Research backs up the case for gallium-doped silicon solar modules as hardy and cost-effective performers, writes LONGi Solar product director Dr Fang Hongbin.
In the solar industry, PERC cell technology is achieving record efficiencies of more than 24%. These higher efficiencies are enabling better LCOE for solar installations. While record efficiencies are good, what counts more are conversion efficiency averages in volume production and efficiency stability over time.
Experts have consistently pointed out the challenges that PERC technology faces soon after installation with regard to potential degradation effects. LONGi Solar has been working to address the issue of light-induced degradation (LID) in PERC cells and modules to prevent degradation issues and offer the best quality modules.
In the past few years, another solar cell/module efficiency degradation phenomenon has caught everyone’s attention: light and elevated temperature induced degradation, or LeTID.
LeTID is believed to be caused by interaction between metal impurity and hydrogen in wafers. With gallium-doped wafers it is easier to control LeTID in solar cells, as there is no need to introduce excessive hydrogen in cell processing to mitigate LID as required for boron-doped wafers.
Light-induced degradation is generally considered to be caused by a boron-oxygen complex formed under light illumination, which reduces solar cell efficiency and power over time after installation. To mitigate LID, you can either reduce oxygen concentration in wafers or replace boron (B) with other dopants, such as gallium (Ga). Research carried out jointly by the Institute for Solar Energy Research Hamelin (ISFH) and LONGi has demonstrated that Ga-doping and low oxygen wafers are effective, as demonstrated in figure 1.
With process optimisation at ingot pulling and cell manufacturing stages, solar cells made with Ga-doped wafers demonstrated efficiency improvements of between 0.06-0.12% (abs) compared to B-doped wafers.
Through thorough research and testing, LONGi’s technology experts concluded that LID and LeTID problems could be effectively solved by using gallium-doped monocrystalline silicon wafers in combination with cell process control, without the need for regeneration (light injection or electrical injection) treatment.
Compared with boron-doped silicon wafers, gallium-doped silicon wafers can improve the efficiency of PERC cells. There is no boron-oxygen complex in gallium-doped PERC cells, so there is not the usual phenomenon of boron-oxygen LID. In the recent white paper Gallium-doped monocrystalline silicon fully solves the problem of a PERC module’s LID, LONGi has summarized its findings on the subject, supported by related studies. Research strongly indicates that the application of gallium-doped silicon wafers can effectively mitigate the initial LID from which cells using boron-doped p-type silicon wafers have long suffered.
The LONGi team conducted an LID test of gallium-doped and boron-doped PERC cells. The test used LONGi’s mass-produced bifacial PERC cells (which had a cell efficiency of about 22.7%). Following is part of the test scheme including the test item, type and quantity of cells.
1sun, 75°C: In order to fully reflect the LeTID, LONGi adopted a test temperature of 75°C. Figure 2 shows the 264 -hour test results at 1sun, 75°C. The boron-doped cell degrades to a maximum of 2.3% at 8 hours and then recovers to a stable value of 1.3% at 96 hours. The degradation value of gallium-doped cells is basically stable at 96 hours at 1.2%, and then slowly degraded to 1.3% (216 hours) and then recovered slightly.
×10suns, >100°C: The LeTID process can be accelerated by adopting ×10suns, >100°C. The test results of gallium-doped PERC cells under this method are shown in Figure 3. Using this test method, the gallium-doped cell also experienced a process of first degrading and then returning to stability. The degradation reached the maximum value of 1.05% at 5 minutes and began to stabilize at a fairly low level of 0.3% at 90 minutes.
Results backed by independent research
Tine U. Naerland from Arizona State University (along with other researchers) studied the minority carrier lifetime degradation of indium-doped, gallium-doped and boron-doped silicon wafers without impurities at room temperature 25°C, as shown in Figure 4.
It can be seen that the minority carrier lifetime of gallium-doped silicon wafers basically maintains a constant value of about 300μs after 104s light exposure, while those of boron-doped and indium-doped silicon wafers degrade continuously and greatly. Therefore, under low-temperature light conditions, the gallium-doped silicon wafer is relatively stable and basically has no degradation. However, in the case of actual outdoor exposure, the working temperature of the cell will exceed 60°C, and the gallium-doped cell will also have a certain degree of LeTID under the action of temperature. Her research clearly supplements LONGi’s test results of the LID of gallium-doped PERC cells and regenerated boron-doped PERC cells at different temperatures.
Another related research has been made by Nicholas Grant and John Murphy from the University of Warwick who recently studied the viability of indium doping and found that its relatively deep acceptor level limits its potential. “Gallium-doped silicon has demonstrated very stable and high lifetimes when subject to extended illumination. There have also not been any known detrimental recombination active defects,” said Grant in a recent interaction with a leading solar industry journal.
The application of gallium-doped silicon wafers can effectively mitigate the initial LID from which cells using boron-doped p-type silicon wafers have long suffered. Hence, gallium-doped silicon does not require the additional stabilization steps used to mitigate degradation, unlike the boron-doped status quo. The average efficiency of gallium-doped cells is 0.09% higher than that of boron-doped cells.
“My team performed stabilization testing and no significant degradation of the PERC solar cells utilizing gallium-doped silicon substrate was observed,” he said. “In contrast, we did observe significant degradation for an equivalent PERC solar cell with a boron-doped silicon substrate under the same experimental conditions.”
The way forward
LONGi’s testing of gallium-doped PERC cells at different temperatures shows that compared with boron-doped cells, gallium-doped cells show significantly lower degradation. Combined with credible academic literature on the topic, this shows that LONGi’s new approach to Ga-doped mono wafers can solve LID problems inherent with PERC technology.
As a result, LONGi has acquired licenses from Shin-Etsu Chemical (a leader in gallium-doped silicon growth) to manufacture gallium-based technologies. The technical team at LONGi has reduced high costs in the production of gallium-doped silicon through in-house technological innovation. LONGi is now able to deliver Ga-doped wafers at a similar price to B-doped wafers.
Gallium-doped silicon solar modules will be more cost-effective in practical applications though reduction in degradation, decrease in cost and increases in reliability.
Dr Fang Hongbin is product director at LONGi Solar.