There are three main reasons why the dielectric passivation layer contributes to the increase of efficiency:
The extra dielectric passivation layer reduces electron recombination:
Electron recombination is the tendency of electrons to recombine and basically block the electrons from freely flowing through the solar cell, which means it can’t reach its potential efficiency.Electrons generated near the back of the solar cell are now free to move up to the emitter and contribute to more electrical current.
The extra dielectric passivation layer increases the solar cell’s ability to capture light:
The dielectric layer reflects the light that passes through the solar cell without generating any electrons. By reflecting this light, the photons are given more opportunity to generate electrical current.
The extra dielectric passivation layer reflects wavelengths above 1180 nm out of the solar cell, that would normally create heat:
Silicon wafers stop absorbing wavelengths above 1180 nm. In normal solar cells, such wavelengths are easily absorbed by the backside metallization and turned into heat.
Comparison PERC solar cell and standard solar cell
As you know, heat reduces the solar cell’s conversion efficiency. The dielectric passivation layer reflects wavelengths above 1180 nm out of the solar cell, and helps the solar cell to work more efficiently by maintaining cooler temperatures.
A conventional crystalline silicon (c-Si) solar cell consists of two layers with different electrical properties. The two layers are called the base and the emitter. The point where the base and emitter meet is called the interface.
An electrical field is generated where the two layers touch – this point is called the interface. The interface pulls negatively charged electrons into the emitter once reaching the interface. When light enters the solar cell, the electrons are released from the silicon atoms.
When electrons are released, they can travel freely through the silicon wafer. However the electrons will only contribute to the electrical current if they reach the interface, between the emitter and the base..
Once electrons reach the interface, they’re pulled into the emitter and will create a voltage difference over the solar cell.
1 Light enters the solar cell and peels off electrons from the silicon atoms.
2. The Electrons move freely through the silicon wafer, and can now move up to the junction, between the base and emitter.
3. If the electrons make it to the junction, between the base and emitter, they have pulled into the emitter. This creates a voltage difference over the solar cell. Working of a solar cell
Shorter wavelengths (blue light) mainly generate electrons near the front of the solar cell, while the longer wavelengths (red light), will generate electrons at the back of the cell. Some of the longer wavelengths will pass through the wafer without generating any current. This is where the dielectric layer on the back of the solar cell makes the difference..
The sun emits light in different wavelengths and when the light reaches the silicon cell structure, it generates electrons at various levels of the solar cell structure.
PERC technology increases the cell’s ability to catch longer wavelengths. The longer wavelengths are especially present during mornings and evenings (sun under an angle) or during cloudy days.
Blue light, with shorter wavelengths, is being absorbed by the atmosphere during these times, as it has to travel a longer path to reach the Earth’s surface. Red light is less easily absorbed by the Earth’s atmosphere.
So the main reason why PERC technology shows better energy yields is the reflective dielectric layer on the back of the solar cells which helps to absorb more red light, even during the morning, evenings or during cloudy weather.