Irrespective of their mechanics and manufacturing solar panels have become susceptible to degradation due to their own manufacturing materials. Yes, the material used to dope the wafers is the main cause behind light induced degradation occurring in solar panels. However, some tests during the manufacturing process may reduce it. Which is why is it crucial to test panels for LID. There is an unidentified degradation also that you may have heard about the LeTID degradation.
What is Light Induced Degradation? What is LID?
A loss of performance of solar modules happening mostly in the initial sunrise hours is known as light induced degradation (LID). The real and overall performance of solar modules is also affected by this. Most silicon solar cells are affected by this defect, causing severe loss in power generation. Solar modules show signs of light-induced degradation within a few days of their installation. The percentage of loss can be between 0.5% and 1.5%.
However, not all modules are influenced in the same manner. The thing that differentiates it the most is the crystal structure of solar cells. Namely, monocrystalline or polycrystalline along with their electric properties whether they are P-type or N-type.
1. Solar Cell Structure
Different crystal structure means different production processes of solar cells.
a) Monocrystalline: These cells are formed with the Czochralski process, which produces a uniform crystal structure that is sliced to form solar cells. These solar cells are highly efficient and have higher oxygen concentrations.
b) Multicrystalline: They are produced with a form of vapor deposition that grows silicon into a substitute. There are many crystalline sections that show up as different reflective edges in a solar panel. They are less efficient with less oxygen concentration.
2. Electric Properties of Silicon Wafers
They refer to the properties of silicon wafers that are required to create a voltage difference in a cell when it is exposed to sunlight.
a) P-type: Such silicon wafers have impurities in controlled amounts that are referred to as doping materials. Such materials accept electrons readily, letting the photovoltaic module create a voltage difference for generating power under sunlight. Boron is most commonly used as their doping element, but some also use gallium.
b) N-type: They have impurities with opposite effects and rather than accepting they release electrons. There are no signs of light induced degradation in such n-type silicon wafers.
Also Read: What is Potential Induced Degradation?
What Causes Light Induced Degradation?
Solar cells are made up of silicon wafers and the formation of boron-oxygen compounds in these wafers results in light-induced degradation. Therefore, the presence of boron as raw material or coating material may result in light-induced degradation of solar panels.
Higher oxygen concentrations in monocrystalline solar cells are also a reason behind light-induced degradation. It takes place when oxygen concentration is higher than expected. This should have made you understand what causes light induced degradation.
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What is LeTID Solar and LeTID Degradation?
A novel degradation mechanism in monocrystalline silicon cells that has significantly longer timescales than the boron-oxygen degradation is termed light and elevated temperature-induced degradation (LeTID). This is more pronounced at higher temperatures.
1. Properties of LetID
- There is a reduction in effective minority carrier lifetime and efficiency
- The cycle of degradation and recovery takes years to decades.
- It leads to an identical effect in the dark which is referred to as carrier-induced degradation (CID).
2. Crucial parameters
- Defect strongly depends on the architecture of solar cells.
- Degradation is enhanced in Passivated emitter rear contact (PERC) cells.
- The position of wafers in the parent ingot, gettering process, along with the presence of grain boundaries also influence degradation.
- Thermal treatments significantly influence the process.
- Degradation reaction kinetics are impacted by dark annealing.
- The rate of degradation is altered by temperature firing.
- Surface passivation layers on hydrogen-rich passivated layers are hugely influenced.
3. Postulated causes
Metallic impurities like cobalt, copper, and nickel are possible causes for the degradation.
4. Mitigate
With increased cases of light and elevated temperature-induced degradation (LeTID), mitigation techniques are also proposed.
- Reduce firing temperature: It is highly recommended to modulate firing temperature with variations in cooling rates.
- Accelerating degradation: With accelerated degradation and a second firing step at a lower temperate is also suggested.
- Wafers: It is recommended to change wafer properties and thickness to reduce degradation.
What are the Causes of LeTID in Solar Panel?
The main reason behind leTID is yet unclear. But with continuous research, it has been made clear that oxygen levels are not responsible for the same. As per the current understanding, this degradation is a result of an interaction between passivation layers at higher temperatures during the firing process in manufacturing.
According to the conclusion of a study by Fraunhofer ISE and Freiberg Materials Research Centre, Germany, leTID is caused by mobile hydrogen reacting with intrinsic crystal defects, and its occurrence is influenced by carrier injection conditions and elevated temperatures.
In another research from 2017, by researchers of the University of Konstanz, the impact of temperature and doping on LeTID in PERC solar cells is calculated. They reached the conclusion that, with high temperatures, the pace of degradation also increases. Thus, determining that LeTID increases in strength with firing temperature and the presence of rich hydrogen layers also influence the same.
Also See: How to Connect 3 Solar Panels in Parallel
What is the Difference Between LID and LeTID?
After learning the causes of leTID in solar panels, let’s learn about the difference between LID and LeTID.
| Light Induced Degradation (LID) | Light And Elevated Temperature Induced Degradation (Letid) |
| The most commonly observed degradation | Not commonly observed |
| This occurs in p-type silicon solar cells that are doped with boron | This occurs in wafers with high operating temperatures |
| It occurs when oxygen is combined with boron | It occurs when a high operating temperature is paired with high light intensity |
| It is a fast degradation process | It is a slower process than LID |
| It occurs with the first exposure to sunlight and stays until power stabilizes | It occurs soon after the solar panels are installed but the process of occurrence and stabilization can take years |
Also See: How to Calculate Solar Panel Wattage?
What is Light Induced Degradation Test?
This test is usually carried out during the early stages of manufacturing of solar panels. To test solar panels for quality assurance and reliability, a light induced degradation test is a must. LED techniques, LID stabilization test, and electrical carrier injection are 3 testing techniques mainly used.
1. Performing the Test
A series of light exposure along with intervals of equal irradiance doses above 5 kWh/m square is used. By maintaining a constant temperature of 50° Celsius, the module performs at its Maximum Power Point (MPP) wherein the module is flashed after each interval.
In case, the power difference of the module in the last 3 flashes is smaller than the threshold value defined by the standard, stabilization is considered complete. With this, the accumulated total irradiance dose requirement is also measured.
If the above-mentioned parameters cannot be determined, the module will be put under test once again. If there is a 5% pass in the performance test after stabilization, it clearly means the module failed the test.
Also Read: How Many Volts Does a Solar Panel Produce?
Why is it Crucial to Test Panels for LID?
Solar panels with silicon solar cells mostly witness light induced degradation, particularly in PERC modules. The recombination of active defects during extra carrier injection by illumination results in a loss in the conversion and generation of electricity. It is crucial to test panels for LID because this helps to ensure the performance of modules is maintained throughout their life cycle.
Well, after learning about light induced degradation it is better to choose solar panels with proper certificates. It is better to ensure whether they are tested for LID and LeTID. Why is it crucial to test panels for LID? The answer to this is to maintain their life-long efficiency. Also, LetID degradation is a long-term damage influencing your solar panels thus it is necessary to mitigate the loss with proper measures.
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