Optimized gettering can improve heterojunction solar cell efficiency by 0.21%
A Chinese research team led by the Shanghai Jiao Tong University has evaluated the impact of gettering in the manufacturing of heterojunction (HJT) solar cell technologies and has concluded it can help improve device efficiency.
“We investigated, in particular, the impact of the phosphorus diffusion gettering (PDG) process on n-type HJT solar cells,” the research's corresponding author, Wenzhong Shen, told pv magazine. “We have successfully fabricated PDG-treated HJT cells with different gas flows and discussed the influence of photoelectric properties. Optimizing gas flow to 1,000 sccm improves the balance between carrier lifetime and saturated dark current density, increasing average efficiency by 0.21%.”
Gettering is widely used in the PV manufacturing industry. It involves three steps and is utilized during crystal growth to remove containments and other forms of defects in wafers. Through this process, the impurities are initially released into a solid solution and then undergo diffusion through the silicon. Finally, they are trapped in an area away from the active circuit regions of the wafer.
Gettering is already embedded in most of the current solar cells, through phosphorus diffusion in PERC or PERx devices, as well as in Al-BSF cells. It is also embedded in TOPCon solar cells, although its effectiveness varies a lot, and in silicon heterojunction cells it can be used as a pre-treatment to improve the quality of silicon substrates.
Phosphorus diffusion is, to date, the most utilized and investigated gettering technique in the solar industry. However, its underlying mechanisms are still controversial and unresolved. Boron diffusion is described as very effective only under unconventional process conditions and is recommended for doped-poly-Si/SiOx structures, which provides a route to simultaneously achieve p+ doping and a strong gettering effect.
In the study “Revealing the effect of phosphorus diffusion gettering on industrial silicon heterojunction solar cell,” published in Solar Energy Materials and Solar Cells, Shen and his colleagues explained that they validate the effects of PDG on 182 mm × 91 mm HJT devices with average efficiencies exceeding 25%. “It is well known that the effect of PDG decreases as the efficiency of solar cells increases,” he stressed.
The team found that phosphorous diffusion takes the form of circular channels rather than the whole plane and that the effect of gettering is concentrated in the half-edge region of the substrate rather than in the middle and edge. “To our best knowledge, these results were not reported before,” Shen added.
The PDG process was also found to have an effect on the cell's square resistance, effective minority lifetime and saturated dark current density. Furthermore, free energy loss simulations were carried out to quantify the power loss at different gas flows.
Thanks to the optimized gas flow, the solar cell efficiency reached 25.14%, which compares to 24.93% for a reference cell fabricated without the PDG treatment. The main efficiency gains derived partially from improved open-circuit voltage and short-circuit current and mostly from an enhancement of the fill factor.
“In general, we believe that the present work demonstrates the feasibility of low-cost and high-efficiency industrialHJT solar cells based on the PDG process,” Shen concluded. “Our research proves that the further optimized gettering process is very effective for the production of low-cost silicon solar cells with reducing the dependence on high-purity wafers.”
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