The ultra-thin TOPCon solar cell uses double-sided polysilicon passivated contacts, formed by low-pressure chemical vapor deposition. The champion device, with a thickness of only 80 μm, achieved an efficiency of 19.7% at an open-circuit voltage of 719 mV and exhibited strong passivation, optical performance and mechanical flexibility.
Researchers from the National University of Singapore have manufactured an ultra-thin TOPCon solar cell with polysilicon (poly-Si) passivated contacts on both sides – a so-called byPoly solar cell – using low-pressure chemical vapor deposition (LPCVD).
Poly-Si layers are used in TOPCon devices to improve passivation and carrier transport. In these structures, a thin interfacial oxide reduces surface recombination by electrically and chemically passivating the silicon interface, while the overlying doped polycrystalline silicon layer provides a low-resistance path for majority carriers.
“Due to its simple structure and good low-light performance, the cell is suitable for building-integrated solar photovoltaics (BIPV), indoor low-light energy harvesting, wearable electronics, portable power supplies, and solar quadcopters,” said study lead author Aaron Danner. pv magazine.
“The proposed selective bipoly solar cell is currently in the laboratory-scale research phase and is not yet ready for commercial production,” he continued. “Further work on large area manufacturing, production line compatibility, validation of long-term stability and optimization of flexible packaging is essential for future industrial implementation.”
In the study “Ultra-thin Bipoly solar cells with selective n-Type TOPCon layer on the front and p-Type TOPCon layer on the back”, published in Progress in photovoltaicsthe research team explained that byPoly solar cells, often referred to as double-sided TOPCon, offer substantial improvements in both surface passivation and charge carrier transport, thanks to careful optimization of the doping levels and interfacial properties of the two poly-Si layers, which effectively suppresses recombination losses while maintaining high open-circuit voltage and fill factor.
The solar cells were fabricated using M2-sized Czochralski silicon wafers, which were thinned from 180 μm to 80 μm via a voltage-controlled etching (SDE) process. Standard Radio Corporation of America (RCA) cleaning was performed to remove organic, metallic, and particulate contaminants prior to processing.
A brief immersion with hydrofluoric acid (HF) removed natural oxides, followed by back protection with plasma-enhanced chemical vapor deposition (PECVD) silicon nitride (SiNx) and potassium hydroxide (KOH) texturing on the front, simultaneously eliminating unwanted polycrystalline silicon and forming pyramidal light-trapping structures.
Image: National University of Singapore, Progress in Photovoltaics, CC BY 4.0
A full-surface doped poly-Si layer was then deposited on both sides via low-pressure chemical vapor deposition (LPCVD), followed by HF treatment to remove native oxide. Selective patterning retained poly-Si only under metallic contacts, minimizing optical losses while maintaining carrier transport, after which the SiNx mask was removed and the surfaces re-cleaned for passivation.
A low temperature oxide (LTO) thin layer was grown to improve interface quality, followed by PECVD SiNx deposition as both passivation and antireflection coating. The rear contacts were opened using laser contact opening (LCO), allowing metallization of screen-printed aluminum (Al), while front silver (Ag) contacts were printed over the selective poly-Si areas. The final co-firing at 740 C completed the fabrication of the device.
Using quasi-steady-state photoconductance (QSSPC) and photoluminescence (PL) measurements, the scientists found that the champion device built with the proposed architecture, with a thickness of 80 μm, achieved an energy conversion efficiency of 19.7%, corresponding to an efficiency-to-thickness ratio of 0.25% per μm. The cell also achieved an open-circuit voltage of up to 719 mV and a fill factor of more than 83%.
“The favorable performance of the device is attributed to the optimized thermal treatment of the poly-Si layers, which improves dopant activation and interface quality, thereby reducing recombination losses,” the researchers said, noting that the ultrathin cells also showed noticeable mechanical flexibility, allowing potential use in curved or non-planar applications such as aerospace and wearable systems.
“Although the quantitative cost analysis was not fully performed in this fundamental study, the proposed cell uses fully silicon-compatible manufacturing processes without additional expensive materials or complex procedures,” Danner explains. “The production costs are almost comparable to those of conventional silicon-based solar cells, demonstrating excellent, low-cost industrialization potential for mass production.”
“With our work, we presented a quantitative evaluation of the interplay between surface passivation quality, optical engineering and mechanical stability in ultrathin silicon solar cells. While the results demonstrate the feasibility of biPoly-based device architectures, further optimization is needed in areas such as contact resistance, thermal stability and long-term reliability under cyclic mechanical deformation,” he concluded.
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