cells for the first time. They fabricated a 1.21 cm2 tandem solar cell with a heterojunction bottom device. The cell achieved an efficiency of 19.35%, reportedly the highest value achieved to date for this cell type using galvanic front metallization.
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