RAY OF LIGHTS
- An Expert's Opinion-
#2: LID minimisation using Ga-doped Silicon Wafer
Why did Ga-doped become the mainstream and what are its limitations?
Dr SARANGI is an independent consultant for the PV value chain with more than 20 years experience. He has been supporting the solar PV industry in Ingot / Wafer / PV cell / PV module manufacturing setup including technology selection, tool selection, process setup, future goal setup etc. Dr Sarangi also specialises in the areas of PV cell efficiency and yield improvement, CTM (Cell to module loss) minimisation and production improvement for PV modules with process optimisation, reliability, and quality improvement.
Light-Induced Degradation (LID) and Light and Elevated Temperature Induced Degradation (LeTID) are the main prominent degradation modes for the p-type standard mono and p-type mono PERC based cells. LID in p-type mono cells is due to B-O defects (Boron-Oxygen).
Minimisation of the LID effect from the mono p-type cells is possible either using another Group III element, such as Aluminum (Al), Gallium (Ga) or Indium (In) in place of Boron (B) as a dopant or reduction of the Oxygen (O2) in the silicon. However, it is not possible to grow p-type silicon with the required low concentration of O2 to mitigate LID.
Replacing B with Aluminum and Indium may not guarantee success either for two reasons:
Incorporating Al into the silicon creates recombination-active defects; and
Indium is still in the experimental stage and its relatively deep energy level limits its potential.
The best alternative is Ga, but it is showing low segregation coefficient during ingot growth. Therefore, Ga has a much stronger thermodynamic tendency to not be incorporated into the solid silicon crystal and would prefer to stay in the silicon melt.
Japanese manufacturer, Shin-Etsu Chemical, holds several patents to resolve the issue and use Ga-doped wafers in solar application. JA Solar, LONGi and Trina have bought the patent rights and started producing p-type mono PERC solar cells based on Ga-doped silicon wafers. Mono PERC cells using Ga-doped wafers are demonstrating less LID compared to B-doped p-type mono PERC cells.
LONGi has also validated that the average efficiency of Gallium-doped cells is 0.09% higher than that of Boron-doped equivalents, as shown in Figure 1.
Figure 1. The efficiency distribution of Gallium-doped and Boron-doped cells (Source: LONGi)
Ga-doped mono PERC cells are not completely free from LID or LeTID. Researchers have also observed LeTID on Ga-doped mono PERC cells. However, LONGi recently claimed that they can control LeTID by controlling the hydrogen content in the cell production process.
In summary, Ga-doped mono PERC cells have higher efficiency and better anti-LID and anti-LeTID performance compared to Boron-doped cells. Future field performance will be key in providing more insight on this development.
Dr Debajyoti Sarangi, Independent Consultant, India.
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