Efecto LID: La degradación inducida por la luz

26 de julio de 2021 por
Efecto LID: La degradación inducida por la luz
Techno Sun, SLU*, Hugo Rodrigo Zapata :

What is the LID effect of photovoltaic modules?

Light Induced Degradation (LID) is a degradation in crystalline silicon cells in the first hours of sun exposure caused by a reduction in photovoltaic efficiency, known as ultraviolet light induced degradation (UVID).

The LID effect mainly affects the actual performance of installed photovoltaic modules compared to the nameplate data provided by some manufacturers. It causes a loss of power and efficiency of silicon  solar panels during the first months of exposure to solar irradiation and can reach losses of 10% of their initial power in the first month after installation.

Thereafter, a phenomenon known as power stabilization occurs, in which there will be lower levels of power loss in the following years of use, meaning that in the early years the loss rates will be higher than in later years.

What produces the LID effect in photovoltaic modules?

This effect occurs due to a reaction of the boron present in the silicon cell with other chemical elements, mainly oxygen, but also iron or copper.

During cell fabrication, traces of oxygen can be included in the silicon during the Czochralski process of obtaining silicon crystals. Due to the effect of light exposure, these positively charged O2 dimers can diffuse through the silicon lattice and create complexes with the dopant boron acceptors. The boron-oxygen complexes create their own energy levels in the silicon lattice and can capture electrons and holes that are therefore not available for the photovoltaic effect.

In addition, temperature plays a very important role in this type of degradation, since when it is slightly elevated, the problem becomes even more complex.

How to detect the LID effect of photovoltaic modules?

It is difficult to obtain data on the LID effect on a given module sample, as manufacturers are not required to report it and it is not mentioned in the solar moduledata sheet. In addition, LID varies from one manufacturer to another, and may even vary from one batch of a given production run to another, making it even more difficult to quantify.

In order to find out if our solar panel suffers from this effect, it is necessary to measure how long the electrons persist before recombining with the positive holes of the reticle. To do this we should use specialized electroluminescence equipment that could indicate the damage of our module. One of the first clues we will have will be a drop in its performance for no apparent reason.

Electrominiscence will make visible the defects that cannot be seen with the naked eye. Through special cameras and filters, the radiation emitted by the panel will be visible and we will be able to see where the emission is not correct.

The image above shows the same module after being exposed to irradiation and how the LID effect has caused a decrease in the efficiency of some of its cells, producing less energy.

The LID effect is a well-known and widespread effect, which is why sophisticated simulation programs, such as PV-SYST, explicitly provide a variable for it when simulating the performance of a PV system.

How to avoid the LID effect of photovoltaic modules?

There is no effective way to completely eliminate this problem from the modules, although manufacturers use available treatments that are used in production lines, the problem is not completely eliminated.

Among these is the removal of as much oxygen as possible from the cell, but this is a costly method and is not completely achieved.

Which solar cells are most affected by the LID effect?

The LID effect does not affect all module types equally. This is due to the different cell types: crystalline structure (monocrystalline or polycrystalline) and their electrical properties (P-type or N-type).

In single crystal structures, solar cells are grown using the Czochralski process which produces a uniform crystalline structure that is cut to make solar cells. They tend to have better electrical properties, but they also tend to have higher oxygen concentrations, which can lead to a higher LID effect.

Polycrystalline structures, on the other hand, are solar cells produced by some form of vapor deposition that grows silicon on a substrate. They are less efficient in electricity production compared to a monocrystalline cell, but have less oxygen present and therefore suffer less from the LID effect.

Silicon wafers with P-type electrical properties (PERC) contain a controlled amount of impurities, called doping elements, which more readily accept electrons and allow a photovoltaic module to create a voltage difference to produce power under sunlight. Most PERC cells use boron as a doping element, while others use gallium. Boron can be a trigger for the LID effect.

On the other side are silicon wafers with N-type electrical properties, which contain impurities that have the opposite effect; they release, rather than add, electrons. N-type silicon wafers rarely exhibit the LID effect.

What is the DLID effect of photovoltaic modules?

A derivative of the LID effect is direct light induced degradation (DLID), which means the degradation of PV cells by exposure to direct sunlight during the initial installation period, which can cause the electronic components of the PV cells to deform or bend due to heat.

The most significant difference between the LID effect and the DLID effect is the duration of such degradation. The former can last for a few days or even more than a week, while the latter can last for a few hours.

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