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What Is LID in Solar Panels? (vs. PID + Anti-LID Techs Explained)

As you delve deeper into the specs or performance of solar panels, you will likely come across the term LID, which is easily confused with another term PID.

This post will explain what exactly LID is in solar panels and how it differs from PID. You will also learn some mitigation strategies for this phenomenon as well as some advanced anti-LID technologies employed in panel products.

Light-Induced Degradation (LID) in Solar Panels

What Is LID in Solar Panels?

LID is an acronym for Light-Induced Degradation.

Classified as one type of degradation mechanism, LID typically occurs in p-type crystalline silicon (c-Si) solar panels. It refers to the phenomenon where the performance of panels decreases when they are first exposed to sunlight. 

This degradation usually happens within the first few hours to days of exposure when the panel is unpacked or just installed at the site. During this period, the panel may experience roughly 1%-3% power loss, but can be higher in some cases depending on the type of silicon used and the quality of the manufacturing process.

After this initial period, which is often described as ‘power stabilization,’ degradation continues but the rate will slow down significantly and is generally counted by year.

For most c-Si solar panels, the annual degradation rate due to LID generally ranges from 0.25% to 0.65% per year. While some advanced panel products can have lower annual degradation rates, around 0.25% per year.

What Causes LID?

The primary cause of LID is the formation of boron-oxygen (B-O) complexes1 in boron-doped p-type c-Si panels, which trap electron-hole pairs that would otherwise contribute to energy generation.

The following is a detailed explanation of the process.

Silicon wafers used in the manufacturing of solar cells are often doped with boron (B) to create p-type semiconductors. During the doping process, boron atoms are introduced into the silicon crystal lattice to create ‘holes’ which are positive charge carriers, essential for the p-n junction.

On the other hand, wafers typically also contain a small amount of oxygen (O) which can be introduced during the Czochralski process used in the manufacturing. In contrast to boron atoms, these oxygen atoms occupy interstitial sites in the silicon lattice.

When the cells are initially exposed to sunlight, the photons generate electron-hole pairs in the silicon. This excitation energy allows the boron and oxygen atoms to become mobile and form B-O complexes, which introduce defect states in the silicon lattice.

These defects act as recombination centers for electron-hole pairs photons generate, meaning that when an electron and a hole encounter a B-O complex, they recombine instead of contributing to the electric current. 

In other words, this recombination reduces the number of charge carriers available to generate current, thereby decreasing the overall efficiency and output of the panel.

What About PID? How Does LID Differ From PID?

PID is another panel degradation mechanism, which is an abbreviation for Potential Induced Degradation.

It refers to a phenomenon where electrical currents do not flow along the defined path, but instead move through the cover, coating, encapsulant material or frame, causing degradation in efficiency and output. 

While LID is triggered by initial exposure to sunlight, PID is induced by a combination of high voltage, elevated temperature and high humidity, which leads to ion migration and surface polarization.

In terms of timing and impact, LID occurs within a short period of time and results in an initial power loss that stabilizes over time. Conversely, PID can develop over a longer period and is often not immediately noticeable in the beginning of operation. Moreover, PID can cause significant and progressive power loss if not mitigated in time.

How to Mitigate LID in Solar Panels? (Strategies & Techs)

Although LID may not be as severe as PID, manufacturers and industry experts are working to mitigate its impacts, which is of great significance to optimize the longevity and maximize the efficiency of solar systems.

Pre-condition the Solar Panels

Prior to their installation, expose the solar panels to light in a controlled environment via flash testing or light soaking. This process can help in stabilizing the performance of the panels by inducing and then mitigating the initial degradation in a controlled manner. This eventually helps to realize a stable operation of the system.

Utilize Alternative Doping Materials

One primary cause of LID is the presence of boron in silicon. By utilizing alternative doping materials, the formation of B-O complexes can be avoided. 

Gallium-doped silicon, for instance, does not exhibit the same level of degradation as boron-doped silicon, namely, leading to reduced susceptibility to LID.

Comply With Industry Standards

It is crucial to adhere to industry standards such as IEC 61215 during panel manufacturing processes. This practice ensures that the solar panels meet stringent performance as well as consistency and reliability criteria, including those related to LID.

Optimize Manufacturing Processes

Improving overall manufacturing processes can contribute to lower LID. It involves a careful control of oxygen levels to ensure low oxygen contamination in the wafers, and optimization of temperatures and durations employed for silicon wafer annealing to reduce defect formation.

Block Diagram of the BO LID-Related Defect States and Processes
The BO LID-related defect states and processes. | Vaqueiro-Contreras M, Markevich VP, Coutinho J, et al. Identification of the mechanism responsible for the boron oxygen light induced degradation in silicon photovoltaic cells. Journal of applied physics. 2019;125(18). doi:https://doi.org/10.1063/1.5091759

Go For n-Type Solar Panels

n-type wafers typically benefit from certain manufacturing processes that can result in lower oxygen content, thereby minimizing defect formation. Additionally, n-type panels are doped with phosphorus, which does not form similar defect complexes with oxygen as in p-type panels. These properties largely avoid the main cause of LID in p-type panels.

Moreover, the minority carrier lifetime in n-type silicon is inherently higher compared to p-type silicon. This also contributes to less recombination losses and fewer defects.

Implement Advanced Passivation Technologies

Passivation technologies help to stabilize the silicon surface and reduce recombination losses. Their implementation offers a robust pathway to mitigating LID in solar panels.

By integrating with high-quality surface passivation through materials like silicon nitride (SiNx), silicon oxide (SiO2) and aluminum oxide (Al2O3), and leveraging advanced cell architectures like PERC, PERT, TOPCon and HJT, panel manufacturers can significantly enhance the stability, efficiency and longevity of their products.

Are Thin-Film Solar Panels Susceptible to LID?

Thin-film solar panels are largely immune to traditional LID effects seen in crystalline silicon panels. This immunity is attributed primarily to the absence of B-O complex formation.

However, thin-film panels can still exhibit other forms of degradation.

For example, amorphous silicon (a-Si) thin-film panels can experience another form of light-induced degradation known as the Staebler-Wronski effect. It usually stabilizes after the initial light exposure and generally does not degrade further.

CdTe and CIGS thin-film panels are inherently less prone to light-induced degradation observed in c-Si panels. However, they still experience other forms of degradation over time.

Conclusion

LID appears to be an inherent characteristic of c-Si solar panels, particularly the p-type.

After the initial period, the power loss of panels caused by LID can still accumulate to a not-small figure. The industry continues to make efforts to minimize its impacts through multi-faceted approaches.

While often not explicitly stated in a solar panel's specification manual, manufacturers are responsible for providing clear and transparent information about their panels' LID specifics.

Solar business owners and project developers are recommended to keep an eye on latest advancements in LID mitigation technologies in order to provide more robust panel performance.

  1. Markevich VP et al. Boron–Oxygen Complex Responsible for Light-Induced Degradation in Silicon Photovoltaic Cells: A New Insight into the Problem ↩︎

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