An international research team consisting of scientists from Chinese module manufacturer Longi has examined the resistance distribution of antimony and phosphorus doped wafers used in solar cell production and found that they provide comparable mechanical strength. However, their work also showed that antimony-doped wafers have a more uniform resistivity, with a slight increase near the tip.
An international research team led by the Australian National University and Chinese module manufacturer Longi has investigated antimony (Sb)-doped Czochralski grown silicon rods as an alternative to phosphorus (P)-doped blocks in PV wafer production and found that they exhibit slightly higher mechanical strength.
“Our study has clarified why Sb-doped n-type silicon rods can achieve a uniform resistance distribution despite the very low segregation coefficient of antimony,” said the study’s corresponding author Rabin Basnet. pv magazine. “We have shown that the key factor is the controlled evaporation rate of Sb during Czochralski growth, and not its co-doping with phosphorus as often speculated. This finding helps explain how the industry has managed to produce uniform Sb-doped blocks and provides a scientific basis for optimizing doping uniformity in next-generation wafers.”
“Antimony is a promising alternative n-type dopant for silicon wafers,” said co-author Daniel Macdonald. “It yields a more uniform resistivity profile along the length of the rod compared to the conventional n-type dopant phosphor, yielding very high quality wafers close to the Auger limit, even in the mature state.”
Currently, antimony appears to be more expensive than phosphorus in terms of raw material costs. However, this higher initial price does not necessarily translate into higher overall costs for photovoltaic wafers. When factors such as production yield, overall performance of the wafers, and their long-term reliability are considered, antimony-doped silicon rods can provide benefits that offset the initial costs.
In their work, presented in “Resistance distribution and donor properties of antimony doped N-type Czochralski silicon rods”, published in Solar energy materials and solar cellsthe scientists explained that they had conducted their tests on Sb- and P-doped 182mm x 182mm wafers supplied by Longi itself, which has recently introduced antimony doping into ingot production.
The P-doped and Sb-doped wafers had resistances of 1 Ω·cm and 0.8 Ω·cm, respectively. Both types were laser engraved into 30mm x 50mm coupons with a comb-like structure. These coupons were then etched in tetramethylammonium hydroxide (TMAH) to remove saw damage and any residual laser ablation. After cleaning, a 15 nm thick aluminum oxide (AlOx) layer was deposited via atomic layer deposition (ALD) at 150 °C, followed by annealing at 400 °C for 30 min.
Using the T/CSTM 00587–2023 method, which measures the flexural strength of crystalline silicon photovoltaic cells via a four-point bending setup, the research group performed strength measurements on the M10 wafers. They also used Electron Paramagnetic Resonance (EPR) spectroscopy to analyze defects and doping levels.
The analysis showed that wafers fabricated from Sb-doped blocks exhibited a more uniform resistivity, with a slight increase near the tip, compared to their P-doped counterparts.
For further analysis, the team used EasySpin, a MATLAB toolbox for simulating and fitting EPR spectra, and observed “weaker” hyperfine lines in the Sb-doped wafers, attributed to the presence of Sb atomic clusters.
The team noted that Sb-doped and P-doped wafers currently offer similar mechanical performance. The key findings of their work consist of detecting the high resistivity uniformity and unique EPR signatures of the Sp-doped samples.
“However, ingot yield is expected to increase with Sb doping, leading to significant cost savings potential for n-type wafers,” Basnet concluded.
The research team included academics from the US Department of Energy’s National Renewable Energy Laboratory (NREL) and the Colorado School of Mines (USA).
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