UNSW researchers have increased the efficiency of the TOPCon solar cells by locally thinning the back poly-Si layer, reducing parasitic absorption and maintaining wafer integrity. The champion cell built with this approach achieved 25.10% efficiency with improved bifaciality and maintained strong passivation.
Researchers from the University of New South Wales (UNSW) in Australia have attempted to improve the efficiency of TOPCon solar cells by reducing the thickness of the polysilicon (poly-Si) layers used in these PV devices to improve passivation and carrier transport.
“In our work, the thinning is only applied locally to the poly-Si layer on the back side, while the thickness of the crystalline silicon wafer remains unchanged,” said the study’s lead author, Ning Song. pv magazine. “As a result, the mechanical integrity of the wafer itself is not directly affected. The locally diluted poly-Si regions remain supported by the underlying crystalline silicon substrate, and the process does not introduce additional mechanical fragility at the wafer level.”
Previous studies have shown that thinning the poly-Si layer can reduce parasite absorption. However, excessive thinning increases recombination losses due to metal paste reaching the wafer. To address this, the UNSW researchers used a laser-induced local thinning method to reduce the poly-Si layer in non-metallized areas, while leaving its thickness intact in metallized areas.
The team implemented a three-step local dilution process integrated into the standard TOPCon manufacturing flow to produce low-temperature TOPCon devices (LT-TOPCon). A 1.3 nm interfacial oxide layer was deposited via plasma oxidation in a physical vapor deposition (PVD) chamber, followed by deposition of a 110 nm poly-Si layer at 250 °C. Activation of the dopant was achieved by annealing at 860 °C for 50 min. A 355 nm UV picosecond laser was used to induce amorphization in selected areas of the poly-Si layer.
“After the laser step, the wafers were etched in a 0.1 wt% sodium hydroxide (NaOH) solution at 40 °C to reduce the poly-Si thickness in the non-lasered areas to approximately 30 nm. Then, a layer of aluminum oxide (AlOx) was deposited on the front and layers of silicon nitride (SiNx) were deposited on both sides. The precursors were annealed in the same PECVD chamber to form the layers of silicon nitride (SiNx) passivation of the rear SiNx and front AlOx/SiNx stack,” the researchers explained.
The team also used Quokka 3, a silicon solar cell simulation tool, to evaluate the short-circuit current and overall efficiency of the new devices.
The champion LT-TOPCon cell achieved an energy conversion efficiency of 25.10%, representing an absolute improvement of 0.12% over a 24.98% efficient reference cell manufactured without the new process. The cell also achieved a fill factor of 83.37%, slightly below 83.45% of the reference, while the open-circuit voltage increased marginally from 727.9 mV to 729.8 mV. “This indicates that the LT process maintains or even improves surface passivation despite thinning of poly-Si,” the team noted.
Additionally, the cell showed a 6.6% improvement in bifaciality, and the researchers expect an additional 0.35% absolute efficiency gain through optimized laser-induced doping, improved process uniformity and advanced metallization schemes.
“The proposed process is designed to be compatible with existing TOPCon production flows and is based on industry-relevant steps such as laser processing and wet chemical etching, making it promising from a process integration perspective,” Song concluded.
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