The weekend read: Gallium in the fight against LID

From pv magazine 05/2020

The Holy Grail of PV is to improve efficiency while simultaneously reducing costs. This has resulted in continued pressure on the PV community to drive down costs in cell and module manufacturing. The way to achieve this goal is to pursue increasing PV conversion efficiencies and by decreasing the annual degradation rates of solar modules. There is also an immediate need for silicon materials to be highly stable while offering high excess charge carrier lifetime.

Most PV modules are fabricated using p-type silicon substrates, which are prone to degradation when exposed to sunlight, even at temperatures as low as 70 C – a temperature frequently encountered under typical operating conditions. For decades, this degradation, known as light-induced degradation (LID), has plagued the PV industry.

P-type doping

Doping is a fundamental phenomenon in PV. Impurities are added to silicon crystals during doping to make them conductive. For p-type doping, Group III elements are the ideal choice for dopants. When these dopants are mixed into pure silicon, holes are formed in the silicon, which can accept electrons, making the silicon material p-type. The holes are formed because Group III elements have one less electron than silicon.

PERC technologies on p-type substrates currently enjoy a dominant position among PV manufacturers. Boron-doped silicon is the predominant p-type dopant, but it is highly susceptible to LID and thus requires additional processing steps to mitigate this degradation.

An alternative method for the production of stable lifetime material is to dope silicon with a different Group III element, such as aluminum, gallium or indium. Aluminum doping is not viable because it forms recombination-active defects in the silicon material.

Nicholas Grant and John Murphy from the University of Warwick recently studied the viability of indium doping, and found that its relatively deep energy level limits its potential. This leaves us with the question of whether gallium could be a viable alternative to boron and its potential to lead as the dominant dopant for p-type silicon solar cells.

Why gallium?

According to Grant, “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.”

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,” says Grant. “In contrast, we did observe significant degradation for an equivalent PERC solar cell with a boron-doped silicon substrate under the same experimental conditions.”

Gallium doping enables PV modules to offer long-term performance, stability and potentially improved return on investment. Another benefit of using gallium doping to stabilize the lifetime and thus cell efficiency is that cell manufacturing lines do not require additional fabrication tools or processing steps, which is not the case for a switch to n-type substrates.

Potential issues

A historic disadvantage of gallium-doped silicon is the low segregation coefficient of gallium during ingot growth. Relative to boron, gallium has a much stronger thermodynamic tendency to stay in the melt rather than be incorporated into the solid silicon crystal.

Researchers have noticed large resistivity variations along the length of the ingot, and the variation can easily exceed one order of magnitude. This is not ideal when PV cell manufacturers require large quantities of wafers within a tight resistivity range (e.g. 1–2 Ωcm). It can also lead to significant variations in the electrical parameters and potentially reduce the usable portion of the ingot, which is damaging to yield and could be detrimental in terms of costs.

Manufacturing license

Solar manufactures such as JA Solar, Longi and Trina Solar are focusing their efforts on the production of gallium-doped silicon wafers.

Over the past six months, JA Solar and Longi, the largest monocrystalline silicon manufacturer in the world, have acquired licenses from Shin-Etsu Chemical to manufacture gallium-based technologies. Shin-Etsu is considered a pioneer of gallium-doped silicon growth.

Grant says that the technology acquisition might indicate that the gallium segregation issue during ingot growth may not be a major deterrent in producing high volumes of gallium-doped silicon wafers at a competitive cost.

Earlier this year, JA Solar announced plans to supply mono-PERC modules utilizing gallium-doped silicon material to two 50 MW solar power plants in Alvarado-La Risca, a town in Spain’s Extremadura region. JA Solar believes that the project could profoundly influence the global PV market.

Focusing on the benefits of the technology, JA Solar stated that gallium-doped “silicon wafer technology could help improve the performance of modules in power generation [and] guarantee the stability of the whole power plant.”

Earlier this year, JA Solar’s mono PERC MBB lines were switched to cells utilizing gallium-doped silicon wafers. Integrated with PERC technology, coupled with selective emitters and large-size wafers, gallium-doping technology can improve the performance of both cells and modules. On the back of this JA Solar became the first PV manufacturer globally to mass-produce high-efficiency mono PERC products that utilize gallium-doped wafers.

In terms of pricing, Longi claims to have solved the problem of high cost of gallium-doped silicon through in-house technological progress. It reports that the price of its gallium-doped silicon products are the same as boron-doped silicon.

Next steps

Gallium is becoming the most promising of the alternative Group III dopants and appears to be in the process of being demonstrated as particularly viable from an industrial perspective.

However, Grant warns that “while there are clear advantages of using gallium doping to improve the stability, it is still very important to realize that other impurities such as oxygen, carbon and metals such as iron and copper can still have a negative impact on the lifetime of the material.”

Questions do remain regarding the stability of the material and completed cells after thermal processing. Further studies of how gallium-doped substrates perform in the context of new processes is required. Grant advises PV cell manufacturers to collaborate with universities and institutes to undertake additional defect and stabilization studies.

Longi’s R&D team aims to study the characteristics of gallium-doped silicon in order to obtain a reasonable resistivity range and higher doping accuracy to improve the gallium-doping process, from both a cost and quality standpoint.

The team at the University of Warwick is currently investigating whether a fundamental defect, which is related to the gallium doping, is limiting the lifetime in gallium-doped silicon. “We are also conducting further studies on gallium PERC solar cells and trying to develop an understanding for the origins of light and elevated temperature induced degradation,” Grant reports.

JA Solar forecasts that “gallium-doped silicon mono wafers will be the main choice in the future market. With the decrease of cost, this product will be more cost-effective in the future practical application. We expect that gallium-doped silicon wafers will occupy more than 60% of the market in 2020.”

By Monishka Narayan