A group of researchers from the University of New South Wales (UNSW) in Australia and Chinese PV manufacturer Trina Solar have designed a silver-free silicon (Si) tunnel oxide passivated back contact (TBC) using a dual-polarity aluminum (Al) contact engineering strategy, specialized aluminum pastes and optimized firing conditions.
“The novelty of this study is the demonstration of screen-printed aluminum contacts on both n-type and p-type poly-Si/SiOx passivated contacts for back-contact silicon solar cells, providing a potential pathway toward silver-free back-contact solar cells,” corresponding author Ning Song told pv magazine. “We also found that aluminum behaves very differently on n-type and p-type poly-Si, providing new insights into contact formation and future contact optimization.”
“From an industry perspective, reducing silver consumption is becoming increasingly important for large-scale photovoltaic manufacturing, and aluminum could be a promising low-cost alternative for future high-efficiency back-contact solar cells,” she went on to say.
The research team used, in particular, a non-fire-through (nFT) Al–Si paste designed with engineered Al–Si alloys and a modified glass frit system to suppress excessive Al–Si alloying at the interface and avoid the formation of deep, large-area Al–p⁺ regions. This reportedly helps preserve the excellent passivation quality of the poly-Si/SiOx contact while still enabling low contact resistance.
To evaluate its applicability in TBC architectures, symmetrical lifetime samples were fabricated representing industrial-like n- and p-type poly-Si/SiOx passivated contacts. These consisted of heavily doped phosphorus- or boron-doped poly-Si layers capped with thin SiOx and AlOx/SiNx stacks, closely mimicking real device structures. The samples were divided into two groups: one focused on extracting contact parameters such as recombination current density, and the other on studying contact formation mechanisms.
A comprehensive set of characterization techniques was employed, including optical microscopy, Raman spectroscopy, photoluminescence imaging, and transmission line method measurements for contact resistance. In addition, scanning electron microscopy and atomic force microscopy were used to examine contact morphology and interface features, while electrochemical capacitance–voltage profiling provided doping depth information. Finally, device-level implications were supported by numerical simulations using Quokka 3 to assess cell efficiency potential.
The simulations showed that a 257 nm femtosecond UV laser can selectively remove aluminum oxide-silicon nitride (AlOx/SiNx) stacksdielectric layers for local contact formation without damaging the underlying poly-Si/SiOx passivation. Silver-free TBC solar cells with local Al contacts were then evaluated using thick n- and p-type poly-Si layers and optimized firing at 700 C. This condition purportedly yields low contact resistivity and contact recombination.
In addition, interfacial analysis showed strong polarity dependence, with n-type poly-Si exhibiting limited etching. The specialized Al–Si paste was also found to moderate overall reaction kinetics, but p-type interfaces remain more reactive due to the lack of a counter-doping barrier, making firing optimization especially critical to avoid passivation loss.
Device simulations confirmed feasibility but highlighted limitations from higher contact recombination. Cell efficiency was found to drops from 26.8% in silver-based cells to 25.9% in Al-based devices.
“As contact recombination current density of the metal contact remains the primary performance-limiting factor, the relatively high recombination losses at the Al/poly-Si interface represent a key challenge for industrial adoption and still need to be substantially reduced before Al contacts can be considered a viable replacement for Ag in high-efficiency TBC solar cells,” the scientists said. “Achieving this for both n-type and p-type contacts simultaneously, while maintaining low contact resistivity, will be critical for closing the efficiency gap.”
The new cell design was described in “Toward silver-free back contact silicon solar cells: Dual-polarity screen-printed aluminum contacts on poly-Si/SiOX passivated contacts,” published in Solar Energy Materials and Solar Cells. Looking forward, the researchers want to improve paste engineering, while incorportating interfacial barrier layers.
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