What is light induced degradation in a solar panel?

Solar panels don’t have any moving parts and they last a really long time - but they don’t last forever.

There are a lot of reasons why solar panels go bad, from physical failures such as the layers in the panel separating to chemistry-related phenomenon such as potential-induced degradation.

It may take decades for the drop is power to be significant, but over time all solar panels lose efficiency. (A solar panel’s 25 year power warranty reflects the manufacturer’s confidence in its long-term performance.)

One common reasons why solar panels lose efficiency is called light induced degradation (LID). Sounds a little weird that light can cause a solar panel to degrade, but it’s a well-known phenomenon.

What is light induced degradation (LID)?

LID is a loss of efficiency in a brand new solar cell that happens when its first exposed to light. The process is related to the chemistry of the cell - specifically boron atoms - and it happens relatively quickly.

Within hours or a few days, the exposure of a solar panel to sunlight may result in a drop in efficiency up to a maximum of around 0.5% or so. After that, LID ceases and the panel should be stable from that point on.

What types of solar cells are affected by LID?

Conventional solar panels are made with pure crystalline silicon. Then, as part of the process to make the silicon conductive, they are infused or “doped” with boron, a lightweight element with the atomic number 5.

Boron is a positively charged atom, which is why silicon cells doped with boron are called p-type cells.

It’s the presence of these boron atoms, when combined with oxygen, that causes LID to happen after being exposed to light, making it a little less efficient at generating electricity.

PERC cells are particularly susceptible to LID

Passivated Emitter Rear Cells - or PERC cells - are a type of higher efficiency solar cell that have a particular weakness to LID. This is because p-type PERC cells use higher levels of boron doping, which can lead to more of the defects that cause LID.

Manufacturers try to prevent this by using higher purity processes that leave less oxygen behind in the silicon to react with the boron. It’s a difficult technical challenge, so even with manufacturers' best efforts, PERC cells may experience higher LID than other types.

N-type solar cells don’t experience LID

One technology that avoids the LID issue completely is n-type solar cells. Remember above that I mentioned that p-type solar cells are called that because they are doped with positively-charged boron atoms? N-type cells instead are doped with negatively-charged phosphorus atoms. Without the presense of boron, the LID problem is eliminated.

N-type is a newer technology, but has found its way into products such as the REC N-Peak series.

Bottom line: check your power warranty and monitor your power production

Overall, LID isn’t an issue that solar homeowners really need to worry much about. For various reasons - LID included - solar panels experience an larger efficiency dropoff in their first year. After that, the panel degradation is slower and steady.

This is reflected in the power warranty for your solar panel. A solar panel will have two warranties: a product warranty which covers defects in the product, and the power warranty that guarantees the efficiency of the panel over its lifespan.

The power warranty for every solar panel I’ve reviewed allows a larger power dropoff in the first year than for later years. Most commonly, you’ll see warranties guarantee that a solar panel won’t lose more than 2% efficiency in the first year and about 0.5% or less every year after that.

What does this mean for the solar homeowner? Simply monitor the production of your system over the years, paying particular attention to sunny but cold days. That’s when you’ll generate the most electricity. If you find that your system no longer hits the output levels that it used to, check to see if the drop in efficiency is greater than what the power warranty allows. If it does, think about making a warranty claim.