Light-Induced Degradation (LID) is a well-known problem in the solar industry, slowly eroding the panels' efficiency and significantly reducing performance loss over the years. This phenomenon has been known in the industry for more than 20 years. LID can be seen during the first hours of exposure to panels under sunlight. LID was first observed in the 1970s with boron-doped silicon solar cell and have been extensively studied since then. Researchers have come up with plenty of solutions to mitigate the effects of LID but no none of them seem to be solving the root cause of the problem. Usually, silicon wafers consist of traces of oxygen during the crystal growth (ingot manufacturing process). This creates complexes with boron and reduces the panels' yield. One method to reduce LID is to dope gallium instead of boron and the other straightforward method is to reduce interstitial oxygen defects.
Boron-doped (P-type) Silicon solar cell is the most commonly used semiconductor in manufacturing solar cells. These boron-doped cells upon exposure to light create defects that reduce the performance of the solar cells. Doping refers to the adding of excessive carriers to make them more conductive. For P-type doping group III elements are usually used and the most common element is boron. P-type elements have three electrons in the valence band and thus the holes are the majority carriers and electrons are the minority carriers. Currently, PERC cells are manufactured in p- typed boron doped silicon substrates as they are susceptible to LID, they require additional manufacturing procedures to mitigate this problem. With the problem of LID, more manufacturers have begun to switch to boron for gallium as a p-type doping material instead of aluminum or indium.
Why Gallium?
In the paper “Lifetime instabilities in gallium doped monocrystalline PERC silicon
solar cells” researchers have conducted a study to evaluate the performance limit of modern Ga-doped silicon materials with high-quality surface passivation. They have found that the effective life for the carriers is found to be > 9 ms in 11.2 Ωcm material[A1]. They also monitored the stability of commercial gallium-doped PERC and boron-doped PERC cells under illumination greater than 1000 h using a photoluminescence method, there were inconsistencies in degradation behavior between different solar cell batches. Stabilized boron doped cells did not degrade and PERC devices were subjected to a 200–300 C dark anneal before the light
soaking, significant differences in the cell degradation signatures were observed with degradation taking longer but the severity being less for gallium-doped PERC cells. The paper concluded that although degradation was observed it had better anti-LID performance and there are positive effects in converting to gallium from boron.
The disadvantage of gallium-doped silicon cells is that they have relatively a low segregation coefficient compared to boron as a result they tend to rather not mix with the silicon ingot crystal during the manufacturing of solar cells. There is also a large resistivity variation and this could potentially damage the total yield.
Manufacturers have been trying to incorporate gallium as a dopant instead of boron and have been successful in reducing LID. AE Solar’s AURORA SERIES has a range of monocrystalline gallium-doped PERC modules available in their product list. The modules have a power rating ranging from 530W to 550W with efficiency ranging from 20 % – 21.08%.
