Since about the mid-1980s, engineers have been actively working on the development of technology for reflecting the sun’s rays from the back surface of photovoltaic panels. This would significantly increase the absorption of light without increasing the cost of manufacturing the cells. Unfortunately, the technologies of that time did not allow achieving the desired result. And the passivated emitter of the rear part of solar cells (PERC – Passivated Emitter Rear Cell) was created only in the 21st century.
PERC technology design solution
The difference between the structure of the classic and the modified cell is as follows:
- Traditional photocell. The back of the silicon base is completely aluminum coated. It plays the role of a current collector, and there is no gap between the semiconductor and the metal.
- PERC element. It is not two, but three-layer. A dielectric with microscopic holes burned out by a laser is placed between the silicon “base” and the aluminum contact. This middle layer increases the absorption of long-wave radiation due to its multiple reflections inside the photocell.
Benefits of PERC technology
This design has not one, but several advantages at once.
1. Increase in absorption of photons.
- The first advantage over the classic solar battery is the already mentioned increase in energy absorption, which leads to an increase in specific efficiency. From the point of view of physics, this is explained as follows.
- Short-wave radiation is absorbed mainly by the front, frontal part of the photocell. Long-wave – back, back.
- The rest of the long waves pass through the entire cell, and then are completely absorbed by aluminum.
- The presence of an intermediate dielectric layer prevents long wavelength photons from “leaving” and reflects them back to the front side.
In practice, this allows you to increase the performance of the solar battery in low light conditions – in the morning, in the evening and in cloudy weather. “Short” photons in such conditions are absorbed by the atmosphere almost completely, and “long” ones are captured by traditional modules ineffectively.
The graph below clearly shows the difference in the level of absorption of wavelengths from 1000 to 1200 nm for both types of solar cells – standard and PERC. In the second, it is noticeably higher.
2. Reducing the temperature of the modules
The second difference of the advanced technology is the back reflection of beams longer than 1180 nm. In classic types of photovoltaics, these photons are completely absorbed by the aluminum and heat it a few degrees higher than the PERC cells. This feature increases the effective efficiency by 0.5-1.5%.
3. Ejection of electrons into the p / n transition region
In addition to increasing the number of generated electrons, PERC technology also helps to reduce the amount of “lost” charges. This phenomenon occurs in traditional cells because electrons move chaotically, and some of them are absorbed by the aluminum current collector before they enter the p / n junction zone.
In advanced panels, these charges undergo a process of recombination, or return. Dielectric “foam” throws them into a region where each electron can find a “hole” and participate in the photoelectric effect. It also contributes to the increased efficiency of PERC solar panels. As a result, panels with the presence of a dielectric “gasket”, at almost the same cost, show an efficiency of 24% where a similar “classic” produces no more than 21-22%.